Middle Triassic and middle Permian radiolarians from the Kamitaki Complex in the Sasayama area, Hyogo Prefecture, Southwest Japan: Evidence for Triassic plate subduction along the eastern margin of Paleo-Asia

Abstract

The Kamitaki Complex, situated in the Sasayama area in Southwest Japan, has long been presumed to be a Permian subduction-related accretionary complex based on correlations from previous studies. However, because of the lack of fossil evidence, the exact age of the complex remained uncertain for a long time period. To address this gap in knowledge, a geological survey and microfossil mapping were conducted in the Kamitaki Complex to determine its age and geological context.

A geological survey revealed that the Kamitaki Complex mainly consists of clastic rocks, and a mixture of sandstone, basalt, and chert blocks within the mudstones. The Kamitaki Complex is tectonically intercalated into the Lopingian (Late Permian) accretionary complex of the Ultra-Tamba Terrane and Late Triassic accretionary complex of the Tamba Terrane. The lithological and structural characteristics of the Kamitaki Complex confirm that it is an accretionary complex. Microfossil mapping yielded depositional ages, with radiolarian fauna such as Eptingium nakasekoi, Pseudostylosphaera japonica, Cryptostephanidium japonicum, and Oertlispongus cf. diacanthus identified in mudstones suggesting an Anisian (early Middle Triassic) age. In contrast, radiolarian fauna found in cherts, including Pseudoalbaillella? aff. longicornis, and Follicucullus cf. porrectus, indicate an early Capitanian (late Guadalupian, middle Permian) depositional age. These findings suggest that the Kamitaki Complex records a trenchward migration of the oceanic plate in a pelagic environment from the early Capitanian and an accretion at the trench during the Anisian period.

Conventionally, the plate boundary between the Panthalassa and Paleo-Asia during the latest Permian to Middle Triassic was of the transform type, primarily because no subduction-related accretionary complexes from this period have been identified in the Japanese Islands. However, the discovery of Kamitaki Complex, an Anisian accretionary complex, provides evidence of Middle Triassic subduction activity along the eastern margin of Paleo-Asia. According to the internal structure and age polarity of the Ultra-Tamba and Tamba terranes, an accretionary complex developed over a prolonged period (approximately 120 million years) in a tectonic setting that persisted along the eastern margin of Paleo-Asia from the late Guadalupian to the earliest Cretaceous period.