Inversion of geoid, topography and gravity data to determine the thickness of the crust and lithosphere in the northeastern region of the Iranian Plateau

Document Type : Research Article

Authors

Department of Earth Physics, Institute of Geophysics, University of Tehran, Tehran, Iran.

Abstract

The main purpose of this research is to determine the thickness of the crust and lithosphere in the northeastern regions of the Iranian plateau. There are different methods to determine these two parameters. According to the definitions and how to determine this thickness, each of the methods has advantages and disadvantages. In this research, large-scale satellite data is used, so we do not need data acquisition that makes it a great advantage, and inversion in this way will give us the final model of the crustal and lithospheric structure in the shortest time.
For this purpose, a joint inversion of geoid topography and gravity data is used. First, we obtain a 1D model that is close to reality using the simultaneous modeling of geoid data, topography, basic concepts of physics and mathematics and local isostasy, for the thickness of the Moho and lithosphere. Then we use a 3D inversion method to reduce the difference between the measured and calculated data. The initial model that is given to the program in 3D inversion is the output of 1D modeling.
The studied area, the northeastern region of the Iranian plateau (including Kopeh-dagh) is an area with great potential in natural resources (mainly oil and gas). The highlands of northeastern Iran are formed in the Alpine-Himalayan folds and are similar to the Zagros mountains from a geological point of view. The Kopeh-dagh mountain range starts from the east of the Caspian Sea and enters Afghanistan after passing through Turkmenistan. The Kopeh-dagh mountain range separates the Turan plate from the central Iranian plate (a part of the Eurasian shield) and reaches a maximum height of 3000 meters. Most geologists consider Kupe-dagh to be the southern edge of the Turan shield and a part of the Eurasian plate. The main fault of Kopeh-dagh (Eshgabad) along the N130 direction forms the southern border of the Turan plate with the Kopedagh mountain range. A very small anomaly, the free-air gradient in the northeast of the Ashgabat fault, indicates the subduction of the southwest-trending Turkmenistan plate beneath Kupeh-dagh. In this way, less displacement is observed between the Turkmenistan plate and the southeastern Caspian lowlands compared to the displacement between the Turkmenistan and Iran plates that can be proved from the value of the gravity anomaly in the west.
There are large gas fields shared by the three countries of Iran, Turkmenistan and Afghanistan in the Kopeh-dagh region and its surrounding areas. Huge gas fields in Iran, Daulatabad-Donmez, Ghazli, Shatlik, Mehri and Bayran Ali in Turkmenistan and Gogar in Afghanistan have been discovered in this area. Geographically, Kopeh-dagh is part of the eastern continuation of the Alborz Mountains, but its geological and structural features are different from the surrounding areas. One of the main goals in the exploration of oil resources is to describe the structure of the sedimentary cover and underground topography. Furthermore, oil production is very sensitive to heat storage by the source rock and thus to the tectonic evolution of the entire lithosphere.
Results show that the thickening of the Moho is observed under the Kopeh-dagh mountain range and the thickness decreases gradually when moving towards the northeast and southwest of the Kopeh-dagh mountain range. The depth of the Moho in the studied area varies from 40 to 60 km. According to the modeling results, the lithosphere-asthenosphere boundary in the southwestern part of the studied area (Central Iran) is approximately 100 km, and it reaches approximately 200 km towards the northeast of the this area.

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References

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Journal of the Earth and Space Physics
Volume 49, Issue 3
online publication date: 15 Nov 2023
November 2023
Pages 633-647

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