Asymmetrical melange fabrics as possible indicators of the convergent direction of plates: a case study from the Shimanto Belt of the Akaishi Mountains, central Japan

Abstract

Scaly-foliated melanges in the upper Maastrichtian-lowest Paleogene strata occurring in a zone several kilometers wide in the Shimanto Belt of the Akaishi Mountains, central Japan, often exhibit composite planar fabrics on a mesoscopic scale with distinct monoclinic symmetry. They are characterized by block-in-matrix fabrics including scaly foliations in mudstone matrices and asymmetric lensoidal and/or ellipsoidal clasts of sandstone which were generated by extension parallel to the layers. These fabrics are geometrically very similar to those of foliated fault-related rocks. Another remarkable fabric is the imbricated stacks of lensoidal sandstone clasts forming mesoscopic duplexes by layer-parallel contraction. The co-existence of these extensional and contraction planar fabrics suggests that non-coaxial deformation had considerably progressed and that strain distribution was heterogeneous during formation of the melange. Systematic analysis of attitudes of these melange fabrics in many outcrops indicates that the fabrics have a remarkably consistent preferred orientation, suggesting that a south-trending overthrust shear about 40°–70° counterclockwise with respect to the trend of strata prevailed during melange formation. This direction is fairly consistent with the relative plate motion deduced from the regional tectonic framework of this age, which indicates the oblique subduction of the Pacific plate relative to the eastern margin of the Eurasian continent. Thus, the melange fabrics with monoclinic symmetry probably reflect the relative motion of consuming plates, and may become excellent geological indicators for determining the direction of convergence of plates during melange formation, since data on melange fabrics are directly and easily available in the field.