Download Structure of the crust and uppermost mantle of Iceland from

Earth and Planetary Science Letters 181 (2000) 409^428
www.elsevier.com/locate/epsl
Structure of the crust and uppermost mantle of Iceland
from a combined seismic and gravity study
Fiona A. Darbyshire *, Robert S. White, Keith F. Priestley
Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK
Received 23 February 2000; accepted 6 June 2000
Abstract
We present a map of the depth to the base of the upper crust and the total crustal thickness across Iceland
constrained by seismic refraction results, receiver function analysis and gravity modelling. Upper crustal thicknesses (as
è .G. Flövenz, J. Geophys. 47 (1980) 211^220) lie in the range of approximately 2^11 km, with the thinnest
defined by O
upper crust below active and extinct central volcanoes and the thickest upper crust close to the flanks of the rift zones.
The thickest crust (40^41 km) lies above the centre of the Iceland mantle plume, where active upwelling and high mantle
temperatures enhance melt production. Thick crust (V35 km) is also found in eastern Iceland, between the current
plume centre and the Faroe^Iceland Ridge. Elsewhere, the crust thins away from the plume centre. The thinnest crust
(920 km) is found in the active rift in the northern part of the Northern Volcanic Zone, where melt production has been
affected by a ridge jump, and in the far southwest of Iceland. The uppermost mantle below Iceland is characterised by
reduced densities below the rift zones, suggesting higher mantle temperatures and the possible presence of partial melt in
these regions. ß 2000 Elsevier Science B.V. All rights reserved.
Keywords: Iceland; mantle plumes; crust; gravity survey maps
1. Introduction
Iceland is created by interaction between the
Mid-Atlantic Ridge spreading centre and the Iceland mantle plume, which causes the generation
of anomalously thick igneous crust. Estimates of
the mantle temperature anomaly in the plume
compared to normal mantle temperatures vary
from 150³C [2,3] to 300³C [4]. The Mid-Atlantic
Ridge is expressed in Iceland as three volcanic rift
* Corresponding author. Tel.: +44-1223-337176;
Fax: +44-1223-360779; E-mail: ¢[email protected]
zones composed of central volcanoes transected
by rifts and ¢ssure swarms (Fig. 1). In southern
Iceland, two subparallel rift zones, the Western
Volcanic Zone (WVZ) and the Eastern Volcanic
Zone (EVZ), are active, separated by a transform
fault system, the South Iceland Seismic Zone
(SISZ). The Northern Volcanic Zone (NVZ) extends northwards from beneath the Vatnajo«kull
icecap to the Tjo«rnes Fracture Zone (TFZ), which
links the Icelandic rift system to the Kolbeinsey
Ridge.
The positions of the Icelandic rift zones have
changed with time; rifting in southern Iceland is
presently being transferred from the WVZ to the
EVZ [5] and at least two major shifts of the north-
0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 2 0 6 - 5
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
410
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
crustal structure. Gravity modelling is used to
complement the crustal models derived from seismic experiments, in order to provide crustal structure information where seismic data are sparse.
We combine the results of the seismic and gravity
models to produce a map of the Moho depth
across Iceland and to investigate variations in
the density of the uppermost mantle.
2. Development of the crustal model
2.1. Seismic data
Fig. 1. Tectonic map of Iceland [44] showing the locations of
long seismic refraction pro¢les (bold lines: SIST [8]; FIRE
[9]; ICEMELT [10]; B96 [11]), stations used for receiver
function analysis (black triangles, labelled with station codes:
[17], open triangles: [16]) and pro¢les GP1^7 (dashed lines)
along which gravity is modelled. RR, Reykjanes Ridge;
WVZ, Western Volcanic Zone; EVZ, Eastern Volcanic
Zone; NVZ, Northern Volcanic Zone; SISZ, South Iceland
Seismic Zone; TFZ, Tjo«rnes Fracture Zone; KR, Kolbeinsey
Ridge; VJ, Vatnajo«kull icecap. Central volcanoes: kr, Kraë r×fajo«£a; ba, Bärdarbunga; gr, Gr|¨msvo«tn; as, Askja; or, O
kull; ka, Katla.
ern rift zone in the last 15 Myr have been identi¢ed. Eastward jumps of the rift axis are believed
to arise from the westward drift of the Mid-Atlantic Ridge with respect to the mantle plume [6],
though the mechanism for this interaction is
poorly understood. The ridge jumps serve to
keep the rift axis above the plume centre. At
present, the centre of the plume is believed to lie
beneath the Vatnajo«kull icecap (see Fig. 1). This
area has the highest topography in Iceland and
marks the centre of the mantle low-velocity anomaly imaged by Wolfe et al. [4].
Two contrasting models have been proposed
for the structure of the crust below Iceland: either
a hot, 10^15 km thick crust overlying a partially
molten upper mantle with anomalously low seismic velocities [7] or a cooler, 20^40 km thick crust
with the thickest crust directly above the plume
centre [8^11]. Previous seismic studies [1,12] also
indicate a large variation in the thickness of the
upper crust across Iceland.
In this paper, we review the information available from published seismic models of Icelandic
A large amount of information about the Icelandic crust has been gained in the last few decades, using a variety of seismic methods. The majority of the studies carried out have been wideangle refraction pro¢les, with numerous short
pro¢les (generally 6 60 km) used to constrain
the structure of the uppermost few kilometres of
the crust (e.g. [1,12^14]) and a few long (typically
s 100 km) pro¢les used to provide information
about the deeper crust and the depth to the
Moho (e.g. [8^11]). Analysis of teleseismic earthquakes has also been used to study Icelandic crustal structure. Tryggvasson [15] used surface wave
dispersion to give an early model of the crust
below Iceland and, more recently, teleseismic receiver function analysis has been used to study the
structure of central, northern and northwestern
Iceland [16,17]. The crustal structure o¡shore Iceland and the surrounding region has also been
studied by seismic refraction pro¢ling (e.g. [18^
21]). The full set of seismic results used in this
study, as well as some previous key publications
about Icelandic crustal structure, are shown in
Table 1.
The results from seismic experiments de¢ne the
depth to the base of the upper crust and to the
Moho, as well as providing information on the
nature of the Moho in Iceland. The transition
from upper to lower crust is de¢ned by a pronounced decrease in the velocity gradient from
s 0.2 s31 in the upper crust to 6 0.05 s31 in the
lower crust (e.g. [1]). The transition occurs at
depths where the P wave velocities lie between
6.3 and 6.7 km s31 . The depth to the Moho as
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
411
Table 1
Sources of information about Icelandic crustal structure from seismic studies; 1960^2000
Method
Location
Authors
Information
Used for map? (Y/N)
RF
RF (HOTSPOT)
WA (SIST)
WA (ICEMELT)
WA (B96)
WA (RRISP)
WA (FIRE)
WA (FIRE)
WA (FIRE)
WA
WA
WA
WA (SIGMA)
WA (RRISP)
WA (RRISP)
WA (NASP)
WA
WA
WA
WA
SW
Central/N Iceland
Western fjords
SW Iceland
Across Iceland
W £ank of NVZ
S of Askja, NVZ
NE Iceland
Kra£a volcano, NE Iceland
Faroe^Iceland Ridge
Axarfjo«rdur, NE Iceland
Katla volcano, S Iceland
Reykjanes Ridge
Iceland^Greenland Ridge
Across Iceland
O¡ S coast of Iceland
Faroe^Iceland Ridge; NE Iceland
Reyfarfjo«rdur
Across Iceland
W Iceland
Faroe^Iceland Ridge
Across Iceland
[17]
[16]
[8]
[10]
[11]
[11]
[9]
[32]
[21]
[45]
[14]
[19,46,47]
[20]
[7,48]
[18,49]
[50]
[51]
[1,12,13]
[52]
[53]
[15]
U,
U,
U,
U,
U,
M
U,
U,
U,
U
U
U,
M
U,
U,
U,
U
U
U,
U,
U,
Y
Y
Y
Y
Y
Y
Y
Y
N
Y
Y
Y
Y
N
Y
Y
N
Y
N
N
N
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
Where the experiments have names, these are given in brackets after the seismic method. Abbreviations are as follows: RF, receiver function analysis; WA, wide-angle seismic refraction pro¢ling; SW, surface wave dispersion analysis; U, information about
upper crustal structure; M, information about Moho depth.
modelled from refraction pro¢les is de¢ned
mainly by wide-angle Pm P re£ections, and the
Moho is assumed to be either a sharp discontinuity or a relatively thin ( 6 V2 km) transition zone
[22]. Receiver function analysis of broadband
data from the Icelandic SIL network [17] suggests
that the nature of the Moho varies across Iceland,
from a sharp discontinuity in some places to a
transition zone several kilometres thick in others.
Where the Moho is gradational in nature, Darbyshire et al. [17] de¢ne the Moho depth as the
depth to the base of the transition zone, below
which shear wave velocities reach 4.4 km s31 or
higher. Du and Foulger [16] do not assign Moho
depths in their study of receiver functions in the
western fjords but instead present a list of the
depths at which the shear wave velocity rises
above 4.1 km s31 , which they assume to be the
base of the crust. In our study we use the velocity
depth models reported by Du and Foulger [16]
but, for consistency, de¢ne values for the Moho
depths from these models in the same manner as
Darbyshire et al. [17].
The seismic results currently published cover
several di¡erent regions of Iceland (long refraction pro¢les and locations where receiver
function analysis has been carried out are shown
in Fig. 1). Trends in the crustal structure can be
seen; these mainly correlate with the past and
present tectonic structure of Iceland and with
proximity to the centre of the Iceland mantle
plume. However, there are still many gaps in the
seismic coverage of Iceland. The crustal structure
of areas such as the EVZ, central southern Iceland, much of northwestern Iceland and the
northeast region (east of the NVZ) must be constrained before we can produce a map of upper
crustal thickness and Moho depth across all of
Iceland.
Gravity modelling may be used to constrain
crustal structure, but care must be taken that possible variations in the density structure of the
uppermost mantle across Iceland are not mapped
into crustal structure. We ¢rst investigate the possibility of mantle density variations by carrying
out gravity modelling in regions where the crustal
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
412
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
structure is well constrained by seismic refraction
pro¢les.
2.2. Gravity modelling along the ICEMELT
refraction pro¢le
where b is the density in kg m33 and VP is the P
wave velocity in km s31 . The density of highervelocity (4.5 km s31 9 VP 9 6.6 km s31 ) crustal
material is given by Carlson and Herrick [25] as:
b ˆ 1000…3:8136:0=V P †
…2†
We investigate the density structure of the crust
and uppermost mantle along the ICEMELT
pro¢le using two-dimensional (2D) gravity modelling. The crustal P wave velocity model of Darbyshire et al. [10] was extended V100 km laterally
at each end and converted to a density model
using three di¡erent relationships. For P wave
velocities 9 4.5 km s31 , at shallow depths, the
following relationship calculated by Zelt [23]
from the data set compiled by Ludwig et al. [24]
was used:
These relationships were used in preference to
Christensen and Wilkins' [26] data from the Reydarfjo«rdur borehole, since the latter present sparser data at densities of 6 2800 kg m33 , and the
borehole was drilled in a region where dyking is
intense, biasing the results away from basalt
£ows. For crustal material of VP v 6.6 km s31 ,
the density is given by a relationship calculated
by Staples [22] using data for gabbro, diabase
and garnet granulite taken from [25,27]:
b ˆ 1000…30:6997 ‡ 2:2302V P 30:598V 2P ‡
b ˆ 1000…5:23315:38=V P †
0:07036V 3P 30:0028311V 4P †
…1†
…3†
The density of the mantle below the ICEMELT
Fig. 2. Map of the Bouguer gravity anomaly across Iceland, contoured at 10 mGal intervals. The gridded data set was supplied
by Eysteinsson and Gunnarsson [29], Orkustofnun Ièslands.
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
pro¢le cannot be calculated directly from the seismic model, since no data are available for subMoho P wave velocities. Instead, a mantle density
of 3260 kg m33 is used in the modelling. This
value is derived from the estimated average mantle (harzburgite) composition below Iceland at 40
km depth and 1000^1100³C (Maclennan, personal
communication, 1999). It is lower than the densities generally assigned to `typical' mantle since it
includes the e¡ect of depletion caused by melt
extraction from the mantle plume. Systematic errors in the velocity^density relationships we have
adopted have little e¡ect on the results of this
study as their main e¡ect would be to cause a
consistent o¡set in the calculated gravity ¢eld.
Since we are modelling gravity anomalies rather
than the absolute gravity, such an o¡set would
make little di¡erence to the crustal thickness variations we model as responsible for those anomalies. We assume that the relationships between
velocity and density do not vary from place to
place across Iceland.
The gravity data set used in this study is a
combination of onshore and ship track measurements obtained during the period 1967^1985 [28].
A Bouguer correction is applied both o¡shore and
onshore. However, because the ICEMELT pro¢le
crosses the Vatnajo«kull icecap, a standard Bouguer correction everywhere onshore is inappropriate for our models. Instead we use a data set for
which the reduction density used in the Bouguer
corrections is modi¢ed in the glaciated regions.
A density of 900 kg m33 is used for the ice and
2600 kg m33 is used for the subglacial bedrock
[29]. The dominant feature in the gravity data
along the ICEMELT pro¢le is a prominent minimum in the Bouguer gravity anomaly which coincides with the presumed location of the plume
centre (Fig. 2).
The ¢rst ICEMELT density model (Fig. 3a)
contains no changes to the sections which are directly constrained by seismic data. The crustal
structure at either end of the pro¢le was modelled
using the changes in the gravity anomaly. A constant density mantle was assumed. The ¢t to the
observed gravity anomaly is extremely poor in
central Iceland (dashed line, Fig. 3c).
The calculated gravity anomaly from our ¢nal
413
Fig. 3. Results of gravity modelling along the extended ICEMELT seismic pro¢le [10]. (a) Initial model with constantdensity mantle. (b) Final density model. In (a) and (b) the
Moho is marked by a bold line and regions of reduced mantle density are shaded. Since the density model used in the
computations consists of 300 polygons, we plot a simpli¢ed
version for clarity. Density values are given in kg m33 .
(c) Observed gravity anomaly (crosses), calculated gravity
anomaly for model (a) (dashed line) and ¢nal calculated
gravity for model (b) (solid line).
density model (Fig. 3b) matches the observed
Bouguer gravity anomaly to within one contour
interval on the original gravity map. In order to
achieve this degree of ¢t (solid line, Fig. 3c), small
changes to the crustal structure within the error
bounds of the seismic model of Darbyshire et al.
[10] were made. In addition, because the seismic
model has poor constraint of the uppermost crust
in the Vatnajo«kull region (60^140 km along the
pro¢le), we changed the density of the uppermost
layer of the density model in accordance with the
model of Gudmundsson and Milsom [30] which
investigates the upper crustal structure of the
Gr|¨msvo«tn region (V100 km along the ICEMELT pro¢le). A small increase in mantle density
(to 3265 kg m33 ) was made at the southeastern
end of the pro¢le (close to the Iceland shelf) and
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
414
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
Fig. 4. (a) ICEMELT density model showing upper crustal
thickness required to ¢t the observed gravity anomaly if no
lateral mantle density variations are allowed. (b) Observed
(crosses) and calculated (solid line) gravity anomaly. (c) Results of ray tracing through the resultant velocity model
from (a). Note the lack of ¢t of the calculated arrival times
(solid lines) to the travel time data (vertical bars).
the density of a 120 km wide region of the mantle
below Vatnajo«kull and the central highlands (20^
140 km) was reduced. Small changes (20 kg m33 )
were made in central and southern Iceland, with a
narrower (V40 km) region of lower density mantle beneath the axial rift zone. Here, the mantle
density reduction is 60^90 kg m33 .
The model ¢ts the observed Bouguer gravity
anomaly well (the RMS residual is 4 mGal), except for a short-wavelength gravity minimum at
distances of +90 to +100 km. The short wavelength of the feature suggests that its origin lies
in the upper crust. There are several possible explanations for the mis¢t here. First, the details of
the upper crustal structure below northwest Vatnajo«kull are not well resolved by the ICEMELT
pro¢le due to the sparse spacing of the seismic
stations. The upper crust in this region may consist of a set of separate high-velocity domes asso-
ciated with the central volcanoes, instead of the
single large-scale smeared dome modelled here
(see Section 2.2.1). In this case the gravity minimum may coincide with a thicker section of upper
crust between the domes. The track of the ICEMELT pro¢le passes between the three central
volcanoes beneath northwest Vatnajo«kull. It is
also possible that a low-density body such as a
volume of hot rock or a melt body exists in the
upper crust below the Gr|¨msvo«tn region. The detailed gravity study of Gudmundsson and Milsom
[30] allows a model containing a magma chamber,
and much volcanic activity has occurred at
Gr|¨msvo«tn during the last century, including major eruptions in 1938, 1983 and 1998. The e¡ect
on the gravity anomaly of a melt body in the
upper crust is likely to be small, due to the relatively small density contrast between molten and
solid basalt [31], but it may contribute to the mis¢t observed.
In order to assess the validity of the mantle
density variations modelled above, we conducted
a set of tests on the ICEMELT crustal model to
investigate whether it was possible to match the
observed gravity anomaly using changes in crustal
structure alone, keeping the mantle density constant along the pro¢le.
Fig. 4 shows the crustal structure required to
match the Bouguer gravity anomaly for a constant-density mantle if the Moho depth is ¢xed
to within þ 1 km and the thickness of the upper
and mid crust is allowed to change freely in the
interval between +40 and +160 km distance along
the pro¢le. To achieve the match between the observed and calculated gravity anomaly it was necessary to increase the depth to the base of the
upper crust by a factor of 4, to 10^12 km below
sea level and to increase the depth to the base of
the middle crustal region to 15 km. The altered
upper crustal thicknesses cannot ¢t the seismic
data from the ICEMELT refraction pro¢le (Fig.
4c) and lie well outside the maximum error estimated for the upper crustal thickness of the seismic model.
If the upper crustal structure is constrained to
be the same as that for the seismic model and the
Moho depth is allowed to vary, then a minimum
crustal thickness of 55 km would be required to
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
match the gravity anomaly for a model with a
constant-density mantle. This crustal thickness
lies well outside the uncertainties of the ICEMELT seismic model, and is also inconsistent
with the crustal thickness estimate of 37 km at
the nearby broadband seismic station SKR [17].
The mid and lower crust beneath Vatnajo«kull
and other Icelandic rift zones may contain partially molten regions undetected by seismic refraction pro¢ling. A prominent shear wave low-velocity region at 10^15 km depth below Kra£a (see
Fig. 1) has been inferred from receiver function
analysis and interpreted to arise from partial melt
in the crust [17]. However, such regions are likely
to contain only a small percentage of melt (indicated by the propagation of S waves below the
Kra£a region), and the di¡erence in density between the melt and the solid material is relatively
small [31]. Hence we expect little di¡erence to be
made to the gravity anomaly by the presence of
these zones.
We conclude that it is not possible to ¢t the
seismic data and the gravity data simultaneously
unless lateral density variations are introduced
into the mantle below the rift zone. The long
wavelength of the gravity low in central Iceland
also suggests a deep source rather than an artefact
of anomalous or three-dimensional (3D) structure
in the upper crust, which would give rise to a
short-wavelength anomaly.
2.2.1. E¡ects of 3D structure
In central Iceland, particularly in the northwestern part of the Vatnajo«kull icecap, the gravity
anomaly is 3D over a relatively short-length scale
(a few tens of kilometres). It is therefore important to assess the e¡ect of 3D structure on the
results of the ICEMELT gravity modelling, which
uses a 2D approximation. 3D structure in the
upper and middle crust is likely to be most signi¢cant in regions close to central volcanoes. The
ICEMELT pro¢le passes between several volcanoes as it crosses Vatnajo«kull. Results from seismic pro¢les and detailed gravity models (e.g.
[30,32]) suggest that each central volcano in Iceland has a dome of high-velocity material beneath
it. However, the resolution of the ICEMELT seismic pro¢le is such that a dome on a larger scale
415
than a single volcano is modelled, instead of several individual volcanic domes.
We used the method of Kuo and Forsyth [33]
to model the gravity anomaly due to topography
on the boundary between the upper and lower
crust in order to investigate the e¡ect of 3D structure associated with the Vatnajo«kull central volcanoes. The results are shown in Fig. 5.
Synthetic 3D topography of the upper/lower
crust boundary below northwest Vatnajo«kull
(Fig. 5a) was input to a 3D gravity modelling
program. The depth to the base of the upper crust
away from the volcanoes was estimated from seismic results [10] and an estimate of the average
upper crustal thickness across Iceland taken
from Flövenz [1]. Below each central volcano,
the base of the upper crust rises in a dome shape
of similar dimensions to those reported by seismic
studies of other central volcanoes (e.g. [32]). A
density contrast of 200 kg m33 across the boundary was assumed, based on the di¡erence between
average upper and middle crustal densities in the
ICEMELT density model. The resulting gravity
anomaly follows the input topography (Fig. 5b).
Gravity sampled along the position of the ICEMELT line shows local highs where the pro¢le
passes close to volcanic domes (Fig. 6, solid line).
In order to assess the e¡ect of a 2D approximation to the full 3D structure, the topography of
the upper/lower crust boundary from the 3D
model was sampled along the ICEMELT line
and extended perpendicularly on each side of
the line to simulate 2D structure (Fig. 5c). The
resulting gravity anomaly (Fig. 5d) shows some
di¡erences to the full 3D solution, particularly
where the pro¢le passes between the central volcanoes, but the di¡erences are not greater than
V3 mGal (Fig. 6, dashed line). This value lies
within the uncertainties of the Iceland gravity
data set; therefore we believe that a 2D approximation to the 3D structure in the vicinity of central volcanoes in Iceland remains valid.
Further modelling was carried out to assess the
e¡ect on the gravity anomaly of a larger-scale
dome which smears out the 3D structure in the
region of the central volcanoes. The topography
of the upper/lower crust boundary along the ICEMELT line was modelled from the results of the
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
416
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
417
Fig. 5. Plan views of the results of gravity modelling tests for 3D structure. The region modelled is northwest Vatnajo«kull.
(a) Synthetic topography of the upper/lower crust boundary. The central volcanoes are labelled as for Fig. 1 and the ICEMELT
pro¢le is marked as a line. (b) Calculated gravity anomaly for 3D topography. (c) Topography of the upper/lower crust boundary sampled along the ICEMELT line and extended laterally to either side. (d) Calculated gravity anomaly for 2D topography.
(e) Topography of the upper/lower crust boundary sampled along the ICEMELT seismic model for northwest Vatnajo«kull and
extended laterally to either side. (f) Calculated gravity anomaly for ICEMELT seismic model of the upper/lower crust boundary.
In each case, point `X' marks the position of zero distance along the ICEMELT pro¢le, used in Fig. 6.
6
ICEMELT seismic model [10], and extended perpendicularly on either side of the pro¢le (Fig. 5e).
The resulting gravity anomaly shows a broadened
region of high gravity (Fig. 5f) in the position
which corresponds to a point of relatively low
gravity between two central volcanoes in the full
3D model (Fig. 6, dotted line). The magnitude of
the di¡erence in gravity between this model and
the full 3D model, and the position of the feature,
is similar to the mis¢t between the observed and
calculated gravity at +90 km along the ICEMELT
density model (Fig. 3). We believe that it is this
smearing out of the volcanic domes along the
ICEMELT pro¢le as a result of the limited resolution of the wide-angle seismic data which gives
rise to the mis¢t between the observed and calculated gravity anomaly.
We also carried out tests using 2.5-dimensional
gravity modelling (i.e. using a ¢nite strike width
for structures along a 2D pro¢le) to assess the
e¡ect of long-wavelength 3D structure at the
Fig. 6. Calculated gravity anomalies sampled along the ICEMELT pro¢le for northwest Vatnajo«kull. Solid line, full 3D
solution. Dashed line, 2D solution for data sampled from
3D topography. Dotted line, 2D solution for smeared out
dome resolved by the ICEMELT seismic model.
Moho and within the mantle. At strike widths
comparable to the length of the structures along
the ICEMELT pro¢le, the 3D e¡ect contributed
less than 10 mGal to the calculated gravity anomaly, which again is within the uncertainties in the
data.
2.3. Gravity modelling along other seismic pro¢les
in Iceland
Gravity modelling along two seismic refraction
pro¢les, FIRE (for diagrams and models, see Staples et al. [9]) and ICEMELT (see above), shows
reduced mantle densities below the rift zones of
central and northern Iceland. We investigate mantle density variations across Iceland further by
considering the gravity anomalies along two other
published seismic refraction pro¢les, SIST [8] and
B96 [11]. The purpose of this study is not to reinterpret the published results, but to use them to
gain extra information about the nature of the
mantle below Iceland.
2.3.1. The SIST pro¢le
The SIST pro¢le runs NW^SE across southwestern Iceland and crosses the Western Volcanic
Zone almost perpendicular to the strike of the rift
zone. We converted the seismic velocities reported
by Bjarnason et al. [8] to densities using the method described above and compared the calculated
and observed Bouguer gravity anomaly (Fig. 7).
In the southeastern section of the model (distances v70 km) the general pattern of the calculated
gravity anomaly matches the data well. However,
across the WVZ (distance V0^40 km) the shallowing of the lower crust creates a broad maximum in the calculated gravity anomaly which is
not observed in the data.
The crustal velocity model of Bjarnason et al.
[8] is derived from seismic data with high shot and
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
418
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
(Fig. 8). We estimate that if the uncertainties in
Moho depth for the B96 crustal model are similar
to those for the FIRE [9] and ICEMELT [10]
models, we have changed the crustal structure
only within the uncertainty bounds of the seismic
model in order to give an adequate ¢t to the observed gravity anomaly (the RMS residual is 11
mGal).
2.4. Use of isostasy to investigate variations in
mantle density structure
Fig. 7. Results of gravity modelling along the SIST seismic
pro¢le [8]. (a) SIST crustal model with low mantle densities
below the WVZ. (b) Observed gravity anomaly (crosses), calculated gravity anomaly for model with constant-density
mantle (dashed line), and calculated gravity anomaly for
model with low mantle densities below the WVZ (solid line).
Gravity modelling along the FIRE [9], ICEMELT and SIST pro¢les indicates lateral variations in mantle density in Iceland. We investigated
this result further by considering the isostatic balance across Iceland. The topography, Moho
depth, average crustal density and average crustal
velocity were sampled at 10^20 km intervals along
receiver density, and the region in which there is a
gravity mismatch is constrained both by numerous crustal diving rays and by Moho re£ections.
We therefore made no changes to the crustal
structure derived from the seismic analysis. We
found that reducing the mantle densities below
the WVZ (Fig. 7) in a similar way to that required
beneath the rift zone in the ICEMELT and FIRE
[9] density models allowed us to improve the ¢t of
the calculated gravity anomaly to the observed
gravity anomaly to an acceptable degree of error
(the RMS residual is 6 mGal).
2.3.2. The B96 pro¢le
The B96 seismic pro¢le [11] runs north to south
along the western £ank of the Northern Volcanic
Zone. The density model created using the published B96 crustal velocity model gives a relatively
poor ¢t to the observed Bouguer gravity anomaly
(Fig. 8), but we found that it was not necessary to
reduce mantle densities below any section of the
pro¢le in order to improve the ¢t to the gravity
data. Instead, changes in upper crustal thickness
and Moho depth at the far ends of the crustal
model where seismic constraints are poor were
su¤cient to provide a ¢t to the gravity data
Fig. 8. Results of gravity modelling along the B96 seismic
pro¢le [11]. (a) Density model derived directly from the seismic model of Menke et al. [11]. (b) Model with thinner crust
at the ends of the pro¢le where the Moho is not constrained
by seismic arrivals. (c) Fit of initial (dashed line) and modi¢ed (solid line) model to the observed (crosses) gravity
anomaly.
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
Fig. 9. (a) Land elevation above mean sea level against
Moho depth below mean sea level for points along the FIRE
[9], ICEMELT [10], B96 [11] and SIST [8] refraction pro¢les
and measurements from receiver function analysis [17].
(b) Average crustal density derived from velocity^density
conversions against Moho depth for points along the refraction pro¢les. Symbols as for (a). (c) Average crustal velocity
derived from seismic models against Moho depth for points
along the refraction pro¢les. Symbols as for (a).
each of the four seismic refraction pro¢les. Data
points were divided into three di¡erent sections :
crust lying well outside the rift zones, crust lying
within the rift zones and crust lying at the edges
of the rift zones or between two closely-spaced
sections of rift. Fig. 9a shows topography at the
sampled points against Moho depth, for the refraction pro¢les and for the receiver function results of Darbyshire et al. [17]. Although there is
419
scatter in the data, a clear linear relationship between land elevation and Moho depth can be seen
for the crust away from the rift zones, similar to
that reported by Menke [34]. Crust within the rift
zones shows a signi¢cantly di¡erent relationship,
with anomalously high topography for the measured crustal thickness. This is an indication that
the mantle below the rift zones has a di¡erent
character from the mantle elsewhere in Iceland.
Plots of land elevation against Moho depth for
crust at the edges of the rift zones lie mostly in
the ¢eld of data for regions away from the rifts,
though some points lie close to the rift zone data.
Plots of average crustal density and velocity for
the four refraction pro¢les (Fig. 9b,c) show some
scatter, which may arise from variations in the
proportion of upper crust to lower crust along
the pro¢les. However, a trend of increasing crustal density and velocity with increasing Moho
depth is apparent in the plots. This suggests that
the proportion of dense lower crust to the less
dense upper crust increases as the total crustal
thickness increases.
We invoke isostatic equilibrium across Iceland
to predict the average density of the uppermost
mantle and to calculate deviations from this average along the refraction pro¢les, using crustal
densities obtained from the seismic velocity models. If we consider the pressure at an arbitrary
depth of compensation H, for a crust of mean
density b c and a mantle of mean density b m,
the relationship:
he b c ‡ H b m ‡ hc … b c 3 b m † ˆ constant
…4†
is obtained, where he is the land elevation above
mean sea level and hc is the depth below mean sea
level to the Moho. From this relationship, the
average mantle density for Iceland can be calculated:
b c …he ‡ hc † ˆ b m hc 3 b m H ‡ constant
…5†
We plot b c …he ‡ hc † against hc for each data
point from the refraction pro¢les, and ¢t a
straight line through the points. Since b m H is
assumed to be a constant, the gradient of the
line gives the value of b m . The average value of
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
420
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
the mantle density beneath Iceland was found to
be V3170^3190 kg m33 . This value is similar to
the density of 3150 þ 60 kg m33 predicted by
Menke [34] who used the relationship between
Moho depth and land elevation to calculate the
average mantle density, assuming a simple threelayered crust in isostatic equilibrium.
In order to calculate mantle density variations
along the seismic refraction pro¢les, we assume
perfect isostatic balance and zero £exural
strength, then use Eq. 5 for each of the points
constrained by seismic data along the pro¢les,
having obtained the value of the constant in Eq.
5 for the full data set from our straight line ¢t.
For this we assume that the compensation depth
H is 60 km, the same as assumed for the gravity
modelling, since this corresponds to the maximum
likely depth of the lithosphere beneath Iceland.
The results of this analysis are shown in Fig. 10.
Variations in mantle density exist for all four pro¢les, but are small for the B96 pro¢le, which does
not intersect with a rift zone, compared to the
variations along the FIRE, ICEMELT and SIST
pro¢les. For these pro¢les, away from the rift
zones, the mantle density lies above the average
value by as much as 70 kg m33 . The positions of
the rift zones along the pro¢les coincide with regions in which the density of the uppermost mantle is signi¢cantly reduced, by as much as 90 kg
m33 below the average.
We note that the average mantle density obtained by this analysis is low compared to normal
mantle densities, and is somewhat lower than expected from geochemistry-based calculations (see
Section 2.2 for details) of an average mantle
density beneath Iceland. However, the calculated
values of mantle density in the oldest regions of
Iceland (e.g. Fig. 10; FIRE pro¢le, distances s 60
km) are less than 30 kg m33 smaller than the
density predicted from geochemistry, and therefore lie within the likely uncertainties of the calculations. In addition, the analysis described
above assumes perfect isostatic equilibrium across
Iceland and zero £exural strength in the lithosphere. In reality, there are several other factors
which may perturb the results of our calculations
of mantle density. First, some degree of dynamic
support from the underlying mantle plume is
Fig. 10. Predicted uppermost mantle densities relative to the
average value for Iceland derived assuming perfect local isostatic equilibrium (see text for details), for the four refraction
pro¢les. The average density is shown as a broken line and
the positions of the rift zones along the FIRE, ICEMELT
and SIST pro¢les are marked.
likely in Iceland, and the degree of this dynamic
support varies on a long-wavelength scale across
the island. Second, the older parts of the lithosphere beneath Iceland are likely to have some
£exural strength and, third, variations in lithospheric thickness and hence in mean mantle density across the island are expected. The exact nature of the variations in £exural strength and
lithospheric thickness is impossible to predict
due to the shifts in the positions of the rift zones
over time and the construction of the crust from
magmas emplaced at widely di¡ering ages as a
result of the rift jumps.
The factors described above will have an e¡ect
on the long-wavelength characteristics of the iso-
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
421
static balance across Iceland, and are therefore
likely to bias the result of any calculation of average mantle density beneath the island. However,
the assumption of local isostatic equilibrium is
valid when analysing the variations in mantle density on a short-wavelength scale of up to a few
tens of kilometres. We therefore consider the
abrupt decreases in the density of the mantle beneath the rift zones (e.g. Fig. 10; FIRE pro¢le,
distances 320 to +20 km) to be real and signi¢cant features.
2.5. Further gravity modelling across Iceland
Results from gravity modelling and considerations of isostatic balance along the FIRE [9], ICEMELT and SIST refraction pro¢les give an important result for further gravity modelling. The
density of the uppermost mantle is not constant
across Iceland but is reduced by 70^90 kg m33
beneath the NVZ, the WVZ and the central rift
Fig. 11. Results of gravity modelling along pro¢le GP1.
(a) Model constrained only by seismic velocity models at
their intersections with GP1. (b) Final density model. (c) Observed Bouguer gravity anomaly (crosses), calculated anomaly for model (a) (dashed line) and calculated anomaly for
model (b) (solid line).
Fig. 12. Results of gravity modelling along pro¢le GP2.
(a) Model constrained only by seismic velocity models at
their intersections with GP2. Regions of reduced mantle density are shaded. (b) Final density model. (c) Observed Bouguer gravity anomaly (crosses), calculated anomaly for model
(a) (dashed line) and calculated anomaly for model (b) (solid
line).
below Vatnajo«kull. We use this result in constructing the gravity models described below. In
addition, we assume that the behaviour of the
mantle beneath the EVZ is similar to that below
the NVZ and WVZ. Mantle densities below the
EVZ are therefore reduced in the gravity models
for pro¢les which cross this region.
Gravity modelling was carried out along seven
2D pro¢les (Fig. 1) in order to constrain the
upper crustal thickness and Moho depth in areas
where there is little or no seismic constraint. The
results are shown in Figs. 11^17. One-dimensional
(1D) velocity models derived from the seismic
pro¢les and receiver functions were used to constrain the structure along the gravity pro¢les
where they intersect with the refraction lines,
and additional seismic information was input to
the 2D models. Models derived from gravity
along pro¢les well constrained by the seismic results were also used as input to subsequent, inter-
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
422
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
secting pro¢les. The 2D structure consisted initially of a simple interpolation between the points
constrained by seismic and earlier gravity pro¢les
(Figs. 11a^17a). Seismic velocities were converted
to density using the velocity^density relationships
described in Section 2.2. The background mantle
density was taken as 3250 kg m33 and a density of
3180 kg m33 was used for the mantle below the
rift zones. The exception was for the rift directly
above the plume centre, where results from the
ICEMELT gravity modelling gave a density decrease of 90 kg m33 . In this region, we used a
mantle density of 3160 kg m33 in the models.
Long-wavelength variations in the gravity
anomaly were ¢tted by changes to the depth of
the Moho. In some cases, a simple interpolation
between the seismic models at the intersection
points was su¤cient to provide a reasonable ¢t
to the longest-wavelength gravity signals, with
depth changes made to the lower crustal section
at the ends of the pro¢les in order to ¢t a general
trend of increasing positive Bouguer anomaly towards the Icelandic coast (e.g. GP1, GP3; see Fig.
1). In other cases (e.g. GP2, GP6, GP7; see Fig.
Fig. 13. Results of gravity modelling along pro¢le GP3. Refer to Fig. 12 for plotting conventions.
Fig. 14. Results of gravity modelling along pro¢le GP4. Refer to Fig. 12 for plotting conventions.
Fig. 15. Results of gravity modelling along pro¢le GP5. Refer to Fig. 12 for plotting conventions.
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
1) changes in the depth to the Moho in positions
between the constrained points were necessary in
order to match the positions of maxima and minima in the observed gravity anomaly. Short-wavelength variations in the observed gravity anomaly
were matched by making changes to the upper
crustal structure. Many of the local maxima in
gravity are associated with central volcanoes
ë r×fajo«kull
(e.g. Askja at 100 km along GP2 and O
at 350 km along GP3). Results from seismic pro¢les show that the upper crust below most central
volcanoes is anomalously thin compared to the
average thickness in Iceland [1], and the density
models were adjusted accordingly to ¢t the local
gravity anomaly maxima. Although, in principle,
the density models could be ¢nely tuned to provide a perfect ¢t to the observed gravity anomalies, we believe that the insertion of ¢ner structure
is unwarranted. Limitations on the models are
imposed by the assumption of two-dimensionality
of structure along the pro¢les and by the likely
errors in the gravity data set. We consider the 2D
approximations to be valid across Iceland, since
tests on the ICEMELT model (see Section 2.2.1)
423
Fig. 17. Results of gravity modelling along pro¢le GP7. Refer to Fig. 12 for plotting conventions.
suggest that 3D structure is likely to contribute
less than 10 mGal to the calculated gravity
anomalies, even in the most highly 3D regions.
The models all show a good degree of ¢t to the
observed gravity anomaly, with RMS residuals of
less than 6 mGal.
3. Discussion and conclusions
3.1. The upper crust
Fig. 16. Results of gravity modelling along pro¢le GP6. Refer to Fig. 12 for plotting conventions.
Results from seismic refraction pro¢les and receiver function analysis show that the average
upper crustal thickness across Iceland is V5 km
[1], but substantial variations from this value are
observed, with a range of upper crustal thickness
of V2^11 km. Fig. 18a shows the results from the
compiled seismic data set. The base of the upper
crust is contoured where there are su¤cient data
points, but there is insu¤cient seismic coverage to
produce a detailed contour map for the whole of
Iceland due to the short wavelength of some of
the variations in upper crustal thickness. The thin-
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
424
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
nest upper crust occurs below central volcanoes
such as Kra£a, the Bärdarbunga-Gr|¨msvo«tn reë r×fajo«kull. Thicker than avergion, Askja and O
age upper crust is found at the edges of the rift
zones, e.g. east of the NVZ [9], in the northern
central highlands [10] and in the far south of Iceland [1]. The increase in seismic velocity with
depth within the upper crust arises from compaction and secondary mineralisation of basaltic lava
£ows [1,35]. The transition from upper to lower
crustal velocities is likely to be due to a combination of increased alteration of the basalts and an
increased proportion of intrusive material. Beneath the central volcanoes, the high-temperature
gradients at the time of emplacement may allow
mineral alteration to occur at shallower depths
than normal, and higher proportions of intrusives
also occur at shallower depths, resulting in an apparent thinning of the upper crust. The thick
upper crust at the edges of the rift zones is probably a result of ridge jumps. When the spreading
axis jumps into old crust, new lava £ows spread
out over a large distance from the ¢ssures at the
rift axis. The temperature gradient within the old
upper crust is lower than that within the upper
crust in the rift zone, so it can be buried deeply by
new lava £ows without as much thermal alteration, creating a thicker seismic upper crust.
3.2. Moho depth variation across Iceland
Fig. 18b shows the crustal thickness across Iceland. The map was created by compiling all the
available Moho depth results from analyses of
seismic data (receiver functions and wide-angle
pro¢les; see Table 1), together with the crustal
thicknesses obtained by gravity modelling along
the `GP' pro¢les (see Fig. 1 for locations of seismic constraint and gravity pro¢les). The resulting
data set was input to the GMT [36] software
package for conversion to a gridded format which
was used to form the map shown.
The thickest crust (V40 km) is found directly
above the centre of the plume (Fig. 18b). This
thick crust probably arises from enhanced melting
due to a combination of high mantle temperatures
and active upwelling [2] in a narrow ( 6 200 km
diameter [37,38]) plume core. Along the Northern
Volcanic Zone, the crust thins rapidly away from
the plume centre as the e¡ect of active upwelling
becomes less signi¢cant with increasing distance
from the plume core. At Kra£a (see Fig. 1),
120 km from the plume centre, the mechanism
of melt generation is thought to be purely passive
decompression of the hot mantle in response to
plate separation, from the observation that geochemical and seismic estimates of the crustal
thickness are in agreement [9,39]. Across the
northern half of the NVZ, the crust thins to
V20 km below Kra£a. This may result from a
combination of changes in melt productivity in
this section of the rift and the e¡ect of the ridge
jump at V7^3 Ma [9]. Melt productivity in the
NVZ is probably reduced by the e¡ect of conductive heat loss into the adjacent older lithosphere.
In addition, a southerly component of the postulated eastward drift [6] of the Iceland mantle
plume may occur, since the Icelandic rift zones
appear to have shifted both eastwards and southwards over time [40]. A southerly drift of the
plume centre would cause the crustal thickness
at a given latitude within the NVZ to be less
than that at the same latitude in the surrounding
older crust, since the melt productivity at a point
on the rift decreases with increasing distance from
the plume centre. East of the Vatnajo«kull icecap,
a band of thick crust (V35^36 km) joins the
plume centre to the westernmost extent of the
Faroe^Iceland Ridge. Whether this thick crust
represents the track of the plume centre is uncer-
C
Fig. 18. (a) Map of upper crustal thickness measurements across Iceland, compiled from seismic observations. The data are contoured where a su¤cient density of data points (small black circles) is available. Sparser data points are plotted as large circles
and coloured according to the same scale as the contoured data. (b) Contour map of depth to the Moho, using a combination
of results from seismic pro¢les, receiver function analysis and gravity modelling. The contour interval is 5 km. Locations of
the seismic pro¢les, broadband seismograph stations and gravity pro¢les from which Moho depth data were taken are shown in
Fig. 1.
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
425
426
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
tain, since the numerous ridge jumps make it dif¢cult to track the past positions of the plume
centre across Iceland. A general west to east trend
of thicker crust across central Iceland compared
to the south and north of the island is apparent
(Fig. 18b). This is probably caused by the eastward drift of the underlying mantle plume with
respect to the Mid-Atlantic Ridge [6].
Studies of the structure of the Faroe^Iceland
Ridge (FIR ; e.g. [21]) and Greenland^Iceland
Ridge (GIR) [20] suggest crustal thicknesses of
25^30 km along the FIR and 32^38 km along
the GIR, with the thickest crust close to the continental margins. The crust beneath Vatnajo«kull is
signi¢cantly thicker than that beneath the aseismic ridges. One reason for the di¡erence in crustal
thickness may be that some of the crust on the
aseismic ridges has been eroded during low sea
levels in the past. Alternatively, an increase in
the temperature of the Iceland plume would enhance melt production. Using the model of [41],
an increase in crustal thickness of 5 km may be
attributed to a change of 50³C in the temperature
of the mantle, assuming passive upwelling. Above
the core of the plume, where active upwelling
plays a part in melt generation, the same increase
in crustal thickness could be produced by a signi¢cantly smaller temperature change. Studies of
the structure of the Reykjanes Ridge [46] suggest
that the temperature of the plume £uctuates by
V30³C on a time scale of 5^10 Myr [42,43]. Crustal thickening may also be achieved by the addition of new material (intrusive and extrusive) to
existing crust. This may occur beneath Vatnajo«kull due to changes in the positions of the rift
zones. Vatnajo«kull lies at the northern limit of
the EVZ, where melt generation began at V2
Ma, but little spreading is thought to have occurred so far.
3.3. Structure of the uppermost mantle
Few direct measurements of the seismic velocity
of the uppermost mantle beneath Iceland have
been made. Bjarnason et al. [8] report an apparent
velocity of 7.6^7.7 km s31 for ray paths crossing
the WVZ, whereas Menke et al. [11] report an
apparent velocity of 8.0 km s31 for a path cross-
ing central Iceland. Synthetic seismogram modelling of the FIRE refraction data [9] suggests a
di¡erence of 0.3 km s31 in the uppermost mantle
velocity between the NVZ and older parts of eastern Iceland.
Gravity modelling along the ICEMELT and
SIST (see above) refraction pro¢les and along
the FIRE [9] refraction pro¢le shows that the
mantle below the rift zones is characterised by
lower densities than those found elsewhere beneath Iceland. In addition, mass balance calculations require the uppermost mantle to be less
dense below the rift zones than below older crust
if we assume that the whole of Iceland is in local
isostatic equilibrium. Typically, the density anomaly in the uppermost mantle beneath the rift zones
is 60^90 kg m33 , corresponding to a temperature
increase of 450^700³C below the rift zones if all of
the density reduction is assumed to arise from
temperature variations alone. The anomaly may
be explained by the di¡erence in temperature between the hot asthenospheric mantle which lies
directly beneath the crust in the rift zones and
the cool lithospheric mantle which lies directly
beneath the crust under older regions of Iceland.
The contrast is likely to be most pronounced in
southeast Iceland, where the active rift zone cuts
through lithosphere of V25 Myr age.
Partial melt in the mantle below the rift zones
may also contribute to the density anomaly. However, studies based on geochemical data and
mathematical models of melt transport (e.g. [9])
suggest that the amount of melt resident in the
mantle is less than 1%, so the e¡ect on mantle
density is likely to be small.
Acknowledgements
Gunnar Gudmundsson (Icelandic Meteorological O¤ce) supplied the GMT [36] scripts used in
Fig. 1, using data taken from Einarsson and
S×mundsson's map [44]. Hjalmar Eysteinsson
(Orkustofnun Ièslands) supplied the gridded Bouguer gravity data set. We also thank John Maclennan, Nick Weir and Brynd|¨s Brandsdöttir
for useful discussions, and G. Clitheroe, I. Grevemeyer and an anonymous reviewer for their help-
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
ful comments. Dept. of Earth Sciences, Cambridge, contribution number 6000.[AC]
References
è .G. Flövenz, Seismic structure of the Icelandic crust
[1] O
above Layer Three and the relation between body wave
velocity and the alteration of the basaltic crust, J. Geophys. 47 (1980) 211^220.
[2] R. White, D. McKenzie, Mantle plumes and £ood basalts,
J. Geophys. Res. 100 (1995) 17543^17585.
[3] R.S. White, Rift-plume interaction in the North Atlantic,
Philos. Trans. R. Soc. Lond. A 355 (1997) 319^339.
[4] C.J. Wolfe, I.Th. Bjarnason, J.C. VanDecar, S.C. Solomon, Seismic structure of the Iceland mantle plume, Nature 385 (1997) 245^247.
[5] H. Jöhannesson, Evolution of rift zones in western Iceland, Nättürufrdingurinn 50 (1980) 13^31.
[6] W.J. Morgan, Hotspot tracks and the opening of the Atlantic and Indian Oceans, in: C. Emiliani (Ed.), The Sea,
7: The Oceanic Lithosphere, Wiley, New York, 1981.
[7] H. Gebrande, H. Miller, P. Einarsson, Seismic structure
of Iceland along RRISP-Pro¢le 1, J. Geophys. 47 (1980)
239^249.
è .G. Flövenz, D. Caress,
[8] I.Th. Bjarnason, W. Menke, O
Tomographic image of the Mid-Atlantic plate boundary
in southwestern Iceland, J. Geophys. Res. 98 (1993) 6607^
6622.
[9] R.K. Staples, R.S. White, B. Brandsdöttir, W. Menke,
P.K.H. Maguire, J.H. McBride, Faroe-Iceland Ridge Experiment 1. Crustal structure of northeastern Iceland, J.
Geophys. Res. 102 (1997) 7849^7866.
è .G. Flö[10] F.A. Darbyshire, I.Th. Bjarnason, R.S. White, O
venz, Crustal structure above the Iceland mantle plume
imaged by the ICEMELT refraction pro¢le, Geophys.
J. Int. 135 (1998) 1131^1149.
[11] W. Menke, M. West, B. Brandsdöttir, D. Sparks, Compressional and shear velocity structure of the lithosphere
in northern Iceland, Bull. Seism. Soc. Am. 88 (1998)
1561^1571.
[12] G. Pälmason, Crustal Structure of Iceland from Explosion Seismology, Soc. Sci. Isl. Reykjav|¨k, 1971, 187 pp.
[13] G. Pälmason, Seismic refraction investigation of the basalt lavas in northern and eastern Iceland, Jo«kull 13
(1963) 40^60.
è . Gudmundsson, B. Brandsdöttir, W. Menke, G.E. Sig[14] O
valdason, The crustal magma chamber of the Katla volcano in south Iceland revealed by 2-D seismic undershooting, Geophys. J. Int. 119 (1994) 277^296.
[15] E. Tryggvasson, Crustal structure of the Iceland region
from dispersion of surface waves, Bull. Seism. Soc. Am.
52 (1962) 359^388.
[16] Z.J. Du, G.R. Foulger, The crustal structure beneath the
northwest fjords, Iceland, from receiver functions and surface waves, Geophys. J. Int. 139 (1999) 419^432.
427
[17] F.A. Darbyshire, K.F. Priestley, R.S. White, R. Stefänsson, G.B. Gudmundsson, S.S. Jakobsdöttir, Crustal structure of central and northern Iceland from analysis of teleseismic receiver functions, Geophys. J. Int., in press.
[18] P. Gold£am, W. Weigel, B.D. Loncarevic, Seismic structure along RRISP - Pro¢le I on the southeast £ank of the
Reykjanes Ridge, J. Geophys. 47 (1980) 250^260.
[19] J.R. Smallwood, R.S. White, T.A. Minshull, Sea-£oor
spreading in the presence of the Iceland plume: the structure of the Reykjanes Ridge at 61³40PN, J. Geol. Soc. 152
(1995) 1023^1029.
[20] I.D. Reid, T. Dahl-Jensen, W.S. Holbrook, H.C. Larsen,
P.B. Kelemen, J.R. Hopper, J. Korenaga, R. Detrick, G.
Kent, 32^38 km thick ma¢c igneous crust beneath the
Greenland-Iceland Ridge, EOS Trans. AGU 78 (Fall
Meet. Suppl.) (1997) F656.
[21] J.R. Smallwood, R.K. Staples, K.R. Richardson, R.S.
White, FIRE Working Group, Crust generated above
the Iceland mantle plume: from continental rift to oceanic
spreading center, J. Geophys. Res. 104 (1999) 22885^
22902.
[22] R.K. Staples, Crustal structure above the Iceland mantle
plume, Ph.D. thesis, University of Cambridge, Cambridge, 1997, 253 pp.
[23] C.A. Zelt, Documentation of `tramp' and related programs, at: http://zephyr.rice.edu/department/faculty/zelt/
rayinv.html, 1992.
[24] W.J. Ludwig, J.E. Nafe, C.L. Drake, Seismic refraction,
in: A.E. Maxwell (Ed.), The Sea, 4, Part 1: New Concepts
of Sea Floor Evolution, Wiley, New York, 1970, pp. 53^
84.
[25] R.L. Carlson, C.N. Herrick, Densities and porosities in
the oceanic crust and their variations with depth and age,
J. Geophys. Res. 95 (1990) 9153^9170.
[26] N.I. Christensen, R.H. Wilkins, Seismic properties, density, and composition of the Icelandic crust near Reydarfjo«rdur, J. Geophys. Res. 87 (1982) 6389^6395.
[27] A. Hynes, D.B. Snyder, Deep-crustal mineral assemblages
and potential for crustal rocks below the Moho in the
Scottish Caledonides, Geophys. J. Int. 123 (1995) 323^
339.
[28] G. Thorbergsson, I.Th. Magnüsson, G. Pälmason, Gravity data and a gravity map of Iceland, in: OS-93027/JHD07, Orkustofnun Ièslands, 1993.
[29] H. Eysteinsson, K. Gunnarsson, Maps of gravity, bathymetry and magnetics for Iceland and surroundings, in:
OS-95055/JHD-07, Orkustofnun Ièslands, 1995.
[30] M.T. Gudmundsson, J. Milsom, Gravity and magnetic
studies of the subglacial Gr|¨msvo«tn volcano, Iceland: Implications for crustal and thermal structure, J. Geophys.
Res. 102 (1997) 7691^7704.
[31] R.S.J. Sparks, P. Meyer, H. Sigurdsson, Density variation
amongst mid-ocean ridge basalts: implications for magma
mixing and the scarcity of primitive lavas, Earth Planet.
Sci. Lett. 46 (1980) 419^430.
[32] B. Brandsdöttir, W. Menke, P. Einarsson, R.S. White,
R.K. Staples, Faroe-Iceland Ridge Experiment 2. Crustal
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart
428
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
F.A. Darbyshire et al. / Earth and Planetary Science Letters 181 (2000) 409^428
structure of the Kra£a central volcano, J. Geophys. Res.
102 (1997) 7867^7886.
B.-Y. Kuo, D.W. Forsyth, Gravity anomalies of the
ridge-transform system in the South Atlantic between
31³ and 34.5³S: Upwelling centres and variations in crustal thickness, Mar. Geophys. Res. 10 (1988) 205^232.
W. Menke, Crustal isostasy indicates anomalous densities
beneath Iceland, Geophys. Res. Lett. 26 (1999) 1215^
1218.
è .G. Flövenz, K. Gunnarsson, Seismic crustal structure
O
in Iceland and surrounding area, Tectonophysics 189
(1991) 1^17.
P. Wessel, W.H.F. Smith, Free software helps map and
display data, EOS 72 (1991) 445^446.
R.M. Allen, G. Nolet, W.J. Morgan, K. Vogfjo«rd, B.H.
Bergsson, P. Erlendsson, G.R. Foulger, S. Jakobsdöttir,
B.R. Julian, M. Pritchard, S. Ragnarsson, R. Stefänsson,
The thin hot plume beneath Iceland, Geophys. J. Int. 137
(1999) 51^63.
G. Ito, Y. Shen, G. Hirth, C.J. Wolfe, Mantle £ow, melting and dehydration of the Iceland mantle plume, Earth
Planet. Sci. Lett. 165 (1999) 81^96.
H. Nicholson, D. Latin, Olivine tholeiites from Kra£a,
Iceland: evidence for variations in melt fraction within a
plume, J. Petrol. 33 (1992) 1105^1124.
K. Smundsson, Outline of the geology of Iceland, Jo«kull
29 (1979) 7^28.
J.W. Bown, R.S. White, Variation with spreading rate of
oceanic crustal thickness and geochemistry, Earth Planet.
Sci. Lett. 121 (1994) 435^449.
R.S. White, J.W. Bown, J.R. Smallwood, The temperature of the Iceland plume and origin of outward propagating V-shaped ridges, J. Geol. Soc. Lond. 152 (1995)
1039^1045.
C. Richardson, D. McKenzie, Radioactive disequilibria
from 2D models of melt generation by plumes and ridges,
Earth Planet. Sci. Lett. 128 (1994) 425^437.
[44] P. Einarsson, K. Smundsson, Earthquake epicentres
1982^1985 and volcanic systems in Iceland (map), in:
Th.I. Sigfürsson (Ed.), Iè hlutarins edli, Festschrift for
Thorbjo«rn Sigurgeirsson, Reykjav|¨k, Iceland, 1987.
[45] E. Sturkell, B. Brandsdöttir, H. Shimamura, M. Mochizuki, Seismic crustal structure along the Axarfjo«rdur
trough at the eastern margin of the Tjo«rnes Fracture
Zone, N-Iceland, Jo«kull 42 (1992) 13^23.
[46] J.R. Smallwood, R.S. White, Crustal accretion at the
Reykjanes Ridge, 61³-62³N, J. Geophys. Res. 103 (1998)
5185^5201.
[47] H. Shimamura, R. Stefansson, M. Mochizuki, T. Watanabe, H. Shiobara, G. Gudmundsson, P. Einarsson,
Northern Reykjanes Ridge microseismicity revealed by
dense OBS arrays, in: B. Thorkelsson (Ed.), ESC - Papers
presented at the XXV General Assembly, September 9^14,
1996, p 491.
[48] W. Menke, B. Brandsdöttir, P. Einarsson, I.Th. Bjarnason, Reinterpretation of the RRISP-77 Iceland shearwave pro¢les, Geophys. J. Int. 126 (1996) 166^172.
[49] M. Ritzert, W.R. Jacoby, On the lithospheric seismic
structure of Reykjanes Ridge at 62.5³N, J. Geophys.
Res. 90 (1985) 10117^10128.
[50] S.M. Zverev, I.P. Kosminskaya, G.A. Krasilstchikova,
G.G. Mikhota, The crustal structure of Iceland and the
Iceland-Faeroe-Shetland region, Soc. Sci. Isl. Greinar V
(1976) 73^93.
[51] K. Mackenzie, J. McClain, J. Orcutt, Constraints on crustal structure in eastern Iceland based on extremal inversions of refraction data, J. Geophys. Res. 87 (1982) 6371^
6382.
[52] M. Ba®th, Crustal structure of Iceland, J. Geophys. Res. 65
(1960) 1793^1807.
[53] M.H.P. Bott, K. Gunnarsson, Crustal structure of the
Iceland-Faeroe Ridge, J. Geophys. 47 (1980) 221^227.
EPSL 5562 16-8-00 Cyaan Magenta Geel Zwart