Crustal and Upper Mantle Imaging of Botswana Using Magnetotelluric Method

Abstract

We used magnetotelluric data from 352 sites in Botswana to derive a country-wide electrical conductivity model of the crust and upper mantle structure. A robust methodological scheme and 3D inversion were used to derive a 3D electrical conductivity model with unprecedented spatial coverage. The model results show interesting features, including the major cratonic blocks and the mobile belts in Botswana. A distinctive resistive structure was imaged in southwest Botswana, which suggests the existence of the Maltahohe microcraton as a separate cratonic unit as proposed by other studies. Furthermore, the model gives new insight into the extension of the East African Rift System to Botswana and the incipient rifting in the Okavango Rift Zone. In northern Botswana, the electrical conductivity model shows a highly conductive structure beneath the Okavango Rift Zone, which connects with a deeper conductive structure that we attribute to the East African Rift System due to its vicinity to Lake Kariba, the last surface expression of the rift system. We suggest that ascending fluids or melt from the East African Rift System causes the weakening of the lithosphere and plays a significant role in the incipient continental rifting in the Okavango Rift Zone.

Mantle Structure Beneath the Damara Belt in South‐Central Africa Imaged Using Adaptively Parameterized P‐Wave Tomography

“…In particular, there have been disagreements regarding the upper mantle structure beneath the Neoproterozoic Damara Belt, a tectonic region that is situated between the Congo and Kalahari cratons (Figure 1). Some prior studies advocate for a seismically slow upper mantle beneath this area (e.g., Akinremi et al, 2022;Begg et al, 2009;Ortiz et al, 2019;Yu, Gao, et al, 2015, 2017, which could possibly indicate an extension of the African LLVP into the upper mantle beneath southcentral Africa. Conversely, others have argued that there is no evidence for anomalously slow mantle beneath the Damara Belt (e.g., Afonso et al, 2022;Celli et al, 2020;Pandey et al, 2022;Priestley et al, 2008;White-Gaynor et al, 2020; rather, the lithosphere in this region may just be thin compared to the thick lithosphere beneath the surrounding cratons (Afonso et al, 2022;White-Gaynor et al, 2020.…”

“…Anomalous structure beneath the Damara region has also been suggested by studies employing magnetotelluric data. For example, Akinremi et al (2022) imaged highly conductive materials beneath the ORZ and attributed them to ascending fluids or melt associated with continental rifting. If anomalously slow upper mantle structure is present beneath the Damara Belt and the ORZ, it could indicate that this region is influenced by the African LLVP, similar to what is thought to be occurring within the EARS.…”

“…Further, the shallow portion of the LLVP may significantly influence the complex continental rift pattern seen throughout eastern Africa (Bastow et al, 2008;French & Romanowicz, 2015;Hansen et al, 2012); however, such rifting activity is not observed throughout most of the Damara Belt. The nature of rifting occurring in the ORZ is still a matter of debate (e.g., Pastier et al, 2017), but if the rifting is associated with the southwestern termination of the EARS, as suggested by some prior studies (Akinremi et al, 2022;Begg et al, 2009;Ortiz et al, 2019;Yu, Gao, et al, 2015, 2017, then it must be influenced by extensional processes driven by something other than the LLVP (e.g., Khoza et al, 2013).…”