Evolution of structures and fabrics in the Barbados Accretionary Prism ; Insights from Leg 110 of the Ocean Drilling Program

The microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo-feldspathic mylonitic shear zone (Torridon, NW Scotland) have been investigated using SEM electron channelling. Our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history. Microstructural variation is particularly pronounced at low shear strains: significant intragranular deformation occurs via a domino-faulting style process, whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures. A sudden reduction in grain size defines the transition to medium shear strains, but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin. Grains within these bands have somewhat steeper preferred dimensional orientations. These domains persist into the high shear strain mylonitic region, where they are oriented subparallel to the shear zone margin and consist of sub-20 μm grains. The microstructures suggest that the principal deformation mechanism was intracrystalline plasticity (with contributions from grain size reduction via dynamic recrystallization, grain boundary migration and grain boundary sliding). Crystal fabrics measured from the shear zone vary with position depending on the shear strain involved, and are consistent with the operation of several crystal slip systems (e.g. prism, basal, rhomb and acute rhomb planes) in a consistent direction (probably parallel to a and/or m). They also reveal the presence of Dauphine twinning and suggest that this may be a significant process in quartz deformation. A single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed. Rather, the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation. The mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution. The stability of some microstructures to higher shear strains, the exploitation of others at lower shear strains, and a continuously evolving crystal fabric, mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient

Contribution à l'étude stratigraphique des terrains secondaires et tertiaires affleurant en Manche occidentale

Contribution n° 15 du Groupe d'étude de la marge continentale de la Faculté des sciences de RennesNational audiencepas de résum

Pré reconnaissance géologique du Sud du Massif armoricain : Affleurements de dépôts aquitaniens

Séance du 11 mars 1968National audiencepas de résum

La couverture sédimentaire tertiaire à l'Ouest et au Sud-Ouest du Massif Armoricain

National audienceUne série de carottages dans la couverture secondaire et tertiaire du Massif Armoricain, complétée par quelques dragages, a permis de dessiner un schéma géologique du plateau continental et du haut de la pente continentale à l’Ouest et au Sud-Ouest de la Bretagne1. Le Crétacé apparait à l’entrée de la Manche, et, très localise, au large del’Ile de Sein2. Le Paleogène. constitue surtout par des terrains du Lutetien supeneur, forme une auréole continue tout autour du Massif Armoricain.3. Ce Paleogène est recouvert par du Néogène :a) localement au Nord de Plie d'Ouessant,b) partout au Sud et au Sud-Ouest de la Bretagne (Aquitanien, Vlelvétien, ou Néogène indifférencié. Plus au large, le Pliocène affleure à la limite du plateau, dans la partie supérieure de la pente continentale).D une façon générale, les terrains sont d’autant plus récents que l’on s'éloigne du socle hercynien vers le Sud-Ouest, c'est-à-dire, vers la pente continentale, Cette disposition confirme la structure mono- cîmale du plateau continental dans cette région

Le Campanien stratotypique : étude lithologique et micropaléontologique

The Campanian was studied in the «Champagne charentaise» and in Aubeterre area. No continuous sections exist, so the material come from two wellcores, one situated in Gimeux enabled us to place the Santonian-Campanian boundary and the second located near Archiac (both localities were named by Coquand). The upper levels of the last are equivalent to the lower levels of Aubeterre section. Between these two wells field sections were sampled. There exists a stratigraphie overlap between each section making it very easy to establish the correlations between them. For each section a detail of lithology and sedimentology is given as the range of the main species selected in benthic and planktonic Foraminifera, calcareous nannofossils, Dinoflagellates and Ostracods. Comparative biozonations are proposed.Le Campanien a été inventorié dans la région stratotypique dans les localités mentionnées par Coquand (1856, 1858). Cette synthèse repose sur l'étude de matériaux provenant d'une part de sondages carottés à Gimeux et à Archiac et d'autre part, de coupes de terrain effectuées à Gimeux, Le Renclos-la Brande en Champagne charentaise et à Aubeterre, de sorte que le Campanien a pu être exploité dans son ensemble depuis la limite Santonien-Campanien. L'étude est fondée sur une analyse lithologique et sédimentologique détaillée et sur un inventaire micropaléontologique concernant les Foraminifères benthiques et planctoniques, les Nannofossiles calcaires, les Dinoflagellés et les Ostracodes. Des tableaux de répartition comparée sont proposés.Neumann M., Bellier Jean-Pierre, Damotte Renée Rivière, Lambert B, Masure Edwige, Monciardini Christian, Platel J. P., Andreieff Patrick. Le Campanien stratotypique : étude lithologique et micropaléontologique. In: Géologie Méditerranéenne. Tome 10, numéro 3-4, 1983. Colloque sur les étages coniaciens à Maastrichtien. Marseille, 26-28 septembre 1983. pp. 41-57

7. Foraminifères

Andreieff Patrick, Anglada R., Aubert J., Blondeau Alphonse, Bourdon M., Caralp M., Le Calvez Yolande, Lorenz C., Magné Jean, Poignant A., Pujol Claude. 7. Foraminifères. In: Documents des Laboratoires de Géologie de la Faculté des Sciences de Lyon, n°59, 1974. Étude biostratigraphique des gisements d’Escornebéou (Aquitaine méridionale, France) pp. 31-56

Tectonics and hydrogeology of the northern Barbados Ridge:Results from Ocean Drilling Program Leg 110

Drilling near the deformation front of the northern Barbados Ridge cored an accretionary prism consisting of imbricately thrusted Neogene hemipelagic sediments detached from little-deformed Oligocene to Campanian underthrust deposits by a décollement zone composed of lower Miocene to upper Oligocene, scaly radiolarian claystone. Biostrati-graphically defined age inversions define thrust faults in the accretionary prism that correlate between sites and are apparent on the seismic reflection sections. Two sites located 12 and 17 km west of the deformation front document continuing deformation of the accreted sediments during their uplift. Deformational features include both large- and small-scale folding and continued thrust faulting with the development of stratal disruption, cataclastic shear zones, and the proliferation of scaly fabrics. These features, resembling structures of accretionary complexes exposed on land, have developed in sediments never buried more than 400 m and retaining 40% to 50% porosity. A single oceanic reference site, located 6 km east of the deformation front, shows incipient deformation at the stratigraphic level of the décollement and pore-water chemistry anomalies both at the décollement level and in a subjacent permeable sand interval. Pore-water chemistry data from all sites define two fluid realms: one characterized by methane and chloride anomalies and located within and below the décollement zone and a second marked solely by chloride anomalies and occurring within the accretionary prism. The thermogenic methane in the décollement zone requires fluid transport many tens of kilometers arcward of the deformation front along the shallowly inclined décollement surface, with minimal leakage into the overlying accretionary prism. Chloride anomalies along faults and a permeable sand layer in the underthrust sequence may be caused by membrane filtration or smectite dewatering at depth. Low matrix permeability requires that fluid flow along faults occurs through fracture permeability. Temperature and geochemical data suggest that episodic fluid flow occurs along faults, probably as a result of deformational pumping

Anatomie et physiologie d'un prisme d'accretion; premiers resultats des forages du complexe de la ride de la Barbade, Leg ODP 110

During the Ocean Drilling Program Leg 110, six sites, on the lower eastern slope of the Barbados ridge accretionary complex and in front of it on the Atlantic Abyssal Plain, were drilled. These sites permitted the analysis of Eocene to Present sedimentation, and the structures and fabrics induced by offscraping and overthrusting of a part of the sedimentary series that covers thb oceanic floor. The main, basal, décollement, as well as several thrust-faults and asymmetrical to isoclinal folds were intersected. Chemical studies of pore water demonstrates that thrust-fault planes and the décollement are preferential pathways for dewatering processes and associated fluid circulation