Download pdf

The rift-to-drift transition in the North Atlantic:
A stuttering start of the MORB machine?
Oliver Jagoutz
Institut für Geologie, Universität Bern, CH-3012 Bern, Switzerland
Othmar Müntener
Gianreto Manatschal Centre géochimie surface- École et observatoire des sciences de la Terre, Université Louis Pasteur,
F-67084 Strasbourg, France
Daniela Rubatto Research School of Earth Sciences, Australian National University, Canberra 0200 ACT, Australia
Gwenn Péron-Pinvidic Centre géochimie surface- École et observatoire des sciences de la Terre, Université Louis Pasteur,
F-67084 Strasbourg, France
Brent D. Turrin Department of Geological Sciences, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA
Igor M. Villa Institut für Geologie, Universität Bern, CH-3012 Bern, Switzerland, and Universita di Milano Bicocca, 20126 Milano, Italy
ABSTRACT
We report U-Pb and 39Ar-40Ar measurements on plutonic rocks recovered from the Ocean
Drilling Program (ODP) Legs 173 and 210. Drilling revealed continental crust (Sites 1067
and 1069) and exhumed mantle (Sites 1070 and 1068) along the Iberia margin and exhumed
mantle (Site 1277) on the conjugate Newfoundland margin. Our data record a complex igneous and thermal history related to the transition from rifting to seafloor spreading. The
results show that the rift-to-drift transition is marked by a stuttering start of MORB-type
magmatic activity. Subsequent to initial alkaline magmatism, localized mid-oceanic ridge
basalts (MORB) magmatism was again replaced by basin-wide alkaline events, caused by
a low degree of decompression melting due to tectonic delocalization of deformation. Such
“off-axis” magmatism might be a common process in (ultra-) slow oceanic spreading systems,
where “magmatic” and “tectonic” spreading varies in both space and time.
Keywords: Ocean-Continent transition, MORB, rifting, magma-poor margins, age determinations.
INTRODUCTION
It is generally accepted that rupturing of continents is followed by localized seafloor spreading
at mid-ocean ridges (MOR), which are considered, on geological time scales, to be symmetric
and steady state. The continuity of this process
is documented by the correspondence of crustal
accretion ages (dated by magnetic anomalies)
and isotopic ages. While these processes are
well studied at present-day MOR, little is known
about how stable these systems are during their
embryonic stage. Even though our understanding of the mechanisms associated with extension
and rifting of continents has improved in the last
decade (Whitmarsh et al., 2001), the processes
that ultimately start the MOR basalt (MORB)
machine and the switch from rifting (delocalized
deformation) to spreading (localized deformation and accretion) are still poorly constrained.
A striking discovery of ODP drilling along the
Iberia-Newfoundland conjugate margins was
the scarcity of effusive magmatism and only
minor volumes of intrusive rocks in areas with
weak magnetic anomalies (Sibuet et al., 2007).
In this paper, we present new U-Pb and 40Ar-39Ar
measurements across the conjugate IberiaNewfoundland margins to document the magmatic and retrograde history of igneous rocks
related to the final rifting and onset of seafloor
spreading in the North Atlantic. The data show
that the initiation of magmatic seafloor spreading
is more complex than previously anticipated and
is transitional in time and space.
THE IBERIA-NEWFOUNDLAND
RIFTED MARGINS SYSTEM
The conjugate, magma-poor Iberia-Newfoundland rifted margins resulted from polyphase Late
Triassic to Early Cretaceous rifting and separation of the North America and Iberia plates.
Rifting migrated from proximal to distal parts
of the future margin, leading to fault-bounded
basins and local thinning of the crust to less than
10 km before final breakup (Whitmarsh et al.,
2001). The Iberia-Newfoundland margins are the
only example of rifted margins where scientific
drilling has sampled basement rocks from the
Ocean-Continent Transition (OCT). The 18 sites
drilled during Deep Sea Drilling Project (DSDP)
Leg 47B and ODP Legs 103, 149, 173, and 210,
combined with a relatively dense geophysical
data set, document the existence of a Zone of
Exhumed Continental Mantle (ZECM) up to
200 km wide, which separates thinned continental crust from oceanic crust. Based on magnetic
analyses and refraction studies (e.g., Russell and
Whitmarsh, 2003; Lau et al., 2006), crust oceanward of the first unambiguous magnetic anomaly
(M3, ca. 128 Ma; Gradstein et al., 2004) is generally referred to as oceanic. However, based on
its proximity to the continental margins and its
highly variable basement structure (Tucholke et al.,
2007), we refer to it in this paper as embryonic
oceanic crust, to emphasize the difference from
“normal” slow-spreading oceanic crust.
Due to the polyphase evolution of rifting,
subdued magmatic activity, and poorly con-
strained magnetic anomalies, the age of final
breakup and the onset of seafloor spreading
between Iberia and Newfoundland are debated.
Based on the first undisputed oceanic magnetic
anomaly, M3, Whitmarsh and Miles (1995)
suggested a Barremian age (128 Ma), whereas
Tucholke et al. (2007) and Péron-Pinvidic et al.
(2007) proposed a late Aptian to earliest Albian
age (ca. 112 Ma), based on drilling results and
seismic stratigraphic arguments.
EMBRYONIC OCEANIC CRUST
DRILLED AT ODP SITES 1070 AND 1277
Embryonic oceanic crust was drilled at a basement high near magnetic anomaly M1 at Site
1070 on the Iberia margin and at a basement high
a few kilometers oceanward of magnetic anomaly
M1 at Site 1277 on the Newfoundland margin.
The locations of both sites, relative to seafloor
spreading magnetic anomalies, suggest a similar
crustal accretion age of ca. 127 Ma (Fig. 1).
At Site 1070, drilling penetrated tectonic
breccias with gouge horizons grading downhole
into massive serpentinized peridotite intruded
by an E(enriched)-MORB–derived pegmatitic
hornblende gabbro (Hébert et al., 2001; Beard
et al., 2002) (see Fig. DR1 in the GSA Data
Repository1). A breccia overlying the pegmatite consists of serpentinite and rare gabbro and
albitite clasts. Sediments immediately on top are
Late Aptian (Whitmarsh et al., 1998).
At Site 1277, drilling penetrated a 40-m-thick,
volcano-sedimentary unit composed of
three basalt flows with a T-(transitional) to
N(normal)-MORB composition (Tucholke et al.
2004) interlayered with debris flows and sandstones that are separated from the underlying basement by a tectonic contact (see Fig.
DR1). Clasts in the sediments are composed
of serpentinized peridotites and gabbros identical to rocks forming the underlying basement.
1
GSA Data Repository item 2007270, analytical
methods, tabulated and illustrated results, and a
stratigraphic column indicating sample locations of
Sites 1277 and 1070, is available online at www.
geosociety.org/pubs/ft2007.htm, or on request from
[email protected] or Documents Secretary,
GSA, P.O. Box 9140, Boulder, CO 80301, USA.
© 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
GEOLOGY,
December
2007
Geology,
December
2007;
v. 35; no. 12; p. 1087–1090; doi: 10.1130/G23613A.1; 3 figures; Data Repository item 2007270.
1087
Age Constraints from the Distal Margin
(Sites 1067, 1068, and 1069)
A maximum age for the deformation leading to continental breakup is given by tilted
Tithonian sediments (ca. 145 Ma) deposited over
thinned continental crust drilled at Sites 901,
1065, and 1069 (Wilson et al., 2001). Devonian
39
Ar-40Ar muscovite ages (361.5 ± 0.5 Ma; see
Fig. DR1) separated from a conglomerate clast
that was recovered at Site 1069 indicate no significant resetting of muscovite Ar ages during
breakup. Jurassic 39Ar-40Ar ages (see Fig. DR1)
of hornblende (167.3 ± 0.9 Ma, 164.6 ± 0.5 Ma,
160.5 ± 0.8 Ma, and 152.6 ± 0.9 Ma) and plagioclase (141.8 ± 0.4 Ma) were obtained from
amphibolites drilled at Site 1067. Our results
on plagioclase are slightly older than those
from gabbro clasts drilled at Site 900 (136.4 ±
0.3 Ma; Féraud et al., 1996). Amphibolite clasts
from Site 1068 yield hornblende 39Ar-40Ar ages
of 140 ± 2 Ma and 131.7 ± 1.1 Ma and a plagioclase age of 133.1 ± 0.3 Ma.
1088
1070
1276
1067/68
zircon 247*
hbl 167-132
plag 142-133
127*
124
116-101
wr 105-98*
1277
RIA
A.P
.
901
IBE
1000
S. NFLD.
BASIN
1069
0
Geochronological constraints
mus 362
embryonic oceanic crust
Number of ODP Site
127* Age [Ma] of alkaline magmatism
5000
SILLS of MORB magmatsim
Age [Ma]
124
42°
39
40
ArAr
plagioclase
ages
116-101
TAGUS A.P.
[Ma] (see text for interpretation)
*ages from literature (see text for reference)
1070
A
46°
~M3 ~M0
~M1
44°
~M3
~M1
IBERIA
B
Geochronological constraints
distal margins
1069 Number of ODP Site
zircon U-Pb age [Ma] of Zircon
mus, hbl, 39Ar-40Ar ages [Ma] of:
plag, wr muscovite, hornblende
plagioclase, whole rock
40°
42°
Iberia
Continental Transitional Embryonic
Transitional
oceanic crust
(1069) 1067
1276 1277
1070 (Projected) 1068
5
10
48° W
Grand Banks
INT
ERIO
GR A
ND
92-69
GALICIA
897
899
898
1065 398
113
500
0
Figure 1. Reconstruction
of Newfoundland-Iberia
rift at 125 Ma (anomaly M0) (adapted from
Tucholke et al., 2004).
Ages are from this paper;
asterisk (*) denotes previously published data
(Beard et al., 2002; Gardien and Paquette, 2004;
Hart and Blusztajn, 2006).
Geochronological constraints for MORB-type
magmatism are shown
in gray boxes; alkaline
magmatism is shown in
white.
R
128
>Albian
44°
4000
2000
BASIN
BANK
BAN
KS
0
300
1000
A
B
Continental
,
300 km
200
100
0
100
Age Constraints from the Embryonic
Oceanic Crust
Site 1070
Kaersutitic hornblende, separated from an
E-MORB–derived gabbroic pegmatite analyzed at the University of Bern, yielded a
39
Ar-40Ar age of 123.9 ± 1.2 Ma, statistically
indistinguishable from the result from the
Lamont laboratory (124.2 ± 0.7 Ma). Strongly
altered plagioclase separates analyzed from
the same sample in Bern yield a poorly
constrained age of 101 ± 5 Ma, whereas
samples analyzed by Lamont yield less rejuvenated and better-constrained plagioclase
ages (116.9 ± 0.8 Ma, 115.7 ± 0.3 Ma, and
111.0 ± 0.3 Ma). U/Pb dating on zircons from
albitite clasts overlying the pegmatite yields
an age of 127 ± 4 Ma, interpreted as a crystallization age (Beard et al., 2002).
200
300 km
history. The temporal and spatial distribution of
the various magmatic and retrograde events is
summarized in Figure 2.
Magmatic Activity within an Embryonic
Magma-Poor Oceanic Crust
The onset of embryonic oceanic crust formation is accompanied (or triggered?) by alkaline
magmatism documented at Site 1277. A phlogopite 39Ar-40Ar age yields a minimum age for this
alkaline episode of 128 ± 3 Ma, closely corresponding to the crustal accretion age of Site
1277. A contemporaneous magmatic event is
documented on the Iberia margin at Site 1070
by the U-Pb age of a single zircon (127 ± 4 Ma)
separated from a biotite-bearing albitite clast
90
Site 1277
Igneous zircons from a hornblende gabbro
dike yielded an intrusion age of 113.2 ± 2.1 Ma
(95% confidence interval). A phlogopite separate from a different alkaline gabbroic dike
yielded a 39Ar-40Ar age of 128 ± 3 Ma.
Plagioclase separates from four different
brecciated, MORB-derived gabbroic clasts,
present within mass flows found on top of the
serpentinized mantle and within the volcano
sedimentary units, yielded well-constrained,
albeit variable, 39Ar-40Ar ages (91.6 ± 0.3 Ma,
76.1 ± 0.4, 69.1 ± 1.1 Ma, and 69.3 ± 2.1 Ma).
DISCUSSION
Based on assumptions outlined in Figure
DR1, we interpret our U-Pb zircon and 39Ar-40Ar
hornblende and biotite ages to date several discrete phases of alkaline and MORB-type magmatism at Sites 1277 and 1070, and 39Ar-40Ar
plagioclase ages date a prolonged retrograde
Continental margin
Oceanic crust
70
on-axis magmatism
RESULTS
46°
N
BMT.-INVOLVED AND BASIN-BOUNDING FAULTS
OTHER FAULT
ISOBATHS [m]
DRILL SITE
Age [Ma]
Age Constraints for Magmatic and
Post-Breakup Evolution
To shed light on the magmatic and thermal
history of the embryonic-oceanic crust of the
juvenile North Atlantic ocean basin, we dated
several gabbroic clasts and veins from the
embryonic oceanic crust (ODP Sites 1070 and
1277) and from the Iberia distal margin (ODP
Sites 1067, 1068, and 1069) using U-Pb dating of zircons by sensitive high-resolution ion
microprobe (SHRIMP) and 39Ar-40Ar techniques on hornblende, phlogopite, muscovite,
and plagioclase mineral separates. 39Ar-40Ar
measurements were conducted at the Institute
of Geological Sciences at University of Bern,
and at the Lamont-Doherty Earth Observatory.
Details of the analytical techniques and results
are presented as electronic supplement.
EVAPORITES
EMBRYONIC OCEANIC CRUST
Two-way traveltime (s)
Although the breccias have not been dated, it
has been suggested that they were deposited
during or directly after accretion of the basement (Péron-Pinvidic et al., 2007). The Site
1277 basement consists of strongly altered serpentinized harzburgite with a high-temperature
mylonitic foliation and igneous veins. Based on
the mineralogical composition of the magmatic
products (clasts and veins) drilled at Site 1277,
alkaline rocks (characterized by phlogopite,
albite-rich feldspar, monazite, zircon, apatite,
orthoamphibole, rutile, and ± xenotime) can be
distinguished from MORB-derived rocks (characterized by plagioclase, clino-, orthopyroxene,
ilmenite, and Ti-hornblende) (Müntener and
Manatschal, 2006). However, small sample volumes (<1 cm3), coupled with the coarse-grained
mineralogy (~0.5 cm), impede meaningful geochemical whole-rock characterizations.
110
130
150
170
off-axis magmatism
Transitional crust
location of breakup
U-Pb:
Zircon
Titanite/
Zircon
Ar-Ar:
Amphibole
Biotite
Plagioclase
Whole rock
Newfoundland
Fission track:
Zircon
Iberia
Iberia
MTR
1276
1277
1070
Gorringe Bank 1068 1067/69
Sample Site
Figure 2: Illustration of spatial and temporal relationship of various events along
Newfoundland-Iberian margin. Symbol color
code: orange and blue are alkaline and
MORB-type igneous events, respectively.
Black symbols are rejuvenated ages (see text
for details). Post-Albian alkaline events and
contemporaneous plagioclase rejuvenation
ages are tentatively grouped in “basin-wide”
events (transparent boxes), illustrating the
possibility of repetitive, basin-wide alkaline
events. Data from literature are the same as
cited in Figure 1, and from Merle et al. (2006)
and Schärer et al. (2000).
GEOLOGY, December 2007
originally interpreted to date the intrusion of
the underlying E-MORB–derived pegmatitic
gabbro (Beard et al., 2002). However, because
neither location (see Fig. DR1) nor mineral
chemistry (Beard et al., 2002) of the albitite
clasts indicate a genetic relationship with the
pegmatite gabbro, they likely reflect a different pulse of magmatism. Based on the similar
mineralogy of the albitite clast and the alkaline
dike at Site 1277, we relate the 127 ± 4 Ma magmatism at Site 1070 with the contemporaneous
alkaline event at Site 1277.
The pegmatitic gabbro at Site 1070 yields
a 124.2 ± 0.7-Ma hornblende age, indicating
younger igneous MORB-type activity. No direct
age information dating the MORB-type magmatism at Site 1277 is available, but indirect
evidence suggests that the MORB-type magmatism occurred shortly after accretion of the
basement (Péron-Pinvidic et al., 2007). The
earliest magmatic history of the embryonic
oceanic crust is thus characterized by alkali
magmatism followed by MORB-type magmas. The 113.2 ± 2.1-Ma U-Pb zircon age of a
hornblende-plagioclase dike at Site 1277 indicates a restoration of magmatism at the Aptian/
Albian boundary. Multiple emplacements of
alkaline sills at Site 1276 (105 Ma and 98 Ma;
Hart and Blusztajn, 2006) and volcanic activity
at the Madeira-Tore rise (104–88 Ma; Geldmacher et al., 2006; Merle et al., 2006) imply
prolonged alkaline off-axis magmatism on
either side of the nascent mid-Atlantic ridge.
Retrograde History
Structural and sedimentary evidence constrains erosion and sedimentation of gabbro
clasts at ODP Sites 1070 and 1277 to be older
than 112 Ma (Aptian/Albian boundary) (PéronPinvidic et al., 2007). The plagioclase 39Ar-40Ar
ages of these rocks are substantially younger,
ranging from 116 Ma to ca. 101 Ma at Site 1070
to as young as ca. 69 Ma at Site 1277. The nonsystematic age distribution with sample depth
(see Fig. DR1) implies that the ages do not
reflect regional cooling, nor are they related to
the proximity of basaltic flows. Instead, the local
difference in ages requires a sample-specific
control on the meter scale on Ar exchange in
plagioclase. Because fluid-induced recrystallization has a dominant effect on Ar exchange (Villa,
2006), we propose that the plagioclase 39Ar-40Ar
ages record the termination of recrystallization, i.e., the end of hydrothermal activity. We
emphasize that plagioclase 39Ar-40Ar ages within
the oceanic realm cannot, a priori, be linked to
regional exhumation and associated cooling.
Our measurements document rejuvenated
39
Ar-40Ar plagioclase, which overlaps Upper
Cretaceous alkaline magmatism, thus affecting
the Newfoundland and Iberian margin (Schärer
et al., 2000; Hart and Blusztajn, 2006; Merle
et al., 2006) (Fig. 2).
GEOLOGY, December 2007
A Plume Origin of Alkaline Magmatism
in the North Atlantic?
One hypothesis to explain the prolonged and
heterogeneous igneous history recorded at Sites
1277 and 1070 involves the passage of mantle
plumes (Duncan, 1984; Hart and Blusztajn,
2006; Merle et al., 2006). An alternative hypothesis relates Cretaceous volcanism in Newfoundland to decompression melting along reactivated
faults (Pe-Piper et al. 1994). Based on currently
available data, none of these hypotheses can be
excluded. However, several lines of evidence are
not in accordance with the plume hypothesis.
Alkaline sills at Site 1276 and from the Southern
Grand Banks in Canada show an isotopic signature that is different from the known Atlantic
hotspot system (Pe-Piper et al., 1994; Hart and
Blusztajn, 2006). The age distribution observed
at Madeira-Tore rise (Merle et al., 2006) does
not favor a simple passing plume-head model.
The similar magmatic histories at Sites 1070
and 1277 (Fig. 2) for at least 15 myr after onset
of magmatic activity suggest a common process
on either side of the proto–mid-Atlantic ridge.
Assuming conservative average full spreading
rates of ca. 10–20 mm/a (Sibuet et al., 2007),
seafloor spreading would have separated these
two localities by ~150–300 km. Although plumehead radii are considered to range between 200
D ≤112 Ma
and 1000 km, the small volume of igneous
rocks in this part of the Atlantic does not favor
plume activity prevailing for more than 15 myr.
Finally, the alkaline magmatic event at 113.2 ±
2.1 Ma is contemporaneous with a basin-wide,
delocalized, extensional morpho-tectonic event,
recorded in the sedimentary architecture in the
Iberia-Newfoundland margins (Péron-Pinvidic
et al., 2007; Tucholke et al., 2007). Accordingly,
we propose an alternative hypothesis to explain
the revival of intra-plate alkaline magmatism.
A CONCEPTUAL MODEL FOR
EMBRYONIC MAGMA-POOR
SEAFLOOR SPREADING
The early alkaline magmatism (ca. 128 Ma),
followed by younger (>124 Ma) E-MORB–type
magmatism, can be explained by a low degree of
melting of (fertile?) mantle. However, alkaline
magmatism recurred ca. 15 m.y. later, at the
same time as a regional-scale morpho-tectono
event that affected the two conjugate margins
at the Apt-Albian boundary. We propose that
during juvenile, ultra-slow spreading, melt
supply along the Atlantic proto-ridge was subdued, and mantle exhumation was the dominant
process. This magma-starved embryonic ocean
crust accumulated in-plane stresses during further extension (Tucholke et al., 2007). Sudden
MORB PHASE slow spreading mid-ocean ridge
1070M3
M3 1277
0 km
Figure 3. Tectono-magmatic model of initiation
of seafloor spreading in
southern North Atlantic
20
modified after PéronPinvidic et al. (2007) and
C 112 Ma ALKALINE magmatism, basin-wide extension
Tucholke et al. (2007).
1277
1070 M3
A: Asymmetric rifting of
0 km
M3
continental Iberia and
Newfoundland margins
10
results in mantle exhumation associated with
alkaline melts. B: Local20
ization of extension into
a magma-starved, slow
B 124-112 Ma MORB-type magmatism, embryonic oceanic crust spreading zone. AccreM3 Hobby High 1065 901 0 km
M3
tion of embryonic-oceanic
crust (exhumed mantle +
low volumes of E- to
10
N-MORB–type melts). C:
Redistribution of the
20
extensional deformation
across previously accreted
segments. Extension trigStructures
A > 128 Ma ALKALINE magmatism
gers decompression meltIsotherms
Hobby High 1065 901
0 km
ing, resulting in low volTectonic
umes of alkaline melts.
spreading faults
Hydrothermal convection
Rift faults
10
triggered by shallow-level
Thinning
structures
intrusion resets plagioHydrothermal
clase 39Ar-40Ar system20
convection cells
atics. D: MOR-producing
Lithologies
Pre Aptian/Albian
Serpentinized mantle
“normal” oceanic crust.
10
Pre-rift sediments
Upper continental crust
Lower continental crust
Lithospheric mantle
Asthenospheric mantle
Oceanic crust
sediments
Alkaline-type rocks
MORB-type rocks
1089
stress release triggers “tectonic spreading”
episodes that are characterized by the distribution of extensional deformation away
from the proto-ridge over previously accreted
embryonic-oceanic crust, which resulted in the
observed tilted blocks in Newfoundland and
Iberia (Péron-Pinvidic et al., 2007; Tucholke
et al., 2007). Associated low-degree decompression melting results in off-axis magmatism
(Fig. 3). Our rejuvenated 39Ar-40Ar plagioclase ages are roughly contemporaneous with
documented alkaline magmatism (Fig. 2).
Although highly speculative, this basin-wide
correlation between magmatic events and
hydrothermal rejuvenation might imply that
basin-wide, off-axis thermal pulses associated
with alkaline magmatism are not uncommon in
the northern Atlantic.
We speculate that “tectonic spreading” phases
are rather short-lived, isolated events intervening
between periods of “normal” igneous accretion
on a timescale probably beyond the resolution
of magnetic anomalies.
Several studies pointed out that the low magmatic flux along MOR is commonly associated
with the formation of amagmatic segments at
ultra-slow spreading rates (<20 mm/yr) (Dick
et al., 2003). Our observations suggest that the
delocalization of deformation is not restricted to
the vicinity of MORs (tens of km), but might be
distributed over much larger areas (>100 km),
locally triggering off-axis alkaline volcanism as
recorded from fast- (Zou et al., 2002) and slowspreading systems (Meyer et al., 1996).
We conclude that off-axis, low-degree mantle melting is not necessarily related to plume
activity, but might be coupled with low-melt
productivity on axis associated with ultra-slow
spreading rates, which by itself might be related
to “tectonic” spreading. Alkaline magmatism
observed at basement highs (Site 1277, this study;
Madeira-Tore rise, Schärer et al., 2000; Geldmacher et al., 2006; Merle et al., 2006) is readily
explained by contemporaneous normal faulting.
CONCLUSIONS
U-Pb SHRIMP data and 39Ar-40Ar data demonstrate multiple episodes of magmatism along
the Iberia-Newfoundland rift system variably
being between alkaline and E-N MORB. Our
results provide a mechanism to explain how
substantially younger, off-axis alkaline magmatism might be reconciled with a non–steadystate MORB engine. Our new data suggest that
the transition from rifting to magmatic seafloor
spreading is not a well-defined event but, rather,
is transitional, and occurred over a period of
time lasting from Barremian to end-Aptian, i.e.,
over >~15myr.
ACKNOWLEDGMENTS
This research used samples provided by the Ocean
Drilling Program (ODP), which is sponsored by
funding agencies of the participating countries. This
1090
research was supported by Swiss National Science
Foundation grant PP002-102809 and grants from the
French Groupe de Recherche Marge (GDR). Detailed
and thoughtful reviews by Brian Tucholke and Henry
Dick substantially improved the manuscript and are
greatly appreciated.
REFERENCES CITED
Beard, J.S., Fullagar, P.D., and Sinha, A.K., 2002,
Gabbroic pegmatite intrusions, Iberia Abyssal
Plain, ODP Leg 173, Site 1070: Magmatism
during a transition from non-volcanic rifting to
sea-floor spreading: Journal of Petrology, v. 43,
p. 885–905, doi: 10.1093/petrology/43.5.885.
Dick, H.J.B., Lin, J., and Schouten, H., 2003, An
ultraslow-spreading class of ocean ridge: Nature,
v. 426, p. 405–412, doi: 10.1038/nature02128.
Duncan, R.A., 1984, Age progressive volcanism in
the New England seamounts and the opening
of the central Atlantic Ocean: Journal of Geophysical Research, v. B89, p. 9980–9990.
Féraud, G., Beslier, M.-O., and Cornen, G., 1996,
40
Ar/39Ar dating of gabbros from the ocean/
continent transition of the western Iberia margins: Preliminary results, Proceedings of the
Ocean Drilling Program, Scientific results,
149: College Station, Texas, p. 489–495.
Gardien, V., and Paquette, J.L., 2004, Ion microprobe and ID-TIMS U-Pb dating on zircon
grains from leg 173 amphibolites: Evidence
for Permian magmatism on the West Iberian
margin: Terra Nova, v. 16, p. 226–231, doi:
10.1111/j.1365-3121.2004.00554.x.
Geldmacher, J., Hoernle, K., Klügel, A., v.d. Bogaard,
P., Wombacher, F., and Berning, B., 2006, Origin and geochemical evolution of the MadeiraTore Rise (eastern North Atlantic): Journal of
Geophysical Research, v. 111, p. 19 pp.
Gradstein, F.M., Ogg, J.G., and Smith, A.G., 2004,
A geologic time scale 2004: Cambridge, UK,
Cambridge University Press, 610 p.
Hart, S., R., and Blusztajn, J., S., 2006, Age and geochemistry of the mafic sills, ODP site 1276,
Newfoundland margin: Chemical Geology,
v. 235, p. 222–237.
Hébert, R., Gueddari, K., Lafleche, M.R., Beslier,
M.O., and Gardien, V., 2001, Petrology and geochemistry of exhumed peridotites and gabbros at
non-volcanic margins: ODP Leg 173 West Iberia
ocean-continent transition zone: Geological
Society [London] Special Publication, v. 187,
p. 161–189.
Lau, K.W.H., Louden, K.E., Funck, T., Tucholke, B.E.,
Holbrook, W.S., and Hopper, J.R., 2006, Crustal
structure across the Grand Banks-Newfoundland Basin continental margin (Part I)—Results
from a seismic refraction profile: Geophysical
Journal International, v. 167, p. 127–156, doi:
10.1111/j.1365-246X.2006.02988.x.
Merle, R., Schärer, U., Girardeau, J., and Cornen,
G., 2006, Cretaceous seamounts along the
continent-ocean transition of the Iberian margin: U-Pb ages and Pb-Sr-Hf isotopes: Geochimica et Cosmochimica Acta, v. 70, p. 4950–
4976, doi: 10.1016/j.gca.2006.07.004.
Meyer, J., Mercolli, I., and Immenhauser, A., 1996,
Off-ridge alkaline magmatism and seamount
volcanoes in the Masirah island ophiolite,
Oman: Tectonophysics, v. 267, p. 187–208,
doi: 10.1016/S0040-1951(96)00094-7.
Müntener, O., and Manatschal, G., 2006, High
degrees of melt extraction recorded by spinel
harzburgite of the Newfoundland margin: The
role of inheritance and consequences for the
evolution of the southern North Atlantic: Earth
and Planetary Science Letters, v. 252, p.
437–452, doi: 10.1016/j.epsl.2006.10.009.
Pe-Piper, G., Jansa, L.F., and Palacz, Z., 1994,
Geochemistry and regional significance of
the Early Cretaceous bimodal basalt-felsic
associations on Grand Banks, eastern Canada: Geological Society of America Bulletin,
v. 106, p. 1319–1331, doi: 10.1130/0016-7606(1994)106<1319:GARSOT>2.3.CO;2.
Péron-Pinvidic, G., Manatschal, G., Minshull, T.A.,
and Sawyer, D., 2007, The tectono-sedimentary
and morpho-tectonic evolution recorded in the
deep Iberia-Newfoundland margins: Evidence
for a complex break-up history: Tectonics,
v. 26, p. doi:10.1029/2006TC001970.
Russell, S.M., and Whitmarsh, R.B., 2003, Magmatism at the west Iberia non-volcanic rifted
continental margin: Evidence from analyses
of magnetic anomalies: Geophysical Journal
International, v. 154, p. 706–730, doi: 10.1046/
j.1365-246X.2003.01999.x.
Schärer, U., Girardeau, J., Cornen, G., and Boillot,
G., 2000, 138–121 Ma asthenospheric magmatism prior to continental break-up in the North
Atlantic and geodynamic implications: Earth
and Planetary Science Letters, v. 181, p. 555–
572, doi: 10.1016/S0012-821X(00)00220-X.
Sibuet, J.-C., Srivastava, S., and Manatschal, G., 2007,
Exhumed mantle-forming transitional crust in
the Newfoundland-Iberia rift and associated
magnetic anomalies: Journal of Geophysical
Research, v. 112, p. doi:10.1029/2005JB003856.
Tucholke, B.E., Sawyer, D.S., and Sibuet, J.C., 2007,
Breakup of the Newfoundland-Iberia rift, in
Karner, G.D., Manatschal, G., and Pinheiro,
L.M., eds., Imaging, mapping and modelling
continental lithosphere extension and breakup,
Volume 282: Geological Society Special Publication, 488 p.
Tucholke, B.E., Sibuet, J.C., Klaus, A., et al., 2004,
Proceedings of the Ocean Drilling Program,
Initial reports; Leg 210: College Station,
Texas, Texas A&M University, Ocean Drilling
Program.
Villa, I.M., 2006, From nanometer to megameter: Isotopes, atomic-scale processes, and continentscale tectonic models: Lithos, v. 87, p. 155–
173, doi: 10.1016/j.lithos.2005.06.012.
Whitmarsh, R., Beslier, M.-O., Wallace, P.J., et al.,
1998, Proceedings of the Ocean Drilling Program, Initial reports: Return to Iberia, Leg 173:
College Station, Texas, Texas A & M University Ocean Drilling Program, p. 493.
Whitmarsh, R.B., Manatschal, G., and Minshull, T.A.,
2001, Evolution of magma-poor continental margins from rifting to seafloor spreading: Nature,
v. 413, p. 150–154, doi: 10.1038/35093085.
Whitmarsh, R.B., and Miles, P.R., 1995, Models of the
development of the west Iberia rifted continental margin at 40 degrees 30 minutes N deduced
from surface and deep- tow magnetic anomalies: Journal of Geophysical Research, v. 100,
p. 3789–3806, doi: 10.1029/94JB02877.
Wilson, R.C.L., Manatschal, G., and Wise, S., 2001,
Rifting along non-volcanic passive margins:
Stratigraphic and seismic evidence from the
Mesozoic successions of the Alps and Western
Iberia: Geological Society Special Publication,
v. 187, p. 429–452.
Zou, H., Zindler, A., and Niu, Y., 2002, Constraints
on melt movement beneath the East Pacific Rise
from 230Th-238U disequilibrium: Science, v. 295,
p. 107–110, doi: 10.1126/science.1064295.
Manuscript received 18 December 2006
Revised manuscript received 25 July 2007
Manuscript accepted 27 July 2007
Printed in USA
GEOLOGY, December 2007