EXTENSIONAL REACTIVATION OF THE CHOCOLATE MOUNTAINS SUBDUCTION THRUSTIN THE GAVILAN HILLS OF SOUTHEASTERN CALIFORNIA

The NE vergent Chocolate Mountains fault of southeastern California ha s been interpreted as either a subduction thrust responsible for buria l and prograde metamorphism of the ensimatic Orocopia Schist or as a n ormal fault involved in the exhumation of the schist. Our detailed str uctural analysis in the Gavilan Hills area provides new evidence to co nfirm the latter view. A zone of deformation is present at the top of the Orocopia Schist in which lineations are parallel to those in the u pper plate of the Chocolate Mountains fault but oblique to ones at rel atively deep levels in the schist. Both the Orocopia Schist and upper plate contain several generations of shear zones that show a transitio n from crystalloblastic through mylonitic to cataclastic textures. The se structures formed during retrograde metamorphism and are considered to record the exhumation of the Orocopia Schist during early Tertiary time as a result of subduction return flow. The Gatuna fault, which p laces low-grade, supracrustal metasediments of the Winterhaven Formati on above the gneisses of the upper plate, also seems to have been acti ve at this time. Final unroofing of the Orocopia Schist occurred durin g early to middle Miocene regional extension and may have involved a s econd phase of movement on the Gatuna fault. Formation of the Chocolat e Mountains fault during exhumation indicates that its top-to-the-NE s ense of movement provides no constraint on the polarity of the Orocopi a Schist subduction zone. This weakens the case for a previous model i nvolving SW dipping subduction, while providing support for the view t hat the Orocopia Schist is a correlative of the Franciscan Complex.