Evolution pétrologique et déformation des semelles métamorphiques des ophiolites : mécanismes d'accrétion et couplage à l'interface des plaques lors de l'initiation de la subduction

Metamorphic soles are m to ~500 m thick tectonic slices welded beneath most large-scale ophiolites (usually ≤ 20 km thick). They typically show a steep inverted metamorphic structure where the pressure (P) and temperature (T) conditions of crystallization increase upward, from the base of the sole (500±100ºC at 0.5±0.2 GPa) to the contact with the overlying peridotite (800±100ºC at 1.0±0.2 GPa). Soles are interpreted as a result of heat transfer from the incipient mantle wedge toward the nascent slab during the first My of intra-oceanic subduction. Metamorphic soles are therefore direct witnesses of petrological processes during early subduction. Their mineralogical assemblage and deformation pattern provide major constraints on the evolution of the thermal structure, on the migration of fluids and on the effective rheology along the nascent slab interface. We present a detailed petrological, (micro-)structural and experimental study, with refined P–T estimates obtained with pseudosection modelling and EBSD measurements, on the garnet-bearing and garnet-free (natural and synthetized) amphibolite. We suggest a new tectonic–petrological model for the formation of metamorphic soles below ophiolites, which involves the stacking of several homogeneous slivers (without any T gradient) of oceanic crust to form the present-day structure of the sole. These successive thrusts are the result of rheological contrasts between the slab material and the peridotites of the upper plate as the plate interface progressively cools. This model outlines the thermal and mechanical complexity of the early subduction dynamics, and highlights the need for more refined numerical modelling studies.Les semelles métamorphiques sont des unités d’origine océanique (≤ 500 m d’épaisseur) situées à la base des grandes ophiolites obductées (≤ 20 km d’épaisseur). Ces unités sont caractérisées par un gradient métamorphique inverse, où les conditions de pression (P) et de température (T) de cristallisation augmentent de la base vers le contact avec l’ophiolite sus-jacente : depuis 500±100˚C et 0.5±0.2 GPa jusqu'à 800±100˚C et 1.0±0.2 GPa. Formées et exhumées au cours des 2 Ma suivant l’initiation des subductions océaniques, les semelles sont des témoins directs de leur dynamique précoce. Les assemblages minéralogiques qu’elles portent et leur déformation fournissent des contraintes majeures, et rares, sur l’évolution de la structure thermique et sur le comportement mécanique de l’interface de subduction naissante. Au terme d'une étude pétrologique, (micro-) structurale et expérimentale sur les amphibolites naturelles de la semelle de Semail (Oman, UAE) et synthétisées en laboratoire, nous proposons un modèle où la semelle métamorphique résulte d’épisodes multiples d’accrétion d’unités homogènes en P–T (donc sans gradient métamorphique) au cours des premières étapes de subduction océanique. L’écaillage subséquent résulte de changements majeurs dans la distribution de la déformation, du fait des variations des propriétés mécaniques des roches à l’interface de subduction lors de son équilibration thermique et de l’augmentation au cours du temps de la proportion de sédiments entrant en subduction. Ce modèle rend compte d’une grande complexité thermique et mécanique à l’interface de subduction, encore insuffisamment examinée dans les études numériques actuelles

Petrological and deformation evolution of metamorphic soles beneath ophiolites : mechanism of accretion and coupling at the plate interface during subduction initiation

Les semelles métamorphiques sont des unités d’origine océanique (≤ 500 m d’épaisseur) situées à la base des grandes ophiolites obductées (≤ 20 km d’épaisseur). Ces unités sont caractérisées par un gradient métamorphique inverse, où les conditions de pression (P) et de température (T) de cristallisation augmentent de la base vers le contact avec l’ophiolite sus-jacente : depuis 500±100˚C et 0.5±0.2 GPa jusqu'à 800±100˚C et 1.0±0.2 GPa. Formées et exhumées au cours des 2 Ma suivant l’initiation des subductions océaniques, les semelles sont des témoins directs de leur dynamique précoce. Les assemblages minéralogiques qu’elles portent et leur déformation fournissent des contraintes majeures, et rares, sur l’évolution de la structure thermique et sur le comportement mécanique de l’interface de subduction naissante. Au terme d'une étude pétrologique, (micro-) structurale et expérimentale sur les amphibolites naturelles de la semelle de Semail (Oman, UAE) et synthétisées en laboratoire, nous proposons un modèle où la semelle métamorphique résulte d’épisodes multiples d’accrétion d’unités homogènes en P–T (donc sans gradient métamorphique) au cours des premières étapes de subduction océanique. L’écaillage subséquent résulte de changements majeurs dans la distribution de la déformation, du fait des variations des propriétés mécaniques des roches à l’interface de subduction lors de son équilibration thermique et de l’augmentation au cours du temps de la proportion de sédiments entrant en subduction. Ce modèle rend compte d’une grande complexité thermique et mécanique à l’interface de subduction, encore insuffisamment examinée dans les études numériques actuelles.Metamorphic soles are m to ~500 m thick tectonic slices welded beneath most large-scale ophiolites (usually ≤ 20 km thick). They typically show a steep inverted metamorphic structure where the pressure (P) and temperature (T) conditions of crystallization increase upward, from the base of the sole (500±100ºC at 0.5±0.2 GPa) to the contact with the overlying peridotite (800±100ºC at 1.0±0.2 GPa). Soles are interpreted as a result of heat transfer from the incipient mantle wedge toward the nascent slab during the first My of intra-oceanic subduction. Metamorphic soles are therefore direct witnesses of petrological processes during early subduction. Their mineralogical assemblage and deformation pattern provide major constraints on the evolution of the thermal structure, on the migration of fluids and on the effective rheology along the nascent slab interface. We present a detailed petrological, (micro-)structural and experimental study, with refined P–T estimates obtained with pseudosection modelling and EBSD measurements, on the garnet-bearing and garnet-free (natural and synthetized) amphibolite. We suggest a new tectonic–petrological model for the formation of metamorphic soles below ophiolites, which involves the stacking of several homogeneous slivers (without any T gradient) of oceanic crust to form the present-day structure of the sole. These successive thrusts are the result of rheological contrasts between the slab material and the peridotites of the upper plate as the plate interface progressively cools. This model outlines the thermal and mechanical complexity of the early subduction dynamics, and highlights the need for more refined numerical modelling studies

Régulation de l'épissage alternatif par des facteurs d'épissage de la famille des protéines SR dans des situations physiologiques ou pathologiques

Les protéines SR jouent un rôle crucial dans les processus de maturation des précurseurs d'ARNm, et plus particulièrement dans la régulation de l'épissage alternatif. Ce mécanisme permet la synthèse de multiples ARNm à partir d'un unique précurseur, et constitue un moteur essentiel de la diversité protéique chez les eucaryotes supérieurs. L'activité des protéines SR est régulée par la phosphorylation de leur domaine RS, riche en résidus arginines et sérines, par plusieurs familles de protéines kinases et dont l'ADN topoisomérase I (Topo I). En utilisant des cellules résistantes à un inhibiteur de la Topo I, nous avons observé que la résistance à la camptothécine est étroitement liée à l'extinction de l'expression de cette enzyme. L'absence de Topo I est corrélée à une hypophosphorylation des protéines SR et à des défauts d'épissage alternatif, ce qui suggère que l'activité kinase de la Topo I n'est pas redondante avec celle d'autres protéines et est nécessaire pour les processus d'épissage alternatif in vivo. Les protéines SR jouent un rôle essentiel au cours du développement, mais paradoxalement, leur fonction dans ce processus reste élusive. Chez la drosophile, la surexpression de diverses protéines SR, dans les disques imaginaux d'œil et dans le cerveau de larves, induit des défauts de développement de ces tissus. Nous avons identifié les ARNm associés à dASF/SF2 et à B52, et nous avons montré que la surexpression de ces deux protéines entraîne des altérations du profil d'épissage de certains d'entre eux impliqués, en particulier, dans le développement de l'œil et du cerveau. L'analyse du développement de ces deux tissus, au stade larvaire, a révélé que la surexpression de dASF/SF2 perturbe la différentiation de cellules de l'ommatidie, et que l'expression de B52-GFP interfère également avec le développement du cerveau. Les protéines SR régulent l'épissage des précurseurs d'ARNm en interagissant avec des séquences activatrices introniques ou exoniques, dont la fonction peut être altérée par des mutations, dans le cas de maladies génétiques. Nous avons étudié une mutation qui localisée dans l'intron 7 du gène E1a du complexe PDH, chez un patient atteint du syndrome de Leigh. Cette mutation créée une séquence spécifiquement reconnue par la protéine SC35 qui est responsable d'un épissage aberrant de l'ARNm E1aPDH à l'origine de la maladie. Nous avons montré que la réduction de l'expression de SC35 par ARN interférence permet de restaurer un épissage normal du gène E1aPDH dans des fibroblastes primaires de patients. Enfin, nous avons identifié des composés chimiques qui inhibent spécifiquement des évènements d'épissage alternatif dépendants d'une ou plusieurs protéines SR, in vitro et in vivo. De telles molécules pourraient ouvrir la voie à de nouvelles approches thérapeutiques pour traiter des pathologies humaines résultant de mutations génétiques qui affectent les processus d'épissage.SR proteins play a crucial role in messenger RNA maturation processes, and more precisely in the regulation of alternative splicing. This mechanism allows the synthesis of multiple mRNAs from a single precursor and is a major source of proteomic diversity. The activities of SR proteins are regulated by phosphorylation of the RS domain, rich in arginine and serine residues, by several protein kinases, including the DNA topoisomerase I (Topo I). Using cells resistant to a Topo I inhibitor, we have shown that resistance to camptothecin is correlated to the downregulation of the expression of this enzyme. Topo I depletion induces the hypophosphorylation of SR proteins as well as alternative splicing alterations, suggesting thereby that the kinase activity of Topo I is not redundant and is necessary for alternative splicing processes in vivo. As key splicing regulators, SR proteins also play an essential role during development, even if this function remains elusive. In drosophila, the overexpression of several SR proteins, in larval eye imaginal discs and brain, is responsible for developmental alterations of both tissues. We have identified candidate target mRNAs associated to dASF/SF2 and B52, and we have shown that overexpression of both SR proteins induces splicing patterns alterations for some of these mRNAs that are involved in eye and brain development. The analysis of both tissues, at the larval stage, revealed that overexpression of dASF/SF2 impairs cell differentiation within the ommatidia, when the overexpression of B52 also alters brain development. SR proteins regulate the splicing of mRNA precursors through binding of intronic or exonic enhancer sequences, which can be altered by mutations in genetic diseases. We have investigated the consequences of a mutation, located in the intron 7 of the PDH complex E1a subunit encoding gene, in a case of Leigh syndrome. This mutation creates an enhancer sequence specific for SC35, and is responsible for an aberrant splicing pattern of the E1aPDH mRNA causing the disease. Using RNA interference, we were able to restore the normal splicing pattern of the E1aPDH gene by reducing the expression level of SC35. At last, we have characterized small chemical molecules that specifically inhibit, both in vitro and in vivo, alternative splicing events regulated by one or more SR protein. Such molecules open exciting perspectives concerning therapeutical approaches to treat human diseases resulting from genetic mutations that impair splicing processes.MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Long-term evolution of a plume-induced subduction in the Neotethys realm

International audienceThe head of a mantle plume can weaken the oceanic lithosphere through the combined action of its buoyancy forces and excess temperature, which eventually induces intra-oceanic subductions. However, the dynamics of plume-induced subductions, their lifetime and their ability to propagate at the scale of an entire ocean remain unknown. Here, we combine a quantitative synthesis of the ophiolitic record of the Neotethys subduction in the Late Cretaceous with 3D spherical modeling of mantle convection self-generating plate-like tectonics to propose that the pre-Deccan plume initiated the Southern Neotethys Subduction. We suggest that this subduction retreated at 5-10 cm yr−1 until it reached the continental margins of Arabia and India. The sequential segmentation of this subduction during its retreat and the diachronous deactivation of the resulting segments drove the Late Cretaceous-Eocene series of plate reorganization events recorded in the Indian Ocean's fabric

Constraining transformation weakening in plagioclase-pyroxene mixtures

International audienceMafic rocks are a key constituent of the oceanic and lower continental crust. Strain localisation and fabric development in these rocks is controlled by the active deformation mechanisms. From studies of natural rocks it has been established that strain localisation and weakening in mafic rocks is directly related to fluid availability and resultant mineral reactions. Understanding the interplay between reactions, fluid availability, and deformation aids in quantifying the stresses and rates of deformation processes. We have conducted an experimental investigation to constrain the weakening mechanisms in gabbro. Shear experiments were performed in a Griggs-type apparatus at 800-900°C, and 1.2-1.5 GPa with a shear strain rate of 10 ⁵s 1. The starting material consists of mixed powders with <100 μm sized grains of plagioclase and clinopyroxene from an undeformed sample of the Kågen Gabbro in Northern Norway. Experiments have been conducted with 'as is' (dried at 110°C) starting material and with 0.1% added water. The experiments at 800°C are very strong with a peak shear stress ~0.8 GPa whilst the 900°C experiments are weaker, reaching peak stresses of ~0.35 GPa. The 800°C experiments show evidence of mineral reactions with newly formed phases making up 10-25% of the sample. In these reaction zones, plagioclase and clinopyroxene have reacted to produce amphibole and garnet. Additionally S-C' mylonitic fabrics have developed in these samples. The 900°C samples show minimal evidence for mineral reactions (2-5% new material) or crystal-plastic deformation mechanisms. The lack of mineral reactions in the rheologically weak experiments (900°C) and abundance of reaction products in the mechanically strong experiments (800°C) is conflicting to our inferences of natural studies. However, if partial melting takes place in the higher temperature experiments, it may account for the pronounced strength decrease. We plan to conduct EBSD and TEM analysis to determine crystallographic properties and accurate grain size and shape parameters in the fine grained reaction zones. Future experiments will use fully dried natural starting material (dried at 700-800°C) and An60 and end-member diopside, these experiments will be compared with our current experiments and be used to determine the exact weakening properties from impurities in the natural starting material

What are the required conditions to trigger the obduction/subduction initiation?

International audienceThe initial stages allowing for the birth of obduction and/or subduction zones still remain enigmatic and highlydebated. Although there is no doubt that initiation occurs on a preexisting lithospheric-scale thermal or mechanicaldiscontinuity (e.g., ridge, supra-subduction zone or transform fault), no consensus has yet been reached onthis subject. However, more and more data allowing to address this issue are available (coming from field analyses,metamorphic petrology and geochronology). In particular, metamorphic soles located at the base of ophioliticnappes, which formed during the very first stages of initiation, yield increasingly precise P-T-t constraints. By confrontingsuch data to thermo-mechanical models, it should be possible to identify which initial configuration allowsfor metamorphic sole formation and for the emplacement (or not) of an ophiolitic nappe of realistic dimensions.In this study we designed thermo-mechanical models encompassing three initial model configurations, for whichP-T-t data from the Semail ophiolite of Oman are then used for the validation of model outputs. The first configurationencompasses an oceanic ridge type initial thermal perturbation (error function). The second configurationmimics the thermal perturbation caused by the arrival of a mantle plume at the base of the lithosphere (gaussianfunction). The third initial condition corresponds to a case where the obduction/subduction initiates along a transformfault delimitating lithospheres of contrasting thermal ages (step function).Our results show that obduction never initiates in ridge type models, excepted for particular conditions that are notcompatible with the Oman case. They also indicate that the initial thermal anomaly has to be sharp but not necessaryof large amplitude and that the strength of the lithosphere has to be high enough to ensure the establishment ofthin and long ophiolitic nappe without buckling. Our results also highlight the key role of shear heating in enablingobduction/subduction initiation

Dissolution precipitation creep as a process for the strain localisation in mafic rocks

International audienceThe lower crust is, on average, mafic in composition and composed of minerals that remain mechanically strong up to high temperatures. Here we show that dissolution-precipitation creep (as a type of diffusion creep) plays a major role in deformation of gabbro bodies at upper amphibolite facies conditions. The Kågen gabbro, N. Norway, is comprised of undeformed gabbro lenses enclosed by mylonitised margins that deformed at 690 ± 25 °C and 1.0–1.1 GPa. The evolution of the microstructures and fabric of the low strain gabbro to high strain margins were investigated. Original clinopyroxene and plagioclase dissolved during mineral reactions and precipitated as new minerals phases: new plagioclase and clinopyroxene (different compositions relative to the magmatic parents) and additional amphibole and garnet. Microstructural and crystallographic preferred orientation (CPO) data indicate that dissolution-precipitation creep is the dominant deformation mechanism. Amphibole shows a strong CPO that is primarily controlled by orientated growth in the stretching direction. The progression of mineral reactions and weakening is directly connected to a fluid-assisted transformation process that facilitates diffusion creep deformation of strong minerals at far lower stresses and temperatures than required by dislocation creep. Initially strong lithologies can become weak, provided that reactions proceed during deformation

Early subduction dynamics recorded by the metamorphic sole of the Mt. Albert ophiolitic complex (Gaspé, Quebec)

International audienceThe metamorphic sole of the Mt. Albert ophiolitic complex (Gaspé peninsula, Quebec, Canada) is a sliver of Ordovician oceanic crust accreted to the base of an incomplete ophiolitic sequence, along a suture zone throughout the north-eastern Appalachians linked to the Taconian orogeny. A detailed mineralogical study of the rocks in the metamorphic section of the sole is provided in this publication: these rocks record valuable information in terms of petrological processes and conditions of accretion, with limited retrogression. The petrology of the metamorphic sole shows that it originates from ocean floor and that it is the equivalent of the Shick-Shock volcanics group, metamorphosed to granulite/higher amphibolite facies. Presence of aluminosilicate bearing metapelites allows constraining pressure conditions in a more robust way than in the case of the famous Semail ophiolite (Oman). Thermobarometric estimates for peak metamorphic conditions for the metamorphic sole of the Mont Albert ophiolitic complex indicate temperatures above 800 °C close to the contact with the overlying peridotite, decreasing to ~650 °C within <30 m, at pressure around 1 GPa. Structurally lower subunits of the sole metamorphosed in the amphibolite/higher greenschist facies retain evidence of basaltic protolith accreted to lower pressures. Evidence of partial melting in high-grade subunits, contradictory with the apparent lack of melting products, may be linked to preferential accretion of melt-poor lithologies to the overlying plate, melt-rich lithologies being too weak to allow coupling and accretion to non-serpentinised peridotite. The reconstructed petrological evolution is consistent with stepwise accretion of metamorphic soles in nascent obduction/subduction systems as proposed for the Semail example and other metamorphic soles worldwide: the subducted oceanic plate is metamorphosed up to granulite facies by heat transfer from the still hot overlying peridotite, and accreted to the base of the future ophiolite when the rheological contrast between the two is minimum. Thermobarometric estimates indicate clock-wise pressure-temperature paths. The inferred thermal evolution requires burial of the metamorphic sole below warm, young (or rejuvenated) oceanic lithosphere, consistent with subduction initiation close to the ridge followed by rapid accretion and exhumation of the metamorphic sole. In this context, exhumation along higher geothermal gradient than burial shows that cooling of the mantle wedge above the plate interface takes place slowly compared to exhumation of the metamorphic sole, even though the plate interface itself has been cooling between burial and exhumation of the rocks

La semelle métamorphique d’Oman : témoinde la dynamique précoce des zones desubduction

National audienceLes semelles métamorphiques sont des unités d’origineocéanique situées à la base des grandes ophiolites obductées. Cesunités, d’une épaisseur maximale de 500 m environ, sont caractériséespar un gradient métamorphique inverse, dans laquelle lesconditions de pression (P) et de température (T) de cristallisationaugmentent vers le contact avec l‘ophiolite sus-jacente depuis500±100C et 0.5±0.2 GPa jusqu’à 800±100C et 1.0±0.2 GPa.Contrairement aux péridotites des ophiolites, les assemblagesminéralogiques des semelles métamorphiques se prêtent bien àla radiochronologie (méta-pélites et plagiogranites), et les âgesobtenus suggèrent que la formation puis l’accrétion des semellesmétamorphiques a lieu au cours des 2 Ma suivant l’initiationd’une zone de subduction intra-océanique. Les semelles métamorphiquesfournissent donc des contraintes majeures sur la dynamiqueprécoce des zones de subduction océaniques (structurethermique, rhéologie effective le long de l‘interface de subductionnaissante), leur déformation et métamorphisme haute températureétant en général expliqués par le transfert de chaleur depuis lemanteau de la plaque supérieure vers l’unité en cours d’accrétion.Cette étude s’attache à préciser la structure et les conditionsP-T de formation de la semelle métamorphique d’Oman, afinde contraindre les processus d’accrétion des semelles métamorphiqueset l’exhumation des massifs ophiolitiques, ainsi que ladynamique de l’interface de subduction. De nombreux auteursévoquent l’existence, au travers de la semelle, d’un gradient thermiquecontinu très fort (~400C/100m), symptomatique d’un métamorphismede contact, sans toutefois rendre compte du gradientinverse de pression. Au terme d’une étude pétrologique etstructurale détaillée, nous proposons un modèle dans lequel lasemelle métamorphique résulte d’épisodes multiples d’accrétiond’unités homogènes en température (donc sans gradient apparent)au cours des premières étapes de subduction intra-océanique. Ceciimplique qu’à chaque étape d’accrétion, l’interface de subductions’est éloignée de la future ophiolite obductée. Nous proposons queces sauts résultent de changements majeurs dans la distributionde la déformation, du fait 1) des variations des propriétés mécaniquesdes roches à l’interface de subduction lors de son équilibrationthermique et 2) de l’augmentation au cours du temps dela proportion de roches sédimentaires entrant en subduction. Cenouveau modèle rend compte d’une grande complexité thermiqueet mécanique lors de la dynamique précoce de l’interface de subduction,pas suffisamment examinée dans les études numériquesactuelles

The SR Family Proteins B52 and dASF/SF2 Modulate Development of the Drosophila Visual System by Regulating Specific RNA Targets▿ †

Deciphering the role of alternative splicing in developmental processes relies on the identification of key genes whose expression is controlled by splicing regulators throughout the growth of a whole organism. Modulating the expression levels of five SR proteins in the developing eye of Drosophila melanogaster revealed that these splicing factors induce various phenotypic alterations in eye organogenesis and also affect viability. Although the SR proteins dASF/SF2 and B52 caused defects in ommatidia structure, only B52 impaired normal axonal projections of photoreceptors and neurogenesis in visual ganglia. Microarray analyses revealed that many transcripts involved in brain organogenesis have altered splicing profiles upon both loss and gain of B52 function. Conversely, a large proportion of transcripts regulated by dASF/SF2 are involved in eye development. These differential and specific effects of SR proteins indicate that they function to confer accuracy to developmental gene expression programs by facilitating the cell lineage decisions that underline the generation of tissue identities