Basement-cover relations in Shetland

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Abstract
Summary: The relationship between basement and cover can be studied in two places in Shetland. In NW Shetland, orthogneisses with a minimum age of 2900 Ma are followed to the east and southeast by Caledonian metasediments and greenschists and these in turn by schists and gneisses with a minimum age of 860 Ma. The metasediments contain bands of mylonite and blastomylonite. Wide belts of superposed schistosity occur in the schist and gneiss unit and also at the junction of the orthogneisses with the metasediments. This area is interpreted as including the Caledonian front. In NE Shetland, two nappes of ultramafic and basic rocks, one above the other, have been thrust over the surface of the earth onto Caledonian (lower Dalradian?) schists and gneisses. During transport the nappes have suffered erosion, and sediments, including nappe detritus, have been deposited on and in front of them and subsequently been over-ridden and metamorphosed by the nappes. The occurrence forms a typical obducted ophiolite complex.