Kiyoshi Baba, Alan D. Chave, Rob L. Evans, Greg Hirth and Randall L. Mackie, Mantle dynamics beneath the East Pacific Rise at
17S Insights from the Mantle Electromagnetic and
Tomography (MELT) experiment, J. Geophys. Res. , vol 111, 2006
The electromagnetic data from the Mantle Electromagnetic and Tomography (MELT)
experiment are inverted for a two-dimensional transversely anisotropic conductivity
structure that incorporates a correction for three-dimensional topographic effects on the
magnetotelluric responses. The model space allows for different conductivity values in
the along-strike, cross-strike, and vertical directions, along with imposed constraints of
model smoothness and closeness among the three directions. Anisotropic models provide
a slightly better fit to the data for a given level of model smoothness and are more
consistent with other geophysical and laboratory data. The preferred anisotropic model
displays a resistive uppermost 60-km-thick mantle independent of plate age, except in
the vicinity of the ridge crest. In most inversions, a vertically aligned sheet-like conductor
at the ridge crest is especially prominent in the vertical conductivity. Its presence suggests
that the melt is more highly concentrated and connected in the vertical direction
immediately beneath the rise axis. The melt zone is at least 100 km wide and is
asymmetric, having a greater extent to the west. Off-axis, and to the east of the ridge, the
mantle is more conductive in the direction of plate spreading at depths greater than 60 km.
The flat resistive-conductive boundary at 60 km agrees well with the inferred depth of
the dry solidus of peridotite, and the deeper conductive region is consistent with the
preferred orientation of olivine inferred from seismic observations. This suggests that the
uppermost 60 km represents the region of mantle that has undergone melting at the
ridge and has been depleted of water (dissolved hydrogen). By contrast, the underlying
mantle has retained a significant amount of water.
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