Electrical conductivity images across the Namibian passive margin: Tectonic implications for the evolution of the Kaoko Belt, the western Kongo Craton and the Walvis Ridge

Abstract:

High-quality magnetotelluric (MT) data were collected at 167 sites onshore in Northwestern Namibia within the framework of a project (SAMPLE) to investigate processes related to the breakup of Gondwana. The entire study area extents from the Walvis Ridge in the Atlantic Ocean and crosses onshore the Kaoko Belt and the western boundary of the Kongo Craton.

The surface expressions of prominent faults such as the Purros Shear Zone and the Three Palm Mylonite Zone of the Kaoko Belt are expressed in 2D models as zones of high electrical conductivity. A region where the Etendeka flood basalts outcrop in the Western Kaoko Zone appears as a 10-15 km deep reaching highly resistive feature. However, dimensionality and strike direction analysis of the data exhibit 3D effects.

The 3D inversion models reproduce the main structures obtained by 2D modelling but the spatial coherency of structures is better resolved. The Purros Shear Zone and the Three Palm Mylonite Zone appear in the 3D inversion models as zones of high electrical conductivity striking in NW-SE direction in the middle crust. Where the Walvis Ridge intersects with the continent, an upwelling of resistive structures indicates a modification of the middle crust, which seems be caused by a deeper anomaly which is not resolved with MT. The spatial extent and possible emplacement of the highly resistive material associated with the Etendeka flood basalts is more enigmatic as only a relatively confined region at upper- to mid-crustal levels seems to be affected. Most importantly, these Cretaceous flood basalt events did not affect or alter the high conductivity structures associated with the preexisting Proterozoic shear zones. Compared with other cratonic boundaries in Southern Africa and elsewhere the transition from the Kaoko Belt into the Kongo Craton is unusual as it is associated with an area of high conductivity at upper to mid crustal levels. As MT site coverage in this region is sparse, however, this observation may be due to a local anomaly. We also present maps showing major anomalies observed in gravimetric and magnetic data and results using these data as constraints for the 3D electrical conductivity inversions.