The Trans Mojave-Sierran shear zone and its role in Early Miocene collapse of southwestern North America

Abstract

Abstract The relative motion between the Pacific and North American plates in early Miocene time was not parallel to the overall NW strike of the transform, but was instead oblique and transtensional. It has recently been proposed that in response to this divergence, the western edge of the North American plate east of the transform gravitationally collapsed and moved 100–150 km to the southwest (S50°–60°W). The region of collapse covered an area of nearly 10 6 km 2 and included what is now southern California, southwestern Arizona, and northwestern Mexico. A major structure facilitating collapse between 21 and 18 Ma was the Trans Mojave-Sierran shear zone (TMSSZ). This east-west shear zone linked the classic detachment fault terranes and metamorphic core complexes of the Mojave desert, southeastern California, southern Arizona, and Sonora, Mexico, to the transtensional plate boundary. To more fully understand the nature and kinematic significance of the TMSSZ and its role in facilitating early Miocene fragmentation of the North American plate, a palinspastic reconstruction of the Mojave desert was performed to remove the disruptive effects of the TMSSZ and younger tectonic events. Features formed just before movements along the TMSSZ were used as markers to assess the TMSSZ deformation. Our analysis indicates that TMSSZ deformation was distributed across a c. 90 km wide band; restoration of markers to their original positions implies that >80 km of dextral shear occurred along the TMSSZ. First-order dextral shear deformation within the TMSSZ is expressed internally by clockwise vertical axis rotations of large areas that were facilitated by second-order zones of sinistral shear that separated the blocks. These second-order sinistral zones apparently exploited older transfer zones of the 24-21 Ma Mojave Extensional Belt.