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ABSTRACT: Tectonic Implications of Early Miocene Volcanic Rocks, San Joaquin Basin

R. B. Cole, P. G. Decelles

The early Miocene (22 Ma) Tecuya volcanic rocks of the San Emigdio Mountains were erupted during a regional episode of crustal extension in coastal California. These rocks are correlative with similar rocks in outcrops and the subsurface over at least 800 km2 in the southern San Joaquin basin. Initial dacitic eruptions produced laterally continuous subaerial to submarine pyroclastic flows. These facies rapidly buried alluvial fan deposits of the lower Tecuya Formation and marine sandstone of the Temblor Formation farther west. Following dacitic volcanism, massive basalt was deposited subaerially along the southeastern basin margin. Farther west, the basalt triples in thickness across synvolcanic, high-angle faults and consists of hyaloclastite and submarine bas lt flows. Facies associations, thickness and tectural trends, and paleodispersal directions of the volcanic rocks were controlled by synvolcanic extensional faulting throughout the southern San Joaquin basin. The regional extent of these volcanic rocks provides an important chronostratigraphic marker and potential stratigraphic trap in a variety of depositional settings, from nonmarine to outer shelf and slope. Preliminary rare-earth element data indicate that both dacites and basalts exhibit LREE enrichment patterns. The ^egrNd(O) values cluster around +3. Although these data are typical of island arc volcanic rocks, geological data clearly indicate that the Tecuya volcanic rocks were erupted in an extensional tectonic setting near the continental margin. Some other model, pe haps involving crustal contamination of magmas produced along the subducted East Pacific Rise, followed by syntectonic eruption along normal faults, is needed to explain this enigmatic combination of geochemical and geological data.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990