Geophysical Unit of Menlo Park, Calif.

U.S. Geological Survey
Open-File Report 2007-1368
Version 1.0

Preliminary Gravity and Magnetic Data of the Lake Pillsbury Region, Northern Coast Ranges, California

By V.E. Langenheim, R.C. Jachens, R.L. Morin, and C.A. McCabe


photo of small boat pulled up on the lake shore

The Lake Pillsbury region is transected by the Bartlett Springs Fault zone, one of the main strike-slip faults of the San Andreas system north of San Francisco Bay, California. Gravity and magnetic data were collected to help characterize the geometry and offset of the fault zone as well as determine the geometry of the Gravelly Valley pull-apart basin and Potter Valley, an alluvial intermontane basin southwest of Lake Pillsbury. The Bartlett Springs fault zone lies at the base of a significant gravity gradient. Superposed on the gradient is a small gravity low centered over Lake Pillsbury and Gravelly Valley. Another small gravity low coincides with Potter Valley. Inversion of gravity data for basin thickness indicates a maximum thickness of 400 and 440 m for the Gravelly and Potter Valley depressions, respectively. Ground magnetic data indicate that the regional aeromagnetic data likely suffer from positional errors, but that large, long-wavelength anomalies, sourced from serpentinite, may be offset 8 km along the Bartlett Springs Fault zone. Additional gravity data collected either on the lake surface or bottom and in Potter Valley would better determine the shape of the basins. A modern, high-resolution aeromagnetic survey would greatly augment the ability to map and model the fault geometry quantitatively.

Download this report as a 24-page PDF file (of2007-1368.pdf; 9.5 MB).

Read the metadata file (pillsbury_metadata.txt; 56 kB).

Go to the data folder. This folder contains six ASCII data files as well as a .zip file of all of them (6 MB).

For questions about the content of this report, contact Vicki Langenheim

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