BUILDING A GEODYNAMIC MODEL OF ALASKA

Two outstanding questions in Alaskan tectonics are: What triggered the uplift of the central Alaska Range in the Late Miocene, and through what mechanism did this region of uplift become localized approximately 500 km inland from the modern plate boundary to the south. The three-dimensional nature of the plate boundary between the North American and Pacific plates in southern Alaska as well as Alaska's complex history of accreted terranes suggest that multiple factors played a role in both the initiation and localization of the uplift of the central Alaska Range. Our aim is to develop a suite of geodynamic models to test competing hypotheses for the uplift of the central Alaska Range at 5-6 Ma. We use the three-dimensional finite element code, CitcomT, to model the plate boundary system in southern Alaska. The model domain spans over 50 degrees in longitude and 30 degrees in latitude and extends over 1500 km in depth. The lithosphere and upper mantle are modeled as a viscous fluid that deforms on time scales of millions of years. The viscosity is governed by a composite (diffusion and dislocation creep) olivine flow law and a depth-dependent yield stress. The initial temperature is defined by a half-space cooling model and plate age. A smooth surface for the subducted plate, i.e., Aleutian and Wrangell Slab, is constructed from contours of the Benioff zone beneath the Aleutian islands and from existing earthquake hypocenter data and seismic reflection, refraction, and tomographic data from beneath south central Alaska. The top of the modeled slab surface extends to a maximum depth of 300 km. The models presented test the sensitivity of deformation patterns in the overriding plate to the plate margin geometry including the structure of the subducted slab. Subsequent models will test the relative importance of: a) lateral strength variations in the crust and lithosphere of the overriding plate corresponding to the position of the accreted terranes, b) a localized weak zone in the overriding plate corresponding to the position of the Denali Fault, c) collision of a buoyant block (Yakutat terrane), and d) change plate motion between the North American and Pacific plates.