Persistent small-scale seismicity around the western end of Lake Ontario, together with larger events in eastern North America, have focused attention on the seismotectonic and neotectonic conditions of southern Ontario and adjacent New York State. Newly discovered normal faults that offset both bedrock and overlying Quaternary sediments by as much as 1.25 m were discovered along the Rouge Valley in metropolitan Toronto. This location is within a zone marked by the intersection of three major lineaments and an elevated level of seismicity. These faults result from reactivation of east-west-oriented fractures in Ordovician shales. Structures are exposed in section only and have no surface expression, suggesting that others may have gone undetected. The origin of the faults is not yet well established, although they may represent the effects of uplift consequent upon isostatic rebound aided by high horizontal compressive stress. Nevertheless, their development in the recent geologic past has important implications for regional seismic hazard assessment, because they occur within 7 km of the Pickering nuclear power plant.

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