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SUMMARY Palaeosecular variation (PSV) determinations and studies of the geometry of the Earth’s main magnetic field provide important information about the field evolution, and to constrain numerical geodynamo models. Palaeomagnetic directional data from lava flows over the last few million years is of particular interest because the regional and global tectonic effects are minimal. However, the distribution of this type of palaeomagnetic data is uneven where the Southern Hemisphere is the destitute side. Therefore, the better knowledge of the geomagnetic field behaviour depends on the increased availability of high-quality data, especially in the Southern Hemisphere. A PSV and the time-averaged field (TAF) study was then performed in 0–5 Ma lava flows from the Caviahue–Copahue Volcanic Complex located in Northern Patagonia, Argentina (37°0′S, 71°10′W). The magnetic mineralogy of lava flows was investigated through thermomagnetic susceptibility curves, isothermal remanent magnetization (IRM) acquisition curves, hysteresis loops and first-order reversal curves (FORCs). Samples are essentially comprised of titanomagnetite with different Ti contents and magnetic domain structures typical of vortex state particles. A total of 50 volcanic sites were sampled, which provided 42 reliable palaeomagnetic site-mean directions after alternating field and thermal demagnetization. From these 42 sites, 36 are of normal and 6 are of reversed polarity. The mean direction from normal (reversed) sites is D = 356.2°, I = −50.1°, α95 = 4.0° and N = 36 (D = 176.5°, I = 59.5°, α95 = 14.1° and N = 6). Using only site-level data with the precision parameter k ≥ 100, we obtain 26 palaeomagnetic sites for PSV and TAF investigations in the study region. The filtered data set has a mean direction (D = 354.4°, I = −53.2° and α95 = 5.1°) close to the expected direction for a geocentric axial dipole (GAD) field (IGAD = −57.3°). The palaeopole (Plat = 84.4°, Plon = 229.1° and A95 = 5.7°) coincides with the Earth’s spin axis within the 95 per cent confidence interval. Virtual geomagnetic pole scatter ($S_{B} = {15.8^{18.9}_{11.8}}^{\circ }$) and the inclination anomaly ($\Delta I = 4.1_{-1.0}^{{9.2}^{\circ }}$) are both consistent at the 95 per cent confidence level with recent PSV and TAF models, respectively. Our results support the presence of small non-dipole field contributions (

Pyroclastic deposits can cover significant areas and register major geological events. Despite their importance, understanding depositional dynamics of pyroclastic density currents (PDCs) and linking explosive deposits to their emission centers is still a challenge, especially in the case of non-welded, massive ignimbrites. Located in the Southern Andes, the Caviahue Copahue Volcanic Complex (CCVC) comprises one of the most active volcanic centers in the Andean Belt. This volcanic complex hosts massive ignimbrites with both source emplacement poorly constrained, currently grouped in the Riscos Bayos Ignimbrites (RBI). In this contribution, we perform a full magnetic characterization and anisotropy of magnetic susceptibility (AMS) study on the massive RBI of the CCVC. The magnetic characterization was performed using magnetic experiments including isothermal remanet magnetization, thermomagnetic curves, hysteresis loops, first-order reversal curves, and scanning electron microscopy. Magnetic experiments indicate primary, multi-domain, high Curie temperature titanomagnetites as the AMS carriers. Ellipsoids are predominately oblate, with a low degree of anisotropy and east-southeastward imbrication. This fabric arrangement is consistent with PDC sedimentary fabrics deposited under laminar flow conditions. Despite RBI massive structure AMS data reveals changes in transport capacity of the PDC and particle organization. These changes are marked by increasing AMS dispersion and decreasing degree of anisotropy up-section within flow units. Directional statistics of AMS data implies the Las Mellizas Caldera as the emission center of RBI. The reconstructed flow path also suggests the PDC overrun of the Caviahue Caldera topographic rim. This study highlights the application of AMS to the identification of emission centers of explosive deposits, featuring its application to massive ignimbrites.

Fil: Moncinhatto, Thiago R. Universidade de Sao Paulo. Instituto de Astronomia, Geofısica e Ciências Atmosféricas. São Paulo, Brazil. Fil: Haag, Maurício B. Universidade Federal do Rio Grande do Sul. Instituto de Geociências. Porto Alegre, Brazil. Fil: Hartmann, Gelvam A. Universidade Estadual de Campinas. Instituto de Geociências.Campinas. Brazil. Fil: Savian, Jairo F. Universidade Federal do Rio Grande do Sul. Instituto de Geociências. Porto Alegre, Brazil. Fil: Poletti, Wilbor. Universidade Federal dos Vales do Jequitinhonha e Mucuri. Instituto de Ciência e Tecnologia. Diamantina, Brazil. Fil: Sommer, Carlos A. Universidade Federal do Rio Grande do Sul. Instituto de Geociências. Porto Alegre, Brazil. Fil: Caselli, Alberto Tomás Universidad Nacional de Río Negro. Laboratorio de Estudio y Seguimiento de Volcanes Activos. Río Negro, Argentina. Fil: Trindade, Ricardo I. Universidade de Sao Paulo. Instituto de Astronomia, Geofısica e Ciências Atmosféricas. São Paulo, Brazil. Anisotropy of magnetic susceptibility is a petrofabric tool used to estimate the alignment of minerals at the site-scale, the imbrication between the magnetic foliation and the emplacement surface being an indicator of flow direction. However, despite numerous studies examining the flow direction in pyroclastic deposits and lava flows, the effect of magnetic mineralogy and the domain state of ferromagnetic phases on the magnetic fabric remains poorly understood. This paper describes the magnetic mineralogy and its influence on the magnetic fabric of Plio-Pleistocene lava flows and ignimbrites of the Caviahue-Copahue Volcanic Complex in the Andean Southern Volcanic Zone, Argentina. Rock magnetism, anisotropy of magnetic susceptibility and anhysteretic remanent magnetization and petrographic observations were performed on 30 sites of the volcanic complex. Results revealed the extrusive and pyroclastic rocks present varied magnetic mineralogy, formed in different stages of the magmatic evolu- tion. Magnetic mineralogy variations strongly affect the anisotropy of magnetic susceptibility data in volcanic rocks and associated ignimbrites, providing ‘scattered’ fabrics when late Ti- rich titanomagnetite phases dominate the fabric, and ‘inverse’ or ‘intermediate’ fabrics when single-domain grains are present. ‘Normal’ fabrics are typically found when early crystallized pure magnetite is present. Our results highlight the complexity in the interpretation of magnetic anisotropy data in volcanic rocks and ignimbrites. Anisotropy of magnetic susceptibility is a petrofabric tool used to estimate the alignment of minerals at the site-scale, the imbrication between the magnetic foliation and the emplacement surface being an indicator of flow direction. However, despite numerous studies examining the flow direction in pyroclastic deposits and lava flows, the effect of magnetic mineralogy and the domain state of ferromagnetic phases on the magnetic fabric remains poorly understood. This paper describes the magnetic mineralogy and its influence on the magnetic fabric of Plio-Pleistocene lava flows and ignimbrites of the Caviahue-Copahue Volcanic Complex in the Andean Southern Volcanic Zone, Argentina. Rock magnetism, anisotropy of magnetic susceptibility and anhysteretic remanent magnetization and petrographic observations were performed on 30 sites of the volcanic complex. Results revealed the extrusive and pyroclastic rocks present varied magnetic mineralogy, formed in different stages of the magmatic evolu- tion. Magnetic mineralogy variations strongly affect the anisotropy of magnetic susceptibility data in volcanic rocks and associated ignimbrites, providing ‘scattered’ fabrics when late Ti- rich titanomagnetite phases dominate the fabric, and ‘inverse’ or ‘intermediate’ fabrics when single-domain grains are present. ‘Normal’ fabrics are typically found when early crystallized pure magnetite is present. Our results highlight the complexity in the interpretation of magnetic anisotropy data in volcanic rocks and ignimbrites.

Fil: Haag, Mauricio Barcelos. Instituto de Geociências, Universidade Federal do Rio Grande do Sul. Brasil. Fil: Sommer, Carlos Augusto. Instituto de Geociências, Universidade Federal do Rio Grande do Sul. Brasil. Fil: Savian, Jairo Fransciso. Instituto de Geociências, Universidade Federal do Rio Grande do Sul. Brasil. Fil: Caselli, Alberto. Universidad Nacional de Río Negro. Laboratorio de Estudio y Seguimiento de Volcanes Activos. Argentina. Fil: Moncinhatto, Thiago Ribas. Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo. Brasil. Fil: Hartmannd, Gelvam André. Instituto de Geociências, Universidade Estadual de Campinas. Brasil. Fil: Ort, Michael. School of Earth and Sustainability. Estados Unidos. Fil: Poletti, Wilbor. Instituto de Ciência e Tecnologia, Universidade Federal dos Vales do Jequitinhonha e Mucuri. Brasil Fil: Ferreira da Trindade, Ricardo Ivan. Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo. Brasil. Pyroclastic deposits can cover significant areas and register major geological events. Despite their importance, understanding depositional dynamics of pyroclastic density currents (PDCs) and linking explosive deposits to their emission centers is still a challenge, especially in the case of non-welded, massive ignimbrites. Located in the Southern Andes, the Caviahue Copahue Volcanic Complex (CCVC) comprises one of the most active volcanic centers in the Andean Belt. This volcanic complex hosts massive ignimbrites with both source emplacement poorly constrained, currently grouped in the Riscos Bayos Ignimbrites (RBI). In this contribution, we perform a full magnetic characterization and anisotropy of magnetic susceptibility (AMS) study on the massive RBI of the CCVC. The magnetic characterization was performed using magnetic experiments including isothermal remanet magnetization, thermomagnetic curves, hysteresis loops, first-order reversal curves, and scanning electron mi croscopy. Magnetic experiments indicate primary, multi-domain, high Curie temperature titanomagnetites as the AMS carriers. Ellipsoids are predominately oblate, with a low degree of anisotropy and east-southeastward imbrication. This fabric arrangement is consistent with PDC sedimentary fabrics deposited under laminar flow conditions. Despite RBI massive structure AMS data reveals changes in transport capacity of the PDC and particle organization. These changes are marked by increasing AMS dispersion and decreasing degree of anisotropy up-section within flow units. Directional statistics of AMS data implies the Las Mellizas Caldera as the emission center of RBI. The reconstructed flow path also suggests the PDC overrun of the Caviahue Caldera topographic rim. This study highlights the application of AMS to the identification of emission centers of explosive deposits, featuring its application to massive ignimbrites.

The Caviahue-Copahue Volcanic Complex (CCVC, Argentina) composes one of the most active volcanic centers in the Southern Volcanic Zone (SVZ) of the Andes, characterized by the presence of voluminous explosive and effusive deposits. Despite its young age (< 5 Ma), CVCC deposits were strongly affected by two glaciations, leading to the removal of a considerable volume of the original deposits, requiring alternative techniques for the reconstruction of this volcanic center. The Riscos Bayos Ignimbrites (RBI) consist of a sequence of non-welded ignimbrites, located approximately 15 km southeast of the CVCC. This unit is commonly associated with the putative collapse of Caviahue caldera (15 x 20 km, 1 km deep) during the Pleistocene, although the source area and emplacement conditions of RBI still poorly constrained. In this work, we combine fieldwork, anisotropy of magnetic susceptibility (AMS, 23 sites) and rheological analyses (17 samples) in order to trace RBI source region and constrain its emplacement conditions, addressing its relevance to CVCC evolution. Rheological parameters, including viscosity, glass transition temperature, and liquidus temperatures were obtained using numerical models available from the literature, while AMS samples were measured using a Kappabridge MFK1-A (Agico) and the data processed using Anisoft5 (Agico). The magnetic mineralogy was characterized using several experiments, including isothermal remanet magnetization, thermomagnetic curves, hysteresis loops, first-order reversal curves and scanning electron microscopy. Our data indicate liquidus temperatures ranging from 969 to 1100 ºC, glass transition temperatures from 653 to 721 ºC, and viscosity (at liquidus temperature) from 3.4 to 7.3 log Pa.s. The absence of welding features in the samples implies RBI emplacement at temperatures below the glass transition temperature, suggesting a fast and effective cooling of the pyroclasts before their settling. The low crystal content of the samples suggests eruption temperatures close to the calculated liquidus temperature of the melt. AMS directional analyses indicate a consistent transport sense to SSE (Az of approximately 100º), implying the southern rim of the CVCC as the main source region of RBI. Magnetic experiments show primary, multi-domain, high curie temperature (580 ºC) titanomagnetites as the main carriers of the AMS signal. Most ellipsoids display oblate to triaxial geometry, with a low degree of anisotropy (< 5%) and magnetic susceptibility (1.0 x 10-2 SI). The low welding degree of RBI units and its geographic distribution outside the Caviahue depression contributes to the Caviahue caldera hypothesis in the region, suggesting its emplacement as an ‘extra-caldera’ pyroclastic unit.

Since the establishment of direct estimations of the Earth's magnetic field intensity in the first half of the nineteenth century, a continuous decay of the axial dipole component has been observed and variously speculated to be linked to an imminent reversal of the geomagnetic field. Furthermore, indirect estimations from anthropologically made materials and volcanic derivatives suggest that this decrease began significantly earlier than direct measurements have been available. Here, we carefully reassess the available archaeointensity dataset for the last two millennia, and show a good correspondence between direct (observatory/satellite) and indirect (archaeomagnetic) estimates of the axial dipole moment creating, in effect, a proxy to expand our analysis back in time. Our results suggest a continuous linear decay as the most parsimonious long-term description of the axial dipole variation for the last millennium. We thus suggest that a break in the symmetry of axial dipole moment advective sources occurred approximately 1100 years earlier than previously described. In addition, based on the observed dipole secular variation timescale, we speculate that the weakening of the axial dipole may end soon.

Temporal variations of Earth\'s magnetic field provide a great range of geophysical information about the dynamics at different layers of the Earth. Since it is a planetary field, regional and global aspects can be explored, depending on the timescale of variations. In this thesis, the geomagnetic field variations for the last two millennia were investigated. For that, some improvement on the methods to recover the ancient magnetic field intensity from archeological material were done, new data was acquired and a critical assessment of the global archaeomagnetic database was performed. Two methodological advances are reported, comprising: i) the correction for microwave method of the cooling rate effect, which is associated to the difference between the cooling times during the manufactory of the material and that of the heating steps during the archaeointensity experiment; (ii) a test for thermoremanent anisotropy correction from the arithmetic mean of six orthogonal samples. The temporal variation of the magnetic intensity for South America was investigated from nine new data, three from ruins of the Guaraní Jesuit Missions and six from archaeological sites associated with jerky beef farms, both located in Rio Grande do Sul, Brazil, with ages covering the last 400 years. These data combined with the regional archaeointensity database, demonstrates that the influence of significant non-dipole components in South America started at ~1800 CE. Finally, from a reassessment of the global archaeointensity database, a new interpretation was proposed about the geomagnetic axial dipole evolution, where this component falls constantly since ~700 CE associated to the breaking of the symmetry of the advective sources operating in the outer core. Variações temporais do campo magnético da Terra fornecem uma grande diversidade de informações geofísicas sobre a dinâmica das diferentes camadas da Terra. Por ser um campo planetário, aspectos regionais e globais podem ser explorados, dependendo da escala de tempo das variações. Nesta tese, foram investigadas as variações do campo geomagnético para os dois últimos milênios. Para isso, aprimoramentos nos métodos de aquisição da intensidade geomagnética registrada em materiais arqueológicos foram realizados, bem como a aquisição de novos dados e uma avaliação crítica da base de dados arqueomagnética global. Dois novos avanços metodológicos são aqui propostos, sendo eles: i) correção para o método de micro-ondas do efeito da taxa de resfriamento, que está associada à diferença entre os tempos de resfriamento durante a manufatura do material e o das etapas de aquecimento durante o experimento de arqueointensidade; (ii) teste para correção da anisotropia termorremanente a partir da média aritmética de seis amostras posicionadas ortogonalmente umas às outras durante o experimento de arqueointensidade. A variação temporal da intensidade magnética para a América do Sul foi investigada a partir de nove dados inéditos, sendo três provenientes das ruínas das Missões Jesuíticas Guaraníticas e seis de sítios arqueológicos associados a fazendas de charque, ambos localizados no Rio Grande do Sul, Brasil, com idades que cobrem os últimos 400 anos. Esses dados, combinados com o banco de dados regionais de arqueointensidade, demonstram que a influência significativa de componentes não-dipolares do campo magnético na América do Sul começou em ~1800 CE. Finalmente, a partir de uma reavaliação do banco de dados globais de arqueointensidade uma nova interpretação foi proposta a respeito da evolução do dipolo axial geomagnético, sugerindo que essa componente está decrescendo constantemente desde ~700 CE devido à quebra da simetria das fontes advectivas que operam no núcleo externo.

We obtained six new high-quality archeointensity results for the Pelotas city region, in South Brazil with ages ranging from 1790 to 1943 CE. Archeointensity measurements were performed with the double heating technique, including partial thermoremanent magnetization (pTRM) checks and pTRM tail-checks. Measurements were corrected from anisotropy of TRM and cooling-rate effects. Archeological sites providing successful results comprised: Charqueada Santa Barbara (1790–1838 CE; 36.4 ± 0.6 μT), Chacara da Brigada Militar (1830–1851 CE; 32.6 ± 2.9 μT), Casa Numero 08 (1878–1879 CE; 32.1 ± 3.1 μT), Colegio da Familia de Eliseu Maciel (1881–1883 CE; 28.7 ± 2.7 μT), Fundacao Simon Bolivar (1884–1889 CE; 30.1 ± 2.0 μT), Campus Porto UFPel (1942–1943 CE; 27.8 ± 2.0 μT). Two other sites did not provide meaningful results. The new data complement previous results obtained in South and Southeast Brazil, Argentina and Chile and provide additional evidence for the rapid decay of the field in the region from about 1500 to 2000 CE. In addition, they show a gradual increase in the contribution of non-dipolar components in the geomagnetic field of South America since 1800 CE, likely indicating a strengthening of the South Atlantic Anomaly over the continent from this time onward.

The São Francisco Craton is one of the fundamental pieces on the evolutionary history of the Gondwana supercontinent. At its southeast edge, the Southern Espinhaço Range (SER) is built mainly by Paleo- and Mesoproterozoic units of the Espinhaço Supergroup. At its western domain, one of its major components, the Conselheiro Mata Group, occurs as a relatively narrow strip, being composed by a succession of formations related to coastal and shallow marine environments. Aiming to present and discuss an integrated research, detailed fieldwork was carried out, gamma-ray spectrometry data were reprocessed, and ground gravity data were acquired. This integration allows clarifying the regional crustal structure and the general geometry of the Espinhaço basin. The defined gamma-ray spectrometry domains fit well to the lithological types and structural framework of the stratigraphic units. Bouguer residual anomaly highlights an amplitude of 27 mGal, ranging from -12 to 15 mGal, outlining a gravity compartmentation defined by a central negative anomaly (CNA) surrounded by positive anomalies, which is approximately in conformity with the regional tectonic segments. The CNA conforms to the main axis of SER, indicating the residual metasedimentary coverage of the Espinhaço basin. Its center, positioned above the Lamarão anticline, houses the nucleus of the precursor basin of Conselheiro Mata Group.Keywords: gravimetry; Espinhaço Supergroup; Conselheiro Mata Group.Integrando mapeamento geológico e análises geofísicas na Serra do Espinhaço Meridional, borda leste do Cráton São Francisco, BrasilRESUMO. O Cráton São Francisco é uma das peças fundamentais da história evolutiva do Supercontinente Gondwana. Em sua borda sudeste, a Serra do Espinhaço Meridional é formada, principalmente, por unidades Paleo- a Mesoproterozoicas do Supergrupo Espinhaço. Um de seus principais componentes, o Grupo Conselheiro Mata, ocorre como uma faixa relativamente estreita no domínio ocidental da serra, sendo composto por uma sucessão de formações relacionadas a ambientes marinhos rasos e costeiros. Com o objetivo de apresentar e discutir uma pesquisa integrada, foram realizados trabalho de campo detalhado, processamento de dados gamaespectrométricos e aquisição de dados gravimétricos terrestres. Tal integração permite esclarecer a estrutura crustal regional e a geometria geral da bacia Espinhaço. Os domínios gamaespectrométricos definidos ajustam-se bem às litologias e ao arcabouço estrutural das unidades estratigráficas. Anomalias Bouguer residuais demonstram uma amplitude de 27 mGal, variando entre -12 a 15 mGal, concordando aproximadamente com os segmentos tectônicos regionais e esboçando uma compartimentação gravimétrica definida por uma anomalia negativa central (CNA), cercada por anomalias positivas. A CNA concorda com o eixo principal da Serra do Espinhaço Meridional, definindo a cobertura metassedimentar residual da bacia Espinhaço. Seu centro, posicionado sobre o anticlinal do Lamarão, abriga o núcleo da bacia precursora do Grupo Conselheiro Mata.Palavras-chave: gravimetria; Supergrupo Espinhaço; Grupo Conselheiro Mata. How to cite: FÉRES WML, ABREU PAA & POLETTI W. 2021. Integrating geological mapping and geophysical analyses in Southern Espinhaço Range, eastern edge of the São Francisco Craton, Brazil. Brazilian Journal of Geophysics, v. 39, n. 1. doi: 10.22564/rbgf.v39i1.2087.

[1] New microwave (MW) paleointensity data on historical bricks from Northeast Brazil presented a bias toward higher fields when compared to previous cooling-rate corrected double-heating paleointensity estimates; the same relates to the previously reported values for pottery from Southwestern Pacific islands. A simple theoretical approach suggests that the MW bias in both collections is due to a cooling-rate effect on MW estimates. We then experimentally corrected the MW cooling-rate effect on Brazilian fragments, increasing the degree of consistency between the previous and new results (reducing discrepancies from 25% to 8%). Results indicate similar experimental behavior between microwave and thermal procedures despite the different ways in which the energy is transferred into the spin system. Finally, they allow cooling times of less than 90 s to be empirically estimated in most of these MW experiments highlighting the need for systematic cooling-rate corrections to be applied in similar MW paleointensity studies in the future.