Geomorphology, active duplexing, and earthquakes within the Central Himalayan seismic gap

The ~500 km long 'Central Himalayan seismic gap' of northwest India, is the largest section of the Himalaya that has not experienced a very large earthquake (M_w > 7.0) in the past 200-500 years. The slip deficit associated with this seismic quiescence has led many to suggest that the region is overdue for a great earthquake (M_w >8), an event which could be potentially devastating given the region's high population (>10 million). Despite the recognition that the region is under considerable seismic risk, the geometry of active fault structures that could potentially fail during large earthquakes remains poorly defined. This has arisen, to a certain extent, because moderate earthquakes, such as the M_w 6.3 1999 event near the city of Chamoli and the M_w 7.0 1991 earthquake near Uttarkashi (responsible for ~1000 deaths), have not produced obvious surface ruptures and do not appear to coincide with surficially mapped faults. We present new geomorphic and river longitudinal profile data that define a prominent ~400 km long distinctive geomorphic transition at the base of the high Himalaya in the seismic gap, defined as a sharp dividing line north of which there are significant increases in normalized river steepness (k_sn), hillslope angles, and local relief. We interpret the morphologic changes across the geomorphic boundary to be produced due to a northward increase in rock uplift rate, given that the boundary cross-cuts mapped structures and lithologic contacts, yet coincides exactly with: 1) the axial trace of the geophysically-imaged ramp-flat transition in the Main Himalayan Thrust, 2) significant northward increases in instrumentally-recorded seismicity, and 3) an order of magnitude change in published Ar-Ar bedrock cooling ages. The available datasets suggest that such an increase in rock uplift rate is best explained by a ~400 km long by ~50 km wide active duplex along the Main Himalayan Thrust ramp, with the leading edge of the duplex giving rise to the geomorphic boundary. The observation that the geomorphic boundary of the seismic gap lies ~350 km to the west of the analogous PT₂ of Central Nepal (yet there is no such well-defined physiographic transition in the region between them) suggests that the changing along-strike character of the middle/high Himalaya transition could reflect changes, on the order of 10² km, in the geometry and/or kinematics of the plate boundary thrust along-strike. These along-strike variations could segment the plate boundary and effectively restrict the locality and rupture length of large earthquakes. This hypothesis is supported by historical records of seismicity which indicate that the M_w ~7.5 earthquake of 1803 occurred roughly within the section of the seismic gap containing the geomorphic boundary, and the M_w ~7.7 earthquake of 1833 ruptured the ~350 km long section of the plate boundary occupied by the Nepalese PT₂.