ABSTRACT A 200-m-thick, near-vertical, middle Miocene (ca. 14 Ma), gabbroic sheeted intrusion in the Muroto area of the Shimanto accretionary complex of southwest Japan yields anisotropy of magnetic susceptibility (AMS) showing a magnetic foliation for the minimum axis (K min ) oblique (by ~70°) to the perpendicular of the intrusive contact. Assuming the K min axis represents the paleovertical axis, these data suggest that the gabbroic sheet was not intruded into the host sediments horizontally. Paleomagnetic measurements of the gabbroic intrusion show an in situ mean direction of reversed polarity (declination/inclination [Dec/Inc] = 287°/–65°, α 95 = 3°) that is considerably different from the expected, reversed-polarity dipole-field direction of this region (Dec/Inc = 0°/–56°). A structural analysis combining the paleomagnetic and AMS data led to the determination of a unique pole of rotation, around which the dike can be back-rotated to its initial orientation. The magnitude of rotation necessary for the in situ paleomagnetic direction to be back-rotated to the expected direction is ~60°, which is consistent with the rotation required for the K min axis to be vertical. This consistency can be regarded as independent support for our interpretation of the AMS results and the reliability of the paleomagnetic data. Consequently, we propose that the Muroto gabbro was intruded when the paleo–trench-fill sediments had been tilted landward by ~20°, presumably by accretion, and that the gabbro might have been intruded as a sill-like sheet along a structurally weak zone, possibly part of the frontal thrust plane in the Shimanto accretionary prism.

Abstract Void space and permeability are two primary factors controlling the movement and storage of fluids in rock and sediments. To investigate fluid flow anisotropy in Berea sandstone, permeability was measured in three perpendicular directions under effective confining pressure as a function of pore pressure. Permeability anisotropy was observed slightly in the normal and in two parallel directions to the bedding planes. We introduced microfocus X-ray computed tomography (CT) as a non-destructive tool and the three-dimensional medial axis (3DMA) method to quantify the flow-relevant geometric properties of the voids structure. Using this apparatus and structure analysis software, we obtained the distributions of pore size, throat size and the number of connecting paths between two faces in an arbitrary region of Berea sandstone. Using these data, we also evaluated the number of connecting paths between two faces and tortuosity within an arbitrary region, and discussed the relationship between permeability anisotropy and voids geometry.