Transtensional opening of the North Atlantic ocean characterized by magnetic fabric of the Kerforne doleritic dyke (Brittany, France). In : 10 th Castle Meeting on new trends in geomagnetism abstracts, Valtice, czech republic, 02 - 08 sept. 2006

The Kerforne active fault, cross-cutting the south Armorican Shear Zone, is related to the last seismic event of Sept. 30th 2002 (Ms = 5.4) ENE of Lorient (Perrot et al., 2005). It is composed of several branches trending 130°-140°N, some of them have been intruded by Jurassic doleritic dykes. One of these dykes, the Brenterc'h dyke, is located in the northernmost part of the Kerforne fault. It is a vertical 140°N-trending dyke about 10 m thick. As most of the Bretonian doleritic dykes, its eastern margin is weathered. We collected 115 samples from this dyke for paleomagnetic and AMS (anisotrophy of magnetic susceptibility) analyses in order to study the initial opening of the North Atlantic Ocean. Special care was attributed to sampling the borders of the dyke in order to detect possible shear and/or alteration along them. Three samples were also collected for K/Ar dating, two from each border and one from the central part of the dyke. / Preliminary paleomagnetic results obtained by AF demagnetizations show one component of magnetization that is carried by magnetite (Sichler and Perrin, 1993). While this component of magnetization from the western and central parts of the dyke show a Jurassic paleopole, that of the eastern border is much younger than the Jurassic emplacement age of the dyke. The K/Ar dating gives a Jurassic age (190-205 Ma) similar to previous dating by Caroff et al. (1995) and Jourdan et al. (2003) with no significant difference between the eastern (weathered) border and the central and western (fresh) border of the dyke. AMS results indicate that most of the magnetic fabrics are oblate in shape in the central part of the dyke, while they are oblate and prolate in the borders. Analysis of eigenvectors and eigenvalues show that the degree of anisotropy values range between 0.6 to 4.8%, the highest are in the eastern border. While the paleomagnetic and AMS analyses are sensitive to later processes that cause for remagnetization of the eastern border and AMS overprint, the K/Ar age determinations show only minor effects of these processes. Based on these data we suggest the following sequence of events: 1 - a dextral transtensional regime during dyke emplacement, which is related to the Biscay bay opening (Biscay-Labrador fault), 2 - initial axial flow from NW to SE in the Brenterc'h dyke , 3 - multiple injections (pulses) occurred also within the dyke (central part); and 4 - post-emplacement alteration related to fluid circulations appeared mainly in the eastern border of the dyke. Phases 1 to 3 are nearly simultaneous and corroborate the geochemical results given by Caroff and Cotten (2004). / The geodynamic interpretation concerning the initial opening of the North Atlantic Ocean needs more investigations of rock magnetism and paleomagnetism. Further dating of the secondary pulses may help to understand the nature and origin of the change in the opening mode (mechanism) of the Brenterc'h dyke which developed from symmetric opening (pure extension related to symmetric tiling) to dextral transtensional opening (parallel tiling). A 3D model using the preliminary magnetic fabric and paleomagnetic data will be discussed in the light of previous results (Aïfa and Lefort, 2000; Lefort et al., in press).