Faults -- Point San Luis map area [sheet 5]

Title: Faults -- Point San Luis map area [sheet 5]
Abstract:: This part of SIM 3327 presents data for faults of the offshore geologic and geomorphic map of the Point San Luis map area, in the vicinity of Pismo Beach, California. The vector data file is included in "Faults_PointSanLuis.zip," which is accessible from <https://pubs.usgs.gov/sim/3327/data/sim3327_data_catalog.html>.
Faults shown on these maps are near-surface (upper about 200 m) structures that are mapped primarily on the basis of interpretation of minisparker seismic-reflection data (tracklines shown in fig. 2 on sheets 2, 4, 6; see also, Sliter and others, 2009). These fault and fold interpretations were further modified on the basis of multibeam imagery and magnetic data; these laterally continuous datasets allow for more accurate interpolation of structures between the seismic-reflection profiles. However, we do not infer fault activity from these data, as we have no direct age data for surficial sediments.
Offshore faults are symbolized as follows: a solid line is used where the location is certain; a dashed line, where the location is inferred; and a dotted line, where the location is concealed. In addition, queries are added where the existence of a feature is questionable. A fault or fold is mapped as inferred when its location is determined by interpolating between seismic-reflection profiles, as well as in bedrock outcrops where a lineament exists in the multibeam imagery but where clear evidence of fault offset or folded strata is lacking. A fault or fold is mapped as concealed where it can be identified in seismic-reflection data but where no surface deformation is apparent in either multibeam imagery or seismic-reflection data. However, it is important to note that, in areas where multibeam imagery does not exist, a fault mapped as concealed may, in fact, have surface expression that is at a scale undetectable in the seismic-reflection data alone. Therefore, it is possible that the location of a fault or fold mapped as concealed may be just as accurate as that of a fault or fold whose location is mapped with more certainty (that is, with a solid or dashed line).
Magnetic data are particularly useful in guiding structural interpretations where massive Mesozoic bedrock is at or near the seafloor surface because, in these areas, seismic-reflection data often are poor owing to lack of penetration. Such is the case in the nearshore areas from Point Piedras Blancas to Shell Beach, which, with the exception of sediment-covered areas in central Estero Bay, are characterized by bedrock outcrops of predominantly Mesozoic basement rocks that are composed of variably magnetic rock types. Magnetic anomalies represent local spatial variations in the Earth’s magnetic field that indicate the distribution of magnetic minerals—primarily magnetite—in the underlying rocks. In many cases, the volume content of magnetic minerals can be related to rock type, and so it follows that abrupt spatial changes in the amount of magnetic minerals commonly mark lithologic or structural boundaries that would not be detectable in seismic-reflection data alone. For example, the Shoreline Fault is not detectable in seismic-reflection data but, rather, is mapped on the basis of correlation of magnetic-anomaly gradients, as well as on seafloor scarps visible in multibeam imagery.
Reference Cited:
Sliter, R.W., Triezenberg, P.J., Hart, P.E., Watt, J.T., Johnson, S.Y., and Scheirer, D.S., 2009, High resolution seismic-reflection and marine magnetic data along the Hosgri fault zone, central California: U.S. Geological Survey 2009-1100, v. 1.1, <https://pubs.usgs.gov/of/2009/1100/>.

Faults -- Point San Luis map area [sheet 5]

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