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

Several features of the Independence dike swarm (IDS) indicate that dikes o pened obliquely to their walls in a sinistral sense. The IDS is largely lat est Jurassic in age (approximate to 148 Ma) and has been traced for more th an 600 km along its NW-SE strike from east-central to southern California. Field and petrographic observations from the IDS in the Sierra Nevada indic ate that: (i) some, and perhaps all, of the dikes initially opened perpendi cular to their margins and then were sinistrally sheared; and (ii) sinistra l displacement occurred during and shortly after dike injection. Dike empla cement overlapped in time with the formation of wall rock mylonite zones wh ich accommodated shear varying in sense from sinistral to west-side-down re verse. We suggest that the IDS intruded dilatant fractures within a regiona l sinistral shear system. The swarm is located within or along the eastern flank of the Cordilleran subduction-related magmatic are. The IDS and assoc iated wall rock deformation may record partitioning into the magmatic are o f the sinistral component of strongly oblique Late Jurassic subduction. (C) 1999 Elsevier Science Ltd, All rights reserved.