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 forearc 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 (ca. 62–50 Ma) and an oceanic plateau (ca. 40–0 Ma). Samples from strata that post-date spreading-ridge subduction record expansion of forearc depositional systems into the retro-arc region during middle Eocene to late Oligocene time. Therefore, 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 forearc become more predominant over more proximal arc sources. Double-dated (U-Pb and fission track) zircons from these strata record widespread cooling in early Eocene time, which we interpret as a result of the cessation of profuse magmatism and crustal-scale cooling after passage of a slab window. Thus, spreading-ridge subduction 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 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. In addition, increased exhumation and erosion rates reflected by short lag times are interpreted to be a result of crustal thickening that results in widespread surface uplift as well as significant vertical uplift in rheological weak zones, such as along the Denali fault zone. 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.