MODELING CLIMATE, TOPOGRAPHY AND PALAEOGLACIER FLUCTUATIONS IN THE CHILEAN ANDES

A one-dimensional flowline model has been constructed, tested and appl ied to two formerly glaciated valley basins within the Chilean Lake Di strict. The vertically integrated ice flow model is similar to those u sed to study historical fluctuations of European Alpine glaciers and i ncludes terms for internal deformation and basal sliding. In addition, longitudinal deviatoric stresses are computed and velocity terms are correspondingly adjusted. The model is driven through a mass balance t erm forced by a stepped lowering of the equilibrium line altitude (ELA ) through time. Experiments, based on generating equilibrium glacier s urface profiles corresponding to various ELAs, indicate that a lowerin g of at least 1000 m of the ELA from its present-day position is requi red to simulate the glacial maximum. Furthermore, the specific geometr y of the two valleys provides an important control on the extent of th e two glaciers, effectively decoupling them from further climatic dete rioration once they have advanced beyond the constraining influence of their valleys into the piedmont zone. The tight nesting of terminal m oraine loops provides evidence for this topographical control on palae oglacier extent. The modelled response and sensitivity of the two pala eoglaciers to climate change differ markedly as a result of contrastin g valley geometry. Glaciers resting on steeper gradients tend to have thinner profiles, faster mass turnover times and correspondingly short er volume time-scales. Puyehue glacier has a response time of c. 1000 years whereas the Rupanco glacier has a response time of c, 2000 years . Hence,Puyehue is more sensitive to climatic fluctuations occurring o n a time-scale of 500-1000 years. Furthermore, the Rupanco glacier may lag or even fail to respond at all to climatic fluctuations at these time-scales, a conclusion substantiated by field evidence. (C) 1997 by John Wiley & Sons, Ltd.