Morphology and evolution of the Norwegian-Greenland Sea

Deep-Sea Research, 1967, Vol. 14, pp. 755 to 771. Pergamon Press Ltd. Printed in Great Britain.

Morphology and evolution of the Norwegian-Greenland Sea* G. LEONARD JOHNSON~ and BRUCE C. HEEZEN~ (Received24 July 1967) Abstract--Throughout the history of the Norwegian-GreeniandSea, the still active Mid-Oceanic Ridge has alwaysbeen the dominanttectonicfeature. It is cut by WNW-ESE fracture zones which originated early in the history of the basin and now bear evidenceof the directionsof early crustal movements. INTRODUCTION

UNTIL recently knowledge of the bottom configuration of the Norwegian-Greenland Sea was meager. NANSEN'S(1904) bathymetric chart was revised by later expeditions, but still remained schematic. The first systematic echo-sounding profiles of the Greenland and Norwegian Seas were made by S.S. Veslekari in 1937-38 under the leadership of Miss Louise A. Boyd (BoYD, 1948). STOCKS(1950) produced a bathymetric chart on the basis of pre-World War II soundings. LITVIN(1964) incorporating about a decade's accumulation of Soviet data produced an excellent chart of the Norwegian Sea and, JOHNSON and ECKHOFF(1966) utilizing recent icebreaker data mapped the Greenland Sea. During the months of July-September 1966 the USCGC Spar in cooperation with the U.S. Naval Oceanographic Office conducted a 5300-mile magnetic and bathymetric reconnaissance survey of the Mid-Oceanic Ridge north of Iceland and to the east and west of Jan Mayen. Soundings were obtained with a Precision Echo Sounder Recorder (Alpine Model 465F) utilizing a GIFFT ESRTR-48 Precision Transceiver and the echo distances, measured in units of 1/400 sec traveltime are probably accurate to at least 1 part in 3000. The echo sounding tapes were read at every peak, trough or change in slope and the depths plotted and later contoured at a scale of 1 : 500,000. A nuclear resonance magnetometer (Varian 4931 DR) was employed to measure the absolute value of the earth's magnetic field. Magnetic tapes were scaled at 50-gamma intervals and at all magnetic highs and lows. Navigation was by LORAN C and was accurate to within 500 yd or better. Agreement between intersecting lines was excellent. Soundings and topographic profiles from various other sources notably Atka 1962, 1966, Edisto 1964, 1965, Veslekari 1937-38 (BOYD, 1948), P.I.N.R.O. (LITVIN, 1965), Atlantic Seal and Arctic Seal of the Marine Geophysical Program of the U.S. Naval Oceanographic Office, Vema Cruise 23, Chain Cruise 13 and H.M.S. Vidal were utilized in the preparation of this report. In addition, soundings from the U.S. Naval Oceanographic Office collection sheets and various charts and atlases were used to fill in areas with no data. Based on the above data it was possible to delineate *Lamont GeologicalObservatory,Contribution No. 1108. tU.S. Naval OceanographicOffice. SLamont GeologicalObservatory,ColumbiaUniversity,Palisades,N.Y. 755

756

G. LEONARD JOHNSON and BRUCE C. HEEZEN

the general physiographic province boundaries of the Norwegian-Greenland Sea and environs (Fig. 1). 85*

80°

r5 a

ZO*

55?

30 °

20"

IO °

0°

Fig. 1. Physiographic provinces of the Norwegian-Greenland Sea. Black Circles indicate earthquake epicenters.

_%1-........... ,,.,,~ .:~/~-52,ooo L

400

• ~ ii~:~

800

iO00

i~ ~v" --52,000

i

'.

'v'

'vt

J\

.,.oo

800 U~j / s.

!-,4 .,~ = .,-~--

~E_

"

-

......

800

~ ' ' 0 0 0

0

-3000

.ooo %.,

3. 1200--

/

r,

.~'!

,00 KILOMETERS

'i

-

~

-J-A--~; ,o0

~,

'

\

\

6

4 0 0 -

" ~

400 ~

-

-- --

-~-

~.ooo

o,,ooo-lt~17~.~vlv...;~-

I .

4

.ooo,

~__~

.-. /

.o.

~ /~J

70 ~

,o.

.

~

l

j.,.

o.

.

.

.

.

.

o.

.

.

.

.

Fig. 2. Icelandic P l a t e a u six profiles; d o t t e d lines are total m a g n e t i c intensity; solid lines are t o p o g r a p h y ; vertical e x a g g e r a t i o n 100

-....

:

l. l/acing p. 756]

757

Morphology a n d evolution of the Norwegiarl-Greenland Sea MID-OCEANIC

RIDGE

The extension of the Mid-Oceanic Ridge into the Greenland Sea and Arctic Basin has been inferred on the basis of earthquake epicenters and spot soundings (ELMENDORFand HEEZEN, 1957; HEEZENand EWING, 1961). More recently the morphology of the ridge has been further delineated by several investigators (LITVIN, 1965; JOHNSONand ECKHOEF, 1966). The earthquake epicenter belt (SYr~Zs, 1965) striking north from Iceland at 66 ° 40'N lies in a north-south striking depression of the Icelandic shelf (Fig. 2, profile 6). Its walls are irregular in contrast to the generally smoothed sea floor of the continental shelf and continental slope north of Iceland (Fig. 3). Just to the

~

-

:

-

•

. •. . . . ... . ... ".~i'~'_r". . . , . ~ . .. . . . .. ... ... ..... .

66" 3,5'N, '. •

....

I s * 9 ~ ' w

.

.

.

.

.

.

.

: .

.

.

.

'

.:

.

.

i!

:':

..

..'~

• i.

.

.

L

.

.

'

.....

, , , , ~ ' ,

.

.

.

.

•

........

.

.

.

..

.

.

.

.

.

.

.

.

-i . . . . . . . . . . .

.

.

.

.

.

:,

.

.

.

.

.

.

"

.

.

:

"

.

.

.

.

.

. . .

.

, _L~i .

.

.

~ . . . . . . . . . . . . FATHOMS

i e.

,'. . r. ,.: ..... . . .i:~ . ~-

{. . . .

................ '}-

- -

• r

-

......

. ....~o0"

I ........ i

'

i

~,........

;. . . . . .

|~

~

0

..". . . . . . .

!

:,.

;

,i

.,,:..

! ' ""'-:'"

"

l ........ 2

,

. . . . . . . .

'

N~UTICAL

t

. . . . . . . .

=...:....,:

"

;

.

66"o~' - - . : N

18*.28'W

-.;

,

.....

!

'

.

.

i~,.~' "

'~

.

.

I~0.

. . . .

-

•

}.

.

•

.

.~,_,~,,,r.~'..

.

.

:

' ....:......,...... "

".

.

.

. . . . . . . . ~

: ......

'; ...........

~

:

~

""

:

-..-.: .................. MTLES

."

'

.

~ . . . . . . . . . .

......

; ..... !

~-.--~ .

"

. . . . . . . . . . . . . . . . . .

. . . .

, ...........

-~}

-.,.

', ............. I .... :.!....~-.i-:..-~...: ............. "..-.~

Fig. 3. E c h o g r a m across earthquake epicenter belt n o r t h of Iceland. Corresponds with a p o r t i o n of profile 6, Fig. 2.

north of 68°N the strike of the Mid-Oceanic Ridge shifts from north-south to northeasterly. The crest from this region northward is marked by a small rift valley with a large positive magnetic anomaly (Fig. 2, profile 3). Between 68 ° 05'N and 68 ° 45'N it averages only 20 miles in width with the smooth flanks probably buried by detritus from Iceland. However, north of the Spar Fracture Zone at 69°N the crestal zone of the Mid-Oceanic Ridge increases in width to approximately 50 miles with eastern flanks extending some 100 miles The intersection of the Mid-Oceanic Ridge and the western end of the Jan Mayen Fracture Zone is an area of intense tectonic activity (70 ° 50'N, 14° 15'W) (Fig. 2, profile 2; Fig. 4). North of Iceland the development of the western flank of the Mid-Oceanic Ridge is limited by the closely adjacent continental block of Greenland. Since the ridge acts as an effective dam for detritus from Greenland its lowermost western flank is completely buried by the Scoresby Abyssal Plain (Fig. 2, profile 3) which lies at a depth slightly greater than 900 fm (1640 m). The broad smooth Icelandic Plateau blankets the eastern flank of the Mid-Oceanic Ridge. The flat-topped Jan Mayen Ridge lies along the eastern border of the plateau. Seismic reflection profiles show a thin (100-300 m) blanket of deposits overlying truncated eastward dipping layers (Fig. 5). This flat-topped ridge apparently is continuous southward from Jan Mayen to approximately 68 ° 30'N where it is replaced by occasional flat-topped peaks.

758

O. L~ONAROJOHNSONand BRUCEC. HEEZEN

~

geU

oi o~ ~o o •~

j

~\\l

0

--

SONO0~S

:~ "0

..---r. cr --.~_

74*

% ©,

.

.

.°•,

.,-....-~;

.........

~

~

" ... ,

•1

"75*

:, °'"°'i".

•

~.

....

zoo

~;~'-.Y'

\

~,

'72*

.

"-..

O

°--

/

/ll'~

/

F-'~

-'~. ]\~'x

~ o o ~

~o.-8~...~. ">L.__J It~~ . . . .

.

,,, ,

.o

~-..,?~,~_~-.

~--~__~__~>j~ ~ / / ~ _ ~ ,

e9

0 o J

•

8o

,

o~

i"

.

71"

. 5*

7'o. 6-

5°

Fig, 4. Bathymetric sketch of the Jan Mayen Fracture Zone and Mohns Ridge, Depths are in nominal echo-sounding units of 1/400 see travel time• Sounding comrol is indicated by dotted lines Black triangles indicate earthquake epicenters (SYKES,! 965).

6*

7*

8*

9*

N

~

m

0 ¢oi

) I

,I t

~.~.

I

~.~.~ .~.~N I I

(

~ .~. ~_~.n

/,.,

~-E:~ ~ •

~Oc~

0 I:~.,0

[~.

/

~.. I

~-'~

[

J J~

.~"~' ~

/I

\ • I/

~, ~ ¢oi

~"~-.~"

N iiiiiii

760

G. LEONARDJOHNSONand BRUCE C. HEEZEN

To the east of the Jan Mayen Ridge a normal appearing continental slope and rise form is present. To the north the Icelandic Plateau is bounded by the Jan Mayen Fracture Zone. The northeastern corner of the Icelandic Plateau is occupied by the wholly volcanic Jan Mayen Island. The spectacular Mt. Beerenberg (2277 m) in turn occupies the northeastern portion of the island. The rocks of Jan Mayen are predominately alkali-basalt and picrite, similar to other oceanic islands associated with the Mid-Oceanic Ridge (FITCH, GRASTY and MILLER, 1965). Seismic reflection data (Fig. 6, profiles B and C) indicate a sediment layer approximately 500 m thick on the central Icelandic Plateau. That portion of the Mid-Oceanic Ridge which strikes from northeast of Jan Mayen toward Bear Island is commonly called Mohns Ridge. The Mid-Oceanic Ridge turns sharply to the north at its intersection with the Greenland Fracture Zone. Mohns Ridge (Fig. 7, profiles 1-14) resembles in form a typical mid-oceanic ridge. The crest zone, which is from 500 to 1000 fins (900-1800 m) shoaler than the rift valley, averages 40 miles in width with the flanks extending more than 70 miles. A deep axial rift is present on all profiles except Fig. 7, profile 11. A small offset is apparently present in the rift at 1½°E; however, no unusual earthquake activity is noted in this region (Fig. 4). Linear magnetic anomalies similar to those observed on the Mid-Oceanic Ridge throughout the world are clearly shown. FRACTURE

ZONES

Just north of Iceland an east-west fracture zone at 66½°N displaces the epicenter belt 60 miles (Fig. 1). One hundred miles farther to the north (68]~°N) between 18° and 19°N an epicenter duster (SYKES, 1965) possibly suggests the presence of another and at this point the strike of the Mid-Oceanic Ridge shifts from north-south to northeasterly (Fig. 8, profile 2; Fig. 9). The Spar Fracture Zone is just to the south of 69 ° (Figs. 8 and 9). There appears to be no displacement of the crest of the ridge, but to the north of this fracture the ridge crest is much wider (Fig. 1). Scattered soundings suggest that at 69½°N yet another fracture zone may be present. The Jan Mayen Fracture Zone was once thought to strike east-west (SYKES, 1965) but on the basis of the recent data it appears to be a broad band of WNW-ESE en e c h e l o n escarpments and ridges. It apparently extends from the Greenland continental margin (OSTENSO, 1967) to the north flank of the Voting Plateau (Fig. 1). To the west of Jan Mayen (Fig. 10) the scarp and occasional deeps are well defined and are in line with the NW-SE elongation of the Jan Mayen Insular Shelf, and the faults, volcanic rift zones and dykes striking NW-SE on the northern part of Jan Mayen (F. J. FITCH, personal communication). Just to the northeast of Jan Mayen a trench is present flanked by a blocky topographic high shoaler than 500 fms (910 m). The en echelon series of troughs and elongated highs strikes to the southeast bisecting the Norwegian Basin (Fig. 2, profile 5). The Jan Mayen Fracture Zone offsets the Mid-Oceanic Ridge, a distance of over 120 miles in a right lateral sense and is parallel to the Greenland and Spitsbergen Fracture Zones to the north and the Greenland-Iceland-Faeroe Plateau to the south. The Voting Plateau (NANSEN, 1904) is a large marginal plateau at the terminus of the Jan Mayen Fracture Zone. The surface of the plateau lies between 650 and 800 fins (1190-1460 m) and covers approximately 12,000 mil~ (Fig. 11). Seismic reflection

..22. . . . "~ ' "~'~" "~+'......................" ~ " ' " - ' " - 5 2

oo

"

2. -

-

-

_ls _ 5 2-

"~-~..+,e'~;.",. ~,.,,,~... 800--

800 FATHOMS

t

800

800

1200

53,000

~/.~ i~,+o

='~\ --

15.

~,~

-+.,'v/ ~'..... - -

-52,000

800

- -

1200

--

% ................ ~I' ++ q •

~=,"'"X ,. . . .~. . . . . + L.:

,11+11__52

I-~,, ~+oo ""-"~ '"""

~

0

~

-5 2 I

I

I

73"

5.~2oo

+,++,++ ~ - " ¢,~

14.

71+ i 5"

i o"

i 5 °

8003000

2000

ooo

i

0

KILOMETERS I(~0

NAUTICAL MILES

1600 3000 I()0

s:cn 0..-it-

n. -m w I.-..

t~

'5

.~

,.,

Fig. 7. F o u r t e e n profiles across the/violins Ridge sector o f the Mid-Oceanic Ridge. D o t t e d lines are total m a g n e t i c intensity; solid lines are t o p o g r a p h y ; vertical exaggeration 100 : 1. [facing p. 760]

Morphology and evolution of the Norwegian-Greenland Sea

:~.

--

..-7

" "..

5

,

761

7"--

Boo

'~6oo

400"

400 -

I000

-

bJ F-

800

1400

I

I

I

69*"

3

/ 7 _ ~ .

45

400 -

67*

S~

ICELAND~

20 ° 50 0

15°

5 lOON, MILE

--800-

t00 KILOMETERS

Fig. 8. Five profiles across the Spar Fracture Zone and continental margin north of Iceland. Dotted lines are total magnetic intensity; solid lines are topography; vertical exaggeration 100 : 1. profiles reveal at least 1000 m of generally horizontal strata WATSON, 1967). OCEAN

BASIN

(JOHNSON,BALLARDand

FLOOR

The southeastern basin of the Norwegian-Greenland Sea lies sandwiched between the continental rise of Norway and the eastern flank of the Icelandic Plateau. This section of sea floor is characterized by abyssal plains interfingering between seamounts and flat intermontane basins. Sediment thickness of 0.8 see (approximately 800 m) have been recorded in the plain regions (Fig. 6, profile D). The ' seamounts ' appear to be composed of basement which is slowly being covered by a sedimentary blanket. To the north of the Jan Mayen Fracture Zone the well-developed 11,000 mile~ Dumshaf* Abyssal Plain lies in approximately 1740 fins (3200 m). *Dumshaf is the classical name of the Norwegian Sea a r e a

(STEFANSSON,

1962).

762

G. LEONAROJOHNSONand BRUCEC. HEEZEN

tl,

H

O "~

O0

~..~

Morphologyand evolutionof the Norwegian-GreenlandSea

I. ~

763

4 0 0 FATHOMS

400'

o~

,ooo

tL800

1400

400

.1800

,2oo~2oo

3.-)~_t~

4. 53.,-,-.-~, -v-~.,,j,~.,.,,.. ~

V'

~

52".

400

5 3 ,oqo,.._-- .".-.',~,-,,.-..,

5

6 .....

~ k/

-800

" 1~

v.~ - :,/'--~--,-5z

~~ooi

~,71"

20e

15e

0

50 N.MILES

I0"

Fig. 10. Jan MayenFracture Zone (WesternEnd); six profiles. Dotted lines are total magnetic intensity;solid lines are topography;verticalexaggeration100 : l.

-80"

9

75*

-70*

ICELAND

FAEROE I

I

20*

-65*

i

0 °

10 °

Fig. 11. Bathymetric sketch map of the Norwegian-Greenland Sea. Contours north of 74 ° after JOHNSON and E c I ~ O ~ (1966); contours south of 74 ° after LITvir~ (1964), U.S. Naval Oceanographic collection sheets and this study. Depths are in nominal echo-sounding units of 1/400 see travel time. Detailed contours of Mohns Ridge are illustrated in Fig. 4.

Morphology and evolution of the Norwegian-Greenland Sea

765

To the west of the Mid-Oceanic Ridge, is the well-developed Greenland Continental Rise. The basin is poorly sounded; however, the sea floor appears to contain at least a few large seamounts and is most likely swale in form. The Greenland Abyssal Plain' (Fig. 1) lies at the northeastern extremity of the basin at depths of 2000-2060 fms (3700-3800 m) and is bounded on the northeast by the Greenland Fracture Zone. It probably interfingers into the western flank of the Mid-Oceanic Ridge.

EVOLUTION The identification of the median valley in the Mid-Oceanic Ridge as a rift valley and considerations of comparative morphology suggests that the ridge is growing wider through the continuing emplacement of mantle derivatives in to a succession of widening median mid-oceanic rift valleys (HEEzEN, 1957, 1959 and 1960). A prominent axial magnetic anomaly was found to be characteristic of the rift valley (HEEZEN, THARP and EWING, 1959) and a series of parallel anomalies of decreasing amplitude was observed flanking the axial anomaly and extending across the ridge to the adjacent basins. It has recently been shown that the rate of growth of the Mid-Oceanic Ridge may be determined by correlating the linear magnetic anomaly pattern with dated polarity reversals of the earth's field (VINE and MATTHEWS, 1963). A positive axial magnetic anomaly coincides with the rift valley of the NorwegianGreenland Ridge (Figs. 2 and 7). South of the Jan Mayen Fracture Zone the magnetic anomaly exceeds 1000 gammas (OSTENSO, 1967). On the Mohns Ridge sector the rift anomaly is generally 500 gammas or less with an apparent decline in intensity towards the Barents Shelf. In Fig. 7 (profiles 1, 6-9 and 13) this positive anomaly coincides with the rift valley. This parallelism suggests that major tectonic events may coincide with reversals of the magnetic field. However, in profiles 4 and 11, for instance, the axial positive anomaly fails to correlate with a specific valley but rather with a peak bordered by deeps which might be considered an uplifted filling of the rift valley. It has been recently demonstrated that remanent magnetism is a principle factor controlling magnetic anomalies in oceanic areas. The axial positive anomaly apparently represents rock implaced during the latest normal epoch (Brunhes) which commenced 700,000 years ago. The central graben of Iceland of Pleistocene age is approximately 60 km wide and is about the same width as the probable zone encompassing the last three major magnetic polarity epochs on the crest of the ridge north of Iceland. The rugged crest zone of Mohns Ridge as well as the Mid-Oceanic Ridge sector north of Iceland, therefore, were apparently created during the past 107 year. Extrapolating this rate of growth backwards in time we can deduce that the Norwegian-Greenland Sea must be about 1 or 2 × 10s years old (Figs. 12 and 13). The crest zone of the Mid-Oceanic Ridge is extremely rugged with fine-scale short wave length relief which indicates a virtual absence of sediment. With increasing distance from the crest sediment accumulation increases to over 500 m on the Icelandic Plateau (Fig. 6). This may not represent the total thickness for the deepest layer penetrated appears relatively smooth and the possibility of a deeper layer is apparent in Fig. 5. In a mid-oceanic area of moderate depth the principle factors governing the thickness of sediment are productivity and age of the underlying basement. Since the middle of the Norwegian-Greenland Sea has a moderately high and uniform productivity the total thickness can be taken as a measure of the age of underlying basement.

766

C

G. LEONARD ,I'OHNSON a n d BRUCE C. HEEZEN

D

-"--b-----.-_

-'T-"--------

Fig. 12. Evolution of the Norwegian-Greenland Sea. (a) Mid-Mesozoic (b) Mid-Tertiary (c) Late Tertiary (d) Pleistocene

Morphology and evolution of the Norwegian-Greenland Sea

767

'M ®

EVOLUTION OF NORWEGIAN SEA

==

GROWING RIFT

h FORMER RIFT AND AXIS OF GROWTH

FUTURE RIFT LATE TERTIARY GROWTH OF MID-OCEANIC RIDGE EARLIER AXES OF GROWTH

tt

.

f

GREENLAND

o

I

•

.A×,SOFM,O-OO~.,C.,~E

l~llHIIItllllll~l:k\

"--~1

1

,

'7

NORWEGIAN

.

S~i'~:~!'_l

.~

~

NORWAY

Fig. 13. Cross-section illustrating postulated axes of ridge growth in the Norwegian-Greenland Sea.

768

G LEONARD JOHNSON and BRUCE C, HEEZEN

If the rates of deposition in Wisconsin and Recent time approximate 3-5 cm/1000 year (ERICSON, EWlNG and WoLrar~, 1964) then the Icelandic Plateau immediately to the west of the Jan Mayen Ridge must be at least 10 M year old. Radiometric dates of 1.5-10.0 M year for basalts from Icelandhave been obtained (GALE, Moom3Arri, SIMONSand WALKER, 1966). The youngest Tertiary intrusives of Scotland and East Greenland are 50 M year. Thus, Iceland represents an altogether much younger sequence of events than the bordering continents. Jan Mayen Island like Iceland was built by basaltic flows. The fiat-topped seamounts and ridges near Jan Mayen were at one time emergent and wave cut, later to be drowned by subsidence. Russian investigators have dredged phyllitic and quartz-mica slates and granitic gneisses from Louise A. Boyd Bank on the crest of Mohns Ridge and from other locations to the southwest (DEIca~IqlTS~¥A and Dm~R, 1966). If these fragments are not ice-rafted material, they could be extremely significant. With the supposed breakup and separation of the Greenland and European blocks in the Mid-Mesozoic (FITCH, 1965), oceanic expansion proceeded through a succession of stages gradually developing the modern Norwegian Sea. The primeval NorwegianGreenland Sea must have resembled in form the present Red Sea (Fig. 12). AS the continental blocks separated, the ridge system expanded filling the growing sialic void. WNW-ESE fractures formed early in the development of the sea basin. As the ridge developed, adjustments in rate and direction of growth occurred along these primary zones of weakness. These tell-tale scars provide the most important clues to the specific direction of motion of the opposing continental blocks. Their importance can be amply demonstrated by an examination of the equatorial Atlantic (HEEzEN and Tru~gv, 1965). The Mohns Ridge section lies nearly equidistant between the parallel portions of the Greenland and Norwegian continental slopes. It is symmetrical to the axis of the ocean basin and its rugged, linear components are bilaterally symmetrical. Thus it is truly mid-oceanic. The segment of ridge between the Jan Mayen Fracture Zone and Iceland is extraordinarily asymmetrical both in form and its position in the ocean basin. This absence of symmetry apparently dates far back in geological history. It is probable that asymmetrical growth of the basin has proceeded throughout the history of the Norwegian-Greenland Sea. The Jan Mayen Ridge and the present axis of the Mid-Oceanic Ridge converge toward the points where they are both terminated by the Jan Mayen Fracture Zone. Lesser lineations form a fan-shaped network separating pie-shaped portions of midoceanic crust, and indicate an increasing rate of expansion with decreasing latitude. A narrow extremely linear feature runs nearly due north along 8°E from the eastern extremity of Mohns Ridge to the western extremity of the Arctic Mid-Oceanic Ridge. It may be complicated and offset by NW-SE fractures in the region west of Spitsbergen. West of Bear Island where its character is best known, its crest zone resembles a small mid-oceanic ridge, but the reconstruction of the Mid-Mesozoic Norwegian Sea indicates that extensive N-S strike slip movement must have occurred essentially along this line. Therefore, this feature has characteristics of both an extensional mid-oceanic ridge and a fracture zone. One gains the impression that extensional processes may have begun to gain over the transcurrent motion which in earlier years was predominant.

Morphology and evolution of the Norwegian-Greenland Sea

769

For obscure reasons ample outpourings of lava along a broad linear belt between Greenland and Scotland produced and are still building the Greenland-Iceland-Faeroe Plateau. A lesser but still ample outpouring extended north of Iceland producing the prominent Icelandic Plateau. COOPER(1965) suggested the Greenland-Iceland and Iceland-Scotland plateaux are separate ridges caused by the sliding of the Baltic and Canadian shields. The Jan Mayen Ridge may represent a splintered fragment from the Greenland block. In any case, it has apparently profoundly affected the subsequent development of the Mid-Oceanic Ridge in the region immediately north of Iceland (Figs. 12 and 13). The foundation of the Jan Mayen Ridge perhaps is a portion of the original floor of the Norwegian Sea which was buried under prograding sediment from the continental margin (Fig. 13). Its foundation may in fact represent a mid-oceanic ridge structure not unlike the present Reykjanes Ridge (H~mTZLER, LE PICHON and BARON, 1966). On the other hand it could be a fragment of continental crust similar to the linear micro-continents so prominent in the Indian Ocean (HEEz~N and THARP, 1965). The top of the Jan Mayen Ridge is bevelled and tilted in some cases to the west and sometimes to the east. The underlying sediments clearly have been bevelled by erosion. These eastward dipping eroded beds are overlain by 100-400 M of horizontal layers. Thus the Jan Mayen Ridge has apparently subsided and somewhat tilted in the late Tertiary. The horizontal stratified cap must represent at least 1-30 m year of deposition. The bevelled strata are at least 1 km thick and certainly are sedimentary. One interpretation of the seismic reflection profile is that the Jan Mayen Ridge consists almost entirely of a bevelled pile of easterly dipping sediments. The deposition of a linear monolithic ridge of stratified sediment in mid-ocean is difficult to imagine. Thus its interpretation as a fragment detached from the Greenland continental slope becomes more attractive. This would imply that prior to the separation of the Jan Mayen Ridge from Greenland the active zone of crustal extension lay somewhere to the east of the Jan Mayen Ridge. Under this interpretation some time in the MidTertiary the axis of the Mid-Oceanic Ridge shifted from the east to the west of the present Jan Mayen Ridge. However, the basin may have never been symmetrically divided and the active rift prior to the separation of the Jan Mayen Ridge from Greenland may have always been situated near the Greenland block. In the Norwegian-Greenland Sea the continental rise is extremely well-developed west of Norway. Continuing deposition here during late Mesozoic and Tertiary time is represented by well-developed sedimentary prisms both to the north and south of the Voting Plateau. The continental rise between Bear Island and Spitsbergen narrows to the north and laps upon the mid-oceanic rift-fracture and toward the edge of the Barents Shelf. Here the continental slope is gentle and almost merges with the continental rise. A reasonably well developed continental rise is present along the continental margin from northeast Greenland south to the western termination of the Jan Mayen Fracture Zone. Between the Jan Mayen Fracture Zone and the Greenland-Iceland-Faeroe Plateau the rise is poorly developed in the narrow basin which lies between the continental slope and the Mid-Oceanic Ridge. Although appreciable sedimentary accumulation in the basin is demonstrated by the existence of a linear abyssal plain as well as by the shallow depths of the apparently filled depression the accumulation is nevertheless anomalously small. Coupled with this anomaly and perhaps in explanation of it, is the rather unique rise which lies to the

770

G. LEONA~ JOHNSONand BRUCEC. HEEZEN

east of the Jan Mayen Ridge. If the later rise was formerly the continental rise of Greenland, the anomalously small size of the present continental rise of Greenland in this sector might be adequately explained. Several abyssal plains have developed in the deepest parts of the basin. Prominent equi-dimensional plains are found north and south of the Greenland Fracture Zone and to the north o f t h e Jan Mayen Fracture Zone. A linear westwardly fingering abyssal plain laps around the abyssal hills at the base of the continental rise of Norway. While the Norwegian Sea grew to a maximum width of 500 miles the Atlantic expanded as much as six times to a width of over 3000 miles. F r o m Iceland to the Jan Mayen Fracture Zone the crest of the Mid-Oceanic Ridge increase in width from 20 to 50 miles. This pattern is in complete opposition to the pie shaped growth of the Icelandic Plateau where the greater growth was to the south. Unless this pattern is to be understood as evidence of a lesser rate or growth in the vicinity of the Jan Mayen Fracture Zone it suggests that the relative motions of Greenland and Europe have radically changed during the latest geological epochs. In other words the point of pivoting of Greenland which apparently lay well north of Greenland may have shifted to the south in very Recent geologic time. Acknowledgements--The senior author was aided in the field work by the officers and crew of the USC(3C Spar, JOHNWAGEMAN,TERRYM.ALEYand the late AND~w C~M~. ROBERTHIGGSof the U.S. Naval Oceanographic Officeaided in the magnetic interpretations. Soundings taken by H.M.S. Vidal were supplied by the Admiralty Underwater Weapons Establishment, Portland by permission of the Hydrographer of the Navy. JEmxyWATSON,Texas Instruments, provided preliminary Marine Geophysical Program data. We wish t o thank Mr. WILLIAMGSELLand Mr. WILBURTGEDDES for support and encouragement in the preparation of this report. Discussions with MA~E THARP were valuable. The research was supported in part by the Officeof Naval Research (NONR 266-48) and the National Science Foundation (GA880). REFERENCES BOVD L. A. (1948) The coast of northeast Greenland. Spec. Pap. Am. Geog. Soc. No. 30, 339pp. CooPeR L. H. N. (1965) Radiolarians as possible chronometers of continental drift. In: Progress in Oceanography (M. SEARS;Ed.). Pergamon Press, pp. 71-82. DEM~NITSKAYAR. M. and V. D. DmrqER (1966) Morphological structure and the Earth's crust of the North Atlantic region. In : Continental Margins and Island Arcs (W. H. POOLEEd.), Geol. Survey of Canada, paper 66-15, pp. 62-79. ELMENDORr C. H. and B. C. HEEZEN (1957) Oceanographic information for engineering submarine cable systems. Bell Syst. tech. Z, 36, 1043-1093. ElUCSOND. B., M. E w ~ o and G. WoLLrN (1964) Sediment cores from the Arctic and subarctic seas. Science, 144, 1183-1192. FITCa F. J. (1965) The structural unity of the reconstructed North Atlantic continent. Phil. Trans. R. Soe., 258, 191-193. FITCrI F. J., R. L. GRASTYand J. A. MILLER(1965) Potassium-argon ages of rocks from Jan Mayen and an outline of its volcanic history. Nature, Lond., 207, 1349-1351. GALE N. H., S. MOORBATH,J. SIMONSand G. P. L. WALKER(1966) K-Ar ages of acid intrusive rocks from Iceland. Earth Planet Sci. Lett., 1, 284-288. HEEZ~N B. C. (1957) Deep-sea physiographic provinces and crustal structure. Trans. Am. geophys. Un., 38, 394. HEEZENB. C. (1959) Paleomagnetism, continental displacements, and the origin of submarine topography. Preprints, IntL ocean Congress, 1, 26-28. Iq_EEZ~NB. C. (1960) The rift in the ocean floor. Scient. Am., 203, 98-110. t-IE~zENB. C., MARIETmuu' and M. EWINO(1959) The floors of the ocean--I. North Atlantic. Spee. Pap. geol. Soe. Am., 65, 122. H~z~N B. C. and M. E w ~ o (1961) The Mid-Oceanic Ridge and its extension through the Arctic basin. In : Geology of the Arctic ((3. O. RAASCI~,Ed.), University of Toronto Press, pp. 622-642.

Morphology and evolution of the Norwegian-Greenland Sea

771

H~zErq B. C. and M. TrIARe (1965) Tectonic fabric of the Atlantic and Indian oceans and continental drift. Phil. Trans. R. Soc., 258, 90-106. HEmTZI~R J. R., X. LE PICnON and J. G. BARON (1966) Magnetic anomalies over the Reykjanes Ridge. Deep-Sea Res., 13, 427-443. Johnson G. L., J. A. BALLARDand J. A. WATSON(1967) Seismic studies of the Norwegian continental margin. Norsk. Polar. Arbok 1966 (in press). JOHNSON G. L. and O. B. ECKI-IO~ (1966) Bathymetry of the North Greenland sea. DeepSea Res., 13, 1161-1173. Ln'vrN V. M. (1964) Bottom relief of the Norwegian sea. (In Russian). Trudy polyar. nauchno-issled, lnst. morsk, ryb. Khoz. Okernogr. (P.LN.R.O.), 16, 89-109. LITvrN V. M. (1965) Origin of the bottom configuration of the Norwegian sea. (In Russian). Okeanologa, Akad. Nauk SSSR, 5 (4), 692-700. Translation: Oceanology, Am. geophys. Un., 5 (4), 90-96. 1966. NANSEN F. (1904) The bathymetrical features of the North Polar seas. Norwegian North Polar expedition, 1893-96, 4, 1-232. OST~NSON. A. (1967) Geophysical studies in the Greenland sea. Geol. Soc. Am. (in press). ST~ANSSON U. (1962) North Icelandic waters. Rit Fiskideild. 3, 269. STOCKS T. (1950) Die Tiefenverhaltnisse des Europaischen Nordmeeres. Dt. hydrogr. Z. 3, 93-100. SYKES L. R. (1965) The seismicity of the Arctic. Bull. seism. Soc. Am., 55, 501-518. VINE F. J. and D. H. MATa~mWS(1963) Magnetic anomalies over oceanic ridges. Nature, Lond. 199, 947-949.