Abstract

A penetrative secondary foliation and lineation, defined by tectonically flattened and elongated fragments, are present in the Onaping Formation of the South and East Ranges of the Sudbury Basin. These elements formed not by shearing, as previously supposed, but by flowage such that maximum shortening was normal to the foliation and maximum extension was parallel to the lineation. The basin was deformed by a 'push' from the southeast and the forward motion at the surface exceeded that at depth. This caused extensive flowage, tectonic thickening and folding in the Onaping Formation, and uplift and rotation of the South Range. The deformation intensity decreased rapidly to the northwest, so that near the basin axis the rocks yielded by folding, while in the North Range the only vestige of the deformation is a local and weak foliation.Quartz grains in the Onaping Formation are generally recrystallized and the lack of a crystallographic preferred orientation suggests post-tectonic annealing recrystallization related to a thermal event. Locally, some of the rocks escaped recrystallization and the quartz c axes lie on a girdle about the maximum principal stress direction. A late episode of brittle deformation developed joints and formed new faults and reactivated pre-existing faults. The Sudbury Basin was less elliptical than its present form, but it is unlikely that it was ever circular.

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