Seismic anisotropy beneath the western part of the Carpathian-Pannonianregion inferred from combined SKS splitting and mantle xenolith studies

Abstract

<p>Information on mantle anisotropy can be obtained from methods such as<br>studying the lattice-preferred orientation (LPO) in mantle peridotites,<br>or conducting shear-wave splitting (SKS) analyses which allow to<br>determine whether it is a single or multi-layered anisotropy and the<br>delay time of the fast and slow polarized wave can indicate the<br>thickness. In this study we provide a detailed SKS mapping on the<br>western part of the Carpathian-Pannonian region (CPR) using an increased<br>amount of splitting data, and compare the results with seismic<br>properties reported from mantle xenoliths to characterize the depth,<br>thickness, and regional differences of the anisotropic layer in the<br>mantle.<br>According to the combined SKS and xenolith data, mantle anisotropy is<br>different in the northern and the central/southern part of the western<br>CPR. In the northern part, the lack of azimuthal dependence of the fast<br>split S-wave indicates a single anisotropic layer, which agrees with<br>xenolith data from the Nógrád-Gömör volcanic field. In the central<br>areas, multiple anisotropic layers are suggested by systematic azimuthal<br>variations in several stations, which may be explained by two,<br>petrographically and LPO-wise different xenolith subgroups described in<br>the Bakony-Balaton Highland. The shallower layer is suggested to have a<br>‘fossilized’ lithospheric structure, which could account for the<br>occasionally detected E-W fast S-orientations, whereas the deeper one<br>reflects structures responsible for the regional NW-SE orientations<br>attributed to the present-day convergent tectonics. In the Styrian<br>Basin, results are ambiguous as SKS splitting data hints at the presence<br>of multiple anisotropic layers, however, it is not supported clearly by<br>xenolith data.<br>Spatial coherency analysis of the splitting parameters put the center of<br>the anisotropic layer at ~140-150 km depth under the Western<br>Carpathians, which implies a total thickness of ~220-240 km. Thickness<br>calculated from seismic properties of the xenoliths resulted in lower<br>values on average, which may be explained by heterogeneous sampling by<br>xenoliths, or the different orientation of the mineral deformation<br>structures (foliation and lineation).</p>