Formation of Miocene Authigenic Carbonates Within the Shimanto Accretionary Complex, Southwest Japan

Abstract Authigenic carbonates in the exhumed Miocene Shimanto accretionary complex (the Nabae carbonates) were studied in the Cape Muroto area to better constrain the formation mechanisms and tectonic setting during carbonate precipitation. Three main types of carbonates were recognized on the basis of morphology: nodular, bedded, and tubular. The δ 13 C values of the carbonates define two distinct groups with relatively low (−31.9‰ to −20.3‰) and relatively high values (−14.7‰ to −4.7‰). The former suggests carbonate precipitation from methane‐enriched fluids, while the latter indicates mixing of fluid sources in a range of depositional settings. Most carbonates have REE + Y patterns characterized by well‐defined positive Eu and Y anomalies, which we interpret to have formed by mixing between defused hydrothermal fluids and seawater. Other samples are enriched in MREE, a signature that is found in methane‐derived cold seep carbonates. Additionally, the carbonates exhibit positive correlations between δ 13 C and various proxies for detrital input (e.g., REE + Y, Zr content, Y/Ho ratios). Based on comparing our observations with drill core and geophysical measurements from the active Nankai accretionary complex, we conclude that the Nabae carbonates were precipitated from relatively low‐temperature hydrothermal fluids or as cold seeps near the deformation front at the toe of the Shimanto accretionary complex in a high heat flow environment. As the first onshore example of ancient authigenic carbonates from an accretionary complex, the Nabae carbonates have the potential to reveal important information regarding the characteristics and distribution of hydrothermal areas in active accretionary wedges.