Geology of the San Jacinto Fault Zone in the Peninsular Ranges of Southern California

Abstract

The San Jacinto fault zone is one of the major branches of the San Andreas fault system in southern California. The straightness, continuity, and high seismicity of the San Jacinto fault zone suggest that it may be currently the most important member of the system. Although alluvium conceals much of the San Jacinto fault, intrusive rocks of the mid-Cretaceous southern California batholith are exposed together with prebatholithic metamorphic rocks along a 50-mile segment in the northeastern Peninsular Ranges. The prebatholithic terrane on both sides of the fault consists of migmatitic gneiss and minor amounts of amphibolite, quartzite, marble, and metaconglomerate. Between San Jacinto and Clark Valleys, various members of a distinctive sequence of metamorphic rocks and gabbroic, tonalitic, and adamellitic plutons are separated by the fault 13 1/2 to 15 miles in the right-lateral sense. A marker section of relatively marble-rich metamorphic rocks within and parallel to a regionally unique post-intrusion zone of cataclastic deformation exposed at the southern end of the Santa Rosa Mountains and at Coyote Mountain is separated between 8 and 13 miles. Geometric extrapolation of the various contacts suggests the southwestern block has risen between 1/2 and possibly 8 miles near Anza and between 0 and 6 miles near Clark Valley. Small net vertical movement near Clark Valley may correspond to relatively large vertical offsets near Anza. The sense of vertical movements probably has reversed repeatedly throughout the history of the fault. The right-lateral component of the net displacement probably increases southeastward from about 14 miles near San Jacinto Valley to between 14 1/2 and 17 miles, but conceivably as much as 22 1/2 miles, near Clark Valley. North of Anza, Quaternary gravels are offset at least 2 miles, and stream courses are displaced at least 2300 feet and possibly 3200 feet. Drainage lines north of Clark Valley have been offset possibly 3 miles in Quaternary time. The displacement on the San Jacinto fault suggests that (1) the line of major displacement within the San Jacinto fault zone extends southeastward into the central part of Imperial Valley and may connect with the Imperial fault, (2) the Banning fault at the southern margin of the San Bernardino Mountains may be the offset continuation of the Sierra Madre fault on the southern flank of the San Gabriel Mountains, and (3) if the displacement on the San Andreas fault is as large as 160 miles, the San Jacinto fault has not always been as important a member of the larger system as its current activity suggests.