Coeval buckling-necking and tablet boudinage of oblique single layers in bulk flattening strain fields: Results from analogue modelling

We applied a series of scaled analogue experiments to investigate how the initial orientation of a single competent layer influences the development of folds and boudins in a bulk flattening strain regime. The initial angle between the competent layer and the shortening axis ( θ Z(i) ) was systematically adjusted in 11.25° increments from 0° to 90°. As θ Z(i) increased, layer thickening decreased, and the resulting structures evolved from coeval folds and boudins to predominantly tablet boudins. Folding and boudinage may be active coevally if θ Z(i) < 45°, with boudin necks oriented perpendicular to the fold axes. Three distinct stages of overprinting, including folding, necking and refolding, are characteristic for one and the same run. In contrast, when θ Z(i) > 45°, tablet boudins became dominant. Incremental studies indicate that the tablet boudins evolve from previous pinch-and-swell structures by tensile fractures. A striking decrease in their size and aspect ratio with progressive strain reflects strain hardening, which is also common in natural rocks. The experimental structures closely resemble natural examples found in salt rocks and other settings of bulk flattening strain. • Flattening structures range from tablet boudinage to coeval folds and boudins. • Three overprinting stages of folding, necking and refolding occur at one and the same run. • Tablet boudinage dominant when θ Z(i) > 45°. • Tablet boudins evolve incrementally from pinch-and-swell structures by tensile fractures.