Petrological and Geochemical Evidence for a Subducted Off-Axis Seamount Chain Beneath the Japan Trench Plate Boundary Fault Zone

Understanding the structure of the incoming Pacific Plate is essential for reconstructing the tectonic evolution of the Japan Trench. International Ocean Discovery Program Expedition 405, “JTRACK,” drilled through the frontal prism and plate boundary fault zone of the Japan Trench and into the underlying Pacific Plate at Site-C0019, located close to the hypocentre of the 2011 Mw 9.0 Tohoku‑Oki earthquake. Here, we present new petrographic and geochemical results from five basaltic samples recovered from below the plate boundary fault zone (930–946 mbsf), which show the first direct geological evidence for a subducted volcanic body beneath this segment of the margin.Core and image logs from Expedition 405 indicate that the drilled interval comprises a repeated sequence of sheeted dykes, massive basalts, and pillow lavas, with at least one interbedded sedimentary horizon separating the successions. This architecture is incompatible with simple ocean-plate stratigraphy and instead indicates a later phase of volcanic activity interacting with pre-existing crust. Of the five samples analysed, one was taken from below the intercalated sedimentary horizon and four from above it. The four samples from above the sediments are systematically more evolved than the sample below. In the samples above, plagioclase shows anorthitic cores overprinted by albitised rims, pyroxenes are Mg-rich, and ilmenite is abundant. Several samples contain K-rich clays, disseminated sulphides, and Zr-rich domains, including possible baddeleyite. These features suggest interaction with sedimentary cover and progressive evolution of magma chemistry during the later stages of emplacement, alongside post-emplacement hydrothermal alteration. Whole-rock major and trace element data show enrichment in incompatible elements (K, Pb, Cs, Rb, Ba, Th, U) relative to typical MORB. K-metasomatism discrimination diagrams indicate that these signatures are not produced by alteration, supporting a primary magmatic origin. Together, the mineralogical and chemical characteristics point to a volcanic body formed off-axis on the Pacific Plate rather than at a spreading ridge.Integration with seismic interpretations suggests that this volcanic body, likely a seamount chain, was faulted prior to subduction and now lies partially perched on a horst, forming a structural high directly beneath the plate boundary fault zone. These results provide the first physical confirmation of a subducted, faulted seamount chain beneath the Japan Trench, offering new constraints on the structure and evolution of the incoming Pacific Plate.