Structural and metamorphic evolution of the rocks of the Jutogh Group, Chur half-klippe, Himachal Himalayas: A summary and comparison with the Simla area |
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Authors: | Dilip K Mukhopadhyay Tamal K Ghosh Bidyut K Bhadra Deepak C Srivastava |
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Institution: | (1) Department of Earth Sciences, University of Roorkee, Roorkee 247667, U.P., India |
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Abstract: | The rocks of the Jutogh Group in the Himachal Himalayas and their equivalents elsewhere are now considered to represent a
several km thick crustal scale ductile shear zone, the so called Main Central Thrust Zone. In this article we present a summary
of structural and metamorphic evolution of the Jutogh Group of rocks in the Chur half-klippe and compare our results with
those of Naha and Ray (1972) who worked in the adjacent Simla klippe.
The deformational history of the Jutogh Group of rocks in the area around the Chur-peak, as deduced from small-scale structures,
can be segmented into: (1) an early event giving rise to two sets of very tight to isoclinal and coaxial folds with gentle
dip of axial planes and easterly or westerly trend of axes, (2) an event of superimposed progressive ductile shearing during
which a plethora of small-scale structures have developed which includes successive generations of strongly non-cylindrical
folds, several generations of mylonitic foliation, extensional structures and late-stage small-scale thrusts, and (3) a last
stage deformation during which a set of open and upright folds developed, but these are regionally unimportant. The structure
in the largest scale (tens of km) can be best described in terms of stacked up thin thrust sheets. Km-scale asymmetric recumbent
folds with strongly non-cylindrical hinge lines, developed as a consequence of ductile shearing, are present in one of these
thrust sheets. The ductile shearing, large-scale folding and thrusting can be related to the development of the Main Central
Thrust Zone. The microstructural relations show that the main phase of regional low-to medium-grade metamorphism (T ≈ 430–600°C andP ≈ 4.5–8.5 kbar) is pre-kinematic with respect to the formation of the Main Central Thrust Zone. Growth zoned garnets with
typical bell-shaped Mn profiles and compensating bowl-shaped Fe profiles are compatible with this phase of metamorphism. Some
of the larger garnet grains, however, show flat compositional profiles; if they represent homogenization of growth zoning,
it would be a possible evidence of a relict high-grade metamorphism. The ductile shearing was accompanied by a low-greenschist
facies metamorphism during which mainly chlorite and occasionally biotite porphyroblasts crystallized. |
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Keywords: | Himachal Himalayas structure metamorphism ductile shearing Main Central Thrust Jutogh Group |
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