THE GLACIAL BUZZSAW: FACT OR FICTION? SEARCHING FOR AN ANSWER IN THE PATAGONIAN ANDES

The erosive power of alpine glaciers has been hypothesized to be sufficient to control the ultimate height of actively developing mountains regardless of the rate at which uplift occurs. This idea is corroborated by the remarkable correlation observed between glacial equilibrium line altitude (ELA) and mean elevations in several major active orogens independent of highly variable late Cenozoic exhumation rates. However are such observations consistent with current understanding of the physics of glaciation and glacial erosion processes? To answer such a question requires appreciation of the non-linear feedbacks between orogenic wedge mechanics, surface processes, and climate change. Simple conceptual analysis indicates that the erosional response of an orogen will differ dependent on whether ELA lowering due to late Cenozoic climate change is slow and weak, or rapid and strong; and also whether the orogen responds by passive isostatic rebound or active uplift driven by accretion. These conclusions are supported by preliminary results of a coupled geodynamic and surface process computer model that incorporates realistic crustal rheology and a physics-based glacial erosion model. To test the magnitude, rates, and timing of erosion predicted by such models we have undertaken low-temperature thermochronologic analysis at various latitudes along the Main Andean Cordillera of Patagonia: a high latitude active orogen with a well-documented late Cenozoic tectonic, climatic, and glacial history. 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 ca. 8 to 6 Ma to rates of ca. 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 at around 17-14 Ma. This is consistent with our predictions 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. New data presently being acquired will be able to match more closely the resolution of the numeric model, and hence better ascertain the importance of the glacial buzzsaw in limiting orogen development in the Patagonian Andes.