Boudinage and two-stage folding of oblique single layers under coaxial plane strain: Layer rotation around the axis of no change (Y)

Scaled analogue modelling using non-linear viscous plasticine as rock analogue has been carried out to investigate the influence of varying layer obliquity on the geometry of single-layer folds and boudins under bulk coaxial plane strain. Two deformation series with different viscosity ratio between layer and matrix (m = 18 and 82) were carried out. The initial angle between shortening axis and competent layer (theta(Z(i))) was gradually changed from 0 degrees to 90 degrees by multiples of 11.25 degrees. Shortening at theta(Z(i)) < 30 degrees 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 (e(Z) = -70%), the rotated long limbs of asymmetric folds are free from boudinage. Boudins, however, developed by combined necking and tensile fracturing at theta(Z(i)) > 60 degrees. 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 in the present study occur in salt rocks and in crystalline basement deformed at deeper structural levels.