The Seagap Fault offshore Tanzania: Tectonic significance in the microplate framework of the West Somali Basin (Indian Ocean)

The current architecture of the East African continental margin is the result of two major tectonic events: the opening of the West Somali Basin (WSB), which took place between the Middle Jurassic and Lower Cretaceous, and the tectonic processes that, since the mid-Cenozoic, have driven the development of the offshore branch of the East African Rift System (EARS). Offshore Tanzania, tectonic activity associated with EARS is primarily concentrated along two major north-south trending structures: The Davie Ridge, a prominent bathymetric feature that has accommodated east-west extension since the Late Eocene to Miocene. The Seagap Fault, a less documented structure exhibiting left-lateral strike-slip motion since the Late Eocene. Both features are believed to be reactivations of older structures, primarily formed during the opening of the WSB, which also influenced the geometry of the Mesozoic ocean-continent transition (OCT) offshore Tanzania. However, the lack of deep seismic imagery and detailed stratigraphic data limits our understanding of the origin and nature of the Seagap Fault. The interpretation of five deep 2D seismic reflection profiles (10s TWT) and 3D seismic datasets provides new insights into the deep architecture of the crustal basement and the Seagap Fault offshore Tanzania. The results indicate that the Seagap Fault zone consists of large-scale localized structures that affect the seafloor and exhibit growth geometries across most of the Miocene sedimentary sequences. The evidence of continuous tectonic activity observed in our seismic mapping, along with data from previous 2D deep seismic studies, suggests that between the Middle-Late Jurassic and approximately 125 Ma, the Seagap Fault functioned as a regional structure, parallel to and coeval with the dextral Davie Fracture Zone. From a tectonic perspective, the Seagap Fault has played a dual role in the geological evolution of the region. Evidence suggests that, at least between 8° and 9°S, it acted as a continental transform margin during the opening of the WSB, sharply juxtaposing oceanic crust to the east against thinned continental crust to the west. In more recent times, the Seagap Fault appears to have been reactivated within the framework of the offshore East African Rift System, resulting in left-lateral transtensional movements. This reactivation may be linked to the clockwise rotation of the Rovuma microplate.