Description
The Elsinore fault, an important structural element of the San Andreas fault system, trends ~N50W through southern California. Estimated offsets along the 200 km length of the fault vary from 10 - 40 km near Corona, California to 4 -5 km near Ocotillo, California. Southeast of Banner, California, the Elsinore fault juxtaposes the prebatholithic metamorphic terranes and plutonic rocks of Granite Mountain on the east to that of Chariot Mountain on the west by pervasive brittle deformation, resulting in anastomosing, short fault segments. The prebatholithic host rocks juxtaposed across the Elsinore fault have undergone amphibolite-grade metamorphism and consist of stromatic migmatite (Mst), sillimanite-bearing migmatitic gneiss and schist (Mg-s), and quartzo-feldspathic gneiss, mica schist, and quartzite (G). These units are separated by gradational boundaries and occur in the same order in a direction away from the plutonic rocks on both sides of the fault. Roof pendants of calc-silicate quartzite and minor amounts of amphibolite also occur throughout the study region. The stromatic migmatites, located adjacent to the plutonic rocks, consist of well-developed granitic and trondhjemitic leucosomes, melanosomes, and mesosomes and are interpreted to have been generated from in situ partial melting of a heterogeneous(?) metapelitic source via a fractional fusion model during the emplacement of the plutonic rocks. The plutonic rocks juxtaposed across the Elsinore fault have been divided into three distinct mineralogical zones that have gradational internal boundaries. From the margins inward, the zones consist of a metaluminous biotite-hornblende tonalite, a metaluminous hornblende-biotite tonalite, and a peraluminous biotite granodiorite/tonalite. These zones are referred to as the Hornblende-Biotite (HB), Large-Biotite (LB), and Small-Biotite (SB) facies, respectively. Well-developed foliation occurs near the margins of the pluton but decreases rapidly toward the core. This fabric is interpreted to represent primary flow structure that resulted from the emplacement of the pluton. The geochemical, petrographic, structural, and field relationships suggest that the crystalline rocks juxtaposed across the Elsinore fault, in the study area, are portions of the same pluton and that this pluton has been defined as a post-tectonic La Posta-type pluton. The N35W-trending Chariot Canyon fault, located along the western border of the field area, is truncated by the Elsinore fault near the town of Banner and is characterized by both brittle and brittle-ductile deformation. The brittle-ductile deformation is interpreted to represent an older and deeper shear zone that has been exhumed whereas the brittle deformation indicates that recent right-lateral strike-slip movement has occurred along this same fault zone. The right-lateral strike-slip movement of the Elsinore fault has separated the Granite Mountain pluton, the surrounding prebatholithic metamorphic rocks, and the Chariot Canyon fault by approximately 2.5 km or less. Vertical uplift of the northeast side of the Elsinore fault has also occurred. The Chariot Canyon fault, located to the west, and the San Felipe and Earthquake Valley faults, located to the east, have apparently accommodated some of the right-lateral movement of the Elsinore fault zone.
