Faults--Offshore of Salt Point Map Area, California

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Title: Faults--Offshore of Salt Point Map Area, California
Abstract:
This part of DS 781 presents data for faults for the geologic and geomorphic map of the Offshore of Salt Point map area, California. The vector data file is included in "Faults_OffshoreSaltPoint.zip," which is accessible from <https://pubs.usgs.gov/ds/781/OffshoreSaltPoint/data_catalog_OffshoreSaltPoint.html>.

The onshore part of the Offshore of Salt Point 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 extends into the offshore about 5 km south of the map area near Fort Ross, and about 50 km north of the map area on the east flank of Point Arena. The coast between Fort Ross and Point Arena, the northwesternmost exposed section west of the San Andreas Fault, is known as the "Gualala Block" (fig. 1) on the basis of its distinctive geology, which has been widely used to develop paleogeographic reconstructions of coastal California that restore as much as 150 to 180 km of right-lateral slip on the combined San Andreas and San Gregorio Fault systems (see, for example, Wentworth, (1968); Wentworth and others (1998); Jachens and others (1998); Dickinson and others (2005); Burnham (2009). The Gualala Block is underlain by a thick (as much as 9 to 11 km, in aggregate), discontinuous Upper Cretaceous to Miocene stratigraphic section (summarized in Wentworth and others, 1998), however only the Eocene and Paleocene German Rancho Formation (unit Tgr) is exposed onshore and is inferred to form seafloor bedrock outcrops in the Offshore of Salt Point map area. The German Rancho Formation consists of sandstone, mudstone, and conglomerate interpreted as deep-water, submarine-fan deposits. The western boundary of the Gualala Block lies offshore. Using seismic-reflection data, McCulloch (1987; his fig. 14) mapped a shore-parallel fault about 3 to 5 km offshore, which Dickinson and others (2005) subsequently named the Gualala Fault. Jachens and others (1998) evaluated aeromagnetic and gravity data across this zone and modeled this structure as a steep fault within the Salinian basement block, characterized by 3 to 5 km of right-lateral offset. In contrast, Dickinson and others (2005) consider the Gualala fault a Late Miocene strand of the San Andreas fault, separating Salinian and Franciscan basement rocks, with minimum right-lateral slip of 70 km. Our analysis of deeper industry seismic-reflection data within California State Waters shows the Gualala fault as a steep, northeast-dipping structure. Shallower seismic-reflection crossing the Gualala fault reveal a thick late(?) Pleistocene section characterized by recent faulting and gentle asymmetric folding. Hence, the Gualala fault appears to be a recently active "blind" structure that has deformed young sediments. Our mapping also documents a more nearshore zone of deformation that we refer to as the "east Gualala deformation zone." This zone extends through the central and southern parts of the Offshore of Salt Point map area and is similarly charcterized by steep faults and gentle folds that deform inferred late Pleistocene strata.

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 onshore part of the Offshore of Salt Point map area to the south flank of Cape Mendocino (Lawson, 1908; Brown and Wolfe, 1972). Emergent marine terraces along the coast in the Offshore of Salt Point map area record recent contractional deformation associated with the San Andreas Fault system. Prentice and Kelson (2006) reported uplift rates of 0.3 to 0.6 mm/yr for a nearby late Pleistocene terrace (exposed at Fort Ross, about 5 km south of the map area) and this recent uplift must also have affected the nearshore and inner shelf, at least as far west as the Gualala fault.

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

Bolt, B.A., 1968, The focus of the 1906 California earthquake: Bulletin of the Seismological Society of America, v. 58, p. 457-471.

Brown, R.D., Jr., and Wolfe, E.W., 1972, Map showing recently active breaks along the San Andreas Fault between Point Delgada and Bolinas Bay, California: U.S. Geological Survey Miscellaneous Investigations Map I-692, scale 1:24,000.
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>.

Burnham, K., 2009, Predictive model of San Andreas Fault system paleogeography, Late Cretaceous to early Miocene, derived from detailed multidisciplinary conglomerate correlations: Tectonophysics 464, p. 195-208.

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.

Jachens, R.C., Wentworth, C.M., and McLaughlin, R.J., 1998, Pre-San Andreas location of the Gualala Block inferred from magnetic and gravity anomalies, in Elder, W.P., ed., Geology and tectonics of the Gualala block, northern California: Pacific Section, Society of Economic Paleontologists and Mineralogists, Book 84, p. 27-53.

Lawson, A.C., ed., 1908, The California earthquake of April 18, 1906, Report of the State Earthquake Investigation Commission: Carnegie Institution of Washington Publication 87, v. 1, 1451 p. and atlas.

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,

Wentworth, C.M., 1968, Upper Cretaceous and lower Tertiary strata near Gualala, California, and inferred large right slip on the San Andreas fault, in Dickinson, W.R., and Grantz, A., eds. Proceedings of conference on geologic problems of San Andreas fault system: Stanford University Publications, Geological Sciences, v. 11, p. 130-143.

Wentworth, C.M., Jones, D.L., and Brabb, E.E., 1998, Geology and regional correlation of the Cretaceous and Paleogene rocks of the Gualala block, California, in Elder, W.P., ed., Geology and tectonics of the Gualala block, northern California: Pacific Section, Society of Economic Paleontologists and Mineralogists, Book 84, p. 3-26.
Supplemental_Information:
Map political location: Sonoma County, California Compilation scale: 1:24,000 Base maps used are hillshades generated from IfSAR, LiDAR, and multibeam mapping both onshore and offshore (see Bathymetry--Offshore of Salt Point Map Area, California).
  1. How should this data set be cited?

    Hartwell, S.R., Johnson, S.Y., and Manson, M.W., 2014, Faults--Offshore of Salt Point Map Area, California:.

    This is part of the following larger work.

    Johnson, Samuel Y., Dartnell, Peter, Golden, Nadine E., Hartwell, Stephen R., Greene, H. Gary, Erdey, Mercedes D., Cochrane, Guy R., Kvitek, Rikk G., Manson, Michael W., Endris, Charles A., Dieter, Bryan E., Watt, Janet T., Krigsman, Lisa M., Sliter, Ray W., Lowe, Erik N., and Chin, John L., 2015, California State Waters Map Series--Offshore of Salt Point map area, California: Open-File Report OFR 2015-1098, U.S. Geological Survey, Reston, VA.

    Online Links:

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -123.45
    East_Bounding_Coordinate: -123.28
    North_Bounding_Coordinate: 38.64
    South_Bounding_Coordinate: 38.49

  3. What does it look like?

    <https://pubs.usgs.gov/ds/781/OffshoreSaltPoint/images/Faults_OffshoreSaltPoint.jpg> (JPEG)
    Faults offshore Salt Point.

  4. Does the data set describe conditions during a particular time period?

    Beginning_Date: 2007
    Ending_Date: 2010
    Currentness_Reference: ground condition

  5. What is the general form of this data set?

    Geospatial_Data_Presentation_Form: vector digital data

  6. How does the data set represent geographic features?

    1. How are geographic features stored in the data set?

      This is a Vector data set. It contains the following vector data types (SDTS terminology):

      • String (20)

    2. What coordinate system is used to represent geographic features?

      The map projection used is WGS 1984 UTM Zone 10N.

      Projection parameters:
      Scale_Factor_at_Central_Meridian: 0.9996
      Longitude_of_Central_Meridian: -123.0
      Latitude_of_Projection_Origin: 0.0
      False_Easting: 500000.0
      False_Northing: 0.0

      Planar coordinates are encoded using coordinate pair
      Abscissae (x-coordinates) are specified to the nearest 0.0001
      Ordinates (y-coordinates) are specified to the nearest 0.0001
      Planar coordinates are specified in Meter

      The horizontal datum used is D WGS 1984.
      The ellipsoid used is WGS 1984.
      The semi-major axis of the ellipsoid used is 6378137.0.
      The flattening of the ellipsoid used is 1/298.257223563.

  7. How does the data set describe geographic features?

    Faults
    lines representing mapped faults (Source: This report)

    FID
    Internal feature number. (Source: ESRI)

    Sequential unique whole numbers that are automatically generated.

    Shape
    Feature geometry. (Source: ESRI)

    Coordinates defining the features.

    FGDCRefNo
    FGDC Digital Cartographic Standard for geologic map symbolization (Source: This report)

    text field containing the reference number for the FGDC Digital Cartographic Standard for geologic map symbolization

    Shape_Leng
    Length of feature in internal units. (Source: ESRI)

    Positive real numbers that are automatically generated.

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)

  2. Who also contributed to the data set?

  3. To whom should users address questions about the data?

    USGS Pacific Coastal and Marine Science Center
    Attn: Stephen Hartwell
    Geologist
    400 Natural Bridges Drive
    Santa Cruz, CA 95060
    USA

    (831) 460-7814 (voice)
    (831) 427-4748 (FAX)
    shartwell@usgs.gov

Why was the data set created?

To expand geologic mapping to the seafloor within the California's State Waters, to update coastal geologic mapping, and to contribute to a uniform regional geologic database, which can be used in geographic information systems. Additionally, to provide a geologic map for the public and geoscience community to aid in assessments and mitigation of geologic hazards in the Salt Point coastal region and to provide sufficient geologic information for land-use and land-management decisions both onshore and offshore. This information is not intended for navigational purposes.

How was the data set created?

  1. From what previous works were the data drawn?

    S-8-09-NC (source 1 of 1)
    U.S. Geological Survey (USGS) , Coastal and Marine Geology Program (CMGP), 2013, Subbottom survey data of field activity S-8-09-NC in Stinson Beach to Point Arena (CA) from 09/08/2009 to 09/30/2009: U.S. Geological Survey (USGS) , Coastal and Marine Geology (CMG), Menlo Park, CA.

    Online Links:

    Type_of_Source_Media: ASCII lat/long shot point files
    Source_Contribution:
    Digital seismic data used to interpret subsurface geologic structure

  2. How were the data generated, processed, and modified?

    Date: 2013 (process 1 of 1)
    Faults were mapped onto shot lines based on the latitude and longitude of seismic picks from field activity S-8-09-NC.

  3. What similar or related data should the user be aware of?

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?

  2. How accurate are the geographic locations?

    Faults were primarily mapped by interpretation of seismic reflection profile data (see field activity S-8-09-NC).
    Faults lines were digitized by heads-up screen digitization on to shot-point picks from seismic-line navigation data - each shot point has an associated lat/long in the survey data. Error is introduced from layback, as well as from interpretation of oblique features, so horizontal accuracy of fault and fold location is estimated to be on the order of 10 to 20 meters.

  3. How accurate are the heights or depths?

  4. Where are the gaps in the data? What is missing?

    Data are complete: no offshore features that could be accurately identified and represented at the compilation scale of 1:24,000 were eliminated or generalized. The smallest area represented is approximately 100 square meters. All geospatial database elements are attributed.

  5. How consistent are the relationships among the observations, including topology?

    Map elements were visually checked for overshoots, undershoots, duplicate features, and other errors by the lead authors and by the GIS technician(s) who created the digital database. Review drafts of the map were reviewed internally by at least two other geologists for consistency with basic geologic principles and general conformity to USGS mapping standards.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?

Access_Constraints:
If physical samples or materials are available, constraints on their on-site access are described in "WR CMG Sample Distribution Policy" at URL: <http://walrus.wr.usgs.gov/infobank/programs/html/main/sample-dist-policy.html>
Use_Constraints:
This information is not intended for navigational purposes.
Read and fully comprehend the metadata prior to data use. Uses of these data should not violate the spatial resolution of the data. Where these data are used in combination with other data of different resolution, the resolution of the combined output will be limited by the lowest resolution of all the data.
Acknowledge the U.S. Geological Survey in products derived from these data. Share data products developed using these data with the U.S. Geological Survey.
This database has been approved for release and publication by the Director of the USGS. Although this database has been subjected to rigorous review and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, it is released on condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use.
Although this Federal Geographic Data Committee-compliant metadata file is intended to document these data in nonproprietary form, as well as in ArcInfo format, this metadata file may include some ArcInfo-specific terminology.

  1. Who distributes the data set? (Distributor 1 of 1)

    U.S. Geological Survey
    345 Middlefield Rd
    Menlo Park, CA 94025-3561
    USA

    (650) 329-4309 (voice)

  2. What's the catalog number I need to order this data set?

  3. What legal disclaimers am I supposed to read?


    These databases, identified as geology maps of the offshore Salt Point map area, California have been approved for release and publication by the U.S. Geological Survey (USGS). Although these databases have been subjected to rigorous review and are substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, it is released on condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use.
    Although these data have been processed successfully on a computer system at the U.S. Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data on any other system or for general or scientific purposes, nor shall the act of distribution constitute any such warranty. The USGS or the U.S. Government shall not be held liable for improper or incorrect use of the data described and/or contained herein.
    Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.
    These data are not intended for navigational use.

  4. How can I download or order the data?

  5. What hardware or software do I need in order to use the data set?

    The downloadable data file has been compressed with the "zip" command and can be unzipped with Winzip (or other tool) on Windows systems. To utilize these data, the user must have software capable of uncompressing the WinZip file and importing and viewing an Esri ArcMap SHP. Users should download the ArcGIS Project File, OffshoreSaltPointGIS.mxd.zip, a compressed (with the "zip" command) version of the ArcMap document (.mxd) that has all the data layers loaded in the table of contents for "Offshore Salt Point" and has all the data symbolized as on the data release map sheets. Download and save this ArcGIS project file, including all data layers, to the directory the user has created for this GIS.

Who wrote the metadata?

Dates:
Last modified: 30-Jun-2013
Last Reviewed: 14-May-2015
Metadata author:
U.S. Geological Survey, Coastal and Marine Geology Program
Attn: Stephen R. Hartwell
400 Natural Bridges Drive
Santa Cruz, CA 95060-5792
US

831-460-7814 (voice)
831-427-4748 (FAX)
shartwell@usgs.gov

Metadata standard:
FGDC Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)

Generated by mp version 2.9.16 on Thu May 14 13:57:07 2015