(Recasts, updates throughout with details on Ford plants, adds byline)
By Justin Hyde
DETROIT, Aug 14 (Reuters) - The heart of North America's automotive industry was shut down Thursday evening by cascading power outages that struck the northeastern United States and southern Canada.
Officials in southeastern Michigan said late Thursday evening that power was returning to some areas, but it could take much of the weekend before all power was restored.
"We have plants in all the impacted areas and we're still assessing the impact," GM spokesman Pat Morrissey told Reuters.
Ford Motor Co. spokesman Ed Lewis said 23 of Ford's 44 plants in North America were down. Those plants include assembly, stamping, engines and other parts.
Chrysler spokeswoman Debra Nelson said nine of its 13 North American final vehicle assembly plants had been hit by the outage. She said some of the plants may not have lost power, but could have shut down as a precaution.
The massive outage hit Michigan, New York, Ohio, New Jersey and parts of Canada, where much of the domestic auto industry's plants and offices are concentrated. The headquarters of all three automakers lost power shortly after 4 p.m. EDT; GM employees had to walk down from their skyscraper by the Detroit River.
"We are sitting here in the dark," Chrysler spokesman Dave Barnas said from the company's headquarters in Auburn Hills, Michigan.
Traffic was also snarled at the Ambassador Bridge, the major freight connection between Detroit and Ontario, Canada.
All three automakers have more than two months' worth of new vehicles in inventory and on dealer lots, making shortages unlikely. An unplanned plant shutdown can hit earnings directly, since automakers book revenues when a vehicle is built, but short interruptions in production can usually be made up by running overtime.
Michigan Gov. Jennifer Granholm told WWJ-AM radio that the outage was affecting much of central and southeastern Michigan. She said four counties had declared a state of emergency, but the state had not as of yet.