Double-Saloon-Door Tectonics in the North Fiji Basin

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Double-saloon-door tectonics in the North Fiji Basin

Article in Earth and Planetary Science Letters · July 2013 DOI: 10.1016/j.epsl.2013.05.041

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Earth and Planetary Science Letters 374 (2013) 191–203

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Double-saloon-door tectonics in the North Fiji Basin

A.K. Martin n

Repsol, Al Fattan Plaza, PO Box 35700, Dubai, United Arab Emirates article info abstract

Article history: Fiji Platform rotated counter clockwise from at least 10.2 Ma until 1.56 Ma, while Vanuatu Arc rotated Received 3 January 2013 clockwise from 12/10 Ma until the present. A revised model incorporating these opposite rotations Received in revised form explains the distribution of magnetic anomalies in the North Fiji Basin (NFB). The conjugate margin of 21 April 2013 southwest Fiji Platform is southeast Vanuatu Arc. Previous models which associate NW-oriented Accepted 22 May 2013 anomalies off southwest Fiji with similarly oriented anomalies northeast of Vanuatu Arc are therefore Editor: T.M. Harrison 1 1 Available online 14 June 2013 wrong. Secondly, these models propose a NW-oriented spreading ridge extending to 19 S, 177.5 E, almost 500 km southeast of the mapped extension of rifted island-arc crust on Vanuatu Arc. This creates an Keywords: unacceptable overlap with Fiji Platform in pre-rift reconstructions. Thirdly, anomalies off SW Fiji which tectonics are NW-oriented in their present-day position were oriented NNE during initial breakup. Rather than backarc aligning with NW-oriented anomalies in the western NFB, they are matched by NNE-oriented anomalies rotation propagation off SE Vanuatu Arc. With further rotation, these conjugate anomaly sets form a fan shape in the southern arc-perpendicular NFB. Fourthly, previous models which recognise a triple junction only from 3 Ma do not explain early arc-parallel WNW–ESE separation of Vanuatu Arc and Fiji Platform required by well-documented opposite rotations. NFB characteristics which match the double-saloon-door tectonic model include opposite rotations of island arc terranes, backarc seaﬂoor spreading which is both arc-parallel and arc-perpendicular, and rifts propagating north, south, northeast and northwest. Features which do not match the double-saloon- door model include the North Fiji Fracture Zone and the West Fiji spreading centre. Both initiated post– 1.5 Ma, when Fiji Platform stopped rotating, and only one terrane, Vanuatu Arc, continued to rotate. The NFB developed via double saloon door tectonics from 12/10 Ma until 1.5 Ma, whereas post-1.5 Ma opening constitutes single saloon door rotation. & 2013 Elsevier B.V. All rights reserved.

1. Introduction Secondly, NFB development has been ascribed to asymmetric opening, emphasising clockwise rotation of the Vanuatu Arc The North Fiji Basin (NFB) is one of the most active backarc (Schellart et al., 2002, 2006; Mann and Taira, 2004; Richards et al., basins in the world (Auzende et al., 1988a; Pelletier et al., 1993, 2011). This aligns with GPS data (Calmant et al., 1995, 2003 Taylor 2001; Ruellan and Lagabrielle, 2005). Active spreading over the et al., 1995) which confirm present-day CW rotation of Vanuatu Arc last 10–12 Ma (Fig. 1a and b) has been ascribed to the location on (Fig. 1b) where thrust earthquake epicentres are concentrated, the strike–slip boundary between the Pacific and Australian plates whereas strike–slip events occur south of the Fiji Platform on the (Hamburger and Isacks, 1988; Lafoy et al., 1990) or to unusually Hunter Fracture Zone (Hamburger and Isacks, 1988; Louat and hot underlying upper mantle (Lagabrielle et al., 1997). Models of Pelletier, 1989; Pelletier et al., 1998). NFB development (Auzende et al., 1988b, 1995a) show the south- However, this ignores paleomagnetic evidence (Fig. 2) which west margin of the Fiji Platform juxtaposed against the south- clearly demonstrates counter clockwise rotation of the Fiji Plat- eastern flank of the Vanuatu Arc in pre-breakup reconstructions form over the last 10 Ma, during most of the period of NFB opening dated at 10–12 Ma. However, these same models equate NW–SE- (James and Falvey, 1978; Malahoff et al., 1982a; Inokuchi et al., oriented magnetic anomalies off the southwest tip of Fiji Platform 1992; Taylor et al., 2000). with similarly oriented anomalies off the northeastern flank of the Here, I reconcile the paleomagnetic and GPS data by showing Vanuatu Arc (Auzende et al., 1988b, 1990). Despite this internal that the NFB opened via double saloon door tectonics over the inconsistency, these models have been widely quoted or adopted majority of its development period from 12/10 Ma until 1.5 Ma, (Schellart et al., 2002; Garel et al., 2003; Mann and Taira, 2004; whereas only a single saloon door rotation of Vanuatu Arc occurred Ruellan and Lagabrielle, 2005; Faccenna et al., 2010). after 1.5 Ma. Secondly, by incorporating the well documented CCW rotation of Fiji Platform (Fig. 2), I demonstrate that existing reconstruction models (Auzende et al., 1988b, 1995a; Ruellan and

n Tel.: +971 506504357. Lagabrielle, 2005; Schellart et al., 2006; Faccenna et al., 2010)are E-mail address: [email protected] internally inconsistent, or incompatible with the progressive

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Fig. 1. (a) The North Fiji Basin in its regional setting in the southwest Paciﬁc Ocean (after Hall and Spakman, 2002; Mann and Taira, 2004; Schellart et al., 2006; Whattam et al., 2008). Light grey¼oceanic crust. Dark grey¼oceanic plateaus or island-arc crust. NZ¼New Zealand; PNG¼Papua New Guinea. CR¼Colville Ridge; FP¼Fiji Platform; KR¼Kermadec Ridge; LHR¼Lord Howe Rise; LR¼Lau Ridge; MBP¼Melanesian Border Plateau; NFB¼North Fiji Basin; NC¼New Caledonia; NR¼Norfolk Ridge; OJP¼Ontong Java Plateau; S¼Samoa; SCT¼San Cristobal Trench; SI¼Solomon Islands; TR¼Tonga Ridge; VA¼Vanuatu Arc; VT¼Vitiaz Trench. Black arrows show direction and rate in cm/yr of motion of the Paciﬁc Plate relative to the Australian Plate (DeMets et al., 1994; Mann and Taira, 2004). (b) Main tectonic elements of the North Fiji Basin (NFB) (after Auzende et al., 1995a; Lagabrielle et al., 1996; Pelletier et al., 2001; Ruellan and Lagabrielle, 2005). White areas outlined in black¼island arc crust. Black¼Islands: A¼Aneityum; Ef¼Efate; Es¼Espirito Santo; M¼Malekula; Ta¼Tanna. Ridges: Ba, Bl and Br¼Balmoral, Bligh and Braemar (after Jarvis et al., 1994); D¼D'Entrecasteaux; WT¼West Torres Platform. Spreading Ridges: CSR¼Central; FSR¼Futuna; HH¼Hazel Holmes; SP¼South Pandora; Tr¼Tripartite; WFR¼West Fiji; Fracture Zones: EFZ¼Epi (after Greene and Collot, 1994; Raos and Crawford, 2004); HFZ¼Hunter; NFFZ¼North Fiji. Thin arrows show representative GPS convergence rates between the Vanuatu Arc and the Australian Plate (Calmant et al., 1995, 2003; Taylor et al., 1995; Wallace et al., 2005, 2009). In the Aneityum Tanna area rates are 116–124 mm/yr, in Efate 86–94 mm/yr, while in Espirito Santo and Malekula they are 17–43 mm/yr. Small dashed square shows the location of Fig. 10.

130 of magnetic lineations in the NFB are compatible with a model 120 incorporating opposite rotations (Fig. 3a–d).Thirdly,thereconstruc- tions are used to discuss NFB tectonic development, and speciﬁc 110 features such as rift propagation and arc-perpendicular seaﬂoor 100 spreading. Fourthly, driving mechanisms for opposite rotations are discussed. Finally, features which do not match the double-saloon- 90 door model (opposite rotations) are discussed. 80

70 2. CCW and CW rotations of Fiji Platform, and Vanuatu Arc 60 respectively Degrees 50 Although James and Falvey (1978) thought the CCW Fiji Plat- 40 form rotation began 6 Ma ago, later work shows rotation began at least by 10 Ma (Malahoff et al., 1982a; Inokuchi et al., 1992; Taylor 30 et al., 2000). Linear regression through the data implies rotation 20 stopped at 1.46 Ma (Malahoff et al., 1982a; their Table 3 and Fig. 8) or at 2.7 Ma (Taylor et al., 2000; their Fig. 6). However their 10 Ma combined data (Fig. 2) suggest rotation of the Fiji Platform stopped 0 at 1.56 Ma, probably as it collided with the Lau Ridge. 01234567891011 Available paleomagnetic data indicate a CW rotation of Vanuatu Fig. 2. Compilation of paleomagnetic rotations for Fiji platform (Malahoff et al., Arc of 28–301 from 6 Ma or 521 from 4 Ma (Falvey, 1978; Musgrave 1982a, blue diamonds; Taylor et al., 2000, magenta squares). Vertical scale degrees and Firth, 1999), although accepted pre-rift reconstructions (Auzende rotation. Horizontal scale Ma. The largest dated rotation is 126.41 at 10.2 Ma. et al., 1988b, 1995a)aredated10–12 Ma (Section 5.1). The Fiji Platform A linear regression suggests rotation stopped at 1.56 Ma. A regression including therefore rotated CCW during the majority of the CW trajectory of Inokuchi et al. (1992) data (not shown here), which is less precisely tied to paleomagnetic samples or is dated stratigraphically rather than isotopically, Vanuatu Arc from 12/10 Ma to the present. Such opposite movements suggests rotation stopped at 0.96 Ma. (For interpretation of the references to need to be incorporated into reconstruction models. colour in this ﬁgure legend, the reader is referred to the web version of this article.)

3. Double-saloon-door rifting and seaﬂoor spreading model rotation of the Fiji Platform and the opposite rotation of Vanuatu Arc. This is determined through a series of reconstructions from This model, which envisages opposite rotations of two island- pre-12 Ma to the present (Figs. 4–9), which show that the majority arc-related terranes, like double saloon doors revolving about their Author's personal copy

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Fig. 3. Double-saloon-door rifting and seafloor spreading model (after Martin, 2006, 2007). Double wavy lines¼island arc crust created at a pre-existing accretionary wedge/ magmatic arc. Motifs on the rotating terranes¼island arc volcanoes. Dotted area¼extended island arc crust. Thin lines within light shaded area¼isochrons within oceanic crust. Thick line with black triangles¼subduction zone. Plate West 2 rotates clockwise about pole P1, whereas plate East 2 rotates counter clockwise about pole P2. (a) 201 of Rotation: arc-parallel rifts have developed, and an arc-perpendicular rift has initiated as West 2 separates from East 2 in an east west direction. (b)401 of Rotation: oceanic crust has extended both to the east and west (compare Fig. 3b with a), and the oceanic rift tips propagate both east and west, shown by isochrons abutting rifted island arc crust. Relative to the southwesterly motion of the east end of plate West 2, and the southeasterly motion of the west end of plate East 2, the arc-perpendicular oceanic rift tip propagates north. Simultaneously, the arc-perpendicular oceanic rift also extends south (compare the NS extent of oceanic crust in Fig. 3b and c). The arc-perpendicular rift therefore propagates both north and south. (c) 601 of Rotation: similar to the development of the North Fiji Basin (NFB), where the situation described in Fig. 3b above has evolved further. (d) For comparison to the model, a compilation of identified magnetic anomalies and lineations in the NFB (after Malahoff et al., 1982a; Auzende et al., 1988a, 1988b,1990, 1994a, 1995a, 1995b; Charvis and Pelletier, 1989; Maillet et al., 1989; Tanahashi et al., 1991; Pelletier et al., 1993; Huchon et al., 1994; Jarvis et al., 1994; Joshima et al., 1994; Tanahashi et al., 1994; Lagabrielle et al., 1996; Pelletier et al., 2000, 2001; Ruellan and Lagabrielle, 2005). Numbers and letter J¼identified magnetic anomalies. Fracture zones: HFZ¼Hunter, FFZ¼Futuna. Note that Fig. 3a–c is symmetrical, whereas the NFB is asymmetrical: Vanuatu Arc is 1163 km long, whereas Fiji Platform extends 510 km. hinges, was outlined previously (Martin, 2006, 2007). It is shown NFB (Figs. 4–9). The earliest (Fig. 4) rotates the Fiji Platform by here for two reasons (Fig. 3). First, the model shows magnetic 126.41 (the largest reported rotation of Taylor et al. (2000)), about lineations which are not dissimilar to those displayed by the NFB a pole near its northeast apex (15137′S, 1791W). To complete the (compare Fig. 3a–c, with d). Note that Fig. 3a–c is symmetrical, reconstruction, the Vanuatu Arc is rotated 631 about a pole just whereas the NFB is asymmetrical—the Fiji Platform is 510 km long north of Nendo Island, at 10122′S, 1661E, where the San Cristobal while the Vanuatu Arc is 1163 km long (Fig. 3d). Secondly, it was Trench, the Santa Cruz Trough and the Vanuatu Arc intersect. previously highlighted that an arc-perpendicular rift propagates This lies between other reported poles (e.g. 91S, 1661E, Schellart north between the southerly moving oppositely rotating terranes. et al., 2002, 2006;11123′S, 165140′E, Wallace et al., 2005, 2009), It should also be noted that the arc-perpendicular rift propagates and marks the swing from Vanuatu Arc-related seismicity to that south as well. The north–south extent of oceanic crust becomes associated with the San Cristobal Trench (Pelletier et al., 1998; progressively greater in Fig. 3a–c. This is relevant to the NFB, Richards et al., 2011). where a striking feature of the north–south oriented Central A reconstruction parallel to the Vitiaz Trench (Fig. 5) follows Spreading Ridge (CSR) is that it propagates both north and south previous models dated at 10 or 12 Ma (Auzende et al., 1988b, (Maillet et al., 1989; De Alteriis et al., 1993; Joshima et al., 1994; 1995a). Subsequent reconstructions document the progressive Ruellan et al., 1994)(Fig. 3d). opposite rotations of Vanuatu Arc and Fiji Platform (Figs. 6–9).

4.1. Existing models inconsistent, and incompatible with progressive 4. Revised reconstructions of the North Fiji Basin (NFB) movement of the Fiji Platform

Taking into account the well-documented CCW rotation of Fiji In their model, Auzende et al. (1988b, 1990) divided the NFB Platform (Section 2) leads to a set of revised reconstructions of the into three areas of magnetic lineations (inset of Fig. 5). Anomalies Author's personal copy

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Fig. 4. Pre-12 Ma pre-rift reconstruction. Fiji Platform is rotated 126.41 (after Taylor et al., 2000) about a rotation pole at 15137S 1791W(PFP), thereby positioning it NS and aligned with the Lau Ridge. The Vanuatu Arc is rotated 631 about a pole at 10122S 1661E(PVA). Stars with OJP and MBP mark Ontong Java Plateau and Melanesian Border Plateau collisions which choke the SW-directed subduction zone (thick line with triangles). Dotted lines: VT¼Vitiaz Trench; NDZ¼northern deformation zone (after Pelletier and Auzende, 1996). Tonga Ridge, outlined in red, is rotated against the Lau Ridge to its position prior to opening of the Lau Basin. (For interpretation of the referencesto colour in this figure legend, the reader is referred to the web version of this article.) with different orientations are interpreted as successive phases of the Auzende et al. (1988b, 1995a) models (inset of Fig. 5). Note that seafloor spreading (Pelletier et al., 1993; Auzende et al., 1995a). the Auzende et al. (1988a, 1995a) interpretation is inconsistent The model highlighted here (Fig. 3) demonstrates that anomalies with their 12/10 Ma reconstruction which, like Fig. 5, matches of differing orientations may be contemporary. Moreover, Area 2 southwest Fiji Platform with southeast Vanuatu Arc. (inset Fig. 5) comprises two parts with NW–SE-oriented anomalies, one lying southwest of Fiji, and one in the west NFB, northeast 4.1.2. Overlap of Vanuatu Arc and Fiji Platform of Vanuatu Arc. Both areas are interpreted as NW–SE-oriented Secondly, in alignment with rifted crust of the Vanuatu Arc seafloor spreading produced simultaneously in the earliest phase extending to 211S, 1701E(Maillet et al., 1989), NW–SE-oriented of clockwise rotation of the Vanuatu Arc (Auzende et al., 1988b, anomalies northeast of Vanuatu Arc extend to approximately 181S, 1995a). The associated 12/10 Ma and 7 Ma reconstructions have 1731E(Fig. 3d). In contrast, the Auzende et al. (1988b, 1995a) been widely adopted (e.g. Ruellan and Lagabrielle, 2005; Faccenna model (inset of Fig. 5) implies that soon after 12/10 Ma they et al., 2010). The above analysis is inconsistent with a number of extended to around 191S, 177.51E, southwest of the Fiji Platform. considerations set out below. This is inconsistent with their own reconstructions (Auzende et al., 1988b, 1995a), which accept that Vanuatu Arc extends to only 211S, 1701E. Such a 500 km extension of the Vanuatu Arc creates an 4.1.1. Conjugate margin of SW Fiji Platform is SE flank of Vanuatu Arc unacceptable overlap with the Fiji Platform in reconstructions of Firstly, Figs. 5–9 demonstrate that the conjugate margin of the their early rifting and separation (Figs. 5–7). An alternative southwestern flank of the Fiji Platform is the southeastern flank of reconstruction (Schellart et al., 2006) accepts this overlap, posi- the Vanuatu Arc southeast of Aneityum Island. The Vitiaz-parallel tioning Vanuatu Arc parallel to and south of the Fiji Platform. reconstruction (Fig. 5), achieved by pivoting about the proposed However, this implies that the conjugate margin to the northeast rotation poles, closely matches the first magnetic anomaly south- Vanuatu Arc is initially the southeast margin of the Fiji Platform, west of the Fiji Platform with the southeastern limit of the and later the southwest margin of Fiji Platform (Schellart et al., Vanuatu microplate immediately southeast of Aneityum Island. 2006, their Figs. 3l–n, and 8a–c). This limit was proposed by Maillet et al. (1989) because backarc troughs terminate there, the volcanic chain almost disappears, and the width of the shallow interplate seismicity zone rapidly 4.1.3. Orientation of magnetic lineations narrows. Thirdly, NW–SE-oriented magnetic anomalies off southwest Therefore anomalies off southwest Fiji Platform should not be Fiji Platform (Fig. 3d and inset Fig. 5) were aligned NNE–SSW interpreted as colinear and coeval with anomalies on the north- 12/10 Ma ago (Fig. 5). With further basin development, such eastern side of the Vanuatu Arc in the western NFB, as shown in anomalies rotate counter clockwise (Figs. 6–9) until they are Author's personal copy

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Fig. 5. Vitiaz-parallel reconstruction. Vanuatu Arc rotated 521, and Fiji platform 58.41 from their present-day positions. This is similar to the pre-rift reconstruction of Auzende et al. (1988b, 1995a) which they date as 10 and 12 Ma respectively. Dotted line¼Vitiaz Trench. Black line with triangles¼NE-directed subduction zone. Thin arrows indicate initial movements of the Vanuatu Arc and the Fiji Platform. Depending on their relative rate of motion (Martin, 2006), initial movement between the east end of the Vanuatu Arc and the west end of the Fiji Platform is strike–slip, implying an R–R–F triple junction. Dashed line¼first magnetic lineation southwest of Fiji Platform, which in its rotated position is sub-parallel to a magnetic lineation off the tip of the Vanuatu Arc. Inset shows subdivision of magnetic lineations into three regions (after Auzende et al., 1988b, 1990). Double dashed line¼axial anomaly. Note that region 2 is divided into two sub-regions, separated by region 3. oriented NW–SE in their present-day configuration (Fig. 3d). For be correct if the Fiji Platform had reached its present-day position example, 7.5 Ma ago, the first anomaly off SW Fiji Platform is prior to the postulated early-stage NW–SE-oriented seafloor orientated at 111W of North, (dashed line in Fig. 6), whereas an spreading. anomaly off the SE tip of Vanuatu Arc is oriented 251E of North. Although NS-oriented anomalies 4 and possibly 5 were previously identified on the CSR (Falvey, 1975; Malahoff et al., 1982b), 4.1.4. WNW–ENE separation of Vanuatu Arc and Fiji Platform, and these were subsequently discounted when only anomalies out to evolution of a triple junction in the NFB 2 or 2A were recognised (Auzende et al., 1988a, 1988b, 1990, Fourthly, previous models envisaged an initial period of exclu- 1995a; Maillet et al., 1989; Huchon et al., 1994). Despite this, sively NW–SE-oriented spreading followed by exclusively E–W NS-oriented magnetic anomalies exist west of those identified as spreading, and then development of a triple junction only 3 Ma anomalies J, 2 and 2A (inset of Fig. 5). Extrapolating using different ago when NS-oriented anomaly 2A developed on the CSR calculated spreading rates, the oldest NS-oriented positive anom- (Auzende et al., 1988b, 1995a). These schemes do not explain the aly at 171146′E is dated as 4.8–5.6 Ma (average 5.2 Ma). A recon- early WNW–ESE separation of the Vanuatu Arc and the Fiji Plat- struction for 5 Ma (Fig. 7) matches this anomaly on the west flank form necessitated by their opposite rotations. Immediately after of the CSR with a NW–SE-oriented anomaly on the east flank off the Vitiaz-parallel reconstruction (Fig. 5), continued rotation SW Fiji Platform evident in previous compilations and extending required a NNE–SSW-oriented transform margin between them. from 1751E, 181Sto176130E, 19130S (Malahoff et al., 1982b; In the 12–7.5 Ma period SW Fiji Platform and SE Vanuatu Arc Auzende et al., 1988a, their Fig. 3; Auzende et al., 1988b, their evolved into conjugate rifted margins, with an R–R–R triple Fig. 4; Tanahashi et al., 1991, their Fig. 4; Huchon et al., 1994, their junction separating them (Fig. 6). Initially, therefore, an R-R-F Fig. 4). triple junction developed, followed by an R–R–R–triple junction. Similarly, with further opposite rotations of Vanuatu Arc and This form of development is constrained by the geometry of Fiji Platform (Figs. 8 and 9), anomalies 2A and 2 on the CSR can be opposite rotations required by paleomagnetic data (Section 2) matched. This demonstrates that anomalies off SW Fiji Platform and modelled in Fig. 3. which are NW-oriented in their present-day position, are correctly Later (Figs. 7–9), the R–R–R triple junction comprises NS- matched with NE–SW-oriented anomalies east and southeast of oriented anomalies on the CSR (e.g. Huchon et al., 1994), and their the SE flank of Vanuatu Arc. Figs. 5–9 demonstrate that, with EW-oriented equivalents on the South Pandora Ridge (SPR) opposite rotations, these anomalies progressively create a fan (Lagabrielle et al., 1996). shape, matching the double-saloon-door tectonic model (compare Finally, post–1.5 Ma, three active spreading ridges meet at a Fig. 3c and d). The Auzende et al. (1988b, 1995a) model could only triple junction at 16140S (Auzende et al., 1988a, 1988b; Lafoy et al., Author's personal copy

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Fig. 6. Figs. 6–9 are drawn taking the Vitiaz-parallel reconstruction (Fig. 5)tobe12Ma(Auzende et al., 1995a), and assuming 521 rotation of the Vanuatu Arc pro-rata over 12 Ma, and 58.41 of the Fiji Platform pro-rata from 12 Ma to 1.56 Ma (see Sections 2 and 4). 7.5 Ma reconstruction. Vanuatu Arc rotated 32.51 and Fiji Platform rotated 33.21 from their present-day positions. Thin lines are rotated magnetic lineations northeast and southeast of the Vanuatu Arc (Fig. 3d), whereas dashed thin lines are rotated lineations north and southwest of Fiji Platform. Paired thin lines represent a cartoon of the R–R–R triple junction which develops as a result of the WNW–ESE separation of the east end of the Vanuatu Arc and the west end of the Fiji Platform (compare Fig. 3). Note that the ﬁrst lineation to the southwest of Fiji, which was oriented slightly east of due north in Fig. 5, has been rotated and is slightly west of due north.

1990; Price and Kroenke, 1991), and a second triple junction may This scenario needs opposite rotational torques which overlap exist at 14150S (Auzende et al., 1994b, 1995a) with the N 1601E between 1761 and 177.51E. ridge possibly connecting to the Hazel Holmes South Pandora Tripartite ridge system to the north (Figs. 1b and 3d). Recognising anomalies 1 and J on the N 160 ridge to the NW (Pelletier et al., 5. NFB development based on revised reconstructions 1993; Auzende et al., 1995a; although others disagree—Lafoy et al., 1990), this phase began after anomaly 2 (1.95–1.78 Ma) and before 5.1. Pre-12 Ma and Vitiaz-parallel reconstructions anomaly J (1.07–0.99 Ma), possibly related to the re-activation of the NFFZ 1.5 Ma ago. With a second triple junction, and the Fiji The Vitiaz-parallel reconstruction (Fig. 5) is dated differently Platform no longer rotating, the post–1.5 Ma phase does not match based on different data (3.96 Ma, Musgrave and Firth, 1999; the double-saloon-door model. It is discussed further in Section 8. 5.55 Ma, interpolating Taylor et al. (2000); 5.84 Ma interpolating Malahoff et al. (1982a); 10.4 Ma extrapolating Falvey (1978); 9.75– 11 Ma, magnetic anomaly 5, or 12–12.4 Ma, anomaly 5A in the 4.1.5. Geodynamic situation from 1761–177.51E, and north and south NFB, Pelletier et al., (1993), on timescale of Gradstein et al. (2004)). of Fiji Platform Dating the Vitiaz-parallel reconstruction as 10–12 Ma (Auzende Although they propose initial opening exclusively via NW- et al., 1988b, 1995a) suggests that the Fig. 4 reconstruction oriented seaﬂoor spreading south of Fiji Platform, the Auzende (maximum rotation of Taylor et al. (2000)) is pre-12 Ma. In this, et al. (1988b, 1995a) reconstructions show progressive CCW the tip of Fiji Platform extends north of the Vitiaz Trench but rotation of Fiji Platform from 12/10 Ma onwards. This implies a coincides closely with a deep deformation zone at 11118′S, 1801E spreading ridge north of Fiji Platform but their reconstructions which may also mark a former subduction zone (Pelletier and only depict it from 3 Ma or 7 Ma. Similarly, Schellart et al. (2006) Auzende, 1996). Taylor et al. (2000) place Fiji Platform further show rotation of Fiji Platform from 7.5 Ma, with a spreading ridge south, but their rotation pole, approximately at 17142′S, 1791W, to the north of Fiji Platform. By proposing that NW–SE-oriented causes an unacceptable overlap of Fiji Platform and northern Lau spreading extends to 191S, 177.51E off SW Fiji Platform, Auzende Ridge. Southwest-directed subduction was stopped either by et al. (1988b, 1995a) and Schellart et al. (2006) imply CW rotation collision of Ontong Java Plateau (Auzende et al., 1988b, 1990; of Vanuatu Arc and its extension lengthens to 177.51E. Between Pelletier and Auzende, 1996; Hall and Spakman, 2002; Mann and 1761 and 177.51E, this requires simultaneous seaﬂoor spreading Taira, 2004) or the Samoa hotspot and Melanesian Border Plateau north and south of the Fiji Platform but with counter clockwise (Falvey, 1975; Hathway, 1993; Taylor et al., 2000; Schellart et al., rotation to the north and clockwise rotation to the south. 2006; Richards et al., 2011). If the Vitiaz Trench marks the Author's personal copy

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Fig. 7. 5.0 Ma reconstruction. Vanuatu Arc rotated 21.71, Fiji Platform rotated 19.31. Dotted lines¼EW-oriented anomalies (after Pelletier et al., 1993). Other lineations as in Fig. 6. With initial movement having started at 6 Ma, Tonga Ridge and Lau Ridge have begun to separate (after Parson and Hawkins, 1994; Parson and Wright, 1996; Taylor et al., 1996). proto-Vanuatu-Tonga-Lau southwest-directed subduction zone this remnant subducted slab between Fiji Platform and the present (Falvey, 1975; Auzende et al., 1988b, 1995a; Pelletier and Hunter Fracture Zone comes from earthquake hypocentres (Chen Auzende, 1996; Okal and Kirby, 1998; Taylor et al., 2000; Mann and Brudzinski, 2001), slices of a tomographic model at 200 km and Taira, 2004; Schellart et al., 2006, although Hart et al. (2004) and 320 km depth (Hall and Spakman, 2002), and Vp/Vs tomo- consider it a tear in the Pacific Plate), then subduction remained graphic images at depths from 100 to 250 km (Conder and Wiens, southwest directed from the pre-12 Ma reconstruction (Fig. 4)to 2006). the Vitiaz-parallel reconstruction (Fig. 5). Only then, northeast- EW-oriented anomalies (dotted in Figs. 7–9) date from anomaly directed subduction began. This implies that during the arc 3A (Pelletier et al., 1993), and were also recognised on the South inversion process, the leading edge of the overriding plate was Pandora spreading ridge (Lagabrielle et al., 1996). In the northwest tectonically eroded by the subducting plate as proposed, for NFB, they abut NW-oriented anomalies or directly against the example off northeast Japan (Yamamoto et al., 2009). Moving Vanuatu Arc, (Charvis and Pelletier, 1989) demonstrating propaga- from the convex-to-the-NE pre-12 Ma reconstruction to the Vitiaz- tion towards the northwest in post–anomaly 3A times as well. parallel position, Fiji Platform rotates 681, whereas Vanuatu Arc Their emplacement implies a ridge-jump to the north in the only rotates 111. Either Fiji Platform rotated faster, or it started Pelletier et al. (1993) model. earlier before the period of opposite rotations. In the 2.5 Ma reconstruction (Fig. 8), conjugate anomalies 2A are superimposed at 171S but overlap at 17130′–19130′S. In Fig. 9, conjugate anomalies 2 are superimposed from 181 to 19130′S, but 5.2. Reconstructions from 7.5 Ma to anomaly 2 (1.95–1.78 Ma) are under-lapped north of 181S. Spreading rates over the last 1 Ma decrease northwards along the CSR from 7.6 cm/yr at 20130′Sto The 12/10–7.5 Ma period sees the first preservation of crust in 5.9 cm/yr at 17130′S(Huchon et al., 1994). Although this demon- the NFB. Fan-shaped NW–SE-oriented anomalies successively abut strates fan-shaped spreading, this effect is more subdued than in rifted crust of Vanuatu Arc between 16140′S and 12130′S, demon- the model shown in Fig. 3. Note also that in the southernmost strating a rift propagated northwest towards the rotation pole extension of the CSR which is offset to the east, the reconstruction (Figs. 6 and 7). Similarly fan-shaped seafloor spreading anomalies does not superimpose conjugate anomalies 2 (Fig. 9). separate the Fiji Platform from the Vitiaz Trench. A curvilinear On the most easterly segment of the Tripartite Ridge, the ridge- subduction zone extends from the southwestern flank of the parallel fabric of the seafloor narrows from 70 km to only a few km Vanuatu Arc to the southeastern margin of the Fiji Platform between 1771E and 177130E, suggesting easterly propagation (Figs. 6–9). The area presently occupied by the Hunter Fracture (Lagabrielle et al., 1996). Only the axial magnetic anomaly is Zone formed as a subduction zone from initial opening of the NFB identified in this area, but assuming the same spreading rate as (Fig. 6), rather than from anomaly 3A times (6.6–6.0 Ma) or later as the South Pandora ridge immediately to the west (8 km/yr half previously thought (Auzende et al., 1988b, 1995a). Evidence for rate), this propagation may have begun 4.4 Ma ago. Author's personal copy

198 A.K. Martin / Earth and Planetary Science Letters 374 (2013) 191–203

Fig. 8. 2.5 Ma reconstruction. Vanuatu Arc rotated 10.91, Fiji Platform rotated 5.31.2A¼anomaly 2A (3.6–2.6 Ma). Note that anomalies 2A are superimposed at 171S, but are overlapped at 17130′–19130′S.

Fig. 9. 1.5 Ma reconstruction. Vanuatu Arc rotated 6.51, Fiji Platform present-day position. 2¼anomaly 2 (1.95–1.78 Ma). Author's personal copy

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Fig. 10. NE-oriented propagating rift within the V-shaped graben, forming the NE-oriented limb of an R–R–R triple junction. Lines with tick marks¼faulted margins of graben which broadens to the SW, and narrows to the NE. Purple lines mark NE-oriented ridges within the graben. Dash dot purple line marks the central trough which extends the farthest to the NE (after Lafoy et al., 1990, their Fig. 4; Tanahashi et al., 1991, their Fig. 9a and b; Auzende et al., 1994b, their Fig. 3; Huchon et al., 1994, their Fig. 6; Joshima et al., 1994, their Figs. 3 and 4; Lagabrielle et al., 1994, their Figs. 2 and 4; Gracia et al., 1996, their Fig. 4). Thick black lines mark identified magnetic anomalies. A¼axial anomaly. J¼Jaramillo or anomaly J (after Huchon et al., 1994). Jn ¼an alternative interpretation of anomaly J (after Auzende et al., 1995a). Note that on the spreading ridge extending SW from the triple junction, Auzende et al., 1995a interpret the pair of flanking anomaly J's extending further NE than shown here. Discussed further in Section 5.3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

5.3. After rotation of the Fiji Platform stopped (1.56 Ma–Recent) anomaly 2A (Joshima et al., 1994, their Fig. 2). Propagation therefore preceded single-saloon-door opening of the NFB. When Fiji Platform stopped rotating at 1.56 Ma, subduction The N 1601E ridge propagates NNW as shown by spreading towards the southeast stopped, and the Hunter Fracture Zone fabric which abuts pre-existing crust, the existence of anomaly J likely initiated. With continued rotation of Vanuatu Arc, post- near the triple junction and its absence to the NNW, and the 1.56 Ma spreading constitutes single-saloon-door rather than narrowing of the spreading ridge from 100 km at 16140S to double-saloon-door tectonics. o50 km at 14150S (Auzende et al., 1990, 1994b; Price and Well-documented pseudo-faults at the northern and southern Kroenke, 1991; Gracia et al., 1996). It was described as en echelon ends of the CSR (Fig. 3d) demonstrate northwards and southwards propagation by Auzende et al. (1991). propagation (Maillet et al., 1989; De Alteriis et al., 1993; Joshima The northeastern limb of the triple junction at 16140S occupies et al., 1994; Ruellan et al., 1994; Gracia et al., 1996). To the south, a V-shaped graben which narrows to the NE (Lafoy et al., 1990), is an offset ridge also propagates south (Maillet et al., 1989; Huchon 40 km wide near the triple junction (Lagabrielle et al., 1994) and et al., 1994; Danyushevsky et al., 2006). Propagation is reported broadens to the SW (Fig. 10). It was interpreted as a pull-apart on after anomaly 2 and before J (De Alteriis et al., 1993; Huchon et al., the NFFZ, implying that the triple junction is R–R–F(Auzende et al., 1994) but comparison of magnetic anomalies and seaﬂoor fabric at 1988b; Lafoy et al., 1990). However, N 501E-oriented volcanic 19130S, 1741E shows northwards propagation began before anom- ridges and troughs with magnetic signatures (Auzende et al., aly 2 (Joshima et al., 1994, their Fig. 7; Ruellan et al., 1994, their 1990) occupy the graben ﬂoor, with the central trough extending Figs. 6, 11 and 12). Similarly southern propagation began by farthest to the NE, indicating NE propagation over at least the last anomaly 2 (Maillet et al., 1989, their Fig. 8; Auzende et al., 1994a, 250,000 (Lafoy et al., 1990) or 400,000 yr (Lagabrielle et al., 1994). their Fig. 2; Auzende et al., 1995a, their Fig. 4.3), and possibly by The older age is based on applying the spreading rates of the CSR Author's personal copy

200 A.K. Martin / Earth and Planetary Science Letters 374 (2013) 191–203 and N 1601E ridges to a 20 km wide zone of volcanic ridges within (Pelletier and Louat, 1989; Parson and Wright, 1996; Taylor the graben. Ridges and troughs actually extend over 32 km (Lafoy et al., 1996; Zellmer and Taylor, 2001; Ruellan et al., 2003; et al., 1990, their Fig. 4; Tanahashi et al., 1991, their Fig. 9a and b; Watanabe et al., 2010). Auzende et al., 1994b, their Fig. 3; Huchon et al., 1994, their Fig. 6; (g) Fiji Platform stopped rotating 1.56 Ma ago when it collided Joshima et al., 1994, their Figs. 3 and 4; Lagabrielle et al., 1994, with the Lau Ridge. Similarly, Vanuatu Arc collided with their Figs. 2 and 4; Gracia et al., 1996, their Fig. 4), and applying the D'Entrecasteaux Ridge from 3.0 to 1.89/1.58 Ma and with West spreading rate within the graben (Huchon et al., 1994), propaga- Torres Platform from 0.7 Ma (Collot et al., 1992; Greene and tion began 1.78 Ma ago. This aligns with the oldest rocks recovered Collot, 1994; Meffre and Crawford, 2001; Schellart et al., 2002). from the graben ﬂank which are 1.35–1.9 Ma old (Lagabrielle et al., Slow convergence rates at Malekula and Espiritu Santo versus 1994). The NE-oriented ridge within the graben forms the third fast rates at Efate and Tanna Islands (Fig. 1b), are best arm of an R–R–R triple junction (cf. Price and Kroenke, 1991; explained by a dextral fault lying between Malekula and Efate Huchon et al., 1994). (Calmant et al., 1995, 2003; Taylor et al., 1995). As a result, the The E–W-oriented Hazel Holmes spreading ridge in the wes- fast-moving southern part of the Vanuatu Arc is separated tern NFB occupies a V-shaped graben centred on 13130S (Charvis from areas blocked by collision by a pair of sinistral and dextral and Pelletier, 1989; Louat and Pelletier, 1989; Pelletier et al., 1993; strike–slip faults (the Epi and Hunter Fracture Zones—Fig. 1b). Schellart et al., 2002), possibly constituting a ridge jump to the The sinistral Hunter Fracture Zone likely acts, along with the south. E–W-oriented magnetic lineations cross-cut NW–SE-trend- sinistral North Fiji and Futuna Fracture Zones (Pelletier et al., ing anomalies (Pelletier et al., 1993). The magnetic lineations also 2001; Ruellan and Lagabrielle, 2005) as a subduction trans- narrow to the west, as does the graben they occupy suggesting form edge propagator or STEP (cf. Bilich et al., 2001; Govers propagation towards the west. and Wortel, 2005), connecting the southern Vanuatu and A second NS-oriented spreading centre occurs just west of Fiji, northern Tonga subduction zones. at 1761E(Auzende et al., 1993, 1994a, 1995b; Huchon et al., 1994). Originally interpreted as a complex strike–slip zone, it is now recognised as an overlapping spreading centre, extending out to 7. Driving mechanism for opposite rotations anomaly J, and possibly to anomaly 2, with propagation both to the north (Huchon et al., 1994) and south (Auzende et al., 1995b). 7.1. Concave slab with divergent slab pull

The collisions of the Ontong Java and Melanesian Border 6. Features diagnostic of double-saloon-door tectonics Plateaus stopped southwest-directed subduction of the Pacific Plate at the Vitiaz Trench, and northeast-directed subduction of A number of features of the NFB described above match the Australian Plate initiated (Section 4 and Figs. 4–9). characteristics previously identified in double-saloon-door tec- A proposed driving mechanism for opposite rotations of island tonics (Martin, 2006, 2007, 2011). arc terranes relates to divergent slab pull forces of a curved subduction zone (Martin, 2007, 2011). These cause deviatoric (a) NFB development involves opposite rotations of the Vanuatu tensile stresses in the overlying plate which are not planar, but Arc and the Fiji Platform over most of the time period involved rather propel two opposite rotational torques. In conjunction with (Section 2). opposite toroidal flows around lateral slab edges (Schellart, 2004; (b) The NFB evolved backarc to the Vanuatu Arc and Fiji Platform Funiciello et al., 2006; Stegman et al., 2006; Kneller and Van with subduction rollback on the Australian Plate (Auzende Keken, 2008) which concentrate mantle flow towards the centre of et al., 1995a; Taylor et al., 1995; Van der Hilst, 1995; Okal and the slab face, this drives opposite terrane rotations. In this scheme, Kirby, 1998; Hall and Spakman, 2002; Gorbatov and Kennett, the opposite rotational torques abut each other in a central 2003; Mann and Taira, 2004; Schellart et al., 2006; Faccenna position (Martin, 2011, his Fig. 13a). et al., 2010; Richards et al., 2011). Flow around the edge of the Pacific Plate at the north end of the (c) Vanuatu Arc and Fiji Platform crust respectively comprise Tonga Arc is evinced by geochemical markers and anisotropy Oligocene–Recent and Eocene–Recent volcanics, volcanoclas- (Fischer and Wiens, 1996; Turner and Hawkesworth, 1998; tics and marine sediments formed at a pre-existing subduction Fischer et al., 2000; Hall et al., 2000; Smith et al., 2001; Pearce zone/accretionary wedge intruded by a magmatic arc and Stern, 2006; Conder and Wiens, 2007). Before opening of the (Malahoff et al., 1982a; Gill et al., 1984; Collot et al., 1992; Lau Basin, such toroidal flow would have entered the NFB. With Hathway, 1993; Greene and Collot, 1994; Meffre and Crawford, the northeastern tip of the Tonga slab just over 700 km from Fiji 2001; Raos and Crawford, 2004). Platform, and given toroidal flows with radii up to 700–800 km (d) Seafloor spreading ridges are both parallel and perpendicular (Kneller and Van Keken, 2007; Schellart et al., 2007) or even to a curved subduction zone (Malahoff et al., 1982b; Auzende greater (Hall et al., 2000), then flow around the Tonga slab likely et al., 1988a, 1995a; Pelletier et al., 1993; Ruellan and influenced the NFB during much of the opening of the Lau Basin, Lagabrielle, 2005). but with waning effect over time. The influence of the Samoa (e) Rifts propagated northwest, east, northeast, north and south hotspot has been interpreted in 3 Ma yr old alkalic volcanism in (Section 5; Figs. 3d and 10). Fiji (Gill and Whelan, 1989), implying flow around the edge of the (f) Ridge jumps and plate re-organisations separated areas of Tonga slab into the NFB. This is supported by NE–SW-oriented fast older crust. In the West Mediterranean which also exhibits splitting directions south of Fiji (Smith et al., 2001; Wiens et al., double-saloon-door tectonics, areas of thicker crust or rifted 2008), which are arc-parallel for the subduction zone south and continental crust were isolated by this process (Gueguen et al., southeast of Fiji Platform. 1997; Martin, 2006). In the NFB, this mostly involved only At its northwestern end, The NW-oriented Vanuatu slab juxta- oceanic crust, except where the Balmoral, Braemar and Bligh poses the EW-oriented San Cristobal/Solomon Island slab (Fig. 1a). Ridges were rifted away from the Fiji Platform by short offset Richards et al. (2011) map the Vanuatu slab to depths of only spreading ridges along the NFFZ (Jarvis et al., 1994). At a 200 km, and the Solomon slab to only 100 km. Depth slices at 200 broader scale, backarc spreading in the Lau Basin is separating and 320 km (Hall and Spakman, 2002) suggest a gap between the the combined Fiji Platform/Lau Ridge from the Tonga Arc Vanuatu and Solomon slabs. It is therefore possible that toroidal Author's personal copy

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flows enter the mantle wedge above the Vanuatu slab from the spreading centres propagate both north (Huchon et al., 1994) northwest. and south (Auzende et al., 1993, 1994a, 1995b). The WFR has been related to the initiation of the NFFZ 1.5 Ma ago, which it 7.2. Double indenters abuts at its northern end. The sinistral NFFZ may create a large- scale pull apart between the Pacific and Australian plates (cf. In the piercing point premise, (Marshak, 1988; Mann et al., 2002; Huchon et al., 1994). In this view, a left step at the west end of Wallace et al., 2005), where buoyant material collides it chokes the Fiji Platform, between the sinistral NFFZ and the sinistral subduction, whereas continued adjacent rollback exerts a rotational Hunter Fracture Zone is occupied by the extra spreading ridge. torque. This is similar to the second stage of double saloon door (c) Thirdly, only the Vanuatu Arc continued to rotate during the tectonics where rotating terranes collide with adjacent buoyant last 1.56 Ma yr, whereas many features evident in the recent continental crust partially choking the subduction zone (Martin, period are more like the double-saloon-door model. One 2007). Two indenters are required for opposite rotation. As noted reason may be that many of these characteristics were above, in the NFB the Ontong Java and Melanesian Border Plateau established prior to single-saloon-door tectonics, such as collisions not only choked subduction but led to an arc reversal. northerly and southerly propagation on the CSR, propagation These two plateaus provide pivot points but on the Australian Plate northeast of the triple junction at 16140S (Section 5.3), and after arc reversal rather than on the Pacific Plate. This zone therefore easterly propagation on the Tripartite Ridge (Section 5.2). does not match the double indenter model. Propagation towards the NW dates from the 12/10–7.5 Ma Considering northeast-directed subduction of the Australian period (Section 5.2). Plate after arc reversal, collision of the D'Entrecasteaux Ridge and (d) The post-1.5 Ma period where only Vanuatu Arc rotates agrees West Torres Platform has been invoked as the indenter responsible with GPS measurements (Calmant et al., 1995, 2003; Taylor for CW rotation of Vanuatu Arc (Schellart et al., 2002; Wallace et al., et al., 1995; Wallace et al., 2005, 2009) and the single rotation 2009). However the collision may only have occurred at 0.7 Ma, or model (Schellart et al., 2002; Mann and Taira, 2004; Richards 3.0–1.58 Ma (Collot et al., 1992; Greene and Collot, 1994)orpossibly et al., 2011). Moreover, the NFB is asymmetric throughout, 8–10 Ma ago (Meffre and Crawford, 2001). Only the earlier date can with the 510 km long Fiji Platform contrasting with the be considered causal for Vanuatu Arc rotation. Secondly, the 1163 km long Vanuatu Arc. This also explains differences from D'Entrecasteaux Ridge impinges on Vanuatu Arc at 15142S, whereas the double-saloon-door model which shows symmetrical proposed rotation poles for Vanuatu Arc lie 480–740 km to the opposite rotations (Fig. 3). north from 9 to 111S, (Schellart et al., 2002; Wallace et al., 2005; here in Section 4). The collision is near the mid-point of Vanuatu Arc rather than at its rotation pole, where the indenter theory 9. Conclusions would have it. Thirdly, although Wallace et al. (2009) acknowledge the CCW rotation of Fiji Platform, there is no obvious indenter on North Fiji Basin (NFB) evolution is examined through a revised the subducting Australian Plate to have caused its rotation. set of reconstructions, building on previous models. Paleomag- netic, isotopic and magnetic data show that Fiji Platform rotated counter clockwise from at least 10.2 Ma until 1.56 Ma, while 8. Features which do not match the double-saloon-door Vanuatu Arc rotated clockwise from 12/10 Ma until the present. model A revised model incorporating these well-documented opposite rotations explains the distribution of magnetic anomalies in the The NFB also includes characteristics which do not align with NFB. The conjugate margin of southwest Fiji Platform is southeast the double-saloon-door model. They occur in the post–1.5 Ma Vanuatu Arc. Previous models (Auzende et al., 1988b, 1995a) period of single saloon door opening when only one terrane, the which associate NW-oriented anomalies off southwest Fiji with Vanuatu Arc, rotates. similarly oriented anomalies northeast of Vanuatu Arc are therefore wrong. Secondly, these models propose an early NW-oriented (a) From 1.5 Ma, a second triple junction may have existed at spreading ridge extending to 191S, 177.51E, almost 500 km south- 14150S, where the N 1601E ridge possibly connects to the east of the mapped extension of rifted island-arc crust on Vanuatu Hazel Holmes/South Pandora/Tripartite ridge system (Auzende Arc. This causes an unacceptable overlap with Fiji Platform in pre- et al., 1994b, 1995a). With the co-existence of the NFFZ and the rift reconstructions. An alternative model (Schellart et al., 2006) South Pandora Tripartite ridges, there are two generally east- accepts the overlap, placing Vanuatu Arc parallel to and south of oriented spreading systems (Fig. 1b), whereas the model Fiji Platform. However, this assumes the conjugate margin of envisages only one (Fig. 3). northeast Vanuatu Arc is first SE Fiji Platform then SW Fiji Plat- The West Mediterranean developed through four stages of form. Thirdly, anomalies off SW Fiji which are NW-oriented in backarc extension (Lonergan and White, 1997; Rosenbaum their present-day position were oriented NNE during initial et al., 2002). Spreading phases were punctuated by plate re- breakup. Rather than aligning with NW-oriented anomalies in organisations/ridge jumps which isolated areas of thicker the western NFB, they are matched by NNE-oriented anomalies off crust, such as the Balearic Peninsula and the Sardinia/Corsica SE Vanuatu Arc. With further rotation, these conjugate anomaly Block (Gueguen et al., 1997). Although seafloor spreading sets form a fan shape in the southern NFB. Fourthly, previous phases are thought to be successive, there is some overlap in models do not explain early WNW–ESE separation of Vanuatu Arc time, such that extension to the north and south of the and Fiji Platform required by their well-documented opposite Balearic Peninsula, for example, occurred simultaneously rotations. The model preferred here with an R–R–R triple junction (Bouillin et al., 1986; Maillard and Mauffret, 1999; Roca et al., evolving before 7.5 Ma (rather than at 3 Ma as previously thought) 1999; Mascle et al., 2004). The implication is that extra does explain this early WNW–ESE separation. spreading ridges may occur during backarc development via NFB characteristics which match the double-saloon-door tec- double-saloon-door tectonics. tonic model include opposite rotations of island arc terranes, (b) The second striking difference between the NFB and the model backarc seafloor spreading which is both arc-parallel and arc- is a second NS-oriented spreading ridge immediately west of perpendicular, and rifts propagating north, south, northeast and Fiji (the West Fiji Ridge – WFR – Fig. 1b). There, overlapping northwest. Features which do not match the double-saloon-door Author's personal copy

202 A.K. Martin / Earth and Planetary Science Letters 374 (2013) 191–203 model include the North Fiji Fracture Zone and the West Fiji Faccenna, C., Becker, T.W., Lallemand, S., Lagabrielle, Y., Funiciello, F., Piromalli, C., spreading centre. Both initiated post–1.5 Ma, when Fiji Platform 2010. Subduction/triggered magmatic pulses: a new class of plumes? Earth Planet. Sci. Lett. 299, 54–68. stopped rotating, and only one terrane, Vanuatu Arc, continued to Falvey, D.A., 1975. Arc reversals, and a tectonic model for the North Fiji Basin. Bull. rotate. The post–1.5 Ma phase matches GPS data and models Aust. Soc. Explor. Geophys. 6, 47–49. which highlight a single rotation of Vanuatu Arc during NFB Falvey, D.A., 1978. Analysis of palaeomagnetic data from the New Hebrides. Bull. Aust. Soc. Explor. Geophys. 9, 117–123. evolution. Fischer, K.M., Wiens, D.A., 1996. The depth distribution of mantle anisotropy beneath the Tonga Subduction Zone. Earth Planet. Sci. Lett. 142, 253–260. Fischer, K.M., Parmentier, E.M., Stine, A.R., Wolfe, E.R., 2000. Modeling anisotropy fl Acknowledgements and plate-driven ow in the Tonga subduction zone back arc. J. Geophys. Res. 105 (B5), 16,181–16,191. Funiciello, F., Moroni, M., Piromallo, C., Faccenna, C., Cenedese, A., Bui, H.A., 2006. I thank Ana Fernandez Gomez and Sonia Verdasco Garcia for Mapping mantle flow during retreating subduction: laboratory models ana- help with the figures. lyzed by feature tracking. J. Geophys. Res. 111, B03402, http://dx.doi.org/ 10.1029/2005JB003792. Garel, E., Lagabrielle, Y., Pelletier, B., 2003. Abrupt axial variations along the slow to ultra-slow spreading centers of the northern North Fiji Basin (SW Pacific): References evidence for short wave heterogeneities in a back-arc mantle. Mar. Geophys. Res. 24, 245–263. Auzende, J.M., Eissen, J.P., Lafoy, Y., Gente, P., Charlou, J.L., 1988a. Seafloor spreading Gill, J.B., Whelan, P.M., 1989. Post subduction ocean island alkali basalts in Fiji. 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