File:Some Types of Orogen - Cross-section Diagrams.png

English: Cross-section diagrams of some types of orogen.

Principal features of the various types of orogens.
(A) Collision related.
Collision orogens showing delamination of the crust. Delamination, or splitting apart of the crust is typical for collisional orogens where plate convergence brings two continental crustal sections into direct contact. Here the space between the delaminated crustal layers of the lower plate is filled by crust of the opposite, upper plate. This feature, also called “crocodile structure”, is in some sort the archetype of thick-skinned tectonics. Examples include the Alps, the Himalaya-Tibet orogen and the Pyrenees. The indentation occurs in a late stage of the collision and may be caused by the buoyancy of the continental crust resisting subduction. The lower crust of the lower plate is eclogitized, thus denser and subducted into the mantle. Such eclogitic packages might not be resolved by geophysical sounding.
(B) Subduction related.
Here the growth of the orogen in the continental upper plate typically occurs hundreds of kilometers inboard of the subduction trench, independent of the subduction angle. Examples include the Andes of Peru, Chile-Bolivia and Chile-Argentina. These orogens demonstrate that the stresses leading to crustal shortening of the upper plate are transmitted over large distances, with the upper (and possibly lower) crust acting as stress guide.
(C) Rigid slab of continental crust.
Orogens where one of the continental plates is made of a rigid long slab from which the sedimentary cover is detached and telescoped as long thrust sheets. Examples include the Variscides, North American Cordillera and the Southern Appalachians. Whereas in the case of the North American Cordillera it could be argued that the rigidity of the slab is related to the greater thickness of the crust (50 km), in the Southern Appalachians only the crust beneath the Cumberland Plateau-Valley and Ridge provinces is thicker (40 km) compared to the 30 km beneath the Inner Piedmont. The crust in the Variscides has a constant thickness of roughly 30–35 km only. But this is an effect of ductile flow within a hot crustal root heated up by collision-related magmatism.
(D) Deformed continental crust.
Orogens where one or both continental plates are horizontally shortened by crustal-scale thrust faults, the cover being detached and telescoped. Examples include the Caledonides, the Alice Springs orogen of Australia, the Alps, the Apennines and the Pyrenees. These orogens share an initial crustal thickness of around 30 km. The detached sedimentary cover is displaced over much shorter distances than examples with a rigid long slab, and the contracted crust is piled up to an imbricate stack of relatively thick thrust sheets made up of upper crustal rocks (including lower crust in case of the Alice Springs orogen). In this context it is interesting to note that in the case of the orogens with rigid long slabs, thin thrust sheets of crystalline basement are thrust upon the long slab; these derive from the continental margins and thus formed from thinned continental crust. It thus can be argued that crustal contraction by thrusting is more pronounced if the crust is thin and possesses a high geothermal gradient.
(E) Basin inversion.
Orogens in which basin inversion played an important role. A prominent feature in these orogens is that steeply dipping reactivated syn-sedimentary normal faults and new thrust faults that formed in contraction lead to regional uplifts of blocks of upper crust in a thick-skinned tectonic style. Examples include the Eastern Cordillera of the Andes in Colombia, the Malargüe fold-and-thrust belt of the Andes in Argentina, the Pyrenees, the Atlas Mountains or the Klippen nappe in the Central Alps. Basin inversion may be mild or complete e.g. in the Atlas Mountains the top basement contact still forms a depression in the core of the orogen, which signifies a mild inversion. Stronger inversion occurred in the Eastern Cordillera of Colombia, where this top basement contact is uplifted to a symmetric antiform centered on the ancient basin, which itself had also been symmetric. A positive relief was also created in the Pyrenees and in the Malargüe fold-and-thrust belt. In these two examples inversion produced an asymmetric antiformal structure, which mimics the asymmetry of the original basin. In the Pyrenees it is the hyper-extended margin versus the relatively undeformed margin of the upper plate. In the Malargüe fold-and-thrust belt it is the uniform orientation of synsedimentary normal faults across the entire basin.
(F) Detached allochthonous cover.

Orogens containing truly thin-skinned fold-and-thrust belts, where the cover is detached from the crystalline basement along a décollement layer and highly allochthonous. In most cases, the décollement layer is made of evaporite and/or shale sequences. Two end-member types may be distinguished: (1) the detached cover is mechanically strong and shortened by detachment folds, as is the case in the Zagros of Iran, the Salt Range–Potwar Plateau of Pakistan and the Klippen nappe of western Switzerland, or (2) the rheological contrast between décollement horizon(s) and the detached cover is moderate and shortening is accomplished by imbricate thrusting and folding such as observed in accretionary wedges like the Makran of Iran/Pakistan. In the Zagros and Salt Range–Potwar Plateau, the detached cover is laterally homogeneous and the décollement layer is thick such that simple upright folds developed. In the Klippen nappe the detached cover is laterally discontinuous owing to synsedimentary faulting and the décollement layer varies in thickness. As a result, the structures within the detached layer are more complex, involving imbricate thrusting. Provided the décollement horizon is of sufficient thickness, steps within the units below may be overcome. Steps not higher than about half of the thickness of the décollement layer can be overcome by a detachment fault. At this point it has to be distinguished between a mechanically stiff footwall (such as crystalline basement as in Zagros and Salt Range–Potwar Plateau) and a weak footwall consisting of cover nappes (as is the case in the Klippen nappe). In the latter case, the thrust contact may be smoothened by plucking off pieces of the footwall and dragging them along the thrust fault. An example of this process is suspected along the contact of the Glarus thrust.