Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
Deciphering Variscan versus Alpine history in the internal Rif system is a key to constraining the tectonic evolution of the Alboran domain and hence the geodynamics of the western Mediterranean system during the Cenozoic. This study focuses on the evolution of the metamorphic envelope of the Beni Bousera massif and its relation to the underlying peridotites. Combining structural geology, metamorphic petrology, and LA-ICP-MS U-Th-Pb dating of monazite, this study contributes to the understanding of the tectonic history of the western internal Rif. The regional foliation (S2) is characterized by LP/HT mineral assemblages and obliterates a former foliation (S1) developed along a barrovian (MP/MT) metamorphic gradient. The dating of some metamorphic monazite grains from a micaschist and a migmatitic gneiss demonstrates that the crustal envelope of the peridotite recorded two distinct tectonometamorphic episodes. Data from monazite inclusions in S1 garnet suggest that the first event, D1, is older than 250-170 Ma and likely related to the Variscan collision, in agreement with the barrovian type of the metamorphic gradient. The second event, D2, is Alpine in age (at circa 21 Ma) and corresponds to a strong lithosphere thinning allowing subsequent subcontinental mantle exhumation. Such a tectonic context provides an explanation for the LP/HT metamorphic gradient that is recorded in the regional foliation of the western Betic-Rif system. This extension is probably related to a subduction slab roll-back in the western end of the Mediterranean realm during the Oligo-Miocene times. No evidences for a Tertiary HP/LT metamorphism have been identified in the studied area
ISSN: 0278-7407

Droits : info:eu-repo/semantics/OpenAccess
insu-01152517
https://hal-insu.archives-ouvertes.fr/insu-01152517
https://hal-insu.archives-ouvertes.fr/insu-01152517/document
https://hal-insu.archives-ouvertes.fr/insu-01152517/file/gueydan-2015.pdf
DOI : 10.1002/2014TC003769

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : In habitat mosaics, plant populations face environmental heterogeneity over short geographical distances. Such steep environmental gradients can induce ecological divergence. Lowland rainforests of the Guiana Shield are characterized by sharp, short-distance environmental variations related to topography and soil characteristics (from waterlogged bottomlands on hydromorphic soils to well-drained terra firme on ferralitic soils). Continuous plant populations distributed along such gradients are an interesting system to study intrapopulation divergence at highly local scales. This study tested (1) whether conspecific populations growing in different habitats diverge at functional traits, and (2) whether they diverge in the same way as congeneric species having different habitat preferences. Phenotypic differentiation was studied within continuous populations occupying different habitats for two congeneric, sympatric, and ecologically divergent tree species (Eperua falcata and E. grandiflora, Fabaceae). Over 3000 seeds collected from three habitats were germinated and grown in a common garden experiment, and 23 morphological, biomass, resource allocation and physiological traits were measured. In both species, seedling populations native of different habitats displayed phenotypic divergence for several traits (including seedling growth, biomass allocation, leaf chemistry, photosynthesis and carbon isotope composition). This may occur through heritable genetic variation or other maternally inherited effects. For a sub-set of traits, the intraspecific divergence associated with environmental variation coincided with interspecific divergence. The results indicate that mother trees from different habitats transmit divergent trait values to their progeny, and suggest that local environmental variation selects for different trait optima even at a very local spatial scale. Traits for which differentiation within species follows the same pattern as differentiation between species indicate that the same ecological processes underlie intra- and interspecific variation.
ISSN: 0305-7364

hal-01032430
https://hal.archives-ouvertes.fr/hal-01032430
DOI : 10.1093/aob/mct176

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
The age of Ultra-high pressure metamorphism and early cooling remains controversial in the Alps. In a new approach to the problem, samples from the undeformed Hercynian metagranite, Brossasco, were studied using an ultra-violet laser ablation microprobe technique for 40Ar-39Ar dating. The results demonstrate the frequent occurence of excess argon with high 40Ar/36Ar ratios (1000-10000) and a strong relationship between apparent ages and metamorphic textures. The highest excess argon ratios are always associated with high closure temperature minerals or large diffusion domains within single mineral phases. The best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event, then mixed with an atmospheric component during rapid cooling and retrogression, producing a wide range of argon concentrations and 40Ar/36Ar ratios. In the present case, some ages in the range 60-110 Ma could be explained by the presence of excess argon incorporated around 40-50 Ma ago. Similar results found in other high-pressure terrains in the Alps may reconcile the argon geochronometer with other systems such as Rb/Sr, U/Pb or Sm/Nd. This study therefore calls for an increasing use of high resolution in-situ sampling techniques to clarify the meaning of 40Ar/39Ar ages in many high pressure terrains.
ISSN: 0010-7999

hal-00197563
https://hal.archives-ouvertes.fr/hal-00197563
https://hal.archives-ouvertes.fr/hal-00197563/document
https://hal.archives-ouvertes.fr/hal-00197563/file/Excess_UV_Arnaud_and_Kelley_GCA_Pub_preprint.pdf

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
Résumé : International audience
The Rif Chain (Northern Morocco) belongs, with the Betic Cordillera(Southern Spain), to the western termination of the Alpine belt. Deepestunits of the Rif Internal Zones (i.e. Lower Sebtides) outcrop in three areas, from North to South : Ceuta, Cabo Negro and Beni Bousera.The quick exhumation of these units during the Late Oligocene-EarlyMiocene is related with a crustal thinning and the Alboran Sea rifting.Despite Beni Bousera massif has been well investigated, no data areavailable in Ceuta and Cabo Negro areas. This study aims to constrainthe last steps of cooling and exhumation in the Lower Sebtides usingmedium- and low-temperature thermochronological data. Combinationof Ar-Ar on micas, fission tracks and (U-Th)/He on apatites (respectivelyAFT and AHe) from high-grade rocks allows understanding thermalhistory between 450-350°C and surface temperatures. These datasuggest a similar cooling evolution of the Ceuta and Cabo Negro units.Ages obtained with all these methods are comprised in the same interval,between 22 and 18 Ma. These data record an almost instantaneouscooling (between 50 and 200°C/Ma) associated to final exhumation afterthe Oligo-Miocene thermal event affecting these units at shallowdepths (10 km). Low-temperature data suggest that Ceuta and CaboNegro units cooled at surface temperatures before Beni Bousera ones.Since the end of Burdigalian, the Rif internal zones vertical movementsdid not exceed 1km, even during the Messinian Salinity. AFT and AHeages in the whole Sebtides are also slightly older than these obtainedin their Spanish equivalent (Alpujarrides). The Gibraltar arc is dividedinto several independent crustal blocks separated by major fault zonesperpendicular to the coast that could have induced independent verticalmovements during the entire Neogene
24 ème Réunion des sciences de la Terre 2014
Pau, France

insu-01080768
https://hal-insu.archives-ouvertes.fr/insu-01080768

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 Elsevier
Résumé : International audience
The Maghrebides, Betics and some parts of the Calabrian, NE Sicilian and Tuscan allochtons constitute dismembered fragments of the Alpine belt that resulted from the Cenozoic collision between Africa and Eurasia and the opening of the Western Mediterranean basin. Mineral and whole-rock geochemical analyses have been performed on three distinct outcrops of mantle material from the Edough Massif of NE Algeria, namely the Bou Maiza (BM) gabbros, the La Voile Noire (LVN) amphibolites and the Sidi Mohamed (SM) peridotites. In all samples, Sr isotopes are largely affected by seawater alteration (87Sr/86Sract. > 0.70384 and up to 0.70888) and cannot be used to evaluate the nature of the source reservoirs. SM peridotites display variable depleted mantle Nd isotopic signatures (εNdact. from + 7.0 to + 12.2) and geochemical features suggesting no significant chemical depletion as a result of partial melting and melt extraction (Mg# < 90; slightly LREE-depleted patterns with La/YbN = 0.33-0.39). These rocks are interpreted as parts of the subcontinental lithospheric mantle incorporated into the crustal units of the Edough Massif during the early stages of opening of the Algerian basin. BM gabbros and LVN amphibolites show geochemical signatures indicating derivation from a common Depleted Mantle reservoir (εNd > + 7.9) and are likely cogenetic, but without filiation with the SM peridotites. Pb isotopes indicate a contribution of sediments in the source reservoir, which is attributed to contamination solely by hydrous fluids released from a sedimentary component. This observation, together with a LILE-enrichment, suggests a back-arc basin environment. These results indicate that BM and LVN units were obducted onto the North African margin and subsequently fragmented, probably during doming and exhumation of the lower continental crust of the Edough massif. Doming resulted in opposite movements of the overlying oceanic units, southward for the BM units and northward for LVN amphibolites.
ISSN: 0040-1951

hal-01122777
https://hal.archives-ouvertes.fr/hal-01122777
DOI : 10.1016/j.tecto.2014.03.031

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
The objective of this study was to realize a three-dimensional (3-D) geological model of the deep basin structure of the Middle Durance region (of folds and faults) by integration of geological and geophysical data, and to evaluate its fault geometry and tectonic history. All of the available geophysical and geological data were compiled in three dimensions using the gOcad geomodeler. The geological and geophysical data were used to build a 3-D geological model of the Middle Durance region. The data on the 3-D geometry of fault surfaces and stratigraphic horizons and the thickness maps of the main stratigraphic units are supported by the 3-D geological model. We show that the Middle Durance Fault cannot be interpreted as a single fault plane that affected the entire Meso-Cenozoic sedimentary layers and the Paleozoic basement but as a listric segmented faulting system in sedimentary layers, rooted in Triassic evaporites and a normal block faulting system in the basement. This decoupling level in the Triassic layers reveals thin-skin deformation, formed by strong mechanical decoupling between the Mesozoic sedimentary cover and the Paleozoic basement. This study also confirms that the Provence geological structure has resulted mainly from Pyrenean deformation, which was partly reactivated by Alpine deformation. We demonstrate that the Middle Durance Fault Zone is a transfer fault that accommodates deformation of the sedimentary filling of the South-East Basin through modified fold geometry over a zone of 7 km to 8 km around the main segment of the fault zone.
ISSN: 0278-7407

hal-01172221
https://hal.archives-ouvertes.fr/hal-01172221
DOI : 10.1002/2014TC003749

Éditeur(s) : HAL CCSD
Résumé : Article soumis à la revue Ruralia fin 2006. La revue a cessé de paraître en 2007.
La tome des Bauges est un fromage fabriqué dans les Alpes du Nord, qui a obtenu en 2002 une Appellation d'Origine Contrôlée (AOC). Elle fait donc partie de ces produits dits " de terroir " sur lesquels se penchent depuis plusieurs années les chercheurs en sciences sociales et qui accèdent au statut de patrimoine. Elle tend à devenir un emblème pour le territoire des Bauges. Or, cette évolution est remarquable dans la mesure où la tome était, il n'y a pas si longtemps, une production quelque peu dévalorisée, qui n'aurait sans doute pas été choisie pour représenter les Bauges vis-à-vis de l'extérieur. Elle n'est devenue ce qu'elle est aujourd'hui qu'après une évolution marquée par des discussions et des confrontations. Étudier en détail l'histoire du processus de labellisation, qui met en jeu les agriculteurs, les élus locaux, les chargés de mission du Parc et les instances nationales chargées de la labellisation des produits permet non seulement de s'interroger sur les transformations de l'élevage bovin dans les Bauges mais aussi plus largement d'évoquer les négociations entre les agriculteurs et d'autres acteurs intervenant en Bauges. En effet, la tome est sans doute l'un des objets les plus pertinents pour analyser la façon dont les différents groupes envisagent l'avenir du massif et de sa filière agricole. Symbole de la transition vers une nouvelle agriculture, moins productiviste, la tome constitue pour le chercheur un objet-clef pour analyser la place qu'occupent aujourd'hui les éleveurs parmi les autres groupes présents en moyenne montagne.
https://halshs.archives-ouvertes.fr/halshs-00809842

halshs-00809842
https://halshs.archives-ouvertes.fr/halshs-00809842
https://halshs.archives-ouvertes.fr/halshs-00809842/document
https://halshs.archives-ouvertes.fr/halshs-00809842/file/MPalisse_Tome-des-Bauges.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Fault affecting silicoclastic sediments are commonly enriched in clay minerals. Clays are sensitive to fluid–rock interactions and deformation mechanisms; in this paper, they are used as proxy for fault activity and behavior. The present study focuses on clay mineral assemblages from the Point Vert normal fault zone located in the Annot sandstones, a Priabonian-Rupelian turbidite succession of the Alpine foredeep in SE France. In this area, the Annot sandstones were buried around 6–8 km below the front of Alpine nappes soon after their deposition and exhumed during the middle-late Miocene. The fault affects arkosic sandstone beds alternating with pelitic layers, and displays throw of about thirty meters. The fault core zone comprises intensely foliated sandstones bounding a corridor of gouge about 20 cm thick. The foliated sandstones display clay concentration along S–C structures characterized by dissolution of K-feldspar and their replacement by mica, associated with quartz pressure solution, intense microfracturation and quartz vein precipitation. The gouge is formed by a clayey matrix containing fragments of foliated sandstones and pelites. However, a detailed petrographical investigation suggests complex polyphase deformation processes. Optical and SEM observations show that the clay minerals fraction of all studied rocks (pelites and sandstones from the damage and core zones of the fault) is dominated by white micas and chlorite. These minerals have two different origins: detrital and newly-formed. Detrital micas are identified by their larger shape and their chemical composition with a lower Fe–Mg content than the newly-formed white micas. In the foliated sandstones, newly-formed white micas are concentrated along S–C structures or replace K-feldspar. Both types of newly formed micas display the same chemical composition confirmed microstructural observations suggesting that they formed in the same conditions. They have the following structural formulas: Na0.05 K0.86 (Al 1.77 Fe0.08 Mg0.15) (Si3.22 Al0.78) O10 (OH)2. They are enriched in Fe and Mg compared to the detrital micas. Newly-formed chlorites are associated with micas along the shear planes. According to microprobe analyses, they present the following structural formula: (Al1,48 Fe2,50 Mg1,84) (Si2,82 Al1,18) O10 (OH)8. All these data suggest that these clay minerals are synkinematic and registered the fault activity. In the gouge samples, illite and chlorite are the major clay minerals; smectite is locally present in some samples.In the foliated sandstones, Kubler Index (KI) ((001) XRD peak width at half height) data and thermodynamic calculations from synkinematic chlorite chemistry suggest that the main fault deformation occurred under temperatures around 220 °C (diagenesis to anchizone boundary). KI measured on pelites and sandstones from the hanging and footwall, display similar values coherent with the maximal burial temperature of the Annot sandstones in this area. The gouge samples have a higher KI index, which could be explained by a reactivation of the fault at lower temperatures during the exhumation of the Annot sandstones formation
ISSN: 0191-8141

hal-01172423
https://hal.archives-ouvertes.fr/hal-01172423
DOI : 10.1016/j.jsg.2015.03.012

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
In Alpine Corsica (France), deeply subducted metabasalts are well preserved as lawsonite-bearing eclogite (Law-Ecl), occurrence of which is restricted to similar to 10 localities worldwide. The Corsican Law-Ecl, consisting of omphacite + lawsonite + garnet + phengite + titanite, occurs as both single undeformed metabasaltic pillows surrounded by lawsonite blueschist (Law-Bs), and carbonate-bearing eclogitic veins. Law-Bs are found as variably deformed metabasaltic pillows locally cross-cut by eclogitic veins and consist of glaucophane + actinolite + lawsonite + garnet + phengite + titanite. Field evidence and microstructures reveal that both Law-Ecl and Law-Bs are stable at the metamorphic peak in the lawsonite-eclogite stability field. Isochemical phase diagrams (pseudosections) calculated for representative Law-Ecl and Law-Bs samples indicate that both lithologies equilibrated at the same conditions of similar to 520 +/- 20 degrees C and 2.3 +/- 0.1 GPa. Therefore, the coexistence at the same peak metamorphic conditions of Law-Ecl and Law-Bs implies that different portions of deeply subducted oceanic crust may store significantly different H2O contents, depending on bulk-rock chemical composition. In addition, thermodynamic modelling of phase equilibria indicates that the occurring progressive dehydration reactions, which are significantly depending on bulk-rock chemical composition, strongly influence rock densification and eclogite formation in subducting slabs.
ISSN: 0263-4929

hal-00617768
https://hal.archives-ouvertes.fr/hal-00617768
DOI : 10.1111/j.1525-1314.2011.00931.x

Éditeur(s) : HAL CCSD
Résumé : The northern Mirdita ophiolite massifs in Albanian Dinarides formed at a slow-spreading ridge, active during the Jurassic (160–165 Ma). They share a common horizontal Jurassic–Lower Cretaceous sedimentary cover showing that they were not deeply and intrinsically affected by later Alpine thrusting. The western massifs of Mirdita, first oceanic core complex (OCC) and detachment shear zone described in ophiolites, compare with OCCs in slow-spreading ridges and provide continuous exposure of the deep internal structure of this system, revealing its kinematics, thanks to detailed structural mapping in peridotites and gabbros. The Mirdita detachments root in the Moho transition zone (MTZ), a weak zone at the top of asthenospheric mantle, where basaltic melts impregnate dunites. The OCC domes are plagioclase-amphibole–bearing mylonitic peridotites, ∼400 m thick, grading downward within 200 m to harzburgitic mantle. The mylonitic detachments crossed Moho beneath a NNE-SSW–trending ridge. On the western side of OCC domes, the hanging wall of the ridge, crustal gabbros, and basalts are still preserved, despite being deeply affected by hydrothermal alteration. From there, the partially molten MTZ was detached as a shear zone, mixing with lower gabbros. The OCC emerged, migrating upsection and eastward over 5 km. Finally, the OCC front is observed in hornblende-rich syntectonic mylonites derived from upper gabbros and from the overlying former lid. Serpentinization is static within these mylonites. A low-temperature detachment fault is expressed as a sheared antigoritic mélange at the margin of the mylonitic shear zone. Asthenospheric flow in the harzburgitic mantle beneath the ridge of origin has been preserved below the OCC rooting. The dominant asthenospheric flow direction trends parallel to the ridge axis. This mantle flow rotates over 200 m into the low-temperature mylonitic detachments, where OCC motion turns transversal to the ridge. Crystal preferred orientation measurements on six samples point to brown hornblende crystal growth during mylonitic flow and illustrate the change of olivine intra-crystalline slip system in mylonites compared to porphyroclastic harzburgite.
ISSN: 1553-040X

hal-01504440
https://hal.archives-ouvertes.fr/hal-01504440
DOI : 10.1130/GES01383.1

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
ISSN: 0264-3707

hal-00680683
https://hal-brgm.archives-ouvertes.fr/hal-00680683
DOI : 10.1016/j.jog.2011.11.010

Éditeur(s) : HAL CCSD
Rendi Conti della Societa Geologica Italianna, Nuova Serie

hal-00407963
https://hal.archives-ouvertes.fr/hal-00407963

Éditeur(s) : HAL CCSD Universidad de Oviedo
Résumé : In recent years attention has been paid to the major role of surface processes on the evolu- tion of mountain belts on different time scales. In this study, sandbox experiments are used to better characterize the complex interactions between surface processes and tectonics. Models results are then applied to better analyze deformation and kinematics of recent active or ancient orogens (case studies are chosen in the Taiwan belt, the Alps and the Hercynian belt).
ISSN: 0474-9588

hal-00853761
https://hal.archives-ouvertes.fr/hal-00853761

Éditeur(s) : HAL CCSD Institut de Géographie Alpine
Résumé : International audience
The international symposium on «Urban Growth and Natural Hazards, in particular in Developing Countries» was held in the University Blaise Pascal, Clermont-Ferrand (France) on December 2 and 3, 1994. This meeting brought together about 70 research people, Ph.D. students, and professionals from France and 12 countries, representing universities, research agencies, as well as government and private companies. The symposium aimed to gather specialized people who are engaged and acting in programmes on hazard appraisal, risk prevention and urban management in city environment. These people aimed to compare experiences acquired in two different fields: on the one hand urban growth and its consequences, and on the other hand, natural hazards and effects, in particular in Developing Countries.Half-way through the International Decade for Natural Hazard Disaster Reduction, we strove to reconcile two points of view usually distincts: nature-oriented research and society-oriented research on hazards. These convergent approaches should merge, not only to improve knowledge of natural hazards and risks, but also to cope with hazard and risk situation on a day-basis in a city. We also choose to study both approaches in Developing Countries (DC) because most of the large cities of the world already are or will be located there in near future. In addition, city location in DC coincides with tropical areas where natural hazards are most frequent threats to life, in particular in mountains, on seashores, and on active convergent plate margins. We stress the fact that the 3 following topics are often lacking in studies which have been carried out on natural hazard appraisal and management. 1) How should we include social and economical factors, and particularly a sound analysis of vulnerability of people and values at risk ? 2) How should we cope with hazard management in large cities of Developing Countries ? 3) What is the specific task of research people in hazard case studies engaged in urban environment and management?Three themes correspond to the three sessions of the symposium. (1) Natural hazards and environmental factors : inventory, diagnostic and classification, (2) Vulnerability of urban society and environment : concepts, approaches, and classification, (3) Tools for urban risk prevention and risk management in case of crisis and catastrophes.
Ce numéro de la Revue de Géographie Alpine reprend une dizaine parmi les 41 contributions exposées par des chercheurs français et étrangers lors du colloque international « Croissance urbaine et risques naturels dans les pays en développement » qui s'est déroulé à l'Université Blaise Pascal, Clermont-Ferrand, les 2 et 3 décembre 1994. L'objectif général du colloque était de rassembler les diverses compétences existant en France et à l'étranger dans le domaine de l'évaluation et de la gestion des risques naturels et de confronter les expériences acquises à l'interface entre les villes en croissance et le champ d'action des phénomènes naturels créateurs de risques. Le colloque a donc réuni des chercheurs, des doctorants et des praticiens français et leurs collègues étrangers chargés de prévention, de gestion des risques naturels et de gestion urbaine, principalement dans les grandes villes des pays en développement (P.E.D.).Parvenus à mi-chemin de la Décennie Internationale pour la Prévention des Catastrophes Naturelles, nous avons porté des regards croisés sur les approches naturalistes et sociologiques des risques naturels. Il est maintenant nécessaire de faire converger ces approches, non seulement dans la connaissance des menaces et des risques, mais dans la gestion du risque quotidien en milieu urbain. Nous avons concentré ces approches dans les pays en développement ; en effet, le site de leurs grandes villes coïncide souvent avec le champ d'action des phénomènes naturels générateurs de risques, notamment dans la zone intertropicale et en particulier dans les marges continent ales actives (par exemple autour du Pacifique).Nous avons porté l'accent sur 3 points principaux, qui semblent en partie faire défaut dans les études et les méthodes actuellement mises en oeuvre dans la gestion des risques naturels : 1) comment prendre en compte les aspects socio-économiques, en particulier les types d'analyse des vulnérabilités ? 2) comment aborder la gestion des risques en milieu urbain dans les P.E.D. ? 3) quel rôle doit jouer le chercheur en Sciences de l'Homme, de la Société et de la Terre dans les analyses, les expertises consacrées aux risques et surtout dans leur gestion globale, à la charnière des sociétés citadines précaires et des milieux « naturels » contraignants ?Les contributions ont été regroupées, exposées et discutées lors du colloque sous forme de 3 thèmes. (1) Les phénomènes naturels créateurs de dommages (= menaces) : diagnostic, inventaire et typologie, (2) Les vulnérabilités des sociétés et des espaces urbanisés : concepts, typologie et modes d'analyse, (3) Les outils d'aide à la prévention et à la gestion du risque en milieu urbain.
Colloque international : "Croissance urbaine et risques naturels dans les pays en développement"
Clermont-Ferrand, Université Blaise Pascal, France

ISBN : 0035-1121
hal-01165252
https://hal.archives-ouvertes.fr/hal-01165252

Éditeur(s) : HAL CCSD Nature Publishing Group
Résumé : International audience
Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes1. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre2, 3. At temperatures above 300–450 degrees Celsius, usually found at depths greater than 10–15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional–mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades4, 5. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults.
ISSN: 0028-0836

hal-01685757
https://hal.archives-ouvertes.fr/hal-01685757
DOI : 10.1038/nature22355

Éditeur(s) : HAL CCSD NRC Research Press
Résumé : Molecular, cytological, and morphological data support the existence of a hybrid population between Schoenus nigricans and Schoenus ferrugineus. This population was found in northeastern Italy, where S. nigricans is central with respect to its natural range and S. ferrugineus is marginal, being most common in the Alps and in central and northern Europe. Molecular marker data show that the putative hybrid population is genetically intermediate between nearby populations of the parent species. Cytological evidence confirmed the hybrid nature of this population, as does the almost complete sterility of plants within the population. Although no seeds were produced by the hybrid population, some possibly fertile pollen grains were produced; this suggests that the possibility of introgression between the two species through the hybrids cannot completely be excluded.
ISSN: 0831-2796

hal-01032047
https://hal.archives-ouvertes.fr/hal-01032047
DOI : 10.1139/G01-138

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : This study describes normal fault zones formed in foreland arkosic turbidites (the Grès d'Annot Formation, SW French Alps) under deep diagenesis conditions (~200 °C) and highlights the occurrence of two markedly different fault-rock types: (1) the foliated fault rocks of the Moutière-Restefond area; and (2) the dilatant fault rocks of the Estrop area. The deformation of (1) is dominated by intra- and transgranular fracturing, pressure solution of quartz and feldspar grains and syn-kinematic phyllosilicate precipitation resulting from feldspar alteration. The combination of these mechanisms results in a strongly anisotropic strain with intense shortening normal to the foliation (pressure solution) and extension parallel to the foliation (quartz- and calcite-sealed extension veins). This deformation implies local mass transfer that may be achieved without (or with limited) volume change. The deformation of (2) is expressed as dilatant quartz-sealed veins and breccia textures in which the main mechanisms are transgranular fracturing and quartz precipitation. Type (2) implies fault volume increase, isotropy of deformation and mass transfer at distances larger than in type (1). This study discusses the origins of (1) and (2) and shows that the permeability of (1) is anisotropic, with higher values than the host rocks parallel to the Y main deformation axis (i.e. perpendicular to the slip vector), whereas the permeability of (2) is isotropic and equivalent to that of the host rocks.
ISSN: 0954-4879

hal-01115177
https://hal.archives-ouvertes.fr/hal-01115177
DOI : 10.1111/ter.12100

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Diamond-bearing UHP metamorphic rocks witness for subduction of lithospheric slabs into the mantle and their return to shallow levels. In this study we present U–Pb and trace elements analyses of zircon and rutile inclusions from a diamond-bearing garnet megacryst collected in a mélange unit exposed on the northern margin of Africa (Edough Massif, NE Algeria). Large rutile crystals (up to 300 μm in size) analyzed in situ provide a U–Pb age of interpreted as dating the prograde to peak subduction stage of the mafic protolith. Trace element analyses of minute zircons (≤30 μm) indicate that they formed in equilibrium with the garnet megacryst at a temperature of 740–810 °C, most likely during HP retrograde metamorphism. U–Pb analyses provide a significantly younger age of attributed to exhumation of the UHP units. This study allows bracketing the age of UHP metamorphism in the Western Mediterranean Orogen to the Oligocene/early Miocene, thus unambiguously relating UHP metamorphism to the Alpine history. Exhumation of these UHP units is coeval with the counterclockwise rotation of the Corsica–Sardinia block and most likely resulted from subduction rollback that was driven by slab pull.
ISSN: 0012-821X

hal-01622789
https://hal.archives-ouvertes.fr/hal-01622789
DOI : 10.1016/j.epsl.2017.06.043