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

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 1 sigma (YC1 sigma) or 2 sigma (YC2 sigma), and youngest cluster age estimated using the Mixture Models (auto) of the DensityPlotter program (YCM). The YSG and YC1 sigma 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 YC2 sigma and the true depositional age, but YCM is clearly older. For Turonian-Maastrichtian strata, which were deposited during a period of strong magmatism, YC2 sigma and YCM correspond to the true depositional ages, and there is no age gap between YC1 sigma and the true depositional age. However, YSG is clearly younger than the true depositional age for Turonian-Maastrichtian strata. These results indicate that YC1 sigma and YC2 sigma 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.

Automated summary

The U-Pb ages of detrital zircons from the Cretaceous Shimanto accretionary complex in southwestern Japan were compared with true depositional ages inferred from radiolarian fossils. The youngest detrital zircon ages obtained from trench-fill sandstones showed good agreement with true depositional ages for Albian-Cenomanian strata, but discrepancies were found for Turonian-Maastrichtian strata. YC1 sigma and YC2 sigma methods showed the closest correspondence to the true depositional ages.