A model for stratigraphic and metamorphic inversions at Sulitjelma, central Scandes

Abstract

The relationship between disposition of metamorphic isograds and early strain history is examined for the Sulitjelma Fold Nappe of the central Scandes. Evidence for stratigraphic inversion and the widespread inversion of the garnet isograd is reviewed, and new evidence for the nature of early Scandian deformation is presented. A two-stage model is presented which explains the inversions by (i) an initial phase of horizontal compression resulting in crustal thickening by vertical stretching during closure of a marginal basin, followed by (ii) continued horizontal compression and crustal thickening by eastward-directed nappe emplacement. The emplacement of the Sulitjelma Fold Nappe during the latter phase of progressive simple-shear-dominated deformation was accompanied by the development of sheath fold geometries and the overturning of isotherms to produce the widespread inversion of the garnet isograd.

A model for stratigraphic and metamorphic inversions at Sulitjelma (Boyle 1987) – comment

“…In his letter, Dr Findlay correctly summarizes the main stratigraphical and structural parts of my model (Boyle, 1987) for the Sulitjelma Fold Nappe (SFN), and goes on to make three main points on the basis of his mapping with S. Kollung (Findlay, 1980 B a s e m e n t 9 r a n i t e a n d autochthon ("granite") Figure 1. Summary geological map of the region around Sulitjelma (after Boyle, Mason & Hansen, 1985).…”

“…S I R S -I n his contribution to the interpretation of the complex structure of the Sulitjelma district Boyle (1987) argues that: (I) the Sjonsta Group is younger than the Furulund Group; (2) the Skaiti Supergroup is older than the Sjensta and Furulund Groups; and (3) the Sjonsta Group is underlain structurally by the Pieske Marble. The Skaiti Supergroup and the Sjonsta and Furulund Groups would appear to be incorporated into a large fold nappe, the Sulitjelma Fold Nappe, which overlies with a possible thrust contact (Kirk & Mason, 1984, fig.…”

“…If Boyle's interpretation is correct (Boyle, 1987) then it carries the implication that his Sulitjelma Fold Nappe, or at least the lower limb thereof, extends 25 km south to the Balvatn region, and thence west some 10-15 km to or beyond Basshaugen. I would argue that the sole of the Sulitjelma Fold Nappe is the tectonic contact between the Pieske Marble and structurally underlying formations, and have argued (Findlay, 1980, table 6) that this tectonic break extends as far north as Vatnfjell.…”

“…That is, Boyle's Sulitjelma Fold Nappe constitutes the whole or part of the Pieske Nappe (Kautsky, 1953), and should include the Pieske Marble. Boyle (1987) does not discuss in detail the contact between the Skaiti Supergroup and the Furulund Group. Boyle, Griffiths & Mason (1979) regarded the rocks now known as Skaiti Supergroup as belonging to the Gasak Nappe and it is evident from observations to the south of Sulitjelma, in the Baldoaivve region and at Balvatn (Findlay 1980, fig.…”

“…4), that the two units are separated by an extensive thrust. Given the interpretation sketched in Boyle (1987, fig. 2), it would appear that this thrust should be folded by the Sulitjelma Fold Nappe.…”