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Termination of a fossil continent-ocean fracture zone imaged with three-dimensional seismic data: The Chain Fracture Zone, eastern equatorial Atlantic

¹ 3DLab, School of Earth, Ocean and Planetary Sciences, Main Building, Park Place, Cardiff University, Cardiff CF10 3YE, UK
² Veritas DGC Ltd., Crompton Way, Crawley, West Sussex RH10 9QN, UK
³ 3DLab, School of Earth, Ocean and Planetary Sciences, Main Building, Park Place, Cardiff University, Cardiff CF10 3YE, UK

We describe the first three-dimensional imaging of the termination of a continent-ocean fracture zone (COFZ), the Chain Fracture Zone, located offshore of the Niger Delta. The COFZ marks the abrupt transition between extended continental crust, comprising multiple half-graben, and oceanic crust that has a pervasive seafloor-spreading fabric. It preserves a history of continent-continent shearing followed by oceanic crust accretion and continent-ocean shearing during the inception of Atlantic rifting. The termination is marked by steeply dipping faults with sigmoidal planform and thrusts that probably formed as a result of continent-continent or continent-ocean shearing. These are crosscut by the seafloor-spreading fabric that formed during the subsequent phase of oceanic crust accretion. The accreted oceanic crust is cut by listric and planar faults that curve in the direction of the COFZ, where they terminate. The transition from continental to oceanic crust across the COFZ is sharp and resolvable to 100–200 m. Complexes of lava flows emanate from volcanoes along the COFZ, bifurcating and trifurcating down the volcano flanks. The volcanoes are 2–5.5 km wide and 1.4 km in height relative to adjacent oceanic crust and were injected at the COFZ, probably as the spreading center migrated along it.

Key Words: seismic reflection • fracture zone • transitional crust • oceanic crust