NEW ISOTOPIC AGE FROM BASALT NEAR CARMEL, CALIFORNIA, AND ITS TECTONIC SIGNIFICANCE

An age of 24.0 to 23.6 Ma on basalt near Carmel, California, provides support for a tectonic model that relates Neogene volcanism in coastal California to the development of slab windows during collision of the East Pacific Rise with western North America. On the north side of Arrowhead Point (latitude 36.5626, longitude -121.9391), the basalt rests unconformably on marine strata of the lower Eocene Carmelo Formation and is overlain by unnamed Miocene marine sandstone. Three of six samples of basalt collected from sea cliff exposures were not dated because petrographic examination revealed evidence of substantial alteration. Plagioclase separates from the remaining three samples were analyzed using the ⁴⁰Ar/³⁹Ar incremental-heating method. Two of the three samples yielded anomalously old ages of 60 to 30 Ma and U-shaped age spectra characteristic of excess argon. The age spectrum from the final sample showed evidence of modest argon recoil but no excess argon. This sample yielded an isochron age of 24.0 ± 0.2 Ma and a total-gas age of 23.6 ± 0.1 Ma, but no age plateau. An eruption age of 24.0 to 23.6 Ma is inferred for this basalt.

The newly-determined age from the Carmel basalt is younger than previously reported whole-rock K-Ar ages of 27.1 ± 0.8 Ma and 27.0 ± 0.8 Ma from basalt samples collected in the same area. K-Ar ages provide no insight into the Ar distribution in these samples, but the presence of excess argon is inferred based on our ⁴⁰Ar/³⁹Ar results.

The new age determination supports the hypothesis of Wilson and others (2005) that the Carmel basalt erupted from one of several volcanic centers that originated about 24-22 Ma above a slab window that resulted from subduction of the spreading ridge segment between the Mendocino and Pioneer fracture zones along the California coast. This eruptive episode is recorded by volcanic rocks that presently occur over a large area from near Point Arena (Iversen Basalt, about 300 km northwest of Carmel) to the western Transverse Ranges (Neenach Volcanics, about 360 km southeast of Carmel) and were dispersed from their original relative positions by Neogene tectonism that included substantial right-lateral displacements along the San Andreas, San Gregorio, and other strike-slip faults.