WARNING
This is a short report of SOPACMAPS. It was compiled to present SOPACMAPS in a concise form.
It contains the introduction common to all study areas and the individual conclusion of each area.
Note:
• The page numbers are not in order because the pages were extracted from different books.
• The table of content presents the content of every study, not the content of this report.
• All the technical analysis was removed but can be consulted in SOPAC.
• You can find the chapter 6 eight times (one per local study area).
PERSO:-i:-iEL E:\IBAR()l'E O~ BOARD STAFF
Effectlf nav ig ant :\ll (Ill Ofn Crew Scienufiques 25 Scientists
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· Sondeur rnultifaisceaux DUAL EM 12 SI:\IRAD . Multibeam echosounder · Sondeurs scientifiques 3.5112!2(Xl khz . Scientists sounders · Couranrometres Doppler RDI 300 . 75 khz . Doppler courantorneter · Gravirnetre KSS 30 BODENSEE\VERK. Gravimeter KSS 30 · Magnetornetre BARINGER M 2+4 . Magnetometer · Thennosalinographe SIS CfD !OOO . Thennosalinographe · Sonde de temperature eau de mer TQP. Temperature water sea lead · Telecornrnande et base acoustique OCEANO. Remote control and acoustic base · Pinger GENISEA & SUBER . Pinger · Lanceur de sonde XBT SIPPICAN MK 12 . XBT launcher · Reseau large bande infonnatiq. & video . Wide band Process & Video network · Systernes infonnatiques temps riel SUN stations on network . Real time processing systems Navigation integree, acquisition de Integrated navigation, data donnees, centrale technique, archivage. acquisition, Technical station video data storage, video · Systernes infonnatiques temps differ': . Post processing systems Rejeu donnees navig. barhy. et scientif. Bathymetric & scientific data · Messagerie electroruque par satellite . Satellite network Mail · 8 laboraroires . 8 laboratories · Emplacement pour 10 conteneurs labo . Location for 10 labos containers
:-iAVIGA TION·POSITIO:-;:-intE:-iT NA VIGA TION & POSITIONI:-iG
.2 gyro compas SGB I(XlO BROWN . 2 gyrocompass · Loch electrornagneuque 2 axes ALMA CLI 60 . Electromagnetic log 2 axis · Loch Doppler 2 axes THOMSON 5722 . Doppler log 2 axis · Positionnemem dynamique eGA HBS ALCA TEL . Dynamic positioning · Recept. Navig Sat transit MAGNAYOX MX 1107 R . NAV SAT TRANSIT receiver · Recept. Navig. Sal. GPS differenucl SERCEL NR 103 . NAY SAT GPS receiver differentiel · Recept. Navig. Sal. GPS MLR CM 015 . NAY SAT GPS receiver .2 recepteurs Navig. LORA.'1 /o,ILR LRX 22 .2 NAY LORAN e receivers 1 recept. Navig, DECCA NAYSTAR 20m D I NAY DECCA receiver
RADIO RADIO
· Station radio BLU DANSK. BLU radio station · Station radio YHF bi-di ERT 2000 . YHF radio station bi-di · Tel. 11x, fax Sal. INMARSAT A MAGNAYOX MX 2-100 . Tel, Tlx, fax Sal. INMARSAT A · Telex Sal. INMARSAT C C. MATFlSNEC . Telex Sal. INMARSAT C
:'>tETEO METEO
· Station recept. Sal. me tee NOAA METEOR INFORMEDIC . Satellite rneteo station receiver · Recepteur facsimile meteo T AIYO Fax TF 72 I . Facsimile rneteo receiver · Recepteur NA VTEX IRC 300 A . NA VTEX receiver · Station rneteo AANDERAA. Meteo station
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MACHINES ENGINES
· Propulsion 2 moteurs (courant continu) JEUMONT SCHNEIDER CCP 138-53- . 2 engines propulsion (dir. current) 8 2xlO00 Kw 150 Tlmn .3 moteurs dieseValtemateur DUY ANT CREPEL 3x II OOKw . 3 diesel engines/alternator 150T/mn · Combustible 580 m3 . Capacity Bunkers · Eau douce 261 m3 . Fresh water · 2 lignes darbre SCHOTTEL. 2 propeller shafts · 2 gouvernails . 2 rudders · Propulseur detrave I bow thruster · Systerre autornatise de production . Automated system of energy et gestion d'energie MERLIN GERIN production and management · Puissance electrique 3000 Kw . Electric power · Puissance administrative 8460 Kw . Administrative power · Puissance courant regule 2 x 40 kw . Regulated current power · Systerne d'alarme autornarise CMR. Automated system of alarm I EXECUTIVE SUMMARY I
The SOPACMAPS Project was designed to investigate as much area as possible of the EEZs of the SOPAC member countries: Solomon Islands, Vanuatu, Fiji and Tuvalu.The main objectives were to map the bathymetry, investigate the resource potential and to study geological processes, including geological hazards. A total area of approximately 730,000 km? was mapped during a total of 87 days at sea, using the Dual Multibeam Swath echosounder on the research vessel L'Atalante, operated by IFREMER under contract to SOPAC and funded by the European Union.
The data are a very important source of information for the understanding of the nature of the seafloor of the South Pacific, and establish an understanding of seafloor process and resources at a level which could not have been imagined as recently as 20 years ago.
During the three legs of 29 days each, multibeam (bathymetry and acoustic imagery), 6- channel reflection seismic, magnetics, gravimetry and sub-bottom profiler data were collected along 439 profiles totalling 15,680 nautical miles.
All data are of very fair to excellent quality. Preliminary reports and on-board processed charts at 1:250,000 scale were delivered to SOPAC shortly after each leg. Reprocessing of the data by IFREMER produced several sets of charts: navigation, bathymetry and acoustic imaging at 1:250,000 scale, and bathymetry, acoustic imaging and morpho-structural interpretation at 1:500,000 scale.
Interpretation of the data, conducted for SOPAC by IFREMER and ORSTOM, produced comprehensive final reports focused on eight areas:
- Central Solomon Trough, - Malaita Area, - Melanesian Arc Gap Area, - North New Hebrides Back Arc Area, - New Hebrides Intra-Arc Basin, - Pandora Bank Area, - Alexa/Charlotte Banks Area, - South Tuvalu Banks Area. The area studied in this report is the most eastern survey area and named STBA (South Tuvalu Banks Area); it is is located in the southern part of the EEZ of Tuvalu around the Hera- Bayonnaise Banks, the Kosciusko-Martha Banks and the Niulakita Island. A large zone of 60,000 km2 belonging to the Melanesian Border Plateau lying on the Pacific crust, north of the Vitias Trench paleo-lineament has been mapped and imaged. Seismic reflexion, magnetic and gravity data have been also collected.
The almost full coverage bathymetric and imagery charts and the collected geophysical data largely increase our geological understanding of this almost unknown area.
Some of the previously reported banks have been precisely located and contoured (Hera- Bayonnaise Banks, Kosciusko-Manha Banks). Some of previously reported banks do not exist New highs and chains of seamounts have been discovered (Luao Seamounts, South Kosciusko- Martha Seamounts, North Eaglestone Seamounts). These shallow water areas could represent good targets for fishing activities which are considered as the best economic potentialities of the area.
In order to be more efficient in terms of economic potentialities, this first approach, which is mainly based on the bathymetry, has to be followed by fishery campaigns on the top of the identified shallow structures.
Sampling survey (dredging) would be also very useful to assess the resources potential of polymetallic crusts. EXECUTIVE SUMMARY
CONTENTS ••...... •...... •....•.....•...... •...... •...... •...... 1
CHAPTER 1 -INTRODUCTION 1-1
1.1 - SOPACMAPS Project presentation 1-1 1.2 - Preliminary notes 1-2 1.2.1 - Sheet assemblage 1-2 1.2.2 - Specific aspects relative to the coastline 1-3
CHAPTER 2 - GEOLOGICAL FRAMEWORK ...... •...... 2-1
2.1 - Regional setting: the Southwest Pacific 2-1 2.1.1 - Present plate boundaries 2-1 2.1.2 - Subduction of the Pacific Plate 2-1 2.1.3 - Subduction of the Indo-Australian Plate 2-7 2.1.4 - The active marginal basins 2-8 2.2 - The New Hebrides Island Arc 2-9 2.3 - Geological setting of Solomon Islands 2-14 2.3.1 - The Solomon Islands Arc 2-14 2.3.1.1 - Plates Reconstruction and the Solomon Islands Arc 2-14 2.3.1.2 - Morphology 2-15 2.3.1.3 _Seismicity 2-15 2.3.1.4 - Crustal structure 2-17 2.3.1.5 - Geodynamic evolution 2-18 2.3.1.6 - Sedimentary deposition and vertical evolution 2-19 2.3.1.7 - Plio-Pleistocene volcanism 2-20 2.3.2 - The Melanesian Arc Gap (MAG) 2-21 2.3.3 - The North New Hebrides Back Arc Area (NNHBAA) 2-23 2.3.3.1 - Regional Geography 2-23 2.3.3.2 - Geological framework/Previous works 2-23 2.3.3.3 - Petrology and age 2-26 2.4 - Tectonic setting of the Melanesian Borderland 2-28 2.4.1 - The North Fiji Basin 2-28 2.4.2 - The Fiji Platform, The Fiji Fracture Zone and the northern part of the Lau Basin 2-31 2.4.3 - The Vitias Trench lineament 2-33 2.4.4 - The Melanesian Border Plateau 2-36 CHAPTER 3 - DATA ANALYSIS 3-1
3.1 - Previous work 3-1 3.2 - Bathymetry 3-2 3.3 - Imagery 3-10 3.4 - Seismic reflection profiles 3-12 3.5 - Magnetism 3-20 3.6 - Gravimetry , 3-22
CHAPTER 4 - GEOLOGICAL SyNTHESiS ...... •...... •...... 4-1
CHAPTER 5 - POTENTIAL RESOURCES ...•...... •...... •...... 5-1
5.1 - Uving Resources 5-1 5.1.1. - Preliminary considerations 5-1 5.1.2 - SOPACMAPS results 5-4 5.2 - Mineral Resources 5-4 5.2.1 - Polymetallic nodules 5-4 5.2.2 - Polymetallic crusts 5-5 5.2.3 - Hydrothermal sulfides 5-6 5.3 - Oil Potential 5-7
CHAPTER 6 - CONCLUSiONS ...... •...... •...... •...... 6-1
CHAPTER 7 - SURVEY EQUIPMENT 7-1
7.1 - Data processing on board AN L'Atalante 7-2 7.2 - Bathymetry and imagery processing 7-4 7.2.1 - Bathymetry 7-4 7.2.2 - Post-processing navigation 7-5 7.2.3 - Post-processing EM 12 Dual bathymetric data 7-5 7.2.4 - EM12 Dual image processing 7-6 7.2.5 - Setting up of the celerity files 7-6 7.3 - Seismic reflection 7-8 7.3.1 - Seismic Reflection on AN L'Atalante 7-9 7.3.2 - G.1. Gun - Model 150 7-9 7.3.3 - Air gun principle 7-11 7.3.4 - Compressor 7-12 7.3.5 - Streamer Specifications 7-13 7.3.6 - Recording data 7-13 7.4 - Instrumentation 7-14 a) Barringer magnetometer 7-15 b) Bodenseewerk seagravimeter system KSS 30 7-16 c) 3.5 kHz hull-mounted sub-bottom profiler ~ 7-20 d) Ocean-meteorology station 7-22 e) Acoustic Doppler current prof Her (VM-ADCP) 7-27 f) Thermo-salinometer - CTD+ 100 7-29
CHAPTER 8 - ENCLOSURES 8-1
ENCLOSURE 1 - Staff 8-2 ENCLOSURE 2 - Cruise chronology 8-6 ENCLOSURE 3 - Logbook 8-19 ENCLOSURE 4 - Environmental conditions 8-32 ENCLOSURE 5 - Document table 8-41 ENCLOSURE 6 - Profile table 8-46 ENCLOSURE 7 - Bibliography 8-58 LIST OF FIGURES
Fig. 1.1 - General navigation map of SOPACMAPS - Leg 1 1-6 Fig. 1.2 - General navigation map of SOPACMAPS - Leg 2 1-7 Fig. 1.3 - General navigation map of SOPACMAPS - Leg 3 1-8 Fig. 1.4 - General sheet assemblage (1/250,000) 1-9 Fig. 1.5 - General sheet assemblage (1/500,000 and details) 1-10
Fig. 2.1 - General map of the Southwest Pacific region 2-2 Fig. 2.2 - Morphotectonic map of the Southwest Pacific 2-3 Fig. 2.3a - Shallow seismicity of the Southwest Pacific 2-4 Fig.2.3b - Intermediate and deep seismicity of the Southwest Pacific 2-5 Fig.2.4 - Proposed quantitative model for present-day relative motions in the Southwest Pacific 2-6 Fig. 2.5 - Bathymetric map of the New Hebrides Arc and location of the NH lAB sheet 2-10 Fig.2.6 - Sedimentary basins and faults in the central and northern New Hebrides Arc 2-11 Fig.2.7 - Place names, tectonic features and bathymetry of the central and western Solomon Islands area 2-16 Fig.2.8 - Bathymetry of the Melanesian Arc Gap 2-22 Fig.2.9 - Islands forming the Eastern Outer Islands Group, Solomon Islands 2-24 Fig. 2.10 - Sketch map of the Northern New Hebrides Back Arc Area 2-25 Fig.2.11 - Structural sketch 2-27 Fig. 2.12 - Tectonic setting of the South West Pacific 2-29 Fig. 2.13 - Evolution of the North Fiji Basin 2-30 Fig. 2.14 - Petrologic provinces of the South West Pacific 2-35
Fig. 3.1 - Survey location and profile identification 3-3 Fig. 3.2 - Bathymetry in the South Tuvalu Banks Area 3-5 Fig.3.3 - Magnetism in STBA (computer contouring) - Contour interval 1OOnT 3-21 Fig.3.4 - Gravity in STBA (computer contouring) - Contour interval 10 mGaI 3-23
LIST OF PLATES
Plate 1 -Interpreted seismic profiles in STBA (PR 196,197,198,199) 3-13 Plate 2 - Interpreted seismic profiles in STBA (PR 200, 202, 230, 231, 250) 3-14 Plate 3 - Interpreted seismic profiles in STBA (PR 203, 204, 211, 212, 213, 214) 3-15 Plate 4 - Interpreted seismic profiles in STBA (PR 215, 216, 217,218,219) 3-17 Plate 5 - Interpreted seismic profiles in STBA (PR 232, 233, 234) 3-18 Plate 6 - Interpreted seismic profiles in STBA (PR 266, 267) 3-19 CHAPTER 1
INTRODUCTION
1.1 - SOPACMAPS PROJECT PRESENTATION
A contract for the SOPAC (SOuth Pacific Applied geosciences Commission), swath mapping project funded by the EEC (Lome III - European Development Fund n? 6) known as SOPACMAPS, was awarded to IFREMER (Institut Francais de Recherche pour l'Exploitation de la MER), the French government research organization that operates the new high-tech oceanographical research vessel L'Atalante.
The tender process saw six bids received for the work. Of these, three bids stood out. EEC procedures ruled one of these ineligible, narrowing the final selection down to two. IFREMER's bid was found to be the most technically competent and was recommended by SOPAC and accepted by the EEC.
The 85 m R!VL'Atalante, launched in 1990, was built specifically for swath mapping surveys and carries a dual Simrad EMI2 multibeam bathymetric system.
The dual EM 12 system produces 160 cross-track beams of 10 angle which radiate from hull mounted transducers. These provide depths over a swath of up to 5 times the water depth. The ship surveys at a speed of 10 knots, or more under the right conditions. An associated acoustic side-scan imagery is obtained in the same time.
The ultimate purpose of the SOPAC seabed swath mapping project is to gain an accurate and reliable assessment of the potential living and non-living resources of selected areas of the ocean floor within the EEZs of Fiji, Solomon Islands, Tuvalu and Vanuatu.
In order to produce the complete scientific study with interpretative maps and geological reports, acoustic side scan imagery and multi-beam bathymetric profiles of the seafloor were needed, along with high resolution 3.5 khz sub-bottom profiles, 6-channel seismic reflection, gravity and magnetic data. These data and images had to be fully processed and co-registered on mosaics.
The schedule of the cruises was:
* Leg 1- 19 July in Noumea (New Caledonia) to 15 August 1993 in Honiara (Solomon Islands), * Leg 2 - 19 August in Honiara (Solomon Islands) to 16 September 1993 in Suva (Fiji), * Leg 3 - 22 September in Suva (Fiji) to 20 October 1993 in Noumea (New Caledonia). The swath mapping and geophysical data collected by RN L'Atalante were processed and interpreted by teams of French scientists belonging to IFREMER, ORSTOM and French Universities ..
The SOPACMAPS project was managed at IFREMER by Guy PAUTOT (from the beginning of the project to the end of 1993) and Raymond LE SUA vs in 1994, both from the "Departement Geosciences Marines" at IFREMER Brest.
1.2· PRELIMINARY NOTES
1.2.1 • Sheet assemblage
Prior to the at-sea operations, a sheet assemblage (at 1/250,000 scale - see Fig. 1.4) was established and used during the survey and for the processing of the first set of charts (that is: navigation, bathymetry and acoustic imagery).
For the final report, and with the on-board SOPAC Representatives' agreement, a new sheet assemblage was adopted; it covers eight areas at a 1/500,000 scale (see following table and Fig. 1.5). This 1/500,000 sheet assemblage was defined so as to take into account the main structural and regional units and to facilitate the presentation of the data.
For legibility reasons, the Central Solomon Trough is not presented at the 1/500,000 scale, but in three sheets named:
- New Georgia Sound (scale 1/250,000), - Mborokua Basin (scale 1/150,000), - and Iron Bottom Sound (scale 1/100,000).
Note that the legend block was added after the sheet assemblage was done and all the basic bathymetry processing was completed; so that the over-all format of the charts, legend blocks included, exceeds the AO size. CST Central Solomon Trough
MALAIT A Malaita
MAG Melanesian Arc Gap
NNHBAA North New Hebrides Back Arc Area
NHlAB New Hebrides Intra-Arc Basins
PBA Pandora Bank Area
ACBA Alexa/Charlotte Banks Area
STBA South Tuvalu Banks Area
- Abbreviations and names of the 8 studied areas - (see Fig. 1.5)
1.2.2 - Specific aspects relative to the coastline
The basic coastline files were provided by SOPAC, and the files format adapted by IFREMER for the charting processing. It was observed in several places that the coastline was not consistent with the ship positioning accuracy. Assuming that the GPS positioning accuracy during the survey period was around 1()()to 150 metres, a few corrections of land masses were proposed to SOPAC and introduced during charting processing.
Three coastline shift modes have been used:
mode a: when possible, the radar position of at least one remarkable point of the coast was calculated and the correction made;
mode b: when the original landmass position was in obvious overlap with the surveyed area, and considering the GPS positioning accuracy, the best landmass position was decided, taking into account the shape of the coastline and the observed submarine morphology; these considerations resulting in a shift ofthe landmass as a whole;
mode c: in one case (see below), there was not enough room available to move the landmass; in this particular case, it was supposed that the coastline section position concerned is doubtful. Charts concerned by the coastline modifications:
o Melanesian Arc Gap:
Tinakula Island : 3.8 nm shift to N72°E (mode b) Nukapu Island : 4.2 nm shift to N67°E (mode b) Nupani Island : 4.2 nm shift to N67°E (mode b)
o Malaita:
Ulawa island : 2 nm shift to Nl15°E (mode b)
Maramasike : the south termination of Malaita overlaps with the bathymetric survey; as there is no place for a satisfactory shift, the coastline must be considered as doubtful and has consequently been presented as a dashed line (mode c).
o Mborokua Basin:
Mborokua Island : 0.7 nm shift to East (mode a)
o New Hebrides Intra Arc basin:
Aoba Island : 1.4 nm shift to North (mode b)
1) The geodetic positioning of the landmasses is not one of the objectives of the SOPACMAPS survey, and it must be stressed that the proposed new position of a few islands corresponds only to an optimal adjustment based on the three different modes listed above;
2) The landmasses shifted positions are the same for the different charts; nevertheless, and due to specific technical problems (incompatibility between the vector and raster computer formats), it was not possible, within the available time, to bring the same landmass modifications to the acoustic imagery charts. The same difficulty occured for the size of lettering (text of the legend block, positions) which are different for navigation, bathymetry charts and the acoustic imagery chans.
3) The charts produced during the SOPACMAPS survey are not marine charts and these charts must not be used for navigation.
CHAPTER 6
CONCLUSIONS . 6.1 - GEOLOGY
6.1.1 - Iron Bottom Sound
The Iron Bottom Basin is flanked major SE-NW faults. This main SE-NW trend is found all across the basin in the general bathymetric trends. The direction of the submarine canyons illustrates the predominance of this SE-NW tectonic control. The seismic reflection profiles also show SE-NW faults and the axis of the deepest part of the basin. The location of the basin axis is slightly offset to the north-east giving an asymmetric pattern of the sedimentary deposition. The gravity anomalies confirm this configuration with a gravity low close to the Florida Islands border.
The other major feature of Iron Bottom Sound is the Savo Volcano and its submarine prolongation. The volcano is surrounded by concentric spurs and furrows, well evidenced on the imagery, probably related to eroded volcanic flows. The magnetism and seismic reflection data suggest an E-W trending extension of the volcanic edifice. The Savo Volcano is bounded on its eastern side by an important N-S fault identified on the seismic profiles, cutting the central part of the basin. Small (a few tens of meters) volcanic mounds are identified in the central basin between Savo and Guadalcanal. They seem to be aligned along this N-S direction.
The last interesting feature is illustrated both by bathymetry and seismic profiling. It is an area located in the central basin showing a rough topography and superficial deformation of the sediments. This area can be interpreted as related to sedimentary slumps coming from the lateral margins of the sound or from the Savo Volcano.
6.1.2 - Mborokua Basin
Three geological provinces are distinguishable:
* The southern province is closely related to the compressive motions affecting the boundary between the Solomon Islands area and the subduction zone north of the Woodlark Basin. This compressive motion is mainly expressed by the rough topography characterized by rhomboidal deformations.
* The central province is mainly controlled by volcanic features especially represented by the Mborokua Volcano. The area trends NW-SE and is bounded to the south by a graben system showing EW, NW-SE and NE-SW directions. This conjugate faulting illustrates both compressive and strike-slip stresses.
* Except the south-eastern and nonh-western edges linked respectively with the Russell and the New Georgia volcanoes, the nonhern province shows a general slope dipping toward New Georgia Sound. The superficial morphology of the slope shows a succession of NW-SE and SW-NE directions previously described in the two other provinces. A remarkable feature is the flowing-type area located in the north-western part of the survey probably related to the Mbulo Volcano.
The seismic profiles across the basin show buried basement blocks tilting north-westward, This tilting indicates a NNW-SSE extensional motion. Such an extension is compatible with the general E-W left-lateral strike-slip motion affecting the whole Solomon Islands zone.
In conclusion the Mborokua area shows the conjunction of compressive and strike-slip stresses related to 'the geodynamic environment of the Solomon Islands.
6.1.3 - New Georgia Sound
The explored New Georgia Sound area constitutes the junction between New Georgia- Kolornbangara- Vella Recent volcanic province to the south and the older Choiseul Cretaceous- early Tertiary basaltic platform.
The volcanic province is morphologically characterized by steep slopes cut by canyons guiding mass-flows down to the foot of the slope. The trends of faulting are the main NW-SE directions of the limits of the domain.
The volcanic province is separated from the nonhern Choiseul platform by a 1,200 to 1,300 m deep basin cut in two parts (Russell and Shortland) by the Fatu a Moana saddle. To the south, the basins and the saddle are cut by an EW fault recognised on the surficial topography. Its presence in the deep seismic layers is not clear. This fault could be interpreted as created by the left-lateral strike slip motion between both domains. It is underlined by an important gravity low. Other E-W faults of less amplitude exist north of the major one on the Choiseul platform.
The Choiseul platform itself shows the same surficial slope sliding guided by N-S to NE- SW canyons. On the seismic profiles the main structural trends in the deep basin are NE-SW faults.
The sedimentary cover is more than 2 s. thick and shows angular unconformities relative to the vertical evolution of the basin.
The Fatu a Moana mound is an uplifted block of basement and its sedimentary cover, culminating at less than 700 m depth. It is associated with an increase of 0.6/1,000 of the salinity. This increased could be explained by current changes, or upwelling due the presence of the mound. 6.2 - POTENTIAL RESOURCES
The most accessible fishing resources are probably located on seamounts and other bathymetric highs (NW-SE volcanic line cutting the central pan of the Mborokua Basin and Fatu o Moana located in the middle of New Georgia Sound).
Ferromanganese deposits, such as nodules and crusts are of little economic interest in CST Area, due to unfavourable conditions for nodules and to very reduced extent of indurated substrate as a possible crust bearing medium.
The same conclusion can be made for hydrothermal sulfides, as the geological conditions for their occurence are not met in the region.
In the field of mineral resources, only oil potential seems to be interesting: the sedimentary cover is favourable in Iron Bottom Sound, Fatu 0 Moana zone and East Malaita area, but the confmnation of this potential needs additional and specific multichannel seismic survey. CHAPTER 6
CONCLUSIONS
The SOPACMAPS cruise allowed to obtain extensive and precise data in the central and north Vanuatu economic zone. A great deal of data are now accumulated especially over the central basins of Vanuatu: North and South Aoba basins, Big Bay- Cumberland Basin, Banks Basin and Torres Basin.
Those basins were surveyed during the past two decades for scientific and economic purpose, but, owing to the multibeam EM 12 echo sounder, we have now a quite fair bathymetric map of the region. In fact, such a map is the basic tool for funher exploration as well for a better understanding of the regional geology as for applied objectives.
Various domains and specific features have been mapped: sedimentary basins, volcanic regions, canyons and submarine fans. South Aoba Basin reveals strong and recent volcano-sedimentary flows coming from Ambrym Volcano; strike slips, trending N60°, NllO°characterize the basin. The North Aoba Basin, interpreted as the northward continuation of Espiritu Santo basement is affected by N120° normal faults
In the field of living resources, bathymetry is quite essential for prospection of living resources like deep sea fishes., but it is clear that in NHIAB, there is no wide developped flat- topped structures which might constitute interesting targets.
Hard mineral resources are evidenced as of poor potential for manganese nodules, but polymetallic crusts may be present on the flanks of the islands and on submarine reliefs, such as the eastern margin of Malekula and Espiritu Santo. Sulfides deposits generally associated to hydrotherrnalism are also supposed to be of very poor interest in NHIAB.
Oil potential is reponed in Big Bay-Queiros Basin, the southwestern flanks of the Torres Basin located east of the Torres Islands, and possibly the Banks Basin. New high resolution multichannel seismic are necessary for petroleum prospection.
Geological hazards can likewise be better identified using bathymetric and imagery maps. The evolution of volcanic edifices activity can be deduced from mapping of mass wastings and volcanic flows on their flanks. Various geological hazards in NHIAB are: the vicinity of active volcanism, coastal mass wasting and instabilities, activity of faulted zones. Explosive volcanic eruptions (the highest destructive effect if it occurs in Aoba), and massive slumps can produce tsunamis. Lahars emission is probably the most effective hazard near Gaua and Ambrym. CHAPTER 6 CONCLUSIONS
The SOPACMAPS cruise obtained accurate and extensive data in the Solomon Islands economic zone. Over the so-called Malaita Area, located on the ancient boundary of the Australian and Pacific Plates. There is now a fair bathymetric map of the east coast of Malaita. The Indispensable Basin, on the west side is not fully covered but the slopes from 1,000 m to 1,500 m in depth are well documented.
The Malaita Area is interpreted by Coleman and Kroenke (1981) as a piece of the oceanic crust of the Ontong Java Plateau folded and obducted 8 My ago during the collision phase between the Ontong Java Plateau and the old Solomon Arc. The result of this collision was the end of the functioning of the North Solomon Trench and the reversal of the polarity of the subduction from NE-SW along the Nonh Solomon Trench to SW-NE along the late Miocene to present-day active San Cristobal Trench (Kroenke, 1984). The obducted and folded Malaita Area was named Malaita Anticlinorium (Kroenke, 1977). This ovenhrusting phase was accompanied by the emission of basaltic volcanism through the old Pacific crust of Malaita (Hughes and Turner, 1977).
The SOPACMAPS cruise seismic profiles indicate that the Nonh Solomon Trench is infilled by a 1 to 2 s.t.w.t thick sedimentary cover dipping to the south. This dipping could be interpreted as a small amplitude present-day deformation within the southwestward dipping Nonh Solomon Trench. Some indications of gentle folding have been observed at the foot of Malaita margin.
The major part of the Malaita margin is constituted by a deformed alternation of highs and lows joining northern Malaita Island with the Ulawa zone. This deformed zone is cut by an important N-S Fault at 162°E. This fault separates the Malaita Anticlinorium from the 6,000 m deep nodal trough created by the junction between the North Solomon and the Cape Johnson trenches.
The two flanks of the Malaita Anticlinorium are different, for example, the Indispensable Basin and adjacent basins contain a greater amount of sediments than in the eastward basins.
The SOPACMAPS cruise was mainly designed for bathymetric purpose and the profiles were run for the maximum of efficiency of the multibeam EMl2 echosounder of the RN L'Atalante. Nevenheless, seismic reflection profiles recorded continuously, in conjunction with magnetism and gravity, give valuable information allowing to discuss the deep structure of the area in term of potentialities. All those information should be an essential basis for funher explorations using various techniques as multi-channel seismic reflection on selected areas and sampling by dredging, coring or drilling.
In the field of living ressources, it is clear that he major part of the summit of the central ridge (less than 1,000 m deep) constitutes a fair possibility for concentration of deep species fishes, like alfonsinos (B eryx splendens sp.). Those fishes are caught on an industrial basis on longlines or bottom trawls in several places in the Pacific ocean, and a precise bathymetry is essential for prospecting.
Other fishing sites are the slopes of East Malaita and the Ulawa Islands where small mounds and seamoun ts are culminating at less than 1,000 m depth. The slopes around the Indispensable Basin, although uncompletely surveyed, show also good opportunities for deep fishing prospect.
The only sedimentary basin observed in the area is located south east of the Guadalcanal termination, with a water depth of about 2,000 rn, but as it is probably submitted to tenigeneous sedimentation, it does not constitute a good situation for polymetallic nodules.
The central zone of East Malaita Area is constituted by a tectonized zone reflecting the Malaita and the Ulawa Islands structural prolongation. It is mainly characterized by three successive parallel ridges separated by deep basins on the slope. The most proeminent ridge is the central one with a summit culminating at an average depth of 750 m. This central ridge, probably volcanic in origin, constitutes a good candidate for polymetallic crusts formation.
The regional geology clearly indicates that CST Area is not favourable in term of hydrothermal activity. Nevertheless as for any volcanic rocks, there may be a scatttering of sulfides assemblages occurring as trace elements, but in no case it can constitute a potential resource.
Geological hazards are important in Malaita Area; they are related to the central volcanic ridge and the slides triggered by volcanic activity and associated earthquakes. A scarce shallow seismicity is present in the region, but quite less intensive than on the southwest Guadalcanal and south San Cristobal coasts in association with the present subduction. CHAPTER 6
CONCLUSIONS
The Melanesian Arc Gap Area was, before the SOPACMAPS cruise, poorly mapped, and little information as available. That region had not been surveyed during the Tripartite 1982-1984 program. So, the most recent map was the CCOP/SOPAC map of Kroenke, Jouannic and Woodward (1983), but for their compilation, the authors disposed only of the 1974 map of Mammerickx (Scripps).
The bathymetric map available after the SOPACMAPS survey shows a very complex transitional zone between the northernmost New Hebrides Island Arc and the easternmost Solomon Island Arc. Located north of the San Cristobal Trench, the area includes the volcanic islands of Nendo, Tinakula, Reef Islands and Nukapu, but the average depth is 2,000-3,000 m. The nature of the crust is oceanic and the sediment cover generally thin, except in some grabens in the north of the zone .
The potential resources of that zone should be related to hydrothermal mineral resources and fishing for living resources. Samplings are now necessary to confmn the existence of mineral deposits associated with hydrothermal activity in the active volcanic province and on the active inferred accretion zone constituted by the 9°30'S Ridge. Concerning fisheries, the relatively shallow seamounts (less than 1,000 m deep) have to be prospected to test the presence of deep sea fish. It could be interesting to investigate a possible relationship between the sea temperatures anomalies produced by active volcanic centers and the presence of living resources. CHAPTER 6
CONCLUSIONS
6.1 - GEOLOGY
The North New Hebrides Back Arc Area represents the junction between the New Hebrides Arc domain and the northwestern corner of the North Fiji Basin but the relationships between the Melanesian Arc Gap Area and the nonhwestern pan of NNHBAA are evident.
The area located west of the Duff Ridge corresponding to N-S alternation of grabens and highs could be interpreted either as a pan of the New Hebrides volcanic arc or as a pan of the 12 to 10 Myoid North Fiji Basin crust trapped by the recent building of the Duff Ridge. We speculate, as Pelletier et al., (1993 a and b) did that the second hypothesis is more conceivable than the first one. The magnetic anomaly and gravity anomaly pattern showing elongated N-S lineations better than dipolar features favors this second hypothesis. Nonh of the Duff Ridge, all the data converge to confirm the existence of an EW present-day spreading ridge named 9°30'S Ridge by Pelletier et al., (1993a and b).
East of the Duff Ridge extends the oceanic crust of the North Fiji Basin. The most remarkable features of this area is the prolongation of the 9°30'S Ridge in the northern pan and the location of two other EW to ENE-WSW ridges at 10020'S and 10050'S. The magnetic data suggest as a working hypothesis that the three ridges near 9°30'S, 10020'S and 10050'S and the roughly EW deformed zone associated with numerous small volcanic centers in the southern part, may be coupled in an incipient seafloor spreading system associated with the Hazel Holme extensional zone. This configuration is clearly evidenced on the magnetic and gravimetric patterns. At a large scale this zone could be interpreted as an EW volcanic line linked with the Charlotte Bank-Pandora Bank system.
The Melanesian Arc Gap Area has an effect upon the northwestern pan of NNHBAA by the large structure elongated WNW-ESE of the Reef Islands well defined by a high gravity and by two EW major normal faults which are cutting the extreme end of the northern pan of NNHBAA.
6.2 - RESOURCES
The bathymetric chan of NNHBAA shows some sedimentary basins, but under a too great water depth for actual oil investigation. The real resource potential of the zone is be related to hydrothermal mineral resources and fishing. Three zones (the active volcanic island arc, the 9°30'S Ridge and the bach-arc trough) are interpreted as possible active hydrothermal sites but chemical measurements, sampling and photographs observations are now necessary to confmn the existence and location of mineral deposits.
For economical fisheries, it could be interesting to prospect bathymetric highs which are probably good sites for deep sea fish populations. CHAPTER 6
CONCLUSIONS
The studied area is located on the easternmost part of the EEZ of Solomon Islands. A 70 krn wide, 500 krn long WNW-ESE trending zone belonging mainly to the northwestern North Fiji Basin have been mapped and imaged. The area includes Anuta and the Fatutaka Islands, and the Pandora Bank, as well as a small portion of the Vitias Trough. Seismic reflexion, magnetic and gravity data have been also collected.
The bathymetric and imagery charts and the collected geophysical data largely increase our geological understanding of this almost unknown area (see geological synthesis).
Previously reponed different banks (Anuta, Fatutaka, Pandora) have been precisely located and contoured.. New highs and seamounts have been identified, the biggest ones being the Kroenke Seamount and the Atalante Seamount. These shallow water areas could represent good targets for fishing which is probably the best economic resource of the area.
In the field of hard mineral resources, polymetallic nodules may exist in the deepest basin located at the junction of PBA and ACBA, and which corresponds to a section of the Viti as Trench; polymetallic crusts may be present on indurated substrate exposed to sea water, but this remains strictly speculative. Sulfides deposits associated to the volcanic history and hydrothermal activity may be locall y found. CHAPTER 6 CONCLUSIONS
The studied area is located on the nonhern part of the EEZ of Fiji and the easternmost part of EEZ of Solomon Islands. A large zone of about 30,000 km2 belonging partly to the Melanesian Border Plateau and partly to the North Fiji Basin was mapped and imaged. The area includes the Alexa and Charlotte Banks, the Vitias Trough and the domain extending east of the Pandora Bank. Seismic reflection, magnetic and gravity data were also collected.
The almost full coverage bathymetric and imagery charts and the collected geophysical data largely increase our geological understanding of this almost unknown area (see geological synthesis).
Previously reponed different banks (Alexa/Charlotte Banks) have been precisely located and contoured. New highs and seamounts have been found (Kroenke and Atalante Seamounts). These shallow water areas could represent good targets for fishing which is by the far the best economic resource of the area.
In order to more effectively assess economic potential, this first approach, which is mainly based on the bathymetry, have to be followed by fishery campaigns on the top of these mineral shallow potential structures. Sampling survey (dredging) would also be very useful to assess.
As for the South Tuvalu Banks Area, the most obvious economic potentiality evidenced by our mapping survey is by the far the fishing activity. The Alexa/Charlotte Banks Area is mainly characterized by four main highs which represents the best candidates for fishing activities. South of the Vitias Trough and in the EEZ of Solomon Islands, two large volcanoes have been contoured. The largest one (18 x 30 km) named Kroenke Seamount is located more or less close to the previously reported shoals. The second one (12 krn in diameter and named Atalante Seamount) has been discovered during this cruise. The shape of these edifices suggest that they probably rises to very shallow water depths and also constitute potential targets for fisheries.
The only area which might be of potential interest for polymetallic nodules is the deepest basin (4,500 m) located at the junction of the Pandora bank Area and the Alexa/Charlotte Banks Area and which corresponds to a section of the Vitias Trench: on the contrary of the surrounding zones which present a mid grey and heterogeneous acoustic reflectivity, the sediments of this basin are controlled by specific deposition processes as revealed by a continuous and darker acoustic facies which may be related to finer grained deposits under or at the limit of CCD.