THE ROLE OF PREEXISTING BASEMENT STRUCTURES IN THE INITIATION AND PROPAGATION OF RIFTS: INSIGHTS FROM THE OKAVANGO RIFT ZONE, BOTSWANA

Results of remote sensing (GeoEye, Shuttle Radar Topography Mission (SRTM), and Landsat Thematic mapper (TM)) and field investigations of the NE-trending Okavango Rift Zone (ORZ), Botswana indicate that preexisting basement structures influence the initial development and propagation of continental rifts. The ORZ is the youngest rift of the East African Rift System. It occurs at the tip of the southwestern branch in an inter-cratonic zone, underlain by the NE-trending Precambrian Damara and Ghanzi-Chobe orogenic belts. The ORZ forms a ~100 km wide half-graben filled by lacustrine and fluvio-deltaic sediment. The ORZ and surrounding areas are mostly covered by the Kalahari Sand.

The basement is best exposed in the Ghanzi Ridge (the focus of this study) as a series of discontinuous inselbergs of Precambrian units and Mesozoic cover rocks along the SW edge of the ORZ. Remotely sensed structures show bedrock in the Ghanzi Ridge as well-developed NE-trending linear fabric defined by parallel limbs of tight folds. Normal faults parallel to the fabric locally form narrow grabens - the presence of which was confirmed by SRTM topographic profiles. A second set of proposed NW-trending faults displace the prominent NE-trending fabric.

Basement structures prominent in GeoEye images are commonly too subtle or obscured by sand cover to be readily documented in the field. However, where the bedrock is exposed (inslebergs and sparse quarries) many remotely sensed structures were confirmed and the most important of which include: 1. Layering of metasedimentary units (045-090/20-75 ^o) and foliation in meta-rhyolite (230-255/74-90 ^o) define the regional basement fabric; 2. Folding is confirmed by layering-cleavage relationships and rare fold closures; 3. Joints with variable orientations; 4. Normal faults commonly parallel the layering/foliation. These faults are typically associated with development of and offset calcrete breccia; 5. The remotely sensed NW-trending faults correspond to a series of small linear valleys filled with Kalahari Sand.

The combined remote sensing and field studies show that faults associated with the formation of the ORZ have inherited their trends by exploiting zones of weakness defined by preexisting basement fabrics.