Basement Gneiss doming the Uppermost Allochthon in the Bogøy area of Steigen, Nordland, Norway.

Abstract

Steigen lies geographically within the Uppermost Allochthon, which was emplacedduring the Scandian phase of the Caledonian orogeny. A tectonized boundaryseparates a basement culmination of Precambrian granite-gneisses from youngercover metasediments. The latter comprise marbles, quartzites, amphibolites andpelitic to semipelitic schists, which have been divided informally into fourgroups. Caledonian metamorphism reached a peak in the almandine-amphibolitefacies during the second main deformation phase (D2), with some retrograderecrystallization after D2. An early deformation phase (D1) is indicated by thepreservation of discordant fabrics within porphyroblasts, while the majority offolds are contemporaneous with the formation of the penetrative schistosity(D2). D2 fabrics and fold-axes have been folded into a large D3 antiform whichdeveloped during uprise of the basement geniss culmination late in the orogeny.The granite-gneiss basement exhibits a foliation which is parallel to the base-ment's contact with the metasediments. The geochemistry of the gneisses is verysimilar to that of other basement gneisses in Nordland and to rapakivigranites.The gneisses are considered to be Proterozoic rapakivi granites whichrose diapirically during the Caledonian orogeny, causing updoming anddeformation of the metasediments.