Natural, Resources of Tonga, Field Report No. 1 CCOP/ SOPAC Cruise Report 105

A MARINE GEOLOGICAL AND GEOPHYSICAL SURVEY OF THE NORTHERN TONGA RIDGE AND ADJACENT LAU BASIN

By Eiichi HONZA (Geological Survey of Japan) Keith B. LEWIS (New Zealand Oceanographic Institute)

and Shipboard Party

Ministry of Lands, Survey and Natural Resources of the Kingdom of Tonga

S. Tongi1ava, Superintendent

1985

CONTENTS

Chapter Page

I Outline of Natsushima 84 Cruise 1 K.B. Lewis, Eiichi Honza II Tectonic setting of the northern Tonga Arc and Lau Basin : Background to the Natsushima 84 Cruise 19 K.B. Lewis III Seismic profiles from northern Tonga 31 T. Kitekei'aho, D. Tappin K.B. Lewis Eiiahi Honza, Yoshihisa Okuda Teruki Miyazaki, Takanobu Yokokura IV Seismic profiles from the Fiji area 37 Riahard HoLmes, Ambika Prasad Eiiahi Honza V Notes on bathymetric data from the northern Tonga Ridge and Lau Basin C. J. Jenkins 43 VI Magnetic anomaly and heat flow measurements in the Tonga Arc and Lau Basin 45 Masato Joshima, Yoshihisa Okuda Teruki Miyazaki, Takanobu Yokokura VII Deep-towed sonar and camera observations at the southern end of Peggy Ridge 49

Hiroyasu Monma, Takeo Tanaka Toshio Tsuahiya, Katsura Shibata VIII Observations of seismicity by a pop-up type OBS array in the Lau Basin 59 Yukio Fujinawa, Takao Eguahi, Takeji Sasanuma IX Preliminary petrology and geochemistry of igneous rocks from the northern Tonga Ridge and adjacent Lau Basin 63 T.J. FaHoon X Late Cenozoic sedimentary rocks dredged from the north Tonga Ridge - preliminary analysis (With an Appendix on dewatering foliation in a forearc sandstone) 73 C.J. Jenkins XI Preliminary log of northern Lau Basin - Tonga Ridge piston cores C.J. Jenkins 85 XII Remnant magnetisation in piston cores from the Lau Basin. 93 Masato Joshima Chapter Page

XIII Preliminary nannofossil ages of samples from Natsushima 84 Cruise S. Shafik 97

XIV Foraminifera micropaleontology of samples from northern Tonga 99 B. W. Hayward

XV Surface sediment samples from northern Tonga - a preliminary grain size and S.E.M. analysis 103 K.B. Lewis, J.S. Mitchell

XVI Sea bottom photographs at sampling stations in the Tonga Arc and Lau Basin 113 Masato Joshima

XVII positions of Niuatoputapu and Tafahi Islands as measured by satellite navigation systems 123 Eiichi Honza

XVIII Resource potential of new data from northern Tonga 125 T. Kitekei'aho, Yoshihisa Okuda 1. I. OUTLINE OF NATSUSHIMA 84 CRUISE

K.B. Lewis New Zealand Oceanographic Institute Eiichi Honza GeologicaL Survey of Japan

SUMMARY

On 14 November 1984, the M.S. Natsushima sailed from Suva, Fiji, with 21 scientists from Japan, Tonga, Fiji, New Zealand, Australia, and CCOP /SOPAC on the second of three annual cruises in the Indian-Pacific Plate Boundary Arc Study (IPPBAS).

During the next 30 days extensive multichannel and single channel seismic surveys were made of the northern parts of the Tonga Ridge and Lau Basin, with transit lines through the Koro Sea and passages of the Lau Ridge. A proton magnetometer was towed throughout. Ocean bottom seismometer, deep-tow sonar, deeptow camera, piston core, and heat flow programmes were successfully completed in the Lau Basin. Dredging operations collected both sedimentary and igneous sequences on the forearc and northern termination of the Tonga Ridge. A free-fall grab with camera recorded the seafloor sediment at most stations. All equipment operated reliably and none was lost, although attempts at coring and measuring heat flow in compact volcanogenic sands on the Tonga Ridge were unsuccessful.

Preliminary results indicate that a volcanic chain coincides with the axis of a line of deep (> 2 sec) sedimentary basins at the crest of the Tonga Ridge. The basin sediments unconformably overlie older sedimentary sequences, which outcrop at knolls of Lower Pleistocene to Upper Miocene volcanogenic and calcareous sandstone. The northern transform termination of the ridge is characterised by a complex igneous suite of fresh vesicular pillow basalts, pre-Pleistocene pyroclastics and basal ts, and a problematical group of granodiorites and serpentinites. No mineralisation or hydrothermal activity was observed in detailed surveys at the southern end of the Peggy Ridge in the central Lau Basin.

A transit stop was made at Vava'u in northern Tonga, and the ship returned to Suva on 13 December 1984.

INTRODUCTION

The Tonga Arc, with its associated backarc Lau Basin, has become recognised as the type example of a simple arc system. Only the Marianas Arc is closely comparable. Understanding its structure and the processes that control its development are critical in the interpretation of more complex arcs and of fossil arcs that now form fold belts on land. In addition, small seeps of thermally mature hydrocarbons on the central Tonga Ridge and an awareness of the potential for mineralisation at backarc spreading centres has increased the significance of exploration in the Tonga area.

Recent exploration by oil companies, studies by geoscience institutions, and major intergovernmental programmes in resource evaluation have ensured that the structural processes and resource potential of central and southern parts of the Tonga Ridge and Lau Basin are becoming reasonably well known. Research can now concentrate on specific targets. However, the northern end of the Tonga Arc remains largely unexplored and much less well understood.

The present survey was designed to focus on several major problems related to the development of sedimentary basins on the northern Tonga Ridge, the nature and position of fracture zones and active seafloor spreading in the northern Lau Basin, the relationships of structural and stratigraphic elements with contiguous elements to the south, and the nature of the termination of convergence tectonics and conversion to transform at the northern end of 2.

of the arc system. In addition, surveys were made of structures in the Koro Sea and on the Lau Ridge in transit to the principal study area. The project design includes data collection of significance for regional mineral resource studies in both Fiji and Tonga.

The North Tonga Ridge - Lau Basin Survey is a critical part of the Indian-Pacific Plate Boundary Arc Study (IPPBAS), an international programme to define the structure and evolution of key segments of convergent plate boundaries in the southwest Pacific Ocean and northeast Indian Ocean. The study was initiated in 1982, by several Japanese scientific agencies with an interest in the marine geoscience of convergent plate margins, as a cooperative venture with similar agencies in the Southwest Pacific, Australia, and Indonesia. Participation was invited in both the planning and execution of the programme in countries throughout the region.

The IPPBAS Programme involves three major surveys. The first to the Solomon Sea and adjacent Bis- marck Sea, was successfully completed in January 1984 (Honza, Keene and shipboard scientists 1984). It involved geoscientists from Japan, Australia, Papua New Guinea, and the Co-ordinating Committee for Offshore prospecting in the South Pacific (CCOP/SOPAC) Office in Suva, Fiji. The data collected has already provided new insights into this region's evolution and a significant reappraisal of its resource potential and offshore geological hazards. The present study comprises the second phase of the programme. A third study, of the east Java Trench and Sunda Arc is planned for late 1985.

The programme is co-ordinated by the Geological Survey of Japan (GSJ) and funded principally by the Japan Science and Technology Agency. Active support has been received from scientific institutions, government agencies, and intergovernmental bodies throughout the region. Use is made of the Japan Marine Science and Technology Centre (JAMSTEC) submersible support vessel M.S. Natsushima during the period of the annual servicing of its submersible. Onboard facilities include GSJ's single and multichannel seismic systems, magnetometer, sampling and heat-flow equipment, JAMSTEC's deeptow sonar and camera systems, and NRCDP's (National Research Centre for Disaster Prevention, Japan) OBS (Ocean Bottom Seismometers). In the case of the present study, all of the participating institutions have allocated funding for scientific personnel, travel, and post-cruise research projects. In addition, the N.Z. Oceanographic Institute and the Department of Lands, Survey and Natural Resources, Kingdom of Tonga, have allocated resources and funds for compilation and printing of the cruise report.

The results of this survey are relevant, not only to the nations of Tonga and Fiji, within whose waters the work took place, but also to international geoscience programmes outlined in IOC Workshop Report No. 35 "CCOP/SOPAC-IOC-UNU Workshop on Basic Geo-scientific Marine Research Required for Assessment of Minerals and Hydrocarbons in the South Pacific, Suva, Fiji, 3-7 October 1983" and by the proceedings of the Third Session, IOC-WESTPAC Programme Group Townsville, September 1983.

Specifically the IOC Workshop Report lists Project A-2.1 "Forearc and Backarc Processes in the Tonga Lau Region", which emphasizes the importance of understanding the evolution of the Tonga forearc and Lau backarc as a closely interrelated unit before an assessment of hydrocarbon and mineral potential can be completed. Other projects suggest multichannel seismic techniques to define thickness and evolution of sedimentary basins at the northern end of the Tonga Ridge (Project A-l. 4), and deep-tow and sampling work in backarc areas to assess the potential of metalliferous deposits at actively spreading rifts (Project B-4). The present study also supports IOCWESTPAC programmes on Margins of Active Plates (MAP), and its subprogrammes on Back-Arc Tectonics (BAT) and Sedimentary Evolution on Active Margins (SEAM) as described in the WESTPAC report.

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OBJECTIVES

The prime objectives of the cruise were determined at a pre-cruise meeting at Nuku'alofa, Kingdom of Tonga, 13-19 March 1984. They are:

1. To define the structural and stratigraphic framework of the little known North Tonga Ridge north of the island of Vava'u; by

(a) a multichannel seismic reflection survey of the ridge; (b) dredging at outcrops to identify seismic units, including basement and volcanic features; (c) heat-flow measurements in a transect across the ridge; (d) sonobuoy refraction lines in forearc areas for velocity profiles to deep horizons.

2. To study earthquake mechanisms, nature of the seafloor, sedimentation and mode of development in the complex northern Lau Basin; by

(a) recording micro-earthquakes with a network of OBS recorders at the southern end of the Peggy Ridge; (b) deep-tow side-scan sonar and deeptow camera surveys of a possible spreading rift at the southeast end of Peggy Ridge; (c) single channel seismic traverses across the Lau Basin.

3. To learn more of the nature of the transition from subduction tectonics to transform at the northern termination of the Tonga Ridge and Lau Basin; by

(a) multichannel seismic traverses across the Tonga Trench at its northern curvature where geometry of subduction may change; (b) a detailed single channel seismic survey of the northern end of the Tonga forearc where structural effects of the adjacent transform margin may occur; (c) dredge sampling of the northern trench wall and forearc particularly adjacent to where Russian scientists reported unusual igneous suites.

4. To assist definition of the structure of the Lau Ridge and Koro Sea; by

(a) single channel seismic reflection profiling en route to and from the principal study area.

5. To collect additional bathymetric data in poorly mapped areas.

5. To interpret the evolutionary history of the northern part of the Lau-Tonga Arc system.

6. To make a preliminary assessment of the resource potential of the North Tonga forearc and backarc.

6. tinuous accurate positioning was possible using an experimental Global Positioning System (GPS) under test for the Sony Corporation of Japan. When all satellites are operational the GPS system will produce fixes with an accuracy of better than 30 m for 24 hours each day, anywhere on earth.

Bathymetry Depth is recorded continuously by a Nippon Electric Company NEC NS10l precision depth recorder. The transducer has a frequency of 12 kHz and a beam width of 150. It is integrated with an NEC 19-inch chart recorder and automatic depth digitizing system. All depths are recorded in uncorrected meters, assuming a velocity of sound in seawater of 1500 m/sec.

Single Channel Seismic Reflection The single channel system uses a Bolt 1900 c/150 cu.in.airgun without wave shaping kit. It is generally fired at 12 sec intervals at a pressure of 110 kg/cm 2. (1500 p.s.i.). Returning signals are received at a 100 element hydrophone array and recorded on both GSJ's Raytheon and CCOP/SOPAC's EPC chart recorders. Filter settings generally 30-125 Hz.

Multichannel Seismic Reflection The multichannel system is based on a Bolt 1500 c /450 cu. in. airgun without wave shape kit fired at 25 sec intervals at 1700 p.s.i. and a 12-channel hydrophone array with 64 elements in each 50m active section. Hydrophone depth is maintained at about 18 m with preset depressors. Ship's speed is maintained at about 4 knots and manual changes of shot interval are made for variations in ship's speed. Filter setting generally 0-64 Hz and a monitor record is obtained on a Raytheon Line Recorder. A high frequency 30-125 Hz record is also obtained for onboard interpretation. Multiplexed signals recorded in SEG-B format. Onshore processing includes 6-fold COP stacking, deconvolution, velocity analysis, migration and colour image display.

Seismic Refraction Four military-type OKI type OC-O1 sonobuoys were available for this cruise. No launcher was available. Difficulties were experienced with tangling of hydrophone arrays, poor reception from the sonobuoys short antenna, and presumed shark attacks on the sonobuoy's hydrophones. Refraction work was also done using the OBS network.

Magnetometry A GeoMetries Model G801 proton precision marine magnetometer is used routinely on every seismic reflection and bathymetric track. A Hewlett-Packard HP7130A chart recorder provides 10-inch strip chart records and an HP9835-B microcomputer provided digital cassette tape and paper printouts at 1 min intervals.

Ocean Bottom Seismometry Nine pop-up type OBS, manufactured by the Nippon Electric Company to NRCDP specifications were deployed. The master clock of each OBS was calibrated from the standard clock, which was synchronised either by the broadcast standard time signals (WWVH) with corrected path delay or by the precision Rubidium clock. These units will record for about one month and are spaced 20-50 km apart. OBS refraction lines are shot using the large 450 cu. in. airgun.

Deep-Tow The NEC deep-tow sonar system uses 70 kHz sound to "illuminate" a 2 km wide swath of seafloor. It is towed at about 100 m above the seafloor and its 4.8 kHz profiler shows layering to 50 m below the seafloor. The deep-tow camera produces realtime video images and allows still photos on command.

Heat Flow

A Bullard-type heat flow probe, manufactured by Nichiyu Giken Company, Japan, is used to measure earth's heat flow. The instrument has three thermistors on a 2 m long probe at 0.8 m spacing. The probe 7. is generally used as a trip weight for piston coring or on a trip-arm in conjunction with a gravity core. It is generally held above the seabed to reach thermal equilibrium and maintained in the seabed for about 15 min. Readings are recovered through external connections without dismantling the watertight container

Free Fall Sampling At most stations a sample is routinely obtained with a preussag Free Fall Grab and Camera. Immediately before impact the apparatus takes a single photograph of an area of seafloor about 1.5 x 2.0 m. The grab takes a sample of an area about 0.3 x 0.4 m into a net with a mesh of about 10 mm so that sediment finer than this is generally washed out during ascent. A sample of soft sediment is obtained in a small diameter tube on the side of the grab. At the surface the float is located with the aid of either a light or a radio beacon.

Dredging Rock samples of selected outcrops are obtained with a simple rock dredge with a 0 . 4 m diameter aper- ture and a chain mesh bag lined with a net. Heavy weights are dragged in front of the dredge and a pipe dredge behind. Fine control of dredging operations is possible because the winch operator has a continuous reading on line tension on a strip recorder. Dredging is limited to depths of less than 4500 m by wire length.

Coring Cores are obtained with a standard Kuellenberg-type piston corer with either a 4 m or an 8 m barrel. The trip arm clamps to the main wire facilitating retrieval. A heat flow probe is generally used as a trip weight. The same head is also used as a gravity corer with either a 4 m or a 2 m "rock" barrel. Coring is limited to depths of less than 5200 m by available wire.

Laboratory Operations All underway and station data was logged immediately after collection for curation in the various host institutions in Japan. Samples were described on board and recognized lithologies were subsampled. In the sample laboratory, rock and core samples were photographed, igneous lithologies were cut and sectioned for onboard petrological analysis, and some sedimentary lithologies were broken down and sorted for examination of their foraminiferal and nannofossil assemblages.

CRUISE LOG

The cruise began and ended in Suva, Fiji, and included a port stop at Vava 'u, Tonga (Fig. 1.1). Almost all of the planned programme was successfully completed and some preliminary interpretations were made on board during the cruise. The operations were blessed with good weather and a fine, well equipped ship with excellent cuisine.

The M. S. Natsushima left Suva on the afternoon of Wednesday, 14 November, after only 8 hours in port, its arrival having been delayed by a typhoon during transit from Japan. The delay of 32 hours in the planned time of departure necessitated some minor cuts from several projects and some rescheduling of operations. However, almost all of the complex shipboard equipment operated com- pletely reliably and no further time was lost from bad weather or gear failure. Remarkably for any oceanographic cruise, no equipment of any sort was lost.

The single/multichannel seismic equipment and magnetometer operated for four weeks without any significant break in the production of good quality data. On one occasion, shark bites punctured one section of the multichannel seismic steamer, which was replaced immediately with a spare section. Sonobuoy refraction work suffered from entangling of sonobuoys with the seismic hydro-

8. phone array shortly after launch, and shark attacks terminated the useful life of at least one sonobuoy. Nine OBS were deployed at the southern end of the Peggy Ridge in the first extensive OBS experiment in the Lau Basin. The observational period was about 21 days and all OBS were recovered successfully after deployment in water depths of 20S62520 m, earthquake data being processed ashore.

The deep-tow sonar/high-resolution seismic system was navigated along precise tracks using seabed acoustic transponders, while being maintained at 100 m above a complex terrain in 2S00 m deep water by careful monitoring of sensors and skilful manipulation of ship and towing winch. Even more impressive was to view the seafloor as the deep-tow camera was "flown" at 2-3 m above the undulating bottom with only a few gentle impacts when the depth changed suddenly.

Sample collection was not attempted between late evening and early morning, which imposed some limitations on planning. However, a good collection of rock samples was obtained from the eastern forearc and northern termination of the Tonga Ridge, the ship's only rock dredge leading a charmed life thanks to careful co-ordination of tensiometer readings with winch and ship operations. The free-fall grab with camera worked successfully at almost every station and all were recovered Coring and heat flow work proved difficult in the areas of compact, volcanogenic sands on the Tonga Ridge but it was successful in the softer sediments of the Lau Basin. All samples were logged on board and preliminary analyses of texture and mineralogy were made. Selected samples were also dated paleontologically using foraminifera and nannofossils, enabling some preliminary onboard interpretation of sample and seismic stratigraphy.

The distribution of cruise time is shown in Table 1.1. Approximately 28% of working time was devoted to multichannel seismic profiling, 29% to single channel seismic profiling, mainly in transit and in the Lau Basin, l5% to sampling, 16% to OBS deployment and recovery, and 12% to deep-tow surveys.

The underway data is recorded as 40 lines of which 19 are single channel lines and 6 are long, slow, multichannel lines (Table 1. 2). The rest are short, deep-tow and sounding tracks. The total distance travelled on single channel lines is about 3100 km and on multichannel lines about 1300 km. The position and depth of sample stations is listed in Table 1.3. There are 12 dredge stations, 8 coring and heat-flow stations, and 18 free-fall grab and camera stations.

The Natsushima called at the picturesque island of Vava'u on Monday, 26 November. Some 80-100 secondary school children were taken on conducted tours of the vessel and a reception was given on board for government and local officials. The scientists and crew of the Natsushima were given an enthusiastic welcome and were royally entertained by representatives of the Government of the Kingdom of Tonga. This proved a particularly welcome break in the middle of the cruise. The cruise ended at Suva on Thursday, 13 December, where, again, a reception was held for government officials.

POST CRUISE PROCEDURES

Curating During the cruise participating scientists selected sub samples of particular lithologies for later analysis, in general leaving a reference sample on board for curation at Geological Survey of Japan. A full list of lithologies and their location is given in an Appendix.

In addition, photocopies of all seismic profiles, seismic logs, and navigation logs were given to all participants along with magnetometer and depth listings at 15-min intervals. Originals of the seismic profiles will be taken to the N.Z. Oceanographic Institute for copy flow reproduction and distribution to

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interested participants. They will then be lodged with the Geological Survey of Japan. All OBS, deep-tow, magnetometer, and heat-flow records will go directly to Japan for report and curation. Bathymetric compil- ation will be supplied to participating institutions and to relevant hydrographic offices.

Four cores were split on board. Half of each core will be deposited at Geological Survey of Japan, and half will go to University of Sydney. Subsamples of ash horizons will be lodged at N.Z. Oceanographic Institute.

Photographs taken by free-fall camera will be processed in Japan and prints will be supplied to NZOI for correlation with surface sediment analyses.

Priority Data are to remain confidentially available to participating scientists for a period of two years or until accepted for publication, whichever is the sooner. Participants wishing to distribute data to colleagues or extend the limits of their own research should consult the co-chief scientists to avoid duplication of effort. All data collected on the cruise remains the property of Geological Survey of Japan, NRCDP, and JAMSTEC.

Publication of Cruise Results It is proposed that a collection of papers summarising the results of this cruise should be published together, in a CCOP/SOPAC Technical Bulletin. It is considered that manuscripts should be submitted for inclusion in this volume in mid1986. Publication of each paper will be subject to the agreement of the two co- chief scientists. Papers should not be published independently without the agreement of the co-chief scientists.

ACKNOWLEDGMENTS

We wish to express our thanks to Captain Hamanaka and his crew for their helpfulness and skill at every stage of the cruise. The excellent cuisine completed everyone's enjoyment of the work. We also wish to express our gratitude to Mr Sione Tongilava and the Government of the Kingdom of Tonga for a truly memorable welcome in Vava'u.

REFERENCE

HONZA, E.; KEENE, J . B.; SHIPBOARD SCIENTISTS, 1984: Cruise Report, M/S Natsushima, December 4, 1983 - January 5, 1984. Geological and geophysical investigations of the western Solomon Sea and adjacent areas. PRAG Record 84/1: 32 p, 3 tables, 2 figs.

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II. TECTONIC SETTING OF THE NORTHERN TONGA ARC AND LAU BASIN: BACKGROUND TO THE NATSUSHIMA 84 CRUISE K.B. Lewis New Zealand Oceanographic Institute

THE MAIN GEOMORPHOLOGICAL STRUCTURE

The Tongan Trench, Tongan Ridge, and Lau Basin lie near the southern end of the line of trenches, island arcs and backarc basin that characterize the western rim of the Pacific Ocean (Fig. II. I). In many respect they form the classical simple arc system (Packam 1978) Their simplicity is largely due to the fact that the convergent plate boundary controlling their evolution has maintained its essential eastward-facing geometry for most of the Cenozoic.

The Pacific seafloor east of the Tonga Trench is about 5,500 m deep, with the depth, magnetic anomalies and DSP bore-hole data indicating a basement age of about 100 Ma, that is, midupper Cretaceous ( Hawkins 1976a)

Tonga Trench, up to 10,500 m and trending NNE-SSW marks the site of subduction of the Pacific Plate beneath modified oceanic crust of the Indo-Australian Plate. The Pacific Plate underthrusts initially at an angle of 12° but steepens

20. rapidly to about 45 ° (Sykes 1966; Mitronovas et al. 1969). It moves in a direction of N900W relative to the Indo-Australian Plate and is converging at a rate of about 100 mm/yr at its northern end (Eguchi 1984), although the rate at which the Pacific Plate is consumed at the trench is increased by the rate of spreading in the Lau Basin. Subduction of aseismic ridges, the Louisville Ridge in the south and the more diffuse Niue Ridge in the north, appear to affect the seismicity of the down going slab and the morphology of the overlying arc (Hanus and Vanek 1978a, b, 1979a, b). The west-dipping Wadati-Benioff Zone beginning at the Tonga Trench, reaches depths of nearly 700 km, compared with only 300 km for the east- dipping zone that begins at the New Hebrides Trench and underlies Vanuatu.

Near its northern end, at about l5°S, the Tonga Trench bends through more than 90 0, with progressively more oblique subduction, to join the complex left-lateral transform system passing through Fiji to the west-facing New Hebrides Arc.

Fresh boninites have been reported from the recurved north-facing portion of the trench wall (Shara- skin et al. 1983). These rocks may indicate an initial phase of intraoceanic subduction or back-arc spreading (Pearce et al. 1984). The volcanic activity in adjacent Samoa may be related to the flexing and rupturing of the Pacific Plate at the bend (Hawkins and Natland 1975).

In the Tonga Arc, subduction probably began in mid Oligocene times (Fig. II. 2), along apre-existing midlate Eocene oblique strike-slip boundary, as the Indian-Pacific stage pole moved southward from near Samoa (Kroenke and Tongilava 1975). Pre-late Eocene igneous basement is a suite of rocks considered to re- present a segment of modified and serpentinised oceanic crust that outcrop on the trench wall (Fisher and Engel 1969) and on the eastern margin of the Tonga Ridge at the island of 'Eua (Ewart and Bryan 1972; Kroenke and Tongilava 1975). 21.

The Tonga Ridge consists of a forearc platform and an active volcanic arc, the Tofua Volcanic Chain. In the southern and central parts of the ridge, the mainly andesite cones of the Tofua Chain are separated from a shallow, sometimes reef studded, forearc platform by the 1800 m deep Tofua Trough. Forearc sediments are thick with up to 5 km of late Eocene to late Oligocene limestone, and early Miocene to Recent volcanoclastic strata, limestone lenses and reefs, the sediment pile thinning towards the eastern rim of the platform (Kroenke and Tongilava 1975; Maung 1982, Scholl and Vallier 1984).

The upper sequence is interrupted by a westward-dipping unconformity, separating Miocene and Pliocene sediments, that indicates a radical change in tectonic style. Later faulting normal to the arc has broken the ridge into a series of reef-capped horst and deeply channeled grabens (Herzer and Exon, in press). Small seeps of thermally mature petroleum have been reported from the southern forearc (Tongilava and Kroenke 1975; Maung et al.1981).

The northern part of the Tonga Ridge, from its northern limit at 15°S to just north of the island of Vava' u at 18 ° 30 I S is markedly different from the ridge to the south (Fig. II. 3) . The forearc platform is relatively narrow and deeply submerged its irregular surface sloping eastwards at depths of 8001800 m. There is no Tofua Trough. The Tofua Volcanic Chain crowns the crest of the ridge and, except immediately north of Vava 'u, there is no evidence of the transverse channels that segment the ridge further south (Karig 1972).

Information on the geology of the northern ridge is limited. Fonualei is active volcano erupting dacite lava (Brodie 1970; Bryan et ale 1972). Vava 'u is an uplifted Pleistocene reef overlying older, probably Miocene, foraminiferal and coral limestone interpreted as a reef talus deposit (Hoffmeister 1928; D. Tappin, pers. corom.). No references have been found to the geology of Niuatoputapu. Two basaltic andesite samples from the cone of Tafahi were analysed by Ewart (1976). Volcanic activity has been reported from Curacao Shoal, north of Tafahi (Anon. 1973). A suite of ophiolitic and volcanic rocks have been dredged from the transform truncated northern end of the ridge (Sharaskin et ale 1983). An unpublished single-channel seismic profile, collected by CCOP/SOPAC on the chartered vessel Machias, revealed a forearc with outcropping knolls and infilled basins (Halunen 1980).

The forearc plateau of the Tonga Ridge, at least in its wide central and southern segments, is an orphaned structure separated from its parent volcanic arc on the Lau Ridge by rifting and spreading of the Lau Basin (Karig 1970; Packham 1978). This has resulted in the curious anomaly of a forearc that dates back to the Eocene in front of a volcanic arc that is probably wholly Plio-Pleistocene in age (Fig. IJ.4). It means too that the geology of the Lau Ridge and Tonga Ridge should be intimately associated with one another and with the evolution of the Lau Basin that split them apart. One possible example of this association is that late Miocene arc volcanism on the Lau Ridge apparently correlates with volcanoclastic turbidites of the same age and derived from a western source found in Tonga (Gill 1976; Herzer and Exon in press) .

The Lau Basin is generally regarded as an actively dilating (backarc) marginal basin because of its generally thin sediment cover, high heat flow and fresh tholeiitic, MORE-type basaltic floor (Karig 1970; Sclater et ale 1972; Hawkins 1974, 1976b; Lawvers et al. 1976; Eguchi 1984). However, an alternative idea, that it is primarily an immense, graben-like feature, has been proposed and vigorously defended (Katz 1974, 1976, 1978).

The oldest oceanic-type crust in the Lau Basin is probably early Pliocene in age (Burns et al 1973; Coleman and Packham 1976). Changes in volcanism in Fiji between 8 Ma and 5 Ma ago (Gill et al. 1984) and uplift of the Tonga forearc about 5-4 Ma ago (Scholl and Vallier 1984) heralded the initial rift. The Lau Basin generally ranges from 20003000 m deep, its shallow depth a probable indication of its youth compared with the 5000-6000 m deep Pacific Basin to the east. It is 1500 km long. It is less than 150 km wide in the south, over 300 km wide at the latitude of Vava'u and 450 km wide at its northern extremity. Dredging has yielded (MORB), midocean ridge-type tholeiitic basalts (Hawkins 1974, 1976b; Gill 1976). Heat flow is variable but generally high (Sclater et al. 1972; Watanabe et al. 1975). Up to 700 m of volcanogenic-rich sediment, with abundant pumice, infills depressions between volcanic knolls (Hawkins 1974; Griffin et al. 1972; Bertine 1974). The geometry of spreading in the basin is largely conjectural, based on sparse bathymetric, magnetic, and seismological evidence.

In the southern part of the basin, many interpretations based on confused magnetic anomaly patterns and thickness of sediment suggest either a central spreading rift with many transform offsets or several jumps in the axis of spreading (Lawvers

et al. 1976). Recent evidence indicates a magma chamber close to the eastern side of the basin near the island of Ata (Scholl and Vallier 1984).

In the wide northern part of the basin, the NW-SE trending Peggy Ridge is 300 km long, has about 1300 m relief, and well-defined seismicity. Some authors have regarded it as transform (Weissel 1977), while others have considered it to be a spreading ridge (Chase 1971; Cherkis 1980). Focal mechanism solutions clearly indicate right lateral movement and define the feature as a transform (Eguchi 1984). However, hydrothermal rocks and fresh pillow basalts dredged from its flanks suggest that the Peggy Ridge is a leaky transform (Bertine and Keene 1975; Lawvers et al. 1976). Differences in transmission of Pn and Sn waves and differences in heat flow across Peggy Ridge suggest that the crust to the SW of the ridge is much older than the crust to the NE (Sclater et al. 1972; Barazangi and Isacks 1971; Lawvers et al 1976). To the north, the island of Niou Fo'ou has erupted mid-ocean ridge-type basal ts 12 times in the last 150 years (Reayetal 1975), and exceptionally high heat flow has been recorded near Rochambeau Bank (Hawkins 1974). In addition Donna Ridge and Zephyr Shoals are bounded by scarps up to nearly 3000 m high and may be regarded as faults on the basis of both of their morphology and of sheared gabbro and greenstone dredged from their scarps (Hawkins 1974).

Proposed geometries of spreading (Fig. 11.5) include a Y-shaped configuration of spreading ridges and transforms (Weissel 1977; Falvey 1978), a curved spreading ridge (Cherkis 1980) and a uniform system of NW-SE trending transforms and NESW trending spreading axes continuous along the length of the basin (Sclater et al. 1972; Eguchi 1984). None of the published interpretations can be related easily to the confused bathymetry and magnetic patterns northeast of Peggy Ridge. None can clearly document the mechanism for spreading in the NE corner of the Lau Basin adjacent to the North Tonga Ridge.

TECTONIC EVOLUTIONS

For much of the early and mid Cenozoic a continuous are, the east facing Vitiaz Are, extended from Papua New Guinea at least as far as Fiji and the present Tonga Arc. Before opening of the South Fiji Basin in the Oligocene, the Tongan segment of this arc may have been close to the Norfolk Ridge-New Caledonia segment of the New Zealand Plateau. There is some suggestion of an Oligocene hiatus in arc volcanicity, which may coincide with opening of the South Fiji Basin (Colman and Packham 1976; Colley and Hindle 1984). As the arc spread out into the Pacific Ocean, it developed the modified island arc volcanism that outcrops extensively in central Fiji and on the Lau Ridge (Gill et al. 1984).

Disintegration of the Vitiaz Arc began in late Miocene or early Pliocene times with a reversal of subduction in the New Hebrides segment of the arc (Fig. II.2). Back-arc spreading in the North Fiji Basin has migrated the New Hebrides segmentment westward and spreading in the Lau Basin migrated the Tongan segment eastward.

In Fiji, periods of slow change in volcanism beg inning somewhere between 8 Ma and 5. 5 Ma ended with the cessation of typical arc andesite volcanism about 3 Ma (Gill et al. 1984). Since then Fiji has rotated anticlockwise through 90° in a complex transform system between the Tonga and New Hebrides Arcs (Malahoff et al. 1982). Part of the transform system runs north of the Lau Basin and Tonga Ridge and is characterized by left-lateral slip in focal mechanism solutions (Johnson and Molnar 1972; Eguchi 1984).

25.

Vanuatu an volcanic rocks are quite distinct from the more MORB-like rocks of Tonga which alone continue the Vitiaz Arc tradition in rock composition and facing direction.

Before disintegration of the Vitiaz Arc, the northern Tonga Ridge must have lain close to the massif of central Fiji (Falvey 1978). Simple E-W spreading would indicate that it fitted against the northern end of the Lau Ridge, whereas the overall NW-SE spreading (Eguchi 1984) would suggest that the ridge north of Vava' u projected north of the Lau Ridge and lay east of the central Fiji Islands of Viti Levu and Vanua Levu.

The thermal expansion that heralded rifting in the Lau Basin is marked on the central and southern part of the Tonga Ridge by a late Miocene-early Pliocene regional unconformity. The subsequent cooling and collapse as the basin widened and Tongan forearc separated from

26.

Lau Arc is indicated by down-tobasin normal faulting and by submergence and westward tilting of the unconformity, which became deeply buried by volcanogenic sediments from the young Tofua Arc (Herzer and Exon, in press) (Fig. 11.4).

In an interpretation of recent international cruises to the southern Tonga region, Dupont and Herzer (in press) theorise that the reef and island studied eastern part of the forearc has been uplifted and broken into arc-normal horst and grabens as a result of underplating by the subducted Louisville Ridge as its interaction with the arc swept southwards during the PlioPleistocene (Fig. 11.6). They infer that the intersection occurred in the region of Vava'u about 3 Ma ago and near Tongatapu about 1.5 Ma ago. They suggest that prior to under plating the forearc strata would have dipped away from the volcanic arc as they do on the Kermadec Ridge which has never been affected by an aseismic ridge. They point to the anomaly of the northern Tonga Ridge, where bathymetric data shows a deep, gently-dipping forearc superficially similar to the Kermadec Ridge. They infer that either it has never been affected by the Louisville Ridge or it has somehow reverted to its original form. If the northern ridge has not been affected then either the Louisville Ridge only began at the latitude of Vava'u or the northern segment is much younger than the rest of the Tonga Ridge and only began to form after the Louisville Ridge swept through the area between 5 Ma and 3.5 Ma ago.

To understand the geological evolution and mechanisms of the enigmatic northern Tonga Ridge and adjacent trench and backarc, it is necessary first to define the structure of the northern ridge and basin, then to contrast it with the ridge and basin to the south and with the structure of Fiji. It may then be possible to interpret the evolution of the area, the history of the rocks at depth, and finally the potential for mineralization of the northern arc.

ACKNOWLEDGMENTS

Special thanks to Trevor Falloon for supplying me with many references cited in the review, and to Don Tiffin and Eiichi Honza for constructive criticism of initial, onboard, drafts of the manuscript.

REFERENCES

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BARAZANGI, M.; ISACKS, B. 1971: Lateral variations of seismic-wave attenuation in the upper mantle above the inclined earthquake zone of the Tonga Island Arc: deep anomaly in the upper mantle. J. geophys. Res. 75(35) : 8493-8516.

BERTINE, K.K. 1974: Origins of the Lau Basin Rise sediment. Geochim. cosmochim. Acta 38(4): 62-40.

BERTINE, K.K.; KEENE, J.B. 1975: Submarine barite-opal rocks of the hydrothermal origin. Science, N.Y. 188 (4184) : 150-152.

BRODIE, J.W. 1970: Notes on volcanic activity at Fonualei, Tonga. N.Z. Jl Geol. Geophys. 13(1): 30-38.

BRYAN, W.B.; STICE, G.D.; EWART, A. 1972: Geology, petrography and geochemistry of the volcanic islands of Tonga. J. geophys. Res. 77: 1566-1585.

BURNS, R.E.; ANDREWS, J.E.; VAN DER LINGEN, G.J.; CHURKIN, M.J.; GALEHOUSE, J.S.; PACKHAM, G.H.; DAVIES, T.A.; KENNETT, J.P.; DUMTRICA, P.; EDWARDS, A.R.; VON HERZEN, R.P. 1973: Site 203. Initial Rep. Deep Sea Drill. Proj. 21: 17-32.

CHASE, C.G. 1971: Tectonic history of the Fiji Plateau. Bull. geol. Soc. Am. 82: 3087-3110.

CHERKIS, N.Z. 1980: Aeromagnetic investigations and sea floor spreading history in the Lau Basin and northern Fiji Plateau. In Symposium on "Petroleum Potential in Island Arcs, Small Ocean Basins, Submerged Margins and Related Areas", Suva, Fiji, September 1971. U.N. ESCAP, CCOP/SOPAC Tech. Bull. 3 : 37-46.

COLEMAN, P.J.; PACKHAM, G.H. 1976: The Melanesian Borderlands and India-Pacific plates' boundary. Earth Sci. Rev. 12: 197-233.

COLLEY, H.; HINDLE, W.H. 1984: Volcano-tectonic evolution of Fiji and adjoining marginal basins. In Kokelaar, B.P.; Howell, M.F. (eds) Marginal Basin Geology. Spec. Publ. geol. Soc. Lond. 16: 151- 162.

DUPONT, J.; HERZER, R.H. (in press): Effect of subduction of the Louisville Ridge on the structure and morphology of the Tonga Arc. In Scholl, D.W.; Vallier, T.L. (Comps and Eds) "Geology and Offshore Resources of Pacific Island Arcs Tonga Region". Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. 2.

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

EWART, A. 1976: A petrological study of the younger Tongan andesites and dacites, and the olivine tholeiites at Niua Fo'ou island, S.W. Pacific. Contr. Miner. Petrol. 58(1): 1-21.

EWART, A.; BRYAN, W.B. 1972: Petrography and geochemistry of the igneous rocks from Eua, Tongan Islands. Bull. geol. Soc. Am. 83 : 3281-3298.

FALVEY, D.A. 1978: Analyses of palaeomagnetic data from the New Hebrides. Bull. Aust. Soc. explor. Geophys. 9(3): 117-130.

FISHER, R.L.; ENGEL, C.G. 1967: Dunite dredged from the nearshore flank of Tonga Trench on expedition Nova 1967. Trans. Am. geophys. Un. 49(1): 217-218.

FISHER, R.L.; ENGEL, C.G. 1969: Ultramafic and basaltic rocks dredged from the nearshore flank of the Tonga Trench. Bull. geol. Soc. Am. 80 : 1373-1378.

GILL, J.B. 1976: Composition and age of Lau Basin and Ridge volcanic rocks: Implications for evolution of an inter-arc basin and remnant arc. Bull. geol. Soc. Am. 87 : 13841395.

GILL, J.B.; STARK, A.L.; WHELAN, P.M 1984: Volcanism accompanying backarc basin development in the Southwest Pacific. In Carlson, R.L.;

Kobayashi, K. (eds) "Geodynamics of Back-arc Regions". Tectonophysics 102: 207-224.

GRIFFIN, J.J.; KOIDE, M.; HOHNDORF, A.; HAWKINS, J.; GOLDBERG, E. 1972: Sediments of the Lau Basin - rapidly accumulating volcanic deposits. Deep Sea Res. 19: 139-148.

HALUNEN, A.J. 1980: Cruise report: Tonga offshore survey TG-80 (1), 14 January - 4 February 1980. CCOP/ SOPAC Cruise Rep. 32 : 6 p.

HANUS, V.; VANEK, J. 1978a: TongaLau system : Deep collision of subducted lithospheric plates. J. Geophysics 44(5) : 473-480.

HANUS, V.; VANEK, J. 1978b: Structure of the Wadati-Benioff Zone in the Tonga region. Cas. Miner. Geol. 23 (1) : 5-23.

HANUS, V.; VANEK, J. 1979a: Morphology and volcanism of the WadatiBenioff Zone in the Tonga- Kermadec system of recent subduction. N. Z. Jl Geol. Geophys. 22(6): 659-671.

HANUS, V.; VANEK, J. 1979b: Northern part of the Tonga region; a complicated subduction closure. J. geophys. 46(4): 385-395.

HAWKINS, J .W. 1974: Geology of the Lau Basin : A marginal sea behind the Tonga Arc. Pp 505-20 in Burk, C.A.; Drake, C.L. (eds) "The Geology of Continental Margins". SpringerVerlag, New York.

HAWKINS, J • W. 1976a: Tectonic setting and petrology of Samoa-Tonga Fiji region. In Glasby, G.P.; Katz, H.R. (eds) "Marine Geological Investigations in the Southwest Pacific and Adjacent Areas". Papers presented at the I .D.O.E. Workshop, Suva, Fiji, September 1975. U.N. ESCAP, CCOP /SOPAC Tech. Bull. 2 : 141-152.

HAWKINS, J.W. 1976b: Petrology and geochemistry of basal tic rocks of the Lau Basin. Earth Planet. Sci. Lett. 28 : 283-297.

HAWKINS, J.W.; NATLAND, J.H. 1975: Nephelinites and basanites of the Samoan linear volcanic chains - their possible tectonic significance. Earth Planet. Sci. Lett. 24 : 429439.

28. HERZER, R.H.; EXON, N.F. (in press): Structure and basin analysis of the southern Tonga forearc. In Scholl, D.W.; Vallier, T.L. (Comps and Eds) "Geology and Offshore Resources of Pacific Island Arcs - Tonga Region". Circum-Pacific Council for Energy and Mineral Resources. Earth Science Series, Vol. 2.

HOFFMEISTER, J .E. 1928: A geology of Vava'u, Tonga. Unpublished manuscript. Bernice P. Bishop Museum, Honolulu.

ISACKS, B.L.; BARAZANGI, M. 1977: Geometry of Benioff Zones : Lateral segmentation and downwards bending of the subducted lithosphere. Pp 99114 in Talwani, M.; Pitman, W.C. (eds) "Island Arcs, Deep Sea Trenches, and Back-arc Basins". Maurice Ewing Series 1. American Geophysical Union, Washington. 470 p.

JOHNSON, T.; MOLNAR, P. 1972: Focal mechanisms and plate tectonics of the Southwest Pacific. J. geophys. Res. 77 : 5000-5032.

KARIG, D.E. 1970: Ridges and basins of the Tonga-Kermadec Island Arc System. J. geophys. Res. 75 : 239- 254.

KATZ, H.R. 1974: Margins of the Southwest Pacific. Pp 549-565 in Burk, C.A.; Drake, C.L. (eds) "The Geology of Continental Margins". Springer-Verlag, New York.

KATZ, H.R. 1976: Sediments and tectonic history of the Tonga Ridge and the problem of the Lau Basin. In Glasby, G.P.; Katz, H.R. (eds) "Marine Geological Investigations in the Southwest pacific and Adjacent Areas". Papers presented at the I.D.O.E. Workshop, Suva, Fiji, September 1976. U.N. ESCAP, CCOP!SOPAC Tech. Bull. 2: 153-165.

KATZ, H.R. 1978: Composition and age of Lau Basin and Ridge volcanic rocks : Implications for evolution of an inter-arc basin and remnant arc : Discussion and reply - Discussion. Bull. geol. Soc. Am. 89 : 1118-1119.

KROENKE, L. W .; JOUANNIC, C.; WOODWARD, P. (Comps) 1983: Bathymetry of the Southwest Pacific. Chart 1 of the Geophysical Atlas of the Southwest Pacific. Scale 1:6,442,192. CCOP!SOPAC, Fiji.

KROENKE. L.W.; TONGILAVA, S.L. 1975: A structural interpretation of two reflection profiles across the Tonga arc. S. Pacif. mar. geol. Notes 1(2) : 9-15.

LAWVER, L.A.; HAWKINS, J.A.; SCLATER J.G. 1976: Magnetic anomalies and crustal dilation in the Lau Basin. Earth Planet. Sci. Lett. 33 : 27-35.

MALAHOFF, A.; HAMMOND, S.R.; NAUGHTON, J.J.; KEELING, D.L.; RICHMOND, R.N. 1982: Geophysical evidence for post-Miocene rotation of the island of Viti Levu, Fiji and its relationship to the tectonic development of the North Fiji Basin. Earth Planet. Sci. Lett. 57 : 398-414.

MAUNG, T. U. 1981: Assessment of petroleum potential of the southern and northern parts of the Tonga Platform. CCOP/SOPAC Tech. Rep. 18 : 52 p.

MAUNG, T. U.; ANSCOMBE, K.; TONGILAVA, S.L. 1982: Petroleum potential of southern part of Tonga Platform. Bull. Am. Ass. Petrol. Geol. 66 (7): 986.

MITRONOVAS, W.; ISACKS, B.; SEEBER, L. 1969: Earthquake locations and seismic wave propagation in the upper 250 km of the Tonga Trench arc. Bull. seism. Soc. Am. 59 : 1115-1135.

PACKHAM, G.H. 1978: Evolution of a simple island arc : The Lau-Tonga Ridge. In Coleman, P.J. (ed.) Second Southwest Pacific Earthscience Symposium, I.G.C.P. project 110, Sydney, December 1977. Bull. Aust. Soc. explor. Geophys. 9(3) : 133-140.

PEARCE, J.A.; LIPPARD, S.J.; ROBERT~ S. 1984: Characteristics and tectonic significance of supra-subduct- ion zone ophiolites. In Kokelaar, B. P.; Howell, M. F. (eds) Marginal Basin Geology. Spec. Publ. geol. Soc. Lond. 16: 77-94. REAY, A.; ROOKE, J .M.; WALLACE, R.C. WHELAN, P. 1974: Lavas from Niuafo'ou Island, Tonga, resemble oceanfloor basalts. Geology 2(12) : 605606. SCHOLL, D.W.; VALLIER, T.L. 1984: Tonga Ridge. In Greene, H.G.; Wong, 29.

F. L. (eds ) Executive summary of geology and offshore resources of Pacific island arcs - Fiji, Papua New Guinea, Solomon Islands, Tonga, and Vanuatu. CCOP/SOPAC Tech. Rep. 44 : 7-18.

SCLATER, J.G.; HAWKINS, J.W.; MAMMERICKX, J.; CHASE, C.G. 1972: Crustal extension between the Tonga and Lau Ridges; petrologic and geophysical evidence. Bull. geol. Soc. Am. 83 : 505-518.

SHARASKIN, A.Y.; PUSTCHIN, I.K.; ZLOBIN, S .K.: KOLESOV, G.M. 1983: Two ophiolite sequences from the basement of the Northern Tonga Arc. Ofioliti 8(3) : 411-430.

SYKES, L.R. 1966: Seismicity and deep structure of the Tonga-Fiji region. Spec. Pap. geol. Soc. Am. 87 : 172.

TONGILAVA, S.L.; KROENKE, L.W. 1975: Oil prospecting in Tonga 1968-1974. S. Pacif. mar. geo!. Notes 1 (l): 1-8.

WATANABE, T.; LANGSETH, M.G.; ANDERSON, R.N. 1977: Heat flow in backarc basins of the Western Pacific. Pp 137-162 in Talwani, M.; Pitman, W.C. (eds) "Island Arcs, Deep Sea Trenches, and Back-arc Basins" . Maurice Ewing Series 1. American Geophysical Union, Washington. 470 p.

WEISSEL, J .K. 1977: Evolution of the Lau Basin by the growth of small plates. Pp 429-56 in Talwani, M.; Pitman, W.C. (eds) "Island Arcs, Deep Sea Trenches, and Back-arc Basins". Maurice Ewing Series 1. American Geophysical Union, Washington. 470 p. 30

31. III. SEISMIC PROFILES FROM NORTHERN TONGA

T. Kitekei'aho, D. Tappin Ministry of Lands, Survey and Natura L Resources Eiichi Honza, Yoshihisa OkudaTeruki Miyazaki, Takanobu Yokonobu GeoLogicaL Survey of Japan K.B. Lewis New Zealand Oceanographic Institute

INTRODUCTION

This preliminary account of the structure and seismic stratigraphy of the northern Tonga Trench, Tonga Ridge, and Lau Basin is based upon interpretations of onboard single channel profiles and multichannel moni tor records only. More authoritative interpretations may be made when data tapes have been processed. Emphasis is placed on locating sedimentary basins, particularly any which can be identified beneath the northern Tonga platform. A brief analysis is given separately on those aspects relating to petroleum potential of the area (Ch. XVIII).

The structural trends are delineated by the geomorphologies of the seafloor as illustrated by the bathymetric map of Chase et al. (1982) and by the seismotectonic of the lithosphere as interpreted by Eguchi (1984).

Positions of tracks are shown in Fig. II and Table I.2.

LAU RIDGE TO PEGGY RIDGE

Line3 (SCS)

Beginning at the thick sedimentary sequence on Lau Ridge, a series of fault blocks step down to the Lau Basin (Fig. III .1). The Lau Basin has an irregular basement topography with onlap and draped fill in depressions, which are separated by knolls of probable volcanic origin. Adjacent to the Lau Ridge, sedimentary fill is up to 0.3 sec thick but in the central Lau Basin, near Peggy Ridge, it is less than 0.1 sec thick. The decrease in age suggests a decrease in age of the irregular basement towards the centre of the basin.

EASTERN LAU BASIN

Line 19 (SCS)

The profile across the eastern Lau Basin does not resolve any sediment cover on acoustic basement in the central part of the basin near Peggy Ridge, .although a thin covering of pumiceous sediments is suggested by sediments obtained at Stns 31 and 32 and observed in deep-tow work. To the east, sediment cover thickens and can be detected seismically at thicknesses of about 0.02 sec about 100 km east of Peggy Ridge. It increases to approximately 0.5 sec at the eastern margin of the basin where several basement blocks, separated by faults and scarps, are tilted to the east. Again the thickness of sediment cover suggests that the age of the basement may increase from the central Lau Basin towards the edge.

ACROSS TONGA RIDGE

Line 20 (MCS)

A traverse across the Tonga Ridge, from the Lau Basin to the Tonga Trench, at a latitude just north of Vava 'u indicates several important 32. characteristics of the northern arc (Fig. 111.2). Sediments at the eastern margin of the Lau Basin are slightly deformed in contact with a high basement ridge referred to onboard as the "Natsushima Ridge". Between the ridge and the main Tonga Ridge is a trough, the "Natsushima Trough", which is much deeper (2900 m) than the adjacent Lau Basin and has only about 0.3 sec fill despite its proximity to the volcanic arc.

The western margin of the Tonga Ridge is terminated at its back-arc slope by either a fault or a vol- canic ridge. Much of the back-arc slope of the ridge is underlain by thick sediments, up to approximately 1.8 sec in thickness, which cover an irregular basement of possibly volcanic origin. North of the volcano of Fonualei, piercement structures reach the seabed in the middle of a 1.6 sec thick sedimentary sequence. Reflectors converge away from the piercement, so that the profile indicates a section through a volcanic cone.

Acoustic basement gradually becomes obscure to the east, where older sedimentary sequences are recognised beneath the blanketing upper sequence. The older sequence outcrops on the upper trench-slope where late Miocene and early Pliocene sandstones and conglomerates were dredged at Stn 15. The evidence for an accretionary prism on the inner trench wall is not clear on near-trace monitor records but should be clarified by future processing. No thick sedimentary fill exists beneath the axis of the trench. There are several horsts and grabens on the outer trench slope indicating tension in the downgoing plate.

NORTHWEST OF VAVA'U

Line 22 (SCS)

Line 22 begins in the "Natsushima Trough", traverses the volcanic arc near Fonualei and a trough, transverse to the ridge axis, between the northern part of the Tonga Ridge and the Vava'u Islands. It shows a thick sedimentary basin just south of Fonualei with sediments up to approximately 2.0 sec thick. The transverse trough has a thin covering of sediments, without prominent structural deformation, although a

33. large fault may be expected to occur beneath the sediments.

ALONG THE NORTHERN TONGA RIDGE

Line 23 (MCS) Line 23 runs south to north along the forearc of the Tonga Ridge north from Vava'u to the recurved northern end of the Tonga Trench near Samoa. There is an almost continuous forearc sequence, which ranges up to at least 1.8 sec thick. There is at least 1.6 sec of undisturbed fill in the transverse trough just north of Vava' u. At the far northern end of the Tonga Ridge the forearc sequence is truncated at a graben on the upper slope above the recurved part of the trench (Fig. 111.3). The graben has sedimentary fill of 1.0 sec in thickness. A line of knolls or a ridge exists north of the graben where basalts, serpentinites, granites, and other igneous material were dredged (Stns 21, 22, 23). The inner slope of the trench appears to be thrust faulted in several

places. The trench fill is about 0.6 sec thick and has an eroded, possibly channelled surface.

THE NORTHERN GRABEN

Lines 25 and 26 (SCS) Traverses of the graben and knolls at the northern end of the Tonga Ridge suggest that they are curved and aligned subparallel to the curved part of the trench. The profiles show horsts and grabens with normal faults. Vesicular pillow basalts were dredged from the outer knolls (Stns 24, 25). There is up to 1.0 sec of fill in the graben.

NORTHERN MARGIN

Lines 27 and 28 (SCS) The inner slope of the shallow, E-W trending, northern part of the trench is steep with little sediment cover (Fig. 111.3), although deformed sediments are suggested at the toe of the slope. Again there are indications of channeling in the trench axis. Bare rock knolls at the top of the inner trench slope may represent arc volcanism. To the south, in the northeast corner of the Lau Basin, there is up to 0.4 sec of flat-lying sediment in depressions between volcanic knolls.

ACROSS NORTHERNMOST TONGA RIDGE

Lines 29 and 30 (SCS, MCS) Lines that are single channel on the flanks and multichannel at the crest traverse the northernmost part of the Tonga Ridge. Sediment cover is thin in the back-arc basin but ,thickens to 1.5 sec on the back-arc flank on the ridge. Sediment layers thicken towards the volcano of Tafahi on the ridge crest. On the forearc (Fig. III. 4) there is 1. 51.8 sec of fill in wide fore-arc basins, and up to 1.0 sec of fill in small basins between faulted sedimentary blocks on the upper trench slope. Sampling indicates that the blocks are of upper Miocene sediments. Deformed sediments on the trench slope could be a result of thrusting.

TONGA RIDGE OBLIQUE TRAVERSE

Line 31 (MCS)

Above a steep and apparently 34 under-thrust lower slope, there is a wide, gently sloping forearc with up to 1.2 sec of fill in small slope basins confined between faulted rock outcrop? with obscure sedimentary lowering (Fig. III.5). Thick sediments, up to 2.0 sec thick, overlie an irregular acoustic basement on either side of the volcanic arc at the ridge crest. Again the volcano is in the centre of a sedimentary basin but whether the arc was injected into a pre-existing basin or whether extrusion of volcanogenic debris has caused basin subsidence may be clarified by detailed seismic stratigraphic studies of processed data.

Sediments thin westwards away from the arc to a series of outcropping basement pinnacles along the western margin of the ridge. Beyond the western margin, the "Natsushima Trough" contains 0.3 sec of onlap fill and the "Natsushima Ridge" appears to be outcropping basement.

EASTERN LAU BASIN

Lines 32 and 34 (SCS)

There is up to 0.8 sec of sedimentary fill between basement knolls along the eastern margin of the Lau Basin but sediments thin gradually westward until there is no detectable sedimentary cover on the irregular basement topography at the southern end of the Peggy Ridge. Line 33 was an OBS refraction line.

NORTHEASTERN LAU BASIN

Lines 35 and 36 (SCS) The sedimentary cover in the depressions between volcanic knolls thickens away from Peggy Ridge to a maximum of 0.5 sec in the northeastern Lau Basin. However, there is no fill apparent on seismic records in the northwestward continuation of the Natsushima Trough. To the south of Donna Ridge, there is a south- facing scarp and an E-W trending trough with more than 1.1 sec of sedimentary fill, the thickest sediment found in the northern Lau Basin.

35. SUMMARY

1. The slope from the Lau Ridge into the Lau Basin is characterised by downfaul ted blocks, which probably developed in response to the tensional regime at the initiation of back-arc rifting.

2. In the centre of the Lau Basin the sediment cover on irregular oceanic basement is thin. It is thicker towards the edges where fill is up to 0.8 sec (c. 600 m) thick. The thickening of sediment cover towards the edges of the basin suggest that basement gets older towards the edges. Although the crust is younger in the centre of the basin, it is not possible to define an active spreading axis on seismic reflection records. The present spreading is believed to be oblique to the basin axis with Peggy Ridge as a major transform, but this trend may have been superimposed on an older E-W spreading system parallel to the basin axis and exemplified by the deep trough south of Donna Ridge.

3. At the boundary between the Lau Basin and the Tonga Ridge immediately northwest of Fonualei, there is a high ridge and a deep trough referred to here as the "Natsushima Ridge and Trough II. The trough has up to 0.3 sec fill in its axis. The ridge-trough couple follows an arc curving round to the northwest towards Donna Ridge.

4. The Tonga Ridge north of Vava’u differs from the rest of the Tonga Ridge in that:

(a) it is not segmented by major E-W faults except possibly at a channel immediately north of Vava ' u and at a graben at the northern extremity of the ridge. (b) the volcanic arc is at the ridge crest (rather than forming a separate ridge to the west) and it intrudes a thick sedimentary basin. The elongate basin is up to 2.0 sec (c. 2 km) thick, has sedimentary layers increasing in thickness towards the volcanic vents from both sides. It may be inferred that volcanoclastic sediments have accumulated in a (partly isostatically) downwarped basin.

5. Strata that unconformably underlie the ridge-crest volcanoclastic basin outcrop at fault-controlled scarps on a wide, gently trenchward dipping, forearc slope. Samples indicate that these older strata may be upper Miocene rocks (Hayward, Shafik, Jenkins, this volume).

6. The trench-slope break is consistently at about 5250 m and there are indications of compressional tectonics in the trench wall. There are clear tensional horsts and grabens in the downgoing slab as it approaches the trench axis.

7. The far northern end of the Tonga Ridge is truncated at a curved graben that is subparallel to the curve of the northern end of the Tonga Trench. To the north of the graben, knolls of fresh basalt and boninite cap a steep trench-slope of granites and serpentinites (Falloon, this volume).

REFERENCES

CHASE, T.E.; SEEKINS, B.A.; VATH, S.C.; CLOUD, M.A. 1982: Topography of the Tonga region. Scale 4"/degree longitude. Mercator Projection. USGS-CCOP/SOPAC South Pacific Project.

EGUCHI, T. 1984: Seism tectonics of the Fiji Plateau and Lau Basin. Tectonophysics 102: 17-32. 36

37.

IV. SEISMIC PROFILES FROM THE FIJI AREA

Richard 90 Zmes, Ambika Prasad MineraL Resoupces Department

Eiichi Honza GeoLogicaL Survey of Japan

INTRODUCTION

During transit between Suva and the Lau Basin, seismic profiles (mainly single channel) were run across features of specific interest on the margin adjacent to Suva, across the Koro Sea, and across the Lau Ridge. These profiles provide important new insights into the geological history of eastern Fiji.

SUVA BASIN

Lines 1 and 2 Acoustic basement on the margin off Suva, labeled layer 6 (Fig. IV. 1), is correlated with the onshore Wainimala Group, which is considered to be older than middle Miocene in age (Rodda and Kroenke 1984). This interpretation is consistent with interpretations of refraction experiments at the northern margin of the Suva Basin (Everingham, pers. corom.) and with the interblended nature of volcanic and sedimentary lithostratigraphy of Wainimala Group exposures on land.

Extrapolation of marginal Suva Basin refraction data, which give average velocities of approximately 3 km/sec for late Miocene to Pleistocene marine sediments (Everingham pers. corom.), suggests that Suva Basin sediments above layer 6 are of the order of 4 km thick in the modern topographic axis of the Suva Basin. This correlates well with a maximum depth of burial of about 2.4 sec (two-way travel time) in the Natsushima profiles.

Layer 5 is a strongly transgressive unit, in part slumped, that may include reef, volcanogenic, and faulted strata. It is in unconformable contact with layer 4 above and layer 6 below, and it represents a remarkable, rapidly-stacked, facies change. Layer 5 is less extensive than succeeding layers. Its very variable, tectonic, and depositional style appears very similar to that modelled for early to middle Miocene units in the margins of the Bligh Water hydrocarbon prospective basin, north of Viti Levu (Eden and Smith 1985). The evidence for early rapid deposition in a relatively high energy environment is a favorable indicator for hydrocarbon potential

38. in the Suva Basin which has not been previously recognized.

Layer 4 infills the axis of the Suva Basin and on laps northwards. It is probably of Miocene age. The unconformity separating layers 4 and 3 is correlated with the Colo Orogeny, recognised as a regional unconformity on Viti Levu and affecting basin successions of 7-14 Ma age range. It is recognized in commercial boreholes to at least 1800 m below sea level in basins west, north, and east of Viti Levu (Eden and Smith 1985), and is identified in commercial seismic profiles from these basins to a much greater depth. It terminates in the south against a strong reflector correlated with the lava Pliocene to pleistocene Kandavu volcano.

Layers 3 and 2 have been tentatively dated as latest Miocene to Pliocene in age, layer 1 being probably Pliocene-Pleistocene (Brocher and Holmes 1985). Layers 1 and 2 have the parallel bedding commonly associated with turbidite deposition. The stratigraphy of the upper layers reflects the generally later, subduction-related arc volcanism on Kandavu of· 3.4-0.7 Ma (Gill et al. 1984) compared with arc volcanism on Vatulele-Beqa of 3.1-5 Ma, and southeast of Viti Levu of 5-6.3 Ma (dates from Rodda and Kroenke 1984).

Layers 1 and 2 extend (on Lines 2 and 3) beyond the Suva Basin into a series of smaller basins as far east as l79D25'E, where they terminate against an igneous basement high and a former subduction trench marking an extension of the Hunter Fracture Zone (Brocher and Holmes 1985).

BAC WATER BASINS

Line 39 Adjacent to the Suva Basin, three asymmetric basins are developed in water depths of 1300-1500 m on the platform between the Hunter Fracture Zone (adjacent to Nairai Island) and Viti Levu (off Nasilai Reef) (Fig. IV.2). Basement was not penetrated in the deepest section of these basins but over 1 sec of sediment was penetrated. Internal reflectors diverge from SW-NE and the shallow NW-trending basin-strike and seismicity in the outer platform margins and the Koro Sea (Everingham, pers. comm.) point to basin development by half graben formation in parallel with continued movement on the NW extrapolation of the Hunter Fracture Zone. Onlap relationships to a SE-

39. trending ridge connecting the Wakaya Island volcanics to Gau Island volcanics, which have ages of 4-5 Ma (Rodda and Kroenke 1984), suggest that basin infill may be early Pliocene to Pleistocene in age. Some of the uppermost sediments are truncated adjacent to uplifted basement by Pleistocene faulting. Overall these Bau Water marginal basins show evidence for much more intensive tectonic activity since the earliest Pliocene than the Suva Basin to the southwest, possibly as a result of relative anticlockwise rotation, with resultant tension and transcurrent movement, between Viti Levu and Vanua Levu.

SOUTH KORO BASIN AND LAKEBA BASIN (LAU RIDGE)

Line 3 At approximately 18°S the seafloor gently rises eastward from the 3000 m asymmetric trough of the Hunter Fracture Zone to a terrace at 2200 m deep and then rises gently again to the Lau Ridge plateau at approximately 1500 m deep before plunging more steeply to over 2000 m in the Lau Basin (Fig. IV.3). A basin on the 2200 m terrace contains at least 1.5 see (two-way travel time) of layered sediments and is terminated at its western margin by horst faulting against pressured volcanic acoustic basement. It is disconnected from Lau Ridge sediments to the east by several pinnacles of presumed volcanic basement. The horst faulting and basement at the western margins of this basin are in line with the trend of adjacent calc-alkaline volcanism with dates reported in Rodda and Kroenke (1984) for Matuku Island of 3.3-4.2 Ma, and Moala Island of 4.16.4 Ma. Together with Totoya Island (5.0 Ma), these islands and western basin margins comprise volcanic cores and relatively young volcanic basement on oceanic crust. The margin igneous stratigraphy indicates that local sediment supply into the basin may be at least late Miocene in age.

The Lakeba Basin on the Lau Ridge plateau is over 1. 5 see thick and disrupted by faults. Acoustic basement ages will reflect locally variable middle and late Miocene and late Pliocene and Pleistocene volcanicity (Woodhall 1985). Volcanoclastics and epispastics provide local basin infi11. However, most basin infill on the Lau Ridge is probably associated with the Pliocene (c. 3 Ma), initial rifting of the Lau Basin (Woodhall 1985). Much of the basement, -sediment-fill, faulting and even erosion observed on Line 3 may be a result of rift bulge and post-rift deflation.

NANUKA BASIN (LAU RIDGE) AND NORTHERN KORO BASIN

Lines 37, 38, 39

The Nanuku Basin exhibits a broadly similar topography, basement tectonics, and style of sedimentary infill to the Lakemba Basin to the south (Figs IV.4, 5). An apparently narrower Lau Ridge crest and less sediment fill result from line positioning at the southern margin of the basin. The average water depth is significantly greater than that in the Koro Basin.

Line 38 runs across the strike of a NE-trending graben which is approximately 36 km wide with a sediment thickness of at least 800 ms. Upfaul ted older sediments are isolated on terraces on the southeast margin which comprise the lower slopes of a 13 Ma old volcano (Rodda and Kroenke 1984). At approximately 1200 m depth, sediments pre- sumed to be of middle Miocene age and younger, onlap volcanic basement. Buried folds, below approximately 300 ms sediment thickness, in the deepest section of the Nanuku Basin may have originated over block faulted basement. The unconformity separating these folds from the top 300 ms of sediments probably post dates the first and most complex phase of basement block- faulting at approximately 3 Ma. These uppermost sediments wedge out to the Nanuku Basin (SE) where they have been upfaulted at their southern margin. Northwest divergent sub-bottom reflectors and modern seafloor asymmetry suggest late Pliocene to Pleistocene basement tilt towards Tavenui Island where lavas overlying charcoal from an occupied site have been dated as 2050 yrs BP (Rodda and Kroenke 1984). This style of relatively young faulting, which defines the modern rate is expressed not only in the Nanuku Basin - Taveuni basement trend, but also in adjacent

41.

NE-trending grabens between Taveuni and Vanua Levu, and within Vanua Levu in Natgewa Bay. The origin for the tension forming these grabens has been attributed by Woodhall (1985) to counter-clockwise rotation of Viti Levu.

Wedge out of Nanuku Basin sediment occurs with onlap on to Koro Sea bathyal sediments. Some Nanuku Basin reflectors appear to be erosionally truncated at the seabed, suggesting erosion, but for the most part reflector style is compatible with distal turbidite deposition from sources to the northwest, including Taveuni and the Lau Ridge. Therefore a basement stratigraphy pre-dating early Miocene is interpreted beneath the deeper north Koro Basin sediments, with the oldest basement possibly as old at late Oligocene, which is the approximate age of the youngest oceanic crust found by Malahoff et al. (1982) some 90 km south of the Koro Basin.

REFERENCES

BROCHER, T.M.; HOLMES, R. 1985: The marine geology of sedimentary basins south of Viti Levu, Fiji. In Brocher T.M. (ed.) "Geological Investigations of the North Melanesian Borderland". Circum-Pacific Council for Energy and Mineral Resources. Earth Science Series.

EDEN, R.A.; SMITH, R. 1984: Fiji as a petroleum prospect. Mineral Resources Department, Suva. 30 p, 9 figs, 1 map.

GILL, T.B.; STORK, A.L.; WHELAN, P.M. 1984: Volcanism accompanying back-arc development in the Southwest Pacific. Tectonophysics 102 : 202-224.

MALAHOFF, A.; FEDEN, R.H.; FLEMING, H.S. 1982: Magnetic anomalies and tectonic fabric of marginal basins north of New Zealand. J. geophys. Res. 87 : 4109-4125.

RODDA, P.; KROENKE, L.W. 1984: Fiji: a fragmented arc. Ch.7. in Kroenke, L. W. "Cenozoic Development of the Southwest Pacific". U.N. ESCAP, CCOP/SOPAC Tech. Bul. 6 : 87-109.

WOODHALL, D. 1985: The geology of the Lau Ridge. In Scholl, D.W.; Vallier, T.L. (Comps and Eds) "Geology and Offshore Resources of Pacific Island Arcs - Tonga Region". CircumPacific Council for Energy and Mineral Resources. Earth Science Series, Vol. 2. 42

43.

V. NOTES ON BATHYMETRIC DATA FROM THE NORTHERN TONGA RIDGE AND LAU BASIN

C.J. Jenkins Ocean Sciences Institute, University of Sydney

Most of the Natsushima half-hour, underway PDR bathymetry readings are incorporated with existing data for the region, using GEBCO data to July 1982, obtained from the New Zealand Hydrographic Office. The results are only preliminary for instance, they involve no sound velocity in seawater' corrections on the Natsushima data, for which a uniform 1500 m/sec velocity was assumed. Additional soundings will be used and a correction will be applied in later compilations. Since the errors will only be of the order of 10 m in 4000 m water depth, and the contour interval on the current map is 500 m the correction is unlikely to be significant. The map was produced on an original scale of 1:1,000,000 at latitude 33°S, and then reduced.

Although a good deal of correspondence is noted between this map and the earlier (1982) CCOP/SOPAC bathymetric edition by Chase et al. the differences caution against too much weight being put on anyone bathymetric contour map in formulating tectonic models of the area. Ridge features in the Lau Basin particularly, can alter in continuity and form depending on the contouring method used. The frequency of conflicting depth readings at crossing track lines and the wide spacing of data points in some regions leave much room for interpretation.

The revised compilation (Fig. V.1) demonstrates that:

1. An isolated block, which stands at 1500 m depth, lies at the northern tip of the main Tonga Ridge, from which it is separated by a 1 km deep, 25 km wide, 50 km long, E-W graben. The graben appears to have a flat floor (see Kitekei'aho, Honza, Lewis, this volume).

2. In the Lau Basin, a ridge that diverges NNW from the Tonga Ridge at 17-18~S and which has been dubbed the "Natsushima Ridge", is less continuous at the 2000 m isobath than indicated on the map by Chase et al (1982) . Nevertheless, it is an important, large-scale, curvilinear feature of the northeastern Lau Basin.

3. Stretching southwest from this "Natsushima Ridge" at 16 ° 20' S is an 80 km long ridge with crest height of 1000-1500 m. This structure joins to the southern end of the Peggy Ridge. Like the Peggy and "Natsushima" Ridges, this lineament appears to have a fundamental tectonic importance to the development of the northern Lau Basin. It may correspond to a NE-trending ridge proposed in the area by Sclater et a1. (1972) , Hawkins (1974) , Weisse1 (1977), and Eguchi (1984) on diverse grounds.

4. The Donna Ridge and a trough lying to its south are well defined in the new bathymetry, as they were in Chase et al (1982). However, 4 km depths are reached in the trough, which in this region of general 2.5 km depth, is of significance to the tectonic interpretations.

5. Al though no new data over the Donna Ridge or Zephyr Shoal is offered by the Natsushima cruise, re-contouring suggests that neither feature amalgamates with the "Natsushima Ridge".

6. Some submarine canyons are developed high on the east-facing trench-slope of the north Tonga Ridge. They are clearest at the 2000 m isobaths, northeast of Vava'u.

7. The floor of the Tonga Trench, where Samoa approaches most closely and where the trench axis changes from a N-S to E-W trench, is much shallower than normal. There depths are 5500-6000 m, as opposed to 60007000 m along the E-W (strike-slip) section, and 7000-8000 m along the 44. primary, N-S oriented (convergent plate boundary) parts of the trench.

A map is being compiled, which will incorporate additional cruises not yet in GEBCO collectors, and the entire Natsushima data set.

REFERENCES

CHASE, T.E.; SEEKINS, B.A.; VATH, S.C.; CLOUD, M.A. 1982: Topography of the Tonga Region. USGS-CCOP/SOPAC South Pacific Project.

HAWKINS, J .W. 1974: Geology of the Lau Basin, a marginal sea behind the Tonga Arc. Pp 505-520 in Burk, C.A. Drake, C.L. (eds) "The Geology of Continental Margins". SpringerVerlag, New York.

SCLATER, J.G.; HAWKINS, J.W.; MAMMERICKX, J.; CHASE, C.G. 1972: Crustal extension between the Tonga and Lau Ridges : Petrologic and geophysical evidence. Bull. geol. Soc. Am. 83 : 505-518.

WEISSEL, J.K. 1977: Evolution of the Lau Basin by the growth of small plates. Pp 429-456 in Talwani, M.; Pitman, W.C. (eds) "Island Arcs, Deep Sea Trenches, and Back-arc Basins". Maurice Ewing Series L American Geophysical Union, Washington. 470 p.

45.

VI. MAGNETIC ANOMALY AND HEAT FLOW MEASUREMENTS IN THE TONGA ARC AND LAU BASIN

Masato Joshima, Yoshihisa Okuda, Teruki Miyazaki and Takanobu Yokokura Geological Survey of Japan

MAGNETIC INTENSITY MEASUREMENTS

Total magnetic intensity was measured during the Natsushima 84 Cruise using a Geometrix proton precision magnetometer. The sensor was towed 200 m aft from the ship. Measurements were obtained every 1 minute on a PC-820l microcomputer, using a BCD-RS232C converting interface, and were also recorded on a chart recorder. An example of the digital printout is shown in Fig. VI.l. The measurements were correlated with positioning data obtained every 30 mins by a large computer after the cruise. Geomagnetic anomaly data was calculated by deducing IGRF 1980 (International Geomagnetic Reference Field), and the results were plotted on a map as profiles on track lines (Fig. VI. 2).

MAGNETIC RESULTS

There are many short wave length anomalies on the Tonga Ridge that are considered to be caused by the many volcanoes of the Tonga Arc and the volcanic basement beneath it. Many strong anomalies observed in the Lau Basin have some lineations. The trend of the lineations is unknown from the limited amount of data but it looks like it may in an N-S or NW-SE direction from the short wave lengths on E-W track lines.

HEAT FLOW MEASUREMENTS

Heat flow measurements were carried out at every piston or gravity coring station. It was hard for a thermistor probe of 2 cm diameter to penetrate the sediments of

46. the Tonga Arc, because the sediments were coarse at most sites. Three stations (Stns 20, 32, 33) seem to provide useful readings (Fig. VI.3), five others do not. Because of a recording malfunction, it has proved impossible to read out two 2-bits of the l6-bit binary data and to calculate the exact correction for the thermistor data. After omitting the lowest 2-bit of each data set and assuming reasonable values for the higher 2-bit, we have calculated the temperature data assuming a linear relation between the change of resistance and that of temperature between 0 and 30 degrees. Roughly, estimated results are as follows:

Stn Greater than 132 mW/m 20 3.2 HFU* 132 mW/m Stn Greater than 32 0.88 HFU 36 mW/m Stn Greater than 33 1.0 HFU 41 mW/m

* Heat Flow Unit

The high heat flow at Stn 20 occurs at the conspicuous ridge and trough at the eastern edge of the Lau Basin near Fonualei.

49.

VII. DEEP-TOWED SONAR AND CAMERA OBSERVATIONS AT THE SOUTHERN END OF PEGGY RIDGE

Hiroyasu Monma, Takeo Tanaka Toshio Tsuchiya and Katsura Shibata Japan Marine Science and Techology Center (JAMSTEC)

INTRODUCTION

The deep-tow system is a nearbottom observation system that manouvres instrument packages, such as sonar and camera, very close to the ocean bottom. The MPL/DEEP TOW was a well known system developed by the Scripps Institution of Oceanography in the early 1960s.

The advantage of this system is that it is possible to conduct detailed mapping and observations of the deep ocean floor.

The JAMSTEC deep-tow system, which has been developed since 1977, consists of the following sub-systems :

1. Sonar System Underwater unit, installed in a towed, open frame, consists of sidescan sonar (SSS), 4.8 kHz sub bottom profiler (SBP), and a pressure sensor. Onboard unit, consists of a signal controller, a data display, a data recorder, and a power supply.

2. Camera System Underwater unit, installed in a towed, open frame, consists of a real time colour TV, and a photographic camera. Onboard unit, consists of colour monitors, video recorders, a signal controller, and a power supply.

3. Acoustic Transponder Navigation System A precise, deep-water positioning system that relates both surface ship and underwater vehicles to bot- tom transponders.

4. Onboard Towing Gear Onboard towing gear is an A-frame, a gimbaled sheave, tow cable of 4300 m in length, and a hydraulic winch.

SURVEY PROCEDURE

The deep-tow survey area was focused within the ridge and trough topography of the central Lau Basin at the southern end of the Peggy Ridge. Here an active spreading axis and a transform fracture zone are considered to form a junction.

The area for the preliminary survey was between 17°10'S and 18o00'S in latitude, and between 176° 00'W and 176°40'W in longitude (Fig. VII. 1). Primary interests were focused on three areas where available bathymetric and magnetic information suggested that spreading centres may occur.

Procedure of the deep-tow site surveys are as follows :

1. A reconnaissance survey to search for the deep-tow site within the preliminary survey area (Figs VII.2, VII.3).

2. Deployment of three transponders on the sea bottom for positioning references. Calibration of relative and geographic co-ordinates of the transponders, and sounding equipment (Fig. VII.4).

3. Bathymetric survey by precision depth recorder (PDR) to produce a precise topographic map (Fig. VII. 5). 4. Fine-scale topographic and geological observation by side-scan sonar (SSS) (Fig. VII.6), and subbottom profiler (SBP) (Fig. VII. 7).

5. Observation of the microtopography and micro-scale geological features by colour TV and photographic camera (Fig. VII.8).

6. Recovery of transponders by sending release signals to the bottom mounted transponders.

DETAILED BATHYMETRIC SURVEY

During transit between OBS launch- ing sites, reconnaissance surveys were made of the areas of primary interest. These revealed four steep sided valleys (Fig. VII. 3). Two of the valleys "B" and "D" corresponded with zones where magnetic anomaly interpretations indicated possible spreading centers. Valley "B" was chosen for detailed examination and is surrounded by four OBS sites which could assist in interpretation of its structure.

The detailed bathymetric survey was conducted by steaming the ship towards east and west at 4 knots.

The distance between each line was 1000 m. The ship positions were automatically plotted on the X-y plotter. Water depth was also plotted manually every 60 sec and corrected on the basis of measured depth-velocity profiles (Fig. VII.4) The average sound velocity to a depth of 2500 m was 1497 m/s.

Thus a bathymetric chart of 5 nm square was produced at a scale of 1: 15,000 and a contour interval of 25 m (Fig,VII.5). The map is refined by combining the sonar and camera data.

The bathymetric map shows that the valley "B" trends NNW-SSE and is 20 km wide from crest to crest. The deepest floor of greater than 2700 m deep occurs along the foot of the eastern wall. The eastern wall appears more steep and rough than the west wall on PDR records. There are several independent mounds and hills on the valley floor. Their elevation ranges from 50 to 300 m. Side echoes on PDR records indicate that they have a rough topography.

DEEP-TOW SONAR SURVEY

Three lines of the deep-towed sonar were run parallel to the trend of the valley at intervals of 1500 m (Fig. VII. 5). The length of each line was about 9 km. As the range of the side-scan sonar (SSS) was 1000 for each side, the total coverage was 9 km by 5 km. The sonographs show areas of different reflective intensity, very strong reflections being generally from rock outcrops and weak reflections from finer sediments (Fig. VII. 6A). A mosaic of seabed character has been constructed from the sonographs (Fig. VII.6B).

The important observations are:

1. On the valley floor, there are many areas of weak reflections. Some

slightly more reflective patches are 100-200 m wide and have a roughness of a few meters. Micro topography in the valley trends randomly, except at the foot of the steep eastern wall where weakly reflective patches are parallel to the trend of the valley.

2. Knolls and steep slopes are commonly moderately reflective, perhaps indicating much pumice at the seabed. There are small patches, particularly on the slope of the central knoll, of very steep reflection presumably from rocky outcrops. Their size is less than a few tens of metres.

3. Sub-bottom profiles obtained by the deep-tow package, show sediment cover of resolvable thickness is extensive in this valley. In the flat floor of the valley, the thickness of the surface sediments is about 16 m and there is a sediment pond on the axis of the valley. The thickness of its sediment is about 30 m (Fig. VII.7A).

4. Sediments barely cover the slope of the central hill (Fig. VII. 7B) where strong and rough reflectors on the SBP record suggest the exposure of basement rock.

OBSERVATIONS BY THE DEEP-TOWED CAMERA SYSTEM

Some characteristic features of the topography were selected from sonar records for the visual obser- vation by the deep-towed camera. Observation sites for the cameras were the steep and rough central hill, the east wall of the valley, and the flat floor with thick sediments (Fig. VII.5). Total recording time of the video TV was 6.5 hours and 650 pictures were taken by photographic camera at a towing speed of 1 knot (Fig. VII.B).

The observations by the deep towed camera system are summarized as follows:

1. The flat floor of the valley is covered with light brown colored,

54. fine, soft sediments, and scattered subangular to subround pebbles. Judging from the sampling by the free-fall grab, most of those pebbles are thinly Mn-coated pumice. There are patches of rare and abundant pumice fragments.

2. The surface of the central hill is rather rough and several depressions are seen on its slope. The coverage of soft sediments on the central hill are rather less than those of the flat floor. Black breccias, probably of fragments of basal t ranging from 5 to 50 cm in size, are observed in several places on the hill. On the other hand, the depressions on the hill are covered with fine sediments and those surfaces are fairly smooth. It seems that the strong and weak reflection patterns on the central hill observed by the SSS are representations of that rough and smooth topography.

3. No pillow lava or hydrothermal activity was observed in this survey area.

4. There were no sedimentary structures that suggest the existence of a bottom current.

57.

5. The number of benthos and nekton observed was extremely few. Fishes, shrimps, jelly fishes, and sponges were seen only rarely. Several animal tracks were observed on the soft sediments.

CONCLUDING REMARKS

The main findings by the deep-tow site survey are summarized below.

1. A steep-sided valley was found at 170 32'S, 176°l6'W in the Lau Basin south of the Peggy Ridge, the topography of which is shown in Fig. VII.5, together with track lines of sonar and camera.

2. The valley trends NNW-SSE, which is similar to the NW-SE trending Peggy Ridge.

3. Observations of 5 square nautical miles by the deep-towed sonar and camera, show that the valley is thinly but extensively covered with light brown colored soft sediments, o to 30 m thick.

4. Although partial exposures of volcanic rocks were observed on the slope of the central hill, pillow lava and hydrothermal activity indicative of active spreading were not found.

5. Judging from the above observations, this valley seems to be presently inactive. Further investigation, including seismicity and magnetic data, is required to pinpoint the zone of presently active spreading.

REFERENCES

CHASE, T . E.; SEEKINS, B. A.; VATH, S.C.; CLOUD, M.A. 1982: Topography of the Tonga Ridge. USGS-CCOP/SOPAC South Pacific project.

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

WEISSEL, J.K. 1981: Evolution of the Lau Basin by the growth of small plates. Pp 429-456' in Talwani, M.; Pitman, W.C. (eds) "Island Arcs, Deep Sea Trenches, and Back-arc Basins". Maurice Ewing Series l. American Geophysical Union, Washington. 470 p.

61.

Fig. VIII.1. Position of OBS sites and air-gun refraction run across OBS 7. seismic large events, such as those at the Tonga Trench region, might be commonly recorded on nearly all OBS, several hundred micro- and small-earthquakes seem to have occurred locally in and around the OBS array. During the OBS observation, significant earthquake swarm activities were observed on 17-18 November and on 4 December 1984. It is interesting to note that, during the period of earthquake swarms, the background noise level was raised several times higher than that of the ordinal calm state. Hypocentre determination of earthquakes will

REFERENCES

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

KARIG, D.E. 1970: Ridges and basins of the Tonga-Kermadec Island Arc System. J. geophys. Res. 75: 239- 254.

SCLATER, J .G.; HAWKINS, J .W.; MAMMERICKX, J.; CHASE, C.G. 1972: Crustal extension between the Tonga and Lau Ridges; petrologic and geophysical evidence. Bull. geol. Soc. Am. 83 : 505-518.

WEISSEL, J .K. 1977: Evolution of the Lau Basin by the growth of small plates. Pp 429-456 in Talwani, M.; Pitman, W.C. (eds) "Island Arcs, Deep Sea Trenches, and Back-arc Basins". Maurice Ewing Series L American Geophysical Unin, Washington. 470 p.

63.

IX. PRELIMINARY PETROLOGY AND GEOCHEMISTRY OF IGNEOUS ROCKS FROM THE NORTHERN TONGA RIDGE AND ADJACENT LAU BASIN

T.J. Fallon Geology Department, University of Tasmania

ABSTRACT

Igneous lithologies were recovered from six dredge stations on the 1984 cruise of the Natsushima to northern Tonga. Seventy-two samples have been examined petrographically, and include fresh basalts, basaltic andesites, andesites, boninites, dolerite, gabbro, plagiogranite, and serpentinised ultramafics. Preliminary major element geochemistry suggests that the rocks dredged from the northern termination of the Tongan Arc are related to the PlioPleistocene Tofua magmatic arc. Pre mid-Miocene basalts recovered from Stn 15, just north of the Vava'u Group, are distinctly different both petrographically and geochemically and are unrelated to the Tofua Arc. Fresh basaltic glass recovered from Stn 31 in the Lau Basin is petrographically and geochemically identical to mid-ocean ridge basalts.

INTRODUCTION

Igneous rocks were recovered from dredge Stns 15, 21, 22, 23, 24, 25, and 31 on the Natsushima 84 Cruise. A preliminary description of the petrography and geochemistry of a representative sub sample of all igneous lithologies recovered is presented here.

PETROGRAPHY

Igneous lithologies at all stations, except Stn 22, were dominated by very fresh basalts and andesites, which have been assigned to petrographic groups based on their primary phenocryst content. Each station is described separately, as rocks grouped from a single station are more likely to be comagmatic than rocks from different stations.

64.

Dredge 1, Stn 15

This dredge haul recovered several kilograms of small fragments of volcanic and subvolcanic rocks which have been grouped into four different lithologies. lA : Clinopyroxene-plagioclasephyric Basaltic Andesites

Group lA is represented by samples 01-31, 01-33, 01-36, 01-39, 01-30, 01-34, and 01-38. Phenocrysts are euhedral consisting of approximately 5% plagioclase (An 93.7-63.9), displaying melt and fluid inclusions, and 1-2% euhedral clinopyroxene (Mg # 386. 7 -81. 5) *. Plagioclase and clinopyroxene phenocrysts occur both as discrete crystals, and in glomerophyric clusters. The groundmass consists of plagioclase microlites (An 54.5-75.3) and quench pyroxenes (Mg # 67.7- 75.3) forming an intersertal texture, with interstitial brown devitrified to altered glass. The glass has been replaced by a mixture of chlorite and smectites. In 01-38, the groundmass quench pyroxenes display a feathery plumose habit. In 01-34, the groundmass plagioclase and clinopyroxene microlites define a subophitic texture to intersertal texture. 01-39 is distinctive in having abundant Fe-Ti oxide granules in the groundmass. lB : Oolerites

This group is represented by 01-35 and 01-37. 01-35 consists of euhedral plagioclase laths (An 8l.9- 32.?) and clinopyroxene (Mg # 84.0-71.1) in an intergranular to subophitic texture. Chlorite fills cracks and fractures within the plagioclase as well as voids between plagioclase and clinopyroxene. Plagioclase shows signs of slight sericitisation, while clinopyroxene shows alteration to uralite and chlorite. 01-35 contains 5-10% Fe-Ti oxides. 01-37 consists predominantly of interlocking plagioclase laths with minor intergranular olivine? and clinopyroxene. Plagioclase laths contain melt and fluid inclusions. The clinopyroxene * Mg # = 100 x atomic Mg/(Mg + Fe2+) is partially replaced by amphibole and chlorites. 01-37 also contains abundant Fe-Ti-oxides. lC : Gabbro

Sample 01-60 consists of approximately equal proportions of coarse grained plagioclase and amphibole. The amphibole replaces clino(?) pryoxene, as relict cores of clino(?)pyroxene are preserved within the amphibole. The latter consists of both hornblende and actinolite. Plagioclase is altered to serici te. There are also minor amounts « 5% of Fe-Ti-oxides present, interstitial to plagioclase and amphibole.

1D : Aphyric Andesite

Sample 01-32 has suffered extensive alteration. The original groundmass has been totally replaced by chlori te and albi te. Minor plagioclase and clinopyroxene phenocrysts originally present have been pseudomorphed by calcite. 01-32 is also cut by quartz-filled fractures.

Dredge 3, Stn 21

This dredge haul recovered the largest quantity (60-70 kg) of igneous rocks. The rocks are all fresh basalts and andesites, and have been grouped into six different lithologies.

3A : pyroxene-Phyric Basalts

This group is represented by samples 03-49, 03-21, 03-51, and 03-50. 03-50 has been extensively altered, the matrix being replaced by quartz, albite, and chlorite and the original orthopyroxene and clinopyroxene phenocrysts have been pseudomorphed by quartz and zeolites. The other three samples are relatively fresh, with alteration confined to the groundmass, in which chlorite and smectites have replaced volcanic glass. All samples contain abundant euhedral orthopyroxene (Mg # 81. 5-85.2) and clinopyroxene (Mg # 81. 6-85.9) phenocrysts in large glomerophyric clusters. The orthopyroxene contains abundant melt and fluid inclusions. Orthopyroxene 65. is the dominant phenocryst phase. The groundmass consists of approximately 50% altered glass and plagioclase (An 69.8-83.9) and pyroxene microlites in a subophitic texture. The groundmass also con- tains abundant Fe-Ti oxide granules and minor pyrite.

3B: Olivine and orthopyroxene phyric Basalt

Sample 03-44 is very vesicular and contains 20-25% olivine (Mg # 80.892.0) and orthopyroxene phenocrysts (Mg # 85.4-86.1), the opx/ol ratio being approximately 3/1. Orthopyroxene occurs as euhedral crystals both in glomerophyric clusters and as discrete phenocrysts, and contains abundant melt and fluid inclusions. Olivine occurs as large phenocrysts with resorbed marg ins; Cr- spinel euhedra, and both melt and fluid inclusions occur in olivine. The groundmass consists of abundant pyroxene microlites (Mg # 74.7-84.9) with minor plagioclase microlites (An 68.6-83.8) and fresh glass.

3C : Olivine + orthopyroxene + clinopyroxene-phyric Basaltic Andesites

Group 3C is represented by samples 03-39, 03-45, and 03-47, all of which contain abundant phenocrysts and glomerocrysts. Phenocrysts include olivine, clinopyroxene, and orthopyroxene while glomercrysts include four types, with orthopyroxene, orthopyroxene + clinopyroxene, orthopyroxene + oli vine, and olivine + orthopyroxene + clinopyroxene, as the dominant phases. When present in a glomerocryst, olivine occurs in the core, suggesting that pyroxene glomerocrysts may be replacing original olivine. Olivine also occurs as large phenocrysts with resorbed margins, containing Cr-spinel, melt, and fluid inclusions. Olivine phenocrysts with the above characteristics form two compositional groupings based on Mg #. One group has high Mg # of 90.8-92.6, the other has Mg # of 83.9-84.5. Olivine occurring as cores to glomerocrysts forms a third group with Mg # of 78.2-78.4. oifferent compositional groups of orthopyroxene and clinopyroxene phenocrysts can also be distinguished by their Mg #. Orthopyroxene and clinopyroxene surrounding olivine in glomerophyric clusters have Mg # of 71.7-76.6 and 66.3-73.5 respectively. Euhedral orthopyroxene and clinopyroxene phenocrysts not as- sociated with olivine have Mg # of 82.3-84.0 and 83.3-85.1 respectively; occasional orthopyroxenes phenocrysts have cores up to Mg # 89.3. The groundmass in this group consists of abundant pyroxene (Mg # 35.1-79.3) and glass with minor plagioclase (An 69.2-76.0) microlites. In 03-47 groundmass pyroxene has a quench morphology, while in 03-45 and 03-39, the pyroxene forms distinct microlites with plagioclase and interstitial brown glass.

3D: Olivine + orthopyroxene + clinopyroxene + plagioclase-phyric Basaltic Andesites

Group 30 is represented by samples 03-22, 03-24, 03-25, 03-34, 03-41, and 03-46. This group is similar to group 3C except for the presence of plagioclase, which occurs as euhedral, as well as resorbed phenocrysts, and as glomerophyric clusters. Plagioclase dominated glomerophyric clusters commonly include orthopyroxene and clinopyroxene. Plagioclase compositions are very anorthitic up to An 93.6 in the cores while rim compositions range down to An 87.3. As for group 3C, group 30 contains different compositional groupings of olivine, orthopyroxene, and clinopyroxene phenocrysts. Olivine occurs as large resorbed phenocrysts containing Crspinels, melt, and fluid inclusions. Two groups of olivine can be distinguished, one with high Mg # of 89.692.7, the other with Mg # of 83.7- 86.1; the latter commonly display a reaction corona of orthopyroxene and minor clinopyroxene, the pyroxenes having Mg # of 78-83.1, similar to Mg # values of co-existing pyroxene phenocrysts and glomerocrysts which do not display a reaction relationship with olivine. The groundmass in all samples except 03-24 consists of abundant pyroxene and altered glass with minor plagioclase microlites. 03-24 is unique,

66. as the groundmass consists entirely of fresh glass and quench pyroxene, containing no plagioclase.

3E : Orthopyroxene + clinopyroxene + plagioclase-phyric Basal tic Andesites

This group is represented by samples 03-26, 03-28, 03-31, 03-40, 03-52, and 03-53, and is characterised by the absence of olivine as a phenocryst phase. Plagioclase is the dominant phenocryst phase, occurring as euhedral zoned and unzoned phenocrysts and as glomerophyric clusters. Euhedral orthopyroxene and clinopyroxene phenocrysts occur together as in glomerophyric clusters sometimes with plagioclase, as well as discrete large phenocrysts. The groundmass in this group consists of plagioclase and pyroxene microlites form- ing an interstitial texture, with interstitial brown devitrified to altered glass. In 03-40 plagioclase and clinopyroxene display a seriate texture, there being no distinct break in crystal size from phenocrysts to groundmass phases. Rocks in group 3E vary in their degree of vesicularity; for example, 03-52 is very vesicular while 03-40 is nonvesicular; most samples show some slight vesicularity.

3F : Aphyric-andesite and Basaltic Andesites

This group is represented by samples 03-23, 03-27, 03-29, 03-30, 03-36, 03-32, and 03-42. All samples are very similar petrographically, containing sparse «1% microphenocrysts of plagioclase, orthopyroxene and clinopyroxene. In 03-36 plagioclase microphenocryst compositions range from An 81.5-91.6; groundmass plagioclase compositions range from An 63.3-67.6. Groundmass and pyroxene (Mg # 32.9-72.5) microlites form an interstitial texture, with interstitial brown devitrified glass. The plagioclase microlites define a well developed pilotaxi tic texture. The groundmass also contains 2.5% Fe-Ti oxides. Rocks in group 3F contain very few vesicles; where they do occur, form distinct linear bands across the rock parallel to the pilotaxitic texture of the plagioclase, probably due to magma flow.

Dredge 4, Stn 22

This dredge station recovered some large blocks of fresh and altered plutonic rocks, altered basalts, and serpentinite. The rocks are grouped into four lithologies.

4A : Gabbro

Group 4A is represented by samples 04-11, 04-12, and 04-15 which were all originally a one- or two- pyroxene gabbro; however, original pyroxene has been totally replaced by amphibole and chlorites. In sample 04-12, remnant cores of clinopyroxene are preserved, having a Mg # = 85.1. The amphibole in these rocks includes hornblende and actinolite. Plagioclase is unaltered and fresh in sample 04-12, its compositions ranging from An 53.9-91.0; in the other two samples, plagioclase is somewhat saussurited. The gabbros all contain accessory Fe-Ti oxides, and secondary epidote. 04-15 contains quartz veins.

4B : Plagiogranite

04-14 is a medium- to fine-grained holocrystalline rock with a microgranitic texture. Staining shows it to contain equal amounts of quartz and plagioclase with no trace of alkali feldspar. It also contains 5-10% hornblende and accessory sphere, apatite, and epidote. Plagioclase compositions lie in the range An 57.5-62.4.

4C : Serpentinised Ultramafics

Two small pieces of serpentinite, samples 04-16 and 04-17 were obtained from Stn 22. 04-17 contains no relict minerals being entirely composed of serpentine. 04-16 however, contains relict cores of olivine and orthopyroxene, suggesting the original rock was possibly harzburgite.

40 : Altered Basalts

Altered basalts 04-10 and 04-13 were also recovered from Stn 22. 04-13 has been totally replaced by 67. calcite and serpentine; however original phenocryst outlines are preserved, suggesting the original rock was a pyroxene-phyric basal t. D4-10 has been totally replaced by chlorites, serpentine, and calcite. Phenocryst pseudomorphs suggest the rock was originally pyroxeneplagioclase-phyric basalt.

Dredge 5, Stn 23

Thid dredge haul consisted entirely of fresh basalt; samples have been grouped into two lithologies.

5A : Olivine + orthopyroxene + clinopyroxene-phyric Basalts

Group SA is represented by samples D5-24, D5-25, and D5-28. These rocks form a very distinctive group, being extremely vesicular and containing abundant large olivine (20%) and orthopyroxene (16%) phenocrysts; D5-25 also contains clinopyroxene « 5%) phenocrysts. The phenocrysts are set in a fine-grained matrix consisting of fresh glass and quench pyroxenes (Mg # 79.6-85.6). These rocks have the petrographic characteristics of boninite or high Mgandesites, except the orthopyroxene is not clinoenstatite but a high Mg # bronzite-enstatite containing between 1 and 2 wt % CaO.

Olivine phenocrysts vary from euhedral crystals to crystals having resorbed and embayed margins. Both olivine and orthopyroxene contain Cr-spinel, melt, and fluid inclusions. Orthopyroxene phenocrysts are generally euhedral.

5B : Orthopyroxene + clinopyroxene + plagioclase-phyric Basal tic Andesites

This group is represented by samples D5-20, D5-2l, D5-23, and D5-27. Group 5B includes non- vesicular to vesicular rocks. The group as a whole is petrographically very similar to group 3E from dredge 3.

Dredge 6, Stn 24

6A : Olivine-phyric high-Mg Andesites

This group is represented by samples D6-2 and D6-3. Both samples are pieces of the outer parts of very vesicular pillow basalts, containing fresh glassy rinds. The glass rind contains small olivine microphenocrysts (Mg # 88.3-90.8) and minor quench pyroxenes set in a fresh, alteration free, glass. The interior of the pillows is characterised by abundant ('\, 10%) olivine microphenocrysts and occasional phenocrysts, containing Cr-spinel inclusions, set in a groundmass of glass and abundant quench pyroxenes. The groundmass pyroxenes display a well-defined quench trend on the conventional pyroxene quadrilateral (Fig. IX.l). Pyroxenes range from high Mg # 89.6 orthopyroxene to clinopyroxenes with Mg # values ranging down to 80.5; Groups 6A and SA are similar petrographically in that they both contain a groundmass of fresh glass and quench pyroxene; however group 6A differs from 5B in containing no orthopyroxene phenocrysts. Group 6A is similar petrographically to rocks from the Upper Pillow Lavas from the Arakapas Fault region of the Troodos Ophiolite, Cyprus (Cameron 1985); these also include olivine and glan-quench pyroxene rocks.

Dredge 7, Stn 25

7A : Olivine-phyric high Mg-Andesite

Group 7A is represented by samples D7-2 through to D7-18. Group 7A are all very fresh vesicular pillows, with fresh glass rinds. As a group they are all very similar petrographically and are likely to be fragments from a single pillowed lava flow. Group 7A is identical petrographically to Group 6A. The glass rind consists of fresh clear glass containing small olivine microphenocrysts, the interior of the pillows consists of olivine microphenocrysts and occasional larger phenocrysts together with quench pyroxenes and glass. Orthopyroxene phenocrysts are conspicuously absent.

Dredge 11, Stn 31 llA : Olivine + plagioclase-phyric Basalt

Station 31 recovered one small piece of fresh pillow rind. The pillow rind consists of fresh glass with euhedral plagioclase microli tes and euhedral olivine (Mg * 87) microphenocrysts. Plagioclase compositions range from An 68.6-82.

GEOCHEMISTRY

Preliminary major element geochemistry of igneous rocks from each petrographic group are presented in Table IX. 2. The detailed geochemistry of the dredged rocks will be described in a later publication. Where possible, defocused beam electron microprobe analyses of glass rinds or unaltered glass groundmass are presented, as these are likely to represent the actual liquid compositions of magmas involved in the formation of the northern Tonga Ridge and Lau Basin.

DISCUSSION

The dredge rocks from Stns 21 and 25 are unusual in that they contain a significant proportion of olivinephyric basalts. The dominant mineralogy of exposed basalts of the Tofua magmatic arc consists of plagioclase, clinophyroxene, and orthopyroxene (Ewart and Bryan 1972; Ewart 1976; Ewart et ale 1977). The Ti02 contents of the basalts from Stns 21 and 25 overlap with those of Tafahi Island, when plotted against MgO (Fig. IX.2). This suggests that these rocks are related to the Tofua magmatic arc, possibly forming a basement sequence not exposed on the islands of the Tofua magmatic arc. Stations 21 and 22 are in an area extensively sampled by the Russian ship R.V. Kallisto, Leg 16 (Saraskin et ale 1983). In this area, the Russians obtained a greater proportion of ultramafic rocks compared to Natsushima 84. Stations 24 and 25, located further west of Stns 21 and 22, bought up a very uniform collection of fresh very vesicular mafic pillow lavas. Chemically, these lavas are high Mgandesite or boninite. Although K-Ar dating is required to constrain the tectonic significance of these boninites, the very fresh nature of the pillows indicates a probable fairly recent eruption. If the boninites do yield a recent age it may indicate possible rifting of the Tofua Arc at its northern termination. The Russian ship R. V. Kallisto recovered boninites close to Natsushima Stns 24 and 25, which

indicates that boninites are probably quite abundant at the northern termination of the Tofua Arc.

Igneous rocks recovered from Stn 15 occurred as clasts in a canyonfill, debris-flow type deposit (C.J. Jenkins, pers. corom. ). Sediment adherring to some of the basalt cobbles has a preliminary age of mid- Miocene (B.W. Hayward, pers. corom.). This suggests the igneous rocks could not have been derived from the young Tofua magmatic arc. The basalts from Stn 15 show significant differences in petrography and geo- chemistry from basalts from the Tofua magmatic arc. Petrographically the basalts from Stn 15 lack ortho- pyroxene phenocrysts, which are ubiquitous in lava from the Tofua magmatic arc. Geochemically, the basal ts have much higher Ti02 contents (Fig. IX.2) than the Tofua Arc volcanics, and contain higher abundances of Na20. The basalts from Stn 15 may be related to the middle Eocene basement exposed on 'Eua. However, published analyses from 'Eua (Ewart and Bryan 1972) have Tj02 contents intermediate between Str. 15 and rocks from the Tofua Arc. Further geochemical study is required to establish the tectonic significance of these basalts.

The EPM analysis of glass rind from stn 31 shows it to have the major element geochemistry of mid- ocean ridge basalts and back-arc basin basalts, similar to previous rocks dredged from the Lau Basin (Hawkins 1976).

REFERENCES

CAMERON, W.E. 1985: Petrology and origin of primitive lavas from the Troodos Ophiolite, Cyprus. Contr. Miner. Petrol. 89.

EWART, A. 1976: A petrological study of the younger Tongan andesites and dacites and the olivine tholeites 71. of Niua Fo'ou Island, S.W. Pacific. Contr. Miner. Petrol. 58 : 1-21.

EWART, A.; BRYAN, N.B. 1972: Petrography and geochemistry of the igneous rocks from 'Eua, Tongan Islands. Bull. geol. Soc. Am. 83 : 3281-3298.

EWART, A.; BROTHERS, R. N.; MALEEN, A. 1977: An outline of the geology and geochemistry and the possible petrogenetic evolution of the volcanic rocks of the Tonga-Kermadec- New Zealand island arc. J. Volcano geotherm. Res. 2 : 205-250.

HAWKINS, J.W. 1976: petrology and geochemistry of basaltic rocks of the Lau Basin. Earth Planet. Sci. Lett. 28 : 283-297.

SHARASKIN, A.Y.; PUSTCHIN, I.K.; ZLOBIN, S.K.; KOLESOV, G.M. 1983: Two ophilite sequences from the basement of the northern Tonga Arc. Ofioliti 8(3) : 411-430.

72

73.

X. LATE CENOZOIC SEDIMENTARY ROCKS DREDGED FROM THE NORTH TONGA RIDGE PRELIMINARY ANALYSIS (with an Appendix on DEWATERING FOLIATION IN A FOREARC SANDSTONE)

Chris. J. Jenkins Ocean Sciences Institute, University of Sydney

INTRODUCTION

Lithified sediments of Recent to late Miocene / early Pliocene age were dredged from high forearc, volcanic arc and back-arc slope locations at water depths of 14004800 m. Of eight successful dredgings, six yielded consolidated sediments. Most of the successful dredging stations were around the very northernmost Tonga Ridge between 15°S and 16°S.

Dredge operations were carried out using a short, 50 cm wide steel tube opening into a 7 cm- mesh chain bag, a cluster of 3 x 50 kg weights being attached to the dredge/main cable linkage 2 m from the dredge mouth. Dredging sites were chosen on the basis of the single channel seismic records and a PDR run across the site immediately before taking up station. Station locations are shown in Fig. X.1.

ROCK CLASSIFICATION

This report is founded on handspecimen and binocular microscope (sand-fraction) examinations of the sedimentary rocks, conducted on board the Natsushima. It is stressed that the descriptions, classifications, and conclusions are only tentative.

So far as possible, the sediment classification follows the systems used by Pettijohn (1975). As noted in Exon et al. (1984:63) who described south Tonga Arc sediments, mixed volcanogenic/calcareous biogenic sediments are difficult to classify, but an approach which organises lithologies between the three axes of mud, sand, and carbonate is appropriate to the facies on and around the Tonga Ridge. Thus, a primary classification based on dominant grainsize, is modified by descriptors indicating both primary composition (e.g., volcanogenic, lithic, or calcareous biogenic) and any extra components like pumice, foraminifera, or pebbles which attain more than 30% by volume. Use of the term 'marl' differs from that in Exon et al. (1984 : 63) being restricted here to a pelitic grade calcareous rock, with appreciable carbonate in its mud fraction.

THE STRATIGRAPHIC ORDERING OF FACIES

Dredging presents a jumbled and biased sample of seafloor lithologies over a distance of perhaps 0.3-1 km. A bias can develop because thin or softer lithologies may not be sampled. However, the lithologies which are collected may be grouped, and it may prove possible to arrange those groups or 'facies' in some kind of stratigraphic order. Possible methods include :

1. Biostratigraphic dating when agediagnostic fossils are present.

2. The use of 'incorporation relations' whereby clasts of older lithologies have been reworked into younger units.

3. By a scale of increasing compaction or lithification.

None of the methods is universally applicable or infallible. In particular, sequences based on degree of lithification should generally be used in concert with other methods.

The tentative results of these analyses, integrated with biostratigraphic information are presented

75. in Fig. X.2.

The lithologic units or ' facies' distinguished in this report are numbered individually for each dredge site, with Fl as the surface ediment and succeeding F numbers for each older facies. Volcanic and igneous units are labelled FV or FI and are described elsewhere (Falloon, this volume) .

ACKNOWLEDGMENTS

I am grateful to B.W. Hayward and S. Shafik for making available their onboard biostratigraphic results. Bruce Hayward, Keith Lewis, and Trevor Falloon offered helpful comments during the study.

FACIES DESCRIPTIONS

Forearc NE of Vava'u : Stn 15

(17°30'S) The east flank of a small step on the middle trench-slope at l7°37.6'S 172°40.4'W, was dredged in 4325-4860 metres of water. Seismic data in single channel form (Line 20) suggested an outcrop of the 'basement' to modern trench-slope basins. The dredged lithologies however, may derive from a variety of sources there and higher on the slope. Indications that a canyon-fill deposit or scree was sampled are :

(i) the modern sediment is an angular gravel with sandy mud matrix and all the major lithologies of the dredge are represented as clasts;

(ii) many larger grains have been enclosed in younger sedimentary units at some stage; and

(iii) blocks of all ages are strongly ferrugineous-stained.

The fact that several cycles of incorporation into younger sediments have apparently operated, suggests the site has long been accumulating scree.

Dredge 15 Dl

The following units are distinguished:

Fl (youngest) Unlithified surface angular gravel ("breccia") Dark greenish brown; muddy sand matrix; clasts up to cobble size (300 m), but generally pebble grade; clasts subangular; poorly sorted; clasts composed of Facies 2-4 and isolated mafic igneous blocks (see below); slightly compacted and plastic; no bedding observed; no forams, implying deposition below the CCD.

Age: Late Pleistocene to Recent.

Examples : 15 Dl-4l, 12.

F2 Foraminiferal sandstones and siltstones

Brownish-white; fine sand and silt grain sizes; poorly sorted; biogenic carbonate about 50%, lithic grains (volcanogenic) 20%, quartz and/or feldspar about 10%, pumice and pale volcanic glass 20%; lithification is variable - brittle and cemented, plastic to soft; foram content variable, but generally high; the unit can enclose blocks of Facies 4 and rarely, basalt clasts.

Age : Late Pliocene to early Pleistocene.

Examples : Coating on 15 Dl-4, 13; 15 Dl-20.

F3 Pumiceous volcanogenic conglomerates

Greyish-green and white, with some black grains/clasts; medium pebble to medium sand grain sizes, some pebbles up to 45 mm size, and matrix generally medium sand grade; sorting is poor; sand size fraction has 60% pumice, 7% bipyramidal euhedral (B) quartz, 7% ?feldspar, 7% (various colour) volcanic glass shards, 7% ?amphibole, and 10% lithic pieces (e.g., siltstone); large pebbles of basalt and olive-brown siltstone are common; pebbles are subrounded (siltstones) to subangular (basalts) pumice clasts up to 50 cm in size have been crushed into wisps parallel to the bedding; the rock is brittle but very porous; no microfossils detected; the unit incorporates clasts of Facies 4 and basalts.

Age : No fossils.

Examples : 15 Dl-2, 8, 9, 21, 25, 48.

77.

F4 Tectonically deformed volcanogenic sandstones

Reddish-, yellowish- or olive-brown, some deep-olive ; muddy sandstones, siltstones, or coarse to medium sandstones; rare gri ts; sorting usually poor, but good in some coarser olive sandstones; sand fractions composed predominantly of black through to colourless volcanic glass (vesicular, fibrous and solid), with minor « 5%) feldspar and quarts; bedding generally poor, better in the olive sandstones, on scale of 1-3 cm; trace fossil burrows present, not common; forams abundant in some specimens (especially the olive sandstone variation) but have been dissolved from some blocks before incorporation into Facies 2; generally well-lithified, though many sandstones remain friable; tectonic fractures common (Fig. X.3A and appendix); the rare pumice clasts are surprisingly uncrushed.

Age : Late Miocene to early Pliocene

Examples : 15 Dl-l, 5, 6(a,b), 7, 28, 29, 40, 54 (brown variation) ; 15 Dl-ll, 15, 24, 44, 50, 53 (olive types).

The isolated mafic igneous blocks obtained in 15 Dl might be clasts and pebbles freed from Facies 3 conglomerates, or may represent a distinct mafic body in the area which mayor may not have the source for F3. Examples include 15 Dl-lO, 27 30, 39, 43, 52, and 60-69. Falloon (this volume) reports that some of these separate blocks have an andesitic petrography and are altered to greenschist metamorphic facies

The lithification of samples suggests that sedimentary Facies 3 and 4 belong to the 'acoustic basement' in unprocessed seismic records of this southern area.

Forearc E of Niuatoputapu : Stns 26 and 28 (16°S)

The first station (26 D8) sampled an area which single channel seismic records suggested was closely faulted, possibly semi-lithified sediments above shallow acoustic basement. The location at 15 ° 54.2 'S, l72°46.5'W was in 3515-3573 m water depth. Dredge 28 D9, located at 15°53.2'5, l73°09.3'W in 1184-1464 m of water was positioned on the face of fault scarp of seismically well layered sediments.

Dredge 26 D8

Only one semi-lithified unit is identified.

Fl Surface sediment

Age : Late Pleistocene to Recent.

Example : 26 Dl-8.

F2 Foraminiferal volcanic sandstones

Yellowish-brown; coarse to fine sandstones; sorting poor; pumice clasts rounded, up to 1 cm in size; sand fractions composed of forams (and other rare biogenic elements) 30%, medium-brown volcanic glass 20% lumps of micritic carbonate and silt/clay 35%, quartz 5%, and trace olivine (2%); lithology is compacted, but still soft and friable; no bedding observed; foram concentrations quite variable.

Age : Early Pleistocene.

Examples: 26 D8-2, 3, 4.

Dredge 28 D9

Two older units are distinguished.

Fl Surface sediment

Pale brown; fine to medium sandstone.

Age : Late Pleistocene to Recent. Example : 28 D9-6.

F2 Sandy marls, shale breccias and diamictic pumiceous grits

Pale brownish-grey to pale yellowiSh-brown; grain sizes generally coarse silt to fine sand, with sand concentrated in the abundant tracefossil burrows; the grits have abundant 2-10 mm pumice pieces set in sandy marl matrix; moderately good sorting except in grits; sandfraction washings are of unbroken micrite/silt/clay clumps 70%, forams 15%, and minor pyrite and colourless volcanic glass; the lithology is moderately well lithified, but is friable or 50ft when wet; bedding

79. present but vague, on scale of 3- 4 cm and marked by colour changes and trace-fossil levels; shale- breccias with subangular clasts of Facies 3 and up to 50 mm in size are a feature of the unit.

Age : Early Pliocene or early to mid Pliocene.

Examples: 28 D9-1, 2, 3, 10.

F3 Volcanic sandstones, grits, marls and shale-breccias

Olive, yellow-olive and deep olive; medium sandstones and siltstones except for grits an breccias; sandstones frequently well sorted and porous; sandy fractions are almost completely of volcanic glass (wide variety in colours and textures) with trace feldspar and ?quartz; grits have clasts up to 1 cm of grey pumice and apparently fresh basaltic glass and are fairly well sorted; facies is generally well lithified, some marls showing septarian nodule type fractures; bedding frequently visible, on scale of 1-2 cm.

Age: In shale-breccia 28 D9-7, mid Miocene to early Pliocene.

Examples: 28 D9-4, 5, 8, 7, 9.

North end of Tofua Volcanic Arc : Stns 21, 22, 24, and 25 (15°5)

Five dredges (21 D3 - 25 D7) were located around the isolated 30 km wide block which terminates the Tofua Volcanic Chain of the Tonga Ridge at 15°5, 17°40'W. They sampled products of several volcanic and tectonic events.

Dredge 21 D3

A huge recovery of diverse lithologies was obtained, reflecting Pliocene basaltic pyroclastic and extrusive activity and subsequently derived, volcaniclastic deposition. The site is positioned at the top of the N-facing slope descending from the Tonga Ridge into the transcurrent, E-W trending trench segment Depths at the site were 1520-2000"m. Five sedimentary/volcanic units are recognized.

Fl Surface sediment

Greyish-olive; fine sandy-silt; volcanogenic; forams present.

Age : Late pleistocene to Recent. Example : 21 D3-5.

F2 Foraminiferal sandstone

Brownish-white when fresh; medium sandstones to siltstones; poorly sorted; sandy phases consist of foraminifera 50%, pumice-textured, dark volcanic glass 45%, olivine 2%, minor basal tic lithic fragments, and greenish-grey altered glass; poorly lithified; diverse benthic microfossils (Hayward, this volume) .

Age : Late Pliocene to Recent, early Pleistocene, and Pleistocene to Recent.

Examples : 21 D3-4, 12, 13.

F3 Calcareous volcanic sandstones, grits and siltstones

White, grey or greenish-grey, some siltstones reddish-grey; grain sizes to 5 mm in grits, but generally medium sandstones; usually moderately well sorted; and fractions comprised of 98% fresh, colourless volcanic glass with intergrown carbonate; olivine occurs in minor amounts; carbonate matrix probably hydrothermal/diagenetic, sediments well bedded, with common graded bedding; planktonic forams present.

Age: Late Pliocene to early Pleistocene.

Examples: 21 D3-4, 17.

F4 Volcanic sandstones and grits Greyish-bluish-green or deep olive green; mostly coarse or medium sandstones, grit particles to 7 mm size; moderate sorting; sand fractions composed almost completely of white glass or grey-blue-green blistered devitrified material; ?quartz, feldspar and black glass in trace amounts and olivine up to 15% in some samples; the grits bear olivine-rich scoria pieces; grit particles angular to subangular; no bedding observed; no fossils detected.

Age: unknown

Examples : 21 D3-2, 3, 12, 13, 15, 16.

80.

F5 Scoriaceous volcanic tuffs and breccias

Black and olive, to grey; clasts to 70 mm size; matrix a coarse ash to lapilli tuff; sorting poor to moderate, with little silt fraction present; sand fraction composed almost entirely of dark, slightly altered vesicular and solid glass with minor colourless glass, euhedral olivine (up to 8%) and trace pyroxene; carbonate and zeolite alteration in evidence; clasts angular to subangular; no bedding observed; no microfossils found.

Age : unknown

Examples: 21 D3-17, 16, 18.

Dredge 22 D4

This dredge station at 14°49'S, 173°23'W was positioned lower on the same N-facing slope as 21 D3, at 4400-4500 m water depth. An entirely igneous suite of serpentinites (massive and deformed), diorite, plagioclase granite was collected.

One sample, 22 D4-3 (Fig. X.3B) is composed of brecciated serpentinite pieces, ranging from 1-100 mm in size, in a fine, ground mass. Clasts are imbricated parallel with fractions in the ground mass. Some large clasts show fractures normal to the imbrication. The lithology is interpreted as probably a fault breccia, brittle deformation and two sets of fractures suggesting low confining pressures (Wicks 1984).

Dredge 23 D5

The stratigraphy suggested in this dredge haul is strikingly similar to that for 21 D3, reflecting a prelate Pliocene, olivine-phyric volcanic episode on the nearby ridge crest. The station lies in 1600- 2250 m of water at 15°19.8'S, 173°23.9'W, located on the southern scarp of an E-W trending, 1 km deep graben newly discovered in the area during this Natsushima cruise. Seismic line 25 reveals a? talus slope at the base of the scarp. The dredged units can be described as follows.

Fl Surface sediment

Olive-brown; fine, sandy mud.

Age : Late Pleistocene to Recent.

Example : 23 D5-1.

F2 Foraminiferal marl

Pale brown, speckled with white forams; a bimodal sediment with forams up to medium sand grade scattered in medium siltstone matrix; sorting consequently poor; sand fraction almost all forams, with only 5% of volcanic glass (wide variation of textures and colours); clots of carbonate (micrite), silt and clay common in sand grade washings; sediment is only partially compacted; soft when wet; bedding not observed.

Age : Late Pliocene to early Pleistocene.

Example : 23 D5-2.

F3 Medium volcanic sandstones and siltstones

Greenish-white or grey; medium sandstone to siltstone; sand fractions consist of 60% brown glass, 30% colourless pumice-glass and minor content of forams, colourless solid glass, olivine and ?feldspar; soft, lithification much as in unit F2; bedding observed between fine white, and darker sandy layers on scale of 2-5 mm.

Age : Early Pleistocene.

Example ; 23 D5-8.

F4 Coarse volcanic sandstones and siltstones

Dark to yellowish-olive; coarse sandstones and siltstones; larger sand grains are well-rounded; sand grades composed of 90% brownish glass (vesicular, saccharine and fibrous), with minor yellowish glass forams, olivine,? quartz and micritic carbonite clumps; unit is moderately lithified; well-bedded in coarse/fine layers on a scale of 120 mm, with wavy and oblique beds and minor scours in most cases; forams frequently concentrated at 81 the tops of coarse beds; some rip-up clasts present; evidently this facies was deposited in a vigorous traction-current environment.

Age : Late Pliocene to early Pleistocene.

Examples : 23 D5-3, 4, 6, 9.

F5 Scoriaceous volcanic breccia Yellowish-olive with white and black components and some purplish scoria pieces; clasts to 90 cm size, matrix is a coarse sandstone to grit grade; particles are angular to subangular; sorting is very poor; sand separates composed of 80% dark brown through to colourless fresh volcanic glass, 15% ?feldspar, and minor basaltic fragments, olivine and pyroxene; unit is well lithified, though still friable; no bedding observed; no fossils.

Age : unknown

Examples : 23 D5-5, 10, 22.

Dredge 24 D6, and 25 D7

Two dredges carried out in close proximity and west of the peak which terminates the Tonga Ridge (Fig. X. 1) yielded fresh pillow lavas with no sediment cover (Falloon, this volume).

CONCLUSIONS

Since this report embodies only preliminary observations of sedimentary rocks from a few dredge hauls over a large area of the north Tonga Arc, no comprehensive picture of the history of sedimentation can be drawn. However, a few significant conclusions seem to be well-supported even at this early stage.

(a) A late Neogene facies of pale brown to white, foraminiferal marls, siltstones and sandstones with high biogenic carbonate content was encountered in nearly every dredge that yielded sedimentary rocks. Palaeontological ages are late Pliocene to early Pleistocene. At 28 D9 a slightly different development with less biogenic content is somewhat older - middle or early pliocene (28 D9 Facies 2; see Hayward, this volume). Surface samples at most dredge and free- fall grab stations show that a similar facies of tan, foraminiferal mud still accumulates widely over the north Tonga Ridge. An explanation of the facies probably hinges on a decrease of coarse volcanic supply over the ridge in the Pliocene, allowing background muddy and biogenic carbonate deposition to dominate. Different developments of the facies are united on composi- tional, textural, and colour traits. The differences in grain sizes and bedding characters seems to reflect a variation in transporting and depositing processes over the ridge.

(b) Three distinct levels of igneous activity were sampled. The youngest in dredges 26 D9 and 28 D9 near the northern extremity of the Tofua Volcanic Chain seems to represent modern Tofua Chain activity, not yet covered by sediments or showing alteration.

(c) Outcrops of an older (pre-early or late Pliocene), olivinejplagioclase-phyric volcanic episode were intersected in dredged 21 D3 and 23 D5 at the northern end of the ridge. Vesicular and non-vesicular basalts plus associated derived tuffs and epiclastics appear to underlie the soft pale-brown foraminiferal marl facies which is late Pliocene to Pleistocene in age (Hayward, this volume). It is this volcanic period - or perhaps an extension further south along the ridge - which may account for the olivine detected in south Tonga Ridge sediments by Cawood (1984). The mineral is certainly a hallmark of succeeding sediments on the north Tonga Arc.

(d) Evidence for even older volcanism comes from 15 Dl, northeast of Vava'u. Firstly, grits in the lowest facies (F4 in Fig. X.2) contain basaltic material. Facies F4 is late Miocene or early Pliocene (Hayward, this volume). Secondly, an apparently younger facies - F3, the volcano- genic-rich conglomerates - bears

82. large basaltic clasts and pebbles. If the stratigraphic order in 15 Dl is correctly interpreted, Facies 3 is Pliocene in age. Whether isolated blocks of basalt and dolerite brought up in this dredge are similar to those in F3 has yet to be determined by petrological and geochemical analysis.

(e) Well lithified brown or olive sandstones and shales of late Miocene or early Pliocene age were collected in widely separated dredgings (15 Dl, 28 D9) and may be widespread. In the region of 15 Dl, northeast of Vava'u, parts of this facies are tectonically deformed.

(f) There are lithological indications of some unconformable relationships in the stratigraphy, with pliocene shale breccias and conglomerates present in 15 Dl and 28 D9.

(g) Occurrence of pyramidal quartz in Facies F3 of dredge 15 Dl, and of abundant olivine in Facies F4 and F5 in both 21 D3 and 23 D5 will warrant special attention in any explanations of the evolution of magma types along the Tonga Ridge.

REFERENCES

CAWOOD,P.A. 1984: Petrography, phase chemistry and provenance of volcanic debris from the Tonga Forearc : Implications for arc history and magmatism. Rep. Ocean Sci. Inst. 2: 1-30.

CHASE, T.E.; SEEKINS, B.A.; VATH, S.C.; CLOUD, M.A. 1982: Topography of the Tonga region. USGS-CCOP/SOPAC South Pacific Project.

EXON, N.F.; HERZER, R.H.; COLE, J.W. 1984: petrology of rocks dredged from the south Tonga Platform. Pp 59-81 in Scholl, D.W.; Vallier, T.L. (Comps & Eds) "Tonga Ridge Resource Study and Tripartite Cruise Report". CCOP/SOPAC Tech. Rep. 38: 406 p.

PETTIJOHN, E.J. 1975: "Sedimentary Rocks" (3rd Edition). Harper & Row. 628 p.

WICKS, F.J. 1984: Deformation histories as recorded by serpentinites.

Ii. Deformation during and after serpentinization. Can. Mineralogist 22: 197-203.

APPENDIX

DEWATERING FOLIATION IN A FOREARC SANDSTONE

Sample 15 Dl-6A is a moderately lithified rock, inclined to be crumbly and very soft when wet. Dark olive-brown, anastomosing veins of variable width and length are oriented vertical to the bedding (Fig. X. 3A). They are darker than the unaffected rock and have a tapered, wispy form up to 1 mm wide and 3 cm long. A strong parallel orientation of veins is noted which is roughly perpendicular to the bed- ding. Sections parallel to bedding show this orientation in the form of an elongated anastomosing fabric, outlining rhombic prisms of unaltered rock with acute angles of 25° or less. The high degree of orientation of the veins implies that they were formed when the rock was under some type of directed strain or confinement. However, there is no evidence of shortening. Pumice clasts of 1 cm size that are set in the sediment show no crushing parallel to the bedding (as some other lithologies in 15 Dl do) and no shortening across the orientation of the veins. Presumably, vein formation took place in an expansional strain state.

Each vein consists of the same material, predominantly dark olivebrown clay with abundant isolated ('floating') sand/silt grains of the type composing the surrounding rock.

A strong control on vein width, length, and spacing is exercised by the bedding, with coarser grained strata having fewer folia. Another significant observation is that bedding surfaces have the appearance of slickenside; a smoothly undulating surface covered thickly with dark-stained clay accumulations and a gentle striation marking the intersection of the vein structure. The rock also shows some dark stained fracture surfaces perpendicular to both the bedding and the 83. vein network.

Pronounced resemblances exist between this set of mesoscopic fabrics and some described from the trench slope of northern Japan by Carson et al. (1982). These fabrics were obtained deep within the Japan Trench slope during DSDP Legs 56 and 57. Carson et al. document the same type of vein structure from subseafloor depths of 600 m under the uppermost trench slope to only 250 cm under the lower trench slope. Convergences in form include wispy shape, nature of the filling, the suppression of veins at and within coarse layers and a strong orientation mostly at very high angles to the bedding. An occurrence of the same tectonic fabric at the surface in the area of 15 Dl on the north Tonga trench slope, suggests there may be some difference in erosion or uplift styles between the Tongan and north Japan trench slope zones.

Dewatering of the sediments, perhaps under high pore-pressure, and in a state of local, directed strains or confinement is the most likely explanation for this fabric. Carson et al. suggest that one cause may be the injection of water from underlying consolidating oceanic sediments accreted to deeper levels in a deforming sediment pile.

REFERENCE

CARSON, B.; VON HUENE, R.; ARTHUR, M. 1982: Small-scale deformation structures and physical properties related to convergence in Japan trench slope sediments. Tectonics 1: 277-302. 84.

85.

XI. PRELIMINARY LOG OF NORTHERN LAU BASIN - TONGA RIDGE PISTON CORES

Chris J. Jenkins Ocean Sciences Institute, University of Sydney

INTRODUCTION

A coring programme on the northern Tonga Ridge and northern Lau Basin yielded four cores (Fig. XI.I). Two 8 m long piston cores were obtained from the central part of the Lau Basin. Two piston cores, from the Tonga Ridge and from the "Natsushima Trough" at its western edge were less than I m long. Four attempts at gravity coring on the Tonga Ridge were all unsuccessful. At all coring stations on the Tonga Ridge either the core barrel or the heat-flow probe or both, were bent. Reasons for this may include - 1. the compacted nature of the well-sorted, volcanogenic sands that blanket the Tonga Ridge; 2. armouring by pumice clasts at and beneath the seabed, and 3. early cementation of vitrictuff lithologies.

LOG FORMAT

The data presented in this report are derived from shipboard logging of split cores. More detailed analysis will follow and will be published later. The grain sizes

86. were examined only by visual or hand-lens examination, and by feel. The colours were described in open sunshine using an abbreviated Munsell Chart. Following this examination, s. Shafik took samples for the nannofossils identification. The other half of the split cores was preserved intact for magneto- stratigraphic analysis in Japan (Joshima, this volume).

The logs (Figs XI.2-5) show black for vacant parts of the core-liners at the time of logging. Dashed unit boundaries were gradational or indistinct; solid boundaries were sharply defined. Triangles beside the cores indicate the nannofossil sample locations. The following colours were recorded:

CORE LOCATIONS

The first, 17 PCl, was situated on the eastern sector of the Tonga Ridge crest platform, slightly north east of Fonualei Island. Recovery was small, amounting to only 0.71 m of yellowish and olive- grey sands and silts. (Fig. XI. 2)

30 PC2 returned nearly a metre of unusual, partly lithified, bluegreen, vitric tuff from the western side of the NNW-SSE directed cleft, the "Natsushima Trough" on the western side of the Tonga Ridge. (Fig. XI.3)

In the western Lau Basin, 32 PC3 yielded nearly 8 m of fairly homogeneous yellowish-brown volcanogenic and foraminiferal sandy muds. The location, west of the Peggy Ridge, was only 130 km east of the Lau Ridge. (Fig. XI. 4)

33 PC4 sampled at the NNW end of the "Natsushima Ridge" which diverges from the back of the Tonga Ridge between Fonualei and Niutoputapu Islands. The coring site lay slightly northeast of the ridge crest. Almost 8 m of well-bedded, turbidi tic, volcanogenic and foraminiferal sands interbedded with fine to medium muddy sands. (Fig. XI.5)

A full interpretation of the cores is dependent on a synthesis of detailed stratigraphic, grain size, mineralogical, micropalentological, and magnetostratigraphic analysis. This will be attempted later.

REFERENCE

CHASE, T. E.; SEEKINS, B. A.; VATH, S.C.; CLOUD, M.A. 1982: Topography of the Tonga region. USGS-CCOP/SOPAC South Pacific Project.

92.

Fig. XI.5E.

Station 33 PC4 continued 10/12/1984 16°24.7'S, 175°05.0'W 1245 m 8 m piston core

8m

Fine sand; Moderate yellowish-brown to dark yellowish-brown

Fine sand; Olive-grey 8.2m 8.2m

93.

XII. REMNANT MAGNETISATION OF PISTON CORES FROM THE LAU BASIN

Masato Joshima Geological Survey of Japan

SAMPLES

Two 8 m piston cores of sediment in the Lau Basin are examined here. One, Stn 32 (PC3) was obtained in the centre of the basin, and another Stn 33 (PC4) in the "Natsushima Trough" which is located in the east side of the Lau Basin (Fig. XII.I). A brief sample description is given in Table XII.l.

MEASUREMENTS

Measurement and magnetisation was carried out by SCT Cll3 cryogenic rock magnetometer. Natural remnant magnetization (NRM) was measured first and remnant magnetization (RM) after weak (7.5 mT) alternative magnetic field demagnetization was also measured. Fig. XII. 2. shows the results.

DESCRIPTION OF RESULTS

Mean inclination of both cores is approximately -35 degrees, which is almost the same value as the inclination of the present geomagnetic field calculated from IGRF 1980 (International Geomagnetic Reference Field); Int. = 41691 nT, Dec. = 130,

96.

Inc. = -36° at 17°8, 176°W. The core from Stn 32 shows a weak intensity zone at 2.6 m where declination and inclination fluctuate. This weak intensity zone may correspond to the Blake event of approximately 111,000 years ago. If this correlation is correct then the sedimentation rate 23 mm/ka and the bottom is 300 ka years old. The core should record other documented events such as the Biwa events of 180 and 270 ka. However, there are no other apparent weak intensity zones and the Biwa events cannot be detected from the pattern fitting alone.

The thickness of sediments at the centre of the Lau Basin is 20 m at most and the basin is considered to be presently active. These paleomagnetic results also suggest that the basin is young. However, it needs more detailed measurement and normalising by magnetic mineral content to decide if the weak intensity zone is caused by a magnetic event or not.

The core from Stn 33 contains many turbidite layers, which are marked by open triangles in Fig. XII.2. The inclination in this core fluctuates more than in the core from Stn 32. No events can be clearly identified and it is inferred that this core may be younger than that of Stn 32 because much of it has been rapidly deposited from turbidity currents.

REFERENCE

CHASE, T.E.; SEEKINS, B.A.; VATH, S.C.; CLOUD, M.A. 1982: Topography of the Tonga Region. USGS-CCOP/SOPAC South Pacific Project.

99.

XIV. FORAMINIFERAL MICROPALEONTOLOGY OF SAMPLES FROM NORTHERN TONGA

B. W. Hayward New Zealand Geological Survey

FORAMINIFERA FROM SURFACE SEDIMENT SAMPLES

Surface sediment from the following core, dredge, and free-fall grab sites was washed over a 200-mesh (63 pm) sieve to remove the mud, then faunas were selectively picked. All are of late pleistocene or Recent age. Foraminifera are unsuitable for discriminating the late Pleistocene from Recent in warm, subtropical areas. The following samples were processed:

13 FGl-2, 15 Dl-41, 16 D2-3, 17 PC (cc) 1-3a, 17 PC (ccl 1-2, 19 FG6-2, 20 FG7-1, 21 D3-5, 22 D4-2, 23 D5-1, 24 D6-1, 26 D8-1, 28 D9-6, 29 DIO-l, 29 DIO-2, 30 FG14-1, 30 PC (ccl 2- 3, 31 Dll-2, 32 PC (cc) 3, 33 PC (cc) 4 34 D12-1, 34 D12-2.

All except one, contained well preserved, diverse calcareous planktonic and rare benthics. Core 15 Dl-41 from c. 4500 m depth contained no calcareous material but was rich in siliceous radiolaria; it presumably came from below the CCD.

FORAMINIFERAL DATING OF CORE SAMPLES AND DREDGED SEDIMENTARY ROCKS

These samples were crushed, washed and picked. Two core samples and 26 dredged sedimentary rocks were examined. Their age determinations are given in Table XIV.l.

No faunas are older than late Miocene and the majority are Pliocene or Pleistocene. Age determinations within this time span, at these latitudes (15-200S), rely on a com- bination of tropical and warm subtropical planktonic foraminiferal zonal schemes (e.g., Srinivasan and Kennett 1981) , with local time ranges for several taxa modified from the total time ranges for all latitudes (e.g., Kennett and Srinivasan 1983) .

101.

cene) contains a rich bathyal fauna as well as abundant benthic species from inner shelf depths (c. 0-60 m) and high energy environments plus other benthic species from low energy environments at depths of less than 500 m.

REFERENCES

BLOW, W.H. 1969: Late middle Eocene to Recent planktonic foraminiferal biostratigraphy. Pp 199-241 in Bronnimann, P.; Renz, H. (eds) "proceedings of 1st International Conference on Planktonic Microfossils, Geneva, 1967".

KENNETT, J.P.; SRINIVASAN, M.S. 1983 "Neogene Planktonic Foraminifera. A phylogenetic atlas". Hutchinson Ross Publishing Co. 265 p.

SRINIVASAN, M.S.; KENNETT, J.P. 1981 A review of Neogene planktonic foraminiferal biostratigraphy: application in the equatorial and South Pacific. Spec. Publs Soc. econ. Paleont. Miner. 32: 395-432.

102.

103.

XV. SURFACE SEDIMENT SAMPLES FROM NORTHERN TONGA: A PRELIMINARY GRAIN SIZE AND S.E.M. ANALYSIS

K.B. Lewis J.B. Mitchell New Zealand Oceanographic Institute

INTRODUCTION

At 20 stations on the northern Tonga Ridge in the adjacent Lau Basin the superficial seafloor sediment was sampled using free-fall grab, dredge, and corer. The samples represent a traverse across the northern are, plus additional samples from its northern extremity (Fig. XV.l). Details of position, depth, and equipment used are given by Lewis and Honza (this volume).

ANALYTICAL METHODS

The sediment samples are described using the classification of Folk (1965) (Fig. XV.2). Loose biogenic sediments use the same classification. The dominant grain type of each fraction is identified where known.

The grain size of each sediment sample was analyzed using standard sieving and pipette techniques. The principle boundaries between gravel (G), sand (5), silt (Z), and clay (C) being set at 2 mm, 64 ) um, and 4) um. To assist in classification percentages of sand in the nongravel (5 + M) fraction and of silt in the mud (M = Z + C) fraction are

104. calculated for each sample.

The colour of fresh samples was estimated by comparison with the standard rock colour chart (RockColor Chart Committee 1970).

The sand fraction of each sample was examined under a binocular microscope and area estimates were made of the principle components. Grain mounts were made of some sand fractions for examination under the petrological microscope and identifications of mafic minerals was made by Dr W.A.Watters (N.Z. Geological Survey, Lower Hutt) .

Smears of selected mud fractions were examined under an ISI TV Mini

Scanning Electron Microscope at NZOI and their principle constituents identified. Photomicrographs and estimates of bulk chemical composition were made at DSIR's Physics and Engineering Laboratory using a Cambridge 250 Mk II SEM, operated at an accelerating voltage of 20 kV. Analyses were via an attached Link 860 Series II energy-dispersive Xray microanalyser used in ZAF4 fully quantitative mode.

RESULTS

Stn 13 FG1

Description: biogenic sandy mud (in- sufficient for analysis) .

Sand: 80% foram with few dark glass shards.

Stn 15 FG3

Description: sandy, muddy gravel. Analysis: G 58.2%, S 9.6%, Z 19.3%, C 12.9%, S/S+M 23.1%, Z/M 60.0%.

Colour: dark yellow brown 10 YR 3/2. Gravel: predominantly subrounded mudstone, some volcanogenic sandstone pumice, and subangular andesite. Sand: dominant (85%) vesiculated glass and bubble-wall shards with some dark lithic grains (?andesite) . Mud: predominantly vesiculated glass (Fig. XV.3) with some diatoms,radiolaria, and rare coccoliths. Link X-ray Bulk Analysis: mainly silica with some AI, Mn, and Fe. Comment: trench slope, at 4126 m, near CCD.

Stn 16 D2

Description: gravelly, biogenic, sandy, mud (ooze). Analysis: G 5.1%, S 32.0%, Z 41.4%, C 21.5%, S/S+M 33.7%, Z/M 65.85%. Colour: dark yellow brown 10 YR 4/2. Gravell small pieces of mudstone. Sand: dominant (70%) foraminifera with sub-dominant (30%) clear and brownish vesiculated glass. Mud: abundant coccoliths, common diatoms, tintinnids, radiolaria, and rare glass (Fig. XV.3). Link X-ray Bulk Analysis: mainly carbonate, rare Si, Fe, and Mn. Comment: forearc slope.

110.

SYNTHESIS

Al though only a few samples were collected over a vast area, some recognisable patterns do emerge.

All of the samples are "oceanic" sediments. There is no identifiable terrigenous detrital content in any of them. Except for a few samples that contain autochthonous detrital clasts, the sediments consist of different mixtures of only volcanogenic and biogenic grains. A few authigenic Fe-Mn micronodules were identified in the silt fraction of one sample, perhaps indicating slower sedimentation than elsewhere on the ridge (Hishida and Uchio 1981; Stoffers et al. 1984).

The volcanogenic grains are predominantly fine sand and silt-sized ash that includes vesicular and bubble-wall glass shards of various colours from clear to dark brown. In general, the clear grains are silicic and the brown ones are basaltic; large clasts are pumiceous and mineral grains include augite calcic plagioclase, hornblende, and hypersthene (W.A. Watters, pers. comm.). No olivine was identified with certainty although a few grains with straight extinction and high birefringence could be olivine. Most volcanogenic debris is probably derived from explosive airfall eruptions, with subsequent direct settling through the water column, from erosion of pyroclastic flows and from explosive, shallow sub- marine, phreatomagmatic eruptions (Carey and Sigurdsson 1984). Some, particularly the mafic and lithic grains, may be derived from disintegration of pumice and nearby rock outcrops. One sample, from the top of a core from the "Natsushima Trough" contains heavily "weathered" glass.

The biogenic elements are predominantly the calcareous tests of planktonic organisms. These are mainly planktonic foraminifera in the sand fraction and coccoliths in the mud fraction. Less common are the remains of tintinnids, the calcareous shells of ostracods, and the siliceous tests of radiolaria, silicoflagellates, diatoms and sponges.

There is a notable correlation between the ratios of volcanogenic to biogenic grains and colour. Samples with high volcanogenic content are generally olive grey or olive brown, whereas those with a high biogenic content are yellow brown when fresh. Those with a subequal amount of volcanogenic and biogenic grains were commonly dark yellow brown.

The ratio of volcanogenic to biogenic grains (and hence colour) changes markedly across the arc. Volcanogenic sediments naturally predominate at the crest of the northern Tonga Ridge close to the active volcanoes of the "Tofua" volcanic chain. They are also the principal constituent on the back-arc slope and in the eastern back-arc (Lau) basin. These areas are downwind of volcanoes in the SE Tradewind Belt and downcurrent in the prevailing westward drift of surface waters.

Biogenic constituents predominate on the forearc. Although the forearc is close to the volcanoes, it is upwind and upcurrent so that the supply of ash is much less than on the back-arc slope. Calcareous, biogenic sediments also predominate in the central and western Lau Basin. This indicates that the major supply of ash is from the arc rather than from back-arc volcanism.

On the upper trench slope, calcareous biogenic grains are uncommon at depths of 3500 m and almost absent at 4200 m deep. In the Tonga area the carbonate compensation depth (CCD) is considered to be at about 4300 m (Berger et al. 1976) but considerable dissolution occurs above this depth. Thus, although the supply of ash may be less than to the forearc, nevertheless volcanogenic grains are the dominant constituent on the seabed because the overall rain of calcareous tests is more- or-less dissolved before or soon after it reaches the seabed.

It is proposed to complete this study with additional mineralogical, biological, and chemical analyses of both coarse and fine fractions of these samples, and to correlate

111. this data with other available surface sediment data collected from northern Tonga.

ACKNOWLEDGMENTS

We are indebted to Graham Walker of DSIR's Physics and Engineering Laboratory for photomicrographs and bulk analyses of mud fractions, and to Bill Watters of N. Z. Geological Survey for mineralogical identifications of sand-sized grains.

BIBLIOGRAPHY

BERGER, W.H.; ADELSECK, C.G.: MEYER, L.A. 1976: Distribution of carbonate in surface sediment of the Pacific Basin. J. geophys. Res. 81 : 26172627.

CAREY, S.; SIGURDSSON, H. 1984: A model of volcanogenic sedimentation in marginal basins. In Kokelaar, B.P.; Howells, M.F. (eds) Marginal Basin Geology. Geol. Soc. Lond. Spec. Publ. 16 : 37-58.

FOLK, R.L. 1965: "petrology of Sedimentary Rocks". Hempshills, Texas. 159 p.

HISHIDA, H.: UCHIO, T. 1981: Sedimentological and geochemical studies of manganese micronodules and associated sediments in some piston cores in the North Pacific Ocean. J. Fac. Engng Univ. Tokyo 36B : 463522.

LEWIS, K.B.; MITCHELL, J .S. 1980: Cook Strait Sediments. N.Z. oceanogr. Inst. Chart, Cstl Series, 1:200, 000.

ROCK-COLOR CHART COMMITTEE, 1970: "Rock-color Chart". Geological Society of America, Boulder, Colorado.

STOFFERS, P.; GLASBY, G.P.; FRENZEL, G. 1984: Comparison of characteristics of manganese micronodules from the equatorial and South-west Pacific. Tschermaks Min. Petro Mitt. 33: 1-23. 112

113.

XVI. SEA BOTTOM PHOTOGRAPHS AT SAMPLING STATIONS IN THE TONGA ARC AND LAU BASIN

Masato Joshima Geological Survey of Japan

METHOD

To assist interpretation of seabed conditions, a free-fall sampler (boomerang sampler), installed with a camera, was used simultaneously at each dredge and piston core station. We used a Photo- Boomerang made by preussag Co., whose net weight is 33 kg, buoyancy 20 N, gripping surface 0.13 m~, average drift speed 80 m/min. , working depth 7000 m, and has a one-shot 35-mm camera whose angular field in water is 33°.

STATIONS

Whenever sampling, and twice during the interval of transponder recovery, one free-fall sampler with camera was launched (Fig. XVI .1). At dredging stations it was launched when the cliff, which was the target of dredging, appeared on the sounding depth recorder.

RESULTS AND IMPLICATIONS

Figures XVI.2-6 show the photographs. Their interpretations are as follows.

Stn 13 : The station is on the site of a deep-tow camera recovery. The diameter of the weight in the centre of the photo is approximately 10 cm. There are many small pebbles and some cobbles on the seafloor. The grab retrieved rounded pumice, black crusts and a small amount of foraminifera-rich mud. The photo shows a smooth surface with rocky outcrop in its right upper corner. The bottom seems to be hard because the weight does not sink much and may be bouncing up a little (Fig. XVI.2).

Stn 14 : The site is near the recovery place of transponder no.2. There are many pebbles and cobbles on the seafloor but the sediment seems to be soft judging by the depth of burial of the trip weight (Fig. XVI. 2).

Stn 15 : The site is on the landward trench wall. The free-fall sampler was dropped just as the ship passed over a cliff, so it could have dropped on a small bench on the cliff. The grab sampled a small amount of fine brown sand and pumice pieces. There is a dense line of small pebbles, which may be pumices, in the centre of the photo. The lineation may be a small sand wave caused by a bottom current (Fig. XVI.2).

Stn 17 : This site is on the Tonga Ridge. Sediments are volcanogenic, black and white, foraminifera- rich muddy sand. The piston corer could not penetrate the firm sand layer. Some bottom current is suggested by ripple marks (Fig. XVI.2).

Stn 18 : The site is also on the Tonga Ridge, and the sediment seems to be sand with pumices up to 1 cm size. The pebbles scattered on the surface are bigger than those at Stn 17 (Fig. XV1.3).

Stn 19 : The site is close to the volcanic chain of Tonga Ridge. The biggest pumice fragments are 5-10 cm in diameter. The grab sampled two small pumice pieces and some black and white volcanogenic and foraminifera-rich sand. The black material in the lower left side of the photograph may be biogenics (Fig. XVI. 3).

Stn 20 : The site is in the centre of a big trough, the "Natsushima Trough". The grab sampled fine, brown, volcanogenic silt and pumice, which can be seen in the upper end of the photo (Fig. XVI.3).

Stn 22 : The site is on the inner trench slope in the northern end of

115. the Tonga Arc. The sediment is olive grey, fine, sandy silt with pumices appearing as dark shadows in the photo, which shows weak halation possibly caused by small amounts of suspended matter in the bottom sea water (Fig. XVI.3).

Stn 24 : The site is on the northwestern slope of a knoll at the northern end of the Tonga Arc. Many rocks in the photo are considered to be basal t, of which two small pebbles were obtained by the grab. A round rock with white marks may be pillow basalt judging by its shape (Fig. XVI.4).

Stn 27 : The site is in a small basin where sediments were expected to be soft. There are many cobbles and the bottom seems to be a hard surface judging by the lack of penetration of the weight (Fig. XVI.4).

Stn 28 : Tuffaceous sandstone and harder, olive sandstone were obtained by dredge at this station. The bottom in the photo may be sandstone. There are faint lineaments which run obliquely from left to right and other obscure ones running from top to bottom of the photo. They may be conjugate faults with approximately 70° between them. Where the weight has impacted it can be seen that black volcanogenic sands cover the sandstone and that a thin white clay covers the black sand. An epibenthic organism has creeped over the surface leaving a trail of black sand behind it. The bottom looks very hard judging by the lack of penetration and the puffing of soft sediment thrown up by the weight (Fig. XVI.4).

Stn 29 : The site is on the eastern wall of the "Natsushima Ridge". The photo shows an outcrop of rock on the cliff wall. The rock seems to be sandstone judging by the broken surface at the left upper side of the rock. The cliff dips steeply to the right upper side of the photo. There are many obscure patterns, parallel to horizontal plane, which may be layering structures (Fig. XVI. 4).

Stn 30 : The site is in the centre of the "Natsushima Trough". The bottom sediments look soft but the corer penetrated only 30 cm into semi-consolidated mudstone at this station. A small amount of "marine snow" can be seen in the photo (Fig. XVI. 5).

Stn 31 : The site is on a gentle slope in the centre of the Lau Basin. Sediments seem to be soft and olive, foraminifera-rich, sandy mud is obtained by the dredge. The photo shows many pebbles on the sea bottom possibly of rounded pumice (Fig. XVI. 5).

Stn 32 : The site is also in the centre of the Lau Basin where the corer worked successfully. The depth of penetration of the weight indicated that the sediment is very soft. There are many pebble to cobble- size pumice fragments in the field of view. There is an assemblage of needle-like material slightly left from the weight, which may be sponge spicules. Similar needles were obtained by the grab (Fig. XVI. 5).

Stn 33 : The bottom also looks very soft at this station. The corer worked well here and the grab sam- pled brown, foraminifera-rich sandy mud. The short ditch at the upper end of the photo may be biogenics (Fig. XVI. 5).

Stn 34 : Soft, brown, foraminiferarich sandy mud was obtained by the dredge here. The photo shows cobbles and weak current marks on the sea bed, which dips gently to the left top of the photo. The angle may be 10-15° (Fig. XVI. 6).

DISCUSSION Pumices become bigger toward the west of the Tonga Arc from Stn 15 to Stn 19. This fact suggests that the pumices were supplied by eruptions of volcanoes in the volcanic chain of the Tonga Arc. There are many types of pumice in the centre of the Lau Basin. Weak halation at Stn 22 and Stn 33 may be caused by suspend- 116. ed matter in the bottom sea water. "Marine snow" appears in photos at Stns 17, 18, 19, 20, 30, and 33, and is clearly common around the Tonga Arc.

REFERENCE

CHASE, T.E.; SEEKINS, B.A.; VATH, S.C.; CLOUD, M.A. 1982: Topography of the Tonga Region. USGS-CCOP/SOPAC South Pacific Project.

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123.

XVII. POSITIONS OF NIUATOPUTAPU AND TAFAHI ISLANDS AS MEASURED BY SATELLITE NAVIGATION SYSTEMS

Eiichi Honza Geological Survey of Japan

Ship's positions during the seismic profiling survey were obtained by the Navy Navigation Satellite System (NNSS), Global Positioning System (GPS), and additional assistances by sun sites and by radar sites from the islands. It is noted that there are some systematic differences between the satellite positions and radar sites.

Twenty fixes were simultaneously obtained from satellite positions and radar sites, when the ship approached Niuatoputapu and Tafahi Islands (Table XVII.l). The 17 sets of fixes on Tafahi show radar positions 1.2 to 2.8 nautical miles east of the satellite fixes. Three radar sites from Niuatoputapu Island show positions 1.7 to 1.8 nautical miles west of the satellite fixes.

The observed results suggest that most radar sites reliably record the distance from the islands. Eight radar sites from Tafahi Island and the three radar sites from Niuatoputapu Island are selected here to record the difference between the satellite fixes and radar sites. The results show the position of Tafahi Island to be 2.0 nautical miles westward of that listed on navigation charts and that of Niuatoputapu island 1. 7 nautical miles east of that on the charts.

The data presented here is sufficient to suggest that there may be errors in the charted positions of Niuatoputapu and Tafahi Islands. However, more detailed surveys, especially ashore, are required to accurately fix these islands.

125.

XVIII. RESOURCE POTENTIAL OF NEW DATA FROM NORTHERN TONGA

T. Fuka Kitekei'aho Ministry of Lands, Survey and Natural, Resources

Yoshihisa Okuda Geological Survey of Japan

The Natsushima 84 Cruise was a reconnaissance survey of a vast, virtually unexplored, area and the MIS Natsushima was not specifically equipped for hydrocarbon or mineral exploration. Nevertheless, survey results do provide important information to guide future resource exploration in northern Tonga.

Our seismic reflection profiles show that the sediments on the Tonga Ridge north of Vava'u may be sufficiently thick (more than 2 sec) for, hydrocarbon maturation. These sediments are frequently intruded by island arc volcanic rocks. The volcanics are a northern extension of the Tofua Volcanic Chain which is Pliocene and Quaternary in age.

The total span of ages for the sedimentary sequences have not yet been strictly determined. Dredged rocks from the forearc are late Miocene in age and the lower units in the thick basin sequences are probably also Miocene in age.

The presence of a high thermal gradient could encourage hydrocarbon generation. We failed to obtain heatflow measurements, which would indicate thermal gradients across the northern ridge because the seafloor is armoured by pumice and compacted sand. However, we would expect a high thermal gradient because of the adjacent arc volcanism and might expect some hydrocarbon maturation since the start of volcanism in Pliocene times. Since Miocene times there may be sufficient time for kerogen catagenesis to occur.

It is impossible to judge from the results of a exploratory survey whether hydrocarbon fields exist in the area. However, in order to target any possible hydrocarbon concentrations it will be very important to analyse the Miocene sedimentary rocks by geochemical methods the aim being to determine whether they contain sufficient organic material to act as potential source rocks.

Other unknowns are the timing of hydrocarbon migration and the existence of suitable" structures for accumulation. Generally speaking the migration and accumulation of any hydrocarbon generated in the area should occur in a short time. To clarify the problems of migration and reservoir potential the present survey results should be used to select small areas for detailed investigation using single channel and multichannel seismic equipment. The objective would be to define basin shape and the form of potential reservoir structures. There is enough coarse sediment existing in the area to provide good reservoir rocks but good permiability cannot always be expected because of the dominance of volcanic detritus.

No hydrothermal activity or mineralization was detected in the central Lau Basin. However, blue grey mud, quite different from all other samples from the region, was dredged from the "Natsushima Trough" and is to be geochemically analyzed.