THE COEXISTENCE OF STEADY AND NON-STEADY STATE TOPOGRAPHY IN THE SAN BERNARDINO MOUNTAINS, SOUTHERN CALIFORNIA, FROM COSMOGENIC 10BE AND U-TH/HE THERMOCHRONOLOGY

Knowledge of bedrock erosion rates is vital in order to address questions concerning the role of climate in mountain building, the behaviour of faults and to constrain models of landscape evolution. By utilising multiple techniques the concept of steady state, which is central to many models of landscape development but difficult to test, can be addressed. Here we investigate the existence of topographic steady state in the young, active San Bernardino Mountains situated adjacent to the big bend of the San Andreas fault, southern California. Measuring the amount of incision into a reconstructed pre-uplift surface along with U-Th/He thermochronology allows the calculation of erosion rates over the life span of the mountains. We derive shorter term erosion rates along a transect of 22 cosmogenic ¹⁰Be alluvial sediment samples spanning the mountains from the San Andreas fault in the south to the Mojave Desert in the north. This allows us to compare erosion rates averaged over millions of years with those averaged over thousands and in some cases hundreds of years.

In the southern regions the hillslopes of a thin crustal block bounded on either side by strands of the San Andreas fault appear to be at, or near, their angle of repose, implying a steady state between uplift and erosion. We find cosmogenic erosion rates and so, on the basis of assumed topographic steady state, infer crustal uplift rates for this block of 1.0-3.4mm/a averaged over the last several hundred years. This value is compared to the U-Th/He erosion rate of 0.8-1.6mm/a averaged over the last 1.25Ma highlighting a striking similarity in erosion rates measured over different timescales. Further north we find greater disparity between long and shorter term erosion rates and little evidence for steady state. This lack of steady state could be due to insufficient rates of crustal uplift away from the main trace of the San Andreas fault, the influence of localised faulting, or because the mountains are composed of larger tectonic blocks in the north which have not yet attained characteristic slopes. We also find that, although a strong climatic gradient is present along our north south transect, the controls of erosion appear to be dominated by tectonic rather than climatic factors.