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