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-graben ...

zone in Northwestern Ontario,

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

, centered on

Lake Nipigon Lake Nipigon ( ; ; ) is a freshwater lake in Northwestern Ontario. Part of the Great Lakes drainage basin, it drains through the Nipigon River into Lake Superior. It is the largest lake entirely within the Canadian province of Ontario. Ety ...

. It represents an aulacogen of the much larger Midcontinent Rift System, 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'' ...

began to split apart during the

Proterozoic The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...

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