Rocks on the east flank of the Salton Trough are igneous and metamorphic rocks that compose two or more fault-bounded packages, or tectonostratigraphic terranes (see Howell and others, 1985). A complex of metasedimentary and mafic metaigneous rocks described by Dillon (1975) may include two Precambrian terranes, the Joshua Tree and San Gabriel terranes, described farther north by Powell (1981). This complex is intruded by intermediate to felsic Mesozoic plutons and rests on the low-angle Chocolate Mountains thrust fault above the (informal) Pelona-Orocopia schist of Haxel and Dillon (1978; see also Haxel, 1977). The Pelona-Orocopia schist consists chiefly of metagraywacke and lesser metapelite, metabasite, metachert, marble, and serpentinite (albiteepidote-amphibolite facies) of uncertain but probable late Mesozoic or early Tertiary age (Conrad and Davis, 1977; Miller and Morton 1977, 1980). It resembled the Franciscan assemblage but lacks melange.
Many workers have speculated on the depositional environment and origin of the Pelona-Orocopia schist. Haxel and Dillon (1978) postulated formation in an ensimatic rift basin with continent on both sides - not unlike the current Salton Trough. Powell (1981) favored an origin as a parautochthonous continental-marginal deposit. In any case, from its quartz content, the Pelona-Orocopia schist clearly originated near a continent and incorporated continental detritus. It was thrust beneath the continental metasedimentary-metaigneous complex some time after Mesozoic plutonism (80 Ma; Powell, 1981) and before Oligocene volcanism (35 Ma; Crowe 1978; Crowe and others, 1979). The thrust fault may have been reactivated one or more times as a low-angle normal, or detachment, fault (Frost and others, 1982).
Evidence from refraction profiling in the western Mojave Desert across the Rand schist, which has been correlated with the Pelona-Orocopia schist (Ehlig, 1968), indicates relatively low-velocity crust beneath this body (max 6.4 km/s; Fuis and others, 1986) that we infer to be continental crust. We speculate that the Pelona-Orocopia schist also rests on continental crust and that the Rand and Pelona-Orocopia schists were emplaced as a tectonic wedge into continental crust in a manner similar to the Franciscan assemblage of central and northern California. We hypothesize that the metasedimentary-metaigneous complex structurally above the schist is analogous to either (1) rocks of the Coast Range ophiolite/Great Valley sequence which rode passively atop the wedge in central and northern California after being extended during uplift of the Franciscan assemblage, or (2) rocks of the Great Valley sequence which were peeled up along backthrust faults during landward movement of the wedge. In southern California, tectonic wedging clearly occurred before the present transform regime, presumably during subduction of the Farallon plate (or its derivative). The geologic data discussed above indicate that the Salton Trough has undergone extension, rather than compression, for approximately the past 5 Ma (probably even longer; see Humphreys and Weldon, in press).
Crustal thickness is unknown in the Chocolate Mountains; however, the Colorado Desert, to the east and north, has a generally thin (26-28 km) crust (Figure. 8.3) and a local root (32 km deep) under the Whipple Mountains metamorphic-core complex (Fuis, 1981; Jill McCarthy, written commun., 1988).