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Volume 9, issue 4 | Copyright
Solid Earth, 9, 923-951, 2018
https://doi.org/10.5194/se-9-923-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 20 Jul 2018

Research article | 20 Jul 2018

Neoproterozoic and post-Caledonian exhumation and shallow faulting in NW Finnmark from K–Ar dating and pT analysis of fault rocks

Jean-Baptiste P. Koehl1,2, Steffen G. Bergh1,2, and Klaus Wemmer3 Jean-Baptiste P. Koehl et al.
  • 1Department of Geosciences, UiT The Arctic University of Norway in Tromsø, 9037 Tromsø, Norway
  • 2Research Center for Arctic Petroleum Exploration (ARCEx), UiT The Arctic University of Norway in Tromsø, 9037 Tromsø, Norway
  • 3Geoscience Centre, Georg-August-University Göttingen, Goldschmidtstraße 3, 37077 Göttingen, Germany

Abstract. Well-preserved fault gouge along brittle faults in Paleoproterozoic, volcano-sedimentary rocks of the Raipas Supergroup exposed in the Alta–Kvænangen tectonic window in northern Norway yielded latest Mesoproterozoic (approximately 1050±15Ma) to mid-Neoproterozoic (approximately 825–810±18Ma) K–Ar ages. Pressure–temperature estimates from microtextural and mineralogy analyses of fault rocks indicate that brittle faulting may have initiated at a depth of 5–10km during the opening of the Asgard Sea in the latest Mesoproterozoic–early Neoproterozoic (approximately 1050–945Ma) and continued with a phase of shallow faulting to the opening of the Iapetus Ocean–Ægir Sea and the initial breakup of Rodinia in the mid-Neoproterozoic (approximately 825–810Ma). The predominance and preservation of synkinematic smectite and subsidiary illite in cohesive and non-cohesive fault rocks indicate that Paleoproterozoic basement rocks of the Alta–Kvænangen tectonic window remained at shallow crustal levels (<3.5km) and were not reactivated since mid-Neoproterozoic times. Slow exhumation rate estimates for the early–mid-Neoproterozoic (approximately 10–75mMyr−1) suggest a period of tectonic quiescence between the opening of the Asgard Sea and the breakup of Rodinia. In the Paleozoic, basement rocks in NW Finnmark were overthrusted by Caledonian nappes along low-angle thrust detachments during the closing of the Iapetus Ocean–Ægir Sea. K–Ar dating of non-cohesive fault rocks and microtexture mineralogy of cohesive fault rock truncating Caledonian nappe units show that brittle (reverse) faulting potentially initiated along low-angle Caledonian thrusts during the latest stages of the Caledonian Orogeny in the Silurian (approximately 425Ma) and was accompanied by epidote–chlorite-rich, stilpnomelane-bearing cataclasite (type 1) indicative of a faulting depth of 10–16km. Caledonian thrusts were inverted (e.g., Talvik fault) and later truncated by high-angle normal faults (e.g., Langfjorden–Vargsundet fault) during subsequent, late Paleozoic, collapse-related widespread extension in the Late Devonian–early Carboniferous (approximately 375–325Ma). This faulting period was accompanied by quartz- (type 2), calcite- (type 3) and laumontite-rich cataclasites (type 4), whose cross-cutting relationships indicate a progressive exhumation of Caledonian rocks to zeolite-facies conditions (i.e., depth of 2–8km). An ultimate period of minor faulting occurred in the late Carboniferous–mid-Permian (315–265Ma) and exhumed Caledonian rocks to shallow depth at 1–3.5km. Alternatively, late Carboniferous (?) to early–mid-Permian K–Ar ages may reflect late Paleozoic weathering of the margin. Exhumation rates estimates indicate rapid Silurian–early Carboniferous exhumation and slow exhumation in the late Carboniferous–mid-Permian, supporting decreasing faulting activity from the mid-Carboniferous. NW Finnmark remained tectonically quiet in the Mesozoic–Cenozoic.

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We dated the formation of large faults in order to constrain the tectonic and exhumation history of the Barents Sea and northern Norway. Some of the dated faults formed apprx. 1 Ga and are much older than expected. However, most dated faults were active during two periods of extension: 375–325 and 315–265 Ma. The study of minerals along these cracks shows that exposed rocks in Finnmark were exhumed from deep (> 10 km) to shallow depth (< 3.5 km) during the two periods of extension.
We dated the formation of large faults in order to constrain the tectonic and exhumation history...
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