Neogene tectonic reconstruction of the Adelbert-Finisterre-New Britain collision, northern Papua New Guinea

Journal of Southeast Asian Earth Sciences, Vol. I1, No. I, pp. 33-51, 1995 Copyright © 1995 Elsevier Science Ltd 0743-9547(94)00032-8 Printed in Great Britain. All rights reserved 0743-9547/95 $9.50 + 0.00

Pergamon

Neogene tectonic reconstruction of the Adelbert-Finisterre-New Britain collision, northern Papua New Guinea Lon D. Abbott Earth Sciences Department, University of California, Santa Cruz, Santa Cruz, CA 95064 U.S.A.

(Received 18 March 1994; acceptedfor publication 12 July 1994) Abstract--The Finisterre terrane is presently colliding with the Australian continent in northern Papua New Guinea. The terrane is exposed in two distinct blocks - - the Adelbert block and the Finisterre block. A elastic sedimentary sequence in the suture zone provides constraints on the age of initial collision for each block and on paleotectonic reconstructions of New Guinea. Provenance shifts within the Finisterre block sedimentary section date the collision of that block at 3.0-3.7 Ma. Analysis of the Adelbert block sedimentary section has not revealed similar provenance shifts. This absence of temporal trends in source area may be caused by oblique collision of the Adelbert block with an allochthonous terrane composed of oceanic crust. These factors would likely lead to a "soft" collision involving little uplift and hence little disruption of the pre-collisional sedimentation patterns. In contrast, the Finisterre block collided orthogonally with terranes composed of continental crust. This "hard" collision has led to rapid uplift and significant modification of the pre-collisional depositional system. A deep water basin existed between the Adelbert block and the continent in the Late Pliocene. Deep marine sediments deposited in this basin were subsequently overthrust by older lithologies of the Adeibert block. When combined with geochemical, seismic and plate kinematic data published by other workers, these data suggest collision of the eastern portion of the Adelbert block in the Middle to Late Pliocene. The western portion of the Adelbert block probably collided in the latest Miocene. Many tectonic reconstructions of northern Papua New Guinea have favored collision of the Finisterre terrane over a doubly-subducting Solomon Sea Plate in an extension of the modern tectonic configuration of the Solomon Sea. The presence of continentally-derived sediment in the Finisterre accretionary wedge casts doubt on this scenario. The trench of a southward-dipping subduction zone would be likely to block continentally-derived sediment from reaching the Finisterre accretionary wedge. The Maramuni Arc, the igneous association usually attributed to the hypothesized southwarddipping subduction zone, appears to have erupted on allochthonous terranes rather than on autochthonous crust. These observations suggest that collision of the Adelbert block and most of the Finisterre block occurred above a single, northward-dipping subduction zone. The double subduction present in the Solomon Sea probably never extended more than 200 km west of its present location.

Introduction

after the onset of collision? Some proposed models invoke separate collision events for the Adelbert and Finisterre blocks, with the Adelbert block colliding in the Early to Middle Miocene and the Finisterre block colliding in the Pliocene (e.g. Jaques and Robinson, 1977; Johnson and Jaques, 1980; Pigram and Davies, 1987). Other models consider the collision of both blocks to be part of a single, time-transgressive event beginning in the Late Miocene or Pliocene (e.g. Cooper and Taylor, 1987; Francis and Deibert, 1988). Neogene clastic sediments exposed in the suture zone along the south flank of the Finisterre terrane record the pre- and syn-collisional history of the terrane, providing information on the collision ages of each block and on the paleogeography of P N G during the Neogene. This paper discusses the composition of those sediments and the conclusions that can be drawn from those compositions with regard to the collision age of each block of the Finisterre terrane and to the Neogene paleogeography of northern P N G .

Northern Papua New Guinea ( P N G ) is the site of an active collision between the Bismarck volcanic arc and the Australian continent (Fig. 1) that has propagated from N W to SE through time (e.g. Crook, 1989). The Finisterre terrane, an extinct volcanic arc, lies in the Bismarck forearc. Collision has uplifted the Finisterre terrane to form the Adelbert and Finisterre ranges. The Finisterre terrane is exposed as two separate blocks (Fig. 1; Pigram and Davies, 1987). The Adelbert block makes up the Adelbert Mountains. The Finisterre block forms the Finisterre and Sarawaget ranges and the mountainous H u o n Peninsula (referred to here collectively as the Finisterre Range; Fig. 1). The Finisterre block is offset to the SE of the Adelbert block. This offset raises a question: were the Adelbert and Finisterre blocks accreted in separate collision events, or were they part of the same time-transgressive collision event, with the Finisterre block sliding south of the Adelbert block 33

34

L . D . Abbott

Fig. 1. Tectonic map of PNG. Inset map shows location. GF = Gogol Fault, WSC = Woodlark Spreading Center. Asterisks show active volcanoes of the Bismarck Arc. The modern triple junction is at the juncture of the New Britain Trench and the Trobriand Trough. The Finisterre terrane is comprised of the Finisterre block, in stippled pattern, and the Adelbert block, in ruled pattern. The Papuan Ultramafic Belt is shown in cross-hatch pattern (after Davies, 1981). The area of Fig. 3 is marked.

Tectonic S e t t i n g The mainland of PNG can be broken into four tectonic provinces (Fig. 1). The Fly Platform, in the southwestern portion of the country, is composed of undisturbed continental crust of the Australian craton. North of the Fly Platform lies the Papuan Fold Belt, which is an active foreland fold-and-thrust belt composed of para-autochthonous continental crust. The New Guinea Highlands north of the Papuan Fold Belt, as well as the Papuan Peninsula (Fig. 1), consist of a collage of allochthonous terranes (Fig. 2) that were sutured to the continent during the Oligocene and Miocene (Pigram and Davies, 1987). To the north of the New Guinea Highlands, and separated from them by the Markham, Ramu and Sepik river valleys, lie a series of volcanic arc terranes (Figs 1 and 2) that have been sutured to the continent in a diachronous series of Neogene arc-continent collisions. The active Finisterre collision, which is the focus of this paper, is the youngest accretion event in PNG (Pigram and Davies, 1987). In other modem arc-continent collisions (e.g. Taiwan and Timor), the arc is colliding with a passive continental margin. The presence of previously accreted terranes on the PNG continental margin presents a more complex geometry, with several possible choices of where to define the edge of the "continental margin". Suturing of the New Guinea Highlands and Papuan Peninsula ter-

ranes (Fig. 2) to the continent was complete before initiation of the Finisterre collision (Pigram and Davies, 1987). Because of this, I consider the outboard (northern) edge of these terranes to form the "continental margin" with respect to the Finisterre collision. The actual geologic conclusions reached using alternative definitions of the continental margin are likely to be identical, but the application of disparate definitions is liable to lead to confusion. Thus, it is important to keep in mind my definition of the term "continental margin" as you read these pages. The Finisterre collision tip has propagated to the SE with time (Crook, 1989; Gill et al., 1993). At present it is defined by a triple junction between the Australian, Solomon Sea and South Bismarck plates that lies at 148°30'E in the western Solomon Sea (Fig. 1; Silver et al., 1991). Southeast of this triple junction, the Solomon Sea Plate is being consumed at its northern margin along the New Britain Trench and at its southern margin along the Trobriand Trough (Fig. 1). Northwest of the triple junction, the Finisterre terrane, which lies on the South Bismarck Plate, overrides the continental margin along the Ramu-Markham Fault (Fig. 1; Silver et al., 1991). The New Britain Trench is a classic subduction zone, with an active volcanic arc (Johnson, 1976) and a seismic Benioff zone (e.g. Abers and Roecker, 1991) associated with it. The case for activity on the Trobriand Trough

The Adelbert-Finisterre-New Britain collision, N. Papua New Guinea

35

Fig. 2. Terranes of PNG after Pigram and Davies (1987). Black areas are volcanic and plutonic outcrops of the Maramuni Arc, after Dow (1977). Australian continental crust here denotes areas that were part of the continent prior to the Oligocene initiation of terrane accretion. Faults are marked by heavy lines with barbs on the upper plate of thrusts. GF = Gogol Fault, RMF = Ramu-Markham Fault, LFZ = Lagaip Fault Zone, BFZ = Bundi Fault Zone, BiFZ = Bismarck Fault Zone. The Maramuni Arc outcrop near Kompiam mentioned in the text is the large black patch just left of the BiFZ label. Asterisks mark the active volcanoes of the Bismarck Arc. is less convincing. It lacks a seismogenic slab (Abers and Roecker, 1991) and the few Quaternary stratovolcanoes on the Papuan Peninsula that might form an associated volcanic arc are not subduction-related (Johnson, 1979; Smith, 1982). Based on seismic reflection profiles, Kirchoff-Stein (1992) concluded that the Trobriand Trough is presently accommodating very slow convergence (0.6 cm yr- t ), but that this is not true subduction. Ambiguity regarding the existence of modern subduction along the Trobriand Trough has led some workers to discount this feature as the southern boundary of the Solomon Sea Plate. Advocates of this alternative plate geometry place the SE boundary of the Solomon Sea Plate at the Woodlark spreading center and the SW boundary on the Papuan Peninsula, at the Owen Stanley Fault (Fig. 1; see Johnson, 1979 and references therein). This alternative plate geometry has no effect on the location of the collision tip, though, as it is still the junction of the New Britain Trench and the Trobriand Trough were continental thicknesses of buoyant material are first thrust under the Bismarck forearc along the R a m u - M a r k h a m Fault. The Finisterre terrane consists of three southeasttrending lithologic belts (Fig. 3). The core of the terrane is the Oligocene through Early Miocene Finisterre Volcanics. These basic volcanic and volcaniclastic rocks were generated by activity along the intraoceanic

Finisterre volcanic arc (Jaques and Robinson, 1977). The northern lithologic belt consists of Neogene carbonate rocks of the Gowop Limestone and associated units (Robinson, 1974; Robinson et al., 1976). The limestone is well-exposed in the Finisterre block but only a few fragments remain in the Adelbert block (Fig. 3). The narrow southern lithologic belt in both blocks is a sequence of clastic sediments (Fig. 3) that was deposited both prior to and during arc-continent collision. The stratigraphy of the Finisterre terrane is very similar to that of the islands of New Britain, New Ireland, the Solomon Islands and Vanuatu (Fig. 1; e.g. Jaques and Robinson, 1977). This similarity suggests that all of these islands were once part of a continuous Oligocene-Early Miocene arc that Robinson (1974) named the Outer Melanesian Arc. Falvey and Pritchard (1984) provided paleomagnetic data that support this argument. Many workers have suggested that Miocene collision with the Ontong Java Plateau (Fig. 1, inset) caused the demise of the Outer Melanesian Arc and led to an eventual reversal of subduction polarity (Kroenke, 1984; Musgrave, 1990) to form the Bismarck Arc, a 1000 km long chain of active volcanoes that run north of the PNG mainland and along the north side of New Britain (Fig. 1). Subduction was occurring along the New Britain Trench and its extension along the present day Ramu and Markham valleys by the late

L. D. Abbott

36

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Fig. 3. Location map showing the simplified geology of the Adelbert and Finisterre blocks of the Finisterre terrane. The location of Fig. 6 is marked. The letters mark the geologic traverses discussed in the text. A: Oranga Creek and Sopa River Road, B: Marak Creek and Fasi River, C: Mawan and Lowo Roads, D: Ulieg Creek, E: Ohn Road, F: Mount Hansemann. Lithologic boundaries are marked by light lines, faults by heavy lines with barbs on the hanging wall side of thrusts. RMF = Ramu-Markham Fault. WT = Wongat Thrust. The Markham submarine canyon lies along the Ramu-Markham Fault in the Huon Gulf. Miocene ( ~ 10 Ma; Musgrave, 1990; Berger et al., 1992) or Pliocene (Johnson and Jaques, 1980). Subduction polarity reversal caused the Bismarck Arc to form just north of the Finisterre terrane, placing the terrane in the Bismarck forearc. Continued subduction along the New Britain Trench caused eventual collision between the Finisterre terrane and the continent, uplifting the Adelbert and Finisterre mountain ranges.

Stratigraphy of the Finisterre Block Abbott et al. (1994) divided the southern Finisterre Range elastic sequence into three major units; the Sarawaget beds, the Erap Structural Complex, and the Leron Formation (Fig. 4). The Erap Structural Complex is in turn subdivided into the Sukurum, Gorambampan and Nariawang units. These authors interpreted the Oligocene through Early Miocene Sarawaget beds as a elastic apron, shed off of the contemporaneous Finisterre Arc (represented by the Finisterre Volcanics; Fig. 4). The Middle Miocene through Early Pleistocene Erap Structural Complex consists of deep water turbidites interpreted as an accretionary wedge built initially above a westward extension of the modern New Britain Trench (the Sukurum and Gorambampan units) and later, with the onset of collision, above the Ramu-Markham Fault

(Nariawang unit; Abbott et al., 1994). The Leron Formation consists mostly of terrestrial sediments and is interpreted as a molasse-like unit deposited after the ongoing collision had caused substantial foreland uplift (Silver et al., 1991; Liu, 1993). Two shifts in sediment provenance punctuate this sedimentary record. The first occurred at about 16-18 Ma, and separates the Sarawaget beds from the Sukurum unit of the Erap Complex (Fig. 4). The Sarawaget beds have a basic volcanic source (Fig. 5) that Abbott et al. (1994) interpreted to be the Finisterre Arc. The uppermost Lower Miocene through Lower Pliocene Sukurum unit is co-dominated by a basic volcanic source and a continental orogenic source consisting of metasedimentary and acidic plutonic rocks (Fig. 5). The Bena Bena and Owen Stanley terranes (Fig. 3) are the most likely source areas for both of the sedimentary components of the Sukurum unit (Abbott et al., 1994). The Sukurum unit may have been deposited as an extension of the Wogamush beds of the Aure Trough (as defined by Francis and Deibert, 1988; Figs 2 and 3), which were deposited in part during the suturing of the Bena Bena and East Papuan Composite terranes to the continent (Abbott et al., 1994). The second provenance shift in the Finisterre Range sedimentary record came at 3.0-3.7 Ma and divides the Erap Complex's Sukurum and Nariawang units (Fig. 4).

The Adelbert-Finisterre-New Britain collision, N. Papua New Guinea Epoch Age ,

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Stratigraphy of the Adelbert Block To first order, the lithologic composition of the Adelbert block mirrors that of the Finisterre block. Like the Finisterre Mountains, the Adelbert Mountains are composed of Oligocene through Early Miocene Finisterre Volcanics unconformably overlain by Miocene Gowop Limestone (Figs 3 and 4). A Neogene elastic sequence lies to the south of the Finisterre Volcanics in both ranges (Robinson et al., 1976; Jaques

37

and Robinson, 1977). However, no well-defined trend in sediment provenance with age has emerged from study of the southern Adelbert elastics, as it did in the Finisterre Range. Near Madang, the Adelbert Mountains consist of argillite that is identical to argillites of the Finisterre block that Abbott et al. (1994) classified as Sarawaget beds. A sample of this argillite from near the top of Mount Hansemann (Fig. 3) is Late Eocene to Middle Miocene (Table 1), broadly consistent with the age of similar material from the Sarawaget beds of the Finisterre Range (Fig. 4). Robinson et al. (1976) mapped these argillites in both the Finisterre and Adelbert blocks as the Gusap Argillite. Abbott et al. (1994) grouped the Gusap argillite with the Sarawaget beds, and I follow that nomenclature here. Coarser elastics dominate the section farther to the west, in the area from Ulieg Creek to Sopa River (Figs 3 and 6). Plio-Pleistocene mudstones belonging to the Wewak beds (as defined by Francis and Deibert, 1988) crop out along the range front (Figs 3 and 6). They are gently tilted and in places the mudstones are interbedded with reef limestones containing numerous coral heads in growth position. Tables I and 2 summarize the age, depositional environment, and lithology of the unit. To the north of the Wewak beds lies a sandstone sequence that does not fit easily into the classification schemes of either Francis and Deibert (1988) or Abbott et al. (1994). Given the reconnaissance nature of my mapping here, it is inappropriate to propose a formal name for these rocks. However, for the convenience of this discussion, I refer to them here as the "Oranga sandstone". Biostratigraphic ages of the Oranga sandstone range from Lower or Middle Miocene to Plio-Pleistocene (Table 1), making the unit time-equivalent to the Korogopa beds and Wewak beds in the Francis and Deibert (1988) classification. However, several characteristics (Table 2) make the Oranga sandstone unlike those units. Benthic foraminiferal faunas indicate deposition of the Oranga sandstone at bathyal depths, with two samples restricted to lower bathyal or greater depths (greater than 2000 m water depth; Table 1). This is much deeper than the neritic to upper bathyal depths of the Korogopa and Wewak beds. The Oranga sandstone is well indurated and dominated by sandstone beds 20 em to several meters thick (Table 2), with sand:shale ratios of 10:1 common. The Korogopa and Wewak beds are described as friable to firm, with sandstone subordinate to mudstone (Francis and Deibert, 1988). Some sandstone beds in the Oranga sandstone fine upwards but most are massively bedded. When combined with evidence for deposition at bathyal depths, the upwardfining beds imply deposition of the Oranga sandstone as turbidites. To the north of and structurally above the Oranga sandstone, lie outcrops of older Sarawaget beds and Finisterre Volcanics (Figs 3 and 6). A well-exposed section at Ulieg Creek (Fig. 6) was examined in detail. Wewak beds mudstones, dated by Hekel (1986) as Plio-Pleistocene, crop out at the range front. The Oranga sandstone, to the north, is composed of tightly folded, 5-30 cm thick sandstone-mudstone couplets with rare interbedded micrites. North of the Oranga sandstone lie well-indurated, matrix-supported cobble conglomerates belonging to the Sarawaget beds (Table 2). Most cobbles are rounded to subrounded andesites or basalts. Rare,

38

L . D . Abbott

(a)

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Lm

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Fig. 5. Ternary diagrams representing the composition of Finisterre block sandstones. (a) QFL diagram; (b) LmLvL~ diagram; (c) QpLvmLsm diagram. Q--mono and polycrystalline quartz grains, F--potassium and plagioclase feldspar, L--lithic fragments, Qp--polycrystalline quartz, Lyre--volcanic and metavolcanic lithic fragments, L~m--sedimentary and metasedimentary lithic fragments, Lm--metasedimentary lithic fragments, Lv--volcanic and metavolcanic lithic fragments, L~msedimentary lithic fragments. The provenance fields of Dickinson et al. (1983) and Ingersoll and Suczek (1979) are marked with light lines. The symbols mark the mean compositions of each of the units listed, and the heavy lines around each symbol delineate a field that is within two standard deviations of the mean. The compositional data used to arrive at these mean values is from Abbott et aL (1994). angular, l m long boulders of limestone are also clasts in the conglomerate. The volcanic clasts likely were derived from the Finisterre Volcanics and the angular limestone blocks appear to be from the Gowop Limestone. To the north the conglomerate grades into volcanic agglomerate and yet farther north the agglomerate grades into basalt. The agglomerate and basalt are interpreted as Finisterre Volcanics (Fig. 6; Table 1). The presence at Ulieg Creek of north-dipping Oranga sandstone south of older Finisterre Volcanics and Sarawaget beds (Fig. 6) suggests that the older units have been thrust south over the Oranga sandstone. At Marak

Creek (Fig. 6), Oranga sandstone dated as Lower to Middle Miocene lies up-dip of Upper Miocene to Lower Pliocene beds of the same unit, implying that the Oranga sandstone here is internally imbricated along a thrust. Heavy shearing is common in many of the fine-grained beds of the Oranga sandstone and a narrow fault zone melange is present in the Fasi River section (Fig. 6). The Plio-Pleistocene Wewak beds that occupy the toe of the range are younger than the base of the overlying Oranga sandstone, again implying a thrust contact between the two. Seismic reflection profiles indicate substantial Miocene thrusting in the Ramu Basin south of the range

The Adelbert-Finisterre-New Britain collision, N. Papua New Guinea

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40

L. D. Abbott

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Fig. 6. (a) Geologic map of the Oranga Creek through Ulieg Creek area. Location shown in Fig. 3. Areas I have not visited are white. These areas are shown by Francis and Deibert (1988) to consist of Finisterre Volcanics. Thrust faults are marked by heavy lines, with barbs on the hangingwall. Black dots mark selected outcrop locations. Topographic contours are marked every 400 m. Numbers on the figure margin are UTM grid coordinates, marked every 5 km. (b) Geologic cross-sections. The locations of the cross sections are shown in part (a). Sections are drawn with no vertical exaggeration. Patterns are the same as in (a), with the white areas of (a) shown on the sections to be Finisterre Volcanics, following Francis and Deibert (1988). (Fig. 3), with more recent thrusting along several of these faults (Francis and Deibert, 1988). The gentle dip of the Wewak beds indicates a decrease in the intensity of deformation through time. The fact that the Wewak beds are deformed at all, however, attests to the recent activity of the Adelbert thrust belt. The evidence cited above indicates that the south flank of the Adelbert Range (Fig. 6), like the south flank of the Finisterre Range, is dominated by southward-vergent

thrust faults. Thus, the suture zone of the Finisterre terrane/continent collision exhibits a similar structural development for both the Adelbert and Finisterre blocks.

Sandstone petrography I performed systematic 500 grain point counts of seven dated, medium-grained sandstone samples from the

The Adelbert-Finisterre-New Britain collision, N. Papua New Guinea

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L . D . Abbott

Adelbert Range (Table 3), using the Gazzi-Dickinson counting method (Dickinson, 1970; Ingersoll et al., 1984). The thin sections were stained for both plagioclase and potassium feldspar using the method of Haughton (1980). This study revealed no consistent compositional trends with age (Fig. 7), such as those observed in the sandstones of the southern Finisterre Range (Fig. 5; Abbott et al., 1994). Table 3 lists compositional data for these samples and Fig. 7 displays the position of each sample on the QFL, QpL~,,L~, and LmL~L~ diagrams of Dickinson et al. (1983) and Ingersoll and Suczek (1979). Some of the samples are composed almost exclusively of volcanic debris (Fig. 7), and are thus similar to the Sarawaget beds and the Nariawang unit of the Finisterre block (Fig. 5; Abbott et al., 1994). Others contain a substantial amount of quartz and metasedimentary rock fragments (Fig. 7), and resemble samples of the Sukurum unit (Fig. 5). Illustrative of the compositional variability with age present in the data set are the samples from Marak Creek (Fig. 6). These samples are numbered 1--4 on Fig. 7, with 1 being the oldest and 4 being the youngest. The two oldest samples are Lower to Middle Miocene (14.4-17.0Ma), based on nannofossil data (Table 3). The stratigraphically lowest sample (1 in Fig. 7) is quartz-poor, whereas the sample 80 m upsection (2 in Fig. 7) contains abundant quartz. An Upper Miocene to Middle Pliocene sample upsection (3 in Fig. 7) is quartzpoor, and the youngest sample (Middle to Upper Pliocene), has a high quartz content (Fig, 7 and Table 3). The complex pattern of shifting source areas through time exhibited by the Marak Creek samples is mirrored in the other Adelbert Range sandstone samples (Table 3).

The Influence of Collision Style on Sandstone Composition The Adelbert block sandstones appear to have had their sources in the same two terranes, one volcanic and the other metasedimentary and granitic, that provided sediment to the Finisterre block sandstones (Fig. 5; Abbott et al., 1994). However, the clearly defined temporal provenance shifts that mark the Finisterre block are lacking in the Adelbert block (Fig. 7). There are several possible reasons for this difference. The Finisterre block provenance study included data from 69 dated sandstones (Fig. 5; Abbott et al., 1994). The much smaller (seven samples; Fig. 7; Table 3) Adelbert block data set may be too small for coherent temporal trends to emerge. Another problem associated with a small data set is the potential for microfossil reworking to bias the results. The foraminiferal fauna shows little sign of reworking (C. Schneider, written communication, 1992) but reworked nannofossil species are common in the Adelbert block samples (M. Filewicz, written communication, 1992). In the Finisterre block study, the internal coherence of the large data set lent confidence in the results. The smaller Adelbert block sample population provides less control. Although the conclusions must be qualified in light of the small data set, it nevertheless seems unlikely that coherent temporal trends in sandstone composition exist in the Adelbert block, as they did in the Finisterre block. For example, none of the

'~o

~z

O

<

O

t~ e~

m

[-

.= & =-

O

o

6 Z e. o

The Adelbert-Finisterre--New Britain collision, N. Papua New Guinea

(a)

43

Q

L Lm

(b)

Lv

(c)

Ls

QP

Lvm

I.sm

Fig. 7. Ternary diagrams of sandstone compositions for samples from the Adelbert Range. Samples from Marak Creek are noted as numbers 1 through 4, with 1 marking the oldest sample and 4 the youngest sample. Samples from other Adelbert Mountains locations are marked with circles. The data used to generate this plot are listed in Table 2, with the definitions of the triangle apices listed in the caption to Table 2. Compositional fielus are after Dickinson et al. (1983) and Ingersoll and Suczek (1979). (a) QFL diagram; (b) L mLvLs diagram; (c) QpLvmLsm diagram. Finisterre block sandstones younger than middle Pliocene contain substantial amounts of quartz or metasedimentary rock fragments (Fig. 5; Abbott et al., 1994). Table 3 reveals that samples rich in quartz and metamorphic rock fragments occur throughout the Adelbert block section, from the Miocene through the Pleistocene, with two of the most quartz-rich samples dated as middle to Upper Pliocene (90A-237 in Table 3) and Pleistocene (90A-232 in Table 3). The massive middle Pliocene influx of volcanic debris into the suture zone in the Finisterre block and the lack of a similar influx into the suture zone of the Adelbert block is explicable when the relative uplift between the Adelbert Range and the Finisterre Range is considered.

The Adelbert Range reaches a maximum elevation of only 1600 m, in contrast to the Finisterre Range which boasts numerous peaks over 4000 m. The Gowop Limestone caps both ranges (Fig. 3; Robinson et al., 1976; Jaques and Robinson, 1980), indicating that the substantially lower elevation of the Adelbert Range is due to less total uplift of the Adelbert block; it cannot be attributed to greater erosion in the Adelbert Range. As noted above, Abbott et al. (1994) attributed the middle Pliocene influx of volcanolithic sediment into the Finisterre block suture zone to collision-induced uplift of the Finisterre terrane. Vigorous erosion of the highly elevated Finisterre block provided enough debris to overwhelm the contribution of continental detritus from

44

L . D . Abbott

the south. In addition, flexure of the downgoing Australian Plate caused backfilling of river valleys issuing from the Highlands, trapping sand-sized, continentally-derived detritus in swamps that lay at the foot of the mountains. These two processes, working in tandem, led to a post-collisional shift to a volcanic sediment provenance (Abbott et aL, 1994). The more subdued topography of the Adelbert block probably shed a smaller volume of debris into the suture zone. The lower Adelbert Range would also constitute a smaller load on the downgoing Australian Plate and hence would cause less flexural subsidence and ponding of Highlandsderived sediment than would the Finisterre Range. With continental detritus from the Highlands able to flow freely into the suture zone, the syn- and post-collisional sandstones of the Adelbert block would have a mixed provenance. The modern environment lends support to such a theory. The Gogol and western Ramu river valleys south of the Adelbert Range and the extreme western Finisterre Range are fiat, swampy areas traversed by low gradient, meandering rivers. In contrast, the eastern Ramu Valley and the Markham Valley host huge alluvial fans built of debris exclusively from the Finisterre Range (Loftier, 1977). The lower Ramu River receives detritus from both the volcanic terranes of the Adelbert and Finisterre mountains and the metasedimentary and plutonic regions along the river's headwaters. If a similar situation existed in the past, then most sandstones should display a mixed metasedimentary and volcanic provenance, with some purely volcanolithic sandstones and some purely metasedimentary lithic sandstones caused by local variations in the depositional system. Just such a pattern is observed in the Adelbert block sandstones. Differences in the relative strength of collision for the Adelbert and Finisterre blocks may explain the much greater uplift of the Finisterre Range. Milson (1981) postulated that a piece of oceanic crust (likely part of the Marum terrane) is trapped between the Adelbert Range and the continental margin, a hypothesis supported by the drilling of gabbro in the Keram 1 well (Francis and Deibert, 1988). The high density of this trapped oceanic crust would likely lead to a "soft" collision of the Adelbert block, resulting in only moderate uplift. Schouten and Benes (in review) recently presented a kinematic plate reconstruction indicating that collision of the Adelbert block was highly oblique. This obliquity would also contribute to a "soft" Adelbert block collision, with little crustal thickening and hence little uplift. In contrast, collision of the Finisterre block was orthogonal (Schouten and Benes, in review) and the block impinged on true continental crust. These two factors would be expected to result in a "hard" collision, resulting in the substantial uplift and erosion observed in the Finisterre block.

Implications of Adelbert Block Stratigraphy for Collision Age Some authors have concluded that the Adelbert block of the Finisterre terrane had collided with the continental margin by the Early to Middle Miocene, in a collision event separate from the Finisterre collision (Jaques and

Robinson, 1977; Johnson and Jaques, 1980; Pigram and Davies, 1987). Others have favored Late Miocene to Pliocene collision of the Adelbert block in a slightly earlier phase of a single, progressive, Finisterre collision event (Cooper and Taylor, 1987; Francis and Deibert, 1988; Abers, 1989; Cullen, 1991). Although the attempt to use sediment provenance shifts to determine the Adelbert block collision age proved inconclusive, the age and depth data for the Adelbert Range clastic rocks provide constraints that bear on this controversy. Jaques and Robinson (1977) identified an Early Miocene and younger clastic sequence unconformably overlying the Finisterre Volcanics in the Ramu Basin (Fig. 3), south of the Adelbert block. They felt that these sediments form an overlap sequence that records Early Miocene collision, uplift and erosion of the Adelbert block. Francis and Deibert (1988) considered the preUpper Miocene sediments of the Ramu Basin to have been deposited in an arc or back-arc basin rather than as an overlap sequence. They dated collision of the Adelbert block as Late Miocene based on their interpretation of the Tsumba beds, from the Tsumba 1 well (Fig. 3) as a syntectonic molasse. Cullen (1991) interpreted the Late Miocene unconformity at the top of the Tsumba beds to mark the onset of the collision. The Tsumba beds contain sandstone and conglomerate with clasts of metamorphic and igneous rock fragments and quartz. They overlie the Finisterre Volcanics, probably unconformably, in the well (Francis and Deibert, 1988). The quartz and metamorphic rock fragments in the Tsumba beds must have been derived from the previously accreted continental terranes of the Highlands, by the same argument used for the provenance of similar clasts in the Sukurum unit of the Finisterre block (discussed above). The presence of the continentally-derived Tsumba beds depositionally overlying the Finisterre Volcanics in the Tsumba 1 well is good evidence that the western portion of the Adelbert block was in close proximity to the continent, and had likely collided, by the time of their deposition. The age control on the Tsumba beds is poor, but they were likely deposited in the latest middle to late Miocene (Francis and Deibert, 1988). The outcrops of Oranga sandstone discussed here lie 90-100 km southeast of Tsumba 1 (Fig. 3). Many of these contain continentally-derived clasts similar to those of the Tsumba beds. However, in contrast to the Tsumba beds, the Oranga sandstone is overthrust by the Finisterre Volcanics (Fig. 6), indicating that the Oranga sandstone is involved in the deformation rather than forming an overlap assemblage that records the completion of the suturing process. The Tsumba beds are marginal marine to non-marine and the depositional depth of the overlying stratigraphic section in Tsumba 1 never exceeds neritic to upper bathyal (Francis and Deibert, 1988). In contrast, the Oranga sandstone was deposited in a deep water basin that in places reached lower bathyal depths (greater than 2000 m; Table 1). Water depths of collisional foredeep basins typically shallow through time, and shallow water depths such as those observed in the Tsumba 1 well are common in the recent sediments of young collisional foredeep basins (e.g. Finisterre block--Abbott et al., 1994; Taiwan--Dorsey and Lundberg, 1988; Japan-Ukawa, 1991). Timor, where the Timor Trough reaches almost 2500 meters in depth, provides an exception to

The Adelbert-Finisterre-New Britain collision, N. Papua New Guinea this trend (Karig et al., 1987). If collision of the eastern portion of the Adelbert block occurred in the Miocene, then the late stage foredeep basin associated with it was deeper than its lateral extensions to either side in the western portion of the Adelbert block (as shown by the Tsumba beds) and in the Finisterre block (the Leron Formation). It was then more analagous to the Timor foredeep than to the foredeeps of other modern collision zones. Several other observations support a Late Miocene or younger collision age for the Adelbert block. Abers (1989) used the continuity of the dipping seismic slab from 144°E longitude under the Adelbert Range to 152°E under New Britain (Fig. 1) to argue for a 1.4-2.8 Ma collision age for both the Adelbert and Finisterre blocks. There is no relative offset in the Bismarck volcanic arc behind the Adelbert block and the Finisterre block (except for a small offset of the Schouten Islands group, of which Kadovar is a part; Fig. 1; Johnson, 1976), nor in the seismic slab beneath the arc (Abers and Roecker, 1991). Johnson (1976) and Milsom (1981) noted this continuity of deep structures in the collision zone across the block boundary. Milsom (1981) attributed the surface offset between the blocks to shallow detachment faulting in the Finisterre block after the onset of collision. Alternatively, the block offset may predate the collision. In either case, it seems unlikely that the deep structures present in the Adelbert and Finisterre collision zones would be continuous if the collisions were separate events, with one occurring in the Early Miocene and the other not beginning until the middle Pliocene. Continuing volcanic activity in the Bismarck Arc north of the Adelbert Range (Johnson, 1976) also supports a Pliocene or younger collision age for the Adelbert block. I am not aware of any Miocene arc-continent collisions where volcanism in the colliding arc has continued to the present. In both Taiwan (Dorsey, 1988) and Timor (Silver et al., 1983) Pliocene collision events caused the cessation of arc volcanism. Lastly, a plate kinematic reconstruction of the South Bismarck and Solomon Sea plates by Schouten and Benes (1993; in review) also implies a Pliocene collision age for the Adelbert block. Gill et aL (1993) published beryllium 10 data that also bears on the collision age of the Adelbert block. Beryllium 10 is a cosmogenic radionuclide that is introduced into arc magmas only through subduction of sediment. Because it has a half-life of 1.5 m.y., its concentration in arc lavas will drop below detectable levels if sediment subduction ceased before a few million years ago (Gill et al., 1993). Beryllium 10 is present in Karkar volcano north of the easternmost Adelbert block (Fig. 1) and in volcanoes north of the Finisterre block, requiring that collision in these locations began at less than 3 Ma, and possibly began more recently than 1.5 Ma (Gill et al., 1993). Beryllium 10 is absent from Manam and Kadovar volcanoes north of the western portion of the Adelbert block (Fig. 1), indicating that either the western portion of the block collided prior to 3 Ma or that subduction continued after 3 Ma, but the young sediment was accreted before being subducted to the depth of magma genesis (Gill et al., 1993). The slab-derived geochemical component of Manam and Kadovar lavas is identical to that of lavas erupted elsewhere in the Bismarck Arc, save for the absence of 1°Be. Gill et al. (1993) considered this good evidence that the absence of ~°Be from the Manam

45

and Kadovar lavas is due to collision of the western Adelbert block prior to 3 Ma, but depletion of ~°Be through sediment accretion remains a possibility. The presence of the Tsumba beds syntectonic molasse (Francis and Deibert, 1988) and the development of a regional unconformity above these beds (Cullen, 1991) provides the best evidence that the western portion of the Adeibert block collided during the Late Miocene. The J°Be data is consistent with this conclusion. The continuity of the seismic slab, the Quaternary activity of Kadovar and Manam volcanoes and the kinematic model of Schouten and Benes (in review) all favor collision of the western portion of the Adeibert block in the latest Miocene or later. Cullen (I 99 !) documented an increase in the subsidence rate in the Tsumba 1 well (Fig. 3) at about 5.0Ma (his Fig. 7) and a further acceleration of subsidence that was accompanied by rapid uplift of the adjacent Adelbert Range at about 3 Ma. These data could be interpreted as evidence for a latest Miocene to Early Pliocene collision age for the western portion of the Adelbert block. Age control on the Tsumba beds and the overlying unconformity in the Tsumba 1 well is poor (Francis and Deibert, 1988). Given the importance of the age of the Tsumba beds and their overlying unconformity for constraining the timing of collision in the westernmost Adelbert block, the identification and dating of Tsumba beds equivalents in outcrop is highly desirable. It is likely that the eastern portion of the Adelbert block did not collide until the middle Pliocene or later. Evidence in support of a middle Pliocene or younger age comes from the overthrusting of the Oranga sandstone by the Finisterre Volcanics, the great depth of the middle Pliocene-Pleistocene basin south of the Adelbert block, the continuity of the seismic slab under both the Finisterre and Adelbert blocks, the Quaternary activity of the Bismarck Arc volcanoes north of the Adelbert block, the results of a recent kinematic plate reconstruction and the presence of ~°Be in lavas from Karkar volcano.

Paleotectonic Implications of the Finisterre Terrane Stratigraphy Many tectonic reconstructions of PNG, envision a Miocene extension of the present-day Trobriand Trough (Fig. 1) westward into Irian Jaya (Cooper and Taylor, 1987; Francis and Deibert, 1988; Cullen and Pigott, 1989), with southward-dipping subduction occurring under the Australian continent throughout the Neogene (Fig. 8). Evidence cited in support of a south-dipping proto-Tobriand subduction zone is an inferred southward-dipping seismogenic zone under the New Guinea Highlands (Ripper and McCue, 1983: Cooper and Taylor, 1987) and the Miocene volcanism and plutonism in the Highlands known as the Maramuni Arc (Figs 2 and 3; Dow, 1977; Francis and Deibert, 1988). In these scenarios, collision of the Finisterre terrane with the continent occurred after consumption of a very large, doubly-subducting Solomon Sea Plate (Fig. 8), analogous to the opposing convergence zones seen today in the Solomon Sea (Fig. 1). Although this simple extension of the modern regime into the past is geometrically appealing, it has not been universally adopted by students of PNG geology. The presence of continentally-derived detritus in the

46

L . D . Abbott

Dow (1977) commented on the fact that no trace of volcanic debris from Maramuni Arc volcanoes is observed in shelf carbonates deposited on the continental margin during the Miocene. He found this "almost inconceivable" and suggested that the continental slope "formed an effective barrier to the southward migration of volcanic detritus". It is difficult to envision the continental slope forming an effective barrier to volcanic detritus if the arc which produced the debris was built on the continent. These observations suggest that accretion of the central New Guinea Highlands terranes (Bena Bena, Schrader, Jimi and Marum terranes) and the East Fig. 8. Interpretation of the earliest Middle Miocene paleo- Papuan Composite terrane (Fig. 2) did not involve geography of PNG redrawn after Francis and Deibert (1988). subduction beneath the continental margin. Figure 9 The outline of the present southern coast of the island of New presents an alternative model in which the central New Guinea is shown for reference. The shaded area is Australian Guinea Highlands were accreted in a collision event over continental crust. The inverted-V pattern delineates blocks of a north-dipping subduction zone after their amalgamathe impinging volcanic arc, with the relative positions of present day geographic features noted. The asterisks denote tion outboard of the continent (Fig. 9a and b). The East volcanoes of the Maramuni and Bismarck arcs. Note the long Papuan Composite terrane was accreted by collision westward extension of the Trobriand Trough, marked "New along the Aure Trough (Fig. 9b) over an east-dipping Guinea-Trobriand Trench". SCT = approximate location of subduction zone. Collision of the central Highlands terranes was likely oblique, involving left-lateral the Sepik Composite terrane. strike-slip motion (Pigram and Davies, 1987). In the model shown in Fig. 9, the Maramuni Arc was develSukurum unit and the Oranga sandstone of the oped on the amalgamation of central Highlands terranes Finisterre terrane causes additional difficulties for and on the East Papuan Composite terrane. The trench models that include a significant westward extension of separating these terranes and their overlying arc from the Trobriand Trough in the past. Here I discuss objec- the continent blocked volcanic debris from reaching the tions other workers have raised to such models an~d limestones of the continental shelf. The 18-13.5 Ma age analyze the difficulties these models have in explaining of the main Maramuni volcanic episode is likely the age the provenance of the Sukurum unit and the Oranga of this subduction event (Rogerson et al., 1987; Page, sandstone. 1976). Such an age is consistent with Pigram and Davies' Johnson and Jaques (1980) and Jaques and Robinson (1987) conclusion that the central Highlands terranes (1977) felt that the geologic evidence for Neogene south- were sutured to the continent in the Middle Miocene and ward subduction under the New Guinea mainland is not that the East Papuan Composite terrane docked in the compelling. The reconstructions of Pigram and Symonds Middle to Late Miocene. The brief duration of wide(1991) show only northward-directed subduction and spread volcanic activity in the Maramuni Arc suggests Page (1976) favored formation of the Maramuni Arc that this subduction event involved the closing of a small above a north-dipping subduction zone. He noted the ocean basin between the continent and the central middle Miocene age (15-12 Ma) and brief duration of Highlands terranes rather than a major ocean basin the major Maramuni Arc igneous event. Rogerson et al. between the continent and the Finisterre terrane, as (1987) reported a slightly longer duration (4.5 m.y.) for several models imply (e.g. Fig. 8). Closure of a small the massive, culminating Maramuni Arc igneous event ocean basin is consistent with the model of Pigram (18-13.5 Ma). If the Maramuni Arc is the sum of the and Symonds (1991), in which the central Highlands volcanic product resulting from Trobriand subduction terranes remained in close proximity to the Australian under the Australian Plate from the Early to Middle continental margin throughout their history. Miocene until the Late Miocene or Pliocene as these The second line of evidence cited in support of a models invoke (Fig. 8; Francis and Deibert, 1988; former westward extension of the Trobriand Trough is Cooper and Taylor, 1987), then the proto-Trobriand the' identification of a southward-dipping seismogenic subduction zone was not associated with a volcanic arc slab beneath the Papuan Peninsula and the central for most of its life. Highlands (Ripper and McCue, 1983; Cooper and Figure 2 shows a comparison of the outcrop pattern Taylor, 1987). Abers and Roecker (1991) contested the of the Maramuni Arc (after Dow, 1977) with the al- presence of such a seismic zone, based on their careful lochthonous terrane boundaries of Pigram and Davies relocations of earthquakes and waveform modeling of (1987). This comparison shows that, where contact re- larger events. The relocations caused the sparse seismiclations are not obscured by the sedimentary overlap ity under the Highlands and the Papuan Peninsula to sequence of the Aure Trough, the Maramuni Arc igneous plot as random clouds rather than as a southward-diprocks lie on the allochthonous terranes rather than on ping Benioff zone. Although this finding does not preautochthonous continental crust (i.e. material that was in clude the existence of a south-dipping slab under the place prior to the Oligocene development of the New Highlands, it indicates that any such slab is largely Guinea orogen). A possible exception is the outcrop of aseismic. Tarua Volcanics (Davies, 1983) near the town of KomPresent convergence along the Trobriand Trough is piam (Fig. 2). Complex faulting in this area (Davies, 1983) extremely modest (0.6 cm yr -l, Kirchoff-Stein, 1992) makes definition of terrane boundaries difficult, and this and the trough is not associated with a volcanic outcrop may well lie on a terrane as well. arc (Johnson, 1979; Smith, 1982). Kirchoff-Stein (1992)

47

The Adelbert-Finisterre--New Britain collision, N. Papua New Guinea

(b)

(a) Oligocene

Late Miocene A~lbeft I

bl~k Flnlsterm block

~~~ssssss~s¢¢ss~ s¢ s~ss~ss¢~ ¢~¢¢¢¢s¢¢¢s¢¢/¢¢s ¢~/¢¢¢¢¢¢¢~s¢¢~¢¢ss¢¢¢¢¢¢¢¢¢

(d)

(C) Early Pliocene ~

Early Pleistocene

i

Basin

Fig. 9. Paleotectonic reconstruction cartoons of Papua New Guinea. Patterns are the same as in Fig. 2. Asterisks denote the Bismarck Arc. Solid lines denote subduction and collision zones, with the barbs on the overriding plate. Dashed lines denote inactive subduction zones. The shaded oval areas schematically depict depocenters for clastic units now exposed in the suture zone of the Adelbert and Finisterrc blocks. Arrows show source areas for sediment deposited in these basins. The stippled pattern depicts areas of continental crust now covered by sedimentary overlap sequences. Terrane shapes roughly correspond to their present outcrop pattern (after Pigram and Davies, 1987). In all diagrams the locations of terranes that had not yet docked with the continent at that time are purely schematic. (A) Oligocene; (B) Late Miocene. The formation of the Maramuni Arc on the colliding terranes during the Middle Miocene has been omitted for clarity. (C) Early Pliocene; (D) Early Pleistocene.

concluded that true oceanic subduction is not presently occurring along the Trobriand Trough. The trough is long enough to sustain subduction, as its length is roughly equal to that of the New Britain Trench, where vigorous subduction is still occurring. It seems unlikely that modern activity along the Trobriand Trough would be so meagre if it is the remaining portion of an extensive Neogene subduction zone (e.g. Fig. 8). As noted above, the source of the Sukurum unit lay in the New Guinea Highlands, as did at least one of the sources for the Oranga sandstone (Fig. 9b). If a Miocene through Pliocene extension of the Trobriand Trough stretched westward along the continental edge past the Highlands terranes, it would likely prove an effective sediment trap, preventing sediment shed off of those terranes from reaching the Sukurum unit and Oranga sandstone depositional basins to the north (Fig. 9b). Models invoking the presence of such a trough in the Miocene-Pliocene must explain the presence of

continental detritus of that age in the Finisterre/New Britain accretionary wedge in one of three ways: 1. All sediment transport was longitudinal, with sediment sources far to the west, past the western termination of the proto-Trobriand Trough. 2. The continental sediments were deposited in the Trobriand Trough and were incorporated first into the Trobriand accretionary wedge. Only after collision caused the Finisterre/New Britain wedge to override the Trobriand wedge (as is occurring now in the western Solomon Sea; Fig. 1; Silver et al., 1991) did the sediments end up in the New Britain/Finisterre wedge. 3. The proto-Trobriand Trough was completely sediment filled, allowing sediment to bypass the trough and end up in the oceanic basin to the north. The first of these possibilities seems unlikely. Garnets, which are rare in PNG, are present in the Sukurum unit.

48

L. D. Abbott

The only known sources in PNG for these garnets are the Bena Bena terrane, Owen Stanley terrane and the Sepik Composite terrane (Fig. 2; Abbott et al., 1994). If the Sukurum unit garnets are from the Bena Bena terrane and/or the Owen Stanley terrane then the amount of longitudinal transport was likely to be minimal, because Pigram and Davies (1987) concluded that both terranes had reached their present positions relative to the stable Australian continent (the Fly Platform) by the Middle Miocene. Both terranes were being sutured to the continent in the Middle to Late Miocene (Pigram and Davies, 1987), contemporaneous with Sukurum unit deposition. It is likely that uplift and erosion of these terranes accompanied suturing. This Miocene activity in the Bena Bena and Owen Stanley terranes and their present close proximity to the Sukurum unit depocenter make them the favored sources for the Sukurum unit garnets (Abbott et al., 1994). Longitudinal transport of the garnets from metamorphic sources in the Sepik Composite terrane, 400 km to the west (Fig. 2), is also possible. However, most reconstructions show the paleoTrobriand Trough extending far to the west of the Sepik Composite terrane (Fig. 8), in which case, sediments derived from the Sepik terrane would encounter the same difficulty crossing the Trobriand Trough discussed previously. Also, the Sepik terrane would not be expected to be a major Miocene sediment source, given that its suturing was completed in the Oligocene (Pigram and Davies, 1987). Transport of Sukurum unit sediment from unidentified sources even farther to the west, in poorly mapped areas of Irian Jaya (Fig. 8), would require over 1000 km of submarine transport. Garnetmica schist cobbles found in the Sukurum unit are as large as 15 cm. The longest documented submarine gravel transport distance of which I am aware is 300 km (Piper et al., 1988) making it unlikely that the cobbles in the Sukurum unit could have been transported 1000 km by submarine processes. The second possibility above is also unlikely. This scenario requires incorporation of the Sukurum unit thrust sheets into the Finisterre/New Britain wedge after collision, at a time of substantial Nariawang unit deposition (Abbott et al., 1994). Interthrusting of Sukurum and Nariawang units would be expected if the Sukurum thrust sheets were added to the wedge after collision, but mapping reveals sequential accretion of first Sukurum unit thrust sheets and then Nariawang unit thrust sheets (Abbott et al., in press). In addition, all thrust panels of Sukurum unit I have observed have a dominant north dip (Abbott et al., in press). Seismic reflection images of the modern Trobriand accretionary wedge (Silver et al., 1991; Kirchoff-Stein, 1992) show the dominance of south-dipping thrust panels there. Landward-vergence has been documented in some modern subduction zones (e.g. Cascadia; Silver, 1972; Mackay et al., 1992), allowing for the possibility of a north dip to some thrust sheets in the proto-Trobriand wedge, but it is unlikely that all Sukurum unit thrust sheets would be north-dipping if they were originally formed in the south-dipping Trobriand subduction zone. The most plausible scenario involving southward-directed subduction is the third possibility, that the Trobriand Trough was sediment-filled, allowing the continental detritus to be transported over it to the northern basin. I know of no evidence contradicting this possibility. Such a scenario is only likely if convergence at the trough

was slow, allowing sedimentation to outpace flexural subsidence. Such slow convergence seems inconsistent with the vigorous volcanic outpourings along the Maramuni Arc in the Middle Miocene (Page, 1976). Although none of the above arguments directly precludes the existence of a western extension of the Trobriand Trough along the continental margin, the body of evidence, when taken together, strongly suggests that such an extension did not exist. The two main observations cited in support of its existence have either been questioned (the presence of a southward-dipping slab) or can be more effectively explained without a southwarddipping subduction zone (Maramuni Arc). The Miocene sedimentology of the Finisterre terrane (this paper) and the Australian platform (Dow, 1977) can be explained far more easily by models that omit southward-dipping subduction along the continental margin (Fig. 9).

Proposed Neogene Tectonic Reconstruction Pigram and Davies (1987) considered the late Oligocene accretion of the Sepik composite terrane as the birth of the New Guinea orogen (Fig. 9a). This event was followed by the suturing of several more allochthonous terranes to the continent during the Miocene. All of the terranes of the central PNG Highlands were in place by the Middle to Late Miocene (Pigram and Davies, 1987). These central Highlands terranes were likely accreted to the continent in a collision event over a north-dipping subduction zone after their amalgamation outboard of the continent and following the closure of an intervening small ocean basin (Fig. 9b). The collision was probably oblique, with significant left-lateral motion (Pigram and Davies, 1987). The East Papuan Composite terrane (EPCT) also collided at this time (Fig. 9b; Pigram and Davies, 1987). Sediments shed off uplifted landmasses resulting from this double collision were deposited along the suture zone (in the Aure Trough) as the Wogamush beds (sensu Francis and Deibert, 1988). Some orogenic detritus, rich in mica schists derived from the Owen Stanley and Bena Bena terranes (Fig. 3), was shed to the north into an ocean basin and became the Sukurum unit of the Finisterre block and the Oranga sandstone of the Adelbert block (Fig. 9b; Abbott et al., 1994). The paleo-Trobriand Trough probably did not extend any farther west than the Aure Trough, 200 km west of the Trobriand Trough's present western terminus (Fig. 9b). The main period of Trobriand Trough activity likely predated the Middle to Late Miocene accretion of the EPCT. Modest reactivation of the trough (KirchoffStein, 1992) is probably a very recent phenomenon. The former westward extension of the Trobriand Trough likely parallels the structural gain of the EPCT, as revealed by such features as the Papuan Ultramafic Belt (PUB) and the Owen Stanley Fault (Fig. 1). These features turn north into the Huon Gulf near the western end of the Papuan Peninsula, and possible scraps of the PUB are exposed on the Huon Peninsula (Davies et al., 1987). The Trobriand Trough also turns north at its western end (Fig. 1), and its former extension likely lies even farther north, under the New Britain accretionary wedge. The Finisterre terrane was formed in the Paleogene, far from the Australian continent (Fig. 9a; Jaques and

The Adelbert-Finisterre-New Britain collision, N. Papua New Guinea Robinson, 1977), as part of the much larger Outer Melanesian Arc (Robinson, 1974). Volcanism ceased in the Early Miocene, and the Gowop Limestone was deposited on top of the arc as it cooled and subsided. Sometime between the Early Miocene deposition of the first Gowop Limestone and the middle Pliocene collision of the terrane with the continent, the southern Finisterre terrane was subjected to a tectonic event that resulted in the heavily sheared, veined nature of the southern exposures of Finisterre Volcanics and Sarawaget beds, and in the coarse foliation of the southernmost Gowop Limestone (Abbott et al., 1994). Exposures of these units on the north flank of the Finisterre Range are not heavily tectonized, suggesting that the event was localized. The nature and cause of this tectonic event are speculative. Northward-directed subduction began on the New Britain Trench in the Late Miocene ( ~ 1 0 M a , Musgrave, 1990; Berger et al., 1992) or Pliocene (Johnson and Jaques, 1980), probably as a means of accommodating Pacific-Australia convergence after collision of the Ontong Java Plateau halted subduction along the West Melanesian Trench (Fig. 9a; Kroenke, 1984; Musgrave, 1990). Subduction at the New Britain Trench created the Bismarck volcanic arc (Fig. 9c). Back-arc spreading began in the Manus Basin after 3.5Ma (Fig. 9d; Taylor, 1979). Due to the polarity reversal between the West Melanesian Trench and the New Britain Trench, the Finisterre terrane occupied the forearc of the new subduction zone (Fig. 9c and d). The kinematic model of Schouten and Benes (1993; in review) predicts that the length of subducted plate should increase from less than 100 km long beneath the westernmost Adelbert block to about 300km long beneath the easternmost Finisterre block. These lengths are nearly identical to the length of the seismogenic slab present under the collision zone at these locations (Abers and Roecker, 1991). The agreement between the model and the seismic slab lengths suggests that at the onset of New Britain Trench subduction in the Late Miocene to Early Pliocene, the Finisterre terrane was separated from the continental margin by an ocean basin that widened from about 100 km at the terrane's northwestern end to 300 km at its southeastern end (Fig. 9c). The Kubor anticline area (Fig. 3) has been a northward-projecting continental salient since before the Miocene (Hobson, 1986). The western portion of the Adelbert block probably collided with this salient in the latest Miocene, soon after initiation of subduction along the New Britain Trench. The eastern portion of the block collided in the middle to Late Pliocene. Collision here was highly oblique (Schouten and Benes, in review) and the Adelbert block overrode the oceanic Marum terrane rather than true continental crust (Pigram and Davies, 1987; Francis and Deibert, 1988). These two factors probably caused a relatively "soft" collision that involved only modest uplift of the Adelbert Mountains. The short duration of subduction beneath the Adelbert block prior to collision and the oblique plate motion probably contributed to the smaller volume of the Bismarck Arc volcanic pile north of the Adelbert block relative to that north of the Finisterre block (Johnson, 1976), where subduction was orthogonal and continued longer. As the collision progressed to the SE, it passed beyond the eastern edge of the Kubor salient. The thickly

49

sedimented Aure Trough to the east formed an embayment in the Australian Plate that offered less resistance to southward movement of the Finisterre terrane. This reduced resistance allowed the Finisterre block to slide about 50 km south of the Adelbert block into its present position north of the Ramu-Markham Valley (Fig. 9d). This movement was accommodated along a NE trending strike-slip fault inferred to run through Astrolabe Bay (Figs 3 and 9d; Robinson et al., 1976; Jaques and Robinson, 1977). Milsom (1981) suggested that the southward translation of the Finisterre block relative to the Adelbert block involved only the upper crust of the Finisterre block. Abbott et al. (in press) presented evidence for substantial southward thrusting of the shallow portion of the Finisterre block, lending support for Milsom's (1981) hypothesis. During Late Miocene-Early Pliocene subduction along the Finisterre/New Britain Trench, the northernmost Aure Trough sediments (Fig. 3) and scattered seamounts, were scraped off of the downgoing oceanic plate. These formed, respectively, the Sukurum and Gorambampan units of the Erap Complex. Collision of the NW corner of the Finisterre block with the continent at 3.0-3.7 Ma caused rapid uplift of the Finisterre block. Sediment shed off this uplift was transported longitudinally along the collision zone (Fig. 9d), probably in an extension of the Markham River-Markham submarine canyon system that is active today (Fig. 3). These volcanolithic sediments formed the Nariawang unit of the Erap Complex (Abbott et al., 1994). Continued uplift after passage of the collision tip exposed the suture zone as the subaerial Ramu and Markham valleys, where the Leron Formation was deposited (Silver et al.. 1991; Liu and Crook, 1991; Liu, 1993). Collision of the Finisterre block progressed to the SE at about 110-240 km Ma -~ (Silver et al., 1991; Abbott et al., 1994), causing the Finisterre terrane to override the Papuan Ultramafic Belt by the early Pleistocene. Blocks of ultramafic rocks found on the Huon Peninsula and the peninsula's high gravity may be the result of this overthrusting (Davies et al., 1987). The recent encroachment of the collision over the western end of the reactivated Trobriand Trough produced the contemporary setting, with the Solomon Sea Plate being progressively consumed at its western margin as the collision continues its southeastward migration.

Conclusions The sedimentary rocks, deposited in the suture zone between the Adelbert and Finisterre blocks of the Finisterre terrane and the Australian continental margin, record important data regarding the age of Finisterre terrane collision and the Cenozoic paleogeography of New Guinea. Two important conclusions can be drawn from this record. 1. The western portion of the Adelbert block probably collided with the continent during the latest Miocene, as seen by a possible syntectonic molasse present in the Tsumba 1 well (Francis and Deibert, 1988) and a regional Late Miocene unconformity (Cullen, 1991). The lack of ~°Be in lavas from Manam and Kadovar volcanoes is consistent with Late Miocene collision here. Collision of the eastern portion of the Adelbert block is likely to have occurred during the

50

L . D . Abbott

Middle to Late Pliocene, as evidenced by thrusting of the Finisterre Volcanics over deep water, Plio-Pleistocene sedimentary rocks of the Oranga sandstone, t°Be concentrations in lavas from Karkar volcano (Gill et al., 1993), Quaternary activity of the Bismarck Arc volcanoes north of the Adelbert block and the continuity of the seismic slab (Abers, 1989). The western end of the Finisterre block first collided with the continent at 3.0-3.7 Ma as the collision propagated to the southeast. 2. No southward-dipping subduction occurred under the leading edge of the Australian continental margin in central PNG during the Neogene. Evidence for this assertion is the presence of continentally-derived detritus in the Finisterre terrane accretionary wedge and maps showing that the Maramuni volcanic arc erupted solely on allochthonous terranes rather than on autochthonous continental crust. The Maramuni Arc was probably developed above a north-dipping subduction zone, along which the amalgam of central Highlands terranes was sutured to the continent after closure of a small ocean basin. This model is similar to conclusions reached by Jaques and Robinson (1977), Johnson (1976), and Pigram and Symonds (1991), but is different from other recent paleogeographic reconstructions for PNG (Cooper and Taylor, 1987; Francis and Deibert, 1988; Cullen and Pigott, 1989), which invoke the presence of southward-dipping subduction beneath the Australian continent during the Neogene. Acknowledgements--This work was funded by grants from BP Australia Ltd and the Geological Society of America, logistical support from the Geological Survey of Papua New Guinea, a National Science Foundation Graduate Fellowship and scholarships from the Society of Exploration Geophysicists and the A R C S Foundation. The interpretations presented here would not have been possible without the biostratigraphic work of Peter Thompson, M a r k Filewicz, Cindy Schneider and Abdoerrias. I appreciate their generous assistance. I have benefitted from numerous fruitful discussions with Eli Silver, and he reviewed an earlier version of this manuscript. Discussions with Rick Rogerson, Kim Kirchoff-Stein and Jim Gill have also been most helpful. The comments of two anonymous reviewers improved the manuscript.

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