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

Marine Geology, 116 (1994) 69 87 Elsevier Science B.V., Amsterdam

69

Kinematics of active spreading in the central North Fiji Basin (Southwest Pacific) P h i l i p p e H u c h o n a, E u l f i l i a G r f i c i a b'c, E t i e n n e R u e l l a n d, M a s a t o J o s h i m a e a n d J e a n - M a r i e A u z e n d e b,1 aCNRS URA 1316, Laboratoire de G~ologie, Eeole Normale Supkrieure, 24 rue Lhomond, 75231 Paris Cedex 05, France bDOpartement Gkosciences Marines, I F R E M E R Centre de Brest, B.P. 70, 29280 PIouzan& France ¢Dpto. Geologia Dinamica, Geofisica y Paleontologia, Universidad de Barcelona, 08028 Barcelona, Spain OC N RS U RA 1279, Institut de G~odynamique, rue Albert Einstein, Sophia Antipolis, 06565 Valbonne Cede x, France eGeological Survey o f Japan, Higashi, Tsubuka, Ibaraki 305, Japan (Received April 26, 1993; revision accepted July 28, 1993)

ABSTRACT Huchon, P., Gr~tcia, 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: STARMERFrench-Japanese Program. Mar. Geol., 116:69 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°30'E 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- 1 across the western ridge, 5.5 cm yr 1 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 176°E 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 T h e N o r t h Fiji Basin ( N F B hereafter) was first d e s c r i b e d as a y o u n g oceanic basin by C h a s e (197l) on the basis o f m a g n e t i c a n o m a l i e s . Its oceanic c h a r a c t e r was also evidenced by high h e a t flow values ( W a t a n a b e et al., 1977) a n d by the a t t e n u a tion o f seismic waves in the u p p e r m a n t l e ( D u b o i s et al., 1973; B a r a z a n g i et al., 1974). G e n e r a l l y c o n s i d e r e d as a b a c k - a r c basin, the N F B is in fact l o c a t e d between two s u b d u c t i o n zones o f o p p o s i t e p o l a r i t y : the N e w H e b r i d e s ( V a n u a t u ) to the west a n d the T o n g a to the east (Fig. 1). Therefore, it

Xpresent address: ORSTOM, UR IF, B.P. A5, Noum6a Cedex, New Caledonia

m a y be r e g a r d e d as a large scale left-lateral pulla p a r t basin between the Pacific a n d the I n d o A u s t r a l i a n plates. H o w e v e r , its detailed tectonics are still d e b a t e d . C o n t r a s t i n g i n t e r p r e t a t i o n s o f available d a t a have been given the same year by H a m b u r g e r a n d Isacks (1988) who e m p h a s i s e d the i m p o r t a n c e o f diffuse d e f o r m a t i o n a n d A u z e n d e et al. (1988b) w h o a p p l i e d plate tectonics concepts. The N F B is generally c o n s i d e r e d as o p e n i n g in two stages. It first o p e n e d following the clockwise r o t a t i o n o f the V a n u a t u island arc a w a y from the Vitiaz island arc (Falvey, 1978). This nearly 90 ° r o t a t i o n o c c u r r e d from 10 to 3 M a a n d resulted in a f a n - s h a p e d p a t t e r n o f m a g n e t i c lineations. It was also a c c o m p a n i e d by an anticlockwise r o t a t i o n o f the Fiji p l a t f o r m (James a n d Falvey, 1978). The

0025-3227/94/$07.00 © 1994 - - Elsevier Science B.V. All rights reserved. SSDI 0025-3227(93)E0096-0

70

P Ht'CHON ET AL,

PLATE

PACIFIC

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©

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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 STARMER 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 STARMERproject 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 ( N F F Z hereafter). This paper pres-

ents a new interpretation of the kinematics of the central part of the NFB, including the western part of the N F F Z (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

Bathymetrv The first multibeam (Seabeam) bathymetric map was obtained in the NFB during the Seapso 3 cruise of the R/V Jean Charcot (Auzende et al., 1986a,b, 1988a). During the STAR~aER 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).

71

K I N E M A T I C S O F A C T I V E S P R E A D I N G : N O R T H FIJI BASIN

Side scan sonar and bathymetric data were also obtained in 1986 and 1987 by the R/V Moana Wave of the Hawaii Institute of Geophysics (Kroenke et al., 1991), as well as Seabeam bathymetric data by the R/V Sonne (Von Stackelberg et al., 1985).

Magnetic anomalies In addition to the data from the STARMERproject, all available information was extracted from the GEODAS C D - R O M (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, Eva 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 and McCamy, 1972). The processing includes several steps: (1) identification of profiles of sufficient length and of proper orientation with respect to the spreading axis projection: we used a minimum length of 10 30 km and retained profiles whose direction is not oblique to the ridge axis by more than 45°; (2) projection of each profile on the selected direction; (3) interpolation at a constant step (generally l km), necessary to be able to perform the FFT; (4) F F T and band-pass filtering between 5 and 100 km. The filter is tapered at both ends of the gate by a linear function returning to zero over 5 km. At an average ship's speed of 10 knots, the 100 km cut-off value corresponds to about 6 hours in the time domain, so that it effectively removes the diurnal variation (24 hours) and its main harmonics (12 and 6 hours). The tapering of the filter between 0 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 data 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 computing 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 done 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 emu cm-1, respectively. We did not try to reproduce the peak observed over the spreading axis by using a larger magnetisation, although magnetisation up to 0.03 emu cm 1 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 a. This

72

P. HUCHONET AL TABLE 1

List of cruises used in this study Institution

Cruise

Year

France Japan HIG

Seapso 3 GH7801 MAHI 2-70

1985 1978 1970 1971 1971 1971 1972 1982 1982 1986 1987 1968 1969 1969 1971 1971 1971 1975 1977 1977 1978 1979 1963 1976 1976 1977 1978 1981 1983 1967 1967 1967 1967 1967 1967 1967 1971 1974 1986 1987 1987 1988 1988 1989 1989 1991 1991 1982 1971

K K71-04-26-02

Lamont D G O

New Zealand ORSTOM

SIO

STARMER

L'SGS WHO]

KK71-04-26-04 KK71-04-26-05 KK72-11-08-02 KK82-03-16-02 KK82-03-16-03 MW8612 MW8701 C12-05 C13-54 ELT40 V28-11 V28-12 V28-13 V32-14 V34-1 V34-14 V35-6 V36-3 EI63-02 AUS400 EVA2 EVA3 EVA6 EVA9 EVAI2 NOVA04 AR NOVA05 AR NOVA06 AR NOVA08 AR N O V A I A HO NOVA04 HO NOVA05 HO DSDP20 Euridyce 3 Papatua 6 Kaiyo 87-1 Kaiyo 87-2 Kao'o 88-1 Kaivo 88-2 Kaiyo 89-1 Ka(vo 89-2 Yokosuka 90-2 Yokosuka 91 L582SP CH 100-09

Ship

Magnetic data points

Jean Charcot

23439 415 908 527 443 7038 1511 2602 20940 2756 16373 582 3O4 339 674 707 499 567 459 274 552 565 84 1187 644 422 2664 519 2902 1205 256 123 66 1268 3318 268 529 1694 4994 15989 7859 13671 13527 5001 7773 10639 28790 2794 1278 211938

?

Mahi Kana Keoki

Moana Wave R. Conrad Eltanin Vema

Endeavour Noroit ? ? ? ? ?

Argo

Horizon

Glomar Challenger Thomas Washington Kaiyo

Yokosuka ?

Chain Total

73

K I N E M A [ I C S O F A C T I V E S P R E A D I N G : N O R T H FIJI BASIN

01her c r u i s e s

-T4

-14

15

-I5

16

-16

17

17

-18

-18

-19

19

20

-20

-21

-21

-22

-22 172

173

174

175

I76

177

172

173

174

175

176

177

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

parameter a is related to the width of the transition zone between blocks of opposite polarity. Increasing a thus attenuates the amplitude of short reversals. A value of l to 2 k m 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 orientations (Auzende et al., 1988a, 1992; GrS~cia et al., 1994-this issue; Tanahashi et al., 1994-this issue). These segments are from north to south (Figs. 1 and 3): the NI6°E ridge from 14°30'S to the 16°50'S triple junction, - the N20°E ridge from the triple junction to 18°10'S, the central N S ridge from 18°10'S to 21°S, the southern N - S ridge, south of 21°S. The two northern segments constitute with the N F F Z 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

P. H U C H O N

173 °

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ET AL.

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 SeaMARC II side scan sonar imagery (Price and Kroenke, 1991). This ridge joins the N F F Z at 16c'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 10 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 7.8 cm yr-1 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 7 cm yr-1. 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 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°30'S and a short segment of the eastern N - S ridge.

The central N - S ridge and the 18°lO'S propagating

rift 22 °

I

173 °

I 174 °

Fig. 3. Structural map of the central part of the North Fiji basin. 1 = Spreading axis; 2 = normal faults; 3 = highs; 4 = lows; 5 = axial graben; 6 = off-axis volcanoes.

Magnetic anomaly sequence up to anomaly 2 was recognised between 20°S and 20°30'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-

K I N E M A T I C S O F ACTIVE S P R E A D I N G : N O R T H FIJI BASIN

NFB

fi

Ifeted

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ies

ra ~r o

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m Z

¢n

-

7

-

8

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-20

-21

-22

172

174

175

176

177

Fig. 4. Magnetic anomaly data projected along tracks• Only profiles trending N60~'E to NI20°E are plotted• Dotted line: active spreading axis. Clearly identified magnetic anomalies l, J (Jaramillo), 2 and 2A are shown. N N F Z = North Fiji Fracture Zone.

76

P. H U C H O N ET AL.

E - W m a g n e t i c a n o m a l y p r o f i l e at 20°S ( E V A 12) 2A?

2

~J

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~

i

~ 2

= zoo[

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7.6 enffyr

~I

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5.6 cm/yr

Observed . . . . . . . . . . . . .

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ion

Fig. 5. Example of E-W magnetic anomaly profile at 20°S.

tiffed. Forward modelling leads to a spreading rate of 7.6 cm yr 1 since about 1 Ma (anomaly J) and 5.6cm yr 1 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 6 c m yr-1 instead of 7.6 and 5.6cm yr-1, respectively). Our rates are also in fairly good agreement with the 8.2cm yr -1 computed by Auzende et al. (1988a) on the basis of Seapso 3 data. At 19°20'S, we could identify the axial anomaly as well as anomalies J and 2, with a 7 cm yr- 1 spreading rate. Further north, the magnetic anomaly profiles are more difficult to interpret due to the northward passage of the 18°10'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 allows 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 typical 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 75 km/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 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 6.3cm yr -~ at 18°S and 5.6 cm yr 1 at 17°30'S. However, it is now generally considered that propagating rifts occur in response to a change in direction of plate separa-

77

KINEMATICS OF ACTIVE SPREADING: NORTH FIJI BASIN 172"30

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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 -1 and 5.9 cm yr -1, respectively, when measured along the E W direction. Figure 8 shows the variation in spreading rate along the whole system, from 20°30'S to 17°30'S. The law of Euler rotation relating the relative velocity to the sine of the distance to the pole of rotation can be used to estimate that the rotation pole is located at about 9°S, 173°30'S. This allows us to predict an E - W spreading rate

of about 5.2 cm y r - 1 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-N150°E 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.

78

P. H U C H O N

ET AL.

(-Q o cl o

c~ co O co o o t£3 co

U3

z U3

-17 Q ~n

-18 Fig. 7. Magnetic anomaly data projected along tracks around the 16c50'S triple junction. Dotted line: active spreading axis. Solid line: magnetic lineation. The area formed during the last 1.1 Ma is filled with small "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 m a p (Fig. 6). F r o m 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 ° 15'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 continuation 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

KINEMAFICS

OF ACTIVE SPREADING:

NORTH

79

FIJI BASIN

cm/yr 8'

7'~ 6.

-21

5

T ~

-20 i

-

-19

-18

i

16°50'Striple n

-17 Latitude-16 !

Fig. 8. Variation in spreading rate along the 173°30'E spreading ridge.

domain at about 17°30'S (Fig. 7). The distance between this intersection and the propagator tip indicates a rate of propagation (30kin Ma -1) which is much slower than that of the 18°10'S propagator. Other known propagation rates also show large variations: 50 km M a - 1 for the 95.5°W Galapagos propagator and 150 km Ma-1 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 implications will be discussed below.

difference in depth to the magnetic sources, as shown by inverse modelling (Joshima et al., 1994this issue). Therefore, we did not try to identify these anomalies, although Auzende et al. (199lb) propose a rate of 5cm yr 1, 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 N20°E, following the model proposed by Lafoy et al. (1990). Using an estimated age of 1 to 1.6 Ma for the formation of the N160°E ridge, its 50 km width implies a spreading rate of 3 to 5 cm yr 1, 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 N160°E ridge

The nature of the 16°50'S triple junction

The N160°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 of 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

The bathymetric map of Fig. 9 shows that the western end of the N F F Z 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 Nautile 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

80

P. H U C H O N

B a l h y m e t r ic

ET AL.

map

-15 12r o

o.

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-17

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-19 f72

175

174

!75

176

P77

178

Fig. 9. Bathymetric map (taken from J.P. Maz6, 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 N F F Z . 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 and the "Jean Charcot Fracture Zone" Along the southern N - S ridge, Maillet et al. (1989) identified the positive anomaly centred on 174°05'E as the axial anomaly, giving a spreading rate of about 6 cm yr-1 since 1 Ma. They further record the existence of anomaly 2, with a slightly

lower spreading rate of about 4 cm yr 1 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 magnetic anomaly pattern closely follows the bathymetric one (Fig. 4). The axial anomaly narrows from about 6 0 k m at 20°20'S to zero at 21°S, corresponding to a southward rate of propagation of 75 km Ma 1, identical to the northward rate of the 18° 1O'S propagator. Alternatively, Ruellan et al. (1989) 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

81

K I N E M A T I C S O F A C T I V E S P R E A D I N G : N O R T H FIJl BASIN

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 1 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 1. 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 eastern N S ridge and the 176°10'E overlapping spreading centre The prominent N N E - S S W seismic belt centred on 176°E was interpreted by Chase (1971) and Brocher and Holmes (1985) 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, N10°E trending ridge between 16°55'S and 16°35'S where the ridge intersects the N F F Z . 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 5.6 cm yr 1 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 Seapso 3 cruise between 17°10'S and 18°S, and 173°40'E and 176°40'E. The bathymetric map (Fig. 9) clearly shows two overlapping graben separated by a central plateau with very sinuous trends. This area was first interpreted by Auzende et al. (1986b, 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 10). 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 of 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 magnetic 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 -1)

82

P. H U C H O N

ET AL.

r Do cl o

m

o a m

17

-18

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

compared to the south (5 cm yr 1), as discussed above. We are more confident in one strike-slip event located at 18°20'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 N160°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°10'S with a T-axis trending N70°E. The N F F Z was taken as a pure transform fault with a N85°E trend, at least to the east of 176°E. Structure and kinematics o f the North Fo'i Fracture Zone

The bathymetric map of Fig. 9, taken from J.P. Maz6 (in prep.), reveals that the N F F Z between 174°E and 178°E is formed of several segments.

K I N E M A T I C S O F A C T I V E S P R E A D I N G : N O R T H FIJI BASIN

East of 176°E, it is marked by a N85°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 N F F Z 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 16~50'S 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. l l). Its curvature therefore allows us to locate the pole of rotation at about 16~'10'S, 175°30'E resulting in the N55°E graben in a N155cE 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 N F F Z . 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. l l). We now examine the consequences of this interpretation of the structure and kinematics of the N F F Z between 174°E and 178°E.

Kinematics of the 16°50'S triple junction 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°50'E as about 5.2 cm y r - 1 (Fig. 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

83

kinematics of the N F F Z between 174°E and 178°E. Then, knowing the direction of spreading across the N160°E (N70°E), closure of the velocity triangle implies rates of spreading of 4.7 and 1.8 cm yr ~ on the N160°E ridge and N55°E graben, respectively (Fig. 12). How do these estimated rates compare with the measured data? For the NI60°E ridge Auzende et al. (1991b) inferred a rate of 5 c m yr 1, very close to our estimate. The width of the N160°E domain (50 kin) combined with the 4.7 cm yr 1 rate implies an age of about 1.1 Ma, in fairly good agreement with the age of the ridge jump from N S to N20°E between 1 and 1.6 Ma ago. As for the N55°E graben, its 20 km width and 1.8 cm yr 1 spreading rate also implies an age of about 1.1 Ma for the beginning of its formation. This internal consistency 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 STARMERI 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°50'E triple junction has operated as a RRR-type triple junction since 1.1 to 1.3 Ma, not a R R F one as suggested by Louat and Pelletier (1989) and Lafoy et al. (1990). Note that a R R F triple junction is not stable and should evolve into a R F F one (McKenzie and Morgan, 1969), which is not the case. Our interpretation thus greatly differs from that of Louat and Pelletier (1989) who assume a pure left-lateral motion all along the N85°E trending N F F Z 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 (50 °) direction of opening along the N160°E ridge.

84

P. HUCHON

172”E

ET AL

178’E 1 S’S

PACIFIC

PLATE

TRIPLE

183

176’E JUNCTION

PROPAGATING

19’S Fig. 11,Structural see text.

and kinematic

int erpretation

of the North

Fiji Basin spreading

Consequences for the motion along the North Fui Fracture Zone: The I76”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”25’E (Von Stackelberg et al., 1985). If we assume that the motion along the NFFZ started at about I. 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 - 1 using the RM2 model (Minster and Jordan, 1978). This large discrepancy thus that the suggests Pacific-Australian plates relative motion is taken up either in a more distributed way, as suggested

ridges and North

Fiji Fracture

Zone. Explanation:

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”5O’S triple junction, let us consider the velocity triangle: we know the spreading rate across the 176”E ridge (5.5 cm yr-l) 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. 11). 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”5O’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 SPREADING: NORTH FIJI BASIN

TTT 16°S0'Striplejunction

•

85

I PAC

WNFB ~

~o '~.~5 5.2 cm/yr N90°E /~ m

ENFB

/.J g

I

TRF176°Etriplejunction

ENFB

E 8

Compressiveridge / ~

]

5.5

cm/yr N93OE

.......--r FP 6 cm/Yr NBS°E

~PAC

No""~thFiji Fracturezone

0,9 cm/yr 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 N F F Z at about 6 cm yr-1 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 1 across the 173°30'E ridge, 5.5 cm yr 1 across the 176°E ridge. We have also shown that nearly all the left-lateral motion along the N F F Z 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 Isacks, in press). This would be in agreement with our suggestion that the motion

along the N F F Z (6 cm yr-1) is only about 60% of the total relative motion between the Pacific and Australian plates (10 cm y r - 1). Second, we conclude that several features of the NFB are not older than about 1.1 to 1.3 Ma. The N160°E and N20°E ridges as well as the whole N F F Z between 173°30'E and 178°E appear to 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 1 at 20°S) than at present (8 cm yr 1). Then a question arises concerning the kinematics of the whole NFB: was there an acceleration 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 surveys 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 kindly provided by Yo Iwabuchi (Japan Hydrographic department). Financial support was provided by IFREMERgrant 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 Gbologie de I'ENS contribution no. 269.

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86 J.L., 1988a. Seafloor spreading in the North Fiji basin (Southwest Pacific). Tectonophysics, 146: 317-352. Auzende, J.M., Lafoy, Y. and Marsset, B., 1988b. Recent geodynamic evolution of the North Fiji basin (SW Pacific). Geology, 16(10): 925-929. Auzende, J.M., Honza, E., Boespfiug, X., Deo, S., Eissen, J.P., Hashimoto, J., Huchon, P., Ishibashi, J., lwabuchi, Y., Jarvis, P., Joshima, M., Kizimoto, K., Kuwahara, Y., Lafoy, Y., Matsumoto, T., Maze, J.P,, Mitsuzawa, K., Momma, H., Naganuma, T., Nojiri, Y., Ohta, S., Ohtsuka, K., Okuda, Y., Ondreas, H., Otsuki, A., Ruellan, E., Sibuet, M., Tanahashi, M., Tanaka, T. and Urabe, T., 1990a. Active spreading and hydrothermalism in North Fiji basin (SW Pacific). Results of Japanese-French cruise Kaiyo 87. Mar. Geophys. Res., 12:269 283. Auzende, J.M., Honza, E. and the STARMER group, 1990b. Bathymetric map of the North Fiji basin between 16°10'S and 21°40'S. IFREMER DRO/GM, 1/200,000 scale, 6 colored sheets. Auzende, J.M., Urabe, T., Bendel, V., Deplus, C., Eissen, J.P., Grimaud, D., Huchon, P., Ishibashi, J., Joshima, M., Lagabrielle, Y., Mevel, C., Naka, J., Ruellan, E., Tanaka, T. and Tanahashi, M., 1991a. In situ geological and geochemical study of an active hydrothermal site on the North Fiji basin. Mar. Geol., 98: 259-269. Auzende, J.M., Okuda, Y., Bendel, V., Ciabrini, J.P., Eissen, J.P., Gracifi-Mont, E., Hirose, K., Iwabuchi, Y., Joshima, M., Kizimoto, K., Lafoy, Y., Lagabrielle, Y., Marumo, K., Matsumoto, T., Mitsuzawa, K., Momma, H., Mukai, H., Naka, J., Nojiri, Y., Ortega-Osorio, A., Ruellan, E., Tanahashi, M., Tupua, E. and Yamaguchi, L., 1991b. Propagation "en 6chelon" de la dorsale du bassin NordFidjien entre 16°40'S et 14°50'S (YOKOSUKA 90-STARMER). C.R. Acad. Sci., 312: 1531-1538. Auzende, J.M., Honza, H., Maz6, J.P. and the STARMER Group, 1992. Comments on the Seabeam map of the North Fiji ridge between 16°10'S and 21°40'S. Ofioliti, 17(1): 43-53. Auzende, J.M., Hey, R., Pelletier, B. and Lafoy, Y., 1993. Propagation d'une zone d'accr6tion fi l'est de la dorsale du bassin Nord Fidjien (SW Pacifique), C.R. Acad. Sci. Paris, 317: 671-678. Auzende, J.M., Grficia, E., Bendel, V., Huchon, P., Lafoy, Y., Lagabrielle, Y., De Alteriis, G. and Tanahashi, M., 1994. A possible triple junction at 14°50'S on the North Fiji Basin ridge (Southwest Pacific)?. In: J.-M. Auzende and T. Urabe (Editors), North Fiji Basin: STARMER French-Japanese Program. Mar. Geol., 116: 25-35. Barazangi, M., Isacks, B., Dubois, J. and Pascal, G., 1974. Seismic wave attenuation in the upper mantle beneath the South Pacific. Tectonophysics, 24: 1-12. Bourgois, J., Renard, V., Aubouin, J., Bandy, W. Barrier, E., Calmus, T., Carfantan, J.C., Guerrero, J., Mammerickx, J., Mercier de L6pinay, B., Michaud, F. and Sosson, M., 1988. La jonction orientale de la dorsale est-Pacifique avec la zone de fracture de Rivera au large du Mexique. C.R. Acad. Sci. Paris, 307: 617-626. Brocher, T.M. and Holmes, R., 1985. Tectonic and geochemical framework of the Northern Melanesian Borderland: an overview of the KK820316 leg 2 objectives and results. In: T.M. Brocher (Editor), Geological Investigations of the Northern

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