Geologic Setting Southwest of the Mission Creek Strand, San Andreas Fault
In the southeastern San Bernardino Mountains, the Mission Creek strand of the San Andreas Fault separates a narrow terrane of San Gabriel Mountains-type crystalline rocks from Mojave Desert-type rocks in the main mass of the mountains (index map).
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| Photograph of mylonitic and cataclastic rock in the southeastern San Bernardino Mountains, southern California; pencil is about 6 inches (15 cm) long. These rocks occur in the upper plate of the Vincent Thrust, a zone of crustal movement and dislocation that underlies much of the Transverse Ranges Province. The original rock type probably was plutonic tonalite or quartz diorite that has been subjected to high-strain squeezing, stretching, and grinding of the mineral crystals to yield a rock whose original grain-to-grain relationships have been completely modified; such a high-strain rock is known as cataclasite or mylonite, depending on the brittle or ductile nature of the deformation style. Photo by J.C. Matti, USGS, July, 1979. |
The Mission Creek strand enters the San Bernardino Mountains from the Coachella Valley (Matti and others, 1982c), curves west and southwest to the headwaters of San Gorgonio River (Matti and others, 1983), and continues northwest along the margin of the mountains along a trace that is obscured beneath alluvium of the San Bernardino Valley (Matti and others, 1985, 1992a). This older trace has been reactivated by the modern San Bernardino strand of the San Andreas that lies along the base of the San Bernardino Mountains.
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| Photograph of mylonitic and cataclastic rock in the Santa Rosa Mountains, southern California; pencil is about 6 inches (15 cm) long. These rocks occur in the lower plate of the eastern Peninsular Ranges Shear Zone, a zone of crustal movement and dislocation that occurs along the east margin of the San Jacinto and Santa Rosa Mountains and in various locations within and east of the Borrego Desert. The original rock type was plutonic granodiorite and tonalite that has been subjected to high-strain squeezing, stretching, and grinding of the mineral crystals to yield a rock whose original grain-to-grain relationships have been completely modified; such a high-strain rock is known as cataclasite or mylonite, depending on the brittle or ductile nature of the deformation style. The conspicuous streaking parallel to the pencil forms what geologists call a "lineation", in this case the streaking out of light-colored (felsic) and dark-colored (mafic) minerals, quartz and biotite, respectively. The streaking probably reflects a stretching-type stress applied to the mineral crystals after they had cooled from their plutonic magma. Photo by J.C. Matti, USGS, June, 1980. |
Rocks outboard (south and west) of the Mission Creek Fault are not native to the San Bernardino Mountains. Instead, they have been faulted against the main mass of the range by displacements on the San Andreas Fault that brought the rocks into the region from their original position about 140 km farther southeast in the Coachella Valley (Farley, 1979; Matti and others, 1985, 1992; Dillon, 1975; Matti and Morton, 1993). These rocks are like those in the eastern San Gabriel Mountains (???) north of the Icehouse Canyon Fault. They form two distinct terranes separated by a steeply dipping thrust fault that is part of the region-wide Vincent-Orocopia-Chocolate Mountains Thrust system (Ehlig, 1981, p. 266-277; Jacobson, 1990, 1997; Jacobson and others, 1988):
- Lower-plate rocks. Lower-plate rocks of the Vincent-Orocopia Thrust crop out in a restricted area in the headwaters of San Gorgonio River. There, the rocks consist mainly of albite-actinolite-chlorite-epidote greenstone that probably represents basaltic flows and tuffs that have been metamorphosed to greenschist facies. Subordinate lithologies include metachert, metasiltstone, metasandstone, and minor carbonate rock. These rocks are similar to the Pelona Schist of the southeastern San Gabriel Mountains.
- Upper-plate rocks. Upper-plate rocks of the Vincent-Orocopia thrust are a lithologically monotonous assemblage that includes foliated and gneissose granitoid rocks, compositionally layered granitic gneiss, and pegmatite. Abundant epidote characterizes many of these rocks. The granitoid rocks and their gneissose equivalents have a range of compositions that includes leucocratic (light-colored) biotite granodiorite, hornblende-biotite quartz diorite and tonalite, granodioritic orthogneiss (metamorphosed igneous rock), and distinctive hornblende- and potassium-feldspar-bearing porphyritic granodiorite and monzogranite that is lithologically similar to the Mount Lowe Intrusion of the San Gabriel Mountains (Ehlig, 1981, p. 262-263, discusses the petrology and regional correlation of the unit; Joseph and others, 1982; Barth and Ehlig, 1988). Most of the plutonic rocks and most of the plutonic protoliths for the layered gneisses probably are Mesozoic in age, although bodies of Precambrian gneiss may be present.
Upper-plate crystalline rocks have been affected by one or more penetrative deformations that have crushed and sheared the rocks and have produced pervasive planar fabrics-including textural foliation, cataclastic and mylonitic foliation, and gneissose compositional layering. Mylonitic fabrics are especially well developed structurally low in the terrane near the Vincent-Orocopia Thrust. Following the last episodes of deformation, the crystalline rocks were intruded by volcanic dikes of hypabyssal dacite porphyry and porphyritic basalt.
As in the San Gabriel Mountains, the Vincent Thrust and underlying Pelona Schist probably occur deep in the subsurface of the southeastern San Berna rdino Mountains. While probable, this interpretation has not been documented.
Geologic Setting Between the Mission Creek and Wilson Creek Strands, San Andreas Fault
The Mission Creek strand and Wilson Creek strand of the San Andreas Fault bound a narrow slice of crystalline basement rocks and overlying Tertiary sedimentary rocks (index map). The crystalline rocks mainly are gneissose to foliated granodiorite that is associated with texturally massive dioritic to monzogranitic plutonic rock (Matti and others, 1983a, 1992b). These rocks locally are overlain unconformably by upper Cenozoic nonmarine sandstone and conglomerate of the Mill Creek Formation (of Gibson, 1964, 1971, as used by Matti and others, 1992b). The sedimentary rocks are well exposed along both sides of State Highway 38 in Mill Creek Canyon just upstream from the San Bernardino National Forest Mill Creek Ranger Station.
As with rocks outboard (southwest) of the Mission Creek strand, rocks outboard of the Wilson Creek strand are not native to the San Bernardino Mountains. They are generally similar to rocks in the Little San Bernardino Mountains, and appear to have been faulted against the San Bernardino Mountains by about 40 to 50 km of displacement on the Wilson Creek and Mill Creek strands of the San Andreas Fault (Matti and Morton, 1993; but see different interpretations by Hillenbrand, 1990, and Sadler and others, 1993).
Continue to Compressional Faults.
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