Geologic structure and exhumation accompanying Yakutat terrane collision, southern Alaska

Graduation date: 2006The climatic and tectonic framework of the St. Elias orogen makes it an excellent place to study the interactions between tectonic processes such as deformation and erosive processes, in particular glacial erosion. One type of link between these processes, proposed by numerical models of orogenic development, is an effect of orographic rainfall on the distribution of erosion, exhumation, strain, and the motion of rock through an orogen. The St. Elias orogen is investigated with the goal of quantifying exhumation rates, patterns, and magnitudes, while a structural interpretation of the foreland is proposed. New apatite and zircon fission-track data are combined with existing apatite and zircon (U-Th)/He data. Together, the apatite fission-track and (U-Th)/He data show a trend of increasing age with distance from the coast, implying a contrast in the rate of exhumation across terrane boundaries from the windward to leeward sides of the orogen for at least the last 5-6 Myr. Exhumation rates based on these chronometers are as high as -3mm/yr, but are generally modest (< 1 mm/yr), and significantly smaller than short-term estimates based on sediment yields. Both zircon (U-Th)/He and fission-track ages are largely unreset, suggesting that stratigraphic and/or structural burial of the deepest parts of the active wedge did not exceed the zircon partial annealing zone (~10 km depth assuming 25 C/km). This estimate is less than but similar to estimates of current wedge thickness and stratigraphic reconstruction based on a balanced cross section. The cross section also indicates 81 km of shortening, or about 47% shortening has been absorbed within the fold and thrust belt of the Yakutat terrane. A present snapshot of material within this wedge, based on the cross section, indicates approximately 64% of the crosssectional area is part of the actively deforming orogen, while 36% is subducting. Together, the thermochronology and the cross section suggest that the fold and thrust belt has absorbed very little of the total terrane convergence, which is as high as 600 km by some estimates. If total convergence estimates are correct, then a significant amount of material has been subducted, or the stratigraphic influx was low prior to the last 5-6 Myr