Convergent plate margins are currently distinguished as ‘accretional’ or ‘erosional’, depending on the tendency to accumulate sediments, or not, at the trench. Accretion and erosion can coexist along the same margin and we have noticed that this mostly occurs where subduction is oblique. Here we show that at oblique subduction zones, sediments that enter the trench are first buried, and later migrate laterally parallel to the trench and at various depths. Lateral migration of sediments continues until they reach a physical barrier where they begin to accumulate. The accretionary wedge size decreases along the trench moving away from the barrier. We therefore suggest that the gradual variation of the accretionary wedge size and sediment amount at the trench along one single subduction zone, as observed in many active plate margins worldwide, can be explained by the lateral tectonic migration of sediments driven by obliquity of subduction as well.

Lawsonite eclogites are rare rocks and have been described from only a few localities in the world. Lawsonite-bearing assemblages are highly unstable and physico-chemical processes linked to exhumation may destroy them; only aggregates interpreted as pseudomorphs after lawsonite could be often recognized. In this paper, we present a detailed structural and petrological study of an area in the northwestern sector of the metaophiolitic high-pressure Voltri Massif (Ligurian Western Alps, Italy). The study area is characterized by a lawsonite-bearing eclogitic metagabbro associated with carbonated serpentinites and glaucophanic metasediments. The metagabbro body reached eclogitic metamorphic peak conditions atT= 465–477°C andP= 20.9–24.4 kbar, with H₂O continuously supplied to the system. H₂O under-saturated conditions, with the occurrence of both lawsonite and epidote, characterized the exhumation path. Both the low temperature recorded by the body and the occurrence of variously carbonated serpentinites led us to interpret this area as a portion of the top of the subducted slab, coupled with a ‘cool’ mantle wedge, where both aqueous fluids and carbonate-rich fluids were present. The occurrence of rocks belonging to different paleogeographic domains (e.g. continent versus ocean) and the multiple deformations recorded by the metagabbro suggest that this area was nearby the slab–mantle interface. This sector was thus affected by a shear regime that acted in a low-viscosity serpentinite channel, bringing these high-pressure rocks back to the surface.

A blueschist-facies mylonite crops out between two high-pressure tectono-metamorphic oceanic units of the Ligurian Western Alps (NW Italy). This mylonitic metabasite is made up of alternating layers with different grain size and proportions of blueschist-facies minerals.The mylonitic foliation formed at metamorphic conditions ofT= 220–310 °C andP= 6.5–10 kbar. The mylonite shows various superposed structures: (i) intrafoliar and similar folds; (ii) chocolate-tablet foliation boudinage; (iii) veins; (iv) breccia.The occurrence of comparable mineral assemblages along the foliation, in boudin necks, in veins and in breccia cement suggests that the transition from ductile deformation (folds) to brittle deformation (veining and breccia), passing through a brittle–ductile regime (foliation boudinage), occurred gradually, without a substantial change in mineral assemblage and therefore in the overallP–Tmetamorphic conditions (blueschist-facies).A strong fluid–rock interaction was associated with all the deformative events affecting the rock: the mylonite shows an enrichment in incompatible elements (i.e. As and Sb), suggesting an input of fluids, released by adjacent high-pressure metasedimentary rocks, during ductile deformation. The following fracturing was probably enhanced by brittle instabilities arising from strain and pore-fluid pressure partitioning between adjacent domains, without further external fluid input.Fluids were therefore fixed inside the rock during mylonitization and later released into a dense fracture mesh that allowed them to migrate through the mylonitic horizon close to the plate interface.We finally propose that the fracture mesh might represent the field evidence of past episodic tremors or ‘slow earthquakes’ triggered by high pore-fluid pressure.

Oblique subduction zones are complex settings where the simultaneous action of trench‐normal and trench‐parallel components of convergence can produce heterogeneous deformational pattern of the upper plate and affect the accretional/erosional behavior of the plate margin. Here we present three‐dimensional thermomechanical numerical models that highlight some processes occurring in the early history (15–20 Myr) of intraoceanic oblique subduction zones, which so far represent the less studied case. These models have been compared with a simulation of a slab sinking under a continental plate. We test subduction starting in oceans floored by two classes of lithosphere: layered (fast spreading oceans) and serpentinite rich (slow to ultraslow spreading oceans). Two main domains develop along the margin of both type of oceanic plates: (a) a domain with a mostly stable trench, a shortening upper plate, characterized by the formation of a topographic relief, and (b) a domain with retreating trench and extending upper plate. In general, we observed that varying the subduction obliquity, the margin could either (i) record an erosional to a balanced accretion/erosion regime or (ii) be characterized by a predominant balanced accretion/erosion regime. In both cases, even where the sediment amount in the trench is high, the upper plate experiences tectonic erosion. We suggest that the formation of topographic reliefs on the fore arc is possibly related to the low amount of sediment in the trench, affecting interplate friction and promoting the upper plate indentation against the slab. The Puysegur subduction zone and the central Andes can be possibly natural examples of such a regime.

The aim of this work is to present a new georeferenced geological map of an area in the Ligurian Western Alps (Lavagnina Lakes area) that includes both a unique geodiversity and great biodiversity, a peculiar geological heritage, and cultural features. The study area is located in the northern part of the Capanne di Marcarolo Regional Natural Park, occurring in the southern Piedmont Region (Alessandria, NW Italy) and close to the suburbs of Genoa. This area has been studied by multi-disciplinary scientific researchers who, so far, have focused their attention on the occurrence of alkaline springs and investigation of different endemic floral species. Moreover, in the past, the Lavagnina Lakes area has been exploited due to the presence of gold mineralization, and several mining records are still visible. We performed detailed geological mapping at a 1:10,000 scale, and collected data that were later integrated into a digital GIS map. The database associated with the map contains information that may be interesting from different points of view: (i) scientific research; (ii) outreach and dissemination activities; and (iii) geotourism (i.e., trail networks and panoramic viewpoints). The area represents a section of the Jurassic Piedmont Ligurian oceanic lithosphere, showing several geologic processes on different scales, such as the serpentinization process and intense and widespread carbonation of ultramafic rocks; the area is, moreover, characterized by fault systems showing paleoseismic structures. Beyond scientific research activities (i.e., geology, geoarchaeology, and mining archaeology), the area can also be promoted for geotourism, outreach and dissemination activities, field trips for schools, and gold panning activities. Hence, our new digital map and our 3D model could be a useful tool to illustrate the main characteristics of the area, leading a non-expert public to explore different geological features in a relatively “small” area. In this way, our map could help to improve geotourism, be used as a tool for educational activities, and, finally, could also help the Capanne di Marcarolo Regional Natural Park to be recognized as a geopark.

Intermediate-depth seismicity is common in subducting slabs and the seismicity rate shows some statistically significant yet enigmatic global positive correlation with the maximal throw of outer-rise normal faults. Here, we have simulated the formation and subduction of outer-rise faults, using 2D thermomechanical numerical models of intra-oceanic subduction with coupled brittle-ductile damage of bending plates. We observed that outer-rise faults are formed episodically during slab segmentation and their maximal throw grows with time. When been subducted to intermediate depth, these faults are locally reactivated either by i) slab unbending/bending, simultaneous to the formation of new outer-rise faults or ii) episodic interplate coupling related to the rugged morphology of the faulted downgoing plate. Faults reactivation is concurrent with a local, transient deviatoric stress increase in intraslab domains among these structures. We suggest that slab domains affected by stress increase could be the appropriate location where potential brittle deformation can occur, generating intermediate-depth intraslab earthquakes, that are predominantly localized in heterogeneous regions of dense faulting formed within slab-segments boundaries. The temporal coincidence of stress growth at intermediate depths and throw-growth of, newly-formed, outer-rise faults at the surface may possibly explain the observed global positive correlation of intermediate-depth seismicity rate with maximal fault throw.