Laramide Shortening in the Clark Mountain Thrust Complex: Reinterpretation of the Timing of the Winters Pass Thrust, Mojave Desert, SE California

The transition to flat slab subduction, around ~90-80 Ma during the Laramide orogeny, marks a pivotal shift in the style of deformation for the North American Cordillera. Late Cretaceous deformation associated with the change in subduction angle is observed throughout the Mojave, paradoxically manifesting as extensional strain deforming the thickened crust formed during the Sevier orogeny. The Winters Pass thrust, located in the Mesquite Mountains of southeastern California, has long been regarded to be the southern continuation of the Wheeler Pass thrust of the NW Spring Mountains, 50 km to the north, and the northern continuation of the Pachalka thrust, 20 km to the south in the Clark Mountains. This study uses 40Ar 39Ar geochronology on synkinematic muscovite to provide direct geochronologic constraints on mylonitic deformation for the Winters Pass, Powerline Road and Pachalka thrusts. Structural data paired with kinematic and microstructural data from petrographic and SEM observations, and data collected with electron backscatter diffraction on recrystallized quartz allows for temperatures of deformation, the geometry of the thrust complex and transport direction of the upper-plate(s) to be determined. This study provides deformation timing constraints of ~73 Ma for the Winters Pass thrust, ~81 Ma for the Powerline Road thrust, and looser constraints on the Pachalka thrust of ~70-85 Ma. Structural data shows an overall NE-E direction of transport for the upper plate. These data suggest that these faults record a Laramide thrust complex with a backwards-breaking thrust sequence and are not correlative to any thrusts of the southern Sevier fold-thrust-belt in the Spring Mountains. We discuss possible drivers of contractional strain in the Mojave during this time and suggest that subduction of the conjugate Shatsky Rise paired with the proximity of the paleo rift-margin played a critical role in the unique style and age of deformation that is reported herein for this portion of the eastern Mojave Desert.