HR: 0830h
AN: T41D-0261    [PDF]
TI: Potential field Modeling of the 3-D Geologic Structure of the San Andreas Fault Observatory at Depth (SAFOD) at Parkfield, California
AU: * McPhee, D K
EM: dmcphee@usgs.gov
AF: U.S. Geological Survey, MS 989, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: Gravity and magnetic data, along with other geophysical and geological constraints, are used to develop 2-D models that we use to characterize the 3-D geological structure of the San Andreas fault (SAF) zone in the vicinity of SAFOD near Parkfield, CA. The gravity data, reduced to isostatic anomalies, comprise a compilation of three different data sets with a maximum of 1.6 km grid spacing for the scattered data and closely spaced ($\sim$40 m) stations along one SW-NE profile crossing the SAFOD pilot hole. Aeromagnetic data were flown at a nominal 300 m above the terrain along SW-NE flight lines perpendicular to the San Andreas Fault. Data were recorded at $\sim$50 m spacing along flight lines approximately 800 m apart. Ground magnetic data recorded every 5 m along lines $\sim$300 m apart cover a 3 x 5 km area surrounding the SAFOD pilot hole. Previous modeling showed that magnetic granitic basement rocks southwest of the SAF are divided by an inferred steep fault sub-parallel to the SAF. We compute 2-D crustal models along 5 km-long southwest-northeast profiles, one of which extends through the SAFOD pilot hole near and along the high-resolution seismic refraction/reflection survey completed in 1998 (Catchings et al., 2002). Our models are constrained by pilot hole measurements, where we see a boundary between sediment and granitic basement at $\sim$770 m and an order of magnitude increase in magnetic susceptibility at $\sim$1400 m, possibly the same depth at which the SW dipping Buzzard Canyon Fault intersects the pilot hole. Regional gravity, magnetic and geologic data indicate two very distinct basement blocks separated by a steeply dipping SAF. The shallowly dipping sedimentary section SW of the SAF coincides with the low velocity zone observed with seismic measurements. Shallow slivers of magnetic sandstone on the NE side of the SAF explain higher frequency features in the magnetic data. In addition, we show a flat lying, tabular body of serpentinite sandwiched between 2 blocks of Franciscan rock on the NE side of and truncating at the SAF. The Salinian granitic rocks to the SW of the SAF contain a magnetic body of unknown origin further to the SW and overlay a high density granitic root, possibly a deeper, denser phase of granitic rock. Furthermore, a shallow magnetic body SE of the SAF, possibly a sliver of granitic rock or serpentinite, gives rise to a significant NW trending high on the ground magnetic map parallel to the SAF. We use 2-D models both NW and SE of the profile that extends through the SAFOD pilot hole to explore the 3-D nature of the unknown magnetic bodies SW of the SAF and their significance to future drilling at SAFOD as well as to other faults sub parallel to the SAF. Our models are the basis for a 3-D digital model of the upper crust surrounding SAFOD that will act as a tool for directly comparing quantitative subsurface interpretations based on various methods including seismic refraction and reflection, seismicity, magnetotelluric, gravity, magnetic, and geologic techniques throughout the lifetime of the SAFOD project.
DE: 1219 Local gravity anomalies and crustal structure
DE: 1517 Magnetic anomaly modeling
DE: 7230 Seismicity and seismotectonics
DE: 8015 Local crustal structure
DE: 8110 Continental tectonics--general (0905)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting