Boundaries of Microcontinents Revealed By Changes of Seismic Anisotropy In The Mantle Lithosphere

The alignment of olivine crystals is the dominant source of large-scale seismic anisotropy in the subcrustal lithosphere and asthenosphere. Both the surface waves and body waves indicate "frozen-in" dipping anisotropic structures in the mantle litho- sphere and a mostly sub-horizontal anisotropy due to the present-day flow in the as- thenosphere. From lateral variations and magnitude of the observed seismic anisotropy we deduce a "frozen-in" origin of the continental mantle fabric created most probably in a stress field acting during the formation of individual lithosphere domains. The dipping structures may represent remnants of accreted pieces of ancient oceanic litho- sphere and paleosubductions inferred also from dipping seismic reflectors extending into the mantle. Joint analysis of shear-wave splitting parametres and patterns of tele- seismic P-residual spheres provide a good resolution for identification of boundaries of lithospheric domains with different fabric orientations, e.g., in the Variscan massifs of central and western Europe and in the Baltic Shield. The domains characterised by consistent orientation of anisotropies probably represent individual cycles of "accre- tion" process. Boundaries of these domains are zones of potential weakness for later processes like rifting, volcanism and intraplate earthquakes. While most methods of continental reconstructions study the uppermost crust, seismic anisotropy allows us to contribute to modelling of the structure and development of the mantle lithosphere which is in many aspects decisive for crustal tectonics.