Crustal-Scale Structure of the Nasr-Abad Buried Salt Diapir in Northwest Central Iran from a Profile Magnetotelluric Dataset

Crustal-Scale Structure of the Nasr-Abad Buried Salt Diapir in Northwest Central Iran from a Profile Magnetotelluric Dataset | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Crustal-Scale Structure of the Nasr-Abad Buried Salt Diapir in Northwest Central Iran from a Profile Magnetotelluric Dataset Mansoure Montahaei, Elham Zare This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5255696/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Great Kavir is the largest salt desert in Iran, located in the northern part of the Central Iran depression. Groups of clustered salt diapirs exist in the northwestern part of the Great Kavir. The Nasr-Abad salt diapir in the Shurab diapiric group is the largest buried salt diapir in this region, whose geometry at depth and surrounding structure are rarely known. In this study, we investigate a broadband magnetotelluric (MT) dataset recorded at 37 closely spaced stations distributed along a SW-NE profile to characterize the geometry, substratum, and overburden of the Nasr-Abad salt diapir. The spatially distributed MT responses noted in this study are associated with geological structures at depths of less than 30 km.The measurements were used to generate a crustal-scale resistivity model of the study area, which correlates well with the known lithostratigraphy of the region. The resistivity model reveals a dipping resistive body that has been uplifted from a deep resistive layer to shallower depths. The geometry of this body indicates a well-defined base and a northeastward dip, suggesting that it corresponds to the Nasr-Abad buried salt diapir. Geophysics Magnetotelluric Electrical resistivity Central Iran Salt diapir Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 1. Introduction Superlative salt domes in the Great Kavir basin in Central Iran (CI) provide an ideal laboratory for observing the evolution of salt diapirism (Jackson et al., 1990). The Shurab diapiric group in the Qom basin, a western peripheral embayment of the Great Kavir basin, consists of four exposed and one buried diapir, and developed in the western CI. Surface studies prove that diapirs Nos. 1, 2, 3, and 5 are extruded, whereas diapir No. 4 forms one of the largest buried salt diapirs in the CI basin and is commonly named Nasr-Abad salt diapir (Baikepour et al., 2016; Roosta et al., 2019). Multiple reflections and energy scattering limit the ability of seismic methods to properly delineate the boundaries of salt formations (Newman et al., 2002; Key et al., 2006; Rubinat et al 2010; Leville et al., 2011). Accordingly, previous investigations of 2D seismic lines provide an approximate assessment of the salt structure in the Nasr-Abad region (Baikepour et al., 2016). However, salt diapirs are ideal targets for MT studies, since they are mainly composed of evaporitic rocks (with high electrical resistivity) enclosed by porous sedimentary rocks (less resistive), which provides an environment where electrical resistivity varies over a wide range. (Rubinat et al., 2010; Avdeeva et al., 2012; Key et al., 2006). In this study, we investigate an MT data set recorded along a profile over the Nasr-Abad buried salt diapir with a high density of measuring stations. The study aims to delineate the subsurface extension of the buried salt diapir and also to define variations in lithology and fluid content of the overburden structures. We re-analyze MT measurements along profile 21, including a detailed dimensionality and directionality assessment of regional geo-electric structures. We also update preliminary inversion results (Baikepour et al., 2016) by employing distortion-corrected MT data in this procedure. 2. Geological setting of the study area The MT data set investigated in this study has been acquired over the buried Nasr-Abad salt diapir, one of the constituents of the Shurab diapiric group in the western Central Iran Basin. Central Iran (CI), the Zagros Fold and Thrust Belt (ZFTB), the Sanandaj-Sirjan Magmatic Belt (SSMB), the Elburz Mountains, Kopetdagh, Makran, and Sistan-Baluchestan are the main tectonic blocks constituting the Iranian plateau (Fig. 1 a) and emerged due to the collision between the Arabian and Eurasian plates. Coinciding with the present-day morphological depression, the CI block has a triangular form bounded by the Elburz Mountains in the north and the Zagros Mountains in the south and undergoes a north-south shortening between the two ranges. As a consequence, 6–7 km thick evaporative deposits accumulated in the sedimentary basin of the Great Kavir have surfaced in the form of several diapirs in the Central Iran Basin. The Shurab diapiric group, composed of four exposed and one buried diapir, has developed in the western CI (Jackson et al., 1990). Strike-slip fault systems and thrust faults are highly evolved at the margins of the CI block. The Saveh-Qom area with a NW-SE trend and the Great Kavir Desert with a NE-SW trend are the two main constituents of the CI basin. The Shurab diapiric group lies in the Nasr-Abad area, southeast of Qom. The main geological structure in this region follows the major trend of the ZFTB and is aligned with the NW-SE direction (Roosta et al., 2019). Several Quaternary strike-slip and thrust faults have developed in the west of the CI block, where the Avaj, Kushk-Nosrat, Indes, Qom-Zefreh, and Dehshir faults (Fig. 1 b) are the most important ones and play a significant role in the tectonic evolution and structural development of this region. They have a prevailing right-lateral component of slip and most often develop in en-echelon arrays, producing restraining stepovers (containing thrusts, folds, and anticlines) between the fault segments (Babaahmadi, et al., 2010). The Khurabad fault is the largest normal or transtensional fault in the Saveh-Qom area, developed between the Elburz and Sarajeh anticlines, with outcrops extending to the south along a line. Stratigraphical studies have inferred that the Khurabad fault is the main factor controlling the large volume of local halite deposition during Lower Red Formation (LRF) time and strata patterns within Upper Red Formation (URF) time. Just to the southeast, the Khurabad fault is oriented north-northwest to south-southeast and forms the Abshirin-Shurab fault zone, exhibiting a significant normal component of offset. The Shurab diapiric group in the Nasr-Abad region extends to the northeastern margins of the Abshirin-Shurab fault outcrops (Morelley et al., 2009). The structural studies identify the Abshirin and Sen-Sen faults as crucial structural elements controlling diapirism in the study area (Moradi et al., 2019). The typical stratigraphic sequence in the Nasr-Abad region in the south of western CI consists of four sedimentary units in descending order: (i) Pliocene to Pleistocene conglomerates deposited on the (ii) evaporites and mudstones of the Upper Red Formation (URF, dated to the late Miocene). The URF is separated by the (iii) limestone, marls, shales, and sandstones of the Qom Formation (QF, dated to the late Oligocene to early Miocene) from (iv) late Eocene to early Oligocene sediments of the Lower Red Formation (LRF). Evaporitic rocks and sediments of the LRF effectively detach the late Oligocene to Pleistocene cover from the Eocene magmatic basement rocks. Furthermore, the sedimentary cover is deformed by a series of folds, thrusts, strike-slip faults, and halokinetic features that are mainly oriented in the NW–SE direction, coincident with the general geodynamic trend of the Zagros fold and thrust belt (ZFTB) (Morelly et al., 2009; Abbasi et al., 2020). 3. Magnetotelluric data in the west of Central Iran basin The magnetotelluric (MT) method utilizes time fluctuations of natural electromagnetic (EM) fields to reveal the electrical conductivity structure of the subsurface.Complex-valued impedance tensor \(\:\left(\underset{\_}{Z}\right(\omega\:\left)\right)\) and tipper vector \(\:\left(\overrightarrow{W}\right(\omega\:\left)\right)\) data, represent the frequency-dependent (ω = 2πf) MT response functions (transfer functions) of the subsurface geoelectric structure. These data are extracted from simultaneous measurements of the time variation of the horizontal electric \(\:\left(\overrightarrow{E}\right(\omega\:\left)\right)\) and full magnetic \(\:\left(\overrightarrow{H}\right(\omega\:\left)\right)\) field components on the earth surface: $$\:\left[\begin{array}{c}{E}_{x}\left(f\right)\\\:{E}_{y}\left(f\right)\\\:{H}_{z}\left(f\right)\end{array}\right]=\left[\begin{array}{cc}{Z}_{xx}\left(f\right)&\:{Z}_{xy}\left(f\right)\\\:{Z}_{yx}\left(f\right)&\:{Z}_{yy}\left(f\right)\\\:{W}_{x}\left(f\right)&\:{W}_{y}\left(f\right)\end{array}\right]\left[\begin{array}{c}{H}_{x}\left(f\right)\\\:{H}_{y}\left(f\right)\end{array}\right]$$ 1 The magnitude and phase of the Impedance tensor,along with the tipper vector (vertical magnetic transfer functions: VTF) are commonly visualized by the apparent resistivity (ρ a ) and phase (φ) sounding curves,as well as the real induction vectors \(\:\:\left(\overrightarrow{P}\right)\) : The presence of highly conductive phases (e.g., saline fluids, mineralization, and partial melt) and their interconnections could significantly reduce the bulk resistivity of the rocks and strongly influence the measured MT transfer functions. MT data from this study were collected at 37 stations distributed across the study area. The measured periods range from 0.003 to 2512 seconds. The MT sites were located along a SW-NE profile perpendicular to the general NW-SE trend of the ZFTB, managing the development of geological structures in this area (Babaahmadi et al., 2010).The stations were positioned based on the geological structures. Specifically, five sites were placed within the area of the Nasr-Abad buried diapir (diapir No. 4), and at least four additional sites were situated to the west of the Ab-Shirin fault, directly above the outcropping diapir No. 5. A site spacing of approximately 250 meters provides an acceptable resolution for shallow features. However, an investigation of the regional structures is also possible, considering the penetration depth of the MT data set. Applying the Niblet-Bostick method to the impedances of each site yields estimates of the maximum penetration depths of the MT data. The results (Fig. 2 ) indicate that the penetration depth of the MT responses varies along the profile. Beneath the SW portion of the profile, the responses generally penetrate to greater depths compared to the NE part. However, the MT dataset provides good data coverage up to a depth of 25 kilometers in the region. 4. Dimensionality and strike analysis of the MT data The geo-electric structure of the subsurface can exhibit different configurations: it may be horizontally layered (1-D), extend infinitely along the strike direction (2-D), or take on a more complex 3-D form. Information about the dimensionality and strike of the regional conductivity structure is typically derived from the internal structure of the MT transfer functions, including impedance tensor and tipper vector data. For the MT sites along Profile 21, dimensionality analysis of the impedance data is performed using Bahr’s phase-sensitive approach (Simpson and Bahr, 2005) and the phase tensor method (Caldwell et al., 2004). Figure 3 a displays the phase-sensitive skew (η) values within the measurement period range for all sites. Generally, the η skew values fall below the threshold of 0.3, beyond which the 2-D assumption for the regional structure is invalidated. The distribution of all calculated η skew values in Fig. 3 b reveals that 97% of the data points are smaller than 0.3. A more comprehensive assessment of the calculated phase-sensitive skew indicates that large values occur at longer periods (> 5 seconds) for a few stations (Fig. 3 c). Despite the theoretical assumption that both techniques (Bahr’s and phase tensor methods) provide equivalent estimates of dimensionality and strike direction, in the case of noisy and strongly distorted data, they may exhibit incompatible resolution and estimation properties (Chervatova, 2014). The novelty of the phase tensor technique lies in its immunity to galvanic distortions caused by unresolvable small-scale conductors. Additionally, it does not rely on assumptions about the regional conductivity structure for dimensionality and directionality analysis (Booker, 2014). Figure 4 presents a pseudo-section and the statistical distribution of phase tensor skew values (β skew angles) for the entire dataset. It appears that β skew angles are consistently less than 3° (a necessary but not sufficient condition for two-dimensionality) across most periods for all stations. A wide region beneath the northeastern part of the profile reveals low skew values throughout the entire period range. In the southwestern part of the profile, the regional conductivity structure appears more complex, and high estimates of β skew angles occur at long periods (> 5 seconds) in this region. Figure 4 b reveals that less than 14% of all data points deviate from those expected for a 2D regional conductivity structure. Overall, the phase tensor skew values are consistent with a 2D approach for further modeling and interpretation of our dataset. Geoelectric strike directions, determined from the phase tensor method for different period bands at all MT stations, are presented as cumulative values in the form of rose diagrams in geographic coordinates (histograms in the first row of Fig. 5 ). Strike estimations reveal significantly greater variability at the shortest periods (0.003-1 sec). This outcome may stem from the heterogeneity of localized structures within the small sampling hemisphere of EM fields during these time intervals. However, MT data indicate that 1-D induction occurs at these periods (see Figs. 3 and 4 ), resulting in an undefined preferred azimuth for the regional conductivity structure. As the period increases, the sampling depth of MT signals also increases, and the orientation of the regional azimuth ranges between N30˚W and N60˚W (with a 90˚ ambiguity). Real induction vectors (IVs) point perpendicularly away from the strike of the conductive anomaly and could be used as external information to resolve the strike ambiguity estimated from impedance data alone (Simpson and Bahr, 2005). The rose diagrams of the IVs in the same period range of impedance data are presented in Fig. 5 b. Generally, the induction vectors along profile 21 indicate geoelectric strike directions between N15˚W and N30˚W. 5. Distortion decomposition and removal Galvanic distortions of regional electric fields due to small near-surface conductivities may cause unreliable modeling and interpretation of the subsurface electrical conductivity structure (Chave and Jones, 2012). Exempt from the galvanic distortions, the phase tensor method provides information about the dimensionality and directionality of the regional geoelectric structure, as discussed in the previous section. However, this technique cannot remove galvanic distortions from measurements or recover regional impedances. Furthermore, it does not take into account experimental data errors when estimating dimensionality parameters and strike direction. In this section, we applied the Groom-Bailey (GB) decomposition, as extended by McNeice and Jones (2001), to examine the regional geoelectric strike and distortion matrix more rigorously and extract regional responses from MT impedances contaminated with galvanic scatter. The GB decomposition method assumes a 3D/2D model composed of a 2D regional geoelectric structure, whose electric field responses are galvanically distorted by 3D small near-surface conductivities. The method employs seven parameters: two complex-valued regional impedances, the strike of the regional conductivity structure, and shear and twist angles (distortion angles) for model characterization. Next, it applies a least-squares approach, weighted by measurement errors, to fit the 3D/2D decomposition model to the observed impedances within a frequency interval for a group of sites. The root mean square (RMS) misfit values from this procedure serve as an obvious indicator for regional 3D induction. Large values of this measure reveal that the assumption of a 2D regional conductivity structure (as adopted by the GB method) cannot be used for proper modeling and interpretation of the observed MT data. However, the 3D/2D distortion model inherent in the GB decomposition is valid if, by constraining decomposition parameters to specific values, it becomes possible to identify a sufficiently wide frequency band where the RMS misfit errors are acceptable (McNeice and Jones, 2001). The major advantage of the GB method is that it utilizes the entire information content of the measured impedances, taking their errors into account within a statistically robust framework to determine decomposition model parameters. The only disadvantage is the assumption of a 2D regional conductivity model intrinsic to this method (Sprat et al., 2009 and 2014). In the present study, the GB decomposition model is first determined for each period (unconstrained GB analysis) and then for one-decade period bandwidths. Finally, period-independent estimates of decomposition model parameters that fit the measured impedances are determined. The RMS misfit, strike azimuth, and the phase difference between the TE and TM modes—whose small values (< 15˚) indicate a low order of multidimensionality—are presented in Fig. 6 for exemplary stations along profile 21. The results are based on a low value of the impedance error floor (0.5%) and suggest a low level of multidimensionality, along with distortion angles (shear and twist angles).Most of the stations along the profile show small phase differences (indicating one-dimensionality) up to intermediate periods of 5 sec (figure S1). However, there is an exception for a group of three sites (21-3500 to 21-4000) located to the west of the Abshirin fault; over the outcrops of diapir number 5 (see Fig. 1 c). Furthermore, the RMS plots from decomposition models for each site indicate that, except for the aforementioned sites (21-3500 to 21-4000), the misfit estimates are less than 2 in all these experiments (Figure S2), implying a 2D regional geoelectric structure. Figure 7 presents histograms of the distortion parameters and the RMS misfit errors obtained from an unconstrained GB analysis of the measured impedances at all stations. The results indicate a low level of galvanic distortion in the study area. The results in Figs. 8 (a-c) indicate the pattern of strikes across the study region, as obtained from an unconstrained GB analysis of all measuring stations along profile 21. At short periods, where the regional conductivity structure is deemed 1D, it becomes difficult to determine a preferred strike azimuth. However, the resulting strike direction varies between N60°E at the intermediate periods (Fig. 8 b) and N30°E at the long periods (Fig. 8 c). It’s important to note that directionality analysis results based on the GB decomposition inherently have a 90° ambiguity (McNeice and Jones, 2001).We further investigate frequency-independent estimates of the regional geoelectric strike azimuth more rigorously by constraining its value to specified angles between − 45° and 45° while fitting distortion parameters. The distribution of RMS misfit errors calculated for individual sites at the specified regional strike angles (Fig. 9 ) may define the contribution of different sites to the constraint of regional strike (Adetunji et al., 2015). The misfit values for individual sites along profile 21 suggest that geological noise caused by near-surface inhomogeneities is distributed unevenly between the sites. The stations that are distributed close to diapirs No. 4 and No. 5 are more sensitive to the strike direction on profile 21. However, the misfit levels are reasonable at all regional strike azimuths, indicating that a 2D approach could be adopted for further modeling and interpretation of the subsurface resistivity structure. Furthermore, we calculate the mean value of all station misfits for each specified strike angle. The results show that the minimum overall misfit of the GB decomposition occurs for strike angles between − 15° and − 30° (dashed lines in Fig. 9 ). The results are consistent with the strike azimuths estimated by the phase tensor method and the directions of the induction vectors (Fig. 5 ). Furthermore, multi-site and frequency-independent GB analysis of all measured impedances (Fig. 8 d) yields an N30°W strike azimuth for all MT stations along profile 21, which is justified by the directions of real induction vectors (Fig. 5 b). Based on the above consistencies, we constrained the geoelectric strike direction to N30°W azimuth and set the galvanic distortion parameters (the shear and twist angles) to 0° in order to remove distortion effects and recover regional impedances. To illustrate locations where data are sensitive to the predefined distortion parameters, we plotted the RMS misfit values obtained from frequency-independent GB analysis against increasing periods for each site along profile 21. The results are obtained assuming a default error floor of 3.5% for impedance data. For this error level, misfit values smaller than 2 indicate that the constrained 3D/2D GB distortion model accurately represents the measurements (Sprat et al., 2009 and 2014). The results (Fig. 10 ) show that an acceptable RMS misfit below two is obtained at almost all sites and for almost all periods. The main exceptions are the long-period (> 100 sec) responses of a few stations and also the short-period responses of station 21-3500, where the misfit ranges between 2.0 and 3.0. 6. 2-D resistivity model Using a non-linear inversion approach, we employed distortion-corrected MT data that had been decomposed along the regional strike direction (N30˚W) to model the crustal conductivity structure of the study region. Inaccurate data, represented as scattered points with large error bars, as well as data points whose corresponding apparent resistivities and phases did not align with the D + assessment (Parker and Whaler, 1981), were removed from the MT dataset prior to inverse modeling. The galvanic distortions that contaminated the apparent resistivity curves were exceptionally removed using the GB decomposition method. However, the static shift factor—an amplitude component that shifts the TE or TM mode apparent resistivities in parallel—remains unknown. To better understand the data and the static shift range, we estimated the shift magnitude as the ratio of the median ρ TE to ρ TM for the 1D segment of the curve at each station. The proportion varies between 0.5 and 1.16, with a clustering around 0.8 to 1.1 for all stations along profile 21 (Figure S3). Next, we calculated the deviation between the median TE and TM resistivities at each site. This analysis reveals that for all stations, the median TE and TM resistivities are within 33% of each other and according to the Kirkby et al., (2019) they remain largely unaffected by the static shift factor. 6.1. Qualitative information from MT data pseudosections Significant features of the regional conductivity structure can be discerned from the pseudosections of MT data, using a coordinate system aligned parallel and perpendicular to the estimated strike direction (Fig. 11 ). The most striking feature in all pseudosections of impedance data is the discontinuity of the resistivity structure induced by the Nasr-Abad salt intrusion beneath stations 21-7000 to 21-8000. In this region, the clean white salt of the LRF and the multicolored salt of the URF result in high apparent resistivities (Fig. 12 a), which coincide with low values of impedance phases (Figs. 11 b, c). There is also a clear indication of a resistive salt body in the TM mode data beneath the southwestern end of the profile, near Shurab Diapir No. 5 (Figs. 11 a, b). However, the massive deep salt deposits in the Qom Basin are also evident from the low values of impedance phases at periods greater than 10 seconds (Figs. 11 b, c). Furthermore, the TE and TM mode pseudosections are influenced by the fine-grained Quaternary sediments in the upper crust. Coincidently, low apparent resistivities and elevated phases occur at short periods between 0.1 and 1 second. The strong positive response of the tipper data at periods longer than 10 seconds beneath the southwestern part of the profile indicates significant conductivity contrasts between the Shurab diapirs and the surrounding sediments (Fig. 11 d). 6.2 Modelling approach In this stage, a 2-D Earth model is sought to appropriately explain the MT measurements. The result is constructed using a regularized inversion scheme that fits the MT responses from an over-parameterized model space to the apparent electrical resistivities and phases of both impedance modes, as well as the tipper data (which have been corrected for distortion effects). The MT data from 37 stations along profile 21, covering the period range T = 0.0034-2512 s, are imported into the WingLink software. We utilize a non-linear conjugate gradient (NLCG) algorithm developed by Rodi and Mackie (2001) to solve the MT inversion problem.The algorithm minimizes a Tikhonov-regularized objective function (Φ) composed of a data misfit norm weighted by the data variances (Φ d ) and a model roughness functional (Φ m ) to generate 2-D models that are maximally smooth: Φ = Φ d + τ Φ m (4a), Φ m = L || m || (4b) τ is usually referred to as the regularization parameter, controlling the weight of the Φ m and Φ d functionals throughout the minimization of the objective function. m is the model vector containing the logarithm of the resistivity of each model cell. In the following experiments, L has been chosen as the integral of the weighted Laplacian squared of the model parameters, calculated using the actual model mesh. Regularization imposed on the model during inversion might be further increased both horizontally and vertically by the α and β factors (increasing horizontal and vertical derivatives, respectively), as well as by a predefined size of the model block dimensions (H 0 /V 0 ). 6.3 Inversion procedure In order to satisfy the boundary conditions, the model space expands 6600×2720 km in the –y and –z directions, much larger than the study region of profile 21. It is discretized into 162×83 rectangular cells whose conductivities are constant. The cell sizes are smallest at the surface just below the sites and enlarge with increasing depth and distance from the profile. Here, the smallest cells have an area of 70×3 m², corresponding to 0.24×0.01 of the EM field skin depth (503√ρT ≅ 293 m) at the shortest periods of 0.0034 seconds, assuming an average resistivity of 100 Ωm. We applied an L-curve study to estimate the optimal value for the regularization parameter (τ). Initial inversions with different τ values were conducted, and for individual experiments, the data misfit was plotted against the model roughness. The resulting trade-off curve is presented in Fig. 12 . According to this figure, the value of τ = 3, located at the corner of the L-curve and minimizing the Φ m and Φ d terms simultaneously, is adopted as the optimal estimate of the regularization parameter in the following inversions. Further numerical experiments show that weighting the regularization term in a manner where both horizontal and vertical derivatives increase by a factor equal to the size of each model cell leads to a more stable inversion result. In order to assess how the inversion outcome is influenced by the choice of the initial model, we tested two different kinds of starting models: homogeneous half-spaces with resistivities purely based on assumptions (100 or 1000 Ωm) and a double-layer starting model composed of a 2-kilometer-thick upper layer with 100 Ωm resistivity overlying a half-space with 1000 Ωm.The latter starting model enhances the sharp discontinuity between conductive surficial sediments and deeper resistive diapiric rocks. The results (Figs. 13 a-c) converge to reasonable RMS misfits of 1.56–2.06. Visually, the models recover similar conductive and resistive features beneath the profile. However, our preferred starting model in the subsequent numerical experiments is the half-space with 100 Ωm resistivity, which provides the lowest RMS misfit across the profile for individual stations. We initiated the inversion procedure based on the TM mode phases by allocating error floors of 2.5% (0.7°) and 10,000% for the TM phases and resistivities, respectively. Then, we incorporated TE mode data into the inverse modeling and sequentially reduced the error floors of the TE phases, TM resistivities, and TE resistivities. Finally, we included the tipper data in the inversion procedure. The error floors for the final inversion result (Fig. 14 a) were set to 2.5% (0.7°) for both mode phases, 5% for the TM mode apparent resistivities, and an absolute value of 0.03 for the tipper data. We assigned a high error floor of 200% to the TE mode apparent resistivity to mitigate the destructive influence of the static shift effect. By closely fitting the phase data, this procedure leads to ρ TE responses that are shifted equally at all frequencies while maintaining the curve shape (Chave and Jones 2012; Comeau et al. 2020). For comparison, we present the results of separate 2D inversions of the TE and TM mode impedances, as well as the TP data and a joint TE + TM + TP inversion, in Fig. 14 . The individual mode responses of the inversion models fit the measurements reasonably well. The TE and TM mode impedances generally produced comparable results, consisting of a conductive upper layer and a massive resistive zone in the middle crust. Furthermore, the vertical magnetic field is zero above a conductive layer (Brasse et al., 2009), leading to a weakly recovered surficial conductor in the inversion result of the tipper data (Fig. 14 d). A test inversion run of joint TE and TM mode impedances shows that including induction vectors in the inversion procedure has a minor influence on the result (Figure S4). The final inversion model (Fig. 14 a) reveals that, besides the disconnected surficial conductive layer, the entire crust in the study region is generally characterized by relatively high resistivities in the range of 200 Ωm. Careful comparisons indicate that the MT responses of the final inversion model reasonably fit the measurements, with the largest RMS occurring for the TE mode phases (Fig. 15 and left column in Fig. 11 ). 7. Results and interpretation In this section, magnetotelluric analysis result (Figure 16a) and a detailed geological cross-section (Figure 16b) obtained from field data (Moradi et al., 2019) are combined to interpret the main structures of the study region recovered in the shallow part of the MT profile. Considering the evident uncertainty in the size and lithology of each geologic unit, we overlaid the MT model with the geological cross-section in Figure 16a. The model confirms the existence of the main tectonic units (faults and the buried diapir No. 4) and shows that the resistivity model correlates well with the known lithology of the study area. The main similarities between the geology and the MT model with increasing depth are as follows: (i) Surficial Pliocene conglomerates (PL c1 ) show resistive responses in the inversion model, except at their bottom, where they appear more conductive. This conductivity could be caused by fluid circulation beneath their contact with the underlying permeable mudstones. (ii) Outcropping highly saline mudstones and marly sandstones (Miocene in age) of the URF (M c ) are correlated with an outcropping conductive surficial layer beneath the profile (the conductor C 1 ). (iii) Eocene evaporites interbedded with red sandy shales and limestones in the LRF (O s ) appear as a resistive plug (resistor R 1 ) and a deeper resistive layer (resistor R 2 ). (iv) More resistive levels beneath the Miocene (O s ) mudstones have been related to the marly limestones and shaly sandstones of the Qom Formation (OM 1 ) and also to the marl stones in the upper part of the LRF (O c ). (v)Beneath this variation of resistive and conductive layers related to lithological alterations in the sedimentary cover, the highly resistive layer in the deeper part of the model (resistor R 3 in Figure 14a) has been correlated with the basalt, dacite, and andesitic lava flows of the basement rocks (E v ). Accordingly, the MT inversion results show that the resistivity variation in this region is mainly controlled by lithology. The highest resistivity values correspond to the salts of the LRF and volcanic basement rocks. The medium resistivity values are associated with the marlstones and sandstones of the QF, while the lowest resistivity values correspond to the mudstones of the URF. In the study area, the lithology of the outcropping diapiric rocks (halite-dominated evaporites) shows a good correlation with the resistivity model and appears as a highly resistive body beneath the most southwestern part of the profile. 8. Conclusions We enhance our understanding of the subsurface structure in the Nasr-Abad region based on the lithostratigraphic image suggested by the MT model, extracting information that is inaccessible through geological surface data. The main features suggested by the MT model are: (i) The Abshirin and Sen-Sen faults affecting the overburden correlate well at depth with abrupt variations in the resistivity pattern in the MT model. Specifically, they appear as the lateral loss of resistive and conductive features. This is well illustrated beneath the southwest of the profile, where the high-resistivity structure (R 2 ) related to the LRF evaporites disappears to the east of the Abshirin fault (figure 16a). (ii) The top of the Eocene volcanic basement exhibits depth variations throughout the study area (R 3 in figure 14a). Accordingly, the thickness of the sedimentary overburden varies along the profile. The URF and QF parts of the sedimentary cover (3-72 Ωm) seem to vanish beneath the southwestern part of the profile, whereas they are still visible at a depth of 4 km in the northeastern part of the profile. This is correlated with the structural studies and radar interferometry information in this region (Morley et al., 2009 and Roosta et al., 2019) and shows that the Eocene volcanic rocks are locally thrusted and uplifted by as much as ~5 km above the adjacent basins. (iii) Sedimentary layers of the Tertiary overburden are tilted near the Nasr-Abad diapir and exhibit more complexity on the western flank than on the eastern one (figure 16a). (iv) The Nasr-Abad diapir is nearly axisymmetric, with the eastern and western walls dipping approximately 50°. The slight asymmetry is attributed to salt rising up a steeply inclined stem, being guided to the northeast. This is consistent with the structural studies indicating that hillocks are distributed at the surface along the northeastern boundary of the dome No. 4 (Baikpour et al., 2016). (v) Evaporites of the LRF appear as a continuous resistive layer (resistor R 2 ) in the deeper part of the MT model. This supports the presence of a salt canopy, which was previously suggested by Jackson (1990) based on the tightly clustering diapirs in this region. As a consequence, we successfully established a correlation between the MT inversion model and lithology information to characterize the deep geometry of the Nasr-Abad buried salt diapir and its adjacent overburden. Specifically, we unraveled: (1) the nearly symmetric shape of the Nasr-Abad diapir, (2) the northeastward direction of salt movement into the overburden, and (3) the top of the volcanic basement, thrusted and uplifted by the Abshirin fault. Declarations Acknowledgements The authors express their gratitude to Iran Gas Engineering and Development Company for kindly providing the field data used in this work and partially funding Elham Zare during her research. References Abaie IL, Ansari HJ, Badakhshan A, Jafari A (1963) History and development of the Alborz and Serajh fields, Central Iran. In: Proceeding, 6th world petrol conference, Frankfurt, pp 1–111. Abbasi, G., Motamedi, H., Orang, K., & Nickandish, A. A. (2020). Petroleum geology of the western part of the Central Iran Basin. Journal of Petroleum Geology , 43 (2), 171-190. Adetunji, A. Q., Ferguson, I. J., & Jones, A. G. (2015). Reexamination of magnetotelluric responses and electrical anisotropy of the lithospheric mantle in the Grenville Province, Canada. Journal of Geophysical Research: Solid Earth , 120 (3), 1890-1908. Arian, M. (2012). Clustering of diapiric provinces in the Central Iran Basin. Carbonates and Evaporites , 27 , 9-18. Avdeeva, A., Avdeev, D., & Jegen, M. (2012). Detecting a salt dome overhang with magnetotellurics: 3D inversion methodology and synthetic model studies. Geophysics , 77 (4), E251-E263. Babaahmadi, A., Safaei, H., Yassaghi, A., Vafa, H., Naeimi, A., Madanipour, S., & Ahmadi, M. (2010). A study of Quaternary structures in the Qom region, West Central Iran. Journal of Geodynamics , 50 (5), 355-367. Baikpour, S., H. Motiei, and K. Najafzadeh. "Geological and geophysical study of salt diapirs for hazardous waste disposal." International journal of environmental science and technology 13 (2016): 1951-1972. Becken, M., Ritter, O., Park, S. K., Bedrosian, P. A., Weckmann, U., & Weber, M. (2008). A deep crustal fluid channel into the San Andreas Fault system near Parkfield, California. Geophysical Journal International , 173 (2), 718-732. Bedrosian, P. A. (2007). MT+, integrating magnetotellurics to determine earth structure, physical state, and processes. Surveys in geophysics , 28 , 121-167. Booker, J. R. (2014). The magnetotelluric phase tensor: a critical review. Surveys in Geophysics , 35 , 7-40. Brasse, H., Kapinos, G., Li, Y., Muetschard, L., Soyer, W., & Eydam, D. (2009). Structural electrical anisotropy in the crust at the South-Central Chilean continental margin as inferred from geomagnetic transfer functions. Physics of the Earth and Planetary Interiors , 173 (1-2), 7-16. Caldwell, T. G., Bibby, H. M., & Brown, C. (2004). The magnetotelluric phase tensor. Geophysical Journal International , 158 (2), 457-469. Chave, Alan D., and Alan G. Jones, eds. The magnetotelluric method: Theory and practice . Cambridge University Press, 2012. Cherevatova, M., Smirnov, M., Korja, T., Kaikkonen, P., Pedersen, L. B., Hübert, J., ... & Kalscheuer, T. (2014). Crustal structure beneath southern Norway imaged by magnetotellurics. Tectonophysics , 628 , 55-70. Comeau, M. J., Becken, M., Käufl, J. S., Grayver, A. V., Kuvshinov, A. V., Tserendug, S., ... & Demberel, S. (2020). Evidence for terrane boundaries and suture zones across Southern Mongolia detected with a 2-dimensional magnetotelluric transect. Earth, Planets and Space , 72 , 1-13. Jackson, M. P. A., Cornelius, R. R., Craig, C. H., Gansser, A., Stöcklin, J., & Talbot, C. J. (1990). Salt diapirs of the Great Kavir, central Iran. Key, K. W., Constable, S. C., & Weiss, C. J. (2006). Mapping 3D salt using the 2D marine magnetotelluric method: Case study from Gemini Prospect, Gulf of Mexico. Geophysics , 71 (1), B17-B27. Kirkby, A., Heinson, G., Holford, S., & Thiel, S. (2015). Mapping fractures using 1D anisotropic modelling of magnetotelluric data: a case study from the Otway Basin, Victoria, Australia. Geophysical Journal International , 201 (3), 1961-1976. Leveille, J. P., Jones, I. F., Zhou, Z. Z., Wang, B., & Liu, F. (2011). Subsalt imaging for exploration, production, and development: A review. Geophysics , 76 (5), WB3-WB20. Li, S., Unsworth, M. J., Booker, J. R., Wei, W., Tan, H., & Jones, A. G. (2003). Partial melt or aqueous fluid in the mid-crust of Southern Tibet? Constraints from INDEPTH magnetotelluric data. Geophysical Journal International , 153 (2), 289-304. McNeice, G. W., & Jones, A. G. (2001). Multisite, multifrequency tensor decomposition of magnetotelluric data. Geophysics , 66 (1), 158-173. Moradi, M., Oskooi, B., Pushkarev, P., Smirnov, M., & Esmaeili Oghaz, H. (2019). Cooperative inversion of magnetotelluric and seismic data on Shurab diapirs in Central Iran. Environmental Earth Sciences , 78 , 1-14. Morley, C. K., Kongwung, B., Julapour, A. A., Abdolghafourian, M., Hajian, M., Waples, D., ... & Kazemi, H. (2009). Structural development of a major late Cenozoic basin and transpressional belt in central Iran: The Central Basin in the Qom-Saveh area. Geosphere , 5 (4), 325-362.. Newman, G. A., Hoversten, G. M., & Alumbaugh, D. L. (2002). Three-dimensional magnetotelluric modeling and inversion: Application to sub-salt imaging. In Methods in Geochemistry and Geophysics (Vol. 35, pp. 127-152). Elsevier. Parker, R. L., & Whaler, K. A. (1981). Numerical methods for establishing solutions to the inverse problem of electromagnetic induction. Journal of Geophysical Research: Solid Earth , 86 (B10), 9574-9584. Rodi, W., & Mackie, R. L. (2001). Nonlinear conjugate gradients algorithm for 2-D magnetotelluric inversion. Geophysics , 66 (1), 174-187. Roosta, H., Jalalifar, H., Nasab, S. K., & Ranjbar, M. (2019). Seven years of surface deformation above the buried Nasr-Abad salt diapir using InSAR time-series analysis, Central Iran. Journal of Geodynamics , 130 , 1-11. Rubinat, M., Ledo, J., Roca, E., Rosell, O., & Queralt, P. (2010). Magnetotelluric characterization of a salt diapir: a case study on Bicorb–Quesa Diapir (Prebetic Zone, SE Spain). Journal of the Geological Society , 167 (1), 145-153. Simpson, Fiona, and Karsten Bahr. Practical magnetotellurics . Cambridge University Press, 2005. Schwalenberg, K., Rath, V., & Haak, V. (2002). Sensitivity studies applied to a two-dimensional resistivity model from the Central Andes. Geophysical Journal International , 150 (3), 673-686. Spratt, J. E., Jones, A. G., Jackson, V. A., Collins, L., & Avdeeva, A. (2009). Lithospheric geometry of the Wopmay orogen from a Slave craton to Bear Province magnetotelluric transect. Journal of Geophysical Research: Solid Earth , 114 (B1). Spratt, J. E., Skulski, T., Craven, J. A., Jones, A. G., Snyder, D. B., & Kiyan, D. (2014). Magnetotelluric investigations of the lithosphere beneath the central Rae craton, mainland Nunavut, Canada. Journal of Geophysical Research: Solid Earth , 119 (3), 2415-2439. Additional Declarations The authors declare no competing interests. Supplementary Files supplementary.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5255696","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":365432144,"identity":"434f8e19-aa8e-4491-b5bb-17a9d23fa2d5","order_by":0,"name":"Mansoure Montahaei","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAtElEQVRIiWNgGAWjYPCCAwz8UBYPsRoOMEg2MJOqxeAAM5Eu0p12+NjjDzV35I1v5B9g+FHDIGPeQECL2e20dIMDx54ZbruRzMDYc4yBR+YAQS05ZhIH2A4zgrQw8DYw8EgQchhEy7/D9ptnAG35S7SWg22HEzdIJDMwE2lLWprE2b7DyTPOPDY4LHNMghgtycckKr4dtu1vT3z48E2NjT1BLSjgAAMDaRpGwSgYBaNgFOAAAMNCP+YG0YkYAAAAAElFTkSuQmCC","orcid":"","institution":"institute of geophysics, university of Tehran","correspondingAuthor":true,"prefix":"","firstName":"Mansoure","middleName":"","lastName":"Montahaei","suffix":""},{"id":365432176,"identity":"97d11c1e-56de-4e4a-abb3-2c82fb79e43d","order_by":1,"name":"Elham Zare","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Elham","middleName":"","lastName":"Zare","suffix":""}],"badges":[],"createdAt":"2024-10-13 14:21:55","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-5255696/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5255696/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":66648087,"identity":"bd0c2600-c275-41d6-b386-93054319ede8","added_by":"auto","created_at":"2024-10-15 07:15:30","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":745493,"visible":true,"origin":"","legend":"\u003cp\u003e(a) structural map depicting tectonic units of Iran plateau. (b) regional topography map of northwestern central Iran. (c) Location of the study area marked by MT stations (indicated by inverted triangles) along profile 21.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/32f915701c28566a77581547.png"},{"id":66646522,"identity":"3b43d1e0-e3ac-45cf-bdb3-49f12d409e89","added_by":"auto","created_at":"2024-10-15 06:59:30","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":444335,"visible":true,"origin":"","legend":"\u003cp\u003eNiblet-Bostick penetration depths of MT response functions as estimated separately for the xy(circular symbols) and yx (square symbols) components of the apparent resisitivity soundings for all sites along the profile 21.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/93c2aa56f080efdbec1d0ec3.png"},{"id":66646525,"identity":"e3fb6d3e-9397-48e8-a3a0-3cc82e2d15f2","added_by":"auto","created_at":"2024-10-15 06:59:30","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":84526,"visible":true,"origin":"","legend":"\u003cp\u003eDistribution of the phase sensitive skew values for the entire data set (a). about 97% of all points have absolute values less than 0.3 (indicated by the red lines (b)) and skew values of the data set are shown over period (c).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/00342a0eb2e13c870041a893.png"},{"id":66646521,"identity":"70948d8e-f0da-4428-ae41-60bc43327491","added_by":"auto","created_at":"2024-10-15 06:59:30","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":74814,"visible":true,"origin":"","legend":"\u003cp\u003eDistribution of the phase tensor skew angles for the entire data set (a). About 86% of all points have absolute values less than 3˚ (indicated by the red lines (b)) and β skew angle values of the data set are shown over period (c). Large β skew angles mostly happen at periods\u0026gt;5 sec.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/4aef403bc810c4ea92d4919c.png"},{"id":66647081,"identity":"31f447f5-e859-45fc-92f4-1b6d0248cad0","added_by":"auto","created_at":"2024-10-15 07:07:30","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":46388,"visible":true,"origin":"","legend":"\u003cp\u003eRose histograms of (a) regional strike angles calculated at different period bands from phase information of impedance data (phase tensors). (b) orientation of real induction vectors at the same period range.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/5f8ceb7b66fa9f6c43d3cd6d.png"},{"id":66647083,"identity":"8f83cc89-02b6-4fba-ba66-7c57db23813d","added_by":"auto","created_at":"2024-10-15 07:07:30","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":70040,"visible":true,"origin":"","legend":"\u003cp\u003eDecomposition analysis results at four representative stations along profile 21 showing the phase differences of the TE and TM modes, shear and twist angles as well as the RMS misfit and strike direction at four exemplary sites for (i) the unconstrained data (ii) data constrained to a single frequency band and (iii) frequency independent data.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/d968dc628aeb10faa5ddbe9b.png"},{"id":66648783,"identity":"3125f8c7-eee3-4041-9d72-92fb9ffda965","added_by":"auto","created_at":"2024-10-15 07:23:39","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":16012,"visible":true,"origin":"","legend":"\u003cp\u003edistribution of the absolute value of shear and twist angles and the average RMS misfits from unconstrained GB analysis.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/a77435237224930893deb490.png"},{"id":66646535,"identity":"c207c450-80dd-4bfc-bdaf-5e882be7aa14","added_by":"auto","created_at":"2024-10-15 06:59:30","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":49085,"visible":true,"origin":"","legend":"\u003cp\u003eResults of the GB strike analysis using one-decade period bandwidths (a-c) and the whole period range data (d).\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/5eb3109761bf853a62a706ab.png"},{"id":66647086,"identity":"7beaf880-c680-483f-94a2-92eb0ac0f9a5","added_by":"auto","created_at":"2024-10-15 07:07:30","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":41430,"visible":true,"origin":"","legend":"\u003cp\u003eThe RMS misfit values obtained for individual sites based on the GB fit at the specified strike azimuths. The dashed lines define the range of strike angles where the minimum of the misfit occurs overally for all stations along the profile 21.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/607285b0e330981e94eff9ea.png"},{"id":66648748,"identity":"c4b122eb-0197-4c27-9b8c-af10109b918e","added_by":"auto","created_at":"2024-10-15 07:23:38","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":48418,"visible":true,"origin":"","legend":"\u003cp\u003ethe RMS misfit values obtained for individual sites based on the GB fit for each site over the whole recorded frequency range. MT responses are recalculated at a geoelectric strike azimuth of N30°W and distortion angles of 0°.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/2bfac8f641e831cf6299a1fe.png"},{"id":66648786,"identity":"6ce4d83f-74cf-4832-8187-b17832c26a5f","added_by":"auto","created_at":"2024-10-15 07:23:39","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":613176,"visible":true,"origin":"","legend":"\u003cp\u003ePseudo-section of the TE (a, b), TM (c) and tipper (d) responses for the MT profile 21. Results are shown for the observed data (left column) and the responses of the inversion model (right column).Pseudosections of the TE mode apparent resistivities are missing due to the additional complexity aroused by the static shift effect.\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/e8619632b39a986fa1163d7c.png"},{"id":66646538,"identity":"70c5ebfc-94ca-4fe6-84d4-88c24a568a13","added_by":"auto","created_at":"2024-10-15 06:59:31","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":16571,"visible":true,"origin":"","legend":"\u003cp\u003eDiagram illustrating trade off between data RMS misfit and model roughness of the inversion results with different values of the Lagrange multiplier (τ). The tau value of each model is given by the number at each data point.\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/33847aacc6bc27badb3faabf.png"},{"id":66647091,"identity":"77ef7c0f-4e17-4771-83ea-7ffada8039c7","added_by":"auto","created_at":"2024-10-15 07:07:31","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":397113,"visible":true,"origin":"","legend":"\u003cp\u003einversion results obtained for different starting models: homogenous half spaces of 100 Ωm (a), 1000 Ωm (b) and a double layer composed of an upper layer with 100 Ωm resistivity, overlying a half space of 1000 Ωm.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/1f7e9eded9640c632b18d49f.png"},{"id":66648089,"identity":"b2c5b191-dc77-4ab0-9261-9779d8307f2f","added_by":"auto","created_at":"2024-10-15 07:15:30","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":390539,"visible":true,"origin":"","legend":"\u003cp\u003eTwo-dimensional models obtained from inversion of (a) joint TE+ TM+ TP, (b) TM, (c) TE, (d) TP magnetotelluric data.\u003c/p\u003e","description":"","filename":"14.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/222d66167197d22857b20f50.png"},{"id":66648090,"identity":"ed253593-e767-4ad2-9cd1-b6471bde4c5f","added_by":"auto","created_at":"2024-10-15 07:15:30","extension":"png","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":18343,"visible":true,"origin":"","legend":"\u003cp\u003eDetailed analysis of the RMS values color-coded for each mode at all stations along the profile 21.\u003c/p\u003e","description":"","filename":"15.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/c68f81860dc1fe19ee15dff7.png"},{"id":66646529,"identity":"41707c5f-ee83-438c-9728-18d0c920ceb8","added_by":"auto","created_at":"2024-10-15 06:59:30","extension":"png","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":318713,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Resistivity model along profile 21 and its interpretation superimposed on the geological cross section of the study area. (b) geological cross section along a profile in the study area, modified from Moradi et al., 2019.\u003c/p\u003e","description":"","filename":"16.png","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/b76c6bfb73f21c88bd279402.png"},{"id":66649871,"identity":"164a5a1e-7097-492f-ac2b-98638c82d781","added_by":"auto","created_at":"2024-10-15 07:31:56","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3446771,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/db1a42ff-0d90-4080-a67f-0486d4efc262.pdf"},{"id":66648770,"identity":"6a67a706-c531-4479-86d3-f7456d535bfb","added_by":"auto","created_at":"2024-10-15 07:23:38","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":513591,"visible":true,"origin":"","legend":"","description":"","filename":"supplementary.docx","url":"https://assets-eu.researchsquare.com/files/rs-5255696/v1/563992b448c24314175e6b03.docx"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eCrustal-Scale Structure of the Nasr-Abad Buried Salt Diapir in Northwest Central Iran from a Profile Magnetotelluric Dataset\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSuperlative salt domes in the Great Kavir basin in Central Iran (CI) provide an ideal laboratory for observing the evolution of salt diapirism (Jackson et al., 1990). The Shurab diapiric group in the Qom basin, a western peripheral embayment of the Great Kavir basin, consists of four exposed and one buried diapir, and developed in the western CI. Surface studies prove that diapirs Nos. 1, 2, 3, and 5 are extruded, whereas diapir No. 4 forms one of the largest buried salt diapirs in the CI basin and is commonly named Nasr-Abad salt diapir (Baikepour et al., 2016; Roosta et al., 2019).\u003c/p\u003e \u003cp\u003eMultiple reflections and energy scattering limit the ability of seismic methods to properly delineate the boundaries of salt formations (Newman et al., 2002; Key et al., 2006; Rubinat et al 2010; Leville et al., 2011). Accordingly, previous investigations of 2D seismic lines provide an approximate assessment of the salt structure in the Nasr-Abad region (Baikepour et al., 2016). However, salt diapirs are ideal targets for MT studies, since they are mainly composed of evaporitic rocks (with high electrical resistivity) enclosed by porous sedimentary rocks (less resistive), which provides an environment where electrical resistivity varies over a wide range. (Rubinat et al., 2010; Avdeeva et al., 2012; Key et al., 2006).\u003c/p\u003e \u003cp\u003eIn this study, we investigate an MT data set recorded along a profile over the Nasr-Abad buried salt diapir with a high density of measuring stations. The study aims to delineate the subsurface extension of the buried salt diapir and also to define variations in lithology and fluid content of the overburden structures. We re-analyze MT measurements along profile 21, including a detailed dimensionality and directionality assessment of regional geo-electric structures. We also update preliminary inversion results (Baikepour et al., 2016) by employing distortion-corrected MT data in this procedure.\u003c/p\u003e"},{"header":"2. Geological setting of the study area","content":"\u003cp\u003eThe MT data set investigated in this study has been acquired over the buried Nasr-Abad salt diapir, one of the constituents of the Shurab diapiric group in the western Central Iran Basin. Central Iran (CI), the Zagros Fold and Thrust Belt (ZFTB), the Sanandaj-Sirjan Magmatic Belt (SSMB), the Elburz Mountains, Kopetdagh, Makran, and Sistan-Baluchestan are the main tectonic blocks constituting the Iranian plateau (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea) and emerged due to the collision between the Arabian and Eurasian plates. Coinciding with the present-day morphological depression, the CI block has a triangular form bounded by the Elburz Mountains in the north and the Zagros Mountains in the south and undergoes a north-south shortening between the two ranges. As a consequence, 6\u0026ndash;7 km thick evaporative deposits accumulated in the sedimentary basin of the Great Kavir have surfaced in the form of several diapirs in the Central Iran Basin. The Shurab diapiric group, composed of four exposed and one buried diapir, has developed in the western CI (Jackson et al., 1990).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eStrike-slip fault systems and thrust faults are highly evolved at the margins of the CI block. The Saveh-Qom area with a NW-SE trend and the Great Kavir Desert with a NE-SW trend are the two main constituents of the CI basin. The Shurab diapiric group lies in the Nasr-Abad area, southeast of Qom. The main geological structure in this region follows the major trend of the ZFTB and is aligned with the NW-SE direction (Roosta et al., 2019).\u003c/p\u003e \u003cp\u003eSeveral Quaternary strike-slip and thrust faults have developed in the west of the CI block, where the Avaj, Kushk-Nosrat, Indes, Qom-Zefreh, and Dehshir faults (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) are the most important ones and play a significant role in the tectonic evolution and structural development of this region. They have a prevailing right-lateral component of slip and most often develop in en-echelon arrays, producing restraining stepovers (containing thrusts, folds, and anticlines) between the fault segments (Babaahmadi, et al., 2010).\u003c/p\u003e \u003cp\u003eThe Khurabad fault is the largest normal or transtensional fault in the Saveh-Qom area, developed between the Elburz and Sarajeh anticlines, with outcrops extending to the south along a line. Stratigraphical studies have inferred that the Khurabad fault is the main factor controlling the large volume of local halite deposition during Lower Red Formation (LRF) time and strata patterns within Upper Red Formation (URF) time. Just to the southeast, the Khurabad fault is oriented north-northwest to south-southeast and forms the Abshirin-Shurab fault zone, exhibiting a significant normal component of offset. The Shurab diapiric group in the Nasr-Abad region extends to the northeastern margins of the Abshirin-Shurab fault outcrops (Morelley et al., 2009). The structural studies identify the Abshirin and Sen-Sen faults as crucial structural elements controlling diapirism in the study area (Moradi et al., 2019).\u003c/p\u003e \u003cp\u003eThe typical stratigraphic sequence in the Nasr-Abad region in the south of western CI consists of four sedimentary units in descending order: (i) Pliocene to Pleistocene conglomerates deposited on the (ii) evaporites and mudstones of the Upper Red Formation (URF, dated to the late Miocene). The URF is separated by the (iii) limestone, marls, shales, and sandstones of the Qom Formation (QF, dated to the late Oligocene to early Miocene) from (iv) late Eocene to early Oligocene sediments of the Lower Red Formation (LRF). Evaporitic rocks and sediments of the LRF effectively detach the late Oligocene to Pleistocene cover from the Eocene magmatic basement rocks. Furthermore, the sedimentary cover is deformed by a series of folds, thrusts, strike-slip faults, and halokinetic features that are mainly oriented in the NW\u0026ndash;SE direction, coincident with the general geodynamic trend of the Zagros fold and thrust belt (ZFTB) (Morelly et al., 2009; Abbasi et al., 2020).\u003c/p\u003e"},{"header":"3. Magnetotelluric data in the west of Central Iran basin","content":"\u003cp\u003eThe magnetotelluric (MT) method utilizes time fluctuations of natural electromagnetic (EM) fields to reveal the electrical conductivity structure of the subsurface.Complex-valued impedance tensor \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left(\\underset{\\_}{Z}\\right(\\omega\\:\\left)\\right)\\)\u003c/span\u003e\u003c/span\u003e and tipper vector \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left(\\overrightarrow{W}\\right(\\omega\\:\\left)\\right)\\)\u003c/span\u003e\u003c/span\u003edata, represent the frequency-dependent (\u0026omega;\u0026thinsp;=\u0026thinsp;2\u0026pi;f) MT response functions (transfer functions) of the subsurface geoelectric structure. These data are extracted from simultaneous measurements of the time variation of the horizontal electric \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left(\\overrightarrow{E}\\right(\\omega\\:\\left)\\right)\\)\u003c/span\u003e\u003c/span\u003e and full magnetic\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left(\\overrightarrow{H}\\right(\\omega\\:\\left)\\right)\\)\u003c/span\u003e\u003c/span\u003e field components on the earth surface:\u003c/p\u003e\n\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e$$\\:\\left[\\begin{array}{c}{E}_{x}\\left(f\\right)\\\\\\:{E}_{y}\\left(f\\right)\\\\\\:{H}_{z}\\left(f\\right)\\end{array}\\right]=\\left[\\begin{array}{cc}{Z}_{xx}\\left(f\\right)\u0026amp;\\:{Z}_{xy}\\left(f\\right)\\\\\\:{Z}_{yx}\\left(f\\right)\u0026amp;\\:{Z}_{yy}\\left(f\\right)\\\\\\:{W}_{x}\\left(f\\right)\u0026amp;\\:{W}_{y}\\left(f\\right)\\end{array}\\right]\\left[\\begin{array}{c}{H}_{x}\\left(f\\right)\\\\\\:{H}_{y}\\left(f\\right)\\end{array}\\right]$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eThe magnitude and phase of the Impedance tensor,along with the tipper vector (vertical magnetic transfer functions: VTF) are commonly visualized by the apparent resistivity (\u0026rho;\u003csub\u003ea\u003c/sub\u003e) and phase (\u0026phi;) sounding curves,as well as the real induction vectors\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\:\\left(\\overrightarrow{P}\\right)\\)\u003c/span\u003e\u003c/span\u003e:\u003c/p\u003e\n\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\u003cimg 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\"\u003e\u003cbr\u003e\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eThe presence of highly conductive phases (e.g., saline fluids, mineralization, and partial melt) and their interconnections could significantly reduce the bulk resistivity of the rocks and strongly influence the measured MT transfer functions.\u003c/p\u003e\n\u003cp\u003eMT data from this study were collected at 37 stations distributed across the study area. The measured periods range from 0.003 to 2512 seconds. The MT sites were located along a SW-NE profile perpendicular to the general NW-SE trend of the ZFTB, managing the development of geological structures in this area (Babaahmadi et al., 2010).The stations were positioned based on the geological structures. Specifically, five sites were placed within the area of the Nasr-Abad buried diapir (diapir No. 4), and at least four additional sites were situated to the west of the Ab-Shirin fault, directly above the outcropping diapir No. 5.\u003c/p\u003e\n\u003cp\u003eA site spacing of approximately 250 meters provides an acceptable resolution for shallow features. However, an investigation of the regional structures is also possible, considering the penetration depth of the MT data set. Applying the Niblet-Bostick method to the impedances of each site yields estimates of the maximum penetration depths of the MT data.\u003c/p\u003e\n\u003cp\u003eThe results (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e) indicate that the penetration depth of the MT responses varies along the profile. Beneath the SW portion of the profile, the responses generally penetrate to greater depths compared to the NE part. However, the MT dataset provides good data coverage up to a depth of 25 kilometers in the region.\u003c/p\u003e"},{"header":"4. Dimensionality and strike analysis of the MT data","content":"\u003cp\u003eThe geo-electric structure of the subsurface can exhibit different configurations: it may be horizontally layered (1-D), extend infinitely along the strike direction (2-D), or take on a more complex 3-D form. Information about the dimensionality and strike of the regional conductivity structure is typically derived from the internal structure of the MT transfer functions, including impedance tensor and tipper vector data. For the MT sites along Profile 21, dimensionality analysis of the impedance data is performed using Bahr\u0026rsquo;s phase-sensitive approach (Simpson and Bahr, 2005) and the phase tensor method (Caldwell et al., 2004). Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea displays the phase-sensitive skew (η) values within the measurement period range for all sites. Generally, the η skew values fall below the threshold of 0.3, beyond which the 2-D assumption for the regional structure is invalidated. The distribution of all calculated η skew values in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb reveals that 97% of the data points are smaller than 0.3. A more comprehensive assessment of the calculated phase-sensitive skew indicates that large values occur at longer periods (\u0026gt;\u0026thinsp;5 seconds) for a few stations (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDespite the theoretical assumption that both techniques (Bahr\u0026rsquo;s and phase tensor methods) provide equivalent estimates of dimensionality and strike direction, in the case of noisy and strongly distorted data, they may exhibit incompatible resolution and estimation properties (Chervatova, 2014). The novelty of the phase tensor technique lies in its immunity to galvanic distortions caused by unresolvable small-scale conductors. Additionally, it does not rely on assumptions about the regional conductivity structure for dimensionality and directionality analysis (Booker, 2014).\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e presents a pseudo-section and the statistical distribution of phase tensor skew values (β skew angles) for the entire dataset. It appears that β skew angles are consistently less than 3\u0026deg; (a necessary but not sufficient condition for two-dimensionality) across most periods for all stations. A wide region beneath the northeastern part of the profile reveals low skew values throughout the entire period range. In the southwestern part of the profile, the regional conductivity structure appears more complex, and high estimates of β skew angles occur at long periods (\u0026gt;\u0026thinsp;5 seconds) in this region. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb reveals that less than 14% of all data points deviate from those expected for a 2D regional conductivity structure. Overall, the phase tensor skew values are consistent with a 2D approach for further modeling and interpretation of our dataset.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eGeoelectric strike directions, determined from the phase tensor method for different period bands at all MT stations, are presented as cumulative values in the form of rose diagrams in geographic coordinates (histograms in the first row of Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Strike estimations reveal significantly greater variability at the shortest periods (0.003-1 sec). This outcome may stem from the heterogeneity of localized structures within the small sampling hemisphere of EM fields during these time intervals. However, MT data indicate that 1-D induction occurs at these periods (see Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), resulting in an undefined preferred azimuth for the regional conductivity structure. As the period increases, the sampling depth of MT signals also increases, and the orientation of the regional azimuth ranges between N30˚W and N60˚W (with a 90˚ ambiguity).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eReal induction vectors (IVs) point perpendicularly away from the strike of the conductive anomaly and could be used as external information to resolve the strike ambiguity estimated from impedance data alone (Simpson and Bahr, 2005). The rose diagrams of the IVs in the same period range of impedance data are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb. Generally, the induction vectors along profile 21 indicate geoelectric strike directions between N15˚W and N30˚W.\u003c/p\u003e"},{"header":"5. Distortion decomposition and removal","content":"\u003cp\u003eGalvanic distortions of regional electric fields due to small near-surface conductivities may cause unreliable modeling and interpretation of the subsurface electrical conductivity structure (Chave and Jones, 2012). Exempt from the galvanic distortions, the phase tensor method provides information about the dimensionality and directionality of the regional geoelectric structure, as discussed in the previous section. However, this technique cannot remove galvanic distortions from measurements or recover regional impedances. Furthermore, it does not take into account experimental data errors when estimating dimensionality parameters and strike direction. In this section, we applied the Groom-Bailey (GB) decomposition, as extended by McNeice and Jones (2001), to examine the regional geoelectric strike and distortion matrix more rigorously and extract regional responses from MT impedances contaminated with galvanic scatter.\u003c/p\u003e \u003cp\u003eThe GB decomposition method assumes a 3D/2D model composed of a 2D regional geoelectric structure, whose electric field responses are galvanically distorted by 3D small near-surface conductivities. The method employs seven parameters: two complex-valued regional impedances, the strike of the regional conductivity structure, and shear and twist angles (distortion angles) for model characterization. Next, it applies a least-squares approach, weighted by measurement errors, to fit the 3D/2D decomposition model to the observed impedances within a frequency interval for a group of sites. The root mean square (RMS) misfit values from this procedure serve as an obvious indicator for regional 3D induction. Large values of this measure reveal that the assumption of a 2D regional conductivity structure (as adopted by the GB method) cannot be used for proper modeling and interpretation of the observed MT data. However, the 3D/2D distortion model inherent in the GB decomposition is valid if, by constraining decomposition parameters to specific values, it becomes possible to identify a sufficiently wide frequency band where the RMS misfit errors are acceptable (McNeice and Jones, 2001). The major advantage of the GB method is that it utilizes the entire information content of the measured impedances, taking their errors into account within a statistically robust framework to determine decomposition model parameters. The only disadvantage is the assumption of a 2D regional conductivity model intrinsic to this method (Sprat et al., 2009 and 2014).\u003c/p\u003e \u003cp\u003eIn the present study, the GB decomposition model is first determined for each period (unconstrained GB analysis) and then for one-decade period bandwidths. Finally, period-independent estimates of decomposition model parameters that fit the measured impedances are determined. The RMS misfit, strike azimuth, and the phase difference between the TE and TM modes\u0026mdash;whose small values (\u0026lt;\u0026thinsp;15˚) indicate a low order of multidimensionality\u0026mdash;are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e for exemplary stations along profile 21. The results are based on a low value of the impedance error floor (0.5%) and suggest a low level of multidimensionality, along with distortion angles (shear and twist angles).Most of the stations along the profile show small phase differences (indicating one-dimensionality) up to intermediate periods of 5 sec (figure S1). However, there is an exception for a group of three sites (21-3500 to 21-4000) located to the west of the Abshirin fault; over the outcrops of diapir number 5 (see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). Furthermore, the RMS plots from decomposition models for each site indicate that, except for the aforementioned sites (21-3500 to 21-4000), the misfit estimates are less than 2 in all these experiments (Figure S2), implying a 2D regional geoelectric structure. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e presents histograms of the distortion parameters and the RMS misfit errors obtained from an unconstrained GB analysis of the measured impedances at all stations. The results indicate a low level of galvanic distortion in the study area.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe results in Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e (a-c) indicate the pattern of strikes across the study region, as obtained from an unconstrained GB analysis of all measuring stations along profile 21. At short periods, where the regional conductivity structure is deemed 1D, it becomes difficult to determine a preferred strike azimuth. However, the resulting strike direction varies between N60\u0026deg;E at the intermediate periods (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb) and N30\u0026deg;E at the long periods (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec). It\u0026rsquo;s important to note that directionality analysis results based on the GB decomposition inherently have a 90\u0026deg; ambiguity (McNeice and Jones, 2001).We further investigate frequency-independent estimates of the regional geoelectric strike azimuth more rigorously by constraining its value to specified angles between \u0026minus;\u0026thinsp;45\u0026deg; and 45\u0026deg; while fitting distortion parameters. The distribution of RMS misfit errors calculated for individual sites at the specified regional strike angles (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e) may define the contribution of different sites to the constraint of regional strike (Adetunji et al., 2015). The misfit values for individual sites along profile 21 suggest that geological noise caused by near-surface inhomogeneities is distributed unevenly between the sites.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe stations that are distributed close to diapirs No. 4 and No. 5 are more sensitive to the strike direction on profile 21. However, the misfit levels are reasonable at all regional strike azimuths, indicating that a 2D approach could be adopted for further modeling and interpretation of the subsurface resistivity structure.\u003c/p\u003e \u003cp\u003eFurthermore, we calculate the mean value of all station misfits for each specified strike angle. The results show that the minimum overall misfit of the GB decomposition occurs for strike angles between \u0026minus;\u0026thinsp;15\u0026deg; and \u0026minus;\u0026thinsp;30\u0026deg; (dashed lines in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). The results are consistent with the strike azimuths estimated by the phase tensor method and the directions of the induction vectors (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Furthermore, multi-site and frequency-independent GB analysis of all measured impedances (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ed) yields an N30\u0026deg;W strike azimuth for all MT stations along profile 21, which is justified by the directions of real induction vectors (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003eBased on the above consistencies, we constrained the geoelectric strike direction to N30\u0026deg;W azimuth and set the galvanic distortion parameters (the shear and twist angles) to 0\u0026deg; in order to remove distortion effects and recover regional impedances. To illustrate locations where data are sensitive to the predefined distortion parameters, we plotted the RMS misfit values obtained from frequency-independent GB analysis against increasing periods for each site along profile 21. The results are obtained assuming a default error floor of 3.5% for impedance data. For this error level, misfit values smaller than 2 indicate that the constrained 3D/2D GB distortion model accurately represents the measurements (Sprat et al., 2009 and 2014). The results (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e) show that an acceptable RMS misfit below two is obtained at almost all sites and for almost all periods. The main exceptions are the long-period (\u0026gt;\u0026thinsp;100 sec) responses of a few stations and also the short-period responses of station 21-3500, where the misfit ranges between 2.0 and 3.0.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"6. 2-D resistivity model","content":"\u003cp\u003eUsing a non-linear inversion approach, we employed distortion-corrected MT data that had been decomposed along the regional strike direction (N30˚W) to model the crustal conductivity structure of the study region. Inaccurate data, represented as scattered points with large error bars, as well as data points whose corresponding apparent resistivities and phases did not align with the D\u003csup\u003e+\u003c/sup\u003e assessment (Parker and Whaler, 1981), were removed from the MT dataset prior to inverse modeling.\u003c/p\u003e \u003cp\u003eThe galvanic distortions that contaminated the apparent resistivity curves were exceptionally removed using the GB decomposition method. However, the static shift factor\u0026mdash;an amplitude component that shifts the TE or TM mode apparent resistivities in parallel\u0026mdash;remains unknown.\u003c/p\u003e \u003cp\u003eTo better understand the data and the static shift range, we estimated the shift magnitude as the ratio of the median ρ\u003csub\u003eTE\u003c/sub\u003e to ρ\u003csub\u003eTM\u003c/sub\u003e for the 1D segment of the curve at each station. The proportion varies between 0.5 and 1.16, with a clustering around 0.8 to 1.1 for all stations along profile 21 (Figure S3). Next, we calculated the deviation between the median TE and TM resistivities at each site. This analysis reveals that for all stations, the median TE and TM resistivities are within 33% of each other and according to the Kirkby et al., (2019) they remain largely unaffected by the static shift factor.\u003c/p\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e6.1. Qualitative information from MT data pseudosections\u003c/h2\u003e \u003cp\u003eSignificant features of the regional conductivity structure can be discerned from the pseudosections of MT data, using a coordinate system aligned parallel and perpendicular to the estimated strike direction (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e). The most striking feature in all pseudosections of impedance data is the discontinuity of the resistivity structure induced by the Nasr-Abad salt intrusion beneath stations 21-7000 to 21-8000. In this region, the clean white salt of the LRF and the multicolored salt of the URF result in high apparent resistivities (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ea), which coincide with low values of impedance phases (Figs.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb, c).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThere is also a clear indication of a resistive salt body in the TM mode data beneath the southwestern end of the profile, near Shurab Diapir No. 5 (Figs.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea, b). However, the massive deep salt deposits in the Qom Basin are also evident from the low values of impedance phases at periods greater than 10 seconds (Figs.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb, c).\u003c/p\u003e \u003cp\u003eFurthermore, the TE and TM mode pseudosections are influenced by the fine-grained Quaternary sediments in the upper crust. Coincidently, low apparent resistivities and elevated phases occur at short periods between 0.1 and 1 second. The strong positive response of the tipper data at periods longer than 10 seconds beneath the southwestern part of the profile indicates significant conductivity contrasts between the Shurab diapirs and the surrounding sediments (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ed).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e6.2 Modelling approach\u003c/h2\u003e \u003cp\u003eIn this stage, a 2-D Earth model is sought to appropriately explain the MT measurements. The result is constructed using a regularized inversion scheme that fits the MT responses from an over-parameterized model space to the apparent electrical resistivities and phases of both impedance modes, as well as the tipper data (which have been corrected for distortion effects).\u003c/p\u003e \u003cp\u003eThe MT data from 37 stations along profile 21, covering the period range T\u0026thinsp;=\u0026thinsp;0.0034-2512 s, are imported into the WingLink software. We utilize a non-linear conjugate gradient (NLCG) algorithm developed by Rodi and Mackie (2001) to solve the MT inversion problem.The algorithm minimizes a Tikhonov-regularized objective function (Φ) composed of a data misfit norm weighted by the data variances (Φ\u003csub\u003ed\u003c/sub\u003e) and a model roughness functional (Φ\u003csub\u003em\u003c/sub\u003e) to generate 2-D models that are maximally smooth:\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Taba\" border=\"1\"\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eΦ\u0026thinsp;=\u0026thinsp;Φ\u003csub\u003ed\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;τ Φ\u003csub\u003em\u003c/sub\u003e (4a),\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eΦ\u003csub\u003em\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;\u003cb\u003eL\u003c/b\u003e||\u003cb\u003em\u003c/b\u003e|| (4b)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eτ is usually referred to as the regularization parameter, controlling the weight of the Φ\u003csub\u003em\u003c/sub\u003e and Φ\u003csub\u003ed\u003c/sub\u003e functionals throughout the minimization of the objective function. \u003cb\u003em\u003c/b\u003e is the model vector containing the logarithm of the resistivity of each model cell. In the following experiments, \u003cb\u003eL\u003c/b\u003e has been chosen as the integral of the weighted Laplacian squared of the model parameters, calculated using the actual model mesh. Regularization imposed on the model during inversion might be further increased both horizontally and vertically by the α and β factors (increasing horizontal and vertical derivatives, respectively), as well as by a predefined size of the model block dimensions (H\u003csub\u003e0\u003c/sub\u003e/V\u003csub\u003e0\u003c/sub\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e6.3 Inversion procedure\u003c/h2\u003e \u003cp\u003eIn order to satisfy the boundary conditions, the model space expands 6600\u0026times;2720 km in the \u0026ndash;y and \u0026ndash;z directions, much larger than the study region of profile 21. It is discretized into 162\u0026times;83 rectangular cells whose conductivities are constant. The cell sizes are smallest at the surface just below the sites and enlarge with increasing depth and distance from the profile. Here, the smallest cells have an area of 70\u0026times;3 m\u0026sup2;, corresponding to 0.24\u0026times;0.01 of the EM field skin depth (503\u0026radic;ρT\u0026thinsp;\u0026cong;\u0026thinsp;293 m) at the shortest periods of 0.0034 seconds, assuming an average resistivity of 100 Ωm.\u003c/p\u003e \u003cp\u003eWe applied an L-curve study to estimate the optimal value for the regularization parameter (τ). Initial inversions with different τ values were conducted, and for individual experiments, the data misfit was plotted against the model roughness. The resulting trade-off curve is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eAccording to this figure, the value of τ\u0026thinsp;=\u0026thinsp;3, located at the corner of the L-curve and minimizing the Φ\u003csub\u003em\u003c/sub\u003e and Φ\u003csub\u003ed\u003c/sub\u003e terms simultaneously, is adopted as the optimal estimate of the regularization parameter in the following inversions. Further numerical experiments show that weighting the regularization term in a manner where both horizontal and vertical derivatives increase by a factor equal to the size of each model cell leads to a more stable inversion result.\u003c/p\u003e \u003cp\u003eIn order to assess how the inversion outcome is influenced by the choice of the initial model, we tested two different kinds of starting models: homogeneous half-spaces with resistivities purely based on assumptions (100 or 1000 Ωm) and a double-layer starting model composed of a 2-kilometer-thick upper layer with 100 Ωm resistivity overlying a half-space with 1000 Ωm.The latter starting model enhances the sharp discontinuity between conductive surficial sediments and deeper resistive diapiric rocks. The results (Figs.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003ea-c) converge to reasonable RMS misfits of 1.56\u0026ndash;2.06. Visually, the models recover similar conductive and resistive features beneath the profile. However, our preferred starting model in the subsequent numerical experiments is the half-space with 100 Ωm resistivity, which provides the lowest RMS misfit across the profile for individual stations.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWe initiated the inversion procedure based on the TM mode phases by allocating error floors of 2.5% (0.7\u0026deg;) and 10,000% for the TM phases and resistivities, respectively. Then, we incorporated TE mode data into the inverse modeling and sequentially reduced the error floors of the TE phases, TM resistivities, and TE resistivities. Finally, we included the tipper data in the inversion procedure. The error floors for the final inversion result (Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003ea) were set to 2.5% (0.7\u0026deg;) for both mode phases, 5% for the TM mode apparent resistivities, and an absolute value of 0.03 for the tipper data. We assigned a high error floor of 200% to the TE mode apparent resistivity to mitigate the destructive influence of the static shift effect. By closely fitting the phase data, this procedure leads to ρ\u003csub\u003eTE\u003c/sub\u003e responses that are shifted equally at all frequencies while maintaining the curve shape (Chave and Jones 2012; Comeau et al. 2020).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFor comparison, we present the results of separate 2D inversions of the TE and TM mode impedances, as well as the TP data and a joint TE\u0026thinsp;+\u0026thinsp;TM\u0026thinsp;+\u0026thinsp;TP inversion, in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e. The individual mode responses of the inversion models fit the measurements reasonably well. The TE and TM mode impedances generally produced comparable results, consisting of a conductive upper layer and a massive resistive zone in the middle crust. Furthermore, the vertical magnetic field is zero above a conductive layer (Brasse et al., 2009), leading to a weakly recovered surficial conductor in the inversion result of the tipper data (Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003ed). A test inversion run of joint TE and TM mode impedances shows that including induction vectors in the inversion procedure has a minor influence on the result (Figure S4).\u003c/p\u003e \u003cp\u003eThe final inversion model (Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003ea) reveals that, besides the disconnected surficial conductive layer, the entire crust in the study region is generally characterized by relatively high resistivities in the range of 200 Ωm. Careful comparisons indicate that the MT responses of the final inversion model reasonably fit the measurements, with the largest RMS occurring for the TE mode phases (Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e and left column in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"7. Results and interpretation","content":"\u003cp\u003eIn this section, magnetotelluric analysis result (Figure 16a) and a detailed geological cross-section (Figure 16b) obtained from field data (Moradi et al., 2019) are combined to interpret the main structures of the study region recovered in the shallow part of the MT profile. Considering the evident uncertainty in the size and lithology of each geologic unit, we overlaid the MT model with the geological cross-section in Figure 16a. The model confirms the existence of the main tectonic units (faults and the buried diapir No. 4) and shows that the resistivity model correlates well with the known lithology of the study area. The main similarities between the geology and the MT model with increasing depth are as follows:\u003c/p\u003e\n\u003cp\u003e(i) Surficial Pliocene conglomerates (PL\u003csup\u003ec1\u003c/sup\u003e) show resistive responses in the inversion model, except at their bottom, where they appear more conductive. This conductivity could be caused by fluid circulation beneath their contact with the underlying permeable mudstones.\u003c/p\u003e\n\u003cp\u003e(ii) Outcropping highly saline mudstones and marly sandstones (Miocene in age) of the URF (M\u003csup\u003ec\u003c/sup\u003e) are correlated with an outcropping conductive surficial layer beneath the profile (the conductor C\u003csub\u003e1\u003c/sub\u003e).\u003c/p\u003e\n\u003cp\u003e(iii) Eocene evaporites interbedded with red sandy shales and limestones in the LRF (O\u003csup\u003es\u003c/sup\u003e) appear as a resistive plug (resistor R\u003csub\u003e1\u003c/sub\u003e) and a deeper resistive layer (resistor R\u003csub\u003e2\u003c/sub\u003e).\u003c/p\u003e\n\u003cp\u003e(iv) More resistive levels beneath the Miocene (O\u003csup\u003es\u003c/sup\u003e) mudstones have been related to the marly limestones and shaly sandstones of the Qom Formation (OM\u003csup\u003e1\u003c/sup\u003e) and also to the marl stones in the upper part of the LRF (O\u003csup\u003ec\u003c/sup\u003e).\u003c/p\u003e\n\u003cp\u003e(v)Beneath this variation of resistive and conductive layers related to lithological alterations in the sedimentary cover, the highly resistive layer in the deeper part of the model (resistor R\u003csub\u003e3\u003c/sub\u003e in Figure 14a) has been correlated with the basalt, dacite, and andesitic lava flows of the basement rocks (E\u003csup\u003ev\u003c/sup\u003e).\u003c/p\u003e\n\u003cp\u003eAccordingly, the MT inversion results show that the resistivity variation in this region is mainly controlled by lithology. The highest resistivity values correspond to the salts of the LRF and volcanic basement rocks. The medium resistivity values are associated with the marlstones and sandstones of the QF, while the lowest resistivity values correspond to the mudstones of the URF.\u003c/p\u003e\n\u003cp\u003eIn the study area, the lithology of the outcropping diapiric rocks (halite-dominated evaporites) shows a good correlation with the resistivity model and appears as a highly resistive body beneath the most southwestern part of the profile.\u003c/p\u003e"},{"header":"8. Conclusions","content":"\u003cp\u003eWe enhance our understanding of the subsurface structure in the Nasr-Abad region based on the lithostratigraphic image suggested by the MT model, extracting information that is inaccessible through geological surface data. The main features suggested by the MT model are:\u003c/p\u003e\n\u003cp\u003e(i) The Abshirin and Sen-Sen faults affecting the overburden correlate well at depth with abrupt variations in the resistivity pattern in the MT model. Specifically, they appear as the lateral loss of resistive and conductive features. This is well illustrated beneath the southwest of the profile, where the high-resistivity structure (R\u003csub\u003e2\u003c/sub\u003e) related to the LRF evaporites disappears to the east of the Abshirin fault (figure 16a).\u003c/p\u003e\n\u003cp\u003e(ii) The top of the Eocene volcanic basement exhibits depth variations throughout the study area (R\u003csub\u003e3\u003c/sub\u003e in figure 14a). Accordingly, the thickness of the sedimentary overburden varies along the profile. The URF and QF parts of the sedimentary cover (3-72 Ωm) seem to vanish beneath the southwestern part of the profile, whereas they are still visible at a depth of 4 km in the northeastern part of the profile. This is correlated with the structural studies and radar interferometry information in this region (Morley et al., 2009 and Roosta et al., 2019) and shows that the Eocene volcanic rocks are locally thrusted and uplifted by as much as ~5 km above the adjacent basins.\u003c/p\u003e\n\u003cp\u003e(iii) Sedimentary layers of the Tertiary overburden are tilted near the Nasr-Abad diapir and exhibit more complexity on the western flank than on the eastern one (figure 16a).\u003c/p\u003e\n\u003cp\u003e(iv) The Nasr-Abad diapir is nearly axisymmetric, with the eastern and western walls dipping approximately 50\u0026deg;. The slight asymmetry is attributed to salt rising up a steeply inclined stem, being guided to the northeast. This is consistent with the structural studies indicating that hillocks are distributed at the surface along the northeastern boundary of the dome No. 4 (Baikpour et al., 2016).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e(v) Evaporites of the LRF appear as a continuous resistive layer (resistor R\u003csub\u003e2\u003c/sub\u003e) in the deeper part of the MT model. This supports the presence of a salt canopy, which was previously suggested by Jackson (1990) based on the tightly clustering diapirs in this region.\u003c/p\u003e\n\u003cp\u003eAs a consequence, we successfully established a correlation between the MT inversion model and lithology information to characterize the deep geometry of the Nasr-Abad buried salt diapir and its adjacent overburden. Specifically, we unraveled: (1) the nearly symmetric shape of the Nasr-Abad diapir, (2) the northeastward direction of salt movement into the overburden, and (3) the top of the volcanic basement, thrusted and uplifted by the Abshirin fault.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThe authors express their gratitude to Iran Gas Engineering and Development Company for kindly providing the field data used in this work and partially funding Elham Zare during her research.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAbaie IL, Ansari HJ, Badakhshan A, Jafari A (1963) History and development of the Alborz and Serajh fields, Central Iran. In: Proceeding, 6th world petrol conference, Frankfurt, pp 1\u0026ndash;111.\u003c/li\u003e\n \u003cli\u003eAbbasi, G., Motamedi, H., Orang, K., \u0026amp; Nickandish, A. A. (2020). 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Cambridge University Press, 2005.\u003c/li\u003e\n \u003cli\u003eSchwalenberg, K., Rath, V., \u0026amp; Haak, V. (2002). Sensitivity studies applied to a two-dimensional resistivity model from the Central Andes. \u003cem\u003eGeophysical Journal International\u003c/em\u003e, \u003cem\u003e150\u003c/em\u003e(3), 673-686.\u003c/li\u003e\n \u003cli\u003eSpratt, J. E., Jones, A. G., Jackson, V. A., Collins, L., \u0026amp; Avdeeva, A. (2009). Lithospheric geometry of the Wopmay orogen from a Slave craton to Bear Province magnetotelluric transect. \u003cem\u003eJournal of Geophysical Research: Solid Earth\u003c/em\u003e, \u003cem\u003e114\u003c/em\u003e(B1).\u003c/li\u003e\n \u003cli\u003eSpratt, J. E., Skulski, T., Craven, J. A., Jones, A. G., Snyder, D. B., \u0026amp; Kiyan, D. (2014). Magnetotelluric investigations of the lithosphere beneath the central Rae craton, mainland Nunavut, Canada. \u003cem\u003eJournal of Geophysical Research: Solid Earth\u003c/em\u003e, \u003cem\u003e119\u003c/em\u003e(3), 2415-2439.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"institute of geophysics, University of Tehran","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Magnetotelluric, Electrical resistivity, Central Iran, Salt diapir","lastPublishedDoi":"10.21203/rs.3.rs-5255696/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5255696/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eGreat Kavir is the largest salt desert in Iran, located in the northern part of the Central Iran depression. Groups of clustered salt diapirs exist in the northwestern part of the Great Kavir. The Nasr-Abad salt diapir in the Shurab diapiric group is the largest buried salt diapir in this region, whose geometry at depth and surrounding structure are rarely known. In this study, we investigate a broadband magnetotelluric (MT) dataset recorded at 37 closely spaced stations distributed along a SW-NE profile to characterize the geometry, substratum, and overburden of the Nasr-Abad salt diapir. The spatially distributed MT responses noted in this study are associated with geological structures at depths of less than 30 km.The measurements were used to generate a crustal-scale resistivity model of the study area, which correlates well with the known lithostratigraphy of the region. The resistivity model reveals a dipping resistive body that has been uplifted from a deep resistive layer to shallower depths. The geometry of this body indicates a well-defined base and a northeastward dip, suggesting that it corresponds to the Nasr-Abad buried salt diapir.\u003c/p\u003e","manuscriptTitle":"Crustal-Scale Structure of the Nasr-Abad Buried Salt Diapir in Northwest Central Iran from a Profile Magnetotelluric Dataset","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-10-15 06:59:25","doi":"10.21203/rs.3.rs-5255696/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"5efae895-726c-4f83-beab-18df40f315ce","owner":[],"postedDate":"October 15th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":38876705,"name":"Geophysics"}],"tags":[],"updatedAt":"2024-10-15T06:59:25+00:00","versionOfRecord":[],"versionCreatedAt":"2024-10-15 06:59:25","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5255696","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5255696","identity":"rs-5255696","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}