Use of detrital zircon U–Pb ages to assess the timing of deposition of Cretaceous trench-fill deposits in the active continental arc along the East Asian margin

Abstract

Abstract The Cretaceous Shimanto accretionary complex in southwestern Japan consists mainly of trench-fill deposits and formed in an active continental margin setting along the East Asian margin. Here, we present U–Pb ages of detrital zircons from the complex and compare youngest detrital zircon ages obtained from trench-fill sandstones with true depositional ages inferred from radiolarian fossils to assess the accuracy of zircon ages for estimating the timing of deposition of the trench-fill sediments. Four different methods were used to calculate the youngest detrital zircon ages, giving four different measures: youngest single-grain age (YSG), weighted mean age of the youngest cluster overlapping in age at 1σ (YC1σ) or 2σ (YC2σ), and youngest cluster age estimated using the Mixture Models (auto) of the DensityPlotter program (YCM). The YSG and YC1σ show good agreement with true depositional ages for Albian–Cenomanian strata, which were deposited during weak magmatic activity, and we find no age gap between YC2σ and the true depositional age, but YCM is clearly older. For Turonian–Maastrichtian strata, which were deposited during a period of strong magmatism, YC2σ and YCM correspond to the true depositional ages, and there is no age gap between YC1σ and the true depositional age. However, YSG is clearly younger than the true depositional age for Turonian–Maastrichtian strata. These results indicate that YC1σ and YC2σ show the closest correspondence to the true depositional ages, which suggests that these two measures best represent the true depositional age of trench-fill deposits in an active continental margin.