TECTONICS, VOL. 6, NO. 3, PAGES 215-232, JUNE …horizon.documentation.ird.fr/.../divers20-05/010028321.pdf216 Collot et al.: Geophysics of New Caledonia Ophiolite TASMAN SEA 10 S

TECTONICS, VOL. 6, NO. 3, PAGES 215-232, JUNE 1987

OVERTHRUST EMPLACEMENT OF NEW CALEDONIAOPHIOLITE: GEOPHYSICAL EVIDENCE

J. Y. COll~t,1,2 A. Malahoff,3 J. Recy,lG. Latham, and F. Missegue1

Abstract. Geophysical studies supportinferences from outcrop geology thatduring the Late Eocene an ophiolite sheetexposed on New Caledonia was thrustsouthward over a rock complex consistingof sedimentary, volcanic, and metamorphicrocks that range in age from Pre-Permianto Eocene. The outcropping ultramaficcomplex consists of a layered sequence,approximately 3000 m thick, includingharzburgite, dunite, wehrlite,serpentinite, and gabbro. The absence ofpillow basalts and sheeted dikes on landsuggests that during or afteroverthrusting, these units were removedeither by thrust faulting or by erosion.Seismic refraction profiles, collectedover the adjacent Loyalty Basin, show thatthe lower crust and upper mantle arecharacterized by velocities of 4.7, 5.8,6.8, and 8.1-8.4 km/s, suggesting the

1Institut Francais de RechercheScientifique Pour le Developpement enCooperation, Noumea, New Caledonia.

2presently at the U.S. GeologicalSurvey, Branch of Pacific Marine Geology,Menlo Park, California.

t~~~;::S~:~v~fo~~W:i~~sH~:~~~;~~ion,Tulsa, Oklahoma.

Copyright 1987by the American Geophysical Union.

Paper number 7T0200.0278-7407/87/007T-0200$10.00

presence of a complete oceanic sectionthat includes sediment, basalt, gabbro,and peridotite. The lower crust isprobably 6.5 km thick beneath the LoyaltyBasin. A seismic reflection profile thatextends across the Loyalty Basin isinterpreted to show the oceanic crustrising toward New Caledonia. We suggestthat the ophiolite on New Caledonia iscontinuous with this rising oceaniccrust. Studies of gravity anomaliesobserved both on land and offshoreindicate the presence of a shortwavelength, high-amplitude (+180 mGal) ,asymmetrical, free-air gravity anomalyalong the northeastern coast of NewCaledonia. A gravity anomaly profile,calculated for a geological modelcharacterized by a 10-km-thick slab ofoceanic crust and mantle materialextending continuously from the ophioliteon New Caledonia to the oceanic crust ofthe Loyalty Basin, successfully matchesthe observed gravity data. We interpretthe extension of the high-gravity anomalyover the whole length of the easternlagoon as evidence for the lateralextension of the root zone of theultramafic complex. A tentativegeodynamic reconstruction suggests thatthe proto-Loyalty Basin crust formed amarginal basin along the eastern margin ofGondwanaland prior to Upper Cretaceoustime; after the opening of the NewCaledonia Basin during Upper Cretaceous toearly Paleocene time a subduction zonedeveloped along the western margin of New

216 Collot et al.: Geophysics of New Caledonia Ophiolite

TASMAN

SEA

10°S

20°

OCEAN

175°W

Fig. 1. Location of New Caledonia in the Southwest Pacifie.

Caledonia. While this zone was active,the Loyalty Basin lithosphere overthrustNew Caledonia in a southeasterly directionand locally rotated clockwise. Thiscompressional tee tonie event wasterminated by upper Eocene time.

INTRODUCTION

Ophiolite exposures of mafic andultramafic rocks of the oceaniclithosphere incorporated into orogeniebelts are of great scientific interestsince they provide direct access todetailed petrographie and structuralstudies of the oceanic crust and theuppermost mantle. Large ophiolitecomplexes were interpreted to have beenoverthrust or "obducted." In the light ofplate tectonic theory, several processesof ophiolite emplacement have beenproposed, including those by Davis andSmith [1971], Coleman [1971], Christensenand Salisbury [1975], Moores [1973], Dewey[1976], and Brookfield [1977]. In manyareas studied, the geological history ofemplacement has been complicated or

obscured by later tectonic events.Geological and geophysical studiesconducted on and around relativelyundeformed ophiolite belts have shown thatthese rock bodies fall into two differenttypes: a slice of oceanic lithospherethat has been separated from its roots, asis the case with the Samail ophiolite[Coleman, 1981; Manghnani and Coleman,1981J, or a section of oceanic lithospherethat can be traced to an ocean basin as adense, continuously dipping slab. Theevidence of ophiolites dipping downwardinto the surrounding lithosphere appearsto be weIl supported by geophysicalstudies of the Troodos massif of Cyprus[Gass and Masson-Smith, 1963) and theEastern Papua New Guinea belt [Milsom,1973; Finlayson et al., 1977] and in thecurrent study of the New Caledoniaultramafic complex. New Caledonia (Figure1) is considered to be a part of the innerMelanesian arc by Glaessner (1950) andAvias [1973]. The New Caledoniaultramafic complex has been interpreted byseveral authors [Aubouin et al., 1977;Parrot and Dugas, 1980] to be part of a

Collot et al.: Geophysics of New Caledonia Ophiolite 217

discontinuous ophiolite belt that extendsfrom New Guinea to New Zealand.

The ultramafic complex of New Caledoniais a relatively undeformed peridotiticmass, covering Many different formationsexposed over a lateral distance of morethan 400 km. Geological observationssuggest that the ultramafic mass couldhave been derived from the north. Todate, only limited geophysical studies ofits root zone have been undertaken. Thispaper focuses upon the results of acombined seismic refraction and gravitystudy of the eastern margin of NewCaledonia and its ultramafic complex.

GEOLOGICAL SETTING

Early geological field studies of theultramafic exposures and their underlyingbasement rocks of New Caledonia byRouthier [1953], Avias [1967], Lillie andBrothers [1970], Guillon [1975], Rodgers[1976], Paris et al. [1979], and Paris[1981] suggest that the peridotite rocksare the remnant of a single overthrustsheet of oceanic crust and mantle that wasemplaced during upper Eocene time over themargin of a continental fragment.

The continental affinity of NewCaledonia Island is suggested by thenature of outcropping basement rocks,which are made up of Pre-Permian toJurassic greywacke that are closelyrelated to those of New Zealand [Lillieand Brothers, 1970] and probably to thoseof New Guinea [Brothers and Blake,1973]. In addition, studies of earthquakewave propagation by Dubois [1969], as weIlas a gravity study by Collot and Missegue[1977], indicate that New Caledonia has adeep crustal root with a thickness of 30to 35 km. This root thins to 21 km southof New Caledonia Island, along the Norfolkridge, as shown by refraction data [Shoret al., 1971]. Rocks forming this rootappear to have affinity with bothcontinental and oceanic crust. TheNorfolk ridge, therefore, MOSt likelyencompasses a continental fragment thathas been complicated by subsequentmagmatic or tectonic events, whereas thecrust beneath the surrounding basins hasoceanic characteristics. Griffiths [1971]suggests that this ridge is underlain bycontinental rocks, probably representing apart of the eastern margin of Gondwanalandthat separated and migrated eastward fromGondwanaland at the time of opening of theTasman Sea during Late Cretaceous toPaleocene [Hayes and Ringis, 1973] and theCoral Sea during Eocene [Falvey and

Mutter, 1981]. In addition, Kroenke[1984] interprets the intercalations ofreworked Coniacian-Campanian rhyolitictuffs in the tholeiitic basaIt of the westcoast of New Caledonia as evidence for anearly stage of rifting; he concludes thatthe Norfolk and Lord Howe ridges breakupinitiated during Late Cretaceous time.

The ultramafic rocks of ~ew Caledoniacover approximately 6000 km. They arerepresented principally by the "GrandMassif du Sud," the west coast massifs,and Many strings of serpentines, scatteredover structural units of the island(Figure 2). The three principalstructural units of New Caledonia include(1) the northern region showing theeffects of Tertiary metamorphism, (2) thecentral mountain chain, which consists ofthe oldest rocks of New Caledonia, and (3)the west coastal area that includes widelyexposed basalts and a deformed Tertiarysedimentary basin.

The rocks studied by Guillon [1975] andParis [1981] show the ultramafic exposuresto consist of a serpentinized base and a"main peridotitic unit" composed ofmassive harzburgites containing dunite andpyroxenite intercalations. In the GrandMassif du Sud the peridotitic unit islocally topped by a dunite-gabbro unit[Moutte and Paris, 1977] commonly showingcumulate textures. Although pillow lavas,sheeted dike swarmS, and the upper layeredgabbros are not exposed here, Coleman[1971] and Aubouin et al. [1977] suggestan ophiolitic origin for this ultramaficmass. Guillon [1975] suggests fromgeological evidence that, on land, theultramafic section May be as thick as 3000m.

The age of the ultramafic mass has beententatively determined by Prinzhofer[1981] to fall between two K/Arradiometric age groups. The first groupranges between 100 and 80 Ma and wasobtained from undeformed dole rite veinscutting peridotite and from plutonicrocks, ranging from hornblendites togranodiorites, that were obtained from theGrand Massif du Sud and from a gabbrosampled on Bogota Peninsula on the eastcoast (Figure 2). The second group, witha radiometric age of 52 Ma, was determinedfrom both whole-rock samples and amphibolesepara tes from a dolerite in the GrandMassif du Sud. These two agedeterminations mark two periods ofintrusion into the peridotites whichoccurred before overthrusting took placebut after the peridotites were formed.Therefore the age of formation of the


167°

~~~ NORTH LOYALTY

~\ ~ \~ BASIN

:: ".}

v( 0 J- "7 o~vj.~l>\/

(l' ~~ \

J- LlFOU

~CD uCIJ DG) D© I>'«+:~I:~. -- :I@fC.:: -:: ::: ::: -,_ c;- - 1 ®

s

Fig. 2. Simplified geological map of New Caledonia after Paris [1981].Bathymetry is in kilometers. (1) Pre-Permian basement: tuffs, cherts andvolcanics; (2) Perrnian to Jurassic: tuffs, volcanics and terrigenous rocks;(3) Ante-Senonian pluto volcanic complexes; (4) Upper Cretaceous to Eocene:terrigenous rocks, volcanics, radiolarian cherts, flyschs with limestonelenses; (5) BasaIt of the west coast, Senonian to rniddle Eocene; (6)Ultrabasic cornplex; (7) Reef complex; (8) Undifferentiated volcano sedimentaryprisrn Permian to Eocene; (9) Mont Panie Eocene blueschists.

peridotite is not known precisely but isprobably greater than 80-100 Ma.

The ultramafic sheet exposed on landappears to be rooted to the northeast ofNew Caledonia Island and to haveoverthrust the underlying basement rocksfrom north to south. The location of theroot is deduced from the followinggeological observations. The basalcontact of the sheet is subhorizontal inthe western and central part of the islandand gradually dips to the northeast andsteepens along the northeast coast. Thebase of the sheet is marked by adiscontinuous and strongly sheared andmylonitized zone of serpentine.Slickensides and foliations within the

serpentinite are parallel with the basalcontact and dip to the northeast [Rodgers,1976]. Analyzing structures, formedduring overthrusting of the ophiolite,Paris [1981] demonstrates that theophiolite sheet was initially transportedfrom the north. Overthrusting intensivelydeformed the basal tic complex and thesedimentary sequences exposed along thewest coast, where upper Eocene rocksreveal the youngest evidence for shearingdeformation, and the main direction ofdisplacement was oriented NNW-SSE. Then,in southern New Caledonia, a late westwardrnovement of the ophiolite occurred asindicated by the axes of the folds in theunderlying basement rocks, near Noumea,


which are overturned to the west [Avias,1967]. In northern New Caledonia, anHP/LT metamorphism accompanied theoverthrusting event. Sediments located innortheastern New Caledonia range in agefrom Eocene to possible Permian and weresupposed by Brothers [1974] and Blake etal. [1977] to have been deposited into adeep trench near a plate junction. Theserocks underwent high-pressure metamorphismduring the upper Eocene time [Blake etal., 1977] to form a belt including rocksof blueschist and amphibolite facies;metamorphic grade increases northnortheastward across the belt. Themetamorphic event was dated by Blake etal. [1977] as being between 39.3 and 36.2Ma. Therefore the age of metamorphism ofthe underlying sedimentary basementprobably is contemporaneous with theoverthrusting of the ophiolite. Thecorrelation between the age ofmetamorphism of the basement sedimentaryrock and the age of overthrusting of theophiolite is substantiated by the studiesof Paris et al. [1979], who dated theoverthrusting stratigraphically as 38 Ma.

In order to develop a structural modelof the overthrust ophiolite sheet, we usedgeologic, and seismic refractioninformation [Pontoise et al., 1982], asweIl as marine and land gravity data[Collot et al., 1982].

MARINE SEI SMIC DATA

The bathymetry of the Loyalty Basin(Figure 2) shows it to be an elongatedtrough (2400 m deep) bounded on thesouthwest by New Caledonia and on thenortheast by the Loyalty Islands, whichencompass a volcanic ridge covered byQuaternary coral reefs [Chevalier,1968]. Multichannel seismic reflectiondata over the Loyalty Basin were obtainedduring the Austradec l cruise in 1972aboard the R/V Coriolis. These seismicdata show that the Loyalty Basin containsabout 4 s of sediment that thickens towardthe Loyalty Islands. Dubois et al. [1974]interpreted the sediment to have beenpartly derived from the erosion of NewCaledonia and partly from Loyalty Islandsvolcanism. A seismic cross section of theLoyalty Basin is shown by profile AUS 114(Figure 3). In this profile, the acousticbasement has been designated as the Dreflector by Bitoun and Recy [1982] andinterpreted on the basis of magneticmodeling to be the top of the basaltic

layer which extends upwards from the basintoward the faulted New Caledonia margineIn the Loyalty Basin, the basaltic layeris overlain by a sedimentary unit boundedabove by the C reflector, which Dubois etal. [1974] interpreted as an uncomformityformed by erosion that post-dated theemplacement of ultramafic rocks. Thissedimentary unit, whose history ofdeposition is not known in detail, appearsto thin toward the margin of NewCaledonia. At the eastern margin of thebasin, a prominent normal fault extendsalong the length of the Loyalty IslandsRidge [Dubois et al., 1974]. The faulthas been interpreted as a boundary betweenthe sediment of the basin and an olderfaulted margin of the Loyalty Ridge[Bitoun and Recy, 1982].

Interpretation of a shipboard seismicrefraction profile (station N1 in Figure 4and Table 1) collected south of MareIsland suggests that the acoustic basementunderlying the sediments of the basin isigneous oceanic crust [Shor et al.,1971]. If so, then both the rise ofhorizon D toward New Caledonia and thesuggested oceanic nature for the deepcrust of the basin led us to propose thatlithologies and structures in the LoyaltyBasin and those in the onshore ophioliteare continuous. In order to examine thestructural relationships between the rocksof the eastern margin of New Caledonia andthe rocks of the Loyalty Basin crust, aseismic refraction survey using oceanbottom seismometers (OBS) was carried outduring the EVA VII cruise in 1978, andmarine gravity data were collected duringboth EVA VI and VII cruises. Thesecruises were a part of the cooperativegeophysical program conducted by ORS TOM(the Institut Francais de RechercheScientifique pour le Developpement enCooperation), the University of Texas,Marine Science Institute, the NationalOcean Survey of the National Oceanic andAtmospheric Administration (NOAA) , andCornell University.

SEI SMIC REFRACTION DATA

Two reversed seismic refraction lines,collected with an OBS located at each endof the lines [Latham et al., 1978], wereobtained in the central (line 7R6 inFigure 4) and western (line 7R7 in Figure4) parts of the Loyalty Basin. The lineswere 80 km long, oriented parallel to theaxis of New Caledonia, and were shot over


11std

12<nc~ 13u~

"" la1

NE-1

-2

3

-4

-5

-6

7

8

-9

-la

7R6

4.9---L;;

-t _ 5:8

8i1 68 ..

Bl4--

7R7

aSW

-,-2

3

4.

5

""czcu

7~

""18

:>;:::

9 10 ""

la1 AUS 114

Fig. 3. Top, 12-channel seismic reflection profile AUS 114 [Dubois et aL,1974]. Location of the line is in Figure 4. Bottom, the interpretedprofile: The main reflectors have been outlined in white. Columns 7R6 and7R7 indicate refraction results plotted as time sections; velocities are inkilometers/second.

a flat ocean floor. Explosives were firedat intervals of 5 min for small charges«10 kg) and 10 min for large charges.The explosive sources were alternated with15-1 air gun shots. To obtain optimumresults, the air gun was modified for useat 9 knots and was towed at a water depthof 30 m. Reduced travel time curves forlines 7R6 and 7R7 were drawn for each OBS(Figures 5 and 6, respectively). Twovelocity interpretations for each line arepresented in this paper. Oneinterpretation assumes horizontal laye ringand is designated in the figures by HN forthe northern OBS and HS for the southernOBS. A dipping-layer rnodel is alsopresented in the figures between HN andHS. Velocity estirnates derived from thetwo rnethods agree well. The data setpresented in the two figures shows thatthere is no significant difference in thethickness or velocity variations in thestrata of the Loyalty Basin in the

direction parallel to the axis of thebasin. Five main velocity units arerecognized and listed in Table 1. Averagevelocities of rocks in these layers wereused to estimate layer densities andthicknesses in the modeling of gravit ydata.

To show the correlation between thevelocity in layers and the reflections,the seismic velocity and depthinterpretation for lines 7R6 and 7R7 areplotted on the seismic reflection sectionAUS 114 in Figure 3, bottom. Five majorreflections (A, B, C, cr, and D) can berecognized on the seismic section AUS114. Except for very deep reflections,the se horizons correlate well with thevelocity boundaries derived from therefraction results. The combined seismicdata from Figure 3, bottom, are convertedinto a depth section (Figure 7) and may beinterpreted in a three-unit crustal model.

1. The first unit is located above the


Fig. 4. Location of stations and profiles: Dots are land gravity stations;solid lines are marine gravity profiles, dashed lines are seismic reflectionprofiles, and stars indicate refraction profiles. Nl-N2 and NIO are from Shoret al. [1971]. Heavy dashed line is the seismic section shown in Figure 3.

reflector D and probably represents threestages of sedimentation. Although horizonct is not evident in seismic refractiondata, we used seismic reflection data tosubdivide this unit into three layers:lA, lB, and lC (Figure 7). Velocities inthis unit range from 2.1 to 4.9 km/s.Reflections within layer lA are roughlyhorizontal and paraI leI and reveal noevidence for basin tilting. However,strata above the C reflector revealwestward displacement of the axis of theLoyalty Basin (Figure 3). The sedimentwedge forming layer lB (between horizons Cand Cf) returns discontinuous reflectionsand probably represents a period of basintilting that took place during or justafter the overthrusting. Layer lC May beinterpreted as indurated sedimentary rockswhose velocity ranges from 4.67 to 4.95km/s; their thickness is approximately 1

km. These rocks may have been depositedprior to the overthrusting event. Theacoustic basement, located at the base ofthe layer lC, extends upward from thebasin toward the margin of New Caledoniaat a 10° angle.

2. Unit 2 (Figure 7) underlies horizonD and is represented by velocities between5.8 and 6.0 km/s. We interpret unit 2 tobe the basal tic layer.

3. The third unit has a velocity of6.8 km/s, and we presume that this unit isgabbro of the oceanic crust. As revealedby refraction line 7R6, unit 3 isunusually thin relative to the thicknessof layer 3 in standard oceanic crust.Although refraction arrivaIs from unit 3are not obvious in the data of line 7R7,the thickness of this unit compares weIlwith the thickness of mafic crusts closeto the Vema and Kane fracture zones

TABLE 1. Location of 5eismic Refraction Profiles and Their Results

NNN

Water 5ediment Basement Oceanic Mantle MohoDepth h, v, t, h, v, t, h, v, t, v, t, Depth,

5 taUon Position km km km/s s km km/s s km km/s s km/s s km

N 1 22°29'5 2.09 1.29 2.15* 2.0 5.16 4.0 6.92 8.04 13.9168°15'E 1.8 3.48 2.8 5.95

(")

N 2 21°39'5 2.20 1.02 2.15* 3.6 5.16 5.5 6.92 8.04 20.2 0......

167°20'E 2.2 3.48 5.6 5.95 ......0

N 10 23°00'5 3.62 1.10 2.15* 1.4 4.95 5.0 6.83 8. Il 14.3 rt

164°57'E 1.3 3.20 1.9 6.02 (1)rt

7R6 N 20°52'5 2.32 1.02 2.08* 0.72 3.46 4.91 3.87 2.43 6.66 5.80 8.45 6.90 16.3III

166°10'E 2.92 3.03 2.05 4.17 5.81 4.80 .......7R6 5 21°22'5 2.27 0.47 2.07* 0.70 2.91 4.95 3.75 4.15 6.90 5.60 8.31 6.70 17.0

166°45'E 3.67 3.05 1.65 3.59 5.70 4.50Ç)

7R7 N 21°02'5 2.30 1.08 2.19 1.27 1.56 4.82 3.47 5.46 6.06 4.04 8.10 5.49 12.4 (1)0

166°02'E 2.01 2.86 1.94 '0;:r'

7R7 5 21°30'5 2.27 1.22 2.30 1.32 1. 76 4.67 3.00 6.73 6.00 3.67 8.03 5.42 13.7 '<[fl

166°37'E 1. 75 3.25 2.11 ,..()[fl

0......

Definitions: h, thickness; v, velocity; t, intercepte z*Assumed velocities. (1)

~

NI, N2, and N10 from 5hor et al. (l971]. (")III......(1)p..0;:l,..III

0'0;:r',..0......,..rt(1)


N S

....1+---,----,----,----,--------,,----------,,----------,----,--

o 40 aOKm.

5.70· 4.95·

6~~

4

40---,-,-------,---r-----.------,-----,---,----,-----+l

oKm. ao

HNO~

1.5..2.08

4 ~D3

2.1

3.0

HS,----0

1.5..2.07 4

~.05

a ~.91 4.9 ~.95 a

6.9C 16

-f--B.31

Km.8.4

6.8

12 5.81 5.8 5.70 12

_J==~~=~~~~==-~==-=-=.=..::.=.==-='-=-=-=.=o=.=..::.=-=c::--=--::..:- -~'--

6.6616 _

8.45

Km.

Fig. 5. Seismic refraction results for line 7R6. Top, Travel time curves foreach OBS; bottom, HN and HS are the homogeneous and horizontal layered models;between HN and HS the dipping layered model is plotted.

[Detrick et al., 1982; Cormier et al.,1984]. However, the combined thickness ofunits 2 and 3 is constant on lines 7R6 and7R7, suggesting that these units representoceanic crust that has been tilted.

Rocks with typical upper mantlevelocities of 8.1 to 8.4 km/s lie atshallow depths beneath unit 3, close toNew Caledonia Island. We assume thatthese velocities represent ultramaficrocks of the upper mantle. If so, thenthe Moho rises from the Loyalty Basintoward New Caledonia, as does the oceaniccrustal section between reflector C' andthe Moho. This Interpretation suggeststhat the oceanic crust of the LoyaltyBasin and the ophiolite section exposed on

the Island of New Caledonia arecontinuous.

GRAVITY DATA

High-amplitude Bouguer gravityanomalies were previously mapped over NewCaledonia by Crenn [1953]. Recent gravitystudies in the New Caledonia area haveallowed a more complete compilation ofgravity data [Collot et al., 1982] (Figure8).

More than 2000 land gravity stationswere established by ORSTOM (Figure 4),using the Worden gravimeter No. 313 forroutine gravity measurements, and theLacoste-Romberg No. 402, in collaboration


8OKm.401+----.-----.------.------.------.------.-----.------rr-o

a.03 .

---=6

3

40--..-----,-----,--------,------.----.-----.----~---+I

oKm. 80

HNOr-

lS ~I

H5-0lS

8

6.006.0

4 ~.B6 __---.:2;:..;9 ~~ 4

14.82 - - - - - - -- - - - - 5.7'---- ----{4:::.I.6:.:J7r-r------------------------------------ _8

~6

8.r

12 I--j::-::=:::-:::--:::_=-:::-_::c_::c_:-::_:=::::::--=::::::::c::c::c::c - - - - _ _ __ _ __ _ _ _ _ _ _ _ _ _~IO

Km.

12

803

Km.

Fig. 6. Seismic refraction results for line 7R7; same legend as in Figure 5.

with the Bureau International des MareesTerrestres of Paris and the RoyalObserva tory of Belgium, for the basestation network. The local network istied to the International GravityStandardization Net (IGSN) 1971 by theInternational Gravity base of NoumeaORSTOM (G = 978865.33 mGal [Jezec,1976]. The elevations of land gravitystations were obtained in two ways: 60%of the stations were observed directly onInstitut Geographique National benchmarks,while others were determined usingaltimeters. Simple Bougujr anomalies werecomputed with a 2.67-g/cm Bouguer densityand were not terrain corrected. Crenn[1952] has estimated the terraincorrecti~ns using two densities, 2.67 and3.0 g/cm , for the whole island. Eighty-

two percent of the stations have terraincorrections less than 6 mGal; 18% haveterrain corrections between 6 and 12 mGal,and less than 0.5% of the stations haveterrain corrections greater than 12mGal. These observations give an idea ofthe general accuracy of the simple Bougueranomalies.

Marine gravity data were collected onboard the R/V Coriolis using the Lacosteand Romberg gravimeter S-51 during EVA VIand VII cruises jointly conducted byORSTOM and the National Ocean Survey(NOAA). The navigation was by radarduring EVA VI cruise and by satelliteduring EVA VII cruise. Gravity data tiedto the Noumea ORSTOM base were correctedfor cross coupling, gravity-meter drift,and Eotvos correction and then reduced to

~~~~~ ,lOkm

o

5

10

15

km

sw 7R7

1.5

~2.1

~ ®

7R6

15

___ 2.11- --3.0

1

-1A

1 B

le

2

3

NEo

5

10

15

F'~g. 7. AUS llayers lA 14 profileis an ind' lB, and lC' 1 converted to dunit 2 is urhated sedim~nt ayers lA and ~B epth section't ,ba 1 acy lay ace th . unit 1sa tic layer' er deposited p i

esedirnentary f' comprises

, unit 3 r or to ~llin, the ocea ' the overth g, layer lCn~c layer. rustingevent;


..

free air anomalies. The analysis of 25crossing errors indicates a meandiscrepancy of 3.38 mGal.

The gravity field in the NewCaledonia-Loyalty Island area is dominatedby five major features (Figure 8) that aredescribed from NE to SW:

1. The Loyalty Ridge gravity highs(LRH): This trend of positive anomaliesis paraI leI to New Caledonia and shows adouble line of high values reaching +150mGal on Ouvea, +170 mGal on Mare, and +180mGal on Lifou.

2. The Loyalty Basin gravity low (LBL)is a negative anomaly reaching -40 mGalthat is largely correlated withbathymetry.

3. The eastern lagoon gravity highs(ELH) represent a continuous positivegravity feature lying along the east coastof New Caledonia between longitudes164°45'E and 167°30'E; this gravityfeature shows a series of highs, locallyreaching +180 mGal, that are characterizedby a short wavelength.

4. The relative gravity low along theaxis of New Caledonia (NCL) trends in anorthwest-southeast direction and issharply terminated near longitude166°35'E, where the low is replaced bylocal gravity highs. The trend of theaxial low is largely inde pendent of thestrike of surface rocks. To the west,this axial low is limited by adiscontinuous positive gravity featurethat shows a well-delineated high reaching+100 mGal in the Noumea region.

5. The New Caledonia Basin gravity low(NCBL) shows values as low as -80 mGal;this anomaly trends paraI leI to the axisof New Caledonia and appears to be locallyinde pendent of the bathymetry.

INTERPRETATION

A NE-SW gravity transect, shown inFigures 8 and 9, cuts across the easternlagoon highs in the Bogota Peninsulaarea. Figure 9 shows the gravity model asweIl as the observed and calculatedgravity anomalies. Computations were madeusing a two-dimensional gravity modelingmethod [Talwani et al., 1959], which fitsweIl with the New-Caledonia Loyaltyelongated structures. A 32-km-thick,standard crust was used as reference, witha 2.~0-g/cm3 crust density and a 3.33g/cm mantle density. The density modelwas constructed using the gravity profileEVA 656 and constraints provided by

seismic refraction results and seismicreflection profile AUS 114. In the NewCaledonia Basin, gravity profile EVA 767and seismic profile AUS 101 were projectedonto the gravity transect (Figure 4); theN10 refraction results from Shor et al.[1971] were extrapolated and used toconstrain this part of the model (Table1) •

The density of sorne rocks were measuredin the laboratory [Collot et al., 1982];other densities were obtained from seismicvelocities using the experimental curve byLudwig, et aL [1970]. Recentmeasurements compared with those given byCrenn [1953] allow us to assign a 2.67g/cm3 density to the basement rocks of NewCaledonia; densities of 2.8 and 2.9 g/cm3

were assumed for the deepest structures ofthe island. Ultramafic exposures aregenerally serpentinized, and samples ofbarely serpentinized rocks have 1ensitiesranging between 3.0 and 3.1 g/cm ,although the value of the velocity-deriveddensity f~r the Loyalty Basin mantle .is3.33 g/cm. The compressibility anddensity of rocks rich in olivine dependsmostly on their degree of serpentinization[Christensen, 1966], and theserpentinization is supposed to decreasewith depth; on the other hand, thepresence at the base of the overthrustsheet of an irregularly developedserpentinized sole reduces the averagedensity of the peridotite. Theseconsiderations lead us to adopt anintermediate density of 3.2 for thedipping dense slab.

Computed gravity anomaly derived usingthe density distribution described abovefits weIl the observed anomaly (Figure9). The most striking feature of thismodel is the dense slab rising up from theLoyalty Basin. The slab consists of the6.5-km-thick Loyalty Basin oceanic crustunderlain by a 4-km-thick mantle sheet of3.2-g/cm3 density, as deduced from gravitycomputation. The ophiolite sequence canbe recognized within that dense slab asshown in Figure 10. The existence of ahigh-velocity root under the easternmargin of New Caledonia has recently beenconfirmed by an unpublished ORSTOMrefraction experiment conducted in theeastern lagoon that shows a velocity of7.0 km/s at a depth of a few hundredmeters. By introducing a low-density bodyunder this slab, a good fit is obtainedfor the eastern lagoon gravity high. Thelow-density body appears to be the


150mgol

) ) A

.0-------

. sc

..' ............

i.'

LI

B

~I1

2 <3

/ '" 103

//

• 50') Kg Irrr 2

-x x-x x_x-x x.............. x )( x-x /.......... x_ x .--x- --x_x- --x

---- -~--~ ~~~~ .......... 7/ x ...... "'x~x<::~"----.2.'-'X------------1x-x~x~x x

-30C 50 IOfl 200 250

Fig. 9. Gravit y model (this transect is shown in Figure 8): Solid curve isthe observed free air anomaly, dashed curve is the observed Bouguer anomaly,and dots represent the calculated anomaly. NCB, New Caledonia Basin; WL,western Lagoon; NC, New Caledonia; EL, eastern lagoon; LB, Loyalty Basin; LI,Loyalty Islands; 7R6 and 7R7 are the refraction lines. Beneath the proposeddensity structure are the mass anomalies calculated in vertical columns.

continental part of New Caledonia that wasthrust beneath the ophiolite. Thethickness of the continental crust beneathNew Caledonia locally reaches 32 km. Theattitude and position of the dipping slabwith respect to the island suggest acontinuous sheet of obducted ophiolite.This ultrabasic sheet appears to haveoverthrust aIl of New Caledonia, as it is

shown by the NW-SE extension of the rootzone along the whole length of the easternlagoon (Figure 8). According to theseresults, the numerous ultramafic exposuresof New Caledonia are remnants of a broadcontinuous ophiolite sheet that waspreviously the upper part of the LoyaltyBasin lithosphere. The northeastwarddipping slab shown in the model does not

Fig. 10. Geological interpretation of the eastern margin of New Caledoniabased on geological and geophysical data. (1) Sedimentary fill postdating theoverthrusting event, (2) layers lC and 2, (3) gabbroic layer, (4) mantlerocks, (5) volcanic rocks?, and (6) New Caledonia metamorphosed rocks pre-/Permian to Eocene.


necessarily imply that the ophiolite wasemplaced in a NE to SW direction. Infact, as shown by Paris [1981], analysisof the evolution of the deformation whichaffected the ophiolite basement rockswhile overthrusting occurred suggests thatthe overthrusting began in a north tosouth direction and then rotatedsouthwestward when the ophiolite reachedthe southern part of New Caledonia.

Assuming an original 10-km constantthickness for the ophiolite sheet and anupper Eocene age for the overthrustingevent, the locally complete erosion of theophiolite would suggest a mean mechanicaland chemical erosion rate of 25 cm/1000yr. This rate is 7 times greater thanthat given by Baltzer and Trescase[1971]. The major part of the hugesedimentary fill of the surrounding basinsprobably consists of the products of thiserosion. Regional isostatic readjustmentsthat occurred after the release of theregional compressional forces, createddifferential vertical movements alongfaults that can be observed mainly alongthe eastern coast of New Caledonia [Dugasand Debenay, 1978]. These movements haveoccurred since the middle Miocene[Coudray, 1977], and, combined with strongerosion of the ophiolite sheet, they maybe responsible for the exposure of theblueschist belt.

East of the Loyalty Basin the LoyaltyIslands have a crust of intermediatethickness. The parallel positive gravit yanomalies that characterize the LoyaltyChain (Figure 8) are not investigated bythe gravity transect because it traversesthe Loyalty Chain where only a singlepositive gravity anomaly is present.However, the parallel gravity anomalies ofthe Loyalty Island may be caused byparallel volcanic chains, which maycorrespond to different volcanic events.West of New Caledonia the gravit y low thatappears to extend along the western coastis interpreted as the gravity effect ofthe New Caledonia Basin crust which bendsdownward and is overlain by a thick wedgeof sediments (Profile AUS 105 by Dubois etal. [1974]).

DISCUSSION

The gravit y anomalies caused by the NewCaledonia ophiolite are similar inmagnitude and wavelength to those observedover the ultramafic complexes of Papua NewGuinea, the Ivrea Zone of Italy and theTroodos massif of Cyprus. As noted by

Finlayson et al. [1977], their gravityinterpretation, guided by refractionresults, is compatible with a high-densityophiolite slab that overthrusts a lowdensity continental edge. However, eachof these ophiolites apparently has aunique tectonic setting. In the IvreaZone, the ophiolite dips at either 32°[Kaminski and Menzel, 1968] or 42°[Berckhemer, 1969] and is squeezed betweentwo thick (50 km) crustal bodies; thisgeological situation emphasizes acontinental collision process. Thetectonic setting of the Papuan and Troodosophiolites remains difficult to interpretbecause of the intermediate thickness (30km) of the crust that flanks both sides ofthe ophiolite sheet. In New Caledonia thesimple model proposed in this papersuggests that the ophiolite emplacementtook place in an oceanic environment butprobably involved a continentalfragment. The overthrust terminatedwithout any continent-continent or arccontinent collision process.

We next propose a speculative model forthe geodynamic setting that prevailedbefore the emplacement of the ophiolite inNew Caledonia. One of the major elementsneeded for the understanding of thegeodynamic evolution of this area isunderstanding the evolution of the LoyaltyChain. Therefore, we first must examinethe nature of the Loyalty Islands and thetiming of events that may be related tothe geological history of the LoyaltyChain and Loyalty Basin, and then wereconstruct the overthrusting event asoccurring along a subduction zone thatextended along the western margin of theNew Caledonia Island.

The Loyalty Basin and Loyalty Islands

The islands of the Loyalty Chain thatare capped with Quaternary coral reefsshow outcrops of volcanic rocks in MareIsland. Analysis of a few samples ofwithin-plate basaIt [M. Monzier and P.Maillet, personal communication, 1986]indicate an age of 9-11 Ma [Baubron etal., 1976]. However, Bitoun and Recy[1982] suggested, on the basis of seismicprofile analysis, that the basement of thewestern margin of the Loyalty Islands mayhave existed prior to upper Eocene time.Thus the basalts exposed on Mare IslandcouId represent the final volcanicactivity of the Loyalty Islands, probablyrelated to Miocene extensional tectonics.

The Loyalty Chain, which is a linear


feature, parallels the trend of theNorfolk-New Caledonia system and the LordHowe Rise. This suggests that, similarlyto the Norfolk-New Caledonia Ridge, theLoyalty chain was rifted from theAustralian margin. If the minimum age(80-100 Ma) of the Loyalty Basin crustgiven by Prinzhofer [1981] is correct,this may indicate that this basin was amarginal basin developed prior to LateCretaceous time along the eastern marginof Gondwanaland. In this hypothesis thecore of the Loyalty Chain consists of afragment of thinned continental crust thatmay have been overlain by island arcvolcanics related to a possible subductionof the Pacifie Plate beneath the Gondwanamargin prior to the Late Cretaceous.

Hyaloclastites were dredged at a sitealong a faulted topographie high locatedat the northern termination of the LoyaltyBasin. Although their lithology is notknown, fission track dates provide a 36.5+ 5.2 Ma age for those rocks (Site G0325 Din the work by Maillet et al. [1983]).These authors believe that this agerepresents the time of formation age ofthe oceanic crust. However, this age isclearly different from the age of theLoyalty Basin lithosphere proposed byPrinzhofer [1981] but is very similar tothe age of the ophiolite emplacement. Weinterpret the rock age as evidence fordike intrusion related to breakage of theoceanic crust that occurred during theophiolite overthrusting.

The Overthrusting Event and the WesternMargin of New Caledonia

Kroenke [1984] proposed that NewCaledonia was rifted from the Lord HoweRise during the Late Cretaceous and thatthe formation of the crust of the NewCaledonia Basin was completed by the earlyPaleocene. Referring to the trenchlikemorphology in the basement of the NewCaledonia Basin, off the west coast of NewCaledonia [Dubois et al., 1974], and tosorne evidence for Eocene island-arcvolcanism in New Caledonia [Paris, 1981],Kroenke [1984] suggested that anortheastward subduction zone was formedalong the western margin of New Caledoniaduring the early Eocene when breakup ofthe Australia-Antarctic plates occurred.At this time, the North Loyalty Basindeveloped behind the Loyalty Islands. Theoccurrence of sorne boninite in theSenonian to middle Eocene basaltic complexof New Caledonia substantiates Kroenke's

interpretation (J.P. Eissen, personalcommunication, 1986); however, the age ofthese volcanics is unknown.

During Eocene time, tectonic eventsproposed by Kroenke [1984], such ascollision or partial obduction, occurredat the site of the subduction zone. Theyprobably caused either a weak intraoceaniczone extending into the Proto LoyaltyBasin or a possible transcurrent faultextending along the rear arc of NewCaledonia to reactivate as a major thrustfault along which the proto Loyalty Basinlithosphere overthrust New Caledonia.

In conclusion:Geological studies of the ultramafic

rocks exposed along the length of NewCaledonia suggest that they form anortheastward dipping sheet consisting ofbasal harzburgite, dunite, wherlite, andlayered gabbros with sorne serpentine.

The total exposed sequence is underlainby Permian to Eocene strata, along alocally steeply dipping thrust fault.

Radiometrie dating of the ophiolitesuggests a minimum age of 80-100 MY.

Stratigraphie dating of the overthrustsuggests an upper Eocene age for theoverthrusting event.

Examination of the crustal velocitystructure derived from ocean bottomseismic refraction and multichannelseismic reflection data strongly suggestsan oceanic origin for the Loyalty Basincrust.

The combined study of geology, gravity ,and seismic data allows the constructionof a geophysical model which suggests thepresence of a complete oceanic sectionoverthrusting the northeastern edge of NewCaledonia.

This model gives evidence for thecontinuity between ultramafic exposuresand the crust-mantle structures of theLoyalty Basin.

The observed free air anomaly of up to+180 mGal extending over the whole lengthof New Caledonia suggests that the rootzone of the complex may underlie aIl ofthe eastern lagoon.

These results lead us to conclude thatthe ultramafics of New Caledonia areremnants of a 10-km-thick, obductedophiolitic complex, whose upper basalticand gabbroic layers have subsequently beenremoved either by erosion or by thrustfaulting. The products of this erosionmay lie within the surrounding basins,where the sedimentary thickness reaches upto 8 km in the Loyalty Basin.

Our geodynamic reconstruction suggests


that the proto-Loyalty Basin crust formedas a marginal basin, along the easternmargin of the Gondwanaland prior to 80-100Ma. During the Upper Cretaceous and earlyPaleocene and after the opening of the NewCaledonia Basin, while a subduction zonewas active along the western margin of NewCaledonia, the ophiolite was emplaced in ageneral north/south direction over NewCaledonia. In this hypothesis, thedifference between the time of formationof the ophiolite and the age of theoverthrusting is a minimum of 40-60 Ma.

Acknowledgments. This work wassupported by the Institut Francais deRecherche Scientifique Pour leDeveloppement en Cooperation (ORSTOM), theNational Science Foundation (Grant EAR75-14815), the U.S. Department of thelnterior (Contract 14-08-0001-16703), theHenry L. and Graee Doherty CharitableFoundation, the National Oceanic andAtmospheric Administration (NO-AOl-78-004120), the Office of Naval Research(N00014-75-C-1121), and the InstitutFrancais de Recherche pour l'Exploitationde la Mer (IFREMER). We are grateful toJ. Dubois for assistance in planning theexperimental strategy, as weIl as to J.Goguel and to M.A. Fisher of the U.S.Geological Survey for reviewing themanuscript. We would also like to thankJ. Daniel, B. Pontoise, and B.M. Larue forassistance in collecting data and foruseful discussion; R. Moody, W. Dewhurst,P. Roppert, and F. Jamet for theirinvaluable assistance in the maintenanceof the equipment, M. Yellin for helpfulassistance in data reduction; and thecrews and the captains of the two ships,the N/O Coriolis (IFREMER), and the Galibi(Service des Phares et Balises deNouvelle-Caledonie) for their cooperationduring the field experiment.

REFERENCES

Aubouin, J., M. Mattauer, and C. Allegre,La couronne ophiolitiqueperiaustralienne: Un charriageoceanique representatif des stadesprecoces de l'evolution alpine, C. R.Seances Acad. Sei. Ser. D 285, 953-956,1977 •

Avias, J., Overthrust structure of themain ultrabasic New Caledonia massives,Tectonophysics, 4, 531-541, 1967.

Avias, J., Major fË!atures of the New

Guinea-Louisiade - New Caledonia Norfolk Arc system, in The Western.Pacific Island Arcs, Marginal SeasGeochemistry, edited by P. J. Coleman,pp. 113-126, University of WesternAustralia, Nedlands, 1973.

Baltzer, F., and J. J. Trescase, Erosion,transport et sedimentation lies auxcyclones tropicaux dans les massifsd'ultrabasites de Nouvelle-Caledonie.Premiere approche du bilan general del'alteration, de l'erosion et de lasedimentation sur peridotite en zonetropicale. Cah. ORSTOM Ser. Geol., 2,221-244, 1971. -

Baubron, J. C., J. H. Guillon, and J.Recy, Geochronologie par la methode K/Ardu substrat volcanique de l'ile de Mare,archipel des Loyaute (Sud-OuestPacifique), Bull. BRGM, 2, 165-176,1976.

Berckhemer, H., Direct evidence for thecomposition of the lower crust and theMoho. Tectonophysics, 8, 97-105, 1969.

Bitoun, G., and J. Recy,-Origine etevolution du Bassin des Loyaute et deses bordures apres la mise en place dela serie ophiolitique de NouvelleCaledonie, Trav. Doc. ORSTOM, 147, 505-539, 1982. --

Blake, M. C., R. M. Brothers, and M. A.Lanphere, Radiometrie ages ofblueschists in New Caledonia, inInternational Symposium on Geodynamicsin South-West Pacifie, Noumea, 1976, pp.279-282, Technip Paris, 1977.

Brookfield, M. E., The emplacement ofgiant ophiolite nappes. l, MesozoieCenozoie examples, Tectonophysics, 37,247-303, 1977. -

Brothers, R. N., High-pressure schists innorthern New Caledonia, Contrib.Mineral. Petrol., 46, 109-127, 1974.

Brothers, R. N., and~. C. Blake, Jr.,Tertiary plate tectonies and highpressure metamorphism in New Caledonia,Teetonophysics, 17, 337-358, 1973.

Chevalier, J. P., Expedition francaise surles recifs coralliens de La NouvelleCaledonie: l'ile de Mare, 3, 158 pp.,Fondation Singer-Polignac, Paris, 1968.

Christensen, N. l., Elasticity ofultrabasic rocks, J. Geophys. Res., 71,5921-5931, 1966. -

Christensen, N. l., and M. H. Salisbury,Structure and constitution of the loweroceanic crust, Rev. Geophys., 11.,57-86,1975.

Coleman, R. G., Plate tectonic emplacement

\ .


of upper mantle peridotite alongcontinental edges, J. Geophys. Res., ~,1212-1230, 1971.

Coleman, R. G., Tectonic setting forophiolite obduction in Oman, J. Geophys.Res., 86, 2497-2508, 1981-

Collot, ~ Y., and F. Missegue, Crustalstructure between New Caledonia and theNew Hebrides, International Symposium onGeodynamics in South-West Pacific,Noumea, 1976, pp. 135-144, Technip,Paris, 1977.

Collot, J. Y., F. Missegue, and A.Malahoff, Anomalies gravimetriques etstructure de la croute dans la region dela Nouvelle-Caledonie: enracinement desperidotites, Trav. Doc. ORSTOM, 147499-564, 1982. ----

Cormier, M. H., R. S. Detrick, and G. M.Purdy, Anomalously thin crust in oceanicfracture zones: New seismic constraintsfrom the KANE fracture zone, J.Geophys. Res., 89, 10249-10266~ 1984.

Coudray, J., The "main events of thesedimentary and structural history ofNew Caledonia from the Oligocene to thepresent day, International Symposium onGeodynamics in South-West Pacific,Noumea, 1976, pp. 217-228, Technip,Paris, 1977.

Crenn, Y., Itineraires gravimetriques deNouvelle-Caledonie, Rap. Inst. Fr.d'Oceanie, Noumea, New Caledonia, 1952.

Crenn, Y., Anomalies gravimetriques etmagnetiques liees aux roches basiques deNouvelle-Caledonie, Ann. Geophys., 2...fasc. 4, 291-299, 1953.

Davies, H. L., and I. E. Smith, Geology ofeastern Papua, Geol. Soc. Am. Bull., ~,3299-3312, 1971-

Detrick, R. S., M. H. Cormier, R. A.Prince, D. W. Forsyth, and E. L. Ambos,Seismic constraints on the crustalstructure within the VEMA fracture zone,J. Geophys. Res., 87, 10599-10612, 1982.

Dewey, J. F., Ophiolite obduction,Tectonophysics, 31, 93-120, 1976.

Dubois, J., Contribution a l'etudestructurale du Sud-Ouest Pacifiqued'apres les ondes sismiques observees enNouvelle-Caledonie et aux NouvelleHebrides, Ann. Geophys., ~, 923-972,1969.

Dubois, J., C. Ravenne, F. Aubertin, J.Louis, R. Guillaume, J. Launay, and L.Montadert, Continental margins near NewCaledonia, in The Geology of ContinentalMargins, edited by C. A. Burk, and C. L.Drake, pp. 521-535, Springer Verlag, NewYork, 1974.

Dugas, F., and J. P. Debenay, Interferencedes failles-flexures littorales et del'erosion karstique sur lesconstructions coralliennes: Le lagon deNouvelle-Caledonie, C. R. Seances Acad.Sei. Sere D, 287, 1091-1094, 1978.

Falvey, D. A:, and J. C. Mutter, Regionalplate tectonics and the evolution ofAustralia's passive margins. BMR, J.Aust. Geol. Geophys., 6, 1-29,~81-.-

Finlayson, D. M., B. J. ïDrummond, C. D. M.Collins, and J. B. Connelly, Crustalstructures in the region of the Papuanultramafic belt, Phys. Earth Planet.Inter., 14, 13-29, 1977.

Gass, I. G~ and D. Masson-Smith, Thegeology and gravit y anomalies of theTroodos massif, Cyprus, Philos. Trans.R. Soc. London,Ser. A, 255, 417-467,1963. ----

Glaessner, M. F., Geotectonic position ofNew Guinea, Bull. Am. Assoc. Pet. Geol.,34, 856-881, 1950.

Griffiths, J. R., Reconstitution of theSouth-West Pacific margin ofGondwanaland, Nature, 234, 203-207,1971-

Guillon, J. H., Les massifs peridotitiquesde Nouvelle-Caledonie. Type d'appareilultrabasique stratiforme de chainerecente, Mem. ORSTOM, 76, 1-120, 1975.

Hayes, D. E., and J. Ringis, Seafloorspreading in the Tasman sea, Nature,243, 454-458, 1973.

Jezec, P. A., Gravity base stations inIndonesia and in the South-West Pacific,Woods Hole Oceanogr. Inst. Rep., WoodsHole, Mass., 1976.

Kaminski, W., and H. Menzel, Topographiedes "Ivrea-Korpes" abgeleitet ausseismischen und gravimetrischen daten,Schweiz., Mineral. Petrogr. Mitt., 48,235-246, 1968. -

Kroenke, L. W., New Caledonia: TheNorfolk and Loyalty Ridges; the NewCaledonia and Loyalty basins, chap 2,Cenozoic Tectonic Development of theSouthwest Pacific With a Contribution byPeter Rodda, Tech. Bull. 6, pp. 15-28,Committee for Co-Ordination of JointProspecting for Mineral Resources inSouth Pacific Offshore Areas, Suva,Fiji, 1984.

Latham, G., P. Donoho, K. Griffiths, A.Roberts, and A. K. Ibrahim, The Texasocean bottom seismograph, paperpresented at the Offshore TechnologyConference, Soc. of Explor. Geophys.,Houston, Texas, 1978.


.1

Lillie, A. R., and R. N. Brothers, Thegeology of New Caledonia, N. Z. J. Geol.Geophys., 13, 145-183, 1970.

Ludwig, W. J~ J. E. Nafe, and C. L.Drake, Seismic refraction, in The Sea,vol. 4, edited by E. C. Bullard, and A.E. Maxwell, part l, pp. 53-84, WileyInterscience, New York, 1970.

Maillet, P., M. Monzier, M. Selo, and D.Storzer, The d'Entrecasteaux Zone (SouthWest Pacifie) - A petrological andgeochronological reappraisal, Mar.Geo!., 53, 179-198, 1983. --

Manghnani~M. H., and R. G. Coleman,Gravity profiles across the Samailophiolite, Oman, J. Geophys. Res., ~,2509-2525, 1981.

Milsom, J. S., Papuan ultramafic belt:Gravity anomalies and the emplacement ofophiolites, Geol. Soc. Am. Bull., 84,2243-2258, 1973. -

Moores, E. M., Geotectonic significance ofultramafic rocks, Earth Sei. Rev., 9,241-258, 1973. -

Moutte, J., and J. P. Paris, Anatomy andstructure of the great southern massif(New Caledonia), in InternationalSymposium on Geodynamics in South-WestPacifie, Noumea, 1976, pp. 229-234,Technip, Paris, 1977.

Paris, J. P., Geologie de la NouvelleCaledonie: Un essai de synthese, Mem.BRGM, 113, p. 279, 198!.

Paris, J~., P. Andreieff, and J.Coudray, Sur l'age Eocene Superieur dela mise en place de la nappeophiolitique de Nouvelle-Caledonie,unite du charriage oceanique periaustralien, deduit d'observationsnouvelles sur la serie de Nepoui, C.R.Seances Acad. Sei. Ser. D, 288, 1659-1661, 1979. -

Parrot, J. F., and F. Dugas, The disruptedophiolitic belt of the SouthwestPacifie: Evidence of an Eocenesubduction zone, Tectonophysics, ~,349-372, 1980.

Pontoise, B., J. Y. Collot, F. Missegue,

and G. Latham, Sismique refraction dansle Bassin des Loyaute: Resultats etDiscussion, Trav. Doc. ORSTOM, 147,541-548, 1982. --

Prinzhofer, A., Structure et petrologied'un cortege ophiolitique. Le massif duSud (Nouvelle-Caledonie). Latransition, mateau-croute en milieuoceanique, These Doct. ing., 185 pp.,Ecole Nationale Superieure des Mines deParis, France, 1981.

Rodgers, K. A., Ultramafic and relatedrocks from southern New Caledonia, Bull.BRGM, 2, 33-55, 1976. ---

Routhier~P., Etude geologique du versantoccidental de la Nouvelle-Caledonieentre le col de Boghen et la pointed'Arama, Mem. Soc. Geol. Fr., 32, 1-127,1953. -

Shor, G. G., H. K. Kirk, and H. W. Menard,Crustal structure of the melanesianarea, J. Geophys. Res., 76, 2562-2586,1971. -

Talwani, M., J. L. Worzel, and M.Landisman, Rapid gravity computationsfor two-dimensional bodies withapplication to the Mendocino fracturezone, J. Geophys. Res., ~, 49-59, 1959.

J. Y. Collot, U.S. Geological Survey,Branch of Pacifie Marine Geology, 345Middlefield Road, MS-999, Menlo Park, CA94025.

G. Latham, Cities Service Oil and GasCorporation, P.O. Box 300, Tulsa, OK74102.

A. Malahoff, University of Hawaii, 1000Pope Road, MSB 226, Honolulu, HI 96822.

F. Missegue and J. Recy, InstitutFrancais de Recherche Scientifique Pour leDeveloppement en Cooperation, BPA 5,Noumea, New Caledonia.

(Received November 20, 1986;revised February 23, 1987;accepted February 24, 1987.)

Documents

TECTONICS, VOL. 6, NO. 3, PAGES 215-232, JUNE …horizon.documentation.ird.fr/.../divers20-05/010028321.pdf216 Collot et al.: Geophysics of New Caledonia Ophiolite TASMAN SEA 10 S