NO UNIQUE MAIN FRONTAL THRUST IN THE HIMALAYA?

The Main Frontal thrust (MFT) is recognized as the frontal fault in the Himalaya that transports Siwalik rocks over Quaternary sediments and defines the Himalayan topographic front as the surface equivalent of the basal décollement of the Himalayan wedge or the Main Himalayan thrust (MHT). The MFT was originally defined in the western Himalaya and then recognized to be a unique frontal fault all along the length of the Himalayan arc. However, there are several examples in the Himalaya where the classical MFT definition breaks down. In the Dehradun recess, northwest Himalaya, the blind Piedmont fault and not the MFT, is the frontal fault. In the Pinjor Dun of the Kangra recess, the Jhajara fault north of the MFT carries Siwalik over Quaternary sediments making the Siwalik over Quaternary definition of MFT non-unique. The Ramgarh thrust, and not the MFT, defines the frontal topography in the Gorubathan recess, eastern Himalaya, and the MFT is blind south of the mountain front. In the adjacent Dharan salient, the frontal Himalaya is characterized by an imbricate fan consisting of 11 faults that repeat the Middle Siwalik section. The MFT can only be loosely defined in the absence of Siwalik over Quaternary contact in the salient and was recognized as the southern-most emergent imbricate carrying Siwalik rocks in its hanging as well as footwall. Correlation of MFT in the Dharan salient and Gorubathan recess across the salient-recess boundary was not possible as the structure south of the Ramgarh thrust in the recess was blind. These observations rule out the presence of a unique MFT all along the Himalayan arc and suggest that the frontal fault in each salient and recess in the Himalaya may not be the same. Kinematically, a high seismic hazard is attributed to the MFT as it is activated only during great earthquakes when slip along the MHT reaches the mountain front. The amount and distance of slip propagated along the locked MHT during a décollement earthquake is variable and may or may not reactivate older thrusts. Therefore, décollement earthquakes can transfer slip to the wedge-front both north and south of the MFT. These observations indicate that the MFT does not correlate as a unique fault across different salient-recess boundaries along the Himalayan arc and contemporary models on its continuous geometry and kinematics need to explain this.