Structural geology and petrology of the Sygnefjell area, northwest Jotunheim, Norway

Abstract

The Sygnefjell area straddles the northwestern margin of the Jotun Nappe. It has a tectonically stacked sequence of four fundamental rock units, tentatively dated from regional correlations: 1. Basal Gneiss: Precambrian autochthonous granodioritic orthogneisses. 2. Parautochthonous cover: Cambro-Ordovician quartz schists, quartzitic psammites and limestones. 3. Supracrustal allochthon: Precambrian and possibly Cambrian arkosic psammites, metavolcanics, conglomerates and limestones. 4. Jotun Nappe: Precambrian strongly tectonized orthogneisses overlain by, and correlated with, meta-gabbros and monzonites preserving igneous textures.

All units have undergone polyphase metamorphism in the greenschist and epidote-amphibolite facies during the Caledonian orogenic cycle. This imposes marginal tectonic and metamorphic concordance with the supracrustals upon the enveloping crystalline rocks which have undergone earlier tectonometamorphic events.

Accompanying metamorphism are several phases of deformation, of which three produce the pervasive textures and tectonic superposition of the Sygnefjell sequence. These are: D₁ and D₂: Isoclinal folds producing a composite (and often mylonitic) penetrative schistosity and banding. The D₂ folds face northwest where strain is lowest, but rotate towards the D₂ finite extension direction (southeast at 25 degrees) with progressive shear.

D3: Early structures are mostly small, asymmetric, northwest verging buckle folds developing, with continuing deformation, northwesterly directed thrusts in the forelimb. These early folds are cross-cut by later brittle thrusts which sometimes follow D₂ mylonite zones. Footwall cut-offs,imbricate geometry and direction of climb are all indicative of northwesterly displacement. Later deformation events (D₄ to D₆) are a response to tectonic thickening of the succession during D₃.

From a knowledge of the deformation sequence, amount of ductile strain and direction and amount of tectonic movement a tentative restoration of Stratigraphic thicknesses and relationships can be made. This restoration and the structural data used in its construction imply a reinterpretation of the provenance of the Jotun Nappe and underlying supracrustal rocks.