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Earth and Planetary Science Letters 272 (2008) 553–566
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l
Do fracture zones define continental margin segmentation? — Evidence from the
French Guiana margin
C.J. Greenroyd a, C. Peirce a,⁎, M. Rodger b, A.B. Watts b, R.W. Hobbs a
a
b
Department of Earth Sciences, Durham University, Science Laboratories, South Road, Durham, DH1 3LE, UK
Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK
A R T I C L E
I N F O
Article history:
Received 24 July 2007
Received in revised form 16 May 2008
Accepted 19 May 2008
Available online 30 June 2008
Editor: C.P. Jaupart
Keywords:
crustal structure
continental margins
oceanic crust
fracture zones
A B S T R A C T
Plate reconstructions suggest that the French Guiana margin in the west equatorial Atlantic is a highly
segmented margin with both rift- and transform-style features. We describe here the results of modelling
coincident multi-channel and wide-angle seismic, gravity and magnetic data acquired along two transects of
this margin. The resulting models not only highlight the degree of structural segmentation but also
demonstrate the effect of trans-tension on margin evolution.
As a whole, the margin is characterised by 35–37 km thick pre-rift continental crust which is separated from
unusually thin oceanic crust (3–4 km) by thinned continental and/or transitional regions. To the north, the
margin exhibits a 320 km wide zone of thinned continental crust adjacent to a narrow ocean–continent
transition, and is interpreted as a transform margin where the wide zone of thinned crust is a result of profile
orientation being highly oblique to the direction of rifting. Approximately 240 km to the south, the margin is
characterised by a 70 km wide zone of thinned continental crust which is wider than typical for transform,
and narrower than typical for rifted margins. This crustal structure is interpreted to reflect a “leaky”
transform formed by trans-tensional extension.
These observations suggest that fracture zone influenced geometry of equatorial Atlantic rifting, did not
produce a well-defined margin crustal structure, but instead resulted in margin segments which display
characteristics of both rift and transform tectonic processes. The associated abundance of fracture zones has
likely also affected the post-rift evolution of the margin, and provided topographic basement highs which
acted as sediment dams to the northwards flux of sediment from the Amazon.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
A diverse range of rift-related structures are observed at the
passive continental margins of the Atlantic; a consequence of
variation in mantle temperature, rate and extent of rifting, rift
geometry and lithospheric thickness. Observed structures have
resulted in margins being classified according to: the orientation of
the rifting direction relative to that of subsequent oceanic spreading –
rift to transform (e.g. Whitmarsh et al., 1996; Edwards et al., 1997); the
degree of rift-related magmatism – volcanic to non-volcanic (Mutter,
1993); the distance over which continental crustal thinning occurs –
wide to narrow (Davis and Kusznir, 2002); and whether or not
transition zones of exhumed mantle or intruded crust are observed
(Dean et al., 2000; Hopper et al., 2006). While the two-dimensional
aspects of the mode of formation of these structural end-members is
relatively well understood, to better understand their three-dimensional inter-relationships, or the margin segmentation, study of alongstrike continuity of features is required, together with correlation of
features between conjugate margin pairs.
⁎ Corresponding author.
E-mail address: [email protected] (C. Peirce).
0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2008.05.022
Large-scale segmentation is observed along the margins of the
eastern Atlantic, e.g. in their magmatic characteristics which vary,
north to south, from volcanic (Rockall – Morgan et al., 1989) to nonvolcanic (Iberia – Dean et al., 2000; Congo-Zaire-Angola – Contrucci
et al., 2004) to volcanic (Namibia – Bauer et al., 2000). However the
nature of the along-strike transition between these end-member
types is poorly known. Similarly, these margins are structurally
segmented by rift- and transform-style structures; the latter related to
long-lived first- and second-order offsets in the Mid-Atlantic Ridge
(MAR) which can be traced to each margin as fracture zones or
fracture-zone-like features. The relatively narrow width of these
zones suggests that the transition from rift- to transform-style
structures is similarly abrupt. However, to date, existing studies
have focused on better understanding of the origin and evolution of
end-member margin characteristics, rather than addressing the
nature of the transition between them.
In order to understand the origin and development of along-strike
segmentation, the western equatorial Atlantic margin was chosen for
study because the satellite-derived gravity anomaly indicates that the
crustal basement fabric is dominated by fracture zones (Fig. 1), whose
intersection with the margin is reflected in the free-air gravity
anomaly as small offsets in the ‘edge effect’ high. In addition, this
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Fig. 1. French Guiana-Northeast Brazil margin. Top: Lineations observed in the first derivative of the satellite-derived gravity FAA (Sandwell and Smith, 1997) showing the locations of
fracture zones which can be traced from the MAR to the continental margin. Middle: Summary of the observed fracture zones traced (black dashed) and interpolated (black dotted) towards
the margin. Bottom: Müller et al.'s (1997) seafloor age isochrons, with heavy contours marking 30 Ma intervals. The location of Profiles A and D at the French Guiana margin are annotated.
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margin has extensive multi-channel seismic data coverage (both
hydrocarbon industry and academic – Fig. 2), and several whole
crustal wide-angle seismic transects acquired by the Amazon Cone
Experiment (ACE – Watts and Peirce 2004).
Here we extend the separate interpretations of ACE seismic and
gravity profiles acquired across the French Guiana margin (Fig. 3), to
develop a geodynamic model of margin evolution and investigate
along-margin structural segmentation. The acquisition and modelling
of the data, upon which this study is based, are described in detail in
Greenroyd et al. (2007a,b).
2. Regional tectonic setting
The passive continental margins of French Guiana and its
conjugate offshore Liberia, are unique amongst the Atlantic margins
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as they represent the point of final continental break-up and opening
of the Atlantic Ocean. Plate reconstructions (Unternehr et al., 1988;
Nürnberg and Müller 1991 – Fig. 4) and commercial seismic data
(Pereira da Siva, 1989; Mello et al., 2001; Cobbold et al., 2004) show
that French Guiana and Northeast Brazil rifted from Guinea, Sierra
Leone, Liberia, Ivory Coast and Ghana during the early Cretaceous at
~110 Ma; this final rifting event being preceded by the formation of
both the North and South Atlantic.
The tectonic evolution of the equatorial Atlantic has been highly
influenced by a series of transform faults and fracture zones
(henceforth referred to as transform features) which cross-cut the
entire ocean basin, evident in the gravity anomaly (Fig. 1) and seabed
bathymetry (Fig. 2). The intersection of these transform features with
the MAR is associated with large lateral offsets in the ridge trend,
which accommodate a total offset of over 3000 km within the
Fig. 2. Bathymetry of the western equatorial Atlantic (Sandwell and Smith, 1997), showing the location of large-offset transform faults along the MAR. Also highlighted are prominent
bathymetric features: the Demerara Plateau to the north; and the Amazon Cone deep-sea fan system and Ceara Rise aseismic ridge to the south. The coincident MCS and WA profiles
that form the basis of this study (A and D) are highlighted in red, while the remaining seismic profiles (B to G) which comprise the Amazon Cone Experiment, are also shown. The red
dashed lines surrounding Profiles A and D mark the extent of the lower panels which show the locations of OBSs (red triangles), OBHs (blue triangles) and land stations (green
triangles). The green box adjacent to Profile D shows the location of Erbacher et al.'s (2004) seismic stratigraphic reference profiles which are shown by the solid green lines in the
corresponding inset. DSDP and ODP wells are marked by red and blue stars respectively. The Guyaplac profiles, traversing the margin between Profiles A and D, are marked by thin
blue lines. See text for details. Bathymetric contours are plotted at 20 m, 50 m, 100 m (dashed line), 500 m (dotted line) and then at every 1000 m (solid line) intervals.
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Fig. 3. Models of the French Guiana-Northeast Brazil margin (after Greenroyd et al., 2007a,b). Simplified illustrations of the interpreted crustal units are shown above and below
corresponding models. Triangles mark OBS/H locations (see Fig. 2). Velocities are colour-coded in km s− 1, where blue colours represent sedimentary layers, dark green the upper
crust, and light green the lower crust. P-wave velocities were converted into density and the free-air gravity anomaly calculated (solid red line) for comparison with that acquired
whilst seismic surveying (dashed) for both profiles. The satellite-derived (Sandwell and Smith, 1997), longer-wavelength anomaly is included (blue). Model parameters are
summarised Greenroyd et al. (2007a,b). Upward arrows locate fracture zone intersections. Interpreted crustal units are annotated: CC — continental crust; TCC — thinned continental
crust; TZ — transition zone crust; OC — oceanic crust. See text for details.
equatorial Atlantic region. Generally, where these transform features
intersect with the continental margin it would be expected that the
margin is characteristically transform in type, while in between riftstyle characteristics predominate. Further evidence for this segmented pattern is provided by studies of the West African margin where
adjacent rift (Liberia – Mascle 1976; Ivory Coast – Peirce et al., 1996)
and transform (Ghana – Edwards et al., 1997) margins are observed.
Post-rift, the equatorial MAR appears to have been slowspreading, although estimates of half-spreading rate vary between
9 mm yr− 1 (Le Pichon and Hayes, 1971) and 28 mm yr− 1 (Nürnberg
and Müller, 1991). At ~ 80 Ma, spreading was accompanied by the
formation of the Ceara Rise offshore Northeast Brazil and the Sierra
Leone Rise offshore West Africa; two oceanic plateaus which, given
their conjugacy, were most likely formed at or close to the MAR
(Kumar and Embley, 1977). Between ~ 115 and ~ 100 Ma syn- to postrift, coarse clastic sediment sequences were deposited in the region
adjacent to the Amazon Cone (Fig. 2) which were later overlain by
post-rift fan-delta and platform carbonate sequences (Brandão and
Feijó, 1994). A major increase in sediment flux from the Amazon
River occurred at ~ 10 Ma (Damuth and Kumar, 1975; Supko et al.,
1977) as a result of the uplift and erosion of the Bolivian Andes.
These vertical movements altered drainage patterns across South
America and resulted in the catchment area for the Amazon River
extending across most of the continent. The overall result of this
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Fig. 4. Plate reconstruction of the equatorial Atlantic (after Nürnberg and Müller,1991). In this reconstruction the North Atlantic rifted prior to 118.7 Ma with rifting in the South Atlantic
progressing northward at this time. The equatorial Atlantic was the last region of the Atlantic to open at ~ 110 Ma. The continents of South America and Africa are denoted by + symbols.
Plates are labelled: AFR — southern Africa; NWA — northwestern Africa; SAM — South America; PAR — Parana; SAL — Salado; and COL — Colorado.
deposition is a sediment column N8 km thick offshore Northeast
Brazil (Houtz et al., 1977; Rodger et al., 2006).
To the north, offshore French Guiana, the bathymetric trend
observed along much of the margin alters from a relatively narrow
continental shelf and steep continental slope dipping towards the
deep abyssal plain, to a wide, gradual sloping shelf terminated by a
near-vertical slope. The latter is the Demerara Plateau, which
Benkhelil et al. (1995) suggest, with its conjugate the Guinea Plateau,
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evolved by trans-tensional rifting and was the last part of the margin
to break-up. Gouyet et al. (1994) conclude that pre-rift sedimentary
deposition occurred in an inner shelf environment and that post-rift
deposition was open marine. Sediment lithology and seismic
stratigraphy of the Demerara Plateau have been obtained from
boreholes and site survey data (ODP Sites 1257–1261 – Erbacher
et al., 2004) and this is used as the primary stratigraphic reference for
the ACE. Gouyet et al. (1994) summarise the structures observed, and
divide its geological history into two main stages:
2.) Post-Aptian (113 Ma–present) — Initiated during the Albian (112–
97 Ma), the break-up between the Demerara and Guinea Plateaux
represents the final opening of the South Atlantic. This break-up
incorporated a combination of perpendicular rifting and dextral
shearing, segmenting the French Guiana and northeast Brazil
margin into a series of rift- and transform-type structures. Postrift, the depositional environment shifted from shallow to open
marine sediments.
3. Datasets
1.) Liassic to Aptian (213–113 Ma) – Prior to the equatorial rifting of
South America and Africa at 118.7 Ma, the Demerara and Guinea
Plateaux were adjacent parts of the southern Central Atlantic
margin (Benkhelil et al., 1995 – Fig. 5). Sedimentation at that time
occurred primarily in an inner shelf environment, with significant
continental influxes. The Neocomian period (145.6–131.8 Ma) also
represents the start of the progressive northward opening of the
South Atlantic Ocean (rifting of Argentina and South Africa); and
As part of the ACE, seismic transects across the continental margin
of French Guiana were acquired, together with bathymetry, gravity
and magnetic data (Watts and Peirce, 2004). This seismic subset
comprises two coincident multi-channel and wide-angle seismic
(MCS and WA) profiles, A and D, and example data sections can be
found in Greenroyd et al. (2007a,b). Here we interpret the results from
Profiles A and D in terms of the mode of evolution of the French
Fig. 5. Initial rift geometry prior to opening of the equatorial Atlantic, for the Demerara Plateau and adjacent areas offshore French Guiana (after Benkhelil et al., 1995). This model
suggests that the initiation of continental break-up is a result of trans-tensional motion between the African and South American plates during the Early Cretaceous which resulted in
the inception of seafloor spreading by the Late Cretaceous. Locations of the two profiles modelled in this study are shown by solid lines.
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Guiana margin, and if and how the along-margin pattern of
segmentation reflects its evolution. Further details of the dataset
and modelling approaches can be found in Greenroyd et al. (2007a,b),
and profile location is shown in Fig. 2.
Several other datasets have been acquired in this region complementing the ACE, which collectively enable a comprehensive
evolutionary history of the margin to be determined. Fig. 2 shows the
location of the datasets which have been used to define sedimentary
and structural characteristics of the region (industry data – Gouyet
et al., 1994), and to develop an understanding of the 3D distribution of
sediment and the orientation and extent of basement structures
(Guyaplac data – F. Klingelhöfer and W. Roest pers. comm., with 2D
profile interpretation by the French Petroleum Institute).
The Guyaplac survey was conducted over the French Guiana
continental margin as part of the French Extraplac (e.g. Loncke et al.,
2006) programme. Of the MCS profiles acquired, ten are margin transects,
oriented northeast-southwest, and one is oriented east-west (Fig. 2). These
profiles, therefore, provide a pseudo-3D structural reference framework
extending from ACE Profile D to, and ~100 km beyond, Profile A.
Collectively, the MCS data image the sediment column and
basement across most of the margin and the corresponding
interpretation suggests that oceanic crust is observed oceanward of
the toe of the Demerara Plateau, which is interpreted as comprising
sediments overlying volcanic sills and intrusives. Significant faulting
of the oceanic crust is also observed, resulting in a rough basement
surface. Some faulting is identified within the sections, primarily on
the Demerara Plateau itself, although also in the abyssal sediments.
More extensive faulting is observed at the basement surface. In
addition, several regions of basement are interpreted as volcanic sills
and intrusions. In particular, two sharp rises in the basement surface
are observed at the toe of the Demerara Plateau on Profile D and are
interpreted as volcanic in nature.
Whilst a detailed 3D description of the basement is not possible
with the MCS data available, a pseudo-3D description can be
approximated from the 50 km separated Guyaplac 2D profiles. Thus,
only large scale features crossing the study area can be mapped.
However, given the general trend of lineations in this region (Fig. 1),
significant fracture zones are likely to be among these.
4. Summary of previous modelling
The modelling of the seismic and gravity data from Profiles A and D
is described in Greenroyd et al. (2007a,b). However, a brief summary
of results and a comparison of similarities and differences between
the two are included here (Fig. 3). Both models comprise nine
subsurface interfaces which vertically partition the models as follows:
an intra-water column thermocline; the seafloor; four intra-sediment
boundaries; the crustal basement; an intra-crustal boundary; and the
Moho. The depth to and velocity between these interfaces also varies
across the models such that they can be subdivided laterally into
regions characteristic of the continental and oceanic crusts, which are
further divided into Upper Crust and Lower Crust and Layer 2 and
Layer 3 respectively, to be compatible with the standard models of
oceanic and continental crustal structure (e.g. Spudich and Orcutt,
1980; Bratt and Purdy, 1984; Christensen and Mooney, 1995).
4.1. Profile A
Profile A exhibits 35–37 km thick pre-rift continental crust which
thins by a factor of ~6.4 over a distance of ~ 70 km. The P-wave velocity
within the Upper Crust ranges from 5.6 to 6.0 km s− 1 while that of the
Lower Crust ranges from 6.4 to 6.7 km s− 1 although ray coverage
within this region of the model is sparse compared to that further
offshore for the OBS/Hs. Oceanward of the thinned continental crust is
a 45 km wide region interpreted as a transition zone between
continental and oceanic crust.
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Further oceanward the crust is characteristically oceanic in style,
with a hummocky basement reflection imaged in the MCS data and a
two layer velocity structure. Layer 2 velocities range from 4.6 to 5.7 km
s− 1 and Layer 3 velocities range from 6.4 to 7.5 km s− 1, both noticeably
different from the adjacent continental crust. The combined Layer 2
and Layer 3 thickness is just 3-4 km which, compared to the global
average of 7.1 ± 0.8 km s− 1 (White et al., 1992), is remarkably thin.
4.2. Profile D
Profile D also exhibits 35–37 km thick pre-rift continental crust.
The continental crust here thins over a total distance of 317 km,
although the thinning occurs mainly in two regions; over a wide zone
between 70 and 235 km offset where the crust thins from 34.4 to
21.7 km thickness and over a narrower zone between 320 and 387 km
offset with thinning from 21.0 to 10.6 km. P-wave velocities are
similar to those observed along Profile A although modelling suggests
a lower velocity in the shallow crust. The Upper Crust velocities vary
from 4.2 to 6.2 km s− 1 and the Lower Crust from 6.2 to 6.9 km s−1. The
wider zone of crustal thinning is interpreted to result from profile
orientation in a direction sub-orthogonal to that of rifting, while the
more rapid, narrower, oceanward zone of thinning is interpreted to
reflect intersection with a transform feature. No transitional crust is
observed which suggests a very narrow ocean–continent transition
(OCT) at this location.
The interpretation of oceanic crust oceanward of 387 km offset is
based on the hummocky nature of the MCS basement reflection and
also the two layer crustal velocity structure suggested by WA
modelling. Layer 2 velocities range from 4.3 to 6.2 km s− 1 and Layer
3 from 6.4 to 7.4 km s− 1, similar to those modelled along Profile A.
Again, the combined Layer 2 and Layer 3 thickness is just 3–4 km,
suggesting that this may be a margin-wide feature.
4.3. Crustal thinning and the ocean–continent transition
The two profiles, despite being separated by only ~240 km alongstrike and both being oriented similarly relative to the coastline,
exhibit dramatic differences in their style of continental crustal
thinning. While the crust along Profile A thins relatively sharply over a
zone ~ 70 km wide, the crust along Profile D thins over ~ 317 km,
although much of this thinning occurs adjacent to the OCT. This
variation in width of the zone of thinning (the margin width) alone
demonstrates significant structural variation exists along this margin.
In addition, the characteristics of the adjacent OCT also vary
dramatically between profiles from being a narrow, sharp boundarylike feature along Profile D, to being a transition zone up to 45 km
wide along Profile A.
Both of these features are interpreted to demonstrate the role
played by transform features not only in margin evolution, but also as
a primary control on along-margin segmentation. For Profile A, a
mode of margin evolution is proposed in which a component of transtensional extension results in a margin with typical extensional
characteristics, but where the margin width and OCT are considerably
narrower than would be expected for a rifted margin. For Profile D, the
model is interpreted to reflect a multi-component evolution in which
large-offset transform faults, bound purely extensional, rifted regions.
The structures imaged on Profile D thus reflect the oblique trend of
this transect across the extensional part of the margin relative to the
direction of rifting, and its intersection with the much narrower
transform-type margin which is interpreted to bound the Demerara
Plateau to the north.
5. Model resolution
The χ2 fit to the data and the resolution on each interface and layer
velocity are tabulated in Greenroyd et al. (2007a,b). Subsequent
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testing of model resolution and uniqueness has been undertaken via
independent inversion of (Fig. 6), and a statistical analysis (Fig. 7) of
model fit to observed traveltimes. An inversion-based approached was
chosen, as it also provides an opportunity to assess modeller bias
inherent in the forward approach to seismic and gravity modelling
used in Greenroyd et al. (2007a,b). An additional source of uncertainty
within the models is their primary dependence on seismic data. Thus,
further tests of the uniqueness of the resulting models have been
undertaken using the magnetic anomaly (Fig. 8).
In Greenroyd et al. (2007a,b), gravity modelling provides additional constraint on the variation in crustal thickness and Moho
geometry beneath the continental slope and shelf where the ray
coverage is limited and, consequently, the P-wave velocity-depth
models are poorly constrained. Modelling of magnetic anomaly data is
used here primarily to identify true oceanic crust, and consequently
better define the location of the OCT, and identify any lineations which
might reflect the location of fracture zones.
5.1. Inverse modelling
For tomographic inverse modelling of the WA data, tomo2d was
used (Korenaga et al., 2000). Using Profile A to demonstrate the
outcome, the inverted P-wave velocity-depth model (Fig. 6) matches
the observed data to a χ2 of 1.26 and clearly shows a velocity-depth
Fig. 6. Results of inverse modelling of the observed traveltimes for Profile A. See text for details. A simple starting P-wave velocity-depth model (top) was inverted using OBS first arrival
traveltimes to produce a best-fit velocity model (centre). 1D velocity-depth profiles (bottom) through the inverted model (blue) and the forward model (black) are compared. The velocitydepth profiles are calculated at 250, 300 and 350 km offset, a region identified from the MCS data as most likely oceanic in nature. The profiles show distinct similarities between the velocity
models produced by the forward and inverse modelling techniques, although the inverse model is more inherently smoothed. OBS locations are highlighted by red triangles.
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Fig. 7. Results of Metropolis uncertainty analysis on Profile D. Top: The distribution of model boundaries produced by the Metropolis algorithm, where 66% of the boundaries lie within
one standard deviation (dark grey) and 86% within two (light grey). The input model (red lines) is the best-fit forward model, sub-sampled every 10 km. Bottom: One standard
deviation velocity errors — where 66% of models created by the Metropolis algorithm lie within the mean plus or minus the velocities shown.
profile in which velocity increases with depth. Within the region of
dense ray coverage beneath the OBSs, the correlation between the
inverse and forward modelled velocity profiles is good (cf. Figs. 3
and 6), with the inverse model being a highly smoothed version of the
forward model with no obvious discrepancies. Thus, for Profile A, the
inverse modelling is complimentary to the forward modelling,
suggesting that between 190–390 km offset the forward model is
relatively independent of modeller bias.
5.2. Metropolis
To further assess the resolution of the P-wave velocity-depth
models, the Metropolis approach was adopted (Pearse, 2002; Tarantola, 2005; Hobbs, 2006). The algorithm is not an inverse technique,
although it is computationally intensive and modeller independent,
and is particularly suited to this problem as it generates random
samples from a probability distribution that is difficult to estimate
directly. The approach is, in effect, a more detailed version of the
forward modelling approach to resolution testing described in
Greenroyd et al. (2007a,b), which is able to test lateral and vertical
velocity and depth variations simultaneously.
Metropolis is designed to quantify uncertainty within a model by
extensively testing models close to the modeller-defined, best-fit, final
forward model, by applying small, random perturbations to that
model and assessing each resulting model in terms of the statistical fit
to observed traveltimes, effectively ‘searching’ the model space
around the final forward model. As a consequence, this approach
extracts quite different information from the data when compared
with the inverse modelling approach. While the inverse modelling has
shown that the large-scale structural features of the models are not
manifestations of modeller bias, this approach will test the resolution
of these features. If, for example, a particular layer is poorly resolved
due to a lack of traveltime picks, then a variety of models, each with a
slightly different velocity or depth structure, may also fit the data.
As part of the Metropolis approach, approximately 20,000 models
were created for Profiles A and D, and from these the statistical mean
and standard deviation (σ) were calculated. Fig. 7 demonstrates an
example of the results for Profile D, with error estimates for the model
boundaries and the P-wave velocities, and the best-fit forward model
lying approximately in the centre of the suite of models created. The
uppermost layers are well constrained across the model. Errors
increase with depth, but are not excessively large and at the Moho the
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1σ error is ±0.2–0.5 km. These errors increase landwards, partly as a
result of the lack of MCS control on the basement surface and partly
due to the lack of ray coverage at the most landward OBS. Similarly,
the velocities are well constrained and 1σ errors are estimated at 0–
0.2 km s− 1 in the sediment layers and 0.2–0.4 km s− 1 in the underlying
crust. Errors in the mantle appear slightly banded, primarily as a result
of the reduction in ray coverage at depth. The banding also highlights
an issue regarding how model parameterisation can affect the results.
A typical upper mantle ray path is 20–60 km in length, whereas the
model is laterally sampled every 10 km. The banding therefore shows
that the Moho boundary and upper mantle velocity may be over
sampled, resulting in instabilities on a wavelength of ~20 km. Hence,
this demonstrates that the ray-trace approach to modelling cannot
resolve anomalies in the mantle of less than ~ 20 km in size.
6. Magnetic modelling and crustal lineations
Magnetic data offer a non-seismic approach by which to constrain
crustal properties at a continental margin. In previous studies, crossstrike variations in crustal magnetization have been observed at
several margins (e.g. Ghana – Edwards et al., 1997; Nova Scotia – Wu
et al., 2006). They are generally used to locate magnetized oceanic
crust and also to assess spreading rate with respect to magnetic field
reversals. However, changes in magnetization may also be associated
with serpentinization and the presence of fracture zones (Lin et al.,
2005). As such, magnetic modelling of the French Guiana profiles was
conducted with three aims: to analyse structural variations which
may resolve the cause of minor misfits in the seismic and gravity
models; to distinguish between oceanic and non-oceanic crust, i.e. the
location of the OCT; and to correlate magnetic anomalies along-strike
the margin to assist spreading rate calculations and the mapping of
fracture zone trends.
The modelling results (Fig. 8) proved to be highly non-unique,
although a region of increased magnetization was observed. This
anomaly is suggested to result from a fracture zone in the oceanic
crust and, consequently, it is assumed that using its magnetic
characteristics the areal extent of the major offset fracture zones
may be mapped in the regional magnetic data.
Fig. 8 shows the Guyaplac regional data and an interpretation of
possible lineations within it. A distinct low runs approximately eastwest at ~ 8°N. Using this trend, three areas of positive anomaly can be
identified between 8° and 10°N. These areas are displaced from one
another by ~90–130 km in an east–west direction and, in conjunction
with the 2D magnetic modelling, support the inference of fracture
zones since they also correlate with offsets in the basement surface
interpreted in the MCS data and lineations in the FAA.
Two of the interpreted features correlate with the fracture zones
which intercept Profile D at ~ 385 and ~ 440 km offset. The first of these
corresponds to the location of the OCT in the WA model, which
suggests that the thinned continental crust along Profile D terminates
at a fracture zone and, despite the wide zone of thinned crust, the
margin here should be termed a transform margin. The second of these
zones intersects the profile at the site of the large topographical change
in the basement surface identified in both the WA and MCS data. Two
further fracture zones are interpreted to lie at ~ 530 and ~470 km offset.
No fracture zones are interpreted to cross Profile A, although one is
extrapolated, from the gravity anomaly further oceanward, to
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intersect the margin at 5.5°N, close to the continental slope on Profile
A. The location of fracture zones in relation to the structural variation
observed, along Profile D in particular, highlights their importance in
the evolution of the French Guiana margin.
7. Discussion
The aim of the ACE was to study the along-margin-strike variation
in crustal structure at the French Guiana-Northeast Brazil continental
margin. Comparison of the results of modelling of data from ACE Profile
A (Greenroyd et al., 2007a) with that of Profile D (Greenroyd et al.,
2007b), coupled with interpretation of the Guyaplac MCS profiles,
suggests significant along-strike variation exists within the ~ 240 km of
the margin imaged by these transects. Both of the whole crustal WA
profiles are interpreted to extend from pre-rift continental to post-rift
oceanic crust and thus may also be used to inform understanding of the
geometry and mode of opening of the equatorial Atlantic.
7.1. Margin evolution
The crustal structure of the equatorial Atlantic margins reflect its
tectonic evolution from the onset of continental thinning to the
establishment of seafloor spreading which continues to the presentday. Oblique rifting of South America from West Africa resulted in
trans-tensional extension which is manifest along-strike by structural
segmentation, whose boundaries define adjacent margin regions
which do not display distinctly rifted or transform characteristics but
instead show elements of both.
The margin as a whole evolved amagmatically, most likely as a
result of the long rift duration and, perhaps, an anomalously cool
underlying mantle. Following final break-up, oceanic crustal accretion
resulted in unusually thin oceanic crust margin-wide, most likely a
result of a combination of factors including a slow spreading rate and a
high density of fracture zones. Post-rift, sediment deposition and
accumulation occurred slowly until the mid-Miocene, when the uplift
of the Bolivian Andes altered drainage patterns across South America
and resulted in the catchment area for the Amazon River extending
across most of the continent. The uplift was accompanied by erosion
which resulted in increased rates of terrigenous sediment flux into the
Atlantic where it was, and still is, transported northwestwards by
deeper ocean water circulation. The bulk of the sediment influx is
deposited as the Amazon Cone deep-sea fan system, with the
remaining deposition being controlled by basement topography
associated with fracture zones offshore French Guiana and by the
Ceara Rise aseismic ridge to the east.
Distinct structural segmentation is observed along the French
Guiana margin (Fig. 9). The key structural features (continental,
thinned continental, and oceanic crust and transition zone) from
Profiles A and D, and B and F (Rodger et al., 2006) have been used to
map out this segmentation. In addition, the OCT is traced along the
margin in two ways. Firstly, to the south, the location of the 8 km
depth-below-surface contour, i.e. the base of the sharp increase in
depth to basement, is taken from the 3D basement grid of Rodger et al.
(2006). Secondly, to the north of this basement grid, the zero crossing
of the gravity FAA was traced along the margin. Both of these have
been observed to be good indicators of the OCT location at the points
at which they intersect the ACE profiles. In addition, the fracture zones
Fig. 8. Interpretation of magnetic lineations within the regional magnetic anomaly. Top: The Guyaplac magnetic anomaly data (F. Klingelhöfer and W. Roest – pers. comm.} shows a
clear east–west trending low at 8°N and a series of positive anomaly blocks between 8° and 10°N. Lineations (solid black), block divisions (dotted black) and the locations of Profile A
and D (red) are shown. Bathymetric contours are shown to illustrate the current seafloor topography. Bottom: Results of magnetic modelling along Profile A (left) and D (right). In
each case the model is divided laterally into several blocks, each assigned a remnant magnetization in A m− 1, with a corresponding inclination of 13° and declination of 6° calculated
from the palaeomagnetic pole at the time of crustal accretion of 84°N 224°E (Gordon and Van der Voo, 1995) and the current survey location of 6.5°N 309°E. The calculated anomaly
(red) is compared with the ACE shipboard (black dotted) and Guyaplac (black solid) data. The seaward extent of the corresponding WA model is shown by the vertical blue line.
Bathymetric contours are plotted at 20 m, 50 m, 100 m (dashed line), 500 m (dotted line) and then at every 1000 m (solid line) intervals.
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C.J. Greenroyd et al. / Earth and Planetary Science Letters 272 (2008) 553–566
Fig. 9. Interpretation of structural segmentation along the French Guiana-Northeast Brazil margin. Bottom: Zones of rifted and thinned continental crust are shown by green shading,
thin oceanic crust by orange shading and transform margins by purple shading. ACE Profile A, D (red), B, E and F (blue) are shown. Fracture zone traces are marked by black solid and
dashed lines (cf. Fig. 1). Top: Reconstruction of the equatorial Atlantic conjugate margins. Interpreted margin orientations are rotated about a pole located at 60°N 35°W. The inset
compares this pole (red dot) with those of Rabinowitz and LaBrecque (1979) showing total rotation poles as green circles and early poles of opening as green triangles. Pole labels are
in Myr. The distance that the margin spread varies along strike. The southern part of the margin has spread ~ 370 km further that the northern (red; spreading path in blue) which, at a
full spreading rate of 40 mm yr− 1, suggests that the southern margin rifted 9 Myr prior to the northern margin. Bathymetric contours are plotted at 100 m (dashed line), 500 m (dotted
line) and then at every 1000 m (solid line) intervals.
picked from the gravity and magnetic data were added and used to
guide interpretation of rift and transform segments of the margin.
Within Fig. 9, two rifted segments of the margin lie roughly northsouth. To the far south of these rifted segments is a transform margin
which, given that this segment of the margin lies approximately
parallel to local transform faults, is likely to be a ‘standard’ transform
margin. The segment of margin modelled by Profile A lies in between
the two rifts, and is interpreted as a “leaky” transform margin,
although this term is used here to describe purely structural aspects
and no implications are inferred as to associated magmatic activity.
The strike of the northern and southern rifts differs by ~ 15°, a
change which is most likely accommodated by the trans-tensional
motion observed along Profile A, i.e. the plate rotation required to shift
from the southern rift direction to the northern rift direction is the
cause of a degree of oblique motion along the transform fault
separating the two. This rotation may be part of a very long lasting
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C.J. Greenroyd et al. / Earth and Planetary Science Letters 272 (2008) 553–566
shift in the plate orientations or may represent a ‘shimmy’ which
occurred over a shorter time scale, possibly related to the release of
tension when the South American and African plates finally parted, at
or close to the Demerara Plateau (Gouyet et al., 1994).
The northernmost transform segment on Fig. 9 is oriented
similarly to the segment which is crossed by Profile A. However, the
structure appears to be more similar to a ‘standard’ transform margin
than the Profile A segment. For example, a marginal ridge is observed
(Gadd and Scrutton, 1997), which is possibly a consequence of the
greater incidence of first-order fracture zones intersecting this
segment of the margin, or it may be a flexural feature associated
with differential buoyancy forces following the development of the
Ivory Coast transform margin.
The obliquely rifted section of the margin described above is not
easily explained by current models of margin evolution and, hence, a
revised model is proposed, based largely on the model of Peirce et al.
(1996); which was developed from Mascle and Blarez (1987) and
Mascle et al. (1997).
The revised model created to explain the oblique rifting observed
in the equatorial Atlantic comprises five main stages:
1) Initial intra-continental rifting begins between South America and
Africa;
2) Rifting continues and the lithosphere extends, thinning the
continental crust. However, stresses are trans-tensional, i.e. rifting
is oblique to the rift axis, and rather than forming strike-slip
transform faults, crustal thinning occurs parallel with the rift axis.
Consequently, segments of highly thinned continental crust are
offset by regions of sharply thinned crust rather than abrupt
transform faults;
3) Crustal thinning proceeds to break-up and oceanic spreading
centres form;
4) As the two lithospheric plates drift apart, strike-slip motion occurs
within the sharply thinned segments of crust. This results in the
juxtaposition of old continental lithosphere against young oceanic
lithosphere at a “leaky” transform margin;
5) The continental plates continue to drift apart, with the spreading
accommodated by strike-slip motion along fracture zones stemming from the “leaky” transforms. The structure associated with
rifting has now developed, showing typical rifted segments and
atypical “leaky” transform segments. The margin then continues to
evolve as increased sedimentation causes a progressively larger
degree of subsidence.
Post-rift, the continents of South America and Africa spread apart,
with associated accretion of oceanic crust. This spreading may be
described by movement of the continents relative to poles of rotation
which, for the Atlantic, have been described by several authors (e.g.
Rabinowitz and LaBrecque 1979) and are generally located within the
North Atlantic between 40–70°N 15–45°W.
Prior to ~107 Ma the poles of rotation are often placed further
south, in West Africa (Rabinowitz and LaBrecque, 1979). Using the
margin orientations shown in Fig. 9, the French Guiana and northeast
Brazil margin may be projected eastwards toward the conjugate West
African margin using such poles of rotation. Fig. 9 shows a
reconstruction of the conjugate margin using a single pole of rotation
positioned at 60°N 35°W and suggests that the conjugate margins can
be located accurately using projection about a single pole.
The reconstruction also suggests that the southern margin has
spread ~ 370 km further than the northern margin within the ACE
study area which, at a full-spreading rate of 40 mm yr− 1, corresponds
with a 9 Ma difference in the time of final break-up between the two
sections of the margin or asymmetric margin evolution. This gradual
break-up may suggest that the trans-tensional features observed at
this margin may be a result of the gradual northward motion of both
the rift and the stress field which, given the progressive development
of the margin, is oriented obliquely to the rift direction.
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7.2. Effect of sediment thickness
The assessment of the basement morphology and seismic velocity
structure within the equatorial Atlantic has shown that the oceanic
crust along Profile D is transected by at least two, possibly three
transform features. The most distinct of these, located at 440 km
offset, is associated with a 1.9 km change in basement topography
along Profile D. Furthermore, the fault can be traced across several of
the Guyaplac MCS data sections in which associated basement
topography gradually decreases eastward, i.e. oceanward. In addition
to the basement topography the fault is also clearly observed in the
gravity FAA.
This first-order transform fault is associated with a N100 km offset
in the MAR and, hence, appears to be very significant in the post-rift
evolution of the region. To the north of the fault, several further
fracture zones are clearly identified, some of which can be traced over
1800 km from the MAR toward the Caribbean at ~ 54°W. To the south
no fracture zones are observed in the FAA to the west of 46°W,
although several hundred kilometres south of the Amazon Cone, they
can once more be traced to the margin. Such an observation would be
readily explained if the margin in this region was formed by purely rift
type processes.
However, as previously established, this is not the case and rifted
and transform structures are observed. A possible explanation for the
lack of fracture zones observed within the regional gravity data (Fig. 1)
is the thick sediment cover which, despite their close proximity,
differs significantly between Profiles A (6.5 km thickness) and D
(3.9 km). The extra ~3 km of sediment observed along Profile A may
result in higher densities within the lower sediment column, which
would reduce the density contrasts and, hence, reduce the FAA
characteristics associated with fracture zones. The reason for this
change in sediment thickness along-margin-strike is partly related to
the Amazon River, the major local source of suspended sediment
(Cobbold et al., 2004). However, the fracture zones observed in this
study may also play a role in controlling the distribution of sediment.
It is likely that the large rise in basement topography, observed at a
fracture zone along Profile D, has dammed the sediment which has
been carried northward up the coast from the mouth of the Amazon
River by prevailing currents (Johns et al., 1998). The Ceara Rise has had
a similar effect, damming the eastern edge of the thick Amazon Cone.
This has resulted in an unusually thick sediment column not just
within the Amazon Cone fan, but also within the region south of 8°N
and west of ~ 46°W, as shown in Fig. 9.
8. Conclusions
In this paper two models of the deep crustal structure of the
passive continental margin of French Guiana-Northeast Brazil have
been interpreted in the context of margin evolution and along-strike
margin segmentation. The models were created by modelling
coincident MCS and WA seismic data, acquired as part of the Amazon
Cone Experiment.
As a whole, the margin is characterised by 35–37 km thick pre-rift
continental crust which is separated from unusually thin oceanic crust
(3–4 km thick) by thinned continental and/or transitional crustal
regions. To the north, Profile D exhibits a 320 km wide zone of thinned
continental crust adjacent to a narrow OCT, and is interpreted as a
transform margin where the wide zone of thinned crust is a result of
profile orientation highly oblique to the direction of rifting. Profile A,
~ 240 km to the south, shows a 70 km wide zone of thinned
continental crust, wider than typical for transform and narrower
than typical for rifted margins which generally thin over b40 km and
N100 km respectively. The crustal structure along this profile is
interpreted to reflect a “leaky” transform formed by trans-tensional
extension. The contrasting crustal features between the models
demonstrate that this margin is highly segmented structurally and
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C.J. Greenroyd et al. / Earth and Planetary Science Letters 272 (2008) 553–566
that the pattern of segmentation defines regions in which the margin
displays characteristics of both rift and transform evolution. The
location of fracture zones are demonstrated to strongly influence the
variation in along-margin-strike structural characteristics, with the
origin of these fracture zones being related to the initial break-up
geometry.
Acknowledgments
We wish to thank the master, officers and crew of the RRS
Discovery, together with the sea-going staff of NERC's UKORS, Tom
Oliva of Seamap UK, and Anne Krabbenhöft and Cord Papenburg who
operated the IFM-Geomar seabed instruments during the cruise.
Land-based assistance was provided by Dr Jesus Berrocal, Prof.
Cleverson Silva, and Prof. Alberto Figueiredo in Brazil, Andy Louch in
NERC's RSU, Lourenildo Leite in Belem and Phillippe Weng and Pierre
Laporte in French Guiana. This research was funded by the NERC
through the Ocean Margins LINK programme and by an Advanced
Research Fellowship awarded to RWH. The GMT and Seismic Unix
software packages (Wessel and Smith, 1998; Cohen and Stockwell,
2000 respectively) were used to create the figures, and ProMAX to
process the MCS data, for this paper. Finally, we also thank the two
reviewers for their helpful and positive comments on this paper.
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