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Tectonic elements of western Canada. Dark blue lines denote seismic reflection profiles and the dashed red line shows the northern limit of sedimentary cover.

We recognize the old Archean cratons, the Superior, Rae and Hearne in pink. Pay attention to the Snowbird Tectonic Zone which we again will encounter in maps of the Alberta Basement. The Snowbird Tectonic Zone (STZ) is a very prominent, very important, once very active, now very old tectonic feature along which truly gigantic tectonic battles were fought between contending continental plates.
Follow this line southwest, into the Alberta Basement transect area, where the STZ abruptly ends at the eastern edge of the Rocky Mountains, cut by a mighty thrust fault (line with teeth) which we can see today from the air and on the ground.

The Cordillera represents the youngest addition to our continent, one which, really, still is ongoing. Remember when we stood at the Pacific coast, at Long Beach on Vancouver Island, looking west to where, under the ocean, a Pacific oceanic plate is moving (or subducting) under our continent? It is moving now, while we are looking at the map.

Anyway, the STZ can be followed from the Rocky Mountain foothills northeast, and then east, all the way to Hudson Bay ...... Earth scientists can pick this tectonic zone out in many outcrops on the ground, on aeromagnetic and gravity maps, and, sometimes, on seismic reflection lines which cross it.

We remember that this is a map from which we have stripped the sedimentary cover where it exists today (in most of Alberta, say), hiding the crystalline underground and the old tectonic units from our eyes. The only orogens (mountain belts) which still are visible to the eye today as a picture similar to that when they were formed are those which have not yet been fully eroded or covered by later sediments, those which still are mountains, like the Appalachians in the east and the Rocky Mountains in the west.

How do we know that the basement below the sediment cover is there? Good question! That's what the LITHOPROBE project is all about, that is to see not only where rocks are exposed but also far below them, down to their roots. On this map, for instance, we can follow the tectonic and rock units from the Canadian Shield into the areas where the Shield becomes covered with sediments. One way we do this is by using an aeromagnetic map such as this one.

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