A direct evidence for disturbance of whole sediment layer in the subducting Pacific plate by petit-spot magma–water/sediment interaction

Recently, small volcanic knolls known as petit-spot are highlighted from seismic viewpoint for a functional necessity to suppress the coseismic displacement during the 2011 Tohoku-Oki earthquake. To better understand physical mechanism of shallow oceanic plate disturbance by the petit-spot magma intrusion, we aim at elucidating the interaction of petit-spot magma and wet sediment in subseafloor volcanic environment. We used petit-spot peperites that are mixtures generated by mingling of magma and wet sediment. The peperites were collected at a petit-spot knoll located east of the outer trench swell in the northwestern Pacific (offshore of northern Japan). The sedimentary portions in the peperites are hardly to poorly consolidated due to heat supply from the magma body, and in places include radiolarian microfossils and zircon grains. Mineral phases of the sedimentary portions were identified to illite, quartz and hydrated sodium aluminum silicate. The radiolarian species and zircon grains were dated at ca. 10.6 to 15.6 Ma and ca. 1 to 108 Ma, respectively. The wide age range can be interpreted that whole sediments on the Early Cretaceous basaltic basement were entirely disturbed and transported to the surface via recent (younger than 1 million years) violate petit-spot magmatism. The subduction of petit-spot with annealed sediment “armor” functioned to prevent coseismic tsunamigenic slip propagation during the 2011 Tohoku-Oki earthquake as the seismic data predicted.