Long-period Magnetotelluric Data Research Articles

Fast 3D Magnetotelluric Forward Modeling Based on a Regularization Approach in Spherical Coordinate

The earth’s curvature shouldn’t be ignored for the large-scale magnetotelluric (MT) modeling. A forward code of spherical coordinate MT method based on staggered grid finite difference method is developed. In addition, the forward problem is usually solved numerically, mostly based on iterative solution. Krylov subspace method is widely used to solve electromagnetic forward modeling problems in frequency domain. However, as the period increases, the convergence of these methods becomes significantly slower (especially for long-period MT data). We added a scaled grad-div operator to the double curl equation of the electric field to perform the nondivergence condition (CCGD). The results show that CCGD method is more efficient than other methods, which is an efficient method for our code.

Magnetotelluric insights into the formation and reactivation of trans-crustal shear zones in Precambrian basement of the eastern U.S. Midcontinent

Three-dimensional inversion of regional long-period magnetotelluric (MT) data reveals the presence of two distinct sets of high-conductivity belts in the Precambrian basement of the eastern U.S. Midcontinent. One set, beneath Missouri, Illinois, Indiana, and western Ohio, is defined by northwest−southeast-oriented conductivity structures; the other set, beneath Kentucky, West Virginia, western Virginia, and eastern Ohio, includes structures that are generally oriented northeast−southwest. The northwest-trending belts occur mainly in Paleoproterozoic crust, and we suggest that their high conductivity values are due to graphite precipitated within trans-crustal shear zones from intrusion-related CO2-rich fluids. Our MT inversion results indicate that some of these structures dip steeply through the crust and intersect the Moho, which supports an interpretation that the shear zones originated as “leaky” transcurrent faults or transforms during the late Paleoproterozoic or the early Mesoproterozoic. The northeast-trending belts are associated with Grenvillian orogenesis and also potentially with Iapetan rifting, although further work is needed to verify the latter possibility. We interpret the different geographic positions of these two sets of conductivity belts as reflecting differences in origin and/or crustal rheology, with the northwest-trending belts largely confined to older, stable, pre-Grenville cratonic Laurentia, and the northeast-trending belts largely having formed in younger, weaker marginal crust. Notably, these high-conductivity zones spatially correlate with Midcontinent fault-and-fold zones that affect Phanerozoic strata. Stratigraphic evidence indicates that Midcontinent fault-and-fold zones were particularly active during Phanerozoic orogenic events, and some remain seismically active today, so the associated high-conductivity belts likely represent long-lived weaknesses that transect the crust.

Geoelectric model of the Earth’s crust and upper mantle of the Dniester-Bug megablock of the Ukrainian Shield

A network of long-period magnetotelluric and magnetovariational data (124 sites) in the period range of 9—16 to 2500—6400 s made it possible to explore the geoelectric structure of the Erth’s crust and upper mantle of most of the Dniester-Bug and adjacent megablocks of the Ukrainian shield. Based on the resistivity cross-sections along the profiles (with 2D inversion), a three-dimensional matrix was created for the territory limited by coordinates 27.7—30.4° E and 47.7—49.4° N, which included the spatial coordinates of each grid node on each profile, the power of the model cells, and the resistance value in the cell. As a result, geoelectrical anomalous structures were identified at different depths from 3 to 100 km.
 The entire block of rocks 200×200 km down to a depth of 100 km is characterized by high resistivity, against which objects of reduced resistivity (LRO) are identified.
 The resulting distribution of high-resistivity rocks over the entire depth of the model is in good agreement with the laboratory dependencies obtained both for the rocks of the Ukrainian Shield and other data. Model data show a significant difference in resistivities in the upper 14—16 km (above 105 ohm), lower crust (about 104 ohm), and upper mantle (103 ohm). Against a general decrease in resistance with depth in the Earth’s crust, three regions were identified in which anomalously high (for a given depth) resistances extend to the entire thickness of the crust. These high-resistivity objects are consistent with positive Bouguer anomalies.
 Against the background of high-resistivity rocks, LROs stand out, the resistance of which does not exceed 120 ohm·m. The spatial dimensions of the LRO zones indicate their locality and do not form a continuous layer. An analysis of the distribution of LROs in space and depth suggests a genetic relationship between mantle LROs and crustal LROs. Comparison of mantle LROs with the Beltska zone of modern activation on the territory of Ukraine shows their good agreement both vertically and horizontally. To explain the lower LRO resistivity in the upper mantle, overheating of the rocks to solidus and 2—3 % melting and/or the presence of fluids is necessary [Gordienko, 2017]. In recent studies discussing the influence of thermobaric conditions and the fluid content necessary to explain the presence of increased conductivity in the upper mantle, the authors of [Blatter et al., 2022] concluded that an anomalously large amount of volatiles is needed with small amounts of melt. The assumption that mantle LROs are related to crustal LROs has been tested by comparing LROs with fault zones.
 The presence of LROs in the mantle, their vertical extent, and their connection with rejuvenated fault systems can serve as a basis for the deep migration of fluids enriched in volatiles.

Effect of the Western Pacific plate subduction on upper mantle in the eastern segment of the Central Asian Orogenic Belt: Revealed by long-period magnetotelluric data

Ever since the Mesozoic era, the eastern segment of the Central Asian Orogenic Belt (CAOB) has been significantly affected by the subduction of the Pacific plate. In order to better understand the subduction effect of the Pacific plate at different distances, we set up 65 magnetotelluric stations to acquire a three-dimensional inversion resistivity structure. The results illustrate that: there are three large-scale high-conductivity anomalies C1, C2 and C3 affecting the crust and mantle of the eastern segment of the Central Asian Orogenic Belt. The high-conductivity anomaly C1 extends from the surface to the asthenosphere and is connected with C2, which exists in the asthenosphere below the Xing'an block. The high-conductivity anomaly C3 under the Songliao block extends downward from the surface to the asthenosphere. We interpret that these anomalies are caused by aqueous fluid and partial melting based on petrophysical analysis. In the light of the high-temperature high-pressure theory, the estimated water content and melting fraction of C3 is considerably higher than that of C2. We infer that the lateral heterogeneity of the fluid-related properties is caused by the variable distance to the Western Pacific subduction zone. The C3 anomaly is located above the stagnant plate and controlled by the medium-distance effects of Pacific plate subduction, which have been influenced by intense dehydration and seriously transforming. The C2 anomaly is located at the end of the stagnant plate and controlled by the long-distance effects of Pacific plate subduction with slight dehydration and transformation.

Controls on the metallogenesis of the Lhasa–Mozugongka district, Gangdese Belt, Tibetan Plateau: Constraints on melt distribution and viscosity from the 3-D electrical structure of the lithosphere

Some of the largest and most significant Miocene porphyry copper systems in China are within the Gangdese metallogenic belt on the southern Tibetan Plateau. It has been recognized that the crustal architecture and rheology, derived from regional tectonic events, has direct implications for the evolution and transport of fluids and magmas, and thus for the metallogenesis and prospectivity. Using data from a magnetetolluric array, which intersects the Lhasa–Mozugongka district of the Gangdese metal belt, a 3-D electrical resistivity model was generated, with the goal of investigating the tectonic and rheological controls on the magmatic mineral system. The lithospheric temperature distribution was estimated by applying the Arrhenius equation to conductivity profiles generated from 1-D Monte-Carlo models of long-period magnetotelluric data. The conductivity of partial melts in the lower and middle crust (30–60 km depth) was estimated for local conditions by applying the experimentally-derived equation of X. Guo et al. (2018). Subsequently, we estimated the melt fraction required to explain the observed bulk resistivity in each part of the study area. Variations in the effective viscosity of the lower and middle crust were constrained by the electrical resistivity model by applying the empirical relation of Liu and Hasterock (2016). Beneath the Miocene Cu–Mo deposits in the Lhasa terrane, conductive features in the lower and middle crust are attributed to partial melt fractions of more than 5% and viscosity reductions of 1–2 orders of magnitude. These conductive features may represent the signatures of (ore-controlling) melt/fluid migration channels and deeper melt/fluid source regions in the form of extensive crustal reservoirs of partial melt. Based on the interpretations of the geophysical model, and other available geological and geochemical evidence, a model of the metallogenic dynamics of the Miocene porphyry Cu–Mo deposits is proposed. Overall, the study highlights the applicability of electromagnetic geophysical methods to reliably link resistivity structures to melt/fluid transport channels and sources within a mineral system and supports the hypothesis that crustal rheology exerts a major control on the distribution of ore deposits.

Three-dimensional crustal and upper-mantle resistivity structure of Alberta, Canada: implications for Precambrian tectonics

SUMMARY This paper describes the first detailed 3-D study of the Alberta basement using the magnetotelluric (MT) method. Long-period MT data were used to generate a 3-D electrical resistivity model of the crust and upper mantle beneath Alberta. The Western Canada Sedimentary Basin was imaged as a low-resistivity layer. A number of crustal and upper-mantle conductors were imaged and include: (1) the Red Deer conductor, (2) the Kiskatinaw conductor, (3) a conductor coincident with the Kimiwan oxygen isotope anomaly, (4) the southern Alberta–British Columbia conductor, (5) the Loverna block conductor and (6) a conductor beneath the Birch Mountain kimberlite field. Conductors (3) and (6) are newly identified in this study. All of these conductors can be related to past tectonic events associated with the assembly of Laurentia and subsequent kimberlite magmatism. The depth of the lithosphere–asthenosphere boundary across Alberta varies from 150 to 300 km as defined from the 100 Ω m contour. Furthermore, the Snowbird tectonic zone in Alberta is characterized by a thick, resistive lithosphere. Similarly, the diamondiferous Buffalo Head Hills kimberlite field is imaged with a thick, resistive lithosphere, both may indicate depletion. In contrast, the Birch Mountain kimberlite field is underlain by a pronounced conductor that extends through the lithosphere and may be the result of metasomatism.

Crustal magnetotelluric imaging of a Paleoproterozoic graphitic suture zone, Curnamona Province, Australia

The Curnamona Province is a Paleoproterozoic to Mesoproterozoic domain that hosts the Broken Hill Pb-Zn ore body and a number of IOCG type Cu-Au deposits, but is mostly covered in Neoproterozoic to Quaternary cover. Three-dimensional resistivity modelling of 55-km gridded long-period (0.1–0.0001 Hz) magnetotelluric (MT) data collected as part of the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) previously revealed the presence of the Curnamona Conductor that exhibits extremely low resistivities <1 Ω.m at upper-crustal depths (<15 km) which has a lateral extent of >200 km in a north–south orientation. The conductor appears to dip to the west, centred under the large igneous province Benagerie Suite Volcanics with resistivity ∼10–100 Ω.m in a broader lower crustal region. To yield higher resolution of the Curnamona Conductor, a 56-site broadband (200–0.001 Hz) MT transect was undertaken with 2 km spacing. The east- west line intersected the observed conductive structures orthogonally. We show consistency between long-period MT AusLAMP 3D models, and new 2D and 3D broadband MT inversions, but with higher spatial and vertical resolution compared to the AusLAMP model.The linearity, depth extent and western dipping morphology of the Curnamona Conductor is remarkably similar to other Paleoproterozoic conductors in Australia along the eastern margin of the Mount Isa Province in northern Australia, and the eastern margin of the Gawler Craton in southern Australia. Plate reconstructions of Paleoproterozoic Nuna supercontinent suggests that these now disparate continental cratons were originally contiguous, with extensive subduction along their eastern margin between 2200 and 1850 Ma. Marine sedimentation in accretionary wedges prior to continental collision occurred during the Lomagundi-Jatuli Event, which recorded a global increase in δ13C (2300–2000 Ma) with a significant increase in burial of organic carbon. We argue that these Paleoproterozoic conductors thus represent graphitic suture zones that are uniquely representative of a time of enhanced carbon burial in Earth’s history.

Complex tectonic setting and deep crustal seismicity of the Sikkim Himalaya: An electrical resistivity perspective

Tectonic setting of the Himalayan arc is dominated by thrust-type earthquakes mainly associated with rupture of the décollement surface, also called the Main Himalayan Thrust (MHT). However, the region encompassing eastern Nepal, Sikkim and western Bhutan departs from this general tectonic model of the Himalaya and experiences deep crustal seismicity of strike-slip nature with focal planes oblique to the convergence direction of the collision front. The 2011 Sikkim earthquake (Mw 6.9) is an example of such transverse tectonic deformation. Several geophysical studies have been carried out in Sikkim to understand the complex tectonics of this region. Among these, a crustal model derived from magnetotelluric (MT) data revealed contrasting crustal structure across the Main Central Thrust Zone (MCTZ) beside a small-scale MHT ramp. Here, we revisit the MT model by including long-period MT data (up to 10,000 s) at six sites along the profile, invert the distortion corrected impedance data, and analyse the results in conjunction with other published geophysical and structural information. Based on this study, we propose a tectonic model for the Sikkim Himalaya that reveals the complex nature of the lithospheric structure including two major contacts beneath the MCTZ and the Main Frontal Thrust (MFT), respectively. The contact beneath the MCTZ is a NW-SE trending lithospheric-scale seismogenic fault that separates two geologically and compositionally distinct blocks of the Indian plate underthrusting the Himalaya. The other contact beneath the MFT could be responsible for the necking of the Main Central Thrust (MCT) at this location.

Lower crustal resistivity signature of an orogenic gold system

Orogenic gold deposits provide a significant source of the world’s gold and form along faults over a wide range of crustal depths spanning sub-greenschist to granulite grade faces, but the source depths of the gold remains poorly understood. In this paper we compiled thirty years of long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) data across western Victoria and south-eastern South Australia that have sensitivity to the electrical resistivity of the crust and mantle, which in turn depend on past thermal and fluid processes. This region contains one of the world’s foremost and largest Phanerozoic (440 Ma) orogenic gold provinces that has produced 2% of historic worldwide gold production. Three-dimensional inversion of the long-period MT and GDS data shows a remarkable correlation between orogenic gold deposits with > 1 t production and a < 20 Ω m low-resistivity region at crustal depths > 20 km. This low-resistivity region is consistent with seismically-imaged tectonically thickened marine sediments in the Lachlan Orogen that contain organic carbon (C), sulphides such as pyrite (FeS2) and colloidal gold (Au). Additional heat sources at 440 Ma due to slab break-off after subduction have been suggested to rapidly increase the temperature of the marine sediments at mid to lower crustal depth, releasing HS− ligands for Au, and CO2. We argue that the low electrical resistivity signature of the lower crust we see today is from a combination of flake graphite produced in situ from the amphibolite grade metamorphism of organic-carbon in the marine sediments, and precipitated graphite through retrograde hydration reactions of CO2 released during the rapid heating of the sediments. Thus, these geophysical data image a fossil source and pathway zone for one of the world’s richest orogenic gold provinces.

Lithospheric resistivity structure of the 2001 Bhuj earthquake aftershock zone

SUMMARY The Bhuj area, in the Kutch region of western India, is a unique intraplate seismic zone in the world where aftershock activity associated with a large magnitude earthquake (7.7 Mw Bhuj earthquake on 26 January 2001) has persisted over a decade and up till today. We studied the lithospheric resistivity structure of the Bhuj earthquake aftershock zone to gain more insight into the structure and processes influencing the generation of intraplate seismicity in broad and, in particular, to detect the deep origin and upward migration channels of fluids linked to the crustal seismicity in the area. A lithospheric resistivity model deduced from 2-D and 3-D inversions of long-period magnetotelluric (MT) data shows low resistive lithospheric mantle, which can be best explained by a combination of a small amount of interconnected melts and aqueous fluid in the upper mantle. The MT model also shows a subvertical modestly conductive channel, spatially coinciding with the Kutch Mainland Fault, which we interpret to transport fluids from the deep lithosphere to shallow crust. We infer that pore pressure buildup aids to achieve the critical stress conditions for rock failure in the weak zones, which are pre-stressed by the compressive stress regime generated by ongoing India–Eurasia collision. The fluidized zone in the upper mantle beneath the area perhaps provides continuous fluid supply, which is required to maintain the critical stress conditions within the seismogenic crust for continued seismicity.

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.

Implications for the lithospheric structure of Cambay rift zone, western India: Inferences from a magnetotelluric study

Broad-band and long-period magnetotelluric (MT) data were acquired along an east-west trending traverse of nearly 200 ​km across the Kachchh, Cambay rift basins, and Aravalli-Delhi fold belt (ADFB), western India. The regional strike analysis of MT data indicated an approximate N59°E geoelectric strike direction under the traverse and it is in fair agreement with the predominant geological strike in the study area. The decomposed transverse electric (TE)- and transverse magnetic (TM)-data modes were inverted using a nonlinear conjugate gradient algorithm to image the electrical lithospheric structure across the Cambay rift basin and its surrounding regions. These studies show a thick (~1–5 ​km) layer of conductive Tertiary–Mesozoic sediments beneath the Kachchh and Cambay rift basins. The resistive blocks indicate presence of basic/ultrabasic volcanic intrusives, depleted mantle lithosphere, and different Precambrian structural units. The crustal conductor delineated within the ADFB indicates the presence of fluids within the fault zones, sulfide mineralization within polyphase metamorphic rocks, and/or Aravalli-Delhi sediments/metasediments. The observed conductive anomalies beneath the Cambay rift basin indicate the presence of basaltic underplating, volatile (CO2, H2O) enriched melts and channelization of melt fractions/fluids into crustal depths that occurred due to plume–lithosphere interactions. The variations in electrical resistivity observed across the profile indicate that the impact of Reunion plume on lithospheric structures of the Cambay rift basin is more dominant at western continental margin of India (WCMI) and thus support the hypothesis proposed by Campbell & Griffiths about the plume–lithosphere interactions.

Magnetotelluric imaging of a fossil oceanic plate in northwestern Xinjiang, China

Abstract Because an oceanic plate colliding with a continental plate will usually be subducted and recycled into the deep mantle, a fossil oceanic plate after the closure of an ancient ocean has rarely been imaged in the subcontinental lithospheric mantle. This has led to a long-standing debate about the fate of subducted ocean plates. The problem can be addressed by imaging the lithosphere in a continental accretion zone with past ocean subduction. We present a study using long-period magnetotelluric data that reveals a large shallow-mantle conductor in a Phanerozoic accretion area in northwestern Xinjiang, China. This conductor extends &amp;gt;300 km laterally at depths from 120 to 220 km and resembles a segment of a fossil oceanic plate. The reduced resistivity is ascribed to the volatile-bearing metasomatic minerals, based on its relatively fertile nature and low temperature. Our results demonstrate that an oceanic plate can be trapped in continental lithosphere, underscoring the significance of oceanic plate subduction to continental accretion, and shedding new light on our understanding of continental formation and evolution.

Imaging Yellowstone magmatic system by the joint Gramian inversion of gravity and magnetotelluric data

Gravity and long-period magnetotelluric data are combined in a joint Gramian inversion to obtain structurally-similar 3D density and electrical resistivity models of the Yellowstone magmatic feeding system. Structural constraints are enforced through a correlation of the model gradients. Gravity data are sensitive to upper-crustal structure, whereas long-period magnetotelluric data are more sensitive to deeper structure. By combining these complementary data, the jointly inverted models provide a consistent image of the partially molten structure underlying the Yellowstone Caldera from the surface to the Moho. Discrete zones of inferred partial melt in the upper and lower crust are fed by a southwest trending plume, and satisfy expectations from modern mantle plume models.

The Missouri High-Conductivity Belt, revealed by magnetotelluric imaging: Evidence of a trans-lithospheric shear zone beneath the Ozark Plateau, Midcontinent USA?

Crust in the central Midcontinent of the USA, part of North America's intracratonic platform, consists of Precambrian crystalline basement overlain by a cover of Phanerozoic sedimentary strata. Analysis of EarthScope long-period magnetotelluric (MT) data provides an opportunity to characterize lithosphere-scale structures of this region. Inversion of these data reveals a distinct belt of high electrical conductivity (≤30 Ωm) traversing Missouri. This belt strikes northwest-southeast, dips steeply to the northeast, and extends at least from the mid-crust into the lithospheric mantle. Significantly, this “Missouri high-conductivity belt” (MHCB) spatially coincides with the Missouri Gravity Low (a distinctive feature on Bouguer anomaly maps), lies adjacent to the southwestern edge of Precambrian bedrock exposure in the Ozark Plateau, trends parallel to both magnetic-anomaly lineaments and mapped fault traces, and aligns with lateral steps (transfer or transform faults) in the Midcontinent rift and the Reelfoot rift. We suggest that the MHCB initiated as a lithosphere-scale transtensional shear zone, which linked rift segments that were active during Proterozoic extensional events. Conceivably, pull-aparts within this shear zone localized felsic igneous activity and produced depressions that filled with sediment in association with the development of the 1.5–1.3 Ga Eastern Granite-Rhyolite Province, yielding a region of less-dense crust that could account for the Missouri Gravity Low. The shear zones likely also provided pathways for fluid migration. High conductivity in the MHCB may reflect deep crustal and lithospheric mantle alteration in response to the passage of fluids during dilatational shearing. We postulate that fluid movement accompanied by shear resulted in graphite and perhaps sulfide concentration along the shear zone in the lower crust, and hydration and/or grain diminution and crystallographic preferred orientation in the lithospheric mantle. This alteration also may have weakened lithosphere along the MHCB, explaining the spatial association of the belt with Paleozoic reactivated faults and with contemporary seismicity.

Lithospheric thinning under the Araripe Basin (NE Brazil) from a long-period magnetotelluric survey: Constraints for tectonic inversion

The lithospheric architecture of the western Borborema Province and northern São Francisco craton of NE Brazil has been investigated through analysis of long-period magnetotelluric data acquired along a 700 km long survey, using 12 instruments. The survey samples several tectonic terrains in the Province and penetrates into the adjacent São Francisco craton after crossing the Araripe Basin, an aborted rift basin filled with Mesozoic sediments that peak at ∼1000 m above mean sea level. High conductivities are observed at shallow depths under the main Precambrian shear zones that pervade the Province – consistent with tectonic reactivation – and as a small patch embedded within the high resistivities that characterize the São Francisco craton. High conductivities (∼25 Ωm) are also observed below 120 km depth between the Patos and Pernambuco lineaments – right under the Araripe Basin – flanked by resistive (>120 Ωm) material immediately to the north and south. This deep, highly conductive body is found consistent with the presence of melt and aqueous fluids, and is interpreted as shallow asthenospheric mantle bounded by thicker lithosphere. We propose that extensional stresses in the Mesozoic stretched and thinned the lithosphere under the Araripe Basin, causing passive upwelling of asthenospheric material and lateral flow of the overlying lithosphere, and resulting in thickening of the lithosphere under the flanks and uplift of the Araripe Basin. We also hypothesize that thermal weakening of the lithospheric mantle – perhaps sustained by channeling of asthenospheric flows under the basin – would have caused regional stresses to concentrate in the brittle upper crust and contribute to basin inversion. We thus propose that a combination of localized horizontal stresses and vertical buoyancy from underlying asthenospheric material are ultimately responsible for the actual topography of the Araripe Basin.

Why do we Need to Know the Electrical Resistivity Structure of Oceanic Lithosphere?

Regional-scale continental magnetotelluric (MT) programs such as AusLAMP are naturally bounded by the continental shelf and electrically-conducting seawater. Within a few hundred kilometres of the coastline, long-period MT data may be strongly influenced by induction in the seawater, a phenomenon known as the coast-effect. Thus, 3D inversion of gridded long-period MT data for continental lithosphere models requires good constraints on the resistivity of the seawater, oceanic crust and upper mantle, and into the asthenosphere.In this paper, we discuss the concept of a horizontal adjustment distance. This is the horizontal distance away from a major contrast in electrical conductance at which the anomalous electric fields are attenuated by a factor of 1/e from a 1D response, and is somewhat analogous to the more widely-known skin-depth concept. For seafloor MT, this adjustment distance can be thousands of kilometres. Inland, the effect depends on the conductance of the sedimentary cover, and the depth-integrated resistivity of the upper crust, and can vary from a few kilometres to hundreds of kilometres. We discuss the implications in terms of 3D smooth inversion that inherently minimises gradients in subsurface resistivity and suggest that the coast effect may be significantly underestimated in some continental models.

Geoelectric structure of northern Cambay rift basin from magnetotelluric data

Broadband and long-period magnetotelluric data were acquired over the northern part of the Cambay rift zone along an east–west profile ~ 200 km in length. The decomposed TE- and TM-mode data were inverted using a 2-D nonlinear conjugate gradient algorithm to obtain the lithospheric structure of the region. A highly conductive (~ 1000 S) layer was identified within the Cambay rift zone and interpreted as thick Quaternary and Tertiary sediments. The crustal conductors found in the profile were due to fluid emplacement in the western part, and the presence of fluids and/or interconnected sulfides caused by metamorphic phases in the eastern part. The demarcation of the Cambay rift zone is clearly delineated with a steeply dipping fault on the western margin, whereas the eastern margin of the rift zone gently dips along the NE–SW axis, representing a half-graben structure. A highly resistive body identified outside the rift zone is interpreted as an igneous granitic intrusive complex. Moderately conductive (30–100 Ω-m) zones indicate underplating and the presence of partial melt due to plume–lithosphere interactions.

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.

AusLAMP MT over Victoria: New insight from 3D modelling highlights regions of anomalously conductive mantle and unexpected linear trends in the crust

The Australian Lithospheric Architecture Magnetotelluric Program (AusLAMP) is a multi-year collaborative project aimed at resolving the first order electrical structure of the Australian continental lithosphere through the acquisition of long-period magnetotelluric data at ~55×55 km spacing. Here we present the results of the first deployment of AusLAMP which was in Victoria in 2014. Previous MT coverage over Victoria comprised limited 2D profiles in the western portion of the State. Three-dimensional inversion of AusLAMP data provides a context for these isolated profiles while revealing interesting and unexpected results with evident correlations with the mapped geology and well established seismic tomography trends. Along the eastern and southern edge of Victoria, the resistivity structure is resolvable into the asthenosphere, in contrast beneath the central and western part the State the base of the thicker lithosphere is not resolved. Resistivity of the asthenosphere beneath the Victorian eastern highlands conforms with global values (~1,000 Ωm) and becomes more conductive (~200 Ωm) beneath the Newer Volcanic province. The seismologically defined lithospheric mantle beneath the central and western part of the State is relatively resistive (~200 Ωm) compare to the east (~20 Ωm). This anomalously conductive lithospheric mantle we tentatively attribute to metasomatism during Palaeozoic accretion of oceanic terranes. Vertically, this conductive lithospheric mantle merges upwards into a series of northeast trending conductive anomalies within the mid to lower crust. These trends correspond with the surficial distribution of Devonian granite intrusions suggesting they represent fossil metasomatised ascent pathways of the granitic melts, which cross cut the older dominant north-south structural trend. The western limit of these linear conductive trends maps out the boundary of the Delamerian and Lachlan Orogens.