A post-orogenic collapse is the loss of height and lateral spread out of mass of an orogen as consequence of the cessation or overcoming of the tectonic forces that formed the orogeny. It has been argued that extension during orogenic collapse is a more effective mechanism of lowering the mountains than erosion.
The Scandinavian Caledonides is an example of an orogeny and mountain chain that reached heights of 8–9 km and then collapsed in the Devonian. The collapse was such that the modern Scandinavian Mountains do not owe their height to the former orogeny but to other processes that occurred in the Cenozoic.
References
Post-orogenic collapse Wikipedia(Text) CC BY-SA