Kinematic evolution of the Araçuáı-West Congo orogen in Brazil and Africa : nutcracker tectonics during the Neoproterozoic assembly of Gondwana.

Abstract

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.