WEIGAND, PETER, California State University, Northridge, CA

ABSTRACT: Middle Miocene Volcanic Rocks of the South Coast Region California

Volcanic activity in the South Coast region of California began in south-central San Diego County about 21 Ma and spread west. Between about 17 and 13 Ma, volcanism occurred along the coast from northern Baja California to the Santa Monica Mountains. Scattered activity continued in the area until about 8.5 Ma. Middle Miocene volcanic rocks crop out in the South Coast region at the following twelve locations: Palos Verdes Peninsula, El Modeno, San Joaquin Hills, Santa Rosa, Laguna Beach, Santa Barbara Island, Santa Catalina Island, San Clemente Island, Anza, Alverson, Jacumba, and Rosarito Beach. Andesite is the predominant rock type. Major-oxide data available from nine of these areas show that eight are calc-alkaline; the Alverson rocks are mixed tholeiitic and alkalic. Initial {87}S /{86}Sr averages 0.7034 +/-0.0003 (7 measurements from 4 areas) and does not vary with SiO2 content. The South Coast centers erupted contemporaneously with a group of five volcanic areas located along the southern margin of the Transverse Ranges and which has generally similar geochemical and isotopic characteristics, but is distinguished by low concentrations of K2O and incompatible trace elements.

The tectonic environment of generation of the magmas parental to these rocks is poorly understood. Subduction presumably ceased in this area about 25 Ma and was succeeded by transform motion which included a major period of crustal rotation that was contemporaneous with much of the volcanic activity. The 87Sr/86Sr data suggest an origin from the mantle or primitive crustal source, and the calc-alkaline signature suggests a subduction environment. Were both suites erupted south of the Rivera triple junction, then translated some 1500 km north to their present position? Or did some sort of young, hot, oceanic slab remain in this area after active subduction ceased and melt 8 to 12 m.y. after San Andreas strike-slip movement began? Did extension from the crustal rotation initiate melting in the lower crust or mantle? Finally, what environments can produce typical calc-alkaline and low-K calc-alkaline magmas that overlap in space and time?

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.