Deep-imaging reflection seismic profiles offshore the Gulf of Guinea, West Africa constrain the structure and composition of a major fracture zone accommodating obliquely divergent intra-continental rifting and eventual break-up along the Gulf of Guinea margin. Interpretation of the seismic data reveals a c. 70 km wide fracture zone comprising fault-bounded blocks of hybrid ‘proto-oceanic’ crust. Gravity modelling of the seismic profiles allows us to propose a testable interpretation in which the fracture zone is composed of a central block of oceanic crust between blocks of partly serpentinized mantle. A synoptic model for the middle Cretaceous break-up of this margin suggests it was accommodated by a left-lateral transtensile shear zone in which fault-bounded blocks underwent continuous counter-clockwise rotation. Serpentinization of the upper mantle was promoted by the embrittlement and fracturing that resulted from progressive stretching and thinning. Domino-style back-rotation of early-formed faults within the shear zone meant that they attained progressively gentler dips before eventually locking up. Consequently, overprinting of faulting led to later faults dissecting – and translating in their hanging walls – a mixed assemblage of stretched continental crustal material and partly serpentinized mantle peridotite. With progressive strain, oceanic crust was produced at releasing bends within the shear zone and a leaky transform evolved.