Geophys. J. zyxwvutsrqponm Inl. zyxwvutsrqpon (1991) 105, zyxwvutsrqpo 71-83 Deep crustal features of the Celtic Sea from complementary processing on the SWAT data zyxw J6r6me Dyment and Maksim Ban0 Laboratoire de Giophysique Marine, EOPGS (CNRS zyxwvutsrqp LIRA 323), 5 rue R. Descartes, 67084 Strasbourg Cedex, France Accepted 1990 September 10. Received 1990 September 10; in original form zyxwvu 1990 May 3 SUMMARY Complementary processing developed by Ban0 (1989), including automatic extrac- tion 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 FrDnt 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. Key words: basin formation, Celtic Sea, deep seismic reflection, reflective lower crust, seismic processing, SWAT profiles. INTRODUCTION The SWAT (South West Approaches Traverse) deep seismic reflection profiles (Fig. 1) were acquired in the Celtic Sea and Western Channel in 1983 by an English-French joint venture including BIRPS (British Institutions Reflection Profiling Syndicate) and ECORS (Etude de la CroQte continentale et Oceanique par RCflexion et &fraction Sismique). These good-quality data aim to investigate the formation of the Celtic Sea and Western Channel sedimentary basins, the structure of the Variscan orogen and the origin of the lower crust (BIRPS zyxwvu & ECORS 1986). Numerous results and hypotheses about these data are already published, including controversial interpretations about the formation and evolution of the Mesozoic basins and their relation to the Variscan structures (Gibbs 1987; Pinet et zyxwvu af. 1987b; Dyment 1989), but also various assumptions about the observation of a strongly reflective lower crust (Klemperer & BIRPS 1987; Hobbs et al. 1987; Bois et af. 1988). It is likely that neither the formation of the sedimentary basins nor the reflectivity of the lower crust can be resolved in a simple way. Complementary compilations of existing data, such as gravity and magnetics, have been completed to provide additional constraints on the interpretation (Setto & 71 by guest on January 23, 2015 http://gji.oxfordjournals.org/ Downloaded from