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Petroleum Geoscience; 1 November 2009; v. 15; no. 4; p. 305-311; DOI: 10.1144/1354-079309-846
© 2009 Geological Society of London
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Article

Structure and composition of the ocean–continent transition at an obliquely divergent transform margin, Gulf of Guinea, West Africa

Jonathan P. Turner1,2,* and Paul G. Wilson1,3

1 School of Geography, Earth & Environmental Sciences, University of Birmingham , Birmingham B15 2TT, UK
2 Present address: BG Group, Thames Valley Park, Reading, Berkshire RG6 1PT, UK
3 Present address: ECL-RPS Energy, 309 Reading Road, Henley-on-Thames, Oxon RG9 1EL, UK

* Corresponding author (e-mail: jonathan.turner{at}bg-group.com)

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.

KEYWORDS: continental break-up, Gulf of Guinea, ocean–continent transition, transtension, serpentinization






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Copyright © 2009 by Geological Society of London