Faults--Offshore of Fort Ross Map Area, California

This part of DS 781 presents data for faults for the geologic and geomorphic map of the Offshore of Fort Ross map area, California. The vector data file is included in "Faults_OffshoreFortRoss.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreFortRoss/data_catalog_OffshoreFortRoss.html.

The Offshore of Fort Ross map area is cut by the northwest-trending San Andreas Fault, the right-lateral transform boundary between the North American and Pacific tectonic plates. The San Andreas extends across the inner shelf in the southern part of the map, then crosses the shoreline at Fort Ross and continues onland for about 75 km to the east flank of Point Arena (fig. 8-1). Seismic-reflection data are used to map the offshore fault trace, and reveal a relatively simple, 200- to 500-m wide zone typically characterized by one or two primary strands. About 1500 m west of the San Andreas Fault, the mid shelf (between water depths of 40 m and 70 m) in the southernmost part of the map area includes an about 5-km-wide field of elongate, shore-normal sediment lobes (unit Qmsl). Individual lobes within the field are as much as 650-m long and 200-m wide, have as much as 1.5 m (check with Steve) of relief above the surrounding smooth seafloor, and are commonly connected with upslope chutes. Given their morphology and proxmity to the San Andreas fault, we infer that these lobes result from slope failures associated with strong ground motions triggered by large San Andreas earthquakes.

Movement on the San Andreas has juxtaposed different coastal bedrock blocks (Blake and others, 2002). Rocks east of the fault that occur along the coast and in the nearshore belong to the late Tertiary, Cretaceous, and Jurassic Franciscan Complex, either sandstone of the Coastal Belt or Central Belt (unit TKfs) or melange of the central terrane (unit fsr). Bedrock west of the fault are considered part of the Gualala Block (Elder, 1998) and include the Eocene and Paleocene German Rancho Formation (unit Tgr) and the Miocene sandstone and mudstone of the Fort Ross area (unit Tsm).

This section of the San Andreas Fault onland has an estimated slip rate of about 17 to 25 mm/yr (Bryant and Lundberg, 2002). The devastating Great 1906 California earthquake (M 7.8) is thought to have nucleated on the San Andreas Fault about 100 kilometers south of this map area offshore of San Francisco (e.g., Bolt, 1968; Lomax, 2005), with the rupture extending northward through the Offshore of Fort Ross map area to the south flank of Cape Mendocino. Emergent marine terraces along the coast in the Offshore of Fort Ross map area record recent contractional deformation associated with the San Andreas Fault system. Prentice and Kelson (2006) report uplift rates of 0.3 to 0.6 mm/yr for a late Pleistocene terrace exposed at Fort Ross, and this recent uplift must also have affect the nearshore and inner shelf.

Previously, McCulloch (1987) mapped a nearshore (within 3 to 5 km of the coast) fault zone from Point Arena to Fort Ross (Fig. 8-1) using primarily deeper industry seismic-reflection data. Subsequently, Dickinson and others (2005) named this structure the "Gualala Fault." Our mapping, also based on seismic-reflection data, reveals this structure as a steep, northeast trending fault and similarly shows the fault ending to the south in the northern part of the Offshore of Fort Ross map area. We have designated the zone of faulting and folding above this structure the "Gualala Fault deformation zone."

Faults were primarily mapped by interpretation of seismic reflection profile data (see field activity S-8-09-NC). The seismic reflection profiles were collected between 2007 and 2010.

References Cited

Blake, M.C., Jr., Graymer, R.W., and Stamski, R.E., 2002, Geologic map and map database of western Sonoma, northernmost Marin, and southernmost Mendocino counties, California: U.S. Geological Survey Miscellaneous Field Studies Map 2402, scale 1:100,000.

Bolt, B.A., 1968, The focus of the 1906 California earthquake: Bulletin of the Seismological Society of America, v. 58, p. 457-471.

Bryant, W.A., and Lundberg, M.M., compilers, 2002, Fault number 1b, San Andreas fault zone, North Coast section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, accessed April 4, 2013, at http://earthquakes.usgs.gov/hazards/qfaults.

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.

Elder, W.P., ed., 1998, Geology and tectonics of the Gualala Block, northern California: Pacific Section, Society of Economic Paleontologists and Mineralogists, Book 84, 222 p.

Lomax, A., 2005, A reanalysis of the hypocentral location and related observations for the Great 1906 California earthquake: Bulletin of the Seismological Society of America, v. 95, p. 861-877.

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.

Prentice, C.S., and Kelson, K.I., 2006, The San Andreas fault in Sonoma and Mendocino counties, in Prentice, C.S., Scotchmoor, J.G., Moores, E.M., and Kiland, J.P., eds., 1906 San Francisco Earthquake Centennial Field Guides: Field trips associated with the 100th Anniversary Conference, 18-23 April 2006, San Francisco, California: Geological Society of America Field Guide 7, p. 127-156.

本数据集781（DS 781）的该部分提供了加利福尼亚州罗斯堡近海（Offshore of Fort Ross）制图区域的地质与地貌图相关断层数据。矢量数据文件包含于"Faults_OffshoreFortRoss.zip"中，可通过http://pubs.usgs.gov/ds/781/OffshoreFortRoss/data_catalog_OffshoreFortRoss.html访问。

罗斯堡近海制图区域被北西向的圣安德烈亚斯断层（San Andreas Fault）切割，该断层是北美板块与太平洋板块之间的右旋走滑转换边界。圣安德烈亚斯断层穿过制图区南部的内陆架，随后在罗斯堡处穿越海岸线，并向陆延伸至阿雷纳角东翼以东约75千米处（图8-1）。研究利用地震反射数据绘制了近海断层迹线，其展现出相对简单的200~500米宽的断裂带，通常以1~2条主断裂段为特征。

在圣安德烈亚斯断层以西约1500米处，制图区最南端的中陆架（水深40~70米之间）发育一处约5千米宽的长条状、垂直岸线分布的沉积叶状体（Qmsl单元）。该区域内单个叶状体最长可达650米、宽200米，相较于周围平坦海底的地形高差可达1.5米（需与Steve核实），且通常与上坡水道相连。鉴于其形态特征与紧邻圣安德烈亚斯断层的位置，研究推断这些叶状体为大型圣安德烈亚斯地震触发强地面运动所引发的斜坡失稳产物。

圣安德烈亚斯断层的错动使不同的海岸基岩块体发生了错位（Blake等，2002）。断层东侧沿海及近岸分布的岩石属于晚第三纪、白垩纪及侏罗纪的弗朗西斯科混杂岩（Franciscan Complex），包括海岸带或中央带的砂岩（TKfs单元），以及中央地体的混杂岩（fsr单元）。断层西侧的基岩被归为瓜拉拉地块（Gualala Block）（Elder，1998），包括始新世与古新世的德国牧场组（German Rancho Formation，Tgr单元），以及罗斯堡地区的中新统砂岩与泥岩（Tsm单元）。

圣安德烈亚斯断层该陆段的估算滑动速率约为17~25毫米/年（Bryant与Lundberg，2002）。破坏性的1906年加州大地震（震级M 7.8）被认为发源于制图区以南约100千米处的旧金山近海圣安德烈亚斯断层（如Bolt，1968；Lomax，2005），破裂带向北延伸穿过罗斯堡近海制图区，直至门多西诺角南翼。罗斯堡近海制图区沿海的抬升海蚀阶地记录了圣安德烈亚斯断层系统相关的近期收缩变形。Prentice与Kelson（2006）报告称，罗斯堡出露的晚更新世阶地的抬升速率为0.3~0.6毫米/年，该近期抬升同样影响了近岸与内陆架区域。

此前，McCulloch（1987）主要利用深部工业地震反射数据，绘制了从阿雷纳角至罗斯堡的近岸（距海岸3~5千米范围内）断层带（图8-1）。随后，Dickinson等（2005）将该构造命名为“瓜拉拉断层（Gualala Fault）”。本研究的制图同样基于地震反射数据，结果显示该构造为一条陡立的北东向断层，且其南端终止于罗斯堡近海制图区的北部。研究将该构造上方的断层与褶皱带命名为“瓜拉拉断层变形带（Gualala Fault deformation zone）”。

断层主要通过地震反射剖面数据解译进行制图（参见野外活动S-8-09-NC）。地震反射剖面采集于2007至2010年间。

参考文献
Blake, M.C., Jr., Graymer, R.W., 与Stamski, R.E., 2002, 加利福尼亚州索诺马县西部、马林县最北端及门多西诺县最南部的地质图与地图数据库：美国地质调查局（U.S. Geological Survey）野外研究杂图2402，比例尺1:100,000。
Bolt, B.A., 1968, 1906年加州地震的震源：美国地震学会公报，第58卷，第457-471页。
Bryant, W.A. 与Lundberg, M.M., 编译, 2002, 美国第四纪断层与褶皱数据库中的北海岸段圣安德烈亚斯断层带1b号断层：美国地质调查局官网，2013年4月4日访问，http://earthquakes.usgs.gov/hazards/qfaults。
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., 与Brabb, E.E., 2005, 加利福尼亚海岸新近纪圣格雷戈里奥-霍斯格里断层带的净右旋滑动：地质证据与构造意义：美国地质学会特刊391，共43页。
Elder, W.P., 主编, 1998, 加利福尼亚北部瓜拉拉地块的地质与构造：太平洋分会，经济古生物学家与矿物学家协会，第84号专著，共222页。
Lomax, A., 2005, 1906年加州大地震的震源位置与相关观测的再分析：美国地震学会公报，第95卷，第861-877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., 与Vedder, J.G.主编的《北美西部大陆边缘及邻近海洋的地质与资源潜力——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委员会，地球科学系列第6卷，第353-401页。
Prentice, C.S. 与Kelson, K.I., 2006, 索诺马县与门多西诺县的圣安德烈亚斯断层，收录于Prentice, C.S., Scotchmoor, J.G., Moores, E.M., 与Kiland, J.P.主编的《1906年旧金山地震百年野外指南：2006年4月18-23日旧金山百年会议配套野外考察》：美国地质学会野外指南7，第127-156页。