“EVALUATION OF DIFFERENT GRAVIMETRIC METHODS TO MOHO

The complexity of geological units in Iran because of several unique events like tectonics and orogenic activities in this region led to extensive investigations for Moho recovery by seismic methods therein. In this research, three gravimetric methods have been evaluated by some point-wise seismic data. We applied collocation method as an iterative process as well as modified forms of Sjöberg and Jeffrey’s theory of isostasy for local Moho depth recovery. The gravity data has been generated by GOCO03S model reduced by topography/bathymetry, sediment and consolidated crust effects. Although the iteration process in collocation approach only slightly changed the estimated depths, this method led to a better agreement with seismic data rather than others. Differences between collocation, Jeffrey and Sjöberg’s solutions with seismic studies are similar but Jeffrey and Sjöberg’s methods displayed a systematic bias. The standard deviations of the residuals among seismic data and gravimetric solutions are around 6 km. Overall, the evaluation of these approaches indicated that Moho from gravimetric approaches reduced only slightly the standard deviation of seismic Moho estimates. Significant discrepancies with seismic data have been detected in Makran subduction zone, Oman Sea, Persian Gulf and Caspian Sea. The explanation of such inconsistency can be partially due to the poor quality of CRUST1.0 data in these areas, as this model has been used to correct the gravity values that were input in the inversion procedures. the Gravity Recovery and Climate Experiment (GRACE) [Tapley et al., 2004a; Tapley et al., 2004b] and the Gravity field and steady-state Ocean Circulation Explorer (GOCE) [Floberghagen et al., 2011]. These two missions have provided global gravity field with an accuracy and a resolution which is suitable for investigating the Moho structure. Among successful seismic estimates, the early results dated back to Beloussov et al. [1980]. The most widely known crustal model based on seismic refraction is CRUST5.1 model [Mooney et al., 1998]. Bassin et al. [2000] further upgraded it and called the new version, CRUST2.0. The CRUST1.0 [Laske et al., 2013] including ice layers, water, sediments and consolidated crustal layers, is the most recent version compiled with a 1×1 arcdeg spatial resolution. As already pointed out, because of insufficient seismic data coverage over large areas, gravimetric or combined gravimetric/seismic solutions have been utilized. Based on some isostasy hypotheses on compensating the Earth’s topographic masses, gravity data can be used to determine the Moho depth. Several basic theories were suggested to explain the mechanism of isostasy like those proposed by Pratt-Hayford [Pratt, 1855; Hayford, 1909] and Airy-Heiskanen [Airy, 1855; Heiskanen, 1931]. Vening Meinesz [1931] modified the Airy-Heiskanen theory by considering a regional instead of a local compensation based on a thin plate lithospheric flexure model [Watts, 2001]. Vening Meinesz theory had been modified by Parker [1973] in an iterative approach for Moho determination and Oldenburg [1974] made an attempt to stabilise this method by applying a low-pass filtering technique. The combination of these two methods was known as a ParkerOldenburg method and it has been generalized for the 3-D gravity inversion by Gómez-Ortiz and Agarwal [2005] and Shin et al. [2007]. Braitenberg et al. [2000] developed a similar method with integration of seismic data; also they estimated variation of the Moho under the Tibet plateau by an iterative inversion method. Moritz [1990] improved the Vening Meinesz inverse problem for a global compensation by adopting the spherical approximation model of the Earth. Since there were some theoretical deficiencies in this isostatic method, Sjöberg [2009] reformulated Moritz’s theory and called it as the Vening Meinesz Moritz (VMM) problem. In this approach he solved a non-linear Fredholm’s integral equation of the first kind. Bagherbandi and Sjöberg [2012] made a comparison between the gravimetric VMM and local Airy-Heiskanen methods in the determination of Moho depth. Another approach based on the inversion of gravity data to determining the Moho depth is collocation method [Krarup, 1969; Tscherning, 1985; Moritz, 1990]. Barzaghi et al. [1992] proposed an approach based on collocation principle, which propagate the covariance structure of the depth interface to the covariance function of the observed gravity field. Barzaghi et al. [2015] applied collocation method by combining the global gravity-gradient information of GOCE and local gravity data to Moho recovery. Barzaghi and Biagi [2014] implemented then a further version of the collocation method by including seismic Moho depths as input data. Braitenberg and Ebbing [2009] studied the structure of the crust by combination of GRACE and terrestrial gravity data. Some other scientists made attempt to estimate Moho depth by applying GOCE gravity gradient data [Braitenberg et al., 2010; Sampietro, 2011; Sampietro et al., 2014]. Reguzzoni et al. [2013] combined seismic and GOCE data to obtain a new global Moho model. Tenzer et al. [2009, 2011, 2012a, 2012b] proved that applying the crustal density-contrast stripping corrections is appropriate for a gravimetric Moho recovery. They showed that gravitational contributions of topography and major known crustal density structures have a large spatial correlation with the Moho geometry. In the area under investigation in this paper, several studies based on seismic data have been performed for the determination of the regional Moho model. Most of these studies concentrated on a specific area, for instance profiles between Shiraz-Mashhad, Tehran-Mashhad and Mashhad-Tabriz [Asudeh, 1982], the southern part of the Caspian Sea [Mangino and Priestley, 1998], Tehran region [Hatzfeld et al., 2003], Mashhad [Doloei and Roberts, 2003; Javan Doloei, 2003], Central Alborz and the northern Iran [Sodoudi et al., 2009. Radjaee et al., 2010], central Zagros [Paul et al., 2006, Shad Manaman et al., 2011], Kopeh-Dagh [Nowrouzi et al., 2007], Naein [Nasrabadi et al., 2008], the northwest Iran [Taghizadeh-Farahmand et al., 2015], and Sanandaj-Sirjan zone [Sadidkhouy et al., 2012]. Furthermore, based on terrestrial gravity data in this area with a quite homogenously coverage, Dehghani and Makris [1984] combined gravimetric and seismic data to determination of the crustal structure of Iran. Abbaszadeh et al. [2013] compared the effective elastic thickness of the lithosphere estimated by terrestrial and satellite data in Iran. Eshagh et al. [2017] reformulated two isostatic methods for Moho recovery by considering contributions of mean Moho over the whole Moho spectrum. In this paper, we adopt collocation method as well as two isostatic approaches to determine the Moho depths inverting gravity data over Iran. The collocation inversion method for a two-layer model devised by Barzaghi and Biagi [2014] is applied. In addition, the generalized form of Jeffrey and Sjöberg’s method proposed by EsEBADI ET AL.