Folds--Offshore of San Francisco Map Area, California

This part of DS 781 presents data for folds for the geologic and geomorphic map of the Offshore of San Francisco map area, California. The vector data file is included in "Folds_OffshoreSanFrancisco.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreSanFrancisco/data_catalog_OffshoreSanFrancisco.html. The Offshore of San Francisco 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 Holocene sediments (mostly units Qms, Qmsb, Qmst) with no seafloor expression, and are mapped using seismic-reflection data. The San Andreas Fault is the primary plate-boundary structure and extends northwest across the map area; it intersects the shoreline 10 km north of the map area at Bolinas Lagoon, and 3 km south of the map area at Mussel Rock. 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 offshore of San Francisco within the map area (Bolt, 1968; Lomax, 2005). The San Andreas Fault forms the boundary between two distinct basement terranes, Upper Jurassic to Lower Cretaceous rocks 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 at and north of Point Lobos adjacent to onland exposures. The Franciscan is divided into 13 different units for the onshore portion of this geologic map based on different lithologies and ages, but the unit cannot be similarly divided in the offshore because of a lack of direct observation and (or) sampling. Folds were primarily mapped by interpretation of seismic reflection profile data (see field activities S-15-10-NC and F-2-07-NC). The seismic reflection profiles were collected between 2007 and 2010. 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. Tectonophysics, 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/.本数据集为DS 781的一部分，提供加利福尼亚州旧金山外海制图区地质与地貌图的褶皱相关数据。矢量数据文件收录于"Folds_OffshoreSanFrancisco.zip"，可通过http://pubs.usgs.gov/ds/781/OffshoreSanFrancisco/data_catalog_OffshoreSanFrancisco.html访问。 旧金山外海制图区横跨北美板块与太平洋板块之间的右旋走滑转换边界，被多条活动断层切割，这些断层共同构成分布式剪切带，包括圣安德烈亚斯断层（San Andreas Fault）、圣格雷戈里奥断层东支、金门断层（Golden Gate Fault）以及马铃薯田断层（Potato Patch Fault）（Bruns等，2002；Ryan等，2008）。这些断层被全新世沉积物（主要为Qms、Qmsb、Qmst地层）覆盖，无海底地表出露，通过地震反射数据完成地质填绘。圣安德烈亚斯断层是主要的板块边界构造，向西北方向延伸穿过制图区；其与海岸线的交点分别位于制图区以北10公里的博利纳斯潟湖，以及制图区以南3公里的贻贝岩。该段圣安德烈亚斯断层的估算滑动速率为17~24毫米/年（美国地质调查局（U.S. Geological Survey），2010），1906年加利福尼亚特大地震（震级M 7.8）的震源被认为位于制图区内旧金山外海数公里处的圣安德烈亚斯断层上（Bolt，1968；Lomax，2005）。 圣安德烈亚斯断层构成两个截然不同的基底地体的边界：东侧为上侏罗统至下白垩统的弗朗西斯科杂岩（Franciscan Complex），西侧为晚白垩世花岗岩及更古老变质岩组成的萨利尼亚地块（Salinian block）。弗朗西斯科杂岩（未划分的KJf地层）在洛博斯角及其以北区域形成海底露头，与陆上露头相邻。本次地质图的陆上部分基于岩性与形成时代差异，将弗朗西斯科杂岩划分为13个不同地层，但由于缺乏直接观测或采样，该地层在近海区域无法进行同等划分。 褶皱主要通过解释地震反射剖面数据完成地质填绘（参见野外工作S-15-10-NC与F-2-07-NC）。相关地震反射剖面采集于2007年至2010年间。 参考文献 Bolt, B.A., 1968，1906年加利福尼亚地震的震源：《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与圣格雷戈里奥断层带结构，收录于Parsons, T. 主编，《加利福尼亚州沿海与近海旧金山湾区地壳结构》：美国地质调查局（U.S. Geological Survey）专业报告1658，第77-117页。 Lomax, A., 2005，1906年加利福尼亚大地震的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。 Ryan, H.F., Parsons, T. 及 Sliter, R.W., 2008，加利福尼亚州旧金山外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）与加利福尼亚地质调查局（California Geological Survey），2010，美国第四纪断层与褶皱数据库，2012年4月5日访问，源自美国地质调查局网站：http://earthquake.usgs.gov/hazards/qfaults/.