Motion of Iberia since the Late Jurassic: Results from detailed aeromagnetic measurements in the Newfoundland Basin

229

Tectonophysics, 184 (1990) 229-260 Elsevier Science

Publishers

B.V., Amsterdam

Motion of Iberia since the Late Jurassic: Results from detailed aeromagnetic measurements in the Newfoundland Basin * S.P. Srivastava a, W.R. Roest a, L.C. Kovacs b, G. Oakey a, S. LCvesque ‘, J. Verhoef a

and R. Macnab a “Atlantic Geoscience Centre, Geological Survey of Canada, Bedford Institute of Oceanography, P.O. Box IO06 Dartmouth, N.S. B2Y 4A2 (Canada) bNaval Research Laboratory, Washington, D.C. (U.S.A.) ‘Blue Vajra Computing. Halifax, N.S. (Canada) (Received

by publisher

March 20.1990)

ABSTRACT

Srivastava, S.P., Roest, W.R., Kovacs, L.C., Oakey, G., Levesque, S., Verhoef, J. and Macnab, R., 1990. Motion of Iberia since the Late Jurassic: Results from detailed aeromagnetic measurements in the Newfoundland Basin. In: G. Boillot and J.M. Fontbote (Editors), Alpine Evolution of Iberia and its Continental Margins. Tectono~hysics, 184: 229-260.

A detailed aeromagnetic survey carried out in the Newfoundland Basin shows well developed seafloor spreading anomalies 24 to 34. Comparison of these anomalies with the corresponding anomalies in the Northeast Atlantic suggests asymmetric spreading from anomalies 31 to 34, with slower spreading in the Newfoundland Basin. Anomaly MO is a very weak anomaly in the Newfoundland Basin compared to south of the Newfoundland Fracture Zone where it forms a prominent low within the large amplitude “J” anomaly. A similar behaviour of this anomaly is observed off Iberia. In the Newfoundland Basin it does not continue as far as the Flemish Cap but terminates in the vicinity of the Newfoundland Seamounts. The position of this anomaly as obtained here differs from previous id~tifications. The shapes of magnetic lineations in the Newfoundland Basin are significantly different from the corresponding lineations off Iberia. This has been interpreted as arising from shifts in the plate boundary between Africa and Eurasia during the time when Iberia was moving as part of the African plate. By combining the present data with other detailed survey data to the north we have been able to derive a plate kinematic solution for Iberia which shows that from the middle Cretaceous to the Late Eocene Iberia moved as part of the African plate and then as an independent plate until the Late Oligocene. Since then it has been moving as part of the Eurasian plate. During these times the boundary between Eurasia and Africa jumped successively from the Bay of Biscay accretion axis to the King’s Trough-North Spanish Trough lineament to the AzoresGibraltar Fracture Zone. The kinematic solution for Iberia so derived, from chron MO to the present, not only explains the formation of some prominent bath~et~c features in the oceanic regions, such as King’s Trough, but equally well the formation of geological features on land, such as the Pyrenees. The difficulties in deriving a kinematic solution for Iberia for times earlier than chron MO are discussed and a speculative position of Iberia at the time of its initial separation from the Grand Banks of Newfoundland is proposed. Furthermore, with the avaiiability of a well-constrained model for the motion of Iberia, it should now be possible to relate more accurately the relative motions among Eurasia, Iberia and Africa to the history of the Mediterranean region.

Introduction

Charlie Gibbs Fracture Zone in the north and the Azores-Gibraltar Fracture Zone in the south (Fig. 3) by Pitman and Talwani (1972), a number of other solutions have emerged, mostly based on the fit of magnetic anomalies and fracture zones (Kristoffersen, 1978; Srivastava, 1978; Olivet et al., 1984; Srivastava and Tapscott, 1986). Most of

FolIowing an early plate kinematic solution for the region of the North Atlantic bounded by the

*

Geological

Survey of Canada

0040-1951/90/$03.50

contribution

0 1990 - Elsevler

50389.

Science Publishers

B.V.

230

S.P. SRIVASTAVA

ET AL

MOTION

OF IBERIA

SINCE

these solutions regions

depict

reasonably

factorily

for

geological

the

moved

the evolution

of comparable

as one

plate

this time as a result of northward Attempts

motion

rocks

also

indicate

Iberia

and

Europe.

of Iberia

rotation

made

Europe

remove

this

separate

plate, or as a small plate that was caught

treating

like a ball bearing

in the relative

the large Eurasian

and African

Iberia

motion plates,

to

as a

between

of Iberia

Maastrichtian 1980;

relative

derived for Iberia. The continental

den

Iberia

of the large un-

strongly

could not have moved

the formation

so

of the Pyrenees.

suggests

with Eurasia

that

during

On the other hand,

kinematic models for present-day plate motions (Minster and Jordan, 1978; Argus et al., 1989) and the distribution now

moving

question

of earthquakes as part

which needs

show that Iberia

of’ Eurasia.

Therefore,

the

movement

between

show an anticlockwise 35” relative

or Aptian

to

and the

Berg

and

Zijderveld,

1982).

shown that a

large part of this rotation

(30”) took place during

the

interval

Hauterivian-Aptian of Iberia

the Early Cretaceous, not

a real

part

palaeomagnetic vide

enough

solution

and

was linked of rotation according

the

of Iberia during

to them, was larger

which implies

of the African measurements

constraints

that

to the movement

that Iberia plate.

alone

to derive

was

However,

do not proa kinematic

for Iberia.

There are, thus, several alternatives for the motion of Iberia before it started to move with Eurasia as it is doing

today.

Either

it. moved

indepen-

is how did

dently,

or as part of the African

Iberia move in the past? To solve this problem continent-based geological information alone is

nation would

of both. To decide between these solutions necessitate using a relatively undisturbed

not

record

of the relative

sufficient

because

structures present. does not provide Iberia

to be answered

is

of continental

et al. (1989) have recently

than that of Africa, geology

ob-

(Van der Voo, 1969; Van den Berg,

Van

of Africa. The amount

because

provide

the geological

the Barremian

remains

unsolved

regions

of approximately

between

Galdeano

(Le Pichon

which exist in the poles of rotation

spreading

oceanic

They

movement

largely

for Iberia.

to explain

et al., 1977; &later et al., 1977; Olivet et al., 1984; Savostin et al., 1986). However, the problem still certainties

solution on seafloor

measurements

Early

to Eurasia.

by

required

Palaeomagnetic

relative

ambiguity

have been

for the adjacent

during

the

based

servations.

and Eurasia

during

kinematic

solutions

the context

plate-kine-

were formed

plate

Acceptable models

prominent

early

that Iberia

Yet, the Pyrenees

satis-

age on adjacent

the

postulated

an accurate

of the oceanic

of some

example,

together

Cenozoic.

231

JURASSIC

formation

For

matic solutions

LATE

well, but fail to account

feamres

landmasses.

THE

of the complexity

of the

At the same time, the geology a well constrained position of

for any given time in the past, due largely

to the fact that no universally

acceptable

deforma-

involved.

motions

Such a record

most of the Cenozoic

among

can be found,

the plates at least for

and later part of the Meso-

zoic, in the seafloor-spreading and fracture

plate, or a combi-

zone patterns

magnetic

lineations

in the North

Atlantic

tion history of the Pyrenees exists. For example, Le Pichon et al. (1971, 1977) have suggested a

Ocean.

transcurrent motion between Europe and Iberia in the late Early Cretaceous as the history of the Pyrenees for this period was dominated by strikeslip motion along the ENE-trending North Pyrenean Fault. On the other hand, others (e.g.

the seafloor-spreading history between plates, one can determine their kinematics independently and thereby obtain the kinematics for Iberia relative to Europe and Africa. It is impossible to derive a well-constrained kinematic solution for the relative motion between an independent Iberia and North America from seafloor-spreading data because of the short length

Peybernes and Souquet, 1975; Souquet et al., 1977, 1980) have argued that the Early Cretaceous evolution of the Pyrenees was dominated by compression and NE-trending transcurrent faults, indicating rotational rather than transcurrent movement of Iberia relative to Europe. Thus continental geological information alone cannot constrain

tions

By matching and fracture

synchronous zone systems

magnetic

linea-

that characterize

of the magnetic lineations and the lack of well-defined fracture zones. Furthermore, the lack of detailed seafloor spreading data, especially in the Northwest Atlantic, has made it difficult to do so.

232

The Azores-Gibraltar Fracture Zone (including the Gloria Fault centred at 37.3”N, 22”W; Laughton et al., 1972; Laughton and W~tmarsh, 1974; Searle, 1979) which lies south of Iberia (Fig. 1) is the present plate boundary between Eurasia-Iberia and Africa (e.g. Argus et al. 1989). Part of this fracture zone was also a plate boundary between Iberia and Africa during the opening of the Central Atlantic from the Jurassic to the middle Cretaceous (Klitgord and Schouten, 1986) and again from the Eocene to the Middle Miocene as is evident from the strong tectonism seen in the Iberian Betic and African Rif mountain systems (Hovarth and Berckhemer, 1982). This more recent activity along this fracture zone makes it difficult to use it as a constraint for a plate kinematic solution for Iberia, although some authors have used it (Olivet et al., 1984). Similarly, in the north, the prominent series of bathymetric features extending from King’s Trough to the Pyrenees along the Azores-Biscay Rise and North Spanish Trough (Fig. 1) has been suggested as a plate boundary between Eurasia and Iberia from the Late Cretaceous to the Palaeocene (Le Pichon and Sibuet, 1971; Searle and Whitmarsh, 1978; Kidd et al., 1982; Whitmarsh et al., 1982; Grimaud et al., 1982; Schouten et al., 1984; Klitgord and Schouten, 1986). However, because of the difficulty in estimating the timing and magnitude of relative motion across this boundary, it cannot be used as a constraint in plate kinematic modelling. A better solution to the problem would be to consider Iberia as part of Africa, as suggested by palaeomagnetic measurements and later adopted by Schouten et al. (1984). This has the advantage that well-constrained poles of rotation can be obtained because of the size of the plate involved (thousands of kilometres versus hundreds of kilometres) and the presence of numerous fracture zones. The motion along the King’s TrouglPyrenees boundary can then be determined independently from the differential motion between the North America-Eurasia and the North America-Africa plate systems (Klitgord and Schouten, 1986). The idea of Iberia moving as part of the African plate was originally explored by Schouten et al. (1984). This was subsequently used by Srivastava and Tapscott (1986) and Klitgord

S.P. SRIVASTAVA

ET AL.

and Schouten (1986) who showed that from the Early Cretaceous to the Oligocene Iberia moved as part of Africa with a plate boundary between Eurasia and Africa extending from King’s Trough to the Pyrenees. After a jump of the plate boundary to the present Azores-Gibraltar Fracture Zone, Iberia started to move as part of Eurasia. In 1987 a detailed aeromagnetic survey was flown in the Newfoundland Basin by the Geological Survey of Canada and the US Naval Research Laboratory. Comparison of these data and previously collected detailed data in the western North Atlantic with the existing compilations in the eastern North Atlantic have been used by Srivastava et al. (1990) and Roest and Srivastava (submitted) to modify the Schouten et al. (1984) model. It was shown that the boundary along King’s Trough was only active from the Middle Eocene to the Late Oligicene. Before that, the boundary between Iberia and Eurasia was located further north in the Bay of Biscay region. The purposes of the present paper are: to document the results of the aeromagnetic survey in detail, to quantify the motions that took place in this part of the North Atlantic by using the results from this survey in conjunction with a detailed compilation of the magnetic data from the eastern North Atlantic (Verhoef et al., 1986), and to discuss the implications of these motions for the syn-rift evolution of this and the neighbouring regions. Magnetic data

Figure 2 shows the flight paths of the 1987 aeroma~etic survey in the Newfoundland Basin together with flight paths and ship’s tracks from previous magnetic surveys. Throughout the 1987 aeromagnetic survey the navigation was maintained using the Global Positioning and Inertial Navigation Systems, resulting in a positional accuracy of better than 1 km. Figure 3 shows the ma~etic anomalies calculated relative to the International Geomagnetic Reference Field (IAGA, 1986) along tracks from the 1987 survey and from an earlier survey to the north (Srivastava et al.,

MOTION

55O

OF IBERIA

SINCE

THE

LATE

233

JURASSIC

60°

survey discussed network

500

550

60° Fig. 2. Map showing

the distribution

here is bounded

of SE-NW

450

of ships’ and aircrafts’ by a box. Numbers

flight lines in the south-central

tracks with magnetic

at the eastern part

represents

Naval Research

carried out by the US Naval Research Laboratory in 1977 (Vogt, 1986) over the I-anomaly south of the Newfoundland shows the presence

Fracture Zone. of well-developed

The figure anomalies

caused by seafloor spreading which can easily be correlated from flight path to flight path. The identification of the magnetic anomalies, shown in Fig. 3, was by a comparison against synthetic anomalies based on the DNAG (Decade of North American Geology) time scale described

350

end of the box refer to flight line numbers an aeromagnetic

Laboratory

1988). Also shown in this figure are the anomalies along some tracks from an aeromagnetic survey

400 ~ data. The region of the detailed survey

1987 aeromagnetic used in Fig. 4. The

flown over the J-anomaly

by the US

(Vogt, 1986).

by Kent and Gradstein (1986). Figure 4 shows the identified anomalies and their correlation between tracks. There are noticeable variations in distances between anomalies due in part to the fact that the tracks for these anomalies are not projected along the mean direction

of spreading

and the anomalies

are not reduced to the pole. As can be seen in Fig. 3, south of the Flemish Cap the tracks intersect the anomalies at different angles because they fan out from west to east. In addition there is a variation in spreading rate. Average spreading rates corresponding to selected anomalies were

234

Fig. 3. Plot of magnetic

S.P. SRIVASTAVA

anomalies

The data south of the Newfoundland

along

flight paths.

Fracture

Identifications

of anomalies

Zone are from a US Naval

Research

based on this data are shown Laboratory

aeromagnetic

ET AL.

in Figs. 1, 4 and 6. survey

carried

out in

1971 (Vogt, 1986).

calculated on the basis of their relative distances from anomaly 330 (old) (Fig. 4), and are plotted in Fig. 5. These rates of spreading were used for the three models shown in Fig. 4. To examine the possibility of asymmetric spreading between the two sides of the Atlantic the spreading rates obtained from the stage poles of rotation between

anomalies (dashed lines) are also shown in Fig. 5. Points younger than anomaly 31 along the continuous line in Fig. 5 can be fitted with the spreading rates obtained from the poles of rotations (dotted lines), suggesting asymmetric spreading with slower spreading for anomalies 31 to 33 on the western flank of the ridge axis. As shown later

MOTION

OF IBERIA

SINCE THE LATE JURASSIC

235

7.66 mm*0

_

MO

34 13.00

600 nT

0

km

mm/y+42

330

mmS;;j

33Y

&

mm/y

31

mm,yr.36

218

26

25

mm,y%zt*

mm/y

248 mm/y

100

600 L---

s

I-J-1 Fig. 4. Correlation

between

(old). For location,

see flight numbers

as obtained

for the top ten profiles

as the intensity

remanent

-21”

to south mainly J-anomaly

anomalies

off Newfoundland.

in Fig. 2. The first two model profiles time scale (Kent

belonging

of magnetisation.

field and 60” and

field and 66” and north

and computed

from Fig. 5 based on the DNAG

rates obtained A/m

observed

Angles

to North

- 19” for the present

for the present

survey south of the Newfoundland

Fracture

Computations

for the southern

model

relative

to anomaly

for the spreading

were carried were 43” and

model these angles were 51* and

show that the difference

in palaeolatitude.

are aligned

were calculated

330 rates

1986). The top model is based on the mean spreading plate system.

and declination

field. For the nor&em

field. The models

arises due to differences

and Gradstein,

America-Eurasia

of inclination

The profiles

from the bottom

The bottom-most

in the skewness five profiles

Zone and show the characteristic

-lSO

observed

in the left-hand

shape of anomaly

out using 6.5 -IfSo

for the

for the remanent

in anomaly comer

34 from

are from the

MO in this region.

S.P. SRIVASTAVA

o!O-

O-

Fig. 5. Spreading

rates (continuous

lines) as calculated

from

the data shown in Fig. 4. Also shown are the rates of spreading calculated

from

the differential

poles

of rotation

between

anomalies (circles) for Iberia relative to North America (dashed lines). The dotted tines are drawn parallel to the dashed lines to show that the observations spreading for anomalies

can be fitted with these rates of

younger than 31. Vertical bars are the

standard deviations.

this may be related to the differential motion of Iberia relative to Eurasia creating excess material not only along the boundary between these two plates but also on the eastern flank of the ridge axis. The distance of anomaly MO from anomaly 330 decreases gradually from south to north (Fig. 4 and Fig. 1). The same is observed in the Northeastern Atlantic (Fig. 1). However, the distance between anomaly MO and anomaly 34 is larger on the western side compared to on the eastern side, again showing an asy~et~c spreading with a higher rate of spreading on the western side. To examine these and other characteristics in the data from this region as well as to create a coherent data base for use in the reconst~ction of the North Atlantic, all available data in this region were compiled, adjusted and gridded. Observations along the track and flight lines were gridded at twenty points per degree of latitude and longi-

ET AL.

tude by means of an algorithm that combined an adjusted digital filter with a weighting method (Verhoef et al., 1986). A variety of display and ma~pulation techniques was used to locate and eliminate erroneous data. The details of the technique are described in Verhoef and Macnab (1988). The resulting data set was then reduced to the pole (for details see Srivastava et al., 1988) and is displayed as a colour shaded relief image in Fig. 6. The compiled data show very clearly the seafloor-spreading anomalies 34 to 21 (Figs. 3 and 6). The anomaly trends change si~ificantly from south to north. In the south they show a NNESSW trend, changing gradually to NNW-SSE north of Mime Seamount (Fig. 6). This is similar to the pattern observed on the eastern side of the Atlantic where the change occurs at the mouth of the Bay of Biscay (Fig. 1). The most noticeable Iineation, formed by the negative anomaly between anomalies 34 and 33 (Figs. 3 and 4), changes its character considerably from south to north. In the south it is very asymmetric with a larger negative lobe tending towards anomaly 33. To the north it becomes more and more symmetric. A similar variation in skewness is observed in the eastern North Atlantic. The model calculations also show a clear change in shape (Fig. 4), but the observations appear to indicate some anomalous skewness (cf. Arkani-Hamed, 1990) on both sides of the ridge. Such a difference in the shape of this anomaly is less noticeable in Fig. 6 where the data have been reduced to the pole. Anomaly MO is not as well developed north of the Newfoundland Fracture Zone as it is south of this zone, where it is defined as a prominent low within the large-amplitude anomaly J (Figs. 3 and 4). A similar observation was made by Rabinowitz et al. (1979). Seismic reflection measurem~ts across MO show the presence, in places, of prominent basement highs forming a ridge under MO (Tucholke et al., 1989; Tucholke and Ludwig, 1982), thereby giving rise to large variations in the shape of the J-anomaly in the northern region. The high density of data in the Newfoundland Basin has resulted in a more accurate location of anomaly MO (Figs. 1 and 6) than was previously possible (Srivastava and Tapscott, 1986).

MOTION

OF IBERI4

Fig. 6. Shaded

SINCE

THE LATE

relief map of magnetic

from Fig. 3. The anomalies data whose track control and gridding, feature

A.

anomalies

have been reduced

in the Northwestern

and Macnab

Seamount;

TR =

Atlantic

together

to the pole and are illuminated

is shown in Fig. 2. For a description

refer to Verhoef

MS = Mime

237

JURASSIC

(1988). Thulean

of the data and of the procedures

Also shown are 1000, Rise;

with the location

from the northwest.

CGFZ

3000 and 4000

= Charlie

Zone.

Gibbs

employed

m isobaths,

Fracture

of the anomalies

Zone;

as obtained

The map was compiled

from the

for compilation,

the locations

levelling

of boundary

NFZ = Newfoundland

B and

Fracture

238

S.P. SRIVASTAVA

North

of the

Newfoundland

Fracture

lineations

are offset across a lineament

3, 4 and

6). Another

feature

labelled

the latter

disruption

“B” (Fig.

corresponds

of semi-lineated

between

farther

west,

anomalies

34 and

long-wavelength,

occur

the semi-lineated

boundary.

(Figs.

Quiet

fea-

Zone.

anomalies

anomalies

low-amplitude,

west of anomaly

14O

12O

of

34 is

Magnetic

Th ough comparable in amphtude younger than 34, these anomalies

not share the characteristics

can

MO, while

3, 4 and 6). The cause

not clear, as they lie in the Cretaceous

Seamount

anomalies

be seen anomalies

a

that

between

with Milne

16O

42'

along

6). We will show

in anomalies

tures A and B is coincident (Fig. 6). A number

“A” (Figs.

occurs

to an old plate

Part of the disruption

Zone,

ET AL.

of those produced

lo"vv

to do by

42'

38O

38'

36'

36'

16O

14O

Fig. 7. Plot of magnetic

anomalies

eastern

(A) limits of the ocean crust generated

(B) and western

of the spreading

along

track, showing

centre to the west (Mauffret

IO0 w

12O correlation

and identification

of anomaly

in this region between

anomalies

et al., 1989). The location Klitgord

of the Azores-Gibraltar

and Schouten

(1986).

MO off Iberia.

Also shown

MlO(?) and M21(?) before

Fracture

Zone ( AGFZ)

are the a jump

is taken

from

MOTION

OF IBERIA

SINCE

THE

LATE

JURASSIC

reversals in the Earth’s magnetic field; it is possible that they are caused by variations in the basement topography. Sea-~neated anomalies can also be seen in the region south of Newfoundland Fracture Zone and east of anomaly MO, where they are much smaller in amplitude (Fig. 3). One such anomaly was identified as anomaly “X” by Masson and Miles (1984) on both sides of the North Atlantic. Large-amplitude anomalies lie over the Newfoundland Fracture Zone, reflecting the massive amount of volcanic flows in this region. The large-amplitude anomaly associated with the Janomaly south of the Newfoundl~d Fracture Zone (Figs. 3 and 6) seems to swing to the east, forming the southeast end of the Newfoundland Ridge. It has been suggested that part of the ridge is underlain by foundered continental crust (Grant, 1979).

The identification of magnetic anomalies in the eastern North Atlantic by a number of workers has been summarized by Srivastava and Tapscott (1986) and Srivastava et al. (1988). Figure 1 shows the locations of various anomalies as published in these papers. Selected profiles off Iberia (Fig. 7) show that south of the Azores-Gibraltar Fracture Zone in the vicinity of 37’N, anomaly MO can easily be identified as a prominent low. As in the western North Atlantic north of the Newfoundland Fracture Zone, anomaly MO is not well developed off Iberia and lies within the high-amplitude J anomaly. A number of semi-lineated anomalies similar to those in the western North Atlantic can also be seen west of anomaly MO (Fig. 7) . Figure 7 also shows the boundaries mapped by Mauffret et al. (1989) in the Tagus Abyssal Plain, where spreading is supposed to have taken place in the Late Jurassic prior to a jump to the west. However, no recognizable seafloor spreading anomalies can be seen in this region. Iberian plate kinematics

The lineations in the eastern North Atlantic (Fig. 1) show that anomaly 34 can be identified to

239

the north and south of the Bay of Biscay as well as within it, suggesting the existence of a triple junction at the mouth of the Bay of Biscay at anomaly 34 time (Kristoffersen, 1978). It seems then likely that at chron 34 Iberia was moving relative to Eurasia, thereby opening the Bay of Biscay (Kristoffersen, 1978; Montadert et al., 1979). The absence of anomaly 33 and younger in the Bay of Biscay shows that there was negligible opening in the Bay of Biscay at subsequent times. By this time, either Iberia had started to move as part of Eurasia or it was moving separately or as part of Africa. The possibility that Iberia was moving as part of Eurasia can be ruled out because of the difficulty in obtaining for each chron a satisfactory and simultaneous match between the western anomalies, when rotated to the east, with the corresponding eastern anomalies off Eurasia and Iberia (Fig. 8). The results clearly show that anomalies 33 to 21 cannot be matched individually by single rotations. A similar mismatch between eastern and western anomalies 33 was also observed by Kristoffersen (1978). However, when separated into three zones, as shown in Fig. 9, the segments of each anomaly can be matched very well. This is not due to the presence of three microplates, one for each zone, but is instead due to successive shifts of the boundary between Eurasia and Iberia. In Zone 1, north of the Bay of Biscay (Fig. 9), the western anomalies north of boundary “B” (Fig. 6) can be satisfactorily matched with the corresponding eastern anomalies off Eurasia, suggesting that these anomalies were formed as part of the North America-Eurasia plate system (Srivastava and Tapscott, 1986; Srivastava et al., 1988). Such a match to the north of the Bay of Biscay and the lack of it to the south (Fig. 8) led Srivastava et al. (1990) to adapt the model of Schouten et al. (1984) of treating Iberia as part of the African plate and the boundary between these two plates as jumping successively to discrete locations. Using the detailed data from both sides of the Atlantic (Fig. 1) Srivastava et al. (1990) were able to demonstrate that such is indeed the case. They showed that boundary B, which extends west of the Bay of Biscay (Figs. 8 and 9), was the main boundary between Eurasia

S.P. SRIVASTAVA

240

and Africa

from chrons

late as chron 6 when Iberia started to move with Eurasia. Such a model explains not only the for-

33 to 19 when it jumped

to the King’s Trough region. The boundary along King’s Trough remained in existence possibly as

mation

200 w Fig. 8. The average western

anomalies

fit between

western

have been rotated

the east (blue lines) by assuming better

fit between

AGFZ

= Azores-Gibraltar

western

and

(dotted

eastern

Fracture

and Eurasia

lineations

Zone (after

moved

the best overall as one plate.

can be obtained, Klitgord

as shown

and Schouten, 1986); GB = Galicia Bank.

and the Pyrenees

(shown

100

(heavy lines) magnetic

to the east (red dots) to obtain

that Iberia

of King’s Trough

15O

lines) and eastern

ET AL.

lineations.

In this figure the locations

fit with the corresponding

By division

of the lineations

in Fig. 9. CGFZ = Charlie KT = King’s

Trough;

magnetic

of the

anomalies

in

in three zones, a much Fracture

Zone;

ABR = Azores-Biscay

Gibbs

Rise;

blOTION

OF IBER1.4

SINCE

THE

LATE

241

JURASSIC

later) but also the cause of the differences in the shapes of the anomalies on the two sides of the Atlantic. For example, anomalies 33 to 21 in zones 2 and 3 were originally formed as part of one zone, but when plate boundary B jumped south to Ring’s Trough the anomalies in Zone 2 underwent additional rotations relative to those in Zone 3 due to their movement with the Eurasian plate. As a result, when we rotate the western anomalies, co~esponding to those in Zones 2 and 3, to the east we find that they cannot be matched completely with the eastern anomalies.

TABLE

The proof that anomalies in zones 2 and 3 (Fig. 9) were formed as part of the North AmericaAfrica plate system lies in the fact that by using NAM/AFR poles (Klitgord and Schouten, 1986; Roest, 1987) (Table 1) the western anomalies can be matched with the corresponding eastern anomalies off Iberia after one additional rotation (pole Pr, Table 1) for Zone 3 and two additional rotations (poles P, + Pz, Table 1) for Zone 2. The additional rotation (about pole Pi) needed for Zone 3 would then be the total differential motion between Iberia and Africa along the Azores-

1

Reconstruction Anomaly

poles for Africa, Age

’

Cap relative

Africa

(Ma) 6

Iberia and Flemish

20

to North

America

Iberia

Lat

Long

(+“N)

(+”

81.07

Angle E)

(+”

56.51 2.22

E)

Flemish

Lat

Long

Angle

(+“N)

(+“E)

(+”

- 5.21 s

68.00

138.20

E)

-4.75

13

35.5

16.28

-9.96

3

76.34

117.33

- 7.98 ’

49.5

73.69

-6.11

-15.46

3

74.70

126.96

- 11.05 6

24

55.6

78.33

- 2.64

- 16.91 ’

72.98

133.28

- 12.94 6

25

59.0

80.02

-0.73

- 18.11 3

73.29

133.88

- 14.25 6

31 4

69.0

82.51

-0.63

-20.96

’

74.96

135.34

- 17.19 6

33 old

80.2

78.30

- 18.35

-27.06

’

85.49

110.28

- 22.41 ’

34

84.0

76.55

- 20.73

-29.60

*

87.18

57.43

- 24.67 6

MO

118.0

66.09

- 20.18

- 54.45 3

68.88

-15.00

- 50.62 ’

Ml0

131.5

65.95

- 18.50

-57.40

a

68.57

- 13.11

- 53.64 ’

M25

156.5

66.70

- 15.85

-64.90

3

66.90

- 12.93

- 60.45 ’

BSA

170.0

67.02

- 13.17

-72.10

’

ECMA

175.0 2

65.97

- 12.76

- 76.44 3

Correction 0

31.43-18.58 40.89-15.55

-4.10

PI +

34.73-17.90

- 11.93

Note:

p2

Reconstruction

Magnetic

poles

Anomaly;

Most anomalies,

describe

relative

with the exception polarity

interval

and Schouten

positions

Magnetic

of anomalies

of Africa

and

Iberia

with

respect

34 and 33, were picked

at their positive

of 0.5 Ma. For example,

the normal

20 Ma.

(1986).

Best fit. Anomaly

30 (Khtgord

and Schouten,

1986).

Ohvet et al. (1984). Calculated

using the African

Based on fit of geological Correction

45.35

- 47.62

- 19.96 ’

E)

poles and pole P,.

boundaries

poles give the total motion

and not isochrons, relative

to North

America.

BSA = Blake Spur

Anomaly.

with an accuracy

19.35 to 20.45 Ma giving a mean of about Klitgord

Angle (+”

- 7.87

ECMA = East Coast

middle of the normal

Long (+“E)

poles 6

P1 44-23.5

P,44-

Lat (+“N) 5

21

Duration

Cap

to Africa

approximate during

age.

the indicated

periods.

peaks. polarity

Their interval

ages correspond for anomaly

to the 6 is from

242

S.P. SRIVASTAVA

Gibraltar

Fracture

boundary

B became

additional

rotation

for Zone

2 would

between

Iberia

Trough-Azores boundary

Zone as a plate boundary extinct. (about

Similarly

pole P2, Table 1) needed

be the total differential and

Eurasia

along

19 to possibly

motion

the

Biscay Rise-Pyrenees

from chron

after

the second

King’s

as the plate 6.

If we had chosen Iberia as an independent plate during the formation of anomalies 33 to 21, such a well constrained Iberia

could

sons: (1) no fracture Gibraltar

solution

for the motion

not have been obtained

Fracture

used as a constraint

zone (except Zone

(AGFZ))

in deriving

200

150

of

for two rea-

for the Azoreswhich

can be

the pole positions

A NAM / AFR + corr. 1 + corr. 2

25O

ET AL.

1oow

MOTION

OF IBERIA

for Iberia

SINCE

THE

lies west

large uncertainty positions

LATE

JURASSIC

of the peninsula,

Iberian

plate. Also, as explained be used as a constraint of recent

1974; ellipses

activity

earlier, the AGFZ because

1979).

of over-

on this fracture

et al., 1972; Laughton

Searle,

(2) a

the pole

of the small size of the

cannot

(Laughton

and

would lie in determining

(Fig. 10) because

printing

243

Figure

zone

and Whitmarsh,

10 shows

the large

of error for each pole as calculated

the method average

of Stock

fit of the

small

ellipses

part

of Africa.

path

remains

and

Molnar

anomalies,

of error

together

when

using

(1983)

Iberia

for an

with

the

is treated

as

As can be seen, the average the same

except

that

pole

in the latter

\

case it is much more constrained. Evolution of the Iberian plate boundaries Here we describe briefly the spatial and temporal evolution of the different plate boundaries

Fig.

10. Positions

relative

to North

uncertainty method

(reconstruction) Large

in the pole

positions

of Stock and Molnar

an independent spond

of finite America.

plate.

to pole positions

treated

as part

great

at the centre

circle

The three partial

reconstruction

from

Because

have significant

anomalies

Fig. 9. Fit between (for clarity,

plate allowed

western

and

plate

uncertainty

and eastern

(lines) lineations

(Zone 2; for clarity, in a perfect

boundary

were rotated

resulting

(Zone 3). To improve

to the east (red squares)

show a perfect

perfect

fit when rotated

part of the Eurasian

match

using the NAM/POR

Similarly,

when they are considered

KT-ABR

before

boundaries, dashed

represented

(previous

eastern

lineations

of the 21 lineations across

by poles Pz and P, respectively, marked

at the time of the nearest

except

boundary

Arrows

to the east

1). Circles move

of - 7.87” about

of -4.1° anomalies

(Srivastava

et al., 1988; Srivastava

for chron

that by chron

between

the instantaneous

plate system in this 6 which

only show a

6, Iberia had started

of the 13 lineations

show that boundary

across other boundaries

pole Rise

rotation

and the Azores-Gibraltar B show

rotation

as part of the AFR/NAM

and the match system

(Table

(in Zone 2) a second

for the lineations

are shown by the displacement

along

isochron.

plate KT-ABR

were first rotated

of the Bay of Biscay the western

plate) poles of rotation

plate system. This clearly indicates

motions

lineations

south of the King’s Trough/Azores-Biscay

B when treated

in

(submitted).

poles of rotation

of King’s Trough

(Porcupine

as part of the NAM/EUR

lines. Arrows

north

lineations

and black lines). North

south of boundary

the mismatch

13. The cumulative

positions)

boundary

lineations

with the corresponding

boundary

chron

(solid blue triangles

as part of the NAM/EUR

plate.

of western

from the

of this can be found

circles not shown here) when a further

fit with the eastern

the fit for lineations

15.5”W was applied

and Roest, 1989). Notice that all western fashion

where locations

them as obtained

Details

Roest and Srivastava

are shown in Zone 3 with black circles) using NAM/AFR

18.6”W is applied,

pole Pz at 40.8”N,

along

poles of rotation.

poles for

to red dots (Zone 3) and to open triangles

about

and the motion

do not

P, at 31.43”N, (KT-ABR)

i

to the

of 7.5 km

a shift of 30 km.

(symbols)

only a few locations

I

the actual

is orthogonal

on the Iberian

120” E

pole is the

displacement

offsets, the second partial

the Iberian

is

rota-

The first pole

the second

pole, and the third rotation

as

when Iberia uncertainty

this point

first two. For each pole a maximum was allowed.

the the

with lines corre-

as follows:

of the anomaly,

pole determined

show

following

when Iberia is treated

ellipses shown

tions for the ellipses were determined is located

dots),

with their uncertainties

of Africa.

for Iberia

ellipses

(large

(1983)

Smaller

poles

shaded

north

B must have moved Fracture

Zone

continuous motion

show compression

took

to location

(AGFZ)

(present

that

to move as

of the KT-ABR as plate

positions) place

and extension.

along

and this

244

The Bay of Biscay plate boundary B

The seafloor magnetic anomalies in the Bay of Biscay show that it mainly opened between the Aptian and the late Campanian (Montadert et al., 1979; Srivastava et al., 1988). However as the above interpretation (Fig. 9) suggests, it was the locus of a plate boundary (B) between Eurasia and Africa for a much longer period of time. Figures 1 and 9 show offsets in anomalies 31 to 21 along this boundary which change from dextral at chron 31 to sinistral at chron 21. The fact that no corresponding offsets are observed on the western flank of the Mid-Atlantic Ridge points to differential motion across this boundary. Such motions, obtained from the differential poles of rotation between Eurasia and Africa, are shown in Fig. 9 with arrows. (We have used the poles of the Porcupine plate for the oceanic regions of the Eurasian plate throughout this paper. Srivastava and Tapscott (1986) showed that this part of the Eurasian plate underwent a slight rotation relative to the continental part of the Eurasian plate in the Late Eocene.) The motion was mainly extensional between chrons 33 and 3I, and changed gradually to strike-slip (with still a small component of extension) between chrons 31 and 21 (see also Fig. 18). Thus, a triple junction which was ridgeridge-ridge (R-R-R) in nature gradually changed to ridge-ridge-transform (R-R-T). The fact that such a triple junction (R-R-T) is unstable (McKenzie and Morgan, 1969) may partly explain the shift of plate boundary B to King’s Trough by chron 19. To examine the trace of this triple junction on the North American plate we carried out a reconstruction of the North Atlantic at chron 21 (Fig. 11) using gridded magnetic and bath~et~ data (ETOPOS, 1986) from both sides of the North Atlantic following the technique of Verhoef et al. (1989). The trace of the triple junction on the North American plate can only be seen as a series of disruptions in the magnetic anomalies 34 and 33 (Fig. llb) that matches well with boundary B. Such a trace in the triple junction is not observed in the bathymetry (Fig. lla). Also, we do not see evidence of boundary B on the Iberian side west of 31”W in the reconstruction framework. This

S.P. SRlVASTAVA

ET AL.

may be partly due to the strike-slip nature of the motion along boundary B from chron 31 to 21. However, large ba~ymet~c features are formed along the eastern part of boundary B because the earlier (pre-chron 31) motion along the boundary was extensional. In the area between boundary B and King’s Trough, the total correction pole P, + P2 improves the fit for anomalies 34 to 21 (Fig. 9) but not for anomaly 13. This suggests that boundary B between Africa and Eurasia became inactive after chron 21 but before chron 13. From the calculation of the differential poles of rotation between Africa and Eurasia it is found that it was at about chron 19 that boundary B became inactive. It was at this time that boundary B jumped to the King’s Trough region along a boundary that extended from King’s Trough to the Pyrenees. When motion along this boundary stopped, Iberia became part of Eurasia as it remains today. Roest and Srivastava (submitted) have suggested an independent motion of Iberia between chrons 19 and 6c. Serious problems arise if Iberia is considered as part of Africa at chron 13. For example, an overlap between Eurasia and Iberia is found along the Pyrenees in the plate reconstruction for this time. This implies that extension would have taken place along the Pyrenees in subsequent times, which is unacceptable in view of the known compression in this region. Furthermore, the formation of the Iberian Betic and African Rif mountain systems during the Tertiary suggests that there was motion between Iberia and Africa at chron 13. King’s Trough-Azores-Biscay

Rise-North

Spanish

Trough plate boundary

A plate boundary between Eurasia and Iberia, linking King’s Trough, the Azores-Biscay Rise, the North Spanish Trough and the Pyrenees, has been postulated by several authors (Le Pichon and Sibuet, 1971; Le Pichon et al., 1977; Searle and Whitmarsh, 1978; Grimaud et al., 1982; Schouten et al., 1984; Srivastava and Tapscott, 1986; IUitgord and Schouten, 1986). However, until now, the motion along this boundary was poorly constrained because of the uncertainties in the kine-

MOTION

OF IBERIA

SINCE

matics

of Iberia.

THE

lies from

compilation

this region

decrease

significant

younger

offset

anomaly

6c are the first anomalies continuation

which

across this region.

Ring’s Trough

may have become

considered be

21 to 6.

Fig.

slip

motion

with

some

Rise in the plate

Whitmarsh

presence

of

a V-shaped

that

north

shown

the Azores-Biscay

and south

character

of Ring’s

changes

in

Rise Trough,

from a continuin

strike-

along

the

The Azores-Gibraltar complex and prominent

Azores-Biscay Rise. The amount of extension agrees well with the present width of Ring’s

tures in the North

plate boundary follows a series of bathymetric fea-

Atlantic.

This area has been the

Trough (about 70 km). Kidd and Ramsay (1986) suggested that most of Ring’s Trough was formed

locus of a plate boundary at different the motion across it has been complex.

by intraplate

discussed,

volcanism

et al. (1982)

trace, which seems to

Azores-Gibraltar plate boundary

extension

compression

the

We note both

linked

ous ridge in the north to individual seamounts the south (fig. 2 of Whitmarsh et al,. 1982).

by chron

and mainly

by

but its seismic

that

unclear.

high in the reconstruction

lla.

extends

show a definite

(Fig. 9). It shows a maximum

of 50 km across Ring’s Trough

supported

6 and possibly

inactive

was directly

phase of the Pyrenees.

it to be a hotspot

bathymetric

6c at the latest. Rotation pole P2 (Table 1) gives the integrated motion across the Ring’s Trough boundary

remains

for anomalies

This suggests

et al., 1982) that the

Trough

The role of the Azores-Biscay boundary

in the anomalies

is observed

than 10, although

chron

a gradual

going from anomaly

of King’s

with the compressive

anoma-

12) shows

in the offset (or bends)

across Ring’s Trough No

19 to perhaps

1971; Grimaud

formation

suggest that this

of the magnetic

(Fig.

and Sibuet,

poles of rotation

submitted)

existed from chron

6. A detailed

245

JURASSIC

The differential

(Roest and Srivastava, boundary

LATE

(32 Ma) followed

by ex-

it was a plate boundary

times, and As will be

during

the early

20 of

evolution of the central North Atlantic when Africa slid along the Grand Banks and Iberia. It

Ring’s Trough which suggests

seems to fit well with our model that a boundary extending from

again became a plate boundary during the early Palaeogene (chron 19) when Iberia started to move

Ring’s

to the Pyrenees

tensional subsidence and rifting between about and 16 Ma ago. Such timing for the formation

Trough

was in existence

from 44 to 25 Ma. The model indicates tive compression North Spanish The

main

ended

phase

of orogenesis

1974). Another

in the

phase of Pyrenean

observed

folding

extension

Alboran

Berckhemer,

speculations

Fig. 11. Shaded relief maps of reconstructions from the northwest. and Iberian

(pole 74.69”N,

has been closed relative

to Iberia

regions

show

bathymetric

The reconstructions 126.96”E,

by rotating

lack

prior

high formed

-11.05’)

Pichon

of the North

are obtained

the portion

to its rotation

of data.

(Le

formed

to the west. White

(a) Bathymetry

Atlantic

by rotating

reconstruction

between regions,

rises (TR).“A”

is a discontinuity

boundary

and

trace

Eurasia

Iberia.

The

data

boundaries north

GB = Galicia

of the triple

basin

section

of Galicia

relative

triple junction

Bank indicate

anomaly

anomalies

is shown

(Hovarth

140.81”E,

due to extension pole Pz (40.9”N, later compression.

(ETOPOS,

and

the sense of mo-

The maps are illuminated

(pole 61.06’N,

formed

bathymetry

the Azores-Biscay

junction

Bank (b) Magnetic

America.

B and KT around

in the magnetic

region

1982). Furthermore,

for the Eurasian

present-day

(THS) which formed

by the trace of hotspot

of the total

at the Azores

at chron 21 relative to North gridded

using

the East and West Thulean between

back-arc

plates to the west. The King’s Trough

of crust

6c it

(Searle, 1980) and with the timing, magnitude and direction of compression east of 19”W in the

model

earlier

chron

motion between the Iberian and African plates since chron 19 (Fig. 9). This agrees well with the

and

occurred in the Late Oligocene to Early Miocene (30-23 Ma). Both of these events seem to fit well with our overall compressive phase (Fig. 9). The supports

Since about

ble 1) gives us an estimate

Pyrenees

(37 Ma, Mattauer

plate.

has remained an active plate boundary between Africa and Eurasia including Iberia. Pole P, (Ta-

or subduction of 40 km along the Trough between 44 and 25 Ma.

in the latest Eocene

Henry,

as an independent

a cumula-

1986);

note

-10.26’)

after chron

21

1555’W,

4.1’)

Other

white

the V-shaped

Rise and Milne Seamount, and the fit of and “B” is the location of the plate

with

reconstruction.

the dotted

line.

FC = Flemish

Cap;

S.P. SRIVASTAVA

ET AL.

MOTION

OF IBERIA

SINCE THE LATE JURASSIC

247

S.P. SRIVASTAVA

248

26O

25O W

24O

23O

22O

21°

26O

25O W

24O

23O

22O

21°

Fig. 12. Correlation

of magnetic

anomalies

across

King’s Trough.

the trough

A gradual

from older to younger

tion agrees with the focal mechanism solutions of present-day earthquakes along the Azores-Gibraltar Fracture Zone (Fig. 13) (McKenzie, 1972; Udias, 1982; Argus et al., 1989), suggesting that the sense of motion along it has remained the same since anomaly 19 time. There is, however, some indication that the pole of motion between Eurasia and Africa is moving in a southward

decrease

in the offsets of anomalies

ET AL.

can be seen across

anomalies.

direction (Roest, 1987), pointing to a gradual reduction in the compressional motion to the east and the extension to the west. Motion of Iberia before and at chron MO

The misfit of the western and eastern MO anomalies (Fig. 9) shows that they were not formed as part of the North American-African plate sys-

MOTION

OF IBERlA

SINCE

THE

LATE

249

JURASSIC

20’

30”

40”

35”

300

Fig. 13. Cumulative

motion

20’

along the Azores-Gibraltar

PI. Also shown are the focal mechanisms

Fracture

of some of the deep earthquakes

from Klitgord

tern, contrary

to the earlier

and Schouten

interpretation

(Schou-

ten et al., 1984; Srivastava and Tapscott, Klitgord and Schouten, 1986; Srivastava

1986; et al.

1988) which suggested that Iberia was moving with Africa at this time. This interpretation follows from the new identification from high-density (Figs.

3 and

of anomaly

data in the Newfoundland

4). This

Zone (AGFZ)

identification

MO Basin

is different

since chron

in this region

19 as obtained

(after Udias,

from the correction

1982). Location

pole

of AGFZ

is

(1986). TJ = triple junction.

the southern tip of Iberia parallel to the Newfoundland Fracture Zone. Figure 14 shows the position of Eurasia, Iberia and Africa relative to North

America

rotation Eurasia Africa

at chron

as given

and by Klitgord (Table

MO using

by Srivastava

the poles

et al. (1988)

and Schouten

1). The position

(1986) for

of Iberia

here is slightly south of that obtained

of for

as shown

by Srivastava

from that obtained earlier (Srivastava and Tapscott, 1986). Also in view of the start of the opening of the Bay of Biscay in the late Aptian

et al. (1988). It becomes Iberia’s position

(Montadert

no seafloor-generated magnetic anomalies can be recognized either in the Newfoundland Basin or in

et al., 1979)

Iberia

moved as part of the Eurasian and therefore must have been arate plate. Except for the Newfoundland

could

not

have

plate at that time moving as a sep-

the abyssal

plains

measurements Fracture

Zone

there are no other fracture zones in the region which can be used as constraints in deriving the pole position for Iberia relative to North America at anomaly MO time. The Azores-Gibraltar Fracture Zone is heavily overprinted with Cenozoic motion and cannot be used for this purpose. Thus, the pole of rotation for Iberia relative to North America for anomaly MO (Table 1) was determined by obtaining the best possible fit between anomalies MO and between the ocean continent boundaries across the Bay of Biscay (Montadert et al., 1979; Deregnaucourt and Boillot, 1982; Boillot and Winterer, 1988) and at the same time maintaining the direction of motion of

the small regions

more problematic to determine for pre-chron MO. This is because

off Iberia,

are available.

amplitudes

even though

of the anomalies

or to the fact that

detailed

This may be due to these regions

in these are not

truly oceanic in nature. The Newfoundland Basin has been interpreted by Tucholke et al. (1989) to be largely underlain by thinned continental crust. According to Sullivan (1983) and Tucholke et al. (1989)

the ocean-continent

boundary

(OCB)

in

the Newfoundland Basin lies slightly west of the J-anomaly or MO, while Keen and De Voogd (1988) on the basis of their deep multichannel measurements across the margin, place the OCB along the base of the slope and 160 km west of the MO anomaly (Fig. 14). Thus, great uncertainty exists about the position of the OCB in the Newfoundland Basin.

250

S.P. SRIVASTAVA

0

*oz

ET AL

MOTION

OF IBERIA

SINCE

THE

LATE

251

JURASSIC

On the Iberian side, Mauffret et al. (1989) have interpreted from their multichannel seismic reflection data that the Tagus Abyssal Plain is largely underlain by oceanic crust with an extinct ridge axis located in the centre of the region. They estimate that this region was formed between chrons M21 and Ml0 prior to a jump of the ridge to the west. This implies that the part of the crust in the Newfoundland Basin west of anomaly MO, if oceanic, should be younger than chron MlO. To examine this possibility and to see what problems arise if we close the Tagus Abyssal Plain along the lines suggested by Mauffret et al. (1989), we plotted in the MO reconstruction (Fig. 14) the mapped position of the OCB off Galicia (Boillot and Winterer, 1988), its expected continuation to the south (W~tmarsh et al., 1989), a boundary A in the Tagus Abyssal Plain across which sharp changes in the depth to basement are observed (Mauffret et al., 1989), and the OCB (eastern limit of the old spreading regime) in the Tagus Abyssal plain (boundary B, from Mauffret et al., 1989). On the North American plate we have shown the locations of the OCB (boundary B’) along the Newfoundland margin (Keen and De Voogd, 1988) and south of the Flemish Cap (Todd and Reid, 1989) and a boundary A’ where changes in basement characteristics are noticed (Srivastava et al., in prep) from the seismic reflection data. We find from Fig. 14 that boundaries A and A’ lie at about equal distances from anomaly MO. Both boundaries correspond to where significant changes in the basement character are noticed and could have similar ages. Mauffret et al. (1989) estimated that boundary A forms the western limit of the spreading regime in the Tagus Abyssal Plain, and as such its age should be close to chron MlO. Immediately to the west of boundary A lies crust which must be younger than chron Ml0 if the ridge jumped to

Fig. 14. Shaded

relief reconstruction

Fig. 11. The topography (Mauffret

map of the bathymetry

is illuminated

from the northwest.

et al., 1989) and B’ off Grand

characteristics

are observed

Plain respectively.

Banks (Keen

on each side of the North

The regions where plates overlap to meet (regions

the west at about chron MlO. Taking the younger end of this scale we have assumed the age for boundary A to be about chron MlO. Except for the Newfoundland Fracture Zone and some minor fracture zones further to the north noted by Tucholke et al. (1989), no other fracture zones lie in the Newfoundland Basin. However, a prominent NW-SE oriented trend centred at 41S”N, 11S”W is seen south of Galicia Bank on the magnetic map of this region (Verhoef et al., 1986). This trend is also apparent further to the south in the depth to basement compilation of the region (Mauffret, pers. commun., 1989). When rotated to the west at MO time (shown by C in Fig. 14) it is parallel to the Newfoundland Fracture Zone. Such parallelism suggests that perhaps Ibe~a-North America motion prior to anomaly MO was close to that of Africa-North America. Two other fracture zones mapped by Mauffret et al. (1989) trend ENE-WSW in the Tagus Abyssal Plain and according to them correspond to the direction of motion of Iberia during the opening of the plain. When rotated at chron MO, these fracture zones lie in an E-W direction (L), Fig. 14). To match boundary A’ with boundary A, a pole of rotation was then determined using trend C as the direction of motion between plates (Table 1). Figure 15 shows the reconstruction for boundary A-A’ (approximately chron MlO). To avoid an overlap between the Flemish Cap and Galicia Bank we moved the Flemish Cap northwest of its present position (pole given in Table 1) to close the Flemish Pass (a deep trough west of the Flemish Cap). Seismic refraction measurements across the Flemish Pass show the presence of a thinned continental crust (Keen and Barrett, 1981) beneath it, suggesting that it was formed due to the movement of the Flemish Cap away from the

of the North

and De Voogd, Atlantic,

(regions

Atlantic

at chron

MO using the procedure

Also shown are: ocean-continent 1988), boundaries

and fracture

of later extension)

of later compression)

boundaries

(dashed

as described

A and A’ where changes

zones C and D to the north

in the basement

and in the Tagus

are shown by white stipple and regions

are shown as blanks.

in

lines) B off Iberia Abyssal

where they fail

252

S.P. SRIVASTAVA

0

5:

0 =:

0

oz *

ET AL.

MOTION

OF IBERIA

SINCE

THE

LATE

253

JURASSIC

Newfoundland

Basin is oceanic

the

was

spreading

Abyssal

Plain

only

between

chrons

plies that the region between in the Newfoundland chron

in nature

confined

Basin

M21. This would

Ml0 must

the

neighbouring

position tried

Newfoundland

the Tagus Abyssal sow

Fig.

16. Positions

4ow

of Africa America.

and

Iberia

relative

to North

fracture

zones C and D. A large overlap

time of its separation chron

Also shown

from North

in the direction

at different

times

are the positions between

America,

of

Africa (at the

CL) and Iberia (at

M21) would result if Iberia were moved in the direction

of fracture

zone D relative

the spreading Magnetic

in the Tagus

Anomaly;

to North Abyssal

FC = Flemish

America Plain.

to account

for

BSA = Blake Spur

Cap; GB = Galicia

Bank.

mate

match

chron

Iberia

relative

stretching

shows a gradual increase in the OCB’s off Iberia and

Newfoundland

north

from

to south,

suggesting

that the opening in this region propagated gradually from south to north as has previously been proposed (Srivastava et al., 1988). problems

to close the Tagus zones E-W

would Abyssal

arise if we attempted Plain

along

fracture

D of Mauffret et al. (1989). This requires motion for Iberia which would result in

obtaining a large overlap the time of its initial America. positions

of

to

the

the OCB’s in the

Basin

with the OCB on

Plain (B-B’).

of lineament

between

We moved Iberia

C until

boundaries

an approxi-

B and

B’ was

obtained (Fig. 18a). In the absence of other information which can be used to justify further movement

either

of the

Flemish

Cap

or of Galicia

Bank, we have merely left them in their positions as obtained in Fig. 15 and show the amount of overlap

which arises in doing

so. This would

then

America

Discussion

of the crust in this region.

The reconstruction the distance between

Serious

jump

M21. To

to match

be the position of Iberia relative to North at some pre-anomaly M21 time. shelf. However, we are not sure how far west one can move the Flemish Cap to account for the

than

plates at the time of their separation,

we have merely southern

of

B’ and A’

be older

another

the ridge axis to the east at about obtain

and M21 im-

boundaries

require

and that

to the Tagus

with Africa’s position at separation from North

This is illustrated in Fig. 16, where the of Africa are shown at various times

together with that of Iberia at chron MO. As can be seen, moving Iberia along direction D will create a large overlap between the southern part of Iberia and the northern part of Africa not at chron Ml6 or M21 but at earlier times when Africa occupied a more northerly position. Another problem arises in deciding which boundaries to match in the reconstruction. Assuming that the region west of boundary A’ in the

Analysis

of seafloor-spreading

data between

Charlie Gibbs and Azores-Gibraltar zones has given us the history of motion

the

fracture of Iberia

relative to its neighbouring plates from anomaly MO to the present. We have shown from this analysis that since chron 34, Iberia has largely moved either as part of Africa or Eurasia, with successive jumps of the plate boundary Eurasia and Africa. For a brief period chrons

19 and

6c, it may

have

moved

between between as an

independent plate but the lack of seafloor spreading data for this period throughout the North Atlantic makes nitely. Such an the differential and Eurasia as

it difficult to establish this defiinterpretation is based mainly on poles of rotation between Africa well as on the geological evidence

on land which requires crustal shortening between Africa and Iberia during this period. The resulting motion of Iberia not only explains the formation of some very prominent bathymetric features on the adjacent ocean floor but equally well the formation of some prominent geological features on land. At the same time it

254

raises some important questions concerning the history of the large abyssal plains west and east of anomaly MO (i.e., the Newfoundl~d Basin off Newfoundland and the Iberia and Tagus abyssal plains off Iberia whose origins have so far remained an enigma). Based on the suggested locations of OCB’s in the Newfoundland Basin and on the Tagus Abyssal Plain, we have derived positions of Iberia prior to chron MO. However, these positions are mainly based on the assumption that the locations of the OCB’s off Newfoundland and Iberia are correct and ignore the possibility that these regions may have formed due to thinning of the continental crust. Seismic refraction measurements, as summarized by Mauffret et al. (1989) for the Tagus Abyssal Plain and for the Newfoundland Basin by Sullivan (1983), suggest abnormally thin crust in these regions. If this crust is indeed continental in nature the question then arises as to how was it formed and what were the positions of the plates involved. Until the true nature of the crust in the Newfoundland Basin and the Tagus and Iberia abyssal plains can be established, any discussion of their evolution, including the one given here, can only be considered speculative in nature. In view of the new solution for the motion of Iberia we will now briefly discuss the effects on the termination of the Tethys Ocean and evolution of the Mediterranean Sea. Implications of the motion of Iberia in the Mediterranean region

It has long been recognized that the evolution of the Mediterranean and termination of Tethys are related to the motion of the surrounding plates (e.g. Dewey et al., 1973). Nevertheless it has been difficult to relate the geological history of this region to plate kinematic models due to the lack of accurate plate kinematic solutions. Others have, nonetheless, used the existing plate kinematic solutions and related them to the tectonic development of the Mediterranean (e.g. Dewey et al., 1973; Biju-Duval et al., 1977; Dercourt et al., 1986; Savostin et al., 1986). Because we have been able to obtain a more constrained kinematic solution for the Iberian plate, it is worthwhile to look

S.P. SRWASTAVA

ET AL.

Fig. 17. Motion of three locations on Africa relative to Eurasia, based on AFR/EUR differential poles of rotation. An abrupt change at chron 25 in the smooth path traced by these poles may be related to the drastic changes in the motion of these and other plates in the North Atlantic at this time. L&4 = Blake Spur Magnetic Anomaly; EC&f,4 = East Coast Magnetic Anomaly.

briefly at the implications of this solution to the Mediterranean region. It is beyond the scope of the present paper to discuss the development of the Mediterranean Sea in detail; this will be done elsewhere. Figure 17 shows the motion of three locations on Africa relative to Eurasia during the evolution of the North and Central Atlantic. It should be borne in mind that these motions are very susceptible to small variations in the pole positions of the plates involved. Even though the poles of rotation may fit the observations for individual plates well, when combined with the poles of rotation for other plates the net result may show large fluctuations which can be quite unrealistic. Many of the models (e.g., Dewey et al., 1973; Biju-Duval et al., 1977; Smith and Woodcock, 1982; Savostin et al., 1986) have been marked by drastic changes in the direction of motion of Africa relative to Eurasia. In some of the models, however, there has been a tendency to obtain a smoother and less eccentric motion for Africa. The motion in Fig. 17, with one exception, shows a fairly smooth path without sudden jumps or drastic changes in the direction of motion. A small change occurs at about chron 25, when a small kink in the direction of motion takes place. Such a change would have very little effect in the region between Iberia and Africa because they were mov-

MOTION

OF IBERIA

SINCE

THE

LATE

ing together

as one plate

during

amount

of

compression

between

Eurasia,

from chron

33 to chron

we move to the eastern relates

to the

mainly

on the location

Eurasia

east of the Pyrenees. 13, when north

and

the

of this

that

and

earlier,

as

Banks

21, increases

How this

region

of Iberia,

in the central

and

the

located be-

west of the Strait of

effect

taking

Mediterranean

Iberia

and North

amount

of stretching after

comparison

that

on the relative

America

in Fig.

south

features

indicative

18a, 18b and

in the overlap in the

a

18~ shows

a

with

time

gradually

Sea. The kink at chron 25 is (probably) related to the large-scale volcanic episodes which occurred

propagated

throughout the North Atlantic at this time (for details, see Srivastava and Tapscott, 1986). Africa

was taking place in the south Newfoundland

seems to have moved relatively smoothly to the southeast from the time of its initial separation from North America to about anomaly 31 time. At this time slight changes in its motion started to take

place.

The motion then gradually resulting in compression

became between

northward, Eurasia and Africa and closure of the Tethys. can then be related

to Alpine

This

The reconstruction

Ml0

time (136 spreading Basin,

Thus, compared to that in the south, there was more stretching between the Flemish Cap and Galicia Bank over a long period of time. This resulted in the separation of the Flemish Cap from the continental block to the west. This may partially account

for the differences

in the Galicia

By anomaly

The detailed seafloor spreading data in the North Atlantic have provided for the first time a

at anomaly

the region to the north between the Flemish Cap and Galicia Bank was still undergoing stretching.

the south of it (Boillot Summary

et al., 1988).

Ma, Fig. 18b) shows that while seafloor

observed

tectonism.

from

suggestions

Atlantic

(Srivastava

were

Similarly,

earlier

North

to the north

of

of the

to which these regions

strengthening

spreading

it is

prior to M25. The large

18a is merely

of Figs.

to north,

as

positions

M25 time (156 Ma).

decrease

but

(such

be used in such reconstructions

overlap

gradual

1987)

on either side of the Atlantic

to speculate

subjected

started

on the Grand

and Welsink,

of additional

lineations)

difficult

that stretching

observations

(Tankard

absence

which could

the compression

eastern

suggest

between

perhaps

with the maximum

It is very likely

as geological

magnetic

At a later time, e.g., at chron were

in

time.

depends

The entire curve in Fig. 17 shows a and rotational motion of Africa rela-

tive to Eurasia, place

The

at this time extended

came very small immediately Gibraltar. translation

Africa

of the boundary

which

boundaries

south

this time.

Mediterranean.

tectonics

and Africa,

255

JURASSIC

in the structures

Bank region from those to and Winterer,

1988).

MO time (118 Ma, Fig. 18~) active

seafloor spreading had started throughout the Newfoundland Basin, but the region to the north between

Eurasia

and North

America

was still being

detailed kinematic solution for the motion of Iberia from the Late Jurassic to the present. The motion

stretched. Active seafloor spreading started in the Bay of Biscay post chron MO, when the north

of Iberia during the evolution of the North Atlantic is depicted in a set of reconstructions carried out for the region between the Charlie Gibbs and Azores-Gibraltar fracture zones (Fig. 18) in which Eurasia has been held fixed at its present location.

Iberian margin margin, creating this region for part of the Bay

It shows

that

during

the

Late

Jurassic

(M25),

when active spreading was taking place in the Central Atlantic between North America and Africa (Fig. 18a), the region between Iberia and the Grand Banks (shown by the overlap pattern) was undergoing stretching. A plate boundary between Iberia and Africa aligned with the Newfoundland Fracture Zone seems to have existed at

separated from the north Biscay a triple junction that remained in a long period of time. The major of Biscay opened between chrons

MO and 33 (negative polarity). About the time of the late Albian (110 Ma) and pre-chron 34, active seafloor spreading had started between Eurasia and North America. Iberia, which was moving as an independent plate during the Cretaceous Magnetic Quiet Period, now started to move with Africa. The E-W plate boundary in the Bay of Biscay between Eurasia and Iberia now became the main

256

S.P. SRIVASTAVA

ET AL.

Y

Fig. 18. ~~onst~ctions Motions relative

of North

along boundaries plate motion.

between

Shaded

American

and Iberian

plate motions

plates from the previous

areas are the regions

of overlaps

with gaps between

relative

to a fixed Eurasian

time are shown by small arrows between

plates implying

plate from chron

while large arrows

later extension

plates imply later compression.

M25 to chron 6.

show the direction

in those regions.

of

The regions

MOTION

OF IBERIA

SINCE

THE

LATE

257

JURASSIC

plate boundary between Eurasia and Africa. In the beginning the motion along this plate boundary, in the oceanic region, was extensional (chrons 33 to 31, Fig. 18e) but it gradually became strike-slip in nature (chrons 31 to 21, Figs. 18f-h). This seems to have continued until chron 19, when Iberia started to move as an independent plate. This may have resulted from the change in motion between Eurasia and Africa in the present Mediterranean when the Italian peninsula (Apt&an promontory), which was perhaps moving as part of Africa as palaeomagnetic measurements suggest (e.g. Van der Berg and Zijderveld, 1982), collided with Eurasia. Also, the compressive motion, which had been taking place across the Pyrenees until this time, slowed down as a result of this collision. These changes in the motion of Iberia brought into existence two new plate boundaries (Fig. 18i), one to the north linking Ring’s Trough and the Azores-Biscay Rise to the North Spanish Trough and the Pyrenees, and the other to the south located along the present Azores-Gibraltar Fracture Zone. Iberia was thus caught between two massive plates, Eurasia in the north and Africa in the south. Motion was extensional in the Ring’s Trough region, and by strike-slip with some compression along the Azores-B&cay Rise. The compressional motion in the Pyrenees continued during this period. Thus, the compressional motion in the Pyrenees and the formation of Ring’s Trough during the early to mid-Tertiary can now be accurately related to plate motions. At about anomaly 6c time, motion along the Ring’s Trough/Azores-Biscay Rise boundary became very small, and Iberia started to move as part of the Eurasian plate (Fig. 18j) and has been doing so since. Conclusions

The following conclusions can be drawn from the present work: (1) The use of high-quality seafloor spreading magnetic data in the North Atlantic has led to a well-constr~ned kinematic model for Iberia. Very often, more data lead to more complex models, but in this case the additional data were used to confirm and modify the simple model of a jump-

ing plate boundary originally proposed by Schouten et al. (1984). (2) Plate kinematic models must be derived by taking into account the evolution of large areas. In this case, we used a self-consistent model for the central North and North Atlantic and the Arctic. It is only after defining as accurately as possible the motion of the large plates that one can predict with a greater accuracy the movements of small plates and the motion across their boundaries. (3) The accuracy of the plate kinematic models for the North and Central Atlantic has improved in such a way that we can now start to make predictions based on differential poles of rotation. Although these poles are in general very unstable, the poles we calculated for the differential motion between Africa and Eurasia follow a smooth path. As a result, we predict gradual changes in the movement of the plates surrounding the present Mediterranean. (4) By constraining the positions of the plates surrounding Iberia in the pre-rift configuration, we can now speculate on the rifting stage of the evolution of the Grand Banks and the Iberian margins. Note added in proof

By identifying additional magnetic anomalies in the Central and North Atlantic, Roes: and Srivastava (sub~tted) have shown that boundary B jumped to the King’s Trongh region at chron 17 and that Iberia acted as a separate plate from chrons 18 to 6c. Acknowledgements

We thank Francois Marillier, Pat Ryall, Matthew Salisbury and Jack Malod for their comments. Discussions regarding the early evolution of this part of the North Atlantic with Alain Mauffret and Denis Mougenot during their visit to the Atlantic Geoscience Centre initiated a reidentification of anomaly MO off Iberia and Newfoundland. B.J. Collette allowed the use of unpublished magnetic anomaly data collected by the Veining Meinesz Laboratorium in Utrecht. Walter Roest was supported by the Natural Sciences and

S.P. SRIVASTAVA

258

Engineering Visiting

Research

Council

Fellowship.

under

This

the Frontier

Geological Section

study

a out

Programme

of the

Drafting

of the

of Canada.

done by the Drafting

of the Bedford

under

was carried

Geoscience

Survey

illustrations,

of Canada

Institute

and Illustration

Mediterranean IAGA

Division

Reference

Geophys.

Union,

C.E.

and

crustal Keen

References

Closure

R.G.,

DeMets,

C. and Stein,

of Africa-Eurasia-North

circuit

and tectonics

America

of the Gloria

S., 1989.

Plate

motion

fault. J. Geophys.

Res.,

94: 5585-5602. Arkani-Hamed,

J., 1990.

Magnetisation

of the oceanic

crust

beneath

the Labrador

Sea. J. Geophys.

Res.

Biju-Duval,

B., Dercourt,

J. and Le Pichon,

X., 1977. From the

Tethys

Ocean

to the Mediterranean

model of the evolution Biju-Duval

and L. Montadert

of the Mediterranean Boillot,

G.

and

(Editors),

Structural

Basins. Technip,

Winterer,

margin:

retrospect

Winterer

et al. Proc.

E.L.,

and

Drill.

In: B. History

and

Drilling In:

G.

on

Galicia

Boillot,

Program,

E.L.

Sci., Results,

J., Zonenshain,

Pichon,

L.P., Ricou,

X., Knipper,

I.M., Geyssant, Sibuet, J.-C.,

AI.,

C., Sbortshikov,

C., Pechersky,

D.H., Boulin, J.,

L.A., Sorokhtin,

Lauer,

J.P.

and

O., Westphal,

Bazhenov,

M.L., evolution

the Pamirs

since the Lias. Tectonophysics,

Biju-Duval,

M.,

B., 1986,

du golfe de Gascogne.

G., 1982. Structure

Bull. BRGM,

W.C., III, Rayan,

1973. Plate tectonics

123: 241-

to 315.

geologique

2(1, 3): 149-178. W.B.F.

and the evolution

and Bonnin,

of the Alpine

J., sys-

tem. Bull. Geol. Sot. Am., 84: 3137-3180. ETOPOS,

1986. Relief map of the earth’s

Am. Geophys. Galdeano,

A., Moreau,

Malod.

M.-G.,

sediments

the rotation

near

surface.

Eos, Trans.

Earth

P.-Y. and

results from Creta-

(Portugal)

of Iberia.

and

Planet.

implications

Sci. Lett.,

94:

95-106. Grant,

origin

observations

bearing

Ridge.

Tectonophysics,

of the Newfoundland

upon

the 59:

71-82. Grimaud. and

S., Boillot, G., Collette, Roberts,

D.G.,

1982.

Iberian-European

plate

convergence:

B.J., Mauffret, Western

boundary

A., Miles, P.

extension

during

a new model.

of

the

basins.

Atlantic

In: H. Berckhemer

and

Region.

a

history.

The ocean-continent

off Eastern

reflection

Canada:

studies.

new

Tectonics.

Drilling

1986. A Jurrasic

America.

7:

Project

to present

(Editors),

Vol. M.. The Western

Geol. Sot. Am., Boulder, A.T.S.,

tion of the King’s Trough

Colo, pp. 45-50.

1986. The geology complex

Site 608 drilling.

The North

and forma-

in the light of Deep Sea Init. Rep. Deep-Sea

Drill.

Proj., 94: 1245-1261. Kidd,

R.B., Searle,

R.C.,

Ramsay,

northeast

Atlantic

Atlantic.

Geology

North

and

ocean.

K.D. and Schouten,

central

A.T.S.,

J., 1982. The geology

Trough, Klitgord,

Pritchard,

formation

Mar. Geol., 48: l-30.

H., 1986. Plate kinematics

of North

America.

Region.

H. and of King’s

In: P.R. Vogt and B.E. Tucholke

Atlantic

Kristoffersen,

Y.,

opening

of the

(Editors),

Vol. M, The

Geol. Sot. Am., Boulder,

1978.

Sea

of the North

Western Cola., pp.

floor

Atlantic.

A.S.

and

Alpine

and

Planet,

R.B..

1974.

Whitmarsh,

Gibraltar

plate boundary.

dynamics

of Iceland

Hingham,

Mass., pp. 63-81.

Laughton,

the early

Sci. Lett.. 38:

and the North

AS., Whitmarsh,

Le Pichon,

X. and

Pyrenean Le Pichon,

Atlantic

Ridge. Nature, J.C.,

European

Reidel,

Orogeny.

Earth

Planet.

east-west

327: 217-220.

1971. Western and Iberian

extension

of

plates during

the

Sci. Lett., 12: 83-88.

X., Sibuet, J.C. and Francheteau.

physics,

Geo-

Area.

BS, 1972. A continuous

Sibuet,

around

Azores-

(Editor),

R.B., Rusby, J.S.M., Somers, M.L..

between

the continents

The

In: L. Krisjansson

Revie, J. and McCartney,

the North

J., 1977. The fit of

Atlantic

Ocean.

Tectono-

12: 169-209.

D.G. and Miles, P.R., 1984. Mesozoic

ing between

Iberia,

Europe

seafloor

and North America.

spread-

Mar. Geol..

56: 279-287. M. and Henry,

J.. 1974. Pyrenees.

(Editor),

Mesozoic-Cenozoic

Orogenic

Studies.

1989.

backarc

spreading Earth

273-290. Laughton,

Mauffret,

K. Hsii (Editors),

subsided Canada;

and subsidence

B., 1988. margin

F.M.,

R. and Ramsay,

Mar. Geol., 45:

H., 1982, Mediterranean

and

of eastern

In: P.R. Vogt and B.E. Tucholke of North

the early Ceno-

63-77. F. and Berckhemer,

Am.

Sot., 65: 443-465.

Gradstein,

Geology

Mattauer,

zoic (Pyrenean) Hovarth.

D. and

Masson,

A.C., 1979, Geophysical

evolution

deep seismic

chronology.

boundary

Pozzi, J.-P., Berthou,

Lisbon

thermal

De Voogd,

from

Geo-

1985. Eos. Trans.

1981. Thinned

fault on the Azores-Gibraltar

67: 121.

J.-A.. 1989. New paleomagnetic

ceous for

Union,

1, 1986. International

351-378.

of the Tethys belt from the Atlantic

D. and Boillot,

Dewey, J.F., Pitman,

G., Le

Grandjacquet,

J., Lepvrier, Savostin,

L.E., Kazmin,

Geological Deregnaucourt,

Kent,

The

103: 809-828. Dercourt,

D.L.,

at the rifted

Mitchell,

Paris, pp. 143-164.

1988,

prospect.

Ocean

tectonic

Alpine system.

Geo-

107-124.

Kidd,

Sea: a plate

of the western

Barrett,

J. R. Astron.

C.E.

results Gordon,

Union

67: 523.

structure,

boundary

D.F.,

Group

Field Revision

crust on the rifted margin

Geophys.

Argus,

I Working

magnetic

continental

is acknowledged.

Am. Geophys.

dyn. Ser., 7: 141-173.

Keen,

of Oceanography,

Geodynamics.

ET AL.

spreading

centre

Portugal):

results

Earth

Geol. Sot. London

A.. Mougenot, Cenozoic

Orogenic

D., Miles,

deformation in

the

Tagus

of a multichannel

Sci., 26: 1112-1123.

Belts:

Data

for

Spec. Publ., 4: 3-22.

P.R.

and

In: A.M. Spencer

and

Malod,

Mesozoic

Abyssal

J.A.,

abandoned

Plain

seismic survey.

(west

of

Can. J.

MOTION

OF IBERIA

Minster,

SINCE

THE

J.B. and Jordan,

tions. J. Geophys. McKenzie,

LATE

T.H.,

Geophys.

McKenzie,

tectonics

J. R. Astron.

D.P. and Morgan,

junction.

Nature,

Montadert.

1978. Present-day

plate

mo-

Souquet,

Souquet,

Roberts,

D.G.,

Guennoc

and

passive

margins:

rift-

Atlantic

ing and subsidence

Am. Geophys.

J.L., Bonnin,

Cinematique Exploit.

Paris, Rapp.

B. and Souquet,

rtsulte

pas

d’un

Pyreneenne”. Pitman,

J., Beuzart,

Oceans,

Peybemes,

le long

d’une

“Faille

111: 204- 210.

anomalies

North

M.T.

in the

Inst. Oceanogr.

and

northeast

North

Atlantic

ocean.

Init. Rep.

379-404

Scale, Rep.

Atlantic

between

spreading

10” and

pattern

40”N.

Geol.

North

S.P., submitted.

between

Atlantic

Eurasia,

Kinematics

of the

from the Late Cretaceous

to Present.

L.A., Sibuet,

and Roulet, belt

M.-J., 1986. Kinematic

from

Triassic. Schouten,

J.-C., Zonenshain,

the Atlantic

Ocean

Tectonophysics,

to ball-bearing

to the

X.

of the Tethys

Pamirs

since

the

tectonics.

an alternative

Eos, Trans. Am. Geophys.

Hellinger,

paleobathymetry the present. Searle, Searle,

of the Atlantic

Union,

C.R.,

1977.

The

Ocean from the Jurassic

1979.

Sidescan

studies

of the North

1980. Tectonic

and

triple

pattern

junction.

Earth

Planet.

and Trough,

seismic

and

Whitmarsh,

R.B.,

northeast

gravity

studies.

1978.

The

Atlantic,

from

Geophys.

J. R. Astron.

of

sion Udias,

bathymetric,

the Alpine-Mediterranean Hsii

(Editors),

Am. Geophys.

Union

1982. Tectonic

region.

In: H. Berckhemer

Alpine-Mediterranean Geodyn.

syntheses

Ser., 7: 15-38.

of and

Geodynamics.

Van

basin:

seafloor

ocean-conti-

Hsii

history.

extension

and

Banks, Newfound-

Pet. Geol., 71: 1210-1232. Constraints

Can. J. Earth Ridge,

and

boundary

from seismic refraction

Sci., 26: 1392-1407. W.J., 1982. Structure

western

North

and origin of

Atlantic

Jr., J.A. and Uchupi,

rift-drift

Tectonics

Iberian

of

Earth

spreading

oil field, Grand

Cap:

B.E., Austin.

den

aspects

reconstructions.

Ocean.

J.

Res., 87: 9389-9407

and

evolution

(Editors),

Berg,

of the

North

and seismotectonic region.

ExtenAtlantic

Geodyn. New

and

Geodynamics.

Ser., 7: 75-82 paleomagnetic

from

the

H.. 1982. Paleomagnetism

in

GeoI. Mijnbouw,

Van den Berg, J. and Zijderveld,

stress field in

In: H. Berckhemer

Alpine-Mediterranean

Union

the Mediterranean.

Newfoundland

Pet. Geol. Mem., 46: 247-263.

J., 1980.

peninsula.

E., 1989. Crustal

of the

and H.R. Balkwill (Editors), Stratigraphy

Am. Assoc.

Am. Geophys. N.H.,

K.D.,

between Eurasia

H.J., 1987. Mesozoic

of Hibemia

A., 1982. Seismicity

K.

53: 259-287. Smith, A.G. and Woodcock,

plate

the Alpine-Mediterranean

Sot.,

Klitgord,

Sci. Lett., 62: 321-339.

basin. In: A.J. Tankard

51:

structure

5:

R., 1990. Iberian

plate boundary

Mesozoic

B.E. and Ludwig,

Tucholke,

spreading

W.R.,

Macnab,

B.J. and Reid, I., 1989. The ocean-continent

structure

Sci. Lett.,

and

the J-Anomaly

136: 283-292.

of the Azores

Pet. Geol..

P., 1983. Some geometrical

A.J. and Welsink,

Margins.

R.C.

King’s

K.

Planet.

Tucholke,

to

Atlantic

Results

the northeast

344: 756-759. combined

south of Flemish

415-434. Searle,

Earth

Geophys.

zones. J. Geol. Sot. London,

R.C.,

centre

Tapscott,

Colo.. pp.

R., 1988. across

I and II. Mar.

H., Roest.

land. Bull. Am. Assoc.

J. Geol., 85: 509-552.

R.C.,

fracture

S. and

Macnab,

1983. The Newfoundland

boundary

and gravity.

J.G.,

of

(Edi-

Sci. Lett., 69: 401-412.

nent

Todd,

65: 190. Sclater,

in

K.D.,

Tankard,

K., 1984. Iberian

plate boundaries,

N.S., pp.

Vol. M. The Western

survey

J. and

a jumping

Nature,

stratigraphy

S.P. and Klitgord,

jumping

Sea. Geol.

1986. Plate kinematics

America.

Part

L.C., Verhocf,

Planet.

123: l-35.

H., Srivastava.

plate kinematics:

evolution

margin.

uncertainties

Geol-

L.P., Le Pichon,

Labrador Dartmouth,

Geol. Sot. Am., Boulder, J. and

Stock, J.M. and Molnar,

in the

Sullivan,

ogy. Savostin,

Series: Cent..

of North

plate kinematics:

48:

Iberia and Africa

spreading

In: J.S. Bell (Coordina-

CR..

S.P., Schouten,

Kovacs,

and Africa.

plate boundaries

Geo-

306-337.

1-121. Roest, W.R. and Srivastava,

1989. Sea floor

aeromagnetic

Newfoundland 1

of the North Ultriectina,

Sea and its Atlantic.

In: P.R. Vogt and B.E. Tucholke

Region.

a detailed

Srivastava,

W. R., 1987. Seafloor

W.R., Atlas

S.P., Verhoef,

from

1:2,400,000. 207, Sheets

of the North

Geosci.

Atlantic.

Atlantic

Srivastava, P., 1985. Magnetic

U.K.,

Basin

tors), The Geology

1979. The

Atlantic.

Sci., Wormley,

of the Labrador

1:2,000,000.

S.P. and Tapscott,

D.E.,

Hunter,

Roest,

Scale, Atlantic

the North

and 2. Roest,

Srivastava,

Hayes,

and

de Ia France.

100-109.

Nord-

Drill, Proj., 43: 879-885. Jones,

Surv. Can.,

ne

M., 1972. Sea floor spreading

SC.

II-VI.

tor), East Coast

des Pyrenees

dans le cycle

Geologiques

Sot., 52: 313-357.

S.P. and

history

Geol. Sot. Am. Bull., 83: 619-646.

Cande,

in the central

D.G.,

In: Evolutions

on the early evolution

Srivastava,

Natl.

Sci. Tech. 54, 108 pp.

coulissage

Atlantic.

P.D.,

Deep-Sea Roberts,

J.M., 1984. Cent.

Bull. Sot. Hist. Nat. Toulouse,

in the North J-anomaly

P. and Auzende, nord

et central.

E.-J..

P., 1980. Tectonogt-

de sedimentation

S.P., 1978. Evolution

bearing

Union Maurice

P., 1975. La chine

III. W.C. and Talwani,

Rabinowitz,

E.-J. and Laurens,

phys. J. R. Astron.

de I’Atlantique

Deboras,

Bur. Rech. Geol. Min. Mem.. 107: 213-233. Srivastava,

Ewing Ser., 3: 154-186. Olivet,

M. and

des PyrCnCes. Geol. Alp., 53: 193-

des bassins

alpin des Pyretrees.

Sibuet, J.C., 1979. Northest processes.

P., Deboras,

ntse et evolution

of triple

224: 125-133. 0..

B., Bilotte,

Alpine

216.

of the Mediterranean

Sot., 30: 109-185.

W.J., 1969. Evolution

L.. De Chat-pal,

P., Peybemes,

1977. La chaine

Res., 83: 5331-5354.

D.P.. 1972. Active

region.

259

JURASSIC

data

59: 49-60.

In: H. Berckhemer

and K. Hsti (Edi-

260

S.P. SRIVASTAVA

tom), Alpine-Mediterranean Union

Geodyn.

Geodynamics.

of the Iberia peninsula. J. and

continental data Verhoef,

margin

base

anomaly

and

evidence

Tectonophysics, R., 1988.

of eastern

construction

for the rotation

Magnetic

Canada: of a 1 :5

data

over

preparation million

J., Collette,

and Williams,

the of a

magnetic

C.A., 1986. Magnetic

J., Roes&

reconstructions

Mar. Geophys.

W.R.

and

and gridded

anomalies

in the north-

S.P.,

1989.

ogy of the North Atlantic

Eos, Trans.

Plate

data. A new tool in decipher-

Am. Geophys.

anomalies

and crustal

Region.

America.

(Editors),

magnetisaThe Geol-

Vol. M, The Western

Geol. Sot. Am., Boulder,

Cola.,

North

pp. 229-

256. structure Atlantic Whitmarsh,

Res., 8: l-25.

Srivastava,

oceans.

Vogt, P.R., 1986. Magnetic

Whitmarsh,

B.J.. Miles, P.R., Searle, R.C., Sibuet, J.C.

Ocean.

across

70: 609 and 618.

tion. In: P.R. Vogt and B.R. Tucholke

7: 5-56.

map. Geol. Sot. Can. Open File 1504, 97 pp.

east Atfantic Verhoef,

Macnab,

ing correlations Union,

Van der Voo, R., 1969. Paleomagnetic Verhoef,

Am. Geophys.

Ser., 7: 83-112.

ET AL.

R.B., G&burg, and origin Ocean. R.B.,

bean-continent l(1): 27.

A. and

Geophys. Miles,

Searle,

R.C.,

of the Azores-Biscay J. R. Astron.

P.R.

boundary

and

1982. The

Rise, northeast Sot., 70: 79-107.

Mauffret,

west of Portugal.

A..

1989. The

Terra

Abstr..