Structural and metamorphic evolution of the northern Himachal Himalaya, NW India - (Spiti-eastern Lahul-Parvati valley traverse)


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Structural and metamorphic evolution of the northern Himachal Himalaya, NW India - (Spiti-eastern Lahul-Parvati valley traverse)
Eclogae Geologicae Helvetiae
Wyss M., Hermann J., Steck A.
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The Himalayan orogen is the result of the collision between the Indian
and Asian continents that began 55-50 Ma ago, causing intracontinental
thrusting and nappe formation. Detailed mapping as well as structural
and microfabric analyses on a traverse from the Tethyan Himalaya
southwestward through the High Himalayan Crystalline and the Main
Central Thrust zone (MCT zone) to the Lesser Himalayan Sequence in the
Spiti-eastern Lahul-Parvati valley area reveal eight main phases of
deformation, a series of late stage phases and five stages of
metamorphic crystallization. This sequence of events is integrated into
a reconstruction of the tectonometamorphic evolution of the Himalayan
orogen in northern Himachal Pradesh.
The oldest phase D-1 is preserved as relies in the High Himalayan
Crystalline. Its deformational conditions are poorly known, but the
metamorphic evolution is well documented by a prograde metamorphism
reaching peak conditions within the upper amphibolite facies. This
indicates that D-1 was an important tectonometamorphic event including
considerable crustal thickening. The structural, metamorphic and
sedimentary record suggest that D-1 most probably represents an early
stage of continental collision.
The first event clearly attributed to the collision between India and
Asia is documented by two converging nappe systems, the NE-verging
Shikar Beh Nappe and the SW-verging north Himalayan nappes. The D-2
Shikar Beh Nappe is characterized by isoclinal folding and top-to-the NE
shearing, representing the main deformation in the High Himalayan
Crystalline. D-2 also caused the main metamorphism in the High Himalayan
Crystalline that was of a Barrovian-type, reaching upper amphibolite
facies peak conditions. The Shikar Beh Nappe is interpreted to have
formed within the Indian crust SW of the subduction zone. Simultaneously
with NE-directed nappe formation, incipient subduction of India below
Asia caused stacking of the SW-verging north Himalayan Nappes, that were
thrust from the northern edge of the subducted continent toward the
front of the Shikar Beh Nappe. As a result, the SW-verging folds of the
D-3 Main Fold Zone formed in the Tethyan Himalaya below the front of the
north Himalayan nappes. D-3 represents the main deformation in the
Tethyan Himalaya, associated with a greenschist facies metamorphism.
Folding within the Main Fold Zone subsequently propagated toward SW into
the High Himalayan Crystalline, where it overprinted the preexisting D-2
After subduction at the base of the north Himalayan nappes, the
subduction zone stepped to the base of the High Himalayan Crystalline,
where D-3 folds were crosscut by SW-directed D-4 thrusting. During D-4,
the Crystalline Nappe, comprising the Main Fold Zone and relies of the
Shikar Beh Nappe was thrust toward SW over the Lesser Himalayan Sequence
along the 4 to 5 kms thick Main Central Thrust zone. Thrusting was
related to a retrograde greenschist facies overprint at the base of the
Crystalline Nappe and to pro-grade greenschist facies conditions in the
Lesser Himalayan Sequence. Simultaneously with thrusting at the base of
the Crystalline Nappe, higher crustal levels were affected by
NE-directed D-5 normal extensional shearing and by dextral strike-slip
motion, indicating that the high-grade metamorphic Crystalline Nappe was
extruded between the low-grade metamorphic Lesser Himalayan Sequence at
the base and the north Himalayan nappes at the top. The upper boundary
of the Crystalline Nappe is not clearly delimited and passes gradually
into the low-grade rocks at the front of the north Himalayan nappes.
Extrusion of the Crystalline Nappe was followed by the phase D-6,
characterized by large-scale, upright to steeply inclined, NE-verging
folds and by another series of normal and extensional structures D-7+D-8
that may be related to ongoing extrusion of the Crystalline Nappe. The
late stage evolution is represented by the phases D-A and D-B that
indicate shortening parallel to the axis of the mountain chain and by
D-C that is interpreted to account for the formation of large-scale
domes with NNW-SSE-trending axes, an example of which is exposed in the
Larji-Kullu-Rampur tectonic window.
Create date
07/12/2012 15:56
Last modification date
20/08/2019 14:04
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