Journal of the Geological Society, London, Vol. 147, 1990, pp. 989-997, 7 figs. Printed in Northern Ireland The structure and stratigraphy of SE Zanskar, Ladakh 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 India 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 Zangla 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/6/989/4890753/gsjgs.147.6.0989.pdf by guest on 26 September 2021 R. McELROY ET AL. Ordovician to Silurian agefor the KarshaFormation according to the age 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) indicate marine 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, Dras; 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 Phe Formation 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 River 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