SLOW GLACIAL EROSION AND DEEP VALLEY INCISION IN THE PATAGONIAN ANDES

The central part of the Patagonian Andes has been actively glaciated since at least 6 Ma. Our modeling of ice coverage at that time indicates that the earliest glaciations started as large ice caps, rather than as small valley glaciers. This conclusion is required to explain the eastward extent of mountain glaciers into the dry eastern foothills of the Andes, and by the widespread occurrence of granitic cobbles derived from the Patagonian batholith, which underlies the western side of the range. Our ice-cover modeling indicates that an ice cap of the same size as the last glacial maximum would have formed at 6 Ma if the valley relief of the range was smaller by about 30 to 50 percent relative to modern.

A dense thermochronological dataset is available in this area, and indicates relatively slow erosion at the regional scale. This conclusion is best illustrated by the fact that the present bedrock surface exposes (U-Th)/He apatite ages no younger than ~6 Ma ages. The closure depth for this area is ~2.5 km depth, which indicates an average erosion rate of 0.4 km/Ma during the interval 6 Ma to present. We suspect that much of that erosion occurred by valley incision rather than by summit lowering, as advocated by the glacial buzzsaw hypothesis. We have set up a thermochronologic inverse calculation, using the PECUBE program, to test this idea. We are using pre-existing thermochronologic data to estimate the amounts of valley incision and summit lowering that has occurred over the last 6 Ma. The modeling is also constrained by admittance estimates of the flexural rigidity of the lithosphere in this area, and by tilted plateau basalts exposed on the east side of the range.

Our work suggests that: 1) erosion by mountain glaciations may have started long before the onset of the Quaternary “ice-house” climate, 2) glacial erosion rates have been much slower than previously though, given the longer time frame for glaciation, and 3) valley incision is a much more important and widespread result of erosion by mountain glaciers.