oa Pliocene Graben Development Led the Reorganization of the Deep-Water Sediment Routing Systems Offshore Tanzania

The distribution and timing of Neogene extensional features along the Tanzania margin as well as their influence on sediment dispersal pathways remain poorly constrained, limiting our understanding of the propagation of the East African Rift System (EARS) in the offshore domain. In this contribution, we unveil the presence of a previously unidentified NNW-SSE-oriented graben offshore Rufiji River delta, which is currently being infilled by a sinuous turbidite channel. We hypothesize that the opening of this graben resulted in a significant reconfiguration of the submarine channel network offshore Tanzania. Our goal is to investigate the genesis of this structure and its impact on the evolution of slope channel systems over time, to provide new constraints on the structural and stratigraphic evolution of the margin.

Horizon maps and 3D seismic reflection data reveal that, from the Miocene to the Pliocene, the slope was intersected by west-to-east oriented turbidite channels. In the Pliocene, these channels underwent a reorientation of approximately 90 degrees southward, likely due to the opening of the graben. Using horizon flattening, we quantify the timing of development of this graben, which occurred during the Pliocene, in agreement with our stratigraphic evidence. The opening of the graben fundamentally altered the sediment delivery system, which was directed eastward for millions of years. Our observation of the timing of development of the graben, combined with knowledge of the chronology of the Neogene rifting along the margin (both onshore and offshore), suggest a potential link between the graben and the tectonics of the EARS. This study provides new evidence of the propagation of the EARS to the western Indian Ocean, highlighting a common trend in the evolution of offshore extensional structures and their control on slope-to-deep water channel system. Tectonic activity offshore Tanzania began in the middle-late Miocene - Pliocene, clearly altering mechanisms of slope-to-deep sediment transport, with important implications for the delivery of coarse-grained materials along the margin.