the structure of the south fiji basin
TRANSCRIPT
Elsevier Scientific Publishing Company. Amsterdam-Printed in The Netherlands
Marine geology and geophysics
THE STRUCTURE OF THE SOUTH FIJI BASIN
F.J. DAVEY
Geophysics Ditkion, L)epartment of Scientific and Industrial Research, WeUingtm (New Zeulmd)
(Final version received May 12, 198 1)
ABSTRACT
Davey, F.J., 1982. The structure of the South Fiji Basin. In G.H. Packham (Editor), The Evolution of the
India-Pacific Plate Boundaries. Tecronoph,vsrcs. 87: 185-241.
New magnetic, seismic and bathymetric data show that the South Fiji basin I:\ a structurally complex
marginal basin. A gap in the identifiable magnetic anomaly lintations exist over the central part of the
basin and prevents the unequivocal linking of the anomaly lineations (anomalies 7A to 12) asbociated with
the ridge-ridge--ridge triple junction in the north with an apparent single aprcading ccntre of the same age
in the south. This gap, which makes a detailed synthesis of the historical development of the basin
difficult. may arise from post-spreading intraplate tectonics. If symmetric spreading is assumed. part of
the oceanic lithosphere formed during the Oligocene episode of seafloor spreading has suhxquentlv hcen
consumed. presumably by subduction westward under Three Kings rise.
INTRODUCTION
The South Fiji basin is one of a series of marginal basins west of the Tonga-
Kermadec island arc-trench system (Karig, 1970). It lies to the north of New
Zealand and is bounded by the Lau-Colville ridge to the east, the Fiji plateau to the
north and the Loyalty rise and Three Kings rise to the west (Fig. 1). The basin is
underlain by oceanic crust (Shor et al., 1971) and was considered by Karig (1970) to
be formed by crustal extension in Oligocene times, with the Three Kings rise and
Loyalty rise being interpreted as remnant arcs. DSDP sites 205 and 285 (Burns et al.,
1973; Andrews et al., 1976) indicate an age of at least middle Oligocene to early
Miocene for the formation of the basin. Magnetic anomalies have been studied by
Weissel and Watts (1975) and Watts et al. (1977) and anomalies 7A through 12
(26.--33 m.y. B.P.) have been identified in the northern part of the basin. They were
interpreted as having been generated by a ridge-ridge-ridge (RRR) triple junction
centered near the northern end of the Three Kings rise. Subdued ridges in the
bathymetry were tentatively identified as the extinct spreading centres. Packham and
0040- 195 1,‘82,‘0000-0000/$02.75 G 1982 Elsevier Scientific Pubiishing Company
25’S
d
40”s 1 I 16O’E 165OE 170’E
Fig. I. Location of the South Fiji basin.
-7 , I 175OE ltw 17vw 170°w
Terrill (1976) have studied the results of DSDP sites 205 and 285 with bathymetric and some seismic reflection data and derived a tentative geologic history for the region. They suggest that the northwestern part of the basin is Eocene in age, in conflict with the magnetie results of Watts et al. (1977), and that middle and upper Miocene volcano&&c sediments eover the centre of the South Fiji basin and thin
towards the margins. In this paper the earlier study of Watts et al. (1977) is extended to include the
southern part of the basin and it also discusses in more detail the geological structure and history of the region.
BATHYMETRIC
A new bathymetric map of the South Fiji basin has been prepared (Fig. 2) and includes a significant amount of new data, obtained with satellite navigation control,
187
primarily in the deeper parts of the basin (Lamont-Doherty Geological Observatory
cruises ELT40, V3215, and V33 14; NZDSIR cruise Tui73). Outside the deeper parts
of the basin, where there are few new data, the bathymetry is based primarily on
Packham and Terrill (1976), Cullen (1975) and data contained on New Zealand
Oceanographic Institute compilation sheets. Part of the northern half of the map has
been presented and discussed by Watts et al. (1977) where the subdued morphologi-
cal ridges coinciding with the extinct spreading centres of the postulated RRR
spreading system were noted.
The margins of the South Fiji basin will not in general be discussed here as they
have been adequately described by Packham and Terrill (1976) and there are few
new data in these regions. However, the map brings out the ridge and trough
morphology to the west of Three Rings rise. This structural grain to the morphology
has a strike of north-south in the east and northeast-southwest in the west. Marked
troughs occur in this region which has a mean depth of about 2000-3000m. Also
noteworthy is the subsidiary ridge which lies about 150 km west of the Lau ridge,
near longitude 180”, between latitudes 24”s and 27”s and rises to within 1700 m of
sea level.
The northwest part of the basin is marked by a series of ridges aligned in an
ENE-WSW direction with wavelengths of about 50 km and amplitudes of about
600m. These ridges are intersected in places by north-south trending ridges. They
do not occur further east than 174S”E and their western limit is marked by a low
irregular north-south ridge lying about 100 km to the east of the Loyalty rise. To the
east of this ridge province the seafloor of the basin is essentially smooth, decreasing
gradually in depth to the north and east and with shallow NNE trending channels in
the north. Isolated small seamounts occur and the region is traversed by a subdued
basement ridge, the Bounty ridge of Watts et al. (1977) which further data have
shown to be narrower and more dissected than previously thought. A southeast
trending ridge, interpreted by them as marking the spreading centre of the SSE arm
of the triple junction system, is a narrow, straight feature traceable from 27”S,
174.5’E to 285”S, 176”E where it is aligned with the eastern part of a conspicuous
ridge, the Central ridge of Packham and Terrill (1976). At 28”s a large elongate
seamount with a least depth of 7OOm, the Marion seamount (new name), abuts the
western flank of the ridge. The deepest seafloor in the South Fiji basin lies along the
eastern side of the narrow ridge reaching depths in excess of 4800 m, and having the
morphology of a downfaulted block about 100 km wide. Minor ridges trend
orthogonally to the southeast ridge on its western side. A ridge marking the western
arm of the triple junction is not obvious, but a low rise was noted by Watts et al.
(1977) which lies in a suitable position, separated by a linear shallow through from
the northern end of the Three Rings rise.
The smooth seafloor of the northeastern South Fiji basin gives way to a more
irregular shallower seafloor at about 27.5’S where a deep terrace extends out from
the Lau ridge towards the west to the Central ridge and Marion seamount and
southwards to about 29.5% To the south of 29.5”s the deep seafloor of the southern South Fiji basin rises gently southwards from about 4300m to about 3500 m a~ 33.5”s where it rises sharply to the Northland Plateau at 2000m. It is generally smooth with some isolated seamounts and ridges trending approximately east-west. Conspicuous lines of elongate seamounts extend southeastwards from the Three Kings rise at about 31”s to the centre of the southern South Fiji basin and eastwards from the Three Kings rise at about 30’S The eastern flank of the Three Kings rise south of 29.5’S is marked by a number of large seamounts, the Sarah seamounts (new name). The rise itself is remarkably straight, lying north-south and abruptly terminated in the north at 28”s.
SEISMIC DATA
The coverage of the seismic reflection data is shown in Fig. 3. Seismic refraction data for the region include sonobuoy data and published two ship refraction measurements (Shor et al., 1971). The positions of these measurements are shown in Fig. 3 and the results of the sonobuoy measurements given in Table I.
Over most of the South Fiji basin a basement reflector can be picked on the reflection data and is confirmed as basement where the sparse refraction data are available. It is uncertain in places along the western flank of the Lau-Colville ridge
and in some regions in the southern South Fiji basin where it lies at depths greater than 1 s two way travel time. The data have been used to produce the sediment
isopach map (Fig. 3) where the sediment thickness is given in tenths of a second of two way travel time. Unless otherwise stated, sediment thicknesses are given in terms of the vertical two way travel time for seismic energy through the sediments. The sediment cover increases in thickness eastwards across the South Fiji basin to over 1 s of section along the flank of the Lau-Colville ridge from the Hunter fracture zone to the New Zealand continental margin. The southern South Fiji basin is underlain by thicker sediments than the northern part. The sediments on the flank of the Lau-Colville ridge are banked up by a low ridge lying subparallel to the Lau-Colville ridge.
It is difficult to separate the sedimentary sequence into separate units of regional extent from the reflection data. As noted by Packham and Terrill (1976) at DSDP site 285, the reflection data show a gradual change in character with depth, from being stratified near the surface to acoustically transparent at depth in the sequence of Miocene volcanogenic sediments, and no major reflector of regional extent. However, at DSDP site 205 stratified Miocene volcanogenic sediments overlie acoustically transparent Oligocene pelagic sediments. Thus there appears to be no sure way of separating the volcanogenic sediments from older pelagic sediments using reflection data. A boundary close to the top of the volcanogenic sediments and below the thin upper layer of Mi~ene-Pli~ne ooze and “red clays” can, however, be traced over a wide area.
SOUTH FIJI BASIN
Bathymetry (metresf
170*E 175*
Fig. 2. Bathymctry of the South Fiji basin. Depths in corrected metres. contour interval=~X4l m. IWX) m contows shorn by heavy line
pp. 197-204
20”s
35”s
Fig. 3. Sediment thickness in the South Fiji basin. Isopachs at intervals of 0.2 s two-way travel time
presented seismic reflection profiles (Figs. 4-9 and 12) are shown by heavy lines. Solid circles mark
and SC from Shot et al. (1371) and solid triangles labelled 1-i’ the sonobuoy measurements listed
labelled X, Y and Z and for the position W on profile L. The solid squares mark the position of IX
id
5"E 180' ip tracks are shown by thin lines and the localion of
location of seismic refraction measurements, !?A, SB
Table 1. See text for the explanation of dashed lines
sites 285 and 205.
205
TABLE I
Sonobuoy results
Sonobuoy Vl Tl v2 T2 v3 T3 V4 T4 v5
station (km/s) (km) (km/s) (km) (km/s) (km) (km/s) (km) (km/s)
I 52 Cl2 1.5 4.27 (1.8) 0.48 4.0 0.68 5.6 0.95 6.4
2 178A V32 1.5 4.39 1.8 * 0.52 5.8
3 179 V32 I.5 4.41 (1.8) 0.68 5.6
4 180 V32 1.5 4.38 (1.8) 0.78 5.8
5 181 v32 1.5 4.23 2.0 * 0.75 5.6
6 182 V32 15 4.17 (1.8) 0.66 3.2 0.87 4.7 0.55 6.3
7 v33 1.5 2.84 (2.0) 0.70 (2.5) 0.90 3.6 1.16 4.6
( ) = assumed velocity; * = interval velocity.
In the northwest of the South Fiji basin thick sediments showing few internal
reflections lie at the base of the Loyalty rise (Fig. 4). The ridge itself has a
sedimentary cover up to 0.6 set thick. The sediments at the base of the slope are
contained to the east by the low irregular morphological ridge (X in profile A,
Fig. 4). The sediments generally conform with basement although there is some
evidence of small scale folding and faulting in the sediments. Younger flatlying
sediments overlie these conformable sediments along the margin of the southern
New Hebrides trench in the north (Fig. 4, profile B). To the east of this region thin
sediments about 0.4-0.2 s thick, lie conformably over a ridge and trough basement
(Fig. 5, profile D) suggesting either a pelagic origin or post deposition deformation
perhaps associated with tectonic movements along the Hunter Fracture Zone. This
ridge and trough province ends fairly abruptly along the line marked Y in Fig. 3.
The northeast of the South Fiji basin is underlain in the west and centre by
flatlying sediments with thicken eastwards. Basement has an irregular low amplitude
surface suggesting back-tilted fault blocks (Fig. 5, profiles E and F). Along the
northern and eastern margins of this region, recent folding and faulting has occurred
in the sediments (Fig. 6, profile G) and results in a less strongly stratified sedimen-
tary sequence. The southern and western boundaries of this tectonically disturbed
zone are delineated by the line marked X in Fig. 3. A shallow highly absorptive
reflector (marked 2 in profile H, Fig. 6) occurs in the sediments underlying part of
the west central South Fiji basin making it difficult to pick a basement reflector in
this region. This reflector, whose extent is delineated by line Z in Fig. 3. may mark a
shallow extensive sill perhaps associated with the group of seamounts as its northern
limit. The Bounty ridge is marked by thin or no sedimentary cover but locally thick
sediments, in excess of 0.6 s, occur between segments of the ridge where it is cut by
transcurrent faults, and in deep narrow valleys along the western margin of its
northern portion (Fig. 6, profile I). As seen in the bathymetry the southeast ridge
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Fig. 5. Profile D across the ridge province of the northwestern South Fiji basin. The sediments show few
reflectors and are general@ conformabfe with basement, Profiles E and F are across the northern South
Fiji basin and show an irregular basement, possibly block-faulted. overlain by flatlying sediments.
marks the western Bank of a shallow do~faulted block about 100 km wide containing, in places, relatively thick sediments (Fig. 7, profile J, Fig. 13, profile T). The marked change in basement depth across this feature and its linear form suggests that it is a fracture zone, hereafter called the Julia Fracture Zone. DSDP site 285 lies within this downfaulted block.
The deep terrace extending westwards from the central Lau-Colville ridge is partially built up of thick sediments (Fig. 7, profileK) which are contained by a low ridge running subparallel to the Lau ridge from W’S to 3O”S. The southern South
3
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7
S
0
K ~ 1
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255km
Fig. 7. Profile J crosses the Julia fracture zone and the downfaultcd block lying to its east. f’wfilc I(
crosses the terrace at the east of the central South Fiji basin.
Fiji basin contains thick sediments which are flatlying over a highly faulted base-
ment (Fig. 8). The structural trend in the basement is approximately east-west over
the central part of the southern basin. The profiles in Fig. 8 show the pinching out of
a deep transparent layer to the south of W on Figs. 3 and 8 with more stratified
sediments occurring towards the New Zealand coast. These layers may correspond
to the pelagic Oligocene sediments and volcanogenic post Oligocene sediments
respectively as found at DSDP site 205 and may indicate erosion or non deposition
of the older sediments to the south. At the southeast end of profile L, Fig. 8, the
...I
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110 km
Fig. 9. Profile N is across a graben to the west of Three King rise and shows back tilted fault blocks
underlying the sediments. Profile 0 shows the thick sediments built up on the western flank of Three
Kings rise.
strong reflector is probably a sill associated with the intrusion occurring within the
sediments. Post depositional faulting and folding is also apparent along the eastern
margin of the southern basin, profile M, Fig. 8. In the west numerous seamounts, the
Sarah seamounts (profile M, Fig. 8) occur along the eastern flank of Three Rings rise
resulting in variable basement topography especially along the “Glomar Challenger”
30 profile presented by Packham and Terrill (1976). Along the margins of the
southern basin the sediments have only faint reflectors perhaps indicating post
depositional tectonic activity as well as a more pelagic sedimentation.
To the west of the Three Rings rise the seafloor shows a marked rifted form with
several major grabens trending approximately north-south. The seismic reflection
data (Fig. 9, profileN) shows the tilted fault block structure to basement. the blocks
being back-tilted to the east. Thick sediments, however, cover the western flank of
the Three Rings rise (Fig. 9, profile 0) where sonobuoy data show at least 2500 m of
sediments. the source of which is now remote.
MAGNETIC AND GRAVITY DATA
Magnetic data recorded since the work of Watts et al. (1977) has enabled the
identified magnetic anomaly pattern in the northern South Fiji basin to be extended
further to the south and allows a clearer definition of the triple junction. The
magnetic data available and the identified anomalies are shown in Fig. 10. The
anomaly lineations to the east of Bounty ridge, the NNE trending ridge of the
postulated RRR triple junction, and to the east of the SE trending spreading ridge
have all been extended. Anomaly identification for selected profiles across each arm
of the triple junction together with synthetic profiles are shown in Fig. 11 and the
parameters given in Table II. Note that no anomalies have been positively identified
over the southwest plate, a region occupied by the Three Rings rise and its flanks.
Anomalies 7A through 12 (26-33 m.y. B.P.) have been identified with perhaps
anomaly 7 in the south.
Magnetic anomalies occur over the southern part of the South Fiji basin with a
similar wavelength and amplitude to the identified anomalies in the northern part.
However, we have been unable to identify unequivocally any anomaly sequence in
the southern region. There is some indication of lineations in the south which trend
approximately NNE-SSW, subparallel to the Lau and Colville ridges but these
lineations cannot be traced over more than two or three profiles. Malahoff et al.
(1977), A. Malahoff (pers. commun., 1978) have identified anomalies 7 through 12 in
the southernmost part of the South Fiji basin using data from very closely spaced
aeromagnetic tracks. Their lineations trend subparallel to the Lau and Colville ridges
with anomaly 7 abutting the Three Rings rise. These anomalies may be tentatively
identified on our southernmost lines but our data are not detailed enough for
reasonable confidence in the identification.
The magnetic data over the margins of the South Fiji basin show a consistent
2o”s
25’
I- + + I/’ :
30”
SOUTH PIJI BASIN
Fig. It Magnetic profiles along tracks (dotted) in the South Fiji basin. Positive anomalies are shown by
the thi solid lines, negative anomalies by the thin broken lines. Identified anomalies and transform faults
are m: ked by thick lines and the anomalies annotated with the anomaly number.
pp. 2
2 I-
228
E-W
L
INE
AT
ION
S
GL
30
V32
15-2
NE
-SW
--
TU
65-2
V3215-3
N
c3 N
W
Fig. 1 1. Selected
magnetic
anomaly
profiles across
the three
arms
of the
triple Junction
with
the synthetic
anomalies
ca
_ -
- __
_
NW
-SE
LI
NE
AT
ION
S
V32l
S
e =8
0’
SR=
2.6
cm/y
r
W 2
229
TABLE II
Parameters used in the magnetic block models
Line&ions
NE-SW E-W NW-SE
Azimuth 123” 178” 400
Remanent magnetism (A/m) 5.0 7.0 5.0
Inclination of prcscnt field -46” --48” -460
Declination 12.Y 12.5” 12.7° Average B - 290” 260” xw
Half bpreading rate (cm/y) 2.6 2.6 2.6
Layer top (km) 4.9 4.5 4.9
Layer bottom (km) 5.4 5.0 5.4
Profile on Fig. I2 R S T
positive anomaly (ZOO-500 nT) marking the Three Kings rise although the ridge and
trough province immediately to the west is, in general, magnetically subdued.
Lapouille (1977) suggests that the rise is a volcanic ridge, a suggestion with which we
would concur. In the east the Lau and Colville ridges and the Havre Trough are
marked by large positive anomalies of short wavelength. Some seamounts in the
southwestern South Fiji basin show interesting negative magnetic anomalies indicat-
ing formation during a geomagnetic reversal.
Gravity data in the region of the South Fiji basin (Watts et al., 1981) show a
regional gravity high of about 20-30 mGa1 over the South Fiji basin with isolated
large amplitude positive anomalies occurring over seamounts (e.g. profile S, Fig. 12).
The regional gravity increases over the Lau basin-Havre trough to about 60-80
mGa1, tending to be largest over the southern part of this young marginal basin
(Weissel, 1977; Malahoff et al., 1977). Similar high values occur over the Fiji Plateau
to the north, also an area of relatively young crust at shallow depth (3000 m) having
been formed during the last 10 m.y. (Chase, 1971). The large positive and negative
anomaly pair associated with the Tonga-Kermadec island arc system pass eastwards
into a well developed outer gravity high. Although a typical crustal thickness, 6-8
km (Shor et al., 1971), occurs in the South Fiji basin the regional gravity anomaly is
above average for deep ocean basins. This may be due to the relatively shallow depth
of the basin compared with the deep Pacific Ocean basin.
THE SOUTH FIJI BASIN TRIPLE JUNCTION
Representative profiles across the three sections of the South Fiji basin triple
junction are shown in Fig. 12. Magnetic, gravity, bathymetric and seismic reflection
S ,,,_MAGNETIC ANOMALY N
!lT
o-
FREE AIR GRAW
231
250 MAGNETIC ANOMALY
nT
0
-:%, FREE AIR GRAVITY ANOMALY
n
km
1 LOCATION
Fig. 12. Representative profiles showing magnetic. gravity. bathymetric and seismic data acro\h the three
arms of the triple Junction. The postulated spreading axes are marked by an arrow. Profile T shoua the
downfaulted block alongside the Julia Fracture Zone. Another minor fracture zone is assumed to lie along
the eastern margin of this block.
data are shown, along with the postulated axes of the extinct spreading centres.
Profiles R and S show a regional decrease in depth and increase in gravity towards
the South New Hebrides trench corresponding to the outer high of the trench
system. The trench system probably extends as far east as 175”E.
The NW-SE spreading centre apparently lies subparallel to a linear narrow
bathymetric ridge now identified as a major fracture zone, the Julia Fracture Zone.
This fracture zone lies along the western margin of a downfaulted block, some 100
km wide, which runs from the triple junction to the Marion seamount and the
spreading centre apparently lies within this block (Fig. 12, profileT). Both margins
of the downfaulted block coincide with large sharp changes in gravity and magnetic
anomalies. The fracture zone is collinear with the transform faults deduced from the
offset of the magnetic anomalies of the east-west trending lineations. The move-
ments on these transform faults occurred primarily post anomaly 11 (32 m.y. B.P.).
The age of downfaulting of the block, however, must post date anomaly 7A (26 m.y.
B.P.). Reworking of early Miocene-Oligocene sediments at the end of the Miocene
at DSDP site 285, which lies within this downfaulted block, indicates that the age of
downfaulting is probably late Miocene.
When looked at in detail a number of complexities exist with the postulated triple
25%
30% i7l”E 175’E 180’
Fig. 13. The identified magnetic anomaly lineations (medium solid line) of the South Fiji basin triple
junction plotted over the bathymetry. The postulated spreading axes (dots) and the Julia Fracture Zone
(heavy solid line) are also shown. Theoretical anomaly lineations, assuming the directions and rates of
spreading given in Table II, are shown for anomaly IO by the thin solid lines with the magnetic “bights”
by thin double lines.
junction. The northeast limb is the only well-defined system but here the morpho- logical ridge assumed to mark the extinct spreading centre coincides closely in places with the youngest anomaly of the eastern plate and is about 40 km from the corresponding anomaly on the northwestern plate. The age of the youngest anomaly decreases to the southwest along the ridge suggesting that spreading died away in a southwesterly direction, Fig. 13. The anomalies over the eastern plate have a fan-like orientation suggesting, as mentioned above, that the pole of rotation lay close to the northern end of the spreading centre or that the northeast spreading centre has rotated clockwise, whereas the anomalies to the northwest are parallel and exhibit a definite bight in their orientation at the triple junction. The western and southeast- em limbs of the system are poorly defined. Both are based on data only on one side
233
of the spreading centre and are comparatively short in length, terminating adjacent
to low morphological rises. The magnetic anomaly lineations are shown superim-
posed on bathymet~ in Fig. 13. Theoretical lineations are also shown based on the
interpreted spreading centres and the rate and direction of spreading for each centre
(Table II). The axis of the magnetic bight in the northern plate coincides closely with
a change in the seafloor morphology with low ridges lying to the west and a smooth
seafloor to the east.
DISCUSSION
Any synthesis of the mode of origin of the South Fiji basin is complicated by the
effect on the region of later geologic events. The Fiji plateau has encroached on the
northern part of the South Fiji basin during the last 10 m.y. (Chase, 1971). However,
the occurrence of anomaly 12 adjacent to the Lau ridge and in most of the northern
part of the South Fiji basin, although apparently cut off at the east of the northern
part, suggests that this encroachment has been minimal and that the Hunter
Fracture Zone and southern South New Hebrides trench may coincide with an older
discontinuity. In the south the geological history of northern New Zealand during
the Oligocene is still subject to speculation and gives little assistance in providing
constraints to the synthesis of the development of the South Fiji basin. Apart from
the data defining the triple junction in the northern South Fiji basin the only
reported magnetic lineation data are the NNE trending anomalies in the southern
part of the South Fiji basin (Malahoff et al., 1977) and there is a large gap in data
over the central South Fiji basin. What data are available show that the basin was
primarily formed during Oligocene times, between 26-33 m.y. B.P. DSDP data show
minimum ages of 15 m.y. B.P. at site 285, where basement was not intercepted, and
30 m.y. B.P. at site 205 for sediments overlying basement and are thus consistent
with the magnetic anomaly data.
In Fig. 14 the lineations for the triple junction in the north and for the southern
anomalies (A. Malahoff, pers. commun., 1978) are shown. The southern end of the
lineations of the southeast arm of the triple junction appears to abut against a low
morphological terrace extending westwards from the Lau-Colville ridge. The north-
ern limit of the well-defined southern anomalies coincide with a line of seamounts
extending in a southeasterly direction, normal to the strike of the lineations, from
Three Kings rise. This line of seamounts probably marks a transform fracture zone,
the Lindsey Fracture Zone, Fig, 14. The southern limit of the southern anomaly
group extends onto the flanks of the Northland plateau, a marked terrace at about
2000 m depth along the southern margin of the South Fiji basin, suggesting that the
Northland plateau is formed partially of uplifted oceanic crust.
During the Oligocene the convergence between the Pacific and Indian plates was
about 35 mm/yr along an azimuth of 060”. The spreading in the South Fiji basin
was therefore probably back arc spreading with the Pacific Indian plate boundary
Fig. 14. The identified magnetic anomaly lineations for the South Fiji basin shown relative to bathymet?
(contour interval is 1 km). SlvNT marks the South New Hebrides trench. HFZ the Hunter Fracture Zone.
JFZ the Julia Fracture Zone, LFZ the Lindsay Fracture Zone and VMFZ the Vening Meinesz Fracture
Zone
235
being a subduction boundary along the present Tonga-Kermadec trend. To enable a
detailed synthesis of the development of the basin to be carried out the triple
junction lineations in the north and the lineations in the south must be linked by
some means. A second ridge-ridge-ridge triple junction could be postulated to
occur in the central South Fiji basin to enable this to be done. However, although
there are large anomalies in this region no identifications were possible and therefore
a detailed reconstruction has not been attempted. Only the interpretation of the
geophysical features seen will be discussed.
If symmetric spreading is assumed for the spreading centres of the triple junction
in northern South Fiji basin and for the spreading centre of the southern lineations,
then this implies that all the southwestern plate of the triple junction system and the
western plate of the southern spreading centre have been removed, presumably
subducted under the Three Kings rise. The Three Kings rise thus appears to play an
important part in the development of the present South Fiji basin. The seismic data
discussed earlier (Fig. 9) show a rifted extensional morphology between Three Kings
rise and Norfolk ridge suggesting that these two ridges were originally much closer
together bringing the Three Kings rise into a closer alignment with Loyalty rise. The
Three Kings rise and Loyalty rise have similar structural characteristics. Both have
thick sediments underlying their western flanks and they have similar magnetic
characteristics (Lapouille, 1977) suggesting a volcanic origin. The Three Kings rise is
interpreted as a relic island arc under which the southwestern part of the South Fiji
basin has been subducted. Geological sampling on the rise obtained a deep water
coral (Usculjnff oirgasa Squires) in the south but, more importantly. obtained
andesitic volcanics at the northern end of the rise (Appendix I).
A reconstruction of the South Fiji basin at the start of the Oligocene (Fig. 15A)
can be achieved by superimposing the anomaly 12 lineations of the northern triple
junction and the southern lineations, assuming symmetric spreading. Other assump-
tions made are that the south end of the southern spreading centre is terminated by
a transcurrent fracture zone-the Vening Meinesz Fracture Zone, the extensional
movements postulated earlier between the Three Kings rise and the Norfolk ridge
have occurred and that the central part of the basin is formed of oceanic crust of
Oligocene age. The schematic reconstruction indicates that the South Fiji basin
possibly contained oceanic crust older than Oligocene in age which either exists in
the northwest of the basin or has since been subducted under Three Kings rise. The
older oceanic crust is probably Eocene in age, based on magnetic anomaly data from
the New Hebrides basin (Lapouille and Weissel, 1979) and the Coral Sea (Weissel
and Watts, 1979).
This model suggests that during the development of the South Fiji basin the
Norfolk ridge has been displaced westward relative to the Tonga-Kermadec island
arc along the Vening Meinesz Fracture Zone by the width of the southern spreading
episode. The Three Kings rise may have been originally displaced by the same
amount unless the postulated subduction and spreading were contemporaneous, in
35m.y BP 26 my 0.P
C.
Fig. 15. Schematic representation of the development of the South Fiji basin (A) and the development
(two models) relative to northern New Zealand (B. C). NC =New Caledonia. LR =Lovalt?; rise.
NR = Norfolk ridge, TKR = Three Kings rise, TKIA = Tonga-Kermadec island arc. VMEZ = Vening
Meinesz fracture zone, NZ =New Zealand, WNR = West Norfolk ridge. Arrowhead indicates relative
motion between plates or crustal blocks. Postulated spreading centres are marked by hea? lines with
divergent arrowheads, subduction zones by heavy lines with open triangles. In 15A anomaly lineations are
shown by tight lines. in ISB and C the relative motion of the Pacific plate to the Indian plate for the
period 35-21 m.y. B.P. is shown by tight arrows starting east of TKIA and the andesitic volcanics of
western-north New Zealand by solid circies.
237
which case the westward displacement of the Three Kings rise would be about half
that amount. The Norfolk basin, formed during rifting of Norfolk ridge from Three
Kings rise, is thus interpreted as a region of oceanic crust of probably Oligocene or
perhaps younger age. At the northern end of the Three Kings rise there appears,
from morphology, to be an element of north-south extension between the Three
Kings rise and the Loyalty rise and the proximity of this break to the western arm of
the northern triple junction suggests they are related. This indicates that the
postulated separation of Three Kings rise from Norfolk ridge was contemporaneous
with the spreading of the northern triple junction and hence with the spreading in
southern South Fiji basin. Extension along the Tonga-Kermadec boundary is also
required to take up the component of north-south extension generated by the triple
junction and the present narrowing of the ridge southwards may be a result of this.
Alternatively simultaneous transcurrent movement and subduction along the Vening
Meinesz Fracture Zone is required but there is no evidence offshore for subduction
along this region during Oligocene times.
The relationship of the South Fiji basin to New Zealand during this period is not
clear. Two possibilities are shown in Figs. 15B and C. The finite pole of rotation for
the Pacific-Indian plate boundary during the Oligocene lies east and close to New
Zealand and shows convergence along the Tonga-Kermadec boundary (Walcott,
1978. The convergence is highly oblique in the south near New Zealand becoming
primarily transcurrent through New Zealand.
In Fig. 15B the Norfolk ridge is assumed to have been collinear with northern
New Zealand prior to the major opening of the South Fuji basin thus providing a
direct link-up of New Zealand with New Caledonia, two areas of continental crust
with similar Mesozoic geological history. This assumption results in the present
alignment of the Tonga-Kermadec ridge with northeastern New Zealand remaining
unchanged and the Indian-Pacific plate boundary is assumed to have been in a
similar position to its present one. In this model the position of the Indian-Pacific
plate boundary remains unchanged during the development of the South Fiji basin
and the Norfolk ridge is displaced westwards, relative to New Zealand, along the
Vening Meinesz Fracture Zone to its present position. The amount of displacement
equals the width of the new oceanic crust formed at the southern spreading centre.
The Vening Meinesz Fracture Zone extends from Norfolk ridge to the western side
of the North Island volcanic zone and has acted as a sinistral transform fault
terminating the south end of the southern spreading centre. This transcurrent
movement does not appear to extend further west than Norfolk ridge and as no
subduction occurs on or between Norfolk ridge and Lord Howe rise, the relative
motion of the Norfolk ridge and Lord Howe rise and New Caledonia basin to the
west could lead to rifting in the eastern south New Caledonia basin, perhaps leading
to the formation of the West Norfolk ridge as a “leaky” transcurrent fault. The West
Norfolk ridge is markedly different to the Norfolk ridge, being highly magnetic and
formed by basic volcanics. Extensional movements through New Zealand south of
the West Norfolk ridge during the Oligocene would also need to be postulated.
In the second model, Fig. 1X, the apparent displacement of Norfolk ridge from
northern New Zealand is assumed to have occurred prior to the Oligocene. The
Indian-Pacific plate boundary at the beginning of the Oligocene would lie along the
eastern side of the Tonga-Kermadec island arc and it is suggested that it would
continue south through New Zealand as shown in Fig. 15C, implying that the
eastern part of North Island, New Zealand, was located on the Pacific plate, lying
perhaps some distance to the east of its present position. During the formation of
the South Fiji basin the Tonga-Kermadec island arc was displaced eastwards, along
the Vening Meinesz Fracture Zone, relative to northwest New Zealand. The Vening
Meinesz Fracture Zone on this model would run from eastern Norfolk ridge to the
Kermadec trench.
Both of these models (Figs. 15B and C) have problems which warrant further
study, in particular the evidence for approximately east-west extension in central
and southern New Zealand or southern Lord Howe rise during Oligocene times for
the model in Fig. 15B and the nature of the plate boundaries in the vicinity of
northeast New Zealand in the model in Fig. 15C. The requirement in the first model,
Fig. 15B, for subduction in the Lord Howe rise-Norfolk ridge region or large scale
extension through New Zealand in the Oligocene, for which there is little evidence,
would favour the second model, Fig. 15C.
Subduction along the northeast margin of northern New Zealand during the early
Miocene has been postulated by Brothers (1974) to explain the existence of lower
Miocene andesitic volcanics found in a restricted region along the western margin of
northern New Zealand. The subducting episode associated with these andesites
would have had to commence in Oligocene times in order for the necessary 100-200
km of oceanic crust to be subducted and would thus be contemporaneous with the
postulated subduction under Three Kings rise. The position of these volcanics and
the amount of Indian-Pacific plate convergence during the Oligocene are shown in
Fig. 15. In the model shown in Fig. 15B the oceanic crust of the South Fiji basin
would be subducted under northern New Zealand to be the source of the Lower
Miocene andesites found there and a continuous subduction zone would presumably
have existed from the northern end of Three Kings rise to the eastern end of
northern New Zealand. In the model shown in Fig. 15C the subducted oceanic crust
giving rise to the lower Miocene volcanics would be subducted Pacific plate and the
northward continuation of these volcanics may now be under the present Colville
ridge. Andesites associated with the postulated Three Kings rise subduction episode
in the Oligocene would thus be a separate feature.
Intraplate deformation has apparently occurred in the South Fiji basin since
spreading ceased at the end of the Oligocene. In the central South Fiji basin a large
block has apparently been downfaulted, probably in the late Miocene. The western
margin of this downfaulted block is marked by a sharp linear ridge which runs into
the large seamounts, Marion seamount and Central ridge, at the centre of the South
239
Fiji basin suggesting that these features may have been partially formed since
spreading ceased. This intraplate deformation may be a possible cause of the lack of
identifiable magnetic anomaly lineations in the central region.
CONCLUSIONS
Magnetic, seismic and bathymetric data have shown that the South Fiji basin is a
structurally complex marginal basin. A large gap in identifiable magnetic anomalies
occurs in the central part of the basin and may be caused by post spreading
intraplate tectonic activity. One, possibly two, edge-ridge-edge triple junctions
occur in the basin. Some postulated spreading centres coincide with basement ridges
but at others no basement ridge is apparent. Part of the oceanic crust formed during
the spreading episode has been subsequently consumed by westward dipping sub-
duction under the Three Kings rise. Andesite has been dredged from the north end
of this rise.
Magnetic anomaly data give an age of 26-33 m.y. B.P. for most of the formation
of the basin. DSDP data are consistent with this age range. Older crust, probably
Eocene in age, may occur in the northwest of the basin.
Basement morphology is irregular. It has a small amplitude (( 500 m) in the
north and a large amplitude in the south where it apparently trends east-west. A
large downfaulted block trends northwest-southeast across the central part of the
basin. It was probably formed during late Miocene times, perhaps along existing
zones of weakness as its western margin is aligned with the transform fractures of
the western arm of the northern triple junction. The sedimentary cover, primarily
volcanogenic from DSDP data, is thin over the central northern area but thickens to
over 1000 m in the south and east where it is closest to its presumed source, the
volcanic centres of the Tonga-Kermadec ridge complex and the west Northland
volcanic centres.
ACKNOWLEDGEMENTS
I am very grateful to A.B. Watts and J. Weissel for making available data used in
this study and for the magnetic anomaly identification and synthetic magnetic
anomalies computation. The help of Captain H.C. Kohler and the officers, crew and
scientists on board R.V. “Vema” cruise 32, leg 15 and cruise 33, leg 14, during which
most of the data used in this study were obtained, is appreciated. W.A. Watters and
I. Keys kindly provided the data given in Appendix I. I am grateful for a review of
the manuscript by RI. Walcott.
This research was supported by National Science Foundation Grant OCE 76-
22034.
APPENDIX 1
Dredged rock samples from Three Kings rise
A Position 3 l”22’S 172”42’E Depth 1~-12~ m Rock sample Coral Limestone. Coral is Osculina uirgasu Squires in a matrix of
calcium mud with minor clay silicate and rich in pteropods.
Age Lower Miocene to Upper Pliocene (in New Zealand). B Position 2g”ll’S 173O09’E
Depth 1800-2200 m Rock sample Basaltic andesite. Phenocrysts of plagioclase (sodic labradorite to
calcic andesine) and augite aggregates are set in an altered very fine-grained groundmass partly composed of pale-brown glass. The groundmass encloses numerous tiny skeletal feldspar crystals and is clouded by minute opaque rods in many places.
Analysis SiO,
Al,03 Fe,O, Fe0
MgO CaO Na,O
RIO H,O+ H,O- TiO,
P205 MnO total
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