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Journal of Geophysical Research NOVEMBER 2007, VOLUME 112 (B11102) : 1-18 http://dx.doi.org/10.1029/2007JB005093 © 2007 American Geophysical Union An edited version of this paper was published by AGU.

Archimer http://www.ifremer.fr/docelec/ Archive Institutionnelle de l’Ifremer

Crustal structure of the basin and ridge system west of New Caledonia

(Southwest Pacific) from wide-angle and reflection seismic data.

F. Klingelhoefer1, *, Y. Lafoy2, J. Collot1, E. Cosquer1, L. Géli1, H. Nouzé1, R. Vially3

1Dep. of Geodynamics and Geophysics, IFREMER, Centre de Brest, B.P. 70, 29280 Plouzané, France. 2Bureau of Geology and Mines, Department of Industry, Mines and Energy of New Caledonia, B.P. 465-98845, Nouméa, New Caledonia 3IFP, 1 & 4, avenue de Bois-Preau 92852 Rueil-Malmaison Cedex, France *: Corresponding author : F. Klingelhoefer, email address : [email protected]

Abstract: During the Zoneco 11 marine geophysical survey (September 2004), two deep reflection seismic profiles recorded by ocean bottom seismometers were acquired in the offshore domain west of New Caledonia. The northern profile crosses the New Caledonia Basin, the Fairway Ridge, the Fairway Basin, and the Lord Howe Rise. The southern profile crosses the Norfolk Rise south of New Caledonia, the New Caledonia Basin, the Fairway Ridge and Basin, and ends at the foot of Lord Howe Rise. On the northern profile the Lord Howe Rise has a crustal thickness of 23 km and exhibits seismic velocities and velocity gradients characteristic of continental crust. The crust thins to 12–15 km in the neighboring Fairway Basin, which is interpreted to be of thinned continental origin based on the seismic velocities. The crustal thickness of the Fairway Rise is 22 km, and it is also interpreted to be of continental origin. The New Caledonian Basin is underlain by crust of 10 km thickness, which shows unusally high velocities (between 7.0 and 7.4) uncharacteristic for either thinned continental or oceanic crust. On the southern profile the Norfolk Rise is also found to be of continental nature. Here, the New Caledonia Basin shows velocities, crustal thickness, and basement roughness characteristic of typical oceanic crust. The crust in the Fairway Basin shows higher velocities than on the northern profile, which could be caused by volcanic intrusions into the crust during extension. A deep reflector in the upper mantle was imaged underneath the New Caledonian Basin on the northern profile. Keywords: wide-angle seismic; SW Pacific; crustal structure.

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KLINGELHOEFER ET AL.: CRUSTAL STRUCTURE WEST OF NEW CALEDONIA X - 3

1. Introduction and previous work

The geodynamic history of the Southwest Paci�c since the Cretaceous has been char-27

acterized by the fragmentation of Gondwanaland. During an extensional period from28

late Cretaceous to early Eocene three subparallel marginal basins were created, the Tas-29

man Sea, the New Caledonia Basin and the Loyalty Basin (Figure 1) separated by two30

aseismic ridges, Lord Howe Rise and the New Caledonia-Norfolk Ridge [Dubois et al.,31

1974; Ravenne et al., 1977; Willcox et al., 1980; Eade, 1988; Symonds et al., 1996; Gaina32

et al., 1998; Willcox et al., 2001; Crawford et al., 2003; Exon et al., 2004; Lafoy et al.,33

2005a; Pelletier , 2006]. This opening was followed by a compressional phase in the late34

Eocene/early Oligocene at the end of which ophiolites were obducted onto New Caledonia35

and Lord Howe Rise became eroded subaerially [Avias , 1967; Paris , 1979; Collot et al.,36

1987; Aitchison et al., 1995; Cluzel et al., 2001; Exon et al., 2007].37

Deep crustal data are generally scarce in this area. During one of the �rst wide-angle38

seismic studies carried out in the Melanesian islands area, two seismic pro�les were ac-39

quired crossing the Lord Howe Rise and the New Caledonia Basin [Shor et al., 1971].40

From modelling of the expanding-spread pro�les acquired during the cruise, the authors41

concluded that the Lord Howe Rise is underlain by 18 to 25 km thick continental crust42

and the Norfolk Ridge by 21 km thick crust. Woodward and Hunt [1971] determined the43

crustal structure across the Tasman Sea from gravity measurements interpreted together44

with the �rst deep seismic results from Shor et al., [1971] . They con�rmed the continental45

nature of Lord Howe Rise and proposed volcanic intrusions to explain short wavelength46

D R A F T June 16, 2007, 1:41pm D R A F T

X - 4 KLINGELHOEFER ET AL.: CRUSTAL STRUCTURE WEST OF NEW CALEDONIA

gravity anomalies. They obtained a 9 km thick crust in the New Caledonia Basin from47

gravity modelling.48

During the Sonne SO-36 cruise, magnetic and gravimetric data were acquired on the49

Lord Howe Rise and Dampier Ridge and in the Lord Howe Basin [Schreckenberger et al.,50

1992]. On the basis of modelling of magnetic data the authors concluded that continen-51

tal crust with a highly magnetized lower crust and slightly less magnetized upper crust52

provides a better �t to the data than does a model with oceanic crust.53

A major compressive phase during the upper Eocene and Middle Oligocene has been54

proposed from DSDP drilling and seismic pro�ling [Burns et al., 1973]. This lead to the55

subaerial exposure of the Lord Howe Rise and created a regional erosional unconformity.56

This erosion is contemporaneous with the obduction of the ophiolite onto New Caledonia.57

A volcanic event associated with the post Oligocene subsidence of the area is identi�ed58

on the basis of imagery and seismic data [de Beuque et al., 1998; Exon et al., 2004]. The59

authors con�rmed the intermediate (continental intruded by volcanism) nature of the60

Lord Howe Rise and proposed on the basis of the magnetic lineations, an oceanic origin61

of the Fairway Ridge and Basin.62

The Austradec I and II marine seismic surveys were the �rst to discover the Fairway63

Ridge, a major structural feature which divides the New Caledonian (NC) basin into the64

NC basin senso stricto and the Fairway Basin. they also found, that it plays an important65

role as a barrier in the distribution of sediments [Ravenne et al., 1977]. The Fairway ridge,66

previously interpreted as a ridge of oceanic nature [Ravenne et al., 1977; Mignot , 1984]67

and as an oceanic piece of the New Caledonia basin crust overthrust along the Lord How68

Rise [Lafoy et al., 1994; de Beuque, 1999; Auzende et al., 2000] is now considered to69



be thinned continental crust [Vially et al., 2003; Lafoy et al., 2005b]. The origin of the70

Fairway Basin remains controversial to this day. Mignot [1984], Eade [1988], Uruski and71

Wood [1991], van de Beuque [1999] and Auzende et al [2000] interpreted the substratum72

of Fairway Basin as oceanic in nature.73

The Moho depth underneath the south New Caledonia Basin has been found to be74

slightly deeper than priviously thought from expanded spread pro�les [Shor et al., 1971].75

The sub-sea�oor basement depth of the New Caledonia Basin was found to increase to-76

wards the west, from 2 km near Lord Howe Rise to 3 km near the Norfolk Ridge [Woodward77

and Hunt , 1971]. Origin of the basin is controversial from oceanic type [Shor et al., 1971;78

Dubois et al., 1974; Weissel and Hayes , 1977; Willcox et al., 1980; Kroenke, 1984; Mignot ,79

1984; Sutherland , 1999; Auzende et al., 2000] to thinned continental type [Etheridge et al.,80

1989; Uruski and Wood , 1991; Sdrolias et al., 2003; Vially et al., 2003].81

The western margin of New Caledonia was previously interpreted as a subduction zone82

active before the late Eocene and the emplacement of the New Caledonia ophiolite [Dubois83

et al., 1974; Kroenke, 1984; Collot et al., 1987]. In 1987 during the ZOE 400 cruise the84

margin was investigated by 14 seismic pro�les [Rigolot and Pelletier , 1988]. The com-85

pressional sedimentary structures observed have been interpreted as representing a zone86

of deformation active between the Eocene compressional phase and the Upper Miocene87

or Piocene, and as having formed in response during the obduction of the peridotites on88

New Caledonia, which never attained full subdcution [Rigolot and Pelletier , 1988].89

This would therefore also explain the absence of a fully developed volcanic arc on the90

New Caledonia island [Rigolot and Pelletier , 1988]. The presence of a fully developed91

short-lived subduction zone west of New Caledonia could also be indicated by the presence92



of some magmas related to an active margin found on the island [Cluzel et al., 2001; Collot93

et al., 1987].94

From modelling of the source-receiver functions of 15 large earthquakes from a land95

station in Noumea Regnier [1988] constructed a simpli�ed model of the crustal and mantle96

structure beneath southern New Caledonia. The �nal velocity-depth function shows a97

Moho depth of 26 +- 1 km. Two low velocity zones are observed in the mantle underneath98

Noumea, the �rst between 40 and 50 km depth and the second from 60 to 65 km depth.99

The second low velocity zone dips towards the North. Due to the absence of a volcanic arc100

on the island, the authors o�er as the most plausible explanation that the two low velocity101

zones result from west coast underthrusting, which perhaps reached the early stages of102

subduction, with a contemporaneous east coast obduction of the ophiolite nappe.103

The main aim of the deep re�ection and wide-angle seismic pro�ling performed during104

the Zoneco 11 cruise in the basin and ridge region system west of New Caledonia was105

the determination of the crustal structure in the region of the New Caledonia Basin, the106

Fairway Ridge and the Fairway Basin. These geophysical data provide a new basis for107

reconstructing the geological history of the region.108

2. Data acquisition and quality

During the Zoneco 11 cruise in 2004 two combined wide-angle and re�ection seismic109

pro�les of 500 km length each were shot across the basin and ridge region system west of110

New Caledonia. The seismic Pro�le N (Figure 1) is located around 23o S, starting from111

the coast o� New Caledonia and crossing the New Caledonia Basin, the Fairway Ridge,112

the Fairway Basin and the Lord Howe Rise and has a total length of 565 km. It was shot113

in two parts, with all 15 available ocean-bottom seismometers (OBS) deployed on both114



parts, o�ering a dense coverage of 10 nm between instruments. All OBS except OBS 20,115

in which the hard disk failed, provided useful data. Four landstations where installed on116

the mainland by the IRD (Institut de Recherche pour le Developpement) in the extension117

of the pro�le, of which one gave useful data. A total of 3923 shots were �red on the pro�le118

by a 8530 in3 airgun array tuned to single-bubble mode to enhance the low frequencies and119

allow deep penetration [Avedik et al., 1993]. The second deep seismic pro�le (Pro�le S) is120

located around 25o S (Figure 1) and has a length of 538 km. It crosses the Norfolk Ridge,121

the New Caledonia Basin, the southern extension of the Fairway Ridge, the Fairway Basin122

and ends on the Lord Howe Rise. The pro�le was shot in two parts the �rst part using 15123

OBS and the second 12 OBS. All OBS provided useful data, except OBS50, which could124

not be recovered. A total of 3922 shots were �red on this pro�le using the same airgun125

array as on Pro�le N. Multichannel seismic (MCS) data were acquired along the pro�les126

using a 4.5 km long, 360 channel digital streamer.127

Processing of the multichannel seismic data was performed using the Geovecteur pro-128

cessing package. It included spherical divergence correction, FK-�ltering, bandpass �l-129

tering (3-5-50-60 Hz), internal mute and dynamic corrections. Velocity analysis was per-130

formed every 200 CDP for the �nal stack. The last processing step included applying131

an automatic gain control and a Kircho� time migration. The sedimentary layers are132

well imaged in the re�ection seismic section and the acoustic basement re�ector is clearly133

distinguishable throughout the pro�les. Clear re�ections from the Moho are found in the134

New Caledonia Basin and other regions.135

On both pro�les, additional gravimetric data were acquired using a BGM5 gravimeter,136

which o�ers a precision between 0.4 and 1.6 mGal depending on the state of the sea,137



and an instrument drift less than 2 mGal per month. The data were corrected for the138

instrument drift and the Eötvös correction was applied to allow the calculation of the139

free-air anomaly. A SeaSpy proton magnetometer towed behind the seismic streamer at140

a depth of 6 m was used for magnetic data acquisition. Its precision is about 0.2 gamma.141

Preprocessing of the OBS data included calculation of the clock-drift corrections to142

adjust the clock in each instrument to the GPS base time. The individual time drifts143

were between 0.4 and 8.2 ms per day with a mean of 2.46 ms. Instrument locations were144

corrected for drift from the deployment position during their descent to the sea�oor using145

the direct water wave arrival. The drift of the instruments never exceeded 200 m.146

Picking of the onset of �rst and secondary arrivals was performed without �ltering147

where possible (mostly between o�sets of 0 - 40 km). Di�erent �lters were applied to148

the instruments where necessary, depending on the quality of the data and the o�set to149

the source. Arrivals from longer o�sets are of lower frequency compared to short o�set150

arrivals, so the �lter frequencies were chosen correspondingly. On some instruments with151

a higher noise level, a narrow �lter was used to pick arrivals from longer o�sets.152

Data quality on Pro�le N is generally very good on all four channels. On the Lord Howe153

Rise and Fairway Ridge useful data were recorded at o�sets between the ship and the154

instrument of up to 200 km. OBS sections from the New Caledonia basin show several155

arrivals from sedimentary layers with high amplitudes (Figure 2). The crustal arrivals156

show one phase only with a velocity around 7 km/s. Clear high amplitude arrivals from the157

re�ection on the Moho and slightly lower amplitude arrivals from turning rays in the upper158

mantle are visible on most sections from the basin. A deep re�ected arrival from the upper159

mantle is observed underneath the New Caledonia basin (Figure 2). Arrivals from OBS160



sections from the Fairway Ridge and Lord Howe Rise include fewer sedimentary phases161

and high amplitude crustal arrivals with velocities between 6 and 6.8 km/s. Re�ected162

arrivals from the Moho and turning rays from the upper mantle arrive later than in the163

New Caledonian basin indicating a larger crustal thickness (Figure 3). Data from the164

Fairway Basin show several sedimentary arrivals, crustal arrivals at a velocity between165

6-6.8 km/s, as well as re�ections from the Moho (Figure 4).166

Along Pro�le S the data quality is equally high as along Pro�le N. Late PmP and Pn167

arrivals underneath the Norfolk Ridge indicate a large crustal thickness (Figure 5). OBS168

sections from the New Caledonia Basin show high amplitude crustal arrivals (Figure 6).169

The crossover distance between crustal and upper mantle arrivals is about 35 km, which170

is typical for oceanic crust. Finally, OBS sections from Lord Howe Rise show similar171

characteristics as those from the northern part of the Rise, including crustal arrivals at172

a velocity between 6.4 and 6.8 km/s and late re�ections from the Moho indicating high173

crustal thickness (Figure 7).174

3. Velocity modelling

The data were modelled using a two-dimensional iterative damped least-squares travel-175

time inversion from the RAYINVR software [Zelt and Smith, 1992]. Modelling was per-176

formed using a layer-stripping approach, proceeding from the top of the structure towards177

the bottom. We used a two-dimensional iterative damped least-squares inversion of travel178

times [Zelt and Smith, 1992]. Upper layers, where not directly constrained by arrivals179

from within the layer, were adjusted to improve the �t of lower layers. For the model pa-180

rameterization we used the minimum-parameter/minimum structure approach, to avoid181

inclusion of velocity or strucural features into the model unconstrained by the data [Zelt ,182



1999]. Lateral velocity changes are included into the model only if required by the data,183

and layers are only included if re�ected arrivals or changes in the velocity gradients are184

necessary to explain all arrivals. Arrival times of the main sedimentary layers and base-185

ment were picked from the re�ection seismic data. These were converted to depth using186

the OBS data and seismic velocities from velocity analysis of the re�ection seismic data.187

The depth and velocities of the crustal layers and the upper mantle were modelled from188

the OBS data only. Velocity gradients and the phase identi�cation in the velocity model189

were further constrained by synthetic seismogram modelling using the �nite-di�erence190

modelling code from the Seismic Unix package [Cohen and Stockwell , 2003; Stockwell ,191

1999].192

Picking uncertainties for each phase were de�ned by the ratio of the amplitudes 250 ms193

before and after onset of the picked arrival. A mean error was calculated for each phase194

of each station and then converted to predicted travel-time picking errors between 20 and195

125 ms using the table of [Zelt , 1999]. Using this procedure the �nal χ2 travel-time error196

of all modelled travel-time picks should be close to 1.0, ensuring a good quality of the �t197

of the model and without over interpretation of arrivals on traces with a low signal to198

noise ratio. All arrivals on the landstation data were assigned a picking error of 125 ms,199

to take into account the unreversed nature of the shots on land and the data quality. The200

number of picks, RMS traveltime residual and the χ2 -error for all phases are listed in201

Tables 1 and 2.202

The Pro�le N velocity model comprises 8 layers: the water layer, four sedimentary203

layers, two crustal layers and the upper mantle layer (Figure 8). Each layer is de�ned by204

depth and velocity nodes. Water velocity is a constant 1500 m/s throughout the model.205



Sea�oor bathymetry was determined from the echosounder logs. The sea�oor model layer206

includes depth nodes at a spacing of 1.5 km (Figure 8). The sedimentary layers are207

modelled using the re�ection seismic data for layer geometry converted to depth using208

velocities from the OBS data and sampled at the the same node spacing as the sea�oor209

layer. Sediment velocities range from 2.15 m/s - 2.70 m/s, 2.80 - 3.15 km/s, 3.20 - 4.60210

km/s and 5.40 - 5.50 kms. The crust was modelled by two layers of 6.4 - 6.6 km/s and211

6.6 - 7.0 km/s. The top of the basement was modelled with a spacing of 8 km, as it is not212

well resolved in the re�ection seismic data, due to the relatively small velocity increase213

between this layer and the overlying acoustic basement. The mid-crustal layer, the top of214

the underplate layer and the Moho are imaged with a lower resolution, and a depth node215

spacing of 10 - 12 km was adequate. The mantle velocity was set to gradient 8.00 km/s216

at the top of the layer and 8.40 km/s at the bottom of the model. A deep re�ection from217

the upper mantle is modelled as a �oating re�ector with no associated velocity increase.218

The velocity model of Pro�le S includes 7 layers: the water layer, three sedimentary219

layers, with velocities of 2.16 - 3.20 km/s, 3.20 - 4.30 km/s and 5.40 - 5.70 km/s, two220

crustal layers with velocities between 6.40 - 6.60 km/s and 6.60 - 7.25 km/s (Figure 9).221

As for Pro�le N the geometry of the sedimentary layers is taken from the re�ection seismic222

pro�les and converted to depth using the interval velocities calculated from the OBS data.223

Node spacing is the same as in Pro�le N: 1.5 km for the sea�oor and sedimentary layers,224

5 and 10 km for the basement and crustal layers.225

4. Error analysis

Two-point ray-tracing between source and receiver (Figure 10) shows the well-resolved226

and the unconstrained areas. Ray coverage is generally very good on both pro�les due to227



the excellent data quality. On Pro�le N the upper two sedimentary layers are well resolved228

throughout the model. The third sedimentary layer is not well sampled between 0 and 150229

km model distance. Basement and crustal layers are well resolved throughout the model230

except in the area of New Caledonia. The Moho is well constrained by re�ections and by231

turning rays into the upper mantle. The re�ector in the mantle is well de�ned between232

400 and 550 km model distance. All sedimentary layers are well sampled by re�ected233

and turning waves rays in the Pro�le S (Figure 10). The crustal layers are well sampled234

except for the lower crustal layer at model distances of less than 100 km. As for Pro�le235

N, the Moho is well constrained throughout the model by re�ections and turning waves.236

On both pro�les, the Moho has been additionally constrained by gravity modelling at the237

ends of the pro�les.238

Additional information about the quality of the velocity model is o�ered by the reso-239

lution parameter (see Figure 11) [Zelt and Smith, 1992]. Resolution is a measure of the240

number of rays passing through a region of the model constrained by a particular velocity241

node and is therefore dependent on the node spacing. Nodes with values greater than242

0.5, corresponding to white and light grey areas in the model, are considered well resolved243

(Figure 11). Only few regions of Pro�le N show a resolution less than 0.5. First, the crust244

beneath New Caledonia is not well resolved as the landstations did not yield high quality245

data. Second, the deeper sedimentary layers are less well resolved than the rest of the246

model as only few turning rays from these layers could be picked. On pro�le S no region247

with a resolution parameter value less than 0.5 can be discerned. The deeper sedimentary248

layers and the acoustic basement in the Fairway Basin show slightly reduced resolution249

values, as only few arrivals could be modelled for these layers.250



The �t between predicted arrival times and travel-time picks provides information about251

the quality of the model (Figure 10). The χ2 is de�ned as the root-mean-square traveltime252

mis�t between observed and calculated arrivals normalised to the picking uncertainty. The253

number of picks, the picking error, the values for the χ2 parameter and the rms mis�t for254

the most important phases of the models are listed in Tables 1 and 2.255

In order to estimate the velocity and depth uncertainty of the �nal velocity model a256

perturbation analysis was performed. The depth of the key interfaces was varied and257

an F-test was applied to determine if a signi�cant change between the models could be258

detected. The 95% con�dence limit gives an estimate of the depth uncertainty of the259

interface (see Figure 11). On Pro�le N the Moho is resolved to 0.5 and -0.4 km depth260

and the mid-crustal re�ector to +0.3 and -0.5 km. Depth constraints on the basement261

are between +0.04 and -0.1 km. The Moho is less well constrained on the margin-parallel262

pro�les, between 0.4 and 0.7 km. For all other interfaces the re�ection seismic data263

additionally constrain the geometry of the re�ectors, and thus the F-test does not predict264

a realistic value. On Pro�le S the Moho is constrained to +0.6 and -0.3 km, the mid265

crustal re�ector to +0.8 and -0.3 km and �nally the basement to +0.2 and -0.2 km.266

5. Gravity modelling

Gravity anomalies in the study region show a close correlation of gravity highs with the267

ridges and lows with the basins with the lowest values found southwest of New Caledonia268

in the New Caledonian Basin (Figure 12). Since seismic velocities and known densities269

for oceanic crust are well-correlated, gravity modelling provides an important additional270

constraint on the seismic model. Areas unconstrained by the seismic data can be modelled271

by comparing calculated gravity anomalies with those observed. Average P-wave velocities272



for each layer of the seismic models were converted to densities using the relationship of273

Christensen and Mooney [1995] for crustal layers and that of Ludwig, Nafe and Drake274

[1970] for sedimentary layers. Upper mantle densities were set to a constant 3.32 g/cm3.275

The lower crustal layer was divided into 3 regions on Pro�le N, of which the middle region276

underneath the New Caledonian Basin is characterized by slightly higher densities of 3.05277

g/cm3 (Figure 13) as compared to 2.84 g/cm3 along the rest of the pro�le. The density278

of the lower crust on Pro�le S was divided into 5 regions of which the crust underneath279

the Fairway Basin and New Caledonia Basin have higher densities around 2.91 g/cm3 as280

compared to the densities underneath the rises (Figure 13) corresponding to the higher281

seismic velocities found in those the basins.282

The gravity data were forward modelled using the gravity module of the software of283

Zelt and Smith (1992). To avoid edge e�ects both models were extended by 100 km at284

both ends and down to a depth of 95 km. The calculated anomalies can be compared with285

the shipboard measured gravity anomaly (Figure 13). The predicted anomalies generally286

�t the observed data well. The largest mis�t on Pro�le N is observed at around 225287

km model distance and might be caused by three-dimensional e�ects of the basement288

topography. The largest mis�ts on Pro�le S are located on both pro�le ends where289

the seismic constraints are weaker. An additional mis�t around 450 km model o�set is290

probably due to 3-D e�ects, as indicated by the positive gravity anomaly in the vicinity291

of the pro�le close to the location of OBS 36 (Figure 12).292

6. Comparison to re�ection seismic data

The wide-angle seismic models converted to two-way travel-time show good agreement293

with the re�ection seismic section (Figures 14 (A) and (B)). The most prominent sed-294



imentary re�ectors were taken from Lafoy and Ship. Sci. Party [2004] with slight ad-295

justments where necessary to �t the OBS data. However, only sedimentary re�ectors296

discernible in the OBS data and therefore necessary for the modelling were included to297

avoid over-parametrization of the inversion. Velocities of these main sedimentary layers298

were constrained by wide-angle seismic data, but some additional layering is imaged by299

the re�ection seismics. Depth of the acoustic basement is in very good agreement along300

the complete model.301

On Pro�le N the �rst sedimentary layer incorporated into the velocity model corresponds302

to the sequence from Mid-Miocene to present (Figure 14 (A)). The second layer comprises303

the Lower Miocene to post Upper Eocene sequences [Lafoy and Ship. Sci. Party , 2004].304

The base of this layer is marked by a regional unconformity which was related to the305

obduction of the ophiolites onto New Caledonia and the corresponding compressional306

phase. The third sedimentary layer corresponds to the Cretaceous to Paleocene sequences.307

Lastly the fourth layer represents the acoustic basement. The re�ection seismic data show308

two di�erent facies, the �rst one di�ractive and interpreted to be intrusive, and the second309

one re�ective with possibly sedimentary structures. This facies could be interpreted as310

series from an ancient volcanic arc metamorphosed during the Rangitata orogeny in the311

Upper Jurassic or Lower Cretaceous [Lafoy and Ship. Sci. Party , 2004].312

On Pro�le S only two instead of three sedimentary layers were included into the �nal313

velocity model (Figure 14 (B)). The �rst layer corresponds to the Miocene to present314

series and the second to the Cretaceous to Miocene series. In the New Caledonian Basin315

the acoustic basement shows a very di�ractive facies and might correspond to the top316

of the oceanic crust. The Moho discontinuity, from wide-angle seismic data modelling317



converted to two-way travel-time, corresponds to the base of a series of re�ectors in the318

corresponding re�ection seismic section from the New Caledonia basin (Figure 15).319

7. Results and discussion

Both wide-angle seismic pro�les (Figures 8 and 9) show large variations of the crustal320

thickness, from 23 km beneath the Lord Howe Rise and the Fairway Ridge to 8 km in321

the basins. The crustal thickness underneath the Norfolk Rise on Pro�le S is around 17322

km. The Moho depth of 26.5 km beneath Lord Howe Rise corresponds well with the323

29 km maximum depth found from expanded spread seismic pro�ling [Shor et al., 1971].324

All three rises are covered by sedimentary sequences ranging from several hundred of325

meters up to 3 km in thickness. The deep crustal structure of all three rises is similarly326

characterised by 3 seismic layers. The �rst layer underneath the acoustic basement shows327

variable velocities between 4 and 5.8 km/s, and may correspond to either volcanic or328

sedimentary rocks. The second and third layers are characterized by velocities of 6.4329

to 6.6 and 6.6 to 6.8 km/s. The interface between these layers is modelled as a second330

order velocity boundary, as no re�ections are observed along this interface but simply a331

change in velocity gradient. A comparison of the velocity-depth pro�les from the Lord332

Howe Rise and Fairway Ridge reveals a close resemblance to other continental fragments333

such as Lousy Bank and the Seychelles Plateau (For compilation see Carlson et al. [1980]334

and references therein) (Figure 16 (A)). Oceanic plateaus generally show steeper velocity335

gradients and a smaller crustal thickness than continental fragments. Volcanic islands336

are characterised by even steeper gradients and smaller crustal thickness. For the Lord337

Howe Rise a continental origin found in this study is in good agreement with results from338

expanding spread pro�les [Shor et al., 1971] and magnetic modelling [Schreckenberger339



et al., 1992]. On the basis of the velocity gradients and crustal thicknesses obtained from340

our dataset it can be deduced that the Fairway Ridge and Norfolk Ridge are of continental341

origin like the Lord Howe Rise. The correlation between positive magnetic anomalies and342

basement highs in this region, which is found on both pro�les (Figures 8 and 9) could343

be explained by continental crust with a considerable magnetization in the lower crust344

[Schreckenberger et al., 1992].345

In the basins of Pro�le N the sedimentary layer thickness increases to up to 5 km in the346

eastern part of the asymmetric New Caledonia Basin and 4 km in the more symmetric347

Fairway Basin. Sedimentary thickness is lower on Pro�le S, with up to 3 km in both basins.348

The crustal thickness decreases on Pro�le N to 8 km in New Caledonia Basin and to 10 km349

in the Fairway Basin. The Fairway Basin crust on Pro�le N is thicker than normal oceanic350

crust. The seismic velocities and the relative thickness of the crustal layers are identical351

to the Lord Howe Rise and Fairway Ridge. This indicates that the Fairway Basin results352

from thinning of continental crust during extension. On Pro�le S the crustal thickness353

of the Fairway Basin is around 12 to 15 km and the lower crust exhibits elevated lower354

crustal velocities as compared to the bordering Lord Howe Rise and Fairway Ridge. As in355

the north the crustal and the relative layer thicknesses correspond to thinned continental356

rather than oceanic crust. A possible explanation for the elevated lower crustal velocities357

and densities could be intrusions into the lower crust emplaced during the rifting of the358

basin.359

Comparison of crustal velocity-depth pro�les from the New Caledonia Basin on Pro�le360

N and Pro�le S to typical oceanic crust [White et al., 1992] indicates that, although361

crustal thickness on both pro�les is only slightly thicker than for oceanic crust, the seismic362



velocities found on the northern pro�le are not representative of oceanic crustal velocities363

(Figure 16 (B)). The upper crustal layer exhibits velocities higher than those usually found364

in normal oceanic crust. No typical magnetic oceanic anomalies are found in the basin365

(Figure 8 (C)).366

In the Parece Vela back-arc basin in the northeast Paci�c Ocean it has been shown from367

sampling that even intermediate to fast spreading oceanic crust can contain high percent-368

ages of upper mantle material of residual but still fertile composition, which otherwise369

is found on segment ends of medium fast spreading mid-ocean ridges or on very slow370

spreading ridges [Ohara, 2006; Ohara et al., 2002]. This fact might be explained by lower371

mantle temperatures in the back-arc basins due to the presence of the cold subducting372

slab [Abers et al., 2006]. Seismic velocities of serpentinised peridotites depend on their373

degree of serpentinisation, but are generally higher than those found typically in lower374

crustal layers (6.00-7.00 km/s) and lower than normal mantle velocities (8.00-8.40 km/s).375

We therefore propose that the crust in the New Caledonia Basin on Pro�le N could con-376

sist to a high degree of serpentinised upper mantle peridotites generated during an amag-377

matic phase of sea�oor spreading similar to crustal accretion at ultra-slow spreading cen-378

tres [Coakley and Cochran, 1998; Michael et al., 2003; Jokat and Schmidt-Aursch, 2006].379

This hypothesis is additionally strengthened by the absence of oceanic magnetic anoma-380

lies, which are mainly produced by the basaltic layer of magmatically formed oceanic381

crust and crustal densities higher than those found in either normal oceanic or continen-382

tal crust. The weak amplitude Moho re�ection in the basin might then originate from the383

serpentinisation front, as has been proposed in very slow spreading ridge environements384

[Jokat and Schmidt-Aursch, 2006].385



On Pro�le S the velocity-depth pro�le from the New Caledonia Basin is indicative of386

normal oceanic crust. The slightly elevated crustal densities in the New Caledonia Basin387

can by explained by the presence of oceanic crust. Also, the basement character in the388

basin changes from the northern Pro�le N, where it is characterised by large blocks, to the389

southern pro�le where it shows a roughness characteristic for oceanic crust, with highs 15-390

20 km spaced about apart (Figure 17 (a) and (b)). In the middle of the basin a pronounced391

basement high correlates with a 200 nT magnetic signature, which might correspond to392

an extinct spreading centre (Figure 17 (b)). The modelling of the magnetic anomalies in393

New Caledonian Basin indicates a crust of basaltic origin which is clearly correlated with394

the topography of the basin and the absence of magnetic anomalies [Collot , 2005]. The395

absence of magnetic anomalies in the basin can be explained by the fact that the basin396

is too narrow to span several magnetic reversals or that the crust was constructed during397

the Cretaceous quiet time [Collot , 2005].398

Neighbouring segments along the spreading axis in back-arc basins can display highly399

contrasted axial morphologies as found in the North Fiji Basin [Garel et al., 2003]. Bathy-400

metric studies of the Fiji back-arc basin demonstrate that both axial highs typical of fast401

spreading, and valleys typical of slow and very slow spreading, can be found along a sin-402

gle segment. This fact can be explained by spatial variations in the distribution of upper403

mantle convection cells below accretion centers superimposed on the thermal anomaly404

located under the whole basin [Garel et al., 2003]. We propose that theNew Caledonia405

Basin crust on Pro�le S is generated by a magmatically spreading segment, and there-406

fore shows features typical for normal oceanic crust: a division into one two seismically407

distinguishable layers (the basaltic and the gabbroic layer), a basement roughness typical408



for oceanic crust, and an axial high corresponding to the spreading center. The much409

stronger Moho re�ection (Figure 15) here originates from the base of the gabbroic layer.410

Modelling of the wide-angle seimic data resulted in imaging for the �rst time a strong411

re�ector in the upper mantle of the New Caledonia Basin (Figure 8). Documentation of412

mantle structures can help improve our understanding of how the subcrustal lithosphere is413

deformed and the relative behaviour between the crust and the upper mantle. The most414

widely studied mantle re�ector is the Flannan re�ector beneath the Devonian-Triassic415

West Orkney Basin o� Scotland [McBride et al., 1995], which has been imaged to a depth416

of about 80 km. Among the proposed mechanisms for the origin of this re�ector are the417

Caledonian orogeny [Brewer and Smythe, 1984] as well as the subsequent basin extension418

[Reston, 1993; Stein and Blundell , 1990]. A later study proposed either di�erential ex-419

tension of crust at least partially coupled to the uppermost mantle, or a broad zone of420

simple shear that was largely con�ned to the upper mantle but reactivated to deform the421

crust, as the origin for the Flannan mantle re�ector [McBride et al., 1995].422

The top of one of the two low velocity zones underneath New Caledonia proposed from423

modelling of the source-receiver functions [Regnier , 1988] may correspond to the mantle424

re�ector found in this study. In this case, the mantle re�ector would represent to the425

top of a subducted plate from an earlier eastward subduction underneath New Caledonia.426

However, the westward deepening of the re�ector does not �t to a proposed eastward dip427

of the low velocity zone. A strong mantle re�ector at about 35 km depth has also been428

imaged beneath a 40 km-wide, back-arc extension zone on North Island, New Zealand, a429

geologic setting which may be similar to that of the New Caledonia Basin. The authors430

propose the origin of this re�ector to be the top of a reservoir of partial melt [Stratford431



and Stern, 2004]. As no excessive volcanism has been found in the New Caledonia Basin432

this hypothesis seems unlikely for the New Caledonia Basin.433

Although the landstation data do not allow to trace the mantle re�ector in the vicinity434

of New Caledonia it general inclination suggests, that it gets in contact with the Moho435

underneath New Caledonia. Therefore, the most likely explanation of the origin of the436

mantle re�ector is that it formed during the extensional phase of the New Caledonian437

basin and represents a shear zone in the mantle. No deep re�ection has been modelled438

on Pro�le S, which might either be caused by the data quality or by the fact, that the439

re�ector is con�ned to the middle segment of the New Caledonian Basin.440

8. Conclusions

Modelling of the wide-angle seismic data from the ZONECO 11 cruise con�rmed of441

the continental nature of the Lord Howe Ridge, and enabled the determination of nature442

of the crust of the Fairway Ridge and the Norfolk Rise, which are also shown to be443

continental. This strengthens the hypothesis that all three ridges formed by fragmentation444

of Gondwanaland, caused either by widespread extension or in a backarc setting.445

The Fairway Basin shows relative layer thicknesses and upper crustal velocity gradients446

typical for thinned continental crust. Its lower crustal velocities indicate a continental447

origin on Pro�le N. Velocities are slightly elevated on the southern pro�le, which can be448

explained by intrusions into the lower crust. This would also explain the slightly higher449

density found from gravity modelling.450

The crust in the New Caledonia Basin on Pro�le N was modelled using velocities which451

are higher than either typical oceanic or continental crust. We propose that the crust452

in this region consists to a high degree of serpentinised upper mantle material generated453



during an amagmatic phase of opening of the basin, as has been found on ultra-slow454

spreading centres. This implies the absence of oceanic magnetic anomalies and of an455

axial high, which would be consistent with magmatic spreading. On Pro�le S the basin456

shows typical oceanic seismic velocities and relative layer thicknesses. Its basement is also457

characterised by typical oceanic roughness. We therefore propose that extension in this458

area resulted in magmatic sea-�oor spreading.459

Amantle re�ector dipping towards the west has been identi�ed beneath the New Caledo-460

nian Basin. Its dip does not correspond to the top of a low velocity zone found underneath461

New Caledonia from receiver function analysis, which has been associated to an eastward462

subduction underneath the island [Regnier , 1988]. The absence of large amounts of vol-463

canism seems to exclude its origin as the head of a melt-bearing zone as proposed for a464

similar re�ector on the North island of New Zealand [Stratford and Stern, 2004]. One465

explanation could be that it probably originates from the extensional tectonics during466

opening of the New Caledonian Basin.467

Acknowledgments.468

We would like to thank the captain and the crew of the N/O Atalante. Acquisition469

and processing of the data was funded by the ZoNéCo program of the ADECAL (Agence470

de Développement Economique de la Nouvelle Calédonie) in cooperation with Ifremer471

(Institute Français de Recherche pour l'Exploitation de la mer) and the IFP (Institut472

Français du Pétrole). The GMT [Wessel and Smith, 1995] and Seismic Unix software473

package [Stockwell , 1999] were used in the preparation of this paper. We would like to474

thank B. Pontoise (IRD) for deploying the landstations and A. Nercessian and S. Rouzo475



for help in the data acquisition. We thank M.-A. Gutscher for help with the English.476

Many thanks also to B. Pelletier and N. F. Exon for reviews which helped improve the477

manuscript.478

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Woodward, D. J., and T. M. Hunt, Crustal structure across the Tasman Sea, N. Z. J.671

Geol. Geophys. 14(1), pp. 39�45, 1971.672

Zelt, C. A., Modelling strategies and model assessment for wide-angle seismic traveltime673

data, Geophys. J. Int. 139, pp. 183�204, 1999.674

Zelt, C. A., and R. B. Smith, Seismic travel time inversion for 2-d crustal velocity struc-675

ture, Geophys. J. Int. 108, pp. 16�31, 1992.676

9. Figure Captions

Figure 1: (A) Predicted bathymetry from satellite altimetry [Smith and Sandwell , 1997]677

of the study region in the southwest Paci�c. Bold line show wide-angle pro�les and major678



physiographic features are annotated. (B) Location map of the wide-angle seismic pro�les679

(Box in A). Circles show OBS locations and inverted triangles IRD landstations. Contours680

are predicted sea�oor bathymetry from satellite gravity [Smith and Sandwell , 1997] in an681

1000m interval.682

Figure 2: (a) Bandpass �ltered (3-5 Hz, 24-36 Hz) vertical geophone data section from683

OBS 04 located in the New Caledonia Basin on Pro�le N. The data are displayed with a684

gain proportional to source-receiver o�set and are reduced at a velocity of 6 km/s. PmP685

(re�ection from the Moho), Pm2P (re�ection from the deep mantle re�ector) and Pn686

(turning waves fromt he upper mantle) are annotated (b) Synthetic seismograms calcu-687

lated from the velocity model for the same station using the �nite-di�erence modelling688

code from the Seismic Unix package [Cohen and Stockwell , 2003; Stockwell , 1999]. The689

synthetic seismograms are calculated every 100 m with a source frequency centred around690

5 Hz. The deep mantle re�ection is not reproduced because no velocity increase is asso-691

ciated to the re�ector.692

Figure 3: (a) Data from the vertical geophone data section from OBS 13 located on the693

Fairway Ridge on Pro�le N. The same gain, �lter and scaling have been applied as in694

Figure 2 a. (b) Corresponding synthetic seismograms calculated from the model using the695

same method as in Figure 2 b.696

Figure 4: (a) Data from the vertical geophone data section from OBS 16 located in the697

Fairway Basin on Pro�le N. The same gain, �lter and scaling have been applied as in698





Figure 5: (a) Data from the vertical geophone data section from OBS 35 located on701

the Norfolk Ridge on Pro�le S. The same gain, �lter and scaling have been applied as702

in Figure 2 a. (b) Corresponding synthetic seismograms calculated from the model using703

the same method as in Figure 2 b.704

Figure 6: (a) Data from the vertical geophone data section from OBS 42 located in the705

New Caledonia Basin on Pro�le S. The same gain, �lter and scaling have been applied as706

in Figure 2 a. (b) Corresponding synthetic seismograms calculated from the model using707

the same method as in Figure 2 b.708

Figure 7: (a) Data from the vertical geophone data section from OBS 53 located on the709

Lord Howe Rise on Pro�le S. The same gain, �lter and scaling have been applied as in710



Figure 8: (A) Final velocity model of Pro�le N including the model boundaries used713

during inversion (solid lines) and isovelocity contours every 0.25 km/s. Positions of OBSs714

(red circles) and landstations (orange inverted triangles) are indicated. Shaded areas are715

constrained by rays from the modelling. (B) shipboard measured magnetic anomaly along716

Pro�le N.717

Figure 9: (A) Final velocity model of Pro�le S including the model boundaries used718

during inversion (solid lines) and isovelocity contours every 0.25 km/s. Positions of OBSs719

(red circles) are indicated. Shaded areas are constrained by rays from the modelling. (B)720

shipboard measured magnetic anomaly along Pro�le S.721

Figure 10: (A) Ray coverage of the sedimentary layers of Pro�le N with every tenth722

ray from two-point ray-tracing plotted. Lower panel: Observed traveltime picks and723



calculated travel times (line) of the sedimentary layers for all receivers along the Pro�le724

N. (B) Same as (A) but for the crustal layers (C) Same as (A) but for the Moho and725

upper mantle layers. (D) Same as (A) but for Pro�le S (E) Same as (D) but for crustal726

layers (E) Same as (D) but for the Moho and upper mantle layers.727

Figure 11: (a) Resolution parameter for all depth nodes of the velocity model of Pro�le728

N. OBS positions are marked by black circles. Contour interval is 0.1. Depth uncertainty729

of the most important model boundaries as determined by 95% con�dence limit of the730

F-test during depth perturbation of the boundary is given in the framed boxes. (b)731

Resolution parameter for all depth nodes of the velocity model of Pro�le S. OBS positions732

are marked by black circles. Contour interval is 0.1. The depth uncertainty of the most733

important boundaries calculated from the 95 % con�dence limit of the f-test is given in734

the framed boxes.735

Figure 12: Free air gravity anomaly in the study area from satellite altimetry [Sandwell736

and Smith, 1995], contoured every 10 mGal. The gravity anomaly north of Pro�le S causes737

a discrepancy between measured gravity by satellite and predicted gravity anomaly from738

modelling (marked by arrow).739

Figure 13: (A) Upper panel: Shipboard measured free-air gravity anomaly (black line)740

and predicted anomaly from modelling (dashed line) along Pro�le N. Lower panel: Gravity741

model of Pro�le N. Positions of OBSs (circles) and landstations (inverted triangles) are742

indicated. Italic numbers give densities used for gravity modelling in g/cm3. Black line743

indicate layer boundaries from seismic modelling and dashed lines additional boundaries744

for gravity modelling. (B) Upper panel: Shipboard measured free-air gravity anomaly745

(black line) and predicted anomaly from modelling (dashed line) along Pro�le S. Lower746



panel: Gravity model of Pro�le S. Positions of OBSs (circles) are indicated. Italic numbers747

give densities used for gravity modelling in g/cm3. Black line indicate layer boundaries748

from seismic modelling and dashed black lines addition boundaries for gravity modelling.749

Figure 14: (A) Migrated multichannel seismic section of Pro�le N. OBS positions are750

indicated by black circles and layer boundaries from wide-angle data modelling by black751

lines. Blow-up of �gure 17 is marked by squares. (B) Migrated multichannel seismic752

section of Pro�le S. OBS positions are indicated by black circles and layer boundaries753

from wide-angle data modelling by black lines. Blow-up of �gures 17 and 15 is marked754

by square.755

Figure 15: Migrated multichannel seismic section of re�ections from the Moho in the New756

Caledonia Basin of Pro�le N. Predicted arrival time from wide-angle seismic modelling is757

indicated by the broken black line.758

Figure 16: (A) Comparison of crustal velocity-depth pro�les from the New Caledonia759

basin on Pro�le N (1) and Pro�le S (2) with typical oceanic crust (grey shade area gives760

envelope of velocities from the compilation of White et al., 1992 ). (B) Comparison761

of velocity depth pro�les for di�erent oceanic plateaus (broken black lines), continental762

fragments (black lines) and volcanic islands (dotted black lines): (1) Fiji Plateau [Sutton763

et al., 1971] (2) Shatsky Rise [Den et al., 1969] (3) Broken Ridge [Francis and Raitt ,764

1967], (4) Fairway Ridge (5) Lord Howe Rise (6) Lousy Bank [Klingelhoefer et al., 2005]765

(7) Seychelles Plateau [Francis et al., 1966] (8) Tenerife [Watts et al., 1997] (9) Réunion766

[Charvis et al., 1999] (10) Jasper Seamount [Hammer et al., 1994] (11) Marquesas [Caress767

et al., 1995] (12) Hawaii [Watts et al., 1985] (13) Ascension [Klingelhoefer et al., 2001].768



Figure 17: Comparison of re�ection seismic sections from the New Caledonia Basin along769

Pro�le N (A) and Pro�le S (B).770

Figure 18: Blow-up re�ection seismic section of the sedimentary layers in the New771

Caledonia Basin on Pro�le N and the parallel re�ection seismic Pro�le Z11-02 about 25772

km to the north (see Figure 1 for location).773



Phase No of picks RMS traveltime residual chi-squared

Water 1 4440 0.029 0.086

Sediments 1 2 19 0.025 0.068

Sediments 1 re�ection 4 1029 0.079 0.619

Sediments 2 3 609 0.096 0.927


Sediments 3 9 271 0.062 0.391

Basement re�ection 17 224 0.146 2.136

Upper crust 14 7528 0.134 1.778

lower crust 11 4703 0.157 2.473

PmP 7 4342 0.182 3.293

Pn 8 346 0.149 2.227

Mantle re�ection 13 355 0.772 59.695

All Phases 27005 0.159 2.529

Table 1. Traveltime residuals and chi-squared error for all phases and the complete

model of Pro�le N.



Phase No of picks RMS traveltime residual chi-squared

Water 1 3374 0.024 1.428

Sediments 1 2 94 0.076 2.629


Sediments 2 3 789 0.095 0.896

Basement re�ection 17 250 0.121 9.808

Upper crust 14 8114 0.107 1.147

lower crust 11 2247 0.111 10.148

PmP 7 3283 0.130 13.476

Pn 8 1011 0.108 2.690

All Phases 22051 0.100 4.655

Table 2. Traveltime residuals and chi-squared error for all phases and the complete

model of Pro�le S.



150˚ 155˚ 160˚ 165˚ 170˚ 175˚

-40˚

-35˚

-30˚

-25˚

-20˚

-15˚

-10˚

-8000

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

Bathymetry

m

North New

Caledonia Basin

Lord How

e Rise

Norfolk R

idge

Fairway Ridge

Tasman Sea Basin

LoyaltyBasin V

anuatuAustralia

North Fiji Basin

South New Caledonia Basin

Central N

ew C

aledonia Basin

Coral Sea

South Fiji Basin

North

Norfolk Basin

South

Norfolk Basin

160˚ 161˚ 162˚ 163˚ 164˚ 165˚ 166˚ 167˚ 168˚

-26˚

-25˚

-24˚

-23˚

-22˚

-21˚

-20˚

-3000

-3000

-2000

-2000

-2000

-2000

-1000

-1000

-1000

-1000

-1000

-1000

0

0

0

Lord Howe Rise

Norfolk R

idge

Fairway Rise

New Caledonia

Central NewCaledonia Basin

Profile Nord

Profile Sud

A

B

0102030405060708091011121314151617181921222324252627282930

3132333435363738394041424344454647484951525354555657

PINNEK

GOI

MON

Figure 1.



1

2

3

4

5

6

7

8

9

10

1

2

3

4

5

6

7

8

9

10

0.10.61.11.62.12.63.13.64.14.65.15.66.16.67.17.68.18.69.19.6x10 4

0123456789

10

Tim

e -

X/6

[sec

]-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0

Offset [km]

Water

Sediments

Crust

PmP

Pm2P

Pn

A

B

SW NE

0123456789

10

Tim

e -

X/6

[sec

]

-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0Offset [km]

Figure 2.



1

2

3

4

5

6

7

8

9

10

0.1 0.6 1.1 1.6 2.1 2.6 3.1 3.6 4.1 4.6 5.1 5.6 6.1 6.6 7.1 7.6 8.1 8.6 9.1 9.6x10 4

1

2

3

4

5

6

7

8

9

10

0123456789

10

Tim

e -

X/6

[sec

]0 10 20 30 40 50 60 70 80 90 100

Offset [km]

Water

Crust

PmP

SW NE

0123456789

10

Tim

e -

X/6

[sec

]

0 10 20 30 40 50 60 70 80 90 100Offset [km]

Figure 3.



1

2

3

4

5

6

7

8

9

10

1

2

3

4

5

6

7

8

9

10

0.10.61.11.62.12.63.13.64.14.65.15.66.16.67.17.68.18.69.19.6x10 4

0123456789

10

Tim

e -

X/6

[sec

]-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0

Offset [km]

Water

Crust

PmP

SW NE

0123456789

10

Tim

e -

X/6

[sec

]

-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0Offset [km]

Figure 4.



1

2

3

4

5

6

7

8

9

10

1

2

3

4

5

6

7

8

9

10

0.10.61.11.62.12.63.13.64.14.65.15.66.16.67.17.68.18.69.19.6x10 4

0123456789

10

Tim

e -

X/6

[sec

]-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0

Offset [km]

Water

Sediments

Crust

PmP

Pn

A

B

SW NE

0123456789

10

Tim

e -

X/6

[sec

]

-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0Offset [km]

Figure 5.



1

2

3

4

5

6

7

8

9

10

1

2

3

4

5

6

7

8

9

10

0.10.61.11.62.12.63.13.64.14.65.15.66.16.6x10 4

0123456789

10

Tim

e -

X/6

[sec

]-70 -60 -50 -40 -30 -20 -10 0

Offset [km]

Water

Crust

PmP

Pn

A

B

SW NE

0123456789

10

Tim

e -

X/6

[sec

]

-70 -60 -50 -40 -30 -20 -10 0Offset [km]

Figure 6.



1

2

3

4

5

6

7

8

9

10

0.1 0.6 1.1 1.6 2.1 2.6 3.1 3.6 4.1 4.6 5.1 5.6 6.1 6.6 7.1 7.6 8.1 8.6 9.1 9.6x10 4

1

2

3

4

5

6

7

8

9

10

0123456789

10

Tim

e -

X/6

[sec

]0 10 20 30 40 50 60 70 80 90 100

Offset [km]

Water

Crust

PmP

SW NE

0123456789

10

Tim

e -

X/6

[sec

]

0 10 20 30 40 50 60 70 80 90 100Offset [km]

Figure 7.



35

30

25

20

15

10 50

Depth (km)

050

100

150

200

250

300

350

400

450

500

550

600

650

Dis

tanc

e (k

m)

SW

NE

8.0

- 8.

2 km

/s

New

Cal

edo

nia

Bas

inF

airw

ay B

asin

Fai

rway

Ris

eL

ord

Ho

we

Ris

e

7.0

7.25

6.5

6.5

5.0

5.0

5.75

5.254.

755.

52.

753.

03.

502.

15-2

.25

2.10

-2.1

52.

5-2.

7

A1.60

2.22

2.94

3.66

4.38

5.10

5.82

6.54

7.26

8.00

8.40

Vel

ocity

km/s

OBS01

OBS02

OBS03

OBS04

OBS05

OBS06

OBS07

OBS08

OBS09

OBS10

OBS11

OBS12

OBS13

OBS14

OBS15

OBS16

OBS17

OBS18

OBS19

OBS21

OBS22

OBS23

OBS24

OBS25

OBS26

OBS27

OBS28

OBS29

OBS30

PIN

NEK

GOI

MON

5000

500

Magnetic Anom. (nT)

050

100

150

200

250

300

350

400

450

500

550

600

650

Dis

tanc

e (k

m)

B

5000

500

Magnetic Anom. (nT)

050

100

150

200

250

300

350

400

450

500

550

600

650

Dis

tanc

e (k

m)

Figure 8.



35

30

25

20

15

10 50

Depth (km)

050

100

150

200

250

300

350

400

450

500

550

Dis

tanc

e (k

m)

SW

NE

8.0

- 8.

2 km

/s

Lo

rd H

ow

e R

ise

Fai

rway

Ris

eF

airw

ay B

asin

No

rfo

lk R

ise

New

Cal

edo

nia

Bas

in

7.0

6.75

7.0

6.5

6.5

5.5

4.5

4.0

5.5

3.5

4.0

6.75

2.0

5.5

A

1.60

2.22

2.94

3.66

4.38

5.10

5.82

6.54

7.26

8.00

8.40

Vel

ocity

km/s

OBS31

OBS32

OBS33

OBS34

OBS35

OBS36

OBS37

OBS38

OBS39

OBS40

OBS41

OBS42

OBS43

OBS44

OBS45

OBS46

OBS47

OBS48

OBS49

OBS51

OBS52

OBS53

OBS54

OBS55

OBS56

OBS57

5000

500

Magnetic Anom. (nT)

050

100

150

200

250

300

350

400

450

500

550

Dis

tanc

e (k

m)

B

5000

500

Magnetic Anom. (nT)

050

100

150

200

250

300

350

400

450

500

550

Dis

tanc

e (k

m)

Figure 9.



-40

-30

-20

-10

0

Dep

th (

km)

W E

A0

4

8

Tim

e -

Offs

et/8

[s]

-40

-30

-20

-10

0

Dep

th (

km)

B0

4

8

Tim

e -

Offs

et/8

[s]

-40

-30

-20

-10

0

Dep

th (

km)

C4

8

Tim

e -

Offs

et/8

[s]

0 100 200 300 400 500 600

Distance [km]

-40

-30

-20

-10

0

Dep

th (

km)

W E

D0

4

8

Tim

e -

Offs

et/8

[s]

-40

-30

-20

-10

0

Dep

th (

km)

E0

4

8

Tim

e -

Offs

et/8

[s]

-40

-30

-20

-10

0

Dep

th (

km)

F4

8

Tim

e -

Offs

et/8

[s]

0 100 200 300 400 500 600

Distance [km]

Figure 10.



35

30

25

20

15

10

5

0

Dep

th (

km)

0 50 100 150 200 250 300 350 400 450 500 550

Distance (km)

SW NE

B

Lord Howe Rise Fairway RiseFairway Basin Norfolk RiseNew Caledonia Basin

+0.6/-0.3 km

+0.8/-0.3 km

+0.2/-0.2 km

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Resolution

OB

S31

OB

S32

OB

S33

OB

S34

OB

S35

OB

S36

OB

S37

OB

S38

OB

S39

OB

S40

OB

S41

OB

S42

OB

S43

OB

S44

OB

S45

OB

S46

OB

S47

OB

S48

OB

S49

OB

S51

OB

S52

OB

S53

OB

S54

OB

S55

OB

S56

OB

S57

35

30

25

20

15

10

5

0

Dep

th (

km)

0 50 100 150 200 250 300 350 400 450 500 550

Distance (km)

35

30

25

20

15

10

5

0

Dep

th (

km)

0 50 100 150 200 250 300 350 400 450 500 550 600 650

Distance (km)

SW NE

A

New Caledonia BasinFairway Basin Fairway RiseLord Howe Rise

+0.5/-0.4 km/s

+0.3/-0.5 km/s

+0.04/-0.1 km/s

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Resolution

OB

S01

OB

S02

OB

S03

OB

S04

OB

S05

OB

S06

OB

S07

OB

S08

OB

S09

OB

S10

OB

S11

OB

S12

OB

S13

OB

S14

OB

S15

OB

S16

OB

S17

OB

S18

OB

S19

OB

S21

OB

S22

OB

S23

OB

S24

OB

S25

OB

S26

OB

S27

OB

S28

OB

S29

OB

S30

PIN

NE

K

GO

I

MO

N

35

30

25

20

15

10

5

0

Dep

th (

km)

0 50 100 150 200 250 300 350 400 450 500 550 600 650

Distance (km)

Figure 11.



160˚

161˚

162˚

163˚

164˚

165˚

166˚

167˚

168˚

-27˚

-27˚

-26˚

-26˚

-25˚

-25˚

-24˚

-24˚

-23˚

-23˚

-22˚

-22˚

-21˚

-21˚

Lord Howe Rise

Norfolk Rise

Fairw

ay R

ise

New

Cal

edon

ia

Prof

ile N

ord

Pro

file

Su

d

160˚

161˚

162˚

163˚

164˚

165˚

166˚

167˚

168˚

-27˚

-27˚

-26˚

-26˚

-25˚

-25˚

-24˚

-24˚

-23˚

-23˚

-22˚

-22˚

-21˚

-21˚

-80

-60

-40

-20

020

4060

80

Gra

vity

Ano

mal

y

mG

al

Figure 12.



35

30

25

20

15

10

5

0

Dep

th (

km)

0 50 100 150 200 250 300 350 400 450 500 550

Distance (km)

SW NE

B

2.20 2.412.58

2.80

2.842.91

2.84

2.91

2.84

3.32

40 200

2040

Gra

vity

Ano

m. (

mG

al)

40 200

2040

Gra

vity

Ano

m. (

mG

al)

35

30

25

20

15

10

5

0

Dep

th (

km)

SW NE

A

2.22.252.392.61 2.51

2.80

2.84

3.06

2.84

3.32

50

100

150

200

Gra

vity

Ano

m. (

mG

al)

0 50 100 150 200 250 300 350 400 450 500 550 600 650

Distance (km)

50

100

150

200

Gra

vity

Ano

m. (

mG

al)

0 50 100 150 200 250 300 350 400 450 500 550 600 650

Distance (km)

Figure 13.



0 Time [s]

x107Offset [k

m]

0 Time [s]

x107Offset [k

m]

0 1 2 3 4 5 6 7 8 9 10 11 12Two-Way Travel-Time [s]0 1 2 3 4 5 6 7 8 9 10 11 12

Two-Way Travel-Time [s]

2040

6080

100

120

140

160

180

200

220

240

260

280

300

320

340

360

380

400

420

440

460

480

500

520

540

560

Offs

et [k

m]

Fig

ure

17

(A)

Lo

rd H

ow

e R

ise

Fai

rway

Bas

inF

airw

ay R

idg

eN

ew C

aled

on

ia B

asin

SW

NE

A

OBS01

OBS02

OBS03

OBS04

OBS05

OBS06

OBS07

OBS08

OBS09

OBS10

OBS11

OBS12

OBS13

OBS14

OBS15

OBS16

OBS17

OBS18

OBS19

OBS21

OBS22

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0 Time [s]

x107Offset [k

m]

0 Time [s]

x107Offset [k

m]

0 1 2 3 4 5 6 7 8 9 10 11 12Two-Way Travel-Time [s]

0 1 2 3 4 5 6 7 8 9 10 11 12

Two-Way Travel-Time [s]

020

4060

8010

012

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018

020

022

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0O

ffset

[km

]

Fig

ure

17

(B)

Fig

ure

15

Lo

rd H

ow

e R

ise

Fai

rway

Ris

eF

airw

ay B

asin

No

rfo

lk R

ise

New

Cal

edo

nia

Bas

inS

WN

E

B

OBS32

OBS33

OBS34

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OBS36

OBS37

OBS38

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OBS41

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OBS45

OBS46

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OBS52

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OBS54

OBS55

OBS56

OBS57

Figure 14.D R A F T June 16, 2007, 1:41pm D R A F T


0

Time [s]

x10 7Offset [km]

8.5

9.0

9.5

10.0

Tw

o-W

ay T

rave

l-Tim

e [s

]

260 270 280 290 300 310 320 330 340 350

Offset [km]

Multiple

Wide-angle Moho

Figure 15.



0

2

4

6

8

10

Dep

th b

elow

bas

emen

t [km

]

3 4 5 6 7 8 9Velocity [km/s]

(2)

(1)

A B

0

2

4

6

8

10

Dep

th b

elow

bas

emen

t [km

]

3 4 5 6 7 8 9Velocity [km/s]

0

5

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15

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25

30

35

Dep

th b

elow

sur

face

of t

he e

dific

e [k

m]

2 3 4 5 6 7 8 9Velocity [km/s]

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

0

5

10

15

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25

30

35

Dep

th b

elow

sur

face

of t

he e

dific

e [k

m]

2 3 4 5 6 7 8 9Velocity [km/s]

Figure 16.



4.00

4.05

4.10

4.15

4.20

4.25

4.30

4.35

4.40

4.45

4.50

4.55

4.60

4.65

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4.80

4.85

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4.95

5.00

5.05

5.10

5.15

5.20

5.25

5.30

5.35

5.40

5.45

5.50

5.55

5.60

5.65

5.70

5.75

5.80

5.85

5.90

5.95

6.00

6.05

6.10

6.15

6.20

6.25

6.30

6.35

6.40

6.45

6.50

6.55

6.60

6.65

6.70

6.75

6.80

6.85

6.90

6.95

7.00

7.05

7.10

7.15

7.20

7.25

7.30

7.35

7.40

7.45

7.50

7.55

7.60

7.65

7.70

7.75

7.80

7.85

7.90

7.95

8.00

8.05

8.10

8.15

8.20

8.25

8.30

8.35

8.40

8.45

8.50

8.55

8.60

8.65

8.70

8.75

8.80

8.85

8.90

8.95

Tim

e [s

]

4.300 4.305 4.310 4.315 4.320 4.325 4.330 4.335 4.340 4.345 4.350 4.355 4.360 4.365 4.370 4.375 4.380 4.385 4.390 4.395 4.400 4.405 4.410 4.415 4.420 4.425 4.430 4.435 4.440 4.445 4.450 4.455 4.460 4.465 4.470 4.475 4.480 4.485 4.490 4.495 4.500 4.505 4.510 4.515 4.520 4.525 4.530 4.535 4.540 4.545 4.550 4.555 4.560 4.565 4.570 4.575 4.580 4.585 4.590 4.595 4.600 4.605 4.610 4.615 4.620 4.625 4.630 4.635 4.640 4.645 4.650 4.655 4.660 4.665 4.670 4.675 4.680 4.685 4.690 4.695 4.700 4.705 4.710 4.715 4.720 4.725 4.730 4.735 4.740 4.745 4.750 4.755 4.760 4.765 4.770 4.775 4.780 4.785 4.790 4.795 4.800 4.805 4.810 4.815 4.820 4.825 4.830 4.835 4.840 4.845 4.850 4.855 4.860 4.865 4.870 4.875 4.880 4.885 4.890 4.895 4.900 4.905 4.910 4.915 4.920 4.925 4.930 4.935 4.940 4.945 4.950 4.955 4.960 4.965 4.970 4.975 4.980 4.985 4.990 4.995 5.000 5.005 5.010 5.015 5.020 5.025 5.030 5.035 5.040 5.045 5.050 5.055 5.060 5.065 5.070 5.075 5.080 5.085 5.090 5.095 5.100 5.105 5.110 5.115 5.120 5.125 5.130 5.135 5.140 5.145 5.150 5.155 5.160x10 5Offset [km]

4.00

4.05

4.10

4.15

4.20

4.25

4.30

4.35

4.40

4.45

4.50

4.55

4.60

4.65

4.70

4.75

4.80

4.85

4.90

4.95

5.00

5.05

5.10

5.15

5.20

5.25

5.30

5.35

5.40

5.45

5.50

5.55

5.60

5.65

5.70

5.75

5.80

5.85

5.90

5.95

6.00

6.05

6.10

6.15

6.20

6.25

6.30

6.35

6.40

6.45

6.50

6.55

6.60

6.65

6.70

6.75

6.80

6.85

6.90

6.95

7.00

7.05

7.10

7.15

7.20

7.25

7.30

7.35

7.40

7.45

7.50

7.55

7.60

7.65

7.70

7.75

7.80

7.85

7.90

7.95

8.00

8.05

8.10

8.15

8.20

8.25

8.30

8.35

8.40

8.45

8.50

8.55

8.60

8.65

8.70

8.75

8.80

8.85

8.90

8.95

Tim

e [s

]

2.740 2.745 2.750 2.755 2.760 2.765 2.770 2.775 2.780 2.785 2.790 2.795 2.800 2.805 2.810 2.815 2.820 2.825 2.830 2.835 2.840 2.845 2.850 2.855 2.860 2.865 2.870 2.875 2.880 2.885 2.890 2.895 2.900 2.905 2.910 2.915 2.920 2.925 2.930 2.935 2.940 2.945 2.950 2.955 2.960 2.965 2.970 2.975 2.980 2.985 2.990 2.995 3.000 3.005 3.010 3.015 3.020 3.025 3.030 3.035 3.040 3.045 3.050 3.055 3.060 3.065 3.070 3.075 3.080 3.085 3.090 3.095 3.100 3.105 3.110 3.115 3.120 3.125 3.130 3.135 3.140 3.145 3.150 3.155 3.160 3.165 3.170 3.175 3.180 3.185 3.190 3.195 3.200 3.205 3.210 3.215 3.220 3.225 3.230 3.235 3.240 3.245 3.250 3.255 3.260 3.265 3.270 3.275 3.280 3.285 3.290 3.295 3.300 3.305 3.310 3.315 3.320 3.325 3.330 3.335 3.340 3.345 3.350 3.355 3.360 3.365 3.370 3.375 3.380 3.385 3.390 3.395 3.400 3.405 3.410 3.415 3.420 3.425 3.430 3.435 3.440 3.445 3.450 3.455 3.460 3.465 3.470 3.475 3.480 3.485 3.490 3.495 3.500 3.505 3.510 3.515 3.520 3.525 3.530 3.535 3.540 3.545 3.550 3.555 3.560 3.565 3.570 3.575 3.580 3.585 3.590 3.595 3.600x10 5Offset [km]

4

5

6

7

8

9

Tw

o-w

ay tr

avel

-tim

e [s

ec]

430 440 450 460 470 480 490 500 510Distance [km]

A New Caledonia Basin, Profile N

B New Caledonia Basin, Profile S

Basement

Basement

4

5

6

7

8

9

Tw

o-w

ay tr

avel

-tim

e [s

ec]

280 290 300 310 320 330 340 350 360Distance [km]

Figure 17.D R A F T June 16, 2007, 1:41pm D R A F T

crustal structure of the basin and ridge system west of...

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