8This study represents the first shear-wave splitting investigation of the Oka-9 vango rift zone (ORZ), an incipient continental rift belonging to the East 10 African rift system in northern Botswana. Analysis of broadband seismic 11 data recorded along a 750 km long profile of 22 stations traversing the ORZ 12 and adjacent Congo and Kalahari cratons and several Precambrian orogenic 13 zones reveals dominantly NE-SW fast orientations, which are parallel to both 14 the absolute plate motion direction (based on the NNR-NUVEL-1A model) 15 and the trend of most tectonic boundaries, including that of the ORZ. Spatial 16 coherence analysis of the splitting parameters and correspondence between 17 the observed fast orientations and the trend of tectonic features indicate that 18 the main source of observed anisotropy is most likely in the upper astheno-19 sphere, probably due to simple shear associated with the relative movement 20 of the lithosphere against the asthenosphere. The presence of consistently 21 rift-parallel fast orientations and normal splitting times in the ORZ and 22 most parts of southern Africa implies that neither an upper mantle plume 23 nor small-scale convection is the dominant source for rift initiation and de-velopment. The first SWS measurements in the vicinity of the ORZ favor a 25 model in which continental rifting develops in response to intra-plate relative 26 movement of continental blocks along zones of weakness produced by ancient 27 tectonic events.28

Rifting incorporates the fundamental processes concerning the breakup of continental lithosphere and plays a significant role in the formation and evolution of sedimentary basins. In order to decipher the characteristics of rifting at its earliest stage, we conduct the first teleseismic crustal study of one of the world's youngest continental rifts, the Okavango Rift Zone (ORZ), where the magma has not yet breached the surface. Results from receiver function stacking and gravity modeling indicate that the crust/mantle boundary beneath the ORZ is uplifted by 4–5 km, and the initiation of the ORZ is closely related to lithospheric stretching. Possible decompression melting of the subcrustal lithosphere occurs beneath the ORZ, as evidenced by a relatively low upper mantle density based on the gravity modeling.

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.

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