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Géosciences, UPR CNRS 4661, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France
Field observations, and an analysis of the strain field, have been carried out in Archaean terranes of the Hebei province in the Dharwar craton of India, the Sino-Korean craton in China, and the Man shield in the Ivory Coast.
Two broadly different situations can be recognized. In the Dharwar and Man Shields the deformation shows a range of characteristic features. The foliation trajectories outline dome-and-basin structures in which supracrustal rocks of greenstone belts invariably occupy the basins. The foliation trajectories are perturbed only by bands of transcurrent shearing. Variations in deformation directions depend essentially on the geometry of plutonic or migmatitic bodies within which the foliation invariably has a domal form. Changes in the intensity of deformation are essentially limited to boundaries between granitoids and supracrustal series. These changes are largely horizontal. Characteristics of the finite strain ellipsoid (the parameter k) vary extremely rapidly.
The second situation is manifested in the Hebei province where the Archaean crust has deformed without the preservation of domes and basins. Here the deformation is characterized by the presence of a foliation and a very strong vertical flattening which extends into catazonal domains. This deformation is homogeneous. To explain these observations it is proposed that this homogeneous deformation was superimposed on pre-existing dome-and-basin structure, obliterating the original geometry. The Hebei example represents a stage of deformation greater than that of the Ivory Coast, which in turn is greater than that in the India craton.
It is apparent that the Archaean crust has specific characteristics that controlled the manner in which it deformed. During accretion, or reworking, it was dominated by body forces related to differences in the densities of the two major Archaean lithological units: granitoid and greenstones. Deformation resulting from boundary forces then led to a second type of behaviour controlled by a vertical planar anisotropy induced during the first stage.
These characteristics are never reported in modern orogenic belts, which generally are marked by a uniform structural trend and vergence. It is proposed that Archean cratons completely lacked a rigid element during the accretion events responsible for their formation, that the thermal regime in these periods was quite unlike that of today, and that the continental crust formed or was reworked during episodes of enhanced mantle plume activity.
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