First amphibian magnetotelluric experiment at the passive continental margin in northern Namibia

An amphibian magnetotelluric (MT) study across the passive continental margin of northern Namibia was conducted in December/January 2010/2011 and October/November 2011 to image the subsurface electrical conductivity structure. The MT experiment is part of the interdisciplinary SAMPLE project (South Atlantic Margin Processes and Links with onshore Evolution) which focusses on imaging and understanding processes related to rifting and the breakup history of the supercontinent Gondwana, in particular the opening of the South Atlantic and the post breakup evolution of the continental passive margins of Africa and South America. The onshore MT data were acquired in the Kaoko Mobile Belt at 167 sites in a ~140 km wide and ~260 km long EW extending corridor, from the Atlantic Ocean onto the Congo Craton. The Kaoko Mobile Belt is a transpressional strike slip orogen with NNW striking sinistral shear zones, folds and thrusts, which was formed during the Pan-African orogeny and the amalgamation of West Gondwana. This onshore network is extended offshore with MT measurements along 2 transects parallel and perpendicular to the Walvis Ridge - an approximately 3400km long seamount volcanic chain, trending NE-SW, from Africa to the Middle Atlantic Ridge, thought to be formed by the volcanic activity of the Tristan da Cunha Plume since the early Cretaceous. The onshore impedances and vertical magnetic transfer functions are generally of excellent quality but indicate significant three-dimensional structures in the crust and upper mantle, particularly in the Western Kaoko Zone, in the vicinity of the prominent shear zones. 2-D inversion of a sub-section of the entire data set, where two-dimensional modeling is consistent with the MT data revealed spatial correlations of a resistive zone and the Archean Congo Craton as well as of conductive structures and surface expressions of prominent faults.