Journal of the Geological Society, London, Vol. 147, 1990, pp. 989-997, 7 figs. Printed in Northern Ireland

The structure and stratigraphy of SE , Himalaya

R.McELROY', J. CATER', I. ROBERTS3,A. PECKHAM4 & M.BOND' 'Department of Earth Sciences, Downing Street, Cambridge CB2 3DS, UK 'Earth Sciences and Resources Institute, Department of Geology, University of Reading, Whiteknights, Reading RG62AB, UK 320 Lon-y-bryn, Bangor, Gwynedd LL572LH, Wales, UK 435Brook Green, London W6 7BL,UK '17Summersdale Court, The Drive, Chichester P0194RF, UK

Abstrpd. A fieldstudy of over 4 km thicknessof Cambro-Ordovician to Cretaceoussediments deposited on the passive margin of northern has provided significant new data on the thickness, age and depositional environments of these deposits. The first detailed structural map of the Phuctal areaand a regional map of EasternZanskar are presented, together with sequentially restored structural cross sections. An imbricate thrust duplex and a lateral ramp stack, which formed in the Phuctal area during SW-directed thrust propagation, were later deformed by NE-directed structures probablyproduced during gravitational collapse of the thrust stack. Collapse occurred along the previously documented Zanskar shear zone, and also by reactivation of the basal detachment of the nappe. This dorsal collapse of the orogen was probably related to an increase in uplift rate during the Neogene, and may have coincided with the initiation of the Main Boundary Thrust farther south.

The interest recently expressed in the complex problems of blueschist-grademetamorphic rocks and ophiolitic mel- NW Himalayangeology, for example by Fuchs (1979), anges. Tothe south of Zanskar,the High Himalayan Srikantia & Riizdan (1980), Thakur (1980), Baud et al. Crystalline complex (HHC, Fig. 1) was thrustsouthwest- (1984) andSearle (1986), promptedEdinburghan wards over mainly Palaeozoic cover rocks along the Main University expedition to the Zanskar mountains during the Central Thrust (MCT, Fig. 1) during the Oligo-Miocene, its summer of 1986. Thispaper presents the results and erosion generating the Siwalik molasse. The Siwaliks were conclusions of our fieldwork, together with some implica- progressively overridden by the older cover rocks along the tionsfor previous interpretations of the geology of the Main BoundaryThrust (MBT, Fig. 1)during the late Ladakh Himalaya. Miocene (Burbank et al. 1986; Mascle et al. 1986). Further Zanskar is an arid mountainous area located to the north southward, thrust propagation into the Indo-Gangetic Plain of the High Himalaya in NW India(Fig. 1). A series of continues at present. Late stage gravitational collapse of the N-S-trending glaciated valleys provideacross-strike ex- unstable inner parts of the orogen was probably responsible posures of thePhanerozoic Tethyan sedimentary cover, for normal faulting along the Zanskar shear zone (ZSZ,Fig. deposited on the NW edge of the Indian craton. 1; Searle 1986), as well as in central Nepal (Cabyet al. 1983) The stratigraphic scheme adopted by Baud et al. (1984) and in southern Tibet (Burg et al. 1984). and the structural map by Gaetani et al. (1985) are useful The pre-collisional cover rocks south of the crystalline introductions to the geology of Eastern Zanskar. New field complex are generally unfossiliferous and poorly exposed. data and regionala synthesis by Searle (1986) have However, SE Zanskar provides excellent exposuresof richly emphasizedthe particular significance of thispart of the fossiliferous sediments which recordthe pre-collision Himalayanorogen tocurrent research intothe processes development of thenorthern Indian passive margin.This involved during collision tectonics. Searle (1986) also points sequence is one of the fewwell exposed segments of the out the urgent need for more detailedfield data, particularly formersouthern margin of Tethys,and also provides a to helpto in producingadequate balanced structural detailedrecord of thetectonic evolution of theinternal cross-sections in this region. The main aim of the present Himalayan orogen. study was to provide such datafor SE Zanskar, with emphasison the little-knownarea along theTsarap Chu Stratigraphic evolution of Zanskar valley near Phuctal. SE The Phanerozoic cover sediments exposed in SE Zanskar represent the lower 4 kmof the Zanskar Supergroup and Geological setting range from Cambro-Ordovician to Early Cretaceous in age, Northward subduction of Tethyan oceanic crust below Tibet together with Quaternary drift deposits.The younger led tothe collision of Indiaand southern Tibet in the deposits of the Zanskar Supergroup are not exposed in SE Eocene, at c. 50 Ma (Besse et al. 1984; Patriat & Achache Zanskar, but have been described by Gaetani et al. (1983, 1984; Searle et al. 1987). The Indus-Tsangpo suture zone to 1985), Searle (1986) and Searle et al. (1988). The base of the thenorth of Zanskar (ITS, Fig. 1)marks the boundary ZanskarSupergroup may equivalentbe in age to betweenthe Indian and Asian plates, with associated 'Infra-Cambrian'sedimentary protoliths within themeta- 989

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Ordovician to Silurian agefor the KarshaFormation according to theage ranges for these Families quoted by Cocks (1985). The depositional environment of the Phe Formation has not been interpreted by previous authors although wave and A current ripples and desiccationcracks werereported by Gaetani et al. (1985), while the trilobites reported by Gupta & Shaw(1982) indicatemarine influence. Examination of the Phe Formation exposed south of Phuctal shows that it consists of a single coarsening-upward unit which includes hummocky cross-stratified sandstones in the upper part of thesequence and siltstones withNE-SW trending wave ripples at the top of the sequence (Fig. 2a). Current ripples in this sequencerecord a dominantlynorthwestward flow km direction. We intemretshallowregressive this as a - marine shelf or coastal sequence consisting of a clastic wedge which Fig. 1. Tectonic setting of the study area. Abbreviations on the map: NS, Northern suture zone; ITS, Indus-Tsangpo suture zone; prograded towards the NW. MCT, Main Central Thrust; MBT, Main Boundary Thrust; MFT, In the conformably overlying, dolomitic Karsha Forma- Main Frontal Thrust;ZSZ, Zanskar shear zone. Ophiolitic klippen: tion we observed domal stromatolites >20 cm in amplitude, D, ; S, Spontang; T, Tso Morari. On the key: TB, separated by channels filled with desiccation breccias capped Trans-Himalayan batholith and associated volcanic rocks; KB by reworked ooid grainstones. The stromatolites had been Karakorum Belt; HHC, High Himalayan Crystalline Complex;OK, recorded previously by Baud et al. (1984). The dolostones ophiolitic klippen; LH, Lower Himalayan sedimentary, metamor- are vuggy andform thetop of repeatedmudstone- phic and granitic units; OC, Outer Crystalline klippe;TS, Tethyan stomatolite-dolostone cycles in theTsarap section.This sedimentary units; IGP, Indo-Gangetic Plain; SW,Siwaliks. appears to be a peritidal mudflat sequence deposited in the Modified after Windley(1984). latterstages of the regression recorded by thePhe Formation. The vuggy dolostones may representreplaced evaporite-bearing horizons. The presence of these tidal flat morphiccomplex (Baud et al. 1984),which probably also deposits capping the regressive PheFormation supports a includes migmatized Phanerozoic rocks equivalent to parts shallow marine interpretation for the main body of the Phe of the Zanskar Supergroup. Formation. The stratigraphy of SE Zanskar has been described by The 150 cm thick ?Upper Silurian Kurgiakh Formation is previous authors (Baud et al. 1984; Gaetani et al. 1985). The absentfrom the Phuctal area(Gaetani et al. 1985). An intention of the present studywas to verify the thickness and erosional unconformity separatesthe Karsha Formation age of the stratigraphicunits as a basis forstructural from an 800 m thick sequence divided by previous authors mapping,and toreport any new observations which intothe Thaple, Muth, PO andLipak Formations. A substantiate the existing interpretations or justify new ones. Devonian to Carboniferous age is generally accepted for this The thickness of each of the stratigraphicunits sequence based on Early Carboniferous faunas found in the encounteredduring the present study is shown in Fig. 2, Lipak Formation (Baud et al. 1984). An Ordovician age was together with the main sedimentary structures observed and proposedfor theThaple Formationconglomerates by palaeocurrent data taken from the section exposed in the Hayden (1904), based on the presence of a Caradoc fauna Tsarap valley. A summary of the depositional history found in the Spiti region. However, the Thaple Formation of thearea basedon previous publications is presented exposed in SE Zanskar consists of conglomeratic red beds, below, together with relevant new data provided by the supporting the continental alluvial environmentproposed present study. forthe Thaple Formation by Gaetani et al. (1985). Presumably the Spiti fauna was reworked from underlying rocks.We foundno evidence of penetrativetectonic Summary of the depositional history of SE Zanskar deformation associated with the basal Thaple unconformity. Between 1 km and 4km thickness of LowerPalaeozoic Strainedpebbles of KarshaFormation dolostone in the marine sediments known as the Phe and Karsha Formations conglomerateshave associated calcite pressure fringes, are present in SE Zanskar (Fig. 2a). Their basal contact is a showing thatthe strain waspost-depositional (presumably normalfault north of the High HimalayanCrystalline Himalayan). The transition from alluvial to shallow marine Complex (Searle 1986). The age of these units is uncertain. and lagoonal environmentsin the overlying Muth, Lipak Nanda & Singh (1977) and Srikantia et al. (1980) suggest a and PO Formations (Baud et al. 1984; Gaetani et al. 1985) Cambrianage for theentire sequence, whereas Gupta & implies a gradual lowering of an initial regional topography. Shaw(1982) reportthe occurrence of Middle toUpper Coupled with the lack of tectonic deformation, this suggests Cambriantrilobites in thePhe Formationand suggest an that the unconformity was extension-related and may have Ordovician to Silurian agefor the overlying Karska formed during an undocumented mid-Palaeozoic continental Formation. We foundpoorly preserved fragmented rifting event.Subsequent basin deepening led to marine brachiopods (including pentamerids,orthids and rhyncho- incursions duringLipak and PO (Carboniferous)times, nellids) within the lower KarshaFormation south of the possibly reflecting thethermal subsidence phase of this Baralacha La Pass, 40 km SE of Tanze.Although these rifting event. fossil fragments were too poorly preserved for identification The Palaeozoic sedimentary sequence is overlain by the to better than Family level, their occurrence supports a Late Permian Panjal Traps volcanics which are 150-200m thick

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4 n v) v) Sedimentary 'alaeofiow data PROBABLE NAME g- Sedimentary PROBABLE facies and 30° interval plots1 AGE SE facias and AGE structures c structures NAME y- (weathering l profile - +l Pwmian P.njrl Traps Eocene to Cretaceous ? '0--- Fmn - (Post-Giumall not seen) Carboniferour .ipak Fm Aptian to ;iumal Oxfordian? Sst Fm Kioto Lst Fm n-20 Wuth Gm Devonian Oxfordian to Spiti Callovian? Shale Fm rhaple ?Devonian ?'n Callovian (Brachiopods: _hL /- pentamsrlds Bathonian rhynchonelllds etc) Silurian Karsha Early 3ioto Fm Jurassic U.Phe Fm

Rhaetian Tsatsa ca. Cambro- Fm 200 OrdovicianI I Tsatsa Fm n-13 Phe Fm ca. Norian Zozar 150 f to Fm c n-26 =) 300 0 (PCL+curr.ripsl -a - Ladinian Hanse \/I not Fm o, z- Anisianl Tamba Scythian Kurkur 40 mean 3 wave-rlpple ETriassic) crest wend (n-20) v U.Phe Fm Late Permian Zorar Fmn Fm 20 n-27 Permian (PANJAL l APS)

LITHOLOGY STRUCTURES mextraclastic r-1 erosion surtaca 0.' 0.' conglomerate intraclastic Bconglomerate msandstone Bmudrock @$ limestone

dolostone (81 ammonite

'herrin$one' she4 debris cross- eddlng ____ evaporite evaporites 1 MAS] massive bedding pseudomorph Fig. 2. (a) Pre-Panjal sedimentary record inSE Zanskar. Compiled m from observationsof the Phe and Karsha Formationsin the Tsarap mcrinoid ossicle valleys and south of the Baralacha La pass, with palaeocurrent data wave ripples goniatiticammonoid from the Tsarap valley section, and observationsof the post-Karsha mburrows units of the Baralacha La. (b) Post-Panjal sedimentary recordin SE Zanskar. All palaeocurrent data are from the Tsarapvalley section. j/l coarsening-up Observations of the post-Kioto sequence are from the Shingri-Chu mlining-up valley. (c) Key to stratigraphic sections.

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and mark a regional volcanic episode causedby rifting of the marinesequence. The thicknesses recordedin the Tsarap SouthTethyan margin(Trommsdorff et al. 1982). We valley section are shown in Fig. 2b. observedcopper-bearing mineralized veins with epidote, quartz, calcite, chlorite, talc and tremolite/chrysotile, which Structural evolution of Eastern Zanskar isconsistent with observations by Baud et al. (1984) and indicates that the volcanics are the source of native copper Structural units placer deposits worked by the local people. We havelargely adoptedthe names usedby Baud et al. The Permian rifting event was followed in SE Zanskar (1984) in describing the structural units present in Eastern by deposition of a 1.6 kmthick Permian toCretaceous Zanskar.From south to north, the units encountered are sequence (Fig. 2b), consisting of the Permo-Triassic Lilang given below. Group, JurassicKioto Limestone Formation and Fer- ruginous OoliteFormation, and Cretaceous Spiti and TheHigh Himalayan Crystalline Complex. This is a Giumal Formations. The names andages of these units have c. 70-150 km wide, WNW-ESE trending belt of high-grade been established by the work of previous authors which is metamorphic and igneous rocks forming the High Himalaya summarized by Baud et al. (1984). rangeon thesouthern border of Zanskar (Figs 1 & 3). Our observations of the Zozar Formation show that it Protoliths include Precambrian basement and Palaeozoic to thickens northwestwards from c. 150 m near Tame to 300 m Mesozoic sediments, possibly as young asthe Cretaceous thick north of Phuctal, where it consists of 2-6 m thick cyclic (Searle et al. 1987). Mid-Tertiarymetamorphism, packageswhich coarsenupwards from muddy marine migmatization and Miocene granite intrusion took place at limestones through thinly-bedded intraclastic marine wack- depths of 15-20km (Searle & Fryer 1985). Structures estones to cross-bedded bioclastic packstonescontaining include S or SW verging recumbent folds, thrust sheets and coral,crinoid, bivalve and brachiopodremains (Fig. 2b). ductile shear zones. Thenorthern contact of theserocks Reversed-palaeoflow ‘herring-bone’ cross bedding (N and S with theTethyan Zanskar Supergroup tothe north and directed)and hummocky cross-stratification are well- northeast is a normal fault (the Zanskar Shear Zone; Fig. exposed in the cliff sectionsalong the west side of the l), dipping c. 30-35” NE near (Searle 1986; Herren TsarapRiver, north of Phuctal. The hummocky cross- 1987) and becoming shallower to the SE(E. Garzanti, pers. stratification occurs as 2 m-wavelength, 20 cm thick bundles comm. 1987). of coarse bioclastic packstonebeds within thicker lime mudstone units. Farther SE, oolitic and sabkha facies have Phuctal Nappe. Thisunit consists of ?Cambrian to beenrecorded within thisunit by Gaetani et al. (1985). Carboniferoussediments. Internal deformation includes These facies andthickness variations, combined with our bedding-parallel thrustfaults and recumbent S-verging palaeoflow data (Fig. 2b), indicate that the Zozar Formation isoclinal folds. Thesouthern margin of thisunit is the was deposited on a NE-SW trending shelf which deepened Zanskar Shear Zone (Fig. 3). The overall geometry of this towards the NW. The Phuctalexposures record storm- structural unit is an antiform affecting the Phe and Karsha influenced open marine shelf conditions. Formations. This structure is likely to be related to a blind The TsatsaFormation, renamed from the ‘Quartzite thrustculmination (Figs 3 & 4 andextreme SW end of Series’ (Baud et al. 1984)by Searle et al. (1988), is about Section 1-1’ in Fig. 5). 200 m thick in the Tsarap valley, where it consists of three packages of siliciclastic terrigenous rocks which coarsen and Zanglaandhigher nappes Theentire Permian to thicken upwards (Fig. 2b). The basal units contain scattered Cretaceous succession, representingsedimentation on the marine shell debris, in thin (1-2 cm) fine sandstone beds. southern margin of Tethys, rests on a low angle fault which These thicken up within themajor cycles into 10-30cm juxtaposesLower Paleozoic rocks tothe south against thick,medium-grained, well-sorted quartzarenites. These Permianand younger rocks tothe north (Figs 3 to 6), display loaded, horizontally-burrowed bases, tool marks and implying major noma1 displacement down to the north. This normalgrading. Climbing-ripple cross-lamination records boundary may be a former SW-verging thrust flat which was palaeoflow towards the west andnorth (Fig. 2b).This subsequently exploited as an extensional fault.The evidence sequence records the first influx of siliciclastic sediment onto for this interpretation is that this is the level from which the the passivemargin of northernIndia after the Permian lateral splays of a major lateral ramp exposed in the Tsarap rifting event. valley branch (Figs 3 to 6), i.e. it must have been a basal The base of the Kioto Limestone Formation (Fig. 2b) is detachment level when the lateral splays formed. A second marked by1-2 mthick beds of bioclastic packstone stratigraphicinterval which actedas a detachment level domianted by articulatedand fragmented megalodontid duringthrusting is the base of thecompetent Kioto bivalves up to 50cm across. This part of the sequence also LimestoneFormation. This can bedemonstrated by the contains ooids and anhydrite pseudomorphs, and low-angle structural repetition of the limestones, where they form the cross-stratified, heterolithic channel-fills withwave-ripped lowest units of several thrust sheets in the northern part of tops. The channel fills are probably of tidalorigin and, the area (Figs 3 to 6). together with the evaporite pseudomorphs and ooids, this suggests thatthe unusual accumulations of megalodontid bivalves formed in shallow,possibly stormbeach, Compressional structures conditions, rather than in a deep (e.g. reef talus) setting. The highest thrust sheet observed, the Khurna nappe (Fig. Our observations of the post-Kioto successions (Fig. 2b) 3),has a frontal anticline formedin Kioto Limestones, support the previous interpretations by Baud et al. (1984) possibly representing ahangingwall ramp anticline formed and Gaetani et al. (1985) that this represents a deepening as the basal thrustcut up-section intothe Spiti Shale

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Fig. 3. Geological map of part of SE Zanskar. Note orientation relative to north, also that the Thaple and Lipak Formations are exposed only in the area north of Tanze, at theSE edge of the mapped area. Based on traverses along the Tsarap-Chu and Shingri-Chu valleys, via Thonde, to Padum. Geological boundaries were extrapolated into other areas by using LANDSAT imagery.

Formation (Figs 4 & 5). A foot-wall syncline is exposed in the west, buteastward itsinverted upper limb is progressively truncated. The next thrust sheet down is called the Zumlung nappe and displaysa geometry similar to that of theKhurna nappe,overthrusting a large foot-wallsyncline at Zangla (Fig. 3). The inverted limb of this synclinealsois progressively truncatedtowards the east. However, the basal thrust is represented still farthereast by the appearance of severalbranch lines which occur withina duplex mid-way between Phuctal and Tantak (Fig. 4). The floor thrust of this culminationclimbs out of the section laterally towards the SSE, probably ontoeroded Spiti Shales, and marks the base of the Zumlung nappe (Fig. 3). As noted by Gaetani et al. (1985), the sole thrust cuts out the limb of the foot-wall syncline mid-way between Zangla and Shade (Fig. 3), and the attitude of the thrust steepens on either sideof this zone, both towards Zangla and towards the Tsarap River. We interpret these geometries as a result of lateral variations in shortening along the faults, directly attributable to tip-line growth of the structures(Elliott 1976~). In the sections exposed on the east bank of the Tsarap River and the upper reachesof the Shingri Chu (Figs 3 & 4), Thrust no thrusts were observed betweenthe upper tectonic contact of the Zanglanappe and thebase of thePanjal traps. - Normal fault m Duplex Fig. 4. Structural map of the Phuctal area, SE Zanskar. The stratigraphy is shown in Fig. 3. Structural units have been extended Anticline beyond the directly observedvalley exposures by using LANDSAT -H Syncline imagery. Direction of thrust motion is derived from boudinage 7P20'E \ " L l-3-- River orientations in the PanjalTraps.

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SECTION I - I'

a. DEFORMEDFINAL STATE

SW C---- Zangla Nappe -+ Zumlung -)C KhurnaNappe .--) NE Nawe Deformed Lilan Gr Tsatsa Fm Kioto Fm Thr

PT b.GRAVITATIONALCOLLAPSE RESTORED (8% shortening)

Footwall cutoffs absent

P

c.THRUSTSRESTORED (mimimum 30% shortening1 Fig. 5. Structural cross-section along line 1-1' (see Figs 3 & 4) parallel to . t thrust motion direction, with sequential palinspastic restorations. Stratigraphy is indicated by the same symbols as those used in Fig. 3, except PT, Panjal Traps.

However, in theThonde area to the west(Fig. 3), we These doubly-plunging hinges possibly resulted from later identifiedimbricate thrust sheet of Panjaltraps volcanic N-S compression which refoldedthe pre-existing lateral rocks which areseparated by tight S-verging recumbent rampstructures (see below). The chevronfolds typically synclines, again with truncatednorthern limbs. The have small hanging-wall rampanticlines above thrusts on synclines contain only the lowermost units of Lilang Group. their eastern limbs. This structural style is also common on The samefault planes affect progressively higherstrati- the northern limbs of folds in the earlier-fomed duplex at graphic levels towards the west (Figs 3 & 7), as seen in the the base of the Zumlung nappe. cliffs north of Karshawhere the tightsynclines of the Thissequence of compressionalstructures can be Thondearea are completelycut out,although large explained in terms of SW-directed piggy-bank thrusting. The hanging-wall anticlines arepreserved. These laterally- first thrusts to affect the area propagated towards the SW climbing thrusts can beregarded as minor lateral ramps, along a basal detachment which occurs at the base of the although we didnot find anydirect evidence of their KiotoLimestone Formation. Motion next occuredon transport direction. thrustspropagating along the base of thePanjal Traps An oblique ramp trending NNW-SSE within the Zangla volcanic rocks, passively transporting the Zangla nappe and nappe crops out on the west bank of the Tsarap River north its internal thrusts toward the SW. Differential movement of Phuctal (Figs 3 to 5). Thrustscut up through the along this decollement may account for the lateral ramps in stratigraphy from the decollement at the base of the Panjal the Thondeand Phuctal areas, and possibly arose because of traps volcanic rocks and branch into a roof thrust along the pre-existing footwall topography. detachment at the base of the Kioto Limestone Formation Thrustsare also seen within thepre-Panjal Traps (Fig. 7). Westward-directedprogressive breaching through stratigraphy, within thePhuctal nappe, for example at the side-wall has uplifted and tilted earlier slices, forming Tanze and south of Phuctal (Fig. 3), indicating that thrust bothupright tight chevron folds and open buckle folds. repetition affected the entire pre-collision stratigraphy. Axial surfacestrend N-S andare vertical or dip steeply SW-directed foreland-propagatingthrust sequences eastward; hinges plunge at low angles to the NNW or SSE. shortened the passive margin on the northern edge of the

n r SW NE Zumlung Nappe 6000 \ Loo Fig. 6. Structural cross-section along line 11-11' (see Fig. 3) perpendicular to the main thrust trend. Stratigraphy is indicated by the same symbols as those 0 10 km used in Fig. 3, except PT, Panjal Traps.

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NW SE ZumlungKhurna Nappe Nappe

Fig. 7. Hanging-wall diagram showing relationships structurally above the normal fault at the base of the Zangla Unit is SE Zanskar; section perpendicu- lar to the thrust transport direction. P1 Stratigraphy is indicated by the same symbols as those used inFig. 3, except Imbricatefan of minor thrusts PT, Panjal Traps. OJ Lateral Tsarap Ramp Zone

Indian Shield from mid-Eocene time onwards (Searle 1986). grey, scaly tectonite. Phyllonitic mark mark an extensional These thrusts formed during episode T2 of Searle (1986), N-directed detachment at this level, which is above the more afterophiolite obduction during theLate Cretaceous to significant detachmentat the base of thePanjal Traps Palaeocene (episode Tl). Tectonic burial probably accounts volcanic rocks. forthe Himalayan metamorphism of Mesozoic protoliths Whether equivalent rocks to thewest were also folded in within the High Himalayan Crystalline complex. Sequential this manner is difficult to ascertain because of the present restoration of thestructures in theTsarap Riversection erosional level, but 35 km farther west, in the cliffs north of (Fig. 5) implies that at least 30% shortening occured during Karsha, upright anticlines affect Triassic rocks which are cut the initial compressionaldeformation of theZanskar bysteeply-dipping thrust planes lying parallel to the axial Supergroup. planes of the folds. This is consistent with Searle's (1986) suggestion that these folds are hangingwall anticlines which have been rotated up from an original recumbent,S-verging Gravitational instability orientation during later gravitational collapse of the Zangla Later modification of the thin-skinnedthrust geometry nappe . occurredduring uplift and gravitationalcollapse of the These folds die out northwards across the Zangla nappe internal parts of the Himalayan orogen. The main focus of north of Phuctal. Their axial planes become progressively this late stage extension was the Zanskar shear zone (Searle steeper from south to north, from 40-50"s through vertical, 1986; Herren 1987). However, the base of the Zangla nappe to steepnortherly dips inthe north. These folds areconfined is also a normal fault, although it dips much more shallowly to the Zangla nappe, whereas structural units farther north NEthan does theZanskar shear zone, and it is retained their original SW-verging geometries. bedding-parallelwith respect tothe hanging-wall strati- Thesestructures within the Zanglanappe indicate graphy. The faultcuts out at least 700m of the footwall topto-N shear, and occur above a N-dipping detachment. stratigraphy near Phuctal, and truncates older thrust faults They are therefore most likely gravitational in origin. They within the Phuctal Unit at Tame (Fig. 3). accommodate NE-directed compression caused bygravity- The hypothesis thatthe current configuration of this induced extension farther SW. They account for some 8% surface is a thrustfault is rejected for tworeasons. (c. 1.5 km) of the total internal shortening of the mapped N-verging folds are present in the hanging-wall strata in the area. The northward change in dip of the fold axial surfaces southern part of the Tsarap valley. There is no evidence that can beattributed to progressive resistance to their thesefolds are associatedwith N-verging thrusts, and we northwardpropagation bypinning of the extensional interpret them as gravity folds associated with normal fault detachment at the base of the Zumlung nappe (Figs. 5 & 6). displacementalong thebase of the Zanglanappe (see The fault at the base of the Zumlung nappe truncates below). The second reason for regarding this contact as a structures in its footwall in the south of the mapped area. normal fault is the fact that if it were a thrust, it would have This implies thatit acted as apassive roof thrust to the to have cut down at least 700 m in the direction of thrusting, underling duplex(Banks & Warburton 1986) as it was which is regarded as highly unlikely. forced totruncate underlying imbricate slices duringthe The amount of extensional displacement along the fault gravity-induced, NE-directedunderthrusting of the imbri- is unknown. The displacement may have been accomodated cate stack.During this process, the base of the Zumlung in part by the N-directed backthrusting observed south of nappe waspassively uplifted, with motionrelative to the the Indus-Tsangpo suture in Ladakh, farther north (see fig. Zangla nappe confined to the Zumlung basal detachment. 6e of Searle 1986). The motion along this fault protected the higher structural The deformation of the hanging-wall of the Zangla units from the type of internal deformation observed in the normal fault is characterized by a series of N-verging, open Zangla nappe. anticlines with short, steep to overturned northernlimbs and The resultant overall geometry is comparable to that of longer, less steeply dipping southern limbs (Fig. 5). Internal triangle zones seen in compressive environments. It differs disharmony is shown in that the folds die out down their in that it accommodated displacement which was generated axial surfaces through incompetent Lilang Group sediments by gravitational collapse, resulting in a more localized area into a level of bedding-planeslip in theTamba Kurkur of effect that that generally seen in compressional tectonic Formation.Pressure solution is more prevalentin environments. The localized compression within the Zangla accommodating theshortening within thecompetent nappe is related tothe gravity slide atthe base of this carbonate sequence higher up the succession. The base of nappe. The whole area wasalso passively dropped down the Lilang Group is represented by sheared slices of a dark along the crustal-scale Zanskar shear zone.

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Discussion gravitationalcollapse and N-directed back-thrustingin southern Tibet was proposed by Burchfiel & Royden (1985). The juxtaposition of the mid-crustal rocks of the High Such speculations can only be supported by further detailed Himalayan Crystalline Complex with un-metamorphosed information onthe relative timing andamount of sedimentary rocks across the Zanskar shear zone within the displacement of the variouscomponent faults whichmay compressive regime of an orogenic belt is a problem which constitute a linked system onthe dorsalside of the has attracted previous study. Although Baud et al. (1984) Himalayan chain. interpreted the Zanskar shear zone as an overthrust contact, Searle (1986) and Herren (1987) interpreted the structure as Conclusions a NE-directed dorsal culmination collapse feature, related to gravitational collapse during uplift of the internal orogen. (1) The presence of Late Ordovician to Silurian Our mappinghas revealed details of a related normal brachiopods in the lower KarshaFormation south of the fault at the base of the Zangla nappe, north of the Zanskar Baralacha La pass supports the Ordovician to Silurian age shearzone, with the observedstructural relationships proposed for this formation by Gupta & Shaw (1982) and demonstratingthat initial overthrusting wasfollowed by implies thatCaradoc faunas recorded in the overlying gravitational collapse. It is likely that the same sequence of ThapleFormation by Hayden (1904) are reworked. The events affected the Zanskar shear zone. Thaple Formation is conglomeratic and overlies a regional Platt’s (1986) development of earlier mechanical models angular unconformity. The lack of deformation associated of orogenic belts (Elliott 19766; Chapple 1978; Davis et al. with this unconformity andthe subsequent progressive 1983) offers an explanation for extensional deformation of increase in water depth suggests an extensional, rift-related the internal parts of an uplifting orogen, such as the Zanskar origin. The Upper Palaeozoic terrestrial to marine sequence Himalaya, in terms of so-called‘wedge tectonics’. Adding may represent an intra-continental syn- to post-rift fill. material tothe base of the orogenic wedgeby basal (2) Shortening of theTethyan margin occurredfrom underplatingincreases the leading taper angle.Once this Palaeocenetimes onwards acrossacomplex piggy-back angle exceeds a critical limit, the wedge is forced to extend system of SW-directed thrusts whichincludes a westward- to reduce the angle. This extension is usually accomplished breaching lateral ramp stack north of Phuctal. At least 30% by means of listric normalfaults down-throwing material (c. 7 km)shortening occurred within theZanskar Super- from structurally high levels within the orogenic wedge. group at this time. Thrust-related stacking of the Zanskar Supergroup and (3) Late extension and collapse-relateddeformation younger sediments was responsiblefor theextreme during the late Neogene was caused by rapid thickening and thickening of theorogen above the High Himalayan uplift of the Himalayan orogen, and was accommodated in Crystalline Complex.This thickening must have been SE Zanskar by listric normalfaulting along re-activated sufficient to bury the crystalline complex deeply enough to former thrusts at the base of the Phuctal and Zangla nappes. reach sillimanite-grademetamorphic conditions (c. 15- Internaldeformation within the Zanglanappe caused 20km asestimated by Searle & Fryer 1985),which was N-verging folds accounting for8% shortening (in addition to accompanied by the emplacement of granitic bodies within theearlier shortening). This internal deformation did not the crystalline complex until the mid-Miocene (Searle & effect structurally higher units, because the fault at the base Fryer 1985). The crystalline complex wasprogressively of the Zumlungnappe was exploitedas apassive roof upliftedalong the MainCentral Thrust until the thrust. The displacementwas probably limited in areal overthickened orogenic wedge underwent extension in the effect, dying out progressively northwards.However, late Neogene. This occurred both by forward propagation of N-directed backthrustingtook place atabout this time the leading taper of theorogen (generating the Main farther north in Ladakh; this may be related to much larger BoundaryThrust) and by internalcollapse of structurally displacementalong the crustal-scale ZanskarShear Zone, high units. The increasing elevation of the Himalayas at this which underlies the units mapped in the present study. time also appears to have initiated or significantly increased the intensity of the Asianmonsoon system, with major The authors would like to thank F. Stewart for his patronage and regional ecological consequences including the replacement Edinburgh University for essential support during the preparation of floodplain forests by grasslands (Quade et al. 1989). for this expedition. We also thank our numerous sponsors and all The basal ZanglaNappe normal fault has a vertical others who gave donations, discounts, advice and physical help. B. stratigraphic displacement of at least 700 m. The Zanskar Windley and the Leicester University Himalayan Research Group shear zone has total displacement estimatedby Searle (1986) provided helpful advice and discussion. The constructive criticisms as 7-10 km.Theoretical calculations indicate aminimum of an earlier draft of this paper by M. P. Searle and M. P. Coward vertical displacementacross theZanskar shear zone of are gratefully acknowledged. 19 km (Herren 1987). This magnitude of throw is consistent with theobserved juxtaposition of migmatites with their References low-grade sedimentary protoliths across the shear zone. Interestingly, the motion of thesefaults seems to be BANKS,C. J. & WARBURTON,J. 1986. Passive roof duplex geometry in the frontal structures of the Kirthar and Sulaiman mountain belts. Journal of broadly contemporaneous witha lateTertiary episode of structural Geology, 8, 229-237. N-directed back-thrusting tothe south of the Indus- BAUD,A., GAETANI,M., GARZANT?,E., Fors, E., Nrcom, A. & TINTOM,A. Tsangpo suture, 70 km farther north (tectonic episode T3 of 1984. Geological observations in southeastern Zanskar and adjacent Searle 1986; cf. Mascle et al. 1986). This compression may Lahul area (northwestern Himalaya). Eclogae Geoiogicae Helvethe, R, 171-197. have resulted from the horizontal component of motion on BESSE,J., COURTILLOT,V., POZZI,J. P., WESTPHAL,M. & ZHOU,Y. X. 1984. the listric normal faults in southern Zanskar, particularly the Palaeomagnetic estimates of crustal shortening in the Himalayan thrusts crustal-scale Zanskarshear zone. Asimilar link between and Zangpo suture. Nature, 311,621-626.

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Received 12 June 1989; revised typescript accepted 10May 1990.

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