Spatial extent of rapid denudation in the glaciated St. Elias syntaxis region, SE Alaska



[1] The ongoing collision of the Yakutat Terrane with the North American Plate formed the St. Elias Range in southeastern Alaska. Previously published detrital zircon fission track dating of sand-size material detected very rapid rock denudation within the syntaxis region of the St. Elias Range, but the size and spatial extent of the rapid exhumation are elusive due to the glacial cover that inhibits direct bedrock studies. Two possible mechanisms were suggested to explain the rapid denudation occurring underneath the Seward-Malaspina Glacier, which differ in their spatial extent of rapidly exhumed rocks. This study evaluates the spatial extent of rapid denudation at the St. Elias syntaxis region. We investigate the cobble-sized material of the Malaspina Glacier. Petrological classifications are used as a provenance tool and combined with zircon and titanite (U-Th)/He analysis to reveal cooling rates of the glacial detritus. In total, 1998 cobbles are classified in the field. Petrographic characterization is conducted on 91 thin sections and the (U-Th)/He ages of 59 cobbles are measured. Fourteen cobbles reveal young cooling ages of 3–2 Ma originating from various lithologies of undeformed and deformed magmatic and metamorphic rocks. Older age populations peak at 12, 27, and 36 Ma. The cooling ages and the mapped known extent of the different lithologies suggest that a larger then previously assumed area of the St. Elias syntaxis region experienced rapid exhumation. Rapid rock exhumation extends beyond the Contact Fault and comprises most of the Seward-Malaspina Glacier catchment.