THE ROLE OF SURFACE PROCESSES AS A DRIVER OF CRUSTAL PROCESSES

In the context of crustal deformation, the key function of surface processes is to redistribute orogenic mass. Erosion and deposition cause unloading and loading that vary in both space and time. In nonglaciated orogens, rates of bedrock incision by rivers typically modulate the rate at which mass is removed from the hinterlands. Overall, climate paces rates of erosion, such that geomorphic rates typically vary at millennial to 10⁵-yr time scales. Whereas our detailed knowledge of temporal change in the climate record has improved greatly in the past few decades, the correlative record of changes in paleo-erosion rates has far less temporal resolution, and few studies have delineated changes in tectonic rates at climatic time scales. Consequently, erosion-tectonic linkages at these shorter geologic time scales remain largely unexplored. At the longer (10⁶-yr) time scales that are typically resolved in tectonic studies, a key question revolves around the extent to which geomorphic processes can “explain” rates of exhumation that are deduced from petrologic, thermochronologic, and geochemical data. In general, maximum rates of sustained erosion appear to be ~10 mm/yr for fluvial and hillslope processes. Glacial erosion may be considerably faster, but few studies have yielded reliable long-term rates. Whereas these rates are compatible with shallow crustal cooling rates in many active orogens, they are insufficient to be the primary driving force for the very rapid exhumation that characterizes many (ultra)high-pressure terranes. Despite spatial correlations among erosion rates, topographic relief, rainfall, and deformation that are suggestive of linkages among these variables in some orogens, causative linkages largely remain to be demonstrated.