Subsurface Structure of the "Petit-spot" Intra-plate Volcanism, in the Northwestern Pacific

Abstract

Single-channel seismic reflection surveys were conducted in the northwestern Pacific to investigate subsurface structure and nature of volcanic eruption of small volcanic knolls considered to be formed by petit-spot intra-plate volcan- ism, and also morphology of a sedimentary basin and oceanic basement around the knolls in the area. Seismic profiles reveal the sedimentary layer is acoustically transparent, having some horizontal reflectors within the layer. Oceanic crust of the Pacific Plate lies flatly beneath the sedimentary layer with nearly constant thickness of ~200-300 m. The layer beneath the small knolls is acoustically opaque, possibly caused by inhomogeneous structure due to feeder dykes or cryptodome and vol- canic deposits. Strong reflectors at base of the sedimentary layer, identified in the vicinity of small knolls, are probably re flec- tions from volcanic sills. The oceanic crust beneath the small knolls is depressed by weight of the volcanic deposits.