CAN STABLE COSMOGENIC ISOTOPES BE USED TO DIFFERENTIATE TECTONIC SETTINGS?

The use of cosmogenic isotope contents of detrital materials to estimate catchment erosion rates and denudation histories has recently become commonplace. These studies have generally used radionuclides, but the potential uses of detrital cosmogenic contents have increased as the measurement of stable cosmogenic isotopes (21Ne, 3He) has become feasible. A sediment grain reaching a sedimentary basin has a cosmogenic isotope content which reflects its particular history of detachment from the rock mass, and hillslope and fluvial transport and storage. The accumulated information provided by a large number of grains provides a signature of catchment tectonic and erosion history. This can readily be demonstrated in ideal cases, relying particularly on the co-variance of key variables (cosmogenic production rate, erosion rate) with altitude and/or relief. However, the practical utility of this concept initially appears limited since individual grains have an almost infinite range of possible histories so requiring unfeasibly large numbers of replicate samples. This implies that cosmogenic contents cannot be used inductively to provide catchment histories, but it may still be possible to use this approach to differentiate competing hypotheses for the evolution of particular catchments. To assess the potential of using cosmogenic isotope contents in this way, we have constructed a 1-Dimensional stochastic model in which the contents acquired by individual grains are computed. The model includes topographic information (elevation, slope), hillslope processes (diffusion, colluvial storage), fluvial processes (advection, storage), and cosmogenic production rate information (elevation, latitude). Large numbers (50-1000) grains are sampled and their cosmogenic isotope contents estimated. Means and variances of these estimates are obtained. The model shows large and systematic differences in the cosmogenic contents obtained under different conditions (altitude, relief, erosion rate, extent and duration of storage). These results are extended to suggest that different tectonic settings can be identified on the basis of cosmogenic isotope contents. We are extending this approach to 3-Dimensional landscape evolution models, and are engaged in preliminary field testing of the model predictions.