In the Sesia-Lanzo Zone, Western Alps, the Rocca Canavese Thrust Sheets (RCT) subunit is characterized by a mixture of mantle- and crust-derived lithologies, such as metapelites, metagranitoids, metabasics, and serpentinized mantle slices with sizes ranging from meters to hundreds of meters. Structural and metamorphic history suggests that the RCT rocks experienced a complex evolution. In particular, two different peak conditions were obtained for the metabasics, representing different tectono-metamorphic units (TMUs), namely, D1a under eclogite facies conditions and D1b under lawsonite-blueschist-facies conditions. The two TMUs were coupled during the syn-D2 exhumation stage under epidote-blueschist-facies conditions. The different rocks and metamorphic evolutions and the abundance of serpentinites in the tectonic mixture suggest a possible subduction-related mélange origin for the RCT. To verify whether a subduction-related mélange can record tectono-metamorphic histories similar to that inferred for the RCT, we compare the pressure-temperature evolutions with the results of a 2-D numerical model of ocean-continent subduction with mantle wedge serpentinization. The predictions of the numerical model fully reproduce the two peak conditions (D1a and D1b) and the successive exhumation history of the two TMUs within the subduction wedge. The degree of mixing estimated from field data is consistent with that predicted by the numerical simulation. Finally, the present-day location of the RCT, which marks the boundary between the orogenic wedge (Penninic and Austroalpine domains) and the southern hinterland (Southalpine domain) of the Alpine chain, is reproduced by the model at the end of the exhumation in the subduction wedge. Therefore, the comparison between natural data and the model results confirms the interpretation of the RCT as a subduction-related mélange that occurred during exhumation within a serpentinized mantle wedge.

Permo‐Triassic remnants (300–220 Ma) of high‐temperature metamorphism associated with large gabbro bodies occur in the Alps and indicate a high thermal regime compatible with lithospheric thinning. During the Late Triassic–Early Jurassic, an extensional tectonics leads to the break‐up of Pangea continental lithosphere and the opening of Alpine Tethys Ocean (170–160 Ma), as testified by the ophiolites outcropping in the Central–Western Alps and Apennines. We revise geological data from the Permian to Jurassic of the Alps and Northern Apennines, focusing on continental and oceanic basement rocks, and predictions of existing numerical models of post‐collisional extension of continental lithosphere and successive rifting and oceanization. The aim is to test whether the transition from the Permo‐Triassic extensional tectonics to the Jurassic opening of Alpine Tethys occurred. We enforce the interpretation that a forced extension of 2 cm/year of the post‐collisional lithosphere results in a thermal state compatible with the Permo‐Triassic high‐temperature event suggested by pressure and temperature conditions of metamorphic rocks and widespread igneous activity. Extensional or transtensional tectonics is also in agreement with the generalized subsidence indicated by the deposition of sedimentary successions with deepening upward facies occurred in the Alps from the Permian to Jurassic. Furthermore, a rifting developed on a thermally perturbed lithosphere agrees with a hyperextended configuration of the Alpine Tethys rifting and with the duration of the extension necessary to the oceanization. The review supports the interpretation of Alpine Tethys opening developed on a lithosphere characterized by a thermo‐mechanical configuration inherited by the post‐Variscan extension which affected Pangea during the Permian and Triassic. Therefore, a long‐lasting period of active extension can be envisaged for the breaking of Pangea supercontinent, starting from the unrooting of the Variscan belts, followed by the Permo‐Triassic thermal high, and ending with the crustal break‐up and the formation of the Alpine Tethys Ocean.

S U M M A R YWe developed a 2-D finite element model to investigate the effect of shear heating and mantle hydration on the dynamics of the mantle wedge area. The model considers an initial phase of active oceanic subduction, which is followed by a post-collisional phase characterized by pure gravitational evolution. To investigate the impact of the subduction velocity on the thermomechanics of the system, three models with different velocities prescribed during the initial subduction phase were implemented. Shear heating and mantle hydration were then systematically added into the models. We then analysed the evolution of the hydrated area during both the subduction and post-collisional phases, and examined the difference in P max -T (maximum pressure-temperature) and P-T max (pressure-maximum temperature) conditions for the models with mantle hydration. The dynamics that allow for the recycling and exhumation of subducted material in the wedge area are strictly correlated with the thermal state at the external boundaries of the mantle wedge, and the size of the hydrated area depends on the subduction velocity when mantle hydration and shear heating are considered simultaneously. During the post-collisional phase, the hydrated portion of the mantle wedge increases in models with high subduction velocities. The predicted P-T configuration indicates that contrasting P-T conditions, such as Barrovian-to Franciscan-type metamorphic gradients, can contemporaneously characterize different portions of the subduction system during both the active oceanic subduction and post-collisional phases and are not indicative of collisional or subduction phases.

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Following the pioneering works carried out in the seventies (e.g., England & Richardson, 1977;Oxburgh & Turcotte, 1970, 1971Toksöz & Bird, 1977) several key advances have been made in numerical modeling methods that have improved the understanding of the thermo-mechanical evolution of the oceanic and continental lithosphere during oceanic subduction and continental collision (e.g.

SUMMARY The anomalous density structure at subduction zones, both in the wedge and in the upper mantle, is analysed to shed light on the processes that are responsible for the characteristic gravity fingerprints of two types of subduction: ocean–continent and ocean–ocean. Our modelling is then performed within the frame of the EIGEN-6C4 gravitational disturbance pattern of two subductions representative of the above two types, the Sumatra and Mariana complexes, finally enabling the different characteristics of the two patterns to be observed and understood on a physical basis, including some small-scale details. A 2-D viscous modelling perpendicular to the trench accounts for the effects on the gravity pattern caused by a wide range of parameters in terms of convergence velocity, subduction dip angle and lateral variability of the crustal thickness of the overriding plate, as well as compositional differentiation, phase changes and hydration of the mantle. Plate coupling, modelled within a new scheme where the relative velocity at the plate contact results self-consistently from the thermomechanical evolution of the system, is shown to have an important impact on the gravity signature. Beyond the already understood general bipolar fingerprint of subduction, perpendicular to the trench, we obtain the density and gravity signatures of the processes occurring within the wedge and mantle that are responsible for the two different gravity patterns. To be compliant with the geodetic EIGEN-6C4 gravitational disturbance and to compare our predictions with the gravity at Sumatra and Mariana, we define a model normal Earth. Although the peak-to-peak gravitational disturbance is comparable for the two types of subductions, approximately 250 mGal, from both observations and modelling, encompassing the highest positive maximum on the overriding plates and the negative minimum on the trench, the trough is wider for the ocean–ocean subduction: approximately 300 km compared to approximately 180 km for the ocean–continent subduction. Furthermore, the gravitational disturbance pattern is more symmetric for the ocean–ocean subduction compared to the ocean–continent subduction in terms of the amplitudes of the two positive maxima over the overriding and subducting plates. Their difference is, for the ocean–ocean type, approximately one half of the ocean–continent one. These different characteristics of the two types of subductions are exploited herein in terms of the different crustal thicknesses of the overriding plate and of the different dynamics in the wedge and in the mantle for the two types of subduction, in close agreement with the gravity data.

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We developed a 2-D finite element model to investigate the effect of shear heating and mantle hydration on the dynamics of the mantle wedge area. The model considers an initial phase of active oceanic subduction, which is followed by a post-collisional phase characterized by pure gravitational evolution. To investigate the impact of the subduction velocity on the thermomechanics of the system, three models with different velocities prescribed during the initial subduction phase were implemented. Shear heating and mantle hydration were then systematically added into the models. We then analysed the evolution of the hydrated area during both the subduction and post-collisional phases, and examined the difference in Pmax–T (maximum pressure–temperature) and P–Tmax (pressure–maximum temperature) conditions for the models with mantle hydration. The dynamics that allow for the recycling and exhumation of subducted material in the wedge area are strictly correlated with the thermal state at the external boundaries of the mantle wedge, and the size of the hydrated area depends on the subduction velocity when mantle hydration and shear heating are considered simultaneously. During the post-collisional phase, the hydrated portion of the mantle wedge increases in models with high subduction velocities. The predicted P–T configuration indicates that contrasting P–T conditions, such as Barrovian- to Franciscan-type metamorphic gradients, can contemporaneously characterize different portions of the subduction system during both the active oceanic subduction and post-collisional phases and are not indicative of collisional or subduction phases.