Mobile-hinge kinking in layered rocks and models : J Struct Geol V13, N3, 1991, P243–259

Abstract

Abstract An unresolved question in structural geology is whether kink folds grow by the gradual rotation of the fold limbs about a fixed hinge, by the nucleation of kink bands that grow by fold-hinge migration or by some combination of these mechanisms. The importance of mobile-hinge kinking has previously been recognized in experimentally deformed slate and phyllite as well as decks of paper cards, but its importance in the folding of multilayers consisting of alternating competent and incompetent layers has been uncertain. This study shows that mobile-hinge kinking can be an important folding mechanism in both natural and experimental multilayers. The natural folds are in pelagic limestones from the Umbria—Marche Apennines of Italy. The geometrical properties of these folds, as well as the small-scale structures in the rocks, indicate that these folds formed by the nucleation and expansion of kink bands with mobile hinges, not by a fixed-hinge folding mechanism. The experimental folds were produced by the deformation of multilayered models consisting of layers of lead separated by layers of wax-impregnated cloth. The progressive growth of these folds by the nucleation and expansion of kink bands was directly observable. A rough estimate of the rate at which fold hinges migrated through the Apennine limestones was obtained indirectly using the displacement history recorded by calcite fibers on a fault, which cuts a kink fold and was active during the formation of the fold. It appears that the rate at which fold hinges migrated was approximately 100 times greater than the displacement rate along a fault that was moving by pressure-solution slip.