Thermal and kinematic modeling of bedrock and detrital cooling ages in the central Himalaya

We introduce a new method that convolves detrital mineral cooling ages with digital elevation models to test numerical models of erosion in collisional orogens. Along a Trans-Himalayan transect in central Nepal, we develop a kinematic and thermal model to predict variations in bedrock cooling ages in modern Himalayan topography. The model assumes a thermal steady state and utilizes a simple ramp-and-flat-style decollement, representing the Main Himalayan Thrust. The model also assumes a topographic steady state, such that overthrusting is balanced by erosion to maintain a constant topographic profile. Erosion rates display strong spatial variations as a function of the angle between the slope of the topographic surface and the trajectories of rock particles approaching the surface. To predict the detrital cooling-age signal, we combine the distribution of bedrock cooling ages within a catchment with the rate of erosion and distribution of muscovite. Predicted cooling-age distributions are compared with detrital 40Ar/39Ar muscovite data to assess varying tectonic and erosion scenarios. Such cooling-age distributions are very sensitive to how much of the total plate convergence is expressed as erosion of the overthrusting plate. The best fit model assigns 4-6 km Myr(-1) of overthrusting ( equivalent to as much as 1.5-2 km Myr(-1) of vertical erosion) to the Asian plate. Although a trade-off exists between ramp geometry along the decollement and the best fit rate, only a narrow range of ramp dips, decollement depths, and erosionally compensated overthrusting rates are compatible with the observed detrital ages.