Faults--Offshore of Bolinas Map Area, California

This part of DS 781 presents data for faults for the geologic and geomorphic map of the Offshore of Bolinas map area, California. The vector data file is included in "Faults_OffshoreBolinas.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreBolinas/data_catalog_OffshoreBolinas.html. The Offshore of Bolinas map area straddles the right-lateral transform boundary between the North American and Pacific plates and is cut by several active faults that cumulatively form a distributed shear zone, including the San Andreas Fault, the eastern strand of the San Gregorio Fault, the Golden Gate Fault, and the Potato Patch Fault (Bruns and others, 2002; Ryan and others, 2008). These faults are covered by sediment (mostly unit Qms) with no seafloor expression, and are mapped using seismic-reflection data (see field activities S-8-09-NC and L-1-06-SF). The San Andreas Fault is the primary plate-boundary structure and extends northwest through the southern part of the map area before passing onshore at Bolinas Lagoon. This section of the San Andreas Fault has an estimated slip rate of 17 to 24 mm/yr (U.S. Geological Survey, 2010), and the devastating Great 1906 California earthquake (M 7.8) is thought to have nucleated on the San Andreas a few kilometers south of this map area offshore of San Francisco (e.g., Bolt, 1968; Lomax, 2005). The San Andreas Fault forms the boundary between two distinct basement terranes, Upper Jurassic and Lower Cretaceous melange and graywacke sandstone of the Franciscan Complex to the east, and Late Cretaceous granitic and older metamorphic rocks of the Salinian block to the west. Franciscan Complex rocks (unit KJf, undivided) form seafloor outcrops adjacent to the shoreline southeast of Stinson Beach that are commonly continuous with onshore coastal outcrops. Faults were primarily mapped by interpretation of seismic reflection profile data (see field activities S-8-09-NC and L-1-06-SF). The seismic reflection profiles were collected between 2006 and 2009. References Cited Bolt, B.A., 1968, The focus of the 1906 California earthquake: Bulletin of the Seismological Society of America, v. 58, p. 457-471. 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. 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. 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, 429 (1-2), p. 209-224. U.S. Geological Survey and California Geological Survey, 2010, Quaternary fault and fold database for the United States, accessed April 5, 2012, from USGS website: http://earthquake.usgs.gov/hazards/qfaults/.本DS 781数据集的此部分提供了加利福尼亚州博利纳斯近海制图区地质与地貌图所需的断层数据。矢量数据文件包含于"Faults_OffshoreBolinas.zip"压缩包中，可通过http://pubs.usgs.gov/ds/781/OffshoreBolinas/data_catalog_OffshoreBolinas.html获取。 博利纳斯近海制图区横跨北美板块（North American Plate）与太平洋板块（Pacific Plate）之间的右旋走滑转换边界，被多条共同构成离散剪切带的活动断层切割，包括圣安德烈亚斯断层（San Andreas Fault）、圣格雷戈里奥断层（San Gregorio Fault）东支、金门断层（Golden Gate Fault）以及土豆补丁断层（Potato Patch Fault）（Bruns等，2002；Ryan等，2008）。这些断层被沉积物（主要为Qms地层）覆盖，无海底地表出露，通过地震反射数据（seismic-reflection data）进行制图（参见野外工作活动S-8-09-NC与L-1-06-SF）。 圣安德烈亚斯断层（San Andreas Fault）是主要的板块边界构造，向西北延伸穿过制图区南部，随后在博利纳斯泻湖处登陆。该段圣安德烈亚斯断层的估计滑动速率为17~24 mm/yr（美国地质调查局（U.S. Geological Survey），2010），而造成严重破坏的1906年加州大地震（震级M 7.8）被认为震源位于制图区以南数公里的旧金山近海圣安德烈亚斯断层上（例如Bolt，1968；Lomax，2005）。 圣安德烈亚斯断层构成了两个截然不同的基底地体的边界：东侧为弗朗西斯科混杂岩（Franciscan Complex）的上侏罗统与下白垩统混杂岩及杂砂岩，西侧为萨利尼亚地块（Salinian block）的上白垩统花岗岩类及更古老的变质岩。 弗朗西斯科混杂岩（Franciscan Complex）岩石（未细分的KJf地层）形成了斯廷森海滩（Stinson Beach）东南近海的海底露头，通常与陆上沿海露头连续分布。 断层制图主要通过地震反射剖面数据解译完成（参见野外工作活动S-8-09-NC与L-1-06-SF）。该地震反射剖面数据采集于2006年至2009年间。 参考文献 Bolt, B.A., 1968，1906年加州大地震的震源：《美国地震学会通报》，第58卷，第457-471页。 Bruns, T.R., Cooper, A.K., Carlson, P.R. 及 McCulloch, D.S., 2002，根据高分辨率地震反射数据推断的法拉隆湾水下圣安德烈亚斯断层与圣格雷戈里奥断层带结构，收录于Parsons, T. 主编的《加利福尼亚州旧金山湾沿岸及海域地壳结构》：美国地质调查局专业报告1658，第77-117页。 Lomax, A., 2005，1906年加州大地震震源位置及相关观测的再分析：《美国地震学会通报》，第95卷，第861-877页。 Ryan, H.F., Parsons, T. 及 Sliter, R.W., 2008，加利福尼亚州旧金山近海圣安德烈亚斯断层带相关的垂直构造变形：《构造物理学》，第429卷（1-2期），第209-224页。 美国地质调查局（U.S. Geological Survey）与加州地质调查局，2010，美国第四纪断层与褶皱数据库，2012年4月5日获取，源自美国地质调查局官网：http://earthquake.usgs.gov/hazards/qfaults/。