Mantle Conductivity Research Articles

Global mantle conductivity imaging using 3-D geomagnetic depth sounding with real earth surface conductivity constraint

The water content in the Earth's interior is of great significance for material circulation and the dynamic evolution of the planet. The water content in mantle minerals significantly affects their conductivities. By measuring the variations in conductivity within the Earth, we can infer the water content in the mantle and study the movement and processes of materials within the Earth. The geomagnetic depth sounding is a widely used method for imaging the mantle conductivity as it has large sounding depth. However, the ocean induction effects can seriously impact geomagnetic data that can't be well corrected using conventional methods. Here, we present a novel three-dimensional inversion method for geomagnetic depth sounding to overcome the ocean induction effects by directly adopt the real earth surface conductivity into the inverse model. In this method, the unstructured tetrahedral grids are used to represent the model in multi-scale and the vector finite-element method is adopted to accurately compute the geomagnetic responses. The synthetic model tests show that the earth surface conductivity has serious effect on the inversion results, but it can be well suppressed by directly modeling it in the inverse model. We further invert the data from 128 geomagnetic stations around the world and obtain a more accurate new model of global mantle conductivity.

Electrical conductivity of the suboceanic upper mantle constrained by satellite-derived tidal magnetic fields: three-dimensional inversion, validation and interpretation

SUMMARY We present the first 3-D upper-mantle conductivity models obtained by an inversion of the satellite-derived tidally induced magnetic fields (TIMFs). We primarily use the M$_2$ period, but the potential benefit of the O$_1$ period is also inspected. The inverse-problem solution is found using the recently developed frequency-domain, spherical harmonic finite-element method based on the adjoint approach. We tested two different TIMF data sets derived from the satellite measurements of the Swarm mission and two different regularizations; the solution is either required to be sufficiently smooth or reasonably close to the a priori 3-D conductivity model WINTERC-e Wd-emax. The reconstructed conductivity models are locally compared with the 1-D conductivity profiles from other studies. If we use one of the available TIMF data sets, the smooth reconstructed model gravitates towards Wd-emax and the TIMF-adjusted Wd-emax model is closer to the reference conductivity profiles than the original Wd-emax model. Finally, we use the obtained 3-D conductivity distributions to calculate the corresponding 3-D water distribution in the upper mantle using thermodynamical and compositional models coupled to the electrical-conductivity laboratory measurement of individual mantle constituents.

Hydrogen Diffusion in the Lower Mantle Revealed by Machine Learning Potentials

AbstractHydrogen may be incorporated into nominally anhydrous minerals including bridgmanite and post‐perovskite as defects, making the Earth's deep mantle a potentially significant water reservoir. The diffusion of hydrogen and its contribution to the electrical conductivity in the lower mantle are rarely explored and remain largely unconstrained. Here we calculate hydrogen diffusivity in hydrous bridgmanite and post‐perovskite, using molecular dynamics simulations driven by machine learning potentials of ab initio quality. Our findings reveal that hydrogen diffusivity significantly increases with increasing temperature and decreasing pressure, and is considerably sensitive to hydrogen incorporation mechanism. Among the four defect mechanisms examined, (Mg + 2H)Si and (Al + H)Si show similar patterns and yield the highest hydrogen diffusivity. Hydrogen diffusion is generally faster in post‐perovskite than in bridgmanite, and these two phases exhibit distinct diffusion anisotropies. Overall, hydrogen diffusion is slow on geological time scales and may result in heterogeneous water distribution in the lower mantle. Additionally, the proton conductivity of bridgmanite for (Mg + 2H)Si and (Al + H)Si defects aligns with the same order of magnitude of lower mantle conductivity, suggesting that the water distribution in the lower mantle may be inferred by examining the heterogeneity of electrical conductivity.

Selection of areas prospective for the search of primary hydrogen in the territory of Ukraine (based on the data of the 3D P-velocity model of the mantle)

The work aimed to identify promising areas for the search for primary hydrogen in Ukraine using a 3D P-velocity model of the mantle under Ukraine and its surroundings. The work uses a 3D P-velocity model of the mantle under the territory of Ukraine and its surroundings from a depth of 50 to 1700 km north of 50° NL and up to 2500 km to the south. The model is derived using ISC bulletin data from 1964 and is presented as horizontal and vertical cross-sections.
 Since the manifestation of hydrogen is primarily associated with the release of ultra-deep fluid tracks in the mantle, a brief analysis of them was presented. A detailed analysis of all nine superdeep mantle fluids isolated on the territory of Ukraine and their velocity characteristics was carried out. A comparison of the location of the routes of passage of superdeep fluids and the velocity structure of the mantle of the considered territory showed that they are confined to tectonically activated areas. In order to confirm the possible pre­sence of primary hydrogen in selected tracks of superdeep mantle fluids, heat flow density maps, a temperature map at a depth of 50 km, a map of the depth of the Moho boundary, a conductivity density map, and a fault map of the Ukrainian Shield were considered.
 A comparative analysis of maps of the location of superdeep mantle fluids and regions on the territory of Ukraine with increased heat flow, increased depth of the Moho boun­dary, and the presence of mantle conductors and faults was carried out. Summarizing the results of the comparative analysis, the most promising areas for the search for primary hydrogen were selected, corresponding to the following routes of superdeep fluids: f12, the northeastern part of f3, the southern part of f4 (the area of the Dnipro-Donetsk basin), the southwestern part of f1 (the area of the Western Ukrainian oil and gas-bearing region) and f9 and f10 (region of the South Crimean oil and gas-bearing province).

Radial shear in the flow at the Earth’s core surface

SUMMARY The Earth’s magnetic field at the core–mantle boundary is the gradient of a harmonic potential function if the mantle is electrically insulating, and the horizontal components of the field can be derived from its radial component in the mantle. Therefore, these components give no further observational information on the core dynamics. However, it can still be envisioned that the horizontal components of the induction equation at Earth’s core surface yield further knowledge on the fluid motions at the top of the core independently of the observations. Here, we show that they provide a linear relationship between the surface velocity and the surface shear (strain shear) that depends on the mantle electrical conductivity. This offers a protocol to calculate the surface shear that we validate with synthetics obtained from dynamo simulations in the limit of a weak mantle conductance. First, using numerical simulations with stress-free boundary condition at the core surface, we retrieve the expected relationship between the horizontal flow uΣ and the shear, ${\bf u}_\Sigma =r\partial _r {\bf u}_{\Sigma }$. Next, we investigate simulations with no-slip boundary condition and insulating mantle, and we obtain the same relationship, even though the shear is not imposed as a boundary condition. Finally, we calculate the flow shear at the top of the core from a magnetic field model based on satellite measurements. The application to geophysical data indicates larger values of the surface flow shear than in the synthetic case, suggesting a possible role of the mantle electrical conductivity. The surface flow shear, in the simulations, much differs from the radial shear in the flow, deeper in the core, which is influenced by the mostly quasi-geostrophic geometry. This implies that we cannot rely on the relationship between the flow and the radial shear for quasi-geostrophic motions to exploit the horizontal components of the induction equation and gain further information on the flow at the Earth’s core surface.

Thermal conductivity of Fe-bearing bridgmanite and post-perovskite: Implications for the heat flux from the core

The thermal conductivity of the lowermost mantle controls the energy transfer from the core to the mantle, and provides insight into the thermal history, evolution, and magnetic field of the Earth. Bridgmanite (Brg) and post-perovskite (PPv) are the most abundant minerals in the lowermost mantle. However, their thermal conductivities, as well as the impact of Fe impurities, are highly controversial. In this study, the thermal conductivities of Fe-free and Fe-bearing Brg and PPv at high pressure and temperature were predicted, through a combination of non-equilibrium molecular dynamics and machine learning potential (MLP) trained with data from first-principles calculations. The thermal conductivities of Fe-free Brg and PPv are in agreement with those calculated by the Green-Kubo method based on the MLP. We found that the presence of 12.5 mol% Fe in the lowermost mantle decreases the thermal conductivities of Brg and PPv by ∼10% and ∼14%, respectively. Furthermore, the phase transition from Brg to PPv increases the thermal conductivity of pyrolite by ∼22%. Incorporating the distribution of minerals, temperature, and iron content obtained through the inversion based on mineral elasticity and seismic tomography models, a global heat flow with substantial lateral variation at the core-mantle boundary was reported. The total heat flux from the core was found to be 7.1 ± 0.5 TW, implying a geologically young inner core of 0.75 ± 0.35 Ga.

Basement and lithospheric structure of the central Ganga Basin between the Bundelkhand craton and the Sharda Deep by magnetotellurics

The geological and tectonic fabric of the Indian plate in the Ganga Basin is masked by alluvial sediments. Geophysical exploration of the basin for hydrocarbons yielded information on the presence of transverse basement ridges and Cenozoic as well as some pre-collision stratigraphy of the basin. However, the depth of the basement especially in the northern part of the basin and the nature of the underlying crust and mantle lithosphere is largely unknown. We have carried out a magnetotelluric (MT) study along a 300-km-long profile between the Bundelkhand craton (BC) and the Sharda Deep (at the Himalayan foothills) across the central Ganga Basin to delineate the basement and the lithospheric structure. The electrical resistivity image obtained by 2-D inversion of broadband MT data reveals significant lateral heterogeneity in the subsurface structure along the profile. The model yields a super-thick sedimentary basin in the Sharda Deep consisting of multiple stages of basin formation, including a Proterozoic paleo-rift, and a highly heterogeneous lithospheric structure of the Indian plate north of the BC. The model consists of two major upper mantle conductors connected to the conductors within the lower crust. It is surmised that enhanced conductivity could be due to the presence of fluids and the upper mantle conductors are feeding to the crustal conductors. However, an in-depth analysis of the genesis of these conductors would require other independent geophysical information which is not available at present. This is the first reporting of such a heterogeneous nature of the Indian continental lithosphere beneath the Ganga Basin at a regional scale. Our model further supports the presence of the electrical Moho in the BC and, together with earlier published MT results along another profile, suggests that the BC extends much farther north than its presently known northern limit.

Simultaneous inversion for source field and mantle electrical conductivity using the variable projection approach

Time-varying electromagnetic field observed on the ground or at a spacecraft consists of contributions from (i) electric source currents, such as those in the ionosphere and magnetosphere, and (ii) corresponding fields induced by source currents within the conductive Earth’s interior by virtue of electromagnetic induction. Knowledge about the spatio-temporal structure of inducing currents is a key component in ionospheric and magnetospheric studies, and is also needed in space weather hazard evaluation, whereas the induced currents depend on the Earth’s subsurface electrical conductivity distribution and allow us to probe this physical property. In this study, we present an approach that reconstructs the inducing source and subsurface conductivity structures simultaneously, preserving consistency between the two models by exploiting the inherent physical link. To achieve this, we formulate the underlying inverse problem as a separable nonlinear least-squares (SNLS) problem, where inducing current and subsurface conductivity parameters enter as linear and nonlinear model unknowns, respectively. We solve the SNLS problem using the variable projection method and compare it with other conventional approaches. We study the properties of the method and demonstrate its feasibility by simultaneously reconstructing the ionospheric and magnetospheric currents along with a 1-D average mantle conductivity distribution from the ground magnetic observatory data.Graphical

New constraints on the structure and composition of the lithospheric mantle beneath the Slave craton, NW Canada from 3-D magnetotelluric data – Origin of the Central Slave Mantle Conductor and possible evidence for lithospheric scale fluid flow

Previously collected magnetotelluric data were used to produce an improved 3-D resistivity model of the lithosphere beneath the Slave craton. A region of low resistivity (<10 Ωm) at ∼100 km depth was imaged beneath the central Slave craton, and identified as the Central Slave Mantle Conductor (CSMC) reported in previous studies. Earlier studies interpreted the CSMC as graphite films but new analysis from laboratory experiments indicates that it is unlikely that graphite films are stable/continuous at this depth. Using recent laboratory experiments we review alternative conduction mechanisms for the CSMC including (a) metasomatic phlogopite, (b) grain boundary sulphides and (c) high density fluids including hydrous carbonatite melts or brines. None of these hypotheses are without flaws requiring either unreasonably high-volume fractions or have petrological challenges. However, brines are the preferred explanation as they (1) can explain geophysical observations, (2) are observed in fibrous diamonds coincident with the CSMC and (3) require small volume fractions (<1%). We hypothesize their emplacement was related to a Mesozoic subduction event. This implies that upper mantle conductors should not be used as an indicator of carbon concentration in the mantle to prospect for diamondiferous regions. Our resistivity model indicates a lithospheric thickness of 210 ± 10 km beneath the Slave craton, consistent with seismic and xenolith studies. A water content of 10–150 ppm at 100–170 km depth in the lithospheric mantle is inferred from resistivity, broadly agreeing with estimates from mantle xenoliths from the central Slave craton lithosphere. Below 170 km the bulk lithosphere is "dry" (<10 ppm), which may explain why the cratonic root has been resistant to erosion by the underlying asthenosphere. Low resistivity fingers extend to surface in multiple locations that may represent paleo-fluid flow pathways through the lithosphere, broadly resembling those observed beneath Olympic Dam, Australia.

Influence of heterogeneous thermal conductivity on the long-term evolution of the lower-mantle thermochemical structure: implications for primordial reservoirs

Abstract. The long-term evolution of the mantle is simulated using 2D spherical annulus geometry to examine the effect of heterogeneous thermal conductivity on the stability of reservoirs of primordial material. Often in numerical models, purely depth-dependent profiles emulate mantle conductivity (taking on values between 3 and 9 Wm-1K-1). This approach synthesizes the mean conductivities of mantle materials at their respective conditions in situ. However, because conductivity also depends on temperature and composition, the effects of these dependencies on mantle conductivity are masked. This issue is significant because dynamically evolving temperature and composition introduce lateral variations in conductivity, especially in the deep mantle. Minimum and maximum variations in conductivity are due to the temperatures of plumes and slabs, respectively, and depth dependence directly controls the amplitude of conductivity (and its variations) across the mantle depth. Our simulations allow assessing the consequences of these variations on mantle dynamics, in combination with the reduction in thermochemical pile conductivity due to its expected high temperatures and enrichment in iron, which has so far not been well examined. The mean conductivity ratio from bottom to top indicates the relative competition between the decreasing effect with increasing temperature and the increasing effect with increasing depth. We find that, when depth dependence is stronger than temperature dependence, a mean conductivity ratio &gt;2 will result in long-lived primordial reservoirs. Specifically, for the mean conductivity profile to be comparable to the conductivity often assumed in numerical models, the depth-dependent ratio must be at least 9. When conductivity is underestimated, the imparted thermal buoyancy (from heat-producing element enrichment) destabilizes the reservoirs and influences core–mantle boundary coverage configuration and the onset of dense material entrainment. The composition dependence of conductivity only plays a minor role that behaves similarly to a small conductivity reduction due to temperature. Nevertheless, this effect may be amplified when depth dependence is increased. For the cases we examine, when the lowermost mantle's mean conductivity is greater than twice the surface conductivity, reservoirs can remain stable for very long periods of time, comparable to the age of the Earth.

Trans-dimensional Bayesian joint inversion of magnetotelluric and geomagnetic depth sounding responses to constrain mantle electrical discontinuities

SUMMARYJoint inversion of magnetotelluric (MT) and geomagnetic depth sounding (GDS) responses can constrain the crustal and mantle conductivity structures. Previous studies typically use either deterministic inversion algorithms that provide limited information on model uncertainties or using stochastic inversion algorithms with a predetermined number of layers that is generally not known a priori. Here, we present a new open-source Bayesian framework for the joint inversion of MT and GDS responses to probe 1D layered Earth’s conductivity structures. Within this framework, model uncertainties can be accurately estimated by generating numerous models that fit the observed data. A trans-dimensional Markov Chain Monte Carlo (MCMC) method is employed to self-parametrize the model parameters, where the number of layers is treated as an inversion parameter that is determined automatically by the data. This adaptability can overcome the under or over-parametrization problem and may be able to automatically detect the conductivity discontinuities in the Earth’s interior. To accelerate the computations, a large number of Markov chains with different initial states can be run simultaneously using the MPI parallel technique. Synthetic data sets are used to validate the feasibility of our method and illustrate how separate and joint inversions, as well as various priors affect the posterior model distributions. The trans-dimensional MCMC algorithm is then applied to jointly invert the MT and GDS responses estimated at the Tucson geomagnetic observatory, North America. Our results not only contain model uncertainty estimates but also indicate two distinct conductivity discontinuities at around 85 and 440 km, which are likely related to the lithosphere-asthenosphere boundary and the upper interface of the mantle transition zone, respectively.

The crustal electrical structure of a tectono-magmatic rejuvenated craton: Insights from magnetotelluric data of the northwestern Qaidam Basin, northern Tibetan Plateau

The Qaidam Basin is a tectono-magmatic rejuvenated craton that plays a key role in the deformation of the northern Tibetan Plateau caused by the far-field effects of the India–Asia collision. However, the deformation mechanism of the crust and upper mantle beneath the basin is still elusive to date. Here we present an electrical resistivity model of the crust and uppermost mantle in the northwestern margin beneath the Qaidam Basin (QB) from magnetotelluric (MT) data. The MT data were collected along a ∼ 300 km long profile and inverted using the MARE2DEM code with the OCCAM algorithm. From the model, four major conductors were revealed in the lower crust of the Qaidam Basin and the crust-mantle transition zones of the Qiman Tagh thrust belts (QTT) and Suganhu Basin, respectively. Constrained by the laboratory measurements, the conductors (∼2–10 Ωm) beneath the western QB can be explained by a small amount of saline fluids (∼0.2%–2%) derived from devolatilization reactions, which indicates ductile deformation of the lower crust of the Qaidam Basin. The upper mantle conductors beneath the Qiman Tagh and Suganhu Basin may be related to asthenospheric upwelling caused by the partial removal of the lithosphere at different stages in the central and northern Tibetan Plateau, respectively. These electrical features suggest that the Qaidam lithosphere has experienced devolatilization, partial melting, delamination of the lower crust, and upwelling of the asthenosphere, resulting in a tectono-magmatic rejuvenated craton.

The study of geomagnetic jerk from 2010 to 2021 based on hourly mean data from global geomagnetic observatories

The secular variation in the global geomagnetic field was analyzed in terms of the annual differences in monthly means by using the hourly mean data from 18 foreign (outside China) observatories of the World Data Center (WDC) for Geomagnetism from January 2010 to January 2020 as well as 9 observatories in the Geomagnetic Network of China from January 2015 to April 2021. In addition, according to the correlation of noisy components from the observatories, a covariance matrix was constructed based on residuals between observations and the CHAOS-7.4 model to remove external contamination. Through a comparison before and after denoising, we found that the overall average standard deviations were reduced by 29.97% in China and by 41.4% outside China. Results showed the correlation coefficient between external noise (mainly the magnetosphere ring current) and the <italic>Dst</italic> index was 0.82, and the correlation coefficient between external noise and the Ring Current (RC) index reached 0.94. A geomagnetic jerk was globally discovered around 2018.0 on the geomagnetic eastward component <italic>Y</italic>. The jerk timing in China was around 2020.0, and the earliest one was in 2018.75, whereas the timing outside China was around 2018.0, and the earliest one was in 2017.67. This 2-year lag may have been caused by the higher electrical conductivity of the deep mantle. After more data were added, this jerk event was found to occur in an orderly manner in the northern hemisphere as the longitude increased and the intensity gradually increased as well. The variations in location of the jerk center were analyzed according to the CHAOS-7.4 model. Results revealed six extreme points distributed nearby the equator. The strongest was near the equator, at 170°E, and the strength gradually decreased as it extended to the northern and southern hemispheres. Another extreme point with the opposite sign was located at the equator, at 20°W, in the south-central part of the Atlantic, and the strength gradually decreased as it extended into Europe. The covariance matrix method can be used to analyze data from the Macau Science Satellite-1 mission in the future, and this method is expected to play a positive role in modeling and separating the large-scale external field.

A global mantle conductivity model derived from 8 years of Swarm satellite magnetic data

Mantle conductivity imaging is one of the scientific goals of the forthcoming Macau ScienceSatellite-1 (MSS-1). To achieve this goal, we develop a data analysis and inversion scheme for satellite magnetic data to probe global one-dimensional (1D) mantle conductivity structures. Using this scheme,we present a new global mantle conductivity model by analyzing over 8 years of Swarm satellite magnetic data. First, after sophisticated data selection procedures and the removal of core and crustal fields, the inducing and induced spherical harmonic coefficients of magnetic potential due to the magnetospheric ring current are derived. Second, satellite <italic>C</italic>-responses are estimated from the time series of these coefficients. Finally, the observed responses are inverted for both smooth and three-jump conductivity models using a quasi-Newton algorithm. The obtained conductivity models are in general agreement with previous global mantle conductivity models. A comparison of our conductivity model with the laboratory conductivity model suggests the mean state of the upper mantle and transition zone is relatively dry. This scheme can be used to process the forthcoming Macau ScienceSatellite-1 magnetic data.

Constraining the Crustal and Mantle Conductivity Structures Beneath Islands by a Joint Inversion of Multi‐Source Magnetic Transfer Functions

AbstractIn this study, we present a tool to simultaneously invert multi‐source magnetic transfer functions (TFs), including tippers, solar global‐to‐local TFs originating from the signals due to ionospheric source, and global Q‐responses originating from the signals due to magnetospheric source. We jointly invert the aforementioned TFs to constrain the local conductivity structures beneath three islands in the Atlantic (Tristan da Cunha), Indian (Cocos), and Pacific (Oahu) Oceans. The recovered conductivity profiles appeared to be consistent with the presence of upper mantle plumes beneath the Tristan da Cunha and Oahu Islands. Our results indicate resistive lithosphere of different thicknesses beneath considered three islands. Besides, new conductivity profiles suggest warmer‐than‐average mantle temperatures and the presence of a small fraction of melt beneath Tristan da Cunha Island. At the same time, the conductivities beneath Cocos Island are in good agreement with estimates expected for ambient mantle conditions.

Synthetic magnetotelluric modelling of a regional fault network – implications for survey design and interpretation

The magnetotelluric (MT) method is increasingly being applied to mineral exploration under cover with several case studies showing that mineral systems can be imaged from the lower crust to the near surface. Driven by this success, the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) is delivering long-period data on a 0.5° grid across Australia, and derived continental scale resistivity models that are helping to drive investment in mineral exploration in frontier areas. Part of this investment includes higher-resolution broadband MT surveys to enhance the resolution of features of interest and improve targeting. To help gain best value for this investment it is important to understand the ability and limitations of MT to resolve features on different scales. Here we present synthetic modelling of continuous, narrow, near-vertical faults 500 m to 1500 m wide with a resistivity 100–200 times less than the background rock resistivity using the ModEM software, and show that for such a situation a station spacing of around 14 km across strike is sufficient to resolve these into the upper crust. However, the vertical extent of these features is not well constrained, with near-vertical planar features commonly resolved as two separate features. This highlights the need for careful interpretation of anomalies in MT inversion. In particular, in an exploration scenario, it is important to consider that a lack of interconnectivity between a lower crustal/upper mantle conductor and conductors higher up in the crust and the surface might be apparent only, and may not necessarily reflect reduced mineral prospectivity.

A relatively dry mantle transition zone revealed by geomagnetic diurnal variations.

The distribution of water within the mantle transition zone (MTZ) has important implications for the material circulation and partial melting of the mantle. Although solubility of hydrogen is very high, leading to speculations that the MTZ plays a key role in the deep-Earth water cycle, the actual water content remains an open question. Electrical conductivity of mantle minerals is very sensitive to water content, so reliable estimates of this physical parameter in the MTZ would provide valuable constraints. Here, we use recently developed joint inversion of geomagnetic diurnal variation for realistic source structure and one-dimensional mantle conductivity profile. Synthetic tests show that the resulting profile is a reasonable proxy for the electrical conductivity distribution of continental mantle over depths where model resolution is best (200 to 600 kilometer), even in the presence of lateral heterogeneity. The inferred water concentration in the MTZ is 0.03 weight %, one to two orders of magnitude below the solubility of wadsleyite and ringwoodite.

A Multi‐Resolution Finite‐Element Approach for Global Electromagnetic Induction Modeling With Application to Southeast China Coastal Geomagnetic Observatory Studies

AbstractWe present a multi‐resolution finite‐element approach for three‐dimensional (3D) electromagnetic (EM) induction modeling in spherical Earth. First, the secondary electric field approach is employed so that both magnetospheric and ionospheric current sources are naturally considered. Second, the multi‐resolution tetrahedral grids are used to approximate the heterogeneous crust and mantle, so that the local ocean effects at coastal and island observatories can be accurately simulated. Furthermore, a parallel goal‐oriented hp‐adaptive finite‐element method with Nédélec vector elements is employed to guarantee the accuracy of solutions for arbitrary 3D conductivity distributions. Finally, two synthetic models are used to verify the accuracy and efficiency of our newly developed forward modeling solver. Results show that accurate solutions can be obtained for problems with several million to hundreds of millions of unknowns in a few minutes using 128 cores on a cluster. We apply this approach to correct the near‐surface ocean effects for several unused Chinese coastal observatories by performing multi‐resolution 3D modeling. The corrected data are inverted for the subsurface layered mantle conductivity structures. The conductivity model beneath southeast China is more resistive than that beneath northeast China by more than half an order of magnitude. By comparing the inverse models with the latest laboratory conductivity‐depth profiles, the estimated transition zone water content is less than 0.01 wt% beneath southeast China irrespective of which laboratory data is used. Considering the low‐velocity anomalies in this region, which suggest high‐temperature structures, less water is expected. We, therefore, infer that the mantle transition zone beneath southeast China is dry.

Constraints on the process and mode of the Paleo-Asian Ocean closure from the lithospheric conductivity structure of the south-eastern Central Asian Orogenic Belt

The Paleo-Asian Ocean eventually closed along the Solonker Suture Zone during the Late Permian and Early Triassic, before the collision between the Siberian Craton and the North China Craton (NCC) formed the eastern Central Asian Orogenic Belt (CAOB). In this paper, Magnetotelluric (MT) profile data across the south-eastern CAOB and the northern margin of the NCC are explored to image the two-dimensional resistivity structure of the region. According to the resistivity model, a north-dipping conductive layer to the north of the Xilinhot fault and a south-dipping conductive layer to the south of the Linxi fault are revealed. The two conductors are interpreted as evidence of the late-stage low-angle bidirectional subduction of the Paleo-Asian Ocean, which constrains the northern and southern deep boundaries of the Solonker Suture Zone. Another couple of mantle conductors with opposite dipping angles are revealed under the Solonker Suture Zone and Bainaimiao Belt, which are interpreted as evidence of the early-stage high-angle bidirectional subduction of the Paleo-Asian Ocean. Compared with the results from previously reported MT profiles in this area, the subduction angle becomes more gentle from west to east. The difference of convergence rate between the NCC and the Siberian Craton is considered to be the major cause of the closure difference along the strike. The northern margin of the NCC has a conductive lower crust, which may result from the magma underplating or crust-mantle decoupling, as it is imaged to be connected with the south-dipping conductive layer beneath the Solonker Suture Zone.

Magnetotelluric Evidence for Distributed Lithospheric Modification Beneath the Yinchuan‐Jilantai Rift System and Its Implications for Late Cenozoic Rifting in Western North China

AbstractThe Yinchuan‐Jilantai rift system (YJRS) is a prominent Cenozoic intracontinental rift zone located along the northwestern margin of the Ordos Block in western North China. Although the absence of volcanism indicates a passive origin, the tectonic driving forces and rift‐related deep processes remain poorly understood. Here we use newly obtained broadband magnetotelluric (MT) data to image the lithospheric electrical structure along a ∼500‐km‐long profile across the YJRS. At middle–lower crustal levels, the resulting model reveals several subvertical, crustal‐scale conductors that spatially correlate with the rift‐parallel, high‐angle normal faults. We attribute these features to a combined effect of saline fluids and partial melt due to recent supply of heat and volatiles into the crust from below. The crustal‐penetrating normal faults that were reactivated during extension serve as permeable pathways for deep fluid migration. In the uppermost mantle, a ∼400‐km‐wide zone of enhanced conductivity is present and requires the presence of partial melt. We interpret this feature as evidence for lithospheric modification through upwelling and decompressional melting of the volatile‐enriched mantle. When compared with the narrow (&lt;100‐km‐wide) mantle conductors imaged beneath the Shanxi rift system along the eastern margin of the Ordos Block, such a feature indicates the YJRS has experienced a larger extent of lithospheric modification. Combined with additional geologic and geophysical observations, we attribute this essential difference to the inherited lithospheric heterogeneity and the eastward decreasing far‐field impacts from the India‐Eurasia collision.