Modelling of noncylindrical folds
Final Report Abstract
In the present project ‘Modelling of non-cylindrical folds’ we carried out experimental studies to constrain the geometrical parameters of non-cylindrical folds. The experiments were focusing on inplane shortening of a competent non-linear viscous layer inside a non-linear or linear viscous matrix under bulk pure constriction. It has been shown that quantification of the geometrical parameters of non-cylindrical folds is not possible using conventional methods, but requires a procedure operating in 3D. Such a new procedure, which includes both new software and a new workflow, has been developed and tested in the present project. In cases of low viscosity ratio between a competent non-linear viscous layer and an incompetent nonlinear viscous matrix, layer-parallel shortening under bulk pure constriction results in dome and basin structures without attaining a regular egg carton pattern. Although these non-rising domes and basins do not move in the vertical direction, they are extremely dynamic structures. Their geometry is changing throughout the entire growth period, and a dominant wavelength is not attained. Even if the magnitude of constrictional strain is very high, such non-rising domes and basins will never attain high amplitudes, because they are overprinted by a second generation of hair-pin type folds, the latter with curved hinge lines and steep axes. Radial in-plane shortening of a horizontal competent layer inside an incompetent matrix under bulk constriction and continues upward flow of the deforming material results in a superior (sample-scale) dome, which is affected by numerous smaller domes and basins (dome-in-dome structures). As during upward flow the bulk strain of the competent layer is accommodated not only by the small-scale domes and basins, but also by the superior dome, the growth rate of rising domes and basins is much lower than the growth rate of non-rising domes and basins, which result from bidirectional flow. In cases of rising structures, both the marginal drag of the layer and the superior dome result from a gradient in the velocity of upward flow of the rock analouge, which is fast in the centre but reduced at the margins due to friction and simple shear. Similar lateral gradients in strain rate should be present in natural domes. Dome-in-dome structures produced in the present project may occur in natural rocks, particularly in the lower part of salt and gneiss domes, where a competent layer is constricted and buckled inside a rising viscous rock matrix. Further constrictional experiments, based on a unidirectional flow regime, are required to show if the domes and basins will be overprinted by hairpin type folds or by constrictional curtain folding and coeval boudinage at very high strain (eY = eZ >> 40%), and to show the degree of layer thickening and the mode of wavelength selection in cases of higher viscosity ratios between layer and matrix. To show the difference in geometrical parameters, particularly of the normalized amplitude and arclength, between 3D structures (domes and basins) and 2D cylindrical folds, both structures should be modeled using the same rock analogues for layer and matrix.
Publications
- (2019) Formation of dome-in-dome structures: Results from experimental studies and comparison with natural examples. Journal of Structural Geology 118 324–339
Zulauf, G.; Zulauf, J.; Thiessen, A.; Hattingen, E.
(See online at https://doi.org/10.1016/j.jsg.2018.11.008) - 2016. Formation of dome and basin structures: Results from scaled experiments using non-linear rock analogues. J. Struct. Geol. 90, 1-14
Zulauf, J., Zulauf, G., Zanella, F.
(See online at https://doi.org/10.1016/j.jsg.2016.07.001) - 2017. Quantification of the geometrical parameters of non-cylindrical folds. J. Struct. Geol., 100, 120-129
Zulauf, G. Zulauf, J., Maul., H.
(See online at https://doi.org/10.1016/j.jsg.2017.06.001)
