The detection of electrical anisotropy in 35 Ma Pacific lithosphere: Results from a marine controlled-source electromagnetic survey and implications for hydration of the upper mantle

Tectonic processes associated with seafloor spreading are known to generate anisotropic fabric in oceanic lithosphere. Anisotropy in seismic refraction data has long been attributed to preferred orientation of the olivine crystal lattice, caused by ductile deformation of the warm uppermost mantle near spreading ridges, which is frozen into place upon cooling. The lithosphere in bulk, however, is several orders of magnitude more electrically conductive than the silicates that dominate its constitution; anisotropic conductivity is therefore indicative of the distribution of electrically conductive minor mineral and fluid phases.; Marine Controlled-Source Electromagnetic (CSEM) sounding data from the Anisotropy and Physics of the Pacific Lithosphere Experiment (APPLE) indicate anisotropic electrical conductivity in 35 Ma lithosphere that had been generated at the fast-spreading East Pacific Rise. 1-D modeling and inversion show that the data require some amount of anisotropy in the uppermost mantle. The preferred model simulates vertical "sheets" of increased conductivity, aligned normal to the fossil spreading direction, more conductive by a factor of at least 2-3. This anisotropic layer begins at the Moho and continues for at least 10 km into the upper mantle - a conclusion supported through the use of two different modeling codes and two different inversion methods applied to independent subsets of the data. Models in which the crust is the only anisotropic layer are fundamentally incompatible with the observed data.; Hydrothermal circulation of seawater into the lithosphere begins at the ridge and may continue for tens of millions of years; ridge-parallel vertical cracks and faults provide conduits through the entire crust and into the upper mantle. Mantle peridotites are serpentinized when hydrated at temperatures below ∼500°C; associated magnetite formation increases bulk electrical conductivity. A mixing law relationship indicates that at least ∼0.1% serpentinization by volume is required to match the preferred conductivity model, with 1% a reasonable estimate. Applied to the vertical sheets model, that corresponds to a 100 m wide zone of serpentinization every 10 km.