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Southern California Geology


Geologic Setting of the Transverse Ranges Province

San Gabriel Mountains

This web-page narrative on the geologic setting of the San Gabriel Mountains is taken or paraphrased from the following sources: Bailey and Jahns (1954); Dibblee (1982a); Ehlig (1981, 1982); Joseph and Others (1982); Matti and others (1992a); Matti and Morton (1993); Miller (1926, 1934, 1946); Morton (1975, 1983); Morton and Matti (1987; 1991a, 1991b).

General Summary

a space-shuttle image of the eastern and central San Gabriel Mountains
Click on this thumbnail to view a space-shuttle image of the eastern and central San Gabriel Mountains (source: NASA photograph STS073-744-051, October, 1995, from the NASA Earth from Space archive of space-shuttle images).

The San Gabriel Mountains are a fault-bounded block of ancient crystalline rocks that rises north of the Los Angeles Basin and the upper Santa Ana River Basin. The eastern end of the mountains rises abruptly to an elevation of over 10,000 feet. To the north, the mountains descend more gradually to the Mojave Desert and to the west to the Sierra Pelona and the Soledad Basin. The range is bounded on the north by the San Andreas Fault zone, on the south and southwest by thrust and reverse faults of the Cucamonga-Sierra Madre fault complex, and on the east by faults of the San Jacinto zone. The interior of the range is complexly deformed by faults of many different ages and tectonic styles.

Most of the crystalline basement rocks that make up the San Gabriel Mountains occur in two packages that are separated by a major geologic structure--the Vincent Thrust . The thrust apparently occurs throughout most of the range, and is a low-angle tectonic dislocation (fault or movement zone) that separates upper-plate rocks above the dislocation from lower-plate rocks below it. Fault movement along this low-angle zone may have been on the order of several tens of kilometers, and is responsible for bringing together the two distinctive packages of rock in the lower and upper plates.

Lower-plate rocks beneath the Vincent Thrust are a complex of metamorphosed sedimentary and volcanic rocks known as the Pelona Schist, a rock unit whose pre-metamorphic protolith consisted of Mesozoic (Jurassic and Cretaceous?) marine deep-water sand, silt, and calcareous and siliceous mud locally interlayered with basaltic flows. These rocks were metamorphosed at low to moderate grades (greenschist to lower amphibolite grade) in late Cretaceous or early Paleozoic time.

Upper-plate rocks above the Vincent Thrust include very old (Proterozoic) metamorphic and plutonic rocks that originally formed part of the ancient North American continental platform (Mendenhall Gneiss, anorthosite-syenite-gabbro complex). These older rocks are most abundant in the western and central part of the range. The ancient rocks have been intruded by various Mesozoic plutonic rocks that occur throughout the range, but are most abundant in the western and eastern parts.

A terrane of metamorphosed sedimentary rock and associated plutonic rocks and high grade (granulite grade) metamorphic rocks in the southeasternmost San Gabriel Mountains is overprinted by a distinctive belt of mylonitic deformation locally intense enough to generate mappable thicknesses of mylonite. Geologists still have not resolved how these rocks relate structurally and provincially to crystalline rocks in the main mass of the San Gabriel Mountains.

The San Gabriel Mountains are traversed by deep, steep-sided canyons cut into highly fractured crystalline basement rocks that form the bedrock underpinnings of the mountains. The sides of most canyons are blanketed by unstable hill-slope rock debris that constantly is being stripped away by slope failures and by runoff and washed out to the range fronts, where sediment is deposited on surfaces and channels of alluvial fans.

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This site last updated September 3, 2004 (ps)

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