Geology of the NW Indian Himalaya


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Geology of the NW Indian Himalaya
Eclogae Geologicae Helvetiae
Steck A.
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This review paper deals with the geology of the NW Indian Himalaya
situated in the states of Jammu and Kashmir, Himachal Pradesh and
Garhwal. The models and mechanisms discussed, concerning the tectonic
and metamorphic history of the Himalayan range, are based on a new
compilation of a geological map and cross sections, as well as on
paleomagnetic, stratigraphic, petrologic, structural, metamorphic,
thermobarometric and radiometric data. The protolith of the Himalayan
range, the North Indian flexural passive margin of the Neo-Tethys ocean,
consists of a Lower Proterozoic basement, intruded by 1.8-1.9 Ga bimodal
magmatites, overlain by a horizontally stratified sequence of Upper
Proterozoic to Paleocene sediments, intruded by 470-500 Ma old
Ordovician mainly peraluminous s-type granites, Carboniferous tholeiitic
to alkaline basalts and intruded and overlain by Permian tholeiitic
continental flood basalts. No elements of the Archaen crystalline
basement of the South Indian shield have been identified in the
Himalayan range. Deformation of the Himalayan accretionary wedge
resulted from the continental collision of India and Asia beginning some
65-55 Ma ago, after the NE-directed underthrusting of the Neo-Tethys
oceanic crust below Asia and the formation of the Andean-type 103-50
(-41) Ma old Ladakh batholith to the north of the Indus Suture.
Cylindrical in geometry, the Himalayan range consists, from NE to SW,
from older to younger tectonic elements, of the following zones: 1) The
25 km wide Ladakh batholith and the Asian mantle wedge form the backstop
of the growing Himalayan accretionary wedge. 2) The Indus Suture zone is
composed of obducted slices of the oceanic crust, island arcs, like the
Dras arc, overlain by Late Cretaceous fore arc basin sediments and the
mainly Paleocene to Early Eocene and Miocene epi-sutural
intra-continental Indus molasse. 3) The Late Paleocene to Eocene North
Himalayan nappe stack, up to 40 km thick prior to erosion, consists of
Upper Proterozoic to Paleocene rocks, with the eclogitic and coesite
bearing Tso Morari gneiss nappe at its base. It includes a branch of the
Central Himalayan detachment, the 22-18 Ma old Zanskar Shear zone that
is intruded and dated by the 22 Ma Gumburanjun leucogranite; it
reactivates the frontal thrusts of the SW-verging North Himalayan
nappes. 4) The late Eocene-Miocene SW-directed High Himalayan or
``Crystalline'' nappe comprises Upper Proterozoic to Mesozoic
sediments and Ordovician granites, identical to those of the North
Himalayan nappes. The Main Central thrust at its base was created in a
zone of Eocene to Early Oligocene anatexis by ductile detachment of the
subducted Indian crust, below the pre-existing 25-35 km thick
NE-directed Shikar Beh and SW-directed North Himalayan nappe stacks. 5)
The late Miocene Lesser Himalayan thrust with the Main Boundary Thrust
at its base consists of early Proterozoic to Cambrian rocks intruded by
1.8-1.9 Ga bimodal magmatites. The Subhimalaya is a thrust wedge of
Himalayan fore deep basin sediments, composed of the Early Eocene marine
Subathu marls and sandstones as well as the up to 8'000 m-thick Miocene
to recent Ganga molasse, a coarsening upwards sequence of shales,
sandstones and conglomerates. The active frontal thrust is covered by
the sediments of the Indus-Ganga plains.
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07/12/2012 16:56
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20/08/2019 16:15
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