Search

After confirming that impoundment of large reservoirs could cause earthquakes, studies on reservoir-triggered seismicity (RTS) have had a considerable scientific incentive. Most of the studies determined that the vertical load increase due to reservoir load, and the reduction of effective force due to the increase in pore pressure, can modify the stress field in the reservoir region, possibly triggering earthquakes. In addition, the RTS is conditioned by several factors such as pre-existing tectonic stresses, reservoir height/weight, area-specific geological and hydromechanical conditions, constructive interaction between the orientation of seismotectonic forces, and additional load caused by the reservoir. One of the major challenges in studying RTS is to identify and correlate the factors in the area of influence of the reservoir, capable of influencing the RTS process itself. A spatial seismicity-triggered reservoir database was created to facilitate the research in this field, based on the specifications of the national spatial data infrastructure (INDE), and to assemble data pertinent to the RTS study in the area of reservoirs. In this context, this work presents the procedures and results found in the data processing of seismotectonic factors (dam height, reservoir capacity, lithology, and seismicity) and compared first to the dams that triggered earthquakes and secondly to the Brazilian dam list. The list has been updated with four more cases, making a total of 30 cases. The results indicate that the occurrence of RTS increases significantly with dam height since dams less than 50 m high cause only 2 % of earthquakes while those higher than 100 m cause about 54 %. The reservoir volume also plays a role, and it was estimated that RTS occurrence requires a limiting minimum value of 1×10-4 km 3. There was no clear correlation between the geology and geological provinces with RTS. The delayed response time of the reservoirs represents 43 % of the total; that is, almost half of them have hydraulic behavior. The highest magnitude, 4.2, was observed at a reservoir with a volume greater than 10 -3 km 3. As a practical outcome, to assist the analysis by the general community, the web viewer RISBRA (Reservoir Induced Seismicity in Brazil) was developed to serve as an interactive platform for Reservoir-Triggered Seismicity Database (BDSDR) data. [ABSTRACT FROM AUTHOR]

After confirming that impoundment of large reservoirs could cause earthquakes worldwide, studies on reservoir-triggered seismicity (RTS) have had a considerable scientific incentive. Most of the studies determined that the vertical load increase due to reservoir load, and the reduction of effective effort due to the increase in pore pressure, can modify the stress regime in the reservoir region, possibly triggering earthquakes. In addition, the RTS is conditioned by several factors such as pre-existing tectonic stresses, reservoir size/weight, area-specific geological and hydromechanical conditions, constructive interaction between the orientation of seismotectonic forces, and additional load caused by the reservoir. One of the major challenges for studying RTS is to identify and correlate the factors in the area of influence of the reservoir, capable of influencing the RTS process itself. To assist the research, it was created a spatial seismicity-triggered reservoir database (BDSDR) based on the specifications of the national spatial data infrastructure (INDE), for gathering data pertinent to the RTS study in the area of reservoirs. In this context, this work presents the procedures and results found in the data processing of seismotectonic factors (dam height, reservoir volume, geology, and seismicity level) and compared with the dams that triggered earthquakes and the Brazilian dam catalog, which was then updated from 26 to 30 cases. The results indicate that the occurrence of RTS increases significantly with dam height since dams less than 50 m high cause only 2 % of earthquakes while those higher than 100 m cause about 54 %. The reservoir volume also plays a role and it was estimated that RTS occurrence requires a limiting minimum value of 1 × 10−4 km³. There was no clear correlation between the geology and geological provinces with RTS. The delayed response time of the reservoirs represents 43 % of the total, that is, almost half of them have hydraulic behavior. The highest magnitude, 4.2, was observed for an event that occurred in a reservoir with a volume greater than 10−3 km³. As a practical result to assist the analysis by the general community, the web viewer RISBRA (Reservoir Induced Seismicity in Brazil) was developed to serve as an interactive platform for BDSDR data. [ABSTRACT FROM AUTHOR]

Abstract The late Neoproterozoic São Rafael pluton, located in the Seridó Belt of the Borborema geologic province, northeastern Brazil, is an unfoliated, metaluminous, epidote-bearing, high-K calc-alkalic granitoid. Three facies are present in its northern lobe: (i) a dominant massive granite to quartz monzonite porphyritic facies containing microcline phenocrysts, (ii) subordinate fine-grained facies, similar to the porphyritic facies except lacking amphibole, that crop out as inselbergs ("Serras"), and (iii) rare pillows of microgranular mafic enclaves (MMEs) that are present within the porphyritic facies. Chemical analyses include 19 bulk-rock data, and 33 microprobe data of epidote and amphibole. Beginning with bulk chemistry and postulated mineral compositions, an iterative procedure of solving matrices characterizes a rock sample. Derived information includes: (i) identifying what minerals are present, (ii) their relative abundances, and (iii) their chemical compositions, even for minerals (such as biotite or amphibole) with complexly variable compositions. Calculated mineral compositions are compatible with microprobe data. The matrix solutions further indicate alternative mineral assemblages, and the compositions of "phantom" minerals that could have existed in the magma source, although absent in the pluton. They establish that olivine, but not clinopyroxene, was a possible phantom mineral and that MMEs are of unrelated magmatic origin. According to matrix analysis, amphibole in an outer zone of the porphyritic facies is solely of neoformed origin, whereas amphibole in an inner zone of the porphyritic facies is of mixed origin—partly neoformed and partly residual (i.e., xenocrystic) "patchy" amphibole derived from MMEs. A qualified geothermometer provides T solidus = 730 ± 6 °C for the outer subfacies, and a qualified geobarometer provides P = 0.78 ± 0.06 kbar. Amphibole in MMEs crystallized at depth, at P~3.3 kbar. Corroded epidote attests to the pressure-sensitive instability of that mineral during rapid ascent decompression. Cathodoluminescence reveals complexly structured zircons with magmatic overgrowths and inherited cores. 207Pb/206Pb ages of inherited components cluster at ~2.0 Ga and ~2.4 Ga, these ages being prominent in the local basement. Oscillatory-zoned magmatic components yield a concordia upper intercept age of 580 ± 9 Ma, interpreted as the age of igneous crystallization. Values of δ18O in zircon are only slightly higher than δ18O in mantle-like zircon. At 0.58 Ga, ε Hf in magmatic zircon was notably unradiogenic (−22 to −26), indicating a magma source in old continental crust as adduced by ages of inherited zircon. A U-Pb titanite date is around 0.55 Ga. For the Rb-Sr and Sm-Nd whole-rock systems, isotopic inheritance from the magma protolith is so extreme and variable that isochrons are actually "scatterchrons" that provide no useable age information. Rb-Sr cooling ages of biotite are 498 ± 1 Ma. Most Sm-Nd depleted mantle ages lie in the range of 2.6 ± 0.13 Ga. São Rafael magma was "unregenerate," not isotopically homogenized when emplaced. Highlights • An unusual epidote-bearing granitoid in the Borborema Province, NE Brazil • Chemical data, and U-Pb, Lu-Hf, Rb-Sr, Sm-Nd, and O isotope data • Matrix calculations of constituent minerals, their abundances and compositions • Matrix analysis of "phantom" mineral assemblages that subsequently had disappeared • A profound degree of isotopic inheritance pertaining to radiogenic isotope systems [ABSTRACT FROM AUTHOR]

The study of the crust using receiver functions can provide valuable geological information, such as average crustal composition, its formation dynamics and the tectonic evolution of a region, as well as serve as an initial reference for the generation of seismic wave velocity models to improve earthquake location. To fill in gaps in information on the crust of the Amazonian Craton and adjacent provinces in Brazil, we used receiver functions and H-k stacking to estimate crustal thicknesses and the V P /V S ratios. The results indicate that the crust of the study region is predominantly felsic, with an average V P /V S around 1.73 and an average thickness of 38.2 km, with a range of 27.4–48.6 km. Minimum curvature interpolation of the crustal thickness values has made it possible to delimitate of the Amazonian Craton, which corresponds to the area with an average thickness equal to or greater than 39 km. In addition, it was possible to identify its potential cratonic blocks, as well as the Paranapanema Block of Paraná Basin. The geometry of the craton, defined by its crustal thickness, is corroborated by the distribution of natural seismicity that accompanies its edges. These are related to suture zones between the Amazonian, São Francisco/Congo and Paranapanema paleocontinents. The sedimentary basins that have undergone rifting processes have a thinner crust, usually less than 37 km thick. Due to the great variability of the results, it was not possible to determine a characteristic value of crustal thickness or V P /V S ratio for each structural province located in the study region. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Jaguar represents an atypical Ni deposit part of the Carajás IOCG system. • Trace elements in magnetite from the Jaguar deposit support a hydrothermal origin. • Pyrite and chalcopyrite have compositions that resemble IOA and IOCG deposits. • The Neoarchean mafic–ultramafic layered intrusions represent potential Ni sources. • Trace element results support that binary discriminant diagrams are not unequivocal. The Jaguar nickel deposit represents an important and unconventional discovery of hydrothermal Ni resources (58.9 Mt @ 0.95% Ni) associated with magnetite and apatite within the Carajás Mineral Province, Northern Brazil. The Jaguar deposit shares several similarities with iron oxide copper–gold (IOCG) deposits of the Carajás Mineral Province, especially regarding the structural control, and the hydrothermal alteration. Although IOCG and Kiruna-type iron oxide-apatite (IOA) deposits are closely associated in several geological provinces, this association has not yet been fully constrained in the Carajás Mineral Province. The Jaguar deposit occurs near mafic–ultramafic intrusions, along a regional fault zone, and is hosted either by granitic (northern portion) or felsic subvolcanic (southern portion) rocks. It is characterized by sub-vertical zones hosting a biotite-chlorite (Bt-Chl) hydrothermal alteration with ductile structures and disseminated sulfides (pyrite and millerite) and magnetite. These zones are overprinted by sulfide-magnetite-apatite-bearing breccias, which represent the main mineralization bodies. The sulfide assemblage in the mineralized zones is dominated by pyrite, pentlandite and millerite, with minor sphalerite and chalcopyrite. We have investigated the concentration of trace elements in the magnetite and sulfides from the host rocks, different alteration facies and the mineralization at the Jaguar deposit by LA-ICP-MS. Magnetite composition ranges from higher Al, Mn, Ti and V contents, for at least part of those in host rocks (granitic and felsic subvolcanic) and Bt-Chl alteration, into lower contents in the magnetite-apatite alteration, associated with the main Ni mineralization. Magnetite with higher Ti and V contents in host rocks is associated with discrete hydrothermal alteration pockets, have high Ni/Cr ratios and plot in high temperature hydrothermal fields in multi-element diagrams. Therefore, we support that these features are compatible with a hydrothermal origin. Anomalously high-Ni contents in magnetite point that the Neoarchean mafic–ultramafic layered intrusions of the Carajás Province represent potential Ni sources for hydrothermal remobilization. Anomalously high V contents in magnetite suggest lower f O 2 conditions upon the formation of the deposit. The concentration of trace elements in magnetite associated with the Jaguar deposit is similar to those from other IOCG deposits from the Carajás Province (but with higher V and Ni contents), especially the Sossego mine in the Southern Copper Belt, indicating that it could represent a Ni-rich member of the regional-scale IOCG mineral system at Carajás. Pyrite and chalcopyrite have a composition similar to those from magmatic deposits, when using previously proposed discriminant diagrams, but this is also the case for other IOA and IOCG deposits. However, low PGE contents (Ru, Rh, Ir and Os) in pyrite from the Jaguar deposit support a hydrothermal origin. Our results highlight that the classifications provided by available discriminant diagrams are not unequivocal. We suggest that multi-element diagrams represent a complementary approach to binary plots and provide a more comprehensive classification for the use of indicator minerals. [ABSTRACT FROM AUTHOR]

Many studies have highlighted dam impacts in the reduction of the transmission of nutrient loads and suspended sediments downstream, increasing coastal erosion (sediment budget) and water residence time, beyond modifying the channel planform on a reach scale. However, the morphohydraulic changes, which are induced by flow regulation, have been little explored. This study has sought to analyze temporal changes in the at-a-station hydraulic geometry from 9 cross-sections along the São Francisco River, comparing the pre and post-dam periods. In addition to that, it was also sought to compare our results with other tropical river systems and assess the relationships of morphohydraulic parameters with channel patterns and functional trends. We have used field data collected by the National Water Agency and the Geological Survey of Brazil covering the middle, lower-middle, and lower courses of the São Francisco River. Potential functions were extracted using the least squares fit method. We have applied the Mann-Whitney non-parametric statistical test to assess any dataset changes. Finally, we have analyzed the data through the ternary diagram. Our results have demonstrated that the São Francisco River has high bank stability and variations in flow are mainly accommodated by changes both in depth and flow velocity. We found changes in the sensitivity of width, depth, and velocity in the post-dam period that imply high mobility of the cross-sections in the ternary diagram. Changes in land use patterns are potentially associated with these observed changes. Despite the São Francisco banks being stable in the analyzed cross-sections, the sensitivity of the channel width increased in the post-dam period. Differences were observed between the response patterns in the post-dam period in the physiographic provinces of the São Francisco River. However, the increase in width sensitivity predominates, with few exceptions. Hydraulic geometry analyses have proved to be essential for the characterization of the functional dynamics of river systems and allowing inferences about sediment transport conditions, flow stage, and channel stability. • Channel width is a conservative parameter in the São Francisco River. • Dam installation primarily affects flow velocity sensitivity and channel depth. • Changes in morphohydraulics are driven by dam implantation and land use alterations. [ABSTRACT FROM AUTHOR]

Amazonian rocks record one amongst the most complete and best-preserved Paleoproterozoic magmatic episodes on Earth. The present contribution documents the extremely well preserved paleoproterozoic architecture of a series of felsic rocks found in the Tapajós Mineral Province (TMP), located in the western part of Pará State, southern Amazon Craton, north of Brazil. These rocks are the first to be investigated to comprehend, based on their textural evidences, their emplacement mechanisms. Textural characterization allowed to identify three main facies with, as following reported, 1) chaotic (“Breccia”) group, 2) eutaxitic (“Eutax”) group and 3) parataxitic (“Paratax”) group vitrophyric textures. Given the superb preservation of our samples, the investigated rocks are grouped, according to their grade of welding, into a wide variety of lithofacies from very low-grade to high-grade and rheomorphic ignimbrites. In the “Paratax group” strong similarities with banding in lava flows are observed. Based on the presented data we discuss the effusive or explosive origin of the observed flow mechanisms. [ABSTRACT FROM AUTHOR]

• Low-cost effective method developed for EGS favorability mapping in Brazil. • Targeting high heat producing granites as hot dry rocks with low porosity. • Knowledge-driven fuzzy logic method integrating geoscientific databases under GIS. • Regional delimitation of six priority zones for future EGS research and exploration. • Complementarity with methods intended to estimate and map global EGS potential. Studies in Brazil related to geothermal resources have been carried out mainly by the National Observatory. Based on the results vast areas of Brazil have been delimited with a surface heat flow of over 60 mW/m2 and an excess temperature above 100 °C at depths between 3 and 6 km. The nature of the heat source and the mode of heat dissipation in these areas are largely unknown. Geothermal resources that might be associated with these areas can occur in hot sedimentary aquifers (HSA) and units of hot dry rocks (HDR) that are hosted, respectively, in intracratonic sedimentary basins and high heat producing granites (HHPG). The focus of our study is on finding resources of high enthalpy (above 150 °C) in HHPG, which regardless of their hydrological properties (porosity, permeability) could be considered as potential targets for enhanced geothermal systems (EGS). The structural provinces of Tocantins and Borborema were chosen as pilot regions based on both the indication of HDR and the wealth of open access georeferenced information. A comprehensive database was compiled and processed under GIS that includes geology, faults, soil typology, radiogenic heat, heat flow and geothermal gradient from wells, thermal spring locations, airborne gamma-ray spectrometry and magnetic surveys, terrestrial gravimetric surveys, and earthquake locations. In parallel, various critical factors were defined primarily to identify and characterize the most promising HHPG under insulating sedimentary rocks for EGS. These critical factors were grouped into three main categories labelled as: 'thermal source', 'thermal insulation' and 'active pathways'. In the absence of exploratory data on existing EGS in Brazil these critical factors were used in this study along three separate work streams to develop a knowledge-driven model based on fuzzy logic for improving the knowledge on the prospectivity of geothermal resources. The developed model was further tested for its validation. The selected approach is expected to be applicable to mapping areas with EGS potential in other regions with similar geology and open access georeferenced databases. [ABSTRACT FROM AUTHOR]

An updated geothermal database is presented by providing a new mapping of terrestrial heat flow over the entire Brazilian territory. New estimates of this geothermal parameter were obtained using an indirect method based on groundwater silica content (silica geothermometry) measured in 4949 wells. The Brazilian Geothermal Database of the Geothermal Laboratory from the National Observatory currently comprises geothermal data obtained directly from temperature-depth profiles mostly in hydrocarbon wells and from several other records in groundwater wells using indirect temperature estimates. The procedure of indirect temperature estimates used here assumes that the amount of silica dissolved in groundwaters depends on the in-situ temperatures of the host rocks or geothermal reservoirs. Following this initial calculation procedure, the heat flow, the depth of the aquifer and the temperature at the reservoir bottom were calculated using a numerical relationship that was specifically modified and adapted to the Brazilian geological context. Measurements of silica content in groundwater wells were obtained from the Groundwater Information System of the Geological Survey of Brazil. When available in the database, the lithological profiles of these wells were additionally used to calculate weighted average thermal conductivity by considering the documented lithotypes and their respective thicknesses. For wells without lithological information, their thermal conductivity was reassessed by including information from the nearest documented well lithotypes and that belong to the same geological context. The results indicate that regions encompassing a system of fractured aquifers, with a mean silica content of 40 ppm, show the highest heat flow values of the order of 70 m W m 2 on average. Regions with karstic aquifers, characterized by a mean silica content of 29 ppm, display most of the lowest heat flow values ≤ 40 m W m 2 . Regions with porous aquifers of mean silica content between 30 and 33 ppm present a range of intermediate heat flow values between 50 and 60 m W m 2 . Taking into account the different associated lithological types, the Brazilian structural provinces display values of thermal conductivity ranging between 2.1 and 2.7 W K. m. The refinement in mapping terrestrial heat flow in Brazil indicates various thermal anomalies at regional scale that can be further studied for prospecting geothermal resources of low-to-high temperature. These regional thermal anomalies are essentially found in the NW region of the Paraná Basin, N of the Tocantins Province (western portion), south-central Tocantins Province (eastern portion), north-central part of the Sáo Francisco Craton and the NE region of the Borborema Province. • Geothermal database of Brazil updated by integrating data from groundwater wells. • Terrestrial heat flow estimated from improved groundwater silica geothermometry. • Average thermal conductivity in wells recalculated and weighted based on lithotypes. • Higher heat flow mapping resolution with new target areas for geothermal exploration. [ABSTRACT FROM AUTHOR]

Abstract: The basal section of the lower Albian Barra Nova Group in the onshore portion of the Regência Platform, a central-eastern geological province of the Espírito Santo Basin, Brazil, comprises a mixed succession characterized by the cyclic alternation of dominant fluvial to shallow marine, siliciclastic rocks and marine carbonate strata. Spectral analysis of gamma-ray logs of four wells has shown consistent evidence of a short eccentricity orbital frequency (100 ka) influence on sedimentation. The spectral analysis points to a time span from 2.95 to 3 Ma (depending on the method employed) for the deposition of the interval studied in the most complete section (), and from 1.84 to 2.1 Ma in the least complete section (), owing to partial erosion. These figures are consistent with the palynological dating of the section (2 Ma), hence showing that cyclostratigraphy can play an important role in assigning a time span for high-frequency cycles (4th and 5th orders). Climatic factors not necessarily linked to eustasy, such as the alternation of dry (carbonate deposition) and mild humid (increased siliciclastic input) climate, are supposed to have controlled the cyclical stratigraphic arrangement. Accommodation as a result of diastrophism, loading and eustatic sea-level rise would not explain such a high mean accumulation rate (29.1 cm/ka) estimated through cyclostratigraphy. Therefore, based on these assumptions and seismic data, the high preservation potential, and resulting well-preserved cyclicity, is herein assigned to halokinesis. Several pieces of evidence obtained from the analysis of seismic data, core description, stratigraphic correlation, palaeontology and spectral data have shown the importance of halokinesis to increasing accommodation, and then the excellent preservation of the cyclical nature of the geological record. This is unusual in sedimentary environments subject to recurrent erosive events such as those related to shallow-marine settings. [Copyright &y& Elsevier]

Abstract: Mineralogical analyses and major and trace element geochemistry combined with iron and sulfur isotopes were applied to banded iron formations (BIFs) and pyrite-bearing sandstones from the Carajás mineral province, Brazil. The BIFs belong to the 2.7Ga old Grão Pará Group and show a rather simple mineralogy (hematite and quartz or cryptocrystalline silica). Their REE+Y patterns suggest deposition under marine sub-oxic conditions. Fe isotopes show highly positive values (δ57FeIRMM-14 =2.06±0.34‰ in average) which can be explained by the partial oxidation of Fe2+ aq in the surface ocean, subsequent to the upwelling of deep anoxic seawater. The resulting iron oxy-hydroxides likely formed in the shallow photic zone as a seasonal process and acquired highly positive iron isotope signature values consistent with a Rayleigh distillation model. The pyrite-bearing sandstones belong to the Aguas Claras Formation of (Palaeoproterozoic) younger age and were cored at the base of the giant Azul manganese deposit. Diagenetic pyrite is characterized by positive δ57FeIRMM-14 =1.02±0.2‰ and δ34SVCTD (11.97±0.12‰) average values, that are different from values of modern marine biogenic pyrite. Results likely indicate that dissolved sulfate was the limiting factor controlling biogenic pyrite formation at that time. Near-zero Δ33S values (0.013±0.003‰) indicate minimal mass-independent sulfur isotope fractionation (MIF-S), consistent with the 2.1Ga age previously obtained from the Aguas Claras pyrites. This result suggests deposition at a time when the oxygen partial pressure in the atmosphere was already high enough (10−5 to 10−2 PAL- or Present Atmospheric Level-) to hinder MIF-S by photochemical reactions. Incidentally, this also confirms that the giant Mn deposit of Carajás is in the same age range (2.1±0.2Ga) as other Mn ores in Gabon and South Africa, pointing to the importance of Mn precipitation in Paleoproterozoic times. [Copyright &y& Elsevier]

Topographic density distribution models are used in many geodetic and geophysical problems. Due to the difficulty in developing a high-resolution three-dimensional density model, many studies are focusing on the development of lateral density models and, despite the existence of global models, it is understood that these are less detailed and need to be improved to be used in regional and local studies. In this context, this research develops a distribution model of the lateral density of topographic masses for Brazil (LTD_Brazil, with 30 arc-seconds grid spacing), associated with their uncertainties, considering more detailed information from geological maps and apparent density values of rock types. The model results indicated an average density of 2459 kg/m³, ranging from 1625 to 3400 kg/m³, with standard deviations ranging from 8 to 351 kg/m³. This average density value has great influence of large Phanerozoic sedimentary basins (Amazonas/Solimões/Acre, Alto Tapajós, Parecis, Parnaíba and Paraná basins) located in Brazil. Furthermore, comparing the LTD_Brazil with a global model (UNB_TopoDensT), it was possible to suggest that the differences (ranging from −500 to 600 kg/m3), more significant in metamorphic and igneous rocks regions, are associated with greater detail of the lithological units used and accuracy of the model. • We compiled data from average density values for each rock type and combined them with lithological units. • We developed a lateral topographic density model. • We analyzed the model considering the Brazilian structural provinces, an average density value and an global model. [ABSTRACT FROM AUTHOR]

Rhyolites compose an important record in the volcanic history of Earth, with significant occurrences in volcanic arcs, large igneous provinces and post-collisional terranes, often associated with explosive events. In several geologic provinces, rhyolites dominate as the most expressive geologic units (e.g., Silicic Large Igneous Provinces - SLIPs). Despite their importance, several aspects related to the emplacement of rhyolite flows are still enigmatic. Recent studies in modern rhyolite lavas suggest similar emplacement mechanisms to basaltic lavas, implying a more dynamic growth model for silicic flows, including outbreak lobes and outpour structures. Despite these advances, studies related to the recognition of these features in ancient flows are still rare. In this work we perform a multi-proxy study of an ancient (Neoproterozoic) rhyolitic lava flow combining fieldwork, petrography, geochemistry, rheology and magnetic fabric analysis. The Cerro do Perau outcrop (CP, southern Brazil) consists of a natural laboratory for the study of rhyolite lavas, presenting excellent exposure of a partially preserved flow with distinct flow features and folds. CP flow consists of a high-silica and low-crystal content rhyolite, suggesting its emplacement as an obsidian flow. Rheology data indicates high liquidus temperatures (>957 °C), with maximum viscosities of 108.5 Pa s and glass transition temperatures (T g) of 750 °C. The absence of brittle features suggests little to none displacement below T g. Structural analysis indicates the predominance of sub-vertical foliation planes, including axial planes of folds, indicative of proximal (near-vent) regions in rhyolite flows. The absence of lineations favors a predominantly planar accommodation of the flow-induced deformation, which is confirmed by the shape of the magnetic fabric ellipsoids. Several of these ellipsoids display a high degree of anisotropy, mostly related to an oblate fabric, indicative of the development of high-strain zones within the flow. Our data suggest that CP flow presents some similarities with recently proposed field-based emplacement models for rhyolitic flows, highlighting the significant data that can be extracted from a combination of magnetic fabrics and rheological analyses. • A multi-proxy study was applied to the reconstruction of an ancient rhyolitic flow. • Rheology data implies original emplacement as an obsidian flow. • AMS and structural data suggests the development of flow-induced high-strain zones. • Structural and AMS data suggest similarities with recent, field-based models. [ABSTRACT FROM AUTHOR]

In this work, we perform a seismological investigation in the central Brazil region for focal mechanisms and stress. This large area (910.000 km2) is surrounded by several geological provinces: Eastern Amazon craton; southwestern Parnaíba basin; western São Francisco craton; northern Paraná basin and almost the whole Tocantins province. The area is crossed, in an SW-NE direction, by the continental-scale discontinuity - Transbrasiliano lineament (LTB). In the Brazilian seismic catalog (1720–2019), it is possible to distinguish a seismic belt following the LTB lineament and some dispersed events in the northern, western and eastern areas. The study area is characterized mainly by low magnitude events (M < 4); only 11 events reported in the catalog with M > 4 occurred, and among them just one with magnitude 5. Based on quality criteria a set of 118 events from 338 present in the catalog were selected for a thorough analysis. This work was divided in three parts: (i) Events relocation with the iLoc code and Regional Seismic Travel Time (RSTT) velocity model; (ii) Focal mechanisms determination of 10 events using waveform envelopes inversion and polarities, and; (iii) Inversion of focal mechanisms for stress field. The study shows that the seismicity of the area is mainly concentrated in two relatively narrow belts and that the major compressional stress axis of the whole zone is well resolved with azimuth about 133° which is compatible with previous studies of stress in central Brazil. [ABSTRACT FROM AUTHOR]