Structure of the European upper mantle revealed by adjoint tomography

Images of the European crust and upper mantle, created using seismic tomography, identify the Cenozoic Rift System and related volcanism in central and western Europe. They also reveal subduction and slab roll back in the Mediterranean-Carpathian region(1-12). However, existing tomographic models are either high in resolution, but cover only a limited area(13,14), or low in resolution, and thus miss the finer-scale details of mantle structure(5,12). Here we simultaneously fit frequency-dependent phase anomalies of body and surface waveforms in complete three-component seismograms with an iterative inversion strategy involving adjoint methods, to create a tomographic model of the European upper mantle. We find that many of the smaller-scale structures such as slabs, upwellings and delaminations that emerge naturally in our model are consistent with existing images. However, we also derive some hitherto unidentified structures. Specifically, we interpret fast seismic-wave speeds beneath the Dinarides Mountains, southern Europe, as a signature of northeastward subduction of the Adria plate; slow seismic-wave speeds beneath the northern part of the Rhine Graben as a reservoir connected to the Eifel hotspot; and fast wave-speed anomalies beneath Scandinavia as a lithospheric drip, where the lithosphere is delaminating and breaking away. Our model sheds new light on the enigmatic palaeotectonic history of Europe.