A LINKED FOLD-THRUST BELT-FORELAND BASIN SYSTEM REVEALS PULSED DEFORMATION AND VARIABLE SLIP RATES

Forward modeling reconstructions and data derived from the Himalayan thrust belt and the foreland basin of far western Nepal tie the erosional unroofing and associated deposition to the kinematics and age of fault motion. The deformation at the surface is modeled using a forward propagating, linked fold-thrust belt-foreland basin system, permitting estimates of the magnitude of erosion at each time step and the extent, depth and age of the associated foreland basin. The units that supplied the sediment to the foreland basin changed through time: 25 to 13 Ma, erosion of the Tethyan Himalaya; ca. 12 Ma, first exposure of the Greater Himalaya; ca. 11 Ma, first exposure of the Lesser Himalaya. In our model, >7 km of Lesser Himalayan stratigraphy moves up and over a 7 km thrust ramp, which focuses erosion and facilitates exposure of Greater Himalayan and Proterozoic Lesser Himalayan rocks. As the thrust ramp propagates southward, more thrust sheets are incorporated into the Lesser Himalayan duplex. These model reconstructions allow us to associate time steps with an age of deposition or exhumation. What emerges is a tempo of deformation that varies with time, marked by periods of rapid shortening during propagation of the Main Central thrust, Ramgarh thrust, and middle stages of the development of the Lesser Himalayan duplex (~25-30 mm/yr). After emplacement of the Ramgarh thrust, early and late stages of Lesser Himalayan duplex development are marked by periods of slow shortening (~13-14 mm/yr). Pulsed deformation has also been found in central Nepal and Bhutan with some rates approaching plate convergence rates punctuated by periods of slow shortening. Thus, the assumption of constant shortening rates of ~20 mm/yr in the Himalaya does not reflect the variability in the system. In western Nepal, shortening through time varied from 4-33 mm/yr but in Bhutan may be as much as 30-50 mm/yr. The timing of fast and slow slip in far western Nepal is similar to, but not identical, to eastern Bhutan. Although time periods of fast or slow slip appear to match along strike, more research is needed determine the temporal range and spatial extent of variability in shortening rates along the Himalayan thrust belt.