Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
B-type olivine fabrics are pervasive within highly depleted dunites of the small-sized Imono peridotite body located within the subduction-type Sanbagawa metamorphic belt of the southwest Japan arc. The dunites contain various microstructures, ranging from porphyroclastic to fine-grained intensely sheared textures. The Mg/(Mg+Fe) atomic ratios (Fo number) of olivine within these dunites are consistently around 0.9, as are the Cr/(Cr+Al) atomic ratios (Cr number) of chromian spinel, suggesting their evolution from a highly depleted magma (boninite). These data provide strong thermal constraints on the formation of the highly depleted dunites,as their formation requires hot, hydrous, shallow mantle (> 1250 degrees C at < 30 km depth) in the mantle wedge. Because the Sanbagawa metamorphic belt finally entrained these peridotites during progressive retrogression, B-type olivine fabrics probably developed in the fore-arc side of the subduction zone, above or along the subducting slab, possibly in association with dehydration fluids derived from the slab. The previously documented small magnitude of S-wave splitting can be explained by the seismic properties of B-type peridotites within an anisotropic layer of approximately several kilometers in thickness, oriented by flow parallel to the subducting slab, under maximum temperatures of 880-1030 degrees C depending on the flow stress. These findings indicate that such a B-type layer could constitute a dominant source of seismic S- and P-wave anisotropy in mantle wedge regions. (c) 2008 Elsevier B.V. All rights reserved.
ISSN: 0012-821X

hal-00412594
https://hal.archives-ouvertes.fr/hal-00412594
DOI : 10.1016/j.epsl.2008.06.014

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A series of two-stage torsion experiments on Carrara marble were conducted to constrain the influence of known prior deformation on rheological response and microstructural evolution. Comparison with previous experimental data on Carrara marble flow and fabric evolution during single-stage torsion deformation experiments provided direct insights into the significance of initial deformation at various conditions. Our experiments were conducted at 727 degrees C temperature and 300 MPa confining pressure, while maintaining a constant strain rate of 3 x 10(-4) s(-1) on the periphery of the cylindrical samples. Under these conditions, the marble is known to deform in power-law (n = 6-10) ductile flow. All torsion experiments were performed with a Paterson type gas-medium testing machine equipped with a torsion actuator module.;Prior (D1) and subsequent (D2) deformation are accomplished by two torsion experiments in sequence on same sample segments. The effect of D1 strain history is investigated during D2 by applying counter-clockwise torsion to a sandwich sample consisting of three segments with different D1 rotation sense. D2 samples experienced continued, first and reversed shearing deformation in top, centre and bottom segments, respectively. D2 bulk strain was chosen equal to D1 strain in top and bottom segments.;D1 experiments followed the typical single-stage deformation behaviour of Carrara marble under the applied experimental conditions. Yielding was followed by strain hardening until a peak stress was reached at a shear strain around 1, after which work softening occurred. Weakening gradually evolved into a constant stress regime. During hardening, a shear microstructure and crystallographic preferred orientation (CPO) developed. Afterwards the volume fraction of smaller dynamic recrystallised grains increased continuously, resulting in a recrystallisation microstructure and CPO at shear strains of 5 and higher. The new D2 experiments displayed a strain variation between homogeneously deformed sample segments that increased with increasing D1 strain. The stress-strain behaviour of the D2 bulk sandwich samples showed less pronounced work-hardening and -softening when compared with single-stage deformation experiments. Furthermore, constant flow stress was attained at increasingly lower strain with increasing D1 strain. In most D2 segments, fabric development is equivalent to single-stage experiments at corresponding absolute strain. The fabrics differed markedly from those in single-stage experiments in the case of reversed two-stage deformation at moderate strain (D1 shear strain 1 and 2.6). Experiments show that grain shape by shearing of relict grains is defined by finite strain and thus affected by strain reversal. Recrystallisation is controlled by absolute strain and not influenced by strain reversal.
ISSN: 0040-1951

hal-00617686
https://hal.archives-ouvertes.fr/hal-00617686
DOI : 10.1016/j.tecto.2010.09.029

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We performed torsional deformation experiments on pre-hydrated fine-grained olivine aggregates using an innovative experimental assembly to investigate water weakening in mantle rocks at high shear strains. San Carlos olivine powder was cold-pressed and then hot-pressed under hydrous conditions, producing aggregates with average grain sizes of 7 or 15 μm. Deformation experiments were performed in a high-resolution gas-medium apparatus equipped with a torsional actuator, under a confining pressure of 300 MPa, a temperature of 1200 °C, and constant shear strain rates ranging from 8 × 10−5 to 1.4 × 10−4 s−1. Maximum shear stresses range from 150 to 195 MPa. These values are 30% lower relative to those determined in previous torsion experiments on dry, fined-grained dunites under similar conditions. Textures and microstructures of the starting and deformed specimens were characterized by scanning and transmission electron microscopy. All deformed aggregates exhibit a shape-preferred orientation marking a foliation and lineation, as well as a reduction in mean grain size from 15 μm down to 3-4 μm due to dynamic recrystallization. Olivine crystallographic fabrics developed rapidly (γ < 0.1), but their strength, characterized by the J-index, is low compared to naturally deformed peridotites or to polycrystalline olivine deformed at similar finite shear strains under dry conditions. The crystallographic fabrics are consistent with deformation by a dislocation accommodated creep mechanism with activation of multiple {0 k l}[1 0 0] systems, among which the (0 1 0)[1 0 0] slip system is dominant, and minor participation of the (0 1 0)[0 0 1] slip system. Transmission electron microscopy confirmed the occurrence of dislocations with [1 0 0] and [0 0 1] Burgers vectors in most grains. Analysis of unpolarized infrared spectra indicates that hydrogen concentration in the olivine lattice is below the saturation level of 18 ppm wt H2O, which is similar to those typically observed in spinel-bearing peridotite xenoliths, and also provide evidence for water-rich inter-granular material trapped in pores and grain boundaries. Seismic properties computed from the CPO observations correspond to those most commonly observed in naturally deformed mantle peridotites with fast P-wave propagation and S-wave polarization subparallel to the shear direction. These torsion experiments on fine-grained olivine polycrystals under hydrous conditions indicate that water weakening under lithospheric conditions is linked to various defects with hydrogen in the olivine structure, as well as with water-derived species in grain boundaries or pores.
ISSN: 0031-9201

hal-00745170
https://hal.archives-ouvertes.fr/hal-00745170
DOI : 10.1016/j.pepi.2012.05.001

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Seismic anisotropy originating within the continental crust is used to determine kinematic flow lines within active mountain belts and is widely attributed to regionally aligned mica. However, naturally deformed micaceous rocks commonly show composite (e.g. S-C) fabrics. It is necessary therefore to understand how both varying mica content and differing intensities of multiple foliations impact on seismic interpretations in terms of deformation fields. An outcrop analogue for granitic mid-crustal deformed zones is used here to calibrate the seismic response against both parameters. Seismic responses are modelled using crystallographic preferred orientations for polymineralic, micaceous granitic gneisses, measured using Electron Back-Scatter Diffraction. The sample results are generalised by modelling the effects of variations in modal composition and the relative importance of deformation fabrics of variable orientation, so-called rock and fabric recipes. The maximum P- and S-anisotropy are calculated at 16.6% and 23.9% for single-foliation gneisses but for mixed (i.e. S-C) foliation gneisses these values reduce to 5.8% and 7.5% respectively. Furthermore, mixtures of multiple foliations generate significant variations in the geometry of the seismic anisotropy. This effect, coupled with the geographical orientation of fabrics in nature. can generate substantial variations in the orientation and magnitude of seismic anisotropy (especially for shear waves) as measured for the continental crust using existing receiver function and teleseismic near-vertical incidence methods. Thus, maps of seismic anisotropy varying with depth in deforming continents need not imply necessarily depth-varying deformation kinematics and tectonic decoupling.
ISSN: 0012-821X

hal-00445247
https://hal.archives-ouvertes.fr/hal-00445247
DOI : 10.1016/j.epsl.2009.09.035

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The late Carboniferous accretionary system of the South Tianshan orogen (North-Western China) underwent complex structural and polymetamorphic evolution. Combined petrological, geochronological and microstructural analysis of (ultra)high-pressure (UHP) metabasites (eclogites and blueschists) enclosed in metapelites show a relict coarse-grained eclogitic fabric S2 surrounded by a dominant fine-grained eclogite and blueschist facies retrograde fabric S2. The S2 fabric is reworked by upright folds F3 that are responsible for a major shortening of the whole accretionary system. For both the eclogite and blueschist, peak and retrograde P–T conditions have been thermodynamically constrained at 25–26 kbar and 425–500 °C and 10–13 kbar and 500−550 °C respectively, suggesting a shared exhumation history. The garnet-whole rock-amphibole isochron in the blueschist yielded Lu–Hf and Sm–Nd ages of 326.0 ± 2.9 Ma and 318.4 ± 3.9 Ma respectively, interpreted to date the prograde to peak metamorphic assemblage. The retrograde path of the eclogite is characterized by heterogeneous omphacite recrystallization into a mylonitic fine-grained matrix and crystallization of blue amphibole. Microstructures in both pristine porphyroclastic and recrystallized fine-grained domains in the eclogite indicate a gradual evolution from constriction-dominated (L>S-type) to flattening-dominated (S>L-type) type of deformation, increase of fabric intensity reflected by gradually growing M-indexes and the development of lattice preferred orientation (LPO) typical for dislocation creep under slightly hydrated conditions. Recrystallization of the matrix in the blueschist is homogeneous, which indicates a matrix dominated channel flow during exhumation. These LPOs evolutions suggest a significant mechanical coupling with the upper plate concomitant with oroclinal bending of the Kazakh orocline. Lock up of Kazakh orocline is responsible for further stress increase resulting in horizontal shortening of South Tianshan accretionary wedge and development of D3 upright folding and steepening of the whole sequence.
ISSN: 1342-937X

hal-01481263
https://hal.archives-ouvertes.fr/hal-01481263
DOI : 10.1016/j.gr.2016.07.007

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
A nested case-control association study was designed to investigate the influence of maternal and fetal copy number variants (CNVs) on reproductive outcomes. Genotypes of ten CNVs encompassing GST and CYP genes were assessed. Significant associations were only found for child CNV genotypes. In particular, the child GSTM1 insertion allele was associated with prematurity protection (odds ratio, 95% CI: 0.67, 0.51-0.89; P < 0.01), whereas the child GSTT2B insertion allele was associated with an increased risk of being small for gestational age (odds ratio, 95% CI: 1.33, 1.07-1.67; P = 0.01). The study highlights the role of the fetal genome in prenatal development and also the need to analyse CNVs in a systematic manner.
ISSN: 1470-0328

hal-00875955
https://hal.archives-ouvertes.fr/hal-00875955
DOI : 10.1111/j.1471-0528.2012.03400.x
PUBMED : 22676722

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The N-S oriented Raghane shear zone (8 degrees 30') delineates the western boundary of the Saharan metacraton and is, with the 4 degrees 50' shear zone, the most important shear zone in the Tuareg shield. It can be followed on 1000 km in the basement from southern Air, Niger to NE Hoggar, Algeria. Large subhorizontal movements have occurred during the Pan-African orogeny and several groups of granitoids intruded during the Neoproterozoic. We report U-Pb zircon datings (laser ICP-MS) showing that three magmatic suites of granitoids emplaced close to the Raghane shear zone at c. 790 Ma, c. 590 and c. 550 Ma. A comprehensive and detailed (158 sites, more than 1000 cores) magnetic fabric study was performed on 8 plutons belonging to the three magmatic suites and distributed on 200 km along the Raghane shear zone. The main minerals in all the target plutons do not show visible preferential magmatic orientation except in narrow shear zones. The AMS study shows that all plutons have a magnetic lineation and foliation compatible with the deformed zones that are zones deformed lately in post-solidus conditions. These structures are related to the nearby mega-shear zones, the Raghane shear zone for most of them. The old c. 793 Ma Touffok granite preserved locally its original structures. The magnetic structures of the c. 593 Ma Ohergehem pluton, intruded in the Aouzegueur terrane, are related to thrust structures generated by the Raghane shear zone while it is not the case of the contemporaneous plutons in the Assode-Issalane terrane whose structures are only related to the subvertical shear zones. Finally, the c. 550 Ma granite group has magnetic structure related to the N-S oriented Raghane shear zone and its associated NNE-SSW structures when close to them, but NW-SE oriented when further. These NW-SE oriented structures appear to be characteristic of the late Neoproterozoic evolution of the Saharan metacraton and are in relation to the convergence with the Murzuq craton. This evolution reflects the rheological contrast existing along the Raghane shear zone marking the western boundary of the Saharan metacraton.
ISSN: 0040-1951

hal-00429354
https://hal.archives-ouvertes.fr/hal-00429354
DOI : 10.1016/j.tecto.2009.04.022

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
New petrographic and microstructural observations, mineral equilibria modelling and U/Pb (monazite) geochronological studies were carried out to investigate the relationships between deformation and metamorphism across the Rehamna massif (Moroccan Variscan belt). In this area, typical Barrovian (muscovite to staurolite) zones developed in Cambrian to Carboniferous metasedimentary rocks that are distributed around a dome-like structure. First assemblages are characterized by the presence of locally preserved andalusite, followed by prograde evolution culminating at 6 kbar and 620 °C in the structurally deepest staurolite zone rocks. This Barrovian sequence was subsequently uplifted to supracrustal levels, heterogeneously reworked at greenschist facies conditions, which was followed locally by static growth of andalusite, indicating heating to 2.5–4 kbar and 530–570 °C. The 206Pb/238U monazite age of 298.3 ± 4.1 Ma is interpreted as minimum age of peak metamorphic conditions, whereas the ages of 275.8 ± 1.7 Ma and 277.0 ± 1.1 Ma date decompression and heating at low pressure, in agreement with previous dating of Permian granitoids intruding the Rehamna massif. The prograde metamorphism occurred during thickening and associated horizontal flow in the deeper crust (S1 horizontal schistosity). The horizontally disposed metamorphic zones were subsequently uplifted by a regional scale antiform during ongoing N–S compression. The re-heating of the massif follows late massive E–W shortening, refolding and retrograde shearing of all previous fabrics coevally with regionally important intrusions of Permian granitoids. We argue that metamorphic evolution of the Rehamna massif occurred several hundred kilometres from the convergent plate boundaries in the interior of continental Gondwanan plate. The tectonometamorphic history of the Rehamna massif is put into Palaeozoic plate tectonic perspective and Late Carboniferous reactivation of (Devonian)–Early Carboniferous basins formed during stretching of the north Gondwana margin and formation of the Palaeotethys Ocean. The inherited heat budget of these magma-rich basins plays a role in the preferential location of this intracontinental orogen. It is shown that rapid transition from lithospheric stretching to compression is characterized by specific HT type of Barrovian metamorphism, which markedly differs from similar Barrovian sequences along Palaeozoic plate boundaries reported from Variscan Europe.
ISSN: 0263-4929

hal-01468009
https://hal.archives-ouvertes.fr/hal-01468009
DOI : 10.1111/jmg.12214

Éditeur(s) : HAL CCSD
Résumé : Fault zones are major discontinuities in sedimentary basins. Understanding their role on fluid migrations is an essential issue to (i) characterize the mechanisms and kinematics of deformation and (ii) to determine the parameters which control the distribution of energetic or mineral resources. This work applies to faulting under a temperature range of 200-250°C, corresponding to that of deeply buried reservoirs as well as potentially seismogenic fault zones. The studied faults are normal faults affecting the “Grès d’Annot” Formation, a Priabonian-Rupelian siliclastic turbidite succession of the Alpine foreland basin. They are located in two distinct areas: the Moutière-Restefond area in the eastern part of the basin and the Estrop area in the western part. Vitrinite reflectance outside fault zones indicates maximal temperatures of 240-260°C at Moutière-Restefond and 170-200°C at Estrop, i.e. burial depths around 8 km and 6 km, respectively, assuming a mean geothermal gradient of 30°C/km. Burial was due to underthrusting below the Embrunais-Ubaye Alpine nappes, the difference in burial between the two areas traducing the westward taper of the nappe front.The studied faults affect alternating arkosic sandstone beds and pelite layers with offsets from centimeters to decameters. Two types of core zones are recognized: (1) the foliated core zones of the Moutière-Restefond area, where deformation dominated by pressure solution and synkinematic phyllosilicate neoformation resulted in a foliated fabric, and (2) the core zones of the Estrop area, characterized by dilatant quartz-mineralized veins and breccia. Fault and host rock petrophysical properties were measured on drill plugs, using the water porosity technique and nitrogen permeability technique under 2 Mpa of confining pressure. In the Moutière-Restefond fault zones, plug axes were oriented in three orthogonal directions corresponding to the main deformation axes (X, Y, Z) of the foliated arkose to characterize the effect of structural anisotropy. In the Estrop fault zones, where deformation does not show equivalent anisotropy, the plug axes were oriented according to the main fault plane orientation (i.e., parallel to azimuth, parallel to dip and normal). In each fault zone, the porosity of fault rocks is equivalent to that of host rocks. In the Moutière-Restefond fault zones, the plugs parallel to the X and Z axes of deformation have a permeability similar to that of the host rocks, whereas permeability parallel to the Y axis (i.e. parallel to the foliation and the main fault plane) is one order of magnitude higher. By contrast, the permeability values in the Estrop fault zones are isotropic and equivalent to the host rock values. This study highlights the importance of various types of deformation mechanisms and related fluid-rock interactions in determining different fault rock types within a similar protolith, ultimately controlling distinct petrophysical properties of fault zones.
Les zones de failles constituent des discontinuités majeures dans la croûte supérieure. Ce mémoire s’intéresse aux failles actives à des profondeurs de 5-8 km, i.e. dans des conditions pouvant jouer un rôle important (i) sur le comportement mécanique de de la croûte, en particulier sur son potentiel sismogénique et (ii) sur la compartimentation des réservoirs géologiques profondément enfouis.Les failles étudiées sont des failles normales (rejet jusqu’à 50 mètres) affectant des alternances de bancs de grès arkosiques et de pélites dans les Grès d’Annot, une formation turbiditique d’âge priabonien-rupélien du bassin d’avant-chaine alpin. Les Grès d’Annot ont été enfouis sous les nappes de l’Embrunais-Ubaye peu de temps après leur dépôt et exhumés au Miocène moyen-supérieur. L’étude a été réalisée dans deux secteurs des Alpes de Haute-Provence, (1) le secteur de l’Estrop (chaine des Trois Evéchés, dans la partie occidentale du bassin) et (2) le secteur de Point Vert-Restefond (alentours du col de la Bonette, dans la partie orientale du bassin). La réflectance de la vitrinite dans les Grès d’Annot indique des températures maximales de 160-190°C dans le secteur (1) et de 240-260 °C dans le secteur (2) , soit des profondeurs d’enfouissement de 5-6 km et 8-10 km, respectivement, dans l’hypothèse d’un gradient géothermique moyen de 25-30°C/km. A partir des observations microstructurales, de données d’inclusions de fluides, de modélisations thermodynamiques sur chlorites syncinématiques et des indices de cristallinité de l’illite, on a établi que les failles du secteur (1) ont fonctionné à une température d’environ 200°C pour une pression de 0.6Kbar et celles du secteur (2) dans une gamme de température de 200-220°C et de pression comprise entre 0.6 et 1.1 Kba. L’activité de la faille de Point-Vert a été datée par la méthode 40Ar/39Ar au Miocène inférieur (21 Ma). Ces failles sont interprétées comme associées aux premiers stades du soulèvement résultant de l’activité des chevauchements dans le socle sous-jacent.Deux types principaux de roches de failles caractérisent les failles étudiées. Les brèches dilatantes sont majoritaires dans les zones de failles du secteur (1) alors que les arkoses foliées sont majoritaires dans les zones de failles du secteur (2). Les brèches dilatantes témoignent de mécanismes de déformation dominés par la fracturation transgranulaire et la pression-solution des silicates. Ce dernier mécanisme se manifeste principalement par d’abondantes précipitations cycliques de quartz, les stylolites restant peu développés. Les géométries structurales sont associées à l’activation de plans T de Riedel déterminant l’ouverture de relais extensifs et un régime localement dilatant. Nous associons ce type de roche de faille à une faible pression effective au cours de la déformation.La déformation cisaillante exprimée dans les arkoses foliées témoigne de mécanismes de déformation dominés par (i) la fracturation intragranulaire et transgranulaire, (ii) la pressionsolution/ précipitation des silicates, principalement le quartz, (iii) la néoformation de micas blancs liée à l’altération des feldspaths et (iv) la précipitation de chlorite. Ce travail montre également que les transformations minéralogiques liées aux interactions fluide-roche dans la zone de faille (forte augmentation de la proportion des phyllosilicates au dépend des feldspaths) changent radicalementla composition minéralogique de la roche. Nous associons ce type de roche de faille à une forte pression effective au cours de la déformation.La pétrophysique montre que les brèches de failles (secteur 1) ont une perméabilité isotrope et équivalente à celle du protolithe (10-3mD sous 2 MPa de confinement) tandis que les roches foliées (secteur 2) montrent une augmentation de perméabilité (10-2mD) selon l’axe Y de la déformation, d’un ordre de grandeur supérieure aux perméabilités suivant les axes X et Z et dans la roche hôte (10-3mD). Ce drain préférentiel potentiel est associé à l’orientation des phyllosilicates dans la foliation.
https://hal.archives-ouvertes.fr/tel-01310382

tel-01310382
https://hal.archives-ouvertes.fr/tel-01310382
https://hal.archives-ouvertes.fr/tel-01310382/document
https://hal.archives-ouvertes.fr/tel-01310382/file/Th%C3%A8seCAVAILHES2012.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We present an integrated study of geochemistry, petrofabrics and seismic properties of strongly sheared eclogites from the Chinese Continental Scientific Drilling (CCSD) project in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. First, geochemical data characterize diverse protoliths of the studied eclogites. The positive Eu- and Sr-anomalies, negative Nb anomaly and flat portion of heavy rare earth elements in coarse-grained rutile eclogites (samples B270 and B295) suggest a cumulate origin in the continental crust, whereas the negative Nb anomaly and enrichment of light rare earth elements in retrograde eclogites (samples B504, B15 and B19) imply an origin of continental basalts or island arc basalts. Second, P-wave velocities (V-p) of three typical eclogite samples were measured under confining pressures up to 500 MPa and temperatures to 700 degrees C. At 500 MPa and room temperature, the mean V-p reaches 8.50-8.53 km/s in samples B270 and B295 but drops to 7.86 km/s in sample B504, and the P-wave anisotropy changes from 1.7-2.7% to 5.5%, respectively. The pressure and temperature derivatives of V-p are larger in the retrograde eclogite than in fresh ones. Third, the electron backscatter diffraction (EBSD) measurements of the eclogites reveal random crystal preferred orientation (CPO) of garnet and pronounced CPO of omphacite, which is characterized by a strong concentration of [001]-axes sub-parallel to the lineation and of (010)-poles perpendicular to the foliation. The asymmetric CPO of omphacite in sample B270 recorded a top-to-the-south shear event during subduction of the Yangtze plate. The calculated fastest V-p is generally sub-parallel to the lineation, but a different deformation environment during exhumation could form second-order variations in omphacite CPO and affect the V-p distribution in eclogites (e.g., the fastest V-p is at similar to 35 degrees from the foliation in sample 13295). Comparison between measured and calculated seismic properties indicates that the CPO of omphacite controls the seismic anisotropy of eclogites at high pressure, and compositional layering and retrograde minerals will increase the anisotropy. Calculated P-wave velocities agree well with velocities measured at 500 MPa and room temperature for fresh eclogites, but much higher than those of retrograde eclogite. As a case study, the laboratory-derived V-p-P and V-p-T relationships were used to estimate P-wave velocities of eclogites and peridotites beneath the Western Superior Province, Canada. The results indicate that besides the fabric-induced anisotropy, the direction dependence of pressure and temperature derivatives of V-p can significantly increase seismic anisotropy of eclogites with depth, which results in eclogites being an important candidate for the seismic anisotropy in the upper mantle. Due to their very high density and velocity, garnet-rich eclogites within peridotite could be detected in seismic reflections in subduction zones.
ISSN: 0040-1951

hal-00463512
https://hal.archives-ouvertes.fr/hal-00463512
DOI : 10.1016/j.tecto.2008.09.027

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : 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.
ISSN: 1525-2027

hal-01243526
https://hal.archives-ouvertes.fr/hal-01243526
DOI : 10.1002/2015GC005776