Electrical Conductivity Images of South African Continental Collision Zones

Within the framework of the German-South African geo-scientific research initiative Inkaba yeAfrica several magnetotelluric (MT) field experiments were conducted along the Agulhas-Karoo transect in South Africa. This 600km long transect is designed to cross the Cape Fold Belt, the Namaqua-Natal Mobile Belt (NNMB), the Karoo Basin and the transition into the Kaapvaal Craton. At the same time, the transect crosses the Beattie Magnetic Anomaly and the Southern Cape Conductive Belt, two of Earth's largest continental geophysical anomalies. In this presentation we will focus on the ~ 1.2-1.0 Ga old convergent margin represented by the Kaapvaal craton-bounding Proterozoic Namaqua-Natal orogenic belt. Along the northernmost segment of the transect, we deployed over 120 MT sites along the 250km profile with a site spacing of 2-4km. With our experiment we focussed on a high-resolution image of the Mobile Belt-Craton transition on lithospheric scale. The MT data show a predominantly 2D behaviour; however, some sites show strong 3D effects, which are consistently observed over a 15km long segment. Anisotropic and 3D modelling studies show that these effects are caused by the unique setting of the extremely resistive rocks of the Kaapvaal Craton in combination with very conductive shallow structures. 2D- and 3D inversion results exhibit a shallow conductivity anomaly: a sub-horizontal, high conductivity band in the upper 5km of the NNMB. In view of other MT data collected in the area, we can map this anomaly over an area of 400km2. Based on deep borehole information, we are able to correlate the conductive band with the black shale and pyrite rich Whitehill formation within the Karoo basin, which seems to wedge out in vicinity of the Craton transition. In mid to lower crustal levels, the MT models show the transition from the untypically conductive rocks of the Proterozoic mobile belt and the extremely resistive rocks of the Kaapvaal Craton.