Thermal implications for the evolution of the spitsbergen transform fault

Thermal implications for the evolution of the spitsbergen transform fault

Tectonophysics, Elsevier 89 Scientific (1982)1-32 Publishing Company, Amsterdam - Printed in The Netherlands THERMAL IMPLICATIONS FOR THE EVOL...

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Tectonophysics, Elsevier

89

Scientific

(1982)1-32 Publishing

Company,

Amsterdam

- Printed

in The Netherlands

THERMAL IMPLICATIONS FOR THE EVOLUTION SPITSBERGEN TRANSFORM FAULT

KATHLEEN

CRANE

’ Lament-Doherty ’ Department

Geological Observoto~,

2, ANNIK

M. MYHRE

2 and EIRIK

SUNDVOR

3

Palisades, N. Y. IO964 (U.S.A.)

of Geology, University of Oslo, Blindern, Oslo 3 (Norway)

3 Seismological (Final

‘, OLAV ELDHOLM

OF THE

Observatory,

University of Bergen, Bergen (Norway)

version received January

1, 1982)

ABSTRACT

Crane,

K., Eldholm,

O., Myhre,

the Spitsbergen

Transform

Tectonophysics,

89: l-32.

Heat Ridge

flow taken

within

Svalbard

between

the Spitsbergen

A.M. and Sundvor,

Svalbard

and Greenland

Transform,

of intrusion

three

Plateau

thermal

for the evolution Structure

provinces:

of

of the Arctic.

(1) the Molloy

and (3) the northeastern

identified

margin

of

zone and part of the Yermak Plateau

of the Yermak Plateau

can be traced

in composition

segment

to anomaly

Fault zone. In response propagated

The propagation

to the latest heating

Plate rotation

aborting

is liberated

boundary

episode

and migration from

with large-scale

of the oblique

continental

transform

stress across the oblique

constraints,

for the western

ridge-transform

of basalt forming Tertiary

volcanic

system,

is still taking

the migration

segment

the Nansen

of the Ridge

position.

system

the Western activity

across Yermak

a zone of Plateau.

on Svalbard.

place. As the transform-ridge

rate will diminish

as orthogonality

approached.

INTRODUCTION

The Fram Strait which composes the areas between the margins of northeastern Greenland and northwestern Svalbard can be divided into several morphological 0 1982 Elsevier Scientific 0040- 195 1/82/0000-ClOOO/$O2.75

Publishing

to

with the Hornsund

to its present

of the ridge-transform

transform

western

of the western

than the northeastern

is associated

trace, and shifted

and migration

the warm

subsidence

20 m.y. younger

that there was no room

the old transform

coincides

lies between

If the thermal

zone

both the visible

then it is likely that the crust is similar

that the original

stress allowed the massive intrusion

phenomenon

Readjustment

shows

7. We postulate

that this propagation

deviatoric

A thermal

and the shelf off Nordaustlandet.

to deviatoric

southwestward

It is suggested extensional

Plateau.

Sea. An additional

of the Molloy Ridge. It straddles

and not older than 13 m.y. (approximately

of the plateau). prior

and the average heat flow is much above the Sclater et

by heat flow lies to the northwest

fracture Yermak

segment

curve but agrees with values from the Norwegian-Greenland

segment

system

reveal

(2) the Yermak

The Molloy Ridge is a short spreading

plateau

implications

and J.F. Sweeney (Editors),

(Nordaustlandet).

al. (1971), cooling

oceanic

E., 1982. Thermal

fault. In: G.L. Johnson

Company

is

t

*

3

regions. The major plate boundaries are defined by the Knipovich Ridge in the Greenland Sea, the Nansen Ridge in the Arctic Ocean and the Spitsbergen Fracture Zone (> 500 km long). There is both morphological

and seismic evidence

that

smaller

the transform

transform located

system

is broken

faults. The actual location and

we, therefore,

into

several

of the plate boundary

use the term

Spitsbergen

spreading

the average width of the entire transform

and

has not yet been entirely

Transform

complex region of the present-day plate boundary between Nansen ridges. According to the latest bathymetric compilations the marginal of Svalbard,

to suggest centers

to describe

the

the Knipovich and (Perry et al., 1980)

system is 140 km. East of the transform

is

Yermak Plateau which forms the westernmost part of the margin north whereas the continental slope north of the island of Nordaustlandet

slopes steeply towards the Eurasian Basin in the Arctic Ocean (Fig. 1). The Yermak Plateau can be divided into two morphological provinces; the northeastern section trends about 150 km NE-SW, narrowing to the northeast. This region is fairly flat topped and is characterized by high-amplitude magnetic anomalies (Feden et al., 1979) (Fig. 2). This segment meets the western portion of the plateau which trends roughly NW-SE. The western segment parallels and is bounded by the Spitsbergen Transform on the southwest and on the northeast by a series of bathymetric highs trending

in the same direction

the NW-SE

trending

the western

Yermak

basalts,

Quaternary

as the western

plateau

Wood and Bock fjords roughly Plateau.

Along

volcanics

North of Greenland,

which define

align with the eastern

these faults there is evidence

and present-day

conjugate

(Fig. 3). Faults

(1982,

spreading

this issue)

showing

to the Yermak

Plateau,

history of the Greenland that

there

flood

hot springs. lies the Morris Jesup Rise,

a flat-topped submarine plateau characterized by high-amplitude magnetic lies similar to those on the northeastern part of the Yermak Plateau. The seafloor

flank of

of Tertiary

Sea is discussed

was continental

sliding

anoma-

by Myhre et al.,

between

northeast

Greenland and Svalbard until anomaly 13 time when the relative plate motion changed. The transform fault system connecting the incipient Lofoten-Greenland and Eurasian basins has often been called the De Geer line. The original zone of continental crustal translation has been responsible for the multiple episodes of compression and shear along numerous parallel faults within the continental mass of western Svalbard (Birkemajer, 1981). However, the actual tectonic history in the Fram Strait remains poorly known despite

the detailed

Fig. 1. Bathymetric values indicate

surveys over the area (Feden

map of the Spitsbergen

are superimposed. seismicity

aeromagnetic

The NW-SE

Transform

line indicates

from Sykes (1965), Horsfield

Fault,

Yermak

Plateau

a heat flow cross-section

and Maton (1970)

Mitchell

et al., 1979; Vogt et

and Svalbard.

Heat flow

(see Fig. 10). Asterisks

et al. (1979) and Perry et al.

(1980). Inset. Regional Arctic

Ocean.

sketch map indicating H. M. = Hinlopen

major structural

margin;

Y. P. = Yermak

features Plateau,

in the Norwegian-Greenland M.J. R. = Morris Jesup Rise.

Sea and the

O0 Fig. 2. Magnetic

anomaly

pattern

(dashed)

of AGS

plateaus

and high-amplitude

better

(1975)

chart

superimposed

of the Arctic

anomalies.

on bathymetric Basin.

Note

Since bathymetric

contours

correlation data

(solid)

between

are sparse,

and rift valley

Yermak-Morris

actual

axis Jesup

correlation

may be

than shown (from Feden et al., 1979).

al., 1979). It is postulated that migration and readjustment of ridge and transform boundaries in the newly evolving ocean may be the cause for the magnetic signatures of the northern Greenland Sea up to and including the southwestern section of the Yermak Plateau. Plate reconstructions suggest that much of the marginal borderlands, evolved Feden

particularly

the Yermak

after or during

Plateau

the initiating

et al. (1979) and Jackson

and

the Morris

stages of rifting

Jesup

Rise,

must

have

(Fig. 2), Vogt et al. (1979);

et al. (in prep.) propose

that the Yermak

Plateau

and the conjugate Moiris Jesup Rise (Figs. 1 and 2) originated as a hot spot during anomaly 13- 18 time (36-42 m.y.) on the newly separating Nansen Ridge. Normal seafloor spreading in the Eurasian Basin later separated the two plateaus. The Yermak Plateau and other aseismic uplifted ocean platforms are enigmatic in their elevated bathymetry and structures (vertical and horizontal). Because of their locations on the ocean-continental boundaries they might be clues to the evolution of initial break up of the continents. In this paper we will discuss,the thermal history of the region and from this data will attempt to apply constraints on the origins of the various morphological and tectonic structures in the Fram Strait region. We will discuss several possibilities for

Fig. 3. Morphology Quaternary structures

and

volcanos

structures

and recent

off of Svalbard

from Sundvor

the origin of the Yermak hotspot

volcanism,

zone of transform HEAT

on

the Yermak

hot spring

Plateau

vs transform migration

activity.

Plateau Structures

and

Svalbard.

on Svalbard

Stars from

indicate Birkenmajer

Tc:&ii (1981),

et al. (1979).

including related

crustal extension

thinning,

stretching,

and compression

splintering, about

a wide

and readjustment.

FLOW

In the fall of 1980, expedition YMER made several crossings of the ocean floor between Svalbard and Greenland, the Yermak Plateau and the margin of Nordaustlandet (YMER, 1981). The heat flow observations were all made with the Ewing thermograd apparatus in tandem with gravity and piston cores. By means of

8 _

8

8

8

15 16

17

18

20

139

143

147

150

142

8

14

138

9

II

133

7

10

10

132

I

I

9

12

7

8

130

131

13

6

6

126

134

5

124

137

14

3

4

123

7

2

#Values

122

-.---_

HI=#

Station

__

(W/m”k)

Conductivity

TABLE I

0.16 0.34 0.04

0.96

0.07

1.0

1.12

0.1

0.89

1.13

0.14 0.09

1.09

a.1

0.99

0.17

1.15

0.08

1.06

1.16

0.03 0.08

0.09

1.12

1.04

0.16

0.05

0.99

1.02

--.-_

_ ._l-l..

0.99

S.D.

~~

Mean

i-O.1562

0.69

0.91

+ 0.352

1-0.1862

1.04 + 0.0992

0.975 -to.1 152

0.83

K(Z)

_-___-... ._^___.__

0.09

0.07

0.08

0.08

0.08

S.D.( Zt

5.06 (2.7 mean)

1.35

I-4

1.46

57.0

43%

61

118 100-121

120

174

147

2.4.-2.9

(2.6 mean)

212 442

2.82

2.87

4.16

3.5

10.55 (2.9 mean)

343

130

8.2

3.1

I

69

229

137

._

174 (poor)

-.-._____ ImW/m’)

173 (3 mean)

._

2 s- 1)

_--_lll_.

4.12

1.65

5.46

3.26

_..._ .

~(X lob Cal. cm

HF

...____.--

7

\

Fig. 4. Temperature Yermak

Plateau.

gradients

for all heat flow stations.

Dot and dashed

profile is from the Greenland

3

profiles

continental

Dashed

are those profiles margin.

profiles

taken

indicate

those stations

off of Nordaustlandet.

In many cases bottom

water fluctuations

on the

The dotted have warmed

or cooled the upper 3-4 m of the sediment.

thermistor probes attached to the core barrel, up to five temperature measurements were taken with each core. Most core penetrations were greater than 6 m with only two stations of 3-3.5 m penetration (Table I). The resultant temperature data show in most cases a linear increase of temperature with depth (Fig. 4). However, seven of the cores clearly

show non-linear

temperature

core. Below that level the temperature true conductive Spitsbergen Nordaustlandet strong bottom

perturbation

increase

heat flow. Two of the perturbed

Transform,

two on the Yermak

show that the upper water temperature

3-4m

variability

in the upper 4 m of the

is linear and probably gradients

Plateau

and

of the sediment

represents

are located all three have been

(Fig. 4). This interpretation

the

along

the

stations

off

subject

to

is based on

the high seasonality of bottom water in the area. Lachenbruch and Marshall (1968) describe the effect of temperature perturbations on cores taken from the Denmark Strait. Their analyses serve as a model for the hydrographic perturbations discussed below. Hydrography Bottom water on the Yermak Plateau and along the shelf of Nordaustlandet is known to be highly variable based on depth, time of the day or season. Tempera-

8 tures can fluctuate

from - 1°C to 5°C on the continental

relative

rates, magnitudes

density

stratified

of current

layers can be defined

(the East Greenland Atlantic

and direction

current

(SCOR,

by: (1) the surface

transpolar

drift

transpolar

1979), (2) the subsurface

water, and (3) the dense Arctic Bottom

The surface

shelf depending

stream

upon the

flow. Along the Svalbard

shelf the

drift stream

northward

flowing

Water.

flows from north

of the Laptev

and

East

Siberian Seas across the length of the Eurasian Basin through the western Fram Strait (Fig. 5) (SCOR, 1979). Its low salinity is probably maintained by the contribution of fresh water principally through river outflow around the Eurasian Basin. Below the surface water lies warm saline Atlantic water (Aagaard and Greisman, 1975). This water mass has its origin in the North Atlantic and flows northward through the Fram Strait hugging the west coast of Svalbard. A portion of this water also enters the Barents Sea both to the south of the Spitsbergen Bank and to the east of Nordaustlandet (after a trip through the Fram Strait). North of Svalbard the Atlantic water enters the Arctic Ocean and flows to the east along the continental slope.

Here the Arctic

Atlantic

water has been cooled

denser than the Arctic surface water. A layer of cold nearly uniform water lies beneath layer is probably

formed

in the Greenland

through

time and space is not known.

through

the Fram

Strait is highly

from 5°C to 1.5”C and is

the Atlantic

Sea but the evolution

The volume and magnitude

variable

(3.557

Sverdrups)

water. This bottom of this water mass of water transport

* probably

depending

on tidal and seasonal variability (Aagaard et al., 1981; Aagaard and Greismann, 1975). There is also considerable horizontal variability in the thickness and the depth of various

components

of the

currents.

Another

large

factor

effecting

current

position and magnitude are the seasonal fluctuations. The icefront in the Fram Strait can vary from year to year and this component of surface cold water can effect the stratification of the water masses on the shelf regions. Also glacial runoff in the summer from Nordaustlandet and Svalbard (Pfirman, pers. commun., 1981) can sweep along the bathymetric contours and thus it is rather easy to encounter a temperature fluctuation of 6°C in only a few days depending upon localized conditions. This of course affects the thermal structure in the upper layers of the sediment. If the temperature of the seawater in contact with the bottom increases, the surficial thermal gradient in the sediments will decrease; if the water temperature decreases, Marshall,

then the surficial thermal gradient will increase (Lachenbruch and 1966, 1968). In Fig. 4 we note that at station 8 on the flank of the Yermak

Plateau and station 16 further south on the plateau, there has been an increase in the bottom water temperature of 0.4”C and 0.25”C. respectively. Station 8 has been warmed to a depth of 3-4m in the sediment and station 16 has been warmed down in the sediment. Off the coast of Nordaustlandet both stations 17 to =2.5-3.5m and 18 have been cooled by 0.2”C to a depth of 3-4 m in the sediment. In contrast. * I Sverdrup=

10’ m3/s

b

Fig. 5. a. Circulation

of Atlantic

Water in the Arctic Ocean (SCOR,

of the Fram Strait and the shelf to the east of Nordauslandet. the shelf (YMER,

1981).

1979). b. Temperature

cross-sections

Note the reverse and steep gradients

along

at

station

increased

20 (much

the warming increase

closer

and cooling

in bottom-water

warm Atlantic

to a depth

or westerly

temperatures

has occurred temperature

bottom

that the isotherms

temperature that The

along the Yermak

the isotherms

water

at the same time over a large region. Plateau

in the water column.

of 1600m where the maximum

It is in this location easterly

the

to the same depth indicates

water core has moved down in the water column

the shelf, thus depressing down

to Nordaustlandet)

by 0.4”C. That all five cores are affected

temperature

lie in nearly vertical.

shift in the warm or cold water

indicates

that the

and eastwards This affects

along

the shelf

fluctuation

is 05°C.

steep gradients.

Any

cores can raise or lower the

on the flanks of the shelf dramatically.

Lachenbruch in bottom-water

and Marshall temperature

(1968) show that for the Denmark Strait step changes of O.l’C will disturb the sediment down to 2.5 m in

one month. The 0.2-0.4”C transients that we notice on the Yermak the sediment down to 3-4m. In a situation where the temperature

Plateau effect fluctuation is

periodic, i.e. a yearly winter-summer signal, then the transient seasonal skin depth in the underlying sediment would be * 3.25 m with a fag time of e 2 months, This would indicate an early summer (late June-early July) warming along the Yermak Plateau and a cooling along the coast of Nordaustlandet. Because of these temperature fluctuations it is apparent that heat flow stations on the shelf must exceed 4m in penetration. Thermal conductivities Thermal conductivity measurements were performed on all of the cores using the transient needle probe technique (Von Herzen and Maxwell. 1959; Balling, 1979). The equipment material (lexan). measured

was temperature calibrated in a thermal conductivity To secure full coverage of all of the cores, conductivities

reference were first

at the top and in the middle of the core (every 75 cm). Later measurements

were made at every 25 cm in the upper

parts. To detect

was placed perpendicular

at the end of each core. Most of the cores

can be described

to the bedding

best by constant

conductivity

any anistrophy

the probe

with depth (Fig. 6). However,

in five

cases a linear increase of conductivity with depth fit the data better than a mean value and these regression lines are also plotted in Fig. 6. In most cores only the upper half of the core was retrieved (5 m) whereas the temperature information extends down to 1Om leaving some question about the regression lines at those depths. Statistics of the data fit to both constant and linear increasing conductivity with depth are given in Table I. Heat flows were calculated using both the mean conductivity as well as the more reliable linearly increasing conductivity. In the latter case the coordinates were transformed from depth to thermal resistance using Bullard’s (1939) equation:

11

Fig. 6. Thermal stations increases

mean

conductivity conductivities

linearly

with depth

of the YMER

sediment

are represented

cores compared

by a dashed

the best fit linear regression

using both the mean fit and the more accurate

regression

line. For

to depth those

in the sediment. cores

lines are also plotted.

where

For all

conductivity

Heat flow is calculated

line fit.

By using the linear expression for conductivity k(z) = k, + b,, this is integrated to: ,withk,,b,andz>O where k, = surface conductivity; z = depth in sediment, b = the slope of the linear increase and R(z) = thermal resistance.

Temperature data plotted against thermal resistance reveal only conductive processes at the s’tations where conductivities increased linearly with depth (stations 9, 10, 11, 12 and 18, Fig. 7). This is revealed in the linear, temperature-resistance curves, Heat flow results from these five stations are very high when calculated using the linearly increasing conductivity model (8.2, 5.1, 10.5, 3.5, and 4 HFU respectively; 343, 212,442, 147 and 174 mW). The last value was determined from a series of conductivities that increased rapidly in the bottom two measurements. Thus, the resultant is probably poor. If we use the much less reliable mean constant conductiv-

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18

20

147

1.12 =0.15x

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7.63

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23”17.89’E

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17

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5.60

7.70

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W/m2)

Conductivity

12.2

05°06.42’E

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130

6

126

325

14’23.62’W

SO”28.99’N

4

124

3170

{ x 10 W/cm)

fm)

fmi

grad

Therm.

PEN

Depth

2952

.-_.

00’47.31’E

3

123

Longitude

at all stations

01°27.i7’E

76’26.75’N

79” 1.5.65/N

2

122

Latitude

HF#

cnnductivitlea

Core

Heat flow data and thermal

TABLE

-~~

13

0 TEMP

Cl5

,“C

TEMP

-2 -=:

.“C

I___ 0I8

i-0

2h

Y’ 2 LL

1

2

01

0

0

“1

05 TEMP

,OC

3+

30

05 TEMP

05

0

IO

,“C

TEMP,OC

Fig. 7. For cores where thermal against

temperature

profile).

Heat flow is calculated

conductivity

to determine

increases

if the heat

linearly

with depth,

flow is conductive

and shown adjacent

(straight

thermal

resistance

is plotted

line) or convective

(curved

to the profiles.

ities then the heat flow at these stations becomes 3, 2.7, 2.9, 2.6 and 1 HFU respectively (126, 113, 121, 109, 43 mW). These results as well as heat flow for the

rest of the stations

are depicted

in Table II and on Fig. 7.

From the heat flow results we can divide the region surveyed into three thermal provinces which are: (1) the Molloy Ridge; (2) the Spitsbergen Transform relay zone (including a portion of the Yermak Plateau); we define this province as a relay zone because

the morphology

has not yet developed

into a full fledged spreading

and (3) the northeastern margin of Spitsbergen (Nordaustlandet). Both regions 2 are part of the greater Spitsbergen Transform domain (Fig. 9).

center; 1 and

Molloy Ridge Eight heat flow stations across continental shelves reveal high heat decay away from the Ridge (Figs. 1, of rotation of the Nansen Ridge we

the Molloy Ridge and out to the bounding flow at the center of the Ridge and a gradual 8, 9). By using the predicted flow lines and rates can plot heat flow vs. age.

14

,” -1

15

The distribution that predicted Molloy

of high heat flow values

by normal

crustal spreading

Ridge and a decreasing

across the Molloy

Ridge is similar

models, that is a thermal

maximum

to

at the

heat flow vs. age.

In Fig. 9 we note that there are two interpretations conductivity

to the data. The better fitting

linearly

increasing

curves yield a much higher heat flow (10.5 HFU)

a point

* 120 km to the west of the Molloy Ridge. A much less accurate

for

value of 2.9

HFU is the resultant of using the mean constant conductivity with depth. These two widely different values obviously point to two different interpretations of the data. In the first high heat flow case we must volcanic

activity

comprised

along the Spitsbergen

of multiple

volcanic

of the transform domain. Compared to the &later

look for an additional

transform.

region

of recent

This would infer that the area is

relay zones or small spreading

et al. (1971) thermal

centers in the middle

decay curve for other accretionary

plate boundaries, the heat flow across the eastern flank of the Molloy Ridge is abnormally high. However, there is a close correlation with the McKenzie theoretical cooling curve with the exception of three points: two lie between SO-120 km to the northwest

of the ridge and one is approximately

120 km to the southeast

of the ridge

t 300

100

200

0

100

200

RIDGE

MC&

+ YMER * OTHER

0

20

LO

60

ixl

100

AGE,my Fig. 9. a. Heat flow vs. distance

away from the Molloy

Ridge. The high values to the west are indicative other values are similar to those measured Sea. All values are compared

by Langseth

to the MacKenzie

Ridge.

b. Heat

of a center of intrusion cooling

and Zielinski

flow vs. age across

(1976)for the Norwegian

curve (dashed

the Molloy

within the larger transform. line).

All

Greenland

near the intersection

of the Knipovich

It is also noteworthy

that the heat flow distribution

and Zielinski

(1974) curve derived

conditions

it was possible

Transform

south

HFU

Ridge and the Spitsbergen

for the Norwegian-Greenland Ridge,

Fault.

to the Langseth

Sea. Because of ice

to take only one heat flow station

of the Nansen

Transform

is very similar within

This was a surprisingly

the Spitbergen

high value of 4.2

(176 mW).

The Yermak

Pkzteau

The Yermak Plateau is also unusually warm relative to its distance from the Nansen Ridge. Four heat flow stations were taken along the plateau from northwest to southeast

(Fig. 1). The region covered

hes to the southwest

of the high-amplitude

9

8

i

& 5” $I32 1 0 ~

I lo

lpo

I I, xf 30 38

3@lkm ,

280 I 51

curve

AGE,my

Fig. IO. a. Heat flow across the Yermak Ridge.

The high heat flow illustrates

Spitsbergen

Transform

b. Seismic refraction

Plateau compared that

up onto a section of the Yermak profile on the Yermak

to the theoretical

there is a new center Plateau

cooling curve for the Nansen

of intrusion

which

extends

from the

Plateau.

with 1900 m of sediment

underlying

heat flow js 16.

17

magnetic

anomalies

mapped

and discussed

by Feden

(in prep.) (Fig. 8). By using the best linear regression

et al. (1979) and Jackson lines for the thermal

ities and applying

them to the stable portion

depth the resultant 343, 212, loo-121

heat flow from NW to SE is 3.1, 8.2, 5.1 and 2.4-2.9. HFU (130, mW) (Figs. 1, 4, 8, 10). Again, as in the Molloy Ridge cross-sec-

tion, if we use only mean 2.4-2.9

HFU

respectively.

conductivities

of the temperature

the values

For the least accurate

will reduce and

lowest

profiles

et al.

conductiv> 4 m in

to 3.1, 3, 2.7 and heat

flow case the

values are twice the heat flow expected on crust as old as 36 m.y. which was theorized by Feden et al. (1979) to be the age of the entire Yermak Plateau. Johnson et al. (1982) postulate the original fragmented

that the plateau

is continental

in origin. If this is the case then

crust must be severely altered by intrusion, stretched by thinning, or by transform activity. We shall discuss these possibilities in a later

section. If the more accurate

conductivity

higher heat flow) the northwestern recent volcanic intrusive northwest of the Molloy Nordaustlandet

activity Ridge.

lines are used (which result in much

which could extend

Plateau

is probably

the site of

to the zone of high heat flow

margin

Three heat flow stations 1.04-1.46 HFU (43.5-61 continental

regression

part of the Yermak

rangeing in depth from 1500 m to 460 m show values of mW) which indicate cold crust of presumably either

or very old oceanic

origin.

Discussion The entire region west of 15”E is unusually warm compared to its distance from plate boundaries. This includes not only the Nansen Ridge-Spitsbergen Transform area but also the western portion of the Yermak Plateau. A thermal boundary somewhere between the Yermak Plateau and the coast off of Nordaustlandet the heat flow is apparently location

quite low. However,

exists where

we do not know the details about the

of this transition.

There are three major interpretations about the origin of the Yermak Plateau: (1) Vogt et al. (1979), and Feden et al. (1979) proposed that the Yermak plateau and the Morris Jesup Rise evolved from anomaly 18 to anomaly 13 as a ridge hot spot phenomena at the Nansen Ridge. The high amplitude magnetic anomalies on both plateaus.may suggest a common origin (Fig. 2). However, the western portion of the Yermak Plateau is characterized by a very smooth magnetic field-quite in contrast to the northeastern section of the plateau. (2) Jackson et al. (1981) and Kovacs et al. (1981) propose that the western part of the Yermak Plateau is continental. They cite dredged gneiss and the smooth magnetic field as evidence for continental crust. (3) Prestvik

(1978) dated

plateau

basalts

on northern

Spitsbergen

(Fig. 3) at an

I I, a. Contmentallithosphere

total.

and continental

lithosphere

toward

all values of y. depth approaches

value.

from Royden

is descripttve

of rifting. proceeds,

ct al

The temperature

GIWO)frx exten\tontl

Long after riftmg.

margtn

lithosphere

thermal

(. ur\c gtxen b$. :

plane

i ~x~rreyx~nd~ I<‘ pun

gradtent

b) dashed I~cc,!nrl. lrtho~phrrr.

(shown

formation.

thinning

For

line).

the

both

slab to be at thermal of rafting. (C) Long after margin

across

which thins seaward

gradient.

stretches

at rrght shov.5 ltthosphertc

in old (l.c.. cool) margin.

continental stgniftcancc

profile

Dikes hec~~mes 100% of the temperature

maternal

along the plane of rifting. ultrabastc

dikes are Intruded when Intruded

ltthosphere thus steepening

the continental

of a wedge of formerly

only. having no btructurai

cxcurs

I\ ratsed considerahl?.

cru\t

ASTHENOSPHERE

LITHOSPHERE

CRUST

by \ertes of dikes whtch ma> not all reach surface

temperature

fixed at base of lithosphere.

As riftmg

m

0 q cracka and ultrabaatc

to pure oceanrc

crust Interrupted

to the tnitiatton

Itself consists

remain

The margin

a constant

calculated

m lithosphere

boundary

surface ele\atton

Theorettcal

pnor

at base of the lithosphere.

lithosphere

1.ransttton

the lithosphere

0b

less steep. hut surface

of continental

becomes

the axis of rifting.

consists

hthosphere

has cooled to equilibrium.

Ocean-continent

i

ASTHENOSPHERE

a

crust. Temperatures

with r, = 1300°C

equilibrium

temperature

LITHOSPHERE

0

The margin

through

of continental

diagram

h. Schematic

gradtent

to equilibrium.

has returned

The temperature

CRUST

IJ

prior to rifting. As rtfttng proceeds.

tncreane in volume and ‘or frequency

Fig.

0a

TEMPERATURE,*C

surface

elevation

_

f. Theoretical

calculted

lithosphere.

.

y=

1 corresponds

model.

model y = 1 corresponds

HEAT FLOW

to pure ocean

100 my.

of heat close to surface,

of y3 that is U(I)=

to curve for y =

to lithosphere;

from Royden

1.0 heat

et al. (1980).

is surface

1 in c. For

the greater

heat flow becomes

y: AI!?(I) where E(t)

and is identical

at which heat is added lithosphere, by factor

to pure oceanic multiplied

WX)fL

floor. Afler about

INTRUSION

High values of initial heat flow reflect addition

is equal to that in deep ocean

from dike intrusion

the rate of subsidence

heat flow of a dike intrusion

of oceanic

all values of y in this figure,

elevation

et al. (1980). Again

OIKE

HEAT FLOW EXTENSION MODEL

initial heat flow is quite tow, which results from great depth

the longer the time until effects are felt at surface.

e. Theoretical

the depth,

from Royden

MODEL

for a11 values of y. For y x0.6

heat flow calculated

flow unit (HFU)

d. Theoretical

INTRUS10N

SUBSIOENCE

SUBSIDENCE EXTENSION h4ocm

z

age of IO-12 m.y. He invoked second DATES

a reactivation

of the Yermak

hot spot to explain

this

stage of volcanism. OF INTRUSION

ON THE YERMAK

PLATEAU

We shall first address the problem of calculating a maximum age of intrusion for the western Yermak Plateau. If we use the models generated by Royden et al. (1980) who theorized a range in heat flow and subsidence values for newly forming oceanic-continental borderlands (Fig. 11) then we can derive a range of age estimates for the Yermak Plateau. If the western section of the plateau is totally oceanic crust then the oldest date of intrusion at the 3.1 HFU site is = 13 m.y. Royden et al. (1980) present a range of theoretical cooling and subsidence for a mixture of oceanic and continental crust. They label the mixture factory

curves and it

represents the percentage of crust that is intruded by basaltic dikes or stretched to oceanic thickness. If y = 1 the crust is purely oceanic, whereas when y = 0 the crust is purely continental

(Fig. 12).

Seismic refraction lines from Sundvor et al. (1979) cover territory only as far north as heat flow station no. 16. Here the heat flow lies between 2.4 and 2.9 HFU and the sediment thickness can be interpreted from the records as being 1.9 km thick and a total depth to basement of 2.4 km (Fig. lob). Using the Airy model for regional isostacy and calculating the amount of subsidence due only to a water loaded plate we can calculate

a thermally

plateau in the following manner: subsidence of a sediment-loaded water-loaded

where

induced

subsidence

of the western Yermak

If we use the Airy model and assume that the plate (U,) is related to the subsidence of a

plate (U, ) by:

p, = density

ment = 2.13 g/cm3,

of the basement-mantle = 3.0 g/cm3, p, = density and p, = density of water = 1.03 g/cm’.

of the sedi-

then lJw= 1.16 km. By using the two heat flow models of Royden to thinned

continental

crust or dike intrusion,

et al. ( 1980) (Fig. 11) which apply

for any range in y = 0 to 1.0 we can

determine a range of possible intrusion ages for this region (Table III). When we superimpose the heat flow determined ages on the subsidence curves of Royden et al. (1980), we note that in order to obtain a subsidence of 1.16 km where the heat flow is 2.4-2.9 HFU, the subsiding crust must be between the ages of 13-22 m.y. depending on the type of model. The only possible solutions are for y = 1.0 (oceanic crust) and the age of’ formation narrows to 16 m.y. B.P. However, the present-day ocean crust could have evolved from the progressive thinning of continental crust or by dike intrusion. This assumes that the southwestern part of the plateau began to subside after the latest intrusion and that none of the plateau had

21

PLATE

RECONSTRUCTIONS SVALBARD

Fig. 12. The plates rotated

back to anomaly

there was no room for any of the western

TABLE

Transform

fault. Prior to anomaly

III

Age of thermal Y

7 along the Spitsbergen plateau.

event

Model

Age of thermal

event (m.y.)

2.4 HFU

3.0 HFU

5.0 HFU

8.0 HFU

1.0

dike

22

14

7

(1

22 13

13 9

5 3


0.8

thinned D T

16

10

_

0.6

D

9

5

0.4

T D T

_

_ _

5

1 _

_

7

22

subsided

before this time. At the warmest

intrusion

is less than

site (8 HFU)

the oldest possible

1 m.y. If we use the less reliable

conductivities

HF of 3.0 then the age of intrusion

would be 13314 m.y. Therefore,

least thermally

activity

clear that intrusive

part of the Yermak present.

Within

Plateau

was occurring

under

age of the

which yield a it becomes

from 16 m.y. to at least 18 m.y. B.P. and probably

this warm region there are detectable

magnetic

at

the southwestern

signatures

to the

aligned

a roughly NE-SW direction. and are thus additional evidence for recent (Fig. 8). This data sheds new light on the evolution of the Yermak Plateau.

in

activitv

Plate rotations and zones of crustal overlap An additional way to constrain the interpretation of the western plateau’s origin is to rotate the plates back to a line of initial shear between Greenland and Svalbard. This will show quite clearly where, because of crustal overlap, it was impossible to have crust of normal continental thickness. If we use the magnetic lineations of Feden et al. (1979) for the region on and adjacent to the Yermak Plateau (Fig. 12) and proceed to rotate the plates back into their previous positions we notice that at the 3000 m contour the coast of Greenland and the southwestern part of the Yermak Plateau begin to overlap at anomaly 5b (Fig. 12). Before anomaly 7 (according to the identified magnetics) it was impossible for the western

section

of the Yermak

Plateau

to have existed. Transform

readjust-

ment probably began around the anomaly 5b period (16 m.y. B.P.) leaving mented crust in the region of the Western Plateau open to off axis intrusion.

frag-

Hot spot reactivation Around the Svalbard-Greenland area there is additional the locus of crustal intrusion after the hot spot episode According

to Prestvik

(1978) plateau

basalts

erupted

evidence for a shift in from anomaly 18-13.

on northern

Svalbard

between

the ages of IO- 12 m.y. B.P. (Fig. 3). Still today, active hot springs exist along the coastline of the Wood and Bock fjords, the sites of Quaternary volcanic activity. No recent volcanic activity has been cited for northern Greenland. Feden et al. (1979) also suggested

a region of increased

volcanic

activity on the Nansen

at anomaly 5b time. The high amplitude magnetic lineaments formed at anomaly symmetric across the Nansen Ridge and thus might be reinterpreted

Ridge beginning 5b time are not as a jump in the

spreading axis causing southward propagation of the ridge and a first readjustment along the highly oblique transform fault. Thus, the western plateau could have been thermally

reactivated

between

16- 10 m.y. B.P.

23

Continental

splintering

and/or

stretching

Johnson et al. (in prep.) propose that the NNW-striking part of the Yermak Plateau may be a continuation of the Caledonian Hecla Hoek landmass of northeastern Spitsbergen. In order to accommodate this region as continental crust they suggest that the plate boundary in the southwestern Eurasin Basin was much further west than today. This requires a. large change in the location to anomalies underwent (1981;

7-13

of the Arctic-Mid-Ocean

to allow space for the western

extension

Fig. 3) then

plateau.

as well as shear and compression the western

plateau

could

However,

as suggested

be the resultant

Ridge prior if this region by Birkenmajer

of a “thinned

and

stretched” continental crust created as the ridge and transform systems migrated across the region. A thinned margin will subside in much the same manner as a piece of cooling oceanic crust, but the residual heat flow will be less for a stretched continental crust than for crust newly intruded by magma. The amount of subsidence coupled with the measured heat flow along the plateau is numerically at odds with the theory that the plateau is continental. However, both the oceanic and continental crustal models can be accommodated by invoking a later emp1acemen.t of continental fragments in the location of the Western Yermak Plateau. The region has undergone extensive shear for more than 60 m.y. and it is possible that multiple faults comprising the original “transform zone” could have splintered off fragments of continental crust and transported them in a NNW direction in a manner similar to the block faulted terrain along the California

borderland.

Deviatoric

could generate massive masking the continental indicates rather

stress across

the migrating

ridge and transform

off axial intrusions along this same system of faults, thus slivers with sections of oceanic crust. Heat flow, however,

that a large percentage

of the plateau

must be very close to “oceanic”

crust

than continental.

Deviatoric

stress and transform-ridge

readjustment

Many well developed oceanic transforms are broad regions encompassing not only the deep transform trough but also elongate and shallow ridges. Some of these transform ridges lie between two transform troughs such as the Siqueiros, the Tamayo and the Charlie-Gibbs (Crane, 1976; Kastens et al., 1979; Macdonald et al., 1979; Tamayo, 1980; Olivet et al., 1974). Others border a single trough. Consideration of the strain ellipse for simple shear on strike-slip faults indicates that thrust faults, folds, normal faults and oblique shears may all occur within a transform fault zone. A summation of the dynamics along a transform must include not only those elements associated with purely strike-slip environment but also those elements that are created because of the derivation of the transform from the ideal state of shear. When

transforms

and ridges are not at their ideal configuration

of

24

right angle alignment,

then tensile

or compressive

stress develops

across the trans-

form zone dependent upon the direction of obliquity (Van Andel et al., 1969: Crane, 1976; Kastens et al., 1979; Bonatti and Chermak, 1981). Compressional and extensional tectonism can also develop around the transform domain where ridges overlap basins,

or where spreading direction changes have occurred new spreading centers in the old transform domain

migration

of both the former ridge and transform

creating pull apart and a subsequent

segments.

Bonatti and Chermak (1981) have documented that several ridges have gone through tremendous vertical tectonics throughout their histories. Some such as the Vema and the Romanche ridges have risen several kilometers in their history, have reached sea level and have since subsided to their present shallow depths. It has been postulated that stress across the transform domain may be responsible for the evolution of these enigmatic ridges (Crane, 1976; Bonatti and Chermak, 1981). Crane

(1976)

creating

postulated

intense

the

compression

This zone of compression

northwards

propagation

across the oblique

of the

and adjusting

East

Pacific

Siqueiros

could have thrust up the shallow median

Rise

Transform.

ridge parallel

to

both the northern and the southern transform troughs. Kastens et al. (1979). using gravity and magnetic profiles over the Tamayo Transform median ridge, showed that

in all likelihood

underlain

the ridge

by a low-velocity

was composed

magma chamber

of either

normal

or by tectonically

oceanic

emplaced

basalts,

serpentinite.

At this same transform Kastens et al. ( 1979). Macdonald et al. ( 1979) Tamayo (1980) and Cyamex and Pastouret (1981) revealed that readjustment of the transform as it broke free of the continental constraints of Baja California and Mexico. allowed for the initiation of new zones of crustal accretion and extension within the former transform domain. These were labeled “relay zones” where new zones of extension cut across old transform median ridges (Fig. 13A). The geographic evolutionary oceanic

corridors.

of intense

setting

of California

Transform.

Both are bordered

migration

to the Tamayo

while the Fram

shall explore the possibilities

Transform

is similar

Both are located

by continental

and readjustment

ever, the ridge adjacent Gulf

of the Spitsbergen

stage of the Tamayo

landmasses

in terms of accretionary lies clearly within readjustment

forming

and both show signs relay zones.

the oceanic confines

Strait is still in a stage of oceanic

of transform

to an earlier

in newly

and transform

infancy.

Howof the We

ridge-plateau

formation in the following paragraphs. The Spitsbergen Transform system offsets the Knipovich Ridge from the Nansen Ridge by 540 km. At the time of opening it was as much as 30” oblique to the Knipovich Ridge (Fig, 14). There are indications that the transform system is broken into numerous relay zones and smaller transforms. Why did the initial transform break into an oblique configuration? Most likely the original fracture along which the continents broke was a compromise between the local zone of weakness in the continental crust and the pole of opening for the Eurasian and American-Greenland plates. As more oceanic crust has formed, the

25

r

36my

DEVIATORIC

STRESS

C

L

Fig. 13. A. Ridge axis extension shear extension propagation

around

and transform

B. Stick diagrams C. Deviatoric

migration

that illustrate

transform

fault. This highly stressed and readjustment

possible

side of the fault is occupied

a growing

region

of

ridge

(Van Andel et al., 1969). transform.

yet the crust to the west of the fault rather

as the eastern

fault generates

area is open to off axis intrusion,

models of ridge propagation

stress about an oblique De Geer-Homsund

of the transform readjustment

along a non-orthogonal

the transform

and transform

migration.

The stress is increased

on both sides

than to the east will be most susceptible

by the Svalbard

continental

to

land mass.

ridge-transform system gradually has become liberated from the confines of predetermined continental zones of weakness. According to LePichon and Hayes (1971), before an entire transform can adjust to the ideal state of orthogonality, the total

26

/

27

width

of the newly

transform

offset.

formed

oceanic

By empirical

basin

deduction

must this

be at least is the

twice

the original

geometrical

configuration

required before changes in spreading may be accommodated entirely within the newly formed malleable oceanic crust. At the present rate of crustal accretion in the Fram

Strait it will be more than 40 m.y. before

system

will be liberated

ever, because

from the constraints

of its oblique

alignment

the entire

Spitsbergen

of the neighboring

Transform

continents.

there must have been tremendous

How-

extensional

deviatoric stress developed across the transform-ridge system. When the component of deviatoric stress is large across a transform system, there will be a constant readjustment along portions of the transform in response to the stress. Fujita and Sleep (1978) modeled the effects of deviatoric stress on ridge-transform systems and they discovered that when intraplate and interplate stresses are at a high angle to the transform there is a marked increase in tensile stress oriented in directions oblique to both the ridge and transform depending on the direction of the deviatoric stress. The effects of off axis deviatoric

stress is observed

at four well surveyed

transform-ridge

intersections (Crane and Gallo, in prep.). In Fig. 13C we notice the enhanced deviatoric stress about an oblique transform system similar to the Spitsbergen Transform fault. The perturbation

ridgeof the

stress field is so large on the inner corner of the transform intersections that extension becomes transform parallel rather than rift parallel and the crust can become subject to large magnitude volcanic intrusions at great distances from the original models

transform and the ridge. Fujita and Sleep (1978) and Fujita (1979) use these to describe the orientation of large seamount chains off the west coast of

North America which are oriented 15” to the transform fault direction The calculations also show that with an obliqueness of 6 = l&4”, the axis of maximum tension is (Y= - 11’ and the dikes begin to propagate into the exterior corners of the transform (1979)

intersections, that

(such

as 20’)

migrates geometric borderlands adjusts

ridge

suggests setting

there

thus is some

the transform

equilibrium

and thus throughout

of an oblique

may undergo

lengthening

angle

the history

ridge transform

system

a cyclic reversal in asymmetric

to a more stable position

throughout

fault

to which

zone.

Fujita

the obliqueness

of an evolving

basin

the

constrained

by continental

spreading

as the transform

time. At the Spitsbergen

Transform

the

Fujita and Sleep (1978) models invoke ridge propagation and a transform jump to the southwest in response to the increased deviatoric stress in that region. The transform landmass

would be unable to migrate to the northeast blocking the way. Deviatoric stress adjacent

because of the continental to the inner corner of the

Knipovich Ridge and Spitsbergen Transform will also be large today and as more oceanic crust is accreted at the ridge, there is more room to readjust the oblique transform ridge intersection. As indication of this kind of readjustment, we point out the following: (1) Plate reconstructions indicate that the western portion of the Yermak Plateau began forming 20 m.y. after the northeastern portion of the Yermak Plateau. This

28

line of demarcation

lies along the eastern

flank of the Plateau

and Wood Fjords on Svalbard, a site of Tertiary volcanism as well as active hot springs. (2) Heat flow across this western of hot spot origin. (3) Heat hottest

First intrusion

half of the plateau

on the Yermak

into the Bock

basalt flows and Quaternary is abnormally

high for crust

ages of 13- 16 m.y. are calculated.

flow is also high directly

location

plateau

trending

across

Plateau

the Spitsbergen

Transform

from the

(Fig. 8).

(4) The Molloy Ridge (a feature in the Spitsbergen Transform system) is apparently a small section of accreting crust. Heat flow across the high lies on the McKenzie cooling plate model peaking directly over the central high. Seismic activity is widely spread in this region. (5) Further evidence of readjustment and translocation of the Knipovich Ridge is apparent in the disconnected. presently inactive Senja-Greenland fracture zone to the south indicating a possible degree of northward readjustment and propagation of the ridge. These extinct transforms may also be continental slivers sheared off the southern Svalbard landmass (Myhre et al. 1982). (6) Along the transform, seismic activity is grouped into four major areas (Fig. 3). The first region is centered at the intersection of the Knipovich Ridge with the Spitsbergen Transform. The most intense activity lies on and around the Molloy Ridge with several epicenters The third region is located This is the site of highest junction

of the highly

with the Nansen

scattered

up onto the Yermak

100 km up the transform

heat flow mentioned

oblique

earlier.

Plateau.

towards

the Nansen

The fourth

trace of the past or present

region

Spitsbergen

Ridge. is at the

Transform

Ridge.

These observations

lead us to suggest:

(1) In response to interplate stress the transform can readjust by large-scale volcanism at acute angles to both the transform and the ridge. Such has probably been the case for the Juan de Fuca Ridge (Fujita and Sleep, 1978) and the Siqueiros Transform fault (Crane, 1976) and is most likely occurring along the Spitsbergen Transform. (2) As more oceanic crust is formed more of the transform ideal state of orthogonality. This model is depicted in Fig.

can readjust to the 13C. where former

sections of the transform trough are cut off, become dormant and are spread out away from the newly formed orthogonal ridge transform system. If such a model is correct for the Fram Strait area, we should expect to see obliquely trending troughs displaced from the present day seismically active system. (3) Readjustment can involve propagation of one or both ridges across a transform domain. In this case the rate of propagation is controlled by the rate of formation of oceanic crust. Former sections of transforms and dormant spreading centers

should

remain

as topographic

signatures

in the ocean floor away from the

propagating ridge (Hey et al., 1980). (4) From the northern intersection with the Nansen

Ridge, heat flow, bathymetry

29

and one interpretation

of the magnetic

anomalies

offset at least once to the east adjacent Plateau.

Plausible

transform

that the Nansen Ridge is

indicate

sector of the Yermak

to the northwestern

traces lie along the sharp NW-SE

aligned contours on

the edges of the Yermak Plateau (Fig. 14). The diagonal trough connecting these two bathymetric highs is no longer seismically active indicating that there might indeed be a series of orthogonal

step-like

ridge and transform

segments

within

the broader

scale Spitsbergen Transform fault. (5) The other distinct regions of high seismic

activity

likely indicate areas of increased shear separated Detailed surveys at other transform faults indicate

by small extensional relay zones. increased seismic activity near the

transform

intersection

Solomon

(1976),

with an accreting

Fox et al. (1976)

along the transform

plate margin-this

has been documented

Burr and Solomon

(1978)

and

Reichle

most

by et al.

( 1976). Extensional relay zones are documented in numerous other transforms where plate readjustments and transform and ridge migrations have forced the present day zone of transform shear to shift accordingly. At the Tamayo Transform (Kastens et al., 1979; Macdonald et al., 1979; Cyamex and Pastouret, 1981) the zone of shear readjusted by 15’ after the neighboring ridges broke free of the continental constraints of Mexico and Baja California. To accommodate this rotation a small zone of extension developed in the transform and the zone of shear broke into two features

allowing

the adjusting

state of orthogonality. The heat flow enigma the plateau response

could

to incipient

along the Yermak

be explained

deviatoric

stress. As a possible

transform

possibly

system to approach Plateau

or it can be connected

regional

intrusion

from in a

with the very large-scale

whereby a broad band of extensional and extrusive to the transform during the period of most intense evolution

of this region we propose

was located along a broad Hornsund

into the present-day

more of an ideal

and across the transform

as a zone of present-day

readjustment

deviatoric stress phenomena activity occurs at an angle

extended

ridge-transform

Yermak

Plateau.

that the original

Fault zone which may have

The major translation

was accompa-

nied by rejuvenation along some of the existing intracontinental faults (Fig. 14). As rifting ensued, the eastern section of the Yermak Plateau and the Morris Jesup Rise were formed by hot spot volcanism. As more oceanic crust formed the proto-MidArctic spreading center broke free from continental constraints and because of deviatoric stress began to propagate to the southwest near anomaly 5 time. This intense deviatoric stress across the transform zone generated massive extrusion and compression across a new region south of the former De Geer-Hornsund transform creating the western Yermak Plateau. With continued propagation the De Geer line became dormant and the new transform relocated to its present position, leaving the Yermak Plateau as a transform-related ridge. The southwestward propagation and jump of the Nansen Ridge coincides with the intense Tertiary volcanics erupted in the Wood fjord on Svalbard.

30

Migration and readjustment is still an ongoing phenomenon formation of the Molloy Ridge and the smaller less developed present

confines

of the Spitsbergen

as is apparent in the relay zones within the

Transform.

However, as more thermal information becomes available across the Yermak Plateau this interpretation will have to be adjusted. In order to accurately delineate the evolving

plate boundaries

in the Fram Strait we need to cover the region with a

much denser network of geophysical and geological investigations. We do not know where the boundary lies between the heat flow on the Yermak Plateau and the low heat flow near Nordauslandet. Does this thermal boundary coincide with the magnetic and structural boundaries on the Yermak Plateau? Detailed heat flow measurements should be taken in those regions that are anticipated to be relay zones in the transform. the structural bard

Further

investigations

relationship

between

oceanic-continental

migrating

transform

of these boundaries

the morphologically

transition

are necessary uplifted

and the possibility

to explain

plateau.

of propagating

the Svalridges and

faults.

ACKNOWLEDGEMENTS

We should like to thank the crew of the H.M.S. “YMER”, the universities of Oslo, Bergen, and Lamont-Doherty Geological Observatory for supplying the necessary equipment to carry out geophysical surveys in the area. N. Balling, P. Petersen and P. Knudsen from the University of Aarhus, Denmark kindly took conductivity measurements on the cores in Stockholm, and K. Bostrtim, J. Thiede and D. Wahlberg were of valuable assistance on expedition YMER. L. Johnson and J. Thiede offered many fruitful discussions. This research was funded by ONR Arctic Research Grant N-00014-80-C-0260, the Norwegian Research Council for Science and Technology and the Norwegian Research Council for Science and the Humanities REFERENCES Aagaard,

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