Finite strain and progressive deformation in models of diapiric structures

Abstract

The distribution of strain within and around gravitationally produced diapiric structures was studied through the use of experimental models which were deformed in a large-capacity centrifuge. A new method of model construction was developed which is equivalent to building the model of initially square 1-mm elements. After deformation the elements assume shapes which approximate parallelograms and their finite strains can easily be calculated. If several initially identical models are deformed to different extents, the finite strain states of an element in each of the models define points on the deformation path of that element. The deformation path can be used to make estimates of the nature of the internal fabric which would be expected in the equivalent element of the natural structure. This method was applied to the study of the finite strain in diapiric ridges. The models demonstrate that the highest strain is always in the region above the diapir. Within the diapir initial vertical stretching is followed by vertical flattening. Large portions of the structure can be seen to suffer what would in natural examples be called polyphase deformation, even though all of the deformation was due to a single buoyant overturn of unstable density stratification. The strain patterns within the models support the contention that in salt diapirs the buoyant salt has a lower viscosity than the overlying sediments, but that in mantled gneiss domes the reverse is true.