The structure and nature of the crust underlying the Northeast Brazilian margins have been investigated based on the interpretation of 12 wide-angle seismic profiles acquired during the SALSA (Sergipe Alagoas Seismic Acquisition) experiment in 2014. In this study, we present the coincident analyses of 2 marine seismic reflection and refraction data, SL07 and SL 08, that have been acquired using 15 and 13 ocean-bottom-seismometers along each profile, offshore the Jacuipe Basin. The SL07 has a 270-km long inland continuation with 46 land-seismic-stations crossing the South Tucano and Reconcavo Basins. Wide-angle seismic forward modellings reveal a narrow necking zone, an intermediate domain with anomalously high crustal velocities interpreted as intruded lower continental crust, and a sharp continent to ocean transition zone, greatly different from the adjacent segments (see companion paper of Evain et al., 2023). Below the Tucano half-grabens, the Moho shows no rise, displaying a flat reflector throughout the landward part of the profile. The high seismic velocities areas in the lower continental crust are interpreted as in- trusions that provide an overloading allowing the necessary #ubsidence for the deposition of the thick sedimentary sequences. Together with the recent information obtained by the analysis of continental basins and aborted ifts, and similar wide-angle experiments on other margins and geodynamic contexts, we propose new paradigm for the thinning process which confirms the crucial role of the lower continental crust and its relation with the upper mantle. We postulate a phase of overloading of the lower continental crust, an exhumation phase of the lower continental crust, and a phase of proto-oceanic crust which involved the lower continental crust and the upper mantle, before the emplacement of a more typical oceanic crust.

Natural Hydrogen Gas (H2) is producing a new golden rush worldwide due to its clean energetic features. The potential of natural hydrogen gas (H2) in the southernmost regions of Brazil, specifically Rio Grande do Sul and Santa Catarina, remains largely unexplored. We found free H2 occurrences identified in the intracratonic Mesozoic Paraná Basin, recognized in four formation tests performed in former exploratory wells. The data revealed H2 concentrations ranging from 0.14% to 8.79%, albeit associated with noncommercial volumes of natural gas. These occurrences of H2 exhibit a curious negative correlation with Helium (He), distinguishing it from many Eurasian occurrences associated with mantle sources. The Paraná Basin hosts the largest Brazilian coal reserves and organic-rich rocks. We posit that the hydrogen gas presence can be attributed to the maturation processes affecting mainly Rio Bonito, Taciba, and Ponta Grossa formations in this basin. Also, we identified hydrogen system elements in the rift-type Precambrian Camaquã Basin, which host potential source rocks, reservoirs, and seals. Until now, no H2 measurements have been carried out in this basin. To solve this lack, it is fundamental to execute a systematic sampling survey; it will enable the identification of the potential of H2 deposits in the Southernmost Brazilian region. This work lays the foundations for future research in the region, which is demanded to realize this economic potential.

The equatorial margin of Brazil is an example of a rift margin with a complex landscape, dominated by an escarpment perpendicular to the continental margin, which testifies to an equally complex rift and post-rift surface and tectonic evolution. This has been the focus of a long debate on the driving mechanism for post-rift tectonics and on the amount of exhumation. This study contributes to this debate with new petrographic and thermochronologic data on 152 samples from three basins, Para-Maranhao, Barreirinhas and Ceara, on the offshore continental platform. Our detrital record goes back to the rift time at ca. 100 Ma ago and outlines three major evolutionary phases of a changing landscape: a rift phase, with the erosion of a moderate rift escarpment, a Late Cretaceous-Palaeogene post-rift phase of major drainage reorganization and significant vertical erosion and a Late Oligocene-to-Recent post-rift phase of moderate vertical erosion and river headwater migration. We estimate that along the equatorial margin of Brazil, over a large onshore area, exhumation since the Late Cretaceous has totalled locally up to 2-2.5 km and since the late Oligocene did not exceed 1 km.

O presente capítulo faz uma introdução geral sobre o tema hidratos de gás naturais, i.e., que ocorrem nos sedimentos, apresentando os tipos existentes, como se formam, os principais gases envolvidos, a sua importância econômica e ambiental (incluindo mudança climática e como geohazards), além de suas ocorrências naturais no mundo. A seguir é apresentado um breve histórico dos estudos e da exploração de hidratos de gás no Brasil, seguido de uma descrição das ocorrências naturais confirmadas no país, no leque do Amazonas (Bacia da Foz do Amazonas) e no Cone de Rio Grande (Bacia de Pelotas). Por fim é apresentada uma breve comparação entre as duas ocorrências e uma discussão sobre o desenvolvimento futuro da exploração de hidratos de gás no país. Palavras-chave: metano, leque do Amazonas, Cone de Rio Grande, recurso energético, mudança climática, geohazard. Abstract This chapter starts with an introduction of the topic of natural gas hydrates, presenting the main existing types, how they form, the main gases involved in their formation, their economic and environmental (including climate change and as geohazards) importance, in addition to their worldwide natural occurrences. Also included is a brief history of the study and exploration of gas hydrates in Brazil, and a description of the two confirmed occurrences in the country: The Amazon fan (Foz do Amazonas basin) and the Rio Grande Cone (Pelotas Basin). The chapter ends with a brief comparison between the two occurrences and a discussion about the future development of the exploration for gas hydrates in the country. Keywords: methane, Amazon fan, Rio Grande Cone, energy resource, climate change, geohazard.

The structure of the North-East equatorial Brazilian margin was investigated during the MAGIC (Margins of brAzil, Ghana and Ivory Coast) seismic wide-angle experiment. This study focuses on the MC5 profile, that spans NW-SE 720 km in length, from the Sa similar to o Paulo Double Fracture Zone to the Barreirinhas margin and continental Borborema province. Its main objective is to understand the fundamental processes which lead to the thinning and finally to the breakup of the continental crust in a specific context of a divergent pull-apart system with two strike-slip borders. The experiment was devised to obtain the 2D structure along this profiles from joint pre-stack depth migration of streamer data and travel-time inversion by forward modeling of 43 Ocean Bottom Seismometers and 21 Land Seismic Stations records. Along the MC5 wide-angle transect, 4 major sectors are identified: 1) the Sa similar to o Paulo Double Fracture Zone presenting a 4.5 km thick volcano-sedimentary Basin on top of a 5.5 km thick basement; 2) a volcanic alignment and intermediate domain SE-ward, formed by the 4.5 km thick Basin III; 3) the 7.5 km thick Basin II, and the 5.5 km thick Basin I composing the continental slope and shelf. While all the offshore basement remains about 6 km thick in the deep-sea domains, acoustic velocity evolves from two-layer 4.8-6 km/s and 6.1-6.8 km/s beneath Basin III to two-layer high velocity 6.1-6.8 km/s and 7.2-7.4 km/s beneath Basin II and I, The necking zone, forming the Parnaiba Platform and associated PiauiCamocim and Cear ' a Basins, is 50 km wide; 4) the Me ' dio Coreau and Cear ' a Central thrust belt, where the unthinned continental crust thickness reaches 32 km. Finally, a schematic kinematic reconstruction that satisfies these observation is argumented.

Passive continental margins depict a wide range of crustal architectures. Such observation mainly come from wide-angle seismic that has proven to be a valuable tool to constrain margins' onshore and offshore crustal structures. Yet, it is still unclear what are the main drivers of such diversity and whether some more general processes do also control their formation. Here, by analyzing coincident marine seismic reflection and refraction data acquired parallel to the coast of northeast Brazil we shed a unique view on the along-strike segmentation of its passive margins. Our two P-wave velocity models add up to 10 other wide-angle velocity models build in the scope of the SALSA (Sergipe ALagoas Seismic Acquisition) project to provide a coherent picture of the crustal and upper mantle architecture of the offshore domains of the Jequitinhonha, Almada, Camamu, Jacuípe, Sergipe and Alagoas margins. In the Camamu-Jacuípe segment, both crustal necking and the continent to ocean transition were shown to be sharp. Along strike, our models show extremely thin crust above a thick anomalous velocity layer and a relatively low density mantle. This architecture is interpreted as made of collapsed continental blocks above either highly intruded lower continental crust and/or serpentinized mantle that is locally exhumed. It greatly differs from the crustal structure imaged to the south, within the Jequitinhonha and Almada segments, where our models support the existence of a vast and homogeneous domain of exhumed lower continental crust. It also considerably contrasts with the offshore architecture of the Sergipe-Alagoas segment to the north. There, the necking and transition to normal oceanic crust was shown to be narrow but gradual and dominated by middle to lower continental crust. On our along-strike models, it is characterized by a thicker but also highly intruded continental crust. Crustal architecture evolves brutally at the frontiers of these segments which coincides precisely with the Camamu Triple Junction and the offshore prolongation of the Vaza Barris fault zone. The former is one of the major geodynamic features in the development of the Central Segment of the South-Atlantic Ocean while the later marks the suture between the São-Francisco Craton and the northeastern Brazilian Orogenic Belt. Therefore, we clearly evidence the first-order control of both geodynamics and lithospheric inheritance on the along-strike segmentation of the northeastern Brazilian passive margins. Our results emphasize the crucial role of plate kinematics and lithospheric strength in shaping their architectures and the exhumation of middle/lower continental crust as a more general process of their formation.

During the SALSA experiment, in 2014, twelve combined wide-angle refraction and coincident multi-channel seismic profiles were acquired in the Jequitinhonha-Almada-Camamu, Jacuipe, and Sergipe-Alagoas basins, NE Brazil. Profile SL09 images the Almada-Camamu basin and the Sa similar to o Francisco craton, with 18 four-channel ocean-bottom seismometers and 22 land stations. The datasets were forward modelled and combined with pre-stack depth migration to increase the horizontal resolution of the velocity models. Our results show that sediment thickness varies between 3.8 km in the oceanward part of the profile, 4.3 km in the Almada basin and 6.5 km in the Camamu basin. Crustal thickness at the north-western edge of the profile is of around 40 km, with velocity gradients indicating a continental origin. The Camamu basin, which corresponds to the triple junction between the aborted N-S oriented Tucano rift, the SW-NE oriented Jacuipe-Sergipe-Alagoas branch, and the N-S Jequitinhonha-Almada branch, presents two crustal layers: a very thin upper layer, about 1.5 km thick, which increases seawards to 3 km in the Almada basin, and an higher velocity (HV) layer (6.8-7.2 km/s) about 4 km thick. This lower layer gradually disappears in the Almada basin. At the south-eastern edge of the profile, the resolution is lower but the thickness of the crust seems to increase up to 5 km. Deep wide-angle reflections indicate upper mantle stratification. Crustal organisation and P-wave propagation velocities in the Almada and Camamu basins indicate a tran-sitional crust domain of exhumed continental crust affinity. In the Camamu triple junction and beneath this thin exhumed continental crust, the HV layer may probably reflect intruded materials. No exhumed upper mantle is observed along the entire profile. The easternmost part of the profile may correspond to a proto-oceanic crust. Typical oceanic crust is never imaged along the 260 km-long offshore profile.

Ocean warming related to climate change has been proposed to cause the dissociation of gas hydrate deposits and methane leakage on the seafloor. This process occurs in places where the edge of the gas hydrate stability zone in sediments meets the overlying warmer oceans in upper slope settings. Here we present new evidence based on the analysis of a large multi-disciplinary and multi-scale dataset from such a location in the western South Atlantic, which records massive gas release to the ocean. The results provide a unique opportunity to examine ocean-hydrate interactions over millennial and decadal scales, and the first evidence from the southern hemisphere for the effects of contemporary ocean warming on gas hydrate stability. Widespread hydrate dissociation results in a highly focused advective methane flux that is not fully accessible to anaerobic oxidation, challenging the assumption that it is mostly consumed by sulfate reduction before reaching the seafloor.

Mapping of natural gas hydrate systems has been performed successfully in the past using the controlled-source electromagnetic (CSEM) method. This method relies on differentiating resistive highly saturated free gas or hydrate-bearing host sediment from a less resistive low-saturated gas or brine-bearing host sediments. Knowledge of the lateral extent and resistivity variations (and hence the saturation variations) within sediments that host hydrates is crucial to be able to accurately quantify the presence of saturated gas hydrates. A 3D CSEM survey (PUCRS14) was acquired in 2014 in the Pelotas Basin offshore Brazil, with hydrate resistivity mapping as the main objective. The survey was acquired within the context of the CONEGAS research project, which investigated the origin and distribution of gas hydrate deposits in the Pelotas Basin. We have inverted the acquired data using a proprietary 3D CSEM anisotropic inversion algorithm. Inversion was purely CSEM data driven, and we did not include any a priori information in the process. Prior to CSEM, interpretation of near-surface geophysical data including 2D seismic, sub-bottom profiler, and multibeam bathymetry data indicated possible presence of gas hydrates within features identified such as faults, chimneys, and seeps leading to pockmarks, along the bottom simulating reflector and within the gas hydrate stability zone. Upon integration of the same with CSEM-derived resistivity volume, the interpretation revealed excellent spatial correlation with many of these features. The interpretation further revealed new features with possible hydrate presence, which were previously overlooked due to a lack of a clear seismic and/or multibeam backscatter signature. In addition, features that were previously mapped as gas hydrate bearing had to be reinterpreted as residual or low-saturated gas/hydrate features, due to the lack of significant resistivity response associated with them. Furthermore, we used the inverted resistivity volume to derive the saturation volume of the subsurface using Archie's equation.

Gas hydrate provinces occur in two sedimentary basins along Brazil’s continental margin: (1) The Rio Grande Cone in the southeast, and (2) the Amazon deep-sea fan in the equatorial region. The occurrence of gas hydrates in these depocenters was first detected geophysically and has recently been proven by seafloor sampling of gas vents, detected as water column acoustic anomalies rising from seafloor depressions (pockmarks) and/or mounds, many associated with seafloor faults formed by the gravitational collapse of both depocenters. The gas vents include typical features of cold seep systems, including shallow sulphate reduction depths (<4 m), authigenic carbonate pavements, and chemosynthetic ecosystems. In both areas, gas sampled in hydrate and in sediments is dominantly formed by biogenic methane. Calculation of the methane hydrate stability zone for water temperatures in the two areas shows that gas vents occur along its feather edge (water depths between 510 and 760 m in the Rio Grande Cone and between 500 and 670 m in the Amazon deep-sea fan), but also in deeper waters within the stability zone. Gas venting along the feather edge of the stability zone could reflect gas hydrate dissociation and release to the oceans, as inferred on other continental margins, or upward fluid flow through the stability zone facilitated by tectonic structures recording the gravitational collapse of both depocenters. The potential quantity of venting gas on the Brazilian margin under different scenarios of natural or anthropogenic change requires further investigation. The studied areas provide natural laboratories where these critical processes can be analyzed and quantified.

In this work, we investigated the molecular stable isotope compositions of hydrate-bound and dissolved gases in sediments of the Amazon deep-sea fan and adjacent continental slope, Foz do Amazonas Basin, Brazil. Some cores were obtained in places with active gas venting on the seafloor and, in one of the locations, the venting gas is probably associated with the dissociation of hydrates near the edge of their stability zone. Results of the methane stable isotopes (δ13C and δD) of hydrate-bound and dissolved gases in sediments for the Amazon fan indicated the dominant microbial origin of methane via carbon dioxide reduction, in which 13C and deuterium isotopes were highly depleted (δ13C and δD of −102.2% to −74.2% V-PDB and −190 to −150% V-SMOW, respectively). The combination of C1/(C2+C3) versus δ13C plot also suggested a biogenic origin for methane in all analysed samples (commonly >1000). However, a mixture of thermogenic and microbial gases was suggested for the hydrate-bound and dissolved gases in the continental slope adjacent to the Amazon fan, in which the combination of chemical and isotopic gas compositions in the C1/(C2+C3) versus δ13C plot were <100 in one of the recovered cores. Moreover, the δ13C-ethane of −30.0% indicates a thermogenic origin.

The structure and nature of the crust underlying the Camamu-Almada-Jequitinhonha-Sergipe-Alagoas basins System, in the NE Brazilian margin, were investigated based on the interpretation of 12 wide-angle seismic profiles acquired during the SALSA (Sergipe ALagoas Seismic Acquisition) experiment in 2014. In this work, we present two 220-km-long NW-SE combined wide-angle and reflection seismic profiles, SL 01 and SL 02, that have been acquired using 15 ocean-bottom-seismometers along each profile, offshore the southern part of the Sergipe Alagoas Basin (SAB), north of the Vaza-Barris Transfer zone. The SL 02 has a 150-km long inland continuation with 20 land-seismic-stations until the Sergipano Fold Belt (SFB). Wide-angle seismic forward modeling allows us to precisely divide the crust in three domains: beneath the continental shelf, a similar to 100 km wide necking zone is imaged where the continental crust thins from similar to 35 km on the Unthinned Continental Domain, which displays a three-layered crust structure, to less than 8 km on the Oceanic Crust Domain. In the necking zone, the upper and the middle layers thin dramatically and almost disappear, while the Moho discontinuity shows clear PmPs. The Continental-Oceanic Crust Boundary (COB) is located at similar to 80 km from the coastline and is marked by intracrustal seismic reflectors and changes in the seismic velocity, showing a sharp transition. On profile SL02, the oceanic crust is perturbed by a volcanic edifice together with an anomalous velocity zone underneath the area.

Twelve combined wide-angle refraction and coincident multi-channel seismic profiles were acquired in the Jequitinhonha-Camamu-Almada, Jacuipe, and Sergipe-Alagoas basins, NE Brazil, during the SALSA experiment in 2014. Profiles SL11 and SL12 image the Jequitinhonha basin, perpendicularly to the coast, with 15 and 11 four-channel ocean-bottom seismometers, respectively. Profile SL10 runs parallel to the coast, crossing profiles SL11 and SL12, imaging the proximal Jequitinhonha and Almada basins with 17 ocean-bottom seismometers. Forward modelling, combined with pre-stack depth migration to increase the horizontal resolution of the velocity models, indicates that sediment thickness varies between 3.3 km and 6.2 km in the distal basin. Crustal thickness at the western edge of the profiles is of around 20 km, with velocity gradients indicating a continental origin. It decreases to less than 5 km in the distal basin, with high seismic velocities and gradients, not compatible with normal oceanic crust nor exhumed upper mantle. Typical oceanic crust is never imaged along these about 200 km-long profiles and we propose that the transitional crust in the Jequitinhonha basin is a made of exhumed lower continental crust. (C) 2018 Elsevier Ltd. All rights reserved.

(1) IFREMER, Dept. Géosciences Marines, Technopôle Brest-Iroise, CS 10070, 29280 Plouzane, France, (2) Instituto Dom Luis (IDL), Faculdade de Ciências da Universidade de Lisboa, 1749-016 Lisboa, Portugal, (3) Instituto Superior de Engenharia de Lisboa (ISEL), Rue Conselheiro Emidio Navarro, 1959-007 Lisboa, Portugal, (4) 4Lablithos, Instituto de Geociências (IG), Universidade de Brasilia, Campus Darcy Ribeiro, 70910900, Brasilia, Brazil, (5) PETROBRAS/CENPES-PROFEXPETROBRAS, Petróleo Brasileiro S.A., Rio de Janeiro, Brazil, (6) JAMSTEC, CEAT, Yokohama, Japan

Five profiles, with coincident multi-chanel and wide-angle seismic, were acquired during the MAGIC (Margins of brAsil, Ganha and Ivory Coast) cruise, in order to image the Maranhão-Barreirinhas-Ceará segment of the Brazilian Margins. The seismic experiment was conducted by Ifremer (Institut Français de Recherche pour l’Exploration de la Mer), UnB (University of Brasilia), FCUL (Faculdade de Ciencias da Universidade de Lisboa) and Petrobras. The main objective of the experiment is to understand the fundamental processes which lead to the thinning and breakup of the continental crust in a specific context of a pull-apart system, limited by two strike-slip borders.

The Santos Basin is the largest offshore sedimentary basin in the southeastern Brazilian margin and originated by breakup of West Gondwana in the Early Cretaceous. We carried out a new thermochronological study by apatite fission track analysis from borehole samples of the Santos Basin and its continental basement to constrain the tectonic history of the southeastern Brazilian margin. Apatite fission track central ages of the basement and borehole samples vary from 21.0±1.8 to 157.0±35.0Ma and from 6.5±1.1 to 208.0±11.0Ma, respectively. From thermal modeling, the basement samples reached the maximum paleotemperatures during the final breakup of South America and Africa. The onshore basement and offshore basin record an early thermotectonic event during the Late Cretaceous linked to the uplift and denudation of the Serra do Mar and Serra da Mantiqueira. Maturation of the organic matter in the offshore basin is related with the progressive increase of the geothermal gradient due to burial. The thermal modeling indicates that the oil generation window started at 55–25Ma. The basement samples experienced the final cooling during the Cenozoic, with an estimated amount of denudation linked to the sedimentary influx in the offshore basin. A rapid cooling during the Neogene becomes evident and it is linked to the reactivation along Precambrian shear zones and change of the Paraíba do Sul drainage system.