Underground salt formations reshape how mountains buckle under pressure

<p>Evaporite layers act as low-friction decollements in many fold-thrust belts and greatly influence their structural evolution. Such evaporite layers also form salt structures that can further affect the development of these belts. To investigate the role of salt structures, the discrete element method was used to simulate the shortening of a sedimentary sequence containing pre-existing salt diapirs, and systematically examine how their presence, position, width, configuration and spacing impact deformation. Model results show that, initially, pre-existing diapirs accommodate shortening through lateral squeezing and salt extrusion, thereby delaying thrust nucleation. Proximal diapirs are incorporated early into the thrust system, promoting the formation of box-folds associated with them near the backstop. Distal diapirs, on the other hand, localise stress, promoting the forward propagation of the decollement. Thrusts use the stems of wide diapirs which absorb significant horizontal shortening. In comparison, thrusts use only the upper or lower parts of narrow diapirs, leading to stem welding, beheading of the diapir, and mechanical coupling. In models with two diapirs, thrusts propagate sequentially from the hinterland to the foreland, with diapir arrangement, width, and spacing controlling the thrust sequence and strain distribution. During model shortening, the pre-existing diapirs also undergo notable lateral translation and some of them are displaced by thrusts forming along their stem. These findings demonstrate that pre-existing salt diapirs redistribute stress and localise deformation, thereby shaping wedge architecture and growth. The strong correlation between our models and natural examples (e.g., salt diapirs in the Zagros Fold-Thrust Belt) supports the simulated mechanisms.</p>