|Western Earth Surface Processes Team|
Geology of the San Gabriel Mountains, Transverse Ranges Province
Table of Contents
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.
San Gorgorio Pass Region
Santa Barbara Coast Plain
San Bernardino Mountains
Return to the Geology of the Transverse Ranges
Note: Information on this website is modified from: Matti, J.C., Morton, D.M. and Cox, B.F., 1992, The San Andreas fault system in the vicinity of the central Transverse Ranges province, southern California: U.S. Geological Survey Open-File Report 92-354 , 40 p., scale 1:250,000.
Crystalline Basement Rocks
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.
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
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:
Western and central San Gabriel Mountains .-Upper-plate rocks above the Vincent Thrust include:
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.
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.
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.
Basement rocks of the Southeastern San Gabriel MountainsIn 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).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.
San Jacinto Fault ZoneAlthough most workers agree about the distribution and geologic history of the San Jacinto Fault south of the San Gabriel Mountains, where it approaches the range the distribution, character, and history of the fault is more complex and less well documented. In part this is because relatively young alluvium of Cajon Creek and the Lytle Creek alluvial fan obscure surficial evidence for the fault. More importantly, the San Jacinto Fault appears to splay into several discrete strands that approach the mountains from the southeast. Much work remains to be done to understand the role each of these strands has played in the overall geologic history of structures assigned to the San Jacinto zone.
Uncertainty regarding the San Jacinto zone in the San Gabriel Mountains is compounded by its equivocal relationship to the San Andreas Fault on the north side of the mountains. Many workers suggest that the San Jacinto either branches directly off the San Andreas Fault on the north side of the Mountains, or merges with the Punchbowl Fault. A connection to the San Andreas via the Punchbowl fault seems unlikely, as the Punchbowl has not been an active ( neotectonic ) member of the San Andreas fault system during the Quaternary period (Barrows, 1975, 1979, 1980, 1987; Barrows and others, 1976, 1985, 1987; Meisling and Weldon, 1989; Weldon and others, 1993). In addition, geologic mapping by Morton (1975; Morton and Matti, 1991a,b; Morton and others, 1983, 1990) indicates that the San Jacinto Fault zone at the surface does not connect either with the Punchbowl or San Andreas faults but instead interacts in some fashion with east-to northeast-striking faults in the interior of the eastern San Gabriel Mountains. Thus, some other connection between the San Jacinto and San Andreas fault zones in the vicinity of the southeastern San Gabriel Mountains must be documented.
Faults of the San Jacinto zone in the mountains
Where it penetrates the southeastern corner of the San Gabriel Mountains near the mouth of Lytle Creek, the fault zone generally identified as the "San Jacinto Fault zone" is a 300-m wide zone consisting of three nearly vertical faults. From west to east, these three faults bound four mappable blocks of biotite gneiss, mylonitic leucogranite, Pelona Schist, and Miocene granodiorite (fig. 3 of Matti and others, 1992a). The fault with the greatest width of crushed rock is overlain by apparently unfaulted alluvium thought to be 200-500 ka old (Morton and Matti, 1987). Four kilometers into the range, the "San Jacinto Fault zone" consists of a relatively homogeneous zone of gouge and crushed rock, 200-300 m thick, bordered on the east by a thrust fault. Here, also, apparently unfaulted alluvium considered to be 200-500 ka (Morton and Matti, 1987), overlies the broad crush zone, but is offset along the eastern edge by the thrust fault. These alluvial relations attest to the antiquity of the "San Jacinto Fault zone" in this part of the mountains--by comparison with it youthfulness south of the mountains.
Six kilometers northwest of the mountain front, the 200- to 300-m wide "San Jacinto Fault zone" diverges into three discrete north-dipping faults, each traversing a separate fork of Lytle Creek (fig. 3 of Matti and others, 1992a). These three faults progressively change in strike counterclockwise until they are all orientated in a northeast direction. These northeast-striking faults converge to the west near the mountain front at the mouth of San Antonio Canyon (fig. 3 of Matti and others, 1992a). Just west of San Antonio Canyon, two faults seem to coincide with the San Gabriel Fault zone in Cow Canyon. The distribution of basement rocks and their bounding contacts allows the displacement sense on these faults to be determined, with surprising results: northwest-striking fault segments appear to have oblique-right-reverse separation; east-striking fault segments appears to have thrust separation; and northeast-striking fault segments appear to have oblique-left-reverse separation. These results are similar to the generalized sense of displacement determined from a recent microearthquake study (Cramer and Harrington, 1987). The overall geometry and sense of displacement of the faults is an antiformal schuppen-like structure.
Within or marginal to the southeastern San Gabriel Mountains, two faults commonly thought to be branches of the "San Jacinto Fault zone" have youthful (neotectonic) fault features. These are the Lytle Creek Fault and the Glen Helen Fault (fig. 3 of Matti and others, 1992a). The Glen Helen Fault , exposed along the west side of Cajon Canyon, is the only fault within the eastern San Gabriel Mountains that has a variety of more-or-less continuous youthful fault features, such as sag ponds and scarps. Morton and Matti (1987) indicate that these fault features are developed in alluvium capped by soils whose degree of development is comparable to the S4 to S5 soil stage of McFadden (1982; Bull, 1991), soils whose age probably falls between 4 and 70 ka. The Lytle Creek Fault forms scarps in alluvial deposits capped by soils that also are correlated to the S4 or S5 soil stage (Morton and Matti, 1987). These are apparently the same as the ones considered to be 50 to 60 ka by Mezger and Weldon (1983). Neither the Lytle Creek Fault nor the Glen Helen Fault can be mapped directly in to the San Andreas fault.
The Punchbowl Fault zone, located just south of the San Andreas Fault in the eastern San Gabriel Mountains, is a deformed early strand of the San Andreas Fault (Barrows, 1975, 1979, 1980, 1987; Barrows and others, 1976, 1985, 1987; Weldon and others, 1993; Matti and others, 1993). It consists of two closely spaced faults separated at most places by a sliver of intensely deformed tonalitic and gneissic rocks (fig. 3 of Matti and others, 1992a). In some places recognizable gneiss and tonalite is missing and the two metamorphic facies of the Pelona Schist are separated by thoroughly sheared basement rock of uncertain parentage. For more detail, see the discussion of the Punchbowl Fault by Matti and others (1992a).
San Andreas Fault
San Andreas Fault On the north side of the San Gabriel Mountains, the San Andreas Fault forms a nearly linear, relatively simple break, slightly concave to the south (fig. 3 of Matti and others, 1992a). Within Lone Pine Valley, the San Andreas forms a linear "rift valley" that reflects long-term activity on the fault. This trace of the fault along the north flank of the San Gabriel Mountains represents the Mojave Desert segment of the San Andreas (Barrows, 1975, 1979, 1980, 1987; Barrows and others, 1976, 1985, 1987); to the southeast, the Mojave Desert segment passes into the San Bernardino strand that lies along the base of the San Bernardino Mountains. For more detail, see the discussion of the San Andreas Fault zone in the San Gabriel Mountains by Matti and others (1992a).
Cucamonga Fault Zone
The Cucamonga Fault zone is located along the southern margin of the eastern San Gabriel Mountains, and marks the eastern end of the frontal-fault system of the San Gabriel Mountains. The fault zone consists of numerous inter-twining, east-striking, north-dipping thrusts that separate crystalline basement rocks of the eastern San Gabriel Mountains from alluvium of upper Santa Ana valley to the south. Some thrust faults of the zone lie entirely within alluvium (Morton and Matti, 1987). Slickensides in the basement rocks are consistently oriented down-dip, indicating the most recent displacements along the Cucamonga Fault zone have been pure thrust.
Faulting has occurred throughout the Quaternary, with individual faulting events estimated at about 6.7 M with a recurrence of about 625 years for the past 13,000 years (Matti and others, 1982; Morton and Matti, 1987). The average north-south convergence across the Cucamonga Fault zone is estimated to have been in the range of 3 mm/yr (Weldon, 1986) to 5 mm/yr (Matti and others, 1982b; Morton and Matti, 1987).
San Gabriel Fault
See the text on the San Gabriel Fault by Matti and others, 1992a.
Other Faults in the San Gabriel Mountains
Located north of the Cucamonga Fault zone between Lytle Creek and San Antonio Canyon are three northwest-striking right-lateral faults, the Duncan Canyon, Morse Canyon, and Demens Canyon faults (Morton and Matti, 1987). They appear to be terminated on the south by the Cucamonga Fault zone and on the north by the South Fork Lytle Creek Fault. All three show right-lateral separation, the 1.5-km separation along the Duncan Canyon Fault being the greatest.
Secondary faults in the area of the San Gabriel Mountains commonly have reverse slip (Weldon and Matti, 1986) and analysis of current seismicity gives a mix of reverse and strike-slip fault-plane solutions (Jones, 1988).
Geologic Setting of the Transverse Ranges ProvinceThe Transverse Ranges Province of southern California is so-named because the mountains, valleys, and geologic structures within this province lie east-west or " transverse to " the prevailingly northwest-trending grain characteristic of southern California. For example, northwest-trending faults of the Peninsular Ranges Province lend a northwest-oriented topographic and structural grain to that province. Likewise, the Coast Ranges and Sierra Nevada Provinces of southern and central California also are prevailingly northwest-trending. The Transverse Ranges lie athwart this northwest grain.
Although referred to collectively as the Transverse Ranges, the province consists of several discrete mountain ranges and intervening valleys, including:
Geologists group these discrete landforms within the Western, Central, and Eastern Transverse Ranges.