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.
The regional traces of folds, faults, and foliations found in many fold-thrust belts are bent in plan view. It proves valuable to distinguish between two types of bent orogens based on their kinematic evolution: oroclines (or rotational arcs) are bent orogens in which segments of the orogen change strike during the evolution of the bend, and nonrotational arcs are bent orogens in which segments of the orogen do not change strike during development of the bend.The kinematic evolution of a bend that forms in a thin-skinned orogen can be described in terms of three parameters: the displacement path trajectories followed by points along the strike of the orogen, the magnitude and distribution of tangential extension along the strike of the orogen, and the change in position of the endpoints of the orogen with respect to a reference line. Only slight differences in the displacement path trajectory pattern determine whether an orogen evolves as a nonrotational arc or as an orocline; thus the distinction between these two types of bends is not always of major tectonic significance. Compressional deformation along irregular continental margins or the impact of indentors on continental margins during collisional orogenies more likely leads to formation of nonrotational arcs (as can be simulated with a sand wedge model). Interaction between a fold-thrust wedge with obstacles in the foreland or with a wrench fault more likely leads to orocline formation. Orocline formation is also associated with noncoaxial reactivation of thrust faults.
Tangential extension is a necessary consequence of certain displacement path trajectory patterns and can accompanyCopyright 1988 by the American Geophysical Union, Paper number 7T0813. 0278-7407/88/007 T-0813510.00 development of either nonrotational arcs or oroclines. Two examples, one from the Umbrian Apennines and one from the Makran Range, demonstrate how movement on cross-strike fault arrays can accommodate tangential extension in thin-skinned bends. Bachtadse and Van der Voo, 1986; Miller and Kent, 1986].
We hypothesize that active tectonic processes in the south polar terrain of Enceladus, the 500-kilometer-diameter moon of Saturn, are creating fractures that cause degassing of a clathrate reservoir to produce the plume documented by the instruments on the Cassini spacecraft. Advection of gas and ice transports energy, supplied at depth as latent heat of clathrate decomposition, to shallower levels, where it reappears as latent heat of condensation of ice. The plume itself, which has a discharge rate comparable to Old Faithful Geyser in Yellowstone National Park, probably represents small leaks from this massive advective system.
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