## 2 Geological Setting  
The NS active faults on the western and eastern margins of the Lut block in eastern Iran are among the fastest-slipping faults [19; 20; 21]. They are currently slipping laterally at a rate of 4.4 \(\pm\) 0.4 and 5.6 \(\pm\) 0.6, respectively [22; 23; 24]. The weighted average direction of the compressional stress in the eastern margin of the Lut, as determined by the seismic strain rate (SSR), focal mechanism stress inversion (FMSI), and GPS-derived strain rates (GSRs), was obtained at about N20\({}^{\circ}\)E [25]. This direction is positively correlated with the faults' mechanism in the region. As a result, the region's NS and EW faults exhibit right-lateral and left-lateral movement in addition to dip-slip movements.  
The character of convergence along the Arabian-Iranian plate boundary changes radically eastward from the Zagros ranges to the Makran region, where the northward subduction of the oceanic part of the Arabian Plate beneath the Lut and Afghan blocks in the northwestern Indian Ocean has mainly affected the seismicity and deformation of eastern Iran [26]. Tectonic activities are relatively high in the Birjand area (Figure 1), which contains a series of NW-SE and EW linear mountain ranges. The ranges are separated by narrow basins with young terraces and gravel fans [27]. According to the geological map [27], alluvial units are Quaternary in age, whereas folded gravel bed sediments and all older alluvial units are Neogene [28]. The sediments' Neogene age indicates that the geological structures in which they are exposed are late Tertiary relics. In the Sistan suture zone, the ranges represent ophiolitic rocks from the Late Cretaceous to the Eocene epochs. The trend of the shear zones and lithological units in the Sistan structural zone runs parallel to the topography [28]. In particular, the trend of the geological structure (North Birjand fault) is EW along the northeastern margin of the Lut in the north and northwest of Birjand (Figure 1). Numerous extension structures (fissure swarms, mostly NS trending) have developed in this area along the North Birjand mountains.  
We initially thought they were formed as a result of subsidence or water erosion; however, field studies revealed that they were formed as a result of tectonic activity. We identified their characteristics, origin, process, and mechanism of formation in this study. The North Birjand fault is the main fault in the study area, which can be assumed as a segment of the splays at the north termination of the Nehbandan fault zone as a dominant structural trend in the east of Iran [28; 29] (Figure 1). Previous studies in the Birjand area [8; 28; 29; 30; 31; 32], have revealed the importance of right-lateral strike-slip and reverse faults in the area's deformation, as well as the observation of various stress distributions related to the regions' plasticity and fracture intensity [28]. The presence of extension structures (fissures) in the center of a compressional zone is an unanswered question addressed in this paper (Figure 2).  
Figure 1: **(a)** Location of the Birjand area (blue cadre) in Iran, and (**b**) Close-up view of the study area (yellow cadre) in the NW of the Birjand city.  
Figure 2: Field examples of the fissures in the study area. Fissures have an average depth of 4 m. (**a**) Parallel fissures; (**b**) fissures with NS, NE–SW trends; (**c**) fissures with zigzag edges; (**d**) fissures with EW trend.