Emplacement of a layered mafic intrusion in the Shimanto accretionary complex of southwest Japan: Evidence from paleomagnetic and magnetic fabric analysis

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 (Kmin) oblique (by ~70°) to the perpendicular of the intrusive contact. Assuming the Kmin 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°, α95 = 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 Kmin 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.