Investigating the Effect of Mantle Flow and Viscosity Structure on Surface Velocities in Alaska Using 3-D Geodynamic Models

We utilize 3-D finite element geodynamic models, incorporating long-term kinematic estimates of upper plate motion, to better understand the roles that viscosity structure and mantle tractions play in generating plate motions and continental interior deformation in Alaska. Surface deformation in the Pacific-North American plate boundary zone in Alaska and northwest Canada is strongly influenced by the complex interactions between flat-slab subduction, gravitational collapse and mantle tractions. Predictions of long-term tectonic block motion derived from recent Global Positioning System datasets (GPS) reveal surface motions atypical of other continental convergent plate boundary zones. Specifically, in northern and northwestern Alaska, southeastward motion is observed directed back toward the plate boundary. Geodynamic models that incorporate a southeastward directed long-wavelength mantle traction of similar to 2.5-3.8 MPa best replicate surface velocities in Alaska. These mantle tractions, in conjunction with the collision of the Yakutat microplate, appear to drive the uplift and deformation in the Mackenzie Mountains. Furthermore, the extent of the northward motion in southern and central Alaska is controlled by the location of the leading edge of the Yakutat flat slab.