LAB as Boundary Between Fossil and Present-day Mantle Seismic Anisotropy

Besides the importance of the crust-mantle boundary discovered by Mohorovicic hundred years ago, the significance of the first-order active upper mantle boundary between the lithosphere and asthenospere has been increasing during several last decades, after a general acceptance of the Earth’s plate-tectonic concept. Topology of the lithosphere-asthenosphere boundary (LAB) and structure of the continental lithosphere record the geodynamic development of outer parts of the Earth. Knowledge of the Moho relief and of crustal velocities is crucial for the LAB and lithosphere modelling, as the upper mantle studies require applying proper crustal corrections. We present a uniform updated model of the European lithosphere-asthenosphere boundary recalculated from data collected during our regional studies of seismic anisotropy and other tomographic experiments, and show results of mapping of large-scale domains of mantle lithosphere characterized by uniform fossil fabrics. Thanks to a long memory of the fabric of the deep continental lithosphere, we define the LAB as a boundary between a fossil anisotropy in the mantle lithosphere and an underlying seismic anisotropy related to present-day flow in the asthenosphere. Analysis of static terms of teleseismic P-wave travel time deviations shows the LAB topology is more distinct beneath the Phanerozoic part of Europe compared with its Precambrian part. The LAB deepens down to ~220 km beneath the two Alpine roots, the South Carpathians and eastward of the Trans-European Suture Zone. Decomposition of the relative residuals into the static and directional-dependent terms of P residuals, shear-wave splitting analysis and joint inversion of the anisotropic parameters, reveal the mantle lithosphere consists of domains with consistent olivine fabrics, which can be modelled by peridotite aggregates with plunging foliation or lineation. Changes of the fossil fabric allow us to map the domain boundaries within the mantle lithosphere. We propose the observed fossil fabric could be created by a stack of successive subductions, by accretions of micro-continent fragments outboard of continental cratons and by a gradual stabilization of the LAB by a mantle flow after a detachment of lower parts of subducting slabs. We advocate importance of combining different methods and data in the Earth structure studies and applying 3D approaches considering seismic anisotropy with general orientation of symmetry axes leading to more realistic models of the large-scale lithosphere structure.