Kinematics and paleogeology of the western United States and northern Mexico computed from geologic and paleomagnetic data: 0 to 48 Ma

Abstract

Fault traces and offsets, cross-section length changes, paleomagnetic inclination and declination anomalies, and stress-direction indicators with ages back to 90 Ma are collected from the geologic literature on the western United States and northern Mexico. Finite-element program Restore simulates paleokinematics by weighted least squares and integrates displacements, strains, and rotations back in time, producing paleogeologic maps, as well as maps of velocity, heave rate, strain rate, and stress direction at 6 m.y. intervals. After calibrating three program parameters against neotectonic velocities from geodesy, all classes of data except inclination anomalies are fit reasonably well. The kink in the San Andreas fault near San Gorgonio Pass has been gradually restored by slip on adjacent faults and automated smoothing. Piercing-point pairs successfully restored along the San Andreas–Gulf of California plate boundary include the Pelona and Orocopia Schists at 6 Ma, the Pinnacles and Neenach Volcanics at 21 Ma, and the Jolla Vieja and Poway conglomerates adjacent to their Sonoran source at 48–42 Ma. During 18–6 Ma, rapid extension on the Oceanside detachment fault system was restored, placing present San Nicolas Island adjacent to present Rosarito, Baja California, at 18 Ma. Since ca. 18 Ma, the western Transverse Ranges have rotated 70° clockwise, restoration of which implies that sinistral faults in this province originated with NNE trends. The first contact between the Pacific and North America plates at ca. 28 Ma was not associated with any dramatic increase in dextral faulting on land; instead, the primary result was extension in the Plush Ranch–Vasquez-Diligencia basins and Colorado River corridor, probably driven by an unstable triple-junction and accelerated by heating and uplift of North America above enlarging slab windows.