Moho depth and Poisson's ratio in the Western-Central Alps from receiver functions

Current knowledge about deep crustal structure of the Alpine orogen has mainly been derived from P-wave velocity models obtained from active and passive seismic experiments. A complementary S-wave model to provide lithological constraints necessary for unique structural interpretation has been missing to date. In this paper, we present important new information on S-wave velocity model in the Alps. We applied the receiver function method using 6 yr of high quality data from 61 permanent and temporary stations sampling the Western-Central Alps. We determined first-order crustal features Moho depth (H) and average Vp/Vs ratio (kappa) with the H-kappa stacking technique that uses timing of direct and multiple P-to-S converted phases from the Moho interface. Synthetic tests reveal a dipping Moho interface, expected beneath an orogen, causes a systematic bias of H and kappa potentially leading to misinterpretation. We thus applied corrections determined from synthetic data to remove the bias, providing better fit of recovered Moho depths with active seismic estimates. For each site, we also obtained independent H and kappa estimates based on the timing of the strong Ps-phase. Our results show a gently south-southeast dipping European Moho at a depth of similar to 24-30 km beneath the Northern Alpine Foreland, steepening rapidly towards the Europe-Africa suture zone to reach a maximum depth of similar to 55 km. South of the suture, the Moho of the Adriatic crust, promontory of the African plate, is at similar to 35-45 km depth. In the previously ill-constrained Western Alps, we found the European Moho at similar to 30 km depth beneath the more external units dipping east-northeast to reach similar to 50-55 km in the inner core of the Alps. The Poisson's ratio clearly correlates with the tectonic units that comprise the Alps. Average crustal values in the European Alpine Foreland are close to 0.25. In the Alps, we observe low values (0.22) in the highly deformed nappes of the Mesozoic Helvetic and Southern Alps indicating a thickening of felsic upper-crustal material. In contrast, the Poisson's ratio is significantly higher (0.26) in the Penninic and Austroalpine units near the suture zone. This rapid and significant change marks a clear rupture between the Alpine forelands and the suture domain. We assign this high Poisson's ratio to doubling of mafic lower crust consistent with results from previous active seismic experiments. A continuation of the lower crustal wedge into the central part of the Western Alps, however, seems unlikely based on low observed Poisson's ratios.