New insights into the tectonics and geodynamics of Botswana from intracrustal structures revealed by receiver function analysis

Abstract

SUMMARY Recent improvements in seismic coverage allow for an increasingly detailed knowledge of the crustal structure beneath Botswana to be obtained. For the first time, shear wave velocity–depth models throughout Botswana are obtained from genetic algorithm inversion of receiver functions. In this approach, averaging of important structural variations is minimized by separating receiver function waveforms into backazimuth and slowness bins. Division of crustal layers into sediment, upper, middle and lower crust reveals important features that enhance our knowledge of tectonics and geodynamics of Botswana. In the west, beneath the Ghanzi-Chobe belt, as well as at the border region of the Zimbabwe craton with Magondi belt, the crust has been thickened through a thickening of the lower crust, most likely by collisional tectonics during the Damara Orogeny. A significant difference in structure is observed between the western and central-to-eastern Kaapvaal craton. Western Kaapvaal craton has a thin mafic lower crust (<10 km) that has survived the extensive melting of the Ventersdorp tectonomagmatic event at 2.7 Ga, contrary to previous reports of a lack of a mafic lower crust. Meanwhile in central-to-eastern Kaapvaal craton, a lower average ${{{}{V}}}_{{}{s}}$ (∼3.3 km s−1) in the upper crust suggests the presence of a suture zone that separates the two blocks of the Kaapvaal craton of different ages and crustal thicknesses. A relatively low ${{{}{V}}}_{{}{s}}$ in the middle crust from the Okavango rift zone to central and SE Botswana suggests reactivation of Palaeoproterozoic shear zones along a thin and weak crust by intraplate relative motion has enabled fluid infiltration from the mantle which causes seismicity in the region. Furthermore, a NW–SE trending region of high Vs in crustal layers is likely related to Karoo flood magmatism that formed the NW–SE trending Okavango dyke swarm. There is also an indication that a dedicated study is required to further investigate shallow converted phases from mobile belts and sedimentary basins in Botswana, however, the results from this study can provide a good starting point for such a study.