Sheath folds are widely believed to develop by the passive geometric amplification of folds in which layering has no mechanical influence during non-coaxial deformation. Where layering becomes rheologically significant, then active sheath folds may form in which the inner nose of the sheath decouples along detachments, and undergoes a translation relative to the outer folded layers that surround and envelope the nose. We present a systematic 3-D analysis of multiple folded layer geometries in a serially-sectioned natural sheath fold. Weak layers, which are reactivated as detachments, define different-shaped sheath folds relative to other layers, with the sense of cut-off along detachments reversing across the axial surface as the inner fold nose has “protruded” into the surrounding sheath envelope. Detachment layers are more tightly folded meaning that such active sheath folds are non-similar shapes. The obliquely oriented, bifurcating, en-echelon nature of the fold hinges developed in adjacent layers suggest that pre-cursor folds formed with pronounced 3-D obliquity relative to the subsequent shear plane. Mineral lineations folded around sheath closures display asymmetric “star-burst” patterns consistent with recrystallisation during active folding and hinge rotation. We show that the eye-fold shapes exposed in any 2-D y-z slice can be used to predict the geometry of marker horizons back along the x-axis in the third dimension. This self-similarity may be of value when tracing stratiform mineralised horizons in large-scale sheath folds.
This work was presented at the Tectonic Studies Group meeting in January 2011 and we thank participants for useful comments. Thanks also to the Carnegie Trust and Edinburgh Geological Society for supporting this study, and to Sudipta Sengupta and an anonymous referee for their thoughtful comments.
- sheath fold