Numerical modeling of basement inheritance and salt decoupling effects on the structural evolution of the Zagros Fold-Thrust Belt

Fold-thrust belts are complex tectonic domains formed in response to near- or far-field compressional stress fields in the Earth’s crust. The complexity of structural style in theses belts is controlled by multiple factors. The presence of mechanically weak layers, defined by rocks that exhibit lower strength than their surroundings, play a significant role in the formation and evolution of fold-and-thrust belts. In this research, a two-dimensional numerical finite difference model incorporating a visco-elasto-plastic/brittle rheology is utilized to investigate the structural evolution of the Fars arc in the southeast of the Zagros fold-thrust belt. The modeling includes the tectonic inversion process, from Permian-Triassic rifting of the Neo-Tethys Ocean to the Oligocene-to-present continental collision between the Arabian and Eurasian plates. The Eulerian grid dimensions were considered as 500 km in length and 60 km in thickness, respectively, comprising 1101×121 nodes. Each cell contains 16 Lagrangian markers that carry the information and properties of each layer. According to the stratigraphic column of the Fars arc, there are 30 km of basement overlain by a 2-km-thick salt layer with a 3-kilometer-thick Palaeozoic sedimentary sequence above the salt layer. Furthermore, to replicate the interplay between Earth's internal and surface dynamics, a 25-kilometer-thick layer representing air is incorporated at the uppermost part of the model to approximate a free surface. The experiments include three inherited basement faults. The role of pre-existing weak zones in extensional tectonics is the shaping of rift basin geometry, especially the formation of half grabens. In the convergence phase, fold-thrust-belts with a basal detachment layer and pre-existing faults produce folding at two distinct scales. The faulting in the basement develops long wavelength folds in the sedimentary cover, while the presence of a salt layer shapes the smaller wavelength folds and thin-skinned thrust faults that extend from the detachment layer to the surface. The reactivated faults play an important role in stress transfer, leading to the emergence of new faults and seismic events. The results indicate that deformed listric faults show a meaningful correlation with the depth distribution of earthquakes throughout the Fars arc. The outcome associated with the presence of a thick basal salt layer and involvement of the basement exhibit a correlation with the structural style of the Fars arc.