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.

日本西南部的白垩纪四万十增生复合体主要由沟渠填充物组成，形成于东亚边缘活跃的大陆边缘。在这里，我们介绍了复杂的碎屑锆石的U–Pb年龄，并将从填海砂岩中获得的最年轻的碎屑锆石年龄与从放射虫化石推断出的真实沉积年龄进行了比较，以评估锆石年龄的准确性，以估算沟槽的沉积时间填充沉积物。四种不同的方法用于计算最小的碎屑锆石年龄，给出了四种不同的度量：最小的单粒年龄（YSG），最小的簇的加权平均年龄（年龄在1σ（YC1σ）或2σ（YC2σ）重叠）和最小的群集年龄是使用DensityPlotter程序（YCM）的混合模型（自动）估算的。YSG和YC1σ与在弱岩浆活动期间沉积的Albian-Cenomanian地层的真实沉积年龄有很好的一致性，我们发现YC2σ和真实沉积年龄之间没有年龄差距，但是YCM明显更老。对于在强烈岩浆作用时期沉积的Turonian-Maastrichtian地层，YC2σ和YCM对应于真实沉积年龄，并且YC1σ和真实沉积年龄之间没有年龄差距。但是，YSG显然比Turonian–Maastrichtian地层的真实沉积年龄年轻。这些结果表明，YC1σ和YC2σ显示出与真实沉积年龄最接近的对应关系，这表明这两种措施最能代表活跃大陆边缘的沟槽填充沉积物的真实沉积年龄。