Download Peculiarities of roughness and thickness of oceanic crust in the

Geophys. J. Int. (2001) 145, 505–516
Peculiarities of roughness and thickness of oceanic crust in the
Eurasian Basin, Arctic Ocean
Estella Weigelt and Wilfried Jokat
Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, 27568 Bremerhaven, Germany. E-mail: [email protected]
Accepted 2000 December 4. Received 2000 December 4; in original form 1999 November 19
SUMMARY
The Gakkel Ridge, northern boundary of the American and Eurasian plates, has the
slowest spreading rate of the global ridge system. Therefore, it provides an excellent
opportunity to study any dependence of crustal fabric on spreading rate. Two parameters, crustal surface roughness and crustal thickness, at the super-slow-spreading
Gakkel Ridge (<20 mm yrx1 full rate) are the subject of the following study. Seismic
and gravity data acquired across the Gakkel Ridge and the adjacent Nansen and
Amundsen basins during the ARCTIC’91 expedition are used.
The surface of the basement, imaged along the seismic multichannel profiles, is very
rough and varies in its topography from several hundreds of metres up to 1000 m. Its
RMS-roughness ranges from 450 m in the central Amundsen Basin to 584 m in the
southern Eurasian Basin. These values agree reasonably well with RMS-roughness
values derived by an empirical model from spreading rates.
The gravity models reveal a 5–6 km thick oceanic crust (density of 2900 kg mx3) in
the central part of the Amundsen Basin, increasing to 9 km towards the Gakkel Ridge.
At the southwestern end of the Eurasian Basin, the oceanic crust is only 2–5 km thick
and thickens towards the Gakkel Ridge. In our model the ridge is composed of a 2 km
thick upper layer with a density of 2600 kg mx3, underlain by an 8 km thick zone with a
density of 2900 kg mx3. The increase of crustal thickness does not confirm theoretical
models for the relation between spreading rate and crustal thickness. The results indicate
that the super-slow spreading rate of the Gakkel Ridge may have caused lateral
variations in the crustal thickness of the Eurasian Basin.
Key words: Arctic Ocean, crustal structure, Gakkel Ridge, gravity anomalies,
mid-ocean ridges.
INTRODUCTION
The Gakkel Ridge, extending over 1800 km through the Arctic
Ocean, is an active mid-ocean ridge and forms the northern
boundary of the Eurasian and American Plate (Fig. 1a). The
Spitzbergen Fracture Zone (SFZ) connects it to the MidAtlantic Ridge system. From a ridge-transform junction at the
northeastern continental margin of Greenland, the ridge crosses
the Eurasian Basin towards the Siberian shelves, where its topography vanishes under a thick sedimentary cover (Sekretov
1998). Aeromagnetic investigations (Karasik 1974; Vogt et al.
1979) provide a well-defined pattern of seafloor spreading magnetic anomalies parallel to and symmetric about the Gakkel
Ridge (Fig. 1b). The oldest identifiable anomaly is Chron 24,
but a broad magnetic negative anomaly along the southwestern
Lomonosov Ridge indicates that the formation of the Eurasian
Basin started about 60 Ma. Spreading models reveal a slow
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spreading rate of 22 mm yrx1 at the initial opening, decreasing
to the super-slow spreading velocity of 6 mm yrx1 full rate for
the Gakkel Ridge between 35 and 25 Ma. The Gakkel Ridge is
currently the slowest spreading segment of the world mid-ocean
ridge system.
Investigations at various ridges worldwide emphasize the
influence of spreading rate on crustal genesis. For example,
rough ridge topography and a pronounced median rift (e.g.
Sclater & Francheteau 1970; Hayes & Kane 1991; Malinverno
1991) seem to be typical characteristics of slow-spreading ridges
(20–40 mm yrx1 full rate). Furthermore, few data indicate that
the oceanic crust thins significantly beneath super-slow ridges
(<20 mm yrx1 full rate, e.g. Bown & White 1994). However,
all empirical models concerning spreading rate and its influence
on crustal structure rely on investigations at ridges with a
full-spreading rate of more than 20 mm yrx1, for example
the Mid-Atlantic Ridge or the fast-spreading (>80 mm yrx1)
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Figure 1. (a) The study area in the Eurasian Basin of the Arctic Ocean with the ships’ track of the ARCTIC’91 expedition (thin line). Along lines 1
and 2 (thick line segments) seismic and gravity data were acquired during the cruise; along lines 3 and 4 (thick line segments) only gravity was
measured. The thick broken lines show seismic refraction observations of the LOREX, and FRAM expeditions: (line a) from Kristoffersen et al.
(1982); (line b) from Jackson et al. (1982); (line c) from Jackson et al. (1984); (line d) from Duckworth & Baggeroer (1985); (line e) from Weber (1979);
and (line f) from Duckworth & Baggeroer (1985). (b) Pattern of the magnetic anomalies and the Chron numbers for the Amundsen and Nansen basins
(modified after Kovacs et al. 1985). The thick black lines show the locations of the seismic lines 1 and 2. Here, the ages of the magnetic anomalies are
used to calculate the standard subsidence curves for oceanic crust of the Gakkel Ridge.
East Pacific Rise. Therefore, the Gakkel Ridge provides an
important opportunity to improve our knowledge of ridge
systematics.
Thermal models of mid-ocean ridges imply a faster cooling
of magma and therefore an increasing viscosity of the melt
below slow-spreading ridges (Sleep & Rosendahl 1979). This
effect would lead to a lower basalt productivity and a thinner
oceanic crust made rugged by faults. Presumably, a change
between volcanic activity and normal faulting of crustal blocks
due to tectonic strain during drifting supports the roughness
of basement surface (Louden et al. 1996). These characteristics
are supposed to be strongly developed along the super-slow
Gakkel Ridge.
Knowledge of the crustal structure in the Eurasian Basin is
sparse due to its permanent ice cover, which limits access for
research vessels. Since 1958, US and British nuclear submarines
have provided information about the bathymetry of the Gakkel
Ridge and the adjacent basins (e.g. Dietz & Shumway 1961;
Johnson & Heezen 1967). The data document a rough seafloor
topography close to the Gakkel Ridge and a deep rift valley.
In the older parts of the Amundsen and Nansen basins the
basement structure is buried under sediments.
Various seismic, magnetic, bathymetric and gravimetric experiments were carried out from drifting ice floes as the ARLIS II
project (Kutschale 1966) or the FRAM I-IV expeditions
(e.g. Duckworth et al. 1982; Jackson et al. 1982; Kristoffersen
et al. 1982). Some of the seismic refraction measurements
(Fig. 1a, line b) indicate the presence of anomalously thin crust
(2–3 km) at the westernmost part of the Eurasian Basin
(Jackson et al. 1982). Other investigations (Fig. 1a, lines a, d
and f) yield a crustal thickness of 6–7 km (Kristoffersen et al.
1982; Duckworth et al. 1982). These results are too sparse in
geographical distribution for any systematic match with global
models to be perceived. This sparsity is mainly due to the
unpredictable course of the drifting ice, and logistic reasons
which restricted the expeditions to the southwestern Eurasian
Basin. A large shooting distance and single-channel records
also restricted the quality of the seismic data. Furthermore, the
available submarine data from those early cruises are too
sparse for any detailed investigation of the ridge’s topography.
This situation dramatically changed in 1993 through the
initiation of the SCICEX programme, within which systematic
submarine surveys across the ridge were conducted. In the 1998
and 1999 seasons a side-scan sonar system was added to provide a 3-D image of the ridge (Kurras et al. 1999). The studies
on-board the US Navy nuclear submarine USS POGY along
the Gakkel Ridge show a rough ridge morphology with axial
depths of 4600–5100 m. Analyses of the gravity data indicate a
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2001 RAS, GJI 145, 505–516
Oceanic crust in the Eurasian Basin
crustal thickness in the rift valley of less than 4 km (Coakley &
Cochran 1998), which is supposed to be typical of super-slow
spreading ridges (Bown & White 1994). However, the profiles extend only 40–50 km from the ridge axis, and therefore
no information on the basement variations in the adjacent
Amundsen and Nansen basins is available to be tied with the
seismic refraction data acquired during the FRAM drift island
expeditions.
Information about off-axis parameters such as basement
topography, sedimentary and crustal thickness were acquired
during the ARCTIC’91 expedition (Fütterer 1992). Two research
vessels, Polarstern and Oden, crossed the entire Eurasian Basin
in a convoy, from the Barents Shelf across the Gakkel Ridge
to the Lomonosov Ridge and back to Svalbard (Fig. 1a).
Geological sampling, multichannel seismic reflection experiments, wide-angle and gravity recordings were carried out
on-board Polarstern. The combination of seismic reflection and
wide-angle data provides the first reliable information on the
thickness of the entire sedimentary cover of the southwestern
Eurasian Basin (Fig. 2; line 1 in Fig. 1a) (Jokat et al. 1995b)
and central Amundsen Basin (Fig. 3; line 2 in Fig. 1a) (Jokat
et al. 1995a). These data were used to map the topography of
the basement surface and thus to establish a roughness study
for the Gakkel Ridge. Furthermore, knowledge about the sedimentary thickness and seismic velocities provides important
constraints on the crustal gravity models.
In this contribution, the results of the gravity modelling
and the roughness calculations for the oceanic basement will be
presented along several transects in the Eurasian Basin. The
results are compared with existing models for crustal thickness
versus spreading rate.
507
ACQUISITION OF SEISMIC AND
GRAVITY DATA
In total, 1500 km of multichannel seismic reflection data were
acquired during the ARCTIC’91 expedition across the Eurasian
Basin. For the seismic investigations a cluster of eight airguns
(each 3 l in volume) and a 300 m long streamer (12 channels) were
used. In addition, wide-angle data were gathered using sonobuoys. The maximum signal range recorded by the buoys was
15 km. This set-up allowed us to image the whole sedimentary
cover down to the basement with reasonable quality (Jokat
et al. 1992). The resulting depth sections (Figs 2 and 3) allowed
us to calculate the RMS-roughness of the basement surface and
to compare the results with values for other ridges (Fig. 4).
The gravity data recorded along the whole cruise track were
acquired with a KSS31 gravimeter (Bodenseewerke) every 10 s.
With an average ship speed of 5 knots, the sample distance is
25 m. The ship’s navigation system supplied GPS data for time,
course, position and ship’s speed over the ground to calculate
the free-air gravity (Jokat 1992). The gravity data (Figs 5 to 8)
show high-frequency spikes resulting from the irregular movements of the ship during ice breaking. The noise along these
transects from ice breaking is higher, since Polarstern operated
without support from Oden. Nevertheless the records reveal very
clearly the much longer wavelength anomalies of the important
structures such as the ridge and basins. No de-spiking or
further filtering was applied to the data: the records are shown
here in their original quality. Low-pass filtering would result in
a negative offset of the data because of the mainly negative
acceleration during ice breaking. De-spiking has poor success
because of the high number of spikes.
Figure 2. Depth transect along line 1 (Fig. 1a) from seismic reflection and refraction measurements. Black arrows mark the position and Chron
number of the magnetic anomalies crossed (Fig. 1b). The references to determine the basement surface differences are marked by thick black lines.
Here the subsidence depth of oceanic crust defined by Parson & Sclater (1977) was taken. The upper curve shows the residual roughness. The
RMS-roughness values R for the basement surface are noted for the Amundsen and Nansen basins.
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2001 RAS, GJI 145, 505–516
E. Weigelt and W. Jokat
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Figure 3. Depth transect along line 2 (Fig. 1a) from seismic reflection and refraction measurements. Black arrows mark the Chron number
and position of the magnetic anomalies crossed (Fig. 1b). The references to determine the basement surface differences are marked by thick
black lines. From Chron 21/20 to Chron 6 the regional basement trend is the subsidence depth of oceanic crust defined by Parson & Sclater (1977);
from Chron 25–21/20 it is a linear function. The upper curve shows the residual roughness. The RMS-roughness values R of the basement surface are
noted for the two segments.
For this study four profiles acquired during the expedition
were chosen (Fig. 1): a transect through the southwestern
Eurasian Basin from the Morris Jesup Rise to the Yermak
Plateau (line 1), a transect through the central Amundsen Basin
from the rim of the Lomonosov Ridge towards the Gakkel
Ridge (line 2), a transect almost parallel to the ridge axis along
the Amundsen Basin (line 3), and a profile from the edge of the
Barents Shelf through the Nansen Basin towards the Gakkel
Ridge (line 4). Along lines 3 and 4 only gravity and bathymetric
data are available.
RMS-Roughness [m]
800
700
Gakkel Ridge, this study
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Malinverno, 1991
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TOPOGRAPHIC ROUGHNESS OF THE
OCEANIC BASEMENT
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Full Spreading Rate [mm/y]
Figure 4. The relation between full-spreading rate and the RMSroughness of basement surface established by Malinverno (1991) (black
line). The black circles show the database of Malinverno’s empirical
model with average values for ridges with the same spreading rate. The
stars mark the included values for the RMS-roughness at the Gakkel
Ridge resulting from this study. The grey shaded area marks the range
of the full-spreading rate for the Gakkel Ridge.
The seismic reflection profiles across the Eurasian Basin show a
rough basement surface with differences in elevation of up to
1000 m. Analyses of four sonobuoys in the southwestern part
(Fig. 1a; line 1) and 10 buoys in the central Amundsen Basin
(Fig. 1a; line 2) provide seismic velocities ranging from 1.6 to
4.5 km sx1 for the sedimentary layers. No seismic velocities
for the upper oceanic crustal layer were identified in the data
(Jokat et al. 1995a). Whereas in the southwestern part of the
basin the oceanic crust is mostly exposed at the seafloor (Fig. 2),
the basement in the central Amundsen Basin is buried under a
thick sedimentary cover of 3200 m at maximum (Fig. 3).
For calculating the topographic roughness we determined
the RMS deviation of the basement surface from a regional
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2001 RAS, GJI 145, 505–516
Oceanic crust in the Eurasian Basin
85.10˚N
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—— Observed Freeair Gravity Anomaly
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Figure 5. Crustal model from free-air gravity data for line 1 (Fig. 1a). For calculating the density and depth values, seismic velocities for the
sediments along the transect were taken from Jokat et al. (1995b). The densities are annotated in kg mx3. The stars show thickness values based on
observations by Kristoffersen et al. (1982) for the Gakkel Ridges centre (reference a: FRAM-I); from Jackson et al. (1982) for the Amundsen Basin
(reference b: FRAM-I); and from Jackson et al. (1984) for the Nansen Basin towards the flank of the Yermak Plateau (reference c).
depth trend (Malinverno 1991).
R ¼ ½&ðhðiÞ 2 Þ=n1=2 for5 i ¼ 1, n; R ¼ roughness ½m;
a number of ridge systems with full-spreading rates between
14 (Southwest Indian Ridge) and 153 mm yrx1 (East Pacific
Rise).
h ¼ depth deviations ½m :
For a regional trend we take the theoretical subsidence depth
of oceanic crust given by Parson & Sclater (1977). They defined
the relation between age and depth of oceanic crust as
dðtÞ ¼ 2500 þ 350t1=2 for5 d ½m; t ½Ma :
For this study the age of the crust is derived from magnetic
anomalies as reported by Vogt et al. (1979) for the Amundsen
and Nansen basins.
Along the transects, the roughness values were calculated
with a spacing of 1 km. The observed roughness values are
compared with calculated values derived from spreading rates
as given by Malinverno (1991):
R ¼ 1296l0:539 for5 R ¼ roughness ½m,
l ¼ spreading rate ½mm yr1 :
Malinverno’s empirically determined relationship between
roughness and spreading rate is based on 101 profiles across
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2001 RAS, GJI 145, 505–516
Line 1: Southwestern Eurasian Basin
The depth section through the southwestern Eurasian Basin
(Fig. 2) shows strong basement elevations scarcely covered by
sediments. Sediment pockets only a few hundred metres thick
exist between the topographic highs.
The topographic differences for the Amundsen Basin are
determined along a 246 km long profile perpendicular to the
axis of the Gakkel Ridge and extending from the northern ridge
shoulders to Chron 13 (34 Ma) (Figs 1b and 2). The resulting
RMS-roughness value for the basement topography amounts
to 584 m.
The RMS-roughness value for the Nansen Basin is determined for a 201 km long profile from the southern ridge
shoulders to the corresponding southern magnetic anomaly
Chron 13 (34 Ma) and amounts to 545 m (Figs 1b and 2).
Spreading rate models based on magnetic anomaly interpretation imply a total opening rate of 6–12 mm yrx1 (34 Ma
to recent) for the Amundsen Basin, and 6–14 mm yrx1
E. Weigelt and W. Jokat
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Free-air Gravity [mGal]
89.76˚N
-48.72˚W
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9.40˚E
------ Calculated Freeair Gravity Anomaly
—— Observed Freeair Gravity Anomaly
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Figure 6. Crustal model from free-air gravity data for line 2 (Fig. 1a). For calculating the density and depth values, seismic velocities for the
sediments along the transect were taken from Jokat et al. (1995a,b). The stars show thickness values based on observations of the LOREX expedition
after Weber (1979) for the northern Amundsen Basin (reference e), and from the FRAM-II expedition after Duckworth & Baggeroer (1985) for the
Amundsen Basin (reference f).
(37 Ma to recent) for the Nansen Basin (Karasik 1974; Vogt
et al. 1979). Applying the roughness–spreading rate relation
of Malinverno (1991), these opening rates would result in
roughness values from 493 (corresponding to 6 mm yrx1) to
312 m (corresponding to 14 mm yrx1).
Line 2:
Central Amundsen Basin
The depth section across the central part of the Amundsen
Basin runs obliquely to the Gakkel Ridge axis from the North
Pole towards the Ridge (Fig. 1a). It reveals a similar rough
basement surface (Fig. 3) to that observed on line 1. Here, the
basement is covered by sediments with a maximum thickness
of 3200 m. A pronounced jump in basement depth of about
1000 m occurs between Chrons 20 and 21 (km 470, Fig. 3).
At this point, a significant change in the regional trend can
be observed. This made it sensible to divide line 2 into two
segments for RMS-roughness calculations (Fig. 3).
The observed RMS-roughness for crust accreted between
Chron 20 (46 Ma) and Chron 6 (20 Ma) amounts to 450 m. In
this time range the full-spreading rates decrease from 13 to
6 mm yrx1. The corresponding calculated roughness values
(Malinverno 1991) are 325 and 493 m.
For oceanic crust older than Chron 21 the general basement level is lowered abruptly (Fig. 3, km 450). This cannot be
explained by sediment-corrected standard subsidence curves.
Therefore a statistical approach with a linear regression line
(Malinverno 1991; Minshull 1999) was used to define the
reference level for the crust created between Chron 24 (53 Ma)
and Chron 20 (44 Ma). The resulting RMS-roughness value
here is 540 m. The full-spreading rate in this area was between
13 and 19 mm yrx1 (Vogt et al. 1979) and therefore the calculated roughness values after Malinverno (1991) range from 325
to 265 m.
Along both transects (Fig. 1; lines 1 and 2) the observed
RMS-roughness for the crustal topography reveals high values,
corresponding to the hypothesis of increasing roughness with
decreasing spreading rate. To emphasize this, the new roughness values were plotted on a spreading rate versus roughness
diagram (Malinverno 1991) (Fig. 4). Although the observations
for the empirical equation are derived from ridges with a
full-spreading rate faster than 14 mm yrx1, the theoretical
curve predicts the roughness in our area of investigations. It
should be noted that the full-spreading rates of the Gakkel
Ridge range from 6 to 22 mm yrx1 (Fig. 4), and therefore the
theoretical roughness values can range from 493 to 265 m.
GRAVITY MODELLING OF THE
CRUSTAL THICKNESS
Gravity and bathymetry data were recorded along the whole
cruise track of the ARCTIC’91 expedition (Fig. 1a). In general,
the Nansen and Amundsen basins are characterized by a x20
to x40 mgal free-air anomaly (Figs 5 to 8). Gravity increases to
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2001 RAS, GJI 145, 505–516
Oceanic crust in the Eurasian Basin
100
100
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9.40˚E
------ Calculated Freeair Gravity Anomaly
—— Observed Freeair Gravity Anomaly
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50
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Line2
Amundsen Basin
Free-air Gravity [mgal]
89.76˚N
-48.72˚W
Free-air Gravity [mGal]
511
Gakkel Ridge -----> ~ 100 km
0
0
1030 water
5
1900-2500 sediments
2900 oceanic crust
10
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Depth [km]
Depth [km]
5
3300 mantle
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25
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500
Distance [km]
Figure 7. Crustal model from free-air gravity data for line 3 (Fig. 1a). The sedimentary thickness and density results are from our own seismic
observations. The stars show the reference values for the thickness of the oceanic crust after this study (lines 1 and 2) and after Duckworth &
Baggeroer (1985) for the Amundsen Basin (reference f).
40–50 mgal at the shoulders of the Gakkel Ridge and decreases
to x50 mgal in the central valley (Fig. 5). 2-D crustal density
models were constructed from these data by applying the LCT
software (# 1987–1992 by LCT, Inc., Houston). The basis for
the 2-D modelling is the algorithm of Rasmussen & Pedersen
(1979).
Model parameters
The thickness of the sediment layers and the topography of the
basement surface result from the depth sections interpreted
from the multichannel seismic reflection profiles (Jokat et al.
1995a,b). Densities for the sediment layers were calculated
according to the density–velocity relation of Nafe & Drake
(1957) by applying the seismic velocities from the sonobuoy
recordings. In our model the density for the sediments varies
between 1900 and 2400 kg mx3. For the oceanic crust we mostly
used a density of 2900 kg mx3 according to similar studies
along mid-ocean ridges (Miller & Christensen 1995). In our
model the mantle has a density of 3300 kg mx3 (Christensen
1972). A low-density zone of 3200 kg mx3 for the upper mantle
is introduced below the central part of the ridge to model the
strong negative gravity anomaly. Additional constraints on
the thickness and structure of the oceanic crust were derived
from results of seismic refraction investigations during the
LOREX and FRAM expeditions (Weber 1979; Jackson et al.
1982; Kristoffersen et al. 1982), although their soundings were
not along our transects.
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2001 RAS, GJI 145, 505–516
Gravity model for the southwestern Eurasian Basin
(line 1)
Fig. 5 shows the free-air gravity profile used for this study. The
solid black line is the calculated gravity of the crustal model
below.
The free-air gravity anomaly increases from x30 mgal in the
Amundsen and Nansen basins to + 40–50 mgal on the flanks
of the Gakkel Ridge. In the central part of the Gakkel Ridge it
decreases to x50 mGal.
The thin sediments were modelled with densities of 1900–
2200 kg mx3. As indicated by the seismic reflection profiles
(Jokat et al. 1995b), only a little sediment is present in basement
pockets in the central part of the transect. Therefore, the
seafloor surface represents the basement surface in this part of
the model.
The wide-angle data from the ARCTIC’91 cruise did not
provide any information on the composition and thickness of
the oceanic crust. Therefore we included results of previous
seismic refraction experiments about 50–150 km away from
line 1 (Jackson et al. 1982, 1984; Kristoffersen et al. 1982). For
the location of the profiles see Fig. 1(a). Based on results from
Kristoffersen et al. (1982) the oceanic crust close to the Gakkel
Ridge was divided into a 2.5 km thick upper part with a density
of 2600 kg mx3 and a lower part down to the Moho at 8 km
depth with a density of 2900 kg mx3.
In a first modelling step, only the crustal thickness was varied
until the differences between measured and calculated free-air
E. Weigelt and W. Jokat
100
100
Free-air Gravity [mGal]
86.24˚N
59.24˚E
83.50˚N
29.64˚E
------ Calculated Freeair Gravity Anomaly
—— Observed Freeair Gravity Anomaly
50
Free-air Gravity [mgal]
512
50
0
0
-50
-50
-100
-100
Line 4
60 km <----- Gakkel Ridge
Nansen Basin
0
0
1030 water
5
2300 sediments
2900 oceanic crust
d
10
*******
10
15
15
Depth [km]
Depth [km]
5
3300 mantle
20
20
25
25
500
450
400
350
300
250
200
150
100
50
0
Distance [km]
Figure 8. Crustal model from free-air gravity data for line 4 (Fig. 1a). Values for the sedimentary thickness and density were chosen after
Kristoffersen & Husebye 1984). Studies from Duckworth & Baggeroer (1985) of the FRAM-IV expedition also give parameters for the sedimentary
cover, as well as for the crustal depth, marked as stars in the Nansen Basin (reference d).
gravity did not exceed 5 mgal. The model (Fig. 5) shows in
general an increase of crustal thickness towards the Gakkel
Ridge that is closely correlated with the shallowing of the
basement.
The model confirms in general the crustal thickness determined from the seismic studies mentioned above. The oceanic
crust in the central part of the Amundsen and Nansen basins
actually appears to be only 2–3 km thick (km 140–220 and
km 480–550, Fig. 5). Towards the ridge a smooth increase
of the crustal thickness from 3 km to 6 km from the basins
towards the ridge’s shoulders has been modelled (km 270 and
340, Fig. 5).
A low-density zone beneath the ridge is introduced to avoid
a difference of 40 mgal between observed and modelled freeair gravity across the rift valley. Here, a density contrast of
100 kg mx3 between partial melt and surrounding mantle
seems to be reasonable, as described by the studies of Talwani
et al. (1965). Otherwise, in the case of a constant density
distribution in the mantle the modelling would demand either
an increase of crustal thickness up to 10 km below the ridge’s
flanks or a decrease of density down to 2500 kg mx3 for the
complete crust off to 50 km from the ridge centre. Both
possibilities would differ significantly from other investigations
in the Eurasian Basin (e.g. Kristoffersen et al. 1982; Coakley &
Cochran 1998) and even from known structures of mid-ocean
ridges in general (e.g. Bown & White 1994).
Gravity model for the central Amundsen Basin (line 2)
The free-air gravity anomaly increases from x30 to x40 mgal
in the central Amundsen Basin to more than 10 mgal towards
the Gakkel Ridge (Fig. 6). While the seafloor topography is
almost flat along the whole transect, the free-air gravity shows
some long-wavelength variations. Most of these can easily be
explained by the basement topography as revealed by the
seismic data along this line (Fig. 3; Jokat et al. 1995a). The
general trend in gravity, however, is caused by variations in
crustal thickness.
The only seismic control on the Moho depth of the
Amundsen Basin is that available from the LOREX Expedition
close to the Lomonosov Ridge (Weber 1979; Mair & Forsyth
1982). A depth of 12 km for the Moho about 80 km away from
line 2 (Fig. 1a) was calculated from these seismic refraction
data. In our gravity model we incorporated this information at
the start of line 2, where the line is closest to the Lomonosov
Ridge (reference e, Fig. 6).
The final crustal model for the whole transect reveals a crust
of 5–6 km thickness for the northern part of the Amundsen
Basin (km 0–270, Fig. 6). While the general basement level
shallows to the south the crust thickens up to 9 km towards the
Gakkel Ridge (from km 400 southwards). This seems to be
quite thick for oceanic crust, but a similar thickness for the
oceanic crust close to our area of investigation was calculated
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2001 RAS, GJI 145, 505–516
Oceanic crust in the Eurasian Basin
from the seismic refraction data of the FRAM II expedition by
Duckworth & Baggeroer (1985) (reference f, Fig. 1). For the
Amundsen Basin the authors suggested a 7 km thick oceanic
crust about 280 km away from the centre of the Gakkel Ridge
formed between Chron 20 and Chron 21.
Even if we reduce the density of the oceanic crust from
2900 to 2800 kg mx3, a minimum depth of 8 km for the crust
is still calculated. Taking into account the fact that the two
measurements are almost 280 km apart, the results coincide
well. All combinations of crustal thickness and reasonable
crustal densities tested in this study result in a relative increase
of 3 km for crustal thickness towards the centre of the Gakkel
Ridge, correlating with the general rise of the basement
topography.
Gravity model for the Amundsen Basin parallel to the
Gakkel Ridge (line 3)
Line 3 almost parallels Chron 13 towards the Greenland
continental margin (Fig. 1a). At the end of the transect the
northernmost tip of the Morris Jesup Rise was reached. While
the gravity field along the southwestern Amundsen Basin varies
only from x10 to x20 mgal, the Morris Jesup Rise has a
pronounced positive anomaly of 40 mgal at maximum. The
northern edge of the plateau is marked by a large negative
anomaly of x40 mgal (Fig. 7).
The sediment layer was modelled with a density of
2200 kg mx3, but has no large influence on the final results
as the seismic data indicate only a 100–200 m thick cover of
deposits (Weigelt 1998). Again the results of the FRAM II
seismic refraction experiment (Duckworth & Baggeroer 1985),
situated 100–120 km away from line 3, were incorporated into
the starting model (reference f, Fig. 7). At the start of the
transect the oceanic crust is almost 10 km thick (Fig. 7). Its
thickness decreases to 6 km in the centre of line 3. The Morris
Jesup Rise has a 17 km thick crust, if we apply a uniform density
of 2900 kg mx3 because of the oceanic origin of the plateau
(Feden et al. 1979; Vogt et al. 1979; Dawes 1990) (Fig. 7).
Gravity model for the central Nansen Basin (line 4)
Line 4 represents the easternmost information gathered in
the Nansen Basin during the ARCTIC’91 expedition (Figs 1a
and 8). The gravity data show higher noise levels as Polarstern
operated without a support vessel. The strong disturbances of
the gravity data are caused by heavy ice breaking. Along the
profile the free-air anomaly has a mean value of x20 mgal.
Long-wavelength variations of 20 mgal are observed in the
abyssal plain. Here, no control of the sedimentary thickness
and basement topography is available from our seismic data
(Weigelt 1998). For modelling, a sedimentary cover with a minimum thickness of 1.5 km deduced from previous experiments
(Kristoffersen & Husebye 1984) was included. Furthermore,
we assumed that the deposits thin towards the Gakkel Ridge,
as observed in the Amundsen Basin (Fig. 3, line 2). Between
km 220 and 350 we assumed a crustal thickness of 4–5 km with
the Moho at 10 km depth, according to the results of the
FRAM IV experiment (Duckworth & Baggeroer 1985) about
175 km southwest of line 4 (reference d, Figs 1a and 8).
For the final crustal model (Fig. 8), no major changes in
crustal thickness are required for the central part of the Nansen
Basin to fit the observed data. The wavelength of about
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2001 RAS, GJI 145, 505–516
513
20 mgal can be modelled by variations of the basement topography of several hundreds of metres only. So, the basement
has a rough surface similar to that imaged by seismic studies
for the central Amundsen Basin (Fig. 6) and the southwestern
Eurasian Basin (Fig. 5). Towards the Gakkel Ridge the
crustal thickness increases from 4–5 km to more than 8 km
(km 450–550, Fig. 8). This corresponds to an ascending basement level of more than 1 km. The model shows that the
characteristics of the oceanic crust in the Nansen Basin are
similar to those of the central Amundsen Basin (Fig. 3, line 2).
Anonymous Soviet studies suppose a 4–5 km thick sedimentary cover for the Nansen Basin (Jackson & Oakey 1990).
If the sediment thickness were included in our gravity model,
the oceanic crust would become extremely thin (less than 2 km).
Such a result no longer satisfies the seismic refraction results
of Duckworth & Baggeroer (1985), which are the only and
closest-situated constraints for modelling.
DISCUSSION
The combination of the seismic and gravity observations in this
study indicates that the crustal structure of the Eurasian Basin
is influenced by the extremely slow spreading rate of the Gakkel
Ridge as expected. Variations in the basement topography of
more than 1000 m and the presence of 2–3 km thin crust are
supposed to be typical of super-slow spreading ridges (e.g. Reid
& Jackson 1981; Malinverno 1991; Coakley & Cochran 1998).
The observed roughness along the transects in the Eurasian
Basin reveal values of 450–584 m, which are significantly
higher than the 100–300 m roughness at faster spreading ridges
with a total opening rate of more than 20 mm yrx1. The new
roughness data calculated for the Gakkel Ridge fit predictions
based on the empirical equation for roughness versus spreading velocity introduced by Malinverno (1991). However, care
is necessary because the empirical relation is exponential, and
therefore a broad range of roughness values for the range of
super-slow spreading rates is predicted. Some control can be
exerted by comparing the spreading rates calculated from the
roughness values with those defined from magnetic anomalies
(Karasik 1974; Vogt et al. 1979).
Whereas the results of the roughness study are consistent
with published models, the gravity modelling on crustal thickness
in the Eurasian Basin reveals surprising results.
Our gravity models indicate a crustal thickness of almost
4 km in the rift valley (Fig. 2), confirming the results of
Coakley & Cochran (1998). These authors suggested an oceanic
crust less than 4 km thick if the crustal density is lower than
2900 kg mx3 along the rift valley. However, their profiles,
orientated perpendicular to the axis, are only 80–90 km long
and cannot provide any information about the crustal fabric of
the adjacent basins.
Theoretical off-axis crustal thicknesses calculated for a
full-spreading rate of 6 mm yrx1 range from 2.5 km (Reid &
Jackson 1981) to 4.5 km (Bown & White 1994) (Fig. 9). The
crustal thickness of 2–3 km modelled from our data for the
centre of the southwestern Nansen and Amundsen basins (line 1)
satisfies the predictions.
Towards the Gakkel Ridge, however, the crustal thickness
increases to 6 km (line 1). Although the spreading rate has
increased to 13 mm yrx1 during the last 10 Ma, the theoretical
models predict a crustal thickness of only 3.5–5.5 km. A
514
E. Weigelt and W. Jokat
12
Crustal Thickness [km]
10
8
6
x
4
2
Su et al., 1994
Bown & White, 1994
Reid & Jackson, 1981
0
0
20
40
60
80
100
120
140
160
Full Spreading Rate [mm/y]
Figure 9. Various empirical models of the relationship between spreading rate and crustal thickness: Bown & White (1994) (dotted), Reid & Jackson
(1991) (dashed), Su et al. (1994) (bold line). The grey circles show the thickness values used for these studies, which are from ridges with spreading rates
mostly faster than 20 mm yrx1. The stars mark the thickness values observed in this study for the Gakkel Ridge, and the triangles are thickness values
for the Gakkel Ridge observed by Jackson et al. (1982), Kristoffersen et al. (1982), Duckworth & Baggeroer (1985) and Duckworth et al. (1982). The
cross shows the crustal thickness at the Southwest Indian Ridge (Muller et al. 1995).
correlation between the crustal thickness and shallowing basement level towards the ridge axis was also observed in gravity
and seismic data acquired during the FRAM I experiment by
Jackson et al. (1982). They interpreted this effect to be a result
of a hot spot (Yermak Hot Spot), which caused the separation
of the Morris Jesup Rise and Yermak Plateau.
The crustal thickness of 5–9 km modelled in this study for
the central Amundsen Basin (line 2) is much more than that
predicted by theoretical models from the spreading rate. The
crustal thickness of 7 km modelled along the southwestern
Amundsen Basin (line 3) is in good agreement with the average
for oceanic crust formed at ridges with a full-spreading rate of
more than 20 mm yrx1.
For the central Nansen Basin (line 4) the gravity models give
crustal thickness values of 4–5 km, increasing to 8 km towards
the flanks of the Gakkel Ridge, which is thicker than oceanic
crust observed at other ridges (e.g. Bown & White 1994).
The increase of crustal thickness towards the Gakkel Ridge
modelled in this study is closely correlated with the rise of
basement level. Even if in some parts the crustal thickening
is not directly supported by seismic refraction, an increase of
3 km in crustal thickness is necessary to satisfy the recorded
free-air gravity.
Our observations conflict with those theoretical models
of oceanic crustal formation that predict a thinner crust for
a slow (<20 mm yrx1) spreading rate (e.g. Reid & Jackson
1981; Bown & White 1994). These models are based on the
assumption that a thinner crust is the result of lower magma
productivity, due to greater conductive cooling at slow-spreading
ridges. Moreover, these models imply steady-state conditions
in seafloor spreading for extremely low rates (<12 mm yrx1)
(Coakley & Cochran 1998).
In contrast, our gravity models show variations in crustal
thickness from 3 to 9 km, which implies changing thermal and
pressure conditions resulting in variable magma production
below the Gakkel Ridge.
One reason for such variable production, suggested by
Louden et al. (1996), may be a pulsed supply of melt, which
does not keep pace with the rate of plate separation. So the
crust could be tectonically thinned and faulted after its accretion.
This would also explain the wider median valley and rougher
basement surface observed at slow-spreading ridges. Mutter
& Karson (1992) also proposed a greater influence of mechanical deformation on crustal fabric for slow-spreading ridges.
Further aspects could be mantle temperature and melt distribution additionally controlling lithospheric accretion (Small &
Sandwell 1992).
Earlier seismic refraction investigations indicated lateral
variations in crustal structure in the southwestern Eurasian
Basin (summarized in Kristoffersen 1990), which are now
confirmed by the ARCTIC’91 expedition for the central part
of the basin. The results imply that the models for crustal
thickness versus spreading rate cannot be transferred simply to
the super-slow-spreading Gakkel Ridge. The currently poor
geophysical and petrological database of the Eurasian Basin
does not permit us to decide which parameters are responsible
for the variations in crustal thickness. At present it is not
known if the described observations are random or systematic
for the Gakkel Ridge. The variations in crustal thickness
derived in this study from the gravity data should therefore be
treated as observations rather than as proven facts.
CONCLUSIONS
The crustal structure and the derived models of the relationship
to spreading rate are based mostly on observations at ridges
with a full-spreading rate greater than 20 mm yrx1. Therefore
the investigations of the ARCTIC’91 expedition at the extremely
slow-spreading Gakkel Ridge enlarge the spectrum of data
to include slow spreading rates, where observations are still
rare. The roughness values presented in this study are the first
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2001 RAS, GJI 145, 505–516
Oceanic crust in the Eurasian Basin
investigations of this kind for the Gakkel Ridge, and fit the
predictions of global models. In contrast to this, the density
models indicate surprising new results about crustal thickness.
(1) A thin crust of almost 4 km was modelled for the rift
valley along line 1, as expected for a super-slow spreading axis.
This is in good agreement with other studies.
(2) The gravity models suggest an increase of crustal
thickness, if we approach the Gakkel Ridge from the Nansen
and Amundsen basins, although the spreading rates remain
almost constantly slow. This is in strong contrast to existing
models for oceanic crust. The current database does not allow
us to introduce a consistent model to explain our observations.
(3) Our data confirm the presence of locally thin oceanic
crust in the Amundsen and Nansen basins. However, this may
be accompanied by density variations of the oceanic crust,
which cannot be tested with the existing data set.
The presented gravity models clearly indicate a lack of
knowledge about the off-axis parameters of the super-slow
Gakkel Ridge. While there is little doubt about the presence of
thin crust along the rift valley of the Gakkel Ridge, no strong
constraints exist on crustal thickness in the adjacent basins at
all. Currently it is not clear whether theoretical thickness versus
spreading-rate models are valid for the super-slow Gakkel
Ridge, or whether the sparse geophysical and petrological data
still provide a biased picture of the crustal fabric of the Gakkel
Ridge. The gravity models have to be confirmed by seismic
refraction data accompanied by petrological sampling, which
hopefully will be acquired in the future.
ACKNOWLEDGMENTS
We are grateful for the excellent support of the captains
and crews of the vessels Oden and Polarstern. Only the joint
forces of the two icebreakers and the close co-operation during
the measurements made these results possible. This is Alfred
Wegener Institute contribution No. n10020.
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