Influence of cap-rock on deformation during extrusion of salt diapir – a numerical study

Growth rate of salt diapirs usually oscillates depending on several factors. The growth can be arrested by depletion of the source layer or diapir burial, conversely, diapir reactivation occurs through erosion of the overburden and/or introduction of tectonic forces. Examples of reactivated diapirs can be observed in the Zagros Mountains in Iran. There, tectonic shortening responsible for development of the Zagros Fold and Thrust Belt simultaneously squeezes the diapirs, which extrude salt onto the surface. The top section of the diapir is usually affected by meteoric water, which dissolves the salt and leaves behind insoluble material embedded within the source layer, forming the so called caprock. This caprock can be assumed to be already present before the reactivation of the diapirs during shortening, hence it may play a role in the development of the salt extrusions. Geometry, composition and mechanical properties of the caprock can vary widely depending on factors such as original composition of diapiric material, dissolution and growth rates, etc. Additionally, exact mechanical properties of any caprock are difficult to determine and are currently largely unknown. 

Hence, we present a series of 2D numerical simulations utilising finite element method to investigate how different geometries and rheologies of the caprock affect the shape of the subaerial extrusions. The analysis was performed with three variable parameters (caprock viscosity, cohesion, and thickness) for three scenarios of diapirism (1 - purely shortening-based, with depleted source layer; 2 - purely buoyancy-based, with preserved source layer and no tectonic forces, and; 3) a combined scenario). We analysed the general deformation patterns as well as quantifiers such as velocities, displacements, strains, strain rates and ratios between vertical and horizontal components of the quantifiers. We investigated variability of averaged values of the quantifiers in time as well as detailed spatial distribution for the finite state of simulation. 

The simulations revealed the strong contrast between less (low cohesion and viscosity) and more competent rheologies in term of deformation patterns. The former tends to result in caprock material being thinly spread over the surface of the salt extrusion, whereas in the latter case the caprock fractures into “rafts” floating on top of the extrusion. The exact geometry of the rafts (size, spacing, distribution) is highly dependent on the geometry and type of diapirism. We also compare the resultant patterns to the quantifiers, especially velocities and their ratios, establishing clear ties between the patterns and deformation dynamics.