CONTENTS

SOPACMAPS PROJECT PRESENTATION

A contract for the SOP AC (South Pacific Applied geosciences Commission) swath mapping project funded by the EEC (Lome III - European Development Fund n" 6) known as SOPACMAPS, has been awarded to IFREMER (Institut Francais de Recherche pour J'Exploitation de la MER), the French government research organisation that operates the new high-tech oceanographical research vessel: L'Atalanie.

The tender process saw six bids received for the work. Of these, three bids stood out. EEC procedures rules 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-meter L'Atalante, launched in 1990, was built specifically for swath mapping surveys and carries a dual Simrad EMI2 multibeam bathymetric system. The dual EMI2 system produces 160 cross-track beams of 1° 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, more under the right conditions. An associated acoustic side-scan imagery is obtained in the same time.

The final 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 EEZ's 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 are needed, along with high resolution 3.5 khz sub-bottom profiles, single 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 is :

* 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) (cf. Fig. 1), * Leg 3 - 22 September in Suva (Fiji) to 20 October 1993 in Noumea (New Caledonia).

CRUISE CHRONOLOGY

c

CHAPTER 2

CRUISE CHRONOLOGY 25 August

End of the New Georgia Sound box at OOh28. Good weather conditions. Transit to East Malaita zone. Two hours stop for Simrad calibration PR 136, 137 and 138.

26 August

Start of the East Malaita box: 02h40. PR 140. Good weather conditions. PR 139 to 140.

27 August

East Malaita box, continuation. Good weather conditions. PR 141 to 143.

28 August

Continuation East Malaita box. Good weather conditions. PR 143 to 150.

29 August

Continuation East Malaita box. Good weather conditions. PR 150 to 157.

30 August

Continuation East Malaita box. Good weather conditions. PR 158 to 164.

31 August

Start of the Melanesian Arc Gap box. Wind 25-30 knots, Sea 4. PR 165 to 166.

1 September

Continuation of the Melanesian Arc Gap. Same weather conditions as 31 August. PR 166 to 167.

2 September

Continuation of the Melanesian Arc Gap. Wind 25 knots, Sea 3. PR 168 to 170. 12 September

Pandora Bank box. Rough sea. Engine problems around 19hOO. PR 185 to 187.

13 September

Pandora Bank box. Rough sea. New problems on the engine around OlhOO. 03h20 decision to stop the profile 187, only one engine is working. Heading to Suva. Maximum speed 8 knots. PR 187 to 188, TI.

14 September

Transit to Suva. T2, D.

15-16 September

Transit to Suva through the North Fiji Basin.

GEOLOGICAL FRAMEWORK

3.1 - Geological Setting of the Solomon Islands (Fig. 2)

This chapter is mainly the result of the compilation of the different synthetic papers especially those published in:

- Vedder, Pound and Boundy, Eds, 986, Geology and offshore resources of Pacific island arcs-central and Western Solomon Islands: Houston, Texas Circum Pacific Council for Energy and Mineral Resources Earth Sciences Series, Vol. 4, and in:

- Vedder and Bruns, Eds, 1989, Geology and offshore resources of Pacific island arcs- Solomon Islands and Bougainville, Papua New Guinea Regions: Houston, Texas, Circum Pacific Council for Energy and Mineral Resources, Earth Sciences Series, Vol. 12.

The other references are in the bibliography chapter.

3.1.1 - Plates Reconstruction and the Solomon arc Islands

Reconstructions of the relative plate motion in the hot spot reference frame for the Cretaceous and Cenozoic times (Gordon and Jurdy 1985; Engebretson et al. 1985; Stock and Molnar, 1982, 1987, Wells, 1989) converge to the conclusion that the double island chain of the Solomon Islands can be interpreted as the result of the complex interactions between the Australian plate and the Pacific plate (Coleman and Packham, 1976; Wells, 1989).

The Australian plate is moving northward at an absolute rate of 7 ern per year (Minster and Jordan, 1978) , and is being consumed beneath the Pacific plate along a series of trenches that flank the southern and western sides of the island arcs. The Pacific plate has a northwestward absolute rate of motion of 10.7 cm per year (Minster and Jordan, 1978) .

The Solomon Islands are on a northwest-trending segment of the leading edge of the Pacific plate where the rate of oblique convergence may exceed 10 cm per year (Johnson and Molnar, 1972). Presumably a combination of subduction and sinistral shear accommodates the rapid convergence along this segment of the plate boundary.

In his study of the Solomon Islands basins, Katz (1980) divided the region into two geologic provinces separated by a major zone of tectonism: the Malaita province and the Main Solomon province. The area which separates the two island chains in the central and western Solomon Islands is called the Central Solomons Trough or Slot.

This structural trough is a part of the Solomon Basin of de Broin., Aubertin, and Ravenne (1977) , and can be divided in four structural basins: the Shortland basin, Russell basin, Iron Bottom basin, and Indispensable basin (Fig. 3).

Katz recognized internal structural complexities in the Central Solomon Trough, including possible fragmentation beneath Manning Strait by a fault zone that caused "en echelon" segmentation of the basin. The sediment thickness increases with water depth, and the axial part of the trough is underlain by 2.5 to 4.5 km of relatively undeformed strata. In their report Maug and Coulson (1983) proposed the term Central Solomons Basin for the entire area located between Bougainville and Malaita. Kroenke (1984), in his regional tectonic synthesis, attributed the development of the Central Solomons Trough to the reversall of arc polarity due to the collision of the Ontong-Java Plateau against the Solomon Islands Zone, which resulted in the transformation of the former back-arc basin into the present intra-arc basin.

3.1.2 - Morphology

Morphologically, the seafloor depression beneath New Georgia Sound is flat bottomed, steep sided and narrow ended. It is slightly sinuous and its floor is disrupted by Savo volcano near the southeast end and by a bathymetric saddle in the constriction between Kolombangara and Choiseul (Fig. 3). Water depths in the broad, nearly flat central part generally range from about 1.000 m to 1.800 m. The area between New Georgia and Santa Isabel is the deepest at slightly more than 1.800 m. The southwestern edge of the depression is borded by islands with steep relief, the highest of which is Guadalcanal where one mountain peak is 2.447 m above sea level (Hackman, 1980). The northeast side of the trough is bordered by islands of lower relief (Choiseul, Santa Isabel and the Florida island group); the highest peak is about 1.220 m on Santa Isabel. Thus, the total structural relief is as much as 9 km.

3.1.3 - Seismicity

Intense seismicity characterizes the northwestern and southeastern parts of the Solomon Islands arc where the trench system is well developed. Conversely, seismic activity is much reduced alond the central part of the arc opposite the active Woodlark spreading ridge where the trench system is weakly expressed bathymetrically. Another relatively quiet zone between the main island group and the Santa Cruz island group is believed to be a transform (de Brain, Aubertin and Ravenne, 1977; Dunkley, 1983).

Clearly defined Benioff zones dip steeply northeastward in the vicinity of Bougainville and the Santa Cruz island group. Near San Cristobal, the Benioff zone is not well defined and may be steeply inclined. Deep-seated earthquakes in the range of 500-700 km occur beneath Bougainville and the Santa Cruz island group. Halunen and Von Herzen (1973) and Kroenke (1972) suggested that these deep shocks may originate along a detached lithospheric slab derived from an old, southwest-directed episode of subduction. Early studies (Denham, 1969, 1971) indicated that most of the seismicity could be ascribed to the northward movement of the Australian plate coupled with sinistral shear along its northern edge. Interpretation of focal mechanisms of large earthquakes for the period 1963 to 1967 showed that slip vectors were roughly orthogonal to both segments of the sharply curved New Britain Trench, a pattern that was not in accordance with simple underthrusting of the Australian plate beneath the Pacific plate along their northwest-trending boundary (Ripper, 1970).

In order to explain the pattern of hypocenters and the direction of slip vectors in terms of plate tectonics, minor plates were postulated along the boundary zone of the two major plates; and these minor plates derived their relative motion from the collision. A complex set of relations including active underthrusting of the Australian plate beneath San Cristobal, the occurrence of deep remnants of lithospheric slabs beneath the central part of the island chain, and underthrusting of the Pacific plate beneath Santa Isabel was proposed by Denham (1975) to account for the spatial distribution of earthquakes. Different authors identified various microplates. Their nomenclature is summarized below (Vedder et al., 1986).

1. Solomon Sea plate (Johnson and Molnar, 1972; Johnson, 1979; Curtis, 1973; Ramsay, 1982, Figure 1), or simply the Solomon plate (Luyendyk, MacDonald, and Bryan, 1973; Weissel, Taylor, and Karner, 1982) lies between the Woodlark spreading ridge in the Solomon Sea and the New Britain Trench.

2. South Bismarck plate (Johnson and Molnar, 1972; Connelly, 1976; Johnson, 1979; Taylor, 1979) is in the South Bismarck Sea between the New Britain Trench and the Bismarck Sea seismic lineament (Denham, 1969; Ripper, 1975a, 1975b, 1977; Connelly, 1976; Curtis, 1973; Krause, 1973) and thus includes the island of New Britain.

3. North Bismarck plate (Johnson and Molnar, 1972) lies north of the Bismarck Sea seismic lineament and south of the Manus Trench.

4. New Britain plate (Curtis, 1973) corresponding to the South Bismarck plate.

5. Manus plate (Curtis, 1973) includes the North Bismarck plate as well as the Solomon Islands archipelago south of the Pacific Province of Coleman (1965a).

Shallow (<80krn), diffuse earthquakes occur in the area of subduction of the active Woodlark spreading system and deep earthquakes that may originate in detached pieces of lithosphere occur in three zones (around 400, 500, and 535 km) (Cooper and Taylor, 1984). Recent seismic activity concentrated at the base of the Ontong Java Plateau suggest active obduction of the Pacific plate in the vicinity of the plateau (Cooper et al.,1986). It is also the illustration of rapid uplift of the area.

3.1.4 - Crustal structure

In the central Solomons, crustal thickness was estimated to vary from about 27 km under Indispensable Strait to 17 km southwest of Guadalcanal (Rose, Woollard, and Malahoff,1968). From five other profiles across the entire Solomons/northern Vanuatu region, they estimated the crustal thickness to vary from 9 to 29 km. Thus, the Solomon Islands appear to have little or no "root", and crustal thickness seems to be similar to that of the oceanic region to the north. Seismic refraction studies by Furumoto et al. (1970) suggested a linear, blocklike character for the Solomons and a crustal thickness that varies from 15 to 20 km. Later work by Furumoto et al. (1976) showed that the crust is as much as 40 km thick beneath the Ontong Java Plateau but only 10 to 12 km in the Solomon Sea south of the arc.

Interpretations of crustal structure from sonobuoy, seismic, and gravity data (Cooper, Bruns, and Wood, 1986; Cooper, Marlow, and Bruns, 1986) indicate that the Central Solomons Trough is composed of three different basins in which as much as 6 krn of Cenozoic sediment may have accumulated. The sedimentary sequence consists of low velocity (1.6-2.6 km/s) strata in the upper part and higher velocity (2.8-4.4 kmIs) strata in the lower part. Volcanic rocks (5.0" 5.5 kmIs) and lower crustal rocks (6.2-7.0 kmIs) underlie the basins (Vedder and Bruns, 1986).

On the basis of preliminary interpretations of seismic and gravity data, two different arc reconstructions are plausible (Cooper et al,1986). One implies a rootless arc in which, highvelocity upper mantle rocks 12 to 15 km beneath the intra-arc basin, are juxtaposed against deformed oceanic crust along the faulted northeast side of the arc. The other, suggests (l) lowdensity lithosphere under the Woodlark Basin, (2) a tongue of lithosphere subducted to a depth of 150 km beneath the modern arc, (3) shallow, possibly relict mantle beneath the intra-arc basin, and (4) a remnant of subducted Pacific lithosphere faulted against old arc rocks.

3.1.5 - Geodynamic evolution

As suggested by Coleman and Kroenke (1981), Kroenke (1984), and Kroenke, Resig, and Cooper (1985), the northeastern part of the Solomon Islands arc is an obducted piece of the oceanic Ontong Java Plateau, and the central islands are the remnants of an early Tertiary northeast-facing arc that collided with the plateau about 20 My. On the contrary Ramsay (1982), demonstrated that the structure of Santa Isabel is not compatible with obduction processes. The modern southwest- facing arc is marked by late Cenozoic volcanic centers that extend from Bougainville to Guadalcanal. This hypothetised reversal in arc polarity (Karig and Mammerickx, 1972; Halunen and Von Herzen; 1973) was a direct result of the older arc-plateau collision. In the Woodlark Basin, seafloor spreading has been active for about 5 My, and the resulting separation of the Pocklington and Woodlark Rises is accompanied by subduction of the spreading system beneath the Solomon Islands arc segment of the Pacific plate at the rate of more than 10 cm/year (Weissel et al., 1982, Taylor, 1984).

To explain the absence of volcanism east of Guadalcanal, Coleman and Kroenke (1981) invoke the effect of cool, thick, depleted oceanic lithosphere of the Ontong Java Plateau being juxtaposed against the downgoing Australian slab. Dunkley (1983), however, asserted that volcanism is not to be expected in the transform area directly east of San Cristobal, the descending slab probably is not in contact with the lithosphere of the Ontong Java Plateau at depth, and the absence of volcanism in eastern Guadalcanal and San Cristobal does not represent an unusual gap in spacing of volcanoes along the arc.

Geologically, the Central Solomons Trough is one of several island-arc-related basins such as Manus, Woodlark, North Fiji and Lau basins, that lie along the complex boundary between the Australian and Pacific plates in the Melanesian Borderlands region. In the vicinity of the Solomon Islands, interaction between these plates since early Tertiary time resulted in two episodes of subduction and arc magmatism that included reversal of arc polarity (Kroenke, 1984).

As suggested by different authors (Vedder et al, 1986; Wells, 1989), the sequence of events leading to basin development seems to have been I) pre-Oligocene, southward-directed subduction of the Pacific plate beneath the Solomons Islands region resulting in deformation and metamorphism of oceanic rocks; 2) Oligocene to early Miocene magmatism and emplacement of large volumes of island-arc tholeiitic volcanic rocks; 3) middle to late Miocene collision with the Ontong Java Plateau resulting in the end of subduction and reversal of arc polarity; and 4) late Miocene to Holocene northward-trending subduction of the Australian plate beneath the Solomons Islands region resulting in a second phase of volcanism together with uplift, extension and the inception of an intra-arc basin. These late Cenozoic events have been complicated by the subduction of the Woodlark spreading system (Exon and Taylor, 1984). According to Dunkley (1983), active subduction of the Woodlark Basin Ridge caused eruption of the unusual magmas in the New Georgia group.

3.1.6 - Sedimentary deposition and vertical evolution

Rapid sedimentation in the Central Solomons Trough probably began in the late Oligocene, either as back-arc volcanoclastic aprons that coalesced southwest of the volcanic centers (Choiseul, Santa Isabel) or as forearc sheets of sediment that collected in now dismembered basins along northeast side of the volcanic centers (Guadalcanal). The arc polarity reversal near the end of Miocene time resulted in the deposition of sediment wedges that were derived from the newly formed volcanic chain to the south of the old arc. These wedges probably spread northward in the incipient intra-arc basin and enlarged in volume as volcanism and uplift increased in the Pliocene. As noted by Katz (1980), the late Oligocene to Pliocene strata in the basin probably are not composed entirely of primary volcanoclastic detritus and probably were derived in part from uplifted older rocks. During episodes of volcanic quiescence, reef limestone and related shelf and slope carbonate beds formated around craters and insular basement highs.

Probable extension accompanied by down warping and faulting during the Pliocene and Pleistocene resulted in a deepening of the basin. At the same time, rapid influx of sediment caused by uplift along the basin margins produced depositional sequences as thick as 3.5 km at places in the trough (Bruns et al., 1986). Katz (1980) estimated about 2 km of downwarping in the trough during the last million years, and Chi vas et al. (1982) documented at least 2 km of Pleistocene uplift on Guadalcanal. Hughes et al. (1984) reported Quaternary uplift rates of 500 to 3.700 m/m.y. in the Rendova-Tetepare Islands area on the southwest side of New Georgia. Paleoecologic interpretations from Quaternary rocks dredged on near Vella Lavella indicate local

indicate local uplift of as much as 700 m during a span of 0.5 m.y. in early Pleistocene time (Resig, 1986). Continued uplift in the same area led to subaerial exposure followed by subsidence of at least 600 m in the late Quaternary (Coulson and Vedder, 1986). Rocks from other dredge sites along the southwest side of New Georgia Sound near the Russell Islands show evidence for uplift of as much as 900 m followed by subsidence of as much as 700 m during a comparable time span. Multichannel seismic-reflection data suggest that as much as 2 km of subsidence has occured along the southwestern side of the Central Solomons Trough during Pliocene and Quaternary time (Bruns et al., 1986). On the northeast side at Choiseul, at least 600 m of uplift has elevated and tilted Pleistocene fringing-reef deposits (Strange, 1981).

Compressive stresses developed in the overriding plate in response to subduction of buoyant oceanic lithosphere (Dunkley, 1983) are responsible of the uplift along the southwest flank of the New Georgia. The estimated amounts of uplift attest to the exceptionally rapid rates of tectonism in the central and western Solomon Islands.

3.1.7 - Plio-Pleistocene volcanism

The Solomon volcanic province as defined by Coleman (1970) includes the reversed modem arc which is dominantly limited to the westernmost, south-facing islands. It incorporates from west towards east, the Shortland islands, a small part of central Choiseul island, the New Georgia islands group, the Russell islands, the north-western part of Guadalcanal island and Savo island. This volcanism results from the northeastward subduction of the Woodlark Basin, an active spreading system of the Australian plate, beneath the over-riding Pacific plate. The following presentation is mainly based on the recent compilation done by Coulson and Vedder (1986).

In the Shortland island, the horseshoe-shaped bay of north Fauro is believed to have been created by a caldera collapse of a large volcano. A minimum 1500 m thick reworked tuff and lava flows sequence is overlain by a 600 m thick sequence (Togha pyroclatics) of volcanic brecchia, tuff and minor lava flows (Turner, 1978). Based on comparison with similar rocks elsewhere in the Solomon islands, the later volcanics, possibly linked to the caldera formation, are assumed to be Plio-Peistocene. On Alu island, some possibly Pliocene andesites are present (Hughes, 1982).

In the central Choiseul island, Mount Maetambe 0060 m) is dominantly composed of andesitic pyroclastic deposits. They consist of tuff, ash and breccia with a minimum thickness of 500 m (Coleman, 1960). No interbedded lava flows was observed. Some interbedded turbidites however indicate that at least part of these deposits were cm placed below sea level. Activity possible began in middle Miocene times and may have been sporadically continuous into Pleistocene times (Hugues, 1981). Geothermal springs are present on Mount Maetambe.

The entire New Georgia islands form a complex of emergent coalescent volcanoes encircled by fringing reefs. It represents also the most active and voluminous volcanic development from late Miocene to Holocene. Volcanic rocks consists of large volumes of porphyritic olivine basalts, picritic basalts, and subordinate hornblende basaltic andesite and andesite flows. Fined-grained aphyric Mg-rich basalts are less common but widespread.

Typically, the volcano flanks display lateral facies changes from massive flows to volcanoclastics, showing an increasing degree of reworking as distance from the volcanic centers increases. Many lavas are chemically atypical of arc suites, this anomaly being attributed to the subduction of the active Woodlark spreading system (Johnson et al., 1984). Among all the New Georgia islands, Kolombangara is an almost circular Pliocene basaltic volcano, 1770 m high, crowned by a 4 km wide, 1000 m deep, very steep crater. On the southern slope, Ndughore Peak is an adventive cone that represents probably the most recent (Pleistocene) volcanic manifestation. The western part of Vella Lavella island consists of a Pleistocene (to Holocene ?) volcanic edifice that culminates at 790 m whereas the bottom of its crater floor is close to sea level. It contains several sulfur steam vents and a few adventive cones surround the main one. A special mention is also made of the very active Kavachi submarine volcano (9°S-158°E). Since 1950 where its activity was noted for the first time, this shallow basalt-andesite volcano has built up seven times a short-lived small islet, up to 150 m long, that is rapidly destroyed after the end of the volcanic activity.

Russell islands are the remnants of a emergent Pliocene volcano composed of basaltic breccia overlain by basalt lavas and is presently encircled by a large fringing reef. On Guadalcanal, dissected Plio-Pleistocene volcanic cones are dominantly composed of hornblende andesite flows with sequence as much as 900 m thick (Gallego lavas; Thompson and PudseyDawson, 1958). Pyroclastic aprons and coarse volcanoclastics flank the extrusive centers. Unstratified tuff breccia interfingers with poorly stratified lithic tuff near the volcanic centers, progressively grades laterally into stratified volcanic wacke. The latest fills in the nearby channels and basins (Wright, 1968).

These deposits also contain air-fall ashes and material derived from the underlying andesite flows. These pyroclastics probably represent ignimbritic sequences linked to explosive activity of the volcanic centers. Volcanic mudflows (lahars) are also present.

Savo island is a Pelean type volcano whose most recent eruptions occured during a period from 1830-1840 (Coleman, 1965). The volcanic edifice consists largely of agglomerates and tuffaceous deposits (Proctor and Turner, 1977). Solfataras are active on the south and eastern slopes of the island. DATA ANALYSIS

4.1- DATA ACQUISITION The complete description of the data acquisition system is given in the technical annex. Here are the specific characteristics of this cruise.

4.1.1· Magnetism- data acquisition and reduction

The magnetic data were acquired at a 6 seconds sampling interval using a BARRINGER M- 244 proton magnetometer, towed 280 meters astern the ship (see detailed description in technical annex). Due to the proximity of the magnetic equator (magnetic inclination of study area is less than 20°), the amplitude of the total field is relatively low: less than 42000 nT. The magnetic anomalies were computed by subtracting the IGRF 90 from the measured total field, but not corrected for diurnal variations. The accuracy of the instrument being equal to about 0.5 n'I', cross-over errors (which are less than about 50 nT, with an average equal to about 20 nT) are thus mainly due to diurnal variations. These errors were taken into account in the hand-made contouring process of the different maps, and do not affect the results we present

4.1.2 - Gravimetry

* Data acquisition

During the SOPACMAPS2 cruise gravity data were collected using the sea gravity meter BODENSEEWERK KSS30. This gravimeter consists of a GSS30 gravity sensor mounted on a KT30-two-axes gyro stabilised platform. The gravity sensor includes a non-astatized spring-mass assembly as basic gravity detector. In calm sea, during the SOPACMAPS2 cruise, the theoretical accuracy of the gravity sensor can be 10.2 mGal.

* Data Processing

Using the on-line-processing system, real gravity is obtained on board the ship, approximately 120 s after the measurement. This system provides values of gravity, Eotvos corrections, Free Air and Bouguer anomalies in mGal. Bouguer anomalies were calculated with a density contrast between the earth crust and the sea water of 1.64 g/cm3.

The starting gravity base station of the cruise was the air terminal building at Honiara Airport, Solomon Islands (G=978273.55 mGal). This absolute value was calculated with respect to the world gravity base station of Postdam, Germany obtained in 1906. The ending base station of the SOPACMAPS2 cruise has been the stairs of the Mineral Resources Division building in Suva, Fiji Islands (G= 978599.56). This absolute value was calculated according to the revised Postdam value that is considered to be 14 mGal lower than the initial Postdam value (Dehlinger, 1978) used to calculate the Honiara basis. To calculate the drift of the gravimeter, we subtracted 14 mGal to the Honiara basis to obtain comparable gravity references in both Honiara and Suva.

During the cruise, gravity data were automatically corrected for spring tension, cross coupling, Eotvos and for latitude, according to the IGSN (International Gravity Standardization net) 1971 ellipsoid.

4.2 . IRON BOTTOM SOUND

After a review of previous data, we will present here a generalised description of the data obtained in the Iron Bottom basin during SOPACMAPS II cruise, between Guadalcanal, Florida and Savo Islands.

4.2.1- Previous data on Iron Bottom Sound

Iron Bottom Sound is a small depocenter, about 50 km long by 30 km wide, between Florida and Guadalcanal Islands, separated from Russell basin by the Savo volcanic Island. The seafloor shoals south-eastward from about 1000m near Savo to about 300m near Sealark Channe1. The flanks of the basin rise steeply toward Guadalcanal and Florida Islands. Maximum sediment thickness in the basin is about 4.5 km (Bruns et al., 1986). According to these authors, Iron Bottom Basin is the geologic continuation of Russell basin, as all the seismic-stratigraphic units are continuous with those in Russell basin on both north and south sides of Savo. The seismic units are also cut by minor faults, probably slump or growth faults. Major sediment sources such as Guadalcanal and the Florida Islands cause rapid deposition, slumping, and movement of debris flows into the basin.

Bruns et al (1986) interpret the development history of Iron Bottom basin to be similar to that of Russell basin as follows:

- Throughout the Miocene and Pliocene time, sediments accumulated in Iron Bottom basin, across western Guadalcanal and across the western part of the Florida island group; - By late Miocene and early Pliocene time, Guadalcanal was increasingly uplifted, probably as part of the regional tectonism of the southern island group; - By Pliocene time, major uplift of western Guadalcanal was largely complete; - In the Quaternary, the rocks forming Savo were intruded through the Miocene and Pliocene strata. Further uplift of Guadalcanal and Florida Islands exposed Pleistocene reefs and the older sedimentary rocks.

4.2.2- SOPACMAPS cruise data

a) Bathymetry

The profiles have been traced parallel to the main SE-NW direction of the Iron Bottom Sound (Fig. 4). Their spacing was calculated to obtain an almost complete coverage. Due to the reduced size of the Iron Bottom sound area the bathymetric data have been plotted in real time and post processed at 1/1 00,000 scale centred at 100S.

The Simrad EMl2 bathymetric map of Figure 5 shows the different morphological elements of the area. The general trend of the Iron Bottom sound is SE-NW. This direction is clearly recognised on both sides of the survey corresponding to the flanks of the basin. These flanks are constituted by steep scarps up to 650 m-high on the south-western side and 800m-high on the north- eastern side. The walls of the basin are characterized by undulated morphology representative of "bad land" type erosion.

The central basin shows a relatively flat bottom at 7oo-750m-depth, slightly dipping toward the north-west. In the shallowest eastern area, it is crossed by a sinuous submarine canyon running parallel to the SE- NW general trend of the sound. In the axial part of the basin a rough topography indicates possible slumped deposits coming from the lateral scarps. In the same area, small mounds, few tens meters high, can be observed.

The other characteristic feature is the surrounding slopes of the Savo Volcano at the north- western tip of the Iron Bottom Sound. These slopes are remarkably regular except a NS spur on the northern flank. The Savo volcano is partially flanked by two curved canyons running down toward the Central Solomon Trough.

b) Imagery

The Simrad imagery map shows the same main morphological features as the bathymetric map. The sinuous canyon of the north-eastern part of the basin is especially well marked. The surroundings of the Savo volcano exhibit concentric small scale spurs and furrows all few tens meters wide aligned along the slope up to the foot of the volcano. Between the spurs the triangular shape areas occupying the lower part of the slope can be interpreted as sedimentary slumps. Both flanks of the sound are also cut off by the succession of spurs and furrows perpendicular to the main slope. Some of the canyons cutting the basin have their origin in these marginal furrows.

c) Seismic reflection data

Single Channel Seismic reflection (S.C.S.) has been collected along the profiles. The sedimentary series, more than I s.t.w.t. thick, although hidden by the multiple of the bottom, are made up of medium amplitude - medium continuity reflectors cut by faults. The upper unit is locally affected by slumps, especially in the eastern part of the surveyed area. These slumps could correspond either to compressional tectonics or to the proximity of sediment sources (Guadalcanal, Florida Islands) resulting in rapid deposition. Northeast of Savo, a submerged horst limited by a major west-facing normal fault was identified.

A major NS lineamentcan be traced from the eastern side of the Savo volcano running to the south trough the central part of the Iron Bottom Sound.

d) Magnetism

The general geological setting of the zone is highly complex and characterized by the existence of many volcanic constructs and tectonic faulting (see Chapter I). It is well known that a magnetized body of finite dimensions in the earth magnetic field tends to generate an induced magnetic field, depending on its shape and on the intensity of its magnetization. A 45°S dipping dipole polarised in the earth's field will for instance produce an induced field characterized by two peaks of positive and negative amplitude respectively (Fig. 6a). A faulted magnetized slab will also produce a similar field, with two peaks of opposite sign (Fig. 6b). As a result, the pattern of magnetic anomalies on the different contoured maps exhibits a series of highs and lows allover the study area that appears to be essentially controlled by the heterogeneity of the basement

Results: Magnetic anomaly values in the Savo volcano area range between -350 and +400 nT, while cross-over errors do not exceed 40 nT. When viewed at the regional scale, the onboard contoured map shows up two parallel stripes of highs with positive values alternating with two parallel stripes oflows with negative values striking in the East-West direction. This direction is consistent with the regional structural trends associated with the absolute motion of the Pacific Plate. Transverse to the arc the Iron Bottom is characterised by a flat magnetic anomaly at 0 nT, flanked North and South by southward decreasing gradient At the local scale, the different highs and lows clearly appear as a series of dipole patterns associated with the volcanic constructs on the crust. The major feature consists in a dipole centred on Savo volcano, with a positive maximum of 400 nT, and a negative relative minimum of -100 nT. East of Savo volcano, a new dipole appears, with a positive high of 300 nT near S 9°03, E 159°53. This dipole is most probably associated with a buried volcanic intrusion, which can clearly be inferred from the seismic records (see profile 100, at 16h45, and profile 102, at 21hlO). In the southernmost half part of the area, the magnetic anomaly pattern consists in a positive gradient northwards, with minimum values reaching -350 nT near S 9°18, E 160°03. This pattern is probably merely associated with the seafloor and basement topography, which dips down northwards.

e) Gravimetry

Gravity data were obtained in the Iron Bottom Sound between Honiara and the Savo volcano along lines 94 to 103. The quality of the preliminary data is medium according to the mean discrepancy in gravity measurements of 6.4 mGal calculated at 4 trackline intersections. This mean discrepancy allows to contour the Free Air Anomalies at 10 mGal intervals. The medium quality of the data may be due to a navigation error for lines 99 and 100, which will be post processed later. This navigation problem was also reported when plotting the EM 12D bathymetric data.

Free air anomalies obtained within the study area range from -10 mGal over the Iron Bottom Sound to +50 mGal over the surrounding submarine island slopes. Free air anomalies can be separated into three groups: the Iron Bottom minimum (-10 mGal), the Savo volcano and north- eastward high (+10 mGal), and a gradient area that extends over the northern slope of Guadalcanal Island and the south-western slope of the Florida platform.

The area of minimum value (-10 mGal) is located between the Florida platform, Guadalcanal Island and the Savo volcano. This area corresponds to the zone of maximum accumulation of sediment above the acoustic basement (see seismic reflection sections).

The Savo volcano and north-eastward high bounds the Iron Bottom Sound to the North. A + 10 mGal contour surrounds the Savo volcano. This volcano is reported to have a +50 mGal maximum simple Bouguer anomaly (Laudon, 1968). The north-eastward extension of the +10 mGal anomaly correlates with a structural high of the acoustic basement indicated on seismic reflection lines 100 and 102. This correlation suggests a possible structural relationship between the Savo volcano and the Florida platform.

The northern slope of Guadalcanal Island is characterized by a gravity gradient of 5.8 mGallkm and the south-western slope of the Florida platform shows a 8.7 mGallkm gradient. Both gravity gradients correlate with the topography of the seafloor close to the islands.

4.2.3 . Conclusions

The Iron Bottom Sound is comprised between SE-NW major faults constituting its flanks. This SE-NW main trend is found all across the basin in the bathymetric general trends. The direction of the submarine canyons illustrates the predominance of this SE-NW tectonic control. The seismic reflection profiles also allow to trace SE-NW faults and 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 the 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 with eroded volcanic flows. The magnetism and seismic reflection data suggest an EW trending extension of the volcanic edifice. The Savo volcano is bounded on its eastern side by an important NS fault identified on the seismic profiles, cutting the central part of the basin. Small few tens of meters volcanic mounds have been identified in the central basin between Savo and Guadalcanal. They seem to be aligned along this NS 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.

4.3- MBOROKUA BASIN

4.3.1 - Previous data on Mborokua Basin

Mborokua basin is the area located between Russell Islands and New Georgia linking the Central Solomon Trough to the North (Russell Basin) with the Woodlark basin to the South. It is characterized in its central part by an emerged small volcano named Mborokua. The existing bathymetric data on the area are scarce especially in the south-eastern area. Seismic profiles have been shot across the Mborokua basin in the framework of Tripartite Project. They indicate that significant sediment accumulations are present in the Mborokua basin, south-west of Mborokua basin. Mborokua basin lies beneath about 1500m of water and is about 60 km long and 30 km wide. The basin contains strata as much as 1 km thick. The sequence thins seaward and is cut by small normal faults (Bruns et aI., 1986). The south side of the basin is about 0.5 to 1 km higher than the north side, and basin strata dip land-ward, indicating uplift of the south edge of the basin. Dredged rocks from the east edge of the basin indicate that part of the section is Pleistocene (Colwell and Vedder, 1986). Uplift of these strata indicates active late Cenozoic uplift of the slope south of the Russell islands.

4.3.2· SOPACMAPS cruise data

a) Bathymetry

The selected scale for real time plotting and post processing of the bathymetry in the Mborokua Basin is 11150,000. The contour interval is 20 meters. The profiles have been carried out with a NW -SE orientation to cover the area with an almost complete coverage within the minimum time (Fig. 7).

Three main provinces can be identified on the bathymetric map of Figure 8.

* to the North, the area opening toward the New Georgia Sound is characterized by gentle rough slopes from depths ranging from WOO to 1500 m. The slope shows two main perpendicular directions: one is close to EW to ENE-WSW and the other one is NNW-SSE (NI200E). On both margins of the area the morphology is typical of volcano-slope morphology with concentric undulations,

* the central "volcanic" province where the bathymetry is more complicated and mainly related to the volcanic topography. A succession of circular highs are aligned along a general Nl200E trend. The Mborokua volcanic island is one of these features. In the detail the whole area is cut by conjugate faults oriented N120, EW and ENE-WSW,

* the southern province shows a very different topography with a large graben limited to the South by two arcuate ridges. South of the ridges the morphology is chaotic and probably represents a deformation zone guided by ENE-WSW and Nl20 faulting.

Free air anomalies obtained within the study area range from -30 to -35 mGal over the Russell basin to + I 00 to + 110 mGal over the western shoal of the Russell islands and the south- eastern shoal (Brougham shoal) of Georgia islands. Free air anomalies were separated into two groups of values corresponding to two different geographic areas: the Russell basin minimum (030 mGal) that extends over the northern half of the survey area, and a set of parallel, elongated gravity highs and a low near and south of the Mborokua volcano.

The Russell basin minimum (0 to -35 mGal) correlates with water depths of 1500-1600 m and with the maximum thickness of sediment in this area (2500 m after Bruns et al, 1986). In the north-west part of the study area, the gravity contours deviate from their general East-West orientation to form a north-north-eastward trending, elongated gravity minimum. This minimum correlates with a graben of similar orientation.

The set of gravity lineations near the Mborokua volcano trends Northwest-Southeast This set includes a gravity high of +20 to +50 mGals extending over the Mborokua volcano, a gravity minimum of +5 to +20 mGals and a high of +70 to + 110 mGal extending south of the Mborokua volcano. The elongated gravity minimum appears to coincide with "pull apart" basins that may have resulted from strike-slip tectonics (see tectonic chapter). The gravity high extending south of the Mborokua volcano in water depths of 800-1200 m, correlates with a buried elongated structural high that outlines the boundary between the Mborokua basin and the forearc slope to the south.

4.3.3 - Conclusions

Three provinces are distinguishable:

* The southern one 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 north-western edges linked respectively with the Russell and New Georgia volcanoes, the northern province shows a general slope dipping toward the 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 allow us to distinguish in this province, buried basement blocks tilting north-westward. This tilting indicates a NNW-SSE extensional motion. Such an extension is compatible with the general EW 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.

Preliminary Report. Leg 2 - SOPACMAPS - page 30 4.4· NEW GEORGIA SOUND

4.4.1- Previous data on New Georgia Sound

The New Georgia Sound is one part of the central Solomon Trough (SLOT). It is interpreted as an intra-arc basin bounded by a double islands chain. To the South the chain composed of Guadalcanal, Russell, New Georgia, up to Bougainville constitutes the active volcanic arc. To the North the Choiseul, Santa Isabel and Florida chain is composed by a tectonized platform of Cretaceous-Early Tertiary oceanic basalts locally metamorphized into greenschist and amphibolite facies. The islands' basement is overlain by Oligocene to Early Miocene arc volcanics and younger shallow water carbonates. The New Georgia Sound is a relatively flat topography basin containing more than 5 km-thick, late Oligocene to present-day sedimentary cover. Multichannel seismic profiles carried out across the New Georgia Sound within the Tripartite project show a more than 3 s.t.w.t, thick sedimentary sequence with two main series: the deepest one (1-1.5 s. thick) is interpreted as Oligocene and Miocene. It is overlain by Pliocene and younger layers with an angular discrepancy. Seismic refraction data obtained on the area (Cooper et al., 1989) evidence the existence of a basement uplift in the saddle between New Georgia and Choiseul Islands with sediments overlying 5.3 kmls basement rocks at about 4.5 km-depth, During the Tripartite project volcanic and sedimentary rock samples have also been dredged along the scarps of the Islands slopes.

4.4.2 – SOPAC MAPS cruise data

a) Bathymetry

The objectives of the survey were the southern part of the New Georgia Sound and to the Northwest, the saddle between New Georgia and Choiseul Islands (Fig. 9). The bathymetric map of Figure 10 can be subdivided in three main zones. The southern zone is the margin of the New Georgia volcanic province. It is characterized by steep slopes dissected by numerous submarine canyons perpendicular to the main direction of the slope.

In the south-eastern corner of the survey, spurs and seamounts are aligned along the slope. Two of the seamounts are culminating at less than 1000m depth. This topography is typical of volcanic origin. The areas that comprised New Georgia, Kolombangara and Vella Lavella volcanoes show the same volcanic type morphology. They are cut in their middle part by sinuous canyons running to the deepest part of New Georgia Sound.

The axial part of the New Georgia Sound has been surveyed in the area where a saddle separates the Russell and Shortland basins. This zone is relatively flat and exhibits a gentle slope toward the deepest part of the adjacent basins. The eastern slope is cut in its middle by an EW canyon The only peculiar morphologic feature of this area is a horst with a square shape previously known and named Fatu O Moana. The summit of the horst is less than 700 rn-deep and shows in its northern part a rounded depression 30 m-deep.

The northern margin of the New Georgia Sound shows a morphology similar to the southern one with the alternation of spurs and canyons perpendicular to the slope trend.

b) Imagery

The imagery map can also be divided in three domains: the southern and northern marginal domains and the central New Georgia Sound domain.

The two marginal domains are illustrated on the imagery by numerous reflective flow zones associated with the volcanic slopes. As in the Mborokua area these zones can be interpreted either as volcanic sheet flow or as debris-flows incorporating volcanic material running down the slopes.

The canyons dissecting the slopes and the areas located between New Georgia, Kolombangara and Vella volcano are also well identified on the images.

The central part of the New Georgia Sound is essentially characterized by the Fatu O Moana mound flanked by highly reflective scarps especially on its eastern side. The Fatu 0 Moana mound seems to be linked with the northern margin of the New Georgia Sound.

c) Seismic reflection

All along the NE margin of the New Georgia Group a set of active submarine canyons and volcano-sedimentary fan deposits have been mapped from 3 seismic profiles (116-119). The canyons are bordered by steep faults which are often related to buried major faults and fractures. These structures mainly trend NE-SW and WSW-ENE. Vedder and Bruns (1986), based on multichannel seismic profiles, also show an important Plio-Pleistocene layer of volcanoclastics and flows (New Georgia wedge) associated with slope unstability of the margin.

On the other hand, seismic profiles located east of Kolombangara Island and in VellaLavella Gulf (lines 120-128) reveal the existence of another system of canyons and flows associated with buried faults trending NE-SW as well. This set of faults ends northward on a WNW -ESE-trending left-lateral fault located at the edge of the New Georgia margin.

Seven seismic reflection profiles (PR129 to PR135) that trend mainly WNW-ESE have been carried out within the northern part of the New Georgia Sound. The water-bottom multiple obscures primary reflections below about 1.5 to 2 s. subbottom time. Seismic reflections in the study area are defined by high lateral continuity and amplitude. The interpretation of the seismic reflection profiles reveals the existence of two major structural trends, WNW-ESE and NE-SW. On the study area, the WNW - ESE-trending features crosscut and offset the NE-SW directions and correspond to two main sinistral shears: between Lines 131 and 132, the sedimentary axis of a buried basin is offset along a left- lateral strike-slip fault; the second sinistral shear, that can explain the poor continuity between Lines 128 and 129, is overlain toward the East by a major WNW-ESE-trending canyon centred at 7°52'S. The NE-SW-trending structures are dominant around the Fatu O Moana structural high, interpreted as a tilted structural block bounded westward and eastward by buried faults that affect the basement which was presumably uplifted

along these faults. The Fatu 0 Moana western fault extends southward and is overlain by a deep canyon that lies within the Vella Gulf (west of Kolombangara), west of 157°E. The northward extension of the Fatu 0 Moana buried eastern fault is marked by the transverse canyon that lies south of Choiseul, east of 157°30'E. The 18 km-wide EW-trending graben that lies at 7°50'S is structurally controlled westward by the buried eastern fault and northward and southward by WNW- ESE sinistral strike-slip faults. Westward of Fatu 0 Moana, NE-SW directions are also present along the outcropping horst centred at 157°E (Line 130) that becomes subdued to the Northeast, bounded by buried normal faults.

d) Magnetism

Magnetic anomaly values in the New Georgia Sound area range between -400 nT and 400 nT. The onboard magnetic map contoured at the 50 nT isovalues shows up lineations trending approximately east-west, that is consistent with the major regional tectonic trend associated with the WNW absolute motion of the Pacific Plate. In the southern part of the survey area, a -lOOn T parallels the average direction of the New Georgia Island coastline. In the northern half of the survey area, this general E- W trend is cross-cut by a N-S accident which can be inferred as a fault from the seismic record (see profile 132, on August 24th at 7h30; profile 131, on August 24th, at 5h50; profile 130, on August 23th, at 18h35). This fault which is overburden by a sedimentary cover, does not show up on the bathymetric map, indicating a probable deep crustal origin. West of the fault, the magnetic response is smooth: the 1000 meters deep basin in the central northwestern part of the map (north of 7°55 S and west of 157°30 E) has a relatively flat signature, with anomaly values ranging between -100 nT and -50 nT. The eastern side of the fault is characterized by a series of well-defined dipoles associated with the Kolombangara volcano and Fatu 0 Moana seamount system.

e) Gravimetry

During the SOPACMAPS2 cruise, gravity data were obtained along lines 116 to 136 in the sedimentary basin of the New Georgia Sound. This elongated basin is bounded to the North by the Choiseul island and to the South by the Vella Lavella and the Kolombangara volcanoes and the New Georgia group of islands including the Vangunu island. The basin forms a saddle shallower by 300 m than the Russell basin to the South and the Shortland basin to the North. The quality of the gravity data could not be determined because no line crossings were made available. However, Free Air anomalies were contoured at 10 mGal intervals.

Free air anomalies obtained within the study area range from -50 mGal over the northeastern flank of the Vangunu island to + 138 mGal over the southern flank of Choiseul island. The gravity pattern over the basin is controlled by two major gravity lineations trending approximately east- west: a gravity minimum of +10 to +30 mGal extending along the southern flank of the New Georgia Sound basin. This minimum may be associated with an E-W trending fault or a greater thickness of low-density sediment. A major gravity high of + I 00 to + 138 mGal extends along the southern slope of Choiseul island where the water depth is 500 to 900 m. This gravity high shows a south-west, locate extension reaching +70 to +80 mGal that can be associated with Fatu 0 Moana, a structural high reaching 700 m in the middle of the New Georgia basin. A gravity map published by Beyer (1986) indicates elongated gravity anomalies as high as +210 mGal over the western coast of Choiseul island; one such anomaly of +170 mGaI in amplitude is located immediately north of our gravity high. These gravity highs could have a common origin: the exposed serpentinite and serpentinized harzburgite, which overlies unaltered basalt flows and schists in the southern part of Choiseul island.

4.4.3 . Conclusions

The New Georgia Sound area explored constitutes the junction between the New Georgia, Kolombangara, 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 northern Choiseul platform by a 1200 to l300m- deep basin cut in two parts (Russell and Shortland) by the Fatu 0 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 EW 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 NS to NESW canyons. On the seismic profiles the main structural trends in the deep basin are NE-SW faults.

The sedimentary cover is more than 2s thick and shows angular unconformities relative to the vertical evolution of the basin.

The Fatu O Moana mound is an uplifted block of basement and its sedimentary cover. It is culminating at less than 700 m-deep. It is associated with an increased of 0.6/1000 of the salinity. This increased could be explained by current changes, or upwelling due the presence of the mound.

4.5 -TRANSIT BETWEEN NEW GEORGIA SOUND AND EASTERN MALAITA This long seismic profile corresponds to the transit between the New Georgia Sound and the Eastern Malaita study areas (Fig. II). The profile runs all along the northwestern edge of the Central Salomon Trough, always parallel to the southern margin of Santa Isabel Island.

The profile shows typical basin-filling units, more than 3 s.t.w.t, thick at the center of the basin. The margin of the profile presents a thinning of the units which onlap the rock basin basement.

The central basin series clearly show four different sedimentary units, from base to top, A, B, C and D:

Seismic Unit A, is characterized by discontinuous to continuous seismic reflections. Highly erosive reflectors can be present within the Unit. The edges of this Unit are formed by faulted blocks. An erosive reflector gentle ondulated and very continuous laterally, has been· identified as a regional discontinuity (Bruns et al., 1986) separating seismic units A and B.

Seismic Unit B, presents highly discontinuous reflectors, acoustically transparents to semi- transparents, and poorly continuous laterally. The top of the unit is marked by a high amplitude and laterally continuous reflector.

Seismic Unit C shows reflections generally medium to high amplitude and laterally continuous. At the top of the Unit C and separing from the upper Unit, there are a series of wedge pockets with chaotic reflections, probably due to channels filling. The seismic horizon between C-D marks an important and ondulated unconformity, that sometimes erodes the top of the Unit C. Seismic Unit D, is formed by a flat lying, very continuous, undeformed reflections lying in conformity with the previous Unit. Occasional channels and faults affecting the very recent sedimentary layers are present.

This seismic units can be age-correlated with those identified by Bruns et aI., (1986): - Unit

A, Late Oligocene and older - Unit B, Miocene and late Oligocene - Unit C, Pliocene - Unit D, Quaternary

4.6 - EAST MALA ITA ZONE

4.6.1 - Previous works

The East Malaita zone is constituted by a NW trending system of structures prolongating the post-early Miocene en echelon fold belt of the Malaita Islands (Kroenke, 1972). At the northern and southern tips of the Malaita Island the the vergence of the folds became southwards with a reverse polarity compared with the main part of the island.

Steep submarine scarps observed on the East Malaita margin suggest the existence of important faulting. To the north this folded system abuts against the North Solomon Trench wich is interpreted by Koenke (1984) as a structural hinge line or a regional syncline. It is a relatively shallow (3000 m deep) , poorly define and sediment-infilled (Vedder and Coulson, 1986) trench which separate the Solomon Islands from the Ontong-Java Plateau. For Coleman and Kroenke (1981) it represents the Late Eocene to early Miocene site of the subduction zone related to the motion of the Ontong-Java Plateau.

Multichannel profiles carried out during the R/V S.P.Lee (1982) allow to operate a correlation between the Ontong-Java Plateau and the Malaita system. The deep troughs bounding the northeast of Malaita are constituted by early Miocene deformed sediments

4.6.2· SOPACMAPS cruise data

a) Bathymetry

The surveyed area corresponds to the entire eastern side of Malaita and Ulawa Islands. (Fig.12). It can be divided in three main zones showing different structural characteristics (Fig.13).

The northern zone, north of 8°45'S is the simplest one with a NI60E trending scarp bounding Malaita island. The scarp is constituted by a succession of parallel steps up to the North Solomon Trench. It is cut in two places by deep square depressions open toward the east. These depressions centered on 8°25'S and 8°45'S are limited to the east by steep scarps. At the foot of these scarps the North Solomon Trench is a triangular shaped basin with a maximum depth reaching 4250 m. The trench is bounded to the east by a regular flat southwest dipping area trending N140E.

The central zone is constituted by a recognized zone reflecting the Malaita and 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 ridge like the two other and the intermediate basins, shows changes of trends limited by N120E, NI40E and N160E faults. The northern tip of this central ridge is formed by the 8°45'S 2500m-deep square depression of the northern zone. The eastern ridge seems to be different in nature that the two other, and could be

associated to the sea-floor topography, and that it does not indicate the presence of magnetized body at shallow crustal levels. The prism that abuts against Ulawa Island is likely to be made of accreted sedimentary material.

Further north, off the east coast of Malaita, the sea-floor topography consists in a series of three, well defined ridges trending NNW, alternating with basins parallel or sub-parallel to the coast line of the island. The major feature is an arc shaped ridge which trends N-S in the south and bends N310 in the north, in the direction of the major left-lateral strike slip fault that affected the northern part of Malaita island. It is flanked by very steep slopes, and its depths is as shallow as 750 meters in some places. Magnetic anomalies allover the area range between -500 nT and +500 nT. They generally trend N340° to N31O°, and reflect sea-floor topography. Their high amplitudes indicate the presence of magnetized material of volcanic origin. This supports the conclusion of the structural interpretation, which considers that the ridges off Malaita are the underwater continuation of Eocene volcanic basement that is known to outcrop on Ulawa Island.

e) Gravimetry

Gravity data were obtained along the eastern side of Malaita and Ulawa islands along lines 140 to 163. The mean discrepancy in gravity measurements calculated at 6 trackline intersections is 1.2 mGals without considering a 17 mGals discrepancy obtained at one intersection. This mean discrepancy allows to contour the Free Air Anomalies at 10 mGals intervals.

Free air anomalies obtained within the study area range from -167 mGal over the 6000 m deep trench east of Ulawa island to + 110 mGal over the shallow water sea floor off eastern Malaita island. From east to west, Free air anomalies can be separated into three groups corresponding to three arcuate structural domains trending roughly NNE-SSW: an east gravity low reaching -63 mGals, an arcuate gravity high reaching locally +60 mGals and a double western gravity as low as - 167 mGals.

The western gravity low that reaches -63 mGal superimposes on a series of 2500-3000 m deep, bathymetric lows trending NE-SW.

The arcuate gravity high extends from Ulawa island obliquely toward the northeastern part of Malaita island. This high coincides with a major topographic ridge that culminates near 700 m and crosscut obliquely the deformation wedge. +50 to +60 mGals values of gravity anomalies suggest that the ridge does not consist only of accreted pelagic sediment, although gravity modelling would be necessary before concluding. From the geology of Ulawa island this ridge appears to be made of a volcanoclastic basement of Eocene-Oligocene age overlain by Miocene carbonate sediment and Pleistocene emerged reefs.

The eastern gravity low shows two minima centred over the trenches. One gravity low reaches -150 mGals and coincides with the 4250 m deep flat-bottomed trough that separates the OJP from the Malaita deformation wedge. The second low (-167 mGals) coincide with the NS 6000 m, deep trench east of Ulawa island. The eastern slope of Ulawa Island is characterized by a gravity gradient of 12.1 mGallkm.

4.6.3 . Conclusions

The survey of the East Malaita area allows to refine the previous observations made in particular by Kroenke (1984). The East Malaita area is one part of the Malaita anticlinorium resulting from the conjunction of shortening processes along the North Solomon and Cape Johnson Trenches and strike-slip to subduction along the San Cristobal Trench.

The northern part of the area shows a quite direct contact between the Malaita island margin and the "subducting" Pacific-Ontong Java plate. The NW-SE Malaita slope abuts against the 4000m- deep sediment-infilled North Solomon Trench flanked to the east by a N140E trending dipping plane.

The central area is essentially represented by the submerged prolongation of the Ulawa- Malaita geologic features. The most remarkable feature of this area is the central ridge underlined by a strong magnetic and gravimetric anomaly. This magnetic anomaly suggests at least for a part a volcanic nature for the ridge. The central ridge structured by N140E, Nl60E and NS faults is separated from the Malaita island by an elongated 3000m-deep graben structured by the same directions. East of this area the North Solomon Trench disappears and is occupied by a deformed sedimentary cover forming a succession of elongated N140E ridges and depressions. The whole area can be interpreted as an old compression zone affected in the present stage by vertical movements.

On 162°E longitude the whole East Malaita domain is cut by a major NS fault up to the southern tip of the Ulawa margin. East of this fault a 6000m-deep square trough constitutes the "nodal" basin at the junction of North Solomon and Cape Johnson Trenches. Recently this trough has been cut by N400E ridges parallel to the Cape Johnson Trench.

At the southern tip of the surveyed area, EW ridges and depressions correspond to the structures related to the San Cristobal strike-slip/subduction zone.

4.7 - MELANESIAN ARC GAP

4.7.1 . Previous works

The Solomon Islands arc ends abruptly northeast of San Cristobal island. The far north of the New Hebrides island arc is also sharply interrupted northwards and westwards at the Reef Islands and Tinakula volcano. Between those two arcs, lies a basin with an average depth of 4000m, apparently unnamed, although its western part is occasionnaly named UIawa Deep, from the name of the Ulawa island north of San Cristobal. We propose to name this region the "Melanesian Arc Gap".

This rectangular-shaped basin is about 300 km long and 200 km wide and bounded by two deep trenches: the Cape Johnson Trench northwards and the over 8000m deep San Cristobal Trench southwards. The sea floor of the basin is uneven, riddled with submarines volcanoes whose the sizes range from 1-2 km to 20 km for the largest, and the depth can be less than 2000m. In the center and on the eastern side of the basin, two huge features are observed: a very large (about 55 km) seamount occurs north of the San Cristobal Trench and a massive NNE-SSW feature is separated from the New Hebrides island arc by a 4500 deep narrow channel. In this basin, a few data only are available on its morphology and origin. In the central part, a narrow (about 40 km width) zone, trending close to north-south, was surveyed by SSI (Seafloor Surveys International, Inc.), in the frame of PacRimWest project (Telecommunications cable route survey). SSI used the Seamarc Sys09 and provided bathymetric, acoustic imagery and structural interpretation charts.

4.7.2 - SOPACMAPS cruise data a)

Bathymetry

Globally the bathymetry and structure of the whole area is complex and sometimes difficult to decipher. Without enter in the very detail, four main provinces can be distinguished (Fig. 14 and 15).

A western province up to 165°E characterized by more than 3000m-deep NS to NE-SW graben cutting from south to north an ondulated plateau flanking to the east and to the south the 4000m-deep edge of the Vityaz trench. This domain is clearly structurally related to the Cape J ohnson- Vityaz trenches.

South of 100S, the second domain is characterized by predominant N75°E trending features. The most remarkable of them are: on the northern edge of the domain, a volcanic line running from the western limit of the survey up to the NS Tinakula-Nupanui volcanic system. The N75°E trending of the volcanic line is parallel to the vector of the relative motion of the plates along the San Cristobal trench.

The same directions are observed at the southern edge of the area constituted by the 75OOm- deep San Cristobal trench. Around 165°E the trench direction changes drastically from N75°E to NS. This new trench is the New Hebrides Trench.

The third domain located east of 165°E and south of g030'S, is characterized by large volcanic plateaus guided by EW to WNW-ESE directions. The plateaus are crowned by emerged Islands or reefs (Ndende, Reefs Islands, Tupanui, Tinakula). Out of these Islands the depths are shallow and range between 1000 and 2ooom. The domain constitutes the northern tip of the New Hebrides Arc.

The fourth domain is comprised between the Vityaz trench to the north and the volcanic province to the south. It shows a spectacular EW structure constituted by a linear narrow ridge 25OOm-deep extending from 167°E up to 165°E. This ridge is flanked to the south and to the north by two elongated more than 4ooom-deep depressions.

b) Imagery

The bottom image of the area confirms the existence of four different provinces with peculiar structural characteristics. The major part of the surveyed area seems to be covered by sediments except the scarps recognized on the bathymetric map. But two domains show highly reflective zones suggesting the absence or scarcity of the sedimentary cover.

The first one is the southwestern domain characterized by the N75°E trending structuration. The whole plateau and the San Cristobal trench bottoms are reflective and probably not covered with sediments. That phenomenon could be related to the steepness of the scarp and the permanent tectonic reactivation of the area.

The second domain without sedimentary cover is the linear ridge located at g030'S. This domain was previously interpreted as incipient active spreading ridge by Pelletier et al. (1993) and Auzende et al. (1993). The volcanic province shows slumps or lava flows at the foot of the recent volcanoes.

c) Seismic reflection

Single Channel seismic reflection lines 165 to 171 were carried out north of Ndende Island in order to complete northward the seismic data acquired during the SOPACMAPS Leg I survey. The study area is dominated by E-W-trending structural directions, mainly located in the eastern part of the study area, and by WSW -ENE directions in the central part. Between 164°lO'E and about 165°E, a WSW-ENE-trending ridge centred at g050'S, is surrounded by two 35 krnwide grabens bordered by fault scarps. The northern graben, 4100 m deep, is filled up by a thick sedimentary series (2 S.t. W.t. on Line 170). The southern graben, characterized by a bulging bottom, shows a sedimentary thickness of about 0.4 to 0.5 s.t.w.t. The WSW-ENE-trending ridge, that shoals to 2500 m deep and is underlain by a thin sedimentary cover (about 0.3 s.t.w.t.), probably represents the westward- dipping western extension of a major structure, the E-Wtrending ridge centred at g030'S in the eastern part of the study area. This ridge, previously mapped by monobeam echosounder, has been called the g030'S Ridge (Pelletier et al, 1993b).

Due both to the shallowness of the ridge and that the lines are almost parallel to it, the narrowness of the EM 12 swath could not map the summit of the 9°30'5 ridge that rises up to at least 1500 m. To the south, the 9°30'5 ridge is flanked by an E-W-trending 2000 m-high fault scarp that corresponds to the northern flank of a parallel graben, 18 km-wide with a bulging bottom covered by a sedimentary series 0.3 to 0.4 s.t.w.t, thick.

The 9°30'5 ridge is very narrow, 10 km, and is characterized by the lack of sediments (Line 170), although sedimentary series are thick (0.8 to 1 s.t.w.t.) immediately north and south of the ridge (profiles 171 and 169, respectively). To the west, the 9°30'5 ridge characterized by a rough topography, ends at 15:45 on Line 170. As the sea-bottom deepens westward, the sedimentary thickness rapidly increases to reach about 2 s.t.w.t. within the WSW-ENE-trending, 4100 rn-deep graben. Line 170 ends westward on a major 1000 m-high, NNE-SSW-trending fault scarp that corresponds to the western flank of a "plateau-like" structure underlain by a thin (0.3 to 0.5 s.t.w.t.) sedimentary cover.

d) Magnetism

Amplitudes of magnetic anomalies in the Melanesian Arc Gap area range between -750 nT and 750 nT. The area is clearly divided in two different provinces, separated by a transect running from l63°30E, to 9°30S, l67°E.

North of this transect, the pattern of magnetic anomalies is characterized by smooth gradients and well defined lineations trending NNE. This indicates that the sea-floor in the northern province is probably made of "normal" oceanic crust, which is not contaminated by back arc material. Along latitude 9°30 S, amplitudes of magnetic anomalies are positive, and mostly greater than 100 nT, and the magnetic pattern correlates with an elongated rise culminating at a depth of about 2100 meters. The magnetic data thus supports the work of Auzende et al (1993) who interpreted the 9°30S rise as an active spreading center located at the northernmost end of a ridge-transform-ridge system associated with the Hazel-Holmes extensional zone (Charvis and Pelletier, 1989).

South of this transect, the magnetic anomaly pattern consists in a series of dipoles associated with the numerous volcanoes present in the area. This signature suggests that the southern province consists in "back arc crust". Near 10025S, 163°44, a proeminent volcano as shallow as 1000 meters generates anomalies with positive amplitudes exceeding 300 nT, and negative amplitudes less than - 500 nT. Near 10020S, 166°E, the group of active volcanoes associated with the Ndende and the Tinakula volcanic islands creates a series of magnetic dipoles with sharp gradients and amplitudes varying from -750 nT up to 750 nT.

In conclusion, the magnetic data supports the structural interpretation : the Melanesian Arc Gap area is separated in a domain of "normal oceanic crust", extending approximately north of latitude 9°30S, and a "domain of back arc crust", south of latitude 9°30 S.

e) Gravimetry

During the SOPACMAPS2 cruise, gravity data were obtained along lines 165 to 171 across the Melanesian Arc Gap. This area located in the North Fiji Basin (NFB) extends between

the Santa Cruz islands and the Viyaz trench West of Duff islands. The quality of the gravity data could not be determined because no line crossings were made available. However, Free Air anomalies were contoured at 10 mGals intervals. The North Fiji basin as a whole area coincides with one of the major geoid highs of the earth and consequently this basin is the location of elevated gravity anomalies.

Free air anomalies obtained within the study area range locally from -45 mGal to +110 mGals. Both these extreme values are located over the Northeastern corner of the surveyed area. However, the average gravity value over this entire surveyed area is in the order of +40/+50 mGals, that is a relatively high value considering the average water depth of 3500 m. This gravity value is consistent with values reported in other regions of the North Fiji Basin, which indicate that the NFB is a young oceanic basin underlain by subducting slabs colder than the surrounding mantle.

Free air anomalies were separated into two groups of values corresponding to two different geographic areas: the western area located between longitude 163° 30 and 165° E where gravity anomalies range between +20 and +70 mGals and the Eastern area east of longitude l65°E where gravity shows the greatest amplitude variations.

In the western area gravity anomalies correspond to bathymetric highs, which may be horst of oceanic crust or volcanic edifices and lows, which may represent grabens. In the Eastern area, gravity highs of + 60 and +110 mGals outline a narrow, E-W trending ridge of probable volcanic nature, whereas the minimum value of -45 mGals coincide with a 4000 m deep flatbottomed depression. Gravity modelling of these strong anomalies would be necessary to decipher the nature of the ridge and associated graben as well as the physical properties of the underlying upper mantle.

4.7.3 - Conclusions

The Melanesian Arc-Gap is structurally constituted by the juxtaposition of different domain.

To the west the oceanic bottom is predominantly structured by l\'E-SW and NS highs and grabens reflecting the Cape Johnson- Vityaz trench trends. The magnetic and gravimetric anomalies maps corroborate these structural trends and suggest the oceanic origin for the basement of this area.

To the southwest and the south (south of 100S), the predominants structures are trending N75°E. They are essentially represented by the "en echelon" northern edge of the San Cristobal New Hebrides Trench dipping under the Melanesian Arc Gap with this N75°E direction. This area is limited to the north by a remarkable volcanic line.

To the southeast the domain is mainly occupied by the volcanic centers emerged and submerged constituting the northern tip of the New Hebrides volcanic arc. The oceanic bottom is the shallowest of the Melanesian gap. The magnetic anomalies pattern evidences the complexity of the volcanic emissions with a succession of dipolar anomalies.

According to Pelletier et al. (l993b), the g030'S ridge, identified from l65°45'S to l68°l5'S north of the New Hebrides Arc platform and the Duff Islands, corresponds to a spreading ridge. EW- trending magnetic lineations, disrupted by NS transform faults, have been identified as anomalies 3A to 2A (7 to 2.5 My). The g030'S ridge is interpreted as linked to the Hazel Holmes Extentional zone trough a NS transform fault (Auzende et aI.1993). On the seismic data, the lack of sediments on the NlOooE trending g030'S ridge and the existence of thick sediment deposits immediately around it, corroborates the interpretation as a spreading ridge. This is also in agreement with our magnetic data that show that the g030'S ridge is characterized by magnetic wave-lengths which are characteristic of oceanic crust, in contrast with the dipolar magnetic signature of the structures that lie south of the g030'S ridge's southern graben and which probably result from arc-related (New Hebrides Arc) volcanism.

4.8 - NORTH NEW HEBRIDES BACK-ARC AREA

4.8.1· Previous Works

The NNHBA includes a group of small scattered islands some 450 km east of the main Solomon Islands chain, and about 130 km north of the islands of Vanuatu. They are the nonhern extension of the New Hebrides island arc system. Four islands, Tinakula, Nendo, Utupua and Vanikoro (Santa Cruz group), form a well-defined chain in the west of the group, while the Duff Islands, Anuta and Fatutaka form a discontinuous chain to the east. Tikopia Island lies midway between the two groups. All the islands are volcanogenic with well preserved volcanic cones on Vanikoro, Utupua and Tikopia and an active volcano on Tinakula. The Reef Islands (NE of Nendo) are composed of coral and sand cays upon a probable volcanic basement (Hughes et al., 1981; Coulson, 1985).

Seismicity defines a Wadati-Benioff zone reflecting the eastward subduction of the Autralian plate along the northern New Hebrides Trench. The Vityaz Trench is seismically inactive, and is thought to be a fossil trench representing a former south dipping subduction zone. In the northern NNHBA region, seismicity, focal mechanisms, and bathymetry allow identification of three zones (Louat and Pelletier, 1989):

II Numerous events clustered along a N1200E trending zone near 11 "S, 166°30'E coincide with a depression separating islands of the Santa Cruz archipelago. Focal mechanisms indicate normal and strike-slip faulting. These solutions have a NE trending I-axis. 2/ At lOo20'S, 165°E, a N-S trend is indicated by normal and strike-slip solutions sharing a common I-axis orientated NlO° E. 3/ A small group of events at 12°30'S, 167°30'E extend along a N1700E trending zone. The zone is correlated with the nonhern New Hebrides back-arc troughs. Two events with normal fault solutions are located just west of the troughs and they display a N125°E I- axis.

The islands form distinct volcanic piles of submarine and subaerial lava flows and volcanic rocks ranging in age from Oligocene to present-day. Basalts predominate and include tholeiitic, high alumina, and alkaline types, together with high and low silica andesites and dacitic rocks. Analyses of rocks suggest that predominantly tholeiitic basalts are found in the western chain and in samples from the Vitiaz Trench. In the eastern chain and Tikopia, high alumina and alkali basalts are common, along with andesites and dacites (Hughes, 1978). A later petrological and geochemical study by Monjaret et al. (1991) which includes the back-arc Jean Charcot and Coriolis Troughs, shows that the northern zone basalts have geochemical characteristics intermediate between MORB and island-arc tholeiites and acid lavas near primitive island-arc lava. These results suggest that the volcanics were emplaced at the time of the initiation of the arc in this area, so the New Hebrides back-arc troughs must be considered as intra-arc troughs and are back-arc structures only because of their location at the rear of the active arc.

It is possible to explain the geology of the NNHBA and the differentiation of the rocks there, by a single subduction reversal model proposed for the Solomon and New Hebrides chain by Coulson (1985). Previous data from the vicinity of the Eastern Outer Islands shows that, no volcanic arc lies immediately south of the Vitiaz paleo-trench.

The northwestern comer of the North Fiji Basin is created by oceanic style accretion along two successive spreading systems trending NW-SE and EW respectively, as far north as 8°30'S and 90S. The Tikopia Ridge could either correspond to, or be superimposed on, the youngest EW spreading axis. The eastern part of the back-arc troughs domain, which in fact forms the western edge of the North Fiji Basin, is a continuous and 400 km-long volcanic ridge, called the Duff Ridge. The western part of the back-arc troughs domain is the edge of the New Hebrides Arc, its northern end being an EW intra-arc graben, the Santa Cruz Trough. The existence of the back-arc troughs is mainly due to the construction of the volcanic Duff Ridge which isolated a piece of the old North Fiji Basin oceanic crust west of it (Pelletier et al., 1993a, 1993b).

4.8.2 - SOPACMAPS cruise data a)

Bathymetry

The North New Hebrides Back arc area has been surveyed during both legs 1 and 2 (Fig. 16 and 17). The southwestern part of the area was partiaJly covered during leg 1 and the northern and eastern area during the leg 2.

The northern zone, north of 100S is characterized by EW predominant trends. West of Duff islands these EW directions corresponds to the 9°30'S ridge and its southern basin previously described in the Melanesian Arc Gap box. East of Duff islands, in the prolongation of the 9°30'S ridge a volcanic line shows the same EW major trends associated with NE-SW and NW-SE directions constituted by adjacent volcanic lineations.

Between 9°30'S and 11 "S the Duff islands ridge constitutes a shallow major feature trending NBS and separating two drastically different domains.

The western domain is constituted by the northern slope of the volcanic southwestern province toward the deep basin flanking the 9°30'S ridge. In this domain characterized by depths ranging from 2000 to 3000m, the observed geological features are parallel NI200E ridges and depressions.

The eastern domain is constituted by a more than 3500m-deep abyssal plain with a quite flat bottom. It is cut around 10020'S by a N700E, 200m-high ridge and in its eastern side by NWSE elongated few hundred meters high lineations.

South of 11oS the Duff ridge becomes deeper (around 2000m) and constituted by a double ridge trending NS. This ridge shows very steep flanks especially on its eastern side. It separates two different domains.

The western domain is characterized by a 2500-3000m deep alternation of NS ridges and depressions. Numerous small circular volcanoes are present in this area.

The eastern domain is a diffuse 3500m-deep deformation zone with small amplitude NlOO- 1200E ridges and lows. separated from the Duff Ridge by a more than 3800m-deep NS elongated basin. One can notice that this deformation zone is occupied by numerous few hundred meters high small cones which could be interpreted as small volcanoes. At a large scale this zone is interpreted as an EW volcanic line linked with the Charlotte Bank-Pandora Bank system.

b) Imagery

The imagery of the whole area reflects the main structural observations described from the bathymetric map. Different zones can be identified.

In the north, the 9°30'S ridge and its associated graben show a high reflectivity bottom with EW tectonic trends. These characteristics prolongates on the eastern side of the Duff ridge related to the predominantly EW volcanic line.

South of 100S, the western part of the survey up to 168°E indicates high reflective features marked by intense NS frustration. Numerous volcanoes have been identified in this area. The Duff ridge steep slopes are cut by perpendicular furrows guiding debris flows toward the abyssal plains.

East of Duff ridge, the flat abyssal plain is cut by numerous small amplitude NW-SE lineations indicating a present-day active deformation of the domain.

In the southern part of the abyssal plain south of II "S, a predominantly EW reflective zone shows at detailed scale numerous small amplitude volcanic cones probably linked with the large volcano located at the foot of the NS prolongation of the Duff ridge.

c) Seismic reflection

NW-SE-trending Single Channel Seismic Lines 172 to 178 were carried out to complete eastward the SOPACMAPS Leg I survey that ended along the southwestern flank of the Duff Ridge. The Duff Ridge was identified by Pelletier et al (l993b) as a 400 km-long volcanic structure. The NW-SE-trending northern part of the Duff Ridge ends northward along the coalescent NlO00E- trending 9°30'S Ridge that was surveyed during the Melanesian Arc Gap box.

The interpretation of the seismic reflexion lines enables to identify three main areas east of the Duff Ridge:

- the northern area, north of 100S, dominated by the presence of sediment-free volcanic features surrounded by a thick sedimentary series (l s.t.w.t.). Between 9°30'S and 100S, a NlO00E-trending ridge that rises up to less than 1500 m probably corresponds to the eastern extension of the 9°30'S Ridge. South of the NlO00E-trending ridge, two main volcanoes aligned along the same trend are present, the easternmost one shoaling to 500m. East of the NlOOOE-trending ridge lies a sediment-free N500E-trending ridge centred on longitude 168°E. Water depths within the northern area can reach 4000 m.

- the central area, between 100S and about 11°IO'S, 3500 m-deep in average, is characterized by a smooth and flat topography. Thick sedimentary series that can reach 1.5 s.t, W.t. are cut by a series of normal faults that mainly affect the upper sequences and rarely the basement. The seismic interpretation indicates NW-SE and WSW-ENE directions, the latter being well marked along a transcurrent fault that ends westward north of the volcano centred at 11°15'S- 168°30'E. Burried faults have been identified that seem to trend N-S along 168°2°'E between 10020'S and 11 °15'S.

- the southern area, that shows water depths averaging 3000 m, starts south of about 11°IO'S were WNW-ESE directions mark the transition with the central area. Southward, the structural fabric of the southern area is not very well expressed, and the irregular topography probably reflects the effects of volcanic intrusions within sedimentary series 0.2 to 0.6 s.t.w.t, thick.

The interpretation of seismic reflection Line 175 illustrates the general structure of the NNHBA, characterized by a sedimentary series that thin from NW to SE. The sediment-free NlO00E-trending ridge that lies at 21hOO is flanked by 1500 to 2000 m-high fault scarps. At 19hOO, another sediment-free structure corresponds to the westernmost of the two seamounts that lie south of the NIOOOE-trending ridge. The sedimentary series that surround the volcanic features thin southeastward and are cut by numerous normal faults, some of them affecting the basement. From 11hOO to the beginning of Line 175, the profile crosses the southern area characterized by a rough topography due to volcanism, and by a thin (0.2 to 0.3 s.t.w.t.) sedimentary cover.

d) Magnetism

The magnetic map of the North New Hebrides Back Arc Area include data collected during both legs, 1 and 2. Amplitudes range between -600 nT and 900 nT. The data were automatically contoured at the 50 nT isocontour, using a 2 nautic miles grid mesh size.

West and South of Duff Ridge, the magnetic pattern consists of a series of dipoles generated by the different volcanic structures present in the area. An outstanding feature is that the Duff Ridge has no magnetic significant signature, suggesting that there is probably no significant magnetic contrast between the Ridge and the adjacent areas. Also important is the fact that there is no preferential directions in the back arc basin west of Duff.

East and North of Duff Ridge, two magnetic lineations trends appear, in the E-W and the NE- SW directions respectively. An outstanding feature is the presence of two, well defined magnetic lineations trending approximately E-W, along 9°30S and along 10055S respectively. The magnetic lineation along 9°30S runs from the westernmost part of the area. It shows decreasing amplitudes east of 168°E and correlates with a series of aligned volcanoes. The magnetic lineation along 10050S abuts against the Duff Ridge near 168°E. It is settled in a very different morphological context than the ridge along 9°30'S : south of 11 "S, the seafloor topography is actually rough, and shows numerous closed highs that are 100-500 m high, 3-7 km in diameter. Seismic reflection data indicate that these highs pierce through the sedimentary cover, suggesting that they are volcanic intrusions (the structure near II °18S, 168°30E is probably a large volcano, 20 km in diameter). The lineations trending NE-SW appear between 100S and 10030S, and between 11 °30 S and 12°S. The absence of well defined, separate volcanic structure suggests that these lineations result from linear magnetized sources aligned in the N60 direction, rather than from a series of aligned magnetic dipoles.

The magnetic data suggest as a working hypothesis that the two ridges near 9°30'S, and 10050'S may be coupled in an incipient sea-floor spreading system associated with the Hazel Holme extensional zone. The presence of large, well defined negative magnetic lineations shows that this hypothesis must be checked, through a careful analysis including all the available data at the local and regional scale.

e) Gravimetry

During the SOPACMAPS cruise, gravity data were obtained across the North New Hebrides Back Arc Area. This area extends South and East of Duff island between the northern termination of the New Hebrides back arc area and the North Fiji Basin (NFB). The free air anomaly map includes gravity data collected during Leg 1 and 2 of the SOP ACMAPS cruise. Free Air anomalies were contoured at 10 mGals intervals.

Free air anomalies obtained within the study area range locally from - 54 mGal to +125 mGals. Both these extreme values are located over the Northwestern corner of the surveyed area.

Free air anomalies were separated into two groups of values corresponding to two different geographic provinces: the southwestern province located southwest of the Duff Ridge an arcuate bathymetric feature that extends from the northwest corner of the map southeastward

to longitude to 168° 30E where gravity anomalies range between +125 mGals to -54 mGals and an Eastern province located cast of the Duff Ridge where gravity shows generally much less variation.

Southwest of Duff islands, gravity anomalies show numerous positive peaks corresponding to bathymetric highs, which represent either volcanic edifices or volcanic and volcano sedimentary horsts of oceanic crust. These gravity highs are separated by lows, which may locally reach -20 mGals. These lows correlate with flat-bottomed troughs or grabens. In the Eastern province, the gravity anomaly if flat ranging between +20 and +40 mGals.This flat anomaly coincides with a 3500 m-deep flat-bottomed seafloor However in the northern part of the province two major gravity highs of +80 mGals and + 120 mGals coincide with volcanic complexes. An elongated gravity minimum reaching -20 mGals extends in the Eastern Province along the Duff ridge. This minimum correlates with a flat 3500 m-deep seafloor, suggesting that this seafloor is underlain by low density material.

4.8.3 . Conclusions

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.

The area located west of the Duff Ridge corresponding to N-S alternation of grabens and highs could be interpreted either as a part of the New Hebrides volcanic arc or as a part of the 12 to 10 MyoId North Fiji Basin crust trapped by the recent building of the Duff islands ridge. We speculate as Pelletier et aI. (1993 a and b) that the second hypothesis is more receivable than the first one. The magnetic anomaly and gravity anomaly pattern showing elongated N-S lineations better than dipolar features favors this second hypothesis. North of the Duff islands ridge, all the data converge to confirm the existence of an EW present-day spreading ridge named by Pelletier et aI. (l993a and b), 9°30'S Ridge.

East of the Duff island 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 part and a roughly EW deformed zone associated with numerous small volcanic centers in the southern part. This configuration is clearly evidenced on the magnetic and gravimetric patterns.

4.9· PANDORA BANK

4.9.1- Previous Works

The northwestern North Fiji basin is separated from the remaining part of the basin by a complex series of E- W trending ridges and troughs, firstly named the Hazel Holme Fracture Zone by Chase (1971). Right-lateral or left-lateral strike slip or even compressional motions have been proposed along this feature (Luvendick et al, 1974; Halunen, 1979; Eguchi, 1984; Hamburger and Isacks, 1988 and unpublished). The Hazel Holme Ridge, which corresponds to the western part of the Hazel Holme Fracture Zone of Chase (west of 171°E), has been partly surveyed during the Eva 14 and Santa Cruz cruises and interpreted as an active extensional zone (Pelletier et al., 1988; Charvis and Pelletier, 1989, Louat and Pelletier, 1989, Pelletier et al, 1993b). The feature is seismically active and is composed of several parallel narrow ridges and deep troughs trending N80- 100 and extending over a maximum width of 120 Ian. The troughs are 3500 to 4000 m-deep in average and are bounded by 500 to 1000-m high scarps. The deepest trough (4500 m-deep around 169°E) is located in the axial part of the feature and appears to be rightlaterally offset. West of 168°30'E, the width of the ridge decreases, lateral troughs disappear and a 2500 to 3500-deep E- W trough is bounded by symmetrical ridges rising to 1700 m. This trough ends at 168°IO'E where it connects with the southern tip of the northern New Hebrides back-arc troughs. The bathymetric profiles across the area exhibit a slow spreading ridge morphology.

The South Pandora Ridge, which corresponds to the eastern part of the Hazel Holme Fracture Zone of Chase (1971), was explored between 171°E to 173°30'E during a R.V. Kana Keoki reconnaissance cruise (1982) and near 174°E by a Sea Marc survey during the R.V. Moana Wave cruise (1987), and is interpreted as an EW trending active slow spreading ridge within a broad transform domain (Kroenke et al., 1991; Price and Kroenke, 1991). The ridge is 110 km wide and is cut by numerous transform offsets. It is constituted by a series of EW trending segments cut along strike by a central trough which is 10 to 20-km wide and 500 to 4500 m-deep. The axial trough is flanked by 1000 to 2000 m-high scarps and is filled in some place by large volcanoes rising sometimes the sea level. Fresh pillow basalts recovered on the South Pandora Ridge have geochemical characteristics intermediate between MORBs and oms (Price et al., 1990; Price et Kroenke, 1991; Sinton et al., unpublished).

4.9.2- SOPACMAPS cruise data

a) Bathymetry

The western part of the Pandora box has been previously described in the North New Hebrides Back-Arc area chapter. It is characterized by a very rough topography due to faulting and small amplitude volcanic mounds (Fig. 18 and 19). The contact with the Pandora Bank zone is marked by a general change of the average depth of the oceanic bottom, deeper than 3000m in the NNHBA area, less than 3000m in the Pandora Bank zone. The contact follows a N45 trending scarp. This trend seems to be significative of the whole area. It is found in the direction of the east Fatutaka volcanic line and also on the western side of the Pandora bank.

The major feature of the area, excepted the Pandora Bank which has not be fully surveyed, is the Anuta volcanic system crowned by the Anuta island. It is a large volcanic edifice centered on 11o30’ and 169°30'E characterized by a rough topography due to adventives craters and lava flows.

The Anuta system is separated from the N45 volcanic line by a 3000m-deep flat sedimentary basin limited to the south by the Fatutaka volcano which is probably one arm of the Anuta system. The N45 volcanic line is constituted by four volcanoes culminating at about 2000m depth. They are crowned by rounded craters.

The east of the survey corresponds to the Pandora Bank system centred on 172°E and 12°S, culminating few meters beneath the sea surface. It is avery large, 60km in diameter circular volcanic edifice.

b) Imagery

The imagery evidenced the importance of the Anuta and Pandora Bank volcanic manifestations. Both edifices are surrounded by high reflective rough topography areas. These areas are characterized by rounded small amplitudes zones which could be interpreted as located lavas flows emitted by the volcanic centers.

Another observation which can be made on the image is the difference in color of the sedimentary cover. Some areas are whiter than others. The white sediments are mainly located at the foot of Anuta and Fatutaka volcanoes. This change of color can be explain either by a remobilization of the sediments due to recent movements or by a difference in nature related to the proximity of the volcanoes.

c) Seismic reflection

WNW-ESE-trending Single Channel seismic lines (179 to 187) were carried out in the vicinity of Pandora Bank. Due to the shallowness of Pandora Bank, Lines 179 and 181 only crossed the northern and southern edges of the structure. From west to east the study area, structurally controlled by WNW-ESE and NE-SW-trending fault scarps, shows a guyot-like large structure that corresponds to the submarine extension of Anuta Island, a large basin intruded by a major volcanic structure, and the western flank of Pandora Bank.

The interpretation of seismic reflection Line 179 illustrates the general structure of the Pandora Bank area, characterized by a sedimentary series that thin from NW to SE. Between 23h30 and 01h30, a large structure characterized by a flat top, is interpreted as a guyot-like structure that shoals up to about 1000 m. Between the guyot and the Pandora Bank, a 25 km-wide graben is present, characterized by sedimentary series that thicken toward the southeast, and the presence of a tilted sedimented horst. The eastward tilted sedimented (about 0.2 s.t.w.t.) horst is affected by intense faulting. Between the tilted horst and the volcanic feature centered at 3hOO that bounds the basin to the west, the western part of the basin shows sedimentary series locally disturbed by a few normal faults and that thicken eastward from 0.3 to about 0.5 s.t.w.t, In contrast, the eastern part of the basin is characterized, east of the tilted horst, by numerous westward-facing faults that affect the sediments that increase eastward to reach about 0.8 s.t.w.t, at 13hoo. At 13hOO, a major westward-facing normal fault marks a change in the sediment pattern, with the presence of westward-dipping sedimentary series east of the fault. The tilted horst of the central part of the basin on Line 179, probably corresponds to the northward extension of the NW -SE-trending feature outlined by Fatutaka Island and the series of volcanoes that intrude the central part of the basin.

d) Magnetism

The map of magnetic anomalies in the Pandora Bank Area is based on 5 parallel, 240 Nautic miles profiles, spaced 10 Nm apart, and running WNW-ESE. Amplitudes range between -750 nT and + 850 nT. The data were contoured at the 50 nT isocontours with an automatic procedure using a 2 Nm grid mesh size.

The magnetic anomalies is dominated by a series of magnetic highs apparently aligned in the N100° direction. The major volcanic structure topped by Anuta Island (near 11035'S, 169°49'E) is neither associated with a magnetic high, neither with a low. North of Fatutaka Island (near 12° 03'S, 170015'E) there is a magnetic dipole, with negative anomalies less than -700 nT , and positive anomalies exceeding 800 nT. The positive high is correlated with the presence of two, 25 km wide, seamounts near latitude 11 °30S.

e) Gravimetry

Five gravity profiles were used to obtain the Free air gravity map within an Eastwest trending transect East of Pandora Bank. The Free air anomalies are positive and range between + 14 mGals and + 119 mGals. The average gravity value over the study area is 40 mGals and coincide with a mean seafloor depth of 3000-3500 m. Free air anomalies show several positive peaks corresponding to bathymetric highs that are interpreted as submarine volcanoes or volcanic massifs. The highest anomaly (the Anuta gravity high, +119 mGals) coincides with a major volcanic massif topped by the island. The maximum gravity value, however, may be located over the Pandora Bank because this bank is the biggest volcanic massif in the surveyed area; unfortunately, the summit of this bank could not be surveyed during the cruise. West of Anuta gravity high, two other gravity highs (+95 and +110 mGals) coincide with two volcanic edifices, the southern one being topped by Fatutaka volcanic Island. Between Anuta Island and Pandora Bank lies a basin that shows the lowest gravity value (+14 mGa1s). Four secondary gravity highs that coincide with four volcanic edifices were recognized in this basin.

4.9.3 - Conclusions

The Pandora Bank zone is characterized by the importance of volcanic manifestations. Three main centers have been identified: the first one is the Anuta-Fatutaka system with a five branch Star morphology, showing on its slopes lobated lava flows.

The second is the N45 volcanic line characterized by four flat topped aligned seamounts

The third is the 60 km in diameter Pandora Bank also bounded by lobated lava flows and adventive craters.

The three volcanic centers are separated by fat bottom sedimentary basins, 3000m deep, cromprising a 200 to 400m-thick sery.

The gravity and magnetic anomalies confirm this configuration but also evidence a well marked EW trend.

PRELIMINARY GEOLOGICAL SYNTHESIS The objective of the SOPACMAPS 2 cruise was the survey of the boundary between the Australian and Pacific plates. At a large scale this boundary can be considered as a deformation zone between both plates. The deformation is considerably dependant of the geometry of the boundaries and in particular of the position of the Ontong Java plateau which is, since more than 10 My, colliding against the Australian plate. The observed and measured motion along this boundary is either of compressive or of strike- slip type. The opening of back-arc basins at the rear of the collision is also one of the main characteristic of the area.

The results obtained on the different zones surveyed during the SOPACMAPS 2 cruise can be synthetised as follows:

5.1 - THE SOLOMON AREA

Data obtained during the cruise provide evidence for extensional tectonics with left lateral strike-slip component. This deformation may have affected the arc summit at two periods of time: the first period during the Oligocene-Miocene may have controlled the development of the Central Solomon Trough and the second one during the Pliocene-Pleistocene may be related to the emplacement of the New Georgia volcanic line (Vedder et al. 1989).

The first period of extensional tectonics is derived from the observation of tilted crustal blocks beneath the northern province of our study area i.e. along the southern flank of the central Solomon trough. The observed NI200E and N700E structural trends and V shape topography together with structural correlations between seismic reflection lines suggest that the buried blocks were tilted toward the northwest indicating that extensional tectonics occured in a NW direction. This direction is oblique to the general trend of the are, suggesting that rifting occured in a left-lateral transtensional tectonic environment. The tilted blocks are burried beneath 500 to 1000 m of sedimentary fill. Based on ages provided by Bruns et al. (1986), the sedimentary fill is dated Pliocene- Pleistocene suggesting that rifting may have occured during Late Oligocene-Early Miocene time. This phasis of left-lateral rifting is in good agreement with the hypothesis of Wells (1989), who suggested that the central Solomon trough formed as an early Tertiary pull apart basin during oblique subduction of the Pacific plate.

The second period of sinistral transtensional tectonics is derived from the analysis of the detailed bathymetry and seismic reflection data in the central and southern provinces. Elongated, narrow flat-bottom troughs and ridges bounded by well delineated scarps trending NI20 °E

across the Mborokua basin as well as the two large tilted blocks flanking the central province to the south are evidence for a major rift zone. Rhombic depressions, left-laterally offsetted faults and small-scale crustal blocks tilted northwestward together with the small- scale rhombic tectonic pattern evident in the southern province support a sinistral strike-slip component of the deformation. Although the two large blocks appear to be tilted to the north small-scale burried blocks are tilted northwestward.

The arc swell and the left-lateral transtensional rift zone extends westward along the arc summit. The arc swell may be due to the subduction of the hot Woodlark basin spreading center (Cooper and Taylor, 1989) beneath the New Georgia islands, whereas the sinistral component of the rifting is related to the oblique convergence motion between Australia and Pacific plates.

The transtensional geodynamic context may account for the alkaline component of the lavas emitted from the New Georgia volcanoes, while the calc-alkaline lavas are related to the subduction processes along the San Cristobal trench.

5.2 - THE EAST MALAIT A AREA

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 North 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 overthrusting 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 North Solomon trench is infilled by a I to 2 kin thick sedimentary cover dipping to the south. This dipping could be interpreted as a small amplitude present-day deformation within the soutwestward dipping North Solomon trench. Some indication of gentle folding have been observed at the foot of Malaita margin. The folded area constitutes a parallel to the slope 3000m-deep ridges alignment.

The major part of the Malaita margin is constituted by deformed alternation of highs and lows joining the northern Malaita Island with the Ulawa zone. This deformed zone is cut by an important NS fault at 162°E. This fault separates the Malaita Anticlinorium from the 6000m-deep nodal trough created by the Junction between the North Solomon and Cape Johnson trenches.

The folded area of the Malaita Anticlinorium is marked by a linear NW -SE volcanic line underlined by strong magnetic and gravimetric anomalies. It could correspond to the youngest basaltic emission (Hughes and Turner, 1977) coinciding with the beginning of the collision of the Ontong Java plateau in the North Solomon Trench.

ENVIRONMENT CONDITIONS

POTENTIAL RESOURCES

7.1 - IRON BOTTOM SOUND, MBOROKUA BASIN, NEW GEORGIA SOUND

Oil potentialities: several Pliocene and Miocene reefal leads have been previously identified in the Iron Bottom Sound. They are considered as potential oil reservoirs. They are for the major part overlain by an important sedimentary cover as confirmed by the SOPACMAPS2 seismic survey. However, in the central part of the basin the bathymetric map shows two small size mounds cropping out above the flat bottom of the Iron Bottom Sound. They could be interpreted either as basement tilted blocks or reefs. In the same way one can notice that the maximum gravity low measured during SOPACMAPS2 cruise is located on the Eastern side of the Iron Bottom Sound. This low confirms the existence of a deep sedimentary basin at that place. Others deep sedimentary basins mapped during the cruise, could be potential oil reservoirs. They are located in the Central Solomon basin where the sedimentary cover is more than 2 s.t.w.t. thick. The Fatu 0 Moana structure (basement horst, reef, ?) is also a good candidate for oil potentiality.

Geolo~cal hazards: the Solomon Islands area due to its position at the convergent boundary between Pacific and Australian plates, is the location of important seismicity along the San Cristobal subduction zone (Dunkley, 1983). The registered earthquakes are either shallow or deep earthquakes depending of the dipping of the subducted plate. At the same time and for the same reasons, important volcanism occured since Miocene up to the Pleistocene. The resulting volcanic line extends from Guadalcanal to New Georgia Islands. Submarine volcanoes have been mapped during the SOPACMAPS cruise mainly within the Mborokua basin. The combination of seismicity and volcanism implies an important unstability of the slopes. Debris flows and mass wasting have been observed all along the explored slopes of Savo, Russel, Mborokua and New Georgia islands. The most spectacular feature is the debris flows appearing of the bottom image of the northwestern corner of the Mborokua basin.

Fishing potentialities: the SOPACMAPS survey allows the discovery and mapping of several new seamounts culminating at less than lOOOm-depth. The majority of them are located along the NW -SE volcanic line cutting the central part of the Mborokua basin. They are good sites for concentration of deep fishes species such as Orange roughy, etc. The Fatu 0 Moana mound located in the middle part of the New Georgia Sound is 750m- deep. It is also associated with an increase of 0.6/1000 of the salinity probably due to current circulation or upwelling. This could be an excellent site for fishing activities.

7.2 - EAST MALAITA AREA

Oil potentialities: The East Malaita area shows three distinct areas: the first one is the 400Om-deep NW -SE depression related to the North Solomon trench. The second one is the "nodal" basin resulting from the junction between the Cape Johnson and North Solomon trenches. The third one is the east Malaita domain which probably represents the submerged junction between the Ulawa Island and the northern part of Malaita which is a large scale anticlinorium resulting from folding during a previous phase of deformation. This area is considered to be a compression zone between the Solomon Islands and the Ontong Java-Pacific plate. The seismic profiles carried out during SOPACMAPS cruise do not provide evidences of present-day compression but evidence of vertical motions. The 3000m-deep graben located immediately east of Malaita island between the central ridge and the Malaita slope, could be a site of oil exploration with adapted means such as multichannel seismic.

Geolo&ical hazards: the central ridge culminating at less than l000m-deep is interpreted as a volcanic basement line. Active slidings have been observed along the ridge slopes. They are constituted by debris flows probably of volcanic origin.

Fishin& potentialities: The major part of the summit of the central ridge is less than 1000m-deep. This constitutes a good possibility for fishing activities. Another site for fishing is represented by the slopes of Malaita and Ulawa islands where small mounds and seamounts are culminating at less than lOOOm-depth.

7.3 - MELANESIAN ARC GAP

Geolo&:ical hazards: the major part of the Melanesian Arc Gap exhibits depths comprised between 2000 and 3000 m affected by vertical or strike-sleep tectonism. The area centred on 166°E and 100S, north of Ndendo is characterized by intense volcanic activity. Some of the surveyed volcanoes could be active like the emerged Tinakula active center. They are fringed as shown in the imagery where steep flanks are affected by flows, debris flows and slumping.

Hydrothermalism: two different zones could be the sites for active hydrothermalism. The first one is the active volcanic province previously invoked and the second could be the 9°30'S ridge interpreted by Auzende et al. (1993) as a possible present-day accretion zone related to the active NFB spreading ridge by a NS transform. Chemical in situ measurements, dredging and photographs observations would be necessary to argument this hypothesis.

5shin&: potentialities: the whole volcanic province exhibits shallows (less than 1000mdeep) seamounts. They are probably excellent sites for the colonization by deep species fishes.

7.4 - NORTH NEW HEBRIDES BACK-ARC AREA

Geological hazards: the volcanic province comprised between Duff ridge and the southwestern part of the survey is the eastern continuation of the volcanic province described in the previous Melanesian Arc Gap box. The volcanoes and associated features show active fracturation related either to the volcanic activity or tectonic present-day deformation. Active volcanism and tectonism results in important slidings along the slopes.

Hydrotheunalism: the conclusions are the same than for the Melanesian Arc Gap box and concern the 9°30'5 ridge and its eastern volcanic prolongation and also the active southwestern volcanic province.

Fishing potentialities: see above the comments done for the Melanesian Arc Gap

box.

7.5 - PANDORA BANK

Due to the volcanic nature of the whole area the only economic potentiality evidenced by out survey concern the fishing activity.

Fishing potentialities: The Pandora bank area is essentially characterized by great number of very shallow seamounts and reef banks. All those features constitute excellent site for fishing deep species. PRELIMINARY CONCLUSIONS

The SOPACMAPS 2 cruise allows to obtain precise and extensive data on the Solomon Islands economic waters. The major part of the Solomon area is located at the boundary of the Australian and Pacific plates. The complex relative motion between both plates and the geometry of their boundaries implies different types of geologic environments and manifestations.

The main conclusions resulting from the SOP ACMAPS 2 survey are:

* The general geological framework of the area has been considerably precised. More than 120000 square kilometers of the Solomon economic zone have been mapped with a quite complete multi beam coverage. About 10 000 km of geophysical (seismic, magnetic and gravimetric) profiles have been carried out during the second leg.

* Geophysical profiles allow to discuss the deep structure of the area in terms of oil potentialities. The first conclusion arising is that these potentialities exist in the Iron Bottom and New Georgia sound.

* On all the surveyed boxes, volcanic effects have been evidenced in terms of geological hazards (active volcanism and related mass wasting).

* The Simrad map allows to localised hundred of new shallow water seamounts, which could be sites for fishing activities.

The survey carried out during the SOPACMAPS cruise represents a first approach mainly based on the mapping. To be effectively useful in terms of economic potentialities this survey must be followed by a systematic sampling survey (dredging, coring, water sampling, etc.). ENCLOSURES