Formation of Young Fold Mountains

Young fold mountains are formed due to a convergent plate margin where the more dense oceanic plate is descending beneath the less dense continental plate. As the oceanic plate is subducting beneath the continental plate, rocks are scraped off the descending oceanic plate. This along with foldong of the continental crust helps to create young fold mountain chains on the leading edge of continental masses.
E.G: The Nazca plate subducting beneath the South American plate forming the Andes.