User:Kate.broo/Fold and thrust belt

Fold and thrust belt
A fold and thrust belt or fold-and-thrust belt is a series of mountainous foothills adjacent to an orogenic belt, which forms due to compressional tectonics. Fold-and-thrust belts commonly form in the forelands adjacent to major orogens as deformation propagates outwards. Fold-and-thrust belts usually comprise both folds and thrust faults, commonly interrelated. Several applied geological science disciplines have been used to construct both quantitative and qualitive models. These models can include structural, tectonic, stratigraphic, and petrological factors surrounding the mechanics and dynamics of fold-and-thrust belts. Fold-and-thrust belts are widely researched by structural geologist due to their influence on crustal shortening and geophysical deformation.

Types
Fold-and-thrust belts are known to be geological diverse and range in regional and tectonic characteristics. Structural interpretations are used to reconstruct various fold-and-thrust belt characteristics. Fold-and-thrust belts can be found in orogenic belts dominated by compressional tectonics, plate collision boundaries, rift faults and boundaries , subduction zones , and intraplate boundaries.

Foreland fold-and-thrust belts
Foreland fold-and-thrust belts form on the external boundaries of orogens and commonly present a wedge geometry in cross-sectional view. One of the most widely known foreland fold-and-thrust belts is known as the Cordilleran foreland fold-and-thrust belt that extends along the west coast of the North American plate boundary spanning from southern Canada to central Utah. This belt, like many foreland fold-and-thrust belts, developed through deformation due to extensional wedging and formed a decreased thickness towards the foreland after depositing over a metamorphic and igneous basement.

Crystalline thrusts
Crystalline thrusts are fold-and-thrust belts that form on the thick, external sides of orogenic belts that involve metamorphic and/or igneous rocks that have formed under ductile-brittle transitions. Famous examples of these thrusts include both the Alps and Appalachians regions that can be considered some the largest crystalline thrust sheets (CTS) in the world. Crystalline thrust sheets range in variable including; ophiolites, tectonic slides, basement uplifts, composite thrust sheets, and thin-skinned, basement-cover sheets. Models of these CTS show that their thickness is most likely developed based on the geothermal gradient and crustal lithology of a given area.

Thin-skinned fold-and-thrust belts
Thin-skinned fold-and-thrust belts are those that have experienced a major amount of shortening resulting in a thickened wedge and weaker basal layer. Deformation in these belts are dependent on the resistance between the mass of the sediment of the belt and the basal detachment. These fold-and-thrust belts are often seen forming near water-saturated shale or salt horizons. This leads to basal layers commonly composed of evaporites or shale.

Mechanics
The mechanics of fold-and-thrust belts are commonly analyzed through the use of geophysical modeling. Investigating on both tectonically active and inactive fold-and-thrust belts have found dominate mechanical controls on growth include: erosion, syntectonic sedimentation  , thickness of basal layer of belt   , and compressional tectonics. Deformation caused by fold-and-thrust belts is commonly exhibited as wedging and visualized through the metaphor of sand being pushed uphill by a moving bulldozer.

Critical taper
To determine the angle between the base and surface of a wedge formed by the fold-and-thrust belt reaching its critical threshold, also known as the critical taper, structural geologist use the models based on the Coulomb criterion and pore-fluid pressure effects. These principles allow geologists to investigate shear traction at failure found in the wedge and build analogue models to represent these fold-and-thrust belts.

Geometry[edit]
Fold and thrust belts are formed of a series of sub-parallel thrust sheets, separated by major thrust faults. The two basic models for deformation. The first is deformation advancing from the interior of a mountain belt outward; this is known referred to hinterland to foreland. The second model of deformation is a sole thrust into the foreland. As the total shortening increases in a fold and thrust belt, the belt propagates into its foreland. New thrusts develop at the front of the belt, folding the older thrusts that have become inactive. This sequential propagation of thrusts into the foreland is the most common. Thrusts that form within the belt rather than at the thrust front are known as "out-of-sequence". The geometry of a fold-and-thrust belt are often investigated to understand the belt's deformation conditions and tectonic evolution.

Out-of-sequence Thrusts
Out-of-sequence thrusts do not follow the typical foreland propagating or in-sequence deformation patterns. Instead, these thrust form both hindward of the thrust as well as develop sequences of breaching thrust sequences from foreland to hinterland. Out-of-sequence thursts are often have two end members that are commonly identified as 1) a reactivated, older in-sequence now forming the out-of-sequence thrust and 2) a younger thrust cutting through the older endmember. Four causes associated with out-of-sequence thrusts include 1) deformation sequence in overriding continental plate 2) local obstacles to thrust propagation 3) synchronous thrusting and 4) maintaining critical taper.

Map view[edit]
In map view, fold and thrust belts are generally sinuous rather than completely linear. Where the thrust front bulges out in the direction of tectonic transport, a salient is formed. Between the bulges the areas are known as recesses, reentrants or sometimes embayments.

Thrust belts[edit]
Profile through the Pyrenees. In the south a fold and thrust belt exists as sediments are folded and stacked (thrust) on top of the other. An example of thin-skinned thrusting in Montana. The white Madison Limestone is repeated, with one example in the foreground (that pinches out with distance) and another to the upper right corner and top of the picture.

North America[edit]
Much of this table is adapted from Nemcok et al., 2005