Imprints of Cretaceous magmatism on the oldest Pacific lithosphere: evidence from seismic anisotropy

Hotspot chains and Cretaceous large igneous provinces (LIPs) in the southern Pacific are spatially associated with the South Pacific Superswell and have been linked to the possible presence of a “superplume” in the deep South Pacific mantle, potentially rooted near the Pacific LLSVP at the core–mantle boundary. Compared with the long-lived, age-progressive Hawaiian-type chain, many South Pacific intraplate volcanic chains appear short-lived and/or discontinuous, which is inconsistent with key assumptions of the classical Wilson–Morgan hotspot hypothesis. Nevertheless, geophysical observations remain sparse, limiting our understanding of plate thermal evolution and the underlying mantle dynamics. To decipher the impact of Cretaceous magmatism and to further improve our understanding of the thermal evolution of oceanic plates, we constrained the lithospheric seismic structure using data recorded by ocean-bottom seismometer arrays. As part of the Pacific Array, an ongoing transnational collaboration, the Oldest-2 deployment was jointly carried out by research teams from Taiwan and Japan. We integrated Oldest-1 data to expand the spatial coverage across the oldest Pacific seafloor, sampling the Magellan Seamount and two adjacent Large Igneous Provinces, the East Mariana Basin and the Pigafetta Basin. We applied the ambient noise tomography method to constrain the three-dimensional isotropic and anisotropic shear-wave velocity structure of the oldest Pacific lithosphere. The resulting radial anisotropy exhibits distinct characteristics between the Magellan Seamount and the two adjacent LIPs. The seamount shows strong radial anisotropy from the crust down to ~30 km depth, indicating well-developed, horizontally oriented crystallized sills. In contrast, the LIPs exhibit negative radial anisotropy within the crust and uppermost mantle. We interpret this anisotropic signature as reflecting former magma conduits, where large volumes of magma were transported vertically from deeper sources to the surface over a relatively short timescale. These findings suggest that, although the seamounts and LIPs beneath the southern Pacific seafloor were likely formed by secondary magmatic sources, the oceanic plate has remained affected by these magmatic processes and continues to preserve clear seismic signatures of such activity, providing valuable observational constraints on the oceanic lithosphere–asthenosphere system.