Fold development in the Jura

The term “folding” encompasses a wide range of processes, most of which are poorly understood. Jura folds, though comparatively simple, have developed by a superposition of different types of instabilities both in space and time. They are never periodic and sinusoidal and are more realistically approximated by kink bands with rounded hinges. Thrusting and kinking instabilities had closely similar thresholds, with kinks usually following and deforming thrusts. An analysis of embryonic folds shows that instabilities in the sedimentary cover were initiated primarily at inherited flaws of the basal décollement layer. They thence spread upward, often following stratigraphically higher incompetent layers in secondary décollement and there nucleating secondary instabilities before reaching the surface (disharmonic folding). Embryonic folds thus are usually narrow, emanating from secondary décollement layers that are connected with the basal décollement zone by thrusts nucleated at inherited obstacles. These are eventually overcome, permitting basal décollement to coalesce with kinking instabilities that grow downward from nuclei in higher décollement intervals. In this way folds centered in the basal décollement layer, and consequently of normal width, may be superposed on the narrow embryonic folds. The sequence and importance of the different elements may vary from place to place to result in a vast catalog of fold shapes.