PROVENANCE OF THE COOK INLET FOREARC BASIN, SOUTHERN ALASKA: INVESTIGATING THE EFFECTS OF CENOZOIC SPREADING-RIDGE AND FLAT-SLAB SUBDUCTION ON SEDIMENT TRANSPORT ALONG CONVERGENT MARGINS

The Cook Inlet basin is the active part of the forearc basin system on the modern southern Alaska convergent margin. New U-Pb geochronology, rare earth elements in mudstone, and conglomerate compositional data from Cenozoic strata in the basin and modern fluvial systems adjacent to the basin demonstrate the profound changes in forearc sediment sources linked to two major Cenozoic tectonic events: spreading-ridge subduction and flat-slab subduction of an oceanic plateau. In Late Cretaceous time, a series of marine forearc basins extended east from the Cook Inlet to the Wrangell-St. Elias Mountains in eastern Alaska and southwestward along the Alaska Peninsula, with the majority of sediment input from Jurassic-Cretaceous volcano-plutonic belts linked to arc magmatism. Following Paleocene-Eocene subduction of a spreading ridge, nonmarine depositional systems transported sediment from east-central Alaska, including areas north of the present eastern Alaska Range, and from the exhumed eastern part of the Cretaceous forearc system to the basin. Our data indicate that from middle Eocene to middle Miocene time, one major sediment conduit was located along the western margin of the basin and drained local arc sources as well distal Paleozoic source terranes in east-central Alaska. Another key conduit was situated along the eastern margin of the basin and also derived sediment from local arc and accretionary prism sources in addition to Paleozoic sources in east-central Alaska. Flat-slab subduction of the Yakutat oceanic plateau initiated along the outboard margin of southern Alaska in early Oligocene time, and began affecting the forearc basin in middle to late Miocene time. Surface uplift of the region east of the Cook Inlet basin associated with shallow subduction gradually sequestered east-central Alaskan sediment sources. By late Miocene time, the provenance signatures from both the western and eastern margins of the Cook Inlet basin closely resembled the detrital signatures of the modern day rivers in those regions. Therefore the sediment source region for the basin probably resembled the modern drainage system by late Miocene time. Subduction processes such as ridge subduction and flat-slab subduction of oceanic plateaus can significantly modify the sediment transport and provenance of forearc basin systems.