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

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 of flat 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 (Site 1174), 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 75 m slip on the decollement. Ductile shortening is associated with porosity loss, implying partly drained conditions above the decollement.