DEGEO - Departamento de Geologia
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Item Transamazonian orogeny in the Southern São Francisco craton region, Minas Gerais, Brazil : evidence for paleoproterozoic collision and collapse in the Quadrilátero Ferrı́fero.(1998) Alkmim, Fernando Flecha de; Marshak, StephenThe Paleoproterozoic Transamazonian orogeny yielded a series of discrete orogens in Brazil. The present field study in the Quadrilátero Ferrı́fero (QF) indicates that the southern São Francisco craton region of the Brazilian highlands preserves a portion of one of these orogens. Two sets of Transamazonian structures occur in this region. The first consists of northwest-verging folds and thrusts affecting supracrustal sequences. It is suggested that this set formed in a fold-thrust belt setting shortly after 2.125 Ga, during the closure of a passive-margin basin that had initiated along the margins of a preexisting continental mass at ca 2.5 Ga. The second set consists of structures defining the prominent dome-and-keel architecture of the QF. This set, a consequence of the emplacement of basement domes against supracrustal rocks at 2.095 Ga, may reflect the consequences of orogenic collapse. Narrow, conglomerate-filled intermontane basins may have formed coevally with dome emplacement. Formation of an ocean basin east of the present-day São Francisco craton eventually occurred in Late Mesoproterozoic. In effect, the Transamazonian orogen of the QF represents the collision and collapse stages of a Paleoproterozoic Wilson cycle. The contractional phase of the Transamazonian orogeny probably represents accretion of an offshore arc to the eastern and southeastern margin of the present-day São Francisco craton region. The arc, and an associated suture, may be traced in the Brasiliano (Pan African) orogen east of the São Francisco craton, northwards into the northeastern lobe of the São Francisco craton. Clearly, initial assembly of crustal blocks to form a larger continent involving South America occurred during the Paleoproterozoic (2.1 Ga). Post-Transamazonian rifting of this continent created the basins which were later inverted during the Brasiliano assembly of Gondwana.Item Structural style of basin inversion at mid-crustal levels : two transects in the internal zone of the Brasiliano Araçuaí belt, Minas Gerais, Brazil.(1996) Cunningham, Dickson; Marshak, Stephen; Alkmim, Fernando Flecha deThe Araçuai belt is the orogenic belt that directly borders the eastern margin of the São Francisco Craton in eastern Brazil. Detailed structural investigations in the Governador Valadares region of Minas Gerais indicate that the amphibolite-to granulite-grade internal zones of the Araçuai belt contain several major, west-vergent, crystalline overthrust sheets. These thrust sheets contain approximately homoclinal east-dipping gneissic banding and are separated from one another by zones of isoclinally and sheath-folded, ductiley sheared, metasedimentary units that behaved as mechanically weak glide horizons during deformation. We interpret this regionally imbricated sequence of basement and cover to be the mid-crustal level manifestation of closure of a mid-Neoproterozoic rift basin that existed to the east of the São Francisco Craton. The major thrusts, which are all cratonvergent, are of Brasiliano/Pan-African age (650-450 Ma) becuase they cut the Neoproterozoic Galiléia batholith. Older fabrics are locally preserved in the basement slices, and these fabrics may be relicts of the Transamazonian orogeny (2.0 Ga). Discrete zones of ductile-brittle extension that were identified in several localities in the study area suggest the occurrence of postorogenic collapse following Brasiliano overthrusting. Alternations of rigid crystalline thrust sheets and highly deformed metasedimentary sequences, such as those of the Governador Valadares region, may be a common structural geometry at a depth of 15–20 km in modern regions of collision and basin closure.Item A structural transect across the Coastal Mobile belt in the Brazilian Highlands (Latitude 20°S) : the roots of a Precambrian transpressional orogen.(1998) Cunningham, Dickson; Alkmim, Fernando Flecha de; Marshak, StephenWe present results of a detailed structural analysis from a 250 km-long, east–west-trending transect crossing the Coastal Mobile Belt, a part of the Precambrian orogen which lies between the eastern edge of the São Francisco craton and the Atlantic coast. The region exposes amphibolite–granulite grade metamorphic rocks and migmatites which formed at mid-lower crustal depths during the Brasiliano orogeny (0.63–0.52 Ga). This event marked the closure of the northernmost Adamastor Ocean and collision between the São Francisco and Congo Cratons during West Gondwana assembly. Brasiliano deformation resulted in W-vergent structures including thrust faults which accommodated kilometer-scale transport of crystalline basement, overturned kilometer-scale folds, sheath folds and penetratively developed gneissosity and schistosity. Isolated relics of an older folded fabric occur locally and may represent Transamazonian (2.2–2.0 Ga) deformation. The orogen is kinematically partitioned with the eastern 175 km dominated by moderately- to steeply dipping north-trending dextral strike-slip and oblique-slip faults and associated flower structures, whereas the western 75 km is dominated by W-vergent shallowly- to moderately east-dipping thrust faults. The boundary between these two provinces may mark a Brasiliano suture. Throughout the transect, quartzite and metasedimentary belts form strongly deformed zones between massive crystalline basement thrust sheets. The granulite-cored Serra do Caparaù massif, the highest mountains in South America outside of the Andes and Guyana shield, occupies a restraining bend between two Brasiliano dextral shear zones. The W-vergent Coastal Mobile Belt formed contiguously with the E-vergent Pan-African West Congo orogen now exposed along the conjugate margin of Africa. Thus an important late Precambrian boundary between structurally linked but kinematically opposed structural provinces must lie hidden in the extended offshore continental margins of either continent. Cretaceous opening of the South Atlantic and separation of the West Congo belt from the Coastal Mobile Belt may have been structurally influenced by this boundary.