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Título

PROTEROZOIC PRE-COLLISIONAL SEDIMENTARY BASINS OF THE ARAÇUAÍ OROGEN: A PROTRACTED TAPHROGENIC HISTORY ALONG THE ESPINHAÇO RANGE, MINAS GERAIS, BRAZIL

Texto do resumo

Supported by detailed geological mapping of an extensive area along the southern Espinhaço range, this abstract aims to identify the stratigraphy of the Proterozoic pre-collisional basins of the Araçuaí orogen, and to propose a tectonic evolutionary model for these basins. They are younger than the Archean basement, reworked with lesser or greater intensity by the Rhyacian orogeny, and older than the Brasiliano orogeny. The following pre-collisional basins were mapped: (1) Orosirian Costa Sena sineclisis; (2) Statherian Espinhaço rift system; (3) Stenian Sopa-Brumadinho graben system; (4) Stenian Galho do Miguel sag; (5) Stenian Conselheiro Mata graben and (6) Tonian-Cryogenian Macaúbas rift/passive continental margin. The Orosirian Costa Sena sineclisis was developed by crustal stretching of the Archean basement, absorbed by flexural subsidence. Slow down-warping of the basement allowed the sea to invade continental areas, giving rise to a T/R sedimentary cycle. Banded iron formations occur along the N-S axis of the sineclisis, as an expressive chemical-sedimentary upper-layer. The sineclisis evolved from symmetry to acquire an asymmetrical topographic section. The Statherian Espinhaço rifts were limited by N-S normal faults, which were connected below to an extensional detachment fault, which crops out at the eastern border of the cordillera and is genetically related to volcanic and plutonic alkaline magmatic suites of variable compositions. This taphrogenic process began due to an asthenosferic heat flow, coming from underneath the São Francisco Craton, directed towards the east, below the Gouveia complex. Increasing conditions of temperature and pressure of the heat flow met the mechanical rupture of the continental lithosphere, giving rise to the Espinhaço rift system. Towards the west, from the N-S detachment, the Gouveia complex keeps its original thickness, but, eastwards from the detachment, the exhumed Guanhães complex must exhibit reduced lithospheric thicknesses. After a geological hiatus greater than 400 Ma, the evolution of the pre-collisional basins resumed when the temperature below the Gouveia complex was high enough to create a thermal heat flow, directed toward the east, underneath the Guanhães complex. The displacement of the heat flow, below a younger detachment, gave rise to the remaining pre-collisional basins. The Stenian Sopa-Brumadinho graben system developed due to thermal subsidence, mechanically absorbed by normal faults, probably after reactivations along older Espinhaço faults. The Galho do Miguel sag is an asymmetric flexural basin formed by the coupled effects of depocenter subsidence and shoulders uplift. This framework enabled the preservation of extensive eolian sandstones. The Conselheiro Mata graben evolved from thermal subsidence absorbed by normal faults. The Macaúbas passive margin comprehends a glaciogenic continental platform linked to a glaciomarine slope, which passes to a large marine basin, developed above the Guanhães complex. It represents block tilting and an abrupt change towards the east of the basin depocenter. The Proterozoic pre-collisional basins record the existence of two long-lived taphrogenic processes, related to the development, progression and continued eastward movement of two distinct thermal heat flows. Around 700 Ma ago, the complete breakup of the Archean basement and the generation of new oceanic crust occurred. This scenario marks the end of the pre-orogenic phase.

Palavras Chave

Neoproterozoic Araçuaí orogen; pre-orogenic basins; pre-orogenic stratigraphic evolution; geological mapping of the southern Espinhaço range