The Chugach-St. Elias orogen, Southeast Alaska/Southwest Yukon, formed in response to the ongoing Yakutat microplate subduction-collision with the North American Plate. Due to heavy glaciation, the region is a prime location to study active convergent orogenesis and climate-tectonic interactions. This study focuses on the long-term distribution of deformation in the St. Elias syntaxis area, where dextral motion along the plate-bounding Fairweather Fault transitions into convergence. We present 2718 new zircon fission-track single-grain ages from 26 glaciofluvial outwash samples. The grain ages range from 293 Ma to 0.2 Ma, and each sample contains two to five age populations with peaks between 267 ± 64 Ma and 1.2 ± 0.7 Ma (1σ). The rocks of the Yakutat microplate are dominated by latest Cretaceous and Eocene fission-track ages, while the rocks of the North American Plate in the syntaxis region reveal two exhumation phases at~5.1 Ma and 2.7 Ma. The spatial pattern of ≤5 Ma cooling ages shows that the area of rapid and deep exhumation at the St. Elias syntaxis is more extensive than previously known and confined to the south by the Fairweather Fault, to the west by the Seward Glacier catchment, and to the east possibly by the inferred Connector Fault. The area seems to be~4800 km 2 large and may extend farther to the north. The new zircon fission-track data further suggest a transpressional plate boundary since~30 Ma and the onset of plate collision 15-12 Ma.

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This study presents the long-term exhumation history of the Wrangellia composite terrane of the remote and ice-covered northern St. Elias Mountains in southwest Yukon, northwest British Columbia, and adjacent Alaska. Detrital zircon and apatite fission-track age distributions are presented from 21 glacial catchments. The detrital sampling approach allows for a large spatial coverage (~30,000 km 2) and access to material eroded beneath the ice. An additional five bedrock samples were dated by zircon fission-track analysis for a comparison with detrital results. Our new thermochronology data record the Late Jurassic-mid-Cretaceous accretion of the Wrangellia composite terrane to the former North American margin and magmatism, which reset the older thermal record. The good preservation of the Jurassic-Cretaceous record suggests that Cenozoic erosion must have been limited overall. Nonetheless, Eocene spreading-ridge subduction and Oligocene-Neogene cooling in response to the ongoing Yakutat flat-slab subduction are evident in the study area despite its inboard position from the active plate boundary. The results further indicate an area of rapid exhumation at the northern end of the Fairweather fault ca. 10-5 Ma; this area is bounded by discrete, unmapped structures. The area of rapid exhumation shifted southwest toward the plate boundary and the center of the St. Elias syntaxis after 5 Ma. Integrating the new data with published detrital thermochronology from the southern St. Elias Mountains reveals an evolving concentration of deformation and exhumation, possibly within a large-scale, transpressional structure providing important constraints for geodynamic models of syntaxes.

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.

We investigate the spatiotemporal evolution of exhumation in the ice-covered St. Elias syntaxis area, southeast Alaska, using multiple thermochronometers and geochronometers from cobble-sized glacial detritus. Multiple thermochronometers reveal the cooling histories from 500 to 60°C of 27 glacially transported cobbles from the two largest catchments of the syntaxis. Cobble lithologies and 21 zircon U-Pb ages (~277-31 Ma) were examined to determine sample provenance. Furthermore, eight amphibole and seven biotite 40 Ar/ 39 Ar ages (~276-16 Ma and~50-42 Ma, respectively), four zircon and six apatite (U-Th)/He ages (~35-4.8 Ma and~4.2-0.6 Ma, respectively), and four apatite fission track ages (~17-1.6 Ma) were used to reconstruct the individual cobble cooling histories. An additional four bedrock samples from the Fairweather Range yielded three biotite 40 Ar/ 39 Ar ages between~42 and 5 Ma. A compilation of published bedrock and new cobble cooling histories from the St. Elias Mountains and Fairweather Range reveals the regional Cenozoic cooling and exhumation history, emphasizing the position of the St. Elias syntaxis as a transitional zone between transpression and subduction settings. The new cobble and bedrock data indicate an onset of rapid exhumation at~5 Ma that was limited in duration (2-3 Myr) and amount (~10 km) in the syntaxial region. This study also demonstrates the usefulness of cobbles for revealing thermal histories of otherwise inaccessible regions as cobble analysis combines advantages of bedrock and detrital thermochronology.