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

Abstract:

The Carpathian-Pannonian region (CPR) is the northeastern end of the Alpine mountain belt. This study is aimed at evaluating the seismic anisotropy of the upper mantle beneath the CPR concentrating on the eastward elongation of the fast anisotropic pattern of the Eastern Alps. We use data recorded by the temporary stations, set up for the Carpathian Basin Project (CBP) extending from the Vienna basin through Hungary into Serbia, together with the petrologic indicator of deformation in basalt-hosted upper mantle xenoliths from Szentbékkála. While some of the results agree with another recently published anisotropy analysis of the Eastern Alps and earlier results for the Pannonian basin, some don’t. We reprocess data from 45 temporary CBP stations consisting recorded waveform of teleseismic events with magnitude greater than 6.0 Mw that have occurred in the distance range from 90° to 130° between 2005 and 2007. The dominant fast polarization orientation is NW-SE for the Hungarian part of the stations as well as at the stations located in the easternmost of the Pannonian Basin. This orientation matches well with the anisotropy structure under the Eastern Alps, which has been attributed to asthenospheric flow. The NW-SE trend turns more into WNW-ESE at the stations situated in Austria, mostly NW of the Vienna basin. Apart from predominately NW-SE fast orientation, there are a few measurements showing NE-SW and almost E-W trend. The SKS measurements most likely originate mainly from the asthenosphere and consequently past (Miocene) asthenospheric flow in the upper mantle may still have a significant contribution to the fast azimuth directions, however, the effect of other factors cannot be excluded.

Xenoliths from the deeper lithospheric mantle have LPO structures of minerals as A-type of olivine in which the fast axis azimuth may correspond to the shear direction, possibly present-day asthenospheric flow in the upper mantle. Large delay times (>1.0 s) might also show the contribution of asthenosphere origin of anisotropy. Considering anisotropy magnitude between 5 to 6% from petrological measurements, and 1.0 s delay time would refer to 100 to 75 km thickness of the anisotropy layer.