CORRELATION OF STRAIN AND THE ANISOTROPY OF MAGNETIC-SUSCEPTIBILITY IN THE ONAPING FORMATION - EVIDENCE FOR A NEAR-CIRCULAR ORIGIN OF THE SUDBURY BASIN

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