Electrical mantle anisotropy and crustal conductor: a 3-D conductivity model of the Rwenzori Region in western Uganda

P>Magnetotelluric measurements were performed east of the Rwenzori Mountains, Uganda. At 23 sites transfer functions between magnetic and telluric fields and phase tensor elements were estimated in the period range 10-10 000 s. The transfer functions indicate a complex 3-D conductivity structure within the crust predominantly at the contact zone of the Rwenzori Mountains and the rift shoulder to the northeast. An alternative representation of the phase tensor ellipses in form of bars is introduced. For periods above 100 s, the major phase tensor bars of all sites strike SSW-NNE and the phase tensor invariants (min) and (max) exhibit a difference of a least 20 degrees. This conspicuous behaviour and the small vertical magnetic field in the same period range can be explained by an anisotropic conductivity in the depth range 30-50 km with the conductive direction perpendicular to the Rift axis. The anisotropy might originate from orientated Olivine crystals in the most upper mantle, whereby the preferred orientation might be motion induced due to delamination of the lower crust beneath the Rwenzoris. A high conducting layer is found to the southeast of the Rwenzoris at 15-km depth matching a seismic low-velocity zone. Possibly it indicates a zone of partial melt within the lower crust connected to volcanic fields nearby. Generally, the phase tensor elements (min) increase above 45 degrees for periods longer than 2000 s denoting an increase of conductivity towards greater depth.