Structure of the North Indian continental margin in the Ladakh-Zanskar Himalayas: Implications for the timing of obduction of the Spontang ophiolite, India-Asia collision and deformation events in the Himalaya

The collision of India and Asia can be defined as a process that started with the closing of the Tethyan ocean that, during Mesozoic and early Tertiary times, separated the two continental plates. Following initial contact of Indian and Asian continental crust, the Indian plate continued its northward drift into Asia, a process which continues to this day. In the Ladakh-Zanskar Himalaya the youngest marine sediments, both in the Indus suture zone and along the northern continental margin of India, are lowermost Eocene Nummulitic limestones dated at similar to 54 Ma. Along the north Indian shelf margin, southwest-facing folded Palaeocene-Lower Eocene shallow-marine limestones unconformably overlie highly deformed Mesozoic shelf carbonates and allochthonous Upper Cretaceous shales, indicating an initial deformation event during the latest Cretaceous-early Palaeocene, corresponding with the timing of obduction of the Spontang ophiolite onto the Indian margin. It is suggested 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 and earliest Palaeocene, prior to the closing of Tethys. The major phase of crustal shortening followed the India-Asia collision producing spectacular 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 detachments including both normal faults (e.g. Zanskar, Karsha Detachments), southwest-directed thrusts reactivated as northeast-directed normal faults (e.g. Zangla Detachment), breakback thrusts (e.g. Photoksar Thrust) and late Tertiary backthrusts (e.g. Zanskar Backthrust). The normal faults place younger rocks onto older and separate two units, both showing compressional tectonics, but have no net crustal extension across them. Rather, they are related to rapid exhumation of the structurally lower, middle and deep crustal metamorphic rocks of the High Himalaya along the footwall of the Zanskar Detachment. The backthrusting affects the northern margin of the Zanskar shelf and the entire Indus suture zone, including the mid-Eocene-Miocene post-collisional fluvial and lacustrine molasse sediments (Indus Group), and therefore must be Pliocene-Pleistocene in age. Minimum amounts of crustal shortening across the Indian continental margin are 150-170 km although extreme ductile folding makes any balancing exercise questionable.