Refraction/wide-angle reflection investigation of the Cadomian crust between northern Brittany and the Channel Islands

A seismic refraction experiment has been carried out off the coast of north ern Brittany (NW France) to investigate the deep structure of the North Arm orican block. The seismic source was an array of airguns and the shots were recorded simultaneously both by ocean bottom seismometers and land station s. We present 2D velocity models computed from two crossing seismic refract ion profiles (labelled P1 and P2 and each about 135 km long) oriented NE-SW and NNW-SSE, i.e. parallel and perpendicular, respectively, to the regiona l basement structures. Wide-angle reflection profiles were also performed t o extend the 2D structures to a 3D crustal interpretation. Observed and cal culated P-wave traveltimes were matched by forward modelling prior to their inversion in order to produce two crustal velocity models. The model which best fit the data consists of: (1) high basement velocities at the top of the crust, except in an area with shallow seismic velocity which can be as low as 3.0 km/s at the surface and possibly related to the existence of sha llow Paleozoic sedimentary basins; (2) the mean crustal velocity ranges fro m 5.5 to 6.8 km/s at a depth of 15 km depth; (3) lower crustal velocities r ange from 7.0 to 7.5 km/s at a depth of 35-38 km and then from 7.5 to 8.1 k m/s at the crust-mantle boundary located between 35 and 38 km depth. Compar isons with the crustal section imaged by the coincident seismic vertical re flection profile SWAT 10 show a good correlation between velocity gradients and lower crustal reflectivity. When compared to surrounding basement area s, the North Armorican Cadomian crust displays a slightly thicker crust and also a thicker reflective lower crust which can be related to the relative preservation of this Upper Proterozoic block from later Paleozoic and Meso zoic tectonic episodes. The crustal structure of the North Armorican block thus appears to be an intercalated feature between the surrounding Paleozoi c areas and older shield or platform domains. (C) 2001 Elsevier Science B.V . All rights reserved.