IS THERE A CASE FOR A MAJOR LATE JURASSIC SHEAR ZONE IN THE PENINSULAR RANGES?

Near Winchester, CA, strongly foliated rocks crop out among foliated and unfoliated plutonic rocks of the southern California batholith (Morton, 2003). The plutons intrude two distinct rock suites separated by a major “suture” or fault, which strikes north-northwest. To the east the rocks include mafic and intermediate schist, amphibolite, and augen gneiss derived from porphyritic granodiorite. To the west the exposed rocks consist of quartzose sandy and silty rocks and matrix-rich conglomerate probably derived from debris flows. Southwest of Winchester in French Valley and near Julian, the youngest interpreted ages of detrital zircon from metasedimentary units are Late Triassic (Kimbrough et al., 2007). The structural relations near Winchester resemble those along the Mojave-Sonora megashear in NW Mexico where Late Triassic and older strata of the Caborca block crop out west of mid-Jurassic volcanic rocks. However, the “suture” has not been mapped beyond the Winchester quadrangle nor has the linkage between inferred fault segments been shown. Nevertheless, thinly laminated gneisses and schists formed by recrystallization of mylonitic rocks extend south where they are mapped as part of the Julian Schist. Locally, the fine-grained, strongly foliated, metasediment containing rootless folds comprises inclusions in moderately foliated Late Jurassic granitoids (Todd, 1995). Southeast of Julian, much deformation in the schist is attributed to Cretaceous tectonism (Thomson and Girty, 1994). However, diverse orientations of foliation in the inclusions and moderately plunging fold hinges commonly recording sinistral shear show rotation of previously deformed rocks. Powell (1993) has shown that carbonate-rich sedimentary sections and associated basement in the Transverse Ranges and Mojave Desert distinguish domains that may be recognized in the Santa Rosa-San Jacinto Mts. These domains provide constraints on the position of major pre-Tertiary fault structures. Restoration of displacement along the San Andreas fault reveals a significant westward jog in the inferred MSms trace, which may reflect a Jurassic releasing bend, southwestward-directed Cretaceous faulting, Tertiary extension or some combination of these.