Mantle structure beneath the incipient Okavango rift zone in southern Africa

Abstract

Numerous investigations of the mature segments of the East African rift system (EARS) have significantly improved our understanding of the structure and processes associated with well-developed continental rifts. In contrast, knowledge of rifting processes at their early stage is still significantly limited. Here we present results from a teleseismic P-wave tomography investigation of the incipient Okavango rift zone (ORZ), which is located at the southwestern terminus of the EARS. P-wave relative travel-time residuals recorded by 17 recently deployed portable seismic stations were manually picked and inverted for three-dimensional upper-mantle and mantle transition-zone tomographic images beneath the ORZ and its adjacent areas. High-velocity anomalies probably representing cratonic lithosphere are visible under the Congo and Kalahari cratons, extending to depths of ~250-350 km. The tectonic boundary of the Congo craton is observed along the western edge of the ORZ. A localized low-velocity anomaly of about-1% in magnitude is revealed in the upper astheno sphere beneath the ORZ, which is interpreted to represent decompression melting induced by lithospheric thinning. The results support the notion that the initiation and early-stage development of the ORZ are mostly due to lithospheric stretching resulted from the relative motion between the Archean Congo and Kalahari cratons along preexisting ancient orogenic zones.

Upper Mantle P and S Wave Velocity Structure of the Kalahari Craton and Surrounding Proterozoic Terranes, Southern Africa

“…The velocity variations seen in our model beneath regions spanning the SASE network are almost identical to the models published by Fouch et al () and Youssof et al (). In northwestern Botswana, our model shows upper mantle structure beneath the Okavango rift similar to the Yu et al () model, and in Zambia, upper mantle structure in our model is almost indistinguishable from structure in the Mulibo and Nyblade () model.…”

“…However, Yu et al () attributed the region of lower velocities under the ORZ to a rift‐related thermal anomaly and not the edge of the cratonic lithosphere. It is possible that the lower velocities beneath the Damara‐Ghanzi‐Chobe Belt could reflect thinner off‐craton lithosphere as well as a thermal perturbation to the upper mantle related to rifting.…”

“…A large number of seismic studies have investigated crust and mantle structure in southern Africa over the past few decades using body wave tomography (e.g., Fouch et al, 2004;Youssof et al, 2015;Yu et al, 2016), surface wave tomography (e.g., Chevrot & Zhao, 2007;Priestley et al, 2008;Fishwick, 2010;Adams & Nyblade, 2011), receiver functions (e.g., Fadel et al, 2018;Hansen et al, 2009;Kgaswane et al, 2009;Nguuri et al, 2001), and waveform modeling (e.g., Brandt et al, 2011;Moorkamp et al, 2019;Zhao et al, 1999). Four relevant body wave tomography studies are briefly summarized.…”

Lithospheric Deformation and Asthenospheric Flow Associated With the Isabella Anomaly in Southern California

“…Rift‐parallel azimuthal anisotropy induced by oriented magmatic cracks in the mantle is also observed beneath other continental rifts such as the Kenya rift (Gao et al, ). Thus, the Salton Trough seems to follow a passive rifting model that rifting is activated by the oblique strike‐slip plate motions and the decompressed melt pocket is directed to align horizontally instead of upwelling, which is similar to the incipient Okavango rift in southern Africa where the APM plays the primary role (Yu et al, ).…”

Crustal Structure and Dynamics of Botswana

“…Bounding the Damara Belt toward the southeast is the Ghanzi‐Chobe Zone that consists of tightly folded metasediment sequences (Wright & Hall, ). At the northern tip, cutting across the Damara Belt and Ghanzi‐Chobe Zone, is the incipient Okavango Rift, often considered to be the southwestern terminus of the East African Rift System (e.g., Leseane et al, ; Yu et al, , and references therein).…”