Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Pushpendra Vir ¹, Jai K. Misra ¹, Dilip K. Mukhopadhyay ¹

Affiliations
1 Department of Earth Sciences. University of Roorkee, Roorkee, U.P. 247667, IN

Abstract

We report an unusual association of kyanite-phlogopite-clinochlore from the psammo-pelitic rocks occurring in the Higher Himalayas along the Satluj river between Shongtong and Morang, Himachal Pradesh. These rocks supposedly belong to the Vaikrita Group and the Haimanta Formation.

Keywords

Metamorphism, Haimanta Formation, Higher Himachal Himalayas.

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Dilip K. Mukhopadhyay ¹

Affiliations
1 Department of Earth Sciences, University of Roorkee, Roorkee 247667 U.P, IN

Abstract

Well-preserved small-scale deformational structures in the banded ferruginous quartzite in the Kolar Schist Belt indicate four phases of folding (F₁- F₄ ) , and a shearing movement at low angle to the foliation planes. Both the F ₁ and F ₂ folds are very tight to isoclinal with attenuated limbs and sharp hinges. They are nearly (but not strictly) coaxial, resulting in type-3 interference pattern. The F₃ folds are open and recumbent or gently plunging reclined with axial trend towards NNE-SSW. The folds of the fourth generation (F₄) are of the nature of warps with vertical axial planes striking NE through E to SE. It is suggested that the F₁ folds were originally isoclinal and recumbent/gently plunging reclined with northerly axial trend, coaxially refolded by upright and isoclinal F₁ folds. As the F₂ folds became isoclines further squeezing in the same direction was accommodated by a subhorizontal shearing movement at low angle to the foliation planes. resulting in wide variation in plunge of F₁-F₂ fold axes. The F₁-F₂ folds and mesoscopic shear zones developed due to an E-W subhorizontal compression acting over a protracted period of time. The F₃ folds developed due to gravity induced vertical compression on subvertical foliations. whereas the F₄ folds indicate a longitudinal shortening during the relaxation period at the waning phase offolding episodes.

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Dilip K. Mukhopadhyay ¹, Bidyut K. Bhadra ¹, Tamal K. Ghosh ¹, Deepak C. Srivastava ¹

Affiliations
1 Department of Earth Sciences, University of Roorkee, Roorkee - 247667, U.P., IN

Abstract

In the Himalayan orogenic belt the rocks of the High Himalaya Crystalline Zone (HHCZ) have been thrust over the rocks of the Lesser Himalaya Zone along the Main Central Thrust (MCT). Around the Chur peak in the Lesser Himachal Himalaya the frontal part of the HHCZ is preserved in a half klippe. Analyses of structures from thin section to outcrop scales in this area show that a progressive ductile shearing has been superimposed on two generations of pre-shearing coaxial folds (F₁ and F₂ ). The F₁ and F₂ are tight to isoclinal and recumbent to gently plunging reclined/inclined folds with E or W axial trend. A penetrative cleavage (S₁) parallel to the axial planes of F₁ folds is the common planar structure, and a crenulation cleavage (S₂) parallel to the F₂ axial planes has sporadically developed. In the ductile shear zones the rocks have been extensively mylonitized and a plethora of small-scale compressional and extensional structures have developed. A set of very open and upright folds (F₃) has affected all these structures. The last episode of deformation is represented by a set of subvertical fractures some of which are normal faults. Detailed mapping in selected areas and stereographic analyses of small-scale structures in the shear zones show that the earlier interpretation of large-scale recumbent folding on stratification is not tenable. Four large-scale thrusts cut up the crystalline rocks of the area into a pack of four thin thrust slices in large scale. The MCT in this area can be considered to be a 5-6 km thick ductile shear zone in which two phases of preshearing coaxial folding and late-stage thrusting can be demonstrated.

Keywords

Structural Geology, Main Central Thrust, Himachal Himalaya.

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Dilip K. Mukhopadhyay ¹, Premanand Mishra ²

Affiliations
1 Department of Earth Sciences, Indian Institute of Technology, Roorkee - 247 667, Uttaranchal, IN
2 present address: HOEC, Vadodara, formerly University of Roorkee, IN

Abstract

The Himalay an foreland fold-thrust belt supposedly overrides the alluvium of the Indo-Gangetic plain along the Main Frontal thrust (MFT) at the mountain front. Although the MFT is taken to extend for more than 2500 km all along the Himalayan mountain front, its surface trace is not observed in the field in many areas. In order to constrain the trajectory of the MFT, we have modelled the geometries of the folds occurring in the Siwalik rocks immediately north of the mountain front at five localities in the northwestern Himalayas using the techniques of cross-section balancing. In four of these areas, the MFT has flat-ramp-fiat trajectories with the ramp anticlines being fault-bend folds. The upper flat of the MFT is buried below the forelimb of the ramp anticlines andlor alluvium derived from the same anticline. Only at one locality the ramp anticline is a fault-propagation fold and the MFT breaks through to the surface. We suggest that the usual lack of surface exposure of the MFT in major parts the Himalayan mountain front may be a consequence of flatramp- flat trajectory of the MFT with the upper Rat buried. We also suggest that the intermontanne valleys (locally called Duns) in the Himalayan foothills formed on the trailing synclines of the MFT-related fault-bend folds.

Keywords

Main Frontal Thrust, Balanced Cross Sections, Himalayan Fold-Thrust belt, Siwalik Hills.

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Premanand Mishra ¹, Dilip K. Mukhopadhyay ²

Affiliations
1 HOEC, Vadodara, IN
2 Department of Earth Sciences, Indian Institute of Technology, Roorkee - 247 667, Uttaranchal

Abstract

Two anticlines, viz., Mohand and Santaurgarh, are present in the Dehra Dun re-entrant of the Himalayan foreland fold-thrust belt. We describe balanced fault-related folding models that adequately reproduce the fold geometry of these two anticlines as deduced from the surface orientation data. The Mohand anticline can be modelled as a multibend fault-bend fold related to the Main Frontal Thrust. A multi-bend fault-propagation folding model with about 47% forelimb thinning is appropriate for the Santaurgarh anticline. The Santaurgarh anticline is related to a blind thrust with anticlinal breakthrough structure (Santaurgarh thrust). Uniformly tapering layers are required in both the models. We also present balanced and restored cross sections across the foreland belt and calculate a shortening of 6.4 km or 17.2% within the Tertiary sedimentary belt.

Keywords

Mohand Anticline, Santaurgarh Anticline, Balanced Structural Models, Northwestern Himalayas, Dehra Dun, Uttaranchal.

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