26. results of leg 30 and the geologic history of the southwest … · hebrides trench east of site...

26. RESULTS OF LEG 30 AND THE GEOLOGIC HISTORY OF THESOUTHWEST PACIFIC ARC AND MARGINAL SEA COMPLEX

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

James E. Andrews, Department of Oceanography, University of Hawaii, Honolulu, Hawaii

INTRODUCTION

To a large degree, Leg 30 was concerned with theorigins of the western Pacific marginal basins, lyingwithin the confines of the India plate (Figure 1). On thebathymetric map of the southwest Pacific there is abewildering pattern of basins and ridges with smallisland chains that lie near the junction between the Indiaand Pacific plates. These features evolved in lateMesozoic and Cenozoic.

In the last few years, several hypotheses for the originof marginal seas have been proposed (Karig, 1971;Packham and Falvey, 1971; Moberly, 1972). All of thesehypotheses propose that arc migration is in some wayrelated to the formation of the new oceanic crust of thebasins, and that the genesis of this crust was differentfrom that of mid-ocean ridges although the seismiclayering of the crust was similar to that of the oceanbasins. Karig (1971) proposed that the splitting of islandarcs was responsible for the formation of the basins, thedriving force being a "thermal diapir" rising from thebenioff Zone associated with the arc. Packham andFalvey (1971) suggested that the crust was formed by theinjection of the asthenosphere into the region behind thearc moving the arc forward. The spreading wouldprobably be asymmetrical with the spreading centerbehind the arc. Moberly (1972) proposed that the arcmoves forward due to the gravitational sinking andretreat of the inflexion point of the lithospheric slab onthe oceanic side of the arc. That is, the trajectory of thesinking plate is steeper than the angle of the plate.

Although the mechanism remains unclear, in our viewthe Paleogene arc and basin systems of the southwestPacific can be explained by contributions of materialfrom the asthenosphere developing areas of new seafloor along the interacting plate boundaries. The migra-tion of island arcs is a function of these processes.

Analysis of the geometry of plate motion is dependenton the mapping of magnetic anomalies and fracturezones related to spreading. Apart from in the TasmanBasin, which is not regarded by us as a marginal sea, nosuch magnetic anomalies have been mapped in thePaleogene basins of the region. But anomaly patternshave been detected in some of the younger basins of theregion: the Lau Basin (Sclater et al., 1972); the FijiPlateau (Chase, 1971; Luyendyk et al., 1974); and theWoodlark Basin (Luyendyk et al., 1973). In all cases theanomaly pattern is not as simple as that found adjacentto mid-ocean ridges.

Because magnetic anomaly patterns capable ofkinematic analysis are either absent or poorly knownwithin the marginal basins drilled, the geologicalhistories of the basins have to be based largely on litho-

logical sampling, seismic profiling, and major plate mo-tions. Drilling by the Deep Sea Drilling Project has beencarried out in the region on Legs 21 and 30 (Figures 1and 2). The results of Leg 21 have been presentedpreviously (Burns, Andrew, et al., 1972, Burns, An-drews, et al., 1973). A preliminary account of the resultsof Leg 30 has been presented by Andrews, Packham, etal., 1973. Details of the drilling are presented in thisvolume. After reviewing the tectonic implications of Leg30 drilling results, it is our present purpose to present aninterpretation of the regional geological history andmake some comments on the origin of the marginalbasins.

DISCUSSION OF DRILLING RESULTSThe sequence at Site 285 (Chapter 3, this volume) in

the South Fiji Basin parallels that drilled to thenortheast at Site 205 in that a coarse ashy succession isfollowed by a biogenic section and then by abyssal clay.However, the rhythmic volcaniclastic sequences of Site285 do not occur at Site 205. Structural ridges seencrossing the seismic profiles from New Zealand to Site285 (Figure 3, in pocket at back of volume) pond clasticsedimentation from the south. It is suggested that thesource of the volcanic sediment at Site 285 was the LauRidge. This also conforms to the present slope of the seafloor.

It would appear that the South Fiji Basin may haveformed in the Eocene and Oligocene. Its present base-ment pattern may have been enhanced or developed bypre-middle Miocene tectonism (see Packham andTerrill, this volume). Late Oligocene ashy biogenicsediments deposited near the depth of total solution ofplanktonic forams at Site 205 may be present beneathintrusive diabase at Site 285. The early Miocene recordis not preserved at either site. In middle to late Miocenetime, intermediate to acid volcanism developed, andpyroclastics were deposited near the Lau Ridge (Site205) and transported to the west (Site 285) anddeposited in the deeper parts of the basin. Biogenicdeposits appear in the latest Miocene, and coincidenttectonism exposing older material resulted in redeposi-tion of Oligocene to early Miocene material in the earlyPliocene section at Site 285. Later sediments (latestPliocene to Recent) were deposited below nannofossilsolution depth (depth of total solution of nannofossils,which is in turn below foram solution depth), probablyas a result of basin subsidence caused by lithospherecooling. Continued minor faulting is apparent onseismic profiles in some of the local basins.

In the New Hebrides Basin at Site 286 (see Chapter 4,this volume) basaltic flows were extruded in middleEocene time and rapid sedimentation began, probably

691

G. H. PACKHAM, J. E. ANDREWS

DEPTH IN FATHOMS• LEG 30 DRILL SITES

O PREVIOUS DRILL SITESo170

30c

282

Figure 1. Location of drill sites, DSDP Leg 30 and previous DSDP sites in the region.

692

RESULTS AND GEOLOGIC HISTORY

in the form of a submarine fan at the base of a volcanicridge with active andesitic volcanism until the lateEocene. Sea-floor depth was above the foram solutiondepth. Much of the material was derived from nearbyshallow water. Mainly biogenic sediments with minorash were deposited in the later late Eocene andOligocene on a sea floor which has subsided belowforam solution depth, but not below nannofossil solu-tion depth. By latest Oligocene time the depths werebelow nanno solution depths with clay and glass shardash accumulating. This subsidence is again attributed tocooling of the lithosphere. A period of nondeposition orerosion intervenes before the Pliocene, to be followed inearly Pliocene to Pleistocene time by a continuous influxof glass shard ash from fairly distant sources. Reworkedfossils, including shallow-water Miocene and Pliocenebenthonic neritic species that appear in beds of earlyPleistocene age near the top of the section, suggest ero-sion of nearby older shelf deposits (probably around theNew Hebrides) and indicate the blocking of the NewHebrides Trench east of Site 286 by the early Pleisto-cene. Deformation of the sea floor, seen in seismicprofiles, occurred after the deposition of the Eoceneclastic sediments but was probably pre-middle Mio-cene. This deformation can be detected as far south asSite 285 in the South Fiji Basin. Packham and Terrill(this volume) favor a late Eocene or early Oligocene age.The Eocene/Oligocene regional unconformity (Kennettet al., 1972) does not occur at this site indicating that itwas structurally isolated probably by the NorfolkRidge, from the Coral Sea, Lord Howe Rise, and NewCaledonia Basin to the west.

In the Coral Sea Basin (Site 287) the section (Chapter5, this volume) is quite similar to that sampled at Site210 (42 km to the west-northwest at Site 287), but showsthe effects of the greater distance from the source of thePliocene and younger elastics (the turbidites are finergrained), and the effect of the basement ridge on whichSite 287 was drilled (turbidites appear later due to eleva-tion, the Eocene/Oligocene regional unconformityspans a larger interval, and pelagic sections are thinnerat Site 287). The basement ridge that lies immediatelysouth of the site trends northwest-southeast and appearsto have developed shortly after the formation of thebasin crust in the early Eocene. At first the depth ofsedimentation was above foram solution depth, butpassed below nannofossil solution depth possibly by lateOligocene and certainly early Miocene. The green siltyclay deposited perhaps from the middle Mioceneonwards may represent the distal ends of turbiditycurrents which were at that time beginning to depositgraded rhythms at Site 210. The turbidites built to thelevel of the sea floor at Site 287 in early Pliocene time.The thickness of Pleistocene turbidites (about 90 m) issimilar at both sites, as is the frequency of deposition(about one flow per 5000 yr).

On the Ontong-Java Plateau basement was notreached at Site 288 (see Chapter 6, this volume), but acomparison to Site 289 suggests that the older sedi-ments (Aptian) may not have been far above it.

Following crustal formation in pre- or early Aptiantime, biogenic and volcanogenic sediments ac-cumulated. A mid-Cenomanian-early Turonian cycle of

volcanogenic sediments may have been derived from thesouthward-migrating spreading center which formed thecrust at Site 289 west of a north-south fracture zone (seebelow). Maximum depth was reached in the Campa-nian.

After the reappearance of planktonic foraminifera inthe Maestrichtian sediment, the depositional environ-ment remained relatively constant till late Oligocenetime. The section is discontinuous, with a major hiatusin the Eocene and lower Oligocene. Reworked sedi-ments suggest that the site has been an unstable surfaceof gentle inclination—probably a topographic low—which had been subject to current scour and minorslumping from Aptian to Miocene. More intense distur-bances have occurred from the late Miocene on.

Ash in the upper Pliocene is probably related tovolcanism on the Stewart Arch and the RoncadorHomocline. The Miocene/Pliocene hiatus may markslumping associated with tectonism and may reflectcollision of the plateau with the Australian plate to thesouthwest.

At Site 289 (see Chapter 7, this volume) thePleistocene to early Oligocene is continuous, and con-tains a diverse microflora and microfauna with good toexcellent preservation.

At least six substantial stratigraphic breaks are pres-ent in the Lutetian (middle Eocene) and Ypresian; Ypre-sian and Thanetian (upper Paleocene); Thanetian andDanian (lower Paleocene); lower Danian andMaestrichtian; and Aptian and Campanian. TheEocene/Oligocene break is similar to that reported byKennett et al. (1972) in the Tasman and Coral seas.

Very minor chert was detected in the lower Miocenewith the major appearance in the upper Eocene accom-panied by the loss of Radiolaria from the sediments.Less chert was observed at this site than at Site 288. Thesiliceous and chert-rich strata (Unit 2 of Sites 288 and289) can be traced between the sites and thickens only atthe margin of the plateau (see below).

Plateau elevation has been relatively constant abovethe foram solution depth, with the exception of a deeperinterval, as seen also at Site 288, in the Campanian.

The Early Cretaceous basement age for Site 288 fitswell at the western end of the original Phoenix spreadingcenter (Larson and Chase, 1972). The age at Site 289 re-quires a fracture zone between Sites 288 and 289 whichappears to have been crossed very near Site 288 (seebelow).

DISCUSSION OF UNDERWAY GEOPHYSICALDATA

Seismic reflection (airgun) and magnetic profiles wererecorded between sites during Leg 30. The continuousreflection profiles are presented (Figure 3) in threesegments as foldouts inside the rear cover of thisvolume. Segment one illustrates profiles from GreatBarrier Island, New Zealand, to Site 286 in the NewHebrides Basin. An essential feature of the structure ofthe South Fiji Basin can be seen here in the quite regularoccurrence of basement ridges and seamounts at about a65-km interval. The topography and structure of thebasin have been discussed in detail by Packham andTerrill (this volume). The north-northeast grain of the

693


285

LOWER-*"PLIOCENE

100-

200-

300-

400-

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SOUTH?

UPPERMIOCENE

MIDDLEMIOCENE

•>

FIJI BASIN

zz|

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\ '_

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II ^

» ' » " •

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1

3

4A

4B

5

286NEW HEBRIDES

100 —

200-

300 —

4 0 0 -

5 0 0 -

6 0 0 -

700-706

PLEIST.

7MI0CENE

UPPEROLIGOCENE

LOWEROLIGOCENE

UPPEREOCENE

MIDDLEEOCENE

Λ < ? . ? ? M

3A

3B

287CORAL SEA BASIN

0-

100-

U. OLIGOCENE—*'200-

252

UPPERPLEIST.

LR. PLEIST.

LOWERPLIOCENE

PLIOCENE

' MIDDLEEOCENELOWEREOCENE

I

1 i *

i i

—

w

1

2

—4•

EXPLANATION

Nannofossil Ooze Radiolarian Ooze

Foraminiferal Ooze

Nanno-foram Ooze

Sand

Sandy Siltand Silty Sand

Nanno-foram Chalk

Graded Beds withBioqenic Ooze

Clay

Silty ClayClayey Silt

Nannofossil Chalk Stratigraphic Hiatus

Micronodules

Figure 2. Stratigraphic columns of holes drilled on Leg 30.

Siliceous Limestone

Conglomerate

ZeoliteZ Z Z Z Z

z z z z zz z z z z

A••••• A•AA

Vi t r ic Tuff

Gabbro and Diabase

Basalt

694

288ONTONG-JAVA PLATEAU

TOO-

200-

300-

400-

500-

600-

800-

PLEISTOCENE

UPPERPLIOCENE

UPPERMIOCENE

MIDDLEMIOCENE

LOWERMIOCENE

UPPEROLIGOCENE

LOWEROLIGOCENE?IntermittentDeposi tion

UPPER PALEOCENE(THANETAIN)

LR. PAL.(DANIAN)

2A

Figure 2. (Continued).


289ONTONG-JAVA PLATEAU

100-

2 0 0 -

300-

5 0 0 -

7 0 0 -

800-

1000-

1271-

PLEIST

UPPERMIOCENE

MIDDLEMIOCENE

LOWERMIOCENE

UPPEROLIGOCENE

MIDDLEEOCENE

L. EOCENE

UPPERPALEOCENE

MIDDLEMAEST.

I~MAE~TT

CAMPANIAN

APTIAN

695


basin may in fact follow from transform patterns relatedto the spreading episode which formed the basin,although the basement ridges postdate crustal formationin the northwestern part of the basin.

The middle to late Eocene thick volcanogenic se-quence at Site 286 can be traced on the profile from Site286 almost to Site 285. From Site 286 southward itforms a thinning wedge in the northwestern part of theSouth Fiji Basin. Further, the sediments can be tracedwell up the steep flank of the Loyalty Island Ridgewithout significant thinning. The implications of this arethat the New Hebrides Basin and the South Fiji Basinare part of the same body of sea floor, and that theLoyalty Islands were probably not the source of thevolcanic detritus, having been uplifted later. Un-doubtedly, they were the source of the sediment infilladjacent to them in the northwestern corner of theSouth Fiji Basin and the southern New Hebrides Basin.

Segment two of the profiles shows the recordsbetween Sites 286 and 287 and between Site 287 and theSolomon Islands. Several major fracture zones are pres-ent between 286 and 287. From east to west they are thed'Entrecasteaux, Rennell, and Louisiade fracture zones.The first of these separates the easterly deformed part ofthe New Hebrides Basin from the western part in whichthe structures are much broader. This fracture zone mayhave been part of the Pacific-India plate boundary dur-ing the obduction of sea floor onto New Caledonia inthe early Oligocene (Landmesser et al., this volume).The other two fracture zones may have developed dur-ing the formation of the Coral Sea. Alternatively theRennell Fracture Zone could have been formed as aresult of the relocation of the Pacific-India plate bound-ary after the obduction took place in New Caledoniaand been associated with the subduction of part of theTasman sea floor under eastern Australia. Evidence forsubduction was found by Hayes and Ringis (1973). TheEocene to Oligocene pelagic strata seen at Site 287appear to be continuous over most of the area, support-ing the regional interpretation of an Eocene crustal agebetween these two sites (Landmesser et al., this volume).

One interpretation of the profile from Site 287towards the Solomon Islands tends to support thehypothesis of marginal basin formation by symmetricalsea-floor spreading from the Coral Sea/New HebridesBasins (Landmesser et al., this volume). The profilecrosses the Louisiade Rise parallel to and south of thePocklington Trough. The rise has a regular and rathersymmetrical cross-section, and the sediment cover thinstowards the crest, partly as a result of erosion. Thepelagic sediments of the rise can be traced beneath theturbidite fill of the Coral Sea Abyssal Plain, and thus in-dicate an Eocene age for the sediment cover of the rise.The continuous magnetic profile along this track showsa seemingly symmetrically disposed pattern of magneticanomalies (Figure 4) and while realizing the limitationsof it as a single profile, it is possible to suggest that this isthe ridge which generated the Coral Sea crust andpossibly the New Hebrides Basin crust as offset by aseries of transform faults. A best fit on the anomaliessuggests a spreading interval from anomaly 21 toanomaly 17 (50 to 46 m.y.B.P.). If this is correct, the age

of the sea floor adjacent to the Queensland Plateaucould be as old as early Paleocene. The disposition ofthis ridge is such that the pole of rotation is similar tothat for the spreading for the Tasman Sea (Hayes andRingis, 1973), but at a significant angle to the structuralgrain of the eastern New Hebrides and South Fiji basins.These differences in direction may imply a sequential orindependent development of the marginal basins.

A second interpretation of the Louisiade Rise is that itis composed of continental crust that has been rifted offthe Australian continental margin. The seismic profilesresemble typical profiles on the Queensland Plateau(Falvey and Taylor, 1974), and the oldest sediments thatfill graben structures could be interpreted as rift valleyfills formed at the commencement of rifting. On thishypothesis sea-floor formation would have been essen-tially confined to the Coral Sea Abyssal Plain area andthe region southwest of the eastern section of theLouisiade Rise (Landmesser et al., this volume).

The third segment of the Leg 30 profiles illustratesprofiles from the Solomon Islands to Site 288, to Site289 (via Site 64 of Leg 7, Winterer, Riedel, et al., 1971)and across the Caroline Ridge and Mariana Basin toGuam. Lithostratigraphic continuity is apparent acrossthe plateau, with Units 1 and 2 of Site 288 being trace-able to Site 289 and beyond. Unit 2 thickens markedly atthe slope break at the margin of the plateau, being near-ly three times thicker at Site 288 than at Site 289. Theseunits are time transgressive, being related to chert for-mation within the section. Between Sites 288 and 289 itappears that a fracture zone was crossed. This featurefits well with the interpretation of the spreading historyof the plateau. To the north the plateau deepens beforereaching the Caroline Ridge. Crust in the MarianaBasin, north of the Caroline Ridge, is characterized by athick unresolved basement reflector overlain by thinpelagic sediments. Ages established in the region (as fareast as the Mid-Pacific Mountains) by Deep Sea DrillingProject cruises are generally younger than 85 m.y.,although it may be that none of the sites have reachedtrue basement. All sites surrounding the region of theMariana Basin have ages of greater than 100 m.y. Thiswould seem to define the region as an isolated section ofyounger crust which possibly formed in the same inter-val as the crust of the Tasman Basin.

Although the profile is interrupted by several ridgesand seamounts between Site 289 and the Mariana Basin,it is apparent that the sediment section is thinning con-tinuously away from the margin of the plateau. This ispresumably due to the removal of carbonate material atgreater depths. Taking some liberties in interpretation, itcan be suggested that Unit 2 of the plateau is equivalentto the section which has been sampled in the MarianaBasin. This would imply a greater age for the basin thanhas been established by drilling.

PLATE MOTIONSMagnetic anomaly patterns of the standard oceanic

type have been described from the Tasman Sea by Hayesand Ringis (1973) demonstrating that the spreading inthe basin occurred from 80 to 60 m.y. B.P. This basinhad been regarded by some authors as a marginal basin.

696

Figure 4. Seismic profile over the Louisiade Rise with observed magnetic anomalies.


The work of Hayes and Ringis demonstrates that its for-mation is contemporaneous with spreading on the Alba-tross Cordillera which commenced between Antarcticaand the Campbell Plateau south of New Zealand at thesame time (Christoffel and Falconer, 1973). Thedifference between the anomaly trends as they are atpresent observed (about 90°) is largely if not entirely dueto later spreading patterns. The Tasman Sea is on the In-dia plate and the Campbell Plateau is on the Pacificplate. The commencement of spreading of the Indian-Pacific Ridge between Australia and Antarctica at 55m.y. B.P. (Weissel and Hayes, 1972) formed oceaniccrust south of the Tasman Sea crust with anomalytrends approximately perpendicular to those of theTasman. This new spreading pattern isolated thesouthwest Pacific region from the major spreadingsystem of both the Indian and Pacific oceans.

Calculation of stage poles for the India-Pacific platerelative motions from the India-Antarctica stage polesof Weissel and Hayes, 1972, and the Pacific Antarcticstage poles of Molnar et al. (1975) with the India plateheld stationary reveals that from 55 m.y. onwards, thepoles have moved southwards with time (Packham andTerrill, this volume) (Figure 5, Table 1). The poles backto 29 m.y. are located south of New Zealand, while theolder ones are located further north with the 45 to 55m.y. stage pole located near 160°W and 10°S. The rota-tion of the Pacific plate is counterclockwise and thesepole locations should mean that the amount of subduc-tion and presumably the amount of volcanic activity onthe northern part of the arc should decrease with time.From the mid-Eocene to the end of the early Oligocenethe line of the present Tonga-Kermadec Trench ap-proximated a transform direction. From the late Oligo-cene the northern boundary of the arc complex shouldhave become closer to a transform direction. Subduc-tion into the Tonga Trench should have become signifi-cant in late Oligocene time and been increasing in rate.

Initially then, plate convergence was greatest on thenorth side of the arc complex, where Eocene volcanicsare best developed and early Oligocene plate con-vergence is indicated by obduction. In the later historyof the region, volcanic activity and convergence shouldhave shifted to the south.

The geological evidence presented is roughly in ac-cord with this except that the changes in trends of thearcs relative to interplate motions have complicated thepattern. So too has the apparent swapping of arcs fromone plate to another as arcs have changed theirpolarities. One of the most conspicuous implications ofthe motions suggested is that the Ontong-Java Plateau, aregion of thick oceanic crust on the Pacific plate(Kroenke, 1972), has moved "westwards" at an in-creasing rate during the last 55 m.y. from a positionnorth-northwest of the 45-55 m.y. pole (Figure 5). Thecollision of the plateau with the arc system demanded bythe geometry of the movements has had a profoundeffect on the present distribution and polarity of arcs aswell as the disposition of subduction zones in the laterCenozoic. It has been suggested that the great crustalthickness of the plateau prevented its subduction(Kroenke, 1972; Packham, 1973). Almost all of the

"westward" motion has taken place in the last 38 m.y.(since the end of the Eocene).

Molnar et al. (1975), who have provided the most re-cent analysis of the South Pacific anomalies, attempteda series of reconstructions assuming that the Pacific-India plate boundary ran through New Zealand. Up to38 m.y. deformation of the New Zealand region is in-dicated, progressively straightening out the New Zea-land geosyncline and bringing the trends of the TasmanSea and South Campbell Plateau anomalies closer.Their 45-m.y. reconstruction requires substantial over-lap between the Campbell Plateau and the Lord HoweRise. As an alternative, they suggest that openingbetween east and west Antarctica took place and thatthere was no interaction along the India-Pacific plateboundary. To put it another way they are suggestingthat there was a collision between east and west Antarc-tica around 38 m.y. ago.

This, however, does not seem to be in keeping withgeological observations and seismic profiles in the RossSea region (P. Barrett, personal communication), andfurther their model implies that no active plate bound-ary existed between the India and Pacific plates prior to38 m.y. On the contrary, abundant evidence exists in theregion for an active Pacific-India plate boundary in theform of island volcanism and the formation of new seafloor in marginal basins. The supposed overlap of theLord Howe Rise and the Campbell Plateau may be anartifact resulting from Molnar et al. (1975) using thepresent plate boundary through New Zealand in theirreconstructions.

Apart from relocations of subducting plate bound-aries associated with suggested reversals of arcpolarities, the Pacific-India plate boundary appears tohave undergone other modifications: (a) the boundaryhas been obductive in the Papuan Peninsula (PapuanUltramafic Belt), New Caledonia, and possibly theSolomon Islands; (b) it has been interpreted as a zone ofarc continental margin collision and subsequently aregion of folding and strike slip faulting in northernNew Guinea; (c) one interpretation of the Rennell Frac-ture Zone is that it was a short-lived transform faultbetween the Pacific and India plates.

INTERPRETED GEOLOGICAL HISTORY

Against the background of the plate motions dis-cussed above, the data collected on Legs 21 and 30 of theDeep Sea Drilling Project, and the regional geology out-lined in Chapter 1, the following geological history isproposed. In general, there was extension of the area ofthe India plate in the early Cenozoic by marginal seadevelopment and reduction of its area in the laterCenozoic due to arc polarity reversals and subsequentplate motions. Most of the additions of new crust in thissecond phase were to the Pacific plate. This follows thescheme outlined by Packham (1973). It is possible at thepresent state of knowledge to present only a qualitativeaccount of the plate tectonics.

Cretaceous-Pal eoceneAbout the middle of the Cretaceous or possibly a little

later the Phoenix/Pacific spreading center was located

698

in the Bellinghausen Sea and the subduction of thePhoenix plate into a trench on the coast of west Antarc-tica ceased. The Phoenix plate then became part of theAntarctica plate (Larson and Chase, 1972). Thespreading axis at this time was nearly parallel to the


Antarctic Peninsula. When the oceanic crust becamefixed to Gondwanaland, the spreading axis was momen-tarily stationary, then as spreading continued the axisand the Pacific plate moved northwards fracturing thecontinental margin on the Pacific plate side of the

30°S

45-55 m.y.

Pacific Plate

38-45 m.y.

29-38 m.y.

21-29 m.y.

10-21 m.y.

Contours in fathoms 0-10 m.y.60°S 150°E 180f 150°W

Figure 5. Poles of rotation for Pacific/India plate with the India plate held stationary and reconstructed locations of theOntong-Java Plateau.

699


TABLE 1Pacific/India Poles of Rotation

(India Plate held stationary)

Age(m.y.)

0-1010-2121-2929-38384545-55

Location

59.4°53.2°51.6°43.9°27.2°9.5°

S, 175.7°S, 174.6°S, 175.5°S, 156.2°S, 153.6°S, 159.7°

WW

wwww

Anticlockwiserotation of

Pacific Plate

11.8°9.9°7.5°7.0°6.6°7.6°

spreading axis. Northward motion suggested by Larsonand Chase (1972) on the basis of paleolatitude estimatesfor magnetic anomalies is supported by paleomagneticmeasurements of sediments from Site 289 by Hammondet al., this volume. Drilling at Sites 288 and 289 es-tablished that the Ontong-Java Plateau basement is lateEarly Cretaceous in age. This age is a little younger thanthe youngest of the Phoenix anomalies identified by Lar-son and Chase to the east of the plateau. Winterer et al.(1974) suggest that they may be separated by transformfaults. We are treating the Ontong-Java Plateau as partof the Pacific plate from the Late Cretaceous onwards.Fracturing of the Gondwanaland continental margininvolved the rifting of the Campbell Plateau from Ant-arctica and the Lord Howe Rise from eastern Australia(Christoffel and Falconer, 1973; Hayes and Ringis,1973). Magnetic anomalies establish that these twoevents were synchronous commencing 80 m.y. ago, butthe present trends of the anomalies are quite different asindicated above. The Campbell Plateau has apparentlybeen rotated with the Pacific plate since the commence-ment of rifting of Australia from Antarctica (late Paleo-cene, 55 m.y. ago) while the Tasman has remained essen-tially part of the India plate. If the formation of theTasman Sea from 80 to 60 m.y. (Late Cretaceous to themiddle of the Paleocene) is attributable to the westwardextension of the Albatross Cordillera, then there was asingle spreading axis active in the region and the Tas-man Sea is not a marginal sea in the generally acceptedsense. The region east of the Tasman spreading ridgewould have been part of the Pacific plate. There is noevidence of volcanic evidence of a convergent plateboundary in New Caledonia or northern New Zealand.Tectonically the Tasman Sea resembled the Gulf ofCalifornia or the Red Sea. Even if the axes did not lineup, the two sets of spreading are closely related and theanalogy holds. The New Caledonia Basin to the east ofthe Lord Howe Rise is probably of similar age to theTasman Sea as is suggested by Leg 21 deep-sea drillingresults. Its mode of formation then is different, since itwas physically separated from the Tasman Seaspreading center at its time of formation.

Prior to the formation of the Coral Sea the Mesozoicclastic rocks that form the metamorphic spine of theOwen Stanley Range in the Papuan Peninsula occupieda position adjacent to the northern end of the Queens-land Plateau. The age of metamorphism was Eocene orolder (Davies and Smith, 1971).

Eocene Phase of Marginal Sea Formation

Eocene sea-floor ages have been obtained in the CoralSea and the New Hebrides Basin. It appears from thestudy of seismic profiles that the Eocene episode couldinclude the western part of the South Fiji Basin (Pack-ham and Terrill, this volume). By extrapolation, theNorfolk Basin might also have been formed at this time.The crust between the Coral Sea, the northern TasmanSea, and the New Hebrides Basin is intersected byseveral large fractures discussed above, and has in placesa thick sediment cover (Landmesser et al., this volume).While it cannot be shown unequivocally that all thecrust is oceanic as indicated previously, it is consideredby them that the oceanic crust present is probablyEocene. No data are available as to the age of the Solo-mon Sea floor. Eocene island arc volcanism is known inthe region; it occurs in the northern part of New Guinea,New Britain, Fiji, and on Eua (Tonga Islands) andpossibly in the New Hebrides (see Chapter 1).

Volcaniclastic sediments and associated shallow waterlimestones occur in New Guinea, New Britain, inferredfor the New Hebrides, and present in Fiji and Eua(Figure 6). Island arc volcanism was associated with theformation of the New Hebrides Basin as indicated by athick sequence of volcaniclastic rocks resting on the sea-floor basement. If the Solomon Sea is of similar age, theEocene volcanics in New Britain and northern NewGuinea may be related to its formation. The location ofthese various volcanic arcs was probably rather differentfrom what it is now as a result of later plate motions andplate boundary relocations.

From the geometry of motion of the Pacific and Indiaplates relative to the Antarctic plate, it is clear that fromthe time of commencement of rifting of Australia fromAntarctica, there had to be an active plate boundarybetween the plates. During this time the relative pole ofrotation for the India and Pacific plates was apparentlynear 10°S and 160°W. The boundary had to be con-vergent along the northern margin of the complex fromNew Guinea to Fiji where there is the corroboratingevidence of the existence of island arcs. Their locationsat the time cannot be determined. At the same time asvolcanism was taking place, sea floor was being formedin the Coral Sea, the New Hebrides Basin, and possiblythe northwestern South Fiji and Norfolk Basins, but inthe last (and possibly the last two) the structural fabric isdifferent from the Coral Sea. The only relatively simple,geometric arrangement to explain these relationships isfor the subduction zone to be on the northern side of thevolcanic arcs and for the arcs to migrate northwardswith the velocity of convergence of the two major platesplus an added velocity due to sea-floor formation withinmarginal seas. The eastern boundary was probably atransform fault. It appears that the width of the newcrust produced increased towards the pole position, i.e.,as the convergence rate between the Pacific and Indiaplates decreased.

The oldest sea floor dated on Leg 30 within the arccomplex is in the Coral Sea and just over 50 m.y. This isonly a few million years after the commencement of rift-ing of Australia from Antarctica and less than 10 m.y.

700

NORTH

NEW GUINEA

NEW

BRITAIN

NEW

IRELAND-

BOUGAINVILLE GUADALCANAL

NEW

HEBRIDES FIJI

LAU

RIDGE

TONGA

RIDGE

u —

10—

20—

30—

40—

50 —

60 —

70 —

80 —

PLEIS

PLIO.

LU^ "

MIOC

EI

LU

IGOC

E

o

EOCE

NEPA

LEOC

ENE

COi ^oo•<

CRE

L

~T~L

M

E

L

E

L

M

~E~"

L

E

}?*>"'''

?

Λ A I • V Λ V'}:?>/•

COLLISIONWITHCENTRALNEW GUINEA

TSUBDUCTI0N

FORMATION

TRANSFORM

7M0TI0N

11CAROLINE BASIN

FORMATION CORAL SEA

FORMATIONTASMANSEA

SUBDUCTIONFROM S or N

TRANSFORM7M0TI0N

SUBDUCTIONFROM ?S

•S

IÜAW ^•;

OCEANIC'CRUSTS

SUBDUCTIONFROM S

SUBDUCTIONFROM WEST

SUBDUCTIONFROM EAST

HPDEDUCTION ~ rSANTA DEFORMATIONISABEL

SUBDUCTIONFROM ?N

DEDUCTION NEW CALEDONIA

FORMATION NEW HEBRIDES BASIN

FORMATIONFIJIPLATEAU

DETACHMENT

Figure 6. Summary of stratigraphic columns for selected islands in the southwest Pacific.

FOLDINGAND

INTRUSION

GAP

FORMATIONSOUTH FIJIBASIN

LAU-TONGARIDGE

VOLCANICS

LIMESTONE

CLASTIC SEDIMENTS

o

H>Zσowor

53

3


younger than the youngest sea floor in the Tasman Sea.As the age of Coral Sea site probably does not representthe greatest age in the basin, spreading in the regioncould have been continuous temporally but notnecessarily geographically. The development of thesemarginal seas had to be independent of the Pacific-Antarctic spreading center since sea-floor formation hadceased in the Tasman Sea and the connection severed.

The initiation of the convergent plate boundary, andthe commencement of the development of the arcs andthe marginal seas all appear to have taken place within ashort time span. The details of the process involved aredifficult to visualize. The Coral Sea does not have avolcanic arc directly related to it. Its outer limit,represented by the Papuan Peninsula and the LouisiadeIsland chain, contains metamorphics of continentalorigin. These same rocks may form the basement of theRennell Ridge and link up with the Mesozoic meta-morphics of New Caledonia (Landmesser et al., thisvolume). The Eocene volcanic arcs that lie furtheroceanward have no continental rocks associated withthem.

Oligocene Sea-Floor Formation and TectonismSea floor of Oligocene age is known in the Caroline

Basin, north of New Guinea, and in the eastern part ofthe South Fiji Basin, but it is not known what time spanis represented in the formation of these areas of seafloor. An early Oligocene date has been obtained in theCaroline Basin (Winterer, Riedel, et al., 1971) and a dateof about the middle of the Oligocene on the eastern sideof the South Fiji Basin (Burns, Andrews, et al., 1973).

Two important tectonic events occurred in theOligocene. The older is the obduction of sea floor ontothe island of New Caledonia (almost certainly in the ear-ly Oligocene) and perhaps contemporaneous with theobduction of the Papuan Ultramafic Belt onto the Pa-puan Peninsula. Recent unpublished work by theAustralian Bureau of Mineral Resources indicates that.this probably took place in the early Oligocene ratherthan the Eocene as suggested by Davies and Smith(1971). The second event at about the end of theOligocene is what has been interpreted as a collision ofan island arc with northern New Guinea. This arc haslate Oligocene volcanics associated with it. Develop-ment of the Caroline Basin as an accretion onto thePacific plate could have been responsible for thesouthward migration of the Eocene-Oligocene volcanicarc comprising New Britain, northern New Guinea, andperhaps including Manus, New Ireland, and ?Bougain-ville (Figure 6). The velocity of motion of the arc wouldbe the sum of the velocity of convergence of the Pacificand India plates plus that due to spreading in theCaroline Basin. The suggested collision of the arc withnorthern New Guinea in the Oligocene and the absenceof volcanics on the southern block indicate a sub-duction zone dipping to the north under the volcanicarc. This would appear then to be a mirror image of thesituation in the Eocene and implies arc polarity reversalbetween the two episodes. If the arc polarity of theeastern end of the arc complex (New Hebrides, Fiji, andTonga) has remained unchanged, the two arc sectorswould have been separated by a transform fault. The

western section would have been part of the Pacific plateand the eastern section of the India plate.

The eastern and probably the southern parts of theSouth Fiji Basin are late Oligocene, and their formationmay have been initiated after the obduction in NewCaledonia. Its development would have carried the LauRidge eastwards together with Fiji. The New Hebridesmay have also been displaced to the east as part of thesame arc (cf. Gill and Gorton, 1973; Packham, 1973) bya new subducted northward extension of this crust.Drilling in the New Hebrides Basin has revealed thatthere was no deposition of volcanic products in quantityat the site in the Oligocene or the Miocene. The oldestshallow water formations in the New Hebrides are latestOligocene to basal Miocene. This suggests that buildingof the arc may have commenced considerably beforethat time. Since the preponderance of rocks on theislands are early Miocene and possibly older volcanicrocks, and since volcanic debris is absent from theOligocene and Miocene of the New Hebrides drill site,the New Hebrides are assumed to have been far re-moved from their present location at that time. Theirpolarity was probably the opposite of that at present(Gill and Gorton, 1973; Colley and Warden, 1974).

Linking the New Hebrides to the western part of thearc complex are the Solomon Islands in which thePacific Province typified by Malaita and northern SantaIsabel (Coleman, 1970) consist of oceanic basaltsoverlain by oceanic sediments (see below). The SolomonIslands of the central province of Coleman (1970) fromChoisel to San Cristobal also have a volcanic basement.The basement lavas on Guadalcanal are chemicallysimilar to oceanic tholeiites rather than part of an islandarc (Hackman, 1971). It is suggested below that theywere part of the Pacific plate either forming part of (oradjacent to) the Ontong-Java Plateau. These islands andthose of the Solomon Pacific Province differ from othersin the arc complex to the east and west in this regard.The others are built of lavas, volcaniclastics, andshallow water carbonate deposits. The location of theSolomon Islands in the Oligocene is uncertain, but platemotions outlined above would place them substantiallyto the east of their present position (Figure 5). It issuggested below that they were inserted into the arc atthe time of the Oligocene arc reversal suggested above.

Convergence between the Pacific and India plates isindicated by the plate motion vectors in the Oligocene(Figure 5), but there is an increasing westerly compo-nent as the pole of rotation moved southward relative tothe India plate. As the pole moved south, the zone ofcrustal accretion to the India plate also moved southinto a region of lower convergence rate. The EmeraldBasin was added to the Pacific plate at this time also.Although the mode of formation of the eastern part ofthe South Fiji Basin is not clear, arc migration to theeast would have involved only a small amount (around400 km) of subduction (equal to the basin width). Dur-ing the early Oligocene the interplate motion along theboundary marked by the present Lau-Tonga Ridgetrend would have been transcurrent in the south, butwith increasing convergence to the north. This may ex-plain why volcanic material is not abundant in the lateOligocene section at Site 205.

702


Miocene and Later

The early to middle Miocene was an interval in whichthere was widespread deposition of limestone in NewBritain and New Ireland and Bougainville, but in theSolomons elastics were common including detritalserpentinites (Coleman, 1970). In the Pacific province ofthe Solomon Islands as exemplified by Malaita, theoceanic basaltic basement is overlain by Albian toPliocene deep water sediments (Deventer and Postuma,1973). Upper Miocene and younger sediments of theprovince contain increasing amounts of terrigenousdetritus, but this detritus is missing from the otherwisesimilar section on the Ontong-Java Plateau (Sites 288and 289). The Miocene of the central province containsabundant shallow water deposits. The juxtaposition ofthese two features and the presence of large bodies ofserpentinite along the Korigole Thrust and the presenceof flat-lying sheets of serpentinite on Choisel make itlikely that part of the crust of the Ontong-Java Plateauand its sedimentary cover have been obducted from thenorth onto the central province (L. Kroenke, personalcommunication). This probably occurred in the earlyMiocene and may indicate a further relocation of thePacific-India plate boundary. Uplift of the centralprovince and the deposition of elastics in the Miocenecould be regarded as a response to the obduction. Thepresence of the late Oligocene or basal Miocene SutaVolcanics on Guadalcanal may indicate that the obduc-tion was preceded by some subduction.

The absence of volcanics and elastics from the early tomiddle Miocene on the islands from Bougainvillewestwards may indicate that they were oriented parallelto the transform direction between the India and Pacificplates. The location of the plate boundary would deter-mine whether the islands were being moved with thePacific or the India plate, but it appears that they wereessentially moving with the Pacific plate (see below).Change of arc orientation may have been the result ofthe development of subplates in the region following thelate Oligocene collision of the arc with northern NewGuinea. These small plates may have been the precur-sors of the ones identified in the region by Johnson andMolnar (1972). The part of the arc west of Manus Islandwas probably originally attached to New Britain. Strikeslip faulting is still taking place in the Bismarck Sea.

By late Oligocene time, the pole of relative rotationfor the Pacific and India plates had moved south of NewZealand (Figure 5), and there was an increasingly rapidrelative westward motion of the Pacific plate carryingthe Ontong-Java Plateau with it. The supposed obduc-tion in the Solomon Islands could have been in someway a response to their interaction with the remainingprobably north-facing part of the arc complex, namelythe New Hebrides and Fiji islands (see below). Alter-natively, and more likely, it was related to plate bound-ary readjustments following the collision of the arcsystem with northern New Guinea in the Oligocene, oc-curring immediately before or contemporaneously withthe suggested detachment of the end of the arc west ofManus Island from New Britain. Subsequently the arcsegment has moved westward on the Pacific plate pro-

gressively overlapping New Britain and northern NewGuinea.

Following the collision of the Oligocene arc and ac-companying the suggested westward motion of theremainder of the arc, elevation of the northern part ofNew Guinea took place, and widespread clastic deposi-tion occurred accompanied in the middle Miocene byvolcanic and plutonic activity. Probably as a conse-quence of the small plate interactions, elevation of thePapuan Peninsula commenced in the early Miocene,resulting in an inflow of clastic sediment into the AureTrough, and in the middle Miocene into the Coral Sea.Continuation of motion probably resulted in the foldingof the Aure Trough in the Pliocene. Arc reorientationand spreading in the Woodlark Basin accompanied bysome change in relative India-Pacific plate vectors leadto renewed subduction and further volcanic activity inthe arc from the Solomons to New Britain with subduc-tion occurring on the south side of the arc.

As is pointed out by Johnson et al. (1973), all of thevolcanic activity in the late Cenozoic especially in NewGuinea cannot be related to presently recognizable sub-duction zones. An anomalous association ofcalcalkaline and shoshonitic volcanics occurs innorthern and eastern Papua.

In the more easterly part of the arc system in the NewHebrides it has been argued by Mitchell and Warden(1972) and others that the present polarity of the arc wasdeveloped in the middle Miocene possibly as a result ofinteraction with the Ontong-Java Plateau-SolomonIslands (Packham, 1973), then as the younger sea floorof the Fiji Plateau formed the New Hebrides movedwestwards to its present position severing its connectionwith Fiji (Chase, 1971) and blocking the central part ofthe New Hebrides trench at least by the earlyPleistocene. At present spreading rates this would havetaken place when the New Hebrides were about 200 kmwest of their present position. The probability that theEast Rennell Island Ridge continued further to the eastis high. It is likely then that the present configuration ofthe trench with the ridge intersecting it and physicallyblocking its central region is a steady-state situation.This is supported by the lack of break in seismicity. Itwould appear then that tectonic erosion was takingplace with segments of the ridge being subducted. Thisprocess may have commenced by the early Pliocenewhen the first sediments derived from the New Hebrideswere deposited at Site 286. At that time the arc wouldhave been about 400 km to the east.

The change from calcalkaline to shoshonitic volcan-ism in Fiji during the Pliocene (Gill and Gorton, 1973)has been attributed to the increased distance of the is-lands from the trench to the east as the Lau Basinformed splitting the arc.

Contemporaneous with the suggested reversal of thepolarity of the New Hebrides Arc, subduction rates inthe Tonga Trench increased and accompanying volcan-ism resulted in the thick volcanogenic accumulation ofsediment in the Minerva Abyssal Plain of the South FijiBasin being deposited. The sediment of the KupeAbyssal Plain of the southern part of the South FijiBasin is probably attributable to the volcanic activity in

703


Northland in northern New Zealand in the early Mio-cene and the late Miocene to Pliocene on the Cora-mandel Peninsula.

The Fiji Plateau has been clearly accreted onto thePacific plate, and the New Hebrides arc has movedwestward consuming some of the Eocene and possiblyOligocene sea floor of the New Hebrides Basin. Theremay be some analogy between the accretion of theCaroline Basin and the Fiji Plateau onto the Pacificplate.

The Lau-Havre Trough has been documented byKarig (1970) and is a good example of arc migration byaddition to the India plate. The Woodlark Basin(Luyendyk et al., 1973) may be different from the otherfeatures discussed here. It is a small area of crustdeveloped in a region of complex motions and incipientcollisions. The crust generated there appears to be aresponse of the motions of the surrounding plates. It isimportant to note that the formation of the crust is not afunction of arc development.

MECHANISM OF MARGINAL SEA GENERATIONAlthough drilling on Leg 30 has provided key data

enabling the foregoing tentative tectonic synthesis of thesouthwest Pacific arc complex to be presented, it has notdirectly contributed to the resolution of the question ofthe mechanism of generation of marginal seas. Whetherthe sea floor is generated symmetrically orasymmetrically cannot be demonstrated with the limiteddrilling information. There are indications of possiblesymmetrical spreading centers in several basins. In par-ticular one interpretation of the structure of theLouisiade Rise is a spreading center for the Coral SeaBasin. In the South Fiji Basin the pronounced andregular trend of the basement ridges suggests transformstructures with a north-northwest trending ridge. Al-though the basement ridges postdate the formation ofthe crust (Packham and Terrill, this volume), they mayparallel the original structure. Arguments were pre-sented for either a two phase symmetrical or a one sided(asymmetrical) spreading history. In the latter case thecrust of the basin would have been developed by an east-wards moving ridge. The patterns of basin developmentare sufficiently similar to each other in trend and to theridge which generated the Tasman Sea to suggest possi-ble continuation of activity of the Tasman Ridge in-dependent of the spreading in the Pacific. Closely spacedintrusive structures and fracture zones may easilyobscure magnetic anomaly patterns that would moreclearly define spreading patterns, accounting for pooranomaly correlation in the basins. On the present datakinematic solutions are not possible. The wavelength ofmagnetic anomalies in the marginal basins suggests thatsome regular process is involved.

It appears from the preceding discussion that arcmigration is usually associated with marginal seadevelopment, although not in the sense popularlyaccepted. That is, that arc migration should be viewed asan effect of the development of marginal seas (cfPackham and Falvey, 1972), rather than as a cause ofthis development (cf. Karig, 1971). Zones of expansionof the India plate tended to migrate southwards with the

pole of rotation for the Pacific and India plates. Itappears to be greater where convergence rates are loweror even transform or divergent relative motions occur.Concurrent expansion of the Pacific plate has notfollowed as regular a pattern (see Table 2). In Eocenetime while the Coral Sea, New Hebrides Basin, SolomonSea, and perhaps the western South Fiji Basin were add-ed to the India plate there were no additions to thePacific plate. While accretion on the Indian plate movedsouth in the Oligocene to the South Fiji Basin, accretionon the Pacific plate occurred in the Caroline Basin to thenorth, and on a much smaller scale in the Emerald Basinto the south. From late Miocene to Recent the Lau-Havre Basin and the anomalous (?hernia-like)Woodlark Basin have been added to the India plate,while the Fiji Plateau has been added to the Pacificplate. Thus the two major additions to the Pacific platealso show the southward displacement of activity withtime, paralleling the motion of the relative pole of mo-tion.

Generation of the basin crust by a thermal diapir(Karig, 1971) would be expected to produce asymmetricspreading or a basin of limited size. It also implies thatthe basin is a function of the prior existence of a subduc-tion zone, and thus that it is a direct consequence of con-vergence of major crustal plates. This same cause/effectrelationship is implied if arc migration is described asthe result of migration of the flexure point of the sub-ducted crust through the mechanism of heavy litho-sphere sinking on a trajectory steeper than the angle ofthe plate (Moberly, 1972).

Marginal sea development as a consequence of arcmigration calls for maximum growth of the basins im-mediately adjacent to zones of maximum convergencerates. In contradiction, we see the location of maximumbasin development tending to follow the pole of rota-tion, i.e., moving towards the zone of lesser con-vergence. Further, the orientation of the plate boundaryat the onset of marginal basin growth may range fromnormal to the major plate motion (convergence occur-ring—Lau-Havre Trough) to parallel to them (trans-form motion along the boundary—South Fiji Basin).

It does not appear valid that the basins are a directresult of island arc development, particularly as themarginal seas can develop on either side on a convergentplate boundary independent of the plate boundary

TABLE 2Areas and Times of Crustal Accretion to the

India and Pacific Plates

Eocene

Oligocene

E. Miocene

L. Miocene-Present

India Plate

Solomon Sea. Coral Sea,New Hebrides Basin,W.S. Fiji Basin (?)

E.S. Fiji Basin

-

Lau Havre Basin,Woodlark Basin

Pacific Plate

—

Caroline Basin,Emerald Basin

-

Fiji Plateau

704


orientation, and may appear in successive episodes toproduce arc polarity reversals as with the suggestedCoral Sea-Caroline Basin succession. Because of thevariety of patterns described, the crust of the marginalbasin is seen as being a result of movement within theasthenosphere and its interaction with lithosphere ofvarying (and changing) structure and buoyancy in thevicinity of a convergent or transform plate boundary.

REFERENCESAndrews, J.E., Packham, G.H., et al., 1973. Southwest Pacific

structures; Leg 30 Deep Sea Drilling Project: Geotimes, v.18, p. 18.

Burns, R.E., Andrews, J.E., et al., 1972. Glomar Challengerdown under: Deep Sea Drilling Project, Leg 21: Geotimes,v. 17, p. 14.

Burns, R.E., Andrews, J.E., et al., 1973. Initial Reports of theDeep Sea Drilling Project, Volume 21: Washington (U.S.Government Printing Office).

Chase, C.G., 1971. Tectonic history of the Fiji Plateau: Geol.Soc. Am. Bull., v. 82, p. 3087.

Christoffel, D.A. and Falconer, R.K.H., 1973. Changes in thedirection of sea floor spreading in the south-westPacific: Oceanography of the South Pacific 1972: NewZealand National Commission for UNESCO, Wellington,p. 241.

Coleman, P.J., 1970. Geology of the Solomon and NewHebrides Islands, as part of the Melanesian Re-entrant,southwest Pacific: Pacific Sci., v. 26, p. 289.

Colley, H. and Warden, A.J., 1974. Petrology of the NewHebrides: Geol. Soc. Am. Bull., v. 85, p. 1635.

Davies, H.L. and Smith, I.E., 1971. Geology of easternPapua: Geol. Soc. Am. Bull., v. 82, p. 3299.

Deventer, J. van and Postuma, J.A., 1973. Early Cenomanianto Pliocene deep sea sediments from north Malaita: Geol.Soc. Australia J., v. 20, p. 145.

Falvey, D.A. and Taylor, L.W.H., 1974. Queensland Plateauand Coral Sea Basin: structural and time-stratigraphicpatterns: Australia Soc. Explor. Geophys., v. 5, p. 123.

Gill, J.B. and Gorton, M., 1973. A proposed geological andgeochemical history of eastern Melanesia. In Coleman, P.J.(Ed.), The Western Pacific: island arcs, marginal seas andgeochemistry: Nedlands, Western Australia (WesternAustralia University Press), p. 543.

Hackman, B.D., 1971. Regional geology of Guadalcanal: acontribution to the study of fractured island arcs: Ph.D.Thesis, University of Western Australia.

Hayes, D.E. and Ringis, J., 1973. Seafloor spreading in theTasman Sea: Nature, v. 243, p. 454.

Johnson, R.W., Mackenzie, I.E., Smith I.E., and Taylor,G.A.M., 1973. Distribution and petrology of late Cenozoicvolcanoes in Papua New Guinea. In Coleman, P.J. (Ed.),The Western Pacific: island arcs, marginal seas andgeochemistry: Nedlands, Western Australia (WesternAustralia University Press), p. 523.

Johnson, T. and Molnar, P., 1972. Focal mechanisms andplate tectonics in the southwest Pacific: J. Geophys. Res.,v. 77, p. 5000.

Karig, D.E., 1970. Ridges and basins of the Tonga-KermadecIsland arc system: J. Geophys. Res., v. 75, p. 239.

, 1971. Origin and development of marginal basins inthe western Pacific: J. Geophys. Res., v. 76, p. 2542.

Kennett, J.P., Burns, R.E., Andrews, J.E., Churkin, M.,Davies, T.A., Dumitrica, P., Edwards, A.R., Galehouse,J.S., Packham, G.H., and van der Lingen, G.J., 1972.Australia-Antarctica continental drift, paleocirculationchanges and Oligocene deep-sea erosion: Nature Phys.Sci., v. 239, p. 51.

Kroenke, L.W., 1972. The geology of the Ontong-JavaPlateau: Hawaii Inst. Geophys. Rept. HIG-72-5.

Larson, R.L. and Chase, C.G., 1972. Late Mesozoic evolutionof the western Pacific Ocean: Geol. Soc. Am. Bull., v. 83,p. 3627.

Luyendyk, B.P., Bryan, W.B., and Jezek, P.A., 1974. Shallowstructure of the New Hebrides Island arc: Geol. Soc. Am.Bull., v. 85, p. 1287.

Luyendyk, B.P., MacDonald, K.C., and Bryan, W.B., 1973.Rifting history of the Woodlark Basin in the southwestPacific: Geol. Soc. Am. Bull., v. 84, p. 1125.

Mitchell, A.H.G. and Warden, A.J., 1972. Geological evolu-tion of the New Hebrides Island Arc: J. Geol. Soc, v. 127,p. 501.

Moberly, R., 1972. Origin of lithosphere behind island arcs,with reference to western Pacific: Geol. Soc. Am. Mem.,132, p. 35.

Molnar, P., Atwater, T., Mammerickx, J., and Smith, S.M.,1975. Magnetic anomalies, bathymetry and tectonic evolu-tion of the South Pacific since the Late Cretaceous:Geophys. J. Roy. Astron. Soc, v. 40, p. 383.

Packham, G.H., 1973. A speculative Phanerozoic history ofthe southwest Pacific In Coleman, P.J. (Ed.), The WesternPacific: island arcs, marginal seas and geochemistry:Nedlands, Western Australia (Western Australia Universi-ty Press), p. 369.

Packham, G.H. and Falvey, D.A., 1971. An hypothesis for theformation of marginal seas in the western Pacific: Tectono-physics, v. 11, p. 79.

Sclater, J.G., Hawkins, J.W., Mammerickx, J., and Chase,C.G., 1972.. Crustal extension between the Tonga and LauRidges; petrological and geophysical evidence: Geol. Soc.Am. Bull., v. 83, p. 2562.

Weissel, J.K., and Hayes, D.E., 1972. Magnetic anomalies inthe Southeast Indian Ocean. Antarctic Res. Ser., v. 19, Ant-arctic Oceanölogy II: The Australian-New Zealand Sec-tor: Am. Geophys. Union, p. 165.

Winterer, E.L., Riedel, W.R., et al., 1971. Initial Reports ofthe Deep Sea Drilling Project, Volume 7: Washington(U.S. Government Printing Office).

Winterer, E.L., Lonsdale, P.F., Matthews, J.L., and Rosen-dahl, B.R., 1974. Structure and acoustic stratigraphy of theManihiki Plateau: Deep-Sea Res., v. 21, p. 793.

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26. results of leg 30 and the geologic history of the southwest … · hebrides trench east of site...

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