Lithospheric Electrical Structure Research Articles

A Review of Relationship between the Metallogenic System of Metallic Mineral Deposits and Lithospheric Electrical Structure: Insight from Magnetotelluric Imaging

In development over 70 years, magnetotelluric (MT) sounding, a high-resolution technique for subsurface electrical resistivity imaging, has been widely applied in resource exploration in the Earth. The key factors of the metallogenic system of metallic mineral deposits can be closely correlated to the electrical anomalies of the lithosphere. In this paper, we review the relationship between the electrical resistivity model of the lithosphere and the metallogenic system. At the beginning, we indicate why the electrical parameters relate to the metallogenic system in all geophysical parameters. The advantage of MT sounding in sketching an electrical resistivity model of the lithosphere is subsequently discussed, and some methods of data processing, analysis and inversion are also introduced. Furthermore, we summarize how to bridge the relationship between the electrical resistivity model of the lithosphere and metallogenic system, and analyze the influence of the rheological variation estimated from conductivity in the lithosphere on mineralization. In the end, we list some typical cases of the application of MT sounding in mineral exploration, and also give some suggestions for future work. This study is aimed at providing guidance in discussing the metallogenic system using an electrical resistivity model.

Lithospheric electrical structure of the Alxa Block, NW China, southern central Asian Orogenic Belt, revealed by 2D inversion of magnetotelluric data

The Alxa Block, situated between the Tarim and North China cratons, is essential to understand the tectonics of northwest China. It has undergone several tectonic cycles, including rifting from northern Gondwana in Neoproterozoic, subduction-accretion of the Paleo-Asian Ocean during Paleozoic, and intracontinental deformation in Mesozoic. Broadband magnetotelluric data covering a frequency range of 320 Hz to ∼ 10000 s was collected along two NW-SE trending lines across major tectonic units of the Alxa region to investigate the deep structures of the Alxa Block. By applying the nonlinear conjugate gradient (NLCG) inversion approach, two-dimensional (2D) electrical resistivity models were obtained at a 100 km depth. These findings indicate that the Zhusileng-Hangwula and Zongnaishan-Shalazhashan tectonic zones have high resistivity in their lower crust and upper mantle, with an overall layered high-low–high resistivity pattern. The upper crust of the Nuru-Langshan tectonic zone is primarily characterized by high resistivity values, whereas the lower crust and upper mantle contain a large-scale low-resistivity (or conductive) zone that is likely the result of partial melting. The large-scale Early–Middle Jurassic nappe structures may be connected to sub-horizontal low-resistivity anomalies in the shallow strata, which are located up to 20 km below the Yagan and Zhusileng–Hangwula tectonic zones. Below the Engger Us fault, two low-resistivity bands are interpreted as the product of bidirectional subduction and final closure of the Paleo-Asian Ocean. The Alxa Block may have subducted southwards beneath the Ordos Block, as suggested by a clear low-resistivity anomaly between the two blocks. Furthermore, partial melting may have happened for high-conductivity bodies in the upper mantle based on the modified Archie formula’s evaluation of their melting degree.

Lithotectonic architecture of the Paleoproterozoic intracratonic Kaladgi rift basin, southwestern India: Inference from a magnetotelluric study

Peninsular India is traversed by several major Paleoproterozoic rift basins, among which the Kaladgi rift basin is a prominent one. Here, we present results from Magnetotelluric (MT) studies in the eastern section of this rift basin. The data were acquired along a ∼120 km long profile passing through Belavanik-Bagalkot-Ukkali regions covering the Paleoproterozoic intracratonic Kaladgi rift basin, the Cretaceous Deccan volcanic province and the Archean gneisses/granites/greenstones of the Dharwar craton. The lithospheric electrical structure was obtained from joint inversion of TE- and TM-modes data using a 2-D nonlinear conjugate gradient algorithm. The results reveal that the Kaladgi sediments are ∼500–1000 m in thickness overlying a highly fractured crystalline basement. Proterozoic sediments are exposed beneath the Deccan basalt cover and in the shallow fractured zones. The conductive-resistive transitions in the model at the crustal depths correspond to major tectonic faults and deformed crustal rocks. The transition boundary at ∼50 km depth defines the electrical Moho. At the same time, a ∼25 km wide, steep conductive feature from ∼75 km to upper mantle lithosphere depths (∼180–200 km) is interpreted as the trace of the Chitradurga Suture Zone (CSZ). The observed crustal heterogeneity, conductivity variations, and resistive lithosphere are attributed to the geological history of the region, including Archean collisional events and subsequent reactivation associated with the Reunion hotspot. The crustal conductors are associated with mafic intrusions from the underplated basalts, and also the high conductivity may be due to the sulfide mineralization in the fault zones. The MT study provides valuable insights into the deep tectonic framework of the region.

Divergent double subduction of Bangong-Nujiang Ocean revealed by high-resolution magnetotelluric data at 86°E in the northern Tibetan Plateau

The Tibetan Plateau was mainly produced by the assembly of several terranes resulting from ocean closure and continental collision. The formation and evolution of the Bangong-Nujiang Ocean (BNO) played a prominent role in the Tibetan Plateau accretion before the Cenozoic collision between the Indian and Eurasian continents. The closure mode of the BNO between the Lhasa and Qiangtang terranes has been controversial, involving subduction polarity, i.e., regarding whether there was only northward subduction beneath the Qiangtang terrane or both southward and northward subduction beneath the Lhasa and Qiangtang terranes. To resolve this issue, a dense broadband magnetotelluric (MT) profile transecting the Bangong-Nujiang suture zone (BNS) at 86°E was completed. After standardized data processing and analysis, geological interpretation can be performed on the two-dimensional (2-D) inversion results of the MT data. Through a large number of inversion experiments, we obtained a reliable lithospheric electrical structure beneath the BNS and its adjacent regions. Our results show that the upper crust in our study area is featured by high resistivity overall, but it is cut by several fault zones, indicating that it may be influenced by the late strike-slip fault caused by the India-Asia collision. The middle-lower crust of the northern Lhasa terrane is highly conductive and may represent juvenile crust. The existence of a northward and southward inclined electrical gradient zone in the BNS may indicate that the BNO was closed by north-south bidirectional subduction.

Lithospheric electrical structure across the Bangong-Nujiang Suture in northern tibet revealed by magnetotelluric

Competing hypotheses have been proposed to explain the subduction polarity of the Bangong-Nujiang Tethyan Ocean and the formation of the high-conductivity anomaly beneath the Qiangtang terrane. However, the lithospheric architecture of the northern Tibetan Plateau is still poorly understood due to inhospitable environments and topography. Therefore, in the winter of 2021, a 440 km long, SN-trending broadband magnetotelluric (MT) profile was recorded in northern Tibet to detect its regional lithospheric structure. The nonlinear conjugate gradients algorithm is conducted to invert the individual TM mode data. A reliable 2D electrical model was obtained by ablation processing and analysis of broadband magnetotelluric data to test the lithospheric electrical structure and dynamics between the northern Lhasa and Qiangtang terranes. The inversion results reveal the lithospheric structure at a depth of 100 km in northern Tibet, which synthesizes geological, geochemical and deep seismic reflection evidence and firmly identifies that the trace of the south-dipping conductor mainly resulted from the southward subduction of the Bangong-Nujiang Tethyan Ocean under the Lhasa terrane and the trace of the north-dipping conductor likely due to the northward subduction of the Bangong-Nujiang Tethyan Ocean under the Qiangtang terrane. In addition, the magnetotelluric profile also images a high-conductivity lithospheric-scale anticline beneath the central Qiangtang terrane, which may correspond to the upwelling of postcollisional magmatism triggered by northward subduction of the Bangong-Nujiang Tethyan Ocean under the Qiangtang terrane, aqueous fluid and/or partial melting.

Three-Dimensional Lithospheric Electrical Structure beneath the Handan-Xingtai District, North China: Implications for Tectonic Control of Skarn-Iron Mineralization

In this study, we determined the lithospheric electrical structure beneath the Handan-Xingtai district and its adjacent regions using magnetotelluric sounding data. To the west of the Handan-Xingtai district, the crust and upper mantle beneath the Taihang Mountains are mainly characterized by high resistivity (>1000 Ωm, which we interpreted to be the relic cratonic lithosphere. In contrast, the lithosphere beneath the North China Plain to the east shows high-conductivity features (<100 Ωm) overall, which may indicate that it has suffered significant modifications. Additionally, other geological and geophysical studies suggested that this district was located in a significant boundary zone where the lithospheric thickness, temperature and geochemistry properties sharply changed. Combined with our resistivity model, we attributed this to the different degrees of lithospheric modification. Specifically, since the late Mesozoic, the subduction, roll-back and dehydration of the Pacific slab caused an unsteady asthenospheric flow and upwelling; therefore, the deep-derived melts and fluids concentrated within the uppermost mantle had even underplated or intruded into the crust, while this process had a negligible effect on the Taihang Mountains. Small-scale mantle convection and upwelling are likely to occur in this kind of transfer zone of lithospherice properties, leading to mantle-derived melts and fluids transporting upwardly near the surface, which was confirmed by the significantly enhanced conductivity beneath the ore district in our resistivity model. During this process, Fe derived from mantle-source magma or relic Precambrian metamorphic basement beneath the Taihang Mountains was extracted and emplaced along with the Yanshanian magmatism.

Lithospheric electrical structure and its implications for the evolution of the middle Qinling Orogenic Belt, Central China: Constraints from 3-D magnetotelluric imaging

The Qinling Orogenic Belt is located at the central part of the China Central Orogenic Belt and links the Qilian and Kunlun Orogenic Belts to the west with the Dabie-Sulu Orogenic Belt to the east, which is sandwiched between the South China Block and North China Block. There have been several geodynamic models developed to explain the evolution process of the Qinling Orogenic Belt, but they remain controversial. The magnetotelluric (MT) method can study subsurface electrical structures. In order to have a better understanding of the tectonic evolution of the Qinling Orogenic Belt, a study of broadband and long period MT profile across the middle Qinling Orogenic Belt is carried out. The MT profile starts from the northeastern margin of the Sichuan basin, crosses the Qinling, and finally reaches the Weihe basin at about 108.5°E. Based on data analysis results, we have finally obtained a lithospheric electrical resistivity model of the middle Qinling Orogenic Belt through three-dimensional (3-D) inversion of magnetotelluric data. The MT result reveals that the Sichuan and Weihe basins may have remnants of the ancient craton basement in our study area. The electrical structure characteristics of the South Qinling Belt and North Qinling Belt are obviously different, the South Qinling Belt is relatively low resistivity, while the North Qinling Belt is relatively high resistivity. This may indicate that the Southern Qinling Belt is affected by the later strike slip extrusion and the Northern Qinling Belt is a relatively stable block. The lithospheric mantle of middle Qinling Orogenic Belt is characterized by overall high resistivity, which may indicate that the lateral extrusion effect of the Tibetan Plateau material is negligible in this region.

3-D electrical structure of the lithosphere beneath the Yanshan Belt and its adjacent areas, North China

The North China Craton is one of the oldest continental nuclei in the world. However, extreme lithospheric modification has occurred since the Mesozoic, of which the most typical area is the Yanshan orogenic belt. In this research, a 3-D lithospheric electrical structure beneath the Yanshan belt and adjacent areas was obtained using magnetotelluric array data, which reveals the regional heterogeneity. Notably, the crust and upper mantle (< 100 km) within the Yanshan belt, especially in its southern part, are characterized by high resistivity (> 1000 Ωm). These features are interpreted to be a relic cratonic lithosphere mainly composed of highly depleted olivine. A lithosphere-scale relatively high-conductivity zone (~ 100 Ωm, near EW-trending), which roughly lies beneath the Miyun-Xifengkou-Qinglong fault coincides with the boundary of Archean greenstone belt and also the Mesozoic granitic zone, probably indicating a reactivated Precambrian lithospheric weak/shear zone within Yanshan belt. In contrast, the lower crust and upper mantle of the Bohai Bay basin show large areas of high conductivity, which may result from the modification of the crust and mantle by the upwelled asthenospheric materials. Additionally, the epicenters of several historic destructive earthquakes, including the 1976 Tangshan earthquake (M7.8), are mainly located near the high- and low-resistivity boundary at the conjunction of the Yanshan belt and the Bohai Bay basin. The fragmentation of brittle crustal rocks mainly happened in where the tectonic stresses concentrated, which might result from the rheological difference between the high-resistivity ancient and the high-conductivity modified lithosphere. In these places, intense seismicity would be induced.

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.

Lithospheric electrical structure beneath the Cathaysia Block in South China and its tectonic implications

To better constrain the heat source of the geothermal system and lithospheric thinning mechanism of the Paleo-Pacific subduction zone, data from 35 broadband magnetotelluric (MT) stations were collected in 2020 along a 180-km-long NW-trending profile in the Cathaysia Block. Based on the measured data, we obtained the lithospheric electrical structure of the Cathaysia Block in South China using the 3D MT inversion code of ModEM. The final resistivity structure along the profile shows a series of highly conductive anomalies and highly resistive anomalies. The highly resistive anomalies in the upper crust correspond well with the distribution of magmatic rocks, which extend downwards to depths of 20 km. The highly resistive anomalies with a depth of 20–40 km are interpreted as crustal mafic sills derived from the mantle. The crustal conductors at depths of 5–15 km are typically interpreted as accumulations of aqueous fluids, which originate from the dehydration of the slab during the flat-slab subduction of the Paleo-Pacific Plate and are confined in the crust by impermeable layers. Fluid fractions of 0.8–9% would be required to explain the observed conductance values. The MT results indicate that there is no magma chamber or partial melt beneath the geothermal system. Therefore, the heat source of the geothermal system in the Cathaysia Block is radiogenic heat produced by the decay of radioactive elements in the crust. Thermal erosion has played a central role in lithospheric thinning and crust–mantle interaction of the Cathaysia Block.

Lithospheric electrical structure between the Erguna and Xing'an blocks: Evidence from broadband and long period magnetotelluric data

The Central Asian Orogenic Belt (CAOB) is the largest accretionary orogen on Earth, and its evolution has been controversial for a long time. Northeast China and adjacent regions form the eastern portion of the CAOB, which is an important area for exploring the tectonic evolution of both the Northeast Asia and the CAOB. The Erguna and Xing’an blocks are two micro-blocks in the west of Northeast China. The magnetotelluric (MT) method can study subsurface electrical structures. A dense broadband and long period magnetotelluric (MT) profile across the collision belt of the two blocks has been completed. Based on the results of data analysis, a three-dimensional (3-D) resistivity model of the crust and upper mantle has finally been obtained. The MT results show that the Derbugan Fault is not a suture zone, whereas the Xinlin-Xiguitu Fault is considered the boundary of the Erguna and Xing’an blocks by westward subduction. The conductive bodies in the middle-lower crust of the Erguna and Xing’an blocks may have been formed in an extensional environment and imply that the crust is weak. The collision belt includes a prominent high-conductivity anomaly from the lower crust to the upper mantle and indicates the existence of mantle upwelling associated with probable delamination.

Deep thermal state on the eastern margin of the Lhasa-Gangdese belt and its constraints on tectonic dynamics based on the 3-D electrical model

Since the Mesozoic, the Lhasa-Gangdese block has undergone several stages of tectonism-magmatism caused by the closure of the New-Tethys Ocean and the collision between the Indian and Eurasian plates, which led to the lithospheric shortening and crustal uplift. Through the careful processing and analysis of the magnetotelluric data on the eastern margin of the Lhasa-Gangdese block near 92°E, a reliable 3D electrical model was obtained to examine the relationship between the lithospheric electrical structure and geodynamics. Some conductive and resistive layers were obtained, which may be formed in some tectonic processes. The thermal model of the lithospheric uppermost mantle (75–100 km) were calculated using the Arrhenius equation and the Hashin-Shtrikman (HS) boundary condition. Combined with the terrestrial heat flow point and curie isothermal in the study area, the temperature distribution at the 30–100 km depths was calculated. The melt fractions of the mid-lower crust calculated by the classical Archie's law, pressure calculated by density and temperature at different depths can be obtained in a meantime. The resistor beneath the Tethys-Himalaya terrane represented the subduction of the Indian plate with a deep angle. The subduction of the Indian crust may not exceed the Indus-Yarlung Zangbo suture, while the Indian lithospheric mantle may not reach the Luobadui-Milashan fault. Additionally, our electrical and thermal structure indicated that the Indian lithospheric mantle probably detached from the Indian crust. Our study further discussed the underplating of the mantle-derived materials and the southern extrusion along the Main Himalaya Trust, and offered a subsidiary evidence for the possible “south-eastward crustal flow” in a local area. Finally, the relationship between the electrical structure and the deep metallogenic mechanism was briefly established.

A plume-modified lithospheric barrier to the southeastward flow of partially molten Tibetan crust inferred from magnetotelluric data

Broadband and long-period magnetotelluric (MT) data were used to obtain a continuous image of the lithospheric electrical conductivity structure across the southeastern Tibetan Plateau margin, where long-wavelength crustal thickening and surface uplift have been attributed to the influx of weak crustal material from Tibet into the adjacent Yangtze Block. The resulting resistivity model reveals two major conductive features in the middle–lower crust beneath southeastern Tibet and the Chuxiong basin, both of which are consistent with the presence of fluids and likely enhanced by shear deformation along large faults. The horizontally extensive conductor beneath southeastern Tibet could be interpreted as a partially molten layer that represents topography-driven crustal flow from central Tibet, whereas the anomalously high conductivities observed beneath the Chuxiong basin require the addition of substantial amounts of saline aqueous fluids. These two conductors are separated by a lithospheric-scale, high-resistivity body that situated beneath the geologically inferred core of the Emeishan large igneous province (ELIP). Together with other geophysical and geological evidence, we interpret this anomaly as remnants of a plume-modified lithosphere that originally formed during the Permian Emeishan volcanism, and speculate it acts as an obstruction to the southeastward crustal flow from Tibet since Late Cenozoic. These results indicate that topography-driven crustal flow may contribute to crustal thickening and uplift in southeastern Tibet, but not in the adjacent foreland. Instead, magmatic underplating associated with the Permian plume activity could account for both the elevated topography and thickened crust of the adjacent Yangtze Block without the need for crustal flow.

Lithospheric electrical structure in the central Tibetan Plateau and its tectonic significance

The Himalaya-Tibetan Plateau that is situated in the southwest China is an ideal natural field laboratory for continental dynamics. In this study, through the careful processing and analyzing of the magnetotellurics data, the reliable 2-D electrical model was obtained to examine the relationship between the lithospheric electrical structure and dynamics in the central Himalaya-Tibetan Plateau. Six conductors mainly result from the partial melting and aqueous fluids, and three resistors are related to different tectonic evolution in the model. Combined with other observations, we summarize that the upwelling hot material (partial melting), aqueous fluids, subduction erosion and crushed zone contribute to the error between the electrical location and the geographic location of the Indus-Yarlung Zangbo suture. The subducted Indian plate is shown by the different angles and different locations of its frontier in different areas. The electrical structure beneath the Lhasa terrane corresponds well with the geological features of an old, locally reworked crust dominantly in the central Lhasa terrane with two young juvenile crustal blocks surrounding the central terrane, and it also shows the relationship among the dynamics, magmatism and mineralization in the Gangdise metallogenic belt. Meanwhile, we indicate that the subducted polarity of the Bangong-Nujiang Tethyan Ocean lithosphere was double-sided, and the smooth subduction of the Longmu Tso-Shuanghu Tethyan Ocean lithosphere may cut through the Moho surface approximately 20 km. Above all, these results can provide helpful constrains to understand the evolution mechanism of the Himalaya-Tibetan Plateau.

Magnetotelluric investigation of lithospheric electrical structure beneath the Dharwar Craton in south India: Evidence for mantle suture and plume-continental interaction

Broad-band and long period magnetotelluric measurements made at 63 locations along ∼500 km long Chikmagalur-Kavali profile, that cut across the Dharwar craton (DC) and Eastern Ghat Mobile Belt (EGMB) in south India, is modelled to examine the lithosphere architecture of the cratonic domain and define tectonic boundaries. The 2-D resistivity model shows moderately conductive features that intersperse a highly resistive background of crystalline rocks and spatially connect to the exposed schist belts or granitic intrusions in the DC. These features are therefore interpreted as images of fossil pathways of the volcanic emplacements associated with the greenstone belt and granite suite formation exposed in the region. A near vertical conductive feature in the upper mantle under the Chitradurga Shear Zone represents the Archean suture between the western and eastern blocks of DC. Although thick (∼200 km) cratonic (highly resistive) lithosphere is preserved, significant part of the cratonic lithosphere below the western DC is modified due to plume-continental lithosphere interactions during the Cretaceous–Tertiary period. A west-verging moderately conductive feature imaged beneath EGMB lithosphere is interpreted as the remnant of the Proterozoic collision process between the Indian land mass and East Antarctica. Thin (∼120 km) lithosphere is seen below the EGMB, which form the exterior margin of the India shield subsequent to its separation from East Antarctica through rifting and opening of the Indian Ocean in the Cretaceous.

Some Insights into the Lithospheric Electrical Structure in the Western Ghat Region from Magnetotelluric Studies

Magnetotelluric (MT) studies along a few traverses, some cutting across the Western Ghats, during the last few years have provided basic insights into the shallow as well as the deeper electrical structure in the regions near and east of the Western Ghat belt. The MT models broadly show a two layered lithospheric electrical structure with an upper high resistive layer (several thousands of Ωm) and a lower moderately conductive layer (a few tens to a few hundred Ωm). The depth of the interface between the two layers is found to vary from about 120–160 km in the south in the SGT to around 80 km in the north in the northern DVP. Another impressive feature that could be noticed in these electrical models is the presence of well-defined major near vertical crustal conductive feature associated with the region of Western Ghat belt, presumably associated with the tectonic evolution of the Western Ghats. Further, these models also brought out several other well-defined conductors that might be linked to structural features like faults, shear zones, etc., in the region. These conductors pierce through the crustal column and some of these, particularly those oriented in NW-SE direction, i.e., oriented transversely with respect to the ambient compressive stress direction of the Indian shield, assume significance in understanding the seismicity of the region.

Lithospheric electrical structure of the middle Lhasa terrane in the south Tibetan plateau

The Lhasa terrane in southern Tibetan plateau is a huge tectono-magmatic belt and an important metallogenic belt. Its formation evolution process and mineralization are affected by the subduction of oceanic plate and subsequent continental collision. However, the evolution of Lhasa terrane has been a subject of much debate for a long time. The Lithospheric structure records the deep processes of the subduction of oceanic plate and continental collision. The magnetotelluric (MT) method can probe the sub-surface electrical conductivity, newly dense broadband and long period magnetotelluric data were collected along a south-north trending profile that across the Lhasa terrane at 88°–89°E. Dimensionality analyses demonstrated that the MT data can be interpreted using two-dimensional approaches, and the regional strike direction was determined as N110°E.Based on data analysis results, a two-dimensional (2-D) resistivity model of crust and upper mantle was derived from inversion of the transverse electric mode, transverse magnetic mode and vertical magnetic field data. Inversion model shows a large north-dipping resistor that extended from the upper crust to upper mantle beneath the Himalaya and the south of Lhasa Terrane, which may represent the subducting Indian continental lithosphere. The 31°N may be an important boundary in the Lhasa Terrane, the south performs a prominent high-conductivity anomaly from the lower crust to upper mantle which indicates the existence of asthenosphere upwelling, while the north performs a higher resistivity and may have a reworking ancient basement. The formation of the ore deposits in the study area may be related to the upwelling of the mantle material triggered by slab tearing and/or breaking off of the Indian lithosphere, and the mantle material input also contributed the total thickness of the present-day Tibetan crust. The results provide helpful constrains to understand the mechanism of the continent-continent collision and the regional exploratory prospect of the deep resources.

A Review of Recent Developments in the Study of Regional Lithospheric Electrical Structure of the Asian Continent

The Asian continent was formed through the amalgamation of several major continental blocks that were formerly separated by the Paleo-Asian and Tethyan Oceans. During this process, the Asian continent underwent a long period of continental crustal growth and tectonic deformation, making it the largest and youngest continent on Earth. This paper presents a review of the application of geophysical electromagnetic methods, mainly the magnetotelluric (MT) method, in recent investigations of the diverse tectonic features across the Asian continent. The case studies cover the major continental blocks of Asia, the Central Asian orogenic system, the Tethyan orogenic system, as well as the western Pacific subduction system. In summary, most of the major continental blocks of Asia exhibit a three-layer structure with a resistive upper crust and upper mantle and a relatively conductive mid-lower crust. Large-scale conductors in the upper mantle were interpreted as an indication of lithospheric modification at the craton margins. The electrical structure of the Central Asian orogenic system is generally more resistive than the bordering continental blocks, whereas the Tethyan orogenic system displays more conductive, with pervasive conductors in the lower crust and upper mantle. The western Pacific subduction system shows increasing complexity in its electrical structure from its northern extent to its southern extent. In general, the following areas of the Asian continent have increasingly conductive lithospheric electrical structures, which correspond to a transition from the most stable areas to the most active tectonic areas of Asia: the major continental blocks, the accretionary Central Asian orogenic system, the collisional Tethyan orogenic system, and the western Pacific subduction system. As a key part of this review, a three-dimensional (3-D) model of the lithospheric electrical structure of a large portion of the Tibetan Plateau is presented and discussed in detail; the model indicates tearing of the underthrusting Indian slab as well as complex crustal conductor geometries, which are not obviously consistent with the hypothesis of a continuous, eastward channel flow. These studies have greatly enhanced our knowledge of the formation and deformation processes of the Asian continent. Lastly, future research to expand field data coverage, improve related techniques, and integrate data from other disciplines is suggested.

Electrical conductivity structure of southeastern North America: Implications for lithospheric architecture and Appalachian topographic rejuvenation

We present the first three-dimensional view of the lithospheric electrical conductivity structure beneath southeastern North America. By inverting EarthScope long-period magnetotelluric (MT) data, we obtain an electrical conductivity image that provides new insights into both the architecture of the Appalachian Orogen and the cryptic post-rifting geodynamic history of the southeastern United States. Our inverse solutions reveal several elongate electrically conductive features that we interpret as major terrane sutures within the Appalachian Orogen. Most significantly, we resolve a highly electrically resistive layer that extends to mantle depths beneath the modern Piedmont and Coastal Plain physiographic provinces. As high resistivity values in mantle minerals require cold mantle temperatures, the MT data indicate that the sub-Piedmont thermal lithosphere must extend to greater than 200 km depth. This firm bound conflicts with conclusions from seismic results. The boundary between the anomalously thick, resistive sub-Piedmont lithosphere and the relatively thin, moderately conductive sub-Appalachian lithosphere corresponds within resolution to the modern Appalachian topographic escarpment. This newly recognized contrast in lithospheric properties likely has important implications for Appalachian topographic rejuvenation.

Lithospheric electrical structure of the Great Xing’an Range

In order to study the lithospheric structure and survey deep resources perspective of the Great Xing’an Range area, a magnetotelluric sounding profile across the central Great Xing’an Range has been completed. The profile starts from the eastern margin of the Hailaer basin, through the Great Xing’an Range to the western margin of the Songliao basin. An electrical structure model of the crust and the upper mantle was finally obtained after data processing, qualitative analysis and 2D inversion of the observed data. The electrical structure can be divided into three tectonic units: the Hailaer basin, the Great Xing’an Range and the Songliao Basin, which is consistent with the regional geology. The research shows that: (1) The high resistivity in the upper crust of the Great Xing’an Range may represent spatially multiple overlapping volcanic rocks, in addition, the low resistivity bodies in the middle-lower crust may reflect hot and weak materials; (2) The lower crust of the Great Xing’an Range may have experienced delamination; (3) There is a lithospheric scale of west-dipping structural transfer zone between the Great Xing’an Range and the Songliao Basin, inferring that the Songnen Terrane firstly subducted to the Xing’an Terrane, then sutured in the form of soft collision, and finally modified by volcanic activities later.