Transient Glacial Incision in the Patagonian Andes from 6 to 3 Ma

Studies of present-day erosion suggest that glaciers may erode faster than rivers at a local scale, raising interesting questions about the onset of glacial conditions associated with late Cenozoic cooling and the timescale of landscape response. We report long-term, mountain-scale erosion rates from >10 Ma to present, through the transition to glacial conditions, in the central Patagonian Andes. Apatite (U-Th)/He ages of glacially-transported detrital granitic cobbles show a two- to three-fold increase in the spatially-averaged erosion rate of the source region after the onset of alpine glaciations, while the most rapidly exhumed samples indicate a greater than tenfold increase in local erosion rates followed by a sharp decrease over the last 3 Ma. We ascribe the early transient fast erosion to both local deepening and widening of valleys, as is characteristic of alpine glaciated landscapes. The subsequent decline in the fastest eroding cobbles from 3 Ma to the present may represent a return to a steady-­state balance between rock uplift and erosion.