Measurements of finite strain, the anisotropy of magnetic susceptibility, the magnetic carrier of the susceptibility, and fieldwork have been carried out in the Onaping Formation of the Sudbury Basin, Ontario. The magnetic fabric indicates that changes in the magnitude and type of magnetic susceptibility ellipsoids closely mirror the changing pattern of deformation from northwest to southeast across the Onaping Formation outcrops. A quantitative correlation has been established between finite strain measurements and the magnetic susceptibility results and this allows the number of available strain measurements to be extended by more than 500 samples.The finite strain results suggest a deformation path in which the Onaping Formation was first affected by NW-SE-directed, horizontal, layer-parallel shortening which produced an upright tectonic cleavage. Subsequently, components of overthrust, NW-directed simple shear were superimposed upon the already deformed rocks of the Onaping Formation. The result of removing the strain, according to the sequence of superimposed deformations revealed by the deformation path, suggests that the Sudbury Basin was nearly circular in shape prior to its deformation. Consequently, a major objection to the meteorite impact theory for origin of the Sudbury Structure is eliminated.
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