structural development of the continental shelf offshore...

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IntroductionThe Lofoten–Vesterålen continental margin, situated between the Bivrost Lineament and the Senja Fracture Zone represents the transition between the passive and much wider continental Vøring margin and the sheared West Barents Sea margin (Fig. 1). The Lofoten–Vesterålen margin segment is approximately 400 km long, typified by a narrow continental shelf with a steep slope. It is in contrast to adja-cent margin segments in terms of crustal structure, break-up magmatism and sediment thickness (e.g. Mjelde et al. 1993; Eldholm & Grue 1994; Blystad et al. 1995; Tsikalas et al. 2001; Steltenpohl et al. 2004).

The Lofoten-Vesterålen shelf has not been opened to petroleum exploration and accordingly no commercial wells have been drilled. Shallow wells drilled in the area have however, proven potential Jurassic reservoir and source rocks (Hansen et al. 1992), and it has been speculated that reservoirs of this generation may contain major oil and gas resources similar to prolific petroleum provinces off-shore Mid Norway. In the absence of deep borehole infor-mation there is still uncertainty regarding the thickness and

the areal distribution of Jurassic as well as older sediments throughout the study area. In most of the area a thin interval is mapped between top basement and the base Cretaceous reflector (Fig. 2), and workers have treated this interval as an unspecified sequence to which they have assigned a Juras-sic to Late Paleozoic age (Jørgensen & Navrestad 1981; Bøen et al. 1984; Mokhtari & Pegrum 1992; Blystad et al. 1995; Løseth & Tveten 1996; Mjelde et al. 1996; Tsikalas et al. 2001; Tsikalas et al. 2005; Bergh et al. 2007; Faleide et al. 2008).

The area under consideration is characterized by basement highs with an overall NE-SW orientation and large Cretaceous basins (Fig. 1), but becomes structurally more complex beneath the basins (e.g. Mokhtari & Pegrum 1992; Blystad et al. 1995; Bergh et al. 2007). Structural elements in the offshore area are bounded by normal faults with orienta-tions similar to the basement structural grain that has been studied onshore Lofoten–Vesterålen (Gabrielsen & Ramberg 1979; Gabrielsen et al. 2002; Bergh et al. 2007). Authors have considered Permo-Triassic, Permo- Jurassic, Late Jurassic -Early Cretaceous, Middle Cretaceous and Late Cretaceous -Early Tertiary rifting as the dominant post-Caledonian tectonic events in the structural evolution of this margin

NORWEGIAN JOURNAL OF GEOLOGY Structural development of the continental shelf offshore Lofoten–Vesterålen, northern Norway

Structural development of the continental shelf offshore Lofoten–Vesterålen, northern Norway

Roald B. Færseth

Færseth, R.B.: Structural development of the continental shelf offshore Lofoten–Vesterålen, northern Norway. Norwegian Journal of Geology, vol. 92., pp 19-40, Trondheim 2012. ISSN 029-196X.

A striking feature of the continental shelf along the Lofoten–Vesterålen margin is the relatively thin sequence of Jurassic-Triassic sediments not only on structural highs, but also in parts of some of the sub-basins. This reflects limited sedimentation as well as periods of uplift and erosion, factors that are crucial for the hydrocarbon potential of the area. Structures in the shelf area off Lofoten-Vesterålen result primarily from major rift episodes in the Middle-Late Jurassic and Late Cretaceous- Paleocene, whereas this area appears unaffected by large faults resulting from the Late Permian-Early Triassic rift episode. This is in strong contrast to near shore and well-explored areas to the south, where large fault-bounded basins filled with thick Permo-Triassic sediments developed during and following this rift episode. At this stage of development, the Lofoten-Vesterålen archipelago and the shelf area are interpreted as parts of a huge NE-SW oriented basement high characterized by a peneplained basement relief, later progressively covered by thin Triassic to Middle Jurassic sedi-ments. Jurassic faults in the Lofoten–Vesterålen margin segment are basement-involved, and both their orientation and listric geometry demonstrate the strong basement-governing control. A major along-strike change in structural pattern takes place across an accommodation zone at a high angle to the NNE-SSW trend of the Jurassic rift structures, and this zone acted as a rift propagation barrier during Jurassic crustal stretching. The accommo-dation zone reflects an E-W basement grain, and the change in dip direction of the Jurassic faults and the tilt of major fault-blocks across this zone took place without evidence of strike-slip motion.The Jurassic rift topography became overprinted by NE-SW to ENE-WSW striking faults resulting from a Campanian-Paleocene rift episode and by the overlying and NE-SW oriented Late Cretaceous Ribban Basin. A major uplift with NE-SW elongation and a maximum width of c. 70 km is located in the shelf area. The uplift is interpreted to result from tectonic compression related to the continent break-up to the NW, the main growth occurring during Oligocene-Miocene times.

Roald B. Færseth, Ryenbergveien 75A, 0677 Oslo, Norway (e-mail: [email protected])

20 R. B. Færseth NORWEGIAN JOURNAL OF GEOLOGY

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Figure 1. Structural elements of the Nor-wegian Sea (slightly modified from Blys-tad et al. 1995). The Lofoten-Vesterålen archipelago with the shelf area outlined. Late Devonian extensional shear zones onshore Norway are shown. NSZ: Nesna Shear Zone with the interpreted offshore extension at basement level (Olesen et al. 2002). MTFC: Møre-Trøndelag Fault Complex, a prominent structural feature both onshore and offshore Norway.

Færseth Fig. 2

QuaternaryTertiaryUpper CretaceousLower Cretaceous

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Figure 2. The Lofoten Ridge is a deeply eroded and narrow Precambrian basement horst that separates the oppositely tilted Ribban and Vest-fjorden basins. The relative thin sequence of Jurassic-Triassic sediments overlying the Precambrian basement is a characteristic feature in the shelf area beneath the Cretaceous Ribban Basin. The cross section is based on seismic reflection profile LO-08-87 (see Fig. 4 for location of the cross section).

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segment (Mokhtari & Pegrum 1992; Blystad et al. 1995; Løs-eth & Tveten 1996; Lundin & Doré 1997; Doré et al. 1999; Tsikalas et al. 2001; Wilson et al. 2006; Bergh et al. 2007). However, proposed models for the structural evolution, in the context of timing and importance of tectonic events, remain open to questioning. This is due to uncertain ty regarding the age and thickness distribution of the pre- Cretaceous sediments and to periods of deep erosion that have removed significant parts of the sediment ary succes-sion over considerable areas. As a consequence, the develop-ment of this margin segment should be seen in a regional context as influenced by post-Caledonian tectonic episodes

documented offshore Mid Norway (Fig. 3), in the western Barents Sea (e.g. Faleide et al. 1984; Gabrielsen et al. 1990; Gudlaugsson et al. 1998) and East Greenland (e.g. Surlyk 1990; Hartz et al. 2002; Hamann et al. 2005).

The present study focuses on the structural development of the Lofoten-Vesterålen continental shelf to explore the effect and timing of latest Paleozoic to Cenozoic tectonic episodes proven to be important in adjoining and well-explored areas, and furthermore to investigate the sedimentological response to the structuring in the area under consideration.


Sakmarian

Kimmeridgian

Figure 3. Tecto-n o s t r a t i g r a p h i c frame work of the Norwegian Sea. Post- Caledonian structures result pri-marily from three extensional episo-des (Corfield et al. 2001; Færseth & Lien 2002; Ren et al. 2003; Müller et al. 2005) followed by compression and late uplift (Gómez & Vergés 2005). The stratigraphy is repre-sentative of areas offshore Mid Nor-way where a large number of commer-cial wells have been drilled.

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Offshore data The structural development presented in this paper is based on the interpretation of 2D seismic sections offshore Lofoten- Vesterålen. The seismic profiles are recorded to a depth of 5-8 s TWT, which is well below the basement-cover contact. The seismic used in this study has been the found-ation for seismic mapping and previous structural analysis along the Lofoten-Vesterålen margin (Mokhtari & Pegrum 1992; Blystad et al. 1995; Løseth & Tveten 1996; Doré et al. 1999; Tsikalas et al. 2001; Wilson et al. 2006; Bergh et al. 2007). Ages assigned to seismic reflectors in this study as well as in published profiles are based on long distance ties to wells S-SW of the study area, shallow wells NW of Lofoten -Vesterålen (Hansen et al. 1992) and assumed ana logy with outcrop data from the island of Andøya (Fig. 4) (Dalland 1975, 1981). Mokhtari & Pegrum (1992) published time structural maps of base Tertiary, intra Turonian and base Cretaceous horizons in the shelf area off Lofoten, whereas Bergh et al. (2007) presented a depth-converted structural contour map of the top basement reflection for the shelf area NW of Lofoten-Vesterålen.

The regional fault pattern (Fig. 4), which forms the basis for the structural analysis presented below, is largely consistent with major faults in the fault maps published by Tsikalas et al. (2001) and Bergh et al. (2007). The profiles presented in this study are based on seismic sections, which have been applied also by previous workers and presented as line draw-ings (Mokhtari & Pegrum 1992; Blystad et al. 1995; Løseth & Tveten; Tsikalas et al. 2001; Tsikalas et al. 2005; Wilson et al. 2006; Bergh et al. 2007; Carstens 2009).

The Lofoten-Vesterålen margin in a regional context

In the well-explored Vøring Basin margin there is general consensus that the opening of the NE Atlantic Ocean close to the Paleocene-Eocene transition (e.g. Eldholm et al. 1989; Mosar et al. 2002) marked the culmination of a c. 340 Ma history of repeated extensional deformation since the Late Silurian-Early Devonian Caledonian collision climax. Fol-lowing Late Devonian backsliding (Fossen 1992; Doré et al. 1999; Osmundsen et al. 2002), the structural develop-ment has been described in terms of broadly defined rift episodes in the Carboniferous, Permo-Triassic, Jurassic-Early Cretaceous, Middle Cretaceous and Late Cretaceous-Early Tertiary (e.g. Bukovics et al. 1984; Bukovics & Ziegler 1985; Ziegler 1988; Blystad et al. 1995; Lundin & Doré 1997; Swiecicki et al. 1998; Brekke et al. 1999; Doré et al. 1999; Gabrielsen et al. 1999; Brekke 2000; Osmundsen et al. 2002, Faleide et al. 2008). Evidence of Carboniferous normal fault-ing is found both in the coastal area of Greenland (e.g. Hartz 2002; Hamann et al. 2005) and onshore Central northern Norway (e.g. Braathen et al. 2002; Eide et al. 2002). Authors have suggested the effects of Carboniferous as well as Late Devonian crustal stretching also beneath the Cretaceous

Vøring Basin (Ziegler 1989; Blystad et al. 1995; Doré et al. 1997; Roberts et al. 1999; Brekke 2000; Osmundsen et al. 2002, 2003; Faleide et al. 2008), but due to the depth of burial this remains uncertain.

In publications over the last decade, authors have related main structures offshore Mid Norway to three exten-sion episodes (Fig. 3) (e.g. Corfield et al. 2001; Færseth & Lien 2002; Ren et al. 2003; Müller et al. 2005) followed by compres sion and late uplift (e.g. Gómez & Vergés 2005). The effects of the extension episodes that lasted some 20-30 Ma are evident on a regional scale as major normal faults bounding basins and highs (Fig. 1), and likely to have influ-enced also the area under consideration. As discussed below, authors have also emphasised the importance of Early and Middle Cretaceous tectonism to explain structures at Mesozoic level in the Møre and Vøring basins (e.g. Blystad et al. 1995; Lundin & Doré 1997; Doré et al. 1999; Brekke 2000; Osmundsen et al. 2002).

As a result of the rifting process, the continental crust was repeatedly stretched and a series of important basins were created above normal fault systems that apparently have rooted in the ductile zone of the middle crust (Mosar 2000; Mosar et al. 2002; Osmundsen et al. 2002). In the Vøring Basin a total crustal extension in the order of 200% (β=2) is estimated as the cumulative effect of rift episodes prior to the Tertiary break-up (Reemst & Cloething 2000; Skogseid et al. 2000; Mosar et al. 2002; Gómez et al. 2004). During

R. B. Færseth NORWEGIAN JOURNAL OF GEOLOGY

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Figure 4. Major faults offshore Lofoten-Vesterålen are interpreted to result from Middle-Late Jurassic and Late Cretaceous-Early Tertiary rift episodes. Faults of the youngest generation (green colour) are oblique to the Jurassic faults. The map shows the E-W oriented accommodation zone at 68°30’ N. Two locations with shallow cores are indicated.

23NORWEGIAN JOURNAL OF GEOLOGY Structural development of the continental shelf offshore Lofoten–Vesterålen, northern Norway

the successive rift episodes (Fig. 3), the locus of maximum extension migrated in a westerly direction towards the zone of the future continent separation, and accordingly renewed rifting did not necessarily follow earlier rifts.

Based on several approaches, authors have concluded that extension was close to E-W during a Late Permian-Early Triassic rift episode, both in the northern North Sea (Færs-eth 1996; Valle et al. 2002) and between the two conjugate margins of Greenland and Norway (Doré et al. 1999; Mosar et al. 2002; Torsvik & Cocks 2005). During the Jurassic rift episode however, both E-W and NW-SE extension have been suggested. Jurassic E-W extension on a regional scale was proposed by Lundin & Doré (1997) and Doré et al. (1999), and Mosar et al. (2002) from plate reconstruc-tions interpreted a broadly E-W Jurassic opening direction between Greenland and Norway. Tsikalas et al. (2005) and Faleide et al. (2008) inferred NW-SE extension in the NE Atlantic during a Late Jurassic-earliest Cretaceous rift epi-sode. Based on observations from the northern North Sea, Færseth et al. (1997) interpreted the Jurassic stretching direction to be NW-directed. As a consequence this area was characterized by oblique rifting and the sequential develop-ment of Jurassic normal faults striking N-S and NE-SW respectively, governed by basement heterogeneities. Based on experimental clay models, Clifton et al. (2000) supported the interpretation of Færseth et al. (1997), and stated that the two Jurassic fault populations were compatible with a consistent NW-SE extension.

In the Lofoten-Vesterålen area, Bergh et al. (2007) proposed that the stretching direction through time was orthogonal to the strike of mapped faults of different generations and they inferred a WNW-ESE Permo-Jurassic extension, changing to NNW-SSE during Cretaceous to Paleogene. Wilson et al. (2006), performed digital mapping to link onshore and offshore structures in the Lofoten area, and interpreted a NW-SE extension associated with Late Jurassic -Early Cre-taceous rifting. In this study, it will be argued that the post-Caledonian structures that developed in the study area are best explained according to the extension directions derived from plate reconstructions. This implies an E-W directed extension during the Late Permian-Early Triassic as well as the Jurassic rift episode, but from the Late Cretaceous the stretching direction became NW-SE oriented (Mosar et al. 2002; Torsvik & Cocks 2005). Accordingly, basement zones of weakness striking predominantly NNE-SSW and NE-SW to ENE-WSW (Gabrielsen & Ramberg 1979; Olesen et al. 1997, 2002; Gabrielsen et al, 2002; Bergh et al. 2007) were close to orthogonal as well as oblique to the stretching direction during the successive rift episodes and therefore became variably utilized.

The age, thickness distribution, composition and maximum burial of pre-Cretaceous sediments, which represent poten-tial reservoir and source rocks are crucial for the hydrocar-bon potential offshore Lofoten-Vesterålen. In this study, the Lofoten-Vesterålen archipelago and the shelf area are interpreted as remnants of a regional basement paleo-high.

Above this high Jurassic sediments are relatively thin, and pre-Jurassic sediments are thin or absent. A thin pre-Creta-ceous succession is in strong contrasts to the area offshore Mid Norway where Middle Triassic-Lower Jurassic sedi-ments with a thickness of some 2500-3000 m occur beneath the Trøndelag Platform and the Halten and Dønna terraces (Blystad et al. 1995; Osmundsen et al. 2002; Gómez et al. 2004; Müller et al. 2005). Another 2000-3000 m of Perm-ian-Lower Triassic sediments may be present at depth in the Froan and Helgeland basins (Fig. 1) (Osmundsen et al. 2002; Müller et al. 2005). In the shelf area off Lofoten-Vesterålen, several observations support the idea of relatively thin pre-Cretaceous sediments in most of the area. Seismic mapping indicates that Cretaceous sediments apparently rest directly on crystalline basement rocks over a considerable part of the area west of Lofoten (Mokhtari & Pegrum 1992). Løs-eth & Tveten (1996) published depth converted geoseismic profiles in the northern Ribban Basin, offshore Vesterålen, and mapped the thickness of the seismic unit between top basement and the base Cretaceous reflector to vary from 0 to 450 m. In shallow wells drilled off Vesterålen and in the Mesozoic basin onshore Andøya (Fig. 4), Triassic sediments are absent and at both locations Middle Jurassic sediments overlie a weathered Precambrian basement (Sturt et al. 1979; Hansen et al. 1992; Mørk et al. 2003). The eroded and smooth basement topography prior to deposition of Trias-sic to Middle Jurassic sediments is in strong contrast to the dramatic basement topography seen beneath the Trøndelag Platform (Osmundsen et al. 2002).

Basement influence in the structural evolution of the Lofoten-Vesterålen margin

The crustal thickness is 26 km at the most beneath Lofo-ten–Vesterålen and c. 30 km beneath the mainland to the SE (Drivenes et al. 1984; Mjelde et al. 1993). The outer shelf, NW of the Utrøst Ridge, represents the transition to the average 6-7 km thick Eocene oceanic crust (Eldholm et al. 1979; Mjelde et al. 1993). An anomalous mantle elevation resides beneath the Lofoten islands and the crustal thickness is c. 20 km (Mjelde et al. 1993; Tsikalas et al. 2001; Stelten-pohl et al. 2004). Steltenpohl et al. (2004) interpreted the SW part of the Lofoten Ridge (Fig. 1) as a metamorphic core complex that was tectonically unroofed due to a Permian, N-dipping and low-angle detachment. On a crustal scale, the Lofoten Ridge is a narrow horst bounded by fault zones (Fig. 2) that sole at the transition from the middle to lower crust at depths of c. 15 km (Tsikalas et al. 2001; Steltenpohl et al. 2004). At top basement level the maximum throw of these fault zones exceeds 8 km.

The Lofoten-Vesterålen archipelago is considered as the western margin of the ancient Baltic craton (e.g. Olesen et al. 1997) and exposes some of the oldest rocks recorded in Norway. The bedrock is composed of Archaean (c. 2700 Ma) high-grade migmatitic gneisses, Paleoproterozoic plu-tonic rocks and some meta-supracrustals (Griffin et al. 1978;

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Sigmond et al. 1984). Middle-Upper Jurassic and Lower Cretaceous sediments that overlie the Precambrian base-ment outcrop in a localized and fault-bounded basin along the east coast of Andøya (Fig. 4) (Dalland 1975, 1981). This basin represents the only Mesozoic basin preserved onshore Norway.

Post-Caledonian uplift exhumed a deep crustal level in the area under consideration, and shows crustal cooling from temperatures higher than 550° C at maximum Caledonian metamorphism to surface temperature in earliest Triassic time (e.g. Løseth & Tveten 1996). The relatively high Cretaceous seismic velocities across the Lofoten margin (Mjelde et al. 1996) suggest that most of the area experi-enced a deep burial during and after the Cretaceous period, and that significant post-Cretaceous uplift and erosion have taken place. Late uplift of the Lofoten-Vesterålen archi pelago and also of major parts of the continental shelf (Hansen et al. 1992; Løseth & Tveten 1996) is explained partly by iso-static effects following glacial erosion (Riis & Fjeldskaar 1992), similar to the western Barents Sea (e.g. Cavanagh et al. 2006). Quantification and timing of uplift in the study area based on shallow cores indicate a series of uplift and subsidence periods, but with the main uplift and erosion in the Late Pliocene-Pleistocene (Hansen et al. 1992). Maxi-mum burial at the drill site west of Lofoten was estimated to be at least 1 km deeper than at present and increased to the NE where the late uplift may exceed 2 km at the location off-shore Vesterålen (Fig. 4).

Onshore lineaments and previously mapped offshore lineaments

The aeromagnetic and gravity anomalies of the area under consideration trend NNE-SSW and NE-SW (Olesen et al. 1997, 2002). Compilations of lineaments onshore Lofoten -Vesterålen using satellite and aerial photograph images (Gabrielsen & Ramberg 1979; Gabrielsen et al, 2002; Wilson et al, 2006; Bergh et al. 2007), demonstrate three dominant trends. Bergh et al. (2007), based on cross-cutting relation-ships in the onshore area, presented a temporal succession of fault movements. They interpreted a NNE-SSW strik-ing linea ment set as the oldest and a NE-SW to ENE-WSW striking lineament set of intermediate age. The third and youngest, WNW-ESE striking lineament set is less promi-nent in a regional context compared to the two other linea-ment sets.

The strike of major normal faults in the shelf area along the Lofoten-Vesterålen margin is remarkably similar to linea-ments observed onshore Lofoten–Vesterålen, and NNE-SSW striking faults represent the dominant fault popu-lation (Fig. 4). In the same area, two major fault zones are oriented NE-SW; one bounds the Lofoten Ridge to the NW and the other represents the boundary between the Utrøst Ridge and the deep basin to the NW. The WNW-ESE strik-ing basement lineament trend mapped onshore Lofoten-Vesterålen (Bergh et al. 2007) is not present as major faults in the offshore area.

Several NNW-SSE to NW-SE structures referred to as trans-fer zones have been introduced to give a lateral segment-ation of the Lofoten–Vesterålen margin (Eldholm et al. 1979; Mokhtari & Pegrum 1992; Myhre et al. 1992; Løseth & Tveten 1996; Olesen et al. 1997, 2002; Tsikalas et al. 2001, 2005; Wilson et al. 2006). The Norwegian continental mar-gin is a well documented example of a segmented passive margin, intersected by lineaments with a NW-SE orientation (Fig. 1) (Blystad et al. 1995; Doré et al. 1997; Brekke et al. 2001; Tsikalas et al. 2001, 2005; Olesen et al. 2002; Mjelde et al. 2003; Mosar 2003; Faleide et al. 2008). Lineaments in the extended continental crust apparently have pre- determined the location of the seaward prolongation as fracture zones (Blystad et al. 1995; Tsikalas et al. 2001, 2005). The Bivrost Lineament, the SW boundary of the study area, represents the transition between the Vøring margin and the along Lofoten-Vesterålen margin segment that becomes progres-sively more narrow to the NE (Fig. 1). Changes in Moho depths across the Bivrost Lineament and the abrupt SW ter-mination of a basement fault-block (Røst High) are regarded as evidence of a deep-seated feature (Mjelde et al. 1993; Ole-sen et al. 2002). Olesen et al. (2002) suggested that the Biv-rost Lineament at basement level represents a detachment dipping 5-15° to the west that constitutes the offshore exten-sion of the Late Devonian Nesna Shear Zone (Fig. 1) (e.g. Braathen et al. 2000, 2002; Skilbrei et al. 2002). A NW and landward prolongation of the Bivrost Fracture Zone as a lin-eament in the East Greenland continental margin has been suggested by Tsikalas et al. (2005).

The interpreted transfer zones along the Lofoten-Vesterålen margin have been used to explain lateral offset of older struc-tures as well as fault polarity changes in this margin segment. However, there are disagreements both concerning the ori-entation and the exact position of these structures (e.g. Ole-sen et al. 1997, 2002; Tsikalas et al. 2001, 2005). The Jennegga and Vesterålen transfer zones that subdivide the margin into the Lofoten, Vesterålen and Andøya segments (Tsikalas et al. 2001) are most commonly referred to, and these trans-fer zones are said to strike NNW-SSE or NW-SE. Olesen et al. (2002), placed the Jenegga transfer zone of Tsikalas et al. (2001) further NE between Vestvågøy and Austvågøy (Fig. 4), and termed it the Melbu transfer zone. These authors also introduced the Mosken transfer zone as another NW-SE striking lineament further SW in the study area.

Significant strike-slip motion has been invoked especially in the development of the Bivrost Lineament, but also for inferred transfer zones in the Lofoten-Vesterålen mar-gin. In this area, transfer zones are said to be active during a Late Jurassic-Early Cretaceous rift episode (e.g. Tsikalas et al. 2001). Workers have linked the transfer zones across the shelf and into the Lofoten-Vesterålen archipelago as zones with strike-slip displacement (e.g. Doré et al. 1997, 1999; Olesen et al. 1997; Tsikalas et al. 2001). However, aero-magnetic and gravity data do not support the view that transfer zones cut across the shelf area (Olesen et al. 2002). Bergh et al. (2007), based on field work onshore Lofoten-Vesterålen, were not able to demonstrate NNW-SSE to


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NW-SE striking lineaments that coincided with the posi-tion of the suggested transfer zones. Maps provided by these workers illustrate that faults and fractures among their youngest population strike WNW-ESE. Accordingly, there is no obvious connection between onshore basement lin-eaments and the interpreted transfer zones. However, Wil-son et al. (2006), referred to the Vesterålen transfer zone as a major deep seated structure in the study area. These authors suggested that the previously inferred Jennegga trans-fer zone can be explained using a model for oblique exten-sion/transtension along the Lofoten-Vesterålen archipelago, and that this zone may represent soft linkage between areas characterized by different fault geometries.

An east-west oriented accommodation zoneBased on the present study of the continental shelf along the Lofoten-Vesterålen margin and applying published onshore observations (Bergh et al. 2007), NNW-SSE to NW-SE ori-ented transfer zones are disregarded as important elements in the structural evolution of the Lofoten–Vesterålen mar-gin. Instead of invoking transfer zones with inferred strike-slip motion, polarity changes along strike are in this paper explained in terms of an E-W oriented accommodation zone governed by basement heterogeneities, and similar to typical accommodation zones in well-studied rift sys-tems (Rosendahl et al. 1986; Rosendahl 1987; Colletta et al. 1988; Færseth 1996; Larsen & Olaussen 2005). Rift systems are characterized by rift segments with alternation of half-graben asymmetry across accommodation zones at a high angle to the overall trend of the rift. Changes in dip direc-tion of faults and the tilt of rift segments across such zones generally take place without evidence of major strike-slip motion (e.g. Rosendahl 1987; Colletta et al. 1988; Morley et al. 1990; Færseth et al. 1997). Accommodation zones act as rift propagation barriers, selectively directing rift segments in a left/right stepping en échelon pattern.

In the study area, an accommodation zone is defined at 68°30’ N (Fig. 4). As described below, this E-W lineament represents a true accommodation zone in the offshore area where major Jurassic faults terminate within this zone and the dip of faults and the tilt of Jurassic rift segments change across the zone. Authors have previously attributed these along-strike variations in structural style to an inferred NNW-SSE to NW-SE striking Jennegga transfer zone (Eldholm et al. 1979; Mokhtari & Pegrum 1992; Løseth & Tveten 1996; Tsikalas et al. 2001, 2005). In the shelf area, fault segments are aligned approximately E-W, and they define a zone that is the seaward prolongation of a base-ment grain represented by the E-W oriented Hadselfjord between Austvågøy and Hadseløy (Fig. 4). This change in fault orientation from a NNE-SSW strike offshore Lofoten to a preferred E-W trend at 68°30’ N is also evident in the base Cretaceous time structural map published by Mokhtari & Pegrum (1992). In the offshore area the accommodation zone coincides with a distinct E-W trending magnetic gra-dient that can be traced westward from Hadselfjord, seen as

a magnetic high in the north and a low in the south (Verhoef et al. 1996; Olesen et al. 2002). A seismic refraction profile across Hadselfjord shows a low-velocity zone below the low-est topographic part of the fjord suggesting fault zone mate-rial at depth (Løseth & Tveten 1996), and indicates that the accommodation zone at this location is associated with a zone of weakness at basement level. A prominent E-W basement line ament has been mapped along the continua-tion of Hadselfjord and to the east (Gabrielsen & Ramberg 1979; Gabrielsen et al. 2002; Bergh et al. 2007), and east of the study area Ofotenfjorden forms a conspicuous E-W oriented bathymetric lineament.

The Late Permian-Early Triassic rift episode and paleogeography

Plate reconstructions demonstrate that Greenland was close to Mid Norway and Central northern Norway in the Perm-ian (Mosar et al. 2002), before the episodes of crustal stretch-ing documented offshore Mid Norway (Fig. 3). Late Perm-ian-Early Triassic rifting is evident as a series of important basins created above major normal faults and is well-docu-mented in the coastal area of East Greenland (Surlyk 1990; Stemmerik et al. 2000; Hartz et al. 2002; Wignal & Twitchett 2002; Hamann et al. 2005), western Barents Sea (Berglund et al. 1986; Wood et al. 1989; Gabrielsen et al. 1990; Gud-laugsson et al. 1998), offshore Mid Norway (Blystad et al. 1995; Brekke et al. 2001; Osmundsen et al. 2002; Müller et al. 2005) and in the northern North Sea (Leirvik et al. 1989; Steel & Ryseth 1990; Færseth 1996).

South of the study area, thick pre-Middle Triassic sedi-ments are localized at depth in the Froan and Helgeland basins (Blystad et al. 1995; Osmundsen et al. 2002; Müller et al. 2005). These NE-SW oriented basins (Fig. 1) exhibit half-graben architecture and are interpreted to contain Late Permian-Early Triassic syn-rift successions as thick as 2000-3000 m (Blystad et al. 1995; Osmundsen et al. 2002; Müller et al. 2005). A total of 750 m of Upper Permian-Lower Trias-sic sediments is proven by two wells at the east margin of the Helgeland Basin (Bugge et al. 2002). The basin-bounding faults are pre-Middle Triassic as they pre-date late Ladinian-Carnian evaporites, and at depth the faults sole into a flat-lying reflection sequence situated between 6 and 8 s TWT (Osmundsen et al. 2002). Müller et al. (2005) subdivided the syn-rift succession into two parts; Upper Permian sedi-ments were interpreted as an initial rift stage whereas Lower Triassic sediments were deposited during a rift climax stage (Fig. 3). As discussed below, the thick Permo-Triassic sed-imentary succession offshore Mid Norway was deposited in a basin surrounded by basement highs. On the Nord-land Ridge (Fig. 1), which represented a basement high to the NW, Upper Permian carbonates resting directly on basement rocks are interpreted as platform or ramp depos-its with build-ups and reefs (Müller et al. 2005). SE of the basin, the Frøya High in the footwall of the Froan Basin, was part of a basement high during Late Permian-Early Triassic


26

rifting, and syn-rift sediments of this generation are absent above the high (Blystad et al. 1995; Osmundsen et al. 2002).

Late Permian-Early Triassic rifting in the Lofoten-Vest-erålen area

Authors have advocated Permo-Triassic rifting and sedi-mentation in the Lofoten–Vesterålen area. In paleo-geo-graphic reconstructions (Müller et al. 2005; Nystuen et al. 2006), the present Lofoten–Vesterålen shelf was interpreted as an area with Late Permian-Early Triassic syn-rift sedi-mentation, being part of a large intra-continental and con-tinuous rift basin between Norway and Greenland. How-ever, Permian sediments are not proven in the shelf area, and Triassic sediments are absent off Vesterålen (Hansen et al. 1992) and in the basin onshore Andøya (Dalland 1975, 1981). Steltenpohl et al. (2004), observed brittle structures and fabrics in basement rocks in SW Lofoten associated with E-W trending and low-angle faults, which they inter-preted as the result of a significant Permian extension event. However, E-W orientated dip-slip faults are not consistent with the regional and well-established E-W extension dur-ing the Late Permian-Early Triassic rift episode (e.g. Mosar et al. 2002). In the shelf area, workers have mapped the local thickening of pre-Cretaceous sediments in the hang-ing wall of some NNE-SSW striking faults, which have been regarded as evidence of Late Paleozoic-Early Mesozoic tec-tonism as well as a significant thickness of pre-Jurassic sedi-ments (Tsikalas et al. 2001; Bergh et al. 2007). A critical issue regarding the timing of initiation of this potential oldest fault group is the age of the expanded sequence of pre-Cretaceous sediments. The sediments have been interpreted as Upper Paleozoic-Lower Mesozoic (Tsikalas et al. 2001; Faleide et al. 2008), and as Permian-Jurassic (Bergh et al. 2007). As faults were active during Jurassic rifting, the expanded pre-Cretaceous sequences are likely to contain unsubstantiated thicknesses of Jurassic sediments and accordingly the actual thickness of Triassic and possibly older sediments remain uncertain.

Beneath the Cretaceous Vestfjorden Basin (Fig. 1), the over-all half-graben architecture at depth has been attributed to pre-Cretaceous rifting, and a post-Devonian to pre-Cre-taceous age has been inferred for an expanded sequence of sediments above basement (Fig. 2) (Jørgensen & Navrestad 1981; Bøen et al. 1984; Blystad et al. 1995; Mjelde et al. 1996; Steltenpohl et al. 2004). Faults associated with the Vestfjor-den Basin project on to land into the NNE striking Vestf-jorden-Vanna Fault Complex (Andresen & Forslund 1987; Gabrielsen et al. 2002) that constitutes the contact between a Precambrian terrain to the WNW and the Caledonian nappe sequence to the ESE (Sigmond et al. 1984). Permian activity has been reported along this fault complex (Andre-sen & Forslund 1987; Olesen et al. 1997).

The modern geophysical signature of the Lofoten Ridge is considered to reflect Late Cretaceous to Tertiary extension and uplift (e.g. Hansen et al. 1992; Løseth & Tveten 1996). However, isotopic data suggest that uplift took place also in

the Permian (Steltenpohl et al. 2004). Observations from the study area indicate that pre-Triassic uplift to create a base-ment high did not only affect the Lofoten-Vesterålen archi-pelago, but also major parts of the present shelf area. The oldest sediments encountered in this area are Early Triassic in age, and were drilled west of Lofoten (Fig. 4). The cored succession that rest directly onto the Precambrian basement was c. 175 m thick, and interpreted as the erosional product from the exposed basement (Hansen et al. 1992). In the shelf area off Vesterålen and in the basin onshore Andøya, Middle Jurassic sediments overlie basement and in both areas the thin conglomerate at the base was derived from the under-lying basement (Dalland 1975; Hansen et al. 1992) indi-cating that Triassic sediments were not deposited in these areas. The eroded and smooth top basement surface, NW of the Vestfjorden Basin, shows deep weathering (e.g. Sturt et al. 1979; Hansen et al. 1992; Mørk et al. 2003) and indi-cates that basement rocks were peneplained and sub-aerially exposed for a long time period. Later, the area was flooded and sedimentation was initiated in Triassic to Middle Juras-sic times.

The Lofoten-Vesterålen area in a regional paleo-geographic context

Based on the arguments above, it follows that the expanded pre-Cretaceous sedimentary succession beneath the Cre-taceous Vestfjorden Basin is the only indication of a large Permo-Triassic rift basin in the study area. The Vestfjorden, Helgeland and Froan basins might have constituted an en échelon array of rift-basins in the near shore area (Fig. 5), as the result of a well-defined NE-SW structural grain where the basins are located that was oblique to the E-W directed extension. In this context the Lofoten-Vesterålen archipel-ago and the shelf area are interpreted as parts of a regional basement high (Fig. 5) that continued to the SW. West of the study area, and presently located along the conjugate Green-land margin, large basins bounded by E to ESE-dipping faults developed during the Late Permian-Early Triassic rift episode (Surlyk 1990; Hartz et al. 2002; Hamann et al. 2005).

The paleo-topography in the study area combined with observations in adjoining areas suggest that the present day western coastline of Norway reflects the Permo-Trias-sic structural configuration, and was caused by the interac-tion between Late Permian-Early Triassic E-W extension and the regional N-S and NE-SW structural grain (Fig. 1). The Permo-Triassic evolution of the study area should be seen as a part of this regional paleo-geographic framework. The study area and the Møre margin are characterized by a prominent NE-SW structural grain (Fig. 1), and in both areas there is no compelling evidence of large-scale Late Permian-Early Triassic faulting. The two areas are inter-preted as peneplained basement paleo-highs (Fig. 5), above which Triassic sediments are thin or absent (Fig. 6). Adja-cent to these basement highs, Permo-Triassic successions achieved thicknesses of 2500-4000 m relatively close to the present day coastline (Figs. 5 and 7) (e.g. Steel & Ryseth 1990; Blystad et al. 1995; Færseth et al. 1995a; Osmundsen


27

et al. 2002; Gómez et al. 2004). The east margin of these rift basins are represented by Late Permian-Early Triassic fault zones with a predominant N-S orientation. Across the NE-SW trending basement highs, the east margin of Late Permian-Early Triassic rift basins was shifted progressively to the east; from the Øygarden Fault Complex off West Nor-way (Figs. 5 and 7a), via major faults offshore Nordland (Fig. 7b) to the Ringvassøy-Loppa Fault Complex (Gabrielsen et al. 1990) in the western Barents Sea.

The two Permo-Triassic basement highs might be termed relay zones or high relief accommodation zones. The north-ern basement high encompassed NE-SW oriented structural elements in the study area, such as the Lofoten-Vesterålen archipelago and the Utrøst Ridge, but also a prominent

structural feature to the SW, i.e. the Utgard High (Fig. 1), which is a large basement high buried beneath the Creta-ceous Vøring Basin (Osmundsen et al. 2002; Johannesen & Nøttvedt 2006). The basement high to the south was asso-ciated with the Møre-Trøndelag Fault Complex, which is a prominent and long-lived structural feature both onshore and offshore Norway (Fig. 1) (e.g. Blystad et al. 1995). This basement high encompassed the Frøya High, the SE Møre Basin (Fig. 6a) and also the continuation to the south along the east flank of the Jurassic Sogn Graben (Fig. 1) where thin Triassic sediments onlap the Precambrian basement to the east (Færseth et al. 1995a; Færseth 1996). This regional base-ment high acted as a very important accommodation zone also during the Jurassic crustal stretching, when the locus of rifting was shifted from the Sogn Graben in the northern North Sea and some 100 km to the east to the oppositely dip-ping Rås Basin offshore Mid Norway, which is bounded to the east by the N-S trending Klakk Fault Complex (Fig. 1).

The regional paleo-geography (Fig. 5) is representative of a long-lived structural setting with deposition of Triassic and older sediments in a restricted basin between the two NE-SW oriented basement highs. Upper Permian coarse clastic sediments have been drilled at the east margin of the Helgeland Basin, which proved that fault-bounded basins were located close to the Norwegian coastline at this stage, with the Baltic Shield as the main sediment source area (Bugge et al. 2002). Based on seismic mapping, thick Upper Paleozoic sediments are interpreted to be deposited offshore Mid Norway (Fig. 7b) (Blystad et al. 1995; Osmundsen et


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Figure 5. The regional Late Permian-Early Triassic syn-rift paleo-geography is interpreted to reflect the interplay between E-W crustal stretching and the prevailing N-S and NE-SW basement grain. The Lofoten-Vesterålen area and the Møre margin are represented by two NE-SW trending basement highs that linked areas dominated by N-S trending fault zones and associated deep Permo-Triassic basins close to the Norwegian coastline. Late Devonian extensional shear zones are also shown. MTFC: Møre-Trøndelag Fault Complex and HSZ: Hardangerfjord Shear Zone. ØFC: Øygarden Fault Complex represents major Late Permian-Early Triassic faults striking N-S.

Figure 6. The two profiles across NE-SW trending Permo-Triassic basement highs exhibit good correlation regarding Permo-Triassic and Jurassic structural development and sedimentation. Location of profiles in Fig. 5 and colours as in Fig. 2. (a) Cross section based on seismic reflection profile GGW-91-418 (Jongepier et al. 1996). Red colour: Thin Triassic sediments above basement in the hanging wall. (b) Cross section based on seismic reflection profile LO-04-88 repre-sents the shelf area NW of Lofoten.

28

al. 2002). Breivik et al. (2011), based on velocity modelling from wide-angle seismic and potential field data, identified a 2-3 km thick succession beneath the Trøndelag Platform, which they interpreted as Devonian sediments. As argued below, this paleo-geography also existed during deposition of Middle Triassic to early Middle Jurassic post-rift sedi-ments (Fig. 3), i.e. over a time period of 75 Ma, following Late Permian-Early Triassic rifting.

The regional model suggested in this account contrasts with previous paleo-geographic reconstructions in which Permo-Triassic sediments, mapped onshore and in near shore areas, have been associated with an intra-continen-tal and continuous rift basin between Norway and Green-land (Dalland 1981; Gage & Doré 1986; Ziegler 1988, 1989; Swiecicki et al. 1998; Brekke et al. 2000; Müller et al. 2005; Nystuen et al. 2006). Instead, it is proposed that major parts of the area now buried beneath the deep Cretaceous Vøring Basin were dominated by huge basement highs above which

Permo-Triassic sediments were thin or absent. The rela-tive thin sedimentary succession between crystalline base-ment and the base Cretaceous beneath large parts of the Vøring Basin (e.g. Skogseid et al. 2000; Raum et al. 2002) indicates that a thick Permo-Triassic succession is unlikely on a regional scale, a notion that is in favour of this model. This model further implies that although comparable sedi-ments were deposited offshore Mid Norway and along the conjugate Greenland margin (e.g. Müller et al. 2005), these two depositional areas did not represent the margins of a large and continuous Permo-Triassic rift basin with its axis beneath the Cretaceous Vøring Basin.

Middle Triassic–Middle Jurassic paleo-geography and the post-rift sedimentation

Middle Triassic to earliest Middle Jurassic sediments off-shore Mid Norway are regarded as post-rift sediments (e.g. Blystad et al. 1995), deposited during a time period of c. 75 Ma (Fig. 3). During this period, the Lofoten–Vesterålen archipelago and the shelf area apparently remained parts of a regional basement high (Figs. 6b and 8). In the study area, sediments of this generation are only proven in a shal-low core west of Lofoten. A c. 45 m thick Lower Jurassic sequence represents fluvial channels and floodplain deposits on an upper delta plain (Hansen et al. 1992). To the south, the Frøya High, the SE part of the Møre Basin and the east flank of the Sogn Graben were parts of a basement high, above which post-rift sediments are thin or absent (Figs. 6a and 8) (Blystad et al. 1995; Færseth et al. 1995a; Jongepier et al. 1996).

The area between the two NE-SW oriented basement highs, which at Mesozoic level is represented by the Trøn-delag Platform and the Halten and Dønna terraces, appears as a restricted basin at this stage of development (Fig. 8). Middle Triassic to earliest Middle Jurassic sedi-ments (Fig. 3) have a thickness of 2500-3000 m in most of the basin (Fig. 7b)(Blystad et al. 1995; Osmundsen et al. 2002; Gómez et al. 2004; Müller et al. 2005). Middle Trias-sic (Anisian-Ladinian) sediments, with a drilled thickness that may exceed 1000 m, mark the transition from a domi-nantly marginal marine to a continentally-dominated facies (Müller et al. 2005). The Late Ladinian-Carnian evaporite sequence, which is restricted to this basin area, has a maxi-mum recorded thickness close to 900 m. A 600-800 m thick Norian-Rhaetian sequence overlie the evaporites, and repre-sents an overall coarsening upward trend from a mudstone-rich lacustrine flood-basin to a sand-dominated fluvial envi-ronment (e.g. Müller et al. 2005). The Early-Middle Jurassic Båt and Fangst groups have a total thickness exceeding 1000 m in most of the basin (Dalland et al. 1988). Fluvial to shal-low marine sandstones of the two groups (Fig. 3) represent the main reservoirs on the Halten and Dønna terraces (e.g. Brekke et al. 1999; Blystad & Søndenå 2005; Østvedt et al. 2005).


Fig. 7 a)

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Færseth Figure 7. Two profiles in areas where thick Permo-Triassic sediments occur close to the Norwegian coastline. Basins are bounded by major Late Permian-Early Triassic N-S striking faults. Location of profiles in Fig. 5. Green colour represents Cretaceous sediment, blue colour Jurassic sediments and red colour Triassic sediments. Other colours as in Fig. 2. (a) East margin of the northern North Sea. (Steel & Ryseth 1990; Færseth et al. 1995a). The dashed line (top Scythian) is interpreted as the syn-post rift transition. Well 31/6-1 was drilled into basement and is projected to the cross section. (b) Cross section offshore mid Norway (Blystad et al. 1995). The dashed line intra-Triassic represents the top of latest Middle-Upper Triassic salt. Lilac colour is interpreted as Upper Paleozoic sediments (Blystad et al. 1995; Osmundsen et al. 2002).


The Jurassic rift episodeThe North Atlantic region experienced significant crustal stretching in the Middle to Late Jurassic, and at the end of the Jurassic period deep intra-continental rift basins existed from the Rockall Trough to the SW Barents Sea, with a sep-arate branch into the North Sea (e.g. Ziegler 1988; Lundin & Doré 1997; Roberts et al. 1999). South of the study area, Jurassic extension structures are described from the Halten and Dønna terraces (e.g. Blystad et al. 1995; Corfield et al. 2001). A major part of the proven hydrocarbon resources offshore Mid Norway is located on the two terraces in traps formed by Jurassic extension, these have Jurassic sandstones reservoirs and hydrocarbons mainly generated from Juras-sic source rocks (e.g. Brekke et al. 1999; Blystad & Søndenå 2005; Østvedt et al. 2005). In the Lofoten–Vesterålen shelf area, Jurassic sediments presently occur at depths compa-rable to the Halten and Dønna terraces, although they have experienced a deeper burial (Hansen et al. 1992; Riis & Fjeldskaar 1992; Løseth & Tveten 1996).

A major part of the study area is interpreted as a NE-SW oriented, eroded and peneplained, Permo-Triassic basement high, similar to the the Møre margin (Figs. 5 and 8). Juras-sic extension and normal faulting resulted in the fragmen-tation of the basement highs, and the two areas show com-mon characteristics associated with the Jurassic rift episode. Hence, a major fault-block at the SE margin of the Møre Basin, drilled by three wells, represents a useful analogue (Fig. 6) both regarding the time of Jurassic rift initiation and termination as well as the style of deformation. A NE-SW striking fault zone at c. 63° N, which is part of the Møre-Trøndelag Fault Complex, has a vertical offset of 5-6 km and separates the Slørebotn Sub-basin from a narrow base-ment platform to the SE (Fig. 1). Like the shelf area along the Lofoten-Vesterålen margin, the block-bounding master faults have listric geometries with intra-basement detach-ments, and a thin Triassic succession overlie Precambrian basement in the hanging wall. In the crestal part of the fault-block, Triassic sediments are absent and a Bajocian-Batho-nian sandstone that represents an initial rift stage over-lies basement (Fig. 6a), a configuration similar to the shelf area off Vesterålen (Hansen et al. 1992) and in the basin on Andøya (Dalland 1975, 1981). Well 6205/3-1R penetrated and partly cored Lower Cretaceous-Upper Jurassic sedi-ments in the hanging wall and provides excellent data for the reconstruction of fault-block rotation and thereby to constrain the duration of Jurassic rifting. By combining seis-mic interpretation and dip magnitude from the dipmeter log and core measurements, Jongepier et al. (1996) concluded that Jurassic extension that initiated in the Bajocian reached a rift climax stage during the Kimmeridgian-middle Titho-nian, whereas latest Tithonian-Cretaceous sedimentation was characterized by passive infill and the progressive onlap of relief created during the Jurassic rifting.

The drilled Middle Jurassic sandstone along the SE mar-gin of the Møre Basin (Fig. 6a) has good reservoir qual-ity, but there was the lack of commercial accumulations of

Based on sedimentological evidence, including heavy min-eral studies (e.g. Morton & Chenery 2009; Morton et al. 2009), a major basement high existed west of the Halten Terrace during the Early-Middle Jurassic and represented an active and important sediment source area (e.g. Johan-nesen & Nøttvedt 2006). As a consequence, the coarsest grained and most fluvial influenced sediments of this gen-eration are located along the western part of the Halten Ter-race and indicate that this could be close to a western basin margin (Fig. 8). This supports the model introduced above that major parts of the area now buried beneath the Cre-taceous Vøring Basin were dominated by a huge basement high prior to the Jurassic rift episode. The Jurassic extension and normal faulting left the Sklinna Ridge as a remnant of the easternmost part of this basement high in the footwall of the Klakk Fault Complex (Figs. 1 and 8).

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Figure 8. Middle Triassic-early Middle Jurassic post-rift paleo-geography. The Lofoten–Vesterålen archipelago and the shelf area remained parts of a huge NE-SW striking basement high. The Triassic-Lower Jurassic sediments that are relatively thin or absent above the topographic highs have a thickness of 2500-3000 m in the restricted basin surrounded by basement highs. Late Devonian extensional shear zones are shown. MTFC: Møre-Trøndelag Fault Complex and HSZ: Hardangerfjord Shear Zone. ØFC: Øygarden Fault Complex is characterized by major Late Permian-Early Triassic faults striking N-S.


by fjords and sounds (Johannesen & Nøttvedt 2006). This is supported by the sampling of shallow marine Bathonian-middle Callovian sandstone in Vesterålsfjorden NW of Hadseløy (Fig. 4) (Smelror et al. 2001). Middle-Late Juras-sic successions with a total thickness of 370 and 375 m have been documented off Vesterålen (Hansen et al. 1992) and on Andøya (Dalland 1975, 1981), respectively. The Bajocian-middle Callovian sandstone and mudstone, 120 and 125 m thick at the two locations, are in this study interpreted to be deposited during the initial rift stage. These sediments onlap deeply weathered basement rocks (Sturt et al. 1979; Dalland, 1981; Hansen et al. 1992; Mørk et al. 2003) and a thin con-glomerate at the base consists almost entirely of weathered basement showing a local provenance. The Middle Jurassic sediments proven in the study area may correspond to the Bajocian-Bathonian Garn Formation (Fig. 3) (Gjelberg pers. comm.), which is widely distributed offshore Mid Norway. The formation was deposited during the initial rift stage (e.g. Corfield et al. 2001), and the thickness of the formation varies considerably. It is usually less than 100m, but with a maximum of 120m in well 6506/11-3.

Observations from the basin onshore Andøya and from the nearshore area off Vesterålen document progressive deepen-ing of the basin during the Jurassic rift episode. On Andøya, fine to coarse, fluvial to shallow marine sandstone and mud-stone representing the initial rift stage, became overlaid by 250 m of Upper Jurassic sediments that represent shal-low marine prodelta to relatively deep marine anoxic con-ditions (Gjelberg pers. comm.). In shallow wells drilled off Vesterålen, there is a hiatus between the Middle Juras-sic sandstone and the overlying silt to mud sequence (Han-sen et al. 1992). The Upper Jurassic sediments, interpreted to be deposited during a rift climax stage are c. 250 m thick and the uppermost (Tithonian) part of the succession was deposited under deep marine anoxic conditions.

The change from a topographic flat region to topogra-phy characterized by a series of tilted fault-blocks at Meso-zoic levels (Figs. 2 and 9), is in this paper interpreted to result from the Jurassic and Late Cretaceous-Paleocene rift episodes. The influence of Jurassic rifting is clearly differ-ent and stronger NW of the Lofoten-Vesterålen archipel-ago compared to the structuring beneath the Cretaceous Vestfjorden Basin, which is also dominated by much larger structural elements (Fig. 9). Beneath the Cretaceous Rib-ban Basin, major Jurassic fault-blocks are typically 15-25 km wide and bounded by basement-involved faults with throws in the order of 3-3.5 km, but locally reaching a maximum of c. 5 km across the Vesterdjupet Fault Zone (Tsikalas et al. 2001). Block-bounding faults have listric geometries, similar to the Jurassic structuring along the SE margin of the Møre Basin (Fig. 6). This fault style is interpreted to result from the thin pre-rift sediments above basement in the two areas and demonstrates the strong influence of basement hetero-geneities, likely to represent Caledonian thrusts and/or Late Devonian low-angle extensional shear zones.

The major Jurassic faults along the Lofoten-Vesterålen

hydrocarbon. This might be seen as a negative observation regarding the hydrocarbon potential of the shelf along the Lofoten-Vesterålen margin.

Jurassic rifting and structures in the Lofoten-Vesterålen area

With the documented effects of Jurassic crustal stretching on the Halten and Dønna terraces (e.g. Blystad et al. 1995), interpreted to reach a maximum further west beneath the Cretaceous Vøring Basin (e.g. Skogseid et al. 2000; Gómez et al. 2004), Jurassic rifting and large-scale faulting are likely to become important also in the structuring of the Lofoten –Vesterålen margin segment. Shallow wells drilled in the study area (Fig. 4) encountered Triassic to Jurassic sedi-ments above crystalline basement, and the cored stratigra-phy was tied to seismic sections (Hansen et al. 1992). How-ever, due to the moderate quality of the seismic combined with the mostly thin Jurassic and Triassic sequences (Fig. 2), it is impossible to map the presence or the thickness and geometry of Jurassic syn-rift sediments within the study area. Workers have mapped Jurassic sediments as part of an undifferentiated Triassic-Jurassic (Blystad et al. 1995; Løseth & Tveten 1996), Upper Paleozoic-Lower Mesozoic (Tsikalas et al. 2001; Faleide et al. 2008) or Permo-Jurassic (Bergh et al. 2007) succession beneath the Cretaceous Ribban Basin.

Whereas a Middle Jurassic rift initiation is established off-shore Mid Norway (Fig. 3), in the northern North Sea (Færseth et al. 1995b; Færseth & Ravnås 1998) and in the coastal area of East Greenland (e.g. Surlyk 1978; Hartz 2002; Hamann et al. 2005), authors have stated that this rift episode commenced in the Late Jurassic in the Lofoten–Vesterålen shelf area (Mokhtari & Pegrum 1992; Blystad et al. 1995; Doré et al. 1999; Reemst & Cloetingh 2000; Tsikalas et al. 2001; Wilson et al. 2006; Bergh et al. 2007). Due to the expanded and wedge-shaped Cretaceous sediments in hang-ing walls, these authors have further claimed that rifting continued into the Cretaceous. A shallow well at the drill site west of Lofoten (Fig. 4) contained thin (16 m) Bathonian-middle Callovian sediments that overlie the Early Jurassic Åre Formation, and in turn are overlain by late Baremian sediments (Hansen et al. 1992). In this account, the Batho-nian-middle Callovian sediments are interpreted as an ini-tial syn-rift succession, and at this specific location, both the base and the top of the syn-rift sediments are unconformi-ties. Hence, it is proposed that the Jurassic rift initiation was Middle Jurassic, contemporaneous with adjoining areas, and was followed by a Late Jurassic rift climax stage that resulted in maximum rotation of fault-blocks.

During the Middle Jurassic rift initiation, and coeval with a relative sea level rise, the eroded and peneplained Pre-cambrian basement was transgressed, and Løseth & Tveten (1996) interpreted transgression from the north due to the apparent decrease in thickness of the Jurassic unit to the south. Most likely the present Lofoten-Vesterålen archi-pelago became submerged during the initial rift stage with deposition of Middle Jurassic syn-rift sediments in the deep-est parts of the rift topography, which today are represented


by a zigzag pattern along strike (Fig. 9), it is suggested that NNE-SSW striking fault segments of these fault zones were initiated during Jurassic rifting and later became linked by NE-SW to ENE-WSW striking fault segments during the Late Cretaceous-Paleocene rift episode, to result in the over-all NE-SW orientation of the fault zones.

Bergh et al. (2007) assumed a WNW-ESE directed Permo-Jurassic extension, orthogonal to NNE-SSW striking faults of the oldest generation. Wilson et al. (2006) using digi-tal mapping to link onshore and offshore structures in the Lofoten area, suggested a NW-SE extension associated with Late Jurassic -Early Cretaceous rifting. These authors sep-arated the Lofoten Ridge into three domains character-ized by variations in trend and structural complexity. In the northern domain, which encompasses Vestvågøy and Aust-vågøy (Fig. 4), the ridge becomes ENE-WSW trending. As a consequence of the inferred NW-SE extension, oblique to the structural trend of this domain, they stated that fault-ing in this area is dominated by transtensional dip-slip and oblique-slip movements. In this account, the orientation of major normal faults (Fig. 4) is interpreted to be consistent

margin strike consistently NNE-SSW and oblique to the NE-SW trend of the overlying Cretaceous Ribban Basin and to the NE-SW striking fault zones having a signifi-cant downthrow resulting from the Late Cretaceous-Paleo-cene rift episode (Fig. 4). The Jurassic fault-blocks define an array of NE-stepping basins and ridges, confined between the Lofoten-Vesterålen archipelago and the Utrøst Ridge (Fig. 9). The Jurassic master faults are parallel with a main lineament trend onshore Lofoten-Vesterålen(Gabrielsen et al. 2002; Bergh et al. 2007) and with narrow fjords and sounds with this orientation, likely to have hosted normal faults (Smelror et al. 2001; Wilson et al. 2006; Bergh et al. 2007). Along the east coast of Andøya, NNE-SSW striking faults, down-to-the east, represent the western boundary of exposed Jurassic-Cretaceous sediments. The offshore exten-sion and the deepest part of this basin are below Andfjor-den to the east of Andøya, whereas the island became part of a major Jurassic basement horst (Fig. 9) (Dalland 1975; Johannesen & Nøttvedt 2006). Beneath the Ribban Basin, some major NNE-SSW striking Jurassic faults tend to merge into the NE-SW oriented fault zones that bound the Lofo-ten and Utrøst ridges. As these fault zones are characterized

Andø

ya

Tromsø

Harstad

Bodø

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inVest

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Basins resulting fromJurassic rifting

Thin Jurassic - Triassic sediments

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Ridge

Basin

Fig. 10a

Fig. 11

Fig. 10b

Andf

jord

en

Mar

mæ

la

Spur

High

Accommodation zone

Figure 9. Jurassic structural elements offshore Lofoten-Vesterålen. Fault-blocks boun-ded by basement-involved faults striking consistently NNE-SSW are parallel with narrow fjords and sounds with this orientation. The NE-SW oriented fault zones bounding the Lofoten Ridge and the Utrøst Ridge are interpreted as the result of both Jurassic and Late Cretaceous-Paleocene rif-ting. Major Jurassic faults ter-minate within an accommoda-tion zone at 68°30’ N and the dip of the faults as well as the tilt of major Jurassic rift seg-ments change across this zone.


variations in structural style to an inferred and NNW-SSE striking Jennegga transfer zone (Eldholm et al. 1979; Mokh-tari & Pegrum 1992; Løseth & Tveten 1996; Tsikalas et al. 2001, 2005; Olesen et al. 1997, 2002). Beneath the SW part of the Ribban Basin, large Jurassic fault-blocks are tilted consis-tently ESE (Fig. 10a) and the tilt at Jurassic and deeper levels is some 10-15° (Løseth & Tveten 1996; Tsikalas et al. 2001). To the NE, however, there is a change across the accommo-dation zone and fault-blocks are tilted 16-25° WNW (Løs-eth & Tveten 1996; Tsikalas et al. 2001).The cross section (Fig. 10b) reveals the structure as a low relief accommoda-tion zone census Rosendahl (1986) due to partly overlap-ping and oppositely dipping normal faults.

The style of deformation, characterized by significant rota-tion of fault-blocks and the large heave across major normal faults (Fig. 10), demonstrate that Jurassic faults along the Lofoten–Vesterålen margin accommodated large magnitude crustal stretching in the same way as Jurassic faults beneath the deepest Cretaceous sub-basins in the Vøring Basin (e.g. Skogseid et al. 2000; Færseth & Lien 2002; Osmundsen et al. 2002). Pronounced tilting of fault-blocks in the study area, following the rift climax stage resulted in the tectonic relief, and significant water depths may have existed in deep parts of half-grabens, similar to Jurassic rift-basins offshore Mid Norway (Nelson & Lamy 1987; Jongepier et al. 1996; Færs-eth & Lien 2002) and in the northern North Sea (Young 1992; Rattey & Hayward 1993; Færseth et al. 1995b). In half-grabens along the Lofoten-Vesterålen margin Jurassic syn-rift sediments, together with eventual older sediments, rep-resent only a minor constituent of the total sedimentary infill (Figs. 2 and 10). This is consistent with observations in several rift basins, which typically contain sediment-starved, thin syn-rift successions deposited in deep-water condi-tions (e.g. Mutter & Larsen 1989; Prosser 1993; Færseth et al. 1995b; Ravnås & Steel 1998; Færseth & Lien 2002).

Cretaceous sedimentation and the influence of the inherited Jurassic rift architecture

Thick Cretaceous sediments were deposited in the Vestf-jorden and Ribban basins and also in the deep basin NW of the Utrøst Ridge (Fig. 2). The large basins flanking the Lofo-ten Ridge, but also second-order structures in the study area exhibit wedge-shaped Cretaceous geometries (Figs. 10 and 11), which have been considered as evidence of Early Cre-taceous extension and normal faulting (Mokhtari & Pegrum 1992; Løseth & Tveten 1996; Lundin & Doré 1997; Doré et al. 1999; Smelror et al. 2001; Tsikalas et al. 2001; Wilson et al. 2006; Bergh et al. 2007). As argued in the following, Creta-ceous sediments are in this account interpreted to represent a post-rift stage with the passive infilling of half-grabens, which following Jurassic rifting and syn-rift sedimenta-tion remained deep-water basins. Due to water depth, Cre-taceous sediments accumulated slowly and are assumed to be deposited predominantly as mudstone and claystone (e.g. Lien 2005). Similar development, with a low sedimentation

with the extension directions derived from plate recon-structions of the Norwegian-Greenland Sea (Mosar et al. 2002; Torsvik & Cocks 2005). This implies an E-W directed Jurassic extension, slightly oblique to the NNE-SSW orien-tated major faults of this generation that was governed by a prominent basement grain (Fig. 9). If Jurassic extension was NW-SE directed, as claimed by previous authors (e.g. Tsika-las et al. 2001, Wilson et al. 2006; Faleide et al. 2008), it is likely that the prominent NE-SW to ESE-WSW basement grain in the area should be utilized for major Jurassic nor-mal faults. However, this structural grain became dominant during Late Cretaceous-Paleocene rifting (Fig. 4) when the stretching direction changed to become NW-SE directed (Mosar et al. 2002; Torsvik & Cocks 2005).

As described above an E-W trending accommodation zone is located at 68°30’ N, and this zone became very impor-tant during Jurassic rifting (Fig. 9). The accommodation zones acted as a rift propagation barrier and major Jurassic faults terminate within this zone and the dip of faults and the tilt of Jurassic rift segments change across the zone (Fig. 10). Authors have previously attributed these along-strike

Færseth Fig. 10

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MarmælaSpur

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LofotenRidge

Ribban BasinLRAZ

Veste

rdjup

et F

ault

Zone

a)

b)

NW SE

Figure 10. The two cross sections demonstrate the change in rift geometry across the accommodation zone at 68°30’ N. Location of cross sections in Fig. 9 and colours as in Fig. 2. (a) Beneath the SW part of the Cretaceous Ribban Basin, large Jurassic fault-blocks are tilted to the ESE. The cross section is based on seismic reflection pro-file LO-04-88. (b) NE of the accommodation zone, fault-blocks are tilted to the WNW. The cross section is based on seismic reflection profile LO-18-86. This cross section shows the structure as a low relief accommo dation zone (LRAZ) due to partly overlapping and oppositely dipping normal faults.


i.e. some 45-55 Ma after the cessation of Jurassic rifting. Variation in sedimentation rate, type of sediment deposited and the thickness distribution were interpreted to reflect variation during infilling of the inherited Jurassic rift archi-tecture, but also responses to post-rift thermal subsidence, sediment loading and differential compaction during burial. Several authors have demonstrated that ignoring the impor-tance of the remnant rift topography in the creation of over-all wedge-shaped geometries and also the possibility of divergent reflector configurations induced by compaction, can lead to overestimation of the duration of active faulting (Bertram & Milton 1989; Prosser 1993; Færseth et al. 1995b; Doglioni et al. 1998).

Patterns similar to those characteristic of the Cretaceous infilling of half-grabens in the Vøring Basin (Færseth & Lien 2002; Gómez et al. 2004) as well as in the northern North Sea (Færseth et al. 1995b; Gabrielsen et al. 2001) are evi-dent also in the study area (Fig. 11). As stated above, Jurassic and older sediments occupy only a minor part of the total amount of sediments in half-grabens along the Lofoten-Vesterålen margin (Figs. 2, 10 and 11). This is also illustrated in a number of cross-sections published from the study area (Mokhtari & Pegrum 1992; Blystad et al. 1995; Løseth & Tveten 1996; Tsikalas et al. 2001; Bergh et al. 2007; Faleide et al. 2008). The Early-Middle Cretaceous sediments, depos-ited in half-grabens with a significant water depth assumed the wedge-shaped geometry of the remnant Jurassic rift topography. In this study, onlap surfaces against fault-scarps later deformed by compaction (Fig. 11) are interpreted as the progressive sedimentary infill of these deep-water basins. Onlap of Early Cretaceous reflectors along pre-exist-ing fault scarps is well-displayed in cross-sections published by previous authors, both as seismic sections (e.g. Bergh et al. 2007, their Fig. 8) and profiles presented as line drawings (Tsikalas et al. 2001; Wilson et al. 2006; Bergh et al. 2007), although these authors have interpreted the Early Creta-ceous infilling package as syn-rift deposits. It was not until the Late Cretaceous that the Jurassic rift topography in the study area was levelled causing the different depocentres, initially represented by NNE-SSW oriented half-grabens, to merge into a wider and NE-SW trending depocentre, i.e. the Ribban Basin.

The Campanian–Paleocene rift episodeThe Jurassic rift architecture NW of Lofoten–Vesterålen was modified by post-Jurassic faulting (Fig. 4). This is seen as rejuvenation of dip-slip movements within major fault zones, which were initiated during the Jurassic rifting, but also as faults that transect large Jurassic fault-blocks to cre-ate smaller compartments. The Lofoten Ridge as a nar-row and uplifted basement horst flanked by two large and oppositely tilted Cretaceous half-grabens (Fig. 2) is a struc-tural configuration that is partly attributed to post-Jurassic crustal stretching. A latest Cretaceous-Early Tertiary age has generally been assigned to young generations of faults both onshore and offshore Lofoten–Vesterålen (Mokhtari

rate and the passive infilling of half-grabens during the Early-Middle Cretaceous has been interpreted in the Vøring Basin (Færseth & Lien 2002; Gómez et al. 2004), along the SE margin of the Møre Basin (Nelson & Lamy 1987; Jonge-pier et al. 1996) and in the northern North Sea (Færseth et al. 1995b; Gabrielsen et al. 2001).

In the Vøring Basin, Cretaceous sediment thicknesses 7-9 km at the most have been mapped in some of the sub-basins. Upper Cretaceous as well as Paleogene deep-marine sandstone reservoirs, sourced from East Greenland (e.g. Lien 2005), have been in focus for exploration over the last 15 years within this basin. The Cretaceous evolution of the Vøring Basin is very controversial and a number of tectonic events are interpreted to have influenced the area, primar-ily involving extension, but also compression and strike-slip (Gowers & Lunde 1984; Price & Rattey 1984; Caselli 1987; Ziegler 1988; Grønlie & Torsvik 1989; Blystad et al. 1995; Bjørnseth et al. 1997; Lundin & Doré 1997; Brekke et al. 1999; Doré et al. 1999; Gabrielsen et al. 1999; Pascoe et al. 1999; Brekke 2000; Skogseid et al. 2000; Brekke et al. 2001; Osmundsen et al. 2002; Fjellanger et al. 2005; Faleide et al. 2008). The pronounced thickness variation of Cretaceous sediments, wedge-shaped sedimentary bodies, onlap sur-faces and the observation that the base Cretaceous reflec-tion is offset by large faults have been regarded as evidence of Early and Middle Cretaceous tectonism.

In contrast to previous interpretations, Færseth and Lien (2002) argued that the Cretaceous sedimentation in the Norwegian Sea, following Jurassic rifting and until the onset of the Campanian-Paleocene rift episode, represented a post-rift thermal subsidence stage, i.e. a time period of c. 60 Ma characterized by overall tectonic quiescence (Fig. 3). This interpretation was supported in a later contribution from the area (Gómez et al. 2004). The model introduced by Færseth and Lien (2002) included significant tectonic relief and water depths at the Jurassic-Cretaceous time transition, and as a consequence the topography created during Juras-sic rifting was not levelled until the Cenomanian-Santonian,

Færseth Fig. 11

TWT

(s)

0

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UtrøstRidge

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ApAl

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NW SE

Figure 11. Lower Cretaceous sediments onlap the fault scarp to the SE and sediments of this generation are interpreted as the passive infill of a half-graben that was characterized by significant tectonic relief and water depth following Jurassic rifting. The cross section is based on seismic reflection profile LO-32-87. Location of cross sec-tion in Fig. 9 and colours as in Fig. 2.


Eldholm et al. 1989; Skogseid et al. 2000). The NW part of the Vøring Basin was totally dominated by the extrusion of basaltic lavas at this stage of its evolution, as expresed by seaward-dipping reflector sequences, but with a decrease in magma volume to the NE across the Bivrost Lineament and into the study area (Eldholm & Grue 1994; Tsikalas et al. 2001; Faleide et al. 2008). The Eocene epoch reflects the early passive margin evolution characterized by differential subsidence caused by thermal cooling, sediment loading and compaction (e.g. Tsikalas et al. 2001; Lien 2005).

Sediments in the continental shelf area along the Lofoten-Vesterålen margin are uplifted and eroded as shown by a regional sub-crop map (Sigmond 1992). The uplift and ero-sion were post-Eocene as sediments of this age are trun-cated. In the study area a late uplift is mapped as a NE-SW elongated dome with a maximum width of c. 70 km (Fig. 12). In map view this structure largely coincides with the Cretaceous high outlined by Blystad et al. (1995) (Fig. 1). However, in the present account this uplift is interpreted as a post-Eocene compressional structure. A major part of the Tertiary-Upper Cretaceous sedimentary fill of the Rib-ban Basin was uplifted and truncated at or near the seabed along the SE flank of the dome (Fig. 11). Also at the SW ter-mination of the structure, Tertiary-Upper Cretaceous sedi-ments, which are thick in the northern Træna Basin, became uplifted and successively truncated. The uplift is well-dis-played in a base Cretaceous time structural map west of Lofoten (Mokhtari & Pegrum 1992), and is also evident in several cross-sections presented by Tsikalas et al. (2001) and Bergh et al. (2007). However, these authors have not com-mented on the origin of this structure.

& Pegrum 1992; Blystad et al. 1995; Løseth & Tveten 1996; Doré et al. 1999; Tsikalas et al. 2001; Bergh et al. 2007). However, in the absence of Upper Cretaceous-Tertiary sed-iments over major parts of the study area the exact age of faulting cannot be categorically stated.

In the Vøring Basin, the Cretaceous post-rift development was terminated by a new rift episode initiated in the intra-Campanian at 83-80 Ma (Færseth & Lien 2002; Ren et al. 2003), which culminated with the final opening of the North Atlantic 54-55 Ma ago (Fig. 3) (e.g. Eldholm et al. 1989). In contrast to the Vøring Basin, the break-up along the Lofo-ten-Vesterålen margin segment occurred on the SE flank of the wide Cretaceous basin between Norway and Greenland. As the study area was located relatively close to the future continent break-up, significant crustal stretching and fault activity are anticipated. As a corollary, it is considered likely that observed post-Jurassic faulting is related to the Campa-nian-Paleocene rift episode (Tsikalas et al. 2001).

As illustrated in Fig. 4, faults in the shelf area, interpreted to result from Campanian-Paleocene rifting, strike NE-SW to ENE-WSW and the major faults dip NW. The faults with the largest throw bound the Lofoten Ridge, and also represent the boundary between the Utrøst Ridge and the deep basin to the NW (Fig. 2). These faults are clearly oblique to the NNE-SSW striking Jurassic faults. Accordingly, prominent base-ment lineament trends oriented NNE-SSW and NE-SW, evi-dent both onshore and offshore Lofoten-Vesterålen (Gabri-elsen & Ramberg 1979; Olesen et al. 1997, 2002; Gabrielsen et al. 2002; Bergh et al. 2007), were utilized during the Juras-sic and Campanian-Paleocene rift episodes, respectively. A preferred NE-SW to ENE-WSW orientation for Campan-ian-Paleocene faults is consistent with the change to the NW-absolute plate directions around 85 Ma (Torsvik et al. 2001; Mosar et al. 2002; Torsvik & Cocks 2005).

Regarding the hydrocarbon potential of the shelf area, the Campanian–Paleocene rift episode might have been of sig-nificant importance. Due to proximity to the continent break-up and the associated magmatic activity (Skogseid & Eldholm 1989; Eldholm & Grue 1994; Tsikalas et al. 2001), a high paleo-geothermal gradient is anticipated. Hansen et al. (1992) suggested a paleo-geothermal gradient of 50° C/km in this area. As a consequence, a relatively early matura-tion of potential Jurassic source rocks is likely, which might be detrimental to the present hydrocarbon potential of the shelf area.

Tertiary evolution in the shelf area NW of Lofoten-Vesterålen

The rift-drift transition between Norway and Greenland occurred in the earliest Eocene prior to anomaly 24B (Fig. 1). The onset of crustal separation was associated with extensive magmatic activity (Skogseid & Eldholm 1989) and rapid thermal subsidence (Bukovics & Ziegler 1985;

Lofote

n Rid

ge

Nordland R

idge

Figure 12. A NE-SW elongated dome along the Lofoten–Vesterålen margin is interpreted to result from Oligocene-Miocene compression following the continenalt break-up to the NW.


SummaryPost-Caledonian structural elements onshore and offshore Lofoten-Vesterålen developed due to rift episodes character-ized by E-W and NW-SE directed extension and the inter-action with the prevailing NNE-SSW and NE-SW basement grain. The basement zones of weakness, close to orthogonal as well as oblique to the stretching direction were variably utilized during the successive rift episodes.In the shelf area along the Lofoten-Vesterålen margin there is no compelling evidence of Late Permian-Early Trias-sic large-scale faulting. This contrasts with adjoining areas where a Late Permian-Early Triassic extension episode became very important and is well-documented. Also in contrast to well-explored areas offshore Mid Norway, Tri-assic sediments are relatively thin and in places completely absent due to non-deposition. Within the regional Late Permian-Early Triassic paleo-geographic setting, the Lofo-ten-Vesterålen archipelago and the shelf are interpreted as parts of a major NE-SW oriented basement high that linked N-S trending fault zones and associated deep Permo-Tri-assic basins, respectively north and south of the study area. This eroded, peneplained and deeply weathered Precam-brian basement high was later transgressed and progres-sively covered by thin Triassic to Middle Jurassic sediments. The rift topography and the expanded thickness of pre-Cre-taceous sediments beneath the Vestfjorden Basin, appear as the only indications of a potential major Late Permian-Early Triassic rift basin in the study area. On a regional scale, the Vestfjorden, Helgeland and Froan basins might have consti-tuted an en échelon array of important Late Permian-Early Triassic rift-basins in the near shore area.

The Middle Triassic to earliest Middle Jurassic succession is regarded as post-rift sediments. Relatively thin sediments of this generation in most of the study area demonstrate that the Lofoten-Vesterålen archipelago and the shelf remained parts of a NE-SW oriented basement high also at this stage of evolution. This contrasts with areas offshore Mid Norway, where post-rift sediments deposited in a restricted basin have a thickness of 2500-3000 m.

The change from a topographic smooth and eroded top basement surface to topography dominated by a series of tilted fault-blocks and uplifted basement blocks, typ-ical of the study area, is interpreted to result from Jurassic and Campanian-Paleocene rift episodes. Pronounced rota-tion of fault-blocks and the large heave across major normal faults, demonstrate that Jurassic faults along the Lofoten–Vesterålen margin accommodated large magnitude crustal stretching. The Jurassic rifting is interpreted to have initi-ated in the Middle Jurassic with a rift climax stage in the Late Jurassic, similar to areas offshore Mid Norway. The Juras-sic faults strike consistently NNE-SSW, parallel with a main lineament trend onshore Lofoten-Vesterålen and with nar-row fjords and sounds with this orientation. Rotated Jurassic fault-blocks are bounded by basement-involved faults with listric geometries. This fault style demonstrates the strong influence of basement heterogeneities, likely to represent

The crestal part of the uplift encompasses the Utrøst Ridge with the Røst and Jennegga highs (Fig. 12) and crystalline basement rocks have been sampled at the seabed on the Utrøst Ridge (Rokkoengen et al. 1979). These NE-SW ori-ented basement highs, bounded by normal faults related to the Jurassic and Campanian-Paleocene rift episodes (Fig. 4), have apparently pre-determined the location of the late uplift. Major reverse reactivation of original normal faults, which is associated with some of the post-Paleocene uplifts in the Vøring Basin (Blystad et al. 1995), was not impor-tant along the Lofoten-Vesterålen margin. However, Løs-eth & Henriksen (2005) published one example of a minor, post-Paleocene reverse reactivation along a steep fault in the Ribban Basin.

Large domes and elongated arches are characteristic features of the Norwegian Sea (Fig. 1) (Blystad et al. 1995). These structures formed after the continental break-up, and were located in the previously thinned continental crust of the Vøring and Møre basins. The mechanism behind the forma-tion of these late uplifts has been much debated (Bukovics & Ziegler 1985; Stuevold et al. 1992; Blystad et al. 1995; Doré & Lundin 1996; Vågnes et al. 1998; Brekke 2000; Mogensen et al. 2000; Pacal & Gabrielsen 2001; Lundin & Doré 2002; Kjeldstad et al.2003; Imber et al. 2005). However, in a com-prehensive study based on uplifts in the Vøring Basin by Gómez & Vergés (2005), they concluded that growth of the structures is best explained by a combination of tectonic compression and sediment loading. The contribution from each of these mechanisms as well as the timing of growth is however, variable between structures observed. In general, tectonic compression seems to have initiated the structures to produce a significant part of the dome amplification from Early Oligocene to Miocene (Fig. 3). A later and shorter period of growth (2.6 to 1.0 Ma) occurred because of differ-ential compaction related to the deposition and loading of a prograding wedge (e.g. Stuevold et al. 1992; Kjeldstad et al. 2003; Gómez & Vergés 2005).

In the absence of Tertiary sediments over much of the study area, the time of growth of the mapped uplift cannot be accurately dated. A smaller uplift termed the Hedda Dome is located at the Bivrost Lineament in the SW part of the study area (Løseth & Henriksen 2005). Paleocene to Oli-gocene sediments were folded to define the dome, and the dome amplification was prior to an intra-Miocene uncon-formity. In the outer shelf, NW of the Utrøst Ridge, Mokh-tari & Pegrum (1992) identified a significant domal uplift where sediments interpreted to be Oligocene-Miocene in age onlapped the flanks of the structure during growth. These observations support the idea that tectonic compres-sion affected also the Lofoten-Vesterålen margin, and fur-ther suggest that dome amplification in the study area was contemporaneous with tectonically induced growth of domes and arches in the Vøring Basin. Hence, the dome in the study area (Fig. 12) is interpreted to result from com-pression related to the continent break-up to the NW, with the main growth taking place in the Oligocene-Miocene.


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Caledonian thrusts and/or Late Devonian low-angle exten-sional shear zones.

Beneath the SW part of the Cretaceous Ribban Basin, large Jurassic fault-blocks are tilted some 10-15° ESE at Juras-sic and deeper levels. To the NE, however, there is a change across the E-W trending accommodation zone at 68°30’ N and fault-blocks are tilted 16-25° to the WNW. The struc-ture is seen as a low relief accommodation zone due to partly overlapping and oppositely dipping normal faults.

The tilting of Jurassic fault-blocks and the associated tec-tonic relief resulted in significant water-depth in the half-grabens. Apart from near shore areas, these deep-water basins contain sediment-starved and mostly thin Jurassic syn-rift successions, and consequently there was still signifi-cant relief and water-depths at the Jurassic-Cretaceous time transition. Hence, major parts of the Cretaceous succession that constitute the infilling of half-grabens is interpreted as a post-rift stage of basin evolution, and the infilling package assumed the wedge-shaped geometry of the remnant Juras-sic rift topography. It was not until the Late Cretaceous that the Jurassic rift topography was levelled causing the differ-ent depocentres, initially represented by NNE-SSW oriented half-grabens, to merge into a wider and NE-SW trending depocentre, i.e. the Ribban Basin.

The Jurassic rift topography became overprinted by NE-SW to ENE-WSW striking faults related to post-Jurassic exten-sion. Faults with this orientation and with the largest throw bound the Lofoten Ridge and also represent the boundary between the Utrøst Ridge and the deep basin to the NW. This young generation of faults is interpreted to result from a Campanian-Paleocene rift episode and became influential in the study area due to the narrow shelf and the proximity to rifting that culminated with the continent break-up along this margin segment.

A dome with a NE-SW elongation and a maximum width of c. 70 km is located in the shelf area along the Lofoten–Vesterålen margin. At the flanks of the structure, Tertiary-Upper Cretaceous sediments are uplifted and truncated at or near seabed. The uplift is in this article interpreted to result from tectonic compression related to continent break-up to the NW, with growth of the structure taking place in the Oli-gocene-Miocene.

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