Parasitic folds with wrong vergence: How pre-existing geometrical asymmetries can be inherited during multilayer buckle folding. (June 2016)

• Record Type: Journal Article
• Authors: Frehner, Marcel; Schmid, Timothy
• Is Part Of: Journal of structural geology. Volume 87(2016:Jun.)
• Page Start: 19
• Page End: 29
• Publication Date: 2016-06
• Subjects: Parasitic folds -- Buckle folds -- Fold vergence -- Asymmetric folds -- Structural inheritance -- Finite-element method; Geology, Structural -- Periodicals; Géomorphologie structurale -- Périodiques; Geology, Structural; Periodicals; 551.805
• DOI: 10.1016/j.jsg.2016.04.004
• Languages: English
• ISSNs: 0191-8141

Abstract

Abstract: Parasitic folds are typical structures in geological multilayer folds; they are characterized by a small wavelength and are situated within folds with larger wavelength. Parasitic folds exhibit a characteristic asymmetry (or vergence) reflecting their structural relationship to the larger-scale fold. Here we investigate if a pre-existing geometrical asymmetry (e.g., from sedimentary structures or folds from a previous tectonic event) can be inherited during buckle folding to form parasitic folds with wrong vergence. We conduct 2D finite-element simulations of multilayer folding using Newtonian materials. The applied model setup comprises a thin layer exhibiting the pre-existing geometrical asymmetry sandwiched between two thicker layers, all intercalated with a lower-viscosity matrix and subjected to layer-parallel shortening. When the two outer thick layers buckle and amplify, two processes work against the asymmetry: layer-perpendicular flattening between the two thick layers and the rotational component of flexural flow folding. Both processes promote de-amplification and unfolding of the pre-existing asymmetry. We discuss how the efficiency of de-amplification is controlled by the larger-scale fold amplification and conclude that pre-existing asymmetries that are open and/or exhibit low amplitude are prone to de-amplification and may disappear during buckling of the multilayer system. Large-amplitude and/or tight to isoclinal folds may be inherited and develop type 3 fold interference patterns. Graphical abstract: Highlights: We model multilayer buckle folding with a predefined geometrical asymmetry. If inherited, the asymmetry resembles a parasitic fold with wrong vergence. In the larger-scale fold, layer-perpendicular flattening and flexural flow occurs. Both work against the asymmetry, leading to de-amplification and unfolding. Hence, a pre-existing wrong vergence is difficult to inherit during buckling.