Rates of erosion and their implications for exhumation

Abstract

At time-scales of 10 2 to 10 5 years, erosion by rivers, landslides and glaciers can exceed 5 mm/y. Sustained denudation at these rates is sufficient to account for many of the rapid rates of unloading or cooling that are revealed by geobarometric or thermochronologic studies. Because feedbacks exist among many surface processes, determinations of erosion rates on a few geomorphic processes can be adequate to estimate mean rates across an entire landscape. Rates of fluvial and glacial incision exert a dominant control on landscape lowering, because these rates set the local base level and modulate the flux from adjacent hillslopes. When rates of deformation are sufficiently rapid and sustained, a collisional orogen approaches a dynamic equilibrium or topographic steady state. Due to variations in erosion rates as a function of Late Cenozoic climate changes, such a steady state should be defined at time-scales longer than one climate cycle. During dynamic equilibrium, the geomorphic system displays a predictable configuration of interacting rivers, hillslopes and glaciated process zones. A dynamic equilibrium appears to prevail near Nanga Parbat, Pakistan, and the Southern Alps of New Zealand, where spatial variations in geomorphic erosion rates mimic variations in bedrock cooling rates at time-scales of 10 6 y.