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The importance of incompetence

¹ Hans Ramberg Tectonic Laboratory, Institute of Geology, Uppsala University, Box 555, S-751 22 Uppsala, Sweden

Metabasaltic dikes that are known to be competent and to deform more slowly than granitoid country rocks in the upper crust are likely to be incompetent and deform faster than granitoid hosts in the higher temperatures of the lower crust. Layers that deform as competent strain markers develop inward-pointing cusps. These deformation structures are called folds in shortened layers and necks in extended layers and are reasonably well understood. By contrast, layers that deform as incompetent markers develop strong internal foliations and outward-pointing cusps. These structures are currently known as mullions in shortened contacts and inverse folds in extended contacts but are comparatively poorly understood. Here we emphasize the importance of the deformation structures in the incompetent, hydrated, metabasaltic intrusions and enclaves that must be among the structures that seismic reflection profiles attempt to image deep in the continental crust.