HR: 09:30h
AN: T11F-07    [Abstracts]
TI: Crustal Structure Across the San Andreas Fault at the SAFOD Site, California, From Gravity and Magnetic Studies
AU: * McPhee, D K
EM: dmcphee@usgs.gov
AF: U.S. Geological Survey, MS 989 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Tilden, J
EM: jtilden@usgs.gov
AF: U.S. Geological Survey, MS 989 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Jachens, R C
EM: jachens@usgs.gov
AF: U.S. Geological Survey, MS 989 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Wentworth, C M
EM: cwent@usgs.gov
AF: U.S. Geological Survey, MS 973 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: Newly compiled gravity, magnetic and geologic datasets from the Parkfield region around the San Andreas Fault Observatory at Depth (SAFOD) help define the structure and geophysical setting of the San Andreas Fault (SAF). A previous 2-D model incorporating gravity, ground magnetic, and geology data along a profile passing within 200 m of the SAFOD pilot hole suggests that, as drilling proceeds NE toward the surface trace of the SAF, it will encounter a broad zone separating granitic basement on the SW and Franciscan basement rocks on the NE. Further to the NE the drill will probably encounter interleaved Franciscan rocks and serpentinite, and possibly a mass of serpentinite itself. Of particular significance are two magnetic bodies located just SW of the SAF which appear to be steeply dipping, fault-parallel, and likely fault-bounded slivers, although the exact nature and depth of these bodies is unclear. The zone between SAFOD and the SAF is also characterized by a dipping Tertiary sedimentary section overlying granitic basement rocks, which coincides with the low velocity zone observed with seismic measurements. This 2-D model was constrained by pilot hole measurements, where we observe a boundary between sediment and granitic basement at ~770 m depth and an order of magnitude downward increase in magnetic susceptibility at ~1400 m depth, which coincides with the steeply dipping northeastern edge of a magnetic granitic body within the granitic basement. New gravity data collected during the fall of 2003 along a 50-km-long seismic reflection/wide-angle refraction profile across the SAF extend through the SAFOD site. Stations were spaced 100 m apart along a 5-km-long portion of the profile centered about the surface trace of the SAF and then 300 m apart along the rest of the line; data were reduced to isostatic residual gravity anomalies. We use these data to produce a 2-D model which further constrains the geometry of the dipping sedimentary section just NE of SAFOD in order to determine if the drill will encounter sediments as it heads toward the SAF. Preliminary modeling shows that the dipping Tertiary sedimentary section NE of SAFOD is likely shallow ($<$ 2km); however, it is difficult to constrain the dip angle of the base of the section. These data will also address the extent of shallow Franciscan rocks NE of the SAF. Although Franciscan rocks crop out immediately NE of the SAF, previous gravity data suggests that the Franciscan rocks overlie low-density Cenozoic deposits. New ground magnetic data will be collected during the fall of 2004 along the profile extending through SAFOD and regions of the SAF just NW and SE of existing ground magnetic data. These observations will allow us to explore the extent of the unknown magnetic bodies along the SAF, their 3-D geometry, and thus their significance to future drilling at SAFOD. If these bodies are indeed steeply dipping, fault-parallel slivers, then as drilling proceeds NE toward the fault zone it is likely that the drill will encounter at least 4 faults that bound the two magnetic slivers.
DE: 8015 Local crustal structure
DE: 8150 Plate boundary--general (3040)
DE: 7205 Continental crust (1242)
DE: 1219 Local gravity anomalies and crustal structure
DE: 1517 Magnetic anomaly modeling
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting