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International audience; Strain localisation is a key process that allows for the emergence of tectonic plates and controlstheir long-term deformation. Upper crustal levels are relatively cold and their rheology is thusgoverned by frictional plasticity. In order to predict the formation of tectonic plates and quantifythe deformation of the Earth's upper shell, geodynamic modelling simulation tools must reliablyaccount for deformation in the frictional plastic realm.Nevertheless, the simulation of frictional plastic strain localisation poses severe issues.Commonly employed implementations (visco-plastic and visco-elasto-plastic) often fail toaccurately satisfy force balance and suffer from a lack of convergence upon mesh refinement.These problems are intimately linked to the fact that commonly employed models do notencompass any characteristic spatial or temporal scales of localisation. Various regularisationtechniques can thus be used as a remedy. Here we investigate three popular regularisationtechniques, namely viscoplasticity, gradient plasticity and the use of a Cosserat medium, anddiscuss their potential application for geodynamic modelling.Powered by

International audience; Metamorphic reactions can lead to drastic changes in rocks mechanical properties. Indeed, duringsuch transformations, the nucleation of new phases with different strength, grain size and/ordensity compared to the primary phases is enhanced, and transient processes due to the ongoingreaction are then activated.Eclogitization of lower crustal rocks during continental subduction constitutes an emblematictransformation illustrating these processes. In such tectonic context, it has been shown thateclogitization seems to be closely associated with the occurrence of seismogenic events. However,the mechanisms that trigger brittle failure in such high pressure environments remain highlydebated. Indeed, whether the change in density or the change in rheology can lead toembrittlement is still enigmatic.By using 2D compressible mechanical numerical models we studied the impact of the strongnegative volume change of the eclogitization reaction on the rocks rheological behaviour. We showthat eclogitization-induced density change occurring out of equilibrium can, by itself, generatessufficient shear stress to fail the rocks at high-pressure conditions.Rupture initiation at depth in continental subduction zones could therefore be explained byvolume changes, even without considering the modifications of the rheological properties inducedby the transformation. Our results also indicate that the negative volume change associated withbrittle failure can enhance the propagation of the eclogitization process by a runaway mechanismas long as the reaction is not limited by the lack of reactants.Powered by

International audience; The Western Gneiss Region in Norway is constituted by a crustal nappe stack that comprises someof the best-preserved exhumed ultra-high pressure (UHP) terranes on Earth. The UHP rocks resultfrom the subduction of the western edge of the Baltica craton beneath Laurentia during theCaledonian orogeny. Mafic eclogites form lenses within granitoid orthogneisses and show the bestrecord of the pressure and temperature evolution. Their exhumation from the UHP conditions hasbeen largely studied, but the prograde evolution has been rarely quantified in the eclogitesalthough it constitutes an important constraint on the tectonic history of this area. This studyfocused on an unaltered eclogite sample from Vågsøy in the Nordfjord region. This sample wasinvestigated using a large panel of methods including phase-equilibria modelling, trace-elementanalyses of garnet, trace- and major-element thermo-barometry and quartz-in-garnet barometryby Raman spectrometry. The eclogite comprises omphacite, garnet, white mica, epidote andamphibole and accessory rutile, quartz, zircon, carbonates and kyanite. Garnet shows a grossularrichcore with inclusions of quartz, epidote, white mica and amphibole, while grossular-poor rimsare enriched in pyrope and middle rare-earth elements and include omphacite and rutile.Inclusions in garnet core point to crystallisation conditions in the amphibolite facies at 550–600 °Cand 11–15 kbar, while chemical zoning in garnet suggests growth during isothermal compressionup to the peak pressure of 28 kbar at 600 °C, followed by near-isobaric heating to 640–680 °C.Isothermal decompression to 8–13 kbar is recorded in fine-grained clinopyroxene-amphiboleplagioclasesymplectites. The absence of a temperature increase during compression seemsincompatible with the classic view of crystallization along a geothermal gradient in a subductionzone and may question the tectonic significance of eclogite-facies metamorphism. Two maintectonic scenarios are discussed to explain such an isothermal compression: (1) either the maficrocks were originally at deep level within the lower crust and were then buried along theisothermal part of the subducting slab, or (2) the mafic rocks recorded significant tectonicoverpressure at constant depth and temperature conditions during the collisional stage of theorogeny. A multi-chronometer geochronological study is currently performed and expected tobring additional, discriminant constraints on this P–T evolution.

International audience; Eclogitization reactions in mafic rocks involve large volume changes, porosity evolution and fluidtransfer. They impact many important geological processes such as the localization of deformationand fluid channeling at intermediate depth in subduction zone. The study of exhumed eclogiticbodies in orogens shows that eclogitization of the oceanic crust is heterogeneous from both astructural and metamorphic point of view. For example, in the European Alps, the Allalinmetagabbro shows high strain areas, consisting of hydrous metagabbros, fully equilibrated undereclogite-facies conditions during the Alpine orogeny. Conversely, large volumes of low strain, fluidundersaturatedgabbros remained largely unaffected by the high-pressure (HP) metamorphism,locally preserving igneous textures and even, occasionally, relics of their magmatic mineralogy.The intensity of deformation as well as the degree of eclogitization in the metagabbro have beenshown to be directly related to the extent of pre-Alpine hydration during high-temperaturehydrothermal alteration [1]. However, the influence of this degree of hydration on (1) reactionkinetics and/or (2) enhancing rheological contrasts leading to heterogeneous deformationpatterns and metamorphic conditions is still debated.In order to address this issue, we propose a multidisciplinary study involving petrographic andmicrotextural observations combined with 2D thermo-mechanical numerical models allowing todiscuss the role of pre-Alpine hydrothermal alteration on the development of HP metamorphicassemblages.We present petrographic and textural data from three different types of rocks from the Allalinmetagabbros: i) undeformed and mostly untransformed metagabbros, with relics of igneousaugite and plagioclase, ii) coronites, with olivine pseudomorphs showing different levels ofhydration, rimmed by a garnet corona, and iii) eclogitized metagabbros, with olivine andplagioclase sites fully replaced by high-pressure assemblages.The role of protolith hydration on the observed range in metamorphic facies is then tested byusing 2D thermo-mechanical models that allow to simulate the deformation of a strong and dryrock with several randomly oriented weak and hydrous zones. Our results show that the shearingof heterogeneous rock can lead to the formation of localized ductile shear zone within a matrixthat remains relatively undeformed but where plastic deformation can occur. The associated P field is also highly heterogeneous, with P ranging from 1 to 3 GPa. The deformation patterns and Pmodelled may suggest that locally hydrated portions of the gabbro acted as rheologicalperturbations sufficiently efficient in producing the structural and metamorphic record nowobserved in the field.[1] Barnicoat, A. C. & Cartwright, I. (1997) Journal of Metamorphic Geology 15, 93–104

International audience; Polymorphic transformations are key tracers of metamorphic processes, also used to estimate thepressure and temperature conditions reached by a rock. In particular, the quartz-coesite transitionis commonly used to define the lower boundary of the ultrahigh-pressure (UHP) metamorphicfield. The partial preservation of coesite included in garnets from UHP rocks bring considerableinsights into the burial and exhumation mechanisms of the continental crust involved inconvergent zone. Coesite was first described in the Western Alps by Chopin[1], in the Dora-Mariawhiteschist, one of the most emblematic UHP rock worldwide. Although the partial preservation ofcoesite inclusions in garnet has long been attributed to the pressure vessel effect, theinterrelationship and relative timing between fracturing and retrogression is still contentious.Here we study the reaction-deformation relationships of coesite inclusions initially enclosed ingarnet and transforming into quartz during the decompression process. We combine 2Dnumerical thermo-mechanical models constrained by pressure-temperature-time (P-T-t) estimatesfrom the Dora-Maira whiteschist. The model accounts for a compressible visco-elasto-plasticrheology including a pressure-density relationship of silica based on thermodynamic data. Thisallows us to study the effect of reaction-induced volume increase during decompression. Ourresults capture the typical fracture patterns of the host garnet radiating from retrogressed coesiteinclusions and can be used to study the relative role of volume change associated with a change ofP-T conditions on the style of deformation during decompression.The mechanisms of the coesite-quartz transformation and geodynamic implications are presentedand validated against geological data. The effect of fluids on the phase transition and theconditions of access of fluids during the transformation are discussed in the light of the results ofthe thermo-mechanical models.This study demonstrates the high potential of thermo-mechanical modelling in enhancing ourunderstanding of the processes involved in the formation and evolution of metamorphic minerals.[1]Chopin (1984) Contributions to Mineralogy and Petrology 86, 2, 107-118

Intermediate-depth earthquakes are registered in convergence zones where crustal rocks are expected to deform by ductile flow. This paradox is also evidenced in exhumed crustal rocks where brittle structures (e.g., pseudotachylytes and breccias) associated to high-pressure metamorphism have been documented. If the link between brittle deformation and metamorphic reactions appears obvious today, the mechanism involved is still a burning issue. We propose that the initial heterogeneity of rocks, by itself, is sufficient to trigger both metamorphic reaction and brittle deformation. Based on a mechanically consistent dynamic model, we show that local pressure variations due to pre-existing heterogeneities can be high enough to reach the thermodynamic conditions required for reaction initiation. Brittle behaviour is then controlled by the strength difference between the untransformed host rock and its reaction product. This continuous process also explains the higher pressures recorded in eclogite facies rocks of ductile shear zones compared to their brittle host rock. Our results, constraint by natural data, have therefore significant implications for intermediate-depth seismicity.

International audience; Depending on observed parageneses, the pressure-temperature (P-T) conditions experienced by rocks can beevaluated and quantified by using thermodynamic calculations. Time (t) is obtained by dating mineralsinvolved in symptomatic parageneses, considered at equilibrium. The resulting P-T-t paths are then used toconstrain the tectonic history, most of the time, by assuming that the pressure recorded by metamorphicrocks directly depends on burial depth. However, in regions where deformation is large and heterogeneous,like in the Alps, the above assumption is questionable. I will show that the pressure history, including acatastrophic pressure drop at the onset of exhumation, as recorded in most well documented high-pressuremetamorphic rocks worldwide (including those from the Alps) fit remarkably well with a two-fold tectonichistory, from compressional during subduction to extensional during exhumation.This contradicts the common assumption that (ultra)high-pressure rocks exhume during continentalcollision. However, numerous natural examples of (ultra)high-pressure rocks, from early Paleozoic to lateTertiary, document that exhumation occurred in extension either driven by slab rollback prior to continentalcollision (e.g. in numerous backarc basins) or by eduction, after continental collision (e.g. NorwegianCaledonides). The so-called “Alpine collision” appears to correspond to two successive events of slabrollback leading to the extensional exhumation of (ultra)high-pressure rocks, prior to continental collision.

International audience; Ore deposit formation requires the synchronisation of multiple processes with transportation of fluids and heat. Ancient inherited structures found within the basement of the Athabasca Basin were formed as a result from of orogenic processes (i.e nucleation and accretion processes). It is established that their reactivation played a significant role for the formation of the unconformity-related uranium deposits. The time gap between the inherited structures' inception and mineralisation event is sufficient for any traces of hydrothermal systems to reach steady state. The aim of this study is to determine the role for these inherited structures affecting the hydrothermal dynamics, by using a pre-existing tectonic structure generated from a thermo-mechanical study and incorporating fluid-thermal processes until steady-state conditions. The models are then experimented with permeability fields of increasing complexity in three types of tectonic scenarios. Results indicate that deep-seated structures operate as fluid-thermal conduits that bring fluids and heat towards the upper portions of the crust. In addition, thermal convection occurs at favourable permeability conditions. The experimentation of models in different permeability fields provides a general framework for the Athabasca Basin suggesting the transportation of basement-associated fluids and heat to the surface.

International audience; Since the discovery of ultra-high pressure (UHP) metamorphic rocks in the Alps and the Norwegian Caledonides,more than three decades ago, the exhumation of high pressure metamorphic rocks has been most often consideredto occur in the frame of continental collision. However, numerous natural examples of (U)HP rocks, from earlyPaleozoic to late Tertiary, document that exhumation occurred in extension either driven by slab rollback prior tocontinental collision (e.g. in backarc basins like in Mediterranean, Papua New Guinea, Carribean) or by eductionafter continental collision (e.g. in Norway). We first summarize the mechanisms of extensional exhumation in slabrollback and eduction modes, using numerical models and natural examples. We then present a reappraisal of thetectonic history of some famous natural UHP rocks examples (e.g. Alps and Himalayas) either showing, or stronglysuggesting, that exhumation occurred prior to continental collision likely driven by slab rollback. In addition, werecently showed that the pressure history, including a catastrophic pressure drop at the onset of exhumation, that isrecorded in most well documented (U)HP rocks worldwide fits remarkably well with a two-fold tectonic history,from compressional during subduction to extensional during exhumation. On the basis of all above argumentswe strongly suggest that the extensional exhumation of (U)HP metamorphic rocks, instead of being one mode ofexhumation among others, is more probably the rule.

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