The Caledonian bedrock layer cake in S Norway

Cake model

Cake model

In the South Norway Caledonides – the Himalaya-style mountain chain that formed ca. 420 million years ago when Greenland and Scandinavia smashed into each other – there are three principally different rock units that are stacked on top of each other (top to bottom):

Crystalline thrust nappes (Strong, partly mylonitized)

Phyllite (décollement zone) Lower Paleozoic, very weak and micaceous

Proterozoic basement (Granitoid, Proterozoic structures, very strong)

Basement

Basement

These are theologically very different units: The phyllite-dominated middle layer is very weak compared to the others and acted as a weak décollement on which the overlying Jotun Nappe and other nappes moved. Most of the nappes are thought to have moved several hundred kilometers!

Because the phyllites localized deformation very effectively, the basement was barely deformed during the Caledonian continent-continent collision in many places, at least at the outcrop scale. Proterozoic cross-cutting relations are well preserved!

Phyllite

Phyllite. Shear bands dipping to the right indicate top-to-the-NW (left) sense of shear.

A primary unconformity is locally preserved between the phyllite unit (décollement) and the basement. A conglomerate and weathering arkose can be found in places, showing that the phyllite layer was deposited on the basement during the Cambrian transgression (rise in sea level). Hence the phyllite was mud deposited in a shallow continental ocean.

Conglomerate with phyllitic matrix

Conglomerate with phyllitic matrix

The nappes are also crystalline basement rocks, such as migmatites, granites and gabbros/anorthosite, probably ripped off the basement somewhere to the northwest of the present coastline. But the base of the nappes (base of the orogenic wedge) is strongly mylonitized (sheared), with a strong banding or schistosity. The mylonite zone is typically something like 200 m thick.

Unconformity with basal conglomerate on top of the Proterozoic basement. Phyllite in upper part of the picture

Another view of the unconformity with basal conglomerate on top of the Proterozoic basement. Phyllite in upper part of the picture.

What is perhaps surprising is that the kinematic indicators (asymmetric structures) in both the mylonite zone and the décollement phyllites consistently show top-to-the-NW sense of shear, contrary to the expected collision-related nappe translations. The explanation for this is that the orogenic wedge (nappes) moved back toward the hinterland shortly after the collision finished, suggesting a change from convergent to divergent plate motions shortly before 400 Ma.

Mylonites at the base of the nappe layer (Jotun Nappe)

Mylonites at the base of the nappe layer (Jotun Nappe). Top-to-the-NE (right) sense of shear.

You can read more about the  transgression in “The making of a land  Geology of Norway” (http://www.geologi.no/the-making-of-a-land) and about tectonic aspects in my papers, for example in publications 47 and 86 in my publication list: http://folk.uib.no/nglhe/Publications.html)

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About Haakon Fossen

Professor of structural geology, University of Bergen. Author of book Structural Geology, published at Cambridge University Press
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2 Responses to The Caledonian bedrock layer cake in S Norway

  1. Hi professor,

    Just passing by to say hi and make you know that is great to review your papers from 1994, 1997 about strain modeling in transpression and transtension. I’m enjoing seen it again for put some informations in my thesys. Thank you!

    Romario Mota, from Brazil

  2. Zbynek says:

    I especially like that basement with veins, would be nice to slice them and analyse. There might be a load of info about partial melting and fractioantion which interests me. And with such great outcrop and visible tectonics, whoa! Thanks for sharing 🙂

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