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Deep Structure of Central California Margin from EDGE Seismic Profile RU-3

K. D. McIntosh, E. A. Silver

Deep-penetration seismic reflection profile RU-3 crosses the central California margin southwest of Morro Bay and shows the Paleogene subduction zone as well as compressional and extensional features of the overlying accretionary wedge. Lower plate rocks are indicated by deep, gently dipping reflectors that extend beneath the continental margin for at least 36 km at a depth of about 15 km. A band of closely spaced folds and thrust faults, 1-2 km thick, with a basal decollement, occurs at the outer edge of the Santa Lucia Bank. These structures appear to have been formed at the toe of an accretionary wedge and subsequently uplifted to their present position. A zone of out-of-sequence thrusting and underplating has thickened the wedge and disrupted most of the original bedd ng between the shallow thrusts and deep reflectors.

Compressional structures in the offshore Santa Maria basin appear to be controlled by an earlier extensional tectonic framework. Tilted surfaces on large, coherent, basement blocks indicate that they have rotated, at least in vertical planes, to accommodate extension and compression during basinal evolution. Extension is indicated by stratigraphic units that thicken toward large faults with normal offsets. Prominent compressional features are apparent in the sedimentary section at the edges of the blocks, although less than 20% shortening is indicated. The Hosgri fault area appears to be the most tectonically active part of the margin at present, with uplift occurring here and in neighboring onshore areas.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.