UnB-Instituto de Geociências-Produção Científica: ABSTRACTS

UnB - Instituto de Geociências - Produção Científica
RESUMOS/ABSTRACTS

The belt of Neoproterozoic granulites in the inner part of the Brasília Belt:
Implications for Collision Tectonics and Exhumation Processes

Luiz José Homem D’el-Rey Silva

In: Congresso Brasileiro de Geologia, 41, João Pessoa, 2002, SBG. Anais, 1: 341-341.

Palavras chave: Terrenos Granulíticos, Análise Tectônica, CICLO BRASILIANO, Crustal Deformation, Uplift of High-P Rocks, FAIXA BRASILIA

RESUMO

Uplift of the granulite belt found in the Internal Zone of the Brasília Belt requires a special process and provides clues for further understanding on collisional tectonics and exhuma tio n of high-P rocks. Approximately at the latitude of Brasília, the Internal Zone and the granulite belt bend around an imaginary E-W axis and stretch, together with the Goiás Magmatic Arc and the External Zone, along a NE-trending northern segment and a SE-trending southern segment. In the northern segment, granulite facies metamorphism is 790-780 Ma old and is mostly found in three M-UM Paleoproterozoic layered intrusions and associated metavolcanics. In the southern segment, granulite facies metamorphism is just 650-630 Ma old and also affects metasediments with Sm-Nd isotopic signature compatible with the Neoproterozoic Araxá Group (metasediments and metavolcanics of a back-arc basin / accre tio nary prism). In despite of such differences, the whole granulite belt should have been exhumed as a whole, by the end of the 650-600 Ma old continental collision that ultimately resulted in the Brasília Belt. This conclusion, also supported by the structural evolu tio n of the M-UM layered intrusions, is specially based on two other key facts: 1 – The granulite belt perforates the (particularities out) »750 Ma old Araxá nappe that emplaced the amphibolite-greenschist facies Araxá Group onto the low-greenschist facies Paranoá Group of the External Zone, as seen around the latitude of Brasília and from there to the South; and 2 – The whole granulite belt is bounded to the East by a system of low-angle, West-dipping frontal ramps. This system coincides with a crustal-scale gravity anomaly, and stands for the sole fault for exhuma tio n. During exhuma tio n, a ~35 km-thick crustal wedge (X) splits in two parts, the eastern part and its bottom (sub-wedge Y) squeeze-up in the hangingwall of the sole fault, whereas the western part (sub-wedge Z) also moves upward, however at a lower velocity than Y. Exhuma tio n of the bottom of Y is possible because, during collision, the lower crust of the São Francisco Plate flows to the West, underneath the sole fault. Sub-wedge Y moves faster because its bottom lies closer to the sole fault than the bottom of Z, so the lower crust coming from the East pushes the sole fault and the bottom of Y upwards and also against Z. Because wedge X is pushed by the Amazonian Plate coming from the West, sub-wedges Y and Z are both under sub-horizontal compression, therefore the Z/Y boundary must be a compressional fault, but the process of exhuma tio n may lead to a complex interplay of kinematics indicators along the boundary, and may also imply that, within Y, some slices may move more than others and, if so, they are entrapped within slices of the Araxá nappe. Further shortening of this imbricated system may create an even more complex interplay of kinematics indicators in the field. Even after detailed field work, cau tio n is recommended in areas of high-P rocks before interpreting faults as extensional or contrac tio nal simply based on kinematics indicators.