Rotation of the Microplates Within the Plate Boundary in Southwestern United States

Abstract

AbstractI investigate the long‐term, rigid motions of the 20 microplates identified by McCaffrey (2005, https://doi.org/10.1029/2004jb003307) within the Pacific‐North America plate boundary in southwestern United States. Those motions are described by the Euler vectors ( for the ith microplate) given by McCaffrey for each microplate. McCaffrey noticed that the Euler poles for those microplates were aligned along the great circle that connects the geometric center of the microplate distribution with the PACI pole, the pole of rotation of the Pacific Plate (PA) about the North American Plate (NA). To explain that alignment, Thatcher et al. (2016, https://doi.org/10.1002/2015jb0126678.0) proposed replacing each by two, vertical‐axis rotations of the microplate, one describing the trajectory (orbit) of its center of mass (CM) and the other its rotation (spin) about that CM, where . Moreover, they suggested that the orbital motion was being driven by drag from the rotating PA, which suggests that the poles coincide with the PACI pole. Then rotation vectors consistent with the given can be found for 17 of the microplates; the other 3 microplates are apparently affected by Basin‐and‐Range extension as well as PA relative motion. The long‐term motion of each of the 17 microplates then can be described as an orbital rotation about the PACI pole plus spin about the CM of the microplate. The closer the microplate CM is to the PA, the more nearly its orbital rotation rate approaches the rotation rate of the PA about the PACI pole.