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Late Cenozoic tectonic development of the Southeast Asian continental margin in the Banda Sea area
Tecro~ophysics, 181 (1990) 267-276 Elsevier
Science Publishers
267
B.V., Amsterdam
Late Cenozoic tectonic development of the Southeast Asian continental margin in the Banda Sea area H.M.S. Hartono Marine Geological institute, .Jalan Dr. Junjunan 236, Bandung 40174 (Indonesia) (Received
May 6,1989;
revision
accepted
November
3,1989)
ABSTRACT Hartono, H.M.S., 1990. Late Cenozoic tectonic development of the Southeast Asian continental In: M. Kono and B.C. Burchfiel (Editors), Tectonics of Eastern Asia and Western
margin in the Banda Pacific Continental
Sea area. Margin.
Tectonophysics, 181: 267-276. The late Cenozoic tectonic development of the Southeast Asian cont~ent~ margin around the Banda Sea is ~mp~cated by interaction with external geological elements. Southeast Asian internal elements are the Banda Volcanic Arc and previous older arcs. External elements are the Australian continental crust, and the Indian and Pacific oceanic crusts. These external elements are now trapped behind the Banda Volcanic Arc. Three main geological events are responsible for the present configuration of the Banda Sea and adjacent areas: (1) collision between the Banda Volcanic Arc and Australian continental crust; (2) emplacement of the Banda Sea oceanic crust; and (3) emplacement of microcontinents now present in and around the Banda Sea. The geology of Timor is fundamental to tectonic interpretations of the collision between the Banda Volcanic Arc and Australia. Differences exist in the tectonic interpretation of Timor and include either overthrusting, upthrusting or m&nge fo~ation. The present paper follows geological data contained in geological maps published by the Geological Survey of Indonesia in which overthrust structures are clearly shown. Paleomagnetic and paleontologic data indicate that the overthrust units were from southern paleolatitudes. Depth and magnetic lineation data support the interpretation that the Banda Sea is underlain by old oceanic crust. Similar orientation of magnetic lineations in the Banda Sea and the Argo Abyssal Plain suggests that they have the same origin. Geological data from northern Banda microcontinents, dredged samples from the Banda/Lucipara ridges and comprehensive geological development of the northern Banda Arc support the interpretation that the microcontinents were translated left-laterally westward from Irian Jaya.
Introduction This report presents a summary of geological studies in the Banda Sea area. It focuses on the Southeast Asian active continental margin which forms a part of the eastern Eurasian continental margin. In the Banda Sea area, the Banda Volcanic Arc is a principal tectonic element along this extensive active continental margin which has evolved continuously since Triassic time. The late Cenozoic to present volcanic arc development has been complicated by its interaction with external elements, namely the Australian continental crust with its rifted components and
portions of the oceanic crust of the Indian Ocean. Parts of both elements are now trapped behind the Banda Volcanic Arc. An attempt is made to explain the entrapment mechanism within the overall development of the late Cenozoic tectonic evolution of the Southeast Asian continental margin. The area can be classified into a number of geological entities which are geologically distinct or form remarkable physiographic features (Fig. 1). They are, from hinterland outwards: the Banda Sea, Banda Volcanic Arc, intervening seas between Banda Volcanic Arc and Banda Outer Arc, the Banda Outer Arc, trench, and the Australian
268
H.EA.S. HARTONO
BANDA
RIDGES
SOUTH
BANDA
SEA
0
na
WETAR SEA FLORES
SEA
*
.
i
7 ,
AUSTRALIA
I
12’ 121
l25* NORTHERN
SANDA
MICROCONTINENTS
13w t-j
SANDA
ARC
1350 SANDA
VOLCANIC
136’
ARC
Fig. 1. Location map of the Banda Sea area.
shelf. A sharp break occurs at the inner edge of the Banda Outer Arc where basement changes from continental to oceanic. This paper presents selected data obtained from published and unpublished sources which support the tectonic interpretation. These data are: seismic refraction results, gravity, magnetics, heat flow, paleomagnetics, land geology (mainly Timor), and analysis of dredged samples from the Banda/Lucipara ridges. Discussions of the data are also presented to develop the tectonic interpretation. Three main geological events are responsible for the present configuration of the Banda Sea area: (1) Collision between the Australian continental crust and the Banda Volcanic Arc.
(2) Emplacement of the Banda Sea oceanic crust. (3) Left-lateral transcurrent faulting along the Sorong faults and the consequences of the emplacement of continental fragments. Seismic refraction data Results of seismic refraction studies carried out by Bowin et al. (1980) show that the South Banda Sea is underlain by oceanic crust. The oceanic crustal character has been identified from shallow mantle depths along refraction lines 23 of Bowin et al. (1980) and MI2 of the Scripps Institution of Oceanography (S.I.O.) Monsoon Expedition in 1960.
THE SOUTHEAST ASIAN CONTINENTAL
MARGIN IN THE BANDA SEA AREA
Based on these data, surficial sediments (velocity 2.15 km s-r) are 0.4 km thick, more consohdated sediments (velocity 5.1 km s-t) are 1.3 km thick, oceanic crust (velocity 6.6 km s-‘) is 4.3 km thick, and mantle (velocity 8.0 km s-r) starts at a depth of 10.4 km (= Moho depth). The refraction lines 6-11 of Bowin et al. (1980), located between the Banda Volcanic Arc and the Banda Outer Arc, give mantle depths of lo-21 km. Oceanic crust is assumed under these lines and includes oceanic crustal characteristics under the Weber Deep and the Savu Sea. There is a narrow passage in this zone, such as the sea between Timor and Wetar, where oceanic basement is also expected. Refraction lines on the Australian shelf and in Timor and Ant troughs show deeper Moho depths and indicate the presence of continental basement. Under the Plio-Pleistocene sediments of the Timor and Tanimbar troughs, 30-40 km of continental basement is assumed. From these studies it is concluded that the Australian continental crust extends up to the inner part of the Banda Outer Arc ridge. This conclusion implies also that the Timor and Aru troughs are underlain by continental basement.
Gravity data Bowin et al. (1980) indicate that gravity data support the seismic refraction results; the Banda Sea basement is oceanic in nature and the Banda Outer Arc ridge and the trenches surrounding it are underlain by continental basement, Gravity modelling by Schluter and Fritsch (1985) crossing the arc near Weber Deep indicates that the Banda Outer Arc and the Aru Trough are underlain by continental basement. The section gives an impression of a deep Moho, at a depth of more than 30 km, beneath the outer arc and the trough. The Banda Sea; Weber Deep and the volcanic arc are underlain by oceanic crust. The deptb to the Moho is less than 10 km under the Banda Sea, 20 km under the volcanic arc and 14 km under the Weber Deep. Gravity modelling by Dwiyanto (1985) crossing the North Banda/Lucipara ridges, Buru/Lucipara
269
basins and Banda Sea indicates a shallow Moho, 8-9 km deep. Sumba appears from a Bouguer gravity anomaly of + 160 to +200 mgal to be underlain by a continental type crust. The crust was estimated by Chamalaun et al. (1981) to be at least 24 km thick. Chamalaun et al.‘s (1976) gravity studies in Timor show that the island is characterized by an unusually steep positive gravity gradient northward. He interpreted this gradient to be caused by a sharp transition from continental crust on the southeastern side of the island to an oceanic crust on the northwestern side. Preliminary computer modelling shows that a sharp change is obvious at appro~mately the northern coastline of Timor from a density of 2.7-3.4 g cmm3, indicating that beneath Timor the crust is thick and under the Wetar Strait the crust is thin. Chamalaun et al.‘s tectonic implication is that subduction is located along the northern side of the island. Magnetic data Bowin et al. (1980) showed the presence of magnetic lineations in the South Banda Sea and indicated that they are of possible sea-floor spreading origin. No assigned geomagnetic polarity ages were given; however, they are compared with similar trending magnetic lineations from the Argo Abyssal Plain, N60-70” with M25-22 magnetic polarity ages (147-142 Ma). Bowin et al.‘s study indicated that the magnetic anomalies of the Australian continental shelf are generally of longer wavelength and usually of lower amplitude than the oceanic crustal anomalies. The appro~mate boundary between the region of low amplitude magnetic anomalies and crust having large-amplitude anomalies is at the inner margin of the Banda Outer Arc. Lapouille et al. (1985) reported the presence of magnetic lineations in the South Banda Sea. Their studies also covered the North Banda Sea. Assigned geomagnetic polarity ages for the South Banda Sea are between Ml4 (131 Ma) and M6 (121 Ma), whereas for the North Banda Sea they are between M3 (117 Ma) and MO (112 Ma). Similarly, Lee and McCabe (1986) show the same lineation trend for the Banda Sea. Both observa-
H.M.S.
270
HARTONO
II
0’
0’
..
mw KALIMANTAN
‘m
.CGZ??i-,
20
2’
7
40
40
6’
69
Q
8”
S=
100
10’
IP
120
r
ARGO
14’
le.*
ABYSSAL
I44
PLAIN
69 1140
I (6’
It@.*
1205
122*
124*
126”
128-
1304
1321
138
IO
Direction
and
Heatflow
values
degrees
---ACMES
Magnetic
lineation
0
Location
of
in
dredge
of
motion
mW me2; and
and
, In
geomagnetic
age
of
formotion
parenthesis time
in heotflow
units
(peal
cm-‘)
scale
site
Fig. 2. Map of the Banda Sea and adjacent regions showing magnetic line&ions (labelled MO through M25), pal~ma~etic rotations shown by the arrows, dredge location and heat flow data (in parentheses). Direction and degree of motion and age of formation from Haile (1987) Otofuji et al. (1979), Chamalaun et al. (1981) and Wensink et al. (1987). Heat flow data from Anderson et al. (1978), Bowin et al. (1980) and Van Gool et al. (1987). Magnetic lineations and geomagnetic time scale from Heirtzler et al. (1978) and Lapouihe et al. (1985). Dredge sites: Dl = basalt, 0.40 f 0.01 Ma; D5 = phylhte, 22.50 + 0.5 Ma; D8 = pyroxene andesite, 6.16 f 0.08 Ma; D9 = pyroxene andesite, 7.03 f 0.17 Ma; DlO = amphibolite, 10.5 f 0.1 Ma; D16 = pyroxene andesite, 0.75 f 0.02 Ma; D19 = metadiabase, 8.75 f 1.48 Ma (data from Silver et al., 1985).
tions show that the Banda Sea oceanic crust becomes younger towards the north. It is remarkable that there is an absence of magnetic Iineations in the area between the North and South Banda seas which coincides approximately with the Banda ridges. Silver et al. (1983,
identified the Banda ridges as continental fragments based on dredge samples from the ridges which are of continental provenance. Also, volcanic rocks related to tbe Banda Volcanic Arc have been obtained from the dredge samples. Some of the magnetic lineation lines cross the Banda
THE
SOKJTHWST
ASIAN
CONTINENTAL
NARGIN
IN THE
BANDA
ridges, which indicates that the nature of the basement inferred from the magnetic lineation data and the dredge samples are incompatible. The presence of magnetic lineations in the Banda Sea has not been unanimously accepted by geologists working on East Indonesian geology. However, there is general acceptance that the Banda Sea is underlain by an old oceanic crust. Land geological, paleomagnetic and pakontological data The geology of Timor is critical to the understanding of the collision processes between the Banda Volcanic Arc and Australian crust. The important features of Timor geology may be summarized as follows. (1) Overthrust structure-from geological maps published by the Geological Survey of Indonesia (Rosidi et al., 1979), it can be observed that an important tectonic event took place during middle Tertiary time when Australia began to collide with the Banda Volcanic Arc. This event is reflected on the maps, which clearly show overthrust structures. The overthrust units range in age from Permian to early Tertiary and are thrust over formations that range from Permian to Jurassic in age. (2) Australian origin of the ~l~h~ons-it is now generally accepted that the over-thrust units or the allochthons are of Australian origin. This conclusion is based mainly on paleomagnetic results (Wensink et al., 1987) and paleontology, in which brachiopods from the allochthonous Maubise Formation has Australian affinities (Archbold and Barkham, 1988). The autochthonous rocks are of Australian origin. Dredged samples from the Banda Sea Silver et al. (1985) analyzed dredge samples from the Banda and Lucipara ridges (Fig. 2) and these revealed continental basement and volcanic rocks. The continental basement rocks are elastic sedimentary and metamorphic rocks. Whole-rock IS-Ar dating yielded an age of 22.5 f 0.5 Ma for a phyllite, and 10.8 f 0.1 Ma for an ampbibolite. These basement rocks are similar to those found
SEA
AREA
271
in nei~bou~ng areas: Buton, eastern Sulawesi, Seram, Misool, Banggai-Sula and Irian Jaya. The volcanic rocks from the Banda and Lucipara ridges and from Gunung Api north of Wetar are talc-alkalic andesites. Based on their alkalinity, the Banda Sea volcanics are similar to those from Damar rather than to voleanics from the other island volcanoes. Radiometric ages of the Banda Sea volcanic rocks are consistent with ages of volcanic rocks from the Banda Volcanic Arc. The chemical composition of the volcanic rocks also indicates contamination by continental silicic rocks. Heat flow data Heat flow data in the Banda Sea are from Bowin et al. (1980), Anderson et al. (1978) and Van Go01 et al. (1987). Generally the Banda Sea has a low heat flow, between 0.4 and 1.7 HFU (Fig. 2). Some higher heat flow values (3.21 and 4.19 HFU) in the area are interpreted to be the result of faulting. The unusual low heat flow values and ocean depths are compatible with the Banda Sea being old oceanic crust. Collision between the Banda Vokanie Arc and Australia The Banda Volcanic Arc is an eastward extension of the Sunda Volcanic Arc (Sumatera, Java, Bali, Lombok, Sumbawa, Flores) and is composed of the volcanic islands of Damar, Teen, Nila, Serua, Manuk and Banda Api. It has an arcuate shape. Internally it is bounded by the Banda Sea oceanic basement and externally by intervening seas (Savu Sea, Wetar Strait and Weber Deep) which are underlain by oceanic crust. The intervening seas are bounded on one side by the Banda Volcanic Arc and on the opposite side by a row of non-volcanic islands (Sumba, Timor, Tanimbar, Kai and Seram) which form the Banda Outer Arc. Australian continental crust Iies to the south of Timor and extends to the east northward to Irian Jaya. Seismic refraction studies and gravity modelling have delineated the extent of the crust to the inner margin of the Banda Outer Arc.
H.M.S. HARTONO
272
The
arc-continent
subduction sion
of oceanic
commenced
reached
collision
and
crust.
when
collided
was preceded Arc-continent
the Australian with
the
by colli-
continent
Banda
Model
Overthrust
The oldest
age of igneous
rocks
in the Banda
Volcanic
Arc, which occurs in the island
consists
of 12 Ma old granite
the minimum The
Australia
which
started
gan to separate
age
would
Position
of
models
Origin of
subduction
geological
north of
autochthonous
Timor
a result of abortive
date
upthrust
north of
all geological
Timor
Australia,
of subducbe
to move northward from Antarctica.
of the collision
units from Australia rifting
of Wetar,
should
age of commencement maximum
1
Characteristics
Volcanic
Arc.
tion.
TABLE
the
formations
time
when it be-
According
units are from
except post-erogenic
Imbricate
to the
south of
mixture
Timor
slab which later becomes
of scraped
and subducted mtlange
magnetic polarity time scale this began during reversal 34 (= 82 Ma). Collision between the Banda Volcanic Arc and the Australian when Australia transition
from
continental crust proper occurred entered the subduction zone. This subduction
of oceanic
crust
be-
parts remained close to the northwestern Australian continent. They all moved northward together and collided with the Banda Volcanic Arc during
neath the volcanic arc to actual collision between the volcanic arc and continental crust took place about 3 Ma ago, based on radiometric ages of
collision
volcanic
crust. The continued northward caused continued compression
rocks contaminated
by continental
crust
from Wetar and Atauro. Dates of similar volcanic rocks from the easternmost part of the arc, from Ambon and Kelang, indicate that collision took place there at about the same time (Abbott and Chamalaun, Different
1981). tectonic
interpretations
of the colli-
sion come from different interpretations structural style, environment of deposition
of the of the
sediments, and seismic reflection/refraction data from Timor. Three models prevail: overthrust, upthrust and imbrication/mClange models. The relationship elements
of the models with associated is shown in Table 1.
tectonic
Recent studies on fossil brachiopods from the allochthonous Maubise Formation of Timor indicate that the fossils are similar to fossils of the same age from Australia (Archbold and Barkham,
between
the volcanic
arc and
the con-
tinental crust. The collision caused overthrusting of the rifted parts onto the Australian continental
This interpretation structures present Outer Arc.
drift of Australia and overthrusting.
best explains on the islands
the overthrust of the Banda
Origin of the Banda Sea and geological implication Hamilton (1979) considered the Banda Sea to be a marginal sea formed by back-arc spreading that created and caused
extension of the Tertiary volcanics the present separation of the West
Sulawesi Volcanic Arc and the Banda Volcanic Arc. Consequently, the age of the Banda Sea oceanic crust should be young and at least the same age as the oldest volcanic or granitic rocks of the Banda Arc, which the available data show to be 12 Ma. Hamilton indicated that the youthful bathy-
1988). Similarly, paleomagnetic studies on the allochthonous Nakfunu Formation indicate that it
metric
originated from a southern paleolatitude (Wensink et al., 1987). The present author suggests that the allochthonous overthrust units originated on the northwestern Australian margin which was rifted during Jurassic time. The rifting was of short dura-
ing a young age for the Banda Sea basement. New data presented by Lapouille et al. (1985) on the magnetic lineations in the Banda Sea indicate the lineations can be correlated with Ml4 (130 Ma) to MO (113 Ma), and become younger towards the north. Similarly, Lee and McCabe (1986) reported the same magnetic lineation trends.
tion,
presumably,
an abortive
rift, and the rifted
expression
of the Banda
Sea, in particular
the Banda Ridges, is one of the criteria
for propos-
THE SQtTTHuCST ASIAN CONTINENTAL MARGIN IN THE BANDA SEA AREA
ARAFfrRA PLQRES
@
BAND4
a
NORTHERN
-
AUSTRAl.lAN
is nutewurthy
r2w
I2W
122*
AUTO~N?~QNOUS
BANDA
MKRQCQNTINENTS
CONTINENTAL
I3P
ki
UNifS
CRUST
: BUTQN,SUtA,
BURU,
EIANDA 4LLocNTNoNous
BANDARiDSES,
BQCAN,
UNlW
081.
BOUNDARY
that the Iineations are continuous with those in the Argo Abyssal Plain in northwestern Australia which indicate a gradation in lineation ages from M25 (147 Ma) to Ml6 (133 Ma) (Fig. 2). An inference that can be drawn from the magnetic lineatiun data is that oceanic crust from the Argo Abyssal Rain and the Banda Sea were originaIly one continuous oceanic crust. The identities of magnetic Iineations of the Argo Abyssal Plain have been confirmed radiometrically from two DSDP h&es; they establish that the Argo Abyssal Plain owes its origin to sea&xz spreading fveevers, 1984). It
SF.4
SEA
Ac~crrding to Lapouille et al. (1985), the ma& in the northern Banda Sea south of Sula, east of Sulawesi and west of Buru, range from M3 to MO (1X3-117 Ma) and are part of a cantirm~us gradation from the southern Banda Sea. However, the uverzkll geolqkal setting, espe= eially the emplacement of ~~r~~~~ents and their intervening seas, suggests a different interpretation for the origin of the magnetic polarity ages of the northern Banda Sea. I accept the magnetic polarity ages and strikes of the magnetic Iineations of the southern Banda Sea (M14-M& = f31-121 Ma), but suggest that Lapouik’s m&gnetic lineations
H.M.S.
214
SUNDALAND Lote
HARTONO
AUSTRALIA
Mesozoic Arc
Bondo Allochthon
Jurassic
Late CretoceausEarly
Tertiory
Tronscurrent fOtIlf
Arc
Lots Cre toceous
Bonda Volcanic
c
Tertiory
A
Bondo Outer Arc
Austrolio
D
Pliocene -Trench
Fig. 4. Tectonic Australia
moved
allochthons sea-floor
evolution
of the Banda Sea and the Banda Arc between
away from Southeast
Asia. The oldest magnetic
Sundaland
lineation
were formed by abortive rifting and the Timor Sea was initially spreading,
consequence
during
transcurrent
magnetic
reversal
34, Australia
separated
and Banda volcanism
commence.
D. Collision
formed.
A. During late Mesozoic
B. After quiescence
from Antarctica
faulting took place which led to the emplacement
Volcanic Arc. C. Subduction
and Australia.
record is M25 and the youngest and began
time
is MO. The Banda
of plate movements
to move
northward.
by
As a
of the Banda Sea plate and the birth of the Banda
commences
between the Banda Outer Arc and the Banda
Volcanic Arc.
netic lineations from the northern Banda Sea should be considered with reservation. From the relative positions of the southern Banda Sea and Argo Abyssal Plain, it is obvious that some offset should have taken place for the Banda Sea crust to be placed into its present position. The Banda Sea crust should have been shifted eastward by right-lateral transcurrent faulting. The timing of such faulting should be younger than the youngest magnetic lineation age of the Banda Sea, which is M6 (121 Ma), and also the faulting could not have taken place during the magnetic quiet period (MO to 34 = 113-82 Ma). Therefore the maximum age for the faulting is at geomagnetic reversal 34 (82 Ma). The intervening magnetic anomaly zone between Ml6 (133 Ma) and Ml4 (130 Ma) is located
at the position of the Banda Volcanic Arc. The birth of the volcanic arc should be closely related to the transcurrent faulting or the commencement of subduction. In the magnetic polarity time scale between MO and 34 there is the Cretaceous magnetic quiet zone. After this event Antarctica and Australia began to separate, and a spreading center developed between Australia and Antarctica. The oceanic crust north of Australia created by sea-floor spreading should have also participated in the northward motion of Australia. However, the transcurrent displacement and the subsequent birth of the Banda Volcanic Arc or associated subduction zone indicates that the Banda Sea crust apparently did not participate in the northward motion.
THE SOUTHEAST
ASIAN
CONTINENTAL
MARGIN
IN THE BANDA
The oldest Banda volcanic or granitic rocks are 12 Ma, whereas the time when Australia started to move northward is at magnetic anomaly 34 (82 Ma). The time span between magnetic anomaly 34 (82 Ma) and 12 Ma should be the time when the Banda Volcanic Arc came into existence. This difference in time is 70 million years. Eventually other data must be sought to narrow this time lapse in order to more precisely date the birth of the Banda Volcanic Arc. Emplacement
and origin
of mi~~ontinents
in
northern Banda The microcontinental slivers, consisting of Buton, Sula, Buru, Obi, Bacan and the Banda ridges, are present in the northern part of the Banda region. There are two main interpretations presented for the emplacement and origin of the ~cr~ontinents. One is advocated by Silver et al. (1985) and other workers who consider that the microcontinents originated from the northern part of Irian Jaya and were emplaced by left-lateral trancurrent faulting along the Sorong Fault. By this mechanism the northern part of the Banda Sea became closed by emplacement of continental slivers and oceanic basement. The Banda ridges have been specifically called by Silver et al. a submerged and displaced continental borderland. This hypothesis considers the age of the southern Banda Sea to be pre-Tertiary and of Indian Ocean origin. The northern Banda Sea and the intervening seas between the microcontinents were interpreted to be derived from the Pacific Ocean. Because the microcontinents were interpreted to be derived from northern Irian Jaya, they are of Gondwanan origin (Fig. 3). Together, they, or at least the most western parts, collided with the Sulawesi Arc. Structural features supporting this interpretation are the To10 thrust, the Hamilton fault and the thrust fault separating the Sula platform and Sulawesi in the northeastern arm of Sulawesi. The second interpretation advocated by Pigram and Pangabean (1983) considers the microcontinents to have originated from northern Australia, and separation of these continental pieces commenced during Jurassic time. This inte~retation is
275
SEA AREA
based on stratigraphic analysis and the timing of the separation is based on magnetic lineations in the Argo Abyssal Plain with magnetic polarity ages M22-M25 (143-147 Ma). The present author considers that the allochthonous formations of the Outer Banda Arc islands (Timor, Tanimbar, Seram) originated from northwestern Australia (Fig. 4). This assumption is supported by paleomagnetic data (Wensink et al., 1987) from the Nakfunu Formation of Timor. The separation should have taken place after the deposition of the Permo-Triassic Maubise Formation and an Early Jurassic of separation is assumed. The allochthonous microcontinents did not travel far from northwestern Australia; they remained close to the continent. Later these allochthonous microcontinents were thrust southward over the autochthonous Australian continental shelf. From this discussion it is apparent that Timor, Tanimbar and Seram have a different geologic origin and emplacement history than that of the microcontinents of northern Banda (Buton, Sula and Banda ridges). The northern Banda microcontinents were derived from northern Irian Jaya. Buton and Sula including the Banda ridges were displaced quite far by the left-lateral tr~scu~ent faulting along the Sorong fault system. The relationship between Seram and the Banda ridges needs further explanation. The Banda ridges are presently south of Seram; however, they have a different origin and history of emplacement from the allochthonous formations of Seram, Tanimbar and Timor. The Banda ridges probably were emplaced first and subsequently the Banda Sea was closed by emplacement of Seram along the Aiduna left-lateral transcurrent fault and anti-clockwise rotation, Traces of the Sorong fault system that emplaced the Banda ridges has been obliterated by the later left-lateral faulting.
References Abbott, M.J. and Chamalaun, F.H., 1981. Geochronology of some Banda Arc vohxnics, sujono (Editors),
In: A.J. Barber and S. Wiryo-
The Geology and Tectonics of Eastern
Indonesia. Geol. Res. Dev. Centre, Spec. Publ., 2: 253-272.
276 Anderson, R.N., Langseth, M.G., Hayes, D.E., Watanabe, T. and Mashui, 1978. Heatflow, thermal conductivity, thermal gradient, In: D.E. Hayes (Editor), A Geophysical Atlas of East and Southeast Asian Seas. Geol. Sot. Am., Map Chart Ser., MC-25. Archbold, N.W. and Barkbam, S.T., 1988. Gondwanan relationships of new Permian brachiopod fauna from Timor. Dep. Geol. Univ. London/Univ. Melbourne, Rep., 24 pp. Audley-Charles, M.G., 1986. Thor-T~bar Trot@: the foreland basin of the evolving Banda orogen. Spec. Publ. Int. Assoc. Sedimentol., 8: 91-102. Bowin, C.O., Purdy, G.M., Johnston, Ch., Shor, G., Lawver, L., Hartono, H.M.S. and Jezek, P., 1980. Arc-continent collision in the Banda Sea region. Am. Assoc. Pet. Geol. Bull., 64: 868-915. Chamalaun, F.H., Lockwood, K. and White, A., 19’76. The Bouguer gravity field and crustal structure of eastern Timor. Tectonophysics, 30: 241-259. Chamalaun, F.H., Grady, A.E., Von der Borch, C.C. and Hartono, H.M.S., 1981. Banda Arc tectonics: the tectonic significance of the Sumba Island, In: J.S. Watkins and C.L. Drake (Editors), Studies in Continental Margin Geology. Am. Assoc. Pet. Geol., Mem., 30: 361-376. Dwiyanto, B., 1985. Marine geology and geophysics of the northern Banda Sea. M.Sc. Thesis, University College London, London, 94 pp. {unpub~sh~). Haile, N.S., 1978. Paleomagnetic evidence for the rotation of &ram, Indonesia. J. Phys. Earth, 26(Suppl.): SlYl-S198. Hamilton, W., 1979. Tectonics of the Indonesian region. U.S. Geol. Surv., Prof. Pap., 1078: 345 pp. Hartono, H.M.S. and Tjokrosaputro, S., 1984. Preliminary account and reconstruction of Indonesian tenanes. Proc. Indones. Pet. Assoc., pp. 185-226. Heirtzler, J.R., Cameron, P., Cook, P.J., Powell, T., Roeser, H.E., Sukardi, S. and Veevers, J.J., 1978. The Argo Abyssal Plain. Earth Planet. Sci. Lett., 41: 21-31. Katili, J.A. and Hartono, H.M.S., 1983. Complications of Ceonozoic tectonic development in eastern Indonesia. In:
H.M.S. HARTONO
T.W.C. Hilde and S. Uyeda (Editors), Geodynamics of the Western Pacific. American Geophysical Union, Washington, D.C., pp. 387401. Lapouille, A., Haryono, H., Lame, M., Pramuwijoyo, S. and Lardy, M., 1985. Age and origin of the seafloor of the Banda Sea (eastern Indonesia). Qceanol. Acta, 8: 379-389. Lee, C.S. and McCabe, R, 1986. The Banda-Celebes-Sulu basin: a trapped piece of Cretaceous-Eocene oceanic crust? Nature, 2444: 1. Otofuji, Y.I., Sasajima, S., Nishimura, Yokoyama, I., Hadiwisastra, S. and Hehuwat, F., 1981. Paleomagnetic evcidence for the paleoposition of Sumba island, Indonesia. Earth Planet. Sci. Lett., 52: 97-100. Pigram, C.J. and Pangabean, H., 1983. Age of the Banda Sea, eastern Indonesia. Nature, 301: 231-233. Prasetyo, H., 1989. Marine geology and tectonic development of the Banda Sea region, eastern Indonesia. Ph.D. Dissertation, University of California, Santa Cruz, Calif.; published as Mar. Geol. Inst. Indones., Spec. Publ., 1: 427 pp.. Rosidi, H.M.D., Suwitodirdjo, K. and Tjokrosaputro, S., 1979. Geologic Map of the Kupang-Atambua Quadrangles, Timor. Geol. Res. Dev. Centre, Geol. Map Ser., Bandung. Schluter, H.V. and Fritsch, J., 1985. Geology and tectonics of the Banda Arc between Tanimbar island and Aru island. Geol. Jahrb., Reihe E 30: 3-41. Silver, EA., Gill, J.B., Schwartz, D., Prasetyo, H. and Duncan, R.A., 1985. Evidence for a submerged and displaced continental borderland, North Banda Sea, Indonesia. Geology, 13: 687-691. Van Gool, M., Huson, W.J., Prawirasasra, R. and Gwen, T.R., 1987. Heat flow and seismic observations in the Northwestem Banda Arc. J. Geophys. Res., 92: 2581-2586. Veevers, J.J. (Editor), 1984. Phanerozoic Earth History of Australia. Clarendon Press, Oxford, 413 pp. Wensink, H. Hartosukorahardjo, S. and Kool, K., 1987. Paleomagnetism of the Nakfunu Formation of Cretaceous age, western Timor, Indonesia. Geol. Mijnbouw, 66: 89-99.
Science Publishers
267
B.V., Amsterdam
Late Cenozoic tectonic development of the Southeast Asian continental margin in the Banda Sea area H.M.S. Hartono Marine Geological institute, .Jalan Dr. Junjunan 236, Bandung 40174 (Indonesia) (Received
May 6,1989;
revision
accepted
November
3,1989)
ABSTRACT Hartono, H.M.S., 1990. Late Cenozoic tectonic development of the Southeast Asian continental In: M. Kono and B.C. Burchfiel (Editors), Tectonics of Eastern Asia and Western
margin in the Banda Pacific Continental
Sea area. Margin.
Tectonophysics, 181: 267-276. The late Cenozoic tectonic development of the Southeast Asian cont~ent~ margin around the Banda Sea is ~mp~cated by interaction with external geological elements. Southeast Asian internal elements are the Banda Volcanic Arc and previous older arcs. External elements are the Australian continental crust, and the Indian and Pacific oceanic crusts. These external elements are now trapped behind the Banda Volcanic Arc. Three main geological events are responsible for the present configuration of the Banda Sea and adjacent areas: (1) collision between the Banda Volcanic Arc and Australian continental crust; (2) emplacement of the Banda Sea oceanic crust; and (3) emplacement of microcontinents now present in and around the Banda Sea. The geology of Timor is fundamental to tectonic interpretations of the collision between the Banda Volcanic Arc and Australia. Differences exist in the tectonic interpretation of Timor and include either overthrusting, upthrusting or m&nge fo~ation. The present paper follows geological data contained in geological maps published by the Geological Survey of Indonesia in which overthrust structures are clearly shown. Paleomagnetic and paleontologic data indicate that the overthrust units were from southern paleolatitudes. Depth and magnetic lineation data support the interpretation that the Banda Sea is underlain by old oceanic crust. Similar orientation of magnetic lineations in the Banda Sea and the Argo Abyssal Plain suggests that they have the same origin. Geological data from northern Banda microcontinents, dredged samples from the Banda/Lucipara ridges and comprehensive geological development of the northern Banda Arc support the interpretation that the microcontinents were translated left-laterally westward from Irian Jaya.
Introduction This report presents a summary of geological studies in the Banda Sea area. It focuses on the Southeast Asian active continental margin which forms a part of the eastern Eurasian continental margin. In the Banda Sea area, the Banda Volcanic Arc is a principal tectonic element along this extensive active continental margin which has evolved continuously since Triassic time. The late Cenozoic to present volcanic arc development has been complicated by its interaction with external elements, namely the Australian continental crust with its rifted components and
portions of the oceanic crust of the Indian Ocean. Parts of both elements are now trapped behind the Banda Volcanic Arc. An attempt is made to explain the entrapment mechanism within the overall development of the late Cenozoic tectonic evolution of the Southeast Asian continental margin. The area can be classified into a number of geological entities which are geologically distinct or form remarkable physiographic features (Fig. 1). They are, from hinterland outwards: the Banda Sea, Banda Volcanic Arc, intervening seas between Banda Volcanic Arc and Banda Outer Arc, the Banda Outer Arc, trench, and the Australian
268
H.EA.S. HARTONO
BANDA
RIDGES
SOUTH
BANDA
SEA
0
na
WETAR SEA FLORES
SEA
*
.
i
7 ,
AUSTRALIA
I
12’ 121
l25* NORTHERN
SANDA
MICROCONTINENTS
13w t-j
SANDA
ARC
1350 SANDA
VOLCANIC
136’
ARC
Fig. 1. Location map of the Banda Sea area.
shelf. A sharp break occurs at the inner edge of the Banda Outer Arc where basement changes from continental to oceanic. This paper presents selected data obtained from published and unpublished sources which support the tectonic interpretation. These data are: seismic refraction results, gravity, magnetics, heat flow, paleomagnetics, land geology (mainly Timor), and analysis of dredged samples from the Banda/Lucipara ridges. Discussions of the data are also presented to develop the tectonic interpretation. Three main geological events are responsible for the present configuration of the Banda Sea area: (1) Collision between the Australian continental crust and the Banda Volcanic Arc.
(2) Emplacement of the Banda Sea oceanic crust. (3) Left-lateral transcurrent faulting along the Sorong faults and the consequences of the emplacement of continental fragments. Seismic refraction data Results of seismic refraction studies carried out by Bowin et al. (1980) show that the South Banda Sea is underlain by oceanic crust. The oceanic crustal character has been identified from shallow mantle depths along refraction lines 23 of Bowin et al. (1980) and MI2 of the Scripps Institution of Oceanography (S.I.O.) Monsoon Expedition in 1960.
THE SOUTHEAST ASIAN CONTINENTAL
MARGIN IN THE BANDA SEA AREA
Based on these data, surficial sediments (velocity 2.15 km s-r) are 0.4 km thick, more consohdated sediments (velocity 5.1 km s-t) are 1.3 km thick, oceanic crust (velocity 6.6 km s-‘) is 4.3 km thick, and mantle (velocity 8.0 km s-r) starts at a depth of 10.4 km (= Moho depth). The refraction lines 6-11 of Bowin et al. (1980), located between the Banda Volcanic Arc and the Banda Outer Arc, give mantle depths of lo-21 km. Oceanic crust is assumed under these lines and includes oceanic crustal characteristics under the Weber Deep and the Savu Sea. There is a narrow passage in this zone, such as the sea between Timor and Wetar, where oceanic basement is also expected. Refraction lines on the Australian shelf and in Timor and Ant troughs show deeper Moho depths and indicate the presence of continental basement. Under the Plio-Pleistocene sediments of the Timor and Tanimbar troughs, 30-40 km of continental basement is assumed. From these studies it is concluded that the Australian continental crust extends up to the inner part of the Banda Outer Arc ridge. This conclusion implies also that the Timor and Aru troughs are underlain by continental basement.
Gravity data Bowin et al. (1980) indicate that gravity data support the seismic refraction results; the Banda Sea basement is oceanic in nature and the Banda Outer Arc ridge and the trenches surrounding it are underlain by continental basement, Gravity modelling by Schluter and Fritsch (1985) crossing the arc near Weber Deep indicates that the Banda Outer Arc and the Aru Trough are underlain by continental basement. The section gives an impression of a deep Moho, at a depth of more than 30 km, beneath the outer arc and the trough. The Banda Sea; Weber Deep and the volcanic arc are underlain by oceanic crust. The deptb to the Moho is less than 10 km under the Banda Sea, 20 km under the volcanic arc and 14 km under the Weber Deep. Gravity modelling by Dwiyanto (1985) crossing the North Banda/Lucipara ridges, Buru/Lucipara
269
basins and Banda Sea indicates a shallow Moho, 8-9 km deep. Sumba appears from a Bouguer gravity anomaly of + 160 to +200 mgal to be underlain by a continental type crust. The crust was estimated by Chamalaun et al. (1981) to be at least 24 km thick. Chamalaun et al.‘s (1976) gravity studies in Timor show that the island is characterized by an unusually steep positive gravity gradient northward. He interpreted this gradient to be caused by a sharp transition from continental crust on the southeastern side of the island to an oceanic crust on the northwestern side. Preliminary computer modelling shows that a sharp change is obvious at appro~mately the northern coastline of Timor from a density of 2.7-3.4 g cmm3, indicating that beneath Timor the crust is thick and under the Wetar Strait the crust is thin. Chamalaun et al.‘s tectonic implication is that subduction is located along the northern side of the island. Magnetic data Bowin et al. (1980) showed the presence of magnetic lineations in the South Banda Sea and indicated that they are of possible sea-floor spreading origin. No assigned geomagnetic polarity ages were given; however, they are compared with similar trending magnetic lineations from the Argo Abyssal Plain, N60-70” with M25-22 magnetic polarity ages (147-142 Ma). Bowin et al.‘s study indicated that the magnetic anomalies of the Australian continental shelf are generally of longer wavelength and usually of lower amplitude than the oceanic crustal anomalies. The appro~mate boundary between the region of low amplitude magnetic anomalies and crust having large-amplitude anomalies is at the inner margin of the Banda Outer Arc. Lapouille et al. (1985) reported the presence of magnetic lineations in the South Banda Sea. Their studies also covered the North Banda Sea. Assigned geomagnetic polarity ages for the South Banda Sea are between Ml4 (131 Ma) and M6 (121 Ma), whereas for the North Banda Sea they are between M3 (117 Ma) and MO (112 Ma). Similarly, Lee and McCabe (1986) show the same lineation trend for the Banda Sea. Both observa-
H.M.S.
270
HARTONO
II
0’
0’
..
mw KALIMANTAN
‘m
.CGZ??i-,
20
2’
7
40
40
6’
69
Q
8”
S=
100
10’
IP
120
r
ARGO
14’
le.*
ABYSSAL
I44
PLAIN
69 1140
I (6’
It@.*
1205
122*
124*
126”
128-
1304
1321
138
IO
Direction
and
Heatflow
values
degrees
---ACMES
Magnetic
lineation
0
Location
of
in
dredge
of
motion
mW me2; and
and
, In
geomagnetic
age
of
formotion
parenthesis time
in heotflow
units
(peal
cm-‘)
scale
site
Fig. 2. Map of the Banda Sea and adjacent regions showing magnetic line&ions (labelled MO through M25), pal~ma~etic rotations shown by the arrows, dredge location and heat flow data (in parentheses). Direction and degree of motion and age of formation from Haile (1987) Otofuji et al. (1979), Chamalaun et al. (1981) and Wensink et al. (1987). Heat flow data from Anderson et al. (1978), Bowin et al. (1980) and Van Gool et al. (1987). Magnetic lineations and geomagnetic time scale from Heirtzler et al. (1978) and Lapouihe et al. (1985). Dredge sites: Dl = basalt, 0.40 f 0.01 Ma; D5 = phylhte, 22.50 + 0.5 Ma; D8 = pyroxene andesite, 6.16 f 0.08 Ma; D9 = pyroxene andesite, 7.03 f 0.17 Ma; DlO = amphibolite, 10.5 f 0.1 Ma; D16 = pyroxene andesite, 0.75 f 0.02 Ma; D19 = metadiabase, 8.75 f 1.48 Ma (data from Silver et al., 1985).
tions show that the Banda Sea oceanic crust becomes younger towards the north. It is remarkable that there is an absence of magnetic Iineations in the area between the North and South Banda seas which coincides approximately with the Banda ridges. Silver et al. (1983,
identified the Banda ridges as continental fragments based on dredge samples from the ridges which are of continental provenance. Also, volcanic rocks related to tbe Banda Volcanic Arc have been obtained from the dredge samples. Some of the magnetic lineation lines cross the Banda
THE
SOKJTHWST
ASIAN
CONTINENTAL
NARGIN
IN THE
BANDA
ridges, which indicates that the nature of the basement inferred from the magnetic lineation data and the dredge samples are incompatible. The presence of magnetic lineations in the Banda Sea has not been unanimously accepted by geologists working on East Indonesian geology. However, there is general acceptance that the Banda Sea is underlain by an old oceanic crust. Land geological, paleomagnetic and pakontological data The geology of Timor is critical to the understanding of the collision processes between the Banda Volcanic Arc and Australian crust. The important features of Timor geology may be summarized as follows. (1) Overthrust structure-from geological maps published by the Geological Survey of Indonesia (Rosidi et al., 1979), it can be observed that an important tectonic event took place during middle Tertiary time when Australia began to collide with the Banda Volcanic Arc. This event is reflected on the maps, which clearly show overthrust structures. The overthrust units range in age from Permian to early Tertiary and are thrust over formations that range from Permian to Jurassic in age. (2) Australian origin of the ~l~h~ons-it is now generally accepted that the over-thrust units or the allochthons are of Australian origin. This conclusion is based mainly on paleomagnetic results (Wensink et al., 1987) and paleontology, in which brachiopods from the allochthonous Maubise Formation has Australian affinities (Archbold and Barkham, 1988). The autochthonous rocks are of Australian origin. Dredged samples from the Banda Sea Silver et al. (1985) analyzed dredge samples from the Banda and Lucipara ridges (Fig. 2) and these revealed continental basement and volcanic rocks. The continental basement rocks are elastic sedimentary and metamorphic rocks. Whole-rock IS-Ar dating yielded an age of 22.5 f 0.5 Ma for a phyllite, and 10.8 f 0.1 Ma for an ampbibolite. These basement rocks are similar to those found
SEA
AREA
271
in nei~bou~ng areas: Buton, eastern Sulawesi, Seram, Misool, Banggai-Sula and Irian Jaya. The volcanic rocks from the Banda and Lucipara ridges and from Gunung Api north of Wetar are talc-alkalic andesites. Based on their alkalinity, the Banda Sea volcanics are similar to those from Damar rather than to voleanics from the other island volcanoes. Radiometric ages of the Banda Sea volcanic rocks are consistent with ages of volcanic rocks from the Banda Volcanic Arc. The chemical composition of the volcanic rocks also indicates contamination by continental silicic rocks. Heat flow data Heat flow data in the Banda Sea are from Bowin et al. (1980), Anderson et al. (1978) and Van Go01 et al. (1987). Generally the Banda Sea has a low heat flow, between 0.4 and 1.7 HFU (Fig. 2). Some higher heat flow values (3.21 and 4.19 HFU) in the area are interpreted to be the result of faulting. The unusual low heat flow values and ocean depths are compatible with the Banda Sea being old oceanic crust. Collision between the Banda Vokanie Arc and Australia The Banda Volcanic Arc is an eastward extension of the Sunda Volcanic Arc (Sumatera, Java, Bali, Lombok, Sumbawa, Flores) and is composed of the volcanic islands of Damar, Teen, Nila, Serua, Manuk and Banda Api. It has an arcuate shape. Internally it is bounded by the Banda Sea oceanic basement and externally by intervening seas (Savu Sea, Wetar Strait and Weber Deep) which are underlain by oceanic crust. The intervening seas are bounded on one side by the Banda Volcanic Arc and on the opposite side by a row of non-volcanic islands (Sumba, Timor, Tanimbar, Kai and Seram) which form the Banda Outer Arc. Australian continental crust Iies to the south of Timor and extends to the east northward to Irian Jaya. Seismic refraction studies and gravity modelling have delineated the extent of the crust to the inner margin of the Banda Outer Arc.
H.M.S. HARTONO
272
The
arc-continent
subduction sion
of oceanic
commenced
reached
collision
and
crust.
when
collided
was preceded Arc-continent
the Australian with
the
by colli-
continent
Banda
Model
Overthrust
The oldest
age of igneous
rocks
in the Banda
Volcanic
Arc, which occurs in the island
consists
of 12 Ma old granite
the minimum The
Australia
which
started
gan to separate
age
would
Position
of
models
Origin of
subduction
geological
north of
autochthonous
Timor
a result of abortive
date
upthrust
north of
all geological
Timor
Australia,
of subducbe
to move northward from Antarctica.
of the collision
units from Australia rifting
of Wetar,
should
age of commencement maximum
1
Characteristics
Volcanic
Arc.
tion.
TABLE
the
formations
time
when it be-
According
units are from
except post-erogenic
Imbricate
to the
south of
mixture
Timor
slab which later becomes
of scraped
and subducted mtlange
magnetic polarity time scale this began during reversal 34 (= 82 Ma). Collision between the Banda Volcanic Arc and the Australian when Australia transition
from
continental crust proper occurred entered the subduction zone. This subduction
of oceanic
crust
be-
parts remained close to the northwestern Australian continent. They all moved northward together and collided with the Banda Volcanic Arc during
neath the volcanic arc to actual collision between the volcanic arc and continental crust took place about 3 Ma ago, based on radiometric ages of
collision
volcanic
crust. The continued northward caused continued compression
rocks contaminated
by continental
crust
from Wetar and Atauro. Dates of similar volcanic rocks from the easternmost part of the arc, from Ambon and Kelang, indicate that collision took place there at about the same time (Abbott and Chamalaun, Different
1981). tectonic
interpretations
of the colli-
sion come from different interpretations structural style, environment of deposition
of the of the
sediments, and seismic reflection/refraction data from Timor. Three models prevail: overthrust, upthrust and imbrication/mClange models. The relationship elements
of the models with associated is shown in Table 1.
tectonic
Recent studies on fossil brachiopods from the allochthonous Maubise Formation of Timor indicate that the fossils are similar to fossils of the same age from Australia (Archbold and Barkham,
between
the volcanic
arc and
the con-
tinental crust. The collision caused overthrusting of the rifted parts onto the Australian continental
This interpretation structures present Outer Arc.
drift of Australia and overthrusting.
best explains on the islands
the overthrust of the Banda
Origin of the Banda Sea and geological implication Hamilton (1979) considered the Banda Sea to be a marginal sea formed by back-arc spreading that created and caused
extension of the Tertiary volcanics the present separation of the West
Sulawesi Volcanic Arc and the Banda Volcanic Arc. Consequently, the age of the Banda Sea oceanic crust should be young and at least the same age as the oldest volcanic or granitic rocks of the Banda Arc, which the available data show to be 12 Ma. Hamilton indicated that the youthful bathy-
1988). Similarly, paleomagnetic studies on the allochthonous Nakfunu Formation indicate that it
metric
originated from a southern paleolatitude (Wensink et al., 1987). The present author suggests that the allochthonous overthrust units originated on the northwestern Australian margin which was rifted during Jurassic time. The rifting was of short dura-
ing a young age for the Banda Sea basement. New data presented by Lapouille et al. (1985) on the magnetic lineations in the Banda Sea indicate the lineations can be correlated with Ml4 (130 Ma) to MO (113 Ma), and become younger towards the north. Similarly, Lee and McCabe (1986) reported the same magnetic lineation trends.
tion,
presumably,
an abortive
rift, and the rifted
expression
of the Banda
Sea, in particular
the Banda Ridges, is one of the criteria
for propos-
THE SQtTTHuCST ASIAN CONTINENTAL MARGIN IN THE BANDA SEA AREA
ARAFfrRA PLQRES
@
BAND4
a
NORTHERN
-
AUSTRAl.lAN
is nutewurthy
r2w
I2W
122*
AUTO~N?~QNOUS
BANDA
MKRQCQNTINENTS
CONTINENTAL
I3P
ki
UNifS
CRUST
: BUTQN,SUtA,
BURU,
EIANDA 4LLocNTNoNous
BANDARiDSES,
BQCAN,
UNlW
081.
BOUNDARY
that the Iineations are continuous with those in the Argo Abyssal Plain in northwestern Australia which indicate a gradation in lineation ages from M25 (147 Ma) to Ml6 (133 Ma) (Fig. 2). An inference that can be drawn from the magnetic lineatiun data is that oceanic crust from the Argo Abyssal Rain and the Banda Sea were originaIly one continuous oceanic crust. The identities of magnetic Iineations of the Argo Abyssal Plain have been confirmed radiometrically from two DSDP h&es; they establish that the Argo Abyssal Plain owes its origin to sea&xz spreading fveevers, 1984). It
SF.4
SEA
Ac~crrding to Lapouille et al. (1985), the ma& in the northern Banda Sea south of Sula, east of Sulawesi and west of Buru, range from M3 to MO (1X3-117 Ma) and are part of a cantirm~us gradation from the southern Banda Sea. However, the uverzkll geolqkal setting, espe= eially the emplacement of ~~r~~~~ents and their intervening seas, suggests a different interpretation for the origin of the magnetic polarity ages of the northern Banda Sea. I accept the magnetic polarity ages and strikes of the magnetic Iineations of the southern Banda Sea (M14-M& = f31-121 Ma), but suggest that Lapouik’s m&gnetic lineations
H.M.S.
214
SUNDALAND Lote
HARTONO
AUSTRALIA
Mesozoic Arc
Bondo Allochthon
Jurassic
Late CretoceausEarly
Tertiory
Tronscurrent fOtIlf
Arc
Lots Cre toceous
Bonda Volcanic
c
Tertiory
A
Bondo Outer Arc
Austrolio
D
Pliocene -Trench
Fig. 4. Tectonic Australia
moved
allochthons sea-floor
evolution
of the Banda Sea and the Banda Arc between
away from Southeast
Asia. The oldest magnetic
Sundaland
lineation
were formed by abortive rifting and the Timor Sea was initially spreading,
consequence
during
transcurrent
magnetic
reversal
34, Australia
separated
and Banda volcanism
commence.
D. Collision
formed.
A. During late Mesozoic
B. After quiescence
from Antarctica
faulting took place which led to the emplacement
Volcanic Arc. C. Subduction
and Australia.
record is M25 and the youngest and began
time
is MO. The Banda
of plate movements
to move
northward.
by
As a
of the Banda Sea plate and the birth of the Banda
commences
between the Banda Outer Arc and the Banda
Volcanic Arc.
netic lineations from the northern Banda Sea should be considered with reservation. From the relative positions of the southern Banda Sea and Argo Abyssal Plain, it is obvious that some offset should have taken place for the Banda Sea crust to be placed into its present position. The Banda Sea crust should have been shifted eastward by right-lateral transcurrent faulting. The timing of such faulting should be younger than the youngest magnetic lineation age of the Banda Sea, which is M6 (121 Ma), and also the faulting could not have taken place during the magnetic quiet period (MO to 34 = 113-82 Ma). Therefore the maximum age for the faulting is at geomagnetic reversal 34 (82 Ma). The intervening magnetic anomaly zone between Ml6 (133 Ma) and Ml4 (130 Ma) is located
at the position of the Banda Volcanic Arc. The birth of the volcanic arc should be closely related to the transcurrent faulting or the commencement of subduction. In the magnetic polarity time scale between MO and 34 there is the Cretaceous magnetic quiet zone. After this event Antarctica and Australia began to separate, and a spreading center developed between Australia and Antarctica. The oceanic crust north of Australia created by sea-floor spreading should have also participated in the northward motion of Australia. However, the transcurrent displacement and the subsequent birth of the Banda Volcanic Arc or associated subduction zone indicates that the Banda Sea crust apparently did not participate in the northward motion.
THE SOUTHEAST
ASIAN
CONTINENTAL
MARGIN
IN THE BANDA
The oldest Banda volcanic or granitic rocks are 12 Ma, whereas the time when Australia started to move northward is at magnetic anomaly 34 (82 Ma). The time span between magnetic anomaly 34 (82 Ma) and 12 Ma should be the time when the Banda Volcanic Arc came into existence. This difference in time is 70 million years. Eventually other data must be sought to narrow this time lapse in order to more precisely date the birth of the Banda Volcanic Arc. Emplacement
and origin
of mi~~ontinents
in
northern Banda The microcontinental slivers, consisting of Buton, Sula, Buru, Obi, Bacan and the Banda ridges, are present in the northern part of the Banda region. There are two main interpretations presented for the emplacement and origin of the ~cr~ontinents. One is advocated by Silver et al. (1985) and other workers who consider that the microcontinents originated from the northern part of Irian Jaya and were emplaced by left-lateral trancurrent faulting along the Sorong Fault. By this mechanism the northern part of the Banda Sea became closed by emplacement of continental slivers and oceanic basement. The Banda ridges have been specifically called by Silver et al. a submerged and displaced continental borderland. This hypothesis considers the age of the southern Banda Sea to be pre-Tertiary and of Indian Ocean origin. The northern Banda Sea and the intervening seas between the microcontinents were interpreted to be derived from the Pacific Ocean. Because the microcontinents were interpreted to be derived from northern Irian Jaya, they are of Gondwanan origin (Fig. 3). Together, they, or at least the most western parts, collided with the Sulawesi Arc. Structural features supporting this interpretation are the To10 thrust, the Hamilton fault and the thrust fault separating the Sula platform and Sulawesi in the northeastern arm of Sulawesi. The second interpretation advocated by Pigram and Pangabean (1983) considers the microcontinents to have originated from northern Australia, and separation of these continental pieces commenced during Jurassic time. This inte~retation is
275
SEA AREA
based on stratigraphic analysis and the timing of the separation is based on magnetic lineations in the Argo Abyssal Plain with magnetic polarity ages M22-M25 (143-147 Ma). The present author considers that the allochthonous formations of the Outer Banda Arc islands (Timor, Tanimbar, Seram) originated from northwestern Australia (Fig. 4). This assumption is supported by paleomagnetic data (Wensink et al., 1987) from the Nakfunu Formation of Timor. The separation should have taken place after the deposition of the Permo-Triassic Maubise Formation and an Early Jurassic of separation is assumed. The allochthonous microcontinents did not travel far from northwestern Australia; they remained close to the continent. Later these allochthonous microcontinents were thrust southward over the autochthonous Australian continental shelf. From this discussion it is apparent that Timor, Tanimbar and Seram have a different geologic origin and emplacement history than that of the microcontinents of northern Banda (Buton, Sula and Banda ridges). The northern Banda microcontinents were derived from northern Irian Jaya. Buton and Sula including the Banda ridges were displaced quite far by the left-lateral tr~scu~ent faulting along the Sorong fault system. The relationship between Seram and the Banda ridges needs further explanation. The Banda ridges are presently south of Seram; however, they have a different origin and history of emplacement from the allochthonous formations of Seram, Tanimbar and Timor. The Banda ridges probably were emplaced first and subsequently the Banda Sea was closed by emplacement of Seram along the Aiduna left-lateral transcurrent fault and anti-clockwise rotation, Traces of the Sorong fault system that emplaced the Banda ridges has been obliterated by the later left-lateral faulting.
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