Tectonic setting for Tsunemori Formation in the Permian accretionary complex of the Akiyoshi Belt, Southwest Japan

Koji Wakita,Ruri Yoshida,Yuki Fushimi

doi:10.1016/j.heliyon.2018.e01084

Abstract

The purpose of this study is to reexamine the age, depositional environment, and tectonic setting of the Tsunemori Formation. This study provides a new tectonic model on the formation of Tsunemori Formation, which is a key formation to understand the Late Permian subduction and accretionary processes in Japan. The sedimentation age of the Tsunemori Formation is late Middle Permian to early Late Permian, based on radiolarians, such as Follicucullus cf. scholasticus Ormiston and Babcock reported in this study. This paper disproves the previous theory of atoll carbonates collapse at the trench for the Akiyoshi Limestone. Most of sedimentary rocks of the Tsunemori Formation is not pervasively sheared, are less deformed compared with sedimentary rocks of the typical accretionary complexes. There is no layer parallel or sub-parallel thrusting in the turbidite sequences of the Tsunemori Formation. Mudstone of turbidite and pebbly mudstone is lack of fissility and scaly cleavages. The styles of deformation, occurrence of reworked fossils, presence of calcarenite and limestone breccia suggest that the Tsunemori Formation was not a part of accretionary wedges, but possibly was fore arc or slope basin deposits. Presence of reworked fossils, calcarenite, limestone breccia suggest that a part of the provenance of Tsunemori Formation are limestone exposed at the arc trench gap, which was moved upward to reach the arc trench gap where the Akiyoshi Limestone provides its fragments and blocks into forearc and/or slope basins. Therefore, Tsunemori Formation is a deposit in the forearc basin and/or slope basin rather than trench deposits.

Highlights

The main purpose of this paper is to propose alternative model for the tectonic setting of Tsunemori Formation, which is a key unit to understand the Permian subduction and accretion processes in Japan
The Tsunemori Formation is intimately associated with “Akiyoshi Limestone” which was formed as atoll carbonates sitting on volcanic island, which was born in the Panthalassan Ocean (Kanmera and Nishi, 1983; Sano and Kanmera, 1988)
The sedimentary sequences of the Tsunemori Formation are well stratified without pinch and swell structures and layer parallel faulting

Summary

Introduction

The main purpose of this paper is to propose alternative model for the tectonic setting of Tsunemori Formation, which is a key unit to understand the Permian subduction and accretion processes in Japan. The Tsunemori Formation is the main target of Sano and Kanmera (1991a, b, c, d) to establish their limestone collapse and accretion model of the Akiyoshi Limestone as well as internal deformation structures, and explained that Tsunemori Formation was a trench deposit, in which Akiyoshi Limestone was accreted as an exotic block. The example of seamount collapse at the trench is observed only at the tectonic erosion margin (e.g., Lallemand et al, 1989). Most of seamounts are subducted into the deeper part of the accretionary prism or into mantle without deformation in the accretion margin (Yamazaki and Okamura, 1989). Seamount accretion occurred in the special case of ocean ridge

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