HR: 0800h
AN: G21C-0679    [Abstracts]
TI: Surface Geometry and Geomorphology of the Rodgers Creek Fault, San Francisco Bay Area
AU: * Hecker, S
EM: shecker@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., M.S. 977, Menlo Park, CA 94025, United States
AB: The Rodgers Creek fault, part of the right-lateral San Andreas fault system in the San Francisco Bay area, is geometrically segmented by bends on multiple scales. North of Sonoma Mountain, along the northern half of the fault, sections of the fault trace trend approximately parallel to the direction of relative plate motion (~N34°W) and display a right-stepping pattern across releasing double bends. Within the releasing bends, the fault trends >5° oblique to plate motion and shows geomorphic evidence of extension. The largest right bend, ~1 km at Santa Rosa, corresponds to the lowest elevations along the fault. To the south, the fault makes a broad restraining double bend around the southwest flank of Sonoma Mountain and trends up to ~13° compressively oblique to plate motion. Long-term uplift (Sonoma Mountain) east of the bend suggests a reduction in slip on the fault to the south. The restraining bend corresponds to the north end of a pronounced aseismic region along the fault that may represent a spatial change in the mode of strain accommodation. Aerial photo analysis (1:6 k) of well-preserved geomorphology at the south end of the Rodgers Creek fault, where the fault makes another left bend with respect to plate motion, reveals a section that is undergoing progressive inversion from localized transtension (at a right bend) to transpression. This inversion is manifest as a northwest- lengthening zone of uplift within the fault zone. The youngest push-ups appear to be overprinting a relict pull-apart and sag pond. This and possibly older sag deposits along the margin of the uplift may mark former positions of a releasing geometry in the fault trace, presently located directly north of the uplift front. Geometric and overprinting relations suggest that the main trace of the fault rotates and translates through the passing bends. This mode of fault-bend migration contrasts with a previously proposed model in which new transverse structures develop progressively in the direction of deformation (Wakabayashi and others, 2004).
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8111 Continental tectonics: strike-slip and transform
DE: 8175 Tectonics and landscape evolution
SC: Geodesy [G]
MN: 2007 Fall Meeting