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

摘要

High-resolution aeromagnetic data from the nascent Okavango rift in northwest Botswana provide an unprecedented view of rift kinematics during the incipient stages of continental extension. Crosscutting relationships between west-northwest-trending 180 Ma Karoo dikes and reactivated northwest-trending Proterozoic basement faults are used to document the kinematics of Cenozoic faulting during the initial stages of rifting. Depth estimates to the top of the dikes using three-dimensional Euler deconvolution solutions have produced the following interpretations. (1) The Okavango rift is a half graben with a downthrow of ～200-300 m. (2) The width of the Okavango rift (100 ± 20 km) is similar to that of more mature continental rifts such as the Tanganyika and Baikal rifts. This suggests that the width of continental rifts is acquired early in their evolution and reflects neither the age and maturity of the rift basin, nor the and subsidence may be a better indicator of the relative maturity of rift basins. (3) Preexisting basement faults exert a major control during rifling, and reactivation processes do not occur synchronously along the entire length of preexisting faults. (4) The Okavango rift is defined by normal faults; there is no evidence of major strike-slip faults, thus excluding a pull-apart tectonic model for this this nascent continental rift stage. (5) The preexisting Sekaka shear zone terminates the Okavango rift to the south, suggesting that such shear zones represent major barriers during longitudinal propagation of rifts. This probably explains why such shear zones commonly evolve into accommodation or transfer zones during further evolution of continental rifts.