THE SEABED MORPHOLOGY OF THE HAZEL HOLME FRACTURE ZONE AND THE NEW HEBRIDES ARC, NORTHERN VANUATU - SOLOMON ISLANDS REGION D.P. Johnson, P.C. Maillet, R.C Rice August 1992 SOPAC Technical Report 138 Marine Geophysical Laboratory, James Cook University, Townsville, Queensland, Australia ORSTOM, France and Department of Geology, LaTrobe University, Bundoora, Victoria 3083, Australia Geology Department, LaTrobe University, Bundoora, Victoria 3083, Australia Prepared for: South Pacific Applied Geoscience Commission (SOPAC) Offshore Program on EC Consultancy Contract no. SP/SOP/04/90 GLORIA data interpretation and reporting Dr David P. Johnson [3] CONTENTS Page ABSTRACT ....................................................................................................................................... 5 ACKNOWLEDGEMENTS .................................................................................................................. 6 INTRODUCTION ............................................................................................................................... 7 REGIONAL GEOLOGY AND GEOLOGICAL SETTING .......................................................... 7 PREVIOUS WORK AND OVERVIEWS ........................................................................ 11 Petrology and Geochronology .............................................. 13 DATA ACQUISITION AND POSITION FIXING ........................................................................... 14 DATA PROCESSING AND COMPILATION ............................................................................... 14 Navigation ......................................................................................................................... 14 Bathymetry ......................................................................................................................... 15 GLORIA ............................................................................................................................... 15 DESCRIPTION AND INTERPRETATION OF THE GLORIA DATA ................................................ 15 Arc Platform ................................................................................................................. 20 Jean Charcot Troughs ................................................................................................. 20 Hazel Holme Fracture Zone ........................................................................................... 21 REMAINING PROBLEMS Back-arc Geology and Hydrothermal Activity .................................................................. 23 New Hebrides Back-arc - Northern Area ......................................................................... 23 Hazel Holme Fracture Zone ................................................................................................. 23 MINERAL RESOURCES. NATURAL HAZARDS AND SEABED MAPPING ............................................ 24 CONCLUSIONS ........................................................................................................................... 25 REFERENCES ................................................................................................................................ 26 [TR138 - Johnson, Maillet & Rice] [4] LIST OF FIGURES Figure Page 1 Location map outlining the western part of the area surveyed by GLORIA ...................................................................... 8 2 Regional bathymetry showing location of the active central chain of volcanism, the regular back-arc slope south of the HHFZ, and the Jean Charcot Troughs to the north .............................................................................. 10 3 The GLORIA mosaic of the northern Vanuatu-southeastern Solomon Islands area ....................................................................................... 16 4 Regional bathymetry for the GLORIA mosaic area of Figure 3 ................................................................................................................... 18 5 Interpretation of the GLORIA data of Figure 3, with some bathymetry from Figure 4 .............................................................. 19 [TR138 - Johnson, Maillet & Rice] [5] ABSTRACT The New Hebrides volcanic arc in northern Vanuatu and southeastern Solomon Islands is a complex area, with subduction of the d'Entrecasteaux Zone into the New Hebrides Trench to the west and a possible micro-plate boundary, the Hazel Holme Fracture Zone, to the east. The back-arc area in this region is deeply faulted, forming the Jean Charcot Troughs, and is also a zone of shallow earthquakes, but the trough area is seismically quieter, compared to adjacent areas to the north and south. A mosaic of GLORIA images of the Jean Charcot Troughs and adjacent arc platform clearly outlines the troughs and associated seafloor features in this region. The troughs are 2400-3000 m deep but show no evidence of recent seabed extrusive lavas, supporting a hypothesis that they are fragmented older crust predating the surrounding volcanic islands and arc, and are not magmatically active. Lineations on the Hazel Holme Fracture Zone (HHFZ) mark horst and graben structures cutting obliquely across the trend of the fault zone. The HHFZ may not be a simple transform, but a series of en echelon ridges forming a relay zone. Y-shaped grabens within the zone could be remnants of spreading zones or rifts, formed in the early history of the western part of the North Fiji Basin. It is not confirmed from this data whether or not the HHFZ is still an active tectonic unit. If not, the origin of earthquakes in this region of the back-arc is solely due to subduction beneath the arc. [TR138 - Johnson, Maillet & Rice] [6] ACKNOWLEDGEMENTS The SOPAC GLORIA survey in parts of the EEZ's of Vanuatu, Solomon Islands, Fiji, Tonga, and Western Samoa resulted from a large effort by many people. It was supported by Lome III funds from the European Community to the South Pacific Applied Geoscience Commission (SOPAC) and by the generous provision of shiptime by the Royal Australian Navy (RAN). The project was coordinated by Dr. Don Tiffin of SOPAC. LCDR Jock Low acted as principal contact with the RAN during the planning stages. The survey was carried out on board the Australian Naval Hydrographic vessel HMAS Cook, commanded by CDR Brian Hunt. The ship's captain, officers and crew provided enthusiastic support during the survey. The following personnel were closely associated and contributed to the success of the scientific work on board : Lt Peter Martin (navigator), Lt Peter Doherty (Data Centre Manager and scientific liaison), and AB Phil Barling (photography). The GLORIA equipment and data acquisition was by Marconi Underwater Systems Limited. Data processing was done by John Hughes Clarke and Guy Carpenter, with supervision by DPJ, under a contract from SOPAC to James Cook University. We thank the Australian Hydrographic Service, Royal Australian Navy, for providing bathymetric data from recent surveys in Vanuatu, and ORSTOM, France, for providing the SEAPSO Leg II Seabeam data. The Seabeam data collected by HMAS Cook during the SOPAC survey are the property of the RAN and are published with the permission of the Hydrographer, Royal Australian Navy. This report has benefitted from constructive reviews by Don Tiffin and Alan Sherwood. [TR138 - Johnson, Maillet & Rice] [7] INTRODUCTION This report describes and interprets data collected within the Exclusive Economic Zones (EEZ) of northern Vanuatu and the southeastern Solomon Islands during a GLORIA survey on HMAS Cook in August, 1989. The survey, carried out by the South Pacific Applied Geoscience Commission (SOPAC), collected wide-swath sidescan sonar and other geophysical data in parts of the EEZ's of five member countries of SOPAC: Vanuatu, Solomon Islands, Fiji, Tonga, and Western Samoa. This report covers the mosaic area of northern Vanuatu and Eastern Solomon Islands, although it draws on data from adjacent mosaic areas to the south and to the east which are reported elsewhere (Price et al, 1992; Jarvis et al, 1991). The aims of the survey in the north-central section of the New Hebrides Arc were: (1) to study the Jean Charcot Troughs and their geological environment, (2) to image other parts of the back-arc region including the eastern limits of the Vanikoro Basin, a sedimentary basin on the New Hebrides platform, and (3) to investigate the seafloor geology in the area, including the juncture of the HHFZ with the arc. A better understanding of the origin of the natural hazards of the region might also be obtained, particularly for earthquakes near the intersection of the arc with the Hazel Holme Fracture Zone (HHFZ) (Figure 1). This report interprets seabed images obtained by the GLORIA long range sidescan sonar system using bathymetry derived from Seabeam and from other echosounding data, together with shallow 3.5kHz sub-bottom profiles and deep seismic airgun data obtained during the survey. No seabed sampling was undertaken on this survey. REGIONAL GEOLOGY AND GEOLOGICAL SETTING The New Hebrides Island Arc (NHA), which includes the islands of Vanuatu and eastern Solomon Islands, lies on the western margin of the North Fiji Basin (Figure 1) and consists of a series of NNW-trending ridges and troughs extending along a strike length of over 1200 km (Kroenke, 1984). The arc lies east of, and adjacent to, the New Hebrides Trench into which the Indo-Australian plate subducts from the west. The trench is 5000-9000 m deep, except in the