THE IMPORTANCE OF GLACIAL EROSION IN CONTROLLING THE LATE CENOZOIC DEVELOPMENT OF THE PATAGONIAN ANDES

The remarkable match between the glacial equilibrium line altitude (ELA) and summit elevations in many high latitude orogens has led to the hypothesis that glaciers act as an erosional “buzzsaw”, whereby the high rates of glacial erosion are able to efficiently remove most topography tectonically uplifted through the ELA. Understanding the importance of glacial erosion is complicated by the fact that development of elevated topographies in active orogens requires appreciation of non-linear feedbacks between orogenic wedge mechanics and surface processes. Simple conceptual analysis indicates that the erosional response of an orogen will differ dependent on whether ELA lowering (glacial buzzsaw) 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 within a critical orogenic wedge. We have made preliminary efforts to augment this analytical model with a more robust coupled geodynamic finite-element model for a generic orogenic wedge, and numeric surface process model incorporating glacial erosion.

The Patagonian Andes - a high latitude active orogen with a well documented late Cenozoic tectonic, climatic, and glacial history - represent an ideal orogen in which to apply low-temperature thermochronology to test the patterns and magnitudes of erosion predicted by our models. Furthermore, we test to what extent the development and form of the chain has been controlled by glacial erosion. Preliminary data from 47-49°S indicate an erosion rate increase of ~60% following the onset of widespread glaciation in the Patagonian Andes. This is significantly lower than expected for a strong glacial buzzsaw acting on an active critical orogenic wedge (ca. 400%), but higher than predicted for a weak glacial buzzsaw acting on either an active or passive orogen, implying that the onset of ELA lowering caused some erosional response at the centre of the orogen. 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 and its role in the development of the present topography of the Patagonian Andes.