From continental extension to seafloor spreading: crustal structure of the Goban Spur rifted margin, southwest of the UK

Abstract

S U M M A R YWe present a 169-km wide-angle velocity model across the Goban Spur rifted margin, southwest of the UK. A 120-km-wide intermediate region is identified between the first clear seafloor spreading magnetic anomaly (anomaly 34r) and thinned continental crust, where velocities increase from 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.Based on these velocities and combined with seismic reflection, gravity and magnetic modelling along this transect we interpret the intermediate region to consist of a 70 km wide zone of exhumed mantle with high Poisson's ratio at top basement (>0.34), an extremely low topographic expression and a high-velocity deep layer. Empirical relationships between velocity and degree of serpentinization suggest that serpentinite content decreases with depth from 100 per cent at top basement to <25 per cent from 5-7 km into basement. The observed magnetic anomaly is best fit by a thin magnetized layer (1 km) of magnetization 2-3 A m −1 . The intense magnetization may be due to magnetite formation during a prolonged interaction of serpentinite with sea water. Between this exhumed mantle and anomaly 34r is a 50-km-wide region raised 400 m above the adjacent exhumed mantle, consisting of a series of basement ridges. Poisson's ratio is not well defined in this region, which may be composed of anomalous oceanic crust or exhumed mantle.

Three-dimensional gravity and magnetic modelling of the Irish sector of the NE Atlantic margin

“…This interpretation differs from the hypotheses that the Rockall Basin is floored by oceanic crust of mid-Cretaceous age (e.g. Chappell and Kusznir, 2005) or that the Porcupine Abyssal Plain adjacent to Goban Spur margin is underlain by exhumed, serpentinised mantle (Bullock and Minshull, 2005). Measurements of the magnetisation of serpentinised peridotite from exhumed mantle on the Iberian margin indicate values that are significantly less than those for oceanic basalts (Zhao, 1996;Zhao et al, 2001;Russell and Whitmarsh, 2003).…”

“…If exhumed mantle occurs within this area we suggest that it may have only limited extent. Such a hypothesis receives some support from the presence of a relative magnetic low which appears to coincide with the anomalous basement identified by Bullock and Minshull (2005) near the western end of the WAM profile (compare their figs. 2a and 9), but further investigation would be necessary to confirm this.…”

“…The oceanic crust in this region nonetheless still appears to be thinner than the global average of about 7 km, as Thinon et al (2003) cite values of 3-5 km for the oceanic crust off the North Armorican margin to the south. Bullock and Minshull (2005) suggest that exhumed mantle may be present in a c. 70…”

Crust and Lithospheric Structure - Global Crustal Structure

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Compressional structures on the West Iberia rifted margin: controls on their distribution

“…Thinon (1999) suggested the existence of exhumed mantle in the Amorican margin and Bullock & Minshull (2005) for the Goban Spur. Interestingly, deformation structures similar to those observed in the Southern Iberia Abyssal Plain are also described from the NW European Atlantic margins.…”