Beyond the Akiyoshi Orogeny: unravelling overturned structures and tectonic processes in the Permian accretionary complex of the Mine-Akiyoshidai area, Yamaguchi, western Japan

Abstract

The concept of the Akiyoshi Orogeny emerged from geological studies of the overturned Akiyoshi Limestone. However, our understanding of the stratigraphy and geological structure of the siliceous–clastic rocks surrounding the Akiyoshi Limestone remains incomplete. We therefore conducted a geological survey within the Beppu unit, which is distributed north of the Akiyoshi Limestone. We found substantial overturning of the strata, even within the siliceous–clastic rocks that form the upper section of the ocean plate stratigraphy within the accretionary complex. The Tsunemori Formation, representing sediments of the forearc or trench slope basin, has also undergone overturning. Extensive overturned structures therefore exist throughout the entire accretionary complex of the Mine-Akiyoshidai area. To understand the mechanism behind these overturned structures within the accretionary complex, we focused on the collision and deformation resulting from the interaction between the accretionary wedge and a massive seamount, along with the back-stop. The overturning of the forearc or slope basin sediments suggests that post-accretion deformation along a large lateral fault may have occurred during the collision between the South and North China blocks. These tectonic events during and after accretion may represent the true nature of the Akiyoshi Orogeny. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists