Upper mantle discontinuities: anisotropic view on the lithosphere - asthenosphere system

This paper stems from a keynote "Inge Lehmann Lecture" during the ESC 2018 General Assembly and presents an overview of the historical development of Earth stratification and the corresponding discontinuities to a broader seismological community. The paper concentrates on the role of seismic velocity anisotropy in modelling the lithosphere-asthenosphere system and presents models of the lithosphere-asthenosphere boundary (LAB), the most significant tectonic boundary from the point of view of plate tectonics. With the use of anisotropic characteristics retrieved from both the surface and body waves, we model the LAB discontinuity as a narrow transition between fossil anisotropy within the mantle lithosphere and present-day flow-related anisotropy in the asthenosphere in different tectonic settings within continental and oceanic plates. The lower plate boundary lies in a relatively broad depth interval, from about 50 km beneath Phanerozoic basins up to about 220 km beneath Precambrian cratons and platforms. Beneath the oceanic plates, the discontinuity shallows up to similar to 30 km or even less. Distinct LAB depth variations at short distances occur particularly in Phanerozoic parts of continents.