Forward and inverse modelling of gravity revealing insight into crustal structures of the Eastern Alps

The crustal structure of the Eastern Alps is less known than that of the Western Alps as participants of the European Geotraverse and particular Swiss projects investigated the Alpine crust there. Recently, the Eastern Alps were subject to seismic research which was conducted in the context of the German, Austrian and Italian TRANSALP project. The 3D modelling of the density structure which is presented in this paper belonged to a series of piggy-back projects which were closely accomplished using the seismic reflection studies. We present a combined gravity-seismic interpretation which is based on the results of both older deep seismic profiles, and the new TRANSALP profile. Special emphasis was laid on geologic and tectonic information which served as model constraints for near-surface structures. A 3D forward modelling of both the Bouguer gravity field and the geoidal undulations provides in-depth insight into the lithosphere and considers the constraints. Due to the uncertainties of used constraints, particularly in depths of the Eastern Alpine Moho, an inversion technique complemented this study and provided insight into the shape and density contrast at the crust-mantle interface. The modelling indicates that the Bouguer gravity field in the Eastern Alps is mainly caused by two sources: the density contrast at the crust-mantle boundary (350 ka/m(3)) and the density inhomogeneities in the upper 10 kin of the crust, which contributes to the overall gravity field by amounts of approximately 30 X 10(-5) m/s(2). The results of the forward and inverse modelling produced a shallower crust-mantle boundary in the (southern) Adriatic area in comparison with the northern part, and differs from the seismic findings of the TRANSALP group to some extent. This observation and observable mismatch in the long wavelength model and the observed geoidal undulations point to density inhomogeneities in sub-crustal and even sublithospheric depths. These deep seated inhomogeneities should have a substantial bearing on the isostatic balance of the Eastern Alps. (C) 2001 Elsevier Science B.V. All rights reserved.