On the interpretation of geoid anomalies in Europe with special regard to the EGT profiles

Abstract

We have studied medium-wavelength geoid anomalies over Europe in particular along three main EGT profiles (FENNOLORA profile, “Central profile” through west Germany, “Alpine profile” across the Alps and the Po Basin) in relation to the lithospheric structure. For a general interpretation of the whole European area we used a simple model prescribing topography, Moho depth undulations, and appropriate densities and applying the concept of Pratt isostasy. With this simple model we could explain most of the larger features in the geoid. While in northern Europe the crustal thickness is crucial, for central and southern Europe it is the combination of both topography and Moho depth. We found a geoid gradient of 4.06 m/km for topography and 0.58 m/km for Moho undulations with a compensation depth of 60 km to be the best to explain the observations. However, also a compensation depth of 27 km for topography and 80 km for crustal thickness undulations fits the data. The negative anomalies over the North Sea and the Po Basin become even more pronounced if crustal effects are considered, which means they can not be explained adequately by crustal thickness of constant density but need additional (thermal or lithological) effects to locally reduce the densities. A remaining negative anomaly in Fennoscandia of about − 4 m might reflect the effect of post-glacial return flow, but can also be explained by increased crustal density compared to Central Europe. Along the FENNOLORA profile the geoid undulations can be explained by the combined effect of the increased thickness of the crust and lithosphere. Lateral density variations in each layer are not necessary to explain the geoid observation and upper and lower crustal density is high. The density decrease at the lithospheric base is small. Along the “Central profile” the effect of topography together with the low-density sediments in the Northern German Lowland already explains most of the geoid anomaly. The existence of a large low-velocity body in the middle crust, which might be a trapped granitic intrusion, is weakly supported by the geoid observation. A clear decrease in density at the transition to the asthenosphere fits the geoid observation. The short, but structurally most complicated “Alpine Profile” is difficult to interpret. Geoid observations can well be explained by the effect of low compensated topography and sedimentary layers. This means that isostasy in the Alps demands considerable lateral intra-crustal density variations and can not be mainly attributed to the Moho deflection. However, isostasy can well be incomplete across the Alps. Two seismically detected crustal bodies penetrating into the upper mantle are best compatible with the geoid observation if their density is around 3 g/cm 3, which agrees with an eclogitic composition of these bodies.