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 ∼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.
Distribution of Active Faults and Lithospheric Discontinuities in the Himalayan-Tibetan Orogenic Zone Identified by Multiscale Gravity Analysis