Browsing by Author "Whittington, Alan"

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    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 Pedrosa
    The 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.
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    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, Alan
    The 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.