Devonian-Mississippian faulting controlled by WNW-ESE-striking structural grain in Proterozoic basement rocks in Billefjorden, central Spitsbergen

but that do not extend into adjacent-overlying, rift-related, Pennsylvanian rocks of the Gipsdalen Group. Structural analysis of field data and aerial photographs suggest that WNW-ESE-striking faults in basement rocks in Billefjorden formed as (sinistral) strike-slip and normal faults during Devonian-Mississippian extension in agreement with previously inferred models of sinistral transtension. The abundance of these faults suggests that their formation was controlled by analogously trending, preexisting structural grain (planar anisotropies) at depth, and their pronounced WNW-ESE strike suggests that the strike of preexisting anisotropies were comparable to recently identified, crustal-scale, WNW-ESE-striking Timanian thrust systems in Svalbard and the northern Barents Sea.


INTRODUCTION
Inheritance exerts a significant control on the strike and geometry of structures formed during subsequent events in various tectonic settings (Fazlikhani et al., 2017;Koehl, 2020;Koehl et al., , 2019Koehl et al., , 2022aLund, 2008;Moreno-Martin et al., 2022;Molnar et al., 2017;Osagiede et al., 2020;Phillips et al., 2016;Schiffer et al., 2020;Thomas, 2005). It is therefore important to constrain the strike and architecture of preexisting structures and their influence on subsequent structures prior to investigating younger structures. In Svalbard for example, Eurekan structures extensively follow preexisting Caledonian grain (Dallmann et al., 1993).
The Svalbard Archipelago in the Norwegian Arctic ( Figure 1A) underwent a complex series of tectonic events in the Paleozoic, including E-W Caledonian contraction Harland et al., 1992;Witt-Nilsson et al., 1998) and related, latest Silurian-Devonian, late-post-orogenic collapse (Friend et al., 1997;McCann, 2000), Late Devonian Svalbardian contraction (Dallmann and Piepjohn, 2020;Piepjohn et al., 1997), and Carboniferous-Permian rifting Cutbill and Challinor, 1965;Cutbill et al., 1976). These events resulted in the development of a well defined N-S-trending structural grain and structures, both in Proterozoic basement and post-Caledonian sedimentary rocks, such as the Caledonian Atomfjella Antiform in northeastern Spitsbergen (Ny Friesland, see Figure 1B for location; Witt-Nilsson et al., 1998) and the Svalbardian and/or Carboniferous Billefjorden fault zone and related brittle faults in central Spitsbergen (Billefjorden, see Figure 1B for location; . These structures were reactivated and overprinted during subsequent events, such as the Eurekan tectonic event, when Greenland and Svalbard collided resulting in the formation of the West Spitsbergen Fold and Thrust Belt (Dallmann et al., 1993;Harland, 1969;Harland and Horsfield, 1974;Maher Jr. et al., 1986).  Despite the widespread, well developed, and longlived character of N-S-trending structural grain throughout Spitsbergen, another trend seems to be increasingly recognized in recent years and includes WNW-ESE-striking structures and fabrics. Most importantly, recent field studies identified late Neoproterozoic Timanian (650-550Ma) thrusts and shear zones in southwestern Spitsbergen (e.g. Vimsodden-Kosibapasset shear zone; Faehnrich et al., 2020;Majka et al., 2008;Mazur et al., 2009) and possibly in central Spitsbergen (Koehl and Muñoz-Barrera, 2018). In addition, structural analysis of regional seismic data showed the presence of several crustal-scale, WNW-ESEstriking, Timanian thrust systems in central Spitsbergen, Storfjorden and the northern Barents Sea (see Figure 1B for location; Koehl, 2019Koehl, , 2020Koehl et al., 2022a).
The present contribution partly builds on previous works by Christophersen (2015) and reports, for the first time, descriptions and structural analysis of a set of abundant brittle faults showing a similar WNW-ESE strike in Proterozoic basement rocks in Billefjorden (see location in Figure 1B), an area that was previously thought to be completely and exclusively dominated by N-S-trending Caledonian grain (e.g. Braathen et al., 2011;Harland et al., , 1992 apart from areas in northern Ny-Friesland (Witt-Nilsson et al., 1998) and in Odellfjellet (Koehl and Muñoz-Barrera, 2018). The study discusses the potential timing of formation of these faults based on inferred kinematics and crosscutting relationships. Finally, the implications of these structures for the tectonic history of the region (e.g. presence of and relationship to preexisting structural grain and influence on subsequent tectonic events) are briefly reviewed.
In the Late Devonian, central Spitsbergen experienced short-lived Svalbardian contraction. This led to the formation of N-S-striking reverse faults such as the Balliolbreen fault segment of the Billefjorden fault zone, which juxtaposed Proterozoic basement rocks in the East against post-Caledonian Lower Devonian sedimentary rocks in the West (Dallmann and Piepjohn, 2020;Piepjohn et al., 1997;Vogt, 1938). However, recent studies of field and seismic data suggest that the Balliolbreen fault formed as a Carboniferous normal fault, which was inverted during Eurekan contraction, and that Svalbardian contraction did not occur in central Spitsbergen (e.g. Koehl, 2021 and references therein;Koehl et al., 2022b).
In the early Cenozoic, Greenland and Svalbard collided during the Eurekan tectonic event, resulting in the  (Dallmann et al., 1993;Harland, 1969;Harland and Horsfield, 1974;Maher et al., 1986) and mild reactivation of major high-angle and moderatelydipping faults and thin-skinned tectonics in the study area in central Spitsbergen (Koehl, 2021).

METHODS
The present study involves observations of and interpretation built from glacially eroded, three-dimensionally discontinuous, local (<a few hundreds of meters wide) field outcrops. The study reports structural field measurements and descriptions of brittle faults in Proterozoic basement rocks in Ebbadalen (Figures 2A-C; 3A-B) and in Adolfbukta-Nordenskiöldbreen ( Figure  4A-E; 5A). Structural measurements of fracture surfaces are plotted in lower hemisphere, equal-area, Schmidt stereonets and characterized by fault strike and dip. Slickenside lineations are characterized by fault strike and dip (American right-hand rule), and lineation plunge. However, due to the poor quality of the few occurrences of slickensides in the field, fault kinematics were mostly studied using fault geometries in cross section and map view, and crosscutting relationships of brittle fault sets. The study also includes structural analysis of onshore escarpments and lineaments on aerial photographs of the Norwegian Polar

RESULTS AND INTERPRETATIONS
Structural field measurements in Proterozoic basement rocks

Observations in Ebbadalen
Brittle faults in Proterozoic basement rocks in Ebbadalen display two major strikes, including a dominant  WNW-ESE-striking, both SSW-and NNE-dipping set, and a subsidiary set of East-dipping, NNE-SSW-to N-Sstriking faults (see stereonet in Figure 2A). Subsidiary East-dipping faults are relatively sparse in basement rocks.
WNW-ESE-striking fracture surfaces are abundant in Proterozoic basement rocks in Ebbadalen (see stereonet in Figure 2A). These fractures display high-angle to subvertical and planar mesoscale geometries. In places, high fracture density (c. 5-10 fracture surfaces per meter) and the presence of up to one-meter-wide fault rock comprised of angular to sub-rounded clasts indicate the presence of major fault zones with highly fractured, c. 10 meters wide core zones ( Figure 2B). Sparse slickenside lineations along major fault surfaces within fault cores suggest oblique-to dip-slip movement along these major faults. However, due to the poorly-preserved character of slickensides in basement rocks, the sense of shear along the main fault zones could not be consistently deduced. For further description of brittle faults in basement rocks in Ebbadalen, the reader is referred to Christophersen (2015).

Interpretations in Ebbadalen
Faults of the subsidiary set are parallel to major Eastdipping post-Caledonian normal faults along the eastern boundary of the Billefjorden Trough (e.g. Lemströmfjellet-Løvehovden fault) and well studied N-S-trending Caledonian grain in Ny Friesland and Billefjorden (e.g. Atomfjella Antiform; Witt-Nilsson et al., 1998). Thus, it is highly probable that East-dipping brittle faults are directly related to the formation of the Billefjorden Trough in the Carboniferous and that their formation may have been controlled by Caledonian grain. These faults will not be discussed further because they are well studied and described in other studies (e.g. Braathen et al., 2011;Maher and Braathen, 2011).
Brittle-ductile faults in the Adolfbukta-Nordenskiöldbreen area strike dominantly WNW-ESE to E-W and subsidiarily ENE-WSW, and dip steeply to moderately ( Figure 4A). These faults truncate the N-S-to NNE-SSW-trending Caledonian foliation at a high angle ( Figure 4A) and are therefore unlikely related to this tectonic event. The faults are largely eroded by Nordenskiöldbreen (glacier), but, in places, fault rocks are preserved, and these occurrences include lens-shaped finely-crushed cataclasite and coarse clasts-blocks in breccia ( Figure 4A, D). Kinematic indicators along WNW-ESE-to E-W-striking faults include centimeter-to meterscale brittle, dominantly left-and subsidiary right-lateral offsets of geological markers such as lithological contacts ( Figure 4D), sigma-clasts, and sub-vertical asymmetric folds ( Figure 4E). Kinematic indicators suggest dominantly sinistral and subsidiary (conjugate?) dextral strike-slip movements along WNW-ESE-to E-W-striking faults in the Adolfbukta-Nordenskiöldbreen area ( Figure 4D-E).

Interpretations in Adolfbukta-Nordenskiöldbreen
Since the NNE-SSW-to N-S-trending folds and foliation are parallel to the N-S-trending fabrics of the Caledonian Atomfjella Antiform (Gee et al., 1992;Witt-Nilsson et al., 1998) we interpret them as Caledonian features. However, older E-W-to WNW-ESE-trending fold structures are yet to be described in the area and their origin will therefore be addressed in the discussion. Nevertheless, their refolding into sub-horizontal, NNE-SSW-to N-S-trending folds in E-W to WNW-ESE cross section ( Figure 4C) suggests that they formed during a discrete, pre-Caledonian tectonic event.

Observations in Ebbadalen and Ragnardalen
Analysis of aerial photographs in basement rocks in Ebbadalen and Ragnardalen reveals the occurrence of two sets of escarpments. Among these, steep, WNW-ESEtrending escarpments dominate, whereas NNE-SSW-to N-S-trending escarpments are subsidiary (respectively G e o l o g i c a A c t a , 2 1 . 7 , 1 -1 6 ( 2 0 2 3 Figures 2A; 5A). Aerial photographs also reveal a set of high-frequency, NNE-SSW-trending lineaments. In places, these high-frequency lineaments bend smoothly in map view and display arcuate geometries (dashed white lines in Figures 2A; 5A). Near snow patches in Ebbadalen, another set of smooth, highfrequency, WNW-ESE-trending lineations is observed locally (yellow lines in Figure 2A). These lineaments also show arcuate geometries in map view and trend slightly oblique to the dominant set of steep, WNW-ESE-striking escarpments (Figure 2A). These smooth, arcuate, WNW-ESE-trending lineations appear to merge with similarly trending, arcuate lineations forming a fan-shaped aggregate in snow patches.

Interpretations in Ebbadalen and Ragnardalen
The high-frequency, NNE-SSW-trending lineaments with arcuate geometries in Ebbadalen and Ragnardalen (dashed white lines in Figure 2A; 5A) appear to follow the dominant ductile bedrock fabrics of the Atomfjella Antiform (Gee et al., 1992;Witt-Nilsson et al., 1998). In addition, these lineaments are slightly oblique to and crosscut by steep, subsidiary, NNE-SSW to N-S-trending escarpments. They are therefore interpreted to represent ductile fabrics in basement rocks (Figure 2A-C).
The set of smooth, high-frequency, WNW-ESE-trending lineations (yellow lines in Figure 2A), which merge with similarly trending, arcuate lineations forming a fan-shaped aggregate in snow in Ebbadalen are interpreted as glacial landforms, such as medial and lateral moraines reflecting the glacier flow.
Furthermore, in Ebbadalen, the two sets of dominant WNW-ESE-and subsidiary NNE-SSW-trending escarpments trend oblique to both glacial features and ductile fabrics and are parallel to the two sets of high-angle brittle faults identified in the field (see stereonet in Figure  2A). Furthermore, several major WNW-ESE-trending escarpments coincide with analogously striking major fault zones and occurrences of fault rocks (green bodies in Figure 2A-C). Hence, we interpret WNW-ESE-and NNE-SSW-to N-S-trending escarpments as high-angle brittle faults in basement rocks. Noteworthy, occurrences of fault rocks and highly fractured fault cores seem to be localized along just a few major faults in the field, some of which align and are connected to each other by two-three major, interconnected, WNW-ESE-striking, SSW-dipping, faultrelated escarpments (Figure 2A). It is suggested that this group of SSW-dipping fault-related escarpments, brittle faults, core zones, and associated fault rocks represent subparallel segments or splays (Biddle and Christie-Blick, 1985;Peacock et al., 2000) of a major fault zone. This newly identified fault zone is hereby named the McCabefjellet fault zone (Figures 2A; 3A). Near Hultberget in the West, the McCabefjellet fault zone is c. 100 meters wide, but the fault widens eastwards to a width of 300 meters as its main segments-splays slightly diverge from each other ( Figure  2A). The McCabefjellet fault zone does not continue into adjacent uppermost Mississippian-lower Permian sedimentary strata of the Gipsdalen Group in Hultberget in the (North-) West ( Figure 2A) and Wordiekammen in the (South-) East ( Figure 3B).

Observations in Adolfbukta-Nordenskiöldbreen and Ferdinandbreen
Aerial photographs of Adolfbukta, Nordenskiöldbreen, and Ferdinandbreen (see Figure 1B for location) show series of pronounced, linear to bending escarpments (Figures 4A; 5B-C) in Proterozoic garnet micaschists of the Smutsbreen unit and in granitic to granodioritic gneisses of the Bangenhuk unit. These escarpments can be divided into two major sets trending WNW-ESE (E-W to NW-SE) and NNE-SSW that both crosscut a set of poorly defined, high-frequency, N-S-trending, undulating escarpments. The E-W-to NW-SE-trending escarpments of the dominant WNW-ESE-trending set are arranged into rhomboid-shaped (duplex-like) patterns and commonly display dominantly anticlockwise-bending (Z-shaped) and subsidiarily clockwise-bending (S-shaped) geometries in map view near or at the intersection with slightly oblique escarpments of the same set ( Figure 4A). All three sets are oblique to a subsidiary set of smooth, ENE-WSW-to E-Wtrending lineaments (Figures 4A; 5B-C).

Interpretations in Adolfbukta-Nordenskiöldbreen and Ferdinandbreen
High-frequency undulating escarpments of the (NNW-SSE-to) N-S-trending set in Adolfbukta-Nordenskiöldbreen and Ferdinandbreen, parallel the main ductile foliation in the field ( Figures 4A; 5A-B) and the main ductile basement fabrics in Ny Friesland (Gee et al., 1992;Witt-Nilsson et al., 1998). N-S-trending escarpments are therefore interpreted to correspond to ductile basement fabrics. In addition, the subsidiary set of smooth, ENE-WSW-to E-W-trending lineaments is well developed in adjacent till and moraine deposits (Allaart et al., 2018) and snow patches. It is therefore believed to represent glacial lineations (possibly supraglacial and medial moraines; The two sets of steep, dominant WNW-ESE-and subsidiary NNE-SSE-trending escarpments crosscut ductile fabric-related escarpments but are crosscut by glacial lineations (Figures 4A; 5B-C). Their steep character and crosscutting relationships with ductile fabrics and glacial lineations, and the occurrence of major WNW-ESEstriking, brittle-ductile faults in the field (Figures 4A, D) suggest that WNW-ESE-trending escarpments correspond to brittle-ductile faults. NNE-SSW-trending escarpments are not well developed in the field, but their steep geometries and the fact that they truncate N-S-trending ductile fabrics suggest that they also correspond to brittle (-ductile) faults. Furthermore, the observed rhomboidal and Z-shaped (and subsidiary S-shaped) geometries in Adolfbukta-Nordenskiöldbreen suggest a component of brittle-ductile sinistral (and subsidiary-conjugate?-dextral) strike-slip movement along major E-W-to NW-SE-striking brittle faults (see white half-arrows in Figures 4A). This is consistent with the kinematic indicators found along WNW-ESE-striking faults in the field ( Figures 4D-E). As mentioned earlier, NNE-SSW-striking faults are parallel to and most likely related to major post-Caledonian, basinbounding brittle faults in the study area and will not be discussed further.  Figures 3A-B). Thus, the faults must have formed prior to the latest Mississippian. Possible timing of formation for WNW-ESE-striking faults in basement rocks in Ebbadalen include the Timanian orogeny, Caledonian orogeny, Devonian extensional collapse, Late Devonian Svalbardian orogeny, and latest Devonian-Mississippian extension.

DISCUSSION
WNW-ESE-striking faults in Proterozoic basement rocks are unlikely to have formed due to Svalbardian tectonism because dominant Svalbardian fabrics and structures trend N-S (Dallmann and Piepjohn, 2020). Noteworthy, in Ferdinandbreen, Proterozoic basement rocks of the Bangenhuk unit crop out West (i.e. in the footwall) of the Balliolbreen fault as observed on aerial photographs (Figures 5C) and as reported in previous field mapping by the Norwegian Polar Institute (Norwegian Polar Institute, 2016). In this area, the Balliolbreen fault was previously thought to juxtapose Proterozoic basement rocks in the East against Lower Devonian sedimentary strata in the West, both of which are unconformably overlain by uppermost Devonian-Mississippian sedimentary rocks of the Billefjorden Group (Dallmann, 2015;, thus justifying the need for a Late Devonian episode of reverse movement along the fault. This is no longer the case. In addition, recent studies in central Spitsbergen show that Svalbardian tectonism did not occur in central Spitsbergen (Koehl, 2021;Koehl et al., 2022b).
The mapped structures are also unlikely to have formed during the Caledonian orogeny since they trend sub-orthogonal to and truncate the well documented, moderately-dipping, N-S-trending Caledonian grain (moderately East-and Westdipping foliation, thrusts and shear zones) of the Atomfjella Antiform in the study area (Gee et al., 1992Gee and Page, 1994;Harland et al., 1992;Witt-Nilsson et al., 1998).

The first Timanian structures and fabrics reported
in Svalbard were only discovered in the past 20-25 years (e.g. Vimsodden-Kosibapasset shear zone in southwestern Spitsbergen;Faehnrich et al., 2020;Majka et al., 2008Majka et al., , 2012Manecki et al., 1998;Mazur et al., 2009). Thus, much work remains in identifying and mapping these old structures. However, WNW-ESE-striking faults and sub-horizontal, E-W-to WNW-ESE-trending folds in Proterozoic basement rocks in Billefjorden (Figures 2A-C; 4A-E; 5A-C) trend parallel to the few potential Timanian structures reported in Svalbard (Faehnrich et al., 2020;Koehl and Muñoz-Barrera, 2018;Mazur et al., 2009) and to recently reported, deep, crustal-scale, Timanian thrust systems in central Spitsbergen, Storfjorden and the northern Barents Sea (e.g. Klitzke et al., 2019;Koehl, 2019Koehl, , 2020Koehl et al., 2022a; Figure 6A). It is possible that the E-W-to WNW-ESE-trending folds formed during late- to post-Caledonian, top-North extensional faulting like in northwestern Spitsbergen Maher et al., 2022) as considered for folds in western Norway (e.g. Wiest et al., 2020). However, the strike of both the faults and the folds is WNW-ESE in the present study, whereas they are orthogonal to each other in western Norway. In addition, the "hyperbolic" folds in Wiest et al. (2020) are symmetrical, whereas the folds in Billefjorden are SSW-verging and are therefore most likely of contractional origin. In addition, the refolding of E-W-to WNW-ESE-trending folds by N-Sto NNE-SSW-trending Caledonian fabrics in Billefjorden ( Figure 6B) suggests that E-W-to WNW-ESE-trending folds in basement rocks in the Adolfbukta-Nordenskiöldbreen area formed prior to the Caledonian orogeny. It is therefore possible that the mapped WNW-ESE-striking faults and E-W-to WNW-ESE-trending folds in basement rocks in Billefjorden initiated during the Timanian orogeny in the latest Neoproterozoic (ca. 650-550Ma). However, unlike E-W-to WNW-ESE-trending folds, WNW-ESE-striking faults do not show any sign of top-SSW thrusting ( Figure  4A, D-E), which is characteristic of the Timanian orogeny. Thus, it is more probable that the WNW-ESE-striking faults formed much after the WNW-ESE-striking folds, which is also supported by the truncation of Caledonian ductile fabrics by the WNW-ESE-striking faults ( Figure 4A).
Another possibility is that the studied faults formed as normal to strike-slip faults during Devonian-Mississippian extension ( Figure 6C). This is supported by i) kinematic indicators (drag-folding, sigma clasts, minor brittle offsets) and Z-shaped duplex-like geometries suggesting dominant sinistral strike-slip movements along WNW-ESE-striking faults in the Adolfbukta-Nordenskiöldbreen area ( Figure 4A, D-E), ii) the presence in Devonian strata of analogously striking, syn-sedimentary, high-angle normal faults showing slickensides indicating top-NNE normal movements Friend et al., 1997;McCann, 2000) and iii) minor left-lateral offsets of basement rocks of the Bockfjorden Anticline in northwestern Spitsbergen (Dallmann and Piepjohn, 2018;Gee, 1972) and in sedimentary rocks of the Billefjorden Group and Hultberget Formation in central Spitsbergen (Koehl, 2021;Koehl and Muñoz-Barrera, 2018). Normal growth faults within strata of the Billefjorden Group in Odellfjellet (Koehl and Muñoz-Barrera, 2018) and Sassenfjorden-Tempelfjorden (Koehl, 2021) die out upwards at or below the base of latest Mississippian sedimentary strata of the Hultberget Formation, which is consistent with fault geometries observed in basement rocks in Billefjorden (Figures 2A-B; 3; 6C-D). A formation as steep, normal to (sinistral) strike-slip faults during Devonian-Mississippian extension is also supported by field data and observations in Ny Friesland where analogous WNW-ESE-striking faults showing both strike-slip and normal kinematics (slickenside lineations) offset Caledonian fabrics in Proterozoic basement rocks (Witt-Nilsson et al., 1998). Furthermore, the brittle to brittle-ductile character of WNW-ESE-striking faults in Proterozoic basement rocks in Billefjorden differs from the dominantly ductile to brittle-ductile character of crustal-scale Timanian thrust systems in the Barents Sea, Storfjorden, central Spitsbergen (Klitzke et al., 2019;Koehl, 2019Koehl, , 2020Koehl et al., 2022a;Figure 6), and southwestern Spitsbergen (Majka et al., 2008(Majka et al., , 2012Manecki et al., 1998;Mazur et al., 2009). This could be due to the burial of the deep ductile portions of Timanian thrusts in central Spitsbergen, where only the upper, overprinted-reactivated, brittle portion is visible in outcrops, and to their uplift and exhumation due to Caledonian and Eurekan tectonism and subsequent erosion in western-southwestern Spitsbergen (Koehl et al., 2022a; Figure 6A -Sikora et al., 2018). It is possible that the Kampesteindalen fault represents the western continuation of major WNW-ESE-striking, fault-related escarpments mapped in basement rocks in Adolfbukta, i.e. that the latter acted as normal faults in the latest Mississippian-earliest Pennsylvanian ( Figure 6D). However, the Kampesteindalen fault is believed to have accommodated exclusively normal movement (Smyrak-Sikora et al., 2018), which contrasts with the (dominantly sinistral) strike-slip sense of shear inferred along WNW-ESE-striking faults in Proterozoic basement rocks in the Adolfbukta-Nordenskiöldbreen area ( Figure 4A). Instead, it is more likely that WNW-ESE-striking basement-seated faults in Adolfbukta-Nordenskiöldbreen formed prior to and controlled the location and strike of subsequent Carboniferous faults like D O I : 1 0 . 1 3 4 4 / G e o l o g i c a A c t a 2 0 2 3 . 2 1 . 7 Devonian-Mississippian faulting in central Spitsbergen 12 the Kampesteindalen fault ( Figure 6D). This is consistent with the interpretation of Koehl and Muñoz-Barrera (2018) in Odellfjellet and Mittag-Lefflerbreen where basementseated WNW-ESE-striking faults controlled the formation of parallel Carboniferous normal faults in the Billefjorden Group and Hultberget Formation in Odellfjellet.
The presence of Timanian grain in Billefjorden would reconcile observations and data of the present study with the model of sinistral transtension during Carboniferous rifting of McCann and Dallmann (1996). In their model, they propose that the obliquity of Carboniferous normal faults (e.g. Ebbabreen faults) to both preexisting (Timanian and Devonian-Mississippian) WNW-ESEstriking faults in Proterozoic basement rocks and major basin-bounding normal faults (e.g. Billefjorden fault zone and the Lemströmfjellet-Løvehovden fault) is directly related to sinistral movements along preexisting structures. Sinistral movement was recorded along inherited Timanian thrusts elsewhere in Spitsbergen (Koehl, 2020;Koehl et al., 2022a;Mazur et al., 2009). Most importantly, Early Devonian post-Caledonian sinistral strike-slip movement occurred at 410Ma along basement-seated mylonitic shear zones in Oscar II Land (western Spitsbergen; Figure  1A), which strike parallel to WNW-ESE-striking faults in basement rocks in Billefjorden (Ziemniak et al., , 2022. Devonian movement along WNW-ESE-striking faults in Billefjorden is further supported by field studies of Piepjohn et al. (1997) in Mimerdalen where parallel brittle faults offset (laterally?) Lower-Middle Devonian (Newman et al., 2019) rocks of the Wood Bay Formation and Fiskekløfta Member of the Tordalen Formation but are unconformably overlain by lower Permian strata of the Wordiekammen Formation. However, ongoing work and recent datesets suggest that the presence of these faults is questionable (Koehl and Stokmo, 2021).
Nonetheless, the WNW-ESE strike, steeply dipping geometry, and normal to sinistral character of the studied faults is compatible with the most likely E-W extension direction during the collapse of the Caledonides in Svalbard (i.e. orthogonal to the N-S-trending orogen; Harland et al., 1992;Witt-Nilsson et al., 1998) during the Devonian-Mississippian (e.g. McCann and Dallmann, 1996). The strike, geometry, and inferred kinematics of the faults also fit a formation due to locally orogen-parallel, N-S-oriented extension (e.g. in northwestern Spitsbergen; Braathen et al., 2018;McCann, 2000).

CONCLUSION
The present study shows that, despite unsuitable strike and geometry with respect to the new tectonic stress direction, new brittle faults (e.g. the McCabefjellet fault zone) develop parallel to prominent preexisting structural trends in the crust. However, instead of reactivating preexisting structures, the new faults developed as overprints, which show the same strike as preexisting structures but developed with different geometry. By contrast, suitably oriented structures have higher chance to be reactivated (e.g. N-S-striking Balliolbreen fault of potential Carboniferous age inverted during the Eurekan event).
The studied, brittle to brittle-ductile, WNW-ESE-striking faults in Proterozoic basement rocks in Billefjorden initiated as sinistral strike-slip to normal faults during Devonian-Mississippian (collapse-riftrelated) extension and overprinted similarly striking, pre-Caledonian (Timanian?) basement fabrics, as suggested by E-W-to WNW-ESE-trending, top-SSW fold structures in the field and crustal-scale Timanian thrust systems on seismic data in Svalbard and the northern Barents Sea. The occasional occurrence of WNW-ESE-striking normal faults within Pennsylvanian sedimentary successions (e.g. Kampesteindalen fault) and their alignment with parallel faults in adjacent Proterozoic basement rocks indicate that some of the studied Devonian-Mississippian faults were reactivated-overprinted during subsequent Pennsylvanian rifting and partly controlled the formation of new faults.