Extension-driven right-lateral shear in the Centennial shear zone adjacent to the eastern Snake River Plain, Idaho

Abstract

We evaluate global positioning system (GPS) surface velocities and gravitational potential energy (GPE) variations to assess the causes of right-lateral shear in the Centennial shear zone, a NE-trending accommodation zone between the extensional Centennial tectonic belt (Montana-Idaho) and volcanic terrain of the eastern Snake River Plain (Idaho). We test the hypothesized “bookshelf” faulting model and find that the normal faults in the Centennial tectonic belt do not accommodate distributed dextral shear. Instead, GPS data reveal that rapid extension in the Centennial tectonic belt adjacent to the much more slowly deforming region of the Snake River Plain drives right-lateral shear between them at rates of 0.3–1.5 mm yr–1. GPE variations support gravitational collapse at a higher rate in the Centennial tectonic belt due to higher topography than in eastern Snake River Plain, which has lower GPE variations due to its low-relief, flat topography and a denser crustal composition. Surface velocity gradients observed in GPS data across the 40–45-km-wide Centennial shear zone reveal distributed deformation due to strike-slip faulting, distributed simple shear, regional-scale rotation, or some combination thereof. In the Centennial shear zone, the fastest lateral shearing is closest to the Yellowstone Plateau, where fault plane solutions with components of right-lateral strike slip are documented within a NE-trending zone of seismicity. Here, two Basin and Range normal faults have Holocene and late Pleistocene slip along their segments that suggest they each may have linked under right-lateral shear. We also propose that right-lateral strike-slip motion may be accommodated on existing NE-trending faults.