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1. The basement: The Precambrian Shield

The Precambrian Shield

Large parts of central and south Greenland are made up by an old ‘shield’, a platform of rocks, which has been stable for a long time, but had previously gone through a long, complex history. A number of events, all of them indeed ‘worldshaking’, have left their traces. Several times continents collided and altered all rocks between them due to the extreme pressure. Sediments were deposited and volcanoes erupted. All these rocks were later exposed to large pressure when they got between colliding tectonic plates and were then deformed, bent and broken, and metamorphosed, and all this even several times through Earth history.

Strongly metamorphosed Precambrian rocks near Kangerlussuaq (West Greenland)

Strongly metamorphosed Precambrian rocks near Kangerlussuaq (West Greenland)

This complex mixture of different, mainly metamorphic rocks then became stable and has largely remained unchanged since then. Rivers and glaciers have altered the shape at the surface, but the rocks as such have not changed. Thus, they provide a solid base for everything that has happend more recently. Such a solid base is termed ‘basement’ or, if it is very old and large, ‘shield’. Everything which is younger and subsequently mostly deposited on top of the basement, is called cover rock to describe the basic structure, at least time-wise.

Most parts of Greenland consist of such an old shield or are at least underlain by one. Precambrian rocks are also exposed in the innermost part of the Scoresbysund, even though only in a quite limited area, at least Precambrian rocks that have not been affected by the Caledonian orogeny. The Precambrian rocks are mostly granites and gneisses. The complex history which has led to their origin can partly be reconstructed. Large-scale tectonic events such as the collision of continents, which have changed rocks strongly and over a wide area, have happened at least three times: 2.6 billion years ago (Archaean), 1.9 billion years ago (lower Proterozoic) and 1 billion years ago (middle Proterozoic).

With some experience you can sometimes see immediately that rocks have such a complex history which involved several phases of movement, pressure, heat etc. This is a bit more complicated and thus beyond the scope of this little overview; for detailed results, even professional geologists need to work for quite some time both in the field and in the laboratory, especially to get absolute ages as those ones given above, which can of course not directly be read in the landscape.

Strongly deformed sediment sample with small veins (vertical). Three steps are necessary to form such a rock: firstly deposition of the original sediment, secondly compressive tectonics for deformation, thirdly intrusion of veins (which are not deformed and thus younger than the deformation). Size ca. 20 cm (left-right).

Yalour rocks 2

In the Scoresbysund, rocks not altered since the Precambrian are exposed only in the innermost Vestfjord and Nordvestfjord in so-called ‘tectonic windows’. These are holes in cover rocks which otherwise cover the Precambrian basement. In wide areas of the central Scoresbysund, there are Precambrian rocks which have been affected by the Caledonian orogeny.

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last modification: 2013-10-12 · copyright: Rolf Stange
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