Testing tectonic concepts for the Seve Nappe Complex, Swedish Caledonides
The recent COSC (Collisional Orogeny in Scandinavian Caledonides) initiative (Gee et al., 2010) motivated new interest in studying the nappe structure in the Scandinavian Caledonides (Gee, 1978). While the thrust at the base of the Seve Nappe Complex (Bergman et al., 2012) records a top-ESE displacement, the upper contact of this thrust nappe stack with the Köli Nappe Complex (Bergman et al., 2012) has a more complicated history and top-ESE and also top-WNW shear sense indicators have been reported (Trouw, 1973; Greiling et al., 2007) from southern Västerbotten and northern Jämtland, north-central Sweden. The goal of this proposal is to look at the various tectonic contacts at the Köli/Seve boundary, within the Seve Nappe Complex and also at its basal contact with the Särv Nappes (Bergman et al., 2012) in the southern part of the Jämtland area close to the proposed COSC drill site. We combine detailed structural field work with a microstructural study and then use a novel approach for dating the mylonitic fabrics in the shear zones (Glodny et al., 2002, 2008a, b; Ring et al., 2007a, b, 2010). This is to evaluate various tectonic models for the emplacement and evolution of the Seve Nappe Complex and will provide critical new data for better understanding the thrust nappe structure and tectonic processes in the Swedish Caledonides, providing expertise and knowledge for SGU. Furthermore, this project will have relevance for interpreting COSC drill core and seismic data.
2. Statement of the Problem
The Caledonides of Scandinavia are, at least in part, compared to the Himalayan orogen (Gee et al., 2010, 2013; Streule et al., 2010; Lorenz et al., 2011). This is a courageous statement that deserves further investigation. One major argument for this comparison is the remarkable presence, in both orogens, of a central nappe complex, composed of granulite facies migmatites with leucogranites, derived from the continental margin of the underthrust slab and extruded great distances onto the continental margin. In the Scandes, these ductilly deformed high-grade rocks, occurring within the Seve Nappe Complex, originated from the continent-ocean transition zone along the outermost part of the Baltoscandian margin and were emplaced c. 400 km eastwards onto the rifted margin and platform preserved in the lower thrust sheets in the Offerdal and Jämtlandian Nappes (Bergman et al., 2012). The Seve Nappe Complex is compared to the High Himalayan Zone (Gee et al., 2013). For the latter, evidence for hot ductile extrusion (Nelson et al., 1996) has been elaborated upon and related to channel flow (Beaumont et al., 2001). Furthermore, a great amount of quantitative structural and geochronological data supporting the extrusion-wedge interpretation has been published (Grujic et al., 1996; Grasemann et al., 1999; Grasemann and Vannay, 1999; Law et al., 2006, 2013, and references therein). The main goal of this project is to use structural and geochronological data to test the extrusion-wedge interpretation for the Seve Nappe Complex. This type of data is critical to either support or contradict proposed tectonic similarities between the Scandes and the Himalayas.