Eugénio P. Almeida

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.

Magnetotelluric survey of the electrical conductivity of the crust across the Ossa Morena Zone and South Portuguese Zone suture

Abstract 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 (

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.

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.

Application of Time-Domain Electromagnetic Method in Investigating Saltwater Intrusion of Santiago Island (Cape Verde)

Santiago Island, the biggest and most populated island of the Cape Verde Republic, is characterised by limited surface waters and strong dependence on groundwater sources as the primary source of natural water supply for extensive agricultural activity and human use. However, as a consequence of the scarce precipitation and high evaporation as well as the intense overexploitation of the groundwater resources, the freshwater management is also in a delicate balance with saltwater at coastal areas. The time-domain electromagnetic (TDEM) method is used to locate the extent of saltwater intrusion in four important agricultural regions in Santiago Island; São Domingos, Santa Cruz, São Miguel, and Tarrafal. The application of this method in Santiago Island proves it to be a successful tool in imaging the fresh/saltwater interface location. Depths to the saline zones and extensions of saline water are mapped along eight TDEM profiles.