2-D SEISMIC MODELING OF THE VARISCAN FORELAND THRUST AND FOLD BELT CRUST IN NW SPAIN FROM ESCIN-1 DEEP SEISMIC-REFLECTION DATA

The foreland thrust and fold belt (Cantabrian Zone) of the Variscan Be lt in NW Spain and the transition to the hinterland (Westasturian-Leon ese Zone) was the location of a seismic experiment in 1991. The seismi c reflection profile (ESCIN-1) is 140 km long and runs in an E-W direc tion. The interpretation and seismic modeling of the main reflective i nterfaces in the profile were made integrating available geological an d geophysical data including surface geology, deep seismic reflection data from ESCIN-1, transmission velocities from a borehole, refraction and laboratory data. The geological and velocity model of the crust w as tested obtaining synthetic seismograms and can be correlated with s urface geology. The velocity model images the main crustal structures interpreted from ESCIN-1. The basal detachment of the foreland thrust and fold belt dips gently from 12 km in the easternmost part of the pr ofile to 16 km in the transition to the hinterland to the west. A new interpretation is proposed for the structure above the basal detachmen t in the eastern end of the profile, where the basal detachment, at a depth of 12 km, is duplicated at 6.5 km by a N-dipping Alpine thrust t hat also duplicates part of the basement. In the western part, two dee p reflective bands dip westward and the most conspicuous one can be fo llowed from 16-27 km depth. The two bands, previously interpreted as c rustal ramps, join a reflective lower crust that is located between 25 and 29 km and fades westwards. A reflective Moho interpreted at the b ase of the reflective lower crust dips and fades in the same direction from 30-34 km.