34. tectonic synthesislo s-0 "2 o lu-4 co-6 iii-1000-500^ o i-500^--i000-80-40 _,-0 o-40...

34. TECTONIC SYNTHESIS

M. Talwani, Lamont-Doherty Geological Observatory, of Columbia University Palisades, New Yorkand

G. Udintsev, P.P. Shirshov Institute of Oceanology, USSR Academy of Sciences, Moscow, USSR

INTRODUCTION

The tectonic evolution of the Norwegian Sea hasbeen complex, and thus seeking evidence of this evolu-tion has represented a unique target for deep-sea drill-ing. The Norwegian Sea lies between Greenland on thewest and Norway and the Barents Shelf on the east.Only a portion of it, that with the Mohns Ridge in thecenter and the Greenland and Lofoten basins on eitherside, is morphologically typical of the North Atlantic.In other parts of it the axis of the mid-ocean ridge doesnot occupy a median position, and areas of unusuallyhigh elevation are distributed asymmetrically. Con-tinental fragments appear to lie within the boundariesof the Norwegian Sea. Iceland, a subareal part of themid-ocean ridge, lies on the southern end of theNorwegian Sea, and two aseismic ridges, the Iceland-Greenland Ridge and the Iceland-Faeroe Ridge, con-nect Iceland with Greenland and Norway, respectively.It is by no means universally accepted that theNorwegian Sea was formed by sea-floor spreading and,in order to assemble details about its evolution, drillinginto the basement was important.

Regardless of what model for its evolution isaccepted, it has been clear that the land connectionbetween Eurasia and North America persisted for along time in the area of the Norwegian Sea. Precisedates for the termination of the land bridge were need-ed. One consequence of the land bridge was that itprovided a barrier in the circulation of water betweenthe Arctic and the Atlantic oceans. This in turn had aprofound influence on faunal differences in the twoareas.

In part, because of its youth, the thickness ofsediments is less in the Norwegian Sea than in the restof the Atlantic and consequenctly the oldest basementrocks are within the reach of the drill in some areas ofthe Norwegian Sea. Leg 38 thus provided a unique op-portunity to sample basement even as old as thatproduced at the time of its first opening (Table 1).

Continental fragments are believed to exist in theNorwegian Sea, and an attempt was made to obtainsamples of basement in those areas. Under thehypothesis of sea-floor spreading, continental pieceswithin the Norwegian Sea imply a jumping of the axisof spreading. By drilling and dating basement onecould indeed verify if and why jumps took place.

Further, it was of interest to inquire into the natureof the chemistry of basement rocks in areas of unusualelevation in the Norwegian Sea.

STRUCTURAL FRAMEWORK

The morphological provinces, important structuralfeatures and the earthquade epicenters which clearlydefine the location of the mid-ocean ridge, are shown inFigure 1. Only the Mohns Ridge portion of the mid-ocean ridge occupies a typical median position with theGreenland Basin to the northwest and Lofoten Basin tothe southeast. Profiles III and IV along whichgeophysical data were obtained across the MohnsRidge are shown in Figure 2. Thereon can be seen thatthe rift valley is associated with a large positivemagnetic anomaly over the crest. In fact, there is a well-developed pattern of magnetic anomalies over all ofMohns Ridge. In particular, attention is called toanomaly 23 on the southeast side of the ridge, lying atthe foot of the V^ring Plateau. Site 343 was located onthis lineation. Sediment thickness is small on MohnsRidge but increases rapidly on either side; in thenorthern part of the Lofoten Basin it is beyond reach ofthe single-channel seismic reflection profiler.

Although Mohns Ridge continues north into theKnipovich Ridge which also has a well-defined rift(Profiles I and II, Figure 2), the Knipovich Ridge clear-ly does not have a median position between Svalbardand Greenland. The Boreas Basin is developed only onthe western side of the Knipovich Ridge; the easternside is buried under a thick sediment cover. The rift axislies closer to the eastern side. A consistent magneticanomaly pattern has not been found over theKnipovich Ridge although many profiles do show amagnetic high over the rift. The question arises as to thekind of prcess that can generate the two ridges whichare so different in their morphology and apparently intheir geophysical structure.

The area lying north of Iceland and the Iceland-Faeroe Ridge is even more complicated. Here the deeppart, represented by Norway Basin, lies on the east side.The presently active mid-ocean ridge, known as theIceland-Jan Mayen Ridge (or the Kolbeinsey Ridge),lies on the west directly north of Iceland. South of theisland of Jan Mayen and west of the northern part ofthe Norway Basin lies the Jan Mayen ridge. The 500-fathom contour defines the highest part of this ridgewhich is believed to be a continental fragment. The arealying west of it that is between the Jan Mayen Ridgeand the Iceland-Jan Mayen Ridge has generally beenincluded in the "Iceland Plateau."

The Iceland Plateau area has been subject of con-siderable discussion in the literature. One view is that it

1213

M. TALWANI, G. UDINTSEV

Site

336

337

338

339

340

341

342

343

344

345

346

347

348

349

350

352

Faunal,

Age Range Determinedfrom Fauna in OldestOverlying Sediment

Late Eocene/middle Eocene38-49 m.y.

Middle Oligocene/late Eocene29-43 m.y.

Early Eocene49-53 m.y.

Basement not reached



Early Miocene0

Early Eocene49-53 m.y.

Pliocene/late Miocene3-10 m.y.

Early Oligocene/late Eocene34-43 m.y.



Early Miocene/Oligocene16-37 m.y.


Late Eocene38-43 m.y.


TABLE 1radiometric, magnetic ages, Leg 38 Sites

RadiometricAge

(m.y.)

40-43

18-25

46.6

44

28.5

3 d

27

18.8

40-44

Magentic LineationObserved or Predicted

: by Model ofTalwani and Eldholm

(in press)

Between #7 and #24

4 m.y. older than #7b

Between #24 and #25

Between #24 and #25

#23

Younger than #5

Between #13 and #20

#6

Between #7 and #20

Corresponding Agea

from Heirtzler et al.Time Scale

Between 27 and 60 m.y.

31 m.y.



58 m.y.

Less than 10 m.y.


21 m.y.


aHeirtzler's time scale is probably too old in the early Tertiary by 5-7 m.y.Assuming a half spreading rate of 0.5 cm/yr.

cEocene and Oligocene sediments are missing from seismic reflection record.dIntruded sill is younger than overlying sediments.

is a part of the Jan Mayen continental fragment; the op-posing view being that a large part of the IcelandPlateau has also been generated by sea-floor spreading.The Iceland-Faeroe Ridge lies east of Iceland andsouthwest of Norway Basin and is flat topped at about400 meters with a rather thin sediment cover. Magneticanomalies over it are high in amplitude, but do notreveal a discernible sea-floor spreading type pattern.Its morphology is unusual compared with the rest ofthe Norwegian Sea.

A number of authors including Johnson and Heezen(1967), Pitman and Talwani (1972), Johnson et al.(1972), Vogt et al. (1970), Talwani and Eldholm (inpress), have suggested that the Norwegian Sea has beenformed by the process of sea-floor spreading. Talwaniand Eldholm (in press) have described the evolution ofthe Norwegian Sea in considerable detail. The drill planfor Leg 38 was chosen to test their model, to supplyvarious details relevant to it, and to present argumentsfor and against it.

Their model depends on the identification ofmagnetic anomalies in the Norwegian Sea (Figure 3).On one hand, an orderly succession of them, from the

axial anomaly through anomaly 24, is identified onMohns Ridge, on its flanks, and in the Greenland andLofoten basins, with anomaly 24 extending onto theV^ring Plateau. This sequence suggests an orderlydevelopment of this part of the Norwegian Sea withoutany jumps in the ridge axis.

On the other hand, south of the Mohns Ridge seg-ment, the ridge spreading axis appears to have jumpedseveral times. The Iceland-Jan Mayen Ridge is the pre-sent axis of spreading. Meyer et al. (1972) and Talwaniand Eldholm (in press), as well as Johnson et al. (1972),have describedTlts magnetic anomaly pattern. Magneticanomaly 5 is continuous on either side (Figures 3 and 8)even though the ridge axis has two small shifts in itwhich presumably have taken place since anomaly 5time. South of Jan Mayen Island lies the (presumably)continental Jan Mayen Ridge. Johnson et al. (1972)have proposed that, prior to the establishment of thespreading axis at the Iceland-Jan Mayen Ridge,spreading took place in an area between this presentlyspreading ridge and the Jan Mayen Ridge. Chapmanand Talwani (in preparation) have identified an axis ofsymmetery which they believe to have the age of

1214

TECTONIC SYNTHESIS

30* 25{ 20* I5C I0* 0 c IOC I5e 20e 25C 30 e

80*

75*

70*

65'

60°

/ X 2910/2397

PLATEAU

CONTOUR DEPTHS ARE IN

NOMINAL FATHOMS (SOUND

VELOCITY θOOfms/sβc.)

6 . ) DEPTH GREATER THAN 1500 fms.

EARTHQUAKE EPICENTERS

i l i i i i i i i i i i i

80°

75°

70°

65°

60 e

30* 25° 2OC 15° IOC Oc I0' 15° 20* 25e 30°

Figure 1. Major structural features and earthquake epicenters (Husebye et al, 1975) in the Norwegian-Greenland Sea. ProfilesI-IV of Figure 2 are located on this map (after Talwani and Eldholm, in press).

anomaly 5D (according to the time scale of Blakely,1974) and that the anomaly pattern in this area extendsfrom 5D to 6A.

Prior to the establishment of this axis of spreading,another axis, now extinct, was present in the NorwayBasin. Therein, anomalies 20 through 23 have been

identified; anomalies younger than 20 exist, but theyhave not been definitely correlated with a magnetictime scale. Talwani and Eldholm (in press) believe thatthis axis became extinct close to anomaly 7 time. Accord-ing to them it is likely that, prior to anomaly 23, stillanother spreading axis existed in the area between the

1215


500 <0 2-500 o

40

o !-40 :

-80

0

2 o•4 co

L6

NWMOHNS RIDGE

AXIS

NW

SE2704tion

SE

1-500 <o i

L-500 <3

r80 _,Uo gLo s

-0

"2 oLu

-4 co-6

III

-1000- 5 0 0 ^

o I-500^

--I000

-80-40 _,-0 o-40 5

--80

-0

-2 .o- 4 co

-6

Figure 2. Composite profiles showing total intensity magnetic data (IGRF removed), isostatic gravity anomalies (two-dimen-sional airy-type with depth of compensation 30 km), and reflection profiler data (basement is shown black). For locationof profiles see Figure 1 (after Talwani and Eldholm, in press).

1216

TECTONIC SYNTHEISIS

30° 25° 20° 15° IOC

oc IOC 15° 20c 25° 30c

80'

75C

70°

65'

60c

>

8OC

75°

65°

F/•:A \ A i .1 t i * i i . ' j I I i i λ i ( i i

60 c

3OC 25° 2OC I5C IOCOc IOC I5C 20° 25C 3OC

Figure 3. Identified magnetic lineations in the Norwegian Sea. For details in the area of the Norway Basin and the IcelandicPlateau, see Figure 8. (After Talwani and Eldholm, in press.)

Faeroe-Shetland Escarpment and the 1500-fathom con-tour in the Norway Basin.

The shift of the ridge axis from the Norway Basin hasbeen discussed by several authors. Johnson and Heezen(1967) first postulated the jump on the basis of theasymmetric position of the presently active Jan MayenRidge between Greenland and Norway. Vogt et al.

(1970) and Le Pichon et al. (1971) also favored thishypothesis of shift in the ridge axis. The three jumpsdescribed above have been detailed by Talwani andEldholm (in press).

The first axis that was active between the time ofopening until shortly before anomaly 23 time lies on theslope east of Norway Basin. The axis then jumped to

1217


form the prominent extinct axis of the Norway Basin.After spreading continued to about anomaly 7 time, theaxis jumped again. In doing so, it broke off a fragmentof the Greenland margin. This fragment now forms theJan Mayen Ridge. Spreading anomalies between 6Aand 5D have been identified with this axis of spreadingwest of the Jan Mayen Ridge. A final jump of thespreading axis to the presently active Iceland-JanMayen Ridge; anomalies from 5 through the presentactive anomalies are identified with it.

Sea-floor spreading type magnetic anomalies havenot been definitely identified on the Iceland-FaeroeRidge; its flat-topped morphology does not suggest atypical mid-ocean ridge. However, the Norwegian Seahas been created by moving apart of Greenland andNorway to produce new sea floor, and because no im-portant transform faults have been identified withinNorway and Greenland perpendicular to their con-tinental margins, it follows, from geometrical con-siderations, that the Iceland-Greenland Ridge, Iceland,and Iceland-Faeroe Ridge also represent new sea floor.Talwani and Eldholm (in press) have suggested thatIceland-Faeroe Ridge was created by sea-floorspreading between the time of opening of theNorwegian Sea to about anomaly 7 time. At the sametime that the axis of spreading shifted from the extinctaxis in the Norway Basin to a new axis west of the JanMayen Ridge, the spreading axis also jumped westfrom the Iceland-Faeroe Ridge to Iceland, which wasformed subsequently. In this hypothesis, the Iceland-Faeroe Ridge is, in a sense, considered to be a sub-marine extension of Iceland. Because of the subsidenceof the mid-ocean ridges with time, it would be expectedthat at one time, the Iceland-Faeroe Ridge was abovesea level. The question of when it subsided below sealevel thus becomes important.

In the north, where despite the fact that the presentKnipovich Rift is continuous with Mohns Ridge, thereseems to be no prominent magnetic anomaly patternassociated with it. In addition, Knipovich Rift liesanomalously closer to Svalbard than it does toGreenland, and no deep basins exist between the riftand Svalbard. Nevertheless geometrical argumentsagain demand that the area between Svalbard andGreenland must also have been created by the sameprocess that created the rest of the Norwegian Sea.

Within the framework of plate tectonics, the move-ment of a rigid plate on a spherical earth can be describedby its motion with respect to a pole of rotation. Todescribe the opening of the Norwegian Sea in quan-titative terms, Talwani and Eldholm (in press) iden-tified four prominent sets of lineations on either side ofthe mid-ocean ridge in the Norwegian Sea. These werelineations corresponding to anomalies 5, 13, 21, and 23.By trial and error the location of a finite pole of open-ing, as well as an amount of rotation, were determinedsuch that if anomaly 5 on the west side was correspond-ingly rotated to the east side, the rotated lineationmatched the lineations on the east side. This gave afinite pole of opening since anomaly 5 time. Similarlyfinite poles of opening since anomaly 13 time, anomaly21 time, and anomaly 23 time were determined. The

finite rotations were subtracted from each other to ob-tain the finite difference poles to represent the motionbetween, for example, anomaly 5 time and 13 time. Byassuming that the pole of opening remained fixed dur-ing each such interval of time, these finite differencepoles were assumed to give instantaneous poles of rota-tion during the corresponding interval. In this mannerinstantaneous poles of rotation were obtained since thetime of opening. A check on the procedure is to obtainthe flow lines corresponding to the poles of rotation.These flow lines would match fracture zones.

The instantaneous poles of rotation thus obtainedcan be used to synthesize the flow lines and theisochrons in the Norwegian Sea. These are shown inFigure 4. To demonstrate the match of the syntheticdata to the observed data, flow lines can be comparedwith the fracture zones and the isochrons with the iden-tified magnetic lineations. It can be noted that thematch is fairly good in the Reykjanes Ridge and theMohns Ridge as well as on the presently active JanMayen Ridge.

In the Norway Basin it can be seen that the observedanomaly 21 cuts across the isochrons showing that thesynthetic motions do not accurately represent the ac-tual motions. Talwani and Eldholm (in press) suggestedthe following explanation for this mismatch. During aportion of the time that the Norway Basin was opening,there was another axis opening to the southwest of theNorway Basin. Thus, for a period of time, twospreading axes, rather than a single opening at theprominent extinct axis of the Norwegian Sea, togetherprovided the total opening. The synthetic model at anyrate provides a basis for comparison of ages actuallydetermined from samples obtained by drilling.

OUTSTANDING PROBLEMS, SELECTIONOF SITES, AND DRILLING RESULTS

IN DIFFERENT AREAS

Iceland-Faeroe RidgeThe bathymetry of the Iceland-Faeroe Ridge ob-

tained from detailed surveys conducted by the GermanHydrographic Office (Fleischer et al., 1974) is shown inFigure 5. The top of the Iceland-Faeroe Ridge is nearlyflat, at about 400 meters depth. Depths increase rapidlynortheast towards Norway Basin and towards thesouthwest into Reykjanes Basin. The map of residualtotal magnetic intensity constructed by Fleischer et al.(1974) (see Site Survey Chapter 2, this volume) showslarge amplitudes, short wavelength magneticanomalies, but without a discernible lineated pattern. Asediment isopach map (Figure 6) of the Iceland-FaeroeRidge has been constructed (Kristofferson andTalwani, in preparation). This map shows that base-ment outcrops at a number of places on the Iceland-Faeroe Ridge. At other places the sediment thickness isless than 0.2 sec double reflection time. The sedimentthickness increases rather notably to the southeast. Italso increases to the southeast and to the west of theridge immediately southeast of Iceland.

We have stated earlier that because of geometricalconsiderations, it appears reasonable to assume that the

1218

TECTONIC SYNTHESIS

60°

Figure 4. Magnetic anomaly locations are as follows: Circles (anomaly 5), squares (anomaly 13), triangles vertex up(anomaly 21), triangles vertex down (anomaly 23), asterick denotes anomaly 6 in the Icelandic Plateau area. Using thetotal opening poles (as developed by Talwani and Eldholm, in press), the anomaly locations on the west side rotated tothe east side are indicated by solid circles, squares, and triangles. As explained in the text, the isochrons and flow linesused are generated from the "finite difference poles." Where the observed anomalies do not agree with the syntheticisochrons, the simplified model is in error (after Talwani and Eldholm, in press).

Iceland-Faeroe Ridge was caused by some form of sea-floor spreading as Greenland and Norway movedapart. However, it is clear that the processes involvedare different from those operating at typical mid-oceanridges. Relief typical of mid-ocean ridge does not existhere. The magnetic pattern also does not show anyidentifiable sea-floor spreading type anomalies.Because of these reasons, no spreading axis could bepostulated for constructing synthetic isochrons shownin Figure 4. However, Talwani and Eldholm (in press)have suggested that an axis of spreading probably ex-

isted on the Iceland-Faeroe Ridge from the time ofopening of the Norwegian Sea to about anomaly 7 time(about 27 m.y. ago) and that at this time the axisjumped westward to form Iceland; simultaneously theaxis in the Norway Basin jumped west to form theIcelandic Plateau. Under this hypothesis the range ofages expected on the Iceland-Faeroe Ridge would bebetween approximately 27 m.y. to about 55 m.y. (timeof initial opening). From the initiation of spreading tothe time of the shift, the spreading axis could haveshifted many times on the Iceland-Faeroe Ridge. We do

1219


Figure 5. Bathymetry of the Ieeland-Faeroe Ridge obtained by detailed surveys conducted by the German Hydro-graphic Office (from Fleischer et al, 1974).

not know whether or not this happened. Perhaps thesimplest assumption is that there was a single axis. Ifthe Iceland-Greenland Ridge was also created byspreading at this single axis, then this extinct axis on theIeeland-Faeroe Ridge lies somewhat closer to Icelandthan the Faeroes.

In choosing sites for drilling, we wanted the sedimentthickness to be small enough so that basement could bereached, but at the same time we wanted a thick enoughsequence of sediments in order to learn as much aboutsediment history as possible. Since it is generally dif-ficult on Glomar Challenger to drill at depths less than500 meters, the sites had to be located off the top flatportion of the Ieeland-Faeroe Ridge. The sedimentthickness at the two sites selected (336 and 352) is seenin the isopach map in Figure 6, as well as in the reflec-

tion profiler sections in Figure 7. Figure 6 shows thatthe sites are located in what appear to be sediment-filled valleys. One may speculate that originally sub-marine canyons existed in both locations and these sitesare located above the walls of the now filled-in canyons.Therefore it would not be surprising if part of thesedimentary section was missing.

Drilling at Site 336 showed the presence of "rubble"immediately above the basalts which suggests subaerialweathering. The basalt, which is petrologically notatypical of mid-ocean ridge basalts, was extruded abovesea level. It now lies at a depth of more than 1300meters below sea level, therefore at least a subsidence ofthis amount has taken place. Although the actual situa-tion may have been considerably more complicated, letus for the sake of simplicity assume that isochrons on

1220

TECTONIC SYNTHESIS

15" IT 13° 12" II" I0" 9 8

Figure 6. Sediment isopach map of the Iceland-FaeroeRidge after Kristoffersen and Talwani (in preparation).

the Iceland-Jan Mayen Ridge have a roughly northeast-southwest strike, that is, perpendicular to the length ofthe ridge. Site 336 lies on the northern flank of theIceland-Faeroe Ridge. The isochron passing through italso passes over the crest of the ridge where basement isnow exposed at nearly 400 meters. Therefore basementat Site 336 lies at a depth more than 900 meters greaterthan the minimum depth of basement of the same ageon the Iceland-Faeroe Ridge. Ignoring erosion anddifferential subsidence due to loading, the point on thecrest of the Iceland-Faeroe Ridge with the same age asbasement at Site 336 was more than 900 meters abovesea level when basement close to sea level was extrudedat Site 336. It took about 30 m.y. for the crest to subsidebelow sea level. The age of basement here is ap-proximately 45 m.y., it, therefore, subsided below sealevel at 15 m.y.

At this time, the points at the crest of the Iceland-Faeroe Ridge at which the age of basement wasyounger than 45 m.y. were still above sea level, thosewith ages older than 45 m.y. had already subsidedbelow sea level. The erosional history of the subaerealIceland-Faeroe Ridge is not known; nor can we saywith certainty that all points of the Iceland-FaeroeRidge, when first formed, stood at the same elevation.Yet if the Iceland-Faeroe Ridge was formed between 55m.y. and 27 m.y., we can hazard a guess that the oldestpoints in the Iceland-Faeroe Ridge probably did notsubside below sea level until about 25 m.y. (55-30 m.y.),and the youngest points subsided even later than 15m.y. ago. Thus at 25 m.y. ago, the time of the jump ofthe ridge axis westward from Iceland-Faeroe Ridge tocreate Iceland, part of Iceland-Faeroe Ridge had begunto subside. This, therefore, can be taken as the best es-timate of the date at which the land bridge was breachedbetween Eurasia and Greenland. Even after thistime, parts of the Iceland-Faeroe Ridge were still

presumably above sea level. It was not until sometimein the Miocene that the entire Iceland-Faeroe Ridgesubsided below sea level.

The faunal evidence at Sites 336 and 352 suggeststhat there was indeed a barrier to circulation betweenthese two sites until late middle Oligocene. The faunalevidence is thus not in conflict with the deduced historyof subsidence of the Iceland-Faeroe Ridge. A hiatus ex-tending through the Miocene and Pliocene exists atboth sites. It is perhaps possible to explain this hiatusby examining the location of the sites on both sides.Both of them lie in areas where the sediment isopachmap (Figure 6) suggests that submarine canyons werepresent at one time, but have been buried by latersediments. The submarine canyons, when active, mayhave, through erosion and nondeposition, been respon-sible for the absence of the missing sediments.

Before the land bridge was breached, the NorwegianSea was cut off from the North Atlantic. According toTalwani and Eldholm (in press), the Greenland Sea didnot open until the early Oligocene. Thus, when NorwayBasin and Lofoten Basin first began to form as a resultof sea-floor spreading in the Norwegian Sea, thesebasins were cut off by the Iceland-Faeroe Ridge fromthe Atlantic to the south. They were also cut off in thenorth from the Arctic because the opening betweenSvalbard and Greenland had not taken place. To thewest lay the Jan Mayen Ridge which was still attachedto Greenland and formed part of its continental slopeand, to the east, Norway. The only circulation was withthe North Sea and through it to the basins in northernEurope which accounts for the faunal similarities in theEocene between the Norwegian Sea with the basins inthe North Sea and in northern Europe.

The age of basement is of considerable interest at Site336 where basement rocks were recovered. Radiometricdeterminations are 40.4 ±3.2 m.y. by the Russiangroup and 43.4 ±3.3 m.y. by the German group.Sediments immediately overlying the basalt (Cores 38to 40) are barren of sediments. The older sediments,dated paleontologically, are at least as old as late Eocene.It is possible to extend this date back to middle Eoceneon the basis of benthic foraminifers. It seems that anage of 45 m.y., close to the làte/middle Oligocene boun-dary, is the best that can be given for this site. The ageof formation of the Iceland-Faeroe Ridge is the same asthat of the Norway Basin as Talwani and Eldholm (inpress) have suggested, that is, from about 55 m.y. to 27m.y. The age determinations are not in conflict withthis hypothesis, but the age is less than that of theyoungest basalt series found on the Faeroes.

Norway BasinThe identified magnetic lineations on the Norway

Basin and the bathymetry contoured at 500-fathom in-tervals are shown in Figure 8. Various topographicprofiles and magnetic profiles across the Norway Basinare shown in Figures 9 and 10, respectively.

Figure 10 clearly shows the presence of magneticlineations of the type associated with sea-floorspreading. The topographic profiles show the lineardepression which has been identified by Talwani and

1221


SITE 336

- o

SITE 352C \V-29

Lu2

0 —

2 —

I-COO OLJZ 4-->Oli-üLüUJ 'Q=co>- 6-

— 3200 8 —

I • i , 'P1300

1 i I i I i i i I i I i I

TIME (HOURS)

— 0

— 800

•1600 O

— 2400

— 3200

0300

09

25°

0100 TIME (HRS.) 0800

3000

SITE 336Profil L

6T3U'

Basalt [ _J FeinstruMuren

631.2,7' N07°00.0' W

METEOR-Fahrten Nr 20und 27

Sparker- Profile

Faroer-Jsland- Rucken

1000

ms

3000

0 50 100 150 200 N.MILES

Figure 7. Seismic reflection profiles over the Iceland-Faeroe Ridge. The location of these profiles is given in Figure 6.

Eldholm (in press) as the extinct rift in the NorwayBasin. Coincident with this extinct rift axis is the axisof symmetry of the magnetic lineations—a magnetichigh. Only the older magnetic anomalies in the NorwayBasin have been identified with certainty. These areanomalies 20, 21, and 22. The magnetic lineations con-verge southwards around the extinct axis (Figure 8).This has made it difficult to identify the magneticlineations at the extinct axis.

We wanted to drill as close to the extinct axis aspossible in order to determine its age. Sedimentthickness in the extinct rift is generally quite large;therefore a site was picked on the "rift mountains" onthe east side of the rift valley. Profiler records obtainedon Vema Cruise 30 (Site Survey chapter) show thethickness and distribution of sediments close to Site337. The sediment thickness is also depicted in theGlomar Challenger records obtained over the site (SiteReport Chapter 3). The eastward continuation ofprofile E-F in Figure 14 shows the sediment distribu-tion on the rift mountains although at a point which issome distance northeast of Site 337. Over the rift moun-tains the sediment thickness is variable, ranging fromnearly zero to a few hundred meters. One has to go aconsiderable distance east of the extinct axis to reach anarea where the sedimentation is not disturbed by

shallow basement relief, but in such areas the thicknessof sediment is so large that basement cannot be reachedby drilling from Glomar Challenger.

As far as the magnetics are concerned, Site 337,located about 20 km east of the extinct axis lies on thesteep slope in the magnetic anomaly between the cen-tral maximum and the first minimum to the east. Base-ment consists of tholeiitic basalt typical of mid-oceanridges. The presence of several horizons of brecciabelieved to represent pillow lavas leaves little doubtthat the basalt here was extruded.

The age of the sediments immediately above base-ment has been difficult to determine. On one hand thebest estimate comes from silicoflagellates which in-dicate a range from middle Oligocene to late Eocene (29m.y. to 43 m.y.). On the other hand, radiometric agesdetermined for basement are much younger—17.5 ± 1.5m.y. by the Russian group and 25.5 ±2.4 m.y. by theGerman group (Kharin et al., this volume). They alsosuggest that argon loss may have occurred leading to asmaller determined age for basement. To reconcile thetwo kinds of data, we have tentatively assigned theyoungest paleontological date to the sites, which is 29m.y. We can use this age to estimate the age at whichthe spreading axis became extinct in the Norway Basin.The half spreading rate in the Mohns Ridge just prior

1222

TECTONIC SYNTHESIS

20

7 TRACK cruise /mileαg

MAGNETIC QUIET ZONE BOUNDARY

MAGNETIC LINEATION

o- GRAVITY HIGH

BOUNDARY OF JAN MAYEN RIDGE AREA

GRAVITY GRADIENT

BASEMENT RIDGES

from seismic reflection profiling

- – ESCARPMENT

l 0 4 from bathymetry

| SONOBUOY SEISMIC REFRACTION PROFILEVema Cruise 29

Figure 8. Map of area between Jan Mayen Fracture Zone and the Iceland-Faeroe Ridge. This area can be defined into fourparts (i) the Norway Basin which roughly lies within the 1500-fathom contour and has an extinct axis; (ii) the Jan MayenRidge region whose east and west boundaries are indicated by dot-dash lines; (iii) the region produced by spreading aboutthe extinct Icelandic Plateau axis. Between roughly 69°N and 70°N, this region is bounded by topographic ridges or escarp-ments denoted by the heavy dashed lines. Spreading took place from between anomaly 6A time to anomaly 5D time;(iv) the now active Iceland-Jan Mayen Ridge. Spreading about roughly the present axis has gone on since anomaly 5 time.Six basement ridges, numbers 1,2. . 6 have also been mapped from seismic reflection and isostatic gravity profiles. Thethree western ridges interrupt the opaque layer and have progressively greater elevation northward, finally coalesce toform the Jan Mayen Ridge block. The three eastern ridges 1, 2, and 3 on the other hand, are covered or formed by theopaque layer as seen in seismic reflection profiles (Eldholm and Windisch, 1974). In our interpretation the area of theseeastern ridges which lie between the southern part of the Jan Mayen Ridge and the area of the prominent anomalies of theNorway Basin, is oceanic; it was formed by sea-floor spreading in a complementary fashion to the fan-shaped lineations ofthe Norway Basin. This map also serves as an index map for the profiles across the Norway Basin (Figures 9 and 10) (afterTalwani and Eldholm, in press).

1223


TOPOGRAPHY

KM (αpprox)

Figure 9. Projected topographic profiles across the Norway Basin and the southern part of the Jan Mayen Ridge area. Seealso caption of Figure 10 (after Talwani and Eldholm, in press).

to anomaly 7 time was 0.5 cm/yr and subsequent toanomaly 7 was 0.8 cm/yr. Ignoring the contem-poraneous axis of spreading at this time, and usingthese spreading rates and the distance of 20 km fromSite 337, an age of about 25 to 27 m.y. would be in-dicated for the spreading axis which agrees well withthe age of anomaly 7 (27 m.y.). Shortly after that timethe spreading axis jumped west of the Jan MayenRidge. It must be pointed out however, that because thelarge range in the paleontologically determined age foroverlying sediments and the discrepancy with theradiometrically determined age of the basalt, our deter-

mination of the age of the spreading axis is not ab-solutely certain.

Jan Mayen Ridge

A physiographic diagram of the area north of Icelandwhich includes the Jan Mayen Ridge is shown in Figure11. A smaller area is outlined within this physiographicdiagram and a detailed bathymetric chart of the smallerarea and the magnetic anomalies plotted along track inthe smaller area are shown in Figures 12 and 13, respec-tively.

1224

TECTONIC SYNTHESIS

MAGNETICS 23'22? 2 I ? 2 0

3OIO5225 3010

5385

3010607

1388

r800

-0

L-800EXTINCT! AXIS KM (αpprox.)

Figure 10. Projected magnetic (total intensity, IGRF removed) profiles across the Norway Basin and the southern part ofthe Jan Mayen Ridge area. Identification of anomalies younger than anomaly 20 in the Norway Basin is uncertain. Thelocation of the extinct axis, as well as ridges I through 6, is based on gravity data. The region of strong gravity gradientson the western side of the Jan Mayen Ridge area is indicated by the bars with a circle in the center. The western andeastern boundaries of the Jan Mayen Ridge region are indicated by the dot-dash lines. Triangles in the Icelandic Plateauarea indicate topographic escarpments (after Talwani and Eldholm, in press).

Gr^nlie and Talwani (in preparation) have dividedthe area near the Jan Mayen Ridge into eight regions(Figures 12 and 13). Region 1, which possesses amagnetic quiet zone, contains the main block of the Jan

Mayen Ridge. This main block contains Sites 346, 347,and 349 and is clearly seen in the physiographicdiagram. South of Region 1 lies Region 2 which has asomewhat more disturbed magnetic field. Regions 1

1225


^~<f ^W>^^l^ , : I =ri

Figure 11. Physiographic diagram of the area north of Iceland including the Jan Mayen Ridge. The outlying area indicatesthe area of the maps in Figures 12 and 13. This map supplied by Gleb Udintsev.

and 2 are separated from each other by a deeptransverse channel. Talwani and Eldholm (in press)believe that Region 2 is a structural continuation of themain block of the Jan Mayen Ridge in Region 1. Theybelieve, together with other authors (including Johnsonand Heezen, 1967; Vogt et al., 1970), that this mainblock is a continental fragment. The ridges in Region 2also represent continental material, but they are moresubsided than in Region 1. However, Region 3,although it appears to be morphologically a part of theJan Mayen Ridge, is believed to have a different origin.

The fan-shaped pattern of magnetic anomalies in theNorway Basin has been mentioned earlier. Fromgeometrical conciderations, the presence of the fan-shaped pattern demands that a contemporaneous sea-floor spreading must also take place in another area.Talwani and Eldholm (in press) believe this area to be

Region 3 immediately southwest of the Norway Basin.In other words, during a part of the time when sea-floorspreading was taking place in the Norway Basin,spreading was also contemporaneously taking place inRegion 3.

Region 5 represents a part of the presently activeIceland-Jan Mayen Ridge. The oldest anomaly iden-tified with this spreading axis is anomaly 5 (Figure 13).Johnson et al. (1972) believed that when the shift of theridge axis took place in the Norway Basin, it did notimmediately move to the present Iceland-Jan MayenRidge, but that there was an intermediate axis ofspreading which produced the anomalies in Region 4.Chapman and Talwani (in preparation) have identifiedthe magnetic anomalies in Region 4 (Figure 13). Thelineations range in age from anomaly 5D to anomaly6A according to the time scale of Blakely (1974).

1226

TECTONIC SYNTHESIS

66°

Figure 12. The bathymetric map of the Jan May en Ridge area is a portion of a larger map constructed byGrΦnlie et al, (in preparation). The depths are in uncorrected fathoms. Track of the Glomar Challengerand the drill sites are indicated. The division into different areas 1 through 8 is discussed in the text.

Anomaly 5D lies on the extinct axis, its age is about18.3 m.y. Anomaly 6A has an age of 24.3 m.y.,therefore the intermediate axis spreading roughly en-compasses the period between 18 and 24 m.y. ago.

The physiography of the northern block-like part ofthe Jan Mayen Ridge is shown in Figures 11 and 12.Seismic reflection records in the area are shown in theSite Report chapter. Reflection records along profile C-

1227


67 e

66 e

Figure 13. Magnetic anomalies plotted along track in the area of the Jan Mayen Ridge (after Grtfnlie andTalwani, in press). The anomaly identification in the area of the extinct Icelandic Plateau axis is fromChapman and Talwani (in preparation).

D are shown in Figure 14. A composite picture of theJan Mayen Ridge from the reflection data and from therefraction data (Figure 15) is also shown in Figure 14.From Figures 14 and 15 we see that on the block-like

Jan Mayen Ridge parallel reflectors exist just un-derneath the bottom. These sediments have a velocityof about 1.85 km/sec associated with them. The refrac-tion results show that below the flat-lying top layers

1228

TECTONIC SYNTHESIS

Figure 14. Seismic reflection records along profiles A-B, C-D, and E-F. These profiles are located in Figure 13.

1229


lower layers exist which dip to the east. These are alsoseen in some of the reflection profiles (Site Surveychapter). Steeply dipping, they are truncated by aprominent reflector which forms the base of the flat-lying sediments on top.

We believe that the layer with a velocity of greaterthan 5 km/sec probably represented basement. Neitherthe reflection records nor the refraction results are clearon where basement lies near the western edge of theblock. Extrapolating the apparent steep slope of thebasement surface westwards, and noting the steeptopographic western slope of the Jan Mayen Ridge, webelieve it possible that basement outcrops on thetopographic slope and that it may represent the westernpart of a basement ridge. Site 346 was located towardthe western edge of the topographic platform. It hadtwo principal objectives: to penetrate through the flat-lying sediments on the top through the unconformityassociated with the prominent reflector and into thedipping reflectors below it. Because of the steep easterlydip, a site on the western edge of the platform was mostlikely to sample the oldest sediments; secondly, if thearguments presented above are correct, it provided thebest opportunity to reach basement. Basement,however, was not reached at Site 346 and therefore, inan attempt to reach it, we located Site 347 even furtherto the southwest, closer to the slope.

At Site 347, as at Site 346, the prominent reflectorwas penetrated and drilling progress was very slowbelow it in a siliceous mudstone. Basement was notreached and from the drilling results it seems quiteprobable that it is the mudstone which probably formsthe steep western boundary of the Jan Mayen Ridgeand not a basement ridge. Site 349 was located on amore central portion of the block and the objective herewas to try again to penetrate the prominent reflectorand drill into the dipping reflectors below.

At all three sites the prominent reflector waspenetrated at about 120 meters. It represents an uncon-formity which is especially clear lithologically in Site349 where it is marked by a basal conglomerate. Abovethe unconformity are "glacial" sediments as well assediments extending from middle Miocene to middleOligocene in age. Below the unconformity aresediments that range in age from early Oligocene to late(and middle?) Eocene. The unit below the unconformi-ty consists primarily of massive terrigenous sandymudstone. Turbudite layers are often present in thelower part of the unit. It is paleontologically barren ex-cept for some arenaceous benthic foraminifers and afew badly preserved calcareous foraminifers. Above theunconformity, the middle Miocene to late/middleOligocene unit consists of sandy mud and biogenicsiliceous oozes that are characterized by a very highpercentage of spine spicules, especially in the lower partof the unit. When the lower unit of Site 349 is comparedwith those at Sites 346 and 347 it is found that, at thesame depth, sediments that are in a stratigraphicallyhigher position are undeformed. This confirms theeastward dip of the older reflectors.

Because basement was not reached at any of thesesites and because the oldest sediments found are notolder than middle Eocene and therefore do not predate

the opening of the Norwegian Sea, one cannot directlydeduce that the Jan Mayen Ridge is a continental frag-ment that existed before the Norwegian Sea opened.However, several factors indicate that basementpossibly much older than Eocene underlies the JanMayen Ridge.

In Figure 15 we note that a large thickness of sedi-ment underlies sediments with velocities of 2.2 km/sec.It was this 2.2 km/sec layer that was penetrated by thedrill holes on the Jan Mayen Ridge and which yieldedEocene fauna. Presumably the sediments at the base ofthe sedimentary column are much older. Elsewhere inthe Norwegian Sea and in the North Sea the boundarybetween the 2.2 km/sec and 3.1 km/sec layers is theboundary between the Tertiary and the Cretaceous, andthe 4.4 km/sec layers represent Mesozoic and perhapseven Paleozoic sediments. A similar situation could ex-ist here.

The age of the older sediments cored on the JanMayen Ridge does not predate the opening of theNorwegian Sea, but if the situation described aboveholds, then some of the older sediments deposited onthe Jan Mayen Ridge predate the time of opening of theNorwegian Sea, and it is quite possible that they lie oncontinental basement. The prominent unconformity atabout 120 meters on the Jan Mayen Ridge can beassociated with the jump of the spreading axis from theNorwegian Sea to west of the Jan Mayen Ridge. Thesediments below the unconformity are mainlyterrigenous in origin and could have been sedimentsdeposited on what was then the eastern continentalslope of Greenland. It is quite likely that just prior tothe development of the new axis of spreading west ofthe Jan Mayen Ridge the area of the Greenland con-tinental slope (including the area that later became theJan Mayen Ridge) was uplifted due to thermal causes.Subsequent erosion can perhaps explain the unconfor-mity at 120 meters. After the creation of the new seafloor between the Jan Mayen Ridge and Greenland,terrigenous sediments from Greenland could not reachthe Jan Mayen Ridge. The increase of the biogeniccomponents at the expense of the terrigenous compo-nent in the flat-lying sediments above the unconformitymay reflect this. The dates check quite well, the oldest

wOr

108 107 106 105 104 118

2

| 3

4

5

650KM

100

Figure 15. Refraction data across the Jan Mayen Ridge(after Talwani and Eldholm, in press). Location of thesestations is indicated in Figure 8.

1230

TECTONIC SYNTHESIS

sediments above the unconformity are late Oligocene inage which corresponds nicely to the shift of thespreading axis from the Norway Basin to the IcelandicPlateau.

Site 350 lies in Region 3 (Figures 12 and 13). As dis-cussed earlier, morphologically, Region 3 has been con-sidered a part of the Jan Mayen Ridge; it lies at anelevation higher than the elevation of the NorwayBasin. However, Talwani and Eldholm (in press) havesuggested that it was probably created by sea-floorspreading, contemporaneously with the development ofthe Norway Basin. The reason for this suggestion is asfollows. The magnetic lineations in the Norway Basin(Figure 8) show, that while anomalies 20 and older areparallel to each other, the younger anomalies tend tocrowd southward into a fan-shaped pattern. Fromgeometrical considerations, therefore, another areamust exist where new ocean crust was created betweenanomaly 20 time and anomaly 7 time (the time that theNorway Basin spreading axis became extinct). It can beseen in Figure 8, that the northern segment of theFaeroe-Shetland Escarpment, which forms the easternboundary of the Norway Basin, is not parallel to the1000-fathom curve which forms the eastern boundaryof the morphological Jan Mayen Ridge, but rather isparallel to the western boundary of Region 3. In otherwords, if Region 3 is included within the Norway Basin,then the Norway Basin does not diminish in width fromnorth to south, a geometrical condition which is im-posed by the motion of rigid plates.

All of these considerations prompted Talwani andEldholm (in press) to suggest that Region 3 was createdcontemporaneously with the Norway Basin by someform of sea-floor spreading. If this hypothesis is right,the age of basement in this area would lie between theages of anomalies 20 and 7, that is between 50 and 26m.y. ago. An "opaque" layer underlies sediment withinRegion 3. This opaque layer, as the seismic reflectionprofile in Figure 14 shows, continues eastward into theNorway Basin.

Drilling revealed the opaque layer at Site 350 to con-sist of basalt. There is considerable scatter in theradiometric ages (Kharin et al., this volume), but valuesof 40-44 m.y. are indicated. These are not in conflictwith the age of late Eocene determined paleontological-ly from the overlying sediments and falls within therange suggested by Talwani and Eldholm (in press) forthis area, assuming of course that the opaque layerforms "true basement" or is very close to it in age.

Site 348 was drilled in the Icelandic Plateau in thearea containing magnetic lineations surrounding the ex-tinct axis of spreading in the Icelandic Plateau. Site 348was located on a prominent positive magnetic anomalywhich Chapman and Talwani (in preparation) haveidentified as anomaly 6. An "opaque" layer was presentin the seismic profiler records at this site (see profile A-B in Figure 14). As at Site 350, the opaque layer herealso was basalt. Its age, determined radiometrically, is18.8 ±1.7 m.y. (Kharin et al., this volume). Paleon-tologically the age for the overlying sediments is earlyMiocene to Oligocene (?). The possible Oligocene age isfrom a single species of foraminifers with a range from

late Oligocene to early Miocene. It is therefore quitepossible that the oldest sediment at this site is earlyMiocene and the age of basement is about 21 m.y., theage of anomaly 6. Correspondingly, the age of themagnetic anomaly 5D at the extinct axis, which givesthe time of extinction of the spreading axis, is 18.5 m.y.

The extinction of spreading and the development of anew axis of spreading at the present Iceland-Jan MayenRidge may be reflected in the prominent lithologicalboundary between the middle Miocene and earlyMiocene sediments used at Site 348. Early Miocene andOligocene sediments have a large terrigenous compo-nent. They are poor in fauna which consists usually ofarenaceous foraminifers and some nannofossils. Thesesediments may reflect a situation where it was possibleto obtain terrigenous sediments directly fromGreenland. After the opening of a new spreading centerto the west, the terrigenous sediment supply decreasedand the biogenic component became more important.This middle Miocene-early Miocene discontinuity isalso very clearly reflected in the pollen/dinocyst ratios(Manum, this volume). The ratios diminish very rapidlyat this boundary implying again the difficulty ofterrigenous sediments from Greenland reaching the siteafter the new spreading center was opened.

V0ring PlateauThe V^ring Plateau is a prominent feature of the

Norwegian Continental Margin. It has a nearly flat sur-face at a depth between about 1200 and 1400 meters.An important buried feature known as the V^ringPlateau Escarpment divides the V^ring Plateau into anInner and an Outer part. The V^ring Plateau Escarp-ment is associated with minimal surface relief, but itdoes not separate two very different geological areas. Abathymetric chart of the V^ring Plateau is given inFigure 16. Magnetic anomalies along selected ships'tracks are plotted in Figure 17. Three seismic reflectionprofiles which cross the escarpment (Figures 18 and 19)show various geophysical data across the escarpmentincluding deep information from seismic refraction aswell as isostatic gravity anomalies.

The magnetic and seismic reflection data give the im-pression that the V^ring Plateau is tectonically con-tinuous with the Lofoten Basin to the north. Magneticlineation 23 in the Lofoten Basin continues to the baseof the V0ring Plateau, and magnetic lineation 24 con-tinues from the Lofoten Basin on up to the V^ringPlateau cutting across the 1500-meter isobath (Figure17). An important negative magnetic anomaly existsjust seaward of the V^ring Plateau Escarpment. It hasbeen explained by Talwani and Eldholm (in press) asarising due to the juxtaposition of highly magnetizedoceanic basement rocks against very weakly magne-tized rocks east of the V^ring Plateau Escarpment.

Basement in the Outer V^ring Plateau dips steeplydown towards the Lofoten Basin. The steeply dippingbasement can be seen in profile E-F in Figure 18. Whilethis profile shows a break in the basement layer in thevicinity of Site 343, other profiles (not shown here) ingeneral, appear to demonstrate a continuity of the V0r-ing Plateau basement in the Lofoten Basin without any

1231


Figure 16. Bathymetric chart of the Voring Plateau. This map is principally based on soundings obtained on Vemasurveys. The track o/Glomar Challenger and Sites 338 through 343 are also shown.

major presence of faulting at the base of the slope of theVoring Plateau. Identification of the smooth prominentreflector on the outer part of the Voring Plateau base-ment was made on the basis of sonobuoy measurements(Figure 19). The refraction measurements yielded avelocity of about 5.2 km/sec. However, results of mul-tichannel seismic reflection profiling available to usafter the drilling program shows that in the vicinity ofthe escarpment on the Outer Voring Plateau the promi-nent reflector is underlain by closely spaced dippingreflectors and that "true basement" lies some distancebelow this prominent reflector. This information hasbeen provided by the Bundesanstalt fur Boden-forschung (K. Hinz, personal communication) and IFP(L. Montadert, personal communication). The resultsavailable to us suggest that the lower series of reflectorsexist in limited area confined between anomaly 24 andthe escarpment (Figure 17); Sites 348 and 342 lie withinthis area.

East of the escarpment in the magnetic quiet zone theprominent reflector is not seen, and the thickness of

sediments increases considerably (Figures 18, 19, and20). By comparison of the seismic velocities shown inFigure 19 with seismic velocities on the NorwegianMargin and in the North Sea, it is believed that pre-Tertiary sediments exist in this area. The boundarybetween the layer of velocities 2.5 and 3.5 km/sec isbelieved to indicate the base of the Tertiary. The layerswith 4.4 km/sec are believed to be Mesozoic or perhapseven Paleozoic sediments. By contrast, on the InnerVoring Plateau, sediments with velocities only underabout 2.2 km/sec are found, thereby indicating a largedisparity in the age of the base of the sedimentary sec-tion in the Inner and Outer Voring Plateau areas. Thesediment layers in the Inner Voring Plateau are general-ly flat over a large area, although the sediment layers doslope up towards the escarpment and they also slopeupwards in the vicinity of diapiric bodies (Figures 18and 20).

The principal objectives of the sites on the V^ringPlateau were to test the salient points of the hypothesisthat has been presented above, that is: (1) Whether the

1232

TECTONIC SYNTHESIS

0°Figure 17

1° 2° 3° 4° 5° 6°Magnetics plotted along selected ship tracks on the Voring Plateau.

shallow reflector under the Outer Voring Plateaurepresents igneous oceanic basement formed im-mediately after the opening of the Norwegian Sea, andif so, why does basement here stand higher thanelsewhere in the Lofoten Basin? (2) Whether the VoringPlateau Escarpment indeed forms "oceanic-continent"boundary. (3) Whether the age of the thick sedimentarycolumn in the Inner Voring Plateau includes Mesozoicor earlier sedimentary accumulations.

The shallow reflector under the Outer Voring Plateauwas found, by drilling at Sites 348, 342, and 343, to con-sist of basalt. Age determination results by thepotassium-argon method are summarized by Kharin etal. (this volume). Ages of 46.6 ±2.5 m.y. were deter-mined for basalts at Site 348, and an age of 24.5 ±2m.y. for the basalt at Site 343. At Site 342, the agedetermined was close to 44 m.y. At Sites 338 and 343the oldest sediments found over the basalt are early

Eocene in age. It is important to emphasize that theyare not middle Eocene, nor are they Paleocene. NoPaleocene sediments have been recovered from theNorwegian Sea either by drilling or by piston coring ofoutcrops. Site 338 lies landward of anomaly 24.Anomaly 25 is not seen on the Voring Plateau oranywhere else in the Norwegian Sea. If the basalt atSites 338 and 342 was extruded between anomaly 24time and anomaly 25, its age, according to the Heirtzlertime scale, lies between 60 and 63 m.y. However, in theearly Tertiary this time scale is too old by perhaps 5-7years (there is no general agreement on exactly what theerror in the time scale is; different estimates have beenmade by different authors). An error of 5-7 m.y. would,from magnetics, place the expectable range at Sites 338and 342 to be 53-58 m.y. A single best guess for the in-itiation of opening of the Norwegian Sea can thereforebe approximately placed at 55 m.y.

1233


- 0

ixJ

6 -

8 -

2400

-3200

3200

2230 0830

Figure 18. Seismic reflection profiles across the V(/>ring Plateau Escarpment.This information was obtained on Vema Cruises 28 and 30.

Thus, the ages at Site 338, determined by differentmethods, roughly agree but we note that radiometricage is the youngest at 46.6 m.y., and the magnetic age isthe oldest lying between about 53-58 m.y. The agedetermined from fossils in the overlying sediments is inbetween, at early Eocene (49-53.5 m.y.). Fine-grained,sandy limestone overlies about 76 cm of basalt brecciaat Site 338. Fauna in the sandy limestone is early

Eocene and appears to be found also in the underlyingbreccia. Although the ages do not exactly match, thebasalts at Site 338 can be characterized as normal oceanfloor tholeiites and can be associated with the earlyopening of the Norwegian Sea.

The age of the basalts at Site 342 is similar, but theoverlying sediments are early Miocene, not Eocene.The profiler record in Figure 20 shows that the layers

1234

TECTONIC SYNTHESIS

MAGNETICS

! Shelf Edge ! Outer High

0 Km. 100 km.

Gammas

IOOO

N7

Mgal. j

100 -

GRAVITY (ISOSTATIC)

I Shelf Edge \ Outer High

0 Km. 100 km

NW100 50 50 100

Sia

2.24 APPARENT VELOCITY[L7 l AVERAGE LAYER VELOCITY i

4.43

V0RING j PLATEAUESCARPMENT

SE150

orooo<M

1

(0I0

J5

σ>

aSS 2C3CM ?3

oCM

00CO

1

27

iS3*

aiL•L

1

4.32 465 4 0 1

Figure 19. Geophysical data obtained across the VΦring Plateau Escarpment (modified from Talwani and Eldholm, 1972).

1235


20 AUGUST

1800

18 AUGUST

0900 0800

Hole343

Hole342

Hole338

(offset 11 km)

r

Hole341

Hole339and340

y

Glacial/Eocene

Miocene/Oligocene

Oligocene/Eocene.

Glacial/Eocene

? Base of Tertiary

H//____- Gl aci al /Miocene

M

LINE DRAWING OF COMPOSITE PROFILE ACROSSV0RING PLATEAU

Figure 20. Line drawing composite of seismic reflection profile across the Vrfring Plateau obtained aboard Glomar Challenger.Reflectors are shown as thin lines, basement is emphasized by oblique shading. Refraction data flayers shown by dottedlines and velocities given in boxes) taken from Talwani and Eldholm (1972), and plotted on the assumption that base of1.8 km/sec layer is equivalent to presumed base of Miocene.

corresponding to Eocene and Oligocene at Site 338 areabsent at Site 342 and therefore one would not expectto reach them by drilling. The age of anomaly 23 at Site343 is 58 m.y. on the Heirtzler time scale. Assuming anerror of 5-7 m.y., the age is 51-57 m.y. This overlaps therange of the age of the overlying early Eocenesediments, but is considerably older than the age whichhas been radiometrically determined—28.5 m.y. AsKharin et al. (this volume), discuss the discrepancy hasto be explained either by stating that the basalt at Site343 is intrusive or by assuming a large amount ofalteration.

We have mentioned earlier the presence of dippingreflectors below the prominent reflector that waspenetrated at Sites 338 and 342. It could be argued thatthe basalts found at Sites 338 and 342 do not representbasement associated with the early opening of theNorwegian Sea, but that they represent basalt flows onan older, perhaps continental crust. We do not supportthis view because anomaly 24 is seen on the V^ringPlateau, at points lying above the 1500-meter isobath.That part of the V^ring Plateau is thus apparentlyformed by sea-floor spreading and it seems un-reasonable to divide the Outer V^ring Plateau into two

parts, one formed by sea-floor spreading, the otherrepresenting older continental crust. We believe that itis more likely that the reflector penetrated at Sites 338and 342, is only slightly younger than the "truebasement" underneath, and the intervening reflectorsmay represent rapidly deposited pyroclastics associatedperhaps with a volcanic activity during the time justafter opening started.

Data on sediments suggest the continued sub-mergence of the Outer Vyiring Plateau since it wasformed. It is quite possible that parts of it, at the time offormation, lay above sea level. One way to explain thelarge sandy component of the muds of the Eocene atSites 338 is to erode nearby subaereal peaks. Sometimeafter early Eocene there is a sudden change in thelithological character of the sediments with a greatdecrease in the terrigenous component suggesting thatthese peaks subsided below sea level. It is pointed outthat this change in sediment lithology is also reflectedvery strongly in the dinocyst/pollen ratio which in-creased markedly and suddenly at this time (seeManum, this volume). It can be noted that thethickness of the Miocene sediments at Site 341 is severaltimes the thickness of Miocene sediments at Site 338,

1236

TECTONIC SYNTHESIS

suggesting that even though the Outer V^ring Plateauwas subsiding, it remained topographically elevatedwith respect to the Inner V^ring Plateau. A shallowerOuter V^ring Plateau, with the consequent erosionaland nondepositional environment associated with fastcurrents, may be used to explain the smaller thicknessof the Miocene sediments there relative to thicknessfound on the Inner V^ring Plateau.

Even though full proof requires drilling through thesubbasalt reflectors in the Outer V^ring Plateau anddrilling to basement, the present drilling resultstogether with earlier geophysical data confirm the greatimportance of the V^ring Plateau Escarpment and donot deny the suggestion that it might define the boun-dary between older continental crust and the newlyformed oceanic crust.

A comparison of the ages and the sediments found atSite 341 compared with the results obtained fromseismic reflection/refraction by Talwani and Eldholm(1972) generally confirm the presence of a thick se-quence of sediments on the Inner V^ring Plateau. Pre-Tertiary sediments were not reached at Site 341 becausedrilling had to be terminated. Confirmation of thepresence of sediments deposited before the time ofopening of the Norwegian Sea requires further drilling.

The presence of diapiric activity was confirmed byrecovering Eocene and Miocene fauna at very shallowdepths at Sites 339 and 340. DisplacedMiocene/Oligocene fauna within a section of thePleistocene at Site 341 suggest that older sedimentshave been obtained from the high-standing diapirs byerosion and slumping. We are not certain about thetime of diapirism. Some diapirs are now covered byflatlying undisturbed Pleistocene sediments suggestingthat they are, at present, not rising. However, as theoutcrops are covered only by a thin sediment veneer,this could suggest continued activity at the presenttime, or possibly that no sedimentation has occurred onthe steep slopes of the diapirs.

Site 345, Jan May en Fracture Zone, andthe Southern End of Mohns Ridge

The location of Site 345, together with the magneticlineations and the bathymetric contours in theneighboring area, is given in Figure 21. Reflectionprofiler records along track ABC which passes throughthe site are shown in Figure 22. Site 345 lies in an areaof complex structure. To the northeast of Site 345, themagnetic lineations corresponding to Mohns Ridge areshown. The two segments of the Jan Mayen FractureZone are indicated. Norway Basin lies southwest of thesouthern section of the Jan Mayen Fracture Zone.Magnetic lineations 21? and 22? in Norway Basin areshown. We have stated earlier that at nearly anomaly 7time the spreading axis in the Norway Basin jumpedwestward.

Gr^nlie and Talwani (in preparation) suggest that, infact, the jumping of the ridge axis was more com-plicated. The jump of the spreading axis actually tookplace in two steps. North of the northern segment ofJan Mayen Fracture Zone, no ridge axis jump occurredand spreading has continued since the opening of theNorwegian Sea. In the area that lies between the two

segments of Jan Mayen Fracture Zone, the spreadingaxis jumped at nearly anomaly 13 time. Thus, betweenanomaly 7 time and anomaly 13 time, there were threesegments of the mid-ocean ridge in the area. One seg-ment lay north of the northern segment of the JanMayen Fracture Zone; a second segment lay betweenthe two segments of the Jan Mayen Fracture Zone, anda third segment coincided with the extinct axis of theNorway Basin. Three lineations, 21?, 22?, and 23?, areshown west of Site 345. They belong to the intermediatearea among the three mentioned above. Site 345 mustalso belong to this intermediate area and, judgingroughly from the tentatively identified lineations to thewest of it, the age of Site 345 lies somewhere betweenanomaly 13 time and anomaly 20 time, that is between38 m.y. and 49 m.y. according to the Heirtzler timescale (the age is somewhat younger if corrections areapplied to the Heirtzler time scale). If a jump in ridgeaxis in this area took place during anomaly 13 time, theJan Mayen Fracture Zone segment to the north of itmust connect anomaly 13 in the Mohns Ridge withanomaly 13 in this intermediate area. In other words,the northern segment to the Jan Mayen Fracture Zonemust extend further to the east than is shown in Figure21.

Track segments C-B lies on the flank of the MohnsRidge. This can probably be appreciated better fromFigures 1 and 2, noting that point B corresponds topoint marked 2703/2315 in Figures 1 and 2. In Figure22, the higher elevation of basement in the vicinity ofSite 345 can be attributed to the proximity of the JanMayen Fracture Zone, but the fracture zone in this areaonly came into existence at anomaly 13 time, while thecrust at Site 345 was emplaced, as suggested earlier,prior to anomaly 13 time. When basement was formedat Site 345, it was formed in an area of normal oceaniccrust away from a fracture zone, but subsequently thesite was elevated due to the establishment of a fracturezone. These considerations might explain thelithologies of the sediments cored at Site 345.

The lowermost sediments, which are Eocene indicatedeposition in deep water with a large number of tur-bidite layers present. On the other hand, the pelagicsediments appear to dominate from the Oligocene on.Turbidites are no longer present. The biogenic compo-nent increases; muds and clays are present, but theyappear to have been deposited as suspensoids in quietwater conditions. This later phase appears to beassociated with the sediment deposition at post anoma-ly 13 time at an elevation that was higher than that ofthe surrounding sea floor.

Tne age of the basement determined radiometricallyis 27 ±3 m.y. (Kharin et al., this volume). This age isyounger than that obtained from faunal evidence in theoverlying sediments. Faunal evidence suggests earlyOligocene or late Eocene. If the late Eocene data are inerror and there are no Eocene sediments present, thiswould not invalidate the arguments regarding initialdeposition in in deep water and subsequent elevation,but simply pushes the whole sequence of events to asomewhat later time. If the above considerations arecorrect, they have an important implication about thegeochemistry and petrology of the basalt. Although the

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71

19

/ If ^

70'

69*

68'5°E5°W

Figure 21. Location of Site 345, the magnetic lineation bathymetric contours in the neighboring area (Gr<j>nlie and Talwani,in preparation; Talwani and Eldholm, in press).

site is close to a fracture zone, the geochemistry andpetrology should be associated with normal ocean crustrather than the fracture zone.

Knipovich Ridge

The continuation of the rift at the crest of the MohnsRidge is shown to continue into the Knipovich Ridge(Figure 1). However, other than the presence of apositive magnetic anomaly in most crossings over theKnipovich Ridge, no identifiable magnetic lineationpattern has been found for this ridge and thereforenone has been shown in Figure 2. Figure 23 showsmagnetic anomalies plotted on track in the vicinity ofSite 344. The Knipovich Rift is also shown in this figureas are topographic peaks rising above 2000 meters. Thedrill site was located on the east flank of a topographichigh—a "rift mountain." The location of Site 344 canalso be seen on the profiler record in Figure 24.

Igneous rocks cored at this site were coarse-graineddiabases and gabbros. The radiometric age of 3 m.y. isyounger than the age of the overlying sediments whichcontain fauna of Pliocene or late Miocene age. The in-tensity of the igneous magnetized rocks is very small.All of these observations indicate that the igneous rocksdrilled form a sill that has been intruded into thesediments. The age of the information of the oceanfloor at this site was therefore not determined.

CONCLUSIONS

Verification of Sea-Floor Spreading Model of Opening ofthe Norwegian Sea

Since igneous rocks reached by drilling in theNorwegian Sea were invariably basalt and the agesdetermined for the basalt were generally in accord with

1238

TECTONIC SYNTHESIS

o -V-27

0800 2330

Figure 22. Reflection profiles along track ABC passing through Site 345. Location of profiles shown in Figure 21.

the model proposed by Talwani and Eldholm (in press),lend support to their model for the tectonic evolution ofthe Norwegian Sea. It should be pointed out, however,that an argument can be made that in some cases that"true basement" was not reached, but that basalt flowsmuch younger than "true basement" were reached. Inparticular, at Sites 338 and 342 on the V^ring Plateaufrom multichannel seismic profiling, there is evidence,of reflectors below the basaltic layer. At Sites 348 and350 the layer penetrated had been termed an "opaque"layer; drilling demonstrated that it was basalt. An argu-ment can be made, therefore, that at some of the sitesthe basalt that was drilled into does not represent theoriginal sea floor but, rather, represents basalt flowssubsequent to the time of formation of a "truebasement." However, the ages determined generallyagree so well with the proposed model that in mostcases it seems that, if basement was not sampled, thematerial actually sampled was not too different in agefrom the underlying basement.

We recognize, of course, that especially in marginalareas such as the V^ring Plateau it is difficult to ab-solutely refute the argument, in view of subbasementreflectors, that the basalt flows reflect volcanic activityover continental basement at the time of early rifting,rather than the age of the sea floor itself. Becauseanomaly 24 is present on the V^ring Plateau, thepresumed continental area could lie between anomaly24 and the V^ring Plateau Escarpment. We believe thatthe preponderance of the geophysical data argueagainst such a hypothesis, but it cannot be absolutelyrefuted.

Time of Opening of the Norwegian Sea

The oldest anomaly found in the Norwegian Sea isanomaly 24. Anomaly 25 has not been seen in theNorwegian Sea. The corresponding ages by theHeirtzler time scale are 60 and 63 m.y., but if theHeirtzler time scale is too old by 5-7 m.y. the age of theopening would perhaps lie between 53 and 58 m.y. Theoldest sediments recovered in the Norwegian Sea areearly Eocene. It would seem then that an age of about55 m.y. represents the best estimate of the time of theinitiation of opening of the Norwegian Sea.

Land Bridges

After the opening of the Norwegian-Greenland Seastarted, two topographic barriers existed between theArctic and the Atlantic. One of these barriers was dueto the high elevation of the Iceland-Faeroe Ridge whichwas created above sea level, but gradually subsided.The model of Talwani and Eldholm (in press) suggestsan age of 55-25 m.y. for the formation of the Iceland-Faeroe Ridge. This age is not contradicted by the age ofthe basalt found at Site 336. Best estimate of the timewhen the oldest parts of this ridge started subsidingbelow sea level is about 25 m.y. ago. We note that manyassumptions have gone into arriving at this figure.However, we also note that this date is substantiated byfaunal evidence from Sites 336 and 352 which lie,respectively, on the north and south side of the Iceland-Faeroe Ridge. Fauna at least as young as late middleOligocene differed at the two sites, conforming the ex-istence of a land bridge at least up to this time.

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77°00'

76°45"

76°30'

76° 15'

76°00'10°

Figure 23. Magnetic anomalies plotted along Vema tracks in vicinity of Site 344 on the east flank of Knipovich Ridge.The light shading corresponds to the Knipovich Rift floor (depths greater than 3200 m). Darker shading corresponds topeaks in the area rising above 2000 meters.

The second land bridge in this area persisted because,in the Greenland Sea, the motion during the first phaseof opening, that is between the time of opening to about38 m.y. ago, was a shearing motion. Greenland slid pastSvalbard and the Barents Shelf without there being anyopening in the Greenland Sea. This opening started, ac-cording to the model of Talwani and Eldholm (inpress), at 38 m.y. after which time the connection wasestablished between the Arctic Sea and the NorwegianSea. The results of the drilling do not bear on theproblem of this second land bridge.

We also point out that the faunal evidence suggeststhat soon after the early opening of the Norwegian Sea,especially the Eocene, faunal similarities of the faunaexisted with northern Europe. The sea connection musthave been through the North Sea and the shallow seaextending over the Norwegian margin and the InnerV^ring Plateau. Continuous sedimentation appears tohave taken place in this area throughout the Tertiary.

Opaque Layer

Eldholm and Windisch (1974) have described anopaque layer in the area north of Iceland and thesouthern part of the Jan Mayen Ridge; it is generallysmooth. Also, in some areas, the opaque layer issuddenly interrupted, leaving "holes." These factors ledto the supposition that the opaque layer was perhaps anash layer, but whereas it might cover basement in someareas, it could not because its lack of relief representsoceanic basement. However, drilling at Sites 348 and350 showed that this opaque layer represents basaltflows. Age determined for the basalt, especially at Site348, agrees so well with the age predicted for a sea-floorspreading type model, that it seems reasonable tosuggest that the basalt layer, if not representing the seafloor at the time of its original formation, was emplacedvery soon thereafter. It is not easy to give a simple ex-planation for the "holes" but it is possible as Talwani

1240

TECTONIC SYNTHESIS

V-3O1Oc/c c V-3010

c/c c/c Dt — 0

1330 1405 2230Figure 24. Seismic reflection profiles obtained in the vicinity of the Knipovich Ridge for Vema

Cruise 30.

and Eldholm (in press) have suggested that the holesrepresent subsided continental crust lying betweenareas in which new ocean floor has been created by sea-floor spreading.

Hot Spots and Mantle Plumes

Several areas in the Norwegian Sea can be consideredto be areas of unusual elevation when considered in thelight of "age versus depth" curves for mid-ocean ridges.These areas include the Iceland-Faeroe and Iceland-Greenland ridges, Iceland, the Iceland-Jan MayenRidge (Kolbeinsey Ridge), and the area lying betweenthe Jan Mayen Ridge and the Iceland-Jan MayenRidge, the latter is the area where spreading took placebetween the time of spreading in the Norway Basin andin the Iceland-Jan Mayen Ridge. The extinct spreadingaxis in this area has been called the extinct spreadingaxis of the Icelandic Plateau. The area towards thesouthwest of Norway Basin, which contains Site 350,can also be considered an area of unusual elevation, if itis accepted that it was formed by sea-floor spreading.The Outer V^ring Plateau is also an area of unusualhigh elevation.

An important objective of the drilling program wasto determine if the chemistry of the basementassociated with sites at unusual elevation wasanomalous. Areas of unusual elevation have beenascribed to hot spots and mantle plumes. Adopting themodel of Schilling (this volume) for the Faeroe-Icelandregion, Raschka and Eckhardt (this volume) havedivided the basalts at the various sites as either "plumederived" or normal "ocean-ridge magma" types. Ac-cording to Raschka and Eckhardt, hot mantle plume-type basalts occur at Sites 342 and 343 on the OuterV^ring Plateau, Site 345 in the Lofoten Basin, and Site350 in the area southwest of the Norway Basin, whichlies on the southern morphological extension of the JanMayen Ridge.

Elsewhere, normal ridge-type basalts are associatedwith Site 336 on the Iceland-Faeroe Ridge, Site 337 onthe extinct axis in the Norway Basin, Site 338 on theOuter VyJring Plateau, Site 344 on the KnipovichRidge, and Site 348 near the intermediate extinct axison the Icelandic Plateau. Schilling^ (this volume)results parallel those of Raschka and Eckhardt (thisvolume).

At Sites 342, 343, 345, and 350 light rare earchenriched patterns, which he believes to be characteristicof plume activity, exist. At Sites 337 and 348 light rareearch depleted pattern is observed. At Site 336 thepattern shows light rare earth depletion for samplesfrom two flows whereas in a sample from a third flow,light rare earth enrichment is shown. At Sites 338 and344 the results can be perhaps considered intermediateshowing relative patterns of rare earch enrichment butan overall low level of RE enrichment exists..

One result is immediately apparent. There is no sim-ple correlation between areas of unusual elevation andbasement geochemistry. For example, Site 348 whichlies in an area of unusual elevation possesses a verycharacteristic rare earth depleted pattern associatedwith normal mid-ocean ridge segments, but the base-ment at Site 350 which, as we have discussed earlier,was formed at normal sea-floor depth and subsequentlyelevated, has a light rare earth enriched pattern.

The models that Schilling has proposed for theFaeroes invoke a variation in time of the mantle plumeactivity to explain the variation of light rare earthpatterns. He has attempted to explain his rare earthanalysis of the samples obtained in the Norwegian Seain a similar fashion. However, it is perhaps fair to saythat the pattern of variation in time and space of mantleplume activity tends to get quite complicated in orderto explain all the observations. For example, on theVyfring Plateau the basalts at Sites 342 and 343 whichare presumably somewhat younger than the basaltsdrilled at Site 338 show a greater light rare earth enrich-

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ment. The opposite is true for the sequence of basaltson the Faeroe platform. If the ages of the V^ringPlateau basalts are, as a whole, somewhat younger thanthe Faeroe basalts then, in order to explain the resultsin terms of the Schilling model, one has to assume thatthe mantle plume activity in the Faeroes decreased intime and very shortly afterwards it increased on theV^ring Plateau. Thereafter it decreased again on theIceland-Faeroe Ridge but it must have resumed againin order to explain the light rare earth enrichment of thesample from the youngest flow. Either such rapidchange in activity of the mantle plumes actually didtake place or some other explanation for the chemistryof the basalts has to be invoked.

ACKNOWLEDGMENTSSupport for the preparation of this report for one of the

authors (M. Talwani) came from Contracts N00014-67-A-0108-0004 and N00014-75-C-0210 with the U.S. Office ofNaval Research and from Grants GA 1434, GP5392,GA17731, GA27281, and DES71-00214-A07 from theNational Science Foundation.

REFERENCESBlakely, R. J., 1974. Geomagnetic reversals and crustal

spreading rates during the Miocene: J. Geophys. Res.,v. 79, p. 2979-2986.

Chapman, M. and Talwani, M., in preparation. An extinctspreading center on the Icelandic Plateau.

Eldholm, O. and Windisch, C. C, 1974. The sediment dis-tribution in the Norwegian-Greenland Sea: Geol. Soc.Am. Bull., v. 85, p. 1661-1676.

Fleischer, U., Holzkamm, F., Vollbrecht, K., and Voppel, D.,1974. Die struktur des Island-Faroer-Ruckens ausgeophysikalischen Messungen: Sonderdruck aus der Det.Zeitschrift, Band 27, H. 3, p. 97-113.

Gr^nlie, G. and Talwani, M., in preparation. A geophysicalstudy of the Jan Mayen Ridge.

Husebye, E. S., Gj^ystdal, H., Bungum, H., and Eldholm, O.,1975. The seismicity of the Norwegian-Greenland Sea:Tectonophysics, v. 26, p. 55-70.

Johnson, G. L. and Heezen, B. C, 1967. Morphology andevolution of the Norwegian-Greenland Sea: Deep-SeaRes., v. 14, p. 755-771.

Johnson, G. L., Southall, I. R., Young, D. W., and Vogt,P. R., 1972. Origin and structure of the Iceland Plateauand Kolbeinsey Ridge: Jr. Geophys. Res., v. 77, p. 5688-5696.

Kristoffersen, Y. and Talwani, M., in preparation.Geophysical study of the Iceland-Faeroe Ridge.

Le Pichon, X., Hyndman, R. D., and Pautot, G., 1971.Geophysical study of the opening of the Labrador Sea: Jr.Geophys. Res., v. 76, p. 4724-2743.

Meyer, O., Voppel, D., Fleischer, U., Closs, H., and Gerke,H., 1972. Results of bathymetric, magnetic andgravimetric measurements between Iceland and 70°N:Deut. Hydrograph. Z., Band 25, H. 5, p. 193-201.

Pitman, W. C, III, and Talwani, M., 1972. Sea-floorspreading in the North Atlantic: Geol. Soc. Am. Bull.,v. 83, p. 619-646.

Talwani, M. and Eldholm, O., 1972. The continental marginsoff Norway: A geophysical study: Geol. Soc. Am. Bull.,v. 83, p. 3575-3608.

, in press. The evolution of the Norwegian-Greenland Sea: Geol. Soc. Am. Bull.

Vogt, P. R., Ostenso, N. A., and Johnson, G. L., 1970.Magnetic and bathymetric data bearing on sea-floorspreading north of Iceland: Jr. Geophys. Res., v. 75,p. 903-920.

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