sopac 1991 - structural fabric in northern north fiji basin - jarvis et al

8/3/2019 SOPAC 1991 - Structural Fabric in Northern North Fiji Basin - Jarvis Et Al

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STRUCTURAL FABRIC IN THENORTHERN NORTH FIJI BASIN

Phillip A. Jarvis, Loren W. Kroenke

Richard C. Price2, Patrick Maillet

July 1991 SOPAC Technical Report 130

Hawaii Institute of Geophysics, University of Hawaii, Honolulu

Geology Department, LaTrobe University, Bundoora, Victoria 3083

Office de la Recherche Scientifique et Technique Outre-Mer (ORSTOM)BP A5, Noumea Cedex

Prepared for: South Pacific Applied Geoscience Commission(SOPAC)Offshore Programme on EC Consultancy Contract no. SP/SOP/01/90GLORIA data interpretation and reporting Dr Phillip A. Jarvis


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TABLE OF CONTENTS

PageABSTRACT .......................................................................................................... 5

ACKNOWLEDGEMENTS ..................................................................................... 6

INTRODUCTION .................................................................................................... 7

HYDROTHERMAL DEPOSITS IN THE NORTH FIJI BASIN ........................ 7

DATA AND TECHNIQUES ........................................................................ 9

SOUTH PANDORA RIDGE

Previous Work ................................................................................. 11GLORIA Sidescan Imagery ................................................................. 12

Seismic Reflection Profile ................................................................ 16

Geological Interpretation .................................................................. 16

CENTRAL NORTH FIJI BASIN TRIPLE JUNCTION

Previous Work .............................................................................. 19

GLORIA Sidescan Imagery .................................................................. 19

Seismic Reflection Profiles ................................................................... 22

Geological Interpretation ......................................................................... 22

CONCLUSIONS ................................................................................................ 26

REFERENCES CITED ............................................................................. 27

[TR130 - Jarvis & others]


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LIST OF FIGURES

Figure Page

1 Location of GLORIA survey areas discussed in this report ....... 8

2 Sketch map of known and proposed major tectonic elements .... 10

3 GLORIA sidescan sonar image of the Jean Charcot Troughsand extreme western end of the South Pandora Ridge .................

a) GLORIA sidescan sonar image of the western section of theSouth Pandora Ridge ....................................................................

b) Bathymetric map of the western section of the SouthPandora Ridge..........................................................

Line drawing interpretation of the single channel seismicreflection profile along the GLORIA track shown on Figure 6 ......

A structural interpretation of the GLORIA image shownin Figure 3 .........................................................................................

a) GLORIA sidescan image of the northern and eastern limbsof the central basin triple junction .....................................

b) Bathymetry of the triple junction area ......................................

Single channel seismic reflection profile across Graben A ..........

Geological and structural interpretation of the GLORIAimagery shown in Figure 7a ...............................................

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ABSTRACT

A GLORIA cruise which included part of the North Fiji Basin covered some of the mosttectonically active and geologically controversial, areas within the entire basin. The South

Pandora Ridge (SPR) is an east-west feature characterised by steep escarpments and very high

seafloor relief. Recent seismicity indicates it is tectonically active but the GLORIA data do not

reveal active volcanism within the surveyed area. Because of this it is considered unlikely to

have significant economic mineral potential. However, because it is still not clear how the SPR

fits into the tectonic framework of the basin, it should be strongly considered for future surveys.

The northern and eastern limbs of the central North Fiji Basin triple junction were also

included in the survey. The northern limb is marked by at least three grabens and many steepnormal faults but very little volcanism. The eastern limb is a single wide graben that extends

about 70 km northeastward from the triple junction. Its connection to the Fiji Transform Fault is

not certain, but the GLORIA data reveal that the area east and north of the eastern limb is

tectonically inactive. The connection is thought to occur in the region southeast of the eastern

limb. While the economic mineral potential of this southeast area may not be as great as the

adjacent triple junction, its relevance to tectonic models of the basin make it a high priority for

future sidescan sonar work.



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ACKNOWLEDGEMENTS

The SOPAC GLORIA survey in parts of the EEZs of Vanuatu, Solomon Islands, Fiji, Tonga,

and Western Samoa was supported by Lome lll funds from the European Community to the

South Pacific Applied Geoscience Commision (SOPAC) and by the generous provision of

shiptime by the Royal Australian Navy (RAN). The project was coordinated by SOPAC Offshore

Co-ordinator Dr Don Tiffin (funded by CIDA), who was Chief Scientist during the survey.

The survey was carried out on board the Australian Naval Hydrographic vessel HMAS

Cook, commanded by Lt Cmdr Brian Hunt. The GLORIA equipment and data acquisition was by

Marconi Underwater Systems Limited. The Seabeam data collected by HMAS Cook are the

property of the RAN and are published with the permission of the Hydrographer, RoyalAustralian Navy.



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INTRODUCTION

In August 1989, the South Pacific Applied Geoscience Commission (SOPAC) conducted ageophysical survey within the Exclusive Economic Zones of Vanuatu, Solomon Islands, Fiji,

Tonga and Western Samoa, using a variety of techniques including GLORIA (Geological Long

Range Inclined Asdic) sidescan sonar, Seabeam wide swath bathymetry, 3.5 kHz sub-bottom

and dual channel seismic reflection profiling to identify potential sites of mineral deposits. This

report discusses the results of the survey within the northwestern and central North Fiji Basin by

examining the modern and relict sea floor fabrics indicative of the history and present geometry

of plate motion in this part of the basin.

HYDROTHERMAL DEPOSITS IN THE NORTH FIJI BASIN

Hydrothermal mineral deposits are now known to be a common occurrence in areas of

seafloor rifting and volcanism. Many deposits, but not all, contain polymetallic sulfide minerals

possessing several valuable metals including iron, zinc and copper. Although first discovered on

major oceanic spreading centers like the East Pacific Rise (Francheteau et al, 1979), they have

now been identified in many of the marginal basins of the southwest Pacific including the Lau

Basin (von Stackelburg et al, 1985), the Western Woodlark Basin, Manus Basin, and the North

Fiji Basin (Auzende et al, 1990). A Lau Basin site has recently been reported to contain thehighest values of native gold yet discovered in submarine hydrothermal deposits (Bush, 1990).

Since 1985, a large number of research cruises have surveyed the rift system of the North

Fiji Basin searching for and investigating the characteristics of hydrothermal mineral deposits.

Anomalously high amounts of methane in the water column provided the first indications of

hydrothermal activity within the basin (Auzende et al, 1990). Active hydrothermal activity was

subsequently found during the French - Japanese STARMER II cruise of the RV Kaiyo which

identified an active hydrothermal vent at the central basin triple junction (Figure 1) producing

chimneys of anhydrite (CaSO). Later work using the submersible Nautile studied this activevent and discovered polymetallic sulfides at an inactive vent site nearby. At the present time,

there are only a few known locations of hydrothermal activity along the North Fiji Basin rift

system although high amounts of methane and total dissolvable manganese in the water

column, indicative of active hydrothermal venting, have been reported from several other

locations in the basin both on and off known rifts. One of the goals of the SOPAC cruise was

to identify other prospective sites of mineralisation within the North Fiji Basin.



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DATA AND TECHNIQUES

The GLORIA cruise track through the North Fiji Basin (Figures 1 and 2) was planned toprovide information on the western South Pandora Ridge, also known as the Hazel Holme

Fracture Zone, and to complement previous expeditions in the northern and eastern arms of the

central North Fiji Basin triple junction. The western South Pandora Ridge was not well surveyed

and had never been surveyed with a sidescan sonar system, while the triple junction area had

been partly covered by SeaMARC II - a dual system that simultaneously obtains both sidescan

sonar and wide swath bathymetric data - during the Tripartite II cruise of the RV Moana Wave

(Kroenke et al, 1987). Several cruises of the STARMER project had also obtained detailed

Seabeam bathymetry in the triple junction region (Lafoy et al, 1990). The GLORIA data cover

the northern limb of the central basin triple junction southward to the triple junction itself, andthen follow the eastern limb to image the Fiji Transform Fault north of Fiji. This latter is the

subject of a separate report (Hughes-Clarke et al, 1991). As well as GLORIA data, Seabeam

bathymetry, dual channel seismic reflection profiles, and 3.5 kHz sub-bottom profiles were

obtained. Full details of the cruise are discussed elsewhere (Tiffin et al, 1989).

Several types of sidescan sonar systems exist, but they all operate on the same principles.

In many respects, a sidescan sonar image may be thought of as an aerial photograph. Instead

of being produced by reflected light, however, it is produced by reflected sound. A full

description of the GLORIA system is given by Somers et al (1978).

GLORIA is capable of being towed at speeds of 7 to 8 knots and can produce an image

of the seafloor as much as 45 km wide. In the middle of each GLORIA swath is a narrow 2-3

km wide band that appears somewhat fuzzy. This is an artifact that results from the GLORIA

equipment being unable to image the seafloor directly beneath it. This fuzzy band therefore

marks the ship's track through the area. At its operating frequency of 6.5 kHz, its resolving

capability would not detect sediment thicknesses less than 0.5 m., that is, the GLORIA signal

could identify an area covered with very thin sediment as 'sediment free'.

Sidescan sonars use the amplitude of the reflected sound wave to determine the

reflectivity and backscatter characteristics of the seafloor. Different seafloor substrates such as

basalt and carbonate sediment have different acoustic properties. Sidescan sonar systems

exploit these differences to produce an image of what the ocean floor "sounds like." The

seafloor signature observed in GLORIA imagery predominantly represents an expression of

seafloor roughness (Mitchell and Somers, 1989). GLORIA imagery is particularly well suited to

delineating steep scarps and discriminating between those regions of the seafloor which are



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mantled by unconsolidated sediment and those which consist of exposed volcanic rocks.

Features which are highly reflective to incident sound energy appear white in the GLORIA

imagery. Typically, such features are steep escarpments often associated with faults, or hard,rough surfaces such as recent lava flows. The three main acoustic facies distinguished on the

GLORIA imagery are: a dark grey image resulting from low backscatter, interpreted as sediment

draped seafloors; a lighter image from moderate backscatter, interpreted as consolidated

basement outcrop, usually on ridges; and high backscatter resulting in a light image, interpreted

as either steep scarps or extrusive neo-volcanics.

While the resolution of sonar images is far too crude to detect mineral deposits directly, it

can be used to identify areas of active volcanism on the seafloor where mineralisation may

occur.

Seabeam is a hull-mounted, multibeam, bathymetric system which provides accurate

contoured bathymetry over a swath width of 0.73 times the water depth. Since the average

depth of the North Fiji Basin is around 3000 m, in this area the average Seabeam swath is

approximately 2250 meters wide. This is much narrower than the sidescan sonar image

provided by GLORIA, and since the bathymetry is an important adjunct to the imagery for

interpretation, the single Seabeam swath obtained across the SPR and along the northern limb

of the triple junction is insufficient to provide all the information necessary to interpret the

GLORIA swath. Other bathymetric data were therefore used to augment the Seabeam data.

In addition to the above data, single channel seismic reflection profiles were collected

using a 140 cubic inch air gun as a sound source. These analog profiles were produced using

a band pass between 30 and 180 Hz. A 3.5 kHz sub-bottom profiler was also operated

continuously during the survey.

SOUTH PANDORA RIDGE

Previous Work

The South Pandora Ridge (SPR) has long been considered to be the plate boundary

separating the Pacific Plate to the north and the microplates of the North Fiji Basin to the

south. In his seminal work on the tectonics of the North Fiji Basin, Chase (1971) identified it as

a left-lateral transform fault. This conclusion was affirmed by Macdonald et al (1973) on the

basis of seafloor morphology and sparse heat flow data. However, an alternative interpretation



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was later put forward by Malahoff et al (in press) who suggested it was an incipient rift on the

basis of magnetic anomalies. This interpretation was supported by Kroenke et al (in press) who

studied the morphotectonics of the eastern SPR during the Tripartite cruises of the RV KanaKeoki and RV Moana Wave (Kroenke and Eade, 1982; Kroenke et al, 1987). Recent volcanic

activity has been reported from separate locations along its length (Sinton et al, in press; Price

et al, 1990), lending further support to this interpretation. Pelletier et al (1988) examined the

extreme western end of the SPR where it intersects the New Hebrides backarc troughs and

called it a northeast-southwest extensional feature on the basis of structural and morphological

considerations.

GLORIA Sidescan Imagery

The GLORIA sidescan imagery reveals the tectonic structural trends in the western part of

the SPR. Its western termination is shown in Figure 3. ENE-trending lineaments in the lower

right part of this figure are identified with the SPR. These trends begin adjacent to the southern

end of the Jean Charcot Troughs (northern New Hebrides backarc), and extend over a zone 75

km wide. However, the trend is not apparent west of 168' 30'E and ENE lineaments do not

impinge upon the arc. Instead, the seafloor near the arc, south of 14'S, is marked by

volcaniform seamounts and features resulting from slumping and arc-related fault processes,

controlled by the proximity of the arc. Johnson et al (1991) describe this area more fully in theirpaper based on the GLORIA data of Figure 3, and other data.

East of 169'E (Figure 4), the GLORIA swath crosses an area marked by strong,

predominantly ENE-trending lineaments. Although there is ample high reflectivity (white areas on

the image, Figure 3), it is due to the extreme ruggedness and large relief typical of the SPR. No

recent volcanism is identifiable along the GLORIA track; the bright reflections are almost all from

steep scarps, mainly without much sediment cover. Most strong lineaments have the same

general ENE trend and these are interpreted as arising from anastomosing fault escarpments.

East of 171' 15'E, the number of reflectors decreases dramatically. This is an artifact due to agreatly reduced GLORIA output power lasting between the gap in the image at 171' 25'E

(Figure 4a) and another at 15' 05'S on the next line (Figure 7a, later). Even with reduced

power, many lineaments with trends of ENE and a few with trends of WNW are still visible

between these points.

Previous workers have suggested that the SPR changes trend at about 170' 30'E and

runs WNW between this point and the arc. In the imagery it is clearly seen that the main



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seafloor fabric throughout the GLORIA area almost to the New Hebrides Arc platform retains the

same ENE trend as the main axis of the SPR in the northeastern NFB. Only in the area west of

168' 30'E do trends on the GLORIA mosaic differ. Pelletier et al (1988) thought the main part ofthe SPR would lay north of the GLORIA track and considered the area to the south was an

older part of the seafloor. Although the GLORIA data show the SPR passes through the region

covered, the WNW trends seen south of the SPR are probably related to older seafloor there.

In the bathymetry of Figure 4b, the SPR is distinguishable as the region of maximum relief

trending N70'E. It can be traced from the northeast corner of the figure into the GLORIA survey

area on the basis of its morphology, passing through a few small basins deeper than 4000 m,

clearly shown in Tripartite l bathymetry data. These basins are arranged in a relay pattern and

have the same ENE trend as the SPR.

Seismic Reflection Profile

While the GLORIA data show the surficial structure and configuration of the seafloor, an

interpretation of an analog seismic reflection record (Figure 5) obtained during the GLORIA

survey provides a structural cross section through the region. The western end of the profile

near the New Hebrides Arc shows uniformly thick sediments (at least 200 m thick) accumulated

in a few depressions in an area of rugged topography, but with comparatively low relief. Incontrast, the SPR itself is marked by numerous faults and extreme relief, with boundary

escarpments more than 1km high but only limited sediment cover. This is typical of much of the

length of the SPR, i.e. the high relief along the SPR is in marked contrast to the relatively

subdued bathymetry characteristic of most parts of the basin.

Geological Interpretation

A geologic sketch map, based on the GLORIA imagery, was constructed to show the

distribution of faults and fault trends (Figure 6). The predominant ENE seafloor fabric is clearly

seen with escarpments covering a wide area of the seafloor of the northwestern NFB. High relief

on some major escarpments indicates a large throw and suggests there may be a component

of extension in the faulting, but there is no clear evidence of an axial rift nor is there any

widespread volcanism apparent in this western area. The sense of slip on the fault escarpments

cannot be determined from these data, but six strike-slip focal mechanism solutions from the

ridge area were reported by Hamburger et al (in press). These had T-axes nearly perpendicular



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to the strike of the ridge, supporting a hypothesis of ridge extension, but the stress axes (and

interpreted fault planes) were all oblique to the strike of the SPR.

South of the SPR, the seafloor fabric has two different trends - ENE and WNW - and is

more subdued, suggesting that this region is less affected by SPR tectonism.

Figure 6 also shows the locations of rock dredges and sediment cores taken during the

Tripartite l cruise of the RV Kana Keoki (Kroenke and Eade, 1982). Although evidence of thin

sediments cannot be seen on GLORIA data, one of the cares taken within the SPR recovered

more than 1.2 m of mud, and all cores recovered more than 1 m of brown pelagic mud

ubiquitous to the North Fiji Basin. Further northeast, however, a rock dredge (Figure 6)

recovered very fresh MORB-like basalt (Sinton et al, in press). Because of its lack of alteration,it is believed that this basalt was emplaced very recently.

Kroenke et al (in press) consider that the presence of fresh basalts, and graben structures

observed on the SPR, indicate that the SPR is a rift. The SPR is described as being an incipient

rift, with the western end thought to be an incipient overlapping spreading centre breaking

through older crust. According to this model, true sea floor spreading has not yet begun in the

area imaged with GLORIA. The location of an axial part of the rift is not easily determined from

the data but, near the centre of the GLORIA image, the imagery can be interpreted as showing

a double rift, with axes at approximately 170' 22'E and 171' 00E. The image shows the latter

rift axis coming from the ENE and terminating in trends that become E-W on the south side of

the GLORIA swath. The western axis carries on to the WSW on the southern side of the

GLORIA swath, but the GLORIA data suggest that this axis does not extend far to the northeast

because the trends on the extreme north side of the swath indicate a swing to the E-W.

Seismic and Seabeam data over both these axial locations show deep and rugged valleys, one

more than 3700 m deep, under the ship's track. These observations suggest overlapping of the

rift axes, or an offset between axial components, or an en echelon character to the axis in this

area.

Nevertheless, the high seafloor relief, strike-slip style of seismicity, and lack of significant

amounts of volcanism are good evidence for the SPR being a predominantly shearing rather

than an extensional boundary. The small amount of recent volcanism observed on the SPR can

be explained as indicating the presence of a small component of extension along with shear.

Not all the authors of this paper have agreed that the SPR is a incipient rift boundary.



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The western termination of the SPR is somewhat problematic. From the GLORIA imagery,

it is clear that deformation associated with the SPR does not extend west of 168' 30'E, that is,

it does not reach the New Hebrides Arc. Apparently the tectonic mechanism controlling the

location of the SPR has affected only the Oceanic type crust of the North Fiji Basin and not the

arc platform crust. This would indicate that the SPR and the northern backarc troughs must

arise from quite different mechanisms.

CENTRAL NORTH FIJI BASIN TRIPLE JUNCTION

Previous Work

The central North Fiji Basin triple junction at 16' 55'S, 173' 55'E (Figure 2) has been the

subject of considerable study since 1985, beginning with the French RV Jean Charcot cruise

under the SEAPSO programme, followed by a Tripartite II cruise by Moana Wave, and later by

several cruises using tho Japanese RV Kaiyo and French RV Noroit with the submersible Nautile

under the STARMER programme. This triple junction marks the northern termination of the

central basin spreading center. The first active hydrothermal site discovered in the North Fiji

Basin was detected at the triple junction in 1988. Despite the wealth of data now available from

along the rift areas, conflicting opinions have still been advanced regarding the tectonic

structure here, especially in regard to the connection of the Fiji Transform Fault with the triplejunction. The eastern limb has often been considered to be the western end of the fault Lafoy

et al, 1987; Lafoy, et al, 1990) although other authors (Jarvis, 1991; Hughes-Clarke, 1991) argue

that the fault terminates east of the triple junction nearer to Fiji.

GLORIA Sidescan Imagery

The GLORIA image covers much of the northern and eastern limbs of the triple junction

as shown in Figure 7a. The axis of the whole northern limb is not imaged by the GLORIA

survey as its northern part falls to the east of the GLORIA swath, but it has been clearly

demarcated by SeaMARC II imagery (Jarvis and Kroenke, in press).

From the triple junction northward to 15' 15'S the GLORIA image shows several

N20'W-oriented parallel lineaments that may be traced continuously for up to 100 km. North of

15' 15'S, the lineaments change direction to N45'W. This northern area has more subdued

topography and fewer lineaments. Volcanism is apparent in only a few places on the northern



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limb, for example, at 16' 22'S, 173' 40'E and at 15' 57'S, 173' 22'E, but a bright area adjacent

to the triple junction, not associated with steep slopes, indicates the presence of widespread

recent basalt effusion there.

The triple junction itself falls within the sweeping turn of the GLORIA equipment and thus

was not able to be processed. It is located at 16' 55' S, 173' 55' E.

The eastern limb is a 50 km-wide rift graben (Figure 7a) that extends N45'E from the

triple junction and narrows rapidly toward its northeastern end. The floor of the graben is seen

on the GLORIA image as a broad bright area, indicative of young volcanics. The graben

reaches depths as great as 3600 m (Figure 7b) but shoals to less than 1900 m near the triple

junction. Near its northeastern terminus, the floor of the graben has several small lineamentswith an east-west trend, and southeast of the end of the graben, a large southeast-trending high

makes a bright reflection. East of this point, for a distance of about 100 km, there are few

lineaments to be seen and the seafloor has very little relief. It is also covered by thick

sediments, indicating it is much older and more stable than the triple junction area.

Seismic Reflection Profiles

A single channel seismic line collected during the 1987 Tripartite ll cruise (Kroenke et al,1987) crosses the northern limb and clearly depicts the rift graben marking the central axis

(Figure 8). The graben is bounded by large, steep normal faults: the fault on the western side

has a throw of nearly 1000 m. Comparatively thick sediment accumulations occur 20 km west

of the northern limb but are not observed within the graben itself. Instead, small hyperbolic

reflectors mark the graben floor. We interpret this to suggest that the northern limb is a young

extensional feature, overprinted in older crust.

Geological Interpretation

As with the SPR, a geological map was produced from the imagery showing the

distribution of faults and recent volcanism (Figure 9). Extension in an ENE - WSW direction

controls the structure of the northern limb. Normal faults predominate, some with throws of

more than 1000 m. While faulting occurs over a 70 km-wide zone, the central axis of the

northern limb is made up of four grabens (Jarvis, 1991) with right-stepping offsets between

them. Two of these grabens are imaged in the GLORIA data, and the third is on the edge of



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the swath. They are indicated as A, B and C in Figure 9. Graben A, just north of, and adjacent

to, the triple junction, has the most volcanic activity associated (indicated by the v-shaped

pattern, Figure 9). Graben 6 extends for 120 km from 16' 15'S to 15' 30'S. Graben C is offset

to the east of graben B and can be seen at the right edge of the imagery at 15' 15'S.

Escarpments trending N20'W, typical of the northern limb, occur east of the area imaged by

GLORIA as far north as 14' 30' (Jarvis and Kroenke, in press), whereas the GLORIA image

north of 15' 10'S shows all the fault trends are N45'W. Because these trends parallel magnetic

anomalies identified with older seafloor spreading (Malahoff et al, in press), and because the

relief is subdued in contrast to the faults of the northern limb, this area is interpreted to be part

of a remnant older seafloor and not related to any recent tectonic activity except possibly

through reactivation of older faults.

The eastern limb is characterised by a short, wide, but deep graben set between two

steep escarpments. The northern escarpment is a steep large normal fault striking N45'W, with

a vertical offset of more than 1000 m. The graben is floored by extensive lava flows, and a few

Small escarpments are present on its floor. Near the northeastern terminus, it narrows between

the two boundary faults and can no longer be traced northeastward in either imagery or

bathymetry. Eastward, the seafloor is covered by thick, relatively undisturbed sediments showing

no trace of any major faulting, and apparently of much greater age than the eastern limb itself.

Most tectonic maps of the area have shown the fault as a continuous left-lateral trace from the

northern Tonga Trench to the triple junction at 174'E. Recent work, in particular this GLORIA

survey, has emphasised that such a model is a distinct oversimplification. There is no direct

fault trace connecting the eastern limb with the Fiji Transform Fault. However, the exact tectonic

mechanism leading from the eastern limb to the fault traces defined west of Fiji (Hughes-Clarke

et ai, 1991) is not clear.

Recent Seabeam data between the eastern limb and 176'E, south of the GLORIA swath,

have shown the area to be a region of high relief marked by ridges trending north and

northeast. Seismicity is distributed over a wide area and epicentres are relatively sparse

compared to the Fiji Transform Fault zone (Hamburger and Isacks, 1991). Lafoy et al (1987)

describe that area and the eastern limb itself as cut by several en echelon strike-slip faults. We

believe that tectonism associated with the eastern limb is offset to the southeast at the high

relief area at the eastern termination of the eastern limb graben. Rather than being a single fault

trace however, the deformation may be distributed over several small fault blocks. This is in

contrast to the Fiji Transform Fault east of 176'E, where the fault is a relatively narrow and well

defined trace.



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CONCLUSIONS

Although the SPR is considered to be the active plate boundary between the Pacific Plate

and the microplates of the North Fiji Basin, it is not clear how it fits into the regional tectonic

framework. In the area surveyed with GLORIA, it is marked by steep escarpments, very high

relief, and little apparent volcanic activity. Thus the western end of the South Pandora Ridge is

not a likely site of hydrothermal mineral deposits. However, northeast of the survey area, recent

volcanism has been reported from several locations along the SPR and this area should be

given a high priority for any future use of wide swath sidescan sonar in the North Fiji Basin.

The northern limb of the triple junction is a zone of recent extension interpreted to be an

incipient rift. It is marked by steep escarpments and graben structures along its axis, and recent

volcanic activity is obvious in certain isolated locations. These locations are likely sites for

hydrothermal venting. Future exploration for mineral deposits should be directed to such areas

where volcanic activity is obvious.

The eastern limb of the triple junction is problematic. It is a wide rift graben but only

extends a short distance from the triple junction. Beyond this, the seafloor is covered by thick

sediments and is considerably older. There is no obvious direct connection between the eastern

limb and the Fiji Transform Fault at its most westerly recognised trace at the Yasawa Trough.

The zone of connecting tectonic activityis

probably located south of the GLORIA survey area.This zone should have a very high priority for any future survey work, both to clarify the

tectonic framework - fundamental to the understanding of the North Fiji Basin northwest of Fiji -

and because there is a high probability of hydrothermal activity and associated sulphide

deposits occurring in that region.


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Auzende, J-M., Eissen, J-P., Caprais, M-P., Gente, P., Gueneley, S., Harmegnies, G., Lagabrielle,Y., Lapouille, A., Lefevre, C., Maillet, P., Maze, J-P., Ondreas, H., Schaaf, A., Singh R. 1986:Accretion Oceanique et Deformation dans la Partie Meridionale du Bassin Nord-Fidjien: ResultatsPreliminaires de la Campagne Oceanographique SEAPSO III du N.O. Jean Charcot (December,

1985). Comptes Rendus de l'Academie des Sciences, Paris, Series ll, 303(1): 93-98.

hydrothermalism in the North Fiji Basin (SW Pacific). Results of Japanese French Cruise Kaiyo87. Marine Geophysical Research 12: 269-283.

Auzende, J-M., Honza, E., and Shipboard scientific party, 1990: Active spreading and

Bush, S. 1990: Gold found in Lau back-Arc sulfides. EOS 71(38): 1083.

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