Active in situ disaggregation of oceanic crust and mantle on Gorringe Bank: analogy with ophiolitic massives

Abstract

The Cyagor II cruise (May 1981) using submersible Cyana SP 3000 on Gorringe Bank off south-west Portugal confirmed the scheme proposed at the end of Cyagor I cruise1,2 that during the middle Cretaceous (110 Myr BP), Gorringe Bank, an old transform fault of Triassic–Liassic age, appears to have tilted to the north-east leading to the exposure of a section of oceanic mantle and crust. Due to its location at the boundary between European and African plates, Gorringe Bank suffered tectonic reactivation and uplifting during compressive and shearing-type events that occurred at the Cretaceous–Tertiary boundary and during the Tertiary (Eocene–Miocene). During the 12 dives of the Cyagor II cruise (Fig. 1), phenomena linked to the disintegration of serpentinites and gabbros in the deep-sea domain have been studied in situ with more detail than before3. Such processes have been suggested from studies on samples dredged or cored in oceans, or from observations in ophiolitic belts but direct observation had not enabled the magnitude of such phenomena to be measured. The observations on Gorringe Bank suggest that the processes described here are active in many oceanic sites. They help us to understand better the significance of many ophiolitic detritic beds more or less closely associated with pelagic sediments. We now compare the observations made at sea, especially on Gorringe Bank with similar observations in ophiolitic zones in mountain belts, which suggests that the ophiolite complexes have formed in a similar way to the breccias seen on the ocean floor.