SEAMOUNT SUBDUCTION AND ATOLL CARBONATE DETACHMENT IN THE PERMIAN CONVERGENT MARGIN IN EAST ASIA

The Akiyoshi Limestone in southwest Japan is atoll carbonate, which developed on the top of a volcanic seamount erupted on the oceanic plate in the Early Carboniferous under the Panthalassa ocean. It has been well-studied in biostratigraphic works using well-preserved fusulinids nearly for 100 years. These research works revealed that half of the limestone beds are overturned. Various tectonic models have been proposed for the mechanism of overturned limestone beds; the major concept to explain the overturned structure of the limestone beds was recumbent folding. An alternative mechanism was limestone collapse along the ancient trench: if the Akiyoshi Limestone was formed as atoll carbonate on a seamount along the ancient convergent margin, it would be difficult for the old, hard rocks to form a recumbent fold by ductile deformation within the soft sediments of the accretionary wedge. However, although atoll carbonate can collapse along the tectonic erosion margin, it is difficult for the atoll carbonate to detach and collapse from the top of the seamount along an accretionary margin with a soft accretionary wedge. Recently, various tectonic models have been proposed on the deformation of accretionary wedges caused by seamount subduction. These are based on deep sea drilling in present accretionary wedges, sandbox analogue experiments and numerical simulations. However, in most of the cases mentioned above, the subducting seamount is smaller than the thickness of the accretionary wedge. The atoll carbonate of the Akiyoshi Limestone continued to form until just before its subduction and accretion, such that the top of the seamount reached the sea surface. The purpose of this research work is to propose a new tectonic model for the deformation of the accretionary wedge and the detachment and accretion of atoll carbonate caused by the subduction of a huge seamount with thick atoll carbonate, using a realistic scale model of a seamount and accretionary wedge.