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Total magnetic field map of the ECSOOT region showing relationship of magnetic images to tectonic features.

When igneous or sedimentary rocks are formed, the minerals with magnetic properties contained in them align themselves parallel to the force of Earth's magnetic field. Intensities of magnetic minerals, and their alignment, will vary from place to place. With the help of computers we can display the different magnetic intensities as colors on a rather striking map, which shows up the different tectonic elements.
On the left of the slide we see the northeastern corner of the Superior Province, and on the upper right the southwestern extent of the Nain Province. These two Archean cratons crunched between them all of the tectonic terranes which are aligned along the roughly north-northwest trending fault lines in the centre and upper portion of the map -- the eastern extension of the Trans-Hudson Orogen. The great Grenville Orogen cuts across it all with a mighty push from the south, wiping out the pre-existing tectonic signatures with its roughly east-west trending features.

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