Is the Okavango Delta the terminus of the East African Rift System? Towards a new geodynamic model: Geodetic study and geophysical review

Abstract

The Okavango Graben (OG) has been considered as the terminus of the southwestern branch of the East African Rift System (EARS) since the 1970's based on fault morphology and early seismic and geophysical data. Thus it is assumed to be an incipient rifting zone, analogous to the early stage of mature rifts in the EARS. Recent geodetic data and geophysical studies in the area bring new insights on the local crust and lithosphere, mantle activity and fault activity. In this study, we computed the velocities for three permanent GPS stations surrounding the graben and undertook a review of the new geophysical data available for the area. The northern and southern blocks of the graben show exclusively a low strike-slip displacement rate of about 1 mm/yr, revealing the transtensional nature of this basin. The seismic record of central and southern Africa is revealed to be instrumentally biased for the events recorded before 2004 and the OG may not represent the most seismically active area in Botswana anymore.Moreover, no significant lithosphere and crustal thinning is found in the tectonic structure as well as no strong negative Bouguer anomaly and surface heat flux. Thus the OG does not match the classical model for a rifting zone.We propose a new geodynamic model for the deformation observed west of the EARS based on accommodation of far-field deformation due to the differential extension rates of the EARS and the displacement of the Kalahari craton relative to the Nubian plate.Keywords: Intraplate tectonics, Okavango, Geodesy, East African Rift System. Highlights : -Time series for three GPS stations astride the Okavango graben have been processed.-The Okavango graben showed main strike-slip displacement over the five past years.-There is no significant evidence for rifting in the area.-Deformation in the region can be explained by far-field deformation accommodation.

Landscape responses to intraplate deformation in the Kalahari constrained by sediment provenance and chronology in the Okavango Basin

“…The white polygon marks the Makgadikgadi-Okavango-Zambezi Basin (MOZB) following the definition of Ringrose et al (2008). The extent the of underlying cratons that bound the MOZB basin are after Pastier et al (2017) and Yu et al (2017) where the Kalahari Craton is consists of the agglomerated Kaapvaal and Zimbabwe cratons. Inset: The white area denotes the Kalahari Basin on the African continent.…”

“…The basin is situated between elevated structural arches (Gumbricht, McCarthy, & Merry, 2001) overlying Proterozoic and Mesozoic rift structures (Dixey, 1956; Doucouré & de Wit, 2003; Modisi, Atekwana, Kampunzu, & Ngwisanyi, 2000; Reeves & Hutchins, 1982; Smith, 1984). Recent seismic, tomographic, and magnetotellurics studies suggest that lithospheric stretching is the result of inter‐cratonic differential tectonic strains of the surrounding Archean cratons (i.e., Congo and Kalahari cratons, Figure 1a; Pastier, Dauteuil, Murray‐hudson, & Moreau, 2017; Yu et al., 2017), where the upper mantle is rheologically weak (Moorkamp, Fishwick, Walker, Jones, & Mt, 2019).…”

“…(b) Geomorphology and structure of the MOZB (marked with a white polygon) superimposed on the sub‐Kalahari Group (Cenozoic sediments) topography map (from Haddon, 2005), illustrating the accommodation space within the semi‐endorheic basin. Location of faults are after Kinabo et al (2008) and Pastier et al (2017). Abbreviations of faults are: SSZ‐ Sekaka Shear Zone; Ky‐ Kunyere; Th‐ Thamalakane.…”

The April 2017 Mw6.5 Botswana Earthquake: An Intraplate Event Triggered by Deep Fluids

“…Although southern Africa experiences diffuse, low‐magnitude seismicity (e.g., Pastier et al, ), no active structure has been identified in the epicentral region of the Botswana earthquake. The nearest active faults are 350 km to the north bounding the isolated Okavango tectonic basin, where a M6.7 earthquake occurred in 1952 (Modisi et al, ; Reeves, ).…”

“…The nearest active faults are 350 km to the north bounding the isolated Okavango tectonic basin, where a M6.7 earthquake occurred in 1952 (Modisi et al, ; Reeves, ). This basin has been interpreted by some as an incipient rift at the western end of a belt of weak seismicity that extends from the Okavango basin through the Kariba graben to the east and connects to the Rukwa region of the East African Rift (Modisi et al, ; Scholz et al, ), although that interpretation has recently been challenged (Pastier et al, ). The 2017 Bostwana earthquake is located well away from these structures.…”