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Southern California Geology


Geologic Setting of the San Gabriel Mountains, Transverse Ranges Province

Geologic Structures

San Jacinto Fault Zone

Although 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.

Continue to Geologic Structures, Punchbowl Fault

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