Relationship between the deformation textures and the AMS fabric of accretionary complex : An example from the Shimamoto Belt, Kochi, Japan.

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AMS (Anisotropy of magnetic susceptibility) fabric has attracted much attention recently for a method of deformation analysis of sedimentary rocks from the accretionary complexes. In this study, AMS fabrics were measured in the Shimanto area from the northern cretaceous units and the southern Tertiary units in Shikoku, and the strain of the sedimentary complexes was examined by the AMS data.The AMS-fabrics of the coherent sandstone and mudstone in the Susaki formation at Ryu-misaki located in the east end of Yokonami peninsula were measures as a representative example for AMS-fabric of non-deformed rocks. Results showed that the characteristic AMS ellipsoids of undeformed sedimentary fabric have oblate component with minimum AMS axes oriented perpendicular to the bedding plane. In the Muroto formation, slaty cleavages are developed in the coherent mudstone as a distinct deformation fabric. The measured AMS ellipsoid in this area have oblate shapes. The minimum AMS axes are slightly shi … More fted to the poles of the slaty cleavage plane from the poles of the bedding plane. This AMS-fabric may be affected by the deformation correlated with the development of cleavage planes. Strain analysis was tried at every point where orientations of cleavage planes and bedding planes were measured. As for strain markers for the analysis, 11 to 15 AMS ellipsoids were obtained at every point. In the analysis, one of the principal directions of the strain ellipsoid was assumed to be the direction to the poles of the slaty cleavage planes. The calculated strain ellipsoids have oblate shape suggesting that the deformation associated with the deformation of AMS ellipsoid was uni-axial shortening. This result is consistent with the result of strain analysis previously reported for different slate areas.Uni-axial compression experiments of synthetic granular samples showed that the degree of the preferred orientation of grains increased due to grain rotation and this caused increase of the degree of AMS fabric. Less