The deformation of caprock on extruding salt diapirs – insights from analog and numerical modelling

Growing salt diapirs emerging at the sedimentary surface can produce outflowing salt extrusions, as observed, for instance, in many locations in the Zagros fold-and-thrust belt (Southern Iran). Flow patterns of such salt extrusions are controlled by gravity spreading and gliding. Furthermore, internal structures and shapes of salt extrusions are affected by factors like the local topography, the width of the diapir and the tectonic stress field. Many field examples of outcropping diapirs reveal, however, that the highly soluble evaporites (mainly halite) are already dissolved at the surface and that extrusions are covered by a ‘caprock’ layer, which is built of a multi-compositional residuum of less soluble minerals and rocks. Thickness, composition and mechanical properties of the caprock (density, shear strength, etc.) strongly vary between individual diapirs depending on the original composition of the salt layer, overlying host rock sediments, erosion rate, etc. Hence, the influence of such a caprock on the dynamics of the salt extrusion might also be highly variable and has not yet been investigated. It is unclear, if the caprock deforms by ductile shearing similar as viscous rock salt or if it acts as competent, brittle cover layer deforming by fracturing and brecciation during flow of the underlying salt.

We present a series of 3D analogue experiments and 2D numerical models in which we systematically investigated deformation patterns of the caprock layer during diapiric extrusion of a viscous material. In the analog experiments, we tested different types of granular materials as caprock equivalent to simulate different rheologies. Specifically, a fine-grained powder was used to mimic a competent, high-cohesive rock and a coarse-grained, low-density granulate for a less competent rock. In the numerical models, we tested a wide range of caprock parameters, such as thickness, viscosity, and shear strength. Our study is specifically focused on salt extrusions of the Iranian Zagros fold and thrust belt. Thus, the extrusion patterns in both, analog and numerical models, were tested on 1. passively growing diapirs and 2. diapirs reactivated by lateral shortening.

The results of this modelling study provide insights into the conditions (e.g. minimum thickness or strength) under which a caprock layer has a significant influence on the style of the salt extrusion or only acts as a passive veneer floating on top of the flowing salt. The model results show that a competent or thick caprock forms a polygonal fracture pattern at the beginning of the extrusion, while the separated blocks slide downslope during later stages of the extrusion. An incompetent or thin caprock rather deforms by flow, shear thinning and folding coupled to the flow of the underlying salt. These characteristics can help us to interpret deformation structures observed on natural salt extrusions, in terms of thickness and deformation behavior of the caprock.