Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Interactions between deformation and melt percolation yield important consequences for the evolution of the mantle lithosphere, controlling its composition and mechanical behavior. In the Lherz massif (Pyrenees, France), the analysis of structural relationships between harzburgites, lherzolites and pyroxenites and of the crystal-preferred orientations (CPO) of olivine and pyroxenes highlights a strong feedback between percolation of basaltic melts and deformation under near-solidus conditions at the lithosphere–asthenosphere boundary. Elongated harzburgite bodies up to tens of meters wide, which are the remnants of an old lithospheric mantle, preserve a constant foliation. This foliation is locally outlined by cm-scale flattened websteritic lenses. At the contact with the enclosing lherzolites, the harzburgite foliation is crosscut by the lherzolites foliation and by cm-wide websterite bands parallel to the contact. Strain intensity in the lherzolites increases with distance to the harzburgites. Based on these observations, we propose that reactive percolation was synchronous to the deformation and propose that variations in instantaneous melt fraction, due to pyroxenes and spinel crystallization during reactive melt transport, guided strain localization. Accordingly, the observed decrease in olivine CPO intensity and change in CPO patterns from harzburgites to distal lherzolites are interpreted as recording changes in the relative contribution of dislocation glide and diffusion processes, which is ruled by a balance between the instantaneous melt fraction and the local strain rate. We also propose that the pervasive websteritic layering in the refertilized lherzolites may result from deformation-assisted melt segregation in a system with decreasing permeability due to refertilization reactions. Finally, we discuss the possible timing and geodynamical context of the refertilization episode.
ISSN: 0012-821X

hal-00411724
https://hal.archives-ouvertes.fr/hal-00411724
DOI : 10.1016/j.epsl.2008.07.053

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The presence of two regional seismic networks in southeastern France provides us high-quality data to investigate upper mantle flow by measuring the splitting of teleseismic shear waves induced by seismic anisotropy. The 10 three-component and broadband stations installed in Corsica, Provence, and western Alps efficiently complete the geographic coverage of anisotropy measurements performed in southern France using temporary experiments deployed on geodynamic targets such as the Pyrenees and the Massif Central. Teleseismic shear waves (mainly SKS and SKKS) are used to determine the splitting parameters: the fast polarization direction and the delay time. Delay times ranging between 1.0 and 1.5 s have been observed at most sites, but some larger delay times, above 2.0 s, have been observed at some stations, such as in northern Alps or Corsica, suggesting the presence of high strain zones in the upper mantle. The azimuths of the fast split shear waves define a simple and smooth pattern, trending homogeneously WNW–ESE in the Nice area and progressively rotating to NW–SE and to NS for stations located further North in the Alps. This pattern is in continuity with the measurements performed in the southern Massif Central and could be related to a large asthenospheric flow induced by the rotation of the Corsica–Sardinia lithospheric block and the retreat of the Apenninic slab. We show that seismic anisotropy nicely maps the route of the slab from the initial rifting phase along the Gulf of Lion (30–22 Ma) to the drifting of the Corsica–Sardinia lithospheric block accompanied by the creation of new oceanic lithosphere in the Liguro–Provençal basin (22–17 Ma). In the external and internal Alps, the pattern of the azimuth of the fast split waves follows the bend of the alpine arc. We propose that the mantle flow beneath this area could be influenced or perhaps controlled by the Alpine deep penetrative structures and that the Alpine lithospheric roots may have deflected part of the horizontal asthenospheric flow around its southernmost tip.
ISSN: 0040-1951

hal-00109313
https://hal.archives-ouvertes.fr/hal-00109313
https://hal.archives-ouvertes.fr/hal-00109313/document
https://hal.archives-ouvertes.fr/hal-00109313/file/barruol_TGRS_Tphy2004_hal.pdf
DOI : 10.1016/j.tecto.2004.08.002

Éditeur(s) : HAL CCSD Springer
Résumé : The analysis of three different regions of the South- Pyrenean fold-and-thrust belt reveals that during the Tertiary compression the hydrological system was compartmentalised in time and space. During the early-middle Eocene, when the thrust front affected marine soft-sediments in the Ainsa basin, the thrust fault zones were dominated by formation fluids derived from Eocene marine waters trapped in the underlying Eocene marls, although influences of meteoric waters were also present. During the middle-late Eocene, when the thrust front emplaced marine rocks over continental redbeds in the eastern Catalan basin (L'Escala thrust), the thrust fault zones were dominated by meteoric fluids. These fluids flowed preferentially along these faults, draining laterally the meteoric fluids and acting as barriers hindering their flowing towards more external parts of the belt. During the Oligocene, the most external part of the fold-and-thrust belt in the eastern Catalan basin developed on top of a salt detachment horizon. The thrust front affected continental materials of late Eocene-Oligocene age. At this moment, the thrusts were conduits for meteoric fluids arriving from the surface and also for evolved meteoric fluids migrating over short distance upwards after being in contact with the underlying evaporitic beds. Most of the fractures show a similar sequence of microfractures. Microfractures of stage 1 formed when the sediment was poorly lithified. Microfractures of stage 2 represent the main episode of vein formation which developed when a progressive induration of the host sediment occurred. During microfracture stage 3, formed in an extensional regime, the host sediment was more indurated. The repetition of this sequence of microfractures in different fracture generations of the same outcrop indicates that the sediment induration was restricted to the vicinity of the vein. Away from the veins, the sediment remained poorly lithified during the entire deformation cycle. Calcite cement within the host rock precipitated later than the syn-compressive veins, when the sediment was more indurated. Elemental geochemistry and stable isotopes of the calcite veins indicates that early generation of microfractures is infilled by local fluids (either marine or meteoric), whereas external fluids (meteoric or evolved meteoric) infilled the main compressive stage of microfractures. The hot temperature of these fluids (157°C to 183°C in the Atiart-Arro example) indicates their circulation through deep parts of the thrust belt. The progressive increase of the 87Sr/86Sr ratio through time is due to the progressive uplift, exposure and erosion of the internal Pyrenean Axial Zone.
Thrust belts and foreland basins, from fold kinematics to hydrocarbon systems

hal-00408001
https://hal.archives-ouvertes.fr/hal-00408001
DOI : 10.1007/978-3-540-69426-7_5