Testing Tectonic Concepts in the Seve Nappe Complex, Jämtland, Sweden

Sammanfattning: When continental plates collide, one of the involved continents is subducted beneath the other one. As a consequence, the lithosphere thickens along the convergent plate boundary and causes the formation of a mountain belt. High-grade metamorphic rocks, formerly parts of the subducted continent, can be exhumed back to the surface and are commonly found in the central domain of mountain belts. The leucogranite- bearing Seve Nappe Complex in the central part of the Scandinavian Caledonides is a good example of such commonly migmatic rock units, which hold the key to understanding the tectonic evolution of a mountain range. This study aims to develop a tectonic model for the Swedish Caledonides, which integrates new structural data, collected during extensive fieldwork, and new geochronological constraints. Here we present the results of the first part of this study. We show how top-to-the-foreland directed shearing affected the migmatic part of the Seve Nappe Complex from bottom to top at amphibolite-facies conditions. Subsequently, the entire Caledonian nappe stack underwent a greenschist-facies overprint, associated with pervasive, again, top- to-the-foreland directed shearing. This last tectonic event resulted in the assembly of the presently observed nappe architecture, which is characterized by the excision of large sections of the lithosphere.