Koji Wakita

Tectonic processes, from rifting to collision via subduction, in SE Asia and the western Pacific: A key to understanding the architecture of the Central Asian Orogenic Belt

We review the processes of accretion of continental blocks during the Tertiary in SE Asia and the western Pacific with the aim of better understanding the evolution of the Central Asian Orogenic Belt, which is a Neoproterozoic to mid-Phanerozoic orogenic collage surrounded by the East European, Siberian, Tarim, and North China cratons. In the western Pacific, there is abundant evidence of sequential plate-tectonic processes from accretion to continent-arc/continent collision, via exhumation and suprasubduction. Early processes involve seafloor spreading, subduction, accretion, arc formation, and back-arc extension. Two important types of tectonic setting and evolution are recognized along the present Pacific convergent margin: sediment/crust accretion and tectonic erosion. Five major accretionary complexes are well illustrated in the Japanese Islands. Tectonic erosion removes material by underscraping the lower part of the upper plate. Subduction is also associated with back-arc extension, particularly in Indonesia and the SW Pacific region. Arc-arc collisional complexes are present in Taiwan, the Philippines, and Japan. The geological record of SE Asia and the western Pacific provides a robust modern analogue for the geological and tectonic history of the Central Asian Orogenic Belt, before it was affected by high-temperature metamorphism.

OPS mélange: a new term for mélanges of convergent margins of the world

Ocean plate stratigraphy (OPS) is essential to understanding accretionary wedges and complexes along convergent plate margins. Mélanges within accretionary wedges and complexes are the products of fragmentation and mixing processes during and following OPS accretion. A new term, ‘OPS mélange’, is proposed here for mélanges composed mostly of blocks of OPS with an argillaceous matrix, and for a mixture of mélanges of multiple origins with either broken or coherent formations. An OPS mélange results from the fragmentation and disruption of OPS, without admixing of other components. Three major types of OPS mélange can be distinguished on the basis of their components: turbidite type, chert–turbidite type, and limestone–basalt type. These three types potentially form similar mélanges, but they are derived from different parts of the OPS, depending on the level of the decollement surface. The concept of ‘OPS mélange’ can be applied to most of the mélanges in accretionary prisms and complexes worldwide. In addition, this proposal recognizes a distinction between processes of fragmentation and mixing of OPS components, and mixing of ophiolite components, the latter of which results in serpentinite mélanges, not OPS mélanges. Mélanges composed of OPS sequences occur worldwide. The recognition of OPS mélanges is a key aspect of understanding tectonic processes at convergent margins, which result in mélange formation in orogenic belts globally.

Geology of the Japanese Islands: An Outline

The geological history of the Japanese Islands began from the breakup of the Super-continent Rodinia about 750 Ma. After a passive margin setting of about 250 million years, the Paleo-Pacific Ocean started to subduct beneath the Paleo-Asian continent about 500 Ma. The tectonic setting of this archipelago is an active convergent margin, where an oceanic plate has been continually subducting for over 500 million years. Plate subduction formed accretionary complexes based on sediment supply from mountains developed in the active continental margins. The accretionary complex is characterized by “ocean plate stratigraphy” which is basalt, limestone, chert, siliceous shale, and terrigenous turbidite in the ascending order. Non-metamorphosed accretionary complexes range from Carboniferous to the present in age. The accretionary complexes are metamorphosed into metamorphic rocks of low-temperature and high-pressure type and of high-temperature and low-pressure type. Extensive igneous activities occurred during Cretaceous to Paleogene, and arc volcanism has been very active throughout Cenozoic. The major tectonic events in Cenozoic are back-arc spreading and arc–arc collision. These events formed the Sea of Japan as a back-arc basin and a complicated island arc system around Japan. Plate subduction also causes frequent earthquakes and volcanic activities in the Japanese Islands. Although Japanese Islands suffer various geological hazards, these geological processes are fundamental for the formation of land for us to live on and for beautiful sceneries like Mt. Fuji.