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The interpretation of the Northumberland Trough geomagnetic variation anomaly using two-dimensional current models

1986; Oxford University Press; Volume: 87; Issue: 2 Linguagem: Inglês

10.1111/j.1365-246x.1986.tb06640.x

1365-246X

R. J. Banks,

earthquake and tectonic studies

The electromagnetic fields at the surface are non-linear functional of the electrical conductivity of the Earth. However, the anomalous magnetic fields which are created by lateral variations in the conductivity are linear functional of the internal current density. The inverse problem analysed in this paper is that of using the spatial structure of the vertical field, defined along a profile perpendicular to the strike of a 2-D structure, to infer the lateral and vertical distribution of current parallel to the strike. In the Northumberland Trough survey, magnetometers at some sites are operated simultaneously with a reference instrument at Durham, and interstation vertical field transfer functions were fitted to the data. They are subsequently used to predict the vertical field at each site which is associated with a specific horizontal field at the reference. Other magnetometers were run separately, and only single-station transfer functions could be computed. These yield predictions of the vertical field which are biased by anomalous horizontal fields. When the spatial structure of the anomaly is known, the horizontal component of the field can be predicted from the vertical by using the constraint that the field derives from a potential which satisfies the Laplace equation. The corrections that are required to convert from single-station to inter-station transfer functions can be estimated from the maps of the horizontal field. This approach is used to generate vertical field values at sites spaced at 10 km intervals along a 100 km profile traversing the Alston Block and Northumberland Trough in northern England. The map of the horizontal component anomalies relates more simply to the structure than the vertical component map. The surface magnetic fields do not define a unique distribution of current density within the Earth. The non-uniqueness is removed by fixing the direction and bounding the magnitude of the current density. The inversion of the set of constraints is solved by linear programming methods. The current density in the Carboniferous sedimentary rocks filling the Northumberland Trough is calculated by combining estimates of their conductivity based on Archie's Law with measurements of the electric field at a magnetotelluric site. When only the surface conductor is allowed, the current densities required to fit the magnetic fields are unacceptably large, indicating that a deeper conductor must be present beneath the trough. Its depth is explored by bounding the current density within it and investigating how the misfit changes as its upper surface is moved up or down. I conclude that the Northumberland Trough magnetic variation anomaly is largely due to a conductive body within the middle and lower crust at depths between 10 and 30 km. The same conductor has been detected in a magnetotelluric survey at three sites around the Trough as a northward-dipping slab with a conductivity—thickness product of 500–1000 S (Beamish). Extrapolation along strike from the WINCH seismic reflection profile (Beamish & Smythe) indicates that the conductor coincides with the Iapetus suture, and is likely to be a permeable zone containing saline fluids which marks the thrust surface. The magnetic variation survey shows that, in the upper crust, the suture lies directly beneath the axis of the Northumberland Trough. Its effect on the strength of the crust must have played a crucial role in controlling the development of the Solway and Northumberland sedimentary basins.