Electrical Conductivity and Anisotropy in Pacific Lithosphere: CSEM Results from APPLE

Emplacement of the sheeted dyke complex and 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 as a function of depth, 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 in February/March 2001 approximately 1000 km west of San Diego, California. Twenty seafloor electric field sensors were deployed (and recovered with data) during this experiment. The transmitter (DASI), a 100 m horizontal electric dipole, was deep-towed in a 30 km radius circle around a central core and perimeter array of receivers. An additional radial tow to 70 km total range and a 15 km radius semi-circular tow around a perimeter receiver supplemented the geometry of the main tow. DASI transmitted a 4 Hz square wave throughout the CSEM phase of the experiment. Smooth (and layered) inversion of short-offset (2-20 km) data, using 1-D isotropic modeling, generates models with upper-crustal resistivities ~10 Ω m, varying by about an order of magnitude across the survey area. Lower crustal resistivities are on the order of 10³ Ω m. Smooth inversion of the long radial tow data indicates upper mantle resistivities of ~10⁴ Ω 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 10⁹ Ω m². A factor of two is observed in the electric fields measured during the circular tow, in a pattern that qualitatively resembles forward modeling results over an uniaxially anisotropic halfspace with enhanced conductivity in the vertical and fossil spreading directions. This supports earlier results from the PEGASUS experiment, which advocated dipping ridge-perpendicular mineral lineaments (e.g. oxides, graphite) of enhanced conductivity within the lithosphere.