Electrical resistivity image of the South Atlantic continental margin derived from onshore and offshore magnetotelluric data

Abstract

We present a deep electrical resistivity image from the passive continental margin in Namibia. The approximately 700 km long magnetotelluric profile follows the Walvis Ridge offshore, continues onshore across the Kaoko Mobile Belt and reaches onto the Congo Craton. Two‐dimensional inversion reveals moderately resistive material offshore, atypically low for oceanic lithosphere, reaching depths of 15–20 km. Such moderate resistivities are consistent with seismic P wave velocity models, which suggest up to 35 km thick crust. The Neoproterozoic rocks of the Kaoko Mobile Belt are resistive, but NNW‐striking major shear‐zones are imaged as subvertical, conductive structures in the upper and middle crust. Since the geophysical imprint of the shear zones is intact, opening of the South Atlantic in the Cretaceous did not alter the middle crust. The transition into the cratonic region coincides with a deepening of the high‐resistive material to depths of more than 60 km.

Comparison of Different Coupling Methods for Joint Inversion of Geophysical Data: A Case Study for the Namibian Continental Margin

“…Impedance calculation and dimensionality analysis were carried out with the algorithms described by Chave and Thomson (2004), Egbert (1997), andMartí et al (2009). The onshore stations' processing was conducted by Kapinos et al (2016) with procedures described in Becken and Burkhardt (2004), Ritter et al (1998), andWeckmann et al (2005). Due to limited survey time, the stations of profile P100 include data only up to 10,000 s and the eight land stations up to 1,000 s. However, the MT stations along profile P3 cover periods up to 50,000 s. We interpolated all data (impedance matrix elements and errors) for 16 periods ranging from ∼30 to 5⋅10 4 s and replaced missing data, particularly large period data for land and profile P100 stations, by dummy values with large errors.…”

“…The stations were deployed along two orthogonal seismic profiles to image the structure along (profile P100) and across (profile P3) Walvis Ridge (Figure 1). Also, we have included data from eight land‐based MT stations, deployed by GFZ Potsdam in October and November 2011 (see Kapinos et al., 2016) to expand the investigated area onshore and account for the electromagnetic coast effect, which arises due to the big resistivity contrast of seawater adjacent to continental crust (Ferguson et al., 1990; Worzewski et al., 2012).…”

“…The gravity modeling study by Maystrenko et al (2013) imaged high density lower crustal intrusions. Inversion of magnetotelluric (MT) data by Kapinos et al (2016) and Jegen et al (2016) shows high resistivities in the middle and deep crust indicative of magmatic processes. While all of these models show similarity concerning the lateral extent of imaged magmatic features, little coherence exists concerning the vertical extent and depth of the geological targets (Jegen et al, 2016).…”

Comparison of Different Coupling Methods for Joint Inversion of Geophysical data: A case study for the Namibian Continental Margin

“…Impedance calculation and dimensionality analysis were carried out with the algorithms described by Chave & Thomson (2004);Egbert (1997), andMartí et al (2009). The onshore stations' processing was conducted by Kapinos et al (2016) with procedures described in Becken & Burkhardt (2004); Ritter et al (1998); Weckmann et al (2005). Due to limited survey time, the stations of profile 100 include data only up to 10000 s and the eight land stations up to 1000 s. However, the MT stations along profile 3 cover periods up to 50000 s. We interpolated all data for 16 periods ranging from ~30 to 5 104 s ⋅ and replace missing data, particularly large period data for land-and profile 100 stations, by dummy values with large errors.…”

Conductivity structure of the lithosphere‐asthenosphere boundary beneath the eastern North American margin

“…Geophysical studies frequently use the magnetotelluric (MT) method to image the conductivity structure of the lithosphere and the upper mantle, both in continental [e.g., Singh et al, 1995;Wannamaker et al, 1996;Fullea et al, 2011;Vozar et al, 2014;Yang et al, 2015] and oceanic regions [e.g., Cox, 1981;Lizarralde et al, 1995;Evans et al, 1999;Baba et al, 2013;Key et al, 2013;Sarafian et al, 2015]. Cold oceanic lithosphere typically exhibits high resistivity values of 10 3 210 5 Xm [e.g., Cox et al, 1986;Evans et al, 2005;Kapinos et al, 2016], whereas the resistivity of the warm upper mantle at greater depths is reduced to 10-100 Xm [e.g., Baba et al, 2006;Naif et al, 2013;Key et al, 2013;Sarafian et al, 2015].…”