The European Alps are the site where classic geologic concepts such as nappe theory, continental subduction and slab breakoff have been first proposed. However, the deep tectonic structure of the Alps has long been poorly constrained by independent geophysical evidence. This review paper summarizes the main results of the CIFALPS passive seismic experiment, that was launched by Chinese, French and Italian scientists in the 2010s to provide new insights on the deep tectonic structure of the Alpine region. The application of a wide range of tomographic methods to the analysis of a single fossil subduction zone makes the CIFALPS experiment a potential reference case for the analysis of other orogenic belts. Major results include: (i) the first seismic evidence of European continental crust subducted into the Adriatic upper mantle, beneath the place where coesite was first recognized in continental (U)HP rocks in the Alps; (ii) evidence of a major involvement of the mantle wedge during (U)HP rock exhumation; (iii) evidence of a serpentinized plate interface favoring continental subduction; (iv) evidence of a continuous slab beneath the Western and Central Alps, ruling out the classic model of slab breakoff magmatism; (v) evidence of a polyphase development of anisotropic fabrics in the Alpine mantle, either representing active mantle flows or fossil fabrics inherited from previous rifting stages. Detection of these major tectonic features allows to propose interpretive geologic cross‐sections at the scale of the lithosphere and upper mantle, providing a baseline for future analyses of active continental margins.

Quantifying geological processes has greatly benefited from the development and use of thermochronometric methods over the last fifty years. Among them is the (U-Th)/He dating method, which is based on the production and retention, within a crystal structure, of radiogenic ⁴He atoms associated with the alpha decay of U, Th and Sm nuclei. While apatite has been the main target of (U-Th)/He studies focusing on exhumation and burial processes in the upper levels of the continental crust (~50–120 °C), the development of (U-Th)/He methods for typical phases of igneous and metamorphic rocks (e.g., zircon and titanite) or mafic and ultramafic rocks (e.g., magnetite) over the last two decades has opened up a myriad of geological applications at higher temperatures (>100–300 °C). Thanks to the understanding of the role of radiation damage in He diffusion and retention for U-Th-poor and rich mineral phases, the application of (U-Th)/He thermochronometry to exhumation processes and continental evolution through deep time is now mainstream. This contribution reviews the (U-Th)/He thermochronometer principle and the influence of radiation damage in modifying the diffusion behavior. It presents applications of (U-Th)/He dating to problems in tectonic and surface processes at shallow to middle crustal depths (>100–300 °C). New and promising applications using a combination of methods will stimulate a research avenue in the future.

In collision belts, the first-order role of the mantle in localizing deformation has remained elusive, as the resolution of geophysical imaging remains too low to constrain crustal geometry. To address this issue, we geologically interpret a recent high-resolution shear-wave velocity model from ambient-noise tomography of Western Alps. We show that the lower crustal Alpine geometry is highly variable at depth, evolving from a preserved European crustal slab in the South to a smooth crustal root in the North. Moho morphology is controlled by numerous pre-existing major faults reactivated during the Alpine orogeny. Two mantle indenters located above the subducted European plate at different depths appear to control the locus of active deformation. The rigid nature of Adria mantle explains the localization of brittle deformation that is transferred towards the upper crust. The strain-field partitioning results in a combination of strike-slip with either shortening or extension controlled by the anticlockwise rotation of Adria.

We derive a three‐dimensional shear‐wave velocity model of the Ligurian‐Provence back‐arc basin (Northwestern Mediterranean Sea) using ocean‐bottom seismometers (AlpArray OBSs) and land stations from permanent and temporary seismic networks. The quality of OBS continuous records is enhanced by a specific processing that reduces instrumental and seabed‐induced noises (transients, tilt, compliance). To further improve the resolution of ambient‐noise tomography in the offshore area, we compute the Rayleigh‐wave part of the Green functions for OBS‐OBS pairs by using onshore stations as virtual sources. 2‐D group‐velocity maps and their uncertainties are computed in the 4–150 s period range by a transdimensional inversion of Rayleigh‐wave travel times. The dispersion data and their uncertainties are inverted for a probabilistic 3‐D shear‐wave velocity model that includes probability densities for V_s and for the depth of layer interfaces. The probabilistic model is refined by a linearized inversion that accounts for the water layer in the Ligurian Sea. Our S‐wave velocity and layer boundary probability models correspond well to a recent, high‐resolution P‐wave velocity cross‐section derived from controlled‐source seismic profiling along the Ligurian‐Provence basin axis. A joint interpretation of the P‐ and S‐wave velocity sections along this profile reveals a thin, anomalous oceanic crust of low P‐wave velocities but high S‐wave velocities, intruded by a gabbroic body. The illuminated part of the upper mantle appears to be devoid of serpentinization.

Between August 2017 and March 2019, an intense seismic swarm was recorded in the Maurienne valley in the north of the Belledonne massif (Western French Alps). In order to study the spatiotemporal evolution of the Maurienne swarm, we created a high‐resolution catalog based on template matching, double‐difference relocation, and moment magnitudes. The catalog includes 71,064 events with a maximum moment magnitude of 3.5 and a magnitude of completeness of 0.7. The seismic activity is interpreted as the reactivation of an N80 strike‐slip fault system called Fond de France. Moreover, earthquake relocation reveals the presence of a shallower fault system with the same strike, but opposite dip direction and smaller size. The presence of two distinct fault systems may explain the observed variation of the b‐value with depth. The seismicity migrated asymmetrically in all directions during the course of about 15 months. Shorter migrations lasting 2–3 days are also observed. The different migration patterns suggest that the swarm is driven by several mechanisms, possibly pore‐pressure diffusion, aseismic slip, and earthquake interactions.