THE ROLE OF BASEMENT FABRICS ON THE DEVELOPMENT OF CONTINENTAL RIFTS

The incipent stages of continental rifting were investigated through a high-resolution aeromagnetic survey of the Okavango Rift Zone (ORZ), an embryonic continental rift in northwest Botswana that is linked to the East African Rift System. This data set reveals the presence of three main fracture orientations: 1) northeast-southwest (045-055), 2) northwest-southeast (310-315), and 3) westnorthwest-eastsoutheast (280-290). The 045-055 structures occur both within the rift zone and throughout the surrounding basement terrane. They also form the main bounding fault system of this incipient rift. The main subsidence of the crust, as interpreted from differences in the depths to the tops of mafic dikes associated with the Karoo Dike swarm, occurs along fault systems in this direction. The 310-315 structures are near orthogonal to the 045-055 lineaments. These lineaments occur only within the confines of the graben and are more prevalent in the southeastern edge of the rift zone. The geometry of these lineaments, at least within the plane of the map, is consistent with an origin as Riedel shears (R’). The 280-290 structures are common in the southern half of the rift zone. All lineaments within the rift zone terminate against a major shear zone oriented in this direction. The 045-055 orientations of the fold axes of the Proterozoic Ghanzi-Chobe fold and thrust belt (i.e, the basement cut by the rift) mirrors that of the main bounding faults of the rift basin.

The results of this study show that major bounding faults parallel the pre-existing basement fabric indicating such structures play an important role in localizing the development of faults within the stress regime present during the initiation of continental rifting. Furthermore, the spatial distribution of lineaments interpreted as rift-related faults corresponds to those predicted as Riedel shears that form in association with strike-slip faulting. We suggest that the Okavango rift basin underwent a clockwise rotation of extension direction that has produced a component of strike-slip faulting along the bounding normal faults of the rift basin as described by Strecker et al, 1990.