PALAEOGEOGRAPHY 35BMR RECORD 1991/75

MESOZOIC TO CAINOZOICPLATE TECTONIC RECONSTRUCTIONS

(PRELIMINARY)FOR PAPUA NEW GUINEA

by

HEIKE I.M. STRUCKMEYER

Bureau of Mineral Resources, Geology & Geophysics, AustraliaPetroleum Division of the Australian Mineral Industries Research Association

Phanerozoic History of Australia Project

11111111 111111111 I II* R 9 1 0 7 5 0 1 *

© Commonwealth of Australia, 1991This work is copyright. Apart from any fair dealing for the purposes of study, research,criticism or review, as permitted under the Copyright Act, no part may be reproduced by anyprocess without written permission. Inquiries should be directed to the Principal InformationOfficer, Bureau of Mineral Resources, Geology and Geophysics, GPO Box 378, Canberra,ACT 2601.

ISSN 0811-062XISBN 0 642 16390 1

This record is IN-CONFIDENCE until 31 September 1992.

TABLE OF CONTENTS

LIST OF FIGURES^

ABSTRACT^

1. INTRODUCTION^ 1

2. PLATE TECTONIC SETTING^ 2

3. TECTONIC COMPONENTS OF THEPAPUA NEW GUINEA REGION ^ 4

3.1 Introduction ^43.2 Description of tectonic components ^7

4. OUTLINE OF PLATE TECTONIC HISTORY ^ 14

4.1 Introduction^ 144.2 Palaeomagnetic constraints ^ 154.3 Late Cainozoic arc-reversal and Cretaceous

subduction? - A discussion^ 164.4 Mesozoic to Early Eocene^ 184.5 Middle Eocene to Recent^ 23

5. IMPLICATIONS FOR HYDROCARBON EXPLORATION^ 32

6. CONCLUSIONS^ 34

7. ACKNOWLEDGEMENTS ^ 35

8. REFERENCES ^ 35

ABSTRACT

Ten preliminary computer-generated plate tectonic reconstructions for the Mesozoic toCainozoic of Papua New Guinea were produced as part of the BMR-APIRA PhanerozoicHistory of Australia Project. The model presented is an attempt to integrate detailedreconstructions for the Papua New Guinea region with similar reconstructions for other areasof the Australian Plate margin by taking into account a global palaeomagnetic framework andthe direction of movement of the major bounding plates. The reconstructions will subsequentlybe used as bases for interpretive palaeogeographic maps.

Eighteen tectonic components which have moved relative to the Australian Plate were defined,digitised and, based on a set of geologic and palaeomagnetic constraints and a plate tectonicmodel for the region, rotated to their relative palaeopositions. The Triassic/Permian boundarywas used as a starting point for the reconstructions.

The present-day geology of Papua New Guinea is the product of a number of plate tectonicprocesses which have affected the northern margin of the Australian Plate during its Mesozoicto Cainozoic history. Two major extensional events occurred in the Late Triassic to EarlyJurassic and the Late Cretaceous to Paleocene within a passive margin setting. A major changein the movement direction of the Pacific Plate in the Eocene and rapid northward movement ofthe Australian Plate from the Paleocene onwards resulted in the oblique convergence of the twoplates, and the formation of island arcs above two major northward and southward dippingsubduction zones to the north. From the Oligocene onward the progressive accretion ofallochthonous terranes of mixed and oceanic to island arc affinity to the Australian margincaused the displacement of parts of the former passive margin and the deposition of a thicksyntectonic sequence in the foreland and in basins forming between the accreting terranes.

At the same time subduction to the north continued and volcanism and sedimentation occurredin the arc-trench systems. As a result of the formation of small ocean basins in the mid- to LatePliocene, particularly the opening of the Manus Basin, the northward movement of theAustralian Plate was taken up within the orogen, leading to uplift, crustal shortening andunderthrusting and, consequently, to trap formation in the Papuan Fold Belt.

Ill

1. INTRODUCTION

Ten preliminary computer-generated plate tectonic reconstructions for the Mesozoic toCainozoic of the Papua New Guinea region (Figure 1) are presented in this report. The studyforms part of the BMR - APIRA (Bureau of Mineral Resources, Geology & Geophysics -Petroleum Division of the Australian Mineral Industry Research Association) PhanerozoicHistory of Australia Project. The maps are presently being integrated with preliminaryreconstructions for other regions of the Australian Plate margin (Walley & Ross, 1991;Bradshaw & Ross, in prep., Yeung & Ross, in prep.) and will subsequently be used as a basefor interpretative palaeogeographic maps. The maps are based on the compilation andinterpretation of a large body of published and unpublished geologic data for the Papua NewGuinea region which will be referred to in the text. Detailed data compilations in the format ofstratigraphic summary columns and data maps form the basis for the geologic constraints forthe maps (Struckmeyer, 1990, in prep.)

The New Guinea Orogen is the product of a number of plate tectonic processes which haveaffected the northern margin of the Australian Plate during its Mesozoic to Cainozoic history.Reconstructions for the New Guinea area have been produced by a number of authors (e.g.Crook & Belbin, 1978; ICroenke, 1984; Pigram & Davies, 1987; Francis & Deibert, 1988;Audley-Charles, 1988; Francis, 1990). They differ considerably, both in the timing andmechanisms invoked, which may in part be ascribed to the sparsity of data in a geologicallycomplex area. Thus, most of the models proposed are valid interpretations of possiblescenarios of the plate tectonic development of the region within the geologic constraintsprovided by the data available. The model presented in this report is part of an attempt tointegrate detailed reconstructions for the Papua New Guinea region with similar reconstructionsfor other areas of the Australian Plate margin by taking into account a global palaeomagneticframework and the direction of movement of the major bounding plates. The computer-generated maps were produced using POMP (Paleoceanographic Mapping Project) digitisingand tectonic reconstruction software in combination with Terra MobilisTm (plate tectonics forthe Macintosh - Denham & Scotese, 1988). Apart from palaeomagnetic information specific tothe Papua New Guinea region, which will be referred to later in the text, the globalpalaeomagnetic framework is based largely on Ziegler & others (1982), Cande & Mutter(1982), Scotese & others (1988), Royer & Sandwell (1989), Scotese & Barrett (1990),Scotese & MacKerrow (1990) and Scotese (in prep.). The time scale used for this study isbased on Harland & others (1982) for the Mesozoic and on Berggren & others (1985a, b) forthe Cainozoic. Isotopic ages were, to a great exent, taken from Page (1976).

1

The software used for the reconstructions is based on hierarchical plate tectonic analysis. Thatis, each plate moves relative to a second plate, which moves relative to a third, etc. Thus, themotion of all plates can be traced back to one master plate (Africa). The relative positions ofthe components at the Triassic/Permian boundary have been used as a starting point for thereconstructions. The recognition and definition of tectonic components which have movedrelative to the Australian Plate involved an analysis of the type of sedimentary and/or igneousassociations they consist of, their relative structural relationships, their original tectonic setting,their biogeography, the age of surrounding ocean floor, the timing of accretion and the age ofoverlap sequences. The number of tectonic components chosen for the reconstructions were,in the first instance, dependent on the scale of the maps to be produced (1:10,000,000).Therefore, components that could have been subdivided to a much greater degree, were oftencondensed into one composite block where required. Although generalisations were necessaryfor the final model, the basic data were compiled at a much greater level of detail in order togain an understanding of the basic problems involved. The use of the term terrane was avoidedfor components that are not fault-bounded entities. However, where components correspond topreviously defined terranes the term was retained. Original boundaries of the tectoniccomponents do not necessarily correspond to present-day tectonic boundaries, becauseprocesses such as strike-slip faulting or incipient extension in the last few million years mayhave resulted in the dispersion of tectonic units which originally formed one component orterrane. Also, component size is very likely to have changed with time. For simplicity reasonsthe components were digitised in their present-day shape using isobaths, faults, coastlines or,in some cases, where no geographic or geologic features were available, arbitrary lines asboundaries. Small-scale movements within individual components or between componentswere not considered in view of the final scale of the maps.

2. PLATE TECTONIC SETTING

The present-day plate tectonic setting of the Papua New Guinea region is summarised in Figure1. The mainland of Papua New Guinea can be subdivided into two major structural provinces:i) the autochthonous Australian Craton and ii) the New Guinea Orogen which consists of a para-autochthonous belt of deformed Australian Craton, the Papuan Fold Belt, and a northern belt ofallochthonous terranes (Pigram & Davies, 1987). The autochthonous craton in Papua NewGuinea presently forms the foreland to the orogen (Pigram & Symonds, in press). The islandsof Papua New Guinea (New Britain, New Ireland, Bougainville) are composed of Tertiaryisland arcs and derived sediments.

2

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At present, oceanic crust of the Solomon Sea is subducted northwards at the New BritainTrench (e.g. Tiffin & others, 1987). The Trobriand Trough is regarded by many authors as anactive or recently active subduction zone (e.g. Hamilton, 1979; Davies & others, 1984, 1987;Lock & others, 1987), but could be a small foreland basin (C.J. Pigram, pers. comm., see alsoChapter 4) similar to the Moresby Trough (Pigram & Symonds, 1988). Some authors believethat active subduction is occurring in the New Guinea Trench (e.g. Hamilton, 1979; Kroenke,1984; Franics, 1990). However, very little data are available and there appears to be no majorseismicity or volcanism associated with the trench. Very young, active spreading centres arepresent in the Manus Basin and the Woodlark Basin, where extension is now proceeding intocontinental crust to the west. The West Melanesian or Manus-Kilinailau Trench is inactive butwas a major subduction zone in the Early to Mid-Tertiary.

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Figure 1: Plate tectonic setting of the New Guinea Orogen.

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3. TECTONIC COMPONENTS OF THE PAPUA NEW GUINEA REGION

3.1 Introduction

The tectonic components chosen for the reconstructions are in many instances synonymouswith tectono-stratigraphic terranes distinguished by Pigram & Davies (1987); in these cases,the names and the designation `terrane' were retained. In view of the scale of the finalreconstructions, not all of the twenty four Papua New Guinea terranes described by Pigram &Davies could be used; several geologic entities and geographic areas were combined into singletectonic components and were given the designation 'block' or 'composite block'.

Three major types of tectonic components can be recognised in Papua New Guinea (Figure 2),those of continental, mixed, and oceanic to island arc affinity. The blocks with continentalaffinity typically contain proximal to distal Mesozoic to Palaeogene passive margin sedimentsof the Papuan Basin and underlying basement; they include the Papuan Fold Belt, the EasternPapuan Plateau, and the North Coral Sea, Jimi, Kubor and Bena Bena components. Blockswith mixed affinity typically consist of low to high grade metamorphics with protoliths ofsedimentary, volcanic, mafic and ultramafic rocks, as well as some non-metamorphic rocks.These successions probably represent distal Papuan Basin deposits in fault-contact withoceanic crust and arc volcanics, an assemblage that is typical of subduction complexes; theyinclude the West New Guinea Composite Block, the Schrader Terrane and the Owen StanleyComposite Block. Components with oceanic affinity are ocean floor sediments and oceaniccrust. They include the Marum Terrane, and the greater part of the Kokoda and Kutu-Louisiade Blocks. Components with volcanic arc affinity are island arcs and derivedsediments; they include the Torricelli and Finisterre Terranes, and the South Bismarck, NewIreland and Bougainville Blocks.

The hierarchical model established for the components of the Papua New Guinea region whichforms the basis for the computer-generated reconstructions is presented in Figure 3. Thus,most components move relative to the Papuan Fold Belt which in turn moves relative toAustralia. Changing movement relationships are indicated with dashed lines (e.g. the JimiTerrane moved relative to the North Coral Sea component from 248 to 22 Ma and relative toAustralia from 22 Ma onwards).

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801 - Australia836 - North Coral Sea846 - West Nev Guinea Composite Block847 - Torricelli Team848 - Finisterre Terrane849 - Jimi Terrane850 - Kubor Block851 - Kutu-Louisiade Block852 - Oven Stanley Composite Block853 - Papuan Fold Belt

857 - Marum Terrane858 - Schrader Ten-ane859- Bena Bena Terrane860 - Kokoda. Block861 - South Bismarck Block862 - Hey Ireland Block863 - Bougainville Block901 - Ontong Java Plateau950 - Eastern Papuan Plateau

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5

Kokoda Block860

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Hey Ireland Block862

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Kubor Block850

Figure 3: Plate hierarchy for the Papua Nev Guinea region.

3.2 Description of Tectonic Components

836: North Coral SeaSynonymy: Papuan PlateauDescription: Marginal plateau along the northern margin of the Coral Sea, whichdetached from the Queensland Plateau as a result of the opening of the Coral Sea in thePaleocene to Eocene. The plateau is now partly overthrust by allochthonous terranes ofthe Papuan Peninsula. It consists of fault blocks of subsided Australian continentalbasement with graben fills of pre-rift and rift sediments of Mesozoic age, draped byEocene to Oligocene sag-phase sediments and Middle Miocene to Recent foreland basindeposits. Major references: Ewing & others (1970), Gardner (1970), Falvey & Taylor(1974), Weissel & Watts (1979), Symonds & others (1984), Pigram & Symonds(1988).

846: West New Guinea Composite Block (WNGCB)Synonymy: includes Sepik Composite Terrane, Landslip Terrane, Dimae Terrane,Mount Turu Terrane and Prince Alexander Terrane of Pigram & Davies (1987);Description: Large block in northwestern Papua New Guinea, north and south of theSepik River; it extends westward into Irian Jaya where it includes the Rouffaer terraneof Pigram & Davies (1987). The WNGCB is interpreted as a subduction complexcomprising non-metamorphosed sediments, low and medium grade metamornhics ofcontinental affinity, blueschists, possible arc volcanics and ultramafics ofJurassic/Cretaceous to Eocene age. Collision with the Australian Craton probablyoccurred in the middle Oligocene. Major references: Dow & others (1972), Hutchison& Norvick (1978, 1980), Norvick & Hutchison (1980), Davies (1982b), Davies &Hutchison (1982), Pigram & Davies (1987), Francis & Deibert (1988), Pawih (1989),Doust (1990), Davies (1990), Francis (1990); (note: a frequently quoted reference forthe region is Rogerson & others [1987], however up to the time of submission of thisreport the publication has not been available from the Geological Survey of Papua NewGuinea).

847: Torricelli Terrane (Pigram & Davies, 1987)Synonymy: Torricelli Structural Complex (Hilyard & others, 1988).Description: NNW-SSE trending terrane along the Bewani-Torricelli Mountains andNorth Coast Ranges in northwestern Papua New Guinea. It consists of a LateCretaceous to Eocene and Early Miocene island arc complex comprising mafic andpartly ultramafic intrusives, basaltic and andesitic lavas, associated volcaniclastic rocks

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and bathyal-abyssal sediments, overlain by an Early Miocene syntectonic sequence ofshallow water limestone and coarse elastics. Accretion to the craton probably occurredduring the Early to Middle Miocene. Major references: Hutchison & Norvick (1978,1980), Norvick & Hutchison (1980), Pigram & Davies (1987), Francis & Deibert(1988), Davies (1990), Doust (1990), Francis (1990).

848: Finisterre Terrane (Pigram & Davies, 1987)Synonymy: Finisterre Terrane in part, ?Lokanu Association, ?Papuan Basin (Hilyard &others, 1988).Description: NW-SE trending terrane in northern mainland Papua New Guinea,extending from the Adelbert Ranges in the northwest, to the Huon Peninsula in theeast. Middle to Late Eocene deep marine sediments are overlain by Oligocene to EarlyMiocene island arc volcanics and derived sediments. The Finisterre Terrane probablyaccreted to the orogen during the Late Miocene to Pliocene; during this time shallowwater carbonate deposition was predominant. Major references: Chappell (1974),Robinson (1974), Jaques (1976), Robinson & others (1976), Tingey & Grainger(1976), Jaques & Robinson (1980), Pigrarn & Davies (1987), Davies (1988); Francis& Deibert (1988), Crook (1989), Francis (1990).

849: Jimi Terrane (Pigram & Davies, 1987)Synonymy: Kubor Terrane and Papuan Basin (Hilyard & others, 1988)Description: Located north of the Kubor Block. The terrane consists of a sequence ofTriassic to Eocene sediments and intrusives with strong similarities to the Papuan Basinsequence. It probably formed part of the Australian Craton during the Mesozoic toPalaeogene, but underwent northwestward movement and rotation relative to theAustralian Craton during the Miocene (Klootwijk & others, in prep.). Major references:Bain & Binnekamp (1973), Bain & Mackenzie (1975), Bain & others (1975),Mackenzie (1980), Haig (1981), Haig & others (1986), Pigram & Davies (1987),Pigram & others (1987), Francis & others (1990), Francis (in prep.).

850: Kubor BlockSynonymy: Kubor Terrane in part of Hilyard & others (1988); it was defined by theseauthors as part of the Papuan Composite Terrane, which was described as anamalgamation of Proterozoic to Permian terranes with affinities to the New England-Yarrol Orogen that accreted to the Australian Craton before the Triassic.Description: The Kubor Block is located in central Papua New Guinea, south of theJimi Terrane. It consists of Late Palaeozoic metamorphics intruded by Triassic

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granites, and partly overlain by Triassic to Cainozoic Papuan Basin sediments. Theterrane formed part of the Australian Craton during the Mesozoic to Palaeogene but hasundergone movement relative to the craton. The movement probably took place duringthe Early to Middle Miocene and the terrane was in its present position by the LateMiocene. Major references: Bain & Binnekamp (1973), Bain & Mackenzie (1974,1975), Bain & others (1975), Skwarko & others (1976), Tingey & Grainger (1976),Davies (1982a), Cussen & others (1986), Haig & others (1986), Rogerson & others(1988), Davies (1990), Francis & others (1990), Hill & others (1990).

851: Kutu-Louisiade BlockSynonymy: Kutu Terrane, southern D'Entitcasteaux Terrane and eastern Owen StanleyTerrane (Louisade Archipelago) of Pigram & Davies (1987); Lokanu Association ofHilyard & others (1988).Description: The Kutu-Louisiade Block includes the southeastern tip of the PapuanPeninsula, the southern D'Entrecasteaux Islands, the Louisiade Archipelago and thesurrounding offshore region to approximately 2000 m water depth. The islands werehere included for simplicity reasons as the boundary between the Kutu-Louisiade andKokoda Blocks was chosen to coincide with the present-day position of the Woodlarkrift. The Kutu-Louisiade Block consists of ?Cretaceous to Eocene submarinevolcanics, intrusives, and interbedded elastics with calcilutite lenses; metamorphicrocks similar to those of the Owen Stanley Composite Block are present in the southernD'Entrecasteaux Islands and the Louisiade Archipelago. The component probablyaccreted to the craton (North Coral Sea) during the Middle Miocene together with theKokoda Block and the Owen Stanley Composite Block. Major references: Davies(1973a), Smith & Davies (1973a, b), Smith & Davies (1976), Smith (1976), Pieters(1978), Smith & Compston (1982), Williamson & Rogerson (1983), Pigram & Davies(1987), Davies & Warren (1988).

852: Owen Stanley Composite BlockSynonymy: includes the Owen Stanley Terrane, Port Moresby Terrane and MenyamyaTerrane of Pigram & Davies (1987); Papuan Basin and Lokanu Association of Hilyard& others (1988); Scrapland in part (Rogerson & Hilyard, 1990).Description: The Owen Stanley Composite Block is located in the central and southernPapuan Peninsula and consists of Mesozoic to Eocene non-metamorphic sediments tolow-grade metasediments and metavolcanics, and minor blueschists, granulites, maficto ultramafic rocks, and Late Cretaceous to Eocene deep water carbonate and siliceousrocks. These are intruded by Miocene granitoids and overlain by Pliocene volcanics.

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The low-grade metasediments have similarities with those of the West New GuineaComposite Block. Major references: Davies & Smith (1974), Brown & others (1975),Tingey & Grainger (1976), Brown (1977), Pieters (1978), Haig (1982), Haig &Malagun (1980), Haig & Tamu (1980), Rogerson & others (1981), Cussen & others(1986), Pigram & Davies (1987), Rogerson & others (1988), Worthing (1988),Rogerson & Hilyard (1990).

853: Papuan Fold Belt (Bain, 1971)Synonymy: Papuan Fold Belt, Aure Thrust Belt; Scrapland in part (Rogerson &Hilyard (1990).Description: The Papuan Fold Belt is a northwest-southeast trending belt,approximately 50 to 150 km wide, of deformed, para-autochthonous, mostly non-metamorphic Mesozoic to Cainozoic Papuan Basin sediments. It forms the frontal partof the New Guinea Orogen and represents crustal shortening between approximately 30and 130 km. The fold belt probably extends offshore to the southeast. Majorreferences: Bain (1971), Bain & Mackenzie (1974), Davies & Norvick (1974),Brown & Robinson (1982), Davies (1983), Ripper & McCue (1983), Hobson (1986),Rogerson & others (1987), Abers & McCaffrey (1988), Francis (1990), Hill (1990),Hill & others (1990), Osborne (1990).

857: Marum Terrane (Pigram & Davies, 1987)Synonymy: Marum Ophiolite Complex (Jaques, 1981); Lokanu Association (Hilyard &others, 1988).Description: The Marum Terrane is located in northern Papua New Guinea between theFinisterre and Schrader Terranes. It consists of an ophiolite complex of unknown age(K-Ar ages give both Jurassic and Paleocene ages) which is in fault contact with asequence of ?Eocene pillow basalt and argillite. The Marum Terrane resembles part ofthe Kokoda Block and may have formed part of the East Papua Composite Blockbefore the Miocene. Major References: Jaques (1981), Jaques & others (1978), Pigram& Davies (1987).

858: Schrader Terrane (Pigram & Davies, 1987)Synonymy: Papuan Basin in part, Hilyard & others (1988).Description: The Schrader Terrane is located in northern Papua New Guinea betweenthe Jimi and Finisterre Terranes. It consists of a sequence of ?Jurassic to Eocenemetasediments and metavolcanics (protoliths probably deposited in deep waterenvironments), and minor ultramafic rocks. The sedimentary protoliths were probably

10

deep water equivalents of the Papuan Basin sequence. The terrane may have been partof the East Papua Composite Block before the Miocene. Major references: Bain &Mackenzie (1975), Pigram (1978), Jaques & Robinson (1980), Pigram & Davies(1987).

859: Bena Bena Terrane (Pigram & Davies, 1987)Synonymy: Goroka Terrane in part, Papuan Basin in part (Hilyard & others, 1988).Description: The Bena Bena Terrane is located in northeastern Papua New Guineasouth of the Finisterre Terrane, between the Owen Stanley Composite Block and theJimi Terrane. It consists of pre-Triassic low grade to medium grade metamorphics,which may be correlative with the Late Palaeozoic Omung Metamorphics of the KuborBlock. They are intruded by Early Jurassic and Late Cretaceous granitoids and, inplaces, unconformably overlain by Late Cretaceous tuffaceous mudstones and Eocenelimestones. Similar to the Kubor and Jimi Terranes, the Bena Bena Terrane is likely tohave been part of the Australian Craton during the Mesozoic to Early Cainozoic. Majorreferences: Bain & Mackenzie (1975), Robinson & others (1976), Tingey & Grainger(1976), Mackenzie (1980), Haig (1981), Haig & others (1986), Pigram & Davies(1987), Rogerson & others (1988), Francis & others (1990).

860: Kokoda BlockSynonymy: Bowutu, Dayman, Woodlark and northern D'Entrecasteaux Terranes ofPigram & Davies (1987); Lokanu and Goropu Associations in part, Papuan Plateau inpart (Hilyard & others, 1988);Description: The Kokoda Block is located along the northern part of the PapuanPeninsula; offshore, it includes the Trobriand Islands, Woodlark Island, the northernD'Entrecasteaux Islands and the surrounding offshore area bounded by the 2000 misobath and the Trobriand Trough in the north. The Kokoda Block consists ofCretaceous to Eocene ophiolite, basaltic volcanics and low to high grade metamorphicswith mostly igneous protoliths, Eocene andesitic volcanics and Eocene sediments.During the Eocene, the Kokoda Block probably amalgamated with the Kutu-LouisiadeBlock, Owen Stanley Composite Block and Marum and Schrader Teffanes above asubdution zone to form a composite terrane (East Papua Composite terrane of Pigram &Davies, 1987) which collided with the North Coral Sea component in the Early toMiddle Miocene. Major references: Trail (1967), Davies (1971), Davies & Smith(1971, 1974), Davies (1980a, b), Smith (1976), Smith & Davies (1973a, 1976),Ashley & Flood (1981), Smith & Johnson (1981), Smith & Compston (1982), Walker

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& McDougall (1982), Stewart & others (1986), Francis & others (1987), Davies &others (1987), Pigram & Davies (1987), Davies & Warren (1988), Worthing (1988).

861: South Bismarck BlockSynonymy: Finisterre Terrane in part (Hilyard & others, 1988).Description: The South Bismarck Block includes New Britain, the Schouten Islands,the Witu Islands and the offshore areas around New Britain to approximately 2000 mwater depth in the north and to 7000-8000 m in the south (New Britain Trench). Theblock consists of ?Middle Eocene to Early Miocene island arc vokanics, intrusives andderived sediments. These are overlain by Miocene to Early Pliocene carbonates andelastics and a second sequence of arc volcanics of Late Miocene to Pleistocene age.Oceanic crust of the Solomon Sea is presently being subducted northwards at the NewBritain Trench. Major references: Davies (1973b), Ryburn (1974, 1975, 1976),Johnson (1976, 1977, 1979), Page & Ryburn (1977), Robinson & Jaques (1978),Taylor (1979), Smith & Johnson (1981), Falvey & Pritchard (1984), Durkee & others(1987), Belford (1988), Lindley (1988).

862: New Ireland BlockSynonymy: Finisterre Terrane in part (Hilyard & others, 1988).Description: The New Ireland Block includes the islands of New Ireland, NewHanover and Manus, plus the surrounding offshore areas to 1500-2000 m water depth.Similar to New Britain, the block comprises ?Middle Eocene to Early Miocene islandarc volcanics overlain by Miocene carbonates and elastics and Pliocene volcanics. Innorthwestern New Ireland and New Hanover, volcanism persisted throughout theMiocene, although some carbonates and elastics of this age are present. The NewIreland, South Bismarck and Bougainville Blocks probably formed one single arcduring the Eocene to Miocene. The boundary between the New Ireland andBougainville Blocks is based on Coleman & Packham (1976), however, according tothe model presented in this report no major movement occurred between the twocomponents. In the Pliocene, the New Ireland Block together with the BougainvilleBlock moved northwestwards relative to the South Bismarck Block. Major references:Johnson (1976, 1979), Hohnen (1978), Taylor (1979), Jaques (1980), Brown (1982),Falvey & Pritchard (1984), Exon & others (1986), Stewart & Sandy (1986, 1988),Durkee & others (1987), Exon & Marlow (1988, 1990), Francis (1988), Haig &Coleman (1988), Marlow & others (1988).

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863: Bougainville BlockSynonymy: Finisterre Terrane in part (Hilyard & others, 1988).Description: The Bougainville Block is located to the east of the South Bismarck andNew Ireland Blocks and includes the islands of Bougainville, Feni, Tanga, Lihir, Tabarand Mussau, and the surrounding offshore areas to 1000-2000 m water depth; in thesouthwest, the boundary coincides with the eastern New Britain Trench and theoffshore continuation of the Sapom/Weitin Fault system. The Bougainville Blockconsists of Oligocene to Early Miocene arc volcanics overlain by Early Miocenelimestone. A hiatus is present in the Middle to Late Miocene and deposition resumed inthe Pliocene to Pleistocene with a sequence of vokanics and elastics. Major references:Blake & Miezitis (1967), Wallace & others (1983), Durkee & others (1987), Marlow &others (1988), Bruns & others (1986, 1989), Hilyard & Rogerson (1989), Rogerson &others (1989), Vedder & Bruns (1989).

901: Ontong Java Plateau (e.g. ICroenke, 1972)Description: Plateau of thickened oceanic crust of Cretaceous age which collided withthe West Melanesian Trench during the Early Miocene. Major references: ICroenke(1972, 1984), Wells (1989).

950: Eastern Papuan PlateauSynonymy: Eastern Plateau.Description: Marginal plateau of subsided, Australian continental crust which rotatedanticlockwise as a result of the opening of the Coral Sea. Rift grabens with eitherMesozoic pre-rift sediments or Late Cretaceous to Paleocene syn-rift sediments aredraped by an Eocene to Recent marine sequence of oozes and turbidites. Majorreferences: Ewing & others (1970), Gardner (1970), Taylor & Falvey (1977), Weissel& Watts (1979), Symonds & others (1984), Pigram & Symonds (1988).

13

4. OUTLINE OF PLATE TECTONIC HISTORY

4.1 Introduction

During the Mesozoic to Paleocene the northeastern margin of the Australian Plate was a passivemargin that underwent two major phases of extension (e.g. Hamilton, 1979; Pigram &Panggabean, 1984; Struckmeyer & others, 1990; Pigram & Symonds, in press). A majorhiatus in the Oligocene of the Papuan Basin reflects the conversion of the margin from passiveto convergent.

It has long been recognised that the New Guinea Orogen is the result of the collision orcollisions between the Australian Plate and the Pacific Plate (e.g. Dewey & Bird, 1970; Davies& Smith, 1971; Karig, 1971; Johnson, 1976; Jaques & Robinson, 1977; Hamilton, 1979), butopinions differ considerably as to the mechanisms and the timing involved. Dewey and Bird(1970) used the New Guinea Orogen as the type example of a continent/island arc collision andpostulated post-collisional reversal of arc polarity, i.e. from a north-dipping to a south-dippingsubduction zone, a model which was supported by a number of authors (e.g. Hamilton, 1979;Cooper & Taylor, 1987). Francis & Deibert (1988) suggested continuous southward dippingsubduction underneath the continental margin from the Early Miocene to Late Pliocene. Ripper& McCue (1983) interpreted this slab as the remnant of a doubly subducting Solomon SeaPlate which is continuing to the present day in the New Britain Trench and the TrobriandTrough. In a detailed discussion on possible arc reversal in New Guinea, Johnson (1976),Johnson & others (1978) and Johnson & Jaques (1980) strongly argued against the presenceof a south-dipping slab underneath the New Guinea mainland (see also section 5.3).

Davies (1982a) and ICroenke (1984) recognised that the New Guinea Orogen formed as a resultof several continent/island arc collisions, Silver & Smith (1983) compared the formation of theNew Guinea Orogen with terrane accretion processes in the American Cordillera, and Pigram& Davies (1987) undertook a detailed terrane analysis of the orogen identifying thirty twoterranes of continental, oceanic and volcanic arc affinity. Interpretations of the timing of thefirst collisional event range from Eocene (e.g. Davies, 1971; Davies & Smith, 1971; Davies &Hutchison, 1982), latest Eocene/earliest Oligocene (Davies, 1990) to Oligocene (e.g. Davies1982b; Pigram & Davies, 1987) to latest Early Miocene/Middle Miocene (e.g. Hilyard &others, 1988; Francis & Deibert, 1988) and Late Miocene to Pliocene (e.g. Cooper & Taylor,1987). A review and discussion of the timing of tectonic events in the New Guinea Orogen isgiven in Pigram & Symonds (in press). Following Pigram & Davies (1987), Pigram & others(1989) and Pigram & Symonds (in press), initial collision is here regarded as the time the

14

continental margin first experienced loading of a thrust mass (mid-Oligocene) and,consequently, deposition of elastics in the developing foreland basin.

4.2 Palaeomagnetic constraints

Palaeomagnetic data are available for two regions in mainland Papua New Guinea, the centralhighlands region and the North Sepik region (Klootwijk & others, in prep.). All data showstrong overprints from Miocene igneous activity; primary components could only be indentifiedin seven localities. The data indicate that the Jitni Terrane and the Kubor Block underwentlarge scale counterclockwise rotations of between 30° and 100°, whereas clockwise rotations ofbetween 30° and 50° were interpreted for the Papuan Fold Belt. The similarity of the Mesozoicto early Cainozoic sequence of the Kubor Block area to the Papuan Basin sequence(Struckmeyer, 1990) implies that the terrane formed part of the basin during this time.However, with the exception of the palaeomagnetic data, there is no evidence for its originalposition along the Australian Craton margin.

There is also no clear evidence for the original location of the Jimi and Bena Bena Terranes.However, their present position relative to the West New Guinea Composite Block impliesnorthwestward movement of the Jimi/Bena Bena Terranes after the accretion of the WNGCB.For their pre-Neogene position, these terranes were therefore rotated southeastward to adjointhe northern edge of the North Coral Sea component.

Palaeomagnetic data from the North Sepik region are mostly from the Torricelli Terrane. Thedata indicate that the terrane collided with Australia at approximately 15° south latitude.Northern Australia reached this latitude about 15 million years ago. A latitude of 0.5° has beenobtained from pelagic limestone lenses of probable Paleocene to Eocene age within the BliriVolcanics of the Torricelli Terrane, which suggests that they accreted to the island arc terrane ata southward dipping subduction zone (Klootwijk et al, in prep.). One sample from anOligocene limestone of the West New Guinea Composite Block indicates a 21° south latitudewhich agrees with a collision of the block with Australia at approximately 30 to 35 millionyears. Francis & Deibert (1988) pointed out that the results of this study are doubtful as agecontrol over the units sampled is inadequate. However, as the palaeomagnetic study is the onlyone of its kind so far for the New Guinea mainland and a revision of the age data has not beenpublished, the data are included in this study.

Falvey & Pritchard (1984) interpreted palaeomagnetic data from the island of New Guinea andsuggested a succession of anticlockwise and clockwise rotations for the Melanesian arc

15

components from the Eocene to present. However, these data need to be revised, as newbiostratigraphic results indicate that, for example, samples assigned to the Middle Miocene atthe time of Falvey & Pritchard's (1984) study are now regarded as Late Miocene to EarlyPliocene (Stewart & Sandy, 1986; Belford, 1988).

4.3 Late Cainozoic arc-reversal and Cretaceous subduction? - A discussion

The main arguments for southward subduction underneath Papua New Guinea during theMiocene to Pliocene include (a) the presence of Miocene and Pliocene to Quaternary arc-relatedvolcanics on the mainland and (b) the interpretation from earthquake data of the presence ofboth a northward and southward dipping slab underneath the suture (Ramu-Markham FaultZone) (e.g. Hamilton, 1979; Cooper & Taylor, 1987; Francis & Deibert, 1988). Johnson(1976), Johnson & others (1978), Johnson & Jaques (1980) and Hamilton & others (1983)pointed out a number of arguments against the presence of a south-dipping slab underneath theNew Guinea mainland. Firstly, the Late Cainozoic volcanics do not require subduction butcould be the result of partial melting of previously subduction-modified lithosphere with themagmatism caused by thrust loading and uplift in the Pliocene. A recent iterpretation of thePorgera intrusive complex (Late Miocene) as "alkalic basalts of invaplate affinity" (Richards &others, 1990) indicates that no previous subduction is required at all. However, Rock &Finlayson (1990) questioned the intraplate interpretation and described the Porgera complex asshoshonitic (lamprophyric/appinitic) with strong similarities to other contemporaneous igneousrocks of Papua New Guinea. Secondly, the earthquake data do not clearly support thepresence of a south-dipping seismic zone. Thirdly, the chain of Quaternary volcanics off thenorth coast of Papua New Guinea would have to be on the continent side of the subductionzone and thus be on the Australian Plate; this would imply that they are not part of the NewBritain volcanic arc. However, there is no bathymetric or seismic evidence for the presence of asubduction zone north of the islands. Johnson (1976, 1979), Johnson & others (1978), andJohnson & Jaques (1980) therefore introduced a model of continued northward subduction andgradual steepening of the subducting slab, with the youngest arc volcanics off the north coastrepresenting the final phase of subduction related volcanism.

If the Late Cainozoic volcanics do not require southward dipping subduction, the same modelcould be invoked for intrusives and extrusives of Oligocene to Late Miocene age which havebeen used as evidence for southward dipping subduction during this time (e.g "Maramuni Arc" -Francis & Deibert, 1988; Francis, 1990). There appears to be no reason why the Oligocene toMiocene igneous events could not have been caused by northward subduction - on thecontrary, the fact that they are typically restricted to the allochthon supports the suggestion of

16

northward subduction during this time. Ages of the intermediate to felsic intrusives andassociated extrusives range from Oligocene to Late Miocene with most ages concentrated in theEarly to Middle Miocene. This igneous activity is likely to be post-collisional to syn-collisional.A model of continuous northward subduction is supported by a recent re-interpretation ofearthquake data from New Guinea (Abers & Roecker, in press), which indicates that there isno southward dipping, sinking slab underneath Papua New Guinea, but a very well defined,northward dipping zone of intermediate depth to deep earthquakes beneath the Huon Peninsula.This reinterpretation also casts doubt on a southward dipping subduction zone being present atthe Trobriand Trough, which has repeatedly been described as a continuation of the NewGuinea Trench and the postulated former subduction zone underneath mainland New Guinea(e.g. Lock & others, 1987; Francis, 1990). It is possible that the Trobriand Trough is a smallforeland basin, which developed in response to loading of the Trobriand Platform (C.J.Pigram, pers. comm., June 1991).

The presence of a Cretaceous arc in the New Guinea region has been suggested by a number ofauthors (Johnson & others, 1978; Brown & others, 1980) based on the presence of EarlyCretaceous arc-related volcanics in the Jimi Terrane sequence. There is a strong similaritybetween the Jimi Terrane and the Mesozoic Papuan Basin sequence suggesting that it was partof the passive continental margin during the Mesozoic to Early Tertiary. The only majordifference between the sequences is the presence of lavas, pillow lavas and volcaniclastics ofolivine tholefitic, low silica andesitic and shoshonitic composition (Kumbruf Volcanics) withinthe ?Early Cretaceous sequence of the Jimi Tenane (Pigram, 1978); this composition indicatesan island arc or volcanic arc origin (Pigram, 1978; Mackenzie, 1980). The volcanics areprobably the source of volcaniclastic sediments in the Kondaku Formation of the PapuanBasin. If the volcanics of the Jhni Terrane are stratigraphically in place, they imply subductionunderneath the continental margin of Australia during the Early Cretaceous. If the volcanics arenot in place stratigraphically, they may be entirely allochthonous - for example, a thrust sheetof island arc origin emplaced during the accretion of the Schrader and Marum Terranes.However, this option is thought to be unlikely and can only be resolved by further detailedfield work in the area.

The possibility of the Kumbruf Volcanics being related to extensional tectonics in the Early toLate Cretaceous should not be discounted, particularly as they may have been comagmatic withalkaline lamprophyres described from the northern Papuan Basin (Finlayson & others, 1988).The presence of Late Cretaceous ophiolites within the allochthon and the timing of the openingof the Tasman and Coral Seas also argue for an extensional environment during at least the mid-to Late Cretaceous. Rifting would have occurred along the edge of the Australian Craton,

17

possibly involving previously subduction-modified lithosphere; the generation of volcanics ofKumbruf composition may be conceivable in such a setting. A modern analogue may be thetholeiitic basalts, andesites and rhyolites erupted in association with rifting in the westernWoodlark Basin (e.g. Johnson, 1979; Binns & others, 1989). Also, andesitic-basaltic todacitic-rhyolitic volcanic s of the same age as the Kumbruf Volcanics that occur along theQueensland coast (Whitsunday Volcanics) and have an 'arc-signature', have been interpreted asrift-related and having formed through partial melting of lithosphere modified during LatePaleozoic subduction (Ewart & others, 1990). The same setting could be envisaged for theCretaceous volcanics of the Jimi Terrane. However, Johnson & others (1978) invoked partialmelting of lithosphere modified as a result of Cretaceous subduction for the source of LateCainozoic volcanics of the highlands area, based on the presence of the Kumbruf Volcanicsand mid-Cretaceous Rb-Sr pseudoisochrons. An alternative explanation may be that, similar tothe Whitsunday volcanics of eastern Queensland, both the Kumbruf Volcanics and the LateCainozoic volcanic rocks were sourced by the partial melting of lithosphere modified byeastward subduction in the Late Palaeozoic rather than in the Cretaceous. There is a possibilitythat the extensional basins which formed during the mid-Late Cretaceous developed as a resultof backarc extension to a subduction zone to the east of Norfolk Ridge (which may havepassed into a strike-slip fault northwestwards).

The following sections give an outline of one possible interpretation of the plate tectonic historyof the Papua New Guinea region. Within the constraints discussed above, the simplest modelpossible was employed to develop the reconstructions presented in this report.

4.4 Mesozoic to Early Eocene

During the Mesozoic to Palaeogene, the northern edge of the Australian Plate was a passivemargin which experienced two major phases of extension, in the Triassic/Jurassic and in theLate Cretaceous to Early Eocene (Pigram & Panggabean, 1984; Symonds & others, 1984;Pigram & Davies, 1987; Struckmeyer & others, 1990; Pigram & Symonds, in press). Figure4 shows a reconstruction for the Triassic to Early Jurassic, when rifting and breakup led to thedetachment of part of the craton margin. There is no clear indication on the present location ofthese detached fragments, but it is possible that some of them now form part of Southeast Asia(e.g. Metcalfe, 1990). The extensional event is reflected in the sedimentary sequence of thePapuan Basin, which displays the characteristics of a rift-drift sequence. During the remainderof the Jurassic (Figure 5) to the Early Cretaceous there was deposition of siliclastic sag phasesediments along the northern margin of Australia (e.g. Pigram & Symonds, in press), whereas

18

approximate limit of Australiancontinental crust= components that detached from or movedrelative to Australia in the LateMesozoic to Cainozoic

fault zonerift zone

palaeolalitude4or s

Late Triassic - Early Jurassic

Figure 4: Late Triassic-Early Jurassic reconstruction. Coastlineof southern Nev Guinea and northern Australia shownfor reference.

19

Oxfordian-Kimmeridgian - 155 Ma

approximate limit of Australian continental crust

components that detached from or moved relativeto Australia in the Late Mesozoic to Cainozoic

spreading centre ...0".400 s^PEdaeolatitude

Figure 5: Late Jurassic (Oxfoniien-Kimmeridgien) reconsiruclion.

20

Cenomanian - 95 Ma

autochthon

component that detsched from or movedrelative to Australia in the Late Mesozoicto Cainozoic

fault zonerift zonespreading centce

4opird. palsteolatitude

Figure 6: Late Cretaceous (Cenomanien) reconstruction.

21

autochtbon component vithmixed affinity

111111111111111111111111111component vithcontinental affinity

component vith oceanicto island arc affinity

Late Paleocene - 60 Ma

Vest

ci Ide limes ill.Trench

20°

30

fault zonespreading centre^palaeolatitude

40°0subduction zone

•:•:•:•:•:•:•:• : •:•:•:•:•:•:

•• ••••••■•••••••••• •••••••••

rifting and seafloor spreading occurred along Australia's northwestern margin in the LateJurassic leading to the detachment of a continental fragment (? Mount Victoria Land ofMetcalfe, 1990).

Figures 6 and 7 show reconstructions for the Cenomanian and Paleocene; they are largelybased on recent work by Pigram & Symonds (in press). The maps capture the secondextensional event which, in my opinion, occurred in two phases, firstly involving the openingof the New Caledonia Basin (Walley & Ross, 1991), incipient rifting in the Coral Sea andprobably Tasman Sea as well as the detachment of continental fragments from the northeasternmargin within an oblique, linked spreading system. These fragments have a strong similaritywith the Papuan Basin sequence and underlying basement and are now located in Irian Jayaand southeast Indonesia. The second phase involved spreading in the Coral Sea and TasmanSea, with alkaline basaltic volcanism occurring on the upper continental slope in Papua NewGuinea (spilites within the Chim Formation - Francis & Deibert; 1988). Intrusions of alkalinelamprophyre sills of Late Albin and Campanian age (Finlayson & others, 1988; Rogerson &others, 1988) may be related to the two phases of initial extension. The northeastern AustralianPlate margin during this time was probably a northeast trending sinistral strike-slip fault system(Figure 6) along which northward movement of the North Coral Sea and Eastern PapuanPlateau occurred as a result of seafloor spreading in the Coral Sea (Pigram & Symonds, inpress.). The Late Cretaceous to Paleocene extensional event was associated with uplift alongthe plate boundary and resulted in differential erosion of the Mesozoic sequence of the easternPapuan Basin. Seafloor spreading continued until the Early Eocene as indicated by magneticlineaments in the Coral Sea Basin (Anomaly 24) and the presence of thrust slices of ophiolitesof this age on the mainland of Papua New Guinea. The Solomon Sea Plate may be a remnantof this ocean. Incipient subduction along the West Melanesian Trench to the northeast ofAustralia may have occurred from the Campanian onwards because arc volcanics of this ageoccur in at least one of the island arc terranes (Torricelli Terrane) which accreted to the craton inthe Neogene.

4.5 Middle Eocene to Recent

Increasing rates of northward movement of the Australian Plate from about 55 million yearsago onward (Cande & Mutter, 1982) and a change in movement direction of the Pacific Platefrom northward to westward approximately 45-42 million years ago (Clague & Jarrard, 1973),led to oblique collision of the two plates. This resulted in the successive accretion of

23

allochthonous terranes of continental, oceanic and island arc origin to the Australian Cratonfrom the Oligocene onwards.

From the ?Early to Middle Eocene onwards (Figure 8), oblique convergence between theAustralian and Pacific/Solomon Plates was compensated for by both northeastward subductionof Australian Plate oceanic crust, and southward subduction of Pacific Plate oceanic crust.Palaeomagnetic data (Klootwijk & others, in prep) suggest that the Torricelli volcanic arcformed above a southward dipping subduction zone (Manus/Kilinailau or West MelanesianTrench ); it was probably part of a single arc which included the Finisterre Terrane and theSouth Bismarck, New Ireland and Bougainville Blocks. Island arc terranes of Irian Jaya(Yeung & Ross, in prep.) probably also formed part of this arc.

Distal sediments of the Australian margin, oceanic crust and early arc-derived volcanicsamalgamated above the northward dipping subduction zone, and they would later form thecomposite terranes of mixed to oceanic affinity of central and eastern Papua New Guinea, i.e.the West New Guinea Composite Block (WNGCB) and East Papua Composite Block (EPCB= Kokoda Block, Owen Stanley Composite Block, Kutu Composite Block). For simplicitythese composite blocks are shown in their present shape. However, they should be envisagedas gradually forming above the subduction zone by the scraping off of ocean floor sediments,incorporation of oceanic crust, and their partial subduction. Following Pigram & Davies(1987), the Eocene age of deformation and metamorphism of these terranes (Davies & Smith,1971) is here regarded as reflecting amalgamation of the terranes above the subduction zone.The Schrader and Marum Terranes are included as part of the East Papua Composite Block,however, it is also possible that they are remnants of ocean floor accreted to the craton during alater phase of subduction.

The arrival of continental crust at the north-dipping subduction zone resulted in the accretion offirstly the WNGCB in the Early to Middle Oligocene (Figure 9), and then the EPCB in theEarly to Middle Miocene (Figures 10, 11), as indicated by the age of syntectonic sediments inthe foreland basin (e.g. Pigram & others, 1989; Davies, 1990) and by palaeomagnetic data (seeChapter 5.2). I suggest that the latter collision initiated northwestward movement of the Riniand Bena Bena Terranes within a sinistral strike-slip sytem. These initial collisions arereflected in marked unconformities in the Papuan Basin (e.g. Pigram & Davies, 1987;Struckmeyer, 1990) and subsequently resulted in the deposition of mostly fine-graineddeposits in the foredeep of the advancing thrust load, while deposition of platform carbonatesoccurred further south (Pigram & others, 1989). The foreland deposits became markedlycoarser with time reflecting a gradual uplift of the thrust sheets. Part of these early foreland

24

Middle Eocene - 45 Ma

autochthon

components 'withcontinental affinity

components 'withmixed affinity

components with oceanicto islarud arc affinity

fault zonesubduction zone

palaeolalitude200 S

11111111 1111111111 1111111

Figure 8: Middle Eocene reconstruction.

25

Mid-Oligocene - 30 Ma

components vith oceanicto island arc affinity

autochthon

1111111111111111111111111111

components vithcontinental affinitycomponents vithmixed affinity

fault zonesubduction zone.4116-411116-,

....-'

200 0palaeola.titude

Early Miocene - 20 Ma

component vithoceanic to island arcaffinityfault zonesubduction zone

11111 111111111 11111111111

autochthon

components vithcontinental affinity

1.1717.7747717:17nr

.4116--1111,—

.....'

200 8

comporuents vithmixed affinity

palaeola.titu.de

Figure 10: Early Miocene reconstruction.

27

1111111111111111111111111111

.iilib-dilL.

200 8

components vithconlinentalaffinitycomponents vithmixed affinity

autochthoncomponents withoceanic to island arcaffinity

fault zonesubduction zone

palaeolatitude

Middle Miocene - 15 Ma

Figure 11: Middle Miocene reconstruction.

28

deposits are preserved in the central highlands area and in the Aure Trough where they are nowincorporated in the fold belt. Southwestward subduction along the West Melanesian Trenchprobably ceased between about 20 and 25 Ma due to the arrival of the Ontong Java Plateau atthe subduction zone (ICroenke, 1984). In places, subduction may have persisted throughoutthe Miocene as indicated by continued volcanism in northwestern New Ireland and NewHanover.

Remaining oceanic crust between the WNGCB and Torricelli Terrane was probably alsoconsumed by northward subduction (see also Davies, 1990), and by about 15 Ma, theTorricelli Terrane was close to its present position (Figure 11). A strike-slip component wasprobably also involved in the accretion of the Finisterre Terrane. It is here envisaged asmoving westwards along the northern edge of the East Papua Composite Block, therebyinitiating northwestward displacement of the Schrader and Marum Terranes (Figures 10-12).The Finisterre Terrane collided first in the northwest during the Late Miocene (Figure 12),andlater in the southeast during the Pliocene to Pleistocene (Figure 13). The remaining oceaniccrust was consumed northwards as indicated by the presence of a north-dipping slab beneaththe terrane (Abers & Roecker, in press).

From the Late Miocene onwards (Figure 12), oceanic crust of the Solomon Sea was subductednorthward and eastward at the New Britain Trench resulting in renewed volcanic activity inNew Britain, New Ireland and Bougainville. Seafloor spreading in the Bismarck Sea in themiddle to Late Pliocene (Figure 13) (Taylor, 1979) resulted in relative northwestward motionof New Ireland, the final accretion of the Finisterre Teffane and the impediment to northwardmovement of Australia which, to a great extent, was taken up by crustal shortening andunderthrusting in the Papuan Fold Belt. For this report, an average of about 100kmforeshortening was assumed for the fold belt. Also, in the middle to Late Pliocene, extensioncommenced in the western Woodlark Basin (Luyendyk & others, 1973) dismembering theWoodlark Rise and Cape Vogel Basin from the Peninsula. Left lateral movements within theorogen that occurred in the Pliocene are not depicted on the maps in view of the scale of thereconstructions.

29

component vithoceanic to island arcaffinity

autochthon 1111111111111111111111111111

component vithcontinentalaffinity^Alik-466.components vithmixed affinity^200 8

fe.ult zonesubduction zone

palaeolalitu.de

Late Miocene - 10 Ma

autochthoncomponents 'withoceanic to island arcaffinity

200S

.........• • • • • •••••••••• • •...... . ...•fault zonesubduction zone

palaeola.titu.de

components vithcontinentalaffinitycomponents vithmixed affinity

500km.

20° 8

-PiwtflA

Early Pliocene - 5 Ma

Figure 13: Early Pliocene reconstruction.

31

5. IMPLICATIONS FOR HYDROCARBON EXPLORATION

Figure 14 shows major proven and potential hydrocarbon provinces of Papua New Guinea.Guinea. The convergent tectonism along the northern margin of Australia resulted in asignificant change in the style and type of deposition and has therefore great implications forhydrocarbon exploration in the area. For example, source and reservoir rocks for thehydrocarbon accumulations of the Papuan Basin were deposited in a Mesozoic passive marginsetting. However, it was the younger phase of the convergent tectonics in the Pliocene whichcaused the reactivation of earlier structures, deformation, and formation of the major traps inthe fold belt (Hill, 1990). Further to the south in the present-day foreland, which is stillrelatively unexplored, several oil and gas shows have been recorded, as well as a recentgas/condensate discovery at Elevala. Osborne (1990) pointed out that in view of world-wideanalogues, e.g. the Middle East where the bigger accumulations occur in the foreland and notin the fold and thrust belt, the foreland should have the potential for major oil discoveries.

A number of young sedimentary basins which formed as a direct result of the convergenttectonics may have significant oil and gas potential. These include the Aitape, Sepik and Ramusubbasins of the North New Guinea Basin (Francis & Deibert, 1988; Donaldson & Wilson,1990; Doust, 1990; Francis, 1990; Kugler, 1990; Pawih, 1990), and the Cape Vogel Basin(Bickel, 1976; Stewart & others, 1986). The northern basins typically are depressions whichcoincide with sutures between allochthonous terranes. They contain a mostly Miocenesyntectonic, deep-water but regressive sedimentary sequence, which is overlain by a very thickPliocene, mostly shallow water to terrestrial overlap sequence of the North New Guinea Basin.Oil and gas shows and seeps have been recorded from a number of regions within these youngbasins, but so far no discovery has been made. Further exploration is needed to conclusivelyassess the hydrocarbon potential. The Cape Vogel Basin contains a thick sequence of Miocenemostly deep-water elastics and minor carbonates, overlain by regressive Late Miocene elasticsand younger, mostly shallow marine elastics and carbonates.

Frontier areas with very little or no exploration history are young sedimentary basins associatedwith the northern island arcs. For example, the New Ireland Basin is an Eocene to Miocene arc-trench system overlain locally by up to 3500 m of Miocene to Recent carbonates and elasticsand may have considerable potential (Exon & others, 1986; Durkee & others, 1987; Stewart &others, 1987; Exon & Marlow, 1990). Arc-reversal in the Late Miocene resulted in strongregional subsidence and a thick sediment pile. Source rocks may be present in Miocenevolcanolithics, carbonaceous mudstones and highly carbonaceous mudstones and occasionalcoal. Potential reservoir rocks may be limited because of the predominantly volcanolithic

32

• Oil discover/-* as discovery

A Oil liekien Gas field

Oi/ seep^"scr Oi/ and 9.cvs seep-11- Gas seep^041/-cl-op of a//ochthon

Figure 14: Proven and potential hydrocarbon provinces of Papua New Guinea.

material, but shelfal Miocene limestones, some of which have porosities of up to 25% arewidespread. Abundant seals should be provided by fine-grained volcaniclastic material andpotential traps include onlap, pinchouts, and normal faults caused by Plio-Pleistocene uplift.

Another example is the Bougainville Basin which is located within the present-day arc-trenchsystem of the New Britain/Solomon Trench (Durkee & others, 1987; Stewart & others, 1987;Vedder & Bruns, 1989). Early Miocene platform carbonates are overlain by Late Miocene toPliocene volcaniclastic turbidites and Pleistocene to Recent neritic limestones. No source rockdata are available for the area, but some Miocene volcaniclastic units may have limited sourcepotential (Shaw, 1985). Good reservoirs probably are provided by the Miocene limestonesand potential traps include reef build-ups, stratigraphic traps, faulted anticlines and fault-blockclosures (Shaw, 1985).

6. CONCLUSIONS

During the Mesozoic to early Tertiary, the northern margin of the Australian Plate was apassive margin, which was affected by two extensional events, one in the Late Triassic to EarlyJurassic, and one in the Late Cretaceous to Paleocene. The second event involved the openingof several small ocean basins within an oblique, linked spreading system and resulted in thedetachment of continental fragments which are now located in Irian Jaya and southeastIndonesia. Rapid northward movement of the Australian Plate from the Late Paleoceneonwards and a change in movement direction of the Pacific Plate in the Middle Eocene resultedin the conversion of the passive margin to a convergent margin and the formation of island arcsabove two major northward and southward dipping subduction zones to the north. Theoblique collision of the two plates at the north dipping subduction zone, backstepping of thesubduction zone and renewed collision caused the progressive accretion of mixed, oceanic andisland arc terranes to the Australian margin, the displacement of parts of the former passivemargin and the deposition of a thick syntectonic sequence in the foreland and in basins formingbetween the accreting terranes.

As a result of the formation of small ocean basins in the mid- to Late Pliocene, particularly theopening of the Manus Basin, the northward movement of the Australian Plate was taken upwithin the orogen, leading to uplift, crustal shortening and underthrusting and, consequently totrap formation in the fold belt.

34

7. ACKNOWLEDGEMENTS

These initial reconstructions have been produced as part of the BMR-APIRA PhanerozoicHistory of Australia Project and sponsoring companies are thanked for their input to theproject. Particular thanks are extended to M.I. Ross for his help with the initial reconstructionsand for teaching me how to use Terra Mobilis and Paleomap software; to C.J. Pigram, P.E.Pieters, R.W. Johnson and D.E. Mackenzie for their willingness to discuss thereconstructions; to J.M. Totterdell, A.M. Walley, C.J. Pigram, R.W. Johnson and W.D.Palfreyman for critically reading the manuscript; and to P.J. Brown for drafting some of theillustrations.

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