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International audience; Subduction margins are the loci of a wide range of deformation processes occurring at different timescales along the plate interface and in the overriding forearc crust. Whereas long-term deformation is usually considered as stable over Myr-long periods, this vision is challenged by an increasing number of observations suggesting a long-term pulsing evolution of active margins. To appraise this emerging view of a highly dynamic subduction system and identify the driving mechanisms, detailed studies on now-exhumed, high pressure-low temperature (HP-LT) accretionary complexes are crucial as they open a window on the deformation history from the plate interface to the surface.In this study, we combine structural and petrological observations, Raman spectroscopy on carbonaceous material, Rb/Sr multi-mineral geochronology and thermo-mechanical numerical models to unravel with an unprecedented resolution the tectono-metamorphic evolution of the Late-Cenozoic HP-LT nappe stack cropping out in western Crete (Hellenic subduction zone). A consistent decrease of peak temperatures and deformation ages toward the base of the nappe pile allows us to identify a minimum of three basal accretion episodes between ca. 28 Ma and ca. 15 Ma. On the basis of structural evidences combined with numerical modeling results, we argue that this succession of mass-flux events triggered (i) pulses in the strain rate, sometimes associated with a switch of the stress regime (i.e., compressional/extensional) and (ii) vertical surface oscillations eventually resulting in the growth of a high forearc topography. This accretion-controlled, Myr-scale tectonic and topographic signal plays a part in active deformations monitored at subduction margins, though it may remain invisible to most of geodetic methods because of superimposed shorter- timescale transients, such as seismic-cycle-related events.

International audience; Study of now-exhumed ancient subduction zones revealed km-scale tectonic units of marine sediments and oceanic crust, which have been underplated (i.e. basally accreted) to the overriding plate at more than 10-km depth. However, geophysical observations of this deep process in active subduction zones (e.g. SW-Japan, Cascadia, Chile) are unclear and the dynamics as well as the existence of tectonic underplating along most of active margins remain controversial.Using high-resolution visco-elasto-plastic thermo-mechanical models, we present with unprecedented details the dynamics of formation, preservation and destruction of underplated crustal nappes at 10-40-km depth in subductions zones. Thus, we are able to characterize tectonic underplating from the plate interface where tectonic slicing is triggered, to the surface where topographic variations may be expected in response to such huge mass transfers.Our results show that subduction segments exhibiting an increasing frictional behaviour control deep accretionary dynamics and that the long-term frictional zonation of the plate interface is stable due to a positive feedback between fluid distribution and effective stress. As a result, successive underplating events are maintained during tens of Myr, leading to the formation of a high coastal relief. The rise of this relief is cadenced by an uplift-then-subsidence cycle, which characterizes each underplating event and the subsequent period of wedge re-equilibration. This periodical evolution is significantly modified by changing the rheological properties of the material entering the subduction zone, suggesting that tectonic underplating is likely a transient process active along most of active margins, depending on severe variations of the hydro-mechanical properties of the plate interface at Myr timescale.

International audience; How rocks deform at depth during lithospheric convergence and what are the magnitude of stress levels theyexperience during burial/exhumation processes constitute fundamental questions for refining our vision ofshort-term (i.e. seismicity) and long-term tectonic processes in the Earth’s lithosphere.We here present a set of 2D numerical experiments of eclogitic rock deformation performed at cm-scale. The deformedmedium is composed of two mineral phases: omphacite and garnet. We carried out a series of experimentsfor different background strain-rates and for a range of realistic pressure and temperature conditions. Results showthat fracturing of the entire eclogite rock can occur under HP-LT conditions for strain-rates even smaller thanthose generally expected for seismic events.We also explore the ranges of parameters where garnet and omphaciteare deforming with a different deformation style (i.e. frictional vs. viscous) and discuss our modelling results atthe light of naturally deformed eclogitic samples. Finally, we show that in cases of fracturing, the first event offrictional strain localisation constitutes a precursor for ductile strain localisation and results from the conversionof the mechanical energy into heat (i.e. shear-heating).

International audience; Eclogitic rocks are crucial for our knowledge of the subduction interface because they provide key constraints on both the evolution (P-T-t paths) and the deformation modes sustained by rocks in subduction zones. We herein focus on eclogitised flattened mafic bodies exposed within granulites from the continental basement slice of the Mt. Emilius klippe (W. Alps, Italy).These eclogites exhibit highly deformed garnetite and clinopyroxenite layers. In some places, these deformed rocks (up to mylonitic) can be found as clasts within meter-thick brecciated fault rocks formed close to the metamorphic peak conditions in the lawsonite-stable eclogite facies (at P 2.2-2.4 GPa and T 500-550°C).The garnet-rich layers tend to show brittle features, whereas deformation within clinopyroxene-rich layers is accommodated by both dislocation creep and fracturing. We present a petro-structural study of these eclogites allowing to track the brittle deformation mode associated with the chemical evolutionfrom the outcrop to the microscopic scale. We propose a new tectono-metamorphic model for the deformation of these rocks, related to the alpine eclogitic stage.This model is consistent with the coexistence of both ductile and brittle features that developed in the same metamorphic facies, and closely associated with fluid circulations. Our study shows therefore that the crustal material, along the subduction interface at HP-LT conditions, can record several successive brittle events in places where deformation is classically envisioned as ductile. This brings new constraints in our understanding of the mechanical processes in the subduction zone interface.

EPMA and LA-ICP-MS processed data for sample #217.

EPMA and LA-ICP-MS processed data used to identify and discard data affected by smearing effects: S1.1 garnet from sample #47a; S1.2 garnet from sample #217a; S1.3 epidote from sample #217; S1.4 titanite from sample #217.

Additional X-ray data on a garnet from sample #39c.

Petrological information on Sistan amphibolitized eclogites.

National audience; Eclogitic rocks are crucial in tectonics as they provide key constraintson both the evolution (P-T-t paths) and the deformationmodes sustained by the rocks in subduction zones. We herein focuson eclogitized flattened mafic bodies remnants exposed withingranulites from the continental basement slice of the Mt. Emiliusklippe (Western Alps, Italy). These eclogites exhibit highly deformed(up to mylonitic) garnetite and clinopyroxenite levels foundas clasts within meter-thick eclogitic shear zone formed close tothe metamorphic peak conditions under lawsonite bearing eclogitefacies conditions. The garnet-rich levels tend to behave in abrittle fashion while deformation within clinopyroxene-rich levels ismostly accommodated by creep, as evidenced by the presence ofelongated grains, subgrains boundaries and intense grain size reduction.Especially, crystallographic preferred orientation (CPO)measurements in garnet indicate a quasi-random distribution. Inmost of the clinopyroxene levels nevertheless, the CPO is relativelystrong, with multiples of uniform distribution varying from4 to 5.5 (value of 1 is random texture). This CPO is characterizedby a strong alignment of poles of the planes (001) parallelto the lineation and poles of the planes (100) and directions ofthe planes [010] distributed along girdles cross-cutting the foliationplane. Based on a petrological study, from the outcrop tothe microscopic scale, we propose a tectono-metamorphic historyof the Mt. Emilius eclogites. This history related to the alpineeclogitic event, allow to link the metamorphic P-T path and thedeformation processes in association with the chemical evolutionsthat could lead to the formation of the Mt. Emilius brecciatedeclogites in the Arbole region. This model is consistent with thealternating ductile and brittle events that developed in the samemetamorphic facies, at P~2.0-2.5 GPa and T~500-550C, closelyassociated to metamorphic fluid circulations. Thus, our study atteststhat the material along the subduction interface, at HP-LTconditions, can locally be brittle where deformation is classicallyenvisioned as ductile.

International audience; Eclogitic rocks are important for understanding tectonics at large scale as they provide key constraints on boththe evolution (P-T-t-" paths) and the deformation modes of the crust along the subduction interface. We hereinfocus our study on eclogitized mafic dykes remnants exposed within granulites from the continental basementsilver of the Mt. Emilius klippe (Western Internal Alps, Italy). These eclogites exhibit highly deformed garnetiteand clinopyroxenite levels. In some places, these rocks with a mylonitic aspect can be found as clasts withinmeter-thick brecciated fault rocks formed close to metamorphic peak conditions in eclogite facies. Especially, thegarnet-rich levels tend to behave in a brittle fashion while deformation within clinopyroxene-rich levels is mostlyaccommodated by creep. This is evidenced by the presence of elongated grains, subgrain boundaries and intensegrain size reduction close to rigid garnets. Crystallographic preferred orientation (CPO) measurements in garnetsindicate a quasi-random distribution. In most of the clinopyroxenes levels nevertheless, the CPO is relativelystrong, with multiples of uniform distribution varying from 4 to 5.5 (value of 1 is random texture). This CPO ischaracterized by a strong alignment of poles (001) parallel to the lineation and (100) and [010] distributed alonggirdles cross-cutting the foliation plane. Our study thus attests that the materials along the subduction interfaceat P2.0-2.5 GPa and T500-550C can locally be brittle where deformation is classically envisioned as ductile.In addition to this deformation analysis, we present a petrological study of these eclogites, from the outcrop tothe microscopic scale, tracking the chemical evolution associated to the observed deformation. Based on all thesedata, we finally propose a tectono-metamorphic history for these rocks allowing to explain the co-existence ofductile and brittle features developed in the same metamorphic facies, and closely associated to the circulation ofmetamorphic fluids.

International audience; The presence of km-scale accreted terranes/units in both ancient and present-day subduction zones attests tochanges in strain localization along the plate interface, whereby these terranes/units get detached from the downgoingslab (or, in places, are eroded away from the tip of the upper plate) and either directly exhumed or accreted/underplated below the upper plate before final exhumation. The rock record (P-T-t data) indicates that, for agiven subduction zone, exhumation is episodic: no more than a few My compared to the 100 My lifetime of typicalsubduction zones. Not much is known, however, regarding this process and important open questions remain:what exactly is episodic (i.e. detachment from the slab and/or exhumation?), for how long and where? How is mechanicalcoupling impacted by the initial structure of the incoming plates (structural/lithological heterogeneities,thermo-fluid regime, geodynamic boundary conditions, etc...)?We herein present both new and literature structural and P-T-t data ranging from shallow (i.e. 15-20 km) to intermediatedepths (100 km) along the subduction interface, that span a range from long-term to short-lived eventsof underplating and/or exhumation, and confront them with the recent wealth of geophysical data gathered onsubduction zones. Structural and petrological data indicate that the slicing of km-scale units mostly occurs atspecific depths where major mechanical changes occur along the plate interface: at 30-40 km (downdip of theseismogenic zone) and 70-80 km (where mechanical coupling between the two plates resumes and where eclogitesget critically dense). This suggests that switches in mechanical coupling (i.e. in the rheology of the material)are key in controlling the ability to detach pieces from the slab (and that later exhumation is rather controlledby large-scale, lithospheric-scale boundary conditions). The study of rock remnants detached from the slab andunderplated during subduction infancy (i.e. metamorphic soles) further illustrates how changing rheologies (crustv. mantle) and increased mechanical coupling affect the potential to retrieve rocks/units and emphasize the role offluids (and of the mantle wedge). Fully-coupled thermo-mechanical models taking into account variable hydrationconditions strengthen these conclusions. From these observations, we infer that long-term mechanical couplingprobably varies throughout subduction lifetime, from strong to weak, as a function of the contrast of effective viscosityon either side of the subduction interface: a young and wet interface will promote strong viscous coupling,metamorphic sole accretion and the formation of high-temperature (eclogite) knockers, whereas a fluid-present yetdrier and colder one will mainly promote underplating of metasedimentary material and, occasionally, large-scaleslivers of oceanic lithosphere. We finally discuss the possible link between strain localization mechanisms leadingto tectonic slicing and seismicity (regular or slow earthquakes), particularly in the light of the recent finding ofeclogite breccias.

International audience; The transition zone at the downdip end of seismic coupling along subduction interfaces is often the site of megathrust earthquake nucleation and concentrated postseismic afterslip, as well as the focus site of episodic tremor and slip features. Exhumed remnants of the former Alpine subduction zone found in the Swiss Alps allow analyzing fluid and deformation processes near the transition zone region (30–40 km paleodepth). The Dent Blanche Thrust (DBT) is a lower blueschist-facies shear zone interpreted as a fossilized subduction interface where granitic mylonites overlie a metamorphosed accretionary wedge. We report field observations from the DBT region where multiple, several tens of meters thick foliated cataclastic networks are interlayered within the basal DBT mylonites. Petrological results and microstructural observations indicate that the various cataclasis events took place at near-peak metamorphic conditions (400–500°C, 1.1–1.3 GPa) during subduction of the Tethyan seafloor in Eocene times (42–48 Ma). Some of these networks exhibit mutual crosscutting relationships between mylonites, foliated cataclasites, and vein systems indicating mutual overprinting between brittle deformation and ductile creep. Whole-rock chemical compositions, in situ 40Ar-39Ar age data of recrystallized phengite, and Sr isotopic signatures reveal that DBT rocks also underwent multiple hydrofracturing and metasomatic events via the infiltration of fluids mainly derived from the oceanic metasediments underneath the DBT. From the rock fabrics, we infer strain rate fluctuations of several orders of magnitude beyond subduction strain rates (∼10−12 s−1) accompanied by fluctuation of supralithostatic and quasi-lithostatic fluid pressures (1 ≥ λ > 0.95). DBT brittle-plastic deformation switches highlight the diversity of deformation processes and fluid-rock interactions in the transition zone region of the subduction interface.

International audience; A wide range of geophysical/petrological data indicates that large amounts of water are released in subduction zones during the burial of oceanic lithosphere through metamorphism and associated dehydration reactions. Large volumes of aqueous fluids are expected and observed in the mantle wedge, just below the continental Moho. Recent estimates suggest that the mantle wedge is heterogeneously serpentinized (generally 20-30%). This serpentinization is believed to cause a significant weakening of the mantle wedge and therefore may critically control the depth of interplate seismogenic coupling. However, data constraining mechanisms driving deep (50-200km) fluid circulation are lacking and fluid-rock interaction processes remain weakly constrained at the km-scale. We herein propose a new fluid migration algorithm based on thermodynamic modelling (PerpleX) where fluids are free to migrate, driven by rock fluid concentration, non-lithostatic pressure gradients and deformation. Oceanic subduction is modelled using a forward visco-elasto-plastic thermomechanical code (FLAMAR algorithm) based on previous work by Yamato et al. (2007). After 15 Ma of convergence between the two plates, we show that deformation is accommodated along a low-strength shear zone in the wall of the subduction thrust interface, characterized by a weak (10-25% serp.) and relatively narrow (between 3-6km) serpentinized front/channel. Our results also show that dehydration associated with eclogitization of oceanic crust (60-75km) and serpentinite breakdown (110- 130km) significantly weakens the mantle wedge at these depths, thereby favouring deep sedimentary accretion in the deep mantle wedge. We finally show that dehydration causes significant fluid overpressures in the downgoing oceanic lithosphere. These results bring new critical constraints on the location of intermediate-depth seismicity and dehydration-embrittlement processes reported by geophysical studies.

International audience; The Monviso ophiolite is composed of two main tectonic slices: the Monviso Unit (MU) to the west, which overlies the Lago Superiore Unit (LSU). Our PT estimates show that the MU has been subducted down to 480°C-23kbar (~70km) during Alpine subduction while the LSU reached slightly deeper conditions (540°C-26kbar, i.e. ~80km). This ophiolite, which comprises large (10-20km long) ophiolite fragments therefore does not correspond to a serpentinite mélange, and may be the southern extension of the Zermatt-Saas ophiolite (Angiboust et al., 2009; 2011). The well-preserved LSU constitutes an almost continuous upper fragment of oceanic lithosphere subducted between 50 and 40 Ma and later exhumed along the subduction interface. It therefore provides a unique opportunity to study strain partitioning and deep mechanical behaviour of the subducting lithosphere. The LSU comprises (i) several hundred meters of eclogitized basaltic crust (+ minor calcschist lenses) overlying a 100-400m thick metagabbroic body and (ii) a serpentinite sole (c. 1km thick). We herein focus on eclogite-facies shear zones, which are found at the boundary between basalts and gabbros, and between gabbros and serpentinites, i.e. between material with marked rheological contrasts. Eclogite facies blocks within the shear zones display intense fracturation, fragment rotation and dispersion in the serpentinite schists which line up the shear zones. We also report the first finding of eclogite-facies breccias, constituted of rotated eclogite mylonitic clasts cemented within an eclogite-facies matrix. Local fracturation of garnet within these breccias is attested by the presence of numerous fracture networks within garnet, generally healed by a Mg-enriched composition. The shear zones also preserve clear evidence of pervasive and channelized fluid flow (of variable duration) leading to alteration of bulk rock composition, weakening of the rock and widespread crystallization of lawsonite. Our results provide new constraints for deep mechanical coupling processes and meter-scale fluid-rock interaction occurring at depth in present-day subduction zones. In particular, we emphasize that (i) rheological contrasts in the field qualitatively support those inferred from experimental flow laws, (ii) fluid flow is channelized along deep eclogite facies shear zones (iii) brittle fracturing recorded by eclogite breccias and garnet fractures could be associated with deep interplate seismicity.

International audience

International audience

International audience; Based on examples from the Betics, Oman and the W. Alps, this contribution attempts to (1) critically evaluate the precision with which metamorphic P-T-t histories are retrieved at present and (2) discuss the implications on our understanding of orogenic processes, focusing on those processes taking place along the subduction channel. Thanks to improved interconsistent thermodynamic databases, multiple thermodynamic softwares and analytical tools, numerous quantitative constraints on metamorphic histories are now accessible. We herein assess the merits and shortcomings of our present knowledge. In particular, we stress that, despite impressive improvements, parts of the P-T-time history still remain systematically ill-constrained. Metamorphic histories do not provide a quite continuous record either, and whether mineral reequilibrations are discrete or continuous remains fondamentally ambiguous. We then focus on the early stages of orogenic build-up, namely on processes related with oceanic and continental subduction. After briefly reviewing our current knowledge of metamorphic histories worlwide, we focus on the example of the W. Alps. We present the results of the comparison between the metamorphic evolution of the Schistes Lustrés paleoaccretionary complex and the evolution of some of the major ophiolitic bodies of the W. Alps exhumed from the Alpine subduction (Zermatt-Saas, Avic, Rocciavre, Monviso). We show how this allows placing constraints on the nature and characteristics of the plate interface (eg, the so-called 'subduction channel') and reconstructing the geodynamic processes at work in a subduction zone. We particularly discuss the dimensions of the bodies sliced up and stacked in the subduction channel, the depths and possible mechanisms at which this happens, and the role of fluid transfer. Along strike variations of the exhumation processes within the same subduction zone are also emphasized. In the light of these findings, we finally address other related issues: insights from thermomechanical models combined with petrological techniques, timing of mineral reequilibration with respect to deformation (and implications for rheology), fluid behaviour and kinetic processes.

Topography in forearc regions reflects tectonic processes along the subduction interface, from seismic cycle-related transients to long-term competition between accretion and erosion. Yet, no consensus exists about the topography drivers, especially as the contribution of deep accretion remains poorly constrained. Here, we use thermo-mechanical simulations to show that transient slab-top stripping events at the base of the forearc crust control uplift-then-subsidence sequences. This 100s-m-high topographic signal with a Myr-long periodicity, mostly inaccessible to geodetic and geomorphological records, reflects the nature and influx rate of material involved in the accretion process. The protracted succession of stripping events eventually results in the pulsing rise of a large, positive coastal topography. Trench-parallel alternation of forearc highs and depressions along active margins worldwide may reflect temporal snapshots of different stages of these surface oscillations, implying that the 3D shape of topography enables tracking deep accretion and associated plate-interface frictional properties in space and time., Nature Communications, 11 (1), ISSN:2041-1723

We report the finding of rare eclogite-facies vugs forming millimeter- to centimeter-sized pockets in meta-ophiolites from the western European Alps. Euhedral garnet crystals covering the vug walls display oscillatory chemical zoning for a wide range of major and trace elements, including Cr, Mn, and rare earth elements. Thermodynamic modeling revealed that closed-system fluid production through the breakdown of prograde glaucophane, lawsonite, and chlorite between 505 °C and 525 °C can successfully explain porosity creation of ~4% and the mineralogical properties of the vugs. Available geologic and geochronologic constraints indicate that the eclogitization of the downgoing mafic crust spanned a window of at least 1 m.y. These observations can only be explained by the presence of extremely low permeability values (

International audience; Fluids are ubiquitous in Earth's lithosphere. Reactions between fluids and rocks critically impact petrophysical rock properties and the geochemistry and geodynamics of the Earth at all scales. By enhancing reaction kinetics and mass transport, fluids play a fundamental role in geological processes from the Earth's surface down to the deep lithosphere. Lithospheric deformation in all tectonic settings is strongly influenced by fluids, which contribute to strain localization, earthquake nucleation, as well as potential rheological weakening or strengthening triggered by advective fluid flow and metasomatism. Multiple and complex feedbacks between host rock and percolating fluids are known to lead to the formation of economic mineral deposits in a great variety of tectonic settings from the seafloor to the roots of mountain belts. Fluids can also provide a favorable environment for life development in unexpected settings.

The physical and mechanical processes rooted in the hydrated, serpentinized mantle above subduction zones remain insufficiently explored despite fundamental implications for our understanding of rheology and fluid recycling along subduction interfaces. Through a field-based investigation, serpentinized peridotites and jadeitite samples from a fossil forearc mantle in the Polar Urals (Russia) are studied here to document fluid–rock interaction processes in the high-P field, as well as the long-term evolution of the base of the mantle wedge. Petrographic, geochemical and microstructural observations reveal a complex, protracted evolution of the jadeitite-forming fluid pathway throughout the gradual cooling of the forearc mantle and increasing serpentinization of the host. It is shown that the jadeitite lenses in the studied locality (a) derive for a large part from a trondhjemitic dyke earlier emplaced in a warm subduction environment, and (b) record the cooling of the subduction hangingwall under high-P conditions associated with increasing host serpentinization. In the studied locality, the majority of the jadeitites formed at relatively high temperatures (>600°C) by the influx of Na–Al-rich, slab-derived metamorphic fluids that were drained along the base of the mantle wedge, parallel to the subduction interface. Changes in bulk-rock geochemical signatures and in paragenetic sequences also constrain the compositional evolution of the fluid channelized along this drainage, with an increasing sedimentary component. The phlogopite-bearing walls of the dyke exhibit Rb–Sr and Ar–Ar ages ranging between c. 405 and c. 390 Ma, a range partly overlapping within uncertainty with the previously dated zircons from the jadeitite core (410–⁠400 Ma; U–Pb). This study opens a unique window on the pristine structures formed above the plate interface by melting and fluid–rock interaction in the early subduction stages, as well as their evolution during secular cooling of the base of the mantle wedge., Agence Nationale de la Recherche, Grant/ Award Number: 16C538. Nicolas Rividi and Michel Fialin (CAMPARIS) are acknowledged for assistance with Electron Probe measurements and mapping. Michael Bostock is acknowledged for insightful discussions. This study was supported by the IdEx ANR-18-IDEX-0001 through a chair attributed to SA. Oleg, Firaz and Katia are acknowledged for assistance during the two field expeditions. The article was prepared when working on the state budget topic IGG UB RAS AAAA-A18-118052590032-6. Two reviewers are acknowledged for constructive comments. This is IPGP contribution #40XX.

La relation entre la pression de fluide et les séismes lents et réguliers dans les zones de subduction est essentielle à la compréhension de leurs propriétés mécaniques, puisque les fluides contrôlent la rhéologie, et donc l'occurrence de ces phénomènes sismologiques particuliers. Cependant, les mécanismes à l’origine des glissements lents ne sont que très imparfaitement compris d'un point de vue de l’enregistrement géologique. Cette thèse apporte de nouvelles connaissances sur ce sujet en étudiant des complexes métamorphiques de haute pression formés par roches représentatives des régions où ces séismes se produisent. La première localité étudiée est un fragment du paléo-prisme d'accrétion du centre du Chili. Ce terrain de haute pression (400°C et 0,8GPa) est composé d'une association lithologique cohérente de schistes verts et bleus. Le second terrain étudié, le complexe de Seghin (suture de Zagros) dans le faciès schiste bleu à lawsonite (480°C-1.8GPa, Crétacé supérieur), est un paléochenal de subduction composé de blocs mafiques dans une matrice serpentinitique. Les observations de terrain, les mesures structurales et les données pétro–géochimiques sur les veines et les roches hôtes révèlent que les premières veines ont été formées par des réactions progrades de déshydratation de minéraux de basse pression. Ces veines, ainsi que d'autres formées plus tard, ont évolué et ont été remplies de minéraux de haute pression. Des événements d'hydro-fracturation, caractérisés par la précipitation de carbonates, recoupent toutes les structures précédentes dans des conditions proches du pic métamorphique. Les analyses pétro-géochimiques suggèrent différentes sources de fluides, la plupart pouvant être associées à une source profonde proche de la transition schiste bleu-eclogite. Ainsi, il est suggéré qu'un mélange de fluides chenalisés et poreux soit le mécanisme d'écoulement dominant le long de l'interface de subduction. On note que les lithologies mafiques présentent des veines de lawsonite bréchique contenues dans une matrice (ultra) cataclastique. La caractérisation du système veine-matrice révèle que l'écoulement fragile et l'écoulement par pression-dissolution sont les principaux mécanismes de déformation dans les faciès schistes bleus. Ainsi, un scénario est établi dans lequel des «pulses» de fluides externes affaiblissent la zone de cisaillement et déclenchent un glissement lent et des séismes de basse fréquence. Une étude complémentaire décrit une première occurrence de roches liées à des failles sismiques dans les faciès de schistes bleus, y compris des brèches et des (ultra-) cataclasites. Dans les niveaux cataclastiques, la présence de minéraux de haute pression (20-35 km de profondeur) indique des conditions représentatives de la zone sismogène. Les relations de cisaillement entre les ultracataclasites, les brèches et les veines montrent que les processus de glissement sismique et d'hydrofracturation sont contemporains. Les modèles mécaniques confirment que ces structures ne peuvent s'être formées que dans un régime d'instabilité critique à des pressions de fluides quasi-lithostatique, tel documenté dans les subductions actives. Enfin, un modèle à grande échelle est proposé, dans lequel les fluides expulsés près de la transition schiste bleu-éclogite sont capables de circuler le long de l'interface de subduction en produisant une hydrofracturation trémorgène, des veines bréchiques (associées à un cisaillement lent) et des ultracataclasites, marqueurs de probables évènements sismiques réguliers et de basse fréquence., Entender la relación entre la presión de fluidos y los terremotos lentos y regulares en la zona de subducción es clave para determinar sus propiedades mecánicas, ya que los fluidos controlan su reología, y, por tanto, la ocurrencia de dichos fenómenos sismológicos. Sin embargo, estos efectos aún no han sido validados desde una perspectiva geológica. Esta tesis aporta nuevo conocimiento sobre esta temática al investigar complejos metamórficos de alta presión que exponen rocas representativas de las regiones donde estos terremotos ocurren. La primera localidad estudiada es un fragmento del paleo-prisma de acreción de Chile central. Este terreno de alta presión (400°C y 0.8GPa) está compuesto por una asociación litológica coherente de esquistos verdes y azules. El segundo terreno estudiado, el complejo de Seghin (sutura de Zagros) en facies de lawsonita-esquisto azul (480°C-1.8GPa, Cretácico tardío), es un paleocanal de subducción compuesto por bloques máficos en matriz serpentinitica. Observaciones de campo, mediciones estructurales y datos petro-geoquímicos de las vetas y las rocas de caja revelan que las vetas tempranas se formaron mediante reacciones progradas de minerales de baja presión. Estas vetas, junto con otras formadas posteriormente, evolucionaron y fueron rellenadas con minerales de alta presión. Eventos de hidrofracturamiento, caracterizados por la precipitación de carbonatos, se sobrepusieron a todas las estructuras previas en condiciones cercanas al pico metamórfico. Los análisis petro-geoquímicos sugieren diferentes fuentes de fluidos, en su mayoría, posiblemente asociados a una fuente profunda cercana a la transición esquisto azul-eclogita. Así, se sugiere que una mezcla entre fluidos canalizados y porosos, fue el mecanismo de flujo dominante a lo largo de la interfaz de subducción. Se destaca que las litologías máficas presentan vetas de lawsonita brechificadas y contenidas en una matriz (ultra) cataclastica. Caracterización del sistema veta-matriz revela que el flujo frágil y el flujo por presión-disolución fueron los principales mecanismos de deformación en facies de esquisto azul. De este modo, se establece un escenario en el que «pulsos» de fluidos externos acompañaron y precedieron al flujo frágil en la zona de terremotos lentos, debilitando la zona de cizalle y desencadenando un deslizamiento lento y terremotos de baja frecuencia. Investigaciones posteriores dan cuentan de un primer registro de rocas relacionadas a fallas sísmicas en facies de esquisto azul, incluyendo brechas y (ultra) cataclasitas. En los materiales cataclásticos, la presencia de minerales de alta presión (20-35km de profundidad) indica condiciones representativas de la zona sismogénica. Las relaciones de corte entre las ultracataclasitas, las brechas y las vetas muestran que los procesos de fallamiento recurrente e hidrofracturamiento fueron contemporáneos. Los modelos mecánicos confirman que estas estructuras sólo pueden haberse formado en un régimen críticamente inestable a presiones de fluidos cercanas a las litostáticas, como se observa en márgenes activos. Por último, se propone un modelo a gran escala, en el que los fluidos expulsados cerca de la transición esquisto azul-eclogita son capaces de circular a lo largo de la interfaz de subducción produciendo hidrofracturación tremorogénica, vetas brechificadas (asociadas a cizallamiento lento) y ultracataclasitas, representando así posibles marcadores de señales sísmicas regulares y de baja frecuencia., Understanding the feedbacks between fluid overpressures and subduction slow and regular earthquake-related processes is critical to constrain the physical nature of the megathrust. Recently, the influence of fluids as weakening agents has been proven as a major factor controlling the rheology of the megathrust and thus the occurrence of a variety of earthquakes. Yet, a significant scientific gap in validating this knowledge from a geological perspective exists. This thesis provides new insights into this issue by investigating blueschist-facies metamorphic complexes that expose rocks representative of the slow and regular earthquakes regions. The first chosen locality is a fragment of the late Paleozoic accretionary wedge exposed in central Chile. This high pressure (400 °C and 0.8 GPa) terrane is composed by a coherent greenschist-blueschist lithological association. The second studied terrane is a late Cretaceous lawsonite-blueschist-facies (480 °C-1.8 GPa) segment of the Zagros suture: the Seghin complex, a well-preserved blueschist block-in-serpentinite matrix paleosubduction channel. We herein combine extensive field observations, structural measurements and petro-geochemical data on vein and host rock materials. In both studied localities, early veins formed after prograde breakdown of low-pressure minerals. These veins, along with newly formed ones, have evolved and re-filled by further high-pressure minerals. Subsequent massive carbonate-bearing hydrofracturing events overprinted all previous structures at near-peak conditions. Petro-geochemical analyses suggest different fluid sources for the veining stages, in most cases, likely associated with a deepseated source near the blueschist-to-eclogite transition. In addition, we concluded that a mixture of channelized and porous flow was the dominant fluid flow mechanism along the subduction interface. Interestingly, mafic lithologies exhibit brecciated lawsonite veins hosted in a variably foliated (ultra) cataclastic host. Characterization of the host-vein system reveals that brittle creep and pressure solution processes were the dominant deformation mechanisms at blueschist-facies depths. Thus, supporting a scenario where external fluids accompanied and predated brittle creep in the slow earthquakes window. Consequently, episodic injection of fluid «pulses» weakened the megathrust and triggered subsequent slow slip and low frequency earthquakes. Further investigations led to a first report of blueschist-facies seismic fault-related rocks including breccias and foliated (ultra) cataclasites. In the cataclastic materials, the occurrence of newly-formed lower lawsonite-blueschistfacies (20-35 km depth) minerals point to conditions representative of the seismogenic zone. Crosscutting relationships among fluidized ultracataclasites, breccias and veins suggest that episodic faulting and hydrofracturing were contemporaneous processes. Mechanical modelling confirms that these fault-related structures can only have formed in a critically unstable regime at near lithostatic fluid pressure conditions, promoting recurrent seismic faulting as monitored in active margins. Last, we propose a large-scale scenario, where fluids expulsed near the blueschist-to-eclogite transition travelled upwards along the subduction interface producing massive, tremor-genic hydrofracturing, vein breccias (associated with slow slips) and ultracataclasites, representing a potential record of some of the slow and regular earthquakes phenomena., Tesis Univ. Granada.

EPMA and LA-ICP-MS trace-element maps have been acquired from amphibolitized eclogites from the Diego de Almagro Metamorphic Complex (Chile). Several garnet growth pulses and garnet resorption stages are revealed by major elements chemical zoning and by heterogeneous Y and rare earth element (REE) behaviour, associated with subduction and exhumation of these rocks. Distribution of REE in prograde garnet is texturally and chemically coupled with the breakdown of REE-bearing minerals while formation of epidote and titanite generations during amphibolitization is recorded by complex textures involving new garnet generation and overprinting phases. The latest overprint stage is characterized by fine-grained intergrowth between garnet and epidote micro-veins, phengite, hornblende, albite and titanite. Garnet cracks have been gradually re-equilibrated during this event witnessing short-scale dissolution–transport–precipitation. Pseudosection modelling shows that local variability in water content during amphibolitization controls garnet stability at the expense of epidote. Overprinting microstructures are explained by the effect of locally-derived aqueous fluids that trigger the ‘unlocking’ of elements from the reacting eclogite-facies paragenesis. These findings highlight the microscopic characteristics of amphibolitization processes documented in exhumed eclogite-facies terranes and shed light on the importance of thorough micro-chemical investigations while undertaking pressure–temperature (PT) estimates on rocks with strong textural disequilibrium. Refereed/Peer-reviewed