Gravity and Magnetic Maps of the Santa Maria Province, California

GRISCOM, ANDREW, and PETER E. SAUER, U.S. Geological Survey, Menlo Park, CA

In this paper we display an isostatic residual gravity map, contour interval 5 mGal, and an aeromagnetic map, which is a mosaic of existing data. Two computer-derived maps, also at a scale of 1:250,000, display the locations of the steepest gravity gradients and of the steepest gradients on the pseudogravity transform of the magnetic map. These gradients indicate boundaries between rock masses with differing physical properties. The gradient boundaries, where steep and linear, are presumably cause by faults.

The gravity reduction removes the isostatic effects of the continental margin, a 60 mGal gradient across the Santa Maria basin, leaving a maximum basin low of 40-50 mGal caused by the low-density basin rocks and bounded by various steep gradients at the basin margin faults. Offshore gravity lows indicate local sedimentary basins trending north to north-northwest. The magnetic anomalies are caused primarily by serpentinite in the basement of Franciscan assemblage and form irregular belts trending northwest across the basin and basin margins, terminating against the offshore Hosgri fault. The magnetic patterns west of the Hosgri are more irregular in character and may reflect a different basement.

The Hosgri fault appears to extend south beyond Point Arguello (about 2 km offshore) according to the gravity map. The right-lateral slip on this fault is thus presumably accommodated on north-dipping thrust faults on the north side of the Santa Barbara Channel and perhaps along the south side of the Santa Maria basin as well. Along the east side of the Hosgri fault, high gravity values indicate local uplift.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)