PHANEROZOIC DEFORMATION IN THE STABLE CRATON, MANITOBA, CANADA

An outlier of strongly deformed Ordovician dolomite in Early Proterozo ic gneiss of northern Manitoba belies traditional theories of Phaneroz oic 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, Th e 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 i n Manitoba, indicating that the dolomites are not in place, The exposu re is pervasively folded and the presence of a spaced cleavage is a st rong argument against syndepositional deformation and glaciotectonics. Uniformly parallel glacial striae indicate that deformation predated the last glaciation, Carbonate solution cannot explain all of the feat ures observed, and karstic weathering on this scale is not reported el sewhere in the western Canadian craton, An impact origin for the outli er is not supported by a circular physiographic feature or signs of sh ock metamorphism, The geometry of the outlier and its relationship wit h regional geophysical lineaments suggest, rather, an association betw een deformation and strike-slip faulting, Whatever the cause, the exis tence of Phanerozoic tectonic deformation structures will require reth inking of some well-entrenched ideas of the geologic history of this ' 'stable'' craton.