Concept of OPS melange and its field mapping technique for ancient accretionary complexes : Using some examples mainly from Jurassic accretionary complexes of the Mino and Chichibu belts in Japan

摘要

OPS melange is a melange which is the product of the fragmentation and mixing of Ocean Plate Stratigraphy(OPS). It is a very typical melange in subduction-accretion orogenic belts. The Japanese Islands yield a record of subduction-accretion tectonics since the late Palaeozoic. Non- or weakly metamorphosed accretionary complexes in Japan are of Carboniferous, Permian, Jurassic, Cretaceous-Palaeogene, and Neogene-Quaternary ages. These accretionary complexes include typical OPS melanges. The OPS melanges contain clasts of sandstone, siliceous mud-stone, chert, limestone and basalt within a mudstone matrix. Although the age of each lithology is different, the lithology of every OPS melange is the same in all of the accretionary complexes. OPS melange is formed by various processes such as sedimentary, tectonic, and diapiric processes. However, most are products of multiple deformational processes during the development of accretionary wedges along a convergent margin. In order to investigate the highly deformed or highly metamorphosed melanges in the collision orogenic belts, it is necessary to know the nature of OPS melange in the weakly deformed and unmetamorphosed accretionary complexes of Japan. There are several key points for investigating OPS melange in the field. For geological mapping of OPS melange, we need to imagine the stratigraphic position of the rocks on the outcrop in the OPS stratigraphy. Even though the OPS were disrupted into fragments, we need to check the stratigraphic top within the blocks and slabs in the melange. As most OPS melanges were formed in multi-deformational processes, large slabs and blocks may be formed in different tectonic processes with small clasts. Age determination of sedimentary rocks and chemical analysis of igneous rocks are helpful in understanding the nature of the protolith of the OPS melange. In the field, we need to consider the existence of an unobservable matrix susceptible to weathering, and to observe the outcrop repeatedly with new knowledge from new research works in the literature.