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Discussion papers | Copyright
https://doi.org/10.5194/se-2018-74
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 31 Jul 2018

Research article | 31 Jul 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).

An anticlockwise metamorphic P-T path and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

Carly Faber1, Holger Stünitz1,2, Deta Gasser3,4, Petr Jeřábek5, Katrin Kraus1, Fernando Corfu6, Erling K. Ravna1, and Jiří Konopásek1 Carly Faber et al.
  • 1Department of Geosciences, UiT The Arctic University of Norway, Tromsø, 9037, Norway
  • 2Institut des Sciences de la Terre (ISTO), Université d ́Orleans, 45100, France
  • 3Western Norway University of Applied Sciences, Sogndal 6851, Norway
  • 4Geological Survey of Norway, Trondheim 7491, Norway
  • 5IPSG, Faculty of Science, Charl es University, Albertov 6, 128 43, Prague 2, Czech Republic
  • 6Department of Geosciences & Centre for Earth Evolution and Dynamics, University of Oslo, Norway

Abstract. This study investigates the Caledonian metamorphic and tectonic evolution in northern Norway, examining the structure and tectonostratigraphy of the Reisa Nappe Complex (RNC; from bottom to top, Vaddas, Kåfjord and Nordmannvik nappes). Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and P-T conditions of deformation and metamorphism that formed the nappes and facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P-T path attributed to the effects of early Silurian heating followed by thrusting. An early Caledonian S1 foliation in the Nordmannvik Nappe records kyanite-grade partial melting at ~760–790°C and ~9.4–11kbar. Leucosomes formed at 439±2Ma (U-Pb zircon) in fold axial planes in the Nordmannvik Nappe indicate that compressional deformation initiated while the rocks were still partially molten. This stage was followed by pervasive solid-state shearing as the rocks cooled and solidified, forming the S2 foliation at 680–730°C and 9.5–10.9kbar. Multistage titanite growth in the Nordmannvik Nappe records this extended metamorphism between 444 and 427Ma. In the underlying Kåfjord Nappe, garnet cores record lower P-T (590–610°C and 5.5–6.8kbar) but a similar geothermal gradient as the S1 migmatitic event in the Nordmannvik Nappe, indicating formation at a higher relative position in the crust. S2 shearing in the Kåfjord Nappe occurred at 580–605°C and 9.2–10.1kbar, indicating a considerable pressure increase during nappe stacking. Gabbro intruded in the Vaddas Nappe at 439±1Ma, synchronously with migmatization in the Nordmannvik Nappe. In the Vaddas Nappe S2 shearing occurred at 630–640ºC and 11.7–13kbar. Titanite growth along the lower RNC boundary records S2-shearing at 432±6Ma. It emerges that early Silurian heating (~440Ma), probably resulting from large-scale magma underplating, initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual nappe units. This tectonic style contrasts subduction of mechanically strong continental crust to great depths.

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Short summary
The Caledonian mountains formed when Baltica and Laurentia collided around 450–400 million years ago. This work describes the history of the rocks and the dynamics of that continental collision through space and time using field mapping, estimated pressures and temperatures, and age dating on rocks from northern Norway. The rocks preserve continental collision between 440–430 million years ago, and an unusual pressure-temperature evolution suggests unusual tectonic activity prior to collision.
The Caledonian mountains formed when Baltica and Laurentia collided around 450–400 million years...
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