IDENTIFYING POTENTIALLY SEISMOGENIC STRUCTURES IN THE UTTARAKHAND HIMALAYA, NW INDIA: INSIGHTS FROM TOPOGRAPHY, BASIN-WIDE EROSION RATES, AND LOW-TEMPERATURE THERMOCHRONOLOGY

The absence of a major earthquake within the state of Uttarakhand, India for at least 200 years has led many to the proposition that this heavily population region (>10 million) is overdue for a great earthquake. Despite this seismic risk, the geometry of faults likely to host a large earthquake in this region remain poorly understood. Here we use topographic and river profile analyses, basin-wide erosion rate estimates from ¹⁰Be concentrations and results from low temperature thermochronology to place new constraints on the spatial distribution of fault-related rock uplift and erosion within the lower and high Himalaya of Uttarakhand. Results from our analyses reveal that hillslope morphology and channel steepness are relatively constant for ~400-km parallel to strike but vary significantly across strike, with the most significant variations occurring at the physiographic transition between the lower and high Himalaya (PT₂), near the axial trace of the ramp-flat transition in the Main Himalayan Thrust (MHT). The cross-strike changes in geomorphology across the PT₂ correlate with an order of magnitude increase in basin-wide erosion rates (~0.06-0.8 mm/a) and a corresponding decrease in apatite (~5-2 Ma) and zircon (U-Th)/He (~10-2 Ma) cooling ages. When combined with published geophysical and seismicity data, we interpret these results to reflect spatial variations in rock uplift and exhumation due to a relatively continuous segment of the MHT ramp-flat system that is at least ~400 km long and ~125 km wide. Earthquake scaling laws, and the alignment of this segment with the rupture patch of the M_w ~7.7 AD 1803 earthquake, suggest that this segment of the MHT provides a sufficiently large and coherent structure capable of hosting a great earthquake. We speculate that the along-strike discontinuation of a well-defined PT₂ to the northwest of ~77.5ºE and to the southeast of ~88ºE could reflect lateral variations in the geometry of the MHT, which could restrict the rupture area of large earthquakes and effectively segment the mountain belt. While this hypothesis remains preliminary, it is supported by geologic and isoseismal mapping, which argue for a lateral ramp in the Main Central Thrust near ~77.5ºE, and suggest that the rupture extent of the AD 1905 and AD 1803 earthquakes did not cross the proposed segment boundaries.