Mesozoic–Cenozoic evolution of Australia’s New …searg.rhul.ac.uk/pubs/hill_hall_2003 New...

INTRODUCTION

Australia’s northern margin, in New Guinea, is part of acomplex convergent plate boundary between the Indo-Australian Plate, the Pacific Plate, and several othersmaller plates including the Philippine Sea and CarolinePlates (Figure 1). Plate-tectonic reconstructions suggestthat the relative motions between all of these plates mustbe considered when attempting to reconstruct the history ofAustralia’s New Guinea margin during the Cenozoic. Thesesmall plates did not exist prior to the Cenozoic and north ofAustralia there is a region with indications of a long sub-duction history so that much of the oceanic record of pre-vious events has been lost. Reconstruction of the geologicalhistory relies on the geological record from outcrops in NewGuinea, radiometric and palaeontological dating, regionalpalaeomagnetic data and regional plate-tectonic recon-structions.

Rare basement outcrops suggest a fundamental differ-ence between east and west New Guinea in the Palaeozoic,consistent with the differences between eastern and west-ern Australia further to the south. During the Palaeozoicthere was an important change from the eastern TasmanOrogenic belt to the region of extension associated withGondwana breakup of an older stable craton along thewestern margin of Australia. Following Triassic–Jurassic rift-ing and breakup a passive margin developed in New

Guinea which is preserved in the New Guinea Fold Belt(Figure 1). The Cenozoic was characterised by orogenicevents, including ophiolite emplacement, arc–continentcollision and the development of the major mountainsknown today. However, despite the importance and youthof these events, their timing and significance remain thesubject of much debate.

In this paper, we aim to synthesise knowledge of NewGuinea geology with a plate-tectonic reconstruction ofAustralia’s northern margin for the Cenozoic. Furthermore,we consider the increased understanding of Palaeozoic andMesozoic tectonics along Australia’s northern margin andits implications for the interpretation of Cenozoic platemotions and orogenesis. We suggest that the geology ofAustralia’s northern margin results from the interactionbetween the fabric of the Australian continent and the adja-cent plate motions. Finally, we review some of the main tec-tonic issues and uncertainties in New Guinea still to beresolved. Thus, our paper is divided into four parts: (i) areview and discussion of constraints provided by thePalaeozoic, Mesozoic and Cenozoic geology and architec-ture of the northern Australia continental margin; (ii) areview and discussion of constraints provided by theCenozoic tectonic history of New Guinea in the context ofthe plate-tectonic history of the west Pacific and southeast

Geol. Soc. Australia Spec. Publ. 22, and Geol. Soc. America Spec. Pap. 372 (2003), 265–289

Mesozoic–Cenozoic evolution of Australia’s New Guineamargin in a west Pacific contextK. C. HILL1* AND R. HALL2

1 3D-GEO, School of Earth Sciences, University of Melbourne, Vic. 3010, Australia.2 SE Asia Research Group, Department of Geology, Royal Holloway University of London, Egham,

Surrey TW20 0EX, UK.

The northern Australian margin includes the island of New Guinea, which records a complex struc-tural and tectonic evolution, largely masked by Mio-Pliocene orogenesis and the Pleistocene onsetof tectonic collapse. In the Palaeozoic, New Guinea contained the boundary between a LatePalaeozoic active margin in the east and a region of extension associated with Gondwanabreakup along the western margin of Australia. In the Permian and Early Triassic, New Guinea wasan active margin resulting in widespread Middle Triassic granite intrusions. The Mesozoic saw Triassicand Jurassic rifting followed by Cretaceous passive margin subsidence and renewed rifting in theLate Cretaceous and Paleocene. Since the Eocene, New Guinea tectonics have been driven byrapid northward movement of the Australian Plate and later sinistral oblique convergence with thePacific Plate, resulting in Mio-Pliocene arc–continent collision. Neogene deformation along the mar-gin, however, has been the result of direct interaction with the Philippine and Caroline Plates.Collision with the Philippine–Caroline Arc commenced in the Late Oligocene and orogenesis con-tinues today. We suggest that the New Guinea Mobile Belt comprises a collision zone between anorth-facing Cretaceous indented margin and a south-facing Palaeogene accretionary prism, sub-sequently cut by a Neogene strike-slip fault system with well over 1000 km sinistral displacement thathas alternated between extension and compression. The change in character of the lithosphere inNew Guinea, from thick and strong in the west to thin and weak north and east of the Tasman Line,was also an important influence on the style and location of Mesozoic and Cenozoic deformation.

KEY WORDS Australia, collision zone, New Guinea, plate tectonics, wrench faults

* Corresponding author: [email protected]

266 K. C. Hill and R. Hall

Figure 1 (a) Tectonic map of New Guinea and the southwest Pacific region with principal geographical locations referred to in the text.Barbed lines are active subduction zones and thick lines are active spreading centres. The light blue shaded areas, drawn at the 200 misobath, are the continental shelves of Eurasia and Australia and areas of thickened oceanic crust/arcs. (b) New Guinea showing sim-plified tectonic belts and the principal tectonic features. AB, Amanab Block; AR, Adelbert Ranges; BB, Bintuni Basin; BG, BenaBena–Goroka Terrane; B–T, Bewani–Torricelli Mountains; CM, Cyclops Mountains; COB, Central Ophiolite Belt; DF, Derewo Fault; FR,Finisterre Ranges; G, Gauttier Terrane; GM, Grasberg mine; HG, Huon Gulf; HP, Huon Peninsula; In, Indenburg Inlier; K, Kubor Range;La, Landslip Ranges; LF, Lagaip Fault; MB, Meervlakte Basin; Po, Porgera Intrusive Complex and mine; RB, Ramu Basin; SB, SepikBasin; SG, Strickland Gorge; ST, Sepik Terrane; Wa, Wandaman Peninsula; WT, Weyland Terrane.

Asia; (iii) a tectonic synthesis of Australia’s northern marginintegrating the plate-margin architecture and the plate-tec-tonic motions; and (iv) tectonic issues still to be resolved.

The Cenozoic plate reconstructions are described indetail in Hall (2002), accompanied by computer anima-tions, and are based on a synthesis of a range of data, fromspreading histories obtained from small ocean basins tomore qualitative information such as that obtained fromterrestrial geology. The reconstructions were made with theATLAS computer program (Cambridge Paleomap Services1993) using the Indian–Atlantic hotspot frame of Müller etal. (1993). Full details of the approach and the model areprovided in Hall (2002). The pre-Cenozoic reconstructionsare based mainly on the geological data from New Guineaand the architecture of the margin, as discussed below.

NEW GUINEA GEOLOGICAL ARCHITECTURE

The island of New Guinea has been divided into four tec-tonic regions based on the Miocene to Holocene orogene-sis affecting the northern part of the Australian Plate

(Figure 1) (Dow 1977; Hill et al. 1996). In the south is theStable Platform, which is the northern continuation ofAustralian continental crust that preserves Mesozoic andpossibly Palaeozoic extensional structures (Cole et al.2000; Kendrick 2000). The adjacent Fold Belt, to the north,comprises the same crust deformed into fold and thruststructures following Late Oligocene–Miocene arc–continentcollision. The fold and thrust deformation occurred in theLate Miocene to Holocene (Hill 1991; Hill & Raza 1999).Most of the northern half of New Guinea is made up of theMobile Belt, including ophiolites of Mesozoic to Paleoceneage (Davies & Jaques 1984; Rogerson et al. 1987). TheMobile Belt also includes distal Mesozoic–Tertiary sedi-ments, abundant Miocene and some Cretaceous volcanicand intrusive igneous rocks, and medium- to high-grademetamorphic rocks with common Early Miocene and someMesozoic cooling ages. In places, the Mobile Belt has beenhighly deformed, with a record of local medium- to high-grade metamorphic rocks that record ductile deformation,particularly along strike-parallel shear zones. These havebeen overprinted by low-angle thrusting and sinistral strike-slip structures (Crowhurst et al. 1996, 1997). The northern

Tectonic evolution of New Guinea 267

Figure 2 Simple chronostratigraphic column for the Fold Belt in New Guinea (after Kendrick 2000; Pigram & Panggabean 1984; Homeet al. 1990; Parris 1994), showing the preserved Palaeozoic section west of the Tasman Line and the low-grade Permian to Early Triassicmetasediments to the east, intruded by Middle Triassic and Early Jurassic granites. Following Jurassic breakup, the clastic Mesozoicsection is relatively uniform. Early Tertiary uplift and denudation in Papua New Guinea led to a significant unconformity in the eastprior to deposition of New Guinea Limestone, whilst there was continuous deposition in the west. The Late Miocene marks the onsetof deposition of a molasse sequence.

rim of New Guinea is interpreted to consist of accretedPalaeogene arcs and oceanic terranes including theGauttier and Cyclops mountains, the Bewani–Torricellimountains, the Finisterre Ranges and the Huon Peninsula(Pigram & Davies 1987). The suture between these terranesand the Mobile Belt is partly covered by the Miocene toPliocene Meervlakte, Sepik and Ramu successor basins(Francis 1990). The successor basins are reported to beunderlain by Mesozoic and Early Tertiary igneous andmetamorphic rocks of basic to intermediate composition,inferred to be of island arc or oceanic origin (Doust 1990).Findlay (2003) and Findlay et al. (1997) review recent map-ping of the Ramu–Markham part of the suture zone andsuggest other interpretations.

PROTEROZOIC HISTORY

The Arafura Platform to the south of the Irian Jaya FoldBelt, comprises Mesoproterozoic cratonic basement(Pieters et al. 1983) overlain by Silurian–Devonian toJurassic sedimentary sequences that record rifting of theGondwana margin (Pigram & Panggabean 1981, 1984). Tothe north, the 5 km-high frontal ranges of the Fold Beltexpose a thick, relatively undeformed uppermostProterozoic and Palaeozoic sedimentary section that over-lies Proterozoic marine rift metabasalts and metavolcanicsof the Nerewip Formation exposed near the Grasberg Mine(Figures 1, 2) (Parris 1994). This Neoproterozoic sedimen-tary section may record erosion of the proposed Rodiniansupercontinent from ca 725 Ma, when Laurentia separatedfrom Australia (Powell 1996) giving rise to the Tasman Line,defined by the eastern limit of Proterozoic crust in Australia(Scheibner 1974). If so, then the Tasman Line trendsroughly north–south near the Papua New Guinea – IrianJaya border and then trends approximately west-northwestalong the front of the ranges (Figure 1).

The change in basement across the north–south part ofthe Tasman Line in Australia is consistent with tomo-graphic images of the Australian region that indicate amarked change in the character of the lithosphere (Simonset al. 1999; Debayle & Kennett 2000; Hall & Spakman2003). To the west of Queensland is lithosphere with highvelocities beneath much of northern Australia which corre-sponds to old Precambrian crust, and a cold, very thicklithosphere with great strength. To the east of the westernedge of Queensland are much lower lithosphere velocitiesconsistent with younger, hotter and much weaker conti-nental lithosphere east of the Tasman Line.

PALAEOZOIC HISTORY

The Palaeozoic margin in New Guinea is here interpretedto have been divided into two parts: an eastern active mar-gin and a western rifted margin, with the boundary roughlyalong the Tasman Line. East of the line in Australia, rocksof the Tasman Orogen are thought to largely overlieCambrian or Ordovician oceanic crust (Aitchison et al.1992) and to have been accreted in multiple orogeniesthroughout the Palaeozoic and earliest Triassic (Veevers2000). This east-facing edge of Gondwana is believed to

have been a long-lived active margin, similar to the west-facing Rocky Mountains of North America in the Mesozoicand Palaeogene (Coney et al. 1990). The youngestPalaeozoic event along the eastern margin was the PermianNew England Orogeny recorded in New South Wales andQueensland that continued into the Early Triassic (Korschin press). Upper Permian to Lower Triassic low-grademetasediments intruded by Middle Triassic granites,recorded in Papua New Guinea suggest a similar history(Van Wyck & Williams 2002; Crowhurst 1999). In contrastthe margin west of the Tasman Line underwent substantialextension and rifting, with major microcontinents breakingaway in the Early Palaeozoic and the Permo-Carboniferous(Metcalfe 1996, 1998) (Figure 3). The thick, relativelyundeformed Palaeozoic section in mainland Irian Jaya isconsistent with this rift history (Figure 2).

In contrast to mainland Irian Jaya, the widely exposedbasement of the Kemum Terrane in the Bird’s Head com-prises Silurian–Devonian low-grade metamorphic rocksintruded by Triassic granites. This has led to models sug-gesting that the Bird’s Head was derived from Papua NewGuinea or eastern Australia (Pigram et al. 1985;Struckmeyer et al. 1993). It is suggested here that theTasman Line continued in a more east–west directionthrough Irian Jaya and that to the north of it were rockssimilar to those of the Tasman Orogen, which rifted offAustralia during the Mesozoic. Therefore an alternative toderiving the Bird’s Head from eastern Australia is that dur-ing the Palaeozoic the Bird’s Head was north of the TasmanLine in a position relative to the Australian continent simi-lar to the present.

In the Papua New Guinea Fold Belt, basement cropsout as Triassic granites in the Kubor Ranges and in the


Figure 3 Devonian–Carboniferous setting of the New Guineamargin (after Metcalfe 1996 figure 12). NG, New Guinea.

Strickland Gorge (Page 1976; Van Wyck & Williams 1998,2002). The Kubor Granites intrude the low-grade Omungmetasediments, which were deposited in the Late Permianto Early Triassic and contain Permian to Proterozoic detri-tal zircons showing affinity with Australia (Van Wyck &Williams 1998, 2002). Lying well to the northeast of theinferred Tasman Line, it is likely that east Papua is under-lain by crust no older than late Precambrian. Crowhurst’s(1999) U–Pb age dating of zircons from the Amanab dioriteyielded an Ordovician inherited zircon population, consis-tent with this hypothesis. However, recent U–Pb age datingof zircons in two young intrusive rocks in Papua NewGuinea indicates the presence of Archaean grains. ThePorgera intrusive complex in the western Papua NewGuinea Fold Belt reveals a mixture of Late Miocene andArchaean zircons (Munroe & Williams 1996). Similarly,Pliocene and Archaean U–Pb ages were obtained from zir-cons from gneiss and granodiorite in the D’EntrecasteauxIslands of eastern Papua (Baldwin & Ireland 1995). Thesetwo datasets could indicate underlying Archaean terranes,but alternatively may demonstrate that the underlyingPalaeozoic terrane remained close to Australia andreceived sediments derived from the Archaean ofGondwana.

Northern limit of continental crust

The present northern limit of intact Australian continentalcrust is important as it helps to define the terranes accretedduring the Cenozoic. In addition, it is important to identifythe terranes removed from the margin in the Cenozoic inorder to determine the previous margin geometries.

Davies et al. (1997) and Pigram and Davies (1987) sug-gested that in Papua New Guinea continental crust extendsto the northern limit of the Fold Belt, defined by the LagaipFault and the Kubor Range (Figure 1), and that all terranesto the north were accreted. In contrast, Rogerson et al.(1987) and Simandjuntuk and Barber (1996) proposed thatcontinental crust continues beneath the Mobile Belt as farnorth as the suture with the accreted arcs. Rogerson et al.(1987) suggested that the ophiolites exposed in the westernPapua New Guinea Mobile Belt were transported there asthin nappes. In Irian Jaya continental crust is thought tounderlie the Fold Belt as far north as the Derewo Fault, andis bounded to the north by the Central Ophiolite belt andthe Mobile Belt (Figure 1).

Two terranes constrain the interpretations regarding thelimit of Mesozoic continental crust in Papua New Guinea.The first is the pre-Jurassic metasediments, minor metaba-sic rocks and synkinematic granites of the BenaBena–Goroka terrane (Rogerson & Hilyard 1990) in theMobile Belt to the northeast and east of the Kubor Range.Following Crowhurst (1999), the second, composite, ter-rane in the Mobile Belt is here defined to include themetabasics intruded by Permian–Triassic diorite in theAmanab block of northwest Papua New Guinea, the con-tiguous Landslip block to the south and adjacent Idenburginlier across the border in Irian Jaya (Figure 1).

Van Wyck and Williams (1998, 2002) used U–Pb datingof detrital zircons from the Goroka terrane to correlate itwith the nearby Omung metasediments, i.e. deposited inthe Permian to Early Triassic and derived from the

Australian continent. Further, they dated the Bena Benaterrane as Permian to Late Triassic and concluded thatAustralia’s cratonic margin extends at least as far north asthe northern edge of the Bena Bena terrane, adjacent to theFinisterre Ranges (Figure 1). Crowhurst (1999) confirmed aMiddle Triassic U–Pb age on zircons for the Amanab meta-diorite, indicating that it intruded a Palaeozoic or older ter-rane, consistent with the Ordovician ages from inheritedzircons in the metadiorite. They also reported 16 Nd–Sranalyses, which indicate the type of underlying crust andsource of the intrusions, and concluded that theAmanab–Idenburg–Landslip terrane was probably apromontory of extended continental crust, possibly raftedin, adjacent to an embayment of oceanic crust to the east.

Many tectonic models suggest that some continentalcrust has been moved along the New Guinea margin sincethe Early Miocene by strike-slip faulting (Hall 2002). In theisland of Bacan, just southwest of Halmahera, there arehigh-grade continental metamorphic rocks includingBarrovian kyanite–staurolite–garnet gneisses (Brouwer1923; Hall et al. 1988) which yield very young (<<1 Ma)K–Ar and Ar–Ar ages (Malaihollo 1993; Malaihollo & Hall1996). The chemistry of most of the present-day volcanicrocks of the Halmahera islands, including Bacan, indicatesa typical intra-oceanic arc character but at the southernend of the arc there is evidence of contamination by under-lying continental basement (Morris et al. 1983).Geochemical work on older volcanic rocks (Forde 1997)similarly shows an intra-oceanic arc character for all pre-Neogene volcanic rocks, but continental crustal contami-nation of ?Middle to Late Miocene igneous rocks at thesouth end of the Neogene arc, suggesting that Australianand Philippine Sea arc crust were brought into contact byEarly to Middle Miocene collision and subsequent strike-slip movements. Pb, Nd, Sr and O isotope geochemicalstudies (Forde 1997) of Neogene volcanic rocks on Bacanshow that some of the contamination could have been pro-duced by Permian granites similar to those known fromQueensland and Banggai–Sula. However, extreme Pb iso-tope ratios in some volcanic rocks suggest that an oldPrecambrian component is also present deeper beneathBacan (Vroon et al. 1996). The Bacan rocks are withinstrands of the Sorong Fault zone, bound to the north andsouth by Mesozoic ophiolites with a Philippine Sea plateorigin. They are clearly fault-bounded and must havemoved from the east. Similar small fragments of continen-tal crust are known in Obi (Figure 1). In Obi and in theBanggai–Sula islands there are also Mesozoic sediments ofAustralian margin character. These islands demonstrate thedispersal of continental crust fragments, undoubtedlyderived from northern New Guinea.

Basement fabric

Davies (1991), Corbett (1994) and Hill et al. (1996),amongst others, recognised a northeast structural grain inNew Guinea, here interpreted to be inherited from the fab-ric in the underlying crust. Davies (1991) recognised majornorth–south or northeast-trending alignments of volcanoes,stocks and normal faults cutting across the Fold Belt. Hill(1991) interpreted similar lineaments as lateral ramps com-partmentalising the structures of the Papuan Fold Belt.


Both authors inferred the trends to result from underlyingcrustal or lithospheric features. Corbett (1994) definednortheast-trending transfer structures throughout thePapuan Fold Belt and related them to the copper–golddeposits. Hill et al. (1996) and Kendrick (2000) linked thenorth–south and northeast structural grain with a similarrectilinear pattern of shear zones in northern Australiarecognisable from structural, regional gravity and magneticmapping (Elliot 1994). Hill et al. (2002a) inferred that thenortheast-trending lineaments continued across the MobileBelt, limiting the amount of strike-slip motion that can beinferred between the Mobile Belt and Fold Belt.

MESOZOIC AND PALEOCENE HISTORY

The Mesozoic history of the New Guinea margin is wellrecorded in the sedimentary record. The relatively uniformstratigraphy in the New Guinea Fold Belt (Figure 2) recordsroughly synchronous rifting and subsidence of the margin.Widespread Middle Triassic igneous activity preceded LateTriassic and Early Jurassic rifting, and Mid-Jurassicbreakup, although it is unclear which terrane, if any, sepa-rated from New Guinea. The Valanginian–Aptian sequencein New Guinea is mainly shale recording a flooding eventas the margin subsided into deep water (Pigram &Panggabean 1984; Home et al. 1990). Similar mudstonedeposition associated with subsidence and flooding isrecorded along Australia’s North West Shelf to theCarnarvon Basin (Bradshaw et al. 1998), but not downAustralia’s east coast (Norvick et al. 2001). Widespread vol-canism occurred in the Aptian–Albian and Cenomanian,but there is little evidence of later Cretaceous igneous activ-ity. Late Cretaceous rifting (Home et al. 1990) is inferred inthe Gulf of Papua associated with uplift and erosion ofsouthern Papua New Guinea. Pigram and Symonds (1991)inferred that rifting also occurred along the northern NewGuinea margin with the possible formation of LateCretaceous marginal basins. In order to reconstruct the

Mesozoic margin, the microcontinents derived from north-ern Australia that are now in southeast Asia must also beconsidered. Some of these were separated during theMesozoic rifting but others are thought to have been slicedoff the margin during westward shear in the Cenozoic.

Triassic orogenesis, igneous activity and rifting

EARLY TRIASSIC

Van Wyck and Williams (2002) demonstrated that themetasediments intruded by Middle Triassic granites weredeposited and deformed in the Late Permian to EarlyTriassic, perhaps during an event correlating with the NewEngland Orogeny along Australia’s eastern margin (Korschin press). Metcalfe (1996) inferred subduction beneath theeastern Australia and Papua New Guinea margin toaccount for the orogenesis and volcanism. Early Triassicorogenesis is consistent with the stratigraphic sectionrecorded in the fold belt of Irian Jaya (Kendrick 2000)(Figure 2). There the Palaeozoic section does not show evi-dence of Triassic deformation, but the Triassic sectionappears to be absent indicating non-deposition or regionaluplift and erosion, recorded by an unconformity within theTipuma Formation (Figure 2).

MIDDLE TRIASSIC

Middle Triassic igneous activity has been documented inPapua New Guinea by Page (1976), Crowhurst (1999) andVan Wyck and Williams (1998, 2002). As summarised byCharlton (2001), Late Permian–Triassic igneous activity isalso recorded in the Bird’s Head of Irian Jaya and thenearby continental terranes of the greater Sula Spur, suchas east Sulawesi and Banggai–Sula as well as Sibumasu(peninsula Myanmar, Thailand, western Malaysia andnorthwest Sumatra). The precise age of these intrusives isless well constrained than the Papua New Guinea exam-ples. In New Guinea most of the Middle Triassic intrusiverocks are granitic to dioritic and are interpreted to resultfrom the northwestern extension of a volcanic arc alongAustralia’s east coast related to subduction to the south-west beneath the continent (Figure 4) (Metcalfe 1996). Weinterpret the Permo-Triassic orogenic belt to have extendedfrom eastern Australia to Papua New Guinea and along thenorth coast of New Guinea after Metcalfe (1996).

LATE TRIASSIC

Pigram and Panggabean (1984), Home et al. (1990) andStruckmeyer et al. (1993) inferred rifting in New Guinea inthe Middle to Late Triassic, requiring a rapid change fromcompression to extension in the Middle Triassic. Crowhurst(1999) and Van Wyck and Williams (2002) have datedmagmatic events in Papua New Guinea at 240 Ma and ca220 Ma, consistent with previous dating of Page (1976). Weinfer that these correspond to a widespread Middle Triassicarc at ca 240 Ma, and Late Triassic extension-related vol-canism at ca 220 Ma. Correlating these events with those inthe New England Fold Belt, they suggest that Late Permianto Early Triassic orogenesis followed by Middle Triassic arcvolcanism and Late Triassic extension and rifting may have


Figure 4 Schematic Early to Middle Triassic palaeogeography(after Metcalfe 1996 and Charlton 2001). The dashed linethrough northern New Guinea is the inferred site of Late Triassicto Jurassic rifting. The part of New Guinea shaded red is theStable Platform (see Figure 1).

been common to Australia’s eastern margin and NewGuinea. This is consistent with the interpretation ofStampfli and Borel (in press) that renewed rifting along thenorth Australian margin of Tethys occurred in the LateTriassic, due to subduction of the Palaeo-Tethys ridgebeneath Asia.

SUMMARY

An important issue for Australia’s northern margin is thelocation of the Sula Spur terranes, including the Bird’sHead. Struckmeyer et al. (1993), following Pigram andPanggabean (1984), inferred that, in the Triassic, these ter-ranes lay to the northeast of Queensland in the position ofthe present Solomon Sea. Recent reflection seismic anddrilling data in the Bintuni Basin, immediately south of theBird’s Head, show a rifted Permian and Upper Palaeozoicsection akin to that along Australia’s North West Shelf andin Irian Jaya (Sutriyono 1999). Thus it seems likely that theSula terranes were adjacent to Irian Jaya as indicated byMetcalfe (1996) and Charlton (2001). Here we followMetcalfe in placing the Sula terranes northwest of IrianJaya, accounting for the lack of Triassic igneous rocksrecorded in mainland Irian Jaya (Figure 4).

Ophiolites in New Guinea

The ages and time(s) of emplacement of the ultramafic com-plexes interpreted as ophiolites in northern New Guinea areimportant in any reconstruction, but are not well known andare based largely upon the work completed by Bureau ofMineral Resource’s (now Geoscience Australia) geologists inthe late 1970s and early 1980s. Davies and Jaques (1984)summarised the known geology of the three major ophiolitecomplexes in Papua New Guinea. Based on K–Ar dating ofthree gabbro and basalt samples, they inferred that therocks of the Papuan Ultramafic Belt crystallised in theJurassic and/or Cretaceous, but that two K–Ar dates on theunderlying metamorphic rocks indicated emplacement inthe Eocene. The Marum ophiolite yielded similar EarlyJurassic and Paleocene ages (Page 1976) and is overlain bya sequence of Eocene argillites (Jaques 1981). Davies andJaques (1984) interpreted it to be of Late Mesozoic or possi-bly Early Tertiary age and emplaced after the Middle Eocenebut before the Late Oligocene–Early Miocene. Applyingapatite fission track analysis to two samples, Hill and Raza(1999) confirmed a Palaeogene or older age for the Marumultramafic suite and their analysis indicated that the rockshave not been heated to temperatures greater than 100°Csince then. They also confirmed thrusting of the ophiolite atabout 5 Ma.

The April Ultramafics (Figure 1) were thought to beMesozoic or older (Davies & Hutchison 1982) and to havebeen emplaced by tectonic stacking in an Eocene subduc-tion system followed by further tectonism during Oligocenearc–continent collision (Davies & Jaques 1984). In con-trast, Rogerson et al. (1987) considered the AprilUltramafics to be of Early Palaeogene age, by analogy withthe Papuan and Marum ultramafic suite. The ages of theophiolites in Irian Jaya are even less well known. Pigramand Davies (1987) suggested that the ophiolites areJurassic and Cretaceous and possibly lower Paleocene.

Monnier et al. (1999, 2000) suggested that the CentralOphiolite Belt in Irian Jaya was formed in a Jurassicbackarc basin, although no new radiometric dating waspresented, whereas ophiolites of the Cyclops Mountainswere suggested to have formed in an Oligocene backarcbasin, based on a small number of K–Ar ages.

Mesozoic rifting

The rift history recorded in the sediments of the PapuanBasin was well documented by Pigram and Panggabean(1984) and Home et al. (1990). Both recorded Mid–LateTriassic and Early–Mid Jurassic rifting events followed bybreakup and formation of a passive margin. The effect ofrifting and breakup in what is now the Mobile Belt hasreceived little attention, mainly due to a paucity of data anduncertain terrane provenance. Hill et al. (in press) com-pared Australia’s northern margin in the Mesozoic to theextensional margin preserved along Australia’s North WestShelf (Chen et al. 2002). Hill et al. (in press) noted abrupt~50 km offsets of the ophiolite belts along strike in NewGuinea in compartments 125 or 250 km wide and inferreda rectilinear post-rift margin in the Mesozoic, with promon-tories of extended continental crust separated by embay-ments of newly formed oceanic crust. This interpretation isconsistent with that of Crowhurst (1999) based on isotopicanalyses of crust in the Amanab area, discussed above.

The nature of the terranes rifted away from New Guineain the Jurassic is unclear. Many authors show varioussoutheast Asian terranes, such as those of parts of Sumatra,Sulawesi and the Banda arc, situated to the north of NewGuinea in the Palaeozoic (Audley-Charles et al. 1988;Audley-Charles 1991; Metcalfe 1996). Struckmeyer et al.(1993) inferred separation of the Sula Spur terranes fromnorthern Papua New Guinea and separation of the Bird’sHead from north Queensland in the Cretaceous. In con-trast, Charlton (2001) shows no terranes north of NewGuinea in the Triassic and the Sula Spur area lying to thewest, in its present position, throughout the Palaeozoic.

If the Middle Triassic igneous activity in Papua NewGuinea was subduction related, the fact that the ca 240 Maplutons are preserved at Amanab and in the Kubor Rangeindicates that only the relatively narrow forearc region canhave separated. Allowing a wide arc–trench distance of~300 km, the departing terranes are likely to have been anarrow strip of up to that width. If, as suggested, the SulaSpur terranes and others like West Sulawesi lay to thenorth of Irian Jaya in the Triassic (Metcalfe 1996) thensome of those terranes would have rifted away in the Midto Late Jurassic (Figure 4). The Bird’s Head is an exception.Hill et al. (2002) used the shallow water clastic Cretaceoussediments in that area to show that it was attached tonorthern Australia until the Paleocene. This is further sup-ported by the recognition of Early Palaeozoic detrital zir-cons in the Paleocene Waripi section of Bintuni Bay(Sutriyono 1999) suggesting that the Bird’s Head wasattached to the Australian continent.

Widespread Aptian–Albian volcanism

Northern New Guinea was a passive margin through theLate Jurassic and Neocomian (Home et al. 1990;


Struckmeyer et al. 1993), but in the mid-Cretaceous therewas volcanic activity, as recorded in the Kondaku Tuff andKumbruf Volcanics exposed around the Kubor Range. Hilland Gleadow’s (1990) apatite fission track analyses of mid-Cretaceous clastic samples from the Papuan Basin showedcommon contemporaneous volcanic grains. Since then, zir-con fission track analyses of mid-Cretaceous and youngerformations across New Guinea have revealed commonmid-Cretaceous grains, indicating a widespread and signif-icant volcanic event (Sutriyono 1999; Hill & Raza 1999;Kendrick 2000). This event may have been related to theresumption of volcanism along Australia’s eastern margin(Norvick et al. 2001), which deposited vast amounts ofAptian–Albian volcanogenic sediments in the Surat Basinin Queensland (Exon & Senior 1976; Fielding et al. 1990).The volcanism has been interpreted to be due to subduc-tion beneath Australia’s eastern margin (Jones & Veevers1983; Elliot 1993; Norvick et al. 2001; Sutherland et al.2001), but Francis (1990) reported alkaline magmatismassociated with rifting in northern New Guinea, and Ewartet al. (1992) and Bryan et al. (1997) argued that the vol-canic rocks of east Australia have a rift signature. In anextensive discussion of the origin of the mid-Cretaceousvolcanics in New Guinea, Struckmeyer et al. (1993) alsoargued for a rift-related affinity. If the volcanic activity wasassociated with renewed extension, the cause may havebeen events that occurred at the northern edge of theAustralian Plate between southeast Asia and the Pacific.For example, Müller et al. (2000) drew attention to aregional tectonic event between 100 and 90 Ma related toplate reorganisation between India and Australia at about99 Ma which they postulated was due to the subduction ofa Neo-Tethyan ridge beneath southeast Asia changing thestresses along Australian margins.

Late Cretaceous – Paleocene rifting

Home et al. (1990) inferred that rifting of the Coral Seabegan in the Cenomanian and continued through the LateCretaceous. An important coeval event is the widespreaduplift and erosion of the southern Papuan Basin, inferredby Home et al. (1990) to be due to uplift of the rift-shoul-der. However, similar uplift of the whole eastern margin ofAustralia occurred in the mid to Late Cretaceous, inter-preted by Gurnis et al. (2000) to be due to rebound asso-ciated with the breaking off of a long-lived subductedslab.

Francis (1990) argued that rifting occurred along the north-eastern Australian margin in the Campanian–Maastrichtian,citing as evidence the alkaline basaltic volcanism along theouter shelf and upper slope and common tuffs in the UpperCretaceous mudstone section. He inferred the formation ofmarginal basins, but of unknown distribution. This is similarto the interpretation of Pigram and Symonds (1991, 1993) andDavies et al. (1997) that Late Cretaceous rifting in the CoralSea and along the northern margin led to the formation ofmarginal basins in the latest Cretaceous and Paleocene withthe two systems linked by a transform fault across the westernend of the Papuan Peninsula (Figure 1). Oceanic spreading ofthe Coral Sea occurred in the Paleocene to Eocene (Weissel &Watts 1979; Gaina et al. 1999) but the timing of formation ofmarginal basins along the north coast is less well known due

to poor dating of the ophiolites that record the event, and aNeogene orogenic overprint. On the basis of the evidence fortheir ages discussed above it is possible some of the ophiolitesrepresent marginal basins formed in the New Guinea Marginin the Late Mesozoic or Early Tertiary.

In the Bird’s Head region of Irian Jaya, Brash et al.(1991) recorded an abrupt change in the stratigraphic sec-tion in the Paleocene, from a section which deepens to thewest to one that deepens to the east towards CenderawasihBay. Hill et al. (2002) inferred that this was due to the for-mation of a Paleocene marginal basin floored by oceaniccrust in what is now Cenderawasih Bay, at the same time asextension in other parts of northern New Guinea.

The Late Cretaceous and Palaeogene geological historyis particularly poorly known in New Guinea and severalmodels are possible. The association of mid to LateCretaceous volcanism, rifting, and subsequent oceanicspreading is a pattern that is recognisable from easternAustralia to New Guinea. This suggests some regionalmechanism but it is not clear what this is. The north andeast Australian margins have suffered several episodes ofrifting of thin elongate slivers of crust over hundreds of mil-lions of years. This is a feature of all the reconstructionsmade for the pre-Cenozoic and the best example is the LordHowe Rise. Gaina et al. (2000) and Müller et al. (2001) sug-gested that rifting of the Tasman Sea was initiated by aridge–hotspot interaction, but it is difficult to see how thismechanism could be applied to the north Australian mar-gin, unless there were numerous small plumes. New mod-els for the causes of the rifting are required.

CENOZOIC HISTORY

Sea-floor spreading in the Tasman and Coral Seas ceased inthe Eocene (Gaina et al. 1998, 1999) and Australia beganto move rapidly northwards. Ophiolites and arc terranesnow found in northern New Guinea are here interpreted tohave formed mainly in intra-Pacific oceanic arcs north ofNew Guinea and to have been accreted to the margin.Davies et al. (1997) have summarised these events, includ-ing an Early Eocene collision of the Bena Bena terrane, aLate Eocene to Early Oligocene collision of the Sepik ter-rane, Late Oligocene collision of a Papuan Peninsula ter-rane and Early Miocene collision of the Finisterre terrane,although there is disagreement about the nature and timingof all these events (cf. Abbott 1995; Cullen 1996).

Eocene and Oligocene tectonic events in New Guineaare poorly constrained. Eocene sediments are largely lack-ing in the Papua New Guinea portion of the Fold Belt(Figure 2) but sandstone and limestone were deposited inthe Irian Jaya Fold Belt. In the Papua New Guinea portionof the Mobile Belt (ST on Figure 1), Davies and Hutchison(1982) interpreted the variously metamorphosed LateCretaceous to Eocene clastic and limestone strata to havebeen deposited in a volcanic island arc and trench envi-ronment. Around the Sepik Basin, Wilson et al. (1993)reported strongly recrystallised Middle to Upper Eocenelimestones, consistent with those reported through much ofthe Papua New Guinea portion of the Mobile Belt (Davies1983; Bain & McKenzie 1975). There have been few reportsof Lower–Middle Oligocene strata, but Findlay (2003)


infers that the continentally derived clastics adjacent to theFinisterre Ranges (Figure 1) may range from Eocene toUpper Miocene.

In the Late Oligocene to Early Miocene there was wide-spread shallow-water carbonate deposition across thePlatform and Fold Belt of New Guinea (Figure 2), reflectingregional subsidence, and also carbonate deposition in alow-energy environment around the Sepik Basin (Wilson etal. 1993). However, volcanism is indicated in parts of theMobile Belt. Page (1976) and Rogerson et al (1987) haveidentified Late Oligocene to earliest Miocene gabbroic todioritic intrusions in the Sepik Terrane (ST on Figure 1).Page (1976) and Findlay (2003) reported Late Oligocene toearliest Miocene K–Ar ages on a microsyenite and lava,respectively, from the Ramu River immediately south of theFinisterre Ranges (Figure 1). A further 40 km southwest, aquartz diorite yielded a Late Oligocene zircon fission trackage (Hill & Raza 1999). The carbonates to the south of theSepik Terrane and in and around the Sepik Basin to thenorth of the terrane indicate relatively little volcanogenicdetritus. On the basis of mapping of limestone outcropsand interpretation of seismic data, the Sepik Basin (Figure1) is reported to have been starved of clastic sediment dur-ing the Late Oligocene to Early Miocene (Wilson et al.1993), suggesting limited igneous activity, at least in thatarea.

Metamorphic rocks in the Mobile Belt of northernPapua New Guinea, have previously been dated by K–Aras Late Oligocene to Early Miocene (Page 1976). Using40Ar/39Ar analysis, Crowhurst (1999) redated many of themetamorphic rocks and determined that the samplescooled rapidly from temperatures above 300–500°C mainlybetween 20 and 18 Ma. Based partly on the presence ofadjacent and coeval starved graben and limestone depositsin the Sepik Basin (Wilson et al. 1993; Hill et al. 1993),Crowhurst (1999) inferred that the metamorphic coolingwas due to extensional unroofing of mid-crustal rocks asmetamorphic core complexes. The unroofing was accom-panied by ductile deformation in rocks of the Sepik Terrane(Figure 1) later overprinted by brittle low-angle thrust defor-mation in the Late Miocene and strike-slip deformation inthe Pliocene (Crowhurst et al. 1996, 1997). The 20–18 Mametamorphic cooling ages coincide with a possible tempo-rary cessation of igneous activity as recorded by a gap inthe K–Ar ages for that period (Rogerson et al. 1987).

Carbonate deposition continued throughout the NewGuinea Fold Belt in the Middle Miocene, but widespreadigneous activity is reported in the Mobile Belt of PapuaNew Guinea, the Maramuni Arc (Figure 1) of Dow (1977).Although mainly Middle Miocene in age, Page (1976),Rogerson et al. (1987) and Hill and Raza (1999) reportedthat the igneous activity ranges from late Early Miocene toLate Miocene (ca 17–10 Ma) with minor activity continuingto the Pliocene. The principal products of the arc were sub-aerial pyroclastics and lavas, with thick, widespread vol-canogenic sediments in addition to granodioritic anddioritic intrusions. The Maramuni Arc extends west only tothe Irian Jaya border and there was insignificant volcanismin western New Guinea during the Neogene. The Neogenevolcanic rocks from Irian Jaya that have been studied areLate Miocene and have an unusual chemical character thatis ‘post-collisional’ quite different from Neogene magmatic

rocks elsewhere in New Guinea (Housh & McMahon2000).

The Middle Miocene igneous activity in Papua NewGuinea has generally been interpreted as subduction-related and most authors suggest southwest-dipping sub-duction beneath Papua (Hamilton 1979; Cullen & Pigott1989; Francis 1990; Hill & Raza 1999) although someauthors are uncertain of the polarity (Cullen 1996) or havesuggested north-directed subduction (Abbott 1995). Therehave also been suggestions that volcanic activity of theMaramuni Arc was not subduction-related (Mason &MacDonald 1978; Johnson & Jaques 1980; Findlay et al.1997) but was due to melting associated with uplift follow-ing Oligo-Miocene arc–continent collision. A doubly dip-ping subduction zone similar to that of the Molucca Searegion has been inferred to extend westward under NewGuinea from the Solomon Sea, based on present seismicity(Cooper & Taylor 1987; Pegler et al. 1995). However,tomography does not show a long south-dipping slab (Hall& Spakman 2003). This implies little or no south-directedsubduction or, alternatively, subduction of a slab within afew million years of its formation similar to that on the east-ern side of the present-day Woodlark Basin. Beneath west-ern New Guinea, there is a seismically poorly defined zonesuggesting south-dipping subduction of about 100 km ofocean crust at the New Guinea Trench, but tomographyshows no slab (Spakman & Bijwaard 1998; Hall &Spakman 2003).

The widespread deformation of Miocene and olderrocks throughout the Fold Belt and Mobile Belt of NewGuinea demonstrates the Late Miocene to Pliocene oroge-nesis. Hill and Raza (1999) and Kendrick (2000) usedapatite fission track analyses to infer the onset of compres-sional deformation in the Mobile Belt at the end of theMiddle Miocene, around 14–12 Ma. They demonstratedthat the orogeny propagated south into the Fold Belt in theLate Miocene to Pliocene, but suggested a transition tostrike-slip deformation in the Pliocene. Kendrick (2000)recognised that in Irian Jaya the structural style of the 5 km-high frontal mountains was very different from the low-lying Fold Belt in Papua New Guinea. Hill et al. (in press)suggested that this was due to the orogeny impinging on thestrong Australian lithosphere causing inversion of the largeextensional fault along the continent edge that had beenactive from the Neoproterozoic to the Miocene. In contrast,Cloos et al. (1998) suggested that the crustal-scale upliftwas due to collisional delamination following locking of asubduction zone to the north at ca 7 Ma, for which there isno evidence.

Pliocene igneous activity is mainly manifested aslocalised stocks in the Fold Belt representing considerablysmaller volumes of magma than during the Maramunievent. Davies (1991) pointed out that this igneous activityprogressed southwards across the Fold Belt, coincidentwith the southwards propagation of deformation and uplift,suggesting that it was related to crustal thickening.Pleistocene igneous activity comprises stratovolcanoes inthe New Guinea Fold Belt. Hamilton et al. (1983) carriedout Sr and Nd analyses on six Pleistocene stratovolcanoesand concluded that they were mantle-derived magmas con-taminated by movement through continental crust, butthey could not exclude derivation from subducted crust or


metasomatised mantle. In contrast, the magma generationhas been attributed to partial melting of the lower crust(Mason & Heaslip 1980) or mantle (Johnson et al. 1978) asa consequence of crustal uplift and reduction of overbur-den pressure.

At both ends of the Fold Belt, in the D’EntrecasteauxIslands in eastern Papua New Guinea and the WandamanPeninsula in northwest Irian Jaya (Figure 1), medium- tohigh-grade metamorphic rocks are exposed at surfacewhich have yielded Pliocene–Pleistocene cooling ages (Hill& Baldwin 1993; Baldwin et al. 1993; Bladon 1988). Hilland Baldwin (1993) interpreted the metamorphic rocks ofthe D’Entrecasteaux Islands to have resulted from rapidextension associated with the westward propagation of theWoodlark spreading ridge (Figure 1) (Taylor et al. 1995,1999). The Pliocene–Pleistocene cooling ages reflect rapidexhumation of metamorphic core complexes (Baldwin et al.1993). The Wandaman Peninsula is less thoroughlymapped, but radiometric dating indicates rapid cooling ofmid-crustal rocks during the Pliocene–Pleistocene (Bladon1988). This, too, is here interpreted to result from extensionassociated with orogenic collapse of the ends of the FoldBelt (Hill et al. 2002).

Cenozoic tectonics and plate models

It is generally agreed that there were several rapid changesin tectonic setting during the Cenozoic but there is still dis-agreement about the site of formation of ophiolites, howmany arcs there were, the timing of arc–continent collision,polarity of subduction, the importance of extension and therole of strike-slip faulting. Constraining the geological inter-pretation of the New Guinea margin is only possible byconsidering its tectonic interactions with the plates to thenorth. It is of particular importance in interpreting NewGuinea geology to consider not only Pacific–Australia platevectors, but also relative motions between Australia andsmall plates such as the Philippine Sea and CarolinePlates. Rather than try to summarise the plethora of differ-ent plate-tectonic models, the different inferences that havebeen made, and reconstruct the thoughts of differentauthors from fragmentary interpretations shown in dia-grammatic cross-sections and local plate models, we havetried below to identify the key features of different groups ofmodels.

There are few complete plate-tectonic models for thisregion, largely due to a paucity of data since the regionbetween New Guinea and the Tonga Arc is vast, sparselypopulated, difficult to access, covered in rainforest, andremote. The marginal basins of this region are largelyundrilled, and there has been little marine investigation ofmany of them and their ages and spreading historiesremain uncertain. Early accounts of New Guinea werebased mainly on the work of Dutch geologists that wassummarised by van Bemmelen (1949), and explorationstudies for oil and minerals (Australian PetroleumCompany 1961; Visser & Hermes 1962). These authorsrecognised the eugeosynclinal character of northern NewGuinea to the north of young mountains at the northernedge of the Australian continent which van Bemmelen(1933, 1939) interpreted in a pre-plate-tectonic undationmodel. In contrast, Carey (1958) was one of the few to have

a mobilist view of the region and emphasised the impor-tance of strike-slip faulting: north New Guinea formed partof his Melanesian shear or Tethyan megashear. After platetectonics became the dominant theory it was quickly recog-nised (Davies 1971; Curtis 1973; Packham 1973) thatnorthern New Guinea included volcanic-arc rocks andthese were interpreted as indicating arc–continent collisionfollowed by subduction polarity reversal (Dewey & Bird1970; Hamilton 1970). However, it was not long before thepolarity reversal model was contested (Johnson 1976;Johnson & Jaques 1980).

Plate-tectonic models that have been proposed sincethe late 1970s fall into three groups. First, there are platemodels which consider the regional history of some or all ofthe major plates of southeast Asia, the western Pacific andAustralia in varying detail (Crook & Belbin 1978; Hamilton1979; Wells 1989; Jolivet et al. 1989; Rangin et al. 1990;Smith 1990; Daly et al. 1991; Yan & Kroenke 1993; Lee &Lawver 1995; Hall 1996, 1997, 1998, 2002). Second, thereare local models which have been proposed to account fordetails of the evolution of sections of the Australian margin,for example in eastern New Guinea or the Melanesian arcs(Jaques & Robinson 1977; Johnson & Jaques 1980; Cooper& Taylor 1987; Pigram & Davies 1987; Hill & Hegarty 1987;Hill et al. 1993; Struckmeyer et al. 1993; Abbott 1995;Musgrave & Firth 1999; Hill & Raza 1999; Charlton 2000).Finally, there are many incomplete plate-tectonic modelswhich are typically in the form of diagrams with cross-sec-tions of subduction zones at different stages, or maps whichshow key features, but without portraying the entire historyof the region (Johnson 1976; Falvey 1978; Falvey &Pritchard 1982; Ridgeway 1987; Richards et al. 1990; Beneset al. 1994; Petterson et al. 1997; Monnier et al. 1999;Weiler & Coe 2000; Findlay 2003).

In the New Guinea region almost all models advocatenorthward-subduction of oceanic crust north of Australiabefore collision of the Australian margin with a south-fac-ing arc. A few authors have proposed that the northernAustralian margin was an active margin and there wassouthward-subduction beneath this margin before theactive north Australia margin collided with an arc (Hill &Hegarty 1987; Hill et al. 1993; Monnier et al. 1999, 2000).Early models, and many authors since, suggested a singlearc–continent collision. Variations include opening of mar-ginal basins within the Australian margin with the marginalbasins being destroyed by subsequent subduction, andmultiple subduction zones. In some cases these subductionzones have a consistent polarity, for example, many mod-els suggest more than one subduction zone dipping north-wards north of Australia, but in other models there aresubduction zones which have opposing polarities andwhich appear to have lived for short periods.

The more complex models proposed, or implied, multiplearc–continent collision events. Pigram and Davies (1987)influenced many subsequent ideas on New Guinea whenthey proposed a terrane interpretation which implied a platemodel involving accretion of fragments during subductionand collision. Subsequently, Daly et al. (1991), Lee andLawver (1995) and many others have suggested the additionof arc fragments onto the northern New Guinea margin atseveral periods during the Cenozoic. However, even in thecases where the same arc is discussed there is considerable


divergence of views about the timing of arc–continent colli-sion. For example, ophiolites were emplaced in northernNew Guinea in the Early Cenozoic but it is not clear if thiswas a single event or whether there were multiple obductionevents at different places on the margin (Davies 1971;Hutchison 1975; Davies & Smith 1971; Pigram & Davies1987; Davies et al. 1996; Monnier et al. 1999). Monnier et al.(2000) suggested that parts of the Central Ophiolite Belt inIrian Jaya are Jurassic backarc basin ophiolites. This impliesthat the ophiolites (approximately along-strike from theApril, Marum and Papuan ophiolites) were formed within theAustralian margin which is interpreted by most other work-ers as a passive margin. It is also widely reported that therewas a Late Oligocene or Early Miocene collision of an arcwith the northern New Guinea margin in the region of Papuaalthough many authors have identified this collision asyounger than Early Miocene and have suggested collisionswhich may continue up to the present day (Jaques &Robinson 1977; Abbott 1995; Davies et al. 1996; Hill & Raza1999; Weiler & Coe 2000).

A common theme in many of the models, particularlythose which involve multiple subduction zones and multi-ple collisions, is the diachronous collision of a volcanic arcwith the northern New Guinea margin (Hamilton 1979;Cooper & Taylor 1987; Richards et al. 1990; Weiler & Coe2000). Almost all authors who consider a diachronous col-lision suggest that it started earliest in the west and pro-gressed eastwards to where collision is apparentlyoccurring between the New Britain – Finisterre arc systemand the northern New Guinea margin. To the east of this isthe Solomon Sea and some evidence has been interpretedto suggest that beneath the Papuan end of this segment ofthe collision zone is a doubly vergent subducted slab whichdips to the north and south in similar fashion to the well-known inverted U-shaped configuration of the MoluccaSea (Cooper & Taylor 1987; Pegler et al. 1995). It has beensuggested that both north- and south-directed subductionhave occurred between a volcanic arc and New Guinea anddiachronous collision has progressively added the arc tothe northern edge of New Guinea and closed the interven-ing ocean from west to east leaving the present SolomonSea as a remnant of a wider ocean.

Many of the regional models concerned with NewGuinea terminate at Papua but because the relative motionbetween Australia and the Pacific involves a large westwardcomponent it is important to consider the Melanesianregion east of Papua to determine where the arcs origi-nated. The term Melanesian Arc is given to the system ofarcs extending from New Ireland, through the Solomons, toTonga (Figure 1). The Melanesian Arc originated in theEocene either by rifting away the edge of the Australianmargin above a subduction zone (Crook & Belbin 1978) orby initiation of subduction in an intra-oceanic setting (Yan& Kroenke 1993). Most models agree that from the EarlyCenozoic there was south- or southwest-directed subduc-tion of the Pacific beneath the Melanesian Arc and thiscontinued until the Solomons collided with the OntongJava Plateau. The arc–plateau collision took place in theMiocene but may have occurred over an extended period,with an early soft collision phase involving shortening ofthe Ontong Java Plateau, followed by a hard collision phasewhen the Solomons were transferred to the Pacific Plate

(Kroenke 1984; Petterson et al. 1997). Pacific–Australiaplate convergence by subduction in the Solomons regionthen ceased and subduction transferred to other locationsin the Early to Middle Miocene. It was at this stage that theplate boundary in the Solomons ceased to be a subductionzone and instead became a strike-slip fault zone. For NewGuinea, the importance of this event is that the terminationof subduction beneath the Solomons is suggested to haveled to initiation of new subduction beneath the PapuanPeninsula to form the Maramuni Arc (Hill & Raza 1999).This southwest-dipping subduction zone was active fromthe Early Miocene but by the Late Miocene subductionprobably ceased and instead new north-dipping subductionbegan beneath the New Hebrides and New Britain arcs(Hill & Raza 1999), followed by formation of the WoodlarkBasin and Bismarck Sea (Taylor 1979; Benes et al. 1994;Taylor et al. 1995, 1999).

TECTONIC MODEL FOR THE EVOLUTION OFAUSTRALIA’S NORTHERN MARGIN

A regional plate reconstruction which includes the northernAustralian margin and synthesises information from theocean basins with terrestrial geology has been described byHall (1997, 1998, 2002). Below, we use this reconstruction toprovide the context to interpret the Cenozoic history of theNew Guinea margin. It has features in common with manyof the models outlined above, and proposes several arc–con-tinent collisions, but differs in postulating a dominantlystrike-slip plate-boundary zone in northern New Guinea dur-ing the Neogene. Before 45 Ma ophiolites were formed in theforearcs of intra-oceanic arcs in the Pacific region and thesewere emplaced by diachronous arc–continent collision withthe north Australian passive margin between New Guineaand New Caledonia, probably in the Eocene. Following amajor plate reorganisation at ca 45 Ma, Australia began tomove rapidly northwards and the Philippine Sea, Carolineand Solomon Sea Plates formed as backarc basins. At about25 Ma, collision of the Philippines–Halmahera Arc with theAustralian margin and collision of the Ontong Java Plateauwith the Melanesian Arc led to the loss of two major sub-duction zones. The arcs between the Philippines andMelanesia became a single arc system which rotated clock-wise at the leading edge of the Pacific Plate, accommodatedby intra-plate subduction at the eastern edge of thePhilippine Sea Plate. This created a Neogene strike-slip faultsystem in northern New Guinea with major sinistral dis-placement of the arc relative to New Guinea.

Below is our model for the tectonic evolution ofAustralia’s northern margin (New Guinea) incorporating allthe information reviewed above and the plate-tectonicreconstruction.

Palaeozoic

Northern Australia was characterised by two crustalprovinces, the Tasman Orogen in the east and a stable cra-ton subjected to Gondwana rifting in the west. The TasmanLine, defining the eastern and northern limit of intactProterozoic continental crust, trended north–south ~100km west of the current Irian Jaya – Papua New Guinea bor-


der and then west-northwest near the present mountainfront and through the Bird’s Head (Figure 3). To the eastand north were accreted terranes of the Tasman Orogen,including the northern Bird’s Head. The preservation ofnortheast-trending lineaments through to at least the north-ern limit of the Fold Belt, contiguous with those inProterozoic crust of northeast Australia, may indicate thatextended Proterozoic and older lithosphere underlies theaccreted Tasman terranes.

Triassic

Along Australia’s North West Shelf, Gondwanan post-rift sub-sidence had created large Triassic depocentres whilst insouthern Irian Jaya there was minimal Triassic deposition andperhaps uplift and erosion. In Papua New Guinea, and prob-ably northern Irian Jaya, Upper Permian to Lower Triassicclastics (and unknown older rocks) of Australian affinity weredeformed and metamorphosed in the Early Triassic in a con-tinuation to the northwest of the New England Orogeny ineastern Australia. Orogenesis was probably related to sub-duction to the southwest beneath the northern and easternAustralia margins, that may have continued along the outermargin of New Guinea through the Bird’s Head and SulaSpur, accounting for the widespread Middle Triassic arc(Figure 4). In the Middle to Late Triassic, the tectonic regimeabruptly changed so that the New Guinea margin and theNew England Orogen to the southeast underwent rifting,associated with Late Triassic volcanism.

Jurassic

Mid–Late Triassic and Early–Mid Jurassic rifting in NewGuinea was followed by Middle Jurassic breakup, but it is

possible that only a relatively narrow Triassic arc and fore-arc region separated from the continent (Figure 4). We sug-gest that the underlying crustal architecture stronglyinfluenced the style of rifting and breakup. West of theTasman Line, rifting was confined to the north of theMapenduma Fault, the extensional continent-boundingfault active since the Proterozoic (Figure 5). East of theTasman Line extensional faulting was more widespread. Inaddition, the strong northeast structural grain influencedthe pattern of breakup, creating promontories of extendedcontinental crust separated by embayments of Jurassicoceanic crust.

Early Cretaceous

In the Late Jurassic and Early Cretaceous New Guinea wasa passive margin. Thermal subsidence resulted in deposi-tion of the shelf, shoreline and basin facies which now hostsignificant hydrocarbon reserves, and hence are reasonablywell known. In the Aptian, and particularly the Albian, therewas widespread volcanism in New Guinea as all along theeastern Australia margin. We suggest that this was associ-ated with renewed subduction beneath the margin, asinferred by Norvick et al. (2001) and Sutherland et al.(2001). The subduction beneath New Guinea would havecaused the Jurassic oceanic crust in the embayments alongthe margin to have been deformed into accretionary prisms.

Late Cretaceous and Paleocene

In the Late Cretaceous, rifting was initiated along the north-ern margin of the Australian continent, perhaps associatedwith subduction (Figure 6). The rifting led to formation ofmarginal basins in the Late Cretaceous and Paleocene sep-


Figure 5 3D sketch of the New Guinea margin in the Jurassic, looking west. It is inferred that the strong Proterozoic crust west of theTasman line results in a narrower zone of rifting and that the northeast structural grain influences the pattern of breakup, resulting inrectilinear promontories and embayments as observed on Australia’s North West Shelf.


Figure 6 (a) Latest Cretaceous palaeogeography (after Hall 2002) and (b) a 3D sketch of the margin. Jurassic–Cretaceous subsidenceled to deposition of a thick clastic sequence, including widespread Aptian–Albian volcaniclastics which may indicate subductionbeneath the margin (Norvick et al. 2001) and formation of an accretionary prism in the oceanic embayments. LateCretaceous–Paleocene rifting is inferred to have led to formation of marginal basins, including opening of the Coral Sea (Figure 1).Regional uplift may be associated with rifting (Home et al. 1990) or with dynamic topographic rebound related to movement over aCretaceous subducted slab (Gurnis et al. 2000).

arating slivers of extended continental crust from the mar-gin. In the Paleocene, the Papuan Peninsula separatedfrom Queensland during opening of the Coral Sea.Paleocene marginal basins probably formed along thenorth coast of New Guinea and Paleocene oceanic crustmay have formed in Cenderawasih Bay adjacent to theBird’s Head (Figure 6).

In the Late Cretaceous and Paleocene the platform insouthern Papua New Guinea was uplifted and eroded, pre-viously suggested (Home et al. 1990) to have been associ-ated with rifting in the Coral Sea (Figure 7). Alternatively,this may have been associated with dynamic topographicrebound associated with the movement of Australia over aCretaceous subducted slab as suggested for the easternAustralia margin by Gurnis et al. (2000). WidespreadPaleocene and Eocene sandstones in western Irian Jayaprobably resulted from the uplift and denudation of PapuaNew Guinea and eastern Australia.

Eocene

By the Eocene, spreading in the Coral Sea and other mar-ginal basins had ceased. During the Eocene, and possiblythe Oligocene, some of the hot buoyant crust of the mar-ginal basins, or intra-oceanic forearcs, was obducteddiachronously along the margin, for instance the Papuan,Marum and parts of the April Ultramafics. Southern PapuaNew Guinea was still mainly emergent, but limestone andfine clastics were deposited in the Mobile Belt and northernpart of the Fold Belt and New Guinea Limestone wasdeposited through northern Irian Jaya.

At about 45 Ma there was a major plate reorganisation.Australia began to move rapidly northwards; new north-dipping subduction began about 2000 km north of

Australia beneath the Philippines–Halmahera Arc; newsouth-dipping subduction began northeast and east ofAustralia forming the Melanesian Arc (Figure 8). Between45 and 25 Ma the Philippines–Halmahera Arc remained inapproximately the same position. East of Australia, roll-back of the subduction hinge led to the formation of a widebackarc Solomon Sea Basin as the Melanesian Arc rotatednorth. From about 40 Ma the Caroline Sea opened as abackarc basin by rollback of the Pacific subduction hinge atthe eastern margin of the Philippine Sea Plate. The SouthCaroline Arc was located east of this backarc basin and wasthe site of formation of arc terranes now found in northernNew Guinea.

Oligocene

As Australia moved north in the Oligocene, the residualrifted terranes and marginal basins to the north reached thePhilippines–Halmahera–Caroline subduction zone. Theywere probably incorporated into the Philippines–Halmahera–Caroline subduction complex and accretionaryprism, such that they now occur as basement beneath thenorthern parts of the Meervlakte, Sepik and Ramu succes-sor basins and in the Weyland Terrane (Figure 1). At about25 Ma there was an arc–continent collision when thePhilippines–Halmahera Arc collided with a continentalpromontory along the Australian margin (Figure 9). Weinfer that this event was akin to the Pliocene arc–continentcollision in Timor in that a fold and thrust belt was createdin an island along the oceanic portion of the margin, withrelatively little effect on most of the northern margin, otherthan Miocene subsidence and the local supply of coarseclastic sediment. Low-energy Late Oligocene to EarlyMiocene limestones were deposited southwest of the


Figure 7 3D sketch of the New Guinea margin in the Palaeogene (after Hall 2002). There was significant uplift and denudation insouthern Papua New Guinea, but subsidence in Irian Jaya. Conceptual marginal basins to the north of New Guinea are unproven, butsynchronous with Coral Sea spreading to the southeast (Davies et al. 1996, 1997). The existence of subduction beneath New Guineais uncertain.


Figure 8 (a) Eocene and (b) Oligocene palaeogeography of the New Guinea margin (after Hall 2002). The Eocene onset of convergenceas Australia started moving rapidly to the north may have resulted in obduction of the marginal basins. Oceanic subduction over 1000 kmto the north led to formation of the Philippine and Caroline Arcs and associated backarc basins. Subduction to the southeast beneath thePapuan Peninsula was associated with formation of the Solomon Sea Plate. G, Gauttier terrane, now in northern Irian Jaya (Figure 1).


Figure 9 (a) Palaeogeography and (b) 3D sketch of the margin in the Late Oligocene. The Philippine–Caroline Arc has collided with acontinental promontory along the margin creating an island orogeny as in Timor today. There was a major change in plate motionsaround this time such that the New Guinea margin changed from being convergent to a strike-slip-dominated margin. G, Gauttier ter-rane; BT, Bewani–Torricelli Mountains (Figure 1).

accreted arc, in the Sepik Basin. The origin of the LateOligocene to earliest Miocene gabbros and diorites in theSepik Terrane (Figure 1) is uncertain.

Early Miocene

The Oligocene arc–continent collision, combined with col-lision of the Ontong Java Plateau with the Melanesian Arc,led to the loss of two subduction zones and a major platereorganisation. After these collisions, throughout theMiocene, the arcs between the Philippines and Melanesiabecame broadly a single arc system which rotated clock-wise at the leading edge of the Pacific Plate, accommodatedby intra-plate subduction at the eastern edge of thePhilippine Sea Plate. The Philippine – South Caroline arcterranes, moved along the north Australian margin in acomplex, regionally extensive strike-slip zone. There wastherefore no significant subduction at the northernAustralian margin in Irian Jaya.

We infer that the sinistral strike-slip system in northernNew Guinea had a significant extensional component andthat parts of the extended continental promontories mayhave been rifted away and transported along the marginwith the collided arcs. The extension also led to the forma-tion of metamorphic core complexes that cooled rapidlyfrom ~500°C to near-surface temperatures around 20 Ma(Figure 10). At the same time, the adjacent Sepik Basin wasstarved of sediment and associated with Early Miocene car-bonate deposition. Regionally, the whole New Guinea mar-gin subsided rapidly leading to widespread deposition of

1–2 km of Miocene shallow-water limestone in southernNew Guinea and an abrupt transition into deeper wateralong the northern margin of the Fold Belt.

Middle Miocene

With the loss of the Melanesian subduction zone,Pacific–Australia convergence was accommodated by devel-opment of new subduction zones: southwest-dippingoblique subduction of the Solomon Sea Plate began firstbeneath Papua to form the Maramuni Arc (Figure 11).However, we cannot rule out the alternative, that the arcvolcanism may have resulted from the ongoing obliqueextension that gave rise to the metamorphic core complexesand North New Guinea Basins, accompanied by littleMiddle Miocene subduction beneath the eastern Papuamargin, but by melting of a previously metasomatised man-tle. Strike-slip movement continued in the Middle Mioceneand the widespread volcanism of the Maramuni Arc filledthe North New Guinea basins with volcanogenic sediments.

Late Miocene to Pliocene

When subduction beneath eastern Papua New Guineaceased, the New Hebrides Trench propagated west to startnew north-directed subduction beneath the Solomons andNew Britain Arcs. The inferred cessation of subductionbeneath eastern Papua New Guinea coincided with theonset of compression at around 14–12 Ma, causing defor-mation, uplift and denudation of the Mobile Belt in the Late


Figure 10 3D sketch of the New Guinea margin in the Early Miocene. There was significant subsidence with regional deposition of up to1 km of Lower Miocene carbonates to the south and relatively starved basins to the north. The northern margin is inferred to have been adivergent strike-slip system, giving rise to local metamorphic core complexes with 20–18 Ma cooling ages (Crowhurst 1999) adjacent tostarved basins. The Solomon Sea Plate was obliquely subducted to the west beneath the Papua New Guinea margin (Figures 9, 11).


Figure 11 (a) Middle Miocene palaeogeography (after Hall 2002) and (b) 3D sketch of the margin. Continued oblique subduction ofthe Solomon Sea Plate to the west has given rise to the Maramuni Arc in Papua New Guinea, although, alternatively, it could have beenrelated to ongoing transtension. Volcaniclastic deposition was widespread in northern New Guinea, filling the previously starved basins.G, Gauttier terrane; BT, Bewani–Torricelli Mountains; AR, Adelbert Ranges; FR, Finisterre Ranges (Figure 1).


Figure 12 (a) Pliocene palaeogeography (after Hall 2002) and (b) 3D sketch of the margin. Convergence of the Caroline Arc with NewGuinea from the end of the Middle Miocene created the New Guinea Orogen causing thrusting in the Mobile Belt and Fold Belt, butongoing strike-slip motion between the Mobile Belt and the accreted arc. The Fold Belt was low-lying as it was built on warm, weak andbroken lithosphere. Northward subduction of the Paleocene oceanic crust in Cenderawasih Bay beneath a continental fragment fromnorthern Papua New Guinea caused it to collide with the Bird’s Head (Sutriyono 1999). BT, Bewani–Torricelli Mountains; AR, AdelbertRanges; FR, Finisterre Ranges (Figure 1).

Miocene. We suggest that the continued oblique plate con-vergence was largely partitioned into a sinistral strike-slipfault north of the New Guinea basins and southwest-directed thrusting to the south. Thrust faulting in theMobile Belt included complex juxtaposition of the ophio-lites, accretionary prisms and metamorphic rocks with theMesozoic–Palaeogene distal sediments and Maramuni Arcintrusive, volcanic and sedimentary rocks. In the LateMiocene to Pliocene, the deformation propagated south tothe fold and thrust belt (Figure 12). Throughout NewGuinea this fold and thrust belt is interpreted to have beenlow-lying and lacking a foreland basin because the fold beltoverlay weak, hot, broken and extended continental crustthat subsided beneath it. When convergence recommencedin west Irian Jaya, we infer that a continental sliver, possi-bly rifted from a northern Papua New Guinea promontory,was adjacent to the oceanic crust in Cenderawasih Bay.Subduction of the oceanic material is interpreted to havedrawn this (Weyland) terrane south to collide with theBird’s Head forming the Lengguru Fold Belt.

Pliocene to Holocene

In Irian Jaya the orogenesis impinged on the MapendumaFault marking the northern limit of strong, thick, cold,Proterozoic Australian lithosphere. The fault was invertedand a 15 km thick slab of the crust was thrust to the surfacebuilding the 5 km-high mountains in the Irian Jaya FoldBelt and creating a substantial foreland basin to the south-west (Figure 13). East of the Tasman Line the orogenesiscontinued to produce a low-lying fold belt overlying weaklithosphere. As the crust was now in compression, few mag-

mas were emplaced, except in local areas of dilation at theintersection of old extensional faults with north-northeast-trending fracture zones. This allowed the emplacement ofCu±Au-bearing magmas from deep in the crust or mantle(Grasberg and Porgera on Figure 13). During the LatePliocene, compression probably waned, but did not cease,and strike-slip motion became dominant in the Mobile Beltand Irian Jaya Fold Belt.

To the east of New Guinea, slab-pull forces at the NewBritain Trench led to the formation of the Woodlark spreadingcentre at the former Solomon Sea transform (Figure 12). Veryyoung lithosphere of the Woodlark Basin is now being sub-ducted at the South Solomon Trench. Throughout this period,west-dipping subduction of the Pacific Plate continued at theTonga–Kermadec Trench with rapid rollback of the subduc-tion hinge in the last 10 million years. In the Bird’s Head area,we suggest that the Mobile Belt adjacent to the Lengguru FoldBelt collapsed towards the northeast, giving rise to the 2 Mametamorphic core complex exposed in the WandamenPeninsula and leaving the present Cenderawasih Bay flooredby extended continental crust. The Pleistocene to Holocenestratovolcanoes in the Fold Belt may be related to extensionassociated with the Woodlark and Cenderawasih orogeniccollapse. However, their confinement to the Fold Belt suggestsan underlying feature such as the south-dipping Solomon Seaslab suggested by Davies (1991).

DISCUSSION

Here, we note the rapid changes in tectonic setting associ-ated with the development of New Guinea during the


Figure 13 3D sketch of the New Guinea margin in the Pleistocene. The orogeny impinged on the strong Proterozoic lithosphere westof the Tasman Line, which acted as a buttress causing crustal thrusting. This created 5 km-high mountains in Irian Jaya and an adja-cent foreland basin. During the Pliocene–Pleistocene, local extensional reactivation of northeast-trending lineaments allowed mantle-derived volcanism, including stocks associated with gold mineralisation, e.g. Porgera and Grasberg (Hill et al. 2003). The Fold Belt inPapua New Guinea remained low with no significant foreland basin. Compression abated around 3 Ma and strike-slip faulting becamemore dominant (Crowhurst et al. 1997). MA, Muller Anticline.

Cenozoic. New Guinea cannot be understood simply as theproduct of an arc–continent collision, or the result ofAustralia–Pacific plate interaction, although both are partof the story. Some of the conflicts between interpretationsof New Guinea geology reflect the difficulties of the terrain,and an inadequate knowledge of this large region, but someprobably reflect the complexity of the history of minor platemotions, the differing character of the lithosphere withinthe orogenic belt, and its unpredictable behaviour duringboth contraction and extension.

It is clear from outcrop data and tomographic studies thatstrong, cold Proterozoic lithosphere underlies southwesternNew Guinea and that this had a significant influence onPhanerozoic tectonics. It is likely that the northeast- andnorthwest-trending Proterozoic structural grain influencedJurassic rifting and breakup, resulting in a rectilinear marginof extended continental promontories separated by embay-ments of Jurassic oceanic crust. Furthermore, the extensionalfaulting associated with rifting appears to be confined to thearea north and east of the strong Proterozoic lithosphere,reactivating a continent-bounding extensional fault that hadbeen active since the Proterozoic. To the east, in the accretedPalaeozoic terranes of the Tasman Orogen, the extensionwas more widespread. Finally, during orogenesis, the strongProterozoic litho-sphere had a profound effect when theorogeny impinged on it. This created a foreland basin alongthe New Guinea margin in the Neogene and created moun-tains over 5 km high. In contrast, the area overlying theaccreted Tasman terranes remained a relatively low-lyingfold belt with no adjacent foreland basin.

The change in character of the lithosphere at the inter-preted position of the Tasman Line is also associated withother tectonic features. From about 45 to 25 Ma this wasthe position where there was a change in subduction polar-ity from north-dipping north of New Guinea to south-dip-ping in the Melanesian Arc. There is a strong positive lowermantle anomaly of uncertain origin seen on tomographicimages below Queensland (Hall & Spakman 2003).Together, these observations suggest the Tasman Line is thesite of significant changes in the properties of the mantleand crust which influenced Cenozoic tectonic history.

An important feature of the Cenozoic plate-tectonicreconstruction is the persistent presence of a sinistralstrike-slip fault system in northern New Guinea throughoutthe Neogene at the same time as the Philippine–CarolinePlate rotated clockwise along the margin. These were thedominant features controlling Neogene orogenesis in NewGuinea. We infer that the strike-slip system was transten-sional in the Early Miocene resulting in metamorphic corecomplexes and graben formation and possibly contributingto volcanic activity of the Maramuni Arc. Towards the endof the Middle Miocene, the strike-slip system became trans-pressional resulting in ~100–200 km of shortening alongthe New Guinea margin. The fault system was probablypartitioned into a strike-slip fault along the northern mar-gin, and fold and thrust structures in the Mobile Belt andFold Belt to the south. In the Pliocene, as convergence wasdiminished, strike-slip motion became dominant through-out the Mobile Belt and in the Irian Jaya Fold Belt. At thesame time the areas immediately east and west of mainlandNew Guinea underwent extensional collapse exposingPleistocene metamorphic core complexes.

There is a substantial divergence in the interpretation ofNeogene tectonics between this paper and Findlay (2003)which largely reflects the very different approach taken toaddress the problem. Here, we attempt an analysis on aplate-margin scale incorporating as much regional and localgeological data as possible and inevitably there are discrep-ancies that need testing. In contrast Findlay (2003) and oth-ers in the Geological Survey of Papua New Guinea havecarried out important new mapping in the area between theBena Bena – Goroka Terrane and the Finisterre Ranges.Citing observed interfingering relationships and OligoceneK–Ar dates as critical data, Findlay (2003) concludes that theFinisterre Ranges were close to their present location at theend of the Oligocene and that there has been no significantmotion between the Bena Bena – Goroka Terrane and theFinisterre Ranges since then, ruling out >1000 km of sinistraloffset, as inferred in the model presented here. We acknowl-edge the interfingering of continental and arc detritus estab-lished by Findlay (2003) and the significant thrusting of theFinisterre Range to the south in the last one million years.However, we suggest that prior to that thrusting, there was amajor sinistral strike-slip fault between the Finisterre Terraneand New Guinea and that arc detritus could have beenderived from any of the Philippine – Caroline Arc fragmentsadjacent to the margin. It is worth noting that Liu and Crook(2001) mapped the same area as Findlay (2002) and studiedsediments in the adjacent Huon Gulf and inferred initial col-lision of the Finisterre Terrane with New Guinea in the latestMiddle Miocene to earliest Late Miocene. Furthermore theyestimated propagation of the collision zone to the east at39–55 km/106 y.

Our analysis suggests that the New Guinea Mobile Beltcomprises a collision zone between a north-facingAustralian indented continental margin and a south-facingMesozoic and Palaeogene accretionary prism in front of anarc, all subsequently cut by a Neogene strike-slip fault sys-tem with more than 1000 km sinistral displacement. Thismodel implies the consumption of most Cretaceous andPalaeogene terranes that may have existed to the north ofNew Guinea. Although integration with plate-kinematicsyntheses greatly increases the reliability of the model, itstill relies heavily on the onshore geological record, partic-ularly prior to the Eocene. The dataset is far from completeand considerable new fieldwork and isotopic and palaeon-tological dating need to be done. Some of the main issuesand uncertainties that need to be addressed to test themodel are listed below.

Major issues and uncertainties to be tested

(1) The interpretation of a west-northwest-trendingTasman line through Irian Jaya relies on unpublished dat-ing from Parris (1994). The age and nature of ?Cambrianoceanic crust cropping out in the Irian Jaya Fold Beltshould be confirmed.

(2) Our model assumes a relatively fixed position of theBird’s Head relative to Australia. This can in part be testedby studies of the relationship between the Bird’s Head andNorth West Shelf sediments to the south following the cur-rent acquisition of considerable new, high-quality reflec-tion-seismic data due to the very large gas discoveries inBintuni Bay in the late 1990s. It can also be tested by fur-


ther provenance studies of the Mesozoic sandstones in theBird’s Head region.

(3) Whilst substantial strike-slip motion is inferred innorthern New Guinea, our model infers minimal strike-slipmotion between the Fold Belt and the Mobile Belt, in partbased on northeast-trending lineaments cutting across theboundary, due to inferred continental promontories (Hill etal. 2003). This contrasts strongly with previous models andshould be tested using remote sensing images and furtherisotopic analysis in the Mobile Belt.

(4) A considerable portion of our New Guinea tectonicmodel relies on the dating of the ophiolites, particularlyinterpretations of Paleocene marginal basins. New datingand geochemical analysis of the ophiolites would improveall models.

(5) The arc or rift nature of Triassic and Permian plutonsis unknown and could be tested geochemically.

(6) Similarly, the chemical character of the MaramuniArc magmatic activity is almost unknown and a subduc-tion-related interpretation is not supported by tomography.Geochemical studies may be able to differentiate betweeninterpretations of subduction-related, transtension-related,or post-collision magmatism.

(7) Substantial Neogene strike-slip motion is hereinferred between the Finisterre volcanics and the sedimentsof the Markham Basin to the south, overprinted by rapidPleistocene convergence and several kilometres of uplift.This should be further tested by field mapping and datingand structural restoration of the Finisterre Ranges.

(8) New models need to be developed to explainepisodes of rifting of the north and east Australian marginsto produce thin elongate slivers of crust several times dur-ing hundreds of millions of years.

ACKNOWLEDGEMENTS

This paper greatly benefited from an early informal reviewby Martin Norvick and helpful and constructive refereeingby Heike Struckmeyer, Bob Findlay and Carmen Gainawith additional comments from Dietmar Müller. Theresearch over many years that has contributed to thisreview has been funded by many oil and mining companieswho are gratefully acknowledged, particularly in their sup-port of PhD theses by Crowhurst, Kendrick and Sutriyono.Hill was in part supported by the Australian GeodynamicsCooperative Research Centre. Financial support to R. Hallhas also been provided by NERC, the Royal Society, theLondon University Central Research Fund, and the RoyalHolloway SE Asia Research Group.

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Received 27 August 2001; accepted 11 June 2002

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