Remnants of a Cretaceous intra-oceanic subduction systemwithin the Yarlung^Zangbo suture (southern Tibet)

Jonathan C. Aitchison a;*, Badengzhu b, Aileen M. Davis a, Jianbing Liu a,Hui Luo a, John G. Malpas a, Isabella R.C. McDermid a, Hiyun Wu b,

Sergei V. Ziabrev a, Mei-fu Zhou a

a Tibet Research Group, Department of Earth Sciences, University of Hong Kong, Pokfulam Rd, Hong Kong, SAR, Chinab Geological Team No 2, Tibetan Geological Survey, Lhasa, Tibet, China

Received 10 April 2000; received in revised form 21 September 2000; accepted 23 September 2000

Abstract

Extensive field investigations along the Yarlung^Zangbo suture zone in southern Tibet reveal the presence of nowfragmented remnants of a south-facing intra-oceanic subduction system. This system developed within Tethys duringthe Cretaceous. The associated arc, forearc ophiolite, and subduction complex were emplaced onto the leading edge ofIndia at the end of the Cretaceous. Rapid sedimentation in oblique-slip basins and disruption of water-saturatedsediments into melange was widespread and concomitant with ophiolite emplacement. We describe the tectonic entitiesthat developed during this previously unrecognized phase of Tethys^Tibet evolution and present a new model for theevolution of this portion of Tibet. ß 2000 Elsevier Science B.V. All rights reserved.

Keywords: Xizang China; Cretaceous; Tethys; Indus^Yarlung Zangbo suture zone; island arcs; ophiolite; subduction zones;melange

1. Introduction

The Yarlung^Zangbo suture zone (YZSZ)marks where the Tethys Ocean was consumed asIndia approached and ultimately collided withAsia. The most widely accepted tectonic modelfor this event is one in which the entire N^S ex-tent of the Tethyan oceanic crust was subductedalong the southern margin of the Lhasa terrane.Existing models for India^Asia collision suggest

the existence of a south-facing subduction systemalong the continental margin of Asia at which thenorthern margin of Tethys was consumed [1,2].We note, the early suggestion by Allegre et al.[3] of the existence within Tethys of a `hypothet-ical island arc which would have to have beencompletely obliterated'. Prost et al., [4], presentedthree models two of which allowed for the possi-bility of intra-Tethyan subduction. Searle et al.,[2], however, suggested that an important di¡er-ence between Tibet and the western Himalaya wasthe absence of evidence for the former existence ofan intra-oceanic island arc in Tibet. Detailed in-vestigation of the YZSZ during the past foursummer ¢eld seasons reveals the presence of rem-

0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 2 8 7 - 9

* Corresponding author. Tel. : +852-28598047;Fax: +852-25176912; E-mail : jona@hku.hk

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nants of an Early to mid-Cretaceous, intra-Te-thys, south-facing subduction system comprisingan intra-oceanic arc, ophiolite and subductioncomplex. We suggest that the presence of threecoeval mid-Cretaceous terranes of intra-oceanica¤nity is unlikely to be a chance occurrence.We describe these rocks and other units withinthe YZSZ and present a model to explain theirco-occurrence.

The YZSZ is the southernmost and youngestamongst the sutures which subdivide the TibetanPlateau into several east^west trending blocks [5].It is geographically located along and/or justsouth of the Yarlung Zangbo (River) in the TibetAutonomous Region of the Peoples Republic ofChina. The YZSZ separates continental rocks ofthe Lhasa terrane to the north from those of theIndian superterrane to the south and is marked bya nearly continuous, but tectonically disrupted,ophiolite belt [6]. Timing of the ¢nal closure ofthis suture is generally accepted to be Eocene[2,7,8]. During the convergence, and ultimatelythe collision, of India and Eurasia much of the

geology which originally developed and lay be-tween these two continental blocks was sub-ducted, smeared out, or otherwise destroyed. Allthat remains of this once extensive oceanic realmnow lies within the few kilometers (maximum)width of the suture zone. This zone provides us,not only with our only evidence of what was onceseveral thousand kilometers of ocean, but alsowith vestiges of what once lay within the Tethys.The study area has been the focus of numerousinvestigations and, because of its location withinthe India^Asia collision zone, is a subject of con-siderable interest. Much of the early complexstructural history of the YZSZ is overprinted byEocene and younger structures associated withthe Himalaya^Tibet orogen [9]. Rocks within theYZSZ are everywhere tectonically disrupted butalmost complete ophiolite sequences are foundat Zedong^Luobusa and Xigaze to the southeastand southwest of Lhasa respectively (see Fig. 1).Several important works synthesizing the resultsof geological traverses across Tibet are alreadypublished (e.g. [2,3,5,10^16]).

Fig. 1. Simpli¢ed geological map of the Zedong area showing the distribution of terranes related to the Early Cretaceous intra-oceanic subduction system (drafted from Tibet Ministry of Geology 1:5000 base maps). All contacts between terranes are faultsand where known the sense of displacement is shown.

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Six lithotectonic units (terranes) can be di¡er-entiated within, and bounding, the YZSZ and weconsider their origins and potential relationshipsin the hope of better constraining the tectonicevolution of the zone as a whole. We assignnames in accord with common usage in the re-gion. In addition, three new names are introducedin order to further discriminate amongst rocksthat may be genetically unrelated. We limit ourassociation of the geographic name Xigaze tothe sedimentary succession present in the XigazeGroup ( = Xigaze terrane, see Fig. 2). Ophioliticrocks near Xigaze are faulted against the southernmargin of this unit and, together with the Luobu-sa ophiolite, are assigned to the newly namedDazhuqu terrane. Two further new names are in-troduced: the Bainang terrane, which encom-passes a fault-bound unit of predominantly redradiolarian cherts and ¢ne-grained siliciclastics ly-ing to the south of the Dazhuqu terrane; and theZedong terrane for an assemblage of intra-oceanicvolcanic arc rocks that lies north of the ophioliticrocks. We describe the terranes and other impor-tant associated units from north to south.

2. Regional geology

2.1. Lhasa terrane

The Lhasa terrane has a variably deformed andmetamorphosed basement of Paleozoic to Creta-ceous sediments and igneous rocks and is subdi-vided into two structural domains [17]. North-ward subduction of Tethyan ocean crust beneaththe Lhasa terrane is inferred to have been contin-uous from at least the mid-Cretaceous until colli-sion between India and Asia in the Eocene [2].Subduction resulted in the production of a hugevolume of magma now represented by the An-dean-type Gangdese batholith and its extrusiveequivalents which are mostly located along thesouthern margin of the Lhasa terrane [2,3]. Clas-sic studies of radiometric ages for rocks of theGangdese batholith indicate an age range of atleast between 94 and 41 Ma [18]. Recent studies,however, suggest the possibility of an even longertime range from as old as 153 þ 6 Ma [19] to

30 Ma [20]. Radiometric dating studies withinthe Lhasa terrane have not been extensive and itis unclear if subduction was continuous from LateJurassic through Eocene or if hiatuses exist. Fieldevidence for Late Jurassic to Early Cretaceoussubduction occurs along the southern margin ofthe Lhasa terrane in the form of an extensive zoneof andesitic volcanics and associated sedimentswithin the Sanri Group [21], a unit which extendsfrom east of Xigaze to east of Zedong.

2.2. Xigaze terrane

The Xigaze terrane incorporates well-exposedsiliciclastic turbidites that ¢lled a sedimentary ba-sin which developed to the south of the Lhasaterrane [1,22]. These mid- to Late Cretaceous sedi-mentary rocks are widely interpreted as forearcbasin deposits that developed in association witha northward subduction of oceanic crust underthe Lhasa terrane. They are best exposed in theXigaze district where they form spectacular con-tinuous outcrops but are not seen in the Zedongarea further to the east. Either they were neverpresent in this region or they have been over-thrust by other rocks and are obscured [23]. Anal-ysis of clast lithologies and paleocurrent data im-plicate the Lhasa terrane as a likely source [1]. Anin£ux of volcaniclastic detritus began at approx-imately the Aptian^Albian boundary [1] coevalwith regionally extensive magmatism within theGangdese Belt. Younger rocks of the Xigaze ter-rane have been removed by erosion in the Xigazedistrict but may exist further to the west betweenLhatse and Sangsang. Fossiliferous Eocene mo-lasse of the Quiwu Formation locally forms anoverlap assemblage with the Lhasa terrane.

2.3. Zedong terrane

The Zedong terrane is a previously undescribedunit that incorporates island arc volcanic and vol-caniclastic rocks comprising basaltic-andesites,andesites, andesitic breccias, rare dacites and oth-er intrusives. This fault-bound unit crops out overapproximately 25 km2 near Zedong extending atleast as far west as Samye. Another occurrence ofthese rocks is located further east near Luobusa.

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Preliminary investigations indicate that tholeiiticisland arc volcanics developed above a basementof basaltic pillow lavas overlain by radiolarianchert. Andesitic breccias intruded by numerousandesitic dikes dominate much of the terrane.This terrane is in tectonic contact with ophioliticrocks to its south and continental margin arc vol-canics and intrusives of the Gangdese batholithand other rocks of the Lhasa terrane to its north.Rare marine fossils occur in volcaniclastic strataand together with radiometric age dating (M.F.Zhou unpublished data) indicate Early Creta-ceous development of this unit. Zedong terranevolcanics are of tholeiitic a¤nity and are thusdissimilar to any calc^alkaline volcanics associ-ated with coeval continental margin magmatic ac-tivity in the Lhasa terrane. These rocks have notpreviously been examined in detail by westerngeologists because the Zedong area lies well tothe east of the Lhasa^Kathmandu highway andhas received little attention although adjacentareas have been mapped on a regional scale[9,23]. We regard this terrane as being highly sig-ni¢cant as it represents a possible intra-oceanicisland arc that we suggest may have similarity tothe Kohistan island arc.

2.4. Dazhuqu terrane

The Dazhuqu terrane incorporates ophiolite se-quences that occur along the YZSZ. These rocksare everywhere tectonically disrupted. However,almost complete sequences are found at Ze-dong^Luobusa to the east and Xigaze to thewest. At Luobusa a well-preserved mantle sectionand a transition zone sequence has been thrustnorthwards over Oligo^Miocene molasse depositsof the Luobusa Formation or onto Cretaceousplutonic rocks of the Gangdese batholith. Thebase of the ophiolite is marked by a tectonic me-lange containing blocks of harzburgite, dunite,gabbro, amphibolite, basaltic-andesite, andesite,siltstone, etc in a serpentinite matrix. The melange

is overlain by dunites and gabbros of the transi-tion zone over which is thrust the mantle sequencewhich consists chie£y of harzburgite with abun-dant podiform chromitites enveloped by dunite.The ophiolite is itself tectonically overlain by Tri-assic £ysch deposits derived from further southand is therefore part of a northward-directedthrust stack. This thrust faulting is associatedwith Miocene development of the Renbu^Zedongthrust system [9]. Petrographic and geochemicaldata [24] suggest that the ophiolitic associationat Luobusa comprised the basement of an islandarc, formed by intra-oceanic subduction in theNeo-Tethyan Ocean during Cretaceous time. Re-melting of depleted mantle peridotites above thesubduction zone formed boninitic magmas thatmoved upward and reacted with the harzburgites,producing podiform chromitites that are of signif-icant economic value [24].

Correlative ophiolitic rocks are well known inthe Xigaze district southwest of Lhasa where theyare exposed over a distance of approximately 150km. Several sections have been studied in detailand this ophiolite has been the focus of consider-able work [6,25^31]. Harzburgite, dunite, gabbro,diabase, basalt and chert are all present. In mostareas the section is north-facing and locally expo-sure is up to 25 km in width. Radiolarian biostra-tigraphy [32] indicates formation of the ophioliticrocks before the late Barremian^early Aptian.Although all elements of an ophiolite are present,the section has been tectonically attenuated byearly normal faulting and later dismembermentalong strike^slip faults. Serpentinite-matrix me-lange along the southern margin of the ophioliteis dominated by harzburgite blocks but locallyincorporates blocks of garnet amphibolite. Pet-rography and geochemistry [14,29,30] are compat-ible with formation in an intra-oceanic setting.Although Cr-mineralization is not of the sameeconomic signi¢cance as that at Luobusa it isknown from outcrops throughout the Xigaze dis-trict and in regions further west along the suture.

CFig. 2. Simpli¢ed geological map of the Xigaze area showing the distribution of terranes related to the Early Cretaceous intra-oceanic subduction system. Note also the distribution of other units implicated in the collision of this system with the Indian pas-sive margin. All contacts between terranes are faults and where known the sense of displacement is shown.

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The presence of dolerite dykes cutting serpenti-nized ultrama¢c rocks suggests more than onephase in the evolution of this ophiolite. It is pos-sible that supra-subduction zone ophiolitic rocksdeveloped within, and upon, a crust of older, pre-existing, oceanic material. Structural data indicatesouthward emplacement of the ophiolite [26].

Supra-ophiolite deposits include chert, siliceousmudstone, felsic tu¡s and ¢ne-grained volcaniclas-tic turbidites. Several sections with di¡erent de-grees of completeness have been examined. De-tailed studies of radiolarians [32] in siliceoussediments from several localities, where deposi-tional contacts with underlying pillow lava or pil-low breccia are unequivocal, indicate late Barre-mian to late Aptian^early Albian deposition of asuccession of supra-ophiolite sediments.

The original relationship of the ophiolitic rocksto other subduction-related rocks located nearbyis uncertain. The existing interpretation [21] sug-gests that turbidites of the Xigaze terrane to thenorth were deposited conformably on ophioliticbasement. However, our detailed ¢eld investiga-tions show that the contact between Dazhuqu ter-rane ophiolitic rocks and Xigaze terrane turbiditesis everywhere either faulted or not exposed.Where exposed the contact is almost always at asouth-dipping, north-directed thrust surface.Where the Dazhuqu terrane supra-ophiolite sedi-ments themselves grade up-section into turbidites(e.g. at Querung) the sandstones are volcaniclasticand locally contain appreciable quantities of de-trital magnetite. As fossils within the overlyingsupra-ophiolite sediments are as young as Albianthey seem to preclude the possibility of a deposi-tional contact with lowermost stratigraphic unitsof the Xigaze terrane if these are older. We alsonote that late Early Cretaceous sediments withinthe supra-ophiolite succession are commonly tu¡-aceous with abundant devitri¢ed felsic tu¡s andvolcaniclastic turbidites. A detailed study of sand-stones of the Xigaze terrane [1], however, demon-strated that a volcanogenic component did notappear until around the Aptian^Albian boundarysynchronous with the initiation of volcanism inthe Lhasa terrane. Leucocratic dikes intrude theXigaze terrane near its boundary with the Dazhu-qu terrane but do not intrude the latter. Units

within the Xigaze terrane are mappable overmany kilometers of strike length whereas tectonicdisruption of rocks within the Dazhuqu terranewhich it supposedly overlies is extensive. We sug-gest that although Dazhuqu terrane supra-ophio-lite sediments may have formed in a forearc basinsetting similar to that in which Xigaze terranesediments accumulated the two sedimentary suc-cessions are genetically unrelated.

2.5. Bainang terrane

The Bainang terrane is an imbricate thruststack containing numerous tectonic slices of redribbon-bedded cherts of Tethyan origin and ¢ne-grained siliciclastics. It crops out in a zone lyingimmediately south of, but in fault contact with,the Dazhuqu terrane. A zone of radiolarites isparticularly well exposed in the Donghla, Xialu,Bainang and Zedong districts. The unit is clearlyfault-bounded and original relations with adjacentunits remain uncertain. We choose to identify thisassemblage as a separate terrane and have namedit after a town located near some of the best ex-posures. A consistent internal stratigraphy can bedetermined from individual thrust slices withinthis terrane. Slices contain all, or parts of, an up-ward younging basalt, red ribbon-bedded chert,siliceous mudstone, ¢ne-grained clastic sequence.In general these slices dip steeply to the north orare overturned. The internal stratigraphy of indi-vidual slices youngs northwards but the overallyounging direction is to the south. Investigationsof conodont and radiolarian biostratigraphy([10,33] and our own research) have revealed theages of some of the material within this zone.Most of the red ribbon-bedded cherts are of lateTriassic to Jurassic and even Early Cretaceousage. Where biostratigraphic control exists (S.Ziabrev, unpublished data) ¢ne-grained clasticsediments interpreted to represent trench-¢ll sedi-ments that overlie the cherts are typically mid-Cretaceous (mid-Aptian).

The structural style of the terrane is reminiscentof subduction complexes [34] and few other loca-tions exist where a succession of rocks such as thismight assemble. However, where we have exam-ined it in detail the Bainang terrane di¡ers from

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well-studied continental margin subduction com-plexes which are dominated by clastic sedimentshed o¡ an eroding continental margin arc. Wesuggest that such Bainang terrane-style assem-blages might develop in subduction complexes as-sociated with intra-oceanic island arc systems. Todate, no such system has been investigated in de-tail by ocean drilling and our hypothesis awaitstesting. It is possible that sections of low-grademetamorphic rocks, such as those located imme-diately south of the Dazhuqu terrane in the Ren-bung valley and south of Bainang terrane radio-larites along the Xianbuqu River, may representthe missing, more clastic-dominated, successionsthat might be expected to have developed in asubduction complex. These sections await detailedinvestigation from this viewpoint. If the Bainangterrane does represent a subduction complex thenit appears that oceanic crust of at least Late Tri-assic to Jurassic age has been subducted north-wards during the mid-Cretaceous beneath a con-vergent margin starved of clastic sediment. Wenote that the ages of radiolarians within the hemi-pelagic siliceous mudstones associated with arc-derived tu¡aceous clastic sediments of the subduc-tion complex are as young as mid-Aptian and atleast partially constrain the timing of subduction^accretion.

2.6. Indian terrane

Continentally derived siliciclastic sedimentaryrocks of the Indian (Himalayan) block are en-countered south of the Bainang terrane subduc-tion complex [35]. Within the study area thesesediments include passive continental margin ma-rine rocks (passive paleo-margin [36]) of Permianto Cretaceous age [35,37]. Sediments become in-creasingly proximal to the south [38] whereas tothe north they are more distal and have oceaniccharacteristics. Local occurrences of limestone-topped basaltic seamounts occur immediatelysouth of the YZSZ and are particularly notablewest of Liuqu.

2.7. Liuqu conglomerate

Numerous zones of rapidly deposited polymict

coarse clastic sediments crop out in a series oftectonically disrupted oblique-slip basins [39]within, and marginal to, the three Early to mid-Cretaceous intra-oceanic terranes. We interpretthese sediments as evidence of latest Cretaceous^Paleocene oblique convergence and translation ofterranes along the YZSZ (Figs. 3 and 4). Thethick successions of rapidly deposited clastic stra-ta are locally referred to as the Liuqu Conglom-erate. Sedimentary facies and clast petrographywithin this unit vary considerably from one areaof outcrop to another and the conglomeratescommonly occur between tectonically distinctunits of the suture zone. The unit is distributedin a series of narrow elongate basins along theYZSZ from Lhatse in the west to Bainang inthe east. Margins of the Liuqu basins are typicallyfaulted although rare depositional contacts existlocally. Liuqu Conglomerate is syn-collisional andaccumulated during the convergence of at leastsome of the terranes found along the YZSZ. Crit-ically, detrital clasts do not include any materialwith compositions consistent with derivation fromany source north of the Dazhuqu or Zedong ter-ranes (e.g. Xigaze terrane or Gangdese batholith).This suggests interaction between the Himalayan(Indian) plate and the intra-oceanic subductionassemblage only and that conglomerate deposi-tion occurred prior to any communication be-tween India and the Lhasa terrane.

The age of the Liuqu Conglomerate is broadlyconstrained by Late Cretaceous to Early Tertiaryfossils and the unit clearly pre-dates the main In-dia-Asia collision. It is dominated by proximalcoarse clastic sediments. Basin margin faults cutoutcrops of Yamdrok melange which contain ra-diolarians as young as late Paleocene (J.B. Lui,unpublished data) yet conglomerates do not con-tain any of the hydro-fractured clasts common inthis melange. We tentatively infer that formationof the Liuqu Conglomerate and Yamdrok me-lange was penecontemporaneous.

Other coarse-grained clastic units occur alongthe length of the YZSZ and are known by a vari-ety of local stratigraphic names. They are notnecessarily coeval and have di¡erent clast con-tents in sedimentary packages that record expo-sure of various source terranes at particular times.

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Fig. 3. Time^space plot showing the development of lithotectonic units distributed along the YZSZ both to the southwest andsoutheast of Lhasa.

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Thus each unit needs to be examined carefullyand caution should be exercised when attemptingcorrelations. The Eocene Quiwu Formation liesbetween Xigaze and Lhasa terranes in the Xigaze

area and the oldest sediments in this unit weremostly sourced from the Lhasa terrane to itsnorth. Dazhuqu, Xigaze, Bainang and Indian ter-ranes to the south of the unit became the domi-

Fig. 4. Simpli¢ed model for the tectonic evolution of rocks exposed along the YZSZ in Cretaceous time. The relative positionsof various terranes are indicated but the scale is arbitrary. Lh = Lhasa terrane; Z = Zedong terrane; D = Dazhuqu terrane;B = Bainang terrane; Ind = Indian terrane.

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nant source terranes later in its deposition. In theZedong^Luobusa area the Luobusa Formationcontains clasts of all terranes near the YZSZ. No-tably, a similar pattern of initial derivation ofsediment from the north then later from moresoutherly terranes is observed. Quiwu Formation(Eocene) and Luobusa Formation (Oligo^Mio-cene) both postdate collision of the intra-oceanicisland arc with India and their developmentchronicles phases of the India^Asia collision.

2.8. Yamdrok melange

A few kilometers south of and sub-parallel to,the mid-Cretaceous intra-oceanic island arc rockslie passive margin and more distal sediments re-lated to the Indian terrane. They are extensivelydisrupted into a region-scale mud-matrix melangeknown as the Yamdrok melange [12]. Various de-grees of stratal disruption range from broken for-mation to areas where huge rafts of sediment ap-pear to be £oating in a pervasively shearedmudstone matrix. These rocks were previously de-scribed as `wild£ysch with exotic blocks' [16]. Themud-matrix of this melange contains Late Creta-ceous foraminifera [40] and locally contains radio-larians as young as late Paleocene (J.B. Lui, un-published data). Strike^slip faults associated withdevelopment of the Liuqu Conglomerate locallytruncate exposures of the melange. We interpretthe melange as having developed in response tocollision of India with an intra-oceanic subduc-tion system during which time water-saturatedsediments along the leading edge of the Indianplate were overridden and structurally telescoped.This resulted in over-pressuring and the develop-ment of both tectonic and diapiric melange.

3. Discussion

Three mid-Cretaceous tectonic entities, of intra-oceanic island arc a¤nity, the Zedong, Dazhuquand Bainang terranes, are recognizable within theYZSZ. As the original relations between theseunits remain indeterminate they are best regardedas discrete terranes. Nevertheless their coeval de-velopment suggests that they once comprised dif-

ferent portions of a single tectonic entity. Thepresent N^S distribution of an arc, forearc ophio-lite, subduction complex, and, in particular, theinternal structural geometry of the subductioncomplex are interpreted to indicate the former ex-istence of a south-facing intra-oceanic island arcwithin Tethys. This interpretation is compatiblewith the suggestion [41] based on seismic tomog-raphy, that such a system once existed within Te-thys in this region. Recognition, and in somecases revised interpretation, of these rocks permitsthe identi¢cation of the products of an intra-oce-anic subduction system, the existence of whichwas suggested previously [2^4].

Available age data indicate intra-oceanic sub-duction had began by the Early Cretaceous. Thetemporal duration of arc activity is uncertain butvolcanism likely ceased with emplacement of thearc assemblage onto the Indian passive margin inlatest Cretaceous^Paleocene time. We note thatonly fragments of this former arc remain in thestudy area as collision-related crustal shorteninghas been extreme [23]. Tomographic imaging ofthe mantle from India northwards across Tibet[41] permits the recognition of slabs of oceaniccrust several thousands of kilometers in lengththat were subducted during Tethys closure andthus may provide clues as to the likely temporalduration of this intra-oceanic island arc system.As a considerable length of lithospheric materialappears to have been associated with this system[41] it may have been long-lived.

Development of the intra-oceanic island arcwas likely complex and is di¤cult to interpret indetail from the highly fragmented record pre-served along the YZSZ. Present data suggestthat subduction and arc volcanism may havecommenced ¢rst. Substantial slab collapse duringphases of subduction likely led to large scale ex-tension in the overriding plate and ophiolite gen-eration. Arc volcanism continued through themid-Cretaceous and is indicated by the presenceof mid-Aptian tu¡s in both Dazhuqu and Bai-nang terranes. Radiolarian data from inferredtrench-¢ll sediments indicate growth of the sub-duction complex during at least the mid-Creta-ceous.

If such an intra-oceanic subduction system ex-

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isted then the questions of when it accreted andwhat it accreted to arise. Key units in, and near,the YZSZ may help to resolve these questions.These units include amphibolite knockers in ser-pentinite-matrix melanges of the YZSZ, LiuquConglomerate and Yamdrok melange. The devel-opment of each of these latest Cretaceous^Paleo-cene units is regarded herein as syn-collisional.Ar^Ar geochronology on amphibole and biotiteseparates from amphibolites (M.F. Zhou, unpub-lished data) in the basal melange zone at Luobusaand near Xigaze yield Late Cretaceous ages. Anamphibolite sample from serpentinite-matrix me-lange developed along the southern margin of theDazhuqu terrane SE of Xigaze, near Bainang, wasdated at 84 Ma [42]. These ages record the timingof cooling during the uplift of high-pressureblocks to upper crustal levels. Similar ages arereported from potentially correlative units else-where in the western Himalaya [43]. The narrowspread of ages for amphibolite samples (70^90Ma) is herein interpreted to represent the timingof emplacement of the intra-oceanic subductionsystem onto the passive margin of northern India.This occurred in the Late Cretaceous when all ofthe oceanic crust that once existed between theintra-oceanic island arc and India had been con-sumed. The timing of the collision of the intra-oceanic island arc with India is also possibly re-corded in major changes in sedimentation pat-terns within Indian passive margin sediments insouth-central Tibet [44,45]. Notably this emplace-ment event signi¢cantly predates closing of theTethys and the initial India^Asia collision eventin the Eocene (55 Ma).

The Liuqu Conglomerate records the rapiddeposition of coarse-grained clastic sediments ina variety of proximal sedimentary environmentsunder an overall oblique-slip regime [39]. Clastswere sourced variously from Indian, Bainang andDazhuqu terranes and indicate that the Indianterrane and intra-oceanic subduction systemwere in close proximity. Despite the present-dayjuxtaposition of Liuqu Conglomerate and the Xi-gaze terrane no clasts that would indicate thepresence of Xigaze or Lhasa terranes within thesource areas have been observed. This unit is in-terpreted [39] as having been deposited in an en-

vironment similar to the present-day Longitudinalvalley of eastern Taiwan where a narrow elongatevalley marks a zone of oblique slip displacementbetween rocks of the recently accreted intra-oce-anic Luzon arc and the continental margin ofChina.

Collision of the intra-oceanic island arc systemwith the Indian continental margin resulted in thedevelopment of SW-directed compressional struc-tures in the Indian terrane [36]. Over-pressuring ofwater-saturated sediments distal to the Indiancontinental margin and the development of thetectonic and diapiric melange seen in the Yam-drok melange was widespread. Similar rocks aretoday seen in southern Taiwan where the Kentingand Lichi melanges have developed in response tothe collision of the Philippine arc with the Chinesecontinental margin [46]. Yamdrok melange isanalogous to other Tethyan melanges where rocksdistal to the margins of Gondwana have beentelescoped during collision. Correlative disruptedzones of sediment occur in the western Himalayaaround Ladakh but reportedly retain more oftheir original internal stratigraphy [47]. Whenoverridden during the ¢nal stages of collisionand loaded by a large volume of clastic sedimentwide tracts of melange developed south of theYZSZ.

The products of an Early Cretaceous intra-oce-anic subduction system that developed within Te-thys are thus partially preserved along the YZSZ.Oceanic crust between India and this system wasentirely consumed and the system was emplacedonto the Indian margin by the end of the Creta-ceous. The remaining oceanic crust that lay to thenorth of this system was subducted under theLhasa terrane resulting in on-going developmentof the Gangdese batholith and associated eruptiverocks during the Late Cretaceous and Eocene.During the Eocene the Indian Plate, upon whichthe intra-Tethyan arc was now a passenger, col-lided with the Lhasa terrane to initiating the Hi-malaya^Tibet orogeny.

The western continuation of the YZSZ suturein Kohistan and Ladakh regions is more compli-cated as it includes the Kohistan^Dras islandarc(s) between Karakoram microplate to thenorth and Indian plate to the south [2]. The

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south-facing nature of the Yarlung^Zangbo arc issimilar to that suggested for the Dras [48] andKohistan [43] arcs in the western Himalaya. TheKohistan^Dras intra-oceanic island arc(s) are fur-ther examples of intra-Tethyan subduction. Ear-lier interpretations of these arc assemblages sug-gest that although they might have been south-facing they docked with continental margins totheir north before colliding with India in the Eo-cene. It has also been proposed that the KohistanArc developed on oceanic crust due to an embay-ment in the oceanic^continental margin [49]. Arecent study [43] has suggested that the Kohistanarc may have accreted to the Indian margin ¢rst.An alternative model [50] envisages an additionalimmature arc developing south of the Kohistanarc and accreting to India. Exposures are moreextensive and reveal di¡erent deeper crustal levelsin the western Himalaya but it appears that theYarlung^Zangbo arc terranes might be fragmentsof a once contiguous system.

Acknowledgements

We thank the Hong Kong Research GrantsCouncil for their on-going ¢nancial support. Con-structive reviews by Philippe Matte and T. MarkHarrison helped to improve the manuscript. Driv-ers and other personnel of Geological Team no. 2of the Tibetan Geological Survey are all thankedfor the many ways in which they have assistedwith our ¢eldwork.[RV]

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