GEOPHYSICAL STUDIES OF CONTINENTAL RIFT INITIATION: AN EXAMPLE FROM THE OKAVANGO RIFT ZONE, NW BOTSWANA

We acquired ~ 660 gravity stations along three transects (1200 km) across the Okavango Rift Zone (ORZ) NW Botswana at an average station spacing of ~2km. The profiles extended from the Kaapvaal craton into the Angolan craton and across Proterozoic mobile belts. In addition, we used high resolution aeromagnetic data obtained from the Botswana Geological Survey to investigate rift initiation processes. Studies on the earliest stages of continental rifting, when key processes such as strain localization occur are lacking. Hypotheses of rift initiation have been proposed including: edge-driven mantle convection occurring at the edges of thick cratonic lithosphere, magma assisted rifting; and tectonic stretching by far-field extensional stresses. Our objective was to test these different hypotheses in order to elucidate the mechanism by which the Okavango rift is forming. The preliminary structural interpretation from the aeromagnetic and gravity data show several sets of trending structures, mainly faults, shear zone and folding. The main bounding faults of the rift zone are predominantly 030-060 concomitant with the basement structural grain suggesting the important influence of the basement fabric on the development of the rift. Additionally, the gravity and magnetic anomaly delineate major tectonic boundaries associated with the cratons as well as major Proterozoic tectonic blocks. Preliminary interpretations suggest that the rift is developing within the Proterozoic mobile belt between the Angolan and the Kapvaal and Zimbabwean cratons. Further modeling is required to determine the thickness of the cratons and the mobile belts as well as the presence of any magmatic bodies at depth.

¹Oklahoma State University, Boone Pickens School of Geology, Stillwater, USA, ² Missouri State University, Spring Field, Missouri State, USA, ³University of Botswana, Gaborone, Botswana.