Alaska crustal deformation: Finite element modeling constrained by geologic and very long baseline interferometry data

Abstract

We compute crustal motions in Alaska by calculating the finite element solution for an elastic spherical shell problem. The method we use allows the finite element mesh to include faults and very long baseline interferometry (VLBI) baseline rates of change. Boundary conditions include Pacific‐North American (PA‐NA) plate motions. The solution is constrained by the oblique orientation of the Fairweather‐Queen Charlotte strike‐slip faults relative to the PA‐NA relative motion direction and the oblique orientation from normal convergence of the eastern Aleutian trench fault systems, as well as strike‐slip motion along the Denali and Totschunda fault systems. We explore the effects that a range of fault slip constraints and weighting of VLBI rates of change has on the solution. This allows us to test the motion on faults, such as the Denali fault, where there are conflicting reports on its present‐day slip rate. We find a pattern of displacements which produce fault motions generally consistent with geologic observations. The motion of the continuum has the general pattern of radial movement of crust to the NE away from the Fairweather‐Queen Charlotte fault systems in SE Alaska and Canada. This pattern of crustal motion is absorbed across the Mackenzie Mountains in NW Canada, with strike‐slip motion constrained along the Denali and Tintina fault systems. In south central Alaska and the Alaska forearc oblique convergence at the eastern Aleutian trench and the strike‐slip motion of the Denali fault system produce a counterclockwise pattern of motion which is partially absorbed along the Contact and related fault systems in southern Alaska and is partially extruded into the Bering Sea and into the forearc parallel the Aleutian trench from the Alaska Peninsula westward. Rates of motion and fault slip are small in western and northern Alaska, but the motions we compute are consistent with the senses of strike‐slip motion inferred geologically along the Kaltag, Kobuk Trench, and Thompson Creek faults and with the normal faulting observed in NW Alaska near Nome. The nonrigid behavior of our finite element solution produces patterns of motion that would not have been expected from rigid block models: strike‐slip faults can exist in a continuum that has motion mostly perpendicular to their strikes, and faults can exhibit along‐strike differences in magnitudes and directions.