Comparison of electrical conductivity structures and 2D magnetic modelling along two profiles crossing the Beattie Magnetic Anomaly, South Africa

Abstract

Two of the Earth’s largest geophysical anomalies, the Beattie Magnetic Anomaly (BMA) and the Southern Cape Conductive Belt (SCCB) extend across the southern African continent for more than 1000 km in an east-west direction. Based on previous electrical and magnetometer array measurements it is believed that both anomalies have a common crustal source with a width of 50 km represented by serpentinized palaeo-oceanic crust. New two-dimensional (2D) electrical conductivity models along a profile from Prince Albert to Fraserburg outline a narrow (2 km wide), southward-dipping zone of high electrical conductivity in the upper crust below the centre of the Beattie Magnetic Anomaly (BMA). Two-dimensional modeling studies of aero-magnetic data show that simple models that can explain the magnetic signature of the BMA, are not consistent with a narrow conductivity anomaly. Thus a common source for the two anomalies is unlikely. A second magnetotelluric (MT) experiment across the BMA, conducted along a 75 km profile centred on Jansenville, 350 km east of the first profile, resolves a sub-vertical and narrow conductivity anomaly below the centre of the BMA. At this location the conductor is reaching deeper to lower crustal levels and is inclined towards the north.