New Caledonia Basin-Fairway Ridge : Structural and Sedimentary Study

INTBRNATIONAL SYMPOSIUM ON GEODYNAMICS IN SOUTH-WEST PACIPIC NOUMEA (NEW CALEDONIA) 27 AUGUST.2 SEPTRMBER 1976 BDITIONS TECHNIP, PAUS 1977, pp. 145-154

NEW CALEDONIA BASIN-FAIRWAY RIDGE : STRUCTURAL AND SEDIMENTARY STUDY

C. RAVENNE(’), C.E. de BROlNt2),J. DUPONT(3) A. LAPOUILLE‘3’and J. LAUNAYl3’ 2 2 SEP, “77 o.w. s. T. o.b9.

Abstract Austradec I and II marine seismic In this paper we first describe the struc- surveys carried out in 1972 and 1973 by the tural elements, and then we show how magnetic IFP-CEPM-ORSTOM group in the southwes tem Pa- data confirm our hypotheses , before examining cifi,c contribüted to the exploration of the the sedimentary series. structural elements located between Lord Howe Rise and Norfolk Ridge, investigated the sedimentary fills in accordance with data from DSDP boreholes, and pointed out the impor- tance of volcanic alignments, GEOGRAPH I C SI TUATION The leading structural element discovered by these surveys was the Fairway Ridge begin- The area investigated is located in the ning at Fairway Reef in the North, Two of us South West Pacific (fig, 1). It is framed by (C.E. de Broin and C. Ravenne) consider that Lord Howe Rise in the West and Norfolk Ridge this ridge prolongs into West Norfolk Ridge in the East. It is bounded in the North at towards the South, the latitude of New Caledonia and in the South by the northermost tip of West Norfolk Ridge. The discovery of this ridge thus leads us to divcde the New Caledonia Basin into an eastem New Caledonia Basin sensu stricto and a wes tem Fairway Basin. GEOTECTONIC SITUATION A study of seismic facies and data from DSDP boreholes enables us to suggest that the This area is located in a mixed marginal fill in the New Caledonian Basin sensu stric- zone context bounded by a continental area, to is mainly of turbidite origin and that in Australia in the West and oceanic zones of re- the Fairway Basin is mainly pelagic except in cent origin in the East, i.e. the Tasman Sea, its southem part. The proportion of elements the South Fiji Basin and the North Loyalty derived from volcanism is considerable in both P1ateau. basins. The crust varies in thickness there, ran- Lastly, we clearly delimit two volcanic ging from a semicontinental thickness of 20 alignments, i.e. one located in the eastem to 30 km under Lord Howe Rise and Norfolk Rid- part of Lord Howe Rise, and the other in the ge to a semioceanic thickness of about 15 km westem part of Norfolk Ridge. under the basin (Shor et al. , 1971).

(1) Institut Français du Pétrole, 1 et 4 avenue de Bois-Préau, 92502 Rueil-Malmaison, France (2) Société Nationale Elf Aquitaine (Production), 26 avenue des Lilas, 64001 Pau, France (3) Office de la Recherche Scientifique et Technique Outre-Mer, B.P. AS, Noumea Cedex, New Caledonia

145 NEW CALEDONIA BASIN-FAIRWAY RIDGE : STRUCTURAI, AND SEDIMENTARY STUDY

PREVIOUS WORK The central part of the ridge, which is the longest part (about 700 km), trends to the North, It abruptly changes its direction It consisted mainly in bathymetry (maps at the level of Fairway reef West of New Ca- by Scripps Institution of Oceanography and New ledonia. In the North it is localized under- Zealand Oceanogr. Inst.) and in seismic re- neath the flank of Landsdm Bank which bounds fraction (Shor et al, , 1971). On the other Lord Howe Rise. Fairway Ridge ends abruptly hand the DSDP boreholes are a valuable con- at 19" S latitude, in a roughly East-West tribution to the understanding of the litholo- trending fracture zone. gy and of marine unconformities (R.E. Burns, J.E. Andrews et al., 1973). Besides the seis- To the South, West Norfolk Ridge extends mic reflection profiles by Mobil Oil Co publi- in the exact prolongation of Fairway Ridge. shed by F.P. Bentz (1974) , no other seismic Two of us (C. Ravenne and C.E. de Broin) con- reflection survey has been done in this area. sider that West Norfolk Ridge is the Southern part of Fairway Ridge. The other three authors divide it into two structural élements of dif- f erent origin.

DATA The axis of Fairway Ridge divides into two sub-parallel alignments of basement in West Norfolk Ridge where they delimit a narrow ba- For this paper, we have made use of B- sin. West Norfolk Ridge curves South East in -LZ'OM seismic lines recorded by ORSTOM and es- its southern tip (fig. 2). pecially of Austradec multitrace seismic-reflection profiles recorded by that Ifl-c~p)!- Fairway Ridge is shallow at both ends and ORSTOM. progressively becomes deeper and buried to An interpretation ot the magnetic anoma- 3,000 m in its piddle part. Under a thin layer lies was attempted, also using data from the of sediments, its substratum has a characte- Austradec and Georstom profiles. tistic seismic facies (fig. 5 a, by c) with a strong reflector overlying a blind zone. In this it differs from Lord Howe Rise and Nor- folk Ridge where the substratum reveals orga- STRUCTURAL FEATURES nized reflections. The presence of these reflections and the thickness of the underlying crust from refraction data (Shor, 1971) ap- 1. Fairway Ridge. pears to prove that both these ridges are of the semicontinental type. Despite the absence This is a new feature revealed by our sur- of seismic refraction on Fairway Ridge, magne- veys on the basis of seismic reflection and tic anomalies and the absence of any organized magnetism data, We began to suspect its exis- reflection suggest that the substratum is tence in 1973 (Dubois et al., 1974). mainly of volcanic origin (cf. the study of magnetic anomalies described hereunder). Fairways Ridge is a major structural feature in the South West Pacific as are Lord The sedimentary cover on this ridge is Howe Rise and Norfolk Ridge (fig. 2 and 3). very thin at either end, up to two to three It divides the New Caledonia Basin into two hundred meters, It gradually becomes thicker sub-basins and plays an important role as a in the middle part where the ridge is buried barrier in the distribution of sediments. under the sedimentary series of the New Cale- donia Basin. ___ __ It appears as a basement ridge (seismic facies , magnetic signature, geologic framework) It is obvious that the origin of this about 70 km wide (fig. 4, 5 a, b, c) and near- ridge lies within the geotectonic evolution ly 1,500 km long. of the South West Pacific, but it is even more difficult to say how it was created, Per- The magnetic signature of this ridge is haps it was a frontal bulge of oceanic crust . characteristic. This aspect will be discussed in front of a subduction zone located to the later in this paper. East as suggested by Dubois et al., (1974). However, perhaps it is a former volcanic The ridge runs almost parallel to Norfolk alignment like the Lord Howe Guyots or Ridge and follows the same directional chan- possibly a former volcanic arc corresponding to one of the subduction stages between the ges. Australian and Pacific plates,

146 .NEW CAMDONIA BASIN-FAIRWAY RIDGE : STRUCTURAL AND SEDIMENTARY STUDY

2. Fairway Basin (fig. 2, 3, 4, G a, by c). of seamounts buried within a sedimentary series. The upper part of this string roughly We call Fairway Basin that part of New Ca- corresponds to the Oligocene unconformity. So- ledonia Basin "sensu lato" which is located me of these volcanoes were later reactivated between Lord Howe Rise and Fairway Ridge. We by subsequent tectonic activity, keep the name South New Caledonia Basin for the southem part of this basin between West Norfolk Ridge and Lord Howe Rise.

Its size and trend are identical to those 3. Piew Caledonia basin Sensu Stricto of the ridges surrounding it because it occu- (fig. 2, 3, 4, 7 a, by c). pies the entire depression between these ridges. What we call New Caledonia Basin sensu stricto is the basin between Fairway Ridge and According to our profiles, it appears to Norfolk Ridge. This basin is much smaller in end in the North at Landsdown Bank. size (about 1,000 km long) than the other

a structures because Wanganella Bank between In its central part, West of Norfolk Is- West Norfolk Ridge and Norfolk Ridge (fig. 3) land, the basin lies under 3,200 m of water. closes it off in its southem part. To the This depth decreases gradually towards its North it is closed off at the fracture zone ends, although leveling off at around 3,000 m bounding Fairway Ridge. The northern end of in front of West Norfolk Ridge. The deepening the. New Caledonia Basin, i.e. the part loca- of Fairway Ridge in its middle part elimina- ted off New Caledonia, has previously been tes its role as a barrier between Fairway and described (Dubois et al., 1970). We should New Caledonia Basins, thus changing the dis- just note the presence of a small basin (fig, tribution of the sediments. 7 c) to the South of New Caledonia, between New Caledonia Basin sensu stricto and Norfolk The basement of this basin is clearly de- R$dge, as was revealed by Dupont et al, , limited in some places by a powerful reflec- (1975). tor revealing considerable diffractions (fig. 6 b) . In other places , the substratum is poor- New Caledonia Basin S.S. has an even flat ly defined because it is masked by diffracting bottom at a depth of about 3,600 m. reflectors (volcanic series ?) or by thick series. Under a sedimentary blanket 2,000 to 3,000 m thick, a basement can be seen that is high- The sedimentary fill (fig. 2) is general- lighted on the seismic profiles by a strong ly about 2,000 m thick. Two much thicker areas reflector (fig, 7 a) on top of a great many were found. One in the northem part of the diffractions. basin right off the southern tip of New Cale- donia is probably linked to the degradation of the reliefs of Landsdown and Fairway Banks as well as to the Volcanic outflows underlining the contact between Lord Howe Rise and Norfolk 4. Norfolk Ridge (fig. 2, 3, 4). Ridge, the other located to the South of West Norfolk Ridge with a thickness of about 3,000 I% of sediments is probably linked to the de- It stretches nearly 1,500 km between the gradation of West Norfolk Ridge and the proxi- North Western tip of New Zealand and New Cale- mity of New Zealand. donia. Two of us (C.R. and C.E. de B.) assume that Norfolk Ridge is linked to the North Wes- A rise of the basement west of the North- tem tip of New Zealand through the Vening em tip of West Norfolk Ridge forms a small Meinesz fracture zone. The corresponding off- threshold that divides the basin in its leng- set might have been caused by the opening up thwise dire ction, of the Tasman Sea farther to the South West, To the North, it extends into New Caledonia. At the contact between Fairway Basin and Norfolk Ridge is dissymmetric. The sedimentary Lord Howe Rise, we found a volcanic alignment series there are tilted towards the East, The on the eastem flank of Lord Howe Rise (high western edge is made up by an alignment of axis in fig. 2). Some evidence of this volca- volcanic intrusions, This alignment appears nism has been presented by F. Bentz (1974). to extend into the western volcanic "sillon" This alignment occurs in the shape of a string (P. Routhier, 1973) of New Caledonia.

147 NEW CALEDONIA BASIN - FAIRWAY RIDGE : STRUCTURAL AND SEDIMENTARY STUDY

The different sedimentary series and their saltic body, Its magnetic characteristics thus age were recently described by J. Dupont et suggest that Fairway Ridge is a magnetic rise. al. (1975). The position of this ridge linking This is shown accross the filtered regional (4 two areas having almost exactly the same geo- profiles which cross it (Lapouille , this volu- logical history (New Caledonia and the Auck- me), and it joins West Norfolk Ridge in the

land region) (A.R. Lillie and R.N. Brothers, South via the Vening Meinesz Fracture Zone. 4 1970) as well as the sedimentary series cha- 4 racteristics from seismic data suggest that This magnetic rise is separated from the this ridge has undergone the same geological continental Lord Howe Rise by a narrow basin history as what is known at its two ends. we call Fairway Basin. This basin is characterized by a negative magnetic anomaly of high Norfolk Ridge separates two very different amplitude , several hundred gannnas , whose wave- sedimentary provinces, i.e. in the West New length is equal to the basin width. Caledonia Basin with its thick sedimentary fill, and in the East a complex region bare of The reduction in the pole of the profile sediments and apparently of recent origin. crossing Lansdown Bank (fig. 9) reeeals the cnaracteristic magnetic minimum of the basin, while the upward prolongation of this profile removes the high frequencies of the variations e ANALYSIS OF MAGNETIC ANOMALIES which are thus due to a topographical effect, since the seafloor is very close to the sea level there. The substratum, as shown by magne- Magnetic anomalies have made a very valua- tic anomalies, is thus higher in the northern -+ ble contribution to the structural knowledge part of the basin. of the unit till now called New Caledonia Ba- sin. The magnetic relief is quite different The New Caledonia Basin sensu stricto, from that over the neighboring Lord How Rise which is then reduced to the area located bet- and Norfolk Ridge. Whereas across those rises ween Fairway Rise and New Caledonia-Norfolk ' the magnetic anomalies are diverse and intense Ridge, is characterized by a lack of well-mar- (Lapouille, this volume; Launay et al, this 4 , ked magnetic anomalies. This area is magneti- volume) , here they are well characterized. cally well contrasted and distinguished from the neighboring ridges which are marked by large positive anomalies outlining Fairway A positive magnetic anomaly with a long Ridge on one hand and basaltic uplifts on the wavelength , more than one hundred kilometers slope between Norfolk Ridge and the New Cale- long and several hundred gammas in amplitude, donia Basin on the other hand. The magnetic extends along the length of New Caledonia Ba- variations here are extremely slight, the am- sin where it can be seen on all the profiles plitudes very low and the wavelengths not well crossing the basin (fig. 8). This positive defined. The great depth of the basaltic base- anomaly, sometimes divided into two parts , ta- ment (overlain by a thick sedimentary layer) kes its origin in the eastern border of Lans- could explain the relative lack of magnetic down Bank. A reduction in the magnetic pole variations, (Galdeano, 1974) of the profile crossing the area above Lansdown Bank, assuming the rema- nent magnetism to be oriented like the pre- sent magnetic field, also induces a slight r phase displacement of the magnetic anomalies h to the East (fig, 9). We can thus suggest that the corresponding geological structure has its SEDIMENTARY SERIES origin on the eastern border of Lansdown Bank, A three kilomete upward prolongation of this profile (Galdeano , 1974) emphasizes the In the basins, seismic profiles enabled breadth of this anomaly whose corresponding us to distinguish up to three main sedimenta- geological structure is thus quite deep-sea- ry series separated by unconformities. Data ted. A geological model defined by its magne- from DSDP boreholes of leg 21, together with tic susceptibilities (Lapouille , this volume) the known land geological history (Lillie and will prove these conclusions that the so-cal- Brothers, 1970) enable us to propose the fol- led Fairway Rise structure should be seated to lowing ages for the main unconformities. the North East and under Lansdowne Bank, It is represented by a magnetic susceptibility ran- - Unconformity 1, which is found through- ging to 13.10-3 c.g.s. corresponding to a ba- out the entire area examined, corresponds in

148 NEW CALEDONIA BASIN-FAIRWAY RIDGE : STRUCTURAL AND SEDIMENTARY STUDY the boreholes to a gap between the Upper Eo- They bend towards the North West in the cene and bower and Middle Oligocene. The un- North and towards the South East in the South. conformity is especially obvious on the rid- These bends appear to have been caused by sprea- ges and on the edges of the basin. In the mid- ding in the North Loyalty and North Fiji Pla- dle of the basins the series are often confor- teaus in the North and by the creation of the mable, and the problem can be raised of the Tasman Sea in the South. possible existence of a more complete series. Three of these elements whose structure is - Unconformity 2, is probably situated in described in this paper - Fairway Basin and tli8 Upper Cretaceous. It is sometimes confused Ridge and New Caledonia Basin S.S. - gradually with the acoustic basement, become deeper in their middle part at the latitude of Norfolk Island. - Unconformity 3, which is not reached in the boreholes , corresponds on seismic profiles Fairway Ridge, which is the structural to different phenomena. Whereas in New Caledo- element that we reveal, is probably an element nia Basin and on Fairway Ridge it overlies an aqsociated with an active fossil margin (fron- obvious acoustic basement, it appears on the tal bulge, former alignment: or volcanic arc). contrary on Lord Howe Rise, Norfolk Ridge and the western border of Fairway Basin to Fairway Basin and New Caledonia Basin S.S., correspond to a strong unconformity separating whose substratum reveals no organized reflec- series of sedimntary origin (Dubois et al,, tion and whose crust is thin, are basins 1974). It is not obvious that this unconformi- probably created in connection with the buil- ty observed on the seismic sections corres- ding of former island arcs (inter-arc or mar- ponds to a synchronous phenomenon throughout ginal basin), The sedimentary fill indicates the whole area. that the age of creation was older than late Cretaceous and at most jurassic. DSDP boreholes, seismic facies and for- mations known on land suggest that sediments are mainly pelagic in Fairway Basin and in the central part of New Caledonia Basin. Near emerged land (New Caledonia) or very shallow ridge (West Norfolk Ridge) the fill looks more We thank ORSTOM, IFP, CP, turbiditic (fig. 7 c), probably with a high for permission to publish proportion of volcanic elements. this paper. We are indebted to the officers and crews of the N.O. Coriolis and Noroit from The two upper series thin out considerably CNEXO and to J.P. Fail and R. Donatien which on the ridges, and unconfomities 1 and 2 are were geophysicists during Austradec II cruise. often found to merge, On the other hand, mainly on Lord Howe Rise, there is a considerable We are grateful to M. Poulet and G. Grau developmnt of the underlying series under un- for their critical reading of this paper. conformity 3 (Dubois et al, , 1974).

BI BLI OGRAPHY

CONCLUSI ON BENTZ (F.P.),- 1974. Marine geology of the Southern Lord Howe Rise, South West Paci- fic, in The Geology of Continental Mar- The area under investigation is a transi- gins. C.A. Burk and C.L. Drake Editors, tion zone in which the convergence of the AUS- SpringerVerZag, New York, pp. 537-547. tral-Indian continental plate and the oceanic Pacific plate is revealed during its evolution BURNS (R.E.), ANDREWS (J.E.) et al.- 1973. by a succession of fossil island arcs with as- Initial reports of the Deep Sea Drilling sociated structures and elements detached from Project, vol. XXI, U.S. Govt. Printing the Australi an margin. Office , Washington , D. C.

The structural elements making up this re- DUBOIS (J,), RAVENNE (C.), AUBERTIN (F.), gion are, for the most part, stretched out LOUIS (J.) , GUILLAUME (R.) , LAUNAY (J.) along a submeridian direction. and MONTADERT (L.) .- 1974. Continental Margins near New Caledonia, in The Geolo-

149 NEW CALEDONIA BASIN-FAIRWAY RIDGE : STRUCTURAL AND SEDIMENTARY STUDY

gy of Continental Margins. C.A. Burk and Seismic traverses across the Northern Lord C.L. Drake Editors, S$T=inge.er Verzug, New Howe Rise and comparison with the Southern York, pp. 521-535. part, in this volume. r.. DUPONT (J.), LAUNAY (J.), RAVENNE (C.) and De BROIN (C.E.) .- 1975. Données nouvelles sur LILLIE (A.R.) and BROTHERS (R,N.).- 1970. The la ride de Norfolk (Sud Ouest Pacifique). geology of New Caledonia. New ZeaZand J. C.R. Acad. Se. , Paris, t. 281 , série Dy &OZ. Geophys. , vol. 13, no I, pp. 145- pp. 605-608. 183.

GALDEANO (A.) .- 1974. Traitement des données ROUTHIER (P.).- 1953. Etude géologique du ver- aéromagnétiqueq : méthodes et applica- sant occidental de la Nouvelle Calédonie tions. Th'he'se 3eme cycZe, Université Paris entre le col de Boghen et la pointe d'Ara- VI. ma, Mem. Soc. geoZ. Fr. NouveZZe Se'&, t. 32, fasc. 1-3, feuilles 1-34 Mem., no LAPOUILLE (A.) .- 1977. Magnetic Surveys over 67, pp. 1-271. the rises and basins in the South West Pa- cific, in this volume. SHOR (G.G.), KIRK (H.K.) and MENAkD (H.W.).- 1971. Crustal structure of the Melanesian area, J. Geophys. Res., vol, 76, no 11, pp. 2562-2586.

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150 NEW CALEDONIA BASIN-FAIRWAY RIDGE : STRUCTURAL AND SEDIMENTARY STUDY

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