NORWEGIAN JOURNAL OF GEOLOGY Onshore and offshore geology in the areo, northern 243

Bridging the gap between the onshore and offshore geology in Nordland,

Odleiv Olesen, Erik Lundin, Øystein Nordgulen, Per Terje Osmundsen, Jan Reidar Skilbrei, Mark A. Smethurst, Arne Solli, Tom Bugge & Christine Fichler

Olesen, 0., Lundin, E., Nordgulen, Ø., Osmundsen, P.T., Skilbrei, J.R., Smethurst, M.A., Solli, A., Bugge, T. & Fichler, C: Bridging the gap between the onshore and offshoregeology in Nordland, northem Norway. Norwegian Journal of Geology, Vol. 82, pp. 243-262. Trondheim 2002. ISSN 029-196x.

We have utilised potential field data in combination with seismic interpretation and bedrock mapping to improve the understanding of basement structures and rifting processes on the continental shelf offshore Nordland, northern Norway. Comparing the Bouguer gravity field to gravity responses from Airy roots at different depths for the northern Scandinavia mountains it is shown that the compensating masses are situated at a relatively shallow depth in the upper crust. The granite within the Tr ansscandinavian Igneous Belt extends to a substantial depth (mini­ mum 22 km) in the crust. These voluminous granites may be related to Proterozoic plate subduction along the western edge of the , analogous to the Sierra Nevada Batholith in California.

The NW-SE trending late Devonian extensional structures (Kollstraumen detachment, and Nesna and Sagfjord shear z.ones) extend from the main­ land north-westwards below the Helgeland, Vestfjorden and Ribban basins. The Bivrost Lineament most likely represents a detachment dipping 5-15° to the southwest and may constitute the offshore extension of the Nesna shear zone. The entire Nordland mainland and offshore area is to a great extent affected by the NE-SW trending late Caledonian gravity collapse. Later (Late-Palaeozoic - Mesozoic) offshore rifting events have resul­ ted in a shallow Moho and rotated fault-blocks separated by transfer zones giving rise to intermediate wavelength gravity and aeromagnetic anoma­ lies. Changes in Moho depths occur across the Surt, Bivrost and Vesterålen transfer zones indicating that these structures are continuous to great depths within the crust. The more local transfer zones separate structural domains characterised by different fault-polarities within the Ribban and Vestfjorden basins. The Myken intrusive complex (Palaeocene age?) to the northeast of the Utgard High gives rise to large gravity and magnetic anomalies comparable to the anomalies originating from the basement structuring.

Odleiv Olesen, Erik Lundin, Øystein Nordgulen, Per Terje Osmundsen, ]an Reidar Skilbrei, Mark A. Smethurst, Arne Solli, Geological Survey of Nor­ way, N0-7491 Trondheim, Norway, Tom Bugge, Norsk Hydro ASA, N0-9480 Harstad, Norway, Christine Fichler, Statoil Research Centre, N0-7005 Trondheim, Norway

lntroduction onshore-offshore divide in the Nordland area ( Olesen et al. 1997b; Fichler et al. 1999). The present paper des­ Potential field data provide continuous coverage of the cribes a new offshore map depicting the depth to crys­ mainland and offshore areas and act as a bridge bet­ talline basement as well as an onshore map of the depth ween areas traditionally investigated by two different to Precambrian basement. The basement structure is methods; field bedrock mapping and seismic interpre­ outlined in a 3D model interpreted from gravity data tation. There is usually a gap of 10-30 km kilometres and constrained by aeromagnetic and petrophysical between the areas mapped by the two methods. The data, seismic interpretations and bedrock mapping. gravity and aeromagnetic data allow basement structu­ The modem aeromagnetic datasets also allow us to res to be traced across this transition zone between the map the continuation of basement faults from the o ns­ mainland and offshore areas. The areas offshore Nord­ hore to the offshore realm. We have concentrated on land are covered with modem aeromagnetic surveys outlining the offshore extension of late Caledonian col­ acquired by the Geological Survey of Norway during lapse structures that have been mapped on the main­ the last 15 years. Several groups of high frequency ano­ land recently (Braathen et al. 2000, 2002; Osmundsen et malies, not discemible in the old dataset, have been al. 2003; Nordgulen et al. 2002). revealed. The improved quality relates to the use of more sensitive magnetometers, doser flight line spa­ We have addressed the mechanism of isostatic compen­ cing, more accurate navigation and improved proces­ sation of the in order to sing and data presentation techniques. define the regional gravity field fo r the 3D modelling. Assuming that the region is dose to isostatic equili­ Previous studies have taken the advantage of potential brium, these uplifted mountainous areas along the field data to extrapolate basement structures across the Norwegian-Swedish national border must be supported 244 O. Olesen et al. NORWEGIAN JOURNAL OF GEOLOGY

Assuming neither nor sedimentation, Neogene o• 10' 20' 20' uplift implieseither that the Moho depth has increased during this time period, or that substantial volumes of low-density rocks have been introduced in the crust or mantle. Rohrman & van der Beek {1996) and Riis {1996) proposed a Neogene uplift of more than 1000 metres in southern Norway from apatite fission track data, modelling of geomorphology and extrapolation of the offshore late Tertiary stratigraphy. Riis {1996) and Hendriks & Andriessen {2002 b) have also propo­ sed a Neogene bedrock uplift of more than 1000 min the -Ve sterålen area, and 600 metres on the mainland to the east.

Geological setting

The bedrock geology (Fig. 2) of the mainland Nordland and Troms is dominated by the Caledonian Upper and Uppermost Allochthons that were thrust onto the Pre­ cambrian basement during the Scandian continent­ continent collision in Silurian and Devonian time metres 100 o 100 -31 61 -1135 -149 -25 55 146 272 4661021 (Roberts & Gee 1985). In the Devonian, the nappes km were dismembered by a late gravity collapse phase of the Caledonian orogen (Rykkelid & Andresen 1994; Fig. l. Compilation of topography!bathymetryfrom Fennoscandia Braathen et al. 2000, 2002; Eide et al. 2002; Nordgulen (Dehls et al. 2000). The yellow lines depict the two mountainous et al. 2002; Osmundsen et al. 2003). The Lofoten-Ring­ areas in Scandinavia. The500 m and l 000 m contours are outlined. TheNordland area is outlined by the black box. vassøya area and the Central Norway basement window in Trøndelag make up the bulk of the Precambrian basement terrains to the north and south, respectively. at depth by substantial volum es of low-density material These geological provinces are of deep-seated origin within the crust or the mantle, or by relief at the with associated regional magnetic and gravimetric crust/mantle or lithosphere/asthenosphere interfaces. anomalies (Figs. 3 - 5). The positive gravity anomalies The fo rmer models represent a Pratt-type isostasy reflect a shallow Moho discontinuity and uplifted, model, while the latter two are consistent with an Airy­ high-grade rocks of intermediate density {Sellevoll Heiskanen type model. 1983; Skilbrei 1988). The exhumation of the deep-crus­ tal rocks in the Lofoten area and the Central Norway The western Scandinavian mountains (Fig. l) are partly basement window has lately been interpreted as a core of Plio-Pleistocene age (Rohrman et al. 1995; Riis 1996) complex fo rmation denudated during large-magnitude and constitute part of a circum Atlantic belt of Neo­ extension along detachments (Hames & Andresen gene uplift including the mountains in Scotland, Sval­ 1996; Braathen et al. 2000). Several minor basement bard and East Greenland (e.g. Japsen & Chalmers windows occur within the Caledonian thrust belt bet­ 2001). As yet, there is no generally accepted hypothesis ween these two larger basement areas (e.g. the Børge­ that explains the Neogene uplift phase of these moun­ fjell and Nasafjall tectonic windows). These tectonic tainous areas. However, several different models have windows have been interpreted in terms of antiformal been proposed: stacks or compressional duplexes, consisting of base­ ment rocks within the Lower Allochthon (e.g. Greiling l) Glacial erosion, isostatic uplift (Dore 1992; Riis & et al. 1998). Recent investigations indicate that the Bør­ Fjeldskaar 1992) gefjell and Nasafjall windows are bordered in the west 2) Migrating p hase boundaries (Riis & Fjeldskaar by ductile to brittle extensional shear zones, and that 1992) the windows represent erosional sections through fo ot­ 3) Pre-subduction instability (Sales 1992) wall culminations {Braathen et al. 2002; Osmundsen et 4) Plate reorganization- intraplate stress (Jensen & al. 2003). Thus, these structures are also candidates fo r Schmidt 1992) extensional unroofingas they appear today_ 5) Mantle convection {Bannister et al. 1991; Stuevold et al. 1992; Vågnes & Amundsen 1993) The basement infrastructure of the Lofoten-Ringvass­ 6) Mantle diapirism (Rohrman & van der Beek 1996) øya area is dominated by Precambrian polymetamor- NORWEGIAN JOURNAL OF GEOLOGY Onshore and offshore geologyin the Nordland area, northern Norway 245

ges (Blystad et al. 1995). At the base Cretaceous level (Fig. 6), the southern Trøndelag Platform corresponds to a seaward dipping, relatively little faulted monocline, which grades northward into the gently synformal Hel­ geland Basin. The Trøndelag Platform is underlain by Upper Palaeozoic and Tr iassic strata. Upper Palaeozoic sedimentary rocks reach their greatest thickness in the central part of the Trøndelag Platform, suggesting a westward shift of the depocentre from Permian to Cre­ taceous time. The Trøndelag Platform terminates along the Nordland Ridge, which changes in orientation from NNE-SSW to alm ost E-W immediately offshore the Meløy area. The Ylvingen Fault Zone makes up the southern margin of the Mesozoic Helgeland Basin. The western boundary of the U trøst Ridge is bound by large faults with down to the northwest throws and forms the landward boundary of the Røst Basin that is covered by Early Tertiary tlood basalts. The Harstad Basin (Gabrielsen et al. 1990) extends from offshore Andøya Con1lnentaJ­ caledonian rod. The belt ture zones extend into the Nordland-Troms area: the of Proterozoic igneous rocks extends to southern Swe­ Bivrost, Jennegga and Senja fr acture zones. den and is characterised by regional magnetic anoma­ lies (Henkel & Eriksson 1987; Fichler et al. 1999).

The Arjeplog-Karesuando and the Bothnian-Senja fault Datasets complexes (Henkel 1991; Zwaan 1995; Olesen et al. A total of seven offshore and two onshore aeromagnetic 1997b) constitute two major shear zones within the Pre­ surveys were compiled (Fig. 4). The surveys were acqui­ cambrian basement (Fig. 6). A prominent set of high­ red by the Geological Surveys of Norway and Sweden, angle, ductile to brittle faults called the Vestfjorden­ the US Naval Research Laboratory and Hunting Ltd. Vanna fault complex (Andresen & Forslund 1987) cont­ Survey details are described by Am (1970, 1975), Ole­ inues from the Vestfjordenarea NNE-wards through the sen & Smethurst (1995), Olesen et al. (1997a,b), Ruo­ sounds to the east of Hinnøya, Senja, Kvaløya, Ringvass­ toistenmaki et al. (1997), Skilbrei & Kihle (1999) and øya and Vanna and into the Lopphavet area (Figs. 2 & Mauring et al. (1999). The modem aeromagnetic sur­ 7). The fa ult complex constitutes the contact between veys (Fig. 5) (Hunting- 1986, LAS-89, NAS-94 and VAS- the Precambrian terrain to the west and the Caledonian 98) were tlown with a line spacing of 2 km and a tie­ nappe sequence to the east. There is evidence of both line separation of S-8 km. The sensor altitudes fo r the Late Palaeozoic and Mesozoic activity along the fault different surveys vary between ISO and 500 metres. The complex (Andresen & Forslund 1987; Olesen et al. NGU-73, LAS-89 and US Naval Research Laboratory 1997b). surveys have been reprocessed applying the new median levelling technique by Mauring et al. (2002). The Nordland offshore area covers the northern Trøn­ Verhoef et al. (1996) and Olesen et al. (1997a,b) publis­ delag Platform, the Træna, Vestfjorden, Ribban and hed previous data compilations from the area, but the Røst basins, and the Nordland, Lofoten and Utrøst rid- 246 O. Olesen et al. NORWEGIAN jOURNAL OF GEOLOGY

used to level the surveys. The amplitudes of the main­ land Bouguer gravi ty field are -140 and + 130·1 o-5 ml s2 (mGal). This contrast of 270·10-5 mfs2 is large even compared with the maximum gravity difference in the Andes and Himalayas of 500 ·10-5 m/s2. The large gra­ vity amplitude points to a large-scale lithospheric dis­ continuity within the area.

Separation of the residual fieldin the magnetic and gra­ vity datasets was carried out in the frequency domain using 8 and 100 km Gaussian filters, respectively. Sha­ ded relief versions of the high pass filtered aeromagne­ tic datasets in grey-tone are superimposed on the colour-contoured total field in Fig. 4. The analytic sig­ nal (total gradient amplitude) of the high-resolution aeromagnetic data was also used to enhance the short wave-length (shallow) content of the data (Fig. 5). The main structural elements of the offshore survey area (Blystad et al. 1995) and the sedimentary subcrop pat­ tern (Rokoengen et al. 1988) are shown on the analytic signal map (Fig. 5).

Petrophysical data (dens i ty, magnetic susceptibility and remanence) of approximately 5100 rock samples from the mainland were used in the interpretation of the potential field data (Table 1). The results from the petrophysical survey are detailed in Mørk & Olesen (1995) and Olesen et al. (1991, 1997b). We have used density data from shallow drilling in the Helgeland­ Brønnøysund basins (Bugge et al. 1993) and from Fig. 3. Compiled Bouguer gravity data from Scandinavia compiled petroleum exploration wells on the Nordland Ridge from Ko rhonen et al. (1999) and Skilbrei et al. (2000). The yellow and the Utgard High ( Olesen et al. 1997b) to constrain lines depict the two mountainous areas in southern and northern the gravity modelling. The densities of sedimentary Scandinavia. CN - Central Norway basement window; LO - Lofoten area; VS - Vestjjorden-Sarek area. The extent of the Nordland area is sequences calculated from density lags are shown in outlined by the black box. Table 2. These estimates were used to constrain the 3D modelling. present dataset is considerably more detailed since it The base Cretaceous map (Fig. 6) was mainly compiled contains both modem high-resolution surveys and from et al. (1992} and Brekke (2000}. The map reprocessed versions of the vintage datasets. has been modified in the Vestfjorden area from re­ interpretation of the NPD seismic reflection data in a The mainland gravity dataset consists of terrain-cor­ SeisVision workstation utilising information from the rected Bouguer data from the University of Bergen, 6610/3-1 well on the northern flank of the Nordland Norway; the University of East Anglia, Great Britain; Ridge and potential field data. A base Tertiary grid the Norwegian Mapping Authority and the Geological adopted from Brekke (2000} has been applied in the Surveys of Norway and Sweden (Svela 1971; Chroston regional 3D modelling. 1974; Henkel 1991; Ruotoistenmaki et al. 1997; Skilbrei et al. 2000}. In addition, marine data were obtained Offshore basement depth estimates have been compiled from the Norwegian Mapping Authority and the Nor­ fr om Olesen & Smethurst (1995} and Olesen et al. wegian Petroleum Directorate, and satellite altimetry in (1997b). These depth estimates were calculated us ing the deep-water areas from the National Survey and the autocorrelation algorithm of Phillips (1979), the Cadastre Denmark (Andersen & Knudsen 1998; Skil­ Euler deconvolution algorithm of Reid et al. ( 1990) and brei et al. 2000}. The complete Bouguer reduction of a least-squares optimising algorithm of Murthy & Rao the gravity data was computed using a rock density of (1989). To further reduce the inherent ambiguity in the 2670 kgfm3 on land. The simple Bouguer correction at modelling of the potential fielddata, petrophysical data sea was carried out using a density of 2200 kg!m3. The (Tables l & 2} from both the basement rocks and the International Standardisation Net 1971 (I.G.S.N. 71} sedimentary succession were used in the forward and the Gravity Formula 1980 fo r normal gravity were modelling. Incorporation of seismic structural inter- NORWEGIAN JOURNAL OF GEOLOGY Onshore and offshore geologyin the Nordland orea, norther n Norway 247

� Detachment -250 -158 -109 -76-60-44-28 -8 15 41 71 105 168 256 403 770

p"' Potential detachment nT 50 o 50 100 � lnterpretation profile iiijji,MM--- km

Fig. 4. Shaded relief of high-pass filteredaeromagnetic data superimposed on the colour map of the total aeromagnetic field referred to IGRF. The violet lines show the interpreted inlineslcrosslinewithin the 3D model. Interpretations along every third inline (thicker line) are displayed in Fig. 12. CN- Central Norway basement window; MI - Myken intrusive complex; UH - Utgard High; UR - Utrøst Ridge; VA - Vikna magne­ tie anomaly. Devonian detachment zones on the mainland (Rykke/id & Andresen 1994; Braathen et al. 2000, 2002; Osmundsen et al. 2003; Nordgulen et al. 2002) have been extrapolatedinto the offshorearea. pretation of the uppermost offshore sedimentary particular, this applies to areas where we expect to find sequences improved the gravity interpretation of the shallow, low-magnetic basement continuing from on­ deeper levels of the basins. In areas where the gravity shore areas, or where down-faulted Caledonian nappes and magnetic datasets gave diverging depth estimates, occur beneath the deep basins. Depth estimates origi­ the gravity interpretation was given highest weight. In nating from high-frequency magnetic anomalies inter- 248 O. Olesen et al. NORWEGIAN jOURNAL OF GEOLOGY

Fig. 5. Shaded relief map of analytic signal (total gradientamplitude) from high reso­ lution aeromagnetic surveys with super­ imposed subcrop pattern (Rokoengen et al. 1988) and ]urassic/Early Cretaceous faults (Blystad et al. 1995). VH-Vega High, YF

- Ylvingen Fault Zone, FB - flow basalts. The map offers a p/ane view of the sub­ crop and fault pattern in the Nordland area. Blue arrows show magnetic discor­ dances in the magnetic basement pattern; Nesna shear zone, Grønna and Hamarøya faults. The red arrows show two offshore ENE-WS W-oriented faults cuttingthrough Late Palaeozoic and Mesozoic sedimentary rocks. The southernmost arrows represent the Ylvingen Fault Zone (Blystad et al. 1995).

O 50km lliiiiiiiiill --w

LEG END ss· - Subctopplng sacllmentary unlts (Rokoengan et al. 1988) - Jurasslc/Early Cnttoceous faults (Biystad at al. 11115)

preted to represent magnetic intra-sedimentary igne­ H = h t h t ( ) coas + topo • p topo l p roo l ous rocks were excluded from the depth to basement contouring. The thickness of the mainland Caledonian where nappes is included in the basement structure map and H = depth to Airy root the 3D modelling (Figs. 7 & 12-13) and has been com­ hcoast = depth to the Airy root at the coast piled fromearlier gravity and aeromagnetic interpreta­ htopo = elevation relative to sea level tions in Sweden (Lind 1986), Troms (Olesen et al. 1990) P root = density con trast a cross the Airy root and Nordland (Sindre 1997). The depth to the Precam­ P topo = density of topography brian basement in Troms and northern Sweden was estimated using the magnetic autocorrelation algo­ We have applied a density of 2670kgfm3 to the moun­ rithm, while fo rward gravity modelling was applied in tains in Norway, a density contrast of 330kgfm3 at the the Nordland area. base of the Airy root, and a crustal thickness of 30 km along the coast. The Moho depth map by Kinck et al. (1993) was upda­ ted by more recent refraction lines in the V øring Basin The gravitational attraction from the 'root' was calcula­ and the Lofoten margin (Mjelde et al. 1992, 1993, ted in the fr equency domain using the AIRYROOT 1998). Existing profiles along the Lofoten-Vesterålen algorithm (Simpson et al. 1983). The isostatic residual islands (Sellevoll 1983), the Blue Road across the Nor­ was achieved by subtracting the gravity response of the wegian-Swedish border (Lund 1979) and Fennolora Airy-Heiskanen 'root' from the 'observed' Bouguer gra­ Profile in Sweden (Guggisberg et al. 1991) were also vity data (Fig. 3). Fig. lO shows the gravity residuals for included in the updated Moho depth map (Fig. 8). the Nordland area.

The main model is based on a 30 km Moho depth along the coast of Norway. These depths agree in general with lnterpretation methods Moho depths (Fig. 8) obtained from refraction seismic studies compiled by Kinck et al. (1993). Applying the Isostasy Airy root equation [ equation (l)] results in a maxi­ An Airy-Heiskanen 'root' (Heiskanen & Moritz 1967) mum Moho depth of 40 km below the mountainous shown in Fig. 9 was calculated from the topographic part of Scandinavia. However, the low altitude areas of and bathymetric dataset using the fo rmula: eastern Scandinavia have an even thicker crust, imply- NORWEGIAN JOURNAL OF GEOLOGY Onshore and offshore gealogy in the Nordland area, northern Norway 249

3D modelling The potential fielddata have been interpreted using the 3D modelling packages ModelVision and Oasis Montaj (Encom 2001; Geosoft 2000 & 200la,b). The existing interpretation described in the section above (base Te r­ tiary, base Cretaceous, offshore depth to crystalline basement and onshore depth to Precambrian basement rocks, Moho depth) and lithosphere thickness (Calcag­ nile 1982) have been applied as the initial model fo r the interpretation. The density of the lithospheric mantle was calculated by assuming a stepwise increase (200 K) in temperature from the Moho (c. 500°C) to the asthe­ nosphere at a temperature of c. 1300°C. The applied thermal expansion fa ctor is 3.2xl0-5K·l, which is adap­ ted from an equivalent study by Breivik et al. (1999) in the southwestern Barents Sea. The 3D model was con­ structed by slicing l O horizons by 17 parall el pro files, spaced 35 km apart. The total gravity response is calcu­

msec lated as the sum of the responses from each individual � �P����l:,"g":; o�% -8266 -6881 -5765 -4446 -3479 -2274 -1785 -1494 -1033 -532 body along all 17 inlines. We have modifiedthe deeper / FauH ,..- Detachment 50 o 50 100 interfaces to fit the regional field, and subsequently

,..- Potential detachment km shallower interfaces to fit the high-frequency part of the gravi ty field.Eve ry third line of the 3D model is dis­ played in Fig. 12. Magnetic data have been included in Fig. 6. Base Cretaceous compiled from Brekke et al. (1992) and the modelling along Crossline l (Fig. 13) intersecting Brekke (2000). The depths in the Vestfjorden area have been reinter­ preted in a seismic workstation environment uti/ising information the Inlines 4-8. from the 6610/3-1 well on the northern flank of the Nordland Ridge, NPD reflection data in the Vestfjorden area and potential field data. Fault zones (Fig. 14) within the basement and partly BP - Bivrost Fracture Zone; JF - Jennegga Fracture Zone; SF - Senja within the sedimentary units were interpreted from the Fracture Zone; SH - Sandflesahigh; YF- YlvingenFault Zone; KDZ geophysical maps. High gravity gradients along the - Kol/straumen detachment zone; NSZ - Nesna shear zone. basin boundaries are generally interpreted to be caused by faults, but may in some cases be related to flexuring ing that the crust and/or the mantle must be denser in and steeply dipping sedimentary bedding. this area. This 'isostatic Moho depth' below the Lofo­ ten-Vesterålen islands and the mountainous areas of northern Scandinavia is significantly different from the Moho depth interpreted from the refraction seismic Results line, e.g. along the Blue Road profile (Figs. 8 & 9). To investigate the potential contribution from intra-crus­ Regional isostasy tal and/or intra-mantle low-density rocks to the iso­ Locating the isostatic compensating low-density rocks stasy and gravity fieldin the region, we (a) simulate an at shallow depth, yields a gravity field that is the most intra-mantle low-density rock by increasing the depth similar to the observed gravity fieldin northern Scand­ to the Airy root (Pcoast in equation (l)] or (b) make inavia (Fig. 11). The calculated RMS is lowest (25.0·10-5 the Airy root shallower (i.e. decrease hcoast) to test a mfs2) fo r the 10 km Airy root model below the nor­ potential intra-crustal low-density contribution to the thern Scandinavian mountains. This differs signifi­ isostasy. The former model produces an anomaly with cantly from the southern mountains where the RMS is longer wavelength and lower amplitude than the latter. lowest (17.6·10-5 mfs2) fo r the 45 km Airy root model We tested various models by calculating the gravity (Fig. 11). A similar comparison between 100 km low­ response from Airy roots with coastal depths (hcoast) pass filtered Bouguer data and the gravity responses varying from 10 to 60 km, including the original 30 km from the various Airy roots yield almost identical model. Root mean squares (RMS) and correlation coef­ results, showing that the correlation is not an artefact ficients (Fig. 11) between the Bouguer gravity grid and caused by the high-frequency component. The results the gravity response were calculated fo r the northern imply that the mountains in northern Scandinavia are and southern Scandinavian mountains. Fig. lla-b partly compensated by low-density rocks within the shows the scatter-diagrams fo r selected models in the crust. The results agree with the findings from the Blue two investigated areas. Road refraction experiment (Lund 1979) that the upper crust extends to larger depths (27 km) below the 250 O. Olesen et al. NORWEGIAN JOURNAL OF GEOLOGY

Table 1.

Rock Unit/Type No. Density Q-value Susceptibility

mean std. mean std; mean std.

Uppermost Allochthon Rodingsfjell Nappe Complex 821 2787 137 3.26 12.8 0.0060 0.034 Helgeland Nappe Complex 488 2811 126 4.41 15.9 0.0032 0.018 Bindal Batholith 304 2738 136 3.30 11.6 0.0040 0.014

Upper Allochthon Nappe Complex 164 2811 130 1.60 4.6 0.0032 0.017

Precambrian Basement Senja, Kvaløy and Ringvassøy, gneiss etc. 384 2716 106 2.11 5.4 0.0078 0.020 Senja, Kvaløy and Ringvassøy, greenstone belts 125 2942 136 0.46 0.6 0.0266 0.059 The Lofoten area 470 2764 130 1.36 5.6 0.0339 0.050 Tysfjord Granite Complex 607 2678 79 1.22 4.0 0.0074 0.024 Sjona-Høgtuva tectonic windows 1140 2643 39 0.0084 0.012 Central Norway basement window (Trøndelag) 419 2729 106 1.04 2.8 0.0091 0.026

Table l. Arithmetical mean and standard deviation of density, susceptibility and Q-values measured on hand specimens from the Nordland area. The measurements are extractedfrom the national petrophysicaldatabase at the Geological Surveyof Norway. Skilbrei ( 1988 ), Diesen et al. ( 1990), Mørk & Olesen ( 1995) and Olesen et al. (1997b) have reported a more detailed presentations of the statistical data. SI units are used. Densityvalues are reported as kgfm3

Table 2.

Well6609/7-l Well6607/5-l Well6507/2-l Density IKUwell IKUwell Deniity Phillips Esso NorskHydro estimates from 6611109-U-01 6611/09-U-02 adapted Petroleum UtgardHigh Dønna Terrace sonl,lbouys Helgeland/ Helgeland/ fur Nordland Univ.of Bergen Brønnøysund Brønnøys\md modelling Ridge Basin Basin

Water depth 250m 368m 381m 352 m 355m 1.03 Start of log 1025m 900m 1050m Pleistocene - 2.21 (!220m) Tertiary 2.10 (1635m) 2.21 (2510m) 2.18 (2005m) 2.25 2.20 Cretaceous 2.25 (1845m) 2.32 (3785m+) 2.37 (3610m) 2.30 2.35 Jurassic - - 2.54 (4400m+) 2.35 2.45 Triassic - - - 2.59 (200m) 2.57 (280m+) 2.58 Permian 2.31 (1920m) - - 2.58 (560m+) 2.58 Basement 2.64 (1960m+) - - 2.70-2.85 2.75 Magmatie

underplating 3.00

Mande 3.25

Table 2. Densityof sedimentary sequences fromdensity logs of wells in the Nordland I, Il and IV areas (Bugge et al. 1993; Olesen et al. 1997b). The estimates (in l OOO*kglm3) are used for the 3D gravitymodelling. Numbers in parenthesis show depth (relative to sea-bed ) of the base of the various sequences. mountains of northern Norway than below the Pre­ The Pratt type of isostasy is consequently deduced to be cambrian shield to the east (13 km), [to the east of important for the isostatic compensation of the Scand­ shotpoints l and 4, respectively, in Figs. 7-8]. The inavian mountains, supporting the condusions of Skil­ mountains in southern Norway are, from our results, brei (1988) for central Norway. The Neogene uplift of partly supported by low-density rocks below the Moho. southern Norway may originate from low-density NORWEGIAN JOURNAL OF GEOLOGY Onshore and offshore geology in the Nordland areo, northern Norway 251

Fig. 7. Regional basement structures within the offshore-onshore Nordland area. The depth to basement surface represents depth to crystalline rocks in the offshore region while it constitutes the depthto Precambrian gneisses and granites in the onshore area. GF - Grønna fault, HF - Hamarøya fault, KDZ - Kollstraumen detachment zone, NSZ - Nesna shear zone, SSZ - Sagfjord shear zone; SH - Sandflesahigh; UH - Utgard High; UR - Utrøst Ridge; NH - Nyk High; VV- Vest­ fjorden-Vanna fault complex; MI - Myken intrusive complex; BF - Bivrost Fracture Zone; JF - ]ennegga Fracture Zone; SF - Senja Fracture Zone; YF - Ylvingen Fault Zone. The Caledonian nappes are most likely down-faulted to a maximum depth of c. 5 km along late Devonian detachments. Black dots along the Blue Road and Fenno­ lora profiles show shotpoints.

Basement depth - km '2.�p. Magnetic sea floor � spreading anomaly -11 -9 -7 -6 -5 -4 -3 -2 -1 -0 •• ••••••. ,!l ,...... Regional fault zone "".. Detachment 50 o 50 100 ,."",. Potential detachment km mantle rocks. The results are in agreement with the southern Scandinavian mountains is much deeper and conclusions of Riis (1996) and Lidmar-Bergstrom may consequently be situated within the lithospheric (1999) that the southern Norwegian was partly mantle or at the lithosphere/asthenosphere interfaces, uplifted in the Neogene, while the northern Scandina­ consistent with the mantle diapirism model by Rohr­ vian mountains originated mainly as rift-shoulders in man & van der Beek (1996). This diapir model, based late Cretaceous to early Te rtiary times. Hendriks & on the Rayleigh-Taylor instability can give explanation Andriessen (2002a) reported that analyses of observed to the equidistant spaced doming ( -900 km spacing) apatite fission track data along a profile from Lofoten along the northern North Atlantic rifted margins. into Sweden fit best with those expected from a retrea­ However, the model cannot explain that the most of the ting scarp model (Beaumont et al. 2000). Neogene upliftoccurred in southern Norway compared to northern Norway. Thus it is possible that a modified This modelling shows that the uplift of the northern mantle diapirism model linking the southern Norwe­ Scandinavian mountains was not caused by low-den­ gian diapir to the Iceland hotspot can explain the sity material within the mantle. The main uplift phase observed uplift. Bijwaard and Spakman (1999) succee­ is therefore neither caused by mantle convection along ded in imaging the Iceland plume to great depths in the the boundary between a warm oceanic asthenosphere mantle using tomographic analysis of teleseismic data. and a colder continental asthenosphere (Bannister et al. Lateral branches of the plume were interpreted to 1991; Stuevold et al. 1992; Vågnes & Amundsen 1993), extend below Great Britain and southern Scandinavia. nor by mantle diapirism (Rohrman & van der Beek 1996). Part of the uplift of the Lofoten area may alter­ natively be explained by the isostatic effect following Transcandinavian Igneous Belt glacial erosion similar to the Barents Sea region uplift The aeromagnetic data (Fig. 4) show that Precambrian (Dore 1992; Riis & Fjeldskaar 1992) and the Svalbard intrusive rocks in the Lofoten-Senja-Ringvassøya base­ area (Skilbrei 1998). The results from the isostatic stu­ ment complex are continuous below the Vestfjorden dies also show that the compensating mass below the Basin and the Caledonian nappes to the tectonic win- 252 0: Olesen et al. NORWEGIAN JOURNAl Of GEOlOGY

Refraction seismics ,.... Detachment 50 50 100 / Blue Road - Fennolora ,.., Potentiel detachment km / Lofoten-Vesterålen ".... Regional fault zone km / OBS Lofoten -47 -46 -45 -40 -32 -29 -25 -23 -17 -16 / OBSVøring

Fig. 8. Depth to Moho compiled from refraction seismic studies Fig. 9. The Airy root calculated from the topography!bathymetry in (Lund 1979, Kinck et al. 1993, Mjelde et al. 1992, 1993, 1998 and Fig. l. A Gaussian 200 km lowpass-filter has been applied to the Sellevoll 1983 ). The black lines show the interpreted inlineslcrossline Airy-root grid to smooth the high-frequency variation. The yellow within the 3D model. Interpretations along every third inline (thic­ lines depict the two mountainous areas in Scandinavia. TheNord­ ker line) are displayed in Fig. 12. The shallow Moho along the main land area is outlined by the black box. Late ]urassic - Early Cretaceous rift axis is deflected by the Surt, Bivrost and Vesterålen transfer zones. dows in Nordland and further into Sweden. They con­ in Sweden. When including a thin layer of granites in stitute part of the Tr ansscandinavian Igneous Belt this area, the deep TIB-body will tilt eastwards and (TIB) (Gaal & Gorbatschev 1987). A regional negative increase in overall thickness. The estimated depth of 22 gravity anomaly occurs in the Vestfjorden-Sarek area km is therefore most likely a minimum value. Analogue (Figs. 3 & 10), immediately to the east of the Lofoten­ deep granites (c. 30 km) within the Sierra Nevada Bat­ Vesterålen positive gravity anomaly, and includes the holith in California are related to Mesozoic plate sub­ area with the highest mountains in northern Scandina­ duction along the western edge of the North American via. The oval, wide (200 km)shape of the anomaly may plate (Ducea 2001). This mechanism also involves indicate a deep-seated source. Low P- and S-wave vel­ large-scale continental shortening and the formation of ocities in the mantle below the mountainous area of voluminous lower crustal residues such as granulites Scandinavia (Bannister et al. 1991) also indicate the and eclogites. The Lofoten-Vestfjorden-Sarek region presence of low-density rocks in the mantle. An itera­ may be in regional isostatic equilibrium due to the tive, least-square optimising algorithm (Marquardt opposing effects of voluminous bodies with positive 1963) shows that to explain the observed steep gradi­ and negative density contrasts. This phenomenon may ents, the low-density body occurs at relatively shallow also contribute to the observed high rock stress in this depths (and is most likely outcropping). Applying a particular area of Norway (Myrvang 1993). density contrast of -100 kg!m3 (Table l) between the granitoid rocks and the intermediate rocks of the Transscandinavian Igneous Belt (equivalent to the gra­ Late Devonian detachments nites in the Tysfjord area and the high-grade rocks in On the assumption that the thickness of the Caledo­ the Lofoten area, respectively) shows that the granitoids nian nappes in Nordland was relatively uniform imme­ must extend to a depth of c. 22 km, making up a signifi­ diately after Silurian nappe emplacement, the strongly cant part of the c. 40 km thick crust in the area (Figs. varying depth to Precambrian basement may be the 12a-c & 15). These deep granites constitute parts of the result of later tectonic deformation and late- to post­ TIB. The bedrock map (Fig. 2) shows that the TIB-gra­ orogenic extensional and strike-slip deformation. nites extend to the east of the negative gravity anomaly NORWEGIAN JOURNAL OF GEOLOGY Onshore and offshore geology in the Nordland area, northern Norway 253

Refraction seismics 50 o 50 100 ------.,/ Blue Road - Fennolora km / Lofoten-Vesterålen 10 -5m/s2 / OBS Lofoten -52-30-22-16-11 -6 -1 357911 14 19 23 28 34 41 50 57 63 / OBSVøring

Fig. 10. Residual gravity after isostatic correctionof Bouguer gravity data. The isostatic correction has been calculated applying the AIRYROOT algorithm (Simpson et al. 1983) to the topographylbathymetry dataset in Fig. l (rock density 2670 kgfm3 on land, 2200 kgfm3 at sea and a crust­ mantle density -contrast of 330 kgfm3 ). The violet lines show the interpreted inlines/crossline within the 3D model. Interpretations along every third inline (thicker line) are displayed in Fig. 12. CN - Central Norway basement window; LO - Lofoten Ridge; MI - Myken intrusive complex; UH - Utgard High; UR - UtrøstRidge.

Notably the Caledonian successions (Figs. 2 & 7), inclu­ A significant portion of the basement in the offshore ding the Uppermost Allochthon, occur in the hanging Helgeland Basin and along the Nordland Ridge is low­ wall block of the observed Devonian detachments and magnetic. The depth to magnetic basement on both shear zones in the Nordland area (Osmundsen et al. flanks of the Helgeland Basin is between 8 and l O km, 2003; Braathen et al. 2002). The Caledonian nappes are i.e. 2-5 km deeper than the depths obtained from gra­ therefore most likely down-faulted to a maximum vity modelling. The low-magnetic nature of the base­ depth of c. 5 km (below the present surface) along late ment is most likely caused by down-faulting of the Hel­ Devonian detachments as illustrated in Fig. 7. geland Nappe along the offshore extensions of the 254 O. Olesen et al. NORWEGIAN jOURNAL OF GEOLOGY

(5-15° to the SW). The Kollstraumen detachment repre­ A. Northern Scandinavian mountains sents the northward termination of the large magnetic

o Corr.coeff0.28 = body situated at a depth of 8-10 km in the Trøndelag -20 fl .. RMS=29.3 Platform. The associated Vikna magnetic anomaly (220 � a. nT ) (Fig. 4) is comparable in size and parallel to the "' .al g "' Roan anomaly on the Fosen Peninsula, l 00 km to the " !!l "' south, and could therefore have a similar cause. The iil -100 < magnetic rocks in Roan were formed during Caledonian � high-grade metamorphism (Moller 1988, Skilbrei et al. 1991), and were most likely uplifted during the Devo­ nian orogenic collapse (Braathen et al. 2000).

Some of the offshore, low-magnetic basement may also B. Southern Norwegian mountains constitute Devonian deposited above the offshore extensions of the Nesna and Kollstraumen o Corr.coelf= 0.62 • Corr.coeff0.65 = fl RMS=20.1 RMS= 17.8 detachments, i.e. similar to the tectonic situation fur­ o � a. ther south along the Nordfjord- and Høybakken "' detachments. Bugge et al. (2002) argued that a shallow­ 40 � � marine, reddish, Permian unit offshore Hel­

-80 l geland represents re-deposited Late Devonian-Early :fl Permian continental sedimentary rocks. This interpre­ tation supports the hypothesis that Devonian sandsto­ Gravilyresponse 50km nes could have been deposited above the Caledonian

-120 -ao o -120 -ao -40 o nappes during gravity collapse of the orogen. An E-W trending section of the potential offshore Kollstraumen c. detachment coincides with the western part of the Ylvingen Fault Zone (Figs. 6 & 7), indicating that this late Mesozoic feature is governed by a deep-seated structure. Interpretation of the basement topography reveals a central north-trending ridge at c. 5 km depth separating the Helgeland Basin into two 6-7 km deep 10 20 30 40 50 60km Depth Airyof Root at sea level sub-basins (Fig. 7). The easternmost sub-basin repre­ sents the Permian Brønnøysund Basin (Dore et al. 1999; Bugge et al. 2002). The 3D modelling of gravity Fig. 11 . Comparison of calculated gravity response of 'Airy roots' at different depths with the observed Bouguer gravity for the mountai­ data along Inline 3 (Fig. 12e) using constraints from nous part (above c. 500 m asl) of northern and southern Scandina­ densities (Table 2), indicates that the basement of the via. A) Northern Scandinavia; scatter-diagrams of the gravity Trøndelag Platform is 2 km shallower than interpreted response of Airy roots at the depths of 30 km and l O km (at the coast) from the seismic profile (4.5 versus 6.5 km). The base­ versus observed Bouguer gravity. The lowermost duster in the dia­ ment reflector is, however, not well defined in these grams represents data from the Vestjjorden-Sarek area. B) Southern profiles. An alternative model to the shallower base­ Scandinavia; scatter-diagrams of the gravityresponse of Airy roots at ment is a higher density than 2580 kg!m3 for the Tr ias­ the depths of 30 km and 50 km (at the coast) versus observed Bou­ guer gravity. C) Root mean squares (RMS) quantify the similarity sic and Permian sedimentary successions. The latter between the observed Bougner gravity and calculated effect of Airy model implies practically no porosity of these sedi­ roots at various depths. The best fit occurs for a shallow depth (l O ments, and we therefore prefer the former model. km) in northern Scandinavia while a deep source (Airyroot of 40-50 km at the coast) seems to compensate for the southern Scandinavian The Sagfjord shear zone (Braathen et al. 2002) farther mountains. Theunits of the gravitydata are l o-s mfs2 (mGal). north may analogously to the Nesna shear zone be extended beneath the offshore Vestfjorden and Ribban Devonian Nesna shear zone and the Kollstraumen basins. A significant part of the basement below these detachment which have been mapped on land by Braat­ basins consists of low-magnetic amphibolite-facies hen et al. (2000), Eide et al. (2002), Nordgulen et al. gneisses that may have been emplaced along Devonian (2002) and Osmundsen et al. (2003). Another alterna­ detachments (Fig. 16). Relatively flat-lying deformation tive is the occurrence of low-magnetic, amphibolite­ zones in the uppermost mountainous part of Hinnøya facies Precambrian gneisses similar to the rocks on are candidates for mainland extensions of these structu­ eastern Hinnøya. However, the model along a crossline res. Low-grade rocks are emplaced on top of high-grade (Fig. 13) through the 3D model suggests that the rocks along these structures (Tveten 1978; E. Tveten Bivrost Lineament is a folded, gently dipping structure pers. comm. 2001). The Nordfjord-Sogn detachment (Norton 1987) constitutes an equivalent boundary NORWEGIAN JOURNAL OF GEOLOGY Onshore and offshore geology in the Nordland area, northern Norway 255

A. lnline 15 D. lnline 6

"'

-00

nT ""'

o o

10

:æ

"'

.., ""'

E. lnline 3 B. lnline 12

1 ���----�------��----�------��� "' 1----=-"""""'""'- ... -100 �,_____ � '------�-----!�":::��=------�-----J nT 1000

10

:æ "'

.., ""' Fig. 12. Five cross sections through the 3D gravitymodel; Inlines 3, 6, 9, 12 and 15. Applied densities are shown in Table 2. Sedimentary succession: yellow - Tertiary, brown - Tertiary flow basalts and oce­ anic crust, light green - Upper Cretaceous, green- Lower Cretaceous, C.lnline 9 blue- Pre-Cretaceous, pale pink- pre-Jurassic; dark green- Caledo­ nian Nappes. Theblack lines show the observed gravityand magnetic

100 ftelds while the blue lines illustrate the gravity response of the total 3D model. The dashed purple line represents the regional gravity

-100 response fromthe mantle. m �----�------�----��----��--�+---� ""'

10

:æ "' ..,

100 200 JOD .... 500 ""'

through the gneisses of . The Hamarøya occurs on the conjugate East Greenland margin (Hartz fault and the Lofoten border faults may have reactivated et al. 2002). the flanks of Devonian detachments extending beneath the Vestfjorden and Ribban basins. This model implies that the Melbu transfer zone is located along the north­ Transfer zones and riftse gmentation eastern termination of the Devonian shear zone. The The Træna - Vestfjorden - Ribban - Harstad basins con­ Bivrost transfer zone may represent an analogous reacti­ stitute the main Late Jurassic-Early Cretaceous rift in vation of the folded offshore Nesna shear zone. A simi­ the Nordland area. Previous seismic studies have shown lar set of NW-SE trending late-Caledonian detachments that the Moho depth varies between 19 and 26 km 256 O. Olesen et al. NORWEGIAN JOURNAL OF GEOLOGY

along the rifted margin (Planke et al. 1991; Mjelde et al. 1993). Moho bulges occur in the Lofoten-Vesterålen area and below the Utgard High/Træna Basin. The major Surt, Bivrost and Vesterålen transfer zones coin­ cide with regional changes in Moho depth (Fig. 8) and are consequently deep-seated crustal features, strongly influencing the segmentation of the rift. These cross­ margin transfer systems have earlier been delineated by Olesen et al. (1997b) and Tsikalas et al. {2001, submit­ ted). In the present study, we have refined the location of the local transfer zones compared to previous stu­ dies. Tsikalas et al. (2001) oriented the Jenegga and Ves­ terålen transfer zones in a NNW-SSE direction which .., 80 120 km differs from the NW-SE oriented Bivrost and Lenvik transfer zones. We argue that these transfer zones have Fig. 13 One crossline (l) through the 3D gravity model. Applied the same NW-SE-orientation as the regional Bivrost densities are shown in Table 2. The black lines show the observed gra­ and Lenvik transfer zones. The Jennegga transfer zone vity and magnetic fields while the blue and red lines illustrate the cannot be extended beyond the Ribban Basin; bedrock gravity and magnetic responses, respectively, of the total 3D model. geology, reflection seismic, aeromagnetic and gravity The gentle dip (5-15°) of the Bivrost detachment is mainly determi­ data do not support that the Jennegga transfer cuts ned from modelling the gradientof the magnetic anomaly. across the middle of the Lofoten Ridge and the inner­ most Vestfjorden Basin. (Figs. 2, 4 & 10). The main structural break across the Lofoten Ridge and Vestfjor­ den Basin occurs along the Mosken sound between Moskenesøy and Værøy (Figs. 4, 7, 10 & 14). The Vest­ fjorden Basin reveals a polarity shift across this transfer zone (Olesen et al. 1997b), which we refer to as the Mosken transfer zone. This zone is most likely a twist­ zone (Colletta et al. 1988) similar to the Vesterålen transfer zone. The Vestfjorden Basin north of the Mos­ ken transfer zone is, however, a shallower (approxima­ tely 2 km deep) half graben with the boundary fault along the eastern margin of the basin (Olesen et al. 1997b). We suggest applying the name Hamarøya fault for this major structure (Figs. 7 & 14) that constitutes a section within the Vestfjorden-Vanna Fault Complex. Detailed seismic interpretations by Tsikalas et al. ( 200 l) to the west of the Lofoten Ridge show that the Mosken transfer zone does not continue into the Skomvær Sub­ basin, and must consequently be of more local charac­ -z.* Magnelicsea tloor ,..,... Regionalfautt zone ter. On the other hand, the northwestern part of the � spreading anomaly Riftzcne - Delachment 50 o 50 100 Jenegga Transfer Zone (Tsikalas et al. 2001) seems to - Potential Transferznne delachment km terminate towards the Lofoten Ridge, since we have not been able to trace this structural discontinuity across Fig. 14. Interpretations of regional basement faults within the Nord­ the Lofoten islands. We suggest applying the name land area. ELBF - Eastern Lofoten border fault; GF - Grønna fault; Melbu transfer zone to this structure since Jennegga has HF - Hamarøya fault; KDZ - Kollstraumendetachment zone; NSZ ­ already been used for the Jennegga High (Blystad et al. Nesna shear zone; SSZ - Sagjjord shear zone; SH - Sandflesa High; UH - Utgard High; UR - Utrøst Ridge;NH - Nyk High; MI.- Myken 1995) and Jennegga Fracture Zone (Mjelde et al. 1993). intrusive complex; W - Vestfjorden-Vanna fault complex; WLBF - The spacing between the transfer zones along the Lofot­ Western Lofoten border fault, BP - Bivrost Fracture Zone; JF- fen­ en margin is 70-140 km. In comparison, corresponding neggaFracture Zone; SF - Senja Fracture Zone. Thenomenclature is transfer zone spacing along the Gulf of Suez rift 1s adapted from Andresen & Forslund (1987), Blystad et al. (1 995), 80-100 km (Colletta et al. 1988). Løseth & Tveten (1996), Olesen et al. (1 997b) and Braathen et al. (2002). Thered box shows the location of the 3D diagram in Fig. 16. The NW-trending Vesterålen transfer zone constitutes a The map illustrates the onshore-offshore links of basement faults in the region exemplified by the Grønna fault, the Vestfjorden-Vanna continuous zone across the onshore-offshore region fault complex, the Bothnian-Senja fault complex and the offshore and separates two regions with differing fault polarity extensions of Devonian detachment and reactivation of the latter (Figs. 7, 14 & 16). This transfer zone links up with an structures as Mesozoic normal faults and transfer zones. apparent (more than 10 km long) offset in the oceanic NORWEGIAN JOURNAL OF GEOLOGY Onshore and offshore geology in the Nordland area, northern Norway 257

continue into Fleinværfjorden and Landegodefjorden to the west of Bodø. We have applied the name Grønna fault (Figs. 7 & 14) to this structure that is making up the southern boundary to a small, near-shore sub-basin (Figs. 6 & 7; Brekke et al. 1992).

Myken intrusive complex Fig. 15. 3D model of the Tysfjorden-Sarek granite intrusion within the Transscandinavian Igneous Belt. The model consists of a total of The western part of interpretation Inline 6 (Fig. 12d) 10 bodies with polygonal cross-sections placed next to each other. intersects a coinciding positive gravity and aeromagne­ Bach body is 35 km wide. The maximum depth extent is 22 km. The tic anomaly located between the Utgard High and the model is displayed with a vertical exaggeration of 2. The perspective Utrøst Ridge (Figs. 4 & 10). The magnetic anomaly is view is from the WSW. restricted to a smaller area than the gravity anomaly which is one of the largest observed gravity anomalies (90·10-5 m/s2) on the Mid-Norwegian continental mar­ Andøya gin. To explain this anomaly, a substantial excess of mass must be present in the crystalline basement and/or the sedimentary sequence. In addition, there is a shallow Moho (22 km) in this area (Mjelde et al. 1998). The depth to the magnetic sources is approximately 3 km below the sea floor (Fig. 7). We conclude that three

Sagflo

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