Search

The Mesozoic geological history of proto-Japan holds a crucial role in understanding the tectonic evolution during this time in East Asia, as a whole. In this context, the evolutionary history of the Median Tectonic Line (MTL) is important but still under debate. Most of the studies on the MTL were carried out on well-preserved clear zonally distributed areas such as the central Shikoku Island, Southwest Japan. However, these clear zonal arrangements are absent and several tectonic units including the MTL appear in close proximity at the westernmost Shikoku Island and the Maana Belt occurs here in a restricted area. Such unclear arrangements of lithounits pose a difficulty for a comprehensive understanding of the large-scale tectonic evolution of Southwest Japan. Hence, the study on and along the Maana Belt is crucial to unravel the holistic tectonic evolution of the MTL. The zircon age dating of the tonalites, geochemically of volcanic arc affinity, revealed 470.6 ± 3.2 and 468.3 ± 2.7 Ma as a magmatic age, which confirmed them as members of the Kurosegawa tectonic zone. In contrast, the scarce chemically-altered leucocratic mylonite shows the zircon age of 151.8 ± 1.8 Ma as a possible timing of magmatism. This may be the exotic block because its age is known as a magmatic hiatus in Southwest Japan. The detrital zircon dating of three (meta-)sedimentary rocks along the Maana Belt confirmed that they can be attributed to the Northern Chichibu Belt or the Kurosegawa tectonic zone. Newly characterized rock unit (= the Northern Chichibu Belt with the Kurosegawa tectonic zone) is in contact with the mid-Cretaceous igneous province of the Oshima metamorphic rocks. The suggested juxtaposition in the present study area is a remnant of the original settings including the initial phase of the MTL but is completely disrupted by the exhumation of the Sambagawa Belt in major parts of the Shikoku Island. Identifying this original setting provides not only the initial arrangement of different lithounits but also a clear understanding of the preexisting large-scale tectonic modification systems at the margin of the East Asian continent. [Display omitted] • Precise distribution of particular terranes in western shikoku was revealed. • Terrane alignment at the latest Cretaceous in Southwest Japan was revealed. • Sinistral faulting stage MTL was disappeared by the exhumation of Sambagawa Belt. • Large scale strike-slip fault occurred at most oceanic side of the East Asia. [ABSTRACT FROM AUTHOR]

• Tsuwano Complex forms the westernmost end of the Permian Maizuru Terrane, Japan. • This newly discovered basement rock complex formed between ca. 2.7 Ga and 1.85 Ga. • Early volcanic arc related granitic crust was formed between ca. 2.7 Ga and 2.5 Ga. • Coeval granitoid magmatism and metamorphism occurred at ca. 1.85 Ga. • Deep to shallow crust (quartzite <2.4 Ga) correlates to Asian continental neighbors. A new rock complex has been discovered from the Tsuwano area of Southwest Japan. This granitic rock complex namely, Tsuwano Complex contains foliated granitic gneiss, massive granitoids, and metaquartzite, forming a distinct unit at the westernmost end of the east-west elongate Maizuru Terrane. The above-mentioned litho-assemblage is characteristically different in appearance from the typical rocks of the Maizuru Terrane as well as the surrounding Jurassic and Permian accretionary rocks. We report here the zircon U-Pb LA-ICP-MS geochronological data of the rocks of this Tsuwano Complex. The comprehensive geochronological data indicate that this complex is much older than the surrounding rocks of the Maizuru Terrane. Zircon cores of the granitic gneiss yield possible magmatic age at ca. 2700 and 2500 Ma, while rim parts show a possible metamorphic age of ca. 1850 Ma. Associated dioritic to tonalitic granitoids also show a comparable age of crystallization at ca. 1850 Ma, indicating Neoarchean granitic rocks suffering a thermal overprinting and associated granitoid magmatism at Paleoproterozoic era. The exposed metaquartzite also yielded Neoarchean detrital zircon grains with the calculated maximum age of deposition at ca. 2430 Ma. While, all the other studied rocks including the grayish granitoids, felsic tuff, and metagabbro yield Paleozoic to Mesozoic ages. This is the first discovery of a Precambrian basement rock complex from the Japanese main islands. Based on the present geochronological dataset, we propose for a correlation and paleo-positioning of these basement rocks on the East Asian cratonic margin and a possible tectonic model of its evolution. [ABSTRACT FROM AUTHOR]