Fold Friday: Norway Amphibolite

Plastic deformation within metamorphic rocks in the Trondheim region of south-central Norway related to the Caledonian Orogeny between 490-390 million years ago.

Credit: John Brady

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You have probably gathered by now, that many folds in geology tend to be caused by some form of tectonic compression. The target of this week is, unsurprisingly, no different. However, the amazingness of these folds still needs to be appreciated.

The beginning of this story starts roughly 500 million years ago with, the volcanic deposition of basalts within a basin that is now part of the Trondheim region in south-central Norway.

Opinions vary as to the metamorphic rock’s true origin; some propose fissure-based eruptions, some say these lavas, and others argue that these volcanic deposits are the result of overlapping shield volcano eruptions. The consensus though is that these rocks originated as submarine volcanics. The argument is valid, as whatever was deposited is now metamorphosed and heavily deformed making it difficult to learn the original rock type.

No matter, one can simplify and say, originally these rocks were volcanic deposits.

Further evidence for submarine deposition is the overlying stratigraphy. Many of the overburden facies are carbonates, sandstones, flysch deposits, and/or turbidites, all of which form commonly in submarine environments. Many of these sedimentary units also contain marine life fossils.

Geologic units deposited in the Trondheim region show four distinct deformational phases. The first two phases generated regional, large-scale folding, the third phase generated smaller-scale folds that are commonly observed at outcrop scales, and the fourth led to kink bands and further minor folds.

In the header photo, the hammer is 24 inches long (60 centimeters), so the deformational phase leading to this outcrop was likely part of the third or fourth phase.

Each of these four events fits into the larger orogenic (collisional mountain-building) sequence that formed many of the mountains in Norway.

The overarching collision is referred to as the “Caledonian Orogeny”, occurring around 500-420 million years ago and shown by the red line in the following image. The Caledonian Orogeny was part of the beginning of building Pangaea when what is now Norway collided with eastern Greenland.