PENINSULAR RANGES PROVINCE

Note: The following is taken verbatim from Cal State Long Beach Natural Science web site.  Please go to their web site for a more in-depth review of the geology of California.

http://seis.natsci.csulb.edu/deptweb/SkinnyCalSites/califoverview.html

The Peninsular Ranges consist of several northwesterly-trending ranges in southwesternmost California that are bounded on the east by the Salton Trough, and on the west by the deeper parts of the Pacific Ocean; the continental shelf and offshore islands (primarily Catalina) are included in this province. The province is truncated to the north by the east-westerly-trending structural elements of the Transverse Ranges; thus the San Fernando Valley is included with the Transverse Ranges because of the dominance of transverse structural elements, but the Los Angeles Basin, located across, and south of, the east-west trending Santa Monica fault system, is included with the Peninsular Ranges because east-west structures are subordinate, and the dominant structures are instead Peninsular in character. Similarly, the boundary between the two provinces offshore lies on Santa Cruz Island in the Channel Islands, expressed as the Santa Cruz fault, and as the Santa Rosa Island fault on Santa Rosa Island; both are extensions of the Santa Monica fault. To the east, the Sierra Madre and Cucamonga faults at the base of the San Gabriel Mountains continue the Santa Monica fault boundary, until this boundary is truncated by the San Andreas fault, which continues as the northeastern boundary of the Peninsular Ranges. These ranges continue southward deep into Mexico, underlying most of Baja California.

While many of the Peninsular Ranges have moderate or subdued relief, Mount San Jacinto in the San Jacinto Mountains rises to over 3000 meters; its northern face drops abruptly to nearly sea level near Palm Springs in the northern part of the Salton Trough, with some of the steepest topography in the United States. Sinilar steep topography is found at the edges of the province farther south, adjacent to the Imperial Valley, and within the province at the northern edge of the Santa Ana Mountains. In contrast, the western part of the province lies at low elevations, such as along the coast, offshore in the borderland, and in the Los Angeles basin.

The bedrock geology that dominates the elevated parts of the Peninsular Ranges Province on land and also underlies the sediments of the western borderland offshore consists of a small volume of high-grade metamorphic rocks intruded by Mesozoic plutons. The metamorphic rocks are difficult to correlate because of their grade and the widely- scattered occurrences of the outcrops. Included here are rocks of Late Paleozoic and Early Mesozoic age, the former largely in Mexico and eastern California, east of the Salton Trough, although some evidence suggests the presence of Upper Paleozoic metamorphic rocks along the western margin of the Salton Trough. Early Mesozoic rocks include the Triassic to Jurassic Bedford Canyon Slate, continent-derived flysch deposits (sandstone-shale with a few interbedded carbonates)in the Santa Ana Mountains, similar to the protolith for the Julian Schist of eastern San Diego County; both units contain zircons of Precambrian age, although Precambrian metamorphic and plutonic rocks are found only over 100 km to the east in the old cratonal area, from which these detrital zircons were apparently derived to form a continent-fringing clastic apron (Gastil and others, 1981). Deposition was followed by Late Jurassic volcanism, typified by the Santiago Peak Volcanics (Larsen, 1948) of the Santa Ana Mountains and other locations, typified by intermediate-composition volcanic strata (150-155 m.y.), sometimes interbedded with marine volcaniclastic sandstones and argillite (Gastil and others, 1981). This volcanic episode predates the large volume of plutonism, which can be grouped into two sequences, an older western group that is more mafic and more compositionally varied with short cooling histories, and an eastern, younger, more sialic group with longer cooling histories. The western group (found, for instance, in the Santa Ana Mountains) consists of gabbros and quartz gabbros, tonalites, and leucogranodiorites that span an age range of 122 to 105 million years based on the U-Pb ages of zircons with no systematic spatial distribution of ages (Woyski and Howard, 1987). The eastern plutonic rocks consist primarily of tonalites and closely-related low K₂0 granodiorites that span an age of 105-89 million years based on U-Pb zircon ages; these latter plutons decrease in age systematically to the east (Woyski and Howard, 1987). The age span for both pluton belts, and the variations seen, are confirmed by K-Ar ages from hornblende and biotite (Silver and others, 1979). Both groups of plutons, collectively called the Peninsular Ranges batholith, are I type granitic plutons, indicating an igneous mantle source. Changes in geochemical parameters eastward (increase in radiogenic Sr, Light Ion Lithophiles, ¹⁸O, as determined by Silver and others, 1979, and a decrease in Nd, based on studies by DePaolo, 1981) indicate that magmas from the western part of the magmatic arc are primarily from a primitive source region, like the upper mantle, while the eastern plutons apparently assimilated small, but increasing eastward, amounts of subducted sediments or highly altered oceanic crust (Woyski and Howard, 1987). In the San Jacinto Mountains, in the eastern part of the Peninsular Ranges batholith, even higher radiogenic Sr and heavy oxygen isotopes, suggest the assimilation of the edge of the North American craton in the magma melting process (Taylor and Silver, 1978). The two pluton types discussed above are occasionally separated by plutons of transitional composition (La Posta pluton), but generally occupy the distinct geographic areas cited above, with one major exception: an outcrop of western-type rocks is located along the eastern margin of the Peninsular Ranges adjacent to the Salton Trough, where it has been thrust over the younger, more sialic plutons along the Santa Rosa mylonite zone.

The western part of the borderland bedrock geology is very different from the eastern borderland near shore that continues onshore as the western pluton belt, discussed above. The rocks of the western borderland consist of Franciscan terrane, as seen exposed on Catalina Island and also on the southern part of Santa Cruz Island, and as have been recovered from dredging the ocean floor (Howell and Vedder, 1981). This terrane is characterized by Mesozoic Franciscan melange and turbidite deposits that have been overthrust by the Jurassic Coast Range ophiolite along the Coast Range thrust; the ophiolite in turn is typically overlain by the flysch deposits of the Great Valley sequence, Jurassic to Cretaceous in age. The Franciscan-type bedrock is separated from the younger plutonic rocks to the east by a major strike slip fault, the Newport-Inglewood fault zone and its southerly extension, the Rose Canyon fault. Major faults west of the Newport- Inglewood fault separate the borderland into three terranes, two of which come on land in (1) the Long Beach-Palos Verdes area, Santa Catalina Island, and the eastern half (and southern half) of Santa Cruz Island in the Channel Islands, and, to the west, the other terrane that comes on land on (2) the western half (and southern half) of Santa Cruz Island, as well as the southern half of Santa Rosa Island (Howell and Vedder, 1981).

The lower elevation parts of the Peninsular Ranges still retain a sometimes-thick sedimentary cover, ranging from Jurassic through Recent rocks. Many of the sedimentary units up through Miocene age are found not only in the western Peninsular Ranges, but also in the southern Channel Islands, and/or in the western Transverse Ranges. This correlation across major province boundaries and/or across long distances (with often an absence of exposures in between) has tectonic implications that will be discussed later. Most notable, but by no means unique, are outcrops of the widespread Oligocene Sespe Formation and other rocks bearing a series of distinctive clasts called the Poway Clasts, and the distinctive San Onofre Breccia, found in coastal Orange County and on Santa Cruz and Santa Rosa Islands.

The Peninsular Ranges have been significantly disrupted by Cenozoic faulting. Offshore and onshore, the Newport-Inglewood fault and its extension separate major basement terranes. Onshore, the Peninsular Ranges are being disrupted by strike-slip faulting along the Elsinore and San Jacinto faults. Both of these active right-lateral faults cut across the Peninsular Ranges and exit into the Salton Trough. . . .