kinematics of active spreading in the central north fiji ...m~riti~ geo/og.v.i 16 (1 991) b9-87...

M ~ r i t i ~ Geo/og.v. I 16 ( 1 991) b9-87 Elsevier Science B.V.. Amsterdam

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Kinematics of active spreading in the central North Fiji Basin (Southwest Pacific)

Philippe Huchona. Eulàlia Gràciab*", Etienne Ruelland, Masato Joshima' and Jean-Marie Auzendeb3

"CNRS URA 1316, Laboraroire de Géologie, EcoZe Normale SupPrieure. 24 rue Lhomond, 75231 Paris Cedex 05, France bDépartement Géosciences Marines, IFREMER Centre de Brest, B. P. 70,29280 Plouzané, fiance

'Dpto. Geologia Dinamica, Geofsica y Paleontologia, Universidad de Barcelona, 08028 Barcelona, Spain %'NRS URA 1279, Instirtir de Géodynamique, rue Albert Einstein, Sophia Antipolis. 06565 Valbonne Cedex, France

'Geological Survey of Japan, Higashi, Tsubuka, Ibaraki 305, Japan (Received April 26, 1993; revision accepted July 28. 1993)

ABSTRACT

Huchon, P., Grkia , E., Ruellan, E., Joshima, M. and Auzende, J.-M., 1994. Kinematics of active spreading in the central North Fiji Basin (Southwest Pacific). In: J.-M. Auzende and T. Urabe (Editors), North Fiji Basin: STARMER French-Japanese Program. Mar. Geol., 1 1 6 69-87.

Based on a synthesis of magnetic and bathymetric data, we re-evaluate the kinematics of the recent opening of the central part of the North Fiji Basin (NFB). The westward motion of the Pacific plate along the left-lateral North Fiji Fracture,Zone (NFFZ) results in the opening of two N-S trending spreading ridges, located at 173"30E and 176'E. Both ridges show complex features such as propagating rifts, ridge jumps and overlapping spreading centres. Their spreading rates are similar: 7.6 to 4 cm yr-' across the western ridge, 5.5 cm yr-' across the eastern one. While the NFFZ is purely strike-slip to the east of the eastern ridge, it becomes more complex to the west: it changes from transpressional to transtensional. then to purely transform and finally joins the western N-S ridge in a RRR-type triple junction. Our kinematic analysis shows that most of the left-lateral motion along the NFFZ is transferred to the eastern ridge at the RTF-type 176E triple junction. It suggests that the western N-S ridge is probably connected to the north with another left-lateral transform. possibly the South Pandora "ridge".

Introduction

The North Fiji Basin (NFB hereafter) was first described as a young oceanic basin by Chase (1971) on the basis of magnetic anomalies. Its oceanic character was also evidenced by high heat flow values (Watanabe et al., 1977) and by the attenuation of seismic waves in the upper mantle (Dubois et al., 1973; Barazangi et al., 1974). Generally considered as a back-arc basin, the NFB is in fact located between two subduction zones of opposite polarity: the New Hebrides (Vanuatu) to the west and the Tonga to the east (Fig. I). Therefore, it

'Present address: ORSTOM, U R IF. B.P. A5, Nouméa Cedex, New Caledonia

c

may be regarded as a large scale left-lateral pull- apart basin between the Pacific and the Indo- Australian plates. However, its detailed tectonics are still debated. Contrasting interpretations of available data have been given the same year by Hamburger and Isacks (1 988) who emphasised the importance of diffuse deformation and Auzende et al. (I988b) who applied plate tectonics concepts.

The NFB is generally considered as opening in two stages. I t first opened following the clockwise rotation of the Vanuatu island arc away from the Vitiaz island arc (Falvey, 1978). This nearly .90c rotation occurred from 10 to 3 Ma and resulted in a fan-shaped pattern of magnetic lineations. It was also accompanied by an anticlockwise rotation of the Fiji platform (James and Falvey, 1978). The

0025-3227/94/S07.00 1994 - Elsevier Science B.V. All rights reserved. SSDl 002 5- 3 227 (93 ) E009 6- O

70 P. HUC'HON FI' AL. 4

Fig. 1. Geodynamic framework of the North Fiji basin. The survey area is outlined.

more recent E-W phase of opening appears to be contemporaneous with the formation of the Lau Basin.

Several qualitative models have been proposed for the recent evolution of the NFB, but these models generally lack constraints of sufficient data or did not consider the system as a whole. The only quantitative kinematic model was proposed by Louat and Pelletier (1989). During the last decade. a considerable amount of data has been acquired in the central NFB by French (IFREMER

and ORSTOM) and American teams, and, since 1987, during the Japanese-French STARYER project. These surveys involved a wide variety of geophysical and geological tools such as multibeam bathymetry, side scan sonar imagery, multichannel seismic profiling. as well as manned submersible observations. The STARMER project mainly focused on the N-S trending active spreading system located at 173'30'E. In this paper, however, we also pay particular attention to another N-S trending ridge recently recognised at 176'E, as well as to another major tectonic feature of the NFB: the North Fiji Fracture zone (NFFZ hereafter). This paper pres-

ents a new interpretation of the kinematics of-the central part of the NFB, including the western part of the NFFZ (west of Viti Levu island). This interpretation is based not only on the analysis of all available magnetic anomaly data but also on bathymetric data which provides control on the identification of magnetic lineations, as well as seismological data.

Overview of the data set and methods

The first multibeam (Seabeam) bathymetric map was obtained in the NFB during the Senpso 3 cruise of the R/V Jenn Clzarco~ (Auzende et al.. 1986a,b, 1988a). During the STARMER project, a considerable amount of Seabeam and Furuno multibeam data have been acquired. All these bathymetric data have been compiled into a 1/200,000 map (Auzende et al., 1990b) and a 1/500,000 map (Urabe et al., 1992) and are also available in a digital format for further processing (see for example Ruellan et al., 1994-this issue).

Side scali sonar and bathymetric data were also obtained in 1986 and 1987 by the R/V Moana Wm*e of the Hawaii Institute of Geophysics (Kroenkc et al., 1991), as well as Seabeam bathymetric data by the R/V Sonne (Von Stackelberg et al.. 19S3.

Magi ler ic au oiiiaiies

In addition to the data from the STARMER project, all available information was extracted from the GEODAS CD-ROM (NOAA/NGDC, 1992). About 50 cruises have been conducted in the central part of the NFB since 1967 (Table 1). It corresponds to about 210.000 magnetic data points. Most of the recent cruises (since 1980) are those of the STARMER

project (about 100,000 magnetic data points) and of the Hawaii Institute of Geophysics (Kana Keoki 1982 and Moana Wave 1986 and 1987 cruises, about 40.000 points). The other recent cruises are Seapso 3. Era 9 and 12 (ORSTOM), Papatua 6 (Scripps Institution of Oceanography) and L582SP (USGS). All those recent cruises provide a total number of 180,000 data points with accurate GPS navigation. A track chart of available data is shown on Fig. 2.

using a Fast Fourier Transform (Schouten an( McCamy, 1972). The processing includes severa steps: (1) identification of profiles of sufficien length and of proper orientation with respect tc the spreading axis projection: we used a minimun length of 10-30 km and retained profiles whost direction is not oblique to the ridge axis by mort than 45"; (2) projection of each profile on tht selected direction; (3) interpolation at a constan step (generally 1 km), necessary to be able tc perform the FFT; (4) FFT and band-pass filterini between 5 and 100 km. The filter is tapered at bod ends of the gate by a linear function returning tc zero over 5 km. At an average ship's speed of I ( knots, the 100km cut-of€ value corresponds tn about 6 hours in the time domain, so that i1 effectively removes the diurnal variation (24 hours) and its main harmonics (12 and 6 hours). The tapering of the filter between O and 5 km allows to remove high frequency variations (the data were sampled every minute, thus every 300 m in the space domain); ( 5 ) back projection of the filtered values on the original profile. The data were then projected along the tracks in order to produce magnetic anomaly maps of Figs. 4, 7 and 10.

Identification of magnetic anomalies Magnetic darri processing

The data were resampled every minute, giving an average spacing along the track of about 300 meters. Magnetic anomalies were obtained by subtracting theoretical values computed using the International Geomagnetic Reference Field (IGRF) from the measured values. For cruises older than 1985, no attempt was made to recompute the correction using the DGRF (Definitive GRF), since it would generally result in very minor changes. All anomalous data points, especially spikes, noise bursts and noise trains were removed by radial amplitude-slope rejection method (Neff and Wyatt, 1986).

Cross track errors are mostly due to the diurnal variation of the magnetic field and to differences between geomagnetic reference fields used for com- puting the magnetic anomalies from measurements of the total magnetic field. In order to remove some of these errors, we applied a band-pass filter

Identification of magnetic anomalies was donc on selected profiles by forward modelling using Schouten and McCamy's method (1972). .synthetic magnetic profiles were computed using the geomagnetic time scale of Harland et al. (1990). In the following, all ages of magnetic anomalies are given accordingly. We used an average depth to the top of the magnetic layer (layer 2) .of 2.7 km. The thickness of the magnetic layer and the magnetisation were taken as 0.5 km and 0.01 emucm-', respectively. We did not try to repro- duce the peak observed over the spreading axis by using a larger magnetisation. although magnetisation up to 0.03 emu cm-' has been measured on several samples from the active part of the axis. Instead of a block model with sharp vertical boundaries between blocks of opposite magnetisation, we have used a model in which crustal material is injected at the spreading ridge with a gaussian distribution of standard deviation Q. This

TABLE I

List of cruises used in this study

Institution Cruise Ycnr Ship Magnetic datn pcints

France Japan HIG

Lamont DGO

New Zealand OKSTOM

SI0

S’rAKhIIIK

CSGS

\t’HO1 ’

sl’rlJ~.so 3 GH7801 MAHI 2-70 KK7 1-04-26-02 KK7 1-04-26-04 KK7 1-04-26-05 KK72-11-08-02 KK82-03- 16-02 KK82-03-16-03 MW8612 MW8701 c 12-05 C 13-54 ELT40 V28-I 1 V28- I2 V28- 13 V32-14 v34- 1 V34-14 V35-6 V36-3 EI63-02 AUS400 EVA2 EVA 3 EVA6 EVA9 EVA 12 NOVA04 AR NOVA05 AR NOVA06 AR NOVA08 AR SOVA I A HO SOVA04 HO NOVAOS HO DSDPZO O t r i c / i w 3 P(l [ l ( l l l~f l 6 fi¿lijY) 87- I kir i jw 87-2 XUij.0 88- I Ktlij .0 88-1 lict;yo 89- I Kiaij.0 89-7 }iiko.siikrr 90-2

L5S2SP CH 100-09

~IJkOSlIktl 9 1

I9S5 1978 I970 1971 1971 1971 1972 1982 1982 1986 1987 1968 1969 1969 1971 1971 1971 1975 1977 1977 1978 I979 1963 1976 I976 1977 1978 1981 1983 1967 1967 I967 I967 I967 I967 I967 1971 I974 1086 1987 1987 198s I 9SX I9S9 I98C) 1991 1991 lox1 1971

2-31-79 415 908 521 443

7038 151 1 2602

20940 2756

16373 581 304 339 674 707 499 567 459 273 552 565 84

I187 643 422

7664 519

7901 1205 256 I23 66

I768 3318 268 519

I694 4991

I5989 7859

I367 I I3577 500 I 7773

i O639 18790 2794 1 27s

21 193s

KINEMATICS O F AC'l-l\'E SPKE.Al)I\C~: \OK'TH FIJI BASIN

- 14

- 1 5

-16

- 1 7

- I 8

- 19

- 20

-2 I

-22

S T A M R c r u i ~ e s

-I4)

- 2 2K - A-- - __h__l

172 173 174 175 176 I

Fig. 2. Track chart of available magnetic data in the central part of the North Fiji basin. Right. STARMER cruises; left, other cruises

parameter CT is related to the width of the transition zone between blocks of opposite polarity. Increasing CT thus attenuates the amplitude of short reversals. A value of 1 to 2 km seems to be appropriate in our survey area. An example of magnetic anomaly identification is given in Fig. 5. On this profile, anomalies 1 to 2A are clearly identified. The misfit between the computed profile and the recorded profile can be attributed to several causes: off-ridge volcanism. as imaged by the seabeam bathymetric map. irregular bottom topography (since the model assumes a planar surface), and variations in magnetisation.

Structure and magnetic anomalies in the central North Fiji Basin

Recent surveys of the central part of the NFB led to the identification of a 800 km long, N-S trending spreading system located between 173"E

and 174"E, and 15"s and 2"s (Fig. 1). It consists of four main ridge segments with different orienta- tions (Auzende et al., 1988a, 1992; GrAcia et al.. 1994-this issue; Tanahashi et al., 1994-this issue) These segments are from north to south (Figs. 1 and 3): - the N16"E ridge from 14'30s to the 16"50S

triple junction, - the N20"E ridge from the triple junction

to 18"10'S, - the central N-S ridge from 18"IOS to 21"s. - the southern N-S ridge, south ,of 2.1"s. The two northern segments constitute with the

NFFZ the 16"50'S triple junction (Lafoy et al.. 1987, 1990), whose geometry and nature will be discussed in a further section. The relationship between the N20"E and the central N-S ridges appears to be a propagating rift, as first defined by Hey (1977) (Auzende et al., 1988a; De Alteriis et al., 1993; Ruellan et al., 1994-this issue), while

74 I'. HUCHON El AL.

173O 174" 14O I I

I I rï30 1740

220

Fig. 3. Structural map of the central part of the North Fiji basin. I =Spreading axis; 2 = normal faults: 3 = highs; 4 = lows; 5= axial graben; 6 = off-axis volcanoes.

the central and southern N-S ridges are offset by about 80 km through a diffuse N45"E trending feature named "Jean Charcot Fracture Zone" (Auzende et al., 1986a).

About 280 km to the east of this system, another N-S trending ridge has been identified at 176"E using SeaMmc II side scan sonar imagery (Price and Kroenke, 1991). This ridge joins the NFFZ at 16"35'S, and appears to be connected to the south near 17'30's with a complex area originally interpreted as an intra-oceanic deformation zone (Auzende et al., 1986b) and recently reinterpreted as a propagating rift (Auzende et al., 1993).

This complex spreading system is considered to be about 3 Ma old and cuts across the fan-shaped pattern of magnetic lineations formed between 1 O to 3 Ma. In the central part of the NFB, Chase (1971) first identified magnetic anomalies 1, 2 and 3, with a (total) spreading rate of 7.8cm yr-'. Using the same data set, Falvey (1975) extended the magnetic anomaly sequence back to anomaly 4 (7 Ma). These conclusions were further confirmed by Malahoff et al. (1982) on the basis of aeromagnetic surveys (Cherkis, 1980). Magnetic anomalies up to 3A, and possibly 4, were identified, with. an average spreading rate of 7cm yr-'. However, more recent surveys did not confirm the existence of N-S trending anomalies 3 to 4 (Fig. 4).

The identification of magnetic anomalies is now facilitated by the good control we have on the location of the actively spreading axis, as identified .L

using Seabeam bathymetry and complemented by in situ observations. However, the only places where the axial anomaly (0.7 Ma to present) and the Jaramillo event (J hereafter) can be clearly identified are the central N-S ridge between 19"s and 20'30s and a short segment of the eastern N-S ridge.

The central N-S ridge und the 18'10'Spropaguting rij ì

Magnetic anomaly sequence up to anomaly 2 was recognised between 20"s and 2W30'S by Maillet et al. (1986). An example of a typical magnetic profile and its interpretation is shown in Fig. 5. Although less clear than younger anomalies, magnetic anomaly 2A (2.45-3.40 Ma) can be iden-

KINEMATICS OF ACIIVE SPKEADING: NORTH FIJI BASIN

NFB Filtered m a g n e t i c anomalies - 14

-15

-16

- 17

-18

- 19

- 20

- 2 I

- 22

Fig. 4. Magnetic anomaly data projected along tracks. Only profiles trending N60"E to N120"E are plotted. Dotted line: active spreading axis. Clearly identified magnetic anomalies 1. J (Jaramillo), 2 and 2A are shown. N N F Z = North Fiji Fracture Zone.

I 6 P. HGCHON ET AL.

E-W magnetic anomaly profile at 20"s (EVA 12)

- - - Maae I - 7.6 c d y r de---- 5.6 c d y r O b s e r v e d

.- -- - ----- A i . n o n ! o ! I o n

Fig. 5. Example of E-W magnetic anomaly profile a t 20%.

tified. Forward modelling leads to a spreading rate of 7.6 cm yr- since about 1 Ma (anomaly J) and 5.6cm yr-' between 3.5 and 1 Ma (Fig. 5). We thus confirm the increase in spreading rate at the time of anomaly J proposed by Maillet et al. (1986), although their rates are slightly higher (8 and 6cm yr-' instead of 7.6 and 5.6cm yr-', respectively). Our rates are also in fairly good agreement with the 8.2cm yr-' computed by Auzende et al. (1988a) on the basis of Seapso 3 data. At 19'2OS, we could identify the axial anomaly as well as anomalies J and 2, with a 7 cm yr-' spreading rate. Further north, the magnetic anomaly profiles are more difficult to interpret due to the northward passage of the 18'1 O'S propagating rift (De Alteriis et al.. 1993). The pattern of magnetic anomalies around the propagator (Ruellan et al., 1994-this issue) shows that the propagation began after anomaly 2 ( 1 -65-2.10 Ma) and before anomaly J (0.91 -0.97 Ma). This observation allo\vs us to bracket the age of the N20'E ridge between 1 and 1.6 Ma, which appears to be slightly older than in previous estimates (Lafoy et al., 1987. 1990). Note that the axial anomaly shows an increase in amplitude toward the tip of the propagating rift, which is the ty pica1 signature of the Fe-Ti enriched basalts formed in this context (Miller and Hey, 1986). Because of the occurrence of the 18'10's propagating rift, the

N20"E ridge was thus much longer about 1.5 Ma ago than at present. Based on bathymetric and magnetic data, its tip was located close to 19"s (De Alteriis et al., 1993; Ruellan et al., 1994-this issue), which leads to a propagation rate of about 75km/Ma, a value which is close to the value given for the Galapagos propagator (Kleinrock and Hey, 1989).

The N20"E ridge and the 16'50's triple junction

L

The axial anomaly is much less clear along the N20"E ridge than along the N-S ridge, because of successive ridge jumps toward the east, especially in the southern part (Ruellan et al., 1994-this issue), These ridge jumps are quite evident when considering the location of the present axis which ~

is shifted eastward with respect to the middle of the N20'E domain (Fig. 6). The width of the axial anomaly decreases from 50 km at 18"s to 30 km at 17"15'S, and then abruptly decreases to 15 km in the vicinity of the triple junction. Anomaly J is difficult to identify north of 17'30's (Fig. 7). The total spreading rate measured for the last million year in the N110'E direction (perpendicular to the N20"E ridge) is about 6.3 cm yr-' a t 18"s and 5.6cm yr-' at 17'30's. However, it is now generally considered that propagating rifts occur in response to a change in direction of plate separa-

Fig. 6. Structural map around the 16"50'S triple junction. Same caption as for Fig. 3.

tion (Hey et al., 1988). Since the N-S ridge is propagating at the expense of the N20"E ridge, we can reasonably assume that spreading is E-W along the N20'E ridge. The spreading rates at 18"s and 17"30'S are then 6.6 cm yr-I and 5.9 cm yr- ', respectively, when measured along the E-W direction. Figure 8 shows the variation in spreading rate along the whole system, from 20'30s to 1T30'S. The law of Euler rotation relating the relative velocijy to the sine of the distance to the pole of rotation can be used to estimate that the rotation pole is located at about 9oS, 173'30's. This allows us to predict an E-W spreading rate

of about 5.2 cm yr-' across the N20"E ridge close to the 16"50'S triple junction (Fig. 8).

The triple junction itself displays large magnetic anomalies whose pattern mainly reflects the topography (Joshima et al., 1994-this issue), although N140"E-N15OoE lineations can still be identified (Fig. 7). This trend parallels that of the normal faults, especially southeast of the triple junction, where perpendicular, N50"E trending structures are also observed (Fig. 6). These N140"E and N50'E trends are thus interpreted as normal faults and transform zones formed during the pre-3.5 Ma stage of opening, respectively.

. .-

P. HUCHON I3 AL.

-17

- 18

Q m

Fig. 7. Magnetic anomaly data projected along tracks around the 16YOS triple junction. Dotted line: active spreading axis. Solid line: magnetic lineation. The area formed during the last 1 . 1 Ma is filled with smail "v". The shaded area is the N30"E trending domain described in the text.

West of the N20"E ridge, both the structural and the magnetic anomaly patterns are highly complex (Figs. 6 and 7). However, several discrete domains with consistent fabrics can be delineated from the structural map (Fig. 6). From east to west, they are: (1) a domain with N20"E trends turning progressively to N30"E toward the west; (2) a 40 km wide complex area with N-S trends, but also curved ridges; (3) a 25 km wide domain with N40"E trends; and. finally (4) a domain with N30"E, N140"E and N-S trending faults. Clear N-S magnetic lineations can be identified in the N-S domain, as well as a particular pattern near

17"S, 173" 1 5'E with symmetric positive anomalies . drawing an inverse V. Since the amplitude of the magnetic anomalies increase toward the tip of the inverse V-shaped area, it is tempting to interpret this pattern as a propagating rift. The fact that the "propagator-like" feature lies in the continua- tion of the well defined western anomaly 2 led us to favour the hypothesis of an extinct propagator. The western limb of the propagator coincides with the boundary of the N40"E trending domain that we interpret as a fracture zone formed during the early stage of opening of the NFB, before 3.5 Ma. Note that anomaly 2A clearly abuts against this

KI\E\l.4TICS OF ACTIVE SPREADING: NORTH .FIJI HASIN 7'

c m l y r

r

1 16"50'S tr iple

6 -

5 , I I I I 1

Latitude-' - 2 1 - 2 0 -1 9 - 1 8 -1 7

Fig. S. %riation in spreading rate along the 173"30E spreading ridge.

domain at about 17'30's (Fig. 7). The distance between this intersection and the propagator tip indicates a rate of propagation (30 km Ma-') which is much slower than that of the 18"1OS propagator. Other known propagation rates also show large variations: 50 km Ma-' for the 95.5"W Galapagos propagator and 150 km Ma-' for the Easter propagator (Phipps Morgan and Parmentier, 1985). A test of the hypothesis of an extinct propagating rift is that anomaly 2 should not exist at 17's to the east of the N20"E ridge: Figure 4 shows that eastern anomaly 2 indeed abuts against the N20'E ridge at about 17'40'E. If this interpretation is correct, it further confirms that the N20'E ridge was formed after a ridge jump that occurred between anomalies 2 and J, thus between 1 and 1.65 Ma. The kinematic impli- cations will be discussed below.

The .V160"E ridge

The NI60'E ridge is marked by a wide axial graben about 4000 m deep and is characterised by an en echelon pattern of the active axis segments (Auzende et al., 1991b, 1994-this issue). The morphology is typical o€ slow spreading ridges. The active axis is marked by a narrow positive magnetic anomaly which is flanked by two large positive anomalies. Since these anomalies follow the flanks of the graben, they probably mainly reflect the

difference in depth to the magnetic sources, as shown by inverse modelling (Joshima et al., 1994- this issue). Therefore, we did not try to identif] these anomalies. although Auzende et al. ( 199 1 b) propose a rate of 5 cm yr-', based on a single profile, where the anomaly identified as the western anomaly J actually lies outside of the N160"E domain. We thus question this identification. Rather we assume that the N160"E ridge formed at the time of the ridge jump from N-S to N2O"E. following the model proposed by Lafoy et al. (1990). Using an estimated age of 1 to I .6 Ma for the formation of the N160"E ridge, its 50 km width implies a spreading rate of 3 to 5 cm yr-', which is in slightly better agreement with the slow spreading morphology. We shall discuss this estimate in a further section by considering the velocity triangle at the triple junction. . -

The nature of the 16"50S triple junction

The bathymetric map of Fig.9 shows that the western end of the NFFZ is occupied by a large N55"E trending basin, more than 3000 m deep. The northern and southern flanks of this graben are only 2100-2500 m deep. During two dives of the submersible Xautile in 1989 (Auzende et al., 1991a), pillow lavas, lava lakes and many open fractures were observed, and fresh basalts were sampled. Following Lafoy et al. (1990), this graben

-15

- 16

-17

-18

-19

Bathymetric map

172 I73 I74 ! 75 I76 I 7 7 I78 Fig. 9. Bathymetric map (taken from J.P. Mazé. in prep.) of the central North Fiji Basin west of Viti Levu island.

has been interpreted by Auzende et al. (1990a) and Tanahashi et al. (1991) as a pull-apart basin connected to the NFFZ. Alternatively, similarities with other intersections of a spreading ridge with a transform fault (Eltanin: Lonsdale, 1986; Rivera: Bourgois et al., 1988) may lead to interpret this feature as an overshot ridge. However, it may also be interpreted as a nascent ridge that plays an important role in the evolution of the triple junction, as discussed below.

The southern N-S ridge und the "Jeun Cliurcot Fracture Zoiie I '

Along the southern N-S ridge, Maillet et al. (1989) identified the positive anomaly centred on 174"OSE as the axial anomaly, giving a spreading rate of about 6 cm yr-' since 1 Ma. They further record the existence of anomaly 2, with a slightly

lower spreading rate of about 4 cm yr-' since 2 Ma, thus showing an increase in spreading rate 1 Ma ago. However, Maillet et al. (1989) interpret the southern N-S ridge as a regressive rift, based on the assumption that the central N-S ridge is propagating toward the south, as suggested by the fan shaped bathymetric pattern (Fig. 3). The mag- . .. netic anomaly pattern closely follows the bathymetric one (Fig. 4). The axial anomaly narrows from about 60 km at 20"20'S to zero at 21% corresponding to a southward rate of propagation of 75 km Ma-', identical to the northward rate of the 18" 1 O'S propagator.

Alternatively, Ruellan et al. ( I 989) consider this southern N-S ridge as recently formed, probably not older than 1 million years based on the width of the axial domain. The spreading axis is marked by a pronounced N-S trending positive anomaly which abuts near 21"s against the N45"E trending

KINEhl.4TICS OF .-\<I'IVE SI'KEALIING: NORTH FIJI BASIN SI

anomalies parallel to the Jean Charcot Fracture Zone. This positive anomaly is centred at 174"6'S, following the axial graben. Its width of about 35 km would thus correspond to a 5 cm yr-l rate of spreading since 0.7 Ma. On both sides but especially to the east, we interpret, following Maillet et al. (1989), the N-S trending positive anomaly as anomaly 2 (Fig. 4), leading to a rate of spreading of about 4.3 cm yr-'.

We conclude that the southern N-S ridge is opening at a much slower rate that the central N-S ridge, which suggests that strike-slip motion occurs along the Jean Charcot Fracture Zone, as shown by several strike-slip fault plane solutions (Hamburger and Isacks, in press).

The emrern N-S ridge and the 176"10E overkipping spreading centre

The prominent NNE-SSW seismic belt centred on 176'E was interpreted by Chase (1971) and Brocher and Holmes (1 985) as a young spreading centre. This area shows a mixture of normal and strike-slip fault plane solutionss with nearly E-W tension. Based on a SeaMARc II side scan sonar survey Price and Kroenke (1991) identified an actively spreading, N 1O"E trending ridge between 16'55's and 16"35'S where the ridge intersects the NFFZ. Further south, magnetic anomaly profiles clearly show the axial and J anomalies (Figs. 4 and 10). The spreading rate measured at 17"s is about 5.6cm yr-' since 1 Ma.

These magnetic anomalies can be traced further south in an area where a detailed multibeam bathymetric map was obtained in 1985 during the Seupso 3 cruise between 17"lO'S and 18"S, and 173"40'E and 176'40'E. The bathymetric map (Fig. 9) clearly shows two overlapping graben sepa- rated by a central plateau with very sinuous trends. This area was first interpreted by Auzende et al. ( I 986b. 1988) in terms of intra-oceanic deformation affecting the oceanic crust formed during the initial stage of opening of the NFB. However, the similarity with the 95.5"W Galapagos propagator (Hey et al., 1986) recently led Auzende et al. (1993) to reinterpret the area in terms of a southward propagating rift. It also shows similarities with large Overlapping Spreading Centres (OSC)

described in several mid-ocean settings such as the East Pacific Rise (MacDonald and Fox, 1983). Clear magnetic lineations are associated with the OSC (Fig. 4 and lo). They progressively narrow southward along the western branch of the OSC while a prominent axial anomaly characterises the eastern branch of the OSC (Fig. 10). It could thus be interpreted as propagating northward, based on the high magnetic anomaly amplitude compared to that on the western branch. However. comparison of bathymetric features with those ot the 95.5'W Galapagos propagator led Auzende et al. (1993) to conclude that the western arm propagates southward.

A kinematic model for the recent E-W opening of the North Fiji Basin

The active E-W opening in the central NFB appears to be distributed on two N-S trending spreading ridges with similar spreading rates. We now examine the kinematic consequences of this observation as well as the relationship with the motion along the NFFZ.

Constraints on plate motions

Because of the complexity of the area and the scarcity of well identified magnetic lineations, no attempt has been made to compute poles of finite rotations by fitting corresponding magnétic anomalies. However, the northward decrease in spreading rate along the central N-S and N20"E ridges (Fig. 8) suggests that the pole of opening of the western N-S ridge should be located rather close to the north of the area. Assuming an E-W direction of opening, we estimated the latitude of the pole of rotation to be around 9"s. To constrain their instantaneous kinematic model, Louat and Pelletier (1989) have used a few strike-slip fault plane solutions along the central N-S ridge. However, we think that the strike-slip mechanisms with T-axes oriented N70"E and located close to the "Jean Charcot Fracture Zone" (Fig. 3) may not be representative of the opening of the N-S ridge but rather indicate that deformation occurs within the Jean Charcot Fracture Zone due to the higher spreading rate to the north (8cm yr-')

82 P. HUCHON ET AL.

- 17

- ! 8

Fig. 10. Magnetic anomaly data projected along tracks around the 176'10E OSC. Dotted line: active spreading axis. Solid line: magnetic lineation. NNFZ= North Fiji Fracture Zone: P R = propagating rift.

compared to the south (5cm yr-'), as discussed above. We are more confident in one strike-slip event located at 18"2O'S, close to the propagator tip, which indicates a N80'E tension axis in fairly good agreement with the strike of the ridge. For the N20"E ridge we also assumed an E-W direction of spreading, because the en echelon pattern of the axial ridge suggests .E-W rather than N110"E opening. The NI 60"E ridge was also considered as opening perpendicular to its strike, an assumption further supported by the occurrence of a large

normal fault event at 16"lOS with a T-axis trending N70"E. The NFFZ was taken as a pure transform fault with a N85"E trend, at least to the east of 176"E.

Structure and kinematics of the North Fiji Fracture Zone

The bathymetric map of Fig. 9, taken from J.P. Mazé (in prep.), reveals that the NFFZ between 174"E and 178"E is formed of several segments.

KINEMATICS OF ACTIVE SPKE.-\DISG: NOK'TH FIJI BASIN 8

East of 176"E, it is marked by a NS5"E trending deep and narrow graben. Its left-lateral motion is clearly shown by the tectonic fabric at the intersection with the 176"E N-S ridge, as revealed by the SeaMARc images (Price and Kroenke, 1991) as well as by a short N-S trending ridge located at 177'25's (Von Stackelberg et al., 1985), interpreted as a pull-apart basin. Unfortunately, the magnetic profiles in this area are oriented parallel to the ridge and do not allow the identification of magnetic lineations (Morton and Pohl, 1990). West of the 176"E triple junction, the deep graben is replaced by an E-W ridge rising about 1000 m above the surrounding seafloor. Then, the NFFZ swings to a N60"E direction and joins a N140"E trending, slightly curved feature interpreted by Tiffin et al. (1990) as a thrust. Finally, it joins the 16"50S triple junction through the N55"E graben. Since this graben is interpreted as a nascent oceanic ridge, the N140"E curved scarp cannot be interpreted as a thrust if it is mechanically linked to the N55"E graben. Based on its curved shape, we prefer to interpret it as a portion of transform fault connecting the N55"E graben to the N60"E leaky transform (Fig. 11). Its curvature therefore allows us to locate the pole of rotation at about I6"10S, I75'30E resulting in the N55"E graben in a N155"E direction of opening, al most perpendicular to the axis of the graben and to left-lateral transtension along the N60"E trending part of the NFFZ. As a consequence, the E-W ridge located right to the west of the intersection with the 176"E ridge appears to be a left-lateral, compressive ridge (Fig. Il). We now examine the consequences of this interpretation of the structure and kinematics of the NFFZ between 174"E and 178"E.

Kinetnutics of the 16'50's triple jutiction

.We now consider the velocity triangle at the triple junction (Fig. 12) with the assumptions concerning the direction of spreading as discussed in previous sections. We estimated the E-W spreading rate across the N20"E ridge at 16"SOE as about 5.2 cm yr-' (Eig. 8). In order to derive the velocity triangle at the triple junction, we use the direction of spreading across the N55"E graben (N155"E) as given by the pole of rotation which describe the

kinematics of the NFFZ between 174"E and I78"E Then, knowing the direction of spreading acros the N160"E (N70"E), closure of the velocity trian gle implies rates of spreading of 4.7 and 1.8 cn yr-' on the N160"E ridge and N55'E graben respectively (Fig. 12). How do these estimated rate. compare with the measured data?

For the N160"E ridge Auzende et al. (1991b inferred a rate of 5 cm yr-', very close to ou estimate. The width of the N160"E domain (50 km combined with the 4.7 cm yr-' rate implies an agc of about 1.1 Ma, in fairly good agreement wit1 the age of the ridge jump from N-S to N20"E between 1 and 1.6 Ma ago. As for the N55"F graben, its 20 km width and 1.8 cm yr- * spreading rate also implies an age of about 1.1 Ma for thc beginning of its formation. This internal consis- tency is a further evidence that the whole system formed almost simultaneously. More direct evidence for the age of formation of the N55"E graben is provided by sampling of the sedimentary cover of pillow lavas on the northern wall of the graben during STARMER I cruise in 1989 (Lagabrielle et al., 1994-this issue). Microfossils assemblages indeed indicate an age of 1.3 Ma, in good agreement with our own estimates.

Based on these simple kinematic considerations, we thus conclude that the 16'50E triple junction has operated as a RRR-type triple junction since 1.1 to 1.3 Ma, not a RRF one as suggested by Louat and Pelletier (1989) and Lafoy et al, (1990). Note that a RRF triple junction is not stable and should evolve into a RFF one (McKenzie and Morgan, 1969), which is not the case. Our interpretation thus greatly differs from that of Louat and Pelletier (1 989) who assume a pure left-lateral motion all along the N85"E trending NFFZ up to the 16"50'S triple junction. Note that their assumption implies that the direction of opening along the N20"E ridge should be oriented more northerly than N85"E (N70"E in Louat and Pelletier's solution), thus highly oblique to the N20"E ridge. In our opinion, the absence of large transform features along the N20"E ridge is contradictory with the very oblique (50") opening it would imply. Note that Louat and Pelletier's solution also implies a N20"E unreasonably oblique (505) direction of opening along the N160"E ridge.

P. HUCHON ET AL.

172'E 178"E 1

P A C I F I C P L A T E

I O' O 1 76"E

TRIPLE JUNCTION I I Rotation pole

I / yfl I

. I NORTH O WNFB PI ATF

TRIPLE ---- I/ JUNCTION 1:

PROPAGATING RIFT

ENFB PLATE

5's

9"s

Fig. 11. Structural and kinematic int erpretation of the North Fiji Basin spreading ridges and North Fiji Fracture Zone. Explanation: see text.

Coiisegztences.for the motion along .the Norrk Fiji Fwture Zone: The 176"E triple junction

Although the direction of motion along the NFFZ is well constrained by fault plane solutions of earthquakes east of 176"E, the velocity is not. The only indirect measure of the velocity is the width of the N-S trending pull-apart basin located at 177"Z'E (Von Stackelberg et al., 1985). If we assume that the motion along the NFFZ started at about 1.1 Ma, synchronous with the ridge jump along the central N-S spreading axis, the 45 km width of the pull-apart basin would indicate a rate of motion of about 3.5 cm yr-'. This rate is much lower than in Louat and Pelletier's model, who assume that the NFFZ corresponds to the Pacific-Australian plate boundary, resulting in a relative velocity of 9.6 cm yr- using the RM2 model (Minster and Jordan, 1978). This large discrepancy thus suggests that the Pacific-Australian plates relative motion is taken up either in a more distributed way, as suggested

by Hamburger and Isacks (1988), or on other discrete plate boundaries.

Another way to constrain the motion along the NFFZ is by considering the triple junction with - the 176"E spreading axis. In the same way as for' the 16"50'S triple junction, let us consider the velocity triangle: we know the spreading rate across the 176"E ridge (5.5 cm yr- ') as well as the direction of motion along the eastern part of the NFFZ (N85"E). Since the direction of spreading across. the 176"E ridge is probably close to E-W, the motion must be small across the E-W ridge that we interpret as transpressional (Fig. I I ) . Since we know the location of the pole of rotation and the velocity at one point along the plate boundary (the N55"E graben), we can easily infer the direction of motion (N30"E) and the velocity (0.9 cm yr-') across the E-W transpressional ridge. Note that any small change in the velocity triangle at the 16"50'S triple junction will not greatly affect these values. In other words, our solution is robust with respect to small error in our previous kinematic

KINEMATICS OF ACTIVE SPRMDISG: NORTH FIJI BASIN

Tm I CSO'S triple junction I

I TRF I76'E triple junction I

ENFB

0.9 c d y r N30'E

Fig. 12. Velocity triangles at the 16"50'S and 176"E triple junctions.

analysis. Finally, closing the velocity triangle leads to estimate the velocity along the eastern part of the NFFZ at about 6cm yr-' and the direction of spreading across the 176"E ridge at N93"E, a reasonable value (Fig. 12).

Conclusion

The previous results have important consequences for the recent geodynamic evolution of the NFB. First, we have shown that two N-S ridges are actually opening with similar rates: 5 to 8 cm yr-' across the 173'30E ridge, 5.5 cm yr-' across the 176"E ridge. We have also shown that nearly all the left-lateral motion along the NFFZ is taken up by the opening of the 176"E ridge. Then, the 173"30'E ridge should be connected to the north to another left-lateral transform zone, possibly the South Pandora ridge, as suggested by the occurrence of strike-slip earthquakes (Hamburger and hacks, in press). This would be in agreement with our suggestion that the motion

along the NFFZ (6cm yr-') is only about 60% of the total relative motion between the Pacific and Australian plates (10 cm yr-I).

Second, we conclude that several features of thc NFB are not older than about 1.1 to 1.3 Ma, Thc N160"E and N20"E ridges as well as the whole NFFZ between 173'30'E and 178"E appear tc have formed almost simultaneously. Nevertheless we have also confirmed that the 173'30'E spreading system formed at about 3.5 Ma and was spreading more slowly (5.6 cm yr-' at 20"s) than at preseni (8 cm yr-I). Then a question arises concerning the kinematics of the whole NFB: was there an acceler- ation in the total rate of E-W opening between the Tonga and the Vanuatu trenches at about 1.1 Ma, or was the deformation distributed elsewhere? The answer obviously requires detailed survey5 over both the southern and northern extremities of the N-S spreading ridges.

Acknowledgements

We thank all the officers, crew, technical and scientific staff who took part to the Japanese-French STARMER Project, as well as its funding agencies. STARMER data were kindIy provided by Yo Iwabuchi (Japan Hydrographic department). Financial support was provided by FREMER grant no. 88-2-410238 and CNRS. We thank Nicolas Chamot-Rooke and Xavier Le Pichon for discussion and Lindsay Parson for his'remarks. Laboratoire de Géologie de l'ENS contribution no, 269.

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Reprinted from

MARINE GEOLOGY

INTERUA'T8OUAL JOURNAL OF MARINE GEOLOGY, GEOCHEMISTRY AND GEOPHYSICS

Marine Geology, 1 16 (1994) 69-87 , ' Elsevier Science B.V., Amsterdam

h

r

Kinematics of active spreading in the central North Fiji Basin (Southwest Pacific)

L

Philippe Huchona, Eulàlia Gràciab*", Etienne Ruelland, Masato Joshima" and Jean-Marie Auzendebvl

"CNRS URA 1316. Laboratoire de Gtologie. Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05. France bDépartement Géosciences Marines. IFREMERCentre de Brest, B. P. 70,29280 Plouzané. France

'Dpto. Geologia Dinamica. Geofsica y Paleontologia, Universidad de Burcelona. 08028 Barcelona, Spain 'CNRS URA 1279, Institut de Géodynamique, rue Albert Einstein, Sophia Antipolis. 06565 Valbonne Cedex, France

'Geological Survey of Japan, Higashi. Tsubuka, Iburaki 305, Japan (Received April 26, 1993; revision accepted July 28, 1993)

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