STRUCTURAL GEOLOGY
Lecture 29
Fold-Thrust Belt Deformation
(Overthrust Belts Reexamined)

Overthrust terrains can be divided into two general classes:  1.)  Those where the decollement surface is parallel to bedding within the sedimentary section.  The central Appalachian Valley and Ridge and the Canadian Rockies are representative examples.  2.)  Those terrains where a horizontal thrust sheet develops in crystalline basement and breaks upward through the sedimentary overburden.  The Wyoming Province of the U.S. Rockies is a type example of structures in sedimentary rocks developing over crystalline upthrusts.  The Brevard fault zone in the southern Appalachians is believed to be another example.

(Fig. 29-1)

 Three general classes of thrust faults develop within thrust sheets whose decollement is within the sedimentary section.  Geiser (1986) has labeled these structures as: Break Thrusts, Fold Thrusts, and Shortening Thrusts (Fig. 29-1).  Each of the three has a different mechanical behavior as explained below.
Break thrusts develop where the Coulomb-Mohr failure criterion is favored over folding and shortening.  Fold-thrust belts developed in a thick but reasonably homogeneous stratigraphic section seem to favor the break thrust style.  The southern Appalachians is one example of the break thrust style.  This style seems to be the most common throughout the upper crust.
Fold thrusts develop where the viscosity contrast between a sheet of sediments (¥) and the underlying rock (h) is large.

	h  >  h1                                                                           

 In this case the Euler Buckling Force is relatively low conpared with the Coulomb-Mohr failure criterion.

	 πEI/L2  <    S0 + µisn                                                         

A fine example of fold thrusts may be found in the Swiss Jura where thin box folds form over a Mesozoic salt.
Shortening thrusts develop in association with volume loss strain.  An example is the Appalachian Plateau where the plateau is folded slightly over blind thrusts but the dominant mechanism for absorbing the decollement slip  layer parallel shortening.  The mechanism for shortening is the development of solution cleavage where the dissolved rock is carried out of the section in circulating water.  Shortening occurs because the yield stress for pressure solution cleavage is less than the Euler Buckling Force and the Coulomb-Mohr failure criterion

	syield   <   πEI/L2

	syield   <    S0 + µisn

(Fig. 29-2)
In map view the structural style of the Appalachians changes from the Central to Southern Appalachians (Fig. 29-2).  In the Central Appalachians the stratigraphic section breaks into two mechanical units:  the lower carbonates are mechanically strong relative to the cover of clastic rocks.  this difference in strength is manifest by blind thrusting where the carbonates duplex below a roof thrust.  The cover folds in response to the change in shape of the underlying duplex.  In contrast, the southern Appalachians consist of a suite of emergent thrusts.  Here the cover is relatively thinner.  This decrease in thickness of the cover is thought to be responsible for the lack in development of a roof thrust and subsequent differential behavior between the basement and cover rocks.
When crystalline rocks break through cover, the folds that develop within cover are called drape folds.  This indicates that the folding is not in response to end loading but rather the folds develop as passive sheets which fall into place over the changing shape of basement rock.  The force for this type of folding derives from the change in shape of the basement as it is uplifted on listric shaped reverse faults. 
In terrain where crystalline thrusts break to the surface two types of behavior are known for the cover rocks (Fig. 29-3).  If the cover rocks consist of thick sections of carbonates and shale, then the drape folds break into blocks which slide on decollement (detachment) surfaces.  If the cover consists of thin clastic sections without an obvious detachment layer, then the cover thins over the crystallin upthrust.  The mechanism for this thinning is often cataclastic flow.

(Fig. 29-3)