The Main Frontal Thrust (MFT), Northwestern Himalayas: Thrust Trajectory and Hangingwall Fold Geometry from Balanced Cross Sections

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.