Integrated acoustic identification of a petit-spot volcanic field in the oldest Pacific plate

Petit-spot volcanism plays a critical role in the metasomatism of oceanic plates prior to subduction and in their recycling into the deep mantle. The extent of metasomatism depends on the number and volume of petit-spot volcanic edifices and intrusions, making precise identification of petit-spot volcanic fields essential. However, conventional methods based on seafloor topography and acoustic backscatter intensity alone have limitations in accurately delineating these features. Here we compiled acoustic survey data obtained using subbottom profilers mounted on both research vessels and submersibles in the region southeast of Minamitorishima (Marcus) Island, the oldest part of the Pacific plate. Analysis of the subbottom profiler data revealed three main stratigraphic facies: O-type (opaque), T-type (transparent), and L-type (layered). Among these types, O_L-type facies, characterized by low slope gradients and high backscatter intensity, indicate petit-spot lava flows. Additionally, M-type facies, marked by multiple discontinuous reflections, occur at boundaries between O_L-type and other facies, representing transitional zones formed by intrusive activity. High-resolution acoustic stratigraphy and ground-truth observations from submersible dives confirmed that M-type facies reflect petit-spot lava intrusions. Integrating seafloor backscatter with subseafloor stratigraphy provides a robust and reliable method for delineating petit-spot volcanism and assessing its geological impact on the abyssal seafloor.