André Danderfer Filho

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.

Quartzito azul com dumortierita e fosfatos de alumínio do Espinhaço setentrional, Bahia: mineralogia e petrogênese

In the Northern physiographic domain of the Espinhaco Ridge, in Bahia State, Brazil, blue quartzite is mined as a valuable dimension stone. The meter- to decameter-sized lenses are irregularly distributed within the whitish quartzites belonging to the Vereda Formation. Optical studies and microprobe analyses have identified dumortierite, kyanite, chromium-bearing muscovite, and lazulite as the main minerals responsible for the bluish and greenish colors. Phosphates, such as trolleite, augelite, svanbergite-goyazite and monazite, occur sporadically. Dumortierite and kyanite, the most abundant color-causing minerals, are mostly concentrated along millimeter-wide, plane-parallel to crossed sedimentary beds. The dumortierite is also found filling discordant fractures, evidencing the high mobility of boron during metamorphism. The estimated metamorphic conditions of P ≥ 3.5 kbar and T = 475 to 560oC are of higher greenschist to lower amphibolite facies. Rounded quartz grains inherited from the sedimentary protolith are conspicuous in the Northern part and absent in the Southern region of the studied area, which indicates increasing deformation southwards. In terms of the depositional milieu, the lithofacies of the Vereda Formation is interpreted as the initial filling product of a Mesoproterozoic rift associated with alluvial planes and lagoons or tidal planes in continental and coastal systems, where the boron that generated dumortierite could have been concentrated in sediments with contribution of evaporitic material.

Petrography, geochemistry and geochronology of the potassic granitoids of the Rio Itacambiruçu Supersuite: implications for the Meso- to Neoarchean evolution of the Itacambira-Monte Azul block

Manuscript ID: 20170131 Received on: 11/01/2017. Accepted on: 01/29/2018. ABSTRACT: In the Itacambira-Monte Azul block (BIMA) the Archean rocks are represented by tonalite-trondhjemite-granodiorite (TTGs) of the Porteirinha Complex and by potassic granitoids of the Rio Itacambiruçu Suite that crop out at the Pedra do Urubu (PUp), Rio Gorutuba (RGp), Barrocão (BAp) and Lagoa Nova (LNp) plutons. This work presents new data for the PUp, RGp and BAp. The petrographic and lithochemical analyses allowed the subdivision of the Rio Itacambiruçu Suite in two groups of granitoids: a medium potassium group, represented by the PUp and RGp, and a high potassium group, in which the BAp rocks are inserted. The geochronological data showed different ages for the medium and high potassium groups, with crystallization ages at ca. 2.92 Ga and 2.65 Ga, respectively. Despite the differences observed between these two groups, the chemical signatures of these rocks are compatible with the signature of biotite-granites generated by the reworking of ancient continental crust. Thereby, the data obtained in this work, added to some previously published results, allowed to the identification of two Archean crustal stabilization events recorded in BIMA, one of Mesoarchean age and the other of Neoarchean age. In addition, we propose the lithodemic reclassification of the Rio Itacambiruçu Suite to Rio Itacambiruçu Supersuite.

Análise de volumes de sal em restauração estrutural: um exemplo na bacia de Santos

A complexidade da halocinese na porcao central da bacia de Santos envolve expressivas estruturas e depocentros deformados, diferenciados ao longo da direcao de deformacao principal. Cinco secoes geologicas, com registro estratigrafico completo na area de estudo, foram restauradas para investigar diferentes estilos de deformacao, incluindo a evolucao da falha de Cabo Frio, coerentemente inserida na evolucao tectonossedimentar da bacia de Santos. O procedimento integrou a restauracao 2D com tratamento e analise 3D, por meio das seguintes etapas: remocao de camadas, descompactacao e compensacao isostatica flexural; restauracao desacoplada da tectonica do sal; conservacao material (inclusive do sal); recomposicao da sobrecarga sedimentar quando diferentes taxas de distensao afetam os dominios desacoplados; calibracao batimetrica do conjunto restaurado e tratamento e analise espacial dos resultados. O detalhamento em 14 etapas de restauracao foi suficiente para tratar a deformacao de forma nao continua, minimizou desvios do metodo e produziu consistencia geometrica e estratigrafica dos resultados no dominio espaco-tempo. A redistribuicao controlada do sal confirmou os efeitos do aporte sedimentar e as estruturas preexistentes sobre a deformacao. Os resultados demonstram a importância da reciprocidade dos efeitos de deslocamento lateral por halocinese sobre isostasia, batimetria e descompactacao, nao considerada nos programas de restauracao existentes.