HR: 0800h
AN: S21A-0246    [Abstracts]
TI: Implications of Preliminary Gravity and Magnetic Surveys to the Understanding of the Bartlett Springs Fault Zone, Northern California Coast Ranges
AU: * Langenheim, V E
EM: zulanger@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025,
AU: Jachens, R C
EM: jachens@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025,
AU: Morin, R L
EM: morin@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025,
AU: McCabe, C M
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025,
AU: Page, W D
EM: WDP7@pge.com
AF: Pacific Gas & Electric, 245 Market St, San Francisco, CA 94105,
AB: We use new gravity and magnetic data in the Lake Pillsbury region to help understand the geometry and character of the Bartlett Springs fault zone, one of the three main strands of the San Andreas system north of the San Francisco Bay area. We collected 153 new gravity stations in the Lake Pillsbury region that complement the sparse regional dataset and are used to estimate the thickness of Quaternary deposits in the inferred Gravelly Valley (Lake Pillsbury) pull-apart basin. We also collected 38 line-km of ground magnetic data on roads and 65 line-km by boat on the lake to supplement regional aeromagnetic surveys and to map concealed fault strands beneath the lake. The new gravity data show a significant northwest-striking gravity gradient at the base of which lies the Bartlett Springs fault zone. Superposed on this major east-facing gravity gradient is a 5 mGal low centered on Lake Pillsbury and Gravelly Valley. Inversion of the gravity field for basin thickness assuming a density contrast of 400 kg/m³ indicates the deepest part of the basin is about 400 m and located in the northern part of the valley, although the inversion lacks gravity stations within the lake. The basin is about 3 km wide and 5 km long and basin edges coincide with strands of the Bartlett Springs fault zone. Our gravity data suggest that Potter Valley, which lies between the Maacama and Bartlett Springs faults, is also as much as 400 m deep in the southern part of the valley, although additional data west of the valley would better isolate the gravity low. Geomorphologic characteristics of the valley suggest that this structure has been quiescent during the late Quaternary. Ground magnetic data are very noisy but the data in conjunction with 9.6 km-spaced NURE aeromagnetic lines suggest that regional analog aeromagnetic data flown in 1962 may suffer from location errors. The regional and NURE data show a northwest-striking magnetic high that extends across Lake Pillsbury. The northeast edge of this anomaly, caused by ultramafic rocks, coincides with the Bartlett Springs fault zone for nearly 15 km. Lake magnetic data indicate as many as three right-stepping strands of the Bartlett Springs fault zone within the gravity- defined pull-apart basin. Two pairs of magnetic anomalies appear to be dextrally offset along the fault, arguing for about 8-9 km of cumulative offset on the fault since the passage of the triple junction at about 3.5 Ma. This estimate is similar to proposed offsets of the Eel River (8.6-10.9 km) at Lake Pillsbury. The minimum long-term slip rate is thus 2.3-3.1 mm/yr, considerably slower than geodetic rates of 5-8 mm/yr. Seismicity forms a 5-km-wide diffuse zone along the Bartlett Springs fault zone in the Lake Pillsbury area, with fewer earthquakes about 5 km northwest of the lake and its associated magnetic anomaly. The McCreary Glade seismicity lineament, located between Potter Valley and Lake Pillsbury, has been attributed to a dike intrusion at depth or reactivation of an older structure. These earthquakes coincide with the northeast edge of a 100-km-long belt of aeromagnetic anomalies and thus appear to have reactivated an older basement feature. The coincidence of the Bartlett Springs fault zone and significant gravity gradients also argues that the much younger fault zone has reactivated older basement features. Our analysis shows that a modern, high-resolution aeromagnetic survey is needed to confirm these preliminary interpretations.
DE: 1219 Gravity anomalies and Earth structure (0920, 7205, 7240)
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 8002 Continental neotectonics (8107)
DE: 8015 Local crustal structure
DE: 8111 Continental tectonics: strike-slip and transform
SC: Seismology [S]
MN: 2007 Fall Meeting