oceanic trench area marginal to a continent in which, according to the theory of plate tectonics, older seafloor underthrusts the continental mass, dragging downward into the Earth's upper mantle the accumulated trench sediments. The subduction zone, accordingly, is the antithesis of the mid-oceanic ridge; (oceanic ridge) new seafloor is generated from the upper mantle at the mid-oceanic ridges, spreads laterally outward, and is eventually subducted, or consumed, at the margins of ocean basins.