Geology makes "to subduct" into an intransitive verb and a reflexive verb. The lower plate itself is the subject. It subducts, in the sense of retreat, or removes itself, and while doing so, is the "subducting plate." Moreover, the word slab is specifically attached to the "subducting plate," even though in English the upper plate is just as much of a slab. The upper plate is left hanging, so to speak. To express it geology must switch to a different verb, typically "to override". The upper plate, the subject, performs the action of overriding the object, the lower plate, which is overridden.