Faults--Offshore of Half Moon Bay Map Area, California

This part of DS 781 presents data for faults for the geologic and geomorphic map of the Offshore of Half Moon Bay map area, California. The vector data file is included in "Faults_OffshoreHalfMoonBay.zip," which is accessible from http://pubs.usgs.gov/ds/781/OffshoreHalfMoonBay/data_catalog_OffshoreHalfMoonBay.html.
The Offshore of Half Moon Bay map area lies about 12 km southwest of the San Andreas Fault, the dominant structure in the distributed, right-lateral, transform boundary between the North American and Pacific plates. The map area straddles the right-lateral San Gregorio Fault, the most important structure west of the San Andreas Fault in this broad zone. The San Gregorio is part of fault system that occurs predominantly in the offshore, extending about 400 km from Point Conception on the south to Bolinas and Point Reyes on the north (Dickinson and others, 2005), intersecting land at a few coastal promontories. In the Offshore of Half Moon Bay map area, the San Gregorio Fault forms a distributed shear zone about 2 to 4.5 km wide that includes two primary diverging fault strands. The eastern strand (also known as the Seal Cove Fault or Coastways Fault) roughly parallels the shoreline, lies onshore for about 3 km at Pillar Point, and locally forms the boundary between outcrops of Cretaceous grantic rocks to the east and Purisima Formation to the west. The western strand (also known as the Frijoles Fault) lies entirely offshore and forms a boundary between the Purisima Formation on the east and undifferentiated Cretaceous and (or) Tertiary rocks (Pigeon Point Formation?) of the Pigeon Point structural block (McCulloch, 1987) on the west. The Pigeon Point block forms a northwest-trending bedrock ridge that extends offshore for about 30 km from Pescadero Point and forms the northwest boundary of the outer Santa Cruz Basin (McCulloch, 1987).
Cumulative lateral slip on the San Gregorio Fault zone is thought to range from 4 to 10 mm/yr in this region (U.S. Geological Survey, 2010). Bathymetric (Bathymetry--Offshore Half Moon Bay, California, DS 781) and seismic-reflection data (see field activity S-15-10-NC) reveal that the offshore outcrops of the Purisima Formation between the eastern and western strands of the San Gregorio Fault Zone are spectacularly folded, faulted and rotated by the strike-slip motion and drag along the faults. The entire map area lies along strike with the young, high topography of the Santa Cruz Mountains and Coast Ranges. This regional uplift has been linked to a northwest transpressive bend in the San Andreas Fault (for example, Zoback and others, 1999). Uplift of nearby marine terraces at rates up to 0.44 mm/yr confirms that this uplift includes the coastal zone (Weber and others, 1995).
Faults were primarily mapped by interpretation of seismic reflection profile data (see field activity S-15-10-NC). The seismic reflection profiles were collected between 2007 and 2010.
References Cited
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.
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 ocean basins - Beaufort Sea to Baja California: Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, v. 6, p. 353-401.
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/.
Weber, G.E., Nolan, J.M., and Zinn, E.N., 1995, Determination of late Pleistocene-Holocene slip rates along the San Gregorio fault zone, San Mateo and Santa Cruz counties, California: Final Technical Report, National Earthquake Hazard Reduction Program, Contract No. 1434-93-G-2336, 70 p., 4 map sheets.
Zoback, M.L., Jachens, R.C., and Olson, J.A., 1999, Abrupt along-strike change in tectonic style: San Andreas fault zone, San Francisco Peninsula: Journal of Geophysical Research, v. 104 (B5), p. 10,719-10,742.

本数据集DS 781的这一部分，提供了加利福尼亚州半月湾近海地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_OffshoreHalfMoonBay.zip"中，可通过http://pubs.usgs.gov/ds/781/OffshoreHalfMoonBay/data_catalog_OffshoreHalfMoonBay.html获取。

半月湾近海制图区位于圣安德烈亚斯断层（San Andreas Fault）西南约12公里处，圣安德烈亚斯断层是北美板块与太平洋板块之间分布式右旋走滑转换边界的主导构造。该制图区横跨右旋走滑的圣格雷戈里奥断层（San Gregorio Fault）——该断层是此宽阔构造带内圣安德烈亚斯断层以西最重要的构造。圣格雷戈里奥断层属于主要分布于近海的断层系统，其展布范围从南部的康塞普申角向北延伸约400公里，直至博利纳斯和雷耶斯角（Dickinson等，2005），仅在少数海岸岬角处与陆地相接。在半月湾近海制图区内，圣格雷戈里奥断层形成了宽约2至4.5公里的分布式剪切带，包含两条主要的离散断层分支。东部分支（又称海豹湾断层或海岸大道断层）大致平行于海岸线，在柱角处存在一段约3公里的陆上段，局部构成了东侧白垩纪花岗岩岩体露头与西侧普里西马组（Purisima Formation）露头的分界。西部分支（又称弗里霍莱斯断层）完全位于近海，构成了东侧普里西马组与西侧鸽子点构造块（McCulloch，1987）的未分异白垩纪和（或）第三纪岩石（疑为鸽子点组？）之间的边界。鸽子点构造块形成一条北西走向的基岩脊，从佩斯卡德罗角向近海延伸约30公里，构成了圣克鲁斯外盆地的北西边界（McCulloch，1987）。

据估算，该区域圣格雷戈里奥断层带的累计侧向滑移速率介于4至10毫米/年之间（美国地质调查局，2010）。水深测量数据（《加利福尼亚半月湾近海水深测量》，DS 781）与地震反射数据（参见野外活动S-15-10-NC）显示，圣格雷戈里奥断层带东西分支之间的普里西马组近海露头，因走滑运动和沿断层的拖曳作用发生了显著的褶皱、断裂与旋转。整个制图区沿走向与圣克鲁斯山脉和海岸山脉的年轻高地形带相连。这种区域性抬升与圣安德烈亚斯断层的北西向压扭弯折存在关联（例如Zoback等，1999）。邻近海成阶地以最高0.44毫米/年的速率抬升，证实海岸带也处于该抬升过程中（Weber等，1995）。

本次断层填图主要通过解释地震反射剖面数据完成（参见野外活动S-15-10-NC），这些地震反射剖面采集于2007年至2010年期间。

参考文献
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., 及Brabb, E.E., 2005, 新近纪圣格雷戈里奥-霍斯格里断层带的右旋总滑移量、加利福尼亚海岸地区：地质证据与构造意义：美国地质学会特刊391，43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., 及Vedder, J.G.编辑的《北美西部大陆边缘及邻近海盆地质与资源潜力——从波弗特海至下加利福尼亚》：环太平洋能源与矿产资源委员会地球科学丛书，第6卷，第353-401页。
美国地质调查局与加利福尼亚地质调查局, 2010, 美国第四纪断层与褶皱数据库，2012年4月5日访问，来自美国地质调查局官网：http://earthquake.usgs.gov/hazards/qfaults/。
Weber, G.E., Nolan, J.M., 及Zinn, E.N., 1995, 加利福尼亚州圣马特奥县和圣克鲁斯县圣格雷戈里奥断层带晚更新世-全新世滑移速率测定：最终技术报告，国家地震减灾计划，合同号1434-93-G-2336，70页，4幅图幅。
Zoback, M.L., Jachens, R.C., 及Olson, J.A., 1999, 构造样式沿走向的突变：旧金山半岛圣安德烈亚斯断层带：《地球物理研究杂志》，第104卷（B5），第10719-10742页。