Using marine magnetotelluric determinant data to study the oceanic upper mantle

Abstract:

The marine magnetotelluric (MT) method plays an increasingly important role in studying the depth variation of electrical lithosphere-asthenosphere boundary (LAB) in oceans. Three-dimensional (3D) inversion of marine MT data is still limited by bathymetry, coastlines, and computer resources. Thus, two-dimensional (2D) inversion of determinant data is introduced to the marine MT method for its advantage in overcoming the 3D effects that often compromise 2D transverse electric/magnetic mode inversions. We motivate the use of determinant data with a synthetic model study of the Hawaiian plume. At the Middle Atlantic Ridge (MAR), 2000 km away from the African coast, 39 marine MT instruments were deployed alongside seismometers to study the LAB’s depth variation at ages 0- 45 My. After MT data analysis, 2D inversion of determinant data is used to derive resistivity models, which show that the thickness of a resistive lid ranges from ~ 20 to 80 km at the study region, generally thickening with age. Punctuated low resistivity anomalies and a low resistivity channel (< 1 Wm) are observed, consistent with the observations from geodynamic modeling. Sensitivity analysis and synthetic tests indicate that the observed anomalies are robust, and they are in good agreement with seismic results. At the Mendocino Fracture Zone (MFZ), 600 km away from the west coast of North America, 17 MT instruments were deployed to study the electrical structure of the lithosphere. The inversion of MT determinant data resolves a resistive lithospheric lid varying from ~ 40 to 140 km, showing a big step in the resistivity model at two sides of the MFZ, generally consistent with expectations from a half-space plate cooling model. Both studies imply that the LAB is more dynamic than previously thought.