MIOCENE INTRA-ARC SEDIMENTARY BASIN DEVELOPMENT WITHIN THE WRANGELL VOLCANIC FIELD, FREDERIKA FORMATION, WRANGELL MOUNTAINS, ALASKA

The Miocene-Holocene Wrangell volcanic field records >25 my of continental arc construction within an accretionary convergent margin in southern Alaska. Whereas previous research focused on <5 Ma volcanic rocks, analysis of Miocene sedimentary and volcanic rocks (Frederika Formation) in the Wrangell Mountains provides insight on the early evolution of the WVF. Measured sections and geologic mapping of the FF document an intermontane basin bound by active volcanic centers. Western outcrops consist of >450 m of cobble-boulder conglomerate, volcanic-lithic sandstone, and lava deposited on southeast-dipping alluvial fans characterized by streamflow, debris flow, and effusive volcanic eruptions. Eastern outcrops consist of >250 m of fluvial-lacustrine pebble-cobble conglomerate, volcanic-lithic sandstone, carbonaceous mudstone, coal, and limestone. Eastern strata include subordinate crystal-vitric tuff, lapilli-tuff, and lithic-tuff-breccia deposited by pyroclastic flows and fallout. Local Miocene volcanic centers and Triassic-Paleozoic metamorphic sources provided FF detritus based on compositional data. Previous age data from the FF were limited to Miocene plant fossils. A SHRIMP U-Pb zircon age of 11.3 ± 0.24 Ma from tuff 109 m above the base of the FF confirms Miocene deposition. FF lavas exhibit geochemical characteristics typical of subduction-related arc volcanic suites. As a group, the FF lavas define elemental relationships indistinguishable from those observed for a subset of <5 Ma western WVF lavas interpreted to have been emplaced in an intra-arc extensional setting. In contrast, Miocene volcanic rocks attributable to magmatism along leaky transform faults in the Yukon Territory exhibit higher Fe/Mg ratios and total alkalies at a given Si content than FF lavas. Collectively, our new data demonstrate that the FF represents a southward extension of alluvial fan-delta conglomerate and lavas exposed in the northernmost Wrangell Mountains (White River “tillite”). Miocene intra-arc extensional basin development was contemporaneous with regional uplift and synorogenic sedimentation outboard in the Yakutat terrane (Yakataga Fm.) and inboard in the Tanana foreland basin (Usibelli Group) in response to northwestward subduction of the Pacific plate and Yakutat terrane.