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 this zone, as well as its present right-lateral strain rate, suggest that currently it may be the most active member of the system in this region.

Both intrusive rocks of the mid-Cretaceous Southern California batholith and prebatholithic metasedimentary and possibly metavolcanic rocks are exposed nearly continuously along a 50-mile segment of the fault zone between San Jacinto and Borrego valleys in the northeastern Peninsular Ranges. The prebatholithic terrane on both sides of the fault zone in this interval consists of locally migmatized banded gneiss and lesser amounts of amphibolite, quartzite, marble, schist, and metaconglomerate. These rocks, together with gabbroic, tonalitic, granodioritic, and adamellitic plutons, constitute distinctive sequences of generally steep-walled bodies which are offset right-laterally by the various breaks in the fault zone. A marker section of relatively marble-rich metamorphic rocks within and parallel to a regionally unique, moderately dipping, postbatholithic zone of cataclastic deformation is also offset near Borrego and Clark valleys.

Extrapolation of critical offset contacts indicates that the cumulative right-lateral movement across the San Jacinto fault zone is about 15 miles. The southwestern side of the fault zone has been relatively raised through a probable 1- to 2-mile vertical component of net displacement, although the San Jacinto and Santa Rosa mountains northeast of the zone are now topographically higher.

North of Anza, distinctive Pleistocene gravels are offset at least 3.2 miles, and stream courses have been displaced 0.45 mile in a right-lateral sense.

The magnitude of displacement and its distribution among the various breaks within the zone indicate that: (1) the San Jacinto fault may connect with the Imperial fault in central Imperial Valley; (2) the positions of the Banning and Sierra Madre faults along the southern margins of the San Bernardino and San Gabriel mountains may reflect the total movement of the San Jacinto fault; (3) if the displacement on the San Andreas fault zone 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; and (4) when compared to the minimum probable Quaternary faulting rate, the displacement suggests the San Jacinto fault zone might be as young as Pliocene.

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