Coeval folding and boudinage in four dimensions

Scaled analogue experiments have been carried out incrementally to demonstrate the growth of constrictional folds and boudins through space and time. Pure constriction acted on a single stiff layer that was embedded in a weak matrix, with the layer trending parallel to the X-axis of the constrictional strain ellipsoid. The viscosity ratio between non-linear viscous layer and matrix was set at ca. 13. Three-dimensional images of the incrementally deformed layer have been obtained using computer tomography. They yielded clear evidence that D1-folds and D1-boudins show striking interactions when growing simultaneously at low to moderate finite strain. The axes of D1-folds are subparallel and the axes of D1-boudins are subperpendicular to the X-axis of the constrictional strain ellipsoid. The normalized wavelengths of D1-folds and D1-boudins, and the amplitudes of D1-folds, are fixed already at low finite strain (−5%) and do not significantly change as deformation proceeds. The normalized wavelength of both structures is not so in line as suggested by theoretical studies. At higher degrees of finite strain, D1-boudins are affected by D2-folds, and rotated limbs of the latter show D2-boudinage, resulting in complex deformation patterns that are difficult to identify in the field. The model results are important for the analysis and interpretation of deformation structures in rheologically stratified rocks undergoing dislocation creep under bulk constriction. Tectonic settings where constrictional folds and boudins may develop simultaneously are stems of salt diapirs, subduction zones or thermal plumes.