CHANGING RATES OF FAULT SLIP AT CLIMATIC TIME SCALES: DRIVERS AND IMPACTS IN THE NEPALESE HIMALAYA

Geomorphologists commonly assert that rates of surface processes change at the time scales of glacial-interglacial cycles. Are these changes independent of tectonic forcing, synchronous with such forcing, or are they in response to or enhanced by this forcing? Abundant geomorphic data point to climatically modulated changes in erosion or sediment delivery rates. Little is known, however, about whether tectonic rates change at similar time scales. Do fault slip rates accelerate or decelerate during glacial cycles? If so, are such changes a predictable response to changes in erosion rates or loading/unloading within the orogen? We hypothesize that changes in the locus and magnitude of erosion during glacial-interglacial cycles drives changes in fault-slip rates. A numerical exploration of a two-fault, imbricated fault system that is broadly analogous to the Main Central Thrust-Main Himalayan Thrust system in Nepal suggests fault slip rates (1) are highly sensitive to the coefficient of friction on each fault and (2) vary as a function of both the magnitude and locus of glacial erosion. The modeling also suggests that, if frictional coefficients on a fault can vary at climatic time scales (due perhaps to changes in fluid fluxes in the upper crust), rates of fault slip could more than double or halve at similar temporal scales. We use preliminary isotopic data to assess whether large scale and spatially varying changes in Himalayan erosion rate may have occurred in the past. Our initial results suggest a multi-fold increase in erosion in the arid hinterlands during glacial times. Based on our models, we would predict a synchronous acceleration of slip on the MCT.