Photograph taken from Anderson Dam looking west across the city of Morgan Hill in the southern Santa Clara Valley towards Loma Prieta. Rocks in foreground are Cretaceous serpentinite and Miocene silica-carbonate rock. Santa Teresa Hills in middle ground are composed of Cretaceous Franciscan Complex melange, greenstone, and serpentinite.
This report depicts the geologic framework and structure of both the upland and adjacent lowland areas in the vicinity of southwestern Santa Clara ("Silicon") Valley and the southern Santa Cruz Mountains. The maps encompass parts of Santa Clara and Santa Cruz Counties and include new and revised geologic mapping of the Los Gatos, Santa Teresa Hills, Laurel, Loma Prieta, and Mount Madonna 7.5' quadrangles and parts of the Morgan Hill and Gilroy 7.5' quadrangles. The mapping was carried out under the National Cooperative Geologic Mapping and Earthquake Hazards Reduction Programs of the U.S. Geological Survey between 1988 and 1997. The mapping was undertaken as part of projects to map the geology of the 1:100,000 San Jose quadrangle and to investigate the geology of the epicentral region of the October 17, 1989 Loma Prieta Earthquake. Some mapping is compiled from older sources. The seven maps are the first detailed depiction of the bedrock stratigraphy and structure along both sides of the San Andreas fault in the southern San Francisco Bay Area.
The description of rock units and structure are arranged by major fault blocks of different basement composition that are separated by the San Andreas fault. The Salinian terrane, which lies southwest of the San Andreas fault, is a regionally extensive Mesozoic rock assemblage, largely of continental affinity, composed of granitic to gabbroic rocks with roof pendants of high temperature metasedimentary rocks. Northeast of the San Andreas fault, the basement is a composite of the Franciscan Complex, the Coast Range ophiolite, and parts of the Great Valley Sequence, a disrupted assemblage of rocks which originally formed the Mesozoic convergent continental margin. The Santa Cruz fault block of this report is comprised entirely of rocks of the Salinian terrane. The Santa Cruz block, in turn, is divided into the subsidiary Ben Lomond and La Honda fault blocks by the Zayante fault. Northeast of the San Andreas fault the Mesozoic composite basement is broken into the subsidiary Sierra Azul and New Almaden blocks along a complex fault boundary consisting of locally merged segments of the Sargent, Berrocal, Sierra Azul, and Aldercroft faults. This complex block boundary is further considered to represent a part of the regionally extensive Coast Range fault, which everywhere separates Mesozoic forearc strata of the Great Valley Sequence from the Franciscan Complex.
The Coast Range fault has been interpreted as a major attenuation fault associated with Late Cretaceous to early Tertiary unroofing of the Franciscan Complex. An alternative model interprets the Coast Range fault as the roof thrust of a thick, regionally extensive system of tectonic wedges. Rocks between the San Andreas fault zone and the Sargent-Berrocal-Sierra Azul-Aldercroft fault zone are a part of the Sierra Azul block. Rocks in the Santa Teresa Hills that have a stratigraphy similar to that of the Sierra Azul area are considered a thrust klippe of the Sierra Azul block.
Files available for downloading:
Index map of 7.5' quadrangles in the 1:100,000 San Jose quadrangle. The area covered by this report is shown in red tint.
Metadata and Readme files
scvmfdb.txt - Text-only version of a file that describes the digital content of this publication and explains how to use the digital database (52 kb).
scvmfdb.pdf - PDF version of a file that describes the digital content of this publication and explains how to use the digital database (36 kb).
mf2373.met.txt - Text-only file of the FGDC-compliant metadata for this publication (304 kb).
mf2373.rev.txt - Text-only file containing the revision list for this publication (8 kb).
mf2373.met.txt - Text-only file of the FGDC-compliant metadata for this publication (304 kb).
Data
mf2373db.tgz Compressed (using gzip) tar file of the digital database (comprised of Arc/Info and supporting database files) for the three maps that make up this publication (17 MB).
Files for viewing and plotting
lgmap.pdf - PDF file containing an image of the entire geologic map and base maps of the Los Gatos quadrangle at a scale of 1:24,000 intended for viewing (4 MB).
lrmap.pdf PDF file containing an image of the geologic map and base maps of the Laurel quadrangle at a scale of 1:24,000 intended for viewing (3.9 MB).
lpmap.pdf - PDF file containing an image of the geologic map and base maps of the Loma Prieta quadrangle at a scale of 1:24,000 intended for viewing (2.7 MB).
sth-mhmap.pdf - PDF file containing an image of the geologic maps and base maps of the Santa Teresa Hills and southwestern part of the Morgan Hill quadrangles at a scale of 1:24,000 intended for viewing (3.9 MB).
mm-glmap.pdf - PDF file containing an image of the geologic maps and base maps of the Mt. Madonna and southwestern part of the Gilroy quadrangles at a scale of 1:24,000 intended for viewing (3.2 MB).
scvexpl.pdf - PDF file containing map keys, index maps, and map units for the southwestern Santa Clara Valley and southern Santa Cruz Mountains, Santa Clara, and Santa Cruz Counties, California (4.9 MB).
scvxsec1.pdf - PDF file containing five cross sections for the Los Gatos and Santa Teresa Hills quadrangles (884 kb).
scvxsec2.pdf - PDF file containing four cross sections for the Laurel and Loma Prieta quadrangles (1 MB).
scvmf.pdf - PDF file of page-sized pamphlet containing detailed unit descriptions and geologic information, plus sources of data, and references (60 kb).
mf2373ps.tgz - Compressed (using gzip) tar file containing PostScript files of the map sheets and pamphlets for this publication (19.7 MB).
For questions about the scientific content of this report, contact Bob McLaughlin
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Please send comments and suggestions, or report problems, to:
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Created: February 6, 2002 (cad)
Updated: April 20, 2007 (bwr, mfd)