Detrital zircon multi‐chronology, provenance, and low‐grade metamorphism of the Cretaceous Shimanto accretionary complex, eastern Shikoku, Southwest Japan: Tectonic evolution in response to igneous activity within a subduction zone

Abstract

Detrital zircon multi‐chronology combined with provenance and low‐grade metamorphism analyses enables the reinterpretation of the tectonic evolution of the Cretaceous Shimanto accretionary complex in Southwest Japan. Detrital zircon U–Pb ages and provenance analysis defines the depositional age of trench‐fill turbidites associated with igneous activity in provenance. Periods of low igneous activity are recorded by youngest single grain zircon U–Pb ages (YSG) that approximate or are older than the depositional ages obtained from radiolarian fossil‐bearing mudstone. Periods of intensive igneous activity recorded by youngest cluster U–Pb ages (YC1σ) that correspond to the younger limits of radiolarian ages. The YC1σ U–Pb ages obtained from sandstones within mélange units provide more accurate younger depositional ages than radiolarian ages derived from mudstone. Determining true depositional ages requires a combination of fossil data, detrital zircon ages, and provenance information. Fission‐track ages using zircons estimated YC1σ U–Pb ages are useful for assessing depositional and annealing ages for the low‐grade metamorphosed accretionary complex. These new dating presented here indicates the following tectonic history of the accretionary wedge. Evolution of the Shimanto accretionary complex from the Albian to the Turonian was caused by the subduction of the Izanagi plate, a process that supplied sediments via the erosion of Permian and Triassic to Early Jurassic granitic rocks and the eruption of minor amounts of Early Cretaceous intermediate volcanic rocks. The complex subsequently underwent intensive igneous activity from the Coniacian to the early Paleocene as a result of the subduction of a hot and young oceanic slab, such as the Kula–Pacific plate. Finally, the major out‐of‐sequence thrusts of the Fukase Fault and the Aki Tectonic Line formed after the middle Eocene, and this reactivation of the Shimanto accretionary complex as a result of the subduction of the Pacific plate.

Timescale of material circulation in subduction zone: U–Pb zircon and K–Ar phengite double‐dating of the Sanbagawa metamorphic complex in the Ikeda district, central Shikoku, southwest Japan

“…Kiminami and Ishihama () predicted the age of the protolith of the Besshi unit (= Minawa unit) to be late Albian to Campanian using the whole‐rock geochemistry of clastic rocks, and their prediction is consistent with our new data. The age population and the youngest cluster age of detrital zircons from sample IK3‐30 of the Minawa unit are similar to those of the Taniyama unit (mélange facies) and Yokoyama mélange in the Shimanto accretionary complex (Hara et al, ; Saito et al, ). Although the Minawa unit is not a mélange, its lithological assemblage, which includes trench‐fill sediments and mafic and siliceous rocks, is the same as that of the Taniyama unit and the Yokayama mélange, and it is possible that the Taniyama unit and the Yokoyama mélange are equivalent to part of the Minawa unit (Figure b).…”

“…Aoki et al, ; Manabe et al, ). It is worth noting that it is not only the depositional ages of the Koboke unit that are quite similar to those of the Hiwasa unit in the Shimanto accretionary complex, but also the detrital zircon age population (Hara et al, ). Moreover, Kiminami et al () noted the similar chemical compositions of the trench‐fill sediments in the Hiwasa and Koboke units, which indicate that the Hiwasa unit is a shallower‐depth counterpart of the Koboke unit (Figure c).…”

Natural and experimental structural evolution of dispersed organic matter in mudstones: The Shimanto accretionary complex, southwest Japan

“…The study area includes the northern section of the belt containing the Cretaceous complex. It includes seven units, from north to south: the Tochidani, Izuhara, Hinotani, Osodani, Taniyama, Hiwasa, and Mugi units based on lithology, biostratigraphy, and dating of detrital zircon (Figure a; Hara et al, ). The Tochidani, Hinotani, Osodani, and Hiwasa units contain coherent turbidite facies of massive and bedded sandstone, interbedded sandstone and mudstone, and mudstone with minor silicic tuff.…”

“…The Izuhara, Taniyama, and Mugi units are mélange facies containing basaltic blocks, chert, and red mudstone in an argillaceous matrix with broken beds of sandstone and mudstone. The Mugi unit is lithologically subdivided into mélange and phyllite subunits (Hara et al, ). The phyllite subunit forms the lower part of the Mugi mélange subunit and is highly sheared and deformed toward the Aki Tectonic Line (ATL), which has a thick shear zone of ~ 500 m width on the hanging wall (Figure ).…”

“…The IC values in the study area have already been investigated in detail (e.g. Hara et al, ) and suggested as a more robust indicator of thermal maturation than Raman analysis of dispersed OM. In contrast, the method also has limitations due to the effect of lithology, fluid activity, contamination of detrital illite owing to bulk analysis (e.g.…”