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

Abstract

Spinel lherzolite xenoliths from the Neogene Kozákov volcano in central Europe, yielding temperatures from 680°C to 1065°C and estimated to originate from depths of 32 to 70 km, provide an exceptionally continuous 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 evaluated from a tectonothermal model for basaltic underplating associated with Neogene rifting. The chemical depletion of sub‐Kozákov mantle decreases with depth, the Mg number in olivine decreasing from ∼91.4 to 90.5 and the Cr number in spinel, decreasing from ∼38.9 to 14.7. Texturally, the sampled mantle consists of an equigranular upper layer (32–43 km), an intermediate protogranular layer (43–67 km), and a lower equigranular layer (below 67 km). Olivine petrofabrics show strong axis concentrations, which change with depth from orthorhombic symmetry in the equigranular upper layer to axial symmetry in the lowermost layer. Calculated compressional and shear wave anisotropies, which average 8% and 6%, respectively, show significant depth trends that correlate with variations in depth of olivine fabric strengths and symmetries. Comparisons of the xenolith anisotropies with field observations of Pn anisotropy and SKS shear wave splitting in the region suggest that foliation is horizontal in the upper layer of the lithospheric mantle and vertical in the middle and lower layers. The depth variation in mantle properties and complexity in central Europe is the result of Devonian to Early Carboniferous convergence, continental accretion, and crustal thickening, followed by Late Carboniferous to Permian extension and gravitational collapse and final modification by Neogene rifting, thinning, and magmatic heating.