A MIOCENE PATAGONIAN ICECAP, PROLONGED GLACIATION, AND THE BREACHING OF THE ANDEAN DIVIDE

The Patagonian Andes have hosted large-scale ice caps since at least the late Miocene and this prolonged glaciation has profoundly affected the morphology of the range. We hope this regional example will inspire reconsideration of the duration and impact of substantial glaciation in other mountainous regions.

Late Miocene glacial sediments in the eastern foothills of the Patagonian Andes have been commonly ascribed to small-scale local glaciations, but the presence of granitic cobbles in these deposits requires long glacier lengths of >80 km in the rainshadow of the Andes. Larger glaciers are expected on the windward, western flank of the range which receives more precipitation. Results from a simple glacier model indicate an icecap at least 85% as large by volume as the Last Glacial Maximum (LGM) patagonian icecap is required to produce the observed deposits. It is only possible for this icecap to grow in the warm late Miocene climate if valleys were less-deeply incised than at present, consitent with our expectation that glacial erosion has deepened the valleys. The presence of LGM-scale glaciation in a relitively warm climate suggests the regular periodic growth of an icecap during most or all of the ~130 cold periods from late Miocene to present, likely with a substatial prior period of alpine glaciation. Not all of these glacial advances have been preserved, but the number of observed glacial deposits is sufficient to support this hypothesis.

In Patagonia, this prolonged and repeated exposure to glacial erosion has been a major factor controlling the landscape morphology. At present, the fluvial divide is surprisingly located to the east of the mountain belt and most of the major rivers in the region drain from east to west through the high topography of the Andes. We suggest the insertion of deep inland fjords resulting from prolonged glacial erosion disrupted the normal fluvial drainage patterns, breaching the topographic divide and driving the fluvial divide to the east.