Seismic stratigraphy and structural characteristics of back‐arc rifts on the Izu‐Ogasawara Arc

Abstract Seven back‐arc rifts are recognized in the Izu‐Ogasawara Arc, namely, the Hachijo, the Aogashima, the Myojin, the Sumisu, the Torishima, the Sofu and the Nishinoshima Rifts from north to south. The acoustic stratigraphy is divided into three units (Units A, B and C) based on the seismic reflection profiles crossing the rifts. The structure of the rifts systematically changes from a half‐graben type to a full graben type in the back‐arc rifts from the Hachijo Rift to the Torishima Rift. The Hachijo and the Aogashima Rifts have a structure of half‐graben, and the Myojin Rift has both structural characteristics of a half‐graben and a full graben. The Sumisu and the Torishima Rifts are an asymmetric full graben. The Sofu and the Nishinoshima Rifts have different structural characteristics from the remaining rifts, from the Hachijo Rift to the Torishima Rift. The boundary faults in the back‐arc rifts from the Hachijo to the Torishima Rifts cut to Unit B. Unit B correlates with volcaniclastic sediments during pre‐rift volcanism between 4 and 2 Ma. The pre‐rift volcanism was probably widespread on the northern Izu‐Ogasawara Arc as is the present arc volcanism. These factors suggest that the beginning of rifting is dated at some time after 2 Ma. The developing process of the rift consists of three stages; (i) a sag stage in the crust at the location of the large offset boundary fault; (ii) a stage of half‐graben formation; (iii) a stage of full graben formation. The offset of the boundary faults becomes larger from the Hachijo Rift to the Torishima Rift and the east‐west width of the rifts also widens to the south. This is presumably because the Hachijo Rift is an earlier rifting stage than the Sumisu and the Torishima Rifts. The more primitive structure in the rifting stage from the Torishima Rift to the Hachijo Rift is probably caused by the propagation of rifting from south to north. The structural difference between the rifts in the northern part and the Sofu and the Nishinoshima Rifts seems to be due to structural differences in the crust between the northern and the southern parts from the tectonic gap.