THE TRANSITION BETWEEN THE ERRIS AND THE ROCKALL BASINS - NEW EVIDENCE FROM WIDE-ANGLE SEISMIC DATA

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 cont roversial. A series of seismic wide-angle experiments carried out duri ng 1988 and 1990, help to clarify the crustal and upper mantle structu re in the region. Results for the crustal structure from a profile whi ch straddles the shelf break between the basin and the Irish Shelf are discussed here. These data together with the available geological inf ormation indicate that the basin probably formed in the late Palaeozoi c to early Mesozoic as part of a regionally linked basin assemblage wh ich includes the Hatton Basin and the shallow sea basins surrounding I reland and Britain. Good data quality has allowed the transition betwe en the relatively unstretched crust of the Irish and British mainland, defined by previous onshore seismic refraction experiments, to be wel l resolved. The Erris Trough, one of the small late Palaeozoic to earl y 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 we stern flank of the horst deepens smoothly towards the trough centre, w here the crust thins to 5 km near the trough margin below a sedimentar y 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 stra in migration to the warmer basin margins as its centre cooled during t he 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 t he lower crust present in onshore Ireland, in particular the presence of a gradient zone defining the crust/mantle transition, is still pres erved in the Trough. This similarity in structure precludes the presen ce of magmatic underplating. The crustal structure observed in the Roc kall Trough can be formed by differential stretching of the lithospher e. In this model the lower ductile crust and mantle lithosphere are st retched over a wide region by beta(2) = 2-3. Strain focusing into a mu ch narrower region of brittle upper crust generates severe amounts of crustal thinning (beta(1) = 8-10), and is responsible for the fusing o f the upper and mid-crustal seismic refraction layers found beneath on shore ireland and Britain. Mediating detachment surfaces, sited at the brittle/ductile transition at any time, served to relay the strain fr om the lower lithosphere into the upper crust. Syntectonic heat loss p lays an important role in controlling the deformation pattern.