Europe from the bottom up: a statistical examination of the Central and Northern European lithosphere-asthenosphere boundary from comparing seismological and electromagnetic observations

The Lithosphere-Asthenosphere Boundary (LAB) is a fundamental boundary in the plate tectonic paradigm - it is the most pervasive on the planet, yet comparatively it is one we know little about. Defined initially on the basis of the mechanical response of the Earth to loading, its usage has become ubiquitous across the geosciences but the natural differences in its definition, due to differences in thermal, physical and chemical parameters, cause confusion. To advance this debate, comparisons are made both qualitatively and quantitatively, between the delineation of the LAB for Europe based on seismological and electromagnetic observations. We examine statistically, using robust methods, the LABs derived from independent datasets and methods.Essentially, all definitions of the LAB, as an impedance contrast from receiver functions (sLABrf), a seismic anisotropy change (sLABa) and an increase in conductivity from magnetotellurics (eLAB), are consistent with a deeper LAB beneath Precambrian Europe, and a shallower LAB beneath Phanerozoic Europe, with some exceptionally deep regions in Phanerozoic Europe, such as the Alps. All three LABs increase in depth significantly and rapidly at a location consistent with the surface expression of the Trans-European Suture Zone (TESZ) which separates Precambrian Europe to the north and east from Phanerozoic Europe in the centre. Two of the definitions, sLABrf and eLAB, are consistent for Phanerozoic Europe with mean values of 90-100. km, compared to an average sLABrf of 135. km. A different two, sLABa and sLABrf, are consistent for Precambrian Europe with mean values of 170-180. km compared to an eLAB mean of 250. km. The deeper eLAB depths for Precambrian Europe are more consistent with body wave and surface wave tomography models than are the sLABa and sLABrf ones, suggesting that the lower lithosphere beneath Precambrian Europe between 170 to 250. km is responding to shearing on the base of the lithosphere from plate driving forces. Taken together the definitions provide strong constraints on the nature of the LAB. For example, the electrical eLAB beneath the TESZ is anomalously thick, to possibly the transition zone, whereas sLABa and sLABrf seismological definitions would place it at far shallower depths. Given the sensitivity of electrical conductivity to partial melt and water content, the thick eLAB beneath the TESZ excludes interpretation of the seismic LAB in that location in terms of a thermal structure that would induce partial melting or of hydration.