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

Abstract

A seismic refraction experiment has been carried out off the coast of northern Brittany (NW France) to investigate the deep structure of the North Armorican block. The seismic source was an array of airguns and the shots were recorded simultaneously both by ocean bottom seismometers and land stations. We present 2D velocity models computed from two crossing seismic refraction profiles (labelled P1 and P2 and each about 135km long) oriented NE–SW and NNW–SSE, i.e. parallel and perpendicular, respectively, to the regional basement structures. Wide-angle reflection profiles were also performed to extend the 2D structures to a 3D crustal interpretation. Observed and calculated 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 3.0km/s at the surface and possibly related to the existence of shallow Paleozoic sedimentary basins; (2) the mean crustal velocity ranges from 5.5 to 6.8km/s at a depth of 15km depth; (3) lower crustal velocities range from 7.0 to 7.5km/s at a depth of 35–38km and then from 7.5 to 8.1km/s at the crust–mantle boundary located between 35 and 38km depth. Comparisons with the crustal section imaged by the coincident seismic vertical reflection profile SWAT 10 show a good correlation between velocity gradients and lower crustal reflectivity. When compared to surrounding basement areas, 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 Mesozoic tectonic episodes. The crustal structure of the North Armorican block thus appears to be an intercalated feature between the surrounding Paleozoic areas and older shield or platform domains.