The Ibero-Armorican arc: indentation versus self-subduction

In the Iberian Variscides it is possible to distinguish major arcuate structures; although highly studied, their characterization and genesis are still a matter of controversy. The main Ibero- Armorican Arc (IAA) is essentially defined by a NW–SE trend in the Iberian branch and an E–W trend in the Brittany branch; however, in northern Spain it is rotated 180º, sometimes known as the Cantabrian Arc (CA). The relationship between these arcs is debatable, being considered either as a single arc generated in one tectonic event, or the result of polyphase bending process. According to the last assumption, there is a later arcuate structure (CA), overlapping a previous major one (IAA). Whatever the proposed models, they must explain the presence of a sinistral transpression regime in Iberia and a dextral one in Armorican branch, and the temporal deformation range of Devonian to Upper Carboniferous (Dias and Ribeiro, 1995)

From granulites to eclogites in the Sesia zone (Italian Western Alps): a record of the opening and closure of the Piedmont ocean

The Sesia zone (Italian Western Alps) offers one of the best preserved examples of pre-Alpine basement reactivated, under eclogite facies conditions, during the Alpine orogenesis. A detailed mineralogical study of eclogitized acid and basic granulites, and related amphibolites, is presented. In these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished. The inferred P-T path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks. In the light of the available geochronological constraints we propose to relate the pre-Alpine granulite and post-granulite retrograde evolution to the Permo-Jurassic extensional regime. The complex granulite-eclogite transition is thus regarded as a record of the opening and of the closure of the Piedmont ocean. -from Author

40Ar/39Ar dating of Penninic Front tectonic displacement (W Alps) during the Lower Oligocene (31-34 Ma)

Direct absolute dating of the Penninic Frontal Thrust tectonic motion is achieved using the Ar-40/Ar-39 technique in the Pelvoux Crystalline Massif (Western Alps). The dated phengites were formed syn-kinematically in shear zones. They underline the brittle-ductile stretching lineation, pressure-shadow fibres and slickensides consistent with underthrusting of the European continental slab below the propagating Penninic Thrust. Chlorite-phengite thermobarometry yields 10-15 km and T similar to 280 degrees C, while Ar-40/Ar-39 phengite ages mainly range between 34 and 30 Ma, with one younger age at 27 Ma. This Early Oligocene age range matches a major tectonic rearrangement of the Alpine chain. Preservation of prograde Ar-40/Ar-39 ages is ascribed to passive exhumation of the Pelvoux shear zone network, sandwiched between more external thrusts and the Penninic Front reactivated as an E-dipping detachment fault. Partial resetting in the Low Temperature part of argon spectra below 24 Ma is ascribed to brittle deformation and alteration of phengites

Airborne magnetic data compared to petrology of crustal scale shear zones from southern Madagascar: A tool for deciphering magma and fluid transfer in orogenic crust,

International audienceThe southern part of Madagascar consists of a granulitic metamorphic belt with a complex Proterozoic shear zone network. Aeromagnetic maps reveal sharp magnetic spatial gradients, especially across shear zones. All shear zones are associated with high magnetic values, except one, the Beraketa shear zone. Based upon relationships between rock magnetic properties, petrographic and aeromagnetic data, we show that the magnetic signal is controlled by variations in proportions of iron-rich oxides. Their nature and texture are variable and complex. Magnetite and ilmenite are often observed together showing intergrowths texture, suggesting possible lamellar magnetism. Detailed petrographic observations of the Zazafotsy shear zone show that a strong magnetic signal is correlated with metamorphic reactions and especially with biotite breakdown to peritectic phases such as orthopyroxene and iron-rich oxides (metamorphic charnockitization). Magmatic material can migrate easily inside the Zazafotsy shear zone and inside the fold hinges close to the shear zone, increasing the kinetics of charnockitic reaction. In opposition, inside the Beraketa shear zone, lower magnetic values are correlated with the absence of iron-rich oxides. This is interpreted as back reaction melting. Thus, peritectic phases, such as iron-rich oxides, react with the water released when magmas crystallise to produce biotite. Inside the Zazafotsy shear zone, iron-rich oxides are stable because part of the migmatite was segregated and escaped with dissolve H2O. In this case, back reaction was no longer possible

Timing of geothermal activity in an active island-arc volcanic setting: preliminary 40Ar/39Ar data from Bouillante geothermal field (Guadeloupe, French West Indies),

International audienceMineral separates of adularia have been extracted from three samples of highly silicified hydrothermal breccias, newly discovered in the active Bouillante geothermal field (Guadeloupe archipelago), and investigated by 40Ar/39Ar geochronology in order to constrain the timing of geothermal activity in this part of the active Lesser Antilles island arc. The inverse isochron diagram indicates an age of 248±50 ka (2σ) for all adularia from one breccia sample (n=8), with an initial 40Ar/36Ar ratio of atmospheric composition (309±12 (2σ)) attesting that this age is valid. This age is concordant with the weighted mean age of 290±40 ka for the same sample. Adularia from other samples yields concordant ages. The obtained 40Ar/39Ar ages can be related either to the magmatic activity of the Bouillante Volcanic Chain (c. 850–250 ka ago) or to the initiation of the volcanic activity of the active Grande Découverte–Soufrière system (200 ka ago–present day). Our results demonstrate that the Bouillante hydrothermal event is coeval with change in the volcanic pulses previously recognized in the magmatic history of the studied area. The possible duration calculated for this hydrothermal activity requires at least two superposed volcanic pulses to be developed