How many subductions in the Variscan orogeny? Insights from numerical models

We developed a 2D numerical model to simulate the evolution of two superposed ocean-continent-ocean subduction cycles with opposite vergence, both followed by continental collision, aiming to better understand the evolution of the Variscan belt. Three models with different velocities of the first oceanic subduction have been implemented. Striking differences in the thermo-mechanical evolution between the first subduction, which activates in an unperturbed system, and the second subduction, characterised by an opposite vergence, have been enlighten, in particular regarding the temperature in the mantle wedge and in the interior of the slab. Pressure and temperature (P-T) conditions predicted by one cycle and two cycles models have been compared with natural P-T estimates of the Variscan metamorphism from the Alps and from the French Massif Central (FMC). The comparative analysis supports that a slow and hot subduction well reproduces the P-T conditions compatible with data from the FMC, while P-T conditions compatible with data of Variscan metamorphism from the Alps can be reproduced by either a cold or hot oceanic subduction models. Analysing the agreement of both double and single subduction models with natural P-T estimates, we observed that polycyclic models better describe the evolution of the Variscan orogeny

Structural analysis of a subduction-related contact in southern Sesia-Lanzo Zone (Austroalpine Domain, Italian Western Alps)

A new foliation trajectory map at 1:10000 scale, represented here with an interpretative structural map, is derived from an original field analysis at 1:5000 scale in the southern Sesia-Lanzo Zone (SLZ). It shows the relative chronology of overprinting foliations, characterised by the mineral assemblages that mark superposed fabrics in each rock type. This map and the associated cross-sections, which synthesise the 3D structural outline of the tectonic contact between the Eclogitic Micaschists Complex (EMC), the Rocca Canavese Thrust Sheets and the Lanzo Ultramafic Complex, allow the correlation of the structural and metamorphic imprints that developed in these crustal and mantle complexes during Alpine subduction. Furthermore, the map and cross-sections allow the immediate perception of the metamorphic environments in which the structural imprints developed in each complex successively under eclogite, blueschist and greenschist facies conditions. The represented structural and metamorphic evolution of the southern end of the SLZ (internal Western Alps) has been inferred based on multiscale structural analysis. The dominant fabrics at the regional scale are two superposed mylonitic foliations that developed under blueschist and greenschist facies conditions, respectively. Metamorphic assemblages underlying the successive fabrics in the different metamorphic complexes allow us to identify contrasting metamorphic evolutions indicating that the tectonic contacts between the EMC, the Rocca Canavese Thrust Sheets and the Lanzo Ultramafic Complex developed under blueschist facies conditions and were successively reactivated during the greenschist facies retrogression

Fluid rock interactions as recorded by Cl-rich amphiboles from continental and oceanic crust of Italian orogenic belts

A number of samples of Cl-rich amphiboles coming from oceanic and sub-continental gabbro bodies has been studied in order to compare their microstructural and compositional peculiarities and to investigate the fluid-rock interactions in different geodynamic contexts. The development of a first group of amphiboles outcropping in the Northern Apennines was the result of a hydration event that has been ascribed to oceanic metamorphism. The second group was found in a slice of continental crust subducted during Alpine collision, in a subcontinental metagabbro from the Sesia-Lanzo Zone of Italian Western Alps. Their development has been ascribed to a hydrothermal event that took place after the exhumation of the metagabbro during pre-Alpine lithospheric extension. The Cl-amphiboles are either found in veins, as granoblastic aggregates in different microstructures or as rims of zoned amphiboles, where brown-amphibole cores (sometimes Ti-rich), and successive green amphibole, are rimmed by the Cl-rich amphibole. All amphiboles show edembergite to pargasite compositions up to glaucophane and crossites when reequilibrated under HP conditions, with a direct correlation between Fe and Na(A) vs. Cl content, and inverse correlation of Mg and Na(M4) vs. Cl. A comparison with other Cl-amphiboles that have been observed both in oceanic and continental settings, allow discussing the role played by Clrich fluids infiltration both in oceanic and continental crust, during lithospheric extension. The large variations in Si, AlIV, AlVI, Fe, Mg, K and Cl may be related to the combination of different factors, such as Cl-content and related cristal-chemical constraints, whole rock composition, PT conditions of reequilibration, the microdomains where the amphibole grows and the variable aHCl/fluid/aH2O/fluid ratio of the fluid in equilibrium with the amphiboles at various stages of the metamorphic evolution. Amphiboles that locally contain extremely high Cl contents (up to 4% wt) could have been in equilibrium with a locally enriched Cl-fluid. As suggested by the fact that the Cl content of amphibole into the veins is generally lower than in amphibole rims far from the veins, these equilibrium conditions probably were reached at places where the system was locally closed. In addition, hydration reaction consumed the H2O component of the fluid, leading to a re-equilibration of the crystallising amphibole with the remaining Cl-enriched fluid. Equilibration temperatures up to 350 \ub0C can be attributed to the Northern Apennines amphiboles, and up to 550 \ub0C to the ones from the Sesia-Lanzo Zone

Influence of subduction geometry and mantle wedge hydration in ocean/continent convergent systems : a numerical simulation

To analyze the effects of hydration mechanism on continental crust recycling a parametric study using a 2D finite elements thermo-mechanical model is here presented. We implemented oceanic slab dehydration and consequent mantle wedge hydration using a dynamic method; hydration is accomplished by Lawsonite and Serpentine break- down. Topography is treated as a free surface. Results of a set of numerical simulations investigate the influence of subduction rate, slab dip and mantle rheology changes on thermal regime variations, exhumation rate and amount of recycled crust. At this purpose subduction rates of 1, 3, 5, 7.5 and 10 cm/yr, slab angles of 30\ub0, 45\ub0 and 60\ub0 and a mantle rheology represented by dry dunite and dry olivine flow laws, have been taken into account during successive numerical experiments. Model predictions pointed out that a direct relationship between mantle rheology and the amount of recycled crustal material exists: the larger is the viscosity contrast between hydrated and dry mantle the larger is the percentage of recycled material into the mantle wedge. A quite impact on recycling is consequent to slab dip variation. Metamorphic evolutions of recycled material are influenced by subduction style: Tmax of Pmax, generally accomplished under eclogite-facies conditions, is sensible to slab dip changing and the increasing subduction rate induces a decrease in Tmax of Pmax values. A direct relationship between subduction rate and exhumation rate results for different slab dips, independent of mantle flow law used. Thermal regimes predicted by different numerical models are compared with PT paths followed by continental crustal slices involved in ancient and actual subduction zones

Numerical simulation of ocean/continent convergent systems : influence of subduction geometry and mantle wedge hydration on crustal recycling

Studies on high-pressure (HP) and ultrahigh-pressure (UHP) rocks exposed in orogenic belts linked to collisional margin show that nappes of oceanic and continental deeply subducted crust can be exhumed to shallow structural levels. In particular, during ocean-continent-type subduction, the crustal material dragged into subduction channel is composed chie\ufb02y by ocean and trench sediments, crustal slices belonging to subducting plate [microcontinent (Ring & Layer, 2003) or linked to early continental collision (Chemenda et al., 1995)] or crustal slices tectonically eroded from the overriding plate (ablative subduction) (Tao & O\u2019Connell, 1992, Marotta & Spalla, 2007). Several models have been developed, during last 20 years, to analyse exhumation of subducted crustal material. They can be resume on \ufb01ve main mechanisms: a) crustal-mantle delamination (Chemenda et al., 1995), b) slab break-off (Ernst et al., 1997), c) slab retreat (Ring & Layer, 2003) and roll-back slab (Brun & Faccenna, 2008) and d) decoupling of two main ductile layers (Yamato et al., 2008), in which the exhumation is mainly driven by negative buoyancy and/or faulting and e) subduction-channel \ufb02ow (Gerya & Stockhert, 2005) in which the exhumation is driven by the upwelling \ufb02ow developed in low-viscosity mantle wedge. Only channel \ufb02ow takes into account recyrculation of crustal slices dragged to high depth by ablation in pre-collisional subduction zones. To study the effects of subduction rate, slab dip and mantle rheology changes on channel flow efficiency a parametric analysis is made. We present the results of a set of numerical simulation with different subduction rates, slab dips and mantle rheology represented by dry dunite and dry olivine \ufb02ow laws. Numerical model predictions are \ufb01nally compare to some PT paths obtained from ancient and actual subduction zones with different slab dips and convergence velocities. A general good agreement between natural data and model predictions emerges from the comparison: exhumation rates obtained from complete PTt-paths (total exhumation rates) are more compatible with natural rates rather than maximum exhumation rates; the thermal states predicted by ablative subduction simulations with a hydrated mantle wedge justify the natural PT estimates obtained on continental crust units involved in ocean/continent subduction systems. For these reasons, we propose ablative subduction of the upper continental plate linked to hydrated mantle wedge as a good alternative pre-collisional mechanism, with respect to the collisional mechamisms as the slab break-off, slab-retreat and roll-back slab

Influenza dell'idratazione del cuneo di mantello sull'evoluzione di un sistema di subduzione oceano/continente : una simulazione numerica

The evolution of an ocean/continent subduction system is simulated by using a 2D finite elements thermomechanical model. The effects of hydration in mantle wedge on mantle flow and on crustal recycling is studied for different hydration rates and maximum depth of dehydration of the oceanic crust for two selected typical subduction velocities (1 cm/a and 5 cm/a). We found a direct relationship between the amount of recycled material and the maximum depth of dehydration. Moreover, the hydration rate and hydration depth have an important impact on peak pressure and temperature of recycled continental crust for a subducion rate of 5 cm/a

Permian geodynamics of the central Southalpine by tectono-thermal record in post-Variscan conglomerates

The central Southern Alps consist of the pre-Alpine basement and Permian-Mesozoic covers, both affected by the Alpine fold-and-thrust belt. The pre-Alpine basement recorded heterogeneous structural and metamorphic evolutions and therefore consists of different tectono-metamorphic units related to different stages of the Variscan evolution. To the east, rocks recorded the effects of the Variscan tectonic burial and escaped the subsequent collision, whereas the units outcropping westward recorded both the effects of Variscan tectonic burial and collision and the westernmost basement rocks even host late-Variscan intrusives and recorded the effects of lithosphere thinning-related Triassic high-temperature (Spalla et al., 2014). Lower Permian volcanoclastic sequences infill intermontane basins and are the oldest sedimentary rocks uncoformably capping the basement (Berra et al., 2016; Zanoni & Spalla, 2018 and refs therein). These sequences consist of volcanites overlaid by lacustrine sandstone and alluvial fan conglomerates. According to radiometric constraints, the age of the conglomerates is more recent westward. These conglomerates contain pebble- to boulder-sized crystalline clasts. The metamorphic evolution recorded in clasts are related to the Variscan orogeny and revealed that the thermal maturity of orogenic traces increases westward, likewise the general record in the metamorphic basement, indicating that conglomerates were fed by the erosion of tectono-metamorphic units similar to those exposed today. In the westernmost conglomerate, clasts recorded high-temperature metamorphism and some derive even from late-Variscan intrusives and later tourmalinite-breccia. Since the conglomerates rejuvenate westward with the increase of orogenic maturity in clasts, we speculate that the post-Variscan lithosphere was affected by westward propagating extension, also responsible for intermontane wrenching. To test this hypothesis, we started 2D numerical simulations on the thermo-mechanical evolution of the lithosphere affected by westward propagating extension

What drives Alpine Tethys opening: suggestions from numerical modelling

We discuss the results obtained for two subsequent numerical models that simulate the evolution of the European lithosphere from the late collision of the Variscan chain to the Jurassic opening of the Alpine Tethys. The first model accounts for the evolution of the crustal lithosphere after the Variscan subduction and collision (300 Ma) up to 220 Ma (Marotta et al., 2009). The second model accounts for the rifting of the continental lithosphere from 220 Ma up to reach the crustal breakup and the formation of the oceanic crust (Marotta et al., 2016). For both models, different initial geodynamic configurations have been tested and we compare the results with natural data of Permian-Triassic metamorphic rocks and Jurassic gabbros and peridotites, to evaluate which configuration best matches the observations. Natural data belong to different structural Alpine domains. Continental rocks are collected from Helvetic and Penninic domains (European paleomargin) and from Austroalpine and Southalpine domains (Adriatic paleomargin) and oceanic rocks are collected from Alpine and Apennine ophiolites. The comparison is made in terms of contemporaneous agreement to lithology, pressure and temperature values, and ages. The comparison between Permian-Triassic to Jurassic natural data from the Alps and the Northern Apennines and two subsequent numerical models simulating the evolution of the lithosphere from the late collision of the Variscan chain to the Jurassic opening of the Alpine Tethys suggests that: i) a forced extension of the lithosphere results in a thermal state that better agrees the Permian-Triassic high temperature event(s) than a solely late-orogenic collapse; ii) 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 up to the oceanization. These results suggest that the Alpine Tethys rifting and oceanization developed on a lithosphere characterized by a thermo-mechanical configuration consequent to a post-Variscan extension affecting the European realm during Permian and Triassic. Therefore, a long-lasting period of continuous active extension can be envisaged for the breaking of Pangea supercontinent, starting from the unrooting of the Variscan belts (300 Ma), followed by the Permian-Triassic thermal peak highlighted by HT-LP metamorphism and gabbros emplacement, and ending with the crustal breakup and the formation of the Alpine Tethys ocean (170-160 Ma). This process could be characterized by alternated period of active extension and stasis, as proposed for the Northern Atlantic rifting or as envisaged for the Ivrea-Verbano Zone based on three metamorphic ages

What drives alpine tethys opening: suggestions from numerical modelling

Continental crustal slices, preserving pre-Alpine metamorphism, are widely described in Alps and Apennine realms (Fig. 1). Variscan-age eclogites (430-326 Ma) generated from continental, oceanic and mantle rocks occur within these slices and suggest a pre-Alpine burial of continental crust at convergent plate margins, in a context of oceanic lithosphere subduction underneath continental upper plate, characterized by a low thermal regime, and followed by continental collision (e.g. Marotta and Spalla, 2007; von Raumer et al., 2013; Spalla et al., 2014). Permian-Triassic remnants (300-220 Ma) of high-temperature metamorphism, mainly occurring within Austroalpine and Southalpine domains (belonging to Adria plate) and associated with widespread basic to acidic igneous activity testi ed by large gabbro bodies (Fig. 1), indicate an increase of the lithospheric thermal regime (e.g. Lardeaux and Spalla, 1991; Schuster and St\ufcwe, 2008; Marotta et al., 2009; Spalla et al., 2014) related to asthenospheric upwelling and lithospheric thinning (e.g. Thompson, 1981; Sandiford and Powell, 1986; Beardsmore and Cull, 2001). During Late Triassic-Early Jurassic an important extensional stage leads to the break-up of the Pangaea continental lithosphere and the opening of the Alpine Tethys Ocean, accounted by the occurrence of ophiolitic sequences in the western Alps and Apennines (Fig. 1). The geodynamic signi cance of the Permian-Triassic high temperature and low pressure metamorphic event has been widely debated and recent numerical models suggest an origin consequent to successive lithospheric extension and thinning events eading to the Mesozoic continental rifting (e.g. Marotta and Spalla, 2007; Marotta et al., 2009; Spalla et al., 2014), whereas on the basis of recent paleogeographic reconstructions it has also been interpreted as engaged by the neo-Variscan late-orogenic collapse (e.g. Spiess et al., 2010; von Raumer et al., 2013). In the northern Atlantic region for instance, a sequence of rift basins from Permian to Cretaceous has been described occurring before the opening of the ocean (e.g. Dor\ue9 and Steward, 2002) making the rifting of the North Atlantic Ocean a long lasting process with several extensional events associated with a migration of eulerian poles as testi ed by the anticlockwise and successive clockwise rotation of superposed rift axes. Based on this idea, we test whether the lithospheric extension can lead the rifting of the Alpine Tethys by comparing numerical modelling of post-collisional extension and successive rifting and oceanization with Permian-Triassic to Jurassic natural data from the Alps and northern Apennines (Fig. 1). In particular, we focus our attention on the thermal state of the pre-rifting (Permian-Triassic in age) lithosphere in order to explore if the opening of the Alpine Tethys started on a stable continental lithosphere or rather developed on a thermally perturbed one. We here discuss the results obtained for two subsequent numerical models that simulate the evolution of the European lithosphere from the late collision of the Variscan chain to the Jurassic opening of the Alpine Tethys. The rst model accounts for the evolution of the crustal lithosphere after the Variscan subduction and collision (300 Ma) up to 220 Ma (Marotta et al., 2009). The second model accounts for the rifting of the continental lithosphere from 220 Ma up to reach the crustal breakup and the formation of the oceanic crust (Marotta et al., 2016). For both models different initial geodynamic con gurations have been tested and we compare the results with natural data of Permian-Triassic metamorphic rocks and Jurassic gabbros and peridotites (Fig. 1), in order to evaluate which con guration best matches the observations. Natural data belong to different structural Alpine domains. Continental rocks are collected from Helvetic and Penninic domains (European paleomargin) and from Austroalpine and Southalpine domains (Adriatic paleomargin) and oceanic rocks are collected from Alpine and Apennine ophiolites (Fig. 1). The comparison is made in terms of contemporaneous agreement to lithology, pressure and temperature values, and ages. The differences between model predictions and natural P-T-age data are synthesized in Fig. 2, where the ages estimate for the rocks are shown using light grey bars for radiometric ages and dark grey for geologically determined ages. For the rst model we compare the results of two different con gurations. The rst one is characterized by a purely gravitational evolution of the lithosphere in order to simulate a late- orogenic collapse. The second con guration instead, is characterized by a forced extension of the lithosphere of 2 cm/yr. With respect to the purely gravitational simulation, for which the t between predictions and observations is obtained for few data only (Fig. 2), the forced extension simulation agrees well with all collected natural data (Fig. 2). The most peculiar character of the Permian\u2013Triassic igneous activity is the widespread emplacement of gabbro stocks at the base of the crust and the occurrence of basaltic products in the volcanics. Therefore, we verify whether the P-T conditions predicted for the lithospheric and asthenospheric mantle by different con gurations cross the solidus of peridotite. Although predictions from all con gurations satisfy the thermal state for mantle partial melting, the latter is attained at 75 km depth for the purely gravitational con guration and at 50 km depth for the simulation with forced extension. Basaltic melt production is thus compatible with all the simulated tectonic settings but, to allow the partial melting of the continental crust, the thermal state must be similar to that suggested by simulation with forced extension. The nal thermo-mechanical setting is very different between the two con gurations. In the purely gravitational simulation both the crustal thickness and the lithospheric thermal state are similar to the initial conditions, while in the forced extension simulation a strong lithospheric thinning occurs together with a hot thermal state. The second model simulates the extension of the continental lithosphere up to reach the crustal breakup and the formation of the oceanic crust. The model also includes the hydration of the uprising mantle peridotite and the extension rate is constant and xed to 1.25 cm/yr on the both sides of the domain (total extension rate of 2.5 cm/yr). Accounting for two different thermal con gurations of the lithosphere allows to constrain two different pre-rifting settings of the Alpine lithosphere (hot and cold simulations with 1600 K isotherm at 80 and 220 km depth respectively). The model results in a symmetric rifting of the continental lithosphere and shows the exhumation of a serpentinized lithospheric mantle (ocean-continent transition zone \u2013 OCTZ). The onset of the lithospheric thinning strongly depends on the initial lithospheric thermal state: for a cold and strong lithosphere, the thinning is very rapid (4.4 Ma) with respect to a hot and weak lithosphere (15.4 Ma). Similarly, the occurrence of the crustal breakup is shorter for a cold lithosphere (7.4 Ma) than for a hot lithosphere (approximately 31.4 Ma). For both the chosen initial thermal con gurations of the lithosphere, the exhumation of the serpentinized mantle starts before the oceanic spreading and the mantle partial melting, making the model compatible with a magma-poor rifting, as suggested for the Alpine case (e.g., Manatschal et al., 2015). In the hot con guration the continental crust thickness sensibly decreases during the extension from 30 km to approximately 5 km close to the OCTZ. In the cold model instead, the crustal thickness decreases from 30 km to approximately 20 km. The comparison between the natural data and the model predictions shows a good agreement with all of the oceanic data for both hot and cold con gurations. Taking into account that a hyperextended system has been proposed for the Alpine Tethys rifting (e.g. Manatschal et al., 2015) and a time span of approximately 30-40 Ma is considered between the rst extensional structures related to the rifting (200 Ma, Mohn et al., 2012) and the oceanic gabbros emplacement (170-160 Ma, see review in Marotta et al., 2009, 2016), a rifting developed on thermally perturbed lithosphere better agrees the natural data available in ophiolites. The comparison between Permian-Triassic to Jurassic natural data from the Alps and the northern Apennines and two subsequent numerical models simulating the evolution of the lithosphere from the late collision of the Variscan chain to the Jurassic opening of the Alpine Tethys suggests that: i) a forced extension of the lithosphere results in a thermal state that better agrees the Permian-Triassic high temperature event(s) than a solely late-orogenic collapse; ii) a rifting developed on a thermally perturbed lithosphere agrees with a hyperextended con guration of the Alpine Tethys rifting and with the duration of the extension up to the oceanization. These results suggest that the Alpine Tethys rifting and oceanization developed on a lithosphere characterized by a thermo-mechanical con guration consequent to a post-Variscan extension affecting the European realm during Permian and Triassic. Therefore, a long lasting period of continuous active extension can be envisaged for the breaking of Pangea supercontinent, starting from the unrooting of the Variscan belts (300 Ma, Fig. 3a), followed by the Permian-Triassic thermal peak highlighted by HT-LP metamorphism and gabbros emplacement (Fig. 3b), and ending with the crustal breakup and the formation of the Alpine Tethys ocean (170-160 Ma, Fig. 3c). This process could be characterized by alternated period of active extension and stasis, as proposed for the Northern Atlantic rifting or as envisaged for the Ivrea-Verbano Zone on the basis of three metamorphic ages (Permian, Triassic and Jurassic; Langone and Tiepolo, 2015). In order to explore this issue a continuous and polycyclic numerical model is necessary to record the thermo-mechanical inheritance of different events during the entire extensional process, and use ages and P-T-t paths of natural data as constraints