TECTONIC CONTROL OF MÉLANGES FORMATION OF SEDIMENTARY AND POLYGENETIC ORIGIN: EXAMPLES FROM PALEOZOIC TO MESOZOIC ACCRETIONARY COMPLEXES OF SOUTHWEST JAPAN

The Paleozoic to Mesozoic accretionary complexes of Southwest Japan includes various types of mélanges. The most of mélanges are polygenetic in origin, being either sedimentary mélanges or diapiric mélanges overprinted by tectonic deformation during subduction processes. The most famous mélanges are tectonic mélanges of the Shimanto Belt. They are tectonically deformed by pervasive shearing during ocean plate subduction after sedimentaion. On the other hand, the mélanges of Jurassic accretionary complex of the Mino Belt are formed as diapiric mélanges or sedimentary mélanges (olistostromal), including the later tectonic deformation during plate subduction. Although most of the mélanges are caused by one or more mechanisms of sedimentary, diapiric and tectonic, some parts of accretionary complexes still preserve the original sedimentary mélanges without tectonic overprint. These sedimentary mélanges are found as the youngest components of the accretionary complexes of the Permian and Jurassic accretionary complexes, respectively. The formation of sedimentary mélanges may occur by means of the termination of accretionary complexes. Another characteristic features of these sedimentary mélanges are dominant inclusions of basalt and limestone within mudstone matrix. The basalt and limestone clasts within the sedimentary mélanges are derived from ancient seamounts. The subducting seamounts forces to deform the pre-existing accretionary wedge, and are hard to accrete within the accretioanry wedge. Therefore, the preservation of seamount fragments forces to require a special tectonic setting, cessation of subduction or accretionary prism formation To stop the formation of Permian accretioanry complexes, subduction and accretion of Maizuru volcanic arc as a collisional ridge may play an important role. On the other hand, subduction and collision of oceanic plateau of the Mikabu Belt caused to stop the Jurassic and Lower Cretaceous accretionary process. The subduction and accretion of volcanic arc and oceanic plateau help to preserve sedimentary mélanges from tectonic overprint by stopping further subduction processes.