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.

本部分属于美国地质调查局（U.S. Geological Survey, USGS）第781号数据集系列（DS 781），提供加利福尼亚州罗斯堡近海制图区地质与地貌图件所需的断层数据。矢量数据文件收录于"Faults_OffshoreFortRoss.zip"，可通过http://pubs.usgs.gov/ds/781/OffshoreFortRoss/data_catalog_OffshoreFortRoss.html获取。

罗斯堡近海制图区被北西向的圣安德烈亚斯断层（San Andreas Fault）切割，该断层为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，随后在罗斯堡穿越海岸线，向陆延伸约75千米至阿雷纳角东侧（图8-1）。研究利用地震反射数据绘制近海断层迹线，结果显示该断层带相对简单，宽度为200~500米，通常以1~2条主断片为典型特征。在圣安德烈亚斯断层以西约1500米处，制图区最南部的中陆架（水深40~70米区间）分布有一片宽约5千米的长条状垂直岸线沉积舌状体（单元Qmsl）。该区域内的单个舌状体最长可达650米、宽200米，相对于周围平坦海底的高差可达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空间位置，研究推断这些舌状体由圣安德烈亚斯断层大地震触发强地面运动引发的边坡失稳形成。

圣安德烈亚斯断层的活动将不同海岸基岩块体错置拼合（Blake等，2002）。断层东侧沿海及近岸区域的岩石属于晚第三纪、白垩纪及侏罗纪的弗朗西斯科杂岩（Franciscan Complex），可分为海岸带或中央带砂岩（单元TKfs），以及中央地体混杂岩（单元fsr）。断层西侧的基岩被归为瓜拉拉地块（Gualala Block）（Elder，1998），包含始新统与古新统的德国牧场组（German Rancho Formation，单元Tgr），以及罗斯堡区域的中新统砂岩与泥岩（单元Tsm）。

该陆上段圣安德烈亚斯断层的估算滑动速率约为17~25毫米/年（Bryant与Lundberg，2002）。破坏性极强的1906年加利福尼亚大地震（震级M 7.8）被认为起源于制图区以南约100千米、旧金山近海的圣安德烈亚斯断层段（如Bolt，1968；Lomax，2005），其破裂带向北延伸，穿过罗斯堡近海制图区直至门多西诺角南侧。罗斯堡近海制图区沿海的抬升海蚀阶地记录了与圣安德烈亚斯断层系统相关的近期收缩变形。Prentice与Kelson（2006）报道，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，该近期抬升作用同样影响了近岸与内陆架区域。

此前，McCulloch（1987）主要利用工业深部地震反射数据，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）"。本研究同样基于地震反射数据开展制图，结果显示该构造为一条陡立的北东向断层，且该断层向南终止于罗斯堡近海制图区的北部区域。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"。

断层制图主要通过地震反射剖面数据解译完成（参见野外作业S-8-09-NC），地震反射剖面数据采集于2007~2010年。

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、马林县最北部及门多西诺县最南部地质图及地图数据库：美国地质调查局杂类野外研究图件2402号，比例尺1:100,000。
Bolt, B.A., 1968, 1906年加利福尼亚大地震的震源：美国地震学会通报，第58卷，第457~471页。
Bryant, W.A. 与 Lundberg, M.M., 编者, 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：美国地质调查局网站，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, 加利福尼亚沿海新近纪圣格雷戈里奥-霍斯格里断层带的纯右旋滑动：地质证据与构造意义：美国地质学会特刊391号，43页。
Elder, W.P., 编者, 1998, 加利福尼亚北部瓜拉拉地块地质与构造：经济古生物学家与矿物学家协会太平洋分会，第84号专著，222页。
Lomax, A., 2005, 1906年加利福尼亚大地震震源位置及相关观测的重新分析：美国地震学会通报，第95卷，第861~877页。
McCulloch, D.S., 1987, 加利福尼亚中部近海区域地质与油气潜力，收录于Scholl, D.W., Grantz, A. 与 Vedder, J.G. 编辑的《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，环太平洋能源与矿物资源协会，地球科学丛书第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. 编辑的《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号，第127~156页。