Seafloor magnetotellurics and their use for constraining mantle structure

Xavier Garcia, Rob. L. Evans and Alan D. Chave.
Woods Hole Oceanographic Institution


Electromagnetic techniques which measure the electrical resistivity of the mantle,
provide a complementary view of the Earth to that offered by other geophysical
approaches. Resistivity provides particularly powerful constraints in areas
where fluids play a key role in shaping mantle processes. Examples of such areas
are at midocean ridges where melt is delivered from the mantle to the seafloor;
subduction zones where water is released from the downgoing slab into the overlying
mantle wedge; and in the asthenosphere where dissolved hydrogen impacts the rheology
of the mantle.

A number of seafloor experiments, using the natural source magnetotelluric (MT)
method, have been carried out. These include: the MELT experiment, which provided
constraints on the melting processes beneath the fast spreading southern East Pacific
Rise;  a deep probing sounding made on an abandoned telephone cable which constrained
the anisotropic, water bearing structure of the asthenosphere beneath the northeast
Pacific plate; and the EMSLAB experiment which featured an onshore-offshore transect
across the Cascadia subduction system.

In addition to these seafloor experiments, MT has been widely used in continental
settings, providing contrasting views of deep continental lithosphere and underlying
asthenosphere, that have helped in the development of tectonic models of continental
accretion and stabilization.

Developments in instrumentation made over the last decade, and an increasing number
of instruments available worldwide, has had a significant impact on our ability to
stage large marine field experiments, and we anticipate the MT will play a significant
role in the proposed new Mantle Dynamics initiative.