Seismological Investigations of Crustal and Mantle Structures Beneath the Incipient Okavango Rift

Abstract:

Rifting plays a significant role in the evolution of sedimentary basins. However, our current understandings on rifting mechanisms are mostly based on studies of mature rifts. Here we report results from the first teleseismic investigations of the incipient Okavango rift zone (ORZ), which is located at the southwestern terminal of the East African Rift System in northern Botswana. Data used in the study were recorded by the 17 broadband seismic stations deployed along a NW-SE profile traversing the ORZ with a recording duration of 2 years starting in the summer of 2012. Receiver function and shear wave splitting techniques have been employed to explore upper mantle thermal anomalies and anisotropy. The resulting dominantly absolute plate motion-parallel fast polarization orientations and normal mantle transition zone thickness ruled out the possible existence of one or more mantle plumes in the upper mantle or mantle transition zone beneath the ORZ. The Moho beneath the Okavango rift zone is uplifted by 4-5 km and is symmetric with regard to the rift axis, favoring a pure shear model of early-stage continental extension. The observations favor a passive model for rift initiation in which rifts develop inside ancient orogenic zones as the result of relative movements between Archean cratonic blocks.