Late Devonian–Carboniferous faulting and controlling structures and fabrics in NW Finnmark

Abstract

In the SW Barents Sea, Devonian-Carboniferous collapse led to the formation of major basins and faults, e.g., the Hammerfest Basin bounded by the Troms-Finnmark Fault Complex, and rhomboid-to sigma-shaped (half-)grabens on the Finnmark Platform. High-resolution aeromagnetic and bathymetry data from the shallow shelf show that analogue fault systems are present in coastal and onshore areas of NW Finnmark. We provide new documentation for the Langfjorden-Vargsundet fault, a post-Caledonian, NW-dipping, zigzag-shaped, margin-parallel fault complex consisting of alternating linear to sub-linear, NNE-SSW-and ENE-WSW-striking, down-NW normal fault segments. This fault formed as an extensional splay-fault of inverted, Caledonian, brittle-ductile thrusts, e.g, the Talvik and Kvenklubben faults, which the fault eventually truncated and offset. In northern Finnmark, the Langfjorden-Vargsundet fault is offset laterally by up to 28 km by a system of WNW-ESE-trending faults notably including potential segments and splays of the Trollfjorden-Komagelva Fault Zone, a reactivated Neoproterozoic, margin-orthogonal transfer fault, separating NW Finnmark from the eastern Finnmark Platform. This fault system likely dies out westwards, and portions of the system's process zone may crop out on the island of Magerøya. Similarly, the WNW-ESE-to ENE-WSW-striking Akkarfjord fault offsets the Langfjorden-Vargsundet fault by c. 2 km left-laterally. This fault may have formed as a conjugate to the Trollfjorden-Komagelva Fault Zone in Neoproterozoic (Timanian?) times. Steeply NW-plunging, upright and gently NE-plunging, inclined folds in Archaean-Palaeoproterozoic basement rocks and margin-parallel Caledonian thrusts may have controlled the formation and geometry of post-Caledonian faults. A Late Devonian-Carboniferous age for the Langfjorden-Vargsundet fault is supported by geochronological dating of onshore dykes and fault gouge, and by syn-tectonic sedimentary wedges along the offshore extension of the Langfjorden-Vargsundet fault. Mini-basins bounded by the Langfjorden-Vargsundet fault on the Finnmark Platform and on the shallow shelf, e.g., the Ryggefjorden trough, may represent analogues to deep, offshore, Devonian-Carboniferous basins, like the Nordkapp Basin, prior to the deposition of late Palaeozoic evaporites.

Late Palaeozoic fault-controlled hydrothermal Cu–Zn mineralisation on Vanna island, West Troms Basement Complex, northern Norway

“…Such an age contrasts with previous workers arguing for a Palaeoproterozoic VMS stringer zone origin of the Cu-Zn mineralisation, linked with the 2.4 Ga mafic dyke swarm (Ojala et al, 2013;Monsen, 2014) and a subsequent spread of metals into sediments now present in the Skipsfjord Nappe (Opheim & Andresen, 1989). The K-Ar dating results are consistent with formation of the VBF and enclosed Cu-Zn mineralised fault rocks/veins as part of an Early Permian rifting event in the North Norwegian continental margin producing NE-SW striking brittle normal faults and associated fracture sets (Gabrielsen et al, 1990;Faleide et al, 2008;Smelror et al, 2009;Hansen et al, 2012), which later on evolved to major fault zones like the Vestfjord-Vanna and Troms-Finnmark fault complexes (Olesen et al, 1997;Indrevaer et al, 2013Indrevaer et al, , 2014Koehl et al, 2018b). Most of these Permian faults, including the VBF, contain features that indicate complex fluid flow and fault-rock interactions; however, very few of them seem to be accompanied by ore mineralisation (cf., Koehl, 2013;Indrevaer et al, 2014).…”

“…The Svecofennian event produced a foreland fold and thrust Figure 1. Regional geological map of the northern Fennoscandian Shield (based on Koistinen et al, 2001;Olesen et al, 2002;Eilu et al, 2008;Bergh et al, 2010;Davids et al, 2013;Koehl et al, 2018b). For the main legend, see Koistinen et al 2001.…”

“…The wide K-Ar illite age range obtained for the VBF (Davids et al, 2013) could reflect a multiphase kinematic and very complex reactivation history of the fault rocks, as outlined in this work. If a minimum Early Permian age is linked to our post-ore movement unconsolidated fault gauge, the main fault zone movement may have initiated as early as in the Carboniferous, even Late Devonian (cf., Koehl et al, 2018b). Carboniferous rifting is known to have produced mafic dyke swarms on the Finnmark portion of the North Norwegian margin (Roberts et al, 2003;Roberts, 2011;Nasuti et al, 2015) and numerous brittle normal faults with extensive hydrothermal alteration and cataclasis (Indrevaer et al, 2013(Indrevaer et al, , 2014Koehl et al, 2018a, b).…”

From widespread Mississippian to localized Pennsylvanian extension in central Spitsbergen, Svalbard

“…Although not always reconstructed in paleo-tectonic reconstructions, in the early Neoproterozoic, the position of Svalbard was probably close to the Timanian margin of northern Baltica prior to the opening of the Asgard Sea and Iapetus Ocean/AEgir Sea (Torsvik et al, 1996;Cawood et al, 2001Cawood et al, , 2010Cawood and Pisarevsky, 2017) and prior to the Timanian Orogeny in the late Neoproterozoic (Roberts and Siedlecka, 2002;Roberts and Olovyanishnikov, 2004). In northern Baltica, similar, steep, abundant, WNW-ESE-striking, margin-oblique (i.e., oblique to the Atlantic margin) brittle faults were mapped on the Varanger Peninsula (Siedlecka and Siedlecki, 1967;Siedlecka, 1975;Siedlecka, 1980) and Magerøya (Koehl, 2018;Koehl et al, 2018c) in northern Norway, and these represent fault segments of a major, inherited, Neoproterozoic subvertical fault, the Trollfjorden-Komagelva Fault Zone, which formed during the Timanian Orogeny and is thought to have accommodated hundreds of kilometers of lateral displacement (Rice, 2013). This fault experienced multiple episodes of reactivation and was last reactivated under transtension; shortly before it was intruded by Mississippian (Visean; Lippard and Prestvik, 1997) dolerite dykes that seal the fault (Roberts et al, 1991;Nasuti et al, 2015).…”

“…A major difference between margin-oblique faults in Odellfjellet (central Spitsbergen) with their counterparts in northern Norway is that the latter accommodated dominantly lateral post-Caledonian (transfer) movement, e.g., the Trollfjorden-Komagelva Fault Zone (Koehl, 2018a;Koehl et al, 2018c), whereas the former accommodated dominantly normal dip-slip to oblique-slip motions (Figs. 4, 8b-c, and 10d).…”