Long-period Magnetotelluric Data Research Articles

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.

Imaging the electrical lithosphere of South Australia - 2D profiles and preliminary results of AusLAMP SA

The Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) has the goal of mapping the electrical resistivity of the Australian lithosphere to constrain the geodynamic framework of the continent. Between August 2014 and May 2015, 125 long-period magnetotelluric (MT) data will be collected across the Gawler Craton and the south-eastern part of South Australia at intervals of 50 km. Results will be compared to existing 3D models highlighting enhanced conductivity in the sub-lithospheric mantle of the Gawler Craton. Initial results of 1D depth transformations show a significant change in resistivity at mantle lithosphere depths (100 km). The results will also be tied to the newly acquired Eucla MT line and a new 1000 km NS MT profile extending north from the central Gawler Craton into the Arunta Province, NT.

Three-dimensional electrical resistivity image of the South-Central Chilean subduction zone

Based on isotropic 3-D inversion, we re-interpret long-period magnetotelluric data collected across the geotectonic structures of the South-Central Chilean continental margin at latitudes 38°–41°S and summarize results of long-period magnetotelluric (MT) investigations performed between 2000 and 2005. The new 3-D conductivity image of the South-Central Chilean subduction zone basically confirms former 2-D inversion models along three profiles and complete the previous results. The models show good electrical conductors in the tip of the continental crustal beneath the Pacific Ocean, the frequently observed forearc conductor at mid-crustal levels, a highly-conductive zone at similar levels slightly offset from the volcanic arc and a – not well-resolved – conductor in the Argentinian backarc. The subducted Nazca Plate generally appears as a resistive but discontinuous feature. Unlike before, we are now able to resolve upper crustal conductors (interpreted as magma reservoirs) beneath active Lonquimay, Villarrica, and Llaima volcanoes which were obscured in 2-D inversion. Data fit is rather satisfactory but not perfect; we attribute this to large-scale crustal anisotropy particularly beneath the Coastal Cordillera, which we cannot include into our solution for the time being.

Effects of near-surface inhomogeneities on MT responses: An analytical model

An analytical model is suggested to describe the electrostatic field produced by near-surface inhomogeneities responsible for galvanic shift in magnetotelluric (MT) apparent resistivity sounding curves. The near-surface inhomogeneities are modeled in thin-sheet approximation with laterally variable longitudinal conductance and transverse resistance. The model accounts for the TM (transverse magnetic) mode secondary electric field in the conductive layered subsurface below the thin sheet. Equations have been obtained to relate the subsurface geoelectric parameters and the spatial harmonics of the secondary electrostatic field. This secondary field, which is the source of galvanic shift in MT data, turns out to be in-phase with the primary field. The equations derived to simulate galvanic distortions are applicable to long-period MT data acquired by a synchronous array.

3D Modelling of magnetotelluric data acquired along the Youanmi deep seismic reflection transects in the Yilgarn, Western Australia

Geoscience Australia (GA) has been acquiring both broadband and long-period magnetotelluric (MT) data over the last few years along deep seismic reflection survey lines across Australia, often in collaboration with the States/Territory geological surveys and the University of Adelaide. Recently, new three-dimensional (3D) inversion code has become available from Oregon State University. This code is parallelised and has been compiled on the NCI supercomputer at the Australian National University. Much of the structure of the Earth in the regions of the seismic surveys is complex and 3D, and MT data acquired along profiles in such regions are better imaged by using 3D code rather than 1D or 2D code. Preliminary conductivity models produced from the Youanmi MT survey in Western Australia correlate well with interpreted seismic structures and contain more geological information than previous 2D models. GA has commenced a program to re-model with the new code MT data previously acquired to provide more robust information on the conductivity structure of the shallow to deep Earth in the vicinity of the seismic transects

The creation of a geophysical observatory at the Aleksandrovka research base of the geological faculty of Moscow State University in the Kaluga region

Since May 2011, long-period magnetotelluric (MT) data have been collected in the non-magnetic pavilion at the geophysical base of Moscow University in the Kaluga Region. The non-magnetic pavilion was constructed in compliance with the following conditions: selection of non-magnetic materials, control of local magnetic field anomalies, and setting the basement for magnetometers separately from the main building. Recording equipment with fluxgate and optomechanical magnetometers with three sets of electric lines with different electrodes was installed. The parallel testing of channels was performed. Currently, the seismo-logical equipment and meteorological station are being prepared for installation.

Crust and upper mantle electrical conductivity beneath the Yellowstone Hotspot Track

Combining long-period magnetotelluric data from the spatially uniform EarthScope USArray and higher-resolution profiles, we obtain a regional three-dimensional electrical resistivity model in the Snake River Plain and Yellowstone areas (Idaho and Wyoming, United States), and provide new constraints on the large-scale distribution of melt and fluids beneath the Yellowstone hotspot track. Contrary to what would be expected from standard mantle plume models, the electromagnetic data suggest that there is little or no melt in the lower crust and upper mantle directly beneath Yellowstone caldera. Instead, low mantle resistivities (10 Ωm and below), which we infer to result from 1%–3% partial melt, are found 40–80 km beneath the eastern Snake River Plain, extending at least 200 km southwest of the caldera, beneath the area of modern basaltic magmatism. The reduced resistivities extend upward into the mid-crust primarily around the edges of the Snake River Plain, suggesting upward migration of melt and/or fluid is concentrated in these areas. The anomaly also shallows toward Yellowstone, where higher temperatures enhance permeability and allow melts to ascend into the crust. The top of the conductive layer is at its shallowest, in the upper crust, directly beneath the modern Yellowstone supervolcano.

The North German Conductivity Anomaly revisited

SUMMARY The North German Conductivity Anomaly was detected already in the early years of electromagnetic deep sounding. It refers to the reversal of induction arrows (as a graphical representation of the tipper transfer function, the ratio of vertical to horizontal magnetic field variations) at the northern and southern margins of the North German Basin. In spite of the many experiments carried out so far, its origin has remained ambiguous; explanations encompass a deep-crustal or even mantle source as well as the simple response to deepening of sedimentary sequences in the centre of the basin. Here, we report on modelling of new long-period magnetotelluric data collected along two profiles in NE Germany and S Sweden, with one transect crossing the Sorgenfrei–Tornquist Zone as the main boundary between Precambrian Baltica and the Palaeozoic belts of central Europe. With the exception of a few sites probably influenced by 3-D salt domes, the data allow a 2-D analysis. Resolution is reduced for large depths due to a well-conducting, saline aquifer, extending across the entire basin. The main result is that the reversal of induction arrows can largely be explained by the resistivity contrast between crystalline basement and sedimentary basin fill. Beneath Rugen island, a southward dipping conductor is interpreted to reflect an alum shale layer in Middle Cambrian–Lower Ordovician sediments. Beneath the southern basin, a moderately conductive upper mantle is modelled (although not very well resolved) which may reflect the thinning of the lithosphere as implied by seismic tomography. As the main anomalously inductive effect is primarily explained by the basin edges and numerous other anomalies exist inside the basin, we suggest not using the term ‘North German Conductivity Anomaly’ any longer.

Deep electrical resistivity structure of the northern Gibraltar Arc (western Mediterranean): evidence of lithospheric slab break-off

Terra Nova, 23, 179–186, 2011 Abstract Uncertainties about the lithospheric structure of the Gibraltar Arc have led to the proposal of several contradictory models to explain its geodynamic setting. Herein, we present a novel 3-D model of the lithospheric electrical resistivity distribution beneath the whole Betic Cordillera obtained by inverting both broad-band and long-period magnetotelluric data. The lithosphere–asthenosphere boundary under SW Iberia is shown to be deeper than under the Alboran Basin. In addition, the sensitivity tests confirm the presence of a N–S oriented low-resistivity anomaly at lithospheric mantle depths east of the 4°W meridian. It coincides with an area of low velocities without earthquake hypocenters and is interpreted as asthenospheric material intruded by the lateral lithospheric tearing and breaking-off of the E-directed subducting Ligurian slab under the Alboran Domain. This scenario suggests that the opening of the Alboran Basin is related to the westward rollback of this E-directed subducting slab.

Joint inversion of long-period magnetotelluric data and surface-wave dispersion curves for anisotropic structure: Application to data from Central Germany

[1] Geophysical datasets sensitive to different physical parameters can be used to improve resolution of Earth's internal structure. Herein, we jointly invert long-period magnetotelluric (MT) data and surface-wave dispersion curves. Our approach is based on a joint inversion using a genetic algorithm for a one-dimensional (1-D) isotropic structure, which we extend to 1-D anisotropic media. We apply our new anisotropic joint inversion to datasets from Central Germany demonstrating the capacity of our joint inversion algorithm to establish a 1-D anisotropic model that fits MT and seismic datasets simultaneously and providing new information regarding the deep structure in Central Germany. The lithosphere/asthenosphere boundary is found at approx. 84 km depth and two main anisotropic layers with coincident most conductive/seismic fast-axis direction are resolved at lower crustal and asthenospheric depths. We also quantify the amount of seismic and electrical anisotropy in the asthenosphere showing an emerging agreement between the two anisotropic coefficients.

Deep deformation pattern from electrical anisotropy in an arched orogen (Betic Cordillera, western Mediterranean)

Long-period magnetotelluric data acquired in the Iberian Massif and the Betic Cordillera arched orogen provide the first evidence of electrical anisotropy in the upper mantle of the Mediterranean region. Strike analysis at different periods reveals preferred structure orientation related to olivine elongation in the mantle, and points to a heterogeneous anisotropy pattern. At deep levels (periods ≥104 s), all the sites show a common north-south geoelectrical strike (∼N170°E), which may represent a low-intensity deformation, possibly related to “frozen” prealpine plate tectonics. For periods between 10 and 103 s, a north-south constant strike (∼N180°E) at the Betic Cordillera sites contrasts with the east-west strike (∼N85°E) in the Iberian Massif. An increase in the magnitude of the induction arrows from the Iberian Massif to the inner part of the Betic Cordillera probably reflects higher deformation toward the axis of the Eurasian-African plate boundary. The integration of electrical anisotropy data with seismic anisotropy allows us to discuss mantle deformation patterns produced by delamination and subduction, suggesting that the latter mechanism may be more suitable for the alpine evolution of the western Gibraltar Arc.

Magnetotelluric evidence for thick-skinned tectonics in central Taiwan

Research Article| August 01, 2009 Magnetotelluric evidence for thick-skinned tectonics in central Taiwan Edward Bertrand; Edward Bertrand 1Department of Physics, University of Alberta, Edmonton, Alberta T6G 2R3, Canada Search for other works by this author on: GSW Google Scholar Martyn Unsworth; Martyn Unsworth 1Department of Physics, University of Alberta, Edmonton, Alberta T6G 2R3, Canada Search for other works by this author on: GSW Google Scholar Chih-Wen Chiang; Chih-Wen Chiang 2Institute of Geophysics, National Central University, Chung-Li, Taiwan Search for other works by this author on: GSW Google Scholar Chow-Son Chen; Chow-Son Chen 2Institute of Geophysics, National Central University, Chung-Li, Taiwan Search for other works by this author on: GSW Google Scholar Chien-Chih Chen; Chien-Chih Chen 2Institute of Geophysics, National Central University, Chung-Li, Taiwan Search for other works by this author on: GSW Google Scholar Francis Wu; Francis Wu 3State University of New York at Binghamton, Binghamton, New York 13902-6000, USA Search for other works by this author on: GSW Google Scholar Erşan Türkoğlu; Erşan Türkoğlu 1Department of Physics, University of Alberta, Edmonton, Alberta T6G 2R3, Canada Search for other works by this author on: GSW Google Scholar Han-Lun Hsu; Han-Lun Hsu 2Institute of Geophysics, National Central University, Chung-Li, Taiwan Search for other works by this author on: GSW Google Scholar Graham Hill Graham Hill 4Institute of Geological and Nuclear Sciences, Lower Hutt 5010, New Zealand Search for other works by this author on: GSW Google Scholar Geology (2009) 37 (8): 711–714. https://doi.org/10.1130/G25755A.1 Article history received: 30 Dec 2008 rev-recd: 20 Mar 2009 accepted: 24 Mar 2009 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Edward Bertrand, Martyn Unsworth, Chih-Wen Chiang, Chow-Son Chen, Chien-Chih Chen, Francis Wu, Erşan Türkoğlu, Han-Lun Hsu, Graham Hill; Magnetotelluric evidence for thick-skinned tectonics in central Taiwan. Geology 2009;; 37 (8): 711–714. doi: https://doi.org/10.1130/G25755A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Taiwan is the type example of an arc-continent collision. Numerous tectonic models have been proposed for this orogen, and include both thin-skinned and thick-skinned lithospheric deformation. These models predict very different structures at middle and lower crustal depths, but insufficient geophysical data exist to unequivocally distinguish between them. Long-period magnetotelluric (MT) data were collected in central Taiwan in 2006–2007 to constrain the crustal resistivity structure. A two-dimensional inversion of these MT data revealed a prominent electrical conductor that extends across the décollement predicted by the thin-skinned model. This feature is interpreted to be due to 1%–2% saline fluids, and is inconsistent with the thin-skinned model. In contrast, the thick-skinned model predicts this feature since fluids are generated in the crustal root through metamorphism. Quantitative correlation of the resistivity and seismic velocity models supports small-volume, high-salinity fluids in a thickened crust as the cause of this conductor. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Electrical structure of the upper mantle beneath Central Europe: Results of the CEMES project

In the years 2001–2003, we accomplished the experimental phase of the project CEMES by collecting long-period magnetotelluric data at positions of eleven permanent geomagnetic observatories situated within few hundreds kilometers along the south-west margin of the East European Craton. Five teams were engaged in estimating independently the magnetotelluric responses by using different data processing procedures. The conductance distributions at the depths of the upper mantle have been derived individually beneath each observatory. By averaging the individual cross-sections, we have designed the final model of the geoelectrical structure of the upper mantle beneath the CEMES region. The results indicate systematic trends in the deep electrical structure of the two European tectonic plates and give evidence that the electrical structure of the upper mantle differs between the East European Craton and the Phanerozoic plate of west Europe, with a separating transition zone that generally coincides with the Trans-European Suture Zone.

Lithospheric structure of the Arabia-Eurasia collision zone in eastern Anatolia: Magnetotelluric evidence for widespread weakening by fluids

Eastern Anatolia is the location of a young continent-continent collision between the Arabian and Eurasian plates. Long-period magnetotelluric data have been used to image the electrical resistivity of the crust and upper mantle in this region. The Anatolian block is being extruded to the west and is characterized by a low-resistivity (fluid rich) lower crust underlain by relatively normal upper mantle structure. The Anatolian Plateau has a lower crust that contains pockets of very low resistivity that may indicate local accumulations of melt. This is underlain by an upper mantle with an anomalously low resistivity that can be accounted for by an asthenosphere containing a few percent partial melt. The presence of fluids may weaken the crust and mantle sufficiently to permit lateral flow, and may also allow a decoupling of the upper and lower portions of the lithosphere. The lithospheric structure of the Anatolian Plateau is similar to that of the northern Tibetan Plateau, with zones of elevated fluid content. However, low resistivity in the Anatolian crust is found in isolated pockets, rather than the widespread regions observed in Tibet.

Flow and electrical anisotropy in the upper mantle: Finite-element models constraints on the effects of olivine crystal preferred orientation and microstructure

Long-period magnetotelluric (MT) data shows that electrical conductivity in the upper mantle is highly anisotropic. Agreement between high electrical conductivity directions and seismic anisotropy fast directions suggests that anisotropic diffusion of hydrogen along oriented olivine crystals controls the anisotropy of electrical conductivity in the upper mantle, since both seismic waves and hydrogen diffusion are faster along the [1 0 0] axis of olivine. Thus, MT electrical anisotropy data, like seismic anisotropy, may be used to map flow patterns in the upper mantle. However, observed electrical anisotropies are significantly higher than seismic ones. To quantify the influence of strain-induced crystal preferred orientations of olivine on upper mantle bulk electrical conductivities, we calculate the macroscopic electrical conductivity anisotropy of a series of naturally and experimentally deformed peridotites using an anisotropic finite-element model. These models, which fully take into account the microstructure: crystal and shape preferred orientations, based on orientation maps obtained by indexation of electron back-scattered diffraction (EBSD) patterns, show macroscopic electrical anisotropy factors ranging from 3 to 16. The intensity of electrical anisotropy depends to first order on the intensity of the olivine crystal preferred orientations, but the relation saturates for strong crystal preferred orientations. In addition, the spatial distribution of the various crystal orientations may significantly enhance influence the anisotropy in strongly textured mantle rocks. The strongest anisotropy factors (>10) occur in mantle rocks in which deformation by dislocation creep has produced not only crystal but also strong shape preferred orientations, even if the latter is masked by recrystallization. Higher anisotropy factors (>100) observed in a few MT experiments imply however an additional, as yet unknown, mechanism controlling electrical conduction at asthenospheric depths in these regions.

Deep electrical structure of the northern Cascadia (British Columbia, Canada) subduction zone: Implications for the distribution of fluids

Research Article| January 01, 2006 Deep electrical structure of the northern Cascadia (British Columbia, Canada) subduction zone: Implications for the distribution of fluids Wolfgang Soyer; Wolfgang Soyer 1 Institute of Geophysical Research, University of Alberta, Edmonton, Alberta T6G 2J1, Canada Search for other works by this author on: GSW Google Scholar Martyn Unsworth Martyn Unsworth 1 Institute of Geophysical Research, University of Alberta, Edmonton, Alberta T6G 2J1, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Wolfgang Soyer 1 Institute of Geophysical Research, University of Alberta, Edmonton, Alberta T6G 2J1, Canada Martyn Unsworth 1 Institute of Geophysical Research, University of Alberta, Edmonton, Alberta T6G 2J1, Canada Publisher: Geological Society of America Received: 11 Jun 2005 Revision Received: 04 Sep 2005 Accepted: 18 Sep 2005 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2006) 34 (1): 53–56. https://doi.org/10.1130/G21951.1 Article history Received: 11 Jun 2005 Revision Received: 04 Sep 2005 Accepted: 18 Sep 2005 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Wolfgang Soyer, Martyn Unsworth; Deep electrical structure of the northern Cascadia (British Columbia, Canada) subduction zone: Implications for the distribution of fluids. Geology 2006;; 34 (1): 53–56. doi: https://doi.org/10.1130/G21951.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Long-period magnetotelluric data have been used to image the deep electrical structure of the Cascadia subduction zone in British Columbia, Canada. Zones of elevated electrical conductivity were found in both the forearc and backarc regions and are interpreted as a consequence of the fluid release from subducting slab. A shallow zone of high conductivity beneath Vancouver Island is likely due to fluids that are trapped above the subducting plate. East of this structure is a conductive (∼0.03 S/m) forearc mantle wedge that also exhibits low seismic velocities and may be serpentinized. A free fluid phase is required to account for this enhanced conductivity. Elevated conductivities are observed in the upper mantle throughout the backarc (∼0.01 S/m) and strongly support the hypothesis of a shallow, convecting asthenosphere. This enhanced upper mantle conductivity can be explained by either hydrogen ion diffusion in olivine minerals, or by a few percent partial melting (<4%). You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Hydrogen diffusivity and electrical anisotropy of a peridotite mantle

SUMMARY Long-period magnetotelluric (MT) data have indicated that electrical conductivity in the upper mantle is highly anisotropic. Rates and anisotropies for self-diffusion of hydrogen in single crystals of mantle minerals are related to electrical resistivity by the Nernst‐Einstein relationship. Assuming that the dominant mechanism for electrical conduction in the mantle is hydrogen diffusion, the electrical anisotropy of a peridotite should be controlled by its mineral composition and by the lattice-preferred orientation (LPO) of its constitutive minerals. Macroscopic electrical anisotropies arising from diffusion of hydrogen in upper mantle rocks displaying strain-induced LPO of olivine, enstatite and diopside are calculated using resistor networks in which each resistor has a statistical probability of representing a mineral grain with a particular misorientation relative to the olivine [100] maximum density direction. The orientations of the grains are defined by angular distribution functions describing LPO (1) generated by viscoplastic self-consistent modelling at a range of shear strains and (2) measured in a naturally deformed peridotite. The naturally deformed peridotite displays a strong LPO, but the predicted mean electrical anisotropy factor is less than 3. Geophysical data indicate higher electrical anisotropies for the mantle. This suggests that grain boundary processes that are controlled by shape-preferred orientation of crystals and/or macroscopic heterogeneities further enhance the electrical anisotropy of the mantle. Ambiguities in the conduction mechanism highlight the need for direct laboratory measurements of ionic conductivities in mantle assemblages that can be compared with those calculated from the Nernst‐Einstein equation.

Magnetotelluric imaging of the fault rupture area of the 1999 İzmit (Turkey) earthquake

Wide-band (320– 0.001 Hz) and long period (0.01–0.0001 Hz) magnetotelluric (MT) data were acquired along two profiles crossing the western part of the North Anatolian fault zone (NAFZ), Turkey, which consists of two main fault branches. The first profile (İzmit profile) crosses the epicentral area of the 17 August 1999 İzmit earthquake. In fact, the MT measurements along this profile started just a few weeks before the occurrence of the mainshock and four instruments happened to be in operation in the vicinity of the fault when the earthquake took place. The second profile (Adapazarı profile) is located about 30 km east of the first profile. Two-dimensional modeling shows the following results. First, the hypocenters of mainshock and aftershocks are located on the highly resistive side near the edge of a conductive zone. Second, the long-period MT data show a low resistivity zone extending down to 50 km between the two fault branches. Such a deep conductive zone is interpreted as representing partial melting resulted from the past complex tectonics in this region, and it is related to the non-seismic after-slip in the layer below the seismogenic zone characterized by the highly resistive layer.

Sensitivity studies applied to a two-dimensional resistivity model from the Central Andes

SUMMARY Long-period magnetotelluric (MT) data have been collected along an E‐W profile across the Central Andes in Northern Chile and Southern Bolivia. A 2-D resistivity model explaining these data was found by inverse modelling. The model includes a prominent conductivity anomaly in the backarc of the South American subduction zone below the Bolivian Altiplano. Another, comparatively localized anomaly occurs beneath the Precordillera in the forearc. The enhanced conductivity in the backarc correlates with reflective zones, low seismic velocities and high seismic attenuation. In contrast, no correlation can be found in the forearc. A strong seismic reflector in context with the down-going slab, and another reflector in the middle crust below the Pre- and Western Cordillera do not coincide with anomalous conductivity. Other 2-D models may also be consistent with the data. We applied different techniques to find the range of relevant models. One simple but useful approach is the direct use of the sensitivity matrix containing the partial derivatives of the data with respect to the model parameters. The sensitivity of the model parameters to the data or data subsets can be visualized by calculating columnwise sums from the sensitivity matrix. These sensitivities may be used to indicate model parameters that are less resolved by the data and thus should not form part of an interpretation. Another approach is to perform systematic studies to establish the validity of geologically important features of the model. These studies comprise forward modelling and the use of a priori information. Combining all approaches generates a more complete assessment of the principal model features. We conclude that a minimum depth extent of the Altiplano conductor can be specified as approximately 50 km. Low sensitivity in the forearc beneath the Longitudinal Valley indicates a limited structure resolution, and therefore may explain the absence of a conductive slab in the model. The existence of the localized conductor below the Precordillera can be established by high sensitivity. Finally, we included the two seismic reflectors in the forearc as a priori information in the inversion process. As a result the absence of a conductive slab in the model is probably a result of low sensitivity, but cannot be totally excluded. A conductive zone coinciding with the strong reflector below the Pre- and Western Cordillera is not manifested by the MT data.

Electromagnetic measurements in the vicinity of the KTB drill site. Part II. Magnetotelluric results

In addition to the magnetovariational measurements across an array in Western Bohemia, close to the KTB ultradeep borehole (Germany), discussed in part I of this paper [1], magnetotelluric results from pivot sounding point Ostrůvek within the array are presented here. Good quality of long-period magnetotelluric data (period range from 30 s to about 1 hour) allowed structural dimensionality of the medium to be analysed in detail. The geoelectrical structure was identified as a slightly distorted two-dimensional regional substratum, with dominating E - W strike, overlaid by a heterogeneous subsurface layer with extremely strong and anisotropic galvanic distortion effect on the magnetotelluric data. Estimating the total static shift distortion tensor by fitting the local magnetotelluric curve to the curve of the global magnetovariational soundings (for the European continent), the static distortions were identified as of generally multidirectional origin. The resulting telluric ellipse is, however, strongly anisotropic, indicating an approximately SW - NE apparent local strike, which is in the approximate agreement with remote reference magnetovariational results. Finally, the magnetotelluric results from the station Ostrůvek are compared with long-period data from the immediate neighbourhood of the KTB borehole on the German territory.