Modeling mountain building and the seismic cycle in the Himalaya of Nepal

A host of information is now available regarding the geological and thermal structure as well as deformation rate across the Himalaya of central Nepal . These data are reconciled in a two-dimensional mechanical model that inco rporates the rheological layering of the crust which depends on the local t emperature and surface processes. Over geological timescale (5 Ma) the simi lar to 20 mm/yr estimated shortening rate across the range is accommodated by localized thrust faulting along the Main Himalayan Thrust fault (MHT). T he MHT reaches the surface along the foothills, where it is called the Main Frontal Thrust fault (MFT). The MHT flattens beneath the Lesser Himalaya a nd forms a midcrustal ramp at the front of the Higher Himalaya, consistent with the river incision and the anticlinal structure of the Lesser Himalaya . Farther northward the MHT roots into a subhorizontal shear zone that coin cides with a midcrustal seismic reflector. Aseismic slip along this shear z one is accommodated in the interseismic period by elastic straining of the upper crust, increasing the Coulomb stress beneath the front of the Higher Himalaya, where most of the microseismic activity dusters. Negligible defor mation of the hanging wall requires a low apparent friction coefficient (mu ) less than similar to 0.3 on the flat portion of the MHT. On the ramp, mu might be as high as 0.6. Sensitivity tests show that a rather compliant, qu artz-rich rheology and a high radioactive heat production in the upper crus t of similar to 2.5 mu W/m(3) is required. Erosion affects the thermal stru cture and interplays with crustal deformation. A dynamic equilibrium is obt ained in which erosion balances tectonic uplift maintaining steady state th ermal structure, topography, and deformation field. Using a linear diffusio n model of erosion, we constrain the value of the mass diffusivity coeffici ent to 0.5-1.6x10(4) m(2)/yr. This study demonstrates that the data are int ernally consistent and compatible with current understanding of the mechani cs of crustal deformation and highlight the role of viscous flow in the low er crust and of surface erosion in orogeny processes on the long term as we ll as during interseismic period.