Structure and evolution of the Lofoten continental margin, offshore Norway

MOHAMMAD MOKHTARI & RICHARD M. PEGRUM

Mokhtari, M. & Pegrum R. M.: Structure and evolution of the Lofoten continental margin, offshore Norway. Norsk Geologisk

Tidsskrift, Vol. 72, pp. 339-355. Oslo 1992. ISSN 0029-196X.

The continental margin of westeni Norway, in the Lofoten area between 67°N and 69°30'N, is geologically complex. The continental shelf is narrow and dominated by rotated extensional fault blocks. The inner shelf has a series of deep half grabens infilled by thick sedimentary sequences, primarily of Cretaceous age. The outer shelf is dominated by a complex structural high, the Utrøst Ridge, where crystalline basement rocks are at the seabed or covered by a thin veneer of sediments. Seaward of the Utrøst Ridge, beneath the continental slope and extending out to water depths of approximately 3000 m, a possible Mesozoic rift basin is buried by Paleogene tlow basalts and Cenozoic sediments several kilometers thick. The Lofoten Escarpment marks the approximate edge of the Tertiary oceanic crust. West of the Lofoten Escarpment a zone of seaward dipping retlectors is observed within the oceanic basalts, interpreted to be controlled by a series of landward dipping listric faults. The oceanic basalts are overlain by a sequence of Eocene and younger Cenozoic sediments. This paper discusses the observed geological structure and presents a general model for this part of the Norwegian margin.

Mohammad Mokhtari, Seismological Observatory, University of Bergen, 5014 Bergen, Norway; Richard Michael Pegrum, Statoil,

4000 Stavanger, Norway.

This paper describes the geology of the area located off the western coast of Norway between 67°N and 69°30'N and extending westwards from the Lofoten-Vesterålen Archipelago to approximately 8°30'E (Fig. 1). In this area the margin is typified by a narrow continental shelf, a steep continental slope and a ftat-ftoored oceanic basin with a water depth of around 3000 m.

Figure 2 shows the main structural elements (Blystad et al., in prep.). The shelf area consists of rifted and rotated basement fault blocks, 1ocally with a thick sedi­mentary cover. The Skomvær and Havbåen Sub-basins have a sedimentary fill, up to 5000 m thick, believed to be predominantly of early Cretaceous age. Together they comprise the Ribban Basin. The Træna Sub-basin, in the southwestern part of the study area is dominated by middle-late Cretaceous and Cenozoic sediments. It deep­ens towards the southwest. The Harstad Basin, beyond the study area towards the northeast, also has a thick Cretaceous and Cenozoic sedimentary fill.

The outer shelf is occupied by a complex structural high, the Utrøst Ridge. At its southwestern end (Røst High) and northeastern end (Jennegga High) crystalline basement rocks are at or very near the sea bed. Else­where the Utrøst Ridge has a thin sedimentary cover of Cretaceous and/or Neogene rocks. The Utrøst Ridge plunges steeply towards the southwest and its termina­tion appears to coincide with the landward projection of the Lofoten Fracture Zone (herein referred to as the Lofoten lineament). A second, poorly documented oceanic fracture zone appears to offset the Utrøst High at the southern end of the Jennegga High. This previ­ously unnamed fracture zone is herein informally re­ferred to as the Jennegga lineament.

The deep marine part of the study area has been subdivided into an inner zone, the Røst basin, and an outer zone, the Lofoten basin, their boundaries being marked by the Lofoten escarpment. The Røst basin has a sequence of Paleogene and younger sediments, up to several thousand meters in thickness, overlying a thick succession of Paleogene ftow basalts. Gravity, refraction and limited seismic reftection data indicate that the ftow basalts are underlain by a thick sedimentary sequence. This is of presumed Mesozoic, most likely Cretaceous, age but could extend up into the early Tertiary (Pale­ocene). The structural elements referred to herein as the Røst basin, Lofoten basin and Lofoten escarpment are not formally defined and fall outside the area considered in Blystad et al. (in prep.) concerning the structural nomenclature of the Mid-Norway region.

The Lofoten escarpment marks the approximate land­ward termination of oceanic volcanic rocks emplaced by active sea-ftoor spreading in the Norwegian-Greenland Sea, which was initiated between anomaly 24 and 25 time - between 59 and 56 my, la test Paleocene to earliest Eocene (Skogseid & Eldholm 1989). Immediately north­west of the Lofoten escarpment, 'seaward dipping reftec­tors' are seen within the oceanic basalt sequence. Similar features have been identified further south, on the V øring Margin and on other rifted continental margins (Hinz 1981). In the Lofoten area these are tentatively inter­preted to result from extension and rotation along land­ward dipping listric faults, detaching deep within the oceanic crust, which were active during the initial phase of sea-ftoor spreading. An 'intra oceanic crustal reftec­tor', observed locally on the reftection data, appears to be offset by this faulting. The Lofoten escarpment is

340 M. Mokhtari & R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFT 72 (I992)

Fig. l. Main physiography and structural features of the Norwegian-Greenland Sea. Modified after Eldholm et al. 1989. Study area is outlined by box.

interpreted to mark the transttlon from continental to oceanic crust within this area.

In the Lofoten basin a ftat-bedded sequence of Eocene and younger bathyal sediments overlies the oceanic basalts which young progressively towards the north­west, towards the presently active mid-ocean ridge (Mohns Ridge, Fig. I).

Seismic database The seismic reftection database utilized in this study is shown in Fig. 3. Approximately 4500 km of multichannel seismic reftection data has been acquired in this area by the Seismological Observatory, University of Bergen, since 1982. These data have been processed at the Seis-

mological Observatory. In addition, data acquired by other academic institutions have been incorporated (Sell­evoll et al. 1986, 1988; Mokhtari et al. 1989; Mokhtari 1991). The Norwegian Petroleum Directorate also kindly allowed access to part of their extensive seismic database covering the Lofoten Shelf.

Gravity, marine magnetic, sonobouy refraction data, OBS data and seabed sub-crop data have also been incorporated in to this study.

The continental shelf In this section we discuss the continental shelf area, out to approximately the 500 m bathymetric contour. Fig. 2 shows the main tectonic elements (Blystad et al. in

NORSK GEOLOGISK TIDSSKRIFf 72 (1992)

LOFOTEN MARGIN

Main structural elements

Fig. 2. Main structural elements off Lofoten Continental Margin.

prep.). The reftection data (Fig. 3) were picked and tied in the conventional manner and up to seven reftectors have been mapped. There is no well control within the study area. Dating of the shallow reftectors is dependent on lang distance ties to wells further south and the deeper reftectors can only be speculatively dated by analogy with the limited sedimentary outcrop data from the island of Andøy in the northern Lofoten-Vesterålen Archipelago (Dalland 1981). Same additional control has been obtained by integrating the results of seabed sampling carried out by IKU (Lien 1976). Considerable doubt remains as to the dating of the reftectors in the deeper part of the Ribban Basin and Træna Sub-basin.

One of the strongest and most continuous seismic reftectors has been correlated with the Base Cretaceous. The dating is dependant on lang distance ties to wells south of the Træna Sub-basin which are apen to alterna­tive interpretation, this dating is therefore considered to be only provisional. The reftector can, however, be car­ried throughout the data set on the shelf and provides the basis for the definition of the principal structural elements. Fig. 4 is a two-way-time contour map of the Base Cretaceous reftector. The main structural elements defined by this map are discussed in more detail in the following.

Lofoten continental margin 341

The Træna Sub-basin

The present study ara includes only the northeastern part of the Træna Sub-basin (Fig. 2), which thickens consider­ably to the southwest. Fig. 5 is a line drawing illustrating the observed stratigraphic relationships.

The Base Tertiary reftector dips monoclinally towards the southwest, from its sea-ftoor subcrop at around 67°45'N, to below 2.0 sec TWT (Figs. 5, 6). Intra-Tertiary reftectors successively subcrop the sea-ftoor, indicating that the present northern limit is truncational and results from post-depositional uplift. The timing and magnitude of this uplift, and the original extent of Tertiary sediments across the Lofoten shelf, are presently unknown.

The Intra-Turonian Reftector (Figs. 5, 7) is provision­ally dated by ties to boreholes southwest of the study area. The reftector is sub-parallel to the Base Tertiary reftector and sub-crops the sea ftoor northwards, indicating post­depositional uplift and erosion. It can be traced downdip to below 4.0 sec.TWT. This reftector, unlike the Base Tertiary, is cut by a number of faults, suggesting a mild phase of late Cretaceous or early Tertiary tectonism. There is insufficient stratigraphic control to date this more precisely. Brekke & Riis ( 1986) described a similar phase of tectonism in the Nordland Ridge area to the south, which they ascribe to 'the Lower Tertiary and perhaps as early as in the Upper Cretaceous'.

The deepest reftector mapped in the Træna Sub-basin is the 'Base Cretaceous' (Fig. 4). It can be traced south­westwards to below 5.0 sec.TWT. Locally, there is evi­dence in the seismic data of a deeper sedimentary sequence (?Jurassic-Triassic) but the reftectors are weak and discontinuous and cannot be mapped on aur current sparse data grid. Over considerable areas it appears that the Cretaceous sedimentary sequence rests directly on crystalline basement rocks. The reftector is cut by several major normal faults and early deposition may have been accompanied by extensional faulting.

The relationship between the thick Cretaceous sediments of the Træna Sub-basin and the Paleogene ftood basalts of the Røst basin is unclear. It appears that the Cretaceous sequence passes beneath the ftood basalts and may make up the bulk of the suspected 'sub-basalt' sediments in the Røst basin. We have insufficient data coverage in the critical area to confirm this relationship, however.

The deepest part of the Træna Sub-basin, and the steeply plunging southward termination of the Røst High, coincide with a landward projection of the Lofoten Frac­ture Zone. This strongly suggests that there was a north­westerly zone of crustal weakness which predated the Tertiary opening of the Norwegian-Greenland Sea and which predetermined the location of the oceanic fracture zone - we refer to this trend as the Lofoten lineament, as its nature and genesis are presently uncertain.

The Utrøst Ridge

The Utrøst Ridge (Blystad et al. in prep.) is a crystalline basement high extending along the outer margin of the

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Lofoten shelf (Fig. 2). The basement high is structurally complex and cut by a number of faults. Two culmina­tions, where crystalline basement rocks may subcrop the seafloor, have been formally named by Blystad et al. (in prep.) the Røst High and Jennegga High (Fig. 4). Re­fraction, reflection and seabed sampling data all confirm the presence of shallow crystalline basement in the Utrøst Ridge (Mokhtari 1991).

Fig. 8 is a line drawing of seismic line L0-8-87 and shows the relationship between the sedimentary fill of the Skomvær Sub-basin, the Marmæle Spur and the Røst High. The Røst High appears to have only a thin and discontinuous sedimentary cover. Because of high inter­val velocities at shallow depth, sea-floor multiples are very strong on seismic lines crossing the crest of the Røst High and it is difficult to identify and follow the Base Cretaceous reflector at shallow depths. It appears on several lines, however, that the reflector is truncated at the seafloor, indicating post-depositional uplift and erosion.

The Jennegga High was earlier considered to be an extension of the Røst High ( Mokhtari et al. 1987). The Røst High and Jennegga High are, however, separated by a faulted 'saddle' and are now treated as separate elements within the Utrøst Ridge (Blystad et al. in prep.). The Jennegga High appears to be faulted on both

flanks and may have the form of a horst. Seismic line L0-18-86 between shotpoint 800 and 900 (for location see Fig. 3) indicates a possible sea-floor basement sub­crop and Lien (1976) has reported crystalline basement outcrops from the same area ( 68.8°N/13.SOE).

The saddle between the Røst High and the Jennegga High and a switch in the vergence of major faults at the northern end of the Havbåen Sub-basin (Fig. 4) approx­imately coincide with the landward projection of the Jennegga fracture zone (Fig. 2). This suggests that the position of the fracture zone, active during early Ceno­zoic opening of the Norwegian-Greenland Sea, was predetermined by a pre-existing zone of structural weak­ness. We refer to this herein informally as the Jennegga lineament.

A similar conclusion was discussed above regarding the Lofoten lineament, which coincides with the steep plunge and termination of the Utrøst Ridge in a south­westerly direction. The Utrøst Ridge appear to terminate abruptly northwards at 69°N. The nature of this termina­tion is uncertain, as there is presently poor seismic con­trol in this area.

The earliest sedimentary fill of the Ribban Basin is thought to be early Cretaceous in age. The lowest part of the sequence clearly onlaps the south-eastern flank of the Utrøst Ridge (Fig. 8) and this relationship indicates that

NORSK GEOLOGISK TIDSSKRIFr 72 (1992) Lofoten continental margin 345

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NORSK GEOLOGISK TIDSSKRIFT 72 (1992) Lofoten continental margin 347

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the basement high existed at this time - possibly as an elevated footwall block to the presumed Cretaceous sedi­ments of the Røst basin. The apparent absence of a significant thickness of Triassic and Jurassic sediments on the Lofoten shelf, in strong contrast to the Trøndelag Platform to the south and the Hammerfest Basin to the north, suggests that the area may have been regionally elevated during the earlier Mesozoic and that the Utrøst Ridge may have been active in pre-Cretaceous times. A similar history and similar age stratigraphic fill is inter­preted for the Vestfjorden Basin, located landward of the Lofoten archipelago (Brekke & Riis 1986).

The upper part of the sedimentary fill of the Ribban Basin (?middle-late Cretaceous) is uplifted and trun­cated at the sea floor above the southeastern ftank of the Utrøst Ridge. This relationship is illustrated in Fig. 8 for the Skomvær Sub-basin, a subelement of the Ribban Basin. This clearly indicates post-Cretaceous rejuvenated uplift and erosion of the U trøst Ridge. In the absence of a Tertiary sedimentary section over much of the Lofoten shelf the timing of this uplift cannot be accurately dated. It may well have been multiphase.

The Ribban Basin

The Ribban Basin is located between the Utrøst Ridge and the emergent basement ridge of the Lofoten­Vesterålen Archipelago, the Lofoten Ridge. Its south­eastern boundary is a NE-SW trending fault zone down­throwing towards the northwest (Fig. 2). The fault zone has a downthrow locally exceeding 5000 m, the footwall forming the Lofoten Ridge. Southwestwards the Lofoten

Ridge is bounded towards the southeast by a second fault system forming the northwestern boundary of the Vestfjord Basin (Brekke & Riis 1986). The Lofoten Ridge terminates towards the southwest at the approxi­mate position of the landward projection of the Lofoten lineament. Northeastwards the fault zone limiting the Ribban Basin extends beyond our study area and its relationship to the fault zone forming the landward boundary of the Harstad Basin is uncertain.

The Ribban Basin is subdivided into two partially discrete depocenters, the Skomvær Sub-basin and Hav­båen Sub-basin (Blystad et al. in prep.) by a northwest­erly trending basement nose (Fig. 4). In both sub-basins the Base Cretaceous seismic reflector can be mapped to below 3.0 sec.TWT. The sedimentary fill thins onto the flank of the Utrøst Ridge giving it the overall geometry of a half-graben. Southwards the Skomvær Sub-basin is limited to the west by a shallow basement ridge termed the Marmæle Spur. The western flank of the Marmæle Spur is formed by a zone of large faults, throwing down to west, named the Vesterdjupet Fault Zone by Blystad et al. (in prep.). Further to the south the Vesterdjupet Fault Zone forms the eastern boundary of the Træna Sub-basin (Figs. 2, 8).

The age of the sedimentary fill of the Ribban Basin is poorly controlled. Four seismic reflectors have been mapped in the deeper parts of the basin, referred to as Base Cretaceous, lntra Lower Cretaceous l, lntra Lower Cretaceous Il and lntra Lower Cretaceous Ill (Figs. 5, 8). The lntra Lower Cretaceous reftectors are restricted to the deeper parts of the Skomvær and Havbåen Sub­basins and onlap the ftanks of the Utrøst Ridge and

348 M. Mokhtari & R. M. Pegrum

Marmæle Spur. They appear not to extend into the Træna Sub-basin and therefore cannot be dated from well control further south. A tentative correlation has been made with the outcrop sequence of Andøya (Dal­land 1981 ), suggesting that they are of earl y Cretaceous age. The disposition and geometry of the reflectors sug­gest that the early Cretaceous sediments were deposited in tectonically active, fault controlled basins, formed during a period of extensional rifting. Brekke & Riis (1986) reach a similar conclusion regarding the Vestfjord Basin. The apparent absence of the earliest part of the lower Cretaceous sequence in the Træna Sub-basin indi­cates that there was differential subsidence at this time and that the Lofoten shelf was a stronger subsident area.

On some seismic lines a sequence of reflectors beneath the Base Cretaceous reftector has been noted. These possibly represent sediments of earlier Mesozoic (Juras­sic/Triassic) or late Paleozoic age. Because of their dis­continuous nature and the sparse seismic grid available at present, no attempt has been made to map the pre­Base Cretaceous seismic reflectors. In many places the Cretaceous sediments appear to rest directly on crys­talline basement rocks and the presence of a thick early Mesozoicjlate Paleozoic sedimentary sequence on the Lofoten shelf is thought to be unlikely. This is in strong contrast to the Norwegian margin further to the south. (Trøndelag Platform) and further to the north (Ham­merfest Basin).

The Røst basin The continental slope off the Lofoten margin is steep (up to 10°) and narrow (down to 25 km). The o u ter margin of the Utrøst Ridge more or less coincides with the top of the continental slope and the 500 m bathymetric con­tour (Fig. 2). The seaward boundary of the U trøst Ridge appears to be fault controlled, with large faults throwing down to the northwest and forming the landward boundary of the Røst Basin (Figs. 2, 8).

The Røst basin is typified by a sequence of basalt flows of probable late Paleocene age. They extend for a dis­tance of 60-70 km landward from the Lofoten escarp­ment to the faulted boundary of the Utrøst Ridge. The uneven surface of the flow basalts generates a character­istic hyperbolic reflection pattern in the seismic data which can be followed with some confidence. The high reftectivity and uneven surface of the basalts effectively masks observations of deeper, sub-basalt reftectors and these are only occasionally observed. The evidence of a sub-basalt sedimentary sequence is therefore largely de­pendent upon the interpretation of additional seismic data derived from OBS, refraction and gravity studies.

The flow basalts of the Røst basin are overlain by Tertiary and younger sediments (Figs. 9, 10). These vary in thickness from approximately 1.0 sec.TWT close to the southeastern margin of the basin to more than 2.0 sec.TWT adjacent to the Lofoten escarpment. Be-

NORSK GEOLOGISK TIDSSKRIFT 72 (I992)

neath the continental slope the Cenozoic sediments show a complex internal stratigraphy with channeling, turbid­ite fan deposition and slumping. There are no well penetrations to accurately date these sediments and the sparse data grid and complex reflector geometries render correlations front line to line uncertain. Lang distance seismic ties to hetter dated sequences, such as on the Vøring Margin (Mutter 1984; Skogseid et al. 1989; Eld­holm et al. 1989), are doubtful due to the sparse seismic grid available.

Fig. l O illustrates the main features of the Røst basin in the study area as depicted by the multichannel reftec­tion seismic data (line UB-82-350). At the foot of the continental slope the sea ftoor is flat and featureless, at a depth of approximately 4.0 sec.TWT (more than 3000 m). The upper 500 ms.TWT of sediments (Pliocene and younger) are seismically transparent and maintain an even thickness until they approach the continental slope where they progressively thin and onlap the under­lying sequence. There is little evidence of sediment input from the present Lofoten shelf, which appears to be a sediment bypass area. On same lines crossing the conti­nental slope there is evidence of recent slumping and downslope gravity gliding.

The top of the landward flow basalts is the discontinu­ous, high amplitude seismic reflector dipping northwest­wards from approximately 5.0 sec.TWT beneath the continental slope to approximately 6.5 sec.TWT at the Lofoten escarpment. The flow basalts are overlain by a thick sequence of sediments which laterally interfinger with the oceanic basalts at the Lofoten escarpment (be­tween shotpoints 1050 and 1250). Emplacement of these basalts began between magnetic anomalies 24 and 25 (59-56 my) late Paleocenejearly Eocene (Skogseid & Eldholm 1989). We therefore interpret the laterally equivalent sediments to be late PaleocenejEocene in age and this clearly indicates that the landward flow basalts, which underlie the sediments, are somewhat older (pre­late Paleocene).

The early Tertiary sediments, together with the un­derlying flow basalts, are clearly ftexured close to the Lofoten escarpment, and younger Tertiary sediments (?Oligocene/Miocene) onlap the ftanks of the ftexure. Mapping of this flexure indicates that it is domal in shape and coincides with the re-entrant in the Lofoten escarpment which occurs at approximately 69°N, 10°E. It is notable that in this area the Lofoten escarpment is not fault controlled, in contrast to the Vøring Plateau escarpment further south (Eldholm et al. 1989).

The mechanism generating the updoming is unclear, but may be related to the transpressional rejuvenation of a deep-seated fault zone rather than to a mechanical response to loading by the oceanic basalt pile. A hetter understanding of the relationships between the updoming and the Lofoten escarpment in this area requires a denser seismic grid and more detailed stratigraphic analysis.

Based on seismic refraction data from an OBS array tied to a regional reftection survey, Mjelde et al. (1991)

NORSK GEOLOGISK TIDSSKRIFf 72 (1992) Lofoten continental margin 349

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Fig. 9. Seismic retlection profile L0-88-08 and its interpretation (see for location Fig. 3).

suggest that the flow basalts in the Røst basin consist of two distinct layers. An upper layer, with an interval velocity of 4.4 km/sec and a maximum thickness of l km, and a lower layer, with interval velocities of 5.2 km/sec, which is approximately 1.5 km thick. The flow basalts of the Røst basin cover an area of at least 3000 km2• With an average thickness of 2.5 km, they represent an enormous volume of volcanic material ap­parently emplaced within a very short time period, pos-

sibly as little as l my (Skogseid & Eldholm 1989). It is notable that in the Lofoten area the late Paleocene flow basalts extend considerably further landward than in the V øring area to the south. The landward swing in the termination of the flow basalts approximately corre­sponds to the position of the Lofoten lineament, but the genetic relationship between these observations is presently uncertain and falls within an area of relatively sparse data control.

350 M. Mokhtari & R. M. Pegrum

Seismic Line: UB 82 350

Line drawing: Line UB - 82 - 350 NW

NORSK GEOLOGISK TIDSSKRIFT 72 (1992)

SE

SEA FLOOR 4 PLIOCENE

5

6

7

8

Fig. JO. Seismic reftection profile UB-82-350 and its interpretation (see for location Fig. 3).

The OBS data further suggest that the flow basalts are underlain by sediments which may reach 2.5 km in thick­ness (Mjelde et al. 1991). This is in agreement with gravity modelling which also indicates the presence of thick sub-basalt sediments (Sellevoll et al. 1988). Nor­mally the high velocity flow basalts effectively mask deeper reflectivity and the presence and structural atti­tude of sub-basalt sediments cannot be resolved on con­ventional seismic reftection data. However, on several of the lines used in this study, especially strike lines oriented parallel to the continental margin, discontinuous dipping reflector segments have been observed beneath the basalts which may represent faulted sediments or sills intruded into the sedimentary sequence (Fig. Il , shot­point 410 at approx. 6.7 sec).

The regional relationship between the thick Cretaceous sediments of the Træna Sub-basin and the Paleocene flow basalts has been discussed earlier. It is postulated that the sub-basalt sediments are primarily of Cretaceous

age and were deposited in a hanging wall basin related to the Utrøst Ridge, the latter representing a footwall uplift to the Røst basin. Sedimentation could have extended up into the early/middle Paleocene, immediately prior to the flow basalt emplacement. Subsequently the sedimentary basin has foundered and been buried beneath the flow basalts which have extended landwards as far as the faulted margin of the U trøst Ridge. It is considered likely that the Røst basin is underalin by a thinned and ex­tended continental crust which extends northwestward as far as the Lofoten escarpment.

The Lofoten escarpment and Lofoten basin Fig. 12 illustrates the seismic expression of the Lofoten escarpment and the oceanic area immediately to the north west (Lofoten basin) in the center of the study area. The most prominent seismic reflector is the high ampli-

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352 M. Mokhtari & R. M. Pegrum

tude 'acoustic basement' horizon at approximately 5.9 sec.TWT which marks the boundary between the Tertiary sedimentary cover and the underlying oceanic basalts. Immediately northwest of the Lofoten escarp­ment a series of seaward dipping reflectors is observed within the basalts. Similar seaward dipping reflector se­quences have been observed on many rifted margins (Hinz 198 1). Our interpretation of the dipping reflectors as resulting from rotation along landward dipping listric faults associated with the early opening of the Norwe­gian -Greenland Sea is discussed further below.

At a depth of 7-8 sec.TWT (Fig. 12) an uneven, discontinuous zone of reflections is observed both be­neath and seaward of the zone of seaward dipping reflec­tors (Sellevoll �t al. 1986; Sellevoll & Mokhtari 1988). In

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NORSK GEOLOGISK TIDSSKRIFT 72 (1992)

some places this reflective interval can be resolved into two distinct reflectors, one at approximately 7.3 sec.TWT and one at 8.0 sec.TWT. A strong water bottom multiple effects the data at approximately 8. 1 sec.TWT making it difficult to detect deeper arrivals. However, Sellevoll et al. ( 1988) believe there is evidence that the water-bottom multiple is disturbed by the signal from a deeper primary reflector at a depth of 8.3-8.5 sec.TWT.

The reflective sequence observed is considered to be too shallow to be derived from the Moho (Drivenes et al. 1984) and has been interpreted as Intra-Oceanic Crustal Reflectors (Sellevoll & Mokhtari 1988), representing the deeper part of Oceanic Layer 3 (Sellevoll et al. 1988). Mutter and the NAT Study Group ( 1985) have observed similar intra-crustal reflectors in old oceanic crust of the

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Fig. 12. Seismic reftection profile UB-82-350 and its interpretations (see for location Fig. 3).

PALEOCENE FLOW BASALTS

5

7

8

NORSK GEOLOGISK TIDSSKRIFT 72 (1992)

North Atlantic Ocean. They interpreted these reflectors as the top of the cumulate sequence in an old magma chamber. The velocity of 7.4 km/sec obtained for this layer from OBS refraction data is comparable to velocity analyses of rock samples from ophiolite suites (Chris­tensen 1978).

The Intra-Oceanic Reflector sequence is observed to be discontinuous and formed of discrete dipping segments (Fig. 12). This segmentation is most noticeable in the zone beneath the seaward dipping reflectors and can be interpreted as a result of deeply penetrating faults. Harper ( 1982) has described abundant normal faulting in the upper oceanic crust and discussed the influence it can have on the development of tilted sheeted dykes and pillow lavas. Dyke rotations up to 50° along listric or domino-style faults are described. Gibson & Love ( 1989) discuss a listric fault model for the formation of seaward dipping reflectors. They postulate that the listric faults detach on a surface which may root back into older detachments formed during extensional faulting of the pre-volcanic continental crust. Eldholm et al. ( 1989) have recently suggested that crustal thinning just prior to breakup was achieved by the pervasive injection of intru­sives and listric normal faulting on the Vøring margin. Our interpretation, illustrated in Fig. 12, is that the dipping reflectors observed within the basalts west of the Lofoten escarpment are controlled by landward facing listric faults which penetrate deeply into the crust and offset the Intra-Oceanic Crustal Reflector sequence. We postulate that these faults may have a common detach­ment surface to the seaward facing faults which thin and extend the continental crust beneath the Røst basin, a relationship similar to that proposed by Gibson & Love ( 1989). The oceanicjcontinental crustal boundary is in­terpreted to be transitional and located approximately beneath the Lofoten escarpment.

Towards the southern and northern limits of the study are the marked flexure dose to the Lofoten escarpment diminishes and a more 'normal' relationship, similar to that described at the Vøring Escarpment (Skogseid & Eldholm 1989), is observed. In these areas there is a 'marginal high', seaward dipping reflectors and a 'K' reflector defining the boundary between Upper and

Lower Volcanic Series. A switch in polarity of the listric faults controlling asymmetric rifting of the Norwegian/ Greenland margins, postulated to coincide with the posi­tion of the Lofoten Fracture Zone by Eldholm et al. ( 1989), is not clearly observed in our data.

Evolution of the Lofoten margin The following evolutionary sequence for the development of the Lofoten margin is a tentative interpretation of our observations. The lack of bore hole information and accu­rate dating of the stratigraphic sequences involved places considerable constraints on our model. The conclusions are based primarily on the gross stratigraphical relation-

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Lofoten continental margin 353

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Fig. 13. Proposed geological model for the Lofoten Continental Margin.

354 M. Mokhtari & R. M. Pegrum

ships observed along a transect of the Lofoten margin from the inner shelf to beyond the Lofoten Escarpment. These relationships are summarized in Fig. 13.

• Paleozoic: no sediments of certain Paleozoic age have been observed on the Lofoten margin. Pre-Triassic ro­tated fault blocks and thick sedimentary sequences have been observed beneath the Trøndelag Platform but ap­pear not to extend north of the Træna Sub-basin. If ever present this sequence has been eroded from the Lofoten area by pre-Mesozoic uplift.

• Triassic/Jurassic: a relatively thin pre-Cretaceous se­quence has been observed locally beneath the Lofoten shelf on a few seismic profiles. Typically this sequence is only observed

' in the deeper parts of the sedimentary

basins and may represent erosional remnants of a once more widespread Triassic/Jurassic sedimentary cover. The thick Triassic/Jurassic sedimentary sequences pres­ent in the Trøndelag Platform/Haltenbanken areas are apparently absent from the Lofoten area. Whether this part of the margin remained structurally high or was uplifted and deeply eroded by post-Jurassic movements, remains unknown. Speculatively, the northern boundary of the thick Triassic/Jurassic sequences of mid-Norway may be placed at the landward projection of the Lofoten lineament.

• Cretaceous: thick sedimentary sequences of Cretaceous age are preserved on the Lofoten shelf. Within the Rib­ban Basin they locally exceed 5000 m in thickness. They are preserved in half-grabens controlled by northwest facing normal faults. The earliest sediments are located in the basin centers and there is a progressive onlap onto the flanks of rotated basement fault blocks. The principal basement fault blocks are the Lofoten Ridge, which brings crystalline basement rocks to the surface in the Lofoten-Vesterålen archipelago, the Marmæle Spur, which forms the eastern boundary to the Træna Sub­basin, and the Utrøst Ridge.

Seismic correlations suggest that early Cretaceous sedi­ments dominate the Ribban Basin whereas alte Creta­ceous sediments dominate the Træna Sub-basin - indic­ating differential subsidence between the two areas. The relatively abrupt southerly termination of the basement fault blocks of the Lofoten shelf appears to coincide with the position of the Iofoten lineament, suggesting it is a structural feature of fundamental importance.

The presence of a thick sedimentary sequence beneath the flow basalts of the Røst basin is indicated from refraction, gravity, and to a lesser extent, reflection data. We consider it most likely that these sediments are primarily of Cretaceous age.

The Lofoten shelf is therefore typified by a period of extension, fault block rotation and strong differential subsidence during the Cretaceous. We tentatively sug­gest that the principal faults are listric at depth and sole out in a speculative intra-crustal detachment zone (Fig. 13).

NORSK GEOLOGISK TIDSSKRIFT 72 (1992)

• Paleogene: Paleogene sediments are present in the Træna Sub-basin but thin northwards and are absent from much of the Lofoten shelf. It is uncertain whether this is a result of post-depositional uplift and erosion or whether the Lofoten shelf remained emergent through much of the Paleogene. The progressive truncation of intra-Tertiary reflectors northward from the Træna Sub­basin shows that a certain amount of post-Paleogene uplift and erosion has occurred.

West of the Utrøst Ridge subsidence continued throughout the Paleogene. Landward flow basalts extend from the Lofoten escarpment to the faulted western boundary of the Utrøst Ridge. Refraction data indicate that the lavas are up to 2.5 km thick. The flow basalts are overlain by a sedimentary sequence which thickens northwestwards and interfingers with oceanic basalts of latest Paleocene/earliest Eocene age. These sediments, together with the underlying flow basalts, are strongly flexured in the central part of the study area. Later sediments onlap the flanks of this flexure.

The Lofoten escarpment appears to be built up to an edifice of oceanic basalts which interfinger landwards with sediments. In the central part of the area the Lofoten Escarpment appears not to be fault controlled, as is the case further to the south at the Vøring Plateau Escarpment. Active spreading was initiated along the Lofoten margin between anomalies 24 and 25. We inter­pret the seaward dipping reflectors immediately west of the Lofoten escarpment to result from crustal thinning, extension and rotation along a series of landward facing listric faults (Figs. 12, 13). Speculatively, these faults penetrate deep into the crust and may sole out at the same level as the seaward facing faults initiated during the Cretaceous extension.

• Neogene: younger Cenozoic sediments are very thin or absent over the Lofoten shelf. Seaward of the Utrøst Ridge they form a continuous blanket extending across the Røst and Lofoten basins. Internal geometries suggest there was little sediment input from the Lofoten shelf, which appears to have been a sediment by-pass area. The effects of late Tertiary uplift of the Lofoten shelf are difficult to assess in the absence of a preserved sedimen­tary record.

Acknowledgements. - We acknowledge the assistance of our colleagues, both at the University of Bergen and in Statoil, for several fruitful discussions during the preparation of this paper. Especially, we acknowledge the comments and sugges­tions of Markvard A. Sellevoll, Bjørn T. Larsen and Jan Vollset. Statiol are acknowledged for their continuing financial support for this research project and the Norwegian Petroleum Directorate for their permission to use selected data from the Lofoten shelf.

Manuscript received April 1991

References Blystad, P., Færseth, R. B., Larsen, B. T., Skogseid, J. & Tørudbakken, B. (in

prep.): Structural elements of the Norwegian continental Shelf. Part Il, The Norwegian Sea. NPD Bull. 6.

NORSK GEOLOGISK TIDSSKRIFf 72 (1992)

Brekke, H. & Riis, F. 1986: Tectonics and basin evolution of the Norwegian Shelf between 62°N and 72°N. NPD contribution no. 25.

Christensen, N. I. 1978: Ophiolites, seismic velocities and oceanic crustal struc­ture. Tectonophysics 47, 131-157.

Dalland, A. 1981: Mesozoic sedimentary succession at Andøy, northem Norway, and relatipn to structural development of the North Atlantic area. In: Kerr, J. W. & Fergusson, A. J. (eds.) : Geology of the North Atlantic Borderlantls. Can. Soc. Petr. Geo/. Mem. 7, 563-584.

Drivenes, G., Sellevoll, M. A., Renard, V., Avedik, F. & Pajchel, J. 1984: The continental marginfcrustal structure off the Lofoten Islands, Northem Norway. In: Spencer, A. M. et al. (eds.) : Petroleum Geology of the North European Margin, 211-216. Graham and Trotman, London.

Eldholm, 0., Thiede, J. & Taylor, E. 1989: Evolution of the Vøring Volcanic Margin. In: Eldholm, O. et al. (eds.) : Proceedings of the Ocean Drilling Program, Scientific Resu/ts, Siles 642-644, Norwegian Sea 104, College Station, Texas, 1033-1065.

Gibson, I. L. & Love, D. 1989: A Jistric fault model for the forrnation of dipping

reftectors penetrated during the drilling of hole 642E, ODP LEG 104. In: Eldholm, O. et al (eds.) : Proceedings 'O! the Ocean Drilling Program, Scientific Resu/ts, Sites 642-644, Norwegian Sea 104, College Station, Texas, 979-983.

Harper, G. D. 1982: Evidence of large-scale rotations at spreading centers from the Josephine Ophiolite. Tectonophysics 82, 24-44.

Hinz, K. 1981: A hypotheses on terrestrial catastrophes. Wedges of very thick oceanward dipping layers beneath passive continental margins - their origin and paleoenvironment significance. Geo/ogisches Jahrbuch, Reihe E. 22, 3-28.

Lien, R. 1976: Ingeniørgeologisk kartlegging på kontinentalsokkeln utenfor Lofoten-Vesterålen. !KU Report 78, 36 pp.

Mjelde, R., Sellevoll, M.A., Shimamura, H., Iwasaki, T. & Kanazawa, T. 1991:

Lofoten continental margin 355

Preliminary results from a seismic refraction study off Lofoten, Norway using

3-component ocean bottom seismogtaphes (OBS). Seismo/ogical Observatory, University, Bergen, Seismo-Ser., 39.

Mokhtari, M. 1991: A geological model for the Lofoten continental margin. Unpublished Dr. Scient thesis, University of Bergen, 230 pp.

Mokhtari, M., Pegrum, R. M. & Scllcvoll, M.A. 1989: A geophysical study of the Norwegian continental margin betwecn 67° and 69°N. Seismo/ogical Observa­tory, University, Bergen, Seismo-Ser, 28, 18 pp.

Mokhtari, M., Sellevcill, M. A. & Olafsson, I. 1987: Seismic study of Lofoten margin N. Norway. Seismological Observatory, University, Bergen, Seismo-Ser. JO, 25 pp.

Mutter, J. C. 1984: Cenozoic and late Mesozoic stratigraphy and subsidence history of the Norwegian margin. Bull. Geo/. Soc. Am. 95, 1135-1149.

Mutter, J. C. & NAT Study Group. 1985: Multichannel seismic images of the oceanic crust's intema1 structure: evidence for a magma chamber beneath the Mesozoic Mid-Atlantic Ridge. Geo/ogy /3, 629-632.

Sellevoll, M.A. & Mokhtari, M. 1988: An intra-oceanic crustal seismic reftecting zone off Lofoten, N. Norway. Geology 16, 666-668.

Sellevoll, M. A. Mokhtari, M., Furnes, H. & Pedersen, R. B. 1986: The oceanic crust and continental-oceanic transition off Lofoten, N. Norway. Seismo/ogical Observatory, University, Bergen, Seismo-Ser. 8.

Sellevoll, M. A., Olafson 1., Mokhtari, M. & Gidskehaug, A. 1988: Lofoten margin, North Norway: crusta1 structure adjacent to the ocean-continent transition. Norges geologiske undersøkelse Specia/ Publication, 3, 39-48.

Skogseid, J. & Eldholm, O. 1989: Vøring Plateau continental margin: seismic interpretation, stratigraphy and vertical movements. In: Eldholm, O. et al. (eds.) : Proceedings of the Ocean Drilling Program, Scientific Resu/ts, Sites 642-644, Norwegian Sea 104, College Station, Texas, 993-1030.