BASINS AND PLUTONS ALONG THE SAN JACINTO FAULT ZONE, SAN BERNARDINOVALLEY TO SAN JACINTO VALLEY, SOUTHERN CALIFORNIA

Gravity and magnetic anomalies delineate structures and plutons along the San Jacinto fault zone (SJFZ), the youthful and most seismically active member of the San Andreas system in southern California. South of San Bernardino basin, 24-25 km of right slip has been well documented on the various faults associated with the fault zone.

The Lakeview Mountains, immediately west of the Casa Loma strand of the SJFZ, consist of a compositionally zoned tonalite pluton, which produces positive gravity and magnetic anomalies. These anomalies are located at a bend in the regional gravity and magnetic gradient produced by the change from mafic Western Peninsular Ranges Batholith (PRB) to less mafic Eastern PRB basement; this gradient does not cross, but parallels the SJFZ northwest of the Lakeview pluton. 2-D and 3-D modeling of gravity and magnetic anomalies indicates that the Lakeview pluton extends to midcrustal depths of 10-15 km. The edges of the pluton dip steeply and the eastern edge lies near the Casa Loma fault. There is no equivalent gravity or magnetic high on the eastern side of the SJFZ anywhere southeast of the Lakeview Mtns. Seismicity projected onto a profile perpendicular to the SJFZ and across the pluton forms a diffuse cloud that terminates at the eastern edge of the pluton. We speculate that the location of the SJFZ may have been influenced by the presence of this relatively dense Mesozoic pluton.

The San Jacinto graben lies between the Casa Loma and Claremont strands of the SJFZ and is commonly cited as an example of a simple extensional strike-slip basin. The depth extent of this graben was modeled previously using 2-D seismic and gravity data. Here we use gravity data to produce a preliminary 3-D model of basin thickness, utilizing the density contrast between crystalline basement rock and basin fill. The 3-D model shows two ~10-km-long subbasins beneath the flat alluvial floor of the valley. The deepest basin is north of the Lakeview pluton and is 4 km thick. The other basin, centered beneath the town of San Jacinto, is 2.5-3 km deep. The basin model implies a more complex evolution of the "classic" extensional strike-slip basin beneath San Jacinto Valley.