Importance of basement faulting and salt decoupling for the structural evolution of the Fars Arc, Zagros fold-and-thrust belt: A numerical modeling approach

Abstract. Understanding the tectonic evolution and crustal-scale structure of fold-thrust belts is crucial for exploring geological resources and evaluating seismic hazards. We conducted a series of finite-difference two-dimensional thermo-mechanical numerical models with visco-elasto-plastic/brittle rheology to decipher how the interaction of inherited basement faults and salt décollement levels control the deformation process and structural style of the Fars Arc in the Zagros fold -thrust belt, during tectonic inversion. Results indicate that initial rifting is controlled by the geometry of inherited faults. During the convergence phase, fold-and-thrust belts display folding at two scales: large wavelength folds induced by basement deformation in the form of fault-propagation faults, and small wavelength folds and thrust systems emerge above the salt layer as detachment folds. Reactivated faults can serve as pathways for stress transfer, resulting in the emergence of new faults and thus seismic activity. The tectonic events in orogenic belts like the Zagros do not adhere to a fixed pattern; they are shaped by factors such as the properties of basement rocks and the orientation of faults. Shallow earthquakes predominantly occur along décollement anticlines, while deeper and larger ones are associated with basement faults. Additionally, we observe variations in resistance to deformation based on salt rheology and fault geometry, with listric faults minimizing resistance. The degree of basement involvement in deformation directly influences the model's resistance, with greater involvement facilitating easier deformation. Our results showing the temporal-spatial relationship between thin- and thick-skinned tectonics can work as an analogue for similar orogenic belts worldwide, such as Taiwan, the Pyrenees, the Alps, the Appalachians, and the Kopet Dagh.