Continent—arc collision and reversal of arc polarity: New interpretations from a critical area

Tectonophysics, 63 (1980) 111-124 @ Elsevier Scientific Publishing Company,

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CONTINENT-ARC COLLISION AND REVERSAL OF ARC POLARITY: NEW INTERPRETATIONS FROM A CRITICAL AREA

R.W. JOHNSON

and A.L. JAQUES

Bureau of Mineral Resources, P.O. BOX 378, Canberra City, A.C.T. 2601 (Australia) Department of Geology, University of Tasmania, Box 252C, G.P.O., Hobart, Tasmania 7001 (Australia)

ABSTRACT Johnson, R.W. and Jaques, A.L., 1980. Continentic collision and reversal of arc polarity: new interpretations from a critical area. In: M.R. Banks and D.H. Green (Editors), Orthodoxy and Creativity at the Frontiers of Earth Sciences (Carey Symposium). Tectonophysics, 63 : 11 l-l 24. Northern New Guinea has been regarded as a region where the polarity of an island arc reversed following collision with the Australian continent in the Tertiary. However, the evidence for this reversal is not compelling. Because present-day volcanism off the north coast of mainland Papua New Guinea is associated with a steeply northward-dipping Benioff zone (almost vertical), and late Cainozoic volcanoes in the central highlands to the south cannot be related to any present-day Benioff zone, a more acceptable interpretation is that, following collision, the northward-dipping slab beneath the arc became suspended nearly vertically. The active marginal basin lying to the north of the arc is unlikely to be subducted southwards beneath the mainland, because the lithosphere beneath marginal basins appears to be neither thick nor cold enough for the initiation of subduction. Polarity reversal, therefore, may not be the inevitable consequence of continent-arc collisions. Instead, the downgoing slab may steepen, equilibrate with the surrounding mantle, and lose its identity. Continuing convergence may be taken up at other plate boundaries, and the accreted arc may never again become active.

INTRODUCTION

The widely accepted proposal that the direction of subduction beneath an island arc may reverse after collision with a continent (Fig. 1) was first made by McKenzie (1969). However, to our knowledge, the northern part of New Guinea island (Fig. 2) is the only well documented example cited in the literature of a region where this type of reversal is thought to have taken place (e.g., Dewey and Bird, 1970; Hamilton, 1973). In this paper we compile information from part of eastern New Guinea (Fig. 3) which substantiates the concept of a continent-c collision, but which does not support the notion of polarity reversal or “flipping”. We sug-

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Fig. 1. Hypothetical reversal of arc polarity (adapted from ,McKenzie, 1969). A. A plate carrying a continent (solid) is subducted beneath an island arc. B. The continent reaches the arc, but unable to descend into the subduction zone because of its buoyancy, it collides with the arc, and the formerly over-riding plate is subducted beneath the arc and continent.

APUA

Iww

Fig. 2. Selected geological features of New Guinea island and adjacent areas. Stippling repreaenb approximate extent of earthquake zones and plate bound&es which are zones of deformation. Plate boundaries defined by axes of submarine trench in northern Solomon Sea are shown by thick lines (trench divides into two valleys in the w&), and direction of subduction by short arrows. Submarine valley in north Irian Jaya is represented by 6000 m isobath. Triangles are main Late Cainozoic eruptive centres of Bismarck volcanic arc (filled) and Highlands volcanic province (open). PUB = Papuan Ultramafic Belt.

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Fig. 3. Tectonic elements of western Papua New Guinea (see Fig. 2; after D’Addarlo et al., 1975). Stippling represents erogenic belt. Ophidite complexes shown by short-line pattern. Thick lines represent major faultslate Cainozoic volcanoes of Bismarck volcanic arc shown solid. M = Menyamya settlement, and location of two most easterly Highlands volcanoes (cf. Fig. 2); HP = Huon Peninsula; KZ = Karkar Island; LZ = Long Island. Lines A, B,and C represent surface traces of planes shown in Fig. 4, and dashed lines limit areas of earthquake epicentres projected normally onto each solid central line.

gest that polarity reversal island-arc collisions. PLATE BOMBARDS

is not

a necessary

consequence

of continent-

AND TECTONIC EXTRAS

In the part of Papua New Guinea east of line X-Y in Fig. 2, there are at least two minor plates between the larger Indo-Australian and Pacific plates (Johnson and Molnar, 1972; Curtis, 1973; Krause, 1973). To the west, mainly in the Irian Jaya province of Indonesia, a zone of major seismicity along the north coast appears to represent the Indo-Australian-Pacific plate boundary (Fig. 2), but other minor plates may exist in the region (e.g. Johnson and Molnar, 1972). The region of Papua New Guinea shown in Fig. 3 is divisible into three broad tectonic elements (D’Addario et al., 1975):

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(1) In the southwest, a stable platform of pre-Mesozoic metamorphic and granitic rocks forms the northeastern corner of the Australian continent, and is overlain by Mesozoic to Cainozoic shelf carbonate and shallow marine elastic sediments. (2) A strongly faulted and folded erogenic belt flanks the northeastern edge of the stable platform. The belt consists predominantly of Late Mesozoic and Tertiary geosynclinal sediments several times the thickness of the platform sediments, together with Cretaceous, Middle Miocene, and Late Cainozoic andesites and related rocks. The northeastern flank of the orogenie belt is extensively metamorphosed, mostly to lower greenschist facies, and an ophiolite belt lies near its northeastern margin. (3) A Tertiary island-arc sequence in the north-coast ranges is clearly separated from the erogenic belt by the alluviated Ramu and Markham valleys. The arc is within a major zone of seismicity, and an active chain of Quaternary volcanoes lies immediately to the north. The oldest rocks exposed in the Tertiary arc are Eocene pelagic and hemipelagic sediments, which were probably deposited on oceanic crust, and dominantly Oligocene (Late Eocene to Early Miocene) island-arc-type volcanic rocks (Jaques and Robinson, 1977). New Britain to the east is also a Cainozoic island arc, characterised at the present day by a well defined northward-dipping Benioff zone, a submarine trench off the south coast, and Quaternary volcanoes to the north (Fig. 2), and probably representing the offset eastern extension of the north-coast-ranges island arc. The Bismarck Sea floor north of the Tertiary island arcs is considered to be an active marginal basin. The western part of the basin at least is probably Tertiary, and present-day sea-floor spreading is believed to be taking place in parts of the east-west seismic zone that crosses the Bismarck Sea floor (Connelly, 1974, 1976; Taylor, 1975). CONTINENT-ARC

COLLISION

The ophiolite belt and metamorphic rocks of the erogenic belt immediately south of the Ramu and Markham valleys are regarded as the site of collision between the Tertiary island arc and the stable continental platform and its peripheral belt of thick sediments (Jaques and Robinson, 1977). The collision is thought to have begun in the Late Oligocene to Early Miocene, corresponding with: (a) southward emplacement of the ophiolites; (b) metamorphism and thrusting of parts of the erogenic belt; and (c) cessation of volcanism in the arc. Subduction beneath the island arc then ceased, but is thought to have restarted in the Late Pliocene (see below). Geological events throughout the collision in the area of Fig. 3 were in marked contrast to those preceding the collision, and were governed by interactions between the Indo-Australian plate and a minor plate to the north, possibly the minor plate existing at the present day - termed “South Bismarck” by Johnson and Molnar (1972; Fig. 2). South of the collision

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zone Middle Miocene regional uplift was accompanied by extensive andesitic and dioritic magmatism in a chain along the present-day axis of New Guinea (e.g. Page and McDougall, 1970). In contrast, the island arc to the north was volcanically quiescent, and thick sheets (up to about 2000 m) of Middle Miocene to Late Pliocene reef limestone were deposited in the eastern half (Jaques and Robinson, 1977). Farther west, about 4000-5000 m of Neogene arc-derived elastic sediments were distributed on the flanks of the island arc. After the period of sedimentation the island arc was uplifted, faulted, and tilted, and the present-day north-coast ranges were formed (Fig. 3). Uplift appears to have been both greater and younger in the east than in the west. The ranges are highest in the east (up to about 4000 m in Huon Peninsula), and have a greater development of limestone and less arc-derived elastic sediments than those to the west. At least 2500 m of uplift has taken place in the Huon region since the Late Pliocene (Jaques and Robinson, 1977), and a set of raised Quatemary reefs is especially well developed on the northeastern coast of Huon Peninsula (Chappell, 1974). SEISMICITY AND LATE CAINOZOIC VOLCANISM

The collision and subsequent plate convergence also determined the nature of the Late Cainozoic volcanism and present-day earthquake activity in this area. The Late Cainozoic volcanism is mainly basaltic and andesitic, and has taken place in two provinces (Fig. 2): (a) in the Highlands, where a large cluster of volcanoes overlies the northern edge of the stable platform, and where the two most easterly volcanoes (Fig. 2), at Menyamya (Fig. 3), overlie part of the erogenic belt; and (b) as the line of active island volcanoes in the south Bismarck Sea, north of and parallel to the Tertiary island arc. The line of island volcanoes is part of the “Bismarck volcanic arc” at the southern margin of the South Bismarck plate (Fig. 2). The eastern half of the Bismarck volcanic arc (in New Britain) is associated with the boundary between the South Bismarck and Solomon Sea plates, and is chemically distinguishable from the western half. By comparing rocks with the same silica content from the volcanic line off the mainland north coast, Johnson (1976, 1977) showed systematic changes in composition along the island chain; the volume of volcanic rocks also appears to increase eastwards along the chain. In the Highlands province pronounced compositional differences exist between individual volcanoes, but no large-scale systematic changes in chemistry have been found (Mackenzie, 1976) analogous to those in provinces overlying inclined lithospheric slabs (e.g., Dickinson and Hatherton, 1967; Ninkovich and Hays, 1972). There appears to be a widespread belief that the area represented in Fig. 3 is at present underlain throughout by a southwest-dipping Benioff zone (e.g., Jakei and White, 1969; Johnson and Molnar, 1972; Karig, 1972), despite claims to the contrary (Johnson et al., 1971), and despite the absence of a

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Fig. 4. Cross sections through mainland Papua New Guinea showing 1969-74 earthquake foci. After Dent and Johnson (Appendii 1 in Johnson, 1977) who give selection criteria for earthquakes and a map showing epicentres. See Fig. 3 for areas of earthquake epicentres and locations of sections. Solid horiaontal bars indicate land masses. Horizontal and vertical scales are equal and in kilometres.

submarine trench north of the coastal ranges. The existence of this postulated southwestiipping Benioff zone is not supported by the distribution of 1969-74 earthquakes shown in Fig. 4. One conspicuous feature of the earthquake cross-sections is the well defined zone of in~rmedia~depth events that dips steeply norih~ard~ beneath the north coast, and which is vertical at depths greater than about 150 km, beneath the volcanic-island chain (Fig. 4B,C). Intermediate-depth earthquakes appear to be absent beneath the volcanoes west of Karkar Island, and appear to be most abund~t a few kilometres south of Long Isiand. Steep dip-slip overthrust foal-rnech~~rn solutions have been obtained for some of these events (Racks and Molnar, 1971; Ripper, 1975a). Crudely defined northward and southward dipping zones beneath the mainland are also shown in Fig. 4A, B. The events defining these “zones” are 125 km deep or less (mostly less than 100 km), and as the thickness of lithosphere beneath the southern part of the platform (Fig. 3) has been determined as about 125 km (Brooks, L909), we believe that the earthquakes cannot be assumed to represent penetration of subducted lithosphere into the underlying asthenosphere. Some of the shallow events beneath the north coast ranges are associated with strike-slip faulting (e.g., Everingham, 1976; Robinson et al., 1976). Other earthquakes, part&G&y those south of the collision zone, may indicate ove~h~sting, possibly within li~osphere that

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may be thicker than normal (cf. Andes; James, 1971). Focal-mechanism solutions do not appear to clarify interpretations of the tectonic setting, as strike-slip, dip-slip over-thrust, and normal solutions have all been obtained (e.g., Ripper, 1975a). A few intermediate-depth earthquakes localized in the vicinity of Menyamya (Fig. 3) give the impression of a south-southwest dipping structure in Fig. 4C. However, Dent (1976) examined the distribution of these events in detail by plotting them on several planes with different azimuths, and concluded that a localised Menyamya seismic zone dips at about 30” towards N230-240E. Dent (1976) postulated that the earthquakes were related to rare intermediate-depth events that form a diffuse band extending southsoutheastwards beneath the Papuan Ultramafic Belt into the western Solomon Sea area (Fig. 2). Although the significance of the Menyamya earthquakes remains obscure, Dent’s interpretation is consistent with those of Curtis (1973) and Ripper (1975b) who considered that the events are part of a generally vague seismic zone dipping away from the Solomon Sea area. There is, however, no justification in assuming that the localized Menyamya earthquakes beneath the erogenic belt are evidence for an extensive seismically active slab extending westwards beneath the stable platform and the Highlands volcanic province. PREVIOUS

INTERPRETATIONS

Several previous interpretations of the tectonic evolution of Papua New Guinea contain the suggestion that northeastward subduction of oceanic lithosphere took place beneath the island arc in early Tertiary times, and that subduction reversed direction, becoming southwestwards, after the continent-arc collision (Dewey and Bird, 1970; Johnson and Molnar, 1972; Karig, 1972; Hamilton, 1973). Johnson and Molnar (1972) discussed the kinematics of the suggested polarity reversal in detail. They pointed out that if the “flipping” mechanism proposed by McKenzie (1969) had taken place in the area, the active volcanoes off the mainland north coast should be on the Indo-Australian plate, and off-set from the other volcanoes of the Bismarck volcanic arc to the east. Because there is no off-set, Johnson and Molnar considered the McKenzietype mechanism unplausible. They suggested three other interpretations for polarity reversal, all dependent upon the existence of a southwest-dipping lithospheric slab at the present day. Johnson and Molnar were unable to evaluate these three models using available geological data, and they concluded that “a more complex model may be required in the future” (p. 5015). FAVOURED

INTERPRETATION

Our model (Fig. 5) is simple. Its main feature is that as a result of the collision the northeast-dipping slab associated with the Tertiary arc was steep

Fig. 5. Interpretation of the continent--arc collision in section roughly between and parallel to lines A and B in Fig. 3. A. As in Fig. 1, except the over-riding plate ls thin. B. The continent collides with the arc, and the downgoing slab is steepened and surpended; continued sinking of the hanging slab leads to island-arc-type volcanism; because the “overriding” plate is too thin, southwestward subduction does not take place.

ened, and that it now hangs vertically beneath the late Cainozoic volcanicisland chain, providing conditions favourable for the present-day genesis of primary arc-trench-type magmas (Johnson, 1976, 1977). The Ramu and Markham valleys are regarded, not as a presentday plate boundary, but as part of the continent-arc collision zone which is at present a broad zone of foreshortening between the converging Indo-Australian and South Bismarck plates. We suggest that the collision first took place at the western end of the Tertiary arc (Fig. 3) in the Late Oligocene to Early Miocene, and that the remaining gap eastwards to Huon Peninsula began closing in the Late Pliocene. Two important events are thought to have resulted from the initial collision. Firstly, the western coastal ranges were elevated; they shed arc-derived detritus onto their flanks, restricting limestone development. Secondly, the rate of descent of the northeast-dipping slab was greatly reduced; the slab became steepened, and suspended or “hung up”, and began to lose its thermal identity relative to the surrounding mantle. The area of initial collision in the region of Fig. 3 is regarded as the eastern end of a longer zone that extends into Irian Jaya. This entire collision zone is considered to be an expression of a major tectonic event that led to cessation of subduction and volcanism in the Miocene throughout the outer Melanesian arcs, including New Britain (Fig. 2), where limestone began to be deposited (e.g. Robinson, 1973; Coleman and Packham, 1976). In the area of Fig, 3, volcanism off the north coast appears to have started

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in the Late Pliocene. Subduction is therefore considered to have resumed, and the gap between the arc and continent to have begun closing progressively eastwards. This motion between the South Bismarck and Indo-Australian plates is thought to be governed by instantaneous poles of rotation in or near the northwestern part of mainland Papua New Guinea (Johnson, 1976, 1977; after Krause, 1973). As the collision progressed eastwards, the more easterly parts of the arc were elevated, exposing the Neogene limestone sheets, and rates of subduction of successively eastward parts of the downgoing slab were reduced. The more easterly parts of the slab were therefore subducted at greater rates over a longer period. This interpretation is consistent with the known distribution of presentday earthquakes: the earthquakes are deeper in the east (Fig. 4B,C), because the slab there has had less time to thermally equilibrate with the mantle, compared to the western part, where no intermediate-depth earthquakes have been recorded beneath the Late Cainozoic volcanic chain (Fig. 4A). Because the volcanic chain is straight in the west and curved in the east, a similar curvature may exist in the underlying slab, and the apparent abundance of intermediate-depth earthquakes south of Long Island (Fig. 4C) may have been produced by stresses set up where the slab is strongly bent. This model also accounts for the lateral changes in rock compositions along the Late Cainozoic volcanic-island chain. If rates of convergence between the Indo-Australian and South Bismarck plates were progressively greater eastwards throughout the Late Cainozoic, then the patterns of mantle isotherms are likely to be different beneath different parts of the volcanic chain. Conditions of magma genesis may therefore change laterally beneath the volcanic line, and are reflected in the differences in volcanicrock compositions along the chain (Johnson, 1976, 1977). Moreover, compared to the more westerly parts of the hanging slab, those in the east may have had greater opportunity to introduce greater volumes of water and partial melts from the subducted, hydrated, oceanic crust, and this may explain why a correspondingly greater amount of volcanism seems to have taken place in the east than in the west. There are no obvious seismic gaps in the earthquake cross-sections (Fig. 4), and therefore no indication that parts of the slab have become detached, The overall impression is that of a hanging slab which is slowly sinking and heating up by thermal equilibration with the su~ounding mantle. The hanging slab and the accompanying Late Cainozoic volcanism are regarded as short-lived geological phenomena. The slab is a vestige of the former arctrench system, and in time it may be totally assimilated by the mantle by detachment and sinking, or thermal equilibration, or both. To account for the Middle Miocene and Late Cainozoic magmatism south of the collision zone, our model incorporates the concept that the base of the underlying lithosphere in this area was chemically modified by the introduction of water and melts from a slab that dipped beneath the continental margin in the Late Mesozoic (Johnson et al., 1978). Diapirism and partial

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melting of the modified source may have been initiated during periods of uplift following the mid-Tertiary collision, and the magmas are therefore not regarded as a direct result of contemporaneous subduction. Evidence that the mantle modification may have taken place in the Cretaceous is provided by the existence of andesitic rocks of this age in the erogenic belt (see above), and possibly by a Cretaceous “pseudoisochron” (Brooks et al., 1976) found in rocks from several Highlands volcanoes (Johnson et al., 1978). Excluded from this in~rpretation are the two most easterly Highlands volcanoes (Fig. 2) which overlie the erogenic belt and Menyamya seismic zone, and which are therefore possibly related directly to a downgoing slab. OTHER PARTS OF THE COLLISION

ZONE

The segment of New Guinea discussed above is only part of a much longer zone characterized by collision-type tectonics, emplacement of ophiolites, and present-day plate convergence. There appear to be major differences in the tectonic evolution of other segments of the zone. In southeast New Guinea, the Papuan Ultramafic Belt (Fig. 2) is an ophiolite sequence thought to have been emplaced in Eocene--Oligocene times (Davies, 1971) when thick crust of the northward-moving Indo-Australian plate collided with a postulated island are on another plate (Davies and Smith, 1971). In common with the region shown in Fig. 3, there is no convincing seismological evidence for present-day southwestward subduction beneath southeast New Guinea, and no bathymetric evidence for an active submarine trench off-shore. Polarity reversal after collision has never been proposed for this region, and there is no new evidence to alter this view. Geological similarities between Irian Jaya (Fig. 2) and western Papua New Guinea are suggested by the limited geological data (Visser and Hermes, 1962). A pronounced difference, however, is the absence of extensive Late Cainozoic volcanic fields in Irian Jaya. Like western Papua New Guinea, the seismicity is intense, but well located earthquakes deeper than about 100 km are rare *. There is no compelling evidence for a southwest-dipping Benioff zone, though deeper events appear to have taken place only in the south (e.g. Denham, 1969). A submarine valley off the no~hwestem coast off Irian Jaya (Fig. 2) may indicate a subduction zone, but it is not associated with the most intense earthquake activity (see Hamilton, 1974a,b). A continentarc collision appears to have taken place in Irian Jaya, but the fate of the downgoing slab is not known.

* 371 earthquakes between l-6% and 134-142’E, recorded between January 1966 and December 1975, and located by ten or more stations, are listed in the BMR Earthquake Data File. Only four of these eventa are deeper than 100 km, and only one is deeper (149 km) than 113 km.

121 DISCUSSION

If, as proposed, the polarity of the Tertiary island-arc in the north-coast ranges of Papua New Guinea has not reversed, could a new convergent plate boundary form off the north coast, and lead to southwestward subduction of the Bismarck Sea floor? Prerequisites for the initiation of subduction seem to be that the lithosphere of the newly subducting part of a plate be cold and thick (e.g., Ringwood, 1975; Fyfe and Leonardos, 1977). Thereis, however, no evidence that these are the characteristics of the lithosphere beneath the Bismarck Sea. On the contrary, if the Bismarck Sea lithosphere is typical of that beneath other active marginal basins, characterized by high heat flow, it will be thin (e.g., Uyeda and Vacquier, 1968; Barazangi and Isacks, 1971) *. We therefore consider that the Bismarck Sea marginal basin is unlikely to be subducted beneath mainland Papua New Guinea while it is active, and while the hanging slab and its thermal anomaly persist. Future subduction of an inactive Bismarck Sea marginal basin is possible, but we believe future plate movements are just as likely to be taken up at other plate boundaries in or near Papua New Guinea. If northern New Guinea can be regarded as the type example of a continent--arc collision (see Dewey and Bird, 1970), then polarity reversal may not be the inevitable consequence. We believe that subducted slabs do not necessarily “flip” when arcs and continents collide, instead, they are more likely to hang, and die. ACKNOWLEDGEMENTS

Much of our geological work in Papua New Guinea was accomplished in conjunction with colleagues in the Bureau of Mineral Resources and Geological Survey of Papua New Guinea. We wish to acknowledge their contributions, and especially to thank the following of them for criticism of an early draft manuscript - J.H.C. Bain, R.J.S. Cooke, D.S.’ Hutchison, D.E. Mackenzie, J.S. Milsom, J.C. Mutter, G.P. Robinson, R.J. Rybum, and R.J. Tingey. The paper is published with the permission of the Director of the Bureau of Mineral Resources, Canberra. ADDENDUM

The Cainozoic development of the sea floor north of the Tertiary island arc (Fig. 3) is probably more complex than is implied in the above account;

* Heat-flow data from the Bismarck Sea is limited to two valuee (Halunen and Von Herzen, 1973) - one high (107 HFU), but the other much lower (38 HFU). Karig (1973) also characterized the Bismarck Sea as a region of high heat flow.

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indeed, its marginal-basin character is not yet clearly established. Taylor (1979) defined more precisely the nature of the plate motions in the east-west earthquake zone crossing the Bismarck Sea floor (Fig. 2). Most of the late Cainozoic sea-floor spreading appears to be confined to the eastern Bismarck Sea (Manus Basin) where, presumably, the lithosphere is thinner than that beneath the western Bismarck Sea north of the Tertiary island arc. However, because some extension is also thought to be taking place in the western third of the earthquake zone crossing the Bismarck Sea (Connelly, 1976; Taylor, 1979), heat-flow values throughout the Bismarck Sea area are unlikely to be as low as those corresponding to the thickened lithosphere apparently necessary for subduction. Hamilton (1978, 1979) presented a more complex interpretation of the Cainozoic geological development of New Guinea than that proposed above. His interpretation is based on the belief that a Benioff zone dips southwards or southwestwards beneath the Highlands volcanoes from the north-coast ranges, and that an additional collision suture exists between the north-coast ranges and the chain of off-shore Quaternary volcanoes (Fig. 3). Hamilton regarded this suture as having formed from the collision of a postulated southward dipping subduction system to the south (produced by polarity reversal after the Tertiary continent-arc collision) with a more northerly, northw~d-dipping subduction system. If this were so, the deformed sediments of two trenches should exist beneath the straits between the New Guinea north coast and the volcanic chain. However, the existence of these trenches - either active or fossil - is not supported by the currently available seismic reflection profiles, and the existence of an undetected, paired, trench complex beneath a strait which in places is only 12 km wide (Fig. 3) seems highly unlikely. Re-examination of earthquake distribution in New Guinea using the BMR Earthquake Data File, and plotting epicentres of earthquakes 125 km deep, or deeper, recorded by 10 or more stations between 1 January 1964 and 31 December 1977, has not substantiated the existence of a Benioff zone beneath the Highlands volcanoes. However, the Menyamya earthquakes are clearly recognisable as a zone trending about N330”E - that is, at rightangles to the direction of dip proposed by Dent (1976). As emphasised above, there is no justification in interpreting these as part of a more extensive zone extending westwards beneath the entire Highlands province. REFERENCES Barazangi, M. and Isacks, B., 1971. Lateral variations of seiunic-wave attenuation in the upper mantle above the inclined earthquake zone of the Tonga irdand arc: deep anomaly in the upper mantle. J. Geophys. Res., 76: 8493-8616. Brooks, J.A., 1969. Rayleigh waves in southern New Guinea I. Higher mode group velocities. Bull. Seismol. Sot, Am., 59: 946-958. Brooks, C., James, D.E., Hart, S.R. and Hofmenn, A.W., 1976. Rb-Sr mantle isochrons. Annu Rep. Dep. Terr. Mag., Carnegie Inst. Washington, D.C.. 76: 176-20’7.

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