FEEDBACK BETWEEN STRUCTURAL INHERITANCE, MAGMA-PLUMBING AND FAULTING IN A MULTIPHASE RIFT: THE SHIRE GRABEN, EAST AFRICAN RIFT

Here, we investigate the feedback between structural inheritance, faulting and magma-plumbing along multiphase continental rift basins. We focus on the Shire Graben, a seismically-active rift in southern Malawi, East African Rift that initially underwent extension in the Late Paleozoic to Late Mesozoic. We use satellite DEM to map surface rift faults, filtered aeromagnetic data to resolve Precambrian basement fabric, buried faults and mafic magmatic intrusions, and field data to support our analyses. Integration of SRTM and aeromagnetic data shows dominance of NW-SE trending basement fabrics along the rift shoulders, and rift-related faults that align with and are parallel to the fabrics. Our results reveal that border faults of both the earlier (Permo-Triassic to Cretaceous) and current (Cenozoic) phases of rifting align parallel to the basement fabric. The mafic dikes predominantly strike NW, NE and ~N-S; where the NE dikes show greatest segmentation and the NW dikes show least segmentation. The early-phase mafic dikes align with and cross-cut both the border faults and intrabasin faults. Intrabasin faults resolved in aeromagnetic data strike parallel to the border faults that follow the basement fabric. The Shire Graben features a synthetic border fault system near the Ngabu area, in which the inner, basin-ward fault (Mwanza Fault) is older and inactive, and the outer fault (Thyolo Fault) is younger and active. Further, SRTM-DEM and aeromagnetic data show that the Mwanza Fault system, which transects the hanging wall block of the Thyolo Fault, is composed of three decoupled segments. The two bounding segments of the Mwanza Fault (S1 and S3) are exposed, and the central segment (S2) is downthrown and buried beneath sedimentary sequences in the hanging wall block of the Thyolo Fault. The S1-S2 and S2-S3 segment boundaries coincide with the mapped NE-SW trending early phase mafic dikes. Also, our data reveal that the Thyolo-Muona border fault system shows major segment terminations and side-stepping at cross-cutting dikes. Thus, we interpret that during early rift phases, the basement fabrics controlled fault geometries and propagation, and these faults served as conduits for magmatic intrusions. In the current phase, the early phase dikes facilitate the decoupling and segmentation of the border fault systems.