Domes and basins are evidence for vertical movements in both compression and
extension tectonic environments. They are thus evidence for interplay between gravity
and tectonic forces in structuring the continental crust. We employ analytical and
numerical techniques to investigate the respective roles of gravity and compression
during the growth of crustal-scale buckle anticlines and diapirs submitted to instantaneous
erosion. The analytical perturbation method, which explores the onset of both
types of instability, yields a âeurooephase-diagramâeuro˝ discriminating eight folding-diapirism
modes, fi ve of which are geologically relevant. Numerical simulations show that the
phase diagram is applicable to evolved, fi nite amplitude stages. Calculated strain
fi elds in both diapirs and folds show normal sense of shear at the interface if the
upper layer is thick compared to the lower layer. We conclude that the present-day
structural techniques applied for distinguishing diapiric domes and folds are ambiguous
if detachment folding and intense erosion take place during deformation, and that
domes displaying extensional structures do not necessarily refl ect extension.