Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
The Pyrenean peridotites (lherzolites) form numerous small bodies of subcontinental mantle, a few meters to 3 km across, exposed within the narrow north Pyrenean zone (NPZ) of Mesozoic sediments paralleling the north Pyrenean Fault. Recent studies have shown that mantle exhumation occurred along the future NPZ during the formation of the Albian-Cenomanian Pyrenean basins in relation with detachment tectonics. This paper reviews the geological setting of the Pyrenean lherzolite bodies and reports new detailed field data from key outcrops in the Bearn region. Only two types of geological settings have to be distinguished among the Pyrenean ultramafic bodies. In the first type (sedimented type or S type), the lherzolites occur as clasts of various sizes, ranging from millimetric grains to hectometric olistoliths, within monogenic or polymictic debris flow deposits of Cretaceous age, reworking Mesozoic sediments in dominant proportions as observed around the Lherz body. In the second type (tectonic type or T type), the mantle rocks form hectometric to kilometric slices associated with crustal tectonic lenses. Both crustal and mantle tectonic lenses are in turn systematically associated with large volumes of strongly deformed Triassic rocks and have fault contacts with units of deformed Jurassic and Lower Cretaceous sediments belonging to the cover of the NPZ. These deformed Mesozoic formations are not older that the Aptian-early Albian. They are unconformably overlain by the Albian-Cenomanian flysch formations and have experienced high temperature-low pressure mid-Cretaceous metamorphism at variable grades. Such a tectonic setting characterizes most of the lherzolite bodies exposed in the western Pyrenees. These geological data first provide evidence of detachment tectonics leading to manle exhumation and second emphasize the role of gravity sliding of the Mesozoic cover in the preorogenic evolution of the Pyrenean realm. In the light of such evidence, a simple model of basin development can be inferred, involving extreme thinning of the crust, and mantle uprising along a major detachment fault. We demonstrate coeval development of a crust-mantle detachment fault and generalized gravitational sliding of the Mesozoic cover along low-angle faults involving Triassic salt deposits as a tectonic sole. This model accounts for the basic characteristics of the precollisional rift evolution in the Pyrenean realm.
ISSN: 0278-7407

hal-00512818
https://hal.archives-ouvertes.fr/hal-00512818
DOI : 10.1029/2009TC002588

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Brecciated and fractured peridotites with a carbonate matrix, referred to as ophicalcites, are common features of mantle rocks exhumed in passive margins and mid-oceanic ridges. Ophicalcites have been found in close association with massive peridotites, which form the numerous ultramafic bodies scattered along the North Pyrenean Zone (NPZ), on the northern flank of the Pyrenean belt. We present the first field, textural and stable isotopic characterization of these rocks. Our observations show that Pyrenean ophicalcites belong to three main types: (1) a wide variety of breccias composed of sorted or unsorted millimeter- to meter-sized clasts of fresh or oxidized ultramafic material, in a fine-grained calcitic matrix; (2) calcitic veins penetrating into fractured serpentine and fresh peridotite; and (3) pervasive substitution of serpentine minerals by calcite. Stable isotopic analyses (O, C) have been conducted on the carbonate matrix, veins and clasts of samples from 12 Pyrenean ultramafic bodies. We show that the Pyrenean ophicalcites are the product of three distinct genetic processes: (1) pervasive ophicalcite resulting from relatively deep and hot hydrothermal activity; (2) ophicalcites in veins resulting from tectonic fracturing and cooler hydrothermal activity; and (3) polymictic breccias resulting from sedimentary processes occurring after the exposure of subcontinental mantle as portions of the floor of basins which opened during the mid-Cretaceous. We highlight a major difference between the eastern and western Pyrenean ophicalcites belonging, respectively, to the sedimentary and to the hydrothermal types. Our data set points to a possible origin of the sedimentary ophicalcites in continental endorheic basins, but a post-depositional evolution by circulation of metamorphic fluids or an origin from relatively warm marine waters cannot be ruled out. Finally, we discuss the significance of such discrepancy in the characteristics of the NPZ ophicalcites in the frame of the variable exhumation history of the peridotites all along the Pyrenean realm.
ISSN: 1437-3254

insu-00944405
https://hal-insu.archives-ouvertes.fr/insu-00944405
https://hal-insu.archives-ouvertes.fr/insu-00944405/document
https://hal-insu.archives-ouvertes.fr/insu-00944405/file/clerc-etal-IJES-Review.pdf
DOI : 10.1007/s00531-013-0927-z

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
For the first time, an albite orthogneiss has been recognised and dated within the HP-LT blueschist facies metabasites and metapelites of the Ile de Groix. It is characterised by a peraluminous composition, high LILE, Th and U contents, MORB-like HREE abundances and moderate Nb and Y values. A U-Pb age of 480.8 ± 4.8 Ma was obtained by LA-ICP-MS dating of zircon and titanite. It is interpreted as the age of the magmatic emplacement during the Early Ordovician. Morphologically different zircon grains yield late Neoproterozoic ages of 546.6-647.4 Ma. Zircon and titanite U-Pb ages indicate that the felsic magmatism from the Ile de Groix is contemporaneous with the acid, pre-orogenic magmatism widely recognised in the internal zones of the Variscan belt, related to the Cambro-Ordovician continental rifting. The magmatic protolith probably inherited a specific chemical composition from a combination of orogenic, back-arc and anorogenic signatures because of partial melting of the Cadomian basement during magma emplacement. Besides, the late Devonian U-Pb age of 366 ± 33 Ma obtained for titanite from a blueschist facies metapelite corresponds to the age of the HP-LT peak metamorphism.
ISSN: 1437-3254

insu-00714570
https://hal-insu.archives-ouvertes.fr/insu-00714570
DOI : 10.1007/s00531-011-0732-5

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Iranian plateau is a flat ~ 1.5–2 km high plateau thought to result from the collision between the Arabian and Eurasian plates since ~ 30 ± 5 Ma, and may represent a young analogue to the so far better studied Tibetan plateau. In order to constrain the exhumation history of the internal Zagros and of the Iranian plateau, we herein present apatite (U-Th)/He (AHe) and apatite (AFT) and zircon fission-track (ZFT) data on plutonic rocks from the Sanandaj–Sirjan Zone (SSZ), Urumieh–Dokhtar magmatic arc (UDMA), Central Iran and Kopet Dagh. Thermochronologic data show that the SSZ was exhumed early in the collision process (essentially before 25–20 Ma), with a likely acceleration of cooling during the late Eocene, from 0.04 to 0.3 mm/year. Results suggest that cooling of the internal Zagros migrated from the SSZ to the UDMA during a more mature stage of the continental collision, after ~ 17 Ma (i.e., coeval with the outward propagation of deformation and topography fronts in the external Zagros). Constant exhumation rates in the UDMA (~ 0.3 mm/year) suggest that no significant variation of erosion rates occurred since the onset of continental collision. In Central Iran, the overlap of ZFT, AFT and AHe ages from gneissic samples points to rapid cooling during the late Eocene (~ 42 °C/Myr), which is consistent with previous reports on the formation of Eocene metamorphic core-complexes.
ISSN: 0024-4937

hal-01115691
https://hal.archives-ouvertes.fr/hal-01115691
DOI : 10.1016/j.lithos.2014.07.021

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
A NE-SW magnetotelluric 110 km-long profile including 18 sites was acquired across the western Rif Cordillera along the Eurasian-African plate boundary, allowing to constrain its poorly known deep structure. It extends from the Internal Zones, close to the Alboran coast, crossing the External Zones and up to the Gharb foreland basin. The periods recorded range from 0.001 s to 1000 s. The combination of magnetotelluric data with available geological data provides new insight regarding the relationship between deep and shallow crustal structures of the Rif Cordillera. Analyses of structural dimensionality suggest a preferential NW-SE direction, and a 2D joint inversion was performed. A 3D inversion extending the 2D model along the strike confirmed the reliability of this approach. The magnetotelluric model shows a heterogeneous upper crust in agreement with the geological structures observed at surface. The Internal Zones correspond to resistive (metamorphic rocks) and conductive (peridotites) bodies, while the External Zones and the foreland basin are characterized by large conductive bodies of variable thickness. A crustal detachment level separating shallow geological units from a probable variscan basement was inferred. At depth, the most relevant feature consists of large resistive bodies with a shallow irregular top, located below the frontal part of the Rif. The outcrops of exotic metapelitic, granitic and gneissic blocks in the frontal part of the Cordillera suggest that these large resistive bodies may correspond to a gneissic or granitic basement surrounded by metapelitic rocks. Late contractive thrust and diapiric processes were responsible for their uplift and shallow emplacement. The Rif constitutes an active southwestward vergent orogenic wedge, oblique to the present-day NW-SE convergent Eurasian-African plate boundary.
ISSN: 1525-2027

hal-00670999
https://hal.archives-ouvertes.fr/hal-00670999
DOI : 10.1029/2011GC003783

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
Dating the timing of motion on crustal shear zones is of tremendous importance for understanding the assembly of orogenic terranes. This objective is achieved in this paper by combining petrological and structural observations with novel developments in in situ U-Th-Pb geochronology of allanite. A greenschist facies shear zone within the Mont Blanc Massif is documented. Allanite is synkinematic and belongs to the mylonitic assemblage. LA-ICP-MS U-Th-Pb isotope analyses of allanite reveal high contents and highly radiogenic isotopic compositions of the common-Pb component. The use of measured Pb-isotope compositions of associated minerals (feldspars and chlorite) is critical for accurate common-Pb correction, and provides a powerful mechanism for linking allanite growth to the metamorphic assemblage. A mean 208Pb/232Th age of 29.44 ± 0.95 Ma is accordingly taken for synkinematic allanite crystallisation under greenschist facies conditions. This age reflects the timing of the Mont Blanc underthrusting below the Penninic Front and highlights the potential of directly dating deformation with allanite.
ISSN: 0954-4879

hal-00963545
https://hal.archives-ouvertes.fr/hal-00963545
DOI : 10.1111/ter.12066

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Microstructures, U-Pb SHRIMP zircon ages and temperature estimates were used to constrain the fabric evolution and timing of mylonitisation in the Patos shear zone (NE Brazil). Melt-bearing HT-mylonitic orthogneiss displays solid-state fabrics with coarse quartz ribbons and sutured grain boundaries. K-feldspar is often fractured and shows peripheral myrmekite. The textures typically become magmatic at the contacts with the Espinho Branco anatexite. Zircon crystals from two samples of the leucosome show Paleoproterozoic (ca. 2.2 Ga) inherited cores that are enveloped by recrystallised Neoproterozoic rims. These zircon grains define a discordia with lower intercept ages of 558 and 562 Ma but with large analytical errors. A leucogranite with transitional contacts with host diatexites provides a well-constrained zircon mean age of 566 ± 6 Ma (n = 15, 95% conf.), which is considered the best estimate for the peak metamorphic conditions. The transition from HT-mylonites to fine-grained mylonites and ultramylonites is marked by a progressive grain size reduction assisted by recrystallisation. Quartz crystallographic fabrics display [0 0 0 1] concentrations between Z and Y. K-feldspar and plagioclase fabrics record the activity of the (0 1 0)[0 0 1] and (0 1 0)[1 0 0] slip systems and, in the lower-temperature tectonites, the (1 0 0)[0 1 0] slip system. These results suggest a diachronic evolution in which an initial HT northern domain was deformed by intracrystalline slip and melt-assisted granular flow, followed by medium- to low-temperature solid-state creep in the southern shear zone domain. The mylonites therefore record the rheological heterogeneities responsible for the formation of a late continental shear zone that nucleated under partially molten conditions and was later reactivated through melt-absent strain localization following exhumation.
ISSN: 0301-9268

hal-01053497
https://hal.archives-ouvertes.fr/hal-01053497
DOI : 10.1016/j.precamres.2013.12.020

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

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : The Oman-Emirates is the largest and best-exposed ophiolite; consequently, it has attracted significant interest among scientists, together with serious conflicts. Most geologists regard this ophiolite as having formed in an intra-oceanic subduction zone before being accreted to the Arabian continent. Here, we propose an alternative scenario, supported by detailed field observations and integrated geophysics. The smaller Emirates part of the ophiolite was forced into a nearby continent, in the pre-collision stage of Tethyan closure. The contraction led to the exhumation of the mantle floor of segmented basins accreted in a rifted system similar to the present-day Gulf of California. The implied high temperature–high pressure metamorphism and the range of geochemical signatures were introduced during the process of rifting, whereas the larger Oman ophiolite was emplaced by obduction onto and along the subducting continental shore. This Ridge–Trench–Transform system might call for a new process to obduct over continents in particular Tethyan ophiolites.
ISSN: 0954-4879

hal-01526919
https://hal.archives-ouvertes.fr/hal-01526919
DOI : 10.1111/ter.12256

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Serpentinites are one of the major components of the oceanic lithosphere and are stable in the slab and the mantle wedge up to 100-150 km depth in subduction zones. During oceanic mantle hydration and alteration, they trap trace and fluid mobile (FME: B, Li, As, Sb, Rb, Ba, Cs, Sr, U and Pb) elements that participate to elemental transfer occurring between the dehydrating slab and the mantle wedge in subduction context. The Lanzo massif is an eclogitized oceanic lithosphere that preserved its oceanic structure and recorded different steps of serpentinization/de-serpentinization from oceanic lizardite to prograde antigorite in subduction context, up to its dehydration and secondary olivine crystallization, and finally retrograde antigorite during massif exhumation. It constitutes a suitable place to study trace element behavior during serpentinization/de-serpentinization processes and associated chemical transfers between the different envelopes of the oceanic lithosphere and the mantle wedge. Geochemical analyses of serpentine and associated minerals show that the serpentinization/de-serpentinization of the Lanzo massif took place in a relatively closed system without significant trace element transfer between the different parts of the oceanic lithosphere. In the deeper part of the lithosphere, from the slightly serpentinized mantle peridotites (SSP, < 20% serpentinization) to the paleo-Moho, composed of massive serpentinites (MS, 80% serpentinization), the trace elements mobility is reduced. The chemical composition of lizardite and antigorite is homogenized with the local degree of serpentinization: in SSP, serpentine veins composition is inherited from the host mineral while, in MS, their composition is homogenous between destabilized phases at the scale of the outcrop (~ 5 m). In the shallowest part of the oceanic lithosphere, from the paleo-Moho to the oceanic paleo-seafloor, the serpentinites are foliated (FS, > 90% serpentinization). In that zone, the alpine deformation enhances the mobility of trace elements and permits their redistribution and the homogenization of antigorite composition at massif scale. Locally, in the SSP and MS, the crystallization of metamorphic veins of ~ 1-2 m corresponds to channelized fluid flows that allowed fluid transfers - and thereby trace elements - to longer distance. The successive crystallizations of antigorite and then olivine are accompanied by a diminution of some FME (B, Li, As, Sb, Ba, Rb) and Eu contents attesting that these elements are removed from slab to mantle wedge during subduction.
ISSN: 0009-2541

hal-00937661
https://hal.archives-ouvertes.fr/hal-00937661
DOI : 10.1016/j.chemgeo.2013.08.025

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 Masson
Résumé : International audience
Although they are famous among Earth scientists, the Lherz peridotites are exposed within geological formations of the North Pyrenean Zone (NPZ) still lacking detailed investigations. Our study focuses on the metasediments of the Aulus basin hosting the Lherz peridotite body and associated ultramafic fragments of smaller size. The new data set comprises of structural analysis and detailed geological mapping of the massive Mesozoic marbles that form the prerift sequence typical of the NPZ and of the ultramafic-rich clastic breccia formations surrounding the peridotite bodies. The massive marbles display an evolution from hot and ductile to cold and brittle deformation, indicative of an exhumation process ending with the sedimentary reworking of both the deformed Mesozoic metasediments and the exhumed ultramafic rocks. Crystal Preferred Orientations (CPO) measured in the marbles support a deformation mechanism by dislocation creep of calcite, which is dominant between 400 8C and 600 8C; these deformation temperatures are within the range determined earlier by Clerc et al. (2015), using RSCM (Raman Spectroscopy of Carbonaceous Material) geothermometry. As a consequence, we better describe the transition from ductile to brittle deformation in the prerift marbles and clarify the origin of the syn-rift breccias. Due to continuous exhumation along detachments' faults, the brecciated metamorphic carbonates of the prerift NPZ sedimentary cover were passively uplifted towards shallower levels and progressively unroofed, while transported passively on the back of the exhumed ultramafic footwall. These results are consistent with the recent interpretations of the North Pyrenean peridotites as remnants of subcontinental mantle rocks exhumed within the pre-Pyrenean rift system. We emphasize the importance of tectonic decoupling between the Mesozoic sedimentary cover and the Palaeozoic basement, which leads to the juxtaposition of metamorphosed and deformed Mesozoic Please cite this article in press as: Lagabrielle Y, et al. Very high geothermal gradient during mantle exhumation recorded in mylonitic marbles and carbonate breccias from a Mesozoic Pyrenean palaeomargin (Lherz area, North Pyrenean Zone, France). C. R. Geoscience (2016), http://dx.
ISSN: 1631-0713

Droits : http://creativecommons.org/licenses/by-nc-nd/
insu-01285079
https://hal-insu.archives-ouvertes.fr/insu-01285079
https://hal-insu.archives-ouvertes.fr/insu-01285079/document
https://hal-insu.archives-ouvertes.fr/insu-01285079/file/1-s2.0-S1631071315002060-main.pdf
DOI : 10.1016/j.crte.2015.11.004

Éditeur(s) : HAL CCSD Elsevier
Résumé : Kyanite and zoisite eclogites from the Limousin area in the Variscan French Massif Central (FMC) have been formed by deep subduction to 100 km depth at peak pressure-temperature conditions around 660 degrees C and 2.9 GPa. They belong to a thin tectonic unit comprising ophiolitic bodies devoid of HP-UHP markers and forming a transported suture zone now separating the two main lithotectonic units of the FMC with contrasted tectono-metamorphic features (namely the lower and upper allochthons). Composition of kyanite-eclogites is similar to supra-subduction-zone basalts found in back-arc basins or island arcs (LREE depleted, negative Nb-Ta anomalies, epsilon Nd: +6 to + 9). Zoisite eclogites have the major-element fingerprint of ultrabasic iron-rich plagioclase cumulates common in continental intrusions or forming Fe-Ti oceanic gabbros. The distribution of immobile trace-elements is however more comparable to alkaline Si-undersaturated rocks, but the Sr-Nd isotopic composition (Sr-87/Sr-86: 0.706; epsilon Nd: -3 to -5) suggests the intervention of an old radiogenic crustal component during their petrogenesis. The chemical composition of hydrous zoisite eclogites does not correspond closely to a common magmatic precursor. It is thus suggested that, by analogy with well exposed worldwide HP-UHP terranes, at least a part of the trace-element signature and the isotopic compositions are controlled by channelled fluid-rock interaction at UHP conditions, leading to localised segregation of zoisite that hosts nearly all Sr and LREE budgets of the bulk eclogite. Sr-Nd isotopic composition of these samples is thus controlled by zoisite and probably represents the isotopic composition of the fluid phase present during UHP recrystallisation. Zircons extracted from a zoisite eclogite were dated in-situ by LA-ICP-MS. Crystallisation of the magmatic precursor is bracketed between 489 and 475 Ma and the UHP event is dated at 412 +/- 10 Ma, in agreement with published ages for the Eo-Variscan HP stage in the FMC. A late resetting of the U-Pb isotopic system at 382 +/- 7 Ma is related to an anatectic high pressure-medium pressure event, well known in the Limousin area. The oceanic eclogites dated in this study are tentatively linked to the Galicia-South Brittany Lower Paleozoic Ocean; they show evidence of the existence of an intra-oceanic subduction zone at the Lower Ordovician and of partial closure of this oceanic domain by ocean-continent subduction during Early Devonian.
ISSN: 0024-4937

hal-00512835
https://hal.archives-ouvertes.fr/hal-00512835
DOI : 10.1016/j.lithos.2010.05.013

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
In the Pyrenees, the lherzolites nowhere occur as continuous units. Rather, they always outcrop as restricted bodies, never more than 3 km wide, scattered across Mesozoic sedimentary units along the North Pyrenean Fault. We report the results of a detailed analysis of the geological setting of the Lherz massif (central Pyrenees), the type-locality of lherzolites and one of the most studied occurrences of mantle rocks worldwide. The Lherz body is only 1.5 km long and belongs to a series of ultramafic bodies of restricted size (a few metres to some hundreds of metres), occurring within sedimentary formations composed mostly of carbonate breccias originating from the reworking of Mesozoic platform limestones and dolomites. The clastic formations also include numerous layers of polymictic breccias reworking lherzolitic clasts. These layers are found far from any lherzolitic body, implying that lherzolitic clasts cannot derive from the in situ fragmentation of an ultramafic body alone, but might also have been transported far away from their sources by sedimentary processes. A detailed analysis of the contacts between the Lherz ultramafic body and the surrounding limestones confirms that there is no fault contact and that sediments composed of ultramafic material have been emplaced into fissures within the brecciated carapace of the peridotites. These observations bear important constraints for the mode of emplacement of the lherzolite bodies. We infer that mantle exhumation may have occurred during Albian strike-slip deformation linked to the rotation of Iberia along the proto-North Pyrenean Fault.
ISSN: 0954-4879

hal-00412602
https://hal.archives-ouvertes.fr/hal-00412602

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
The current structure of the South Armorican Domain in the French Variscan Belt, provides an excellent record of the late orogenic evolution, as documented by the complex interactions between extensional structures and partial melting. In the Golfe du Morbihan, the South Brittany Migmatite Belt (SBMB), tectonically overlain by non-melted metamorphic units, was studied by combined structural and 40Ar/39Ar methods. The recognition of successive deformation stages points to a strain continuum initiated at depth at the end of a partial melting event, and continued to near-surface brittle faulting, encompassing ductile strain localization over a major top-to-the-ESE extensional shear zone (SSZ) consistent with an overall WNW-ESE orogen-parallel stretching. This multidisciplinary study making the link between structure, texture, microchemistry and 40Ar/39Ar geochronology allow distinguishing between cooling, recrystallization and syn-tectonic crystallization ages. The first extensional insights, witnessed by dyke intrusion were dated at ~319 ± 6 Ma immediately after the SBMB partial melting. Strain localization along the SSZ, occurred until ~302-298 Ma when SBMB crossed the ductile-brittle transition. Moreover, in situ 40Ar/39Ar analysis, of syn-tectonic white micas sampled in the ductile and brittle structures yield similar ~300 Ma. Formation or reactivation of the SSZ may have occurred during the rapid cooling of the SBMB settled at ~305 Ma by conventional 40Ar/39Ar analysis. Large-scale late orogenic extensional event of the previously overthickened Variscan crust thus lasted over ~20 ± 3 Myr in the same extensional tectonic regime guided by the South Armorican Shear Zone.
ISSN: 0278-7407

insu-00628699
https://hal-insu.archives-ouvertes.fr/insu-00628699
https://hal-insu.archives-ouvertes.fr/insu-00628699/document
https://hal-insu.archives-ouvertes.fr/insu-00628699/file/2010TC002788.pdf
DOI : 10.1029/2010TC002788

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
The ODP/IODP multileg campaign at ODP Site 1256 (Cocos plate, eastern equatorial Pacific) provides the first continuous in situ sampling of fast spreading ocean crust from the extrusive lavas, through the sheeted dikes and down into the uppermost gabbros. This paper focuses on a detailed petrographic and microanalytical investigation of the gabbro section drilled during IODP Expedition 312. The marked patchy and spotty features that can be observed in many Hole 1256D gabbros is mostly due to a close association of two different lithological domains in variable amounts: (1) subophitic domains and (2) a granular matrix. Major and trace element mineral compositions, geothermometry, and petrological modeling suggest that subophitic and granular domains follow one single magma evolution trend formed by in situ fractionation. The subophitic domains correspond to the relative primitive, high-temperature end-member, compositionally similar to the basalts and dikes from the extrusive unit upsection, while the granular domains fit with a magma evolution by crystal fractionation to lower temperatures, up to a degree of crystallization of similar to 80%. Our results support the following scenario for the fossilization of the axial melt lens at ODP Site 1256: relatively primitive MORB melts under near-liquidus conditions fill the melt lens and feed the upper, extrusive crust. Near the melt lens-sheeted dike boundary at lower temperatures, crystallization starts with first plagioclase before clinopyroxene in a mushy zone forming the subophitic domains. At decreasing temperatures, the subophitic domains continue to crystallize, finally forming a well-connected framework. Evolved, residual melt is finally trapped within the subophitic network, crystallizing at near-solidus conditions to the granular matrix. Another important textural feature in Hole 1256D gabbros is the presence of microgranular domains which are interpreted as relics of stoped/assimilated sheeted dikes (transformed to "granoblastic dikes" by contact metamorphism). All these different domains can be observed in close association, often at the thin section scale, demonstrating the extremely complex petrological record of combined crystallization/assimilation processes ongoing in the axial melt lens. Very similar gabbros with a marked spotty/patchy appearance, and bearing the same close association of lithological domains as observed at Site 1256, are known in the so-called "varitextured gabbro" unit from the Oman Ophiolite located at the same structural level, between cumulate gabbros and granoblastic dikes. The close petrological similarity of the gabbro/dike transition between both IODP Hole 1256D and the Oman ophiolite suggests that in situ fractionation and dike assimilation/contamination are major magmatic processes controlling the dynamics and fossilization of the axial melt lens at fast spreading oceanic ridges.
ISSN: 1525-2027

hal-00644798
https://hal.archives-ouvertes.fr/hal-00644798
DOI : 10.1029/2011GC003655