Abstract

Well developed flights of marine terraces along the central California coastline, provide horizontal datums that record the neotectonics of the coastal area. The San Gregorio fault zone, a major branch off of the San Andreas fault deforms and offsets these marine terraces in coastal San Mateo County, and unambiguously offsets the marine terrace shoreline angles horizontally at Point Ano Nuevo. The fault zone is undergoing right lateral strike slip movement with 10-15 degrees of convergence, with the majority of the movement occurring along the two primary faults within the zone, the Frijoles and Coastways faults. Analysis of the offset of the two lowest marine terraces (Santa Cruz and Western terraces) indicates the rate of movement has been essentially constant throughout most of the late Pleistocene (past 230,000 years) at 6-10 mm/yr. Analysis of deranged stream patterns of two small creeks at Point Ano Nuevo suggests that horizontal slip has ranged between 4-8 mm/yr over the past 105,000 years.