sentences:  In general, the oldest components are farthest west, but a complete record of evolution is preserved in the forearc.The IBM subduction zone began as part of a hemispheric-scale foundering of old, dense lithosphere in the Western Pacific .The beginning of true subduction localized the magmatic arc close to its present position, about 200 km away from the trench, and allowed the sub-forearc mantle to stabilize and cool.