Fold duplexes

摘要

Fault duplexes were recognized as early as the late nineteenth century and have been identified in most fold-thrust belts. Duplexes appear to provide a mechanical advantage to the emplacement of large thrust sheets by transferring displacement upward from one detachment horizon to another. However, faulting is not the only mechanism for displacement transfer; pressure-solution cleavage and folding may play the same role. Folds or cleavage within a duplex absorb the shortening of the underlying detachment, so that the displacement on the floor thrust decreases toward the foreland. As displacement on the floor decreases, displacement on the upper detachment (roof thrust) increases. In fold duplexes, the geometries of the detachment folds vary from open, concentric and kink-band folds to vertical and recumbent isoclines. A range of duplex styles, from fault duplexes to cleavage and fold duplexes, can be found within a single a single thrust belt, and this variation raises several important questions to guide future study. They include what controls the geometry of the folds within fold duplexes; and, perhaps more importantly, what conditions (lithology, pressure, temperature, strain rate) determine the type of duplex (fault, cleavage or fold) that will develop.