The Ailao Shan-Red River shear zone (Yunnan, China), Tertiary transform boundary of Indochina
Philippe Herv6 Leloup a, Robin Lacassin a, Paul Tapponnier a, Urs SchS.rer b, Zhong Dalai c, Liu Xiaohan c, Zhang Liangshang b, Ji Shaocheng c,
Phan Trong Trinh d Laboratoire de tectonique et m£canique de la lithosphkre, URA 1093, CNRS, IPG-Paris, 4 place Jussieu, 75252 Paris Cedex 05, France
b Laboratoire de g£ochronologie, Universit£ Paris 7 and CNRS, lPG-Paris, 75252 Paris Cedex 05, France Institute of geology, Academia Sinica, PO Box 634, 100 011 Beijing, China
~1 Institute of geological Sciences, National center for natural sciences and technology, Nghia Do, Tuliem, Hanoi, Vietnam
Received 24 March 1994; accepted 19 June 1995
Abstract
The Red River Fault zone (RRF) is the major geological discontinuity that separates South China from Indochina. Today it corresponds to a great right-lateral fault, following for over 900 km the edges of four narrow ( < 20 km wide) high-grade gneiss ranges that together form the Ailao Shan-Red River (ASRR) metamorphic belt: the Day Nui Con Voi in Vietnam, and the Ailao, Diancang and Xuelong Shan in Yunnan. The Ailao Shan, the longest of those ranges, is fringed to the south by a strip of low-grade schists that contain ultramafic bodies. The ASRR belt has thus commonly been viewed as a suture.
A detailed study of the Ailao and Diancang Shan shows that the gneiss cores of the ranges are composed of strongly foliated and lineated mylonitic gneisses. The foliation is usually steep and the lineation nearly horizontal, both being almost parallel to the local trend of the gneissic cores. Numerous shear criteria, including asymmetric tails on porphyroclasts., C-S or C ' -S structures, rolling structures, asymmetric foliation boudinage and asymmetric quartz ( c ) axis fabrics, indicate that the gneisses have undergone intense, progressive left-lateral shear. P - T studies show that left-lateral strain occurred under amphibolite-facies conditions (3-7 kb and 550-780°C). In both ranges high-temperature shear was coeval with emplacement of leucocratic melts. Such deformed melts yield U / P b ages between 22.4 and 26.3 Ma in the Ailao Shan and between 22.4 and 24.2 Ma in the Diancang Shan, implying shear in the Lower Miocene. The mylonites in either range rapidly cooled to = 300°C between 22 and 17 Ma, before the end of left-lateral motion.
The similarity of deformation kinematics, P - T conditions, and crystallization ages in the aligned Ailao and Diancang Shan metamorphic cores, indicate that they represent two segments of the same Tertiary shear zone, the Ailao Shah-Red River (ASRR) shear zone. Our results thus confirm the idea that the ASRR belt was the site of major left-lateral motion, as Indochina was extruded toward the SE as a result of the India-Asia collision. The absence of metamorphic rocks within the 80 km long "Midu gap" between the gneissic cores of the two ranges results from sinistral dismemberment of the shear zone by large-scale boudinage followed by uplift and dextral offset of parts of that zone along the Quaternary Red River Fault. Additional field evidence suggests that the Xuelong Shan in northern Yunnan and the Day Nui Con Voi in Vietnam are the northward and southward extensions, respectively, of the ASRR shear zone, which therefore reaches a length of nearly 1000 km.
Surface balance restoration of amphibolite boudins trails indicates layer parallel extension of more than 800% at places where strain can be measured, suggesting shear strains on the order of 30, compatible with a minimum offset of 3(10 km
4 P.H. Leloup et al. /Tectonophysics 251 (1995) 3-84
along the ASRR zone. Various geological markers have been sinistrally offset 500-1150 km by the shear zone. The seafloor-spreading kinematics in the South China Sea are consistent with that sea having formed as a pull apart basin at the southeast end of the ASRR zone, which yields a minimum left-lateral offset of 540 km on that zone. Comparison of Cretaceous magnetic poles for Indochina and South China suggests up to 1200 + 500 km of left-lateral motion between them. Such concurrent evidence implies a Tertiary finite offset on the order of 700 + 200 km on the ASRR zone, to which several tens of kilometers of post-Miocene right-lateral offset should probably be added.
These results significantly improve our quantitative understanding of the finite deformation of Asia under the thrust of the Indian collision. While being consistent with a two-stage extrusion model, they demonstrate that the great geological discontinuity that separates Indochina from China results from Cenozoic strike-slip strain rather than more ancient suturing. Furthermore, they suggest that this narrow zone acted like a continental transform plate boundary in the Oligo-Miocene, governing much of the motion and tectonics of adjacent regions. 700 and 200 km of left-lateral offset on the ASRR shear zone and Wang Chao fault zone, respectively, would imply that the extrusion of Indochina alone accounted for 10-25% of the total shortening of the Asian continent. The geological youth and degree of exhumation of the ASRR zone make it a worldwide reference model for large-scale, high-temperature, strike-slip shear in the middle and lower crust. It is fair to say that this zone is to continental strike-slip faults what the Himalayas are to mountain ranges.
1. Introduction
Field studies of Quatemary faults and compila- tions of earthquake moment tensors help provide a more precise understanding of the active deformation of continents (e.g. Jackson and McKenzie, 1988; Armijo et al., 1989).
In Asia for example, the fact that slip rates on the largest active faults turn out to be on the order of centimeters per year (e.g. Armijo et al., 1986; Kidd and Molnar, 1988; Peltzer et al., 1989) implies that movements along a few narrow zones absorb much of the present-day convergence between India and Eurasia (e.g. Avouac and Tapponnier, 1992, 1993; Avouac et al., 1993). The increasing number of studies of active faults also leads to more quantita- tive bounds on the relative importance of shortening mechanisms such as strike-slip faulting and over- thrusting. The slip rates on faults in western Tibet and the Tien Shan, for instance, suggest that 30 -50% of the present-day convergence between India and Siberia is taken up by strike-slip faulting along the northern and southern edges of Tibet (Armijo et al., 1989; Peltzer et al., 1989; Avouac and Tapponnier, 1993).
In general, however, present-day tectonic styles and rates cannot be extrapolated for long into the past. Because the deformation of Asia started with the onset of collision, prior to -- 50 Ma (e.g. Besse et al., 1984; Patriat and Achache, 1984; Jaeger et al., 1989), instantaneous rates derived from earthquake moment tensors (e.g. Holt et al., 1991) concern at
most = 2 .10-4% of the collision span, while rates deduced from morpho-tectonic studies (e.g. Armijo et al., 1989) characterize only the last few percent of that span. A full, quantitative understanding of the finite strain induced by collision evidently requires analysis of pre-Quaternary movements and deforma- tion.
Although seafloor-spreading reconstruction (e.g. Molnar and Tapponnier, 1975; Patriat and Achache, 1984) and paleomagnetism (e.g. Achache and Cour- tillot, 1984; Besse et al., 1984; Besse and Courtillot, 1988) provide estimates of the convergence between India and Siberia since collision began (3500-2000 km), regional finite strains and offsets within the collision zone, whose surface area covers about l07 km 2, are still poorly known. Field studies of the styles, amounts, and ages of Cenozoic brittle and ductile deformation throughout that zone, coupled with more paleomagnetic studies such as those un- dertaken by various groups in the last decade (Achache et al., 1983; Achache and Courtillot, 1984; Enkin et al., 1992; Chen Yan et al., 1993; Gilder et al., 1993; Huang and Opdyke, 1993; Yang and Besse, 1993), are needed to discriminate between various finite deformation scenarios, among which the large-scale polyphase extrusion model proposed by Tapponnier et al. (1982, 1986) and Peltzer and Tap- ponnier (1988).
We present here a detailed study of Tertiary deformation and metamorphism along the Red River Fault zone (RRF), in Yunnan province (China), that sets preliminary results discussed by Tapponnier et
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6 P.H. Leloup et al./Tectonophysics 251 (1995) 3-84
al. (1990), Sch~er et al. (1990) and Zhong Dalai et al. (1990) on firmer ground. After an outline of the geologic and tectonic histories of regions surround- ing this great fault zone, we describe in detail the kinematics, P - T conditions, and timing of strain events along the Ailao Shan-Red River (ASRR) metamorphic belt. Our observations show, beyond doubt, that large-scale left-lateral shear followed by a reversal to right-lateral occurred along the ASRR zone in the mid-late Cenozoic. This brings support to the basic tenets of the two-phase extrusion model; namely, the early, collision-driven escape of In- dochina towards the SE, and the subsequent change to accommodate the present-day escape of Tibet and South China. We examine quantitative constraints on the amount of extrusion and rotation consistent both with the results of our field study and with South China seafloor magnetics (Briais et al., 1993). Fi- nally, we discuss the bearing of such constraints on the mechanical behavior of the continental litho- sphere.
2. Outline of the geology and tectonic history of Yunnan
Yunnan is the southernmost large province of China. Morphologically, it is a mountainous region that blends with the Tibetan highlands towards the northwest, where the elevation of many ranges ex- ceeds 4000 m. Towards the east it forms a broad plateau, about 1800 m high, with more gentle relief, that resembles that of Guizhou province. Geologi- cally, Yunnan straddles three of the main domains that Chinese geologists have long distinguished in
this part of Asia: the Yangzi paraplatform to the northeast, the Sichuan-Eastern Tibet (Songpan Ganzi, Sanjiang) fold belts, to the northwest, and the Indochina " b l o c k " or Sundaland to the south (e.g. Huang, 1960; Wang Hongzhen et al., 1985). The Red River Fault zone (RRF) separates the later block from the two previous regions, which belong to South China.
This zone has long been recognized as one of the main geological discontinuities of Southeast Asia (Fig. 1) (e.g. Huang, 1960; Huang, 1978; Hutchin- son, 1982; Wang Hongzhen et al., 1985). It follows the Ailao Shan-Red River (ASRR) metamorphic belt, that comprises four principal ranges, or massifs, in which mid-crustal rocks have been unroofed: the Ailao Shan, Diancang Shan, and Xuelong Shan, in Yunnan, and the Day Nui Con Voi in Vietnam (Fig. 2). The active Red River and Qiaohou faults (Fig. 5) also follow this zone, which extends more than 1000 km, from the Gulf of Tonghzing to the Hengduan mountains (Fig. 1). Farther north, towards Tibet, the NW-SE-str iking RRF zone bends clockwise, es- pousing the more northerly trend of the Sanjiang ("Three Rivers") mountain ranges (Fig. 2).
Any assessment of the nature and role of a zone of that length requires knowledge of the geological history of the regions that it crosses. In particular, it is important to understand how the zone relates to major regional tectonic episodes from the Precam- brian to the present, and what similarities and differ- ences in paleogeography or strain styles exist on either side of it along strike. Hence, before focussing on strain and metamorphism along that zone we broadly discuss the large-scale geology and tectonics of regions located north and south of it.
Fig. 2. Structural sketch map of Yunnan province and neighboring areas (box on Fig. 1), resulting from compilation of following maps: for Yunnan area, Geological Bureau of Yunnan Province (1979), Bureau of Geology and Mineral Resources of Yunnan (1983), Bureau of Geology and Mineral Resources of Yunnan Province (1990); for Tibet, Tang Yaoqing et al. (1988); and for surrounding regions, General Geological Department of the Democratic Republic of Vietnam (1973), Earth Sciences Research Division (1977). X.L.S, = Xuelong Shan; D.C.S. = Diancang Shan. Cities: B.Y. = Bao Yen (Vietnana); Ch. = Chengdu (Sichuan); C.M. = Chiang Mai (Thailand); Chu.= Chuxiong (Yunnan); C.R.= Chiang Rai (Thailand); D. = Deqen (Yunnan); D.B.P. = Dien Bien Phu (Vietnam); H. = Hanoi (Vietnam); J. = Jinghong (Yunnan); K. = Kunming (Yunnan); L. = Lijiang (Yunnan); Le. = Leshan (Sichuan); Li. = Litang (Sichuan); M. = Mandalay (Burma); Ma. = Markam (Tibet); My. = Myitkyina (Burma); Q. = Qamdo (Tibet); S. = Simao (Yunnan); T. = Tengchong (Yunnan); V. = Vientiane (Laos); X. = Xiaguan (Yunnan); Y. = Yuanmou (Yunnan). Boxes A, B and C indicate areas of Figs. 3a,b and 6, respectively.
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 7
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8 P.H. Leloup et al./Tectonophysics 251 (1995) 3-84
2.1. South China block: Yangzi parapla(form and southern end of the Sanjiang belt
2.1.1. Yangzi paraplatform The Yangzi paraplatform corresponds to the area
north of the Red River Fault zone and east of the Jianchuan fault (Figs. 2 and 3). High-grade metamor- phic rocks, whose protolith might include the base- ment of the Yangzi platform, are exposed only along the Yuanmou and Red River faults (Figs. 2 and 3) (Lu Liangzhao, 1989). Elsewhere, sediments ranging in age from mid-upper Proterozoic (Sinian) to Eocene, up to 10 km thick (Wang Hongzhen et al., 1985), cover the basement. Proterozoic and Paleo- zoic rocks crop out in two main areas, the Kunming high and the Lijiang mountains that flank the Chu- xiong Mesozoic basin to the east and west, respec- tively (Figs. 2 and 3).
2.1.1.1. Kunming high and Chuxiong basin. South of Kunming, the lowermost sediments are folded, schis- tosed Proterozoic shales, carbonates and volcaniclas- tics. They are unconformably covered at places by Lower Devonian conglomerates and sandstones, fol- lowed by the Upper Devonian, Carboniferous and Permian shallow-water carbonates that give rise to the south China tower karst (Geological Bureau of Yunnan Province, 1979; Karst Institute of Guilin, 1981; Bureau of Geology and Mineral Resources of Yunnan, 1983). The unconformity testifies to the proximity of the "Caledonian" belt of south China, whose existence is well established in Guangxi province where metamorphic Siluro-Ordovician schists are capped by old red sandstones (Karst Institute of Guilin, 1981). Near Kunming, basalt flows coeval with the Emei Shan basalts of Sichuan are interbedded with the Upper Permian limestones. The basalts are particularly abundant west of Kun- ming where they include thick pillow-lava units (e.g. north of Midu). Such volcanism indicates regional extension and localized rifting of the carbonate plat- form at the end of the Permian (e.g. Wu Genyao, 1993).
Mesozoic deposits cover much of the platform in Yunnan. Southeast of Kunming, Lower-Middle Tri- assic, mostly shallow-marine carbonates are gener- ally conformable on the Permian (Bureau of Geology and Mineral Resources of Yunnan, 1983). West of
Kunming, in the Chuxiong basin (Fig. 2), Upper Triassic gray sandstones and marls grade upwards into Jurassic and Cretaceous red sandstones. The lower red sandstones display decametric cross-bed- ding. Near Lufeng (Fig. 3), they contain fossil Lower Jurassic dinosaurs including the prosauropods Lufen- gosaurus and Yunnannosaurus (Sun and Cui, 1986). The youngest red beds, north of Chuxiong, are of Eocene age (Fig. 3) (Bureau of Geology and Mineral Resources of Yurman, 1983). To the west, the maxi- mum thickness of the Chuxiong Mesozoic-Tertiary continental sequence is about 6 km (Wang Hongzhen et al., 1985). In spite of the existence of conglomer- atic horizons, there is no clear evidence of an impor- tant regional unconformity within that sequence, whether in the field or on published maps (e.g. Bureau of Geology and Mineral Resources of Yun- nan, 1983). On the other hand, the Upper Triassic, the Jurassic and even the Eocene red beds are uncon- formable, often at a low angle, upon the Paleozoic and Proterozoic rocks of the Kunming region, which appears to have formed a broad N-S-trending high or horst since the lower Mesozoic. We conclude that, in Yunnan, the Yangzi platform remained essentially stable from the Upper Triassic to the lower Tertiary. Around Kunming, the basal unconformity of the Mesozoic-Tertiary continental sequence seems to be a distal onlap over a region mostly structured by late Paleozoic extensional block-faulting.
Cenozoic deformation has been strong. Regional, mostly NE-ENE-oriented shortening has folded the red beds over much of northern Yunnan after the Paleocene (Bureau of Geology and Mineral Re- sources of Yunnan, 1983; Tapponnier et al., 1986, 1990). Folds with NW-NNW-trending axes, clear on geological maps and satellite images, are well devel- oped in the Chuxiong basin (Fig. 3). In the western part of this basin, the NE-facing limbs of anticlines are often vertical or inverted, suggesting NE-verging thrust d6collements at depth (section Ib, Fig. 4). On retrodeformable sections drawn from the 1:500,000 map (Geological Bureau of Yunnan Province, 1979) and field observations, the corresponding shortening is moderate, on the order of 20% (e.g. 20-25 km along section I between Kunming and Midu; Fig. 4). The Tertiary fold axes bend counterclockwise to more westerly trends as they near the NW-SE-strik- ing Chuxiong-Nanhua fault (Tapponnier et al.,
14 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
1990), indicating drag by left-lateral strike-slip along that fault (Fig. 3). A steep, NW-NNW-striking, axial plane cleavage is often observed in the core of the anticlines (section Ib, Fig. 4). Locally, cleavage planes in flattened red shales bear a down-dip lin- eation, marked by green, slightly elongate reduction spots. The average elongation ratios of the spots, measured at one site west of Chuxiong, are 1.46 in the X / Y cleavage plane and 2.41 in the X Z plane ( X > Y > Z being the strain ellipsoid axes). Ap- proaching the Red River Fault zone from the north, the cleavage becomes stronger. North of Gasa, it grades from fracture to intense flow cleavage as the Mesozoic rocks become epimetamorphic slates. The cleavage, which is nearly vertical, bends counter- clockwise from a N170°E average strike to N150°E, parallel to the Red River Fault zone and to the foliation in the Ailao Shan gneisses, consistent with left-lateral movement on that zone (Tapponnier et al., 1990). At one site, about 15 km northwest of Gasa and 2 km northeast of the Red River fault, two steep superimposed cleavages may be observed in purplish, probably Triassic, slates and conglomer- ates. The first, nearly bedding-parallel cleavage (So+ ~) strikes N140°E, parallel to the Red River fault and bears a clear down-dip stretching lineation. The second cleavage (S 2) strikes N160 _+ 10°E, and defines sigmoidal lenses between the S0+ l planes, consistent with left-lateral shear on So÷ ~. These microstructures, which give the slates the appearance of C-S tectonites, seem to result from folding and flattening due to ENE shortening, giving rise to So+ ~, followed by left-lateral shear, also due to ENE shortening, giving rise to S 2.
Folds with NE-ENE-trending axes, almost per- pendicular to the predominant folds in the Chuxiong basin, also exist, mostly adjacent to the southern splays of the Xianshuihe fault (Xiaojiang, Anning He and Yuanmou faults; Figs. 2 and 5). North of Kun- ruing, such folds affect concordant Jurassic and Permo-Carboniferous beds. South of Kunming, steep, E-W- to NE-SW-striking cleavage is found locally
in the Mesozoic sandstones. Northwest of Yuanyang (Fig. 3) along the Red River, E - W cleavage in Paleozoic rocks is parallel to folded, steeply dipping Neogene conglomerates. We interpret these NE-SW to E - W folds and cleavage to result from late Neo- gene-Quaternary NNW-SSE shortening, compatible with left-lateral slip on the Xiaojiang fault, right- lateral slip on the RRF (e.g. Tapponnier and Molnar, 1977; Allen et al., 1984) (Fig. 5), and with the SSE-directed impingement of the Kunming- Chuxiong block against the northeastern edge of Indochina (the Red River Fault).
This region is presently undergoing active defor- mation, as attested by the fairly high level of histori- cal and current seismicity (Institute of Geophysics, 1976). Extant fault-plane solutions show that, as in the late Neogene, this deformation is consistent with roughly NNW shortening, but with roughly ENE extension as well (Fig. 5). Right-lateral slip occurred on the NW-SE-striking Qujiang fault during the Tunghai earthquake (M = 7.7, 4 / 1 / 1 9 7 0 ) (e.g. Tap- ponnier and Molnar, 1977) (Fig. 5a). The focal mechanisms of the 1966 earthquakes on the N-S- striking Xiaojiang fault (Fig. 5) imply left-lateral slip along it (e.g. Tapponnier and Molnar, 1977). A normal component of slip on the roughly N-S faults south of Kunming has created several Quaternary half-grabens, some of them filled by lakes (Fig. 5) (e.g. Tapponnier and Molnar, 1977; Allen et al., 1984).
2.1.1.2. Lijiang mountains. The Lijiang region, be- tween the Cheng Hai and Jianchuan faults (Figs. 3 and 6), is characterized by a thick sequence of Siluro-Ordovician-Triassic sedimentary rocks, mostly clastics and shallow-water carbonates with some interbedded Upper Permian basalts (Bureau of Geology and Mineral Resources of Yunnan, 1983). The lowermost part of the sequence is well exposed north of Lijiang, in the core of a N-S-trending antiform that rises to over 5000 m in the Yulong (5596 m) and Haba Xue (5396 m) Shan (Fig. 6).
Fig. 5. Active tectonics of Yunnan. (a) Map of active faults of Yunnan and surrounding regions based on LANDSAT and St,or satellite image analysis. Focal mechanisms are from Tapponnier and Molnar (1977), Le Dain et al. (1984), Wu and Wang (1988) and Chen and Wu (1989). (b) Interpretative sketch. Over much of Yunnan, active tectonics are compatible with = NNW-SSE-striking maximum stress (o" I ), and minimum stress (o" 3) striking --- ENE-WSW.
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 15
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Quaternary basin ~ ' ~ International border
16 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
There, along the narrow Tiger leap-gorge (Hutiao Xia), the Jinsha river (upper Yangzi), whose course must be antecedent to the growth of the antiform, has cut a ~ 3500 m deep "cluse" into the folded sedi- mentary sequence. Micaschists, derived from lami- nated, crossbedded sandstones and volcaniclastics (Siluro-Ordovician-Lower Devonian?), crop out in the deepest part of the antiform core. They are overlain by white, phlogopite-bearing marbles (Up- per Devonian?), in turn overlain by a series of black schists, light-gray calcschists and marbles containing horizons of greenschists and boudins of metabasalts and amphibolites (Carboniferous-Permian?). Most of the marbles and schists are sheared parallel to bedding, the bedding-parallel foliation being folded by the antiform (Lacassin et al., 1993a, 1995). A prominent stretching lineation, folded with the folia- tion, may be restored to a mean N40°E direction after unfolding about the antiform axis (N175°E). Shear indicators in either limb of the antiform indi- cate a top-to-the-SW sense of shear prior to folding. The phlogopite-bearing marbles yield a Rb /S r age of 35.9 + 0.3 Ma (2 o', isochron defined by several fractions of phlogopite and whole rock, initial 87Sr/86Sr ratio of 0.712) (Lacassin et al., 1995). This late Eocene age probably dates SSW-directed transport (in the present geographical coordinates) on an originally flat dEcollement, in all likelihood a result of the penetration of India into Asia (Lacassin et al., 1993a, 1995). The well-preserved antiformal shape and the elevation ( = 5600 m) of the Yulong mountain, which still stands high above the sur- rounding country, implies that the folding of that drcollement results from the same = ENE directed Tertiary shortening that folded the red beds of the Chuxiong basin (Fig. 3). Since transport on the drcollement predates the antiformal folding, ENE shortening must postdate 36 Ma in northeast Yun- n a n .
North of the Diancang Shan, most of the NNE- striking faults that dominate the crustal fabric appear to be presently active (Figs. 5 and 6), as attested by
the sharpness of their traces on satellite imagery and in the field and by the seismicity, which includes at least one great historical event (Yongsheng, 1515) (e.g. Institute of Geophysics, 1976; Allen et al., 1984; Wu and Wang, 1988). The faults have oblique, normal left-lateral slip (Tapponnier and Molnar, 1977; Allen et al., 1984) (Fig. 5a). Such movement probably results from bookshelf faulting due to pre- sent NW-SE right-lateral shear parallel to the Red River Fault (Fig. 5b). While ongoing movement on those faults is consistent with the currently = E -W regional extension, several of them appear to be former W-dipping thrusts related to the ENE, mid- Tertiary shortening phase. The most prominent of these former thrusts, the Chenghai Fault, places Per- mian volcanics on top of Jurassic sandstones (Bureau of Geology and Mineral Resources of Yunnan, 1983) (Figs. 2, 3 and 6). Inversion of these faults may account for their present-day listric geometry (Wu and Wang, 1988).
In spite of the apparent complexity of the Ceno- zoic deformation, however, and of stress changes in part related to large-scale clockwise rotations, Ter- tiary folding of the Yangzi platform appears to result mostly from two successive, nearly orthogonal phases of shortening, oriented ENE and NNW in the pre- sent-day geographic coordinates. These phases post- date deposition of the Paleocene red beds and of the Neogene conglomerates, respectively.
2.1.2. Southeastern Sanjiang and the Benzilan- Jinsha suture
West of the Jianchuan fault, northwestern Yunnan lies in the southern termination of the Sanjiang ("Three Rivers") mountain belts of western Sichuan and southeastern Tibet. The belts, which strike roughly N-S, channel the flow of the Jinsha, Lan- cang and Nu Jiang (Yangzi, Mekong, and Salween rivers, respectively) (Figs. 2 and 6). The high, rugged relief indicates that the Cenozoic deformation of this area, which lies only --- 500 km from the eastern end
Fig. 6. Geological map of Sanjiang area (box C in Fig. 2), from our fieldwork and compilation of the following maps: Geological Bureau of Yunnan Province (1979), Bureau of Geology and Mineral Resources of Yunnan (1983), Yan et al. (1985), Bureau of Geology and Mineral Resources of Yurman Province (1990). Box corresponds to Fig. 30a. S, T and U refer to the Jianchuan-Madeng (Fig. 7a), Judian-Weixi (Fig. 7b), and Benzilan-Deqen (Fig. 8) traverses, respectively.
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 17
18 P.H. Leloup et aL / Tectonophysics 251 (1995) 3-84
of the Himalayan range (Fig. 1), has been particu- larly strong (e.g. Wang and Chu, 1988). The regional crustal fabric is characterized by NNW-SSE- to N-S-striking faults bounding long and narrow tec- tonic stripes that involve mostly Triassic or late Paleozoic rocks (Figs. 2 and 6) (Bureau of Geology and Mineral Resources of Yunnan, 1983). Southwest of the Jinsha river, these roughly parallel fault-stripes bend or abut against the NW-SE Red River Fault zone, whose extension is marked here by the active Qiaohou fault (Allen et al., 1984). This latter fault may be followed northward from the northern tip of Diancang Shan to Weixi (Fig. 6), and farther to Deqen, near the border between Yunnan and Tibet (Bureau of Geology and Mineral Resources of Yun- nan, 1983) (Figs. 2 and 6). The N-S fault-stripes are cut at places by shorter NW-SE-striking faults. One such fault (the Zhongdian fault) extends from Deqen to the southeast of Zhongdian (Figs. 5 and 6). The city of Zhongdian is located in a Quaternary basin on a right step along that fault (Figs. 5 and 6) implying that the fault may play a role analogous to that of the Poqu fault, as part of a right-lateral en 6chelon array connecting the Karakorum-Jiali fault zone of south- ern Tibet (K.J.F.Z., Fig. 1) to the Red River Fault (Fig. 1) (Armijo et al., 1989).
Three roughly E - W traverses (Jianchuan-Lanp- ing, Judian-Weixi, and Benzilan-Deqen sections; S, T, and U on Fig. 6, respectively), described below, throw light on the stratigraphy and structure of the tectonic stripes located between the Jinsha and Lan- cang rivers.
The Jianchuan-Lanping section (S on Figs. 3, 6 and 7a) starts in a = 2000 m thick series of conti- nental molasse of mid-Upper Eocene age, overlying Lower Eocene red beds (Bureau of Geology and Mineral Resources of Yunnan Province, 1990). The molasse, composed of massive polymict conglomer- ates with well rounded cobbles (limestones, red sandstones, volcanics, etc.), is intruded by alkaline, subvolcanic monzonites (Fig. 6) (Bureau of Geology and Mineral Resources of Yunnan, 1983). Titanite and zircon fractions in one intrusion yield an U /P b cristallisation age of 35 ___ 0.1 Ma (SchSrer et al., 1994). The conglomerates, which have been uplifted by the W-dipping normal fault that bounds them to the west (Figs. 3, 6 and 7a), form much of the high mountain west of Jianchuan (Laojun Shan, 4247 m).
They are only gently warped and rest unconformably on all other rocks, including metamorphic Paleozoic schists to the northeast. To a degree, they resemble the roughly coeval Kailas-Hemis-Noijinkangsang conglomerates of the Indus and Zangbo valleys in southern Tibet. They are the oldest, thickest coarse Cenozoic conglomerates we have seen in Yunnan. Elsewhere, similar rocks are of Neogene age. Impor- tant tectonic movement thus occurred prior to the Upper Eocene in this part of Yunnan.
The section continues westwards across folded, mostly W-dipping Triassic rocks. The rocks include green-purple pelites and sandstones with interbedded horizons of green dacitic and pink rhyolitic flows and tufts, overlain by thinly bedded, fossiliferous (Halobia sp.) limestone breccias and black shales, followed by more massive greywackes, dark gray limestones and finally epimetamorphic calcschists. The characteristic association of purple Triassic pelites and green dacites observed here is also found near Luchun, south of the southeasternmost part of the Ailao Shan (Fig. 3). On the whole, this Triassic sequence also resembles that seen in northern Thai- land. The rocks apparently belong to the Weixi group (Jen Chi Shuen and Chu Ching Chuan, 1966). That group, of Lower-Middle Triassic age (up to Carnian), is chiefly composed of folded and slightly metamorphic shales and calcschists capped by lime- stones and sandstones of Norian age. Where well developed, the cleavage in the Triassic rocks strikes = N-S, consistent with roughly E - W shortening. This cleavage becomes particularly strong in the steep, SSE-striking epimetamorphic calcschists just east of the active Qiaohou fault. Close to the fault, the cleavage is folded and affected by numerous kink bands. By contrast, although they dip steeply, the greenish, fossil-plant beating sandstones and lignite beds that fill the small Neogene Madeng basin just west of that fault (Figs. 3 and 7a) show no cleavage at all. West of this Neogene basin, the section ends near Lanping in the tightly folded, Jurassic-Creta- ceous to Lower Eocene limestones and red beds of the Yangbi basin. The NNW-striking fold crests that form the summits of the Xuebang Shan (4295 m) west of the Madeng basin (Fig. 6) imply that these rocks have suffered strong Tertiary shortening in an ENE direction, in present-day geographic coordi- nates.
P.H. Leloup er al . / Tectonophysics 251 (/995) 3-84 19
The Judian-Weixi traverse (T on Figs. 6 and 7b) first crosses several kilometers of a monotonous sequence of gray, quartz-rich schists (Cambro- Ordovician-Devonian; Bureau of Geology and Min- eral Resources of Yunnan, 1983). The schistosity strikes N-S and dips 20-50°W in general. The schists include pyrite nodules, boudinaged quartz veins and a few tens of meters of fine-grained, leucocratic gneisses (probably deformed acidic- volcanic tuffs) with SSW-plunging lineation, and top-to-the-SSW shear senses, as in the Yulong Shan d6collement. Westwards, the schists grade into darker carboniferous slates (Bureau of Geology and Mineral
Resources of Yunnan, 1983), bounded to the west by a prominent, = N10°E-striking, W-dipping fault (Fig. 7b). The fault has a sharp morphological trace and dams east flowing rivers, creating a series of small Quaternary basins clear on LANDSAT images and air photographs. We interpret this fault to be a Quater- nary, apparently active, normal fault (Fig. 5), north- ward extension of the fault bounding the Eocene molasse farther south (Figs. 6 and 7). Folded, steeply N-dipping, N75°E-striking greenschists, calcschists and thinly bedded limestones, crop out west of the fault. These rocks, which may be of Permian or Lower Triassic age, are intruded by an undeformed,
Section T / SW / E NE
Mekong Weixi 1 / / ~ / ~ 2-3 | Xuelong Shah I ~ ~ Judian
m d x \®~ ? , / / ~ (Yank_
4000 ®~ \ ~'~ II I I / / ~ ~ooo t00oO E
, , k m , L ,,ass,c _/ ,- , / ®
W Section S / j ' E
/ L aojun Jianchuan Oiaohou Fault zone / <4:~,~nm) basin
Madeng I / i I bas,n j I I J m
r t I 1 I ~sooo ,~p , / / 4000
. . . . . , , I ~ ' / / -/'1 ' l \ /71/ ~ _11000
Triassic--~// (~
N.oQ.neand U ..sozoc Voc.ncs I .O.von.n.nd Quaternary to Tertiary red ~_. .~Limestone carboniferous Eocene to beds J ̂ ^ ^ ^JDacite Triassic Sandstone Schists,Gneisses,Marbles
Fig. 7. Geological traverses. (a) Jianchuan-Madeng traverse (S on Fig. 6). (b) Judian-Weixi traverse (T on Fig. 6). Geology from field observations and topography from Operational Navigation Charts at a scale of hi,000,000.
20 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
coarse-grained granite. The western rim of this gran- ite intrudes steeply dipping, massive volcanic rocks including both mafic (dolerites, spilites, and vertical, flattened pillows that strike N110°E) and acidic terms (rhyolites). These bimodal volcanics occur in a sedi- mentary sequence that comprises folded limestones, red sandstones and purplish pelites (Bureau of Geol- ogy and Mineral Resources of Yunnan, 1983). Like that on the Jianchuan-Lanping traverse, this se- quence probably belongs to the Triassic Weixi group. The section ends in the Neogene Weixi basin, whose western limit is an E-dipping Quaternary normal fault, bounding the metamorphic core of the Xuelong Shan. The structure and deformation of that range, northernmost gneiss massif along the Red River Fault zone, will be described later.
Between the Mekong and Deqen, section U2 (Figs. 6 and 8) crosses a sequence reportedly Permo-Triassic in age (Bureau of Geology and Min- eral Resources of Yunnan, 1983) that comprises flyschoid schists with volcanic rocks of andesitic affinity (including agglomerates and breccias) and typical flyschs. The rocks display steep cleavage and are cut by brittle fault zones. Near Deqen, a nearly vertical N-S-striking fault zone juxtaposes kilomet- ric slices of schists, calcshists, deformed volcanic rocks, micaschists and gneisses, red-purple pelites, and granodiorite, separated by gouge and cataclasite (Fig. 8b). The fault zone, which is a few kilometers across, forms the western boundary of a 5-10 km wide granodiorite massif (Baimaxue Shan, 5137 m; Figs. 6 and 8). Farther east, flyschoid rocks and thick volcanics including pillow lavas are overlain by Tri- assic limestones. The whole assemblage is strongly folded and unconformably capped by red conglomer- ates and sandstones of probable Eocene age (Fig. 8b) (Bureau of Geology and Mineral Resources of Yun- nan, 1983).
Still farther east (section U1 on Figs. 6 and 8), near Benzilan, an outstanding ophiolitic melange is exposed in a N-S-striking, 3 -4 km wide zone. The overall dip of the zone is to the W. From E to W (i.e. from bottom to top) one finds deformed gabbros, a tectonic melange of mafic volcanics and limestone slices, red or purple cherts, serpentinized harzbur- gites, more limestones alternating with mafic vol- canics including pillows encrusted by cherts, and finally flyschoid sandstones (Fig. 8a). The rocks
form slivers separated by cataclastic zones that lo- cally show normal/right-lateral senses of shear, par- ticularly in the serpentinized ultramafics. East of Benzilan, a strongly deformed, steep sequence of marbles and calcschists with interbedded horizons of flattened pillow basalts, greenschists and acidic tuffs, probably Permian in age, is exposed along the Jinsha river. The rocks display bedding-parallel foliation, with a = N-S strike, dips ranging between 60 and 80°W, and a strong down-dip stretching lineation. West side-up shear senses imply that this zone was a major ductile E-directed thrust (Fig. 8a).
The Benzilan-Deqen section thus clearly cuts a major suture zone. To the east, one finds the de- formed passive margin of the Yangzi platform with the characteristic pillow basalts associated with Per- mian extensional faulting. To the west, one finds an active margin, complete with the effusive and intru- sive rocks characteristic of a calcalkaline volcanic arc and the flyschoid sediments apparently deposited in the back and fore-arc basins. Dismembered slices of a complete ophiolitic sequence attest to the former presence of deep sea floor. The eastward vergence of the thrust affecting the cover of the Yangzi platform is consistent with collision following W-directed subduction under the arc (in the present geographic coordinates), in keeping with paleogeographic evi- dence (e.g. SengiSr, 1984). Final suturing apparently occurred in the Lower Triassic (Liu Baotian et al., 1983). Subsequent shortening and shearing of the suture zone, likely in the Tertiary, could have formed the N-S-striking dextral cataclastic zones along it. The Benzilan suture zone appears to represent the southernmost extension of the Jinsha suture, which separates the Qiangtang block from the Song Pan terrane in western Sichuan and Tibet (e.g. Chang Chenfa et al., 1986; Mattauer et al., 1992) (Fig. 1).
Between Benzilan and Deqen (Fig. 2), small folds and thrusts cut a mid-Late Pleistocene colluvial apron. The thrusts dip 25-50 ° to the northwest and strike between 40 and 90°E, consistent with NNW, late Quaternary shortening, as observed elsewhere in Yunnan (Fig. 5).
2.2. Indochina and the "lndosinian" sutures
Blocks and terranes located south of the Red River Fault zone form the Indochina "Block" or
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 21
5000- m
2000-
O-
Section U2
W Baimaxue Shan E Deqen granodiorite
iekona I I Baimaxue Shan I I , - r ' I j 51~rm ~ I I
l i ~ P i l l ° w basalts interbedded / = K : ~ Cherts with cherts I ~ .-- [ "11 ~llBasalts ) ~._o I x x x IGabbros t ~
Peridotltes ~ alO
2000m i 3 1 9
Fig. 8. Geological traverse of Sanjiang belt between Yangzi and Mekong at = 28°20'N. (a) Traverse UI of Fig. 6 showing Benzilan ophiolites. (b) Traverse U2 of Fig. 6. Geology from field observations and topography from Operational Navigation Charts at a scale of 1:1,000,000.
Sundaland (Fig. 1) (Hutchinson, 1982). As on the western Yangzi platform, Mesozoic continental red beds cover older sediments and basement over wide areas. West of the Mekong river (Lancang Jiang) a large granite batholith (the Lincang batholith), dated at 270 + 59 Ma (Rb/Sr on whole rock; Liu Chang- shi et al., 1989) intrudes a N-S-striking belt of micaschists, metavolcanics and gneisses, about 60 km wide and 400 km long (Figs. 2 and 3). The rocks are interpreted to represent the northern extension, in Yunnan, of the metamorphic and granitic belt of northwestern Thailand (Figs. 1 and 2) (Academy of Geological Sciences of China, 1975; Department of Mineral Resources, 1982; Hutchinson, 1983). In
Thailand, the granites are = 200-240 Ma (Rb/Sr on whole rocks; Beckinsale et al., 1979). North of the Simao basin, micaschists and quartzites, intruded by a granite, form the core of the Wuliang Shan dome, northernmost prong of the Lincang belt (Fig. 3). Permo-Carboniferous carbonates, and Lower Tri- assic limestones and volcaniclastics crop out south of the Ailao Shan, in the core of anticlines near Pu Erh, and along the Mekong valley (Fig. 3) (Bureau of Geology and Mineral Resources of Yunnan, 1983). Undeformed pillow-lavas, of probable Permian age, are well exposed near Mojiang.
The Mesozoic red beds of the Simao basin, whose ages range from Upper Triassic to Eocene, reach a
thickness of 8 km (Wang Hongzhen et al., 1985). They are affected by folds and thrusts that trend NNW-SSE (Figs. 3 and 4). The thrusts are particu- larly well developed in the eastern part of the basin, between Simao and the Ailao Shan (Fig. 3). The folds appear to be ramp anticlines on generally W- directed thrusts (Fig. 4) and usually correspond to topographic highs, which suggests fairly recent fold growth. NW-SE- to E-W-striking strike-slip faults branch on the thrusts, forming lateral ramps. The fact that anticline cores expose Jurassic red beds near Simao and Permo-Carboniferous limestones more to the east (Fig. 3), suggests progressive eastward deep- ening of a regional drcollement. Near Simao, red Upper Cretaceous-Paleocene shales lie in the hinges of synclines (Bureau of Geology and Mineral Re- sources of Yunnan, 1983). From first-order retrode- formation of sections H and III, shortening and W-directed transport of the Mesozoic cover appear to be on the order of = 20 km (24%) and = 40 km (33%), respectively (Fig. 4). In the western part of the Simao basin, the vergence of folds and thrusts is opposite, toward the east. At the southern boundary of the basin, near Jinghong, the Lincang batholith is offset at least 40 km by NW-SE left-lateral faults (Fig. 3) (Bureau of Geology and Mineral Resources of Yunnan, 1983).
Ultramafic bodies and rocks with ophiolitic affini- ties are exposed along several narrow belts within the Indochina "block". They have generally been interpreted to mark suture zones (e.g. Hutchinson, 1975; SengiSr, 1984, 1987; Hutchinson, 1989), the Song Ma zone, in Vietnam, and the Uttaradit zone, in Thailand, being the most prominent (Fig. 1).
In north Vietnam, about 120 km south of the Red River, serpentinized gabbro-dolerites, harzburgites and dunites from which chromite is mined, mark the NW-SE-striking Song Ma suture zone for at least 170 km (Figs. 1 and 2). The ultramafic rocks, which appear to have been thrust northwards, follow a belt of metamorphic, reportedly Paleozoic rocks. South of that belt, reportedly Siluro-Devonian flyschoid shales and acidic volcanic tufts are intruded by granodiorites and granites. The age of the Song Ma suture zone is unknown, even though the paleogeog- raphy may be taken to imply SW-directed subduc- tion (in the present geographic coordinates) possibly in the mid-Paleozoic. The existence of a regional
Upper Carboniferous unconformity (Deprat, 1914) might suggest an upper Paleozoic collision of this active margin (Khorat-Kontum block) with the South China block (Fontaine and Workman, 1978; Helm- cke, 1985; SengiSr et al., 1988). North of the Song Ma suture, however, in the Song Da belt, Permian basic volcanics, Triassic shales and marine lime- stones, as well as Cretaceous red beds are tightly folded (General Geological Department of the Democratic Republic of Vietnam, 1973), which im- plies significant crustal shortening in the Mesozoic and Cenozoic.
In northeastern Thailand, ultramafic (serpentines, peridotites and pyroxenites with talc, magnetite and chromite pods) and mafic rocks (metagabbros, am- phibolites, and metavolcanics of basaltic-andesitic composition) are exposed for ~ 150 km along the NE-SW-striking Nan-Uttaradit suture zone (Barr and Mac Donald, 1987) (Figs. 1 and 2). Northeast of Uttaradit, these rocks, together with micaschists and black calcschists, form meters thick tectonic slivers separated by nearly vertical, NE-SW-striking, right- lateral shear zones. Blueschists, of epidote-blueschist facies, crop out a few kilometers to the west (Barr and Mac Donald, 1987). Along the southeast side of the zone, ultramafic rocks and sericite schists with large lenses of deformed andesites and polymict conglomerates, appear to have been thrust southeast- wards onto a zone of steep, folded flyschs that locally display the typical Bouma sequence. The Nan-Uttaradit suture zone may continue into Laos to Luang Prabang and farther towards the NE, roughly parallel to the Dien Bien Phu strike-slip fault (Figs. 1 and 2) (e.g. SengiSr and Hsii, 1984; Tapponnier et al., 1986, Hutchinson, 1989).
Whatever its northwards extension, that suture is usually interpreted to mark an important separation between the Kontum-Khorat and the Shan-Thai blocks. To the northwest, folded sedimentary and volcanic series include fossiliferous Lower Permian to Carnian-Norian shales, with sandstone and lime- stone intercalation, unconformably covered by Upper Triassic-Lower Jurassic acidic to intermediate vol- canic effusives, tufts, and volcaniclastics (Hess and Koch, 1975; Baum and Hahn, 1977; Hahn and Siebenhi~ner, 1982). These series have suffered at least two strong phases of shortening. One took place before the Upper Triassic, and the other, responsible
P.H. Leloup et al . / Tectonophysics 251 (1995) 3-84 23
for relatively tight folds with axes trending N-S to NE-SW (Baum and Hahn, 1977; Department of Mineral Resources, 1982; Hahn and Siebenhilner, 1982) after the Jurassic-Cretaceous. The regional geology, and the fact that the Permo-Triassic Thai- Lincang granodiorite and the Nan-Uttaradit zone parallel each other (Fig. I), has led most authors to interpret the southeastern margin of the Shan-Thai block as the site of northwest directed subduction in the upper Permian (in the present geographic coordi- nates) (e.g. Hutchinson, 1989; Barr et al., 1990, SengiSr, 1984), followed by collision with the passive margin of the Kontum-Khorat block in the Middle Triassic (e.g. Bunopas and Vella, 1983; SengiSr and Hsii, 1984). Helmcke (1985), however, among oth- ers, inferred that the collision occurred earlier. The fact that similar uppermost Triassic sediments and volcanic rocks lie unconformably on more deformed Paleozoic schists on either side of the Nan-Uttaradit zone (Baum and Hahn, 1977; Department of Mineral Resources, 1982; Hahn and Siebenhi~ner, 1982) sup- ports the former view.
Small outcrops of reportedly ultramafic rocks are found at three places west of the Lincang batholith between Changning and Menglian in Yunnan (e.g. Bureau of Geology and Mineral Resources of Yun- nan, 1983; Zhang Qi et al., 1985) (Figs. 2 and 3). These rocks, which are associated with Early Devo- nian-Middle Permian cherts and limestones, have been interpreted to outline a W-verging suture of Paleotethys, between the Bao-Shan and Simao blocks (Wu Haoruo et al., 1994). According to the latter authors, this suture continues into northwestern Thailand near Chiang Mai and Chiang Rai where basaltic rocks and, locally, ultramafics occur within the Thai-Lincang granitic belt (Department of Min- eral Resources, 1982; Barr et al., 1990) (Fig. 2). On a petrological and geochemical basis, however, Barr et al. (1990) inferred that the Chiang Mai/Chiang Rai mafic and ultramafic rocks should be interpreted as the remnants of a back-arc basin located west of the "Nan-Uttaradit subduction".
Toward the west, the Shan-Thai block is limited by Cenozoic fault zones: the Indoburman-Myitkyina suture, the Shan scarp, and the active Sagaing fault (e.g. Le Dain et al., 1984; Armijo et al., 1989) (Fig. 1). Southeast of the Nan-Uttaradit suture zone and Dien Bien Phu fault, the Khorat plateau undoubtedly
remained more stable, particularly since the Upper Triassic. In the Upper Carboniferous and Pex~ian, normal faulting affected the basement of the Khorat plateau (Mouret, 1994). Along the northeastern edge of the plateau, SW-verging thrusts, clearly imaged on seismic reflexion profiles (Mouret, 1994), affect Upper Carboniferous-Lower/Middle Triassic: sedi- ments and are unconformably capped by Upper Tri- assic rocks (Indosinian orogeny). During most of the Mesozoic and Paleocene, the Khorat basin formed a broad depocenter, comparable to the Sichuan basin, in which a thick sequence (up to 4500 m; Hutchin- son, 1989) of mainly continental sandstones accumu- lated. That sequence was subsequently folded, proba- bly in the Tertiary, into brachy anticlines and syn- clines oriented either roughly parallel to the NE-SW Nan-Uttaradit zone or to the NW-SE belts of An- nam. Thus, as much of Yunnan, the Khorat suffered two roughly orthogonal shortening episodes after the Cretaceous/Paleocene. The orientations of shorten- ing, however, appear to have been somewhat differ- ent, and the amounts much less.
As north of the Red River fault zone, the present- day deformation of northern Indochina is consistent with roughly N-S compression, but with more con- jugate strike-slip faulting and only minor normal faulting (Fig. 5). Near the border between Burma and Yunnan, for instance, several NE-SW-1;rending faults that appear to be active on LANDSAT images (e.g. the Mengla, Mengxing, Jinghong and Nanting faults; Fig. 5) (Le Dain et al., 1984) offset, by 6-15 km, left-laterally, the Mekong and Salween rivers. On 6 November 1988, two earthquakes with M s =
7.6 and 7.2 occurred near the northwestern tip of the Lincang fault (Fig. 5). The focal mechanisms of those shocks are compatible with right-lateral slip on the latter fault within a N-S compressional regime (Chen and Wu, 1989). In Vietnam, the NNE-SSW- trending Dien Bien Phu fault shows clear geomor- phic evidence of active left-lateral slip, on satellite images, topographic maps, and in the field (Tappon- nier et al., 1986).
The Red River Fault itself has long been known to be active, with a combination of right-lateral and normal faulting (e.g. Tapponnier and Molnar, 1977; Allen et al., 1984; Guo Shunmin et al., 1986; Tap- ponnier et al., 1990). As the longest and morphologi- cally clearest fault in the area between Tibet and the
24 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
South China sea, it is also presumably the fastest slipping, with minimum slip rate estimates ranging between -- 3 and 7 m m / y r (Allen et al., 1984; Guo Shunmin et al., 1986). The fault zone is rather linear and simple only between Midu and Lao Cai (Fig. 5), with at most two strands (Range Front and Mid valley Faults), a few kilometres apart (Fig. 9) (Allen et al., 1984). South of Lao Cai, in Vietnam, the fault splays into parallel strands along and north of the Day Nui Con Voi, and within the Tonghzing Alps. Motion appears to be oblique (normal-right-lateral) on several of those strands. North of Midu, the situation is even more complex. While oblique left- stepping, en-rchelon normal faulting continues northwestwards, along either flank of the Diancang Shan range, followed by fight-lateral faulting along the Qiaohou fault, bookshelf faulting on = N-S- striking faults north of that range probably transfers right-lateral motion to the Zhongdian fault. The kine- matics of faulting along the Red River and other regional active faults are nonetheless consistent with the regional stress orientations that characterize the current deformation of Yunnan (Fig. 5b) (Tapponnier and Molnar, 1977; Allen et al., 1984; Le Dain et al., 1984; Guo Shunmin et al., 1986; Wu and Wang, 1988; Tapponnier et al., 1986, 1990; Leloup, 1991; Leloup et al., 1993).
3. Tertiary deformation within the Ailao Shan- Red River metamorphic belt
In Yunnan, the Ailao Shan-Red River metamor- phic belt is composed of two main massifs, the Ailao Shan and Diancang Shan (Figs. 2 and 3) (e.g. Academy of Geological Sciences of China, 1975). We describe below the structure, deformation, P - T conditions, and age of metamorphism in these two massifs, targets of our most detailed field work.
3.1. The Ailao Shan metamorphic massif
The Ailao Shan metamorphic massif is less than 20 km wide and more than 300 km long. It juxta- poses two belts of rocks with contrasting metamor- phic facies. Along the northeast side of the massif, high-grade gneisses make a continuous range that culminates at 3137 m (Figs. 3, 9 and 10). Lower-
grade schists flank the gneisses along the southwest side of that range (Figs. 3, 9 and 10). Between Mojiang and the northern tip of the Ailao Shan, the schists contain small, dismembered bodies of ultra- mafic rocks elongated parallel to the range (Figs. 3 and 9). These rocks probably represent remnants of obducted oceanic crust and mantle. H P / L T meta- morphism is reported in the lower-grade schists (Huang, 1978; Bally et al., 1980; Duan Xinhua, 1981; Fan Chengjing, 1986). The Ailao Shan has thus long been interpreted to represent a suture be- tween the Yangzi platform and Indochina. Various suturing ages have been proposed, ranging from Sinian (Precambrian; Fan Chengjing, 1986) to Paleo- zoic (Helmcke, 1985) or Indosinian (Middle-Upper Triassic; e.g. Li Chun-Yii et al., 1979; Bally et al., 1980). The steep limit between the gneisses and the schists, called the "Ailao Shan fault" (Figs. 9, 10, 11 and Fig. 12), has been interpreted to be a thrust, the gneiss belt thus forming an upthrust slice of the Precambrian basement of the Yangzi platform (e.g. Duan Xinhua, 1981; Cheng Yuankung, 1987). Hence, high-grade metamorphism in the Ailao Shan has generally been considered to be Precambrian. In this context, the existence of some Tertiary K / A r ages was only taken to imply retrograde Cenozoic meta- morphism (Bally et al., 1980; Fan Chengjing, 1986; Wang and Chu, 1988). Tapponnier and coworkers have proposed a radically different interpretation (Tapponnier et al., 1986, 1990; SchS.rer et al., 1990, 1994; Zhong Dalai et al., 1990; Leloup, 1991; Leloup et al., 1993; Leloup and Kienast, 1993), in which the Ailao Shan gneisses result from large-scale Cenozoic strike-slip shear, the schists and ultramafics having being smeared along the range by such motion. We present here most of the evidence that led us to this interpretation.
3.1.1. The high-grade gneisses We were able to study the structure of the Ailao
Shan, and the characters of ductile strain along seven sections spanning a length of 300 km (Fig. 9). Three sections cut the entire massif, with nearly continuous outcrops (sections Ab, C and Db, Fig. 9). Evidence from 14 sites, spread over a distance of 90 km along the northeast flank of the massif, complement the sections (zones E and G, Fig. 9). The fact that the northernmost section, located near the northern tip of
P.H. Leloup et al. / Tectonophy:
\ ® L
Jingd'ong
S
f ,
I I 40km
Fig. 9. Structural map of Ailao Shan massif with location of field obser Yunnan (1983), modified according to field observations and LANDSAT a diagrams (lower hemisphere) show direction density diagrams of stretch Maximum density areas are black. Local trend of metamorphic belt is cross-sections of Fig, 12. B and F correspond to partial sections startin~ of river-polished outcrops, with each studied outcrop shown by star. boundary of Ailao Shan. Elongated crosses indicate deformed granite b,
field observations: drawn from Bureau of Geology and Mineral Resources of L~',tDSA'r and SPOT satellite images analysis. Box a refers to Fig. 11. Schmidt
Ls of stretching lineations (L) and of foliations poles (S) in mylonitic gneisses. ~hic belt is shown by straight lines on each diagram. Ab, C and Db refer to ons starting at northern boundary of Ailao Shan. E and G correspond to zones n by star. H and J correspond to partial cross-sections starting at southern d granite bodies. M.V.F. = Mid Valley Fault; R.F.F. = Range Front Fault.
/
\
L
/ J
pp. 25-28
. : : : 077 : : : : . i . i -~ : : : : : : . "?
\ Quaternary and Neogene I G n e i s s ~ Norite
Mesozoic ~ Low grade , , schists -~i,,Ultramafics
~] Paleozoic ~ Granite \
P.H, Leloup et al. / Tectonophysics 251 (1995) 3-84 29
Allao Shan sheer zone SW o ~) i I N E
A.F. R_F.F_ M_V.F_
-2
~4 " km km
(~) (~) Ailao Shan shear zone (~)
(~I(~ Red River
~. I R.R.F. I t i 4 / I ,,1/. . , YUANYANG ~)IC) ~ ~, I ~ I ,
o "::~E::!!!! ..... " : ~ i ..... " ~+
km 4km
. ooge . e Ou ,e,ns,y Ma,b,e
Mlarnatite
Fig. 10. Geological sections across Ailao Shan shear zone (sections A and D on Fig. 3). Parts of sections within Ailao Shan high-grade gneisses are detailed on Fig. 12 (sections Ab and Db). R.F.F. = active Range Front Fault (mainly normal slip); M.V.F. = active Mid Valley Fault (purely strike slip); A.F. = Ailao Shan Fault. Sections drawn from fieldwork and Bureau of Geology and Mineral Resources of Yunnan (1983).
the Ailao Shan, lies 300 km away from the southern- most one (section Ab) close to the Vietnam border, and that neither geological maps nor satellite images reveal significant change of attitude or fabric along the gneiss belt, implies that our data set characterizes fairly well the overall structure and deformation of the belt.
3.1.1.1. Structure of the gneissic belt. High-grade gneisses form the elongated topographic backbone, 8-10 km wide core of the Ailao Shan (Figs. 9 and 10). The rocks include thinly banded, biotite-sil-
limanite-garnet-bearing paragneisses, orthogneisses, augengneisses with large feldspar porphyroblasts, migmatites, deformed leucocratic veins, and intru- sions of anatectic leucogranites and granodiorites. The laminated paragneisses, which form a steep, continuous band along the northeastern border of the belt, are particularly well exposed in small canyons incising the steep range-front (zones E and G, Fig. 9). They contain stretched amphibolite layers and leucocratic veins, that form spectacular boudin trails, and rare boudins of gamet-pyroxenites, apparently derived from leached calc-silicates (Leloup, 1991;
30 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
~ Gneisses and mylonites 1
Anatectic granites I
Arnph~olite boudins 1 Marble boudins ]
~ Granites
~ Active strike slip fault
m Serpentinites and ultrabaslc rocks
Low grade schists
Neogene [ I Sedimentary rocks
Active normal fault
~ Tertiary strike slip fault Structural fabric from satemite images
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 3l
Leloup and Kienast, 1993). Large, up to several hundreds meters wide boudins of intensely deformed marbles, with complex shapes, are found in the paragneisses, close to the border of the massif, along sections B, C, D, F and H (Fig. 12). Au- gengneisses, orthogneisses (deformed granites), and more or less deformed anatectic rocks are more common in the inner part and along the southwestern border of the high-grade core (e.g. sections C and Db, Fig. 12). Migmatites, some with complex flow folding, are exposed at places in the middle of the core. Light-gray, fine-grained gneisses (probably de- formed rhyolites, or hypovolcanic granites), often with rod structure, are found along the southwestern part of the core along sections B, C and D. In zones E and G, and along sections B, C, D, and F (Figs. 9 and 12), the high-grade rocks form spectacular, steeply NE-dipping slabs, roughly parallel to the trend of the range. Between Mosha and Yuanjiang, such slabs are clear on SPOT images and on topo- graphic maps, as they control the drainage direction of many streams (Fig. 11).
To the southeast, along section A (Figs. 10 and 12), paragneisses containing large amphibolite boudins and lenses of leucocratic gneisses maintain a steeply dipping foliation only near the northern limit of the core (sites A1, A2 and A3, Fig. 12). Along this section, a --- 500 m thick band of recrystallized marbles, containing decimetric siliceous pods, is ex- posed 5 km south of the Range Front fault. South of these marbles, towards Yuanyang, the steeply foli- ated gneisses give way to flat-lying, altered micas- chists embedding layers of gneisses, migmatites and pegmatites (Figs. 10 and 12). South of Yuanyang, the rocks dip again more steeply towards the north. Garnet-bearing micaschists containing lenses of leu- cocratic rod gneisses are found at the southern limit of the metamorphic core. Farther south, stripes of sedimentary rocks (mainly Paleozoic and Triassic), at most only slightly metamorphic, are affected by steep faults with structural evidence of left-lateral slip. Several boudins of marble (= 50 m thick) are
found within the northernmost of these fault zones (Fig. 10).
3.1.1.2. Macroscopic characters of deformation. Most rocks in the Ailao Shan gneissic core display pervasive mylonitic textures (Higgins, 1971:; Bell and Etheridge, 1973; Hobbs et al., 1976). The small-scale mylonitic banding, the small size of ma- trix-grains and the strong deformation of porphyro- clasts, which are often elongated and exhibit long tails, are qualitative indicators of the intensity of ductile strain (Figs. 13, 14, 15 and 16). The banding, defined by the strong elongation of layers with var- ied petrology, is particularly spectacular on stream- polished outcrops in zones E and G (Tapponnier et al., 1990), and along sections C, D and F (e.g. Fig. 13a,b). In the paragneisses of the northeastern border of the massif for example, thin dark layers of biotite-micaschist or amphibolite, alternate with lay- ers bearing quartz-feldspar-biotite-sillimanite + garnet. Deformed leucocratic veins are also part of the macroscopic banding. The different layers are generally affected by pervasive boudinage (Fig. 14).
The foliation, which is marked by the preferred orientation of planar minerals (micas) and by flat- tened quartz or feldspar ribbons, is parallel to the macroscopic compositional banding (Fig. 13d). The foliation planes bear a prominent stretching mad min- eral lineation (Fig. 13c). The lineation is marked by elongate quartz and feldspar ribbons, long tails of feldspar porphyroclasts, pressure shadows on gar- nets, and by the alignment of metamorphic minerals (biotite, amphibole, sillimanite). As in most strongly deformed rocks, the foliation plane and the lineation appear to be nearly parallel to the XY plane and to the X-axis of the finite strain ellipsoid (X > Y > Z), respectively (Flinn, 1965; Nicolas and Poirier, 1976). Most of the rocks display L-S tectonite fabrics, in which both foliation and lineation are well ex- pressed, a fact suggestive of nearly plane-strain. Local, constrictional strains in leucocratic gneisses of the southwestern border of the Ailao Shan is attested
Fig. 11. Structural map of central part of Ailao Shah (box a on Fig. 9). Drawn from field mapping (bold drawing) and SF~rr satellite image analysis (dotted lines). Elevations from hl00,000 Chinese topographic maps, are in meters. R.F.F.=active Range Front Fault; M.V.F. = active Mid Valley Fault. C and Db refer to sections of Fig. 12.
SW
"'" [~
~ R~
~ ~ - ~
....
~ ~ ~
Slte
D1~ _ R
.F.F.
NE
M.V.F
. ®
1®
~°°°r
Q 1 ~
SW
NE
2000
~
/i ~U
~o St
le C2
~ YS
11 20
00
YUANYANG
t 2k
in
f
N S N
W-
~-
E
8
NN
E
Stte A
~ Yu
sl _ sit
e A2
N
W~
-E
S .~
oN
m¢
RO
CKS O
F ~
~rs
s
~ N
Am
.ES
~
~m
OU
gC
eo
um
s AIL
AO SH
AN SI-EA
R ZOIE
GNEm
SES
~ M
/~MA'
TTTE
S ~
ROD O
R~
~
PYR~
EIb~
IITE B
OUO
iN$ (S
ITAR
N)
~]
SC
ItmTS
Fig.
12.
Geo
logi
cal
cros
s-se
ctio
ns o
f A
ilao
Sha
n hi
gh-g
rade
myl
onit
es (
loca
ted
on F
igs.
9 a
nd l
l).
Arr
ows
wit
h nu
mbe
rs i
ndic
ate
obse
rvat
ion
site
s an
d sa
mpl
es (
Figs
. 13
-18)
. O
n C
and
Db,
top
ogra
phy
is d
raw
n fr
om C
hine
se 1
:100
,000
top
ogra
phic
map
s; d
ashe
d li
ne i
s pr
ojec
tion
of
cres
t li
nes
on s
ides
of
road
sec
tion
map
ped
in f
ield
. On
sect
ion
A, a
n ap
prox
imat
e to
pogr
aphy
has
bee
n dr
awn
from
the
Ope
rati
onal
Nav
igat
ion
Cha
rts
at a
sca
le o
f h1
,000
,000
and
fro
m f
ield
obs
erva
tion
s. D
iagr
ams
show
fol
iati
on (
grea
t ci
rcle
s)
and
stre
tchi
ng l
inea
tion
tre
nds
(arr
ows)
in
myl
onit
ic g
neis
ses,
alo
ng e
ach
sect
ion
(Sch
mid
t di
agra
ms,
low
er h
emis
pher
e).
Arr
ows
poin
t to
war
ds s
hear
dir
ecti
on (
mov
emen
t of
ha
ngin
g w
all)
. D
ashe
d li
ne i
ndic
ates
ove
rall
dir
ecti
on o
f co
rres
pond
ing
sect
ion.
t~
t-~ I
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 33
by a rod fabric with strong lineation but ill defined foliation. Folds with axes parallel to the lineation (a-folds; e.g. Malavieille, 1987) often affect the my- lonitic banding, while sheath folds (e.g. Quinquis et al., 1978) parallel to the lineation are locally ob- served (e.g. site G1, Fig. 9). Both type of folds are characteristic of progressive deformation in highly strained rocks (e.g. Cobbold and Quinquis, 1980; Lacassin and Mattauer, 1985; Malavieille, 1987).
The foliation, which is parallel to the slabs of gneisses visible on satellite images and in the land- scape (Figs. 11 and 13e), is either parallel to the trend of the metamorphic belt or slightly more N-S striking (Fig, 9). Average foliation strikes are N124°E along section A, N133°E south of Yuanjiang (section B, Fig. 9), N128°E along section C, N149°E along section D, N146°E between Mosha and Ejia (zones E, G and section F), and N175°E east of Jingdong (section H) (Fig. 9). At the northern tip of the Ailao Shan, the foliation geometry seems to be rather complex with more scattered foliation poles. The foliation is generally steep with dips between 60 and 90 ° towards the NE (65 ° on average) (Figs. 9, 12 and 13). Along sections C and D, locally flatter folia- tions seem to be related to the complex three-dimen- sional geometry of large-scale marble boudins (Fig. 12). Migmatites also display foliations with various strikes and dips. In general, however, the foliation strikes parallel to the belt (Tapponnier et al., 1990).
The stretching lineation is in general nearly hori- zontal in the foliation plane and thus strikes nearly parallel to the metamorphic belt (Figs. 9 and 12). On sections A, C and D, the average lineations strike N110°E, N127°E and N148°E, respectively (Figs. 9 and 12). In the streams of the northeast flank of the Ailao Shan, it strikes N131°E between Mosha and Chunyuan (zone E), N140°E along section F, and N148°E between Chunyuan and Ejia (zone G). East of Jingdong, along section H, the lineation strikes N169°E (Fig. 9). The pitch of the lineation in the foliation plane is generally between 0 and 20 ° south- wards. In zones where the foliation dips more gently (large-scale boudins of sections C and D, and the southern part of section A), the lineation remains parallel to the belt (Fig. 12) (Tapponnier et al., 1990). Rod lineations in leucocratic gneisses are also nearly horizontal and parallel to lineations in the surrounding gneisses. Despite some scatter, lin-
cations at the northern tip of the Ailao Shan strike NW-SE to N-S.
3.1,1.3. Shear criteria. In all outcrops studied, we found impressive shear-sense indicators of various types (e.g. Berth6 et al., 1979b; Simpson and Schmid, 1983; Hanmer and Passchier, 1991). At the outcrop scale, river-polished surfaces provide nearly horizon- tal sections close to the XZ plane of the finite strain ellipsoid (i.e. perpendicular to foliation and parallel to lineation). Such sections display particularly clear shear criteria. Asymmetric boudinage structures af- fecting amphibolite or leucocratic layers (e.g. Han- mer, 1986; Hanmer and Passchier, 1991), together with asymmetric foliation boudinage (Platt and Vis- sers, 1980; Gaudemer and Tapponnier, 1987; La- cassin, 1988), are widespread in the gneisses (Figs. 13a,b and 14). Left-lateral shear bands, typical of C-S or C ' -S mylonitic fabrics (e.g. Berth6 et al., 1979a, b), are also abundant, particularly in the orthogneisses of the southwest border of the core (Fig. 15) (Tapponnier et al., 1986, 1990). Feldspar porphyroclasts are usually strongly deformed, with elongate, asymmetric tails (Fig. 16c) that often form well-developed rolling structures (Fig. 16) (e.g. La- garde, 1978; Simpson and Schmid, 1983; Passchier and Simpson, 1986; Van Den Driessche and Brun, 1987; Hanmer and Passchier, 1991). The wings or tails may be very long (typically 8 cm long for rotated porphyroclasts less than 1 cm wide), a fact that suggests very large shear strains (Van Den Driessche and Brun, 1987). It is often possible to observe rolling structures with clear stair-step geom- etry of the wings (Passchier and Simpson, 1986; Hanmer, 1990) mixed on the same outcrop with others with opposite wings lying in a single plane parallel to the shear plane ("in plane"; Hanmer, 1990; Hanmer and Passchier, 1991). Some isolated boudins also form rolling structures. In the au- gengneisses, feldspar porphyroblasts that grew dur- ing deformation often display snowball smactures, reminiscent of those of garnets, with small rotated tails (Fig. 16b,e).
On each studied outcrop, a left-lateral shear sense, usually consistent with several of the above,, indica- tors, was clear. The different types of indicators were always mutually consistent. That the shear senses are consistent within each site and from one site to
t-o t
Fig.
13.
Def
orm
atio
n in
the
Aila
o Sh
ah s
hear
zon
e. (
a) T
ypic
al r
iver
pol
ishe
d ou
tcro
p di
spla
ying
ban
ded
myl
onit
es a
nd m
igm
atit
ic a
ugen
-gne
isse
s w
ith
seve
ral
gene
ratio
ns o
f st
retc
hed
leuc
ocra
tic v
eins
. N
earl
y ve
rtic
al f
olia
tion
stri
kes
NI2
0°E
; vi
ew t
owar
ds N
(si
te C
1 on
Fig
s.
11 a
nd 1
2).
(b)
Lam
inat
ed p
arag
neis
s m
ylon
ites
at
site
GI
(Fig
. 9)
; fo
liatio
n di
ps s
teep
ly t
o N
E a
nd s
trik
es
= N
145°
E,
view
tow
ards
NN
W.
(c)
Sub-
hori
zont
al s
tret
chin
g lin
eatio
n on
nea
rly
vert
ical
fol
iati
on i
n gn
eiss
es;
view
tow
ard
SW (
site
A2,
Fi
g. 1
2).
(d)
Polis
hed
slab
of
band
ed m
ylon
ite
(sam
ple
YU
42 f
rom
site
C2,
Fig
. 12
); n
earl
y ho
rizo
ntal
sec
tion
, vi
ew f
rom
abo
ve,
see
corr
espo
ndin
g qu
artz
c-a
xes
fabr
ic o
n Fi
g.
18d.
(e)
Ver
tical
am
phib
oliti
c pa
ragn
eiss
es a
t si
te A
1 (F
ig.
9);
folia
tion
stri
kes
-- N
110°
E;
view
tow
ards
SE.
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 35
7 : "
Fig. 13 (continued).
another implies remarkably uniform kinematics at the regional scale. We were, in fact, unable to find a single outcrop with criteria opposite to the bulk left-lateral sense. Together with other inferences at a larger scale, discussed later, such remarkable consis- tency suggests that deformation was probably close to simple shear.
3.1.1.4. Boudinage and bounds on finite strain. Boudinage of centimetric to decametric scale is widespread, as a result of stretching of the layers that form the mylonitic banding. In the paragneisses, boudins of amphibolite often form trails tens of meters long. Several generations of leucocratic veins emplaced during the deformation (Leloup and Kien-
36 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
SE
/
, __
-=_-
---
NW
~:
, lm
,
Fig.
14.
Met
er-s
cale
bou
dina
ge in
the
Aila
o S
han
shea
r zon
e. (a
) S
tretc
hed
]euc
ocra
tic v
ein
at s
ite C
1 (F
igs.
9 a
nd 1
3a),
view
tow
ards
NW
. P
enci
l gi
ves
scal
e. (b
) La
min
ated
~-
. pa
ragn
eiss
myl
onite
s sh
owin
g le
ft-la
tera
l sh
ears
. Stre
tche
d m
icac
eous
laye
rs (
inse
rt) s
how
dis
conn
ecte
d bo
udin
s lin
ked
by e
long
ated
tai
ls (
site
GI,
Fi
gs.
9 an
d ]3
b).
(c,d
) ,~
S
tretc
hed
amph
ibol
ite la
yers
, le
ft-la
tera
l sh
ears
sep
arat
e cl
ockw
ise
rota
ted
boud
ins;
site
GI
(Fig
s. 9
and
]3b
) an
d si
te D
I (F
ig.
12),
resp
ectiv
ely.
(e
) S
ketc
h of
tra
ils o
f am
phib
olit¢
bou
dins
at s
ite G
I (F
igs.
9 a
nd 1
3b).
Per
vasi
ve le
ft-la
tera
l sh
ears
and
cons
iste
nt a
sym
met
ry o
f bou
dins
impl
y bu
lk le
ft-la
tera
l she
arin
g, in
itial
ly a
djac
ent b
oudi
ns a
re
,~
ofte
n di
scon
nect
ed,
wit
h ga
ps l
arge
r th
an b
oudi
n si
ze (
see
for
exam
ple
a an
d b)
; b
e ar
e su
b-ho
rizo
ntal
out
crop
sur
face
s vi
ewed
fro
m a
bove
. I
38 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
P.H. Leloup et at. / Tectonophysics 251 (1995) 3-84 39
NW SE
Fig. 16. Asymmetric deformation of feldspar porphyroclasts or porphyroblasts implying left-lateral shearing in Ailao Shan zone. All are nearly horizontal sections, viewed from above. (a,b,d,e) Examples of rolling structures at site C1 (Figs. 12 and 13a). Boxes show location of inserts. (c) Asymmetric tails on feldspar porphyroclast at site GI (Figs. 9 and 13b). Coin gives scale. (f) Laminated paragneiss with asymmetric feldspar porphyroclast at site A2 (Fig. 12). Coin gives scale. (a) and (d) show well-developed rolling structures, which suggest large values of shear strain. On (a), note also the sigmoidal shape of the large feldspar porphyroclast in the center of photograph. (b) and (e) show synkinematic feldspar porphyroblasts in migmatitic augen-gneisses, forming incipient rolling structures.
Fig. 15. Left-lateral C-S and C ' -S mylonitic fabrics in the Ailao--Shan shear zone. (a) Orthogneisses (site C2, Fig. 12), view towards NW~ Lens gives scale. (b) Paragneisses (site D2, Fig. 12), horizontal section, view from above. Lens gives scale. (c) Migmatitic augengneiss with leucocratic bands (site GI, Figs. 9 and 13b), horizontal section, view from above. (d) Polished slab of sample YU43 (Site C2, Fig. 12).
40 P.H. Leloup et aL / Tectonophysics 251 (1995) 3-84
ast, 1993) are also stretched. Boudins are generally asymmetric and often connected by left-lateral shears striking more easterly than the foliation plane (Fig. 14).
A detailed study of such amphibolitic and leuco- cratic boudins trails (Fig. 14) shows that initially adjacent boudins are usually well separated, with gaps often larger than the boudins themselves, sug- gesting very large strain (e.g. Gaudemer and Tap- ponnier, 1987). The restoration of such stretched layers using surface-balancing (Lacassin et al., 1993b) provides minimum layer-parallel extension e (Ramsay, 1967):
e = ( l t - l o ) / l o
where 11 is the length of the stretched layer, and I 0 is the restored layer length. The values thus obtained range between 590 and 870% for the amphibolite layers, and between 270 and 660% for the leuco- cratic veins (Lacassin et al., 1993b). Such values are only lower bounds for the total finite extension, since the layers first formed during the early stages of progressive deformation (transposition of the amphi- bolite bands parallel to the mylonitic foliation) or were emplaced late in the deformation regime (syn- tectonic leucocratic veins).
There are several instances of boudinage at a much larger scale (100 m to 1 km). Along section A, asymmetric foliation boudinage may be inferred to occur at a scale of at least 100 m, from detailed mapping of the foliation (Leloup, 1991) (Fig. 17). The asymmetry of such large-scale boudinage is consistent with left-lateral shearing. Kilometer-scale boudins of marbles are scattered along the northeast- em flank of the gneiss core (Fig. 12). Careful study of existing regional geological maps (e.g. Bureau of Geology and Mineral Resources of Yunnan, 1983), LANDSAT and SPOT images also reveals that the southwestern border of the high-grade gneiss core is offset by left-lateral shear zones several tens of kilometers long (l_~loup, 1991) (Figs. 9 and 11).
3 .1 .1 .5 . M i c r o s t r u c t u r e s a n d q u a r t z c - a x e s p r e f e r r e d
o r i e n t a t i o n . The study of the Ailao Shan mylonites at the microscopic scale confirms the observations made at larger scales. In thin sections, the foliation and the lineation are marked by metamorphic miner-
I 50m I
Micaschists,Migmatitic Gneisses i TM ,'~dS
Leucocratic Gneisses YUANYANG ~ll
Fig. 17. Sketch map of 100 m-scale foliation boudinage at site A3 (Fig. 12). Asymmetry of amphibolite boudins and of large-scale foliation boudinage in gneisses is consistent with left-lateral shear- ing in N110/120°E direction. R.R.F. indicates average strike of active Red River Fault (border of Ailao Shan) located = 1 km farther north.
als (biotite, muscovite, chlorite, sillimanite, amphi- boles) and by quartz or feldspar ribbons. Both the ribbons and the gneiss matrix are made of small crystals usually elongated in the foliation plane. Some minerals (e.g. feldspars, garnets, sillimanites), often elongated, have been fractured and stretched parallel to the lineation during late stages of the deformation.
Various types of shear criteria are observed in thin sections (Leloup, 1991; Leloup and Kienast, 1993). These criteria include (1) pervasive C or C' shear planes, (2) sigmoidal mica crystals (Eisbacher, 1970; Lister and Snoke, 1984), (3) feldspar porphy- roclasts with very elongate asymmetric tails made of small, recrystallized quartz or feldspar, (4) rolling structures, and (5) asymmetric pressure shadows (e.g. Malavieille et al., 1982) of chlorite-muscovite on garnet porphyroclasts.
The quartz forms very long polycrystalline rib- bons generally made of recrystallized grains (30-200 /zm long on average). The grains have an irregular shape, sutured boundaries, often with undulose ex- tinction, subgrains and deformation bands, attesting to internal deformation. In some samples, the matrix contains small elongated quartz grains with aspect
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 41
ratios of up to = 10. Plastic deformation combined with dynamic recrystallisation thus appears to have been the dominant deformation mechanism in quartz (e.g. White, 1976).
Quartz c-axis fabrics, measured using the U-stage microscope, are well defined, with generally oblique girdles punctuated by several maxima (Fig. 18). Depending on the maxima positions, several types of fabrics may be distinguished: (1) fabrics with only one maximum on the Y-axis (e.g. sample GR3), (2) an oblique girdle with maxima close to the Y-axis (e.g. sample R17), (3) an oblique girdle with maxima in the XZ plane (e.g. sample R47), and (4) fabrics with crossed girdles (e.g. sample AY90). The max- ima close to the Y-axis, found in most fabrics, sug- gests activation of the prismatic ( a ) slip system in many crystals (Burg and Laurent, 1978; Bouchez and Prcher, 1981). Such activation has often been con- sidered indicative of relatively high-temperature con- ditions (T> 400°C) (Blacic, 1975; Nicolas and Poirier, 1976). The maxima in the XZ plane, and slightly oblique to Z are typical of the basal ( a ) slip system (e.g. Burg and Laurent, 1978; Bouchez and Prcher, 1981). The fact that, in some samples, the c-axis fabric only displays one maximum rather than girdles, may suggest one predominant slip system (e.g. prismatic ( a ) in GR3, Fig. 18). Fabrics with oblique girdles, as in most samples, display constant asymmetry in the XZ plane consistent with left lateral shear (e.g. Laurent and Etchecopar, 1976; Burg and Laurent, 1978; Lister and Hobbs, 1980; Bouchez and P~cher, 1981; Etchecopar and Vasseur, 1987). The rare fabrics with crossed-girdles always show one predominant girdle and an asymmetric disposition of the c-axes maxima (e.g. AY90, Fig. 18). They are thus also consistent with the left-lateral shear regime.
One particular sample (YU29, Fig. 18) yields an unusual c-axis fabric with a maximum close to X and two secondary maxima in the XZ plane. The quartz c-axes were measured in one ribbon made of slightly elongated crystals of relatively large size (250/zm) with rectilinear or lobate boundaries. Since, according to Gapais and Barbarin (1986), such mi- crostructures suggest high mobility of the grain boundaries, hence high temperatures during deforma- tion, the fabric obtained on this sample might reflect activation of the prismatic (c ) glide system (Gapais
and Barbarin, 1986; Mainprice et al., 1986), which may only occur at very high temperatures (Nicolas and Poirier, 1976).
3.1.2. The low-grade schists South of the Ailao Shan fault, the low.-grade
schist belt runs parallel to the gneiss core from the northern tip of the range to at least Mojiang (Figs. 3 and 9). There, the belt reaches a maximum width of = 10 km (Fig. 3). The low-grade schists are signifi- cantly less deformed than the high-grade gneisses. The schistosity strikes are more dispersed than is the foliation in the gneisses. They range between N110°E and N180°E, with an average direction of N130°E along section C, and between N140°E and N200°E, with an average direction of N160°E, along section D. The schistosity is thus parallel to, or more northerly trending than the gneissic foliation. It is usually steep and rarely bears a stretching lineation. Where present, the lineation strikes parallel to the stretching lineation in the high-grade gneisses.
The largest bodies of ultramafic rocks, up to 2 km wide and 8 km long, are exposed in the central part of the schist belt, northwest of Mojiang and near Shuang Gou (Figs. 3, 9 and 11). They are mostly composed of serpentinized harzburgites and gabbros with ophitic structure. In general they are separated from surrounding upper Mesozoic sandstones by steeply dipping, cataclastic faults. On geological maps these elongate ultramafic bodies form trails parallel to the Ailao Shan and are separated by left-lateral faults like those offsetting the southeast- ern border of the gneissic core (Figs. 3, 9 and 11). Southwest of section C, near Anding, a small serpen- tinite massif lies in thrust contact with Mesozoic red beds. Near section D, the southwestern border of the low-grade schists corresponds to a spectacular align- ment of small serpentinite bodies along a fault strik- ing = N135°E (Fig. 11). The serpentinite is affected by a steeply dipping, N170°E-striking, schistosity. The schistosity is in turn cut by two sets of steep shear planes (N30°E-striking dextral planes and, more numerous N 135°E-striking left-lateral planes), indi- cating N75°E shortening. The obliquity of th~s direc- tion to the N135°E-striking fault, and the predomi- nance of left-lateral planes implies bulk left-lateral shear, parallel to the Ailao Shan range. We therefore
42 P.H. Leloup et al./ Tectonophysics 251 (1995) 3-84
interpret these ultramafic bodies as large-scale boudins resulting from post-Mesozoic left-lateral shear along the range (Figs. 9 and 11).
3.1.3. Summary o f deformation in the Ailao Shah
Foliations, stretching lineations, and shear criteria in the strongly deformed Ailao Shan high-grade
Xq
Z
P.H. Leloup et al . / Tectonophysics 251 (1995) 3-84 43
gneisses, coherent from the microscopic to the re- gional scale, suggest that the deformation regime was close to simple shear. The intense strain recorded in the rocks implies that the foliation lies at a low angle to the bulk shear-plane, and that shear occurred in a direction close to the stretching lineation (e.g. Escher and Watterson, 1974; Mattauer, 1975). The mylonitic rocks of the Ailao Shan thus formed in a large-scale horizontal left-lateral shear regime, paral- lel to the range, under high-temperature conditions.
When approaching the Ailao Shan core from the south, the regional N-S axial plane cleavage of the folded red beds of the Simao basin, blends with the cleavage in the low-grade schists, which progres- sively bends to become NW-SE, parallel to the foliation direction in the high-grade gneisses (Figs. 3, 9 and 11). We interpret this change to result from progressive deformation during a single phase of E-W shortening, responsible for both the N-S- trending folds with N-S schistosity in the Simao and Chuxiong basins, and for the NW-SE-trending my- lonites within the Ailao Shan left-lateral shear zone (Tapponnier et al., 1986, 1990; Leloup, 1991).
3.2. The Diancang Shan massif
The Diancang Shan ("mountain of changing sky cloud") is a more bulky mountain massif than the Ailao Shan. About 15 km wide and 80 km long, it culminates at 4122 m. Its jagged crest towers above the surrounding country, which stands at an average elevation of 2200 m (Fig. 19). The Diancang Shan lies along the projected northwestward trend of the Ailao Shah (Figs. 2 and 3). The two massifs are separated by the "Midu Gap", an almost 80 krn long stretch without metamorphic rocks (Fig. 3) (Tapponnier et al., 1990). The Diancang Shah forms a rising horst mostly composed of high-grade rocks, bounded on either side by en-rchelon active normal
faults (Figs. 5 and 19) (e.g. Allen et al., 1984; Leloup et al., 1993). Folded Mesozoic rocks fill the Yanghi basin to the west (Figs. 2 and 3), while mostly Paleozoic rocks are exposed to the.. east. Neogene and Quaternary deposits lie unconformably on the older, folded rocks on the hanging walls of the active normal faults. The largest Neogene- Quaternary basin is that occupied by Er Hai lake along the eastern flank of the Diancang Shah (Fig. 19).
We have studied three sections across the: meta- morphic rocks of the Diancang massif in some detail. The first one is along the river gorge west of Xia- guan, near the southern end of that massif (P2 and P3, Fig. 19). The second one reaches the 4092 m high summit west of Dali (section Q, Fig. 19). The last section (section R, Fig. 19) is across the,. north- ern tip, only 2.5 km wide, of the massif near Qiao- hou.
3.2.1. High-temperature deformation The Diancang Shan metamorphics comprise rock
types very similar to those seen in the Ailao Shan, including paragneisses, augengneisses, skarns, leuco- cratic melt layers, micaschists, hornblende schists and marbles. The Dali white and gray marbles have made that city famous in all of China for at least 1200 years ("Dali Shi" is the Chinese word for marble). Along the east flank of the mountain (sec- tion Q, Fig. 20), marbles, mylonitic paragneisses, orthogneisses and amphibole-bearing micaschists form spectacular, vertical or steeply ENE- or WSW- dipping slabs striking parallel to the massif (Figs. 20 and 21). The gneisses contain stretched leucocratic veins (generally pegmatites) and rare boudins of skarns. The western part of the section exposes W-dipping paragneisses, migmatitic gneisses and mi- caschists. At the southern end of the massif (P, Fig.
Fig. 18. Quartz c-axis preferred orientations of Ailao Shan shear zone (Schmidt diagrams, lower hemisphere). Measurements made on U-stage are presented in XZ plane. L is stretching lineation or X-axis. Contours at 1, 2, 3 and 5% density, if not specified below. Dashed lines correspond to shear planes observed in thin section. Small Schmidt diagrams on lower right side of each c-axis fabric show attitude of foliation (great circle), lineation (dot), and thin section plane (dashed great circle), relative to geographic axes. (a) Sample GR3 (site GI, Figs. 9 and 13b). (b) Sample R17 (section Db, Fig. 12). (c) Sample R47 (section Db, Fig. 12). (d) Sample YU42 (section C, Fig. 12), 144 c-axes. (e) Sample AY 90 (section C, Fig. 12). (f) Sample YU44a (section C, Fig. 12), 465 c-axes. (g) Sample Y3 (section A, Fig. 12). (h) Sample YU61 (site A3, Fig. 12), 521 c-axes, contours 1, 3.5, 4, 6%. (i) Sample YU29 (site D2, Fig. 12), 180 c-axes, contours 1, 2, 3, 4.5, 6%.
44 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
Lake
River
o,, ,
I I Cross section
Sedimentary rOCkS
Quaternary
Neogane
Eocene
Jurassic - Cretmceous
Taas
Permian basalt
P~mian
Devonian
Plutonic rocks
.o,,,e
Metamorphic rocks
TerUary Gnelsses
o,~.r o., .oo,0nio rooks
Folialion strike and dip tmeasur~'d)
HT LInestion (measured)
HT f o(latlon
\ \ \ LT foHatlon
\
"~ LT L~neatlon (me~s~ed)
P.H. Leloup et al./ Tectonophysics 251 (1995) 3-84 45
19), the gneisses dip more gently towards the west or south. The northernmost section displays only a nar- row stripe of fresh paragneisses, micaschists and calcschists.
3.2.1.1. Macroscopic characters of deformation. As those of the Ailao Shan, most Diancang Shan rocks show evidence of intense ductile strain. Widespread mylonitic textures include small-size matrix grains, strongly deformed porphyroclasts, and small-scale banding. The banding is parallel to a prominent foliation that bears a spectacular, nearly horizontal lineation. The foliation, marked by the preferred orientation of planar minerals (biotite, ___ muscovite) and by the flattening of quartz or feldspar ribbons, is parallel to the large-scale gneiss slabs visible in the landscape and to the petrographic boundaries (Figs. 20 and 21a,c). The lineation is marked by the elonga- tion of quartz or feldspar ribbons, by the tails of feldspar porphyroclasts in the gneisses, and by the alignment of metamorphic minerals (biotite, amphi- bole, tourmaline) (Fig. 21b,c,d). Locally, marbles and gneisses are affected by folds with nearly hori- zontal axes and by sheath folds (Fig. 21e).
In places, the high temperature (HT) foliations and lineations are overprinted by late, retrograde (LT) microstructures (Leloup et al., 1993). In partic- ular, in the eastern part of section Q, the HT folia- tion bears striations marked by chlorite (e.g. at site Q1, Fig. 20b). West of Xiaguan, at the mouth of the river gorge (sites P1 and P2, Fig. 19), chlorite and sericite mark penetrative east-dipping foliation and lineation in the zone of LT mylonites that forms the eastern boundary of the gneisses.
Along section Q (Figs. 19 and 20) the HT folia- tion remains everywhere parallel to the massif core. On the eastern and western flanks of the range it strikes N160°E and N165°E, respectively, on average (Fig. 22a). Along the eastern flank, it dips steeply (60-90 °) towards the NE (Fig. 20b). Locally more gentle dips seem to result from large-scale boudinage (Fig. 20c,d). From site Q3 to the summit (Fig. 20b), the foliation attitudes form an inverted fan. It swings to dips of ~- 50°W on average at the mountain crest (Fig. 20), and on its western flank (site Q6, Fig. 20). Farther north, along section R the foliation strikes N140°E on average (Fig. 22b) with vertical or south- westward dips.
The foliation wraps around the southwestern end of the massif (Tapponnier et al., 1990) (Fig. 19). At site P3 (Fig. 19), the HT foliations dip :~ 30 ° towards the SSE with strikes ranging between N20°E and N100°E (Figs. 19 and 22c). In contrast, the LT mylonites observed at sites P1 and P2 display E- to ENE-dipping foliations whose strikes range between N125°E and N190°E (Figs. 19 and 22d). In addition to the differences in metamorphic grade, the average orientations of the HT and LT foliations are statisti- cally different (Leloup et al., 1993). Moreover, in the landscape, the LT mylonites forming the eastern boundary of the massif (sites P1 and P2) cut across the H T foliations.
Despite the complexity of the foliation geometry, the trend of the HT lineations remains nearly paral- lel to the trend of the Diancang Shan (Figs. 19 and 22a,b,c). The HT lineations strike N163°E on aver- age in the eastern part of section Q (Figs. 19 and 22a), and N165°E in its western part (Figs. 19 and 22a). Along section R (Fig. 19), the lineations strike = N 143°E (Fig. 22b). In zone P, the H T lineations, which strike N120°E on average, are slightly oblique to the trend of the range (Figs. 19 and 22c). They are statistically distinct from the LT lineations that strike N97°E on average at site P1, and N91°E at site P2 (Figs. 19 and 22d) (Leloup et al., 1993). The LT lineations have average pitches of 70°S on the LT foliation planes (Fig. 22d).
At one site along section Q (site Q4), we have been able to estimate the minimum finite elongation with the boudinage-restoration technique used in the Ailao Shan (Lacassin et al., 1993b). The restoration of syntectonic leucocratic veins at this site yields a mean layer parallel extension value of 540%. The occurrence of sheath folds (Fig. 21e), and of rolling structures and mylonitic textures, all suggestive of large strain, are in keeping with such large finite extension along this section.
3.2.1.2. Macroscopic shear criteria, microstructures, and quartz c-axes preferred orientation related to H T deformation. As in the Ailao Shan, we found ubiquitous evidence for non-coaxial deformation in most rock types of the Diancang Shan. Left-lateral shear criteria at the outcrop scale include C-S or C'-S mylonitic fabrics, elongated asymmetric tails and rolling structures next to deformed porphyro-
46 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
3 o01 , +o2L 1:3+ / '":++'~++:+~+LI t + t * + } , v , t ' ~ , X + I
3000
250(
I 500m I Marbles
®
Micaschists
Paragneisses
..=b ~ Skarns
Granitic veins
Or thogneisses
Amphibolitic schists
Dark schists with amphlbolite
WSW EN E
._...__......,~t e Site Q2 Q3
Marble ~ ' ~ I Amphibolites \\~ ~\~ \~=~q'°+ 0
Fig. 20. Geological section across the Diancang Shah massif (section Q located on Fig. 19). Arrows with numbers show location of studied sites or samples. (a) Whole section. (b) Detailed section of upper part of eastern flank of Diancang Shan [box on (a)]. (c) Detail showing geometry of foliation between sites QI and Q3. (d) Sketch of 3-D geometry of foliation and lineation. Quartz c-axis preferred orientation of samples YU6 and D30 are shown on Fig. 24b and c.
!i!i
!~ii
i~i
• ~i
! ii
l ̧¸¸
t~
t~
e~
60
L,q
Fig.
21.
HT
def
orm
atio
n in
Dia
ncan
g Sh
an. (
a) S
ub-v
erti
cal
folia
tion
in
gnei
sses
, vi
ew t
owar
ds S
SE o
f cr
est
sout
h of
sec
tion
Q (
Fig.
19)
. (b
) V
erti
cal
folia
tion
in
mar
bles
wit
h cl
ear
stre
tchi
ng
linea
tion
(pi
tch
= 10
°N)
(sit
e Q
2 on
Fig
. 20
). (c
) L
ands
cape
sho
win
g sl
abs
of s
teep
ly d
ippi
ng g
neis
ses
wit
h su
b-ho
rizo
ntal
lin
eati
on (
pitc
h =
12°N
), vi
ew
tow
ards
NW
(si
te Q
I on
Fig
. 20
b).
(d)
Stee
ply
dipp
ing
gnei
sses
wit
h ne
arly
hor
izon
tal
linea
tion
at
Dia
ncan
g Sh
an s
umm
it,
view
tow
ards
N (
site
Q5
on F
ig.
20b)
. (e
) E
ye f
olds
an
d m
ushr
oom
-lik
e fo
lds
in p
arag
neis
ses
at s
ite
Q4
(Fig
. 20
b);
such
str
uctu
res
are
likel
y to
be
sect
ions
of
shea
th f
olds
. K
nife
giv
es s
cale
.
48 P.H. Leloup et al . / Tectonophysics 251 (1995) 3-84
clasts, and asymmetric foliation boudinage in micas- chists (Fig. 23). At the microscopic scale, similar shear criteria also include deformed mica crystals of sigmoidal shape, often limited by C or C' shear planes. The quartz c-axis preferred orientations, which resemble those obtained in the Ailao Shan, also imply relatively high-temperature conditions
(maxima close to Y). Left-lateral shear senses are confirmed by the asymmetry of oblique girdles (Fig. 24).
At each site along section Q (Fig. 20), the differ- ent types of shear criteria are mutually consistent and indicative of left-lateral shear. They are particularly outstanding on the eastern flank of the range (Fig.
Fig. 22. Lineations and foliations in Diancang Shan. Schmidt diagrams, lower hemisphere projection. Sections and zones are located on Fig.
19.
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 49
23) and scarcer, but unambiguous, on the western flank (site Q6). It is important to note that the shear-senses remain left-lateral all along the section up to the Diancang Shan summit, despite the change from E- to W-dipping foliations.
In the gorge west of Xiaguan (site P3, Fig. 19), consistent shear criteria (asymmetric tails of de- formed feldspar porphyroclasts, C or C' shear planes) in vertical sections parallel to the HT lineation indi- cate top-to-the-SE shear, hence oblique (normal left- lateral) movement. The average pitch of the shear directions in the foliation planes is 56°E.
3.2.1.3. Variations in foliation dips at I00 m scale. Along section Q (Figs. 19 and 20), a = 200 m wide zone with more gently dipping foliations may be followed in the landscape (Fig. 20c). This zone lies between the generally steep gneiss or marble slabs that make up much of the eastern flank of Diancang Shan (Fig. 20b,c). HT lineations in the gently dip- ping gneisses (---N155°E) are parallel to the range and to lineations in adjacent steeper gneisses. Asym- metric tails and rolling structures on feldspars indi- cate top-to-the-NW shear. Such shear is compatible with that observed in the steeper gneisses, and hence with left-lateral movement, in spite of the change in foliation dip (Fig. 20d). The complex structure of the zone may result from three-dimensional, asymmetric foliation boudinage during progressive left- lateral/normal shear (Fig. 20d). In that zone, as in much of section Q, the lineations have small, but constant pitches towards the north (10-20 ° on aver- age; Figs. 21b and 22a), which is consistent with a normal component of movement on NE-dipping planes in a left-lateral regime (Fig. 20d). Similar structures, with changes in foliation dip without changes in shear direction, also exist along section C in the Ailao Shan (Fig. 12). In either range, such structures appear to be associated with marbles.
3.2.2. Low-temperature chlorite-bearing slickensides and lineations
The chlorite-bearing slickensides overprinted on the HT foliations in the eastern part of section Q are nearly down-dip (pitches ranging between 70°S and 70°N) on steep planes. Asymmetric chlorite crystal- lizations indicate normal movement, down to the ENE, on such planes, posterior to HT left-lateral
shear (Leloup et al., 1993). Farther southwards, west of Xiaguan (sites PI, P2, Fig. 19), the LT chlorite- bearing mylonites, with E-trending lineations (Fig. 22d), display asymmetric feldspar porphyroclasts and asymmetric mica fish. Such shear criteria are also indicative of LT ductile normal shear (with a small right-lateral component) down to the east in this mylonite zone (Leloup et al., 1993). Both the LT mylonites and the chlorite-bearing slickensides ap- pear to result from movement on the still active normal fault that bounds the Diancang Shan to the east. Such ductile to brittle structures probably have been progressively uplifted in the footwall of that fault (Tapponnier et al., 1990). The direction of extension consistent with the chlorite slickensides or with the LT lineations is compatible with the Qua- ternary strain field deduced from the active fault pattern in the area (e.g. Allen et al., 1984; Tappon- nier et al., 1990).
3.2.3. Summary and discussion of deformation char- acters in Diancang Shan
The gneisses of the Diancang Shan core display spectacular evidence of intense high-temperature strain. Consistent shear criteria are found at most sites and, given the intense strain in the rocks, shearing must have occurred on planes nearly' paral- lel to the foliation, in a direction close to the lin- eation (Escher and Watterson, 1974; Mattauer, 1975). Thus, as in the Ailao Shah, the gneissic core of the Diancang Shan appears to have been the site of large-scale, left-lateral, ductile shear in a ~ N160°E direction parallel to the range. All the structural evidence suggests that the major deformation episode in both massifs is the same and that they both belong to a single crustal shear zone.
The southern termination of the Diancang Shan core (site P3, Fig. 19), shows more complex HT deformation geometry and kinematics than the center of the range. The fact that more gently dipping foliations wrap around that termination implies 3-D strain (Fig. 19). Top-to-the-SE shear, parallel to N120°E-trending lineations with both left-lateral and normal components of movement, has occurred there. Such shear is compatible with, if oblique to, the left-lateral shear senses within the steep inner core of the range (Fig. 19). We have inferred that the shape of the H T foliation at the southern termination of the
J
Fig.
23.
Exa
mpl
es o
f sh
ear
crite
ria
on t
he e
aste
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P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 51
Diancang Shan massif results from late dissection of the Diancang Shan-Ailao Shan shear zone by a large left-lateral/normal shear plane dipping to the SE (Leloup et al., 1993) (Fig. 25b). Such oblique motion would have contributed to unroof and uplift the gneissic core of the Diancang Shan, while burying the northern extremity of the Ailao Shan under the Midu gap.
3.3. P-T conditions and age of deformation in the Ailao Shan and Diancang Shan
A striking character of the Ailao Shan-Red River metamorphic belt is the rather sharp horizontal meta- morphic gradient found on either side of its gneiss core. Unmetamorphosed rocks crop out between 3 and 10 krn away from the high-grade gneisses (e.g.
D YU6
iiii i iiiiiiiii!iiiJ ' ,
) Fig. 24. Quartz c-axis preferred orientations in Diancang Shan (Schmidt diagrams, lower hemisphere). Measurements made on U-stage are presented in XZ plane. L is lineation and X-axis. Contours 1, 2, 3, and 5%. Small Schmidt diagrams on lower right side of each c-axis fabric show attitude of foliation (great circle), lineation (dot), and thin section plane (dashed great circle), relative to geographic axes. (a) Sample Dl3 (located on Fig, 19). (b) Sample YU6 (section Q, located on Fig. 20a), 527 c-axes. (c) Sample D30 (section Q, located on Fig. 20b).
52 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
Bureau of Geology and Mineral Resources of Yun- nan, 1983; Bureau of Geology and Mineral Re- sources of Yunnan Province, 1990; Tapponnier et al., 1990). Such steep thermal gradients might result from two causes: (1) high-temperature metamor- phism restricted to the shear zone in close relation with the shear event, and (2) a large amount of differential uplift between the gneisses and the sur-
rounding sedimentary rocks. Petrological (Leloup et al., 1993; Leloup and Kienast, 1993) and geochrono- logical (Scharer et al., 1990, 1994; Harrison et al., 1992a, 1995; Liu et al., 1992; Leloup et al., 1993) data suggest that both processes took place.
Geological and structural evidence for high-tem- perature deformation in the gneisses is compelling (Leloup and Kienast, 1993). The banded parag-
®
®
\
- l O k m
-1Okra
Fig. 25. (a) Sketch of the 3-D geometry of Diancang Shan, Midu gap, and northern part of Ailao Shan. Moho depths are from Yan et al. (1985). (b) Sketch of the 3-D geometry of the Ailao Shan-Diancang Shan shear zone in final stages of left-lateral shearing. Large oblique left-lateral shear plane dismembers ASRR shear zone at southern extremity of Diancang Shan (zone P3). Ellipses on that plane show lineation attitude in zone P3).
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 53
neisses along the eastern flank of the Ailao Shan (i.e. zones E and G, Fig. 9) are amphibolite-facies rocks with biotite, garnet and frequent sillimanite defining the stretching lineation (Bureau of Geology and Min- eral Resources of Yunnan Province, 1990; Leloup and Kienast, 1993). Some migmatitic rocks and leu- cocratic melts emplaced during progressive shearing are strongly deformed (with layer parallel extension of up to 660%; Figs. 13a,b and 14a) (Lacassin et al., 1993b). The common occurrence of prismatic ( a ) glide in quartz suggests temperatures in excess of 400°C during deformation. Furthermore, activation of prismatic ( c ) glide in YU29 (Fig. 18) implies, at least locally, temperatures close to the granitic solidus. Strong recrystallization of feldspar porphy- roclasts in gneisses (Fig. 16) also testifies to high- temperature deformation.
The active Range Front normal fault (RFF) sharply bounds the Ailao Shan high-grade gneisses to the northeast. North of the fault, Mesozoic sediments are only slightly metamorphosed. Such metamorphic contrast appears to be due, in part, to uplift of the Ailao Shan gneisses along the RFF (Tapponnier et al., 1990). Toward the SW, although the Ailao Shan fault follows the boundary between the gneisses and the low-grade schists, the metamorphic gradient is less steep. Electron microprobe analysis performed on the blue amphiboles found in reportedly H P - L T rocks in those schists, has revealed either winchite fringed by actinolite rims (Zhang Ru Yuan et al., 1990), or magnesio-riebeckite (Leloup and Kienast, 1993), but no glaucophane. P - T conditions in the schists thus appears to have been only around 4 kb and 170°C (Zhang Ru Yuan et al., 1990).
A sharp metamorphic contrast exists on either side of the Diancang Shan. Since the mountain is bounded by two active normal faults, a significant part of that contrast is probably due to recent exten- sional uplift.
A detailed petrological study of pelite-derived gneisses in zone E (Fig. 9) confirms high-tempera- ture coeval with left-lateral shearing (Leloup and Kienast, 1993). There were two successive, stable parageneses during deformation. The most prominent paragenesis (P1) is characterized by amphibolite fa- cies assemblages (biotite, garnet, sillimanite, quartz, K-feldspar, plagioclase). The second one (P2), found only in cracks or pressure-shadows affecting the
porphyroblasts of P1, shows retrograde minerals typ- ical of greenschist facies (chlorite, muscovite, + bio- tite). Left-lateral shear criteria are coeval with either paragenesis, implying a retrograde deformation path (cooling during shearing). Together with biotite- garnet and plagioclase-garnet thermobarometry, the compositional zoning of P1 garnets suggests a tem- perature increase during crystallization of the garnets from ~ 660°C and ~ 5kb (Pla) to 710 + 70°C and 4.5 + 1.5 kb (Plb, close to the granitic solidus) (Fig. 26) (Leloup and Kienast, 1993). P2 P - T conditions are estimated to be near 500°C and under 3.8 kb (with large uncertainties) (Fig. 26). The metamorphic grade seems to be slightly lower in the Diancang Shan than in the Ailao Shan, but the scarcity of originally pelitic rocks precludes precise P - T esti- mates in much of the massif. So far, only one sample from the southern border of the massif (DC3, site P3, Fig. 19), a micaschist containing staurolite and garnets, has permitted P - T estimates. They imply nearly isothermal evolution from - 7 kb (550°C) to = 5 kb (570°C) (Fig. 26) (Leloup et al., 1993), with higher pressure and lower temperatures than in the Ailao Shan. The rocks exposed at the southern end of Diancang Shan were thus deformed deeper (be- tween 25 and 18 km), albeit at lower temperatures than those along the northeastern flank of Ailao Shan (= 18 km) (Fig. 26). In both cases however, the P - T estimates correspond to a relatively high geothermal gradient.
Rb-Sr ages obtained by Chinese geologists on metamorphic rocks of the Ailao and Diancang Shan mostly range between 840 and 1700 Ma, with a cluster between 840 and 900 Ma (Bureau of Geology and Mineral Resources of Yunnan Province, 1990; Fan Chengjing, 1986; Zhong Dalai, pers. commun., 1988). K / A r ages on micas range between 11.5 and 230 Ma, with most of them comprised between 11.5 and 50 Ma (Wang and Chu, 1988). However, with- out a precise description of the sampling locations, samples, and of the analytical method used, it is difficult to assess which of these age groups, if any, relates to the deformation observed in the ranges. More recent geochronological results obtained on accurately located samples in the Diancang and Ailao Shan gneisses (Sch~rer et al., 1990, 1994; Harrison et al., 1992a, 1995; Liu et al., 1992; Leloup et al., 1993) yield a much clearer picture.
54 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
12
10
40
r ~
8
7- DC3
P l a
4
P2
2
'4-
rq.. co
w DC3
P l b Ky / fe ' : l l Iii P l b
And
30
e4 II
20
~la E
P2 gl,
10-~
0 - L. 0 P2 ~ ~ P l b
DC3 P l a
300 400 500 600 700 800 900 1000 Temperature (°C)
Stability domain for Pla ~ Stability domain for Plb according to thermobarometry ~ according to thermobarometLT
Stability domain for P2 according to thermobarometry
Fig. 26. P - T evolution of Ailao Shan and Diancang Shan gneisses. Data from Leloup et al. (1993) and Leloup and Kienast (1993). la, lb and 2 correspond to successive parageneses observed in the paragneiss of the NE border of Ailao Shan (site G1 and zone E, Fig. 9). DC3 is a sample from zone P3 at the southern extremity of Diancang Shah (Fig. 19). As = aluminosilicate (Ky = kyanite, And = andalusite, Sill = sillimanite); Bi = biotite; Cd = cordierite; Ch = chlorite; Ct = chloritoid; Gt = garnet; K f = K-feldspar; Mu = muscovite; Qz = quartz; St = staurolite.
U / P b ages on z i rcons , m o n a z i t e s and x e n o t i m e s
f rom leucogran i t i c veins , paral le l to the fo l ia t ion and
a f fec ted by lef t - la teral shear are s imi la r in bo th
mass i f s (Fig. 27). In the cent ra l A i l ao Shan, the ages
ob t a ined f rom two d i f fe rent ve ins (YS9, site E1 and
YS 11, sec t ion C ) are 23 + 0.2 M a (2 ~r) ( S c h ~ e r et
al., 1990). N e w ages on z i rcon and t i tani te f rac t ions
f rom m o n z o n i t e s , m o n z o s y e n i t e s and pegma t i t e s
P.H. Leloup et al./ Tectonophysics 251 (1995) 3-84 55
range between 26.3 + 0.3 and 24.1 + 0.2 Ma (Sch~er et al., 1994). In the Diancang Shan, monazites, xeno- times and zircons from a leucocratic layer along section Q (YS35, Fig. 20b) yield ages between 24.4 + 0.2 and 22.3 + 0.3 Ma (20-) respectively (Liu et al., 1992; Schiirer et al., 1994). The longer time separating the monazites and xenotime closure ages probably reflect slower cooling than in the Ailao Shan (Liu et al., 1992), in keeping with P - T esti- mates. Such ages date the crystallization of the leucogranitic melts (Sch~er et al., 1990, 1994; Liu et al., 1992). Since the melts are deformed, the ages imply that left-lateral shear was in progress in both massifs from at least before 26 Ma until after 22 Ma.
The U / P b ages have been complemented by more than 40 new 4°Ar/39Ar measurements on biotites, white micas, amphiboles and K-feldspar from the Ailao Shan and the Diancang Shan (sections A, C, D, E, F, G, P and Q) (Harrison et al., 1992a, 1995; Leloup et al., 1993). Thermal histories have been retrieved from K-feldspar age spectra using an exten- sion of the single diffusion domain/activation en- ergy closure theory (Dodson, 1973) that applies to minerals with a discrete distribution of domain sizes or activation energies (Lovera et al., 1989, 1991, 1993; Harrison et al., 1991; Fitz Gerald and Harri- son, 1993). None of these studies yields ages older than 27 Ma. The 4°Ar/39Ar measurements from samples collected close to the U / P b dated samples (sections C, Q and zone E) systematically give ages younger than the U /Pb ones (Fig. 27) (Harrison et al., 1992a; Leloup et al., 1993).
The cooling histories of zone E, section C (Ailao Shan) and zone P and section Q (southern part of Diancang Shan) have been retrieved from K-feldspar thermal histories (4°At/39Ar data), and from ages and closing temperatures of monazite and xenotime (U /P b ages), amphibole, white mica and biotite (a°Ar/39Ar ages) (Fig. 27). For each zone, results from different laboratories and with different tech- niques, all outline simple and coherent cooling histo- ries. The ages corresponding to the higher tempera- tures (U /Pb ages) are significantly older than the end of the left-lateral shear episode. In zone E, the age obtained on the FA2 amphibole (Harrison et al., 1992a), whose closing temperature is comparable to that estimated for paragenesis P2 in the same zone (Leloup et al., 1993), suggests that left-lateral shear
was still going on around 22 Ma (Fig. 27b). In zone E and section C (Ailao Shan), temperatures dropped below those compatible with quartz plasticity (= 300°C) around 20 Ma. This suggests that ductile shear came to an end at that time in the sampled rocks, although brittle left-lateral slip could have continued. In the Diancang Shan, temperatures of
300°C were reached around 17 Ma but maintained until about 4.7 Ma (Leloup et al., 1993) (Fig. 27c). More recent evidence suggest that cooling was di- achronous along the belt. In fact, temperature dropped below =300°Ca t = 22 Ma on section A, at =2 0 Ma on section C, at = 19 Ma on section /3, (Fig. 27a), and at = 18 Ma for sections E, F and G (Harrison et al., 1992a, 1995; Leloup et al., 1994) (Fig. 27a,b).
Hence, with the new evidence at hand, the overall picture that emerges is as follows. The left-lateral shear episode that formed the Ailao and Diancang gneiss cores is neither of Precambrian, nor of Meso- zoic, but of Oligo-Miocene age. It must have started after the lower Tertiary (50-60 Ma), since Upper Cretaceous-Paleocene red beds are affected by kine- matically consistent strain, and in all likelyhood before 26.5 Ma. Ductile shear was still in progress at 22 Ma. Fast uplift subsequently caused the tempera- ture to drop and ductile shear to stop around 20 Ma in the gneisses now exposed at the surface',, even though, at the same structural level, brittle left-lateral faulting probably continued along zones now eroded, buried or juxtaposed with the active Red River Fault traces. The change to the present day stress field (NNW-SSE shortening) and the onset of righMateral slip on the active Red River Fault took place later (Allen et al., 1984; Tapponnier et al., 1990), possibly around 5 Ma (Leloup et al., 1993).
3.4. Uplift mechanisms
The occurrence at the surface of the young, high- grade metamorphic rocks of the Ailao and Diancang Shan massifs implies great uplift. It seems that two distinct processes have led to the exhumation of the gneisses (Tapponnier et al., 1990) but with a signifi- cant difference between the two massifs.
In the Diancang Shan, assuming a crustal density of 2.8, the highest pressure recorded (sample DC3) corresponds to a depth of approximately 25 km. The
56 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
T°C 800
700
600
500
400
300
200
100
®o 15
T°C 800
7OO
600
500
400
300
200
100
@
~ect ion C..J I I t I ' I I I 1 I
20 25 Age (Ma)
T°C 800
700
600
500
400
300
200
100
®o 15 20 25
Age (Ma)
~zDiancang Shan h one P and section QJ
YS35
DC10c b,\\\',,,\~ C DC2a
II// Dc~t oc2~ f J
DC9c
DC16 ~ '~
DC9c
0 " ~ ! I I I I I ~ ! ! I I I I I I I I 1 l f I
0 5 10 15 20 25 Age (Ma)
I " j Kfm°del ~:~i whitemica'~39 40 xen°time~,u/ bl Q biotite [ ] amphiboleJ Ar/ Ar [ ] [ ] monaziteJ P
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 57
cooling history (Fig. 27c) suggests two successive uplift episodes (Leloup et al., 1993). Rapid cooling starting at 4.7 Ma may correspond to the activation of the normal fault bounding the range to the east. Such faulting could account for = 10 km of ex- humation and the formation of 4-5 km of structural relief (Tapponnier et al., 1990; Leloup et al., 1993). Cooling between 24 and 17-19 Ma (Fig. 27) could correspond to movement on, and denudation of the Diancang Shan by, the large oblique shear plane that bounds the massif to the south (Leloup et al., 1993). This movement produced large-scale boudinage of the shear zone during late stages of left-lateral shear. It might also account for about 10 km of uplift.
In the Ailao Shan, given the maximum pressure estimate (paragenesis Plb, 5 + 1.5 kb), the gneisses have been uplifted less, about 18 km since crystal- lization of that paragenesis. The cooling history found in zone E implies that temperatures lower than those corresponding to PI ( = 700°C) had been reached at = 23.5 Ma (Fig. 27b). If uplift had been uniform since 23.5 Ma, it would have occurred at a minimum average rate of the order of 0.8 m m / y r . The small pressure drop between Plb and P2 suggests that uplift started at the time of P1, during left-lateral deformation (Leloup and Kienast, 1993). Although such a pressure drop is within thermobarometers uncertainties, the fact that rapid cooling ( = l l 0 ° C / M a , Fig. 27b) took place at this time sup- ports this inference. Along section C and in zone E, the cooling histories retrieved from the 39At/4°Ar data (amphiboles and K-feldspars) show only one cooling phase (Fig. 27a,b) (Harrison et al., 1992a, 1995) apparently roughly coeval with the older cool- ing phase in the Diancang Shan (Fig. 27). In con- trast, however, temperatures in the Ailao Shan dropped below 200°C at ~ 19 Ma, = 100°C less than in the Diancang Shan at the same time, and the thermochronological techniques used do not give information on the thermal history below 200°C.
Since the ductile strain and left-lateral offsets appear to be large, small deviations in the shear direction from the horizontal can induce significant vertical movement along parts of the shear zone (Tapponnier et al., 1990). This led Harrison et al. (1992a) to interpret rapid cooling around 20 Ma as a result of unroofing, due to a component of normal throw and oblique extension during the overall left- lateral shear regime. Reconstruction of the opening of the south China Sea, which implies that left-lateral movement on the ASRR zone results in transtension along the southeastern part of that zone, southeast of the Diancang Shan (Briais et al., 1993), is consistent with this interpretation.
Although the cooling history of the Ailao Shan after 19 Ma has not been deciphered yet, significant uplift of its gneiss core must have occurred on the active Range Front Fault, which bounds it to the east, even though the present structural relief (2-2.5 km), a lower bound of such uplift, appears to be less than that in the Diancang Shan.
3.5. Separation of the Ailao Shan and Diancang Shan: origin of the Midu gap
Although the same left-lateral shear episode is clear in both the Ailao Shan and Diancang Shan, which are elongated and aligned parallel to the shear direction, the two ranges are separated by the = 80 km long Midu gap, in which no metamorphic rock is exposed (Fig. 3) (Bureau of Geology and Mineral Resources of Yurman, 1983; Tapponnier et al., 1990). The two ranges are connected across that gap by the active fault traces that form the northwest extension of the Red River fault zone (Fig. 5) (Allen et al., 1984). Between the southern tip of the Midu basin and section M east of Nanjie (Figs. 3 and 28a), those traces are lined with 1-2 km wide zones of dark,
Fig. 27. Thermochronology of gneisses of Ailao Shah and Diancang Shan cores. (a) Thermochronology of Ailao Shan gneisses along section C (Figs. 9 and 12). (b) Thermochronology of Ailao Shan gneisses in zone E (Fig. 9), Samples YS9 and YXI6 are from site El, sample FA2 from site E2 (Fig. 9), In (a) and (b), 4°Ar/39 Ar data axe from Harrison et al. (1992a, 1995) while monazite and xenotime U/Pb results are from Sch~er et al. (1990). (c) Thermochronology of Diancang Shan gneisses in zone P and along section Q (Figs. 19 and 20),4°Ar/39mr data from Leloup et al. (1993). Monazite an xenotime U/Pb results from Liu et al. (1992) and Seh'm'er et al. (1994). Sample DC9c is from site P2, samples DC5 and DC2 are from zone P3 (Fig. 19). Samples DC10, DC16, DC28 and YS35 are from section Q (Figs. 19 and 20).
58 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
clay-rich gouges (Leloup, 1991). Southwest of the gouges, Mesozoic red beds are strongly folded. Ten kilometers south of the fault zone, these steep sand- stone beds exhibit no cleavage. Closer to it, they become more deformed and display spectacular, steep, fracture cleavage striking N140°E. Between those schistosed sandstones and the gouges, a = 1.5 km wide stripe of calcschists affected by strong flow cleavage is exposed (Fig. 28a). At site M1 (Fig. 28a), the flow cleavage strikes N-S (__+ 20 °) and is cut by two sets of conjugate shear planes consistent with N100 + 10 ° directed shortening (Fig. 28b,c,d). The more prominent shear planes are left-lateral and strike N170°E on average (Fig. 28b,c). Minor, right- lateral shear planes strike N50°E on average (Fig. 28c). The progressive increase in strain intensity towards the NE, the size of the gouge zones, the predominance of left-lateral shears and the shorten- ing direction are all consistent with large left-lateral movement, rather than with recent fight-lateral slip (Allen et al., 1984) along the RRF zone. We con- clude that the left-lateral Ailao Shan-Diancang Shan shear zone crosses the Midu gap, but at a higher structural level. North of section M, the brittle left- lateral shear zone, and the corresponding gouges appear to lay buried under the Quaternary fill at the edge of the Midu Basin (Leloup, 1991). This brittle zone is thus not strictly aligned with the Diancang Shan (Fig. 3). Such observations led Leloup et al. (1993) to propose the following structural history: (1) until = 23 Ma, the Diancang Shan and Ailao Shan formed a continuous shear zone; (2) at least
20 km of left-lateral movement subsequently took place along the oblique shear plane at the southern tip of Diancang Shan between --~ 23 and = 17 Ma, accounting for the left step between that tip and the Midu brittle shear zone (Fig. 25b) and for = 10 km of denudation and uplift of the Diancang Shan; (3) tectonic activity along the ASRR was minor, at least in this area, between = 17 and 5 Ma; (4) starting at 4.7 Ma, slip on the active faults now bounding the Diancang Shan, leading to further rapid uplift of this massif and fight-lateral offset of parts of the left- lateral shear zone, followed activation of the RRF in a right-lateral sense. This scenario accounts both for the two distinct uplift phases documented in the Diancang Shan and for the different structural levels now exposed along this part of the ASRR-RRF zone.
3.6. Could the Ailao Shan and Diancang Shan gneiss cores result from processes other than left-lateral shear?
When taken at face value and in its simplest acceptance, the evidence presented in this paper is compatible with the interpretation first proposed by Tapponnier and coworkers (Tapponnier et al., 1986, 1990), namely, that the Ailao Shan-Diancang Shan gneisses mark the trace of a large, left-lateral Ter- tiary shear zone between Indochina and South China. Is this interpretation uniquely constrained by the data, however? Could other, more complex scenarios account equally well for the observations? In particu- lar, could the present attitude of the gneisses be very different from when they formed? Might they not correspond to a deformed drcollement between base- ment and shortened sedimentary cover or within the middle-lower crust?
In that case, initially flat-lying gneisses (Fig. 29a) would have been subsequently bent into a steeper, apparently strike-slip attitude. In the Diancang Shan, along section Q, the foliation planes do display an inverted fan geometry (Fig. 20), with a common intersection parallel to the mean lineation (Fig. 22a). This geometry could reflect folding of an initially flat foliation about an axis parallel to the lineation. But that mechanism would require structural symme- try, and inversion of the shear criteria from one limb of the fold to the other (Fig. 29b). This can be ruled out, since the observed shear senses are left-lateral irrespective of the dip of the foliation. The inverted foliation fan thus probably reflects upwards thinning of the shear zone.
Warping by drag of a flat foliation near the normal faults that bound the gneisses could also account for their present attitude (Fig. 29c). This, however, would also result in dip-dependent shear senses: left-lateral along NE-dipping faults (Ailao Shan, east flank of Diancang Shan), right-lateral near SW-dipping faults (western flank of Diancang Shan), which is not observed (Fig. 29c). Besides, in the Ailao Shan, there is no evidence that the gneisses plunge under the low-grade schists towards the southwest.
The Ailao Shan gneisses could be interpreted as initially flat mylonites upthrusted onto the schists along the Ailao Shan fault (Fig. 29d). There is,
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 59
Section M Active N El
3ooo SW Red River fault -~ [- Site M1 1 !
1kin I Nanjie=2k m t ~ l (a~
~ R e d sandstones ~ Schistosed sandstones ~ Calcschists ~ l Fault
Gouge
d
i
L_
~ N170
Fig. 28. Deformation in Midu gap, between Ailao Shan and Diancang Shan. (a) Cross-section M (located on Fig. 3); site M1 refers to photograph and sketch on (b) and (c). (b) Left-lateral shears in calcschists at site M1, shear planes and foliation strike NI40°E and N20°E, respectively. Horizontal plane, view from above. (c) Sketch of conjugate shear bands observed at site M1. Dominant left-lateral shears imply rotational deformation. (d) Shaded zones represent respective ranges of observed left- and right-lateral shears, NI00°E shortening direction inferred from conjugate shears is compatible with left-lateral shear along N140°E-trending ASRR zone.
however , little strain evidence for large-scale over- thrusting (dip slip l ineat ions, top- to- the-SW shear criteria) in the Ailao Shan, and upthrust ing cannot account for the structure of the Diancang Shan.
All al ternative hypotheses discussed above would imply warping of a flat d6col lement about an axis parallel to a preexis t ing l ineat ion for a length in excess of 500 km, which seems quite improbable.
60 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
®
®
~ - . - / / 1
Fig. 29. Alternative hypotheses to interpretation of Ailao Shah and Diancang Shan as left-lateral shear-zones. All hypotheses involve a regional drcollement level (a), later folded (b), affected by normal faults (drag folding), (c) or by thrust (d).
Such a lineation would trend NW-SE, a direction markedly different from that of the E - W shortening compatible with folding of the cover rocks, or that obtained by unfolding the lineation on the Yulong Shan drcollement (Lacassin et al., 1993a, 1995). The sense of transport, for at least 500 km, would be towards, rather than away from the India-Asia colli- sion front, also an improbable circumstance. We conclude that the interpretation of the Ailao Shan- Red River zone as a major left-lateral shear zone is both the simplest and most plausible one.
3.7. Continuation of the Ailao Shan-Red Riuer shear zone towards the N and S
3.7.1. The Xuelong Shan and Sanjiang fault zone
3.7.1.1. The "Madeng gap" and the Xuelong Shan. A = 90 km long hiatus separates the northern tip of the Diancang Shan from the northernmost high-grade metamorphic massif identified so far along the RRF, the Xuelong Shan (Fig. 6). By analogy with the Midu gap, we refer to this zone as the "Madeng gap". This gap is crossed by the trace of the active Qiaohou fault, along which Mesozoic calcschists exhibit strong schistosity (section S, Figs. 3 and 7a), as on section M in the Midu gap (Figs. 3 and 28).
North of section T', west of the Weixi basin and east of the N-S Mekong valley, the Xuelong Shan ("Mountain of the snow dragon") forms a NW- SE-directed range about 50 km long and l0 km wide, culminating at 3909 m (Fig. 30a). As in the Ailao and Diancang Shan, the steep NE topographic flank of the range (Fig. 30b) is marked by conspicu- ous triangular facets attesting to recent uplift along the active normal fault bounding the Weixi Neo- gene-Quaternary basin. Until recently, the rocks of the Xuelong Shan had been mapped as slightly meta- morphosed Paleozoic sediments (Bureau of Geology and Mineral Resources of Yunnan, 1983), possibly because the area was remote and the range, covered with abundant vegetation, not accessible by road. More recent geologic maps (Bureau of Geology and Mineral Resources of Yunnan, 1984) show that this range is made of NW-SE-striking, metamorphic strips: epimetamorphic Triassic rocks (mostly schists and marbles) to the east, and high-grade gneisses (gneisses and amphibolite schists) to the west (Figs. 7b and 30a). East of the Weixi basin, Triassic vol- canics and sedimentary rocks are sliced by NW-SE faults, parallel to the range (Fig. 30a). Toward the west, N-S folds affect Mesozoic-Tertiary red beds (Figs. 6 and 30a).
We were able to study the deformation of the metamorphic rocks along an incomplete section only
4 km long, near the southern tip of the massif (section T', Fig. 30). Approaching the range from the north, the Triassic volcanics and limestones are tightly folded, steeply dipping, strongly brecciated and sliced by cataclasite zones. A 40 m wide gouge
P.H. Leloup et al./ Tectonophysics 251 (1995) 3-84 61
zone separates the massif core from these deformed rocks. This gouge zone, which is aligned with the steep NE flank of the range, probably links the recent normal fault along it with the Qiaohou fault farther south (Fig. 30a,b). West of the gouge zone, one finds schists, strongly deformed rhyolites, fine- grained gneisses, micaschists with leucogranite and amphibolite boudins, and migmatitic gneisses (Fig. 30b). All the metamorphic rocks display mylonitic texture and evidence of intense non-coaxial deforma- tion. The foliation dips 37°E and strikes N161°E on average, nearly parallel to the N150°E strike of the Xuelong range (Fig. 30c). A strong stretching lin- eation is observed along the whole section (Fig. 31a), striking N170°E on average with a small pitch to the north (Fig. 30c). As in the Ailao and Diancang Shan, numerous shear-sense indicators are visible, including (1) C-S structures (Fig. 31b,c), (2) por- phyroclasts with asymmetric tails (Fig. 31d), (3) asymmetric foliation boudinage, and (4) rolling structures. All these indicators are consistent with top (east side)-to-the-north or left-lateral shear.
Our preliminary study of the Xuelong Shan meta- morphic rocks thus implies a structure and strain regime similar to those described along the Ailao and Diancang Shan. The temperature during defor- mation may also be comparable since deformed migmatitic paragneisses crop out at the southeast end of section T'. One notable difference is the attitude of the foliation, which is flatter than in most of the Ailao and Diancang Shan, with the exception of regions near their southern ends (e.g. section A, Fig. 12, and zone P3, Fig. 19). It is possible, however, that the foliation becomes steeper in the inner core of the central and northern Xuelong Shan, as suggested by the rather straight petrographic boundaries on either side (Fig. 30a) (Bureau of Geology and Min- eral Resources of Yunnan, 1984). Like the other two ranges, the Xuelong range is bounded to the east by an active normal fault that probably accounts partly for the uplift of the gneisses. For these reasons, we conclude that, in all likelihood, the Xuelong Shan belongs to the Ailao Shan-Red River shear zone. Preliminary Eocene-Oligocene U/Pb ages (Sch~er et al., in prep.) support this contention.
3.7.1.2. The Sanjiang fault zone. There appears to be no other comparable metamorphic massif northwest
of the Xuelong Shan, at least in Yunnan, on extant large-scale geologic maps (e.g. Bureau of Geology and Mineral Resources of Yunnan, 1984; Bureau of Geology and Mineral Resources of Yunnan Province, 1990), although a N-S zone of "dynamo-metamor- phism" reportedly exists along the upper Mekong valley, approximately between 27°40'N and 29°N (Wang and Chu, 1988, fig. 2). North of the Xuelong Shan, the Sanjiang (three rivers) area is densely sliced by N-S-striking fault zones (Fig. 6). One such prominent zone passes west of Deqen, at the western extremity of section U2 (Figs. 6 and 8a). There, vertical, kilometric slivers of various rocks are sepa- rated by gouge and cataclasite zones possibly associ- ated with strike-slip movement. The sense of slip on that fault is not known, however, and both the Benzilan suture, and the Gaoligong Shan gneiss belt along the Salween valley (Zhong Dalai, pers. com- mun.), which strike N-S, show evidence of right- lateral shear. Besides, at the rim of the east Hi- malayan syntaxis, strike-slip deformation in the three rivers area has probably been overprinted by intense shortening. Thus, at this stage of our study, it seems equally possible that the RRF continues north of Xuelong Shan past Deqen, or that it has been offset right-laterally by the N-S-striking fault zones that follow the Mekong and Salween for hundreds of kilometers.
3.7.2. The Day Nui Con Voi South and east of the Ailao Shan, the Day Nui
Con Voi ("elephant back mountain range") stretches for about 300 km along the north bank of the Red River (Fig. 2). With a width of 10-15 km, it: forms the southernmost massif of the ASRR metamorphic belt. In southern Yunnan, that range nearly :merges with the Ailao Shan (Bureau of Geology and Mineral Resources of Yunnan, 1983), being separated from it by the active Red River valley fault (Fig. 21). Near the Yunnan-Vietnam border, this fault splays into two roughly parallel strands, the Song Chay and Song Hong faults, which bound the Day Nui Con Voi to the north and south, respectively (Figs. 5 and 32a). Currently, both fault-strands appear to slip mostly right-laterally, with variable components of normal slip.
As for the Ailao Shan, the Day Nui Con Voi metamorphic rocks (mostly gneisses) are mapped as
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 63
Proterozoic (General Geological Department of the Democratic Republic of Vietnam, 1973; Bureau of Geology and Mineral Resources of Yunnan, 1983). South of the Song Hong fault, the Ailao Shan termi- nates in the reportedly Paleogene Phang Si Pang granite massif (General Geological Department of the Democratic Republic of Vietnam, 1973). The only available geochronological study of that granite has provided K / A r ages between 80 and 29 Ma (Izokh et al., 1964). Our preliminary mapping and sampling of the Day Nui Con Voi gneisses across one nearly complete section and at four sites outside that section shows that most rock types are mylonitic with a well-defined foliation, striking nearly parallel to the belt, as in the Ailao Shan (Fig. 32b). Along the Bao Yen section, the foliation is generally steep, dipping either towards the SW or NE, as in the Diancang Shan (Fig. 32a,b). As in the Ailao and Diancang Shan the stretching lineation is everywhere well defined and nearly horizontal (Fig. 32b). Shear criteria are left-lateral irrespective of the foliation dip (Fig. 32c,d). Preliminary U /Pb and 39Ar/*OAr radio- metric dating yields Eocene-Oligocene ages (Sch~er and Leloup, work in progress).
Hence, we interpret the Day Nui Con Voi to be part of the ASRR shear zone, which brings this narrow belt of Tertiary, left-laterally sheared, meta- morphic rocks to a total outcrop length of 900 km, almost to the shore of the South China Sea (Figs. 1 and 2).
4. Summary and discussion
4.1. The Ailao Shan-Red River shear zone, a refer- ence model for H T lower-crustal strain in plate-scale strike-slip faults
The results of the comprehensive study presented here firmly establish the existence of a narrow zone of intense Tertiary deformation and metamorphism that can be traced across much of continental south- east Asia. While this zone, dubbed Ailao Shan-Red River (ASRR) shear zone by Tapponnier et al. (1986, 1990), follows the better known right-lateral Red River Fault (RRF), it is now demonstrated to have been the site of prominent left-lateral movement in the Oligo-Miocene.
The evidence for such movement at that time is both impressive and remarkably consistent, for nearly 1000 km along strike, throughout Yunnan and north Vietnam. Though misinterpreted or overlooked, sur- prisingly, by previous workers (e.g. Bally et al., 1980; Duan Xinhua, 1981; Allen et al., 1984), such evidence is magnificently exposed in the --- ~0 km wide cores of four elongate ranges (Xuelong Shan, Diancang Shan, Ailao Shan, and Day Nui Con Voi) in which high-grade, amphibolite-facies gneisses stand exhumed.
Together with the aspect ratio (1/100) of the metamorphic zone, all the measurements at hand rule out an origin by processes other than longlasting, localized strike-slip motion. The gneisses were de- formed at depths of 15-25 km under peak tempera- ture and pressure of 710 + 70°C and -~ 7 Kb, respec- tively. The steep, usually N-dipping, mylonitic folia- tions bear near horizontal stretching lineations every- where parallel to the local trend of the zone. Left- lateral senses of ductile shear are ubiquitous and clear. Crystallization ages and cooling histories of various minerals in the gneisses demonstrate that such shear went on unabated for at least = 10 Myr (between ---27 and 17 Ma) (SchSrer et al., 1990, 1994; Harrison et al., 1992a; Leloup et al., 1993). Since the location, chemical composition and age (35 + 0.1 Ma) of the Jianchuan monzonites imply that they derive from anatexy in the shear-zone continuation under the Madeng gap, the duration of shear was more likely longer than 18 Myr (= 35-17 Ma) (Sch~er et al., 1994).
There is no geochronological evidence for mag- matism and metamorphism during the Paleozoic and Mesozoic along the Ailao and Diancang Shan, as ought to be the case if the ASRR zone had been the site of subduction followed by suturing and collision during the Indosinian orogeny (e.g. Bally et al., 1980; Duan Xinhua, 1981). Regressions of discor- dant zircon ages only yield upper intercepts between 670 and 1600 Ma, implying a Proterozoic protolith for the gneisses (Sch~er et al., 1994). Hence, con- trary to that fashionable view (e.g. Dewey et al., 1989; Duan Xinhua, 1981; Wu Haoruo et al., 1994), and no matter how profound a geologic, tectonic and geomorphic discontinuity, that zone is not a Phanero- zoic suture. In keeping with this conclusion, the dismembered ultramafics and low-grade schists ex-
64 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
P.H. Leloup et al. / Tectonophysics 25l (1995) 3-84 65
posed along the southern Ailao Shan can only be remnants of a suture transverse to it, offset and smeared a long distance by transcurrent motion (Tapponnier et al., 1986, 1990). The final effect of strike-slip shear and "parasi t ic" uplift has in fact been to juxtapose two belts of metamorphic rocks with highly contrasting grades: one composed of recent, high-temperature rocks generated in the lower crust from an ancient protolith and locally uplifted, the other made of older low-temperature rocks trans- ported at shallow depth in the upper crust. Such a simple mechanism probably accounts best for the formation of paired metamorphic belts in which lin- cation is parallel to trend.
To date, not only is the ASRR shear zone the only prominent zone of well-documented Tertiary meta- morphism in Asia outside the Karakorum and Hi- malayas, but it appears to be the only exposed strike-slip shear zone of that size and age, with that degree of metamorphism and anatexy, worldwide.
Among other zones with a similar strain style, that which follows the Alpine Fault of southern New Zealand bears perhaps the closest resemblance, but it seems less developed and is much less exhumed. The alpine schists, which are interpreted to derive from trench greywackes (Landis and Bishop, 1972), form a narrow (10-15 km wide) metamorphic stripe along the central segment of that fault, mostly between Mount Aspiring and Arthur's pass. That distance, about 300 km, is shorter than the length of just the Ailao Shan in Yunnan. The schists appear to have reached only the lowermost amphibolite grade (garnet-oligoclase, T < 550°C), and this mostly within = 5 km south of the fault. Although some mylonites with lineations parallel to fault-strike, and isolated, concordant pegmatite dykes are found adja- cent to the fault ( < 1-2 kin) (Grindley, 1963; Reed, 1964), there is no description of the steep, many kilometers thick, horizontally lineated, mylonitic gneiss slabs, or of the widespread synkinematic in- trusives and often pervasive anatectic melting that
prevail in the cores of the Ailao and Diancang Shan. Originally, most of the strain and metamorphism of the alpine schists had been interpreted to have taken place during the Mesozoic (Rangitata orogeny, be- tween 140 and 80 Ma), concurrent with 3 / 4 of the total dextral displacement (480 km) that the offset between the Otago and Marlborough arc terranes implies on the fault (Wellman, 1955; Wellman and Cooper, 1971; Scholz et al., 1979). Only slight retro- grade metamorphism, together with at most --- 12 km of uplift, was reported to have accompanied the remaining horizontal displacement (120 km) in the late Cenozoic (Kaikoura orogeny, 5 Ma-present). Shear heating during the latter event was interpreted to have been enough to reset K - A r and Rb-Sr ages near the fault (e.g. Sheppard et al., 1975; Scholz et al., 1979). More strain is now thought to have oc- curred since the mid-Tertiary, in better agreement with plate tectonic reconstructions (e.g. Weissel et al., 1977; Stock and Molnar, 1987). But where the Alpine schist belt is transcurrent, it is clear that such strain has not exposed hundreds of kilometers long, tens of kilometers wide massifs of mid-lower-crustal rocks that suffered prograde, high-temperature Ter- tiary strike-slip shear.
Data from the Ailao Shan-Red River zone thus best document what ought to be the typical thermo- mechanical regime and style of deep-crustal strain in large continental strike-slip faults, for which it pro- vides a reference example. The recent age and high grade of the zone, in particular, has made it possible to begin dating thermal changes resulting from tran- scurrent deformation with unprecedented accuracy (Fig. 27).
The most constant structural character is the trend of the lineation, which is locally parallel to relative movement, everywhere and irrespective of the more variable dips of the foliation. Although such my- lonitic foliation is usually steep, variations in its dip probably result from rheological layering of the crust and from asymmetric boudinage, an important and
Fig. 31. Deformation along section T' across Xuelong Shan (Fig. 30b). (a) Aspect of foliation (dipping 40 ° towards ENE) and nearly horizontal lineation (pitch 20°N) at site 02. Pencil indicates scale. (b,c) Left-lateral C-S structures in micaschists at sites 03 and 04 (Fig. 30b), respectively. View from above perpendicular to foliation and parallel to lineation, north on left side, approximate scale shown by coin. (d) Polished slab of mylonitic meta-volcanic rock viewed from above (sample YN100 from site 01). Asymmetry of feldspar porphyroblasts indicates left-lateral sense of shear. Slab is approximately 7 cm long.
66 P.H, Leloup et al. / Tectonophysics 251 (1995) 3-84
ubiquitous process at various scales and development stages of the shear zone. That process eventually contributed to dismember the zone (Leloup et al., 1993).
The evidence for long-lasting high-temperature spells, with anatectic melting of crustal rocks and, apparently, some degree of mixing with rising man- tle melts, is compelling. In the northern Ailao Shan, the temperature was high enough to cause unan- abated melting for at least = 4 Myr (26.3-22.4 Ma), while it remained above the solidus for nearly 2 Myr (24.2-22.4 Ma) in the Diancang Shan (SchSxer et al.,
1994). Such steady, high temperatures are consistent with shear heating at a sustained slip-rate of several centimeters per year (Nicolas et al., 1977; Fleitout and Froidevaux, 1980), under a depth-averaged shear stress on order of 50-100 MPa (Scholz et al., 1979), with none of the ambiguities that make the evidence from other shear zones, particularly in thrust ter- ranes, inconclusive (e.g. Scholz, 1990).
The synoptic model that emerges (Fig. 33) is of a mature shear zone, comparable to that discussed, for instance, by Scholz (1990, fig. 3.26), but with mag- matic melts contributing to weakening and strain-
I J \~ ' B~oY~o Loo oJ , .000m , F 7 °u= ..... ' o~..a. Oo°Vo, I ~ ~n°'°ch,,' . . . . ~ V; '~ ~moh'h°"te
"~ Neogene metamorphic rocks ~ Marble ®
N
/
it Fig. 32. Structure and deformation of Day Nui Con Voi (DNCV), located on Fig. 2. (a) NE-SW cross-section south of Bao Yen (Fig. 2). Drawn from field observations. Topography is from 1:250,000 Vietnamese topographic maps. (b) Schmidt diagram (lower hemisphere) of foliation (lines) and lineafion (dots) along Bao Yen section (Fig. 32a). (c,d) S-C' mylonites at site DNCV1 [located on (a)] indicate a left-lateral sense of shear. View from above.
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 67
Fig 32 (continued).
68 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
softening at and below the crustal brittle-ductile transition, and with downward extension into the mantle. The particularly high temperatures at rela- tively shallow depth, higher than suggested by sim- ple shear-heating models (Fig. 33; Leloup and Kien- ast, 1993; Leloup et al., 1995), require upward trans- port of heat by magmatic fluids. The length and slenderness of the ASRR zone, the amount of dis- placement, discussed later, the location and alkalic chemistry of the Jianchuan monzo-syenites, which imply involvement of mantle magma sources (Sch~er et al., 1994), require localized shear in the litho- spheric mantle, roughly beneath that in the crust, as documented, for instance, along the Precambrian Norde-Stromfjord shear zone in Greenland (Soren- sen, 1983). Finally, the fact that hypabyssal rocks intrude the shallow red beds of the Madeng and Midu gaps (Fig. 3) suggests volcanism along the zone in the Eocene, a circumstance impossible to establish in more ancient, deeply eroded terranes. Hence, as shown on Fig. 33, the ASRR shear zone may have acted for significant periods of time as a more or less continuous melt conduit, or cleft, all the way from the asthenosphere to the surface, which might ultimately account for its long lifespan as a weak deformation zone. The common occurrence of such a process within the continental lithosphere would help explain why models of deformation based upon bulk, solid-state flow-laws of rocks (e.g. Eng- land and Houseman, 1986) yield results that bear such little resemblance with the large-scale features observed in the India-Asia collision realm.
4.2. Concurrent evidence for a finite left-lateral off- set > 500 km across the ASRR zone
4.2.1. Right- and left-lateral offsets The finite, geological offset across the Red River
Fault zone has long been a subject of debate. Confu- sion has arisen in part because motion along the zone switched from left-lateral in the Oligo-Miocene to fight-lateral in the Plio-Quatemary.
Estimates have ranged from --200-250 km right-lateral to more than 1500 km left-lateral. Be- fore starting a critical discussion of the data at hand, let us recall that, given plate tectonics and scaling laws, it would be foolish to search for finite displace-
ments smaller than a few tens of kilometers on a strike-slip shear zone 1000 km long. Nor should there be any hope to map such displacements by studying the zone in detail over a length of less than
100 kin. The idea that the fault zone has offset dextrally
the Yangbi and Chuxiong basins (Figs. 2 and 34b) by = 200-250 km (e.g. Cobbold and Davy, 1988; Dewey et al., 1989), a speculation based on the apparent, present-day separation of their western edges on 1:5,000,000 scale maps, can be dismissed at once. For such a finite geological offset to subsist, fight-lateral strain would have had to overwhelm left-lateral strain along the zone, which is clearly not the case, the latter dwarfing the former by at least one order of magnitude (e.g. Allen et al., 1984; Lacassin et al., 1993b). That inference is all the more unfounded that the Chuxiong basin is just one of several upper Mesozoic basins north of the fault zone (Figs. 1, 2 and 34b).
In fact, all the prominent dextral offsets visible along the fault are of geomorphic, not geologic, nature. The best documented offset (= 6 km) is that of the Red River southeast of Daqiao (Fig. 35) and northwest of Atu, and of many of its tributaries in between (Allen et al., 1984). This offset, which has been estimated, from inferences on incision and re- gional uplift rates, to have accrued since sometime between the mid-Pleistocene (750 ka) and the Late Pliocene (3 Ma), has been used to place bounds on the current dextral slip-rate (2-8 mm/yr; Allen et al., 1984). It probably falls short of the total dextral offset on the fault, however. Larger valley offsets that help account for the complex geometry of the Red River's northern catchment where it crosses the Midu gap, can be found on satellite images and topographic maps. The clearest offset, about 20 km (Fig. 35), is between the deeply incised, = NE- trending Shuitian valley west of the fault (e.g. Allen et al., 1984, fig. 8), possibly a former, abandoned course of the Red River, and its entrenched, NE- trending channel east of the fault, past the bend downstream from Daqiao. Greater offsets (about 43 and 57 km, respectively) are obtained by matching the Daqiao or Shuitian valleys, west of the fault, with the Red River east of the fault and upstream from Danuo, where it tangents the Ailao Shan range- front (Figs. 3 and 35; Allen et al., 1984).
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33.
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e. B
ecau
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1993
). M
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ross
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ar
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) D
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tal
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ctur
e. D
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ide
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70 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 71
• Midu MIDU~:" "B,~IN ,~..,~
Shui
~01!00'E eNanhua
i 25"00'N
25"00'14
Nanjie
101 20= 301 401 501 60kml ~ / / / ~ ~
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Fig. 35. Plausible right-lateral offsets of drainage across active Red River Fault trace (bold line) south of Midu.
That the uplifted Ailao Shan metamorphics form a barrier generally impassable to drainage, guiding much of the Red River course, accounts for the fact that few offsets larger than 6 km have been found, and so far mostly near the Midu gap. Although the evidence for such offsets is limited, we infer 2 0 - 5 0 km to be a plausible range of values for the finite late Cenozoic dextral displacement on the active Red River Fault (Fig. 36). If such amounts of displace- ment had accrued at a uniform slip rate since ---5 Ma, consistent with the onset of the second uplift phase in the Diancang Shan, that rate would be on the order of 7 _ 3 m m / y r , slightly greater than that (5 + 3 m m / y r ) inferred by Allen et al. (1984).
There is, by contrast, no shortage of convincing clues for very large cumulative sinistral offsets on the ASRR shear zone. When taken together, several
lines of independent evidence point to tile in- escapable conclusion that the finite amount of left- lateral displacement taken up by that zone is in excess of 500 km, even though quantitative data are presently insufficient to constrain the exact value.
4.2.2. Strain rate and finite strain in the gneisses
A shear strain rate of 2 -3 c m / y r for only = 4 Ma, probably a lower bound to account for a contin- uous melting span of that duration in the northern Ailao Shan (e.g. Scholz, 1990), would require a finite left-lateral offset on order of 100 km. If main- tained for what we infer to be the likely minimum duration of left-lateral shear ( = 18 Ma), such a rate would have produced a finite geological offset of 360-540 km (Fig. 36).
72 P.H. Leloup et al . / Tectonophysics 251 (1995) 3-84
An offset of that size is consistent with the strain features observed and, where feasible, measured in the Ailao and Diancang Shan gneiss cores. The pervasive compositional banding, the overall my- lonitic strain style, the ubiquitous occurrence of asymmetric foliation boudinage, and the frequent presence of sheath folds (Figs. 12-17, 20, 21, 31 and 32), all qualitatively indicate shear strains (~/) in excess of 20 (e.g. Bell and Etheridge, 1973; Quin- quis et al., 1978; Lacassin and Mattauer, 1985; Gaudemer and Tapponnier, 1987; Malavieille, 1987; Lacassin, 1988). Quantitative support for this infer- ence is provided by measurements of the layer-paral- lel extension of stretched amphibolites within the paragneisses. Surface-balanced restoration of several foliation-parallel amphibolite boudins trails at one site in the northern Ailao Shan yields ~ = 33 + 6 (Lacassin et al., 1993b). Portions of the shear zone are less deformed. This is mostly the case of anatec- tic melts and intrusives, which are generally late- comers within various parts of that zone. Accord- ingly, the same restoration technique, applied to stretched melt veins at two distant sites along the Ailao and Diancang Shan, yields "y = 7 + 3. On the other hand, tens of meters wide ultramylonitic corri- dors, in which strain is beyond measurement, com- monly occur in the gneisses. In addition, as shear went on for millions of years, progressive strain kept obliterating many anterior deformation features, so that those now visible in the rocks testify to only a fraction of the bulk finite strain. This suggests that the finite deformation observed across the 10 km wide ASRR zone is compatible with a minimum amount of left-lateral displacement of perhaps 200 km, if allowance is made for minor departures from a simple shear regime, or of 330 + 60 km, if the strain measured in the amphibolites is taken to be a typical average value (Lacassin et al., 1993b).
4.2.3. Geological offsets Given the nature and strain style of the ASRR
shear zone, examination of the way in which it relates to the main features of the regional geology and tectonics, discussed in the first part of this paper, brings forth first-order constraints on its finite left- lateral offset (Figs. 1, 2, 3, 6 and 34). That there is no geochronological record of major tectono-meta- morphic events along the zone between the late
Proterozoic and the Tertiary simplifies the task of looking for offsets since, save for differential strain within regions on either side, one should expect most transverse Phanerozoic markers to be offset by roughly the same amounts.
To begin, recall that the Late Eocene-mid- Miocene sinistral motion recorded in the high-grade gneisses of the ASRR zone is in full kinematic agreement with the main phase of regional shorten- ing to the south and north, a compatibility in large- scale strain that we have long pointed out (e.g. Tapponnier et al., 1986, 1990; Peltzer and Tappon- nier, 1988; Leloup, 1991). The maps and sections of Figs. 3, 4, 6-12, 28 and 30 document the style of that phase, which is shown to have prominently affected the Late Cretaceous-Paleogene red beds and more ancient rocks over a widespread area. The manner in which the deformation of those rocks south of the zone blends with shear within it brings further support to an intimate transpressive link. The cause of such shortening and transpression in the Eocene-Oligocene is unambiguous: there exists no other event to invoke than the collision between India and Asia.
Three main types of geological structures north and south of the ASRR zone may be used to estimate finite sinistral offsets: upper Mesozoic red-bed "basins" , upper Paleozoic-lower Mesozoic "su- tures", and belts or outcrops of intrusive or extrusive rocks in that same age range (Fig. 34). As is com- monly the case, there are problems with relying upon any such category of structures and rocks as large- scale displacement markers. First, while deeply un- derthrust tectonic imbricates, as in a suture zone, or intrusive batholiths, as in a magmatic arc, may be considered rather perennial transcrustal markers, ef- fusive volcanics or sedimentary deposits are little more than surface coatings, the initial attitudes of which are easily modified by shallow-crustal pro- cesses.
One problem with the red-bed "bas ins" , for ex- ample, is knowing whether they actually represent localized zones of subsidence and deposition or, instead, vestigial patches of a cover of formerly greater extent. The typical thickness (4-5 km) of the sequences in several such "bas ins" implies that the former is usually the case (e.g. Fig. 4), but the location of the original depocentres is often impre-
P,H. Leloup et al. / Tectonophysics 251 (1995) 3-84 73
cisely known. Whether the present limits of the basins coincide with their initial depositional bound- aries, are of a tectonic or erosional nature, or have significantly changed shape since deposition, raises additional problems. The western boundaries of the Chuxiong, Yangbi (or Lamping) and Khorat basins, for instance, are clearly tectonic. Such is the case, also, of all the boundaries of the Simao, Dongba, Qamdo-Markham, and Sichuan basins. The eastern limit of the Chuxiong basin and the southeastern limit of the Khorat basin, on the other hand, appear to be mostly depositional/erosional. In the Sichuan basin, the greatest thickness of red beds (-- 10 km) lies in the east, along the Lungmen Shan, implying a Mesozoic flexural origin. The majority of the red-bed "patches" of eastem Tibet, South China and In- dochina thus appear to be both basins, mostly of foreland type, and large synclinoria of Mesozoic- early Tertiary age. If, as our study of the regional tectonics suggests, the principal factor that changed the shape of those basin boundaries is the mostly Eocene (-- 55-30 Ma) folding and thrusting caused by the early impingement of India into Asia, then they may safely be used as first-order markers of Oligo-Miocene offsets.
Other difficulties arise from the lack of continuity of marker-features at the scale involved. Constrain- ing offsets on a fault zone with piercing points requires that fairly narrow, markedly oblique, and well-defined features be followed to near intersection with that zone. Given the geologic history and the accuracy of extant mapping, this condition is rarely met. The sutures and intrusive belts, potentially the best markers, crop out in discontinuous fashion be- cause the younger red beds tend to cover them. In addition, because Tertiary strain caused tectonic up- lift along the ASRR zone and involved left, or right-lateral motion on faults parallel, or at high angle to it, respectively, neither such markers nor all the boundaries of the red beds basins are easily traced to the very edges of that zone.
Once such difficulties are surmounted, wherever possible, two more sources of uncertainty occur. The first, a common pitfall, comes from the possible occurrence of several features of the same type on either side of the zone. This typically arises if the marker rocks are common, which is obviously the case here with the Mesozoic red beds and Permian
basalts (Fig. 34b,c), but also with the Phanerozoic sutures (Fig. 34a), because there are several of them and they are generally poorly dated. For instance, potential counterparts of the Song Ma suture of Tongking (Figs. 1 and 2), whose age is still con- tentious, are unclear northwest of the ASRR zone. The second source of uncertainty is due to smearing along the shear zone, which results in smaller appar- ent offsets. That such a process took place is implied by the presence of low-grade schists and ultramafics along the southern Ailao Shan. In all likelihood, this process also affected the Permian basalts and Trias- sic andesitic-dacitic tuffs between Deqen and Luchun, as well as rarer rocks found only near the shear zone.
Given all the possible sources of error and uncer- tainty discussed, and in spite of the correspondingly large error bars and dispersion, the sinistral offsets depicted in Fig. 34 and summarized in Fig. 36 and Table 1 show a fair degree of consistency.
The minimum left-lateral separation of three dif- ferent rock types that have been sinistrally smeared along the Ailao Shan-Red River zone indicates that the lowermost bound of such offsets is greater than 400 km. First, the belt of intermediate Triassic tuffs and agglomerates that terminates near the Jianchuan-Madeng section north of the zone'., only resumes south of it, west of the Yuangjiang-Mojiang road, at a distance greater than 400 km (Bureau of Geology and Mineral Resources of Yunnan, 1983) (Figs. 3 and 34c). Second, the thick Permian basalts last exposed between Midu and Lijiang north of the zone, are not found again south of it before Jlinping near the Vietnam border, at a distance of 460 km (Leloup, 1991; Wu Genyao, 1993) (Figs. 3, 6 and 34c). The typical, mostly alkalic petrology and chemistry of those rift basalts, whose pillows imply shallow-marine emplacement, is identical (Wu Genyao, 1993). Third, the only equivalent of the northern Diancang Shan and Jianchuan norites (N 3) (Fig. 18), a distinctive, rarer rock type, is found at the extremity of the Ailao Shan near the Vietnam border, 500 _+ 50 km to the southeast (Bureau of Geology and Mineral Resources of Yunnan. 1983; Leloup, 1991)(Fig. 34d).
Because outcrops of norites exist south of the Phang Si Pang, and both the tufts and basalts con- tinue along one side or the other of the ASRR zone,
74 P.H. Leloup et a l . / Tectonophysics 251 (1995) 3-84
the actual offset of these three rock formations is probably larger. The total offset of the Triassic tuffs and arc volcanics between Deqen and the Dien Bien Phu fault, for instance, more likely ranges between 650 and 780 km (Figs. 2 and 34c). More Permian basalts are found near Benzilan (Fig. 6). The offset between these basalts and those at the Yunnan- Vietnam border would amount to 580 km or more (Fig. 34c). The other prominent patch of Permian basalts that extends south of the Sichuan basin to Chengjiang, southeast of Kunming, on the other hand, stops too far from the Red River to be used as a reliable marker, even though a match with basalts of that age in southern Tongking would yield compa- rably large numbers.
Matching the Benzilan-Jinsha suture of eastern Tibet with the Nan-Uttaradit suture of Thailand, inferring that the latter continues into Laos to Lu- ang-Prabang, then along the dextral Dien Bien Phu fault, would yield an offset of 650-780 km (Fig. 34a), with smearing along the ASRR zone account- ing for this range of values, as for the Triassic arc volcanics. Although such values are somewhat greater than those inferred by extending the northern end of the Nan-Uttaradit suture under the Simao
basin (e.g. Peltzer and Tapponnier, 1988; Briais et al., 1993), the fact that we found clear evidence of dextral motion along the Benzilan suture may be taken as further justification for that match. Another possible match, and the same offset range, would be obtained by assuming that the Benzilan-Jinsha ultra- mafics mark the continuation of the Song Ma suture, first offset dextrally 100-200 km (Peltzer and Tap- ponnier, 1988) by the Dien Bien Phu fault. This alternative match would bring some elements of the late Paleozoic-early Mesozoic paleogeography into an acceptable framework, but would require the Jin- sha and Song Ma sutures to have the same age, which remains to be established. Even if the Song Ma ultramafics were ultimately found equivalent to those reported in the Litang area (e.g. Mattauer et al., 1992) (Fig. 2), a sinistral offset of --- 500 km on the ASRR shear zone would still be needed.
The most characteristic and continuous belts of intrusive rocks trending transverse to the Ailao Shan-Red River zone are the Permo-Triassic, Thai-Lincang range (granitic and granodioritic batholits), and the Cretaceous granites that follow the coasts of South China, in Guangdong province, and of South Vietnam, between Vung Tau and Nha
Table 1 Left-lateral offsets of geological markers across the Ailao Shan-Red River shear zone
South China Indochina Left-lateral offset along the ASRR zone (km)
Jinsha-Benzilan Song Ma or Uttaradit 650-780 Ophiolites
Mesozoic red-bed basins
Upper Permian basalts (P~)
Triasic arc volcanics (T 2 _ 3)
Cretaceous granites
Norites (N2)
West Markam West Yangbi 500 + 50 Dongba West Yangbi > 600 East Markam East Simao 700 + 50 West Sichuan West Khorat 800 4- 200 East Chuxiong East Khorat 1050 + 100
Near Jianchuan South of Yuanyang > 460 Near Benzilan (section U1) South of Yuanyang > 580
West of Jianchuan (section S) Southeast of Yuanjiang > 400 Northwest of Weixi Dien Bien Phu 650-780
Western boundary Western boundary 700 + I00
North of Diancang Shan Near Jinping 500 + 50
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 75
Trang (Fig. 1). Southwest of the Red River, the Lincang batholit is both offset by the Nan Ting fault and interrupted by a narrow zone of high-grade mylonitic rocks (Chongshan metamorphic belt, Fig. 3) (Wu Haoruo et al., 1994), parallel to the Ailao Shan, that may have been a similar, Tertiary sinistral shear zone. This precludes the use of that batholith as a marker for offsets along the ASRR zone (Figs. 1, 2 and 34d). The granite belts of coastal Guang- dong and South Vietnam, on the other hand, can be traced to within = 150 and 50 km of that zone and of its extension offshore, respectively (Figs. 1 and 34d), without the occurrence of any large intervening tectonic feature. The origin of those granites, presently under study, is contentious, but the re- gional stratigraphy leaves little doubt on their Creta- ceous age. Although the southernmost part of the Guangdong granite belt appears to have been rifted away from the mainland in the early stages of open- ing of the South China sea (e.g. Briais et al., 1993), matching the northwestern edges of the intrusive belts on either side of the ASRR shear zone yields an offset of 700 + 100 km (Fig. 34d).
The most prominent red-bed basins of western Yunnan and eastern Tibet are abruptly truncated by the Ailao Shan-Red River shear zone, and thus have boundaries that define acceptable piercing points. On the northeast side of the zone, the closest possible counterparts of the Yangbi and Simao basins are the Dongba and Qamdo-Markham basins (Figs. 1 and 2). The Yangbi and Dongba basins contain similar sequences of Cretaceous-Eocene limestones and red sandstones with comparable styles of Tertiary fold- ing. The separation between the piercing points of the western boundaries of the Markham and Yangbi basins is 500 +__ 50 km (Fig. 34b). This offset value is the smallest for this group of basins. Matching the eastern boundaries of the Simao and Markham basins, for instance, would yield an offset of 700 + 50 km. Matching the western boundary of the Yangbi basin with that, less well defined, of the Dongba basin, would yield an offset in excess of 600 km (Fig. 34b).
Even greater offsets are suggested by the separa- tion of the largest red-bed basins of Southeast Asia, farther away from the ASRR zone. Those offsets are more uncertain because the corresponding markers, except one, cannot be simply traced to the vicinity of the shear zone. Also, such offsets probably integrate
significant finite sinistral displacements on other Ter- tiary faults, the most prominent of which being the Xianshuihe, Song Da-Song Ma, and Song Ca fault zones (Fig, 2). With this in mind, a displacement of 800 + 200 km, for instance, is needed to realign the westem, tectonic boundaries of the Sichuan and Khorat basins (Figs. 2 and 34b), as long advocated by some of us (Tapponnier et al., 1986; Peltzer and Tapponnier, 1988). Matching the eastern, deposi- tional/erosional boundaries of the Chuxiong and Khorat basins would imply as much as 1050 + 100 km of sinistral displacement (Fig. 34b).
The geological evidence at hand (Table 1 and Fig. 36) is thus consistent with a minimum Tertiary left- lateral offset on the Ailao Shan-Red River shear zone (sensu stricto) greater than 400 km. On a broader scale, if other Cenozoic sinistral faults roughly parallel to this shear zone are included, the maximum amount of Tertiary displacement between the stable parts of the Yangzi and Khorat-Kontum platforms might have reached about 1200 kin. At a more detailed level, the evidence discussed implies a minimum of = 500 km and a most likely value of 700 __+ 200 km for the Tertiary sinistral offset on the ASRR zone per se (Table 1 and Fig. 36). Note that the Plio-Quaternary dextral offset on the Red River fault, if on order of several ten kilometers, would substantially increase the latter values. Given the extant geochronological constraints, which imply that the shear zone was active for about 20 Myr, a finite displacement of about 700 km would correspond to a uniform shear-strain rate of 3.5 cm/yr . Such a rate would be compatible with the mechanical inferences made earlier.
4.2.4. Paleomagnetic evidence and seafloor spread- ing
Systematic sampling of red beds in the basins of South China and Indochina, still in progress, has begun to yield a fairly dense upper Mesozoic paleo- magnetic database, with which assessment of large- scale latitudinal Tertiary motions and rotations has become feasible. Several different studies have shown, for instance, that South China as a whole has not rotated significantly since the Cretaceous (e.g. Enkin et al., 1991, 1992; Gilder et al., 1993; Huang and Opdyke, 1993; Ma et al., 1993). Since that time, on the other hand, the Khorat red beds, on the stable
76 P.H. Leloup et al . / Tectonophysics 251 (1995) 3-84
core of Indochina, appear to have experienced 14.2 -t-7.1 ° of clockwise rotation relative to those of Sichuan in South China (Yang and Besse, 1993). Such studies thus imply significant sinistral motion of Indochina with respect to a relatively stable South China block. From the differences between observed and predicted Cretaceous paleolatitudes, Huang and Opdyke (1993) have inferred several degrees of southward translation of the Simao basin with re- spect to stable Eurasia, an amount unfortunately within measurement uncertainties. Using an im- proved, more complete database, Yang and Besse (1993) and Yang et al. (1995) have concluded that the post-Cretaceous southward motion of Indochina relative to China amounted to 7.9 + 2.5 ° in latitude, which may be taken as evidence for as much as 1200_ 500 km of left-lateral displacement on the
Red River fault system (Fig. 36). While this particu- larly large value is compatible, within uncertainties, with the offset we find most likely on the ASRR shear zone (700 + 200 kin), we suspect that, like the largest geological offsets listed in Table 1 and for the same reasons, it integrates finite sinistral displace- ments on other faults roughly parallel to that zone (Figs. 2 and 34) (Yang and Besse, 1993).
One final, independent way to constrain the amount of offset on the ASRR shear zone, quantita- tively pioneered by Briais et al. (1993), is based on the idea that seafloor-spreading in the South China sea, which the Ailao Shan-Red River shear zone enters south of Hanoi, has absorbed most of the left-lateral movement along that zone (e.g. Tappon- nier et al., 1982, 1986; Peltzer and Tapponnier, 1988). Particularly strong support for this idea comes
................................................ >33°+6° i i i :i.L.....i ..................................................................................................................................... n t e i ...... ] -
I :~i~a9 Red beds
KhOra t -- S i c h u a n 3 ~ OSO~ i 0 0
I K h o r a t - C h u x i o n g _ . i ........................................................... :...~i~ ~ : ' . : ~ - : : : ~ : ~ i~. .~. : !~+-~+.~- .~, :+.~.- ! . . - - . . : - - .......................................................................................... 0 ~, I
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i ~ d % g U t r a m a f c s
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Fig. 36. Summary of geomorphic (right-lateral) and geologic (left-lateral) offsets across the Ailao Shan-Red River shear zone.
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 77
from the new radiometric ages found in the ASRR metamorphics, which establish that the duration of shear on the zone (35-17 Ma; Leloup et al., 1993; SchMer et al., 1994; Harrison et al., 1995) was nearly coeval with that of spreading in the sea (32-15.5 Ma; Briais et al., 1993). By simply assum- ing the shear zone and the spreading ridge to form a single boundary between two plates (Indochina and South China), the rotation parameters derived from the magnetic anomalies on the seafloor can be used to reconstruct motion along that zone (Briais et al., 1993). The reconstruction is consistent with the ASRR shear zone being mostly strike-slip, with transpression in the northwest and transtension in the southeast (Briais et al., 1993, fig. 14). For a point situated between Yuanjiang and Yuanyang (Fig. 3), the sinistral shear-strain rate is found to range be- tween 3.0 and 3.7 cm/yr , and the finite amount of left-lateral movement is ---540 + 50 km (Briais et al., 1993) (Fig. 36). This latter value must be consid- ered a lower bound since it relates to seafloor- spreading only, Including the strain related to rifting and crustal attenuation on either margin of the sea prior to spreading would plausibly increase the amount of movement on the ASRR shear zone by 100-200 km, assuming the mechanism to have re- mained the same. Hence, both the rate and the finite sinistral offset derived from seafloor-spreading pa- rameters appear to be in good agreement with those inferred from geological evidence onland.
4.3. Role of the ASRR shear zone in the tectonics of India-Asia collision
Overall, the observations and results presented and discussed in this paper thus strengthen the long advocated view that the Ailao Shan-Red River shear zone is the greatest Cenozoic tectonic feature of continental Southeast Asia east of the Assam syn- taxis. That it absorbed many hundreds of kilometers of sinistral motion in the Oligo-Miocene, as In- dochina was extruded southeastwards, relative to a then more stationary South China, by the penetration of India into Asia, is inescapable. Even though it slices deep into the interior of a continent, the ASRR zone has in fact most of the attributes of a great transform plate boundary. It is thus no surprise that it played a key role in orchestrating the tectonics of a
vast region and the rearrangement of large litho- spheric blocks, all the way from Tibet to Borneo.
A measure of the importance of that rearrange- ment, and of its bearing on collision mechanics, is readily obtained by estimating the surface of Asian lithosphere removed by such transform faulting from regions facing the Indian indenter (e.g. Tapponnier et al., 1986; Peltzer and Tapponnier, 1988; Armijo et al., 1989). To a first order, that surface loss (extru- sion) may be derived from the quantitative recon- struction of Briais et al. (1993), with a couple of amendments. First, we adjust Briais et al.'s finite rotation parameters - - pole at 7.9°N, 85.7°E,, oJ = 10.8 °, from anomaly 10 (30.3 Ma) only - - to in- clude anomaly 11 (32 Ma) and to allow for = 150 km of crustal extension across the South China Sea margins prior to seafloor spreading. The slightly different parameters we obtain (pole at 8.2°N, 85.6°E, o~ = 15.2 °) (Fig. 37) hence fall in better agreement with 700 km of sinistral motion along the ASRR zone and with the mean paleomagnetic rotation (14.2 °) between the Sichuan and Khorat red beds. Second, we take 150-200 km of Eocene sinistral slip on the Wang Chao-Three Pagodas Fault zone (Figs. 1 and 37) (Peltzer and Tapponnier, 1988; Lacassin et al., 1993b, in prep.) into account by assuming such motion to correspond to 6.5 ° of additional clockwise rotation of South Indochina about the adjusted In- dochina/South China pole (Fig. 37). The surface of the roughly triangular zone between the ,current western boundary of Indochina and its position prior to rotation, = 40 Ma (shaded on Fig. 37), is then computed on the sphere. The area thus lost te, extru- sion amounts to about 830,000 km 2 with ~- 640,000 and = 190,000 km 2 due to movement along the Ailao Shan-Red River and Wang Chao-Three Pago- das zones, respectively. Prominent field evidence for shortening and thickening of the crust of that triangu- lar region, which lies in the path of India's eastern tip, both during and after extrusion (Tapponnier et al., 1990; Lacassin et al., 1995), implies that this amount must be considered a lower bound. Using stage rotation poles located farther southwest than on Fig. 37 (Briais et al., 1993) would also increase the extruded area. Hence, the comparison of the area obtained here (0.83 × 10 6 km 2) to various estimates of the total Asian surface loss due to collision - - 6-8.5 X 10 6 km 2, according to Tapponnier et al.
78 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
(1986); 3 .5-4.2 X 10 6 k m 2, according to Le Pichon et al. (1992) - - requires that 17 + 7% of the penetra- tion of India into Asia, at least, was absorbed by the extrusion of Indochina alone.
Much still remains to be elucidated. The signifi- cant differences, for instance, between the various offsets estimated from the regional geology and pale- omagnetism need be resolved by finer-scale field- work in the mountainous countries that straddle the shear zone, many of which have remained until recently beyond reach. Much more detailed mapping,
analysis, and radiometric dating of mafic-ultramafic and granitic rocks need be done in order to convinc- ingly assess the exact correspondence between su- tures and magmatic arcs that have been offset by the shear zone. An even more daunting task lies ahead to determine the precise offshore connection, outside the realm of hand-sampling and accurate satellite or aerial imaging, of the various strands of the Ailao Shan-Red River system with the extensional basins and spreading centers of the South China sea. The limited shallow seismic reflection data, mostly
OMa / i
/ / 17Ma!. / 1-,
/ + ~e
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X " . . . + t
t
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\ * . "
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t
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o
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Fig. 37. Minimum estimate of extruded area. Present-day coastlines of South China, Indochina and India (continuous lines) are plotted together with their position before Indochina extrusion ( --- 40 Ma, dotted lines).
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 79
"classified", and the marine geophysical evidence acquired to date are clearly inadequate to address that facet of the problem.
At least does it seem safe to conclude, with the facts gathered onland at this point, that all interpreta- tions or models of Southeast Asian tectonics that do not allow for, or predict, the existence of the Ailao Shan-Red River shear zone, with at least 500 km of Oligo-Miocene sinistral motion on it and, by way of consequence, an intimate causal link between that zone and the opening of the South China sea (e.g. Dewey et al., 1989; England and Houseman, 1986; Cobbold and Davy, 1988; Sornette et al., 1993; Rangin et al., 1995) ought to be regarded as funda- mentally flawed, be it in terms of the geological or of the physical inferences upon which they rest.
Acknowledgements
This work summarizes most of the tectonic results of the Hong He-Jinsha Jiang cooperative program between the Centre National de la Recherche Scien- tifique (CNRS), the National Science Foundation of China and Academia Sinica. Joint field study in Yunnan took place between 1988 and 1991. Since 1991, cooperation between CNRS and the National Center for Natural Science and Technology of Viet- nam permitted preliminary study of the continuation of the Ailao Shan-Red River shear zone in Vietnam. That cooperation was directed by Pr. Nguyen Trong Yem, who is gratefully acknowledged. We also ac- knowledge financial support by programs Tecto- scope, Dynamique et Bilan de la Terre (DBT) and Mecalith, of Institut National des Sciences de l'Uni- vers (INSU) and by Mission des Relations Interna- tionales (MDRI), of CNRS. We benefited from nu- merous discussions on the tectonics of Asia with R. Armijo, J. Besse, Chen Yan, B. Meyer, R. Weldon, Yang Zhenyu. A. Briais, J.R. Kienast and Y. Ricard helped address specific points discussed in this pa- per. T.M. Harrison, F.J. Ryerson and Chen Wenji from UCLA contributed greatly in producing and discussing argon ages. Guy Aveline and Evelyne Lesur are responsible for the quality of the figures, and Maryvonne Le Court de Beru for the organiza- tion of field trips and administrative support. Finally,
H. Leloup expresses his deepest thanks to Asia Emergency Assistance, and to the staffs of St. Theresa hospital in Hong Kong and of University of Paris VI who enabled him to survive long enough to finish this paper. This is IPGP contribution No. 1371.
References
Academy of Geological Sciences of China, 1975. Geological Map of Asia 1:5000000. Geographic Map Publ., Beijing.
Achache, J. and Courtillot, V., 1984. Paleogeographic and tecton- ics evolution of southern Tibet since middle Cretaceous time: new paleomagnetic data and synthesis. J. Geophys. Res., 89: 10,311-10,339.
Achache, J., Courtillot, V. and Besse, J., 1983. Palaeomagnetic constraints on the late Cretaceous and Cenozoic tectonics of southeastern Asia. Earth Planet. Sci. Lett., 63: 123-136.
Allen, C.R., Gillespie, A.R., Han, Y., Sieh, K.E., Zhang, B. and Zhu, C., 1984. Red River and associated faults, Yunnan province, China: Quaternary geology, slip rates and seismic hazard. Geol. Soc. Am. Bull., 95: 686-700.
Armijo, R., Tapponnier, P., Mercier, J.L. and Han Tonglin, 1986. Quaternary extension in southern Tibet: field observations and tectonic implications. J. Geophys. Res., 91: 13,803 - 13,872.
Armijo, R., Tapponnier, P. and Han Tonglin, 1989. Late Cenozoic fight-lateral strike-slip faulting in southern Tibet. J. Geophys. Res., 94: 2787-2838.
Avouac, J.-P. and Tapponnier, P., 1992. Cinrmatique des drfor- mations actives en Asie centrale. C.R. Acad. Sci. Paris, 315: 1791-1798.
Avouac, J.-P. and Tapponnier, P., 1993. Kinematic model of active deformation in central Asia. Geophys. Res. Lett., 20: 895-898.
Avouac, J.P., Tapponnier, P., Bai, M., You, H. and Wang, G., 1993. Active thrusting and folding along the northern Tien Shan and late Cenozoic rotation relative to Dzungaxia and Kazakhstan. J. Geophys. Res., 98: 6755-6804.
Bally, A.W., Allen, C.R., Geyer, R.B., Hamilton, W.B., Hopson, C.A., Molnar, P.H., Oliver, J.E., Opdyke, N.D., Plafker, G. and Wu, F.T., 1980. Notes on the Geology of Tibet and Adjacent Areas. Report American Plate Tectonics Delegation to the People's Republic of China. US Geol. Surv., Rep.
Barr, S.M. and Mac Donald, A.S., 1987. Nan fiver suture zone, northern Thailand. Geology, 15: 907-910.
Barr, S.M., Tantisukrit, C., Yaowanoiyothin, W. and Macdonald, A.S., 1990. Petrology and tectonic implications of upper Pale- ozoic volcanic rocks of the Chiang Mai belt, northern Thai- land. J. Southeast Asian Earth Sci., 4: 37-47.
Baum, F. and Hahn, L., 1977. Geological Map of Northern Thailand, 1:250000, sheet 3. Fed. Inst. Geosci. Nat. Resour., Hannover.
Beckinsale, R.D., Suensilpong, S., Nakapadungrat, S. and Walsh, J.N., 1979. Geochronology and geochemistry of granite mag-
80 P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84
matism in Thailand in relation to a plate tectonic model. J. Geol. Soc. London, 136: 529-540.
Bell, T.H. and Etheridge, M.A., 1973. Microstructure of mylonites and their descriptive terminology. Lithos, 6: 337-348.
Berth6, D., Choukroune, P. and Gapais, D., 1979a. Orientations pr6f~rentielles du quartz et orthogneissification progressive en r6gime cisaillant: l'exemple du cisaillement sud-armoricain. Bull. Min6ral., 102: 265-272.
Berth6, D., Choukroune, P. and Jegouzo, P., 1979b. Orthogneiss, mylonites and non coaxial deformation of granites: the exam- ple of the South Armorican shear zone. J. Struct. Geol., 1: 31-42.
Besse, J. and Courtillot, V., 1988. Paleogeographic maps of the continents bordering the Indian Ocean since the early Jurassic. J. Geophys. Res., 93: 11,791-11,808.
Besse, J., Courtillot, V., Pozzi, J.P. and Zhou, Y.X., 1984. Paleomagnetic estimate of Cenozoic convergence in the Hi- malayan thrusts and Zangbo suture. Nature, 311: 621-626.
Blacic, J., 1975. Plastic deformation mechanisms in quartz: the effect of water. Tectonophysics, 27: 271-294.
Bouchez, J.L. and P6cher, A., 1981. The Himalayan main central thrust pile and its quartz-rich tectonites in central Nepal. Tectonophysics, 78: 23-50.
Briais, A., Patriat, P. and Tapponnier, P., 1993. Updated interpre- tation of magnetic anomalies and seafloor spreading stages in the South China Sea, implications for the Tertiary tectonics of SE Asia. J. Geophys. Res., 98: 6299-6328.
Bunopas, S. and Vella, P., 1983. Tectonic and geological evolu- tion of Thailand. Paper presented at Stratigraphic Correlation of Thailand and Malaysia. Geol. Soc. Thailand, Bangkok/Geol. Soc. Malaysia, Kuala Lumpur.
Bureau of geology and mineral resources of Yunnan, 1983. Geo- logical Map of Yunnan, scale 1:1000000. Kunming.
Bureau of geology and mineral resources of Yunnan, 1984. Geo- logical Map of Yunnan, scale 1:200000, sheet G-47-X (Weixi). Kunming.
Bureau of Geology and Mineral Resources of Yunnan Province, 1990. Regional Geology of Yannan Province, Geol. Publ. House, Beijing, Geol. Mere., 21,728 pp.
Burg, J.P. and Laurent, P., 1978. Strain analysis of a shear zone in a granodiorite. Tectonophysics, 47: 1-42.
Chang Chenfa and 26 others, 1986. Preliminary conclusions of the Royal Society and Academia Sinica 1985 geotraverse of Tibet. Nature, 323: 501-507.
Chen, Y. and Wu, F.T., 1989. Lancang-Gengma earthquake, a preliminary report on the November 6, 1988 event and its aftershocks. EOS Trans. Am. Geophys. Union, 70: 1527-1540.
Chen Yan, Courtillot, V., Cogn6, J.-P., Besse, J., Zhenyu, Y. and Enkin, R., 1993. The configuration of Asia prior to the collision of India: Cretaceous paleomagnetic constraints. J. Geophys. Res., 98: 21,927-21,941.
Cheng Yuankung, 1987. A cognitive basis and discussion on the nappe structure of Ailao Sban-Dian Cang Shan. Geol. Yun- nan, 6: 291-297.
Cobbold, P. and Quinquis, H., 1980. Development of sheath folds in shear regime. J. Struct. Geol., 2: 119-126.
Cobbold, P.R. and Davy, P., 1988. Indentation tectonics in nature
and experiment. 2. Central Asia. Bull. Inst. Uppsala, 14: 143-162.
Department of Mineral Resources, 1982. Geological Map of Thai- land, scale: 1:1000000. R. Thai Sure. Dep., Bangkok.
Deprat, J., 1914. Etude des plissements et des zones d'6crasement de la moyenne et basse rivi~:re noire. M6m. Serv. Geol. Indochine, 3: 1-59.
Dewey, J.F., Cande, S. and Pitman, W.C.I., 1989. Tectonic evolu- tion of the India/Eurasia collision zone. Eclogae Geol. Helv., 82: 717-734.
Dodson, M.H., 1973. Cooling temperature in cooling geochrono- logical and petrological systems. Contrib. Mineral. Petrol., 40: 259-274.
Duan Xinhua, Z.H., 1981. The Ailaoshan-Tengtiohe fracture, the subduction zone of an ancient plate. Acta Geol. Sin., 55: 258-266.
Earth Sciences Research Division, 1977. Geological Map of the Socialist Republic of the Union of Burma, scale 1:1000000.
Eisbacher, G.H., 1970. Deformation mechanisms of mylonite rocks and fractured granulites in Cobequid Moutains, Nova Scotia, Canada. Geol. Soc. Am. Bull., 81: 2009-2020.
England, P. and Houseman, G., 1986. Finite strain calculations of continental deformation, 2. comparison with the India-Asia collision zone. J. Geophys. Res., 91: 3664-3676.
Enkin, R.J., V. Courtillot, Xing Lisheng, Zhuang Zhonghai Zhang Zhenhai, Zhang Jinxing, 1991. The stationary Cretaceous Pale- omagnetic pole of Sichuan (South China Block). Tectonics, 10: 547-559.
Enkin, R., Zhenyu Yang, Chert Yan and Courtillot, V., 1992. Paleomagnetic constraints on the geodynamic history of the major blocks of China from the Permian to the present. J. Geophys. Res., 97: 13,953-13,989.
Escber, A. and Watterson, J., 1974. Stretching fabrics, folds and crustal shortening. Tectonophysics, 39:223-231.
Etehecopar, A. and Vasseur, G., 1987. A plane kinematic model of progressive deformation in a polycrystaline agregate. Tectonophysics, 39: 121-139.
Fan Chengjing, 1986. The tectonic-metamorphic belt of Mt Ailao in Yurman province. Geol. Yunnan, 5: 281-291.
Fitz Gerald, J.D. and Harrison, T.M., 1993. Argon diffusion domains in K-feldspar I: microstructures in MH-10. Contrib. Mineral. Petrol., 113: 367-380.
Fleitout, L. and Froidevaux, C., 1980. Thermal and mechanical evolution of shear zones. J. Struct. Geol., 2: 159-164.
Flinn, D., 1965. On the symetry principle and the deformation ellipsoid. Geol. Mag., 102: 36-45.
Fontaine, H. and Workman, D.R., 1978. Review of the geology and mineral resources of Kampuchea, Laos and Vietnam. In: P. Nutalaya (Editor), Geology and Mineral Resources of Southeast Asia. Asian Inst. Technol., Bangkok, pp. 538-603.
Gapais, G. and Barbarin, B., 1986. Quartz fabric transition in a cooling syntectonic granite (Hermitage massif, France). Tectonophysics, 125: 357-370.
Gaudemer, Y. and Tapponnier, P., 1987. Ductile and brittle deformation in the northern Snake Range, Nevada. J. Struct. Geol., 9: 159-180.
General geological department of the democratic republic of
P.H. Leloup et al. / Tectonophysics 251 (1995) 3-84 81
Vietnam, 1973. Geological Map of Vietnam (The North Part), scale 1:1000000. Hanoi.
Geological Bureau of Yunnan province, 1979. Geological Map of Yunnan Province, scale 1:500000. Kunming.
Gilder, S.A., Coe, R.S., Haoruo Wu, Guodun Kuang, Xixi Zhao, Qi Wu and Xianzan Tang, 1993. Cretaceous and Tertiary paleomagnetic results from southeast China and their tectonic implications. Earth Planet. Sci. Lett., 117: 637-652.
Grindley, G.W., 1963. Structure of the alpine schists of South Westland, Southern Alps, New Zealand. NZ J. Geol. Geophys., 6: 872-930.
Guo Shunmin, Zhang Jing and Li Xianggen, 1986. Fault displace- ment and recurrence intervals of earthquakes on the northern segment of the Honghe River fault zone, Yunnan province. Seism. Geol., 8: 77-90.
Hahn, L. and Siebenhiiner, M., 1982. Explanatory notes (paleontology) on the geological maps of northern and western Thailand, 1:250000. Bundesanst. Geowiss. Rohstoffe, Han- nover, 76 pp.
Hanmer, S., 1986. Asymmetrical pull-aparts and foliation fish as kinematic indicators. J. Struct. Geol., 8:111-122.
Hanmer, S. and Passchier, C., 1991. Shear-sense indicators: a review. Geol. Surv. Can. Pap., 90-17, 72 pp.
Harrison, T.M., Heizler, M.T. and Lovera, O.M., 1991. 40Ar/39Ar results for alkali feldspar containing diffusion domains with differing activation energy. Geochim. Cos- mochim. Acta, 55: 1345-1448.
Harrison, T.M., Chen Wenji, Leloup, P.H., Ryerson, F.J. and Tapponnier, P., 1992a. An early Miocene transition in defor- mation regime within the Red River fault zone, Yunnan, and its significance for Indo-Asian tectonics. J. Geophys. Res., 97: 7159-7182.
Harrison, T.M., Leloup, P.H., Ryerson, F.J., Tapponnier, P., Lacassin, R. and Chen wenji, 1995. Diachronous initiation of transtension along the Ailao Shan-Red River Shear zone, Yunnan and Vietnam. In: T.M. Harrison and Ann Yin (Edi- tors), The Tectonics of Asia. Cambridge Univ. Press, New York, NY.
Helmcke, D., 1985. The Permo-Triassic "Paleotethys" in main- land southeast-Asia and adjacent parts of China. Geol. Rund- sch., 74: 215-228.
Hess, A. and Koch, K.E., 1975. Geological map of northern Thailand, 1:250000, sheet 1. Fed. Inst. Geosci. Nat. Resour., aannover.
Higgins, M.W., 1971. Cataclastic rocks. US Geol. Surv. Prof. Pap., 687: 1-97.
Hobbs, B., Means, W. and Williams, P., 1976. An Outline of Structural Geology. Wiley, New York, NY, 571 pp.
Holt, W.E., Ni, J.F., Wallace, T.C. and Haines, A.J., 1991. The active tectonics of the eastern himalayan syntaxis and sour- rounding regions. J. Geophys. Res., 96: 14,595-14,632.
Huang, C,C., 1978. An outline of the tectonic characteristics of China. Eclogae Geol. Helv., 71(3): 611-635.
Huang Kainan and Opdyke, N.D., 1993. Paleomagnetic results from Cretaceous and Jurassic rocks of south and southwest
Yunnan: evidence for large clockwise rotations in the in- dochina and Shah-Thai-Malay terranes. Earth Planet. Sci. Lett., 117: 507-524.
Huang, T.K., 1960. Characteristics of the structure of China: Preliminary conclusions. Sci. Sin., 9: 492-543.
Hutchinson, C.S., 1982. Southeast Asia. In: A.E.M. Naim and F.G. Stehli (Editors), The Indian Ocean. The Ocean Basins and Margins, 6. Plenum, New York, NY.
Hutchinson, C.S., 1989. Geological Evolution of SE Asia. Claren- don, Oxford, Oxford Monogr. Geol. Geophys., 13,368 pp.
Institute of Geophysics, 1976. The Catalog of Suong Shocks of China. Acad. Sin., Beijing.
Izokh, E.P., Le Dyn Hyu and Nguyen Van Tien, 1964. New information on magmatic activity in north Vietnam Dokl. Akad. Nauk SSSR, 155: 96-98.
Jackson, J.A. and McKenzie, D.P., 1988. The relationship be- tween plate motions and seismic tensors, and the rate of active deformation in the Mediterranean and Middle East. Geophys. J. Int., 93: 45-73.
Jaeger, J.-J., Courtillot, V. and Tapponnier, P., 1989. Paleontolog- ical view of the ages of the Deccan traps, the Cretaceous/Ter- tiary boundary, and the India-Asia collision. Geology, 17: 316-319.
Jen Chi Shuen and Chu Ching Chuan, 1966. Geosynclinal In- dosinian structure in the Lanp'ing-Weisi region, western Yun- nan. Int. Geol. Rev., 12: 447-463.
Karst Institute of Guilin, 1981. Geological Map of Guilin, scale 1:100000.
Kidd, W.S.F. and Molnar, P., 1988. Quaternary and active fault- ing observed on the 1985 Academia Sinica-Royal Society geotraverse of Tibet. Philos. Trans. R. Soc. London A, 327: 337-363.
Lacassin, R., 1988. Large-scale foliation boudinage in gneisses. J. Struct. Geol., 10: 643-647.
Lacassin, R. and Mattauer, M., 1985. Kilometer-scale sheath fold at Mattmark and implications for transport direction in the Alps. Nature, 316: 739-742.
Lacassin, R., Tapponnier, P., Sch~er, U., Leloup, P.H., Liu Xiaohan and Zhang Liang Sheng, 1993a. Tertiary metamor- phism within the folded tiger-leap detachment in NW Yunnan (China). Terra Nova, 5, Suppl. EUG VII Terra Abstr., p. 260.
Lacassin, R., Leloup, P.H. and Tapponnier, P., 1993b. Bounds on strain in large Tertiary shear zones of SE Asia from boudinage restoration. J. Struct. Geol., 15: 677-692.
Lacassin, R., Sch~rer, U., Leloup, P.H., Arnaud, N., Tapponnier, P., Liu Xiaohan and Zhang Liansheng, 1995. Tertiary defor- mation and metamorphism SE of Tibet: the folded tiger-leap d6collement of NW Yunnan (China). Tectonics, submitted.
Lagarde, J.L., 1978. La d~formation des roches dans les domaines h schistositfi subhorizontale. Univ. Rennes, Rennes. 164 pp. (3~me cycle).
Landis, C.A. and Bishop, D.G., 1972. Plate tectonicsand regional stratigraphic-metamorphic relations in the southern part of the
82 p.H. Leloup et aL / Tectonophysics 251 (1995) 3-84
New Zealand geosyncline. Geol. Soc. Am. Bull., 83: 2267- 2284.
Lanrent, P. and Etchecopar, A., 1976. Mise en Evidence h l'aide de la fabrique du quartz d'un cisaillement simple ~ d6verse- merit ouest darts le massif de Dora Maira (Alpes Occidentales). Bull. Soc. Geol. Fr., 7: 1387-1393.
Le Dain, A.Y., Tapponnier, P. and Molnar, P., 1984. Active faults and tectonics of Burma and surronding regions. J. Geophys. Res., 89: 453-472.
Le Pichon, X., Fournier, M. and Jolivet, L., 1992, Kinematics, topography, shortening, and extrusion in the India-Eurasia collision. Tectonics, 11: 1085-1098.
Leloup, P.H., 1991. CinEmatique des deformat ions "himalayennes" dans la zone de cisaillement crustale de l'Ailao Shan-Fleuve Rouge. Thesis. Univ. Paris 6, Paris, 148
Pp. Leloup, P.H. and Kienast, J.R., 1993. High temperature metamor-
phism in a major strike-slip shear zone: the Ailao Shan-Red River (P.R.C.). Earth Planet. Sci. Lett., 118: 213-234.
Leloup, P.H., Harrison, T.M., Ryerson, F.J., Chen Wenji, Li Qi, Tapponnier, P. and Lacassin, R., 1993. Structural, petrological and thermal evolution of a Tertiary ductile strike-slip shear zone, Diancang Shan, Yunnan. J. Geophys. Res., 98: 6715- 6743.
Leloup, P.H., Tapponnier, P., Lacassin, R., Harrison, T.M., Chen Wenji and Ryerson, F.J., 1994. Diachronic uplift, transtension, and sinistral shear rate along the Red River zone. EOS Trans. Am. Geophys. Union, Nov. 1 (Suppl.): 630.
Leloup, P.H., Lacassin, R., Battaglia, J. and Ricard, Y., 1995. Is shear heating a plausible mechanism to generate granites in lithospheric strike-slip shear zones? Terra Nova, 7, Suppl. EUG VIII Terra Abstr., p. 48.
Li Chun-Yi~, Liu Xueya, Wang Quan and Zhang Zhimeng, 1979. A tentative contribution to plate tectonics of China. Chin. Acad. Geol. Sci. (unpubl.).
Lister, G.S. and Hobbs, B.E., 1980. The simulation of fabric development during plastic deformation and its application to quartzite: the influence of deformation history. J. Struct. Geol., 2: 355-370.
Lister, G.S. and Snoke, A.W., 1984. S-C mylonites. J. Struct. Geol., 6: 617-638.
Liu Baotian, Jiang Yaoming and Qu Jingchuan, 1983. The discov- ery of a palaeoceanic crust strip along the line from Litang to Ganzi in Sichuan and its significance on plate tectonics. Contrib. Geol. Qinhai-Xizang Plateau, 12: 119-127.
Liu Changshi, Zhu Jinchu, Xu Xisheng, Chu Xuejun, Cai Dekun and Pin, Y., 1989. Study on the characteristics of Linchang composite granite batholith in west Yunnan. Geol. Yunnan, 8: 189-204.
Liu Xiaohan, Sch~er, U. and Tapponnier, P., 1992. Timing of large-scale strike-slip movements in the Diancang shan and Ailao Shan shear belts, Yunnan, China. EOS Trans. Am. Geophys. Union, 73(14): 310.
Lovera, O.M., Richter, F.M. and Harrison, T.M., 1989. The 40Ar/39Ar thermochronology for slowly cooled samples hav- ing a distribution of diffusion domain sizes. J. Geophys. Res., 94: 17,917-17,935.
Lovera, O.M., Richter, F.M. and Harrison, T.M., 1991. Diffusion domains determined by 39Ar released during step heating. J. Geophys. Res., 96: 2057-2069.
Lovera, O.M., Heizler, M.T. and Harrison, T.M., 1993. Argon diffusion domains in K-feldspar II: kinetics properties of MH-10. Contrib. Mineral. Petrol., 113: 381-393.
Lu Liangzhao, 1989. the metamorphic series and crustal evolution of the basement of the Yangzi Platform. J. Southeast Asia Earth Sci., 3: 293-301.
Ma Xinghua, Yang Zhenyu and Xing Lisheng, 1993. The lower Cretaceous reference pole from the North China, and its tectonic implications. Geophys. J. Int., 115: 323-331.
Mainprice, D., Bouchez, J.-L., Blumenfeld, P. and TubiE, J.M., 1986. Dominant C slip in naturally deformed quartz: implica- tions for dramatic plastic softening at high temperature. Geol- ogy, 14: 819-822.
Malavieille, J., 1987. Extensional shearing deformation and kilo- meter-scale "a"- type folds in a cordillieran metamorphic core complex (Raft River mountains, northwestern Utah). Tecton- ics, 6: 423-448.
Malavieille, J., Etchecopar, A. and Burg, J.P., 1982. Analyse de la gEomEtrie des zones abritrees: simulation et application a des exemples naturels. C.R. Acad. Sci. Paris, 294: 779-784.
Mattauer, M., 1975. Sur le mEcanisme de formation de la schis- tosit~ dans I'Himalaya. Earth Planet. Sci. Lett., 28: 144-154.
Mattauer, M., Malavieille, J., Calassou, S., Lancelot, J., Roger, F., Hao Ziwen, Zhu Zhiqin and Hou Liwei, 1992. La cha]ne triassique de songpan-Garze (Ouest Sechuan et Est Tibet): une cha]ne de plissement-drcollement sur marge passive. C.R. Acad. Sci. Paris, 314: 619-626.
Molnar, P. and Tapponnier, P., 1975. Cenozoic tectonics of Asia: effects of continental collision. Science, 189: 419-426.
Mouret, C., 1994. Geological history of northeastern Thailand since the Carboniferous. Relations with Indochina and Car- boniferous to early Cenozoic evolution model. In: Int. Symp. Stratigraphic Correlation of Southeast Asia, Bangkok, Thai- land, 15-20 November 1994.
Nicolas, A. and Poirier, J.P., 1976. Crystalline Plasticity and Solid State Flow in Metamorphic Rocks. Wiley, New York, NY, 444 pp.
Nicolas, A., Bouchez, J.L., Blaise, J. and Poirier, J.P., 1977. Geological aspects of deformation in continental shear zones. Tectonophysics, 42: 55-73.
Passchier, C.W. and Simpson, C., 1986. Porphyroclast system as kinematic indicators. J. Struct. Geol., 8: 831-843.
Patriat, P. and Achache, J., 1984. India-Eurasia collision chronol- ogy has implications for crustal shortening and driving mecha- nism of plates. Nature, 311: 615-621.
Peltzer, G. and Tapponnier, P., 1988. Formation and evolution of strike-slip faults, rifts, and basins during the India-Asia colli- sion: an experimental approach. J. Geophys. Res., 93: 15,085- 15,117.
Peltzer, G., Tapponnier, P. and Armijo, R., 1989. Magnitude of late Quatemary left-lateral displacements along the northern edge of Tibet. Science, 246: 1285-1289.
Platt, J.P. and Vissers, R.L.M., 1980. Extensional structures in anisotropic rocks. J. Struct. Geol., 2: 397-410.
P.H. Leloup et al.// Tectonophysics 25! (1995) 3-84 83
Quinquis, H., Audren, C., Brun, J.P. and Cobbold, P., 1978. Intense progressive shear in the ile de Groix blueschists and compatibility with subduction or obduction. Nature, 273: 43- 45.
Ramsay, J.G., 1967. Folding and Fracturing of Rocks. Int. Ser. Earth Planet. Sci., New York, NY, 568 pp.
Rangin, C., Klein, M., Roques, D., Le Pichon, X. and Trong, L.V., 1995. The Red River fault system in the Tonkin Gulf, Vietnam. Tectonophysics, 243: 209-222.
Reed, J.J., 1964. Mylonites, cataclasites, and associated rocks along the alpine fault, south island, New Zealand. NZ J. Geol. Geophys., 7: 645-684.
SchMer, U., Tapponnier, P., Lacassin, R., Leloup, P.H., Zhong Dalai and Ji Shaocheng, 1990a. Intraplate tectonics in Asia: a precise age for large-scale Miocene movement along the Ailao Shan-Red River shear zone, China. Earth Planet. Sci. Lea., 97: 65-77.
Sch~er, U., Lian-Sheng, Z. and Tapponnier, P., 1994. Duration of strike-slip movements in large shear zones: the Red River belt, China. Earth Planet. Sci. Lea., 126: 379-397.
Scholz, C.H., 1990. The Mechanics of Earthquakes and Faulting. Univ. Cambridge, Cambridge, 439 pp.
Scholz, C.H., Beavan, J. and Hanks, T.C., 1979. Frictional meta- morphism, argon depletion, and tectonic stress on the Alpine fault, New Zealand. J. Geophys. Res., 84: 6770-6782.
Sengbr, A.M.C., 1984. The Cimmeride orogenic system and the tectonics of Eurasia. Geol. Soc. Am., Boulder, CO, Spec. Pap., 195, 82 pp.
SengiSr, A.M.C., 1987. Tectonic subdivisions and evolution of Asia. Bull. Techn. Univ. Istanbul, 40: 355-435.
Sengbr, A.M.C. and Hsii, K.J., 1984. The cimmerides of eastern Asia: history of the eastern end of Palaeo-Tethys. Mem. Soc. Grol. Fr,, 147: 139-167.
Sengbr, A.M.C., Altmer, D., Cin, A., UstaiSmer, T. and Hsii, K.J., 1988. Origin and assembly of the Thethyside orogenic collage at the expense of Gondwana Land. In: M.G. Audley-Charles and A. Hallam (Editors), Gondwana and Tethys. Geol. Soc. Spec. Publ., 37: 119-181.
Sheppard, D.S., Adams, C.J. and Bird, G.W., 1975. Age of metamorphism and uplift in the Alpine schist belt, New Zealand. Geol. Soc. Am. Bull., 86:1147-1153.
Simpson, C. and S. Schmid, 1983. An evaluation of shear criteria to deduce the sense of movements in sheared rocks. Geol. Soc. Am. Bull., 94: 1281-1288.
Sorensen, K., 1983. Growth and dynamics of the Mordre Stroen- fjord shear zone. J. Geophys. Res., 88: 3419-3437.
Sornette, A., Davy, P. and Sornette, D., 1993. Fault growth in brittle-ductile experiments and the mechanics of continental collisions. J. Geophys. Res., 98:12,111-12,139.
Stock, I. and Molnar, P., 1987. Revised history of early Tertiary plate motion in the south-west Pacific. Nature, 325: 495-499.
Sun, A.L. and Cui, K.H., 1986. A brief introduction to the lower Lufeng saurischian fauna (lower Jurassic: Lufeng, Yunnan, People's Republic of China). In: K. Padian (Editor), The Beginning of the Age of Dinosaurs. Cambridge Univ. Press, Cambridge, pp. 275-278.
Tang Yaoqing, Liu Zenqian and 10 others, 1988. Geological map
of Qinghai-Xijang (Tibet) plateau and adjacent areas. Geol. Publ. House, Beijing.
Tapponnier, P. and Molnar, P., 1977. Active faulting and tectonics of China. J. Geophys. Res., 82: 2905-2930.
Tapponnier, P., Peltzer, G., Armijo, R., Le Dain, A.-Y. and Cobbold, P., 1982. Propagating extrusion tectonics in Asia: new insights from simple experiments with plasticine. Geol- ogy, 10: 611-616.
Tapponnier, P., Peltzer, G. and Armijo, R., 1986. On the me, chan- ics of the collision between India and Asia. In: M.P. Coward and A.C. Ries (Editors), Collision Tectonics. Geol. Soc. Spec. Publ., 19: 115-157.
Tapponnier, P., Lacassin, R., Leloup, P.H., SchMer, U., Zhong Dalai, Liu Xiaohan, Ji Shaocheng, Zhang Lianshang and Zhong Jiayou, 1990. The Ailao Shan/Red River metamorphic belt: Tertiary left-lateral shear between lndochina and South China. Nature, 343: 431-437.
Van Den Driessche, J. and Brun, J.P., 1987. Rolling structures at large shear strain. J. Struct. Geol., 9: 691-704.
Wang, E. and Chu, J.J., 1988. Collision tectonics in the Cenozoic orogenic zone bordering China, India and Burma. Tectono- physics, 174: 71-84.
Wang Hongzhen and 43 others, 1985. Atlas of the Palaeogeogra- phy of China. Inst. Geol., Chin. Acad. Geol. Sci., Wuhan Coll. Geol., Beijing, 253 pp.
Weissel, J.K., Hayes, D.E. and Herron, E.M., 1977. Plate tectonic synthesis: the displacement between Australia, New Zeland, and Antartica since the late Cretaceous. Mar. Geol., 25: 231- 277.
Wellman, P. and Cooper, A., 1971. Potassium-argon ages of some New Zealand lamprophyre dikes near the Alpine fault. NZ J. Geol. Geophys., 14: 341-350.
White, S., 1976. The effect of strain on the microstnactures, fabrics and deformation mechanisms in quartzites. Philos. Trans. R. Soc. London, 283: 69-86.
Working Group on Resource Assessment, 1991. Total sedimen- tary isopach map offshore East Asia. Co-ordinating Comm. Offshore Prospect., Bangkok.
Wu, Genyao, 1993. Permian basalts in Lijiang and Jinping, West Yunnan: a comparative study and its geological significance. Acta Petrol. Sin., 9: 63-70.
Wu, Francis, T. and Peide Wang, 1988. Tectonics of western Yunnan province, China. Geology, 16: 153-157.
Wu Haoruo, Bouher, C.A., Ke Baolia, Stow, D.A.V. and Wang Zhongcheng, 1994. The Changning-Menglian suture zone; a segment of the major Cathaysian-Gondwana divide in South- east Asia. Tectonophysics, 242: 267-280.
Yan Qizhong, Zhang Guoqing, Karl Rong.ju and Hu Hongxiang, 1985. The crust structure of Simao to Malong profile, Yunnan povince, China. J. Seism. Res., 8: 249-264.
Yang, Z. and Besse, J., 1993. Paleomagnetic study of Permian and Mesozoic sediments from northern Thailand supports the ex- trusion model for lndochina. Earth Planet. Sci. Lett., 117: 525 -552.
Yang, Z., Besse, J., Sutheetorn, V., Fontaine, H. and Buffetaut,
84 P.H. Leloup et al . / Tectonophysics 251 (1995) 3-84
E., 1995. Jurassic an Cretaceous paleomagnetic data for In- dochina: paleogeographic evolution and deformation history of SE Asia. Earth Planet. Sci. Lea., in press.
Zhang Qi, Li Dazhou and Keixu, Z., 1985. Preliminary study on Tongchangjia ophiolite melange from Yun County, Yunnan Province. Acta Petrol. Sin., 1 :1-14 (in Chinese with English abstract).
Zhang Ru Yuan, Cong Bo Lin and Li Yong Gang, 1990. Petrol-
ogy of blueschist in western Yunnan province, China, and its tectonic significance. Sci. China, 33:11 I0- I 123.
Zhong Dalai, Tapponnier, P., Wu Haiwei, Zhang Lianshang, Ji Shaocheng, Zhong Jiayou, Liu Xiaohan, Sch~er, U., Lacassin, R. and Leloup, P.H., 1990. Large-scale strike-slip fault, the major structure of intracontinental deformation after collision. Kexue-Tongbao, 35: 304-309.
