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

Open-File Report 2007-1368

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Abstract

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.

Geospatial Extents

Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Preliminary Gravity and Magnetic Data of the Lake Pillsbury Region, Northern Coast Ranges, California
Series title:
Open-File Report
Series number:
2007-1368
Edition:
Version 1.0
Year Published:
2007
Language:
ENGLISH
Publisher:
Geological Survey (U.S.)
Contributing office(s):
Geophysics Unit of Menlo Park, CA (GUMP)
Description:
Report: 24 p.; Metadata; Data Files
Online Only (Y/N):
Y
Additional Online Files(Y/N):
Y