Earth and Planetary Science Letters 182 (2000) 61^76
www.elsevier.com/locate/epsl
Magnetic evidence for slow sea£oor spreading during the
formation of the Newfoundland and Iberian margins
S.P. Srivastava a; *, J.-C. Sibuet b , S. Cande c , W.R. Roest d , I.D. Reid e
a
Geological Survey of Canada, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, N.S., Canada B2Y 4A2
b
Ifremer, Centre de Brest, P.O. Box 70, 29280 Plouzane, France
c
Scripps Institute of Oceanography, University of California San Diego, La Jolla, CA 92093, USA
d
Geological Survey of Canada, Continental Geoscience Division, Ottawa, Ont., Canada K1A 0Y3
e
Geological Institute, University of Copenhagen, Oester Voldgade 10, DK-1350, Copenhagen K, Denmark
Received 13 April 2000; received in revised form 18 July 2000; accepted 19 July 2000
Abstract
There is considerable debate concerning the nature and origin of the thin crust within the ocean^continent transition
(OCT) zones of many passive non-volcanic continental margins, located between thinned continental and true oceanic
crust. This crust is usually found to be underlain by upper mantle material of 7.2^7.4 km/s velocity at shallow depths
(1^2 km). It has been proposed that such crustal material could have originated either by exhumation of upper mantle
material during rifting of continents or by slow seafloor spreading. One of the examples of occurrence of such a crust
are the conjugate margins of Newfoundland and Iberia. Here we present an interpretation of magnetic data from these
regions to show that their OCT zones are underlain by crustal material formed by slow seafloor spreading (6.7 mm/yr)
soon after Iberia separated from the Grand Banks of Newfoundland in the late Jurassic. Similarities in the magnetic
anomalies and velocity distributions from these regions with those from the Sohm Abyssal Plain, a region lying
immediately south of the Newfoundland Basin and formed by seafloor spreading at a similar rate of spreading, give
further support to such an interpretation. The idea that these regions were formed by unroofing of upper mantle during
rifting of Iberia from Newfoundland may be likely but the presence of weak magnetic anomalies in these regions, which
bear all the characteristics of seafloor spreading anomalies, makes it difficult to ignore the possibility that these regions
could be underlain by oceanic crust formed during slow seafloor spreading. The similarities in velocity structure and the
presence of small amplitude magnetic anomalies both across this pair of conjugate margins of the North Atlantic and
that of the Labrador Sea suggest that this OCT velocity structure may be the norm rather than the exception across
those passive non-volcanic margins where the initial seafloor spreading was slow. Furthermore, the existence of similar
velocity distributions along a few active spreading centers raises the possibility of formation of similar crust across slow
spreading ridges. ß 2000 Elsevier Science B.V. All rights reserved.
Keywords: continental margin; magnetic anomalies; sea-£oor spreading; transition zones; Newfoundland
1. Introduction
* Corresponding author. Tel.: +1-902-426-3148;
Fax: +1-902-426-6152; E-mail: [email protected]
At many non-volcanic rifted continental margins, one ¢nds a zone, between the truly faulted
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 3 1 - 4
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
62
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
and extended continental crust and the region
with undisputed oceanic magnetic anomalies,
where it has been di¤cult to establish the true
nature of the underlying crust. This region, which
can be as much as 170 km wide [2], has often been
called the ocean^continent transition (OCT) zone
[1^4]. Where they have been explored, these zones
are surprisingly similar, not only in their velocity
distributions but also in their tectonic fabrics and
magnetic signatures. Two sets of conjugate margins which have been extensively studied are the
Labrador Sea margins [4,14] and Iberia [2,3,5^
7,13] and Newfoundland [1,8,9] margins located
across the North Atlantic. Understanding the nature of the crust which underlies the OCT of these
margins is important as it can give us some idea
of the processes which have been responsible for
the break up of the lithospheric plates and the
onset of sea£oor spreading. The Iberia^Newfoundland conjugate pair has been the focus of
extensive multichannel seismic re£ection pro¢ling
[1,8,10^12], seismic refraction work [2,7,9,13], detailed magnetic studies [15^17] and drilling across
the Iberia margin by the Ocean Drilling Program
[3,18]. In spite of these detailed studies, the nature
and origin of the crust within the OCT in all these
regions remains in dispute. There are three
schools of thought: (1) it is thinned and intruded
continental crust [3,8], (2) it is neither oceanic nor
continental crust but consists of upper mantle
rocks exhumed by simple shear [4,14,21] or by
pure shear extension [2,12,13,22] and (3) it is oceanic crust formed at an ultra-slow sea£oor
spreading rate [3,19,20].
1.1. The problem and previous work
For the sake of clarity, a brief review of the ¢rst
two ideas is included here before examining the
third idea.
Fig. 1 shows a plot of representative velocity
distributions of the OCTs across Iberia Abyssal
Plain (IAP), Tagus Abyssal Plain (TAP) and Labrador Sea (LB) margins. Also shown here are the
velocity distributions from north and south Newfoundland Basin (NB) and Sohm Abyssal Plain
(SAP) which will be discussed later. For comparison, we have also included velocity bounds for a
59^170 Ma old crust from the Atlantic Ocean [34]
which was formed at spreading rates higher than
10 mm/yr. The important thing to notice here is
the occurrence of a thin (generally 6 3 km), low
velocity, moderate to high gradient layer on top
of a high velocity layer ( s 7.0 km/s) with low
gradient for all the margins under discussion.
For all these regions, the velocity distribution in
the upper layer varies anywhere from 3.5 to
5.0 km/s in the top, increasing gradually in steps
or sharply to more than 7.0 km/s, before gradually changing to mantle velocity. Such a distribution is far from the normal ocean crustal velocity
distributions with distinct layer 2, layer 3 and
mantle velocities as shown for the average Atlantic crust (Fig. 1). Though the velocity values in
the top layer fall within the velocity bounds for
oceanic seismic layer 2, their gradients in a few
cases (Iap(IAM), Snb and Sap) are too high to be
associated either with a normal layer 2 [34] or
with a thinned continental crust. Similarly the velocity distributions for the underlying layer are
too high to be associated with normal layer 3 or
magmatically intruded or underplated lower continental crust [13]. What has been well accepted
and demonstrated by others [2,4,7,13,14] from detailed refraction measurements carried out in
these regions is that this layer is largely peridotite
with some serpentinization due to hydrothermal
alteration. Based on these velocity distributions
and other inferences drawn from seismic re£ection
measurements from these regions, it has been concluded that OCTs under these regions contain
neither extended continental crust nor typical oceanic crust [2,4,7,13,14]. So we can safely rule out
the ¢rst possibility that the crust within the OCT
in these regions is extended continental crust.
That brings us to the second possibility, that
the top layer may be altered serpentinized peridotite upper mantle material which is exhumed at
these shallow depths [2,4,7,13,14]. It was the
high gradient in the top layer in the IAP (as
seen in Iap(IAM) and to some extent in Iap(DT)
lines in Fig. 1) together with the drilling results
across its northern part (where the majority of the
sites drilled, located on highs, showed the presence of serpentinized peridotite) which led some
[2,7,13,14] to suggest that the OCT of the IAP is
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
63
Fig. 1. Velocity versus depth pro¢les from di¡erent regions, with depth measured from acoustic basement showing occurrences of
a high velocity layer under a thin low velocity layer. The sources of these results are: Iberia Abyssal Plain (Iap(DT) and
Iap(IAM)) [2,7], Tagus Abyssal Plain (Tap) [6], South Newfoundland Basin (Snb) [9], Northeast Newfoundland Basin (Nenb)
and Northwest Newfoundland Basin (Nwnb) [Reid, Srivastava and Sibuet, in preparation], Sohm Abyssal Plain (Sap) [25], South
Flemish Cap (Sfc) [38], and Labrador Sea (Lab) and West Greenland (Wgrn) [4]. Their locations in IAP and NB are shown in
the inset. Location of Sap is shown in Fig. 2. Patterned area indicates bounds for 59^170 Ma old Atlantic crust [34].
largely underlain by serpentinized peridotite. In
such a model the high velocity gradient is interpreted as resulting from a high degree of serpentinization aided by hydrothermal circulation
along fractures and faults present in the top
layers. The degree of such circulation would decrease with increasing depth thus resulting in less
serpentinization of peridotites at greater depths
giving rise to lower velocity gradients [2,7,13] in
the deeper layers. No deep shear zones have been
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
64
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
observed in the lower layers from deep seismic
re£ection data in this region to suggest movement
of upper mantle along them [12,13]. Thus the proposed model would involve exhumation of upper
mantle by a pure shear mechanism. Across the
Labrador Sea margins a simple shear mechanism
is supposed to have brought this material within
the OCT to such shallow depths [4,14].
What is less clear from this model is how exhumation of the upper mantle took place across
such a great width of OCT ( s 170 km) without
generation of any melt. As pointed out by
McKenzie (in [14]) and White (in [14]), no matter
how one moves the upper mantle upward it has
the propensity to melt due to decompression as all
theoretical calculations [35,36] suggest. This
would argue the existence of some melt product
together with serpentinized peridotite in the thin
crustal layer in these regions. Recent seismic refraction results across the slow spreading Southwest Indian Ridge (half rate 6^7 mm/yr) show
crust consisting of layer 2 and 3 velocities to be
about 2^2.5 km thick [46]. It is suggested that this
probably resulted from conductive heat loss from
the upwelling mantle at this low spreading rate.
No doubt the amount of melt generated can decrease if the regions are subjected to a long duration of rifting [2,4,30]. The velocity data alone in
the present instances cannot be used to distinguish
serpentinized peridotite from gabbro or basalt.
This is because serpentinization of peridotite can
decrease its seismic velocity [33] to such an extent
that it becomes indistinguishable from that of basalt or gabbro. It is, therefore, possible for this
layer to contain a limited amount of basaltic material together with serpentinized peridotite. One
way to explore this possibility is to consider the
magnetic data from these regions to see if they
contain sea£oor spreading anomalies. Earlier attempts using magnetic data o¡ Iberia were not
very successful [2,16,17] for two reasons. One,
the modelling of sea£oor spreading magnetic
anomalies in IAP was only tried using a constant
spreading rate for the entire region [16] when evidence exists, from regions to the south [26,29]
formed by sea£oor spreading during the same period, that it could be variable and two, the calculation of the depth to the source rocks, utilizing
small amplitude gridded magnetic anomaly data,
used the inversion method, which becomes unstable for such small amplitude anomalies and cannot resolve the depth problem accurately [40]. We
have re-interpreted track data from this region
together with data from NB using a variable
rate of spreading. Here we present a new interpretation of magnetic data from these regions.
2. Magnetic anomalies
It has been known for some time that sea£oor
spreading anomalies, belonging to M sequence
anomalies, lie on both sides of the North Atlantic
[26,29]. O¡ Africa they lie south of the Azores^
Gibraltar Fracture Zone and o¡ North America
they lie south of the Newfoundland Fracture
Zone (Fig. 2). A large amplitude magnetic anomaly, known as the J anomaly [23], which lies between anomalies m0 and m1 [24,25], marks the
beginning of this sequence of anomalies. However, it is not certain if all of them continue north
of these fracture zones o¡ Iberia and Newfoundland. To facilitate visual comparison of these
anomalies north of the fracture zones it was essential to bring these regions close together with
the southern regions, where these anomalies can
be recognized. Reconstruction of the plates at the
time of anomaly m0, which forms the younger
end of the J anomaly, using the Verhoef et al.
[27] compilation of magnetic data for the North
Atlantic, was therefore carried out (Fig. 2) using
the poles of rotation of the Eurasian and African
plates as given by Srivastava et al. [15] and a
modi¢ed pole for the Iberian plate as calculated
here (given in Fig. 2) based on the present identi¢cation of the m0 anomaly. The entire data set
was ¢rst reduced to the pole before data from
di¡erent plates were rotated and merged [28].
The reconstruction (Fig. 2) clearly shows that
the large amplitude J anomaly, which is most
prominent in the vicinity of the two fracture
zones, namely the Newfoundland Fracture Zone
in the western North Atlantic and the Azores^
Gibraltar Fracture Zone (Fig. 2) in the eastern
North Atlantic, continues both north and south
of these fracture zones. Its amplitude, however,
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
65
Fig. 2. Shaded relief map showing reconstruction of the magnetic anomalies in the North Atlantic at chron m0 (118 Ma) relative
to North America. The map is illuminated from the northwest. The reconstruction is obtained by rotating reduced to pole
gridded magnetic anomaly data of Verhoef et al. [27] for the Eurasian (pole 69.67³N, 154.26³E, 323.17³), Iberian (pole 64.71³N,
18.94³W, 358.11³) and African (pole 66.09³N, 20.18³W, 354.45³) plates to the west and keeping the North American plate data
in its present location. Also shown are the 1000 m, 2000 m, and 3000 m isobaths, locations of a set of identi¢ed magnetic
anomalies in the region (e.g. m4, m11, m17, etc.), locations of the magnetic pro¢les Sap 5, Sap 4 and AF 2 (shown in Fig. 3a)
and of the refraction measurements from Sap (square with cross) shown in Fig. 5. FC: Flemish Cap; GAL: Galicia Bank; GB:
Grand Banks; IAP: Iberia Abyssal Plain; TAP: Tagus Abyssal Plain; NB: Newfoundland Basin; SAP: Sohm Abyssal Plain;
AAP: African Abyssal Plain; NFZ: Newfoundland Fracture Zone; AGFZ: Azores^Gibraltar Fracture Zone.
decreases considerably northward until it becomes
exceedingly small in the vicinity of Flemish Cap
o¡ Newfoundland and Galicia Bank o¡ Iberia.
Besides the J anomaly, the other striking feature
in this ¢gure is the parallelism between the NE^
SW trending anomalies which lie on both sides of
the J anomaly (which now forms the ridge axis)
o¡ Africa and eastern Canada south of the Grand
Banks. These have been identi¢ed as M sequence
anomalies [26,29]. Similar anomalies are not as
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
66
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
noticeable to the north, o¡ Iberia and the Grand
Banks of Newfoundland. This may be because
they are very small in amplitude and would not
show up in the gridded data except for a group of
anomalies which can be seen oceanward of the
3000 m isobaths o¡ the Grand Banks and Iberia
(Fig. 2).
To examine if decrease in their amplitude is the
reason for their lack of recognition, we compared
original track data from these regions against
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
67
Fig. 3. Correlation between observed and calculated M sequence anomalies in the Northwest Atlantic o¡ Nova Scotia and Newfoundland. The pro¢les are aligned relative to anomaly m4. The locations of these pro¢les are shown in Figs. 2 and 5. The model anomalies are calculated for a 2 km thick layer, located at 8 km depth in (a) and (b) and at a depth of 5.5 km in c, using
3 A/m as intensity of magnetization except for the J anomaly where values vary between 5 and 8 A/m. Other places where these
values varied are shown below the model. Also shown are the spreading rates for the models. (a) Pro¢les from the Sohm Abyssal
Plain, Sap 5, Sap 4 and the African Abyssal Plain, AF 2. The pro¢le AF 2 (dotted line) has been £ipped end to end to create its
mirror image for comparison. The rates of spreading for pro¢le AF 2 and for Sap 5 and 4 as obtained from model calculations
are shown below and above the pro¢les. (b) Pro¢les from the South Newfoundland Basin whose positions are shown in Fig. 5.
Dotted pro¢le is a portion of Sap 5 pro¢le shown in (a). (c) Pro¢les from the North Newfoundland Basin, Erbl 33, Erbl 56 and
Erbl 54 (projected in the direction of plate motion, parallel to tracks 7^10, Fig. 5). Other parameters used in the models were:
for NB Dr = 319.6³, Ir = 50.5³, for SAP Dr = 320.6³, Ir = 48.3³. Geomagnetic time scale of Kent and Gradstein [47] is used
throughout in model calculation.
6
those from regions to the south o¡ Africa and
Nova Scotia. Figs. 3 and 4 show examples of
observed anomalies along a few of the tracks
from these regions whose locations are shown in
Figs. 2 and 5. Comparison of the anomalies in
Fig. 3a,b clearly shows that such is indeed the
case. For ease of comparison between SAP and
NB data we have included a portion of data along
a track, Sap 5, from SAP (Fig. 3a) and shown it
by a dotted line, among NB data in Fig. 3b where
their spreading rates were comparable. The similarities in the signature of anomalies between SAP
and NB pro¢les clearly show the possibility that
anomalies in NB also belong to M sequence
anomalies and were formed at a comparable
rate of spreading. The small amplitude anomalies,
m4^m17 in south NB (Fig. 3b), are clearly visible
both in the aeromagnetic (tracks 7^10) and in the
shipborne data (CO19) but they are hardly noticeable as linear features in the gridded data except
for a large amplitude positive anomaly lying near
anomaly m17 o¡ the Grand Banks (Fig. 2). This,
as can be seen, is caused by the superimposition
of a long wavelength, large amplitude negative
anomaly on these small amplitude anomalies.
This long wavelength anomaly is not observed
o¡ Iberia, as a result anomalies here appear
slightly di¡erent to those in NB. Furthermore,
the anomalies o¡ Iberia are even smaller in amplitude (Fig. 4), which may arise because the
depth to basement here in general is about
1.0 km deeper compared to that in NB. The magnetic anomalies in SAP and Africa Abyssal Plain
(AAP) are much larger in amplitude than those in
NB (Fig. 3a). The anomalies in the northern NB,
on the contrary, are larger in amplitude and do
not have the long wavelength anomaly superimposed on them and, therefore, show up in the
gridded data as linear anomalies (m4 and m11,
Fig. 2). This we believe is due to lesser source
depth (6 vs. 7.5 km).
The conjugate track data in Figs. 3a and 4c
(shown by dotted lines) have been £ipped over
end to end to create their mirror images for
easy comparison. The locations and identi¢cations of magnetic anomalies both in the SAP
and in the AAP (Fig. 2) are based on correlation
of a fairly dense network of tracks [26]. Here we
have merely shown these anomalies along one
pair of conjugate tracks (Sap 5 and AF 2, Fig.
2) and one additional track (Sap 4) for comparison and to show their rate of spreading in these
regions. Unlike the track data in the SAP (Sap 5,
Sap 4, Fig. 3a), which came from the original
aeromagnetic data collected more or less in the
direction of plate motion, the track data in the
AAP (AF 2, Fig. 3a) are derived from gridded
data (Fig. 2) due to a lack of observations along
long tracks oriented in the direction of plate motion. The situation in NB and o¡ Iberia is di¡erent, in that we do have a fair amount of data in
the NB collected along the direction of plate motion but not o¡ Iberia. Unlike those o¡ Africa,
the gridded data o¡ Iberia and Newfoundland
could not be used in identifying anomalies as
the anomalies in these regions, as shown above,
are too small in amplitude especially in the IAP
and do not show up well in the gridded data (Fig.
4b, top). Furthermore, anomalies in the SAP do
not show up equally well as linear features as they
do in the AAP (Fig. 2). This we believe is caused
not only by the smallness in amplitude of the
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
68
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
anomalies in SAP compared to that in AAP but
equally by their variations between close tracks
(e.g. Sap 5 and Sap 4, Figs. 2 and 3a). A similar
situation exists in the NB where the data density
is even lower.
The use of a magnetic model which showed the
best ¢t to the SAP data everywhere (Fig. 3a) also
showed a satisfactory ¢t, on the whole, to the
data in the NB, IAP, and TAP, but resulted in
a slight shift between some of the observed and
model anomalies, suggesting a slightly di¡erent
rate of spreading. Changing the rate of spreading
between anomalies m20 and m4 slightly for these
regions (NB, IAP, and TAP) resulted in a more
satisfactory ¢t between the models and observations everywhere (Figs. 3b,c and 4a^c). This is
specially noticeable for the deep tow data in the
IAP (Fig. 4b) where the anomalies can easily be
identi¢ed because of better resolution as they are
observed close to the source. An exceedingly good
correlation between deep tow observed and calculated anomalies is obtained for anomalies from
m1 to m8 (Fig. 4b) which rapidly deteriorates
for older anomalies. We had to use a higher magnetization value for anomaly m8 as this is an isolated very prominent anomaly in this region, even
in the surface data [16]. Its exact cause is not
certain as no basement sample could be obtained
at this place [3] but we suspect it to be highly
magnetized serpentinized body. At other places
m8 forms a very small amplitude anomaly (e.g.
Fig. 4a). Comparison between calculated and observed anomalies at the sea surface in this region
(top part of Fig. 4b) shows that even though
anomalies m1^m8 can be recognized in the surface data one would be hard pressed to correlate
them everywhere using gridded data. This is be-
cause gridded data show a further decrease in the
amplitude of these anomalies compared to the
surface data (Fig. 4b) and also smooth out the
short wavelength ( 6 10 km) anomalies because
of 5 km gridding interval used (Fig. 2). We had
to lower the magnetization for anomalies older
than m8 in our model (Fig. 4b) in order to match
the observations. This may be indicative that the
source layer is thinning out here. A comparison
between model calculations, one with topography
and one without, showed a di¡erence of less than
10% for all regions where magnetization remained
low. The maximum di¡erence is observed under
the J anomaly (Fig. 4b) because of the large magnetization which had to be used here to match the
observed values. If the basement highs are caused
largely by serpentinized peridotite bodies then the
topographic e¡ect would be even smaller because
of the low magnetization observed at most of
such highs drilled in this region [41].
The spreading rate o¡ Iberia seems to have remained constant after the m4 anomaly, contrary
to that in NB. Both in the NB and o¡ Iberia, the
spreading rate during this period seems to decrease gradually from south to north (e.g. Figs.
4b,c and 3c). As we will show later, we believe this
was caused by the movement of Flemish Cap as a
micro-plate during this time. In spite of changing
the rate of spreading between some anomalies in
the NB (Fig. 3b) and o¡ Iberia (Fig. 4a), we ¢nd
that the mean rate of spreading of 6.7 mm/yr
obtained between anomalies m4 and m20 in these
regions (Figs. 3b and 4a) remained close to
(6.8 mm/yr) that obtained in the SAP (Fig. 3a).
Unlike between Africa and Nova Scotia (Fig. 3a),
the spreading between Newfoundland and Iberia
remained symmetric during most of this period.
C
Fig. 4. Correlation between observed and calculated M sequence anomalies in the Northeast Atlantic o¡ Iberia. The deep tow
(DT) data from Iberia Abyssal Plain and o¡ Galicia Bank are obtained from [16, 31] respectively and the surface data are from
[17,16]. The data for TAP come from [6], and for IAP (Iam9) from [12]. The data from Newfoundland Basin, Erbl 36, shown in
(c) are the surface ship data which have been projected in the direction of plate motion. They have been £ipped end to end to
create a mirror image for comparison. The locations of these pro¢les are shown in Fig. 5. For other explanations see Fig. 3.
(a) The model anomalies are calculated for a 2 km thick layer, located at 8 km depth with intensities of magnetization as shown.
(b) The 2 km thick layer is located 3 km below the measurement level and 8 km below the sea surface with intensity of magnetization as shown. (c) The 2 km thick layer is located 2 km below the measurement level and 6 km below the sea surface with the
same intensity of magnetization (except for anomaly J, it is assumed to be 4 A/m) as shown in (b). Other parameters used in
model calculations were: Dr = 343³, Ir = 58³. The spreading rates used in each case are shown in the ¢gures.
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
69
70
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
This con¢rms an earlier observation [15] that Iberia, though still moving separately from Africa,
was moving at a rate which was comparable to
that of Africa relative to North America during
pre-m4 times. Soon after m4 time, it seems to
have started moving faster, at least to anomaly
m0 time, as the rates of spreading obtained here
suggest. This could explain why a signi¢cant dislocation is observed between anomalies 34 across
the Azores^Gibraltar and Newfoundland Fracture Zones [15] when only a small o¡set is noticed
in the J anomalies across them (Fig. 2).
Fig. 5 shows the locations of identi¢ed magnetic anomalies from all the shipboard and aeromagnetic data in the NB and only from a few of
the tracks, oriented in the direction of plate motion, in the IAP and TAP. Identi¢cation of these
anomalies throughout IAP and TAP is under way
and will be published together with those from
NB elsewhere.
3. Interpretation and discussion
3.1. Magnetic data constraints
The great similarities in the signature and the
rate of spreading of the magnetic anomalies between the SAP (Fig. 3a) and the southern part of
NB (Fig. 3b) suggest that the anomalies at least in
the southern NB also belong to M sequence sea£oor spreading anomalies. In the northern part of
the NB the anomalies are slightly higher in amplitude and more variable in shape. Nonetheless,
they can be correlated with modelled pro¢les. In
the IAP and TAP the situation becomes even
more di¤cult where the anomalies are further
subdued in their amplitude and can hardly be
recognized in the surface data. It is the correlation
between the deep tow and the modelled anomalies
in this region, based on identical models used to
generate anomalies in the NB (Fig. 4b,c), which
con¢rms the idea that these anomalies too were
formed by sea£oor spreading. Alternatively, these
could be caused by small variations in magnetization of the underlying continental rocks as suggested for the IAP [17], but then one would not
expect to see any correlation between pro¢les
across the North Atlantic. This is contrary to
what we see, especially between the deep tow
data from the IAP (Iap(DT), Fig. 4b) and its conjugate surface data from the NB (Erbl 56, Fig.
3c). A similar correlation is seen farther to the
north between the tracks o¡ Galicia Bank
(Gal(DT), Fig. 4c) and south Flemish Cap (Erbl
36, Fig. 4c).
There are several other observations which
complement our interpretation of these anomalies
as sea£oor spreading generated anomalies at a
very slow rate of spreading, namely: (1) the
mean rate of spreading of 6.7 mm/yr obtained
between anomalies m4 and m20 agrees well with
earlier calculations of 6.8 mm/yr based on the
drilling transect [3]; (2) the presence of oldest
anomalies in the south and youngest in the north,
as we see on both sides of the North Atlantic
(Fig. 5), con¢rms earlier speculation of gradual
separation of the Grand Banks of Newfoundland
from Iberia from south to north. Such a separation could only happen by rift propagation [32]
during sea£oor spreading. For example, along
transects Gal(DT) and Erbl 36 (Fig. 4c) we
interpret anomaly m3 as the oldest anomaly,
although immediately to the south, we can see
the presence of older anomalies, perhaps to anomaly m10‡ /m11 along the drilling transect (Fig. 4b)
and to anomaly m20 in the southern most regions
(Figs. 3b and 4a). This suggests that spreading
propagated gradually to the north in this region.
Furthermore, identi¢cation of anomaly m3 as the
oldest anomaly o¡ Galicia and Flemish Cap
agrees well with the drilling results across Galicia
Bank suggesting that Galicia Bank separated
from Flemish Cap earlier than chron m0 [21];
(3) and lastly, the formation of a chain of seamounts, known as Newfoundland Seamounts
and located immediately north of Snb in Fig. 5,
can be accounted for by the sea£oor spreading
model. This we postulate to have happened due
to the movement of Flemish Cap together with
the region to south and west of it as a micro-plate
during the formation of NB. The movement of
such a micro-plate can only be deciphered precisely once the lineations on both sides of the
Atlantic are established accurately. However, assuming that the spreading during most of the time
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
71
Fig. 5. Reconstruction of the North Atlantic at chron m0 (large open circles belong to the North American Plate while large solid circles are the rotated positions belonging to the Iberian Plate), showing locations of identi¢ed M sequence anomalies o¡ Newfoundland and Iberia. Also shown are the locations of pro¢les shown in Figs. 3 and 4, of the refraction results (squares with
crosses) shown in Fig. 1 and of ODP drill holes (small solid circles). Iap(DT): Iberia Abyssal Plain Drilling Transect; Iap(IAM):
Iberia Abyssal Plain along line IAM-9. For other explanations see Fig. 1.
when NB was formed was symmetric and using
the location of the anomalies identi¢ed o¡ Iberia
and the trends of the lineations in the NB as a
guide, we can decipher the trends of the resulting
lineations o¡ Iberia. Such a pattern of the lineations can then be used to decipher the development the Newfoundland Seamounts as is shown
in Fig. 6. The resulting con¢guration of the magnetic lineations and of the Flemish Cap microplate in the North Atlantic at various times as
shown in Fig. 6 are based on the observation
that spreading has progressed between Iberia
and Newfoundland from south to north. So while
regions in the south were spreading, the regions
north of them were undergoing extension. Thus as
the sea£oor spreading progressed to the north,
with Flemish Cap remaining attached to the Galicia Bank, a large portion of the Grand Banks of
Newfoundland including Flemish Cap underwent
extension, thereby rotating Flemish Cap clockwise
relative to Newfoundland and giving rise to extension in many of the sedimentary basins which
underlie this region. During this time some extension also took place o¡ Iberia. Extension over the
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
72
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
Fig. 6. Schematic diagrams showing development of the Newfoundland Basin, and Iberia and Tagus Abyssal Plains by sea£oor
spreading as Flemish Cap separated from Galicia Bank at di¡erent times. The dashed lines are the assumed positions of magnetic
lineations o¡ Iberia. Flemish Cap micro-plate is shown as hatched region. Dotted lines signify assumed isobaths merely to show
outlines of the region o¡ Iberia and Newfoundland. SM: Seamounts; COB: assumed continent ocean boundary. Notice that a
di¡erent frame orientation is used for (e).
Grand Banks probably took place along several
transfer faults which extended from the oceanic
region landward. The southern limits of these
transfer faults at di¡erent times are shown as discrete boundaries A, B and C in Fig. 6. It is postulated that at some time one of these boundaries
shifted to the south. For example we postulate
plate boundary B of the Flemish Cap micro-plate
shifting to position A post anomaly m11 time in
Fig. 6d, which resulted in magnetic anomalies
north of boundary A rotating relative to those
south of it, giving rise to a slightly di¡erent ori-
entation to these anomalies (e.g. in m15 and m17
immediately north of the Seamounts; Fig. 5).
Where and when these boundaries changed position is not certain. This can only be deciphered
once the locations of all anomalies are known
precisely on both sides of the North Atlantic.
The resulting motion of Flemish Cap would create a slight extensional motion along boundary A,
giving rise to seamounts as well as a slight nonparallelism between anomalies on the two sides of
the ridge axis. This would imply formation of
seamounts prior to chron m0 (118 Myr), contrary
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
to what is found from the dredging result of one
of the seamounts (97.7 þ 1.5 Myr; [42]). On the
other hand, this may be due to later volcanism
at this seamount located along this zone of weakness. Evidence for Flemish Cap acting as microplate also comes from the gradual increase in separation of anomaly m4 from m0 from north to
south (Fig. 5) mainly in the NB. This is because
spreading remained slower in the north when
Flemish Cap was attached to the Galicia Bank.
It only became detached close to m3 time.
3.2. Seismic
The interpretation of magnetic data as described above from the SAP, NB, IAP and TAP
shows a high probability that they were all
formed by sea£oor spreading at similar spreading
rates. This would imply that they should be
underlain by similar crust. To see if this is so we
have included velocity distributions from the NB
[9] (Reid, Srivastava and Sibuet, in preparation,
2000) as well as from the SAP in Fig. 1. The measurements in SAP were carried out along a 60 km
long reversed Ocean Bottom Hydrophone line oriented along m4 chron [25] (Fig. 2) and are particularly signi¢cant in the present instance because
this is a region known to be oceanic and formed
at the same time and at a comparable rate of
spreading of 6.8 mm/yr to that in NB and IAP.
The locations of these measurements are shown in
Figs. 5 and 2. The important thing to notice from
this ¢gure is the occurrence of a thin ( 6 3 km),
low velocity layer, whose velocity lies within layer
2 bounds, on top of a high velocity ( s 7 km/s),
low gradient layer in both regions. The result
from SAP (Sap, Fig. 1) show small depth increments in velocity structure within the thin crust,
but these are artifacts, arising from the use of
sonobuoy data. In spite of this, it also shows a
sharp gradient in velocity distribution in the
upper layer, overlying a 7.7 km/s velocity layer
similar to what is observed in south NB (Snb,
Fig. 1) and across IAP (Iap(IAM), Fig. 1). Such
a velocity distribution along this line seems to
have remained the same on the landward side
[25] where the spreading rate was the same (Fig.
3a) but in older regions farther landward with
73
higher spreading rate (anomaly s m16, Fig. 3a)
it showed a normal oceanic velocity distribution
[25]. Since the velocity distributions in older
( s m4) crust are solely based on sonobuoy measurements they should be treated with some caution.
The similarity in the magnetic data, the rate of
spreading (Fig. 3b) and the seismic velocity structure among SAP, NB and IAP (Fig. 1) argue
strongly that they could all be formed by the
same process, namely, slow sea£oor spreading.
The SAP is not the only region of slow spreading
containing thin crust overlying upper mantle ^
other examples are the Arctic Ocean [43] where
the mean spreading rate was 5 mm/yr, the Labrador Sea Extinct Ridge [37] where the spreading
rate was even lower (3.5 mm/yr) and part of the
Mid Atlantic Ridge [44] with slightly higher
spreading rate (10 mm/yr). Therefore it is possible
for a crust containing largely serpentinized peridotite to be formed at slow spreading rate. Nonetheless, velocity data alone cannot be used to
eliminate the possibility of the existence of some
basalt and gabbro probably on top of, or with,
serpentinized peridotite in these regions.
The exposure of serpentinized peridotite rocks
at the sea£oor is not unique to the IAP, they have
also been observed as exposed rocks together with
ultrama¢cs at the sea bottom across a slow
spreading section of the Mid Atlantic Ridge at
15³N [39]. In some cases, lava £ows lie directly
over mantle peridotite, without intervening gabbroic `lower crust', which these authors interpret
as arising from lack of magma generation. Besides
decrease in melt production with decreasing
spreading rate, amagmatic extension can thin the
crust further as seen across the Labrador Sea [37].
Across the margins, such amagmatic extension
may play an even more important role in controlling the thickness of the crust. In places this may
cause the crust to be so thin that the upper mantle
gets exposed as ridges. This seems to be the case
in the IAP immediately south of Galicia Bank
(along track Iap(DT); Fig. 5) and south of Flemish Cap in the Newfoundland Basin where the
thinnest crust is observed (Sfc, Fig. 1).
Such a thin crust containing mostly serpentinized peridotite raises an important question
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
74
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
about the source of the observed magnetic
anomalies over them : whether it lies mainly in
volcanic part of crust or elsewhere. In modelling
the magnetic anomalies (Figs. 3 and 4) it is assumed that their source, which is not very highly
magnetic, lies within a thin layer. As mentioned
earlier such a layer may contain some basalt and
gabbro besides serpentinized peridotite. It is
known that serpentinized peridotite can contribute enough to the observed anomalies [45] and
therefore it is possible that they may be a source
of some of the large anomalies seen in the IAP
and NB. Because they will have a more di¡use
reversal boundary than quickly cooled basalts of
layer 2 due to the length of time over which the
magnetization was acquired [45] the resulting
anomalies may not be very sharp in their signatures, similar to what we observe in both IAP and
NB.
4. Conclusions
In the absence of other geophysical methods
besides deep drilling to di¡erentiate between igneous crust and altered mantle peridotite, we can
only speculate on the exact nature of the crust
across the margins based primarily on seismic velocity distributions. Nonetheless, the probability
is high that all three regions, NB, IAP and TAP
together with SAP, are underlain largely by serpentinized peridotite upper mantle material with
some gabbroic and basaltic rocks because of the
presence of sea£oor anomalies formed at similar
spreading rates in all these regions. The idea that
IAP and TAP were formed by unroo¢ng of upper
mantle during rifting of continents may be likely
but the presence of weak magnetic anomalies in
these regions which bear all the characteristics of
sea£oor spreading anomalies makes it di¤cult to
ignore the possibility that these regions may be
underlain by oceanic crust formed during slow
sea£oor spreading. If other process such as
changes in magnetization, caused by intrusive
bodies, alone within the underlying rocks, could
produce them, then one would not be able to see
their symmetry relative to the ridge axis, nor their
linear characteristics and correlation with anoma-
lies known to have been formed in oceanic regions
to the south.
Acknowledgements
We thank Charlotte Keen and Matthew Salisbury, all anonymous reviewers and Maurice Tivey
for their comments and observations. Drafting of
the ¢gures done by digital cartography section of
the Atlantic Geoscience Centre is gratefully acknowledged.[RV]
References
[1] C.E. Keen, B. de Voogd, The continent-ocean boundary
at the rifted margin o¡ eastern Canada: New results from
deep seismic re£ection studies, Tectonics 7 (1988) 107^
124.
[2] Discovery 215 Working Group, Deep structure in the vicinity of the ocean-continent transition zone under the
southern Iberia Abyssal Plain. Geology 26 (1998) 743^
746.
[3] R.B. Whitmarsh, D.S. Sawyer, 1996. The ocean/continent
transition beneath the Iberia Abyssal Plain and continentrifting to sea£oor spreading, in: R.B. Whitmarsh, D.S.
Sawyer, A. Klaus, D.G. Masson (Eds.), Proc. ODP Sci.
Results, 149, College Station, TX, pp. 713^736.
[4] D. Chian, Evolution of nonvolcanic rifted margins: New
results from the conjugate margins of the Labrador Sea,
Geology 23 (1995) 589^592.
[5] R.B. Whitmarsh, P.R. Miles, A. Mau¡ret, The ocean-continent boundary o¡ the western continental margin of
0
Iberia. I. Crustal structure at 40³30 N, Geophys. J. Int.
103 (1990) 509^531.
[6] L.M. Pinheiro, R.B. Whitmarsh, P.R. Miles, The oceancontinent boundary o¡ the western continental margin of
Iberia. II. Crustal structure in the Tagus Abyssal Plain,
Geophys. J. Int. 109 (1992) 106^124.
[7] D. Chian, K.E. Louden, T.A. Minsull, R.B. Whitmarsh,
Deep structure of the ocean-continent transition in the
southern Iberia Abyssal Plain from seismic refraction pro¢les: Ocean Drilling Program (Legs 149 and 173) transect,
J. Geophys. Res. 104 (1999) 7443^7462.
[8] B.E. Tucholke, J.A. Austin, E. Uchupi, Crustal structure
and rift-drift evolution of the Newfoundland basin, in:
Extensional Tectonics and Stratigraphy of the North Atlantic Margins, Am. Assoc. Pet. Geol. Mem. 46 (1989)
247^263.
[9] I. Reid, Crustal structure of a nonvolcanic rifted margin
east of Newfoundland, J. Geophys. Res. 99 (1994) 15161^
15180.
[10] S.P. Srivastava, J.-C. Sibuet, A joint AGC and
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
IFREMER geophysical cruise to the Newfoundland and
Orphan Basin, Cruise Report CSS Hudson 92-22, Mission
ERABLE (Unpublished Report), Geological Survey of
Canada, Dartmouth, N.S., and Institut Francais de Recherche pour L'Exploitation de Mer, Centre de Brest,
1992, 121 pp.
M.O. Beslier, M. Ask, G. Boillot, Ocean-continent
boundary in the Iberia Abyssal Plain from multichannel
seismic data, Tectonophysics 218 (1993) 383^393.
S.L.B. Pickup, R.B. Whitmarsh, C.M.R. Fowler, T.J.
Reston, Insight into the nature of the ocean-continent
transition o¡ west Iberia from a deep multichannel seismic re£ection pro¢le, Geology 24 (1996) 1079^1082.
S.M. Dean, T.A. Minshull, R.B. Whitmarsh, K.E. Louden, Deep structure of the ocean-continent transition in the
Southern Iberia Abyssal Plain from Seismic Refraction
0
Pro¢les: The IAM-9 transect at 40³20 N, J. Geophys.
Res. 105 (2000) 5859^5885.
K.E. Louden, D. Chian, The deep structure of non-volcanic rifted continental margins, Phil. Trans. R. Soc. London A 357 (1999) 767^804.
S.P. Srivastava, W.R. Roest, L.C. Kovacs, G. Oakey, S.
Levesque, J. Verhoef, R. Macnab, Motion of Iberia since
the Late Jurassic: Results from detailed aeromagnetic
measurements in the Newfoundland Basin, Tectonophysics 184 (1990) 229^260.
R.B. Whitmarsh, P.R. Miles, Models of the development
of the west Iberia rifted continental margin at 40³30P N
deduced from surface and deep-tow magnetic anomalies,
J. Geophys. Res. 100 (1995) 3789^3806.
R.B. Whitmarsh, P.R. Miles, J.-C. Sibuet, V. Louvel,
Geological and geophysical implications of deep tow magnetometer observations near Sites 897, 898, 899, 900, and
901 on the west Iberia continental margin, in: Proc. ODP
Sci. Results, 149, College Station, TX (1996) 665^674.
ODP Leg 173 Shipboard Scienti¢c Party, Drilling reveals
transition from continental breakup to early magmatic
crust, EOS Trans. Am. Geophys. Union 79 (1998) 173^
181.
S.P. Srivastava, W.R. Roest, Nature of thin crust across
the southwest Greenland margin and its bearing on the
location of the ocean-continent boundary, in: E. Banda,
M. Torne, M. Talwani (Eds.), Proceedings of the NATOARW Workshop on Rifted Ocean-Continent Boundaries,
11^14 May, 1994, Mallorca, Spain, Kluwer Academic,
Dordrecht, 1995, pp. 95^120.
S.P. Srivastava, W.R. Roest, Extent of oceanic crust in
the Labrador Sea, Mar. Pet. Geol. 16 (1999) 65^84.
G. Boillot, D. Maugenot, J. Girardeau, E.L. Winterer,
Rifting processes of the west Galicia Margin, Spain, in:
A.J. Tankard, H.R. Balkwill (Eds.), Extension Tectonics
and Stratigraphy of the North Atlantic Margin, AAPG
Mem. 46 (1989) 363^377.
J.P. Brun, M-O. Beslier, Mantle exhumation at passive
margins, Earth Planet. Sci. Lett. 142 (1996) 161^173.
W.C. Pitman III, M. Talwani, Sea £oor spreading in the
North Atlantic, Bull. Geol. Soc. Am. 83 (1972) 619^646.
75
[24] P.D. Rabinowitz, S. Cande, D.E. Hayes, Grand Banks
and J Anomaly Ridge, Science 202 (1978) 71^73.
[25] B.E. Tucholke, W.J. Ludwig, Structure and origin of J
Anomaly Ridge, western North Atlantic Ocean, J. Geophys. Res. 87 (1982) 9389^9407.
[26] W.R. Roest, J.J. Danobeitia, J. Verhoef, B.J. Collette,
Magnetic anomalies in the Canary Basin and the Mesozoic evolution of the central North Atlantic, Mar. Geophys. Res. 14 (1992) 1^24.
[27] J. Verhoef, W.R. Roest, R. Macnab, J. Arkani-Hamed,
Members of the Project Team, Magnetic anomalies of the
Arctic and North Atlantic Oceans and adjacent land
areas, Open File 3125 of the Geological Survey of Canada, 1996.
[28] J. Verhoef, K.H. Usow, W.R. Roest, A new method for
plate reconstructions: The use of gridded data, Comput.
Geosci. 16 (1990) 51^74.
[29] K.D. Klitgord, H. Schouten Plate kinematics of the central Atlantic, in: P.R. Vogt, N.E. Tucholke (Eds.), The
Geology of the North Atlantic. Vol. M, The Western
North Atlantic Region, Geological Society of America,
Boulder, CO, 1986, pp. 351^378.
[30] J.W. Bown, R.S. White, The e¡ect of ¢nite extension rate
on melt generation at continental rifts, J. Geophys. Res.
100 (1995) 18011^18030.
[31] J.C. Sibuet, V. Louvel, R.R. Whitmarsh, R.S. White, S.J.
Hors¢eld, B. Sichler, P. Leon, M. Recq, Constraints on
rifting processes from refraction and deep-tow magnetic
data: The example of the Galicia continental margin
(West Iberia), in: E. Banda, M. Torne, M. Talwani
(Eds.), Proceedings of the NATO-ARW Workshop on
Rifted Ocean-Continent Boundaries, 11^14 May, 1994,
Mallorca, Spain, Kluwer Academic, Dordrecht, 1995,
pp. 197^217.
[32] G.E. Vink, Continental rifting and implications for plate
tectonic reconstructions, J. Geophys. Res. 87 (1982)
10677^10688.
[33] N.L. Christensen, M.H. Salisbury, Structure and constitution of the lower oceanic crust, Rev. Geophys. Space
Phys. 13 (1975) 57^86.
[34] R.S. White, D.P. McKenzie, R.K. O'Nions, Oceanic crustal thickness from seismic measurements and rare earth
element inversion, J. Geophys. Res. 97 (1992) 19683^
19715.
[35] I. Reid, H.R. Jackson, Oceanic spreading rate and crustal
thickness, Mar. Geophys. Res. 5 (1981) 165^172.
[36] J.W. Bown, R.S. White, Variation with spreading rate of
oceanic crustal thickness and geochemistry, Earth Planet.
Sci. Lett. 121 (1994) 435^449.
[37] S.P. Srivastava, C.E. Keen, A deep seismic re£ection pro¢le across the extinct mid-Labrador Sea spreading centre,
Tectonics 14 (1995) 372^389.
[38] B.J. Todd, I. Reid, The continent-ocean boundary south
of Flemish Cap: Constraints from seismic refraction and
gravity, Can. J. Earth Sci. 26 (1989) 1392^1407.
[39] M. Cannat, Y. Lagabrielle, N.D. Coutures, H. Bougault,
J. Casey, L. Dmitriev, Y. Fouquet, Ultrama¢c and gab-
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart
76
[40]
[41]
[42]
[43]
S.P. Srivastava et al. / Earth and Planetary Science Letters 182 (2000) 61^76
broic exposures at the Mid-Atlantic Ridge: Geological
mapping in the 15³N region, Tectonophysics 279 (1997)
193^213.
S.A. Dehler, C. Lowe, Determination of source depth and
location from potential ¢eld data: Examples from Canadian o¡shore basins, Trans. Am. Geophys. Un. 80 (1999)
F281.
X. Zhao, Magnetic signatures of peridotites rocks from
Sites 897 and 899 and their implications, in: R.B. Whitmarsh, D.S. Sawyer, A. Klaus, D.G. Masson (Eds.), Proc.
ODP Sci. Results, 149, College Station, TX (1996) 431^
448.
K.D. Sullivan, C.E. Keen, Newfoundland seamounts petrology and geochemistry, Geol. Assoc. Can. Spec. Pap. 16
(1972) 461^476.
R.H. Jackson, I. Reid, R.K.H. Falconer, Crustal structure
near the Arctic Mid-Ocean Ridge, J. Geophys. Res. 87
(1982) 1773^1783.
[44] P.R. Kelso, C. Richter, J.E. Pariso, Rock magnetic properties, magnetic mineralogy, and paleomagnetism of peridotites from sites 895, Hess Deep, in: C. Mevel, K.M.
Gillis, J.F. Allan, P.S. Meyer (Eds.), Proc. ODP Sci. Results, 147, College Station, TX, 405^413.
[45] J. Pablo Canales, R.S. Detrick, J. Lin, J.A. Collins, Crustal and upper mantle seismic structure beneath the rift
mountains and across a nontransform o¡set at the MidAtlantic Ridge (35³N), J. Geophys. Res. 105 (2000) 2699^
2719.
[46] M.R. Muller, T.A. Minshull, R.S. White, Segmentation
and melt supply at the Southwest Indian Ridge, Geology
27 (1999) 867^870.
[47] D. Kent, F.M. Gradstein, A Jurassic to present chronology, in: P.R. Vogt, B.E. Tucholke (Eds.), The Geology of
North America. Vol. M, The Western North Atlantic,
Geological Society of America, Boulder, CO, 1986, pp.
45^50.
EPSL 5586 12-9-00 Cyaan Magenta Geel Zwart