We use multifold seismic reflection profiles to constrain the geometry and slip sense of a half graben border fault and the extension direction in the adjacent rift basin. Our data are from the western branch of the East African Rift System, often cited as an analog of early-stage passive continental margin formation. In this case study, classic tilt-block half graben clearly occur along a documented strike-slip fault. The Livingstone Mountains Border Fault bounds the northernmost basin in the Nyasa (Malawi) Rift and is a series of straight, dextral oblique-slip fault segments which mimic the characteristics of a parallel, underlying, prerift ductile shear zone. Structures adjacent to the border fault show extension slightly oblique to the border fault, indicating that the border fault is predominantly a strike-slip fault, consistent with studies of border fault exposures. Data indicate overall extension in the adjacent Livingstone Basin is at a much higher angle to the strike-slip border fault, consistent with the tilt-block half graben architecture. This characteristic is similar to basins bounded by ''weak'' strike-slip faults. Some aspects of Livingstone Basin fault geometry resemble pull-apart, strike-slip duplex, and extensional imbricate fan fault geometries. However, the overall inexact match suggests the Livingstone Basin may be an incipient form of these classic strike-slip basin models.