A study of large-scale lateral variations of P velocity in the upper mantle beneath western Europe

A large-scale three-dimensional inversion of teleseismic P wave arrival times from ISC bulletins is performed over the part of Europe extending from Scandinavia to the Alps, and down to a depth of 700 km. The inferred velocity variations are compared with those obtained previously for North America in a similar study (Romanowicz 1979). The models obtained manage to explain one third of the standard deviation in the data, implying that the complicated short wavelength structure in Europe causes substantial scatter that cannot be accounted for by the largescale model. However, the geographical extent of the large-scale regions of higher and lower velocity anomalies remains stable when the initial specifications of the models are changed, and can therefore be discussed qualitatively in the framework of large-scale tectonic regionalization. In the uppermost 250 km of the crust and mantle, average velocities are up to 4 per cent hgher under the Scandinavian Shield than under the Alps. This is in agreement with previous studies in Europe and may be interpreted primarily in terms of variations of thickness and strength of the low-velocity zone. In this depth range, the average structure for the Hercynian part of western Europe is intermediate between that of the Shield and that of the Alps and Mediterranean region. Velocity fluctuations are not well resolved in the depth range from 250 to 450 km. Between 450 and 700 km, higher velocities are inferred beneath the shield and platform; velocities decrease towards the west and south-west. Under the Alps, velocities appear to be slightly higher than average. The higher velocities under the shield agree with those inferred between depths of 450 and 700 km in the central stable part of North America, suggesting that this may be a general feature of shields and platforms. The relatively higher velocities under the Alps may be compared to the lower velocities obtained beneath the western US, suggesting that the deep structure under major orogenic belts may be related to their mode of formation.