European-Adriatic plate collision in teleseismic tomography of the Eastern Alps

We present potential scenarios of the European and Adriatic plates’ collision that formed the Alps and the neighbouring mountain belts. Our results are based on teleseismic body-wave data from the AlpArray-EASI complementary experiment (2014-2015, Hetényi et al., Tectonophysics 2018) and the AlpArray Seismic Network (Hetényi et al., Surv. Geophys. 2018).  Tomography of seismic velocities in the upper mantle along a ca. 200 km broad and 540 km long north-south transect images steady southward thickening of the lithosphere beneath the Bohemian Massif  and northward dipping East-Alpine lithospheric keel. Thanks to the dense spacing of the AlpArray Seismic Network stations and high-quality data, the high-resolution tomography resolves for the first time two sub-parallel down-going high-velocity heterogeneities beneath the Eastern Alps, instead of a single, thick anomaly. The southern heterogeneity, which we relate to the subducted Adriatic plate, is more distinct than the northern one, which loses its connection with the shallow parts. Moreover, amplitudes and size of this heterogeneity decrease in cross-sections perpendicular to the strike of the Alps when shifting towards the Central Alps. The presented collision scenarios consider the smaller northern heterogeneity as (1) a remnant of a delaminated early phase subduction of the European plate with the reversed polarity relative to that in the Western Alps, (2) a piece of continental and oceanic lithosphere together, or, (3) a fragment of a quite extended lithosphere margin foundering in a preceding phase of the Adriatic subduction.