Éditeur(s) : HAL CCSD Elsevier
Résumé : A Ar-40/Ar-39 geochronological study was performed on amphibole and biotite from some representative units of distinct tectonic domains of the southeastern Guiana Shield, north of the Amazonian Craton, the Amapa Block and the Carecuru Domain. In the AmapA Block, an Archean continental block involved in the Transamazonian orogenesis (2.26-1.95 Ga), the investigated minerals, from rocks of the Archean high-grade basement assemblage, give only Paleoproterozoic ages, indicating their complete resetting during the Transamazonian orogenic event. Amphibole ages vary from 2087 +/- 3 to 2047 +/- 20 Ma, and biotite ages spread mainly between 2079 +/- 18 and 2033 +/- 13 Ma. In the Carecuru Domain, in which the geodynamic evolution is related to Paleoproterozoic magmatic arc setting during the Transamazonian event, calc-alkaline granitoids yield amphibole age of 2074 +/- 17 Ma, and biotite ages of 1928 +/- 19 Ma and 1833 +/- 13 Ma. These data reinforce the importance of the Transamazonian orogenic cycle in the investigated area, and indicate that the rocks were not significantly affected by post-Transamazonian events. When coupled with available U-Th-Pb monazite and Pb-Pb zircon geochronological records and petro-structural observations, the new Ar-40/Ar-39 data delineate contrasting cooling and exhumation histories for the tectonic domains. In the AmapA Block, the data suggest nearly vertical T-t paths that reflect fast cooling rates, which indicate tectonically controlled exhumation, related to collisional stages of the Transamazonian event, between 2.10 and 2.08 Ga. Conversely, in the Carecuru Domain, low cooling rates suggest that the arc-related granitoids underwent slow and monotonous cooling since their emplacement until reaching the biotite isotopic closure temperature.
ISSN: 0895-9811

hal-00420075
https://hal.archives-ouvertes.fr/hal-00420075
DOI : 10.1016/j.jsames.2008.08.010

Éditeur(s) : HAL CCSD
Résumé : In SW Yukon – SE Alaska, the boundary between the Pacific and North America plates is characterized by a syntaxis at the transition between the Aleutian subduction to the W and the Fairweather – Queen Charlotte strike-slip faults to the SE. The relative motion is oblique to the main fault structures, and the area is marked by the Yakutat block collision. From the Chugach – Saint Elias mountains in the plate boundary zone (up to 6 000 m high) to the intraplate strike-slip faults, markers of the present-day deformation give information on its partition in the system.During my PhD, I first measure surface deformation using a dense GPS network, deployed up to 500 km inland the North America plate. After precise processing and corrections of transient effects in the area (postseismic and glacial isostatic rebound), a new residual velocity field is produced for the syntaxis area, from which I derive strain rates. Those data allow me to quantify the fault slip rates for Fairweather and southern Denali strike-slip faults, and to characterize a bi-modal deformation pattern: Along the plate boundary, the deformation is localized on large-scale structures (accretionary prism to the W, Fairweather to the E); In the syntaxis area, strain rates are the highest and the GPS data shows a diffuse intraplate deformation, similar to an indentor pattern. The Yakutat block seems to strongly drive the North America plate deformation.This indentor pattern induces strong lateral variations on the large intraplate faults: the Denali – Totschunda – Duke River system. In a second part, I realize a regional geomorphological study to characterize the role and slip rate of those faults. From very high-resolution Digital Elevation Models (~ 1 m), a detailed cartography is done. On the basis of fieldwork observations, I measure offsets of fluvial and glacial markers, which are sampled for dating. A dextral cumulative deformation is highlighted on the northern Denali Fault, where as all southern Denali is marked by vertical deformation. This study allows me to quantify new slip rates for the system Denali – Totschunda – Duke River, and to show the leading role of the Totschunda (~ 14 mm/a) and Duke River (~ 6 mm/a) faults, contrary to the Denali Fault (~ 1 mm/a) North of the syntaxis.The new tectonic model for the Yakutat collision provides an important case study for the understanding of indentor systems. The concomitance of a rigid-block deformation (to the West) and diffuse deformation (to the East), as well as the near-zero slip rate on the lithospheric-scale southern Denali Fault highlight the major control of boundary conditions and the structural heritage on the orogen deformation.
Au SO Yukon – SE Alaska, la frontière entre les plaques Pacifique et Nord-Amérique est marquée par une syntaxe à la transition entre la subduction des Aléoutiennes à l’O et le décrochement de Fairweather – Queen Charlotte au SE. Le mouvement relatif est oblique et la région est marquée par la collision du bloc Yakutat. De la chaine des Chugach – Saint Elias en frontière de plaque (jusqu’à 6 000 m d’altitude) aux grands décrochements intraplaque, les marqueurs de la déformation actuelle apportent des informations sur sa partition dans ce système.Au cours de cette thèse, je m’intéresse dans un premier temps à mesurer la déformation de surface à l’aide d’un réseau GPS dense, déployé jusqu’à 500 km à l’intérieur de la plaque Nord-Amérique. Après un travail minutieux de traitement des données et de correction des effets transitoires (rebond post-glaciaire et post-sismique), un champ de vitesses résiduelles robuste et inédit est produit au niveau de la syntaxe, dont je dérive des taux de déformation. Ces données me permettent de quantifier les vitesses de glissement les failles décrochantes de Fairweather et Denali au Sud, et également de caractériser une déformation bimodale : En frontière de plaques, la déformation est localisée sur les grandes structures (prisme d’accrétion à l’O, Fairweather à l’E) ; Au niveau de la syntaxe, les taux de déformation sont les plus importants et les données GPS mettent en évidence une déformation intraplaque diffuse, similaire à un champ attendu à l’aplomb d’un indenteur. Le bloc Yakutat semble donc contrôler fortement la déformation de la plaque Nord-Amérique.Ce champ de déformation induit des variations latérales fortes sur les grandes failles intraplaque : le système Denali-Totschunda-Duke River. Dans une seconde partie, je réalise une étude géomorphologique régionale pour caractériser le rôle et la vitesse de ces failles. À partir de Modèles Numériques de Terrain très haute résolution (~ 1 m), une cartographie de détail est réalisée. Puis une mission de terrain me permet de mesurer des décalages sur des marqueurs fluviatiles et glaciaires et collecter des échantillons pour les dater. Je mets en évidence une déformation cumulée dextre sur le segment Nord de la faille de Denali, alors que toute la partie Sud déforme verticalement la surface. Cette étude me permet de quantifier de nouvelles vitesses de failles sur le système Denali-Totschunda-Duke River, et de montrer le rôle prépondérant des failles de Totschunda (~ 14 mm/a) et Duke River (~ 6 mm/a) contrairement à la faille de Denali (~ 1 mm/a) au Nord de la syntaxe.Le nouveau modèle cinématique proposé pour la région de collision Yakutat permet d’apporter un nouvel exemple à la compréhension des systèmes d’indenteur. La concomitance d’une déformation de blocs (à l’Ouest) et diffuse (à l’Est), ainsi que l’absence de déformation sur la faille lithosphérique de Denali Sud met en évidence le contrôle majeur des conditions aux limites et de l’héritage structural dans la déformation des orogènes.
https://hal.archives-ouvertes.fr/tel-01241688

tel-01241688
https://hal.archives-ouvertes.fr/tel-01241688
https://hal.archives-ouvertes.fr/tel-01241688/document
https://hal.archives-ouvertes.fr/tel-01241688/file/manuscrit_anais_v2_optimise.pdf

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
Résumé : 1 p.
International audience
The 12th May 2008 Wenchuan earthquake in the Longmen Shan occurred on a large thrust fault largely inherited from an Indosinian structure itself probably controlled by an older structural heritage of the South China block continental margin. Within the whole northeast Tibet region, such a structural inheritance has had a major impact on the Tertiary deformation. It appears of primary importance to assess the pre-Tertiary tectonic evolution of the main blocks involved to understand the actual deformation in the eastern edge of Tibet. Over the past decades, the Proterozoic to Cenozoic tectonic, metamorphic and geochronologic history of the Longmen Shan and Songpan Garzê area have been largely studied. We present a synthesis of the tectonic evolution of the Songpan Garzê fold and thrust belt from Triassic to present. The Songpan-Garzê belt was formed during closure of a wide oceanic basin ?lled with a thick (5 to 15 km) sequence of Triassic ?yschoid sediments. Closure of the basin due to Triassic subduction involved strong shortening, intense folding and faulting of the Triassic series. A large-scale décollement, that presently outcrops along the eastern boundary of the belt (Danba area), allowed the growth of a wide and thick accretionary wedge. It develops in the Paleozoic and Triassic series and separates the accretionary prism from an autochthonous crystalline basement which shares many similarities with the basement of the Yangtze Craton (0.7-0.9 Ga). To the north and northwest, below the thickened Triassic series of the belt, the composition (oceanic or continental) of the basement remains unknown. During the Indosinian orogeny the emplacement of orogenic granites (220 - 150 Ma) was associated to crustal thickening. The isotopic composition of granitoids shows that their magma source were predominantly derived from melting of the proterozoic basement with varying degrees of sedimentary material and negligible mantle source contribution. In the Danba area, the décollement outcrops in a large tertiary antiform with a NNW-SSE axis. It has been exhumed too in the hanging wall of the NE-SW faults of the Tertiary Longmen-Shan belt that marks the present day transition from the Tibetan plateau to the Sichuan basin. These faults have episodically absorbed signi?cant shortening since the Late Triassic. The amount and precise timing of post-triassic deformation are dif?cult to constrain especially because of the dif?culty to isolate the tertiary thermochronological signal from the protracted late Triassic - Cretaceous thermal history. Nonetheless it is generally accepted that Jurassic - Cretaceous tectonism did not modi?ed the general Triassic architecture of eastern Tibet contrarily to the Tertiary deformation. The long-term cooling histories obtained on Mesozoic granites and on the metamorphic series of the Danba dome are very similar showing a very slow and regular cooling during Jurassic andCretaceous, con?rming the absence of major tectonic event between c.a. 150 and 30 Ma. Low temperature thermochronology data indicate that ?nal exhumation and cooling occurred in theTertiary with an acceleration between 10 and 5 Ma along the major tectonic structures. Within the Longmen Shan range, a total denudation of 7 to 10 km is estimated for the late Cenozoic period. Similar amounts of late Tertiary denudation have been estimated along an east-west section across the Xianshuihe fault
Asian climate and tectonics seminar
Utrecht, Netherlands

insu-00610298
https://hal-insu.archives-ouvertes.fr/insu-00610298
DOI : 10.1016/j.jseaes.2010.03.008

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
New U–Pb monazite, zircon, and xenotime ages date the Late Carboniferous crystallization of the anatectic Vialais granite in the Montagne Noire Axial Zone and the high-temperature deformation and metamorphism of the augen gneiss (Ordovician granite protolith) from which the granite was likely derived. The U–Pb monazite ages obtained from the augen gneiss (308 ± 3 Ma), late kinematic Vialais granite (303 ± 4 Ma), and post-kinematic leucogranite (298 ± 2 Ma), bracket the high-temperature deformation and metamorphism at ~ 310–300 Ma, clearly postdating regional contraction and nappe emplacement (> 320 Ma). The planar–linear and locally linear fabrics in the augen gneiss outline a regional-scale double dome structure (Caroux and Espinouse sub-domes) containing smaller (km-scale) upright folds. Compared to the Caroux sub-dome, the larger, migmatitic Espinouse sub-dome contains abundant leucosome, leucogranite bodies and late-kinematic intrusions (Vialais granite). The Vialais granite displays a weak magmatic foliation that defines the main layering of the Espinouse migmatite. Ellipsoidal quartz–sillimanite nodules concentrated above the roof of the granite recorded an increment of strain (vertical shortening, E–NE elongation) during granite emplacement and crystallization, consistent with the extension event that exhumed the domes. These new geochronological and structural data suggest that the Montagne Noire double dome formed in a pull-apart structure within a dextral strike–slip system. Upright folding in the sub-domes has been traditionally assigned to a regional contraction event; alternatively, we propose that local contraction is associated with the convergence of low-viscosity crust beneath the upper crust pull-apart. Dynamic models of extension of hot crust indicate that contraction at depth is generated by flow of low-viscosity orogenic crust converging and rising to fill the gap created by upper crust extension. This interpretation solves the long-standing problem of apparent coeval contraction and extension in the Montagne Noire double dome and establishes migmatite dome emplacement in strike–slip corridors as a style of late Variscan tectonics.
ISSN: 0040-1951

insu-01112009
https://hal-insu.archives-ouvertes.fr/insu-01112009
DOI : 10.1016/j.tecto.2014.12.002

Éditeur(s) : HAL CCSD Geological Society of London
Résumé : Permo-Triassic rift basins offer important hydrocarbon targets along the Atlantic margins. Their fill is dominated by continental red beds, comprising braided fluvial, alluvial fan, aeolian, floodplain and lacustrine facies. These relatively lightly explored basins span both the Atlantic and Tethyan domains and developed above a complex basement with inherited structural fabrics. Sparse data in offshore regions constrain understanding of depositional geometries and sedimentary architecture, further impeded by their deep burial beneath younger strata, combined with the effects of later deformation during continental breakup. This paper provides results from a multidisciplinary analysis of basins along the Atlantic margin. Regional seismic and well data, combined with geochemical provenance analysis from the European North Atlantic margins, are integrated with detailed outcrop studies in Morocco and Nova Scotia. The research provides new insights into regional basin tectonostratigraphic evolution, sediment fill, and reservoir distribution, architecture and quality at a range of scales. Regional seismic profiles, supported by key well data, indicate the presence of post-orogenic collapse basins, focused narrow rifts and low-magnitude multiple extensional depocentres. Significantly, Permo-Triassic basin geometries are different and more varied than the overlying Jurassic and younger basins. Provenance analysis using Pb isotopic composition of detrital K-feldspar yields new and robust controls on the sediment dispersal patterns of Triassic sandstones in the NE Atlantic margin. The evolving sedimentary architecture is characterized by detailed sedimentological studies of key outcrops of age equivalent Permian-Triassic rifts in Morocco and Nova Scotia. The interplay of tectonics and climate is observed to influence sedimentation, which has significant implications for reservoir distribution in analogue basins. New digital outcrop techniques are providing improved reservoir models, and identification of key marker horizons and sequence boundaries offers a potential subsurface correlation tool. Future work will address source and seal distribution within the potentially petroliferous basins.
Petroleum Geology: From Mature Basins to New Frontiers Proceedings of the 7th Petroleum Geology Conf

hal-00590618
https://hal.archives-ouvertes.fr/hal-00590618

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
The Early Palaeozoic Orogen of SE China consists of three litho-tectonic elements, from top to bottom: a sedimentary Upper Unit, a metamorphic Lower Unit and a gneissic basement. The boundaries between these units are flat lying, south directed, ductile decollements. The lower one is coeval with an amphibolite facies metamorphism (M1). The belt is reworked by migmatite-granite domes, high-temperature metamorphism (M2) and granitic plutons related to post-orogenic crustal melting. We date here the syn-M1 ductile shearing at 453 +/- 7 Ma by U-Th/Pb method on monazite. Previous ages and our new
ISSN: 0954-4879

hal-00420901
https://hal.archives-ouvertes.fr/hal-00420901
https://hal.archives-ouvertes.fr/hal-00420901/document
https://hal.archives-ouvertes.fr/hal-00420901/file/Faureetal-TerraNova-2009.pdf
DOI : 10.1111/j.1365-3121.2009.00888.x

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Geodetic data support a short-term continental deformation localized in faults bounding lithospheric blocks. Whether major "faults" observed at the surface affect the lithospheric mantle and, if so, how strain is distributed are major issues for understanding the mechanical behavior of lithospheric plates. A variety of evidence, from direct observations of deformed peridotites in orogenic massifs, ophiolites, and mantle xenoliths to seismic reflectors and seismic anisotropy beneath major fault zones, consistently supports prolongation of major faults into the lithospheric mantle. This review highlights that many aspects of the lithospheric mantle deformation remain however poorly understood. Coupling between deformation in frictional faults in the uppermost crust and localized shearing in the ductile crust and mantle is required to explain the post-seismic deformation, but mantle viscosities deduced from geodetic data and extrapolated from laboratory experiments are only reconciled if temperatures in the shallow lithospheric mantle are high (> 800 °C at the Moho). Seismic anisotropy, especially shear wave splitting, provides strong evidence for coherent deformation over domains several tens of km wide in the lithospheric mantle beneath major transcurrent faults. Yet it cannot detect narrow strain localization zones or shallowly dipping faults. Seismic profiling images shallow-dipping seismic reflectors in the lithospheric mantle interpreted as the continuation of normal or, less frequently, inverse faults in the mantle. However the nature of these reflectors is poorly understood. There is a large discrepancy between the scale at which deformation processes are studied (a few kilometers, at most, due to the limited size of continuous mantle exposures at the Earth's surface) and the scale inferred for strain localization in the mantle from geophysical observations (tens of kilometers). Combining data on deformation microstructures and crystal preferred orientations in naturally deformed peridotites and seismologic observations allows nevertheless discussing strain localization processes that may play a role on the development of mantle shear zones.
ISSN: 0040-1951

hal-00750314
https://hal.archives-ouvertes.fr/hal-00750314
DOI : 10.1016/j.tecto.2012.06.006

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The basement of central Madagascar displays two contrasted structural patterns. The first one (D1) is characterized by north-striking foliations that are gently dipping to the west and carry W- to WSW-plunging lineations, whereas the second one (D2) is characterized by steeper foliations that are striking to the NNE and lineations that are either subhorizontal or gently plunging to the SSW. The younger pattern is related to late-Panafrican tectonics along the major Angavo shear zone that is about 1000 km in length and 40 km in width with apparently little offset. Deformation in the Angavo zone induced interference folding on both sides. The D2 event is characterized by low pressures (ca 400 MPa) and high temperatures (up to 790 degrees C) responsible for prograde granulite facies conditions, that resulted from heat transfer due to magma and fluid advection in the Angavo shear zone. The D2 event is pinned at 550 +/- 11 Ma by a new monazite age from a reoriented Andringitrean granite near Ankaramena. A new suite of amphibole and biotite Ar-Ar geochronological data enables to retrace the thermochronogical evolution inside and outside the Angavo shear zone. Combined with new structural results from the western interference zone. these ages yield a better understanding of the late-Panafrican history of central Madagascar. No diachronism is observed along the strike of the Angavo shear zone. Conversely, amphibole and especially biotite ages decrease from West to East, i.e. towards the shear zone. These new ages range from 511 to 469 Ma. A 1-D conductive model constrains the thermal effect in relation with the Angavo shear zone to be restricted to a lateral distance of ca 60 km (in map view) for a maximum heating duration of 20 Myr. This is in agreement with the Ar-Ar data and with the width of the observed interference zone. Following this episode of deformation and heat transfer, the estimated cooling rates of the Angavo shear zone range from to 15 to 6 degrees C/Myr, respectively before and after 515 Ma. A post-collision intracontinental setting is suggested for the Angavo shear zone, which is regarded as a remote effect of the Kuunga Orogeny between India and Antarctica.
ISSN: 0040-1951

hal-00463507
https://hal.archives-ouvertes.fr/hal-00463507
DOI : 10.1016/j.tecto.2009.03.009

Éditeur(s) : HAL CCSD
Résumé : Corsica is located in the Mediterranean sea, about 90 km from the Italian coast of Tuscany and 175 km from the French Riviera. This relatively large island stretches for 180 km in the north-south direction and 85 Km east-west. The rugged landscape, with a mean elevation of ca. 500 m and the highest point at 2710 m (Monte Cinto) is interrupted only by a major plain along the east coast. CorseAlp 2011 was held in April 2011. A link to the website is here: CorseAlp 2011. The CorseAlp field trips were designed to introduce the participants to the general features of the geology of Alpine Corsica and to recent specific discoveries. Field trip 1 focused on the geology of the Ocean-Continent Transition preserved in the Monte San Petrone, in Central Corsica, which was subducted to eclogite facies conditions during the Alpine orogeny. The three days post-workshop field trip (Field trip 2) focused on the general architecture of Alpine Corsica in an EW transect between the Balagne region and Bastia. The main topics of Field trip 1 and 2 allow discussion about the overall evolution of Alpine Corsica in the framework of Mediterranean geodynamics, with a focus on the most controversial aspects.
Journal of the Virtual Explorer

hal-00688965
https://hal.archives-ouvertes.fr/hal-00688965

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Seismic anisotropy of the south Iberian upper mantle is investigated using shear-wave splitting of SKS phases. We analyzed teleseismic events recorded by sixteen permanent broadband stations installed on the southern Iberian Peninsula and in northern Africa, and we determined fast polarization directions phi, and delay times delta t between fast and slow components. The area of investigation extends across two important geological structures in the Variscan Iberian Peninsula: the Variscan Iberian Massif in its center, and the Gibraltar arc in the Southeast, that represents the most westerly Alpine belt in the western Mediterranean. Shear-wave splitting measurements from stations in the Betic domain show homogeneous ENE-WSW fast directions nearly parallel to the trend of the mountain belt, and smooth spatial variations. Stations in the North, toward the southern part of the Variscan Iberian Massif show homogeneous fast directions however trending NS to NE-SW, different from those recorded in the Betic. These observations may reflect a post-Hercynian (Variscan) deformation of the Ossa-Morena zone, related to the main stages in the tectonic evolution of this part, namely transpressional stage, transtensional stage and shortening episode, or a deformation related to the posterior Alpine orogeny. Along the Gibraltar arc, we observe a smoothly varying phi trend changing from ENE-WSW in the Eastern Betics to NS in the area of Gibraltar and Ceuta, following more or less the general trend of the mountain belt around the Alboran Sea, and the coastline. Since a similar rotation is also visible in results from Pn anisotropy, this suggests that the anisotropy is vertically coherent starting from just below the Moho. Comparing the anisotropy pattern expected from various geodynamic models with the observed SKS splitting suggests that the anisotropy is best explained by a model of slab rollback, rather than by delamination models.
ISSN: 0012-821X

hal-00412175
https://hal.archives-ouvertes.fr/hal-00412175
https://hal.archives-ouvertes.fr/hal-00412175/document
https://hal.archives-ouvertes.fr/hal-00412175/file/Buontempo_SKS_gibraltar_EPSL2008_hal.pdf
DOI : 10.1016/j.epsl.2008.06.024

Éditeur(s) : HAL CCSD Société géologique de France
Résumé : Upper Carboniferous heating and melting of the middle orogenic crust associated with the emplacement of syntectonic granitoids are documented in the Upper Gneissic Unit of the Livradois area (central part of the French Massif Central). Crustal melting post-dates peak metamorphism conditions (800-625 degrees C, 10-8 kb) dated at 360 +/- 4 Ma (U-Th-Pb on monazite). The P-T evolution of the metamorphic series indicates that Barrovian metamorphism was followed by a decompression (from 10 +/- 1 kbar to 6 +/- 1 kbar) associated with either a decrease in temperature in the southern part of the series or with an increase in temperature (of about 150 degrees C) in the northern part of the series. This evolution records the first step of the exhumation of the series coeval with granitoids intrusion, of which the emplacements were dated at 315 +/- 4 and 311 +/- 18 Ma (U-Pb on zircon). The final stage of the exhumation is associated with an isobaric cooling of the whole series. Similarity of Ar-40/Ar-39 ages for biotite in the paragneisses (307-300 Ma) and K-feldspar in the granitoids (306-300 Ma) document rapid cooling for this stage. Moreover dextral reverse mylonites, at the border and the northern part of the metamorphic series indicate north-south compression coeval with the unroofing of the series. Youngest Ar-40-Ar-39 ages on K-feldspar (274.6 +/- 5 Ma) combined with normal shearing in mylonites limiting the Carboniferous Brassac-les-Mines basin document the late Carboniferous-early Permian stage of extension coeval with the upwelling of the Velay granitic dome.
EISSN: 1777-5817

hal-00617429
https://hal.archives-ouvertes.fr/hal-00617429
DOI : 10.2113/gssgfbull.182.1.5