Document Type : Research Article
PhD. Student of Geodesy K.N.Toosi university of technology
Professor, Department of Geodesy, Faculty of Geodesy and Geomatics Engineering, K.N. Toosi University of Technology, Tehran 19697-64499, Iran
Mohorovičić discontinuity (commonly called Moho) is the boundary between the Earth's crust and mantle. Isostatic-gravimetric and seismic methods can be used to determine the boundary. Moho separates the oceanic and continental crust from the lower mantle. In other words, Moho is simply a physical / chemical boundary between the crust and the mantle that can cause significant changes in geophysical properties, such as seismic wave velocity, density, pressure, temperature, etc. (Bagherbandi et al., 2011). (Martink, 1994) and (Mooney, 1998).
An accurate, high-resolution Moho depth model is important in geodesy, geology, and geophysics, geodynamic modeling, and seismic tomography, seismic hazard assessment, or understanding of seismic source mechanisms and other applications (Guido et al., 2019). In addition, a proper Moho is able to determined crust structure which can provide valuable information on the heterogeneities of the deeper upper mantle layers, which are relevant for the calculation and analysis of gravity, geothermal, and geomagnetic models (Stalk, 2013). There are several Moho models, but their accuracy and resolution are not satisfactory in the Makran subduction zone. On the other hand, in subduction zones, Moho has a complex geometry. (Taghizadeh Farahmand et al., 2014) As a result, existing models do not provide adequate resolution for the Makran subduction zone. Makran region is about 1000 km long and is located in the southeast of Iran and southwest of Pakistan (Prestley et al., 2022), (Dashtbazi et al., 2019), (Shadmannamen et al., 2010) and (Dashtbazi et al., 2023).
Combined methods to determine the depth of Moho in the absence of seismic points with suitable density and coverage are mainly used in geophysical and geodetic studies. These methods include the Parker-Oldenberg method and the Vening Meinesz-Moritz method. In order to improve the existing models of Moho depth in Makran subduction zone as a complex tectonic zone, two different models named BC and SC were developed by gravity-seismic combination method. Global data (CRUST1.0) and (VMM) were used as seismic and gravitational data, respectively, in an appropriate manner with two approaches of filtering and spectral composition and using least squares adjustment. The obtained models have a resolution of 5'×5' arc-degree corresponding to 9×9 km grid size. The obtained Moho accuracy was evaluated with four different regional and local models. RMS of the results are obtained 5.28, 1.55, 4.18 and 1.27km for the BC model and 5.59, 1.17, 3.74 and 3.04 km for the SC model, respectively. The Moho depth model obtained for Western Makran in Iran has significantly improved the accuracy and resolution of Moho depth models in the study area.
SC Moho Model in Makran subduction zone which is proposed in our study has better RMS in comparison BC Combination Moho model so It can be used as the first priority. Although the Moho depth models developed in our study could significantly improve the existing Makran subduction zone models, but, using much more accurate seismic data or Moho model in Makran subduction zone in Iran would improve this study. Finally, it is suggested that the Moho model in eastern Makran in Pakistan be studied in the same way as presented in this study. To compare the situation of Moho between the western and eastern Makran.