The structural re-interpretation of the Lower Shire Basin in the Southern Malawi rift using gravity data

Abstract

In this study, we undertake the re-interpretation of the key structural domains of the Shire Graben-a less-studied segment of the East African Rift System, which holds great scientific and economic potential for its multiphase rifting history and hydrocarbon resources. By utilizing a collocated approach of enhancing fault-related gradients in gravity maps, we identify dominant and subtle structural domains and boundaries within the Shire Graben. Our study confirmed the previously identified normal faults in the basin and we also found some previously-unmapped faults, thus providing a more robust compilation of faults in the Shire Graben. In general, we observe that areas of sediment accumulation in the basin show a gravity-low anomaly among-which is the Lengwe domain. The areas where volcanic deposits overlie the clastic sediments show gravity-high anomalies and these include the Mwabvi domain. However, we identify a NW-SE-striking zone of distinct gravity-low anomaly located along the central axis of the Shire Graben, overlain by recent alluvial deposits, interpreted to be a possible previously-unmapped buried Permo-Triassic (Karoo) Basin. We suggest that this zone, here-in referred to as the “Ngabu Domain”, constitutes one of the three main sedimentary depocenters within the Shire Graben, in addition to the previously-mapped Lengwe and Mwabvi depocenters. We highlight the need for further subsurface investigation of the “Ngabu Domain” as a potential target for economic exploration in southern Malawi.