Abstract: Timing and Cause of Middle Tertiary Magmatism in Onshore and Offshore Southern California

Peter W. Weigand

Numerous suites of middle Tertiary igneous rocks occur in coastal and offshore southern California between the Transverse Ranges and Ensenada, Mexico. Of these, 16 have been dated. With the exception of one ⁴⁰Ar/³⁹Ar determination, all the dates have been by the K-Ar method, and 44 of the 47 dates were published in the 1970s. Twelve of the dated suites cluster around 15 Ma. The exceptions include the San Miguel Island Volcanics (32-25 Ma), a pluton on Santa Catalina Island (19 Ma), the Santa Rosa basalts in the Peninsular Ranges (8.5 Ma), and basalt from Northeast Bank (4.5 Ma). The remaining dates range from 17 to 13 Ma and average 14.8 ± 3 Ma (n = 41). Large areas of the southern California continental borderland are covered by volcanic rocks that may have erupted during this same time span. This age of igneous activity coincides with the most active period of clockwise rotation of the Transverse Ranges as indicated by paleomagnetic results. This rotation resulted as dextral shear between the Pacific and North American plates became distributed across a wide belt, perhaps as the crust was heated by rising sub-continental mantle. Resulting localized extension in this mantle caused by rotations, rifting, and leaking transforms may have initiated decompression melting that produced this intense and widespread igneous event.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994