Tectonic setting required for the preservation of sedimentary mélanges in Palaeozoic and Mesozoic accretionary complexes of southwest Japan

Abstract

Abstract The Palaeozoic to Mesozoic accretionary complexes of southwest Japan include various types of melange. Most melanges are polygenetic in origin, being sedimentary or diapiric melanges that were overprinted by tectonic deformation during subduction. Sedimentary melanges, without a tectonic overprint, are present in the Permian accretionary complexes of the Akiyoshi and Kurosegawa belts and in the Early Cretaceous accretionary complex of the Chichibu Belt. These melanges are characterized by dominant basalt and limestone clasts, within a mudstone matrix. The basalt and limestone clasts within the sedimentary melanges were derived from ancient seamounts. Subduction of a seamount results in deformation of the pre-existing accretionary wedge, and it is difficult to incorporate a seamount into an accretionary wedge; therefore, preservation of seamount fragments requires a special tectonic setting. Oceanic plateau accretion might play an important role in interrupting the processes of subduction and accretion during the formation of accretionary complexes. Especially the Mikabu oceanic plateau might have caused the cessation of accretion during the Early Cretaceous. The subduction and accretion of volcanic arcs and oceanic plateaux helps to preserve sedimentary melanges from tectonic overprinting by preventing further subduction.