Formation and deformation of hyperextended rift systems: Insights from rift domain mapping in the Bay of Biscay-Pyrenees

International audienceThe Bay of Biscay and the Pyrenees correspond to a Lower Cretaceous rift system including both oceanic and hyperextended rift domains. The transition from preserved oceanic and rift domains in the West to their complete inversion in the East enables us to study the progressive reactivation of a hyperextended rift system. We use seismic interpretation, gravity inversion, and field mapping to identify and map former rift domains and their subsequent reactivation. We propose a new map and sections across the system illustrating the progressive integration of the rift domains into the orogen. This study aims to provide insights on the formation of hyperextended rift systems and discuss their role during reactivation. Two spatially and temporally distinct rift systems can be distinguished: the Bay of Biscay-Parentis and the Pyrenean-Basque-Cantabrian rifts. While the offshore Bay of Biscay represent a former mature oceanic domain, the fossil remnants of hyperextended domains preserved onshore in the Pyrenean-Cantabrian orogen record distributed extensional deformation partitioned between strongly segmented rift basins. Reactivation initiated in the exhumed mantle domain before it affected the hyperthinned domain. Both domains accommodated most of the shortening. The final architecture of the orogen is acquired once the conjugate necking domains became involved in collisional processes. The complex 3-D architecture of the initial rift system may partly explain the heterogeneous reactivation of the overall system. These results have important implications for the formation and reactivation of hyperextended rift systems and for the restoration of the Bay of Biscay and Pyrenean domain

Un exemple de tectonique de plate-forme : les causses majeurs (sud du massif central, France)

Dans cet article sont présentés à titre d'exemple les différents types de déformations subies par une région de plate-forme, les Causses majeurs (France méridionale), lors de la phase de compression tectonique d'âge pyrénéen. Les conclusions tirées de cette étude peuvent en effet être appliquées aux bassins pétroliers de même type. On définit la géométrie et l'orientation des structures produites lors de cette phase, failles, plis associés aux failles, fracturation, stylolithes. On s'attache à montrer l'influence d'anciennes structures hercyniennes du soubassement sur la répartition et l'orientation des déformations affectant la couverture jurassique. On montre aussi que des structures présentant des orientations particulières se forment au voisinage des failles apparues lors des premiers stades de la compression : ces structures traduisent une modification locale du champ de contraintes due aux failles précoces, un essai de chronologie des différents types de structures est proposé

Annotated record of the detailed examination of Mn deposits from R/V "Le Suroit" GEOMANCHE75/1 stations

We report here preliminary results from the detailed sampling of granitic basement at a depth of 4.000 m at the lower edge of the Armorican continental margin near 48°N and 12°W. We consider that we have sampled here the extreme lateral limit of the continental crust where local exposures emerge from under a cover of Cainozoic,Mesozoic and probable Palaeozoic sediments

Annotated record of the detailed examination of Mn deposits from R/V "Jean Charcot" GEOMANCHE 76/2 stations

This report presents the preliminary results of the study of rocks and sediments obtained by dredging during four cruises of the R/V Jean Charcot and R/V Le Suroit during a cooperative program between CNEXO and CEPM (CH 58: April 1975; SU 01: December 1975; CH 66: February 1976; and CH 67: March 1976). Several dredges on the continental slope of the Goban Spur recovered "granitic" rocks on two morphological structures, Granite Cliff and "Menez Bihan" (Pautot et al., 1976), in water depths ranging from 3200 to 4200 meters. According to the radiometric age and petrology, the granodiorite appears to have a close affinity with similar igneous facies which have been described in Iberia (Capdevilla et al., 1973) and attributed to the Variscan intrusive episode which is also found in the nearby continental area (southwest Great Britain and Brittany, France). They often are coated by a centimeter-thick layer of ferromanganiferous deposits (Shaaf et al., 1977)

Dredged Rocks from the Armorican and Celtic Margins

This report presents the preliminary results of the study of rocks and sediments obtained by dredging during four cruises of the R/V Jean Charcot and R/V Le Suroît during a cooperative program between CNEXO and CEPM (CH 58 : April 1975 ; SU 01 : December 1975 ; CH 66 : February 1976 ; and CH 67 : March 1976). A bathymetric map of the margins under study, as well as the location of the 27 dredges considered in this report, are presented in Figures 1 and 2, respectively. [NOT CONTROLLED OCR

Seismic constraints on the three-dimensional geometry of low-angle intracrustal reflectors in the Southern Iberia Abyssal Plain

Several lines of evidence suggest that simple shear rifting of the continental crust, in the formof low-angle detachment faulting, occurred during the final stages of continental breakupbetween West Iberia and the Grand Banks. The primary evidence for such faulting is theoccurrence of low-angle, high amplitude reflectors within the basement adjacent to the ocean–continent transition zone. Here we present a series of intersecting, depth migrated seismicreflection profiles that image one such reflector, the H-reflector, located on the southern edgeof Galicia Bank. ‘H’ lies beneath several boreholes drilled during ODP Legs 149 and 173,in a region where the oceanward extent of extended continental crust steps at least 150 kmwestward from its location in the southern Iberia Abyssal Plain to its location off the relativelyshallow Galicia Bank. In our profiles ‘H’ appears to define a surface that extends over a regionof at least 200 km2 and that dips down ?19? to the north, towards Galicia Bank. The profilesshow that a close affinity exists between ‘H’ and the most seaward continental crust. Based ongeophysical data and ODP drilling results, we infer that the basement above ‘H’ is composedof continental crust deformed by extensional faults into a series of wedge-shaped blocks andthin slivers. These basement wedges have a complex 3-D geometry. ‘H’ rises to the basementsurface on a number of the seismic profiles and appears to define locally the oceanward extentof continental fault blocks

Structure and rifting evolution of the northern Newfoundland Basin from Erable multichannel seismic reflection profiles across the southeastern margin of Flemish Cap

We present the results from processing and interpreting five lines from the 1992 Erable multichannel seismic reflection experiment extending from the southeastern margin of Flemish Cap into the northern Newfoundland Basin. These profiles reveal significant along strike variations in the rifting styles experienced by Flemish Cap. In the southwest, a 100-km-wide transition zone is identified between thinned continental crust and thin oceanic crust. Similar to the conjugate Galicia Bank and Iberian margins, this transition zone contains a section of deep basement adjacent to a series of shallower ridges and is interpreted as exhumed serpentinized mantle. Along strike towards the northeast, this transition zone pinches out completely within 100 km and is replaced by thin oceanic crust directly adjacent to thinned continental crust. By interpreting nearby seismic profiles and profiles on the conjugate margins using the same classification criteria, we construct regional maps of the distribution of crustal domains on both sides of the North Atlantic. These maps reveal significant variations in rifting style on the conjugate margins and along strike of each margin and also highlight the role of ancient transfer zones in compartmentalizing these rifting variations into four distinct regions. We propose that the limited localization of shallow topographically high serpentinized peridotite ridges on the Newfoundland-Iberia and Flemish Cap-Galicia Bank conjugate margins, was directly related to an increase in the rate of extension following the separation of Flemish Cap and Galicia Bank which exhumed deeper, less serpentinized mantle