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Demarcating the continental–oceanic boundary (COB) is extremely important as it contributes to an understanding of the tectonic environments of marginal basins through plate reconstructions. In general, the COB is estimated by jointly interpreting the features of the basement geomorphology, bathymetry and geophysical datasets, including gravity, magnetic, seismic and geochemical data. We propose a novel methodology named the tilt-theta angles correlation (TTAC), a second-order filtering approach to estimate the COB using high-resolution satellite gravity data. This method computes the angular difference or correlation angle between the vectors computed from the gradients of the tilt and theta derivative-based first-order filters. Oceanic regions generally exhibit small correlation angles (∼0°), thereby indicating a direct correlation; in contrast, continental regions depict large correlation angles (∼180°) and thus display an inverse correlation. The TTAC method marks the COB at the spatial position where the correlation angles abruptly shift from inverse to direct. Tests using synthetic data and the larger spectral energy content of TTAC for long wavelengths over tilt, theta and minus of the sign of vertical derivative (-SiVD) filters attest the effectiveness of the new methodology over first-order derivative based filters. Additionally, tests utilizing real data over both the West African Transform Margin (WATM) and across the South Atlantic Conjugate Margin (SACM) provide favourable results compared with the previous methodologies used to demarcate the COB. The COB estimates derived using the new methodology are consistent with the COB estimates obtained during recent plate-reconstruction studies. We also compare our results with an earlier interpretation along a conjugate margin encompassing magma-poor and magma-rich regions. In the complex rifted environments of the SACM, in which volcanic rocks are dominant, the TTAC methodology substantially improves the previous COB estimates, thereby establishing it as an efficient edge marker by simultaneously reducing small-scale geological noise and enhancing regional-scale geological contributions. [ABSTRACT FROM AUTHOR]

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]

Abstract: In an attempt to locate the presence of possible groundwater resource regions in the semi-arid North-East Brazil, an integrated survey including aeromagnetic and magnetotelluric (MT) studies have been undertaken in the Guaribas region and only MT survey in the Caracol region. In the Guaribas region the aeromagnetic data, its analytic signal and Euler solutions reveal several subsurface small-scale faults and intrusives that are conducive to be potential groundwater resource regions. A total of about 22 broad-band magnetotelluric (MT) soundings in the period range of 0.006–300 s along two profiles on the marginal arcs of the intra-cratonic sedimentary Parnaíba basin in North-East Brazil have been made across the regional geological strike, the Senador Pompeu Lineament (SPL). SPL trends N40°E and marks a basement high reflecting an irregularity in the original basin geometry. While one of the MT profiles traverses across the SPL, the other lies only in the aeromagnetically surveyed sedimentary region. Two-dimensional inversion of MT data of both profiles shows that the sedimentary basin is conductive (100–150 Ω m) and shows as a thin graben with an average thickness of about 2–3 km beneath both profiles. The basin is located to be at shallow depths (from surface to about 500 m). Based on the facts that the study region falls on sedimentary region having low-to-very low permeability and also in accordance with the subsurface lithology around the study region, the mapped sedimentary basin largely manifests the zone of potential sedimentary aquifer having moderate resistivity of 50–250 Ω m and is located at relatively shallow depths. The identified aquifer zone is believed to have links with the Parnaiba River flowing at a distance of about 300 km NW from the study region. We discuss interpretation of our results of MT and aeromagnetic data sets in the light of hydrological features of the study region. [Copyright &y& Elsevier]

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]

The commonest technique for determination of the continental-oceanic crustal boundary or transition (COB) zone is based on locating and visually correlating bathymetric and potential field anomalies and constructing crustal models constrained by seismic data. In this paper, we present a simple method for spatial correlation of bathymetric and potential field geophysical anomalies. Angular differences between gradient directions are used to determine different types of correlation between gravity and bathymetric or magnetic data. It is found that the relationship between bathymetry and gravity anomalies can be correctly identified using this method. It is demonstrated, by comparison with previously published models for the southwest African margin, that this method enables the demarcation of the zone of transition from oceanic to continental crust assuming that this it is associated with geophysical anomalies, which can be correlated using gradient directions rather than magnitudes. We also applied this method, supported by 2-D gravity modelling, to the more complex Liberia and Cote d'Ivoire-Ghana sectors of the West African transform margin and obtained results that are in remarkable agreement with past predictions of the COB in that region. We suggest the use of this method for a first-pass interpretation as a prelude to rigorous modelling of the COB in frontier areas. [ABSTRACT FROM AUTHOR]

ABSTRACT The magnetotelluric (MT) method has been applied to the determination of the deep resistivity structure of the northeastern margin of the Parnaiba Basin. Transient electromagnetic (TEM) and MT data were collected in early 1999 along a 95 km long N–S line, extending from the coast across the projected subcrop position of a discontinuous fault found to the west of the study area that is believed to be a possible basin-bounding fault. The MT data were processed to yield the TE- and TM-mode responses and then corrected for static shift using central-loop and single-loop TEM data, respectively. Regularized 2D MT inversion was subsequently undertaken using a structured initial model with the near-surface constrained by TEM inversion results. As a consistency check, we performed another set of 2D inversions using different smooth initial models. The various optimal 2D inversion models show clearly the presence of a major basement trough, over 2 km deep, located about 70 km from the coast. We interpret it as possibly marking the main basin margin and suggest that it may have implications for groundwater resource development in the area. [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]

ABSTRACT Marine magnetotelluric measurements using 'free-fall'' instruments without effective compasses suffer from the problem of unknown orientation of the receivers at the seafloor. While past works indicate that marine magnetotelluric orientation of the instruments can be estimated by reference to land deployments of known orientation using the transfer tensor method, there is limited published information on how this is implemented in practice. We document this method and propose a set of new time- and frequency-domain approaches to solve this orientation problem of the seafloor receivers. We test these methodologies in onshore and offshore magnetotelluric data whose orientations are well known and apply these techniques to marine magnetotelluric data with unknown orientation. For the controlled tests, both time- and frequency-domain approaches produce overall comparable results. To investigate the effects of the subsurface structure distribution on the orientation process, a dimensionality analysis of a controlled dataset is carried out. In subsequent analysis using the available disoriented marine magnetotelluric data from offshore Brazil and from the Vassouras magnetic observatory on the mainland for remote referencing, frequency-domain methods yield approximate orientation angles among themselves with low standard deviation each. Time-domain results are consistent for most cases but differ from frequency-domain results for some situations. [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]

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]