Tectonic evolution of Southwest Japan at the Cretaceous time inferred from the zircon U–Pb geochronology along the Maana Belt, western Shikoku, Japan

Abstract

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.