Magnetic and structural fabrics of the melange in the Shimanto accretionary complex, Okinawa Island: Implication for strain history during decollement‐related deformation

Abstract

The magnetic fabrics of the melange in the Shimanto accretionary complex of Okinawa Island were obtained from the anisotropy of magnetic susceptibility (AMS) data. The magnetic fabrics show a marked contrast between the structurally lower and upper parts of the melange; the lower part records deformation in the footwall of the decollement (D1), whereas the upper part documents that in the hanging wall of the decollement (D2). In the lower part of the melange the magnetic fabrics show good correlations with S1 layer‐parallel foliation, reflecting strain geometries under layer‐perpendicular flattening. As S‐C fabrics were locally developed by heterogeneous shear during D1, some of the previous magnetic fabrics were rotated with S1. In this case, the resultant magnetic fabrics reflect “S” surfaces, and maximum AMS axes are reoriented parallel to shear directions without clear change in shape and anisotropy degree of the AMS ellipsoids. Thus “S” surfaces did not act as flattening planes but may represent the deformation through independent particulate flow and mechanical rotation. The lower part of the melange was pervasively sheared during D2, reflecting that it laid immediately above the decollement. However, D1 magnetic fabrics were not modified because shear strain was accomplished by brittle microfaults without development of penetrative strain. In contrast, penetrative strain was significant in the upper part of the melange, resulting in the magnetic fabrics recording incremental strain associated with D2. Therefore deformation mechanism and proximity to a decollement play an important role on development of magnetic fabrics in a melange.