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Magnetotelluric (MT) data from 49 broadband and 23 long period stations, covering periods between 10−3 and 104 s, were measured along a NNW-SSE profile of approximately 470 km, crossing the Parnaíba Basin, Borborema Province and São Francisco Craton in northeastern Brazil. The geotectonic scenario of the study region comprehends a complex structural framework formed during the Brasiliano orogenic collage. The MT data were processed to obtain the impedance tensor estimates for each MT site using a robust technique and then followed by a dimensional analysis via both the rotational invariants (WAL invariants) of the magnetotelluric tensor and the phase tensor. The two approaches indicated a three-dimensional nature of the region, allowing to proceed to 3D inversion, despite the data distribution along a profile. Several inversion trials of the full impedance tensor, tipper and tipper combined with impedance tensor resulted in a final 3D resistivity model, which suggests a division of the profile into three major zones. A sub horizontal resistive crustal block, interpreted as being part of the Parnaíba block, characterizes the western zone. A mosaic of resistive and conductive zones, which integrates the extended fold belt of the Borborema Province, composes the central zone. In the interface between the central and the eastern zones, there is a sub vertical conductive region limiting two resistive blocks, interpreted as a suture zone. This suture divides tectonic provinces and is supposed to be related with a Neoproterozoic collisional system. The 3D MT imaging recovered the structural framework under the Parnaiba Basin and marked the main lineaments. The paper suggests a distinct tectonic evolution for the complex assemblage of blocks in Northeast Brazil. • The 3D MT imaging allowed drawing a crustal model that recovers existing structures in great depth along the study region. • The 3D MT imaging recovered the structural framework under the Parnaiba Basin and marked the main lineaments. • The paper suggests a distinct tectonic evolution for the complex assemblage of blocks in Northeast Brazil. [ABSTRACT FROM AUTHOR]

In this paper we present the results of an investigation on the electric crustal structures depicted by a magnetotelluric (MT) transect cutting through the easternmost sector of the Alta Floresta Gold Province, in the SW Amazon Craton, Brazil. The MT dataset was acquired along a 200 km NNE-SSW transect encompassing 35 broadband and 12 long-period stations, covering a period range of approximately 10 −4 to 10 4  s. We adopted a data processing scheme based on robust estimators over the electromagnetic cross-spectra for combined single and remote reference stations in order to estimate horizontal and vertical transfer functions. The dataset was inverted using a 3-D MT based minimum structure algorithm, considering the data dimensionality, directionality analysis and the irregular station distribution. A set of inversions considering different model parameters have been performed to achieve a model with robust main features and to test the stability of the obtained solutions. A resolution study was also proposed in order to evaluate the average depth of model sensitivity. The electrical structures depicted can improve the understanding of the ancient tectonic features encrypted at the cratonic stable crust, contributing to elucidate the mechanisms that might have controlled its ore genesis. Conductors in the mid-crust correlate with shallow mafic enclave-rich granitic and foliated volcanic facies outcrops, linking the geoelectric anomalies to a possible mid-crustal source correlated to the Statherian magmatism, which is believed to be the main pulse related to gold metallogenesis at this province. [ABSTRACT FROM AUTHOR]

In this work, the Iguatu basin in Northeastern Brazil was imaged by 49 broadband (~0.01–1,000s) magnetotelluric (MT) and transient electromagnetic (TEM) soundings displaced on a 3D arrangement effected by a NE-SW along-basin major axis profile and three NW-SE transversally oriented profiles. The Iguatu basin is an intracratonic sedimentary basin, with genesis related to the opening of the South Atlantic Ocean (160-115 Ma). The basin was implanted over Precambrian basement of the Borborema Province. The sedimentary rocks deposited from Early Cretaceous to Quaternary include medium to coarse sandstones with conglomerate levels at the bottom, siltstones, mudstones, followed by Tertiary similar sedimentary rocks and Quaternary unconsolidated alluvium coverings. The MT responses (impedance and tipper) were robustly estimated and static-shift corrected using transient electromagnetic (TEM) measurements. Both rotational invariant and phase tensor dimensionality analyses revealed a complex subsurface structure, characterized by 2D/1D structures for short period data (<1 s), associated to sedimentary rocks within the basin, and a 3D behavior for longer periods, related to crustal depths. Three-dimensional inversion of full impedance was performed yielding a preferred 3D resistivity model with nRMS error of 1.99, which recovered important geoelectrical structures within the basin and crust. Two remarkably low resistivity anomalies (<1 Ω m) were observed in the crust of the Borborema Province, a feature also observed other MT studies elsewhere in the province. They can be ascribed to either the Mesozoic reactivation of the Aiuaba-Orós-Jaguaribe shear zone or the percolation of aqueous fluids between the zones of weakness presented in the fractured basement beneath the Iguatu basin; or even be a combination of both mechanisms. The resistivity model varies appreciably within the basin alternating low resistivity (<1 Ω m) graben-like and resistive (>300 Ω m) horst-like features, reflecting a structurally complex rift tectonic system of this ~800 m deep pull apart basin. A good correlation between resistivity values and shallow unconfined aquifers were spotted. • 3D MT model of Iguatu basin in NE Brazil revealed prominent resistivity variations for basin and upper crust structure. • Highly conductive anomalies beneath Iguatu Basin can be associated to shear zones Mesozoic reactivation or fluids percolation. • Good correlation between conductivity well log values and the MT imaging suggest areas of potable water. [ABSTRACT FROM AUTHOR]

The interaction between the São Francisco Craton and its surrounding belts during the Gondwana formation produced an intricated structural framework with superposition of tectonic events. This work aims to infer the subsurface geoelectric structures along a cryptic suture between the Brasília and Ribeira belts in order to verify tectonic limits between crustal blocks. A three-dimensional data inversion using the ModEM code was applied in 83 magnetotelluric sites of broad band and long period with remote reference. The processing was done by the robust method and with analysis of dimensionality and directionality through the phase tensor and induction vectors, respectively. Finally, an inversion model was generated, defined in a homogeneous mesh (103 × 103 x 45) and 1000 Ω. m of 2.3 × 2.3 km (x, y) even including the ocean. The full impedance tensor data were inverted and after 224 iterations an adjustment of 1.98 was achieved. The modelled MT North and South profiles show distinct geoelectric frameworks at NW and SE segments. To the NW thin-skinned tectonics is evidenced by low angle dipping conductors below the tectonic domains of the Brasília Belt and the interference zone with the Ribeira Belt. To the SE of the MT profiles the MT model shows two resistive blocks and a continuous conductor that follows a detachment fault below the Cenozoic Taubaté Basin. We interpret the limit between the different geoelectric profile segments as the suture between Brasília and Ribeira Belts. The limit matches the first Ribeira Thrust in the North MT profile or the Buquira shear zone in the South MT profile. The Precambrian geometry of resistive blocks and conductors is modified in the Amparo Antiform by reverse and normal faults with vertical displacement reaching thousands of meters. Deep crustal and mantle geoelectric structures were also evidenced by our 3D MT modelling: a) high conductive domains with resistivity around 400 Ω. m in the middle and lower crust; b) deep vertical conductors (DVC) with roots at the base of the lithosphere. [ABSTRACT FROM AUTHOR]

We present a resistivity model for the boundary of the Paleozoic Parnaíba Basin and the Cretaceous Sanfranciscana Basin (NE Brazil). The boundary underlies the Neoproterozoic Rio Preto belt, whose basement consists of reworked Archean and Paleoproterozoic rocks. In order to characterize the subsurface geoelectrically, 17 broadband magnetotelluric (MT) soundings were acquired and distributed along a ∼90 km profile across Rio Preto belt. The transfer functions were obtained with a robust code, thus obtaining reliable results for all four components of the impedance tensor. Some apparent resistivity and phase curves were shown to be heavily contaminated by local electromagnetic noise at low frequencies, and others showed minimal radio signals associated with the deadband frequency range. In this study, the MT data set was interpreted using 2D and 3D inversion procedures. Comparing the two MT models, the crustal-scale structure is well resolved in the 2D model, but the 3D model explains significantly better the measured data related to deep/shallow structures and proved to be more sensitive to conductive anomalies. The final resistivity image derived from the 3D inversion denotes a resistive basement with vertical conductive anomalies in the SE portion and a conductive domain in the NW portion. The homogeneous conductive block corresponds to the Archean domain of the Rio Preto belt, and the heterogeneous resistive block has the morphology and physical properties of the Paleoproterozoic block. Lateral geological correlation and gravity data support that the basement of the Parnaíba Basin in the study area is formed by the Archean block of the Rio Preto Belt. The boundary between the resistive blocks is a reverse fault that is also the tectonic front of a sub-horizontal detachment fault. In the Paleoproterozoic block, the conductors are associated with graphitic and volcano-sedimentary sequences, the dome and keel structures explain the geometry of the conductors. Additionally, graben-likes features were revealed in this block, interpreted as Cambrian basins filled by Cretaceous sediments of the Urucuia and Areado formations. These structures have a good correlation with those defined in the eastern edge of the Parnaíba Basin in previous studies. Towards the NW end of the section, the resistive blocks abruptly turn into a high conductivity domain (HCD) covered by a shallow conductive layer of Parnaíba Basin sandstones that modifies the electrical properties of the basement by interaction of basal fluids. • A crust resistivity model for the boundary of the Parnaíba Basin and the Sanfranciscana Basin from magnetotelluric inversion. • The resistivity image denotes a conductivity and a resistivity domain. • A Bouguer anomaly constraint that the conductivity domain corresponds to the Archean Basin, beneath the Parnaíba Basin. • A series of conductors located into the resistivity domain interpreted as metavolcano sedimentary sequences. • Grabenform structures were performed and interpreted as Cambrian Basins. [ABSTRACT FROM AUTHOR]

In this study, broadband magnetotelluric (MT) data are acquired from 101 stations in the southeastern edge of the Parnaiba basin in NE Brazil. The MT stations are distributed in 10 semi-parallel profiles, with NW-SE orientation. In order to characterize the internal structure of this area, we propose a geoelectric resistivity model by applying the three-dimensional (3D) inversion of this data. In this way, the time series of the horizontal electric and magnetic fields are processed using a robust technique to evaluate the impedance tensor. The dimensionality analysis that was performed indicates a predominant 3D behavior, and the full tensor components were inverted. The preferred model verified the presence of low-resistivity areas located in the upper crust. The interpretation of these areas is in relation to geodynamic events that happened in the three domains of the Borborema Province: northern, central, and southern. The low-resistivity region located in the northern domain shows that there is an association with graphite precipitation, as defined by the metamorphism in the western zone of the Seridó belt. We propose that the conductive region in the central domain is explained by the reactivation of the Pernambuco shear zone. In the southern domain, it was associated with serpentinization processes by the hydration of mafic and ultramafic rocks from the Brejo Seco complex in the context of subduction in the Neoproterozoic. In addition, the 3D resistivity model revealed the graben-like structures with predominate NE-SW direction observed in previous works. Likewise, upper crust discontinuities were shown to be associated with the Senador Pompeu and Pernambuco shear zones, which were defined previously using aerogravimetric and aeromagnetic data. • We studied the southeast edge of Parnaiba basin through the 3D inversion of magnetotelluric data. • The results showed graben-likes structures along the southeast border of the basin. • The 3D inversion models also showed low-resistivity regions in the upper-mid crust along the southeast edge of the basin. • The geophysical models showed a structural complexity with crust anomalies, like shear zones. [ABSTRACT FROM AUTHOR]