Regional three-dimensional gravity investigations in the black forest, southwest Germany

Abstract

The gravimetric pre-surveys of the continental deep drilling project (KTB) yielded results different to those hoped for at the beginning of the program. Firstly, 2-D calculations based on a refraction seismic interpretation allowed a comparison of gravity effects on a 160 km long profile, but despite the fact that in the 2-D model granitic bodies were added at the earth's surface, we only obtained a regional gravitational fit with strong deviations on a local scale. Secondly, a gravimetric field survey in the region around Haslach yielded rather poor indications of the geological structures in the 5–15 km depth interval which is, in the context of the KTB, of significant interest. Therefore, all available information derived from geological and tectonic hypotheses and from reflection and refraction seismic interpretation were combined in a regional 3-D density model: For the local anomalies, the distribution of granite and gneiss bodies known from surface geology was added (model bodies). From an earlier regional study in southern Germany, which combined gravimetric and seismic results, the Moho undulations and the density of the lower crust and upper mantle were taken as additional model parameters in order to discuss regional features. The Kraichgau anomaly in the north of the Black Forest is modelled from a 2-D hypothesis as a body with an unusually high density and with a considerable lateral extent. This approach demonstrates that evidence concerning the density contrast of anomalous masses can be found if boundary conditions are considered in the construction of the model.