Physical and Compositional Characterization of the Greater Alpine Crust Using Seismic Observables

The crust, despite being the Earth's outermost layer, remains extremely challenging to study given its heterogeneity and complexities. High-resolution integrated tomographic studies at various scales are essential to indirectly obtain robust information on the compositional and physical properties of the crust. The Greater Alpine Crust (GAC), shaped by the Hercynian, Alpine, and Apennine orogenies, provides a natural laboratory for studying geodynamic processes at plate boundaries. These orogenies have driven the continuous evolution of the European crust from the Paleozoic era to the present day.

This study aims to obtain robust seismic constraints withing the GAC to assess lateral physical and compositional variations. Our approach primarily relies on phase velocity measurements of Rayleigh and Love waves derived from Ambient Noise (AN) tomography, and compressional-to-shear wave velocity ratio (Vp/Vs) and crustal thickness measurements obtained from Receiver Functions (RF). We then use a thermodynamic model, together with independent constraints such as petrology and heat flow data to make interpretations in terms of compositional variation of the crust.

46,041 Rayleigh and 40,028 Love dispersion curves were calculated, and maps of phase velocities were obtained from 3 to 35 seconds. Shear-wave velocity (Vs) maps were derived from surface-wave phase velocity measurements, via a Neighborhood Algorithm. The Molasse, Pannonian, Po, Adriatic, and Tyrrhenian basins are characterized by low Vs (< 2.8 km/s) down to 3 km depth. The Po and Adriatic basins are recognized as low velocity zones down to 10 km depth, with velocities below 3.5 km/s. From 15 km depth the highest velocities of the GAC are in the Tyrrhenian basin (> 4.4 km/s), where the Moho is shallow, while the continental crust presents velocities around 3.5 km/s. From 30 km depth the roots of the Alps, Apennines, Dinarides and Carpathians are clearly visible as relative low velocity zones.

Earthquake data recorded from 2015 to 2023 with a minimum magnitude of 5.5 and a maximum of 8.5, with epicentral distances from 25 to 95 degrees of the center of the study area, were used to calculate P-wave RFs at more than 400 seismic stations using iterative deconvolution. H-κ analysis was performed, using a Moho calculated from AN as a prior. Vp/Vs ratio and crustal thickness were obtained beneath each station. Within our study area, the Moho is deepest under the Alps, Apennines and Dinarides (> 50 km), and shallowest under the Hercynian basement and the sedimentary basins. The lowest Vp/Vs are found in the Moldanubian and Saxo-Thuringian belts (average ~1.70), while the sedimentary basins, and the Alpine and Apennine belts present the largest and most variable Vp/Vs (average ~1.78).

Finally, a comprehensive interpretation of crustal composition and temperature was conducted, integrating constraints from petrological data, heat flux measurements, and thermodynamic modeling. This approach resulted in a new, robust physical and compositional characterization of the GAC.