Pacific-North American plate interaction and Neogene volcanism in coastal California

Abstract

Palinspastic and global plate tectonic reconstructions of the southwestern United States, combined with analysis of the distribution of Cenozoic igneous activity suggest that Neogene and younger volcanic rocks in the Continental Borderland and Coast, Transverse and Peninsular Ranges appear to reflect subduction of the Farallon plate south of the Mendocino Fracture Zone and overriding of the Pacific—Farallon ridge. The igneous activity of Neogene age in coastal California appears to be a continuation of fields of magmatism in adjacent areas of southeastern California, Arizona and Baja California. With the close approach of the Pacific—Farallon ridge to the North American continental margin, progressively younger, hotter and thinner Farallon plate underwent subduction beneath California, resulting in more elevated temperatures at shallower depths and closer to the trench. Consequently, the associated volcanic arc apparently shifted to the west. Volcanic activity in coastal California appears to have been most widespread shortly after overriding of the Pacific—Farallon ridge occurred— that is, at that time at which the oceanic lithosphere was thinnest, and hot, partially molten asthenosphere was closest to the subduction zone. The zone of near-coast igneous activity in California was probably limited on the north by the extension of the Mendocino Fracture Zone in the descending Farallon plate. South of the fracture zone the lithosphere was young and hot; to the north it was older and, therefore, colder. Thus, Neogene igneous activity in the Basin-Range Province was associated with more typical subduction of older lithosphere involving a deeper, down-dip locus of melting, resulting in a wider arc-trench gap. The abrupt narrowing of the Neogene arc-trench gap from north to south occurred near the present-day Garlock Fault. The role of oblique and en echelon strike-slip faulting in permitting and localizing middle Cenozoic magmatism in the areas west of the San Andreas Fault is uncertain. Discrepancies in timing of movement on the northern and southern San Andreas Fault seem to require oblique strike-slip faulting, and by association, igneous activity, in the Borderland and Transverse Ranges prior to 10 m.y. B.P. Oblique, divergent strike-slip faulting in the San Francisco Bay area does seem to be responsible for significant igneous activity between 14 m.y. B.P. and present. The Page Mill, Grizzly peak , Sonoma and Clear Lake volcanic fields occur in zones of divergence between faults of the San Andreas, Calaveras and Hayward systems. Further, the age of these volcanic rocks corresponds with the documented displacement history of the San Andreas Fault.