We developed a 2D numerical model that simulates the evolution of a subduction in order to analyze which different metamorphic facies and deformation fabrics can be recorded within a subduction complex and whether the subduction velocity can influence their distribution. Regions with pressure and temperature (P‐T) conditions characteristic of different metamorphic facies have been explored in the different domains of the subduction system to verify the existence of associations that may result as diagnostic of specific areas of the subduction system at different times. Distribution patterns of strain rates and relative deformation fabrics have also been investigated to verify whether they can be recorded under different metamorphic facies conditions. Our results show that three domains characterized by contrasting metamorphic conditions can be simultaneously observed in different regions of the subduction complex, in contrast with the widespread idea that different metamorphic series are representative of peculiar geodynamic scenarios, but in agreement with the commonly observed coexistence of contrasted P‐T conditions in orogenic belts. In addition, we verified that mylonitic fabrics can develop more frequently in metamorphic facies characterized by high Pressure/Temperature (P/T) ratios, while coronitic fabric can be better preserved in metamorphic facies with low‐to‐medium P/T ratios.

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 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.

Lithospheric slices preserving pre-Alpine metamorphic imprints are widely described in the Alps. The Variscan parageneses recorded in continental, oceanic, and mantle rocks suggest a heterogeneous metamorphic evolution across the Alpine domains. In this contribution, we collect quantitative metamorphic imprints and ages of samples that document Variscan tectonometamorphic evolution from 420 to 290 Ma. Based on age distribution and metamorphic imprint, three main stages can be identified for the Variscan evolution of the Alpine region: Devonian (early Variscan), late Devonian–late Carboniferous (middle Variscan), and late Carboniferous–early Permian (late Variscan). The dominant metamorphic imprint during Devonian times was recorded under eclogite and HP granulite facies conditions in the Helvetic–Dauphinois–Provençal, Penninic, and eastern Austroalpine domains and under Ep-amphibolite facies conditions in the Southalpine domain. These metamorphic conditions correspond to a mean Franciscan-type metamorphic field gradient. During the late Devonian–late Carboniferous period, in the Helvetic–Dauphinois–Provençal and central Austroalpine domains, the dominant metamorphic imprint developed under eclogite and HP granulite facies conditions with a Franciscan field gradient. Amphibolite facies conditions dominated in the Penninic and Southalpine domains and corresponded to a Barrovian-type metamorphic field gradient. At the Carboniferous–Permian transition, the metamorphic imprints mainly developed under amphibolite-LP granulite facies conditions in all domains of the Alps, corresponding to a mean metamorphic field gradient at the transition between Barrovian and Abukuma (Buchan) types. This distribution of the metamorphic imprints suggests a pre-Alpine burial of oceanic and continental crust underneath a continental upper plate, in a scenario of single or multiple oceanic subductions preceding the continental collision. Both scenarios are discussed and revised considering the consistency of collected data and a comparison with numerical models. Finally, the distribution of Devonian to Triassic geothermal gradients agrees with a sequence of events that starts with subduction, continues with continental collision, and ends with the continental thinning announcing the Jurassic oceanization.

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.