1-D layered seismic structure of the crust beneath Botswana

Abstract:

In an attempt to improve knowledge of crustal structure beneath Botswana, 1-D shear wave velocity models for layered crust are estimated from genetic algorithm inversion (Sambridge and Drijkoningen, 1992; Shibutani et al, 1996) of radial receiver functions from waveforms recorded by 35 stations in Botswana. The preliminary stacked receiver functions reveal converted phases from the Moho and mid-crustal interfaces, with some backazimuthal variation that may be caused by local heterogeneities or anisotropic and dipping structures. “Delayed” and “double-peaked” direct P arrivals observed at stations in mobile belts and sedimentary basins are caused by high Vs contrasts in the shallow structure. Resulting 1-D models reveal low velocity layers in the thinner crust (33.7 ± 2.4 km) of Okavango rift zone that may be caused by fluid presence in weak zones as suggested by previous studies. Maximum crustal thinning is coincident with the location of the rift zone’s active fault system. While the thin crust seems to extend into central Botswana beneath Nosop Basin, thicker crust (~50km) is observed beneath SW end of Ghanzi-Chobe belt and Western border of Zimbabwe craton. A NW-SE trending region of higher Vs in the upper crust cuts across tectonic regions from northern to central Botswana. Its coincidence with a reported higher gravity anomaly suggests the presence of mafic intrusions in the upper crust that may be related to the intrusions of the nearby Okavango dyke swarm. Similarity of velocity models in Damara belt and Congo craton may suggest an extension of the Damara belt beyond its currently presumed boundary with the Congo craton. Backazimuthal variation of receiver functions as well as "double peaked" and "delayed" direct P phase warrant further investigation of shallow structure beneath mobile belts and sedimentary basins, as well as anisotropic and dipping structures in the crust.