High Conductivity Zone Research Articles

Electrode Activation Sequencing Employing Conductivity Changes in Irreversible Electroporation Tissue Ablation

Irreversible electroporation (IRE) uses high-voltage pulses applied to tissue, which cause dielectric breakdown of cell membranes resulting in cell death. IRE is a promising technique for ablation of nonresectable tumors because it can be configured to spare critical structures such as blood vessels. A consequence of pulse application is an increase in tissue electrical conductivity due to current pathways being opened in cell membranes. We propose a novel IRE method introducing electrode switching and pulse sequencing in which tissue conductivity is first increased using preparatory pulses in order to form high-conductivity zones, which then helps provide higher electric field intensity within the targeted tissue as subsequent pulses are applied, and hence, enhances the efficiency and selectivity of the IRE treatment. We demonstrate the potential of this method using computational models on simple geometries.

Magnetotelluric Studies at the San Andreas Fault Zone: Implications for the Role of Fluids

Fluids residing in interconnected porosity networks have a significant weakening effect on the rheology of rocks and can strongly influence deformation along fault zones. The magnetotelluric (MT) technique is sensitive to interconnected fluid networks and can image these zones on crustal and upper mantle scales. MT images have revealed several prominent electrical conductivity anomalies at the San Andreas Fault which have been attributed to the presence of saline fluids within such networks and which have been associated with tectonic processes. These models suggest that ongoing fluid release in the upper mantle and lower crust is closely related to the mechanical state of the crust. Where fluids are drained into the brittle crust, and where these fluids are kept at high pressures, fault creep is supported. Fluid fluxes from deeper levels, in combination with meteoric and crustal metamorphic fluid inflow, and in response to fault creep, leads to high-conductivity zones developing as fault zone conductors in the brittle portion of crust. In turn, the absence of crustal fluid pathways may be characteristic for mechanically locked segments of the fault. Here, MT models suggest that fluids are trapped at depth and kept at high pressures. We speculate that fluids may infiltrate neighboring rocks and in their wake induce non-volcanic tremor.

Grain size effect on the electrical conductivity of clinopyroxene

Complex impedance spectra of polycrystalline samples (with grain size fractions ~5–63, 63–160 and 160–250 μm) and a single crystal sample (with orientation parallel to b), prepared from a natural megacryst augite, were measured in a piston cylinder apparatus at 10 kbar and 500–1,000°C and with a Solartron 1260 Impedance/Gain Phase analyzer over a frequency range of 0.1–106 Hz. The main charge carriers are attributed to small polarons, and the activation enthalpy is 83 ± 3 to 90 ± 3 kJ/mol. The measured electrical conductivity shows no difference between the polycrystalline and single crystal samples, suggesting independence of electrical conductivity on grain size given a change above ~5 μm. The electrical conductivity of augite is much higher than that of olivine, indicating that, if regionally enriched, augites may lead to zones of high electrical conductivity and electrical anisotropy in the deep lithosphere.

Assessment of contamination by intensive cattle activity through electrical resistivity tomography

Abstract The intensive animal production is considered highly risky for groundwater and soil because of high mobility of some contaminants from animal wastes. The aim of this work was to obtain an electrical conductivity image of unsaturated and saturated zones at a feedlot (cattle feeding field) at the surroundings of Buenos Aires city (Argentina) in order to detect the most critical sectors of the field, with regard to contamination by animal wastes. Dipole–dipole electrical soundings (electrical resistivity tomography) were performed at the corral zone and the surroundings. 2D and 3D models of conductivity were obtained. Even if there is a calcareous plate below some parts of the corrals and soil compaction is high, vertical infiltration or subsurface runoff may have occurred since these sites, with high animal charge, show soil conductivities higher than the surroundings. The models showed higher conductivities of saturated zone increasing in the direction of groundwater flow. These results were taken into account for further designs of soil and groundwater sampling. Groundwater conductivity was three times greater downgradient from the corrals with high concentrations of nitrates and phosphorous. A zone of high conductivity was found below a small channel of effluents from the corrals.

Coupled Mechanical and Hydraulic Analysis of the Effect of a Front High Hydraulic Conductivity Zone on Shield Tunnelling

A localized high hydraulic conductivity zone with high water pressure is denoted as a hazardous zone. This study focuses on the 3D coupled mechanical and hydraulic analysis of the effect of a front hazardous zone on shield tunnelling. The tunnelling process is modelled by a step-by-step excavation. And a series of three-dimensional coupled mechanical and hydraulic analysis are performed to estimate the distribution of the TSR of the ground ahead of the tunnel face. The result show that the tunnelling directly influences on the distribution of the pore water pressure and the TSR. The maximum influence range of the hazardous zone on the face stability is within 4 times the R value. And the high water pressure can induce a failure of the ground at the tunnel face by itself in conjunction with a poor ground condition.

Geophysical signatures of fluids in a reactivated Precambrian collisional suture in central India

The Central India Tectonic Zone (CITZ) marks the trace of a major suture zone along which the south Indian and the north Indian continental blocks were assembled through subduction-accretion-collision tectonics in the Mesoproterozoic. The CITZ also witnessed the major, plume-related, late Cretaceous Deccan volcanic activity, covering substantial parts of the region with continental flood basalts and associated magmatic provinces. A number of major fault zones dissect the region, some of which are seismically active. Here we present results from gravity modeling along five regional profiles in the CITZ, and combine these results with magnetotelluric (MT) modeling results to explain the crustal architecture. The models show a resistive (more than 2000 Ω·m) and a normal density (2.70 g/cm 3) upper crust suggesting dominant tonalite–trondhjemite–granodiorite (TTG) composition. There is a marked correlation between both high-density (2.95 g/cm 3) and low-density (2.65 g/cm 3) regions with high conductive zones (<80 Ω·m) in the deep crust. We infer the presence of an interconnected grain boundary network of fluids or fluid-hosted structures, where the conductors are associated with gravity lows. Based on the conductive nature, we propose that the lower crustal rocks are fluid reservoirs, where the fluids occur as trapped phase within minerals, fluid-filled porosity, or as fluid-rich structural conduits. We envisage that substantial volume of fluids were transferred from mantle into the lower crust through the younger plume-related Deccan volcanism, as well as the reactivation, fracturing and expulsion of fluids transported to depth during the Mesoproterozoic subduction tectonics. Migration of the fluids into brittle fault zones such as the Narmada North Fault and the Narmada South Fault resulted in generating high pore pressures and weakening of the faults, as reflected in the seismicity. This inference is also supported by the presence of broad gravity lows near these faults, as well as the low velocity in the lower crust beneath regions of recent major earthquakes within the CITZ.

Analysis of urinary neurotransmitters by capillary electrophoresis: Sensitivity enhancement using field-amplified sample injection and molecular imprinted polymer solid phase extraction

Capillary electrophoresis (CE) has been investigated for the analysis of some neurotransmitters, dopamine (DA), 3-methoxytyramine (3-MT) and serotonin (5-hydroxytryptamine, 5-HT) at nanomolar concentrations in urine. Field-amplified sample injection (FASI) has been used to improve the sensitivity through the online pre-concentration samples. The cationic analytes were stacked at the capillary inlet between a zone of low conductivity - sample and pre-injection plug - and a zone of high conductivity - running buffer. Several FASI parameters have been optimized (ionic strength of the running buffer, concentration of the sample protonation agent, composition of the sample solvent and nature of the pre-injection plug). Best results were obtained using H 3PO 4–LiOH (pH 4, ionic strength of 80 mmol L −1) as running buffer, 100 μmol L −1 of H 3PO 4 in methanol–water 90/10 (v/v) as sample solvent and 100 μmol L −1 of H 3PO 4 in water for the pre-injection plug. In these conditions, the linearity was verified in the 50–300 nmol L −1 concentration range for DA, 3-MT and 5-HT with a determination coefficient ( r 2) higher than 0.99. The limits of quantification (10 nmol L −1 for DA and 3-MT, 5.9 nmol L −1 for 5-HT) were 500 times lower than those obtained with hydrodynamic injection. However, if this method is applied to the analysis of neurotransmitters in urine, the presence of salts in the matrix greatly reduces the sensitivity of the FASI/CE–UV method.Therefore, a solid phase extraction (SPE) on a dedicated imprinted polymer (MIP) was developed to extract specific neurotransmitters, catecholamines, metanephrines and indolamines, from urine. Matrix salts were thus discarded after sample extraction on AFFINIMIP™ Catecholamine & Metanephrine (100 mg) cartridge. Therefore, lower limits of quantification were determined in artificial urine (46 nmol L −1 for DA, 11 nmol L −1 for 3-MT and 6 nmol L −1 for 5-HT).The application of this protocol MIP-SPE/FASI–CE–UV analysis of neurotransmitters in human urine gave rise to electropherograms with a very good base line and signal to noise ratios above 15.

The structure and current-voltage characteristics of copper-tungsten electric contacts prepared using a Plasma Focus installation

The structure and current-voltage characteristics (CVCs) of copper-tungsten electric contacts deposited using a Plasma Focus facility are studied. It is shown that the reduced contact resistance is ∼0.4 × 10−6 Ω cm2 at 300 K and ∼0.4 × 10−7 Ω cm2 at 80 K, which meets the highest requirements for ohmic contacts in electrical installations and semiconductor electronics. The formation of microcracks on the surface of tungsten exposed to high-speed plasma streams has no significant impact on the contact resistance. Under the effect of high pressure, a melt of copper penetrates deep into tungsten along the cracks, solidifies, and thereby forms zones of high conductivity. The soldering of electric contacts with tin solder leads to CVC deviation from linearity at 80 K. A probable cause of this is the formation of SnO2-copper Schottky barriers on the surface of Cu-W contacts.

Tidal Boundary Conditions in SEAWAT

SEAWAT, a U.S. Geological Survey groundwater flow and transport code, is increasingly used to model the effects of tidal motion on coastal aquifers. Different options are available to simulate tidal boundaries but no guidelines exist nor have comparisons been made to identify the most effective approach. We test seven methods to simulate a sloping beach and a tidal flat. The ocean is represented in one of the three ways: directly using a high hydraulic conductivity (high-K) zone and indirect simulation via specified head boundaries using either the General Head Boundary (GHB) or the new Periodic Boundary Condition (PBC) package. All beach models simulate similar water fluxes across the upland boundary and across the sediment-water interface although the ratio of intertidal to subtidal flow is different at low tide. Simulating a seepage face results in larger intertidal fluxes and influences near-shore heads and salinity. Major differences in flow occur in the tidal flat simulations. Because SEAWAT does not simulate unsaturated flow the water table only rises via flow through the saturated zone. This results in delayed propagation of the rising tidal signal inland. Inundation of the tidal flat is delayed as is flow into the aquifer across the flat. This is severe in the high-K and PBC models but mild in the GHB models. Results indicate that any of the tidal boundary options are fine if the ocean-aquifer interface is steep. However, as the slope of that interface decreases, the high-K and PBC approaches perform poorly and the GHB boundary is preferable.

Experimental study on water rock interactions at temperatures up to 435 °C and implications for geophysical features in upper mid-crust condition

Globally observed zones of high conductivity and low seismic velocity in the mid-crust remain enigmatic despite previous theoretical and experimental investigations. We have carried out a series of dissolution experiments on mineral and rock samples in aqueous solutions using flow-through reactors at temperatures as high as 435 °C and at pressures of 23–36 MPa. These experiments provide useful information for evaluating the behavior of crustal fluids and their role in the formation of geophysical detectible anomalies in the mid-crust. The steady state dissolution rates of minerals or rocks are measured as functions of temperature, flow rate and pressure. The maximum release rates of Si for silicate minerals are always observed at 300 °C (or at 300–400 °C for silicate rocks), which results in strong leaching of Si and the break-up of silicate framework structures in minerals and rocks. From a tectonic perspective, plate motions are ultimately responsible for inducing cracking in rocks, generating porosity, decreasing pressure, and moving fluids across and through continents. These processes of decreasing pressure probably lead to the migration of aqueous fluid in the mid-crust to locations where they are close to their critical state at temperatures ranging from 300 to 435 °C. Therefore, water rock interactions occurring in the crust will cause strong leaching of Si, breakage of silicate framework structures, and rock collapse. The strong water–rock interaction will further lead to the generation of increased rock porosity and also drive fluid flow. These hydrothermal events, therefore, have a significant role in enhancing the conductivity of rocks in the mid-crust.

Introduction to special section on Phenomenology, Underlying Processes, and Hazard Implications of Aseismic Slip and Nonvolcanic tremor

This paper introduces the special section on the “phenomenology, underlying processes, and hazard implications of aseismic slip and nonvolcanic tremor” by highlighting key results of the studies published in it. Many of the results indicate that seismic and aseismic manifestations of slow slip reflect transient shear displacements on the plate interface, with the outstanding exception of northern Cascadia where tremor sources have been located on and above the plate interface (differing models of the plate interface there also need to be reconciled). Slow slip phenomena appear to result from propagating deformation that may develop with persistent gaps and segment boundaries. Results add to evidence that when tectonic deformation is relaxed via slow slip, most relaxation occurs aseismically but with seismic signals providing higher‐resolution proxies for the aseismic slip. Instead of two distinct slip modes as suggested previously, lines between “fast” and “slow” slip more appropriately may be described as blurry zones. Results reported also show that slow slip sources do not coincide with a specific temperature or metamorphic reaction. Their associations with zones of high conductivity and low shear to compressional wave velocity ratios corroborate source models involving pore fluid pressure buildup and release. These models and spatial anticorrelations between earthquake and tremor activity also corroborate a linkage between slow slip and frictional properties transitional between steady state and stick‐slip. Finally, this special section highlights the benefits of global and multidisciplinary studies, which demonstrate that slow phenomena are not confined to beneath the locked zone but exist in many settings.

A target-oriented magnetotelluric inversion approach for characterizing the low enthalpy Groß Schönebeck geothermal reservoir

SUMMARY Electrical conductivity is a key parameter for the exploration and characterization of geothermal reservoirs as hot mineralized formation water of active geothermal areas usually exhibits significantly higher conductivity than the surrounding host rock. Here we present results of a magnetotelluric (MT) exploration experiment carried out in the vicinity of the Gros Sch¨ onebeck geothermal test site in Northern Germany, where a doublet system of two 4.3-km deep boreholes was drilled to establish an in situ laboratory to investigate the potential for geothermal energy production. Classical 2-D smooth inversion of the MT data, recorded along two profiles, reveals a shallow conductive structure in good agreement with information from regional geology and seismic images. However, at the northernmost part of the profiles, the conductivity models reveal deep-reaching conductive structures, which appear uncorrelated with existing (geophysical or geological) data. Incorporating information from seismics as independent constraints for MT inversions allows us to examine the model space rigorously but target oriented. Employing so-called tear-zone inversions we can effectively derive an alternative class of models, which are consistent with the MT observations but also with the other data sets. We speculate that the zones of high conductivity imaged at reservoir depth are related areas of reduced thickness of the overlaying evapourite layer. The enhanced conductivity can be explained by a higher fracture density in anhydritic layers and/or generally lower resistivity of the pore fluid.

Nonvolcanic tremor along the Oaxaca segment of the Middle America subduction zone

The Oaxaca subduction zone is an ideal area for detailed studies of plate boundary deformation as rapid convergent rates, shallow subduction, and short trench‐to‐coast distances bring the thermally defined seismogenic and transition zones of the plate interface over 100 km inland. Previous analysis of slow slip events in southern Mexico suggests that they may represent motion in the transition zone, defining the downdip edge of future megathrust earthquakes. A new deployment consisting of broadband seismometers distributed inland along the Oaxaca segment provide the means to examine whether nonvolcanic tremor (NVT) signals can also be used to characterize the boundary between the seismogenic and transition zones. In this study, we established that NVT exists in the Oaxaca region based on waxing and waning of seismic energy on filtered day‐long seismograms that were correlated across neighboring stations and were further supported by appropriate relative time moveouts in record sections and spectrograms with narrow frequency bands. Eighteen prominent NVT episodes that lasted upwards of a week were identified during the 15 months analyzed (June 2006 to September 2007), recurring as frequently as every 2–3 months in a given region. We analyze NVT envelope waveforms with a semiautomated process for identifying prominent energy bursts, and analyst‐refined relative arrival times are inverted for source locations. NVT burst epicenters primarily occur between the 40–50 km contours for depth of the plate interface, except in eastern Oaxaca where they shift toward the 30 km contour as the slab steepens. NVT hypocenters correlate well with a high conductivity zone that is interpreted to be due to slab fluids. NVT is more frequent, shorter in duration, and located further inland than GPS‐detected slow slip, while the latter is associated with a zone of ultra‐slow velocity interpreted to represent high pore fluid pressure. This zone of slow slip corresponds to approximately 350°C–450°C, with megathrust earthquakes, microseismicity, and strong long‐term coupling occurring immediately updip from it. This leaves NVT primarily in a region further inland from the thermally defined transition zone, suggesting that transition from locking to free slip may occur in more than one phase.

An indirect assessment on the impact of connectivity of conductivity classes upon longitudinal asymptotic macrodispersivity

Solute transport takes place in heterogeneous porous formations, with the log conductivity, Y = ln K, modeled as a stationary random space function of given univariate normal probability density function (pdf) with mean 〈Y〉, variance σY2, and integral scale IY. For weak heterogeneity, the above mentioned quantities completely define the first‐order approximation of the longitudinal macrodispersivity αL = σY2IY. However, in highly heterogeneous formations, nonlinear effects which depend on the multipoint joint pdf of Y, impact αL. Most of the past numerical simulations assumed a multivariate normal distribution (MVN) of Y values. The main aim of this study is to investigate the impact of deviations from the MVN structure upon αL. This is achieved by using the concept of spatial correlations of different Y classes, the latter being defined as the space domain where Y falls in the generic interval [Y,Y + ΔY]. The latter is characterized by a length scale λ(Y), reflecting the degree of connectivity of the domain (the concept is similar to the indicator variograms). We consider both “symmetrical” and “non‐symmetrical” structures, for which λ(Y′) = λ(−Y′) (similar to the MVN), and λ(Y′) ≠ λ(−Y′), respectively, where Y′ = Y − 〈Y〉. For example, large Y zones may have high spatial correlation, while low Y zones are poorly correlated, or vice versa. The impact of λ(Y) on αL is investigated by adopting a structure model which has been used in the past in order to investigate flow and transport in highly heterogeneous media. It is found that the increased correlation in the low conductive zones with respect to the high ones generally leads to a significant increase in αL, for the same global IY. The finding is explained by the solute retention occurring in low Y zones, which has a larger effect on solute spreading than high Y zones. Conversely, αL decreases when the high conductivity zones are more correlated than the low Y ones. Dispersivity is less affected by the shape of λ(Y) for symmetrical distributions. It is found that the range of validity of the first‐order dispersivity, i.e., αL = IYσY2, narrows down for non‐symmetrical structures.

Effect of a front high hydraulic conductivity zone on hydrological behavior of subsea tunnels

This study focuses on the hydrological behavior of the ground ahead of a tunnel face. A localized high hydraulic conductivity zone with high water pressure is denoted as a hazardous zone. To analyze the impact of the hazardous zone on the face stability, a new indicator (TSR: the ratio of the driving force to the resisting force) is proposed on the basis of the “advance core” concept (Lunardi, 2000). A series of three-dimensional steady-state seepage analyses are performed to estimate the distribution of the TSR of the ground ahead of the tunnel face. The analyses results show that the hydrostatic pore water pressure acts on the most of the hazardous zones throughout all of the tunnelling stages. When the distance between the tunnel face and the hazardous zone (dh) value is approximately five times the excavated tunnel radius, the influence of the hazardous zone increases rapidly.

Deep crustal structure of northwestern part of Turkey

The deep crustal structure of the Polatlı–Nallıhan–Karasu (NW Turkey) area was investigated by Magnetotelluric and Transient Electromagnetic measurements. All measurements were acquired as part of a national project called Crustal Structures of Turkey undertaken by the General Directorate of Mineral Research and Exploration of Turkey (MTA). Magnetotelluric data were collected from 44 stations located along a 250 km northwest to southeast profile crossing in succession the İstanbul Zone, Intra-Pontid Suture, Sakarya Continent, İzmir–Ankara Suture, Tavşanlı Zone (the Tauride–Anatolide Belt), Intra-Pontid Suture and Sakarya Continent. After decompositional analysis and strike direction estimation, static shift is corrected by using transient electromagnetic data from MT apparent resistivity data. The processed MT data were inverted to derive a two-dimensional geoelectrical model. The estimated model is interpreted with the help of gravity data observed along the MT lines and using earthquake epicenter which occurred around MT measurement lines. Four layers with distinctive resistivity values dominate the geoelectrical section. The uppermost layer, having resistivity values of 5–35 ohm m can be interpreted as the sedimentary fill. This layer reaches down to about 10 km depth in Haymana Basin. The second layer has resistivity values higher than 110 ohm m and reaches 27 km depth. The first and second layers can be inferred as constituting the upper crust in the general model of the Earth's lithosphere. The third layer underlying the upper crust has resistivity values of 10–30 ohm m corresponding to the lower crust. The fourth layer having high resistivity values (> 225 ohm m) can be considered as the upper mantle. The depth of the boundary separating the crust from the upper mantle (Moho) varies between 20 and 40 km. High conductive zones between the İstanbul Zone and Sakarya Continent, Sakarya Continent and Tavşanlı Zone coincide with high Bouguer gravity anomaly values. It can be inferred from these properties that these zones correspond to the Intra-Pontide Suture and the İzmir–Ankara Suture. Noticeable differences in tectonic characteristics are observed between the southern and northern part of Sakarya Continent in the geoelectrical section. The northern part of the continent is separated by high dip slip faults while the faults in the southern part do not affect the high resistive zone in the deeper part of the section. These faults controlled the development of the Haymana Basin but they do not alter the high resistive zone to form separate blocks. The majority of these faults, which are clearly seen in the geoelectrical section are not evident in the sedimentary fill. These buried faults have not been studied in detail prior to the present study.

Shallow to intermediate resistivity features of the Colfiorito Fault System inferred by DC and MT survey

Over the last decade electromagnetic (EM) measurements have provided new constraints on the upper-crustal structure of the major fault zones in the world, both when they act as conduit and as a barrier, due to strong sensitivity of resistivity to fluids circulation and mineralization. On the track of a high impact magnetotelluric (MT) study performed across the San Andreas Fault, high resolution EM data were collected in the Colfiorito epicentral area along profiles crossing some main fault lineaments. Being the study focussed both on shallow that on intermediate resistivity distribution in the brittle upper-crust, a MT profile was integrated by several electrical resistivity tomographies (ERT). The latter were successful in locating faults even where the structures are buried by a wide covering of Quaternary deposits and in the recognition of different electrical signatures of the faults. MT resistivity model crossing Mt. Prefoglio normal fault clearly imaged the typical thrust structures of the area and a high conductive zone spatially related to the fault. Seismicity seems to be located outside such conductive area, whose behaviour suggests a fluidised and altered zone incapable of supporting significant stress internally.

Mechanisms of strain‐rate‐ and reaction‐dependent permeability in the mid‐crust of the Southern Alps: insight from three‐dimensional mechanical models

AbstractWe used three‐dimensional mechanical modelling to explore the interplay of rheology, modes of permeability creation and fluid flow in the mid‐crust of an oblique orogen. We used the central Southern Alps and the Otago Schist of New Zealand to constrain our models. We also compared our models with the magnetotelluric survey along the Rangitata–Whataroa rivers that imaged a U‐shaped zone of high conductivity, interpreted as interconnected fluids, beneath the central Southern Alps. Modelling was carried out using the numerical code FLAC3D. We used a number of simple assumptions: an initially homogeneous starting material, deformation boundary conditions based on the tectonics of the South Island, the capability of the modelled material to develop an anisotropic permeability structure, strain rate and reaction‐induced permeability increases, initial saturation and lithostatic pore pressures as a basis for our models. The initial isotropic permeability was 10−18 m2. We modelled two possible mechanisms of permeability increase: (i) strain‐rate dependent and (ii) reaction dependent. For a strong mid‐crust, the models showed enhanced permeability and hence fluid interconnectivity in a symmetric region beneath the model main divide, both ends were turned up towards the brittle–ductile transition and fluid flow was the greatest in the across strike direction. For a weak mid‐crust, the region of enhanced permeability was asymmetric and turned up towards the brittle–ductile regime close to the Alpine Fault. The strong mid‐crust model reproduces the features that are common to all interpretations of the MT soundings and is our preferred model for the central Southern Alps.

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

Stratigraphy of the Archean western Superior Province from P- and S-wave receiver functions: Further evidence for tectonic accretion?

The Archean western Superior Province in Canada represents the nucleus of the North American continent whose origin has been speculated to be the result of widespread crustal accretion some 2.7 Ga ago. In this paper, crustal and upper-mantle seismic discontinuities beneath the western Superior Province of the Canadian shield are imaged with teleseismic P-to-S and S-to-P converted phases using the receiver function method. Three crustal discontinuities are observed: the Moho, ranging in depth between 38 and 47 km and dipping to the south; and two intra-crustal discontinuities having depths of approximately 15 and 30 km. The crustal discontinuities undulate laterally and often lose continuity, possibly indicating an imbricated structure and/or regions of velocity gradients. In the shallow lithosphere, a positive discontinuity is imaged at approximately 65 km depth and is consistent with earlier refraction and wide-angle reflection results. Additionally, two zones of negative receiver function amplitudes at 55 km depth are observed and are coincident with a region of anomalous tomographic low P- and S-wave velocities as well as a zone of high electrical conductivity. The images for the crust and shallow upper-mantle, when integrated with previous geophysical studies, are consistent with ideas of continental root formation due to imbrication of Archean subducted material and accretion of island arcs observed in surface geology.