doi:10.1144/GSL.SP.1996.106.01.30 1996; v. 106; p. 483-497 Geological Society, London, Special Publications

 Jeffrey F. A. Malaihollo and Robert Hall  

IndonesiaThe geology and tectonic evolution of the Bacan region, east 

Geological Society, London, Special Publications

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The geology and tectonic evolution of the Bacan region, east Indonesia

J E F F R E Y E A. M A L A I H O L L O & R O B E R T H A L L

SE Asia Research Group, Department of Geological Sciences,

University College London, Gower Street, London WCIE 6BT, UK

Abstract: Bacan is located in the zone of convergence between the Eurasian, Philippine Sea and Australian plates. The oldest rocks in Bacan belong to the Sibela Continental Suite and are probably of Precambrian age. The complex includes continental phyllites, schists and gneisses of upper amphibolite facies. Isotopic dating yielded extremely young ages due to interaction with hydrothermal fluids. Juxtaposed against the continental rocks is the mostly unmetamorphosed, arc-related Sibela ophiolite, probably derived from the Philippine Sea plate. Isotopic dating yielded a Cretaceous age with an Oligocene-Miocene overprint. In north Bacan, the oldest formation is the Upper Eocene Bacan Formation which comprises interbedded arc volcanic and turbiditic volcaniclastic rocks, metamorphosed under conditions between the prehnite- pumpellyite and greenschist facies. A similar Lower Miocene sequence, assigned to the same formation, is exposed in south Bacan. The Oligocene Tawali Formation on Kasiruta, NW of Bacan, consists of arc basalts and volcaniclastic turbidites, metamorphosed to zeolite facies. The Bacan and Tawali Formations represent different parts of an arc, active from Late Eocene until Early Miocene, resulting from northward subduction of the Australian plate under the Philippine Sea plate. There is a major Lower Miocene unconformity, representing collision of the Australian continent with the Philippine Sea plate, above which shallow marine limestones of the Lower- Middle Miocene Ruta Formation were deposited. This deposition was interrupted by sudden influxes of volcaniclastic sands, forming the Amasing Formation. The Upper Miocene- Pleistocene Kaputusan Formation, rests locally unconformably on older rocks, and includes three members. The Goro-goro Member consists of arc andesites, originating from four eruption centres, which erupted from Late Miocene to Pleistocene, with the oldest in south and the youngest in north Bacan. The Pacitak Member consists of shallow marine pyroclastic rocks. The Mandioli Member formed fringing coastal reef limestones. The volcanic rocks of this formation were produced by eastward subduction of the Molucca Sea plate. Quaternary basalts are related to movement along the Sorong fault. It is concluded that most of the Bacan region has been part of the Philippine Sea plate since the Cretaceous; volcanic rocks of different ages all have an arc character and chemistry, and lithological variations reflect different positions within the volcanic arc; and there is evidence for continental crust of Australian origin in the Bacan area by the Early Miocene.

Three major plates, the Eurasian, Philippine Sea and Australian plates, converge in the Bacan region (Fig. 1). The Eurasian plate has a complex eastern margin which includes continental and volcanic arc crusts accreted during collision with the Philippine Sea plate (e.g. Rangin 1991). The Philippine Sea plate has a long arc history (Hall et al. 1995a), extending back at least to the Cretaceous, and is currently converging with Eurasia in the Philippines, and overriding the Molucca Sea plate in the south. The Australian plate has been moving northwards since the Cretaceous. During much of the Palaeogene, oceanic crust of the Australian plate was subducted under the southern edge of the Philippine Sea plate (Hall et al. 1995b). A strike-slip fault system, the Sorong fault, subse- quently developed at this plate boundary.

Bacan contains rocks of continental, ophiolitic and arc affinities (Fig. 2), and therefore an under-

standing of the tectonic evolution of Bacan should greatly enhance our knowledge of the development of this area of complex plate boundaries. This paper briefly describes and summarizes the geology of each of the formations in the Bacan region, discusses them in the light of regional tectonic events, and proposes a synthesis of the tectonic evolution of this region. A detailed description of the li thostratigraphy of Bacan (Fig. 3), with chemical and palaeoenvironmental +analyses can be found in Malaihollo (1993).

Geology and tectonic evolution of Bacan

Sibela continental suite

The oldest rocks in the Bacan region form part of the Sibela Continental Suite. The complex includes continental phyllites, schists and gneisses of upper

From Hall, R. & Blundell, D. (eds), 1996, Tectonic Evolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 483-497.

483

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amphibolite facies (c. 600°C, c. 5 kbar), with strong penetrative fabrics indicative of a polyphase deformation and recrystallization history, typical of Barrovian type dynamo-thermal metamorphism (Brouwer 1923; Hall et al. 1988). There is also evidence for retrograde metamorphism associated with post peak-metamorphism deformation. The protoliths were pelites with minor amounts of sandy and carbonate/marly horizons. Whole rock chemistry of the pelites suggests that they were derived from a cratonic area and were deposited on an active margin underlain by continental crust.

Based on regional stratigraphical arguments (Hamilton 1979) and the Pb isotopic signatures of Quatemary volcanic rocks (Vroon 1992), these metamorphic rocks are postulated to be of Precambrian or Palaeozoic age. This suggestion has not been confirmed isotopically, as K-Ar and Ar-Ar step heating results yielded extremely young ages (< 0.21 Ma; Malaihollo 1993), interpreted to be a result of interaction with a hydrothermal fluid carrying fractionated argon which were subse- quently concentrated between the sheet silicate layers, causing an apparent young age. This mech- anism could account for other unusually young ages from high-grade metamorphic rocks in the region (cf. Linthout et al. 1991).

There are highly foliated metasedimentary rocks

in the south Saleh Islands and the SE comer of north Bacan. These rocks have a similar character to the Sibela Continental Suite, particularly in having brown metamorphic biotite which through- out the whole region is found only in the Sibela Continental Suite, but are of lower metamorphic grade.

This paper interprets the Sibela Continental Suite to be derived from the Australian plate, where these rocks formed either part of the Mesozoic-Tertiary passive margin of north Australia or fragments rifted from Australia during Gondwana break-up. Many authors have suggested or implied that the continental rocks in Bacan were introduced into the region via strike-slip motion of the Sorong fault system from the Bird's Head region of Irian Jaya (e.g. Hamilton 1979; Silver et al. 1985) or central Papua New Guinea (Pigram & Panggabean 1984). 87Sr/86Sr isotopic compositions of volcanic rocks (E. Forde, pers.comm. 1993) suggest that the Upper Miocene Kaputusan Formation on south Bacan was erupted through continental crust, indicating the presence of continental material under the Bacan region by the Late Miocene. The presence of a Lower Miocene post-collisional intrusion (the Nusa Babi Monzodiorite) in Bacan suggests the presence of continental crust by the Early Miocene (as discussed below).

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Kasiruta Bacan Possible Tectonic Events

Alluvium Reef ~ Quaternary volcanism related to movements on Sarong Fault. Uplift of Sibela Block.

Thrusting in Halmahera, movement on splays of Sarong Fault.

aputusan m Kapulusan FB Associated subaerial and shallow marine pyroclastic deposits, and fringing reefs,

Goro-goro Mbr Goro-goro Mbr Four eruptive centres which migrated northwards with time. Pacitak Mbr Pacltak Mbr Start of volcanism in Bacan (-7 Ma), Halmahera and Obi (- 12 Ma). Mandioli Mbr Mandioli Mbr

Ruta Formation

Tawali Fm Marikapal Mbr

Tawali Fm Jojok Mbr

Bacon Formation

Initiation of eastward subducfion of Molucca Sea Plate under Halmahera,

Shallow marine carbonate platform [open platform, patch reef, tidal bar and foreslope facies).

Local shallow marine volcaniclastic and littoral deposit,

Collision of Australia and arc on the Philippine Sea plate.

Arc volcanic and turbidites,

Repeated, high concentration, proximal volcaniclastic turbidites, i Metamorphosed to Prehnite- Inner and middle fan facies, Pumpellylte to Lower

Greenschist facies. I

Volcanism related to the northward movement of

Pillow lavas with volcanic arc chemistry. Australia under the Zeolite facies metamorphism.

Philippine Sea plate, I

I Similar sequence in Bird's Head and southern Philippines,

Fig. 3. Cenozoic stratigraphy of the Bacan region with related local tectonic events. Timescale after Harland et al. (1990).

S i b e l a o p h i o l i t e

Juxtaposed against the continental suite is the Sibela ophiolite (Hall et al. 1988). There are two types of material recognized in this suite: ophiolitic rocks, most of which are of lower crustal plutonic and mantle origin (microgabbros, gabbros and serpentinised harzburgites) with few volcanic rocks, and spatially-related rocks including unusual amphibole cumulates and many rocks with magmatic-tectonic fabrics. This dismembered, incomplete ophiolitic complex is mostly un- metamorphosed, although locally there are mylonitic rocks which have been affected by upper amphibolite-lower granulite facies metamorphism (c. 1000oc, c. 5 kbar) related to ductile deformation of hot rocks at or near their place of formation, and local recrystallization in shear zones (Malaihollo 1993).

Chrome spinel compositions suggest that the peridotites may have formed in an arc setting. This is supported by geochemical evidence from cogenetic metagabbros. Petrographic study reveals that most of the cumulate rocks consist mainly of cumulus amphibole with minor pyroxene and inter- cumulate plagioclase, indicating crystallization

under hydrous conditions, implying an arc-related setting. The cumulate rocks in the Sibela ophiolite are very different from those of the Halmahera ophiolite, which consist of olivine, orthopyroxene and clinopyroxene (Ballantyne 1992). The Halmahera cumulates are genetically related to the ophiolite which is interpreted to have formed in a supra-subduction zone, normally associated with the initiation of a subduction zone, from the intra- Pacific subduction (Ballantyne & Hall 1990; Ballantyne 1991). Preliminary Nd-Sm ages on the Halmahera cumulates are Jurassic (M. E Thirlwall, pers. comm. 1992). Ar-Ar and K-Ar dating of the Sibela cumulates and metagabbro (Malaihollo 1993) yielded Cretaceous (97-94 Ma) and Oligocene-Miocene (37-25 Ma) ages, although amphiboles analysed contain excess argon and therefore plateaux ages will be slightly older than closure ages (Lanphere & Dalrymple 1976). The Cretaceous age may be linked to volcanic activity recorded in the east Halmahera ophiolite (94- 80Ma; Ballantyne 1992), suggesting a link between the Sibela ophiolite and the east Halmahera ophiolite. The Oligocene-Miocene ages are related to the Oligocene volcanism (discussed below).

GEOLOGY • TECTONICS OF BACAN, E INDONESIA 487

There are two possible interpretations for the association of ophiolitic and cumulate rocks. The first possibility is that the ophiolitic rocks represent older crust of possibly Jurassic age (cf. east Halmahera-Gag ophiolite) intruded by Cretaceous arc cumulates related to intra-oceanic subduction. This interpretation is supported by chemical similarities between the cumulates and zoned ultramafic complexes, normally associated with arc magmatism. Alternatively, both the ophiolitic and arc cumulate rocks may be related to the same intra-oceanic subduction during the Cretaceous. The age and nature of the original crust upon which the arc was built is unknown.

Juxtaposition of continental and

ophiolitic rocks

The continental and ophiolitic rocks are juxtaposed in the Sibela Mountains. Differences in meta- morphic character indicate that metamorphism occurred before amalgamation of continental and ophiolitic rocks. The continental suite is interpreted to be part of the Australian continental basement, whereas the ophiolitic rocks represent the Philippine Sea plate. Regional mapping shows that since the Early Miocene, rocks from these two plates have the same geological history, indicating an Early Miocene collision (Hall et al. 1995a). There are three possibilities for the mode of juxta- position of the continental and ophiolitic rocks: (1) the Sibela Mountains represent a suture zone resulting from collision between the Australian and Philippine Sea plates; (2) the continental rocks collided with the Philippine Sea plate and were subsequently translated by strike-slip faulting as part of a composite fragment to their present position; (3) the continental rocks remained part of the north Australian Margin until the late Neogene and were then translated by strike-slip faulting on the Strong fault into the region. These possibilities are discussed below.

erupted in an arc setting. Turbiditic rocks were deposited by dilute, low concentration, possibly distal or low energy currents. The Bacan Formation was affected by a thermal event at c. 15 Ma.

In south Bacan the oldest rocks dated are a very similar sequence of Early Miocene age. These interbedded volcanic and volcaniclastic turbidite rocks were metamorphosed to prehnite- pumpellyite facies (c. 240-330°C, c. 2 kbar) due to burial metamorphism, with local hydrothermal alteration. Whole rock and mineral chemistry indicates an arc origin for the volcanic rocks. Graded volcaniclastic arenites indicate deposition by turbidity currents which dissipated their energy as they travelled down slope. Isotopic dating shows that this formation was affected by a thermal event atc. 8Ma.

Undated metabasites in the Saleh Islands were metamorphosed at conditions between upper prehnite-pumpellyite and lower greenschist facies (c. 250-360°C, c. 4 kbar). The metamorphic character of these rocks suggests static, regional metamorphism. Geochemical and lithological evidence from the metabasites indicates that they represent an arc-related calc-alkaline sequence, possibly formed in a backarc, and they have similar chemical characteristics to the Upper Eocene and Lower Miocene sequences.

The Upper Eocene, Lower Miocene and the Saleh Island sequences are similar in all aspects, except age. More dating is needed, although this may prove difficult, since despite examination of > 200 thin sections, only four samples were found to be suitable for biostratigraphic or isotopic dating. This is attributed to lack of fossils in the original sediments, metamorphism which destroyed fossils, and alteration by thermal overprints of the volcanic rocks. The rocks assigned to the Bacan Formation could represent either different parts of a single, continuously active arc, or separate arcs active at different times between the Late Eocene and Early Miocene.

The Bacan Formation

In north Bacan the oldest rocks exposed belong to the Upper Eocene Bacan Formation. This comprises interbedded basic-intermediate volcanic and volcaniclastic turbiditic rocks. The Bacan Formation was folded and subsequently meta- morphosed under conditions transitional between the prehnite-pumpellyite and lower greenschist facies (250-330°C, c. 2 kbar upwards), with characteristics of burial metamorphism. Hydro- thermal alteration may be related to the Neogene Kaputusan volcanism. Mineral and whole rock geochemistry indicates that the volcanic rocks were

The Tawali Formation

The oldest rocks on Kasiruta belong to the newly defined Tawali Formation, named after the island of Kasiruta (also known as Tawali Besar). Rocks forming part of this formation were originally assigned to the Bacan Formation (Yasin 1980), but here they are assigned to a new formation because of differences in lithology and the character of metamorphism and deformation. The lower part of the Tawali Formation consists of basaltic pillow lavas and interbedded sediments (Jojok Member), and it is overlain by fossiliferous volcaniclastic turbidites (Marikapal Member). This formation is affected by burial metamorphism transitional

488 J.F.A. MALAIHOLLO & R. HALL

between low and high temperature zeolite facies (c. 180°C, < 2 kbar). Whole rock chemistry indi- cates that the Lower Oligocene basalts are highly differentiated arc lavas, erupted in a deep, open marine environment above the CCD. The Upper Oligocene volcaniclastic rocks are products of repeated, high concentration, proximal turbidity currents with associated slumped deposits.

The Tawali Formation is interpreted as an equivalent of the Bacan Formation although there are several differences between them. There is an apparent lack of Oligocene rocks on Bacan, although as noted above, only a few samples of the Bacan Formation have proved dateable. There are also differences in lithology and metamorphic character (particularly the lower grade of meta- morphism suffered by the Tawali Formation rela- tive to the Bacan Formation), and trace element differences between the Tawali and the Bacan Formations. Rocks of similar lithology and age to the Tawali Formation have now been identified throughout the Halmahera-Waigeo region (the Tawali Formation in Halmahera, the Anggai River Formation in Obi and the Rumai Formation in Waigeo; Hall et al. 1991). The Oha Formation of Halmahera is probably not of Upper Cretaceous- Eocene age as previously suggested (Hakim & Hall 1991), but is interpreted as part of the same arc as the Bacan Formation. Despite differences, all these formations range in age from Upper Eocene to Upper Miocene and they all pre-date the c. 22 Ma unconformity (discussed below) interpreted to be related to the arrival of the continental crust in the region. Differences in lithology could be explained if the different formations represent different parts of the arc (e.g. arc slopes, backarc basin, forearc slopes). Differences in the grades of metamorphism

could be a function of different structural positions during collision with the Australian continent or different thermal environments due to different positions in the pre-collision arc.

Thus, the simplest explanation for all these Upper Eocene, Oligocene and Lower Miocene volcanic arc formations is that they represent arc volcanism at the edge of the Philippine Sea plate due to the subduction of the Indo-Australian plate under the Philippine Sea plate (Fig.4). This is supported by the fact that all of these formations record similar palaeomagnetic declinations and inclinations which are characteristic of the Philippine Sea plate (Hall et al. 1995a; Ali & Hall 1995).

Early Miocene unconformity

Above the Bacan and Tawali Formations, there is a major regional unconformity. This is characterized by a major change in lithological, metamorphic and structural character. Rocks below the unconformity are folded arc volcanic rocks and turbidites, meta- morphosed up to greenschist facies, whereas the rocks immediately above it are mostly unfolded, unmetamorphosed carbonates. The age of the unconformity is estimated to be c. 22 Ma (Early Miocene) throughout the Halmahera region (Hall et al. 1995a).

Nusa Babi intrusive rocks

The Nusa Babi Monzodiorite (NBM) includes quartz-monzodiorite dykes and plugs with associ- ated aplite dykes of monzogranite composition. These intrude the Bacan Formation and have been reported to intrude the Sibela Complex (Yasin

Tawali Fm,, Marikapal Mb. & Rumai Fm. (turbidites) Bacan Fm. arc Bacan Fm. & Anggai River Fm,

~ ~ (forearc turbidites

(back-arc pillow basalts) Sibela Continental Suite

. X ~ 4 × × X X X ,

Philippine Sea plate Australian plate

No Scale Intended

Fig. 4. Different positions, in the pre-collisional stack, of the volcanic formations in the Bacan-Halmahera-Waigeo region which were part of the Late Eocene-Early Miocene arc.

GEOLOGY & TECTONICS OF BACAN, E INDONESIA 489

1980), but do not intrude the Ruta and Amasing Formations. The NBM is locally altered, possibly due to auto-metasmorphism. Magma evolution was primarily controlled by fractionation and the addi- tion of water. Whole rock geochemistry indicates that the NBM is of plutonic arc or collisional magmatism origin. K-Ar dating yields an Early Miocene age (Baker & Malaihollo 1996). The geo- chemistry, isotopic age and the fact that the NBM intrudes the Bacan Formation indicate a post- collisional origin, probably related to the arrival of the Sibela Continental Suite in the region.

The Saleh Diorite

The Saleh Diorite intrudes the Sibela Continental Suite in the Saleh Islands and is juxtaposed against it in central Bacan. This intrusion consists of amphibole-bearing diorite and micro-diorite. Locally these rocks have suffered low temperature alteration, probably due to auto-metamorphism, and subsequent hydrothermal alteration. Whole rock geochemistry indicates that the Saleh Diorite is different from the NBM and is of arc or post- collisional origin. K-Ar dates suggest the c. 15 Ma age is related to the initiation of the Halmahera arc.

The Ruta and Amasing Formations

The Lower-Middle Miocene Ruta Limestone lies unconformably above the Bacan and Tawali Formations. There are four microfacies recognized in the Ruta Limestone: skeletal wackestones- packstones, algal boundstones, foraminiferal packstones and bioclastic-lithoclastic packstones. These represent deposition on an open platform, a platform margin build-up (patch reef), a tidal bar downslope from a patch reef (open platform) and foreslope talus respectively, all forming part of a shallow marine carbonate platform. The deposition of the Ruta Formation began in the Early Miocene, predominantly as open platform, platform margin build-up and foreslope talus facies• Between the Early~ Miocene and Middle Miocene, deposition was locally interrupted by sudden and high influxes of volcaniclastic material forming sand- stones of the Amasing Formation. Three facies in the Amasing Formation have been recognized: shallow marine with storm horizons, shoal or estuarine, and beach deposits. These facies represent shallowing upwards from an open carbonate platform to a beach environment. Deposition of the Ruta Formation continued until the Middle Miocene and its upper part is domi- nated by the tidal bar facies. There was widespread development of carbonate platforms at this time throughout the Halmahera-Waigeo region (Hall et al. 1995a), in north Irian Jaya (Pigram & Davies 1987; Pigram et al. 1990) and in the Philippines (Mitchell et al. 1986). Widespread carbonate platform development suggests that most of the Bacan-Halmahera-Waigeo region was in a quiet tectonic setting, in an equatorial position and was part of an extensive shallow marine area.

In north Irian Jaya, the Middle Miocene Moon Volcanics (Pieters et al. 1989) may be linked with the Maramuni arc of Papua New Guinea which Dow (1977) suggested represented post-collision subduction reversal. However, the character and distribution of this volcanism is still poorly known and if an arc existed, it may have been limited to the eastern part of New Guinea (Ali & Hall 1995).

The Kaputusan Formation

The Kaputusan Formation was deposited above the Ruta and Amasing Formations. It consists of three members: the Goro-goro Volcanic, the Pacitak Volcaniclastic, and the Mandioli Limestone Members• Field mapping and aerial photographic interpretation indicates that the Kaputusan Formation rests directly upon the Bacan Formation in some areas implying an unconformable contact at its base.

The Goro-goro Member consists of two- pyroxene andesites (TPAN), hornblende-pyroxene andesites (HPAN), hornblende andesites (HBAN), and hornblende-biotite andesites (HBIAN). Associated with the volcanic rocks are subaqueous pyroclastic flows with related base surge deposits. The petrography, mineral chemistry, whole rock major and trace element chemistry of these rocks are typical of island-arc volcanic rocks. They are mostly fresh, akhough locally they are metamor- phosed to zeolite facies, interpreted as due to hydrothermal activity. Magma diversification of the Goro-goro Member was achieved mainly by fractionation of plagioclase, pyroxene, Fe-Ti oxide, amphibole and biotite, with evidence for replenish- ment, immiscibility, resorption and assimilation. Magmatic conditions were < 8 kbar and 2-10 wt.% H20 with PH '-' < Ptotal" The Goro-goro Member • 2 U includes at least four eruption centres (as defined by bulk trace element ratios): in south Bacan (mostly HBIAN), the Goro-goro area (mostly TPAN), and the north Mandioli and the Kaputusan areas (largely HPAN and HBAN). The Mandioli group are shoshonitic rocks, which may indicate eruption in an extensional setting within a domi- nantly convergent zone, such as a strike-slip zone where similar rocks commonly occur (e.g. Ellam et al. 1988). K-Ar ages indicate that these centres ~ were erupting from the Late Miocene (c. 7.5 Ma) to the Pliocene (c. 2.2 Ma). Each of these centres appears to have been active for c. 2 Ma, with a history of multiple eruptions. The Upper Miocene- Upper Pliocene Pacitak Member consists of

490 J.F.A. MALAIHOLLO ,~ R. HALL

reworked pyroclastic and volcaniclastic material from the Goro-goro Member, deposited in an oxygen-rich, nearshore, shallow marine environ- ment. The Upper Miocene-Lower Pliocene Mandioli Member consists of wackestones and packstones which formed fringing coastal reefs. The Kaputusan Formation is interpreted to be the product of the eastward subduction of the Molucca Sea plate under Halmahera, and can be correlated with the Weda Group of Halmahera and the Woi Formation of Obi (Hall et al. 1995a; A l i & Hall 1995).

Quaternary deposi ts

Although currently there are no active volcanoes on Bacan, Quaternary volcanic rocks are present. These are fresh olivine-, pyroxene- and plagio- clase-phyric basalts. Mineral and whole rock geo- chemistry indicate that these are arc basalts which erupted through a quartz-rich basement (Sibela Continental Suite). There is evidence of a within- plate basalt component from whole rock and mineral chemistry, possibly indicating magmatism related to movements along the sinistral Sorong fault. This is supported by the linear distribution of the volcanic centres, and in accordance with the view of Silitonga et al. (1981). Detritus from the Sibela Continental Suite can be found solely in the Quaternary deposits, indicating that it has become available for erosion only recently, suggesting extremely fast rates of uplift (c. 1 mm a-l).

Discussion

The history and motion of the Philippine Sea plate has been complex, although recent palaeomagnetic results are contributing to a clearer picture. The work of Hall et al. (1995a, b) suggests that between c. 50-40 Ma the plate experienced a c. 50 ° clock- wise rotation with a southward translation; no rotation between 40 to 25 Ma, and from 25 to 0 Ma the plate rotated clockwise 35 ° with northward translation. The first rotation may be related to the change of motion in the Pacific plate (Clague & Jarrard 1973) at about 42 Ma. Australia was moving north throughout the Tertiary and we interpret the regional unconformity at c. 22 Ma as the result of collision of Australia with a volcanic arc at the southern edge of the Philippine Sea plate (discussed below). This may have caused or contributed to renewed clockwise rotation of the Philippine Sea plate.

Pre-Tertiary and early Tertiary evolution of Bacan

There are two types of pre-Tertiary rocks in Bacan: continental crust and ophiolitic/arc material. The continental crust is presumably of Australian margin or micro-continental origin; north of this margin was oceanic crust. Ophiolite/arc material in Bacan records Cretaceous and Oligo-Miocene ages, both of which can be correlated with ages recorded in Halmahera which now forms part of the

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Early Miocene Fig. 5. Simplified tectonic setting of the Bacan region during the Late Palaeogene-Early Miocene. The oceanic crust of the Australian plate was being subducted under the Philippine Sea plate. The Bacan region moved from arc-forearc (Late Eocene) to a backarc (Oligocene) setting and returned to an arc-forearc position (Early Miocene).

GEOLOGY ~ TECTONICS OF BACAN, E INDONESIA 491

Philippine Sea plate. This implies that the ophiolitic rocks of Bacan were part of the Philippine Sea plate since at least the Cretaceous. Plate tectonic recon- structions show that almost all the oceanic floor of the Mesozoic west Pacific has long since been subducted. The ophiolites of Halmahera and Waigeo, which represent the oldest parts of the Philippine Sea plate, were formed in an intra- oceanic arc setting (Ballantyne 1991, 1992) and the Cretaceous Halmahera volcanic arc was situated at sub-equatorial latitudes (Hall et al. 1995a). There is no lithostratigraphic evidence on Bacan for the history of the region between the Cretaceous and Late Eocene.

4 5 - 2 2 M a

Between the Late Eocene and Early Miocene there was extensive arc activity throughout the Bacan- Halmahera region. Sukamto et al. (1981) suggested that Oligocene volcanism in the Bacan-Halmahera region was the result of west-dipping subduction from the Pacific side, implying that the region lay on the Eurasian plate. Recent palaeomagnetic evidence (Hall et al. 1995a) from the Tawali Formation shows that the region was part of the Philippine Sea plate. Oceanic crust of the Australian plate was subducted northwards, forming an arc at the southern margin of the Philippine Sea plate and fragments of this arc extend from northern New Guinea, through the Bacan-Halmahera-Waigeo region, into the east Philippines (Rangin et al. 1990; Hall 1995; Hall et al. 1995a). The Oligocene Tawali Formation is interpreted as part of this volcanic arc.

The Upper Eocene to Lower Miocene Bacan Formation is genetically related to the Tawali Formation and represents temporal and spatial variations of the same arc (Fig. 5). There are two possible correlatable volcanic arcs in the east Indonesia-west Pacific area: the Oligocene- Miocene volcanic rocks of Irian Jaya and Papua New Guinea (e.g. Pigram & Davies 1987) or the Eocene-Oligocene Palau-Kyushu remnant arc (Karig 1975; Sutter & Snee 1980). The Palau- Kyushu and the West Mariana ridges are associated with opening of the Parece Vela Basin (at 30- 17 Ma) and these remnant arcs are attributed to subduction of Pacific ocean (e.g. Uyeda & Ben- Avraham 1972; Seno & Maruyama 1984) and are therefore different from the arc represented by the Tawali Formation. Correlation of volcanic rocks in Bacan and the Bird's Head was suggested by Van Bemmelen (1949) and Verstappen (1960). The arc volcanic rocks of north Irian Jaya, for example the Arfak (Ratman & Robinson 1981; Pieters et al. 1982), Batanta (Sanyoto et al. 1985), Yapen (Atmawinata et al. 1989) and Mandi Formations

(Pieters et al. 1989) and Papua New Guinea, for example the Bismarck Volcanic Province (Dow 1977), are interpreted to be related to subduction between the Australian and Pacific plates (Dow 1977; Pigram & Davies 1987) and may be a continuation of the Bacan-Tawali Formation volcanic arc.

22 M a u n c o n f o r m i t y

A collision between the Australian margin and an island-arc has been implied or suggested by many authors (e.g. Dow 1977; Jaques & Robinson 1977; Pieters et al. 1983; Pigram & Davies 1987), although suggestions of the age of this collision vary. Pigram & Symonds (1991) review estimates of its age ranging from Eocene to Late Miocene; for east New Guinea they argue for a collision un- conformity at c. 30Ma. Charlton et al. (1991) tentatively suggested a mid-Oligocene age for collision on Waigeo, based on a poorly defined Upper Oligocene age of the Mayalibit Formation. However, new isotopic dating of volcanic rocks from this formation indicates a narrow age range, of uppermost Oligocene-lowermost Miocene (our unpublished results). At the south end of Mayalibit Bay on Waigeo these volcanic rocks dip at up to 40 ° beneath Miocene limestones, suggesting they are below the unconformity.

The presence of the Australian continental rocks on Bacan could be due to: (1) Early Miocene collision; (2) Early Miocene collision followed by Neogene strike-slip translation; or (3) Pliocene or younger pure strike-slip translation. Although there is no indisputable evidence, the proximity of the Sibela Continental Suite to the arc, interpreted to be the result of subduction of the Indo-Australian plate under the Philippine Sea plate, and the juxta- position of the continental rocks with the ophiolite interpreted to represent Philippine Sea plate material favours a collisional interpretation (1 or 2). The character of the Nusa Babi Monzodiorite, suggesting post-collisional melting of a continental source, and the evidence of a continental crustal contribution to Late Miocene volcanic rocks also suggest the presence of a continental basement beneath central and south Bacan by the early Neogene.

On Bacan there is a regional unconformity of Early Miocene age which is interpreted as resulting from the collision of Australian and Philippine Sea plates (Fig. 6). There is a change from folded, metamorphosed arc rocks to unmetamorphosed carbonates and a possible stitching intrusion of Philippine Sea plate rocks (the Bacan and Tawali Formations) and Australian plate rocks (Sibela Continental Suite) by the Nusa Babi Monzodiorite which is of Early Miocene age. This interpretation

492 J.F.A. MALAIHOLLO • R. HALL

/ / / / / / / / / / / / / / / / / / / /

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,/

~ - . i ~ / I r e g i o n ," !

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.- ' . : . : . : . ;- ' . : .- ' . ' . . ' . ' . . : . : . : . : . ; . . + ; . ; . : . : . , . ;

Fig. 6. Simplified tectonic setting of the Bacan region during the Early Miocene. Continuation of subduction culminated in collision and thrusting of the continental crust, under the arc-forearc region of the Philippine Sea plate. Following collision, the Philippine Sea plate rotated westward.

is supported by a similar lithostratigraphy through- out the Bacan-Halmahera region (Hall et al. 1988 1991), where most of the Lower to Middle Miocene is represented by carbonate rocks deposited above Upper Eocene-Lower Miocene arc rocks of Philippine Sea plate affinity and rocks of Australian continental affinity. It is therefore concluded that the collision in east Indonesia occurred in the Early Miocene and the authors agree with Ali & Hall (1995) who suggest that much of the evidence from east Indonesia and New Guinea can be understood in terms of pre-Miocene intra-oceanic arc tectonics, an arc-continent collision at about 22 Ma, and Neogene strike-slip tectonics.

1 9 - 1 5 M a

After the Early Miocene collision, the region was uplifted and deposition of shallow marine carbonates followed. A tectonically quiescent period is envisaged at this time. Localized uplift contributed influxes of volcaniclastic material onto the carbonate platform (Fig. 7), probably derived from the pre-Miocene formations.

In i t i a t i on o f M o l u c c a Sea s u b d u c t i o n a n d

s u b s e q u e n t arc v o l c a n i s m

Northward movement of Australia during the Neogene occurred without subduction at the boundary of the Australian and Philippine Sea plates, which was the left-lateral strike-slip Sorong fault system. However, arc volcanism did result from the eastward subduction of the Molucca Sea

plate under Halmahera (the Philippine Sea plate) at the Halmahera trench (Fig. 8). On Bacan there is a local unconformity of Late Miocene age, and in places the Upper Miocene Kaputusan Formation rests directly on the Bacan Formation. The Kaputusan Formation is the equivalent of other Upper Miocene arc sequences on Halmahera and Obi. Although the oldest isotopic age obtained from the Kaputusan Formation is c. 7.5 Ma, there was a thermal event at c. 15 Ma recorded by wide- spread resetting of isotopic ages in the Bacan Formation and rocks on Obi. The Saleh Diorite, interpreted as a precursor to the Kaputusan volcanics, was intruded at c. 15 Ma. The oldest isotopic ages obtained from volcanic sequences on Obi are c. 12 Ma (Baker & Malaihollo 1996). Thus, initiation of subduction possibly started at c. 15 Ma and the first volcanic products were erupted at c. 12Ma.

L a t e N e o g e n e - R e c e n t

At c. 3 Ma there was more than 60 km shortening between east and west Halmahera, attributed to movement along the Sorong fault (Nichols & Hall 1991). Splays of the Sorong fault running through the Bacan region may have contributed to the ending of Kaputusan volcanism. Quaternary volcanism was later reactivated along the fault splays (Fig. 9). These faults are also responsible for the shaping of Bacan coastlines and the creation of deep basins surrounding Bacan. The culmination of collision processes in the Molucca Sea will eventually result in the transfer of the Bacan-

GEOLOGY & TECTONICS OF BACAN, E INDONESIA 4 9 3

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, i i i i I .%1 i i i . . i f .%:.:

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ioput - " ::iiii

Fig. 7. Simplified tectonic setting of the Bacan region during the Middle Miocene. Northward movement of Australia was accommodated by a transform boundary, which moved north with Australia (Hall e t al . 1995a, b). The Philippine Sea plate was rotating clockwise due to subduction on all sides. There was widespread development of platform carbonates in the Bacan region, away from the plate boundary.

Ha lmahera reg ion f rom the edge of the Phi l ippine Sea plate to the Eurasian margin (Hall & Nichols 1990).

Implications for regional tectonics

This study has p rov ided new geological data and e lucidated the tectonic deve lopmen t of the Bacan

region. In a regional context , it has contr ibuted to an unders tanding of the tectonic his tory o f the convergent zone be tween the Austral ian, Phi l ippine Sea and Eurasian plates in east Indonesia . Some of the mos t impor tan t impl ica t ions of this work are:

(1) Mos t of Bacan is in terpreted as part o f the Phi l ippine Sea plate. Cre taceous l inks are indi- cated by the presence of the Sibela ophioli te ,

~. , & : ~ ! ~ F ~ i ~ . " i ! ~ ! # # ! ~ 7 I " VolcQniccentres

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!:!:!:~:!:..':!:!:..':!:!:..':!:!:!:!:!: ~ i i i i~i i i i i i i i i i i i i i i i i~- : : ; : : : : : ;S t : : : : : : ; : : : : : ; : : : : : : : : : : , + : : : : : : : : : : : : : : : : : : "

: , : , : , : , : , : , ; , : ° : , : , : ° : , : , : , : , : , : ° : , : , , + +

~ ~ - ~ ' 3 ~!~!~!~!ii:':.i~i!i!iiiiiiiii~iii~i~iii~i~i::...... x? ~ ' + ~

Fig. 8. Simplified tectonic setting of the Bacan region during the Late Miocene. Westward subduction of the Molucca Sea plate under the Philippine Sea plate started at c. 15 Ma, probably from the south and moved northward. Volcanicity started in Bacan at c. 7.5 Ma. Shallow marine sequences and fringing reefs formed around volcanic centres.

494 J.F.A. MALAIHOLLO ~; R. HALL

Fig. 9. Simplified tectonic setting of the Bacan region during the Late Pliocene-Quaternary. The Sorong fault system transform boundary cut through the Bacan region, contributing to the cessation of Miocene-Pliocene volcanism. Quaternary volcanism erupted through continental crust along strands of the Sorong fault. Faults controlled coast lines and basins developed between strands of the fault.

and younger connections are indicated by the similarity of Eocene-Oligocene arc sequences which also record a similar palaeomagnetic history (Hall et al. 1995a).

(2) The Bacan and Tawali Formations are most likely to be arc-related sequences produced as a result of subduction of oceanic crust of the Australian plate, probably directly linked to the Indian Ocean, under the Philippine Sea plate.

(3) The Sibela Continental Suite arrived in the Bacan region after collision of Australia or a rifted fragment of Australia with the Philippine Sea plate.

(4) The collision of Australia with the Philippine Sea plate can be dated at c. 22 Ma in the Bacan region.

(5) Younger Neogene-Recent movements on strands of the Sorong fault have modified the geology of the region and permitted emplace- ment of younger volcanic rocks.

Several different models for the tectonic develop- ment of NE Indonesia have been proposed, e.g. the indentor model of Charlton (1986); the terrane model of Silver et al. (1985); and the marginal basin model of Ben-Avraham (1978), and all were based upon limited geological data. The Ben- Avraham model considers Bacan-Halmahera as part of the Australian continent, which is wrong as almost all of the region has been part of the Philippine Sea plate since Cretaceous. Both the Charlton and Silver et al. models imply that the Sibela continental block was translated by post- Miocene strike-slip motion to the Bacan region,

which has been disputed by results of this study. Ali & Hall (1995) and Hall (1995) offer alternative interpretations of the region which incorporate the results of this and other recent geological and geo- physical studies.

In a wider context, if the terrane definition of Howell et al. (1985) is followed, the Sibela Continental Suite, the Sibela ophiolite, the Bacan, Tawali and Kaputusan Formations could be con- sidered as five different tectono-stratigraphic terranes. This may lead to interpretations such as those of Struckmeyer et al. 1993, who apparently assign separate histories to each of these 'terranes'. Karig et al. (1986) argued that rocks in the north Philippines, which have a similar character to the Sibela Continental Suite, Sibela ophiolite, Bacan and Kaputusan Formations, were separate allo- chthonous terranes juxtaposed by strike-slip fault- ing. Whilst not arguing against their interpretation, this study has shown that the juxtaposition of rocks of different character does not necessitate the notion of allochthonous terranes. In the Bacan region, the only fragment that may genuinely be an allochthonous terrane is represented by the Sibela Continental Suite. For the most part, the region has always been at the edge of the Philippine Sea plate, and different 'tectono-stratigraphic terranes' reflect different tectonic regimes at the edge of the plate.

The use of stratigraphical similarities is often used in the literature to correlate tectono-strati- graphic terranes and ultimately tectonic history (e.g. Hamilton 1979; Pigram & Panggabean 1984). Using this technique, the Bacan region could be

GEOLOGY • TECTONICS OF BACAN, E INDONESIA 495

correlated with parts of Papua New Guinea (Oligocene pillow lavas against continental base- ment; Dow 1977), the Zamboanga Peninsula of Mindanao (high grade continental rocks juxtaposed against meta-ophiolite unconformably overlain by Middle Miocene volcanic rocks; Rangin 1991) and north Philippines (see above; Karig et al. 1986). This type of correlation may lead to erroneous interpretations since each of these regions has different tectonic affinities (Bacan was and is still part of the Phil ippine Sea plate; North Papua New Guinea was part of the Philippine Sea plate, but now is part of the Australian plate; Zamboanga and north Philippines were and are still part of the Eurasian plate). Similarities in the stratigraphy are consequences of similar tectonic histories (areas recording the collision of continental with oceanic plate), and one should be cautious in correlation and interpretation of movement of terranes along strike-slip fault. All too often tectonic reconstruc-

tion of a region is based on meagre geological data, such as in the Bacan region, resulting in a diversity of interpretations. Al though this study has provided the most comprehensive geological dataset from the Bacan region, the history of the region in the Early Miocene and before the Eocene is still unclear. Bearing this in mind, one should be cautious in interpreting the tectonic evolution of other, similarly complex, older regions.

This work was supported by NERC award GR3/7149, grants from the Royal Society and the University of London SE Asia Geological Research Group, and financial assistance from Amoco Production Company. We thank S. J. Baker, P. D. Ballantyne, E T. Banner, T. R. Charlton, E. M. Finch, G. J. Nichols and S. J. Roberts for their contributions and discussion, and C. C. Rundle and D. C. Rex for guidance and help with isotopic dating. Logistical assistance was provided by GRDC, Bandung and the Director, R. Sukamto with field support by D. A. Agustiyanto, A. Haryono, and S. Pandjaitan.

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