Accelerated erosion of high-latitude glaciated terrain in Alaska since 3-4 Ma

Abstract:

Erosion worldwide is proposed to have intensified due to oscillating climate, cooling, and glaciation in Pliocene-Recent time. Quantifying the initiation and rates of glacial erosion due primarily to climate forcing over geologic timescales is challenging. Reasons include strong tectonic-climatic feedbacks in actively deforming regions and the bias in surficial deposits toward more recent periods. Pleistocene glaciers repeatedly covered the entirety of southern Alaska including tectonically active and inactive regions. Here we present apatite (U-Th)/He [AHe] ages from a vertical transect in a glaciated, high-relief, but tectonically quiescent study area in the western Alaska Range. Samples between 500 and 1600 m have AHe ages that increase from 3 to 4.2 Ma (n=7) and suggest rapid cooling. In contrast, samples between 1600 and 1900 m have a larger range of AHe ages that increase from 4.2 to 7.5 Ma (n=6) and suggest slower cooling. The age-elevation relationship reveals an order-of-magnitude acceleration in erosion at 3-4 Ma with rates of ~1 mm/yr since that time. In addition to this local vertical transect, we also present regional detrital apatite fission track (AFT) ages from 12 modern catchments that span a 450 km length of the Alaska Range. The detrital AFT ages indicate synchronous Pliocene exhumation (3.7-3.3 Ma) exclusively among high relief (2.5-5.5 km), glaciated (20-55% modern area) catchments. There are no known or hypothesized large-scale tectonic changes affecting the entire Alaska Range at 3-4 Ma. The timing of accelerated exhumation is instead coeval with the ~3.6 Ma onset of Northern Hemisphere glaciation. Widespread glaciation in continental Alaska has been suggested at this time based on the onset of loess deposition and boreal pollen. Our results suggest that deep incision into the Alaska Range resulted from accelerated glacial erosion since 3-4 Ma, well after the onset of rock uplift in Oligocene and Miocene time due to tectonics.