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Southern California Geology


Major Faults of Southern California
Inland Empire Region


San Andreas Fault Zone, Mojave Desert Segment

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Table of Contents for San Andreas Fault Zone, Mojave Desert Segment (click on any item to jump to that section)

Mojave Desert Segment, San Andreas Fault

Geologic Setting in the San Gabriel Mountains

Faulting Chronology

Displacement History

Mojave Desert Segment, San Andreas Fault

The Mojave Desert segment of the San Andreas fault extends from Tejon Pass to the San Bernardino valley, where it passes into the San Bernardino strand. The modern break forms a singular trace that runs the entire length of the segment (Ross, 1969) and describes a gently-curving arc having a regional strike of about N 60 W; in Tejon Pass the trace is deflected into the big bend of Hill and Dibblee (1953, p. 453). Ground rupture associated with the 1857 earthquake on the San Andreas fault occurred along the Mojave Desert segment from Tejon Pass to about Wrightwood (Sieh, 1978a), and the modern trace has been the site of recurring Holocene ground rupture (Sieh, 1978b, 1984; Weldon and Sieh, 1985).

The Mojave Desert segment has been described thoroughly by Barrows and others (1985, 1987) on the basis of detailed mapping by the California Division of Mines and Geology (Barrows, 1975; Kahle, 1975; Barrows, 1979, 1980; Barrows and others, 1976; Beeby, 1979; Kahle and others, 1975; Kahle, 1979; Kahle and Barrows, 1980). Their mapping shows that from Tejon Pass southeast to Elizabeth Lake the modern trace coincides with older traces to form a narrow fault zone having a relatively simple faulting history. By contrast, between Elizabeth Lake and Cajon Pass the modern trace is but one of several fault strands that form a zone several kilometers wide (see similar interpretations by Noble, 1926, 1933, 1954a,b; Wallace 1949; Dibblee, 1967a, 1968a, 1975b). Within this wide zone, fault strands like the Punchbowl, Nadeau, and Little Rock faults have evolved sequentially and been abandoned, culminating in the modern trace that apparently evolved no earlier than the Pleistocene (Barrows and others, 1985, p. 105-106).

Geologic Setting in the San Gabriel Mountains

In the San Gabriel Mountains Barrows and others (1985, 1987; Barrows, 1987) recognize five discrete strands of the San Andreas fault including, from west to east, the Punchbowl, Nadeau south, Nadeau north, Mojave Desert, and Little Rock strands (we use the term "Mojave Desert strand" to designate the "main San Andreas trace" of Barrows and others, 1985, 1987). Careful mapping by Barrows and others (1985) shows that the Punchbowl, Nadeau (north and south branches), and Little Rock faults all are truncated on their northwest and southeast ends by the Mojave Desert strand of the San Andreas. Thus, they can be viewed as anastomosed and abandoned strands of the San Andreas that probably are related to strands of the fault farther to the southeast in the San Bernardino Mountains and (or) Salton Trough.

Regional correlation of the Punchbowl fault is of particular interest because the fault appears to be a significant strand of the San Andreas, and yet its isolated position outboard (west) of the main San Andreas trace makes its role in the overall history of the San Andreas difficult to evaluate. The fault extends for about 75 km along the northeast flank of the San Gabriel Mountains (fig. 1; Barrows and others, l987). The strand is characterized by its locally sinuous trace and west-dipping reverse dips, a geometry attributed by Barrows and others (l987) to deformation following its right-slip history. The Punchbowl fault originally was identified by Noble (1953), who interpreted it as a reverse dip-slip fault and implied that it merged with the San Jacinto fault by way of the Glen Helen fault (Noble, 1954a,b). Subsequently, Dibblee (l967a, 1968a) showed that the Punchbowl fault is a right-lateral fault that he interpreted as an old strand of the San Andreas family. Because it is truncated on the southeast and northwest by younger strands of the San Andreas zone, the Punchbowl should be viewed like any other linear structure that has been displaced by a strike-slip fault: its northwest and southeast terminations have been displaced from cross-fault counterparts that should be identifiable after right slip is restored on younger strands of the San Andreas. This interpretation motivated us to search southeastward for a possible displaced counterpart for the Punchbowl fault on the opposite side of the main trace of the San Andreas.

Faulting chronology .--The multiple San Andreas strands in the San Gabriel Mountains evolved sequentially about 5 m.y. ago (Barrows and others, 1985, 1987). The Punchbowl fault appears to be the oldest strand, although Barrows and others (1987, p. 2, 84, 86) indicate that sequencing relations between the Punchbowl and Little Rock strands are not completely clear. According to Barrows (1987, p. 149-153, figs. 6-8), stratigraphic relations between the Punchbowl fault and sediments of the Juniper Hills Formation (of Barrows, 1987) indicate that the Punchbowl fault may have generated right slip continuously between early Blancan (5 Ma) and late Blancan time (2 Ma). However, the age of the Juniper Hills Formation is poorly constrained (Barrows, 1987, p. 129-130), and relevant beds in the unit cannot be confidently assigned to a particular part of the Blancan (A.G. Barrows, oral communication, 1990). The Nadeau faults appear to be Pliocene in age, while the Mojave Desert strand may have originated in middle Pleistocene time.

Displacement history. --Barrows and others (1985, Table 4, 1987 p. 86) were able to document no more than 102 km of right slip on all strands of the San Andreas fault in the eastern San Gabriel Mountains: 21 km on the main or Mojave Desert strand of the San Andreas; 16 km on the Nadeau fault (north and south branches); 21+ km on the Little Rock fault; and 44 km on the Punchbowl fault. Their estimates for the Punchbowl fault are comparable with those proposed by other workers. Dibblee (1967a, fig. 72; 1968a, p. 263-264, fig. 1) recognized between 32 and 48 km of displacement on the Punchbowl based on cross-fault correlations between the San Francisquito and Fenner faults, between marine rocks of the San Francisquito Formation, and between the Sierra Pelona and Blue Ridge windows of Pelona Schist. Farley and Ehlig (1977) and Ehlig (1981, fig. 10-4) proposed about 40 km of displacement based on their suggestion that the Punchbowl fault has displaced strata in Ridge Basin that contain polka-dot granite clasts from strata in the Punchbowl Formation that contain similar clasts. Barrows and others (1985, 1987) acknowledge that their total displacement for the San Andreas (102 km) is considerably less than the widely accepted displacement (240 km), and they point out that displacement on the Little Rock strand may be greater than the 21 km they were able to document. Their displacement estimate for the main San Andreas strand (21 km) also is considerably less than most workers would infer for the strand.

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