Abstract: Constraints from Potential Field Anomalies on Offset Histories of Faults Bounding the Gualala Block

JACHENS, R.C., C. M. WENTWORTH, and R. J. McLAUGHLIN, U.S. Geological Survey, Menlo Park, CA

Apparent minimal offset on the northwest-trending offshore fault bounding the west side of the Gualala block severely limits fault reconstructions in the northern San Andreas fault (SAF) system. Anomalies in new high-resolution aeromagnetic surveys suggest two magnetic bodies associated with the southern half of the Gualala block and truncated at the SAF: a shallow body coinciding with the Black Point spilite that is confined between the San Andreas fault and the offshore fault 10 km to the west, and a larger, deeper body that extends from a 30-km-long edge along the SAF westward beyond survey control (>20 km) at the edge of the continental shelf. The two bodies have nearly equivalent northern boundaries and, unless fortuitously juxtaposed, are most likely related parts of a common mass. The deep body passes beneath the trace of the offshore fault with no significant disruption of its northern or southern boundaries, a fact which denies large strike slip if this fault is steep but would permit unspecified offset if this fault has a shallow dip.

Palinspastic restoration of the Gualala block is constrained by distinctive magnetic and gravity anomalies (combined with geologic relationships) along strands of the SAF system. Restoring 150 km of rightslip along the San Andreas-San Gregorio-Hosgri faults and 25 km along the Rinconada-Reliz fault aligns the Gualala block magnetic anomaly with a comparable anomaly over inferred buried Logan gabbro in coastal San Mateo County. This restoration requires no offset on the offshore fault, but places the Gualala block west of Salinia. Alternatively, restoration of the Gualala block along the SAF proper in the San Francisco Bay area (while also honoring the constraints given above) requires at least 70 km of 175 km of the right-slip accommodated to the south on the San Andreas-San Gregorio-Hosgri and Rinconada-Reliz faults be accommodated to the north on the fault offshore of the Gualala block. Then restoring 100 km along the SAF to align Vizcaino block and Sierra Azul (Santa Cruz Mts) gravity and magnetic anomalies places the Gualala block adjacent to similar but not equivalent Sierra Azul stratigraphy.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California