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.

The Afar Depression is an ideal locale for the investigation of crustal processes involved in the transition from continental rifting to oceanic spreading. To provide relatively high resolution images of the crust beneath the Red Sea rift (RSR) represented by the Tendaho graben in the Afar Depression, we deployed an array of 18 broadband seismic stations in 2010 and 2011. Stacking of about 2300 receiver functions from the 18 and several nearby stations along the ∼200 km long array reveals an average crustal thickness of 22 ± 4 km, ranging from ∼17 km near the RSR axis to 30 km within the overlap zone between the Red Sea and Gulf of Aden rifts. The resulting anomalously high V p ∕V s ratios decrease from 2.40 in the southwest to 1.85 within the overlap zone. We utilize theoretical V p and melt fraction relationships to obtain an overall highly reduced average crustal V p of ∼5.1 km/s. The melt percentage is about 10% beneath the RSR while the overlap zone contains minor quantities of partial melt. The observed high V p ∕V s values beneath most of the study area indicate widespread partial melting beneath the southwest half of the profile, probably as a result of gradual eastward migration of the RSR axis. Our results also suggest that the current extensional strain in the lower crust beneath the region is diffuse, while the strain field in the upper crust is localized along narrow volcanic segments. These disparate styles of deformation imply a high degree of decoupling between the upper and lower crust.

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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To investigate the mechanisms for the initiation and early‐stage evolution of the nonvolcanic southernmost segments of the East African Rift System (EARS), we installed and operated 35 broadband seismic stations across the Malawi and Luangwa rift zones over a 2 year period from mid‐2012 to mid‐2014. Stacking of over 1900 high‐quality receiver functions provides the first regional‐scale image of the 410 and 660 km seismic discontinuities bounding the mantle transition zone (MTZ) within the vicinity of the rift zones. When a 1‐D standard Earth model is used for time‐depth conversion, a normal MTZ thickness of 250 km is found beneath most of the study area. In addition, the apparent depths of both discontinuities are shallower than normal with a maximum apparent uplift of 20 km, suggesting widespread upper mantle high‐velocity anomalies. These findings suggest that it is unlikely for a low‐velocity province to reside within the upper mantle or MTZ beneath the nonvolcanic southern EARS. They also support the existence of relatively thick and strong lithosphere corresponding to the widest section of the Malawi rift zone, an observation that is consistent with strain localization models and fault polarity and geometry observations. We postulate that the Malawi rift is driven primarily by passive extension within the lithosphere attributed to the divergent rotation of the Rovuma microplate relative to the Nubian plate, and that contributions of thermal upwelling from the lower mantle are insignificant in the initiation and early‐stage development of rift zones in southern Africa.

SKS, SKKS, and PKS splitting parameters measured at 34 seismic stations that we deployed in the vicinity of the Cenozoic Malawi Rift Zone (MRZ) of the East African Rift System demonstrate systematic spatial variations with an average splitting time of 1.0 ± 0.3 s. The overall NE‐SW fast orientations are consistent with absolute plate motion (APM) models of the African Plate constructed under the assumption of no‐net rotation of the global lithosphere and are inconsistent with predicted APM directions from models employing a fixed hot spot reference frame. They also depart considerably from the trend of most of the major tectonic features. These observations, together with the results of anisotropy depth estimation using the spatial coherency of the splitting parameters, suggest a mostly asthenospheric origin of the observed azimuthal anisotropy. The single‐layered anisotropy observed at 30 and two‐layered anisotropy observed at 4 of the 34 stations can be explained by APM‐related simple shear within the rheologically transitional layer between the lithosphere and asthenosphere, as well as by the horizontal deflection of asthenospheric flow along the southern and western edges of a continental block with relatively thick lithosphere revealed by previous seismic tomography and receiver function investigations. This first regional‐scale shear wave splitting investigation of the MRZ suggests the absence of rifting‐related active mantle upwelling or small‐scale mantle convection and supports a passive‐rifting process for the MRZ.

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