REGIONAL PALEOCENE-EOCENE EXHUMATION OF THE NORTH AMERICAN FOREARC BASIN MARGIN, SOUTH-CENTRAL ALASKA, RECORDED BY APATITE FISSION-TRACK THERMOCHRONOLOGY

Thirty-six bedrock apatite fission-track (AFT) ages from the margins of the Cook Inlet forearc basin (CIFB), and 6 reconnaissance detrital AFT ages from Cenozoic age strata filling the basin define Paleocene-Eocene cooling of proximal source terranes rimming the CIFB. The timing of this cooling is strikingly similar at the arc-forearc margin along the upper Alaska Peninsula (~56 to 42 Ma) and north-western upper Cook Inlet (~52 to 40 Ma), and the accretionary complex-forearc margin along north-eastern upper Cook Inlet (~55 to 38 Ma) despite the region’s different tectonic settings and separation by up to 500 km. The cooling events are coeval with widespread Cenozoic clastic sedimentation throughout the upper Alaska Peninsula and Cook Inlet regions (Copper River, West Foreland, Wishbone, Arkose Ridge, and Chickaloon formations), suggesting that the cooling interval may reflect a regional exhumation event during latest Paleocene to Middle Eocene time that simultaneously involved the intrusive arc and western-most accretionary prism. Preliminary interpretation of detrital AFT and detrital zircon ages suggests Early Oligocene exhumation of sediment source terranes distal to the CIFB margins. Disparate cooling ages across some fault boundaries (e.g. Bruin Bay and Lake Clark faults) may reflect exhumation of the CIFB margin along those regional-scale structures, but the datasets are either complicated by subsequent structural and thermal overprints, or are too small for robust interpretation. Alternatively, regional Eocene exhumation indicated throughout the forearc region may be related to proposed lithospheric events occurring along the continental margin during Paleocene and Eocene times, such as the subduction of the Kula-Farallon ridge and/or passing of a related slab window, and changes in obliquity and rate of slab subduction. Conspicuously absent is evidence for substantial cooling after ~30 Ma recording collision of the Yakutat block with southern Alaska, suggesting relatively low exhumation of the upper CIFB margin in response to Yakutat underplating.