|Western Earth Surface Processes Team|
San Gabriel Fault
The San Gabriel fault extends southeastward from Ridge Basin to the western San Gabriel Mountains, where most workers recognize north and south branches (fig. 1). The north branch curves eastward through the San Gabriel Mountains to the southeastern part of the range, where its eastward continuation is obscured by a complex network of faults (Morton, 1975a). The south branch traverses the south flank of the San Gabriel Mountains, where the fault is obscured by the Quaternary frontal-fault zone that bounds the south margin of the mountains. Most workers suggest that the south and north branches rejoin in the vicinity of the eastern San Gabriel Mountains, but this structural reunion has not been documented and each worker has suggested a different arrangement of linking faults (contrast the views of Dibblee, 1968a, fig. 1, 1982a, fig. 1; Ehlig, 1973, fig. 1 and 1981, fig. 10-2; Crowell, 1975a, fig. 1; 1975c, p. 208-209; 1982a, fig. 1). According to the generally accepted view, the combined south and north branches of the San Gabriel fault must work their way through the structural complexity of the eastern San Gabriel Mountains and continue eastward.
Continuity of the San Gabriel fault southeast and northwest of its mapped distribution is problematical. Matti and others (1985) proposed that the Banning fault forms the southeastward continuation of the San Gabriel fault. We elaborate this proposal below. Northwestward continuation of the San Gabriel fault beyond Ridge Basin is obscured by Quaternary thrust faults that carry crystalline rocks of the Frazier Mountain region southward over the San Gabriel fault and associated sedimentary rocks of Ridge Basin. There, most workers follow Crowell (1950, 1975a, 1982a, figs. 3, 4) who proposed that the San Gabriel fault at depth continues northwestward beneath the thrust sheets to join the San Andreas fault (Crowell, 1982a, p. 29). We interpret this junction as a meeting point where an older fault (the San Gabriel) intersects a younger fault (the San Andreas, sensu stricto), and we view the northwest end of the San Gabriel fault as a piercing point whose palinspastic position should be identifiable after right-slip is restored on the San Andreas (sensu stricto). When we restore 160 km of right slip on the Mojave Desert segment of the San Andreas, the northwest end of the San Gabriel fault restores to the Cajon Pass region where the Cajon Valley fault (fig. 1) may represent a former continuation of the San Gabriel fault (Matti and Morton, 1992).
Estimates for right slip on the San Gabriel fault range from 0 to 70 km. Crowell (1952) first proposed large lateral displacements based on his recognition that distinctive Precambrian basement clasts in the upper Miocene Violin Breccia of Crowell (1954a) in Ridge Basin and in upper Miocene marine beds of the Modelo Formation have been displaced 30 to 35 km from likely cross-fault sources in the Frazier Mountain region and western San Gabriel Mountains, respectively. However, Paschall and Off (1961) discounted the role of right slip on the San Gabriel fault and instead accounted for the distribution of basement clasts in the sedimentary units purely on the basis of vertical dip-slip movements. Crowell (1962, p. 39-40) reiterated his proposal that clasts in sedimentary rocks require about 32 km of displacement on the San Gabriel fault, and also suggested that basement rocks in the Frazier Mountain region may have been displaced by as much as 48 km from cross-fault counterparts in the western San Gabriel Mountains (Crowell, 1962, p. 41). Carman (1964) called for about 32 km of right slip on the fault based on his proposal that upper Miocene nonmarine deposits of the Caliente Formation in the Lockwood Valley area were part of the same fluvial drainage that deposited the Mint Canyon Formation in Soledad basin; the two parts of the fluvial system since have been displaced by the San Gabriel fault. This proposal was expanded and refined by Ehlig and others (1975), who used stratigraphic patterns of distinctive volcanic clasts and basement-rock clasts to propose that the Caliente-Mint Canyon drainage was disrupted by as much as 56 to 65 km of right slip on the San Gabriel fault. Ehlert (1982b) subsequently increased this estimate to 70 km. Ehlig and Crowell (1982) subsequently refined the basement-rock correlations originally cited by Crowell (1962), and concluded that the San Gabriel fault has displaced these rocks by about 60 km. This figure has become widely accepted as the total displacement on the San Gabriel fault. In the San Gabriel Mountains, the 60-km displacement presumably is split between the north and south branches of the fault: about 22 km on the north branch, as shown by displaced crystalline rock units (Ehlig, 1968a), and the remaining 38 km presumably on the south branch, although this displacement has not been proven by cross-fault correlations. We propose that the San Gabriel fault has no more than about 44 km of displacement: 22 km on the north branch (Ehlig, 1973), and 22 km on the south branch based on our proposal that the fault has displaced the left-lateral Malibu Coast-Santa Monica-Raymond fault from the Evey Canyon-Icehouse Canyon fault in the southeastern San Gabriel Mountains (discussed below).
Crowell (1982a, fig. 12) proposed that right slip on the San Gabriel fault was initiated in the late Miocene (about 10 m.y. ago) and largely was completed by the end of the Miocene (about 5 Ma). Late Miocene onset of faulting is interpreted from interfingering stratigraphic relations between the syntectonic Violin Breccia and marine sediments of late Miocene age. Termination of strike-slip displacements on the San Gabriel fault by earliest Pliocene time is interpreted from relations in Ridge Basin, where the Pliocene Hungry Valley Formation of Crowell (1950) has been mapped as a depositional cap that seals the main displacement history of the San Gabriel fault (Crowell, 1982c). The basal part of the Hungry Valley Formation is about 5 m.y. old, and Crowell (1982a,b) concluded that right slip within the San Andreas transform system switched from the San Gabriel fault to the San Andreas fault at about this time. However, this conclusion has been questioned by some workers, most notably Weber (1982) who indicates that the Hungry Valley sequence is disrupted by the San Gabriel fault and that a few km of right slip on the San Gabriel occurred after the Hungry Valley was deposited.