Abstract

Apatite (U-Th)/He dating from the St. Elias orogen in southern Alaska illustrates a potential association between long-term denudation and glacier sliding. Cooling ages as young as 0.4 Ma (exhumation ~4–5 mm/yr) are concentrated in a narrow band near the glacier equilibrium line altitude (ELA) front, where mean Quaternary ELA intersects the windward flank of the orogen. This band of denudation is not correlated with individual faults, structural trends, or known concentrations of precipitation, and we propose that it is produced by focused glacier sliding at or near the ELA front. This implies that long-term glacial erosion is a maximum at ELA, which corroborates model predictions that glaciers and climate can control the pattern of crust removal from orogens. Denudation rates do not covary with fluctuations in glacier size along the ELA front, suggesting that small glaciers are capable of keeping pace with incision by larger ones and tectonic rock uplift. Ice discharge may thus play a critical, but complex, role in excavating glaciated orogens.

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