Anisotropic Structure of the Upper Mantle in the Pannonian Basin: From SKS Splitting data and Xenolith Constraints

摘要

The Carpathian-Pannonian region (CPR) is the northeastern end of the Alpine mountain belt. In the Western Alps,udavailable results of anisotropy investigations (SKS splitting) show clear belt-parallel anisotropy. In the Eastern Alpsudthis pattern not only is broken but also does not follow the strike of Carpathians and Dinarides. This study is aimedudat evaluating the seismic anisotropy of the upper mantle beneath the Pannonian Basin concentrating on the eastwardudelongation of the fast anisotropic pattern of the Eastern Alps. We use data recorded by the temporary stations, setudup for the Carpathian Basin Project (CBP) extending from the Vienna basin through Hungary into Serbia, togetherudwith the petrological indicator of deformation in basalt-hosted upper mantle xenoliths from Szentbékkála analyzedudwith the EBSD system at Geosciences Montpellier, France (Kovács et al., 2012). We reprocess data from 45udtemporary CBP stations consisting of recorded waveform of teleseismic events. The minimum energy method isudapplied to measure the splitting delay time and fast axis orientation. The dominant fast polarization orientation isudNW-SE for the Hungarian part of the stations as well as at the stations located in the easternmost of the PannonianudBasin. This orientation matches well with the anisotropy structure under the Eastern Alps, which has already beenudattributed to the asthenospheric flow (Qorbani et al., 2015). The NW-SE trend turns more into WNW-ESE at theudstations situated in Austria, mostly NW of the Vienna basin. Apart from predominately NW-SE fast orientation,udthere are a few measurements showing NE-SW and almost E-W trend. The lithospheric thickness in the CPRudregion is estimated about 60 km, the SKS measurements thus most likely originate mainly from the asthenosphere.udLattice preferred orientation of xenoliths from the deeper lithospheric mantle show A-type fabric of olivine, inudwhich the fast axis azimuth may correspond to the shear direction, possibly present-day asthenospheric flow inudthe upper mantle. Considering anisotropy magnitude between 5 to 6% from petrological measurements, and 1.0uds delay time would refer to 100 to 75 km thickness of the anisotropy layer. We observe larger delay times in theudnorthwest (Alpine-Carpathian connection) and to the external domain of the Pannonian basin (close to surroundingudCarpathian), referring to strong or thicker anisotropy beneath these stations.