Answer: 1. Where any piece of "nonsubductable" continental crust, usually thick continental crust, or volcanic-arc crust enters a subduction zone at the edge of a continent, it is thrusted onto the edge of the continent.
2. If a continent follows subducting oceanic lithosphere into a trench, the buoyant continent cannot subduct and instead collides with the volcanic arc on the overriding plate. Then, a new subduction zone forms that faces in the opposite direction. The arc transfers from one plate to another and becomes part of the continent.