Evolution of the Piranshahr pull-apart basin at releasing stepovers of the strike-slip fault systems in Northwest of Iran: Insights from two-dimensional numerical modeling

The Piranshahr pull-apart basin, situated in the northwestern part of the Zagros Main Recent Fault (MRF), is characterized by two right-stepping segments of the dextral MRF. Here, a combination of finite element modeling techniques, especially two-dimensional numerical modeling using ABAQUS software, along with field-based structural geological methods are used to assess the geometry and evolution of the pull-apart along the releasing stepovers in this strike-slip system. The utilized numerical approach applies two-dimensional (2D) finite-element modeling related to elastic Newtonian rheology to evaluate the distribution of stress and localization of strain within the pull-apart basin. This study provides valuable insights into the factors controlling the shape, as well as exploring the interaction between the pre-existing structures in this right-lateral strike-slip releasing stepover, pull-apart basin development in strike-slip systems, and stress-strain behavior by studying the impact of boundary conditions and fault overlap on the deformation pattern. The models consider three representative geometries of fault segment interactions, including underlapping, neutral, and overlapping stepovers, positioned at angles of 30°, 45°, and 60°. The results indicate that increased overlap creates an extensive and elongated deformation pattern, while decrease overlap leads to block rotation and a narrow deformation pattern. In addition, the degree of overlapping between parallel strike-slip faults influences the stress and strain. The mean normal stress within the transtensional basin, located between the fault segments, exhibits an extensional nature, while the region outside the stepover experiences general compressive mean normal stresses. The Piranshahr transtensional pull-apart basin exemplifies the progressive evolution of underlapping stepovers, resulting in displaying an elongated rhomboidal to trapezoidal-shaped geometry over time.