Journal oj Southeast Asian Eanh Sciences, Vol. 6, No.2, pp. 83-92, 1991 Printed in Great Britain

0743-9547/91 $3.00 + 0.00 Pergamon Press pIc

Kurosegawa terrane: disrupted remnants of a low latitude Paleozoic terrane accreted to SW Japan

JONATHAN C. AITCHISON,* SHIGEKI HAD At and SHINICHI YOSHIKURAt *Department of Geology and Geophysics, University of Sydney, NSW 2006, Australia and

tDepartment of Geology, Kochi University, Akebono-cho, Kochi 780, Japan

(Received 26 November 1990; accepted 6 May 1991)

Abstract-The Kurosegawa terrane is an anomalous, disrupted, predominantly Paleozoic lithotectonic assemblage of convergent continental margin affinity located between two Mesozoic terranes in SW Japan. On the basis of Silurian macrofossils in limestones and sparse Devonian plant fossils in overlying volcaniclastic sediments portions of the terrane were considered previously to represent a Silurian through Devonian sedimentary succession. Radiolarian data, together with sedimentological analysis, indicate the possibility that hiatuses may occur in this succession although their position remains indeterminate. Faunal, floral and paleomagnetic data indicate low latitude development during the Paleozoic, probably near the northern margins of Gondwana. Late Paleozoic oceanic crustal rocks are incorporated in a chaotic complex which crops out along the northern margin of the terrane. The chaotic rocks are interpreted to represent remnants of a subduction complex. Spatial relations of weakly-metamorphosed subduction complex rocks distributed along the northern side of the terrane and higher grade blueschist-bearing rocks to the south may indicate that during the Late Paleozoic to Early Mesozoic, subduction was south-directed along the northern margin of the Kurosegawa terrane. In the Jurassic, the Kurosegawa terrane underwent an oblique collision with Japan which was then part of Eurasia. Strike-slip faulting, associated with, and post-dating this collision resulted in dispersal of the Kurosegawa terrane into a narrow, discontinuous belt which transects the outer zone of SW Japan.

INTRODUCTION

KUROSEGAWA TERRANE is a disrupted, composite, of predominantly Paleozoic age which is

by, and separates, two distinctly different jWC'VLV,'v terranes both of which have traditionally been "OOlMH~.U to the Chichibu Belt (Yoshikura et al. 1990).

occurrence, within this belt, of a narrow and ·nJ<,,,nr. t1nuous zone of high-grade metamorphic tocks,

"U"V""'J and volcaniclastic sediments surrounlled by has been recognized since the 1930s

1931). Several distinct lithological assemblages recognized: (1) igneous and high-grade basement . (2) Silurian to Devonian volcaniclastic sediments

contain Silurian carbonate blocks; (3) serpen­(4) a Late Paleozoic-Early Mesozoic chaotic

(5) well bedded Upper Paleozoic-Lower covering sediments. All of these rocks are

anomalous with respect to those in surrounding . The northern margin of the Kurosegawa is defined by the Agekura Thrust where it is

tact with the Chichibu terrane (Is ozaki 1987). southern boundary of the Kurosegawa terrane is

obscured by younger strata but, where mapped, a fault south of which lie rocks of the Sanbosan

. ~, Both ,Chichibu and Sanbosan terranes are ,'W!dely interpreted as Late Mesozoic subduction com­, '~lexes. Not only do Kurosegawa terrane rocks differ

1l);1ithology, they also represent a marked disruption in ~~eoverall southward younging progression of terranes

'\YWch were accreted to, or developed along, the SW J,llargin of Japan. Fhe Kurosegawa terrane crops out intermittently

Wth' }, III an elongate, narrow (max. 10 km wide), E-W striking zone which transects the outer zone of SW

83

Japan for over 600 km (Fig. 1). The westernmost exposures of the terrane are high-grade metamorphic rocks, gabbro, pyroxenite and serpentinite in western Kyushu (Ishikawa et al. 1974). Outcrop is semi­continuous eastwards through Kyushu, Shikoku and Kii Peninsula to the Kanto Mountains. Kurosegawa terrane rocks are not exposed in central Kii Peninsula as they have been overthrust by the Sanbagawa terrane. The litho- and biostratigraphies of tectonically exotic Silurian to Devonian strata in the southern Kitakami and Abukuma regions of NE Japan (Saito and Hashimoto 1982) also bear striking resemblance to strata of similar ages within the Kurosegawa terrane. If these rocks are correlatives, then the Kurosegawa terrane, althougnnarrow, may extend for over 2000 km.

Kurosegawa terrane rocks were first recognized as a discrete geological entity along the Kurosegawa River in Ehime Prefecture, western Shikoku by Ichikawa et al. (1956) who interpreted them as basement uplifted along a major fault zone. Serpentinite-matrix melange, consti­tutes the Kurosegawa Tectonic Zone (KTZ) ofIchikawa et al. (1956) and comprises much of the Kurosegawa terrane. It is characterized by a discontinuous series of composite lenses which generally contain rocks of assemblages 1 and 2 above. Mud-matrix melange (assemblage 4) which contains blocks . of Upper Paleozoic greenstone, limestone, chert, acidic tuff and clastic sediments and occurs predominantly to the north of rocks of the KTZ. Elements of the KTZ and the chaotic complex' are covereu-ay-I::;ower-Mesozoic sedimentary rocks and for this reason we include both within the Kurosegawa terrane.

Traditional structural subdivisions of SW Japan place the Kurosegawa terrane and Mesozoic strata against which it is juxtaposed within the Chichibu Belt. The

84 JONATHAN C. AITCHISON et al.

~ Rycite Metomorpi-uc Belt

r: v: ... 1 Sonbogowo Metamorphic Belt

\- - -1 Chichibu terrane

• Kurosegowa terrane

~~UU{d San boson terrane

~ Shimanto l~ terrane

33'N

'O~--_2~5===K=llO~:~~~TR~ES--~75====~ 133'E

34'N

I ~ South Kitakami ~orea

~ Mino/Tambo ~ terrane

c::::::::J Kurosegowa c..::...-.J terrane

···.··.··'··'1···'

"

"'''~1 .,

:!~ Jl

'"·\1' ,Y

t~ I,; , iij Fig. 1. Lithotectonic terrane map of Shikoku, Japan, indicating locations of outcrops of Siluro-Devonian volcaniclastic

sediments in the Kurosegawa terrane from which radiolarian samples of this study have been collected. I~" "',1,

;:;1

earlier concept of the Chichibu Belt predates many recent advances in understanding of plate tectonics. The Kurosegawa terrane is clearly both, bounded by faults, and records a geological history different to that in Mesozoic terranes against which it is juxtaposed. Recent tectonic models interpret the terrane as a serpentinite­matrix melange zone separating two contrasting terranes (Hada et al. 1979). Strata north and south of the Kurosegawa terrane are assi~~d to the Chichibu and Sanbosan terranes, respectively. Each of the bounding terranes is unique although both contain rock associ­ations indicative of developments in accretionary plate margin settings.

The first indication of a Silurian age for strata within the KTZ was the discovery of the coral fossil Halysites (Kobayashi and Iwaya 1940). Subsequent paleonto­logical and isotopic studies confirmed the Paleozoic nature of much of the terrane. In this paper we present radiolarian data which refine and further constrain stratigraphy of the Kurosegawa terrane. The overall nature of the terrane is also discussed in terms of its likely position and relations to other areas throughout its development.

TECTONO STRATIGRAPHY OF THE KUROSEGA W A TERRANE

Igneous and high -grade basement rocks

Medium to coarse-grained, commonly sheared, calc­alkaline granitic rocks comprise the oldest rocks known within the terrane (Y oshikura 1985). Intermediate­pressure facies, high-grade, metamorphic rocks, includ-

'.!.,~

J ing biotite gneiss and amphibolite, which comprisdl distinctive part of the Kurosegawa terrane in westei

u/

Shikoku, were named the Terano Metamorp~

Rocks by Ichikawa et al. (1956). Gneissic medium:l coarse-grained garnet-clinopyroxene amphibolites oce\ locally, nevertheless, they crop out intermittent throughout the terrane (Yoshikura 1985). Metamorph

.,,1

minerals indicate formation under high pressur granulite facies conditions on the order of arou~ 10 kbj850°C (Yoshikura et al. 1981, Yoshikura 198; Alteration of garnet along cracks and kelyphitic ri~ around garnets adjacent to clinopyroxenes indica subsequent retrograde metamorphism.

Isotopic data provide a wide scatter of radiometric ~ determinations, ranging from Late Precambrian throul Middle Jurassic (Yoshikura et al. 1981, 1990). Th~ is a strong cluster of ages around 400 Ma and a late Ordovician (447 ± 10 Ma) U-Pb zircon age has b~ determined for the Y okokurayama Granite (Premo et; 1988). Yasui (1984) described pebbles and minen derived from the Yokokurayama Granite resediment into the overlying Silurian sequence. Many of the i~ topic data which indicate post-Ordovician ages for roC overlain by the sequence of ?Silurian to Devonian sel ments have been interpreted previously as representi the effects of later metamorphism (Y oshikura 1985).

Siluro-Devonian volcaniclastic rocks

A characteristic element of the Kurosegawa terrane a sequence of continental margin volcanic arc-deri, sediments which locally overlie granitic basement ro( with unconformity. Pyroclastic rocks including igni brites and voluminous vitric, crystal, and lithic tl

Kurosegawa terrane, Japan 85

volcaniclastic sandstones

zone of limestone blocks

tuffaceous sediments

granitic basement

radiolarian locality

I 100 metres

plant fossils (Hirata,1966)

* * .~ .... conodonts (Nikawa, 1986)

* * * * * * * * * ..... vv'v'vv

VV

•••••••••• vvvvvv vvvvvv vvvvvv vvvvvv

++++++ ++++++

* * * * *

" * Silurian macrofossils

Konomori, Jingamori, Kagamigawa, Suzaki

Yokokurayama, Gionyama

Suberidani, Kurosegawa, Terano

Fig. 2. Schematic stratigraphic column depicting typical lithostratigraphic succession of Siluro-Devonian volcaniclastic rocks in the Kurosegawa terrane. Approximate stratigraphic levels sampled for radiolarians at various localities are

indicated together with the level of other faunal and fioraHocalities.

'e the dominant lithologies. The original stratigraphic lccession is commonly well-preserved in the numerous ctonic lenses where this volcanogenic sequence is :posed and it is broadly similar at most localities 'ig. 2). Although locally overturned, strata are typically ,uthward-younging. The local occurrence of welded nimbrite at Yokokurayama (Yoshikura and Sato )76), together with diverse sponge spicule assemblages hich occur in acid residues of samples processed for .diolarians, is interpreted to indicate deposition in a arginal to shallow marine setting. Massive and cally brecciated reef limestones occur in a zone of

_tric-tufLand-¥olcaniclastie- sandstones low in the ratigraphic succession. The horizons containing lime­ones are overlain by silicic vitric tuffs and tuffaceous .ndstones which are, in turn, succeeded by thickly :dded volcaniclastic sandstones and pebbly conglomer­es.

Our field observations, from throughout Kyushu and Shikoku, confirm the previously established litho­stratigraphic successions for these volcaniclastic rocks. On the basis of radiolarian biostratigraphy and sedimen­tological analogy, we reassess aspects of this sequence. Numerous halysitids, favositids, stromatoporids, cephalopods, tribolites and conodonts have been recov­ered from limestones (Hamada 1958, Kobayashi and Hamada 1974, Kuwano 1976, Nakai 1981). These: fossils indicate a mid-Silurian (Wenlockian-Ludlovian) age. We note that no Silurian fossils have been recovered from the enveloping pyroclastic and epiclastic strata and for reasons detailed below we consider that some of the limestones may well be allochthonous. The only diagnostic fossils from volcaniclastic lithologies are lat­est Devonian (Famennian) conodonts reported from vitric tuffs high in the succession at Yokokurayama (Nikawa 1986) and the plant fossils Leptophloeum

86 JONATHAN C. AITCHISON et al.

rhombicum and Lepidodendropsis sp. which occur in volcaniclastic sandstones even higher in the succession (Hirata 1966). On the basis of Australian flora data (Gould 1976) these plant fossils are considered to indicate a latest Devonian to earliest Carboniferous age.

We have reservations about assumptions that the sequence represents a continuous Silurian through De­vonian succession. The volcaniclastic sequences with­in the Yokokurayama Formation and its correlatives are apparently conformable (Yoshikura 1985) with no major hiatuses known. Nevertheless, where thickest, the sequence is less than 1200 m thick and composed pre­dominantly of volcanogenic ejecta derived from proxi­mal centres of explosive calc-alkaline volcanic activity (Yoshikura 1985). By analogy with sedimentation rates in comparable modern tectonic settings it is considered exceedingly unlikely that the entire sequence would have taken 60+ Ma (mid-Silurian through latest Devonian) to accumulate. Even at modest sedimentation rates for volcaniclastic sequences in convergent margin settings of 100 m/106 yr it is unlikely to have taken much more than 10 Ma for the sequence to accumulate. Where best­exposed (e.g. Kurosegawa, Yokokurayama), Silurian limestones occur as olistoliths within volcanogenic sedi­ments and elsewhere volcaniclastic sediments are observ­able both above and below the limestone blocks. Contacts are typically sharp, and transitional facies have not been observed between the limestones and tuffs. Allochthonous submarine glide blocks are not uncom­mon in arc-related environments (Conaghan et al. 1976, Ineson 1985). Some of the Silurian limestones within the Kurosegawa terrane may have been resedimented, during a period of arc volcanism, into a volcaniclastic sedimentary sequence as a direct resttft of tectonic disruption of an existing carbonate reef.

Furutani (1983), Ishiga (1988) and Wakamatsu et al. (1990) assigned Silurian-Middle Devonian ages to radio­larians extracted from vitric tuffs below the level of plant-bearing sandstones at Yokokurayama on the basis of their stratigraphic position between strata con­taining Silurian and Upper Devonian fossils. Radio­larian samples have been processed from the following localities where the Kurosegawa terrane is exposed: Gionyama, Suzaki, Kurosegawa, Terano, Yoko­kurayama, Jingamori, Kagamigawa, Konomori and SuberidanL(Fig. 1) and we have recovered several well­preserved faunas. Most faunas are from the upper 800 m of the stratigraphic succession. The best-preserved material comes from Konomori near Kochi city and includes: Ceratoikiscum avimexpectans Deflandre group, Helenifore sp., Entactinia spp., Entactinosphaera spp., Pylentonema sp., Copiellintra sp., Dejlantrica spp., and Palaeoscenidium cladophorum variants (Fig. 3). Large entactinids with strong three-bladed spines are common in many faunas especially those from Jingamori. Speci­mens resembling Huasha Cheng were reported from Yokokurayama by Wakamatsu et al. (1990). Our samples from similar localities and elsewhere on Yoko­kurayama have yielded similar specimens together

with Helenifore sp., Copicyntra sp. and entactinids with_ three-bladed spines. Faunas we have collected from/ Kurosegawa and Suberidani contain? Copiellintra sp ... and other, as yet undescribed, radiolarians (Aitchison et al. in preparation).

We agree with the statement of Wakamatsu et a/ .. , (1990) that" ... component species of these assemblages· are fairly different from those of previously known Silurian and Devonian faunas of other districts ... " Detailed comparison of Kurosegawa terrane radiolarian ... faunas with well-documented and independently-dated:-. faunas from outside of Japan (Holdsworth and Jones'~~: 1980, Cheng 1986, Nazarov and Ormiston 1983, 1986,:" Nazarov 1988, Goodbody 1986, Renz 1988, Ai 1990) indicates closest affinity to Middle to Devonian radiolarian assemblages for faunas Konomori and other high stratigraphic levels. The sence of any specimens of Holoeciscus may indicate the Kurosegawa terrane faunas are pre-F"u,\.;u_uLClu (Holdsworth and Jones 1980, Cheng 1986). the presence of Helenifore sp., which is common Upper Devonian (Frasnian) faunas from the ,-,UHUU'I'.

Basin of West Australia (Nazarov and Ormiston Aitchison, in preparation), together with nosphaera spp., Copiellintra sp. and entactinids well-developed three-bladed spines indicates Devonian (Frasnian) affinity. Radiolarian faunas rare lower in the succession but those from Kurosega Y okokurayama and Suberidani differ from faunas -higher stratigraphic levels. Details of Silurian to Devonian radiolarian biostratigraphy are sparse. uncertain whether Silurian limestones are thonous or to what degree they may be alloch For reasons given above we consider that the section too condensed to represent a continuous . succession. Tuffs are abundant and radiometric could resolve both the local stratigraphy and constraints for global radiolarian biostra correlation.

Serpentinites

Much of the Kurosegawa terrane is characterized the presence of serpentinite-matrix melange. blocks, which include a wide range of terrane lithologies, are mantled by scaly, lustrous, and are commonly enveloped in a sheared or o,..~"ct,,~P.

serpentinite matrix. These blocks range in size from· few cm3 up to several m3. Schistosity of the serpentinite· and abundance of blocks within it increases towards the-·­margins of the terrane.

Numerous belts of serpentinite-matrix melange and they are concentrated along major faults. They locally up to I km wide and some zones can be

-for-man.y...kilometres._S_erpentinite zones pinch and and are commonly anastomosed around areas with coherent and mappable stratigraphy (e.g. Devonian volcaniclastic sediments at Y which may melange.

Kurosegawa terrane, Japan

Fig. 3. Radiolarians recovered from volcaniclastic sediments of the Kurosegawa terrane in Shikoku, Japan. Specimens A-G are all from localities at Konomori and specimen H is from Kurosegawa. (Length of scale bar indicated in parentheses) (A) Ceratoikiscum sp. (180 lIm), (B) Ceratoikiscum sp. (390 lIm) (C) Helenifore sp. (270 lIm), (D) Helenifore sp. (270 lIm), (E) Dejlantrica solidum Wakamatsu, Sugiyama and Furutani (100 lIm), (F) Copiellintra sp. (130 jlm), (G) undescribed new

species (180 lIm), (H) unidentified spumellarian (180 jlm).

87

Kurosegawa terrane, Japan 89

Late Paleozoic-Early Mesozoic chaotic complex

A chaotic zone of Upper Paleozoic-Lower Mesozoic strata occurs mainly on the northern side of middle Paleozoic rocks of the KTZ and locally within or on the southern side of the KTZ where it is tectonically imbricated. The chaotic rocks include a wide range of blocks of various origins and are appropriately re­ferred to as a melange complex. Although the original mechanism of formation of this tectonostratigraphic unit is equivocal its lithological content indicates ac­cumulation in some form of accretionary wedge which developed along a convergent margin (Is ozaki 1987).

complex continued through into the Triassic. However, we note that some K-Ar age data from muscovites in glaucophane-bearing greenstones appear to indicate an earlier Middle Devonian-Early Carboniferous (394-352 Ma) metamorphic event (Maruyama et al. 1984). Further study is required to resolve the significance of these rocks.

Upper Paleozoic and Mesozoic covering strata

Relatively undeformed, well-bedded, sandstones and mudstones with conglomeratic interbeds appear to unconformably overlie parts of the KTZ and chaotic complex rocks. These sediments were derived from silicic to intermediate volcanic and plutonic rocks, possibly including those of the KTZ upon which they were deposited. Upper Permian sequences occur in western Kyushu and Shikoku and together with Middle Triassic, predominantly muddy, strata in Shikoku over­lie the KTZ, but do not overlie the chaotic complex. However, shallow marine strata with an abundant Up­per Triassic fauna unconformably overlie elements of both the KTZ and chaotic complex. Thus they constitute an overlap assemblage between two elements of the Kurosegawa terrane. The absence of Upper Permian covering strata overlying the chaotic complex is prob­ably related to the timing of its development. An uncon­formity is present between Upper Triassic rocks and uppermost Permian covering strata. Notably, equiva­lents of these strata are not known from adjacent

The dominant lithology in the chaotic complex is polymict mudstone-matrix melange containing a variety of blocks, some of which are of mappable extent. Blocks include feldspathic arenites, Permian limestones and limestone breccias, weakly-metamorphosed greenstones, Upper Carboniferous through mid-Permian chert, mid­to Upper Permian acidic tuff, and rare mylonitic granitic rocks and lawsonite-bearing blueschists. The melange matrix characteristically exhibits a strong foliation which anastomoses around elongate or boudinaged .blocks. Zones of less chaotic broken formation also occur and consist of disrupted interbedded sandstone and mudstone. Sandstones within the complex are massive medium- to coarse-grained feldspathic arenites and graywackes containing feldspar, quartz and felsic to mafic igneous rock fragments. Minor amounts of detrital hornblende, orthopyroxene, biotite, epidote and schis-

terranes. tose lithic fragments are also present. Mudstone in the chaotic complex has yielded uppermost Permian radiolarians (Isozaki 1987) and Upper Triassic l'adio-larians have been recovered from marl clasts CKdachi 1989).

Where distribution of the KTZ is imbricate or other­CORRELATIVES

wise repeated by faulting the chaotic complex commonly An allochthonous terrane, the South Kitakami has a strong regional metamorphic overprint. Some of terrane, which also incorporates Silurian limestones these rocks occur in nappes thrust northwards at a low and Devonian volcaniclastic rocks has been described angle over incipiently metamorphosed chaotic complex from the Kitakami district in northeastern Japan (Saito rocks. Ino and Kagamigawa formations are two and Hashimoto 1982). The lithostratigraphy and faunal examples of such rocks and they crop out in the vicinity content of these rocks (Kato et al. 1981) provide a of Kochi. They are dominated by schistose mudstones compelling support for the postulate that they may which incorporate blocks of greenstone, chert, limestone once have constituted part of the same lithotectonic and acidic tuff (Maruyama 1981). Rare conodonts and unit namely the Kurosegawa terrane. As with the radiolarians indicate Permian-Triassic ages for blocks Kurosegawa terrane in SW Japan, Silurian fossils are within the metamorphosed rocks which accord with their restricted to limestones. Further work is required to interpretation as metamorphosed equivalents of the determine the radiolarian biostratigraphy of the volcani-

?; chaotic complex. clastic sequence. ("I • Greenstones in the chaotic complex contain glauco- Jolivet et al. (1988) proposed a model in which a ':; phane or lawsonite indicating a glaucophane facies or microcontinental fragment, the "Okhotsk micro­- Pumpellyite indicating pumpellyite-actinolite facies continent", was progressively accreted to Eurasia during . metamorphism. Blueschist metamorphism of portions of the Late Mesozoic. This hypothesis seems to accord

~.~. the chaotic complex provides further evidence-of-sub--with-the-likely-Jurassic collision of the Kurosegawa :}<luction zone accretion. Jadeite-glaucophane schists terrane with SW Japan. However, the. available data

.~ within the subduction melange are as young as Early are insufficient to determine the existence or otherwise Triassic (Maruyama et al. 1984). Together with radio- of this microcontinent and whether or not the

,; larians reported by Adachi (1989) they may indicate that Kurosegawa terrane was once part of the postulated !:subduction related to development of the accretionary microcontinent.

, I I

90 JONATHAN C. AITCHISON et al.

TEMPORAL AND SPATIAL EVOLUTION OF THE KUROSEGA WA TERRANE

The present position of Paleozoic convergent conti­nental margin rocks of the Kurosegawa terrane is highly anomalous in relation to strata in the predominantly Mesozoic terranes against which it is now juxtaposed. High-grade Paleozoic blocks are unknown in the surrounding terranes and the development of the Kurosegawa terrane, up until at least the Triassic, appears to have been unrelated to that of the rest of Japan.

formed at low latitudes under tropical to subtropicM\ conditions. If this chaotic complex developed to th~;:,'

north of older crystalline Kurosegawa terrane basemen{\ rocks then they must have been located even south.

Regional deformation related to the collision major continental fragment such as the now extensive Kurosegawa terrane, albeit narrow in surface. expression, and the Japanese portions of Eurasia can be expected to have obvious ,,,,,,,,,, . .,,,,.u Originally coherent elements of the Kurosegawa have been severely disrupted into a zone of matrix melange. Serpentinite detritus found fluvial sediments developed on or near the terrane indicate development and exposure of serpentinite-matrix melange in SW Japan by the --.u,.c.,~ Cretaceous. Further evidence for a major collision should be observable throughout SW Japan.

strike lncre: plate result Kuro (Had; ment: in as~

1981) trans1

Siliceous volcaniclastic sediments of the Kurosegawa terrane developed atop crystalline basement rocks in association with convergent continental margin volcan­ism during the Mid Paleozoic. Tectonism associated with convergence may have disrupted Silurian reef limestones resulting in their redeposition into coeval or slightly younger volcaniclastic sediments. Paleomagnetic data from vitric tuff and welded tuff of the Kurosegawa terrane indicate deposition at low latitudes (from 5 to 15°; Shibuya et al. 1983). A diverse fauna is present both within Silurian reef limestone and overlying tuffaceous sediments. Faunal and floral elements strongly support paleomagnetic data with clear affinity to other low latitude equatorial faunas. Silurian coral and trilobite faunas show closest similarity, at generic level, to those of eastern Australia rather than other, now closer, areas such as South China (Kobayashi and Hamada 1974, Pickett 1982). Upper Devonian plant fossils and radio­larians within Devonian volcaniclastic strata also show close similarity to those from Australia (Gould 1976, Nazarov and Ormiston 1983, Aitchison, in preparation). Portions of the Kurosegawa terrane must have formed far from their present latitude and the availrt15le evidence indicates that they possibly originated from close to the northern margin of Gondwana at a latitude similar to that of Australia.

major N-S shortening has affected many of the tprr.,~''''''···c.··f'''

Th Paleo Radic logic, long Devo nised

Chaotic complex rocks of Late Paleozoic-Early Mesozoic age are interpreted as an accretionary com­plex. They are predominantly developed along the northern margin of the Kurosegawa terrane and they are interpreted by Isozaki (1987) to have developed in response to south-directed subduction. Predominantly Paleozoic oceanic material was consumed and includes oceanic and alkalic basalts, limestones and cherts. A relict stratigraphic sequence of chert, acidic tuff and interbedded mudstones and sandstones indicates the gradual approach of this material towards a subduction zone during the Mesozoic. Triassic blueschist meta­morphism of portions of the Late Paleozoic-Early Mesozoic chaotic complex attests to a subduction zone setting. Jadeite-glaucophane schists within the sub­duction melange are as young as Triassic (Maruyama et al., 1984) and together with the radiolarians reported by Adachi (1989) indicate that subduction related to development of the accretionary complex continued through into the Triassic. Paleomagnetic data (Sasajima 1981) and fusulinid foraminifers (Ishii et al., 1985) indicate that components of the subducting plate

of SW Japan. North of the Kurosegawa Chichibu, Sanbagawa and Mino-Tamba terranes are characterized by large scale southward-vergent structures related to a Late Jurassic compressional (Faure 1985). Lithologies within these terranes generally consistent with their interpretation as developed in marginal basins or otherwise narrow Faure (1985) postulated that nappes in these developed in response to the rapid closure of these associated with the northward advance and of the Kurosegawa terrane with Eurasia during Jurassic. Seismic, gravity and other evidence (Bada et" 1982, Bada and Suzuki 1983, Yoshikura et al. 1 Ozawa et al. 1985, Murakami 1987) indicates although the areal extent of the Kurosegawa __ "._ .. ,_.,_ narrow at the surface it extends as a north-dipping U'-·',',."L,·

to a depth of several kilometres beneath the vUll"lJllUUr,'

terrane. The Chichibu terrane is a possible Late zoic subduction complex which developed along southern margin of the Eurasian margin of SW The Chichibu terrane has been thrust southwards obducted onto and, in some areas, completely over Kurosegawa terrane indicating that Eurasia was the over-riding or upper-plate position relative to Kurosegawa terrane during the Jurassic.

The Mino terrane, which lies to the north Kurosegawa terrane, is widely considered to ret)resen.<;. a Jurassic subduction complex and this' should be critically reassessed in terms of the ~V')~'V'W,

effect of Kurosegawa terrane collision during its deve1.·.·. opment. Collision of the Kurosegawa terrane may had a profound effect on the development of tectonic entities such as the Mino terrane. Notably, are major differences between the structural style overall lithological content and proportions in "''''UU·-.• C.c~

subduction complexes such as the Shimanto which flanks the outer zone of SW Japan, and the terrane.

The narrow, elongate, disrupted nature of Kurosegawa terrane may indicate that collision oblique and it experienced significant.

I that prob:

We Early Kuro sout!: the n dence subdl Late nortr of sa the : Kurc effect detai of th collis

Ackno JSPSI our re from ( Japan discus of fos: Unive anony

Adad

Au~ Am

Kurosegawa terrane, Japan 91

strike-slip dispersal along the margin of Eurasia. Increased strike-slip motion due to a possible change in plate vectors related to the effects of collision may have resulted in further disruption and dispersion of the Kurosegawa terrane during the earliest Cretaceous (Hada 1972, 1974). Numerous transtensional sedi­mentary basins developed along the Kurosegawa terrane in association with this strike-slip regime (Taira et al. 1981) and serpentinite may have diapirically risen along transtensional fractures.

CONCLUSIONS

The Kurosegawa terrane is a disrupted assemblage of Paleozoic rocks of predominantly continental affinity. Radiolarian data and reconsideration of sedimento­logical evidence indicate that volcaniclastic sediments,

'. long considered to represent a continuous Silurian to Devonian sequence, may conceal some as yet unrecog­nised hiatuses. Faunal and paleomagnetic data indicate that these Paleozoic rocks developed at low latitudes, probably close to Gondwana.

We postulate that during the Late Paleozoic and .. ' Early Mesozoic much crustal material between the - .Kurosegawa terrane and Eurasia was consumed at a

south-directed subduction zone which developed along the northern margin of the Kurosegawa terrane. Evi­'dence for this is seen in Late Paleozoic to Early Mesozoic subduction melange and blueschist metamorphism.

· Late Jurassic collision of the Kurosegawa terrane with north-eastern Asia resulted in the obduction of a series 'of southward-vergent nappes, which now characterize the structural framework of SW Japan, onw the Kurosegawa continent. This collision had a profound effect on the geological structure of Japan. Further detailed study is required to elucidate both the full extent of the terrane, and the nature and precise timing of the collision.

Acknowledgements-Jonathan Aitchison gratefully acknowledges a : JSPS Post Doctoral Fellowship (Host Scientist: Prof. S. Hada) to carry . our research in Japan. Support for this research was also obtained

from Grant-in-Aid for Scientific Research of Ministry of Education in Japan (No. 63540611jHada). We are grateful to Dr T. Yasui for

.- discussions about the geology ofYokokurayama and for the provision · of fossiliferous samples. Staff of the Electron Microscope Unit at the University of Sydney assisted with SEM work. Suggestions from two anonymous reviewers helped to improve the manuscript.

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