Problem of folding in ductile shear zones: a theoretical and experimental investigation

The paper describes micro-scale folds within a narrow ductile shear zone of the Peninsular Gneissic Complex, South India. The characteristics of the folds indicate that they have formed by buckling on the mylonitic foliation parallel to the C fabric. This raises the question of how a buckling instability could develop on the mylonitic foliation, as there can be no overall shortening along the shear direction. On the basis of natural observations we show that mylonitization involves sericitization locally along the C fabrics, forming discrete mechanically weak zones, which perturb the homogeneous shear stress field in their neighborhood, and induce shortening along the bulk shear direction, leading to buckling instabilities on the mylonitic foliation. The weak zone model is supported with results obtained from physical experiments. Theoretical analysis shows that the weak bodies with large length-to-thickness ratios perturb the shear stress field in the shear zone significantly, developing compressive stresses along the bulk shear direction, required for buckle instabilities to occur on the mylonitic foliation.