Seismic Anisotropy Beneath the Western Part of the Carpathian-Pannonianregion Inferred from Combined SKS Splitting and Mantle Xenolith Studies

Information on mantle anisotropy can be obtained from methods such asstudying the lattice-preferred orientation (LPO) in mantle peridotites,or conducting shear-wave splitting (SKS) analyses which allow todetermine whether it is a single or multi-layered anisotropy and thedelay time of the fast and slow polarized wave can indicate thethickness. In this study we provide a detailed SKS mapping on thewestern part of the Carpathian-Pannonian region (CPR) using an increasedamount of splitting data, and compare the results with seismicproperties reported from mantle xenoliths to characterize the depth,thickness, and regional differences of the anisotropic layer in themantle.According to the combined SKS and xenolith data, mantle anisotropy isdifferent in the northern and the central/southern part of the westernCPR. In the northern part, the lack of azimuthal dependence of the fastsplit S-wave indicates a single anisotropic layer, which agrees withxenolith data from the Nógrád-Gömör volcanic field. In the centralareas, multiple anisotropic layers are suggested by systematic azimuthalvariations in several stations, which may be explained by two,petrographically and LPO-wise different xenolith subgroups described inthe Bakony-Balaton Highland. The shallower layer is suggested to have a‘fossilized’ lithospheric structure, which could account for theoccasionally detected E-W fast S-orientations, whereas the deeper onereflects structures responsible for the regional NW-SE orientationsattributed to the present-day convergent tectonics. In the StyrianBasin, results are ambiguous as SKS splitting data hints at the presenceof multiple anisotropic layers, however, it is not supported clearly byxenolith data.Spatial coherency analysis of the splitting parameters put the center ofthe anisotropic layer at ~140-150 km depth under the WesternCarpathians, which implies a total thickness of ~220-240 km. Thicknesscalculated from seismic properties of the xenoliths resulted in lowervalues on average, which may be explained by heterogeneous sampling byxenoliths, or the different orientation of the mineral deformationstructures (foliation and lineation).