Tectonostratigraphy in the Velfjord-Tosen Region, Southwestern Part of the Helgeland Nappe Complex, Central Norwegian Caledonides
Tectonostratigraphy in the Velfjord-Tosen region, southwestern part of the Helgeland Nappe Complex, Central Norwegian Caledonides TERJE THORSNES & HELGE UZlSETH Thorsnes, T. & Leseth , H. 1991: Tectonostrat igraphy in the Velfjord-Tos en region, southwestern part of the Heigeland Nappe Compiex, Central Norwegian Caledonides. Nor. geol. unders. Bull . 421, 1-18. In the southwestern part s of the Helgeland Nappe Compl ex, dismembered ophiolites and their cover sequences are imbricated together with nappes consisting dominantly of continentally deri ved rock units. A local nappe sequence comprises three nappes, informally termed the Lower , Middle and Upper nappes . The Lower and Upper nappes con sist of partly migmatitic mica gneis ses and orthogn eisses associated with marbles and calc-silicate gneisses. Between these nap pes, the Middle nappe includes an igneous complex of mafic and ultramafi c rocks of cphiolitic affinity , unconformably overlain by a cover sequence with material derived partiy from the ophio lite substrate. Plutonic rocks , ranging from hornblende gabbros to diorites and granites, were emplaced at seve ral stages during the tectonometamorphic evolution of the region . Based on comparison with nearby areas, most of the regional metamorphism and deform ation would seem to have occurred in the Ordovician , with a later phase of thrusting during the Mid Silurian to Early Devonian Scandi an orogeny. Terje Tttorsnes , Norges geologiske underseketse, Postboks 3006, 7002 Trondhe im, Norway. Helge t.esett; Institutt for
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