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Southern California Geology

Geologic Setting of the Transverse Ranges Province

San Bernardino Mountains

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).

Mylonitic and cataclastic rock Mylonitic and cataclastic rock in the southeastern San Bernardino Mountains; pencil is about 6 in (15 cm) long. Photo by J.C. Matti. Click here for more info and enlarged image.

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.

Mylonitic and cataclastic rock Mylonitic and cataclastic rock in the Santa Rosa Mountains; pencil is about 6 in (15 cm) long. These rocks occur in the lower plate of the eastern Peninsular Ranges Shear Zone. Photo by J.C. Matti, USGS. Click here for more info and enlarged image.

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.

    deformed granite

    This is a view of deformed granitic rock like that occurring on the upper plate of the Vincent Thrust in the southeastern San Bernardino Mountains. This example from Antarctica shows deformed gneissose granitic rock with lensoidal bodies of older rock (boudins) shaped by shearing forces (image source: University of British Columbia Keck Geology Consortium Structural Geology Slide Set photograph Christine Smith)

     
    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.

Continue to Geologic Setting between the Mission Creek and Wilson Creek strands, San Andreas Fault

Selected References

Return to Transverse Ranges Province

Return to Geologic Setting and Geologic History

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