Determining the Elastic thickness of the lithosphere in The Zagros Mountains using the Admittance function

Document Type : Research Article

Authors

1 Ph.D. Student, Department of Earth Physics, Institute of Geophysics, University of Tehran, Tehran, Iran

2 Professor, Department of Earth Physics, Institute of Geophysics, University of Tehran, Tehran, Iran

Abstract

Zagros orogeny is one of the most active orogenic belts among the mountain ranges extending approximately 2000 kilometers from the Anatolian fault in eastern Turkey to the Minab fault in southern Iran. Concerning the importance of this region as well as the essential role of elastic thickness in controlling the rate of deformation under applied loads, determination of Te in Zagros Fold and Thrust belt has been conducted. The lithosphere's elastic thickness (Te) is a convenient measure of the flexural rigidity, which is defined as the resistance to bending under applied loads.
To determine the elastic thickness of the lithosphere, the spectral admittance function is applied. We applied the load deconvolution of the admittance function between free-air gravity and topography data for estimation of Te. The Free air anomalies with a five arc-minute resolution are utilized in this study.
In flexural isostatic studies, the gravity and topography data are compared with theoretical models to estimate several parameters of the lithosphere. In the simplest model, a plate has been flexed by a surface load, with the magnitude of the resulting deflection, which is governed by Te.
Using the random fractal surfaces as the initial surface and subsurface loads applying at lithosphere, the lithosphere is modeled, and the post flexural gravity and topography are determined. Based on these new fields, the predicted admittance function is determined. Finally, the best-fitting Te is one that minimized the misfit between the observed and predicted functions. Additionally, the weighted misfit by the jackknife error is applied to estimate the observed admittance.
The accuracy of the method is checked through synthetic modeling. Two fractal surfaces are used as the two initial surface and subsurface loads applied to the lithosphere. After calculating the corresponding gravity and topography data by the load deconvolution method, the observed and predicted admittance are estimated. The best-fitting Te will be obtained by minimizing the misfit between observed and predicted functions. After confirming the accuracy of the method in Te determination, the technique will be applied to the real data acquired from the NCC as follow.
We consider a three-layered crust during the lithosphere modeling on which the internal loading is applied on the middle crust. To model the lithosphere, the global CRUST 1.0 is applied by treating the lithosphere as a three-layer crust.
The 2D map of Te variations in the target area is depicted by utilizing the load deconvolution of the admittance function between free-air gravity and topography data. High-precision ground gravity data, which is more accurate than satellite data, allows us to detect more details on Te variations in the region.
Based on the obtained results, the estimated range of Te in the survey region can be considered low to intermediate. This predicted range is in good accordance with the area's geology background as it is regarded as a young, active orogeny system. Te range and hence the lithosphere's predicted strength to deformation is supported by the previous studies using different geophysical and seismological studies. The mean value of Te in the area is 37±2 km. The maximum amount is detected in the Sanandaj-Sirjan zone. The overall predicted trend of Te follows the geological background of the region. Additionally, the estimated trend for Te and the strength to the applied load and deformation is in good agreement with the previous geophysical and seismological studies conducted in the region.

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References

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Journal of the Earth and Space Physics
Volume 47, Issue 1
April 2021
Pages 59-75

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