Reconstructing Along‐Strike Variability of Retreating Accretionary Margins Using Detrital Isotopic Proxies in Patagonia and Japan

Abstract

Retreating accretionary systems are common features of convergent plate margins. They are characterized by upper plate extension and backarc basin formation, which often forms diachronously and asymmetrically along‐strike. This study tests the hypothesis that this along‐strike variability in backarc basin formation is reflected in detrital and igneous isotopic variations along the margin. We present new U‐Pb ages and εHf(t) isotopic values from detrital zircon, as well as a compilation of published U‐Pb ages and εNd(t) data from the Jurassic–Cretaceous Rocas Verdes Basin in southern Patagonia and the Neogene magmatic arc in the Japanese Islands. These data serve as proxies for isotopic changes in arc volcanism in response to periods of extensional and contractional regimes. In both regions, detrital zircon εHf(t) values from the retroarc basins are consistent with εNd(t) trends from igneous rocks along‐strike, demonstrating that detrital isotopic records mirror igneous isotopic variations where direct igneous data are lacking. Spatial analyses of isotopic records reflect regional variations in the onset of extension from south to north in Patagonia, with isotopic pull‐ups at ca. 176 Ma and ca. 155 Ma respectively; while in both central and northern Japan, it appears to share similar timing (ca. 20 Ma), matching independent evidence in both regions. Along‐strike variations in isotopic trends are also consistent with backarc basin geometry, as broader, longer–lasting isotopic pull‐ups correspond to wider ocean basins. Together, these results provide a spatially integrated isotopic‐geochronologic framework, which may be useful for interpreting the tectonic evolution of other retreating margins.