Deep crustal features of the Celtic Sea from complementary processing on the SWAT data

SUMMARY Complementary processing developed by Bano (1989), including automatic extraction of reflections and migration of deep seismic data, are applied to the SWAT (South West Approaches Traverse) deep seismic profiles in the Celtic Sea. The automatic extraction of reflection gives a more reliable definition of the basement under the deep Celtic Sea Basins. Using migration, we derive a more accurate geometry of these basins. Upper crustal reflectors, related to Variscan thrusts, are well evidenced on the data. The Variscan Front underlies the North Celtic Sea Basin on the SWAT 4 profile and has been interpreted as a detachment fault which would have initiated the basin. This controversial role is discussed, including the observation, on the processed data, of normal faults which offset the thrust. As observed on many Western European deep seismic profiles, the SWAT profiles display a highly reflective lower crust. This seismic unit is particularly well defined after automatic extraction of reflection. In the Celtic Sea area, the Moho appears flat and the lower crust roughly of constant thickness. Pull‐down effects under the youngest sedimentary basins are clearly resolved and lead to a reasonable estimation of the upper crustal seismic velocity. In regard to the extensional tectonic which affected the Celtic Sea area, the flatness of the Moho and most of the lower crustal reflections suggests a recent origin of these patterns. Differences in character of the lower crustal reflections are enhanced by the automatic extraction of reflection. Dipping events and crossed reflections are observed under the Haig‐Fras and Cornwall batholiths. Most of the dipping discontinuous events can be interpreted as diffraction hyperbolae flanks. A more laterally heterogeneous and complex lower crust, related to the batholiths, is consequently inferred.