Description
The study area is located in the San Vicente Reservoir 7-1/2 minute quadrangle, San Diego County, California, in the western part of the Peninsular Ranges province. Basement rocks in the Peninsular Ranges of southern California include Cretaceous batholithic rocks which intrude earlier metamorphic (prebatholithic) rocks. Prebatholithic rocks in the study area include gneissic granodiorite and metavolcanic rocks. The gneissic granodiorite displays some textural features characteristic of cataclastic rocks. Mafic minerals in the gneissic granodiorite are segregated into lenticular pods; preferred orientation of these mafic pods defines a foliation (gneissosity) and a lineation. The metavolcanic rocks are tentatively correlated to the Santiago Peak Volcanics. The metavolcanic rocks are gently folded in some areas; with increased deformation the folds become tighter and bedding is boudinaged and transposed. Batholithic rocks are undeformed and include gabbroic rocks, tonalitic rocks, and granodiorite. Foliations in the prebatholithic rocks maintain a very consistent north northwest trend and subvertical dip throughout the study area. Orientations of lineations vary considerably. This is interpreted to mean that the stress field which produced these structures was dominated by an east northeast-directed compressive force. Deformation of the prebatholithic rocks was most intense in a north northwest-trending zone about one kilometer wide. The tectonic evolution of the study area and adjacent areas began in Triassic time with an eastward dipping subduction zone east of the present position of the Peninsular Ranges, at the margin of the North American craton. This resulted in intracratonic magmatism in Triassic and Early Jurassic time in southern Arizona and Sonora. At the same time, sediments shed from the craton covered the trench and formed a clastic apron which was protolith for the Bedford Canyon Formation and Julian Schist. Another eastward dipping subduction zone developed west of and subparallel to the continental margin subduction zone. This subduction zone formed within oceanic lithosphere and resulted in the development of an intraoceanic magmatic arc represented now by the gneissic granodiorite and the metavolcanic rocks. Continued convergence at the eastern (continental margin) subduction zone caused the intraoceanic arc to collide with the craton about 150-143 m.y. ago. This resulted in deformation of the clastic apron and the intraoceanic arc, and accretion of these onto the continent. All post-collision convergence occurred at the western subduction zone, and magmas generated above this subduction zone intruded the deformed pile to form the Peninsular Ranges batholith.
