GLACIAL EROSION AND TECTONIC COUPLING IN ACTIVE OROGENS: THE CHARACTERISTIC RESPONSE TIME OF THE SOUTHERN ANDES TO LATE MIOCENE CLIMATE CHANGES

There is considerable interest on the role of climate in controlling topography and local relief in tectonically active landscapes, and we present a theoretical model that shows that the southern (Patagonian) Andes present a clear test case of quantifiable feedbacks between climate and tectonics. Although much of climate/tectonic coupling discussion has focused on the role of fluvial erosion, the debate has more recently turned to include glacial erosion as well. Glacial erosion is probably important in limiting the maximum elevation in the majority of convergent mountain belts, which have hosted alpine ice caps for most of the last few million years. A convergent wedge model of Andean orogen development suggests that the lowering of the Equilibrium Line Altitude (ELA) since the Late Miocene may have lead to a lowering of the relief in the Southern Andes of around 1 km. Coupling this tectonic model with the results of recent analytic and numerical models of glacial erosion, we show that the characteristic response time of the Andes is of the order of 5 Myrs. Climate change over shorter time-scales is heavily attenuated; frequencies associated with Milankovitch cycles do not produce an orogenic scale tectonic response in the Andes. Given this timeframe, low-temperature thermochronologic (such as those produced by AFT and AHE) measurements are required to determine the degree of climate/tectonic coupling. We present analytical predictions of relief, sedimentation rates, and rock uplift rates changing over time for the orogen. Preliminary AFT observations of increases in rock uplift rates are consistent with these predictions, with erosion rates apparently increasing by a factor of two after the onset of glaciation.