Dinarides slab gap - fact or fiction

Ever since the first regional teleseismic tomography images of the central Mediterranean region, one aspect that has stood out in nearly every model is the missing deep slab under the north and central Dinarides. In contrast, concurrent investigations of crustal formation have pointed to a deep crustal root under the whole of the Dinarides, supporting the hypothesis of a laterally continuous slab. In the last decade, several attempts have been made to untangle this conundrum but without much success. Nevertheless, these efforts have yielded some notable new findings, such as possible lithospheric delamination under the central Dinarides. This study aims to utilize all the available seismological results in combination with several new analyses to shed light on the upper mantle structure beneath the central Dinarides. We conducted the SKS shear-wave splitting analysis using 21 stations from the Croatian national seismic network and 7 stations from the AlpArray network. We considered events that occurred between 2010 and 2022 with magnitudes greater than MW = 6.0 and epicentral distances ranging between 85° and 120°. In parallel, a teleseismic Generalized Radon Transform (GRT) migration was conducted along a set of 2D profiles to provide structural insights into the subduction zone within the study area. Data from the Croatian national seismic network, the CRONOS temporary network, and the AdriaArray Temporary Network were used for the migration. For this approach, we considered events that occurred after January 2020 within the epicentral distance range of 30° - 100° and magnitudes greater than MW = 5.5. In addition to these two new analyses, we used other seismological results from previous investigations (including S-receiver functions and ambient noise tomography) to fill in the gaps in our investigation of the lithospheric structure under central Dinarides. Preliminary results exhibit distinctive patterns: the orientation of SKS fast axes, indicative of mantle flow, in the north and central External Dinarides aligns perpendicular to the mountain chain’s strike. However, this orientation abruptly transitions to a NW-SE direction further from the coast and continues in the northern part of Croatia. Results from converted/scattered-waves and ambient noise, for their part, point to a thickened crust under the central and southern External Dinarides, with a high-velocity anomaly reaching at least 100 km depth but a relatively thin lithosphere. Taken together these results suggest that the slab blocks the mantle flow up to depths of 100 – 150 km.