The western Troms margin is characterized by Archean to Paleoproterozoic basement rocks of the West Troms Basement Complex (Bergh et al., 2010) that are preserved and exposed in a horst block formed during post-Caledonian extension (Indrevær et al., 2013). The West Troms Basement Complex consists of tonalitic, trondhjemitic, and granitic gneisses; metasupracrustal rocks; and mafic and felsic igneous rocks (Corfu et al., 2003; Bergh et al., 2010). These rocks were deformed during the Svecofennian orogeny, which resulted in the formation of NW–SE-trending steep foliation, ductile shear zones, and upright and vertical macrofolds, which were only weakly reworked during the Caledonian Orogeny (Corfu et al., 2003; Bergh et al., 2010).

In NW Finnmark, Paleoproterozoic basement rocks occur in several tectonic windows of the Caledonides, e.g., Repparfjord-Komagfjord and Alta-Kvænangen tectonic windows (Zwaan and Gautier, 1980; Pharaoh et al., 1982, 1983; Bergh and Torske, 1988; Fig. 1), and consist of low-grade supracrustal metavolcanics and metasedimentary rocks of the Raipas Group. These Greenstone belts formed as NW–SE-trending rift basins in the Paleoproterozoic during the opening of the Kola Ocean (Bergh and Torske, 1986, 1988), although more recent studies tentatively reinterpret these rocks as foreland basin deposits derived from the Svecokarelian Orogeny (Torske and Bergh, 2004). A thin cover of Neoproterozoic to Cambrian (para-)autochthonous metasedimentary rocks occurs on top of Paleoproterozoic basement rocks in Finnmark (Siedlecki, 1980; Ramsay et al., 1985; Andresen et al., 2014; Corfu et al., 2014). Other Neoproterozoic–Ordovician units in eastern Finnmark include metasedimentary rocks of the Barents Sea and Tanafjorden–Varangerfjorden regions (Siedlecki, 1980; Siedlecka and Roberts, 1992), which are exposed on the Varanger Peninsula (Fig. 1).

The Timanian Orogeny produced major NW–SE-trending folds (Roberts and Siedlecka, 2002) and WNW–ESE-trending fault complexes like the TKFZ (Jonhson et al., 1978; Herrevold et al., 2009). The TKFZ was mapped as a narrow, single-segment fault strand all the way along the Kola Peninsula in Russia in the east, where it merges with the Sredni-Rybachi Fault Zone (Roberts et al., 1997, 2011), to the Barents shelf in the west (Gabrielsen, 1984; Gabrielsen and Færseth, 1989; Gabrielsen et al., 1990; Roberts et al., 2011). We present an alternative model in which the TKFZ splays into multiple fault segments and dies out between the Varanger Peninsula and the Barents shelf. On the Varanger Peninsula, the TKFZ is well displayed on satellite images, but is generally poorly exposed. 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 Cryogenian–Ediacaran (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).

Coastal areas of NW Finnmark are dominated by Caledonian thrust sheets of the Kalak Nappe Complex and Magerøy Nappe (Ramsay et al., 1985; Ramberg et al., 2008; Corfu et al., 2014), formed in the Neoproterozoic through Silurian (Fig. 1). The Kalak Nappe Complex is composed of amphibolite facies schists, metapsammites, and paragneisses and comprises several allochthonous thrust sheets with Proterozoic basement rocks, clastic metasedimentary rocks, and plutonic rocks of the Seiland Igneous Province (Corfu et al., 2014). A major thrust defines the contact with the underlying pre-Caledonian basement (Ramsey et al., 1985). Dominant structures include a gently NW-dipping foliation; NNE–SSW-trending, east-verging, asymmetrical recumbent folds; and low-angle thrusts that accommodated top–ESE shortening (Townsend, 1987a; Kirkland et al., 2005). The Kalak Nappe Complex was previously considered to represent an exotic terrane accreted on the Laurentian margin of Rodinia prior to the rifting of the Iapetus Ocean and to have later been thrusted over Baltica during the Caledonian Orogeny (Kirkland et al., 2008). However, paleocurrent and geochronological data suggest these rocks to be of Baltican origin (Roberts, 2007; Zhang et al., 2016).

The Seiland Igneous Province corresponds to a large, late Neoproterozoic mafic and ultramafic intrusion linked to the early–mid rifting stages of the Iapetus Ocean (Elvevold et al., 1994; Corfu et al., 2014). Recent geophysical studies by Pastore et al. (2016) show that the base of the Seiland Igneous Province defines two deep-reaching roots located below the islands of Seiland and Sørøya constraining the thickness of the Kalak Nappe Complex in this area to a maximum of 10 km. On the Porsanger and Varanger peninsulas, ENE–WSW- to NNE–SSW-trending Ediacaran metadolerite dyke swarms are particularly common, and they are associated with the rifting of the Iapetus Ocean as well (see Roberts, 1972; Siedlecka et al., 2004; Nasuti et al., 2015).

The Kalak Nappe Complex is structurally overlain by the Magerøy Nappe, which consists of Late Ordovician to early Silurian greenschist facies metasedimentary and metaplutonic rocks (Andersen, 1981, 1984; Corfu et al., 2014) that crop out on the island of Magerøya (Fig. 1). The Magerøy Nappe is characterized by asymmetrical NNE–SSW-trending, east-verging folds and low-angle, NW- and SE-dipping thrusts similar in trend to those observed within the Kalak Nappe Complex (Andersen, 1981) and is intruded by granitic and gabbroic plutons, e.g., the Silurian Honningsvåg Igneous Complex (Corfu et al., 2006) and the Finnvik Granite (Andersen, 1981). Remnants of the Magerøy Nappe thrust units are also found in northeastern Sørøya and on the Porsanger Peninsula (Kirkland et al., 2005, 2007; Corfu et al., 2014; Fig. 1).

The SW Barents Sea margin was subjected to multiple episodes of extensional faulting after the end of the Caledonian Orogeny, starting with the collapse of the Caledonides in the Middle to Late Devonian–early Carboniferous, lasting until the early–mid Permian, although evidence of this stage is only preserved onshore in western and mid-Norway (Séranne et al., 1989; Chauvet and Séranne, 1994; Braathen et al., 2000; Osmundsen and Andresen, 2001). During this period, basement ridges in Lofoten–Vesterålen (Klein and Steltenpohl, 1999; Klein et al., 1999; Steltenpohl et al., 2004, 2011; Fig. 1) and in mid-Norway (Osmundsen et al., 2005; Fig. 1) were exhumed as metamorphic core complexes, synchronous with the development of large half-graben basins such as the Vøring and Møre basins in mid-Norway (Blystad et al., 1995) and the Hammerfest, Nordkapp, and Ottar basins in the SW Barents Sea (Gabrielsen et al., 1990; Breivik et al., 1995; Gudlaugsson et al., 1998; Indrevær et al., 2013; Fig. 1). The main rifting events occurred in the Late Jurassic and peaked in the Early Cretaceous, when major offshore basins such as the Tromsø and Harstad basins formed. The rifting ended with full breakup of the North Atlantic Ocean and formation of a transform plate margin in the SW Barents Sea at the Paleocene–Eocene transition (Faleide et al., 1993, 2008).

Off the coasts of western Troms and NW Finnmark, the SW Barents Sea margin is characterized by a relatively shallow area, the Finnmark Platform (Gabrielsen et al., 1990; Fig. 1), which is thought to have remained relatively stable since late Paleozoic times. For example, the inner part of the Finnmark Platform, here referred to as the eastern Finnmark Platform (Fig. 1), was only affected by the formation of minor Carboniferous, ENE–WSW- to NE–SW-trending half-graben and graben structures (Bugge et al., 1995; Samuelsberg et al., 2003; Rafaelsen et al., 2008; Fig. 1). In the hanging wall of the MFC, the western part of the Finnmark Platform (Fig. 1) shows a prominent gravity low, the Gjesvær Low, which was ascribed to the presence of low-density Caledonian rocks (Johansen et al., 1994; Gernigon et al., 2014). We explore and argue for an alternative explanation, i.e., the presence of Devonian collapse basin deposits draped against a low-angle extensional detachment of the SISZ, similar to the Nordfjord-Sogn Detachment Zone, a late-orogenic shear zone that bounds the Middle Devonian Hornelen, Kvamshesten, and Solund sedimentary basins onshore in western Norway (Séranne et al., 1989; Chauvet and Séranne, 1994; Wilks and Cuthbert, 1994; Osmundsen and Andersen, 2001). Ductile detachment surfaces of comparable size, showing analog kinematics and timing of activity contemporaneous with the Nordfjord-Sogn Detachment Zone are documented as far north as the Lofoten–Vesterålen Margin (Klein and Steltenpohl, 1999; Klein et al., 1999; Steltenpohl et al., 2004, 2011), but Devonian collapse basin sedimentary rocks and extensional detachments have not yet been reported along the margins of western Troms and NW Finnmark.

Multiple studies have reported post-Caledonian brittle faults in onshore coastal areas in Lofoten–Vesterålen, western Troms, and NW Finnmark (Roberts, 1971; Worthing, 1984; Lippard and Roberts, 1987; Townsend, 1987a; Rykkelid, 1992; Lippard and Prestvik, 1997; Roberts and Lippard, 2005; Bergh et al., 2007; Hansen et al., 2012; Indrevær et al., 2013; Davids et al., 2013). A common feature is the presence of rhombic, zigzag-shaped fault trends similar in geometry to offshore basin-bounding faults. Dominant fault–fracture trends of the margin strike NNE–SSW, ENE–WSW, and NW–SE (Bergh et al., 2007; Eig, 2008; Eig and Bergh, 2011; Hansen et al., 2012; Hansen and Bergh, 2012; Indrevær et al., 2013). Typical examples are basin-bounding, NNE–SSW- and ENE–WSW-trending brittle normal faults that are part of the Vestfjorden-Vanna Fault Complex, which bounds the offshore Vestfjorden Basin southeast of the Lofoten islands and which can be traced northward to western Troms (Indrevær et al., 2013; Fig. 1), whereas the NNW–SSE to WNW–ESE trend typically reflects margin-oblique, transform fault trends (Faleide et al., 2008). An analog to the onshore Vestfjorden–Vanna Fault Complex in NW Finnmark is the Langfjorden–Vargsundet fault (Fig. 1), described by Zwaan and Roberts (1978) and Worthing (1984) as a major NE–SW-trending, NW-dipping normal fault juxtaposing rocks from the Kalak Nappe Complex and the Seiland Igneous Province in the northwest against Precambrian basement rocks of the Repparfjord-Komagfjord and Alta-Kvænangen tectonic windows in the southeast (Fig. 1).

The NW Finnmark margin is located along the northeastward prolongation of the Lofoten–Vesterålen and western Troms segments of the Norwegian continental shelf (Fig. 1). Similar fault sets and trends as in Lofoten–Vesterålen exist in Finnmark and their interaction is thought to partly have controlled the rhombic geometry of many offshore sedimentary basins (Bergh et al., 2007; Indrevær et al., 2013). A typical example along the western Troms and NW Finnmark margins is the NW-dipping TFFC, which bounds the Harstad Basin to the east and the Hammerfest Basin to the southeast (Gabrielsen et al., 1990; Indrevær et al., 2013). The TFFC defines a system of irregular branching faults trending NNE–SSW and ENE–WSW and terminating as a WNW–ESE-trending fault zone northwest of the island of Magerøya where it merges with the NE–SW-trending, NW-dipping MFC at the southeastern boundary of the Nordkapp Basin (Gabrielsen et al., 1990) and of the triangular-shaped southwesternmost Nordkapp basin (Omosanya et al., 2015; Fig. 1). We address a possible genetic relationship and structural inheritance of the post-Caledonian MFC with the Caledonian SISZ and argue that the MFC may have initiated as an extensional splay during the reactivation of the SISZ as an extensional detachment during the late- to post-orogenic collapse of the Caledonides. Furthermore, we tentatively link basement ridges such as the Norsel High in the footwall of the Nysleppen Fault Complex (Gabrielsen et al., 1990) to bowed segments of the SISZ (Fig. 1).

The Norwegian continental shelf is segmented by transfer fault zones of which the largest is the offshore De Geer Zone (Faleide et al., 1984, 2008; Cianfarra and Salvini, 2015), the main fault segment of which is the Hornsund Fault Zone, an offshore NNW–SSE-trending fault that runs parallel to the west coast of Spitsbergen and separates the SW Barents Sea margin from the Lofoten–Vesterålen Margin (Fig. 1). In the south, the De Geer Zone proceeds through the Senja Fracture Zone and into the Senja Shear Belt on the shore of the island of Senja (Fig. 1). Olesen et al. (1993, 1997) suggested shifts of polarity of the Vestfjorden-Vanna Fault Complex along the Senja Fracture Zone, and they argued that the formation of the Senja Fracture Zone offshore was controlled by a major onshore basement weakness zone, the Bothnian-Senja Fault Complex (Fig. 1), which provided suitably oriented basement heterogeneities for the development of a transfer zone (e.g., Doré et al., 1997). Similarly, Indrevær et al. (2013) proposed the existence of a fault array termed the Fugløya transfer zone to explain offsets and shifts of polarity along the Vestfjorden-Vanna Fault Complex farther northeast in western Troms (Fig. 1). The Fugløya transfer zone trends N–S to NNW–SSE and continues on the shore of western Troms, where it merges with the NW–SE-trending Bothnian-Kvænangen Fault Complex, and offshore where it is thought to merge into the TFFC and the Ringvassøy-Loppa Fault Complex (Indrevær et al., 2013; Fig. 1).

Analogously in NW Finnmark, the WNW–ESE-trending TKFZ seems to merge into a basin-bounding fault, in this case the WNW–ESE-trending, NE-dipping fault segment of the TFFC (Gabrielsen, 1984; Gabrielsen and Færseth, 1989; Roberts et al., 2011). In nearshore areas of NW Finnmark, the TKFZ is thought to proceed offshore and seems to correlate with a large escarpment north of Magerøya and into the Barents Sea (Vorren et al., 1986; Townsend, 1987b). In the area where it terminates, it merges and links up with the TFFC to form triangular-shaped mini-basins (Gabrielsen, 1984; Gabrielsen and Færseth, 1989; Roberts et al., 2011). We explore an alternative origin for the WNW–ESE-trending fault segment of the TFFC and further examine its interaction with the onshore–nearshore TKFZ, which potentially acted as a transfer fault after the Caledonian Orogeny and contributed to offset the LVF near Magerøya and adjacent coastal areas (Koehl et al., 2018; Fig. 1). Other major WNW–ESE-trending faults exist offshore, northeast of the Varanger Peninsula, and these bound the Tiddlybanken Basin, a large WNW–ESE-trending basin that formed in Carboniferous times (Mattingsdal et al., 2015; Fig. 1).

The absolute age of post-Caledonian brittle faults in NW Finnmark is poorly constrained, although a few contributions provide valid insights (Lippard and Prestvik, 1997; Davids et al., 2013; Torgersen et al., 2014; Koehl et al., 2016). Torgersen et al. (2014) performed K–Ar dating of brittle fault gouge in the footwall of the LVF and obtained dominantly Carboniferous to early Permian ages, as well as a subsidiary Early Cretaceous age for one of the faults. Roberts et al. (1991) and Lippard and Prestvik (1997) presented indirect evidence of early Carboniferous dolerite dykes emplaced along and sealing WNW–ESE-trending brittle fault segments of the TKFZ on Magerøya, thus providing a minimum estimate for the latest stage of faulting along this fault. These dykes produce high positive aeromagnetic anomalies (Nasuti et al., 2015) and may be used to further identify brittle faults in NW Finnmark. Late Devonian dolerite dykes emplaced along brittle faults that trend NE–SW and N–S have been identified and dated on the eastern Varanger Peninsula (Guise and Roberts, 2002) and on the Kola Peninsula (Roberts and Onstott, 1995). By comparison, Davids et al. (2013) obtained Late Devonian–early Carboniferous ages from K–Ar dating of illite clay minerals for early extensional faulting along the Vestfjorden-Vanna Fault Complex and related faults in Lofoten–Vesterålen and western Troms.

In seismic data (Fig. 5; see the Supplement for uninterpreted seismic sections), basement rocks typically show chaotic internal reflection patterns, which complicate the task of identifying intra-basement structures and basins, and individualize layered sedimentary sequences. However, kilometer-thick layers bearing strong basement fabrics such as widespread, gently dipping foliation or pronounced mylonitic fabric commonly found along large shear zones may turn out to be resolvable on the seismic scale (see Sect. 4.1.2.; Fountain et al., 1984; Reeve et al., 2013; Phillips et al., 2016; Fazlikhani et al., 2017). For instance, we observed a several-kilometer-thick, curved, shallow-dipping layer that is characterized by moderate-amplitude reflections, which are parallel to the layer's upper and lower boundaries (see “Sørøya-Ingøya shear zone” reflections in Fig. 5c–g). We interpret these pronounced internal fabrics as widespread mylonitic foliation separated by internal thrusts within a large-scale shear zone. Numerous smaller basement shear zones may be present below late Paleozoic–Cenozoic sedimentary rocks on the western Finnmark Platform, and these correspond to steeply to moderately dipping fabrics made of subparallel, moderate- to high-amplitude reflections (see Figs. 5b, e, f, g and 6a–c).

Potential Devonian sedimentary deposits along the SW Barents Sea are sparse and as a result their seismic character is not well constrained (Fig. 3). This sedimentary succession has not been drilled, which makes its interpretation on seismic data rather speculative. However, we believe that the best two candidates to represent Devonian sedimentary deposits analog to those in western and mid-Norway (Braathen et al., 2000; Osmundsen and Andersen, 2001; Fazlikhani et al., 2017) are located at the base of the southwesternmost Nordkapp basin and on the western Finnmark Platform near the Gjesvær Low (Fig. 1). In the southwesternmost Nordkapp basin, possible Devonian sedimentary strata are located at a deep level (below 4 s TWT) and their seismic signature is thus largely masked by overlying sedimentary successions (Fig. 5c and d). By contrast, on the western Finnmark Platform (Fig. 5e) potential Devonian sedimentary rocks are relatively shallower, which makes their seismic pattern easier to distinguish from underlying basement rocks and from overlying Carboniferous sedimentary deposits and seismic artifacts (Fig. 5e). Devonian sedimentary rocks on the western Finnmark Platform display relatively low seismic amplitudes, partly similar to analog deposits in the North Sea (see seismic facies 1 in Fazlikhani et al., 2017). The internal reflection pattern is rather chaotic apart from a few discrete, shallow-dipping, moderate-amplitude reflections that converge towards each other upwards and that we interpret as major sedimentary sequence boundaries (see dotted white reflections in Figs. 5e and 6b and c). Furthermore, Devonian sedimentary deposits are likely separated from underlying basement rocks by an angular unconformity that appears as arcuate, high-amplitude seismic reflections (“base Devonian” reflection in Figs. 5e and 6b and c). We interpret these arcuate, high-amplitude seismic reflections as an erosional unconformity.

Lower Carboniferous sedimentary deposits of the Billefjorden Group, composed of thick clastic sedimentary deposits interbedded with occasional coal-bearing sedimentary rocks (Fig. 3), may produce high-amplitude seismic reflections related to their organic-rich content (Fig. 5a and b). Such sedimentary strata are present on the eastern Finnmark Platform, where they appear to thicken to the southeast near the coast of NW Finnmark (Fig. 6d), whereas they are rather sparse on the western Finnmark Platform, i.e., eroded or never deposited (Fig. 5e and f). On the eastern Finnmark Platform, the transition from basement rocks (see “Top basement” reflection in Fig. 5a and b) to lower Carboniferous sedimentary rocks is difficult to interpret on seismic sections. This is attributable to the strong similarities between high seismic amplitudes displayed locally by both basement rock fabrics such as major shear zones (see yellow dotted lines in Fig. 5b) and lower Carboniferous coal-bearing sedimentary deposits. Low-amplitude reflections also show identical chaotic patterns in both basement rocks and clastic sedimentary rocks of the Billefjorden Group (Fig. 5a and b). In the southwesternmost Nordkapp basin, lower Carboniferous sedimentary strata are believed to be present, although their seismic signature certainly appears to be affected by overlying upper Carboniferous evaporite deposits (Fig. 5c and d). The boundary between lower Carboniferous sedimentary deposits and potential underlying Devonian sedimentary rocks was not identified in the southwesternmost Nordkapp basin. Nevertheless, the maximum thickness of Billefjorden Group sedimentary strata on the eastern Finnmark Platform is ca. 600 m (Bugge et al., 1995), and this suggests that the several-kilometer-thick succession below the mid-Carboniferous reflection and above a thick shear zone in the southwesternmost Nordkapp basin is composed of lower Carboniferous sedimentary rocks probably complemented by thick Devonian sedimentary deposits (Fig. 5c and d). Alternatively, sedimentary deposits of the Billefjorden Group directly overlie basement rocks.

On the Finnmark Platform (Figs. 1 and 2), the base of the upper Carboniferous sedimentary succession is difficult to identify (see “mid-Carboniferous” reflection in Figs. 3 and 5). In places, it appears as a linear, moderate- to low-amplitude seismic reflection that separates subparallel reflections of lower and upper Carboniferous sedimentary rocks, whereas in other places the reflection is irregular and truncates high-amplitude coal-bearing sedimentary deposits of the Billefjorden Group and/or high-amplitude reflections produced by basement rocks (Fig. 6a) and/or low-amplitude reflections in Devonian sedimentary strata (Fig. 6b and c). Nevertheless, this reflection generally corresponds to an angular unconformity (e.g., Fig. 6a–c and e) and is therefore interpreted to correspond to a regional erosion surface.

In the southwesternmost Nordkapp basin, the base of upper Carboniferous sedimentary deposits (see “mid-Carboniferous” reflection in Figs. 3 and 5c and d) appears as a clear, discrete high-amplitude reflection. The strong acoustic impedance contrast producing the high seismic amplitude for the mid-Carboniferous reflection most likely arises from the presence of upper Carboniferous evaporite deposits partly composed of mobile salt (halite), which is significantly less dense than regular sedimentary rocks (see “Top upper Carboniferous evaporites” reflection in Figs. 3 and 5c and d). This evaporite succession was identified by Gudlaugsson et al. (1998) and is restricted to basinal areas located northwest of the MFC and north of the TFFC (Figs. 1 and 2). It is characterized by a highly variable thickness, which is due to the presence of lensoidal bodies bounded to the top and bottom by high-amplitude reflections on the basin edges and to the occurrence of thick bodies made of chaotic reflection patterns near the center of the basin (Fig. 5c). We interpret the lensoidal bodies on the basin edges as pillows of mobile salt and the chaotic bodies near the basin center as small salt diapirs based on similarities with large salt diapirs and evaporite deposits observed in the Nordkapp Basin (Gabrielsen et al., 1992; Jensen and Sørensen, 1992; Koyi et al., 1993; Nilsen et al., 1995). We consider that the presence of analog late Paleozoic evaporite deposits in the southwesternmost Nordkapp basin and in the Nordkapp Basin (Jensen and Sørensen, 1992; Koyi et al., 1993; Gudlaugsson et al., 1998) and the absence of such deposits in the Hammerfest Basin constitute strong arguments to justify a change of name for the “easternmost Hammerfest basin” (Omosanya et al., 2015) into the “southwesternmost Nordkapp basin”. However, this basin shows a large amount of normal displacement along its southern boundary fault, the NW-dipping MFC, which is opposite to the Nordkapp Basin where basin subsidence was dominantly accommodated along the SE-dipping Nysleppen Fault Complex (Fig. 1). Hence, despite their similarities, the Nordkapp Basin and the southwesternmost Nordkapp basin should be treated as two separate basins.

Non-evaporitic, upper Carboniferous and Permian sedimentary deposits are characterized by subparallel, flat-lying to shallow-dipping, homogeneous, moderate- to low-amplitude seismic reflections (see Fig. 5). Permian deposits are relatively thin on the Finnmark Platform and are sometimes difficult to distinguish from upper Carboniferous deposits (Fig. 5a, b, e, f, and g). In the southwesternmost Nordkapp basin, however, late Paleozoic sedimentary deposits are thicker and individual units are therefore easier to identify in seismic data. Thus, we interpreted a thin unit characterized by high-amplitude reflections (see “base Asselian” and “top Asselian evaporites” reflections in Figs. 3 and 5c and d) as Asselian (earliest Permian) evaporite deposits that were evidenced by exploration well 7124/3-1 on the northern flank of the southwesternmost Nordkapp basin (Figs. 2, 5c and d). Where present, this thin Asselian evaporite succession defines the base of the Permian sedimentary succession and therefore serves as an upper boundary for the Carboniferous succession (see “base Asselian” reflection in Fig. 5c and d). However, Asselian evaporites are too thin and too discontinuous to be seismically resolvable on the Finnmark Platform (Bugge et al., 1995). Occasionally, Asselian evaporites are truncated by chaotic reflections of small salt diapirs sourced from deeper upper Carboniferous evaporites in the southwesternmost Nordkapp basin (Fig. 5c).

The base Triassic reflection (see Fig. 5) defines the (near-)top of the late Paleozoic sedimentary succession and is easily interpreted through the whole Barents Sea as it corresponds to a high-amplitude reflection that represents the top of a regionally widespread carbonate unit (Bugge et al., 1995). Other important seismic reflections interpreted in the present study include the base Cretaceous; base Paleocene; the upper regional unconformity, which corresponds to a major erosional unconformity and represents the base of Quaternary sediment cover (Solheim and Kristoffersen, 1984); and the seabed reflection (Fig. 5). These reflections are penetrated by a large number of exploration wells and shallow drill cores both on the Finnmark Platform and in the southwesternmost Nordkapp basin, where they all display consistently high seismic amplitudes (Faleide et al., 1984; Bugge et al., 1995; Gudlaugsson et al., 1998; Omosanya et al., 2015).

In this section, we describe the most important structural elements of the Finnmark Platform and of the southwesternmost Nordkapp basin (see Figs. 1 and 2) based on interpreted key seismic sections (Fig. 5). We also highlight the most dominant fault trends and their interactions with major structures such as the TFFC, MFC, TKFZ, and SISZ to form offshore sedimentary basins.

We identified a several-kilometer-thick, curved (in cross section), shallow-dipping layer of moderate-amplitude reflections that we interpreted to represent a large-scale basement-seated shear zone, which we name the SISZ. The upper boundary surface of the SISZ (Fig. 7) appears to be relatively shallow in coastal areas. On the western Finnmark Platform, the SISZ dominantly dips to the NW but switches to a dominant dip to the northeast on the eastern Finnmark Platform. In the footwall of the MFC and in the southwestern part of the western Finnmark Platform, the SISZ occurs at a relatively shallow depth (< 1.5 s TWT). There it is believed to have been deeply eroded and is now overlain by a very thin sedimentary cover (see Figs. 5c–f and 6d). The SISZ shows significant lateral thickness variations that range from 2.0 to 2.5 s (TWT) near the coastline and in the footwall of the TFFC to 0.5 s (TWT) below the MFC and the TFFC (Fig. 5f). The SISZ deepens to the northwest towards the center of the western Finnmark Platform before bending upwards in the footwall of the TFFC (Fig. 5e and f). The SISZ then curves down where the listric TFFC merges with the shear zone at depth, thus delineating an elongated, NE–SW-trending ridge in the footwall of the TFFC (see “basement ridges” in Figs. 1 and 5e and f). A similar pattern is observed in the southwesternmost Nordkapp basin where the SISZ deepens to the northwest before curving up near the center of the basin and merging with the N-boundary fault of the southwesternmost Nordkapp basin, the Rolvsøya fault, hence giving this basin a characteristic “U” shape in cross-section (Fig. 5c and d). The SISZ also curves down in the footwall of the Rolsøya fault and defines a second elongated, ENE–WSW-trending ridge (see “basement highs” in Fig. 1). Importantly, the two basement ridges located in the footwall of the TFFC and of the Rolvsøya fault (“basement highs” in Fig. 1) are separated by a narrow trough that is bounded to the southwest by the WNW–ESE-trending segment of the TFFC (Fig. 7). Apart from this narrow trough, the attitude of the SISZ is uniform along NE–SW transects on the western Finnmark Platform and within the southwesternmost Nordkapp basin with a gentle dip to the northeast (Fig. 5g).

Notably, the spoon-shaped geometry of the SISZ, with asymmetric, NE–SW-trending, northeastward-broadening NE plunge (Fig. 7) appears to coincide with a basement gravity low on the western Finnmark Platform: the Gjesvær Low (Johansen et al., 1994; Gernigon et al., 2014; Fig. 1). The geometry of the SISZ also matches the trend and shape of the southwesternmost Nordkapp basin (Figs. 1 and 7). Farther south, along the coasts of western Troms and westwards below the Hammerfest Basin, the low quality of available seismic data did not allow us to trace the SISZ more precisely (Fig. 7). On the eastern Finnmark Platform, the SISZ bends from NE–SW into a more WNW–ESE trend and changes in dip from gentle to steep to the northeast (Fig. 7), and as a result the SISZ becomes too deep to interpret on seismic data in the northeastern part of the eastern Finnmark Platform (Fig. 7). The multiple changes of trend, dip direction, dip angle, and thickness of the SISZ gives the shear zone a spoon-shaped geometry (Fig. 7).

On the western Finnmark Platform, subsidiary, steep SE-dipping high-amplitude reflections occur in basement rocks and these are truncated by the mid-Carboniferous reflection and the base Devonian erosional unconformity in the footwall of the TFFC (see yellow dotted lines in Fig. 5e–g). Despite dipping southeast, these reflections resemble the dominant reflection pattern observed within the SISZ (Fig. 5e and f). Thus, we interpret them as SE-dipping, mylonitic shear zones (yellow dotted lines in Fig. 5e–g). The upper boundary of one of these SE-dipping shear zones coincides with an abrupt seismic facies change on the western Finnmark Platform, from moderately dipping, moderate-amplitude reflections in the west to gently dipping to subhorizontal low-amplitude seismic reflections in the east (Fig. 5g). This change also coincides with a ca. 1 s (TWT) deepening of the upper boundary of the SISZ towards the northeast (Fig. 5g) and with a small normal offset of a lensoidal, eastwards-thickening layer of subhorizontal reflections located above the SISZ (see dotted black lines in Fig. 5g). We interpret these changing attributes to be related to the presence of a NNE–SSW-trending, ESE-dipping brittle fault that flattens and merges into the SISZ and which may have developed along a preexisting, steep ductile shear zone (yellow dotted lines in Fig. 5g).

Similar NE–SW-trending but NW-dipping shear zones may exist in basement rocks on the eastern Finnmark Platform, for example in the form of steeply dipping, high-amplitude seismic reflections truncated by the mid-Carboniferous reflection (see yellow dotted lines in Figs. 5b and 6a). These reflections differ from gently dipping, high-amplitude reflections of lower Carboniferous coal-bearing sedimentary deposits (Fig. 6a) and rather resemble the SISZ reflection pattern, though these are located well above the presumed continuation of the SISZ (Fig. 5e and f). We therefore interpret these steep reflections as a NE–SW-trending, NW-dipping shear zone similar to the SISZ (Fig. 5b).

Faults bounding Paleozoic sedimentary strata and basins include the major TFFC and MFC and numerous faults on the Finnmark Platform. The TFFC is made of alternating ENE–WSW- and NNE–SSW-trending, NW-dipping, listric fault segments that form a zigzag pattern and that separate the Hammerfest Basin in the northwest from the western Finnmark Platform in the southeast (Figs. 1 and 5e and f; Gabrielsen et al., 1990; Indrevær et al., 2013). Seismic data below ENE–WSW- and NNE–SSW-trending fault segments of the TFFC show that these fault segments merge with and merge into shallow-dipping reflections of the SISZ at depth (Fig. 5e and f). At the northeast termination of the Hammerfest Basin, the TFFC bends 90 degrees clockwise and continues to the southeast as a WNW–ESE-trending, NE-dipping, listric fault (Figs. 1, 2, and 5g). At depth, this fault merges with the SISZ (see Fig. 5g) near a narrow trough in the top surface of the SISZ, separating two elongated NE–SW- to ENE–WSW-trending basement ridges in the footwall of the TFFC and of the Rolvsøya fault (see “basement highs” in red in Figs. 1 and 7). In map view, the WNW–ESE-trending, NE-dipping segment of the TFFC bends anticlockwise into the main fault segment of the MFC, which corresponds to a linear, NE–SW-trending, NW-dipping fault (Figs. 1, 2, and 8a and b). The interaction of these two faults in map view gives the western Finnmark Platform and the southwesternmost Nordkapp basin triangular shapes (Figs. 2 and 8a and b). The main segment of the MFC defines the southeastern boundary of the southwesternmost Nordkapp basin (Figs. 1, 2, and 5c and d) and of a ca. 25–30 km wide graben structure on the western Finnmark Platform that is believed to be partly filled with Devonian sedimentary deposits (Figs. 1, 2, and 5e and f). Northeastwards, the main segment of the MFC (Fig. 5c–f) is replaced by several minor fault segments with limited vertical throw (Fig. 5a and b) that define the southeastern boundary of the Nordkapp Basin (Figs. 1 and 5a and b). The southwesternmost Nordkapp basin is bounded to the north by an E–W- to ENE–WSW-trending, south-dipping, listric normal fault, the Rolvsøya fault, which flattens at depth and merges into gently dipping reflections of the SISZ (Fig. 5c and d). The Rolvsøya fault separates the southwesternmost Nordkapp basin from the Ottar Basin to the northwest and from the Nordkapp Basin to the northeast (Figs. 1 and 2).

Late Paleozoic grabens on the eastern Finnmark Platform display fault patterns that are analogous to those that shape the southwesternmost Nordkapp basin and the western Finnmark Platform (Figs. 1 and 2). Numerous steeply dipping, listric normal faults made of alternating, zigzag-shaped, ENE–WSW- and NNE–SSW-trending segments bound relatively narrow, few-kilometer-wide graben and half-graben structures that are filled with wedge-shaped, late Paleozoic sedimentary successions (Figs. 2 and 5a and b). In particular, one of these zigzag-shaped faults trends NE–SW to NNE–SSW, dips to the northwest, and can be traced for about 60 km from the northern coast of Magerøya onto the eastern Finnmark Platform (Figs. 1 and 2). Southwestward, this fault roughly aligns with a similarly shaped and oriented, NW-dipping onshore and nearshore fault complex synthetic to the TFFC described as the LVF (Figs. 2 and 5a and b; Zwaan and Roberts, 1978; Lippard and Roberts, 1987; Roberts and Lippard, 2005; Koehl et al., 2018). We tentatively interpret the ca. 60 km long, zigzag-shaped brittle fault on the eastern Finnmark Platform, northeast of Magerøya, as the northeastward continuation of the LVF on the eastern Finnmark Platform (Figs. 1, 5a and b, and 6a).

Below the minor northern segments of the MFC, we identified a large NE–SW-trending, SE-dipping fault that is antithetic to the MFC (Fig. 5a and b). Due to the rather low quality of seismic data at large depths, the interaction of the northern segments of the MFC with the antithetic SE-dipping fault is difficult to evaluate. Our data indicate that the northern segments of the MFC crosscut the NE–SW-trending, SE-dipping in the southwest (Fig. 5b), whereas farther northeast, along strike, the northern fault segments of the MFC seem to merge and die out into upper Carboniferous evaporite deposits (Fig. 5a).

In the following section, fault offsets and thickness variations in the sedimentary successions across brittle faults will be described as a basis to infer timing and sense of shear for brittle faults on the Finnmark Platform and in the southwesternmost Nordkapp basin. Regional stratigraphic thickness maps (Fig. 9a–c) show that late Paleozoic sedimentary strata on the eastern Finnmark Platform thicken from < 0.1 s (TWT) in the southeast to a maximum thickness of ca. 2 s (TWT) in the footwall of the MFC (see also Fig. 5a and b). This gradual thickness increase contrasts with the abrupt thickness increase in Devonian–Carboniferous sedimentary strata in the hanging wall of major normal faults, e.g., the WNW–ESE-trending segment of the TFFC and the main segment of the MFC (Fig. 9a–b), thus separating depositional versus tectonic thickness changes.

The dominant shear zone system within basement rocks on the western Finnmark Platform is the SISZ (Figs. 5c–g, 6b–c and e–f, and 7). A pronounced intra-basement unit made of subhorizontal, high-amplitude reflections occurs above the SISZ (Fig. 5g). The top reflection of the SISZ and the overlying intra-basement unit are offset by a NNE–SSW-trending, gently east-dipping fault, which is accompanied by a thickness increase in the intra-basement unit across the east-dipping fault (see black dotted line in Figs. 5g and 6e). This fault is interpreted to have a top–E normal sense of shear (see dotted black lines in Figs. 5g and 6e) and is itself truncated by the subhorizontal mid-Carboniferous reflection, which constrains its activity to the Middle to Late Devonian–early Carboniferous (Fig. 5g).

In the southwesternmost Nordkapp basin, the Devonian–lower Carboniferous sedimentary succession (Fig. 5c and d) appears to be thickest at the intersection of the TFFC and MFC (Fig. 9a), where vertical displacement along the MFC and TFFC is estimated to be ca. 1.5 s (TWT), based on an offset of the mid-Carboniferous reflection (see Fig. 5d). The overlying upper Carboniferous succession displays a similar attitude as shown by the broad thickening of similar sedimentary strata at the intersection of the TFFC and MFC (Fig. 9b). These observations suggest that the WNW–ESE-trending segment of the TFFC and the main segment of the MFC potentially formed simultaneously in Devonian times and acted as syn-sedimentary normal faults that contributed to the thickening of Devonian–lower Carboniferous and upper Carboniferous sedimentary deposits within the southwesternmost Nordkapp basin (Fig. 5c and d). In this scenario, the Rolvsøya fault likely limits the extent of thickened Devonian–lower Carboniferous and upper Carboniferous sedimentary strata to the north. If we consider the thickness of the seismic package limited upwards by the mid-Carboniferous reflection and downwards by the top reflection of the SISZ in the footwall of the Rolvsøya fault, the maximum thickness of Devonian and lower Carboniferous sedimentary rocks on the northern flank of the basin does not exceed ca. 1 s (TWT). This thickness estimate is significantly thinner than what is observed within the southwesternmost Nordkapp basin, where the Devonian–lower Carboniferous succession reaches a maximum thickness of ca. 2–2.5 s (TWT; see Fig. 5c and d). By analogy, the thickness of upper Carboniferous sedimentary strata on the northern flank of the southwesternmost Nordkapp basin decreases from ca. 1.5 s (TWT) to ca. 0.5–1 s across the Rolvsøya fault (Figs. 5c and d and 9b). Hence, the Rolvsøya fault was active and largely contributed to sediment thickening within the southwesternmost Nordkapp basin during the Middle to Late Devonian–Carboniferous.

On the western Finnmark Platform, potential Devonian sedimentary rocks are characterized by low-amplitude chaotic reflections within which we observed distinct, shallow-dipping, moderate-amplitude reflections that we interpreted as major sedimentary sequence boundaries (see white dotted lines in Figs. 5e and 6b and c). These shallow-dipping reflections diverge from each other downwards and define gently dipping, wedge-shaped layers of low-amplitude chaotic reflections that thicken downwards against arcuate, high-amplitude basement reflections that represent an erosional unconformity (see “base Devonian” reflection in Fig. 5e), and to the northwest against an ENE–WSW-trending, SE-dipping normal fault (Figs. 5e and 6b and c). We interpret these sedimentary units separated by shallow-dipping, moderate-amplitude reflections to represent growth strata deposited along an active ENE–WSW-trending, SE-dipping normal fault, which is parallel to SE-dipping basement shear zones (Figs. 5e and 6b and c). In addition, the main segment of the MFC shows a decreasing amount of vertical displacement to the southwest, accompanied by a simultaneous thickness decrease in the upper Carboniferous succession along strike (Fig. 9b), before the MFC eventually dies out on the western Finnmark Platform (Figs. 1, 2, and 5e and f). Analogously, upper Carboniferous sedimentary deposits on the western Finnmark Platform display a wedge shape that is thickest in the southeast, near the MFC, and gradually thins towards the TFFC in the northwest (Figs. 5e and f, 9b). This upper Carboniferous sedimentary wedge likely formed by syn-tectonic sedimentary growth along the main segment of the MFC.

On the eastern Finnmark Platform, the offshore portion of the LVF (see Figs. 1 and 5a and b) downthrows the mid-Carboniferous reflection by ca. 0.5 s (TWT) to the northwest (Fig. 5b) and bounds a NE–SW-trending graben structure filled with thickened lower Carboniferous and upper Carboniferous sedimentary strata (see Fig. 5a and b). In this graben structure, the lower Carboniferous and upper Carboniferous sedimentary successions thicken against the LVF (Fig. 5b), while thickness variations become negligible farther north where the LVF dies out (Figs. 1 and 5a). Consequently, similar thickness increases of lower Carboniferous and upper Carboniferous sedimentary strata elsewhere within graben and half-graben structures on the eastern Finnmark Platform suggest that syn-tectonic sediment deposition along the LVF and analog ENE–WSW- to NNE–SSW-trending faults mostly occurred in Carboniferous times. Furthermore, in the footwall of the northern segments of the MFC, we recorded anomalously thick upper Carboniferous succession (Fig. 9b) with a thickness comparable to what is observed within the southwesternmost Nordkapp basin (Fig. 9b). This succession shows a half-ellipsoid shape in map view with a NE–SW-trending major axis parallel to the MFC (Fig. 9b). We therefore argue that this thickness change on the eastern Finnmark Platform is the result of syn-tectonic sediment deposition in the hanging wall of a NE–SW-trending, SE-dipping fault antithetic to the MFC (Fig. 5a and b). We suggest that the half-ellipsoid shape of the thickened upper Carboniferous sedimentary deposits on the eastern Finnmark Platform reflects a large offset near the center of the SE-dipping fault and decreasing vertical throw towards the fault tips, a feature characterizing syn-sedimentary, rift-related normal faults (Fig. 9b).

By contrast, depositional sediment wedges may occur on the eastern Finnmark Platform as well, and they differ from fault-controlled thickness changes. One example is the ca. 600 m thick lower Carboniferous succession evidenced by shallow drilling between the Nordkinn Peninsula and Magerøya (see “star” symbol in Fig. 1; Bugge et al., 1995), which we reinterpreted as a prograding Carboniferous sedimentary system (Fig. 6d). The apparent thickening of the lower Carboniferous succession near the coast of NW Finnmark is more likely to be related to sedimentary processes during the formation of large clinoforms in a prograding sedimentary system (Fig. 6d) than to syn-tectonic deposition in the hanging wall of a NE–SW-trending, NW-dipping fault.

In the southwesternmost Nordkapp basin and on the eastern and western Finnmark Platform, the Permian sedimentary succession is thin and shows a relatively constant thickness compared to the underlying Devonian–lower Carboniferous and upper Carboniferous successions (Figs. 5a–d and 9a–c). However, the base Asselian and base Triassic reflections marking the lower and upper boundary of the Permian succession show some offsets across the main segment of the MFC, WNW–ESE-trending segment of the TFFC and Rolvsøya fault, thus accounting for minor thickness variations in the Permian succession across these faults (Figs. 5c, d, and g and 9c). We interpret these small offsets and thickness variations as the product of minor faulting activity in the Permian and mild Mesozoic reactivation of these faults, thus suggesting that the main tectonic activity along these faults was essentially restricted to the Middle to Late Devonian–late Carboniferous (Fig. 5c, d, and g). Moreover, on the western and eastern Finnmark Platform, most brittle faults die out within the upper Carboniferous succession and only a few faults crosscut the Permian succession with a limited amount of offset (Fig. 5a, b, e, and f).

Most faults observed within the late Paleozoic succession on the eastern and western Finnmark Platform and in the southwesternmost Nordkapp basin die out in the upper part of the succession before reaching the base Triassic reflection (Fig. 5). A few exceptions exist where the MFC and the WNW–ESE-trending segment of the TFFC show small offsets of Mesozoic sedimentary strata (Fig. 5c–g). The weak influence of these faults compared to offsets observed within late Paleozoic successions (Fig. 5c–g) suggests that at least some major faults were mildly reactivated in Mesozoic times but in general most brittle faults on the eastern and western Finnmark Platform and in the southwesternmost Nordkapp basin remained inactive after Carboniferous times.