Strain discontinuities within the Seve-Köli Nappe Complex, Scandinavian Caledonides

Abstract

This paper utilizes the results of a regional survey of the state of finite strain carried out in an area built up of stacked thrust sheets to draw conclusions about the relative timing of penetrative deformation and thrust motion, and to place constraints on the deformation models devised to explain the geometry of the individual thrust sheets. A strain survey within the Seve-Köli Nappe Complex reveals abrupt changes which represent discontinuities in the strain pattern. One of these discontinuities coincides with the boundary between the Seve and Köli Nappes and separates intense transversal stretching strain within the Seve Nappe from the longitudinal strains of lower intensity within the Köli Nappe. The weight of geological evidence favours the interpretation of this strain discontinuity as a unconformity with significant deformation of the Seve Nappe occurring before rocks of the overlying Köli Nappe were deposited (? Middle Ordovician). A second discontinuity marks the junction between the Krutfjell Nappe and the underlying lower grade Köli rocks. The strain distribution is incompatible with a previous interpretation of the discontinuous Krutfjell Nappe as a string of mega-boudins—an interpretation which implies considerable post-thrust flattening of the nappe pile. The strain results and other evidence favour earlier deformation of the Krutfjell Nappe before these rocks were thrust over the underlying tectonic units. The lens-shaped outcrop configuration of the Krutfjell Nappe is explicable in some cases if the geometry of this nappe is a westerly thinning wedge. Within the low grade Köli Nappe rocks north of the Grong Culmination the longitudinal stretching is difficult to explain in terms of easterly directed nappe movements whereas the clear transverse strains in other tectonic units appear to predate the main Silurian thrusting event.