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The Salton Trough is the landward extension of a ridge/transform-fault system, the East Pacific Rise, of the Gulf of California (see Figure. 3.13). This system became well established during the late Cenozoic (approx 5 Ma) as the plate boundary jumped inland from offshore Baja California (Atwater, 1970, 1989; Humphreys and Weldon, in press).

The Salton Trough is underlain by upper Cenozoic sedimentary rocks and minor amounts of volcanic rocks, which are exposed chiefly around its edge and are penetrated in wells. Onset of rifting and major subsidence in the Salton Trough was followed by marine incursion during the latest Miocene to late(?) Pliocene, as indicated by the Imperial Formation (Dibblee, 1954; Powell, 1984). The thick Cenozoic sedimentary section 15 offset by Quaternary faults, both exposed and buried, and is intruded by Quaternary volcanic rocks, both silicic rocks that form volcanoes at the two inferred onshore spreading centers (Figure. 8.7) and mafic rocks that are penetrated in geothermal wells (Elders and others, 1972; Robinson and others, 1976). Faulting in the Salton Trough occurs primarily on conjugate northwest- and northeast-striking faults and is largely strike slip (Johnson and Hadley, 1976; Johnson, 1979; Fuis and others, 1982). North-south-striking faults, however, such as the north end of the Imperial fault, the Brawley fault, and north-south-striking seismicity lineaments (that outline inferred spreading centers; Figures. 8.1, 8.7), have normal components and lead to the subsidence that ultimately created the Salton Trough. Earthquake hypocentral depths indicate that brittle fault motion extends to about 12-km depth in the Imperial Valley but deeper in the adjacent Peninsular Ranges along the San Jacinto fault (Doser and Kanamori, 1986).

Detachment faulting on the east flank of the Salton Trough, in the Chocolate Mountains and other ranges, preceded the Pliocene and later basin-forming tectonics in the Salton Trough (Dillon, 1975; Berg and others, 1982; Frost and others, 1982). Similar faulting on the west flank of the Salton Trough, however, may have both preceded and overlapped in time the tectonics in the Salton Trough (Wallace and English, 1982; Schultejahn, 1984; Isaac and others, 1986).

Biehler and others (1964) and Fuis and others (1982, 1984) demonstrated from seismic surveys that the sedimentary rocks (1.8-5.5 km/s) in the central Salton Trough are as much as 5 km thick (Figure. 8.6A). Below 5-km depth, a low-velocity (5.6 km/s) "basement," which is not separated from the overlying sedimentary rocks by a velocity discontinuity, is inferred to be metamorphosed (greenschist facies) sedimentary rocks (Fuis and others, 1982, 1984); this "basement" layer extends to 12-km depth. High heat flow in the Salton Trough (see Lachenbruch and others, 1985) is inferred to cause the metamorphism of the sedimentary rocks. Thus, the entire section of inferred upper Cenozoic sedimentary rocks, metamorphosed and unmetamorphosed, is as much as 12 km thick.

Below 12- to 14-km depth in the Salton Trough, a high-velocity (7.1-7.2 km/s) "subbasement" that is indicated by seismic-refraction data (Figure. 8.6A) is inferred to be gabbro generated at one of the nearby spreading centers (Fuis and others, 1982, 1984). Modeling of seismic-refraction and gravity data indicate that the Moho in the central Salton Trough is 23 to 28 km deep (Fuis and others, 1982, 1984). The central Salton Trough is interpreted to be underlain entirely by late Cenozoic crust (Figure. 8.6B).

Buried scarps separating old crust (plutonic and metamorphic rocks; 5.9-6.0 km/s) from new crust (sedimentary and basaltic rocks; 1.8-7.2 km/s) are visible by seismic methods on both sides of the Salton Trough (Fuis and others, 1982; Fuis and Kohler, 1984). On the west side of the rift, where the new-crust/old-crust boundary is ragged in outline (Figure. 8.7), we interpret normal faults (Figure. 8.6B); on the east side, where this boundary 15 linear, we interpret a strike-slip fault. In our cross section, faults on the west side of the Salton Trough are inferred to have originated by pullaway from the Cerro Prieto spreading center to the southeast; the fault on the east side is inferred to be a largely passive suture (Figures. 8.6B, 8.7; Fuis and others, 1982). A similar rift configuration is seen, for example, in the Gulf of Elat (Gulf of Aqaba, Red Sea; Ben-Avraham, 1985).