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Southern California Geology

 

Major Faults of Southern California
Inland Empire Region

 

San Andreas Fault Zone, Coachella Valley Segment

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

Coachella Valley Segment, San Andreas Fault

Displacement History


View looking east at geologic and landscape features View looking east at geologic and landscape features in the northern Coachella Valley. Photo by J.C. Matti, USGS. Click here for explanation of features and enlarged image.

Coachella Valley Segment , San Andreas Fault

The Coachella Valley segment of the San Andreas fault is a relatively simple fault zone, although locally it is complicated by en-echelon strands and lateral splays (Clark, 1984). The segment is relatively straight and generally has a uniform regional strike of about N 45° W, although Bilham and Williams (1985) identified alternating segments 9 to 14 km long that have trends of N 40° W and N 48° W, respectively. Youthful tectonic landforms and faulted Holocene alluvial deposits indicate that the Coachella Valley segment is a modern neotectonic element (Keller and others, 1982), but the antiquity and tectonic significance of the segment cannot be judged because Quaternary and late Tertiary sediments of the Salton Trough conceal older rocks bearing evidence for its full history.

In the southern Indio Hills the Coachella Valley segment of the San Andreas fault is joined by another right-lateral fault (map sheet 1). Dibblee (1954, p. 26, pl. 2; 1968a; 1975a) referred to these two faults as north and south branches of the San Andreas fault, but Allen (1957, p. 336-339, p. 346) cited geologic relations that discouraged him from assigning the name San Andreas to any of the faults in this region. Accordingly, Allen (1957, fig. 1) applied the names Mission Creek fault and Banning fault, respectively, to Dibblee's north and south branches of the San Andreas.

We share Dibblee's view that the northern of his two branches is the main strand of the San Andreas fault in the Salton Trough, and that the name "San Andreas" can be properly applied to the strand. However, we are not certain that his south branch is a throughgoing strand of the San Andreas. Our nomenclature reflects these interpretations (map sheet 1). We refer to Dibblee's north branch as the Coachella Valley segment of the San Andreas fault, and we apply that nomenclature to the entire extent of the fault in the Salton Trough. Like Allen (1957), we refer to Dibblee's south branch as the Banning fault, and we believe that it is a reactivated segment of the ancestral late Miocene Banning fault (an idea proposed by Dibblee, 1975a, p. 134).

Oblique aerial photograph looking east Oblique aerial photograph looking east at the Coachella Valley trace of the Banning Fault in the northern Coachella Valley, southern California. Photo by J.C. Matti, USGS. Click here for more info and enlarged image.

The southeast and northwest terminations of the Coachella Valley segment of the San Andreas involve complex interactions with other fault zones. To the southeast, surface expression of the segment terminates near the southeast margin of the Salton Sea, where it interacts with the Brawley seismic zone and the Imperial fault (Sharp, 1982; Johnson and Hill, 1982). Some workers view this segment of the San Andreas fault as the northwesternmost of a series of right-stepping transform faults that extend from the Gulf of California onshore into the Salton Trough (Moore and Buffington, 1968, fig. 4; Elders and others, 1972, fig. 1; Crowell and Ramirez, 1979; Lonsdale and Lawver, 1980, fig. 1; Crowell, 1981, fig. 18-4; Johnson and Hill, 1982, fig. 6; Curray and Moore, 1984, figs. 1, 9).

At its northwest end the regional strike of the Coachella Valley segment is deflected westward, and it splays into the Mission Creek and Mill Creek strands of the southeastern San Bernardino Mountains. We cannot prove that these multiple strands merge beneath the Coachella Valley because they are buried by unfaulted Quaternary alluvium where they exit the San Bernardino Mountains. However, their map pattern in the mountains strongly suggests that the strands coalesce southeastward, and by the simplest interpretation they ultimately form a single fault zone--the Coachella Valley segment of the San Andreas fault.

Northwest of Desert Hot Springs the Coachella Valley segment of the San Andreas fault loses its clear surface expression, and Holocene displacements have not been demonstrated for the segment. The fault forms conspicuous scarps in Quaternary alluvium southeast of Desert Hot Springs, and discontinuous scarps can be traced northwestward where they disrupt young (but not youngest) alluvium in the center of town (Clark, 1984). However, to the northwest, latest Quaternary alluvial fans that flank the Little San Bernardino Mountains are not disrupted by the fault. The late Quaternary history of the Coachella Valley segment has not been worked out in this region.

Displacement history .--Although small youthful displacements on the Coachella Valley segment of the San Andreas fault have been recognized on the basis of cross-fault correlations between Quaternary alluvial materials (e.g., Keller and others, 1982; Matti and others, 1985), large older displacements recognized on the basis of cross-fault correlation between Cenozoic and pre-Cenozoic units generally have not been recognized. In part this reflects the fact that older rocks of appropriate age containing evidence for large-scale displacements largely are buried by young Quaternary sediment that has filled the Salton Trough. In addition, the widely cited model for 240 km of Pliocene and Quaternary displacement on the San Andreas fault in effect has dampened the search for pre-Pliocene cross-fault counterparts in the Salton Trough because the 240-km model requires that pre-Pliocene rocks have been displaced completely out of the Salton Trough region.

A study by Dillon (1975) suggests that this may not be the case. In his study of the southern Chocolate Mountains, Dillon (1975, fig. 70, p. 334-365) proposed that rocks in the southeastern San Bernardino Mountains have been displaced from the southern Chocolate Mountains by 180 20 km of right slip on the Coachella Valley strand of the San Andreas (Dillon's north branch of the San Andreas). Dillon's proposal is based on three cross-fault correlations: (1) crystalline rocks of San Gabriel Mountains-type northeast of San Gorgonio Pass correlated with similar rocks in the vicinity of Mammoth Wash in the Chocolate Mountains (Dillon, 1975, p. 59-60, 351-353); (2) the Miocene Coachella Fanglomerate in the Whitewater area correlated with the fanglomerate of Bear Canyon in the southern Chocolate Mountains (Dillon, 1975, p. 341-346); and (3) the inferred strandline position of the marine Imperial Formation in the Whitewater area correlated with the inferred strandline position for the marine Bouse Formation in the Chocolate Mountain region (Dillon, 1975, p. 347-350, fig. 69). Dillon's reconstruction contrasts with that of Peterson (1975), who restores the Coachella Fanglomerate farther south in the Salton Trough and calls for 215 km of displacement on the Coachella Valley strand of the San Andreas. Continue to San Bernardino Mountains Segment, San Andreas Fault

Continue to San Bernardino Mountains Segment, San Andreas Fault Zone
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