Folding and boudinage of an oblique single layer
Final Report Abstract
The present proposal was focusing on the deformation of single competent layers under bulk coaxial plane strain. We carried out three principal experimental studies during which the layer was initially rotated around one of the principal strain axes (X>Y=0>Z). The main aims of these studies were: (1) to show the impact of the degree of layer inclination on the deformation geometry of a single competent layer; (2) to show how folding will change into boudinage (and vice versa) when the layer is rotated around the principal strain axes; (3) to verify if sequential development of folds and boudins is possible under bulk plane strain. All experimental runs were carried out using non-linear viscous plasticine as rock analogue to simulate dislocation creep. In each of our principal studies, we carried out two deformation series with different viscosity ratio between layer and matrix (m = 18 and 82). Deformation was carried out at a strain rate of 5x10^-5 s^-1 until a final shortening strain of eZ = -70% was attained. The initial angle between strain axis and competent layer (θ(i)) was gradually changed by multiples of 11.25°. An additional incremental-strain study was performed to show the growth of folds and boudins with progressive strain. As most of the deformation structures are 3D objects, the geometrical analysis of folds and boudins was carried out using both classical sections and computed tomography. The first study was focusing on a competent layer that was initially rotated around the intermediate Y- axis from the YZ-plane (folds with axes parallel to the Y-axis) to the XY-plane (boudins with axes parallel to the Y-axis). Shortening at θZ(i) <30° was accommodated by layer thickening and two-stage folding. Low-wavelength F1-folds were refolded by large-wavelength homoaxial F2-folds, both with similar degree of tightness. With increasing layer obliquity, the number of folds and the degree of F2- fold asymmetry decrease, and F1- and F2-folds approximate in size. Although bulk shortening was high, the rotated long limbs of asymmetric folds are free from boudinage. Boudins, however, developed by combined necking and tensile fracturing at θZ(i) >60°. Because of the high finite strain and related layer rotation, these boudins are not asymmetric (as expected) but symmetric. Folds and boudins like those produced under these conditions occur in salt rocks and in crystalline basement deformed at deeper structural levels. The second study was dealing with a layer that was initially rotated around the principle stretching axis (X) from the XZ-plane (coeval boudins and extension-parallel folds) to the XY-plane (boudins with axes parallel to the Y-axis). During progressive strain, each layer was crossing different strain fields (shortening, reduced shortening, elongation) resulting in coeval boudinage and F1-folding, the latter with fold axes parallel to X (extension-parallel folds). A rise in θZ(i) led to increase of boudinage and decrease of F1-folding. The amplitude of F1-folds increased with viscosity ratio between layer and matrix but was hardly affected by θZ(i). The arc- and wavelength of F1-folds, on the other hand, increased if θZ(i) >45°. In cases of low viscosity ratio between layer and matrix, boudins were affected by F2-folds with axes subperpendicular to the layer. Both F1- and F2-folds show increasing interlimb angles at rising θZ(i). The folds and boudins produced in this study are frequent in high-pressure/lowtemperature metamorphic rocks of subduction-zone settings and in salt walls. The third of our experimental studies was focusing on a layer whose inclination was incrementally changed by turning the layer around the shortening axis (Z) from the XZ-plane (coeval boudins and extension-parallel folds) to the YZ-plane (folds with axes parallel to the Y-axis). With increasing initial layer inclination (θX(i)) the axes of developing folds rotate from the X- to the Y-direction resulting in extension-parallel, non-cylindrical (periclinal) and parasitic folds; the layer-parallel elongation decreased to zero at θX(i) = ca. 60°, and shortening along the Z-axis was progressively accommodated by buckling. The amplitude of folds increased with the degree of layer obliquity. Non-cylindrical folding of single oblique layers under bulk coaxial plane strain is described for the first time. In contrast to extension-parallel folds, their shape is significantly affected by the viscosity ratio between layer and matrix. There is a striking difference in deformation-induced layer rotation between the three settings mentioned above. In cases where the layer rotates around the Y-axis, the layer rotates slower than a corresponding passive plane. If the layer rotates around the X- or Z-axis, respectively, it rotates like a passive plane. This difference is important for the growth rate of folds and boudins. Further investigations should focus on the behavior of oblique competent single layers deformed under bulk constriction and flattening. Moreover, the deformation behavior of oblique multilayers should also be considered during further studies.
Publications
- 2020. Boudinage and two-stage folding of oblique single layers under coaxial plane strain: Layer rotation around the axis of no change (Y). J. Struct. Geol., 135
Zulauf, J., Zulauf, G., Hattingen, E.
(See online at https://doi.org/10.1016/j.jsg.2020.104023) - 2020. Coeval boudinage and folding of oblique single layers under coaxial plane strain: Layer rotation around the principal stretching axis (X). J. Struct. Geol., 141
Zulauf, J., Zulauf, G., Hattingen, E.
(See online at https://doi.org/10.1016/j.jsg.2020.104217) - 2021. From parasitic via non-cylindrical to extension-parallel folding of oblique single layers under coaxial plane strain: Layer rotation around the shortening axis (Z). J. Struct. Geol. 145
Zulauf, G., Zulauf, J., Hattingen, E.
(See online at https://doi.org/10.1016/j.jsg.2021.104303)
