The high-pressure Voltri Massif outcrops at the eastern border of the Ligurian Western Alps. It e... more The high-pressure Voltri Massif outcrops at the eastern border of the Ligurian Western Alps. It encompasses meta-ophiolites and metasediments recording blueschist (Palmaro-Caffarella Unit) to eclogite (Voltri Unit) syn-kinematic peak conditions. The study of eclogite and blueschist metagabbro lenses wrapped by highly sheared serpentinite or metasediments revealed a strong strain partitioning between host-rock and the lenses and between core and rims of the lenses. The cores still preserve magmatic textures statically overgrown by the high-pressure assemblages, whereas rims display tectonite and mylonite structures. Coupling P-T pseudosections and petrography we obtained clockwise P-T paths for the metagabbro lenses. The Palmaro-Caffarella metagabbro recrystallized at peak conditions of 10-15 kbar and 450-500°C. The Voltri metagabbro reached peak metamorphic conditions in the lawsonite stability field, ranging from about 21 kbar and 450-490°C to 22-28 kbar and 460-500°C. The decompression trajectory is almost isothermal, slightly cooling. In order to constrain the exhumation mechanism of the Voltri Massif, we performed 2D numerical models trying to simulate the intraoceanic subduction started in the Ligurian-Piedmontese branch of the Western Tethys in Mesozoic time. The initial setup of the oceanic basin (amplitude, spreading rate and composition) was derived from literature. So we reproduce a non-layered oceanic lithosphere typical of slow and ultra-slow spreading ridges like the Jurassic Ligurian Tethys: the serpentinized lithospheric mantle includes discrete gabbros bodies and is covered by a discontinuous basaltic layer. The oceanic basin is moreover surrounded by continental margins. Because of slab dehydration, a viscous serpentinite channel forms in the mantle wedge; the models show that its evolution is strongly controlled by rheology of serpentine. Ductile deformation of serpentine enhances, inside the channel, the mixing of sediments, slices of the oceanic crust and mantle that are scraped off during subduction. Slab-derived serpentinite can be therefore closely associated with mantle wedge serpentinite; moreover also slices of the overriding plate can be subducted and included into the channel. Our simulations show that serpentinite rocks were fundamental to decrease the bulk density of the HP terrains below the mantle value, thus causing the exhumation of part of the serpentinite channel. The simulation routine can also supply P-T paths of selected rock volumes of the model and we found remarkable correspondence between the modeled P-T paths of some metagabbro bodies and the P-T path of metagabbro lenses of the Voltri Massif. The preponderance of highly sheared serpentinite, the strong strain-partitioning, the different metamorphic peaks of the Massif units and the agreement between natural and simulated P-T paths suggest that the Voltri Massif could be part of a ``fossil'' serpentinitic channel and buoyancy could have played an important role during the final exhumation stages.
Convergent plate margins are currently distinguished as &... more 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.
The high-pressure Voltri Massif outcrops at the eastern border of the Ligurian Western Alps. It e... more The high-pressure Voltri Massif outcrops at the eastern border of the Ligurian Western Alps. It encompasses meta-ophiolites and metasediments recording blueschist (Palmaro-Caffarella Unit) to eclogite (Voltri Unit) syn-kinematic peak conditions. The study of eclogite and blueschist metagabbro lenses wrapped by highly sheared serpentinite or metasediments revealed a strong strain partitioning between host-rock and the lenses and between core and rims of the lenses. The cores still preserve magmatic textures statically overgrown by the high-pressure assemblages, whereas rims display tectonite and mylonite structures. Coupling P-T pseudosections and petrography we obtained clockwise P-T paths for the metagabbro lenses. The Palmaro-Caffarella metagabbro recrystallized at peak conditions of 10-15 kbar and 450-500°C. The Voltri metagabbro reached peak metamorphic conditions in the lawsonite stability field, ranging from about 21 kbar and 450-490°C to 22-28 kbar and 460-500°C. The decompression trajectory is almost isothermal, slightly cooling. In order to constrain the exhumation mechanism of the Voltri Massif, we performed 2D numerical models trying to simulate the intraoceanic subduction started in the Ligurian-Piedmontese branch of the Western Tethys in Mesozoic time. The initial setup of the oceanic basin (amplitude, spreading rate and composition) was derived from literature. So we reproduce a non-layered oceanic lithosphere typical of slow and ultra-slow spreading ridges like the Jurassic Ligurian Tethys: the serpentinized lithospheric mantle includes discrete gabbros bodies and is covered by a discontinuous basaltic layer. The oceanic basin is moreover surrounded by continental margins. Because of slab dehydration, a viscous serpentinite channel forms in the mantle wedge; the models show that its evolution is strongly controlled by rheology of serpentine. Ductile deformation of serpentine enhances, inside the channel, the mixing of sediments, slices of the oceanic crust and mantle that are scraped off during subduction. Slab-derived serpentinite can be therefore closely associated with mantle wedge serpentinite; moreover also slices of the overriding plate can be subducted and included into the channel. Our simulations show that serpentinite rocks were fundamental to decrease the bulk density of the HP terrains below the mantle value, thus causing the exhumation of part of the serpentinite channel. The simulation routine can also supply P-T paths of selected rock volumes of the model and we found remarkable correspondence between the modeled P-T paths of some metagabbro bodies and the P-T path of metagabbro lenses of the Voltri Massif. The preponderance of highly sheared serpentinite, the strong strain-partitioning, the different metamorphic peaks of the Massif units and the agreement between natural and simulated P-T paths suggest that the Voltri Massif could be part of a ``fossil'' serpentinitic channel and buoyancy could have played an important role during the final exhumation stages.
Convergent plate margins are currently distinguished as &... more 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.
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