From continental thinning to sea-floor spreading : a geophysical study of rifted margins southwest of the UK

Geophysical methods were used to examine the crustal structure of the Goban Spur and North Biscay margins and the evolution from rifting to sea-floor spreading processes. A 169 km wide-angle velocity model across the Goban Spur margin was produced and a 120 km wide region identified between undisputed oceanic crust and thinned continental crust, where velocities increase form 4.5 km s-1 to 6.8 km s-1 in the top 4 km beneath acoustic basement. At depth it can be divided into a region where a 1.5 km thick high velocity layer (7.2-7.6 km s-1) exists and a region where this layer is absent, but velocities of ~ 7 km s-1 are present. Wide-angle PmP arrivals are observed across the whole of the intermediate region, but a normal incidence reflection Moho is not present. The intermediate region is interpreted to consist of exhumed mantle where the high velocity layer is present and slow-spreading oceanic crust further seaward where this layer does not exist. The exhumed mantle region is 70 km wide, has a Poisson’s ratio < 0.34 at top basement and an extremely low topographic expression. The serpentinite content is estimated to decrease with depth from 100% at top basement to <25%, 5-7 km into basement. The observed magnetic anomaly is best fit by a thin, 1-1.5 km, layer of magnetisation 2-3 A m-1 and may be attributed to magnetite during a prolonged interaction of serpentinite with seawater. The region of slow-spreading oceanic crust is 50 km wide, consists of a series of basement ridges raised 400 m above the adjacent exhumed mantle and has a Poisson’s ratio of ~ 0.29 at top basement. Reprocessing and migration of multichannel seismic profiles across the North Biscay margin show that continental crust was thinned from > 24 km to ~ 3 km over a distance of ~ 200 km, at the expense of both upper and lower crust. Preferential thinning of the lower crust is evident in the gravity model and the disappearance of deep layered reflectivity.