Spiral-shaped fabrics in metamorphic rocks: A new example of rotation during progressive deformation

Abstract

This paper deals with a new example of spiral geometry in medium-grade metamorphic rocks. In this example, multi-microlayers are wrapped into millimeter-to-centimeter scale spirals and form pod-shaped structures 3 cm to >10 cm long and 1.5–2 cm across which are outlined by layers up to 3 mm thick and are named spiral pods. Their long axes are oriented at angles of 40–90° to mineral lineations, and are sub-parallel to the average orientation of curvilinear hinge lines of shear-related folds. The wrapping sense coincides with the sense determined by shear sense indicators and shear-related fold vergence. We conclude that the spiral pods developed during a single progressive non-coaxial deformation. Their development began with the formation of buckle microfolds and small shear-related tight to isoclinal folds with curvilinear hinge lines. Shear-related folds experienced refolding and subsequent disarticulating of their refolded hinges. The sense of rotation of hinge lines and rolling of hinges is consistent with the shear sense and these processes led to the formation of spiral pods at all stages of progressive non-coaxial deformation. Thus the spiral pods evolved from refolded folds with curvilinear hinge lines whereas spiral slump folds with a similar structure from cylindrical refolded folds.