INVESTIGATING MODERN GLACIAL EROSION-TECTONIC INTERACTIONS WITHIN THE ST. ELIAS MOUNTAINS, ALASKA USING MARINE SEDIMENT PROVENANCE

The coastal St. Elias Range in Alaska is home to large temperate glaciers, making it an exceptional location to study the interaction between glacial erosion and tectonics in an active orogen. Glacial erosion scales with ice flux and bedrock erodibility that is affected by regional faulting and tectonic strain. Previous work in this region has focused on the Malaspina-Seward and Bering-Bagley ice fields, using exhumation rates derived from detrital and bedrock zircon and apatite thermochronology to constrain regions of increased glacial erosion and uplift. Within the Malaspina-Seward, erosion is concentrated in regions with both fast sliding velocities (Seward Throat), and concentrated tectonic strain (Seward ice field) (Headley et al., 2013; Enkelmann et al., 2015). For the Bering-Bagley, erosion is thought to be concentrated on the windward side of the range based on thermochronolgy (Berger et al., 2008). This study uses geochemical provenance of silt-sized (15-63μm) surficial sediments collected throughout the Gulf of Alaska to test if the same erosion patterns are also observed in the fine-grained fraction. Onshore bedrock elemental data is used to create a Bayesian Composition Estimator (Van den Meersche, 2008) mixing model for Gulf of Alaska sediments offshore the Bering-Bagley and Malaspina-Seward ice fields. Our results show a majority of fine-grained sediment deposited by the Bering Glacier during the 1994-1995 surge event originated from an ultra-mafic metabasalt zone and the low-grade metamorphic Orca-Valdez formation rocks underlying the Bagley ice field. This suggests that the tectonic-erosion coupling within the Bagley ice field, which overlies the Contact fault, may be stronger than what is observed from thermochronology. Sediment deposited by the Malaspina-Seward glacial system appears to reflect a mixed provenance representing the Yakataga formation that underlies the Seward throat, and Chugach Metamorphic Complex and Orca-Valdez rocks that underlie the Seward ice field. Our results support global observations that temperate glacial erosion in an active orogen is concentrated in regional fault zones.