sentences:  The Izu interarc rifts began to form about 2 Ma.The three segments of IBM (figure to right) do not correspond to variations on the incoming plate.Boundaries are defined by the Sofugan Tectonic Line (~29°30’N) separating the Izu and Bonin segments, and by the northern end of the Mariana Trough back-arc basin (~23°N), that defines the boundary between the Bonin and Mariana segments.