The role of backstop shape during inversion tectonics physical models.

O estilo da deforma??o de sequ?ncias sedimentares de bacias submetidas a uma invers?o tect?nica positiva foi discutido a partir da an?lise de modelos laboratoriais, desenvolvidos em caixas de experimentos, com camadas de areia depositadas no espa?o entre dois blocos de madeira. O espa?o simulava est?gios de extens?o crustal que conduziram ? forma??o de (1) um hemi-graben, gerado sobre um descolamento basal l?strico, com os blocos simulando o teto e o muro; e (2) um graben, com os blocos representando as margens externas que se distanciaram ao longo de um descolamento horizontal. Combina??es de dois angulos diferentes foram usadas para simular o mergulho das falhas normais curvas ao longo da face interna dos blocos de madeira. Nos hemi-grabens, os anteparos possu?am geometria convexa, e, nos grabens, geometria c?ncava. No pacote de areia, o encurtamento foi particionado em movimentos dirigidos a p?s e antepa?s, e a cinem?tica da contra??o foi fortemente influenciada pela geometria convexa ou c?ncava das faces internas dos anteparos. Um efeito obst?culo, caracterizado por rota??o do pacote de areia, pr?ximo ao bloco do muro, foi mais elevado junto ?s faces internas dos blocos de mais alto angulo de mergulho. Os resultados foram comparados a outros experimentos f?sicos e aplicados a uma bacia invertida encontrada na natureza.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 diferente 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

Field and geochronological evidence for origin of the Contendas-Mirante supracrustal Belt, São Francisco Craton, Brazil, as a Paleoproterozoic foreland basin

The low-grade Contendas-Mirante meta-volcano-sedimentary belt (CMB) was previously interpreted as an Archean greenstone belt with the uppermost formation deposited in the Paleoproterozoic. New zircon U-Pb LAICP- MS ages on sedimentary groups and granite sheets intrusive in the belt constrain the timing of basin development. Phyllites of the Lower Group and meta-arenites of the Upper Group yielded similar young zircon populations at ca. 2080 Ma, which are interpreted as the maximum deposition age for the entire sequence. The unique exception is the 3273?3338 Ma zircon-bearing quartzite that occurs in spatial association with the 3300 Ma rhyolite. A granite sheet intrusive into metagraywacke of the Lower Group constrains the minimum deposition age of 2045 ? 26 Ma for the CMB. Altogether four main age clusters of detrital zircon grains were recognized, namely 2075?2200 Ma, 2200?2440 Ma, 2500?2770 Ma, 3270?3380 Ma. These age intervals match the ages of igneous and metamorphic rocks in the different terranes that comprise the S?o Francisco craton. Maximum deposition age for both groups indicates that sedimentation occurred immediately prior the highgrade metamorphism in the adjacent Archean-Paleoproterozoic Jequi? Block and Itabuna-Salvador-Cura?? Orogen. These observations, along with coarsening upwards in the sediments, indicate that the CMB represents a foreland basin developed along the eastern margin of the Gavi?o Block during plate convergence. Paleoarchean basement rocks were reactivated and emplaced into the CMB during basin inversion and is timely correlated with the granitic intrusion. Recognition of older supracrustal fragments is ambiguous, one main example is the 3273?3338 Ma zircon-bearing quartzite, which correlates with the northern Jacobina basin along the Contendas- Mirante lineament (e.g., Bahia gold belt) and may represent part of an Archean intracontinental rift sequence