Seismic images and evolution of the Iberian crust in the Pyrenees

Abstract

Crustal images of the Pyrenean range based on coincident wide-angle and vertical deep seismic profiles are presented in this study. Wide-angle experiments in the thickened Iberian crust along strike lines have provided no evidence for reflections from interfaces between layers of different velocities in the middle or lower crust. In contrast, reversed sounding on a line across the Pyrenean range shows wide-angle reflectivity. Its unusual reflectivity pattern points to widespread impedance contrasts of localized continuity embedded in the crust. This observation is consistent with a new image of vertical-incidence reflectivity, obtained by coherency-weighted migration of the coincident Ecors traverse. This profile shows numerous discontinuous and dipping reflections from the middle to the base of the crust. The reflections dip towards the axis of the range. The upper limit and the attitude of the reflective zones are so strongly variable that the crust can hardly be envisaged as the simple, continuous three-layered model previously used for geodynamical reconstructions. The reflective zones cannot be brought back to the horizontal image of a layered-lower crust of proposed extensional origin by plate unbending to the pre-Alpine stage. The dipping reflections may result from compressional deformation, as the south Pyrenean deep crustal domain has undergone both Hercynian and Alpine orogenic deformation. If part of the present reflectivity pattern was due to Alpine remobilization, the corresponding lower crustal deformation would have spread out, thereby accomodating convergence by thickening, even outside the axial zone, where the upper crustal deformation appears to be concentrated.