Fault Systems Marginal to the Møre-Trøndelag Fault Complex, Osen

NORWEGIAN JOURNAL OF GEOLOGY Fault systems marginal to the Møre-Trøndelag Fault Complex 59

Fault systems marginal to the Møre-Trøndelag Fault Complex, Osen – Vikna area, Central Norway

Elin Olsen, Roy H. Gabrielsen, Alvar Braathen & Tim F. Redfield

Olsen, Å.E.V., Gabrielsen, R.H., Braathen, A. & Redfield, T.F.: Fault systems marginal to the Møre-Trøndelag Fault Complex, Central Norway. Norwegian Journal of Geology, vol. 87, pp. 59-73,.Trondheim 2007. ISSN 029-196X.

The NE-SW striking Møre-Trøndelag Fault Complex (MTFC) is a key element in the structuring of Mid Norway. Studies of remote sensing data show that the fault complex has a width that exceeds 50 km. Within this zone, characteristic structures include folded, mylonitic km-wide sinistral detachment zones, truncative, steep, discrete fault strands and numerous smaller fracture-lineaments. Repeated reactivation is revealed through a range of fault rocks, spanning from mylonites to unconsolidated breccias and gouge. Brittle structures fall in two categories, (i) major fault strands, and (ii) wider, intervening damage zones hosting fracture-lineaments that, by field checking, are confirmed to be smaller faults. In the coastal region of Fosen and Vikna, major fault trends that relate to the MTFC are approximately NE-SW striking, whereas smaller faults are seen as three main sets with E-W, N-S, and NW-SE orientations. Outcrop studies show that steeply dipping to vertical NW-SE faults of the Fosen Peninsula (Osen area) are characterised by c. 1-km regular spacing. They host mylonites found as clasts in cataclasites and a late overprint of tensile fractures. These fault rocks probably relate to the latest Caledonian, extensional denudation and unroofing of the region. Studies of N-S faults reveal similar characteristics. Resolved movement on NW-SE faults divides into dextral horizontal separation of some tens of metres at the most, and an additional dip-slip component. In contrast, one studied NE-SW fault strand of the MTFC, the Drag fault of Vikna, reveals extensive deformation in a broad zone of mylonites and cataclasites. These rocks are cut by dm-scale layers of non-cohesive breccia and gouge. The latter fault products likely relate to Juras- sic or even later faulting and associated, near-shore basin formation. We conclude that major NE-SW fault strands of the MTFC have experienced multiphase reactivation in the Phanerozoic, which is in contrast to the intervening, broad damage zones. They are basically unaffected by late to post-Caledonian deformation events.

Elin Olsen & Roy H. Gabrielsen1,2; Geological Institute, University of Bergen, 5007 Bergen, Norway. 1now at Dept. of Geoscience, University of Oslo, P.O. box 1047 Blindern, 0316 Oslo, Norway. Alvar Braathen2 & Tim F. Redfield; Norges geologiske undersøkelse, N-7491 Trondheim, Norway, corresponding author, [email protected]; now at the 2Centre for Integrated Petroleum Research, Allégaten 41, University of Bergen, Norway, now at UNIS, P.O. box 156, 9191 Longyearbyen, Norway.

Introduction shore projects in West (Smethurst 2000) and Central Norway (e.g., Eide 2002). The regional implications of the fault complexes of the Mid-Norwegian continental shelf have frequently been In this contribution, we present detailed analyses of fault the focus of attention because of their potentially great zones from Central Norway, along the northern part of bearing on the hydrocarbon systems in the area (Buco- the regional Møre-Trøndelag Fault Complex (MTFC). vics & Ziegler 1985; Gabrielsen et al. 1984; Withjack et The study areas are located on the Fosen Peninsula and al. 1989; Grunnaleite & Gabrielsen 1995; Lundin & Doré the Vikna archipelago (Figs. 1 and 2). A feature of spe- 1996; Vågnes et al. 1998; Gabrielsen et al. 1999; Pascal cial interest is that the evolution of some of these faults & Gabrielsen 2001). Many of these faults can be traced can be directly linked to near-shore Jurassic basins (Bøe onshore, through the near-shore regions into what con- 1991; Sommaruga & Bøe 2002), whilst others can be stitutes the basement to the nearby sedimentary basins, related to movements along the MTFC. Our goal is to hence offering the possibility for assessing offshore- highlight faulting episodes and structural characteristics onshore structural links. In this light, onshore basement of the regional scale damage zone bounding the northern faults can yield detailed information regarding for exam- margin of the major MTFC. ple transport directions, reactivation history, and depth and age of faulting (e.g., Gabrielsen & Ramberg 1979; Grønlie & Roberts 1989; Fossen et al. 1997; Gabrielsen et al. 1999; Smethurst 2000; Osmundsen et al. 2002). This Structures of the Mid-Norwegian shelf is reflected by research efforts over the last decade, following an increased requirement for more detailed data- A N-S trending Late Palaeoproterozoic(?) structural bases, which motivated cooperative onshore-offshore grain, marked by fracture systems and dyke swarms, projects, bringing industry, academia and Geological is ubiquitous in the western part of the Fennoscandian Surveys together. Examples include major onshore-off- Shield (e.g., Gabrielsen et al. 2002). These systems have 60 E. Olsen et al. NORWEGIAN JOURNAL OF GEOLOGY

Fig. 1. Regional map, emphasising major shear zones that were active during extensional denudation of the Caledonides. They bound basement windows within the present distribution of Cale- donian nappes. Associa- ted Early-Mid Devonian basins are located. Most of the shear zones show indications for later reac- tivations.

been regarded as rift-related precursors to the opening of 1986), but also in connection with the latest Scandian the Iapetus Ocean (Andréasson 1994). Somewhat more extension and exhumation event, which activated both locally, the same trend is important in the shelf areas. E-W (e.g. Chauvet & Séranne 1994) and NW-SE struc- In addition, NNW-SSE and NW-SE striking structures, tural grains (e.g. Braathen et al. 2000; Osmundsen et al. which are prominent throughout western Scandinavia, 2002; Olesen et al. 2002; Skilbrei et al. 2002). including the Gudbrandsdal-Värmland-Västervik tectonic zone (Ramberg et al. 1977; Strömberg 1978; Berth- Bearing in mind the observed dominant offshore struc- elsen & Marker 1986; Gabrielsen et al. 2002), also seem to tural trends, it would be reasonable to suggest that the have counterparts offshore. The offshore NW-SE trend basement structural grain, which includes Palaeoprotero- includes the regionally important but diffuse Bivrost and zoic, Late Neoproterozoic and Early Palaeozoic (Caledo- Surt Lineaments (Blystad et al. 1995; Doré et al. 1997; nian) elements, influenced the Late Palaeozoic to Ceno- Olesen et al. 2002) and the well documented Jan Mayen zoic structuring offshore Mid Norway. Of these trends, Fracture Zone (e.g. Talwani & Eldholm 1977) although the N-S to NNW-SSE and the NE-SW are dominant, no direct link between the onshore and offshore system whereas the subordinate NW-SE trend is still of some has been observed when geophysical signature is con- importance, although its influence is poorly constrained. cerned (Olesen et al 2007). The present paper particularly concentrates on the characteristics of some of the NW-SE trending structures. In It is well known that the main Caledonian, regional addition, one major NE-SW and one N-S structure are structural grain trends NE-SW in Mid Norway. How- presented for comparison. We discuss their potential ever, recent studies have revealed a much more complex implications for the offshore structuring, and their rela- situation, partly arising from polyphase folding (Roberts tionship to the regionally very important MTFC. NORWEGIAN JOURNAL OF GEOLOGY Fault systems marginal to the Møre-Trøndelag Fault Complex 61

Fig. 2. Bedrock map of Central Norway, showing fault strands of the Møre- Trøndelag Fault Complex (MTFC), the Høybakken and Kollstraumen detachments, and Devonian and Jurassic basins. The study area is outlined.

Latest Caledonian to post-Caledonian During extension, rocks in the hangingwall of the detach- template in the vicinity of the Møre ments were affected mainly by low-grade metamorphism Trøndelag Fault Complex and semi-ductile to brittle deformation throughout, and characterised by faulting and associated basin formation. The MTFC is one of several large fault zones that transgress Footwall rocks recorded unroofing from high-grade and from the mainland on to the shelf (Fig. 1). Each of the fault ductile, to low-grade and more brittle conditions (Ander- zones has a long and complex evolution, as seen by the pres- sen & Jamtveit 1990; Osmundsen & Andersen 2001; Braa- ence of mylonites and superimposed cataclasites, breccias then et al. 2004; Kendrick et al. 2004). Early-Mid Devonian and gouge (Grønlie et al. 1991; Watts 2001). These fault of supradetachment basins in general had their provenance rocks probably reflect a gradual unroofing of the region in hangingwall, Scandian nappe rocks (Steel et al. 1985; through a series of tectonic events, spanning from the final Cuthbert 1991; Osmundsen et al. 1998). However, recent stages of the Scandian orogeny until present (Grønlie & results suggest that the uppermost successions in some of Roberts 1989; Grønlie et al. 1994; Gabrielsen et al. 2002; the basins also received an input of sediments from the Braathen et al. 2004; Redfield et al. 2005). The regional shear footwall (Eide et al. 2003; Fonneland & Pedersen 2003). zones possibly initiated as major extensional structures that Hence, progressively deeper parts of the footwall surfaced developed during tectonic denudation of the orogen (e.g., during the orogenic denudation, maybe even revealing Andersen 1998). This is exemplified by the steep MTFC that Proterozoic basement gneisses. In view of the structural interacted with low-angle extensional detachment zones, setting, this unroofing suggests that brittle faulting also both towards the SW (Nordfjord-Sogn Detachment Zone; prevailed in these units. A regional burial and subsidence Osmundsen & Andersen 2001) as well as northwards (Høy- then occurred, as recorded by low-grade metamorphism in bakken and Kollstraumen detachment zones; Sérrane 1992; several of the Devonian basins (Roberts 1983; Sturt & Braa- Krabbendam & Dewey 1998; Braathen et al. 2000, 2002; then 2001). This metamorphic episode is considered to be of Nordgulen et al. 2002; Osmundsen et al. 2003, in press) and Late Devonian to Early Carboniferous age (Steel et al. 1985; at depth (Hurich 1996). Some of the major shear zones were Bøe & Bjerkli 1989; Torsvik et al. 1986; Eide et al. 1997). subject to vertical separation that exceeded 50 km (Dewey et al. 1993; Andersen et al. 1994). The MTFC is of particular significance for the correlation 62 E. Olsen et al. NORWEGIAN JOURNAL OF GEOLOGY

Fig. 3. Lineament map of the Møre- Trøndelag region. Major lineaments are shown as red lines, whereas smaller lineaments are displayed in blue .

of the structural development of Central Norway because (Bøe 1991; Sommaruga & Bøe 2002). The three first basins it clearly affects the distribution of onshore rock units are bounded by an approximately NE-SW trending fault (Lundqvist et al. 1996, Braathen et al. 2002; Osmundsen associated with either the Verran or the Hitra-Snåsa fault et al. 2003) and also the position and geometry of offshore strands of the MTFC (Fig. 2), whereas the basin-bound- basins (Blystad et al. 1995; Gabrielsen et al. 1999; Somma- ing fault in Frohavet can be traced northeastwards into the ruga & Bøe 2002). With its surrounding, regional damage study area of Vikna (Drag fault in Fig. 2). zone, the MTFC is characterized by poly-phase deformation including sinistral ductile shear in Early to Mid The central strands of the MTFC can be traced from off- Devonian time in fairly wide, steeply dipping mylonitic shore the Møre region along the SE margin of the Central zones (Grønlie & Roberts 1989; Grønlie et al. 1991, 1994; Norway basement window and across the Grong-Olden Redfield et al. 2004, 2005). Subsequent retrogression of Culmination towards the Børgefjell basement window mylonites and formation of cataclastites and pseudotachy- (Roberts 1986, 1998). On the Fosen Peninsula, the fault lites (Sherlock et al. 2004) mark the transition into a more zone consists of two marked strands, the Hitra-Snåsa and brittle deformation regime, with narrow fault strands Verran faults, with a surrounding network of accommo- developed along foldlimbs of the much wider and now dation fault zones, which are all well displayed on satel- tightly folded shear zones. The brittle fault strands have lite scenes (e.g. Grønlie et al. 1991). The ENE-WSW to a long reactivation history, including brittle, oblique-slip, NE-SW grain of the MTFC basically affects a crustal sec- dextral strike-slip and normal fault episodes in the Late tion from Trondheim in the south to Vikna and Leka in Devonian, Permo-Triassic, post Mid-Jurassic and Ceno- the north (Rindstad & Grønlie 1987; Titus et al. 2002; zoic times (Grønlie et al. 1991; Gabrielsen et al. 1999; Redfield et al. 2004, 2005), but is best expressed along the Redfield et al. 2005). The temporal framework of faulting segment made up of the major fault strands (Gabrielsen along the MTFC can be further constrained through the et al. 2002), which also marks the locations of the Jurassic existence of fault-controlled Jurassic basins. They appear basins. The main orientations of lineaments within and in NE-SW oriented near-shore basins (Fig. 2) in Beistad- south of the MTFC are ENE-WSW, NNE-SSW and E-W fjorden (inner Trondheimsfjorden), in Edøyfjorden and (Rindstad & Grønlie 1987). Northeastwards, the trends around Griptarane near Kristiansund, and in Frohavet are ENE-WSW, NW-SE and there are subordinate E-W NORWEGIAN JOURNAL OF GEOLOGY Fault systems marginal to the Møre-Trøndelag Fault Complex 63

Fig. 4. Variation/frequency (%) diagram for the greater study area (framed in Fig. 2)

and N-S population (Rindstad & Grønlie 1987; Olsen 2002). The N-S trend is also expressed as faults in the near- shore Jurassic basins, apparently interacting with NE-SW master structures, whereas NW-SE faults form transfer structures within the basin realm (e.g. Sommeruga & Bøe 2003). The NW-SE trend parallels major fracture zones of the North Atlantic, e.g. the Jan Mayen Fracture Zone. However, there seems to be no direct link between these mega-structures and the near-shore, smaller fault zones.

The study area is located to the western part of the Cen- tral Norway basement window (Fig. 2). This tectonic window in the Caledonide allochthon consists mainly of high- and medium-grade orthogneisses (Möller 1998) with infolded tongues of supracrustal rocks of nappe origin. It was unroofed during large-scale movements on opposed, orogen-parallel, extensional shear zones (Braa- then et al. 2000, 2002), namely the Høybakken (Sérrane 1992; Osmundsen et al. 2006) and Kollstraumen extensional detachments (Nordgulen et al. 2002). On the southwestern side of the window, one or more Devonian supradetachment basins are juxtaposed with rocks of the Central Norway basement window. They were sourced from their surrounding nappe rocks, but started to receive additional sedimentary input from the footwall mylonites and gneisses of the detachments at a late stage in their depositional history (Eide et al. 2003, 2005).

Tectonic lineaments of the Fosen Peninsula Landsat and aerial lineament mapping conducted in connection with the present study (Fig. 3) have revealed a pattern that is in general accordance with that obtained from regional lineament studies (Gabrielsen & Ramberg 1979; Gabrielsen et al. 2002) and also from more local fracture studies in the area (Aanstad et al. 1981; Rindstad & Grønlie 1987, Grønlie & Roberts 1989; Redfield et al. 2004, 2005). A frequency orienta- Fig 5. A) Strike of lineaments versus distribution in percent in the tion analysis of the present lineament data shows that NE-SW Osen area (Fig. 6). Gneiss foliation omitted. B) Length distribution of lineaments in the study area, plotted as length versus number of to ENE-WSW (peak at N75E) and NW-SE (peak at N140E) lineaments for he entire dataset. C) As for B, but for the NW-SE trends are dominant. N-S and E-W to WNW-ESE trends are population. D) As for B, but for the N-S population. The N-S linea- significant, but subordinate (Figs. 4 and 5). ments are less abundant, but longer than the NW-SE lineaments 64 E. Olsen et al. NORWEGIAN JOURNAL OF GEOLOGY

Fig. 6. Sha- ded relief map of Osen-area, Fosen Peninsula. Note the distinct NW-SE -oriented topographic lineaments, as well as NE-SW lineaments that are parallel to the regional foliation in the bedrock.

The ENE-WSW to NE-SW trend is, in many areas, SE trending lineaments, which are characterised by a high consistent with the pronounced Caledonian foliation, frequency and relatively short lineament length. This which also corresponds with the regional trend of the population is well displayed in aerial photography (scale MTFC. This makes a detailed interpretation and mean- 1:37,500; Fig. 6), where they appear as slightly curved ingful statistical evaluation of the strain intensity diffi- topographic features of high contrast, several of which cult, although it is possible to distinguish between folia- define drainage systems and fjords. Several of the linea- tion and fracture-lineaments as seen in satellite imagery ments are characterised by an anastomosing or splay- and aerial photography in many places, especially where type geometry, some also with segments arranged in an the foliation is folded and deviates from the grain of en echelon fashion. Others are clean-cut and straight. The the faults. Nevertheless, mapping of the ENE-WSW to lineament length varies between 0,5 and 6 km (Fig. 5c). NE-SW trend is indeed important, because it provides The NW-SE-lineaments are, in some places, seen to off- markers that can be used to determine absolute dis- set the main Caledonian structural grain. placement across faults that are oriented transverse to this trend (see below). The N-S lineament population is not especially well dis- Faults in the vicinity of Osen played in the Landsat lineament data but is, clearly dis- tinguishable in aerial photography. Here, the N-S linea- Our approach to field studies of fracture zones has been ments are clear-cut features of high contrast, and stand that of Gabrielsen et al. (2002): Fracture-lineaments out as regular, straight features in elevated areas (Fig. 6). identified in remote sensing data are, by field control, They are also seen to define a part of the drainage system generally classified as either joint zones with no displace- and some of the fjords. Albeit less frequent, these partic- ment, which are rare, or as fault zones with evidence of ular lineaments tend to be longer than those of the NW- displacement. In the following, we describe the detailed SE population (Fig. 5). architecture of the most significant fault sets of the Osen district and the Vikna islands. The focus has been on the The E-W lineament population (maximum azimuth of c. NW-SE fracture-lineaments, whilst the N-S and NE-SW N110˚E) are rare in the study area. In a regional perspec- to ENE-WSW systems have been studied mainly for pur- tive, Gabrielsen et al. (2002) noted that E-W lineaments poses of comparison. No E-W faults were encountered in (N90˚E – N110˚E) are relatively common in the northern the field. In addition to the field investigation, a number Trøndelag region. However, their significance is not well of thin-sections have been studied in order to provide established, neither on a regional nor a local scale. detailed characteristics of the fault rocks, as well as to confirm the relative age constraints introduced by super- Of particular interest for the present study are the NW- imposed faulting. NORWEGIAN JOURNAL OF GEOLOGY Fault systems marginal to the Møre-Trøndelag Fault Complex 65

Fig. 7. Foliation map for the Osen area and position of distinct NW-SE lineaments. A) Poles to foliation of the bedrock gneisses plotted in lower hemisphere, equal area plot. B) Rose diagram of the strike of foliation for the entire area. C) Poles to foliation in the western part of the study area, close to lineament A, D) poles to foliation around lineament D, E) poles to foliation around lineament E, and F) poles to foliation around lineament C.

NE-SW to ENE-WSW trending faults. NE-SW faults are et al. 1991; Watts 2002; Redfield et al. 2005). Its regional not easily distinguished from foliation in remote sensing importance is also illustrated by a clear link to near-shore data (Fig. 6). However, faults of this set are in some places Jurassic basins (Sommaruga & Bøe 2002) located some well exposed, as for example at Drag. This steep fault zone kilometres to the southwest (Fig. 2). reveals a core of more than 5 metres width, surrounded by a c. 40 m-wide damage zone. A green, epidote-bear- N-S trending faults. N-S lineaments are faults that can be ing cataclasite makes up the bulk of the core, whereas the followed along strike for several kilometres. They display damage zone reveals layers and pockets of protocataclas- a combination of simple-fracture or anastamosing frac- ite in intensely fractured quartz-feldspathic gneiss. There ture-network geometry. Widths of the topographic fea- are also dm-scale layers of poorly consolidated breccia, tures are between 10 and 50 metres, where damage zones cut by continuous zones of non-cohesive breccia and envelop a fault core that varies in width between some gouge. This complexity is one that is commonly encoun- centimetres and a few metres. All mapped N-S-faults are tered along several of the major NE-SW faults, which dis- basically vertical, and are, in general, characterised by a play a whole range of fault rocks ranging from mylonites zonation similar to that described by Braathen & Gabri- to cataclasites and breccias. The steep NE-SW fault zone elsen (1998, 2000); for example, fracture frequencies of at Drag is interpreted as a major structure that has seen the Osen fault vary from 0.5 fractures/m in the distal polyphase activation in accordance with the evolution zones to more the 100 fractures/m in zones near its core. recorded along major fault strands of the MTFC (Grønlie This core contains cataclasite and protocataclasite, with 66 E. Olsen et al. NORWEGIAN JOURNAL OF GEOLOGY

Fig. 8. Typical topographic expression of lineaments in the study area. Fig. 9. A) Fracture profile from fault C, showing the fracture frequ- Notation refers to Figure 7. A) Lineament A seen from the NW. Note ency versus distance to fault core. B) Frequency versus dip of recor- how it splits into two branches: A1 and A2. B) Lineament C as seen from ded fractures around fault A. C) Frequency versus dip of recorded the SSW. C) Lineament D has a sharp topographic expression, which fractures around fault B. D) Frequency versus dip of recorded fractu- basically express the width of the damage zones. View is from the SE. res around fault C. clasts of ultramylonite and mylonite, the latter showing a tively diverse and complex geometrical architecture, includ- semi-ductile style of deformation. In thin-sections, four ing different types of splays and intervening lensoid bodies, different stages of grain-size reducing processes are iden- as well as interference patterns caused by neighbouring linea- tified (Olsen 2002), and the deformation products cover ments. Also, the lineaments vary considerably in topographic the entire range from plastic via semi-ductile to brittle expression; from modest, eroded and overgrown depressions deformation mechanisms. (lineament A; Fig. 8a) via well-defined zones of high fracture frequency without great relief (lineament C; Fig. 8b), to steep NW-SE trending faults. Five well-exposed faults were selected canyons with vertical margins several tens of metres in heigth for detailed study (lineaments A – D; Figs. 7 and 8). Due to (lineament D; Fig. 8c). The widths of topographic lineaments the steep terrain, key localities were selected based on aerial vary somewhat along strike, but are commonly in the range photographs, geological criteria and accessibility. The distance of 5 to 30 metres. between each lineament is strikingly regular and is in the order of 1 km. In more detail, the NW-SE faults display a rela- Despite their variable appearance, all the investigated NORWEGIAN JOURNAL OF GEOLOGY Fault systems marginal to the Møre-Trøndelag Fault Complex 67

Fig. 10. A) Green, epidote-rich cataclasite with red feldspar clasts of fault C. B) Mylonites in quartz-feldspathic gneiss. Note recrystallization of quartz to form polygonal, euhedral stingers of smaller grains (arrows), which is consistent with dynamic recrystallisation. C) Protocataclasite to cataclasite where the matrix is characterized by secondary epidote, with preserved clasts of the host rock. Note two high-strain zones (arrows) that cut through the other fault rock. D) Ultracataclasite laminae (arrow) in cataclasite.

NW-SE faults of the Osen area have a similar general of the core. In places, parts of the core can be seen to split architecture. They contain a core of fault rocks, varying into lenses with their long axes oriented parallel to the in width from less than 10 cm to 1 m, which is enveloped walls of the fault core. Such lensoid features are com- by damage zones. Fracture frequencies are 50 –100 frac- monly intensely fractured. Fracture surfaces of the fault tures/m in the core and diminishing outward to 2-5 frac- core are frequently covered with mineral coatings, mainly tures/m in the distal parts of the damage zones (Fig. 9a). quartz, chlorite, epidote and calcite. Quartz and epidote The majority of fractures are steep, dipping between 90˚ are also seen to fill irregular veins. In other veins, epidote and 60˚ (Figs. 9b-9d). Fractures of the core are generally is found together with prehnite. Very fine-grained quartz short (< 1 m) compared to the damage zones. Analysing veins, up to 20 cm wide, occur in the cores of some faults the damage zones in more detail, it is clear that two main (Fault B). They tend to transect networks of quartz and fracture systems occur. One is subparallel to the first- epidote veins as well as layers of ultracataclasite. Based order structure, and has moderate frequencies (<5-10 on the observed cross-cutting relationships, a relative fractures/m) of long fractures (> 10 m). This set is bound chronology can be established, from oldest to youngest; by, or in places replaced by, a population of two fracture (i) mylonites, (ii) quartz-epidote veins, (iii) cataclasites, sets basically oriented symmetrically around the core of and subordinate (iv) quartz- and calcite-filled veins. the fracture-lineament, with the trend of the lineament within the acute bisector. This population reveals fre- Microscopically (Figs. 10b,c,d), the fault rocks encoun- quencies in the range of 2-5 fractures/m. tered along the NW-SE faults in the Osen area reveal textures and cross-cutting relationships in accord with Despite the narrow fault core, there is a variety of fault the mesoscopic chronology. The mineralogy also has a rocks present. Commonly, fragments of mylonites are bearing on metamorphic conditions during faulting. The encountered among the clasts in the main rock type, a mylonites, mostly present as fragments in cataclasites, cataclasite to protocataclasite (Fig. 10a). Ultracataclasite show evidence of dynamic recrystallisation of especially is commonly found in the most intensely deformed parts quartz, which suggests relatively high-temperature con- 68 E. Olsen et al. NORWEGIAN JOURNAL OF GEOLOGY ditions during deformation. This is further supported by co-existing mineral parageneses (quartz-chlorite-epidote- prehnite) consistent with metamorphic conditions of the lower greenschist facies. In some thin-sections, relicts of mm-wide fractures filled with a similar, subhedral miner- als can be seen. They cross-cut the mylonitic fabric.

The general appearance of cataclasites is that of mm- to cm-thick bands of ultracataclasite that are hosted by layers and laminae of cataclasite and protocataclasite. Deformation is truly brittle, as demonstrated by extensive granulation through intra- and transgranular fracturing. The ultracataclasite reveals a weak granulation fabric defined by tails of abrasion products from larger clasts, and rows of disintegrated clasts. Clast shapes vary from rounded (highly abraded) to irregular and angular (poorly abraded).

Two generations of tensile fractures are present. These fractures cross-cut all cataclasites and they are filled with fibrous quartz and euhedral calcite.

Displacement along the NW-SE faults In the Proterozoic basement of Mid Norway, a lack of good markers generally hampers detection of the sense Fig. 11. A) Fault C in gneiss with vertical granitic dike suggesting of displacement along smaller faults. Commonly, slip- displacement as indicated by arrows. B) Displacement pegmatite analyses are based on slickenside lineations and related dike (arrows) by approximately 4 m, as indicated by red line in a structures (fibrous mineral growth, etc.). The slickenside branch of fault C. method can be used to resolve the final movement, but may be problematic in cases where multi-stage deformation and reactivation has occurred. lineations are scarce at most localities in the Osen area. Due to the modest displacement that characterises the Where present, recorded slickenside lineations are con- NW-SE faults, it has been possible to identify marker hori- sistent with a dominant strike-slip movement. Fig. 12a zons in the gneisses that could be utilised in the displace- displays lineations as recorded from one of the traverse ment analysis. Fault A transects migmatitic, banded gneiss (NW-SE) faults in the Osen area, whereas Fig. 12b shows that contains steeply dipping tabular pegmatite dykes. the situation for a series of small faults associated with Several single dykes in a characteristic group of dykes the Drag fault. The latter is one of the major ENE-WSW- were correlated across one of the splay faults of the main striking strands that splays out from the greater Møre- fault. The determined horizontal component is around Trøndelag Fault Complex. 4 metres in a dextral sense. With similar means, the dextral separation of the master fault strand was established to be nearly 20 metres, whereas total vertical separation is around 10 metres (Fig. 11). Fault B offers similar condi- Discussion tions for observation, revealing a dextral horizontal offset of 20 metres and a vertical (minimum) separation of 8 Fault rocks and regional unroofing metres. For Fault D, correlation of the two steeply-dipping The lack of specific age determinations for fault rocks contacts between a granite and a diorite, suggests a dextral of the Osen area precludes any firm correlations with separation of 20 metres. Fault E has a minimum dextral known regional events. However, as this part of the Fosen component of 8 meters, as determined from correlating a Peninsula lies within the Central Norway basement win- distinct, c. 5 m-wide, vertical granitic dyke. No informa- dow, one can draw some inferences regarding faulting tion regarding the vertical component of movement could conditions and chronology from the established unroof- be determined in the last two cases. ing history of the window. The window was unroofed and possibly partly exhumed during the latest-Scandian, The displacement analyses were in some cases supported late-orogenic extensional denudation of the Caledonides by slickenside data. However, fresh fracture surfaces with (Braathen et al. 2000; Bingen & Nordgulen 2002; Nord- NORWEGIAN JOURNAL OF GEOLOGY Fault systems marginal to the Møre-Trøndelag Fault Complex 69

sional denudation of the Caledonides, which roughly correlates with the Late Devonian to Early Carboniferous (Eide et al. 2003, 2005; Kendrick et al. 2004; Osmund- sen et al. 2006). Considering the deformation style and the abundance of epidote in the cataclasites, which is a regionally widespread fault rock (Grønlie et al. 1991; Watts et al. 2004, Osmundsen et al. 2006), it is reasonable to suggest such a date for this development. Still, some pseudotachylites of the MTFC, that are associated with epidote-bearing cataclisites, yield Permian ages (Sherlock et al. 2004), suggesting that the fault activity of this stage continued well into the Permian. Later fracture events could be consistent with minor reactivation in the Meso- Fig. 12. A) Fault planes with recorded slip-lines (dots) associated zoic. In this context, it is also of interest to note that the with NW-SE-striking fault C at Osen. B) Slip-linear plot of small unconsolidated breccias and fault gouge that characterise faults with recorded slip-lines associated ENE-WSW-trending Drag the NE-SW trending Drag fault have not been encoun- fault. tered in the NW-SE and N-S trending faults of the Osen area. These fault rocks are most likely to be of Jurassic age, but they may also have seen reactivation in the Cenozoic (cf. Grønlie & Torsvik 1989). This suggests a shorter and gulen et al. 2002). During exhumation, the area formed strictly older evolution in the Osen area of the basement the footwall to two major extensional detachments window compared to areas adjacent to major strands of (Braathen et al. 2000, 2002; Nordgulen et al. 2002), and the MTFC. is likely to have been rising in the crust due to unroofing during progressive, ductile, normal fault movements (Kendrick et al. 2004; Osmundsen et al. 2006). At a later stage, into Early Carboniferous time, uplift and related Implications of the NW-SE faults at Osen cooling caused the deformation style of the window to change from dominantly ductile to brittle (Braathen et According to Gabrielsen et al. (2002), the NW-SE faults al. 2000, 2002). Extension continued through the Meso- of the Central Norway basement window differ signifi- zoic as evidenced from the development of near-shore cantly from the regional NW-SE lineament population Jurassic basins. This suggests that the area was in a sur- that occurs elsewhere in the Caledonides and within the face position, or only slightly buried, at this time. Apatite Fennoscandian Shield of southern Norway and Sweden fission track (AFT) data (Redfield et al. 2004, 2005) con- to the east and southeast (Fig. 13A). The Osen NW- firm that the area remained in this position throughout SE faults are shorter and narrower and they terminate the Mesozoic in the vicinity of the MTFC. abruptly against the most easterly strands of the MTFC. Also, the trend of the NW-SE faults of the Osen area is From the established temporal framework, most of the slightly more northerly than that of the sub-parallel observed mylonites formed under low- to medium-grade regional lineaments of the mid Norwegian shelf (Fig. metamorphic conditions, and hence record ductile defor- 13B). Altogether, this indicates that the NW-SE faults of mation that occurred before the final unroofing of the Mid Norway strictly relate to the MTFC, and therefore basement window. These fault rocks are considered to formed as accommodation structures in its regional relate to the Silurian to Early Devonian, Scandian tectono- damage zone. metamorphic event in the area (Møller 1988; Schouenborg et al. 1991; Nordgulen et al. 2002). Alternatively, they may The fault sets encountered in the Osen area are situated reflect shear zones cutting into progressively lower levels as along the northwestern margin of the MTFC, in a posi- the Devonian unroofing developed, following the recently tion that is below the major Devonian detachments, i.e. established unroofing history, spanning into the Late in the northwestern part of the Central Norway base- Devonian and Carboniferous, as discussed by Kendrick et ment window. Recent understanding of the latest-Scan- al. (2004) and Osmundsen et al. (2006). dian extension in the area emphasizes the early, sinistral, transcurrent character of the MTFC, or a branch hereof, The subsequent, brittle activation of the NW-SE faults, which is inferred to have functioned as a boundary fault accompanied by cataclasites, would then be most readily between extensional detachment ramps in Early-Middle correlated with the latest- to post-Scandian structuring Devonian times (e.g. Braathen et al. 2002; Osmundsen of the MTFC. The fault complex offers a host of possi- et al. 2003). Although having a transverse orientation to bilities for tectonic reactivation, spanning from the Late the MTFC, it is tempting to set the NW-SE faults of Osen Palaeozoic to Cenozoic (Grønlie & Roberts 1989; Gabri- into a context of major and successively deeper and more elsen et al. 1999). However, the study area is likely to have localised faulting along NE-SW strands of the MTFC. entered the brittle regime during a late phase of exten- However, the vertical dip of the NW-SE faults of the Osen 70 E. Olsen et al. NORWEGIAN JOURNAL OF GEOLOGY

Fig. 13. Possible explanations for the development of the NW-SE-striking faults of the Fosen peninsula. A) The faults are genetically affiliated with offshore NW-SE regional lineaments. B) The faults are affiliated with the regional NW-SE set of faults that are abundant in the basement of southern and central Sweden and eastern Norway. C) The faults were generated by the latest Caledonian to post-Caledonian NE-SW-oriented collapse and extension. D) The faults were initiated by differential sinistral displacement along master strands of the Møre-Trøndelag Fault Complex. The latter model is considered the most likely (see text for explanation).

area makes them less likely candidates as primary exten- this deformation post-dated the formation of the sional structures affiliated with the latest Caledonian to gneissic fabric of the host rocks. post-Caledonian collapse (Fig. 13C). Although having a slightly more northerly trend, the From the reasoning presented above, a model for the Drag fault shows a similar evolution as other, well-stud- NW-SE faults of the Fosen Peninsula has to take into ied strands of the MTFC (e.g. Grønlie et al 1991; Redfield consideration the following: et al. 2005). This supports an interpretation of this struc- 1) The faults in the Osen area are characterised by a c. 1 ture as a marginal strand of the MTFC, thus having the km regular spacing. In established cases, they indicate status as one of its many fault branches. The outermost modest dextral horizontal separation of some tens of elements probably occur as far northwest as the Vikna metres at the most, with an additional dip-slip com- islands and Leka (Rindstad & Grønlie 1987; Titus et al. ponent. If present offshore, they are of subseismic 2004). In this light, the NW-SE trending faults at Osen scale. could have been activated as Riedel prime (R’) faults dur- 2) The fault zones reveal mylonites, cataclasites and a ing overall, differntial sinistral shearing at a latest-Scan- late overprint of tensile fractures, consistent with a dian stage (approximately Late Devonian), with a status fault rock sequence of progressive exhumation. All as minor accommodation faults between major strands NORWEGIAN JOURNAL OF GEOLOGY Fault systems marginal to the Møre-Trøndelag Fault Complex 71 of the MTFC (Fig. 13D). Synthetic, major faults include Acknowledgements: The project has been supported by Statoil ASA and the Hitra-Snåsa and Verran faults (e.g., Grønlie & Rob- Statoil-Vista through the KISS project, and by the Geological Survey of Norway. The authors are grateful to David Roberts and Per Terje erts 1989; Grønlie et al. 1991, 1994), the Bæverdalen lin- Osmundsen (both at the Geological Survey of Norway) for numerous eament to the southeast (Redfield et al. 2005), and a series good suggestions to improve the manuscript. Roberts is also particu- of fault branches around the Vikna islands, all of which larly thanked for correcting the English text. are likely to have had sinistral displacements (Titus et al. 2004). This pattern also fits a regional strain reconstruction for Mid Norway for the Late Devonian (Gabrielsen et al. 1999). References

The NW-SE faults of the Fosen Peninsula around Osen Aanstad, K.M., Gabrielsen, R.H., Hagevang, T., Ramberg, I.B. & Tor- vanger, O. 1981: Correlation of offshore and onshore structural may have seen only subtle reactivation during post- features between 62˚N and 68˚N, Norway. Proceedings, Norwegian Devonian tectonic events and related uplift, since they Symposium on Exploration, Bergen 1981, Norwegian Petroleum lack the unconsolidated breccia and fault gouge, that are Society, NSE/11, 1-24. otherwise seen along strands of the MTFC. Reactivation Andersen, T.B. 1998: Extensional tectonics in the Caledonides of is only reflected in late, minor fracturing. southern Norway, an overview. Tectonophysics 285, 333-351. Andersen, T.B. & Jamtveit, B. 1990: Uplift of deep crust during orogenic extensional collapse: A model based on field studies in the Sogn- Sunnfjord Region of Western Norway. Tectonics 9, 1097-1111. Andersen, T., Jamtveit, B., Dewey, J.F. & Swensson, E. 1991: Subduc- tioan and eduction of continental crust: major mechanisms during continent-continent collosion and orogenic extensional collapse, a Conclusions model based on the south Norwegian Caledonides. Terra Nova 3, 303-310. Analyses of remote sensing data and outcrop studies of Andersen, T.B. & Osmundsen, P.T. & Jolivet, L. 1994: Deep crustal faults on the Fosen Peninsula and Vikna islands can be fabrics and a model for extensional collapse of the southwest Nor- summarised as follows: wegian Caledonides. Journal of Structural Geology 16, 1191-1203. Andréasson, P.G. 1994: The Baltoscandian margin in Neoproterozoic - 1. The large-scale Møre-Trøndelag Fault Complex (MTFC) early Palaeozoic times. Some constraints on terrane derivation and has a width that exceeds 50 km, with several narrow accretion in the Arctic Scandinavian Caledonides. Tectonophysics, high-strain zones surrounded by damage zones. 231, 1-32. 2. Brittle deformation is seen in two types of settings; (i) Berthelsen, A. & Marker, M. 1986: 1.9-1,8 Ga old strike-slip megashears major, discrete, NE-SW striking fault strands, and (ii) in the Baltic Shield, and their plate tectonic implications. Tectonop- wider, intervening damage zones hosting fracture-lin- hysics 128, 163-181. eaments. Bingen, B. Nordgulen, Ø. & Solli, A. 2002: U-Pb geochronology and Palaeozoic events in the Mid Scandinavian Caledonides. In Eide, E. 3. Fracture lineaments of the Fosen Peninsula and Vikna (ed.): BATLAS. Mid Norway plate reconstruction atlas with global islands are confirmed to be small faults. They appear in and Atlantic perspectives. Norges geologiske undersøkelse 66-67. three main sets with E-W, N-S, and NW-SE orientations. Blystad, P., Brekke, H., Færseth, R.B., Larsen, B.T., Skogseid, J. & Tørud- 4. NW-SE faults of the Fosen Peninsula (Osen area) are bakken, B. 1995: Structural elements of the Norwegian continen- characterised by a c. 1 km regular spacing. They host talss helf. Part II: The Norwegian Sea Region. Norwegian Petroleum mylonites, found as clasts in cataclasites, and a late Directorate Bulletin 8, 45pp. overprint of tensile fractures. These fault rocks prob- Brekke, H. & Riis, F. 1987: Tectonics and basin evolution of the Nor- wegian shelf between 62° and 72° N. Norsk Geologisk Tidsskrift 67, ably relate to differential sinistral movements along 295-322. the master strands of the MTFC and reactivation in Braathen, A. & Gabrielsen, R.H. 1998: Lineament architecture and latest Caledonian to post-Caledonian, extensional fracture distribution in metamorphic and sedimentary rocks with denudation and unroofing of the region. application to Norway. Norges geologiske undersøkelse Report 98.043, 5. Resolved movement on NW-SE faults divides into 78pp. dextral horizontal separation of some tens of metres Braathen, A. & Gabrielsen, R.H. 2000: Bruddsoner i fjell – oppbygning at the most, and an additional dip-slip component. og definisjoner. Gråsteinen 7, 1-20. Braathen, A., Nordgulen, Ø., Osmundsen, P.T., Andersen, T.B., Solli, A. 6. The NE-SW Drag fault of Vikna is a strand of the & Roberts,D. 2000: Devonian, orogen-parallel, oopposed extension MTFC that reveals extensive deformation in a broad in the Central Norwegian Caledonides. Geology 28, 615-618. zone of mainly cataclasites. These rocks are cut by Braathen, A., Osmundsen, P.T. & Gabrielsen, R.H. 2004: Dynamic dm-scale layers of non-cohesive breccia and gouge. development of fault rocks in a crustal-scale detachment: An exam- The latter fault products likely relate to Jurassic fault- ple from western Norway. Tectonics 21. TC4010, 1-21. ing and associated, near-shore basin formation. Dom- Braathen, A., Osmundsen, P.T., Nordgulen, Ø., Roberts, D. & Meyer, inant kinematics are strike-slip. G.B. 2002: Orogen-paralle extension of the Caledonides in northern central Norway: an overview. Norwegian Journal of Geology, 7. Major NE-SW fault strands of the MTFC experienced 82, 225-241. multiphase reactivation in the Phanerozoic, whereas Bukovics, C. & Ziegler, P.A. 1985: Tectonic development of the Mid- intervening, broad damage zones are basically unaf- Norway continental margin. Marine and Petroleum Geology 2, 2-22. fected by post-Caledonian deformation events. Bøe,R. 1991: Structure and seismic stratigraphy of the innermost mid- 72 E. Olsen et al. NORWEGIAN JOURNAL OF GEOLOGY

Norwegian continental shelf: an example from the Frohavet area. of basement structural grain, and the particular role of the Møre- Marine and Petroleum Geology 8, 140-152. Trøndelag Fault Complex. Marine and Petroleum Geology 16, 443- Bøe, R. & Bjerkli, K. 1989: Mesozoic sedimentary rocks in Edøyfjorden 465. and Beistadfjorden, central Norway: implications for the structural Gabrielsen, R.H. & Ramberg, I.B. 1979: Fracture patterns in Norway history of the Møre-Trøndelag Fault Zone. Marine Geology 87, 287- from Landsat imagery: Results and potential use. Proceedings, Nor- 299. wegian Sea Symposium, Tromsø 1979, Norwegian Petroleum Soci- Caine, J.S., Evans, J.P. & Forster, C.B. 1996: Fault zone arcitecture and ety, NSP/1-28. permeability structure. Geology, 24 1025-1028. Grunnaleite, I. & Gabrielsen, R.H, 1995: The structure of the Møre Chauvet, A. & Séranne, M. 1994: Extension-parallel folding in the Basin. Tectonophysics 252, 221-251. Scandinavian Caledonides: implications for late-orogenic proces- Grønlie, A., Naeser, C.W., Naeser, N.D., Mitchell, J.G., Sturt, B.A & Ine- ses. Tectonophysics 238, 31-54. son, P. 1994: Fission-track and K-Ar dating of tectonic activity in Cuthbert, S. 1991. Evolution of the Hornelen Basin, west Norway: new a transect across the Møre-Trøndelag Fault Zone, Central Norway. constraints from petrological studies of metamorphic clasts. In Norsk Geologisk Tidsskrift 74, 24-34. A. Morton, S.P. Todd & P.D.W. Haughton (eds.): Developments in Grønlie,A., Nilsen,B. & Roberts,D. 1991: Brittle deformation history sedimentary Provenence Studies. Geological Society of London Spe- of fault rocks on the Fosen Peninsula, Trøndelag, Central Norway. cial Publication, 57, 343-360. Norges Geologiske Undersøkelse Bulletin 421, 39-57. Dewey, J.F., Ryan, P.D. & Andersen, T.B. 1993: Orogenic uplift and Grønlie, A. & Roberts, D. 1989: Resurgent strike-slip duplex develop- collapse, crustal thickness, fabrics and metamorphic phase changes: ment along the Hitra-Snåsa and Verran Faults, Møre-Trøndelag the role of eclogites. In H.M. Prichard, T. Alabaster, N.B.W. Harris Fault Zone, Central Norway. Journal of Structural Geology 11, 295- & C.R. Neary (eds.): Magmatic Processes and Plate Tectonics. Geo- 305. logical Society of London Special Publication 76, 325-343. Grønlie, A. & Torsvik, T. H. 1989: On the origin and age of hydrother- Doré, A.G. & Gage, M.S. 1987: Crustal alignments and sedimentary mal thorium-enriched carbonate veins and breccias in the Møre- domains in the evolution of the North Sea, north-east Atlantic Trøndelag Fault Zone, Central Norway. Norsk Geologisk Tidsskrift margin and barents shelf. In J.Brooks & K.Glennie (eds.): Petroleum 69, 1-19. Geology of North West Europe, (Graham & Trotman), 1131-1148. Hurich, C.A. 1996: Kinematic evolution of the lower plate during Doré, A.G., Lundin, E.R., Fichler, C. & Olesen, O., 1997: Patterns of intracontinental subduction: An example from the Scandinavian basement structure reactivation along the NE Atlantic margin. Caledonides. Tectonics 15, 1248-1263. Journal of the Geological Society of London 154, 85-92. Hurich, C.A. & Kristoffersen, Y. 1988: Deep structure of the Cale- Eide, E. (ed.) 2002: BATLAS. Mid Norway plate reconstruction atlas donide orogen in southern Norway: new evidence from marine with global and Atlantic perspectives. Norge geologiske undersøkelse, seismic reflection profiling. Norges geologiske Undersøkelse Special Trondheim, 75pp. Publication 3, 96-101. Eide, E.A., Torsvik, T.H. & Andersen, T.B. 1997: Absolute dating of Kendrick, M.A., Eide, E.A., Roberts, D. & Osmundsen, P.T. 2004: The brittle fault movements: Late Permian and Late Jurassic extensional Mid-Late Devonian Høybakken Detachment, Central Norway: fault breccias in western Norway. Terra Nova 9, 135-139. 40Ar/39Ar evidence for prolonged late/post Scandian extension Eide, E. & Solli, A. 2002: 40Ar/39Ar geochronology of Palaeozoic events and uplift. Geological Magazine 141, 329-344. in the Mid Scandinavian Caledonides. In Eide, E. (ed.): 2002: BAT- Krabbendam, M. & Dewey J.F. 1998: Exhumation of UHP rocks by LAS. Mid Norway plate reconstruction atlas with global and Atlantic transtension in the Western Gneiss region, Scandinavian Caledo- perspectives. Norge geologiske undersøkelse, 68-71. nides. In: Holdsworth, R.E., Strachan, R.E. & Dewy, J.F. (eds.): Con- Eide, E.A., Haabesland, N.E., Osmundsen, P.T., Kendrick, M.A., Ander- tinental transpressional and transtensional tectoncs. Geological Soci- sen, T.B. & Roberts,D. 2003. 40Ar/39Ar geochronology and indirect ety of London Special Publication 135, 159-181. dating of continental sedimentary rocks: Late Devonian-Early Car- Lundin, E.R. & Doré, A.G. 1997: A tectonic model for the Norwegian boniferous ’Old Red Sandstone’ discovered in outer Trøndelag. In passive margin with implications for the NE Atlantic: Early Creta- Nakrem (ed.): Abstracts and Proceedings of the Geological Society of ceous to break-up. Journal of the Geological Society, London 154, Norway 1, 2003. Norsk Geologisk Forening, Vinterkonferansen 2003. 545-550. Eide, E.A., Haabesland, N.E., Osmundsen, P.T., Andersen, T.B., Lundquist, T., Bøoe, R., Kousa, J., Lukkarinen, H., Lutro, O., Solli, A., Roberts, D. & Kendrick, M. 2005: Modern techniques and Old Red Stephens, M.B. & Weihed, P. 1996: Bedrock map of central Fenno- problems – determining the age of continental sedimentary depo- scandia. Scal 1:1 000 000. Geological Surveys of Finland (Espoo), sits with 40Ar/39Ar provenance analysis in the west-central Norway. Norway (Trondheim) and Sweden (Uppsla). Norwegian Journal of Geology, 85, 133-149. Möller, C. 1988: Geology and metamorphic evolution of the Roan Fonneland,H.C., & Pedersen.R.B. 2003: Provenance of the Hornelen area, Vestranden, Western Gneiss Region; Central Norwegian Cale- devonian basin; implications for the exhumation history of the donides. Norges geologiske undersøkelse Bulletin 413, 1-31. Western gneiss region, Norway. In Nakrem (ed.): Geological Society Nordgulen, Ø., Braathen, A., Corfu, F., Osmundsen, P.T. & Husmo, T. of Norway, Abstracts and Proceedings 1, 2003, 28-29. 2002: Polyphase kinematics and geochronology of the late-Caledo- Fossen, H. 1992: The role of extensional tectonics in the Caledonides nian Kollstraumen detachment, north-central Norway. Norwegian of south Norway. Journal of Structural Geology 14, 1033-1046. Journal of Geology 82, 299-316. Fossen, H., Mangerud, G., Hesthammer, J., Bugge, T. & Gabrielsen, Olesen, O., Ebbing, J., Lundin, E., Skilbrei, J.R., Torsvik, T.,H., Hansen, R.H. 1997: The Bjorøy Formation: a newly discovered occurrence E.K., Henningsen, T., Midbøe, P., & Sand, M. 2007: An improved of Jurassic sediments in the Bergen Arc System. Norsk Geologisk tectonic model for the Eocene opening of the Norwegian – Green- Tidsskrift 77, 269-287. land Sea: Use of modern magnetic data. Marine and Petroleum Gabrielsen, R.H., Braathen, A., Dehls, J. & Roberts, D. 2002: Tectonic Geology 24, 53-66. lineaments of Norway. Norwegian Journal of Geology 82, 153-174. Olesen, O., Lundin, E., Nordgulen, Ø., Osmundsen, P.T., Skilbrei, J.R., Gabrielsen, R.H., Færseth, R., Hamar, G.P. & Rønnevik, H.C. 1984: Smethurst, M.A., Solli, A., Bugge, T. & Fichler, C. 2002: Bridging the Nomenclature of the main structural features on the Norwegian gap between the onshore and offshore geology in Nordland, nor- continental shelf north of the 62nd parallel. In A.M. Spencer et al. thern Norway. Norwegian Journal of Geology 82, 243-262. (eds.): Petroleum Geology of the North European Margin. Norwegian Olsen, Å.E.V. 2002: Tverrforkastninger (NV-SØ) på nordflanken av Petroleum Society, Graham & Trotman, 41 - 60. Møre-Trøndelag-forkastningskomplekset i Osenområdet. Unpubl. Gabrielsen, R.H., Odinsen, T. & Grunnaleite, I. 1999: Structuring of Cand. Scient thesis., University of Bergen, 138pp. the Northern Viking Graben and the Møre Basin; the influence Osmundsen, P.T. & Andersen, T.B. 2001: The Devonian basins of NORWEGIAN JOURNAL OF GEOLOGY Fault systems marginal to the Møre-Trøndelag Fault Complex 73

western Norway: products of large-scale sinistral transtension? northern North Sea. Journal of the Geological Society of London 157, Tectonophysics 332, 51-68. 769-781. Osmundsen, P.T., Sommaruga, A., Skilbrei, J.R. & Olesen, O. 2002: Sommaruga,A. & Bøe,R. 2002: Geometry and subcrop maps of shallow Deep structure of the Mid Norway rifted margin. Norwegian Jour- Jurassic basins along the Mid-Norway coast. Marine and Petroleum nal of Geology 82, 205-224. Geology 19, 1029-1042. Osmundsen, P.T. & Andersen, T.B. 2001: The middle Devonian basins Steel, R. J., Sidlecka, A. & Roberts, D. 1985: The Old Red Sandstone of western Norway: sedimentary response to large-scale transten- basins of Norway and their deformation. In Gee, D. G. & Sturt, B. sional tectonics? Tectonophysics 332 51-68. A. (eds.): The Caledonide Orogen- Scandinavia and Related Areas. Osmundsen, P. T., Andersen, T. B., Markussen, S. & Svendby, A.K. 1998: John Wiley & Sons, Chichester, 293-315. Tectonics and sedimentation in the hanging wall of a major exten- Strömberg, A. G. B. 1978. Tectonic zones in the Baltic Shield. Precam- sional detachment: The Devonian Kvamshesten basin, western brian Research 6, 217-222. Norway. Basin Research 10, 213-234. Stuevold, L. M., Skogseid, J. & Eldholm, O. 1992. Post-Cretaceous uplift Osmundsen, P.T., Braathen, A., Nordgulen, Ø., Roberts, D., Meyer, G.B. events on the Vøring continental margin. Geology 20, 919-922. & Eide, E. 2003: Late/post Caledonian extension in North-Central Sturt, B.A. & Braathen, A, 2001: Deformation and metamorphism of Norway: The Nesna shear zone and its regional implications. Jour- Devonian rocks in the outer Solund area, western Norway – impli- nal of the Geological Society London 160, 137-150. cations for models of Devonian deformation. International Journal Osmundsen, P.T., Eide, E., Haabesland, N.E, Roberts, D., Andersen, of Earth Sciencs (Geologische Rundschau) 90, 270-286. T.B., Kendrick, M., Bingen, B., Braathen, A., & Redfield, T.F. 2006: Talwani, M. & Eldholm, O. 1977: Evolution of the Norwegian-Green- Exhumation of the Høybakken detachment zone and the Møre- land Sea. Geological Society of America Bulletin 88, 969-999. Trøndelag Fault Complex central Norway. Journal of the Geological Titus, S.J., Fossen, H. & Pedersen, R.B. 2004: Pull-apart formation and Society London 163, 303-318. strike-slip partitioning in an oblique divergent setting, Leka Ophi- Pascal, C. & Gabrielsen, R.H. 2001: Numerical modelling of Cenozoic olite, Norway. Tectonophysics 354, 101-119. stress patterns in the mid Norwegian Margin and the northern Torsvik, T., Sturt, B.A., Ramsay, D.M., Kisch, H.J. & Bering, D. 1986: North Sea. Tectonics 20, 585-599. The tectonic implications of Solundian (upper devonian) magne- Ramberg, I. B., Gabrielsen, R. H., Larsen, B. T. & Solli, A. 1977. Analysis tization of the Devonian rocks of Kvamshesten, western Norway. of fracture pattern in Southern Norway. Geologie en Mijngouw 56, Earth and Planetary Letters 80, 337-347. 295-310. Torsvik, T.H., Sturt, B.A., Ramsay, D.M., Grønlie, A., Roberts, D., Smet- Redfield, T. F., Torsvik, T. H., Andriessen, P. A. M. & Gabrielsen, R. hurst, M., Atakan, K., Bøe, R. & Walderhaug, H. J. 1989: Palaeomag- H. 2004: Mesozoic and Cenozoic tectonics of the Møre Trøndelag netic constraints on the early history of the Møre- Trøndelag Fault Fault Complex, Central Norway: Constraints from new apatite fis- Zone, Central Norway. In C. Kissel, C. Jai (eds.): Palaeomagnetic sion track data. Physics and Chemistry of the Earth 10, 673-682. Rotations and Continental Deformation, 431-457. Redfield, T.R., Braathen, A., Gabrielsen, R.H., Osmundsen, P.T., Vågnes, E., Gabrielsen, R.H. & Haremo, P. 1998. Late Cretaceous- Torsvik,T.H. & Andriessen,P.A.M. 2005: Late Mesozoic to Eraly Cenozoic intraplate contractional deformation at the Norwegian Cretaceous components of vertical separation across the Møre- continental shelf: timing, magnitude and regional implications. Trøndelag Fault Complex, Norway. Tectonophysics 395, 233-249. Tectonophysics 300, 29-46. Rindstad, B. & Grønlie, A. 1987: Landsat TM data used in the mapping Watts, L.M. 2002: The Walls Boundary Fault Zone and the Møre-Trøn- of large-scale geological structures in coastal areas of Trøndelag, delag Fault Complex: a case study of two reactivated fault zones. central Norway. Proceedings, Earthnet Pilot Project on Landsat The- PhD-thesis University of Durham, 550pp. matic Mapper Applications, Frascati, Italy 1987, 169-181. Watts, L.M., Sherlock, S., Holdsworth, R.E. & Roberts, D. 2004: Regio- Roberts, D. 1983: Devonian tectonic deformation in the Norwegian nal implications of Ar39-Ar40 pseudotachylite ages from the reacti- Caledonides and its regional perspectives. Norges geologiske under- vatred Møre-Trøndelag Fault Complex, Central Norway (Abstract). søkelse, 380, 85-96 Tectonics Study Group Meeting, University of Leeds, January 2001, Roberts, D. 1986: Structural photogeological and general features Abstract volume. of the Fosen-Namsos Western Gneiss Region of Central Norway. Withjack, M.O., Meisling, K.E. & Russell, L.R. 1989: Forced folding Norges geologiske undersøkelse Bulletin 407, 13-25. and basement-detached normal faulting in the Haltenbanken area, Roberts, D. 1998: High-strain zones from meso -to macro-scale at dif- offshore Norway. In.Tankard, A.J & Balwill, H.R. (eds.): Extensional ferent structural levels, Central Norwegian Caledonides. Journal of tectonics and the stratigraphy of the North Atlantic Margins. Ameri- Structural Geology 20, 111-119. can Association of Petroleum Geologists, Memoir 46, 567-576. Schouenborg, B., Johansson, L. & Gorbatschev, R. 1991: U-Pb zircon ages of basement gneisses and discordant felsic dykes from Vestran- den, westernmost Baltic shield and central Norwegian Caledonides. Geologische Rundschau 80, 121-134. Séranne, M.,1992: Late-Paleozoic kinematics of the Møre-Trøndelag fault zone and adjacent areas, central Norway. Norsk Geologisk Tids- skrift 72, 141-158. Séranne, M. & Séguret, M. 1987: The Devonian basins of western Nor- way: Tectonics and kinematics of extending crust. In M.P.Coward, J.F.Dewey & P.L.Hancock (eds.): Continental extensional tectonics. Geological Society of London Special Publication 28, 537-548. Sherlock, S., Watts, L.,Holdsworth, R.E. & Roberts, D. 2004: Dating fault reactivation by Ar/Ar laserprobe: an alternative view of apparently cogenetic mylonite-pseudotachylite assemblages. Journal of the Geological Society, London 161, 335-338. Skilbrei, J.R., Olesen, O., Osmundsen, P.T., Kihle, O., Aaro, S. & Fjellan- ger, E. 2002: A study of basement structures and onshore-offshore correlations in Central Norway. Norwegian Journal of Geology 82, 263-280. Smethurst,M.A. 2000: Land-offshore links in western Norway and the