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The Rocca Canavese Thrust Sheets Unit (RCTU) is a subduction-related mélange that represents the eastern-most complex of the Sesia–Lanzo Zone (SLZ), bounded by the Periadriatic (Canavese) Lineament that separates the Alpine subduction complex from the Southalpine domain. The RCTU is limited to the south by the Lanzo Massif (LM) and to the east by the Eclogitic Micaschists Complex (EMC). Particularly the tectonic contact area of the RCTU, adjacent to the neighbouring SLZ and the LM is characterised by a 100–200-m-thick mylonitic to ultra-mylonitic zone (MZ) that was active under blueschist-to greenschist-facies conditions. Despite the dominant mylonitic structure, some rocks (garnet-bearing gneiss, garnet-free gneiss and orthogneiss) still preserve pre-mylonitic parageneses in meter-sized domains. The scarcity of superposed structures and the small size of relicts impose a detailed microstructural analysis supported by chemical investigation to reconstruct the tectono-metamorphic history of the MZ. Therefore, we integrated the classical meso- and microstructural analysis approach with a novel quantitative technique based on the Quantitative X-Ray Map Analyzer (Q-XRMA), used to classify rock-forming minerals starting from an array of X-ray elemental maps, both at whole thin section and micro-domain scale, as well as to calibrate the maps for pixel-based chemical analysis and end-member component maps, relevant for a more robust conventional geothermobarometer application as well for calculating reliable PT pseudosections. Pre-Alpine relicts are garnet and white mica porphyroclasts in the garnet-bearing gneiss and biotite and K-feldspar porphyroclasts in garnet-free gneiss and orthogneiss, respectively, providing no PT constraints. The Alpine evolution of the MZ rocks, has been subdivided in three deformation and metamorphic stages. The first Alpine structural and metamorphic equilibration stage (D1 event) occurred at a pressure of ca. 1.25–1.4 GPa and at a temperature of ca. 420–510 °C, i.e. under blueschist-facies conditions. The D2 event, characterised by a mylonitic foliation that is pervasive in the MZ, occurred at ca. 0.95–1.1 GPa and ca. 380–500 °C, i.e. under epidote-blueschist-facies conditions. The D2 PT conditions in the MZ rocks are similar to those predicted for the blocks that constitute the RCTU mélange, and they overlap with the exhumation paths of the EMC and LM units. Therefore, the RCTU, EMC and LM rocks became coupled together during the D2 event. This coupling occurred during the exhumation of the different tectono-metamorphic units belonging to both continental and oceanic lithosphere and under a relatively cold thermal regime, typical for an active oceanic subduction zone, pre-dating Alpine continental collision.

Within the Zermatt-Saas Zone (ZSZ, northwestern Alps), Ti-chondrodite- and Ti-clinohumite-bearing assemblages in serpentinites indicate UHP conditions. Multiscale structural analysis (1:20 scale ma...

International audience; 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

Geological mapping, multiscale structural analysis, and estimations of the degree of fabric evolution and of reaction progress allow the construction of a 3D quantitative model of structural and metamorphic gradients in a portion of continental crust deeply involved in the Alpine subduction system and mainly structured under eclogite-facies conditions before the continental collision. The investigated and modelled rocks outcrop in the surroundings of Mt. Mucrone, in the central Sesia–Lanzo Zone. The Geomodeller® software allowed a quantitative 3D estimation of domains characterized by homogeneous fabric evolution and metamorphic reaction progress (DFE and DRP, respectively) for two of the seven structural and metamorphic imprints detected in this area: the D2-eclogitic and D5-greenschist stages, which are the most pervasive at km scale. Such a 3D modelling clarifies mutual relationships between fabric and metamorphic gradients and indicates that: (i) DFE and DRP are closely related regardless of the rock type; (ii) the syn-deformational thermal regime can influence the degree of metamorphic transformation, if DFE remains below the 60% threshold; (iii) the phase transitions can not be properly implemented in quantitative geodynamic modelling without considering the heterogeneity of reaction progress and fabric evolution.

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.

In the upper Tesso Valley the folded contact between Piemonte Zone ophiolites and Sesia-Lanzo Zone continental crust is exposed. Here serpentinites, metabasites, calcschists and fine-grained gneisses are deformed by four ductile superposed groups of structures, associated with different mineral assemblages. Different serpentinite lithologies have been recognized and studied in detail. Mylonitic D2 structures are pervasive and mineral assemblages point to re-equilibration at T of 450 &plusmn, 50 ∘C and P of 0.8 &plusmn, 0.3 GPa, under blueschist/epidote amphibolite-facies conditions. Pre-D2 structures and mineral assemblages are relics within S2 and indicate a re-equilibration under eclogite-facies conditions, at T of 570 &plusmn, 50 ∘C and P &gt, 1.8 GPa. Post-D2 occurs under greenschist-facies conditions. Numerical modeling of a subduction zone allows exploration of the geodynamic context in which such PT path could have developed, and to make hypotheses about the possible timing of such a scenario, in agreement with the timing generally proposed for the Alpine subduction and collision. Model predictions indicate that pre-D2 mineral assemblages may have developed during Paleocene at 60&ndash, 90 km depth and 115&ndash, 145 km from the trench, or, alternatively, during lower Eocene at ca. 70&ndash, 90 km depth, and 135&ndash, 160 km from the trench.

In the Southern Steep Belt, Italian Central Alps, relicts of the pre-Alpine continental crust are preserved. Between Valtellina and Val Camonica, a poly-metamorphic rock association occurs, which belongs to the Austroalpine units and includes two classically subdivided units: the Languard-Campo nappe (LCN) and the Tonale Series (TS). The outcropping rocks are low to medium grade muscovite, biotite and minor staurolite-bearing gneisses and micaschists, which include interlayered garnet- and biotite-bearing amphibolites, marbles, quartzites and pegmatites, as well as sillimanite-bearing gneisses and micaschists. Permian intrusives (granitoids, diorites and minor gabbros) emplaced in the metamorphic rocks. We performed a detailed structural, petrological and geochronological analysis focusing on the two main lithotypes, namely, staurolite-bearing micaschists and sillimanite- bearing paragneisses, to reconstruct the Variscan and Permian-Triassic history of this crustal section. The reconstruction of the tectono-metamorphic evolution allows for the distinction between two different tectono-metamorphic units during the early pre-Alpine evolution (D1) and predates the Permian intrusives, which comprise rocks from both TS and LCN. In the staurolite- bearing micaschists, D1 developed under amphibolite facies conditions (P=0.7- 1.1 GPa, T=580-660◦C), while in the sillimanite-bearing paragneisses formed under granulite facies conditions (P=0.6-1.0 GPa, T>780◦C). The two tectono-metamorphic units coupled together during the second pre-Alpine stage (D2) under granulite-amphibolite facies conditions at a lower pressure (P=0.4-0.6 GPa, T=620-750◦C) forming a single tectono-metamorphic unit (Languard- Tonale Tectono-Metamorphic Unit), which comprised the previously distinguished LCN and TS. Geochronological analyses on zircon rims indicate ages ranging between 250 and 275 Ma for D2, contemporaneous with the emplacement of Permian intrusives. This event developed under a high thermal state, which is compatible with an extensional tectonic setting that occurred during the exhumation of the Languard-Tonale Tectono-Metamorphic Unit. The extensional regime is interpreted as being responsible for the thinning of the Adriatic continental lithosphere during the Permian, which may be related to an early rifting phase of Pangea.

In the Sesia-Lanzo Zone (SLZ), the subunit Rocca Canavese Thrust Sheet (RCT) is characterised by a mixture of mantle- and crust-derived lithologies and can be a good candidate to be a former subduction-related melange of the Austroalpine domain. The unit consists of metapelites, metagranitoids, metabasics, metagabbro and serpentinised lherzolite lenses from metre to hundred-metre size.According to the literature, PT peak conditions for all lithologies are 0.8-1 GPa at 300-400°C, in lawsonite-blueschist facies conditions recorded during the Alpine subduction. However recent work describes different mineral assemblages for the metamorphic peak, separating rocks with lawsonite from those with jadeite. Therefore, we refined the meso and microstructural analysis of the tectonic slices of RCT and we performed a detailed thermo-barometry of different metamorphicstages in order to quantify Alpine peak conditions and P-T-d-t paths, and to test whether the RCT represents a subduction-related melange. We focus on metagabbros, Jd-bearing and Lws-bearing glaucophanites since they have the most suitable chemistry allowing to reconstruct the complex evolution of the mixing. Metagabbros and Jd-bearing glaucophanites experienced a D1a metamorphic stage characterised by a pressure of 1.3-1.8 GPa and temperature of 450-550°C, in eclogite facies condition. Lws-bearing glaucophanites experienced a D1b metamorphic stage at a temperature

This work reconstructs the metamorphic evolution recorded in the cobbles of the Ponteranica Formation to be compared with the evolutions of the basements of the central Southern Alps and of the clasts of Aga and Vedello and Dosso dei Galli conglomerates. The Ponteranica Formation is part of the lower volcano-sedimentary cycle of the Permian sequences of the western Orobic basin. These sequences cap the Variscan basement in which different tectonometamorphic units were recognized and contoured. We sampled all the crystalline rock types preserved as clasts such as different types of metapelites, meta-intrusives, granitoids, and tourmalinites and reconstructed the relative chronology of the superposed equilibrium assemblages. Only for metapelitic rocks the PT paths were reconstructed on the ground of mineral chemical analyses and geothermobarometry. The cobbles of the Pontereanica Formation record two types of metamorphic evolution. In Type 1 cobbles, the M1 metamorphic assemblage is characterised by white mica, biotite, garnet, staurolite, ± kyanite and is interpreted as the result of the record of the Variscan collision. This assemblage is followed by M2 assemblage characterised by white mica, chlorite, ± biotite that represents the Variscan exhumation. In type 2 cobbles, relict andalusite and possible cordierite characterise the pre-M2 metamorphic re-equilibration that developed under a high thermal regime. The PT paths recorded in cobbles are consistent with the tectono-metamorphic evolutions recorded in the basement units suggesting that during the deposition of conglomerates, the type of tectono-metamorphic units exposed to erosion were equivalent to that presently exposed in the Orobic Alps.The evolution of type 1 cobbles is similar to the evolution recorded in the clasts of Aga and Vedello conglomerates, whereas the clasts of Dosso dei Galli Conglomerate recorded only the effects of the Variscan tectonic burial. The occurrence of tourmalinite cobbles may imply an age up to Upper Permian / Lower Triassic for the Ponteranica Formation. In this view, the age of the Permian conglomerates of central Southalpine decreases westward with the increase of the thermal maturity of the metamorphic imprints recorded in clasts. These observations are consistent with a westward migration of the extensional tectonics responsible for the basin opening.

International audience; We document structural, geochemical, petrological and 40Ar/39Ar geochronological data performed on Variscan eclogites from the Argentera–Mercantour Massif, southwestern Alps. Based on the high-resolution field mapping, we present new eclogite occurrences and discuss the relationships between eclogites and surrounding migmatites. We recognized for the first time preserved eclogite facies assemblages. Trace elements and REE patterns establish that eclogites protoliths are MORBs contaminated by continental crust. Standard thermobarometry and thermodynamic modeling are consistent with P–T values of 640–740 °C for 1.5 ± 0.25 GPa, coherent with paleo-geotherms predicted for warm subduction of oceanic crust. We inter- pret these eclogites as a dismembered cryptic suture zone. 40Ar/39Ar dating on amphiboles yields an age of 339.7 ± 12 Ma for eclogite retrogression under amphibolite facies conditions. All these data are combined to link the Argentera–Mercantour Massif in the tectonic framework of both pre-Mesozoic Alpine basement and European southern Variscides.

In Val d&rsquo, Ala (Piedmont, Western Alps, Italy), the more interesting rocks for the mineralogical research are represented by rodingites (rich in mineralized veins and fractures) associated with serpentinites in the eclogitized oceanic crust of Piemonte Zone, south of Gran Paradiso Massif. Among the vein-filling minerals, garnets are the most appreciated as mineral specimens and, in less degree despite their vivid and rich colors, for their potential as gem-quality materials. This study provides a complete gemological characterization of five faceted samples and offers new information by means of Synchrotron X-ray computed micro-tomography imaging gem features. Electron-probe microanalysis (EMPA) and laser ablation&ndash, inductively coupled plasma&ndash, mass spectrometry (LA&ndash, ICP&ndash, MS) established that the chemical composition of garnets from different localities, resulted both close to pure andradite, enriched in light rare earth elements (LREE) with a positive Eu anomaly, and grossular-andradite solid solution (grandite), enriched in heavy rare earth elements (HREE). X-ray powder diffraction analyses indicate the possible coexistence of almost pure grossular and andradite. A spectroscopic approach, commonly used with gem-like material, by Raman and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, completes the characterization of the samples. The new data on the textural and geochemical features of the grandite and andradite garnets suggest local growth processes under various chemical and oxidation conditions of metasomatic and metamorphic fluids interacting with the host-rocks. Garnets represent long-lasting mineral records of the complex geological history of the Val d&rsquo, Ala rodingitic dikes during their oceanic- and subduction-related metamorphic evolution.

A tectonic mixing of thin slivers of calcschist, fine-grained gneisses (metasediments and metagranitoids), quartzite, metabasites and serpentinites occurs all along the western margin of central and southern Sesia-Lanzo Zone (SLZ - Austroalpine Domain) at the boundary with Piemonte Zone (PZ) metaophiolites, and extends from Santanel klippe to Lanzo Massif, over a distance of 50 km. All the rocks belonging to SLZ and PZ together underwent four episodes of deformation, giving rise to a complex regional tectonostratigraphy. The metamorphic mineral assemblages marking successive foliations, detected from the central to the southern part of the SLZ western margin, indicate that various metamorphic conditions dominate in different portions of this tectonic mixing. In the southern sector of this “mixing zone” parageneses marking superposed foliations indicate that rocks of SLZ and PZ experienced an early eclogite facies imprint, followed by reequilibration under blueschist facies conditions, and that they were finally widely retrogressed under greenschist facies conditions, during the last two deformational stages. The strong syn-metamorphic deformation of this rock assemblage leaves, at the moment, the interpretation on its origin open: i) the present day configuration could be entirely due to transposition that occurred in a mantle wedge at the early stages of deformation, up to eclogite facies conditions, during active subduction; ii) a detrital origin of these alternating layers of terrigeneous and carbonaceous rocks may correspond to a primary sequence of an extensionally-thinned continental margin reworked in the Alpine subduction system. Similar kinds of “tectonic mélange” of continental and oceanic crustal slices, with variably serpentinized peridotites, are expected to be generated in an hydrated mantle wedge during ablative subduction as it results by 2D numerical simulations of ocean/continent subduction systems.

The Riffelberg-Garten Unit (RGU) is a tectono-stratigraphic unit, belonging to the Zermatt-Saas Zone (ZSZ), characterized by a chaotic mixture of metasediments, metabasites and serpentinites (Bearth, 1967; Dal Piaz & Ernst, 1978; Campari et al., 2004). In particular the RGU is composed by meter to cm-sized clasts of eclogites, glaucophanites, marbles, and serpentinites within a heterogenous matrix of micaschists, marbles and calcschists. This unit mainly occurs in the Valais and in upper Aosta Valley, and its originally complex lithostratigraphy is further complicated by polyphase deformation generated during Alpine subduction, collision, and the subsequent exhumation. Such a complexity makes the origin of this mélange still debated. The multiscale structural analysis was performed over an area of 12km2 in the upper Valtournanche valley (Breuil) and revealed the occurrence of four superposed groups of ductile structures. The new structural map at 1:5000 scale has been integrated by form surface maps performed at more detailed scales (1:100, 1:50, 1:200). Microstructural and petrologic investigations show that the first two groups of structures, D1 and D2, developed under eclogite facies conditions, while D3 and D4 developed under decreasing P and T during the progressive RGU exhumation. New quantitative PT estimates suggest that the thermal regime associated with the development of D1 and D2 structures is compatible with a cold subduction, as the one already inferred for ZSZ. On the contrary the T/P ratio characterizing D3 and D4 results to be higher, indicating that the exhumation occurred in a mature collisional setting.

Multiscale structural analysis and petrological modelling were used to establish the pressure-peak mineral assemblages and pressure–temperature (P–T) conditions recorded in the rodingites of the upper Valtournanche portion of the oceanic Zermatt-Saas Zone (ZSZ; Western Alps, northwestern Italy) during Alpine subduction. Rodingites occur in the form of deformed dykes and boudins within the hosting serpentinites. A field structural analysis showed that rodingites and serpentinites record four ductile deformation stages (D1–D4) during the Alpine cycle, with the first three stages associated with new foliations. The most pervasive fabric is S2 that is marked by mineral assemblages in serpentinite indicating pressure-peak conditions, involving mostly serpentine, clinopyroxene, olivine, Ti-clinohumite and chlorite. Three rodingite types can be defined: epidote-bearing, garnet–chlorite–clinopyroxene-bearing and vesuvianite-bearing rodingite. In these, the pressure-peak assemblages coeval with S2 development involve: (i) epidoteII + clinopyroxeneII + Mg-chloriteII + garnetII ± rutile ± tremoliteI in the epidote-bearing rodingite; (ii) Mg-chloriteII + garnetII clinopyroxeneII ± vesuvianiteII ± ilmenite in the garnet–chlorite–clinopyroxene-bearing rodingite; (iii) vesuvianiteII + Mg-chloriteII + clinopyroxeneII + garnetII ± rutile ± epidote in vesuvianite-bearing rodingite. Despite the pervasive structural reworking of the rodingites during Alpine subduction, the mineral relicts of the pre-Alpine ocean floor history have been preserved and consist of clinopyroxene porphyroclasts (probable igneous relicts from gabbro dykes) and Cr-rich garnet and vesuvianite (relicts of ocean floor metasomatism). Petrological modelling using thermocalc in the NCFMASHTO system was used to constrain the P–T conditions of the S2 mineral assemblages. The inferred values of 2.3–2.8 GPa and 580–660 °C are consistent with those obtained for syn-S2 assemblages in the surrounding serpentinites. Multiscale structural analysis indicates that some ocean floor minerals remained stable under eclogite facies conditions suggesting that minerals such as vesuvianite, which is generally regarded as a low-P phase, could also be stable in favourable chemical systems under high-P/ultra-high-pressure (HP/UHP) conditions. Finally, the reconstructed P–T–d–t path indicates that the P/T ratio characterizing the D2 stage is consistent with cold subduction as estimated in this part of the Alps. The estimated pressure-peak values are higher than those previously reported in this part of ZSZ, suggesting that the UHP units are larger and/or more abundant than those previously suggested.

International audience; In the Argentera-Mercantour Massif, swarms of high-K calc-alkaline lamprophyres intruded into Carboniferous migmatites and early Permian granites, likely in Permian-Triassic times. The dykes collected multiple magmatic injections, the latest of which of alkaline affinity. Syn-intrusive vesicles and straight chilled margins suggest that the lamprophyres emplaced at a shallow crustal level.Lower greenschist facies mineral assemblages replaced lamprophyre igneous minerals as a result of late-intrusive hydrothermal circulation that occurred soon after the dyke emplacement. The hydrothermal event is constrained at T = 300–350°C and P < 0.1 GPa by matching thermobarometry and pseudosection results. During the Alpine collision, the lamprophyres were intersected by upper greenschist facies mylonitic shear zones developed at T = 420–450°C and P = 0.2–0.4 GPa, which are the metamorphic peak conditions of the Argentera-Mercantour Massif in Alpine times.

Co-auteur étranger; International audience; The Gruf complex in the Lepontine Alps is one of the rare occurrences of Phanerozoic ultra-high temperature (UHT)metamorphism in the world, but its age is still a matter of debate. Herewe present LA-ICPMS dating in a petrographic context ofzircon and monazite from a UHT restitic granulite. Zircons and monazites are both included in large crystals and in retrogradesymplectites. In such restitic rocks, partial melting or fluid interactions are unlikely, precluding resetting of the monazitechronometers. Zircon cores yield Permian ages, which are interpreted as the age of charnockitization. They are sometimessurrounded by a narrow rim at 32 Ma. Monazites are strongly zoned, but all yield a 31.8 ± 0.3 Ma age interpreted as the time ofcomplete (re-)crystallization during the UHT paragenesis. We propose that the zircons dated a post-Hercynian metamorphismwhich is responsible for the widespread formation of granulites in the Southern Alps and the crust differentiation. This fluidabsentmelting event produced refractory lithologies, such as restites in charnockites. We suggest that Gruf UHT paragenesis isalpine in age and crystallized from these refractory lithologies.We conclude that the lower restitic crust produced in the Permianhad the ability to achieve UHT conditions during the fast exhumation and heating related to lithospheric thinning in Alpine time.

The Zermatt-Saas Zone was part of the Middle to Late Jurassic Tethyan lithosphere that underwent oceanic metamorphism during Mesozoic time and subduction during Eocene time (HP to UHP metamorphism). In upper Valtournanche, serpentinite, metarodingite and eclogite record a dominant S2 foliation that developed under 2.5±0.3 GPa and 600±20°C during Alpine subduction. Serpentinites contain clinopyroxene and rare zircon porphyroclasts. Clinopyroxene porphyroclasts show fringes within S2 with similar compositions to that of grains defining S2. Zircon cores show zoning typical of magmatic growth and thin fringes parallel to the S2 foliation. These features indicate crystallization of clinopyroxene and zircon fringes during HP syn-D2 metamorphism, related to the Alpine subduction. The U–Pb zircon dates for cores and fringes reveal crystallization at 165±3.2 Ma and 65.5±5.6 Ma, respectively. The Middle Jurassic dates are in agreement with the known ages for the oceanic accretion of the Tethyan lithosphere. The Late Cretaceaous - Paleocene dates suggest that the Zermatt-Saas Zone experienced high-pressure to ultra-high-pressure (HP–UHP) metamorphism at c. 16 Ma earlier than previously reported. This result is in agreement with the evidence that in the Western Alps the continental Sesia-Lanzo Zone reached the subduction climax at least from 70 Ma and was exhumed during ongoing oceanic subduction. Our results are further evidence that the Zermatt-Saas ophiolites diachronically recorded heterogeneous HP–UHP metamorphism.

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.

We develop a two-dimensional thermo-mechanical numerical model in which the formation of oceanic crust and serpentinite due to the hydration of the uprising mantle peridotite has been implemented, with the aim of discussing the behaviour of the lithosphere of the Alps and Northern Apennines during the transition from continental rifting to ocean spreading of the Alpine Tethys. The predictions of the model are compared with natural data related to the Permian–Triassic high-temperature – low-pressure (HT-LP) metamorphism affecting the continental lithosphere and data from the JurassicP–Tevolution of the oceanic lithosphere from the Alps and the Northern Apennines. Our analysis indicates that a thinned continental crust, an ocean–continent transition zone and an oceanic lithosphere characterize the final structure of the system in a poor magma rift pre-Alpine configuration. We also find that mantle serpentinization starts before crustal break-up and that denudation occurs before ocean spreading. The mantle denudation starts several million years before the gabbros/basalt formation, generating an ocean–continent transition zone from the passive continental margin to the oceanic lithosphere of size 160–280 km. The comparative analysis shows that the extension of a hot and weak lithosphere, which promotes the development of hyperextended Alpine margins, better agrees with the natural data. Finally, our comparative analysis supports the hypothesis that the lithospheric extension preceding the opening of the Alpine Tethys did not start in a stable continental lithosphere, but developed by recycling part of the old Variscan collisional suture.

A numerical modelling approach is used to validate the physical and geological reliability of the ablative subduction mechanism during Alpine convergence in order to interpret the tectonic and metamorphic evolution of an inner portion of the Alpine belt: the Austroalpine Domain. The model predictions and the natural data for the Austroalpine of the Western Alps agree very well in terms of P-T peak conditions, relative chronology of peak and exhumation events, P-T-t paths, thermal gradients and the tectonic evolution of the continental rocks. These findings suggest that a pre-collisional evolution of this domain, with the burial of the continental rocks (induced by ablative subduction of the overriding Adria plate) and their exhumation (driven by an upwelling flow generated in a hydrated mantle wedge) could be a valid mechanism that reproduces the actual tectono-metamorphic configuration of this part of the Alps. There is less agreement between the model predictions and the natural data for the Austroalpine of the Central-Eastern Alps. Based on the natural data available in the literature, a critical discussion of the other proposed mechanisms is presented, and additional geological factors that should be considered within the numerical model are suggested to improve the fitting to the numerical results; these factors include variations in the continental and/or oceanic thickness, variation of the subduction rate and/or slab dip, the initial thermal state of the passive margin, the occurrence of continental collision and an oblique convergence.

Petrological investigations supported by multi-scale structural analysis of eclogitized serpentinite in the Zermatt–Saas Zone of the Western Alps allows for the determination of mineral assemblages related to successive fabrics, upon which the P–T–d–t path of these hydrated mantle rocks can be inferred. Serpentinites of the upper Valtournanche, with lenses and dykes of metagabbro and meta-rodingite, display an Alpine polyphase metamorphic evolution from eclogite to epidote-amphibolite facies conditions associated with three successive foliations having different parageneses in these rocks. Serpentinite mainly consists of serpentine with minor magnetite; however, where S1 and S2 foliations are pervasive, metamorphic olivine, together with Ti-clinohumite and clinopyroxene, are also found. The mineral assemblage associated with D1 includes serpentine1, clinopyroxene1, opaque minerals, titanite ± olivine1, Ti-clinohumite1 and ilmenite; the D2 assemblage is the same (±chlorite) but minerals have different compositions. The assemblage associated with D3 comprises serpentine3, opaque minerals, ±chlorite3, ilmenite and amphibole3. Ti-clinohumite is associated with veins that are older than D2 and pre-date D3. Veins that post-date D3 are characterized by amphibole + chlorite or by serpentine. P–T conditions for S2 parageneses evaluated using two pseudosections for different bulk compositions suggest that these rocks experienced pressures >2.5 ± 0.3 GPa at temperatures slightly higher than 600 °C. The late epidote–amphibolite facies re-equilibration associated with D3 and D4 developed during late syn-exhumation deformation related to folding and testifies to a small temperature decrease. These results, which were integrated in the regional framework, suggest that different portions of the Zermatt–Saas Zone registered different P–T peak conditions and underwent different exhumation paths. In addition, the inferred P–T–d–t path suggests that the Valtournanche serpentinites re-equilibrated close to the UHP conditions registered by the Cignana meta-cherts. These results imply that tectonic slices exhumed after UHP metamorphism might be wider than previously reported or that small-size UHP units, tectonically sampled during the Alpine convergence, are more abundant than those that have been detected to date.

Detailed mapping of superposed fabrics and their mineral support allows for reconstruction of the tectonometamorphic evolution of the Ivozio Complex, within the inner portion of the Sesia-Lanzo Zone (Western Italian Alps). The resulting evolution is characterized by a multi-stage structural and metamorphic re-equilibration during Alpine subduction, starting from the pre-Alpine igneous association (Amp0 + Cpx0). The prograde associations begin with S1a marked by AmpI + ZoI which pre-date the growth of GrtI (S1b); successive increase in pressure stabilizes a second generation of Amp + Grt (S1c AmpII + ZoI + GrtII). The growth of prograde lawsonite and omphacite occur during S1d (OmpI + Lws + GrtII + AmpII) within lawsonite-bearing eclogites, while S1e is associated with the break-down of lawsonite, producing the association OmpI + Ky + ZoII + GrtII + AmpII (lws-bearing eclogites); S1d-e stages are associated with AmpII + ZoI + GrtII + OmpI in eclogites. The second generation of penetrative foliation (S2), describing the retrograde evolution, is divided into S2a (AmpII + GrtII + Pg + ZoII) and S2b (Chl + AmpIII + Pg + Ab). The comparison between the reconstructed evolution of the Ivozio Complex and P–T paths inferred in the Southern Sesia-Lanzo Zone suggests a non-uniqueness of the Sesia-Lanzo Zone continental crust, during the Alpine subduction.

Interactions of fabric evolution and chemical parameters driving reaction progress during the amphibolite-to-eclogite transition were investigated in eclogitized amphibolites of the Western Alps. In the Sesia–Lanzo Zone (SLZ), mafic rocks ranging from eclogitized hornblendites to true eclogites occur as layers and boudins within micaschists and are characterized by different modal amounts of amphibole, omphacite, zoisite, garnet, and phengite, constituting the Alpine HP assemblage. Across narrow zones, this array of lithologies displays gradients in planar fabrics characterized by coronitic, S-tectonitic, and mylonitic textures with different extents of eclogitization. Eclogitic parageneses are controlled not only by the bulk rock composition of the protoliths but also by the degree of fabric evolution. This is the case for omphacite occurrence, which is constrained by plagioclase composition and modal amount (NK parameter) in the protoliths, whereas the increase in modal omphacite and the concomitant dec...

Feedback relations between deformation and metamorphic mineral reactions, derived using the principles of non-equilibrium thermodynamics, indicate that mineral reactions progress to completion in high-strain areas, driven by energy dissipated from inelastic deformation. These processes, in common with other time-dependent geological processes, lead to both strain, and strain-rate, hardening/softening in rate-dependent materials. In particular, strain-rate softening leads to the formation of shear zones, folds and boudins by non-Biot mechanisms. Strain-softening alone does not produce folding or boudinage and results in low-strain shear zones; strain-rate softening is necessary to produce realistic strains and structures. Reaction–mechanical feedback relations operating at the scale of 10–100 m produce structures similar to those that arise from thermal–mechanical feedback relations at coarser (kilometre) scales and reaction–diffusion– mechanical feedback relations at finer (millimetre) scales. The dominance of specific processes at various length scales but the development of similar structures by all coupled processes leads to scale invariance. The concept of non-equilibrium mineral stability diagrams is introduced. In principle, deformation influences the position of mineral stability fields relative to equilibrium stability fields; the effect is negligible for the quartz ! coesite reaction but may be important for others. Application of these results to the development of structures and mineral reactions in the Italian Alps is discussed. Metamorphism is commonly associated with deformation and regional thermal gradients, together with chemical reactions, all of which are non-equilibrium situations. In many instances, another non-equilibrium process, namely fluid flow (either of melt or fluids comprised of H2O, CO2 and so on), accompanies the metamorphic process and is intimately coupled to deformation, infiltration, heat advection and chemical reactions. This paper explores the influence of energy dissipated by such processes upon: (i) the development of meso-scale structures commonly observed in deformed metamorphic rocks; (ii) the kinetics of metamorphic reactions; and (iii) the thermodynamic stability of metamorphic assemblages. To focus the discussion we concentrate on metamorphic rocks undergoing deformation and chemical reactions with no large-scale mass infiltration (i.e. metasomatism) so that fluid flow is considered insignificant and also any regional thermal gradients are neglected. As such, the energy dissipated during deformation and metamorphism consists of four parts: (i) that due to mechanical processes; this comprises dissipation arising from inelastic deformation and from the work done in forming chemical components in the deforming–reacting system by mineral reactions; (ii) that arising from the diffusive flux of chemical components across gradients in both chemical potentials and local gradients in temperature; (iii) that arising from chemical reactions; and (iv) that arising from thermal conduction. In the absence of metasomatism at the outcrop scale, many effects arising from mass flux and thermal transport turn out to be unimportant. An important ingredient of the interaction between these dissipative processes is that of spatial scale, so that various processes dominate at different length scales. Thermal feedback effects dominate at scales larger than the outcrop scale, and mass transfer feedback effects dominate at the micro-scale. The dominance of these different processes at specific length scales and the development of similar geological structures at all scales by different coupled processes are the two principles at the heart of the scale invariance of structures observed in geology. This separation of spatial scales also From: SPALLA, M. I., MAROTTA, A. M. & GOSSO, G. (eds) Advances in Interpretation of Geological Processes: Refinement of Multi-scale Data and Integration in Numerical Modelling. Geological Society, London, Special Publications, 332, 189–223. DOI: 10.1144/SP332.12 0305-8719/10/$15.00 # The Geological Society of London 2010. greatly simplifies any study of the coupling effects. We include a brief consideration of the influence of metasomatism in the Discussion section. In many metamorphic rocks there is evidence of non-equilibrium in the form of partially reacted mineral assemblages. An example is the Monte Mucrone–Monte Mars region in the Italian Alps (Sesia–Lanzo Zone: Zucali et al. 2002) where igneous relicts are preserved in a subduction complex; here in rocks immediately adjacent to each other, the metamorphic reactions reach completion only in highly deformed shear zones which are ubiquitous. In such rocks the progress of mineral reactions correlates with the amount of strain and not necessarily with the maximum pressure and temperature (P, T ) conditions reached. The important questions are: (i) What roles do mineral reactions play in promoting deformation and influencing the style of deformation, particularly the development of shear zones, folds and boudinage? (ii) What role does deformation play in promoting metamorphic reactions? (iii) In regions where the metamorphic reactions have not proceeded to completion and deformation accompanies the reactions, are estimates of pressure and temperature conditions derived from equilibrium theory relevant and/or accurate, and what influence does deformation have on the position of the mineral phase boundary in pressure–temperature space established by using equilibrium thermodynamics? The questions are summarized in Figure 1. Although there is a wealth of literature published on these issues, there is no discussion that integrates them within one framework. We attempt to do this within the concepts of modern non-equilibrium thermodynamics. The structure of the paper is as follows. We first outline the observations of partially completed metamorphic reactions and the relationship to deformation observed in the Italian Alps. We then briefly review the development of non-equilibrium thermodynamics as it is relevant to the coupling of geological deformation and metamorphic reactions. Next we consider the influence of metamorphic reactions on constitutive behaviour, together with the influence of mineral reactions on deformation with particular emphasis on localization, folding and boudinage. The concept of non-equilibrium mineral phase diagrams is then introduced, and we discuss the influence of deformation on mineral stability. Finally, we consider the implications of this non-equilibrium framework for metamorphic rocks in general, and the Italian Alps in particular. Terms are defined as they are introduced in the text and in Table 1. Examples of extent of reactions as a function of degree of deformation from the Italian

Preservation of stratigraphic settings in continents is mostly confined to uppermost crustal levels, in which prevailing deformation is translational and internal strain of tectonic units is weak. This does not specially depend on association of metamorphism to tectonic processes, but rather on mechanical properties of deforming multilayers. Scattered findings of sedimentary features, not supported by structural investigation on the actual nature of lithologic layering, may lead to failure in determining size of preserved sedimentary sequences. In the intermediate and lower crust, interaction of metamorphism and deformation down to granular scale facilitates construction of new types of lithologic layers and segregation of thick differentiated mineral layerings that mimic stratigraphic sequences. Critical examples of difficulties encountered in different tectonic contexts when assessing the sedimentary origin and related stratigraphic meaning of variously deformed layered sequences are summarised. In deep subduction-collision zones similar or contrasted lithostratigraphies are of little help in definition of tectonic units; the structural and metamorphic reworking of rocks of contrasted origin constructs tectonic units that repeatedly couple and decouple from similar adjacent sequences and their actual relative mechanical paths may be disclosed by a combined structural and petrologic analytical tool delimiting volumes that experienced equivalent structural histories and metamorphic signatures (contouring of tectono-metamorphic units=TMUs); these units constitute valuable elements of correlation in metamorphic belts and for the investigation of mechanisms of lithosphere dynamics.

When continental rifting does not develop on a stable continental lithosphere, geodynamic interpretation of igneous and metamorphic records, as well as structural and sedimentary imprints of rifting-related lithospheric extension, can be highly ambiguous since different mechanisms can be responsible for regional HT–LP metamorphism. This is the case of the European Alps, where the exposure of Variscan structural and metamorphic imprints within the present-day Alpine structural domains indicates that before the Pangaea break-up, the continental lithosphere was thermally and mechanically perturbed by Variscan subduction and collision. To reduce this ambiguity, we use finite-element techniques to implement numerical geodynamic models for analysing the effects of active extension during the Permian–Triassic period (from 300 to 220 Ma), overprinting a previous history of Variscan subduction-collision up to 300 Ma. The lithosphere is compositionally stratified in crust and mantle and its rheological behaviour is that of an incompressible viscous fluid controlled by a power law.Model predictions of lithospheric thermal state and strain localization are compared with metamorphic data, time interval of plutonic and volcanic activity and coeval onset of sedimentary environments. Our analysis confirms that the integrated use of geological data and numerical modelling is a valuable key for inferring the preorogenic rifting evolution of a fossil passive margin. In the specific case of the European Alps, we show that a relative high rate of active extension is required, associated for example with a far extensional field, to achieve the fit with the maximal number of tectonic units. Furthermore, in this case only, thermal conditions allowing partial melting of the crust accompanying gabbroic intrusions and HT–LP metamorphism are generated. The concordant set of geological events that took place from Permian to Triassic times in the natural Alpine case is justified by the model and is coherent with the progression of lithospheric thinning, later evolving into the appearance of oceanic crust. Igneous and metamorphic records, as well as structural and sedimentary imprints of rifting-related lithospheric extension, are frequently ambiguous. This is the casewith high-temperature–low-pressure (HT–LP) metamorphism, which is undoubtedly related to high thermal regimes and can be consequent to the late orogenic collapse of a collisional belt or to lithospheric thinning, thereby promoting continental rifting (e.g. Thompson 1981; Wickham & Oxburgh 1985; Sandiford & Powell 1986; Platt 1998; Beardsmore & Cull 2001). The same uncertainty holds for the normal faulting that can accommodate the lateral expansion of the thickened axial part of a mountain belt during the final stages of continental collision, when gravitational forces dominate the horizontal stress field or, alternatively, accommodate lithospheric thinning in the axial zone of continental rifting (e.g. Wernicke 1985; Molnar & Lyon-Caen 1988; Keen et al. 1989; Malavieille 1993; Vissers et al. 1995). This interpretative ambiguity is enhanced when continental rifting does not develop on a stable continental lithosphere, but follows the thermal and mechanical instabilities induced by a subduction–collision process in which the rifting precursor signals overprint the markers of a late orogenic extension. This latter case corresponds to that of the European Alps, where the exposure of Variscan structures and metamorphic imprints within the present-day Alpine structural domains indicates that before the Pangaea break-up the continental lithosphere was thermally and mechanically perturbed by Variscan subduction and collision. While the metamorphic and igneous records of Variscan orogeny are widespread in the European continental crust, a diffuse igneous activity associated with HTmetamorphism accounting for a Permian–Triassic high thermal regime is peculiar to the Alpine area. The overprint of HT From: RING, U. & WERNICKE, B. (eds) Extending a Continent: Architecture, Rheology and Heat Budget. Geological Society, London, Special Publications, 321, 33–72. DOI: 10.1144/SP321.3 0305-8719/09/$15.00 # The Geological Society of London 2009. Permian–Triassic evolution on the HP relics of the Variscan subduction and collision has been interpreted as induced either by late-orogenic collapse or by lithospheric extension and thinning leading to continental rifting (e.g. Lardeaux & Spalla 1991; Diella et al. 1992; Gardien et al. 1994; Ledru et al. 2001). Even the interpretation of the geodynamic environment responsible for the development of intracontinental basins hosting the Permian volcanic products allows two possible alternatives, one envisaging a strike-slip dominated regime (Arthaud & Matte 1977; Cassinis & Perotti 1994), which is compatible with the evolution of a mature collisional setting (Molnar & Lyon-Caen 1988), the other a continental rifting tectonic setting (Siletto et al. 1993; Selli 1998; Staehle et al. 2001). In both cases the continental rifting promoting Mesozoic opening of the ocean in a lithosphere thermally softened and thinned by slab break-off processes is generally accomplished in the final stages of continental collision. Trying to resolve the dualistic interpretation of the geodynamic significance of the Permian–Triassic high thermal regime, Marotta & Spalla (2007) and Spalla & Marotta (2007) used numerical models to simulate ocean subduction leading to continental collision, lithospheric detachment and subsequent gravitational thermal relaxation. Comparing predictions from each model with natural data, they concluded that the Permian–Triassic metamorphic and igneous imprints in the Alps cannot simply result from the thermal relaxation consequent to lithospheric unrooting during late orogenic extension, and that a supplementary mechanism, such as hot mantle upwelling under continental plates, was necessary to satisfy the Permian–Triassic geothermal state required to account for the natural metamorphic data. Indeed, the lithospheric thermal detachment was premature (360 Ma) with respect to natural thermal signals (290–225 Ma). Our goal here is to expand the preliminary analysis by Marotta & Spalla (2007) on the effects of a forced extension during the Permian–Triassic period (from 300 to 220 Ma) via new models characterized by extension velocities varying from no extension to 2.0 cm a. The initial configuration corresponds to the geodynamic setting at 300 Ma after simulation of Variscan subduction–collision in Marotta & Spalla (2007). Model predictions of lithospheric thermal state and strain localization at different structural levels are compared to a broader set of metamorphic data corroborated by the emplacement conditions of mafic intrusions, time interval of plutonic and volcanic activity and coeval onset of sedimentary environments. It is worth noting that the present study, however, recovers natural data from a fragmented setting carrying reminiscences of rifted margins after the subduction and collision of the Alpine orogeny, whose hinterland and foreland obviously preserve the rifting imprints better. Orogenic fragmentation of the rifted margins greatly complicates the heterogeneous structural patterns inferred from seismic exploration of present-day passive margins (e.g. Boillot et al. 1995) and, hence, similarly reproduced by analogue modelling (e.g. Brun & Beslier 1996). The integrated use of natural data and numerical modelling may help to infer the puzzled rifting evolution even in a fossil passive margin. Geological framework

New detailed maps of two key areas of the Southalpine basement unravel the superposition of pre-Alpine deformation stages associated with different metamorphic imprints, and distinguish Alpine thrust and fold structures involving basement and Permian-Mesozoic sedimentary cover. Basement rocks consist of metasediments (micaschists, quartzites and paragneisses) and metagranitoids and cover rocks comprise Permian volcanics and sediments. The boundaries between two tectono-metamorphic units have been distinguished at the map scale, thanks to the integrated use of structural and petrological analyses, on the basis of foliation trajectories supported by metamorphic assemblages. Two different pre-Alpine metamorphic evolutions characterise a basement portion that seemingly appears as a monotonous sequence dominantly affected by a greenschist-facies imprint. The volcano-sedimentary sequence of Permian age has been used as the time marker that separates Alpine from pre-Alpine superposed structures; Alpine deformation consists of two superposed groups of structures, the first of which is locally associated with a very low-grade metamorphic imprint and related at the regional scale to a south-verging thrust system.

The evolution of the pre-Alpine Corio and Monastero metagabbros points to strong chemical and mineralogical similarities with that of other Permian gabbro bodies of the Alps, which are concentrated in the Southalpine and Austroalpine domains. The structural and metamorphic pre-Alpine evolution of these gabbros records a re-equilibration following the emplacement in the deep crust (P=0.6–0.9 GPa and T=850±70 °C), exhumation through amphibolite facies conditions (P=0.5–0.35 GPa and T=570–670 °C), followed by a greenschist facies imprint (0.25≤P≤0.35 GPa and T

The microstructural and petrological evolution of pre-Alpine metabasites from the Southern Alps allow to compare the P–T–d–t paths in the three adjacent basement zones of Domaso-Cortafo (DCZ), Dervio-Olgiasca (DOZ) and Monte Muggio (MMZ) for delineating the Variscan and post-Variscan evolutions of the deep and middle crust in the Lake Como region. The DCZ rocks show a P–T evolution characterized by a pre-D2 almost isobaric T-increase up to T = 560–650°C and P = 0.7–1.1 GPa during D2 deformation. A decompressional cooling characterizes the exhumation during which D3 deformation took place. The DOZ amphibolites show a P–T–d–t path in which the physical metamorphic conditions predating D2 were: T = 530–590°C and P = 0.8–1.2 GPa. D2 deformation developed during a decompression accompanied by a T-increase (T = 630–710°C and P = 0.4–0.7 GPa). The final exhumation stage is characterized by cooling occurring during a slight decompression (T

The concept of ‘metamorphic field gradient’ applied to the lithologically homogeneous deep crust of the Southern Alps suggests the existence of two metamorphic units. The comparison of P-T-d-t paths, derived in adjacent portions of this basement and supported by a continuous foliation trajectory map helps to distinguish three tectono-metamorphic units, corresponding to the Domaso–Cortafo, Dervio–Olgiasca and Monte Muggio zones. The degree of granular scale reorganisation of the prevailing planar fabric is considered in the three zones together with the relative chronology of the structural imprints and the metamorphic environments in which they developed. This approach emphasizes that the dominant metamorphic imprint of each unit coincides with that of the most pervasive fabric at the regional scale when the degree of fabric evolution is sufficiently high, and not with the Tmax–PTmax recorded in each tectono-metamorphic unit. In terrains that underwent polyphase deformation and metamorphism the ‘metamorphic field gradient’ concept cannot therefore be utilised to discriminate tectono-metamorphic units.

A review of P-T peaks and paths of eo- and meso-Alpine eclogite fades rocks occurring along the axial part of the Alpine chain shows that rocks re-equilibrated under high- and low- T (group-B and -C eclogites), are, respectively, hosted within a lower and an upper tectonic level of the Penninic nappe system. If P-T estimates for eclogites are considered peak conditions the two crustal portions, otherwise undistinguishable, were sutured during the collision of the European and Adriatic continental plates, which corresponds to the latest tectonic mechanism of eclogitization. Before collision, formation and preservation of eclogitic rocks up to shallow levels was assisted by subduction of the cold oceanic crust. The two lithospheric processes of oceanic subduction and continental collision, though separated in time, contribute to continuous generation of eclogites under thermal conditions that evolve from higher to lower P-T ratios from the end of ocean consumption. Exhumation trajectories are characterized by low- or high thermal regimes in the same structural domain in different parts of the chain (Western and Eastern Austroalpine), in the same part of the chain (Penninic and ophiolites in Western, Central and Eastern Alps), or even within the same nappe (Dora-Maira, Gran Paradiso and Adula). Late orogenic collapse or slab breakoff processes may have caused late heating at very low pressure (0.3 GPa) during exhumation in some units of the Pennine nappes and ophiolites Mechanisms of nappe emplacement are demonstrably multiphase and inferences on palaeogeographic derivation of eclogitic units can be drawn from interpretation of P-T trajectories.

During the annual meeting of Gruppo Italiano di Geologia Strutturale (GIGS), held in Milano on 28–30 October 2013 (Zucali et al., 2013), a poster session was dedicated to structural mapping of the Mediterranean region. The goal was to highlight the structural–geological mapping skills of the GIGS community, in contrast to the tendency of the Italian evaluation agency to disregard, or underestimate, mapping work that is undertaken. The session collected a wide number of highquality contributions, at scales adapted to highlight local problems. Significant attention within the community was drawn to the importance of primary data sources; these control the quality of laboratory results and subsequent geological conclusions. The success of the poster session formed the basis for this special issue focused upon structural geological mapping in the Mediterranean. It includes many different areas of the Italian geological domains, from North to South as well as Islands. The structural maps presented in this collection of 16 contributions represent the tools that have been fundamental to: (i) unravel the tectonic histories across old and recent orogenic belts, from Variscan convergence to different stages of Alpine-Apenninic evolution, across different structural levels; (ii) infer deformation mechanisms and the main tectonic systems acting at various structural levels and under different tectonic regimes; (iii) prospect resources or plan tunneling across mountain chains. Maps encompass various geological studies such as geothermal reservoirs (Montanari et al., 2015), brittle tectonics (D’Adda & Zanchetta, 2015; Festa, Fioraso, Bissacca, & Petrizzo, 2015; Ghiselli, Zucali, & Bini, 2015) and present-day seismotectonic evolution (Romagnoli, Catalano, Pavano, & Tortorici, 2015), continental and oceanic lithosphere-scale histories (Casini et al., 2015; Fazio, Cirrincione, & Antonino, 2015; Federico, Crispini, Malatesta, Torchio, & Capponi, 2015; Filice et al., 2015; Fontana, Tartarotti, Panseri, & Buscemi, 2015; Frassi, 2015; Funedda, Meloni, & Loi, 2015; Maino, Bonini, Dallagiovanna, & Seno, 2015; Ortolano, Cirrincione, Pezzino, Tripodi, & Zappala, 2015; Rebay, Maroni, Siletto, & Spalla, 2015; Schiavo et al., 2015). All maps have been constructed from the integration of structural analysis with stratigraphy, petrology, geochemistry-geochronology and geothermics and synthesizing data from multiscale structural investigation that can be organized in solid frameworks as geo-databases (Ortolano et al., 2015).

In connection with movements between Europe and Adria, the Southern Alps underwent extension leading to the formation of the Middle Jurassic South-Alpine passive continental margin. As a result of mainly S-vergent Alpine thrusting and folding, a substantially preserved Mesozoic crustal section, ranging from the surface to a paleo-depth of ca. 15 km is exposed in the Lugano-Lake Como area. Tectonic processes associated with Permo-Mesozoic extension can thus be investigated at different crustal levels. Rifting began with a thermal anomaly during which rocks at middle-crustal levels were sheared at ca. 650–750°C. Deformation was distributed on a several km wide zone and late-kinematic pegmatites were emplaced at this stage. Temperatures then decreased and beginning in the Norian, extension was accommodated by a major, E-dipping normal fault, the Lugano-Val Grande normal fault. The fault which can be followed down to a paleo-depth of ca. 12 km, was listric and flattened (to 20°) at ca. 7–9 km depth. In the upper 8–10 km, deformation was only brittle, whereas at deeper crustal levels greenschist mylonites were formed. With continued normal faulting the colder hanging wall cooled the fault zone and greenschist mylonites were progressively replaced by lower-temperature ultramylonites and cataclasites. The thickness of the fault zone varies from some tens of meters in the upper 5–6 km to several hundred meters at deeper levels: deformation was thus discrete at a crustal scale.

The effects of the hydration mechanism on continental crust recycling are analyzed through a 2-D finite element thermomechanical model. Oceanic slab dehydration and consequent mantle wedge hydration are implemented using a dynamic method. Hydration is accomplished by lawsonite and serpentine breakdown; topography is treated as a free surface. Subduction rates of 1, 3, 5, 7.5, and 10 cm/yr; slab angles of 30°, 45°, and 60°; 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 exists between mantle rheology and the amount of recycled crustal material: the larger the viscosity contrast between hydrated and dry mantle, the larger the percentage of recycled material into the mantle wedge. Slab dip variation has a moderate impact on the recycling. Metamorphic evolution of recycled material is influenced by subduction style. TPmax, generally representative of eclogite facies conditions, is sensitive to changes in slab dip. A direct relationship between subduction rate and exhumation rate results for different slab dips; this relationship does not depend on the used mantle flow law. Thermal regimes predicted by different numerical models are compared to PT paths followed by continental crustal slices involved in ancient and recent subduction zones, making ablative subduction a suitable precollisional mechanism for burial and exhumation of continental crust.

Subduction-collision zones are characterized by the amalgamation and disaggregation of lithospheric slices; such processes work in competition in constructing the tectonic architecture of metamorphic belts. Determination of contours for thermally and structurally characterized units is crucial to define the variations in sizes of such slices involved in the dynamic evolution of an active margin. The dimensions of these entities change over time and must be reconstructed using the structural and metamorphic evolution of the basement rocks as tracers, rather than by simply relying on lithologic associations. They constitute tectono-metamorphic units (TMUs) and represent discrete portions of the orogenic crust influenced by a sequence of metamorphic and textural changes. Their translational trajectories and shape changes during deformation cannot simply be derived from the analysis of the geometries and kinematics of tectonic units but from a joint reconstruction of quantitative P-T-d-t paths. The TMU investigation tool bears a marked thermo-tectonic connotation and, through modelling, offers the opportunity to test the physical compatibilities of interconnected variables, such as density, viscosity, and heat transfer, with the interpretative geologic history. Comparison between modelling predictions and natural data obtained by this analytical approach has helped solve longstanding ambiguities on the pre-Alpine and Alpine geodynamic evolution of the different continental units of the Central and Western Alps and explore the crustal levels of protolith derivation. Three-dimensional estimation of structurally and chemically re-equilibrated volumes aids in the evaluation of physical parameters chosen for the numerical modelling.