Tectonostratigraphy in the Velfjord-Tosen region, southwestern part of the Helgeland Nappe Complex, Central Norwegian Caledonides.

Abstract

In the southwestern parts of the Helgeland Nappe Complex, dismembered ophiolites and their cover sequences are imbricated together with nappes consisting dominantly of continentally derived rock units. A local nappe sequence comprises three nappes, informally termed the Lower, Middle and Upper nappes. The Lower and Upper nappes consist of partly migmatic mica gneisses and othogneisses associated with marbles and calc-silicate gneisses. Between these nappes, the Middle nappe includes an igneous complex of mafic and ultramafic rocks of ophiolitic affinity, unconformably overlain by a cover sequence with material derived partly from the ophiolite substrate. Plutonic rocks, ranging from hornblende gabbros to diorites and granites, were emplaced at several stages during the tectonometamorphic evolution of the region. Based on comparison with nearby areas, most of the regional metamorphism and deformation would seem to have occured in the Ordovician, with a later phase of thrusting during the Mid Silurian to Early Devonian Scandian orogeny.