Incipient Rifting of the Kalahari Craton: Reactivation of Mesozoic Dormant Rifts in Southern Africa's Cratonic Interior

Abstract

Continental rifts preferentially initiate in orogenic belts and avoid cratons as old, cold cratonic lithosphere resists deformation. Recent earthquakes in eastern Botswana unveil an active incipient rift zone within the Kalahari Craton that hosts ∼80‐km long, >100‐m high fault scarps of the Khurutshe Graben. We investigate the rift structure, stratigraphy, and long‐term evolution to understand the rift‐timing and initiation mechanisms. We utilize 2‐D electrical resistivity imaging, borehole logs, and observations in excavation trenches across the Khurutshe Fault (KF) to delineate fault structure and relationships with subsurface stratigraphy. The resistivity tomography reveals three geoelectrical layers: a shallow ∼10‐m thick resistive unit representing Holocene Kalahari Formation (EL1) which thickens toward and terminates on the KF, an underlying ∼80‐m thick highly‐discontinuous moderate‐to‐low resistivity Jurassic (“Karoo”) unit (basalts and siliciclastics; EL2) which also thicken toward the KF, and basal highly‐resistive Precambrian basement (EL3). Borehole isopach maps confirm a ubiquitous pattern of hanging wall stratigraphic thickening toward the KF. Fault trenching reveals highly cemented Karoo sandstones hosting slickensided calcite‐altered slip surfaces. These observations indicate: (a) normal faulting with syn‐tectonic deposition of Jurassic sequences evidence Karoo rifting in southeast Botswana, and (b) initial phase of rift faulting followed by magmatism, and rift dormancy. Absence of magmatism during “Karoo” rift inception and the development of the rift zone near an intra‐cratonic suture may reflect early structural inheritance. The rift exploited a mechanical weakness in southern Africa's cratonic lithosphere, and is now reactivated at the tip zone of the Southwestern Rift of the Cenozoic African Rift System.