High Conductivity Anomalies Research Articles

Characterisation of sulphide-bearing waste-rock dumps using electrical resistivity imaging: the case study of the Rio Marina mining district (Elba Island, Italy)

Sulphide-bearing mine dumps are potential sources of pollution when acid mine drainage (AMD) occurs. Because the generation of AMD depends on the volume and composition of waste materials, their characterisation is crucial for the evaluation of geochemical hazards and for the design of remediation strategies to minimise their environmental impact. In this paper, a cost-effective strategy for the characterisation of an inactive mine dump in the Rio Marina mining district (Elba Island, Italy) using earth resistivity imaging (ERI) is presented. As no information regarding the nature of waste rocks is found in reports for the mine, five ERI profiles were acquired at the top of the waste pile. The results show that waste rocks are heterogeneous with a maximum thickness of 30m. Due to the large amounts of dispersed sulphide minerals, the waste rocks are characterised by an electrically conductive geophysical signature in comparison to the surrounding resistive metamorphic bedrock. A geostatistical approach was adopted to estimate the elevation of the edges of the mine dump, and the net volume of the waste rocks was computed through a raster analysis of the elevations of the upper and lower boundaries of the mine dump. High-conductivity anomalies were detected within the core of the mine dump. The integration of the hydrogeological, geochemical and geological framework of the Rio Marina mining district suggests that these anomalies could be a geophysical signature of subsurface regions where AMD is currently generated or stored, thus representing sources of environmental pollution.

Crustal and uppermost mantle structure and seismotectonics of North China Craton

We determined a 3-D P-wave anisotropic tomography of the crust and uppermost mantle beneath North China Craton (NCC) using 107,976 P-wave arrival times from 16,073 local earthquakes recorded by 380 seismic stations. Our results show significant lateral heterogeneities beneath NCC. The lower crust and uppermost mantle beneath the North China Basin show widespread low-velocity anomalies which may reflect high-temperature materials caused by the late Mesozoic basaltic magmatism in the NCC. Low-velocity anomalies also exist beneath the Trans-North China Orogen, which may reflect asthenospheric upwelling since late Mesozoic. Large crustal earthquakes generally occurred in high-velocity zones in the upper to middle crust, while low-velocity and high-conductivity anomalies that may represent fluid-filled, fractured rock matrices exist in the lower crust to the uppermost mantle under the source zones of the large earthquakes. The crustal fluids may lead to the weakening of the seismogenic layer in the upper and middle crust and hence cause the large crustal earthquakes. The NW–SE P-wave fast velocity directions seem to be dominant in the uppermost mantle under the central parts of eastern NCC, suggesting that these mantle minerals were possibly regenerated but keep the original fossil anisotropy formed before the new lithospheric mantle was produced during the Mesozoic to Cenozoic.

Electrical conductivity of fluid-bearing quartzite under lower crustal conditions

The electrical conductivity of fluid-bearing quartzite was determined as function of temperature and fluid fraction at 1GPa in order to assess the origin of the high conductivity anomalies observed in the middle to lower crustal levels. Dihedral angles of quartz-fluid-quartz determined from recovered samples were below 60°, suggesting that fluid forms an interconnected network through the quartz aggregate. The electrical conductivity of quartzite increases with increasing temperature, which can be approximately expressed by Arrhenius equation. The apparent activation enthalpy decreases from 0.70 to 0.25eV with increasing fluid fraction in volume from 0.00043 to 0.32. The electrical conductivity (σ) of the fluid-bearing quartzite increased with fluid fraction (ϕ) proportionally to a power law (σ∝ϕ0.56–0.71) within the temperature range of 900–1000K. The electrical conductivity of the aqueous fluid-bearing quartzite with the maximum fluid fraction (0.32) was found to be about three orders of magnitude higher than that of dry quartzite at 1000K. However, its electrical conductivity was definitely lower than the geophysically observed values of high-conductivity anomalies, even if the quartzite contained large fluid fractions (0.32). The present results suggest that fluid-bearing quartzite is unable to account for the high-conductivity anomalies in terms of fluid fraction. A significant amount of other ionic species, such as Na, Cl, and Al in aqueous fluid, in addition to silica phases dissolved in fluid, is required to increase conductivity.

Deuterium‐hydrogen exchange in olivine: Implications for point defects and electrical conductivity

Knowledge about hydrogen self diffusion (DH) is critical for determining mantle hydrogen distribution and understanding point defects. Also, chemical diffusion of hydrogen in olivine, such as redox exchange with polarons (DRedox), depends on DH. In this study deuterium 2H was exchanged into hydrogen 1H saturated single crystals of San Carlos olivine between 750 and 900°C at 2 GPa. We measured and fit the resulting 2H profiles to obtain DH,[100] = 10(−4.9±1.4)*e(−140±30kJ/mol)/(RT) m2/s, which is ∼1 log unit lower than DRedox,[100], with similar activation enthalpy Ha. By comparing these two diffusion coefficients, we estimate the small polaron diffusion coefficient. Additionally, we estimate DH in the [010] and [001] orientations, demonstrating that DH is highly anisotropic in olivine. These DHvalues were used with the Nernst‐Einstein relation to estimate the electrical conductivity by hydrogen in olivine (σH = 101.1*e(−130kJ/mol)/(RT) S/m for 10−2 wt % H2O) that is lower in magnitude than previous measurements. Our results suggest that hydrogen alone cannot account for high electrical conductivity anomalies observed at asthenospheric depths (∼10−2 to ∼10−1 S/m). The maximum anisotropic variation of DH and σHin olivine is ∼2 log units between 750 and 900°C and increases when extrapolated to higher temperature (∼3.3 at 1400°C). Anisotropy observed in the mantle may indicate substantial amounts of hydrogen in olivine with lattice‐preferred orientation.

Origin of High Electrical Conductivity in the Lower Continental Crust: A Review

Electromagnetic measurements have demonstrated that the lower continental crust has remarkable electrical anomalies of high conductivity and electrical anisotropy on a global scale (probably with some local exceptions), but their origin is a long-standing and controversial problem. Typical electrical properties of the lower continental crust include: (1) the electrical conductivity is usually 10−4 to 10−1 S/m; (2) the overlying shallow crust and underlying upper mantle are in most cases less conductive; (3) the electrical conductivity is statistically much higher in Phanerozoic than in Precambrian areas; (4) horizontal anisotropy has been resolved in many areas; and (5) in some regions there appear to be correlations between high electrical conductivity and other physical properties such as seismic reflections. The explanation based on conduction by interconnected, highly conductive phases such as fluids, melts, or graphite films in grain boundary zones has various problems in accounting for geophysically resolved electrical conductivity and other chemical and physical properties of the lower crust. The lower continental crust is dominated by mafic granulites (in particular beneath stable regions), with nominally anhydrous clinopyroxene, orthopyroxene, and plagioclase as the main assemblages, and the prevailing temperatures are mostly 700–1,000°C as estimated from xenolith data, surface heat flow, and seismic imaging. Pyroxenes have significantly higher Fe content in the lower crust than in the upper mantle (peridotites), and plagioclase has higher Na content in the lower crust than in the shallow crust (granites). Minerals in the lower continental crust generally contain trace amounts of water as H-related point defects, from less than 100 to more than 1,000 ppm H2O (by weight), with concentrations usually higher than those in the upper mantle. Observations of xenolith granulites captured by volcano-related eruptions indicate that the lower continental crust is characterized by alternating pyroxene-rich and plagioclase-rich layers. Experimental studies on typical lower crustal minerals have shown that their electrical conductivity can be significantly enhanced by the higher contents of Fe (for pyroxenes), Na (for plagioclase), and water (for all minerals) at thermodynamic conditions corresponding to the lower continental crust, e.g., to levels comparable to those measured by geophysical field surveys. Preferred orientation of hydrous plagioclase, e.g., due to ductile flow in the deep crust, and alternating mineral fabrics of pyroxene-rich and plagioclase-rich layers can lead to substantial anisotropy of electrical conductivity. Electrical conductivity properties in many regions of the lower continental crust, especially beneath stable areas, can mostly be accounted for by solid-state conduction due to the major constituents; other special, additional conduction mechanisms due to grain boundary phases are not strictly necessary.

Unstable graphite films on grain boundaries in crustal rocks

The origin of high electrical conductive anomalies in the lower crust is a long-standing and controversial problem. Although it has been proposed that saturated saline water or partial melt increases electrical conductivity, graphite film has also been recognized as a potential cause of high conductivity since the discovery of fine graphite films on the grain boundaries of high-grade metamorphic rocks. To investigate the stability of graphite film on grain boundary of silicate minerals under lower crustal conditions, electrical conductivity of graphite film on synthetic grain boundaries of quartz bicrystals was measured by means of impedance spectroscopy at 1 GPa and up to 1200 K in a multianvil apparatus. At first heating, the electrical conductivity of the thin graphite film with thickness less than 100 nm was initially very high but decreased with time during annealing. Under high temperature conditions (> 1000 K), the conductivity of a thin carbon film rapidly decreases and approaches the quartz conductivity. This indicates that graphite film on a grain boundary between two quartz crystals is not stable at high temperatures. Optical microscopic observation of the run products suggested a disconnected feature of graphite on a quartz grain boundary. Disconnection of graphite film can be caused by higher interfacial energy between graphite and silicate minerals than that of the grain boundary energy. Therefore, a thin graphite film is not a likely candidate to account for high conductivity anomalies in the middle and lower continental crust.

Signatures of continental collisions and magmatic activity in central Brazil as indicated by a magnetotelluric profile across distinct tectonic provinces

Broadband and long-period magnetotelluric soundings were collected along a 560km E–W profile in a region of central Brazil subjected to Neoproterozoic collision tectonics and Archean to Cretaceous magmatic events. The profile crosses the northeastern portion of Phanerozoic sediments and volcanics of the Paraná basin, the southern extension of the Neoproterozoic metasedimentary rocks of the Brasília belt, locally pervaded by Cretaceous alkaline magmas of the Alto Paranaíba igneous province (APIP), and Neoproterozoic sedimentary cover and exposed Archean basement of the southern São Francisco craton. 2D conductivity structures derived by joint inversions of the TE and TM polarization modes and a separate inversion of the tipper components show signatures of the past tectonomagmatic events that affected the area. A gravity-defined suture zone beneath the Paraná basin is detected in the models as a subvertical conductor extending from crustal to upper mantle depths, which strengthen the interpretation of a Neoproterozoic collision of the São Francisco craton and a continental block beneath the sedimentary basin. Deep underthrusting of organic graphite-bearing metasedimentary rocks composing a fossil suture zone is proposed to explain such increase in electrical conductivity. A similar conductivity signature beneath the sedimentary covered region of the São Francisco craton can be tentatively interpreted as another similar cryptic suture zone. Other isolated high-conductivity anomalies at midcrustal depths below the Paraná and APIP provinces are interpreted as fossil residues representing precipitated graphite and sulfide derived from percolating volatiles during the emplacement of Cretaceous mafic-ultramafic volcanics. A very high conductivity wedge into the lower lithosphere is highlighted in the deep mantle beneath the APIP volcanic complex, coincident with a zone of low velocity defined by seismic tomography. Geochemical evidence indicates that the alkaline magmatism stemmed from a metasomatized upper mantle at only slightly raised temperatures. Thus, interconnected carbonatite melts of low melting point and graphite in the lithospheric mantle are the most likely candidates to explain this high conductivity. The seismic and conductivity anomalies are probably triggered by the same source mechanisms (incipient melting) because they are plausibly related to the same magmatic process. Conductivity anomalies at crustal and mantle depths in the southern segment of the São Francisco craton suggest that its lithosphere was significantly affected by the several tectonomagmatic episodes it has experienced throughout its geological history. Consequently, the enhanced conductivity in the lower crust can be genetically related to upwelling volatile-rich intrusions whereas upper mantle high conductivity can be related to refertilization by infiltrations of low-degree carbonatitic melts from deeper-sourced metasomatic processes.

Distribution of melt beneath Mount St Helens and Mount Adams inferred from magnetotelluric data

anomalous conductivity in this region is due to partial melt associated with the volcanism. Here we test this hypothesis by using magnetotelluric data recorded at a network of 85 locations in the area of the high-conductivity anomaly. Our data reveal that a localized zone of high conductivity beneath this volcano extends downwards to join the mid-crustal conductor. As our measurements were made during the recent period of lava extrusion at Mount St Helens, we infer that the conductivity anomaly associated with the localized zone, and by extension with the mid-crustal conductor, is caused by the presence of partial melt. Our interpretation is consistent with the crustal origin of silicic magmas erupting from Mount St Helens 8 , and explains the distribution of seismicity observed at the time of the catastrophic eruption in 1980 (refs 9, 10). The magnetotelluric method determines the conductivity structure of the crust by using electric currents induced in the earth by natural fluctuations in the Earth’s external magnetic field. More precisely, the conductivity ( ) or resistivity ( D 1= ) is inferred from measurements of the amplitude and phase relationships between the surface electric and magnetic field vectors as a function of period. Previous magnetotelluric data 1 4 from a series of widely spaced lines in southern Washington first showed that a conductive zone, the Southern Washington Cascades Conductor (SWCC), occurs in the region between Mounts St Helens, Adams and Rainier (Fig. 1). The conductance (or conductivitythic kness product) of a body

Evidence for an underground runoff and soil permeability at the Ouled Fayet (Algiers, Algeria) subsurface landfill pilot project from geophysical investigations

Results from geophysical investigations (electrical resistivity, electromagnetic mapping and seismic refraction) on an excavated cell of the Ouled Fayet (Algiers, Algeria) pilot landfill indicated the presence of an underground runoff and permeable soil underneath the cell. These results contradict those obtained by a feasibility study, based, however, only on the analysis of seventy-six 10-m drilling cores. The 1D boreholes information has been proven to be insufficient and to give biased results. The presence of water at depth is evidenced by lower resistivity, high conductivity anomalies and increase of P-wave velocity. Thus, to the contrary of what is claimed in the feasibility study, a threat of leachate pollution is real. This study shows that landfill construction studies cannot give trustful results without geophysical investigations. More specifically, in Algeria, it is imperative to elaborate a landfill construction code, which should include mandatory geophysical prospecting and deeper drilling cores.

Mineral and groundwater exploration with the SkyTEM system

The SkyTEM system is a helicopterborne TEM system (Sorensen and Auken, 2004) originally designed and developed for hydrogeophysical and environmental investigations. The aim was to develop an airborne system that would give the same resolution as conventional ground-based TEM soundings. To deliver both early and late time data of good quality, the SkyTEM system can presently be programmed to deliver the combination of moments that is optimal for the survey aims: low moments at ~12 kAm2 where low current and fast transmitter turnoff provide early time data with good near-surface resolution, and high moments at 120-220 kAm2 where a high current, more transmitter turns and lower base frequency provide late time data with good penetration. Repetition frequencies are chosen to optimally reject the power line frequency. In a sedimentary environment where small resistivity contrasts are significant, it is essential that the system produces data with high accuracy and that very early time gates are measured to provide good near-surface resolution. In its present configuration, the vertical resolution of the upper layers for the SkyTEM system is comparable to that of airborne frequency domain systems while the horizontal resolution is better due to less lateral filtering as the signal-to-noise ratio at early times is generally very good for TEM systems. The high moment provides good signal-to-noise ratio at late times. Because of its high lateral and vertical resolution, the SkyTEM system is increasingly used also in mineral prospecting where accurate definition of high conductivity anomalies is essential. The user-definable moments also makes it possible to include transmitter-off intervals so that background noise can be measured during flight. Before takeoff, a transmitter waveform calibration takes place.

電磁気法探査（ＥＭ法探査）技術の現状と展望―地震探査との統合型解析に向けて―

Electromagnetic (EM) methods have been developed for estimating the apparent resistivity or conductivity of material composing subsurface structure depending on survey objectives, while seismic methods are applied mainly to understand geological structure or stratigraphy. Magnetotelluric subsurface sounding (MT) methods are proven tools detect high-conductivity anomalies in the Earth's interior; controlled source EM (CSEM) methods detect high-conductivity anomalies; and, nuclear magnetic resonance (NMR) methods estimate “the mean free path” of hydrogen atoms in various molecules composing underground materials. Regarding methodologies, EM methods frequently applied in practice are: (1) natural source MT methods, (2) artificial source MT or EM methods, and (3) borehole EM methods. The fundamental principles of these methods can be summarized as measuring the induction effects of a survey target using a natural or artificial electromagnetic source. Finally, we investigate examples of recent EM approaches in connection with seismic methods and discuss the integration of survey data from both methods as a key to exploring underground structures not only in terms of stratigraphic interpretation but also for estimating material physical properties.

Imprints of a Proterozoic tectonothermal anomaly below the 1.1 Ga kimberlitic province of Southwest Cuddapah basin, Dharwar craton (Southern India)

SUMMARY A detailed 2-D modelling of the seismic structure and other geological and geophysical signatures across the Cuddapah basin of the southern Indian shield suggests upwarping of highvelocity and high-density layers, which are observed close to the surface below the southwestern part of the basin. This anomalous feature is constrained by (i) a strong gravity high anomaly of about 55 mGal, (ii) a 100 km wide high conductivity anomaly (resistivity < 100 � -m) extending from surface to a minimum depth of 50 km in the mantle lithosphere and (iii) large scale massive intrusive activity. These features are interpreted to be an expression of a thermal anomaly, which may have acted like a plume during the Proterozoic and could well correspond to a 1.1 Ga kimberlitic activity. Below this region, the thin granitic‐gneissic crust is underlain by well-differentiated, high-velocity layers, possibly due to underplating and densification of much of the crust by extruded magma.

A three-dimensional crustal geoelectric model of the Ukrainian Shield

A 3-D crustal geoelectric model of the Ukrainian Shield (USh) is constructed from magnetic variation data. High electrical conductivity anomalies with resistivities of 1–100 Ω m are located at depths of up to 30 km from the basement surface. A high conductivity layer with ρ = 25 Ω m and with its upper boundary at a depth of 70 km is supposed to exist in the upper mantle of the southwestern USh.

Conductivity Structure of Crust and Upper Mantle Beneath the Northern Tibetan Plateau: Results of Super‐Wide Band Magnetotelluric Sounding

AbstractTo study the conductivity structure of crust and upper mantle as well as the thermal regime of lithosphere beneath the northern Tibetan Plateau, the project INDEPTH (III)‐MT completed super‐broadband magnetotelluric (MT) sounding along Dêqên‐Longwei Cuo (line 500) and Nagqu‐Golmud (line 600) in northern and central Tibet in 1998 and 1999. The result shows that the conductivity structure of crust and upper mantle is quite different across the Kunlun Mountains. North to the mountains the crust and upper mantle is relatively highly resistive. South to the mountains, the conductivity structure of crust and upper mantle is of obvious stratification as described following. The upper crust is dominated by discontinuous high‐resistivity bodies with intercalated low‐resistivity anomalies. And the electric structure of the upper crust in NS direction looks like complicated with discontinuous and block‐shaped features. Meanwhile the intermediate and lower crust are characterized by large‐scale high conductivity anomalies, indicated by separated high‐conductivity bodies of big sizes and differing shapes with resistivity less than 4m. Beneath the Bangong‐Nujiang and Jinshajiang River sutures, the high‐conductivity bodies in the crust tend to extend toward the upper mantle, implying existence of a low‐resistivity conduit between crust and mantle. Based on the observed electric structure of crust and mantle beneath the northern and central Tibetan Plateau, we infer that there are extensive partial melt and thermal fluids there like southern Tibet. Their origin is associated with thermal exchange between crust and mantle beneath the Bangong‐Nujiang and Jinshajiang River sutures which occurred in their own conduits. The thermal exchange conduit below the Bangong‐Nujiang River suture was formed earlier than that beneath the Jinshajiang River suture. Therefore the thermal activity of crust and mantle beneath the study area began from south and west, then spread toward north and east, leading to the current heat flow distribution in middle and northern Tibet which becomes larger from west to east and from south to north.

Seismic evidence for the influence of fluids on the 2005 west off Fukuoka prefecture earthquake in southwest Japan

High-resolution 3-D seismic velocity ( V p and V s) and Poisson's ratio ( σ) structures in the epicentral area of the 2005 west off Fukuoka prefecture earthquake were determined by inverting a large number of high-quality P- and S-wave arrival times and sP depth phase data to better understand the genesis of the main-shock and its aftershock sequence. The hypocenters of the earthquakes used in this study were relocated with a combined method which enables to accurately relocate the earthquakes occurred outside the seismic network by using sP depth phase data together with double-difference travel time data. This new approach of earthquake location can be applied to other offshore regions of subduction zones where few seismic stations exist while numerous suboceanic earthquakes occur. Strong crustal velocity heterogeneities were revealed in the hypocentral area, which might reflect the existence of fluids, geometrical irregularities of the fault zone, and geothermal anomalies in the region. The main-shock hypocenter was located near a distinct zone characterized by low-velocity, high-Poisson's ratio and high-conductivity anomalies, being considered to be associated with the existence of fluids in and around the source area. Such fluids might have reduced the seismic velocity of the crustal materials around the main-shock hypocenter and weakened the mechanic strength of the fractured rock matrix in the source area, and thus triggered the 2005 west off Fukuoka prefecture earthquake.

Dynamics, crustal thicknesses, seismic anomalies, and electrical conductivities in dry and hydrous ridge-centered plumes

3D convection and melting models for ridge-centered plumes in the upper 410 km of the mantle have been computed for dry and hydrous mantles, taking into account the effects of water on melting and viscosity. Dry and hydrous models have fundamentally different flow and melting styles in the melting regime: in the dry model, active upwelling occurs to shallow depth and results in a very thick, narrow plume-generated crust, whereas dehydration stiffening in the hydrous models enforces passive upwelling in much of the melting region and causes the plume crust to be much less thick, but broader in along-ridge direction. The output of the numerical models, i.e. temperature, melting degree, water content, and porosity, is used to compute synthetic models of seismic velocity anomalies and electrical conductivity. For this purpose, combinations of theoretical and empirical relations with recent experimental data are developped. The models predict that the seismic velocity anomaly of the deep, unmolten plume stem is mostly thermal in both water-bearing and water-free plumes, but that the two velocity minima in the anomaly in the melting zone of the plume are much more clearly distinguishable in a water-bearing plume. If one assumes a contribution to the electrical conductivity of hydrous mantle from free protons, and if the water reservoir features large-scale contiguity, one would also expect a strong high-conductivity anomaly in the plume stem which should be visible to magnetotelluric observations. However, comparison with observations suggests that a simple free-proton model may not be applicable.

Fine three-dimensional P-wave velocity structure beneath the capital region and deep environment for the nucleation of strong earthquakes

A detailed 3-D P-wave velocity model of the crust and uppermost mantle under the capital region is determined with a spatial resolution of 25 km in the horizontal direction and 4–17 km in depth. We used 48750 precise P-wave arrival time data from 2973 events of local crustal earthquakes, controlled seismic explosions and quarry blasts. These events were recorded by 123 seismic stations. The data are analyzed by using a 3-D seismic tomography method. Our tomographic model provides new information on the geological structure and complex seismotectonics of this region. Different patterns of velocity structures show up in the North China Basin, the Taihangshan and the Yanshan Mountainous areas. The velocity images of the upper crust reflect well the surface geological, topographic and lithological features. In the North China Basin, the depression and uplift areas are imaged as slow and fast velocity belts, respectively, which are oriented in NE-SW direction. The trend of velocity anomalies is the same as that of major structure and tectonics. Paleozoic strata and Pre-Cambrian basement rocks outcrop widely in the Taihangshan and Yanshan uplift areas, which exhibit strong and broad high-velocity anomalies in our tomographic images, while the Quaternary intermountain basins show up as small low-velocity anomalies. Most of large earthquakes, such as the 1976 Tangshan earthquake (M 7.8) and the 1679 Sanhe earthquake (M 8.0), generally occurred in high-velocity areas in the upper to middle crust. However, in the lower crust to the uppermost mantle under the source zones of the large earthquakes, low-velocity and high-conductivity anomalies exist, which are considered to be associated with fluids, just like the 1995 Kobe earthquake (M 7.2) and the 2001 Indian Bhuj earthquake (M 7.8). The fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the large crustal earthquakes.

New magnetotelluric data through the boundary between the Ossa Morena and Centroiberian Zones

The south-western part of the Iberian Peninsula, including the southern branch of the Iberian Massif, has recently been the subject of several magnetotelluric (MT) studies. This area is made up of three different tectonic terranes: the South Portuguese Zone (SPZ), the Ossa Morena Zone (OMZ) and the Central Iberian Zone (CIZ). The boundaries between these zones are considered to be sutures, which appear as high electrical conductivity anomalies in the MT surveys. The OMZ is characterised by a conductive layer at middle-lower crustal levels. To investigate the continuity of this conductive layer into the CIZ, a new MT profile was carried out. This 75-km long ENE profile goes through the boundary between the OMZ and the CIZ. The results of a two-dimensional magnetotelluric inversion revealed a high-conductivity anomaly in the transition OMZ/CIZ (the so-called Central Unit), which is interpreted as due to interconnected graphite along shear planes. High-conductivity anomalies appeared in the middle crust of the CIZ, whose geometry and location are consistent with the conductive layer previously found in the OMZ, thus confirming the prolongation of the conductive layer into the CIZ. The top of this layer correlated spatially with a broad reflector detected by a seismic profile previously acquired in the same area. This, together with other geological and petrological evidence, points to a common origin for both features.

Crustal heterogeneity and seismotectonics of the region around Beijing, China

A detailed three-dimensional (3-D) P-wave velocity model of the crust and uppermost mantle under the Chinese capital (Beijing) region is determined with a spatial resolution of 25 km in the horizontal direction and 4–17 km in depth. We used 48,750 precise P-wave arrival times from 2973 events of local crustal earthquakes, controlled seismic explosions and quarry blasts. These events were recorded by a new digital seismic network consisting of 101 seismic stations equipped with high-sensitivity seismometers. The data are analyzed by using a 3-D seismic tomography method. Our tomographic model provides new insights into the geological structure and tectonics of the region, such as the lithological variations and large fault zones across the major geological terranes like the North China Basin, the Taihangshan and the Yanshan mountainous areas. The velocity images of the upper crust reflect well the surface geological and topographic features. In the North China Basin, the depression and uplift areas are imaged as slow and fast velocities, respectively. The Taihangshan and Yanshan mountainous regions are generally imaged as broad high-velocity zones, while the Quaternary intermountain basins show up as small low-velocity anomalies. Velocity changes are visible across some of the large fault zones. Large crustal earthquakes, such as the 1976 Tangshan earthquake ( M=7.8) and the 1679 Sanhe earthquake ( M=8.0), generally occurred in high-velocity areas in the upper to middle crust. In the lower crust to the uppermost mantle under the source zones of the large earthquakes, however, low-velocity and high-conductivity anomalies exist, which are considered to be associated with fluids. The fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the large crustal earthquakes.

Syngenetic, thin graphite-rich horizons in lower crustal rocks from the Serre San Bruno, Calabria (Italy), and implications for the nature of high-conducting deep crustal layers

In order to better understand the nature of deep crustal high electrical conductivity, we studied the electrical properties of a tilted section of a former lower continental crust exposed in the Calabrian arc of the Alpine–Apennine mountain system. Geoelectric field measurements and impedance measurements on rock samples showed that these high-grade metamorphic rocks are generally highly resistive as expected for crystalline, electrolytically conducting rocks of low porosity. This holds for graphite-free metabasites as well as for metapelites which generally contain accessory, up to 3% biogenic graphite in the form of isolated grains. Clearly as an exception, a group of thin stratiform black horizons with thicknesses of 1–15 cm within the metapelitic series was detected by means of self-potential (SP) measurements. Rock samples from these horizons exhibit high, quasi-metallic bulk conductivities of up to 50 S/m (0.02 Ωm) in agreeement with up to 20% syngenetic graphite, forming a network of interconnected streaks or crack fillings. The high amount of carbon most probably originates from organic matter of C org-rich black shales. Relative enrichment of the low mobility graphitic matter compared to the carbon content of the assumed protoliths may have been due to pressure solution and partial melting during prograde metamorphism, without major contribution of a fluid phase, resulting in isolated graphite flakes. Although enriched, graphite in this form has little effect on electrical conductivity. For the Calabrian black horizons, microscopic analyses make conceivable that, in a further decisive step, isolated graphite grains were mechanically smeared to continuous pathways during uplift by shearing, producing hereby the observed graphitic network which is needed to generate high conductivity. As C org-rich black shales are common members of sedimentary sequences throughout the earth’s history, good conductors of this type may be expected in the continental crust at any depth depending on tectonic and metamorphic history, with the exception of magmatic protoliths. Regarding the extremely high conductivity of the meta-black shale samples containing syngenetic sheared graphite, a total thickness of a few meters of such rocks is sufficient to explain magnetotelluric high conductivity anomalies in the deep crust.