Crustal Conductivity Anomalies Research Articles

Deep structure of the Salair fold-nappe terrane (NW CAOB) according to magnetotelluric sounding

The Salair fold-nappe terrane (a.k.a. Salair orogen, Salair) is the northwestern part of the Altai-Sayan folded area of the Central Asian Orogenic Belt. It is composed of Cambrian – Early Ordovician volcanic rocks and island-arc sedimentary deposits. In plan, Salair is a horseshoe-shaped structure with the northeast-facing convex side, which is formed by the outcrops of the Early Paleozoic folded basement. Its inner part is the Khmelev basin composed of Upper Devonian – Lower Carboniferous sandstones and siltstones. The Early Paleozoic volcanic rocks and sediments of Salair are overthrusted onto the Devonian-Permian sediments of the Kuznetsk basin. The Paleozoic thrusts, that were reactivated at the neotectonic stage, are observed in the modern relief as tectonic steps. Our study of the Salair deep structure was based on the data from two profiles of magnetotelluric sounding. These 175-km and 125-km long profiles go across the strike of the Salair structure and the western part of the Kuznetsk basin. Profile 1 detects a subhorizontal zone of increased conductivity (100–500 Ohm·m) at the depths of 8–15 km. At the eastern part of Profile 1, this zone gently continues upward, towards a shallow conducting zone that corresponds to the sediments of the Kuznetsk basin. Two high-resistance bodies (1000–7000 Ohm⋅m) are detected at the depths of 0–6 km in the middle of the section. They are separated by a subvertical conducting zone corresponding to the Kinterep thrust. The main features are the subhorizontal positions and the flattened forms of crustal conductivity anomalies. At the central part of Profile 2, there is a high-resistance block (above 150000 Ohm⋅m) over the entire depth range of the section, from the surface to the depths of about 20 km. In the eastern part of Profile 2, a shallow zone of increased conductivity corresponds to the sediments of the Kuznetsk basin. The subhorizontal mid-crust layer of increased conductivity, which is detected in the Salair crust, is typical of intracontinental orogens. The distribution pattern of electrical conductivity anomalies confirms the Salair thrust onto the Kuznetsk basin. The northern part of the Khmelev basin is characterized by high resistivity, which can be explained by abundant covered Late Permian granite massifs in that part of the Khmelev basin. The Kinterep thrust located in the northeastern part of the Khmelev basin is manifested in the deep geoelectric crust structure as a conducting zone, which can be considered as an evidence of the activity of this fault.

Lithospheric architecture of a Phanerozoic orogen from magnetotellurics: AusLAMP in the Tasmanides, southeast Australia

We present a resistivity model of the southern Tasmanides of southeastern Australia using Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) data. Modelled lower crustal conductivity anomalies resemble concentric geometries revealed in the upper crust by potential field and passive seismic data. These geometries are a key part of the crustal architecture predicted by the Lachlan Orocline model for the evolution of the southern Tasmanides, in which the Proterozoic Selwyn Block drives oroclinal rotation against the eastern Gondwana margin during the Silurian period. For the first time, we image these structures in three dimensions (3D) and show they persist below the Moho. These include a lower crustal conductor largely following the northern Selwyn Block margin.Spatial association between lower crustal conductors and both Paleozoic to Cenozoic mafic to intermediate alkaline volcanism and gold deposits suggests a genetic association i.e. fluid flow into the lower crust resulting in the deposition of conductive phases such as hydrogen, iron, sulphides and/or graphite.The 3D model resolves a different pattern of conductors in the lithospheric mantle, including northeast trending anomalies in the northern part of the model. Three of these conductors correspond to Cenozoic leucitite volcanoes along the Cosgrove mantle hotspot track which likely map the metasomatised mantle source region of these volcanoes. The northeasterly alignment of the conductors correlates with variations in the lithosphere-asthenosphere boundary (LAB) and the direction of Australian plate movement, and may be related to movement of an irregular LAB topography over the asthenosphere.By revealing the tectonic architecture of a Phanerozoic orogen and the overprint of more recent tectono-magmatic events, our resistivity model enhances our understanding of the lithospheric architecture and geodynamic processes in southeast Australia, demonstrating the ability of magnetotelluric data to image geological processes over time.

A 3-D conductivity model of the Australian continent using observatory and magnetometer array data

An exploratory 3-D model of the electrical conductivity structure of the Australian continent is presented. The model is derived from the inversion of vertical magnetic-field transfer functions from the Australia-wide Array of Geomagnetic Stations. Crustal conductivity anomalies evident in the model are consistent with those previously mapped by independent magnetometer array studies and new electrical structures are suggested in the upper mantle. The model represents a seamless continent-scale basis for further models likely to be derived from subsequent studies.The model reveals three upper-mantle enhanced-conductivity anomalies beneath Archaean cratonic regions and two upper-mantle anomalies beneath Phanerozoic terranes in eastern Australia. Two of these anomalies have been investigated by recent magnetotelluric (MT) surveys, one in the Yilgarn Craton–Officer Basin–Musgrave Block the other in the Gawler Craton region, and are consistent with the MT results. Across much of central Australia enhanced conductivity at depths of 50–100 km is observed in the model. This region corresponds well with a recognized seismic velocity gradient at 75–100 km. Conductivity differences are also observed beneath Archaean cratons in Western Australia. The Pilbara Craton is represented as an enhanced conductivity anomaly at about 100 km, corresponding well with the lower-velocity anomaly evident in surface wave tomography models. The Yilgarn Craton is imaged as a low-conductivity body, with conductivity two orders of magnitude lower than the Pilbara Craton, continuing to greater depths.

Crustal conductivity anomalies in central Sweden and southwestern Finland

Lateral conductivity variations in the Earth's crust in central Sweden and southwestern Finland were mapped using magnetic field variations recorded simultaneously by 29 Gough-Reitzel-type magnetometers in summer 1985. Three events of 3 or 4 h were digitized and analysed to obtain single-station induction arrows and interstation horizontal field transfer functions. The 1985 magnetometer array covered part of the BEAR (Baltic Electromagnetic Array Research) array operated in 1998 on the Fennoscandian Shield. Results from eight BEAR sites are used to control and to supplement the 1985 array results. The conductivity of the crust was found to increase in the western part of the array in western central Sweden. Crustal conductors were found both in the western part of the array and in the Baltic Sea area in the eastern part of the array. 3G-D modelling based on the induction arrows revealed a major conductivity anomaly in the middle and lower crust under the archipelago of Southwest Finland.

Multisheet modelling of the electrical conductivity structure in the Fennoscandian Shield

Electromagnetic multisheet modelling is a powerful tool for large model areas, if they can be approximated by a multilayered heterogeneous conductivity structure of small vertical dimension in comparison with the penetration depth of electromagnetic fields. In this paper, thin sheet technique is applied to the whole Fennoscandian (Baltic) Shield, whose upper mantle conductivity structure is the objective of the long period electromagnetic array experiment BEAR (Baltic Electromagnetic Array Research). Three thin sheets, each of about 120,000 model cells with the base length of 10 km, describe a-priori crustal inhomogeneities in terms of conductances. The three sheets represent i) upper crust from surface to the depth of 10 km including continental and ocean bottom sediments and seawater, ii) middle crust ranging from 10 km to 30 km and iii) lower crust from 30 km to 60 km. Thus, modelling is taking into account distortions caused by crustal conductivity anomalies. Additionally, distortions due to inhomogeneous external current systems are investigated by introducing an equivalent current system of a polar electrojet model at ionosphere height. Modelling results are illustrated by induced current distribution at different depth levels and by various electromagnetic transfer functions. The latter demonstrate the resolution of crustal conductivity anomalies and their screening effect even at long periods. The predicted behavior of transfer functions in the very complex conductivity structure is compared with the experimental BEAR data, showing qualitatively a first order agreement for most of the sites. Modeled phases for periods of a few thousands of seconds are considerably biased in comparison with experimental data if the background 1-D model has monotonously decreasing resistivity. However, the bias from phases is removed if a conducting asthenosphere having a resistivity of 20 Ωm is emplaced between the depths of 200 km and 300 km. Thus, multisheet modelling indicates a well conducting upper mantle under the Fennoscandian Shield. All modelling has been performed using a multisheet code by Avdeev, Kuvshinov and Pankratov.

Electromagnetic induction in the Australian crust: results from the Australia-Wide Array of Geomagnetic Stations

An array of 57 three-component magnetometers has been deployed over the Australian continent to record geomagnetic field fluctuations at one-minute intervals from November 1989 until June 1990. The inductive response of the continent to external field variations at various frequencies is dominated by a prominent coast effect around the western, southern and eastern coasts. Albany displays a particularly large induction effect. The east and west coast display very similar coast effects that are detectable up to 500 km inland. The northern margin of the continent displays a smaller, more complicated inductive response with contributions from crustal conductivity anomalies in the Canning Basin and the Gulf of Carpentaria, and, possibly, from offshore shallow-water sediments. Contour mapping of the A and B transfer function coefficients reveals evidence for an conductive zone surrounding the North-Central cratonic area. The zone sweeps through the main sedimentary basins that separate the Australian cratonic blocks and includes several of the conductors mapped previously. Electric currents induced in the sedimentary basins appear to be locally concentrated by shear zones and basement highs. An earlier suggestion for an Australia-wide north-south conductor appears to be supported by the data and there is evidence for its continuation into the Gulf of Carpentaria. Our array results suggest a possible east-west continuation of a conductive zone through the Officer Basin and into the Canning Basin. The presently available data are consistent with the existance of a large U-shaped lntercratonic Conductive Zone that curves around the central part of the country. Thin sheet modelling by Corkery and Lilley indicates that the required conductivity contrasts are geologically realistic.

Aspects of the geoelectric models of the baltic shield

The geoelectric structure of the Baltic Shield in Finland has been studied since 1980. Four types of geoelectric model have been considered so far: numerical models for the galvanic distortions of the upper 10 km; horizontal distribution of crustal inductive anomalies; vertical cross-sections above relevant crustal anomalies; and vertical conductivity profiles on a regional scale. The models are based on extensive, on-going measurements being conducted with magnetovariational, magnetotelluric and frequency sounding methods. Geoelectric studies in Finland are a part of both the international ELAS (Electrical conductivity of the asthenosphere) and the EGT (European Geotraverse) projects. In Finland, the most relevant crustal conductivity anomalies south of 67°N are a conductive band surrounding the Central Finland Granite Area in the west and south and the Oulu, Mikkeli and Outokumpu anomalies (N-S, E-W and N-S, respectively). The anomalies are located in the middle crust, typically from 6–12 km downwards. Several regional conductivity boundaries and the existence of a conductive layer in the lower crust, at least in parts of Central Finland, have also been noted. A vertical cross-section across the Oulu anomaly shows a wide crustal conductor, which seems to dip towards the west. The anomaly has exceptionally good conductivity (0.5/5 Ωm) and cannot be attributed to the known sandstone formation or the schist zone on the surface. Along the SVEKA DSS (deep seismic sounding) profile, some increases in conductivity of the upper crust correlate with the Kainuu and the Kuhmo schist zones. The upper crustal parts of the Raahe-Ladoga zone has also a higher conductivity than the surroundings. Some local conductivity anisotropies seem to coincide with deep faults located by DSS. The area east of Kuhmo seems to be more or less free of crustal conductivity anomalies, allowing electric soundings down to the upper mantle. A deep conductivity model based on the measurements in this area differs from earlier models for northern Scandinavia, where the presence of a good conductor at asthenospheric depths has been reported. Variations in the electrical conductivity of the Earth's crust and upper mantle thus yield information about the structural composition and the tectonic development of the Earth. The results now presented are only preliminary and their detailed geological-tectonic importance is not yet fully understood. Later, the models will be completed with the results from work done in both the eastern and the western parts of the Shield.

Are there two types of conductivity anomaly (CA) caused by fluid in the crust?

The different effects (temperature, fluids, tectonics) that play a role in the generation of the crustal conductivity anomalies (CA) are taken into account to be able to distinguish between typical conducting zones. In addition to a general depth distribution of the CA-s described by Ádám's empirical relation (first type CA), the data show that along some linear deep fractures in East-Central Europe the CA-s are at shallower depths probably due to an increase of the crack porosity in these zones (second type CA).

Magnetotelluric and audiomagnetotelluric measurements in Finland

Joint Finnish—Hungarian MT (magnetotelluric) and AMT (audiomagnetotelluric) measurements were carried out in Finland in the framework of the international ELAS project. The conditions for MT measurements are favorable at these latitudes. Five MT and 150 AMT stations gave information on the electrical conductivity distribution in the area: AMT results guided the choice of MT sites with minimal near-surface distortion effects and helped the interpretation of the MT soundings; the MT measurements indicate the presence of large conductivity anomalies and can be best interpreted as lateral induction effects of near-surface dyke structures. This result is confirmed by a certain correspondence between the directions of the maximum impedances and of the tectonic zones of the area. Any information about the upper mantle would require the use of S q harmonics because of the crustal conductivity anomalies detected by the MT measurements.

Electrical conductivity anomalies in the earth

Anomalies of short-period geomagnetic variations have been found in various regions over the world. It is known that such anomalies arise from electromagnetic induction within an electrical conductivity anomaly or from local perturbation of induced electric currents by a conductivity anomaly. In order to investigate a regional electric state in the Earth, conductivity anomaly (CA) studies based on anomalous behaviors of geomagnetic variations have been extensively undertaken, as well as studies based on magnetotelluries in which induced currents are directly used. Some of the geomagnetic variation anomalies, however, turned out to be caused by surface conductors, such as sea water and sediments. Anomalies of this sort have been intensively studied and classified into coast, island, peninsula, and strait effects in the case of sea effects. Three-dimensional conduction or channelling of induced electric currents is sometimes observed in the cases of sediments and some crustal conductivity anomalies. However, anomalies of such surface origins often provide some information of the underground conductivity structure. Electrical conductivity anomalies can be classified into two types: anomalies originating in the crust and in the upper mantle. Many of crustal anomalies are well correlated with metamorphic belts, fracture zones, and hydrated layers, and magnetic and gravity anomalies are also often found over the conductivity anomalies. Most of mantle anomalies have been interpreted mainly in terms of high temperature and partial melting, since conductivity anomalies coincide well with anomalies in heat flow and seismic wave velocities.

The Polarization of Micropulsations and Geo-electric Structure

Summary A study is made of how electromagnetic induction in crustal conductivity anomalies affects the observed polarization characteristics of micro- pulsations. Use is made of a new linear technique of modelling conductivity anomalies in the Earth. This technique allows any curved interface between two layers of different conductivity to be modelled by using Fourier composition of solutions for a sinusoidal interface. Two particular models are studied, one representing a sedimentary basin on the surface and the other a buried fault. It is found that the polarization characteristics of long-period pulsations (>100 s) are only slightly affected by both models. However, although the buried anomaly has little effect on the polarization of medium- and short-period pulsations (< 100 s), anomalies near or on the surface can change their polarization characteristics considerably. It is concluded that more care should be taken in the siting of geomagnetic observatories and in the interpretation of micropulsation data.