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Earth and Planetary Science Letters 224 (2004) 193 – 211
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Regional anomalies of sediment thickness, basement depth and
isostatic crustal thickness in the North Atlantic Ocean
Keith E. Louden a,*, Brian E. Tucholke b,1, Gordon N. Oakey c,2
a
Department of Oceanography, Dalhousie University, Halifax, NS, Canada B3H 4J1
Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
c
Geological Survey of Canada-Atlantic Region, P.O. Box 1006, Bedford Institute of Oceanography, Dartmouth, NS, Canada B3Y 4A2
b
Received 3 November 2003; received in revised form 30 April 2004; accepted 5 May 2004
Abstract
We calculate the anomalous basement topography for the North Atlantic Ocean from 30j to 70jN latitude and from 0j to 70jW
longitude at a resolution of roughly 6 6 km, using grids of total sediment thickness and observed and predicted sea-floor
bathymetry to correct for the effects of isostatic sediment loading and lithospheric age. Plotting this residual topography for various
plate reconstructions during opening of the North Atlantic, we delineate consistent patterns of basement highs related to variations
in hotspot-related volcanism. In addition to Iceland and the Azores, we recognize three centers of excess volcanism at the midAtlantic ridge: the Milne Seamounts and Azores-Biscay Rise ( f 75 – 40 Ma), the Southeast Newfoundland Ridge and MadeiraTore Rise ( f 130 – 110 Ma), and the East and West Thulean Rises ( f 60 – 50 Ma). The duration of volcanic activity ranges from
8 to 10 m.y. (Thulean Rises) to 60 m.y. (Iceland) and thus it appears that both long- and short-lived hotspots coexist, even in
relatively close proximity. In contrast, during the period 110 – 60 Ma we observe little excess volcanism during either continental
breakup or seafloor spreading.
We estimate isostatic crustal thickness from the anomalous basement depths, after first removing dynamic effects
created by mantle flow. Maximum thicknesses of volcanic features, from 30 km beneath the Greenland – Iceland – Faeroe
ridge to f 15 km beneath the Azores-Biscay Rise, are broadly consistent with seismic data and predictions of
decompression melting. Widths of volcanic features indicate that thickening primarily occurs within 100 – 200 km of
hotspots except along continental margins that rifted at the time of the hotspot activity (i.e. East Greenland and the HattonRockall Bank). We observe conjugate structures south of Greenland and Edoras Bank, where excess volcanism appears to
have extended beyond the margin proper and into oceanic crust. Similar conjugate features appear in the Labrador Sea
south of Davis Strait. Finally, we identify anomalous oceanic regions adjacent to some continental margins, where
unusually low values of predicted crustal thickness suggest either additional variations in plate properties or non-isostatic
effects within the mantle.
D 2004 Elsevier B.V. All rights reserved.
Keywords: North Atlantic; hot spots; plate reconstructions; sediment thickness; basement depth
* Corresponding author. Tel.: +1-902-494-3452.
E-mail addresses: [email protected] (K.E. Louden), [email protected] (B.E. Tucholke), [email protected] (G.N. Oakey).
1
Tel.: +1-508-289-2494.
2
Tel.: +1-902-426-3549.
0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2004.05.002
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K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
1. Introduction
In this study, we present and interpret images of
anomalous basement topography for the North Atlantic Ocean from 30j to 70jN latitude and from 70jW to
0j longitude. A digital grid of sediment thickness [1] is
presented and together with the seafloor bathymetry
[2] is used to correct for the load of the sediment cover.
This region contains a wide variety of geological
features that include the Iceland and Azores hotspots,
volcanic and non-volcanic rifted continental margins
with deep sedimentary basins, active and extinct
spreading centers with a range of morphologies, fracture zones, aseismic plateaus, and seamounts. These
features are characterized by anomalous topography,
i.e. by basement depth that indicates departure from
standard oceanic crustal thickness and underlying
mantle thermal structure. Seafloor spreading magnetic
anomalies and poles of plate rotations are well determined in the region of this study. Thus we are able to
reconstruct positions of basement anomalies at the
time of their formation and identify conjugate structures that were created by episodes of anomalous
ridge-crest volcanism.
Long-lasting volcanism that began after f 60
Ma at the Iceland and Azores hotspots is apparent
in this study, but we also find evidence for three
more localized and shorter-lived pulses of volcanism. These pulses formed conjugate structures
comprising the Milne Seamounts and Azores-Biscay Rise (at f 75– 40 Ma), the Southeast Newfoundland Ridge and Madeira-Tore Rise ( f 130 –
110 Ma), and the East and West Thulean Rises
( f 60 – 50 Ma). It is notable that the period
110 –60 Ma is characterized mostly by normal
volcanic activity along the spreading centre and
by formation of non-volcanic rifted margins, in
contrast to the formation of volcanic margins that
followed.
We find that simple isostatic thickening of the
crust can explain basement depth anomalies close
to centers of hotspot volcanism (i.e., within 100 –
200 km), although this does not hold true along
volcanic margins where the crustal thickening
extends much greater distances along the strike of
the margins. In contrast, longer-wavelength depth
anomalies in the ocean basins cannot be explained
by crustal thickness variations; rather, they require
variations in lithospheric thermal structure and/or
dynamic topography in response to deeper mantle
convection.
2. Sediment thickness
A consistent digital grid of sediment thickness
has been produced for the North Atlantic region
from 30jN to 70jN latitude and 70jW to 2jE
longitude [1]. The compilation was constructed from
digitized contours extracted from 21 published maps
of basement depth and sediment thickness [3 –23];
these maps were originally produced from seismic
and well data and they have varying degrees of
resolution. Conversion of basement-depth maps to
sediment thickness was made when required using
ETOPO-5 digital bathymetry [26], with a minimum
sediment thickness set at 1 m. Contours from
individual map sheets were digitized and interpolated using a minimum curvature algorithm [24,25].
These data were then gridded with a cell resolution
of 0.05 0.05 degrees, (a grid of roughly 6 km).
Some sub-areas with closely spaced contours were
gridded initially at a higher resolution and then
regridded before merging with other map areas.
Fig. 1. Location of datasets used to construct the digital sediment
thickness map (after Oakey and Stark [1]). Numbers are keyed to
publications [3 – 23] in the references. Areas where overlapping
maps were merged are indicated by ‘‘M’’.
K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
Each grid file was compared with the original
contours and control points were added so that
interpolated values did not exceed the contour
values of the original map. Where overlaps existed
between separate maps, the overlaps were trimmed
to produce smooth transitions from map to map.
The overlapping segments then were superimposed
and recontoured by hand to avoid any discrepancies
between contours. Generally, the manipulation needed to join map sheets was minimal and well within
195
original interpretation errors. No major discrepancies
were found between overlapping maps. Finally, the
overlapping regions were digitized, interpolated,
gridded and merged into the master file with the
adjacent grids.
Data coverage and references are shown in Fig. 1
and a color display of the gridded sediment thickness
data is shown in Fig. 2. Sediment distribution is
controlled by the following general factors, as previously described by Tucholke and Fry [5]: (a) the
Fig. 2. Map of sediment thickness with values indicated by the color scale bar, plotted using GMT software [82]. See text for explanation of the
gridding technique. Thin lines are 2000 and 4000 m isobaths from gridded bathymetry of Smith and Sandwell [2]. Features discussed in text are:
Erik drift (ED), Feni drift (FD), Gardar drift (GD), LS (Labrador Sea), NR (Southeast Newfoundland Ridge), RR (Reykjanes Ridge).
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K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
tectonic history and age of the crust, (b) the nature
and location of sediment sources, (c) structural trends
in the basement, and (d) processes delivering sediments to depocenters.
The greatest variations in sediment thickness
occur on the continental margins, as a result of
varying amounts of tectonic extension and subsidence (see [28] for a more detailed description of
sediment thickness variations offshore eastern Canada). Near-zero thicknesses occur over unrifted
basement blocks, while sediments more than 10
km thick commonly fill adjacent rift basins. Conjugate margins typically show asymmetric sediment
distributions; thicker sediments occur in basins of
the eastern margins of Labrador, Newfoundland and
Nova Scotia than in the conjugate margins of West
Greenland, NW Europe and Morocco, respectively.
On oceanic crust, sediment thickness generally
increases with crustal age. The youngest crust along
the mid-ocean spreading center is devoid of seismically detectable sediment cover; whereas uppermost
Paleocene and Eocene oceanic crust along southeast
Greenland and Rockall has sediment thicknesses of
1.5– 2.0 km, and the oldest oceanic crust adjacent to
Nova Scotia, Morocco and Newfoundland (Upper
Jurassic to Cretaceous) has thicknesses of 3– 6 km.
The general pattern of increasing sediment thickness with increasing crustal age can be modified
substantially by the availability of sediments. For
example, increased thickness relative to crustal age
in the northernmost basins is caused by significant
sediment input from glaciation, pro-glacial outwash,
and resulting gravity flows to the deep basin (e.g.
Labrador Sea). These glacial effects are more pronounced in the western than in the eastern North
Atlantic. Off eastern Newfoundland and Nova Scotia,
sediments over ocean crust are significantly thicker
than over conjugate ocean crust in the eastern North
Atlantic.
Deep currents also affect sediment distribution on
oceanic crust by depositing often thick drifts downstream from sources of sediment supply. A prominent
example is on the east flank of the Reykjanes Ridge,
where sediments are thick relative to conjugate crust
on the west flank. This thick sediment drift (Gardar
drift) was deposited by westward-intensified, contourfollowing bottom currents emanating from the Norwegian –Greenland Sea [27]. Other examples are the
Erik drift off the southern tip of Greenland, the Feni
drift in Rockall Trough, and thick drift deposits on the
Southeast Newfoundland Ridge (Fig. 2).
At smaller scales, sediment thickness is strongly
controlled by the original structural configuration of
the underlying basement. Structural lows (e.g. fracture
zones) become filled while structural highs (e.g.
seamounts) retain little sediment cover. Sediment
thickness in such areas generally is a direct reflection
of the underlying crustal topography.
3. Basement depth anomaly
The major purpose of this paper is to map variations in basement depth and compare them to a model
basement that has no sediment and no variation in
crust and mantle structure. To do this, we correct the
basement depth by removing the sediment load with a
simple model of isostatic adjustment used by Crough
[29]. Because Crough’s formulation is in reflection
time, we first convert the sediment-thickness map to
travel-time using the relationship H(t) = Vot + 0.5kt2,
where t is the one-way travel-time beneath the seafloor, Vo = 1.63 km/s, and k = 1.14 km/s2, based on
average North Atlantic values [8]. The sediment
velocity and isostatic correction factors are primarily
constrained by sonobuoy and borehole data for sediment thickness < 2000 m [29]. Adjustment of basement depth from the isostatic model amounts to
adding 600 m to seafloor depth for each second of
two-way reflection time through the sediment.
Because sediment thicknesses are substantially
greater than 2000 m on many of the continental
margins, we compare Crough’s [29] borehole constraints and model against results based on seismic
refraction data over regions of thick sediment cover
off the Labrador [30] and Nova Scotian [31] margins
(Fig. 3). This comparison shows agreement for the
thinner sediments but progressively larger adjustment
factors (650 – 750 m/s reflection time) for thicker
sediments. This variation is consistent with a value
of 675 m determined for the COST B-2 borehole [29],
and it probably is an effect of velocities increasing
proportionally faster than densities in the deeper sediments. We compared a grid of basement depths
derived using the 600 m correction factor against
basement depths (5 – 7 km) determined in the refrac-
K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
tion profile offshore Nova Scotia[31]. There is
generally good agreement, although the gridded values are systematically deeper byf 100 ms. Because
the gridded values are already deeper than the refraction depths, we decided that use of a higher correction
factor was not warranted. Thus, all sediment corrections were made using a value of 600 m per second
reflection time. These corrections were added to a grid
of water depths from Smith and Sandwell[2] to
determine adjusted basement depth.
We next corrected for expected variations in
unloaded basement depth predicted by a model of
conductive cooling of the underlying lithosphere as a
function of its age, again assuming local isostasy. We
first produced a grid of basement age using Mueller
et al.’s [32] magnetic isochrons and the time scale of
Cande and Kent [33]. This grid was also used to
define the limits of oceanic crust. Near the continental boundaries this map is only approximate because
it does not consider complex transitional regions that
are known to occur between oceanic and continental
crust, especially at non-volcanic margins (e.g.[34]).
Model depths, shown inFig. 4, were calculated from
the age grid using the lithospheric thermal model
GDH1 of Stein and Stein [35], with an assumed zero
age depth of 2600 m. This model was developed to
match observed worldwide depth and heat flow, in
particular basement depths that are shallower and
heat flow that is higher on old seafloor than predicted from previous models (e.g. the plate model of
Parsons and Sclater[36] or the half-space model of
Davis and Lister [37]). This approach does not
consider the effects of other thermal processes (e.g.
mantle plumes), and thus it minimizes basement
anomalies on old seafloor that might result from
them.
By removing basement depth predicted by the
GDH1 model from sediment-corrected basement
depth, we produced a plot of anomalous basement
depth as shown inFig. 5. As expected, there are
significant regions of elevated basement topography,
most notably in the areas of the Iceland and Azores
hot-spots. Regions of excess basement depth are
70°N
13
21
13 5
25 34
21
5 13
21
60°N
21
34 25
25 34
50°N
25 13
40°N
M
30°N
70°W
-5600
25
M1
6 M0
60°W
-5000
-4500
34 21
21
5 5 13 25
34
34
M1 M
6 0
M2 5
Fig. 3. Depth corrections for removal of sediment load above
basement, assuming local isostasy; corrections are added to seafloor
depth to obtain unloaded basement depth. Data for ODP and COSTB2 boreholes from Crough[29]. Corrections are compared with
results derived from seismic refraction profiles off the Labrador
(LAB-R) [30] and Nova Scotia (NS-R) [31] margins. The data
values fall between correction factors of 600 and 750 m/s of twoway travel time.
197
50°W
40°W
-3500
30°W
-2500
20°W
10°W
0°
G DH1 B athymetry (m)
Fig. 4. Theoretical depth of unsedimented ocean crust derived using
the plate model of Stein and Stein
[35] with a zero-age depth of
2600 m. Ages are determined from the indicated magnetic isochrons
of Mueller et al. [32] and the time scale of Cande and Kent
[33].
Plots for Figs. 4 – 8 and 10, 11 used GMT software [82].
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K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
Fig. 5. Residual basement depth anomaly relative to the GDH1 plate model [35]. Dashed lines are magnetic isochrons shown in Fig. 4, and thin
lines are 2000 m and 4000 m isobaths as shown in Fig. 2. Features discussed in text are ABR (Azores-Biscay Rise), AGFZ (Azores-Gibraltar
Fracture Zone), ATJ (Azores Triple Junction), CGFZ (Charlie-Gibbs Fracture Zone), CI (Canary Islands), CP (Cruiser Plateau), CRS (Corner
Rise Seamounts), DS (Davis Strait), EG (East Greenland Margin), ETR/WTR (East/West Thulean Rise), FIR (Faeroe – Iceland Ridge), GIR
(Greenland – Iceland Ridge), GMS (Great Meteor Seamount), HRB (Hatton-Rockall Bank), KT (King’s Trough), LS (Labrador Sea), MI
(Madeira Island), MS (Milne seamounts), MTR (Madeira-Tore Rise), NAFZ (Newfoundland-Azores Fracture Zone), NES (New England
Seamounts), NR (Southeast Newfoundland Ridge), RR (Reykjanes Ridge), SAP (Sohm Abyssal Plain), and SGH (South Greenland High).
much more limited and they appear primarily beneath the Sohm and Biscay abyssal plains and
offshore Portugal. If we used other thermal models,
they would affect regional anomalies primarily at
crustal ages older than chron 25 (56 Ma) (i.e. the
regions of excess basement depth). For example, use
of Parsons and Sclater’s [36] plate model would
reduce the negative anomalies in the Sohm Abyssal
Plain by about 500 m, although a substantial region
of excess depth would remain. This model would
also increase the regions of elevated topography in
the oldest crust between the Charlie-Gibbs (CGFZ)
K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
and Newfoundland-Azores-Gibraltar (N-AGFZ) fracture zones, although to a lesser degree. However,
our discussion of basement topography in the following section is primarily qualitative and deals
with relative crustal depths, so it matters little which
thermal model is used.
4. Plate reconstructions
Our intent is to discuss basement depth anomalies that formed during the opening of the North
Atlantic Ocean. To do this, we plot the anomalies in
Figs. 6 –8 at reconstructed positions for the magnetic isochrons used to define the age grid, back to
chron M0. A small region north of Iceland is
omitted in the reconstructions to avoid complexities
related to an extinct spreading center (Aegir Ridge)
199
within the Norwegian basin. The map of present
basement anomalies (Fig. 5) was first split into 6
plates (North America, Greenland, Eurasia/Rockall,
Porcupine, Iberia and Africa). The separate plates
were then rotated using poles of rotation relative to
the North American plate (Table 1 and [38 –43])
and they were joined at the relevant isochron. This
result is simplified because the depth data for the
rotated plates are not regridded after rotation. Instead, the rotated depth data are re-scaled to achieve
a best fit of the actual rotated and the re-projected
positions of isochrons and coastlines. Most rotations
are east –west, rather than north – south, so distortion
of the original Mercator grid is not significant for
the large regional scale of this study.
The following sections describe primary features of
the depth anomalies in the reconstructions beginning
with chron M0.
Fig. 6. Basement depth anomaly reconstructed at chrons M0 and 34. Thin lines are magnetic isochrons (shown in Fig. 4) and isobaths (shown in
Fig. 2). Abbreviations for features discussed in text (additional to those given in Fig. 5) are BTJ (Biscay Triple Junction), IAP (Iberia Abyssal
Plain), NGFZ (Newfoundland-Gibraltar Fracture Zone), NB (Newfoundland Basin), NS (Newfoundland Seamounts), TAP (Tagus Abyssal
Plain) and TS (Tore Seamount).
200
K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
Table 1
Poles of rotation relative to North America
Greenland
Eurasia Rockall
Porcupine
Iberia
Africa
Chron
5
13
21
25
34
M0
M16
M25
References
Lat
Long
Deg
Lat
Long
Deg
Lat
Long
Deg
Lat
Long
Deg
Lat
Long
Deg
0
0
0
68
137
2.5
68
137
2.5
68
137
2.5
79.08
77.95
2.41
0
0
0
68
129
7.78
68
129
7.78
48.06
143.5
7.2
76.41
7.12
9.81
62.8
91.95
2.61
67.12
137.28
10.94
62.55
142.01
10.25
73.38
129.05
11.04
74.51
4.83
15.32
24.48
137.25
3.12
63.25
143.89
14.15
57.15
148.19
13.22
72.72
135.5
14.29
80.6
0.5
18.07
65.3
122.45
11
76.23
148.8
21.83
72.82
152.76
20.6
87.18
57.43
24.67
76.55
20.73
29.6
67.5
118.48
13.78
69.67
154.26
23.17
69.67
154.26
23.17
68.88
15
50.62
66.09
20.18
54.45
0
0
0
0
0
0
0
0
0
66.9
12.93
60.45
66.24
18.33
59.71
0
0
0
0
0
0
0
0
0
66.9
12.93
60.45
66.7
15.85
64.9
[40]
4.1. Chrons M0 (120 Ma—early Aptian) and 34 (83
Ma—early Campanian) (Fig. 6)
Major elevations are associated with the Southeast Newfoundland Ridge (NR) and Madeira-Tore
Rise (MTR) at the junction of the Mid-Atlantic
Ridge (MAR) with the Newfoundland-Gibraltar
fracture zone (NGFZ). These depth anomalies are
attributed to a period of enhanced volcanism that
began ca. chron M4 and died out shortly following
chron M0, which coincides with the formation of
the magnetic J-anomaly [44]. Oceanic character of
the NR [45] and a high velocity, locally compensated crustal root beneath Josephine Seamount at the
northern end of the MTR [46] are consistent with
this interpretation.
Anomalous elevations are also associated with
the Newfoundland Seamounts (NS) in the Newfoundland Basin (NB). They are paired with the
conjugate Tore Seamounts (TS) in the Iberia Abyssal Plain (IAP) (Fig. 6, left), and thus they appear to
have been formed by a ridge-centered hotspot. This
volcanism appears to have followed breakup by at
least 20 m.y., based on an age date (97.7 F 1.5 Ma)
for one of the Newfoundland seamounts [62], and
waned by the time of chron 34 (Fig. 6, right).
There is significant asymmetry in basement depth
between the large negative anomaly in the Sohm
Abyssal Plain (SAP) [47] and more normal depths
in the Canary Basin. It is possible that this asymmetry is not an original feature of the basin but was
5 – 34
M0
[41]
[42]
5 – 34
M0
[41]
[42]
5 – 34
M0 – M25
[41]
[42]
5 – 34
M0 – M25
[43]
[44]
created (or enhanced) by regional uplift on the
African plate, possibly associated with more recent
volcanism of the Canary Island (CI) [48] and
Madeira [49] hotspots.
To the north of the NR and MTR, asymmetric
depths also appear in the opening NewfoundlandIberia rift, with basement f 1 km deeper off Iberia.
The Newfoundland Basin was affected by early postrift magmatism [83], but it is not yet clear if the
magnitude of this event was sufficient to account for
the asymmetry. Thus it is possible that the depth offset
may have existed from the time of continental separation. By chron 34, crust forming along the MAR is
relatively symmetric in depth and it is not disturbed by
large anomalies. There is only a slight shallowing of
basement depth at the Biscay triple junction (BTJ) and
northwards.
4.2. Chrons 25 (56 Ma—late Paleocene) and 21 (47
Ma—middle Eocene) (Fig. 7)
During this period there was a general increase in
volcanic activity along the MAR. Elevated basement
in Davis Strait (DS) increases in depth rapidly toward
the central Labrador Sea (LS). Elevated basement is
coincident with the volcanic margins of East Greenland (EG) [50] and Hatton-Rockall Bank (HRB)
[51,52]. The elevated basement extends south of
Greenland (SGH) and Hatton Bank, with a few
isolated highs continuing to the Charlie-Gibbs fracture
zone (CGFZ). Seismic studies have shown basement
K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
volcanics immediately south of Greenland [53] and
Edoras Bank [54]; these features were originally
contiguous with one another and with the SGH. They
most likely were created by an early stage of volcanism associated with the Iceland plume, in which
plume magmatism spread as a convective sheet or
was channeled along the rifting plate boundary
[53,55].
South of the CGFZ, elevated basement is associated
with the East and West Thulean rises and with a
rejuvenated Newfoundland-Milne (or a separate
Milne) hotspot. The Thulean rises (TR) were formed
over a short time interval that ended by the time of
chron 21, and they appear to be spatially separated
from elevated basement associated with Iceland. The
Milne hotspot (MS) may have been active at the MAR
axis shortly following chron 34, and there is a suggestion that it migrated south along the MAR, forming a
V-shaped trail [56]. Subsequently, the Milne hotspot
appears to have migrated back to the north along the
MAR, forming elevated topography at the ridge axis.
The appearance of elevated topography at the
Azores triple junction (ATJ) first occurred immediately following Chron 21, and it later developed a
more robust Azores high. To the south, the Great
Meteor hotspot that earlier had formed the New
England—Corner seamount chain (NES and CRS)
migrated across the MAR at f 76 Ma and formed
the Cruiser Plateau (CP) as an off-axis feature on
the east side of the MAR [57]. Volcanism associated
with this hotspont also may have migrated northward from f 75 to f 30 Ma to form the elevated
areas immediately north of the Hayes FZ (i.e. Plato
and Atlantis seamounts).
4.3. Chrons 13(33 Ma—early Oligocene) and 5 (10
Ma—late Miocene) (Fig. 8)
By chron 13, increasing elevation along the Reykjanes Ridge (RR) near Iceland is apparent. To the
south, most of the region of elevated basement is flat.
It was not until after chron 13 that the elevated, Vshaped topography of the Reykjanes Ridge formed.
There was little anomalous basement elevation from
the Charlie-Gibbs fracture zone (CGFZ) to f 100 km
northward, but the area became shallower by chron 5
as elevated topography developed on the Reykjanes
Ridge.
201
Anomalous basement elevation is reduced south of
Charlie-Gibbs fracture zone. Volcanism stopped quickly at the Thulean rises, and the influence of the Milne
hotspot declined by the time of chron 5. For a period
around chron 13, shallow topography was created east
of the MAR around the Kings Trough (KT) and
Azores-Biscay Rise (ABR) plate boundary [58].
From chron 13 to chron 5, there was a significant
increase in basement elevation associated with Azores
hotspot (ATJ), but it was asymmetrically displaced
toward the east side of the MAR along the AzoresGibraltar plate boundary. This was followed by a
relative decrease in anomalous elevation from chron
5 to present along the MAR [59] (Fig. 5), while
volcanism continued on the Azores Islands to the east
(e.g. [60]).
South of the Azores, elevated topography was
restricted primarily to the Africa plate and was associated with the Great Meteor hotspot. There was
southward relative migration of the Great Meteor
hotspot as it passed beneath Cruiser plateau (CP)
and formed the Great Meteor seamount (GMS) [57].
5. Implications
5.1. Variations in hotspot activity with time
The above analysis of residual basement topography indicates that all of the significant positive depth
anomalies (i.e. excess topography) can be systematically reconstructed to specific periods and locations of
excess volcanism (hotspots). South of the Azores,
hotspots were not fixed at the MAR axis, so magnetic-anomaly ages of basement do not constrain the
timing of volcanism, except during the period when
the Great Meteor hotspot crossed the MAR ( f 75 –
30 Ma) to form the elevated topography on the
African plate [57]. Since that time, all volcanism in
this region has been located on the African plate. It is
noteworthy that the Azores and GMS hotspots may
have interacted during the period f 47– 33 Ma (Figs.
7 and 8); this could complicate interpretation of
isotopic anomalies in ocean crust that formed during
this time along the MAR between the Hayes and
Azores fracture zones (e.g. [61]).
From the Azores northwards, all of the excess
volcanism appears to have been centered on or closely
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K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
Fig. 7. Basement depth anomaly reconstructed at chrons 25 and 21. Thin lines are magnetic isochrons (shown in Fig. 4) and isobaths (shown in
Fig. 2). Abbreviations for features discussed in text (additional to those given in Fig. 5) are HFZ (Hayes Fracture Zone), and TR (Thulean Rise).
K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
203
Fig. 8. Basement depth anomaly reconstructed at chrons 13 and 5. Thin lines are magnetic isochrons (shown in Fig. 4) and isobaths (shown in
Fig. 2). Labels are as given in Fig. 5.
204
K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
adjacent to the MAR axis at five distinct locations.
There is a significant range in duration of volcanism at
these sites (Table 2), from 60 m.y. (Iceland) to as little
as 8 –10 m.y. (Thulean Rise).
The present period of excess ridge-crest volcanism
associated with Iceland, and to a lesser extent the
Azores, is quite distinct from earlier periods of seafloor spreading. In particular, from the end of volcanic
activity associated with formation of the Newfoundland Rise and Madeira-Tore Rise ( f 120 –110 Ma) to
the start of robust volcanism associated with Milne
seamounts and the Azores-Biscay Rise ( f 80 – 75
Ma) and eventually Iceland (60 Ma), there is little
evidence from basement topography for excess volcanism throughout the entire region. One exception,
as noted earlier, is the Newfoundland and possibly
Tore seamounts, which seem to have formed at about
98 Ma. We also note the lack of significant volcanism
during continental breakup over roughly the same
period [62].
5.2. Variations in crustal thickness
Plots of basement depth anomaly versus distance
along isochrons are given in Fig. 9 relative to a fixed
position for the Charlie-Gibbs fracture zone (CGFZ).
Along the present MAR, we observe elevated basement zones with f 1500 km half-widths centered on
the Azores and Iceland hotspots. These features have
often been described in previous work, generally
relating the elevations to geochemical anomalies
(e.g. [63]). The CGFZ is clearly identified with the
boundary between these two large-scale anomalies.
Note also that the maximum amplitude of the Azores
signature is reduced relative to Iceland, in part be-
Table 2
Duration of hotspot activity
Hotspot
Start
Stop
Duration
Reference
Iceland
Azores
Milne
A-BR + KT
Newfoundland Ridge
Madeira-Tore Rise
Thulean Rise West
Thulean Rise East
60
33 – 48
76
79
129
138
59
54
Active
Active
53 – 36
52 – 31
109
118
49
46
60
33 – 48
23 – 40
27 – 48
20
20
10
8
*[51]
*[59]
*
*[54]
*
*
*
*
* This paper.
Fig. 9. Basement depth anomaly from Fig. 5 plotted along isochrons
0, 5, 13, 21, 25, 34 and M0. Depths west and east of the MAR are
shown in black and gray, respectively. Plots are aligned relative to
the Charlie-Gibbs Fracture Zone (CGFZ). Positions of the
Newfoundland-Azores and Azores-Gibraltar fracture zones are
shown by vertical black and gray lines, respectively. Mean depths
(horizontal lines) are shown for the segments delineated by the
fracture zones. Labels of features are as given in Fig. 5.
cause the present Azores hotspot has shifted east of
the MAR.
Along isochrons older than chron 5, wavelengths of basement anomalies (300 – 500 km) related to various hotspots, are much shorter than
those along the present spreading axis. This agrees
with existing models in which anomalous mantle
flow caused by a hotspot is directed for significant
distances along the spreading center [64,65]. The
K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
basement anomaly that is present along the older
isochrons is caused by thickened crust produced by
excess decompression melting at elevated asthenospheric temperatures [66]. Studies from Iceland and
the Reykjanes Ridge show that the extent of
primary excess crustal thickness is much more
limited (i.e., a half wavelength of 100 –200 km)
than the excess elevation along the MAR [67], and
therefore the elevated temperatures are similarly
narrow [68].
Another prominent feature in Fig. 9 is the apparent
asymmetry between average depths on opposite sides
of the MAR, with east-flank depths consistently
shallower than depths to the west (except for the
Newfoundland basin as previously mentioned). There
is some evidence that this long-wavelength feature
may be a dynamic effect of mantle flow. In Fig. 10 we
show a calculation of dynamic topography [69] for
our study area, derived from the seismic tomographic
model of Grand [70]. An obvious feature of the
dynamic topography is uplift associated with the Iceland plume and the African plate. To remove these
effects, we subtract the dynamic topography from our
Fig. 10. Dynamic basement topography [69] predicted by a
geodynamic model of mantle flow derived from the seismic
tomography model of Grand [70].
205
observed anomalous basement depths and then convert the remaining anomalous basement topography to
crustal thickness variations assuming local isostatic
balance (Fig. 11). In this case,
qm qw
tc ¼ Dh
þ ta
qm qc
where tc is the isostatic crustal thickness; Dh is the
observed anomalous basement depth; qw (1.03 Mg/m3)
and qm (3.3 Mg/m3) are the water and mantle densities;
and ta (7 km) is the average oceanic crustal thickness
[71]. For the average crustal density (qc) we use a value
of 2.95 Mg/m3, assuming that most of the excess crust
is produced by underplating of mafic melt with typical
velocities of 6.8– 7.4 km/s and densities of 2.9 –3.0
Mg/m3 (e.g. [72,73]).
Profiles across features of interest, compared with
observed values of crustal thickness from seismic
models, are shown in Fig. 12. The largest predicted
thicknesses of 27 – 29 km (Fig. 12, top left) are
associated with the Greenland – Iceland – Faeroe
Ridge, consistent with observed values of f 30 km
[72,74]. Excess volcanic thicknesses of 17 –18.5 km
observed along the conjugate margins of East Greenland [74,84] and Hatton-Rockall Bank [75,76] also
agree with our predicted thicknesses under the continental slope (Fig. 12, bottom). Further landward, the
greater predicted crustal thicknesses along profiles 12
and 13 probably are related to increasing contributions
from continental crust across the ocean – continent
transitions. As shown in Fig. 11, thick volcanic crust
(15 – 20 km) also is predicted south of Davis Strait and
south of Greenland where it forms the SGH.
For oceanic crust in the Iceland and Irminger
basins, our predicted crustal thicknesses of f 12
km are close to observed values of 9.5– 11 km [74 –
76] (Fig. 12, bottom). However, for the Reykjanes
Ridge predicted values of f 13 –15 km are significantly greater than observed values of 7.5 –10 km
[67,77]. This probably indicates that dynamic uplift
related to shallow-mantle variations in the Iceland
plume has not been completely removed by the
theoretical calculation.
Volcanic features between the Charlie-Gibbs F.Z.
and the Azores have predicted maximum thicknesses
on the order of 20 km. The Milne Seamounts (MS)
and Azores-Biscay Rise (ABR) are somewhat thicker
206
K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
Fig. 11. Adjusted residual basement depth and isostatic crustal thickness computed from the residual depth anomaly (Fig. 5), with the dynamic
topography of Fig. 10 removed. Conversion from residual depth to crustal thickness assumes an average crustal density of 2.95 Mg/m3. See text
for details. Extracted data for the numbered profiles are given in Fig. 12. Note that profiles 12 and 13 approximately connect margin conjugates
but do not follow flow lines.
and thinner than this, f 23 and 15– 16 km, respectively, and the Southeast Newfoundland Ridge is also
thicker at f 26 km. The seismically determined
thickness for the Madeira-Tore Rise (MTR) (16 km)
[46] is only 4 km less than our prediction, but the
ABR seismic thickness (9 km) [56] is much lower
( f 7 km) than predicted. The latter seismic value is
complicated by the unusual interpretation of a lowvelocity zone (6.4 km/s) beneath the ridge, and this
model is not as well constrained as for the other
profiles. Predicted crustal thicknesses at the Azores
range from about 25 –35 km.
K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
207
Fig. 12. Relative isostatic crustal thickness variations for the profiles identified in Fig. 11. The thick grey bars give observed crustal thickness
determined from refraction profiles for the Iceland Ridge [72,74], Madeira-Tore Rise [46], Azores-Biscay Rise [56], Reykjanes Ridge [67,77],
Hatton-Rockall Bank and adjacent basin [75,76], and the East Greenland margin and adjacent basin [74,84].
The widths of the above features vary from 250 to
300 km (Azores, MTR and NR) to 150 km (ABR, MS
and TR). These values suggest less robust volcanism
and lower mantle temperatures than at Iceland. For
instance, a simple passive melting model [66] would
predict temperatures f 50 – 100 jC lower for the
Azores and f 100 –150 jC lower for the ABR and
MS relative to Iceland. Previous studies of Schilling
[78] and Ito and Lin [65] also predict a 50 – 100 jC
difference in mantle temperature between the Iceland
and Azores plumes. More complex dynamic models
of upwelling are also consistent with such narrow
plumes of excesss melt [79].
Removal of the dynamic topography produces a
much more symmetric pattern of depths between the
oldest ocean basins off Nova Scotia and Morocco (cf.
Figs. 5 and 11). In contrast, however, there is an
increased discrepancy between the residual depths of
basins off Newfoundland and Iberia. In addition, there
are significant regions of ocean basins, primarily those
adjacent to some continents (e.g. France, Iberia, NW
Africa, Nova Scotia) where the predicted isostatic
crustal thickness is very small or even negative. Although the crust may be thin adjacent to some nonvolcanic margins (e.g. [34]), these particular values are
certainly not valid, and they most likely indicate either
non-isostatic effects, unmodeled variations in the mantle temperature or flow field, or variations from the
assumed densities of crust and mantle. For instance, the
low values in the Sohm Abyssal Plain off Nova Scotia
could indicate edge-driven convection at the continent –ocean boundary [80] that eventually could con-
208
K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211
tribute to possible subduction [81]. In the future, with
constraints provided separately from seismic crustal
thickness, anomalous basement depth, mantle tomography, and heat flow, it may be possible to better
quantify such effects.
6. Conclusions
A digital map of total sediment thickness for the
North Atlantic Ocean, from 30j to 70jN latitude and
from 0j to 70jW longitude, is described which clearly
depicts the basic characteristics of sediment distribution in the region. This grid in combination with grids
of observed and predicted seafloor bathymetry allows
us to calculate a grid of basement-depth anomalies
corrected for the effects of isostatic sediment loading
and lithospheric age. Grids in GMT format [82] of
sediment thickness, basement depth anomaly and
isostatic crustal thickness (see below) are available
at ftp
tp://ftp.phys.ocean.dal.ca/pub/users/klouden/
EPSL7162.
Using maps of basement depth anomalies plotted
for plate reconstructions representing stages of opening of the North Atlantic, we delineate consistent
patterns in basement depth that are related to hotspot
volcanism. In addition to Iceland and the Azores, we
recognize three centers of excess volcanism at the
Mid-Atlantic ridge: the Milne Seamount and AzoresBiscay Rise (75 –40 Ma), the Southeast Newfoundland Ridge and Madeira-Tore Rise (130 – 110 Ma),
and the East and West Thulean Rises (60 –50 Ma).
The duration of volcanic activity ranges from 8 to 10
m.y. (Thulean Rises) to as much as 60 m.y. (Iceland).
Thus it is possible for both long- and short-lived
hotspots to coexist, even in relatively close proximity.
We note that there was little excess volcanism north of
the Azores during the period from approximately 110
to 60 Ma. Finally, we note that all hotspots north of
the Newfoundland-Gibraltar F. Z. have been located at
or near the spreading center, while those to the south
are not.
From the anomalous basement depths, we estimate isostatic crustal thickness after removing a
model of dynamic effects created by mantle flow.
Maximum predicted thicknesses of volcanic features,
from 30 km beneath the Greenland –Iceland –Faeroe
Ridge to f 15 km beneath the Azores-Biscay Rise,
are broadly consistent with seismic models and with
calculations of decompression melting. Widths of
volcanic features indicate that thickening primarily
occurs within 100 –200 km of hotspots except along
continental margins (i.e. East Greenland and the
Hatton-Rockall Bank) that rifted at the time of the
hotspot activity. We identify conjugate structures
south of Greenland and Edoras Bank, where excess
volcanism appears to have extended beyond the
margin proper and into oceanic crust. Similar features appear south of Davis Strait in the Labrador
Sea. Finally, we locate oceanic regions adjacent to
some continental margins where unusually low values of predicted crustal thickness suggest that there
are additional variations in plate properties or nonisostatic effects within the mantle that were not
modeled. It is hoped that in the future combined
constraints from measurement of crustal thickness,
mantle velocity, and heat flow will help to better
understand these remaining anomalous regions.
Acknowledgements
K.E.L. acknowledges support from the Natural
Sciences and Engineering Research Council (NSERC)
of Canada and from the Southampton Oceanography
Centre during a sabbatical visit in 2003. B.E.T.
acknowledges support from the Henry Bryant Bigelow Chair in Oceanography at Woods Hole Oceanographic Institution. Much of the sediment thickness
and basement mapping on which this work is based
was made possible by support from the National
Science Foundation and the Office of Naval Research.
We thank Walter Roest and John Hopper for helpful
reviews. Woods Hole Oceanographic Institution
Contribution Number 11,154. [BOYLE]
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