U-PB DETRITAL ZIRCON GEOCHRONOLOGY OF PALEOGENE FOREARC BASIN STRATA IN SOUTH-CENTRAL ALASKA: SEDIMENT ROUTING AND PROVENANCE IN A TERRESTRIAL FOREARC BASIN

Paleogene sedimentary and volcanic strata exposed in the Matanuska Valley, southern Talkeetna Mountains, and northern Chugach Mountains record forearc basin sedimentation, magmatism, and deformation. We present new U-Pb detrital zircon dates from the Chickaloon (n=2890) and Wishbone formations (n=505) that build upon detrital zircon dates previously reported from the Arkose Ridge Formation (n=1686). These data are compared with compiled dates from modern rivers and bedrock to provide improved constraints on sediment routing and provenance. Detrital zircon and tuff/lava dates constrain Chickaloon and Arkose Ridge sedimentation to ca. 61-55 Ma; Wishbone strata yield ca. 55-53 Ma maximum depositional ages. Multidimensional scaling analysis and unmixing modeling of detrital zircon dates delineate three provenance groups: (1) northern (arcward) samples with Late Cretaceous to Eocene dates eroded primarily from volcanic-plutonic sources along the northern margin of the basin; (2) southern (trenchward) samples with Jurassic dates sourced chiefly from metasedimentary accretionary prism bedrock along the southern basin margin; and (3) central samples with Jurassic to Eocene dates that reflect mixing of northern and southern sources in axial fluvial systems. Syndepositional exhumation of arcward and trenchward sources is compatible with bedrock thermochronologic data that record rapid rock cooling ca. 61-45 Ma. Syndepositional exhumation and growth of active volcanic centers prompted progradation of gravelly fluvial environments across swampy fluvial environments in a transtensional depocenter that was part of a ~250 km long system of dextral faults. Extensional faults created sediment accommodation and formed conduits for ca. 49 Ma and younger lavas that prograded across the sedimentary basin fill. Transtensional basin development is consistent with tectonic models that invoke oroclinal bending and slab window formation during late Paleocene to early Eocene time.