WHAT IS “A MÉLANGE”? - A COMPARATIVE STUDY ON UNITS OF MÉLANGES IN MINO AND SHIMANTO ACCRETIONARY COMPLEXES IN SOUTHWESTERN JAPAN

Mélange is defined by its texture (block-in-matrix), components (exotic blocks) and size (mappability). This definition does not include its origin. However, various kinds of mélanges exist in the world that are described based on their origin. The definition of a unit of mélange is not clear until now. If mélange is formed by a single process, one unit of mélange must be clear. However, if mélange is formed by multiple processes, how could we define a melange unit? In order to discuss a unit of mélange, I will compare two types of mélanges in Japan. One is the Cretaceous mélanges in Shimanto accretionary complex (AC), and the other is the Jurassic mélanges in Mino accretionary complex (AC). The mélange of Shimanto AC is mainly caused by a single tectonic process in a convergent margin. Since the structural contrast is clear between these mélanges and their surrounding coherent units, a single unit of mélange is very distinct in Shimanto AC. The Oki and Mugi mélanges are good examples. On the other hand, the mélanges of Mino AC were formed by multiple processes such as sedimentary, diapiric and tectonic. It shows block-in-matrix texture in the outcrops, including exotic blocks of limestone, basalt and chert which were derived from ocean plate stratigraphy that were formed far from the trench. However, the Mino AC includes mélange-like parts, broken formations or coherent formations which were caused by multiple accretionary processes. Therefore, mélanges of Mino AC are sometimes too small based on the established definition of a mélange. Festa et al. (2012) recommended the use of the terms small-scale or meso-scale mélanges for such kinds of mélanges. The structure of mélanges of Mino AC is similar to the one drawn by Cowan (1985). Although the mélange unit in the outcrop scale is too small for the mélange definition, the Mino AC yields the mélange texture as a whole, and is big enough to be considered as mélange based on the definition. It would be confusing if we apply the same term “mélange” for the chaotic units at different scales in a single AC. In this study, the term”mélange complex” for the chaotic Mino AC is proposed.