Relationship between magnetic fabrics and shear directions in mélange within the Miyama Formation, Shimanto accretionary complex, Japan

Abstract

The magnetic fabrics of tectonic mélange in the Miyama Formation, Shimanto accretionary complex of Kii Peninsula, Japan, are examined using anisotropy of magnetic susceptibility (AMS) data, and they are compared with shear directions estimated by traditional measurements of P–Y intersections in the mélange. In most cases, AMS data are consistent with the estimated shear directions, and they can be divided into two types: one with Kmax (the maximum magnetic susceptibility) axes oriented parallel to the shear direction on the shear planes, and another with Kmax axes oriented parallel to the P–Y intersections. In both types, the Kmin (the minimum magnetic susceptibility) axes are oriented perpendicular to the mélange foliation. However, some sites exhibit unexpected Kmax axes orientations trending perpendicular to the mélange foliation. Analyses of texture and magnetic properties indicate that such unexpected orientations are caused by a predominance of single magnetic domains of magnetite and/or the existence of ankerite and siderite veins in which the long axes of crystals are oriented perpendicular to the foliation in the mélange. As a consequence, the restored shear direction deduced from AMS data improved the credibility of the kinematics of ancient plate motions. Although AMS data provide useful information on shear directions in mélanges, it is essential to examine the magnetic carriers and their magnetic domain structures for accuracy of the analyses.