HR: 16:15h
AN: GP12B-02    [PDF]
TI: Electrical Conductivity and Anisotropy in Pacific Lithosphere: CSEM Results from APPLE
AU: * Behrens, J
EM: jbehrens@ucsd.edu
AF: IGPP, Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0225 United States
AU: Constable, S
EM: sconstable@ucsd.edu
AF: IGPP, Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0225 United States
AU: Everett, M
EM: everett@geo.tamu.edu
AF: Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843 United States
AU: MacGregor, L
EM: lucym@soc.soton.ac.uk
AF: OHM Ltd., Mariners Suite, Royal Mail House, Terminus Terrace, Southampton, CA SO14 3FD United Kingdom
AB: Strain associated with plate formation at mid-ocean ridge spreading centers may influence electrical conductivity at various depths in the lithosphere, and may leave an anisotropic fabric frozen in place. By measuring lithospheric electrical conductivity and anisotropy, insight may be gained regarding the formation and evolution of oceanic crust and mantle. Controlled-source electromagnetic (CSEM) sounding of 35 Ma Pacific lithosphere was undertaken as part of the Anisotropy and Physics of the Pacific Lithosphere Experiment (APPLE), carried out approximately 1000 km west of San Diego. The transmitter (DASI), with a 100 m horizontal electric dipole antenna, was deep-towed in a 30 km radius circle around an array of receivers. A radial tow to 70 km total range and a 15 km radius semi-circular tow supplemented the geometry of the main tow. DASI transmitted a 4 Hz square wave throughout the CSEM phase of the experiment. Smooth (and layered) inversions of short-offset (2-20 km) data, using 1-D isotropic modeling, generate models with upper-crustal resistivities $\sim$1 $\Omega$m, varying by about an order of magnitude across the survey area. Lower crustal resistivities are on the order of 10$^{3}$ $\Omega$m. Smooth inversion of the long radial tow data indicates upper mantle resistivities of $\sim$10$^{4}$ $\Omega$m, with an increase in conductivity below 20 km depth. This may be due to thermally-activated olivine conduction, indicating that the base of the lithosphere has been detected. The integrated resistivity-thickness product for the top 100 km of our model is 1.1 x 10$^{9}$ $\Omega$m$^{2}$. The electric field is characterized in terms of the polarization ellipse parameters. During the circular tow, the maximum axis varies by a factor of two, while the minimum axis varies by a factor of 5. Forward calculations were run on models with a uniaxially anisotropic layer sandwiched between two isotropic layers. Simulated data from models with increased conductivity perpendicular to the paleo-spreading direction in the anisotropic layer (with a ratio of 10) match the actual data well. In addition, the anisotropy appears to be in the lithospheric upper mantle; an anisotropic sheeted dike complex alone does not replicate the variation in the data.
DE: 0694 Instrumentation and techniques
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting