Anticlockwise metamorphic pressure–temperature paths 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

Solid Earth. 2019;10:117-148 DOI 10.5194/se-10-117-2019


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Journal Title: Solid Earth

ISSN: 1869-9510 (Print); 1869-9529 (Online)

Publisher: Copernicus Publications

Society/Institution: European Geosciences Union (EGU)

LCC Subject Category: Science: Geology: Stratigraphy

Country of publisher: Germany

Language of fulltext: English

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C. Faber (Department of Geosciences, UiT The Arctic University of Norway, Tromsø 9037, Norway)

H. Stünitz (Department of Geosciences, UiT The Arctic University of Norway, Tromsø 9037, Norway)

H. Stünitz (Institut des Sciences de la Terre (ISTO), Université d'Orleans, Orleans 45100, France)

D. Gasser (Western Norway University of Applied Sciences, Sogndal 6851, Norway)

D. Gasser (Geological Survey of Norway, Trondheim 7491, Norway)

P. Jeřábek (IPSG, Faculty of Science, Charles University, Albertov 6, 128 43, Prague 2, Czech Republic)

K. Kraus (Department of Geosciences, UiT The Arctic University of Norway, Tromsø 9037, Norway)

F. Corfu (Department of Geosciences and Centre for Earth Evolution and Dynamics, University of Oslo, Oslo, Norway)

E. K. Ravna (Department of Geosciences, UiT The Arctic University of Norway, Tromsø 9037, Norway)

J. Konopásek (Department of Geosciences, UiT The Arctic University of Norway, Tromsø 9037, Norway)


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Time From Submission to Publication: 28 weeks


Abstract | Full Text

<p>This study investigates the tectonostratigraphy and metamorphic and tectonic evolution of the Caledonian Reisa Nappe Complex (RNC; from bottom to top: Vaddas, Kåfjord, and Nordmannvik nappes) in northern Troms, Norway. Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and pressure–temperature (<span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>T</i></span>) conditions of deformation and metamorphism during nappe stacking that facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise <span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>T</i></span> path attributed to the effects of early Silurian heating (D<span class="inline-formula"><sub>1</sub></span>) followed by thrusting (D<span class="inline-formula"><sub>2</sub></span>). At ca. 439&thinsp;Ma during D<span class="inline-formula"><sub>1</sub></span> the Nordmannvik Nappe reached the highest metamorphic conditions at ca. 780&thinsp;<span class="inline-formula"><sup>∘</sup></span>C and <span class="inline-formula">∼9</span>–11&thinsp;kbar inducing kyanite-grade partial melting. At the same time the Kåfjord Nappe was at higher, colder, levels of the crust ca. 600&thinsp;<span class="inline-formula"><sup>∘</sup></span>C, 6–7&thinsp;kbar and the Vaddas Nappe was intruded by gabbro at &gt;&thinsp;650&thinsp;<span class="inline-formula"><sup>∘</sup></span>C and ca. 6–9&thinsp;kbar. The subsequent D<span class="inline-formula"><sub>2</sub></span> shearing occurred at increasing pressure and decreasing temperatures ca. 700&thinsp;<span class="inline-formula"><sup>∘</sup></span>C and 9–11&thinsp;kbar in the partially molten Nordmannvik Nappe, ca. 600&thinsp;<span class="inline-formula"><sup>∘</sup></span>C and 9–10&thinsp;kbar in the Kåfjord Nappe, and ca. 640&thinsp;<span class="inline-formula"><sup>∘</sup></span>C and 12–13&thinsp;kbar in the Vaddas Nappe. Multistage titanite growth in the Nordmannvik Nappe records this evolution through D<span class="inline-formula"><sub>1</sub></span> and D<span class="inline-formula"><sub>2</sub></span> between ca. 440 and 427&thinsp;Ma, while titanite growth along the lower RNC boundary records D<span class="inline-formula"><sub>2</sub></span> shearing at <span class="inline-formula">432±6</span>&thinsp;Ma. It emerges that early Silurian heating (ca. 440&thinsp;Ma) probably resulted from large-scale magma underplating and initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual thrust slices (nappe units). This tectonic style contrasts with subduction of mechanically strong continental crust to great depths as seen in, for example, the Western Gneiss Region further south.</p>