Pulsed deformation and variable slip rates within the central Himalayan thrust belt

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. We reproduce the deformation identified at the surface through a forward-propagating, linked fold-and-thrust belt-foreland basin system. This approach permits estimates of the magnitude of erosion at each time step and the extent, depth, and age of the associated foreland basin. The model reconstructions reveal that the units that supplied the sediment to the foreland basin changed through time: 25-13 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, exposure of Greater Himalaya and Lesser Himalaya rock is associated with the formation of a thrust ramp that cuts through 7 km of footwall Lesser Himalaya stratigraphy and translates >7 km of Lesser Himalaya rock over the ramp, forming a Lesser Himalaya duplex. An increase in structural relief focuses erosion over the region of the ramp and facilitates exposure of Greater Himalaya and Proterozoic Lesser Himalaya rocks. As the Lesser Himalaya ramp propagates southward, more Lesser Himalaya thrust sheets are incorporated into the Lesser Himalaya duplex. Although uniquely dating thrust events is challenging, 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 Himalaya duplex (similar to 25-30 mm/yr). After emplacement of the Ramgarh thrust, early and late stages of Lesser Himalaya duplex development are marked by periods of slow shortening (similar to 13-14 mm/yr). Although long-term and modern (geodetic) rates of deformation agree at similar to 20 mm/yr, rates of shortening through time have varied from 4 to 33 mm/yr.