THE ROLE OF THE DENALI FAULT IN LOCALIZING EXHUMATION IN THE ALASKA RANGE

Driving mechanisms for intracontinental exhumation and uplift are often ascribed to collision, but the first-order variables that determine where exhumation is focused are not well understood. The Neogene Alaska Range is generally assumed to have formed due to plate boundary processes, including the ongoing collision of the Yakutat block, but the AK Range's location 500 km inboard of the site of collision raises questions as to what is focusing the uplift. The peaks above ~ 1500 m form a relatively narrow, long topographic high that broadly follows the arcuate trace of the Denali fault, an active dextral fault.

We compare low- to moderate-T thermochronology and preliminary field observations with 3D high-resolution regional viscous flow models of the Alaska subduction-transform plate boundary system to examine the role of the Denali fault in focusing the exhumation. The models include the Denali Fault as a low viscosity shear zone (10²⁰ to 10²¹ Pa-s) through the lithosphere. We find a good spatial agreement to observed high topography in the Alaska Range in models that include a composite viscosity structure with non-Newtonian rheology. The model does not include a block with different buoyancy (Yakutat), but rather uses the existing shallow-dipping Pacific plate subduction geometry based on Benioff zone seismicity. These results suggest the flat slab beneath south central Alaska coupled with a weak shear zone in the overriding plate is important for transferring and localizing deformation inboard from the plate boundary.

Low-T thermochronology indicates that over the last 24 m.y. the amount of exhumation is highly variable along strike of the fault. K-feldspar Ar/Ar cooling ages and apatite fission track ages from granitoids in the central Alaska Range (Denali) suggest Miocene to recent exhumation is focused at a bend in the Denali fault, but 120 km to the east little exhumation has occurred since 38 Ma. Further east, in the eastern Alaska Range, a transition from low to high recent rates of exhumation coincides with the eastern termination of Benioff zone seismicity. The location of the youngest AFT ages (~1 Ma) is offset west of the region of greatest exhumation, as determined from K-feldspar and mica Ar/Ar cooling ages, indicating that the location of rapid exhumation along the Denali fault has shifted with time.