sentences:  About 15 Ma, the northernmost IBM began to collide with Honshū, probably as a result of new subduction along the Nankai Trough.A new episode of rifting to form the Mariana Trough back-arc basin began sometime after 10 Ma, with seafloor spreading beginning about 3–4 Ma.Because disruption of the arc is the first stage in forming any back-arc basin, the present Mariana arc volcanoes cannot be older than 3–4 Ma but the Izu-Bonin volcanoes could be as old as ~25 Ma.