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Paper No. 5
Presentation Time: 8:00 AM-4:45 PM
Geoelectrical Imaging of Developing Border Faults of the Okavango Rift Zone (ORZ), NW Botswana: Providing International Research Experience for Students
Our NSF-IRES project is designed to partner US students with peers from the University of Botswana and University of Zambia to conduct field-based research focused on investigating the interplay between neotectonics and surficial processes due to rifting. During the first year of our project, we have focused on the Okavango Rift Zone (ORZ) in NW Botswana, an area of incipient rifting. Previous studies suggest that strain during rifting is accommodated by younger, more active faults near the rift margins, while wanning activity occurs along faults in the interior of the rift zone. During our first field season, 4 US and 2 Botswana students investigated the detailed 2-D structure of the developing juvenile border fault system using electrical resistivity and controlled source audio magnetotelluric techniques. Geomorphologic expression of these faults is very subtle with fault scarps <~ 5-10 m high. The resistivity sections show faults as steeply dipping anomalies characterized by sharp lateral resistivity contrast cutting basement rocks. The resistivity profiles show a thin high resistivity layer (~1000 Ohm.m) ~5-10 m thick, interpreted to be associated with calcretes overlying the basement rocks. The calcrete layer is underlain by a zone of lower resistivity (60-300 Ohm.m) which we interpret to be associated with saturated semi- to fully consolidated sandstone. Below this layer, unweathered basement can be seen to the base of the resistivity sections on the footwall block. However, within the graben both the basement and calcrete layer are absent. Instead the graben is characterized by a very low resistivity layer (< 5 Ohm.m), which we attribute to the presence of possible saline water. Our preliminary investigations suggest that the Kunyere fault zone is characterized by several synthetic and antithetic faults defining a 2 km wide zone of fractured bedrock. Some of these faults do not show any evidence of recent activity.