Upper mantle anisotropy beneath central Europe from SKS wave splitting: Effects of absolute plate motion and lithosphere-asthenosphere boundary topography?

Abstract

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.