عنوان مقاله [English]
The Makran subduction with an approximate length of 900 kilometers has a thick and wide accretionary prism. Two-thirds of the accretionary prism width (around 200 kilometers) is onshore. The wide onshore part of the Makran accretionary prism provides a unique opportunity to study the structure of an accretionary prism. Despite the potential occurrence of large megathrust earthquakes in the region and the presence of major commercial ports like Chabahar and Konarak on the accretionary prism, the Makran region remains one of the less explored areas in Iran. Moreover, the presence of thick, young sedimentary deposits and hydrocarbon seepages has transformed the Makran accretionary prism into one of the promising regions for potential oil reserves. In this study, we have focused on investigating the structure of the accretionary prism in the border of western and eastern Makran (i.e., Central Makran) lying within the Iranian part of Makran, using ambient noise data recorded by a dense local seismic network installed from 2016 to 2020 and ambient noise adjoint tomography method. By improving the initial used shear-wave velocity model, we could reduce the misfit between observed and synthetic forward waveform field by 80%. The final velocity model covers the accretionary prism of Central Makran up to the depth of 60 km. The results reveal that the Makran accretionary prism can be divided into two frontal- and paleo-prism segments. The paleo-prism segment exhibits a higher shear-wave velocity (averaging 4.0 k/s) compared to the frontal-prism segment. The frontal-prism segment, located in the southern part of the accretionary prism, covers less than half of the onshore accretionary prism and its average thickness is about 22 km. The higher shear-wave velocity in the paleo-prism segment implies presence of convoluted structures mixed with pieces of oceanic crust scrapped from the top of the subducting oceanic lithosphere. The thickness of the sedimentary cover within the paleo-prism segment varies between 22 to 30 km but the depth to magnetic basement is less than 8 km over the segment implying presence of magnetized igneous rocks within the paleo-prism segment. These conditions make the existence of hydrocarbon reservoirs in the paleo-prism segment of the accretionary prism highly unlikely. Based on these findings, it is recommended to initially concentrate the exploration stages for hydrocarbon resources in the frontal-prism segment. The lower shear-wave velocity in the frontal-prism segment can potentially lead to the development of a sedimentary basin effect and an increase in the amplitude of seismic waves reaching this region, ultimately increasing the seismic hazard in the frontal-prism segment.