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

Abstract

S U M M A R YCurrent 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 (κ) with the H-κ 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 κ 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 κ estimates based on the timing of the strong Ps-phase. Our results show a gently south-southeast dipping European Moho at a depth of ∼24-30 km beneath the Northern Alpine Foreland, steepening rapidly towards the Europe-Africa suture zone to reach a maximum depth of ∼55 km. South of the suture, the Moho of the Adriatic crust, promontory of the African plate, is at ∼35-45 km depth. In the previously ill-constrained Western Alps, we found the European Moho at ∼30 km depth beneath the more external units dipping east-northeast to reach ∼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.

Constraints on the structure of the crust and lithosphere beneath the Azores Islands from teleseismic receiver functions

“…Wölbern & Rümpker (2017) showed that the method can produce ambiguous results and that, in general, the resulting Vp/Vs ratios are less reliable than the depths estimates. It has been further demonstrated that departures from an assumed 1-D horizontally layered model can yield flawed Hk-stacking result (Julià 2007;Lombardi et al 2008). Ocean islands pose additional problems: reverberations of converted phases beneath an island might be reflected at the seafloor surrounding the island.…”

Erosion-driven uplift of the modern Central Alps

“…The crust of the Central and Western Alps is between 40 to 60 km thick as inferred from depth determinations of the Moho (Waldhauser et al, 1998;Touvenod et al, 2007;Lombardi et al, 2008). A full isostatic compensation of such a thick crustal root would imply a mean elevation between ~1500m and ~3700m (Airy hypothesis).…”

Crustal thickness and images of the lithospheric discontinuities in the Gibraltar arc and surrounding areas

“…0.25 km s −1 ) leading to relatively minor bias in depth for typical crustal thickness values (∼1 km) and v P /v S ratio (∼0.03) (Lombardi et al 2008). In addition, the vertical resolution of the receiver functions is controlled by the wavelength of the Ps-converted phase, which depends on the spectral content of the recorded signal and the velocity of the medium.…”

“…Applying to this ideal backazimuth distribution the Zhu & Kanamori (2000) method results in a crustal thickness of 26.3 km (underestimation of 3.7 km) and a Vp/Vs of 1.82 (overestimation of 0.09). In general, the application of this Zhu & Kanamori (2000) method to dipping structure leads to an underestimation of the Moho depth and an overestimation of the Vp/Vs ratio (see Lombardi et al (2008) for a discussion about Zhu & Kanamori (2000) method and dipping Fig. 1).…”