FOREARC BASIN DETRITAL RECORD OF MESOZOIC-CENOZOIC TECTONICS IN THE NORTHERN CORDILLERA, SOUTH-CENTRAL ALASKA (Invited Presentation)

Upper Jurassic to Pliocene forearc basin strata exposed in south-central Alaska record variations in sedimentary basin development associated with Late Mesozoic terrane accretion and translation of the Insular terranes (Alexander-Wrangellia-Peninsular) along the western North American margin, Paleocene-Eocene spreading ridge subduction, and Oligocene-Recent flat-slab subduction of an oceanic plateau. In Upper Jurassic to Upper Cretaceous strata, detrital zircon U-Pb populations with trends toward more evolved hafnium isotopic data record ~40 Myr orogenic cycles of magmatic flare-up and crustal thickening during the Late Triassic-Early Jurassic and Late Jurassic that can be linked to Cordilleran-scale accretion events. Detrital zircon U-Pb and fission track double-dating and Hf isotopes from Upper Cretaceous to Pliocene strata and modern rivers reveal the effects of two different modes of flat-slab subduction on the evolution of the overriding plate. When a subducting spreading ridge drives slab flattening, our data suggest that after the ridge has passed along strike, retro-arc sediment sources to the forearc become more predominant. Spreading-ridge subduction also results in an elevated geothermal gradient and thermal resetting of rocks in the upper plate that is revealed by thermochronologic data that record the presence of young age populations found in subsequent, thin sedimentary strata in the forearc basin. When a subducting oceanic plateau drives slab flattening, our data suggest that basin catchments get smaller and local sediment sources become more predominant. Crustal thickening due to plateau subduction drives widespread topographic inversion and significant vertical uplift in rheologically weak zones that, combined, create topography and increase rock exhumation rates. Consequently, the thermochronologic signature of plateau subduction has generally young age peaks that generate short lag times indicating rapid exhumation. Our data suggest that the character and obliquity of down-going slabs provides a fundamental control on the nature of depositional systems, location of dominant sediment source regions, and areas of maximum subsidence in forearc basins.