什么是 Frontal accretion, tectonic underplating？

Frontal accretion is the addition of material to a wedge at its toe, as the décollement propagates into the undeformed sediments of the foreland and a new thrust slice is cut. Tectonic underplating is the addition of material to the base of the wedge, behind its toe, as slices of the lower plate are detached and stacked beneath it, often as duplexes.