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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 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 southeastern Brazilian onshore-offshore margin and underlying lithosphere are investigated by three-dimensional magnetotelluric imaging using 202 high-quality broadband and long-period amphibious MT-dataset, orthogonally traversing the regional surface deformation hills of Serra da Mantiqueira and Serra do Mar, as well as the coast-parallel onshore-offshore lineaments. Three-dimensional conductivity model reveals an electrically heterogeneous crust (up to 40 km) in the onshore Brasília and Ribeira Orogenic Belts, and ∼ 2–10 km thick-conductive-sedimentary-wedge beneath the offshore Santos Basin laying on top of a stretched and thinner resistive crust. Regionally, the lithosphere is found ∼130 km thick beneath the most northwestern part of the Brasília Orogenic Belt which is thinning out towards the deep-water Santos Basin (∼70 km). This thinning coincides with the lithosphere-asthenosphere boundary in recent seismic tomography results, reflecting the current state of the uplifted asthenosphere post-West Gondwana break-up. A major steeply dipping sub-lithospheric conductor, in conjunction with a confined asthenosphere upwelling (or lithospheric thinning), has been observed underneath Embu-Paraíba do Sul domain (Ribeira belt), which may be somehow associated with the continental scale rifting, surface uplift and the Cretaceous São Paulo-Rio de Janeiro dykes swarm. Our findings provide valuable insights into the formation of southeast Brazilian highlands and its association with mantle plume-induced lithospheric delamination (or removal) and the occurrence of alkaline magmatism, after reactivation of some pre-existing weak zones. • The crust is thick and electrically heterogeneous in the Brasília and Ribeira belts and becomes relatively thin and uniform in the offshore margin. • The opening of the South Atlantic Ocean raised the lithosphere towards the offshore Santos basin. • Mantle-plume-driven lithospheric delamination has been observed underneath the highlands of southeastern Brazil. [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]

A magnetotelluric (MT) survey was conducted on the Caldas Novas geothermal reservoir located in the state of Goia´s, Central Brazil. The region of Caldas Novas is a popular tourist spot because of the occurrence of hot water springs. The purpose of the first MT survey in this area is to provide more information on the geoelectrical structure of this important geothermal reservoir. The MT method is a frequency domain technique that utilizes naturally occurring magnetic and electric signals as source to obtain a resistivity map of the subsurface. Since temperature and permeability are some of the factors controlling electrical resistivity, MT is widely utilized for surveying geothermal areas such as Caldas Novas. Data were acquired along two profiles crossing the Serra de Caldas (Caldas Mountains) with a total of 25 MT stations. Frequencies of acquisition were in the range from 0.008 to 176 Hz. Spacing between stations were usually around 5 km. Apparent resistivity and phase data from the transverse electric (TE) and transverse magnetic (TM) modes were computed for both profiles. Very high (100,000 Ω m) apparent resistivity values in the TM mode indicate distortion, possibly caused by 3-D resistive structures. In this work, we focus on Profile 2, which was acquired aligned at N40°E, approximately following the direction of weakness N50°E. Two-dimensional (2-D) inversion suggests a basin-like model with very high-resistivity block structures associated with concentric faulting below the Caldas Mountains inside a resistive basin that extends to depths of approximately 25–30 km. This model is in good agreement with gravity data and the available geological information in the area and can help delineate areas of new geothermal reservoirs. The high resistivity associated with the depression in the MT model can be interpreted as caused by the low-permeability quartzites that form the Caldas Mountains. Although the survey configuration does not allow for a full three-dimensional (3-D) interpretation, a 3-D resistivity model was constructed and the MT responses computed. The 3-D synthetic data explains the behavior of both the TE and TM modes in this faulted resistive environment. [Copyright &y& Elsevier]