Geology and geomorphology--Offshore Scott Creek, California

This part of DS 781 presents data for the geologic and geomorphic map of the Offshore of Scott Creek map area, California. The vector data file is included in "Geology_OffshoreScottCreek.zip," which is accessible from http://dx.doi.org/10.5066/F7CJ8BJW. The offshore part of the map area lies southwest of the southwest flank of the Santa Cruz Mountains, extending from the shoreline to water depths of about 60 to 75 m on the gently dipping (about 0.6° to 0.8°) continental shelf. The shelf is underlain by Neogene bedrock and a variably thick (as much as 24 m) late Quaternary sediment cover. Sea level has risen 120 to 130 m over about the last 21,000 years (for example, Stanford and others, 2011), leading to broadening of the continental shelf, progressive eastward migration of the shoreline and wave-cut platform, and associated transgressive erosion and deposition (for example, Catuneanu, 2006). The Offshore of Scott Creek map area is now an open-ocean shelf that is subjected to full, and sometimes severe, wave energy. Shelf morphology and geology are also affected by local faulting, folding, uplift, and possibly subsidence. The offshore of Scott Creek map area straddles the right-lateral San Gregorio Fault Zone, an important structure in the distributed transform boundary between the North American and Pacific plates (see, for example, Dickinson and others, 2005). Regionally, this fault is part of a system that occurs predominantly in the offshore for about 400 km from Point Conception in the south (where it is known as the Hosgri fault; Johnson and Watt, 2012) to Bolinas and Point Reyes in the north (Bruns and others, 2002; Ryan and others, 2008). The San Gregorio Fault in the map area is part of a 90-km-long offshore segment that extends from Point Sur on the south, across outer Monterey Bay to Point Año Nuevo (just one kilometer north of the map area) on the north; Weber and Lajoie, 1980; Brabb, 1997; Wagner and others, 2002). Offshore parts of this fault system are identified on seismic-reflection data based on abrupt truncation or warping of reflections and (or) juxtaposition of reflection panels with different seismic parameters. In this map area, the San Gregorio Fault forms a distributed about 2-km-wide shear zone that includes two main faults. The nearshore eastern part of the zone, which includes the Coastways Fault, partly coincides with a prominent bathymetric lineament on the outer flank of nearshore bedrock outcrops between Waddell Creek and Davenport. The western part of the zone, which includes the Frijoles Fault, cuts across the flat, sediment-covered shelf. Cumulative lateral slip on San Gregorio Fault Zone in this region is thought to range from 4 to 10 mm/yr (Weber, 1994). McCulloch (1987) considered the San Gregorio Fault Zone the eastern margin of the Outer Santa Cruz Basin (fig. 8-1). Farther offshore, outside California's State Waters but within the map area, this basin is cut by the northwest-trending Ascension Fault (Greene and others, 2002; U.S. Geological Survey and California Geological Survey, 2010). Emergent marine terraces on the flanks of the Santa Cruz Mountains between Santa Cruz and Point Año Nuevo (fig. 1-1) are as high as 240 m with estimated uplift rates that range from about 0.2 m/year (for example, Bradley and Griggs, 1976; Lajoie and others, 1991) to as much as 1.1 mm/yr (for example, Perg and others, 2001). This uplift has been attributed to a combination of (1) advection of crust around a bend in the San Andreas Fault, and (2) uplift on the northeast (landward) side of a steep-northeast dipping offshore San Gregorio fault (Anderson, 1990; Anderson and Menking, 1994). The uplifted region in this tectonic model includes the nearshore and shelf of the Offshore of Scott Creek map area, but considerable shore-normal uplift gradients are associated with both processes and offshore uplift rates are not well constrained. Map unit polygons were digitized over underlying 2-meter base layers developed from multibeam bathymetry and backscatter data (see "Bathymetry--Offshore of Scott Creek Map Area, California" and "Backscatter--Offshore of Scott Creek Map Area, California"). The bathymetry and backscatter data were collected between 2006 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. Bruns, T.R., Cooper, A.K., Carlson, P.R., and McCulloch, D.S., 2002, Structure of the submerged San Andreas and San Gregorio Fault zones in the Gulf of Farallones as inferred from high-resolution seismic-reflection data, in Parsons, T. (ed.), Crustal structure of the coastal and marine San Francisco Bay region, California: U.S. Geological Survey Professional Paper 1658, p. 77-117. Catuneanu, O., 2006, Principles of Sequence Stratigraphy: Amsterdam, Elsevier, 375 p. 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., Maher, N.M., and Paull, C.K., 2002, Physiography of the Monterey Bay National Marine Sanctuary and implications about continental margin development: Marine Geology, v. 181, p. 55-82. Johnson, S.Y., and Watt, J.T., 2012, Influence of fault trend, bends, and convergence on shallow structure and geomorphology of the Hosgri strike-slip fault, offshore Central California: Geosphere, v. 8, no. 6, 25 p., doi:10.1130/GES00830.1. LaJoie, K.R., Ponti, D.J., Powell, C.L., II, Mathieson, S.A., and Sarna-Wojcicki, 1991, Emergent marine strandlines and associated sediments, coastal California; A record of Quaternary sea-level fluvtuations, vertical tectonic movements, climatic changes, and coastal processes, in Morrison, R.B., ed., Quaternary non-glacial geology, conterminous United States: Geological Society of America, Geology of North America, v. K-2, p. 190-214. 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. Perg, L.A., Anderson, R.S., and Finkel, R.C., 2001, Use of a new 10Be and 26Al inventory to data marine terraces, Santa Cruz, California, USA: Geology, v. 29, p. 879-882. Ryan, H.F., Parsons, T., and Sliter, R.W., 2008. Vertical tectonic deformation associated with the San Andreas fault zone offshore of San Francisco, California: Tectonphysics, v. 429, p. 209-224. Stanford, J.D., Hemingway, R., Rohling, E.J., Challenor, P.G., Medina-Elizalde, M., and Lester, A.J., 2011, Sea-level probability for the last deglaciation - A statistical analysis of far-field records: Global and Planetary Change, v. 79, p. 193-203. Weber, G.E., and LaJoie, K.R., 1980, Map of Quaternary faulting along the San Gregorio fault zone, San Mateo and Santa Cruz counties, California: US Geological Survey Open-File Report 80-907. Weber, G.E., 1994, Late Pleistocene slip rates on the San Gregorio Fault Zone at Point Año Nuevo, San Mateo County, California, in Lettis, W.R., ed., Field trip guidebook to transpressional deformation in the San Francisco Bay region: Friends of the Pleistocene, Pacific Southwest Cell. 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.本DS 781数据集的这一部分提供了加利福尼亚州斯科特溪近海地图区域的地质与地貌图数据。矢量数据文件包含于"Geology_OffshoreScottCreek.zip"中，可通过http://dx.doi.org/10.5066/F7CJ8BJW获取。 该地图区域的近海部分位于圣克鲁斯山脉西南翼的西南侧，从海岸线延伸至约60至75米水深的平缓倾斜（倾角约0.6°至0.8°）大陆架（continental shelf）。该大陆架覆有新近纪基岩以及厚度不均（最大可达24米）的晚第四纪沉积盖层。在过去约21000年中，海平面上升了120至130米（如Stanford等人，2011），导致大陆架范围扩大、海岸线和浪蚀平台逐步向东迁移，并伴随海侵侵蚀与沉积作用（如Catuneanu，2006）。如今斯科特溪近海地图区域属于开放海洋大陆架，承受着持续且有时强度较高的波浪能量。大陆架地貌与地质还受到局部断裂、褶皱、抬升以及可能的沉降作用影响。 斯科特溪近海地图区域横跨右旋圣格雷戈里奥断裂带（San Gregorio Fault Zone），该断裂带是北美板块与太平洋板块之间分布式转换边界的重要构造（如Dickinson等人，2005）。区域上，该断裂带属于一个主要分布于近海的系统，总长约400千米，南起康塞普西翁角（此处被称为霍斯格里断裂；Johnson和Watt，2012），北至博利纳斯和雷耶斯角（Bruns等人，2002；Ryan等人，2008）。本地图区域内的圣格雷戈里奥断裂属于一段长90千米的近海段，南起苏尔角，穿过蒙特雷湾外区，北至阿尼奥新诺角（位于本地图区域以北仅1千米处；Weber和Lajoie，1980；Brabb，1997；Wagner等人，2002）。该断裂系统的近海部分可通过地震反射数据识别，依据为反射轴的突然截断或弯曲，以及（或）具有不同地震参数的反射段的并置。在本地图区域内，圣格雷戈里奥断裂形成了一个宽约2千米的分布式剪切带，包含两条主要断裂。该带的近岸东部部分包含海岸带断裂（Coastways Fault），部分与瓦德塞尔溪和达文波特之间近岸基岩露头外侧显著的水深地形线理相重合。该带的西部部分包含弗里霍莱斯断裂（Frijoles Fault），横穿平坦的沉积覆盖型大陆架。该区域圣格雷戈里奥断裂带的累计侧向滑动速率被认为介于4至10毫米/年之间（Weber，1994）。 McCulloch（1987）将圣格雷戈里奥断裂带视为外圣克鲁斯盆地的东界（图8-1）。在更远的近海区域，即加州州属水域之外但位于本地图区域内，该盆地被北西向的阿森松断裂（Ascension Fault）切割（Greene等人，2002；美国地质调查局（U.S. Geological Survey）与加州地质调查局（California Geological Survey），2010）。 圣克鲁斯与阿尼奥新诺角之间圣克鲁斯山脉侧翼的出露海成阶地（图1-1）海拔最高可达240米，估算抬升速率范围约为0.2毫米/年（如Bradley和Griggs，1976；Lajoie等人，1991）至高达1.1毫米/年（如Perg等人，2001）。这种抬升被归因于两种作用的组合：(1) 圣安德烈亚斯断裂（San Andreas Fault）弯曲处周围地壳的平流，以及(2) 东北向陡倾的近海圣格雷戈里奥断裂东北侧（陆侧）的抬升（Anderson，1990；Anderson和Menking，1994）。该构造模型中的抬升区域包含斯科特溪近海地图区域的近岸与大陆架，但两种作用均伴随显著的垂直岸线抬升梯度，且近海抬升速率尚未得到精确约束。 地图单元多边形是在基于多波束水深测量与反向散射数据生成的2米基础底图上进行数字化的（详见"Bathymetry--Offshore of Scott Creek Map Area, California"与"Backscatter--Offshore of Scott Creek Map Area, California"）。水深与反向散射数据采集于2006年至2010年之间。 参考文献 Brabb, E.E., 1997，加利福尼亚州圣克鲁斯县地质图：数字数据集，美国地质调查局公开文件报告97-489，比例尺1:62500。 Bruns, T.R., Cooper, A.K., Carlson, P.R., and McCulloch, D.S., 2002，基于高分辨率地震反射数据推断的法拉隆湾淹没圣安德烈亚斯与圣格雷戈里奥断裂带结构，收录于Parsons, T.主编，《加利福尼亚州旧金山湾沿岸与海洋地壳结构》：美国地质调查局专业论文1658，第77-117页。 Catuneanu, O., 2006，《层序地层学原理》：阿姆斯特丹，爱思唯尔出版社，375页。 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，加利福尼亚沿海新近纪圣格雷戈里奥-霍斯格里断裂带的右旋总滑动量：地质证据与构造意义，《地质学会美国专刊》391，43页。 Greene, H.G., Maher, N.M., and Paull, C.K., 2002，蒙特雷湾国家海洋保护区的地貌学及大陆边缘发育意义，《海洋地质》，第181卷，第55-82页。 Johnson, S.Y., and Watt, J.T., 2012，断裂走向、弯曲与汇聚对加利福尼亚中部近海霍斯格里走滑断裂浅部结构与地貌的影响，《地球圈》，第8卷，第6期，25页，doi:10.1130/GES00830.1。 LaJoie, K.R., Ponti, D.J., Powell, C.L., II, Mathieson, S.A., and Sarna-Wojcicki, 1991，加利福尼亚沿海出露海成海岸线与相关沉积：第四纪海平面波动、垂直构造运动、气候变化与海岸过程记录，收录于Morrison, R.B.主编，《第四纪非冰川地质，美国本土》：地质学会美国，《北美地质》，K-2卷，第190-214页。 McCulloch, D.S., 1987，加利福尼亚中部近海的区域地质与油气潜力，收录于Scholl, D.W., Grantz, A., and Vedder, J.G.主编，《北美西部大陆边缘及邻近海洋地质与资源潜力——波弗特海至下加利福尼亚》：得克萨斯州休斯顿，环太平洋能源与矿产资源理事会，地球科学系列，第6卷，第353-401页。 Perg, L.A., Anderson, R.S., and Finkel, R.C., 2001，利用新型10Be与26Al同位素库存测定加利福尼亚州圣克鲁斯海成阶地年代，《地质学》，第29卷，第879-882页。 Ryan, H.F., Parsons, T., and Sliter, R.W., 2008，加利福尼亚州旧金山近海圣安德烈亚斯断裂带相关的垂直构造变形，《构造物理学》，第429卷，第209-224页。 Stanford, J.D., Hemingway, R., Rohling, E.J., Challenor, P.G., Medina-Elizalde, M., and Lester, A.J., 2011，末次冰消期海平面概率：远场记录的统计分析，《全球与行星变化》，第79卷，第193-203页。 Weber, G.E., and LaJoie, K.R., 1980，加利福尼亚州圣马特奥县与圣克鲁斯县圣格雷戈里奥断裂带第四纪断裂图，美国地质调查局公开文件报告80-907。 Weber, G.E., 1994，加利福尼亚州圣马特奥县阿尼奥新诺角圣格雷戈里奥断裂带的晚第四纪滑动速率，收录于Lettis, W.R.主编，《旧金山湾区域挤压变形野外考察指南》：更新世之友，太平洋西南分部。 Wagner, D.L., Greene, H.G., Saucedo, G.J., and Pridmore, C.L., 2002，加利福尼亚州蒙特雷30'×60'图幅及邻近区域地质图：加州地质调查局区域地质图系列，比例尺1:100000。