The Nipigon Embayment is an inactive continental

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...

zone in Northwestern Ontario,

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

, centered on

Lake Nipigon Lake Nipigon (; french: lac Nipigon; oj, Animbiigoo-zaaga'igan) is part of the Great Lakes drainage basin. It is the largest lake entirely within the boundaries of the Canadian province of Ontario. Etymology In the Jesuit Relations the lake is ...

. It represents an

aulacogen An aulacogen is a failed arm of a triple junction. Aulacogens are a part of plate tectonics where oceanic and continental crust is continuously being created, destroyed, and rearranged on the Earth’s surface. Specifically, aulacogens are a ri ...

of the much larger

Midcontinent Rift System The Midcontinent Rift System (MRS) or Keweenawan Rift is a long geological rift in the center of the North American continent and south-central part of the North American plate. It formed when the continent's core, the North American craton, ...

, which formed some 1,100 million years ago when the

North American craton North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography. Etymology The word ''north'' is ...

began to split apart during the Proterozoic eon.Heat flow in the Nipigon arm of the Keweenawan rift, northwestern Ontario, Canada
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References

Geology of Ontario Aulacogens Mesoproterozoic rifts and grabens {{Canada-geology-stub