Exhumation of the ultra high-pressure Tso Morari unit in eastern Ladakh (NW Himalaya): a case study.

Exhumation processes of the ultra-high pressure (UHP) Tso Morari dome (NW-Himalaya) are investigated using structural, petrological and geochronological data. The UHP Tso Morari unit is bounded by the low-grade metamorphic Indus Suture Zone to the NE and Mata unit to the SW. Three deformation phases (D1, D2 and D3) are observed. Only D3 is common to the UHP unit and the surrounding units. In the UHP unit, the first deformation phase (D1) produced upright folds, under eclogitic conditions (> 20 kbar; 580 ± 60 °C). D1 is overprinted by D2 structures related to a NW-SE trending open anticline. This phase is characterized by blueschist mineral associations, and corresponds to the quasi-isothermal decompression from a depth of 90 km (eclogitic conditions) up to 30-40 km. The final exhumation phase of the Tso Morari unit is dominated by tectonic denudation and erosion (D3), associated with a slight temperature increase. Radiochronological analyses indicate that the UHP exhumation process began during the Eocene. Exhumation was fast during D1-D2 and slowed down through D3 in Oligocene time. The change in the deformation style from D1-D2 to D3 in the Tso Morari unit coincides with changes in the exhumation rates and in the metamorphic conditions. These changes may reflect the transition from an exhumation along the subduction plane in a serpentinized wedge, to the vertical uplift of the Tso Morari unit across the upper crust

Diachronous exhumation of HP-LT metamorphic rocks from southwestern Alps: evidence from fission-track analysis

International audienceNew fission-track ages on zircon and apatite (ZFT and AFT) from the southwestern alpine paleo-accretionary wedge document a contrasting cooling history from east to west. In the eclogitic Monviso ophiolites, the ZFT ages are 19.6 +/- 0.8 Ma and the AFT ages are 8.6 +/- 1.7 Ma. In the HT-blueschist eastern Queyras, ZFT ages range from 27.0 +/- 1.5 Ma to 21.7 +/- 1.6 Ma and AFT ages from 14.2 +/- 2.0 to 9.4 +/- 1.1 Ma. In the LT-blueschist western Queyras, ZFT ages are between 94.7 +/- 3.1 Ma and 63.1 +/- 2.9 Ma and AFT ages are between 22.2 +/- 1.6 and 22.6 +/- 1.5 Ma. The Chenaillet ophiolite yields ages of 118.1 +/- 3.7 Ma on ZFT and of 67.9 +/- 8.5 Ma on AFT. These new FT data combined with petrological and geochronological constraints record a diachronous exhumation in the paleo-accretionary wedge during subduction and collision

Modélisation géométrique 3D des granites stéphaniens du massif du Pelvoux (Alpes, France).

International audienceLa modélisation 3D de la géométrie des granites Stéphaniens du Massif du Pelvoux a permis de mettre en évidence le contexte cisaillant associé à leur mise en place. Dans les Massifs Cristallins Externes Français, ces cisaillements se répartissent selon deux directions, N50 et N135, respectivement dextre et sénestre. Ce système décrochant Carbonifère s'intègre dans un contexte d'extension N-S connu dans l'ensemble de la chaîne Varisque

Anisotropic self-affine properties of experimental fracture surfaces

The scaling properties of post-mortem fracture surfaces of brittle (silica glass), ductile (aluminum alloy) and quasi-brittle (mortar and wood) materials have been investigated. These surfaces, studied far from the initiation, were shown to be self-affine. However, the Hurst exponent measured along the crack direction is found to be different from the one measured along the propagation direction. More generally, a complete description of the scaling properties of these surfaces call for the use of the 2D height-height correlation function that involves three exponents zeta = 0.75, beta = 0.6 and z = 1.25 independent of the material considered as well as of the crack growth velocity. These exponents are shown to correspond to the roughness, growth and dynamic exponents respectively, as introduced in interface growth models. They are conjectured to be universal.Comment: 12 page

Diachronous evolution of the alpine continental subduction wedge: evidence from P-T estimates in the Briançonnais Zone houillère (France - Western Alps).

International audienceThe study of continental subduction processes requires detailed Pressure Temperature (P-T) paths to understand the kinematic of burial and exhumation of continental units. In the French Western Alps, the Briançonnais zone is a remnant of the continental subduction wedge. P-T conditions have been estimated in its most internal parts, but there is a lack of data in the western part, known as the "Zone houillère". This Briançonnais Zone houillère is classically divided into two sub-units: the upper and lower Houiller units. This study focuses on both of these in the Clarée valley, north of Briançon. In this low-grade metamorphic terrain, estimation of P-T history is complicated because there are few adapted methods and these rocks have a poor metamorphic mineralogical content, including detrital metamorphic minerals inherited from their hercynian history. Therefore, to acquire accurate P-T estimates a multi-method approach is required, involving qualitative and quantitative Raman study of Carbonaceous Material (RSCM), chemical analysis from quantified X-ray maps and thermodynamic modelling of chlorites and K-white micas. Such multi-approach P-T estimates on a sandstone sample allow distinguishing hercynian peak metamorphic conditions of 371 ± 26°C and 3.5 ± 1.4 kbar and alpine peak metamorphic conditions of 275 ± 23°C and 5.9 ± 1.7 kbar. These results are consistent with our RSCM and Tmax estimates. Raman study conducted on organic-rich schist samples shows an eastward increase of the alpine Tmax in the upper Houiller unit, from 280 to 300°C across the Briançonnais Zone houillère. In contrast, carbonaceous material included in detrital grains of muscovite in the sandstone exhibits higher temperatures. This hercynian Tmax is estimated using thermodynamic modelling at 376 ± 50°C. According to these results and previous work in more internal parts of the Briançonnais zone, a geodynamic reconstruction is proposed, which is characterized by a diachronous evolution of the Briançonnais zone involved in alpine continental subduction at different times. The geothermal gradient in the Briançonnais zone changes from 8°C/km during early continental subduction, to 40°C/km during the collisional event at about 35-30 Ma. The intermediate gradient of 15°C/km estimated in the Briançonnais Zone houillère suggests that this unit was buried later, than the more internal Briançonnais units, after 40 Ma

Source tracing of detrital serpentinite in the Oligocene molasse deposits from the western Alps (Barrême basin): implications for relief formation in the internal zone.

International audienceWe present the first contribution of tracing the source area of ophiolitic detritus in the Alpine molasses by Raman spectroscopy. The lower Oligocene molasse deposits preserved in the Barrême basin, in the SW foreland of the western Alpine arc, are known for the sudden arrival of the first "exotic" detritus coming from the internal Alpine zones. Among them, the pebbles of serpentinized peridotites have so far not been studied. We show that they only consist of antigorite serpentinite, implying that they originate from erosion of HT-blueschists. In contrast, the upper Oligocene/lower Miocene molasse, shows mixed clasts of serpentine including antigorite and lizardite without any evidence of chrysotile. This suggests that they were derived from a less metamorphosed unit such as the LT-blueschist unit. Taking into account the sediment transport direction in the basin and the varied metamorphic characteristics of the other ocean-derived detritus, we constrain the lithological nature of the source zones and the location of the relief zones, identified as the internal Alps, SE of the Pelvoux external crystalline massif. Available structural data and in situ thermochronological data allow reconstructing the Oligocene to early Miocene collisional geometry of the Paleogene subduction wedge. This phase corresponds to two major phases of uplift evolving from a single relief zone located above the Ivrea body during the early Oligocene and persisting up to the early Miocene; then during the late Oligocene/early Miocene a second relief zone developed above the Briançonnais zone. At that time, the internal western Alps acquired its double vergency

The Monviso ophiolitic massif (Western Alps), a section through a serpentinite subduction channel.

The exhumation of subducted lithosphere requires a mechanically weak zone at the interface between the subduction plane and the rigid overlying mantle peridotites with a viscosity greater than 10 20 Pa.s. At shallow depths (< 40-50 km) blueschists are exhumed in accretionary wedge along the interface between the subducting plate and the overriding plate (Platt, 1993). At greater depth, serpentinites plays the role of mechanically weak layer in cool continental subduction and act as the lubricant and produce a return flow for the exhumation of eclogitic rocks. The close association of serpentinites and eclogites in the Monviso massif (Western Alps) allow to discuss the concept of subduction serpentinite channel. We propose that the Monviso ophiolitic massif corresponds to a section of a 50 km long serpentinite channel in where eclogitic blocks were exhumed between 60 and 45 Ma and ended whEuropean continental margin was involved in the southeast dipping subduction zone

Reconstructing the total shortening history of the NW Himalaya.

The onset of India-Asia contact can be dated with both biostratigraphic analysis of syn-collisional sedimentary successions deposited on each side of the Indus Suture zone, and by radiometric dating of Indian crustal rocks which have undergone subduction to great depths in the earliest subduction-collision stages. These data, together with paleomagnetic data show that the initial contact of the Indian and Asian continental margins occurred at the Paleocene/Eocene boundary, corresponding to 55 ± 2 Ma. Such dating, which is consistent with all available geological evidence, including the record of magnetic anomalies in the Indian ocean and decrease of magmatic activity related to oceanic subduction can thus be considered as accurate and robust. The sedimentary record of the Tethys Himalaya rules out obduction of oceanic allochtons directly onto the Indian continental margin during the Late Cretaceous. The commonly inferred Late Cretaceous ophiolite obduction events may have thus occurred in intra-oceanic setting close to the Asian margin before its final emplacement onto the India margin during the Eocene. Granitoid and sedimentary rocks of the Indian crust, deformed during Permo-Carboniferous rifting, reached a depth of some 100 km about 1 Myr after the final closure of the Neo-Tethys, and began to be exhumed between 50 and 45 Ma. At this stage, the foreland basin sediments from Pakistan to India show significant supply from volcanic arcs and ophiolites of the Indus Suture Zone, indicating the absence of significant relief along the proto-Himalayan belt. Inversion of motion may have occurred within only 5 to 10 Myr after the collision onset, as soon as thicker and buoyant Indian crust chocked the subduction zone. The arrival of thick Indian crust within the convergent zone 50-45 Myr ago led to progressive stabilization of the India/Asia convergent rate and rapid stabilization of the Himalayan shortening rate of about 2 cm.yr-1. This first period also corresponds to the onset of terrestrial detrital sedimentation within the Indus Suture zone and to the Barrovian metamorphism on the Indian side of the collision zone. Equilibrium of the Himalayan thrust belt in terms of amount of shortening vs amount of erosion and thermal stabilization less than 10 Myr after the initial India/Asia contact is defined as the collisional regime. In contrast, the first 5 to 10 Myr corresponds to the transition from oceanic subduction to continental collision, characterized by a marked decrease of the shortening rate, onset of aerial topography, and progressive heating of the convergent zone. This period is defined as the continental subduction phase, accommodating more than 30% of the total Himalayan shortening

Anomalie thermique et sous-placage en zone d'avant-arc (exemple du massif Triasique de El Oro, Equateur)

Depuis au moins 540 Ma deux grands systèmes de subduction coexistent sur Terre : d'une part, les systèmes de subduction-collision (chaînes Hercynienne, Himalayenne ou Alpine) et d'autre part, les systèmes de subduction de type péri-pacifique. Pour ces derniers, l'avant-arc constitue une zone clef pour retracer l'évolution de la subduction au cours du temps. En effet ces zones au contact avec le slab peuvent enregistrer des événements tectoniques et/ou des conditions métamorphiques variées (e.g. formation de paired metamorphic belts ), qui sont autant d'indicateurs du contexte géodynamique. Le massif métamorphique de El Oro en Equateur est un exemple exeptionnel où une section complète et basculée de l'avant-arc Triasique est préservée. L'ensemble est constitué d'une série métasédimentaire de bas à haut grade métamorphique intrudée par des granitoïdes de type S, juxtaposé avec un laccolithe gabbroïque et des schistes bleus. Ce travail de thèse s'est concentré sur l'étude du métamorphisme de haute-température basse-pression et ses relations les schistes bleu. Afin de contraindre l'événement tectono-métamorphique affectant l'avant-arc Equatorien au Trias et la formation d'une "paired metamorphic belt", nous avons utilisé des outils structuraux, métamorphiques, géochimiques, géochronologiques et de modélisation thermique. Nos résultats montrent que durant cette période l'avant-arc Equatorien connait un intense épisode de fusion partielle en régime extensif. La base de la croûte est migmatisée sur une épaisseur de 10km. Les estimations Pression-Température indiquent que les conditions de fusion partielle varient de 4.5 kbar et 650C pour la partie supérieure métaxitique et jusqu'à 7.5 kbar et 720C pour la partie inférieure diatexitique. La gradient géothermique inféré est divisé en deux segments : un segment supérieur caractérisé par un gradient de 40C/km et un segment inférieur caractérisé par un gradient quasi-isothermique. L'absence de paragénèse de ultra-haute température est attribuée à la grande fertilité du protolithe métasédimentaire. Les résultats géochimiques montrent que les plutons granodioritiques sont issus d'un mélange entre : (1) les liquides de fusion partielle produit par la réaction de deshydration de la muscovite des métasédiments et (2) un magma basique. Les âges U-Pb sur zircons et monazites révèlent que l'événement anatectique fût bref entre 229 et 225 Ma. La source de chaleur à l'origine de l'événement thermique est attribuée à la mise en place d'un pluton gabbroïque à ~ 230 Ma en base de croûte. Successivement, se sous-plaque les schistes-bleu refroidissant rapidement l'avant-arc. L'événement anatectique observé dans le massif de El Oro au Trias s'insrit à plus grande échelle au sein d'une large anomalie thermique affectant l'ensemble du continent sud Américain entre 260 et 220 Ma. Durant cette période la marge est un soumise à un régime extensif accompagné d'un important magmatisme d'origine crustal, principalement en position d'arc et d'avant-arc. Nous attribuons cette anomalie thermique d'ampleur continental à une "avalanche mantellique". A la lumière du contexte géodynamique globale nous inteprétons la formation de la paired metamorphic belt de El Oro à la rupture du slab.Since about 540 Ma, two subductions systems co-exist on Earth: the subduction-collision systems (Hercynian, Himalayan or Alpin belts) and the circum-pacific subduction system. For the last the forearc region constitutes a key zone to understand the dynamic of the subduction. Indeed the forearc region in contact with the slab may records various tectonics events and/or metamorphic conditions (e.g. formation of paired metamorphic belt). Theses geological records are direct evidences of the linked geodynamical context. The El Oro metamorphic complex in Ecuador is a unique example where a whole Triassic forearc section is tilted and well preserved. The complex is made of low to high grade metasedimentary rocks intruded by S-type granitoids, juxtaposed with gabbroic rocks and blueschists. This study is focused on the high-temperature metamorphism and its retionaships with the high-pressure metamorphism. In order to constrain the tectono-metamorphic affecting the forearc region and the formation of a paired metamorphic belt we used strutural, metamorphic, geochemical, geochronological and themal modeling studies. Our results show that during Triassic times the Ecuadorian forearc underwent a strong episode of partial melting in extentional context. The migmatized part of crust is 10 km thick. Pressure-Temperature estimates indicate that partial melting started at 4.5 kbar and 650C for the upper metatexitic part until 7.5 kbar and 720 C for the lower diatexitic part. The resulting geothermal gradient exhibits two parts: an upper part caracteristed by a thermal gradient of 40C/km and a lower part caractérized by a near-adiabatic gradient. The absence of ultra-high tempetature paragenesis is attributed to the high fertility of the metasedimentary protolith. Geochemical results show that granodiorite made of a miwing between: (1) the melt extacted under muscovite dehydration melting and (2) a basic magma. U-Pb ages on zircon and monazite reveal that the anatectic event was short, between 229 and 225 Ma. The origin of the thermal anomaly is attributed to the emplacement of the gabbroic plutonic unit at ~230 at root level. Successively, the blueschites are underplating triggering a strong coolng of the forearc region. The anatectic recorded in the El Oro metamorphic complex at Triassic times is part of a larger thermal anomaly affecting the whole south american margin between 260 and 220 Ma. During this period the margin is under extentional conditions and exhibit a strong S-type magmatic activity. This magmatism is mainly located in arc and forearc position. We attribute this large-scale thermal anomaly to slab fall in the lower mantle. In the light of the geodynamical context, we suggest that the formation of the El Oro paired metamorphic belt is related to slab breakoff.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

How cracks are hot and cool: a burning issue for paper

International audienceMaterial failure is accompanied by important heat exchange, with extremely high temperature – thousands of degrees – reached at crack tips. Such temperature may subsequently alter the mechanical properties of stressed solids, and finally facilitate their rupture. Thermal runaway weakening processes could indeed explain stick-slip motions and even be responsible for deep earthquakes. Therefore, to better understand catastrophic rupture events, it appears crucial to establish an accurate energy budget of fracture propagation from a clear measure of the various energy dissipation sources. In this work, combining analytical calculations and numerical simulations, we directly relate the temperature field around a moving crack tip to the part α of mechanical energy converted into heat. Monitoring the slow crack growth in paper sheets with an infrared camera, we measure a significant fraction α = 12% ± 4%. Besides, we show that (self-generated) heat accumulation could weaken our samples with microfibers combustion, and lead to a fast crack/dynamic failure/ regime