The Hercynian crust in northwestern Spain: a seismic survey

In summer 1982 a network of crustal seismic profiles was established in NW Spain with shotpoints at sea and on land. Some profiles were designed to capture the general features of the crust whereas others were aimed at resolving the detailed structure of specific geological features. The interpretation of three profiles aimed at the most important features of the earth's crust in northwest Spain is discussed here. The crust consists of three layers with compressional velocities of 6.0–6.1 km/s, 6.2–6.3 km/s and 6.7–6.9 km/s. The thicknesses of these layers vary laterally, the average values under the central part of Galicia being 13 km, 8 km and 10 km. The fine structure of the lower crust has still to be determined. So far, lateral variations in the velocity structure of the lower crust from a constant velocity layer to a gradient layer have been detected along the main profile from NE to SW over a distance of 250 km. The Moho discontinuity is reached at 30–32 km in the central part of the surveyed area: this decreases to 23–27 km to the north and to the west. On the basis of P MP amplitudes, the Moho seems to be rather sharp.