Passive and active seismic studies of lithospheric structure and anisotropy beneath Sudetes (NE Variscides)

The area of Sudetes, located at the margin of the Bohemian Massif, represents the NE-most part of the Variscan internides between the Elbe Fault in SW and the Odra Fault in NE. The lithosphere of the region is a mosaic of several distinct units/terranes with complex tectonic history ranging from the upper Proterozoic till the Quaternary. The crustal and uppermost mantle structure of this region was studied by seismic wide-angle experiment SUDETES 2003 and the results of 2-D isotropic modelling were published. Recently, this dataset, comprising off-line recordings from a net of intersecting profiles, was interpreted using anisotropic delay-time inversion. This resulted in models of 2-D distribution of upper crustal and uppermost mantle anisotropy based on azimuthal variability of the Pg and Pn traveltimes, respectively. The upper mantle of Sudetic region was the target of a passive seismic experiment AniMaLS. The project involved 23 broadband seismic stations deployed in the area of Sudetes and Fore-Sudetic block in SW Poland, supplemented with the data from 6 permanent seismic stations, operating in this area in Czech Republic and Poland. The measurements cover a ~200x100 km large area, with ~30 km inter-station spacing. The stations, deployed for a period of 24 months (2017-2019), provided broadband recordings of local, regional and teleseismic events. The aim of the experiment is to study the structure, seismic velocity variations including anisotropy distribution, and to map the upper mantle seismic discontinuities (Moho, lithosphere-asthenosphere boundary, mantle transition zone). Currently, the AniMaLS data are being interpreted using shear wave splitting method and receiver function method. The analysis of SKS and SKKS splitting was based on cross-correlation, eigenvalue minimization and transverse energy minimization methods. Resulting time delays between slow and fast S-wave components are ~1.2 sec on average, with fast velocity axis oriented largely in WNW-ESE direction, consistently with results of delay-time inversion of Pn phase traveltimes. Crustal anisotropy is characterized by similar fast axis orientation, but with lower amplitude of anisotropy. The orientation of fast axes in the crust and mantle correlates well with surface trends of tectonic units and with strike directions of major fault zones. This suggests vertically coherent deformation throughout the lithosphere, most likely during consolidation of the Sudetic region in Variscan times.