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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150
29”22.52’E
81033.00’N
18
20
147
1.12 =0.15x
0.966 0.545
7.63
502
23”17.89’E
81”25.14’N
17
143
5.60
7.70
1.0 _
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07”05.08’E
16
142
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0.99
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79’ 15.86’N
14
138
13
137
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1.017
7.38
2734
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0.667
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3.94
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7.63
W/m2)
Conductivity
12.2
05°06.42’E
SO”47.18’N
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133
10
132
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03°t6.32’E
8 I 006.46’N
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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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