Print this pageAbstract: Neotectonics of the San Gregorio Fault Zone: Age
Dating Controls on 
Offset
History and Slip Rates
CLARK, JOSEPH C., IUP, Indiana, PA
The San Gregorio fault (SGF) is part of the larger San Andreas fault (SAF) system of central California that forms part of the active boundary between the Pacihc and North American plates. The SGF is located mostly offshore from Monterey Bay to its juncture with the SAF in Marin County, a distance of as much as 200 km. Cross-fault matches of porphyritic granodiorites, lower Eocene conglomerates, and Oligocene basalts on the Monterey and Point Reyes Peninsulas indicate about 150 to 160 km of cumulative right-slip. Correlations and dating of an ash bed in the Monterey diatomite at 10.83+0.03 Ma suggest that this slip was initiated about 10 Ma (late Miocene).
A glauconitic facies of the Santa Margarita that crops out in the central
Santa Cruz Mountains and on the Point Reyes Peninsula is dated at 7.9~0.3
Ma and offset about 100 km. The cross-fault correlation of two pairs of
Purisima sections in the Santa Cruz Mountains suggests about 19 km of offset
in the last 3 m.y. These data indicate the following slip rates for the
SGF.
Time interval Displacement Slip rate
10-8 Ma (late Miocene), 50-60 km, 25-30 mm/year.
8-3 Ma (late Miocene-lower Pliocene), 81 km, 16 mm/year.
3-0 Ma (late Pliocene-Holocene), 19 km, 6 mm/yea.
The post-late Pliocene slip rate of 6 mm/yr falls within the Quaternary slip rate estimate of 5-11 mm/yr of other workers. The addition of slip from the SGF to the northern SAF may explain the postulated greater present rate of slip north of their juncture with that rate suggested for the San Francisco Peninsula segment of the SAF.
AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California