TERMINATION AND STRAIN TRANSFER AT THE NORTHERN TERMINUS OF THE OKAVANGO RIFT ZONE – THE ROLE OF TECTONIC INHERITANCE

The early stages of development of transfer zones associated with continental rifting is investigated utilizing remote sensing mapping and targeted field work near the northeastern terminus of the Okavango Rift Zone (ORZ) in the vicinity of the mid-Zambezi river, southern Zambia. The ORZ appears to terminate within this region raising the possibility that strain is being transferred either to the Luangwa Rift Basin (Zambia) and/or the more evolved Malawi Rift along a nascent transfer zone. The mid-Zambezi river area is underlain by Phanerozoic rocks of the Upper Karoo Supergroup (Batoka Basalts) formed during the breakup of Gondwana in Middle-Jurassic time. Rift-basins formed during this time follow the structural trend of the Pan-African mobile belts. Basalt flows are pervasively cross-cut by multiple joint sets readily recognizable in remote sensing imagery (e.g., Shuttle Radar Topography Mission data). These lineaments define three main sets, with azimuths of 055, 085, and 320. This spatial distribution is similar to Riedel shears; O55 ~ R-shears, 320 ~ R’, and 085 ~ “transfer zone” with right-lateral slip. These lineaments and joints have been etched by the erosive forces of the Zambezi River and exert control on the development of the river channels. Field measurements define three major joint sets; 080/85^o, 358-020/80-85^o, and 285/85^o. Also several cataclastic zones (074/85 ^o and 000/85) were mapped and parallel some joint sets. Comparison of the most important lineament trends mapped in remote sensing (azimuths ~055) and the trends of the main normal faults for the ORZ and the ORZ basement fabric (azimuths ~050), show remarkable similarity even though this joint set was rarely detected in the field. This suggests that even with the basalt flows obscuring the basement structure, younger joint sets may be inheriting an older structural fabric from the Proterozoic basement. Interpretation of the “Riedel shear pattern” of the joints as being related to either the present day regional stress field, or inherited from the basement, or a combination of both remains equivocal.