The transition between the Erris and the Rockall basins: new evidence from wide-angle seismic data

Abstract

The Rockall Trough is a deep-water basin west of Britain and Ireland. The origin of the basin and the nature of the crust beneath it is controversial. A series of seismic wide-angle experiments carried out during 1988 and 1990, help to clarify the crustal and upper mantle structure in the region. Results for the crustal structure from a profile which straddles the shelf break between the basin and the Irish Shelf are discussed here. These data together with the available geological information indicate that the basin probably formed in the late Palaeozoic to early Mesozoic as part of a regionally linked basin assemblage which includes the Hatton Basin and the shallow sea basins surrounding Ireland and Britain. Good data quality has allowed the transition between the relatively unstretched crust of the Irish and British mainland, defined by previous onshore seismic refraction experiments, to be well resolved. The Erris Trough, one of the small late Palaeozoic to early Mesozoic basins which fringe the mainland shelf region north of the Porcupine Basin is a half-graben. Its major bounding fault is located on its western margin and the basin is divided from the Rockall Trough by a narrow horst, the Erris Ridge. The shelf trajectory along the western flank of the horst deepens smoothly towards the trough centre, where the crust thins to 5 km near the trough margin below a sedimentary and water column 8 km thick. Surprisingly, the crustal thickness is slightly greater over a 150 km broad zone at the trough centre (i.e., ca. 6 km). This change in crustal thickness may be due to lateral strain migration to the warmer basin margins as its centre cooled during the deformation. The crustal structure beneath the sediment pile at the trough centre is two layered, as opposed to the three-layered seismic refraction structure found onshore. However, the basic character of the lower crust present in onshore Ireland, in particular the presence of a gradient zone defining the crust/mantle transition, is still preserved in the Trough. This similarity in structure precludes the presence of magmatic underplating. The crustal structure observed in the Rockall Trough can be formed by differential stretching of the lithosphere. In this model the lower ductile crust and mantle lithosphere are stretched over a wide region byβ 2 = 2−3. Strain focusing into a much narrower region of brittle upper crust generates severe amounts of crustal thinning (β 1 = 8−10), and is responsible for the fusing of the upper and mid-crustal seismic refraction layers found beneath onshore Ireland and Britain. Mediating detachment surfaces, sited at the brittle/ductile transition at any time, served to relay the strain from the lower lithosphere into the upper crust. Syntectonic heat loss plays an important role in controlling the deformation pattern.