formed due mainly to the effects of folding
on layers within the upper part of the Earth
. In the time before either Plate tectonic
theory developed, or the internal architecture of thrust belts became well understood, the term was used for most mountain belts, such as the Himalayas
. The term is still prevalent in physical geography
literature but has otherwise generally fallen out of use except as described below. The forces responsible for formation of the fold mountains are called Orogenic
movements. The term orogenic
is derived from a greek word meaning mountain building. These forces act at tangent to the surface of the earth and are primarily involved in plate tectonics.
Fold mountains are generally formed in the less-deformed areas peripheral to areas strongly affected by thrust tectonics
. Typically they are found in the foreland region where a major mechanically weak decollement
horizon is present. The frontal thrust
(or thrusts) propagate(s) a long distance along this horizon and subsequent movement on the thrust can give rise to a sequence of folds as the hanging wall of the thrust effectively crumples. The anticlinal
crests may be high enough to form mountains. Most fold mountains are likely to be relatively young in geological terms as they will start to be eroded
as soon as they are formed.
- The Jura mountains - A series of sub-parallel mountainous ridges formed by folding over a Triassic evaporite decollement due to thrust movements in the foreland of the Alps.
- The Zagros mountains - A series of elongated anticlinal domes, mostly formed as fault bend folds over underlying thrusts in the foreland of the Zagros collisional belt