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

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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