Estudo da estrutura subsuperficial da província Borborema com correlação de ruído sísmico

Ambient seismic noise has traditionally been considered as an unwanted perturbation in seismic data acquisition that "contaminates" the clean recording of earthquakes. Over the last decade, however, it has been demonstrated that consistent information about the subsurface structure can be extracted from cross-correlation of ambient seismic noise. In this context, the rules are reversed: the ambient seismic noise becomes the desired seismic signal, while earthquakes become the unwanted perturbation that needs to be removed. At periods lower than 30 s, the spectrum of ambient seismic noise is dominated by microseism, which originates from distant atmospheric perturbations over the oceans. The microsseism is the most continuous seismic signal and can be classified as primary – when observed in the range 10-20 s – and secondary – when observed in the range 5-10 s. The Green‘s function of the propagating medium between two receivers (seismic stations) can be reconstructed by cross-correlating seismic noise simultaneously recorded at the receivers. The reconstruction of the Green‘s function is generally proportional to the surface-wave portion of the seismic wavefield, as microsseismic energy travels mostly as surface-waves. In this work, 194 Green‘s functions obtained from stacking of one month of daily cross-correlations of ambient seismic noise recorded in the vertical component of several pairs of broadband seismic stations in Northeast Brazil are presented. The daily cross-correlations were stacked using a timefrequency, phase-weighted scheme that enhances weak coherent signals by reducing incoherent noise. The cross-correlations show that, as expected, the emerged signal is dominated by Rayleigh waves, with dispersion velocities being reliably measured for periods ranging between 5 and 20 s. Both permanent stations from a monitoring seismic network and temporary stations from past passive experiments in the region are considered, resulting in a combined network of 33 stations separated by distances between 60 and 1311 km, approximately. The Rayleigh-wave, dispersion velocity measurements are then used to develop tomographic images of group velocity variation for the Borborema Province of Northeast Brazil. The tomographic maps allow to satisfactorily map buried structural features in the region. At short periods (~5 s) the images reflect shallow crustal structure, clearly delineating intra-continental and marginal sedimentary basins, as well as portions of important shear zones traversing the Borborema Province. At longer periods (10 – 20 s) the images are sensitive to deeper structure in the upper crust, and most of the shallower anomalies fade away. Interestingly, some of them do persist. The deep anomalies do not correlate with either the location of Cenozoic volcanism and uplift - which marked the evolution of the Borborema Province in the Cenozoic - or available maps of surface heat-flow, and the origin of the deep anomalies remains enigmatic.

Estrutura crustal e mantélica da província Borborema através de funções do receptor e dispersão de ondas superficiais

The Borborema Province, located in northeastern Brazil, has a basement of Precambrian age and a tectonic framework structured at the Neoproterozoic (740-560 Ma). After separation between South America and Africa during the Mesozoic, a rift system was formed, giving rise to a number of marginal and inland basins in the Province. After continental breakup, episodes of volcanism and uplift characterized the evolution of the Province. Plateau uplift was initially related to magmatic underplating of mafic material at the base of the crust, perhaps related to the generation of young continental plugs (45-7 Ma) along the Macau-Queimadas Alignment (MQA), due to a small-scale convection at the continental edge. The goal of this study is to investigate the causes of intra-plate uplift and its relationship to MQA volcanism, by using broadband seismology and integrating our results with independent geophysical and geological studies in the Borborema Province. The investigation of the deep structure of the Province with broadband seismic data includes receiver functions and surface-wave dispersion tomography. Both the receiver functions and surface-wave dispersion tomography are methods that use teleseismic events and allow to develop estimates of crustal parameters such as crustal thickness, Vp/Vs ratio, and S-velocity structure. The seismograms used for the receiver function work were obtained from 52 stations in Northeast Brazil: 16 broadband stations from the RSISNE network (Rede Sismográfica do Nordeste do Brasil), and 21 short-period and 6 broadband stations from the INCT-ET network (Instituto Nacional de Ciência e Tecnologia – Estudos Tectônicos). These results add signifi- cantly to previous datasets collected at individual stations in the Province, which include station RCBR (GSN - Global Seismic Network), stations CAUB and AGBL (Brazilian Lithosphere Seismic Project IAG/USP), and 6 other broadband stations that were part of the Projeto Milênio - Estudos geofísicos e tectônicos na Província Borborema/CNPq. For the surface-wave vii tomography, seismograms recorde at 22 broadband stations were utilized: 16 broadband stations from the RSISNE network and 6 broadband stations from the Milênio project. The new constraints developed in this work include: (i) estimates of crustal thickness and bulk Vp/Vs ratio for each station using receiver functions; (ii) new measurements of surfassewave group velocity, which were integrated to existing measurementes from a continental-scale tomography for South America, and (iii) S-wave velocity models (1D) at various locations in the Borborema Province, developed through the simultaneous inversion of receiver functions and surface-wave dispersion velocities. The results display S-wave velocity structure down to the base of the crust that are consistent with the presence of a 5-7.5 km thick mafic layer. The mafic layer was observed only in the southern portion of the Plateau and absent in its northern portion. Another important observation is that our models divide the plateau into a region of thin crust (northern Plateau) and a region of thick crust (southern Plateau), confirming results from independent refraction surveys and receiver function analyses. Existing models of plateau uplift, nonetheless, cannot explain all the new observations. It is proposed that during the Brazilian orogeny a layer of preexisting mafic material was delaminated, as a whole or in part, from the original Brasiliano crust. Partial delamination would have happened in the southern portion of the plateau, where independent studies found evidence of a more resistant rheology. During Mesozoic rifting, thinning of the crust around the southern Plateau would have formed the marginal basins and the Sertaneja depression, which would have included the northern part of the Plateau. In the Cenozoic, uplift of the northern Plateau would have occurred, resulting in a northern Plateau without mafic material at the base of the crust and a southern Plateau with partially delaminated mafic layer.

Estudo e aplicação da transformada de Fourier na regularização de dados sísmicos na exploração de petróleo

In the oil prospection research seismic data are usually irregular and sparsely sampled along the spatial coordinates due to obstacles in placement of geophones. Fourier methods provide a way to make the regularization of seismic data which are efficient if the input data is sampled on a regular grid. However, when these methods are applied to a set of irregularly sampled data, the orthogonality among the Fourier components is broken and the energy of a Fourier component may "leak" to other components, a phenomenon called "spectral leakage". The objective of this research is to study the spectral representation of irregularly sampled data method. In particular, it will be presented the basic structure of representation of the NDFT (nonuniform discrete Fourier transform), study their properties and demonstrate its potential in the processing of the seismic signal. In this way we study the FFT (fast Fourier transform) and the NFFT (nonuniform fast Fourier transform) which rapidly calculate the DFT (discrete Fourier transform) and NDFT. We compare the recovery of the signal using the FFT, DFT and NFFT. We approach the interpolation of seismic trace using the ALFT (antileakage Fourier transform) to overcome the problem of spectral leakage caused by uneven sampling. Applications to synthetic and real data showed that ALFT method works well on complex geology seismic data and suffers little with irregular spatial sampling of the data and edge effects, in addition it is robust and stable with noisy data. However, it is not as efficient as the FFT and its reconstruction is not as good in the case of irregular filling with large holes in the acquisition.

Estudo e aplicação da transformada de Fourier na regularização de dados sísmicos na exploração de petróleo

In the oil prospection research seismic data are usually irregular and sparsely sampled along the spatial coordinates due to obstacles in placement of geophones. Fourier methods provide a way to make the regularization of seismic data which are efficient if the input data is sampled on a regular grid. However, when these methods are applied to a set of irregularly sampled data, the orthogonality among the Fourier components is broken and the energy of a Fourier component may "leak" to other components, a phenomenon called "spectral leakage". The objective of this research is to study the spectral representation of irregularly sampled data method. In particular, it will be presented the basic structure of representation of the NDFT (nonuniform discrete Fourier transform), study their properties and demonstrate its potential in the processing of the seismic signal. In this way we study the FFT (fast Fourier transform) and the NFFT (nonuniform fast Fourier transform) which rapidly calculate the DFT (discrete Fourier transform) and NDFT. We compare the recovery of the signal using the FFT, DFT and NFFT. We approach the interpolation of seismic trace using the ALFT (antileakage Fourier transform) to overcome the problem of spectral leakage caused by uneven sampling. Applications to synthetic and real data showed that ALFT method works well on complex geology seismic data and suffers little with irregular spatial sampling of the data and edge effects, in addition it is robust and stable with noisy data. However, it is not as efficient as the FFT and its reconstruction is not as good in the case of irregular filling with large holes in the acquisition.

Evolução dos sistemas de falhas de borda do Rifte Potiguar com base em curvas de crescimento de falhas

The discussion about rift evolution in the Brazilian Equatorial margin during the South America-Africa breakup in the Jurassic/Cretaceous has been focused in many researches. But rift evolution based on development and growth of faults has not been well explored. In this sense, we investigated the Cretaceous Potiguar Basin in the Equatorial margin of Brazil to understand the geometry of major faults and the influence of crustal heterogeneity and preexisting structural fabric in the evolution of the basin internal architecture. Previous studies pointed out that the rift is an asymmetrical half-graben elongated along the NE-SW direction. We used 2D seismic, well logs and 3D gravity modeling to analyze four major border fault segments and determine their maximum displacement (Dmax) and length (L) ratio in the Potiguar Rift. We constrained the 3D gravity modeling with well data and the interpretation of seismic sections. The difference of the fault displacement measured in the gravity model is in the order of 10% compared to seismic and well data. The fault-growth curves allowed us to divide the faulted rift border into four main fault segments, which provide roughly similar Dmax/L ratios. Fault-growth curves suggest that a regional uniform tectonic mechanism influenced growth of the rift fault segments. The variation of the displacements along the fault segments indicates that the fault segments were formed independently during rift initiation and were linked by hard and soft linkages. The latter formed relay ramps. In the interconnection zones the Dmax/L ratios are highest due to interference of fault segment motions. We divided the evolution of the Potiguar Rift into five stages based on these ratios and correlated them with the major tectonic stages of the breakup between South America and Africa in Early Cretaceous.

Evolução dos sistemas de falhas de borda do Rifte Potiguar com base em curvas de crescimento de falhas

The discussion about rift evolution in the Brazilian Equatorial margin during the South America-Africa breakup in the Jurassic/Cretaceous has been focused in many researches. But rift evolution based on development and growth of faults has not been well explored. In this sense, we investigated the Cretaceous Potiguar Basin in the Equatorial margin of Brazil to understand the geometry of major faults and the influence of crustal heterogeneity and preexisting structural fabric in the evolution of the basin internal architecture. Previous studies pointed out that the rift is an asymmetrical half-graben elongated along the NE-SW direction. We used 2D seismic, well logs and 3D gravity modeling to analyze four major border fault segments and determine their maximum displacement (Dmax) and length (L) ratio in the Potiguar Rift. We constrained the 3D gravity modeling with well data and the interpretation of seismic sections. The difference of the fault displacement measured in the gravity model is in the order of 10% compared to seismic and well data. The fault-growth curves allowed us to divide the faulted rift border into four main fault segments, which provide roughly similar Dmax/L ratios. Fault-growth curves suggest that a regional uniform tectonic mechanism influenced growth of the rift fault segments. The variation of the displacements along the fault segments indicates that the fault segments were formed independently during rift initiation and were linked by hard and soft linkages. The latter formed relay ramps. In the interconnection zones the Dmax/L ratios are highest due to interference of fault segment motions. We divided the evolution of the Potiguar Rift into five stages based on these ratios and correlated them with the major tectonic stages of the breakup between South America and Africa in Early Cretaceous.

Expressão geofísica-estrutural do lineamento transbrasiliano na porção central da Bacia do Parnaíba (Maranhão-Piauí)

The objective of this study was to characterize the structural-geophysical expression of the Transbrasiliano Lineament (TBL) in the east-central portion of the Parnaíba Basin. The TBL corresponds to a major Neoproterozoic NE-trending shear zone related to the Brasiliano orogenic cycle, with dextral strike-slip kinematics, underlying (but also laterally exposed in the NE and SW basin edges) the sedimentary section of the Parnaíba Basin. In this study, the interpretation of gravity and magnetic anomaly maps is consistent with the TBL kinematics, the signature of the geophysical anomalies corresponding to the high (plastic behaviour) and subsequent declining temperature (ductile to brittle behaviour) stages during Brasiliano and late Brasiliano times. The pattern of residual gravity anomalies is compatible with an S-C dextral pair shaping the geological bodies of an heterogeneous basement, such as slices of gneisses and granulites (positive anomalies), granitic and low-medium grade metasedimentary rocks (negative anomalies). Such anomalies curvilinear trends, ranging from NNE (interpreted as S surfaces) to NE (C surfaces), correspond to flattening surfaces (S), while the NE rectilinear trend must represent a C band. The narrower magnetic anomalies also display NNE to NE (S surfaces) trends and should correspond to similar (although narrower and more discontinuous) sources in the equivalent anomaly patterns. Pre-Silurian pull-apart style grabens may contribute to the NE negative gravimetric anomalies, although this interpretation demands control by seismic data analysis. On the other hand, the curvilinear anomalies associated to contractional trends are incompatible with their interpretation as pre-Silurian graben, in both maps. In the (reduced to the pole) magnetic anomalies map, most of these are again associated to low-temperature shear zones (C planes) and faults, juxtaposing distinct blocks in terms of magnetic properties, or eventually filled with basic bodies. It is also possible that some isolated magnetic anomalies correspond to igneous bodies of late-Brasiliano or Mesozoic age. The basement late discontinuities pattern can be interpreted in analogy to the Riedel fractures model, with steep dipping surfaces and a sub-horizontal movement section. This study also explored 2D gravity modeling controlled by the interpretation of a dip seismic line as regards to the Transbrasiliano Lineament. The rock section equivalent to the Jaibaras Group occupying a graben structure (as identified in the seismic line) corresponds to a discrete negative anomaly superimposed to a gravimetric high, once again indicating a stronger influence of older crystalline basement rocks as gravimetric sources, mainly reflecting the heterogeneities and anisotropies generated at high temperature conditions and their subsequent cooling along the TBL, during the Brasiliano cycle.

Expressão geofísica-estrutural do lineamento transbrasiliano na porção central da Bacia do Parnaíba (Maranhão-Piauí)

The objective of this study was to characterize the structural-geophysical expression of the Transbrasiliano Lineament (TBL) in the east-central portion of the Parnaíba Basin. The TBL corresponds to a major Neoproterozoic NE-trending shear zone related to the Brasiliano orogenic cycle, with dextral strike-slip kinematics, underlying (but also laterally exposed in the NE and SW basin edges) the sedimentary section of the Parnaíba Basin. In this study, the interpretation of gravity and magnetic anomaly maps is consistent with the TBL kinematics, the signature of the geophysical anomalies corresponding to the high (plastic behaviour) and subsequent declining temperature (ductile to brittle behaviour) stages during Brasiliano and late Brasiliano times. The pattern of residual gravity anomalies is compatible with an S-C dextral pair shaping the geological bodies of an heterogeneous basement, such as slices of gneisses and granulites (positive anomalies), granitic and low-medium grade metasedimentary rocks (negative anomalies). Such anomalies curvilinear trends, ranging from NNE (interpreted as S surfaces) to NE (C surfaces), correspond to flattening surfaces (S), while the NE rectilinear trend must represent a C band. The narrower magnetic anomalies also display NNE to NE (S surfaces) trends and should correspond to similar (although narrower and more discontinuous) sources in the equivalent anomaly patterns. Pre-Silurian pull-apart style grabens may contribute to the NE negative gravimetric anomalies, although this interpretation demands control by seismic data analysis. On the other hand, the curvilinear anomalies associated to contractional trends are incompatible with their interpretation as pre-Silurian graben, in both maps. In the (reduced to the pole) magnetic anomalies map, most of these are again associated to low-temperature shear zones (C planes) and faults, juxtaposing distinct blocks in terms of magnetic properties, or eventually filled with basic bodies. It is also possible that some isolated magnetic anomalies correspond to igneous bodies of late-Brasiliano or Mesozoic age. The basement late discontinuities pattern can be interpreted in analogy to the Riedel fractures model, with steep dipping surfaces and a sub-horizontal movement section. This study also explored 2D gravity modeling controlled by the interpretation of a dip seismic line as regards to the Transbrasiliano Lineament. The rock section equivalent to the Jaibaras Group occupying a graben structure (as identified in the seismic line) corresponds to a discrete negative anomaly superimposed to a gravimetric high, once again indicating a stronger influence of older crystalline basement rocks as gravimetric sources, mainly reflecting the heterogeneities and anisotropies generated at high temperature conditions and their subsequent cooling along the TBL, during the Brasiliano cycle.

Evolução estratigráfica da sequência neocarbonífera-eotriássica da Bacia do Parnaíba, NE do Brasil

The Parnaíba Basin consists in an intracratonic basin whose sucession of rocks is arranged in five supersequences. The Upper Carboniferous-Lower Triassic Sequence represents the third major sedimentary cycle and corresponds to Balsas Group, which is divided into four units: Piauí Formation, Pedra de Fogo Formation, Motuca Formation and Sambaíba Formation, from base to top. Different interpretations have been made by several authors in recent decades to interpreted the depositional system and environments related to each unit that belongs to this sequence. In general way, it is described as a thick pack of siliciclastic sediments deposited under complex conditions, varying from clastic/evaporitic shallow marine to lacustrine and desert environment. Aiming to clarify the sedimentary sequence evolution, this work underwent a stratigraphic analysis of the Upper Carboniferous-Lower Triassic deposits by applying modern concepts of the sequence stratigraphy based on well and seismic database. Three main depositional sequences of higher frequency were identified in each well analyzed. The sequence 1 corresponds to rocks initially deposited by a fluvial system with braided channel characteristics which evolved to shallow marine with coastal sabkha conditions related to a transgressive stage, that later evolved to a deltaic system. The Sequence 2 corresponds to rocks deposited in a lacustrine/desert environment associated with sabkha generated during a period of increased aridity in which the area occupied by the Parnaíba Basin had been suffering. The registration of a major regressive phase is shown in Sequence 2 which evolved to a dominantly desert environment recorded in Sequence 3. Seismic stratigraphy analyses allow to define a series of stratigraphic surfaces and related genetic units, as well as to infer its lateral expression. Seismic facies associated with such sequences are dominantly parallel and sub-parallel, with good lateral continuity, suggesting the sedimentary rate was relatively constant during deposition.

Evolução estratigráfica da sequência neocarbonífera-eotriássica da Bacia do Parnaíba, NE do Brasil

The Parnaíba Basin consists in an intracratonic basin whose sucession of rocks is arranged in five supersequences. The Upper Carboniferous-Lower Triassic Sequence represents the third major sedimentary cycle and corresponds to Balsas Group, which is divided into four units: Piauí Formation, Pedra de Fogo Formation, Motuca Formation and Sambaíba Formation, from base to top. Different interpretations have been made by several authors in recent decades to interpreted the depositional system and environments related to each unit that belongs to this sequence. In general way, it is described as a thick pack of siliciclastic sediments deposited under complex conditions, varying from clastic/evaporitic shallow marine to lacustrine and desert environment. Aiming to clarify the sedimentary sequence evolution, this work underwent a stratigraphic analysis of the Upper Carboniferous-Lower Triassic deposits by applying modern concepts of the sequence stratigraphy based on well and seismic database. Three main depositional sequences of higher frequency were identified in each well analyzed. The sequence 1 corresponds to rocks initially deposited by a fluvial system with braided channel characteristics which evolved to shallow marine with coastal sabkha conditions related to a transgressive stage, that later evolved to a deltaic system. The Sequence 2 corresponds to rocks deposited in a lacustrine/desert environment associated with sabkha generated during a period of increased aridity in which the area occupied by the Parnaíba Basin had been suffering. The registration of a major regressive phase is shown in Sequence 2 which evolved to a dominantly desert environment recorded in Sequence 3. Seismic stratigraphy analyses allow to define a series of stratigraphic surfaces and related genetic units, as well as to infer its lateral expression. Seismic facies associated with such sequences are dominantly parallel and sub-parallel, with good lateral continuity, suggesting the sedimentary rate was relatively constant during deposition.

Medium change monitoring using ambient seismic noise and coda wave interferometry: examples from intraplate NE Brazil and the Mid-Atlantic Ridge

This thesis presents and discusses the results of ambient seismic noise correlation for two different environments: intraplate and Mid-Atlantic Ridge. The coda wave interferometry method has also been tested for the intraplate data. Ambient noise correlation is a method that allows to retrieve the structural response between two receivers from ambient noise records, as if one of the station was a virtual source. It has been largely used in seismology to image the subsurface and to monitor structural changes associated mostly with volcanic eruptions and large earthquakes. In the intraplate study, we were able to detect localized structural changes related to a small earthquake swarm, which main event is mR 3.7, North-East of Brazil. We also showed that the 1-bit normalization and spectral whitening result on the loss of waveform details and that the phase auto-correlation, which is amplitude unbiased, seems to be more sensitive and robust for our analysis of a small earthquake swarm. The analysis of 6 months of data using cross-correlations detect clear medium changes soon after the main event while the auto-correlations detect changes essentially after 1 month. It could be explained by fluid pressure redistribution which can be initiated by hydromechanical changes and opened path ways to shallower depth levels due to later occurring earthquakes. In the Mid-Atlantic Ridge study, we investigate structural changes associated with a mb 4.9 earthquake in the region of the Saint Paul transform fault. The data have been recorded by a single broadband seismic station located at less than 200 km from the Mid-Atlantic ridge. The results of the phase auto-correlation for a 5-month period, show a strong co-seismic medium change followed by a relatively fast post-seismic recovery. This medium change is likely related to the damages caused by the earthquake’s ground shaking. The healing process (filling of the new cracks) that lasted 60 days can be decomposed in two phases, a fast recovery (70% in ~30 days) in the early post-seismic stage and a relatively slow recovery later (30% in ~30 days). In the coda wave interferometry study, we monitor temporal changes of the subsurface caused by the small intraplate earthquake swarm mentioned previously. The method was first validated with synthetics data. We were able to detect a change of 2.5% in the source position and a 15% decrease of the scatterers’ amount. Then, from the real data, we observed a rapid decorrelation of the seismic coda after the mR 3.7 seismic event. This indicates a rapid change of the subsurface in the fault’s region induced by the earthquake.

Medium change monitoring using ambient seismic noise and coda wave interferometry: examples from intraplate NE Brazil and the Mid-Atlantic Ridge

This thesis presents and discusses the results of ambient seismic noise correlation for two different environments: intraplate and Mid-Atlantic Ridge. The coda wave interferometry method has also been tested for the intraplate data. Ambient noise correlation is a method that allows to retrieve the structural response between two receivers from ambient noise records, as if one of the station was a virtual source. It has been largely used in seismology to image the subsurface and to monitor structural changes associated mostly with volcanic eruptions and large earthquakes. In the intraplate study, we were able to detect localized structural changes related to a small earthquake swarm, which main event is mR 3.7, North-East of Brazil. We also showed that the 1-bit normalization and spectral whitening result on the loss of waveform details and that the phase auto-correlation, which is amplitude unbiased, seems to be more sensitive and robust for our analysis of a small earthquake swarm. The analysis of 6 months of data using cross-correlations detect clear medium changes soon after the main event while the auto-correlations detect changes essentially after 1 month. It could be explained by fluid pressure redistribution which can be initiated by hydromechanical changes and opened path ways to shallower depth levels due to later occurring earthquakes. In the Mid-Atlantic Ridge study, we investigate structural changes associated with a mb 4.9 earthquake in the region of the Saint Paul transform fault. The data have been recorded by a single broadband seismic station located at less than 200 km from the Mid-Atlantic ridge. The results of the phase auto-correlation for a 5-month period, show a strong co-seismic medium change followed by a relatively fast post-seismic recovery. This medium change is likely related to the damages caused by the earthquake’s ground shaking. The healing process (filling of the new cracks) that lasted 60 days can be decomposed in two phases, a fast recovery (70% in ~30 days) in the early post-seismic stage and a relatively slow recovery later (30% in ~30 days). In the coda wave interferometry study, we monitor temporal changes of the subsurface caused by the small intraplate earthquake swarm mentioned previously. The method was first validated with synthetics data. We were able to detect a change of 2.5% in the source position and a 15% decrease of the scatterers’ amount. Then, from the real data, we observed a rapid decorrelation of the seismic coda after the mR 3.7 seismic event. This indicates a rapid change of the subsurface in the fault’s region induced by the earthquake.

Petrologia e geoquímica do magmatismo ediacarano Serra do Caramuru, Rio Grande do Norte, NE do Brasil

The Serra do Caramuru and Tapuio stocks, located in the extreme NE of Rio Piranhas-Seridó Domain (RN), are representative of the Ediacaran-Cambrian magmatism, an important magmatic feature of the Brasilian / Panafrican orogeny of the Borborema Province. These bodies are lithologically similar, intrusive in paleoproterozoic gneiss embasement, being separated by a thin belt of mylonitic orthogneiss. The field relations show a magmatic stratigraphy initiated by dioritic facies that coexists with the porphyritic granitic and equigranular granitic I facies, and less frequently with equigranular granitic II facies. These rocks are crosscut by late granitic dykes and sheets with NE-SW / NNE-SSW orientation. The dioritic facies (diorite, quartz diorite, quartz monzodiorites, tonalite and granodiorite) is leucocratic to melanocratic, rich in biotite and hornblende. The granitic facies are hololeucocratic to leucocratic, and have biotite ± hornblende. Petrographic and geochemical (whole rock) data, especially from Serra do Caramuru pluton, suggest fractionation of zircon, apatite, clinopyroxene (in diorites), opaque minerals, titanite, biotite, hornblende, allanite, plagioclase, microcline and garnet (in dykes). The behavior of trace elements such as Zr, La and Yb indicates that the dioritic magma does not constitute the parental magma for the granitic facies. On the other hand, the granitic facies seems to be cogenetic to each other, displaying differentiation trends and very similar rare earth elements (REE) spectra [12.3≤(La/Yb)N≤190.8; Eu/Eu*=0.30-0.68]. Field relationships and REE patterns [6.96≤(La/Yb)N≤277.8; Eu/Eu*=0.18-0.58] demonstrate that the granitic dykes and sheets are not cogenetically related to the Serra do Caramuru magmatism. The dioritic facies is metaluminous (A/CNK = 0.88-0.74) and shoshonitic, whereas the granitic ones are metaluminous to peraluminous (A/CNK = 1.08-0.93) and high potassium calc-alkaline. Dykes and sheets are strictly peraluminous (A/CNK = 1.01-1.04). Binary diagrams relating compatible and incompatible trace elements and microtextures indicate the fractional crystallization as the dominant mechanism of magmatic evolution of the various facies. The Serra do Caramuru and Tapuio stocks have well preserved magmatic fabric, do not show metamorphic minerals and are structurally isotropic, showing crosscutting contact with the ductile fabric of the basement. These observations lead to interpretate a stage of relative tectonic stability, consistent with the orogenic relaxation period of the Brasiliano / Pan-African orogeny. Chemical plots involving oxides and trace elements indicate late to post-collisional emplacement. In this context, the assumed better mechanism to describe the stocks emplacement within an extensional T Riedel joint, with ENE-WSW extensional vector. The U-Pb zircon age of 553 ± 10 Ma allows correlating the Serra do Caramuru magmatism to the group of post-collisional bodies, equigranular high potassium calc-alkaline granites of the NE of Rio Piranhas-Seridó Domain.

Petrologia e geoquímica do magmatismo ediacarano Serra do Caramuru, Rio Grande do Norte, NE do Brasil

The Serra do Caramuru and Tapuio stocks, located in the extreme NE of Rio Piranhas-Seridó Domain (RN), are representative of the Ediacaran-Cambrian magmatism, an important magmatic feature of the Brasilian / Panafrican orogeny of the Borborema Province. These bodies are lithologically similar, intrusive in paleoproterozoic gneiss embasement, being separated by a thin belt of mylonitic orthogneiss. The field relations show a magmatic stratigraphy initiated by dioritic facies that coexists with the porphyritic granitic and equigranular granitic I facies, and less frequently with equigranular granitic II facies. These rocks are crosscut by late granitic dykes and sheets with NE-SW / NNE-SSW orientation. The dioritic facies (diorite, quartz diorite, quartz monzodiorites, tonalite and granodiorite) is leucocratic to melanocratic, rich in biotite and hornblende. The granitic facies are hololeucocratic to leucocratic, and have biotite ± hornblende. Petrographic and geochemical (whole rock) data, especially from Serra do Caramuru pluton, suggest fractionation of zircon, apatite, clinopyroxene (in diorites), opaque minerals, titanite, biotite, hornblende, allanite, plagioclase, microcline and garnet (in dykes). The behavior of trace elements such as Zr, La and Yb indicates that the dioritic magma does not constitute the parental magma for the granitic facies. On the other hand, the granitic facies seems to be cogenetic to each other, displaying differentiation trends and very similar rare earth elements (REE) spectra [12.3≤(La/Yb)N≤190.8; Eu/Eu*=0.30-0.68]. Field relationships and REE patterns [6.96≤(La/Yb)N≤277.8; Eu/Eu*=0.18-0.58] demonstrate that the granitic dykes and sheets are not cogenetically related to the Serra do Caramuru magmatism. The dioritic facies is metaluminous (A/CNK = 0.88-0.74) and shoshonitic, whereas the granitic ones are metaluminous to peraluminous (A/CNK = 1.08-0.93) and high potassium calc-alkaline. Dykes and sheets are strictly peraluminous (A/CNK = 1.01-1.04). Binary diagrams relating compatible and incompatible trace elements and microtextures indicate the fractional crystallization as the dominant mechanism of magmatic evolution of the various facies. The Serra do Caramuru and Tapuio stocks have well preserved magmatic fabric, do not show metamorphic minerals and are structurally isotropic, showing crosscutting contact with the ductile fabric of the basement. These observations lead to interpretate a stage of relative tectonic stability, consistent with the orogenic relaxation period of the Brasiliano / Pan-African orogeny. Chemical plots involving oxides and trace elements indicate late to post-collisional emplacement. In this context, the assumed better mechanism to describe the stocks emplacement within an extensional T Riedel joint, with ENE-WSW extensional vector. The U-Pb zircon age of 553 ± 10 Ma allows correlating the Serra do Caramuru magmatism to the group of post-collisional bodies, equigranular high potassium calc-alkaline granites of the NE of Rio Piranhas-Seridó Domain.

Estudo da sismicidade na barragem do Açu no período de 2012 a 2013

The seismic activity in the Northeastern of Brazil has been a constant target of study, since it is the most active region of the country. However, some areas have their earthquakes related to human action, what means they are induced. The Açu dam is a classic example of reservoir-induced seismicity and it has been the subject of several studies. Recently, after a considerable period of inactivity, the LabSis / UFRN recorded events related to the dam, which led to the installation of a network around the reservoir. From the data provided by this network, it was observed that the seismic activity is related to a new epicental area inside the lake. The epicentral parameters and the focal mechanism were determined. It was found that the events were related to the reactivation of a basement structure in a new seismogenic subvertical fault with NE-SW-striking subparallel to the São Rafael Fault. These results were used in the preparation of a scientific paper, which discussed the relationship between this seismicity with the geology of the region and with the reservoir water level. The paper showed that the diffusion of pore pressure was the main mechanism that controlled the triggering of the induced seismicity at the reservoir.