MANTLE TRANSITION ZONE DISCONTINUITIES BENEATH THE INCIPIENT OKAVANGO RIFT

The Okavango Rift Zone (ORZ) of northern Botswana is an incipient continental rift situated within the Neoproterozoic Damara belt between the Congo Craton to the northwest and the Kalahari Craton to the southeast. Mantle structure and thermal status beneath the ORZ were poorly known, mostly due to a complete paucity of broadband seismic stations in the area. As a component of an interdisciplinary project funded by the United States National Science Foundation, a broad band seismic array was deployed over a 2-year period between mid-2012 and mid-2014, along a profile of 750 km long. Using P-to-S receiver functions (RFs) recorded by the stations, the 410 and 660 km discontinuities (d410 and d660) bordering the mantle transition zone (MTZ) beneath the MTZ are imaged for the first time. When a standard Earth model is used for the stacking of RFs, the apparent depths of both discontinuities beneath the Kalahari Craton are about 15 km shallower than those beneath the Congo Craton. Using teleseismic P- and S-wave travel-time residuals obtained at the stations and lithospheric thickness estimated by previous studies, we conclude that the apparent shallowing is the result of a 100-150 km difference in the thickness of the lithosphere between the two cratons. Relative to the adjacent tectonically stable areas, no significant anomalies in the depth of the MTZ discontinuities or in teleseismic P- and S-wave travel-time residuals are found beneath the ORZ. Theses observations imply an absence of significant thermal anomalies in the MTZ and in the upper mantle beneath the incipient rift, ruling out the role of mantle plumes in rift initiation. We propose that the initiation and development of the ORZ were the consequences of relative movements between the South African block and the rest of the African plate along a zone of lithospheric weakness between the Congo and Kalahari cratons. An area of thinner-than-normal MTZ is found at the SW corner of the study area. This anomaly, if confirmed by future studies, could suggest significant transferring of heat from the lower to the upper mantle.