Early- to mid-Silurian extrusion wedge tectonics in the central Scandinavian Caledonides

Abstract

We present evidence for an extrusion wedge in the Scandian fold-thrust belt of the central Scandinavian Caledonides (Seve nappe complex). Rb-Sr multimineral geochronology in synkinematic assemblages indicates simultaneous movements at the normal-sense roof shear zone and at the reverse-sense floor shear zone between 434 Ma and 429 Ma. A Sm-Nd age of 462 Ma from a mylonitic garnet mica schist documents prograde garnet growth and possible incipient subduction. Pressure-temperature pseudosection calculations provide evidence for eclogite facies metamorphic conditions and nearly isothermal decompression at ~670 ± 50 °C from 17.5 to 14.5 kbar in garnet-kyanite mica schists during reverse-sense shearing, and from 15 to 11 kbar in garnet mica schists during normal-sense shearing. These data and the presence of decompression-related pegmatites dated at 434 Ma and 429 Ma indicate that the Seve nappes form a large-scale extrusion wedge. This wedge extends along strike for at least 150 km and marks an early stage of ultrahigh-pressure metamorphism, exhumation, and orogenic wedge formation in this part of the Scandinavian Caledonides predating the major, post-415 Ma ultrahigh-pressure exhumation processes in southwestern Norway. INTRODUCTIONExtrusion wedges are increasingly recognized as major, lithosphericscale structural features in collisional orogens that exhumed ultrahigh-pressure (UHP) metamorphic rocks during plate convergence, as for example in the Himalayas, Alps, and Hellenides (see Ring and Glodny, 2010, and references therein). They are characterized by simultaneous movements along their normal-sense roof and reverse-sense floor shear zones. We focus on HP metamorphic rocks of the Seve nappes in the Jämtland-Västerbotten segment of the central Scandinavian Caledonides (Fig. 1A). They are bounded by a normal-sense, hinterland-directed shear zone at the roof and a foreland-directed shear zone at the floor (Greiling et al., 1998). This set of opposed-sense shear zones is a structural feature crucial for understanding Scandian orogenic wedge evolution. Based on this geometry, a Himalayantype extrusion of the Seve nappes was hypothesized (Gee et al., 2010). We studied both shear zones to test for simultaneous ductile shear and for their timing within Scandian orogenic wedge evolution.

Replacement reactions and deformation by dissolution and precipitation processes in amphibolites

“…In the Middle Seve Nappe, the granulite and amphibolite facies metamorphism appears to postdate the HP‐UHP stage and produced partial melting at 442–436 Ma (Ladenberger et al., ). A recent study has related the amphibolite facies metamorphic “stage” to the exhumation and lateral extrusion of the SNC (Grimmer, Glodny, Drüppel, Greiling, & Kontny, ). In the Lower Seve Nappe, a pervasive amphibolite facies foliation overprints the (U)HP fabric where present; where not present, it represents the main metamorphic fabric.…”

Evidence from Rb–Sr mineral ages for multiple orogenic events in the Caledonides of Shetland, Scotland

“…In both Norway and East Greenland there is evidence for metamorphism up to eclogite facies and examples of hinterland-directed synconvergent extensional shear zones, perhaps indicative of 'channel flow' (e.g. Hartz et al 2001;Andresen et al 2007;Grimmer et al 2015). In mainland Scotland, the Northern Highland Terrane (Fig.…”

Microdiamond on Åreskutan confirms regional UHP metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides

“…As in central J€ amtland, Ar/Ar data on hornblende in the SNC of northern J€ amtland (Dallmeyer & Gee, 1988) provided evidence of pre-Silurian retrogression of the eclogites. Farther north in southern V€ asterbotten, kyanite-garnet gneisses yielded a Sm-Nd garnet-apatite-feldspar-whole rock age of c. 462 Ma (Grimmer et al, 2015), which was also considered to reflect the prograde or peak pressure stage of metamorphism. In southern Norrbotten, earlier Ordovician (perhaps late Cambrian) ages of 500-480 Ma have been obtained by various methods (Ar/ Ar, Dallmeyer & Gee, 1986;Sm/Nd, Mørk et al, 1988;U/Pb titanite, Essex et al, 1997;U/Pb zircon, Root & Corfu, 2012) for the HP metamorphism, contemporaneous with early Caledonian volcanic-arc magmatism in the overlying K€ oli Nappe Complex.…”