Kinematic evidence for the effect of changing plate boundary conditions on the tectonics of the northern U.S. Rockies

We derive surface velocities from GPS sites in the interior Northwest U.S. relative to a fixed North American reference frame to investigate surface tectonic kinematics from the Snake River Plain (SRP) to the Canadian border. The Centennial Tectonic Belt (CTB) on the northern margin of the SRP exhibits west directed extensional velocity gradients and strain distributions similar to the main Basin and Range Province (BRP) suggesting that the CTB is part of the BRP. North of the CTB, however, the vergence of velocities relative to North America switches from westward to eastward along with a concomitant rotation of the principal stress axes based on available seismic focal mechanisms, revealing paired extension in the northern Rockies and shortening across the Rocky Mountain Front. This change in orientation of surface velocities suggests that the change in the boundary conditions on the western margin of North America influences the direction of gravitational collapse of Laramide thickened crust. Throughout the study region, fault slip rate estimates calculated from the new geodetic velocity field are consistently larger than previously reported fault slip rates determined from limited geomorphic and paleoseismic studies. Plain Language Summary This article uses over a decade of GPS and seismic data to understand the tectonics of the northern Rockies between the Snake River Plain and the Canadian border, east of the limit of the Cascadia subduction zone. Key Points The change in boundary conditions on western North America affects the interior Both tectonic boundary conditions and gravitational potential energy excite deformation in the northern Rockies Fault slip rates in the northern Rockies are higher than suggested by paleoseismic methods