--> Abstract: Late Quaternary Slip Across The San Gregorio Fault Zone, San Mateo County, California; Estimates from Marine Terrace Offsets, by G. E. Weber, J. M. Nolan, and E. N. Zinn; #90920 (1999).

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WEBER, GERALD E., Earth Sciences Dept., UCSC, Santa Cruz, CA; JEFFREY M. NOLAN, Nolan Associates, Santa Cruz, CA; and ERIK N. ZINN, Consulting Geologist, Santa Cruz, CA

Abstract: Late Quaternary Slip Across The San Gregorio Fault Zone, San Mateo County, California; Estimates from Marine Terrace Offsets

Based on recent geologic mapping we have revised the marine terrace stratigraphy on both sides of the San Gregorio fault zone. We delineated 6 marine terraces on the Pigeon Point structural block (west of the San Gregorio fault zone) that correlate well with the 6 of the 7 marine terraces present on the Santa Cruz Mountains structural block (east of the San Gregorio fault zone). Only the Cement Terrace on the Santa Cruz Mountains structural block has no counterpart on the Pigeon Point structural block.

The marine terraces can be used to estimate uplift rates of the structural blocks on opposite sides of the fault. East of the fault (Santa Cruz Mountains block) the Late Pleistocene uplift rate is between 0.4 and 0.55 m/ky. West of the fault (Pigeon Point block) the uplift rate is between 0.15 and 0.30 m/ky, with the latter the most reasonable interpretation.

The late Quaternary uplift rate across the San Gregorio fault zone is the difference between the uplift rates for the two structural blocks: 0.1 to 0.3 m/ky, with 0.15 m/ky the best estimate. Hence, the vertical slip rate across the fault zone during the past 600,000 years is extremely low and lies in the range of long term uplift rates for the central California coastline.

Unfortunately, marine terrace offsets do not constrain horizontal offset as well as the vertical offset. A horizontal slip rate across the San Gregorio fault zone of 4-11 mm/yr is consistent with the offset of both marine terrace shoreline angles and small streams near Point Ano Nuevo. The best estimate is 8-9 mm/yr of horizontal slip. Consequently, the differences in the late Quaternary horizontal and vertical slip rates indicate the San Gregorio fault zone is predominantly a strike-slip fault with a minor vertical component of slip.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California