Evidence for Crustal Extension in the Main Himalayan Thrust Hanging Wall During the Latest Great Himalayan Earthquake in Western Nepal

The largest (M8+) known earthquakes in the Himalayas have ruptured the upper locked segment of the Main Himalayan Thrust (MHT) zone, producing slip at the surface along the Main Frontal Thrust (MFT) at the range front. However, out-of-sequence active structures have received less attention. One of the most impressive examples of such faults is the active fault that generally follows the surface trace of the Main Boundary Thrust (MBT), which has generated a clear geomorphological signature along the Himalayan belt.

The recent activity of this fault is clear along a ̴ 120 km-stretch of this boundary fault between the Siwaliks and the Lesser Himalaya in western Nepal between the towns of Surkhet and Gorahi. We first use a high-resolution Digital Elevation Model generated from triplets of very high-resolution Pleiades images in order to map the fault scarp and its geomorphological lateral variation. Across most of its length, this fault is reactivated as a normal fault and associated to cumulated scarps up to ̴ 25 meters-high. This suggests that several earthquakes ruptured the hanging wall of the MHT accommodating crustal extension. We excavated a 17 meters-long trench at the toe of the scarp in the village of Sukhetal exhuming the fault plane and two colluvial wedges. Radiocarbon dating analysis of 19 detrital charcoals suggests that the last slip event on this fault occurred in the early 16 ^th century. This rupture could be related to the great 1505 earthquake (M 8+) that devastated the region, and was also found along thrust fault escarpments of the MFT. This suggests that several faults branching at depth on the main decollement at the front of the megathrust system could be activated either co-seismically or within a short period of time (e.g. post-seismic related deformation),with opposite sense of motion.