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