Evolution pattern of pull-apart basins related to the continuation of the Main Recent Fault in NW Iran

The continental suturing in Zagros was mainly occurred between the Arabian Plate in the southeast and Central Iran in the northeast along the Main Zagros Thrust. During the Pliocene time, the suture zone was reorganized and the belt-parallel component of the Arabia – Central Iran convergence has been localized along the Main Recent Fault (MRF). The Main Recent Fault is a major active strike-slip fault system on the border between the northern Zagros belt and Central Iran. Both geometry and kinematics of the fault system is rather well known along its central part and at its SE termination, while its possible continuation to the northwest is ambiguous. Moreover, less regard has been paid to possible relationships between this major intracontinental fault system and other strike-slip faults in NW Iran – SE Anatolia. The aim of this study is to describe both the genesis and evolution of Quaternary extensional basins in relation with the present-day geometry and kinematics of the NW continuation of the Main Recent Fault between SE Anatolia and NW Iran. We have used a combined approach including fault-slip data analysis and tectonic geomorphology to investigate active faulting of the MRF in NW Iran. Our results indicate that, to the north of latitude 37°N (NW from Piranshahr town), the main zone of the Main Recent Fault continues northwards along a less known branch of the Neotethyan suture up to the sinistral Khoy – Baskale fault zone in SE Anatolia. The recognized fault network in addition to the well-known NW part of the Main Recent Fault is divided into three distinct southern, central and northern structural zones. The almost 200 km long active fault system affecting the central and northern structural zones is transtensional dextral in character and is constituted by several strike-slip and normal faults and fault zones. The structural linkages of different zones occur through direct structural connections or soft linking releasing fault relay zones between overlapping fault strands. Our results also reveal that the NW continuation of the MRF clearly terminates to a NE-striking sinistral fault zone. At that place, the intracontinental dextral shear dies out and active deformation is transferred to N-S normal fault zones at the intersection zone of the NNW-SSE dextral and NE-SW sinistral faults. The crustal-scale ESE extension induced to these N-S fault zones has produced elongated extensional basins which, in the west of Lake Urmia and SE of Lake Van, resolve the space problem at the termination of the intracontinental dextral shear. These large elongated extensional basins show significant differences in both geometry and structural pattern with respect to the usual pull-apart basins formed in releasing fault relay zones of the dextral Main Recent Fault. According to our observation, the Marivan, Piranshahr, and Sardasht tectonic depressions are among the pull-apart basins formed in this kind of fault relay zones, while the Silvana – Serow and Başkale depressions are the result of crustal extension at the end of the dextral system. Considering the evolution stage of all the investigated extensional basins, those are classified into three distinct groups of active, transitional and inactive basins. Active basins are undergoing active extension and deposition, while inactive basins are transected by shortcut strike-slip faults and have entered the erosional stage. Basins in the transitional stage are filled by recent deposits affected by retrogressive erosion and incision. Active extension ended in the mature basins due to a direct structural connection of the overlapping main fault segments through a shortcut fault zone. This erosion – deposition balance in the extensional basins (whatever their genesis) suggests that the extensional basins are more evolved southwards along the Main Recent Fault implying a probable northwards propagation for the dextral fault system. The distribution pattern of extensional basins described in this study reveals the importance of strike-slip faulting in producing special tectonic geomorphology features that are usually seen in extensional tectonic settings, while a dextral transpression is prevailing over the region.