Gomar, F., Ruh, J. B., Najafi, M., and Sobouti, F.: Importance of basement faulting and salt decoupling for the structural evolution of the Fars Arc (Zagros fold-and-thrust belt): a numerical modeling approach, Solid Earth, 15, 1479–1507, https://doi.org/10.5194/se-15-1479-2024, 2024.

Abstract

Understanding the tectonic evolution and crustal-scale structure of fold-and-thrust belts is crucial for exploring geological resources and evaluating seismic hazards. We conducted a series of two-dimensional finite difference thermomechanical numerical models with a visco–elasto–plastic/brittle rheology to decipher how the interaction between inherited basement faults and salt décollement levels control the deformation process and structural style of the Fars Arc in the Zagros fold-and-thrust belt during tectonic inversion. Numerical experiments with extension and consequent convergence phases indicate that strain accumulation patterns during initial rifting are controlled by the location and geometry of the prescribed faults. During convergence, the inverted basement faults form large-wavelength and foreland-verging fault-propagation anticlines in the sedimentary cover, while the thick salt layer promotes the growth of second-order detachment anticlines accompanied by both fore- and back-limb thrust faults. Experiments without prescribed basement faults result in dispersed brittle/plastic deformation during rifting and convergence and an effective mechanical decoupling along the salt horizon. Overall, reactivated faults can serve as pathways for stress transfer, resulting in the formation of new faults triggering seismic activity. The structural evolution of orogenic belts like the Zagros does not adhere to a fixed pattern; it is shaped by factors such as basement rock properties and inherited fault orientations. Shallow earthquakes predominantly occur along décollement anticlines in the sedimentary cover, while deeper and larger ones are associated with basement faults. We also observe variations in resistance to deformation based on salt rheology and fault geometry, with listric faults minimizing resistance. The degree of basement involvement directly influences the model’s resistance to deformation, with greater involvement facilitating easier deformation. Our results, related to the temporal and spatial relationship between thin- and thick-skinned tectonics, can work as analogues for similar orogenic belts worldwide, such as Taiwan, the Pyrenees, the Alps, the Appalachians, and the Kopet Dagh.