Terrane amalgamation in the Philippine Sea margin

Tectonophysics,

207

181 (1990) 207-222

Elsevier Science Publishers

B.V., Amsterdam

Terrane amalgamation Robert Department

in the Philippine Sea margin

Hall and Gary J. Nichols

of Geological Sciences, University College London, Gower Sireet, London WClE (Received

July 6.1989;

revision

accepted

September

6BT (U.K.)

30,1989)

ABSTRACT Hall, R. and Nichols, (Editors), Tectonics

G.J., 1990. Terrane amalgamation in the Philippine Sea margin. In: M. Kono and B.C. Burchfiel of Eastern Asia and Western Pacific Continental Margin. Tecronophysics, 181: 207-222.

The Philippine Sea plate includes plateaus of thickened crust interpreted as imbricated ophiolite and arc-related terranes of late Mesozoic-early Tertiary age separated by thinner oceanic crust. The arrival of plateaus at the subducting southwest margin of the Philippine Sea plate has caused the Philippine Trench to propagate southward in increments and caused transfer of terranes to the Philippine margin. New data from the Hahnahera region indicate that the position, nature and evolution of plate boundaries have been strongly influenced by the heterogeneous character of the Philippine Sea plate. At present the Philippine Trench terminates at an oceanic plateau which is structurally continuous with an old forearc and ophiolite terrane on Halmahera. The position of this terrane has caused Philippine Sea plate-Eurasia convergence to be transferred from subduction at the Philippine Trench to the Molucca Sea Collision Zone through a broad NE-SW dextral transpressional zone across Hahnahera. This plate boundary configuration is unstable and requires the future development of a new subduction zone to the east of Hahnahera which will result in amalgamation of the Halmahera ophiolite terrane to the Philippine margin. In the Halmahera region amalgamation of terranes to the evolving Philippine microcontinent is currently in process.

1) is rotating

north of the Bird’s Head of Irian Jaya by a strike-slip fault east of Halmahera (Fig. 2A). Cardwell et al. (1980) proposed a transform fault

clockwise with respect to Eurasia about an Euler pole situated close to its northern edge. Although

extending from north of Morotai to the Palau Trench (Fig. 2B) with an extension of this

Introduction The Philippine

Sea plate (Fig.

there are different estimates of the exact position

“Halmahera-Palau

of the pole (Chase, 1978; 1979; Ranken et al., 1984;

Trench

Minster and Jordan, Seno et al., 1987) all

Halmahera Sea plate.

predict that the rate of convergence between Eurasia and the Philippine Sea plate increases southward. The principal expression of this convergence is westward subduction at the Philippine

the Molucca Sea and the Philippine Trench by an east-west fault cutting Morotai (Fig. 2C). A dextral offset on the fault was deduced from the relative motions between the Sang&e Arc, the Philippine Sea plate and Halmahera. Moore and Silver (1983) argued that the Snellius Ridge, northwest of Morotai, was continuous with the Halmahera Arc (Fig. 2D) and considered the Philippine Trench as a young feature which is propagating south and not currently connected to

(about 10 cm y-l), the Philippine Trench terminates abruptly. Published tectonic models propose different links between the trench and other plate boundaries of the region (Fig. 2). Hamilton (1979) suggested that the southern end of the Philippine

0040-1951/90/$03.50

to the Sorong Fault Zone

0 1990 - Elsevier

Science Publishers

Sea. Both models place

on a separate plate to the Philippine

Later models considered Halmahera as part of the Philippine Sea plate. McCaffrey (1982) linked

Trench, yet just north of Halmahera at 2”5O’N, where the relative convergence rate is greatest

Trench was connected

Fault” linking the Philippine

to the Molucca

B.V.

R. HALL

208

AND

G.J. NICHOLS

t’

Fig. 1. Principal

tectonic

elements

and physical

from I = Chase (1978);

features

2 = Minster

of the Philippine

and Jordan

other plate boundaries. Hall (1987) linked the Philippine Trench with a dextral strike-slip zone across northern Halmahera (Fig. 2E) and proposed that southward development of the Philippine Trench and the initiation of the Halmahera Trench were related; a diffuse zone of deformation north of Halmahera was postulated to allow the opposed subduction of the Philippine Sea and Molucca Sea plates. West of Halmahera the Molucca Sea plate has an inverted U-shaped configuration (Hatherton and Dickinson, 1969; Katili, 1975; Hamilton, 1979; Cardwell et al., 1980; McCaffrey, 1982) and

(1979);

Sea region.

3 = Ranken

Eurasia-Philippine

et al. (1984);

Sea plate poles of rotation

4 = Seno et al. (1987).

is dipping east under Halmahera and west under the Sangihe arc (Fig. 3). The two former trenches in the Molucca Sea (Fig. 4) are now buried beneath the collision complex which is interpreted to be thrusting outward onto the two opposed forearcs of Halmahera and Sangihe (Silver and Moore, 1978). The thrust systems bounding the collision complex can be traced southward as far as the Sorong Fault system. Northward these thrusts can be identified up to the latitude of Talaud, but between Talaud and Mindanao there is no evidence of Molucca Sea subduction between the Philippine Trench and the Cotobato Trench. The

TERRANE

AMALGAMATION

IN THE

iamilton,

PHILIPPINE

1979

kcoffrey,

SEA

209

MARGIN

Cordwell

I&Y

1982

et al., 1980

oore and Silver, 1983

iOO

El KEY

CS CELEBES SEA

0’

BS

BANDA SEA

PS

PHILIPPINE SEA

-

SUBDUCTION

-

THRUST

N

COLLISION

ZONE

&

STRIKE-SLIP

FAULT

PT

Phibpp~ne Trench

5 F Sarong Fault M 2 Molucco Sea CoIlwon Zont

M

Mindanao

5

Sulawesi

P

Palau Is.

CT

H

Halmahera

AT Ayu Trough

IJ lrian Jayo

ZONE

N 5 1 N. Sulowes~ Trench Cotoboto

Trench

NGT NewGuma Trench

Iall, 1987 Fig. 2. Published

tectonic

models

for the southern Philippine

end of the Philippine

Trench

Sea plate showing

and other plate boundaries

most detailed interpretation of regional seismicity by McCaffrey (1982) suggests that approximately 200-300 km of lithosphere have been subducted beneath Halmahera and that beneath Morotai the Molucca Sea plate is cut by the subducting Philippine Sea plate (Fig. 3). On the opposite side of the Molucca Sea, the Benioff zone associated with the west-dipping slab can be identified to a depth of approximately 600 km beneath the Celebes Sea (Cardwell et al., 1980). This slab can be recog-

proposed

links between

the end of the

of the region.

nized beneath southern West Mindanao further north.

but not

This paper brings together new information from geological fieldwork in the Halmahera region and new marine survey data which permit the construction of a tectonic model for the southern end of the Philippine Sea plate. The model shows how terrane transfer processes have allowed an unstable system of plate boundaries to have been maintained since the Late Miocene and has impli-

210

R. HALL

cations

for the concepts

ophiolite

emplacement

of the Philippine

of sutures,

collision

and the larger-scale

and

history

pine Trench Cardwell posed

New marine data and the Philippine Trench

metric

have accepted

or seismic

evidence

tween the Philippine and little

evidence

Trench

there is little bathyto indicate

a link be-

and the Sorong Fault,

for a fault

linking

the Philip-

al.,

1980).

the position

east-west

earthquake Trench

All authors

et

termined

Sea plate.

to the Palau Trench

[which]

edge of the central Morotai”. ated zones

with

both

1979; de-

of his prolimit

of

with the Philippine

but linear extends

Molucca

NICHOLS

(1982)

from the southern

The abundant

zone of shallow

from

the

northern

Sea seismic zone into

shallow seismicity

associ-

of the

two opposed

subduction

and the Molucca

Sea collision

complex

this region fault

McCaffrey

associated

and a “sparse

earthquakes

G.J.

(Hamilton,

and orientation

fault

activity

AND

with

makes

it difficult

certainty.

The

to identify

fault

in

such

identified

a

in the

subducted slab (McCaffrey, 1982) has no obvious surface expression; there is no evidence on aerial

5”

photographs fault,

4”

nor

east-west

of Morotai is there

for a major

bathymetric

fault connecting

east-west

evidence

the Molucca

for an

Sea to the

Philippine Trench (Nichols et al., 1989). Instead, new detailed marine data (using GLORIA side-

2”

scan sonar and single-channel seismic profiling) show that the southern end of the Philippine Trench links to a NE-SW fault system (Fig. 5;

0

Nichols et al., 1989). Field data (Hall et al., 1988a, b, c) and aerial photographic interpretation show 2O 3”

tem as a dextral

128’

124O

that the fault system northern Halmahera. servable geometric

offsets

can be traced We interpret

strike-slip (Nichols

arguments

zone, et al.,

of McCaffrey

on land across this fault sysbased

on ob-

1989)

and

the

(1982).

The Philippine Trench is a clear feature as far south as 2”5O’N, where the GLORIA data show that

the Philippine

Trench

terminates

against

a

shallow region within the Philippine Sea plate, the East Morotai Plateau, which is structurally continuous with the Halmahera ophiolite terrane (Nichols et al., 1989). Further south and east the water is deeper (4.5 km, typical of this region of
Fig.

3. Form

of Benioff

zone contours

region and the three-dimensional Sea plate in that region show positions Halmahera Molucca Molucca ilton,

(after

of recently

in the Molucca

configuration McCaffrey,

Sea

of the Molucca

1982). Solid circles

active volcanoes

of the Sangihe

and

arcs. Note that the solid lines with solid teeth in the Sea represent Sea collision

1979);

the outward-directed complex

the former

plate are now buried teeth mark a thrust

trenches

beneath which

thrusts

(Silver and Moore, bounding

the collision

is interpreted

to be within the buried

Molucca

of the

1978; Ham-

the Molucca complex.

by McCaffrey Sea plate.

Sea

The open (1982)

the Philippine Sea plate) but no trench is developed and there is only minor deformation of the sediments.

The

area

between

the

East

Morotai

Plateau and Halmahera is seismically active as far south as l”45’N and sediment deformation is seen on GLORIA records and seismic profiles as far south as 1’20’N. At l”20’N a prominent faultbounded east-west ridge with sediments banked against it marks the northern side of a second plateau. This basement ridge can also be traced

TERLWE

AMALGhhtATtON

IN THE

PHILIPPINE

SEA

211

MARGIN

EURASI

PHILIPPINE

SEA

PLATE PLATE, D

P’

kcrl

CAROLINE

PLATE

AUSTRALIAN

IJ”

120”

Fig. 4. Principal tectonic elements of the region around Halmahera. Solid circles show positions of recent volcanoes of the Sanglhe and H&~&era

arcs. Mayu and Tifore are small islands situated in the centre of the Mofucca Sea.

onland into the H~m~era-Wigs ophiohtic basement terrane. The well developed trench north of 2’50’N gives way southward to a shallower, seismically active area where a subduction zone morphology has not evolved. The trench appears to be a young feature (Karig, 1975; Cardwell et al., 1980) propa8a~g southward (Moore and Silver, 1983; Hall, 1987; Nichols et al., 1989) and the thickened crust of the East Morotai Plateau seems to prevent (or slow) further southward propagation of the trench. Geology of Halmahera The physical continuity of the East Morotai Plateau and basement ridges at the southern end of the Philippine Sea plate with eastern Halmahera (Fig. 5) indicates a probable geological shanty. Proprietary and published seismic data around Halmahera (Letouzey et al., 1983) indicate that strati~ap~c units mapped onland can be traced

offshore. The basement of eastern Halmahera (Fig. 6) consists of a pre-Upper Cretaceous dismembered ophiolite (Hall et al., 1988a) associated with forearc volcanic and sedimentary rocks. Work on the ophiolitic rocks ~~all~t~e and Hall, 1989; PD. Ballantyne, work in progress) indicates an arc setting for the formation of most of the components of the ophiolite; the mineralogy and geochemistry of these rocks indicate hydrous melting, currently widely interpreted as implying a subduction zone-related setting (Bioomer and Hawkins, 1983; Pearce et al., 1984). Similar ophiolitic rocks are found on Waigeo, which lies immediately to the north of the Bird’s Head of Brian Jaya, on Gebe and Gag, and on other small islands between Waigeo and the southeastern arm of Halmahera. These form part of the East Ha~m~era-Wigs Op~o~te Terrane ~S~~to et al., 1981). Upper Cretaceous to Eocene volcanic and sedimentary rocks rest unconfo~ably on the ophio-

212

R. HALL

AND

G.J. NICHOLS

Fig. 5. Tectonic elementsat the southernend of the Philippine Trench (after Nichols et al., 1989).

litic basement (Hall et al., 1988a, 19%). The strati~aphy of this sequence is summarised in Table 1. Upper Cretaceous volcaniclastic sediments vary from deep-water breccio-conglomerates to siltstones, contain fresh talc-alkaline debris, and are locally interbedded with pelagic limestones containing planktonic foraminifera. Within the same sequence are andesitic pillow lavas, lava breccias and minor intrusives of intermediate composition. The sequence has not been deeply buried or strongly deformed; it contains low-grade alteration minerals in veins but virtually all the relatively unstable volcanic debris remains very fresh. Middle Eocene volcaniclastic rocks are similar but differ principally in having a more distal character. Coarse volc~clastics are less abundant, and associated limestones are entirely redeposited and contain abundant material from a shallow water carbonate source. The basement of western Halmahera consists of petrographically and chemically similar arc volcanics and volcaniclastic sediments. These rocks represent material rapidly eroded and transported from an active

volcanic arc. Middle to Upper Eocene sediments also include marginal marine to littoral facies conglomerates, coals and limestones. They rest unconformably on the ophiolitic basement complex and the volcaniclastic sediments. This suggests a similar setting to that observed in the present-day Sunda forearc where volcanic debris is being eroded from Sumatra while carbonate reefs and ophiolites are exposed on a forearc high on Nias and Simulue (Moore et al., 1980). The complexity and dynamic character of this forearc (Beaudry and Moore, 1981) provides an excellent model for the interpretation of the Halmahera ophiolitic basement and its Late Cretaceous-early Tertiary volcaniclastic

cover.

Structurally, the Halmahera basement consists of fault-bounded blocks which were deformed and juxtaposed before the Late Eocene (Hall et al., 1988a). By the Early Miocene the basement was eroded flat and remained close to sea-level throughout the Miocene, during the deposition of shallow-water reef and reef-associated limestones. On Halmahera (Hall et al., 1988b, 1988~) and

TERRANE

AMALGAMATION

IN THE PHILIPPINE

213

SEA MARGIN

8

1

4

Alttium

7J

Reef limestone Late Aeistocene-Recent volcanics

a

128”

t

129”

!

TERTIARY m

Rio-Reistocene volcsnict

a

Mio-Miocene sedimants

1

Miocene limostone

WESTERN ARMS fl

Eocene-Miocene volcanics B sediments

-g

Cunt~nentai basement Basic1‘ultrabasic

/

HAL~AHERA ,:. ....‘..*;

t’

*...

km

0

Fig. 6. Summary map and cross-section showing the principal geological features of the island of Halmahera (based on Hall et al., 1988a, b, c).

R. HALL

214 TABLE 1 Summary of the stratigraphy of Halmahera (based on Hall et al., 1988a, b, c) WESTERN Alluvwn,

Quaternary

and Pliocene Late Miocene

reef

Coarse

and fine shallow

Late

-

Cret.

Island

eroded)

arc volcanics

volcamclastics

and

reef

Carbonate

mudstones

with

serpentinite

rare

gravels

and

and sands

and associated

limestones

Rare _______________~~___II_________

Oligocene

HALMAHERA

Alluvium

Reef

and associated - now

EASTERN

limestone

marine elastics eroded from volcarw arc

limestones

Eocene

limestone

arc voicanics

(Reef

Miocene

HALUAHERA

marls _____.-_..-..

Forearc

sequence

pelagic

and

limestones

of

redeposited and

volcaniclastics -._-ll*l-..l-------Late

Ophlolite

complex

Mesozoic

Waigeo (Van der Wegen, 1963; Supriatna and Apandi, 1982) the limestones are overlain by shallow-marine Upper Miocene and Pliocene sedimentary rocks. Upper Miocene-Pliocene elastic sediments in west Halmahera were derived from a volcanic arc to the west. Our initial fieldwork (Hall et al., 1988b) suggested arc activity began in the Pliocene but fresh volcaniclastic debris in Upper Miocene sediments of southwestern Halmahera (Hall et al., 19%) indicates arc volcanism in western Halmahera began earlier, in the Late Miocene. In eastern Halmahera, the Upper Miocene-Pliocene sediments are mainly calcarenites and marls which conformably overly the Miocene limestones. The present K-shape of the island is controlled by two major fault directions, NE-SW and WNW-ESE, which form a conjugate set (Fig. 7 ). In the northeastern and northwestern arms of the Halmahera and on the island of Morotai, the NE-SW structural trend is dominant. This trend can be traced offshore northeast of Halmahera and Morotai (Nichols et al., 1989). Our structural interpretation of the island indicates an important overthrust component on this fracture set, directed to the northwest. In the northeastern arm thrusting causes a repetition of the basement and cover sequences and has uplifted the ophiolitic basement. Lineaments oriented NW-SE and WNW-

AND

G.J. NfCHOLS

ESE are observed in the southern arms, but in contrast to the northern arms, the WNW-ESE fracture set is dominant. The coastlines of the southeastern arm are fault-controlled and oriented WNW-ESE and this fracture trend can be identified offshore on seismic lines (Letouzey et al., 1983). The WNW-ESE fracture set is interpreted as having predominantly sinistral offset with little overthrust component. It is sub-parallel to the sinistral Sorong Fault zone to the south and the most important faults in southern Halmahera appear to be sinistral spiays off the Sorong Fault system. In central Halmahera a fold-thrust belt forms the boundary between the ophiolitic eastern basement and arc volcanic western basement (Figs. 6 and 8). In the southwestern arm the volcanic basement is overthrust by Neogene sediments with an

Fig. 7. Principa! faults mapped on land in the northeast and southern arms of Halmahera (after Hall et al., 1988~).

TERRANE

A~LGA~TION

IN THE

Fig. 8. The proposed

interpretation

arating

Sea plate,

the Philippine

sian plate in the region between

PHILIPPINE

of plate

Molucca Halmahera

SEX

215

MARGIN

boundaries

sep-

Sea plate and Euraand Mindanao.

eastward dip. Balanced cross-sections indicate at least 40 km east-west shortening between east and west Halmahera in the fold-thrust belt (Hall et al., 1988c). A further 20 km shortening is deduced in the southeastern arm from relationships between op~olitic basement and Neogene cover sediments. The age of thrusting is between 3 and 1 Ma. Much of the northwestern arm of Halmahera is covered by late Neogene to Recent volcanics produced by the Halmahera volcanic arc which extends from the northwestern arm into a chain of volcanic islands parallel to the west coast of Halmahera. The active arc is the product of the eastward subduction of the Molucca Sea plate beneath Halmahera. Present-day

tectonics

Like McCaffrey (1982), Moore and Silver (1983) and Hall (1987) we consider that Halmahera is currently part of the Philippine Sea plate but differ in our interpretation of the character and position of some plate boundaries (Fig. 8). The differences result from interpretation of our new field data from Halmahera, the newly acquired marine data, and our attempt to combine this new data with published information from the region to produce a tectonic interpretation which is consistent with present-day tectonics and can be described in terms of rigid plates. Between Halmahera and north Sulawesi, the Molucca Sea plate is currently almost eliminated by subduction beneath the Eurasian plate (Sangihe Arc) and the P~lippine Sea plate (Halmahera

Arc). To the north, the Molucca Sea plate must either be connected to Mindanao or separated from it by a major tectonic boundary. We propose that the Molucca Sea plate continues north as the part of East Mindanao between the Philippine Trench and the P~lippine Fault. The P~~ppine Fault Zone can be traced offshore south of Mindanao but dies out before Talaud (Moore and Silver, 1983). We suggest that the southward continuation of the Philippine Fault is the West Halmahera Thrust (Silver and Moore, 1978) which corresponds approximately to the leading edge of the Sangihe (Eurasian plate) forearc, currently obducting onto the Halmahera forearc. The East Sulawesi Thrust (Silver and Moore, 1978), bounding the west side of the Molucca Sea collision zone, is interpreted as a backthrust developing within the forearc at the rigid backstop of the Sangihe volcanic arc. The great thickness of deformed material in the Molucca Sea collision complex represents the two compressed forearc sediment wedges, with slivers of Sangihe forearc basement reaching the surface in the Talaud, Mayu and Tifore islands situated along the central axis of the Molucca Sea. The boundary between the Philippine Sea plate and the Molucca Sea plate is the P~lippine Trench, linked to the former Halmahera Trench (now beneath the collision complex) by the dextral strike-slip system of north Halmahera. With these interpretations the recent tectonics of the whole region between Mindanao and the Sorong Fault can be approximated using rigid plates. The intense internal deformation of the collision complex in the Molucca Sea indicates that this rigid plate model is a simplification. Furthermore, the Eurasian plate is deforming in a complex manner which could be described in terms of smaller plates, for example, the Celebes Sea is being subducted beneath the north arm of Sulawesi. Nevertheless, for the purposes of understanding the tectonic development of this region an adequate description can be approached using three rigid plates (Fig. 8): the Eurasian plate (North Sulawesi-West Mindanao), the Molucca Sea plate (Molucca Sea and East Mindanao), and the Philippine Sea plate (Halmahera and the P~lippine Sea).

R. HALL

216

The relative motions of these plates cannot be determined precisely because determinations of motion vectors are not yet fully in agreement, but choosing different sets of data (Chase, 1978; Minster and Jordan, 19’79; Ranken et al., 1984; Seno et al., 1987) makes no substantial difference to this model. South of 17ON Eurasia-Philippine Sea plate oblique convergence is decoupled into strike-slip faulting along the Philippine Fault and normal underthrusting at the Philippine Trench (Fitch, 1972; Cardwell et al., 1980). Decoupling potentially allows oblique convergence to maintam a long term stability; orthogonal subduction observed at boundaries of major plates appears to be dynamically favourable (Scotese and Rowley, 1985) and strike-slip faulting behind the trench takes up the non-orthogonal component of relative motion. North of about 10”N tectonic stability has been largely achieved by such decoupling. At the latitude of Halmahera the P~lippine Sea plate is moving west-northwest (azimuth 290”) relative to Eurasia and the total convergence is distributed between the Eurasian, Molucca Sea and Philippine Sea. Between 10”N and 3*N slip vectors at the Philippine Trench (Ranken et al., 1984) suggest that the convergence direction between the Philippine Sea plate and Molucca Sea plate is close to east-west and the northward component of Eurasia-Philippine Sea plate slip is decoupled into movement on the Philippine Fault. The Molucca Sea plate is therefore moving north relative to Eurasia along the Philippine Fault and east relative to the Philippine Sea plate at the P~lippine Trench. Around Halmahera we suggest that the Quaternary-present NE-SW and WNW-ESE faults form a conjugate system which indicates east-west convergence between the Philippine Sea plate and the Molucca Sea plate. An important implication of this motion direction is that the NE-SW fault system of north Halmahera is highly oblique to the convergence direction and is therefore not a stable plate boundary at present. The relative motion between the Philippine Sea plate and the Molucca Sea plate is absorbed in a zone of faulting, with both dextral strike-slip and thrusting components, and the zone of faulting has shifted with time.

Tectonic development

AND G.J. NfCHOLS

of the region

This tectonic model and the estimated rates of plate convergence provide the basis for reconstructing the late Neogene tectonic history of the region. Further constraints are provided by observations of lengths and positions of subducted slabs in the region (Cardwell et al., 1980; McCaffrey, 1982) and the rate of motion on the Philippine Fault (Acharya, 1980). Using these observations, combined with the geological history of Halmahera based on our fieldwork, it is possible to obtain a reasonably clear picture of the evolution of the region from the present to the Late Miocene. However, extending this model to a wider area, and over a longer interval of time, is handicapped by lack of regional geological information (for example, from parts of Mindanao) and by lack of quantitative data (for example, reliable palaeoma~etic results). We follow Hall (1987) in postulating that development of subduction at the Philippine and Halmahera trenches was linked. Several important features emerge from this model and the Neogene reconstructions: (1) The present system of plate boundaries is not stable and deformation in the region is the expression of the shifting pattern of plate boundaries. (2) Oblique convergence between the Philippine Sea plate and Eurasia has been accommodated by combinations of (a) subduction of the Philippine Sea plate, (b) strike-slip faulting within the margin, and (c) elimination of oceanic lithospheric remnants within the Eurasian margin. (3) The position of subduction zones has been controlled by the presence of regions of thickened crust within the Philippine Sea plate. The role of plateaus within the Philippine Sea plate

The P~lippine Sea plate is not a simple oceanic plate of relatively uniform thickness but has had a complex evolution (Karig, 1975; Mrozowski et al., 1982; Hilde and Lee, 1984; Seno and Maruyama, 1984) and includes oceanic plateaus and ridges as well as areas of more typical oceanic crust. Of particular interest here is the West Philippine Basin for which the many inte~retations of origin and

TERRANE

AMALGAMATION

IN THE

PHILIPPINE

SEA

217

MARGIN

w PRESENT

Fig. 9. Cartoons are based

illustrating

DAY

the tectonic

on an assumption

evolution

of the southern

that the rate of Eurasia-Philippine

day. The Bird’s Head of Irian Jaya is shown for reference is shown

schematically.

Philippine

Other

Sea plate beneath

are all part of Philippine at trench

(E) blocks Philippine

beneath

the Snellius

(shown

by oblique

blocks

subduction,

Philippine between

Trench

are discussed

West Mindanao

and north

subduction

Trench.

Fault

and

Halmahera

lines) becomes and

east and west Halmahera

of oblique

and continues

Trench

grows

by advance

through

and NE-SW

south

strike-slip forearc

north

faulting

plate at the Sorong

(F) shows westward Snellius

Philippine

Ridge (SR)

Late Miocene

Eastward

(E) and Early

(D). In the Pliocene Trench

and internal

in north

is overthrusting

subduction

(C) Snellius

Halmahera

Ridge

deformation forearc.

of the

and Halmahera

(HA)

of East Mindanao

at present

site of Philippine

is initiated

at the same time

Pliocene

(D). Sorong

Ridge arrives to Halmahera. of Halmahera

(B). At present

the Halmahera

Fault zone

subduction

Sea plate. Arrival

on the east side of East Mindanao convergence.

on east side of the Snellius

of Halmahera

and the Eurasian

of present-day

(EM),

The ages

and is the same as the present

of the Australian

reconstruction

East Mindanao

zone is initiated

component

active from the Early Pliocene

the Philippine

is accommodated

Sulawesi;

Sea plate since the Late Miocene.

has been constant

boundary

in the text. Earliest

to plateaus

and new subduction

takes up northward

Ridge

and the northern

assumptions

Sea plate and are analogous

end of the Philippine Sea plate motion

Fault

zone

at subduction

zone,

Subduction

at the

region:

(A) the Molucca

thrusting

Sea is closed

218

development are well reviewed by Hilde and Lee (1984). The central part of the West Philippine Basin includes closely spaced magnetic lineations and opened between about 45 Ma and 35 Ma (Hilde and Lee, 1984). North and south of the Central Basin magnetic lineations are more widely spaced and less confidently identified. On the north side of the Central Basin is the Daito Ridge and Daito province, including the Oki-Daito ridges, and the Amami Plateau, which have been interpreted as remant arcs (Murachi et al., 1968; Karig, 1975; Shiki et al., 1977; Mizuno et al., 1978; Klein and Kobayashi, 1980; Lewis et al., 1982; Tokuyama et al., 1986) and continental fragments (Nur and Ben-Avraham, 1982). South of the Central Basin there is less information. The southern West Philippine Basin, which includes the East Morotai Plateau, is relatively shallow and has an irregular topography (Mrozowski et al., 1982; Nichols et al., 1989). It has been interpreted as either older (Mrozowski et al., 1982; Hilde and Lee, 1984) or the same age as the Central Basin (Mrozowski et al., 1982). Based on our work on Halmahera we suggest that many of the West Philippine Sea plateaus are Late Mesozoic to early Tertiary ophiolitic and arc-related rocks. We propose that East Mindanao, the Snellius Ridge and the East HalmaheraWaigeo Terrane (Fig. 9) are three such plateaus at different stages of amalgamation into the Philippine margin. In the eastern Philippines similar basement terranes are composite and record complex histories of amalgamation but are also generally interpreted as having formed in arc-related tectonic settings before or during the early Tertiary (e.g. Hawkins et al., 1985; McCabe et al., 1985; Karig et al., 1986; Geary et al., 1988). Lewis et al. (1982) suggest that much of the east Philippines developed in the Late Cretaceous in an arc setting (their proto-East Mind~ao-Sag arc. Disorganized inter-arc extension in the Paleocene and Eocene separated the Daito Ridge province from the East Mindanao-Samar arc by opening of the central West Philippine Basin. In discussing the polarity of the subduction system Lewis et al. (1982) suggest that there are no features in the southern West Philippine Basin that could be remnant arc features similar to the Daito Ridge pro-

R. HALL

AND

F.J. NICHOLS

vince. However, our observations on Hahnahera, and the continuation of the Halmahera basement into the East Morotai Plateau (Nichols et al., 1989), suggest that such features do exist. Rocks dredged from the Daito province (Shiki et al., 1977; Mizuno et al., 1978; Tokuyama et al., 1986) and dated (Ozima et al., 1977; McKee and Klock, 1980) are very similar in both ages and lithologies to those we have found in Halmahera. Like Klein and Kobayashi (1981), we suggest that the Daito Ridge province, and the southern West Philippine Basin formed before and during the initial opening of the West P~~ppine Basin. Within the Philippine Sea plate the plateaus are separated by areas of thinner, “normal” oceanic crust which we interpret to have formed by Tertiary spreading. At the southern end of the Philippine Trench this thinner crust, along with seamounts, is subducted whereas the extensive thick piateaus are not. Nur and Ben-Avraham (1982) suggest that arrival of plateaus at subduction zones prevents further subduction and continued convergence requires development of new subduction zones. We suggest that the arrival of plateaus at subduction zones has also resulted in terrane amalgamation in the Philippine margin. The effects of plateau arrival can be observed today in the Halmahera region and in the northern part of the Philippine Sea plate where the Amami Plateau is in collision with the Ryukyu island arc (Tokuyama et al., 1985). Within the Philippine Sea plate, sites of subduction appear to have been controlled by the shape and position of the plateaus. We suggest that younger subduction zones developed oceanward of older subduction zones with orientations approximately perpendicular to the local convergence direction. Oceanward shift of subduction zones caused transfer of plateaus from the Philippine Sea plate to the Philippine margin and was accompanied by shift in the position of related strike-slip faults. Development

of Neogene configuration

If subduction at the Philippine Trench is young, as argued by Karig (197.5) Cardwell et al. (1980), Lewis et al. (1982) and Hall (1987), then before this subduction system developed the east Philip-

TERRANE

AMALGA~TION

IN THE

PHlLtPPlNE

SEA

NARGIN

pines must have formed part of the Philippine Sea plate. Lewis et al. (1982) suggest that the East Mindanao-Samar block formed an old volcanic arc which collided diachronously with West Mindanao resulting in the development of a new subduction zone at the present P~hppine Trench. The similarities in ages and lithologies of rocks of the East Mindanao-Samar fragment, the Daito Ridge province plateaus, and Halmahera lead us to support their interpretation that all these regions formed part of a Late Cretaceous-early Tertiary arc province. We also agree that these fragments became separated by early Tertiary inter-arc extension, and by later opening of the central West Philippine Basin. However, it seems probable to us that the Samar-East MindanaoHalmahera province was not a single intact arc terrain as represented by Lewis et al. (1982) but was more likely to be a region of plateaus separated by thinner oceanic crust similar to the present-day Daito Ridge province. There are problems with the geometry of the reconstruction proposed by Lewis et al. (1982) which postulates that the central West Philippine Basin opened as a backarc basin with a spreading centre parallel to their East Mindanao-Samar arc whereas at present the Central Basin Spreading Centre is almost perpendicular to the East Mindanao-Samar terrane (Hilde and Lee, 1984). The HalmaheraSangihe arc collision is not simply the continuation south of collision between East and West Mindanao. From our reconstructions we suggest that collision in Mindanao (early Miocene according to Lewis et al., 1982, pre-10 Ma according to Moore and Silver, 1983) was the consequence of the arrival at the subduction zone of an East Mindanao “plateau” and was the cause of the development of the opposed subduction system of the Molucca Sea. Our earliest reconstruction (Fig. 9F), assuming the present pole of Eurasia-Philippine Sea plate rotation, and the present rate of motion, shows collision .between East and West Mindanao occurred at 8 Ma, suggesting that the present rate of convergence may be higher than the average rate over the last lo-15 Ma. Nakamura et al. (1984) suggest that the relative direction of convergence between the Philippine Sea plate and the Eurasian plate changed at about 1 Ma which

219

would require a reduction in our assumed convergence direction and an earlier collision in Mindanao, in agreement with Lewis et al. (1982) and Moore and Silver (1983). West-directed subduction at the southern end of the P~pp~e Sea plate has been in process since at least early Miocene as indicated by the age of volcanic rocks in North Sulawesi (Dow, 1976; Effendi, 1976; Apandi, 1977) and West Mindanao (Ranneft et al., 1960; Lewis et al., 1982). Before about 10 Ma the subduction zone was west of East Mindanao which formed part of the P~ppine Sea plate (Fig. 9F). At this stage the Molucca Sea was part of the Philippine Sea plate and was analogous to the areas of “normal” oceanic crust now separating plateaus within the Philippine Sea plate. Arrival of the East Mindanao plateau at the subduction zone occurred in the Miocene and prevented further subduction. Blocking of subduction led to development of new subduction zones (Fig. 9E) within the former area of the Philippine Sea plate and their positions were controlled by the limits of the thickened plateaus within the Philippine Sea plate; subduction developed on the east side of the Mindanao terrane and on the west side of the Snellius edge-Halm~era plateau in opposed directions. This configuration is deduced from the present configuration of Molucca Sea lithosphere. We suggest that at about 5 Ma the Philippine Trench extended only to the southern end of Mindanao (Fig. 9D); south of this latitude east-directed subduction of the Molucca Sea accommodated convergence. We speculate that the Snellius Ridge was the next plateau to reach the subduction zone and incorporate into our model McCaffrey’s (1982) suggestion that eastward subduction beneath the Snellius Ridge ceased when it entered the collision zone. The Snellius Ridge has characteristics of an arc (Moore and Silver, 1983) but forms a flattopped feature about 2 km below sea-level. Identification of this plateau as part of an old arc, analogous to parts of the Daito Ridge province, rather than part of the modem Halmahera arc would explain why it is now at such depth and covered by undeformed sediments. The east-west offset between the Snellius Ridge and Halmahera

220

(Fig. 9D-F) is interpreted in the subducted Molucca Sea slab (McCaffrey, 1982) and accounts for the hanging slab beneath Morotai, the greater amount of subduction beneath Halmahera and the earlier arrival of the Snellius Ridge at the subduction zone. At about 2 f 1 Ma the Snellius Ridge arrived at the subduction zone, blocking the south end of the trench (Fig. SC). A new subduction zone then developed on the east side of the Snellius Ridge and extended south to 2”5O’N where further southward propagation of the trench was prevented by the East Halmahera-Waigeo ophiolite terrane. The Philippine Trench therefore became linked to the Halmahera Trench by the present dextral strike-slip system. We date this event from the age of thrusting on Halmahera which is linked to the dextral strike-slip system. Since about 2 Ma convergence causing subduction north of 2’5O’N has been acco~odated further south by a combination of east-directed subduction of the Molucca Sea plate and internal deformation of the Philippine Sea plate seen as active strike-slip faulting and overthrusting on Halmahera (Fig. 9B). The present NE-SW linkage of Molucca Sea subduction to Philippine Sea plate subduction is unstable (Fig. 9A). This direction is oblique to the east-west convergence and the capacity of the lithosphere in the region between the Snellius Ridge and Halmahera to absorb the strain between the two opposed subduction zones is limited. Furthermore, the Molucca Sea plate has now been completely subducted. Continued motion between the P~lippine Sea plate and Eurasia can only be accommodated by repositio~g of the subduction-strike-slip system. The recent development of the region suggests that a new subduction zone will develop east of Halmahera, beyond the East Morotai Plateau. The ophiohte terrane of East Halmahera will then be transferred into the Philippine-Eurasia margin. Terrane amalgamation

The development of the P~~ppine Sea margin in the Halmahera region offers some insight into the transport and assembly of allochthonous ter-

R. HALL

AND G.J. NICHOLS

ranes. The plateaus amalgamated are already complex structural terranes on arrival at the margin and their shape and position in the P~~ppine Sea plate control plate boundary evolution. Subduction zones and strike-slip faults are intimately linked. North of about lOoN tectonic stability has been largely achieved by decoupling of EurasiaPreppie Sea plate oblique convergence into strike-slip movement at the Philippine Fault and underthrusting at the Philippine Trench. In the Philippines an eastern “buffer” plate bounded by the Philippine Fault and Philippine Trench is a complex region dominated by arc-related and ophiolitic rocks identified as a single terrane by McCabe et al. (1985). However, the similar stratigraphic features of this long and narrow terrane could also result from amalgamation of similar plateaus derived from the Philippine Sea plate over a long period rather than a single collisional event. This could account for the apparent diachronous nature (Lewis et al., 1982; Moore and Silver, 1983) of the collision as well as local stratigraphic differences along the Philippine margin. Decoupling potenti~ly allows oblique convergence between two plates to achieve a long-term stability. At the southern end of the Philippine Sea plate a stable configuration of plate boundaries has not evolved because of the internal heterogeneity of the P~~ppine Sea plate and because the Eurasian and Philippine Sea plates are also in collision with the Australian plate. In this region the concepts of “suture”, “collision” and “ophiolite emplacement” become exceedingly ambiguous and two-dimensional models of erogenic development are likely to be misleading. Terrane transfer has caused repositioning of plate boundaries on time scales of a few millions of years and at each stage old ophiolites and arc-related rocks have been “emplaced”. Observations in the Halmahera region suggest deformation episodes are probable before, during and after terrane transfer. The ophiolite terranes from the southern part of the Philippine Sea plate are in the process of amalgamation in the Philippine margin and the “sutures” between these terranes are thrust zones and strike-slip faults which often do not mark the traces of former subduction zones.

TERRANE AhJALGAMATION

221

IN THE PHILIPPINE SEA MARGIN

Geary.

E.E.,

ages

Fieldwork in Halmahera was funded by the Royal Society, the University of London Consortium for Geological Research in Southeast Asia, Amoco International, British Petroleum, Enterprise Oil, Total Indonesie and Union Texas (SE Asia). We thank GRDC Bandung for support and our field colleagues, M.G. Audley-Charles, P.D. Ballantyne, L.A. Garvie, A.S. Hakim, S. Hidayat, Kusnama, S.L. Tobing. We also thank F.T. Banner, D.J. Carter, A.R. Lord and L. Gallagher for palaeontological support and J. Letouzey of IFP for access to seismic records. We thank the scientific and technical staff of the R.R.S. “Charles Darwin” cruise 30 funded by NERC.

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22: 169 pp.

Geologic 42: 3-12.

van

Waigeo

(W. New