Is the Oceanic Upper Mantle Electrically Anisotropic?

Project Investigator

Doctor Graham Heinson

Project Collaborators

J. Behrens (SIO)

Steve Constable (SIO)

Kerry Key (SIO)

L.M. MacGregor (OHM Surveys, Aberdeen UK)

M.E. Everett (Texas A&M University, College Station, USA

Project details

The APPLE (Anisotropy and Physics of the Pacific Lithosphere Experiment) project was specifically designed to detect and study electrical anisotropy in the oceanic crust and mantle. A total of 20 seafloor electromagnetic recorders were deployed on 30 Ma lithosphere in the Eastern Pacific, recording magnetotelluric (MT) data in the band 20 s to 100,000 s and 4 Hz signals from a controlled source EM (CSEM) transmitter (Fig. 1). CSEM data collected at the center of a 30 km radius circular transmitter tow show an unambiguous signature of anisotropy with a NS conductive direction. These and other CSEM data indicate anisotropy of at least a factor of two over depths of 6–30 km, which we suggest is associated with serpentisation of ridge-parallel normal faults or fractures in the uppermost mantle. There is little or no anisotropy in the crust. The dominant signal in the MT data, which has no resolution in the resistive part of the lithosphere, is that of the resistivity contrast associated with the continent–ocean boundary, nearly 1,000 km distant, evident as a frequency independent electrical strike parallel to the edge of the continental shelf. With the electrically conductive direction about 120 ° from the current plate motion vector, the hypothesis that significant anisotropy is created in the mantle by hydrogen conduction in olivine aligned by plate motion is not supported by our data.

Research Links

Offshore Hydrocarbon Mapping Surveys

College of Geosciences, Texas A & M University

Scripps Institution of Oceanography (SIO)