Investigating geometric and morphologic changes in the Alborz orogenic belt (Qazvin-Rasht area), based on 2D numerical modelling

Deformation of microcontinents occurs during collision with a larger stable continent. Deformation is usually concentrated at the boundaries of the deforming microcontinent and depends on mechanical stratigraphy and pre-existing structures. The Alborz Mountains, located in the northern margin of central Iran underwent various deformations including Cimmerian, Laramide and Alpine compressions since the Triassic and its highest exhumation occurred in Cenozoic, as a result of the Alpine orogeny. Relocation of the Alborz Mountain front is visible in the satellite images in the Qazvin-Rasht area, where the geophysical maps also indicate changes in the basement at the subsurface. The aim of this study is to investigate possible factors responsible for this large displacement of the mountain front, using 2D numerical modeling. We designed and run five 2D models considering different parameters, such as the presence of basal and middle detachments and the effect of their thickness as well as the involvement of basement faults. The experiments show a faster movement of the deformation towards the foreland in the presence of basal detachment compared to the case without basal detachment. The addition of intermediate detachment intensifies this process. Increasing the thickness of the middle detachment increases the exhumation and deformation, especially towards the foreland. The model with a pre-existing basement fault shows very clear localization of deformation on the basement fault which causes developing the deformation towards the foreland. According to the cross sections constructed across the area on both sides of the change in the mountain front, there is an increase in the thickness of the Jurassic shale (Shemshak Formation) as a detachment level in the central Alborz, where higher exhumation and extensive outcrops of Paleozoic rocks is observed compared to the western Alborz. The numerical models with different thickness of intermediate detachments (1 and 2 km) have simulated this difference between the central and western Alborz. Considering the larger displacement of deformation front toward the foreland in the numerical model in the case of a pre-existing fault, the presence of a basement fault in the western Alborz could be assumed to cause the larger southward displacement of the mountain front compared to the central Alborz.