Luı́s A. Mendes-Victor

Are some of the deep crustal conductive features found in SW Iberia caused by graphite

Abstract 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.

Electromagnetic imaging of a transpressional tectonics 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.

A hydrogeological investigation using EM34 and SP surveys

EM34 and SP surveys were used to delineate shallow structures associated with the mineral water springs in the Vilarelho da Raia area located NE Portugal. This spring is part of a set of CO2-rich mineral (hot and cold) waters connected to the main Hercynian NNE-SSW fault systems. The EM34 survey was interpreted using a quasi-three-dimensional inversion approach based on a smooth-regularisation algorithm. The model put into evidence the conductive overburden as well as the fractured granitic formation. A preliminary estimation of the aquifer porosity is made based on the EM34 model. The SP anomalies were interpreted considering as having their sources on the aquifer interfaces. The modelling of two SP profiles allow the characterisation of a fault connected to the spring.

Coast effects on magnetic and magnetotelluric transfer functions and their correction: application to MT soundings carried out in SW Iberia

Abstract 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.

An audiomagnetotelluric survey over the chaves geothermal field (NE Portugal)

Abstract In an attempt to define the resistivity model of the Chaves geothermal field in NE Portugal, a detailed survey with scalar audiomagnetotelluric measurements was performed. The soundings were made in the frequency range from 2300 to 4.1 Hz. Electrical resistivity models were derived from the application of 1-D inversion, 2-D trial and error modeling and 2-D inversion procedures. The resistivities inside the geothermal field are low, reaching not more than 30 Ωm and increasing up to 60–150 Ωm beneath the shallow geothermal reservoir. The conductance in the low resistivity zone (20–35 S) suggests that the geothermal field is behaving as a local conductivity anomaly, with predominant 2-D features. The low resistivity structure is associated with sedimentary and metamorphic rocks saturated with highly mineralized water and with fractured zones allowing upward movements of hydrothermal fluids. The range of porosity (13–27%) of the formations associated with the geothermal field was estimated using the modified Archie law.

Magnetotelluric observations over the Chaves geothermal field (NE Portugal)—Preliminary results

Abstract On the Portuguese mainland the most important low enthalpy field occurs in the graben of the Chaves region, where magnetotelluric measurements have been carried out to determine the crustal electrical conductivity distribution. From the impedance tensor decomposition two distinct regional directions (N-S and N65° ± 10°E) have been found. These directions are consistent with the geology and related to the main faults. The apparent resistivity and phase curves derived from the determinant of the impedance tensor were used to obtain 1-D and 2-D interpretations. The regional models indicate the existence of a conductive layer at a depth between 7 and 12 km (resistivity in the range of 100–300 Ω m, from 1-D modelling and 60 Ω m from 2-D model). The 2-D interpretation provides a first estimate of the regional geothermal gradient (28°C km −1 ) in the Hercynian granite. The 1-D models from the soundings located within the graben suggest that local tectonics could play a significant role in the CO 2 extraction and in migration of the deep CO 2 , as indicated from isotopic data.

Magnetotelluric study of a plio-quaternary tectonic depression: / the Vilarica basin NE Portugal

The Vilarica basin, located northeast Portugal astride a major late-Variscan NNE-SSW reactivated strike-slip fault, is an excellent example of interplate neotectonic activity whose development has been mainly interpreted as a result of left-lateral displacement. Thirty magnetotelluric soundings were carried out in the Sta Comba da VilaricarSampaio region northern of . the tectonic basin in order to investigate the internal structure of the basin and its relationship with the main tectonic faults. Distortions of the impedance tensors were studied using Groom-Bailey decomposition technique. The predominant regional . strike N26E is in good accordance with exposed geology and can be explained for the reactivation of previous structures. Using two-dimensional inversion, three resistivity cross sections were obtained at north, center and south of the studied area. . The graben is revealed as a low resistivity 20-100 V m structure due to the sedimentary filling. The increasing electrical . resistivity from 400 to 4000 V m at depths greater than 2 km is related to the Hesperian basement rocks. The main faults, which controlled the formation and evolution of the basin, are revealed by resistivity gradients within the upper crust. q 2000 Elsevier Science B.V. All rights reserved.

Three-dimensional magnetotelluric modelling of the Vilariça depression (NE Portugal)

Abstract The Vilarica depression is located astride the northern branch of a Late Variscan (NNE–SSW) left-handed strike–slip fault known as the Vilarica Fault Zone (NE Portugal), whose reactivation in Pliocene–Quaternary times is well documented. The magnetotelluric method was applied in order to evaluate the electrical structure of the northern part of this tectonic depression. Thirty magnetotelluric soundings were carried out and a three-dimensional model was constructed. Different segments of the Vilarica Fault Zone, as well as the earlier Variscan thrusts preserved in the studied area, are well displayed in the model. Low-resistivity (30–200 Ω m) structures were revealed in the central part of the depression up to a depth of 1 km. These shallow conductors are mainly associated with the sedimentary infillings of the basin and are bordered and/or interrupted by strongly weathered and fractured blocks of metasediments displaying variable electrical resistivity. Deeper domains show a relatively homogeneous high resistivity (3500 Ω m), being difficult to distinguish the nonfractured and nonweathered metamorphosed autochthonous metasediments (at depths above 2 km) from the gneissic basement. A conductive layer (500 Ω m) was detected at a depth ranging from 8 to 12 km; this feature is tentatively associated with an important crustal detachment that may correspond to the root domain of the major active strike–slip faults in NW Iberia.

On the applicability of joint inversion of gravity and resistivity data to the study of a tectonic sedimentary basin in Northern Portugal

Abstract The Chaves basin is a pull-apart tectonic depression implanted on granites, schists, and graywackes, and filled with a sedimentary sequence of variable thickness. It is a rather complex structure, as it includes an intricate network of faults and hydrogeological systems. The topography of the basement of the Chaves basin still remains unclear, as no drill hole has ever intersected the bottom of the sediments, and resistivity surveys suffer from severe equivalence issues resulting from the geological setting. In this work, a joint inversion approach of 1D resistivity and gravity data designed for layered environments is used to combine the consistent spatial distribution of the gravity data with the depth sensitivity of the resistivity data. A comparison between the results from the inversion of each data set individually and the results from the joint inversion show that although the joint inversion has more difficulty adjusting to the observed data, it provides more realistic and geologically meaningful models than the ones calculated by the inversion of each data set individually. This work provides a contribution for a better understanding of the Chaves basin, while using the opportunity to study further both the advantages and difficulties comprising the application of the method of joint inversion of gravity and resistivity data.