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
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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
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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.
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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.
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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 area west of the islands of Malekula and Espiritu Santo where the d'Entrecasteaux Zone on the Indo-Australian Plate, containing an aseismic ridge and seamount chain (Collot et al, 1985), abuts the arc and fills the trench. The plane of subduction is marked by earthquake foci which dip under the arc at approximately 70' (Pascal et al, 1978) and extend to a depth of more than 300 km (Louat and Pelletier, 1989).
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The arc is thought to have originally been part of an earlier east-west trending Vitiaz arc system under which the Pacific Plate was subducting to the south. Subduction ceased and polarity reversed to begin again in the New Hebrides Trench, possibly after collision with the Ontong Java Plateau (Kroenke, 1984) during a major plate reorganisation (Honza, 1991). A similar polarity reversal occurred in the Solomon Islands to the northwest where two oppositely-dipping Wadati-Benioff zones have been mapped (Cooper and Taylor, 1985). The New Hebrides Arc rotated southwest to accommodate opening of the North Fiji Basin beginning in the late Miocene (about 8 Ma; Kroenke, 1984). Palaeomagnetic evidence indicates the New Hebrides Arc has undergone about 30' of clockwise rotation during the past 6 Ma (Falvey,1978).
The central New Hebrides arc platform is some 125 to 175 km wide, with older rocks along the western and eastern edges forming the sides of a deep (3000 m), central intra-arc basin (Greene and Johnson, 1988). The western and eastern belts are composed of Tertiary rocks, some of which date from the previous Vitiaz paleo-arc system.
There are two linear zones of modern volcanic and tectonic activity: (1) The Central Chain, a line of volcanic centres extending along the central basin and including Epi, Ambrym, Aoba, Banks Islands, and the Santa Cruz Islands (Figure 2), is the locus of modern arc volcanism. The volcanics are mainly tholeitic basalts with minor picrites, but with transitions to calcalkaline volcanics relating to distance from the trench axis (Jakes and White, 1969; Dupuy et al, 1982). Active submarine volcanism with associated hydrothermal iron deposits has been mapped in shallow water (<500 m) on the arc platform near Epi Island (Exon and Cronan, 1983).
(2) In the back-arc about 35-55 km east of the active volcanic chain is a series of narrow (30-35 km wide), extensional troughs (Dubois et al, 1978; Louat and Pelletier, 1989; Charvis and Pelletier, 1989). These are known as the Coriolis Troughs in the southern arc area (Price et al, 1992), and the Jean Charcot Troughs to the north. There are no troughs in the central area opposite the d'Entrecasteaux Zone, instead, the back-arc is a regular, but steep, slope to the floor of the North Fiji Basin. The troughs and their adjacent slope form a borderland between the arc platform and the North Fiji Basin. The north central New Hebrides Arc (northern Vanuatu) is in a compressional regime with a major earthquake zone occuring in the back-arc region east of the islands.
The marginal North Fiji Basin (NFB) to the east has formed since 8-10 Ma (Chase, 1971; Kroenke, 1984; Louat and Pelletier, 1989). The basin forms a triangular area bounded to the north by the Melanesian borderlands, to the southeast by Fiji and the Hunter Fracture Zone, and to the southwest by the New Hebrides Arc. It is a relatively shallow (2500-3000 m) basin compared to normal ocean depths
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(4500-5500 m), and exhibits high heat flow compared to other back-arc and marginal basins (Sclater and Menard, 1967; Sclater, 1972). It is actively opening at a spreading centre with at least one triple junction Taylor and Karner, 1983: Lafoy et al, 1990). Auzende et al (1990) consider the North Fiji Basin is the most evolved of a number of back-arc basins, based on geodynamic and petrological factors,
Sediments from the arc extend some 200 km eastward into the NFB (Kroenke et al, in press(a)), and are up to 1.2 km thick in the immediate back-arc area (Chase, 1971). Sound velocities in the sediments range from 2.0-3.0 km/s, higher than for pelagic materials, and this is attibuted to a high volcaniclastic content. No cores have penetrated through this sediment cover, but its thickness suggests the underlying crust must be several million years old.
Much farther east, in the Tonga Trench, the Pacific Plate is subducting westwards beneath the Indo-Australian Plate, thus the NFB represents a central block of ocean crust across which two opposing subduction motions are transferred, with the lateral movement expressed on at least two complex fracture zones, the Hunter Fracture Zone extending southwest of Fiji, and the Fiji Transform Fault Zone north of Fiji (Hughes Clark et al, 1991).
Other major structural elements, including the Hazel Holme Facture Zone (also called South Pandora Ridge), cut the NFB. Seismicity associated with the HHFZ has been described as having a recumbent Y-shape (Louat and Pelletier, 1989), the western branch of which is included in the area of this study. This branch trends N100'E, and abuts the back-arc at latitude 13'30'S, immediately east of the Banks Islands (Figure 2). Louat and Pelletier (1989) and Hamburger and Isacks (in press) noted that seismicity of the HHFZ begins at the same longitude, but farther north, from where the Fiji Transform Fault Zone leaves off. Thus the HHFZ may be part of the system which transfers motion across the NFB between the Tonga and New Hebrides trenches. However, it is not at all clear whether the HHFZ is a transform fault or an extensional feature within the basin. At the northern end of the NFB, very little mapping has been done along the area of the Melanesian Borderlands, site of the old Vitiaz paleo-arc, but the lack of seismicity there suggests no tectonic movement is present.
PREVIOUS WORK AND OVERVIEWS
Karig and Mammerickx (1972) and Luyendyk et al (1974) presented initial plate tectonic syntheses for the region. The general geology of the New Hebrides Arc in the Vanuatu-Solomon islands region was documented by Mitchell and Warden (1971), Carney and MacFarlane (1982) and Carney et al (1985). Seismicity has been documented by Dubois et al (1978), Pascal et al (1978), and summarised by Louat et
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al (1988). Palaeomagnetic data were reviewed by Falvey (1978). Offshore data collected as part of the 1982 and 1984 Tripartite cruises are contained in a summary volume edited by Greene and Wong (1988). Some of this latter work investigated the origins and nature of troughs on the arc platform (Greene and Johnson, 1988; Falvey and Greene, 1988), although regional tectonic analyses were also included (Greene et al, 1988). This work also indicated the presence of a sedimentary basin, the Vanikoro Basin, just west of the troughs. A recent summary of the back-arc area following the acquisition of Seabeam bathymetric data is given by Recy et al (1990).
The western NFB east of the arc and, in particular, the Hazel Holme Fracture Zone, are less well known. Most data have been collected by research vessels in transit, and there is little detailed bathymetry, only scattered dredging, and no seabed mapping west of longitude 172'E. Active spreading within the NFB lies 500 km east of the present New Hebrides Arc and is unrelated to it. Spreading has also been proposed for the Hazel Holme Fracture Zone (South Pandora Ridge of Kroenke et al, in press (b)). An integrated model for seismotectonics and relative plate motions in the NHA-NFB region was given by Louat and Pelletier (1989), based on earthquake mechanism solutions. A history and relationship between the NHA back-arc and the western NFB is given by Charvis and Pelletier (1989) who presented seismic evidence to show the extensional nature of the northern back-arc troughs and the HHFZ.
The Hazel Holme Fracture Zone was originally named and described by Chase (1971, p.3089) as a "series of ridges and troughs" which trend southwest from the Hazel Holme and Horizon Banks at the southeast end of the Vitiaz Trench (Figure 1). Chase (1971) interpreted it as a transform fault marking the northern limit of spreading in the NFB, and he suggested that crust to the south would be post-Late Miocene, and crust to the north would be considerably older.
More recent work indicates the HHFZ is the central part of an active linear zone about 100 km wide, composed of horsts and grabens (Pelletier et al, 1988). However Louat and Pelletier (1989) point out that an extensional interpretion of this morphology is not supported by earthquake focal mechanism solutions; the solutions indicate right-lateral strike slip faulting with a nodal plane oriented around N25'E. Therefore the HHFZ should be a right lateral transform fault, based on these solutions. But Charvis and Pelletier (1989) argue that the HHFZ and the back-arc troughs are genetically related, and arise from recent extension within the NFB, with the distribution of the troughs modified by compression of the arc by the impacting d'Entrecasteaux Zone.
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Petrology and Geochronology
Monjaret et al (1990) presented detailed petrological and geochronological data from igneous rocks dredged from the northern (Vanikoro), central (Vot Tande) and southern Jean Charcot Troughs. Samples from the northern area range in age from 12.4 Ma to 0.3 Ma, with most samples (10 out of 13) having ages between 2.9 and 1.1 Ma. The oldest age of 12.4 Ma (mid-Miocene) is on a sample from the eastern margin of the troughs. The sample is geochemically similar to mid-ocean ridge basalt (MORB), and is interpreted to be part of the old NFB crust. Most other samples obtained have island arc affinities, however some have chemical affinities that are transitional between MORB and island arc tholeiites. These transitional samples range in age between 3.9 and 1.1 Ma.
Samples from the central Jean Charcot Troughs are arc tholeiites ranging in age from 4.9 to 2 7 Ma. They are therefore older than most of the analysed samples from farther north, but since they also have an island arc affinity, they are believed to be part of the older arc suite.
Analysed samples from the southern part of the troughs adjacent to the HHFZ constitute a bimodal suite of tholeiites and MORB with ages ranging from 5.5 to 3.5 Ma. The suite appears to include material from late eastern belt volcanism, as well as samples from the adjacent or underlying NFB crust.
Monjaret et al (1990) noted that the ages of most of these dredged samples pre-date volcanism on the adjacent islands and in the Central Chain, placed at less than 2 million years (i.e. Pliocene- Holocene; Macfarlane et al, 1988). Thus they may be representative of the older NFB crust adjacent to the arc. Analyses confirm the volcanics have an orogenic affinity, although the most northerly samples are primitive and could signify the first stages of arc formation in that area. The troughs are therefore not related to the modern arc and are back-arc features only in the sense that they lie behind the newly developing arc to the west.
The western limit of the NFB magnetic lineation pattern is 167'40'E (Charvis and Pelletier, 1989). while seismic refraction data indicate the boundary between the NFB and the NHA is narrow (<50 km) wide) and situated between longitudes 167'15 - 167'40'E (Sage and Charvis, 1991). Together these geophysical and petrological data confirm the troughs lie on the western border of the NFB, probably on old crust which is being fragmented by present arc tectonics.
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DATA ACQUISITION AND POSITIONING
Data acquisition is fully described in a cruise report for this survey (Tiffin et al, 1990). Four systems were deployed: a multi-transducer Seabeam echosounding system installed on HMAS Cook; the GLORIA (Geological Long Range Inclined Asdic) system (Somers et al, 1978); a high resolution (3.5 kHz) sub-bottom profiler (SBP), and an airgun seismic reflection profiler (SRP) with a 140 cubic inch airgun as a sound source.
Positioning during the survey was accomplished by two systems, both accessed by a Magnavox MX1102: (1) a real time Global Positioning System (GPS), and (2) a Transit (or Navy Navigation) Satellite system in conjunction with dead reckoning (SATDR). The GPS constellation of semi-stationary satellites at an altitude of about 20,000 km was not complete at the time of the survey and the navigation "window" when three acceptably placed satellites were available to give an accurate position was about 8 hours per day, distributed over a ten hour period. Navigation for the remainder of the day depended upon the Transit system and dead reckoning. As Selective Availability (SA) was not operational at the time, the accuracy of the GPS continuous fixes was better than 25 m, while those of the Transit/DR system were usually within 1 km.
DATA PROCESSING AND COMPILATION
All processing was carried out under contract to SOPAC at the Marine Geophysical Laboratory, James Cook University, by John Hughes Clarke and Guy Carpenter under the supervision of D. Johnson. The following summary briefly outlines the main processing steps. All final plots were made on Mercator projection (WGS84 spheroid) at 1:375,000 scale at 20'S.
Navigation
Using Transit and GPS satellite fixes, a best fit interpolation was made for the cruise track. During data gaps in the navigation file, GLORIA towfish headings were used to estimate ship's heading and constant speeds were interpolated. An initial trackline plot showed some logged Transit fixes gave a poor plotted position. These were rejected and the final ship tracks re-plotted.
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Bathymetry
Seabeam data were edited to remove bad data, then merged with the final navigation file. Consecutive beam data were averaged to allow the along-track data spacing to be made the same as cross-track spacing. The results were then plotted at a 100 m contour interval and 1:375,000 scale. Two additional data sets were plotted at the same scale and merged by hand contouring:
(1) SEAPSO Seabeam data from the RV Jean Charcot consisting of several regional lines and detailed surveys over three areas.
(2) Standard hydrographic soundings collected by the Royal Australian Navy Hydrographic Office Detached Survey Unit (HODSU) at a five nautical mile grid spacing, with extra sounding lines over high ground.
Data from the World Data Centre was not used since the SEAPSO and HODSU data were more recent, on closer line spacings, and demonstrably more reliable. The MGD data were commonly in error by 1-2 water depths returns (750 m) and there was no stated control on position fixing.
GLORIA
The GLORIA data were processed in two stages: (1) A preliminary stage merged the data with navigation and bathymetry files, then removed topographic and radiometric artifacts. (2) Post-processing located the data in geographic space, maximised the contrast in acoustic backscatter, and plotted the result.
The images were checked, final images written to magnetic tapes, and the tapes were sent to the Australian Centre for Remote Sensing (ACRES) where laser-written negatives were prepared and photo prints made at the scale of 1:375,000 for each of the eleven mosaic areas.
DESCRIPTION AND INTERPRETATION OF THE GLORIA IMAGE
The GLORIA image, shown in Figure 3, covers the area east and north of Banks Islands. A gap in coverage at the northern end of the survey is due to shoals centred in the area, which prevented the
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seabed from being imaged there. Bathymetry is shown in Figure 4, and an interpretation of the seabed geology in Figure 5.
The GLORIA sidescan image is similar to an aerial photograph except that the image is produced by reflected sound rather than by reflected light. Different seabed substrates have different acoustic back-scatter properties, and the system records the intensity of the back-scattered sound, producing an image with shades of grey. Along the centre of each image is a narrow zone of no data, which represents the nadir area immediately under the ship not imaged by the sidescanning system.
The seafloor signature observed in GLORIA sidescan imagery is predominantly an expression of seafloor roughness (Mitchell and Somers, 1989). GLORIA imagery is particularly well suited to delineating steep scarps and discriminating between those regions of the seafloor which are mantled by unconsolidated sediment and those which consist of exposed volcanic rocks. The three main acoustic facies distinguished on the GLORIA imagery are: a dark grey image resulting from low backscatter, interpreted as sediment-draped seafloors; a lighter image from moderate backscatter, interpreted as consolidated basement outcrop, usually on ridges; and high backscatter resulting in a light image, interpreted as either steep scarps or extrusive neo-volcanics. Areas of black are beyond the range of the GLORIA signal or may be shadows cast by topographic highs, or areas where the seabed passes into an acoustic shadow zone where the signal has not reached, caused by refraction of the sound waves. Great care must therefore be taken when interpreting the causes of the shades of grey on the image.
Two main types of backscatter response predominate and are readily apparent on the mosaic image of Figure 3:
(1) Bright zones representing high intensity backscatter of acoustic energy from the seabed, which are interpreted to be mainly from hard, irregular rocky seafloor, but can also be reflections from steep dopes, especially those inclined towards the sonar towfish. Sinuous or anastomosing linear features, are interpreted here as mainly arising from scarps and ridges on the seabed.
(2) Medium intensity mid to dark grey zones represent lower intensity acoustic backscatter, usually from relatively flat or gently sloping seabed with sediment cover.
In the far acoustic range, some bright zones display repeated imaging with up to 5 parallel images decreasing in intensity away from the towfish. These are artifacts whose origin is not well understood, but they may be due to multiple paths of acoustic rays leaving the near-surface sound channel before
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descending below the thermocline, hence producing multiple images at increasing times, and therefore distances, from the towfish.
The GLORIA mosaic displays numerous bright zones cutting across areas of low intensity, with many shadow zones, confirming the complexity of the region. The bright zones mainly represent scarps along the edges of individual troughs, and the low intensity areas represent sediment-covered ridges or trough floors. In some cases the brighter zones are paralleled by black zones which are acoustic shadows, where GLORIA has been unable to image or "see" over the edge of a scarp.
The study area can be described in three parts: the arc platform, Jean Charcot Troughs, and the Hazel Holme Fracture Zone (HHFZ).
Arc Platform
The GLORIA image shows the sedimented upper surface and slope of the arc platform. The region south of latitude 13'45'S and west of 168'15'E is the slope south of the HHFZ. It displays relatively featureless seabed with minor bright zones which are probably scarps, trending around N-S. Water depths in the imaged area are in the range of 1500-2500 m. The slope appears fairly uniform, but ridges, possibly bare of sediment, are present as bright reflectors in some areas. An active volcanic island, Mere Lava, lies on the western edge of the swath at 14'27'S.
Jean Charcot Troughs
The Jean Charcot troughs lay north of the HHFZ, in the northern pari of the mosaic area. Bathymetry shows them to be a series of complex horst and graben structures with relief in the order of 1000 m (Monjaret et al, 1990). Six individual troughs can be recognised (Figure 5), each of which is 3-9km wide and 20-35 km long, oriented in a northerly or northeasterly direction. Most troughs are over 3000 m deep, with two at 2400 m. The troughs are separated by linear ridges rising to 2000 m water depth. The troughs occur in a rectangular area 50-55 km wide (E-W), and 100-120 km long (N-S). This area is bounded to the west by a ridge extending along strike from the line of active volcanoes occurring northwards from Ureparapara through the Torres Islands. The ridge is seen in the GLORIA mosaic as a series of linear, anastamosing bright reflectors along approximately 167'30'E. Small, submerged volcanic cones are also found along this trend, and a major volcanic seamount is seen at 12'40'S. This seamount
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may reach close to the sea surface at 35 m depth (pers. comm, 1992). The ridge lies on the eastern side of the Vanikoro Basin and may be its eastern boundary.
On the eastern side of the mosaic, the Eastern Ridge of Charvis and Pelletier (1989) can be identified as a long series of almost continuous bright reflections snaking through the image from the north near longitude 168'05'E, to join up with the slope east of the Banks Islands. These strong reflections disappear just at the junction of the HHFZ with the arc. On the eastern flank of this ridge is a line of large volcanoes rising to water depths as shallow as 1500 m. The most obvious of these on the image is located at 12'33'S, 168'13'E. The slopes of these volcanoes are dissected by radial gullies indicating considerable erosion has taken place. Reflective zones surrounding the peaks of some volcanoes are probably flows, but as they are not highly reflective, they are likely sediment covered. Since no modern lava flows are present, the volcanoes are interpreted to be extinct.
In the northwest part of the mosaic, on the west side of, and adjacent to, the gap in the mosaic, lies an extensive area of approximately N-S trending, linear, closely spaced, reflections, individually continuous over distances of 30 km or more. While some of these at a distance from the sonar vechile could be artifacts as discussed above, they are mainly definite reflections and suggest a seafloor heavily covered in volcanics probably broken by strong tectonic activity. Most of the northern area of troughs and ridges is heavily tectonised.
Hazel Holme Fracture Zone
The western end of the HHFZ is imaged in the southeastern part of the mosaic. Southwest of the HHFZ, between it and the back-arc, regional bathymetry shows relatively flat seabed at depths of around 3000 m. Northeast, the seabed is poorly surveyed but appears to be generally flat with major volcanic prominences at Tikopea and Mitre islands, and a major, linear trough at 4600 m deep trending N60'E along the northern side of the HHFZ (Figure 2). The HHFZ itself is composed of irregular ridges and valleys over an area about 100 km wide and several hundred km long. The little bathymetric data available in the area of the HHFZ requires interpretation for that area to be based almost solely on the GLORIA data.
Regionally, the image shows a grey seabed signature with numerous bright linear zones trending mainly E-W, although one strong series of reflectors branches off the main trend and heads southwestward toward the small island of Mere Lava near the bottom of the mosaic, stopping at about
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168'30'E. East of longitude 169'00'E, only a single GLORIA pass was obtained on a transit to the NFB triple junction area. It shows a series of bright N60-80'E reflectors from ridges with up to 1000 m of relief.
West of longitude 169'00'E four separate E-W trending troughs whose floors, particularly the southern two, exceed 3500 m in depth (well below the general level of the surrounding NFB seabed), mark the junction of the HHFZ with the arc. The troughs do not penetrate into the arc but abut the arc platform at the line of extinct volcanoes along longitude 168'15'- 168'20'E and incise deeply into the lower slope. The two northern troughs, ringed by strong reflections, trend N80'E, and have been imaged eastward for a length of 35 km. These troughs reach almost 4000 m deep. The two southern troughs, clearly visible on the mosaic as dark grey areas, are deeper and more extensive, and join in the east to form a Y with its single arm extending east from 169'00'E. The floors of the troughs are relatively flat at 3500-4500 m water depth. The southern arm, about 2-6km wide, extends N50'E for 50 km from the Y-junction to the arc platform where it terminates against the eastern flank of a volcano with a peak at about 1000 m. Its north side, brightly imaged on the mosaic, is a 2600 m scarp, falling in a single steep drop to the floor of the trough. The northern arm has an overall E-W trend with a gentle curve. A mish-mash of reflections from structures of the HHFZ where it joins the arc, lies on the north side of this arm.
Seismic reflection and 3.5 kHz high resolution profiles collected on this survey and those presented by Charvis and Pelletier (1989) show these troughs are grabens or half-grabens separated by tilted horsts of basement with sedimentary cover up to approximately 300 m (400 ms) thick. Ail troughs but one have flat-lying sedimentary fill up to 225 m (300 ms) thick. If the graben development is in early North Fiji Basin oceanic crust, the extensional tectonics near the arc are much younger than the crust, considered to be 8 to 10 million years old, but their formation must have ceased some time ago to allow the sediment to accumulate without disruption.
Shallow depth, high heat flow, and thin sediment cover in the North Fiji Basin indicate that the basin is young and at least partly of extensional origin (Sclater, 1972), but the HHFZ does not appear to be part of the contemporary rift system, which is located far to the southeast (Auzeande et al, 1990). The GLORIA image shows structural fabric within the HHFZ does not follow the E-W trend of the zone but cuts obliquely across it. This is strong evidence that the HHFZ cannot be a simple transform fault. Strike-slip fault plains derived from earthquake mechanism solutions are also in a direction oblique to the trend of the zone. Motion might therefore be accommodated by a succession of en echelon blocks whose movement may cause small pull-apart basins and the horst and graben structure observed. Further mapping would show whether these oblique trends continue to the southwest of the HHFZ, or whether a different and older tectonic grain is preserved there. The indications are, from a GLORIA swath along the back-arc (Price et al, 1992), that the trends do not reach the vicinity of the back-arc.
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REMAINING PROBLEMS
Backarc Geology and Hydrothermal Activity
Most investigations of modern back-arcs have concentrated on mature back-arc areas such as the Mariana and Bonin arcs. In these places the back-arcs are wide and display more than one phase of back-arc spreading. In contrast, the New Hebrides Arc appears to have a very young back-arc system, despite the age of the North Fiji Basin. Young back-arcs represent a major gap in our understanding of arc evolution. Even with this GLORIA survey, our knowledge of the New Hebrides back-arc is inadequate to properly map or understand this geologically important area. In particular there has been little work to investigate the possibility of hydrothermal venting in this region. Such vents have been recognised in spreading ridges and back-arc regions elsewhere (e.g.-Herzig et al, 1990). More geological, geophysical, and bathymetric mapping is needed in the back-arc.
New Hebrides Back-arc - Northern Area
Charvis and Pelletier (1989) proposed that extensional features in the HHFZ and in the back-arc troughs are genetically related and due to recent extension within the NFB, with trough distribution modified by compression of the arc from the impacting d'Entrecasteaux Zone. Recent extension in the northern back-arc troughs is not supported by the GLORIA data which show no evidence of any neo-volcanics in the troughs, while the eroded nature of at least some of the seamounts behind the arc, as well as the thick sediment fills in the grabens of the HHFZ, suggest these structures are not young. In particular the Y-shaped basins immediately behind the arc could be relicts from earlier spreading. Further seabed mapping should be undertaken in the region west of longitude 169'30'E adjacent to the present GLORIA coverage of the HHFZ. The old ages for dredged basalts, and the geophysical data suggest these troughs result from fracturing of older rocks.
Hazel Holme Fracture Zone
The detailed bathymetry and structure of the HHFZ and its relationship to the surrounding NFB is very poorly known. The area covered by the GLORIA data is the axial zone, and fault blocks and offsets have not been fully mapped over the full width of the zone. The lack of any sign of recent volcanism and the thick sediment cover suggest the western end of the HHFZ is an old feature. If it represents a boundary separating Miocene sea-floor to the north from younger sea-floor to the south, it could
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accommodate new sea-floor to the southeast by right lateral slip. However, it may be an evolving ridge system with extinct grabens at the western end now filled with sediment. Active faulting, graben formation, and seafloor volcanism increases eastward toward the northwestern arm of the central NFB triple junction.
MINERAL RESOURCES, NATURAL HAZARDS AND FUTURE SEABED MAPPING
Back-arcs are considered to be prospective areas for seabed mineralisation, including hydrothermal poly-metallic sulphide deposits (Rona, 1988; Fouguet et al, 1991), and minerals have been found in back-arc rifts and other locations. Results of recent investigations into the nature and occurrence of hydrothermal venting and poly-metallic sulphides in the northern New Hebrides back-arc troughs, including submersible dives undertaken by the France-Japan STARMER programme, have yet to be reported. However, if the Jean Charcot Troughs are old intra-arc features and not young back-arc troughs, it is unlikely that they are prospective areas for exploration. Future seabed mapping in this region should therefore have two main aims:
(1) to define the orientation of seafloor topography and structure north and south of the HHFZ. The relatively small area between the HHFZ and the eastern Solomon Islands is probably the oldest part of the NFB. It is almost unknown, but recent mapping by ORSTOM will help to clarify structures there.
(2) to better define the HHFZ, particularly the size, orientation and nature of faults and basins along it to determine its age and activity.
Hydrothermal deposits have been located in the central rift areas of the North Fiji Basin (Auzende and others, 1989). If the HHFZ is a young rift, then it would also be a prospective area for minerals. However, no neo-volcanism has been positively identified in the part of the fracture zone surveyed by GLORIA, and therefore no target areas can be specified with the present data.
The principal natural hazard in the region covered by this report is earthquake related. The central New Hebrides arc is under compression while the HHFZ further east may be undergoing dextral shear. A better understanding of the tectonic origin of the HHFZ and of its interaction with the back-arc is vital to understand the origin of earthquakes in eastern Vanuatu. Detailed mapping of the HHFZ and the South Pandora Ridge would help to confirm their relationship to the active NFB spreading and to the New Hebrides Arc. Sampling is also needed to verify interpretations of the seabed images. Seabed mapping on
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two scales would resolve problems relating to the history and development of resources in the back-arc region:
(1) Regional coverage north and south of the HHFZ would provide a better understanding of the regional setting.
(2) Detailed mapping with high resolution swath bathymetry and seafloor imaging over the width of the HHFZ would provide a much clearer understanding of its nature, trend and origin.
CONCLUSIONS
(1) The intersection of the HHFZ and the back-arc is much better known as a result of this survey, but more data is needed.
(2) No extensive areas of young volcanism are found on the floors of the Jean Charcot Troughs to suggest a youthful origin by rifting. This supports an earlier hypothesis that these troughs may be relicts from the breakup of a previous arc platform, in which case their history predates volcanism both on the adjacent volcanic islands and on the emerging arc to the west.
(3) The northwestern part of the North Fiji Basin is the oldest part of the basin. The HHFZ may separate it from younger crust to the south. There is a need to elucidate the origins and history of this region, and the nature of its seismic activity.
(4) Horst and graben structures cut obliquely across the trend of the HHFZ, indicating that the HHFZ is not a simple transform fault, Fault movement may be between series of en echelon ridges, forming a relay, however, grabens within the HHFZ at the arc could be remnant spreading zones, formed during early opening of the NFB. If so, this would support the hypothesis that the HHFZ formed as a rift area in the NFB.
(5) The Vanikoro Basin was only mapped on its eastern side, which is marked by a rocky ridge and possibly young volcanism. Mapping on the basin side of the ridge is needed to better define its limits and to understand its structure and seafloor processes.
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