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The magnetotelluric impedance tensor is analysed in the context of Clifford algebras. In this framework, the tensor is broken down into different parts, each one with a particular geometric algebra meaning, the simplicity of which allows us to deduce a number of known properties and opens up many other possibilities. As examples to show its capabilities, some of the algebraic relationships involving the impedance tensor, such as rotations, Mohr diagrams and phase tensor, are shown under this theory. Rotations are analysed in Clifford algebra Cl3 and Mohr diagrams, and phase tensor are expressed in Clifford algebra Cl2. The galvanic distortion matrix and its transformations are also seen in Clifford algebra Cl2, where a number of relationships allow us to recognize the galvanic distortion in a measured two‐dimensional/three‐dimensional impedance tensor. These relationships are useful as constraints to determine the galvanic distortion parameters.

The upper crustal structure of the central Cantabrian Mountains consists of a block of Variscan basement uplifted over the Duero basin as a consequence of a southward displacement along a major thrust during the convergence between Europe and Iberia in the Eocene-Oligocene (Alpine orogeny). We present the results of the 3-D modeling of thirteen new magnetotelluric sites, five of them being completed by long period data, over a 100 km-long, N-S oriented profile across the central Cantabrian Mountains and the Duero basin. Dimensionality analyses indicated a dominant E-W direction but with influence of 3-D structures at long periods and locally in the Cantabrian Mountains. Accordingly, we performed a 3-D joint inversion of the full impedance tensor and the geomagnetic transfer function following a sequential inversion workflow. The inverse model presents similarities with existing lithospheric models. In the southern part of the area, the conductive sediments of the Duero basin over a high resistive and homogeneous Iberian lithosphere are well delineated. Towards the north, beneath the Cantabrian Mountains, the model reveals a heterogeneous and conductive lithosphere, in which various elongated and dipping conductors in the upper and middle crust are associated with major Alpine thrusts, one being the frontal thrust of the Cantabrian Mountains over the Duero basin. At deeper depths (between 20 and 35 km depth), the Iberian crust appears as subducting to the north beneath a conductive zone interpreted as the hydrated mantle wedge of the north-Iberian continental margin.

One of the most remarkable natural events on Earth are the large lateral flank collapses of oceanic volcanoes, involving volumes of rock exceeding tens of km3. These collapses are relatively frequent in recent geological times as supported by evidence found in the geomorphology of volcanic island edifices and associated debris flows deposited on the proximal ocean floor. The Island of Fogo in the Cape Verde archipelago is one of the most active and prominent oceanic volcanoes on Earth. The island has an average diameter of 25 km and reaches a maximum elevation of 2829 m above sea level (m a.s.l.) at Pico do Fogo, a young stratovolcano located within a summit depression open eastward due to a large lateral flank collapse. The sudden collapse of the eastern flank of Fogo Island produced a megatsunami ~73 ky ago. The limits of the flank collapse were deduced as well from geomorphologic markers within the island. The headwall of the collapse scar is interpreted as either being located beneath the post-collapse volcanic infill of the summit depression or located further west, corresponding to the Bordeira wall that partially surrounds it. The magnetotelluric (MT) method provides a depth distribution of the ground resistivity obtained by the simultaneous measurement of the natural variations of the electric and magnetic field of the Earth. Two N-S magnetotelluric profiles were acquired across the collapsed area to determine its geometry and boundaries. The acquired MT data allowed the determination of the limits of the collapsed area more accurately as well as its morphology at depth and thickness of the post-collapse infill. According to the newly obtained MT data and the bathymetry of the eastern submarine flank of Fogo, the volume involved in the flank collapse is estimated in ~110 km3. This volume –the first calculated onshore– stands between the previously published more conservative and excessive calculations –offshore– that were exclusively based in geomorphic evidence. The model for the summit depression proposing two caldera collapses preceding the collapse of the eastern flank of Fogo is supported by the MT data.

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.

In SW Iberian Variscides, the boundary between the South Portuguese Zone (SPZ) and the Ossa Morena Zone (OMZ) corresponds to a major tectonic suture that includes the Beja Acebuches Ophiolite Complex (BAOC) and the Pulo do Lobo Antiform Terrane (PLAT). Three sub-parallel and approximately equidistant MT profiles were performed, covering a critical area of this Palaeozoic plate-tectonic boundary in Portugal; the profiles, running roughly along an NE–SW direction, are sub-perpendicular to the main Variscan tectonic features. Results of the three-dimensional (3-D) modelling of MT data allow to generate, for the first time, a 3-D electromagnetic imaging of the OMZ–SPZ boundary, which reveals different conductive and resistive domains that display morphological variations in depth and are intersected by two major sub-vertical corridors; these corridors coincide roughly with the NE–SW, Messejana strike–slip fault zone and with the WNW–ESE, Ferreira–Ficalho thrust fault zone. The distribution of the shallow resistive domains is consistent with the lithological and structural features observed and mapped, integrating the expected electrical features produced by igneous intrusions and metamorphic sequences of variable nature and age. The development in depth of these resistive domains suggests that: (1) a significant vertical displacement along an early tectonic structure, subsequently re-taken by the Messejana fault-zone in Late-Variscan times, has to be considered to explain differences in deepness of the base of the Precambrian–Cambrian metamorphic pile; (2) hidden, syn- to late-collision igneous bodies intrude the meta-sedimentary sequences of PLAT; (3) the roots of BAOC are inferred from 12 km depth onwards, forming a moderate resistive band located between two middle-crust conductive layers extended to the north (in OMZ) and to the south (in SPZ). These conductive layers overlap the Iberian Reflective Body (evidenced by the available seismic reflection data) and are interpreted as part of an important middle-crust decollement developed immediately above or coinciding with the top of a graphite-bearing granulitic basement.

SUMMARY A new 3-D inversion algorithm for the interpretation of magnetic total field anomalies is presented. Using Bayes' theorem the algorithm allows the inclusion of a priori information of model parameters and to treat this information as well as the measured data in a probabilistic manner. In this way a rather fast and stable method is obtained. The models consist of a system of vertical rectangular prisms with its top, base, susceptibility and remanent magnetization intensity and direction as variable parameters. Several synthetic examples show the possibilities that the algorithm offers. Finally the interpretation of an anomaly measured in NE Spain is presented, which shows a strong remanent magnetization component oblique to the local geomagnetic field.

The Iberian Chain is a Cenozoic intraplate thrust belt located within the Iberian plate. Unlike other belts in the Iberia Peninsula, the scarcity of geophysical studies in this area results in a number of unknowns about its crustal structure. The Iberian Chain crust was investigated by means of a NE-SW refraction/wide-angle reflection seismic transect and two magnetotelluric profiles across the chain, oriented along the same direction. The seismic profile was designed to sample the crust by means of three shots designed to obtain a reversed profile. The resulting velocity-depth model shows a moderate thickening of the crust toward the central part of the profile, where crustal thickness reaches values above 40. km, thinning toward de SW Tajo and NE Ebro foreland basins. The crustal thickening is concentrated in the upper crust. The seismic results are in overall agreement with regional trends of Bouguer gravity anomaly and the main features of the seismic model were reproduced by gravity modeling. The magnetotelluric data consist of 39 sites grouped into two profiles, with periods ranging from 0.01 s to 1000 s. Dimensionality analyses show significant 3D effects in the resistivity structure and therefore we carried out a joint 3D inversion of the full impedance tensor and magnetic transfer functions. The Mesozoic and Cenozoic basins along the Chain are well characterized by shallow high conductive zones and low velocities. Elongated conductors reaching mid-crustal depths evidence the presence of major faults dominating the crustal structure. The results from the interpretation of these complementary geophysical data sets provided the first images of the crustal structure of the Iberian Chain. They are consistent with a Cenozoic shortening responsible of the upper crust thickening as well as of the uplift of the Iberian Chain and the generation of its present day topography., Funding for the research was provided by the I + D + I research projects: BTE-2001-4993-E, CGL2008-04916, and by the Consolider-Ingenio 2010 programme, under CSD 2006-00041 “Topo-Iberia.”

The structure of the western termination of the Alpine Pyrenean-Cantabrian Orogen (NW Iberian Peninsula) has been under debate during the last years. We present new results derived from a new magnetotelluric profile that runs through the western Cantabrian Mountains and the León Mountains, together with gravity data, structural field data and petrological modeling that allows the crustal structure of the sector to be clarified. In the Cantabrian Mountains, the overall Alpine structure is a regional monoclinal flexure that corresponds to the back-limb of E-W trending and north-dipping thrust system. It is formed by a connected thrust-fault splay that overturns the Cenozoic sediments of the Bierzo basin, probably rooted in a detachment at 15 km depth. Further to the South, in the León Mountains, we propose a "pop-up" geometry characterized by high-angle dipping reverse faults that extend toward the lower crust probably causing uplift and the erosion of the early foreland basin. Apatite fission-track and paleontological data suggests that uplift and exhumation of the León Mountains occurred later than in the Cantabrian Mountains. Uplift started in the Middle and Upper Miocene in the León Mountains linked to the activity of the highlighted "pop-up" structure whereas it took place during the Oligocene in the Cantabrian Mountains., This work has been supported by the project Consolider-Ingenio TOPOIBERIA 2010 CSD2006-041.

The western part of the Iberian Peninsula (Iberian Massif) is the best exposed fragment of the Variscan orogen in Europe. Its southern half was generated by an oblique collision between three continental terranes belonging to the margins of Laurassia (Avalonia) – the South Portuguese Zone (SPZ) – and Gondwana – the Ossa Morena Zone (OMZ) and the Central Iberian Zone (CIZ). The boundaries between them are considered to be suture zones. A 200 km long magnetotelluric profile across the three Variscan terranes was done in a NNE direction, approximately perpendicular to the main tectonic features. The results of two-dimensional inversion of the MT dataset reveal high-conductivity zones coinciding with the transitions SPZ/OMZ and OMZ/CIZ. These conductive bodies related to the sutures at depth were interpreted as graphite enrichments along shear planes formed due to the overall transpressive regime. A high-conductivity layer extending along the whole OMZ was found at a depth of 15–25 km, the top of which spatially correlates with a broad reflector detected by a recently acquired deep seismic reflection profile. The high conductivity was interpreted as caused by the Precambrian Serie Negra graphite-rich rocks. Carbon and oxygen X-ray mapping with electron microprobe on polished sections of Serie Negra samples from OMZ revealed the presence of interconnected graphite, which supports the hypothesis that graphite is determinant for the high conductivity. Two graphite types, which help to record the geological evolution, were identified: graphite accumulations in the schistosity surfaces produced by folding and metamorphism, and metallic films of graphite developed along late faults. The conductive layer shows blobs of higher conductivity suggesting macro-anisotropy. Additional mylonitisation and shearing produced by thrusting at depth can be the origin of these zones of enhanced conductivity, given that the detachment level is located within the Serie Negra. Several high-resistivity features were found in the upper crust, related to Devonian and Carboniferous successions and probably to some unexposed plutons in the SPZ and the Palaeozoic series of OMZ plus some granitic intrusions. In the CIZ, a high-resistivity zone extending to the whole crust is correlated with extensive late Variscan granite intrusions.

SUMMARY A study of synthetic anisotropic models that explain phases exceeding 90 ◦ in magnetotellurics is presented. The basic model comprises an anisotropic layer overlain by a shallow (local) anisotropic block, with both structures inserted in a 2-D model. The 2-D strike and the anisotropy strikes (layer and block) differ. The influence of the following parameters was analysed: anisotropy strike, geometry of the block and the layer, and anisotropy ratios of the block and the layer. We show that, according to this model, the anomalous phase effect is limited to those sites above the shallow block and does not influence the regional structure, which can therefore be recovered. These results were applied to field data from a magnetotelluric profile in SW Iberia where phases greater than 90 ◦ occurred in an area in which alternating bands of schist and graphite-rich blackschists crop out, giving rise to strong macroanisotropy.

A new integrated three-dimensional (3D) gravity and geoid forward modeling was used to investigate the lithospheric structure of the northwestern Mediterranean basins, namely the Gulf of Lyons, Liguro-Provençal Basin, Valencia Trough, and South Balearic Basin. Available geological and geophysical data were used to construct preliminary maps of the thickness of the sediments and the base of the crust and lithosphere, which were further refined by 3D modeling. Our model results show that the base of the crust thins from 22 km in the SW areas of the Valencia Trough to less than 12 km at its NE transition to the oceanic domain of the Provençal Basin and from 28 km in the shelf area of the Gulf of Lyons to about 12 km in the deep oceanic part of the basin. Upper mantle densities vary from 3240 kg/m3 beneath the Valencia Trough (where P-wave velocities are anomalously low, suggesting the presence of an anomalous uppermost mantle), to 3320 kg/m3 beneath the Gulf of Lyons and the Provençal Basin, where 'normal' P-wave velocities are recorded. The base of the lithosphere rises from ∼75-80 km at the shoreline of the Valencia Trough and Gulf of Lyons to 60 km in the northern part of the Valencia Trough and 55-60 km at the center of the Provençal Basin. The structure of the crust and lithospheric mantle obtained from the 3D modeling indicates differential thinning. Moreover, other geophysical observations, e.g., the uneven distribution of heat flow values, suggest an asymmetric extensional model. © 2003 Elsevier Science B.V. All rights reserved.

Recent results obtained from two-dimensional inversion of magnetotelluric (MT) data acquired in SW Iberia reveal high-conductive features at the middle–lower levels of the crust. The top of these anomalous structures correlates very well with the depth (10–13 km) of an important seismic interface that has been interpreted as a regional detachment horizon. Very shallow and relatively narrow conductors in the Ossa Morena Zone appear to correspond to small-scale fluid-deposited graphite systems in the preorogenic metasedimentary sequences. Some of the midcrustal conductors can be ascribed to graphite-bearing thrust zones, the formation of graphite with variable crystallinity being a consequence of Variscan shearing processes. Deep-seated conductors are tentatively interpreted as a result of relatively continuous, highly crystalline grain-boundary graphite films presumably preserved in basement, granulite(?) rocks. Assuming that graphite occurs as interconnected films, calculations indicate that a fraction of 0.006–0.02% of this accessory mineral is enough to explain the range of the electrical resistivity estimated on the basis of MT models. The role of graphite on the thermal behaviour of the crust is also discussed. The results show that low contents of graphite do not significantly change the thermal behaviour of earth materials.

Summary In this paper we show some of the effects that appear in magnetotelluric measurements over 2-D anisotropic structures, and propose a procedure to recover the anisotropy using 2-D inversion algorithms for isotropic models. First, we see how anisotropy affects the usual interpretation steps: dimensionality analysis and 2-D inversion. Two models containing general 2-D azimuthal anisotropic features were chosen to illustrate this approach: an anisotropic block and an anisotropic layer, both forming part of general 2-D models. In addition, a third model with dipping anisotropy was studied. For each model we examined the influence of various anisotropy strikes and resistivity contrasts on the dimensionality analysis and on the behaviour of the induction arrows. We found that, when the anisotropy ratio is higher than five, even if the strike is frequency-dependent it is possible to decide on a direction close to the direction of anisotropy. Then, if the data are rotated to this angle, a 2-D inversion reproduces the anisotropy reasonably well by means of macro-anisotropy. This strategy was tested on field data where anisotropy had been previously recognized.

The magnetotelluric signature of the Alpine part of the Cantabrian Mountains was investigated in a N-S magnetotelluric profile, from the Duero basin to the Cantabrian shoreline. A two-dimensional conductivity model was obtained from the joint inversion of TM, TE modes and the geomagnetic transfer functions. A shallow conductor associated with the sediments of the Duero basin, and three north-dipping conductors in the upper and middle crust were detected. Of the latter, one is interpreted as the sole detachment of the Cantabrian Mountains, whereas the others correlate with the Cabuerniga and San Vicente de la Barquera faults. This geometry suggests that the Cantabrian Mountains comprise uplifted Variscan basement. A comparison with a seismic reflection profile shows a good correlation between these conductors and seismic features. The lower crust is characterised by high conductivities.

The coast effect on magnetic and magnetotelluric data acquired near the SW coast in the Iberian Peninsula was investigated using three-dimensional modelling. The ocean was modelled as a layer with constant conductivity (0.3 ohm m) and thickness of 3.5 km. The continental resistivity model was constructed taking into account its electrical structure obtained from previous magnetotelluric studies. The model shows that the ocean-coast effects are important for periods greater than 100–300 s (in this case), affecting both the induction vectors and the impedance tensors. A method for the correction of the coast effects on the impedance tensor is presented. Synthetic data generated from a model including coast effects were corrected using the method and compared successfully with data not showing those effects. As an example the correction has been applied to some magnetotelluric soundings carried out in SW Iberia.

Forty-one magnetotelluric soundings were carried out along a 200 km-long profile (approximately NNE-SSW) across the three major geotectonic units in SW Iberia. A model obtained from two-dimensional inversion of the magnetotelluric data set reveals high conductivity zones in the middle-lower crust (10–30 km). Two of these zones correspond to the transition between the main geotectonic units: one between the South Portuguese Zone and the Ossa Morena Zone, interpreted as having been caused by metasediments, and the other between the Ossa Morena Zone and the Central Iberia Zone associated with a shear zone and metasediments. Another high conductivity anomaly related to black shales with major graphite impregnation was detected within the Ossa Morena Zone. The resistive features, located preferentially in the upper crust (1–10 km), coincide with gabbroic and granitic complexes.

Many types of mixing model are used widely within the earth sciences to determine the electrical properties of porous media consisting of solid and fluid phases of known conductivities, volume fractions and distributions (i.e. phase connectivities). Most models are valid for two or more conducting phases. However, of the simple models only Archie’s law includes a variable term, the Archie cementation exponent m, that describes the connectivity of the phases. Unfortunately, Archie’s law is only valid for one conducting phase distributed within a non-conducting phase, which makes it inapplicable in instances where the rock matrix has a significant conductivity such as for clay-rich rocks and in calculations involving partial melting. More complex models exist which account for multiple conducting phases and control over phase conductivity. We have adapted the conventional Archie’s law to produce a simple modified Archie’s law that can be used with two conducting phases of any conductivity and any volume fraction, while retaining the ability to model variable connectivities within those phases that result from variations in their distribution. The modified model has two exponents (m and p) that describe the connectivity of each of the two phases. The exponents are related through an equation that depends also on the volume fractions of the two phases. The modified and the conventional versions of Archie’s law have been tested on a granular analogue porous medium with a conducting matrix and a pore space saturated with a range of saline fluids with different salinities and conductivities. The new model describes the experimentally determined electrical behaviour of the system extremely well, improving greatly on the conventional Archie’s law.

Recent magnetotelluric (MT) studies have shown that the lower crust in the Pyrenees contains a high conductivity zone consistent with a subducting continental slab, whose conductivity is 0.33 S/m. Partial melting has been interpreted to be the most plausible explanation for this high conductivity. Here we report a two-dimensional conductivity model of the lithosphere by integrating field-scale and laboratory determinations of the conductivity of continental crustal and mantle rocks. The laboratory data provide empirical formulas which allow us to determine the fluid saturated rock and melt conductivity when temperature, pressure and lithology are known. Consequently, we have also calculated the density, lithostatic pressure, and several alternative temperature profiles for use in the model from gravity, seismic and thermal field data. These can be used with a prescribed melt fraction to predict the electrical conductivity at depth, which can be compared with the MT conductivity data. Alternatively, the laboratory data can be combined with the MT conductivity data to predict the melt fraction at depth. The primary outputs of the modelling are conductivity and melt fraction prediction profiles for six mixing models; (i) Waff’s model/Hashin–Shtrikman (HS) upper bound, (ii) HS lower bound, (iii) parallel layers, (iv) perpendicular layers, (v) random melt areas, and (vi) a modified Archie’s law that takes account of the presence of two conducting phases. The modelling results indicate that a good match to the MT data can be obtained along the whole profile by the influence of pressure, temperature and the fluid phase with the only exception being the subducted slab, where a minimum of 4.7% melt fraction is necessary to explain the data.

A 2D modeling of geoid/quasigeoid data across the Pyrenees is consistent with electromagnetic images of the Pyrenean lithospheric structure, showing a subducted Iberian lower crust. Using new magnetotelluric data acquired through the orogen, a 3D electrical conductivity model of the Pyrenean lithospheric structure is obtained. The most outstanding feature is the presence of a high conductivity zone at lower crustal depths along the orogen in the contact zone between the Iberian and European plates. The integration of various geophysical parameters suggests that the simplest and most reasonable mechanism to explain the observed data is partial melting of the subducted Iberian lower crust.

The transition between the South Portuguese Zone (SPZ) and the Ossa Morena Zone (OMZ) is made up of a major geosuture, which is indicated by the Beja-Acebuches ophiolite. Several geological data suggest that this suture is the result of an oblique collision, with a northward propagation, between the SPZ and OMZ. From the structural point of view the OMZ is much more complex than the adjacent Central Iberian Zone and the SPZ. The frequently complicated internal structure is the result of superposition of various structural elements developed during different tectonic events. With the aim of providing new constraints to this complex deep structure a magnetotelluric (MT) survey was designed by the universities of Lisbon and Barcelona in the scope of the bilateral cooperation and the Europrobe programs. A first NNE–SSW profile across the SPZ and OMZ was carried out in September 1997. The profile is 40 km long and consists of nine deep MT soundings with periods ranging from 0.0039 to 4000 s. The time series were processed using a robust algorithm, after visual inspection. The study of the dimensionality and directionality of the regional electrical structures is based on the analysis of the induction arrows and the Groom–Bailey decomposition. A strike of N125E was determined, in accordance with regional geological features. 2-D inversion was undertaken on both TE and TM modes using the RRI method. The main feature of the model is a large conductive body (

Magnetotelluric data with periods of up to 2000 s were collected and the conductivity structure of the crust and upper mantle was imaged in the Central Betic chain. Interpretation of the impedance tensor components and the geomagnetic transfer functions was made in terms of a two-dimensional resistivity model along a NW–SE profile from the Guadalquivir foreland basin and across the boundary between the External and the Internal Betics. Elongated high conductivity zones in the upper and middle crust related to fluid circulation along both sediments and faults were detected. The conductors in the upper crust coincide with the areal distribution of the Cenozoic and Mesozoic sedimentary rocks from the Guadalquivir basin and the External Betics and the conductors in the middle crust with the major Miocene extensional detachments affecting the basement rocks of the Internal Betic zone. A high conductivity zone at lower crustal levels beneath the outer Internal Betics was also detected next to an area where earlier seismic reflection data had imaged a duplication of the reflective Moho. This conductor was interpreted as partial melting of a southeast dipping subducted Iberian lower crust. The geodynamic significance of this subduction depends mainly on whether the overthrusting lower crust belongs to the Alboran domain or to the Iberian plate. In the former case it would correspond to a major plate boundary and in the latter to an intraplate feature.

Several two-dimensional structures are modelled for vertical electrical soundings in arrays parallel and perpendicular to the strike of the structure. The models are a horst and a graben within a three-layer medium, a cliff over two layers, and heterogeneities around the electrodes. Apparent resistivity curves are shown for different model parameters and different distances to the two-dimensional structures. Some of the features on the shape of these curves are inflections that may be misinterpreted as fictitious layers, of slopes greater than 45°; some features are simply anomalous peaks. One-dimensional interpretation of a two-dimensional graben model has been performed, in order to show typical errors when 2D structures are interpreted as one-dimensional. A real case corresponding to a landfill near Barcelona and showing strong lateral and topographic effects is presented. This represents combined effects of the above theoretical 2D models.

SUMMARY An MT profile which roughly coincides with the south side of the ECORS-Pyrenees deep reflection seismic profile was made from the North Pyrenean zone through the central Pyrenees and Ebro basin. The periods recorded ranged from 0.0128s to 2000 s. Analysis of MT data shows a 2-D conductive structure. The strike direction of the structure is E-W, parallel to the Pyrenean chain. Interpretation of the data includes geomagnetic transfer functions and is constrained by some relevant geometric crustal features observed in seismic data. Accordingly, a 2-D resistivity cross-section has been obtained. The most striking results are: (1) detection of subvertical conductors in the upper crust coinciding with major fault zones; (2) detection of a deep, high-conductivity zohe at the contact between the Iberian and the European plates: (3) the fact that the European plate is thicker and more resistive than the Iberian one. Interpretation of the deep high conductivities suggests the existence of partial melting in the Pyrenees related to the subduction of the Iberian plate beneath Europe.

A combined interpretation of magnetotelluric and gravity data recorded onland Mallorca is presented. The magnetotelluric data consist of five stations along a NW-SE profile across the island which periods ranging from 256 Hz to 200 s. An electrical model has been obtained taking into account the 3D sea effect. The gravity data consist of the Bouguer anomaly from which the residual contribution of the sedimentary formations has been obtained. A 3D modelling of this residual anomaly shows a good agreement with the MT model. The results of the combined interpretation of the gravity and magnetotelluric data provide constraints to the crustal structure of the island which is representative of the whole Balearic promontory.

A magnetotelluric profile through the Central Pyrenees indicates the presence of very high conductive zones at lower crustal and upper mantle depths. High conductivity at upper mantle depths is interpreted as partial melts within subducted lower crust, whereas high conductivity at lower crustal depths is interpreted as rising magmas derived from the melting crustal slab. Such melts suggest that the continental lower crust, together with its lithospheric mantle may have been subducted into the mantle during the Pyrenean continental collision. Thus, magma generation can be related to thermal reequilibration of a subducted lower crust, a scenario that may serve as a model for understanding the late evolution of other collisional orogens.

[1] Melange rocks outcrop widely in the central and western frontal sectors of the Betic Cordillera, as in many other collisional orogens where they form part of the accretionary wedges. Extrusion of Triassic plastic clays and evaporites was favored by the progressive accretion of the Betic External Zones, mixing rocks of different provenance and forming a synorogenic frontal melange unit. MT data coupled with gravity data are a valid combined methodology to characterize the geometry of these melange units, since the characterization of plastic rocks geometry is usually uncertain using seismic techniques. The results correlate well with known geological features (sedimentary basins, calcareous ranges, evaporitic rocks) and reveal the deep geometry. A resistive body, slightly dipping toward the SE, points to the continuity of the Iberian Massif below the Guadalquivir basin (2°) and the External Zones (6–8°). To the south, gravity models show the Iberian continental crust subducting below the Internal Zones with a roughly 20–35° slope. The main conductive bodies are related to the location of evaporitic rocks involved in the frontal melange. They overlie the Iberian Massif and, southwards, the frontal Jurassic and Cretaceous limestone sequences of the External and Median Subbetics. In this setting, thick plastic rock units placed above the foreland could act as a lubricant facilitating continental subduction, and being progressively accreted toward a frontal melange.

Scientific discussion and different points of view are a basis of the advancement of knowledge. We acknowledge the comments of Jones et al. [2012] as an opportunity to publicly discuss the structure and origin of the Atlas Mountains. Moreover, we welcome the opportunity to compare our results with those recently published by the group responsible for the comment [Ledo et al., 2011], although it is not pertinent to comment in detail on a paper published in another journal. We also wish to remark that the paper of Ledo et al. [2011] was reviewed and published during the revision period of our contribution [Anahnah et al., 2011]; therefore, they are two different approaches and data sets, measured in different sites and by different instruments for the same region, lending readers the chance to compare different interpretations. The main differences on the data sets are: the profile of Anahnah et al. [2011] compared with the profile of Ledo et al. [2011] is 170 km longer, vertical magnetic data were obtained and lower frequencies were recorded, Departamento de Geodinámica, Universidad de Granada, España, Instituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Científicas, España, Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada, España, Département de Géologie, Université Mohammed V, Marruecos, Unidad de Granada, Instituto Geológico y Minero de España, España, Departament de Geodinàmica i Geofísica, Universitat de Barcelona, España, Institute of Geological & Nuclear Sciences Limited, Nueva Zelanda, Géosciences Montpellier, Université Montpellier, Francia, Département de Géologie, Université Abdelmalek Esaadi, Marruecos, Centro de Geociencias, Universidad Nacional Autónoma de México, México

A NE-SW magnetotelluric 110 km-long profile including 18 sites was acquired across the western RifCordillera along the Eurasian-African plate boundary, allowing to constrain its poorly known deepstructure. It extends from the Internal Zones, close to the Alboran coast, crossing the External Zones andup to the Gharb foreland basin. The periods recorded range from 0.001 s to 1000 s. The combination ofmagnetotelluric data with available geological data provides new insight regarding the relationship

The Atlas Mountains are characterized by high elevations and Quaternary volcanism. Long period magnetotelluric data acquired along a NNW-SSE transect reveal the presence of a conductive anomalous mantle below the High Atlas. Data dimensionality analyses show a preferent N80 degrees E strike of the deep resistivity structure in agreement with the induction vector alignment at long periods. Accordingly, a 2D inversion of the data set was carried out. Large resistive bodies at the crustal basement most likely correspond to batholiths emplaced in more conductive metapelites. They are covered by outcropping conductive sedimentary detritic and carbonate rocks. Lithospheric thinning producing anomalous mantle and basin development in the Atlas probably started during Triassic-Jurassic rifting. Inversion tectonics since the Oligocene produced low shortening on previous lithospheric weak zones, with thrusting of the Atlas above the stable African plate. Melting at the top of the anomalous mantle is connected with Quaternary basaltic volcanism in the Middle Atlas.

International audience; The Iberian Massif is the best exposed segment of the European Variscan Belt. It includes relatively well preserved terranes that were accreted by transpression along time and resulted in a number of geotectonic units that formed part of the Late Paleozoic assembly of the Pangaea Supercontinent. In SW Iberia, these units are the Central Iberian Zone (CIZ), Ossa Morena Zone (OMZ), and the South Portuguese Zone (SPZ). A 210 km long NE-SW magnetotelluric profile was carried out through the CIZ, from the OMZ-CIZ boundary toward the north, reaching the Tagus (Cenozoic) basin. Data dimensionality analysis resulted in a suitable 2-D electrical resistivity structure, allowing a 2-D inversion of the data set. Complementary available geophysical data (deep seismic, gravity and aeromagnetic) and a comparison with a detailed geological cross section led us to constrain the interpretation of the 2-D electrical resistivity structure of the CIZ crust. The results show, for the upper crust, the existence of diverse conductive/resistive bodies that correlate well with known geological features (sedimentary basins, faults, granitic plutons, mineralized systems). A mild but steady conductive band is located along the middle and lower crust that is interpreted as a mafic granulite basement. The upper section of this band connects with several elongated shallow conductors, providing further evidence for the existence, in the Central Iberian Zone, of a complex decollement system where the major faults are rooted. Such a crustal architecture is viewed as the northward continuation of the Variscan large-scale structures previously recognized in the southern sectors (OMZ and SPZ).

Summary An algorithm which allows the combined 3-D inversion of magnetic total field and gravimetric data is presented. Model parameters which include top and base of vertical rectangular prisms, susceptibility, remanent magnetization with its inclination and declination and density can be given as a priori information. the joint inversion serves to reduce the inherent ambiguity, mainly of gravimetric models, in cases when it can be assumed that a magnetic and a gravimetric anomaly are caused by the same source rock and, in general, allows a fast modelling of potential field data. Different synthetic examples are provided to show some of the possibilities that this algorithm offers. Finally, a couple of anomalies measured in southwestern Germany are modelled using this algorithm.

SUMMARY An inversion algorithm which takes into account the varying geometry in magnetotelluric models is considered. The particular problem of estimating the shape of the interface between two different conductivities is studied. This boundary is represented by an implicit function and its parameters are calculated in the inversion process. The assumption of using such a function allows us to simplify the computation of the sensitivity matrix of the model, and gives a stronger numerical stability to the inversion procedure. The finite element method is used to solve the forward problem. The inversion algorithm is based on a Bayesian approach that allows the introduction of a priori information. Several examples with simple geometries show the possibilities offered by the algorithm.

An algorithm for two‐dimensional electrical resistivity modeling using the finite‐element method with mixed boundary conditions can calculate the electrical potential along any arbitrary direction. In the particular case of the direction parallel to the strike of the structure, a numerical singularity occurs. We resolved it by calculating the potential near the singularity and improving the involved interpolation of the transformed potential. Investigating the shape of potential in the Fourier transformed space, we conclude that this interpolation may combine logarithmic and exponential functions.

Seismic and electric data obtained in a recent geophysical survey place new constraints on the structure at depth of the Olot region, the youngest Quaternary volcanic area of the Iberian Peninsula. The upper crust under the Olot Volcanic Zone (OVZ), Southern Pyrenees, was investigated by a dense network of seismic refraction profiles. The profiles running along the main axis of volcanism show P-velocities less than 4 km/s for up to 400 m under the OVZ. Higher velocities are observed in the Eocene sediments, 5.3 km/s in the east and 4.6 km/s in the west of the OVZ. Vertical electrical soundings clearly define the volcanic rocks. Resistivities higher than 1,000 Ω.m are typical of pyroclastics, and values above 10,000 Ω.m are found in basaltic flows less than 140 m thick. An Eocene substratum identified at the base of the soundings is characterized by resistivities of between 150 and 500 Ω.m. The seismic basement, characterized by a velocity of 6.0 km/s, was observed at 2 km depth, with a lateral continuity deepening to the southwest. Velocities higher than 6.6 km/s are suggested in the OVZ below 5 km depth. The particle motion analysis of individual seismograms has provided independent constraints from S-phases and PS-phases converted at the basement-sediment discontinuity. In the OVZ, no indications of the existence of anomalous bodies in the upper 5 km of the crust have been found. A local anomalous zone evidenced at the eastern edge of the OVZ deserves further study. © 1991., The data presented here have been gathered in an experiment supported by the Commission of European Communities under its Geothermal Energy Programme, contract n ° EN3G-0074-E.

[1] In recent years a number of magnetotelluric (MT) surveys have been carried out in the southern branch of the Iberian Variscan Massif, namely in the Ossa Morena Zone (OMZ) and its neighboring areas, the South Portuguese Zone (SPZ) and the Central Iberian Zone (CIZ), in order to determine the geoelectrical structure of this transpressive orogen and gain insight into its internal architecture. New MT data collected across the OMZ complete a dense database that covers the whole belt and its boundaries. This paper presents an integrative three-dimensional model of the available MT data from the apparent resistivities (four components), impedance phases and geomagnetic transfer functions from 61 stations. Given the location of the study area, near the Atlantic Ocean, coast effect on the data was analyzed as well as the stability of the mesh due to the strong resistivity contrasts. A mesh of 80 × 77 × 76 cells was finally used, with an average cell size of 4 × 4 km in the horizontal direction, and increasing thickness in the vertical direction from 25 m in the shallower layers to 5 km in the deepest ones. The most striking feature is, in addition to the conductive anomalies associated with the sutures between the SPZ-OMZ and the OMZ-CIZ, a conductive layer which spans the whole Ossa Morena middle to lower crust. The top of this conductive layer is interpreted as a major decollement between the granulitic basement and the overlying meta-sedimentary pile.

A simple algorithm to solve inverse geophysical problems is presented. It uses a statistical modelling of the forward problem in the presence of an unknown disturbing parameter vector with nonlinear dependence between their components and the model field. The nonlinear dependence is investigated directly in the parameter space and a particular linear approximation is made. Thus, a nearly optimal solution is obtained. The quality of the estimates is statistically predicted, this allows a rational selection of the most informative observation points. The method is especially suitable for many typical inverse problems in geophysical prospecting. A gravity exploration example is presented.

A simple method to contour local inhomogeneities using seismic data is proposed. It formalizes an approximate inversion method which is based on the interpretation of local inhomogeneities as making the differences between an actual seismic data set and a previous reference model. It uses the optimal statistical criteria of parameter estimation and recognition and the ray representation of the waves spreading. Any combination of direct, reflected and/or other types of waves may be used as the database. Inhomogeneities, having a size two times above the wavelength of the seismic waves, can be resolved. Laboratory experiments, using ultrasonic waves and analysis of data from field experiments, confirmed the theoretical results. The method can be used to search for ore bodies, kimberlite cubes, oiltraps, etc.

The Las Cañadas caldera in Tenerife (Canary Islands) is a well-exposed caldera depression in which the active Teide–Pico Viejo complex stands. In addition to its volcanological interest, the Las Cañadas caldera also holds the main groundwater reservoir of Tenerife. An audiomagnetotelluric and magnetotelluric survey was carried out in order to image the interior of the caldera depression. The field campaign consisted of 33 audiomagnetotelluric sites in the period range from 0.001 to 0.3 s and 11 magnetotelluric sites from 0.004 to 200 s. A detailed mapping of the electrical conductivity of the subsurface was obtained. For the long periods a three-dimensional modelling of the island – including the bathymetry – was carried out to study the effect of the ocean. This effect starts to be important at periods longer than 10 s. Accordingly, the sites were arranged into six profiles and a two-dimensional joint inversion of all data until 10 s was performed for each profile. The geometry of the high conductive zones found indicates that the caldera includes two closed depressions in the western (Ucanca) and central (Guajara) sectors, whereas in the eastern sector (Diego Hernández) the top of the main conductive zone shows a gentle inclination towards the northeast (La Orotava valley). A narrow and marked high conductive anomaly, probably caused by the presence of highly fractured rocks and fossil hydrothermal alteration along a fracture zone, runs parallel and close to the present caldera wall, suggesting the position of the structural border of the caldera. The results indicate that there are two main aquifer zones separated by the Roques de García spur, coinciding with the western and central depressions. The eastern depression could be hydrologically disconnected from the central depression by another structural limit not visible at the surface. The saturation zones would have a thickness of more than 700 m starting at a few hundred metres below the present caldera floor. The data are consistent with a multiple vertical collapse origin for the caldera depression rather than a sector collapse origin.

[1] Thirty new MT soundings were carried out in SW Iberia allowing the investigation of the lateral continuity of some major structures and lithological features that are believed to control decisively the deep architecture of the Ossa Morena Zone belts. Results of the two-dimensional inversion of the data reveal high-conductivity zones in the middle-lower crust, interpreted as the signal of a granulitic basement with grain-boundary interconnected films of graphite. The resistive features, located preferentially in the upper-middle crust, are ascribable to igneous complexes of variable age.

[1] Deep seismic reflection data acquired as part of the SW-Iberia EUROPROBE project across the transpressional Variscan orogen sample three tectonic terranes: the South Portuguese Zone, the Ossa-Morena Zone, and the Central Iberian Zone. The seismic data reveal the existence of a mid-crustal reflective body 140 km long and of variable thickness (up to 5 km), the Iberian Reflective body. The conductivity image provided by coincident MT soundings, the amplitude characteristics of the seismics, mineralization studies related to magmatic ore deposits, and the surface geology suggest that the IRB is a mantle-derived mafic intrusion. The geophysical, geological and petrological data suggest that the IRB is most probably an Early Carboniferous (approximately at 350–340 Ma) mantle-derived intrusion possibly linked to plume activity that took place in Europe in the Carboniferous and Permian.

16 pages, 5 figures, The combined analysis of magnetotelluric measurements, tomographic velocity models and deep seismic reflection images confirms that the Betics orogen consists of the juxtaposition of two crustal domains characterized by distinctive physical properties. At depth these data sets show evidence for a non-coincidence of the petrological and the seismic Moho beneath the Betics chain. The data sets reveal the geophysical properties of the Alboran domain (Internal Betics) and the Iberian Massif (External Betics). According to this, the Iberian crust features a relatively high seismic velocity, is seismically transparent in the seismic reflection images and is electrically resistive. The Alboran domain crust is characterized by a low average velocity, displays high reflectivity in the seismic reflection images and is electrically conductive. The outcrops of the metamorphic complexes (Alpujárride and Nevado Filábride), showing relatively high velocities coupled with low Vp/Vs values (1.67) derived from the Wadati slopes, suggest the existence of rocks rich in silica beneath the Alboran domain crust. An interpreted detachment at 12 km depth imaged by deep seismic reflection suggests that these rocks could be related to the Iberian upper crust. Partial melts and fluids are proposed to explain the high conductivity observed at deep crustal levels. These would account also for the reflectivity and the low Vp/Vs ratios mapped beneath the Alboran domain. © 1998 Elsevier Science B.V. All rights reserved

Gravimetric, DC-multi-electrode, Electromagnetic EM34 and VLF measurements were carried out on two profiles.

SUMMARY An analytical solution and its application to compute 2-D geoid anomalies, based on Brun’s formula (Heiskanen & Moritz 1967), was developed by Chapman (1979). In this paper we present the derived solution from a thorough review of Chapman’s algorithm. Apart from some typographical errors in the mathematical expressions, we have found that some of the precautions given by the author are not necessary when the algorithm is applied, in particular those regarding the unit system, topography and density contrast. Recent studies have shown that geoid anomalies, when used with other geophysical anomalies, are able to constrain plausible models of lateral density variations at depth and therefore to determine the Earth’s internal structure. Chapman’s algorithm (1979), based on Stokes’ theorem, is a tool used to calculate 2-D and 3-D geoid anomalies produced by a given body. However, due to the restrictions imposed, the algorithm is limited to the modelling of marine tectonic structures. The goal of this research note is to extend the application of Chapman’s algorithm and to fix some typographical errors found in its formulation. The geoid anomaly for any arbitrary 2-D body is

Geoscientists are becoming increasingly interested in the crustal structure of the Western Mediterranean Sea. Here, is located the island of Mallorca, part of the Alpine system linked with the Betic range. This island is formed by two Neogene imbricated thrust systems stacking Mesozoic rocks (Serra de Tramuntana along the NW coastline, and Serres de Llevant along the SE coastline) with a central part constituted by small post-orogenic basins.

In the eigenvalue eigenvector decomposition used in inversion techniques, we can sometimes obtain eigenvalues close to zero. Related to these eigenvalues close to zero, a set of stratified earth models is generated. These are compatible with the theoretical observations according to a given model. The method is applied to ellipticities and phase velocities of Rayleigh waves, and supply proof of the advantage of working with both sets of data together.

An inversion algorithm for interpreting electrical soundings based on a probabilistic treatment of the a priori information not only includes all the previous ones, but allows consideration of the constraints between the parameters. By introducing the a priori information, a unique solution among all the equivalent ones is obtained which is coherent with the geological background. Several examples dealing with the usual problems in the automatic interpretation of the electrical soundings illustrate the advantages of this algorithm. Good results are obtained with this method.

Cerdanya is a Neogene half-graben basin that lies over the Palaeozoic rocks of the Eastern Pyrenean Axial Zone. Subvertical fault scarps bound the half-graben at the southern and southeastern margins, but Neogene sediments may directly overlap Palaeozoic rocks in the northern margin. Vertical electrical soundings show that the Miocene sequence reaches a maximum thickness of over 700 m and that fine-grained lacustrine deposits from the central part of the basin grade to coarser fluvial deposits toward the margins. Macro- and micro-structural data analyzed in Cerdanya and other related Neogene basins, such as Rossello, Confient, Capcir and Seu d'Urgell, suggest that they formed as a consequence of transtensile motion along an old northeastern-trending fault. The stress situation favoured sinistral movements along east-trending faults.

An alternative inversion method is proposed, the error function of which evaluates an average between the slopes of apparent resistivity more accurately than the classical one of points. Convergency is achieved by this procedure in cases of initial models for which classical error functions fail to converge. The generation of equivalent models with respect to the slopes is also discussed.