RECENT MOVEMENTS ALONG THE MAIN BOUNDARY THRUST OF THE HIMALAYAS - NORMAL FAULTING IN AN OVER-CRITICAL THRUST WEDGE

The Main Boundary Thrust (MBT) is one of the major Himalayan thrusts o ccurring during the Cainozoic, and it is presently incorporated within the Himalayan thrust wedge (Lesser and Outer Himalayas) displaced abo ve the Indian lithosphere. Nonetheless the MBT shows recent normal dis placement along most of its length. We suggest that the orientation of the major principal stress within the Himalayan thrust wedge deviates significantly from the horizontal and when this deviation exceeds the dip of the vectors normal to back-tilted thrusts, the normal componen t of displacement may act along these faults. Steep north-dipping segm ents of the MBT therefore show a normal component of displacement if a geometrical definition is used, but they are faults in a compressiona l regime where the major principal stress axis has deviated from the h orizontal. Micro-structural data recorded along the Surkhet-Ghorahi se gment of the MBT are consistent with a strong deviation of the state o f stress. The presence of such peculiar normal faulting along the MBT is used to calibrate the mechanical characteristics of the belt consid ered as a Coulomb wedge. The following characteristics are suggested: (a) very poor strength contrast between basal decollement and rocks in the wedge body, and (b) a high pore fluid pressure ratio (probably cl ose to 0.8-0.9) and a higher fluid pressure ratio (close to 1.0) along the active normal faults if a high internal friction angle (close to the Byerlee value) is considered. The strong deviation in principal st ress direction may have recently increased, due to a taper of the Hima layan wedge exceeding the stability boundary and may be controlled by erosion and isostatic uplift rebound of the Himalayan range.