Restoration of the external Scandinavian Caledonides

Abstract

-Three models are evaluated for restoring basement rocks coring tectonic windows (WindowBasement) in the Scandinavian Caledonides; parautochthonous (Model I) and allochthonous (models II/III), with initial imbrication of the Window-Basement post-dating or pre-dating, respectively, that in the external imbricate zone (Lower Allochthon). In Model I, the Window-Basement comes from the eastern margin of the basin now imbricated into the Lower Allochthon, while in models II/III it comes from the western margin. In Model II, the Window-Basement formed a basement-high between Tonian and Cryogenian sediments imbricated into the Middle and Lower allochthons; in Model III deposition of the Lower Allochthon sediments commenced in Ediacaran times. Balanced cross-sections and branch-line restorations of four transects (Finnmark-Troms, Västerbotten-Nordland, Jämtland-Trøndelag, Telemark-Møre og Romsdal) show similar restored lengths for the models in two transects and longer restorations for models II/III in the other transects. Model I can result in c. 280 km wide gaps in the restored Lower Allochthon, evidence for which is not seen in the sedimentology. The presence of <3 km thick alluvial-fan deposits at the base of the Middle Allochthon indicates proximal, rapidly uplifting basement during Tonian-Cryogenian periods, taken as the origin of the Window-Basement during thrusting in models II/III. Model I requires multiple changes in thrusting-direction and predicts major thrusts or back-thrusts, currently unrecognized, separating parts of the Lower Allochthon; neither are required in models II/III. Metamorphic data are consistent with models II/III. Despite considerable along-strike structural variability in the external Scandinavian Caledonides, models II/III are preferred for the restoration of the Window-Basement.

Middle to Late Devonian–Carboniferous collapse basins on the Finnmark Platform and in the southwesternmost Nordkapp basin, SW Barents Sea

“…In map view, the TKFZ is irregular, with different structural segments and branching subsidiary faults both across and along strike, locally showing duplex structures (Siedlecka and Siedlecki, 1967;Siedlecka, 1975). The TKFZ formed along the southwestern boundary of the Timanian Orogeny in the late CryogenianEdiacaran (Roberts and Siedlecka, 2002;Siedlecka et al, 2004) and was later reactivated as a strike-slip fault during the Caledonian Orogeny when it accommodated significant lateral displacement constrained to 200-250 km of dextral strike-slip movement (Bylund, 1994;Rice, 2013).…”

“…1; Gayer et al, 1985;Lippard and Roberts, 1987;Rice, 2013). The TKFZ is believed to continue farther west, off the coast, where it is thought to interact with and merge into the WNW-ESE-trending segment of the TFFC (Gabrielsen, 1984;Vorren et al, 1986;Townsend, 1987b;Gabrielsen and Faerseth, 1989;Gabrielsen et al, 1990;Roberts et al, 2011;Bergø, 2016;Lea, 2016).…”

Mid/Late Devonian-Carboniferous collapse basins on the Finnmark Platform and in the southwesternmost Nordkapp basin, SW Barents Sea

“…Our data, however, suggest that 1105 the TKFZ dies out near the coasts of NW Finnmark (present contribution and Koehl et al, submitted), and in this section we review and discuss new evidence obtained from the interpretation of offshore seismic and aeromagnetic data. First, the TKFZ was described onshore eastern Finnmark as a major sub-vertical fault that accommodated dominantly strike-slip movement (Roberts, 1972;Rice, 2013). Farther west, the 1110 TKFZ crops out onshore Magerøya, where it is made of numerous, high-frequency, subparallel, subvertical, WNW-ESE trending faults and fractures that accommodated at least small-scale postCaledonian, strike-slip to oblique-slip displacement (Koehl et al, submitted).…”

“…Senja Shear Zone and Fugløya transfer zone (Indrevaer et al, 2013), which are both sub-parallel to the onshore, Neoproterozoic, WNW-ESE trending Trollfjord-Komagelv Fault Zone (TKFZ) in eastern Finnmark (Siedlecki, 1980;Herrevold et al, 70 2009) and to the Kokelv Fault on the Porsanger Peninsula ( Figure 1; Gayer et al, 1985;Lippard & Roberts, 1987;Rice, 2013) while the coastal Langfjord-Vargsund fault (LVF) trends NE-SW, parallel to the TFFC (Figure 1). The TKFZ is believed to continue farther west, off the coast, where it is thought to interact with and merge into the WNW-ESE trending fault segment of the TFFC (Gabrielsen, 1984;Vorren et al, 1986;Towsend, 1987;Gabrielsen & Faerseth, 1989;Gabrielsen 75 et al, 1990;Roberts et al, 2011;Bergø, 2015;Lea, 2015).…”

“…In map view, the TKFZ is irregular, with different structural segments and branching 160 subsidiary faults both across-and along-strike, locally showing duplex structures (Siedlecka & Siedlecki, 1967;Siedlecka, 1975). The TKFZ formed along the southwestern boundary of the Timanian Orogeny in the late Cryogenian-Ediacaran (Roberts & Siedlecka, 2002;Siedlecka et al, 2004), and was later reactivated as a strike-slip fault during the Caledonian Orogeny when it accommodated significant lateral displacement constrained to 200-250 km of dextral strike-slip 165 movement (Bylund, 1994;Rice, 2013).…”