Correlation of Pinnacles and Neenach Volcanic Formations and Their Bearing on San Andreas Fault Problem
The Pinnacles Volcanic Formation (San Benito County, California) is an early Miocene (23.5 m.y.B.P.) sequence of calc-alkaline andesite, dacite, and rhyolite flows interbedded with pyroclastic and volcaniclastic rocks. Structurally the volcanic rocks form a west-dipping homocline which is truncated on the northeast by the Chalone Creek fault. The Chalone Creek fault is interpreted to be an early Miocene trace of the San Andreas fault. The Neenach Volcanic Formation (Los Angeles County, California) is an early Miocene (23.5 m.y.B.P.) sequence of calc-alkaline andesite, dacite, and rhyolite flows interbedded with pyroclastic and volcaniclastic rocks. Structurally the rocks form a northwest-dipping homocline which is broken by many faults. The formation is truncated on the southwest by the San Andreas fault. The Pinnacles Volcanic Formation and the Neenach Volcanic Formation lie respectively west and east of the San Andreas fault zone and are separated 315 km from each other in a right-lateral sense. Ten rock types with nearly identical field, petrographic, and chemical characteristics are present in each formation in essentially the same stratigraphic order: vitric lapilli-tuff, flow-banded rhyolite, perlite, pumice lapilli-tuff, hypocrystalline hypersthene andesite, augite-olivine andesite, andesite tuff, porphyritic dacite, agglomerate, and porphyritic rhyolite. The Pinnacles Volcanic and Neenach Volcanic Formations were contiguous across the San Andreas fault at the time of their formation and subsequently have been offset 315 km along the San Andreas fault. For several reasons the Pinnacles-Neenach correlation provides some of the most conclusive documentation of large-scale, lateral displacement on the San Andreas fault yet discovered in central California: (1) a precise distance of separation can be determined because both areas are directly adjacent to traces of the fault, thus eliminating the necessity of interpretive projection into the fault; (2) the age of the formations can be determined accurately because the rocks are volcanic and suitable for radiometric dating; (3) the correlation is based on 10 different rock types, each with distinctive petrologic features, thus reducing the likelihood of chance occurrence. Other correlations of pre-Miocene rocks along the San Andreas fault show offsets of the same magnitude as the Pinnacles-Neenach rocks suggesting that 315 km may be the maximum offset along the San Andreas fault proper.