Radiolarian and U–Pb zircon dating of Late Cretaceous and Paleogene Shimanto accretionary complexes, Southwest Japan: Temporal variations in provenance and offset across an out-of-sequence thrust

Abstract

In this study, we describe radiolarian fossils, analyze sandstone petrography and geochemistry, and perform detrital zircon U–Pb dating on the Late Cretaceous and Paleogene Shimanto accretionary complexes. These data are used to constrain temporal variations in provenance, which are related to volcanic arc activity. We also determine the behavior of the Aki Tectonic Line, which represents an out-of-sequence thrust. Based on lithology and radiolarian assemblages, we subdivide the Late Cretaceous to early Paleocene Mugi Unit into the Mg1, Mg2, and Mg 3 subunits, and the Eocene Naharigawa Unit into the Nh1 and Nh2 subunits. Detrital zircon U–Pb ages indicate that sandstones of the Mugi Unit were sourced mainly from Late Cretaceous to Paleocene igneous rocks. Nh1 sandstone records a single peak age in the Late Cretaceous, whereas Nh2 sandstone preserves multiple peaks from the pre-Jurassic to early Paleocene. The Naharigawa Unit lacks syn-depositional detrital zircons; however, felsic tuff within Nh1 records an age of 48.7 Ma, which is consistent with radiolarian ages. The Nh1 and Nh2 sandstones contain slightly higher Ba and Rb concentrations than those of the Mugi Unit, suggesting that basement had been uplifted and eroded in the source region, prior to deposition. We conclude that temporal variations in sandstone composition within the Late Cretaceous to early Paleocene Shimanto accretionary complex resulted from tectonic events that occurred in response to syn-depositional igneous activity. We infer that the evolution of the Eocene Shimanto accretionary complex was influenced by uplift and erosion of pre-Jurassic basement. Late Cretaceous radiolarians were identified in subunit Mg3 that forms the footwall of the Aki Tectonic Line, which was previously interpreted as a boundary fault between the Late Cretaceous and Paleogene Shimanto accretionary complexes. The Aki Tectonic Line is re-interpreted as an out-of-sequence thrust (OST) that was active under low-grade metamorphic conditions. We classify several OSTs close to the Late Cretaceous and Paleogene Shimanto accretionary complexes, based on contrasts in metamorphic grades across the faults. Within the study area, the Aki Tectonic Line is classified as a Type-2 OST, indicating that it developed within the Cretaceous Shimanto accretionary complex at temperatures of 250–270 °C.