Domino boudinage under layer-parallel simple shear

• In simple shear, the tilt and separation of boudin segments is an indicator for high strain.
• The rotation rate of boudin segments largely depends on their aspect ratio and the scaled gap width.
• Boudins with low viscosity ratio develop shapes, which may resemble shear-band boudins forming under the opposite shear sense.
• Shear deformation of the terminal segments of finite length boudin trains lead to a chaotic appearance of the structure.
• Natural examples from Western Cyclades Detachment System (Greece) show many similarities with the modeled structures.

The boudin segments of a torn competent layer experience synthetic rotation in layer-parallel simple shear. As long as the individual segments in a boudin train are constrained by their neighbors, even a highly viscous boudin deforms internally to create the necessary space for rotation. The rotation rate is then much smaller compared to the case of an isolated segment. Hence, a small tilt of boudin segments is not indicative of low strain. The rotation rate at this stage largely depends on the aspect ratio of the boudin segments and the scaled gap width. Once the tilted boudins are no longer constrained by their neighbors, the rotation rate greatly accelerates. In the case of a low viscosity ratio between the boudins and the host, the boudin segments develop complex shapes, which may give an impression of shear-band boudins forming under the opposite shear sense. We furthermore investigate the behavior of boudin trains of finite length. The terminal segments are displaced out of the shear plane, deforming into isoclinal folds, and separate into groups of boudin segments that rotate into the shear direction and eventually lead to an overall chaotic appearance of the structure. Natural examples of domino boudinage from a high shear –strain detachment zone in the Western Cyclades (Greece) show many similarities with the modeled structures suggesting that, under simple shear deformation, the rotation and separation of boudin segments is an indicator for high shear strain.