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

Mesozoic–Cenozoic evolution of Australia’s margin in a west Pacific context

K. 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 onset of tectonic collapse. In the Palaeozoic, New Guinea contained the boundary between a Late Palaeozoic active margin in the east and a region of extension associated with Gondwana breakup along the western margin of Australia. In the Permian and Early Triassic, New Guinea was an active margin resulting in widespread Middle Triassic granite intrusions. The Mesozoic saw Triassic and Jurassic rifting followed by Cretaceous passive margin subsidence and renewed rifting in the Late Cretaceous and Paleocene. Since the Eocene, New Guinea tectonics have been driven by rapid northward movement of the and later sinistral oblique convergence with the , 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 a north-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 that has alternated between extension and compression. The change in character of the lithosphere in New 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, , wrench faults

INTRODUCTION Guinea which is preserved in the New Guinea Fold Belt (Figure 1). The Cenozoic was characterised by orogenic Australia’s northern margin, in New Guinea, is part of a events, including ophiolite emplacement, arc–continent complex convergent plate boundary between the Indo- collision and the development of the major mountains Australian Plate, the Pacific Plate, and several other known today. However, despite the importance and youth smaller plates including the Philippine Sea and Caroline of these events, their timing and significance remain the Plates (Figure 1). Plate-tectonic reconstructions suggest subject of much debate. that the relative motions between all of these plates must In this paper, we aim to synthesise knowledge of New be considered when attempting to reconstruct the history of Guinea geology with a plate-tectonic reconstruction of Australia’s New Guinea margin during the Cenozoic. These Australia’s northern margin for the Cenozoic. Furthermore, small plates did not exist prior to the Cenozoic and north of we consider the increased understanding of Palaeozoic and Australia there is a region with indications of a long sub- Mesozoic tectonics along Australia’s northern margin and duction history so that much of the oceanic record of pre- its implications for the interpretation of Cenozoic plate vious events has been lost. Reconstruction of the geological motions and orogenesis. We suggest that the geology of history relies on the geological record from outcrops in New Australia’s northern margin results from the interaction Guinea, radiometric and palaeontological dating, regional between the fabric of the Australian continent and the adja- palaeomagnetic data and regional plate-tectonic recon- cent plate motions. Finally, we review some of the main tec- structions. tonic issues and uncertainties in New Guinea still to be Rare basement outcrops suggest a fundamental differ- resolved. Thus, our paper is divided into four parts: (i) a ence between east and west New Guinea in the Palaeozoic, review and discussion of constraints provided by the consistent with the differences between eastern and west- Palaeozoic, Mesozoic and Cenozoic geology and architec- ern Australia further to the south. During the Palaeozoic ture of the northern Australia continental margin; (ii) a there was an important change from the eastern Tasman review and discussion of constraints provided by the Orogenic belt to the region of extension associated with Cenozoic tectonic history of New Guinea in the context of Gondwana breakup of an older stable craton along the the plate-tectonic history of the west Pacific and southeast western margin of Australia. Following Triassic–Jurassic rift- ing and breakup a passive margin developed in New * 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 zones and thick lines are active spreading centres. The light blue shaded areas, drawn at the 200 m isobath, are the continental shelves of Eurasia and Australia and areas of thickened /arcs. (b) New Guinea showing sim- plified tectonic belts and the principal tectonic features. AB, Amanab Block; AR, Adelbert Ranges; BB, Bintuni Basin; BG, Bena Bena–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, Sepik Basin; SG, Strickland Gorge; ST, Sepik Terrane; Wa, Wandaman Peninsula; WT, Weyland Terrane. Tectonic evolution of New Guinea 267

Asia; (iii) a tectonic synthesis of Australia’s northern margin (Figure 1) (Dow 1977; Hill et al. 1996). In the south is the integrating the plate-margin architecture and the plate-tec- Stable Platform, which is the northern continuation of tonic motions; and (iv) tectonic issues still to be resolved. Australian continental crust that preserves Mesozoic and The Cenozoic plate reconstructions are described in possibly Palaeozoic extensional structures (Cole et al. detail in Hall (2002), accompanied by computer anima- 2000; Kendrick 2000). The adjacent Fold Belt, to the north, tions, and are based on a synthesis of a range of data, from comprises the same crust deformed into fold and thrust spreading histories obtained from small ocean basins to structures following Late Oligocene–Miocene arc–continent more qualitative information such as that obtained from collision. The fold and thrust deformation occurred in the terrestrial geology. The reconstructions were made with the Late Miocene to Holocene (Hill 1991; Hill & Raza 1999). ATLAS computer program (Cambridge Paleomap Services Most of the northern half of New Guinea is made up of the 1993) using the Indian–Atlantic frame of Müller et Mobile Belt, including ophiolites of Mesozoic to Paleocene al. (1993). Full details of the approach and the model are age (Davies & Jaques 1984; Rogerson et al. 1987). The provided in Hall (2002). The pre-Cenozoic reconstructions Mobile Belt also includes distal Mesozoic–Tertiary sedi- are based mainly on the geological data from New Guinea ments, abundant Miocene and some Cretaceous volcanic and the architecture of the margin, as discussed below. and intrusive igneous rocks, and medium- to high-grade metamorphic rocks with common Early Miocene and some Mesozoic cooling ages. In places, the Mobile Belt has been NEW GUINEA GEOLOGICAL ARCHITECTURE highly deformed, with a record of local medium- to high- grade metamorphic rocks that record ductile deformation, The island of New Guinea has been divided into four tec- particularly along strike-parallel shear zones. These have tonic regions based on the Miocene to Holocene orogene- been overprinted by low-angle thrusting and sinistral strike- sis affecting the northern part of the Australian Plate slip structures (Crowhurst et al. 1996, 1997). The northern

Figure 2 Simple chronostratigraphic column for the Fold Belt in New Guinea (after Kendrick 2000; Pigram & Panggabean 1984; Home et al. 1990; Parris 1994), showing the preserved Palaeozoic section west of the Tasman Line and the low-grade Permian to Early Triassic metasediments to the east, intruded by Middle Triassic and Early Jurassic granites. Following Jurassic breakup, the clastic Mesozoic section is relatively uniform. Early Tertiary uplift and denudation in New Guinea led to a significant unconformity in the east prior to deposition of New Guinea Limestone, whilst there was continuous deposition in the west. The Late Miocene marks the onset of deposition of a molasse sequence. 268 K. C. Hill and R. Hall rim of New Guinea is interpreted to consist of accreted Palaeogene arcs and oceanic terranes including the Gauttier and Cyclops mountains, the Bewani–Torricelli mountains, the Finisterre Ranges and the Huon Peninsula (Pigram & Davies 1987). The suture between these terranes and the Mobile Belt is partly covered by the Miocene to Pliocene Meervlakte, Sepik and Ramu successor basins (Francis 1990). The successor basins are reported to be underlain by Mesozoic and Early Tertiary igneous and metamorphic 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 and suggest other interpretations.

PROTEROZOIC HISTORY

The Arafura Platform to the south of the Irian Jaya Fold Belt, comprises Mesoproterozoic cratonic basement (Pieters et al. 1983) overlain by Silurian–Devonian to Jurassic sedimentary sequences that record rifting of the Gondwana margin (Pigram & Panggabean 1981, 1984). To the north, the 5 km-high frontal ranges of the Fold Belt expose a thick, relatively undeformed uppermost Proterozoic and Palaeozoic sedimentary section that over- Figure 3 Devonian–Carboniferous setting of the New Guinea lies Proterozoic marine rift metabasalts and metavolcanics margin (after Metcalfe 1996 figure 12). NG, New Guinea. of the Nerewip Formation exposed near the Grasberg Mine (Figures 1, 2) (Parris 1994). This Neoproterozoic sedimen- tary section may record erosion of the proposed Rodinian have been a long-lived active margin, similar to the west- supercontinent from ca 725 Ma, when Laurentia separated facing Rocky Mountains of North America in the Mesozoic from Australia (Powell 1996) giving rise to the Tasman Line, and Palaeogene (Coney et al. 1990). The youngest defined by the eastern limit of Proterozoic crust in Australia Palaeozoic event along the eastern margin was the Permian (Scheibner 1974). If so, then the Tasman Line trends New England Orogeny recorded in New South Wales and roughly north–south near the – Irian Queensland that continued into the Early Triassic (Korsch Jaya border and then trends approximately west-northwest in press). Upper Permian to Lower Triassic low-grade along the front of the ranges (Figure 1). metasediments intruded by Middle Triassic granites, The change in basement across the north–south part of recorded in Papua New Guinea suggest a similar history the Tasman Line in Australia is consistent with tomo- (Van Wyck & Williams 2002; Crowhurst 1999). In contrast graphic images of the Australian region that indicate a the margin west of the Tasman Line underwent substantial marked change in the character of the lithosphere (Simons extension and rifting, with major microcontinents breaking et al. 1999; Debayle & Kennett 2000; Hall & Spakman away in the Early Palaeozoic and the Permo-Carboniferous 2003). To the west of Queensland is lithosphere with high (Metcalfe 1996, 1998) (Figure 3). The thick, relatively velocities beneath much of northern Australia which corre- undeformed Palaeozoic section in mainland Irian Jaya is sponds to old Precambrian crust, and a cold, very thick consistent with this rift history (Figure 2). lithosphere with great strength. To the east of the western In contrast to mainland Irian Jaya, the widely exposed edge of Queensland are much lower lithosphere velocities basement of the Kemum Terrane in the Bird’s Head com- consistent with younger, hotter and much weaker conti- prises Silurian–Devonian low-grade metamorphic rocks nental lithosphere east of the Tasman Line. intruded by Triassic granites. This has led to models sug- gesting that the Bird’s Head was derived from Papua New Guinea or eastern Australia (Pigram et al. 1985; PALAEOZOIC HISTORY Struckmeyer et al. 1993). It is suggested here that the Tasman Line continued in a more east–west direction The Palaeozoic margin in New Guinea is here interpreted through Irian Jaya and that to the north of it were rocks to have been divided into two parts: an eastern active mar- similar to those of the Tasman Orogen, which rifted off gin and a western rifted margin, with the boundary roughly Australia during the Mesozoic. Therefore an alternative to along the Tasman Line. East of the line in Australia, rocks deriving the Bird’s Head from eastern Australia is that dur- of the Tasman Orogen are thought to largely overlie ing the Palaeozoic the Bird’s Head was north of the Tasman Cambrian or Ordovician oceanic crust (Aitchison et al. Line in a position relative to the Australian continent simi- 1992) and to have been accreted in multiple orogenies lar to the present. throughout the Palaeozoic and earliest Triassic (Veevers In the Papua New Guinea Fold Belt, basement crops 2000). This east-facing edge of Gondwana is believed to out as Triassic granites in the Kubor Ranges and in the Tectonic evolution of New Guinea 269

Strickland Gorge (Page 1976; Van Wyck & Williams 1998, Australian continent. Further, they dated the Bena Bena 2002). The Kubor Granites intrude the low-grade Omung terrane as Permian to Late Triassic and concluded that metasediments, which were deposited in the Late Permian Australia’s cratonic margin extends at least as far north as to Early Triassic and contain Permian to Proterozoic detri- the northern edge of the Bena Bena terrane, adjacent to the tal zircons showing affinity with Australia (Van Wyck & Finisterre Ranges (Figure 1). Crowhurst (1999) confirmed a Williams 1998, 2002). Lying well to the northeast of the Middle Triassic U–Pb age on zircons for the Amanab meta- inferred Tasman Line, it is likely that east Papua is under- diorite, indicating that it intruded a Palaeozoic or older ter- lain by crust no older than late Precambrian. Crowhurst’s rane, consistent with the Ordovician ages from inherited (1999) U–Pb age dating of zircons from the Amanab diorite zircons in the metadiorite. They also reported 16 Nd–Sr yielded an Ordovician inherited zircon population, consis- analyses, which indicate the type of underlying crust and tent with this hypothesis. However, recent U–Pb age dating source of the intrusions, and concluded that the of zircons in two young intrusive rocks in Papua New Amanab–Idenburg–Landslip terrane was probably a Guinea indicates the presence of Archaean grains. The promontory of extended continental crust, possibly rafted Porgera intrusive complex in the western Papua New in, adjacent to an embayment of oceanic crust to the east. Guinea Fold Belt reveals a mixture of Late Miocene and Many tectonic models suggest that some continental Archaean zircons (Munroe & Williams 1996). Similarly, crust has been moved along the New Guinea margin since Pliocene and Archaean U–Pb ages were obtained from zir- the Early Miocene by strike-slip faulting (Hall 2002). In the cons from gneiss and granodiorite in the D’Entrecasteaux island of Bacan, just southwest of Halmahera, there are Islands of eastern Papua (Baldwin & Ireland 1995). These high-grade continental metamorphic rocks including two datasets could indicate underlying Archaean terranes, Barrovian kyanite–staurolite–garnet gneisses (Brouwer but alternatively may demonstrate that the underlying 1923; Hall et al. 1988) which yield very young (<<1 Ma) Palaeozoic terrane remained close to Australia and K–Ar and Ar–Ar ages (Malaihollo 1993; Malaihollo & Hall received sediments derived from the Archaean of 1996). The chemistry of most of the present-day volcanic Gondwana. rocks of the Halmahera islands, including Bacan, indicates a typical intra-oceanic arc character but at the southern Northern limit of continental crust end of the arc there is evidence of contamination by under- lying continental basement (Morris et al. 1983). The present northern limit of intact Australian continental Geochemical work on older volcanic rocks (Forde 1997) crust is important as it helps to define the terranes accreted similarly shows an intra-oceanic arc character for all pre- during the Cenozoic. In addition, it is important to identify Neogene volcanic rocks, but continental crustal contami- the terranes removed from the margin in the Cenozoic in nation of ?Middle to Late Miocene igneous rocks at the order to determine the previous margin geometries. south end of the Neogene arc, suggesting that Australian Davies et al. (1997) and Pigram and Davies (1987) sug- and Philippine Sea arc crust were brought into contact by gested that in Papua New Guinea continental crust extends Early to Middle Miocene collision and subsequent strike- to the northern limit of the Fold Belt, defined by the Lagaip slip movements. Pb, Nd, Sr and O isotope geochemical Fault and the Kubor Range (Figure 1), and that all terranes studies (Forde 1997) of Neogene volcanic rocks on Bacan to the north were accreted. In contrast, Rogerson et al. show that some of the contamination could have been pro- (1987) and Simandjuntuk and Barber (1996) proposed that duced by Permian granites similar to those known from continental crust continues beneath the Mobile Belt as far Queensland and Banggai–Sula. However, extreme Pb iso- north as the suture with the accreted arcs. Rogerson et al. tope ratios in some volcanic rocks suggest that an old (1987) suggested that the ophiolites exposed in the western Precambrian component is also present deeper beneath Papua New Guinea Mobile Belt were transported there as Bacan (Vroon et al. 1996). The Bacan rocks are within thin nappes. In Irian Jaya continental crust is thought to strands of the zone, bound to the north and underlie the Fold Belt as far north as the Derewo Fault, and south by Mesozoic ophiolites with a is bounded to the north by the Central Ophiolite belt and origin. They are clearly fault-bounded and must have the Mobile Belt (Figure 1). moved from the east. Similar small fragments of continen- Two terranes constrain the interpretations regarding the tal crust are known in Obi (Figure 1). In Obi and in the limit of Mesozoic continental crust in Papua New Guinea. Banggai–Sula islands there are also Mesozoic sediments of The first is the pre-Jurassic metasediments, minor metaba- Australian margin character. These islands demonstrate the sic rocks and synkinematic granites of the Bena dispersal of continental crust fragments, undoubtedly Bena–Goroka terrane (Rogerson & Hilyard 1990) in the derived from northern New Guinea. Mobile Belt to the northeast and east of the Kubor Range. Following Crowhurst (1999), the second, composite, ter- Basement fabric rane in the Mobile Belt is here defined to include the metabasics intruded by Permian–Triassic diorite in the Davies (1991), Corbett (1994) and Hill et al. (1996), Amanab block of northwest Papua New Guinea, the con- amongst others, recognised a northeast structural grain in tiguous Landslip block to the south and adjacent Idenburg New Guinea, here interpreted to be inherited from the fab- inlier across the border in Irian Jaya (Figure 1). ric in the underlying crust. Davies (1991) recognised major Van Wyck and Williams (1998, 2002) used U–Pb dating north–south or northeast-trending alignments of volcanoes, of detrital zircons from the Goroka terrane to correlate it stocks and normal faults cutting across the Fold Belt. Hill with the nearby Omung metasediments, i.e. deposited in (1991) interpreted similar lineaments as lateral ramps com- the Permian to Early Triassic and derived from the partmentalising the structures of the Papuan Fold Belt. 270 K. C. Hill and R. Hall

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

Triassic orogenesis, igneous activity and rifting

EARLY TRIASSIC

Van Wyck and Williams (2002) demonstrated that the metasediments intruded by Middle Triassic granites were deposited and deformed in the Late Permian to Early Triassic, perhaps during an event correlating with the New England Orogeny along Australia’s eastern margin (Korsch in press). Metcalfe (1996) inferred subduction beneath the Figure 4 Schematic Early to Middle Triassic palaeogeography eastern Australia and Papua New Guinea margin to (after Metcalfe 1996 and Charlton 2001). The dashed line account for the orogenesis and volcanism. Early Triassic through northern New Guinea is the inferred site of Late Triassic orogenesis is consistent with the stratigraphic section to Jurassic rifting. The part of New Guinea shaded red is the recorded in the fold belt of Irian Jaya (Kendrick 2000) Stable Platform (see Figure 1). (Figure 2). There the Palaeozoic section does not show evi- dence of Triassic deformation, but the Triassic section appears to be absent indicating non-deposition or regional Both authors inferred the trends to result from underlying uplift and erosion, recorded by an unconformity within the crustal or lithospheric features. Corbett (1994) defined Tipuma Formation (Figure 2). northeast-trending transfer structures throughout the Papuan Fold Belt and related them to the copper–gold MIDDLE TRIASSIC deposits. Hill et al. (1996) and Kendrick (2000) linked the north–south and northeast structural grain with a similar Middle Triassic igneous activity has been documented in rectilinear pattern of shear zones in northern Australia Papua New Guinea by Page (1976), Crowhurst (1999) and recognisable from structural, regional gravity and magnetic Van Wyck and Williams (1998, 2002). As summarised by mapping (Elliot 1994). Hill et al. (2002a) inferred that the Charlton (2001), Late Permian–Triassic igneous activity is northeast-trending lineaments continued across the Mobile also recorded in the Bird’s Head of Irian Jaya and the Belt, limiting the amount of strike-slip motion that can be nearby continental terranes of the greater Sula Spur, such inferred between the Mobile Belt and Fold Belt. as east Sulawesi and Banggai–Sula as well as Sibumasu (peninsula Myanmar, Thailand, western Malaysia and northwest Sumatra). The precise age of these intrusives is MESOZOIC AND PALEOCENE HISTORY less well constrained than the Papua New Guinea exam- ples. In New Guinea most of the Middle Triassic intrusive The Mesozoic history of the New Guinea margin is well rocks are granitic to dioritic and are interpreted to result recorded in the sedimentary record. The relatively uniform from the northwestern extension of a volcanic arc along stratigraphy in the New Guinea Fold Belt (Figure 2) records Australia’s east coast related to subduction to the south- roughly synchronous rifting and subsidence of the margin. west beneath the continent (Figure 4) (Metcalfe 1996). We Widespread Middle Triassic igneous activity preceded Late interpret the Permo-Triassic orogenic belt to have extended Triassic and Early Jurassic rifting, and Mid-Jurassic from eastern Australia to Papua New Guinea and along the breakup, although it is unclear which terrane, if any, sepa- north coast of New Guinea after Metcalfe (1996). rated from New Guinea. The Valanginian–Aptian sequence in New Guinea is mainly shale recording a flooding event LATE TRIASSIC as the margin subsided into deep water (Pigram & Panggabean 1984; Home et al. 1990). Similar mudstone Pigram and Panggabean (1984), Home et al. (1990) and deposition associated with subsidence and flooding is Struckmeyer et al. (1993) inferred rifting in New Guinea in recorded along Australia’s North West Shelf to the the Middle to Late Triassic, requiring a rapid change from Carnarvon Basin (Bradshaw et al. 1998), but not down compression to extension in the Middle Triassic. Crowhurst Australia’s east coast (Norvick et al. 2001). Widespread vol- (1999) and Van Wyck and Williams (2002) have dated canism occurred in the Aptian–Albian and Cenomanian, magmatic events in Papua New Guinea at 240 Ma and ca but there is little evidence of later Cretaceous igneous activ- 220 Ma, consistent with previous dating of Page (1976). We ity. Late Cretaceous rifting (Home et al. 1990) is inferred in infer that these correspond to a widespread Middle Triassic the Gulf of Papua associated with uplift and erosion of arc at ca 240 Ma, and Late Triassic extension-related vol- southern Papua New Guinea. Pigram and Symonds (1991) canism at ca 220 Ma. Correlating these events with those in inferred that rifting also occurred along the northern New the New England Fold Belt, they suggest that Late Permian Guinea margin with the possible formation of Late to Early Triassic orogenesis followed by Middle Triassic arc Cretaceous marginal basins. In order to reconstruct the volcanism and Late Triassic extension and rifting may have Tectonic evolution of New Guinea 271 been common to Australia’s eastern margin and New Monnier et al. (1999, 2000) suggested that the Central Guinea. This is consistent with the interpretation of Ophiolite Belt in Irian Jaya was formed in a Jurassic Stampfli and Borel (in press) that renewed rifting along the backarc basin, although no new radiometric dating was north Australian margin of Tethys occurred in the Late presented, whereas ophiolites of the Cyclops Mountains Triassic, due to subduction of the Palaeo-Tethys ridge were suggested to have formed in an Oligocene backarc beneath Asia. basin, based on a small number of K–Ar ages.

SUMMARY Mesozoic rifting An important issue for Australia’s northern margin is the The rift history recorded in the sediments of the Papuan location of the Sula Spur terranes, including the Bird’s Basin was well documented by Pigram and Panggabean Head. Struckmeyer et al. (1993), following Pigram and (1984) and Home et al. (1990). Both recorded Mid–Late Panggabean (1984), inferred that, in the Triassic, these ter- Triassic and Early–Mid Jurassic rifting events followed by ranes lay to the northeast of Queensland in the position of breakup and formation of a passive margin. The effect of the present Solomon Sea. Recent reflection seismic and rifting and breakup in what is now the Mobile Belt has drilling data in the Bintuni Basin, immediately south of the received little attention, mainly due to a paucity of data and Bird’s Head, show a rifted Permian and Upper Palaeozoic uncertain terrane provenance. Hill et al. (in press) com- section akin to that along Australia’s North West Shelf and pared Australia’s northern margin in the Mesozoic to the in Irian Jaya (Sutriyono 1999). Thus it seems likely that the extensional margin preserved along Australia’s North West Sula terranes were adjacent to Irian Jaya as indicated by Shelf (Chen et al. 2002). Hill et al. (in press) noted abrupt Metcalfe (1996) and Charlton (2001). Here we follow ~50 km offsets of the ophiolite belts along strike in New Metcalfe in placing the Sula terranes northwest of Irian Guinea in compartments 125 or 250 km wide and inferred Jaya, accounting for the lack of Triassic igneous rocks a rectilinear post-rift margin in the Mesozoic, with promon- recorded in mainland Irian Jaya (Figure 4). tories of extended continental crust separated by embay- ments of newly formed oceanic crust. This interpretation is Ophiolites in New Guinea consistent with that of Crowhurst (1999) based on isotopic analyses of crust in the Amanab area, discussed above. The ages and time(s) of emplacement of the ultramafic com- The nature of the terranes rifted away from New Guinea plexes interpreted as ophiolites in northern New Guinea are in the Jurassic is unclear. Many authors show various important in any reconstruction, but are not well known and southeast Asian terranes, such as those of parts of Sumatra, are based largely upon the work completed by Bureau of Sulawesi and the Banda arc, situated to the north of New Mineral Resource’s (now Geoscience Australia) geologists in Guinea in the Palaeozoic (Audley-Charles et al. 1988; the late 1970s and early 1980s. Davies and Jaques (1984) Audley-Charles 1991; Metcalfe 1996). Struckmeyer et al. summarised the known geology of the three major ophiolite (1993) inferred separation of the Sula Spur terranes from complexes in Papua New Guinea. Based on K–Ar dating of northern Papua New Guinea and separation of the Bird’s three gabbro and basalt samples, they inferred that the Head from north Queensland in the Cretaceous. In con- rocks of the Papuan Ultramafic Belt crystallised in the trast, Charlton (2001) shows no terranes north of New Jurassic and/or Cretaceous, but that two K–Ar dates on the Guinea in the Triassic and the Sula Spur area lying to the underlying metamorphic rocks indicated emplacement in west, in its present position, throughout the Palaeozoic. the Eocene. The Marum ophiolite yielded similar Early If the Middle Triassic igneous activity in Papua New Jurassic and Paleocene ages (Page 1976) and is overlain by Guinea was subduction related, the fact that the ca 240 Ma a sequence of Eocene argillites (Jaques 1981). Davies and plutons are preserved at Amanab and in the Kubor Range Jaques (1984) interpreted it to be of Late Mesozoic or possi- indicates that only the relatively narrow forearc region can bly Early Tertiary age and emplaced after the Middle Eocene have separated. Allowing a wide arc–trench distance of but before the Late Oligocene–Early Miocene. Applying ~300 km, the departing terranes are likely to have been a apatite fission track analysis to two samples, Hill and Raza narrow strip of up to that width. If, as suggested, the Sula (1999) confirmed a Palaeogene or older age for the Marum Spur terranes and others like West Sulawesi lay to the ultramafic suite and their analysis indicated that the rocks north of Irian Jaya in the Triassic (Metcalfe 1996) then have not been heated to temperatures greater than 100°C some of those terranes would have rifted away in the Mid since then. They also confirmed thrusting of the ophiolite at to Late Jurassic (Figure 4). The Bird’s Head is an exception. about 5 Ma. Hill et al. (2002) used the shallow water clastic Cretaceous The April Ultramafics (Figure 1) were thought to be sediments in that area to show that it was attached to Mesozoic or older (Davies & Hutchison 1982) and to have northern Australia until the Paleocene. This is further sup- been emplaced by tectonic stacking in an Eocene subduc- ported by the recognition of Early Palaeozoic detrital zir- tion system followed by further tectonism during Oligocene cons in the Paleocene Waripi section of Bintuni Bay arc–continent collision (Davies & Jaques 1984). In con- (Sutriyono 1999) suggesting that the Bird’s Head was trast, Rogerson et al. (1987) considered the April attached to the Australian continent. Ultramafics to be of Early Palaeogene age, by analogy with the Papuan and Marum ultramafic suite. The ages of the Widespread Aptian–Albian volcanism ophiolites in Irian Jaya are even less well known. Pigram and Davies (1987) suggested that the ophiolites are Northern New Guinea was a passive margin through the Jurassic and Cretaceous and possibly lower Paleocene. Late Jurassic and Neocomian (Home et al. 1990; 272 K. C. Hill and R. Hall

Struckmeyer et al. 1993), but in the mid-Cretaceous there to poor dating of the ophiolites that record the event, and a was volcanic activity, as recorded in the Kondaku Tuff and Neogene orogenic overprint. On the basis of the evidence for Kumbruf Volcanics exposed around the Kubor Range. Hill their ages discussed above it is possible some of the ophiolites and Gleadow’s (1990) apatite fission track analyses of mid- represent marginal basins formed in the New Guinea Margin Cretaceous clastic samples from the Papuan Basin showed in the Late Mesozoic or Early Tertiary. common contemporaneous volcanic grains. Since then, zir- In the Bird’s Head region of Irian Jaya, Brash et al. con fission track analyses of mid-Cretaceous and younger (1991) recorded an abrupt change in the stratigraphic sec- formations across New Guinea have revealed common tion in the Paleocene, from a section which deepens to the mid-Cretaceous grains, indicating a widespread and signif- west to one that deepens to the east towards Cenderawasih icant volcanic event (Sutriyono 1999; Hill & Raza 1999; Bay. Hill et al. (2002) inferred that this was due to the for- Kendrick 2000). This event may have been related to the mation of a Paleocene marginal basin floored by oceanic resumption of volcanism along Australia’s eastern margin crust in what is now Cenderawasih Bay, at the same time as (Norvick et al. 2001), which deposited vast amounts of extension in other parts of northern New Guinea. Aptian–Albian volcanogenic sediments in the Surat Basin The Late Cretaceous and Palaeogene geological history in Queensland (Exon & Senior 1976; Fielding et al. 1990). is particularly poorly known in New Guinea and several The volcanism has been interpreted to be due to subduc- models are possible. The association of mid to Late tion beneath Australia’s eastern margin (Jones & Veevers Cretaceous volcanism, rifting, and subsequent oceanic 1983; Elliot 1993; Norvick et al. 2001; Sutherland et al. spreading is a pattern that is recognisable from eastern 2001), but Francis (1990) reported alkaline magmatism Australia to New Guinea. This suggests some regional associated with rifting in northern New Guinea, and Ewart mechanism but it is not clear what this is. The north and et al. (1992) and Bryan et al. (1997) argued that the vol- east Australian margins have suffered several episodes of canic rocks of east Australia have a rift signature. In an rifting of thin elongate slivers of crust over hundreds of mil- extensive discussion of the origin of the mid-Cretaceous lions of years. This is a feature of all the reconstructions volcanics in New Guinea, Struckmeyer et al. (1993) also made for the pre-Cenozoic and the best example is the Lord argued for a rift-related affinity. If the volcanic activity was Howe Rise. Gaina et al. (2000) and Müller et al. (2001) sug- associated with renewed extension, the cause may have gested that rifting of the Tasman Sea was initiated by a been events that occurred at the northern edge of the ridge–hotspot interaction, but it is difficult to see how this Australian Plate between southeast Asia and the Pacific. mechanism could be applied to the north Australian mar- For example, Müller et al. (2000) drew attention to a gin, unless there were numerous small plumes. New mod- regional tectonic event between 100 and 90 Ma related to els for the causes of the rifting are required. plate reorganisation between India and Australia at about 99 Ma which they postulated was due to the subduction of a Neo-Tethyan ridge beneath southeast Asia changing the CENOZOIC HISTORY stresses along Australian margins. Sea-floor spreading in the Tasman and Coral Seas ceased in Late Cretaceous – Paleocene rifting the Eocene (Gaina et al. 1998, 1999) and Australia began to move rapidly northwards. Ophiolites and arc terranes Home et al. (1990) inferred that rifting of the Coral Sea now found in northern New Guinea are here interpreted to began in the Cenomanian and continued through the Late have formed mainly in intra-Pacific oceanic arcs north of Cretaceous. An important coeval event is the widespread New Guinea and to have been accreted to the margin. uplift and erosion of the southern Papuan Basin, inferred Davies et al. (1997) have summarised these events, includ- by Home et al. (1990) to be due to uplift of the rift-shoul- ing an Early Eocene collision of the Bena Bena terrane, a der. However, similar uplift of the whole eastern margin of Late Eocene to Early Oligocene collision of the Sepik ter- Australia occurred in the mid to Late Cretaceous, inter- rane, Late Oligocene collision of a Papuan Peninsula ter- preted by Gurnis et al. (2000) to be due to rebound asso- rane and Early Miocene collision of the Finisterre terrane, ciated with the breaking off of a long-lived subducted although there is disagreement about the nature and timing slab. of all these events (cf. Abbott 1995; Cullen 1996). Francis (1990) argued that rifting occurred along the north- Eocene and Oligocene tectonic events in New Guinea eastern Australian margin in the Campanian–Maastrichtian, are poorly constrained. Eocene sediments are largely lack- citing as evidence the alkaline basaltic volcanism along the ing in the Papua New Guinea portion of the Fold Belt outer shelf and upper slope and common tuffs in the Upper (Figure 2) but sandstone and limestone were deposited in Cretaceous mudstone section. He inferred the formation of the Irian Jaya Fold Belt. In the Papua New Guinea portion marginal basins, but of unknown distribution. This is similar of the Mobile Belt (ST on Figure 1), Davies and Hutchison to the interpretation of Pigram and Symonds (1991, 1993) and (1982) interpreted the variously metamorphosed Late Davies et al. (1997) that Late Cretaceous rifting in the Coral Cretaceous to Eocene clastic and limestone strata to have Sea and along the northern margin led to the formation of been deposited in a volcanic island arc and trench envi- marginal basins in the latest Cretaceous and Paleocene with ronment. Around the Sepik Basin, Wilson et al. (1993) the two systems linked by a across the western reported strongly recrystallised Middle to Upper Eocene end of the Papuan Peninsula (Figure 1). Oceanic spreading of limestones, consistent with those reported through much of the Coral Sea occurred in the Paleocene to Eocene (Weissel & the Papua New Guinea portion of the Mobile Belt (Davies Watts 1979; Gaina et al. 1999) but the timing of formation of 1983; Bain & McKenzie 1975). There have been few reports marginal basins along the north coast is less well known due of Lower–Middle Oligocene strata, but Findlay (2003) Tectonic evolution of New Guinea 273 infers that the continentally derived clastics adjacent to the rocks elsewhere in New Guinea (Housh & McMahon Finisterre Ranges (Figure 1) may range from Eocene to 2000). Upper Miocene. The Middle Miocene igneous activity in Papua New In the Late Oligocene to Early Miocene there was wide- Guinea has generally been interpreted as subduction- spread shallow-water carbonate deposition across the related and most authors suggest southwest-dipping sub- Platform and Fold Belt of New Guinea (Figure 2), reflecting duction beneath Papua (Hamilton 1979; Cullen & Pigott regional subsidence, and also carbonate deposition in a 1989; Francis 1990; Hill & Raza 1999) although some low-energy environment around the Sepik Basin (Wilson et authors are uncertain of the polarity (Cullen 1996) or have al. 1993). However, volcanism is indicated in parts of the suggested north-directed subduction (Abbott 1995). There Mobile Belt. Page (1976) and Rogerson et al (1987) have have also been suggestions that volcanic activity of the identified Late Oligocene to earliest Miocene gabbroic to Maramuni Arc was not subduction-related (Mason & dioritic intrusions in the Sepik Terrane (ST on Figure 1). MacDonald 1978; Johnson & Jaques 1980; Findlay et al. Page (1976) and Findlay (2003) reported Late Oligocene to 1997) but was due to melting associated with uplift follow- earliest Miocene K–Ar ages on a microsyenite and lava, ing Oligo-Miocene arc–continent collision. A doubly dip- respectively, from the Ramu River immediately south of the ping subduction zone similar to that of the Molucca Sea Finisterre Ranges (Figure 1). A further 40 km southwest, a region has been inferred to extend westward under New quartz diorite yielded a Late Oligocene zircon fission track Guinea from the Solomon Sea, based on present seismicity age (Hill & Raza 1999). The carbonates to the south of the (Cooper & Taylor 1987; Pegler et al. 1995). However, Sepik Terrane and in and around the Sepik Basin to the tomography does not show a long south-dipping slab (Hall north of the terrane indicate relatively little volcanogenic & Spakman 2003). This implies little or no south-directed detritus. On the basis of mapping of limestone outcrops subduction or, alternatively, subduction of a slab within a and interpretation of seismic data, the Sepik Basin (Figure few million years of its formation similar to that on the east- 1) is reported to have been starved of clastic sediment dur- ern side of the present-day Woodlark Basin. Beneath west- ing the Late Oligocene to Early Miocene (Wilson et al. ern New Guinea, there is a seismically poorly defined zone 1993), suggesting limited igneous activity, at least in that suggesting south-dipping subduction of about 100 km of area. ocean crust at the New Guinea Trench, but tomography Metamorphic rocks in the Mobile Belt of northern shows no slab (Spakman & Bijwaard 1998; Hall & Papua New Guinea, have previously been dated by K–Ar Spakman 2003). as Late Oligocene to Early Miocene (Page 1976). Using The widespread deformation of Miocene and older 40Ar/39Ar analysis, Crowhurst (1999) redated many of the rocks throughout the Fold Belt and Mobile Belt of New metamorphic rocks and determined that the samples Guinea demonstrates the Late Miocene to Pliocene oroge- cooled rapidly from temperatures above 300–500°C mainly nesis. Hill and Raza (1999) and Kendrick (2000) used between 20 and 18 Ma. Based partly on the presence of apatite fission track analyses to infer the onset of compres- adjacent and coeval starved graben and limestone deposits sional deformation in the Mobile Belt at the end of the in the Sepik Basin (Wilson et al. 1993; Hill et al. 1993), Middle Miocene, around 14–12 Ma. They demonstrated Crowhurst (1999) inferred that the metamorphic cooling that the orogeny propagated south into the Fold Belt in the was due to extensional unroofing of mid-crustal rocks as Late Miocene to Pliocene, but suggested a transition to metamorphic core complexes. The unroofing was accom- strike-slip deformation in the Pliocene. Kendrick (2000) panied by ductile deformation in rocks of the Sepik Terrane recognised that in Irian Jaya the structural style of the 5 km- (Figure 1) later overprinted by brittle low-angle thrust defor- high frontal mountains was very different from the low- mation in the Late Miocene and strike-slip deformation in lying Fold Belt in Papua New Guinea. Hill et al. (in press) the Pliocene (Crowhurst et al. 1996, 1997). The 20–18 Ma suggested that this was due to the orogeny impinging on the metamorphic cooling ages coincide with a possible tempo- strong Australian lithosphere causing inversion of the large rary cessation of igneous activity as recorded by a gap in extensional fault along the continent edge that had been the K–Ar ages for that period (Rogerson et al. 1987). active from the Neoproterozoic to the Miocene. In contrast, Carbonate deposition continued throughout the New Cloos et al. (1998) suggested that the crustal-scale uplift Guinea Fold Belt in the Middle Miocene, but widespread was due to collisional delamination following locking of a igneous activity is reported in the Mobile Belt of Papua subduction zone to the north at ca 7 Ma, for which there is New Guinea, the Maramuni Arc (Figure 1) of Dow (1977). no evidence. Although mainly Middle Miocene in age, Page (1976), Pliocene igneous activity is mainly manifested as Rogerson et al. (1987) and Hill and Raza (1999) reported localised stocks in the Fold Belt representing considerably that the igneous activity ranges from late Early Miocene to smaller volumes of magma than during the Maramuni Late Miocene (ca 17–10 Ma) with minor activity continuing event. Davies (1991) pointed out that this igneous activity to the Pliocene. The principal products of the arc were sub- progressed southwards across the Fold Belt, coincident aerial pyroclastics and lavas, with thick, widespread vol- with the southwards propagation of deformation and uplift, canogenic sediments in addition to granodioritic and suggesting that it was related to crustal thickening. dioritic intrusions. The Maramuni Arc extends west only to Pleistocene igneous activity comprises stratovolcanoes in the Irian Jaya border and there was insignificant volcanism the New Guinea Fold Belt. Hamilton et al. (1983) carried in during the Neogene. The Neogene out Sr and Nd analyses on six Pleistocene stratovolcanoes volcanic rocks from Irian Jaya that have been studied are and concluded that they were mantle-derived magmas con- Late Miocene and have an unusual chemical character that taminated by movement through continental crust, but is ‘post-collisional’ quite different from Neogene magmatic they could not exclude derivation from subducted crust or 274 K. C. Hill and R. Hall metasomatised mantle. In contrast, the magma generation a mobilist view of the region and emphasised the impor- has been attributed to partial melting of the lower crust tance of strike-slip faulting: north New Guinea formed part (Mason & Heaslip 1980) or mantle (Johnson et al. 1978) as of his Melanesian shear or Tethyan megashear. After plate a consequence of crustal uplift and reduction of overbur- tectonics became the dominant theory it was quickly recog- den pressure. nised (Davies 1971; Curtis 1973; Packham 1973) that At both ends of the Fold Belt, in the D’Entrecasteaux northern New Guinea included volcanic-arc rocks and Islands in eastern Papua New Guinea and the Wandaman these were interpreted as indicating arc–continent collision Peninsula in northwest Irian Jaya (Figure 1), medium- to followed by subduction polarity reversal (Dewey & Bird high-grade metamorphic rocks are exposed at surface 1970; Hamilton 1970). However, it was not long before the which have yielded Pliocene–Pleistocene cooling ages (Hill polarity reversal model was contested (Johnson 1976; & Baldwin 1993; Baldwin et al. 1993; Bladon 1988). Hill Johnson & Jaques 1980). and Baldwin (1993) interpreted the metamorphic rocks of Plate-tectonic models that have been proposed since the D’Entrecasteaux Islands to have resulted from rapid the late 1970s fall into three groups. First, there are plate extension associated with the westward propagation of the models which consider the regional history of some or all of Woodlark spreading ridge (Figure 1) (Taylor et al. 1995, the major plates of southeast Asia, the western Pacific and 1999). The Pliocene–Pleistocene cooling ages reflect rapid Australia in varying detail (Crook & Belbin 1978; Hamilton exhumation of metamorphic core complexes (Baldwin et al. 1979; Wells 1989; Jolivet et al. 1989; Rangin et al. 1990; 1993). The Wandaman Peninsula is less thoroughly Smith 1990; Daly et al. 1991; Yan & Kroenke 1993; Lee & mapped, but radiometric dating indicates rapid cooling of Lawver 1995; Hall 1996, 1997, 1998, 2002). Second, there mid-crustal rocks during the Pliocene–Pleistocene (Bladon are local models which have been proposed to account for 1988). This, too, is here interpreted to result from extension details of the evolution of sections of the Australian margin, associated with orogenic collapse of the ends of the Fold for example in eastern New Guinea or the Melanesian arcs Belt (Hill et al. 2002). (Jaques & Robinson 1977; Johnson & Jaques 1980; Cooper & Taylor 1987; Pigram & Davies 1987; Hill & Hegarty 1987; Cenozoic tectonics and plate models Hill et al. 1993; Struckmeyer et al. 1993; Abbott 1995; Musgrave & Firth 1999; Hill & Raza 1999; Charlton 2000). It is generally agreed that there were several rapid changes Finally, there are many incomplete plate-tectonic models in tectonic setting during the Cenozoic but there is still dis- which are typically in the form of diagrams with cross-sec- agreement about the site of formation of ophiolites, how tions of subduction zones at different stages, or maps which many arcs there were, the timing of arc–continent collision, show key features, but without portraying the entire history polarity of subduction, the importance of extension and the of the region (Johnson 1976; Falvey 1978; Falvey & role of strike-slip faulting. Constraining the geological inter- Pritchard 1982; Ridgeway 1987; Richards et al. 1990; Benes pretation of the New Guinea margin is only possible by et al. 1994; Petterson et al. 1997; Monnier et al. 1999; considering its tectonic interactions with the plates to the Weiler & Coe 2000; Findlay 2003). north. It is of particular importance in interpreting New In the New Guinea region almost all models advocate Guinea geology to consider not only Pacific–Australia plate northward-subduction of oceanic crust north of Australia vectors, but also relative motions between Australia and before collision of the Australian margin with a south-fac- small plates such as the Philippine Sea and Caroline ing arc. A few authors have proposed that the northern Plates. Rather than try to summarise the plethora of differ- Australian margin was an active margin and there was ent plate-tectonic models, the different inferences that have southward-subduction beneath this margin before the been made, and reconstruct the thoughts of different active north Australia margin collided with an arc (Hill & authors from fragmentary interpretations shown in dia- Hegarty 1987; Hill et al. 1993; Monnier et al. 1999, 2000). grammatic cross-sections and local plate models, we have Early models, and many authors since, suggested a single tried below to identify the key features of different groups of arc–continent collision. Variations include opening of mar- models. ginal basins within the Australian margin with the marginal There are few complete plate-tectonic models for this basins being destroyed by subsequent subduction, and region, largely due to a paucity of data since the region multiple subduction zones. In some cases these subduction between New Guinea and the Tonga Arc is vast, sparsely zones have a consistent polarity, for example, many mod- populated, difficult to access, covered in rainforest, and els suggest more than one subduction zone dipping north- remote. The marginal basins of this region are largely wards north of Australia, but in other models there are undrilled, and there has been little marine investigation of subduction zones which have opposing polarities and many of them and their ages and spreading histories which appear to have lived for short periods. remain uncertain. Early accounts of New Guinea were The more complex models proposed, or implied, multiple based mainly on the work of Dutch geologists that was arc–continent collision events. Pigram and Davies (1987) summarised by van Bemmelen (1949), and exploration influenced many subsequent ideas on New Guinea when studies for oil and minerals (Australian Petroleum they proposed a terrane interpretation which implied a plate Company 1961; Visser & Hermes 1962). These authors model involving accretion of fragments during subduction recognised the eugeosynclinal character of northern New and collision. Subsequently, Daly et al. (1991), Lee and Guinea to the north of young mountains at the northern Lawver (1995) and many others have suggested the addition edge of the Australian continent which van Bemmelen of arc fragments onto the northern New Guinea margin at (1933, 1939) interpreted in a pre-plate-tectonic undation several periods during the Cenozoic. However, even in the model. In contrast, Carey (1958) was one of the few to have cases where the same arc is discussed there is considerable Tectonic evolution of New Guinea 275 divergence of views about the timing of arc–continent colli- (Kroenke 1984; Petterson et al. 1997). Pacific–Australia sion. For example, ophiolites were emplaced in northern plate convergence by subduction in the Solomons region New Guinea in the Early Cenozoic but it is not clear if this then ceased and subduction transferred to other locations was a single event or whether there were multiple obduction in the Early to Middle Miocene. It was at this stage that the events at different places on the margin (Davies 1971; plate boundary in the Solomons ceased to be a subduction Hutchison 1975; Davies & Smith 1971; Pigram & Davies zone and instead became a strike-slip fault zone. For New 1987; Davies et al. 1996; Monnier et al. 1999). Monnier et al. Guinea, the importance of this event is that the termination (2000) suggested that parts of the Central Ophiolite Belt in of subduction beneath the Solomons is suggested to have Irian Jaya are Jurassic backarc basin ophiolites. This implies led to initiation of new subduction beneath the Papuan that the ophiolites (approximately along-strike from the Peninsula to form the Maramuni Arc (Hill & Raza 1999). April, Marum and Papuan ophiolites) were formed within the This southwest-dipping subduction zone was active from Australian margin which is interpreted by most other work- the Early Miocene but by the Late Miocene subduction ers as a passive margin. It is also widely reported that there probably ceased and instead new north-dipping subduction was a Late Oligocene or Early Miocene collision of an arc began beneath the New Hebrides and New Britain arcs with the northern New Guinea margin in the region of Papua (Hill & Raza 1999), followed by formation of the Woodlark although many authors have identified this collision as Basin and Bismarck Sea (Taylor 1979; Benes et al. 1994; younger than Early Miocene and have suggested collisions Taylor et al. 1995, 1999). which may continue up to the present day (Jaques & Robinson 1977; Abbott 1995; Davies et al. 1996; Hill & Raza 1999; Weiler & Coe 2000). TECTONIC MODEL FOR THE EVOLUTION OF A common theme in many of the models, particularly AUSTRALIA’S NORTHERN MARGIN those which involve multiple subduction zones and multi- ple collisions, is the diachronous collision of a volcanic arc A regional plate reconstruction which includes the northern with the northern New Guinea margin (Hamilton 1979; Australian margin and synthesises information from the Cooper & Taylor 1987; Richards et al. 1990; Weiler & Coe ocean basins with terrestrial geology has been described by 2000). Almost all authors who consider a diachronous col- Hall (1997, 1998, 2002). Below, we use this reconstruction to lision suggest that it started earliest in the west and pro- provide the context to interpret the Cenozoic history of the gressed eastwards to where collision is apparently New Guinea margin. It has features in common with many occurring between the New Britain – Finisterre arc system of the models outlined above, and proposes several arc–con- and the northern New Guinea margin. To the east of this is tinent collisions, but differs in postulating a dominantly the Solomon Sea and some evidence has been interpreted strike-slip plate-boundary zone in northern New Guinea dur- to suggest that beneath the Papuan end of this segment of ing the Neogene. Before 45 Ma ophiolites were formed in the the collision zone is a doubly vergent subducted slab which forearcs of intra-oceanic arcs in the Pacific region and these dips to the north and south in similar fashion to the well- were emplaced by diachronous arc–continent collision with known inverted U-shaped configuration of the Molucca the north Australian passive margin between New Guinea Sea (Cooper & Taylor 1987; Pegler et al. 1995). It has been and New Caledonia, probably in the Eocene. Following a suggested that both north- and south-directed subduction major plate reorganisation at ca 45 Ma, Australia began to have occurred between a volcanic arc and New Guinea and move rapidly northwards and the Philippine Sea, Caroline diachronous collision has progressively added the arc to and Solomon Sea Plates formed as backarc basins. At about the northern edge of New Guinea and closed the interven- 25 Ma, collision of the Philippines–Halmahera Arc with the ing ocean from west to east leaving the present Solomon Australian margin and collision of the Ontong Java Plateau Sea as a remnant of a wider ocean. with the Melanesian Arc led to the loss of two major sub- Many of the regional models concerned with New duction zones. The arcs between the Philippines and Guinea terminate at Papua but because the relative motion Melanesia became a single arc system which rotated clock- between Australia and the Pacific involves a large westward wise at the leading edge of the Pacific Plate, accommodated component it is important to consider the Melanesian by intra-plate subduction at the eastern edge of the region east of Papua to determine where the arcs origi- Philippine Sea Plate. This created a Neogene strike-slip fault nated. The term Melanesian Arc is given to the system of system in northern New Guinea with major sinistral dis- arcs extending from New Ireland, through the Solomons, to placement of the arc relative to New Guinea. Tonga (Figure 1). The Melanesian Arc originated in the Below is our model for the tectonic evolution of Eocene either by rifting away the edge of the Australian Australia’s northern margin (New Guinea) incorporating all margin above a subduction zone (Crook & Belbin 1978) or the information reviewed above and the plate-tectonic by initiation of subduction in an intra-oceanic setting (Yan reconstruction. & Kroenke 1993). Most models agree that from the Early Cenozoic there was south- or southwest-directed subduc- Palaeozoic tion of the Pacific beneath the Melanesian Arc and this continued until the Solomons collided with the Ontong Northern Australia was characterised by two crustal Java Plateau. The arc–plateau collision took place in the provinces, the Tasman Orogen in the east and a stable cra- Miocene but may have occurred over an extended period, ton subjected to Gondwana rifting in the west. The Tasman with an early soft collision phase involving shortening of Line, defining the eastern and northern limit of intact the Ontong Java Plateau, followed by a hard collision phase Proterozoic continental crust, trended north–south ~100 when the Solomons were transferred to the Pacific Plate km west of the current Irian Jaya – Papua New Guinea bor- 276 K. C. Hill and R. Hall

Figure 5 3D sketch of the New Guinea margin in the Jurassic, looking west. It is inferred that the strong Proterozoic crust west of the Tasman line results in a narrower zone of rifting and that the northeast structural grain influences the pattern of breakup, resulting in rectilinear promontories and embayments as observed on Australia’s North West Shelf. der and then west-northwest near the present mountain possible that only a relatively narrow Triassic arc and fore- front and through the Bird’s Head (Figure 3). To the east arc region separated from the continent (Figure 4). We sug- and north were accreted terranes of the Tasman Orogen, gest that the underlying crustal architecture strongly including the northern Bird’s Head. The preservation of influenced the style of rifting and breakup. West of the northeast-trending lineaments through to at least the north- Tasman Line, rifting was confined to the north of the ern limit of the Fold Belt, contiguous with those in Mapenduma Fault, the extensional continent-bounding Proterozoic crust of northeast Australia, may indicate that fault active since the Proterozoic (Figure 5). East of the extended Proterozoic and older lithosphere underlies the Tasman Line extensional faulting was more widespread. In accreted Tasman terranes. addition, the strong northeast structural grain influenced the pattern of breakup, creating promontories of extended Triassic continental crust separated by embayments of Jurassic oceanic crust. Along Australia’s North West Shelf, Gondwanan post-rift sub- sidence had created large Triassic depocentres whilst in Early Cretaceous southern Irian Jaya there was minimal Triassic deposition and perhaps uplift and erosion. In Papua New Guinea, and prob- In the Late Jurassic and Early Cretaceous New Guinea was ably northern Irian Jaya, Upper Permian to Lower Triassic a passive margin. Thermal subsidence resulted in deposi- clastics (and unknown older rocks) of Australian affinity were tion of the shelf, shoreline and basin facies which now host deformed and metamorphosed in the Early Triassic in a con- significant hydrocarbon reserves, and hence are reasonably tinuation to the northwest of the New England Orogeny in well known. In the Aptian, and particularly the Albian, there eastern Australia. Orogenesis was probably related to sub- was widespread volcanism in New Guinea as all along the duction to the southwest beneath the northern and eastern eastern Australia margin. We suggest that this was associ- Australia margins, that may have continued along the outer ated with renewed subduction beneath the margin, as margin of New Guinea through the Bird’s Head and Sula inferred by Norvick et al. (2001) and Sutherland et al. Spur, accounting for the widespread Middle Triassic arc (2001). The subduction beneath New Guinea would have (Figure 4). In the Middle to Late Triassic, the tectonic regime caused the Jurassic oceanic crust in the embayments along abruptly changed so that the New Guinea margin and the the margin to have been deformed into accretionary prisms. New England Orogen to the southeast underwent rifting, associated with Late Triassic volcanism. Late Cretaceous and Paleocene

Jurassic In the Late Cretaceous, rifting was initiated along the north- ern margin of the Australian continent, perhaps associated Mid–Late Triassic and Early–Mid Jurassic rifting in New with subduction (Figure 6). The rifting led to formation of Guinea was followed by Middle Jurassic breakup, but it is marginal basins in the Late Cretaceous and Paleocene sep- Tectonic evolution of New Guinea 277

Figure 6 (a) Latest Cretaceous palaeogeography (after Hall 2002) and (b) a 3D sketch of the margin. Jurassic–Cretaceous subsidence led to deposition of a thick clastic sequence, including widespread Aptian–Albian volcaniclastics which may indicate subduction beneath the margin (Norvick et al. 2001) and formation of an accretionary prism in the oceanic embayments. Late Cretaceous–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 a Cretaceous subducted slab (Gurnis et al. 2000). 278 K. C. Hill and R. Hall

Figure 7 3D sketch of the New Guinea margin in the Palaeogene (after Hall 2002). There was significant uplift and denudation in southern Papua New Guinea, but subsidence in Irian Jaya. Conceptual marginal basins to the north of New Guinea are unproven, but synchronous with Coral Sea spreading to the southeast (Davies et al. 1996, 1997). The existence of subduction beneath New Guinea is uncertain. arating slivers of extended continental crust from the mar- Australia beneath the Philippines–Halmahera Arc; new gin. In the Paleocene, the Papuan Peninsula separated south-dipping subduction began northeast and east of from Queensland during opening of the Coral Sea. Australia forming the Melanesian Arc (Figure 8). Between Paleocene marginal basins probably formed along the 45 and 25 Ma the Philippines–Halmahera Arc remained in north coast of New Guinea and Paleocene oceanic crust approximately the same position. East of Australia, roll- may have formed in Cenderawasih Bay adjacent to the back of the subduction hinge led to the formation of a wide Bird’s Head (Figure 6). backarc Solomon Sea Basin as the Melanesian Arc rotated In the Late Cretaceous and Paleocene the platform in north. From about 40 Ma the Caroline Sea opened as a southern Papua New Guinea was uplifted and eroded, pre- backarc basin by rollback of the Pacific subduction hinge at viously suggested (Home et al. 1990) to have been associ- the eastern margin of the Philippine Sea Plate. The South ated with rifting in the Coral Sea (Figure 7). Alternatively, Caroline Arc was located east of this backarc basin and was this may have been associated with dynamic topographic the site of formation of arc terranes now found in northern rebound associated with the movement of Australia over a New Guinea. Cretaceous subducted slab as suggested for the eastern Australia margin by Gurnis et al. (2000). Widespread Oligocene Paleocene and Eocene sandstones in western Irian Jaya probably resulted from the uplift and denudation of Papua As Australia moved north in the Oligocene, the residual New Guinea and eastern Australia. rifted terranes and marginal basins to the north reached the Philippines–Halmahera–Caroline subduction zone. They Eocene were probably incorporated into the Philippines– Halmahera–Caroline subduction complex and accretionary By the Eocene, spreading in the Coral Sea and other mar- prism, such that they now occur as basement beneath the ginal basins had ceased. During the Eocene, and possibly northern parts of the Meervlakte, Sepik and Ramu succes- the Oligocene, some of the hot buoyant crust of the mar- sor basins and in the Weyland Terrane (Figure 1). At about ginal basins, or intra-oceanic forearcs, was obducted 25 Ma there was an arc–continent collision when the diachronously along the margin, for instance the Papuan, Philippines–Halmahera Arc collided with a continental Marum and parts of the April Ultramafics. Southern Papua promontory along the Australian margin (Figure 9). We New Guinea was still mainly emergent, but limestone and infer that this event was akin to the Pliocene arc–continent fine clastics were deposited in the Mobile Belt and northern collision in Timor in that a fold and thrust belt was created part of the Fold Belt and New Guinea Limestone was in an island along the oceanic portion of the margin, with deposited through northern Irian Jaya. relatively little effect on most of the northern margin, other At about 45 Ma there was a major plate reorganisation. than Miocene subsidence and the local supply of coarse Australia began to move rapidly northwards; new north- clastic sediment. Low-energy Late Oligocene to Early dipping subduction began about 2000 km north of Miocene limestones were deposited southwest of the Tectonic evolution of New Guinea 279

Figure 8 (a) Eocene and (b) Oligocene palaeogeography of the New Guinea margin (after Hall 2002). The Eocene onset of convergence as Australia started moving rapidly to the north may have resulted in obduction of the marginal basins. Oceanic subduction over 1000 km to the north led to formation of the Philippine and Caroline Arcs and associated backarc basins. Subduction to the southeast beneath the Papuan Peninsula was associated with formation of the Solomon Sea Plate. G, Gauttier terrane, now in northern Irian Jaya (Figure 1). 280 K. C. Hill and R. Hall

Figure 9 (a) Palaeogeography and (b) 3D sketch of the margin in the Late Oligocene. The Philippine–Caroline Arc has collided with a continental promontory along the margin creating an island orogeny as in Timor today. There was a major change in plate motions around 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). Tectonic evolution of New Guinea 281

Figure 10 3D sketch of the New Guinea margin in the Early Miocene. There was significant subsidence with regional deposition of up to 1 km of Lower Miocene carbonates to the south and relatively starved basins to the north. The northern margin is inferred to have been a divergent strike-slip system, giving rise to local metamorphic core complexes with 20–18 Ma cooling ages (Crowhurst 1999) adjacent to starved basins. The Solomon Sea Plate was obliquely subducted to the west beneath the Papua New Guinea margin (Figures 9, 11). accreted arc, in the Sepik Basin. The origin of the Late 1–2 km of Miocene shallow-water limestone in southern Oligocene to earliest Miocene gabbros and diorites in the New Guinea and an abrupt transition into deeper water Sepik Terrane (Figure 1) is uncertain. along the northern margin of the Fold Belt.

Early Miocene Middle Miocene The Oligocene arc–continent collision, combined with col- With the loss of the Melanesian subduction zone, lision of the Ontong Java Plateau with the Melanesian Arc, Pacific–Australia convergence was accommodated by devel- led to the loss of two subduction zones and a major plate opment of new subduction zones: southwest-dipping reorganisation. After these collisions, throughout the oblique subduction of the Solomon Sea Plate began first Miocene, the arcs between the Philippines and Melanesia beneath Papua to form the Maramuni Arc (Figure 11). became broadly a single arc system which rotated clock- However, we cannot rule out the alternative, that the arc wise at the leading edge of the Pacific Plate, accommodated volcanism may have resulted from the ongoing oblique by intra-plate subduction at the eastern edge of the extension that gave rise to the metamorphic core complexes Philippine Sea Plate. The Philippine – South Caroline arc and North New Guinea Basins, accompanied by little terranes, moved along the north Australian margin in a Middle Miocene subduction beneath the eastern Papua complex, regionally extensive strike-slip zone. There was margin, but by melting of a previously metasomatised man- therefore no significant subduction at the northern tle. Strike-slip movement continued in the Middle Miocene Australian margin in Irian Jaya. and the widespread volcanism of the Maramuni Arc filled We infer that the sinistral strike-slip system in northern the North New Guinea basins with volcanogenic sediments. New Guinea had a significant extensional component and that parts of the extended continental promontories may Late Miocene to Pliocene have been rifted away and transported along the margin with the collided arcs. The extension also led to the forma- When subduction beneath eastern Papua New Guinea tion of metamorphic core complexes that cooled rapidly ceased, the New Hebrides Trench propagated west to start from ~500°C to near-surface temperatures around 20 Ma new north-directed subduction beneath the Solomons and (Figure 10). At the same time, the adjacent Sepik Basin was New Britain Arcs. The inferred cessation of subduction starved of sediment and associated with Early Miocene car- beneath eastern Papua New Guinea coincided with the bonate deposition. Regionally, the whole New Guinea mar- onset of compression at around 14–12 Ma, causing defor- gin subsided rapidly leading to widespread deposition of mation, uplift and denudation of the Mobile Belt in the Late 282 K. C. Hill and R. Hall

Figure 11 (a) Middle Miocene palaeogeography (after Hall 2002) and (b) 3D sketch of the margin. Continued oblique subduction of the Solomon Sea Plate to the west has given rise to the Maramuni Arc in Papua New Guinea, although, alternatively, it could have been related 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). Tectonic evolution of New Guinea 283

Figure 12 (a) Pliocene palaeogeography (after Hall 2002) and (b) 3D sketch of the margin. Convergence of the Caroline Arc with New Guinea from the end of the Middle Miocene created the New Guinea Orogen causing thrusting in the Mobile Belt and Fold Belt, but ongoing strike-slip motion between the Mobile Belt and the accreted arc. The Fold Belt was low-lying as it was built on warm, weak and broken lithosphere. Northward subduction of the Paleocene oceanic crust in Cenderawasih Bay beneath a continental fragment from northern Papua New Guinea caused it to collide with the Bird’s Head (Sutriyono 1999). BT, Bewani–Torricelli Mountains; AR, Adelbert Ranges; FR, Finisterre Ranges (Figure 1). 284 K. C. Hill and R. Hall

Figure 13 3D sketch of the New Guinea margin in the Pleistocene. The orogeny impinged on the strong Proterozoic lithosphere west of 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 in Papua New Guinea remained low with no significant foreland basin. Compression abated around 3 Ma and strike-slip faulting became more dominant (Crowhurst et al. 1997). MA, Muller Anticline.

Miocene. We suggest that the continued oblique plate con- mas were emplaced, except in local areas of dilation at the vergence was largely partitioned into a sinistral strike-slip intersection of old extensional faults with north-northeast- fault north of the New Guinea basins and southwest- trending fracture zones. This allowed the emplacement of directed thrusting to the south. Thrust faulting in the Cu±Au-bearing magmas from deep in the crust or mantle Mobile Belt included complex juxtaposition of the ophio- (Grasberg and Porgera on Figure 13). During the Late lites, accretionary prisms and metamorphic rocks with the Pliocene, compression probably waned, but did not cease, Mesozoic–Palaeogene distal sediments and Maramuni Arc and strike-slip motion became dominant in the Mobile Belt intrusive, volcanic and sedimentary rocks. In the Late and Irian Jaya Fold Belt. Miocene to Pliocene, the deformation propagated south to To the east of New Guinea, slab-pull forces at the New the fold and thrust belt (Figure 12). Throughout New Britain Trench led to the formation of the Woodlark spreading Guinea this fold and thrust belt is interpreted to have been centre at the former Solomon Sea transform (Figure 12). Very low-lying and lacking a foreland basin because the fold belt young lithosphere of the Woodlark Basin is now being sub- overlay weak, hot, broken and extended continental crust ducted at the South Solomon Trench. Throughout this period, that subsided beneath it. When convergence recommenced west-dipping subduction of the Pacific Plate continued at the in west Irian Jaya, we infer that a continental sliver, possi- Tonga–Kermadec Trench with rapid rollback of the subduc- bly rifted from a northern Papua New Guinea promontory, tion hinge in the last 10 million years. In the Bird’s Head area, was adjacent to the oceanic crust in Cenderawasih Bay. we suggest that the Mobile Belt adjacent to the Lengguru Fold Subduction of the oceanic material is interpreted to have Belt collapsed towards the northeast, giving rise to the 2 Ma drawn this (Weyland) terrane south to collide with the metamorphic core complex exposed in the Wandamen Bird’s Head forming the Lengguru Fold Belt. Peninsula and leaving the present Cenderawasih Bay floored by extended continental crust. The Pleistocene to Holocene Pliocene to Holocene stratovolcanoes in the Fold Belt may be related to extension associated with the Woodlark and Cenderawasih orogenic In Irian Jaya the orogenesis impinged on the Mapenduma collapse. However, their confinement to the Fold Belt suggests Fault marking the northern limit of strong, thick, cold, an underlying feature such as the south-dipping Solomon Sea Proterozoic Australian lithosphere. The fault was inverted slab suggested by Davies (1991). and a 15 km thick slab of the crust was thrust to the surface building the 5 km-high mountains in the Irian Jaya Fold Belt and creating a substantial foreland basin to the south- DISCUSSION west (Figure 13). East of the Tasman Line the orogenesis continued to produce a low-lying fold belt overlying weak Here, we note the rapid changes in tectonic setting associ- lithosphere. As the crust was now in compression, few mag- ated with the development of New Guinea during the Tectonic evolution of New Guinea 285

Cenozoic. New Guinea cannot be understood simply as the There is a substantial divergence in the interpretation of product of an arc–continent collision, or the result of Neogene tectonics between this paper and Findlay (2003) Australia–Pacific plate interaction, although both are part which largely reflects the very different approach taken to of the story. Some of the conflicts between interpretations address the problem. Here, we attempt an analysis on a of New Guinea geology reflect the difficulties of the terrain, plate-margin scale incorporating as much regional and local and an inadequate knowledge of this large region, but some geological data as possible and inevitably there are discrep- probably reflect the complexity of the history of minor plate ancies that need testing. In contrast Findlay (2003) and oth- motions, the differing character of the lithosphere within ers in the Geological Survey of Papua New Guinea have the orogenic belt, and its unpredictable behaviour during carried out important new mapping in the area between the both contraction and extension. Bena Bena – Goroka Terrane and the Finisterre Ranges. It is clear from outcrop data and tomographic studies that Citing observed interfingering relationships and Oligocene strong, cold Proterozoic lithosphere underlies southwestern K–Ar dates as critical data, Findlay (2003) concludes that the New Guinea and that this had a significant influence on Finisterre Ranges were close to their present location at the Phanerozoic tectonics. It is likely that the northeast- and end of the Oligocene and that there has been no significant northwest-trending Proterozoic structural grain influenced motion between the Bena Bena – Goroka Terrane and the Jurassic rifting and breakup, resulting in a rectilinear margin Finisterre Ranges since then, ruling out >1000 km of sinistral of extended continental promontories separated by embay- offset, as inferred in the model presented here. We acknowl- ments of Jurassic oceanic crust. Furthermore, the extensional edge the interfingering of continental and arc detritus estab- faulting associated with rifting appears to be confined to the lished by Findlay (2003) and the significant thrusting of the area north and east of the strong Proterozoic lithosphere, to the south in the last one million years. reactivating a continent-bounding extensional fault that had However, we suggest that prior to that thrusting, there was a been active since the Proterozoic. To the east, in the accreted major sinistral strike-slip fault between the Finisterre Terrane Palaeozoic terranes of the Tasman Orogen, the extension and New Guinea and that arc detritus could have been was more widespread. Finally, during orogenesis, the strong derived from any of the Philippine – Caroline Arc fragments Proterozoic litho-sphere had a profound effect when the adjacent to the margin. It is worth noting that Liu and Crook orogeny impinged on it. This created a foreland basin along (2001) mapped the same area as Findlay (2002) and studied the New Guinea margin in the Neogene and created moun- sediments in the adjacent Huon Gulf and inferred initial col- tains over 5 km high. In contrast, the area overlying the lision of the Finisterre Terrane with New Guinea in the latest accreted Tasman terranes remained a relatively low-lying Middle Miocene to earliest Late Miocene. Furthermore they fold belt with no adjacent foreland basin. estimated propagation of the collision zone to the east at The change in character of the lithosphere at the inter- 39–55 km/106 y. preted position of the Tasman Line is also associated with Our analysis suggests that the New Guinea Mobile Belt other tectonic features. From about 45 to 25 Ma this was comprises a collision zone between a north-facing the position where there was a change in subduction polar- Australian indented continental margin and a south-facing ity from north-dipping north of New Guinea to south-dip- Mesozoic and Palaeogene accretionary prism in front of an ping in the Melanesian Arc. There is a strong positive lower arc, all subsequently cut by a Neogene strike-slip fault sys- mantle anomaly of uncertain origin seen on tomographic tem with more than 1000 km sinistral displacement. This images below Queensland (Hall & Spakman 2003). model implies the consumption of most Cretaceous and Together, these observations suggest the Tasman Line is the Palaeogene terranes that may have existed to the north of site of significant changes in the properties of the mantle New Guinea. Although integration with plate-kinematic and crust which influenced Cenozoic tectonic history. syntheses greatly increases the reliability of the model, it An important feature of the Cenozoic plate-tectonic still relies heavily on the onshore geological record, partic- reconstruction is the persistent presence of a sinistral ularly prior to the Eocene. The dataset is far from complete strike-slip fault system in northern New Guinea throughout and considerable new fieldwork and isotopic and palaeon- the Neogene at the same time as the Philippine–Caroline tological dating need to be done. Some of the main issues Plate rotated clockwise along the margin. These were the and uncertainties that need to be addressed to test the dominant features controlling Neogene orogenesis in New model are listed below. Guinea. We infer that the strike-slip system was transten- sional in the Early Miocene resulting in metamorphic core Major issues and uncertainties to be tested complexes and graben formation and possibly contributing to volcanic activity of the Maramuni Arc. Towards the end (1) The interpretation of a west-northwest-trending of the Middle Miocene, the strike-slip system became trans- Tasman line through Irian Jaya relies on unpublished dat- pressional resulting in ~100–200 km of shortening along ing from Parris (1994). The age and nature of ?Cambrian the New Guinea margin. The fault system was probably oceanic crust cropping out in the Irian Jaya Fold Belt partitioned into a strike-slip fault along the northern mar- should be confirmed. gin, and fold and thrust structures in the Mobile Belt and (2) Our model assumes a relatively fixed position of the Fold Belt to the south. In the Pliocene, as convergence was Bird’s Head relative to Australia. This can in part be tested diminished, strike-slip motion became dominant through- by studies of the relationship between the Bird’s Head and out the Mobile Belt and in the Irian Jaya Fold Belt. At the North West Shelf sediments to the south following the cur- same time the areas immediately east and west of mainland rent acquisition of considerable new, high-quality reflec- New Guinea underwent extensional collapse exposing tion-seismic data due to the very large gas discoveries in Pleistocene metamorphic core complexes. Bintuni Bay in the late 1990s. It can also be tested by fur- 286 K. C. Hill and R. Hall ther provenance studies of the Mesozoic sandstones in the the Geological Society of Australia 8, 1–133. Bird’s Head region. AITCHISON J. C., IRELAND T. R., BLAKE M. C. JR & FLOOD P. G. 1992. 530 Ma zircon age for ophiolite from the New England Orogen: oldest (3) Whilst substantial strike-slip motion is inferred in rocks known from eastern Australia. Geology, v. 20, 125–128. northern New Guinea, our model infers minimal strike-slip BAIN J. H. C. & MACKENZIE D. E. 1975. Ramu 1:250 000 geological map motion between the Fold Belt and the Mobile Belt, in part and explanatory notes, sheet SB/55-5. Geological Survey of Papua based on northeast-trending lineaments cutting across the New Guinea, Port Moresby. boundary, due to inferred continental promontories (Hill et BALDWIN S. L. & IRELAND T. R. 1995. The tale of two eras: Pliocene–Pleistocene unroofing of Cainozoic and late Archean zir- al. 2003). This contrasts strongly with previous models and cons from active metamorphic core complexes, Solomon Sea, should be tested using remote sensing images and further Papua New Guinea. Geology 23, 1023–1026. isotopic analysis in the Mobile Belt. BALDWIN S. L., LISTER G. S., HILL E. J., FOSTER D. A. & MCDOUGALL I. (4) A considerable portion of our New Guinea tectonic 1993. Thermochronologic constraints on the tectonic evolution of active metamorphic core complexes, D’Entrecasteaux Islands, model relies on the dating of the ophiolites, particularly Papua New Guinea. Tectonics 12, 611–628. interpretations of Paleocene marginal basins. New dating BENES V., SCOTT S. D. & BINNS R. A. 1994. Tectonics of rift propaga- and geochemical analysis of the ophiolites would improve tion into a continental margin: western Woodlark Basin, Papua all models. New Guinea. Journal of Geophysical Research 99, 4439–4455. BLADON G. M. 1988. 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