Exhumation of an eclogite terrane as a hot migmatitic nappe, Sveconorwegian orogen

Abstract

We demonstrate a case of eclogite exhumation in a partially molten, low-viscosity fold nappe within high-grade metamorphosed crust in the Eastern Segment of the Sveconorwegian orogen. The nappe formed during tectonic extrusion, melt-weakening assisted exhumation and foreland-directed translation of eclogitized crust, and stalled at 35-40 km depth within the collisional belt. The eclogites are structurally restricted to a regional recumbent fold in which stromatic orthogneiss with pods of amphibolitized eclogite make up the core. Hightemperature mylonitic gneiss with remnants of kyanite eclogite (P N 15 kbar) composes a basal shear zone 50 km long and b 4 km wide. Heterogeneously sheared and partly migmatized augen gneiss forms a tectonostratigraphic marker in front of and beneath the nappe, and is in turn structurally enveloped by a composite sequence of orthogneisses and metabasites. The entire tectonostratigraphic pile underwent near-pervasive deformation and recrystallization under high-pressure granulite and upper amphibolite conditions. U-Pb SIMS metamorphic zircon ages of eclogite and stromatic orthogneiss constrain the time of eclogitization at 988 ± 6 Ma and 978 ± 7 Ma. Migmatization, concomitant deformation, and exhumation are dated at 976 ± 6 Ma, and crystallization of post-kinematic melt at 956 ± 7 Ma. Orthogneiss protoliths are dated at 1733 ± 11 and 1677 ± 10 Ma (stromatic gneiss) and 1388 ± 7 Ma (augen gneiss in footwall), demonstrating origins indigenous to the Eastern Segment. Eclogitization and exhumation were coeval with the Rigolet phase of the Grenvillian orogeny, reflecting the late stage of continental collision during construction of the supercontinent Rodinia.

Role of strain localization and melt flow on exhumation of deeply subducted continental crust

“…Weakening of rocks by interstitial melt plays a significant role in the dynamics of orogenic systems (e.g., Vanderhaeghe 2009;Jamieson et al, 2010) and can result in large displacements along melt-bearing shear zones bounding regions of rapid uplift (Hollister and Crawford, 1986), extrusion of lower crustal rocks along orogenic sutures (Konopásek and Schulmann, 2005;Möller et al, 2015), or channel flow of partially molten lower crust during intracrustal indentation (Beaumont et al, 2001;Vanderhaeghe et al, 2003;Culshaw et al, 2006;Jamieson et al, 2007;Schulmann et al, 2008a;Maierová et al, 2012). Labrousse et al (2015) suggested that melting of subducted upper continental crust at ultrahigh-pressure (UHP) conditions triggers its subsequent exhumation.…”

Hallandian 1.45Ga high-temperature metamorphism in Baltica: P–T evolution and SIMS U–Pb zircon ages of aluminous gneisses, SW Sweden

“…High-pressure metamorphism is lacking. This is in striking contrast to the 1.1-0.9 Ga Sveconorwegian orogen which comprises crustal-scale continental blocks displaced out of their original context, as well as deeply depressed, highpressure metamorphic parts of Baltica (Möller et al, 2015).…”

“…1; Söderlund et al, 1999;Möller et al, 2007;Stephens et al, 2009;Petersson et al, 2013). Eclogite-facies metamorphism of parts of the Eastern Segment attests to deep tectonic burial (>55 km) of continental crust at 0.99-0.98 Ga (Möller, 1998(Möller, , 1999Johansson et al, 2001;Austin Hegardt et al, 2005;Möller et al, 2015). It was followed by high-grade metamorphism, ductile deformation and partial melting at 0.98-0.96 Ga (op.…”

“…1 and 2; Berthelsen, 1980;Bingen et al, 2008a,b;Möller et al, 2015). Today the Sveconorwegian province forms a c. 500 km wide orogenic belt extending across southwestern Scandinavia, the remainder of a 1.14-0.90 Ga large hot orogen.…”

“…1) are composed of bedrock principally formed along 1.64-1.52 Ga Gothian (Bingen et al, 2005;Åhäll and Connelly, 2008) and 1.52-1.47 Ga Telemarkian (Bingen et al, 2008a) active margins. Their pre-Sveconorwegian locations and relation to the pre-Sveconorwegian Baltica is unclear Möller et al, 2015).…”