Asymmetric Electrical Structures Beneath Mid-Ocean Ridges

Abstract

A comparative study of deep mantle structures beneath spreading centres revealed asymmetry of lithosphere and mantle. Multidisciplinary marine expeditions were conducted at two survey sites : one around a super-fast spreading ridge in the East Pacific Rise, and the other around a slow and stable ridge segment of the continental breakup region, the Gulf of Aden. At the former site, a Mantle ELectromagnetic and Tomography experiment (MELT) was conducted with a large number of seismometers and electromagnetometers deployed at the seafloor to image the shape and the geometry of partial melt bodies below the fast spreading centre. At the latter, a seafloor magnetotelluric array was deployed to obtain a twodimensional electrical conductivity structure beneath the continental breakup region, and the structure was compared with precise swath mapping and sea surface gravimetry. At both sites, the deep electrical conductivity structures showed remarkable asymmetry with respect to the ridge crest. Partial melt bodies that correspond to the seafloor-producing factory at the surface could not be observed right below the ridge crest, but they significantly shifted by several tens of kilometers from the ridge crest. The asymmetry also appeared in bathymetry, gravity and seismic structures. This implies that the mid-ocean ridges are created at the contacts of different oceanic mantles rather than right above the symmetric partial melt bodies.