Geophysical constraints on the crustal structure of the Olot Volcanic Area, northeastern Iberian Peninsula

Seismic and electric data obtained in a recent geophysical survey place new constraints on the structure at depth of the Olot region, the youngest Quaternary volcanic area of the Iberian Peninsula. The upper crust under the Olot Volcanic Zone (OVZ), Southern Pyrenees, was investigated by a dense network of seismic refraction profiles. The profiles running along the main axis of volcanism show P-velocities less than 4 km/s for up to 400 m under the OVZ. Higher velocities are observed in the Eocene sediments, 5.3 km/s in the east and 4.6 km/s in the west of the OVZ. Vertical electrical soundings clearly define the volcanic rocks. Resistivities higher than 1,000 Ω.m are typical of pyroclastics, and values above 10,000 Ω.m are found in basaltic flows less than 140 m thick. An Eocene substratum identified at the base of the soundings is characterized by resistivities of between 150 and 500 Ω.m. The seismic basement, characterized by a velocity of 6.0 km/s, was observed at 2 km depth, with a lateral continuity deepening to the southwest. Velocities higher than 6.6 km/s are suggested in the OVZ below 5 km depth. The particle motion analysis of individual seismograms has provided independent constraints from S-phases and PS-phases converted at the basement-sediment discontinuity. In the OVZ, no indications of the existence of anomalous bodies in the upper 5 km of the crust have been found. A local anomalous zone evidenced at the eastern edge of the OVZ deserves further study. © 1991.
The data presented here have been gathered in an experiment supported by the Commission of European Communities under its Geothermal Energy Programme, contract n ° EN3G-0074-E.