Does Karakorum Fault Cut Across the Great Himalaya? Findings of Strike Slip Active Fault Along the Humla-Karnali River in the Northwestern Part of Nepal

It is well known that collision between the Indian and the Eurasian plates does not cause only mega thrusting along the Himalayan front but also great strike-slip shear or N-S extensional zones in the Tibetan block. Active tectonics of Tibet is presumed to be extrusion of Tibetan block along great strike-slip faults such as the Altyn Tagh and the Kunlun faults and consequent evolution of normal faults of N-S direction in the tensional stress field. The Karakorum fault seems to be terminated by Himalayan range on its southeastern edge (Searle,1996). As a southeastern extension of the fault, we found a new fault named Yari fault that was strike-slip fault across the Great Himalayan Range along Humla-Karnali River using a method of photo interpretation of aerial photographs and US spy satellite images. Nakata (1989) showed that MCT (Main Central Trust) active fault system and Bari Gad fault which is located within the Lesser Himalaya of western Nepal are right-stepping echelon striking NW- SE for 170km based on photo interpretation and fieldwork, and also pointed out that those could connect to the Karakoram fault system. There was, however, a gap for about 100 km between the Karakoram fault and the northernmost of the MCT active fault system. In 1990_fs, the Topological Survey of Nepal took vertical air photographs covered all the territory with the aim of publishing topographical maps. With using those new aerial photographs, we recognized an active fault cutting the Great Himalaya in the northwestern Nepal up to the international border of China as NW extension of the MCT active fault system. It is named Yari fault after nearest village name, which has single trace for 50km striking NW-SE generally. In the east of Yari village, alluvial fans and river terraces of different age have been displaced along the fault trace continuously with marked by north-facing scarp and right-lateral offset stream. Yari fault should play very important role as a bridge connecting Karakorum fault on the north of Himalaya and MCT (Main Central Trust) active fault system that is located along the topographical boundary of Higher Himalaya and Lower Himalaya. We would like to establish a hypothesis that not only Tibetan block but also Himalayan ranges are being divided by a series of strike-slip faults such as those strike-slip faults and extruded eastward.