Trench basins related to an almost entirely subducted oceanic lithosphere are filled by deposits sourced from either the passive continental margin or the accretionary wedge. They also include large-scale submarine landslides often triggered by the inherited rough topography of the subducting plate. This mechanism provides the deposition of material by mass-transport processes from the wedge to the trench, resulting in highly heterogeneous trench sedimentation. The role of the mass-transport deposits in the accretion mechanisms, as well as their influence on seismic behaviour at the plate interface is difficult to assess in the active trenches, because of their limited accessibility. This study reports on a well-preserved fossil trench-fill sequence exposed in the Northern Apennines. A multidisciplinary survey including fieldwork, stratigraphy, biostratigraphy, geochemistry, and zircon provenance, allows reconstructing the anatomy of these deposits. The heterogeneities derived by distinct sediment packages made of mass-transport deposits and deep-sea turbidites reflect a setting with two feeding source areas: the accretionary wedge and the continental margin, respectively. Alongside the roughness of the subducting plate, this deposits association favours coherent underplating processes developed at depth. Such accretion mechanisms, linked to thrust faults at the base of the wedge, can generate both major earthquakes and new mass movements, reinforcing the complex role of trench-fill processes in subduction dynamics.

In the Northern Apennines, the Internal Ligurian Units are considered deformed and metamorphosed fragments of the Ligure-Piemontese oceanic basin. In this paper, we report on the temperature and pressure conditions of the metamorphic peak for four Internal Ligurian Units, estimated using different geothermometers and geobarometers based on the white mica and chlorite compositions. These minerals were formed during the D1 deformation phase in the pre-Oligocene. The results indicate that the Portello and Gottero units are both characterized by metamorphic conditions pertaining to low blueschists facies, while the Colli-Tavarone and Bracco-Val Graveglia Units show a lower metamorphic imprint that produces assemblages of prehnite-pumpellyite facies. The estimated geothermal gradient for the metamorphic peak achieved by the analyzed Internal Ligurian Units during the D1 phase is 7–15 °C/Km, which is indicative of deformation in a subduction setting. Under these conditions, the D1 phase developed in these units as a result of underplating at the base of the accretionary wedge during the closure of the Ligure-Piemontese basin. These data indicate a close geodynamic correlation among the Internal Ligurian Units and the ophiolite-bearing units of the Alps.

In the Western Alps, while many P–T constraints are available for the high‐grade units, the dataset for the low‐grade units located along the westernmost edge of the belt remains scarcer and incomplete. To face these uncertainties, we quantify for the first time the metamorphic peak conditions of the Marguareis Unit, a fragment of European margin (Briançonnais Domain) regarded till now as very‐low‐grade unit, exposed along the boundary between Maritime and Ligurian Alps (South‐Western Alps), whose deformation history is well constrained. The Marguareis Unit is regarded as a thinned portion of the European continental margin involved in the Alpine collision. Metamorphic peak conditions reconstructed using the chlorite–phengite multi‐equilibrium approach on the Eocene siliciclastic deposits indicate that the Marguareis Unit recorded high‐pressure and low‐temperature conditions during its subduction, accretion and subsequent exhumation into the Alpine wedge.

Along the Western Alps, the oceanic units showing blueschists to eclogite facies metamorphic imprint are classically regarded as fragments of the Ligurian-Piedmont Ocean. These units recorded a strongly deformation related to their subduction, accretion and subsequent exhumation into the Alpine wedge, developed during the convergence between the Europa and Adria Plates. However, some of these units, for example the Moglio-Testico Unit, are less pervasively deformed, providing evidence of their sedimentary evolution as well as the tectono-metamorphic history. Therefore, we present original stratigraphic, structural and thermo-barometric data to characterize the tectono-metamorphic history and the sedimentary evolution of the Moglio-Testico Unit, performing different techniques including fieldwork, structural analysis and chlorite-phengite multiequilibrium thermobarometry. Our dataset indicates that the Moglio-Testico Unit can be considered as a fragment of oceanic cover whose sedimentary evolution reflects that of a portion of oceanic lithosphere approaching to the subduction zone. Structural analysis combined with the thermobarometry indicate that this unit recorded a polyphase deformation history developed under High Pressure-Low Temperature metamorphic conditions (D1: 1.2–1.0 GPa and Tpeak: 330–260 °C; D2: 0.4–0.7 GPa and 230–170 °C) during its underthrusting, accretion into the Alpine wedge and subsequent exhumation up to the shallower crustal levels.

We investigated the kinematics of the coupled Helminthoid Flysch–Marguareis Unit outcropping in the High Tanaro Valley in order to constraint the tectonic evolution of the Western Ligurian Alps. Field mapping highlights a tectonic pile composed of, from top to bottom: the Marguareis Unit (Briançonnais Domain); the Helminthoid Flysch Unit (Ligurian Domain) and several continental-derived tectonic slices (Chambeuil Slices) showing affinity to the Briançonnais Domain. Each unit recorded different deformation histories represented by pre-, syn- and post-stacking structures. The syn-stacking structures are represented by thrusts and folds, both responsible for the coupling of the units and for their current structural relationships. In other sectors of the Alpine chain previous authors have highlighted similar structural architectures. We maintain that syn-stacking deformation events played a key role in the tectonic evolution of the Western Ligurian Alps.