Caroline Janette Souza Gomes

Experimental models of 'Basement'-controlled salients - application to the proterozoic fold-thrust belt of the quadrilátero ferrífero (Minas Gerais, Southeastern Brazil)

Scaled sandbox models are used to simulate the development ofbasement´-controlled, salients. We investi- gate the controlling factors on the development of closed curvatures in map-view, considering constant both the sand pack thickness and the space between obstacles. These models are compared with the Fundao- Cambotas Fault System in the Proterozoic fold-thrust belt in the Quadrilatero Ferrifero region, along the southeastern margin of the Sao Francisco craton. In the experiments, a pronounced curvature resulted from the margin-controlled salient process in the presence (i) of a basal ductile detachment, or (ii) of pre-existing structures. The results suggest that the convex-to-the-foreland, west-vergent Fundao-Cambotas Fault System that borders the Archean basement highs and displaced older Transamazonian structures westward, is partly a consequence of interaction of propagating thrusts with obstacles in the foreland.

The role of backstop shape during inversion tectonics physical models

The style of deformation of rocks from basin-infilling sequences in positively inverted natural basins was discussed upon the results of laboratory experiments carried out in sandboxes with sand packs laid down in the space between two wooden blocks. The space simulated stages of crustal extension leading to (1) a half graben due to extension above a listric extensional detachment, with the blocks simulating the footwall and hanging wall, or (2) a graben, with the blocks simulating the external margins that drifted apart above a horizontal detachment. Combinations of two different angles were used to simulate the dip of curved normal faults along the internal face of the wooden blocks. Backstops in the half graben had a convex up internal face. Backstops in the graben had a concave up internal face. Shortening was partitioned in forward and backward movements within the sand packs, and the kinematics of contraction was largely influenced by the convex or concave internal faces. A buttress effect characterized by rotation of the sand pack close to the footwall was stronger for footwall with steeper-dipping internal faces. The results were compared to other physical experiments and applied to an inverted basin found in nature.

Geometry and kinematics of experimental antiformal stacks

Sandbox experiments with different boundary conditions demonstrate that antiformal stacks result from a forward-breaking thrust sequence. An obstacle blocks forward thrust propagation and transfers the deformation back to the hinterland in a previously formed true duplex. In the hinterland, continued shortening causes faults to merge toward the tectonic transport direction until the older thrusts override the younger thrusts. In experiments using thin sand layers or high basal friction, shortening is accommodated by a cyclic process of thrusting, back rotation of the newly formed thrust combined with strong vertical strain, and nucleation of a new thrust. Continuous deformation produces an antiformal stack through progressive convergence of branch lines.

Positive inversion of extensional footwalls in the southern Serra do Espinhaço, Brazil - insights from sandbox laboratory experiments

Analogue experiments were carried out to get insights into the processes governing positive inversion during the foreland propagating thrust tectonics in the southern Serra do Espinhaço, a Brasiliano/Panafrican foldthrust belt in southeast Brazil. In particular, model listric half-grabens were inverted by applying contractional displacement to the footwall blocks. We investigated two different inversion conditions in listric half-grabens: (i) extensional and contractional detachments at the same level and (ii) at different positions. The models revealed that the development of a forward-breaking thrust system occurs in the basin synrift deposits, by contractional translation of the extensional footwall block when the extensional and contractional master faults do not coincide. Our experiments show the tectonic imbrication between basement and synrift sequences which characterizes the southern Serra do Espinhaço, and support the location in the eastern mountain range domain of the Espinhaço rift master fault system, which is not exposed at the surface.

Testing the influence of a sand mica mixture on basin fill in extension and inversion experiments

We compare the deformation patterns produced by sand and a sand mica mixture (14:1 ratio of sand to mica by weight) while simulating basin fill in extension and inversion models to analyze the potential of the sand mica mixture for applications that require a strong elasto-frictional plastic analogue material in physical models. Sand and the sand mica mixture have nearly equal angles of internal friction, but the sand mica mixture deforms at a significantly lower level of peak shear stress. In extension, the sand mica mixture basin fill experiments show fewer normal faults. During inversion, the most striking difference between the sand and the sand mica mixture basin fill experiments is related to the internal deformation in fault-propagation folds, which increases with an increase in the basal friction. We conclude that our strongly elasto-frictional plastic sand mica mixture may be used to simulate folds in experiments that focus on mild inversion in the brittle crust.

Structural evolution of Au, Ag and Pb lode occurrences of Serrita and Parnamirim, Pernambuco, Brazil

Um levantamento estrutural e microestrutural foi realizado para analisar a evolucao das ocorrencias filoneanas de Au, Ag e Pb, localizadas entre as cidades de Serrita e Parnamirim, PE. Essas ocorrencias ocorrem encaixadas em metassedimentos, correlacionados ao Grupo Salgueiro, e em granodioritos associados a Suite Serrita, no Dominio Pianco-Alto Brigida, sudoeste da Zona Transversal, Provincia Borborema. Foram identificadas quatro fases de deformacao nos metassedimentos, que registram a mudanca de uma tectonica compressiva para direcional. As duas primeiras geraram uma trama de baixo ângulo, enquanto a terceira e a quarta fases formaram dobras, com clivagem de crenulacao NE-SW/ESE-WSW, e zonas de cisalhamento regionais, respectivamente. Nos metassedimentos, os veios formaram-se a partir de fraturas anastomosadas de direcao E-W e N-S/NNW-SSE, cuja propagacao e dilatacao ocorreram em curtos intervalos de tempo. Apos sua cristalizacao, os veios foram submetidos a novas etapas de cristalizacao mineral concomitante a processos de deformacao e mineralizacao, os quais marcam a transicao do regime ruptil-ductil para o ruptil. Os veios nos granitoides exibem deformacao incipiente e similar a rocha encaixante. Concluiu-se que, nas duas encaixantes, os veios se formaram sob o mesmo campo de tensao, durante o final da quarta fase, sob um tensor de compressao maxima de direcao NW-SE. As diferencas resultariam de um processo de refracao de fraturas em funcao do contraste reologico das encaixantes. Essa refracao induziu a formacao de fraturas de cisalhamento direcionais com direcoes E-W/ESE-WNW e N-S/NNW-SSE na litologia menos competente, o metassedimento, e fraturas de tracao de direcao NW-SE na litologia mais competente, o granodiorito.

As propriedades friccionais de areias de quartzo (natural e colorida): medidas efetuadas em experimentos físico-analógicos e em um ring-shear tester

To improve knowledge of the quartz sand used in analogue modeling, two laboratory tests were carried out to investigate the frictional properties of colored sand. The first one estimated the coefficient of internal friction from the reverse fault dip simulated in a pack of different colored dry sand layers. The second test consisted of measuring stress-strain curves separately for different colored sand using a ring-shear tester. The coefficient of internal friction determined from fault dips is significantly lower and less precise than that determined on the same material with a ring-shear tester. Comparisons with previous granular material tests reveal that although our ring-shear test results are high, they are within the range of values reported in literature. Our results also show that natural and colored sands have different angles of internal friction, and that the Acrilex dye produces a distinct mechanical behaviour for different colored sand. The variation is related to the chemical composition of sand dyes and should be carefully evaluated in future sandbox experiments.

Analog Models of Flanking Structures and a Natural Example in the Quadrilátero Ferrífero, Minas Gerais

The aim of this study is to present analog models of flanking structures and to analyze the Fábrica Nova synform, Quadrilátero Ferrífero, Minas Gerais, from a geometric point of view. We set up seven models using a linear viscous silicone and produced flanking structures with a shear velocity of 2 cm h-1. At different initial orientations with respect to the shear zone boundary, a rigid cross-cutting element with lubricated boundaries was deformed via sinistral bulk flow at a shear strain of γ = 1.28. The most interesting features of our experiments are the geometries of the different marker lines, which are heterogeneous and resulted from thickening and thinning of the silicone at the cross-cutting element terminations. To compare our analog models and the Fábrica Nova synform, we analyzed the outermost marker line of the analog models and the top surface of the Cauê Formation in the Paleoproterozoic metasediments. The best comparisons between the experiments and the natural example were obtained by our CIS90 model in terms of the flexure shape near the cross-cutting element and the cross-cutting element orientation. Thus, we suggest that the cross-cutting elements in both situations act as obstacles and consequently produce local perturbations in laminar flow.

Glass Microbeads in Analog Models of Thrust Wedges

Glass microbeads are frequently used in analog physical modeling to simulate weak detachment zones but have been neglected in models of thrust wedges. Microbeads differ from quartz sand in grain shape and in low angle of internal friction. In this study, we compared the structural characteristics of microbeads and sand wedges. To obtain a better picture of their mechanical behavior, we determined the physical and frictional properties of microbeads using polarizing and scanning electron microscopy and ring-shear tests, respectively. We built shortening experiments with different basal frictions and measured the thickness, slope and length of the wedges and also the fault spacings. All the microbeads experiments revealed wedge geometries that were consistent with previous studies that have been performed with sand. However, the deformation features in the microbeads shortened over low to intermediate basal frictions were slightly different. Microbeads produced different fault geometries than sand as well as a different grain flow. In addition, they produced slip on minor faults, which was associated with distributed deformation and gave the microbeads wedges the appearance of disharmonic folds. We concluded that the glass microbeads may be used to simulate relatively competent rocks, like carbonates, which may be characterized by small-scale deformation features.

O cinturão epidérmico de antepaís da Bacia de Irecê, Cráton do São Francisco: principais elementos estruturais e modelagem física analógica

Located in the central portion of Bahia state, Irece Basin displays the best exposures of neoproterozoic sedimentary cover at Northern Sao Francisco Craton. Despite of the large amount of geological studies performed there, some questions remain unsolved, especially concerning the tectonic evolution of the thin-skinned fold-and-thrust belt that involves the rocks of the basin. In order to contribute to the understanding of such evolution, the present study reviews the main structural elements of the basin and surroundings, and present new data acquired through sandbox physical analog modeling. The Thin-skinned Fold-and-thrust Belt of Irece Basin is a great curved feature, confined in the homonymous syncline, whose genesis is related to the development of orogenic belts north of Sao Francisco Craton. Its evolution was conditioned by a N-S tectonic vector, responsible by the nucleation of E-W folds and thrusts. At basin boundaries, the deformation is accommodated by strike-slip faults, which locally rotated early structures. Towards south, the belt gradually loses its expression, only remaining structures related to the Chapada Diamantina thrust-and-fold system. The sandbox analog model successfully simulated the development of the Thin-skinned Fold-and-thrust Belt of Irece Basin, and indicates that its map-view curve results from the interaction with the syncline borders, as well as substrate geometry of the foreland belt. The propagation was made through a low-friction detachment, probably conditioned by the rheological contrast between the Una Group carbonates and the underlying Espinhaco Supergroup siliciclastic rocks.