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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.

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.

The origin and evolution of the

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. key words: basement structures, slightly dipping basal detachment, sand layer thickness, basal friction.

Resumo A mina de Piteiras, localizada no distrito esmeraldífero de Itabira-Nova Era, Minas Gerais, foi estudada em termos de geologia, mineralogia e gênese. A mineralização concentra-se num corpo tabular com dois a quatro metros de largura e centenas de metros de extensão composto de xistos metaultramáficos com variáveis teores de f1ogopita e Mg-hornblenda. O corpo apresenta-se fortemente cisalhado e intrudido por pegmatitos sintectônicos ricos em albita. As encaixantes são paragnaisses pertencentes a uma seqüência metavu!canossedimentar de possível idade arqueana, metamorfizados em condições da fácies anfibolito em temperatura em torno de 600°C e pressão de 5,5kbar. Esmeralda é encontrada em f1ogopita xistos e veios de quartzo nas proximidades dos pegmatitos. Durante um evento tectonometamórfico regional desenvolveu-se um zona de cisalhamento pervasiva numa camada ultramáfica da seqüência metavu!canossedimentar.A deformação concentrou-se preferencialmente nessa camada devido ao contraste reológico. Pegmatitos intrudiram o corpo metaultramáfico durante o cisalhamento e forneceram os fluidos que causaram o processo metassomático que levou à f1ogopitização e à geração da esmeralda por meio do Se e Si contidos nos fluidos residuais e do cromóforo Cr dos xistos. A percolação dos fluidos teve controle estrutural.

Scaled sandbox models are used to simulate the development of´basement´-controlled, salients. We investigate 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 Fundão-Cambotas Fault System in the Proterozoic fold-thrust belt in the Quadrilátero Ferrífero region, along the southeastern margin of the São 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 Fundão-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.