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Geologic Setting of the San Gabriel Mountains, Transverse Ranges Province

Crystalline Basement Rocks

Basement rocks of the Southeastern San Gabriel Mountains

In the southeasternmost San Gabriel Mountains between San Antonio Canyon and Lytle Creek Canyon, the pattern of crystalline basement rocks is different than elsewhere in the range. Here, an east-trending fault known as the Icehouse Canyon Fault zone (Evans, 1982; Morton and others, 1983) seems to be an important geologic structure that separates upper- and lower-plate rocks of the Vincent Thrust to the north from rocks to the south that may or may not be bottomed by the Thrust (fig. 3 of Matti and others, 1992a). The Icehouse Canyon Fault that separates the northern and southern basement-rock groups is a late Cenozoic right-lateral strike-slip fault that apparently has re-occupied a major lithologic boundary created earlier during the late Mesozoic to early Cenozoic (May, 1986, 1988; May and Walker, 1989). The Icehouse Canyon zone thus may be an important structural boundary in the San Gabriel Mountains (May, 1986, 1989; May and Walker, 1989).

Metasedimentary suite .--These rocks include metasedimentary rocks (marble, schist, quartzite) that Powell (1993) placed within his regionally extensive Placerita suite and Dibblee (1982a) placed within his San Antonio terrane. The rocks consist of high-grade (amphibolite-grade) metaquartzite, marble, biotite-sillimanite schist, and graphitic schist. On Ontario Ridge in the vicinity of San Antonio Canyon, the metasedimentary rocks form large mappable masses; however, to the east toward Lytle Creek, the metasedimentary rocks are progressively fragmented and enveloped by tonalitic and monzogranitic rocks of the Cretaceous granitoid suite.

Cretaceous granitoid suite. --These plutonic rocks mainly are tonalitic in composition, but include mappable bodies of monzogranite and granodiorite (Morton and Matti, 1987, 1990a,b).

a polished slab of tonaliteClick on this thumbnail to view a polished slab of tonalite like that occurring south of the Icehouse Canyon Fault zone in the southeastern San Gabriel Mountains. This example is from South Africa (geologic association unknown) (image source: University of Cape Town Rock Art Gallery of rocks from South Africa)  

 

a polished slab of a slightly foliated equigranular granitic rock This is a polished slab of a slightly foliated equigranular granitic rock similar to late Mesozoic granitic rock occurring south of the Icehouse Canyon Fault zone in the southeastern San Gabriel Mountains. This example (geologic association unknown) probably is monzogranitic in composition (image source: Marble and Granite, Inc )

 

Some of the tonalitic rocks are gneissose, and have a mylonitic foliation produced by ductile deformation of the rocks. The degree of mylonitic deformation increases southward toward the mountain front where, in places, tonalitic rocks have been uniformly converted to mylonite belts as much as 1000 ft thick (the "black-belt" mylonite belt of Alf, 1948; fig. 3 of Matti and others, 1992a). Dikes and small masses of relatively undeformed granitoid rock that are late Cretaceous in age (about 78 Ma according to May and Walker, 1989) intrude the deformed tonalitic rocks, thus providing a late Cretaceous upper seal on the age of mylonitic deformation.

outcrop photograph of mylonitic granitic rock
This is an outcrop photograph of mylonitic granitic rock similar to late Mesozoic mylonite of the "black belt" in the southeastern San Gabriel Mountains. This example from the Bonavista Bay, Canada shows mylonitic rock that formed within the deep-seated part of a fault zone (image source: Newfoundland and Labrador Geological Survey website on Geological Features and its link to Structural Geology )
 

 

South of the Placerita metasedimentary suite and the intrusive tonalitic rocks is a heterogeneous metamorphic and plutonic assemblage consisting of two main units (Hsu, 1955; Morton, 1975; May, 1986; Morton and Matti, 1987; May and Walker, 1989):

  • Older metamorphic rocks consist of multiply deformed, compositionally layered, garnet-pyroxene-bearing quartzofeldspathic gneiss and minor marble and calc-silicate rock that originally were metamorphosed to high-grade granulite conditions, but subsequently were retrogressively metamorphosed to greenschist and amphibolite conditions. The age of the pre-granulite protolith is unknown, but usually is interpreted either as (1) Precambrian, based largely on the granulite grade of metamorphism, or (2) Paleozoic, based on the occurrence of marble that regionally is interpreted to be Paleozoic or late Precambrian in age. The granulite metamorphic event itself probably is mid Cretaceous based on a 108 Ma U/Pb radiometric-age determination from pegmatite injected contemporaneous with late-stage granulitic metamorphism (May and Walker, 1989, p 1260)

hand sample of garnet-bearing compositionally-layered metamorphic rock
This is a hand sample of garnet-bearing compositionally-layered metamorphic rock similar to granulites in the southeastern San Gabriel Mountains. Although this example from the Coast Plutonic Complex of British Columbia is texturally similar to San Gabriel granulites, it may not be metamorphosed strongly enough to reach granulite grade (image source: University of British Columbia Earth and Ocean Science (OES) Image Collections , photograph © M.L. Bevier)

  • Younger plutonic rocks consisting of tonalitic granitoid rocks and pyroxene-bearing granitoid rock (charnockite) have intruded the older granulitic metamorphic terrane.

Both suites of metamorphic and plutonic rocks were mylonitized contemporaneous with igneous emplacement of the tonalitic and charnockitic rocks. Mylonitic tonalite associated with the granulitic rocks yields U-Pb ages of about 88 Ma (May and Walker, 1989).

hand sample of dynamically metamorphosed porphyritic granitoid rock
View a hand sample of dynamically metamorphosed porphyritic granitoid rock, probably tonalitic in composition similar to mylonitic tonalitic rock in the southeastern San Gabriel Mountains. This example is from southeastern British Columbia (image source: University of British Columbia Earth and Ocean Science (OES) Image Collections , photograph © C.A. Giovanella)

The mylonitic rocks along and near the southeastern margin of the San Gabriel Mountains are deep parts of major tectonic zones of dislocation. The mylonitic fabric and minor structures within the mylonitic rocks are primarily oriented east; linear structures are subhorizontal or plunge at small angles. Vergence of minor folds suggests a movement direction of the San Gabriel Mountains westward relative to the valley to the south. For a recent structural analysis of basement rocks of the southeastern San Gabriel Mountains, see May (1986, 1989), Morton and Matti (1987), and May and Walker (1989).

The affinity of basement rocks south of the Icehouse Canyon Fault relative to basement rocks elsewhere in southern California is not obvious. Ehlig (1981) and Dibblee (1982) are among those who recognize their uniqueness; however, a home for these rocks has not been established definitively. Matti and others (1985, 1992; Matti and Morton, 1993) speculated that they are correlative with Peninsular Ranges rocks in the lower and upper plates of the Eastern Peninsular Ranges mylonite belt in the Santa Rosa and San Jacinto Mountains (also see May,1986, 1989).


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