Antonio L. Padilha

Characteristics of the Equatorial Electrojet determined from an array of magnetometers in N-NE Brazil

An array of 29 vector magnetometers was operated in N-NE Brazil from November 1990 until March 1991. We present the analysis of 16 selected quiet days, for which a simple model of an equivalent current distribution for the Sq and EEJ, fits the observed maximum amplitude of the daily variation at midday.In equatorial regions the precise latitude profile of the Sq field is masked by the EEJ. This uncertainty is resolved by assuming that the EEJ, obtained after subtracting the Sq from the daily ranges, should present a ratio of 0.3 for the westward to eastward current. With this constraint, a combined non-linear least squares inversion of Sq and EEJ was used to estimate the parameters of Onwumechili’s model of the EEJ current distribution. The H and Z components of the EEJ are jointly inverted and good agreement obtained between the calculated and observed data for all 16 days.The EEJ’s main parameters averaged for 16 quiet days were: A total positive current intensity equal to 67 ± 20 (103 A) for diurnal range M4 (or 80 ± 20 (103 A) for M3) and a half-width of 403 ± 67 km. The EEJ centre was located at 21 ± 16 km south of the dip equator. The Sq was estimated from several permanent observatories and found to be centred at a mean latitude of 5.5 ± 2 degrees south.

Global Pc5 geomagnetic pulsations of March 24, 1991, as observed along the American Sector

Analysis of the globally coherent Pc5 geomagnetic pulsation event of March 24, 1991, from a chain of stations extending from the auroral oval to the equatorial region along the American sector, has shown unequivocal evidence on the enhancement of pulsation amplitude in the narrow equatorial band centered at the dip equator. The fine spatial structure of this equatorial enhancement, documented observationally for the first time, emphasizes the importance of an ionospheric component associated with enhanced Cowling conductivity. The presence of an ionospheric component is also indicated at mid-latitudes by the sharp increase in the amplitude of pulsations, more dominantly in the Y than in the X component, on the sunward side of the dawn terminator. The sunrise effect also causes a reversal of the sense of rotation of the wave polarization across the dawn terminator. Propagation of the magnetospheric inducted polar electric field to low and equatorial latitudes through the ionosphere is invoked to account for the equatorial enhancement and the sunrise effect seen in these Pc5 pulsations.

Electromagnetic constraints for subduction zones beneath the northwest Borborema province: Evidence for Neoproterozoic island arc-continent collision in northeast Brazil

The Borborema province in northeast Brazil occupies a crucial position in the complex Neoproterozoic West Gondwana reconstruction puzzle. However, correlation attempts between northeast Brazil and West Africa have been hampered because key links in the internal structure of the Borborema province have yet to be identifi ed. To aid such an correlation, a magnetotelluric study was undertaken along two subparallel profi les to image the deep electrical structure in the northwestern part of the province. Despite the occurrence of recurrent tectonothermal episodes that affected the region in the past, two-dimensional models show that a large-scale signature of the assembled terrane during the Neoproterozoic accretion and collision is plausibly preserved in the area. Two resistive features dipping from the upper crust into the upper mantle in downward convergence (opposite directions) are defi ned beneath one of the profi les that are interpreted to be related to remnants of former subduction slabs, since the observed high-resistivity zone is consistent with a dehydrated oceanic lithosphere depleted of sediments. On the basis of geological and geochemical information, a model of collision of an intraoceanic magmatic arc coalesced into an earlier passive margin is proposed for the Neoproterozoic tectonic evolution of the province, involving processes of reversal of subduction polarity and oceanic slab breakoff.

2-D geoelectrical model for the Parnaiba Basin conductivity anomaly of northeast Brazil and tectonic implications

Abstract A magnetometer array study in the north-northeast of Brazil has revealed a roughly NE–SW-trending conductive structure in the southeastern part of the intracratonic Parnaiba Basin. The magnetovariational response functions of this structure are numerically modelled to constrain its geometry to facilitate its geological and tectonic interpretation. The 2-D numerical model that incorporates the ocean effect and can account for the spatial and period dependence of the observed response locates the source regions of enhanced conductivity in a graben structure in the basement as well as in a block confined to the central part of the basin with an embedded resistive body. The anomalous electrical character of the sediments in the central part of the basin is consistent with the magnetotelluric data, the graben structure in the basement is corroborated by the aeromagnetic data. The formation of the graben structure is considered to be a manifestation of the extensional tectonics associated either with the Brasiliano orogeny or with the Jurassic–Cretaceous magmatic events. The diabase dikes intruded in the basin in association with the Jurassic–Cretaceous magmatic activity are shown to be accountable for the mapped resistive body entrapped in the conducting Paleozoic sediments. The thermal effects associated with magmatic activities are invoked to produce enhanced conductivity by the generation of carbon through the pyrolysis of hydrocarbon-saturated sediments.

Upper crustal structure of the northeast Paraná Basin, Brazil, determined from integrated magnetotelluric and gravity measurements

Eight magnetotelluric (MT) soundings were performed as a continuation of an earlier audiomagnetotelluric (AMT) survey conducted in the northeastern border of the Parana basin, a large intracratonic basin located in central eastern South America and constituted by Silurian to Jurassic sedimentary rocks with Lower Cretaceous sill-type magmatic intrusions and overlying volcanics. Two of the MT stations were carried out near two deep petroleum wells. The remaining MT soundings were done on a profile traversing two important gravity features: a positive anomaly near the border of the states of Sao Paulo and Minas Gerais and a strong (trending NW-SE) linear gradient. Major findings of an integrated interpretation of the MT survey and of available gravity data are as follows: (1) inhomogeneities and/or strong magnetization of the superficial volcanics and diabase intrusions in the Paleozoic sedimentary rocks appearing to distort the MT results; and (2) identification of important structural discontinuities, including a possible different crustal structure beneath the Parana basin compared to the region on the north, a thickening of the crust toward the NE, and the probable existence of a trough (graben?) within the basin, characterized by a thick accumulation of sediments and basalts.

Geophysical constraints on tectonic models of the Taubaté Basin, southeastern Brazil

Abstract Magnetotelluric, gravity and geothermal flux data are analysed to constrain the tectonic evolution of the Taubate Basin, a SW-NE 20 km wide and 170 km long rift-like Tertiary basin in southeastern Brazil. The basin is characterized by half-grabens and contains up to 1000 m of sediments cut by normal faults. The magnetotelluric measurements show the presence of two layers: an upper highly conductive one (less than 10 Ωm from 1-D inversions), associated with Tertiary sediments, and an extremely resistive (more than 10000 Ωm) lower layer, related to the crystalline basement. Gravity anomalies in the region are ascribed to both near-surface sources, sediments and granites, and a deep-seated source associated with crustal thinning towards the continental margin. Available geothermal flux values within the basin are anomalously high; however, they have probably been affected by convective transport because they were determined in water boreholes. These results suggest a stable thermal regime at present and, viewed together with the absence of magmatism and thermal subsidence, indicate that the mechanism related to the basin formation did not originate a deep-seated thermal anomaly in the region. The event can be seen as a shallow one, involving a small area in the upper part of the crust. The region can be characterized typically as a strike-slip mobile zone, with horizontal displacements between juxtaposed blocks. A transtensional model is suggested for the Taubate Basin to explain some of its principal geological features.

Magnetotelluric soundings across the Taubaté Basin, Southeast Brazil

Thirteen magnetotelluric (MT) soundings were carried out in Neoproterozoic crystalline terrains and Tertiary sediments of the Taubaté basin, southeastern Brazil. The soundings were deployed in a cross-strike profile bisecting the basin along one of its thickest sub-basins and extending over mountain plateaus to the southeast and the northwest. Occurrences of numerous alkaline plugs and aligned tectonic grabens in the region are records of intense Mesozoic-Cenozoic tectonic activity. MT analysis techniques were used to evaluate data quality, infer strike direction, and correct for near-surface distortion. As the studied area is located only some tens of kilometers away from the Atlantic Ocean and in one of the most densely populated regions of the country, the data are severely distorted by industrial interference and the coast effect. Because of such effects, the data are modelled using a 2D inversion scheme within periods shorter than 0.1 s for the Taubaté Basin sites and shorter than 1–10 s for the off-basin sites, the latter depending on the distance of the site from the coast. The main result observed in the modelling is the identification of a conducting zone below 10 km depths beneath the region. There is poor resolution in the data of structures below this conductor, which is also not imaged beneath the basin. Studies carried out in different tectonic regions of the world have also reported conductive layers at about the same depth but in the studied area it is impossible to reach any conclusion about the total conductance of the layer with the available MT data.

Disturbances on magnetotelluric data due to DC electrified railway: A case study from southeastern Brazil

Magnetotelluric (MT) soundings were carried out in the period range of 20 to 6000 s along profiles roughly orthogonal to the Campos do Jordão Railway (CJRW), in the Brazilian southeastern region. The profiles were located over two adjacent regions with contrasting conductivity, the conductive sedimentary region of the Taubaté Basin and the resistive crystalline region of the Serra da Mantiqueira. The railway operates with DC current that produces an intense electromagnetic noise but only during diurnal periods, being turned off at night. The objective of this study is to characterize the CJRW noise in order to verify its effect on MT parameters. It was inferred that the entire length of the Taubaté Basin is probably affected by the noise, whereas in the crystalline terrains the noise reaches distances in the range of 76 to 126 km. The electric channels show a strong dependence on geology which is suggestive of the potential application of the CJRW as a controlled source in geophysical studies. The data were processed with modern techniques presently available to the scientific community. Under the conditions of the present study, it was observed that the robust Single Station technique is as efficient as the robust Remote Reference to remove the kind of noise generated by the CJRW, an intense perturbation that affects only some well-defined portions of the time series. Finally, the analyses reaffirm the necessity of a careful choice of the station to be used as reference in the Remote Reference technique.

Effects of the equatorial electrojet on magnetotelluric surveys: Field results from northwest Brazil

The distortion effect that the nonuniform equatorial electrojet (EEJ) currents might have on the plane-wave assumption in magnetotelluric (MT) appli- cations is investigated under experimental conditions. A broad-band (0.0006-2048 s) MT survey, consisting of 8 stations aligned at right angles to the magnetic equator and probing diverse geologic terrains, was car- ried out during the daytime, under the presence of the EEJ nonuniform source, and at nighttime, when the EEJ flux almost vanishes. A comparison between the daytime and nighttime soundings did not show any sig- nificant divergence, contrary to expectation from some theoretical calculations. Thus, the results assure that the traditional plane-wave formula employed in MT can provide reliable subsurface conductivity structures even under the influence of the EEJ, at least up to periods of 2048 s. An additional modelling exercise conside- ring the two models most commonly used to simulate the EEJ sources was also performed. It indicates that the line-current model generates departures from the plane-wave results not observed in.;practice and that the Gaussian model only agrees with the experimental data, within their error bars, if the geoelectric section approaches its most conductive limit defined from a sta- tic shift evaluation.

Imaging three‐dimensional crustal conductivity structures reflecting continental flood basalt effects hidden beneath thick intracratonic sedimentary basin

A large-scale array of long-period magnetic data and a deep-probing magnetotelluric profile were recorded in the intracratonic Parana sedimentary basin in central eastern South America, which presents a thick and extensive sedimentary-magmatic sequence that allows its basement to be investigated only by indirect methods. Integration of the results from both methods showed that the crust beneath the basin presents several quasi-linear highly conductive channeled zones with limited lateral extent, in coincidence with some of the main tectonic structures recognized at the surface, and a moderate but pervasive lithosphere conductivity enhancement beneath its central part. Upward movement of CO2-bearing volatiles and magmas precipitating highly conducting mineral phases along discrete subvertical fault zones that served as feeder conduits for Early Cretaceous voluminous continental flood basalts was a likely process responsible for the localized conductivity enhancements. Correlation between some of the linear conductive zones and elongated magnetic anomalies and between the maximum depth occurrence of most of these conductive anomalies and the Curie depth at which crustal rocks lose their magnetism gives strong support to interconnected iron oxides (especially magnetite) and iron sulfides (such as pyrrhotite) as the main conductive sources. The moderate bulk conductivity increase in the crust and upper mantle beneath the central part of the basin is unexpected for a postulated cratonic basement and is tentatively associated with impregnation of the lithosphere by conducting minerals related either to widespread tectonic events in the Ordovician or Late Precambrian or to dispersed magmatic residues of an Early Cretaceous magma differentiation contaminating the entire lithosphere.