tectonica del cenozoico de nueva guinea

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los principales descubrimientos de hidrocarburos se encuentran al oeste y este de nueva guinea , esta conformado por la formación sirga a lo largo de la nueva guinea de un evento de colisión de arco - continental .

Transcript of tectonica del cenozoico de nueva guinea

  • AUTHORS

    Andrew Quarles van Ufford Depart-ment of Geological Sciences, University ofTexas at Austin, Austin, Texas 78712

    Andrew Quarles van Ufford earned a B.A. de-gree in geology from Haverford College, Hav-erford, Pennsylvania, in 1989 and a Ph.D. fromthe University of Texas at Austin in 1996. Heworked as a geologist on the Asia-Pacific explo-ration for ARCO until 2000. He obtained anM.B.A. degree from Northwestern Universityin 2002 and has since been manager of planningat Pioneer Natural Resources, U.S.A. in Irving,Texas.

    Mark Cloos Department of GeologicalSciences, University of Texas at Austin, Austin,Texas 78712; [email protected]

    Mark Cloos earned a B.S. degree in geologyfrom the University of Illinois, Champaign-Urbana (1976) and a Ph.D. from the Universityof California-Los Angeles (1981). He joined thefaculty at the University of Texas at Austin in 1981as a structural geologist and is now professorand Getty Oil Company Centennial Chair. Hisresearch interests involve all aspects of thegeology of convergent plate margins.

    ACKNOWLEDGEMENTS

    We thank James R. Moffett of Freeport McMoRan,Inc., whose idea and support made the ErtsbergProject possible. Dave Potter, Steve Van Nort,Dave Mayes, Tom Collinson, Mark Gilliam, GaryOConnor, Kris Hefton, Jay Pennington, KeithParris, Bambang Trisetyo, Peter Sedgwick, ImantsKavalieris, and Art Ona provided discussions anddirect assistance. Special thanks to AmeliusBeanal, Julianus Magal, Etinus Tabuni, Domin-ikus Mom, Tiranus Beanal, Benny Dolame, andthe Tembagapura helicopter operations crewfor assistance during fieldwork. We also thankour University of Texas colleagues Robert E.Boyer, William R. Muehlberger, Sharon Mosher,Rich Weiland, Stefan Boettcher, Paul Warren,Benyamin Sapiie, Eric Beam, Tim McMahon,Eric James, and Stacey Tyburski for discussionsand assistance. Reviews by Eli Silver, W. R.Dickinson, E. A. Mancini, and A. Tripathy aregreatly appreciated. This is Ertsberg ProjectContribution No. 21.

    Cenozoic tectonics ofNew GuineaAndrew Quarles van Ufford and Mark Cloos

    ABSTRACT

    Major hydrocarbon discoveries have been made in eastern and

    westernmost New Guinea, and there is great potential for additional

    discoveries. Although the island is a type locality for arc-continent

    collision during the Cenozoic, the age, number, and plate kine-

    matics of the events that formed the island are vigorously argued.

    The northern part of the island is underlain by rocks with oceanic

    island arc affinities, and the southern part is underlain by the Aus-

    tralian continental crust. Based on regional sedimentation patterns,

    it is argued herein that the Cenozoic tectonic history of the island

    involves two distinct collisional orogenic events.

    The first Cenozoic event, the Peninsular orogeny of Oligocene

    age (3530 Ma), was restricted to easternmost New Guinea.Emergent uplifts that shed abundant detritus resulted from the

    subduction of the northeastern corner of the Australian continent

    beneath part of the Inner Melanesian arc. This collision uplifted the

    Papuan ophiolite and formed the associated mountainous uplift

    that was the primary source of siliciclastic sediments that largely

    filled the Aure trough. Between the Oligocene and Miocene, the

    paleogeography of the region was similar to present-day New Cale-

    donia. The continental crust under central and western New Guinea

    remained a passive margin.

    The second event, the Central Range orogeny, began in the

    latest middle Miocene, when the bulldozing of Australian passive-

    margin strata first created emergent uplifts above a north-dipping

    subduction zone beneath the western part of the Outer Melanesian

    arc. The cessation of carbonate shelf sedimentation and widespread

    initiation of siliciclastic sedimentation on top of the Australian con-

    tinental basement is dated at about 12 Ma. This collision emplaced

    the Irian ophiolite and created the present mountainous topography

    forming the spine of the island.

    AAPG Bulletin, v. 89, no. 1 (January 2005), pp. 119140 119

    Copyright #2005. The American Association of Petroleum Geologists. All rights reserved.

    Manuscript received July 10, 2003; provisional acceptance October 8, 2003; revised manuscript receivedAugust 11, 2004; final acceptance August 30, 2004.

    DOI:10.1306/08300403073

  • INTRODUCTION

    New Guinea is a type locality of island arc-continent

    collision during the Cenozoic (Dewey and Bird, 1970).

    The northern half of the island is underlain by a crys-

    talline basement of ocean crust with arc affinities de-

    rived from the floor of the Pacific basin (Figure 1). The

    southern half of the island is composed of passive-

    margin strata overlying the Australian continental base-

    ment. Debate exists, however, regarding the number

    and timing of the events that created the Central Range,

    the approximately 1300-km (800-mi)-long mountain-

    ous spine of the island.

    Significant oil and gas accumulations have been

    discovered in eastern and westernmost New Guinea,

    and the region has significant potential (McLennan

    et al., 1990). Several significant hydrocarbon accumu-

    lations, including the giant Hides gas field (5 tcf ), have

    already been developed in the fold and thrust belt of

    Papua New Guinea (Figure 1) (Carman and Carman,

    1990, 1993). The highlands in the Indonesian half of

    the island are much less explored. In westernmost

    New Guinea, significant oil production has come from

    the Salawati and Bintuni basins (Katili, 1986, 1991).

    The early 1990s discovery of the Tangguh gas field

    (14+ tcf ) in the Bintuni basin proves that at leastone supergiant gas accumulation is present. All of the

    hydrocarbon discoveries known to us are in structures

    produced during Cenozoic tectonism.

    The outline of the island of New Guinea has been

    described as similar to a bird flying westward (Figure 1).

    As a result, the island is commonly geographically divided

    Figure 1. Tectonic map of New Guinea, modified from Hamilton (1979), Cooper and Taylor (1987), and Sapiie et al. (1999).Spreading centers northwest and southeast of New Guinea are slow (

  • into the Birds Head and Tail regions. The central

    portion of New Guinea, the Birds Body, can be divided

    into four lithotectonic provinces from south to north:

    the foreland basin, the Central Range fold and thrust

    belt, a metamorphic belt with an overlying ophiolite

    complex, and an accreted oceanic arc complex.

    Tectonic models for the origin of the Central

    Range have been based primarily on field relationships

    on the eastern half of the island, the nation of Papua

    New Guinea. In this paper, new biostratigraphic infor-

    mation is reported for the Cenozoic strata near the

    Ertsberg (Gunung Bijih) mining district, which is lo-

    cated in western New Guinea near the Puncak Jaya

    (labeled with box on Figure 1, 4884 m [16,023 ft]),

    the highest part of the Central Range of Papua (for-

    merly Irian Jaya), a province of Indonesia. This infor-

    mation is integrated with published stratigraphic studies

    and other geologic data from across New Guinea to

    identify the major Cenozoic orogenic events that formed

    the island.

    Tectonic Models

    The number and timing of Cenozoic orogenic events

    in New Guinea has been debated. In part, this is be-

    cause the geologic history of the area is complex, be-

    cause it is located near the junction of the Pacific,

    Australian,and Philippine plates. The primary obser-

    vations for which the plate interaction models must

    account are the timing and locations of arc volcanism,

    patterns of deformation, and the ages of regional meta-

    morphism, as well as type and thickness of sedimentation.

    Many theories exist for the Cenozoic tectonic

    evolution of New Guinea. Based on geologic relation-

    ships in eastern New Guinea, Dow et al. (1972) and

    Dow (1977) concluded that there was evidence for two

    distinct orogenic events. To explain the origin of the

    New Guinea trench, the bathymetric depression north

    of the island, and volcanism in the highlands, Hamil-

    ton (1979) proposed that the island is the result of the

    collision of the Australian continent with a south-facing

    arc in the early Miocene, followed by subduction re-

    versal in the middle Miocene (Figure 2A). In a regional

    tectonic synthesis, Kroenke (1984) proposed that there

    were two major arc-continent collisions, and that the

    New Guinea trench is a recently reactivated relict of

    an older oceanic subduction zone (Figure 2B). Milsom

    (1985) proposed that an Eocene collision was followed

    by subduction reversal in the early Miocene to form the

    New Guinea trench, which changed into left-lateral

    transform faulting in the late Miocene (Figure 2C).

    Cooper and Taylor (1987) proposed a doubly dipping

    oceanic plate, separating two active volcanic arcs on

    the Australian and Pacific plates, zippered shut from

    west to east since the Oligocene (Figure 2D). In this

    model, the New Britain trench is a part of the north-

    dipping subduction zone, and the Trobriand trough is

    a relict of the south-dipping zone. Dow and Sukamto

    (1984a, b) and Dow et al. (1988) proposed that New

    Guinea is the product of two distinct islandwide arc-

    continent collisions (Figure 2E). One is in the Oligo-

    cene (Oligocene orogeny), and the other is in the latest

    Miocene (Melanesian orogeny).

    Most of the recent models for the Cenozoic tec-

    tonic history of New Guinea are significantly different

    from the ones just mentioned. Pigram and Davies (1987)

    proposed that New Guinea formed as the result of ac-

    cretion (docking) of at least 32 distinct tectonostrati-

    graphic terranes along the northern Australian margin

    from the middle Oligocene to the Pliocene. Audley-

    Charles (1991) argued for multiple collisions in the

    Miocene. Struckmeyer et al. (1993) present paleogeo-

    graphic maps that explicitly illustrate a complex his-

    tory of accretion. Pigram et al. (1989, p. 199) believe

    that the amalgamation of several arc complexes, oce-

    anic plateaus, and microcontinents began northeast

    of the present-day New Guinea at about 30 Ma and

    continues to the present (Figure 2F). They argued that

    the northern Australian margin changed from a passive

    margin to a foreland basin setting in the Oligocene

    (30 Ma, at least as far west as 135j longitude). Ac-cording to their maps (see also Pigram and Symonds,

    1991), the front of the south-verging foreland thrust

    belt advanced about 100 km (62 mi) in 17 m.y. (at a

    rate of 0.5 cm/yr [0.2 in./yr]). It is important torecognize that the approximately 100-km (62-mi)

    advancement they show for the thrust front can ac-

    count for only a small fraction of the total convergence

    (>1000 km [>620 mi] at 12 cm/yr [5 in./yr] alongan azimuth of 245j) between the Pacific and Aus-tralian plates during this time span (see plate recon-

    structions of Scotese et al., 1988; Jolivet et al., 1989).

    For this tectonic model to be correct, most of the

    PacificAustralian plate convergence must have been

    accomodated somewhere else.

    Hall (1996) presented a kinematic model for the

    region that was based on poles of rotation he deduced

    for the Philippine plate with respect to the Pacific, Aus-

    tralian, and Eurasian plates. He concluded that the island

    of New Guinea margin was one of strike-slip tectonism

    from about 25 to about 5 Ma. This kinematic model

    is not consistent with the pattern of magmatism and

    Quarles van Ufford and Cloos 121

  • sedimentation across the island, but the inferred Philip-

    pine plate movements are compatible with the regional

    tectonic model presented at the end of this paper.

    In sum, tectonic models of New Guinea range from

    one discrete collisional event to prolonged and piece-

    meal accretion. None of these workers have evaluated

    the details of how the change from steady-state subduc-

    tion to collisional orogenesis would manifest itself in

    the rock record; this will be discussed in some detail.

    Our starting point is a reappraisal of the Cenozoic stra-

    tigraphy of western New Guinea, supplemented with

    new data from the core of the highlands.

    Figure 2. Schematic diagramsillustrating various tectonic mod-els proposed for the Cenozoicplate-tectonic history of NewGuinea. (A) Modified from Ham-ilton (1979); (B) modified fromKroenke (1984); (C) modifiedfrom Milsom (1985); (D) mod-ified from Cooper and Taylor(1987); (E) modified from Dowet al. (1988); (F) modified fromPigram et al. (1989).

    122 Cenozoic Tectonics of New Guinea

  • REGIONAL SEDIMENTATION

    Biostratigraphy

    Determining absolute ages from biostratigraphy is some-

    what problematic because correlations have changed over

    time. In their classic regional report on Irian biostra-

    tigraphy, Visser and Hermes (1962, enclosure 7) report

    ages in terms of numbered T-stages (e.g., T26T27).

    More recent workers use lettered T-stages (e.g., Ta2;

    Adams, 1984; Simon Petroleum, 1992, personal com-

    munication). This paper uses Adams (1984) to place

    the lettered T-stage notation in an absolute timescale.

    Dow et al. (1988, their figure 28) proposed a corre-

    lation between numbered to lettered T-stage notations

    that has most of the boundaries as nearly the same age

    as in this paper, but a few differ by as much as 3 m.y.

    (for example, base of T27).

    Regional Cenozoic Stratigraphy

    Stratigraphic columns were compiled from the lit-

    erature for Cenozoic strata deposited on the north-

    ern Australian margin that is now exposed in the

    Lengguru, Irian, Papuan, and Aure fold and thrust

    belts and from the stratigraphy of wells in the Sala-

    wati, Bintuni, and southern Central Range foreland

    basins (Figures 3, 4). Included in the regional strat-

    igraphic synthesis is new data (Quarles van Ufford,

    1996) based on biostratigraphic analyses of the 1700-m

    (5600-ft)-thick section of Cenozoic limestone exposed

    in the glaciated areas near Puncak Jaya (Figure 1), the

    highest mountain peak in all of southeast Asia (Figure 4,

    column R).

    The stratigraphic columns show that during the

    Cenozoic, carbonate shelf deposition occurred across

    most of the southern part of western and central New

    Guinea (Figure 4, columns AT). Along the north-

    westernmost slope and rise of the Australian margin

    (Birds Head and Neck regions), deep-water carbon-

    ates of the Imskin Formation accumulated and are now

    uplifted and exposed in the Lengguru fold belt (Figure 4,

    columns I and N).

    A disconformity, locally overlain by sandstone of

    Oligocene age, is recognized across much of western

    and central New Guinea (Figure 4, columns D, E, H,

    and KT). As many workers have placed great em-

    phasis on this disconformity, we report in some detail

    Figure 3. Tectonic map of New Guinea showing the location of the Cenozoic chronostratigraphic section in Figure 4. See Figure 1for explanation of map symbols.

    Quarles van Ufford and Cloos 123

  • on the nature of the associated siliciclastic sediments

    we found exposed at this stratigraphic level in the core

    of the Central Range near Puncak Jaya (Figure 1).

    Oligocene Siliciclastic Strata in Western New Guinea:Sirga Formation

    The only siliciclastic unit in central and western New

    Guinea between the Eocene and middle Miocene that

    is of sufficient thickness and continuity to warrant for-

    mation status overlies the Oligocene disconformity. This

    unit, the Sirga Formation (Visser and Hermes, 1962,

    p. 8485), is a 10100-m (33330-ft) or so thick quartz

    sand-rich unit (Figure 4, columns D, E, H, and KR).

    This unit was called the Adi Member by Pigram and

    Panggabean (1983) and Pieters et al. (1983).

    The Sirga Formation was deposited during a pe-

    riod of subaerial exposure, as indicated by plant fos-

    sils and coal films in the type locality in the Birds Head

    ( Visser and Hermes, 1962, p. 8485). Near Puncak

    Figure 4. Cenozoic chronostratigraphic cross section. The location is shown in Figure 3. Shallow-marine limestone is defined ascarbonate rock deposited on a shelf (

  • Jaya, Quarles van Ufford (1996) found that the Sirga

    Formation is characterized by (1) a lower 1020-m

    (3366-ft)-thick, nonfossiliferous, fine-grained to gran-

    ule quartz sandstone with cross-beds; (2) a planar, or

    a 12-m (3.36.6-ft)-thick lower boundary with re-

    worked clasts of the underlying Eocene shallow-marine

    carbonate Faumai Formation; (3) a distinctive coal seam,

    up to 30 cm (12 in.) thick, which is present within 1 m

    (3.3 ft) of the base of the formation at two of the seven

    localities where the contact outcrops; (4) a lower con-

    tact, which marks a biostratigraphic gap (only fossil

    zone Tc is missing); and (5) an upper boundary, which

    is gradational more than 20 m (66 ft) and has layers of

    quartz sand grading upward into glauconitic quartz

    foraminiferal sand to glauconitic marly foraminiferal

    packstone and finally into foraminiferal packstone.

    Overall, this section is similar to those described by

    Rossetter (1978), Pieters et al. (1983), Pigram and Pang-

    gabean (1983, p. 78), Brash et al. (1991), and Lunt and

    Djaafar (1991) for the Sirga Formation in the Birds

    Head region.

    The Sirga Formation in the Puncak Jaya region is an

    extremely mature quartz sandstone. Dozens of hand

    specimens were examined with a hand lens to identify

    the best representatives of the different variants of sand-

    stone. A dozen samples were thin sectioned and exam-

    ined petrographically. Four representative samples were

    stained for feldspar and point counted (Figure 5A).

    The samples are clean quartz arenites (>95% quartz).

    Using the criteria of Dickinson and Suczek (1979), all

    of the samples would be classified as derived from a

    craton interior or continental block provenance on a

    QFL ternary provenance plot. Three of the four point-

    counted samples would be classified as from the same

    provenance on a QmFLt ternary provenance plot. The

    fourth sample is very coarse grained, with 28% granules

    of polycrystalline quartz. Using the petrographic cri-

    teria of Dickinson (1985), this sample is inappropriate

    for provenance discrimination.

    Tabular cross-beds are abundant in the Puncak Jaya

    region. After correcting for local bedding tilt caused by

    folding, 64 cross-bed measurements indicate a north-

    easterly flowing depositional current (Figure 5B). This

    indicates a source in the direction of the Australian

    craton. The presence of coal, the glauconitic character

    of the quartz-sand layers, cross-bedding, paleocurrent

    directions, and contact relationships led to the inter-

    pretation that the Sirga Formation near Puncak Jaya

    was deposited in a transgressive fluvial and/or beach

    environment (Figure 6A).

    Oligocene Disconformity

    The disconformity of Oligocene age that is present

    across most of the Australian continental shelf that

    underlies southern New Guinea (Figure 4, west of col-

    umn U) has been given profound tectonic significance.

    Dow et al. (1988, p. 199200) argue the unconformity

    formed as a result of continental basement uplift

    and deep erosional incision during their islandwide

    Figure 5. (A) Ternary petrofacies diagrams of the OligoceneSirga Formation (four samples) in the Puncak Jaya area (Figure 4,column R). Provenance interpretation is modified after Dickin-son and Suczek (1979). Q (total quartz) = Qm (monocrystallinequartz) + Qp (polycrystalline quartz); F = total feldspar; L =unstable lithic fragments; and Lt = L + Qp. Three hundred pointcounts per sample (Quarles van Ufford, 1996). CB = continentalblock. (B) Cross-bedding orientation in the Sirga Formation nearPuncak Jaya.

    Quarles van Ufford and Cloos 125

  • Oligocene orogeny. Pigram et al. (1989, 1990) and

    Pigram and Symonds (1991) argue the Oligocene un-

    conformity in the carbonate section atop the continen-

    tal basement of southern New Guinea resulted from a

    flexural forebulge of the basement (after Jacobi, 1981)

    migrating southward approximately 100 km (62 mi) in

    advance of a landmass emerging from the Pacific basin.

    However, a global phenomenon that must ac-

    count, at least in part, for the origin of the Oligocene

    disconformity is the largest eustatic fall in sea level in

    the Cenozoic (Haq et al., 1987). This event, now well

    dated as between 33 and 30 Ma (Vakarcs et al., 1998),

    involved a sea level fall of about 90-m (300-ft). It has

    been detected in cores drilled into mid-Pacific atolls

    (Schlanger and Premoli Silva, 1986) and is now well

    known to have had a dramatic effect on deep-sea sed-

    imentation patterns in the region (Fulthorpe et al.,

    1996). An Oligocene disconformity, similar to that re-

    corded in the stratigraphy of southern New Guinea is

    present in carbonate strata on the northwest (Apthorpe,

    1988) and northeast shelves of Australia (Davies et al.,

    1989). These nearby areas did not undergo tectonic

    movements in the middle Cenozoic.

    In New Guinea, the Oligocene transgression and

    deposition that followed the sea level low created a

    variable pattern of unconformity because of variable

    preexisting relief and depth of erosion. In the Birds

    Head region, Mesozoic to middle Miocene formations

    onlap the Kemum high (Figure 1), indicating that it

    was a basement exposure and minor source of silici-

    clastic detritus throughout the Cenozoic (Dow et al.,

    1988, p. 3134, 194).

    In the western highlands, the unconformity typ-

    ically spans a range of about 5 m.y. In the Puncak Jaya

    section, which must have been in an outer shelf envi-

    ronment, only one foraminiferal zone (Tc) has not

    been found, and the cessation of deposition may be as

    short as 1 m.y. (Figure 4).

    In central New Guinea around the Arafura high,

    the unconformity is overlain by the lower Miocene

    Darai Limestone and juxtaposes rocks as old as Me-

    sozoic. It appears that the Arafura high was also a

    Figure 6. Interpretedblock diagram for SirgaFormation terrestrial toshallow-marine deposi-tion. (A) Ertsberg miningdistrict depositional set-ting. (B) Regional depo-sitional setting.

    126 Cenozoic Tectonics of New Guinea

  • pre-Cenozoic feature because no Permian to Jurassic

    strata occurred beneath onlapping Cretaceous strata

    or Neogene carbonates. In six out of nine wells that

    penetrate the basement in the Arafura high, more

    than 400 m (1300 ft) of Cretaceous sediments rest

    disconformably on top of what is correlated as either

    the Silurian to Devonian Modio-Brug Formation or Pre-

    cambrian or early Paleozoic Kariem Formation (Visser

    and Hermes, 1962, enclosure 8; Simon Petroleum, 1992,

    personal communication). Part of the Arafura high

    apparently rose hundreds of meters during the Paleo-

    cene as the area was gently tilted during movements

    associated with the opening of the Coral Sea basin

    from 62 to 56 Ma (Weissel and Watts, 1979; Davies,

    1990; Davies et al., 1991). In addition, it is likely that

    the Arafura high was similarly uplifted, and the flanks

    were again gently tilted during the about 3530 Ma

    Peninsular orogeny (discussed below).

    In sum, we conclude that the widespread Oligo-

    cene unconformity in central and western New Guinea

    is primarily the result of the short-lived, but large, sea

    level fall. The Sirga Formation is simply the basal trans-

    gressive unit deposited during sea level rise and does

    record evidence of a major, local tectonic event.

    Source of the Quartz Sand

    The primary source of the quartz grains is uncertain.

    Grains may have migrated from Australia as shore-

    lines transgressed northward. Probable sources for

    quartz are the Kemum and Arafura highs. We empha-

    size that some, and perhaps most, quartz must have

    been released during exposure and dissolution of early

    Cenozoic carbonate formations. Scattered grains and

    thin quartz sand beds are present in Eocene limestone

    in the western Central Range and in the Birds Neck

    region (Figure 4, columns K and P). Quarles van Ufford

    (1996) found that in the Puncak Jaya area, nearly 60 m

    (200 ft) of the approximately 400-m (1300-ft)-thick

    Paleocene to Eocene Waripi Formation dolomites con-

    tains more than 10% quartz. The rest of the Waripi

    as well as the lower part of the Eocene to Oligocene

    Faumai Formation limestones contain scattered grains

    of mostly silt-sized quartz.

    Imskin Formation: Pelagic Carbonate Sedimentation

    Carbonate strata directly indicate that the northern

    and eastern edges of the Birds Head region was a pas-

    sive margin until the middle Miocene. The Imskin

    Formation, a pelagic limestone that grades shelfward

    (south and west) into shallow-water carbonate forma-

    tions (Figure 4, columns I and N), outcrop in the Leng-

    guru fold and thrust belt in the Birds Neck region at

    about 135j300E (Visser and Hermes, 1962, p. 7779;Koesoemadinata, 1978; Pieters et al., 1983; Brash et al.,

    1991). The Imskin Formation consists of marl, chalk,

    chert, and abundant pelagic foraminifera. In the Leng-

    guru area, the Imskin Formation lacks siliciclastic beds,

    except during the period of Sirga Formation deposition

    (Figure 4, column N) (Brash et al., 1991). The regional

    setting in the Birds Head region at the time of Sirga

    deposition is illustrated in Figure 6B.

    Synorogenic Sedimentation on Continental Basement:Central Southern New Guinea

    There are two distinct ages for the initiation of imma-

    ture siliciclastic sedimentation on top of the conti-

    nental basement of New Guinea (Figure 4). The evi-

    dence is (1) an older Oligocene event recorded in the

    Aure trough of easternmost New Guinea and (2) a

    younger late Miocene event recorded across the width

    of New Guinea.

    The regional synthesis reveals that from the Oligo-

    cene to middle Miocene, immature siliciclastic sedi-

    mentation atop Australian basement was restricted to

    easternmost New Guinea, east of 144jE (Figure 4, col-umn Papua New Guinea (PNG) 2 and eastward). The

    oldest synorogenic sediments are found in the Aure

    trough near the Papuan Peninsula. Up to 7 km (4 mi) of

    sediment has accumulated in the depression since the

    middle Oligocene (Te14, 32 Ma; Edwards, 1950;Brown et al., 1975; Slater et al., 1988). They were de-

    rived from sedimentary, igneous, and metamorphic ter-

    ranes exposed in the Papuan Peninsula. Similar sedi-

    mentation occurred northeast of the Papuan Peninsula

    in the Cape Vogel depocenter (Davies et al., 1984).

    Synorogenic sediments deposited on the continen-

    tal basement of western New Guinea are significantly

    younger. Kilometer-thick sequences of siliciclastic

    strata have accumulated in the Salawati, Bintuni, Aki-

    meugah, and Iwur basins (Figure 7) since the latest

    middle Miocene (Figure 4) (Visser and Hermes, 1962,

    p. 8899; Dow et al., 1988, p. 170178). The propor-

    tion of different siliciclastic lithologies varies signifi-

    cantly from basin to basin, and different names have

    been used for the formations overlying middle Miocene

    carbonate strata. The Klasaman, Steenkool, Akimeugah,

    Buru, and Iwur formations are reported to contain shale,

    siltstone, sandstone, lignite, fossiliferous marl, and lo-

    cally, a basal conglomerate containing New Guinea

    Quarles van Ufford and Cloos 127

  • limestone group clasts (Figures 4, 7) (Visser and Hermes,

    1962, p. 9698). The oldest siliciclastic deposits are

    found in the bottom of the Iwur basin of central New

    Guinea (Tf1 stage, 14 Ma; Dow, 1977), but the Tf2age (12 Ma) is when the siliciclastic deposition be-came widespread on top of the continental basement

    of Papua.

    The youngest pulse of coarse synorogenic sedi-

    mentation on top of continental basement appears to

    be slightly younger in Papua New Guinea. The oldest

    strata related to the generation of the Papuan fold and

    thrust belt that caused the imbrication of Darai Lime-

    stone (Hobson, 1986) appear to be Pliocene (Figure 4,

    PNG columns 26). In the Aure trough area, silici-

    clastic sediments derived from the Papuan Peninsula

    region had been continuously accumulating since the

    Oligocene. However, a distinct change to conglomeratic

    nonmarine deposition indicates that a substantial new

    uplift of the source terrane occurred in the Pliocene

    (Era beds, Figure 4, column PNG1) (Slater et al., 1988;

    Davies, 1990; Klimchuk, 1993; Kugler, 1993).

    Synorogenic Sedimentation on Oceanic Basement:Central Northern New Guinea

    In northern Papua, coarse clastic material had accumu-

    lated in the North Coast basin (also known as the

    Meervlakte; Figure 1) since the early middle Miocene.

    These deposits bury most of the collided arc complex.

    The oldest siliciclastic material, the Makats Forma-

    tion, which blankets this oceanic basement, appears

    to be dated as early middle Miocene (1614 Ma;

    Visser and Hermes, 1962, p. 100111). These silici-

    clastic deposits on the north side of the island appear

    to predate, by several millions of years, the beginning

    of widespread synorogenic sedimentation on top of

    Australian continental basement. The Makats Forma-

    tion contains clasts of metamorphic rocks, mica schist,

    [and] slates (Visser and Hermes, 1962, p. 100106),

    indicating deep denudation of the source landmass

    emerging to the south.

    Interbedded clastic and carbonate sediments in the

    Sepik basin of northern Papua New Guinea (Figure 1)

    were deposited on top of deformed sediments and meta-

    morphic rock that correlates with the Owen Stanley

    metamorphic belt that underlies the Papuan Penin-

    sula (Dow, 1977; Doust, 1990). Geochronology of the

    metamorphic belt indicates recrystallization before

    about 25 Ma, in the latest Oligoceneearliest Miocene

    (Hill et al.,1993). Clastic strata (including the poly-

    mictic Amogu conglomerate) of early Miocene age

    accumulated along the southern margin of the Sepik

    basin (near the northern edge of the present-day Pap-

    uan Peninsula). At the same time, carbonate strata

    accumulated in the rest of the basin (Doust, 1990;

    Wilson et al., 1993).

    Miocene Volcanism: Maramuni ArcEastern New Guineaand the Trobriand Trough

    A period of southwest-dipping subduction occurred

    along northeastern New Guinea at the Trobriand

    trough (Figure 1) between 20 and 10 Ma and gen-

    erated the Maramuni arc (Page, 1976; Dow, 1977;

    Figure 7. Simplified chronostratigraphic section since the early Miocene located on the Australian shallow-water carbonate platform andnear the modern southern deformational boundary for the Central Range. Based on Figure 4 and modified from Visser and Hermes (1962).NGLG = New Guinea limestone group; cgl = conglomerate; ls = limestone; ss = sandstone; terr = terrigenous; and v = volcaniclastic.

    128 Cenozoic Tectonics of New Guinea

  • Davies et al., 1984; Davies, 1990). This magmatic arc

    was emplaced into the Australian continental crust east

    of 140.5jE (Figure 3). The westernmost occurrence ofprobable Maramuni intrusives, just west of the inter-

    national border, are undated (marked with a question

    mark in Figure 3) (McMahon, 2000a). Maramuni-age

    igneous rocks farther to the west are only found em-

    placed into allochthonous Pacific plate terranes, the

    Weyland overthrust (Utawa batholith), and the west-

    ernmost Irian ophiolite belt next to the Birds Neck

    (Figure 3) (Dow et al., 1988, p. 149154; McMahon,

    2000b).

    Maramuni arc plutons intruded the largely sub-

    merged belt of deformed and metamorphosed conti-

    nental strata in easternmost New Guinea, but volca-

    nism provided detritus to the Sepik and Ramu basins

    that locally overwhelmed carbonate sedimentation (Wil-

    son et al., 1993). Only in the northwestern Papuan fold

    and thrust belt (Star Mountains region, near the In-

    donesia and Papua New Guinea border) did Maramuni

    arc volcanism become recorded in the middle Miocene

    shelf sediments (Pnyang and Lai siltstones) deposited

    atop continental crust (Davies, 1990). These partly

    volcanogenic strata are located north of the section in

    Figure 3 and are not reported in the wells south of the

    Papuan fold and thrust belt.

    The formation of the Maramuni arc is a distinct

    tectonic event that all tectonic models for New Guinea

    must account. It is the product of a short-lived period

    of subduction at the Trobriand trough. Its northern ex-

    tension is actively being overridden by the Finisterre

    Huon forearc terrane (Silver et al., 1991).

    Middle Cenozoic Tectonism: Westernmost New Guinea(Birds Head)

    The Cenozoic history of the Birds Head region of

    westernmost New Guinea should now be well under-

    stood because of major hydrocarbon discoveries in the

    Salawati and Bintuni basins. The Kemun high was a

    basement exposure and a minor source of siliciclastic

    detritus throughout the Cenozoic. Most importantly,

    the extensive Kais Formation reef complexes along

    the southern and western margin of the Kemum high

    (Figure 3) indicate that rapidly eroding highlands

    were not present near the Bintuni and Salawati basins

    until the latest Miocene (Visser and Hermes, 1962,

    p. 85; Dow et al., 1988).

    Based on the literature known to us, it appears

    that the deposition of the partially siliciclastic Klasafet

    Formation in the Bintuni and Salawati basins of the

    Birds Head region began in the early middle Miocene

    (early Tf1, 1716 Ma; Figure 4, column E and H)

    (Visser and Hermes, 1962, p. 8892; Froidevaux, 1978).

    The only direct structural evidence for Oligocene de-

    formation of strata deposited on the continental base-

    ment of western New Guinea is found near the island

    of Misool (Figure 1; columns A and B in Figure 4). In

    this area, an angular unconformity with up to about

    15j of discordance separates tilted Oligocene ZaagLimestone and older formations from the near-

    horizontal Miocene Kasim marl and Openta Limestone

    (Pigram et al., 1982; Robinson et al., 1988). We believe

    that the Oligocene tilting recorded near Misool, the

    renewed uplift of the Sele high, and perhaps the ini-

    tiation of Klasafet sedimentation are the easternmost

    manifestation of collisional plate interactions occurring

    west of the present-day New Guinea in the Sulawesi

    area (Silver et al., 1985; Bergman et al., 1996).

    TWO OROGENIES: PENINSULAR ANDCENTRAL RANGE

    Regional stratigraphic patterns and fieldwork reported

    in Quarles van Ufford (1996) and summarized in this

    paper indicate that in the vicinity of the Papuan Penin-

    sula, two Cenozoic orogenic events affected the Aus-

    tralian continental basement, but only the younger

    event is evident in central and western New Guinea. In

    eastern New Guinea, two collisional orogenic events

    were separated by an episode of southwestward sub-

    duction along the Trobriand trough, which emplaced

    the Maramuni arc into the Australian continental base-

    ment. In this paper, the orogenies are named for the

    largest mountainous uplift each created: the Peninsular

    Range and Central Range.

    Several tectonic enigmas near New Guinea must

    be considered in any plate model for the history of the

    island. These include (1) the northwestern and south-

    eastern limit of Trobriand troughMaramuni arc mag-

    matism (Figure 3); (2) the origin of the New Guinea

    trench (Figure 1); and (3) the western limit of arc

    magmatism associated with subduction at the New

    Britain trench (near 145jE, Figure 3). The discussionthat follows is a hypothesized sequence of plate-tectonic

    adjustments in the southwest Pacific that account for

    these three issues as well as the geologic history of

    the island as described in this report. The reconstruc-

    tions are based on the fact that the overall motions of

    the Pacific and Australian plates are well constrained

    Quarles van Ufford and Cloos 129

  • (Scotese et al., 1988). They are an attempt to place

    the New Guinea region into a more complete plate-

    tectonic context than is illustrated by reconstructions

    such as those shown in Figure 2. Complete justifica-

    tion requires the analysis of the history of magmatism

    and deformation in the islands to the east and west of

    New Guinea and must be the subject of another paper.

    Major Change in Pacific Plate Motion at 43 Ma

    A major Eocene change in Pacific plate motion is evi-

    dent from the distinct bend in the HawaiianEmperor

    seamount chain that is dated at 431 Ma (Clague and

    Dalrymple, 1989). This change is either the result or

    the cause of the formation of new subduction zones

    in the western Pacific basin (Hilde et al., 1977; Kroenke,

    1984) and corresponds to a large increase in spread-

    ing rate between the Australian and Antarctic plates

    (Veevers et al, 1990).

    North of the equator, the west-dipping IzuBonin

    Mariana subduction system was established. To the

    south, two subduction systems were started. One was

    northeast-dipping at the PapuanRennellNew Cale-

    donian trench system and generated the Inner Mela-

    nesian arc. The other, far to the northeast (1500 km;900 mi), was a southwest-dipping subduction zoneand generated the Outer Melanesian arc that is now

    the inactive New GuineaManusKilinailauSolomon

    trench system (Figure 8A). Whether subduction started

    at the same time at both of the Melanesian arc systems

    is uncertain, but major events along the inner sub-

    duction zone (southern arc) soon caused all the Pacific

    Australian plate convergence to become concentrated

    at the outer subduction zone (northern outer arc).

    Subduction Accretion and Collisional Orogenesis

    Before further synthesis is discussed, it is important to

    differentiate the generally short-lived collisional oro-

    genesis processes from the commonly long-lived, near-

    steady-state, subduction processes of offscraping and

    underplating. The terms subduction and collision have

    been used as synonyms in textbooks and many papers.

    From the perspective of describing fundamental con-

    vergent margin processes, formal differentiation of

    these terms is desirable (Cloos, 1993). We restrict the

    term collision for subduction zone events that lead to

    some kind of change in plate motions and the uprooting

    of crystalline basement or thick-skinned deformation.

    Subduction can (but not always) cause the accumula-

    tion of an accretionary prism that is the product of

    prolonged tectonism (see Cloos and Shreve, 1988a, b

    for discussion of the mechanics of subduction accretion

    and nonaccretion and erosion). Subduction accretion is

    a thin-skinned deformational process that commonly

    occurs steadily for many tens of millions of years,

    forming an accretionary prism without the detachment

    of the underthrusting layer of ocean crust. Offscraping

    widens accretionary prisms, whereas underplating

    thickens them. By themselves, these processes cause

    tectonism but not orogeny (which historically was

    defined as the generation of mountains that are subject

    to erosion). All active subduction zones with large

    accretionary prisms are nearly entirely underwater.

    Accretionary complexes can, of course, become parts

    of mountainous uplifts when they are involved in

    continent-arc and continent-continent collisions. Col-

    lisions, in our definition, involve the jamming of a sub-

    duction zone with consequent deformation involving

    the crystalline top of the descending plate and some

    kind of rearrangement of plate motions. Discrimina-

    tion of subduction, offscraping, underplating, and

    collisional tectonism provides insight into the tectonic

    controls on sedimentation and the significance of ter-

    rane boundaries in New Guinea and elsewhere.

    The basic physics of subduction is obviously related

    to but fundamentally different from collision. Steady-

    state subduction can continue as long as the bulk den-

    sity of the downgoing lithosphere (crystalline crust

    and underlying lithospheric mantle) is greater than the

    bulk density of the underlying asthenospheric mantle

    (Figure 9A). With little modification, it can continue

    for many tens of millions of years and result in subduc-

    tion erosion truncating a margin or subduction accre-

    tion growing an accretionary prism. Collisional oro-

    genesis, however, begins when incoming lithosphere

    that is less dense than the asthenospheric mantle begins

    to turn downward to subduct. Incoming lithosphere

    is positively buoyant when it has a sufficiently thick

    capping of crystalline continental crust or a large oce-

    anic arc complex (Figure 9C). Depending primarily on

    the speed of convergence and other factors as well,

    collisional orogenesis can be prolonged, but it is com-

    monly a short-lived event (a few million years) that

    ends when there is a change in plate motions, a re-

    arrangement of plate boundaries, or both (Cloos, 1993).

    One common manifestation is the contraction of the

    overriding plate in the area of the arc because this

    is a thermally weakened lithosphere. Where oceanic

    arcs are involved in a collision, lithospheric rupture

    and subduction reversal, with the line of the old arc

    becoming the axis of the new trench, are common.

    130 Cenozoic Tectonics of New Guinea

  • Three episodes of subduction reversal occurred in the

    New Guinea region in the Cenozoic. Once, it occurred

    immediately following the jamming of a subduction

    zone, and twice, it was delayed, and the result of later

    tectonic reorganizations caused subduction to initiate

    in the still warm and weak arc environment.

    The Papuan and Irian ophiolite complexes are up-

    lifted forearc terranes. They overlay the Owen Stanley

    and Ruffaer metamorphic belts that are composed

    largely, if not entirely, of the Australian continental

    rise, slope, and outer shelf protoliths. Any oceanic ac-

    cretionary prism composed of pelagic and trench axis

    materials that are scraped off the Pacific plate is very

    minor, if any is present. The subduction zones above

    which these ophiolites were uplifted because of con-

    tinental margin underthrusting and collision was sim-

    ilar to the intraoceanic Mariana convergent margin (a

    sediment-poor trench with active subduction erosion)

    north of New Guinea. Only a small prism could have

    accumulated, and thermal thinning of the lithosphere

    beneath the coeval arc was limited because the periods

    of subduction before the collisions were short.

    Along northern Australia, the outer portions of

    the continental margin were bulldozed, and the top of

    the deforming pile formed small islands prior to colli-

    sional orogenesis and the uprooting of crystalline base-

    ment. This precollision complex involves the sequen-

    tial deformation of first the rise, then slope, and finally

    shelf strata (Figure 9B). In the Puncak Jaya region of

    western New Guinea, crystalline basement became

    involved in the deformation at about 8 Ma (Weiland

    and Cloos, 1996). The regional field relations indicate

    that collisional tectonism involving crystalline basement

    began about 4 m.y. after the approximately 12-Ma

    initiation of the Central Range orogeny as marked by

    widespread siliciclastic sedimentation.

    Collision Event 1: The Peninsular Orogeny

    The Peninsular orogeny of New Guinea is an Oligo-

    cene event (3530 Ma) restricted to the vicinity ofthe Papuan Peninsula. This event was caused by the

    underthrusting of the northeastern edge of the Aus-

    tralian continent and marked the end of a 1015-m.y.

    episode of northeast-dipping subduction (Figure 8A, B).

    Total convergence at the Papuan subduction zone was

    probably only a few hundred kilometers because only

    a minor volcanic arc was generated in the Trobriand

    Sea (Kroenke, 1984; Davies et al., 1984; Davies and

    Warren, 1988). By about 30 Ma, the Papuan segment

    was fully jammed (Figure 9C). The underthrusting

    of bulldozed sediments (Owen Stanley metamorphic

    belt) and the Australian continental margin caused the

    uplift and exposure of the crystalline oceanic forearc

    block, the Papuan ophiolite (Davies, 1971; Davies and

    Jaques, 1984). The Oligocene to Pliocene paleogeog-

    raphy in the area of the Papuan Peninsula was probably

    quite similar to present-day New Caledonia, which

    formed at about the same time and in a similar manner.

    The Eocene to Oligocene subduction zone form-

    ing the Inner Melanesian arc continued eastward from

    the Papuan Peninsula to the Rennell trench and arc

    complex and from there southward to New Caledonia

    Norfolk ridge (Figure 9B) (Parrot and Dugas, 1980) and

    to New Zealand, where it is known as the Kaikoura

    orogeny (Brothers, 1974; Hayward et al., 1989). The

    combination of nearly coeval collisional orogenesis at

    the Papuan, New Caledonia, and New Zealand trenches

    stopped northeast-dipping subduction along the entire

    length of the Inner Melanesian arc. This jamming

    caused all Pacific and Australian plate convergence to

    become accomodated far offshore at the outer Mela-

    nesian trench system, and the oceanic lithosphere be-

    tween the arc was effectively welded to the Australian

    plate.

    The Peninsular deformational belt extended as a

    submarine terrane north of the present-day Papuan

    Peninsula into the Sepik region and beyond. It was

    blanketed by Miocene mudstone and limestone and

    then intruded by the magmas of the Maramuni arc

    (Dow, 1977; Doust, 1990; Wilson et al., 1993).

    Since the Oligocene, the Peninsular uplift has been

    the source of immature sediment (Slater et al., 1988;

    Davies, 1990) and more than 7 km (4 mi) of siliciclastic-

    rich strata, the Aure Group, accumulated in the Aure

    trough (Figure 4). This depression, the trench of the

    extinct Papuan subduction zone, was a sediment trap

    protecting the Australian carbonate shelf from the

    influx of siliciclastic detritus from the Peninsular high-

    lands. Much of the Aure Group is composed of fine- to

    medium-grained turbidite and mass flow deposits that

    grade upward from deep to shallow marine (Kugler,

    1967; Brown et al., 1975; Francis et al., 1986). The

    Miocene part of the Aure Group contains abundant

    carbonate clasts of Eocene age (Carman, 1990), as well

    as some ophiolite and blueschist detritus. Basement

    detritus became much more abundant in Pliocene and

    younger strata (Francis et al., 1986; Klimchuk, 1993).

    The stratigraphic succession records the progressive

    erosional unroofing of the Papuan Peninsula.

    Marine geophysical studies in the eastern plateau

    region (Figure 1) indicate that the Oligocene jamming

    Quarles van Ufford and Cloos 131

  • 132 Cenozoic Tectonics of New Guinea

  • of the Papuan subduction zone had some comparative-

    ly subtle, regional effects that generated structures that

    could be hydrocarbon traps. As much as 1 km (0.6 mi)

    of reverse slip during the late Oligocene to early Mio-

    cene occurred along preexisting basement normal faults

    more than 200 km (120 mi) southwest of the Papuan

    Peninsula (Davies et al., 1989). It seems probable that

    the collision caused similar movements in the Arafura

    Figure 8. Tectonic evolution of New Guinea consistent with regional deformation and magmatic and sedimentation patternssummarized in this paper and from the literature for the surrounding region. AUS = Australian plate; BT = BewaniTorricelli arc;EQTR = equator; FH = FinisterreHuon arc; IMA = Inner Melanesian arc; IOB = Irian ophiolite belt; K = Kilinailau; LHR = Lord Howerise; MB = ManusBismarck arc; NGT = New Guinea trench; NR = Norfolk ridge; NS = north Solomon arc; OMA = Outer Melanesianarc; OJP = Ontong Java Plateau; PAC = Pacific plate; PHS = Philippine plate; POB = Papuan ophiolite belt; SS = South Solomon. T1, T2,T3 = postulated transform faults. Approximate paleolatitudes are modified from Scotese et al. (1988) and Veevers et al. (1991).

    Figure 9. Lithospheric-scale cross sections through time of the AustralianPacific, arc-continent collision. Northern Australia was a passivemargin since rifting in the Triassic (Pigram and Panggabean, 1984). See Figure 3 for line of section. (A) Prior to collisional orogenesis, theoceanic portion of the Australian plate is subducted toward the north. The passive continental margin is not involved. (B) Initiation oforogeny at about 12 Ma from contractional thickening of bulldozed passive-margin strata and the underthrusting of Australian continentalbasement. Passive-margin strata on the Australian plate are bulldozed and contracted to such a point that a subaerial high underlain by aprecollision complex is formed. Erosional detritus from this precollision complex accumulates nearby and records the beginning of theCentral Range orogeny in the stratigraphic record. (C) Initiation of collisional orogenesis at about 8 Ma. The point of neutral buoyancy on theAustralian plate has reached the subduction zone. Convergence between the Australia and Pacific plates is no longer accommodated byconvergence. Positive and negative lithospheric buoyancy are with respect to the asthenospheric mantle. NGT = New Guinea trench.

    Quarles van Ufford and Cloos 133

  • high (Figure 1). This region was the site of modest

    tilting and uplift during the Paleocene opening of the

    Coral Sea (Weissel and Watts, 1979; Davies, 1990).

    Near-vertical uplift of a few hundred meters in the

    late Oligocene could fully account for the Paleocene to

    Miocene unconformity centered on the Arafura high

    (Figure 4, columns U to PNG2). Exposure of Mesozoic

    siliciclastic formations (Kembelangan Group), such as

    that detected on the eastern plateau, could have been

    a source for the quartz sands in the Sirga Formation.

    Except around the uplifted area centered on the

    present-day Papuan Peninsula, carbonate sedimenta-

    tion occurred elsewhere on the Australian continental

    shelf (Figure 4). The Oligocene sea level drop of about

    90-m (300-ft) at about 3330 Ma caused large areas

    of the Australian shelf to become emergent. This

    created the regional disconformity over what is now

    western and central New Guinea that was overlain by

    the transgressive Sirga Formation during the middle to

    late Oligocene (Figure 4, columns D, E, H, and KR).

    Pelagic limestone deposition north of the Birds Head

    indicates that a complete passive-margin setting (shelf-

    slope-rise) existed in western New Guinea until the end

    of the middle Miocene.

    Subduction at the Trobriand Trough andMaramuni Magmatism about 2010 Ma

    A mechanical cause for the short-lived subduction

    event forming the Trobriand troughMaramuni arc is

    unaccounted for in existing tectonic models. We be-

    lieve a simple explanation for the initiation of sub-

    duction at the Trobriand trough is the jamming of

    southwest-dipping subduction at the KilinailauNorth

    Solomon trench and Outer Melanesian arc segment by

    the collision of the Ontong Java Plateau. A profound

    change in plate interactions was caused by the sub-

    duction of the leading edge of the more than 30-km

    (18-mi)-thick oceanic crust underlying the Ontong

    Java Plateau (Kroenke, 1984). To account for the east-

    ern limit of the New Guinea trench and the north-

    western and southeastern limits of the Trobriand trough

    and Maramuni arc and the western limit of the New

    Britain trench, we postulate that three major transform

    fault zones (T1T3 in Figure 8C) formed between 25

    and 20 Ma. Movements postulated for transform fault

    T3 are compatible with the late Cenozoic motions of

    the Philippine plate deduced by Hall (1996).

    We postulate that southwest-dipping subduction

    occurred at the Trobriand trough between two major

    transforms (T1 and T2 in Figure 8C). This was con-

    current with the initiation of northeast-dipping con-

    vergence by the immediate subduction reversal behind

    the western part of the Outer Melanesian arc between

    the northern transform and another still farther north

    (T2 and T3 in Figure 8C). The Trobriand trough is

    located where the Inner Melanesian arc would have

    been located during the period of subduction at the

    Papuan trench from the Eocene to about 30 Ma. Sub-

    duction to accommodate the AustralianPacific plate

    convergence probably started here because it was still

    a belt of thermally weakened lithosphere. This was a

    delayed subduction reversal.

    Contractile deformation of Aure trough strata

    began to form the Aure fold and thrust in the middle

    Miocene (Slater et al., 1988). It seems likely that some

    of this deformation was the result of the short-lived,

    minor reactivation of the old Papuan subduction zone

    before subduction was fully established at the Tro-

    briand trough. Subsidence analysis of wells in the Gulf

    of Papua shows that the rate of sediment accumula-

    tion became more rapid at about 25 Ma (Pigram and

    Symonds, 1991; Wang and Stein, 1992). Uplift of the

    present-day Papuan Peninsula and faster erosion is

    another manifestation of crustal movements caused by

    the initiation of subduction at the Trobriand trough.

    The Cape Vogel basin was a forearc basin that ponded

    substantial sediment (Davies and Smith, 1971).

    Collision Event 2: Central Range Orogeny

    The name Central Range orogeny is proposed for the

    event that generated the about 1300-km (800-mi)

    mountainous spine of New Guinea that stretches from

    the Birds Neck (135jE) up to the Papuan Peninsula(146jE). This chain includes the Sneeuw Mountains(Hamilton, 1979, his figure 119), also known as the

    Pegunungan Maoke or Central Range (Allison and Pe-

    terson, 1989) of Papua, as well as the New Guinea

    Highlands and Papuan foothills (Dow, 1977), also

    known as the Central Cordillera of Papua New Guinea

    (Davies, 1990).

    Fast, north-dipping subduction along an approx-

    imately 500-km (300-mi) length of the Outer Mela-

    nesian arc (Figure 8D) began between 30 and 25 Ma

    (Figure 9A). This soon caused submarine tectonism and

    metamorphism of the Australian continental margin

    deposits. The first evidence of land emergence caused

    by the approach of this subduction zone comes from

    the early middle Miocene (early Tf1, 1614 Ma)Makats Formation in the North Coast basin (Visser and

    Hermes, 1962, p. 103105). This basin is underlain by

    134 Cenozoic Tectonics of New Guinea

  • the Irian ophiolite (at the time a forearc terrane) and

    the oceanic island arc complex along the Irian north

    coast. The oldest Makats Formation appears to be re-

    stricted to the eastern part of the basin and seemingly

    was sourced from small early middle Miocene islands

    near the modern international border. The Makats

    Formation reflects the development of isolated islands

    formed by the progressive tectonic bulldozing of the

    sedimentary sequence deposited on oceanic basement

    (continental rise) and then transitional Australian crust

    (continental slope and outer shelf; Figure 9B). The

    paleogeography of the eroding bathymetric high(s) was

    probably similar to present-day subduction zones off

    Sumatra forming Nias Island and in the Lesser Antilles

    forming Barbados Island.

    The lower Iwur Formation on the south side of

    the Central Range contains the oldest siliciclastic de-

    posits known to have been deposited on continental

    basement outside of the Aure trough. This occurrence

    probably marks the filling of the Aure trough to the

    east, but it could mark the first debris shed southward

    from an emerging landmass to the north. In any case,

    the Central Range orogeny did not begin until the

    latest middle Miocene because carbonate shelf sedi-

    mentation continued across the Australian continental

    shelf until about 12 Ma. At that time, the regional

    change in depositional patterns records that a sub-

    stantial landmass underlain by bulldozed continental

    margin deposits was elongated east-west. The Klasa-

    man, Akimeugah, Iwur, and lower Buru formations

    contain voluminous shale, siltstone, and sandstone,

    which indicates a siliciclastic-rich landmass extended

    more than 500 km (300 mi) (Figure 4, columns CU).

    A pronounced sedimentological change occurred

    in the PliocenePleistocene (5 Ma), when the coarse-ness of deposits along the flanks of the Central Range

    increased dramatically for hundreds of kilometers along

    strike (Figure 7). The Sele, Steenkool, Dakebo, upper

    Buru, Birim, and Era formations contain boulder beds

    that appear to date when the Central Range attained a

    topography similar to that of today. Shortly before,

    crystalline Australian basement first became involved

    in the deformation. Field relations and fission-track

    analysis studies in the Puncak Jaya region indicate that a

    30-km (18-mi)-wide basement block, the Mapenduma

    anticline, was pushed southward with unroofing be-

    ginning at about 8 Ma (Weiland and Cloos, 1996).

    Collisional orogenesis in western New Guinea must

    have begun at that time (Figure 9C).

    The collisional jamming that formed the Central

    Range changed the force balance on the plates, and we

    believe this explains the cause of major tectonic events

    to the west and east. Subduction began along trans-

    form T3 (Figure 8c), extending the Java subduction

    system eastward forming the Banda trench. This seg-

    ment is now undergoing collisional tectonism resulting

    from subduction of the northwestern part of the Aus-

    tralian continent near Timor. Northward-dipping sub-

    duction also began by delayed subduction reversal along

    the easterly segment of the still warm Outer Melane-

    sian arc. This created the present New BritainSolomon

    arc system. As PacificAustralian convergence became

    accommodated at this zone, subduction at the Tro-

    briand trough ceased. We emphasize that this geom-

    etry and sequence of events accounts for the location

    of the western end of the New Britain arc (Figure 1).

    The most northwestern segment of the Outer

    Melanesian arc complex, the product of southwest-

    dipping subduction between about 40 and 25 Ma and

    the northeast-dipping subduction from 25 to about

    10 Ma, is built on Mesozoic oceanic basement. With

    the initiation of northeast-dipping subduction between

    transform faults T2 and T3 at about 25 Ma, the older

    arc complex became a forearc basement terrane. A

    large piece of this terrane, the Irian ophiolite belt, was

    uplifted during the Central Range orogeny, as Austra-

    lian continental margin sediments (Ruffaer metamor-

    phic belt) and the crystalline basement were underthrust.

    The associated arc (Maramuni time equivalent) was

    parked near the present north coast of New Guinea at

    about 10 Ma.

    Ongoing Collisional Orogeny in Eastern New Guinea

    The Central Range orogeny is a slightly younger and

    ongoing event in eastern New Guinea. From about 20

    to 10 Ma, PacificAustralian convergence was accom-

    modated by the southwest-dipping subduction at the

    Trobriand trough (Figure 8C, D). The jamming of

    the subduction zone in western New Guinea corre-

    sponds to the slowing and eventual cessation of fast

    subduction at the Trobriand trough. The initiation

    of northeast-dipping subduction began beneath the

    extinct but still warm eastern segment of the Outer

    Melanesian arc. Since about 10 Ma, nearly all Pacific

    Australian convergence has been accommodated at the

    New BritainSolomon Arc system (Figure 8D). The

    forearc terrane for this system is now exposed in the

    Adelbert and Finisterre Ranges and Huon Peninsula

    and eastward-forming New Britain Island. As with the

    Irian ophiolite, these terranes contain remnants of the

    Quarles van Ufford and Cloos 135

  • Outer Melanesian arc volcanism that was the product

    of southwest-dipping subduction between about 40

    and 25 Ma.

    In eastern New Guinea, progressive jamming of

    the north-dipping subduction zone is underway. This

    uplift of the Adelbert Range and FinisterreHuon

    Ranges has formed a mountain belt along the north

    coast of eastern New Guinea. In the Sepik basin re-

    gion, collisional tectonism deforms the shallow-marine

    Miocene strata (Dow, 1977; Doust, 1990) that blan-

    keted rocks deformed in the Peninsular orogeny.

    The change in force balance from the collision

    that formed the Central Range orogeny caused pro-

    found plate-tectonic changes in the immediate area

    and probably the entire Pacific basin. Cox and Engeb-

    retson (1985) and Pollitz (1986) delineate a change in

    Pacific plate motion at that time. It appears that the

    prong of the Pacific plate directly north of New Guinea

    was temporarily detached and moved as its own kine-

    matic entity from about 5 to 3 Ma (Figure 1) (the

    Caroline plate of Weissel and Anderson, 1978). The

    back-arc area of the New Britain arc ruptured, forming

    the Bismarck microplate with strike-slip faulting and

    sea-floor spreading along its northern boundary (Figure 1).

    This piece of lithosphere has been a distinct kinematic

    entity since about 3.5 Ma (Taylor, 1979). The trans-

    form zone extends westward, emerging onland as the

    BewaniTorricelli fault zone (BTFZ in Figure 1),

    which links to the Yapen and Sorong faults (SYFZ in

    Figure 1) in western New Guinea (Sapiie et al., 1999).

    Transform faulting was localized in this region be-

    cause the lithosphere was locally thin beneath the

    recently extinct arc. The relict arc, largely buried be-

    neath the deposits of the North Coast basin, has been

    dismembered as a result of about 300 km (190 mi) of

    left-lateral, postcollision, transform offset.

    Another tectonic result of the change in force

    balance is found to the east of New Guinea. The north-

    ern corner of the Australian plate ruptured, forming

    the Woodlark spreading center. The Solomon Sea mi-

    croplate is just a tear in the Australian plate that has

    been moving northward into the New Britain trench

    since about 3.5 Ma (Weissel et al., 1982).

    Major uplift in eastern New Guinea is well dated

    from a change in provenance from continental to

    volcanic-rich materials near the FinisterreHuon

    forearc terrane at about 4 Ma ( Abbott et al., 1994a, b;

    Abbott, 1995). Directly south, collisional deformation

    caused the initial unroofing of basement-cored up-

    lifts in the Papuan fold and thrust belt at about 4 Ma

    according to the apatite fission-track analysis of Hill

    and Gleadow (1989). Farther south, renewed uplift in

    the Papuan Peninsula is also evident by the appearance

    of conglomeratic, molasse-type, deposits (Era beds) in

    the Aure trough region (Brown et al., 1975; Pigram

    et al., 1989; Davies, 1990), the renewed faulting and

    folding in the Aure trough (Kugler, 1993), and a five-

    fold increase in sediment accumulation rates in the

    Gulf of Papua (Pigram and Symonds, 1991; Wang and

    Stein, 1992).

    Collisional deformation in eastern New Guinea is

    ongoing. Thrust-type earthquakes are found beneath

    the FinisterreHuon terranes and along the southern

    flank of the Papuan highlands (Abers and McCaffrey,

    1988; Sapiie et al., 1999). The juncture between the

    active deformation front at the New Britain trench and

    the inactive front at the Trobriand trough (Figure 1)

    is propagating eastward at a rate between 110 and

    210 km/m.y. (68 and 130 mi/m.y.) (Silver et al., 1991;

    Abbott et al., 1994a). The active involvement of the

    Australian continental basement in collisional orogen-

    esis is dated at about 10 Ma in western New Guinea

    (dated at 8 Ma near Puncak Jaya) and at about 5 Manear the international border. The Central Range col-

    lisional orogenesis has propagated along the length

    of the entire island at a rate of about 150 km/m.y.

    (93 mi/m.y.).

    Implications for Hydrocarbon Exploration in New Guinea

    The implications of the model for the Cenozoic tec-

    tonics of New Guinea presented in this paper center

    on the timing of uplift and erosion, thick sedimenta-

    tion, deep burial and heating, and structural trap for-

    mation in the different parts of the island. On the

    Papua New Guinea side of the island, there have been

    two distinct collisional orogenic events since about

    35 Ma. In the western half of the Birds Body, one col-

    lisional orogenic event since about 12 Ma considers the

    regional tectonic relationships.

    Collisional tectonism causes complex folding and

    thrusting in the mountainous core zone and common-

    ly causes modest movements in the basement of the

    foreland, where sediments eroded from the rising moun-

    tains are deposited. Movements in the foreland base-

    ment can be located 100 km (62 mi) or more from the

    collision-generated mountain front. The Oligocene tec-

    tonic event that caused minor folding in the western

    edge of the Birds Head block is probably the east-

    ernmost manifestation of collisional tectonism in the

    Sulawesi region.

    136 Cenozoic Tectonics of New Guinea

  • In central and western New Guinea, the Oligo-

    cene sea level fall led to widespread exposure of the

    shelf. The Sirga Formation, a well-sorted transgressive

    quartz sandstone unit should have, at least locally,

    good reservoir characteristics. This unit is highly var-

    iable in thickness but has widespread distribution in

    both the highly deformed highlands and beneath the

    southern foreland basin.

    CONCLUSIONS

    The Cenozoic tectonic history of New Guinea records

    two major orogenic events (emergent mountain build-

    ing and erosion) related to arc-continent collision at

    northward-dipping subduction zones. The first event,

    the Peninsular orogeny, caused the cessation of con-

    vergence that began in the Eocene, when the northern

    corner of the Australian continental crust jammed the

    subduction zone in the Oligocene. The effects are re-

    stricted to eastern New Guinea. The Papuan ophiolite

    was emplaced above the Owen Stanley metamorphic

    belt, and the orogeny generated the Papuan Peninsula.

    The uplifted area, similar to present-day New Cale-

    donia, was the source of abundant siliciclastic sedi-

    ment deposited in the Aure trough.

    The Oligocene disconformity and the transgressive

    deposition of the quartzose Sirga Formation in western

    and central Papua is the result of the about 90-m (300-ft)

    drop in sea level between 33 and 30 Ma. Regional

    stratigraphic relationships indicate that the Australian

    margin in central and western New Guinea remained

    a carbonate shelf until the late middle Miocene.

    The Central Range orogeny was the event that

    formed the present-day shape and topography of New

    Guinea. Before the orogeny began, the Australian

    margin rise and slope sediments were bulldozed and

    metamorphosed. The top of the deformed sediment

    pile was locally emergent and eroded in the middle

    Miocene (1614 Ma). Uplifts causing widespreadsiliciclastic sedimentation above the Australian conti-

    nental basement formed in the latest middle Miocene

    at about 12 Ma. Collisional orogenesis involving crys-

    talline basement probably began beneath the west-

    ernmost Central Range at about 10 Ma, propagated

    eastward reaching central New Guinea at about 5 Ma,

    and is ongoing in eastern New Guinea. The eastern

    edge of the collisional orogenesis has propagated

    along the length of the entire island at a rate of about

    150 km/m.y. (93 mi/m.y.). Cenozoic tectonism form-

    ing the island of New Guinea has generated a network

    of folds, faults, and stratigraphic complexities that host

    a major hydrocarbon province in the eastern highlands

    but is still largely untested in the western highlands.

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