Plio-Pleistocene Slip Rates on San Andreas Fault in Coachella Valley from Clast Provenance Studies in Mecca Basin

C. J. Lewis¹, S. Dougherty², A. G. Sylvester¹, and C. J. Busby^1
1Department of Geological Sciences, Univ of California Santa Barbara, University of California, Santa Barbara, CA
²ExxonMobil Production Company, New Orleans, LA

We use unique clasts in Pliocene-Pleistocene conglomerate beds to infer the rate of right-lateral displacement of the Mecca basin from its source areas across the San Andreas fault in Coachella Valley. The Mecca basin on the northeast side of the San Andreas fault in Coachella Valley contains Pliocene-Pleistocene nonmarine sedimentary rocks, including distinctive conglomerates displaced right-laterally from their source areas in the Santa Rosa Mountains on the southwest side of the fault. All of the conglomerates in the Pliocene- Pleistocene Palm Spring Formation consist primarily of granitic clasts and have source areas on both sides of the San Andreas fault. Distinctive chlorite-albite schist, vein quartz, and granite and gneiss clasts have a probable source area in the Orocopia and Little San Bernardino Mountains on the northeast side of the fault. One conglomerate horizon, however, contains a distinctive assemblage of mylonitic meta-carbonate, mylonitic granite, tourmaline-bearing pegmatite, garnet-bearing aplite, and biotite schist clasts with an apparent source area in the Santa Rosa mylonite zone on the southwest side of the fault. Quantitative establishment of the clast provenance in each of the conglomerate beds included in the Pliocene- Pleistocene nonmarine strata permits an assessment of the predominant source area during its period of deposition.

Preliminary conclusions indicate that the Mecca basin lay north of the Santa Rosa mylonite zone before ~2mybp, and then lay adjacent to the mylonite zone during deposition of the unique conglomerate clasts in the upper member of the Palm Spring Formation ~1mybp. Since that time the Santa Rosa mylonite zone has been displaced to its present position 35km north of the Mecca Hills, requiring a slip rate of 30 to 35 mm/yr. This rate is the same order of magnitude as published estimates of slip rates on the San Andreas fault, >20 mm/ yr measured geodetically in the Coachella Valley, the 30–35 mm/yr from creep measurements in central California, and the >12 mm/yr from correlation of the Pinnacles and Neenach Volcanic Formations.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California