Fold mountains

Zagros Mountains,seen from space.

Fold mountainsare formed by the effects offoldingon layers within the upper part of theEarth's crust.Before the development of the theory ofplate tectonicsand before the internal architecture ofthrust beltsbecame well understood, the term was used to describe most mountain belts but has otherwise fallen out of use.

Formation

Fold mountains form in areas ofthrust tectonics,such as where twotectonic platesmove towards each other atconvergent plate boundary.When plates and the continents riding on themcollideor undergosubduction(that is – ride one over another), the accumulated layers of rock may crumple and fold like a tablecloth that is pushed across a table, particularly if there is a mechanically weak layer such assalt.Since the less densecontinental crust"floats" on the densermantlerocks beneath, the weight of any crustal material forced upward to form hills,plateausor mountains must bebalanced by the buoyancy forceof a much greater volume forced downward into the mantle. Thus the continental crust is normally much thicker under mountains, compared to lower-lying areas.Rock can fold either symmetrically or asymmetrically. The upfolds areanticlinesand the downfolds aresynclines.Severely folded and faulted rocks are callednappes.In asymmetric folding there may also be recumbent and overturned folds. The mountains such formed are usually greater in length instead of breadth.

Examples

• TheJura mountains– A series of sub-parallel mountainous ridges that formed by folding over aTriassicevaporitedecollementdue to thrust movements in theforelandof theAlps
• The 'Simply Folded Belt' of theZagros Mountains– A series of elongated anticlinal domes, mostly formed asdetachment foldsover underlying thrusts in the foreland of theZagros collisional belt,generally above a basal decollement that formed inevaporiteof the lateNeoproterozoictoEarly CambrianHormuz Formation
• The Akwapim-Togo ranges inGhana
• TheRidge-and-Valley Appalachiansin the eastern part of United States.
• TheOuachita Mountainsof Arkansas and Oklahoma.

See also

• Fault-block mountain– Mountains formed by vertical or tilt displacement of fault blocks
• Mountain formation– Geological processes that underlie the formation of mountains
• Aravalli Range– Mountain range in western India