Boudinage and folding of oblique single layers in bulk constrictional strain fields: Results from analogue modelling

We conducted scaled analogue modelling to show the influence of varying single layer initial orientation on the geometry of folds and boudins in a bulk constrictional strain field. The initial angle between the plane of shortening and the competent layer (θ_Z(i)) was incrementally increased from 0° to 90° by multiples of 11.25°. While the amount of layer thickening decreased with increasing θ_Z(i), the deformation structures produced range from pure dome-and-basin folds to coeval folds and boudins. Based on the attitude of fold axes, there are extension-parallel (F_EPR) and extension-perpendicular (F_EPP) folds, with axes subparallel and subperpendicular to the principal stretching axis (X), respectively. Coeval growth of F_EPR folds and boudins occurred when θ_Z(i) > ca. 25°. The F_EPP folds can be subdivided into a first type which affect the entire layer (if θ_Z(i) ranges between 11.25 and 78.75°) and a second type, referred to as FB_EPP folds, which are affecting pre-existing boudins if θ_Z(i) > 45°. The interlimb angle of all types of folds increases with increasing θ_Z(i). Folds and boudins similar to the ones produced in this study can be found in salt domes and in tectonites of subduction zones.