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).