DEPOSITION, PROVENANCE, AND EXHUMATION OF NEOGENE STRATA IN THE SYNTAXIS OF THE CHUGACH-ST. ELIAS RANGE, SOUTHEAST ALASKA

More than 5 km of Neogene sedimentary strata are exposed in the Chugach-St. Elias Range along the southeastern coast of Alaska. New stratigraphic, U-Th/He thermochronologic, and zircon U-Pb geochronologic data from these strata provide insights on basin configuration, provenance, and exhumation within a tectonically active and glaciated continental margin. Evaluation of the Neogene basin configuration reveals angular unconformities restricted to the eastern margin of the basin near the Malaspina Glacier, where 1000s m of underlying Paleogene sedimentary strata was deformed and eroded followed by Neogene sedimentation in proximal fan-delta and fluvial systems. The central and western parts of the basin contain ~10 km of stratigraphically conformable Neogene sedimentary strata deposited in more distal shelf and upper slope settings. U-Pb sandstone detrital zircon analyses and U-Pb dating of zircons from igneous and metamorphic clasts in conglomerates indicate that Neogene sediment was mainly sourced from Early Cenozoic - Late Mesozoic (~50-70 Ma) plutons of the Chugach-Prince William terrane and Coast Plutonic Complex, with secondary sources from the Wrangellia terrane (> ~130 Ma). Apatite and zircon U-Th/He data from detrital igneous and metamorphic clasts suggest that Neogene sediments were buried no deeper than ~2 km in eastern parts of the study area, and that temperatures did not exceed ~40-45°C. Sediments were buried more deeply (~7.5 km) in western parts of the study area, and may have reached temperatures between ~120-160°C. Apatite ages imply young exhumation to the east within the St. Elias syntaxis, whereas exhumation ages are older in the west where sediments were deeply buried in distal parts of the thrust belt near the Aleutian subduction zone. The integration of these data allows for a revised depositional model for Neogene strata in southeastern Alaska. Sediments were mainly locally sourced from Early Cenozoic – Late Mesozoic plutons and deposited in a southwest-plunging foreland basin situated parallel to the strike of the Aleutian subduction zone. These strata were subsequently incorporated into a heavily glaciated, southward-propagating fold and thrust belt.