LATE CENOZOIC EROSIONAL HISTORY OF THE PATAGONIAN ANDES: A KEY TO UNDERSTANDING THE IMPORTANCE OF CLIMATE CHANGE AND GLACIAL EROSION IN CONTROLLING MOUNTAIN DEVELOPMENT

The Andean mountain chain has long been recognized as having remarkably correlative latitudinal variation between morphology and climate; the central Andes showing arid conditions, low erosion, and high elevation, while the southern Andes have a colder, wetter climate, higher erosion, and a north to south decrease in maximum elevation that matches remarkably with the height of the glacial equilibrium line altitude (ELA). These observations have led to the proposition that, along with tectonics, latitudinal variation in erosion related to climate and the glacial erosion 'buzzsaw' has had a first order control on the topographic development of the Andes, and perhaps more provocatively, that Cenozoic climate change was actually the predominant cause of Andean uplift, for example by limiting the amount of sediment entering the subduction channel, increasing plate boundary stresses to support high topography. Better understanding the late Cenozoic erosional history of the Patagonian Andes is critical to testing some of these ideas. Their high latitude and relief means that interactions between climate, tectonics, glacial erosion, and mountain development are more pronounced than in the central Andes. Also any climate-induced increase in erosion since the onset of major glaciation between ~7.0 and 5.0 Ma should have been sufficient to be recorded by low-temperature thermochronometric ages presently at the surface. Preliminary apatite (U-Th)/He and fission track data from elevation transects at 39°S, 44°S, and 48°S reveal a marked and consistent acceleration in erosion at ~8 to 6 Ma to rates of ~0.5 to 0.6 mm/yr, coeval at all latitudes with onset of major glaciation in the region, but well after initial surface uplift sometime around 17-14 Ma. This is consistent with predictions of analytical and numerical models for moderately efficient glacial erosion acting on an active orogen, or very efficient glacial erosion (strong buzzsaw) acting on an inactive orogen responding by passive isostatic rebound. Our data also imply that increased sediment flux into the trench in the southern Andes (south of ~37°S) has only been significant since about 8 Ma. Thus climatically controlled trench sediment fill as a mechanism for differential uplift of the Andes should only be invoked to explain the last ~8 Myr of Andean development.