Seismic upper mantle anisotropy beneath the Polish Sudetes from SKS splitting — Effect of present-day asthenospheric mantle flow deflected by cratonic keel?

Abstract

To better characterize the structure and evolution of the upper mantle beneath the Polish Sudetes (NE margin of the Variscan internides), we performed shear-wave splitting analysis using teleseismic data from 23 temporary and 8 permanent broadband seismic stations from a period of about two years. SKS and SKKS phases were analysed using the minimum energy and the rotation-correlation method to determine average splitting parameters (φ, δt), characterizing orientation and strength of mantle anisotropy. The observations were interpreted in terms of one-layer anisotropy with a horizontal symmetry axis. The average fast polarization azimuth for this region is equal to ∼113°, and the delay time to ∼1.1 s. The results document a large-scale trend of the upper mantle fabric with NW-SE to WNW-ESE orientation, coinciding with fast velocity axes obtained from the shallower lithospheric studies of the Pn and Pg phases. At smaller scale, anisotropy parameters seem to vary between the SW and NE regions, suggesting moderate differences in orientation of the mantle fabric. Small-scale variations are likely to reflect anisotropy changes related to differences between lithospheric blocks, while more uniform large-scale trend may be related with fabric induced by asthenospheric flow. Estimated thickness of the anisotropic layer exceeds the thickness of the mantle lithosphere, supporting plausibility of substantial asthenospheric contribution. According to previous studies, the large-scale trend of fast polarizations observed in the Sudetes extends also over broader area located to the SW of the Teisseyre-Tornquist Zone, independently of the orientation of the tectonic sutures and of lithospheric inhomogeneity, suggesting its asthenospheric origin. The trend runs obliquely to the plate motion direction, but is sub-parallel to the NW-SE-striking margin of thick East European Craton lithosphere located to the NE of the study area, supporting the concept of the mantle flow re-oriented to ∼NW-SE direction by a thick cratonic keel.