Geophysical Characterization of Faults of the Okavango Rift Zone, Northwest Botswana, Africa

Abstract

To examine the geophysical characteristics of the faults associated with the OkavangoRift Zone (ORZ) in northwest Botswana, electrical resistivity tomography (ERT), magnetotellurics (MT), total field magnetic, and gravity surveys were conducted along selected faults across the rift. MT results show that the Tsau and Lecha faults have propagated into the sedimentary cover and are interpreted to be active conduits for fluid flow from the Okavango Delta despite their lack of surface expression. ERT and MT data show that tectonic activity along the Kunyere Fault has significantly waned and strain is being accommodated by the tectonically active Thamalakane Fault and Mababe Fault. Impermeable fault cores and an abundance of minor faults, fractures, and splays within the damage zones characterize the faults within the ORZ as both barriers and conduits to fluid flow. Ground and aeromagnetic data display some of the faults as magnetic highs while others are depicted as magnetic lows in the basement. The differences in magnetic character may be attributed to the 1.) commingling of fluids and basement rocks of contrasting redox potentials along fault planes or 2.) extreme variation of magnetic susceptibilities of the rocks juxtaposed on either side of the faults. Fluids may also play a role in increasing pore pressure through cracks associated with interconnecting faults and fractures, causing the ORZ to have a lack of severe earthquakes and allowing easier slip to occur between fault planes which may enhance lengthening the widening of the rift.