The Surna, Rinna and Orkla Nappes of the Surnadal-Orkdal district, southwestern Trondheim
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In the district between Surnadal and Orkdal within the 'Gula', the rock sequence between the Late Precambrian metasandstones of the Tingvoll Nappe and assumed Lower Ordovician metavolcanites of the Støren Nappe, three major nappes have been distinguished. These tectonic units are here termed the Surna Nappe, the Rinna Nappe and the Orkla Nappe and are correlated with the Essandsjø (Seve), Øyfjell (Køli) and Gula Nappes, respectively. Metapelites represent the most widely distributed lithology, with basic metavolcanites occurring in the Surna and Orkla Nappes. Metamorphic grade generally decreases upwards; amphibolite facies in the Surna Nappe, garnet grade in the Rinna Nappe, and garnet to biotite grade in the Orkla Nappe. The litholigies of the Orkla Nappe are divided into two formations; the Hoston Formation below, com- posed mainly of greenschists and phyllites and the overlying Svorkmo Formation which consists mainly of phyllites.