Rift evolution in regions of low magma input in East Africa

Magmatism is often invoked as critical for assisting strain focusing during continental rift development, and in driving lithospheric thinning and continental rupture. Accordingly, models of rift basin evolution in the East African Rift System (EARS) have previously focused on magma-rich basins; however, a complete understanding of how magmatism drives rift evolution requires analyses of both magma-rich and magma-poor extensional environments. We investigate a ∼10-million-year history of fault development in the magma-poor Lake Tanganyika Rift of the Western branch of the EARS, utilizing reprocessed legacy and high-resolution commercial seismic reflection datasets. Unlike the magma-rich Eastern branch, fault-strain does not focus into the basin floor along the rift axis within the first ∼10 million years of rift development. Instead, large rift-bounding faults (border faults) accommodate the majority of extensional strain (∼90% of an estimated 11.5 km of extension over the 52 km-wide rift) from rift inception through to present-day rifting. The Western branch of the EARS is also characterized by crustal structure and fault geometries that differ from those observed for successfully rifted magma-poor margins (e.g., listric normal faulting with mid-crustal detachments). In the absence of voluminous magmatism, we suggest that either (1) complete continental rupture cannot be achieved in the Western branch of the EARS, or (2) continental break up in the Western branch will proceed under a model that contrasts with that invoked for both the Eastern branch and magma-poor rifted margins generally.