Macro- and Micro-deformation Features of the Creeping Strand of the San Andreas Fault, and Evidence for Changes in Historic Creep Rate, Flook Ranch, Bitterwater Valley, CA

Evidence of active right lateral creep on the San Andreas Fault (SAF) at Flook Ranch (latitude 36.3982° ) includes offset cultural features, prominent tectonic geomorphology, macro- to micro-scale deformation of late Holocene alluvium, and historic variations in rate of activity. At Flook Ranch, the fault zone consists of two N35° W-striking faults that bound a fault-parallel, linear swale that is bordered by two 0.5-m-high scarps. Trenches excavated at the site exposed massive silt and clay loam overlain by <1,100-year-old thin-bedded silt and gravel, and two (western and eastern) fault zones bounding, in part, the margins of the linear swale. Creep-related structures at the fault zones include 4-m-wide zones of abundant fractures extending across the eastern and western fault zones, broad synformal warping, and truncated alluvial deposits. Initial observations of faulted sand samples at the microscopic scale show many distinct, narrow (<0.5mm) faults that consist of ultra-fine grained (clay size) material and bound silt and silty sand units. Sand units immediately adjacent to faults are notable for their lack of textural features that might record faulting; no porosity reduction, grain size reduction, or localized preferred grain orientation is observed at fault contacts. At the eastern fault zone: (1) apparent vertical offsets range from <0.2 m to ∼1.0 m, (2) Reidel shears dip steeply to the east, strike 10° -30° clockwise from the main SAF zone, and flower upward into pervasive extensional fractures, and (3) Reidel shears, rotate clockwise into extensional fractures as they approach the ground surface. In comparison, the western fault zone is characterized by: (1) apparent vertical offsets of as much as 2 m, (2) steep west dipping Reidel shears that strike 0° -50° clockwise from the fault zone, and (3) fractures extending to the ground surface show no progressive clockwise rotation up section. A March 2003 survey of an offset ∼95 year-old fence line yielded a dextral offset of 1.19-1.51 m and a creep rate of 22-28 mm/yr. This creep rate is: (1) lower than a 1966 reported creep rate of 32-33 mm/yr of the same fence line (Brown and Wallace, 1968), and (2) lower than a 1983 reported historic creep rate of 28-32 mm/yr of the fence line (Cotton et al., 1986), but (3) greater than the average 26 year (1969-1995) creep rate of 16-17 mm/yr of a U.S.G.S creep meter located <10 m south of the fence (Schulz, 1989). In summary, structural relations exposed in trenches, coupled with survey data, indicate that creep is concentrated primarily within the two fault zones, and that these fault zones record different faulting styles and variability in activity and deformational histories.