LONG-TERM FOREARC BASIN EVOLUTION IN RESPONSE TO CHANGING SUBDUCTION STYLES IN SOUTHERN ALASKA

Detrital zircon U-Pb and fission track double-dating and Hf isotopes from the Mesozoic and Cenozoic strata in the southern Alaska fore-arc basin system reveal the effects of two different modes of flat-slab subduction on the evolution of the overriding plate. The southern margin of Alaska has experienced subduction of a spreading-ridge (~62–50 Ma) and an oceanic plateau (~40–0 Ma). When a subducting spreading ridge drives slab flattening, our data suggest that after the ridge has moved along strike retro-arc sediment sources to the fore arc become more predominant over more proximal arc sources. Spreading-ridge subduction also results in thermal resetting of rocks in the upper plate that is revealed by thermochronologic data that record the presence of young age peaks found in subsequent, thin sedimentary strata in the fore-arc 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 surface uplift 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. The cessation of volcanism associated with plateau subduction limits the number of syndepositional volcanic grains that produce identical geochronologic and thermochronologic ages. This study demonstrates the merit of double-dating techniques integrated with stratigraphic studies to expose exhumational age signatures diagnostic of large-scale tectonic processes in magmatic regions