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.

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

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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Rifting of stable continents is a key element of plate tectonic cycles. In spite of numerous studies, the mechanism responsible for the initiation and evolution of rift valleys such as the East African Rift System (EARS) is still poorly understood, partly because most previous investigations focused on rift segments that were in the mature stage. Geodynamic modeling [Huismans et al., 2001] suggests that upwelling of the asthenosphere ubiquitously observed beneath mature rifts can either originate from thermal or dynamic anomalies in the deep mantle (active rifting) or be induced by thinning of the lithosphere from far‐field stresses (passive rifting) [Sengor and Burke, 1978].

The Mesoproterozoic Midcontinent rift (MCR) in the central US is an approximately 2000 km long, 100 km wide structure from Kansas to Michigan. During the 20-40 million years of rifting, a thick (up to 20 km) layer of basaltic lava was deposited in the rift valleys. Quantifying the effects of the rifting and associated volcanic eruptions on the structure and composition of the crust and mantle beneath the MCR is important for the understanding of the evolution of continental lithosphere. In this study we measure the crustal thickness (H), and the sharpness of the Moho (R) at about 24 portable and permanent stations in Iowa, Kansas, and South Dakota by stacking Pto-S converted waves (PmS) and their multiples (PPmS and PSmS). Under the assumption that the crustal mean velocity in the study area is the same as the IASP91 earth model, we find a significantly thickened crust beneath the MCR of about 53 km. The crustal V p /V s ratios increases from about 1.80 off rift to as large as 1.95 within the rift, which corresponds to an increase of Poisson's ratio from 0.28 to 0.32, suggesting a more mafic crust beneath the MCR. The R measurements are spatially variable and are relatively small in the vicinity of the MCR, indicating the disturbance of the original sharp Moho by the rifting and magmatic intrusion and volcanic eruption.

Soil magnetic properties measurements are relatively fast and inexpensive but have been proved to be sufficient for preliminary investigations in diverse socio-developmental issues. This paper presents results of a reconnaissance study of soil colour and magnetic susceptibility (χ) in eastern Botswana, where ~80% of the population resides. The work is a first step to creating a database of rock and soil magnetic properties and to document spatial variations in magnetic properties in the country. These measurements are important as constraints for interpretation of available aeromagnetic data and can also be exploited for environmental soil research (pollution) and land-use planning (agriculture). The soils sampled include derivatives of varying types and provenance such as Archean gneissic granitoids, metamorphosed rocks (granulites), volcano-sedimentary assemblages, Karoo basalts, and alluvial sediments. A soil colour chart was used since soil colours and magnetic properties are diagnostic of its parent rock sources and weathering profiles. Soil magnetic susceptibilities were measured at both low frequency (0.46 MHz, χlf) and high frequency (4.6 MHz, χhf), thus allowing calculation of frequency-dependent susceptibility (χfd, χfd%) for detecting ultra-fine ferromagnetic minerals.It was found that soils with Hues ranging from 7.5YR to 10YR have appreciable amount of magnetic materials and soils with Hues of 2.5YR are generally nonmagnetic. The results of soil magnetic susceptibility profiles show spatial variation closely related to the variation in basement rocks, which provides excellent evidence that the magnetic susceptibility variation reflects basement rocks or bedrock composition (soil parent material). In relation to the Botswana physiographic units, soils from the hardveld (Precambrian) show the highest susceptibilities, followed by those from the sandveld, with the lowest values being from the alluvial. The frequency dependent magnetic susceptibilities indicate the presence of ultra-fine super-paramagnetic minerals such as magnetite/maghemite. It is suggested that a systematic and continuous programme of rock and soil magnetic measurements would benefit various socio-economic and development priority sectors of Botswana. This also applies to many developing countries in Africa where soil physics and measurement of soil susceptibility in particular, is generally still at an embryonic stage.