Key-ring structure gradients and sheath folds in the Goantagab Domain of NW Namibia

The concept of deformation phases is one of the corner stones of structural geology but, despite its simplicity, there are situations where the concept breaks down. In the Goantagab Domain of NW Namibia, structures in an area of complex deformation can be subdivided into at least four sets, attributed to four deformation phases on the basis of overprinting relations. Three of these sets of structures, however, formed during the same tectonic event under similar metamorphic circumstances but slightly different flow conditions. These sets of structures show gradational transitions in space that can be understood by a concept of “key-ring structure gradients”, where older DA structures are reoriented and overprinted by new structures DA+1 that have similar orientation, and seem to grade into DA structures outside the overprinted area. This kind of behaviour may be common in inhomogeneous non-coaxial flow. In the core of the Goantagab Domain, D2 structures are thus reoriented and overprinted by local D2b folds and foliations that have the same orientation and style as D2 structures outside the domain core.