DEGEO - Departamento de Geologia
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Item Proterozoic crustal extension and generation of dome-and-keel structure in an Archean granite-greenstone terrane.(1992) Marshak, Stephen; Alckmim, Fernando Flecha de; Evangelista, Hanna JordtItem Dome-and-keel provinces formed during Paleoproterozoic orogenic collapse—core complexes, diapirs, or neither? : Examples from the Quadrilátero Ferrífero and the Penokean orogen.(1997) Marshak, Stephen; Tinkham, Douglas; Alkmim, Fernando Flecha de; Brueckner, Hannes K.; Bornhorst, TheodorePaleoproterozoic dome-and-keel provinces, in which troughs of deformed and metamorphosed Paleoproterozoic supracrustal rocks surround domes of Archean basement, continue to puzzle geologists. In current literature, some authors refer to the domes as diapirs (implying basement flowed vertically upward) and others consider them to be Cordilleran-type metamorphic core complexes (implying that the contact between basement and cover is an upwarped detachment). Geochronological studies suggest that dome emplacement occurred during extensional collapse of contractional orogens, seemingly supporting the core-complex analogy. However, structural analyses in the Quadrilátero Ferrífero (Brazil) and the Penokean orogen (Michigan) demonstrate that the domes resemble diapirs in shape and in terms of surface kinematics. The domes differ from diapirs, however, in that they did not flow penetratively within but were emplaced by movement along steeply dipping shear zones. We suggest that contrasts between domeand- keel and core-complex provinces reflect contrasts between Paleoproterozoic and Phanerozoic crustal structure. Specifically, during the Paleoproterozoic, when crust was warmer and supracrustal assemblages denser, core-complex–like detachment faulting and associated plutonism resulted in juxtaposition of hot basement beneath a denser, tectonically thickened supracrustal layer. The resulting viscosity contrast and density inversion triggered vertical rise of diapir-shaped basement domes by slip on steep shear zones.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.Item 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, StephenThe 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.Item Kinematic evolution of the Araçuáı-West Congo orogen in Brazil and Africa : nutcracker tectonics during the Neoproterozoic assembly of Gondwana.(2006) Alkmim, Fernando Flecha de; Marshak, Stephen; Soares, Antônio Carlos Pedrosa; Peres, Guilherme Gravina; Cruz, Simone Cerqueira Pereira; Whittington, AlanThe Neoproterozoic Araçuaí-West Congo (A-WC) orogen is one of many Brasiliano/Pan-African orogens that developed during the assembly of West Gondwana. This orogen was split apart in Mesozoic time, due to opening of the South Atlantic—the Araçuaí orogen now underlies eastern Brazil, whereas the West Congo belt fringes central Africa's Atlantic coast. Significantly, at the time it formed, the A-WC orogen was bounded on the west, north, and east by the São Francisco-Congo craton, a crustal block that had the shape of a lopsided, upside-down ‘U’. Thus, the orogen was “partially confined” during tectonism, in that it occupied an enclave surrounded on three sides by cratonic crust. Formation of the A-WC orogen resulted in kinematically complex deformation, substantial crustal shortening, and production of a large volume of magma. How such features could develop in this particular setting has long been a mystery. Our field studies in the Araçuaí orogen, together with published data on the West Congo belt, characterize the kinematic picture of the A-WC orogen, and lead to a tectonic model that addresses its evolution. In our model, the A-WC orogen formed in response to closure of the Macaúbas basin. This basin was underlain by oceanic crust in the south, but tapered northward into a continental rift which terminated against the cratonic bridge linking the eastern and western arms of the São Francisco-Congo craton. Closure occurred when the western arm (now the São Francisco craton) rotated counterclockwise towards the eastern arm (now the Congo craton). This closure may have been driven by collision of the Paranapanema, Amazonian, and Kalahari cratons against the external margins of the São Francisco-Congo craton, rather than by slab-pull associated with subduction of the Macaúbas basin's floor. Thus, the process of forming the A-WC orogen resembled the process of crushing of a nut between two arms of a nutcracker. Such “nutcracker tectonics” led to a series of kinematically distinct deformation stages. Initially, internal portions of the orogen flowed northwards. Then, substantial crustal thickening occurred in the orogen's interior, and the deformation front migrated outwards, producing thrust belts that overlapped the internal margins of the craton. With continued closure, space in the enclave became restricted and the orogen's interior underwent lateral escape to the south. Late-stage extensional collapse triggered both production of late- to post-collisional granites and exhumation of high-grade rocks from mid-crustal levels.Item Extensional collapse in the Neoproterozoic Araçuaí orogen, eastern Brazil : a setting for reactivation of asymmetric crenulation cleavage.(2006) Marshak, Stephen; Alkmim, Fernando Flecha de; Whittington, Alan; Soares, Antônio Carlos PedrosaThe Araçuaí orogen of eastern Brazil is one of many Brasiliano/Pan African orogens formed during the Neoproterozoic assembly of Gondwana. Its western edge, bordering the São Francisco craton, is the Serra do Espinhaço fold-thrust belt, in which top-up-to-the-west (reverse-sense) faults, west-verging folds (F1), and east-dipping spaced to phyllitic cleavage (S1) developed. We have found that the kinematics of deformation changes markedly at the hinterland margin of this fold-thrust belt. Here, beneath a plateau known as the Chapada Acauã, metadiamictite and fine-grained pelitic schist comprise an east-dipping belt that contains an assemblage of structures indicative of top-down-to-the-east (normal-sense) shear. This assemblage includes a cascade of F2 folds that refold F1 folds and verge down the dip of the belt's enveloping surfaces, vertical tension gashes, and top-down-to-the-east rotated clasts. Based on the presence of these structures, we propose that the plateau exposes a regional-scale normal-sense shear zone, here called the Chapada Acauã shear zone (CASZ). Because F2 folds refold F1 folds, normal-sense shear in the CASZ occurred subsequent to initial west-verging thrusting. Considering this timing of motion in the CASZ, we suggest that the zone accommodated displacement of the internal zone of the Araçuaí orogen down, relative to its foreland fold-thrust belt, and thus played a role in extensional collapse of the orogen. The CASZ trends parallel to preserved thrusts to the west, and thus may represent an inverted thrust fault. Notably, throughout the CASZ, S1 schistosity has been overprinted by a pervasive, west-dipping asymmetric crenulation cleavage (S2). The sigmoid shape of S1 surfaces in S2 microlithons require that slip on each S2 surface was top-down-to-the-west. S2 cleavage is axial-planar to the down-dip verging F2 folds. Based on its geometry, we suggest that S2 cleavage initiated either as an antithetic extensional crenulation cleavage during reverse-sense shear, or as a near vertical asymmetric crenulation cleavage formed during east–west shortening of a preexisting east-dipping schistosity. Subsequent normal-sense shear in the CASZ reactivated this cleavage, causing clockwise rotation of S2 domains (as viewed looking along-strike to the north), in a manner similar to that of rotational ‘bookshelf faults’. Such movement could have accommodated concomitant vertical flattening of the CASZ during extensional collapse.