Origin of long-wavelength magnetic anomalies associated with subducting oceanic lithosphere

Abstract:

Many subduction zones correspond to strong long-wavelength magnetic signatures that are not predicted by forward models in which magnetisation is confined to the continental crust and unsubducted oceanic lithosphere. Previous studies have proposed this signature to be sourced within hydrated, serpentinised mantle wedge. Alternatively, thermal models of subducting oceanic lithosphere suggest slab material cool enough to hold magnetisation may persist >200 km below the Earth’s surface. Here we investigate the origin of long wavelength magnetic anomalies by constructing alternative predictions of the lithospheric geomagnetic field, which we compare to the MF7 model based on satellite observations. We compare 2 models for subduction zone magnetisation, one in which the wedge is magnetised and one in which the subducted slab is magnetised. Magnetisation deeper than the Moho of the overriding plate helps to reconcile the observed anomalies for a number of subduction zones, including Sumatra, the Aleutians, and along the Pacific margin of North and Central America. North-south trending arcs, especially those near to the magnetic equator, have magnetisations within the null space that produces little or no external magnetic field, and are less distinct in the observations. A magnetic anomaly persists along the Baja and Mexican west coast, indicating that the fossil subduction zone continues from California through Central America despite limited seismic evidence of a slab there. Distinguishing between the two competing models is difficult because it depends critically on the trend in magnetic anomaly away from the trench.