GEOMETRY AND KINEMATICS OF RECENT DEFORMATION OF THE HIMALAYAN HINTERLAND, SOUTHWEST TIBET AND NORTHWEST NEPAL

The Karakoram fault system has long been thought to be a narrow zone (<20 km-wide) zone of right-lateral shear that has been interpreted to facilitate eastward extrusion of the Tibetan plateau. We present results from a study that incorporates earthquake epicenters, structural, and remote sensing data (Landsat 7, Corona photography, and ASTER) that demonstrates that the Karakoram fault system is significantly wider than previously interpreted. The study region (~60km x 120km) is situated between 81 degrees E and 8 degrees E longitude within the hinterland of the Himalayan fold-thrust belt straddling the China-Nepal border. The active structural system of this area is dominated by WNW-striking right-slip faults and NNW-striking normal faults that are a part of the Karakoram fault system. Three lakes, inculding Late Mansarovar, occur along the a large (~40 km-long) SSE striking right-stepover that transect the middle of the study area. Two of these lages (~25km2) are found at the northwest corner of the stepover adjacent to the Indus-Yalu suture zone. A large basin (~25,000km2) lies immediately west of the stepover. The southern end of the stepover is characterized be a system of en echelon normal right-slip faults that offset river channels, alluvial fans, and ridges. The total slip along faults at the northern end of the stepover does not feed entirely into the stepover. There is geomorphic evidence that suggests right-slip faulting continues east of the stepover along the Indus-Yalu suture zone. Earthquake epicenters complement this data: in the study area, over 20 earthquake epicenters ranging in Richter magnitudes 2 and 4 have been recorded by Pandey et al. (1995). 5 epicenters are located along the stepover; another 15 are concentrated adjacent to the southern WNW-striking branch of the stepover in northwest Nepal. Field data from northwestern Nepal indicates normal faulting to be the dominant control on basin formation: N to NNW-striking normal faults define fault-bounded blocks ranging in size from 10km to 50km-wide. These fault-bounded blocks are bordered on the north by right-slip faults oriented 280 degrees and contain subhorizontal striations. These faults deform rocks belonging to the Greater Himalayan Crystallines. Our results demonstrate that the Karakoram fault system is significantly wider than previously interpreted.