Browsing by Author "Cunningham, Dickson"

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    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 de
    The 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.
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    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, Stephen
    We 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.
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    Tectonic implications of precambrian Sm-Nd dates from the southern São Francisco craton and adjcent Araçuaí and Ribeira belts, Brazil.
    (2000) Brueckner, Hannes K.; Cunningham, Dickson; Alkmim, Fernando Flecha de; Marshak, Stephen
    The Archean and Paleoproterozoic São Francisco craton of eastern Brazil is surrounded on all sides by Brasiliano (=Pan African) orogens. The N–NE trending orogen that separates the eastern edge of the southern São Francisco craton from the Atlantic coast can be divided into the largely greenschist and amphibolite facies Araçuaı́ belt on the west and the largely granulite facies Ribeira belt on the east. A pronounced linear gravity and magnetic anomaly, the Abre Campo discontinuity, defines the boundary between these two belts. We obtained Sm–Nd mineral ages and whole-rock Sm–Nd model ages for garnet-bearing metamorphic rocks along an E–W transect across the southern São Francisco craton, the southern Araçuaı́ belt, and the Ribeira belt at about latitude 20°S. A recrystallization age of 2.1 Ga from metasediments recrystallized during the development of the classic dome-and-keel province of the southern São Francisco craton (the ‘Quadrilátero Ferrı́fero’) indicates that dome emplacement occurred during the waning stages (extensional collapse) of the Transamazonian collisional orogeny. Seven mineral ages from the southern Araçuaı́ and Ribeira belts date the thermal peak of metamorphism at between 538 and 589 Ma, confirming that these belts were pervasively remobilized during the Brasiliano event. Samples from the Araçuaı́ belt yield either Archean (>2.6 Ga) or Transamazonian (2.1–2.3 Ga) TDM model ages, indicating that the protoliths were either fragments of the São Francisco craton crust or were sediments derived from that craton, which presumably had been stretched to form a thinned continental margin during Meso- and Neo-Proterozoic rifting events. Notably, a mixed meta-pelite and metabasite sequence in the southern Araçuaı́ belt, the Dom Silvério Group, has Transamazonian ancestry and thus may represent oceanic sediments deposited on or east of this stretched margin and then thrust back onto the continent to mark a collisional suture between the São Francisco block and an Archean (?) crustal sliver to the east. The rocks of the Ribeira belt (i.e. the region east of the Abre Campo discontinuity) have younger model ages (TDM=1.6–2.0 Ga), indicating that this belt was not originally part of the São Francisco craton — it may represent an accreted Transamazonian terrane. Therefore, the Abre Campo discontinuity marks an important crustal boundary, possibly a suture. Brasiliano shear zones appear to steepen and root into the Abre Campo discontinuity, suggesting that it formed in Brasiliano time.