North American dynamics and Western US tectonics

Interest and controversy exist on the origin of forces that move and tectonically deform plates, especially regarding the relative importance of loads applied to the plate margins and base and those created internally (e. g., by elevated potential energy in uplifted regions). To quantify these loads, we evaluate predicted interplate stress through two-dimensional finite element analysis of the North American plate, finding that boundary loads are most important, followed by internal and basal loads. Craton root basal drag of similar to 4 MPa opposes absolute plate motion, compared to basal tractions elsewhere that average similar to 0.4 MPa, suggesting that North America is separated from a relatively static deep Earth mantle by a weak asthenosphere. San Andreas shear (similar to 1.5 TN/m), gravitational collapse, and southern Cascadia pull all contribute importantly to western U. S. deformation; the region also is relatively weak. Important future work includes incorporating three-dimensional plate structure onto global flow calculations and including the global set of plates.