PROVENANCE RESPONSE TO FLAT-SLAB SUBDUCTION AS RECORDED IN DETRITAL ZIRCON SIGNATURES FROM THE SOUTHERN ALASKAN FOREARC BASIN

Sediment deposited in the southern Alaskan forearc basin contain a complete record of Late Cretaceous subduction of normal oceanic crust, Paleocene-Eocene flat-slab subduction of a spreading ridge, and Oligocene-present flat-slab subduction of an oceanic plateau. We track the evolving provenance of sediment in response to documented tectonic effects associated with different subduction styles through U-Pb detrital zircon geochronology. The detrital zircon signature of the Late Cretaceous Matanuska Formation, which was deposited coeval to normal subduction, is dominated by Late Cretaceous and Jurassic ages interpreted to be derived from the adjacent Late Cretaceous arc in the Alaska Range and rocks of the accreted Talkeetna arc. After Paleocene-Eocene subduction of a spreading ridge, however, the detrital signature of the Eocene West Foreland Formation shows an increase in Paleozoic and Precambrian grains interpreted to be derived from distal terranes found in interior Alaska. Beginning in Oligocene time, the progressive insertion of an oceanic plateau is interpreted to result in a progressive decrease in Paleozoic and Precambrian grains in the Oligocene Hemlock, upper Oligocene-middle Miocene Tyonek, upper Miocene Beluga, and Pliocene Sterling formations. An increase in input from more proximal and younger sources in these strata are interpreted as a result of exhumation in the Alaska Range and Talkeetna Mountains.

Overall, this study documents that the tectonic effects of three different subduction styles resulted in three discrete provenance signatures in the southern Alaska forearc basin. Proximal sources dominate the detrital zircon signature for strata deposited during subduction of normal oceanic crust. Subduction of a mid-ocean ridge resulted in an increase in detrital zircon populations reflective of far reaching inboard terranes that are north of the present-day Alaska Range. Flat-slab subduction of an oceanic plateau resulted in progressive exhumation of the area above the flat-slab and an overall contraction of the sediment source region. These results offer insight into the evolution of a forearc basin developed during three different subduction styles and strengthens provenance models for forearc basins affected by normal and flat-slab subduction processes.