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Aeromagnetic map with colour-shaded relief, set parallel to Matachewan dykes to suppress the dyke signature and emphasize local structure. White lines show seismic reflection profiles.

This artificial sunshine and its calculated expression in shadows, as if the magnetic values shown were an actual surface relief instead of colours, is a wonderful usage of computer power. It highlights structural or depositional trends and features. In this case, the predominant structural expression of the numerous, roughly north-south trending dykes in the eastern part of the map was diminished by lining up the imagined sunshine with the direction of the dykes, so that the dykes themselves would not throw a predominant shadow.
We can trace the Ivanhoe Lake Fault Zone from the upper right to the lower left corner of the map. This line is the surface expression of the west-dipping fault plane along which the rocks from the left were heaved onto those on the right of the fault line. As we can see, there are many more features on the aeromagnetic map which represent the varying magnetic intensities of various faults and rock units and their orientation. To understand them all would help get you close to a degree in geophysics.

Before we leave Precambrian times, we should look at another giant tectonic event, one which almost tore our continent apart. It lies in the south, in the Great Lakes region, and is called the Keweenawan Rift System (KRS).

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