An outlier of strongly deformed Ordovician dolomite in Early Proterozoic gneiss of northern Manitoba belies traditional theories of Phanerozoic stability in the Canadian shield. Despite the perception that this part of the craton has been tectonically quiescent since Precambrian time, the observed structures appear to have had a tectonic origin. The margins of the outlier are not parallel to bedding, and the outlier is up to 150 m lower than the projected base of Phanerozoic deposits in Manitoba, indicating that the dolomites are not in place. The exposure is pervasively folded and the presence of a spaced cleavage is a strong argument against syndepositional deformation and glaciotectonics. Uniformly parallel glacial striae indicate that deformation predated the last glaciation. Carbonate solution cannot explain all of the features observed, and karstic weathering on this scale is not reported elsewhere in the western Canadian craton. An impact origin for the outlier is not supported by a circular physiographic feature or signs of shock metamorphism. The geometry of the outlier and its relationship with regional geophysical lineaments suggest, rather, an association between deformation and strike-slip faulting. Whatever the cause, the existence of Phanerozoic tectonic deformation structures will require rethinking of some well-entrenched ideas of the geologic history of this “stable” craton.

You do not currently have access to this article.