Lmnont -Doherty Geological Observatory of Columbia University, Pabsades, N. Y. ... Geological Observatory contribution ...... University of Tasmania, Hobart, pp.
Tectonophysics,
87 (1982) 253-277
Elsevier Scientific
Publishing
253
Company,
Amsterdam-Printed
in The Netherlands
THE OPENING OF THE WOODLARK BASIN, SUBDUCTION OF THE WOODLARK SPREADING SYSTEM, AND THE EVOLUTION OF NORTHERN
JEFFREY
MELANESIA
K. WEISSEL,
Lmnont-Doherty
BRIAN
SINCE MID-PLIOCENE
TAYLOR
Geological Observatory
’ A /so at Department
‘.* and GARRY
TIME *
’
D. KARNER
of Columbia
University,
Pabsades,
of Geological Sciences, Columbia
University,
New York, N. Y. IO02 7 (I/. S.A
N. Y. 10964 (U.SA.j
)
’ Now at Hawaii Institute of Geophysics, Honolulu, HI 96822 (U.S.A.) (Final
version received
July 16, 1981)
ABSTRACT
Weissel, J.K., Taylor. Woodlark G.H.
B. and Kamer,
spreading
Packham
system,
(Editor),
G.D.,
1982. The opening
and the evolution
The Evolution
of the Woodlark
of northern
Melanesia
of the India-Pacific
Plate
Basin, subduction
since mid-Ptiocene
Boundaries.
of the time. In:
Tectonoph,ysics,
87:
253-277. Magnetic
anomaly
and seismological
magnetic
lineations
trending
6 cm/yr
for the last 1 m.y. Spreading
spreading
system
spreading
in the basin has apparently
at successively
implying
that eventually
d’Entrecasteaux
Islands
with the northern decoupling young
margin
Woodlark
Analysis
of the relative motions spreading
Islands
the opening
during
of interaction
ridge subduction.
and basaltic
of an unusual tectonic
Basin plate
boundary
section
and geological
Basin are undergoing
Hitherto
evidence
suggest
tensional
will propagate
unreported
through
seismicity
the
associated
Rise probably
reflects
difficulties
in subducting
due to mechanical
that
deformation.
westward
shallow
of the Woodlark
basins, possibly
of seafloor
prior to 3.5 m.y. in the east, and
partial the
between
of the Woodlark
of the Woodlark
volcanic
Indo-Australian,
Basin. Several tectonic
Basin with the Solomon hypocenters;
extremely
and geological
Trench
and voluminous
close to the trench
suite with ridge subduction
and Pacific plates shows that
at high rates (=‘ 10 cm/yr)
include high heat flow in the Solomon
and only shallow andesites
the Solomon,
has been subducted
beneath features
the Solomon limited
to the
and arc may be symptomatic Trench,
eruptions
of high K,O/TiO,
axis. This close association
implies a strong dependence
of
which shoals to 4 km; low olivine
in space and time
of the petrogenesis
on the
regime.
A combination (Taylor,
system
These features
levels of seismicity basalts
and Solomon
to the west. Commencement beginning
focal mechanisms
peninsula.
of the NE-trending
and associated
lithosphere.
the Woodlark region
the Woodlark
of the Woodlark
15”-20”
end of the Woodlark
into the Papuan
spreading
rate has been approximately
by over 10% from east to west along the Woodlark
opening
been time-transgressive.
the western
of active seafloor
Basin. The total opening
rates diminish
a pole of current
later times to the west. Earthquake
the land areas bounding We believe
data define segments
ENE in the Woodlark
of this study
1979) provides
* Lamont-Doherty
of the Woodlark
a reconstruction
Geological
Observatory
0040- 19.5!,‘82/0000-0000/%02.75
Basin and the previous
of the positions
contribution
of the continents,
study
of the Bismarck
No. 3316.
0 1982 Elsevier Scientific
Publishing
Basin
ocean basins, and island chains
Company
in northern explain
Metanesia
for mid-Pliocene
the trench-tike
structure
Cenozoic
talc-alkaline
volcanism
seismicity
beneath
igneous
activity
aiong
the north Papuan
New Ireland-Solomons
chain
observed
time. In accepting
off the Trohriand the Papuan
peninsula.
the existence
margin
of a Solomon
of New Guinea,
peninsula,
and the presence
The rapid changes
the spatial
we can of Late
of intermediate
in relative motions
over the past 3.5 m.y. may explain
plate.
the occurrence
depth
along or across the
and temporal
changes
in
on these islands.
INTRODUCTION
To unravel the tectonic evolution of the Melanesian island chains and small ocean basins that extend from New Guinea in the northwest to New Zealand in the southeast is a difficult undertaking. In a tectonic regime dominated during the Cenozoic by overall oblique convergence between the major Pacific and Indo-Australian plates, several small marginal basins have formed at extensional plate boundaries. Often, parts of these basins were subsequently subducted. We attempt to reconstruct the tectonic evolution of the region through knowledge of present plate boundaries and geologic and geophysical evidence activity preserved in island arcs, continental fragments Melanesia. patterns
On the one hand, seafloor
and can provide these
spreading
in the small ocean basins determine basins
quantitative
(e.g., Weissel
distinctive physiograp~c the land regions provide, histories
of convergent
reconstructions and
Watts,
magnetic
histories
for past pIate and marginal hneatidn
and fracture
of extensional
of land
1979; Taylor,
masses
boundary basins of
along
1979). On
zone
plate boundaries the margins the other
of
hand,
features in the ocean basins and the geological records of in a more qualitative way, information on the locations and plate boundaries.
Paleomagnetic
measurements
such as those
from the New Hebrides Islands and Fiji (Falvey, 1978; James and Falvey. provide additional constraints on paleogeographic reconstructions.
1978) may
The work reported in this paper concerns the post-Miocene evolution of northern Melanesia (Fig. 1). We will (a) present magnetic anomaly data which define the opening history of the Woodlark Basin since approximately the geophysical and geological consequences of subduction ing system restore
beneath
the continental
the New Georgia fragments,
island
Islands chains
section
3.5 m.y. ago, (b) discuss of the Woodlark spread-
of the Solomon
and oceanic
basins
Islands,
(c)
to their config-
uration at 3.5 m.y. B.P., and (d) discuss the variations in time and space of Late Cenozoic volcanism along the Pacific plate margin in northern Melanesia. EVOLUTION
OF THE WOODLARK’BASIN
The Woodlark Basin is bounded on the north by the Woodlark rise and on the south by the Pocklington rise. The basin is bounded on the east by the Solomons island arc-trench system and appears closed in the west by the Trobriand shelf (Fig. 1). As neither the Woodlark rise nor the Pocklington rise are recognized as
255
\
3 b
\
q
P
257
active island
arcs, the Woodlark
Basin cannot
the sense of the Lau Basin or Mariana Pocklington Papuan
and Woodlark
peninsula
Trough.
rises appear
(Davies,
to NE
occurs
basin,
near
154’E
on both
adjacent
area appears
to the Woodlark
and Pocklington
al., 1974). In the west, a thicker
change oceanic
part of the rises deepen
crust
within
the
part of the
cover but a thin cover is observed
rises (Luyendyk
sediment
in
in strike from ESE
of 3.0 to 4.0 km. In the eastern
free of sedimentary
basin
to the fold belt of the
and Woodlark
and an abrupt
rises (Fig. 1). The
at a depth
a “back-arc”
on the western
similar
1977). Both the Pocklington
Basin is generally
the central
Islands
geologically
to the east away from Papua New Guinea Woodlark
be considered
et al., 1973; Johnson
cover is observed
(Luyendyk
et
et al., 1973)
probably because of sediment transport from nearby Papua New Guinea. A diffuse zone of shallow teleseismicity extends across the Woodlark
Basin
between 9 and 10”s (Fig. 2). East of 154”E, these events are located along the center of the basin, whereas most of the events recorded west of 152”E occurred along the south margin of the Woodlark rise. There is an apparent seismic gap between about 152 and 154”E. The western Woodlark Basin events appear continuous with a belt of activity extending through the d’Entrecasteaux Islands and along the Papuan peninsula (Fig. 2). The NE-trending eastern branch of the Woodlark rise is also associated
with a zone of sparse seismicity.
is discussed
The significance
of this new observation
in more detail below.
Previous studies Carey (1958) was the first to suggest an extensional Basin. Milsom (1970) pointed out that the bathymetric within
the basin
are indicative
Reconnaissance to identify
seafloor
located Woodlark suggested
near
lines across the basin
spreading
anomalies
154S*E
enabled
spreading. Luyendyk
1, J and 2 of the geomagnetic
et al., 1977) repeated (Fig. 3B). They inferred
Basin has been the configuration
3B. They proposed
of an active center of seafloor
magnetics
scale (e.g., LaBrecque
origin for the Woodlark and seismicity patterns
about
a segment
et al. (1973) reversal
of spreading
that most of the oceanic
time center
crust in the
accreted during the past 3 m.y. Luyendyk et al. ( 1973) of plate boundaries in the Woodlark Basin shown in Fig.
that the current
direction
of spreading
in the basin is about N-S,
and confined extensional tectonics west of 154“E to the Woodlark rise in accordance with the seismicity patterns. From a study of Woodlark Basin seismicity, Curtis (1973b) proposed the plate boundary configuration shown in Fig. 3A. While his spreading segments have about the same trends as those of Luyendyk et al. (1973), both the direction of spreading (NNW-SSE) and the locations of transform faults are significantly different. Curtis (1973b) also confined the current extensional plate boundary in the western part of the basin to the Woodlark rise area except for a southward offset in the spreading center just east of the d’Entrecasteaux
Islands
(Fig. 3A).
Fig. 3. Nature and configuration of plate boundaries in the Woodlark Basin area according to: (A) Curtis ( 1973b); and (B) Luyendyk et al. ( 1973). Double lines indicate extensional plate boundaries. sawtooth line indicates the convergent plate boundary along the Solomon Trench, and single lines are strike-slip or unspecified boundaries. The 2&m isobath is shown (dotted).
Petrologic evidence suggests that rifting, probably related to seafloor spreading in the Woodlark Basin, may be occurring in the d’Entrecasteaux Islands. Smith ( 1976) found that mildly per-alkaline rhyolites are the most abundant lava type of Quaternary age along Dawson Strait which exists between the two largest of the d’Entrecasteaux Islands. Smith noted that such rock types are not normally associated with island arc volcanism but are similar to rocks from known continental rift zones. An earthquake focal mechanism in~ca~ng normal faulting just east of the d’Entrecasteaux Islands (Fig. 2) was determined by Ripper (1975). Further west, focal mechanisms (Figs. 2 and 12) suggest that normal faulting is occurring within the Papuan peninsula. Part of the uplift of the mountain ranges of the Papuan
259
peninsula reflected
described
by Davies (1977) may be due to crustal
by the tensional
focal mechanisms
and Mime)
on the extremity
and Smith,
1975).
of the Papuan
extensional
processes
as
(Fig. 2). The large bays (Goodenough peninsula
are flooded
graben
(Milsom
Our working model We have examined most available marine geophysical Basin held by Lamont-Doherty Geological Observatory,
data from the Woodlark the Australian Bureau of
Mineral Resources in Canberra, and the National Solar-Terrestrial Data Center in Boulder, Colorado. Magnetic anomalies plotted along ships’ tracks in the Woodlark Basin are shown in Fig. 4. In Fig. 5, magnetic profiles located in Fig. 4 are compared to a theoretical seafloor spreading anomaly profile based on the geomagnetic reversal time scale of LaBrecque et al. (1977). The anomaly identifications and lineation magnetic spreading
trends shown in Fig. 4 agree with those of Luyendyk anomaly data obtained since the previous work segments
in the western
part of the Woodlark
et al. (1973). However, reveal presently-active
Basin. Track uu’ in Fig. 4
crosses a well-defined central anomaly with anomaly J on the south and anomalies J, 2 and perhaps part of 2’ on the north. The other anomaly identifications in Fig. 4 suggest also
dextral
suggested
D’Entrecasteaux For reasons
offsets of the spreading by the strike-slip
center
further
to the west. Such offsets
1 (Figs. 2 and
mechanism
12) just
are
east of the
Islands. yet unknown,
the amplitude
of the central
anomaly
is greater
in the
western than in the eastern part of the basin (Figs. 4 and 5). Apparently more crust has been accreted to the north flank of the Woodlark spreading system than to the south flank. This can be seen from Fig. 5 where the theoretical profile which matches the observed profiles was computed using a spreading rate of 3.6 cm/yr for the north flank and 2.4 cm/yr for the south flank. Figure 6 illustrates the relationship between
free-air
gravity,
magnetics.
and
bathymetry along profile bb’ (Figs. 4 and 5). Sediment thickness in the basin is very small near this track. The asymmetry of spreading is readily apparent. The Woodlark spreading
center
free-air
anomalies.
gravity
in conjunction From
and the margins
with active and extinct
the seismicity
of the basin
Short wavelength patterns
and
free-air
spreading
are associated
with distinctive
gravity
are often found
centers
focal mechanisms
“lows”
(Watts.
1982).
(Figs. 2 and
12) and
the
observed seafloor spreading magnetic lineations (Figs. 4 and 5) we have delineated the plate boundaries in the Woodlark Basin area (Fig. 7). The transform which bounds the eastern extremity of the spreading system is drawn so as to coincide with a north trending, west facing bathymetric scarp, and to be consistent with the nearby strike-slip focal mechanism. The easternmost spreading segment, which has not been mapped by magnetics, is made coincident with a band of shallow seismicity activity in Fig. 7. The segment of spreading center between 154 and 155”E is well-mapped
-
0_
261
Fig. 5. Comparison of observed with a model magnetic
profile
magnetic
anomaly
profiles
based on the geomagnetic
the model, the depth to the top of the magnetic
from the Woodfark reversal
iayer is 3.S km, its thickness
rn~g~e~~z~t~~~ is 0.008 e.m.u. The model profile was computed cm/yr
for the north
an azimuth
of 345”.
physiographic
margins
by magnetic seismicity
flank, and 2.4 cm/yr approximately
to the iineation
of the basin on each magnetic
Iineations
(Fig.4
is systematically
profiles pattern.
spreading
rates;
have been projected Solid
triangles
of 3.6
onto
incatc
et al., 1973), but associated
magnet&.
West of 154aE we are now able to define (Fig. 4) and thus our model workers
is 0.5 km, and the intcnsitv
asymmetric
km north of the spreading
magnetics
from those of the previous
in Fig. 4)
et al.. 1477). In
the
profile.
and Luyendyk
20-30
using
for the south. The obscrvcd
perpendicular
Basin (located
time scale (LaBrccque
center inferred
spreading
for the plate boundary
for the western
segments differs
part of the Wo~dlark
shallow from the from the
significantly Basin (Fig.
3A and 3B). Crustal extension in the form of seafloor spreading has produced oceanic iithosphere in the Woodtark Basin east of the d’Entrecasteaux Islands. Further west. crustal extension in the form of rifting is occurring as suggested by peralkaline rhyofitic volcanism in the d’Entrecasteaux Islands (Smith, 1976), abundant but diffuse shallow seismicity (Figs. 2 and 7), earthquake focal mech~isms from the Papuan peninsula showing normal faulting (Figs. 7 and 12), and the presence of drowned graben along the northeast coast of the peninsula. Four
heat
flow measurements
have been
made
in the eastern
Woodlark
Basin
Fig. 6. Magnetic
anomaly
across
the Woodlark
center
is defined
(top),
free-air
gravity
anomaly
(center).
and
Basin at 154S”E (profile bb’ in Fig. 4). On this profile,
by a local gravity
low as well as the magnetic
anomalies
topographic the location
(bottom)
profiles
of the spreading
and morphology.
(Halunen and Von Herzen, 1973; Fig. 9). The low value (74 mW/mZ) near the inferred position of the spreading center probably reflects heat loss by convection in the oceanic crust. Two of the three measurements taken along the axis of the Solomon Trench (Fig. 9) are relatively high ( 143 and 126 mW/m* ). These values reflect the very young age of the Woodlark Basin crust in the trench. Figure7 shows that the Woodlark Basin is wider in the east than in the west (- 300 km at 155S”E compared to only 60 km at 151.5”E). Furthermore, the oldest anomaly consistently recognized in the eastern part of the basin, anomaly 2’ (3.5 m.y. B.P.), is absent in the western part (see Figs. 4 and 7). These features suggest that the separation of the Woodlark and Pocklington rises (which constitute the margins of the Woodlark Basin) has been time transgressive, commencing just prior to 3.5 m.y. ago east of 154OE, but at later times in the western part of the basin. A possible explanation is that the Woodlark spreading system is propagating westwards with time. When anomaly 2’ formed (3.5 m.y. P.B.), seafloor spreading was occurring east of 154”E (in terms of present geographic coordinates) while further west, crustal extension was accomodated by widespread rifting of continental crust. This is similar to the situation envisaged for the opening of the Red Sea by Cochran (1981). Over the past 3.5 m.y., the locus of seafloor spreading has progressed westward and we speculate that the Papuan peninsula may eventually rift apart. In this evolutionary model. it is possible that spreading activity in the original (but now subducted) eastern portion of the basin began even earlier than the presently
263
264
preserved
lineations
been opening, Solomon
suggest (slightly
the ridge flanks
Trench
before 3,5 m.y.). While the Woodlark
and the spreading
(Fig, 8). Thus. any evidence
has been lost. Note from Fig. 5 (profiles magnetic
anomaly
the finite rotation
decreases
by about
which describes
a pole which lies 15-20’
center
have moved
for crustal
accretion
the opening
the
prior to 3.5 m.!.
au’ and cc.‘) that the width
10% between
Basin has towards
of the central
155 and 153”E. This implies that
of the basin over the last -. 1 m.y. has
west of the Woodlark
Basin.
Two additional tectonic features related to the northern margins of the Trobriand Shelf and Woodlark rise area (Fig. 7) warrant comment here. The first is the Trobriand “Trench” which Hamilton (1979) suggests is the site of current subduction of the Solomon Sea crust suggestion on (a) the occurrence Papua and the d’Entrecasteaux appearance
of the northern
seen in seismic
reflection
beneath the Trobriand Shelf. Hamilton based his of Late Cenozoic andesitic volcanism in southeast Islands
margin records.
(Johnson
et al., 1978), and (b) the trench-like
of the Trobriand
Shelf-western
Since the level of historical
Woodlark
seismicity
rise as
associated
with the Trobriand Trench is low, we believe it is currently inactive but may have been active in the recent geological past as discussed in more detail below. The
pig. 8.
Vector diagram
for the Solomon
lines give the trend of relative (Molnac Woodlark
et al., 1975). The S-i (T.J.)
(Solomon/Indo-Australian spreading Junction. transform Solomon
46 km,/m.y. segment
along
Trench
cm our interpretation
depending
the
bathymetric
trends, extremity
as the transform
Trench. and predicts
triple junction is subducted.
only
of the Woodiark is moving
and predicts
assumes spreading
in the
spreading
center
the triple junction
migration system
(15 cm/yr)
Solid motion
relative motions
the easternmost
5 km/m.y.
rapidly
anomalies
Two different
of the Woodlark
spreading T.J.2
triple Junction.
pole of relative
of the magnetic
on the trend
orthogonal
Solomon
if the eastern
(e.g. Fig. 7), the S-I-P
from the P-l
lines show the trend of plate boundaries.
are depicted,
NW
(I), Pacific (P) plates
vector is derived
ridge). T.J. I assumes
follows
Alternatively,
(S), Indo-Australian The P-I
vector is based
Basin (Fig. 7). Dashed
of the triple junction migrating
motion.
is
Woodlark of the triple
is bounded
southeast
along
by a the
265
second
feature
margin
of the northeast-trending
determined
of interest
a strike-slip
we interpret
is the association part
mechanism
as reflecting
of shallow
seismicity
of the Woodlark
with the northern
rise (Figs. 2 and
7). We
for one of these events (5 in Fig. 2; Fig. 7) which
right-lateral
slip parallel
to the northern
margin
of the rise.
We suggest that the Solomon and Woodlark Basins are partially decoupled along the northern margin of the Woodlark rise. This may result from the relatively buoyant lithosphere deeper
of the Woodlark
and presumably
SUBDUCTION
Basin and rise being more difficult
denser
Solomon
OF THE WOODLARK
to subduct
than the
Basin lithosphere.
SPREADING
SYSTEM
During the 3.5 m.y. history of seafloor spreading in the Woodlark Basin, rapid oblique convergence has characterized the relative motion between the major Pacific and Indo-Australian crust formed, Basin,
have
tectonic
been
situation
subduction
plates (Fig. 8). Consequently,
and correspondingly subducted is rare
at the Solomon in that
the
case of current
but there the ridge segments parallel manner. One of the variables
enter
in~uencing
of younger Trench.
spreading
zone with a high angle between
only other documented
more than 350 km of the earliest
lesser amounts
crust in the Woodlark
At the present
system
is entering
the ridge segments
ridge subduction
a continental the possible
time. an
and the trench.
is along the margin
subduction temporal
this
oceanic The
of Chile,
zone,
and in a near-
and spatial
effects of ridge
subduction is the rate of triple-junction migration (e.g., Marshak and Karig, 1977). The current rate and direction of migration of the Solomon, Pacific and IndoAustralian (S-P-I) ties in the location
triple junction is poorly constrained (Fig. 8) owing to uncertainof the ridge and transform segments in the eastern Woodlark
Basin (Fig. 7). However, whatever the details of the spreading geometry, migration of the S-P-I triple junction during the past 3.5 m.y. is probably 200 km (Fig. 8). Thus the Woodlark the New Georgia Group. Several important Woodlark
spreading
symptomatic (1) shoaling
tectonic system
of the Solomon
center has been subducted
and geological
features
limited
is subducted
beneath
the Solomon
of ridge subduction.
(2) low levels (3) voluminous axis in the New (4) complex, Solomon Islands
spreading
These features Trench
to about
the total less than adjacent
to
to the region where the Islands
may
be
include: 4 km (Figs. 7 and 9),
of seismicity and only shallow focus events (Figs. 2 and 7), eruptions of chemically unusual magmas very close to the trench Georgia Islands, and and as yet poorly understood differential vertical movements in the (Hackman, 1973).
266
267
Seismicity Figure2
shows
that the level of seismicity
subducting
the Woodlark
subducting
deeper,
feature
and,
greater
compared
presumably,
of the earthquake
depths Basin
Basin
older
distribution
(Curtis,
crust
1973a; Ripper.
the segment
arc subducting
1975, 1977). Forsyth
than
suggested
underthrust
the warm oceanic
plate may buckle
is
activity
at
the Woodlark
(1975) attributed
similar lack of seismicity associated with subduction of the Chile relatively warm, and hence less rigid nature of the young subducted Forsyth
which
Sea. Another
lack of earthquake
of island
of trench
to the north
of the Solomon
is the near
than 80 km along the section
lithosphere
is low along to the segment
or deform
and sink, and may be too soft to allow rupture
a
Rise to the lithosphere.
plastically
rather
to occur in large
earthquakes. The lack of intermediate and deep focus events could also be explained by rapid thermal equilibration between any young lithosphere that is underthrust and the surrounding mantle material. As mentioned above, the seismicity along the north side of the Woodlark rise (Fig. 7) may reflect decoupling between the Solomon and Woodlark Basins, possibly due to mechanical difficulties of subducting the Woodlark lithosphere. The rightlateral strike-slip underthrusting Basin.
Vokunism
focal mechanism the Solomon
Trench
at a somewhat
slower
has
been
of Bougainville,
concentrated
Plio/Pleistocene
in the region
volcanism
adjacent
Dominated by the New Georgia Islands, this “volcanic also includes the Russels, Savo, western Guadalcanal high-Al,
low-Ti andesites
1978) set this island andesites
rate than
Basin is
the Solomon
in the New Georgia Islands
With the exception Islands
(5 in Fig. 2; Fig. 7) suggests the Woodlark
on Bougainville
apart
are not common.
from the other Diorite-tonalite
to the Woodlark province” and part
(Blake and Miezitis, Plio/Pleistocene stocks
in the Solomon
ranging
Basin.
(Coleman, 1970) of Choiseul. The
1967; Bultitude volcanic
centers
et al., where
in age from 3.5 to 1.5
m.y. occur on ‘Bougainville, Guadalcanal, and one occurs in eastern New Georgia. The voluminous assemblage of Late Cenozoic volcanics in the New Georgia Group (Fig. 9) range from high-Mg picritic basalts through olivine basalts to basaltic andesites
(Stanton
and Bell, 1969). All the volcanism
in the New Georgia
Group
has
occurred within 100 km of the (extrapolated) trench axis. The recently active Cook, Kavachi. and Simbo submarine volcanoes (Fig. 9) are even closer (20-40 km); implying a narrowing with time, not the expected widening (Dickinson, 1973) of the arc-trench distance when subduction of very young lithosphere occurs. In tectonic position, these recent volcanoes are at the top of the trench inner wall. Bathymetry (Fig. 9) suggests the small forearc basin seen on the “Glomar Challenger” seismic
268
reflection
profile
(Fig.
10) may
not be present
along
most
of the New Georgra
Group. All the published analyses of rocks K ,O/TiO, ratios compared to “average” marginal
basin basalts
from the New Georgia Group have high island arc tholeiites, basaltic andesites, and
(see Table I), The picritic
their high MgO and Cr,O,,
and olivine
and slightly low Al,OJCaO
basalts
contents.
are unusual
for
These basalts
are
strongly porphyritic in forsteritic olivine and green chromian diopside or diopsidic augite (Ramsey and Stanton, 1979). Much of their observed compositional range can be explained
by fractionation
spine1 (Ramsey normative ratios
TABLE
of olivine + clinopyroxene
1979). The New Georgia
and have “talc-alkaline
of the New Georgia
similar
and accumulation
and Stanton,
affinities”
volcanics
to those from other island
(Stanton
(0.7030-0.7043,
basaltic
+ chrome
andesites
are quartz
and Bell, 1969). The X7SrjXhSr Gill and Compston,
1973) are
arcs.
I
Major-element
chemical
analyses
Sample
New Georgia
lavas ’
number picritic
basalts
143,‘2
olivine basalt
364
Av. 414.
Av. 450,
367.372
443,413
basaltic 111/2
andesites
536
Av. 377. 428
SiO,
44.99
46.29
47.19
48.87
49.95
51.00
TiO,
0.33
0.35
0.45
0.48
0.65
0.52
0.70
Al&
6.54
8.43
9.01
12.37
14.01
19.10
18.28
53.02
Fe203
2.67
3.57
3.41
4.00
6.41
4.70
3.46
Fe0
7.29
7.17
6.41
6.09
5.59
4.27
4.55
MnO
0.19
.0.20
0.15
0.19
0.2 1
0.18
0.18
M@ CaO
28.10
23.01
21.06
12.28
6.57
4.15
4.31
6.56
7.58
8.71
11.03
10.16
8.82
8.85
Na,O
0.92
1.16
I .25
1.88
2.65
3.80
3.20
K2O
0.57
0.82
0.83
1.30
1.75
1.84
2.16
H,O’
1.06
0.54
0.45
0.86
1.33
0.62
0.4 1
0.30
0.64
0.71
0.47
0.7 1
0.44
0.62
0.05
0.04
0.06
0.20
0.2 1
0.21
0.38
0.4 1
H,O-
N.A.
CO,
0.08
p205
0.34
0.29
0.12
0.08
100.09
100.30
100.12
1.73
2.34
1.84
0.7040
0.7037
0.7034
Cr203
Total K,O/TiO, “Sr/
Yjr e
a Stanton
and
Compston
(1973).
Bell (1969);
b Ewart
(1976);
’ Hawkins
N.A. 0.29
0.09
N.A.
N.A.
0.03
100.17
100.28
99.82
100.26
2.71
2.69
3.54
3.09
0.7043
0.7039
(1976);
d Luyendyk
et al. (1973);
‘Gill
and
Marshak igneous
and Karig (1977) and others have suggested
activity
is likely to occur when a spreading
to the trench. The New Georgia One concept continued
divergence
asthenospheric Marshak
volcanics
of near-orthogonal material
and Karig,
are a present-day
subduction
of the ridge occupying
the
downdip resulting
of such activity.
system
would
in the subduction void
the magmatism
near-trench
at a high angle
example
of a spreading
flanks
1977). However,
that anomalously
ridge is subducted
(DeLong
and
Fox,
arc suites,
with
the exception
with 1977;
is not silicic as Marshak
Karig (1977) predicted, possibly because of the absence of an accretionary Rather, the major element and strontium isotope chemistry is not unlike other island (1.7-4.0).
suggest
zone,
of the very
high
K,O/TiO,
and
prism. that of ratios
High K,O/TiO, ratios (- 0.5-2.0) are a characteristic of island arc volcanics compared, for example, to MORB (- 0.05-0.5) (Bryan et al., 1976; Ewart, 1976). However,
the mechanism
for this relative
enrichment
Av. Island Arc ’ basalt
of K,O
over TiOz is not
Av. Lau
Woodlark
basin ’
basin ’
basaltic andesite
Av. 230.
349
STA.68
104/2
325 53.98
55.60
56.98
51.16
53.41
48.8
0.58
0.62
0.97
0.88
0.79
1.2
21.08
16.90
15.90
17.37
17.75
16.4
14.7
Al,&
4.05
4.65
1.78
3.43
3.24
2.0
N.A.
1.30
2.27
6.27
6.16
6.25
6.9
8.94
Fe,% Fe0
0.09
0.15
0.18
0.17
0.17
0.2
0.19
MnO
2.37
5.44
3.80
6.14
4.78
8.6
7.73
8.70
6.64
5.23
10.27
9.54
12.6
M@ CaO
3.70
3.63
4.5 1
2.63
2.60
2.4
2.52
Na#
2.28
1.61
3.34
0.84
0.68
0.18
0.07
KzO
0.83
1.31
0.87
0.55
0.48
0.30
N.A.
0.7 1
1.21
0.05
0.23
0.23
0.27
N.A.
HzO-
0.14
N.A.
CO*
0.08
N.A.
p&h Crz03
N.A.
0.05 0.43
0.23
0.73
N.A. 0.19
N.A. 0.14
N.A.
N.A.
N.A.
N.A.
N.A.
N.A.
100.15
100.26
100.61
100.02
100.06
100.07
3.44
0.95
0.86
0.15
3.93 0.7036
2.60 0.7038
50.3 1.47
11.7
0.08
SiO TiO*
H,O’
Total 0.05
KzO/TiO,
50 km Fig.
IO. Sin&channel
obtained
seismic
on “Glomar
scdimentaq
reflection
Challenger”
section is obsened
profile
across
in the Solomon
known.
That other
the New Georgia island
spreading unusual
center tectonic
volcanics
arc volcanics,
isotope chemistries, temporal association
of the Solomon A thick.
Basin. A small “fore-arc’.
of the inner trench wall and the ~;uad~canal-Rus~ell~
even
part
Leg 30 (see Fig. 9 for location).
Trench
and
acousticall!
basin is located
island
arc
rcverherent
bcturen
the top
ridge
have much higher
yet have
similar
major
KzO/Ti02 element
ratios and
than
strontium
is without current explanation. Nevertheless, the spatial and of this unusual volcanic suite with the subduction of an active
implies
that
magma
genesis
is controlled
in some
way by the
setting.
Vertical movements in the Solomon lsknds
Hackman (1973) noted that rapid differential block uplift has been a feature of the post-Pliocene history of the Solomon archipelago. On Guadalcanal, Pleistocene surfaces of marine erosion are observed 800 m above sea level (Hackman, 1973). Choiseul, Santa Isabel. New Georgia and Guadalcanal show evidence of recent uplift and tilting towards the Solomon Basin (Neef, 1978). The Solomon Basin (Figs. 9 and lo), a fault-bounded sedimentary basin with over 3 set of sediment (DeBroin et al., 1977) extends from the Florida Group to just southeast of Bougainville. The Florida Group has suffered late Quaternary submergence (Neef, 1978).
271
DeLong
and Fox (1977) and others predicted
with consequent of a spreading Basin
in sedimentation
ridge. While it is tempting
in the Solomon crust,
Solomons
changes Islands
further
to subduction
work
needs
MID-PLIOCENE
RECONSTRUCTION
should
of the arc and forearc. accompany
to relate the post-Pliocene
of young
to be done
arc before this suggestion
that shoaling
patterns,
and presumably to quantify
subduction
vertical buoyant
the uplift
tectonics Woodlark
history
of the
can be fully explored.
OF NORTHERN
MELANESIA
Figure 11 presents a reconstruction of the northern Melanesian region with respect to a fixed Pacific plate at 3.5 m.y. B.P. The diagram was constructed by (a) closing the Bismarck Sea (Taylor, 1979) (b) closing the Woodlark rise against the Pocklington rise using the observed spreading directions for the Woodlark Basin (this
study,
Fig.
7)
and
(c) rotating
the
Indo-Australian
reconstructed Woodlark and Pocklington rises, Australian rotation pole (Molnar et al., 1975). Comparing
the reconstruction
about
for 3.5 m.y. (Fig.
plate,
the present
including
the
Pacific/Indo-
11) with the present
situation
(Fig. I), we see that the Solomon Sea has become much smaller in area since mid-Pliocene time. By accepting the existence of the Solomon plate, we are forced to look for additional plate boundaries, evidence for which is not compelfing. However, subduction of the Solomon plate along the Trobriand margin of New Guinea 3.5 m.y. ago may explain (a) the trench-like structure of this margin as observed in seismic reflection records (for example, Hamilton, 1979). (b) the occurrence of Pleistocene and Recent talc-alkaline volcanism along the Papuan peninsula and the d’Entrecasteaux Islands (excluding Dawson Strait) (Johnson et al., 1978), and (c) several intermediate depth earthquakes which have occurred beneath the north Papuan
peninsula
underthrust sinistral
(Ripper,
1975).
rates were probably strike-slip
Australian Western
section
If this convergent
of the
boundary
low and the dip of the downgoing
of the
boundary
plates would link the Trobriand sections
plate
mid-Tertiary
between
Trench New
the
island
exist,
slab shallow. The
Solomon
with the Solomon Britain
did
arc
and
Indo-
Trench. collided
with
northern New Guinea beginning in the early Miocene, the collision progressing eastwards with time (Jaques and Robinson, 1977). Jaqucs and Robinson proposed that the collision was accompanied by (a) a component of strike-slip motion along the collision zone, (b) crustal shortening across the zone, and (c) fracturing, faulting and uplift of the mid-Tertiary arc terrain, as demonstrated by the Holocene uplift of flights of coral terraces along the northeast coast of the Huon peninsula (Chappell. 1974). An implication of our reconstruction (Fig. 11) is that prior to the opening of the Woodlark and Bismarck basins, convergence across the plate boundary south of New Britain is not required. While the existence of early Pliocene and presumed Late Miocene volcanics on New Britain (Page and Ryburn, 1977) implies some
,/-._
--.‘,
273
subduction,
we nevertheless
suggest that the plate boundary
was dominantly
strike-
slip. The Ontong Java Plateau erly subduction) tion at this trench
collided
in the middle
system (Kroenke.
as a result
of this collision,
subduction
of oceanic
with the West Melanesian
or late Miocene,
plate
lithosphere
effectively
1972; Coleman boundary along
Trench
blocking
and Packham,
readjustments
the southwest
(southwest-
further
subduc-
1976). Probably
led to (a) northeasterly
margin
of the New Ireland-
Solomon Island archipelago (Fig. 1 l), (b) abduction of oceanic lithosphere to form Malaita and other parts of the “Pacific Province” of the Solomon Islands and (c) the sinistral mega-shear system shown in Fig. 11. We believe that the strike-slip motion along this shear zone has continued through to the present as suggested by shallow, left-lateral
strike-slip
earthquakes
near Santa
Isabel
(Fig. 2). As total displacement
across the shear zone is unknown, we have used the present geography Solomon Islands as their relative positions in the mid-Pliocene (Fig.
of the
Pacific
plates over the past 3.5 m.y. It follows from the reconstruction
TABLE
(Fig. II)
that there was subduction
of oceanic
II
New focal mechanism solutions No.
(I
)
Date
Lat.
Long.
Depth
Azirnuth/phmgc
Axis of
Axi:, of
(d. m. yr)
(“S)
(‘E)
(km)
p&s to nodal
compre~-
tcnslon
pIZinC>
slon
of
iV&JA BLcIitI
po
1
06/01
9.62
151.50
x
25x/1 5
2
2x/ 10/i?
7.33
I4b.83
2
32xj
3
Ol/O5j73
10.00
150.20
15
180/32
4
02/05/73
9.96
150.2
I
29
03x/35
15x/35
IOX/Sb
Ixx;oI
5
23/10/74
8.40
154.03
1X
309/ 13
2 lh/O3
2bOj12
35’.‘04
6
25,/
9.16
156.70
33
329104
05Y/OO
0 14/O?
2x4 io3
Z/03/72
3.59
150.16
39
294,/0X
026/16
070/05
33X,‘lX
30/08/72
3.51
144.92
12
260/09
350/00
215/06
305,/06
I I /OY/72
3.54
149.M
0
2X4/12
0 I7/02
24
19/i l/74
3.20
150.59
IX
299,‘Ob
03 I/ox
075 /02
.1?2:‘10 34.5 i’ IO
148.00
39
294,/‘02
OX/IO
070/M
33Y/WX
146.03
33
33x/10
06X/20
023/w
2x/
(II)
&S~XJJ’ck
1l/75
II
354/x
305/x
232,‘30
IX?/12
784/,2Y
05x/40
I24/56
02x/‘o4
037/O?
&U
I pi
16/06/75
3.10
06/08/75
2.5
1i/10/75 06/O I /77
3.39
14X.61
33
205/07
Ii5/00
25O/Ob
IhO~Oi
3.64
144.45
33
ooo/cw,
090/00
045/(K)
Ii5 /oo
1
05
w
w
775
lithosphere
towards
mid-Pliocene.
the NE beneath
The
throughout
occurrence
the New
subject
in the region, An
to Recent
although
volcanism
Ireland
mid-Tertiary
suite
along
et al..
regime of plate
in these earlier
volcanic island
to the
volcanics
(Johnson
tectonic
directions
of alkaline
the Tabar-Feni
arc prior
arc-related regions
for a long-standing
subduction
interesting
This suite, which is composed equivalents, is not represented
to
Ireland-Bougainville
1979) is evidence
to debate.
Miocene
early
Britain-New
1976, 1978; Johnson, convergence
the Solomon-New
of
rocks
times are
characterizes
(Figs. I and
chain
I I ).
of alkali basal&, olivine nephelinite and intermediate in New Ireland or the Solomon island chain. The low
TiO, values and trace-element data of the Tabar-Feni rocks suggest arc type volcanism (Johnson, 1979). Most of these rocks were emplaced during the Quaternary. It is tempting
to attribute
underthrusting moved
southeast
Though
composition
or to a complete
relative
not entirely
observed
their unusual
direction
to New
equivalent,
Ireland
Pacific
plate
during
of subduction
to its present
changes
in petrology
on Fiji and the Lau ridge following
subducted
to either a radical
cessation
the opening
position
(Taylor,
of volcanic
isolation
change
in
as New Britain 1979).
rocks have been
of these features
of the Lau Basin (Gill.
from the
1976a.b).
The
Tabar-Feni Islands and the Fiji/Lau ridge are similar in that they occur in tectonic settings where eruptive centers have been removed from a position above subducted lithosphere
which was favorable
for the generation
of talc-alkaline
magmas.
ACKNOWLEDGMENTS
We thank P. Coleman, M. Perfit for constructive Bureau
of Mineral
S. DeLong, D.A. Falvey, R.W. Johnson, C. Langmuir, and comments on this manuscript. Geophysical data from the
Resources
is published
with permission
of the Director.
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