The distribution of detrital mineral cooling ages in river sediment provides a proxy record for the erosional history of mountain ranges. We have developed a numerical model that predicts detrital mineral age distributions for individual catchments in which particle paths move vertically toward the surface. Despite a restrictive set of assumptions, the model permits theoretical exploration of the effects of thermal structure, erosion rate, and topography on cooling ages. Hypsometry of the source‐area catchment is shown to exert a fundamental control on the frequency distribution of bedrock and detrital ages. We illustrate this approach by generating synthetic 40Ar/39Ar muscovite age distributions for two catchments with contrasting erosion rates in central Nepal and then by comparing actual measured cooling‐age distributions with the synthetic ones. Monte Carlo sampling is used to assess the mismatch between observed and synthetic age distributions and to explore the dependence of that mismatch on the complexity of the synthetic age signal and on the number of grains analysed. Observed detrital cooling ages are well matched by predicted ages for a more slowly eroding Himalayan catchment. A poorer match for a rapidly eroding catchment may result from some combination of large analytical uncertainties in the detrital ages and inhomogeneous erosion rates within the basin. Such mismatches emphasize the need for more accurate thermal and kinematic models and for sampling strategies that are adapted to catchment‐specific geologic and geomorphic conditions.

The character and distribution of cooling ages in modern river sediment provide useful constraints on rates and patterns of uplift and erosion within actively deforming mountain ranges. Such sediment effectively samples all locations within the catchment area, irrespective of remoteness. We evaluate how successfully detrital cooling ages may be used to constrain hinterland erosion rates by examining the modern catchment of the Marsyandi River in central Nepal. Over the 100-200-km-length scale of the catchment, laser fusion 40 Ar/ 39 Ar data for detrital muscovite collected from 12 separate sites illustrate the downstream development of a detrital cooling-age signal that is both systematic and representative of the contributing area. Comparisons of paired samples indicate that, at short spatial (tens of meters) and temporal (hundreds of years) scales, the detrital cooling-age signal is consistent. The distribution of bedrock cooling ages in a subcatchment and the resulting detrital signal at the catchment mouth can be modeled as a function of the erosion rate, relief, hypsometry, catchment area, and muscovite distribution. Given that independent constraints are available for most of these variables, the detrital age signal should be a robust indication of the spatially averaged erosion rate. In the Marsyandi, our model predicts erosion rate differences of approximately twofold, with higher rates (>2 mm/yr) along the southern topographic front of the Himalaya.

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[1] 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 40 Ar/ 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 À1of overthrusting (equivalent to as much as 1.5-2 km Myr À1 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.