Abstract: ⁴⁰Ar/³⁹Ar Geochronology of Southern California Neogene Volcanism

LUYENDYK, B. P., P. GANS, and M. J. KAMERLING, Univ. of California, Santa Barbara, CA 93106

Mafic rock samples from the northern Channel Islands (western Transverse Ranges) and the Peninsular Ranges and offshore islands were dated by ⁴⁰Ar/³⁹Ar incremental heating experiments on groundmass and plagioclase concentrates (21 total). Many of the ages are substantially different from published K-Ar dates. A 17.7 +/- 0.3 Ma lava from San Miguel Island agrees with the paleontology control here (Saucesian) and conflicts with prior older K/Ar dates from units above the one we dated. Hypabyssal intrusions from Santa Rosa Island are around 19 Ma. The top of the Santa Cruz Island volcanic section is 16.33 +/- 0.26 Ma. Anacapa ages near 16 Ma are the youngest found for the northern Channel Islands. Published ⁴⁰Ar/³⁹Ar dates from the Tranquillon volcanics are 17.8 Ma suggesting a widespread volcanic event between 19 and 16 Ma. We interpret this as timing the beginning of rifting of the northern borderland. The apparent age progression of volcanism from west (19-17 Ma) to east (16-15 Ma) is suggestive of a propagating rift prior to major extension and rotation of the western Transverse Ranges. The El Modeno volcanics in the Peninsular Ranges give ages of about 11 Ma, several million years younger than published K/A dates. The Rosarito Beach Formation comprises Miocene basalts, tuffs, and sediments that accumulated in two basins between Tijuana and Ensenada. The section spans from 15.51 +/- 0.14 to 16.19 +/- 0.28 Ma. This formation was derived from a western (offshore) source that has since been translated away, or submerged, or eroded. It is possible that part of the northern Channel Islands platform was adjacent here before rifting and rotation. Ages from San Clemente Island are younger the Rosarito section. The base of the exposed section is 15.32 Ma. The combined sections on San Clemente and at Rosarito span at least eight reversed magnetic polarity intervals. More ages from these sections could aid in calibrating the magnetic reversal time scale.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California