a sag in the earth's crust created by subduction in the ocean near a continent. The subduction usually creates an island arc which, with the continental shore, forms the edge of the geocline. The sag is created from a combination of back-arc spreading and the weight of sediment that accumulates in it from both the continent and the island arc. The sediments so accumulated form much of a suture zone (orogen) during any continent-continent collision from plate-tectonic motions.