The basement region in SW Norway called Western Gneiss Region contains high-strain zones (mylonite zones) with deformation fabrics suggesting top-to-W or sinistral movements, particularly in the coastal parts. This was the leading edge of the Baltica craton during the Caledonian collisional history, subducted and shortly after obducted in a Himalayan-style setting, making it a classical area for studies of high-P metamorphism of continental crust and extensional deformation/orogenic collapse. The high pressures are indicated by widespread occurrences of eclogite, locally with coesite and micro-diamonds.
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| Eclogite, Nordfjord. Red-brown garnet, green omphacite, bluish kyanite. |
I took pictures of these porphyroclasts, showing examples of sigma- and delta-style porphyroclasts and porphyroclast imbrication (tiling).
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| Sigma-type porphyroclasts with tails not crossing the line of symmetry (reference line in lower figure). Top-to-left (W) sense of shear. Locality at Sandane. |
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| Delta-type porphyroclast where thin tails of recrystallized feldspar (suggesting high temperatures) crosses the line of symmetry. Top-to-left (W) sense of shear. Locality at Finnøy. |
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| Porophyroclast tiling or imbrication. Top-to-left (W) sense of shear. Locality at Sandane. |
See Figure 15.28 in my textbook for a closer explanation:
