ONSET OF GLACIATION CAUSES RAPID INCREASE IN EXHUMATION RATE BEFORE RELAXATION TOWARDS STEADY-STATE: DETRITAL APATITE (U-TH)/HE RECORD 6 MA TO PRESENT IN CENTRAL PATAGONIA

The understanding and quantification of the transition between fluvial and glacial erosive regimes in alpine landscapes is explored using a novel application of detrital thermochronometry. We present long-term, basin-scale rates of erosion in central Patagonia from 6 Ma to the Last Glacial Maximum. A series of four Argentine glacial deposits near Lago Buenos Aires contains apatite-bearing granite and granodiorite boulders sourced from the Patagonian batholith in Chile. The difference between deposition time (determined by independent chronometers) and measured apatite (U-Th)/He thermochronometric age yields a “lag time” for a given rock, which scales inversely with erosion rate. This natural experiment provides a first-order understanding of the relationship between the onset of glaciation, the changing rate of erosion, and the development of topography in the central Patagonian Andes. The analysis of 206 grains from 72 cobbles in four deposits shows a rapid increase in erosion rate associated with the onset of glaciation in the region between ~6 and 3.3 Ma, followed by a relaxation of erosion rate approaching presumed steady-state conditions today. This detrital record indicates that glacial topography is formed quite rapidly after the onset of glaciation in Patagonia, likely due to an increase in valley relief, and is not the result of a climatic signal, such as the mid-Pleistocene transition observed in shorter-duration Northern Hemisphere glacial records.