Download Newly identi¢ed segments of the Paci¢c^Australia plate boundary

Earth and Planetary Science Letters 193 (2001) 347^358
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Newly identi¢ed segments of the Paci¢c^Australia
plate boundary along the North Fiji transform zone
Bernard Pelletier a; *, Yves Lagabrielle a , Mathieu Benoit b , Guy Cabioch a ,
Stëphane Calmant a , Erwan Garel c , Christe©le Guivel d
a
d
UMR 6526, IRD, Laboratoire de Gëologie-Gëophysique, P.O. Box A5, Noumëa, New Caledonia
b
UMR 6538, IUEM, Place Nicolas Copernic, 29280 Plouzanë, France
c
UMR 6538, UBO, 6, Avenue le Gorgeu, P.O. Box 809, 29285 Brest, France
Laboratoire de Planëtologie et Gëodynamique, 2 rue de la Houssinie©re, P.O. Box 92208, 44322 Nantes, France
Received 15 March 2001; received in revised form 18 June 2001; accepted 25 September 2001
Abstract
The North Fiji transform zone, a 1500 km long and 200 km wide transform segment of the Pacific^Australia plate
boundary, is one of the major transform fault systems of the Earth. New data collected during the ALAUFI cruise
(March 2000) on board the R/V L'Atalante make it possible to define more accurately the geometry and kinematics of
this transform plate boundary. Three spreading centers or extensional zones (the North Cikobia spreading center, the
Futuna spreading center and the southeast Futuna volcanic zone) and a strike-slip fault zone (the Futuna transform
fault) have been discovered over a distance of 500 km along the eastern North Fiji transform zone, from the north of the
Fiji platform to the east of the Futuna archipelago. The Futuna transform fault oriented 100³ has been mapped over a
distance of 250 km. It must be considered to be an important tectonic element of the transform plate boundary. Pure
strike-slip as well as transpression and transtension motions are responsible for the complex morphology of this feature.
The uplifted Futuna^Alofi ridge represents a major compressional relay along the Futuna transform fault. The Futuna
spreading center trending 20^30³ is composed of a series of en ëchelon left-stepping spreading segments. It represents a
200 km long extensional relay between the Futuna transform fault and the western part of the North Fiji transform
zone, the Fiji transform fault, which bounds the Fiji platform to the north. The opening rate at the Futuna spreading
center is estimated at 4 cm/yr. Although the North Cikobia spreading center and the southeast Futuna volcanic zone
have been only partly mapped, bathymetric and reflectivity data clearly reveal that active extension also takes place
along these two features. A spreading rate of 2 cm/yr is inferred at the North Cikobia spreading center. Therefore, the
North Fiji transform zone appears to be composed of two main overlapping transform segments relayed by parallel
extensional zones. The three active extensional zones have an ENE^WSW to NNE^SSW orientation, while compressive
features along the Futuna transform fault are NW^SE to NNW^SSE oriented, in accordance with the present-day leftlateral transform motion along this part of the Pacific^Australia plate boundary. ß 2001 Elsevier Science B.V. All
rights reserved.
Keywords: transform faults; spreading centers; Paci¢c plate; Australia Plate Fiji
* Corresponding author. Tel.: +687-26-0772; Fax: +687-26-4326.
E-mail address: [email protected] (B. Pelletier).
0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 5 2 2 - 2
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B. Pelletier et al. / Earth and Planetary Science Letters 193 (2001) 347^358
1. Introduction
In the region north of the Fiji platform, between the northern end of the Tonga trench and
the central North Fiji Basin, the vector of relative
motion between the Paci¢c and the Australia
plates is roughly parallel to the plate boundary.
This corresponding transform boundary segment,
the North Fiji transform zone (NFTZ), is almost
1500 km long (Fig. 1). It represents one of the
major transform boundaries on the Earth's surface [1] and has always been regarded as playing
a major role in the recent tectonic development of
the Lau and North Fiji basins [2,3]. The NFTZ is
characterized by a 200 km wide belt of shallow
seismicity with predominant left-lateral strike-slip
focal mechanism solutions [4,5]. Thrust-type
events also occur in the region of the Futuna^
Alo¢ islands [6,7].
Although the NFTZ is a key feature of the SW
Paci¢c tectonics, its detailed outline has remained
poorly known. Previous surveys have shown that
the western part of the NFTZ consists of successive long transform segments and short extensional relay zones, such as the west Cikobia volcanic
zone (WCVZ) and the Yasawa^Yadua volcanic
zone [8^10] (Fig. 1). Surveys in the northern
Lau basin, south of the NFTZ, revealed that extension occurs along three neovolcanic zones,
namely, the northeast Lau spreading center, the
northwest Lau spreading center and the east Cikobia volcanic zone (ECVZ) [9,11,12]. However,
the northward extensions of these three neovolcanic zones are unsurveyed. In addition, the eastern part of the NFTZ between the Fiji platform
and the Tonga trench, where these structures
should be connected, has never been surveyed in
detail. Scarce available data suggest a complex
structural pattern, involving transform faults,
compressional relay zones and probable active
spreading centers [13]. During a transit of the
R/V L'Atalante from Papeete to Noumëa in October 1999, a preliminary, short survey around
the Futuna and Alo¢ islands revealed that a
WNW^ESE elongated feature, the Futuna^Alo¢
ridge, must be interpreted as a piece of uplifted
oceanic crust along a compressional relay zone of
the NFTZ [14].
We report here the main results of the ¢rst
large survey devoted to the study of the eastern
part of the NFTZ, northeast of the Fiji platform
and o¡shore the islands of Futuna and Alo¢ (Fig.
1A). This survey, the ALAUFI cruise, was conducted in March 2000 on board the French R/V
L'Atalante. Scienti¢c operations included bathymetric mapping and acoustic imagery using a
SIMRAD EM12 dual multibeam echo sounder,
gravimetry and magnetic pro¢ling, six channel
seismic re£ection pro¢ling and dredge hauls. We
also used additional bathymetric, imagery and
magnetic data obtained during a N^S transit of
the R/V L'Atalante SOPACMAPS Leg 3 cruise
from Fiji to Tuvalu in 1990, and during the aforementioned transit of the R/V L'Atalante from Papeete to Noumëa in October 1999 [14].
2. Newly discovered spreading centers and
extensional zones north of the Fiji platform
One of the major results of the ALAUFI cruise
is to show that well organized spreading centers
or extensional zones are active within the 200 km
wide NFTZ transform boundary. Between
178³40PE and 176³20PW, that is along a distance
of 500 km from west to east, three active extensional zones have been evidenced on the basis of
bathymetry, acoustic imagery and magnetic pro¢ling. These are: the north Cikobia spreading center (NCSC), the Futuna spreading center (FSC)
and the southeast Futuna volcanic zone (SEFVZ)
(Fig. 1B).
2.1. The north Cikobia spreading center (NCSC)
The NCSC, located north of the Fiji platform,
has been mapped over 80 km along-strike between 178³45P and 179³30PE and 14³10P and
14³35PS during the transit from Vanuatu to the
study area (Figs. 1 and 2). This feature appears
on the general bathymetric map as a narrow
trough trending 60³ (Fig. 1) and, although unknown in detail, it has been speculatively interpreted as an active extensional zone [13].
Multibeam bathymetry reveals that the NCSC
is characterized by a deep graben (2250^3200 m
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349
Fig. 1. Tectonic setting of the NFTZ and location of the area surveyed by the R/V L'Atalante during the March 2000 ALAUFI
cruise. (A) Location of the surveyed area at the Paci¢c^Australia transform plate boundary. Ship tracks include a Fiji^Tuvalu
N^S transit of the R/V L'Atalante (SOPACMAPS Leg 3, 1990 cruise) and the Papeete^Noumëa transit of the R/V L'Atalante
and its short survey around Futuna in October 1999 [14]. (B) Tectonic sketch showing spreading segments and transform faults
segments including those discovered during the ALAUFI cruise.
depth) varying in trend towards northeast from
45³ to 65³ then 70³ (Fig. 2A). Activity of this
feature is indicated by scarps and volcanism delineated by a strong acoustic re£ectivity of the
sea£oor (Fig. 2B). Between 179³E and 179³10PE,
a 1 km wide ridge culminating at 1500 m depth is
present in the axial part of the graben, and is
interpreted as a neovolcanic ridge. The deepest
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Fig. 2. Bathymetry (A), acoustic imagery (B) and magnetic (C) maps of the part of the NCSC surveyed during the ALAUFI
cruise. The magnetic anomaly map also includes previous aeromagnetic data along N^S tracks [15]. The bathymetric map has a
100 m isocontour. Deep orange and red above 1500 m, yellow from 1800 to 2100 m, light blue from 2700 to 3000 m, dark blue
deeper than 3000 m.
part of the graben is found at 179³18PE^179³25PE,
north of a large volcano with a summital caldera,
in a zone of possible axial discontinuity. Indeed
north of this deepest part, a linear, narrow (500 m
wide) ridge trending 73³ likely represents the axial
part of the NCSC east of 179³15PE. The system
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Fig. 3. Bathymetric map of the main area surveyed during the ALAUFI cruise. Contours: 1000 m. Shallow to deep from red to
blue. Red and orange from 0 to 1250 m, yellow from 1250 to 1750 m, green from 1750 to 2750 m, light blue from 2750 to 3250
m, dark blue deeper than 3250 m (maximum 3750 m).
appears thus to be o¡set towards the north, and
abuts to the east on a transverse 150³-trending
structure at 14³20PS^179³35PE. East of 179³45PE
the oceanic fabric trends NNE^SSW parallel to
the FSC bathymetric fabric (see below).
Compilation of magnetic data acquired during
the ALAUFI cruise with data from a previous
aeromagnetic survey [15] indicates that magnetic
lineations clearly parallel the sea£oor bathymetric
fabric. A positive anomaly that correlates with the
Brunhes anomaly is centered on the graben (Fig.
2C). Identi¢cation of anomalies 1 and 2 as proposed on the Fig. 2C leads to a full spreading rate
of 2 cm/yr (20 km wide zone for the last 1 Ma
Jaramillo Chron, and 40 km wide zone for the
last 2 Ma anomaly 2 Chron).
In conclusion, the bathymetry, acoustic imagery
and magnetic data collectively support the interpretation that the graben with an elongated volcanic axial ridge corresponds to an active spreading center. Although the NCSC has not been fully
mapped all along its strike, it likely extends both
towards the northeast and the southwest, where it
may connect to the northern extension of the
WCVZ [9], and to the eastern extension of the
Tripartite spreading ridge (TSR) [16]. Indeed a
plausible interpretation is that the ENE^WSWtrending NCSC, the NNE^SSW-trending WCVZ
and the WNW^ESE-trending TSR join together
in a kind of Ridge^Ridge^Ridge triple junction
near 15³S^178³30PE, north of the Fiji platform
(Fig. 1B).
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Fig. 4. Acoustic imagery map of the main area surveyed during the ALAUFI cruise.
2.2. The Futuna spreading center (FSC)
The FSC has a general 30³ orientation and extends over more than 200 km from 15³40PS at the
northeastern tip of the Fiji platform to 13³35PS
northwest of the Futuna^Alo¢ islands (Fig. 3).
Active spreading is revealed by a 15 km wide
band of continuous strong acoustic re£ectivity
of the sea£oor (Fig. 4). Very fresh basaltic lavas
(pillows and massive £ows) have been recovered
from nine dredge hauls regularly spaced along the
axis (Fig. 5), thus con¢rming active spreading.
The FSC deepens southward; it is composed of
three main parts (Figs. 3, 5 and 6).
The northern part lies northwest of Futuna
from 13³35PS to 14³25PS, and is o¡set about 20
km westward relative to the central part. It consists of two main segments trending 35^40³ and a
relay zone composed of three small (6 km long) en
ëchelon segments. Segments have a pronounced
dome-shaped transverse pro¢le (Fig. 6). The
sharpest axial ridge, close to 14³S, culminates at
600 m below sea level, 1300 m above the surrounding sea£oor. On the western side of the
ridge the oceanic fabric strikes 40^50³. Such obliquity as well as the peculiar morphology of the
northern part of the ridge and the o¡set relative
to the central part suggest a recent ridge jump and
a young emplacement of the northern part of the
FSC, at the time of the initiation of the Futuna
transform fault. Bathymetry and imagery data of
the sea£oor area around 14³S^178³30PW suggest
that a relict spreading axis is present north of the
Futuna transform fault, with the same orientation
as the central part of the FSC but slightly o¡set to
the east (Fig. 5).
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Fig. 5. Structural map of the main area surveyed during the ALAUFI cruise.
The central part of the FSC extends from
14³15PS to 15³15PS and is composed of three,
30^40 km long, left-stepping, en ëchelon segments
trending 25^30³. The dome of the axial part of the
two northern segments disappears both to the
north close to the junction with the Futuna transform fault and to the south where it is replaced in
the southern segment by a series of small troughs
and ridges that deepens southward (Figs. 4 and
6). The sea£oor mapped over a distance of 55 km
on both sides of the center of the axial zone is
characterized by a fault fabric which trends 30³,
parallel to the highly re£ective axial part. This
indicates that a steady-state spreading has occurred in a 110^120³ direction for long enough
to produce a 110 km wide domain of oceanic
crust. The magnetic map clearly reveals a sea£oor
spreading-type pattern (Fig. 7). The lineations of
the magnetic anomalies parallel the sea£oor
bathymetric fabric (Fig. 7, top). A wide positive
anomaly is centered along the axial zone, and is
£anked on both sides by a series of negative and
positive anomalies (Fig. 7, bottom). These anomalies are interpreted as anomalies 1, J and 2, giving
a full spreading rate of 4 cm/yr (Fig. 8).
The boundary between the central and the
southern parts of the FSC is marked by a slight
change in trend from 30³ to 15³ of the axial zone
in the vicinity of a 600^1400 m high scarp trending 135³ (Fig. 3). This scarp is located in the extension of the Peggy ridge, a major transform
fault of the northern Lau basin [9,12]. Curvature
of the oceanic fabric close to the scarp suggests
that recent right-lateral strike-slip motion occurred along it.
The southern part of the FSC, south of
15³15PS, includes the ECVZ, partly mapped during a previous survey [9]. It consists of a spectacular V-shaped rift (deepest point at 3739 m depth)
bounded to the west by a series of arcuate ridges
and troughs ranging in trend from N^S to 135³
and, to the east by a 1000 m high 20³-trending
scarp (Figs. 3^6). East of this scarp, slices of sedimented crust are displaced along a set of parallel,
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Fig. 6. Bathymetric pro¢les across the Futuna spreading center (1^4) and the Futuna transform fault (5). Lateral extension of the active volcanic zone revealed by strong acoustic
re£ectivity is shown by a bold line above each pro¢le.
140^160³-trending right-lateral strike-slip faults
showing rectilinear to slightly curved outline.
This pattern suggests active southward propagation of the FSC into older oceanic crust north of
the Fiji platform. Basalts, dolerites and greenschist facies gabbros belonging to the rifted old
oceanic lithosphere were recovered by dredging at
the tip of the propagator (dredge site 8, Fig. 5).
2.3. The southeast Futuna volcanic zone (SEFVZ)
East^southeast of the Futuna^Alo¢ Islands, a
30 km wide zone, trending WSW^ENE between
176³30PW and 177³25PW and centered at 14³45PS,
is delineated by a strong acoustic backscattering
(Fig. 4). This zone, marked by a slightly positive
bathymetry and studded with numerous small
volcanoes, is characterized by morphologic lineations trending 80^90³ in the northeastern part and
50³ in the southwestern part (Figs. 3 and 5). A
positive magnetic lineation lies in the middle of
the southwestern part (Fig. 7). Bathymetry and
imagery clearly reveal that this zone, here called
the SEFVZ, is an active volcanic zone. There are
insu¤cient data to map completely the southward
extension of the SEFVZ. However, it is likely that
the SEFVZ joins the highly re£ective NNE^SSWtrending zone found at 15³15PS^177³45PW and
then, further south, abuts on the NW^SE-trending transverse structures associated with the
northwest extension of the Peggy ridge transform
fault (Fig. 1B).
The southwestern part of the SEFVZ is parallel
to the FSC and located south of the Futuna
transform fault. Its northeastern part connects
to the Futuna transform fault and is therefore
oblique to the E^W left-lateral relative motion
between the Paci¢c and Australia plates. According to its orientation and geometry, the SEFVZ
can be interpreted as an active extensional zone or
spreading center. However, the lack of clear axial
topographic lineament suggests di¡use magmatism rather than axially focused magmatic accretion.
3. Strike-slip faults and compressional relay zones
along the NFTZ
The second major result of the ALAUFI cruise
is the discovery of E^W-trending fault zones that
connect orthogonally to the FSC, namely the Futuna transform fault and the north Futuna fault
zone (Fig. 5). The Futuna transform fault, along
which lie the Futuna and Alo¢ islands, represents
the main segment of the eastern part of the NFTZ
and likely extends to the northwestern end of the
Tonga trench (Fig. 1).
3.1. The Futuna transform fault
The Futuna transform fault has been mapped
over 250 km near 14³20PS, from 176³20PW to
178³45PW (Figs. 3^5). It can be divided into three
parts.
The western part, from the northwestern tip of
Futuna island to the northern end of the central
segment of the FSC at 14³10PS^178³45PW, consists of a deep E^W-trending trough (2500^3200
m depth, Fig. 3) in which 100³-trending en ëchelon lineations are interpreted as active strike-slip
faults. A relay at 178³25P^178³30PW corresponds
to the deepest part of the trough, and can be
regarded as a small pull-apart basin. Left-lateral
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355
Fig. 7. Magnetic anomaly map (isocontour 100 nT, positive anomalies in yellow and red, negative anomalies in blue) and pro¢les
(positive anomalies in black). Pro¢le underlined in yellow is modelled in Fig. 8.
strike-slip motion along this fault zone is expected, in accordance with the spreading direction
of the FSC.
The central part of the Futuna transform fault
zone consists of the Futuna^Alo¢ ridge and its
fault-bounded boundaries (Fig. 5). The emerged
parts of the Futuna^Alo¢ ridge, the Futuna and
Alo¢ islands, are composed of submarine basaltic
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Fig. 8. Example of E^W magnetic anomaly pro¢le across the
Futuna spreading center (1) and synthetic model (2). Location of pro¢le in Figs. 5 and 7.
sequences and rare gabbros, capped by a series of
uplifted reef terraces of presumably Quaternary
age and reaching 300 m above sea level in Alo¢
[17]. The tectonics is still active as revealed by the
0.5 m maximum uplift of the modern reef along
the southern coast of Futuna during the last major earthquake in March 1993 [7,18]. Di¡erences
in altitude of reef terraces between both islands
suggest that uplift rates vary from one island to
another, and that the ridge basement is composed
of independent tectonic blocks [14]. Our survey
reveals that the Futuna^Alo¢ ridge extends
WNW^ESE over 50 km between 177³45PW and
178³15PW. It shallows westward, the highest portion of the ridge corresponding to the Futuna and
Alo¢ islands (northwest Futuna being therefore
the highest part, 524 m at Mount Puke). The
southwest £ank of the ridge is arcuate southwestward, and small curved ridges lie at the toe of the
slope west of Futuna. This morphology, characteristic of accretionary prisms, con¢rms that active compression is occurring along the western
side of the ridge. Thrust-type focal mechanism
solutions of shallow earthquakes are restricted
to the westernmost part of the ridge [6,7]. The
eastern part of the ridge (less than 1000 m depth)
is a¡ected by NE^SW- and NW^SE-trending
fractures, and is bounded to the south and to
the north by 100³-trending troughs and lineations,
which are interpreted as active strike-slip faults, in
agreement with the occurrence of strike-slip fault
type shallow earthquakes along the southern fault
zone [14]. The northern fault zone should extend
westwards in the channel between Futuna and
Alo¢ islands, and joins the frontal thrust zone
south of Futuna. The progressive change in the
orientation of the ridge leads to a northwestward
increase of the compressional component. The
presence of strike-slip fault across the ridge would
explain the di¡erence in uplift rate between adjacent blocks.
The eastern part of the Futuna transform fault,
east of 177³45PW, is characterized by a complex
pattern of ridges and troughs limited by faults
(Fig. 5). Geometry of structures suggests intense
shearing and block rotation along left-lateral
strike-slip faults. Between 177³10PW and
177³40PW, a series of NW^SE-trending ridges,
which culminate at 400^1000 m below sea level,
likely result from compression-related uplift, in a
way similar to that of the Futuna^Alo¢ ridge.
Further east, from 177³10PW to 176³20³W, E^W
lineaments, ENE^WSW-trending fabric and evidence of active volcanism suggest that transtension occurs along this segment of the Futuna
transform fault which bounds the SEFVZ to the
north.
3.2. The north Futuna fault zone
The north Futuna fault zone is located at
13³45PS between 178³17PW and 178³40PW. It corresponds to a 20 km wide deformed area exhibiting a series of E^W-trending lineations which perpendicularly abut against the northernmost
segments of the Futuna spreading ridge (Figs.
3^5). Numerous small volcanoes are emplaced
along these lineations. At its eastern end, the fault
zone connects to a series of three NNW^SSE
ridges culminating between 1200 and 1500 m
depth (Fig. 3). These ridges are arcuate westward
and converge on the northern tip of Futuna (Fig.
5). By comparison with the origin of the Futuna^
Alo¢ ridge, these ridges can be regarded as pieces
of oceanic crust, uplifted and tilted toward the
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east. Therefore active transpression and block rotation may take place within a 60 km wide area
around the Futuna^Alo¢ islands.
4. Conclusion
New data collected during the ALAUFI cruise
make it possible to de¢ne more accurately the
anatomy and the functioning of the NFTZ, a
key element of the Paci¢c^Australia plate boundary (Fig. 9). Three spreading centers or extensional zones (the NCSC, FSC, SEFVZ) and one major strike-slip fault zone with transpressional and
transtensional segments (the Futuna transform
fault) have been revealed along the eastern part
of the NFTZ over a distance of 500 km.
The Futuna transform fault, surveyed over a
distance of 250 km, must be considered an important tectonic element of the NFTZ. Strike-slip,
compression and transtension motions are responsible for the complex morphology of this 100³trending feature along which the western tip of
the uplifted Futuna^Alo¢ ridge formed the Futu-
357
na archipelago. The 30³-trending FSC, composed
of a series of en ëchelon left-stepping spreading
segments, represents a major 200 km long extensional relay between the Futuna transform fault
and the western part of the NFTZ which bounds
the Fiji platform. Active oceanic spreading along
the FSC is collectively revealed by high re£ectivity
of the sea£oor, by the occurrence of very fresh
basaltic pillow lavas and by the presence of magnetic lineations parallel to the bathymetric fabric.
A spreading rate of 4 cm/yr at the FSC is deduced
from the interpretation of magnetic anomalies.
The NCSC and the SEFVZ have been only partly
mapped during the cruise. However, bathymetric
and re£ectivity data clearly reveal that active extension also occurs along these two features. A
spreading rate of 2 cm/yr is inferred at the NCSC.
The NFTZ, along which the Paci¢c plate moves
westward at a rate of 8.5 cm/yr relative to the
Australia plate, is thus composed of two main
transform segments between which extension occurs (Fig. 9). The Futuna transform fault to the
east and the Fiji transform fault to the west overlap and are separated by a series of parallel exten-
Fig. 9. Sketch showing the newly identi¢ed segments discovered during the ALAUFI cruise within the Paci¢c^Australia plate
boundary.
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sional zones. Other additional spreading centers
or extensional zones may also exist in the nonsurveyed easternmost portion of the NFTZ, between 177³W and the western termination of the
Tonga trench at 175³W. The three newly identi¢ed active extensional zones have an ENE^WSW
to NNE^SSW orientation, while the compressive
features are oriented NW^SE to NNW^SSE, in
accordance with the present-day left-lateral transform motion along this part of the Paci¢c^Australia plate boundary. When considering the plate
boundary as a major, 200^300 km wide transform
system, the active spreading centers display the
expected orientation for tensional features, while
the uplifted ridges are found with the expected
orientation for pressure features.
Acknowledgements
[7]
[8]
[9]
[10]
[11]
[12]
The ALAUFI cruise was a joint project between IRD and INSU/CNRS, in the framework
of the Programme National sur les Risques Naturels (PNRN). We thank the captain and the
crew of the R/V L'Atalante for their e¤ciency
during the cruise. UMR Gëosciences Azur contribution no. 376.[AC]
[13]
[14]
[15]
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