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

Abstract

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 g1/4g200-250gkm 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 g1/4g100-200gkm 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 Babuska, who coined innovative views on the European lithosphere and died on 30 March 2021.