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Geomorphic and structural features of the creeping San Andreas Fault and long term creep rate, Flook Ranch, Bitterwater Valley, CA


, : Geomorphic and structural features of the creeping San Andreas Fault and long term creep rate, Flook Ranch, Bitterwater Valley, CA. Abstracts with Programs - Geological Society of America 36(4): 38

Structural and geomorphic features at Flook Ranch (latitude 36.3983 degrees 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 degrees W oriented fault traces that bound a fault-parallel swale and merge to the northwest into a single N45 degrees W striking trace near the southern end of a 1 km-long pressure ridge. We excavated a approximately 20-m-long trench (WLA T-1) across a prominent linear swale ( approximately 12 m wide) bounded by low 0.5-m-high east- and west-facing topographic escarpments. A approximately 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 approximately 95 year old fence 100 m south of WLA T-1 indicates the creeping zone is approximately 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 approximately 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 approximately 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 approximately 1,000 years.

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