In this study, two-dimensional finite element modeling with Newtonian elastic rheology was conducted using ABAQUS software to investigate the geometry and evolution of a rhomb-shaped transtensional basin formed along releasing step-overs in the right-lateral strike-slip Dehshir–Baft–Qom–Zefreh fault system, located within the Urumieh-Dokhtar magmatic arc. The models examine three geometries of fault releasing steps: underlapping, neutral, and overlapping configurations. The geometric analysis of these models revealed spindle-shaped, open Z-shaped, and rhomb-shaped basins, respectively. Our findings indicate that increased overlap between faults leads to an extended and diffuse deformation pattern, while the absence of overlap results in block rotation and a more localized deformation style. The transtensional basin exemplifies the progressive evolution of overlapping step-overs, gradually transitioning from an elongated rhomboidal to a trapezoidal geometry over time. Furthermore, the results demonstrate that the degree of overlap between parallel strike-slip faults significantly affects the distribution of stress and strain. Within the releasing step-over, average normal stress concentrates as extensional in the fault center and as compressional near the fault tips. Paired zones of maximum and minimum stress values are concentrated at the inner and outer edges of the fault tips, respectively. By examining the releasing step-over of the right-lateral Dehshir–Baft–Qom–Zefreh fault and analyzing the stress-strain distribution within the transtensional basin, this research offers valuable insights into the geodynamic processes and tectonic evolution of the Qom–Zefreh and Dehshir–Baft regions.