Analysis of rotation of folds during progressive deformation

Abstract

In the course of progressive deformation, fold hinges inclined to the principal strain axes undergo rotation within the X—Y plane toward the X axis (X > Y > Z). The amount of rotation depends on the initial angle of divergence from Y (ψ 2 ), and the nature and magnitude of the strain. Initial values of ψ 2 as low as 5° can produce significant deviation of the fold hinges at the level of strain attending orogenic folding, and the initial variability of fold hinges can be magnified to complex patterns. This rotation has been computed as a function of initial hinge orientation and the magnitude and type of strain. Where fold hinges were initially parallel or close to Y, they undergo small rotations, even for large strains. In host folds with assemblages of incongruous parasitic folds, the high strains in those limbs that approach parallelism with the axial surface are associated with dramatic rotations of initially divergent parasitic fold axes. In the crests of the host, however, lower strains and smaller initial deviation result in little change in the minor fold orientation. In this way the characteristic congruous and incongruous patterns of aberrant folds result. Slight periclinal form in the initial folds provides a range of divergent axial orientations relative to the Y axis. In the course of subsequent straining, this variability becomes magnified, and a marked degree of noncylindricality may result. In regions of recumbent folding, the aerial extent of limbs approaching the orientation of the axial surface and exhibiting high layer-parallel extension predominates over hinge areas. In consequence, the parasitic folds in these limbs can develop the widespread patterns of divergent axes which are characteristic of such terranes.