Mapping Extensional Structures in the Makgadikgadi Pans, Botswana with remote sensing and aeromagnetic data: Implication for the continuation of the East African Rift System in southern Africa

Abstract:

We used Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) and aeromagnetic data to map extensional structures in the Makgadikgadi Pans in northeastern Botswana. These pans are a major morphological feature in Southern Africa characterized by the presence of low lying and flat topography with the highest elevation of 945 m. This topography was a result of multiple filling and desiccation of paleo-lakes that accompanied alternation of wetter and dryer climate during the Late Quaternary period. The objective of our study was to map the extent and distribution of normal faults using their morphological expression and magnetic signature, and examine their relationship with paleo-shorelines of the pans. We: (1) Created a hill shade relief map from the SRTM DEM; (2) Extracted regional NW-SE trending topographic profiles across the pans; (3) Constructed displacement profiles for major normal faults; and (4) Created tilt derivative images from the aeromagnetic data. We found that: (1) The northeastern part of the pan is dissected by three morphologically-defined NE-trending normal faults. The along strike continuity of these faults is in the range of 75 and 170 km and they are spaced at ~30 km apart from each other. (2) The topographic profiles suggest that the exposed minimum vertical displacement (EMVD), defined by poorly developed escarpments, is in the range of 0 m and 49 m. (3) The displacement profiles of the faults is characterized by maximum EMVD in the middle of the faults and that it decays towards the fault tips. These faults are also apparent in the aeromagnetic maps where they seem to displace E-W trending Karoo-age dikes. (4) At least the outer paleo-shoreline of the pans is modified by the NE-trending faults. This suggests that the faults are younger than the paleo-shorelines, which is suggested to have been developed between 500 and 100 ka. Traditionally, the southwestern extension of the East African Rift System has been assigned to the Okavango Rift Zone to the northwest of the Makgadikgadi Pans. Our results suggest that the southwestern continuation of this rift system might be a 400 km wide and diffused zone of extension that encompasses both the Okavango Rift Zone and the Makgadikgadi Pans. Our study has implication for understanding within-plate kinematics and seismicity of the African continent.