Benioff Zones, Absolute Motion and Interarc Basin

While in the Honshu region, the Benioff zone exhibits a straight profile, dipping west at about 30°, the Mariana arc has a curved zone changing into a very steeply dipping zone at depth, and the Peru-Chilean arc has a very extended and gently dipping zone at 16°. These can be interpreted as three type-cases where, correspondingly, the trenchline is at rest with respect to the deeper mantle and the trenchline is moving in the dip direction with respect to the deeper mantle. Where the direction of relative motion has changed, we can then expect sharper bends as in the cases of Central Bonin and the Peru-Chilean arcs.This interpretation is consistent with the concept of the decoupling of plate motion across the asthenosphere and enables us to use the Benioff Zone to decipher the recent history of absolute plate motion.The steepening of the Marianas Zone is closely related to the westward motion of the Philippine Sea Plate. The separation of the plates due to anchoring of the trenchline, after the subducted lithosphere reaches the mesosphere, and the continued westward motion of the Philippine Sea Plate is probably responsible for the formation of the back-arc basin and the lack of large shallow thrust-type earthquakes there. This could be one of the several mechanisms for the formation of the back-arc basin.