Long-term evolution of an accretionary prism: The case study of the Shimanto Belt, Kyushu, Japan

The Shimanto Belt in SW Japan is commonly described as a paleo-accretionary prism, whose structure is explained by continuous accretion like in modern accretionary prisms such as Nankai. We carried out a structural study of the Cretaceous to Miocene part of the Shimanto Belt on Kyushu to test this hypothesis of continuous accretion. Most deformation structures observed on the field are top-to-the-SE thrusts, fitting well the scheme of accretionary wedge growth by frontal accretion or underplating. In particular, the tectonic m lange at the top of the Hyuga Group records a penetrative deformation reflecting burial within the subduction channel. In contrast, we documented two stages of extension that require modifying the traditional model of the Belt as a "simple" giant accretionary wedge. The first one, in the early Middle Eocene, is mostly ductile and localized in the foliated bases of the Morotsuka and Kitagawa Groups. The second one, postdating the Middle Miocene, is a brittle deformation spread over the whole belt on Kyushu. Integrating these new tectonic features to existing data, we propose 2-D reconstructions of the belt evolution, leading to the following conclusions: (1) Erosion and extension of the margin in the early Middle Eocene resulted from the subduction of a trench-parallel ridge. (2) The Late Eocene to Early Miocene evolution is characterized by rapid growth of the prism, followed by a Middle Miocene stage where large displacements occurred along low-angle out-of-sequence thrusts such as the Nobeoka Tectonic Line. (3) From middle Miocene, the strain regime was extensional.