GEODYNAMIC MODELS OF SLAB-DRIVEN UPPER PLATE DEFORMATION IN AN OBLIQUE SUBDUCTION ZONE SETTING

Geologic observations, geochemical constraints, and geophysical data indicate a two-dimensional paradigm for plate tectonic boundaries is no longer adequate to explain the observations. Although, many three-dimensional (3D) models have explored the effect of an indentor on upper plate deformation, the role of the slab in driving 3D deformation from below the upper plate is less understood. Results from high-resolution 3D geodynamic models of flat slab subduction in the oblique subduction zone setting of Alaska are presented. The geographically referenced numerical models examine the relative role of the driving force of the underlying flat slab and the resisting force of a lithospheric shear zone. In particular, the models vary the (a) slab-upper plate coupling and (b) strength, length, and width of the Denali fault shear zone and investigate how these factors modulate the along strike variations in surface plate motion, sense of motion on the Denali fault, and along strike variations in mountain building. Models with a Denali fault viscosity (10¹⁷ Pa s) four orders of magnitude lower than plate boundary viscosity (10²¹ Pa s) produce the fastest motion, with whole-scale motion of up to 10 mm/yr in the Wrangell block forearc sliver. The models predict laterally variable slip rates across the Denali fault, with the highest rates occurring along the central fault, consistent with observations. The results suggest the driving force of the flat slab contributes a minimum of 25% of long-term slip rates along the Denali fault. The predicted fault motion changes from fault parallel in the eastern segment, where the slab subducts parallel to the Denali fault, to convergent motion along the central segment of the fault, where the slab subducts at an oblique angle to the fault. In this way, the models show the flat slab drives the Wrangell block forearc sliver into the North American continental backstop of the Alaska interior, with the Alaska Range at this juncture.