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Science 2 September 1994:
Vol. 265. no. 5177, pp. 1436 - 1439
DOI: 10.1126/science.265.5177.1436
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Articles

Seismic Evidence for a Lower-Crustal Detachment Beneath San Francisco Bay, California

Thomas M. Brocher 1, Jill McCarthy 1, Patrick E. Hart 1, W. Steven Holbrook 2, Kevin P. Furlong 3, Thomas V. McEvilly 4, John A. Hole 5, and Simon L. Klemperer 5

1 U. S. Geological Survey, Mail Stop 977, 345 Middlefield Road, Menlo Park, CA 94025, USA.
2 Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
3 Department of Geoscience, 439 Deike Building, Penn State University, University Park, PA 16802, USA.
4 Seismographic Station, Department of Geology and Geophysics, University of California, Berkeley, CA 94720, USA.
5 Department of Geophysics, Stanford University, Stanford, CA 94305, USA.

Results from the San Francisco Bay area seismic imaging experiment (BASIX) reveal the presence of a prominent lower crustal reflector at a depth of sim15 kilometers beneath San Francisco and San Pablo bays. Velocity analyses indicate that this reflector marks the base of Franciscan assemblage rocks and the top of a mafic lower crust. Because this compositional contrast would imply a strong rheological contrast, this interface may correspond to a lower crustal detachment surface. If so, it may represent a subhorizontal segment of the North America and Pacific plate boundary proposed by earlier thermo-mechanical and geological models.

Submitted on May 9, 1994
Accepted on July 11, 1994


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