Upper mantle anisotropy at the transition zone of the Saxothuringicum and Moldanubicum in southeast Germany revealed by shear wave splitting

The main structures of the mid-European lithosphere, crossing Europe from t he Iberian peninsula to the Bohemian Massif, are predominantly formed by th e Variscan orogeny. To investigate the anisotropy of the mantle at the tran sition zone between the two Variscan units, the Saxothuringicum and the Mol danubicum, we carried out a field experiment in SE Germany in 1995-1996: 23 mobile broad-band stations were installed for 6 months in the Vogtland-Obe rpfalz-Bavarian Forest area. The station profile crossed the suture zone of the Saxothuringicum and Moldanubicum near the KTB borehole (German Contine ntal Deep Drilling Program). With a mean station spacing of about 10 km? we intended to obtain a high lateral resolution of the anisotropy parameters and to resolve possible changes when passing the suture zone. The analysis of the observed birefringence of SKS phases shows E-W directions for the fa st polarization. Therefore, the directions deviate only slightly from the s trike of the Hercynian mountain belt and from the direction of the absolute plate motion in that region. Indications for the transition zone come from a rotation of the fast polarization direction from 86 degrees +/- 13 degre es in the northern part of the profile (Saxothuringicum) to 110 degrees +/- 15 degrees in the southern part (Moldanubicum) as well as from strong vari ations of the splitting parameters with respect to the azimuths of the inco ming waves in the middle of the profile. We interpret these variations as a n expression of a complex mantle structure formed either by several anisotr opic layers with inclined symmetry axes in at least one layer or by a model consisting of inhomogeneous anisotropic layers. A comparison of the azimut hal variations of the splitting parameters in the middle of the profile wit h those observed at the Grafenberg station GRAl-situated in the central par t of the transition zone approximately 100 km to the west-shows remarkable differences, which may reflect lateral variations in the direction and incl ination of the symmetry axes in the transition zone even on a small scale. Both observations-the change in the fast polarization direction from the no rthern to the southern part as well as the variations with respect to diffe rent azimuths in the middle of the profile-suggest that the transition zone between the Saxothuringicum and the Moldanubicum continues down into the u pper mantle.