Folds--Drakes Bay and Vicinity Map Area, California

This part of DS 781 presents data of folds for the geologic and geomorphologic map of the Drakes Bay and Vicinity map area, California. The vector data file is included in "Folds_DrakesBay.zip," which is accessible from http://pubs.usgs.gov/ds/781/DrakesBay/data_catalog_DrakesBay.html.

Folds in the Drakes Bay and Vicinity map area are identified on seismic-reflection data based on abrupt truncation or warping of reflections and (or) juxtaposition of reflection panels with different seismic parameters such as reflection presence, amplitude, frequency, geometry, continuity, and vertical sequence. The Point Reyes Fault Zone runs through the map area and is an offshore curvilinear reverse fault zone with predominantly north-side-up motion (Hoskins and Griffiths, 1971; McCulloch, 1987; Heck and others, 1990; Stozek, 2012) that likely connects with the western San Gregorio fault further to the south (Ryan and others, 2008), making it part of the San Andreas Fault System. The Point Reyes Fault Zone is characterized by a 5 to 11 km-wide zone of deformation in the shallow subsurface that is associated with two main fault structures, the Point Reyes Fault and the western Point Reyes Fault. Near the Point Reyes headland, vertical displacement of granitic basement across the Point Reyes Fault is at least 1.4 km (McCulloch, 1987). Offshore Double Point, vertical displacement on the Point Reyes Fault is difficult to assess because subsurface age constraints from nearby wells are lacking, and there are few offset horizons across the fault imaged on available seismic data. However, warping and folding of Neogene strata are clearly visible on high-resolution seismic data (see field activity S-8-09-NC). The western Point Reyes Fault is defined by a broad anticlinal structure visible in both industry and high-resolution seismic datasets that exhibits that same sense of vergence (north-side-up) as the Point Reyes Fault.

Folds were primarily mapped by interpretation of seismic reflection profile data (see field activity S-8-09-NC). The seismic reflection profiles were collected between 2006 in 2009.

References Cited

Heck, R.G., Edwards, E.B., Kronen, J.D., Jr., and Willingham, C.R., 1990, Petroleum potential of the offshore outer Santa Cruz and Bodega basins, California, in Garrison, R.E., Greene, H.G., Hicks, K.R., Weber, G.E., and Wright, T.L., eds. Geology and tectonics of the central California coastal region, San Francisco to Monterey: Pacific Section, American Association of Petroleum Geologists Bulletin GB67, p. 143-164.

Hoskins E.G., Griffiths, J.R., 1971, Hydrocarbon potential of northern and central California offshore: American Association of Petroleum Geologists Memoir 15, p. 212-228.

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.

Ryan, H.F., Parsons, T., and Sliter, R.W., 2008, Vertical tectonic deformation associated with the San Andreas Fault offshore of San Francisco, California: Tectonophysics, v. 475, p. 209-223.

Stozek, B.A., 2012, Geophysical evidence for Quaternary deformation within the offshore San Andreas fault system, northern California: Masters Thesis, San Francisco State University, 141 p.

本数据集隶属于DS 781系列，包含加利福尼亚州德雷克湾及周边制图区的地质与地貌图褶皱相关数据。矢量数据文件收录于"Folds_DrakesBay.zip"，可通过http://pubs.usgs.gov/ds/781/DrakesBay/data_catalog_DrakesBay.html获取。

德雷克湾及周边制图区的褶皱可通过地震反射（seismic-reflection）数据识别，识别准则为反射波的突然截断、弯曲畸变，或不同地震参数（包括反射波是否存在、振幅、频率、几何形态、连续性以及垂向层序）的反射面板并置。雷耶斯角断裂带（Point Reyes Fault Zone）贯穿本制图区，为一处近海弧形逆冲断裂带，主体表现为北盘上升运动（Hoskins与Griffiths, 1971; McCulloch, 1987; Heck等, 1990; Stozek, 2012）；该断裂带向南可能与西侧的圣格雷戈里奥断裂（San Gregorio fault）相连（Ryan等, 2008），因此隶属于圣安德烈亚斯断裂系统（San Andreas Fault System）。

雷耶斯角断裂带以浅地表5至11千米宽的变形带为特征，该变形带与两个主要断裂构造——雷耶斯角断裂（Point Reyes Fault）及西雷耶斯角断裂（western Point Reyes Fault）相关联。在雷耶斯角海岬附近，雷耶斯角断裂两侧花岗质基底的垂向位移至少达1.4千米（McCulloch, 1987）。在双角近海区域，由于缺乏邻近钻井提供的地下年代约束数据，且现有地震数据中可识别的该断裂错断地层较少，雷耶斯角断裂的垂向位移难以估算。但在高分辨率地震数据中可清晰观察到新近纪（Neogene）地层的弯曲畸变与褶皱（详见野外作业S-8-09-NC）。西雷耶斯角断裂以广泛发育的背斜构造为特征，该构造在工业地震及高分辨率地震数据集均可观测到，其运动指向（北盘上升）与雷耶斯角断裂一致。

褶皱的制图主要通过地震反射剖面数据解译完成（详见野外作业S-8-09-NC），该地震反射剖面采集于2006年至2009年间。

参考文献
Heck, R.G.、Edwards, E.B.、Kronen, J.D. Jr. 与 Willingham, C.R., 1990，加利福尼亚州近海外圣克鲁兹与博德加盆地的油气潜力，收录于Garrison, R.E.、Greene, H.G.、Hicks, K.R.、Weber, G.E. 及 Wright, T.L. 主编，《旧金山至蒙特雷的加利福尼亚中部沿海区域地质与构造》：美国石油地质学家协会太平洋分会公报GB67，第143-164页。
Hoskins E.G. 与 Griffiths, J.R., 1971，加利福尼亚北部及中部近海的油气潜力：美国石油地质学家协会专刊15，第212-228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. 及 Vedder, J.G. 主编，《北美西部大陆边缘及邻近海域（波弗特海至下加利福尼亚）地质与资源潜力》：得克萨斯州休斯顿，环太平洋能源与矿产资源理事会，地球科学丛书第6卷，第353-401页。
Ryan, H.F.、Parsons, T. 与 Sliter, R.W., 2008，加利福尼亚州旧金山近海圣安德烈亚斯断裂相关的垂向构造变形：《构造物理学》，第475卷，第209-223页。
Stozek, B.A., 2012，加利福尼亚北部近海圣安德烈亚斯断裂系统内第四纪变形的地球物理证据：旧金山州立大学硕士论文，共141页。