The Miocene Blanca Formation, Santa Cruz and Santa Rose Islands, California--Stratigraphic Correlation and Tectonic History

SAVAGE, KAREN L., PETER W. WEIGAND,* BARBARA D. CHIN, STEPHANIE L. DESPARD, and JENNIFER L. SHELTON, California State University, Northridge, CA

Geochemical studies of volcanic clasts from the middle Miocene Blanca Formation exposed on Santa Cruz and Santa Rosa Islands confirm earlier studies that suggested correlation of these two occurrences on the basis of stratigraphic position and lithology. Clasts from both areas are calc-alkaline dacite and rhyolite that define smooth trends on element-variation diagrams and possess unusually low incompatible trace-element concentrations and Sr-isotopic ratios when compared to other typical calc-alkaline volcanic suites: Rb ranges from 6 to 60 ppm, Sr from 260 to 575 ppm, and 87Sr/86Sr averages 0.7033. The main exposure of this formation on Santa Cruz Island has been divided into three members. Based on elemental similarities, clasts from the Santa Cruz Near Point exposure are most simi ar to those from the lower two members, whereas clasts from the Santa Rose exposure are more similar to those from the upper member. Clasts from all three exposures likely were derived from the same volcanic complex before being deposited in a single submarine-fan complex. We conclude that these rocks were erupted and deposited onto the Baja Borderland allochthon by subduction-related volcanism that was active south of the Rivera triple junction. The fan complex was then translated on this allochthon northward relative to the North American craton and rotated ~90 to 128 degrees CW to its present position.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)