Oblique opening and noncoaxial emplacement of the Jurassic Independence dike swarm, California

Several features of the Independence dike swarm (IDS) indicate that dikes opened obliquely to their walls in a sinistral sense. The IDS is largely latest Jurassic in age (≈148 Ma) and has been traced for more than 600 km along its NW–SE strike from east-central to southern California. Field and petrographic observations from the IDS in the Sierra Nevada indicate that: (i) some, and perhaps all, of the dikes initially opened perpendicular to their margins and then were sinistrally sheared; and (ii) sinistral displacement occurred during and shortly after dike injection. Dike emplacement overlapped in time with the formation of wall rock mylonite zones which accommodated shear varying in sense from sinistral to west-side-down reverse. We suggest that the IDS intruded dilatant fractures within a regional sinistral shear system. The swarm is located within or along the eastern flank of the Cordilleran subduction-related magmatic arc. The IDS and associated wall rock deformation may record partitioning into the magmatic arc of the sinistral component of strongly oblique Late Jurassic subduction.