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Geological Society, London, Special Publications
Present and past influence of the Iceland Plume on
sedimentation
Stephen M. Jones, Nicky White, Benjamin J. Clarke, Eleanor Rowley and Kerry Gallagher
Geological Society, London, Special Publications 2002; v. 196; p. 13-25
doi:10.1144/GSL.SP.2002.196.01.02
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Present and past influence of the Iceland Plume on sedimentation
S T E P H E N M. JONES 1, N I C K Y W H I T E 1, B E N J A M I N J. C L A R K E 1'2,
E L E A N O R R O W L E Y I ' 3 & K E R R Y G A L L A G H E R 1'4
tBullard Laboratories, Madingley Rise, Madingley Road, Cambridge CB3 0EZ, UK
(e-mail: jones @esc.cam, ac. uk)
2present address: Department of Geology and Geophysics, University of Edinburgh,
King's Building, West Mains Road, Edinburgh EH9 3JW, UK
3present address: Shell Eygpt, 6 Hassan El-Sherley Street, PO Box 2681 El Horreya,
Heliopolis, Cairo, Eygpt
4TH Huxley School of Environment, Earth Science, and Engineering,
Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AS, UK
Abstract: The Cenozoic development of the North Atlantic province has been dramatically
influenced by the behaviour of the Iceland Plume, whose striking dominance is manifest by
long-wavelength free-air gravity anomalies and by oceanic bathymetric anomalies. Here, we
use these anomalies to estimate the amplitude and wavelength of present-day dynamic uplift
associated with this plume. Maximum dynamic support in the North Atlantic is 1.5-2 km at
Iceland itself. Most of Greenland is currently experiencing dynamic support of 0.5-1 km,
whereas the NW European shelf is generally supported by < 0.5 kin. The proto-Iceland
Plume had an equally dramatic effect on the Early Cenozoic palaeogeography of the North
Atlantic margins, as we illustrate with a study of phime-related uplift, denudation and
sedimentation on the continental shelf encompassing Britain and Ireland. We infer that
during Paleocene time a hot subvertical sheet of asthenosphere welled up beneath an axis
running from the Faroes through the Irish Sea towards Lundy, generating a welt of magmatic
underplating of the crust which is known to exist beneath this axis. Transient and permanent
uplift associated with this magmatic injection caused regional denudation, and consequently
large amounts of clastic sediment have been shed into surrounding basins during Cenozoic
time. Mass balance calculations indicate agreement between the volume of denuded material
and the volume of Cenozoic sediments deposited offshore in the northern North Sea Basin
and the Rockall Trough. The volume of material denuded from Britain and Ireland is
probably insufficient to account for the sediment in the Faroe-Shetland Basin and an excess
of sediment has been supplied to the Porcupine Basin. We emphasize the value of combining
observations from both oceanic and continental realms to elucidate the evolution of the
Iceland Plume through space and time.
The free-air gravity field over the northern
hemisphere is dominated by a long-wavelength
high centred on Iceland, stretching from the
Azores to Siberia and from Baffin Island to
Denmark (Fig. 1). Both theoretical considerations and observations from the world's oceans
suggest that long-wavelength ( > 1000km) positive anomalies are generally associated with
mantle upwelling and dynamic uplift (Sclater
et al. 1975; McKenzie 1994). If these inferences
also hold in the North Atlantic province, the areal
extent of the gravity high suggests that the
continental margins of NW Europe and eastern
Greenland are dynamically supported at present.
In the first part of this paper, we compare
estimates of dynamic support derived from the
bathymetry and gravity field of the North
Atlantic oceanic realm to constrain the present
magnitude of dynamic support of both oceanic
crust and the adjacent continental margins.
Next, we consider the region that was affected
by the plume in the past. Temporal and spatial
variation of plume-related uplift has played an
important role in the evolution of both margins,
especially in controlling the generation and
distribution of clastic sediments. Here, we
consider the specific example of sediment mass
balance around Britain and Ireland. Geochemical
evidence has proved that this region experienced
magmatic underplating of the crust during
From: DOR~, A.G., CARTWRIGHT,J.A., STOKER,M.S., TURNER,J.P. & WHITE,N. 2002. Exhumationof the
North AtlanticMargin: Timing,Mechanismsand Implicationsfor PetroleumExploration. Geological Society,
London, Special Publications, 196, 13-25. 0305-8719/02/$15.00 © The Geological Society of London 2002.
14
S.M. JONES E T A L .
mGal
-300-60-50-40-30-20-10
0
10 20 30 40
50 60 400
Fig. 1. Free-air gravity anomaly over part of the northern hemisphere, displayed using a Lambert azimuthal equal
area projection centred on Iceland. The long-wavelength gravity high centred on Iceland, which extends from
south of the Azores to Spitzbergen and from Baffin Island to Scandinavia, should be noted.
Paleocene time, leading to permanent uplift,
which drove denudation (Thompson 1974;
Brodie & White 1995). Mass balance provides
a means of comparing and verifying complementary measures of Early Cenozoic denudation, estimated from the onshore record of
denudation and the offshore record of sediment
accumulation.
Two forms of surface uplift are associated with
mantle plumes. Dynamic support always results
when abnormally hot mantle is emplaced
beneath the lithosphere. This support is transient
and disappears when the thermal anomalies in
the asthenosphere and lithosphere dissipate by
convection and conduction. The North Atlantic
region is dynamically supported today and it
must also have been dynamically supported
during Early Cenozoic time, as the volcanic
record attests to abnormally hot mantle at that
time. Permanent uplift may also occur if mantle
thermal anomalies induce melting and the melt is
injected into or just beneath the crust. Permanent
uplift affected Britain and Ireland during
Paleocene time, and the regions flanking the
line of continental separation at the PaleoceneEocene boundary. Comparison of the two themes
we present in this paper therefore demonstrates
the relative spatial extent and magnitude of
transient and permanent uplift. The term
epeirogenic uplift refers to uplift that could be
permanent and/or transient. In general, epeirogenic uplift refers to uplift that is not generated
by horizontal plate motions, and the term need
not imply any particular mechanism. However,
the present and past epeirogenic uplift of the
North Atlantic region we discuss in this study is
generated by the Iceland Plume.
Present-day dynamic support
The North Atlantic Ocean is anomalously
shallow in the vicinity of Iceland. Anomalous
topography culminates at Iceland itself, which
rises to c. 2 k m above sea-level, or c. 4.5km
above the average depth of the global mid-ocean
ridge system. Two methods can be employed to
ICELAND PLUME PAST AND PRESENT
investigate dynamic support of the region of
oceanic crust round Iceland. The first method
exploits the fact that the depth of oceanic crust
away from the influence of mantle plumes varies
with age in a well-understood manner, by
comparing the present-day bathymetry around
Iceland with this reference depth. The second
method involves establishing a link between
dynamic support and the long-wavelength freeair gravity anomaly. Analysis of gravity
anomalies should always be treated with caution,
as any gravity field can be explained by an
infinite number of density distributions, each
with different implications for dynamic support.
In this section, we first establish that estimates of
dynamic support derived from bathymetry and
gravity are in general agreement over oceanic
crust around Iceland. This result then gives us
confidence in estimating dynamic support of
the adjacent continental margins using gravity
alone.
-1
0
15
Estimates from bathymetry
Figure 2 is a plot of anomalous topography in the
North Atlantic, calculated by subtracting the
well-known age-depth model of Parsons &
Sclater (1977) from the bathymetry. Unfortunately, the anomalous topography in Fig. 2
cannot be interpreted solely in terms of presentday dynamic support because it also includes a
component of permanent topography caused by
spatial variations in the thickness of oceanic
crust. However, the magnitude of this permanent
topography can be estimated given determinations of oceanic crustal thickness from wideangle seismic experiments. In the oceans
surrounding Iceland but away from the continental margins and the Greenland-IcelandFaroes Ridge, anomalously hot mantle has
generated crust 7 - 1 0 k m thick (White 1997).
Isostatic balancing shows that this variation of up
to 3 km greater than the thickness of standard
1
2
Residual topography (km)
Fig. 2. Anomalous topography of the North Atlantic Ocean, calculated by subtracting the age-depth cooling
relationship of Parsons & Sclater (1977) from the ETOPO5 bathymetry grid. The age of oceanic lithosphere was
taken from Mailer et a l. (1997); the Greenland-Iceland-Faroes Ridge was excluded from the calculation because
its age is not well known. Anomalous topography is corrected for sediment loading of oceanic basement using the
method of Le Douaran & Parsons (1982) and the sediment thickness map of Laske & Masters (1997). It should be
noted that the anomalous topography displayed here contains both a component of present-day dynamic support
and a permanent component caused by crustal thickness variations.
16
S.M. JONES ETAL.
oceanic crust (7 km, White et al. 1992) generates
permanent topography of 0-0.5 km. This value
of 0.5 km is smaller than the total variation in
anomalous topography across these regions,
suggesting that most of the anomalous topography is generated by present-day dynamic
support. However, close to the continental
margins and the Greenland-Iceland-Faroes
Ridge, the thickness of oceanic crust is
10-15 km, varying over distances of < 50 km
(Barton & White 1997; Weir et al. 2001). In these
regions, permanent topography probably
accounts for most of the anomalous topography
in Fig. 2.
To summarize the discussion above, anomalous topography of those regions away from the
continental margins and the GreenlandIceland-Faroes Ridge can be interpreted as an
estimate of dynamic support with an error range
of 0.5 km. However, short-wavelength variations
in anomalous topography associated with continental margins and the Greenland-IcelandFaroes Ridge are most likely to reflect permanent
topography associated with crustal thickness
variations. Bearing in mind these caveats, three
important results concerning present-day
dynamic support can be gleaned from Fig. 2.
First, the amplitude of dynamic support close to
Iceland itself is 1 . 5 - 2 k m . Secondly, the
amplitude of dynamic support is partially
controlled by the active mid-ocean ridge system.
This fact is clearly seen both to the north of
Iceland, where the active Kolbeinsey Ridge is
more anomalously elevated than the extinct
Aegir Ridge, and to the south of Iceland, where a
tongue of anomalous uplift extends along the
Reykjanes Ridge. Thirdly, the continental
margins of eastern Greenland and NW Europe
are currently experiencing at least 0.5kin of
dynamic support, implying that the adjacent
continental shelves are also experiencing
significant dynamic support at present.
Estimates f r o m gravity
The free-air gravity field over a convecting layer
results from two competing effects: density
variations within the layer and surface deformation induced by the convective circulation. In
the case of upwelling plumes, hot mantle has a
lower density than the surrounding mantle, which
acts to reduce the gravity. However, an upwelling
plume deforms the Earth's surface upwards,
which acts to increase the gravity. The sign of the
total free-air gravity anomaly depends on the
relative magnitude of these effects. At Rayleigh
numbers appropriate to Earth, the effect of
surface deformation outweighs the effect of the
reduction in density, so upwelling regions in the
mantle are characterized by positive free-air
gravity anomalies at the surface.
The transfer function between the free-air
gravity anomaly g and topography h is called the
admittance and is given by
Z(k) = g(k)/h(k).
Admittance is a function of wavenumber k. At
wavelengths shorter than c. 500 km, admittance
is controlled by the mechanical properties of the
lithosphere. Admittance decreases systematically with increasing wavelength (i.e. decreasing
wavenumber), and the rate of decrease is
dependent upon the effective elastic thickness
of the lithosphere. McKenzie (1994) and
McKenzie & Fairhead (1997) exploited this
behaviour to estimate the effective elastic
thickness of the lithosphere in the Pacific and
Indian Oceans from the short-wavelength part of
the gravity and topography fields. At wavelengths above c. 500km the admittance calculated from the observed gravity and topography
diverges from that calculated from theoretical
models assuming an elastic plate. At these long
wavelengths, the flexural strength of the lithosphere plays no part in supporting topography.
Instead, topography is supported by stresses
exerted on the base of the lithosphere by the
convecting mantle and by long-wavelength
variations in the density structure of the
lithosphere, which are isostatically compensated.
Thus, dynamic support produces long-wavelength anomalous topography Ahconv that correlates with the long-wavelength free-air gravity
anomaly. Isostatically compensated topography,
such as the permanent anomalous topography
that arises from crustal thickness variations in the
North Atlantic, is not correlated with a measurable gravity anomaly.
The behaviour of the admittance function
within the wavelength band 5 0 0 - 3 0 0 0 k i n
suggests a linear relationship between gravity
and topography. If this simple relationship,
observed in the Pacific and Indian Oceans, also
holds in the North Atlantic then anomalous
topography caused by dynamic convective
support can be calculated from the longwavelength free-air gravity field using
Ahconv = g/Z.
The value of Z to be used can be constrained by
observations. It has long been recognized that Z
~ 3 5 m G a l k m -1 is appropriate for Earth's
oceans (Sclater et al. 1975). Another measure
of the admittance to be used to calculate
dynamic support can be obtained from numerical
ICELAND PLUME PAST AND PRESENT
convection experiments. McKenzie (1994) summarized several numerical models of axisymmetric plumes, including the model of Watson &
McKenzie (1991) that successfully matched the
observed gravity, topography and melt production of the Hawaiian Plume. These convection models are characterized by admittances in
the range 34.4 + 2 . 2 m G a l k m - ~ . Hence,
observed and theoretical values for the admittance between topography and gravity of the
oceans at long wavelengths are in good
agreement.
The free-air gravity dataset used here is a
compilation of point measurements over land,
together with the satellite gravity dataset of
Sandwell & Smith (1997) over the oceans, as
described by McKenzie & Fairhead (1997). The
short-wavelength part of the gravity field that is
influenced by the flexural strength of the
lithosphere was removed using a low-pass filter.
The region of interest has dimensions comparable with the radius of Earth, so low-pass filtering
was carded out using a spherical harmonic model
of the gravity dataset. The long-wavelength
gravity model was generated using spherical
harmonic coefficients of degrees 0 < l --- m ~<
53 (equivalent to a low-pass filter of c. 750 km)
with a suitable taper at the upper cut-off to
prevent ringing.
I
-2
I
I
t
17
Figure 3 shows the dynamic component of
anomalous topography Ahconv estimated from the
free-air gravity field using two different values of
Z. These estimates are simply a scaled version of
the long-wavelength gravity field and resemble
the original gravity field fairly closely (compare
Figs 1 and 3). In the region surrounding Iceland,
estimates of convective support from the gravity
field agree reasonably well with independent
estimates of convective support from topography
(compare Figs 2 and 3). In particular, the
dynamic support estimates from gravity
reinforce the three observations derived from
Fig. 2 and noted in the previous section. First,
peak dynamic support is c. 1.8km at Iceland.
Secondly, the active spreading axis exerts an
important control on the long-wavelength gravity
field and on the magnitude of dynamic support.
Dynamic support is centred on the Mid-Atlantic
Ridge, and to the north of Iceland the active
Kolbeinsey Ridge is experiencing greater support
than the extinct Aegir Ridge. Thirdly, the
continental margins are currently experiencing
significant dynamic support of 0.5-1 km.
It is important to note that estimates of
present-day dynamic support calculated directly
from long-wavelength gravity anomalies are not
always in agreement with estimates calculated
from the bathymetry. Figure 3 suggests that
I
0
p!iiiiiii#iiii~
Residual Topography (km)
2
Fig. 3. Estimates of present-day dynamic support in the North Atlantic region, calculated by dividing the longwavelength free-air gravity field by a constant admittance, as discussed in the text. Bold continuous lines indicate
continent-ocean boundaries. (a) Dynamic support predicted using an admittance of Z = 35 mGalkm -1
(appropriate for subaqueous regions); (b) dynamic support predicted using an admittance of Z = 50 mGal km-1
(equivalent value for subaerial regions). K, Kangerlussuaq; S, Scoresby Sund.
18
S.M. JONES E T A L .
significant dynamic support occurs from Iceland
to the Azores. However, Fig. 2 indicates no
measurable dynamic support just south of the
Charlie Gibbs fracture zone. Thus, the amplitude
of dynamic support calculated from gravity data
is unlikely to be correct in detail and should be
treated with some caution. Nevertheless, the
agreement between dynamic support estimates
based on bathymetry and gravity in the vicinity
of Iceland suggests that Fig. 3 provides a
reasonable estimate of dynamic support of the
G r e e n l a n d - I c e l a n d - F a r o e s Ridge and the
adjacent continental shelves.
Free-air gravity anomalies may be used to
estimate dynamic support of the continents in the
same way as for the oceans. Whereas it is
appropriate to employ an admittance of
Z = 35 mGal km -1 to estimate dynamic support
of regions covered by water, an equivalent airloaded value of Z = 50 reGal k m - 1 should be
used for subaerial regions. Dynamic support
estimates calculated using both values of
admittance are shown in Fig. 3. Greenland is
currently experiencing dynamic support of
0.5-1 km. Dynamic support is greatest on the
east coast, notably in the region between
Kangerlussuaq and Scoresby Sund, adjacent to
the Greenland-Iceland-Faroes Ridge. In contrast, the magnitude of dynamic support beneath
the NW European shelf seldom exceeds 0.5 km.
Dynamic support increases westwards from
Scandinavia across the Norwegian continental
shelf. The North Sea and southern England do
not appear to be dynamically supported.
Cenozoic denudation of Britain and Ireland
In this section, we focus on one aspect of the
problem of determining the Early Cenozoic
shape of the Iceland Plume, namely the
relationship between Cenozoic uplift, denudation
and sedimentation on the continental shelf
surrounding Britain and Ireland. Our long-term
goal is to use mass balance calculations in
conjunction with estimates of permanent uplift
caused by magmatic underplating to determine
the behaviour of the Iceland Plume throughout
Cenozoic time.
We have chosen to carry out mass balance
calculations for the British Isles for three
important reasons. First, geochemical evidence
collected from rocks of the British Cenozoic
Igneous Province proves that the crust beneath
much of Britain and Ireland has been thickened
by a substantial amount of igneous material as
we explain below, implying significant permanent uplift during Paleocene time. Secondly, a
substantial body of information about the
denudation of Britain and Ireland is available
based on modelling of subsidence, vitrinite
reflectance, apatite fission-track and sonic
velocity datasets. Thirdly, the products of
Cenozoic denudation have been carefully
mapped in all of the surrounding sedimentary
basins. Although detailed work has been carried
out previously on both onshore denudation and
offshore deposition, no attempts have yet been
made to check these estimates by constructing a
mass balance on a regional scale.
The evidence for magmatic underplating of the
crust beneath Britain and Ireland is not widely
recognized, despite the fact that the consequences of such an igneous addition, in terms of
uplift, denudation and sedimentation, can explain
many features of the surface geology and thus
have obvious implications for the hydrocarbon
industry. For decades it has been recognized that
the composition of flood basalts from the
Hebrides can be explained only by fractional
crystallization of up to 70% of their original
liquid mass (Thompson 1974). The crystallized
residuum must therefore remain at depth. From a
surface processes point of view the argument
ends here, because if such material is added
anywhere within the upper half of the lithosphere
its density will be less than that of the
asthenosphere it displaces, and isostatic balancing shows that permanent uplift will result. The
depth at which crystallization occurred, and at
which the crystallized residuum remains, can be
established using geobarometry techniques,
which determine the pressure at which the
major-element compositions of both flood
basalts and the phenocrysts they contain are in
equilibrium. Pressures of c. 1 GPa are estimated,
equivalent to a depth of around 30 km, i.e. the
depth of the Moho (Thompson 1974; Brodie &
White 1995). This result is not surprising, as the
fact that basaltic melt is less dense than the
mantle lithosphere but roughly the same density
as the lower crust means that the melt should rise
through the mantle and pond at the base of the
crust, giving rise to the concept of magmatic
underplating. Wide-angle seismic experiments
across North Atlantic volcanic continental
margins have imaged high-velocity bodies at
the base of the continental crust, which are
usually interpreted as pods of igneous material
underplated beneath the crust at the time of
continental separation (Barton & White 1997).
Preliminary results from modelling of a wideangle seismic line spanning the Irish Sea
suggest that a high-velocity zone exists near
the Moho, which probably represents the
igneous material that we expect to see
underplated beneath the onshore part of the
ICELAND PLUME PAST AND PRESENT
British C e n o z o i c Igneous Province (S.
A1-Kindi, pers. comm.). However, as this
discussion implies, seismic evidence alone can
never directly reveal the age or nature of these
high-velocity layers, so it is always necessary to
interpret seismic evidence in conjunction with
geochemical evidence. To conclude, it should be
noted that crustal magmatic underplating is not a
peculiar feature of the North Atlantic but is
common to all continental flood basalt provinces
(Cox 1993).
19
Mass balance calculations
The mass balance calculation has three
parts: the extent and amount of denudation
of the sediment source area or catchment
(Fig. 4); the mass of sediment accumulated
through time in the sedimentary basins
immediately adjacent to the British Isles
(Fig. 5); and the mass of material lost from
the system by solution and by escape to the
deep ocean.
#~f'}o
50 °
-10 °
.5o
0o
-]0
.5 o
0o
I
0.0
Denudation
0.5
1.0
1.5
2.0
(km)
2.5
Fig. 4. Estimates of Cenozoic denudation of Britain and Ireland based on modelling subsidence histories and
apatite fission-track length distributions. +, locations of well sections in extensional sedimentary basins used for
subsidence modelling. The amount of missing post-rift basin fill was predicted by fitting a theoretical subsidence
curve to the remnant synrift stratigraphy, assuming the standard lithospheric stretching model. This technique has
been described fully by Rowley & White (1998), and results have been tabulated by Hall (1995) and Rowley
(1998). ©, locations of apatite samples. Apatite fission-track length distributions were modelled to find
Mesozoic-Cenozoic thermal histories by K. Gallagher using the Laslett et al. (1987) annealing model for
Durango apatite and the method described by Gallagher (1995); the results were reported by Rowley (1998). The
amount of Cenozoic cooling was converted to a range of denudation estimates by Monte Carlo modelling using
the range of geothermal gradients observed in the North Sea today (Rowley 1998). Data covering Ireland are from
Allen et al. (2002). (a) Minimum denudation estimate found by contouring results derived from subsidence
analysis, the lower bounds of the Rowley (1998) denudation estimates derived from modelling apatite fissiontrack length distributions, and the minimum denudation estimate for Ireland of Allen et al. (2002, fig. 10b). (b)
Maximum denudation estimate found by contouring the modes of the Rowley (1998) denudation estimates and
the maximum denudation estimate for Ireland of Allen et al. (2002, fig. 10a). These contour plots were generated
by taking the mean of all estimates within blocks of dimension 1° longitude x 30t latitude (c. 50 km x 50 km) and
gridding the resulting values using the continuous curvature spline method of Smith & Wessel (1990). It should be
noted that a further set of Cenozoic denudation estimates based on modelling of vitrinite reflectance profiles from
a subset of the wells used for subsidence analysis yields a denudation estimate that lies between the minimum and
maximum estimates illustrated here (Rowley 1998).
20
S.M. JONES ETAL.
50 °
-10"
0.1
0.5
1.0
-5"
1.5
2.0
2.5
3.0
315
Solid Sediment Thickness (km)
Fig. 5. Map of the NW European continental shelf showing the Cenozoic solid sediment thickness. This isopach
map was constructed from a database of 2D and 3D seismic reflection surveys calibrated with well-log
information. Solid thickness was calculated from the observed thickness by subtracting the pore-space volume
predicted by the standard exponential relationship between fractional porosity q~ and depth z given by
~b= q~0exp(-z/A), where ~b0 = 0.6 is the depositional porosity and A = 2 km is the compaction length scale. The
present-day canyon system at the southern end of the Porcupine Basin is shown. Contours on land show
topography smoothed using a filter of width 50 km and used to define the drainage catchments supplying each
offshore sediment sink. Segments of catchment boundaries that are offshore at present were positioned using
present-day bedload transport patterns. Inset shows central igneous complexes, flood basalts and dykes of the
North Atlantic Igneous Province, mostly emplaced during Paleocene time.
The best-constrained element in the mass
balance problem is the mass of Cenozoic
sediment in the offshore basins. Here, the
volume of solid sediment is used in place of the
mass of sediment. The solid volume of the
offshore sediment pile is calculated by removing
the volume accounted for by porosity, predicted
by the standard exponential porosity model. The
principal source of error in calculating the solid
volume is the depth conversion procedure.
However, the error introduced by this uncertainty
can be realistically quantified by using a range of
velocity-depth functions, and the resulting error
estimates are illustrated in Fig. 6. Parameterization of the porosity-depth relationship used in
the compaction calculation is relatively well
determined for the basins surrounding Britain
and Ireland. For example, using parameters for
the end-member lithologies of shale and sand
determined by Sclater & Christie (1980) for the
North Sea yields a variation in solid volume that
is only 14% of the error range associated with the
depth conversion calculation in that region.
Figure 5 shows the solid sediment thickness
accumulated around Britain and Ireland during
Cenozoic time. During this time the main
sediment sinks were the Porcupine Basin, the
Rockall Trough, the Faroe-Shetland Basin and
the North Sea Basin. The solid volume of
Cenozoic deposits in southern England and
offshore southern Ireland is negligible.
The total Cenozoic denudation is shown in
Fig. 4, based on three independent lines of
evidence. The first line of evidence depends on
our knowledge of extensional sedimentary
basins. Many extensional basins around Britain
and Ireland contain a fault-controlled syn-rift
stratigraphy but the anticipated post-rift stratigraphy is partially or entirely absent (Brodie &
White 1995). The amount of missing post-rift
stratigraphy can be predicted, based on our
knowledge of the kinematics of extensional
ICELAND PLUME PAST AND PRESENT
Solid volume (x 10 3 km 3)
~
o
0
0
•
~
1
~
0
I
,
,
,
,
I
O
Porcupine
i-I
Rockall
i
FaeroeShetland
~]
•
D
N North Sea
.
.
.
.
i
~
T ....
Fig. 6. Summary of mass balance calculations for NW
European continental shelf. Open bars represent the
amount of solid sediment accumulated offshore,
calculated by integrating Fig. 5 over each offshore
depocentre. Error ranges reflect uncertainties in depth
conversion. The solid sediment volume for the North
Sea plotted here has been halved to account for the fact
that Scandinavia has also supplied sediment to the
North Sea. Filled bars represent volume of rock
denuded from onshore catchments, calculated by
integrating the minimum and maximum estimates
given in Fig. 4 over the regions marked in Fig. 5. The
Porcupine sediment sink is supplied by two catchments: SI, southern Ireland and the Irish Sea; WI,
western Ireland. The other sediment sinks have one
catchment each.
basins (Rowley & White 1998). The other two
methods of estimating denudation rely on
thermal indicators within the sediment pile that
retain a m e m o r y of their burial history.
Reflectance of vitrinite increases with rising
temperature by means of a non-reversible
reaction, so vitrinite retains a memory of the
highest temperature it has experienced. Denudation estimates can be obtained by comparing
vitrinite reflectance profiles with a global
reflectance dataset from non-inverted basins
(Rowley 1998). Analysis of fission tracks in
apatite also provides temperature histories that
can be interpreted in terms of denudation through
time. The most prominent feature of Fig. 4 is the
peak in denudation centred on NW England. This
region suffered denudation of 1-2.5 km during
Cenozoic time. A peak in denudation centred on
21
NW England has also been suggested by
previous studies based on apatite fission-track
data alone (Lewis et al. 1992). The magnitude of
Cenozoic denudation generally decreases
towards the present offshore areas. Another
important message from Fig. 4 is that the error
range on the denudation estimate for any
particular region is c. 1 km.
It is difficult to quantify loss of mass from the
system. Loss by solution depends strongly on the
rock type being eroded. It would be difficult to
include the amount of loss by solution in the
mass balance calculations because of the
uncertainty over the original lithologies being
eroded and the variety of lithologies within each
catchment. Here we apply no corrections to
account for loss of mass from the system. When
loss of mass from the system is neglected, the
solid volume of sediment measured in offshore
basins provides a lower bound on the denudation.
If the solid sediment volume measured offshore
is found to be greater than the onshore estimate
of denudation then an additional, unidentified
source of sediment to the offshore basin would be
implied. On the other hand, if the offshore solid
sediment volume is found to be significantly less
than the onshore estimate of denudation then the
difference between the two estimates may
provide an estimate of the amount of material
lost from the system.
Mass balance results
Figure 6 summarizes the result of balancing the
volume of solid sediment accumulated offshore
with direct estimates of denudation made
onshore for the Porcupine, Rockall, F a r o e Shetland and northem North Sea systems. The
drainage catchment matched with each offshore
sediment sink is shown in Fig. 5. The boundaries
of these catchments are based on the present-day
topography and bedload transport patterns.
Palaeotopographic reconstructions of Britain
and Ireland during earliest Eocene time, at the
time of maximum dynamic support by the
Iceland Plume (see discussion later), suggest
that the north-south drainage divide running
through Scotland and England has remained
relatively static since Paleocene time (Jones
2000). The positions of the west-east-oriented
drainage divides are more likely to have altered
position during Cenozoic time, and we consider
the effect of such migration in the following
discussion.
In the Rockall Trough and northern North Sea
systems, the volume of Cenozoic sediment
accumulated offshore is the same as the
denudation measured onshore, within error.
22
S.M. JONES ETAL.
However, in the Faroe-Shetland system, the
volume of sediment accumulated offshore is
greater than the denudation measured onshore by
at least 2 x 104km 3. This discrepancy implies
that the Faroe-Shetland Basin has another
source of Cenozoic sediment in addition to the
catchment in the region of NW Scotland. The
additional sediment source may be a catchment
in the region of the Faroes or Greenland.
Alternatively, the boundaries of the drainage
catchment shown in Fig. 5 covering NW
Scotland may be incorrect. The southern
boundary of that catchment is not well
constrained and it is possible that sediment was
also sourced from western Scotland and channelled between the Outer Hebrides and the
Scottish mainland towards the Faroe-Shetland
Basin. A third possibility is that additional
sediment was transported into the Faroe-Shetland Basin by currents running along the basin
axis. Strong bottom currents flowing southwestwards along the Faroe-Shetland Trough
were initiated in Oligocene time and persist to
the present day (Davies et al. 2001).
Lack of a significant difference between the
two estimates of denudation in the Rockall and
northern North Sea systems suggests that little
mass has been lost from those systems. However,
the total sediment supplied to the Porcupine
Basin by both the catchment covering western
Ireland and the catchment covering southern
Ireland and the Irish Sea is greater than the
material accumulated offshore by at least
4>< 104km 3. Thus, a significant mass of
sediment has been lost from this system. The
magnitude of this discrepancy shown in Fig. 6 is
a lower bound because no account has been taken
of sediment supplied to the western side of the
Porcupine Basin by the Porcupine Bank and
Ridge. Three effects have probably contributed
to the loss of mass from the Porcupine system.
First, Carboniferous limestone crops out over a
large area of the Irish catchments, and erosion of
this limestone produces negligible clastic sediment. Secondly, it is likely that most of the
sediment supplied to the southern end of the
Porcupine Basin has escaped directly to the deepsea sediment fan observed on the North Atlantic
oceanic abyssal plain via the canyon system in
the SE of the Porcupine Basin (Fig. 5). This
canyon system has been active since Early
Oligocene time (Jones 2000). Thirdly, a variety
of evidence from both the continental shelf
surrounding Britain and Ireland and adjacent
oceanic crust suggests that the head of the
Iceland Plume expanded rapidly during earliest
Eocene time, causing dynamic uplift centred NW
of Britain and Ireland (see discussion below).
The resulting southeasterly tilt of Britain and
Ireland probably caused most of the material
eroded from southern Ireland and the Irish Sea
during Eocene time to be shed southwards into
the Bay of Biscay, rather than westwards into the
Porcupine Basin.
Cenozoic permanent and transient uplift
In this section, we discuss evidence for temporal
and spatial variation in the Iceland Plume
throughout Cenozoic time.
Paleocene p e r m a n e n t uplift
The history of sediment flux into the basins
surrounding Britain and Ireland during Paleocene time adds detail to the denudation history
established using the mass balance. The rate of
sediment flux into an offshore basin is related to
the size and the rate of denudation of the
corresponding drainage catchment. The lag time
between an increase in denudation rate and the
corresponding increase in sediment flux offshore
is likely to be < 100ka (Reading 1991; Burgess
& Hovius 1998). Therefore, sediment flux
histories in the basins surrounding Britain and
Ireland can be directly related to uplift of their
sediment source regions. Calculation of the
volumes of Paleocene and Eocene sediment
sequences in the northern North Sea and FaroeShetland Basins has shown that sediment flux
into these basins grew through Paleocene time to
a maximum at 5 9 - 5 8 Ma and then decreased into
Eocene time (Reynolds 1994; Clarke 2002).
Thus we may infer that epeirogenic uplift of
Scotland and northern England was initiated in
Early Paleocene time and the rate of uplift
peaked during mid-Late Paleocene time. In
contrast, sediment flux into the Porcupine Basin
remained low throughout Paleocene time,
suggesting that uplift rates were lower away
from the region of Scotland and northern
England at this time (Jones 2000).
The denudation history of Scotland inferred
from the history of sediment flux around
Scotland correlates with the history of onshore
igneous activity, which also peaked at 59-58 Ma
(White & Lovell 1997). As we have seen, the
sediment flux data suggest that Paleocene
epeirogenic uplift was confined to the region
close to the present-day surface expression of
onshore igneous activity. We therefore suggest
that the Paleocene igneous activity, epeirogenic
uplift and denudation were initiated when a
subvertical sheet of unusually hot asthenosphere
was injected beneath the present Faroes-Irish
Sea-Lundy axis. The surface expression of
ICELAND PLUME PAST AND PRESENT
onshore igneous activity is concentrated in
western Scotland and Ulster, but this igneous
activity is offset from the locus of maximum
denudation, which is centred on northern
England and the Irish Sea. If the denudation
shown in Fig. 4 is driven mainly by permanent
uplift resulting from magmatic underplating, the
greatest amount of melt must have been added to
the crust beneath this region. Preliminary
modelling of a wide-angle seismic line crossing
Ireland, the Irish Sea and northern England has
imaged a high-velocity pod near the Moho,
which is thickest beneath the centre of the profile
and thins towards western Ireland and towards
the North Sea (S. A1-Kindi, pers. comm.). This
pod may represent the layer of magmatic
underplating beneath the crust that is predicted
by both petrological and sedimentological
evidence.
P a l e o c e n e - E o c e n e dynamic support
The sedimentary records in all basins surrounding the denuded area of Britain and Ireland
indicate a major regression-transgression cycle,
with maximum regression corresponding to the
upper Flett Formation and its lateral equivalents,
which were deposited during earliest Eocene
time (e.g. Milton et al. 1990; Ebdon et al. 1995).
It is now generally believed that this regressiontransgression cycle is related to a phase of
transient dynamic uplift that peaked in earliest
Eocene time (Nadin et al. 1997; Jones et al.
2001). The magnitude of dynamic support can be
quantified within extensional sedimentary
basins, where we can isolate epeirogenic vertical
motions from tectonic vertical motions, which
are ultimately caused by horizontal plate
motions. Given the synrift subsidence history of
an extensional basin, the post-rift subsidence
history can be calculated using the wellestablished lithospheric stretching model. Postrift marker horizons with sedimentologically
well-constrained water depths are then compared
with the anticipated post-rift subsidence curve to
reveal the magnitude of dynamic support through
time. The most important of these marker
horizons around Britain and Ireland are the
earliest Eocene delta-top coals of the upper Flett
Formation and its lateral equivalents. The results
of this subsidence analysis show that peak
dynamic support was c. 0.5 km in the Porcupine
and Faroe-Shetland Basins, and dynamic support decreased in a southeasterly direction to zero
across southern England (Nadin et al. 1997;
Jones 2000; Jones et al. 2001).
Peak dynamic support at the PaleoceneEocene boundary was coeval with voluminous
23
intrusive and extrusive igneous activity offshore
NW of Britain and Ireland that was associated
with break-up of Europe and Greenland above
unusually hot asthenosphere. White (1997)
collated oceanic crustal thickness measurements
and showed that the hot asthenosphere of the
Iceland Plume head extended at least 1000km
along the continent-ocean boundaries to the
north and south of the present GreenlandIceland-Faroes Ridge. Dynamic support estimates from subsidence analyses around Britain
and Ireland suggest that the plume head extended
a similar distance inboard of the NW European
continental margin.
We might expect that as maximum transient
dynamic support of the basins surrounding
Britain and Ireland occurred during earliest
Eocene time, the acme of denudation and
offshore Sediment flux should also have occurred
at this time. However, as we discussed above,
maximum sediment flux into the Faroe-Shetland and North Sea Basins actually occurred
3 - 4 Ma previously, during Late Paleocene time.
Only in the Porcupine Basin, west of Ireland, was
the peak sedimentation rate coeval with peak
dynamic support in earliest Eocene time (Jones
2000). This discrepancy in timing of several
million years between the two different measures
of epeirogenic uplift is an important observation
that has yet to be explained. One possible
explanation is that there were two separate
phases of mantle plume activity, the first an
upwelling hot vertical sheet that led to magmatic
underplating of the crust beneath the FaroesIrish Sea-Lundy axis, and the second the growth
of a mushroom-shaped plume head beneath a
region over 1000 km in radius.
O l i g o c e n e - R e c e n t dynamic support
Variations in crustal thickness and structure
around Iceland record changes in the size of the
thermal head of the Iceland Plume, measured at
the Mid-Atlantic Ridge, through Cenozoic time.
As we have already discussed, the radius of the
plume head was > 1 0 0 0 k m at the time of
continental separation between Europe and
Greenland in earliest Eocene time. However,
the distribution of normal-thickness oceanic
crust south of Iceland suggests that during Late
Eocene times, the part of the plume head beneath
the Mid-Atlantic Ridge extended < 300 km from
the present centre of Iceland (White 1997). The
portion of the plume head beneath the ridge then
increased to its present radius of c. 1000km
between Oligocene time and the present (Figs 2
and 3). Studies of plate motion with respect to the
hotspot reference frame suggest that the centre of
24
S.M. JONES ETAL.
the Iceland Plume lay beneath Greenland during
mid-Cenozoic time and has effectively moved
eastwards towards Europe through time (Lawver
& Mtiller 1994). We suggest that this relative
motion between the Mid-Atlantic Ridge and a
plume of relatively constant mass flux may
account for the apparent increase in the size of
the head of the Iceland Plume between Oligocene
time and the present. This history of motion also
agrees with our observation that Greenland is
currently experiencing greater dynamic support
than NW Europe (Fig. 3). The reason is that a
greater volume of hot plume-head material
accumulates beneath the plate that the plume
stem is moving away from than accumulates
beneath the plate that the plume stem is
approaching (Ribe & Delattre 1998). As
Rohrman & van der Beek (1996) have suggested,
relative movement of the Iceland Plume towards
Europe may also provide an explanation for the
Miocene-Recent epeirogenic uplift and consequent increase in denudation that is known to
have affected Scandinavia.
Conclusions
The Cenozoic evolution of the North Atlantic
province has been dominated by interaction
between the Iceland Plume convective system
and sea-floor spreading between Europe and
Greenland on a hierarchy of spatial and temporal
scales. The continental sedimentary record
seems to be influenced by the relative importance
of dynamic support, which has varied through
through time, and permanent uplift, which was
driven by magmatic underplating of the crust.
Further progress in understanding this relationship will depend upon measuring the distribution
of Paleogene magmatic underplating and upon
an improved quantitative understanding of
Cenozoic denudation.
Our conclusions concerning the present shape
and size of the Iceland Plume and concerning
Cenozoic mass balance are:
(1) maximum dynamic support in the North
Atlantic is 1.5-2 km at Iceland itself.
(2) The magnitude of dynamic support in the
North Atlantic is influenced by the location of the
active spreading centres.
(3) The continent-ocean boundary of NW
Europe is currently experiencing dynamic uplift
of c. 0.5km, which decreases to zero across
Scandinavia and the North Sea. The continentocean boundary of eastern Greenland is currently
experiencing dynamic uplift of c. 1 km.
(4) Cenozoic mass balance validates onshore
denudation estimates by showing they are
compatible with the solid volume of sediment
accumulated offshore.
S.M.J. was supported by an NERC studentship and
B.J.C. was supported by a BP studentship. We are
indebted to B. Mitchener and J. Perry of BP for
providing data and support. A. Carter and T. Hurford
provided the fission-track data that were modelled to
produce Fig. 4, and P. Allen allowed us to use results
reported elsewhere in this volume in the same figure.
We thank H. Walford for help in producing Fig. 3, and
M. Shaw-Champion for help in producing Fig. 5. A.G.
Dor~, J.-I. Faleide, B. Lovell and M. Rohrman
provided helpful reviews. This paper is Department
of Earth Sciences Contribution ES.6677.
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