Interpretation of tomographic images of uppermost mantle structure: Examples from the western and central alps

Abstract

Three-dimensional seismic tomography results are normally calculated and displayed by use of reference models. The former is referred to as an initial reference model and the latter as a final reference model. Conventionally, these two radial or one-dimensional models are identical. The use of the initial reference model also for display purposes (as final reference model) is convenient and well-suited to test data and inversion procedure. For tectonic and petrological interpretation, however, the use of the initial model as a final reference model is sometimes a poor choice, as it may suppress reliable and important information of the actual three-dimensional velocity field. The PM2 one-dimensional model for Europe (Spakman et al., Phys. Earth Planet. Int. 79, 3–74, 1993) was derived by one-dimensional inversion of the same seismic data later used for three-dimensional tomographic inversion. The PM2 model differs from the global Jeffreys-Bullen (JB) reference model by a pronounced layer with reduced P-wave velocities associated with the asthenosphere. For mapping the lithosphere-asthenosphere boundary (LAB) the radial velocity distribution in the uppermost mantle is as important as the lateral velocity variation. To better image the lithosphere-asthenosphere system beneath the Alps, therefore, we re-display the three-dimensional tomographic results obtained by Spakman et al. (1993) relative to the JB model. The usage of this final reference model with a constantly increasing velocity in the depth range from 80 to 200 km leads to images with enhanced vertical velocity variation across the LAB. When defining the LAB as the depth where the vertical gradient of P-wave velocity assumes zero or negative values, the thickness of the lithosphere in the central European foreland outside the Rhinegraben is approximately 100 km. The deep-seated volume of high-velocity beneath the Po plain and northern Appennines found by several other studies, denotes the lithospheric slab connected with the European lithosphère in the central Alps and likely in the northern part of the western Alps. In contrast, the European lithosphere in the southern parts of the western Alps is disconnected from this slab. The overriding lithosphere has a thickness of about 100 km beneath Corsica and Sardinia and thins to only about 50 km beneath the Ligurian Sea. For the Adriatic lithosphere a thickness of about 90 km is obtained.