The three dimensional shape and localisation of deformation within multilayer sheath folds

► Active sheath folds form where layering plays a mechanical role during deformation. ► First systematic 3-D analysis of an multilayer active sheath fold. ► Inner nose of sheath decouples and translates relative to outer folded layers. ► Eye-folds in 2-D (y-z) slice may be used to predict fold geometry along the x-axis. ► Self-similarity is useful if tracing stratiform mineralisation in large sheath folds.