Anisotropy of electrical conductivity record of initial strain at the toe of the Nankai accretionary wedge

Pierre Henry (1) , Laurence Jouniaux (2) , Elizabeth Screaton (3) , Sabine Hunze (4) , Demian Saffer (5)

1 Collège de France - Chaire Géodynamique
2 IPGS - Institut de physique du globe de Strasbourg
3 Department of Geology
4 Geowissenschaftliche Gemeinschaftsaufgaben-GCA, Hannover
5 Department of Geology and Geophysics [Laramie]

An approach based on March's theory is applied to measurements of the anisotropy of electrical conductivity on samples and is used to quantify initial strain at the toe of the Nankai accretionary wedge. A quantitative determination of strain is possible from simple assumptions: passive reorientation offlat pores forming the porous network and existence of a linear relationship between fabric tensor and electrical conductivity tensor. We show that this simple model correctly accounts for the increase of anisotropy with compaction at a reference site located in the trench (Ocean Drilling Program drill Site 1173). At the toe of the accretionary wedge (Site1174), development of anisotropy in the horizontal plane and concurrent reduction of vertical plane anisotropy are observed. This can be explained by 12%horizontal ductile shortening, occurring after decollement initiation but before slip on imbricate thrust faults. Anisotropy in the underthrust sequence is correctly described by vertical compaction, consistent with decoupled stress states across the decollement. At Site 1174 the magnitude of ductile strain implies at least 75m slip on the decollement. Ductile shortening is associated with porosity loss, implying partly drained conditions above the decollement.

Mots clés: 8105 Tectonophysics: Continental margins and sedimentary basins (1212); 3022 Marine Geology and Geophysics: Marine sediments-processes and transport; 8094 Structural Geology: Instruments and techniques; 5109 Physical Properties of Rocks: Magnetic and electrical properties; anisotropy of electrical conductivity; decollement; Nankai; strain; accretionary wedge

Mots clés: Géodynamique; Géomateriaux; Géosciences marines; Milieux poreux; Tectonique; tectonophysique; accretionary wedge; nankai; strain; electrical conductivity; ODP

Domaines: Géophysique [physics.geo-ph]; Géophysique [physics.geo-ph]; Tectonique

Pierre Henry, Laurence Jouniaux, Elizabeth Screaton, Sabine Hunze, Demian Saffer. Anisotropy of electrical conductivity record of initial strain at the toe of the Nankai accretionary wedge. Journal of Geophysical Research, 2003, 108, pp.doi:10.1029/2002JB002287. ⟨10.1029/2002JB002287⟩. ⟨hal-00108280⟩