Elastic thickness of the Iranian lithosphere from gravity and seismic data

Highlights

• We estimated the lithospheric elastic thickness map of Iran based on combination of gravimetric and flexure theories of isostasy.

• We found large elastic thickness for central Iranian block.

• We observed a low lithospheric strength along the Zagros and Kopeh Dag fold and Thrust Belts.

• Our model reveals low lithospheric strength of the Arabian shield containing different thermal state of the Arabian lithosphere.

• Makran accretionary complex is characterised by a weak lithospheric strength.

Abstract

We estimate the (effective) elastic thickness of the Iranian lithosphere (and adjoining tectonic plates) by using the approach that combines the Vening Meinesz-Moritz's (VMM) regional isostatic principle with the isostatic flexural model formulated based on solving a flexural differential equation for a thin elastic shell. To model the response on a load more realistically, we also consider the lithospheric density structure. The resulting expression describes a functional relation that links gravity field and mechanical properties of the lithosphere. The Young modulus and the Poisson ratio are computed from seismic velocity data in prior of estimating the lithospheric elastic thickness. The presented results reveal that the estimated elastic thickness closely resembles a regional tectonic configuration associated with the extensional tectonism along the Red Sea-Gulf Rift System, the continental collision of the Arabian and Eurasian plates, and the subduction along the Makran Subduction Zone. Seismically and volcanically active convergent tectonic margins of the Zagros and Kopeh Dagh Fold and Thrust Belts further extending along the Makran Accretionary Complex are characterised by a low lithospheric strength, with the elastic thickness typically less than ∼30 km. These small values of the elastic thickness are in a striking contrast to much larger values within most of the Central Iranian Blocks. According to our estimate, local maxima there reach ∼70 km in the Tabas micro-block. The elastic thickness of the Turan and Arabian Platforms reaches maxima of ∼100 km. These results generally support the hypothesis that tectonically active zones and orogens have a relatively low strength, resulting in a significant response of the lithosphere on various tectonic loads, compared to a significant strength of old cratonic formations. Interestingly, however, we observe a striking contrast between a low strength of the Arabian Shield compared to a high strength of the Arabian Platform. A possible explanation of this finding could be given by a different thermal regime of the Arabian lithosphere, controlled mainly by a mantle upwelling and a consequent extensional tectonism along the Red Sea-Gulf Rift System.

Introduction

The strength and (effective) elastic thickness of the lithosphere govern its response to tectonic and surface processes (e.g. Tesauro et al., 2012). The knowledge of strength distribution contributes to a better understanding of mechanisms behind deforming processes, such as rifting, orogenesis, isostasy, postglacial rebound, origin and evolution of sedimentary basins, seismicity, and volcanism. The effective elastic thickness of the lithosphere defines a thickness of elastic layer that would respond to applied loads in the same way as a heterogeneous lithospheric plate. This parameter thus provides a valuable information about the stress state of the lithosphere caused by tectonism, such as subductions and orogenic formations that create loads on the surface and within the lithosphere (e.g. Watts, 2001; Tassara et al., 2007).

The effective elastic thickness T_e of the lithosphere depends on many factors such as a composition and density structure, stresses and thermal states, and the geometry and mechanical properties of the lithosphere (e.g. Lowry and Smith, 1995; Burov and Diament, 1995, Burov and Diament, 1996; Pérez-Gussinyé et al., 2004; Pérez-Gussinyé and Watts, 2005). Whereas a thermal gradient mainly controls the oceanic lithosphere, the continental T_e depends not only on a thermal gradient, but equally also on a continental crustal thickness and composition. The continental T_e estimates show large variations in plates of the same thermotectonic age (cf. Burov and Diament, 1996). In general, old tectonic provinces (>1.5 Gyr) have the lithosphere that is colder and thicker, more depleted in basaltic constituents, and therefore, more dehydrated than younger ones (e.g. Jordan, 1979). Recent studies of T_e for large continental areas (e.g. Audet and Bürgmann, 2011; Flück et al., 2003; Pérez-Gussinyé and Watts, 2005; 2009 Pérez-Gussinyé et al., 2009; Simons et al., 2003; 2003b; Swain and Kirby, 2003b; Tassara et al., 2007; Tesauro and Kaban, 2013) revealed high values of T_e (>60 km) located within older provinces where the lithospheric thickness is much larger than in younger provinces. These findings suggest that continental cratonic interiors are more resistant to deformations (e.g. Tesauro and Kaban, 2013). In active orogenic belts, for instance, the lithosphere becomes weaker with time due to a crustal thickening as well as flexural stresses that are caused by a lithospheric bending due to topographic and horizontal tectonic loads. Consequently, a thick continental crust becomes sufficiently hot to reduce its strength significantly. This process results in a mechanical decoupling between the upper crust and the lithospheric mantle, and consequently in a large reduction of T_e (e.g. Burov and Diament, 1996). Other factors, such as reheating and hydrating of the lithospheric mantle, occurring in the continental mobile mountain belts located in back-arc regions, could decrease the continental T_e (e.g. Hyndman et al., 2005). The extensional tectonism along continental rift zones can also weaken the lower crust, leading to a subsequent crust-mantle decoupling and a reduction of T_e (cf. Cloetingh and Burov, 1996; Burov, 2011).

Methods of estimating T_e from gravity and topography are divided into forward and inverse techniques (Watts, 2001). In the forward modelling, load structures are known (for example seamounts and sedimentary basins), and T_e is estimated based on applying a trial-and-error principle. The continental T_e is typically estimated indirectly by using a cross-spectral analysis (i.e. admittance or coherence) of gravity and topography data, especially in case when the lithospheric strength is unknown. A number of authors investigated the continental T_e. Forsyth (1985) modelled T_e based on the coherence method in the Fourier domain (e.g. Watts, 2001, p. 195). The coherence analysis was later applied, for instance, by Djomani et al. (1995), Doucouré et al. (1996), McKenzie and Fairhead (1997), Ojeda and Whitman (2002), Swain and Kirby (2003a, 2003b), McKenzie (2003), Audet and Mareschal (2004), Gómez-Oritz et al. (2005), Tassara (2005), and Tassara et al. (2007). Other authors applied the admittance analysis. Among these studies, we could mention work by McGovern et al. (2002), Pérez-Gussinyé et al. (2004), Pérez-Gussinyé et al. (2007, 2009), or Galán and Casallas (2010). A review of inverse spectral methods can be found in Kirby (2014).

Results from a cross-spectral analysis often provide inconsistent estimates of T_e; for a more detailed discussion we refer readers to studies by Artemjev and Kaban (1991) and McKenzie (2010). Estimates of T_e for cratons based on the transfer function (admittance) between the free-air gravity and topography are, for instance, in some cases significantly lower than values obtained from the coherence analysis between the Bouguer gravity anomalies and topography (e.g. McKenzie, 2003). This discrepancy has been attributed to limitations in applying the admittance method for elevated and actively deforming continental regions (cf. Artemjev and Kaban, 1991; Burov, 2011). McKenzie (2010) compared the coherence and admittance methods, and demonstrated that both results differ by as much as an order of the magnitude in those continental regions where topographic variations are small. At wavelengths larger than 500 km, the continental admittance is often controlled by a dynamically supported gravity and topography, mantle convection, or postglacial rebound. The elastic thickness will be overestimated, if these effects are not removed. For shorter wavelengths, however, elastic forces in the crust and the lithosphere control the admittance. When the coherence is large, the coherence and admittance results are similar, but over flat regions, the coherence does not deliver reliable estimates of T_e.

Combined gravity and topographic information alone might not be sufficient enough to provide realistic estimates of T_e. To address this issue, some authors considered additional parameters or different theoretical approaches. Tesauro and Kaban (2013) presented a global model of T_e while considering variations of the Young's modulus within the lithosphere. Chen et al. (2015) applied the fan-wavelet method to estimate T_e from gravity, topography, and recent models of sedimentary basins. Tesauro et al. (2017) took into consideration temperature, composition, and strain rates of the lithosphere. Isostatic theories have also been used to study the lithospheric properties (e.g. Turcotte et al., 1981; Calmant et al., 1990; Filmer et al., 1993; Burov and Diament, 1995; Stewart and Watts, 1997; Johnsson et al., 2000; Braitenberg et al., 2002; Jordan and Watts, 2005). Employing isostatic theories, Eshagh (2018) derived a new technique to estimate T_e based on combining flexural and gravimetric isostatic models in the spherical harmonic domain. This method involves the information about a lithospheric density structure (including sediments, underlying crystalline crust, and lithospheric mantle), and crustal thickness variations. In addition, he considered mechanical properties of the lithosphere (Young's modulus and Poisson's ratio) computed from seismic velocity data (see also Eshagh et al., 2018; Eshagh and Pitoňák, 2018).

In this study, we applied the method developed by Eshagh (2018) to estimate T_e of the Iranian lithosphere, characterised by a rather complicated geological structure. Two studies of T_e in Iran were published, providing quite contradicting results. Abbaszadeh et al. (2013) used the admittance method and two sets of terrestrial and satellite-based gravimetric data. They concluded that both gravity datasets are equally adequate for estimating of T_e. Their finding is obviously not surprising because high frequencies of gravity and topographic data are compensated by the lithospheric strength over their selected areas. More recently, Zamani et al. (2014) used the wavelet-coherence method of the Bouguer gravity anomalies and topographic data, and compared their T_e estimates with the result presented by Abbaszadeh et al. (2013). According to their comparison, the coherence analysis provides typically larger estimates of T_e than the admittance. This finding agrees with McKenzie (2010) who demonstrated that the admittance method underestimates the continental T_e.

According to the result by Abbaszadeh et al. (2013), maxima of T_e were detected along the High Zagros and Sanandaj-Sirjan Zone and the Alborz Mountains, and minima in the Persian Gulf, the south part of the Zagros Fold and Thrust Belt, and the East Iranian Belt. These results are not fully consistent with the expected low lithospheric strength along active convergent margins (including orogenic belts). The result presented by Zamani et al. (2014) closely mimics the geological configuration of Iran, but not necessary its tectonic evolution. According to their result, maxima of T_e (>60 km) are located in the Caspian Sea Basin and minima of T_e (<15 km) in the Tabas micro-block (including the East Iranian Belt). These findings generally agree with the expected spatial behaviour of T_e. Maxima of T_e are in agreement with the fact that the Caspian Sea Basin underlain by a rigid lithospheric block (cf. Jackson et al., 2002; Chen et al., 2015). Nevertheless, small values of T_e in the Tabas micro-block might not be fully justified. Stöcklin (1968) and Nabavi (1976), for instance, proposed that this block is rather rigid. Similarly, (1984, 1988); Jackson and McKenzie, 1988 suggested mostly on the base of seismological observations that the Central Iranian Blocks can be regarded as rigid. Thus, this region might be characterised by a more pronounced strength of the lithosphere. To address these inconsistencies, we closely examined both solutions by comparing them with our estimate of T_e. Moreover, our elastic thickness estimate extends over adjoining tectonic plates in order to get a broader picture of the lithospheric strength of the whole region that involves major geological features such as the Turan, Caspian, and Arabian Platforms and the Arabian Shield.

The study is organised into six sections, starting with a review of the method developed by Eshagh (2018) to compute T_e in Section 2. Tectonic setting of the study area and input datasets are described in Section 3. Results presented in Section 4 are compared with existing studies in Section 5. Major findings are discussed in Section 6 and concluded in Section 7.