High-Resolution 3D Imaging of Crustal and Upper Mantle Structure in the Alps from Full Waveform Inversion of Teleseismic P Waves

Abstract

The northward movement of the African and European plates since the Late Cretaceous has led to the slanted subduction of the Tethys oceanic lithosphere beneath the Adriatic microplate, followed by an asynchronous continental collision between the European plate and continental microplates (Iberia and Adria) during the Cenozoic. This dynamic interaction has given rise to the creation of intensely deformed mountain chains, encompassing the Alps, Apennines, Dinaric, and Carpathian ranges. Furthermore, the convergence of these colliding continental plates triggers crustal shortening, playing a substantial role in the development of orogenic systems and mountains. This process has accreted crustal regions with distinctive properties, resulting in the formation of intricate and varied tectonic units. The objective of this study is to develop high-resolution seismic models of the crust and upper mantle in the Alps, considering P-wave velocity (VP), S-wave velocity (VS), and density. This is achieved with Full Waveform Inversion (FWI) method, utilizing teleseismic earthquakes recorded by the European permanent seismological broadband stations and supplemented by data from temporary stations, including AlpArray, SWATH-D, and CIFALPS2. We employed a semi-automated data selection method, incorporating a rigorous process to ensure data quality. We built a P-wave dataset for Full Waveform Inversion (FWI) that encompasses approximately 91 teleseismic events, whose magnitude (MW) range from 6 to 7.4. These events are characterized by depths less than 20 km or exceeding 120 km. We used the AK135 velocity model as the initial model in our inversion process and applied iterative inversions on the Z, N, and E components of P-waves. The P-waves were filtered within the 6-25 second period range. The optimization algorithm utilized the limited-memory BFGS. The time windows considered during the inversion process were set to 40 seconds (20 seconds before the P-onset). We derived comprehensive models for VP , VS, and density beneath the Alps, enabling us to investigate the lithospheric and upper mantle structures beneath the Western, Central, and Eastern Alps simultaneously. Our models effectively capture key Alpine features, including the thick low-velocity sedimentary basins of Molasse Basin (MB), the Po Basin (PB), and the Southeast-France Basin (SFB), alongside the high-velocity Ivrea body (IB). Moreover, we identify small high-velocity anomalies in the Central and Eastern Alps along the Periadriatic line, corresponding with Permian magmatic rocks observed in these areas. Our model depicts the underthrusting of the low-velocity European crust beneath the Ivrea body mantle wedge in the southwestern Alps and investigates its variation along the strike of the Alps. Additionally, we prepare a Moho topography map from the VS model by considering the iso-velocity of 4.3 km/s as a prox