Thinning of the Goban Spur continental margin and formation of early oceanic crust: constraints from forward modelling and inversion of marine magnetic anomalies

Abstract

The deep seismic reflection profile Western Approaches Margin (WAM) cuts across the Goban Spur continental margin, located southwest of Ireland‐ This non‐volcanic margin is characterized by a few tilted blocks parallel to the margin. A volcanic sill has been emplaced on the westernmost tilted block. The shape of the eastern part of this sill is known from seismic data, but neither seismic nor gravity data allow a precise determination of the extent and shape of the volcanic body at depth. Forward modelling and inversion of magnetic data constrain the shape of this volcanic sill and the location of the ocean‐continent transition. The volcanic body thickens towards the ocean, and seems to be in direct contact with the oceanic crust. In the contact zone, the volcanic body and the oceanic magnetic layer display approximately the same thickness. The oceanic magnetic layer is anomalously thick immediately west of the volcanic body, and gradually thins to reach more typical values 40 km further to the west. The volcanic sill would therefore represent the very first formation of oceanic crust, just before or at the continental break‐up. The ocean‐continent transition is limited to a zone 15 km wide. The continental magnetic layer seems to thin gradually oceanwards, as does the continental crust, but no simple relation is observed between their respective thinnings.

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

“…These anomalies cannot be produced by basement topography alone; highs of 0.5–1 km observed at A34 in this region produce magnetic anomalies an order of magnitude smaller than observed. The shipborne data support the gradual decrease in magnetic anomaly across the intermediate zone from 200 to −50 nT seen in the gridded data instead of a rapid trend to ∼0 nT west of the continental slope shown by Louvel et al (1997). The final fit to these data is not very good, however a simple, minimum structure, model is preferred to a complex one for interpretation of the major magnetic features.…”

“…The zone between A34r and the continental slope has a different magnetic character north and south of this study line: northwards is a sharp, well‐defined magnetic anomaly (∼400 nT) whereas to the south lies a magnetically quiet zone (±100 nT) up to 200 km wide. 2‐D magnetic modelling coincident with WAM predicts a simple structure: a magnetized body at the foot of the continental slope becoming 2 km of magnetized crust further seaward (Scrutton 1985; Louvel et al 1997). This structure and magnetization (5.5 A m −1 ) are compatible with a simple model of normal oceanic crust; however, it should be noted that due to the variation of magnetic anomaly along strike, north and south of the line, the anomaly these authors have fitted is substantially different to that used here (; Verhoef et al 1996).…”

“…Existing studies at the Goban Spur margin have been concentrated over thinned continental crust and have been interpreted as an abrupt transition from continental to oceanic crust at the foot of the continental slope (e.g. Peddy et al 1989; Horsefield et al 1994; Louvel et al 1997). The aim of this study was to reassess the crustal structure of the Goban Spur margin in light of recent discoveries and to examine in greater detail some inconsistencies of the previously published interpretation.…”

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

“…In the south (e.g. on the Goban Spur margin) seaward-dipping reflectors and underplating appear to be absent, although basaltic lava flows have been proven at DSDP Site 551 (de Graciansky et al, 1985) and are responsible for a local magnetic anomaly (Scrutton, 1985;Louvel et al, 1997). Kimbell et al (2004Kimbell et al ( , 2005 presented results of regional 3D gravity modelling of the lithospheric structure of the NE Atlantic margin, which included the Irish sector.…”

Seismic geomorphological reconstructions of Plio-Pleistocene bottom current variability at Goban Spur

“…2a). These terraces correspond to half-graben structures inherited from the early Cretaceous north Atlantic rift, which gradually steps down towards the Porcupine abyssal plain (Naylor and Shannon, 1982;Louvel et al, 1997). The buoyant Hercynian basement is believed to constitute the southwestern prolongation of the Cornubian Ridge (Dingle and Scrutton, 1979;Fig.…”