Constraints on Rift Basin Structure and Border Fault Growth in the Northern Malawi Rift From 3-D Seismic Refraction Imaging

Abstract

We present new constraints on rift basin structure in the northern Malawi Rift from a 3-D compressional velocity model to investigate border fault geometry, accommodation zone structure, and the role of preexisting structures underpinning this rift system. The velocity model uses observations from the first wide-angle refraction study conducted using lake-bottom seismometers in one of the East African great lakes. The Malawi Rift is flanked by basin-bounding border faults and crosses several significant remnant structures, making it an ideal location to investigate the development of normal faults and their associated basins. The 3-D velocity model reveals up to ~5 km of synrift sediments, which smoothly transition from eastward thickening against the Livingstone Fault in the North Basin to westward thickening against the Usisya Fault in the Central Basin. Greater than 4 km of sediment are imaged within the accommodation zone pointing to the early development of the border faults. We use new constraints on synrift sediment thickness to construct displacement profiles for both faults. Both faults accommodate large throws (>7 km) consistent with their significant lengths. The dimensions of these faults are close to or larger than the maximum size predicted by models of fault growth. The presence of an intermediate velocity unit with velocities of 3.75–4.5 km/s is interpreted to represent sediment deposits beneath Lake Malawi from prior rifting in the Permo-Triassic (Karoo) and/or Cretaceous-Paleogene. The distribution of preexisting basins implied by these sediments may help account for changes in intrabasinal faulting and border fault development between the two basins.