the opening of the woodlark basin, subduction of the ... - ScienceDirect

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.

REFERENCES

Blake, D.H. and Mietzitis, Miner. Resour., Bryan.

W.B.. Thompson,

tion in basal& Bultitudc.

chemical

Geol. Geophys.. Carey,

R.W.

Cochran.

movements J.R..

Coleman,

P.J..

approach

J. Geophya.

B.W..

New Guinea.

Aust., Bur.

geologic

scttinga

and diffcrentia-

Res.. Xl : 42X5-4304.

197X. Andesitea

andesite

to continental

of Tasmania,

of coral

terraces.

and sea-level changes.

1981. The Red Sea: Implications

EOS. Trans.

Re-entrant,

Project.

and a reference

University

J.. 1974. Geology

margins.

and Buka Islands.

of Bagana

composition.

volcano.

Bur. Miner.

Papua

Resour..

Ncu

J. Aust

3: 2X1-295.

Drift. A Symposium. tectonic

Drilling

and Chappell,

stratigraphy,

S.W.. lY58. The tectonic

Chappell.

of Bougainville

G.. Frey, F.A. and Dickey. J.S., 1976. Inferred

from the Deep-Sea

R.J.. Johnson.

Ciuinea:

Y.. 1967. Geology

Bull., 93.

Am. Geophys.

1970. Geology Southwest

Union,

of the Solomon

Pacific.

drift.

Hobart, Huon

In: S.W. Carey

(Convener),

Continental

pp. 177-3.55.

Penmsula.

New Guinea:

a study

of Quaternarc

Geol. Sot. Am. Bull., X2: 30X7-3 IOY. for the early development

of Atlantic-type

continental

62: 407. and New

Pac. Sci.. 24: 2X9-314.

Hebrides

Islands.

as part

of the Melaneaian

216

Coleman,

P.J. and Packham,

Earth Curtis.

G.H..

1976. The Melartesian

borderlands

and India-Pacific

plates’ boundarv

Sci. Rev,, 12: 197.-233.

J.W., L973a. The spatial

seismicitv

of Papua

New Guinea

and the Solomon

Island