STRUCTURE OF THE NORTH INDIAN CONTINENTAL-MARGIN IN THE LADAKH-ZANSKAR HIMALAYAS - IMPLICATIONS FOR THE TIMING OF OBDUCTION OF THE SPONTANGOPHIOLITE, INDIA-ASIA COLLISION AND DEFORMATION EVENTS IN THE HIMALAYA

The collision of India and Asia can be defined as a process that start ed with the closing of the Tethyan ocean that, during Mesozoic and ear ly Tertiary times, separated the two continental plates. Following ini tial contact of Indian and Asian continental crust, the Indian plate c ontinued its northward drift into Asia, a process which continues to t his day. In the Ladakh-Zanskar Himalaya the youngest marine sediments, both in the Indus suture zone and along the northern continental marg in of India, are lowermost Eocene Nummulitic limestones dated at simil ar to 54 Ma. Along the north Indian shelf margin, southwest-facing fol ded Palaeocene-Lower Eocene shallow-marine limestones unconformably ov erlie highly deformed Mesozoic shelf carbonates and allochthonous Uppe r Cretaceous shales, indicating an initial deformation event during th e latest Cretaceous-early Palaeocene, corresponding with the timing of obduction of the Spontang ophiolite onto the Indian margin. It is sug gested here that all the ophiolites from Oman, along western Pakistan (Bela, Muslim Bagh, Zhob and Waziristan) to the Spontang and Amlang-la ophiolites in the Himalaya were obducted during the late Cretaceous a nd earliest Palaeocene, prior to the closing of Tethys. The major phas e of crustal shortening followed the India-Asia collision producing sp ectacular folds and thrusts across the Zanskar range. A new structural profile across the Indian continental margin along the Zanskar River gorge is presented here. Four main units are separated by major detach ments including both normal faults (e.g. Zanskar, Karsha Detachments), southwest-directed thrusts reactivated as northeast-directed normal f aults (e.g. Zangla Detachment), breakback thrusts (e.g. Photoksar Thru st) and late Tertiary backthrusts (e.g. Zanskar Backthrust). The norma l faults place younger rocks onto older and separate two units, both s howing compressional tectonics, but have no net crustal extension acro ss them. Rather, they are related to rapid exhumation of the structura lly lower, middle and deep crustal metamorphic rocks of the High Himal aya along the footwall of the Zanskar Detachment. The backthrusting af fects the northern margin of the Zanskar shelf and the entire Indus su ture zone, including the mid-Eocene-Miocene post-collisional fluvial a nd lacustrine molasse sediments (Indus Group), and therefore must be P liocene-Pleistocene in age. Minimum amounts of crustal shortening acro ss the Indian continental margin are 150-170 km although extreme ducti le folding makes any balancing exercise questionable.