ISSN.0110-0408

SOUTH PACIFICMARINE GEOLOGICAL NOTESTECHNICALCCOP-SOPACSUVA

SECRETARIATESCAP

VO!. 1 No. 8August 1978

MARINE GEOLOGY OF EASTERN CORAL SEA

(EASTERN MARGIN OF INDO-AUSTRALIAN PLATE, NORTH OF NEW CALEDONIA)

J. Daniel*, C. Jouannic*, B.M. Larue-, J. Recy*

ABSTRACT

The major structures of the eastern part of theCoral Sea analysed from the concept of plate tectonicsillustrate three fundamental types of fossil plateboundaries: the island arc, the spreading ridge andthe transform fault. Imprecise knowledge of the rela­tive ages of the structures makes an integrated geo­dynamic reconstruction of the region very speculative.

The Rennell Basin and associated small chain is asediment filled fossil subduction zone. Following thecessation of subduction the island arc subsided andwas capped with coral reefs.

The South Rennell Trough is a spreading featurewhich began opening at the eastern end of the struc­ture in late Oligocene, the spreading propagating west­ward in time forming a fan-shaped opening.

The d'Entrecasteaux Fracture Zone was a transi­tional feature connecting the inferred New Caledonia ­Loyalty Islands Subduction Zone and the West-SantoFracture Zone. This "transition" zone is similar tothe present New Hebrides - Hunter Transition Zonewhich connects the New Hebrides Subduction Zonewith the Hunter Transform Fault.

* ORSTOM B.P. A.5 NOUMEA - NEW CALEDONIA.

81

INTRODUCTION

Between the Australian continent and the boundaryof the Indo-Australian and the Pacifie plate (Fig. 1)represented by the Tonga-Kennadec, the New Hebridesand the Solomon Subduction Zones, is a presently in­active marginal zone from 2,000 to 3,500 km wide,characterized by complex morphology and a large di­versity in crustal thickness and nature.

Morphologically, south of 18° S. latitude, struc­tures such as the Lord Howe Rise, the New CaledoniaBasin and the Norfolk Ridge are found successivelyfrom south to north alonl; more than 2,000 km and far­ther to the east, the Three Kings Ridge and the SouthFiji Basin are also weIl defined.

North of 18° S. latitude the Coral Sea Basin isweIl determined, but the eastern part of the Coral Seaconsists of many distinct morphological features withvarious trends.

The literature contains many discussions inter­preting the individual structures in this marginal zonein the light of plate tectonics, notably: spreadingcentres have been found in the Tasman Basin (Hayesand Ringis 1973), in the Coral Sea Basin (Weisseland Watts 1977) and the South Fiji Basin (Weisseland Watts 1975); fossil subduction zones have beendiscovered in New Caledonia (Geze 1963; Dubois1971) and in the Vitiaz and Pocklington Troughs(Chase 1971; Karig 1972 and 1974). Landmesser(1975) discussed the origin of the eastern Coral Sea.Coleman and Packham (1976) discussed the struc­tures adjacent ta the present boundaries of the Iodo'Australian and the Pacifie plates"

10-·

.,

OC E A N"...

SOUTHFIJI

BASIN

". .'... ' i

r \

NORTHFIJI

PLATEAU G· ))'I;~" \(~

1\1

PACIFIC

)0. ~

r-i"'\~le)I~

( ~

. '- l '\~.

)~/ ~

( :1 ~.\ ""

TASMAN SEA

AUSTRALIA

10"

zoO

FIG. 1. Bathymetry and major structural leatures 01 the Southwest Pacific(Offshore structures delimited by 1000 lathom isobath).

82

Recent magnetic, bathyrnetric and continuous seis­mic refiection data collected by ORSTOM - Noumeafrom 1972 to 1976 during the AUSTRADEC'" and GE­ORSTOM cruises, suggest a new interpretation forthe Rennell Basin and Ridge, the South RennellTrough and the d'Entrecasteaux Fracture Zone (Fig.2).

RENNELL BASIN AND SMALL CHAIN

Rennell and Bellona Islands together with Indis­pensable Reefs and the adjacent basin (Rennell Basin)form a structural unit about 200 km long (Recy et al1975). The sea bottom in the Rennell Basin is fiat(Figs. 3 and 4) and generally less than 2,400 fathomsdeep. The western fiank of the basin is an inclinedplane with a widely curved bulge (Fig. 3, profile AA').The northeastern boundary is formed by the relativelysteep slope of the submarine plateau 600 to 800fathoms deep on which Rennell and Bellona Islandslie as uplifted atolls 200 and 80 m above sea level(Grover 1960; Taylor 1973). On Rennell Island thereef is 500 m thick.

In the Rennell Basin, three sedimentary formationsare observed: the upper formation (formation 1) with athickness varying north to south from 0.5 to 1.5 sec­onds (two way refl;ection tim~) lies unconformably overthe next lower umt. The mlddle unit (formation 2) is0.3 seconds thick and abuts against the IndispensableReefs bas~ment. A strong .acoustic refiector sepa­r~tes the mlddle and lower umts (formation 3). Forma­tlOn 3 thickens eastward (1.0 seconds on the profileED) and appears to have been deposited in a widerand less distinct basin than the present RennellBasin.

The magnetic anomalies along the western borderof the basin appear to result from volcanic intrusi vesand do not correlate from one profile to another (Fig.4);, the northwestern p~rt of the plateau (profile AlAl ) . has lower amplltude anomalies, suggestingmatenal with a lower magnetic susceptibility (vol­caniclastic rocks for instance).

Comparison of these data with those of Legs 21and 30 of the Deep Sea Drilling Project (Andrews et al1~7.3~ .Andrews, Packham et al 1973) lead to two pos­S.l b111tIes: the strong refiector found between forma­tIOns 3 and 2 in the Rennell Basin may correlate to

refiector 4 in (Andrews et al 1973), which was shownto be Middle Eocene in age. If this is the case thediscontinuity between formations 2 and 1 results fromthe Upper Eocene - Oligocene tectonic disturbance.

Refiector 3 - 2 in the Rennel1 Basin may correlateto the Upper Oligocene refiector 3 of Andrews et al(1973), in which case the discontinuity 2 - 1 in theRennel1 Basin would mark the end of the Upper Mio­cene tectonic phase.

Whatever its age, the general features of this struc­ture suggest a fossil subduction zone (Recy et al1975; Recy et al 1977). The trench was filled withhorizontal deposits during or after the subductionactivity. This kind of filling excludes the assump­tion of the basin having been formed by mere subsi­dence. The plateau and the islands are the remnantso~ the island arc. After the subduction ended, thedlsappearance of the driving stresses were fol1owedby negative isostatic readjustment which resulted insubsidence of the islands and growth of corals. ThePleistocene uplift of the atolls, might be due to thebulge associated with the present subduction of theSolomon Islands (Recy et al 1975).

SOUTH RENNELL TROUGH

South of the Rennell Basin, a 30 km wide and atleast 700 km long trough trends N 30° E. Packham(1973) and Terrill (1975) suggest the trough is a frac­ture zone associated with the opening of the TasmanBasin in Paleocene time. Landmesser (1975) asso­ciated the trough with the opening of the Coral SeaBasin in Eocene time.

The trough decreases in depth from south to north(Larue et al 1977) until reaching a saddle betweenprofiles HH' and II' (Fig. 2). Sedimentation occursonly in the deepest part of the trough where sedimentthickI:lesses up to 1 second are found (profile KK',Fig. 5).

The trough is bordered by two submarine ridgeswhich get wider from south to north. At the northernend these ridges form an une ven plateau overlain bya thin sediment cover.

The magnetic anomalies associated with the troughfeature (Larue et al 1977) have the longest wavelengths and the highest amplitudes in the region (pro­files JJ' and LL') and can be correlated from one pro­file to another (Fig. 6),

, AUSTRADEC cruises hav~ been sponsored by Office de la Recherche Scientifique etTechnl~ue Outre Mer, InstItut Français du Petrole, Céntre National pour l'Exploitationdes Oceans and French petroleum Companies.

83

FIG. 2. Location of seismic reflection Iines. Cross sections presented areindicated by heavy lines.

84

SWA h.

3 •.

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c B c oSE

h.

3 •.

NWNE ~'i<i;~}..~- ;

3 •.

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FIG. 3. Seismic refIection profiles across the RenneIl Basin. For this figure and aIl other cross sections, depths are in seconds of',"n "",,, rnf/"rlinn limp nnrl iripntifir.:lrinn letter refer to FiR. 2.

-1.,00.

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FIG. 4. Interpreted seismic reflection (solid lines) and magnetic profiles (dotted lines) across the Rennell Basin.

L '

K'

G'13 G4 Gl,,7

2

3

4

,,7

3 J4

!l

•7

2 Kl,4

,•7

2

,L

4

,•7

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~I~~~~5iiiiiiiiiiiiiZ:~~1

FIG. 5. Schematic cross section of South RenneII Trough.

87

1\

-00

+100

j:aJ-100

+"

-~[

G G' +, !IJ

\l~oo Il

H

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0

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:[1 1

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FIG. 6. Correlations of magnetic anomalies on South Rennell Trough.

88

A matrix inversion analysis Hutton (1970) gi vessusceptibility profiles with symmetries associatedwith the trough axis (Fig. 7). Those profiles may al­so be correlated between themselves, with a bettercorrelation towards the east than west.

The trough and its border may be interpreted asa spreading centre. The spreading would have begunat the eastern end and propagated later to the wes t.Northward, the crust generated at the spreading centreis triangular in shape as shown by the 2000 fat!lomisobath (Fig. 2). It could represent a "sphenochasm"after Carey (1958). A similar scheme was suggestedby Luyendyk et al (1973) for the opening of the Wood­lark Basin.

In the eastem part of the area, three heat fiow mea­surements have a mean value 1.86 H.F.U. (Halunenand Von Herzen 1973). Applying the heat fiow ­depth - age relationship to these data suggests an Oli­gocene age for the feature (Larue et al 1977; Sclateret al 1971: Sclater and Francheteau 1970).

D'ENTRECASTEAUX FRACTURE ZONE

The d'Entrecasteaux Fracture Zone extends fromthe northern extremity of the New Caledonian Ridgeto Espiritu Santo and Malekula Islands in the NewHebrides. Menard (1969) defined the d'EntrecasteauxFracture Zone as a major structural feature extendingfrom the New Hebrides to the Coral Sea Basin. Luyen­dyk et al (1973) having given it less importance, 10­cated this fracture zone farther north. Lapouille andDugas (1975) suggest that the area west of EspirituSanto and Malekula Islands is a fossil subductionzone. A more detailed description has been given byDaniel et al (1977). The major structural feature isa SSW-NNE trending fault seen on profiles PP', RR'and SS' (Fig. 2) with the NW fault block being upliftedvertically 1000 to 1500 m high (Fig. 8). The sedimentcoyer in the region reaches its maximum thicknessnear the upthrown block of the fault. Three formationscan be observed:

1. The relatively thin upper layer best seen onprofile UU':

2. The second layer which is up to 2 secondsthick near the fault and thinning to the NW;

3. The third layer generally of constant thickness(about 0.6 seconds thick).

The distribution of sediments in the area suggestthe following structural interpretations. The upperlayer is a recent deposit. The fault displaces thedeposits of both lower formations. Removing the ef­fect of this fault the upper part of the lower formationhas the shape of a narrow arched trough, filled with

89

horizontal sediment layers. The direction of fillingprecludes any subsidence similar to the RennellBasin (Recy et al 1975; Recy et al 1977). The ori­ginal structure has the shape of an oceanic trench butno associated island arc is seen. The morphology ofthe feature is not unlike the Hunter Fracture Zone(Dubois et al 1971).

In the New Hebrides the subduction zone and is­land arc are weIl defined. Further east, however, isa zone of transition· eventually joining the HunterFracture Zone. In this transition zone the arc' ispoorly developed and marked only by isolated vol­canic peaks. Similarly the d'Entrecasteaux FractureZone forms the bent extremity of the fossil New Cale­donia - Loyalty Islands Subduction Zone. During orat the end of the subduction process, the d'Entre­casteaux "Trench" was filled with sediments of thesecond layer. Later, the structure was warped as aresult of the South Rennell Trough spreading and, inits western part, by movement on the d'EntrecasteauxFracture Zone.

The d'Entrecasteaux structure abuts New HebridesSubduction Zone at the point where the trench dis­appears in front of Espiritu Santo and. M~lekul~ ~s­lands. Furthermore, a maximum of selsmIc actlVltywhich exists at the intersection of the d'EntrecasteauxFracture Zone and the Benioff Zone beneath EspirituSanto Island (B. Isacks and G. Pascal, pers. comm.)might be due to this old litospheric fracture.

CONCLUSION

In the eastern part of the Coral Sea three fundamen­tal fossil plate boundaries are seen; a spreading cen­tre (South Rennell Trough), a subduction zone (Ren­nell Basin) and a strike-slip fault (d'EntrecasteauxFracture Zone). The relative importance of thosethree structures must be studied in more detail. Fur­thermore. if the d'Entrecasteaux Fracture Zone andthe New Caledonian Subduction Zone were, on theworld scale, connected with the network of plateboundaries with ridges, subduction zones and trans­form faults, as a continuous and closed network, theRennell Basin and the South Rennell Trough wouldappear as structures more isolated and limited inspace.

If we consider a "first order" network of coherentplates around the earth, we might then consider a"second order" autonomous network with less spread­ing. This assumption raises the problem of rigidityand lack of distortion of plates in the vicinity of theirboundaries. We can, indeed. either assume" secondorder limits" to define .. second order plates" as rigidas the main plates, or assume those secondary struc­tures, containing sorne comparable characteristics ofthe "first order limits", as traces of plate distortionsin the vicinity of their ooundaries, connected with thereadjustment required by the plate geometry.

l'

IL'--'--

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0

0.01

t.0.02

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-0.02

I~~....._-----~=-~ ------

o l-----rmdllllJllllll.LLD=nrrITTT1rn:"'llilJlillWll.llll.LLW.ll.I.1JWJlJ.mrTTTTmrITTTT~L..U.LI..LL.LLI.~9lT1

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FIG. 7. Distribution o[ apparent susceptibility.1. Sea bottom.2. Lower Iiroit o[ the magnetized layer.3. Apparent susceptibility computed [or 20 prisms in e.m.u.

L

90

.. " ~ ... ". .. " . '.

p p'Or----------------------.

•

Iii L.... ---"

s.f.d.

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5'

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:1

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T T'

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s.f.d.

o 50 100 le",.

~H~iiiiiiiiii~§iiiiiiHIIIIIIIIII'_ïi~~iiiiiiiil!

FIG. 8. Schematic cross sections of d'Entrecasteaux Fracture Zone. Numbers refer to formationsdescribed in text.

91

M

· ': .. ,'

M'

.. ', ~ <..

0,...---------------------------- _

10 ~ .....I

N N'0r--------------------------...

• c.._-:::::::=::::~

10 ~ ..J

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10

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FIG. 8 (continued). Schematic cross sections of d'Entrecasteaux Fracture Zone

92

........

REFERENCES

ANDREWS, J.E. et al 1973: Deep Sea Drilling Pro­ject: Leg 21, Tasman Sea - Coral Sea (PreliminaryResults). Pp. 185-99 in R. Fraser (Comp.) "Oce­anography of the South Pacific, Wellington, NewZealand, 1972". National Commission for UNES­CO, Wellington.

ANDREWS, J.E., PACKHAM. G.H. et al 1973: Leg30 Deep Sea Drilling Project. Geotimes 18(9):18-21.

CAREY, S.W. 1958: The tectonic approach to conti­nental drift. Pp. 177-355 in S.W. Carey (ed.)"Continental Drift: a symposium". Univ. of Tas­mania, Hobart.

CHASE, C.G. 1971: Tectonic history of the FijiPlateau. Bull. geoi. Soc. Am. 82: 3087-110.

COLEMAN, P.J.; PACKHAM, G.H. 1976: The Mela­nesian Borderlands and India - Pacific Plates'Boundary. Earth-Sc. Rev. 12: 197-233.

DANIEL, J.; JOUANNIC, C.; LARUE, B.M.; RECY,J. 1977: Interpretation of d'Entrecasteaux Zone(North of New Caledonia). Pp. 117-24 in "Geo­dynamics in the South-West Pacific". SymposiumInternational, Nouméa, 27 August - 2 September1976. Editions Technip, Paris.

DUBOIS, J. 1971: Propagation of P waves and Ray­leigh waves in Melanesia: structural implications.J. geophys. Res. 76: 7217-40.

DUBOIS, J.; LARUE, B.M.; LAUNAY, J.; RECY,J.; REICHENFELD, C. 1971: Profils bathy­métriques et magnétiques à travers le BassinNord-Fidjien et la dorsale de Hunter. UnpublishedReport. ORSTOM, Nouméa. 31 pp.

GEZE, B. 1963: Observations tectoniques dans lePacifique (Hawaii, Tahiti, Nouvelles Hébrides).Bull. Soc. geoi. Francel7: 154-64.

GROVER, J.C. 1960: The geology of Rennell andBellona, the uplifted atolls on the edge of theCoral Sea. Pp. 103-19 in "The British SolomonIslands. 3. Natural History of Rennell Island",Danish Science Press, Copenhagen.

HALUNEN, A.J.; VON HERZEN, R.P. 1973: Heatfiow in the western equatorial Pacific Ocean.J. geophys. Res. 78(23): 5195-208.

HAYES, D.E.; RINGIS, J. 1973: Seafioor spreadingin the Tasman Sea. Nature 243: 454-8.

HUTTON, M.A. 1970: Interpretation of oceanic mag­netic anomalies using a linear inverses technique.Unpubl. Thesis. Univ. of Durham.

93

KARIG, D.E. 1972: Remnant arcs. Bull. geoi. Soc.Am. 83: 1057-68.

KARIG, D.E. 1974: Evolution of arc systems in theWestern Pacific. Ann. Rev. Earth Planet. Sei. 2:51-75.

LANDMESSER, C.W. 1975: Submarine geology of theEastern Coral Sea Basin, South-West Pacific.Unpubl. M.Sc. Thesis, Univ. of Hawaii. 64 pp.

LAPOUILLE, A.; DUGAS, F. 1975' Geologicalevolution of New Hebrides and Loyalty areas.Bull. Aust. Soc. explor. Geophys 6 (2/3): 52.

LARUE, B.M.; DANIEL, J.; JOUANNIC, C.; RECY,J. 1977: The South Rennell Trough: evidencefor a fossil spreading zone. Pp. 51-62 in "Geo­dynamics in South-West Pacific". SymposiumInternational, Nouméa, 27 August - 2 September1976. Editions Technip, Paris.

LUYENDYK, B.P.; MACDONALD, K.C.; BRYAN,W.B. 1973: Rifting history of the Woodlark Basinin the Southwest Pacific. Bull. geoi. Soc. Am. 84:1125-39.

MENARD, H.W. 1969: Anatomy of an expedition.McGraw Hill, New York.

PACKHAM, G.H. 1973: A speculative Phanerozoichistory of the Southwest Pacific. Pp. 369-88 inP.J. Coleman (ed.) "The Western Pacific: islandarcs, marginal seas, geochemistry". Univ. W.Aust. Press, Perth, W. Aust.

RECY, J.; DANIEL, J.; LARUE, B.M.; HAWKINS,L.V. 1975: De l'existence d'une zone de subduc­tion fossile dans la région de Rennell (Sud-OuestPacifique). C.R. Acad. Sei. Paris, 281: 489-92.

RECY, J.; DUBOIS, J.; DANIEL, J.; DUPONT, J.;LAUNAY, J. 1977: Fossil subduction zones.Examples in the South West Pacific. Pp. 345-56in "Geodynamics in South-West Pacific". Sympo­sium International, Noum~a, 27 August - 2 Septem­ber 1976. Editions Technip, Paris.

SCLATER, J.G.; FRANCHETEAU, J. 1970: Theimplication of terres trial heat fiow observationson current tectonic and geochemical models ofthe crust and upper mantle of the earth. Geophys.JI. 20: 509-42.

SCLATER, J.G.; ANDERSON, R.N.; BELL, M.L.1971: Evolution of ridges and evolution of theCentral Eastern Pacific. J. geophys. Res. 76:7888-915.

TAYLOR, G.R. 1973: Preliminary observations ofthe structural history of Rennell Island, SouthSolomon Sea. Bull. geaI. Soc. Am. 84: 2795-806.

TERRILL, A. 1975: Depositional and tectonic pat­tern in the northern Lord Howe Rise - Mellish Risearea. Bull. Aust. Soc. explor. Geophys. 6(2/3):37-9.

WEISSEL, J.K.; WATTS, A.B. 1975: Tectonic corn­plexities in the South Fiji marginal Basin. EarthPlanet. Sei. LeU. 28: 121-6.

WEISSEL, J.K.; WATTS, A.B. 1977: Evolution ofthe Coral Sea Basin. Second S.W. Pacifie Work­shop Symposium, 6-8 Dec. 1977. Univ. of Sydney.(Unpubl. Abstr.).

Publications in this series of Notes are intended toinclude results of local studies, brief reports dealingwith field observations, and short summaries and data

relevant to the aims of CCOP/SOPAC.

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