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The Fiji Island Group consists of 320 islands situated within the complex boundary zone between the Pacific and Indo-Australian plates in the SW Pacific, comprising a broad region of active and inactive back arc basins, island arcs, ridges, transform faults and subduction zones (Figure 4. 1A). In total, this boundary is a mosaic of microplates whose boundaries have been reorganised repeatedly through time as the microplates accrete to, or are detached from the major plates. The margin of the oceanic part of the Pacific Plate is well defined as lying along the Tonga-Kermadec Trench, and continues westward along the seismically quiescent Vitiaz Trench lineament which juxtaposes Pacific Cretaceous oceanic crust against arc and back arc crust of Tertiary age (Taylor et aI., 2000). At present, the boundary between the Pacific. and Indo-Australian plates lies to the south of the Vitiaz lineament, but north of Fiji along the seismically active Fiji Fracture Zone (Figure 4. 1B), south along the North Fiji Basin spreading centres, and westward via the New Hebrides (Vanuatu) Trench (eg. Louat and Pelletier, 1 989). Two subduction zones of opposing polarity currently face each other (Figure 4. 1B). At the Tonga-Kermadec Trench, Pacific lithosphere is being subducted in a westerly direction, whereas younger Indo­ Australian lithosphere is being subducted beneath the Pacific Plate at the Solomon and New Hebrides Trenches. The Fiji Platform (Figure 4. 1B) is an area of relatively shallow water « 2000m), situated at the northern end of the 2400km long, mainly submarine Lau-Colville Ridge, flanked by the actively spreading North Fiji Basin in the west and the Lau back arc basin to the east (Figure 4. 1A). The Hunter Fracture Zone (also known as the Hunter Ridge) is a well defined curvilinear belt of ridge-

Figure 4.1A. Southwest Pacific and major morphotectonic elements, modified after Crawford et al. (2003). NFB - North Fiji Basin; SFB - South Fiji Basin; HFZ - Hunter Fracture Zone.

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VANUA LEVU

KADAVU GROUP �

Figure 4.1B. Fiji Platfonn detail, showing main island groups and simplified bathymetry after Gill and Whelan (1989a). FFZ - Fiji Fracture Zone

A.

E.

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incipient ridge axis transform fault

l flow lines

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active subduction zone subduction zone 7Ma B. D. F. - - 3Ma Present day FFZ HFZ

Figure 4.2A-F. Geodynamic evolution of the North Fiji Basin, after Azunde et al., (1995). VA - Vanuatu Arc; FP -Fiji Platfonn; NC - New Caledonia; HFZ - Hunter Fracture Zone;

FFZ - Fiji Fracture Zone. See text for further explanation.

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trough topography extending from the Koro Sea to the southern termination of the New Hebrides Trench (Launay, 1 982). The currently inactive 3 1 -25 Ma South Fiji Basin (Weissel, 1 98 1 ; Davey, 1 982; Malahoff et aI., 1 982; Sdrolias et aI., 2002; Crawford et aI., 2003) is located to the south of the Hunter Fracture Zone (Figure 4.1A).

4.2 Tectonic history

Most authors concur that Fiji was once a component of a near-continuous Vitiaz (or proto-Melanesian) island arc comprising what are now the older parts of the Tonga­ Fiji-Vanuatu and Solomons arc, that was initiated in the early Eocene with subduction of the Pacific Plate in a west to southwest direction (eg. Chase, 1 971; Gill and Gorton, 1 973; Packham, 1973, Coleman and Packham, 1 976; Hilde et aI., 1 977; Falvey, 1 978; Colley at:td Hindle, 1 984; Gill, 1987; Gill and Whelan, 1 989a; Inokuchi et aI., 1992; Hathway and Colley, 1994; Begg and Gray, 2002; Crawford et aI., 2003). The main basis for this interpretation results from the somewhat tentative correlation of the Late Eocene basement rocks of Fiji, Tonga and Vanuatu. The oldest known rocks in Fiji, the Yavuna Group of western Viti Levu (Rodda,

1994) are unconformably overlain by Late Eocene limestones (Hathway, 1 994) and have been interpreted to represent a fragment of a late Eocene to Early Oligocene volcanic arc (Hathway and Colley, 1 994). On the island of Eua, Tonga, arc-like lavas dated at 46-40Ma (Ewart et aI., 1 977; Duncan et aI., 1985) underlie Middle Eocene limestones (Tappin and Balance, 1994). In addition, ODP Site 841 recovered low-K arc-tholeittic rhyolites dated at 44±2 Ma (Crawford et aI., 2003 and references therein). Carney and Macfarlane (1 978) reported Lower Miocene conglomerates from Maewo, Vanuatu, containing reworked Eocene limestones and undated volcanics with low-K arc tholeiitic affinities similar to the Eocene �olcanics in Tonga and Viti Levu. Continued subduction alo?g the Vitiaz arc post 25 Ma is well supported by numerous radiometric dates on lavas in at least the Vanuatu and Fijian sections of the Vitiaz arc (Crawford et aI., 2003 and references therein).

Subduction along the Vitiaz arc ceased in response to the collision of Melanesian Border Plateau, immediately north of Fiji (Falvey, 1975; Gill and Whelan, 1 989a), and the Ontong Java Plateau (OJP) to the north of the Solomons section of the arc (packham, 1 973; Falvey, 1 975; Kroenke, 1 984). The initial collision of the OJP with the Vitiaz arc may have occurred as early as 25-20 Ma (Kroenke, 1 984; Yan and Kroenke, 1 993; Petterson et aI., 1 997), with collision of the Melanesian Border Plateau east of Fiji occurring as late as 7.5 Ma (Gill and Whelan, 1 989a; Hathway, 1 993). However, the main episode of blocking and compression related to these collisions was probably between 1 0 and 5 Ma (Crawford et aI., 2003 and references therein). In Fiji, uplift, folding, intrusion of trondjhemite ± gabbro (Colo Plutonic Suite), and accompanying low-temperature metamorphism occurred - 1 0 Ma (Rodda, 1967; Rodda, 1 974; Rodda and Kroenke, 1 984;Whelan et aI., 1985), while further uplift associated with defonnation seems to have occurred at - 7.5 Ma (Begg and Gray, 2002 and references t

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erein). At approximately the same time (1 1 -8 Ma), the New Hebrides Western Belt Province had ceased volcanic activity and was undergoing widespread uplift and erosion (Carney and Macfarlane, 1 982).

The tennination of subduction along the Vitiaz trench led to the reversal of arc polarity to northwest of Fiji along the Vanuatu-New Hebrides segment (Figure 4.2A) possibly as early as 12 Ma (eg. Auzende et aI., 1 996; Pelletier et aI., 1 993), or, potentially as late as 8 Ma (eg. Hamburger and Isacks, 1 987; Gill and Whelan,

1 989a; Crawford et aI., 2003). Clockwise rotation of the Vanuatu arc (Figure 4.2B) resulted in the initial opening of the North Fiji Basin (NFB) along a NW-SE spreading axis (Auzunde et aI., 1996). By -7 Ma (Figure 4.2C) rifting along the NW -SE spreading axis had ceased and was replaced by an E-W trending spreading centre from the northwestern tip of the basin to the north of the Fiji platfonn. Spreading along the E-W trending axis resulted in the antic10ckwise rotation of the Fiji Platfonn and led to a situation of "forced" subduction of South Fiji Basin crust along the Hunter Fracture Zone (HFZ). Although the HFZ has been generally considered to mark the trace of a transfonn-system, linking the opposing Vanuatu- New Hebrides and Tonga-Kennadec arcs (eg. Kroenke, 1 984), the observation of obliquely truncated magnetic anomalies (Malahoff et aI., 1982) and well preserved subduction morphology (Brocher and Homes, 1 985) suggests that it may have

constituted a short lived subduction zone that accommodated the rotation of the Fiji Platform between 7-3 Ma (eg. Auzende et aI., 1996; Crawford et aI., 2003). Between -4 Ma and -2 Ma the N-S trending east Lau spreading centre had propagated south and was spreading at - 1 00mmlyr (Taylor et aI., 1 996). The initiation of spreading in the Lau Basin coincides with a major change in stress direction from N-S to E-W in the NFB and the development of a triple junction (Figure 4.2D) between the former E-W axis and the newly created N-S trending spreading centre (Auzende et aI., 1996). The development of the Fiji Fracture Zone (FFZ) to the north of the Fiji Platform at -1.5 Ma and subsequent left-lateral displacement along it, induced a reorganisation of both E-W and N -S trending spreading axis (Figure 4.2E). At present, spreading in the NFB occurs along the central and west Fiji propagating ridges (Figure 4.2F), isolating an intermediate microplate (Auzende et al., 1 994; Auzende et aI., 1 996). In the south, the central spreading ridge is connected to the New Hebrides trench by a large left-lateral transform fault and north of the 1 6° 50' S triple junction, spreading occurs along the N160° segment and the Hazel Holme Ridge (Auzende et aI., 1 996).

Following Auzende et aI. (1 996), the geodynamic evolution of the NFB can be

summarised into three main stages: Initial spreading in a NE-SW direction from 12-

7 Ma, followed by N-S directed spreading from 7 to 3 Ma, and finally E-W orientated spreading from 3 Ma to the present.

During"the opening of the NFB, the Vanuatu-New Hebrides arc and Fiji Platform rotated clockwise and anticlockwise respectively, in the manner of opening double gates, away from their sites of formation on the Vitiaz arc (Falvey, 1 975, 1 978; Taylor et al., 2000). However, the magnitude and timing of these rotations, particularly the rotation of the Fiji Platform, has been a contentious issue.

The clockwise rotation of the Vanuatu-New Hebrides arc is indicated by the triangular shape of the North Fiji Basin and the paleomagnetic results of Musgrave and Firth (1 999), from Nendo Island in the Solomons, suggest a total clockwise rotation of the New Hebrides arc of 52°±12° since -10 Ma.

The complementary anticlockwise rotation of the Fiji Platform has been investigated by a number of paleomagnetic studies. James and Falvey (1978) deduced that Viti Levu had rotated antic10ckwise � 21 ° between 6 Ma and 2-1.5 Ma. Malahoff et al. (1982), using new paleomagnetic and KI Ar data from the Tavua volcanic centre, interpreted the rotation to be � 90°, commencing at � 7 Ma and resulting in a rate of rotation of 13.7° Myr-l. However, Malahoff et ai. (1982) speculated that the rotation of Viti Levu may have commenced somewhat earlier at 1 0 Ma with total rotation as high as 1 1 5°. Inokuchi et al. (1 992) suggested that Fiji had undergone a clockwise rotation of 45° during the Oligocene, related to the development of the South Fiji basin, with a subsequent antic10ckwise rotation of 75° occurring since the end of the middle Miocene in response to the opening of the NFB. They described the rotation of Viti Levu as a "ball-bearing"-type rotation that is accommodated by antithetical transform faults and the spreading of the NFB. Using data from 126 sites from the Fiji Island Group, Taylor et aI., (2000), indicate that the Fiji Platform rotated 135

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±1 7° beginning at, or before 1 0Ma and ceasing abruptly at 3 Ma with the development of a well-defined N-S aligned spreading centre in the NFB (Figure 4.2D). Based on this larger magnitude of rotation, the reconstruction of Taylor et al. (2000) differed from previous models (eg. Falvey, 1 978; Gill et al., 1 984) in that Fiji, Tonga and Lau form a continuous N-S aligned arc, as opposed to a co-linear arrangement, with Fiji forming a hinge zone between the Vanuatu-New Hebrides and Lau-Tonga arcs. Taylor et ai. (2000) favoured a non-continuous Vitiaz arc on the basis of their paleomagnetic results and suggested that stratigraphic correlations between Fiji and the Vanuatu-New Hebrides basement should be treated with caution. Using stress analysis of dykes and kinematic analysis of faults within the Tavua Volcanic centre on Viti Levu Begg and Gray, (2002) indicate that the Fiji platform had undergone �50° of rotation since -5 Ma. Given that rotation apparently stopped at 3 Ma (eg. Taylor et aI., 2000) this suggests that particularly rapid rotation of -25° Myr-l occurred from �5 to 3 Ma.

In summary, it would appear that the main contentious issue regarding the plate

tectonic history of Fiji surrounds the existence, or otherwise, of a continuous pre­ Mid-Miocene Vitiaz or proto-Melanesian arc. It is apparent that a satisfactory geometric solution (ca. -12 Ma) is highly dependent upon; 1) the amount of rotation

suffered by both the Vanuatu-New Hebrides arc and the Fiji Platform, and 2) the age of the southern Vanuatu arc and Hunter Ridge basement. The second point is of

parti,cular importance, as any satisfactory geometric reconstruction involving a co­

linear Vitiaz arc requires that the southern third of the Vanuatu arc, including the Mathew-Hunter Ridge, is Pliocene in age or younger. Older basement rocks (eg. Miocene or older) in the southern Vanuatu arc would result in significant overlap between the Fiji Platform and the Vanuatu-New Hebrides basement.

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