(1) Geophysical Institute, Academy
of Science of the Czech Republic, Prague
(2) EOST - Institut de Physique du
Globe Strasbourg, France
Systematic preferred orientation of olivine crystals in the continental mantle lithosphere results in large-scale seismic anisotropy as almost ubiquitous property observable by both body- and surface-wave velocities. Orientations of generally dipping (a-c) olivine foliations, derived from spatial variation of P-wave delay times and S-wave splitting, are systematic within large domains which are separated by sutures cutting the entire lithosphere thickness. The transitions between domains with different orientation of anisotropic structures are distinct and relatively narrow, indicating thus that major part of the anisotropy is frozen in the subcrustal lithosphere. The overall pattern of large-scale anisotropy can be modified by minor effects originating in the crust, by deformations along boundaries of colliding blocks and by a present day flow in the asthenosphere. We observe detachments of the rigid upper crust from the mantle lithosphere, which are indicated by lateral shifts across the sutures, e.g., in the Bohemian Massif (BM) and the French Massif Central (MC). Our observations of abrupt changes of seismic anisotropy indicate that the Variscan sutures are at some places burried beneath allochtonous crust detached from its original mantle lithosphere. We suggest that the burried Variscan sutures created a predisposition for the Cenozoic Limagne Graben in the MC and the Eger Graben in the BM and provided open channels for volcanic activity.
Not available.