Structural evolution of the Gurla Mandhata detachment ...manning/pdfs/murph02.pdfsystem, to which it...

For permission to copy, contact [email protected] 2002 Geological Society of America428

GSA Bulletin; April 2002; v. 114; no. 4; p. 428–447; 15 figures; 5 tables; Data Repository item 2002048.

Structural evolution of the Gurla Mandhata detachment system,southwest Tibet: Implications for the eastward extent of the

Karakoram fault system

M.A. Murphy*An YinP. Kapp†

T.M. HarrisonC.E. ManningDepartment of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California,Los Angeles, California 90095-1567, USA

F.J. RyersonLawrence Livermore National Laboratory, Livermore, California 94550, USA

Ding LinGuo JinghuiInstitute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China

Figure 1. Simplified geologic map of south-west Tibet compiled from mapping by Au-gusto Gansser (Heim and Gansser, 1939),Tibetan Bureau of Geology and MineralResources (Cheng and Xu, 1987), Yin et al.(1999b), and our own observations.

M

ABSTRACT

Field mapping and geochronologic andthermobarometric analyses of the GurlaMandhata area, in southwest Tibet, revealmajor middle to late Miocene, east-west ex-tension along a normal-fault system,termed the Gurla Mandhata detachmentsystem. The maximum fault slip occursalong a pair of low-angle normal faults thathave caused significant tectonic denudationof the Tethyan Sedimentary Sequence, re-sulting in juxtaposition of weakly metamor-phosed Paleozoic rocks and Tertiary sedi-mentary rocks in the hanging wall overamphibolite-facies mylonitic schist, marble,gneisses, and variably deformed leucogran-ite bodies in the footwall. The footwall ofthe detachment fault system records a lateMiocene intrusive event, in part contem-poraneous with top-to-the-west ductile nor-mal shearing. The consistency of the meanshear direction within the mylonitic foot-wall rocks and its correlation with struc-turally higher brittle normal faults suggest

*Present address: Department of Geosciences,University of Houston, Houston, Texas 77204-5007,USA; e-mail: [email protected].

†Present address: Department of Geosciences,University of Arizona, Tucson, Arisona 85721-0077.

that they represent an evolving low-anglenormal-fault system. 40Ar/39Ar data frommuscovite and biotite from the footwallrocks indicate that it cooled below 400 8Cby ca. 9 Ma. Consideration of the originaldepth and dip angle of the detachment faultprior to exhumation of the footwall yieldstotal slip estimates between 66 and 35 kmacross the Gurla Mandhata detachmentsystem. The slip estimates and timing con-straints on the Gurla Mandhata detach-ment system are comparable to those esti-mated on the right-slip Karakoram faultsystem, to which it is interpreted to be ki-nematically linked. Moreover, the meanshear-sense direction on both the Karako-ram fault and the Gurla Mandhata detach-ment system overlap along the intersectionline between the mean orientations of thefaults, which further supports a kinematicassociation. If valid, this interpretation ex-tends previous results that the Karakoramfault extends to mid-crustal depths.

Keywords: Himalayas, kinematics, meta-morphic core complex, strike-slip faults, Ti-betan plateau.

INTRODUCTION

The extent to which we can reconstruct oro-genic systems is governed by how well the

boundaries between tectonically distinct do-mains are defined. Although several insightfulstudies have been conducted to understand theprocesses controlling contemporaneous east-west extension and distributed strike-slipfaulting within the Tibetan plateau and north-south shortening in the Himalayan thrust belt(Armijo et al., 1989; Molnar and Lyon-Caen,1989; Avouac and Tapponnier, 1993; Molnaret al., 1993; England and Molnar, 1997;McCaffrey and Nabelek, 1998; Seeber andPecher, 1998; Yin et al., 1999a), the evolutionand even location of the boundaries betweenthese two structural regimes remain insuffi-ciently understood to make quantitative recon-structions since the initial collision betweenIndia and Asia at ca. 50–70 Ma (Klootwijk etal., 1992; Rowley, 1996; Hodges, 2000; Yinand Harrison, 2000).

The boundary between the western Tibetanplateau and the Himalayan fold-and-thrust beltis defined by the Karakoram fault (Fig. 1). It

Geological Society of America Bulletin, April 2002 429

STRUCTURAL EVOLUTION OF THE GURLA MANDHATA DETACHMENT SYSTEM, SOUTHWEST TIBET

430 Geological Society of America Bulletin, April 2002

MURPHY et al.

has been previously proposed that the Kara-koram fault system accommodates (1) north-ward indentation of the Pamir promontory(Tapponnier et al., 1981; Burtman and Molnar,1993; Strecker et al., 1995; Searle, 1996), (2)radial expansion of the Himalayan arc (Mol-nar and Lyon-Caen, 1989; Ratschbacher et al.,1994; Seeber and Pecher, 1998), and (3) east-ward lateral extrusion of the Tibetan plateau(Tapponnier et al., 1982; Peltzer and Tappon-nier, 1988; Armijo et al., 1989; Pecher, 1991;Avouac and Tapponnier, 1993). Assessing therelationships between deformation of the re-gions emphasized in the three proposed causesof the movement along the Karakoram faultsystem continues to be problematic because ofuncertainties in its initiation age, the net slipon it, and the location of its potential lateralcontinuation across the Himalayan-Tibetan or-ogen. From the southern Pamirs southward toMount Kailas, the Karakoram fault is well de-lineated. East of Mount Kailas, however, thelocation of the fault is speculative (e.g., Ar-mijo et al., 1989; Ratschbacher et al., 1994).

Hypotheses for the geometry and extent ofthe Karakoram fault system at its southeastend define two groups. Peltzer and Tapponnier(1988) proposed that the Karakoram fault sys-tem transfers slip to the Indus-Yalu suturezone in the Mount Kailas–Gurla Mandhata re-gion and extends across the entire length ofsouthern Tibet. Alternatively, Pecher (1991)suggested that the Karakoram fault systemlinks with the South Tibet detachment system.He noted a systematic deflection in the ori-entation of mineral-stretching lineations at thetop of the Greater Himalayan Crystalline Se-quence, which he interpreted to indicate aright-lateral shear. Opposing both of these in-terpretations is the view that the Karakoramfault system terminates into the north-trendingPulan basin adjacent to Gurla Mandhata(Ratschbacher et al., 1994) or merges and ter-minates into the Indus-Yalu suture zone in theMount Kailas area (Searle, 1996) (Fig. 1). Wehave tested these different interpretations bycombining field mapping with geochronologicand thermobarometric analyses of rocks in theMount Kailas–Gurla Mandhata region, wherethe Karakoram fault system is postulated toeither terminate or transfer slip farther to theeast. Here, we document the existence of alarge-scale normal-fault system, termed theGurla Mandhata detachment system, whichhas exhumed mid-crustal rocks belonging tothe Greater Himalayan Crystalline Sequence.We interpret the faults defining the Pulan ba-sin to be genetically related to the Gurla Man-dhata detachment system. Together they rep-resent an evolving extensional fault system.

On the basis of timing constraints, kinematicdata, and slip estimates for both faults, we ad-vocate a kinematic link between the Karako-ram fault system and the Gurla Mandhata de-tachment system.

Regional Geology

The Himalayan-Tibetan orogen is builtupon a complex tectonic collage that was cre-ated by sequential accretion, from north tosouth, of several microcontinents, flysch com-plexes, and island arcs onto the southern mar-gin of Eurasia since the early Paleozoic (Al-legre et al., 1984; Chang et al., 1986; Sengorand Natal’in, 1996; Yin and Nie, 1996; Hodg-es, 2000; Yin and Harrison, 2000). From northto south, the major tectonic elements in centraland south Tibet consist of a middle Paleozoicmicrocontinent known as the Kunlun block, aPermian–Lower Jurassic flysch sequence re-ferred to as the Songpan Ganzi terrane, a mid-dle Paleozoic–Jurassic microcontinent, re-ferred to as the Qiangtang block, and aPaleozoic–Mesozoic microcontinent known asthe Lhasa block (Allegre et al., 1984; Changet al., 1986; Sengor and Natal’in, 1996; Mur-phy et al., 1997). Discontinuous belts of ultra-mafic rocks separate each of these tectonic el-ements and are interpreted to representobducted oceanic crust now defining the su-ture between them (Allegre et al., 1984; Gi-rardeau et al., 1984; Pearce and Deng, 1988).The final assembling of this Tibetan collageoccurred with the docking of the Lhasa blockin the Late Jurassic (Yin et al., 1988). Togeth-er with the previously accreted blocks, theLhasa block constitutes the southern extent ofAsia prior to accretion of the Indian subcon-tinent at ca. 60–50 Ma (Klootwijk et al., 1992;Rowley, 1996). The Indus-Yalu suture sepa-rates these accreted blocks from Indiansubcontinent.

The Himalaya lies between the Indianshield to the south and the Indus-Yalu sutureto the north. It consists of three lithotectonicunits bounded by three north-dipping late Ce-nozoic fault systems: The Main Boundarythrust, the Main Central thrust, and the SouthTibetan detachment system. The Lesser Him-alaya is the structurally lowest slice. It isbounded at the base by the Main Boundarythrust and at the top by the Main Centralthrust and consists of Middle Proterozoic me-tasedimentary, sedimentary, and volcanicrocks, and Cambrian–Ordovician graniticrocks (Parrish and Hodges, 1996). The GreaterHimalaya Crystalline Sequence is bounded bythe Main Central thrust below and the SouthTibetan detachment fault above (Burg and

Chen, 1984; Burchfiel et al., 1992) (Fig. 1)and is composed of sedimentary, granitic, andvolcanic rocks of Late Proterozoic–EarlyCambrian age (Parrish and Hodges, 1996;DeCelles et al., 2000), which were metamor-phosed in the Tertiary. The North (or Tethyan)Himalaya lies between the South Tibetan de-tachment system and the Indus-Yalu suturezone. It consists of upper Precambrian–lowerPaleozoic sedimentary and metasedimentaryrocks (Gansser, 1964; Yin et al., 1988; Gar-zanti, 1999) and thick Permian–Cretaceouscontinental-margin sequences (Cheng and Xu,1987; Brookfield, 1993; Garzanti, 1999). Theentire sequence is commonly referred to as theTethyan Sedimentary Sequence.

The active trace of the Karakoram fault cutsobliquely across the Songpan-Ganzi terraneand the Qiangtang and Lhasa blocks; at thesoutheast end of the trace in the Mount Kailas–Gurla Mandhata region, the active trace of thefault cuts across the Indus-Yalu suture into theNorth Himalaya (Fig. 1). Four fault systemsmerge in this region: (1) the South Kailasthrust, the site of north-directed movement(Gansser, 1964; Yin et al., 1999b); (2) theTethyan fold-and-thrust belt, the site of south-directed folding and offset; (3) the right-slipKarakoram fault; and (4) the down-to-the-westGurla Mandhata detachment system.

Geologic Setting and Previous Work

The Mount Kailas area was first investigat-ed by Augusto Gansser in 1935 (Heim andGansser, 1939; Gansser, 1964), who recog-nized both a thrust system with south-directedslip involving the Tethyan Sedimentary Se-quence and a thrust system with north-directedslip along the Indus-Yalu suture zone, referredto as the South Kailas thrust (Fig. 1). This arearemained unmapped following Gansser’s re-connaissance work until 1987 when the Ti-betan Bureau of Geology and Mineral Re-sources completed a two-year mapping projectat a scale of 1:1 000 000 (Cheng and Xu,1987). Their work focused chiefly on regionalstratigraphic and age correlation with south-central and southeastern Tibetan rocks. Sincethen, four geologic expeditions have beenmade to the Mount Kailas–Gurla Mandhataregion (Ratschbacher et al., 1994; Miller et al.,1999; Yin et al., 1999b; Murphy et al., 2000).Miller et al. (1999) investigated the compo-sition and age of volcanic rocks exposed inthe Mapam Yum Co area (Fig. 1). Yin et al.(1999b) investigated the deformation historyalong the Indus-Yalu suture, focusing on theslip history of the South Kailas thrust. Asmapped by Ratschbacher et al. (1994) and



Murphy et al. (2000), a system of active brittleright-slip faults belonging to the Karakoramfault system extend into Mapam Yum Co andLa’nga Co areas, also known as Lakes Man-asarovar and Raksas, respectively (Fig. 21).What has remained uncertain is whether thisfault system extends eastward past MountKailas adjacent to the Indus-Yalu suture zoneor, instead, links to active east-west extension-al structures immediately south of MountKailas.

Armijo et al. (1989) and Liu (1993) noteda significant east-west extensional componentto the Karakoram fault system in Gar Valley(Fig. 1). Ratschbacher et al. (1994) extendedthese observations southward to the Pulan ba-sin and interpreted that it is kinematicallylinked to the Karakoram fault system, whichthey postulated may terminate there. Althoughit is clear from the pattern of active seismicity(e.g., Chen and Molnar, 1983) that the Kara-koram fault and faults bounding the Pulan ba-sin are both active and most likely linked ina neotectonic sense (Ratschbacher et al.,1994), long-term (tens of millions of years)linkage of these faults remains undeterminedbecause of poor constraints on net slip andtiming of the two fault systems. Murphy et al.(2000) estimated 66 6 5.5 km of dominantlyright slip on the southern part of the Karako-ram fault on the basis of the offset of theSouth Kailas thrust. Yin et al. (1999b) inter-preted the South Kailas thrust to have beenactive at 13 Ma on the basis of thermal mod-eling of its footwall, which places an upperlimit on the time of slip for the Karakoramfault. If the Karakoram fault is linked to ex-tension in the Pulan basin, then this slip esti-mate and timing constraint require compatibleestimates for basin bounding structures. How-ever, the small size and uncertain age of thePulan basin have made it difficult to feed sucha large magnitude of slip into normal faultsbordering the basin. We outline next the re-sults of our field investigation of the GurlaMandhata area that documents a previouslyunrecognized low-angle normal-fault system,referred to as the Gurla Mandhata detachmentsystem.

GEOLOGY OF THE GURLAMANDHATA AREA

Rocks within the Gurla Mandhata area weremapped during the summers of 1995, 1997,and 1998 at a scale of 1:100 000 (Fig. 2). Thegeologic framework may be viewed as con-

1Loose insert: Figure 2 is on a seperate sheet ac-companying this issue.

sisting of four different components, eachwith a unique deformational history. They are,from west to east, the Tethyan fold-and-thrustbelt, the Pulan basin, the two Gurla Mandhatadetachment faults, and an extensional ductileshear zone directly below the Gurla Mandhatadetachment faults (Fig. 3).

Tethyan Fold-and-Thrust Belt

Exposed in the western part of the studyarea is a fold-and-thrust belt with southwardvergence and offset, which exposes a .9.1-km-thick section of the Tethyan SedimentarySequence (Fig. 4A). It consists of Upper Pro-terozoic through Lower Cretaceous marinesedimentary to low-grade metasedimentaryrocks (mainly quartzite and phyllite). A largethrust sheet composed of mantle-type rocks(norite, dunite, and harzburgite) is exposedalong the southern edge of La’nga Co and Ma-pam Yum Co. Gansser (1964) refers to thispackage of rocks as the Kiogar-Jungbwa)ophiolite. South of this thrust sheet, the styleof folding varies from broad and concentricfar from major thrusts to tight and overturnedadjacent to them (Fig. 4B). The largest strati-graphic throw occurs along a north-dippingthrust ;10 km southwest of Pulan (Figs. 1and 2). It places Ordovician phyllite in itshanging wall over Permian–Upper Jurassicstrata in its footwall (Gansser, 1964; Chengand Xu, 1987). Because thrust faults in ourstudy area and adjacent areas in the fold-and-thrust belt along strike do not contain crystal-line basement rocks in their hanging walls(Cheng and Xu, 1987) (Figs. 1 and 2), we in-terpret this part of the fold-and-thrust belt,south of the Kiogar (or Jungbwa) ophiolite, tobe restricted to the Tethyan Sedimentary Se-quence (Fig. 4B). However, because no sub-surface data exist, we cannot rule out the pos-sibility that the thrust faults involvestructurally deeper rocks as suggested by fieldstudies in Ladakh (Corfield and Searle, 2000)and south-central Tibet (Ratschbacher et al.,1994).

Pulan Basin

Deposited unconformably above the Teth-yan Sedimentary Sequence is a .400-m-thicksection of Tertiary sedimentary rocks that de-fine the Pulan basin (Fig. 3). The basin post-dates contractional deformation related to theTethyan fold-and-thrust belt. The dimensionsof the Pulan basin, as defined by the spatialextent of the basin sedimentary fill, are 40 kmlong by 12 km wide (Fig. 2). It forms a localtopographic low, flanked on the east by the

Gurla Mandhata peak (7728 m). The MajaRiver flows through the basin from north tosouth and slightly off center along its easternmargin, which in turn is connected to the Kar-nali River in far-western Nepal. Steeply tomoderately dipping brittle normal faultsbound the eastern and western margins of thebasin.

The stratigraphy of the basin was investi-gated along seven east-west traverses alongriver-cut exposures. We divided the stratigra-phy into three map units based on lithologyand degree of deformation (Figs. 2 and 5A).The stratigraphically lowest unit (Tcg1) con-sists of interbedded white to tan sandstone andsiltstone with minor pebble-gravel conglom-erate lenses. Its thickness is .100 m. The baseof the lower unit was not observed. Tcg1 isbroadly folded along north-trending fold axes.Unconformably above Tcg1 lies an ;200-m-thick, upward-coarsening sequence of white totan sandstone, siltstone, and boulder-pebbleconglomerates (Tcg2). The lower part of thesequence is dominated by interbedded silt-stones and sandstones. Sandstone beds arefine- to medium-grained, poorly sorted, and20–100 cm thick. Siltstone beds are 1–10 cmthick and contain plant debris. The upper partof the sequence is dominated by coarse-grained sandstone and boulder-cobble-pebbleconglomerate. Conglomerate beds range from1 to 2 m thick and are clast supported. Clastsare almost exclusively gneiss, leucogranite,and foliated leucogranite (Fig. 5B). Debris-flow deposits occur along the eastern marginof the basin within Tcg2 and are interpreted tohave been deposited in an alluvial-fan envi-ronment. Both planar and trough cross-bedswere observed in the finer-grained lithologiesof the middle unit. The paleo–flow direction,as deduced from measurements of imbricatedclasts, shows two preferred directions, south(1758) and west (2618) (Fig. 5C). The impliedpaleodrainage system with south- and west-flowing rivers is similar to the present drain-age network (Fig. 2).

Capping Tcg2 is an ;100-m-thick sequenceof boulder-pebble conglomerate (Tcg3). Theconglomerate is more completely lithified thanthe underlying Tcg2 unit and forms resistantledges near Pulan. Although it contains a mi-nor component of metamorphic clasts, it isdominated by clasts of sandstone and lime-stone (Fig. 5D). The paleo–flow direction, asdeduced from measurements of imbricatedpebbles, is dominantly eastward (Fig. 5E).Tcg3 dips homoclinally to the east at ;58.This unit defines a paleopeneplain in the cen-tral and southern parts of the basin that hassubsequently been incised by the Maja River.

Structural evolution of the Gurla Mandhata detachment system, southwest Tibet: Implications for the eastward extent of the Karakoram fault system

M.A. Murphy, An Yin, P. Kapp, T.M. Harrison, C.E. Manning, F.J. Ryerson, Ding Lin, and Guo JinghuiFigure 2

Supplement to: Geological Society of America Bulletin, v. 114, no. 4

Figure 2. Geologic map of the Gurla Mandhata area. GMDF 1 and 2–Gurla Mandhata detach-ment faults 1 and 2.


MURPHY et al.

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Figure 4. (A) Stratigraphic column of theTethyan Sedimentary Sequence in the Gur-la Mandhata area, after Cheng and Xu(1987), Gansser (1964) and our own obser-vations. (B) Cross section C–C9 across theTethyan fold-and-thrust belt (see Fig. 2 forlocation). Cross section is based on map-ping by Heim and Gansser (1939) and ourown observations. Lithologic symbols cor-relate to those in Figure 2.

The southern limit of exposed basin sedimen-tary fill is immediately south of Kejia near theChina/Nepal border where a brittle normal-fault system defining the basin changes strikefrom a north trend to a more westerly orien-tation (Fig. 2).

North-trending normal faults broadly definethe eastern and western limits of the Pulan ba-sin strata (Fig. 2). The fault system consistsof east- and west-dipping sets of discontinu-ous, subparallel normal faults (Fig. 6A). Bothsets cut the upper Tcg3 unit of the Pulan basinstrata (Tcg3). The east-dipping set can betraced ;20 km. Near Pulan, it branches intodiscontinuous en echelon normal faults (Fig.2). The mean slip direction of the east-dippingfault set is 1208 6 208 (Fig. 6B). The maxi-mum stratigraphic throw, based on offset ofthe Tcg2/Tcg3 contact, is 150 m (Fig. 3; crosssection B–B9). The west-dipping set is trace-able for ;60 km from the southwest cornerof Mapam Yum Co in the north to the townof Kejia in the south (Figs. 2 and 6A). Indi-vidual faults cannot be traced more than ;8km. Together, these faults have a curved mappattern that closely follows the flanks of theGurla Mandhata massif. The mean slip direc-tion on these faults is 2748 6 108 (Fig. 6B).The maximum stratigraphic throw, based onoffset of the Tcg2/Tcg3 contact, is ;200 m.The magnitude of east-west extension of Tcg2and Tcg3 is small, amounting to ;2% or 300m (Fig. 3; cross section B–B9). The strati-graphically lowest unit (Tcg1) is clearly moredeformed (Fig. 7A) and has probably been ex-tended more. However, limited exposure ofthis unit precludes a quantitative estimate ofits extensional strain.

Two fold sets—north-northwest trendingand east-northeast trending—were observed inthe Pulan basin sedimentary fill (Fig. 2).North-northwest–trending folds are spatiallyassociated with north-northwest–trending nor-mal faults and are interpreted to be related toslip on these faults possibly due to downdipbends in the fault surface. The north-trendingfold in Figure 7A is interpreted as a rollover


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Figure 6. (A) Structure map of the Gurla Mandhata detachment system. (B–F) Lower-hemisphere, equal-area stereonet plots of structuraldata collected from the Gurla Mandhata detachment system.

structure, which implies that it formed by slipalong a listric-shaped normal fault. East-northeast–trending folds are common near thetips of small magnitude (,200 m) north-trending normal faults (Fig. 2). We interpretthese folds to reflect strain at accommodationzones between en echelon normal faults (e.g.,Faulds and Varga, 1998).

Gurla Mandhata Detachment Faults

The eastern edge of the Pulan basin isbounded by a pair of west-dipping low-angle(,458) normal faults, termed the Gurla Man-dhata detachment fault 1 (older) and GurlaMandhata detachment fault 2 (younger) (Fig.

7B). The curviplanar traces of both faultscross the flanks of the north, northwest, andwest sides of the Gurla Mandhata massif (Fig.2).

The structurally higher detachment, GurlaMandhata detachment fault 2 (GMDF 2 in thefigures), juxtaposes the lower (Tcg1) and mid-dle (Tcg2) units of the Pulan basin in thefault’s hanging wall against phyllite and mar-ble in its footwall (Fig. 7A). Footwall unitscorrelate with Ordovician rocks exposed to thewest in the Tethyan fold-and-thrust belt (Fig.2). Where exposed, the fault zone is definedby an ;2–10-cm-thick layer of foliated black,orange, and white clay gouge containing an-gular clasts of the footwall rocks. Well-

developed sets of P foliations and R1 shearsin the clay gouge indicate top-to-the-westshear (e.g., Rutter et al., 1986). Low strain isindicated by preserved crinoids in the rocksimmediately beneath the detachment, but thereis a weak stretching lineation that is definedby aligned muscovite and smeared quartzgrains. Shear-sense direction on the fault wasdetermined from tool marks, chatter marks,and P foliation–R1 shear pairs in the claygouge. The mean shear-sense direction onfault 2 is 2808 6 48 (Fig. 6C).

Gurla Mandhata detachment fault 2 locallyincises a structurally lower detachment fault,the Gurla Mandhata detachment fault 1(GMDF 1 in the figures). The fault juxtaposes


MURPHY et al.

Figure 7. (A) Upper (younger) and lower (older) Gurla Mandhata detachment faults 1and 2 (GMDF 1 and GMDF 2), respectively. View is to the north. Buildings in the centerof the foreground are 5–10 m tall and lie ;1 km from the base of the mountain. Fault 2juxtaposes the lower (Tcg1) and middle (Tcg2) stratigraphic units of the Pulan basin inthe hanging wall against phyllite and marble in the footwall. Fault 1 places myloniticschist, marble, gneiss, and variably deformed leucogranite dikes in the footwall againstphyllite and marble in the hanging wall. Fault symbols: open box—GMDF 1, box onhanging wall; double-tick—GMDF 2, ticks on hanging wall; bar and ball—normal fault,bar and ball on hanging wall. (B) Eastern margin of the Pulan basin defined by the GurlaMandhata detachment system. View is toward the east. The approximate width of field ofview is 12 km.

lower Paleozoic phyllites and marbles in itshanging wall over a thick (.2 km) section ofsillimanite-bearing mylonitic schists, gneisses,marbles, and variably deformed granite dikesand sills in its footwall. The hanging-wallrocks are commonly preserved as kilometer-scale klippen on the western lower slopes ofthe Gurla Mandhata massif. Where exposed, asharp, striated contact defines the detachment.

Locally, a thin layer (,3 cm) of foliated,black-to-brown clay gouge with small angularmylonitic porphyroclasts derived from thefootwall is present (Fig. 8A). In the footwall,immediately beneath this contact, is an ;2–5-m-thick cataclastic zone with calcite and epi-dote veins that overprint an older myloniticfoliation. Locally, fault 1 cuts this cataclasticzone and the footwall mylonitic foliation. Sev-

eral west-dipping and east-dipping, meter- tocentimeter-scale brittle normal faults are pre-sent within this zone. Thin (;1–3 cm thick)cataclasite zones are present in this zone andlie subparallel to the attitude of the trace offault 1. Often, they consist of a black aphaniticmatrix (;80%) containing white feldspar por-phyroclasts. The attitude of fault 1 partiallydefines a dome and in general dips ;188 to308 away from the Gurla Mandhata massif(Fig. 6A). The mean shear-sense direction offault 1 is 2798 6 58 (Fig. 6D).

Footwall of the Gurla MandhataDetachment System

The footwall rocks of the Gurla Mandhatadetachment faults 1 and 2 were investigatedalong 11 traverses. Two traverses extended farinto the footwall (;16 km) and were con-ducted in order to characterize the rock se-quence and style of deformation. These twotraverses are located in deeply incised valleysin the Gurla Mandhata massif (west side,Ronggua gorge; north side, Namarodi gorge)(Fig. 2). A 2.1-km-thick section of the foot-wall rocks is exposed along Ronggua gorge(Fig. 3; cross section A–A9), and in Namarodigorge, an ;1-km-thick section is exposed. Im-mediately below fault 1 is a sequence of my-lonitized garnet-biotite-muscovite schist (bs)and marble (mbl) that reaches a thickness of1.2 km. Below this unit is an ;900-m-thicksequence of quartzofeldspathic biotite-garnetgneisses and biotite schist (gn). The structur-ally deepest rocks consist of quartzofeldspath-ic migmatitic gneiss (mig) that is .50 mthick. It consists of leucocratic bodies (ellip-tical bodies ;1–3 m in diameter and dikes upto 2 m thick), biotite-rich zones, and bandedgneiss that contains a folded mylonitic fabric.

The characteristic mineral assemblage ofthe footwall rocks is (Bt 1 Ms 1 Grt 1 Plag1 Sil). Inclusions in the garnet include biotite,plagioclase, muscovite, quartz, monazite, ap-atite, and ilmenite. Subhedral to anhedral gar-nets contain a nonrotational growth historyand normally do not preserve a fabric.

Except for the migmatite unit, the entire se-quence shares a common penetrative myloni-tic fabric. Primary and secondary foliations(S, C, and C9 foliations) are defined by alignedsillimanite (fibrolite), biotite, muscovite, andrecrystallized quartz, whereas quartz rods,feldspar grains, and sillimanite grains definethe lineation. The mean trend of the stretchinglineations in the footwall rocks from thesouthwest, west, northwest, and north sides ofthe Gurla Mandhata massif is 2808 6 48 (Fig.6F). Several fabric elements in outcrop and in



Figure 8. (A) Fault zone of Gurla Mandhata detachment fault 2, typified by sheared silty-clayey gouge exhibiting P foliations, R1 shears,and top-to-the-west duplexes. View is toward the south. Photograph is taken from an outcrop immediately east of the town of Kejia.(B) Biotite-garnet gneiss in the footwall of the Gurla Mandhata detachment system in Namarodi gorge. View is toward the north. Noteshear bands and asymmetric augen structures indicating west-directed shearing. (C) Cliff exposure along the north side of Rongguagorge; 1–2-m-thick leucocratic granite dikes cut the mylonitic foliation at lower structural levels, but are deformed at higher structurallevels and swing toward the west into parallelism with the shear zone. View to the north.

thin sections oriented parallel to lineationshow asymmetry (asymmetric biotite andmuscovite grains, domino-tilted and asym-metric feldspar grains and porphyroclasts, andshear banding), indicating a significant simple-shear component of deformation of the foot-wall (Fig. 8B). Most (;70%) indicate domi-nantly west-directed shearing in the footwallrocks, although some are consistent with top-to-the-east motion.

The most prominent structures in the foot-wall rocks are ductile normal faults. They ex-ist at several scales, traceable for distancesranging from 10 cm to 1 km. Nearly all ofthese faults strike parallel to Gurla Mandhatadetachment faults 1 and 2 (Fig. 6E) and dis-play variable-slip magnitudes of tens of cen-timeters to hundreds of meters on the basis of

offset lithologic units. The largest of these isa top-to-the-west normal fault that cuts a 100-m-thick marble unit ;200–300 m below thedetachment at the mouth of Ronggua gorge(Fig. 2).

The mylonitic foliation is folded at allscales. The most dominant folds are thosewith axes oriented subparallel to the stretchinglineation, which we refer to as corrugations.The largest corrugations expressed in the my-lonitic foliation, and by the traces of GurlaMandhata detachment faults 1 and 2, have awavelength of ;18 km (Figs. 6A and 7B).Fault 1 is commonly exposed in the core ofsynforms, suggesting that it had been foldedmore than the younger detachment (fault 2).Brittle normal faults along the eastern marginof the Pulan basin follow closely the curved

geometry of fault 1. The fact that these faultsare not folded suggests they formed after thedevelopment of the corrugations.

The timing of formation of the east-trendingfolds (corrugations) is not clear. Three possi-bilities for the development of the folds are(1) the observed fold geometry could be in-herited, possibly from north-south shorteningrelated to the Tethyan fold-and-thrust belt; (2)the folding may also have developed synchro-nously with motion on the Gurla Mandhatadetachment system in a constrictional strainfield (e.g., Yin, 1991), and (3) they potentiallydeveloped late in the evolution of detachmentfaults 1 and 2. However, field observations fa-vor that the corrugated fault geometry formedby north-south compression that was synchro-nous with east-west stretching. First, the im-


MURPHY et al.

Figure 9. Thermobarometric results from rocks within the footwall of Gurla Mandhatadetachment fault 1. Parallelograms indicate where pressure and temperature estimatesfrom GARB and GASP overlap for a particular sample. Aluminum silicate stability fieldis after Holdaway (1971).

Figure 10. Backscattered-electron image showing textural relationship between monaziteinclusions (dark) and garnet (light) in sample GM-3. Th-Pb monazite ages show a young-ing pattern from core to rim, which we interpret to reflect the age of garnet growth.

plied amplitude of the corrugations decreaseswith time, on the basis of (a) the crosscuttingrelationships between the mylonitic foliationand both detachment faults and (b) the pres-ervation of fault 1–bounded klippen in thetroughs of the synformal corrugations that foldthe fault 2 surface. Second, the middle andupper units of the Pulan basin sedimentaryrocks do not appear to be folded about axesparallel to the large-scale corrugations, sug-gesting that motion on the detachment oc-curred along a corrugated surface while thePulan basin strata were being deposited.Third, the most extensive exposures of Pulanbasin sedimentary rocks occur next to a syn-form, implying that this topographic depres-sion existed during their deposition.

Footwall Leucogranites

Variably deformed leucogranite bodiesmake up ;10% to 20% of the footwall ofGurla Mandhata detachment fault 1. Five li-thologies were recognized: (1) muscovite-bearing granite (2) muscovite-tourmaline–bearing granite (3) muscovite-biotite–bearinggranite (4) biotite-bearing granite, and (5) bi-otite-garnet–bearing granite. We use the term‘‘granite’’ to describe leucocratic (Qtz 1 K-feldspar 1 Plag) rocks in general. The mostabundant lithology is muscovite-tourmaline-bearing granite, and the least abundant is biotite-garnet–bearing granite. Within the upper 2 kmof the shear zone, leucogranite bodies occurexclusively as dikes and sills that are fromtens of centimeters to 2 m thick. They gen-

erally display straight contacts with the coun-try rock. In some cases, a tourmaline-richzone is present along the contact between thedike and the country rock. Two generations ofdikes and/or sills were recognized on the basisof their crosscutting relationship and intensityof deformation. The older set parallels the fo-liation and displays a mylonitic foliation sim-ilar to that of the country rock. The youngerset cuts the older set at lower structural levels,

but is deformed at higher structural levels andswings toward the west into parallelism withthe shear-zone foliation (Figs. 3 [cross sectionA–A9] and 8C). We interpret this relationshipto indicate that the period of time between in-trusion of the older and younger sets overlapswith top-to-the-west shearing.

THERMOBAROMETRY

Metamorphic pressures and temperatureswere calculated for samples from the footwallrocks of the Gurla Mandhata detachment sys-tem by using the THERMOCALC computerprogram, version 2.75 (Powell and Holland,1988), and the data set of Holland and Powell(1998). The plagioclase-garnet-Al2SiO5-quartz(GASP) barometer (Koziol and Newton,1988) and the garnet-biotite (GARB) ther-mometer (Ferry and Spear, 1978) were usedfor the reported estimates (Fig. 9; GSA DataRepository item 1).2 Sample locations are in-dicated in Figure 2. Mineral compositionswere determined by using a Cameca Camebaxelectron microprobe at the University of Cal-ifornia, Los Angeles (UCLA), with a nominalbeam current of 10 nA and an acceleratingvoltage of 15 kV. Standardization was con-

2GSA Data Repository item 2002048, Mineralcomposition, trace element, and 40Ar/39Ar isotopicdata, is available on the Web at http://www.geosociety.org/pubs/ft2002.htm. Requestsmay also be sent to [email protected].



TABLE 1. Th-Pb ION-MICROPROBE MONAZITEAGES FOR GURLA MANDHATA SCHIST GM-3

Grain Diameter Spot 208Pb* 208Pb/232Th age†

(mm) (%) (Ma 6 2s)

1 40 85 14.4 6 0.51 - 1 76 13.7 6 1.02 30 2 76 16.9 6 0.63 50 1 87 12.4 6 0.44 20 1 71 11.6 6 1.05 20 1 93 14.8 6 0.56 20 1 57 10.8 6 1.17 80 1 85 16.4 6 0.57 - 2 90 15.7 6 0.48 60 1 90 13.6 6 0.310 30 1 92 13.8 6 0.311 30 1 92 11.4 6 0.3

*Calculated by assuming common 208Pb/204Pb 536.7.

†Ages based on comparison with monazite 554.

TABLE 2. Th-Pb ION-MICROPROBE ANALYSES OF GM-4 MONAZITE

Grain Spot 208Pb*/232Th 208Pb*/232Th 208Pb/204Pb 208Pb* 208Pb/232Th age†

(x 10–4) (6 1 s.e.)(x 10–6) (%) (Ma 6 2s)

2 1 3.28 5.21 402 91 6.6 6 0.23 2 3.29 4.74 388 91 6.7 6 0.23 1 2.27 3.97 455 92 4.6 6 0.24 1 3.41 5.42 482 92 6.9 6 0.27 1 2.63 3.37 444 92 5.3 6 0.27 2 3.39 5.18 460 92 6.9 6 0.27 3 3.37 4.82 411 91 6.8 6 0.211 1 3.36 4.75 476 92 6.8 6 0.2

Note: s.e.—standard error.*Calculated by assuming 208Pb/204Pb 5 36.7.†Ages based on comparison with monazite 554.



(x 10–4) (6 1 s.e.)(x 10–6) (%) (Ma 6 2s)

1 1 5.2 5.8 377 90 10.6 6 0.21 2 6.1 7.0 815 95 12.4 6 0.21 3 6.0 6.7 649 94 12.1 6 0.23 1 5.5 6.6 679 95 11.1 6 0.24 1 5.9 5.9 737 95 12.0 6 0.25 1 4.7 7.0 501 93 9.4 6 0.26 1 5.3 7.0 642 94 10.8 6 0.27 1 5.3 6.9 611 94 10.7 6 0.27 2 4.5 25 753 95 9.1 6 1.08 1 6.1 9.5 725 95 12.3 6 0.48 2 6.0 41 1089 97 12.0 6 1.69 1 5.6 6.1 657 94 11.4 6 0.211 1 6.1 7.1 560 93 12.3 6 0.212 1 5.7 6.3 652 94 11.4 6 0.212 2 6.2 59 1172 97 12.5 6 2.413 1 5.8 7.3 615 94 11.7 6 0.213 2 6.2 18 898 96 12.6 6 0.8




(x 10–4) (6 1 s.e.)(x 10–6) (%) (Ma 6 2s)

1 1 4.07 7.27 413 91 8.2 6 0.21 2 4.15 7.39 526 93 8.4 6 0.21 3 3.96 6.84 377 90 8.0 6 0.22 1 3.68 7.44 357 90 7.4 6 0.42 2 3.72 6.81 501 93 7.5 6 0.23 1 3.57 8.20 375 90 7.2 6 0.43 2 3.67 7.75 616 94 7.4 6 0.44 1 5.44 7.89 484 92 11.0 6 0.44 2 5.25 7.02 681 95 10.6 6 0.25 1 5.11 6.55 450 92 10.3 6 0.25 2 4.55 7.10 389 91 9.2 6 0.25 3 4.43 5.47 653 94 8.9 6 0.27 1 4.01 5.65 544 93 8.1 6 0.2


ducted on natural materials. Sillimanite andquartz were assumed to be pure phases.

Sample GM-1 is a garnet-biotite schist thatcontains garnet, anthophyllite, plagioclase, bi-otite, tourmaline, and quartz. Plagioclase, bi-otite, and quartz occur as inclusions in the gar-net, whereas anthophyllite and tourmalineoccur only in the matrix. If it is assumed thatall were in equilibrium, the reaction antho-phyllite 1 anorthite → pyrope 1 grossular 1quartz yields pressures between 6.5 and 7.5kbar at temperatures between 550 and 650 8C(Fig. 9). The temperature is bracketed by biotite-garnet thermometry calculations (Ferry andSpear, 1978) and the given reaction. BothGM-2 and GM-3 are garnet-sillimanite-biotite-muscovite schists. Sample GM-2 lies;200–300 m above the migmatite/gneiss con-tact, and GM-3 lies ;400–800 m above thesame contact. X-ray mapping and electron-microprobe traverses of garnets in both sam-ples show that they are homogeneous exceptnear rims, which display strong increases inXsps (spessartine component) and Fe/(Fe 1Mg) and decreases in Xalm (almandine com-ponent) (GSA Data Repository item 2; seefootnote 1). We interpret the rim compositionas the result of diffusive zoning associatedwith declining temperature. Biotite-garnetthermometry (GARB) (Ferry and Spear, 1978)using biotite inclusions yields low temperatureestimates between 390 and 450 8C for GM-3and between 450 and 550 8C for GM-2. Sam-ple GM-3 contains sillimanite, although thesetemperatures lie outside of the sillimanite sta-bility field. Therefore, we conclude that biotiteinclusions in the garnet are inappropriate forcalculating peak temperatures. Instead, wehave paired measurements of garnet-corecompositions with those of matrix biotitecrystals that lie on the order of a few milli-meters from the garnet edge (Florence and

Spear, 1992). We should note that the propor-tion of garnet to biotite in rock is very small.Therefore, we would not expect that the bio-tite in the matrix has been significantly alteredby net-transfer reactions involving the disso-lution of garnet (Spear, 1993). GARB calcu-lations using these pairs yield temperaturesthat we interpret to more accurately reflect the

peak temperature the rock was subjected to,given the degree of zoning observed (Florenceand Spear, 1992) and the presence of silliman-ite in both rocks. GASP (Koziol and Newton,1988) calculations for GM-2 yield pressure es-timates between 5 and 7.6 kbar in a temper-ature range defined by GARB calculations be-tween 575 8C and 690 8C (Fig. 9). This result


MURPHY et al.

TABLE 5. Th-Pb ION-MICROPROBE DATA OF GM-7 MONAZITE


(x 10–4) (6 1 s.e.)(x 10–6) (%) (Ma 6 2s)

4 1 1.05 1.89 598 94 21.2 6 0.84 2 1.01 3.91 742 95 20.5 6 1.65 1 1.13 2.21 827 96 22.9 6 0.85 2 0.89 3.33 948 96 18.0 6 1.46 1 1.14 2.18 939 96 23.0 6 0.87 1 0.94 1.12 769 95 18.9 6 0.49 1 0.47 1.88 190 81 9.5 6 0.810 1 0.64 2.41 194 81 12.9 6 1.010 2 1.18 2.37 856 96 23.8 6 1.012 1 0.86 1.95 694 95 17.4 6 0.812 2 0.85 6.60 1172 97 17.2 6 2.6


Figure 11. Histogram of all 49 Th-Pb monazite ages from Gurla Mandhata leucogranitebodies. The peaks at 7 and 11 Ma are interpreted to reflect the age of magmatic monazite.The 11 Ma year peak may also correspond to the age of metamorphic monazite. Thedistribution of older ages (17–24 Ma) is interpreted to reflect incorporation of inheritedmagmatic monazite.

is similar to the conditions estimated for GM-1. GASP and GARB calculations for GM-3yielded pressures between 3.6 and 4.5 kbarand temperatures of 600 8C to 650 8C (Fig.9). These results indicate that GM-2 and GM-1 equilibrated at ;2–3 kbar higher pressurethan GM-3. Given that these two samples arenow only ;500 m apart, perpendicular to theshear zone, we interpret this result to indicatethat the footwall sequence was highly attenu-ated during shearing.

Age Constraints

To address the timing of metamorphism weundertook in situ Th-Pb dating of monazitethat occurs as both inclusions in garnet (Fig.10) and in the matrix of sample GM-3 (Table1). Ten monazite grains identified in a pol-ished thin section were dated in situ by the208Pb/232Th ion-microprobe method describedin Harrison et al. (1995). A single garnet graincontaining six monazite inclusions and fouradditional monazite grains in the adjacent ma-trix were mounted with a standard in epoxyand Au coated for analysis. Analyses wereconducted with an O– primary beam focusedto an ;20 mm spot. Ages are determined bydirect reference to monazite standard 554(Harrison et al., 1999). Monazite inclusions insample GM-3 yielded a range of ages from16.9 6 0.3 Ma to 10.8 6 0.6 Ma (Table 1)that show an apparent correspondence to theirposition within the garnet from core to rim(Fig. 10). Monazite grains in the matrix alsoshow a range of ages between 16.5 and 11.5Ma, a distribution of ages similar to that with-in the garnet. Interpreting these data as crys-tallization ages requires some considerationfor the potential degree of Pb* (radiogenic Pb)loss. GARB thermometry indicates that GM-3 was subjected to temperatures of 600 to 6508C. We do not think that the monazites havehad significant Pb* loss for two reasons: (1)Both monazite inclusions and matrix mona-zites yield a similar spread in 208Pb/232Th ages,and none in the matrix are younger than thosein the garnet; (2) Ar-closure ages of nearbybiotites and muscovites imply that GM-3 hasbeen at ,400 8C since ca. 8.8 Ma, precludingthe possibility of Pb* loss since that time(Smith and Gileti, 1997).

To ascertain the age relationships amongthe different leucogranite bodies, we dated (1)two dikes that cut the mylonitic foliation atdeep structural levels, but are deformed andoriented parallel to the foliation higher in theshear zone (GM-4 [Table 2] and GM-5 [Table3]), (2) a mylonitized sill (GM-6 [Table 4])that is cut by GM-4, and (3) a mylonitized sill

(GM-7 [Table 5]) near the top of the footwallextensional shear zone, immediately beneathGurla Mandhata detachment fault 1 (see Fig.2 for sample location). All the granite bodiesare small, ,1 m thick, except for GM-7 thatis ;30 m thick. We obtained 208Pb/232Th ion-microprobe monazite ages following themethod described by Harrison et al. (1995).For each sample, ;8–20 grains of monazitewere separated from 1 kg of rock by usingstandard rock-crushing and mineral-separationtechniques and were mounted in epoxy with amonazite standard (554) and Au coated.

With the exception of GM-7, all monazitesanalyzed yield an early to late Miocene agepeak. However, interpretation of the data iscomplicated owing to a relatively wide rangeof ages that is observed in every sample. Sam-ple GM-4 (Table 2) yields a weighted meanage of 6.8 6 0.2 Ma with an MSWD (meansquare of weighted deviates) of 0.97, exclud-ing two repeat spot analyses on grains 3 and7. GM-4 is a sample of a dike with an ori-entation of N208W/658NE. A nearby sill (sam-ple GM-6 [Table 4]), which is cut by the GM-4 dike, yields a weighted mean age of 8.7 6



Figure 12. 40Ar/39Ar age spectra and isochron diagrams for muscovite and biotite fromrocks in the footwall of the Gurla Mandhata detachment system.

0.1 Ma. The fact that the GM-4 dike cuts theGM-6 sill indicates that their crystallizationages bracket the timing of ductile shear at thisposition in the shear zone between 6.8 6 0.2Ma and 8.7 6 0.1 Ma. However, we note thatshearing must have continued at structurallyhigher levels in the shear zone because theunfoliated dike (GM-4) is deformed higher upin the shear zone. Nonetheless, we can assertthat shearing between the dike and sill ceasedby 6.8 6 0.2 Ma because the crosscutting re-lationship is preserved. Monazite ages ob-tained from GM-5 (Table 3) yield a weightedmean age of 11.4 6 0.1 Ma. GM-7 (Table 5)shows the widest distribution, yielding agesfrom early to late Miocene.

A plot of all the Th-Pb spot ages of GurlaMandhata granite monazites reveals two dis-tinct groups of ages at ca. 7 and 11 Ma aswell as a broader distribution of ages between17 and 24 Ma (Fig. 11). We interpret the ca.7 and 11 Ma peaks to represent the age ofmagmatic monazite represented by the leuco-granite dikes and sills just discussed. Alter-natively, the 11 Ma peak may reflect inheri-tance of metamorphic monazite grains, suchas those that occur in sample GM-3, alreadydescribed. The presence of 17–24 Ma agesmay indicate a distinct magmatic event, assupported by extensive studies in other partsof the orogen (e.g., Scharer et al., 1986; Nobleand Searle, 1995; Searle et al., 1997; Harrisonet al., 1999). Alternatively, these ages may in-dicate incorporation of inherited magmaticmonazite grains. Our interpretation of thespread of monazite ages favors inheritance ofrestitic grains for two reasons: (1) monaziteinclusion ages from GM-3 overlap with the11.2 Ma peak in monazite ages from the gran-ites, and (2) monazite saturation temperaturesare low. Light rare earth element concentra-tions (La, Ce, Pr, Nd, Sm, Gd) in samplesGM-7 and GM-4 (tabulated LREE concentra-tions are available in the GSA Data Reposi-tory Item 3; see footnote 1) yield a monazitesaturation temperature of ;650 8C (Rapp andWatson, 1986), which is the same as that in-ferred for peak metamorphic temperatures inGM-1, GM-2, and GM-3. We therefore expectthese granites to contain a large number ofinherited monazite grains.

40Ar/39Ar thermochronology was conductedto characterize the time of exhumation ofrocks within the footwall of the Gurla Man-dhata detachment system. Ten samples wereanalyzed for 40Ar/39Ar, of which four were col-lected immediately beneath the lower detach-ment and six were collected at structurallydeeper levels (Figs. 12 and 2; GSA Data Re-pository Item 4; see footnote 1). All ages and

isotopic ratios are reported at the 2s uncer-tainty level. 40Ar/39Ar analyses were per-formed at UCLA on a VG3600 equipped witha quadrupole mass spectrometer. Sampleswere step heated to obtain three-isotope plotsto correct for the composition of the trappedAr.

Sampling and subsequent analyses wereconducted to test for variations in coolingages, both along strike and in the transportdirection of the detachment system. Moreover,samples GM-8 through GM-15 were collectedat an appropriate distance from granite dikesto assure that they were not significantly re-heated during intrusion. All samples analyzed,regardless of location, yield late Miocene ages(Figs. 2 and 12). Samples that lie within theupper 100 m of the footwall extensional shearzone, immediately beneath Gurla Mandhatadetachment fault 2 or locally beneath GurlaMandhata detachment fault 1 (GM-8, GM-9,GM-10, GM-11), yield weighted mean agesbetween 9.9 and 7.4 Ma. Younger ages cor-respond to more northern positions alongstrike of the detachment system near Ronggua

gorge, possibly reflecting northward propaga-tion of the fault system. Parallel to the trans-port direction of the fault, samples alongRonggua gorge (GM-12, GM-13, GM-5)show no apparent pattern in their cooling ages.Considering only the weighted mean ages ofsamples from Ronggua gorge suggests that thestructurally deepest rocks in the footwallcooled below 400 8C by ca. 9 Ma. In two othervalleys, structurally lower rocks (GM-14 andGM-15) cooled 1–2 m.y. earlier than rocksstructurally above them (GM-8 and GM-11)(Fig. 2), suggesting significant telescoping dueto shearing of the footwall rock.

DISCUSSION

Tectonic Evolution of the GurlaMandhata Detachment System

Magnitude of SlipLimits on the minimum net slip on the Gur-

la Mandhata detachment system are set by theextent of the ductile shear zone exposed in adirection parallel to the slip direction, which


MURPHY et al.

Figure 12. (Continued.)

is ;20 km (Figs. 2 and 6A). The maximumslip estimate is defined by the pressure esti-mate for sample GM-3 and GM-2. The pressure-temperature estimates for these two rocks in-dicate that they equilibrated at significantlydifferent peak pressures. If a lithostatic pres-sure gradient of 0.27 kbar/km (average rockdensity of 2750 kg/m3) is assumed, the peakpressures can be translated to depth of equil-ibration relative to the surface. Note that the

uncertainties in the pressure-temperature esti-mates represent their precision and not theiraccuracy, as uncertainties in the calibrationsand activity models are not considered. Sam-ple GM-2 yield depths of 18.5–28.2 km. GM-1 yields pressures between 6 and 7.2 kbar, cor-responding to depths between 22.2 and 26.7km. The pressure estimate for sample GM-3is lower, 3.6–4.5 kbar, and corresponds to adepth of 13.3–16.7 km. The higher-pressure

rocks and the lower-pressure rocks are cur-rently separated by ;500 m vertically, indi-cating significant tectonic juxtaposition andthinning by ductile shearing. Laterally, sampleGM-2 projects along contours of the foliation(see Fig. 6A) to within 3 km of sample GM-3 in the direction of fault slip. Because bothGM-2 and GM-1 are currently at similar struc-tural levels in the footwall and have pressure-temperature estimates that overlap one anoth-er, we assume that the pressure-temperatureestimate for sample GM-1 better reflects theconditions of these rocks prior to slip on theGurla Mandhata detachment system. The pre-ceding discussion indicates that the differencein the equilibration depths between samplesGM-1 and GM-3 is 9.4 6 5.5 km. If it isassumed that these rocks reached their presentposition with respect to one another along aductile shear zone at a dip of 228 (shallowestmeasured dip of the structurally lower detach-ment fault, Gurla Mandhata detachment fault1) (Fig. 2), a minimum of 25.9 6 14.9 km ofslip along the Gurla Mandhata detachmentsystem is required to juxtapose these tworocks (Fig. 13). For translation of these rocksto the surface, an additional 24.7 6 4.9 km offault slip is required. Simply using the higher-pressure estimate from samples GM-2 andGM-1 of 6.5–7.5 kbar (24–27.7 km) requires66 6 5.9 km of slip on a shear zone dippingat 228 to bring them to their present positionin the shear zone. These slip estimates arehighly sensitive to the dip of the shear zone.If the shear zone slipped at a higher angle of458, the magnitude of displacement to bringthe rocks to their position with respect to oneanother decreases to 13.3 6 7.8 km (Fig.13A). Moreover, to bring the deepest rocks tothe surface changes the slip estimate to 34.56 3.1 km. We prefer the interpretation that theshear zone slipped at an angle of ,458, be-cause the hanging-wall strata do not show acorrelative amount of eastward tilting acrossthe basin.

An independent estimate of the depth atwhich these rocks resided prior to their ex-humation by the Gurla Mandhata extensionalsystem comes from considerations of thedepth of the basal thrust in the Tethyan fold-and-thrust belt, which is assumed to lie alongthe contact between the Tethyan SedimentarySequence and the rocks currently exposed inthe footwall of the Gurla Mandhata detach-ment system. Figure 4B predicts that the depthto the basal thrust is ;8.3 km in the south and;12.4 km in the north, relative to the present-day surface. The rocks beneath the basal thrustare now exposed in the footwall of the GurlaMandhata detachment system. If we assume



Figure 13. (A) Geometric model used to estimate magnitude of displacement along Gurla Mandhata detachment system. Estimates shownfor fault dip of 228 and 458. No vertical exaggeration. See text for explanation. (B, C, and D) Kinematic model for juxtaposition andexhumation of samples GM-3 (A) and GM-2 and GM-1 (B).

that the footwall rocks were exhumed along afault oriented at its present dip of 228, then22–33 km of slip is required, correspondingto depths of 8.3 and 12.4 km, respectively. Ifwe assume that the fault slipped at a higherangle of 458, then the total slip changes to 12–18 km, corresponding to the same depths. Toeither slip estimate, 20 km must be added toaccount for the present exposure of the ductileextensional shear zone at the surface (Fig.6A), increasing the slip estimate to a maxi-mum of 53 km and a minimum of 32 km. Weconclude from these different estimates thatthe Gurla Mandhata detachment system hasaccommodated at least 66 km of top-to-the-west normal shear. We view this estimate as aminimum because the detachment system mayhave a ramp-flat geometry (Fig. 13, B–D). Inthis scenario, the depth estimate of 24–27.7km limits the minimum depth of the flat seg-ment of the fault. Although we recognize thatthe Gurla Mandhata detachment system mostlikely does flatten at depth, we do not thinkthat its footwall rocks have traveled far alongit because of arguments for its link with thesouthern part of the Karakoram fault system,which is estimated to have a total of 66 kmof right-slip (Murphy et al., 2000).

Structural ModelThe following model views the footwall ex-

tensional shear zone, the detachment faults(Gurla Mandhata detachment faults 1 and 2),

and the Pulan basin as an evolving extensionalfault system (Fig. 14). Our interpretation islargely motivated by two results of our inves-tigation: the extensional shear zone, detach-ment faults, and normal faults within the Pu-lan basin share a common slip direction, andthe crosscutting relationships and impliedtemperatures at which the different faultsslipped are consistent with younger structuresforming at successively higher levels in thecrust.

Figure 14, A–D, illustrates our interpreta-tion. Figure 14A shows the preextensional set-ting whereby the Tethyan Sedimentary Se-quence has been shortened by south-directedthrusts. The depth to the basal thrust based oncross section C–C9 (Fig. 4B) is predicted tolie between 8.3 and 12.4 km. We interpret thisas the minimum depth from which the foot-wall rocks of the detachment system originat-ed prior to slip. The footwall rocks could havebeen subject to prograde metamorphic condi-tions during this early crustal thickeningevent. We speculate that this tectonic settingprevailed until about middle Miocene, the ageof garnet crystallization. This age constraint isconsistent with results reached by Yin et al.(1999b) on the age of reheating of the SouthKailas thrust in the Mount Kailas area ;40km north.

Figure 14B illustrates the initiation of GurlaMandhata detachment fault 1 (the older de-tachment fault) and the extensional ductile

shear zone currently exposed in its footwall.We suggest that fault 1 represents the upper-crustal equivalent of the ductile shear zone.Shear along this fault system attenuated andassisted in exhumation of the footwall rocks.We rule out the possibility that fault 1 corre-lates with the South Tibetan detachment faultto the south of our study area (Fig. 1) on thebasis of structural observations indicating thatthe South Tibetan detachment exhibits top-to-north shear strain (P. DeCelles, 2001, personalcommunication).

Figure 14C shows continued slip on GurlaMandhata detachment fault 1; this slip trans-lated formerly active parts of the ductile shearzone to shallower levels in the crust. Depend-ing on what the geothermal gradient was atthis time, the footwall rocks could have beenexhumed to shallow enough levels in the crustto start accumulating radiogenic 40Ar, imply-ing a late Miocene age (ca. 9 Ma) for the timeof slip on fault 1 and the ductile shear zone.Within the active ductile shear zone, leuco-granite dikes were emplaced and subsequentlysheared between 8.7 and 6.8 Ma. The corre-lation between the clast composition of themiddle unit in the Pulan basin (Tcg2) andwest- to south-directed paleoflow implies thatmylonitic rocks were exposed at the surfaceand providing detritus to the basin. On the ba-sis of the youngest 40Ar/39Ar mica ages of thefootwall rocks on the west side of the GurlaMandhata massif, the age of this unit is youn-


MURPHY et al.

Figure 14. Structural model for the evolution of the Gurla Mandhata detachment system. (A) Preextensional setting. Tethyan Sedimen-tary Sequence is shortened by southward movement along thrusts belonging to the Tethyan fold-and-thrust belt, possibly resulting inprograde metamorphism of units agn, bs, gn, and mig (lithologic symbols correlate to those in Fig. 2). (B) Initiation of the footwallextensional ductile shear zone and its upper-crustal equivalent, the Gurla Mandhata detachment fault 1. (C) Continued shear on fault1 translates formerly active parts of the ductile shear zone along with sheared leucogranite dikes to shallower levels of the crust.Deposition of Tcg2 is interpreted to have occurred at this time. (D) Initiation of Gurla Mandhata detachment fault 2 and deposition ofTcg3 possibly coeval with initiation with west-dipping normal faults within Pulan basin. Red arrows indicate the relationship wherefault 2 locally cuts across fault 1, and where fault 1 has been excised by fault 2.

ger than ca. 8.1 Ma (weighted mean age ofGM-10) (Fig. 12).

Figure 14D depicts the initiation of GurlaMandhata detachment fault 2, which locallycut across Gurla Mandhata detachment fault 1at an angle ,108. At other localities, fault 1has been excised by fault 2, usually at thecrests of antiformal corrugations. The strati-graphically highest unit is dominated by claststhat correlate with the Tethyan SedimentarySequence. This change in source may reflecteither eastward tilting of the hanging walls ofthe two Gurla Mandhata detachment faults ei-ther as domino-style tilt blocks or along a listric-shaped fault, or it may reflect uplift of theTethyan Sedimentary Sequence in the footwallof an east-dipping normal-fault system alongthe western margin of the Pulan basin. Wesuggest that the Pulan basin represents a su-pradetachment basin (Friedmann and Bur-bank, 1995) that formed on the hanging wallof the Gurla Mandhata detachment system.

Relationship with the Karakoram Fault

One implication of recognizing late Mio-cene slip on the Gurla Mandhata detachmentsystem is that it must interact with the Kara-koram fault system, which has been docu-mented to be active at this time (Arnaud,1992; Searle et al., 1998; Dunlap et al., 1998).We suggest that the Gurla Mandhata exten-sional system and the Karakoram fault systemhave been a kinematically linked fault systemsince the late Miocene on the basis of timingconstraints, slip estimates, and kinematics ofeach fault (Fig. 15). The model we present inFigure 15 would be an example of an anti-thetic shear zone following terminology usedby Faulds and Varga (1998).

KinematicsSouth of Baer, between Menci and Mount

Kailas (Fig. 1), the Karakoram fault system isrepresented by a broad (,45 km wide) system

of right-slip faults (Ratschbacher et al., 1994;Murphy et al., 2000) with a mean orientationof 3108/358SW (Fig. 15). This fault system ex-tends into the Mapam Yum Co area cuttingobliquely across the Tethyan Sedimentary Se-quence where it meets north-trending brittlenormal faults belonging to the Gurla Mandha-ta detachment system. Although the intersec-tion of the two fault systems was not recog-nized in the field, we suggest that the twointersect in the Mapam Yum Co area. In orderfor two faults to be kinematically compatible,either their principal strain axes must lie par-allel to one another (Marrett and Allmendin-ger, 1990), or slip on both faults must be par-allel to their intersection line. The latter caseis true for the Karakoram fault system and theGurla Mandhata detachment system. Figure15 shows a plot of the mean orientation ofright-slip faults between Menci and MapamYum Co and the orientation of Gurla Man-dhata detachment fault 2 along with striations



Figure 15. (A) Kinematic model oflinkage between the Karakoramfault system and the Gurla Mand-hata detachment system. Our pro-posed model shows a system offaults that feed right-slip motioninto north-striking normal faults,forming a series of pull-apart ba-sins. The magnitude of throw alongthe normal faults increases fromnorth to south as the right-slip faultsystem is traversed. This change ex-plains how little denudation is ob-served near Mapam Yum Co, butsignificant denudation is observednear Gurla Mandhata. (B) Lower-hemisphere, equal-area stereonetshowing the mean fault planes forthe Karakoram fault system andGurla Mandhata detachment 2along with striations measured onindividual faults (Fig. 7). As shownby the plot, the close spatial rela-tionship between the mean slip di-rection on both faults and their intersection line suggests kinematic compatibility.

measured on the individual fault planes. Thea95 error ellipses of the mean slip direction ofboth faults overlap and plot close to their in-tersection line. Ratschbacher et al. (1994)reached a similar conclusion for kinematic re-lationships between the Karakoram fault sys-tem and normal faults associated with the Pu-lan basin. Our measurements extend theirconclusion to include the larger-magnitude ex-tensional faults bounding the Gurla Mandhatamassif.

TimingNormal-slip shear on the Gurla Mandhata

detachment system can be demonstrated tohave occurred between 8.7 and 6.8 Ma. Ageconstraints on the southern segment of the Ka-rakoram fault system can be inferred from theage of the South Kailas thrust. 40Ar/39Ar resultsfrom a K-feldspar separate from a volcanic-rock cobble in the Kailas conglomerate im-mediately below the thrust yield an age spec-trum that is consistent with a reheating eventat 13 Ma, which Yin et al. (1999b) interpretedto be due to burial by the South Kailas thrust.Because the Karakoram fault system cuts theSouth Kailas thrust (Murphy et al., 2000), thisresult places an upper age limit on the south-ern part of the Karakoram fault at this loca-tion. K-feldspar from a single leucogranitesample collected along the Karakoram faultsystem in the Zhaxigang area (;80 km northof Namru, northwest corner of Fig. 1) yieldsan age spectrum that indicates the rock was

rapidly cooled by ca. 10 Ma (Arnaud, 1992).Our field mapping of this area suggests thatrapid cooling of these rocks may best be ex-plained by either oblique slip along the Ka-rakoram fault system or exhumation at a right-stepping bend in the master fault.

DisplacementAlthough it has been argued that the Kara-

koram fault system may have accommodated;1000 km of right-slip in the Cenozoic (e.g.,Peltzer and Tapponnier, 1988), a growingbody of field data suggests much smaller dis-placements, ;250 km (Ratschbacher et al.,1994), ,150 km (Searle, 1996; Searle et al.,1998), and ;66 km (Murphy et al., 2000). Asdiscussed earlier, the magnitude of normal slipaccommodated by the Gurla Mandhata detach-ment system is .66 km.

On the basis of these kinematic, timing, anddisplacement constraints for the Karakoramand the Gurla Mandhata systems, we infer thatthey are linked and together represent a singlefault system that has been active since the lateMiocene (Fig. 15). If this interpretation is val-id, a consequence of the observation that theGurla Mandhata detachment system exhumesmid-crustal rocks is that the Karakoram faultsystem extends to a similar crustal depth. Thisinterpretation is supported by field and ana-lytical investigations of migmatitic rocks ex-humed along the central part of the Karakoramfault in the Banggong Co area, approximately300 km northwest of Namru, (Fig. 1) by Sear-

le et al. (1998) and Dunlap et al. (1998).Moreover, this interpretation implies that theKarakoram fault system steps southward 90–50 km south of Mount Kailas area via theGurla Mandhata detachment system and ex-tends eastward into the High Himalaya of far-western Nepal as a major crustal-scale right-slip shear zone.

CONCLUSIONS

Field mapping along with geochronologicand thermobarometry analyses of the GurlaMandhata area in southwest Tibet reveals ma-jor east-west extension along a normal-faultsystem, termed the ‘‘Gurla Mandhata detach-ment system.’’ The maximum structural throwoccurs along a pair of low-angle normal faults(detachment faults), which juxtapose Tertiarysedimentary rocks and Paleozoic rocks be-longing to the Tethyan Sedimentary Sequencein its hanging wall against amphibolite-faciesmylonitic schist, marble, gneisses, and vari-ably deformed leucogranite bodies in its foot-wall. Upper Tertiary strata were deposited un-conformably on the Paleozoic section and areintimately associated with east- and west-dipping brittle normal faults. The footwall ofthe detachment faults records a late Mioceneintrusive event, in part contemporaneous withtop-to-the-west ductile normal shearing. Mac-ro- and microscale sheared mylonitic rocksdefine a .2.1-km-thick ductile shear zone inthe footwall. The consistency of the mean


MURPHY et al.

shear direction within the mylonitic footwallrocks and its correlation with structurallyhigher brittle normal faults suggest that thefaults and ductile shear zone represent anevolving low-angle normal-fault system. 40Ar/39Ar data from muscovite and biotite from thefootwall rocks indicate that they cooled below400 8C at ca. 9 Ma. Considerations of thedepth at which the footwall rocks originated,prior to their exhumation along the normal-fault system, and the angle at which the faultoriginally dipped, yield total slip estimates be-tween 66 and 35 km. These slip estimates andtiming constraints on the Gurla Mandhata de-tachment system are comparable to those es-timated on the right-slip Karakoram fault sys-tem. Moreover, the mean shear-sense directionon both the Karakoram fault and the GurlaMandhata detachment system overlap alongthe intersection line between the mean orien-tation of the faults and suggest that the twoare kinematically linked. If valid, this inter-pretation reinforces previous data suggestingthat the Karakoram fault extends to mid-crustal depths, as indicated by the exhumationof amphibolite-facies mylonitic rocks alongthe Gurla Mandhata detachment system.Ratschbacher et al. (1994) reached a similarconclusion between the Karakoram fault sys-tem and normal faults associated with the Pu-lan basin. Our measurements extend their con-clusion to include the larger magnitudeextensional faults represented by the GurlaMandhata detachment faults. If valid, this in-terpretation reinforces previous data suggest-ing that the Karakoram fault extends to mid-crustal depths, as indicated by exhumation ofamphibolite-facies mylonitic rocks along theGurla Mandhata detachment system.

ACKNOWLEDGMENTS

We thank Richard Law, Laurent Godin, and MikeSearle for thorough reviews, which greatly im-proved the manuscript. This work was supported byNational Science Foundation grant EAR-9628178(to An Yin and T.M. Harrison) and Lawrence Liv-ermore National Laboratory grant 97-GS-001 (toAn Yin and T.M. Harrison).

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MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 25, 2000REVISED MANUSCRIPT RECEIVED AUGUST 22, 2001MANUSCRIPT ACCEPTED SEPTEMBER 26, 2001

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