Paper No. 3
Presentation Time: 9:30 AM
THE ROLE OF TECTONIC INHERITANCE IN THE GEOMETRY AND LOCATION OF CONTINENTAL RIFTS – AN EXAMPLE FROM THE OKAVANGO RIFT ZONE, BOTSWANA
The role of tectonic inheritance in the incipient stages of continental rifting is being investigated using remote sensing studies integrated with field investigations to characterize older basement structures and younger extensional faulting associated with the nascent Okavango Rift Zone (ORZ), Botswana; the youngest branch of the East African Rift system. The ORZ is located between the Congo and Kalahari cratons within the NE-trending Precambrian Damara and Ghanzi-Chobe orogenic belts. It forms a half-graben filled by fluvio-deltaic sediments of the Okavango River. High-resolution satellite imagery (Shuttle Radar Topography Mission (SRTM), and GeoEye) were used to construct geologic maps of subtle basement fabrics (e.g., fold limbs and noses) as well as faults truncating these fabrics. Where exposed, field mapping confirmed remote sensing interpretations and provided important structural orientation data. The regional basement fabric is well defined by limbs of tightly folded metasedimentary units (057/61), and foliation in meta-rhyolites (060/75). Rift-related normal faults are sub-parallel to the basement fabric (054/75). Calcrete was commonly associated with observed faults suggesting failure was enhanced by increased pore fluid pressure. A set of remotely sensed NW-trending faults (azimuth of 340-350) form small linear valleys. Alignment of drainage systems and incipient graben formation suggest the ORZ may be propagating to the W-SW into Namibia.
Topographic profiles of fault scarps (from SRTM data) were used to characterize escarpment evolution and length-displacement of the main fault traces. The amount of displacement and scarp degradation varies between fault traces, as well as along the same fault trace. Scarps in the NE corner typically preserve greater displacement and less degradation. Length-displacement ratios are typically high, and may reflect the influence of pre-existing basement structures as an easy path for lateral fault propagation.
Combined remote sensing and field studies show that faults associated with the formation of the ORZ inherited their trends by exploiting zones of weakness defined by preexisting basement fabrics. This suggests that the localization of continent rifts at the very earliest stages is strongly influenced by the preexisting crustal fabric.