THE NORTHERN LAU BASIN: DIFFUSE BACKARC EXTENSION AT THE LEADING EDGE OF THE INDO-AUSTRALIAN PLATE

L.M. Parson' and D.L. Tiffin'

August 1992 SOPAC Technical Report 141

1 LM. Parson, Institute of Oceanographic Sciences Deacon Laboratory, Brook Road, Wormley, Surrey, UK :2 D.L Tiffin, South Pacific Applied Geoscience Commission, Suva, Fiji

Prepared for:South Pacific Applied Geoscience Commission (SOPAC) [3J

CONTENTS

Page ABSTRACT 5 ACKNOWLEDGEMENTS ...... 6

INTRODUCTION ...... 7 BATHYMETRY 9 GEOPHYSICS 10

DATA 11 GLORIA IMAGERY 11 THE GLORIA SURVEY 11

RESULTS 13 The basement ridge and sedimented Inter-ridge area 16 The Kings Triple Junction and surrounding area 23 The sedimented block terrain in the central northern Lau Basin 26 The Northwest Lau Spreading Centre (NWLSC) 27 • The northern Peggy Ridge 29

DISCUSSiON 29 MINERAL RESOURCE POTENTIAL OF THE NORTHERN LAU BASIN 33

CONCLUSIONS 35 REFERENCES 36

[TR141 • Parson & TlffinJ [4]

LIST OF FIGURES

Figure Page 1 Location map of lau Basin between Fiji and Tonga 8 2 Outline of the northern lau Basin survey area 12 3 GLORIA mosaic of the northern lau Basin survey area 14 4 Line drawing structural Interpretation of Figure 3 15 5 Bathymetry of the northeastern Lau Basin area of study 17 6 A section of seismic reflection profile 18 7 A section of seismic reflection profile 21 8 Preliminary detaJed bathymetry by RV Ke/dysh of a dive area 24 9 A section of seismic reflection profile with line drawing 28 10 Tectonic synthesis of the Lau Basin 31

(TR141 - Parson & Tiffin] [5)

ABSTRACT

GLORIA sidescan imagery of the Lau Basin north of 1tJS reveals several morphoteetonic terrains: a basement ridge and sedlmented Inter-ridge area in the southeast between 1740W' and 1750SOW, 15°50'$ and 1'fJS; a triple Junction in highly tectonlsed sedlmented terrain In the eastern north Lau Basin; an extensional deeply sedlmented terrain in the central north Lau Basin; a newly discovered linear neovolcanlc zone striking NNE from Peggy Ridge In the northwestern basin; and the northern Peggy Ridge. Unlike the central Lau Basin, there Is no evidence for long-term back-arc crustal accretion In the 250 km wide northern Lau Basin. We conclude that the most recent extension affecting the northern lau Basin has been accommodated at the linear neo-volcanlc zone and the extensional area around the triple junction, but In the past has been largely by formation of pull-apart basins and local magmatism. We further speculate that stress transfer from the PacifIc Plate to the Indo-Australian Plate may have caused the initiation of the triple junction in the northern Lau Basin. The nee-volcanic zones are areas where hydrothermal mineralisation can occur, and Inactive chimneys have been found at the triple Junction.

(TR141 - Parson & Tiffin 1 [6J

ACKNOWLEDGEMENTS

This SOPAC GLORIA survey In parts of the EEZs of Vanuatu. Solomon Islands. Fiji, Tonga, and Western Samoa was supported by lome III funds from the European Community to the South

Pacific Applied Geoscience Commission (SOPAC) and by the generous provision of shiptime by the Royal Australian Navy (RAN). The Seabeam data collected by HMAS Cook are the property of the RAN and are published with the permission of the Hydrographer, Royal Australian Navy.

We would like to express our appreciation to the Master, officers and crew of the HMAS Cook, and to the engineers of Marconi Underwater Systems Limited, who ensured that the survey was euccessna All those in the shipboard scientific party, including Quentin Huggett, deserve fUll credit for working so hard to collect this data set. We also acknowledge our debt for constructive criticism and advice offered by many fellow researchers in the tau Basin, amongst whom Bram Murton and Jim Hawkins have been especially helpful. John Wilson and Bob Whitmarsh are thanked for comments on an earlier draft of this paper.

[TRW· Parson & TiffinJ [7)

INTRODUCTiON

The Lau Basin comprises a southerly tapering and actively spreading marginal basin between the eastern Islands of Fiji, and Tonga, occupying In total some 250,000 sq km between about 14°$ and 24°$ (Figure 1). The basin Is characterised by a mainly north-south topographic grain, with depths locally exceeding 3500 m, but typically between 2400-2800 m. The basin Is bounded on its west side by the Lau Ridge, a palaeo-arc supporting numerous Islands and shoals, and on the east side by an active arc, the Tofua Volcanic Arc. The Tofua Arc lies just west of the Tonga platform In the south, separated from it by the narrow Totua Trough, but In the north, the Tofua Arc is central on the Tonga platform.

The lau Basin Is a marginal deep water basIn (Karig 1970) In a lone of back-arc extension at the leading edge of the Indo-Australian Plate. Magnetic anomaly patterns are complex. but show that south of about 1'1'30'8. spreading has taken place along short, almost north-south, ridge segments which lie centrally in the basin north of 19O5, and, south of 1905, in the eastern basin. A number of authors have studied the southern and central Lau Basin In detail (e.g. Hawkins, 1974; Welssel, 1977; von Stackelberg and von Rad, 1990), but few surveys have been reported in the area north of 1f3S. Thus the detailed tectonic framework in the northern area has remained elusive as exemplified by the diverse models purporting to show its structure (Lawver et al, 1976; Cherkls, 1960; Pelletier and Louat. 1989).

The accurate location of some spreading ridge segments in the central and southern Lau Basin back-arc system has been revealed by recent detailed Seabeam mapping by the RV Sonne (von Stackelberg and Scholten, 1990). These data place a NNE trending spreading centre at around 1804O'S, 176015W. More recently, using GLORIA long·range skJescan sonar, Parson et al (199Oa, 1990b) imaged much of the southern basin and found that, south of 1'f>30'S, back-arc extension is presently located along three spreading segments: the Central, Intermediate, and Eastern Lau Spreading Centres. Each of these segments strikes approximately north-south, but towards the north they are progressively offset westward by an increasing distance from the active Totua Arc (Figure 1). At its most distant, the northernmost segment Ohe Central Lau Spreading Centre) abuts the southeastern limit of Peggy Ridge some 150 km to the west of the arc (Parson et al 1990b) whereas In the south, the Eastern Lau Spreading Centre Is only 20-40 kilometres from the arc (Morton and Sleep, 1985; Collier and Sinha, 1990).

Peggy Rk:lge, a NW·SE striking volcano-tectonic lineament cutting across the northwest Lau Basin, had been previously interpreted as a spreading ridge (Chase, 1971), but Scrater at al

[TR141 • Parson & Tiffin]

[91

(1972) Indicated that magnetic data supported a hypothesis that it is a transform fault. Since then, Eguchi (1984) has published earthquake motion tensor solutions which confinn a zone of strike-slip motion along the ridge. Earlier GLORIA reconnaissance (Parson at al 1990a) imaged only the southernmost part of Peggy Ridge and was unable to provide controls on the Interpretations to the north. The subsequent GLORIA survey by Tiffin at aI (1990), which forms the basis of this report, has given a better understanding of the back-arc tectonics in the northern Lau Basin, and allowed a regional synthesis of the back-arc between 15° and 22°$ which demonstrates significant contrasts in the mode of extension between the northern and southern lau Basin.

Recent compilations of bathymetry and sidescan data for the southern lau Basin (Parson at ai, 19928), combined with the deep drilling results of the Ocean Drilling Programme In the south and central parts of the basin, found that a combination of extensional magmatic rifting and phases of localised volcanic construction preceded the initiation of true seafloor spreading there (Parson at ai, 1992b). In that area, a north-south topographic fabric Is dominated by ridges and scattered seamounts. But in contrast to a large data set for the southern basin, data for the northern Lau Basin is scant. Because It has received less attention, previous Interpretations there have been apecuenve. In partiCUlar, north and east of Peggy Ridge, apart from the Island of Nluato'ou and the shoal areas of Rochambeau Bank, Zephyr Shoal and Donna Seamount (Figure 1), there were few available data, and fewer coherent Interpretations of the morphology or tectonics prior to this survey.

BATHYMETRY

Several workers have constructed bathymetric charts to varIous scales (but little detail) since Karig (1970) Initiated Interest In the area. Hawkins (1974) produced the first major regional bathymetry of the Lau Basin by recontouring conventional echo-sounding data. Chase (1985) prepared a series of bathymetric maps for the region incorporating some more recent multibeam bathymetry data. Parson et al (l992a, and work In progress) have synthesised and compiled US, French and German SeaBeam data together with Seabeam data from this survey and all previous published and unpublished conventional bathymetric data. Bathymetry of the northeastern Lau Basin is included in this report.

[TR141 • Parson & Tiffin] [10]

GEOPHYSICS

Models based on magnetic anomaly comrsanons presented by several workers have proposed seafloor spreading processes In the Lau Basin. but estimates on the age of opening have ranged from 5·10 Ma (Selater at ai, 1972) to 2.5..:3Ma (MaJahoff at ai, 1982). tawver et aI (1976) pointed out the complex, discontinuous, and non-parallel nature of the marine magnetic anomalies, and Weisse! (1977) used magnetic anomaly data In the south and central basin to constrain his suggested seafloor spreading model for back-arc opening from the time of anomaly 2' (anomaly 2A of Berggren at ai, 1985; about 3 Ma). While proposing additional spreading centres and transfonn lineaments north of 1'fIS however, Weisse! was unable to constrain these models geophyslcally. Murthy (1990) provided a comprehensive synthesis and provisional Interpretation of combined aeromagnetic and marine magnetic data in the northern Lau Basin to constrain some of the diverse tectonic interpretations there. Malahoff et al (In press) developed complex patterns of Interpreted magnetic anomalies. and produced a moclel involving southward propagating ridges in the central portion of the basin, and a complex of transform offsets and highly segmented ridge systems In the north. Parson et al (1990b) used GLORIA data from the basin south of 1-,ooo'S to define precise positions of axial zones and this framework has largely guided subsequent speculation on the tectonic evolution of the basin. Most recently, Parson et al (1992) have interpreted a recompUed residual magnetic anomaly data set. They propose that the central Lau Basin opened by a rapid, southerly propagation of the East Lau Spreading Centre, commencing at around 5.5 Ma near the present location of Peggy Ridge.

Eguchi (1984) inferred several WNW-ESE striking transform faults from shallow seismicity and mechanism solutions at the northern end of the Lau Basin, but Indicated shallow seismicity had an overall east-west trend across the northern basin. Pelletier and Louat (1989) used seismic mechanism solutions to infer plate boundaries in the Lau Basin and proposed a North Lau microplate. Their data Indicate a sub-east-west resultant motion between the northern parts of the Lau Ridge and the Tonga arc.

Heat flow data from the northwestern lau Basin show a range of values from 0.36 mlcrocal/crrr2 to 1.70 microcal/cm2 - low compared to predicted values for "young" oceanic crust (Scfater et ai, 1971; Sdater, 1972). One unconfirmed heat flow value of 3.2 microcal/cm2 Is reported north of Fonualei Island in a trough between the Tonga platform and Natsushlma Ridge (Joshlma at al. 1985).

[TR141 - Parson & Tiffin] [11]

DATA

GLORIA sidescan Imagery (Somers et ai, 1978), slngle channel seismic reflection, 3.5 kHz sub-bottom profiles, and Seabeam bathymetry along the GLORIA track (Figure 2), were obtained In 1989 (Tiffin et et, 1990) in northern tau Basln north of 1'fIS. The study area in this paper extends from the northeastern flank of the Fiji Islands to the western side of the Tonga platform. Including the northern part of Peggy Ridge. all part of a larger survey covering areas of Vanauatu, Solomon Islands, Fiji, Tonga and Western Samoa. The GLORIA survey In northern Lau Basin was designed to define seafloor tectonic fabrics Indicative of plate motion geometry, Including the complex areas north of Peggy Ridge and around a recenUy found and poorly mapped spreadlngcentre east of Niuafo'ou Island where hydrothermal sediments had been sampled (Hawkins and Helu, 1986). High levels of seismicity recorded In certain parts of the region indicate active structural and tectonic processes (Hamburger and Isaacs, 1988).

GLORIA IMAGERY

The seafloor signature observed In GLORIA sidescan imagery is predominantly an expression of seafloor roughness (Mitchell and Somers, 1989) although other important characteristics such as Incidence angle of the sonar beam on target, and seafloor physical properties, must be considered. GLORIA imagery is thus particularly well suited to delineating steep scarps and discriminating between regions of the seafloor mantled by unconsolidated sediment and those which consist of exposed volcanic rocks. The convention for the display of GLORIA data Is that light tones represent high backscatter (strong echoes), darker tones are lower backscatter. The three main acoustic facies distinguished on our GLORIA Imagery are: a dark grey image resulting from low backscatter, interpreted as sediment-draped seafloors; a lighter image from moderate backscatter, Interpreted as outcrops of basement, often associated with ridges; and high backscatter resulting in a light image, interpreted as either steep scarps or nearly sediment-free lavas.

THE GLORIA SURVEY

The operational details of the SOPAC GLORIA survey, which imaged areas of the exclusive economic zones of Vanuatu, Solomon Islands, Fiji, Tonga and Western Samoa, have been described In detail elsewhere (Tiffin et al, 1990). Tracks of the survey ship HMAS Cook In the

[TR141 - Parson & Tiffin]

[131

Lau Basin were spaced at approximately 24 km throughout the area, which ensured almost 100% lnsonflcatlon of the seafloor in a range of water depths to a maximum of about 3600 m under the ship (and at one point, by misfortune, less than 150 m). Ship's speed was maintained at about 8-8.5 knots. Continuously acquired single channel seismic reflection data were of good quality, with up to 2 km penetration. A bathymetry swath under the ships track with a width of approximately 0.7 times water depth was provided by the hull-mounted SeaBeam mUlti-beam echcrsoundlng system. These latter data were Invaluable in determining the orientation of features In the GLORIA nadir, a band of poorly Imaged seafloor about 2--3km wide directly under the sonar towflsh.

The subset of the whole of the HMAS Cook survey data, used for the preparation of this report, includes survey lines south of 1S015'S and east of 17T'45'W (Figure 2). GLORIA data In this area (FIgure 3) were processed both at 10SDL, UK and at James Cook University, Australia, and are presented here together with a line drawing structural interpretation (Figure 4). GLORIA data In the Western Samoa area to the north are discussed In Hill and Tiffin (1991), and In the north Fiji area to the west, by Hughes Clark et al (1991).

RESULTS

Approximately 60% of the northern Lau Basin has been insonlfled using the GLORIA long range sklescan, and these data have been merged with the multlbeam bathymetry recorded simultaneously during the HMAS Cook survey. Those Seabeam data sets collected by Hawkins and others dUring the PAPATUA expeditions of Scripps Institution of Oceanography in the 1980's (SIO, unpubllshed data) have also been re-assessed. In addition, ail the available 12 kHz and 10 kHz echo-sounding data, and 3.5 kHz high resolution seismic reflection data have been compiled and incorporated with slnwe and multichannel seismic reflection records. With the GLORIA data, this has provided a comprehensive, partially mree-dlmenslonal reconnaissance framework which we here use to subdivide the northern lau Basin area into five morphotectonic domains (MTD), each of which Is characterised by a distinctive topography and surficial structural fabric. This scheme is useful to describe the variations In tectonism and volcanism throughout the basin. The tectonic style of each MTD differs from its neighbour and these contrasts have been interpreted to reflect variations in age and degrees of volcanism and tectonism. The five morphotectonlc domains are defined as follows, and their boundaries are shown In Figure 2. They are discussed In terms of their GLORIA Image, bathymetry, and seismic reflection characteristics.

[TR141 • Parson & Tlfflnl

[16J

1) A basement ridge and sedlmented Inter-ridge area in the southeastern portion of the survey area. 2) A highly tectonised sedlmented terraln In the northeastern Lau Basin located around a recenUy discovered triple junction (Nilsson at ai, 1989). called here the King's Triple Junction.

3) Deeply sedlmented terrain in the central northern tau Basin. 4) A linear neo-volcanlc extensional zone, hereln referred to as the Northwest lau Spreading

Centre, trending NNE-SSW through 16"S 177"30'W. 5) The northern Peggy Ridge.

Figure 5 shows the bathymetry over the northeastern study area.

The Basement Ridge and Sedlmented Inter.ridge Area

Striking contrasts in seafloor topography, reflectivity and geological setting occur In the eastern half of the study area (Figure 3). Eastwards from about 176030'W (excluding a triangular area around the King's Triple Junction, described later), this largest of the MTD's described here covers some 50,000 sq km of the northeastern lau Basin where the GLORIA data (Figure 3) record a series of broad bands with a mottled, generally strong, backscattering texture. The bands trend approximately N100W In the south, but diverge in an Irregular fashion to the north and northwest. These bands of multiple, discontinuous lineaments depict structural features: the brighter bands are usually ridges. These alternate with uniformly low-backscatterlng bands of comparable width, generally representing sedimented troughs which are sometimes punctuated with isolated reflective targets.

Bathymetric profiles show these multiple bright linear bands are usually found on ridges, some of which shoal to less than 1600 m water depth, which are characterised by steeply sloping planar flanks. The linearity of the features may be more apparent when the ship's track Is parallel to them as In the eastern part of the mosaic rather than crossing them orthogonally as In the south. Bathymetry and seismic profiles confirm a complex micro-topography dominated by partially coalesced pinnacles and Irregular highs, locally draped with minor sediment cover (Figure 6). Each of the ridges identified here has a characteristic pinnacled form and strikes approximately north-south

[TR141 - Parson & TlffinJ

[19)

These ridges are separated by flat-floored, deeply-sedimented basins, some more than 2800 m deep, In places interupted by scattered seamounts. In the north and central part of this

MTD, intense defonnatlon is associated with the emerging King's Triple Junction, described separately. The area west of 176°30'W contains ridges which are less well defined, sedimented areas which are less regJlar, and a style of deformation of the sediment basin fill which is less well constrained. We have accordIngly put the arbitrary domain boundary running roughly NNE from about 176030'W (Figure 2).

The ridges are bathymetrlcally complex, range in width between 7 km to more that 30 km, have convoluted, indented margins. and terminate towards the north, locally In a bifurcating and feathered form. For the purposes of this report, the ridges are Informally referred to as ridges A-D from east to west and the corresponding Inter-ridge troughs or basins are annotated according to their flanking highs, (Basin A/B, Basin B/C, etc, Agure 4).

Areas of uniform medium-to-Iow backscatter between the ridges are sediment filled basins, interrupted locally by Isolated, equant, highly backscatterlng targets, similar In size and morphology to some of the small constructional units resolved In the ridges, especially Ridge 0

(Figure 3). These are Interpreted as partially buried seamounts. Sedimentary sequences in the basins are locally up to 1.0 second two way travel time (TWT) thick (approximately 1 km). The upper 0.2 seconds is difficult to resolve due to interference from the airgun signal, but most of the remaining upper section Is seismically transparent to low frequencies. Some weak laminated reflections are observed. Locally, f1at-lying, well-defined events occur in the deepest parts of the basins, but the irregular basement itself is difficult to identify. Little evidence for recent deformation is observed, although minor faulting emerges at the surface of Basin B/C.

Ridge A, at the eastern limit of coverage, Is the least well-Imaged of the elevated topography and its form Is largely interpolated from an irregular line of weakly reflective targets aligned roughly along 174015'W. The ridge Is identified as the western side of the northern Tonga platform. A distinct contrast In acoustic texture between the flank of Ridge A and the more uniform west-sloping floor of the adjacent Basin A/B Is readily apparent. Bathymetry shows the shallowest point of Ridge A imaged is Just under 1800 m water depth.

The fabric of Ridge B extends northwards, diverging slightly from Ridge A, from south of our survey area to at least 15°45'S where it passes imperceptibly into the MTD associated with the King's Triple Junction. It is approximately defined by uneven, N-S striking boundaries traced at the junction between brightly backscatterlng, large-scale hummocky and pinnacled topography, and

[TR141 - Parson & Tiffin] 120J

the flat acoustic response of Basins A/B and BIC (Figure 4). A striking and persistent ridge-parallel Jlnear fabric characterises the central portion and extends over 100 km in a 15 km wide band. Seismic profiles and bathymetric data show these fabrics to be dosely-epeceo. semi-contlnuous fault scarps, with throws of 100m or less. The fauhs scarps alternate between east·facing and west-facing within the ridge, producing a crude micro-horst and graben fabric that we believe may be extensional In mode.

Northerly divergence of Ridges B and D, and the termination of the shorter Ridge C, results In a broadening of the Inter-ridge area BID to more than 90 km In width at around 15°45'8. This broad Inter-ridge area is dominated by structural elements of a triple junction, the second MTD described In the next section. The structural fabric of Ridge B appears to terminate against the triple junction, but to the north of 1Ef'S, a continuation of structural elements similar to Ridge B swings clockwise, thus widening the BID inter-ridge separation still further.

Ridge B near 1504O'S is intersected by a major ENE-striking rift valley which tapers to the west and terminates against Ridge B Just east of the triple junction. Eastward, the valley extends into the Tonga platform beyond our coverage (Figure 4). Steep inward-facing rift walls plunge steeply to the f1at-lying sediment fill on the valley floor. Seabeam and seismic reflection data on our eastern-most N-S transect of the valley (Figure 7), snow Its southern valley wall drops 1000 m vertically over 2 km, producing one of the steepest tectonic slopes identified in the area. The valley is greater than 2600 m deep, but deepens to a maximum depth of locally more than 2800 m In the west.

On the north side of the rift, a broad area of uniformly high backscatter on the GLORIA records is constrained at the rift margin by irregular lobate boundaries. A series of unusual concentric bright reflectors is centred in this area at 1S022'S, 174000'W (Figure 3). We Interpret this bright area as the surface of lava flows emanating from the vicinity of the concentric rings. The rings appear to result from a complex, collapsed volcanic edifice. This volcanic seamount is about 40 km west of the line of the active Tofua Volcanic Are, measured from Curacao Shoal near where Volcanic activity was reported in 19n.

Ridge A and Ridge B are separated in the area of the GLORIA image by a sediment filled, inter-ridge basin (Figure 4) in which the sediment surface slopes westward, away from Ridge A. However, about 20 km southward of the southern mosaic boundary, data from the 1964 cruise of the NBtsushima (Honza et al, 1985) Indicate Basin AlB exceeds 2200 m depth Just before Ridge B merges with Ridge A, the Tonga platform, terminating the basin.

[TR141 - Parson & TlffinJ

(22)

Ridge C Is imaged for less than 75 km, emerging from south of our survey area where It Is less than 1900 m deep. and deepening northward to approximately 1~30'S where it becomes submerged by the sediments of basins BIC and C/O. Morphologically It has similar topographic complexity to Ridge B, although apparently not cut by as dense a faulting fabric. While contrasting view directions of the sonar vehicle may contribute to this apparent difference, we consider the pervasive deformation of northern Ridge B is more likely a function of Its proximity to the evolving King's Triple Junction, discussed below. A sharp. 20 km westward offset of Ridge C appears to occur between lftl45'S and 16°55'8. but is not reflected in the flanking ridges, B or D. Pervasive planar sub-bottom reflections observed in seismic profiles across Ridge C (Figure 6) suggest that the ridge Is, at least In part, a sedimentary structure.

Ridge D follows a complex path as It widens northwards from 1~S, where It Is less than 20 km wide east-west. Curving northwestwards at Hf30'S, it bifurcates into two roughly equldimensional ridges (Ridge D-east and Ridge D-west) separated by a well defined sediment-filled basin opening to a maximum of 5 km In width (Figures 3 and 4). Linear, ridge-parallel reflectors heading roughly NW-SE along the ridge are tentatively Interpreted as fault fabrics, or possibly, aligned volcanic extrusive constructs. The central portion of Ridge D shoals to 1603m, the shallowest part of the MTD, where It corresponds to a highly complex topographic and structural pattern of superimposed peaks and troughs. Ridge D-east appears to veer northerly north of 1SOS and continues Into Nluafo'ou Ridge upon which sits the Island of the same name. This Island has had recent volcanic activity (MacDonald, 1948) and bright reflectors Imaged on the

GLORIA data to the northeast and southeast of the island are tentatively interpreted as Volcanic flows.

GLORIA and seismic data (e.g.- Figure 6) show that Basin C/D separating the ridges In the south Is narrow and sediment-filled, with the floor deeper than 2500 m. At the southern mosaic limit, the GLORIA data suggest that Ridge C merges with Ridge D. This is confirmed by data collected south of the mosaic by the 1984 Natsushima survey (Honza et at, 1985), which shows only one ridge In that area.

Basin B/C, also sediment filled, exceeds 2300 m depth and the 1984 Natsushima data show that It deepens southward to reach almost 3000 m In depth before It too terminates when two sets of merged ridges, C/D and A/B (Tofua Arc merged with Ridge B), meet just north of FonuaJellsianci (Figure 1; Chase, 1985). In the south, the combined ridges C/O, and the trough

B/C, have been Informally named Natsushima Ridge and Natsushlrna Trough In Honza et al (1985).

[TR141 • Parson & Tiffin] [231

The Kings Triple Junction and Surrounding Area

This triple Junction, first identified by Nilsson at al (1989), has been variously named the Nlua Fa'ou Ridge (Hawkins, 1986). Niuaro'ou Triple Junction (Tiffin at ai, 1990). Mangatola Triple Junction (NUsson et ai, 1989) and King's Triple Junction (Usltsl" at ai, 1990; Malahoff and Falloon, 1990). For the purposes of this report, we refer to it as the King's Triple Junction in honour of King Taufa'ahau Tupou IV, the King of Tonga.

The area around the Kings Triple Junetlon is intensely deformed and associated with complex neovolcanism. The MTD covers a triangular area (FIgure 2) of over 5000 sq km between 15°15'$, 174040'W, 16"20'5. 174050'W, and 15°15'5, 17So30'W which on the GLORIA data is traversed either by myriads of fine, parallel, and closely spaced linear reflectors or, more locally, by narrow bands of uniform, intensely backscatterlng seafloor (Figure 3).

South of 1504O'S, an arcuate array of these weakly fanning lineaments, which we interpret as normal extensional fault scarps, converges to the south, gently curves to the east, and heads into Inter-ridge Basin SIC, ending almost against the faulted terrain of Ridge B. The core of this splaying fabric In the north strikes about 195° and is defined by a narrow, jagged, and southerly tapering tongue of uniformly high backscatter (Figure 3). Bathymetric and seismic data dearly show these lineations are derived from many small offset faults with a maximum of about 50 m on Individual throws. SeaBeam data and acoustic profiles (Hawkins 1989, and unpublished Scripps Institution of Oceanography data) confirm the core of this area as axial rift, although Hawkins (pers comm 1990) dredged samples of manganese encrusted basalts from the suggested neovolcanlc area, and not fresh material. West of the axial core area, long, persistent, clearly defined, 165°-trending lineations can be seen In the adjacent sediment covered region, also trending toward Basin S/C.

Near the core area around 15°37'$, 174052'W, a superimposed set of northward strengthening, sub-east-west trending reflections from orthogonal lineaments cuts this southerly-striking fabric, and can be traced more than 30 km to the west. A weaker structural fabric continues into the sedimentary area to the west and reaches almost to Niuafo'ou Ridge, Just $E of the island. This fabric is interpreted as a nascent, possibly propagating, arm of a ridge~ridge-ridge triple junetlon. Within the eastern core segment of this arm, a westerly tapering zone of seafloor about 12-15 km wide has an Intense backscattennq fabric, similar to that described above for the southern arm, and we interpret this narrow enclave as further, east-west striking neovolcanic axial floor. A 9 km x 9 km area centred on 15°37'$, 174051'W (Figure 8),

[TR141 - Parson & Tlffln1

[25]

closely surveyed by the RV Akademik Alexander Keldysh prior to submersible dives (L1sitsin at al, 1990; Malahoff and Falloon, 1990), has bathymetric trends at 0150 crosscut by a valley trending 2550 which may be the central axial rift of the E-W arm. Dives made on two sites at the triple junction by MIR submersibles from the Keldysh recovered very fresh aphyric lavas, glass and dacite, and observed extensive volcanic fields of fresh pillow, tube, and sheet flows cut by an extensive system of fracturing (Malahoff and Falloon, 1990). Although no active hydrothermal venting was seen, manganese coatings on pillow lavas were taken as Indicative of recent nearby hydrothermal activity.

A secondary set of weaker lineations, less well defined and truncated by the later sub-east-west lineations, strikes west-northwest from near the triple junction. These lineatIons appear to cut the Niuafo'au Ridge north of Niuafo'ou Island and offset it to the east, whereas the newer set, described above, does not quite reach the ridge.

North from the triple junction, a pervasive 0250 fabric dominates, extending NE about 70 km. This broad, brightly backscanering band is Interpreted as the third (NE) axial arm of the

King's Triple Junction. The axial fabric of the NE arm, defined by weak, uniformly bright lineations, Is about 18 km wide adjacent to the triple Junction, and narrows to the NE, tapering to a narrow slot near 15°07'8. Throughout the neighbourhood of the triple junction, fault lineations range in frequency and amplitude from finely-spaced bundles less than 100 m apart, with no resolvable throws, to major continuous scarp faces, downthrown more than 500 m towards the adjacent axes. Subparallel reflectors east of the axial zone, possibly a northern continuation of Ridge B with a north-northeasterly trend, continue to the Tonga Trench to eventually intersect the inner wall at a deep re-entrant where bonlnite lavas were dredged by the Natsushirna (Falloon and Crawford, 1991).

Short "whale-back" ridges on the northwestern side of the NE arm raise the axial valley floor by a few tens, locally hundreds of metres. It was on the flank of one of these raised portions of the axial floor that Hawkins and Helu (1986) reported dredging hydrothermal sulphide chimney deposits during petrological sampling. The RV Keldysh made one dive in this vicinity with one of its MIR submersibles and reported an extensive, recently active hydrothermal field of low temperature depos1ts with coalesced chimneys up to 10 m high. Sulphides high in copper, zinc and iron were sampled.

In summary, throughout this MTD a fine, linear fabric is developed approximately subparallel to the flanking ridges. On the GLORIA mosaic, the fabric comprIses discontinuous

[TR141 - Parson & Tiffin] [26]

linear reflectors 10 km or less in length, spaced In the south between 3 and 5 km but becoming more closely spaced to the north. On seismic reflection profiles, these lineaments are confirmed as normal dip-slip fault structures, with throws of a few tens of metres. Overall, the GLORIA data suggest a crescentic shape for the north-south arms 0# the triple junction, with a weak, short, sub-orthogonal WSW arm and an older, short and weak WNW arm.

The Sedlmented Block Terrain In the Central North Lau 8.sln

A topographIcally complex and highly varied area of the northern Lau Basin covering some 12,000 square kilometres is clearly bounded on its northwestern side by a brightly backscatterlng linear zone interpreted as a newly discovered neovolcanic spreading centre (Figure

3), the Northwest tau Spreading Centre (NWLSC), and on its southwestern side by Peggy Ridge, variously described as a transform fault (Sclater at el, 1971), a spreading axis (Chase, 1971) or a leaking transform fault (Lawver et ai, 1976). More recendy, It has been shown by GLORIA data to be a transtensional feature, at least In Its southeastern limit, by Parson et aI (1990b). In the east, the boUndary with the ridge/inter-ridge MTD described previously is not well defined. Even backscatter over most of this MTD is interpreted to represent a sedlmented basin, or Interlinked basins (Figure 3).

Two broad, irregularly shaped areas of hummocky and plnnacled terrain occur withIn the basin. The first occupies much of the southeast of the area and this we provisionally refer to as

Ridge E. On GLORIA Imagery, It has the appearance of a ridge partially buried in sediments, and thus looks older than the ridges described previously. The second, smaller area, extends eastward from the NWLSC as a band of roughly east-west elevated topography which includes Zephyr

Shoal and Donna Seamount.

Little or no linear fault scarp fabrics are observed in these hummocky ridge areas, in contrast to the ridges further east, but, within the sedimented basin, extensive and complex Intersecting fault scarps occur (Figure 4). North of Donna Seamount, around 1a<'00'S, 17a<'20'W, clusters of short fault lineaments at 120° azimuth are truncated by roughly east-west faults. At 1602O'S 176°35'W, east-west faulting cuts obliquely across 060° striking fabrics; and around 16"20'8 1no05W, complex (mutually cross-eutting) intersections between faults trending 135° and 170° are discernible. The most striking faults are the arcuate, concave-northward and north-facing escarpments located in the west and central parts of the basin where they subtend a 3500 m deep area just south of Donna Seamount, centred at 1a<'15'S, 176°4Q'W. cumuenve faut

[TR141 - Parson & Tiffin] [27J

throws exceed 1700 m from the surrounding e'evated areas to the central deep. The deep area is bounded on the west by a sma/I seamount, possibly flat-topped.

Despite 100% insonlflcation of the area, this part of the northern Lau Basin Is one of the

most difficult to interpret satisfactorily. A full Interpretation of the complex tectonic pattern requires more bathymetric and deep seismic data.

The NO

A brlghtly-backscatterlng seafloor striking approximately 04

(Figure 3), bounds the western margin of the block terrain described above. and extends north

and westward beyond this study area. It covers some 4,000 sq km of seafloor within the GLORIA mosaic. and is terminated on the south by the broad, elevated. linear Peggy Ridge, where it is about 40 km wide. The intensity of backscatter Is the highest and most extensive recorded during the lau Basin survey, and we interpret the area as probably bare or lightly seclmented neovolcanlc basement rock. Its very finely IIneated GLORIA Image is identical to typical axial valley

floors of medium- and slow-spreading ridges described elsewhere (Parson and Searle, 1986;

Parson et ai, 1992). Portions of seismic reflection profiles across this area clearly display sub-planar strata at depth (Figure 9) suggesting that magmatism Is at an early stage, and break-Up has not progressed far. Compilations of magnetic data (Malahoff at ai, in press; Murthy, 1990; Parson at ai, in press) show a strong positive magnetic lineament corresponding to this zone. We

suggest this area is a recently initiated spreading segment and reter to it as the Northwest Lau Spreading Centre (NWLSC) (the Northwest tau Basin Volcanic Zone (NWLBVZ) of Hughes Clarke et ai, 1991).

AlthougJ the northern limit is not known, Its southeastern edge, and the fabric within it, strike approximately04Qo, trending directly toward RochambeaU Bank, a large shoal area approximately 75 km farther to the NE (Figure 1). High heat flow in the area of Rochambeau Bank, and recovery there of glassy basaltic pillows With fresh surface textures (Sclater and others 1972) indicate recent volcanism, and suggest that the NWLSC may reach at least that far to the NE, if not to the western end of Tonga Trench itself.

The overall morphology of the NWLSC and the distinctly Iineated character of the main

"axial zone" is not as straightforward to Interpret as the backscattering pattern may initially suggest. In particular, the eastern flank of the NWLSC Is locally marked by a steep weSl-tacing

[TR141 - Parson & Tiffin)

[29]

fault scarp, and this ranges along strIke to a maximum of 1000-1250 m. Outside of the scarp, the axial valley shoulders are characterised by a series of apparenUy unconnected, steep-sided basement(?) blocks, elevated more than 1000 metres above the flanks of the rift. The upper surfaces of these blocks are roughly rounded, rather than pinnacled. and their flanks while steep, are distinctly concave rather than planar. This blocky terrain extends along the eastern margin of the NWLSC, continuing east to link up with the very shallow area of Zephyr Shoal at 15°50'8 176045'W. Fault fabrics seen in the western part of the adjacent sedlmented block MTD, are absent.

The Northern Peggy Ridge

The northern flank of Peggy Ridge Is characterised by highly reflective areas of predominantly fiat-lyIng seafloor (interpreted as sheet flows), punctuated by small volcanic constructional ridges, and Irregular seamounts or "haystack" type pillow constructs. Parson et 81 (1990b) first recognised these features around the southeastern flanks of Peggy Ridge during an earlier GLORIA survey (Parson et ai, 1990a). Their occurrence here supports the earlier suggestions of Peggy Ridge as a transtensional ("leaky") feature.

At our shallowest crossing, Peggy Ridge Is less than 1250 metres deep, and has a ribbed and rugged upper surface characterised by clusters of linear peaks and troughs. The northeastern flank Is a steep, faulted margin, downthrown near1ya kilometre to a valley about 2500 metres deep. The eastern wall of this valley is a steep-sided ridge, striking subparallel to, and a few kilometres distant from the main Peggy Ridge structure, but less elevated, averaging around 1800 m depth. This latter ridge, possibly spa/led from the main structure, appears to continue southeastward from the NWLSC. GLORIA data for these two ridges are subtly but, we feel, possibly significantly different. Although the main Peggy Ridge structure is crudely llneated, it appears to be dominated by a superimposed seamount texture typlfled by the ridges further to the east. The subparallel subordinate feature Is more finely ornamented and Iineated, suggesting some affinity to the locally fresher textures observed in the floor of the NWLSC.

DISCUSSION

While division of the stuely area into five morphotectonlc domains has been to some extent arbitrary, it permits the direct comparison of tectonic style and geological evolution for this

(TR141 - Parson & Tiffin) [30J

highly complex and varied terrain. The results are Illustrated by a morphotectonlc regional summary of the Lau Basin (Figure 10) derived from this GLORIA data and that of Parson at aJ

(l990a), and from Seabeam data, including that by Hawkins (1989) and others.

The evolution and segmentation of the back-arc north of Peggy Ridge has been a subject of conjecture by many authors. but has not previously been recognised. The new data presented

here provide a clearer picture of much of the northern Lau Basin, and indicate a present focussing of magmatic extension at the NWLSC and at the King's Triple Junction.

Uttfe data Is available between 1-,0 and 18°30'8, especially near the Tonga platform, but south of 18°30'$, an approximately north-south fabric has been identified. The interpretations there

(von Stackelberg and von Rad, 1990; Parson et ai, 1990b) suggest that there is a basic difference between the southern and northern areas.

The basement ridges of the north diverge from the Tofua Are, suggesting that they are genetically similar to It, or that extensive volcanic sedimentation from the arc has filled in the

Intervening Valleys and buried the ridges under the arc platfonn. We are uncertain as to the exact origin and evolution of the ridges, but there are some similarities between the tectonic style of

much of the northern lau Basin topographic fabric and that described by Parson at al (1992a) for the western and southern centraiLau Basin. There, broad north·south shoal ridges, often continuous over many tens of kilometres, separate sedimented basins of similar dimensions to those described above. Deep drilling results (Parson et ai, 1992b) have proved that the horst and graben topography there results from extensional rifting of a basement complex that was derived from either a fragmented and remnant forearc or an arc complex, or a hybrid of both. South of Peggy Ridge, this process is thought to have been operable dlJing the earliest stages of evolution

of the Lau Basin (and perhaps many other marginal basins). The comparable topography seen throughout much of the southern Lau Basin Is a possible analogue for this northern rift terrain.

The ridges have dlffering structural styfes. Ridge E; is broad, diffuse and, In Its eastern part, has the appearance of an older and more heavly sedimented ridge than the others. Ridges C and

D are also sedimented, but are not as broad as Ridge E. These two ridges continue south of the study area where they join before eventually merging with the Tonga platfonn. Ridge B has less sediment cover despite its proximity to the active arc and therefore may be younger than the other ridges. It too, merges with the Tooga p1attonn south of the study area.

[TR141 - Parson & Tiffin J

[32J

In the north, the structural fabric of Ridge B terminates against two other features: a deep transverse trough on the east that cuts the Tofua Arc near Its northern end, and the King's Triple

Junction on the west. North of the transverse trough, a fabric slmlar to Ridge B resumes, but with a N1Soe trend. This latter fabric continues to the northern Tonga Trench where It incises the inner wall of the trench.

Ridge A, the Tonga platform, is in this area dominated by the active Tofus Arc.

The data suggest that the r1dgejinter-rldge structures In the eastern half of the study area have been established for some time but are now Inactive, undergoing relatively little current deformation except in the area of the King's Triple Junction, where they are intensely defonned.

The Inter-ridge basins are mostly f1at-lylng, affected mainly by sedimentation from the nearby Tofua Arc and localised mass-wasting from the ridge flanks. Seismic profiles show little evidence of current extensional or compressional strain In the basins.

The diffuse extension in the vicinity of the developing King's Triple Junction appears to

Increase In magnitude towards the rifted margin of the Lau Basin. The Initiation and tectonic evolution of the triple Junction, and In particular the western arm, may be related to a number of factors. These may Include. for example, the Influence of the strike-slip component of motion between the Pacific Plate and the Indo-Australian Plate at the northern margin of the Lau Basin, and Its coupling with the trench-orthogonal back-arc component of extension derived from the descent of the Pacific Plate beneath the Tonga platform at the leading edge of the Indo-Australian

Plate. The sharp westward swing of the Tonga Trench places unsubducted Pacific Plate adjacent to the tau Basin both to the east and to the north (Figure 1). Transform motion between the plates occurs along the northern margin of the Lau Basin; subduction occurs to the east. An increasing amount of deformation, possibly ductile rifting and eventual tearing must take place to the Pacific

Plate Immediately beneath the margin of the NE Lau Basin as the southern part subducts and the adjacent northern part does not.

The effects of this deformation across the complex northern plate junction cannot be predicted, but we suggest It may have a directional Influence on extension of the overriding

Indo-Australian Plate. Interaction of these coupled stress regimes may cause the complex asymmetrical stress that results In the arcuate, concave-eastward form of the triple Junction. An increased extensional component on the western side of the curved deformation zone, due to Its crescentic shape, must have caused the initiation of the third arm. It Is further suggested that this third (western) arm Is propagating In the general direction of Niuafo'ou Island, an island with

[TR141 • Parson & TiffinJ [33J

historic erruptlons of lavas of ocean floor affinity (Reay and others 1974) and clearly associated with recent submarine volcanism.

Seismic data for shallow earthquakes In the northern Lau Basin (Hamburger and Isaacs. 1986) further support an Interpretation of the extension of a tectonic lineament westwards from the King's Triple Junction. A tightly constrained line of events of magnitude 4.5 or more marks the focus of seismic aetivlty from the triple junction to a point roughly located at 15°00'5, 176030'W, near Aochambeau Bank. At this point the activity swings southward to align with our interpreted position of the NWLSC. Tentative Interpretations of magnetic anomalies in the northern-most Lau

BasJn (Parson, In prep) suggest easterly stepping segments of spreading axes lay between the King's Triple Junction and Rochambeau Bank.

Negligible shallow earthquake activity in the central northern Lau Basin Indicates this area is virtually aseismic, presently without neotectonic activity, but it is flanked by active transform structures, neovolcanic zones or propagating rifts (structures associated with the King's Triple Junction, NWLSC, and Peggy Ridge). The extensional mechanisms referred to above, relating to stress possibly transferred from the complex deformation of the subducting Pacific Plate, thus appear to result in the fragmentation of the overriding plate edge Into sub-plates, including possibly one microplate, north of Peggy Ridge.

MINERAL RESOURCE POTENTIAL OF THE NORTHERN LAU BASiN

Neovoicanic zones are normally areas of high heat flow where hydrothermal circulation and minerai sulfides are possibly present. Neovolcanlc areas in the northern Lau Basin occur in at least two of the MTD's described: the Northwest Lau Spreading Centre, and the King's Triple Junction.

Hydrothermal sulfides have already been confirmed In the area of the King's Triple

Junction. Fragments of sulfide chimneys were recovered in a dredge at a site on the northeastern limb of the triple junction (Hawkins and Heiu, 1986) in 2100 m of water, and a submersible dive on the site by a MIR submersible from the RV Akademlk Alexander Ke/dysh (Malahoff and Falloon,

1990) confirmed the existence of lron- and copper-rich (40% Fe, 30% Cu) mineralisation In the area. The extent of mineralisation at the site is not yet known, and other nearby areas of mineralisation probably occur and have yet to be discovered. There may be other sites on the southern limb and near the core of the triple junction itself: two submersibJe dives on the core

[TR141 - Parson & Tiffin] [34]

area did not find active venting but saw evidence for nearby hydrothermal activity. Much more work is needed In the area to determine the nature of the deposits, their size, and similarities or differences with deposits in the southern Lau Basin (von Stackelberg and von Rad, 1990) and other back-arc regions.

The NWlSC has not been investigated for mineralisation. and It Is not known if sulfide mineralisation occurs there or not. This area should be Investigated further by water geochemistry and bottom sampling, and the location of the spreading centre should be mapped northward beyond our present su rvay area.

[TR141 - Parson & Tiffin) [35]

CONCLUSIONS

1) The western arm of the King's Triple Junction links to a proposed extension of the

Northwestern lau SpreadIng Centre via Nluafo'ou Island to subtend a Lau Basin "mlcroplate" bounded on its southern flank by Peggy Ridge. A somewhat similar microplate was Inferred from seismicity by Pelletier and Louat (1989).

2) The western arm of the King's Triple Junction Is a pull-apart rift which Is derived from

convex extension on the two other arms of the triple junction.

3) Peggy Ridge displays a right-lateral strike-slip motion with a component of east-west extension derived from back-arc extension, thus Inducing a leaking aspect.

4) The Northwest lau Spreading Centre is a recent (perhaps only 1 Ma) diffuse zone of magmatic accretion superposed on "old" sedlmented horst and basin terrain evolved

during earliest rifting and formation of the Lau Basin.

S) Sulfide mineralisation known to occur at the King's Triple Junction in the northeastern lau

Basin should be further investIgated. The Northwest lau Spreading Gentre should also be investigated for occurrence of minerals.

6) Northern lau Basin extension is now being accommodated In two main areas of spreading, but in the past was largely by formation of pull-apart basins and local magmatism.

7) MultJ-ehannel seismic profiles are needed in the sedimented block terrain to determine Its structure and depth. Sampling is needed In the areas of neovolcanism and on the ridges to determine their age, nature, and origin.

8) DetaHed bathymetry and Imagery are needed, especially to map the northeastern extent of the NWLSC, and determine Its relationship to the western arm of the King's Triple Junction, and to the northern Tonga Trench. Mapping of the southern extent of the

ridge/inter-ridge terrain Is also needed to Investigate its relationship to the Tofua

ArcjTonga Platform.

[TR141 - Parson & Tiffin] [36]

REFERENCES

Berggren, W.A., Kent, D.V., Flynn, J.J., and van Cowering, J.A., 1985: Cenozoic Geochronology. Geological Society of America, Bulletin 96: 1407·1418.

Chase. C., 1971: Tectonic History of the FIJIPlateau. Geological Society of America, Bulletin 82: 3087-3110.

Chase, T.E. 1985: Submarine topography of the Tonga-FIJI region and the southern Tonga platform area. In Scholl, D. and Vallier, T. J. (ads): Geology and Offshore Resources of Pacific Island Arcs· Tonga Region. Circum-Pacific Council for Energy and Mineral Resources Eat1h Sciences Series, 2: 21-22.

Cherkis, N.Z., 1980: Aeromagnetic Investigations and Seafloor Spreading History In the lau Basin and North FIJIPlateau. UN ESCAP COOPjSOPAC Technical Bulletin 3, pages 37-45.

Collier, J. and Sinha, M., 1990: Seismic Images of a Magma Chamber Beneath the Lau Basln Backarc Spreading Centre. Nature, 346, 6285, pages 646-648.

Eguchl, T., 1984: Seismotectonics of the Fiji Plateau and Lau Basin, Tectonophysics, 102: 17-32.

Hamburger, M.W. and Isaacs, B.L, 1988: Diffuse Backarc Deformation In the Southwestern Pacific. Nature, 332: 599-604.

Hawkins, J.W., 1974; Geology of the Lau Basin, a marginal sea behind the Tonga Arc. In Drake, C, and BUrke, C. (Eds), Geology of Continental Margins, Springer· Verlag, Berlin, 505-520.

Hawkins, J.W., 1988: Cruise aeport . PAPATUA Expedition, Leg 04, RV Thomas Washington. Scripps Institution of Oceanography Reference Series, Reference 88-14.

Hawkins, J.W., 1989: Cruise Report - ROUNDABOUT Expedition, Legs 14, 15, RV Thomas Washington. Scripps Institution of Oceanography Reference Series, Reference 89-13, 51 pages.

HaWkins, J.W. and Helu, 5., 1986: Polymetallic Sulfide Deposit from "Black-smoker" Chimney, Lau Basin. EOS, Volume 67, page 378.

HIli, P.J., and Tiffin, DL, 1991: Marine Geology and Geophysics of the Western Samoan Exclusive Economic Zone: Results of GLORIA, SeaMARC II, and Other Shipboard Studies. SOPAC Technical Repon 135.

Honza, E., Lewis, K.B., and Shipboard Scientific Party, 1985: Marine Geological and Geophysical Survey of North Tonga Ridge and Adjacent Lau Basin. CCOPjSOPAC Cruise Repon 105, 29 pages.

Hughes Clarke. J.E., Jarvis, P., Price, R., Kroenke, L, 1991; Tectonic Activity and Plate Boundaries Along the Northern Flank of the Fiji Platform. SOPAC Technical Repon 127, 45 pages.

Joshlma, M., Okuda, Y., Miyazaki, T., and Yokokura, T., 1985: Magnetic Anomoly and Heat Flow Measurements in the Tonga Arc and Lau Basin. In Honza, E. and Lewis, K B. (Eds.): "A Marine Geological and Geophysical Survey of the Northern Tonga Ridge and Adjacent Lau Basin". Mineral Resources of Tonga Fjeld Report 1, pages 45-47.

[TR141 - Parson & Tiffin) • [37J

Karlg D.E. 1970: RKtges and Basins of the Tonga-Kermadoo Island Arc System. Journal of Geophysical Research 75: 239-254.

Kroenke, L., 1984: Cenozoic Tectonic Development of the Southwest Pacific, with a contribution by Peter Rodda. UN ESCAP, CCOPjSOPAC Technical Bulletin 6. 122 pages.

Lawver, L. Hawkins, J. W., and Selater, J. G., 1976: Magnetic Anomalies and Crustal Dilation in the Lau Basin. Earth and PJanetaJy Science Letters, 33: 27-35.

Usitsln, A" and Shipboard Scientists, 1990: Report of the 21st Cruise of the RV Akademlk AJexandr KeIc/ysh. Moscow (In RUssian). Five volumes.

MacDonald, G.A., 1948: Notes on Niuafo'ou. AmerIcan JoumaJ of Science, 246: 55-n.

Malahoff, A., Faden, A.H., and Fleming, H.S., 1982: Magnetic Anomalies and Tectonic Fabric of Marginal Basins North of New Zealand. Journal of Geophysical Research, 87: 4109-4125.

Malahoff, A" and FaUoon. T., 1990: PreUminary Report on Northern UlU Basin Dive Program with the MIR Submersibles, Project MIRLAU. SOPAC Cruise Report 137, 27 pages.

Malahoff, A., Kroenke, L. W., Cherkis, N., and Brozena, J., (In press): Magnetic and Tectonic Fabric of the North Fiji Basin and Lau Basin, In, Kroenke, L., and Eade, J.V., (Eds) Basin Formation, Ridge Crest Processes, and Metallogenesis in the North Fiji Basin. Circum-Pacific Council for Energy and Mineral Resources, Earth Sciences Series, Houston, Texas.

Morton, J.L, and Sleep, N.H., 1985: Seismic Reflections from a Lau Basin Magma Chamber. In Scholl, D. and Vallier, T.J. (ads): Geology and Offshore Resources of Pacific Island Arcs - Tonga Region. CIrcum-Pacific Council for Energy and Mineral Resources Earth Sciences Serles 2: 441-453.

Mitchell, N.C. and Somers, M.L., 1989. Quantitative backscatter measurements with a long range sidescan sonar. IEEE, Journal of Oceanic Engineering, 14: 368-374.

Murthy, K.S.M. 1990: Seafloor Spreading MagnetJc Uneatlons in Lau Backarc Basin, Southwest Pacific. Institute of Oceanographic Sciences Deacon Laboratory Technical Report, March 1990, 13 pages + figs.

Nilsson, K., Flarendo, F.F., and Hawkins, J.w., et aI., 1989: Petrology of a Nascent Triple Junction, Northeastern Lau Basin. EOS, Transactions of the American Geophysical Union, Abstracts, 70: page 1389.

Parson, LM. and Searle, R.C. 1986: Strike-slip Fault Styles in Slow-slipping Oceanic Transform Faults - Evidence from GLORIA Surveys of Atlantis and Romanche Fracture Zones. Journal of the Geological Society of London, 143: 757-761.

Parson, LM. and Scientific Party, 1990a: RRS Charles Darwin Cruise 33/8,5 May-1 June, 1988. GeophysJcaJ and Geological Investigation of the Lau Back-arc Basin, SW Pacific: Institute of Oceanographic Sciences Deacon Laboratory Cruise Report 206, 28 pages.

Parson, LM., Pearce, JA, Murton, B.J., Hodkinson, A.A. and Scientific Party, 1990b: Role of Ridge Jumps and Ridge Propagation in the Tectonic Evolution of the Lau Backarc Basin, Southwest Pacific. Geology, 18: 470-473.

(TR141 - Parson & Tiffin] [38J

Parson, L M., Hawkins, J. W., and Hunter, P.J., 1992a: Morphotectonics of the Lau Basin seafloor • Implications for the opening history of backarc basins. In: Parson, LM. Hawkins, J.W. Allan, J. and ScientifJc Party, 1992b: Proceedings, Initial Reports (Pt A), Ocean Drilling Program, Leg 135.

Parson, LM. Hawkins, J.W. Allan, J. and Scientific Party, 1992b: Proceedings, Inltlal Reports (Pt A), Ocean Drilling Program, Leg 135.

Parson, LM., Patriat, Ph., Sealte, A.C., and 8rials. A.A., 1992: Segmentation of the Centrallndlan Rktge between 12°12'5 and the Indian Ocean Triple Junction. Marine Geophysical Researches.

Pelletier, B., and Louat, R., 1989 : Seismotectonics and Present-day Belatlve Plate Motions in the Tonga-lau and Kermadec-Havre Region. Tectonophysics, 165: 237-250.

Reay, A" Rooke, J.M., Wallace. R.C., and Whelan, P., 1974: Lavas from Niuafo'ou Island, Tonga, Resemble Ocean-floor Basalts. Geology, December, 1974. pages 605-606.

Sclater, J.G., 1972: Heat flow and elevation of marginal basins of the western Pacific. Journal of Geophysical Research, 77: 5688-5696.

Sclater, J.G., Anderson, R.N., and Bell, M.L., 1971: The elevation of ridges and the evolution of the eastern central Pacific. Journal of Geophysical Research, 76, 7888-7915.

Sclater, J.G., Hawkins, J.W., Mammerlckx:, J., and Chase, C.G., 1972: Crustal extension between the Tonga and Lau Ridges: Petrologic and geophysIcal evidence. Geological Society of America, Bulletin 83: pages 505-518.

Somers, M.L, carson, R.M., Revle, J.A., Edge, R.H., Barrow, B.H. and Andrews, A.G., 1978: GLORIA II - An Improved long-range sidescan sonar. In 'Proceedings, IEEEjlERE Subconterence on offshore instrumentation and communicatIons'. London, B.P.S. Publications Ud., page. J16-J24.

Tiffin, D.L, Oarke, J.E.H., Jarvis, P., Johnson, D., Hill, P., Huggett, c, Parson, L., Price, R., 1990: ceoP jSOPAC GLORIA survey: Port VUa-Apia-Port Vila on HMAS Cook, 1 August - 1 September 1989. SOPAC Cruise Report 130, 45 pages. von Stackelberg, U. and von Roo, U., (Eds.), 1990: Geological and Hydrothermal Activity in the tau and North Fiji Basins. GeoJogisches Jahrbuch. Reihe D. Heft 92. Hannover. 660 pages. von Stackelberg, U. and Scholten, J., 1990: Interpretation of Bathymetry Data and Underwater Photographs In Lau and North Fiji Basins, Southwest Pacific Ocean. In: von Stackelberg, U. end von Aad, U., (Eds.), "Geological end HydrothElfTTlalActMty In the Lau and North Fiji Basins·. Geologlsches Jahrbuch. Reihe D. Heft 92. Hannover, pages 93-109.

Welssel, J.K., 19n: Evolution of the Lau Basin by the growth of smail plates. In: Talwani, M. and Pitman, w.e. (Eds.): "Island Arcs, Deep Sea Trenches and Back-Arc Basins". American Geophysical Union, Washington DC, pages 429-436.

[TA141 - Person & TlffinJ