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Southern California Geology

 

Geologic Setting of the Transverse Ranges Province

San Gabriel Mountains

Vincent Thrust

The Vincent Thrust apparently was first recognized by Levi Noble (unpublished mapping, 1928, not published until 1954), and later was recognized independently by Ehlig (1958) who first appreciated its regional importance. The thrust and its lower- and upper-plate rock masses can be observed directly in the eastern part of the San Gabriel Mountains; elsewhere in the range, it is inferred to occur in the subsurface. The thrust is well exposed in the lower part of Coldwater Canyon in the North Fork of Lytle Creek, and on the north side of Mount San Antonio; in these places, the fault forms a conspicuous, discrete plane between rocks above and below it.

The Vincent Thrust brought the upper-plate and lower-plate rock masses together in Late Cretaceous or earliest Tertiary time (Conrad and Davis, 1977; Ehlig, 1981, 1982). This event apparently was not confined to rocks now exposed in the San Gabriel Mountains but is a regionally-developed zone of crustal decoupling that probably underlies much of the Transverse Ranges and Mojave Desert provinces of southern California (Haxel and Dillon, 1978; Ehlig, 1982; Jacobson, 1990, 1997; Jacobson and Dawson, 1995; Jacobson and others, 1988, 1996; Nourse, 1989).


Lower-Plate Rocks

Pelona Schist
Photograph of metasedimentary schist like Pelona Schist that occurs in the lower plate of the Vincent Thrust in the southeastern San Gabriel Mountains. This example from the Mt. Rushmore area shows schist interlayered with a granitic dike (image source: University of British Columbia Keck Geology Consortium Structural Geology Slide Set, photograph Julie Maxson)
 

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. Where it crops out in the eastern San Gabriel Mountains, Pelona Schist occurs in two distinct blocks separated by the Punchbowl strand of the San Andreas Fault:

  • Blue Ridge block. --Northeast of the Punchbowl Fault, Pelona Schist consists of medium- to coarse-grained white-mica schist of upper greenschist and lower amphibolite grade. Greenstone metabasalt layers are common, and quartzite and metacarbonate layers are rare. Garnet and hornblende are common in mafic rocks, most noticeably in the greenstones.
  • Lytle Creek block .--Southwest of the Punchbowl Fault, Pelona Schist is fine-to medium-grained and is metamorphosed only to greenschist grade. Spotted albite-white mica schist predominates, but locally the schist contains tightly and complexly folded quartzite-carbonate layers. Quartzite commonly contains convoluted layers rich in manganese-aluminum garnet (spessartine) and barium sulfate (barite). Biotite is rare in this part of the Pelona Schist, but the biotite look-alike stilpnomelane is widespread. Blocks of serpentine, actinolite and, in some places, small amounts of chromium mica (fuchsite) are scattered in both the amphibolite- and greenschist-grade schist. Small masses of manganese silicate (rhodonite) and occurrences of manganese epidote (piedmontite) are rare. Beneath the Vincent thrust, structurally highest parts of the Pelona Schist are dominated by greenstone that consists of albite, epidote, chlorite, and amphibole.

Upper-Plate Rocks

Western and central San Gabriel Mountains .-Upper-plate rocks above the Vincent Thrust include:

  • the Proterozoic Mendenhall Gneiss and associated rocks on the order of 1.6 billion years old
  • a Proterozoic anorthosite complex on the order of 1.3 billion years old (Carter, 1980, 1981; Carter and Silver, 1982)
  • various Mesozoic granitoid rocks that intrude these older terranes. The latter include a distinctive suite of Triassic granitic rocks grouped within the Mt. Lowe Igneous Pluton (Joseph and others, 1982).

Eastern San Gabriel Mountains. --In the eastern San Gabriel Mountains, the crystalline-rock complex that directly overlies the Vincent Thrust consists mainly of gneissic (layered) metamorphosed plutonic rocks and associated metasedimentary rocks that have been complexly deformed under deep-seated ductile metamorphic conditions that locally yielded mylonite as the end product. The fact that mylonite is most pervasive and uniformly developed directly above the Vincent Thrust suggests that mylonitic deformation probably is related to tectonic emplacement of the upper plate over lower-plate Pelona Schist along the thrust.

Cape town gneiss
View a polished slab of gneissose rock like that occurring in the upper plate of the Vincent Thrust in the southeastern San Gabriel Mountains; these rocks probably originally were plutonic granitic rocks before they were deformed and metamorphosed. This example from South Africa shows the interlayering of dark (mafic) and light (felsic) mineral layers typical of gneiss (image source: University of Cape Town Rock Art Gallery of rocks from South Africa)
 

granitic gneiss
View an outcrop closeup of gneissose rock like that occurring in the upper plate of the Vincent Thrust in the southeastern San Gabriel Mountains; these rocks probably originally were plutonic granitic rocks before they were deformed and metamorphosed. This example from the Coast Plutonic complex of British Columbia probably has a geologic history similar to that of upper-plate rocks in the southeastern San Gabriel Mountains (image source: University of British Columbia Earth and Ocean Science (OES) Image Set , photograph M.L. Bevier)
 

The mylonitic zone above the Vincent thrust varies in thickness: for example, mylonite overlying the thrust ranges from about 20 ft thick east of San Antonio Canyon to as much as 2,000 ft or more west of San Antonio Canyon. Thickness variation suggests that a once-uniform zone of mylonite has been thinned by later deformation postdating original movements on the Vincent Thrust.

augen gneiss
View an outcrop closeup of mylonitic (ductiley sheared) rock similar to that occurring in the upper plate of the Vincent Thrust in the southeastern San Gabriel Mountains; these rocks probably originally were plutonic granitic rocks before they were deformed and metamorphosed. This example from southeastern British Columbia probably has a geologic history similar to that of upper-plate rocks in the southeastern San Gabriel Mountains (image source: University of British Columbia Earth and Ocean Science (OES) Image Set , photograph C.A. Giovanella)
 

Above the mylonite is a mixture of gneissic-textured pre-Mesozoic metamorphic rocks and Mesozoic granitoid rocks that include older units like the Triassic Mount Lowe intrusion (Barth and Ehlig, 1988) and younger tonalitic rocks of Cretaceous age (Nourse and others, 1998).

The mylonitic and gneissic complex and the underlying Pelona Schist have been intruded by the Miocene to late Oligocene Lytle Creek pluton and related dikes and sills (Hsu and others, 1963; Miller and Morton, 1977). The Lytle Creek pluton consists of massive light colored granodiorite that has a typical granitoid texture except in its marginal parts, where much of the rock has a texture indicative of shallow intrusive (hypabyssal) depths. The pluton is intruded by diabase dikes and one small body of olivine-bearing diabase-gabbro.


Continue to Basement Rocks of the Southeasternmost San Gabriel Mountains
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This site last updated September 3, 2004 (ps)

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