Two subduction-related heterogeneities beneath the Eastern Alps and the Bohemian Massif imaged by high-resolution P-wave tomography

We present high-resolution tomographic images of the upper mantle beneath the Eastern Alps and the adjacent Bohemian Massif (BM) in the north based on recordings from the AlpArray-EASI and AlpArray seismic networks. The tomography locates the Alpine high-velocity perturbations between the Periadriatic Lineament and the Northern Alpine Front. The northward-dipping lithosphere is imaged down to ∼ 200–250 km of depth, without signs of delamination. The small amount of crustal shortening compared to that in the Western Alps and the bimodal character of the positive perturbations with a separation beneath the Tauern Window indicate a dual source of the velocity heterogeneity, most probably formed by a mixture of a fragment of detached European plate and the Adriatic plate subductions. A detached high-velocity heterogeneity, sub-parallel to and distinct from the Eastern Alps heterogeneity, is imaged at ∼ 100–200 km beneath the southern part of the BM. We associate this anomaly with the western end of a SW–NE-striking heterogeneity beneath the south-eastern part of the BM, imaged in models of larger extent. The strike, parallel with the Moldanubian–Brunovistulian mantle–lithosphere boundary in the BM and with the westernmost part of the Carpathian front, leads us to consider potential scenarios relating the heterogeneity to (1) a remnant of the delaminated European plate, (2) a piece of continental-and-oceanic lithosphere mixture related to the building of the BM, particularly to the closure of the old Rheic ocean during the MD–BV collision, or (3) a lithospheric fragment going through to the NW between the Eastern Alps and Western Carpathians fronts in a preceding subduction phase. The study is dedicated to our outstanding and respected colleague Vladislav Babuška, who coined innovative views on the European lithosphere and died on 30 March 2021