Evaluating CRUST1.0 crustal model efficiency for Moho depth estimation in Middle East region

Document Type : Research

Authors

1 PhD student, / Department of Earth Sciences, Science and Research Branch, Islamic Azad University

2 Faculty member / Institute of Geophysics, University of Tehran

3 professor/ Department of Earth Sciences, Science and Research Branch, Islamic Azad University

Abstract

Study of Moho in Middle East and surrounding region is of great importance for scientists, because it has a rich geological history and contains parts of the Eurasian, Indian, African and Arabian plates as the main plates and some small plates. According to complexity and different tectonic structures in Middle East using a proper method that yields a Moho depth model which is in accordance with these structures, has a great importance. In this paper we compare the Moho depth obtained from two different methods, 1) Gravity data inversion of spherical prisms (tesseroids) and 2) Moho depth evaluation using tesseroids and CRUST1.0 crustal model. Determining of Moho depth from gravity data is a nonlinear inverse problem. Regarding the extent of the study area we use an efficient inversion method (Uieda’s inversion method) in order to consider the earth's curvature by using spherical prisms instead of rectangular prisms. In this method one needs to minimize the Γ(p)= ϕ(p)+ μ θ(p) cost function, where ϕ(p) is the fidelity term, θ(p) is the penalty term and μ is regularization parameter. In this method in addition to Moho depth, we need to estimate three hyper parameters namely the regularization parameter (μ), Moho reference level (h_n) and density contrast (∆ρ). They are estimated in two steps during the inversion by holdout-cross validation method .To estimating the relief of the Moho from gravity data, first one must obtain the gravitational effect of the target anomalous density distribution attributed to the Moho relief, this requires eliminating all gravity effects other than that of the target anomalous density from observed data. In the first method tesseroid modeling is used to calculate the gravity effect of the topography and sediments. The effect of topography and crustal sediments are removed using global topography and crustal models. In the second method first we extract Moho depth over the study region from CRUST1.0 model and then evaluate gravity effect arising from this anomalous Moho, then using inversion method to estimate the Moho depth from CRUST 1.0 model. According to the results, the minimum depth of Moho is about 12 km in some parts of Indian Ocean and the maximum depth is about 54 km in the west of Tibetan plateau from the first method which is in accordance with plate boundaries and correlates well with the prominent tectonic features of the Middle East region. The Moho depth obtained from the second method varies between 7.5 and 49 km where the minimum depth is related to the parts of Indian Ocean and maximum depth is appeared in parts of the Zagros in Iran. Comparing the results of two methods demonstrate the acceptable performance of the adapted inversion procedure and utilizing of spherical prisms but the calculated Moho depth from second method failed to estimate acceptable Moho depth especially in divergent boundary at Red sea, Gulf of Aden and Indian Ocean. The results indicated that the CRUST1.0 model, at least over an area with large extent, is not a suitable model for gravity inversion and Moho depth estimation.

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