Seismic lithosphere–asthenosphere boundary beneath the Baltic Shield

Abstract

The problem of the existence of the asthenosphere for old Precambrian cratons is still discussed. In order to study the seismic lithosphere -asthenosphere boundary (LAB) beneath the Baltic Shield, we used records of nine local earthquakes with magnitudes ranging from 2.7 to 5.9. To model the LAB, original data were corrected for topography and Moho depth using a reference model with a 46-km-thick crust. For two northern events at Spitsbergen and Novaya Zemlya, we observe a low-velocity layer, 60 -70-km-thick asthenosphere, and the LAB beneath Barents Sea was found at depth of c. 200 km. Sections for other events show continuous first arrivals of P-waves with no evidence for "shadow zone" in the whole range of registration, which could either be interpreted as the absence of the asthenosphere beneath the central part of the Baltic Shield, or that the LAB in this area occurs deeper (.200 km). The relatively thin low-velocity layer found beneath southern Sweden, 15 km below the Moho, could be interpreted as small-scale lithospheric heterogeneities, rather than asthenosphere. Differentiation of the lower lithosphere velocities beneath the Baltic Shield could be interpreted as regional heterogeneity or as anisotropy of the Baltic Shield lithosphere, with high velocities approximately in the east-west direction, and slow velocities approximately in the south-north direction.

Seismic basement in Poland

“…The edge of the EEC in Poland, the Teisseyre-Tornquist Zone (TTZ), continues in SW Scandinavia as the Sorgenfrei-Tornquist Zone (STZ). This boundary between Baltica and Phanerozoic Europe is well visible in the basement, Moho depth, as well as in lithospheric thickness (e.g., Lassen and Thybo 2012;ScheckWenderoth and Maystrenko 2013;Grad et al 2009Grad et al , 2014Knapmeyer-Endrun et al 2014;Wilde-Piórko et al 2010;Puziewicz et al 2006;Gregersen et al 2002;Geissler et al 2010). The gravity modeling suggests a very small density value in the uppermost mantle 3.11 g/cm 3 below the younger area of WEP, while for the older area of EEC, it is 3.3 g/cm 3 (e.g., Krysiński et al 2009).…”

Mantle Flow and Determining Position of LAB Assuming Isostasy

“…The results obtained using the discussed methods could be verified using other methods; e.g., the depth of the LAB could be verified using seismic methods (e.g., Grad et al 2009Grad et al , 2014Levander and Miller, 2012). However, note that, according to Jones et al (2010), the seismic LAB may sometimes not be the same as the LAB used in the theory of plate tectonics.…”

“…Lithosphere, asthenosphere, and the lithosphereasthenosphere boundary (LAB) are important terms in the physics of the solid Earth, but their meanings are understood differently by different specialists (e.g., Jones et al 2010;Grad et al 2014;Grad 2015, 2018). We apply herein the meaning used in the theory of plate tectonics, where the lithosphere is a mechanically resistant layer divided into tectonic plates, i.e., units that can move relative to each other.…”

The geophysical characteristic of the lower lithosphere and asthenosphere in the marginal zone of the East European Craton

“…Similar LVZ were also observed in other areas of the EEC. Beneath Scania in the southern Baltic Shield, a relatively thin low-velocity layer was found 15 km below the Moho (Grad et al 2014). It can be interpreted as small-scale lithospheric heterogeneity and can be explained by a model with a 16 km thick lowvelocity layer with a P-wave velocity drop of 0.33 km/s.…”