21. ASPECTS OF THE GEOLOGY OF THE EASTERN CORAL SEAAND THE WESTERN NEW HEBRIDES BASIN

Charles W. Landmesser and James E. Andrews, Department of Oceanography,University of Hawaii, Honolulu, Hawaii

andGordon H. Packham, Department of Geology and Geophysics, University of Sydney, NSW,

Australia

ABSTRACTThe submarine geology of the eastern Coral Sea and New

Hebrides Basins in the southwest Pacific is characterized by complexbathymetry, comprising numerous geomorphological and structuralfeatures related to the tectonic setting of the region near the marginof the India plate. Most of the main structural features of the regionappear to be of Eocene age.

Miocene and younger sediments in the eastern Coral Sea are tur-bidity current deposits on the Coral Sea Abyssal Plain and pelagicbiogenic sediments and abyssal clays in other parts of the basin.Eocene and Oligocene sediments are probably biogenic oozes. Theoldest sediments of the New Hebrides Basin are mid Eocene toOligocene biogenic sediments and Miocene abyssal clays. Elevationof various ridges later in the history of the region and the approachof the New Hebrides Ridge have been responsible for the shedding ofelastics into the basin.

The major physiographic features of the region are the LouisiadeRise in the eastern Coral Sea, the Rennell Island and East RennellIsland ridges between the Coral Sea and the New Hebrides Basin, theRennell Trough on the southeast side of the Rennell Island Ridge,and the d'Entrecasteaux Fracture Zone in the New Hebrides Basin.The nature and age of some of these features are ambiguous.

The Louisiade Rise is cut by a fracture system. The rise may beeither submerged continental crust or an old spreading ridge inwhich the crust was thickened during decreased spreading associatedwith its demise as an active feature. The fracture zone is probably atransform fault (system) related to the opening of the Coral Sea.

The Rennell Island and East Rennell Island ridges may befragments of the Mesozoic fold belt that margins much of easternGondwanaland, or they may be in part or entirely built of island arcvolcanics related to the opening of the Coral Sea.

The Rennell Fracture Zone may have been formed during theCoral Sea-New Hebrides Basin development as a transform faultbetween them, or it could have resulted from the relocation of theIndia-Pacific plate boundary that formed in the early Oligocene afterobduction took place in New Caledonia.

INTRODUCTION

The purpose of this chapter is to review the availabledata on the highly complex eastern part of the Coral Seaand the western part of the New Hebrides Basin. Unfor-tunately, the present state of knowledge leaves most ofthe critical questions of the region unresolved, but it ishoped that this review will highlight the more importantproblems and stimulate further investigations into themarine geology of the region. Some of the implicationsof this review are further discussed in the regional syn-thesis chapter (Packham and Andrews, this volume).

The Coral Sea Basin (Figure 1) is one of severalmarginal basins in the complex structure of the

southwest Pacific. As defined by Wiseman and Ovey(1954), the Basin is bounded by the Queensland Plateauand Australian continental margin on the southwest, bythe margin of Papua-New Guinea, the Papuan Plateauand the Louisiade Archipelago on the northwest, and bythe Solomon Islands and South Solomon Trench on thenorth, while a major NNE-SSW trending ridge, theRennell Ridge of Krause (1967), separates the Coral SeaBasin from the New Hebrides Basin to the east.

The Coral Sea Basin presently forms part of thenortheastern margin of the India plate, directly adjacentto the western boundary of the Pacific plate.

In order to more clearly define the structural units ofthe Coral Sea Basin, a detailed bathymetric chart has

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C.W. LANDMESSER, J.E. ANDREWS, G.H. PACKHAM

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been constructed. Depths are based on a sound velocityof 1500 m/sec and are uncorrected for variations. Gard-ner (1970) has presented a chart of the western CoralSea, and the chart presented here encompasses thegreater part of the eastern Coral Sea.

Bathymetric data utilized include the most recentcompilation of soundings by the Hydrographic Office ofthe Royal Australian Navy on their GEBCO 1:1,000,000plotting sheet series, bathymetry along tracks of theM/V Taranui, R/V Mahi, and R/V Kana Keoki ob-tained by the Hawaii Institute of Geophysics,bathymetric data from the NOVA Expedition of ScrippsInstitution of Oceanography, and bathymetry alongtracks of D/V Glomar Challenger during Legs 21 and 30.Seismic profiles used in this work are from HawaiiInstitute of Geophysics cruises of the R/V Kana Keoki,R/V Mahi, and R/V Vema, and from Legs 21 and 30 ofD/V Glomar Challenger.

BATHYMETRY AND PHYSIOGRAPHY

Bathymetry of the eastern Coral Sea Basin ispresented in Chart 1 (in back pocket of volume). Basedon these bathymetric contours and several previousstudies, a number of physiographic provinces have beendefined in the Coral Sea Basin. The location of thesefeatures is shown in Figure 2.

The western Coral Sea Basin is dominated by theCoral Sea Abyssal Plain, as previously discussed byGardner (1970) and Ewing et al. (1970). Thisphysiographic province is approximately defined by the4500-meter contour, and extends in an ESE directionbetween latitudes 12°S and 16°S and between longitudes148°E and 155°E. To the northwest of the Coral SeaAbyssal Plain lies the Papuan Plateau, adjacent to thecontinental slope of Papua. To the southwest, theQueensland Plateau is situated marginal to northeastern

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GEOLOGY OF EASTERN CORAL SEA AND WESTERN NEW HEBRIDES BASIN

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Figure 2. Physiographic provinces in the vicinity of the eastern Coral Sea Basin, southwest Pacific, and locations of seismicprofiles.

Australia. The seaward extent of the Papuan Plateau isdefined approximately by the 2500-meter contour, whilethe Queensland Plateau, separated from the Australiancontinental margin by the Queensland Trough, is de-fined by the 1500-meter contour. Sediment forming theCoral Sea Abyssal Plain has been transporteddownslope from New Guinea (Burns, Andrews, et al.,1973; Burns and Andrews, 1973) via several large sub-marine canyon systems which have been discussed byWinterer (1970).

The greater part of the Coral Sea Basin (east of theabyssal plain), extending northeastward to the SouthSolomon Trench and eastward to the Rennell IslandRidge, is characterized by complex bathymetry and avariety of morphologic features. Terrigenous sedimenta-tion has been generally lacking, and consequently,primary features are only slightly subdued. This is inmarked contrast to their almost complete burial beneaththe Coral Sea Abyssal Plain. Along the northwesternmargin of this area, the Pocklington Trough trends NE-SW along the eastern margin of the LouisiadeArchipelago. This feature was first described by Krause(1967). Depths are in excess of 3500 meters, with someareas exceeding 4000 meters water depth at thesouthwestern and northeastern ends of the Trough.

The bathymetric high between latitudes 10°S and12°S adjacent to the northern Pocklington Trough hasbeen identified as the Pocklington Ridge (Krause, 1967).This ridge trends ENE-WSW. Its extent is defined ap-proximately by the 2000-meter contour.

Continuing along the northern boundary of the CoralSea Basin, the sinuous South Solomon Trench occupiesthe area adjacent to the southern margins ofGuadalcanal and San Cristobal islands. This narrowdeep trends generally ESE from 158°E to 162°Elongitude, at which point the trend changes to ENE un-til termination of the trench at approximately 165°Elongitude. Depths in the South Solomon Trenchgenerally exceed 4500 meters, while maximum waterdepths greater than 8000 meters are encountered southof San Cristobal. Numerous submarine troughs orcanyons are located on both sides of the trench, withfaulting and fracturing playing a dominant role in shap-ing the trench walls.

Separating the Coral Sea and New Hebrides basins isa prominent bathymetric high defined by the 2500-metercontour, which trends NNW-SSE between latitudes11°S and 13°S and continues along a southerly trend toapproximately 16°S. This ridge was first called theRennell Island Arc by Glaessner (1950) and has beenmore recently referred to as the Rennell Island Ridge byKrause (1967). The coral atoll of Rennell Island issituated on this ridge. Other atolls occur along the crestof the ridge, including Bellona Island to the northwest ofRennell Island and the Indispensable Reefs to the south.A number of major and minor offsets which are relatedto major NE-SW trending fracture zones and associatedminor transcurrent faults may be noted along theRennell Island Ridge, as described by Taylor (1973).

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C.W. LANDMESSER, J.E. ANDREWS, G.H. PACKHAM

The numerous minor offsets perpendicular to the majorSSW-NNE trend are characteristic of the fracturepattern developed throughout the area. Several largesubmarine canyons along the flanks of the RennellIsland Ridge exhibit a SSE-NNW trend, also suggestiveof structural control related to the intersecting fractures.

A complex series of ridges and troughs trending SSW-NNE is located to the east of the Rennell Island Ridgeand a second ridge that may be its easterly extension. Tothe east and south of this second ridge is the moreregular sea floor of the New Hebrides Basin. The ridgesare defined by the 2500-meter contour, while the 4000-meter contour outlines the narrow troughs. The easternridge, here called the "East Rennell Island Ridge,"diverges northwards from the southern limit of theRennell Island Ridge. Between 14° and 13°S it trendseast-west and extends eastwards to the western wall ofthe New Hebrides Trench north of Espiritu Santo.

The New Hebrides Basin sea floor between this ridgeand the d'Entrecasteaux Fracture Zone is smoother thanthat to the east of the fracture zone and that to the northof the ridge. Site 286 was located in the rougher sea flooreast of the fracture zone and south of the East RennellIsland Ridge.

The western part of the New Hebrides Basin isseparated from the northern end of the New CaledoniaBasin by a broad ridge extending from the northern endof the Lord Howe Rise to the northern end of the ridgeon which New Caledonia lies.

Parallel to the western margin of the Rennell IslandRidge, a major trough is outlined by the 3000-metercontour. The name "Rennell Trough" is used herein forthis feature, which has maximum depths greater than4000 meters.

The area between the Pocklington Trough to the westand the Rennell Trough to the east is characterized bytwo WNW-ESE trending rises collectively called theLouisiade Rise. The northwestern part is outlined by the2000-meter contour. The northeastern flank of theLouisiade Rise is bounded by the Rennell Trough. Alarge, elongate deep, trending WSW-ENE, terminatesthe northwestern segment of the Louisiade Rise on thesoutheast. This deep is defined by the 4000-meter con-tour. Continuing to the southeast, the other bathymetricsmaller high is encountered, with depths rangingbetween 2000 and 3000 meters. This rise, regarded as thesoutheastern continuation of the Louisiade Rise to thenorthwest, is terminated on the east by the southern partof the Rennell Trough. Its southwestern margin is in-distinct, merging into a series of broad ridges andtroughs.

ANALYSIS OF SEISMIC REFLECTION PROFILESBased on continuous seismic reflection profiles, the

structure and sediment distribution across a number ofthe physiographic features in the eastern Coral SeaBasin have been defined. The results of this analysis arepresented in Chart 2 (in back pocket of volume) and dis-cussed below. In certain areas where profiles are notavailable, structural trends have been inferred fromdetailed bathymetry (Chart 1). In the following discus-

sion, total reflection time to a reflector is used ratherthan subbottom reflection time. The location of seismicreflection profiles presented herein is shown in Figure 2.

Coral Sea Abyssal PlainIn the Coral Sea Abyssal Plain, a thick sequence of

acoustically stratified turbidites overlies a prominentreflector at approximately 6.8 sec. This reflector, iden-tified by Andrews (1973) as the top of mid-Oligocene tolate Oligocene nannofossil chalk cored at DSDP Site210 (Figure 3), shows moderate relief suggestive ofrough basement topography. A deeper reflector appearsin some areas, and may be traced to the basement ridgein the center of Profile AA' (Figure 4). Based on a com-parison of sedimentation at DSDP Sites 210 and 287,this basement high was formed prior to early Oligocene,possibly during initial development of oceanic crust inthe early Eocene (see Chapter 5, this volume). Similarbasement structures appear on other reflection profilesacross the area (Ewing et al., 1970), and the site surveyat DSDP Site 287 suggests an en echelon pattern ofdevelopment.

On the western end of the profile at the foot of theQueensland Plateau, a transparent sequence is overlainby a succession containing a number of reflectors. Inthis region a basement reflector is clearly visible. It risesto the east. This situation along the edge of the plateauhas been interpreted by Falvey and Taylor (1974) largelyon the basis of seismic refraction and gravity data. Theyhave interpreted the basement at the foot of the slope asthe outer edge of the continental crust. The overlyingacoustically transparent section would represent LateCretaceous to Paleocene sediments deposited in riftsformed immediately prior to the development of theCoral Sea. The upper sequence containing the frequentreflectors would then be the Eocene and younger se-quence deposited after breakup and subsidence of thecontinental margin.

The eastern boundary of the Coral Sea Abyssal Plainis marked by a basement ridge, which extends south ofthe Louisiade Rise (Figure 4). In Profile AA' (Figure 4),the thick abyssal plain turbidites appear to overlie aseries of slump deposits at the base of the ridge. This un-conformity suggests that slumping from the ridgepredates deposition of the turbidites (mid Miocene).Moreover, ponded sediments covering the faulted base-ment near the top of the ridge postdate basementfaulting. The date of faulting of this structure is difficultto determine, it might have been after the accumulation ofa significant thickness of biogenic sediments or slum-ping might have taken place from time to time as sedi-ment accumulated on the faulted surface. The faulting isprobably related to a broad fracture zone trending NE-SW toward the Rennell Island Ridge.

Near the northeastern margin of the Coral SeaAbyssal Plain, a NW-SE trending ridge is defined bybathymetric contours on Chart 1. Ewing et al. (1970)have presented a profile across this structure, showing itto be a basement feature, characterized by normalfaulting toward the southwest and possible tilting of thebasement surface and overlying sediments toward the

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GEOLOGY OF EASTERN CORAL SEA AND WESTERN NEW HEBRIDES BASIN

northeast. Its development may be related to down-faulting on the southwestern flank of the LouisiadeRise.

Tectonic activity postdating the initiation of turbiditedeposition is indicated in Profiles AA', BB', and CC,(Figures 4, 5, and 6) in which the surface of the tur-bidites and some of the deeper layers show disturbance.This is particularly apparent in Profile BB' in which tur-bidites extend some distance up the slope of theLouisiade Rise indicating elevation of the rise.

Louisiade RiseThe Louisiade Rise (Figure 5) appears to be both a

major physiographic and structural feature in theeastern Coral Sea Basin. The flanks of the rise arecharacterized by normal faulting away from the risecrest, and piercement structures are present in severalareas. These structures suggest that tensional stress hasplayed the dominant role in forming the Louisiade Rise.

A thick, moderately stratified sedimentary sequenceoverlies basement, and based on interpolation fromDSDP Sites 210 and 287, is probably dominated bycalcareous ooze resulting from pelagic deposition sinceEocene time. The prominent reflector at 0.3 sec belowthe sediment surface may be within the Eocene. Smalltroughs on the rise contain up to 0.7 sec of sediment(two-way travel time). The character of the profilesacross the rise resemble those across the QueenslandPlateau. The small troughs of sediment there have beeninterpreted by Falvey and Taylor (1974) as LateCretaceous to Paleocene clastic sediments.

The late Eocene-early Oligocene regional unconfor-mity reported by Burns and Andrews (1973) wouldalso be expected in the stratigraphic section of theLouisiade Rise. Erosional features characterize the pre-sent surface of the rise and are particularly evident nearits crest where numerous scour channels may be seen.Subsurface reflectors appear to be truncated along thesouthwestern margin, creating an unconformity whichhas probably resulted from bottom current scour orfrom turbidity currents moving downslope from thecrest of the rise toward the Coral Sea Abyssal Plain.Interbeds of nannofossil ooze with Oligocene formsreported in the turbidite sequence at DSDP Site 210(Burns, Andrews, et al., 1973) and Site 287 (see Chapter5, this volume) may possibly be traced to a source areaon the Louisiade Rise.

The extension of the rise to the southeast (Charts 1and 2) appears to be similar in character to thenorthwestern Louisiade Rise (Profile CC, Figure 6 andProfile DD', Figure 7, center). The sedimentary sectionis comprised of moderately stratified deposits, probablydominated by pelagic biogenic components. Reflectorswithin the sedimentary column dip away from the risecrest and appear truncated at the rise crest whereerosional features are dominant. Erosion has left only athin veneer of sediment covering basement near the risecrest, making the area a favorable site for future sampl-ing.

A fault bounded trough trending NE-SW appears toseparate the Louisiade Rise proper to the northwestfrom this rise segment to the southwest (Charts 1 and 2).

Profile DD' (Figure 7) shows a section across thistrough (left) and the flank of the rise (center). A com-plete section across this area is presented in Profile CC(Figure 6) (center). The eastern side of the trough ischaracterized by several basement ridges with"deformed" sediment between.

The NE-SW trending trough may in part represent afracture zone that is herein named the Louisiade Frac-ture Zone. The basement ridges along the troughmargins appear to trend NNE-SSW, their abundanceand distribution suggests that they may be a series of"feather fractures," oblique to the main fracture andsimilar to the type discussed by Andrews (1971) in thenortheastern Pacific. In this case the main fracture,probably a transform fault, probably trends as indicatedon Chart 2, or alternatively the small faults maythemselves represent a series of small transform faults.Geometrically this latter seems more likely (see below).

Normal faults are also abundant, surrounding bothrises (Chart 2), and probably represent secondary struc-tures developed prior to significant sediment accumula-tion. Profile EE'E" in Figure 8 shows a section acrossthe northeastern margin of the Louisiade Rise and thenorthern extension of the Louisiade Fracture Zone. Anumber of well-defined grabens dominate the area, andundeformed sediments suggest that these basementstructures developed early in the geological history ofthe region.

Whether the crust constituting these rises is oceanic orcontinental cannot be determined with certainty on thebasis of the available data. If Falvey and Taylor (1974)are correct in suggesting that continental crust extendsdown to the foot of the Queensland Plateau into waterdepths of around 4000 meters, then it is possible that thewhole of this rise region could be continental. It mightfurther be pointed out that the rises have water depthscomparable with that of the Papuan Plateau. Only thetrough between the two rise segments is of a depth com-parable to that of the Coral Sea Abyssal Plain. Asalready stated, the sediments on the Louisiade Rise havesimilar structure and seismic character to those on theQueensland Plateau. On a continental crust interpreta-tion the Louisiade Rise represents an extension of theQueensland Plateau crust rifted off during the formationof the Coral Sea, and the fracture zone is the result of atransform fault passing between two blocks of continen-tal crust possibly related in some way to complexities inthe development of the Coral Sea crust.

The equally uncertain alternative view is that the risesare underlain by oceanic crust. It was suggested that therise is covered by Eocene sediments and should have anage (if oceanic) generally comparable to that of theCoral Sea. The depth of the rise is , however, very muchshallower than would be expected for its age. Sparsemagnetic data are available from the region and thoseobserved on the crossing of this feature on Leg 30 ofDSDP are shown in Figure 5. The anomalies show somesemblance of symmetry about the ridge crest. If onestarts with the established age of the sea floor at Site 287(about anomaly 21) then the age at the ridge crest coun-ting the anomalies is 17 (42 m.y.). There is a roughmatch between these anomalies and the observed shapes

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GEOLOGY OF EASTERN CORAL SEA AND WESTERN NEW HEBRIDES BASIN

and spacings. While there is a great deal of uncertaintyattached to this interpretation, it provides a speculativeexplanation of the origin of the sea floor. The halfspreading rate would be less than 2 cm/yr. Continuationof spreading at this rate would give the basin a max-imum age of 63 m.y. (early Paleocene). The shallowdepth of the rise may be explained by the formation ofthicker crust associated with the cessation of spreading.However, the argument must be regarded as highlytenuous since when an attempt was made to correlatethese anomalies observed on Leg 30 with the onesavailable in the area from the Nova Expedition, it wasunsuccessful. On the oceanic crust spreading ridge inter-pretation, the two rise segments would form part of anold spreading ridge separated by a transform fault. ThePocklington Trough would probably represent anothertransform feature being essentially parallel to theLouisiade Fracture Zone. The Louisiade Ridge has noequivalent west of the Pocklington Trough, nor is therea symmetrically disposed segment of sea floor north ofthe Louisiade Rise corresponding to the abyssal plainsea floor between the Louisiade Rise and theQueensland Plateau. A fanciful explanation for theserelationships is that spreading ceased in the westernCoral Sea, and the spreading ridge moved to theSolomon Sea north of the Papuan Peninsula whilespreading continued in the western Coral Sea. The crustequivalent to that under the Coral Sea Abyssal Plainmay have been lost by subduction under the RennellIsland Ridge or in the South Solomon Trench. Thepossibility of asymmetrical spreading cannot be ruledout. The Pocklington Trough is probably more com-plicated than a simple transform feature since it hasassociated with it a negative free air gravity anomaly inexcess of 100 milligals (Falvey and Taylor, 1974).

Pocklington Ridge

Sections across the ENE-WSW trending PocklingtonRidge are shown in Profile BB' (Figure 5) northeast ofthe Louisiade Rise, and in greater detail in Profile FF'(Figure 9). This feature is characterized by a faultedbasement surface, overlain by sediment of variablethickness. Basement outcrops appear in a number ofareas. Normal faulting is dominant on the flanks of theridge, with fault planes dipping away from the ridgecrest (Chart 2). The sedimentary sequence is generallymuch thinner than on the Louisiade Rise, suggestingthat the Pocklington Ridge is a more recent structuralfeature or has been subject to more current scouring.

A further problem exists deriving from the form of thewestern Coral Sea. Its shape suggests that for simpleopening the pole of rotation would have lain in centralor western Papua-New Guinea. Transform faults in thevicinity of the Louisiade Rise should trend at about 30°and the spreading ridge at about 120°. These directionsare at variance with those shown on Chart 2. The smallfaults parallel to the trough between the two sections ofthe Louisiade Rise are close to that of the predictedtransform and the Rennell Fracture Zone (see discus-sion below). The axis of the suggested spreading ridgeshown on the chart is based on the topographic ridgetrend.

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GEOLOGY OF EASTERN CORAL SEA AND WESTERN NEW HEBRIDES BASIN

The Pocklington Ridge may be a constructive featureformed as a result of submarine volcanism, alternativelyit may be a submerged extension of the Mesozoicmetamorphics outcropping in the LouisiadeArchipelago west of the Pocklington Trough. The Leg30 DSDP magnetic profile showed low amptitude shortwavelength anomalies over the ridge. This would tend tosupport the latter view.

The northern surface of the Pocklington Ridge ischaracterized by a number of shallow normal faults(Figure 9, Profile FF' right of center). These faults areprobably related to tension in the upper lithosphereproduced by crustal warping in response to subductionalong the South Solomon Trench.

Rennell Island Ridge—East RennellIsland Ridge—Rennell Trough

The Rennell Island Ridge, forming the eastern boun-dary of the Coral Sea Basin, appears on reflectionprofiles to vary in structure and morphology progress-ing from north to south. Sections across the northernpart of the ridge, between 11°S and 12°20'S, are shownin Figures 6, Profile CC, and 10, Profile GG' (right ofcenter). Sediment is generally lacking on the relativelysteep flank of the ridge, while coral growth has occurredalong its crest, as indicated by the presence of the coralatolls of Bellona Island, Rennell Island, and Indispen-sable Reefs. Taylor (1973) has suggested that more than500 meters of carbonate sediments and reef limestoneshave been deposited over basement during general sub-sidence of the ridge since its initial formation. However,in Figures 6 (Profile CC) and 10 (Profile GG'), coralgrowth appears limited on the ridge crest, indicatingthat certain areas have been well below sea level since itsdevelopment or it subsided rapidly. The relatively steepwestern slope of the Rennell Island Ridge is character-ized by normal faulting toward the west.

The Rennell Trough, adjacent to the western marginof the ridge, is floored by flat-lying stratified sediments,over-lying an acoustically transparent sedimentary sec-tion that more closely follows the basement configura-tion. This relationship is especially well defined inFigure 6 (Profile CC right of center) which crossesperpendicular to the NNW-SSE structural trend. Awedge of the deeper sediments outcrops at the base ofthe Rennell Ridge, creating an angular unconformitywith the overlying horizontal sediments so that therelationship resembles that on the eastern side of theCoral Sea Abyssal Plain on Profile AA" (Figure 4). Thestructure of the lower (transparent) sediments may bedue to northeast compression that has taken place sincetheir deposition or could have resulted from slumping ofthe older sequence.

South of 15°S the Rennell Trough is shallower than itis to the north, but a similar relationship may be seen inthe sedimentary section. Figure 7 (Profile DD'D")presents a section across the southeastern Coral SeaBasin, crossing a part of the southern Rennell Trough-Rennell Island Ridge. A thin layer of stratifiedsediments lies unconformably on acousticallytransparent deposits.

The southern Rennell Island Ridge appears to have aslightly different structure than that discussed above forthe northern ridge, probably due to tectonic overprin-ting. A section across the southern part of the ridge isshown in Figure 7 (Profile DD'D"). Here, the ridge isbisected into the Rennell Ridge and the East RennellIsland Ridge by a large graben feature. Normal faultingtoward the axis of the trough is dominant. A piercementstructure may be seen intruding ponded sediments onthe right (ESE) flank of the trough, suggesting magmaticactivity along parts of the ridge. When located on thebathymetric chart of the area (Chart 1), this troughappears to be one of a complex series of troughs andassociated ridges (collectively the Rennell FractureZone) trending NE-SW, toward the South SolomonTrench. The northern part of this complex, north of15°S, is broader than to the south, with several paralleltroughs commonly offset along NW-SE linaments,perpendicular to their NE-SW trend (Chart 1). A sectionacross this region is presented in Figure 6 (Profile CC),east of the Rennell Island Ridge. It crosses the northernflank of the East Rennell Island Ridge. Sedimentswithin the troughs appear to have been deformed show-ing relationships similar to those in the Rennell Trough.

It is apparent from the bathymetric map and theprofiles across the Rennell Trough that only thenorthern part south of Rennell Island reaches oceanicdepths (< 4000 m) the remainder is less than 3500meters deep. The elevation of the shallower part of thetrough relative to New Hebrides Basin is shown inProfiles CC and D'D" (Figures 6 and 7). This being so,only the crust of the northern part can be taken asalmost certainly oceanic. The southern part is subject tothe same ambiguity as the Louisiade Rise—it could beunderlain by oceanic or continental crust.

The Rennell and East Rennell Island ridges are suf-ficiently shallow to make them unlikely to be oceanic orsubstantially so. Two main alternatives that may applyto all or part of the ridge complex can be proposed. Thesimplest possibility is that the ridges represent disruptedfragments of the Gondwanaland continental marginprobably composed of Mesozoic sialic metamorphics,linking the New Caledonia metamorphics to those in theLouisiade Islands and the Papuan Peninsula. The se-cond is that the ridge may represent a segment of theEocene volcanic arc that supplied the volcanogenic tur-bidites to Site 286 in the New Hebrides Basin; the sub-duction zone would have presumably been on thenorthern side.

The Rennell Fracture Zone that separates the RennellIsland Ridge and the East Rennell Island Ridge is afeature that trends at 30° in the area of Chart 1. Thedeep troughs and evidence of magmatic activity along itsuggest that it contains areas of oceanic crust. The mostlikely interpretation of the structure is a transform fault.The trend of this structure to the south is crucial inattempting to determine its age and tectonicsignificance. It is possible that the structure continues onmuch the same trend and links up with the fractures in-tersected on the Leg 21 profile north of the DampierRidge (see figure 3, Burns and Andrews, 1973), or

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that it swings towards a more easterly trend and linkswith fractures seen on the same profile on the southernmargin of the Queensland Plateau.

In the former case the trend is compatible with thetransform direction between the Pacific and India platesfor the interval 29 to 38 m.y. (latest Eocene to earlyOligocene), as determined by Packham and Terrill (thisvolume). If this transform was part of the plate boun-dary during that interval, then the subduction of part ofthe Tasman Sea under eastern Australia as suggested byHayes and Ringis (1973) could be a consequence of sucha location for the Pacific-India plate boundary.

This scheme would be in agreement with their obser-vation that the amount of subduction decreasessouthwards since the pole is located at about 163°W,43°S. South of Australia the motion would probably bea transform fault.

The plate boundary immediately prior to this wasprobably located in New Caledonia and the d'En-trecasteaux Fracture Zone where obduction andtransform motion are suggested (see below).

The second alternative interpretation of the RennellFracture Zone is that it was a transform fault formedbetween two ridge segments during the opening of theCoral Sea, linking a spreading axis in the New HebridesBasin with one in the Coral Sea Basin. In this case theminor faults crossing the Louisiade Rise would also betransform faults and the spreading axis, if located on theLouisiade Rise, should be perpendicular to them. Thewedge-shaped form of the western Coral Sea indicates,as stated previously, that the pole of opening waslocated not far away, probably in central or westernPapua-New Guinea. The trend of the transform faultswould then be expected to trend in a more easterly direc-tion when followed to the south.

The significance of the northern part of RennellTrough is even more difficult to establish than that ofthe other features discussed. A wide number ofpossibilities are open. Two are: (a) it may constitute asmall patch of sea floor developed during the develop-ment of the Coral Sea north of the Louisiade Rise con-tinental crust, or (b) it may be a small patch of deeperoceanic crust corresponding to the Coral Sea AbyssalPlain developed during the earlier more rapid spreadingfrom the Louisiade spreading center.

The tentative suggestion is made on Chart 2, on thebasis of the possible deformed sediment in the trough,that this may be a zone of underthrusting. However, theconfiguration of the basement in the profiles does notconfirm this hypothesis. The fault indicated on the mapis the maximum extent it could have had. Some con-vergence in the northern section of the trough between itand the Rennell Island Ridge is geometrically possible inthe Oligocene if the Rennell Trough was part of thePacific-India plate boundary. It could have been part ofthe convergent plate boundary that may have been pre-sent in New Guinea at this time.

New Hebridges Basin andd'Entrecasteaux Fracture Zone

In the New Hebrides Basin to the east of the d'En-trecasteaux Fracture Zone an Eocene basement age hasbeen established at DSDP Site 286 (Figure 3) near theNew Hebrides Trench.

The sea floor east of the fracture zone has a cover ofEocene volcaniclastic turbidites that have been de-formed. These are overlain by late Eocene to Oligocenebiogenic ooze, then by Miocene abyssal clay. Overlyingthese are clastic Pliocene to Plesitocene sediments. ThePliocene and Pleistocene sediments are flat lying and fillhollows. It is not known whether the oozes and abyssalclays are unconformable on the turbidites. Packham andTerrill (this volume) suggest that the deformation tookplace in the early Oligocene when there was an activeplate boundary in the region as indicated by the obduc-tion of ultramafic bodies onto New Caledonia. Thed'Entrecasteaux Fracture Zone discussed below couldhave been part of this same plate boundary. The latestdate possible for the deformation is Miocene, after thedeposition of the abyssal clay.

The d'Entrecasteaux Fracture Zone is expressed onthe Leg 30 DSDP crossing as an east-facing scarp 2000meters high (see Chart 1). The deepest sediments in thenarrow trough on the east of the scarp dip to the west,suggesting underthrusting from the east. Latersediments in the trough are flat lying. To the west of thescarp, the sea floor and its sedimentary cover slope gent-ly to the west. Flat-lying sediments (presumed to be tur-bidites) occupy some of the depressions. Thebathymetry suggests that the flat-lying sediments hereand those in the trough immediately east of the fracturezone were probably derived from the New Caledoniaand Loyalty ridges. The Pliocene and Pleistocenesediments at Site 287 were derived from the NewHebrides. The quality of the seismic records available isnot sufficiently good to say whether the sedimentsresting on the basement west of the d'EntrecasteauxFracture Zone are an extension of the volcaniclastic tur-bidites drilled at Site 286. The character of the basementon each side of the fracture zone is, however, quitedifferent; on the east the wavelength of basementfeatures is 10 to 20 km while on the west it is over 50 km.This difference may be attributed the location of theplate boundary at the fracture zone at the time of the ob-duction of sea floor onto New Caledonia so that onlythe east was deformed on a small scale.

SUMMARY OF THE TECTONIC HISTORY

Formation of the Coral Sea Abyssal Plain and theNew Hebrides Basin sea floor took place in the Eocene.In the Coral Sea region rifting of the Australian con-tinental margin took place. Spreading may havecentered on the Abyssal Plain extending south into theCato Trough-Mellish Reef area off the Queenslandcoast. Additional small areas of oceanic crust havedeveloped (the Rennell Trough and the area of sea floorbetween the two sections of the Louisiade Rise). TheLouisiade Rise could be an area of submerged continen-tal crust. The Rennell Island Ridge and the East RennellIsland Ridge could also be continental or be partly orcompletely built up of island arc volcanics with a trenchformerly developed to the north. This is suggestedbecause of the thick volcanogenic turbidite sequence inthe New Hebrides Basin. If any part of these ridges wasan island arc, it is more likely to be the East RennellIsland Ridge. The Rennell Fracture Zone could be amajor fracture zone separating the Coral Sea and the

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New Hebrides Basin spreading centers. The geometry ofmotion is apparently generally consistent with the poleof opening of the Tasman Sea.

An alternative interpretation of the Louisiade Rise isthat it is a spreading ridge cut into two main segmentsby transform faults.

The Eocene sediments of the Coral Sea are biogenicwith a minor terriginous content. These biogenicsediments occur on the rises and beneath the abyssalplain. Island arc volcanics provided Eocenevolcaniclastic turbidites to the New Hebrides Basin inthe middle Eocene with the volcanism dying out in thelate Eocene.

OligoceneThe Pacific-India plate boundary was located in New

Caledonia in the early Oligocene and the convergentplate motion obducted oceanic crust and upper mantlematerial onto New Caledonia. The d'EntrecasteauxFracture Zone could have been essentially a transformfault section of this plate boundary with minor un-derthrusting. Possibly contemporaneous with the ob-duction was deformation of the sea floor of the NewHebrides Basin and the elevation of the Loyalty Ridge.The deformation stops abruptly at the d'EntrecasteauxFracture Zone. To the west the convergent plate boun-dary may have gone along the south side of the east-westpart of the Rennell Island and East Rennell IslandRidges as a short-lived subduction zone linking even-tually with the obduction zone of the PapuanUltramafic Belt perhaps via the Pocklington Trough.The overthrusting may have occurred at this time (D.Dow, personal communication) rathe*- than in theEocene as suggested by Davies and Smith (1971).

As an alternative to the origin suggested above, theRennell Fracture Zone may have been formed in theOligocene by relocation of the India-Pacific plate boun-dary after the obduction occurred in New Caledonia.The fracture zone would have extended to the easternAustralian coast and then the plate boundary extendedalong the eastern Australian coast southwards. The ex-pected convergent plate motion that took place with thiscould explain the subduction of part of the Tasman seafloor under eastern Australia suggested by Hayes andRingis (1973).

The sediments deposited at this time werepredominantly biogenic. A hiatus in deposition at-tributable to the distribution of oceanic currents(Kennett et al., 1972) occurs in the Coral Sea-LordHowe Rise-Queensland Plateau region but not in theNew Hebrides Basin presumably due to a physiographicbarrier to deep currents. Some clastic sediments mayhave been deposited near the Rennell Ridges and nearNew Caledonia if they were elevated above sea level.

Miocene and Later

Subsidence of the sea floor of the Coral Sea AbyssalPlain and much of the New Hebrides Basin below thedepth of total dissolution of nannofossils resulted in thedeposition of abyssal clays. In the Coral Sea AbyssalPlain deposits the abyssal clay is followed in the middleMiocene by the onset of turbidite deposition which is

still continuing. The elastics were derived from the up-lifted regions of the Papuan Peninsula and perhaps asfar afield as northern New Guinea. Some uplift of theabyssal plain sediments is evident on the southern flankof the Louisiade Rise and the eastern part of the abyssalplain.

Clastic sediments have also been derived from NewCaledonia, the Loyalty Islands, and Rennell Island anddeposited in the New Hebrides Basin and RennellTrough. These sediments probably contain a significantbioclastic component coming from the reef complexesdeveloped around these islands and on the adjacentridges.

Early Pliocene to Pleistocene sediments also cameinto the New Hebrides Basin from the east with an in-creasing rate of sedimentation as the New HebridesIslands moved westwards due to the convergence of thePacific and India plates and the development of the FijiPlateau. Shallow water detritus in the early Pleistocenesediments indicates the partial blocking of the NewHebrides Trench from at least that time onwards.

ACKNOWLEDGMENTSThe authors are grateful to the many individuals who

devoted their time and effort acquiring and reducing ship-board data and preparation of charts. We would like toacknowledge the benefit we have gained from discussions withcolleagues, notably Drs. L.W. Kroenke and D.A. Falvey.Special thanks are extended to Dr. John I. Ewing of Lamont-Doherty Geological Observatory for seismic reflection profilesobtained during cruises of the R/V Vema in 1966, and to TheHydrographer, Royal Australian Navy, for contributing muchof the bathymetric data used in constructing charts of theeastern Coral Sea Basin. Thanks are also extended to Mr.Richard Rhodes and the staff of the Hawaii Institute ofGeophysics graphic department for drafting charts and il-lustrations. This study was made possible by support from theOffice of Naval Research and the National Science Founda-tion.

REFERENCESAndrews, J.E., 1971. Abyssal hills as evidence of transcurrent

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Gardner, J.V., 1970. Submarine geology of the western CoralSea: Geol. Soc. Am. Bull., v. 81, p. 856-881.

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