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In the study of fold patterns in relation to basement tectonics, the experimental-model approach followed by Mead, Cloos, and others has been elaborated in the development of relevant kinematic models. These models are displayed in motion and have the advantage of arbitrary mechanical reversibility, as well as the capability of showing the effects of deformational influences in both the "before" and "after" stages.
In the application of model results to field examples, the influence of transcurrent basement shearing is invoked to explain the development of extensively repetitive folding over large areas of layered rocks. Thus, pervasive shearing, rather than the application of external lateral stress at a distant point, produced the widely distributed deformation. In this context the kinematic models demonstrate "skin tectonics" and "wrinkle folds."
Various effects in fold patterns can be explained in terms of the varying ratio of horizontal to vertical movement in the shear systems. If the resistance of flexible layers to ductile compression is considered, it may be demonstrated that dilatational areas tend to develop at points of maximum flexure, particularly at crests and troughs of folds. Gravitational stress then augments crestal dilatations and depletes or prevents corresponding dilatations in troughs.
If differential movements occur unevenly within a wide zone of shearing, the effect will be relative reversals of movement (or of shear sense) that would not necessarily involve any actual reversals in the direction of material movement relative to geographic coordinates. This circumstance would be analogous to the manner in which a body may undergo both positive and negative accelerations while maintaining positive velocity.
The effect of relative reversals of movement is equivalent to that of oscillatory shearing, for which the terms "forward" cycle and "reverse" cycle may be used to describe the relative differential movements. A succession of oscillations results in the superimposition of two approximately orthogonal deformation ellipsoids. The ellipsoid generated by the forward cycle maintains a degree of unrecovered strain on which the cross-deformation ellipsoid of the reverse cycle is then superimposed. This process is advanced as an explanation of cross-fold phenomena in which dilatational maxima are located at the tops of domes which mark the intersection of structural cross-trends, or which are aligned en echelon along one particular trend. Where layers are steeply dipping as in an isoclinal seq ence, the cross-deformation effect is expressed in the form of steeply plunging buckles and "drag folds" with intervening tensional ruptures.
Block tectonics may be simulated by introducing discontinuities of shape and viscosity into kinematic models. In this way, oscillatory shearing may be seen to generate "intracratonic mobile belts" in which successive shear cycles produce repeated increments of unrecovered strain which are compounded within the belt. This compounding gives the effect of a deformation that is disproportionately large for the evident amount of net movement between blocks.
The worldwide prevalence of conjugate systems of crossing shear zones or cross-dislocations that lie in the quadrantal northeasterly and northwesterly directions is seen as a result of accelerations and decelerations in the rotational movement of the earth. The effects may be simulated in the appropriate kinematic model.
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