GEOMORPHIC AND STRUCTURAL FEATURES OF THE CREEPING SAN ANDREAS FAULT AND LONG TERM CREEP RATE, FLOOK RANCH, BITTERWATER VALLEY, CA

Structural and geomorphic features at Flook Ranch (latitude 36.3983°N) on the creeping segment of the dextral San Andreas Fault have recorded deformation of late Holocene fan deposits.  A detailed survey of the fault zone shows two N35°W oriented fault traces that bound a fault-parallel swale and merge to the northwest into a single N45°W striking trace near the southern end of a 1 km-long pressure ridge.  We excavated a ~20-m-long trench (WLA T-1) across a prominent linear swale (~12 m wide) bounded by low 0.5-m-high east- and west-facing topographic escarpments. A ~10-m-long trench (WLA T-2) was also excavated across a single low southwest-facing escarpment 60 m northwest of WLA T-1 and on the opposite side of an active fluvial channel.  Abundant extensional fractures extend across a width of 18 m in WLA T-1, and a width of 4 m in WLA T-2; the greatest concentration of fractures coincides with the topographic escarpments in both trenches.  Extensional fractures, likely accommodating deformation associated with aseismic creep, rotate clockwise as they near the ground surface and are oriented 20 to 30 degrees clockwise from the main San Andreas Fault zone.  A survey of an offset ~95 year old fence 100 m south of WLA T-1 indicates the creeping zone is ~13 m wide and the fence is dextrally offset 1.19 to 1.51 m.  The long-term creep rate for this segment of the San Andreas Fault for the last ~95 years is 22-28 mm/yr.  This rate is lower than the previously reported historic creep rate of 28-32 mm/yr based on 1983 measurements (Cotton et al., 1986).  A creepmeter ~25 m south of the fence indicates a 16-17 mm/yr creep rate between 1969 and 1995, but there is some question whether or not the creepmeter extended across the entire width of active deformation (Schulz, 1989). Colluvial wedge deposits and liquefaction-related features were not observed in the trenches; their absence strongly suggests that these deposits have not experienced a large coseismic surface-fault rupture in the last ~1,000 years.