THREE-DIMENSIONAL NUMERICAL MODELS OF FLAT SLAB SUBDUCTION, THE DENALI FAULT, AND OVERRIDING PLATE DEFORMATION IN ALASKA

Early theories of plate tectonics assume that plates are rigid with deformation limited to within a few hundred kilometers of the plate boundary. However, observations indicate most continental plates defy such rigid behavior and deformation can extend over 1000 kilometers from the plate boundary. We use regional 3D numerical modeling of the Pacific-North American plate boundary in Alaska to investigate the relative controls of slab geometry, continental scale faulting, and a non-linear rheology on overriding plate deformation. The shape of the subducting Aleutian slab is defined by Wadati-Benioff zone seismicity and seismic tomography. The thermal structure for both the subducting and overriding plate is based on geologic and geophysical observables, thereby capturing the regional variability in the plate boundary system. Viscosities on the order of 10²⁰ Pa s at the plate boundary which increase to 10²⁴ Pa s inboard emerge in the non-Newtonian models, thus allowing the lithosphere at the plate margin to weaken and plate interior to strengthen. The inclusion of a lithospheric shear zone, the Denali fault, enables the portion of overriding plate between the Denali fault and the trench to partially decouple from the rest of the overriding plate, forming an independently moving region that correlates to what has been described from geologic and geodetic studies as the Wrangell block. Models with the non-Newtonian rheology predict strike-slip motion along southeastern portion of the Denali fault that transitions to compressional motion along the eastern and northern bend in the Denali fault, consistent with thermochronologic data that show significant late Neogene exhumation in portions of the central Alaska Range, including Mt. McKinley which is the tallest mountain in North America. These 3D numerical models of the Pacific-North American margin in Alaska show the subducting Pacific slab is the main driver of overriding plate deformation in south central Alaska.