| Summary: | This part of DS 781 presents data for the folds for the geologic and geomorphic map of the Offshore Aptos map area, California. The vector data file is included in "Folds_OffshoreAptos.zip," which is accessible from http://dx.doi.org/10.5066/F7K35RQB. The shelf in the Offshore of Aptos map area is cut by a diffuse zone of northwest-striking, steeply dipping to vertical faults mapped with high-resolution, seismic-reflection profiles (sheet 8). Faults are mapped on the basis of abrupt truncation or warping of reflections and (or) juxtaposition of reflection panels with different seismic parameters. Seismic profiles traversing this diffuse zone cross as many as 13 faults over a distance of 8 km. Mapped fault lengths in this diffuse zone are typically 2 to 7 km, and the strike of these offshore faults rotates from about 325° to 350° from southwest to northeast. Faults in this diffuse zone cut through Neogene bedrock and locally appear to disrupt overlying latest Quaternary sediments, and the presence of warped reflections along some fault strands suggests there may be both vertical and strike-slip offsets. This broad, distributed zone of deformation resembles the northwest-trending Monterey Bay Fault Zone (Greene, 1977, 1990), which occurs about 10 km farther west in outer Monterey Bay and similarly lacks a lengthy (> 20 km), continuous "master fault." Deformation in both the Monterey Bay Fault Zone and the diffuse zone of faults in the Offshore of Aptos map area is attributable to its location in the 40-km-wide, northward-narrowing structural zone between two major, right-lateral, strike-slip faults, the San Andreas Fault to the east and the offshore San Gregorio Fault to the west (McCulloch, 1987; Brabb, 1997; Wagner and others, 2002; Dickinson and others, 2005). Folds were primarily mapped by interpretation of seismic reflection profile data (see OFR 2013-1071). The seismic reflection profiles were collected between 2007 and 2010. References Cited Brabb, E.E., 1997, Geologic Map of Santa Cruz County, California: A digital database, US Geological Survey Open-File Report 97-489, 1:62,500. Dickinson, W.R., Ducea, M., Rosenberg, L.I., Greene, H.G., Graham, S.A., Clark, J.C., Weber, G.E., Kidder, S., Ernst, W.G., and Brabb, E.E., 2005, Net dextral slip, Neogene San Gregorio-Hosgri Fault Zone, coastal California: Geologic evidence and tectonic implications: Geological Society of America Special Paper 391, 43 p. Greene, H.G., 1977, Geology of the Monterey Bay region: U.S. Geological Survey Open-File Report 77-718, 347 p. Greene, H.G., 1990, Regional tectonics and structural evolution of the Monterey Bay region, central California, in Garrison, R.E., Greene, H.G., Hicks, K.R., Weber, G.E., and Wright, T.L., eds., Geology and tectonics of the central California coastal region, San Francisco to Monterey, Pacific Section American Association of Petroleum Geologists, Guidebook GB-67, p. 31-56. McCulloch, D.S., 1987, Regional geology and hydrocarbon potential of offshore central California, in Scholl, D.W., Grantz, A., and Vedder, J.G., eds., Geology and Resource Potential of the Continental Margin of Western North America and Adjacent Oceans -- Beaufort Sea to Baja California: Houston, Texas, Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series, v. 6., p. 353-401. Wagner, D.L., Greene, H.G., Saucedo, G.J., and Pridmore, C.L., 2002, Geologic Map of the Monterey 30' x 60' quadrangle and adjacent areas, California: California Geological Survey Regional Geologic Map Series, scale 1:100,000.
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