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USGS Banner with Coachella Valley as seen from Keyes View in Joshua Tree National Park
Western Earth Surface Processes Team

San Andreas Fault System in the Inland Empire and Salton Trough

Major Structural Blocks

The diverse basement terranes of southwestern California can be grouped into rocks of Peninsular Ranges-type, San Gabriel Mountains-type, and San Bernardino Mountains-type (fig. 2).

Basement rocks of San Gabriel Mountains -type (fig. 2A) consist of two crustal layers separated by a low-angle tectonic contact the Vincent thrust (Ehlig, 1982). The upper thrust plate consists of Mesozoic plutons of various compositions, ages, and deformational styles that have intruded prebatholithic crystalline rocks. One distinctive Mesozoic granitoid unit is the Lowe pluton of Triassic age (Joseph and others, 1982a). The prebatholithic rocks largely are Proterozoic, and include orthogneisses and the anorthosite-syenite complex that is so well known from the western San Gabriel Mountains (Silver, 1971; Ehlig, 1981; Carter, 1982). The lower plate of the Vincent thrust consists of Pelona Schist Mesozoic quartzofeldspathic sandstone and siltstone, limestone, quartzite, chert, and mafic volcanic rocks that have been metamorphosed to greenschist and lower amphibolite facies, presumably during late Mesozoic to early Tertiary emplacement of the upper plate (Ehlig, 1968b, 1981, 1982). From the Frazier Mountain region southeast to the Salton Trough, Pelona Schist occurs as windows and fault-bounded blocks (Ehlig, 1968b), including: (1) the Sierra Pelona window in the western San Gabriel Mountains; (2) the Lytle Creek window in the eastern San Gabriel Mountains; (3) the Blue Ridge slice between the Punchbowl and San Andreas faults; (4) a large window that mostly has been buried beneath sedimentary fill of the San Bernardino Valley; and (5) the Orocopia Mountains and Chocolate Mountains windows of Pelona-type schist that are referred to as Orocopia Schist and that are separated from rocks of San Gabriel Mountains-type by the Orocopia-Chocolate Mountain thrust. Both lower and upper plates are intruded by high-level Miocene granitoid plutons and dikes that were emplaced after initial juxtaposition of the two plates (Miller and Morton, 1977; Crowe and others, 1979).

Basement rocks of Peninsular Ranges -type (fig. 2B) consist of Jurassic and Cretaceous granitoid rocks (granodiorite, quartz diorite, tonalite, gabbro) that have intruded prebatholithic metasedimentary rocks (pelitic schist, metaquartzite, marble, quartzofeldspathic gneiss and schist). Along its north and northeast edge, the Peninsular Ranges block is bordered by a mylonitic belt of ductile deformation that separates lower plate plutonic and metasedimentary rocks of typical Peninsular Ranges type from broadly similar upper-plate rocks that appear to be parautochthonous equivalents of the more typical Peninsular Ranges suite; the autochthonous and parautochthonous suites have been telescoped along the mylonite zone (Sharp, 1979; Erskine, 1985). The ductile zone is referred to by different names locally, but Sharp (1979) recognized its regional significance and named it the Eastern Peninsular Ranges mylonite zone. The ductile zone probably continues westward into the southeastern San Gabriel Mountains, where mylonitic rocks have been mapped a few km north of the Mountain front (Alf, 1948; Hsu, 1955; Morton and Matti, 1987); if so, then the San Gabriel mylonitic belt may separate lower and upper plate rocks of Peninsular Ranges type a speculative proposal we adopt here. Juxtaposition of Peninsular Ranges-type and San Gabriel Mountains-type rocks in the southeastern San Gabriel Mountains represents either (1) original intrusive relations between the two suites or (2) their tectonic juxtaposition by Neogene strike-slip displacements or (3) their tectonic juxtaposition by Paleogene or latest Cretaceous thrust faulting (May and Walker, 1989), or (4) a combination of some or all of these mechanisms.

Basement rocks of San Bernardino Mountains -type (fig. 2C) are similar to those in the Mojave Desert, and consist of Triassic through Cretaceous granitoid rocks of various compositions that have intruded prebatholithic orthogneiss (Proterozoic) and metasedimentary rocks (late Proterozoic and Paleozoic metaquartzite, marble, pelitic schist, and gneiss). The metasedimentary rocks are comparable with rocks of the Cordilleran miogeocline (Stewart and Poole, 1975). The Mesozoic plutonic rocks include both deformed and undeformed suites that extend southeastward into the Little San Bernardino Mountains, where they intrude rocks of San Gabriel Mountains-type. Strongly deformed Mesozoic granodiorite, tonalite, and quartz diorite form a discrete belt in the southeastern San Bernardino Mountains and along the western margin of the Little San Bernardino Mountains. Like rocks of San Gabriel Mountains-type, rocks of San Bernardino Mountains-type may be a layered terrane with batholithic and prebatholithic rocks in an upper plate separated from Pelona Schist in a lower plate by a low-angle fault comparable to the Vincent thrust.

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