Update Re: Predictive Late Cretaceous to Early Miocene Paleogeography of the San Andreas Fault System Derived from Detailed Multidisciplinary Conglomerate Correlations

Kathleen Burnham
Independent Research, P. O. Box 21087, Oakland, CA 04620-1087

Sixteen upper Cretaceous and Paleogene conglomerates of the California Coast Ranges, from Anchor Bay to Simi Valley, were included in a multidisciplinary study. Detailed analysis; including microscopic petrography (69-135 characteristics), microprobe geochemistry, and SHRIMP U/Pb zircon dating; centered on identification of matching unique clast varieties, rather than on simply counting general clast types, and included analyses of matrices, fossils, diagenesis, paleocurrents, adjacent rocks, and stratigraphy.

San Andreas fault system paleogeographic reconstruction was impeded for three decades by seemingly conflicting geologic correlations. Apparently irreconcilable proposals of dextral offsets across the San Andreas fault included reports of certain younger features showing larger offset than older features across the same fault. These included: 315 km offset of Miocene Pinnacles-Neenach Volcanics (Matthews, 1976), 563 km offset of Cretaceous Anchor Bay-Eagle Rest peak (Ross et al., 1973), ~30 km offset of six pairs of Cretaceous-Pliocene features across the San Andreas fault section between Pinnacles and Point Reyes (Dibblee, 1966), and at least four proposals of Oligocene-Miocene correlatives separated ~315 km across that same section (Addicott, 1968; Nilsen and Clarke, 1975; Stanley, 1987; Graham et al., 1989).

From conglomerate correlations, and using the modern shapes of tectonic blocks, an upper Cretaceous–early Oligocene paleogeographic model was constructed (Burnham, 1998 thesis) that has proved to be predictive. Since its first introduction, in April and June 1998, other authors have reported seven subsequently identified correlative pairs of geological and geophysical features consistent with it. The 1998 model with its regionally expanded 2006 version now synthesizes all of the above, incorporates at least 58 pairs of correlative features, extends from 70 to 21.3 Ma, and reaches from Pelona and Orocopia to the Mendocino triple junction. The model suggests that especially considering geologic timescales, the San Andreas fault is not the principal fault of the San Andreas fault zone.

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California