ALBERT

Description: Fast polarization directions α of split SKS waves in Central Europe change from NE/ENE in the western part to dominatingly E/ESE orientation towards north and east. This coincides strikingly well with the dominating trend of Hercynian deformational crustal features. It hints to frozen anisotropy related to paleo-crustal fabric. But when considering plausible anisotropy values of about 2–3% then only a small fraction (δt 〈 0.3 s) of the rather large observed average delay-times (δt = 0.83 ± 0.31 s) between the two split waves could be attributed to structural anisotropy in the relatively thin Central European crust. Therefore, the main “anisotropy signal” has to be associated with lattice-preferred orientation (LPO) of olivine below the crust. It may be either frozen in the subcrustal lithosphere since Hercynian times or have developed more recently in the asthenosphere. The thickness of the lithosphere varies significantly beneath Europe and the depth contours show systematic changes in trend. The latter varies from dominatingly NE in the southwest to SE in the north and east. The polarization directions α of the fast split SKS waves observed at seismic stations in proximity to the southern and northeastern boundaries of Central Europe are subparallel to the trends of these strong anomalies in lithosphere topography. A causal relationship is assumed and a new model proposed to explain the observations in α and δt. It takes into account the possible effects of paleo-deformational events. They may have produced both anisotropic crustal fabric and probably still preserved and similarly trending frozen LPO in the subcrustal lithosphere. The model also considers the influence of recent absolute motion of the West European lithospheric plate towards NE and the effect of its pronounced lower boundary topography on the formation and trend of LPO in the asthenosphere. Accordingly, the effects of anisotropy of different nature and age at different depth levels but with similar trend may superimpose constructively. This could explain the rather large delay-times observed at Central European stations which are too large to be attributed to frozen anisotropy in the lithosphere alone. The model would even permit the total effect observed to be attributed to asthenosphere flow controlled by absolute plate motion direction and lithosphere-asthenosphere boundary topography.

Description: The plate tectonic concept has nearly revolutionized the geo-sciences in the last decades. Many geodynamical, geophysical and geological phenomena could be understood for the first time. The investigation of the present-day global crustal kinematics is a crucial contribution which can be delivered by space geodesy only. Since the beginning of the MERIT-Campaign continuous good quality laser ranging data to LAGEOS are available. Using these data a precise estimation of present-day global-scale tectonic motion is possible. During the WEGENER/MEDLAS project the worldwide tracking network has been densified by a number of fixed and mobile stations in the southern Europe and eastern Mediterranean. Having now for a number of sites three to four occupations within four years significant site motions can be extracted. The derived motions are essential input information for deformation analyses and the interpretation of plate tectonic parameters.