Isabelle Thinon

Deep structure of the Armorican Basin (Bay of Biscay): a review of Norgasis seismic reflection and refraction data

The Bay of Biscay is bounded to the north by the North Biscay margin, which comprises the Western Approaches and Armorican segments. In the 1970s and 1980s, most researchers considered this margin typical of a non-volcanic passive margin: it is characterized by a striking succession of tilted blocks beneath which occurs the S reflector and the continent–ocean boundary is abrupt. This paper examines the Armorican segment and is based on a study of all early seismic profiles together with new multichannel reflection and refraction seismic data (Norgasis cruise). An important result is the discovery of a 80 km wide ocean–continent transition zone that coincides with the Armorican Basin (a deep sedimentary basin). It is characterized by a high-velocity lower-crustal layer (7.4–7.5 km s−1) overlain by sediments. The other results are: (1) the main crustal thinning occurs exclusively under the narrow continental slope; (2) the tilted blocks and the S reflector are observed only at the base of the continental slope in the narrow domain called the ‘neck area’; (3) the North Biscay Ridge is a large oceanic plateau present only off the NW Armorican margin rather than a long ridge elongated off the whole North Biscay margin.

Déformations pyrénéennes dans le golfe de Gascogne

Abstract A detailed cartography of the maximal Pyrenean deformation dated from Eocene has been made for the northern Bay of Biscay from the interpretation of new reflection seismic data. These new seismic images confirm the occurrence of a primary tectonic event for the Pyrenean phases. This event would underline the initialisation of the Iberia–Europe convergence. The Pyrenean deformations are mainly located at the boundaries of the continental and oceanic domains and affect the oceanic crust. This deformation is principally underlined by compressive and inversed structures.

The syn-rift sedimentary cover of the North Biscay Margin (bay of Biscay): from new reflection seismic data

The Armorican Basin is a deep sedimentary basin lying at the footside of the North Bay of Biscay. From previous scattered inadequate data, the age and nature of this basin, oceanic domain or deep part of the Armorican margin itself were largely speculated. From this new seismo-stratigraphic study based on a dense seismic cover, the sedimentation within the Armorican Basin is beginning in the Aptian times, during the last tectonic rifting episode of the margin. The first sediments formation identified as the « 3B layer » is characterised on the profiles by a chaotic and transparent seismic facies and was emplaced by slumping process when the margin collapsed, at the final rifting phase, just before the oceanic accretion. The new seismic reflection data give also some informations on the polyphased evolution of the North Biscay Margin during the rifting period. Two main events occurred during the Lower Cretaceous times (the first one is pre-Berriasian, the second is Aptian), separated by a quiet tectonic period including the Upper Berriasian and Lower Aptian times. The first event is responsible of the margin tectonic structuration in some blocks, the second of collapsing and the emplacement of the allochthonous sediments (3B layer) in the Armorican Basin.

Impacts des exploitations de nickel sur les taux d'apports sédimentaires au littoral de Thio en Nouvelle-Calédonie

The impact of the mine has strongly shaped the New Caledonian watersheds on rocky massifs of peridotites and caused important phenomena of fattening and siltation of the rivers that drain them. Conflicts of use result between local populations and minors. The watershed of the Thio River is the subject of an ongoing study funded by the National