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 We stern Alps as participants of the European Geotraverse and particular Swiss projects investigated the Alpine crust there. Recently, the Eastern Alps w ere subject to seismic research which was conducted in the context of the G erman, Austrian and Italian TRANSALP project. The 3D modelling of the densi ty 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 bas ed on the results of both older deep seismic profiles, and the new TRANSALP profile. Special emphasis was laid on geologic and tectonic information wh ich served as model constraints for near-surface structures. A 3D forward m odelling of both the Bouguer gravity field and the geoidal undulations prov ides in-depth insight into the lithosphere and considers the constraints. D ue to the uncertainties of used constraints, particularly in depths of the Eastern Alpine Moho, an inversion technique complemented this study and pro vided insight into the shape and density contrast at the crust-mantle inter face. 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 b oundary (350 ka/m(3)) and the density inhomogeneities in the upper 10 kin o f the crust, which contributes to the overall gravity field by amounts of a pproximately 30 X 10(-5) m/s(2). The results of the forward and inverse mod elling produced a shallower crust-mantle boundary in the (southern) Adriati c area in comparison with the northern part, and differs from the seismic f indings of the TRANSALP group to some extent. This observation and observab le mismatch in the long wavelength model and the observed geoidal undulatio ns point to density inhomogeneities in sub-crustal and even sublithospheric depths. These deep seated inhomogeneities should have a substantial bearin g on the isostatic balance of the Eastern Alps. (C) 2001 Elsevier Science B .V. All rights reserved.