Rift kinematics during the incipient stages of continental extension: Evidence from the nascent Okavango rift basin, northwest Botswana

High-resolution aeromagnetic data from the nascent Okavango rift in northwe st Botswana provide an unprecedented view of rift kinematics during the inc ipient stages of continental extension. Crosscutting relationships between west-northwest-trending 180 Ma Karoo dikes and reactivated northeast-trendi ng Proterozoic basement faults are used to document the kinematics of Cenoz oic faulting during the initial stages of rifting. Depth estimates to the t op of the dikes using three-dimensional Euler deconvolution solutions have produced the following interpretations. (1) The Okavango rift is a half gra ben with a downthrow of similar to 200-300 m. (2) The width of the Okavango rift (100 +/- 20 km) is similar to that of more mature continental rifts s uch as the Tanganyika and Baikal rifts. This suggests that the width of con tinental rifts is acquired early in their evolution and reflects neither th e age and maturity of the rift basin, nor the amount of extension. It is su ggested that the cumulative do downthrow (sediment infill included) and sub sidence may be a better indicator of the relative maturity of rift basins. (3) Preexisting basement faults exert a major control during rifting, and r eactivation processes do not occur synchronously along the entire length of preexisting faults. (4) The Okavango rift is defined by normal faults; the re is no evidence of major strike-slip faults, thus excluding a pull-apart tectonic model for this nascent continental rift stage. (5) The preexisting Sekaka shear zone terminates the Okavango rift to the south, suggesting th at such shear zones represent major barriers during longitudinal propagatio n of rifts. This probably explains why such shear zones commonly evolve int o accommodation or transfer zones during further evolution of continental r ifts.