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Western Earth Surface Processes Team

San Andreas Fault System in the Inland Empire and Salton Trough

San Jacinto Fault Zone

Within the Peninsular Ranges Province, the San Jacinto fault zone separates the San Jacinto Mountains and San Timoteo Badlands from the Perris block. In the San Jacinto Mountains the fault forms a series of en-echelen segments that locally are complicated by thrust faults (Sharp, 1967, 1972). A major right step in the fault zone occurs to the northwest in the San Jacinto Valley. There, the eastern right-stepping strand (Claremont fault) is located along the northeast side of the Valley and locally forms scarps in late Quaternary deposits; however, youngest Quaternary units are not broken and the fault largely must be inferred beneath sediment deposited by the San Jacinto River and by alluvial fans derived from the San Timoteo Badlands. To the west, a left-stepping strand of the San Jacinto zone (Casa Loma fault) is located along the southwest side of the San Jacinto Valley. The fault forms a scarp that Sharp (1972, map sheet 2) traced almost continuously from Hemet to north of the Lakeview Mountains. However, to the northwest the fault has no surface expression in youthful alluvium of the northern San Jacinto Valley, and slip must step right (east) onto the Claremont strand of the San Jacinto zone (J.C. Matti and D.M. Morton, in prep.). Within the Perris block, the San Jacinto fault has generated about 25 km of right-lateral displacement since early Pliocene time (Sharp, 1967; Matti and Morton, 1975).

Between the San Jacinto Valley and the San Gabriel Mountains the San Jacinto fault zone traverses Quaternary alluvial units and sedimentary rocks. In the Reche Canyon area the fault displaces strongly folded Quaternary deposits of the San Timoteo Badlands, and has a pronounced bow convex to the west that forms a restraining bend (Morton and Matti, in press); the main trace is flanked by subparallel faults (Morton, 1978a,b) that may be right-lateral in origin. Southeast of metropolitan San Bernardino the main trace displaces older and younger Quaternary units, but southeast and northwest of this break the youngest floodplain deposits of the Santa Ana River and Cajon and Lytle Creeks are not broken.

The distribution and geologic history of the San Jacinto fault in the southeastern San Gabriel Mountains are problematical. The name "San Jacinto" traditionally has been applied to a northwest-oriented fault zone developed in crystalline rocks east of the mouth of Lytle Creek canyon (fig. 3). There, the zone consists of two or more closely spaced fault strands that form impressive shear zones in crystalline bedrock but do not displace Quaternary alluvial deposits (Morton and Matti, 1987, plate 12.1). To the east, the Glen Helen fault forms scarps and sag ponds in alluvial deposits that probably are as young as Holocene (Sharp, 1972, map sheet 3; Morton and Matti, 1987, pl. 12.1), and to the west the Lytle Creek fault forms a scarp in alluvial materials that are latest Pleistocene in age (Morton and Matti, 1987, pl. 12.1). Mezger and Weldon (1983) documented a late Quaternary right-slip rate of about 2 mm/yr for the Lytle Creek fault. The Glen Helen fault probably is the active strand of the San Jacinto fault zone in the San Gabriel Mountain region, but it is unclear if this fault records the total history of the fault that developed southeast of the San Gabriel Mountains.

Models for the long-term history of the San Jacinto fault in the southeastern San Gabriel Mountains depend on how various bedrock faults that occur there are mapped and interpreted. Some workers indicate that the fault zone east of the mouth of Lytle Creek continues to the northwest and joins the San Andreas fault by way of the Punchbowl fault (Noble, 1954b; Ehlig, 1975, fig. 1, 1981, fig. 10-2). This interpretation leads to the idea that the San Jacinto fault feeds into the San Andreas, and that total slip on the San Andreas northwest of this junction includes displacements on the San Jacinto (Ehlig, 1982, p. 375; Weldon, 1984). However appealing, this model is questioned by other workers (Dibblee, 1968a, fig. 1, p. 266; 1975b, p. 156; Morton, 1975a, figs. 1, 2, p. 175) who conclude that the San Jacinto fault cannot be mapped into either the Punchbowl or San Andreas faults; instead, faults attributed to the San Jacinto zone appear to splay into several branches that curve west into the San Gabriel Mountains without joining the San Andreas fault at the surface (Morton, 1975a; Morton and others, 1983). This difference in structural interpretation has led to uncertainty about how the San Jacinto fault participates in the San Andreas fault system.

We propose that fault strands in the southeastern San Gabriel Mountains that originated by Quaternary right-slip on the San Jacinto fault can be understood only in the context of earlier Neogene faults that juxtaposed and rejuxtaposed rocks of Peninsular Ranges-type and San Gabriel Mountains-type. Figure 3 illustrates the pattern of Neogene faults in the southeastern San Gabriel Mountains and our interpretation of basement-rock provincial affinities (discussed below). In our discussion of the San Gabriel and Banning faults, we propose that the east- to northeast-oriented Evey Canyon, Icehouse Canyon, and Stoddard Canyon faults are segments of throughgoing structures that formerly were regionally extensive namely, the middle Miocene left-lateral Malibu Coast-Raymond-Banning fault and the late Miocene San Gabriel-Banning fault. Moreover, we propose that these older faults separate rocks of San Gabriel Mountains-type on the west, north, and east from rocks of Peninsular Ranges-type on the south (fig. 3). In our model, these older structures trend essentially eastward until they enter the Lytle Creek drainage, where they converge and trend southeastward down Lytle Creek Canyon. There, they are represented by the fault zone that occurs east of the mouth of Lytle Creek Canyon. In our view, then, the name "San Jacinto fault" in Lytle Creek Canyon has been applied to an ancient fault zone that has witnessed multiple episodes of strike-slip faulting only the latest of which can be attributed to the movement history of the fault called "San Jacinto" that traverses the Peninsular Ranges Province to the southeast. This composite movement history is consistent with the broad crush zone that marks the trace of the San Jacinto fault between the Icehouse Canyon fault and the mountain front, and accounts for the pronounced contrast in basement-rock types on either side of the crush zone (fig. 3; Morton and Matti, 1987, Plate 12.1).

Thus, earlier Neogene faulting in the southeastern San Gabriel Mountains established a structural template that has been modified by Quaternary right-slip within the San Jacinto zone. But where and how did Quaternary displacements occur in the mountains? To the southeast, within the Peninsular Ranges, 25 km of Quaternary right slip can be documented for the San Jacinto fault on the basis of displaced basement rocks (Sharp, 1967) and displaced Pliocene sedimentary rocks of the San Timoteo Badlands (Matti and Morton, 1975). The San Jacinto thus should be viewed as a strand of the San Andreas system, and an easy solution would have right-slip faults of the San Jacinto in the southeastern San Gabriel Mountains connect with the San Andreas northwest of Cajon Pass. However, we reiterate here that faults attributable to the San Jacinto cannot be mapped beyond the southeastern San Gabriel Mountains. Instead, right-lateral faults that might be attributable to the San Jacinto zone in Lytle Creek Canyon curve westward and southwestward into the San Gabriel Mountains and turn into east- to northeast-trending north-dipping faults that have reverse left-slip displacements (Morton, 1975a, figs. 1, 2). Morton (1975b) viewed the curving fault splays in the southeastern San Gabriel Mountains as a schuppen-like structure within which right slip on northwest-oriented strands of the San Jacinto is transformed into reverse left slip on east- and northeast-oriented faults that appear to be continuations of the San Jacinto zone.

We adopt Morton's (1975b) proposal here, but modify it to fit our notion that these curving faults were not pioneered by Quaternary slip but instead are older Neogene strands of the Malibu Coast-Banning and San Gabriel-Banning fault systems that have been reactivated and deformed during Quaternary time due in part to the 15° angular convergence between the San Jacinto and San Andreas faults (Morton and Matti, in press). We propose that one of these curving structures is the reactivated Stoddard Canyon segment of the San Gabriel fault that originally was a right-lateral structure but now has latest-movement indicators interpreted by Morton (1975a) as evidence for oblique left slip. A second oblique left-lateral fault (the San Antonio) trends down San Antonio Canyon and has displaced three fault segments of the old Malibu Coast-Banning system and the San Gabriel-Banning system (fig. 3): (1) the north branch of the San Gabriel fault about 3 km from the Icehouse Canyon fault, (2) the Evey Canyon fault about 3 km from the Icehouse Canyon fault, and (3) the south branch of the San Gabriel fault about 3 km from the Stoddard Canyon fault.

The oblique left-lateral faults all root eastward or northeastward into the northwest-oriented fault zones of Lytle Creek drainage that traditionally have been assigned to the San Jacinto fault. One of these strands apparently has generated at least 8 to 13 km of Quaternary right slip that has displaced plutonic contacts between Miocene granitoid rock and Pelona Schist (fig. 3; Morton, 1975a). However, even this displacement cannot be mapped northwestward out of the Lytle Creek drainage basin.

The structural complexity in the southeastern San Gabriel Mountains makes it unclear to us exactly how right slip on the San Jacinto has been transferred to the San Andreas. Two extreme interpretations apply: (1) right slip on the San Jacinto is transferred entirely to the San Andreas by way of a right-step that takes place between San Gorgonio Pass and the southeastern San Gabriel Mountains, or (2) right slip on the San Jacinto fault is transformed entirely into oblique left slip along faults within the southeastern San Gabriel Mountains, and thereby does not contribute at all to right slip on the Mojave Desert segment of the San Andreas. We suspect that kinematic interaction between the San Jacinto and San Andreas faults has involved both right-stepping slip transfer and left-stepping slip bypass, but until the relative scale of either process is documented, the amount of slip transfer from the San Jacinto to the Mojave Desert segment of the San Andreas can only be inferred. In the absence of actual data, we arbitrarily infer that 20 km of slip has been transferred between the two faults by way of the right step across the San Bernardino Valley; we infer that the remaining 5 km has been transformed into oblique left slip within the southeastern San Gabriel Mountains, where this transformation has led to convergence, uplift, block rotations, and severe shape changes of blocks bounded by brittle faults.

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