Back-arc basin closure at the East Asian margin during Permo-Triassic boundary: Evidence from geochemistry and U-Pb zircon data of sedimentary breccia from Maizuru Terrane, Southwest Japan

Abstract

The paleo Maizuru back-arc basin, which was located at the East Asian continental margin opened in Early Permian times and closed during the Permo-Triassic (P-T) boundary. The back-arc basin sediments are exposed in the present-day Maizuru Terrane located in Southwest Japan and hold the key to understand the tectonic evolution of both this back-arc basin and that of the East Asian continental margin. The opening of the Maizuru back-arc basin was accompanied by the deposition of a massive black mudstone-dominated formation, presently recognized as the Maizuru Group. The closure of the back-arc basin which was accompanied by the deposition of coarse-grained materials such as the Tonoshiki breccia made up of differently-sized extremely angular clasts implies that the P-T boundary was a tectonically active period in the Maizuru Terrane. Petrographical and geochemical studies revealed that Tonoshiki breccia can be divided into two main types, a first type mainly made up of mafic to intermediate rock clast fragments (Type I) and a second type mostly consisting of felsic rock clast materials (Type II). Both types commonly contain massive black mudstone clasts that were derived from the underlying Maizuru Group. The U-Pb detrital zircon geochronology of those two types of breccia suggests that they were deposited during two phases of the tectonic activity-induced debris flow that occurred at ca. 259 Ma and ca. 251 Ma for the first and second type, respectively. Type I was sourced from the Southern zone, whereas Type II was sourced mostly from the Northern zone of the Maizuru Terrane. This switch of provenance testifies to the intense tectonic activity that reigned during the P-T boundary. The closure of the Maizuru back-arc basin was induced by the subduction of the basement crust of the basin under the East-Asian continent during the P-T boundary.