ISOTOPIC VARIATIONS IN DETRITAL (ΕHF) AND WHOLE ROCK (ΕND) RECORDS ALONG RETREATING ACCRETIONARY MARGINS: INSIGHTS FROM SOUTHERN PATAGONIA AND JAPAN

Retreating accretionary margins, where long-term subduction and slab retreat result in upper plate extension and formation of backarc basins, are common on Earth's surface, notably in the Pacific plate margin and the Mediterranean Sea. Yet their evolution in the geological record is relatively difficult to reconstruct due to deformation and erosion during basin closure, subduction, and accretion. Here, we investigate detrital isotopic trends along retreating margins using U-Pb geochronology and whole rock (εNd) and zircon (εHf) isotopic signatures. Existing models suggest increasing isotopic values as the arc moves away from the craton due to slab retreat and backarc extension, while backarc closure leads to more enriched values from crustal thickening. However, the timing and magnitude of isotopic changes along the basin axis in retreating accretionary margins are unknown. We hypothesize that the timing and relative magnitude of isotopic changes along the basin axis are linked to diachronous opening and relative extension. To test this, we analyze two datasets of whole rock (εNd) and zircon (εHf) isotopic signatures from the Jurassic-Early Cretaceous Rocas Verdes Basin (RVB) in Patagonia and the Western Coast of Japan (WCJ) as a modern analog. By dividing the data into N-S sectors, compiled and new results reveal distinct isotopic responses in the RVB and WCJ regions. In the RVB region, the positive isotopic response initiates earlier in the South and later in the North, matching the well-known evolution of the basin. However, in the WCJ region, the onset of a juvenile isotopic trend is evident mainly in the South, whereas in the North, eNd isotopic signatures in arc rocks remain relatively unchanged during the opening of the Sea of Japan. With respect to the magnitude of the isotopic fractionation in both RVB and WCJ regions, a more pronounced juvenile increase occurs where the magnitude of extension is greatest. Our findings support the hypothesis that isotopic variations along retreating accretionary margins may be related to diachronous opening and the relative extension along the strike of the margin. Linkages between the detrital isotopic signatures and the timing and relative magnitude of backarc extension could be leveraged in other ancient retreating orogens.