| چکیده انگلیسی مقاله |
This paper, benefitting from seismic studies, investigates the lithospheric and upper mantle dynamics of eastern Iran, shedding light on the tectonic evolution of the Sistan mountain ranges from the Cretaceous period to the present. Historically, due to fewer seismic studies focusing on this area, the mountain ranges were only examined from a geological perspective. Previous studies encompassing petrology, geochemistry, and tectonics have provided valuable insights into understanding the dynamic phenomena of this region (e.g., Pang et al., 2013; Jentzer et al., 2022). However, with the expansion and utilization of various seismographic networks, accessing more information about subsurface structures has become feasible, potentially illuminating new dimensions of the geodynamics in this area. Recent seismic studies reveal a warm mantle beneath the crust of this region, indicating relatively active dynamics (e.g., Mahmoodabadi et al., 2023). The presence of a thinner lithosphere compared to surrounding regions, accompanied by high-velocity anomalies observed in the asthenosphere of this area, may indicate lithospheric dripping and separation of lower lithospheric portions. This process has led to the upwelling of the asthenosphere and heating of the remaining lithosphere and crust of the region (Mahmoodabadi et al., 2024). Additionally, receiver function analysis reveals crustal structures inclined towards the west, suggesting the involvement of underthrusting beneath the mountainous region, a phenomenon associated with crustal thickening (Mahmoodabadi et al., 2023). Seismic evidence, along with other geological indicators, contribute to establishing a structural evolution model for this region. The current crustal and lithospheric structure, as well as the topography of the Eastern Iranian Ranges (Sistan), have been shaped by a series of hierarchical events starting from the opening of the Sistan Ocean during the Cretaceous period (e.g., Pang et al., 2013). In the present study, we explain how subsequent intra-oceanic subduction brings the Afghan continental block to the subduction zone, resulting in crustal layering and thickening. Continental subduction is influenced by the buoyancy of the system, eventually reaching a point where buoyant forces acting on the continental lithosphere prevent further downward movement. This positive buoyancy can uplift portions of the overlying oceanic crust, leading to the entrapment of oceanic crust within the continental lithosphere, setting the stage for the existence of ophiolites in the region. The closure of remaining oceanic domains possibly played a significant role in shaping observable geological structures today. Convergence forces, combined with pressure on the remaining oceanic lithosphere, forced it to subduct beneath the Lut Block, initiating another phase of orogenesis and contributing to the formation of ophiolitic belts along the mountain ranges. Following the closure of oceanic domains, further continental subduction likely played a significant role in shaping present observed geological structures. Subsequent convergence forces led to lithospheric thickening, triggering the detachment of weaker portions of the thickened lithosphere. Thinning of the lithosphere, facilitated by asthenospheric melting and ongoing tectonic processes further enabled ductile flow in the lower crust, supported by seismic evidence such as the presence of low shear wave velocities. In summary of the findings, this study presents a new model of the tectonic evolution of the Eastern Iranian Ranges, revealing a complex interplay of diverse geological processes. These processes span from intra-oceanic subduction to continental collision, encompassing crustal thickening and lithospheric delamination. Through seismic studies and analysis of geological evidence, this research provides crucial insights into understanding the structural evolution of this region. Moreover, it offers implications for broader tectonic processes within continental collision zones. |