Magnetotelluric measurements across the Beattie magnetic anomaly and the Southern Cape Conductive Belt, South Africa

Abstract

The Beattie Magnetic Anomaly (BMA) and the Southern Cape Conductive Belt (SCCB), two of Earth's largest continental geophysical anomalies, extend across the southern African continent in an east-west direction. To resolve structural details of the SCCB, a high-resolution magnetotelluric study was conducted in March 2004, along a 150-km-long N-S profile across the Karoo Basin in South Africa. A two-dimensional conductivity model at a scale of the entire crust exhibits three distinct zones of high conductivity: (1) Beneath the surface trace of the axis of the BMA we observe a high-conductivity anomaly at 5 to 10 km depth; however, it remains enigmatic whether both geophysical anomalies have the same source. (2) A shallow, regionally continuous subhorizontal band of high conductivity can be linked to a 50- to 70-m-thick pyritic-carbonaceous marker horizon intersected in deep boreholes. (3) Several highly conductive synformal features in the mid crust are newly imaged near the northern extremity ofthe profile. Our data generally support the existence of a deeper conductivity belt but allow for a much better definition of crustal conductivity in this region. Such laterally confined zones of crustal conductivity can only be resolved with a dense site spacing and broadband recordings. Copyright 2007 by the American Geophysical Union.