RT Journal Article SR Electronic(1) A1 Turner, Jonathan P. A1 Wilson, Paul G.YR 2009 T1 Structure and composition of the ocean–continent transition at an obliquely divergent transform margin, Gulf of Guinea, West Africa JF Petroleum Geoscience, VO 15 IS 4 SP 305 OP 311 DO https://doi.org/10.1144/1354-079309-846 PB European Association of Geoscientists & Engineers, SN 1354-0793, AB ABSTRACT 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. , UL https://www.earthdoc.org/content/journals/10.1144/1354-079309-846