Paper No. 168-3
Presentation Time: 2:05 PM
TERRANES, SPREADING RIDGES, AND OCEANIC PLATEAUS – A TALE OF THE SOUTH-CENTRAL ALASKAN FOREARC (Invited Presentation)
Upper Jurassic to Pliocene forearc basin strata in south-central Alaska record variations in sedimentary basin development associated with Late Mesozoic terrane accretion and translation of the Insular terranes along western North America, Paleocene-Eocene spreading ridge subduction, and Oligocene-Recent oceanic plateau subduction. Detrital zircon U-Pb populations in Upper Jurassic to Upper Cretaceous strata display trends of more evolved Hf isotopic data that record ~40 Myr orogenic cycles of magmatic flare-up and crustal thickening during the Late Triassic-Early Jurassic and Late Jurassic. 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. Slab flattening driven by a subducting spreading ridge results in uplift of the forearc region and more regional retro-arc sources of sedimentThis process also results in thermal resetting of rocks in the upper plate that is revealed by young zircon fission track populations found in subsequent, thin sedimentary strata in the forearc basin. Slab flattening driven by subduction of an oceanic plateau results in diminished basin catchments with local sediment sources more predominant. Crustal thickening 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. One of the difficulties with provenance studies in this region is that with Alaska’s prolonged magmatic history, individual igneous belts are geographically extensive and plutonic belts of different ages often overlap. An integrated approach of modern river sampling and basin strata characterization in conjunction with an inverse Monte Carlo approach of mixture modeling allows for partitioning of widespread and pervasive ages in sediment source terranes. 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.