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.

The western Alpine arc originated during the Cretaceous orogenesis as a consequence of the continental collision between the European and Adriatic pl

The dynamics of continental subduction is largely controlled by the rheological properties of rocks involved along the subduction channel. Serpentinites have low viscosity at geological strain rates. However, compelling geophysical evidence of a serpentinite channel during continental subduction is still lacking. Here we show that anomalously low shear-wave seismic velocities are found beneath the Western Alps, along the plate interface between the European slab and the overlying Adriatic mantle. We propose that these seismic velocities indicate the stacked remnants of a weak fossilised serpentinite channel, which includes both slivers of abyssal serpentinite formed at the ocean floor and mantle-wedge serpentinite formed by fluid release from the subducting slab. Our results suggest that this serpentinized plate interface may have favoured the subduction of continental crust into the upper mantle and the formation/exhumation of ultra-high pressure metamorphic rocks, providing new constraints to develop the conceptual and quantitative understanding of continental-subduction dynamics.

Gephart and Forsyth's method has been applied to estimate stress orientations from earthquake fault plane solutions of the southwestern Alps, a region where the tectonic stress regime is known to be fairly complex. Fault plane solutions have been either taken from the literature or computed using data from local and regional networks. Data refer to seismic events of magnitude in the range 2.5–5.3 which occurred in the last decades at depths between 0 and 25 km. Two zones with a different stress orientation have been identified in the studied area (44.0°–45.5°N, 6.5°–8.5°E): the western zone, corresponding to the crest of the alpine belt, where a high‐dip maximum compressive stress is found, and the eastern zone (Alps chain to Po Plain transition), characterized by an almost horizontal E‐W σ₁ and a nearly vertical σ₃. Hypocenters of earthquakes used for stress inversion lie in the depth ranges 0–15 km and 5–25 km in the western and eastern zones, respectively. The transition between the two stress domains is very sharp, and this is also indicated by space distribution of earthquake individual misfits to the respective stress models. The findings of the present study are a good match for tectonic models which assume E‐W compression derived from the Adria‐Europe interaction and producing: (1) major thrusting processes in the eastern side of the chain and (2) secondary tensional effects at very shallow depth beneath the alpine belt crest (western zone of the area studied in this work).