CALIBRATING RATES OF EROSION AT THE CATCHMENT TO OROGEN SCALE

Through improved quantification of the nature and rate of erosion over discrete spatial and temporal scales, two recent advances have enhanced our understanding of the interactions among climate, tectonics, and landscape evolution. Innovative geochronological techniques now enable us to extract improved age estimates from the landscape, whereas high resolution digital topographic databases allow visualization and quantification of landscape characteristics. Traditional approaches to erosion rates, as estimated via sediment fluxes in rivers draining orogens or sediment volumes in basins next to them, have in many cases been superceded by more direct measurements of erosion within orogens themselves. The digital reconstruction of deformed and dissected geomorphic markers permits direct quantification of erosional volumes, whereas dating the initiation of deformation allows erosion rates to be determined. At the scale of individual catchments, rates of river incision into bedrock or hillslope erosion via landsliding delineate rates of landscape lowering. In rapidly deforming orogens where sustained rates of rock uplift exceed several mm/yr, an approximate, long-term, topographic equilibrium is commonly established in the landscape, such that advection of rock and erosion are balanced. Over shorter time frames, this balance is perturbed by discrete seismic events and by major swings in climate. Thus, measures of erosional processes and rates over intervals shorter than seismic or climatic cycles risk defining transient rates that are atypical of mean, long-term rates. A broader temporal perspective on erosion can often be obtained using low-temperature geochronometers or cosmogenic nuclide dating. Application of such techniques to detrital sediments provides an opportunity to integrate data from an entire swath across an orogen and to delineate regional contrasts in cooling/erosion histories that can be tied to the thermo-mechanical evolution of the orogen.