Depth variation of seismic anisotropy and petrology in central European lithosphere: A tectonothermal synthesis from spinel lherzolite

Spinel Iherzolite xenoliths from the Neogene Kozakov volcano in central Eur ope, yielding temperatures from 680 degreesC to 1065 degreesC and estimated to originate from depths of 32 to 70 km, provide an exceptionally continuo us record of the depth variation in seismic and petrological properties of subcontinental lithospheric mantle. Extraction depths of the xenoliths and thermal history and rheological properties of the mantle have been evaluate d from a tectonothermal model for basaltic underplating associated with Neo gene rifting. The chemical depletion of sub-Kozakov mantle decreases with d epth, the Mg number in olivine decreasing from similar to 91.4 to 90.5 and the Cr number in spinel, decreasing from similar to 38.9 to 14.7. Texturall y, the sampled mantle consists of an equigranular upper layer (32-43 km), a n intermediate protogranular layer (43-67 km), and a lower equigranular lay er (below 67 km). Olivine petrofabrics show strong axis concentrations, whi ch change with depth from orthorhombic symmetry in the equigranular upper l ayer to axial symmetry in the lowermost layer. Calculated compressional and shear wave anisotropies, which average 8% and 6%, respectively, show signi ficant depth trends that correlate with variations in depth of olivine fabr ic strengths and symmetries. Comparisons of the xenolith anisotropies with field observations of Pn anisotropy and SKS shear wave splitting in the reg ion suggest that foliation is horizontal in the upper layer of the lithosph eric mantle and vertical in the middle and lower layers. The depth variatio n in mantle properties and complexity in central Europe is the result of De vonian to Early Carboniferous convergence, continental accretion, and crust al thickening, followed by Late Carboniferous to Permian extension and grav itational collapse and final modification by Neogene rifting, thinning, and magmatic heating.