Abstract: Miocene Volcanic and Volcaniclastic Rocks of Santa Cruz Island: Evidence for Offset Along Santa Cruz Island Fault

Bruce M. Crowe, David G. Howell, Hugh McLean

On Santa Cruz Island, north of the Santa Cruz Island fault, a west-northwest-trending probable strike-slip fault, more than 2,500 m of Miocene volcanic and volcaniclastic rocks overlies unfossiliferous San Onofre Breccia (subsurface) and is overlain by lower Relizian strata of the Monterey Formation. Estimated volume percentages of exposed volcanic rocks are basalt 10 percent, andesite 65 percent, and dacite 25 percent. Andesite and dacite flow rocks contain a distinctive three-pyrozene phenocryst assemblage (coexisting augite, hypersthene, and pigeonite). The volcanic rocks are chemically similar to low-potassium island-arc tholeiites but have unusually high titanium dioxide values.

South of the Santa Cruz Island fault, Saucesian(?) through Luisian-age volcaniclastic deposits of the Blanca Formation locally overlie Saucesian San Onofre Breccia. The Blanca Formation contains voluminous reworked and subordinate primary-pyroclastic deposits interbedded with volcaniclastic sandstone and conglomerate indicating contemporaneous volcanism and sedimentation. Clasts of the lower and middle lithofacies of the Blanca Formation include greenschist and blueschist basement rocks and a texturally variable quartz-hornblende dacite. The upper Blanca lithofacies contains a diverse assemblage of pyroxene andesite and dacite.

The Santa Cruz Island volcanic field is truncated on the south by the Santa Cruz Island fault. The suite of volcanic clasts from the Blanca Formation differs mineralogically and texturally from the volcanic rocks north of the Santa Cruz Island fault. K/Ar dates from the Blanca Formation are slightly younger than those from the top of the northern volcanic section. These relations suggest strike slip of unknown amount and direction along the Santa Cruz Island fault and are consistent with but do not provide definitive evidence that this fault may be the subaerially exposed continuation of the East Santa Cruz Basin fault of Howell and others.

AAPG Search and Discovery Article #90976©1976 AAPG-SEPM-SEG Pacific Section Meeting, San Francisco, California