Abstract

The South West Approaches Traverse comprises 1600 km of profiling to 15 seconds two-way time. The profiles were planned to investigate Variscan structures in the crust and upper mantle. In Ireland, SW England and NW France, Variscan deformation and granite intrusion extend from Middle Devonian to Early Permian time, about 370 to 270 Ma ago. We have imaged a normal fault dipping gently south under the Celtic Sea that corresponds at surface, in position and trend, with the Variscan 'front' and can be traced down to at least 20 km depth. As under Caledonian Britain to the north, the lower crust is everywhere strongly reflective from about 6 seconds two-way time down to the Moho at about 10 seconds. The Haig Fras granite and Cornubian batholith, crossed west of the Isles of Scilly, appear only as unusually shallow portions of the lower crustal reflectors.