TECTONICS, VOL. 10, NO. 5, PAGES 863-874, OCTOBER 1991

COLLISION PROPAGATION IN PAPUA the collision point of about 212 km/m.y. during NEW GUINEA AND THE SOLOMON SEA the past 1 m.y.

Eli A. Silver, Lon D. Abbott, Kimberly S. Kirchoff-Stein, Donald L. Reed, and Barfie Bernstein-Taylor Earth Sciences Board and Institute of Marine

Sciences, University of California, Santa Cruz David Hilyard • Geological Survey of Papua New Guinea, Port Moresby

Abstract. The mountain system in Papua New Guinea (PNG) is growing by accretion of Australian margin strata to the front of the Papuan fold-and-thrust belt in the south and by accretion of exotic terranes along its NE margin. Presently, the Finisterre Terrane is being accreted along the NE margin of PNG, and the collision point is migrating eastward within the Solomon Sea. East of the collision point the New Britain Trench marks the tectonic front between the Solomon Sea

and South Bismarck Sea plates, whereas to the west the tectonic front lies in the southern part of the Papuan fold-and-thrust belt. Present-day sedimentary environments within the western Solomon Sea are inferred from interpretation of seismic reflection data. Similar facies are observed accreted to the front of the Finisterre Terrane in

the Markham valley. The frontal Leron Formation is largely fluvial and shallow marine, and the Erap Complex contains elements of turbidites and pelagic deposits. There appears to be a smaller volume of coarse, proximal marine strata accreted to the front of the Finisterre Terrane than is

expected from interpretation of reflection profiles in the western Solomon Sea. This difference may be attributed to a combination of submarine

erosion in the Markham canyon, partial incorporation of these deposits into the Erap Complex, and a difference in depositional rates and products carded to the Solomon Sea in the past relative to what is observed today. Based on plate motions, alternative collision geometries, and the youngest dates available for the Erap Complex, we determine a rate of progression of

'Now at: Stanmore, Australia.

Copyright 1991 by the American Geophysical Union.

Paper number 91TC00867. 0278-7407/91/91 TC-00367510.00

INTRODUCTION

Papua New Guinea (PNG) (Figure 1) occupies the eastern half of the large island of New Guinea, on the northern margin of the Australian continent. The western part of PNG (west of 147øE) contains a growing mountain system, composed of three distinct tectonostructural provinces. The southern province consists of a fold-and-thrust belt [Bain et al., 1975; Dow, 1977], whereas the central part of the range is an igneous and metamorphic complex [Dow, 1977]. The northern part of the mountain system is the recently accreted Finisterre Terrane (Figure 1) [Bain, 1973; Jaques and Robinson, 1977; Johnson, 1979; Pigram and Davies, 1987].

The Finisterre Terrane was defined by Pigram and Davies [1987] as an Oligocene to early Miocene volcanic sequence overlain by middle Miocene through Pliocene carbonates. The volcanics were inferred to overlie Eocene argillite. Our work indicates that rock units along the southern Finisterre mountains are Neogene and Quaternary in age and were accreted to the terrane during the last 2-4 m.y.

The eastern part of PNG, east of 147øE, has no active foreland fold-and-thrust belt (Figure 1). The Papuan peninsula forms most of eastern PNG and is separated from the Solomon Sea to the north by the Trobriand Trough, a zone of very slow convergence [Davies et al., 1987a; Reed et at., 1988]. The New Britain Trench, bounding the north margin of the Solomon Sea, is associated with both seismicity and active volcanism along the Bismarck island arc. The latter appears to continue westward [Johnson, 1979; Davies et at., 1987a] north of the Finisterre mountains, forming the active volcanic terrane of western PNG.

Study of the active arc-continent collision in Papua New Guinea [Jacques and Robinson, 1977; Johnson, 1979] and the Solomon Sea (Figure 1) allows us to investigate the relationships between convergence, sediment supply, erosion, and deformation in a collision zone. The collision

point forms a triple junction between the South Bismarck, Solomon Sea, and Australian plates. It lies within the western Solomon Sea, at the intersection of the Trobriand Trough, New Britain Trench, and Ramu-Markham fault system [Davies et at., 1987a] (Figures 1 and 2). If such collision points can be recognized in the geologic record, the timing of collision and the rate of progression

864 Silver et al.: Collision Propagation in Papua New Guinea

142 o E 144 ø 146 ø 148 o 150 ø 152 o 154 o I [ I L I I I 2ø

..- • • • S Bismarck Sea

_ Plate

0 Biotaare k --. J

• • • Britain

_ 4 ø

X. '• /'/*ro• • •. New uraa,n ) '1 6 o P•n,,• . •• !Peninsula •/ Solomon • rapua • • • •• sea Plate I • 149øE New Guinea • ••ent "- •

• • • I Australian• "e•.•%• • • • I . Plate % %• • •ustralia 1 Plate % I

Wh lOO Fig. 1. Location map of Solomon Sea and Papua New Guinea, showing major geographical and structural features discussed in text. Location relative to Australia shown in inset, lower fight comer. Area of Figures 2 and 8 are shown by rectangles. NBT, New Britain Trench; TT, Trobfiand Trough; RMFZ, Ramu-Markham Fault Zone.

147øE 148 o 149 o 150 ø 151 ø 152 ø

::::::::::::::::::::::: ß .... :::::::::::::::::::::::::::::::::::::::::::: S:::::::::::::::::::::::::: • ' ..... '"' ':'"•

• Gi_ra-WariaI:.i!i!::;::::i::i::;::!::::!;::::;ii?i• •11Tlll ITTIIIWPTIIIIIII71-1

:'::"'• Volcanic • Slope Arc rocks Basins ?• Owen-Stanley Terrane

Fig. 2. Structural map of western Solomon Sea region, showing major thrusts, structural basins, and large submarine canyons. Located in Figure 1.

of the collision along a mountain front could be determined [see Crook, 1989].

The southern part of the Finisterre Terrane is an accretionary wedge composed of uplifted sediments equivalent to those we interpret from seismic reflection data in the Solomon Sea, as well as uplifted and accreted fluvial deposits of the Markham valley. We first present a brief synopsis of the results of marine geophysical studies in the region of the collision tip, in order to develop a conceptual model of the expected sequence of accretionary materials in the collision zone. That synopsis is followed by a discussion of observations of the accreted sedimentary sequence onshore in the Markham valley area.

DATA SOURCES

Information presented in this paper is based on offshore and onshore studies. The offshore data

consist of multichannel seismic profiles (Figure 3) acquired aboard the R/V Fred Moore of the University of Texas with a 2000 cubic inch airgun and recorded on a 96 channel, 3.3 km long streamer. Common depth point (CDP) spacing is 16.7 m at 48 fold. Processing was done initially at the University of Tulsa and the Hawaii Institute of Geophysics under the direction of G. Moore. It included band-pass filtering, deconvolution, and finite difference (45 ø ) migration. Further processing at the University of California, Santa

Silver et al.: Collision Propagation in Papua New Guinea 865

147 o E 148 ø 149 o 150 o 151 o

Huon Peninsula

New Britain

152 ø 6 ø

S

7 ø

Papuan Peninsula

-•'•..•- Thrust UCSC Track Lines

,, : ,, ,, • • ,, ,, ,, ,,

0 Kilometers 100 I I I I I •

Contour Interval 1 Km I

147 o E 148ø I I I

149 o 150 ø 151 o

8 o

9 o

152 ø

Fig. 3. Map of seismic tracks from University of California, Santa Cruz cruise and bathymetry of Solomon Sea. Hachured parts of labeled UCSC lines are shown in Figure 4. Also shown are New Britain Trench and Trobriand Trough.

Cruz included additional d•onvolution, filtering, partial frequency-wavenumber (F-K) migration, and residual finite difference migration. The main source of previously published marine data is Davies et al. [1987a], who presented the results of a largely single-channel seismic refl•tion survey collated aboard the R/V Natsushima.

Onshore data are geological field studies along the northern margin of the Markham valley, carried out by E.A. Silver and L.D. Abbott in 1989 and by L.D. Abbott in 1990. These studies were initiated as a result of a geological field conference in 1988 [Rogerson et al., 1988]. Here we discuss preliminary results of the field studies; a later, more complete report will be based on paleontologic and petrologic analyses of our samples. We used the 1:250,000 scale geological base maps produced by the PNG Geological Survey in conjunction with the Australian Bureau of Mineral Resources and Energy during the 1970s as a starting point for these studies.

RELATIONSHIPS OFFSHORE IN THE

WESTERN SOLOMON SEA

In the western Solomon Sea the propagating tip of the collision lies between the Papuan peninsula and the Finisterre Terrane [Cooper and Taylor,

1987; Davies et al., 1987b] (Figures 1 and 2). Three zones of thrusting inters•t at a triple junction (Figures 1 and 4) in the western part of the 149øE Embayment. The Trobriand Trough is thickly sedimented and has r•ently undergone convergence [Davies et al., 1987a]. Large thrust sheets, spaced 5 to 7 km apart, form the lower landward slope of the trough and pond slope basins behind them (Line 9, Figure 5).

Along most of its length the New Britain Trench is deeper and has less sediment fill than is seen in the Trobriand Trough [Davies et al., 1987a]. Also, thrust packages are more numerous and individual thrust sheets are thinner than those

of the Trobriand Trough. West of the triple junction the south facing thrust system (Figure 5, lines 10 and 12) continues westward as the Ramu- Markham fault zone (Figure 1), marking the collision front where Australian continental

lithosphere is underthrust to the north, beneath the Finisterre terrane.

The Markham submarine channel system eff•ts thrust development of the Ramu-Markham fault zone (Figure 5, lines 10 and 12) in two ways. It acts to supply large quantities of sediment to the western Solomon Sea and adjacent basins, and it erodes material that has b•n accreted to the thrust

front. The incoming sediment wedge is very thick

866 Silver et al.: Collision Propagation in Papua New Guinea

146 ø E

6 ø

o

148 ø 150 ø 152 ø

Bismarck Sea Plate 'r• •<:•;i:

T, ,9•erre Tri ........ ' ' "" "•

Fluvial Collisional Marine Basin

Trench Turbidites

• Pelagics Carbonate Platform

•• Volcanic Arc

Fig. 4. Map of sedimentary facies, based offshore on seismic data interpretation. Seismic control shown in Figure 3. Open arrow shows location of collision site.

just to the west of the triple junction. In both seismic lines 10 and 12, reflections occur to depths approaching 5 s below the seafloor (approximately 8 km), yet we find no indications of basement.

The thrust front is being eroded by the Markham canyon system on line 10, and a major levee system is developed south of the canyon and thrust front (Figure 5). No thrust front erosion appears in line 12, where the canyon axis is far to the south. As can be seen in Figures 2 and 3, seismic line 10 crosses the edge of a local high (the Duboin spur), whereas line 12 (Figure 6) crosses a restricted basin (the Gira-Waria Reentrant) [Davies et al., 1987a]. The rapid downslope change from open reentrant to narrow constriction at the Duboin Spur produces significant variability in the accreting sedimentary facies.

Distribution of Seismic Facies in the Western

Solomon Sea

Based on the seismic reflection studies of the

IJCSC (Figure 3) and Natsushima cruises [Davies et al., 1987a], we have constructed a map (Figure 4) of the probable distribution of sedimentary environments in the western Solomon Sea. This

map can be compared with observations of sediment facies onshore. We have simplified these environments into pelagic, trench turbidite, collisional marine basin, fluvial, and carbonate platform environments (Figure 4).

Pelagic sediment is acoustically transparent and poorly layered material that tends to drape topography (Figure 7). Trench turbidites are

imaged as well-layered strata that fill basins to level surfaces and form gentle slopes on a channelized fan surface (Figure 7). The collisional marine basin deposits grade into trench turbidites, and generally no sharp boundary separates them. Collisional marine basin strata are thick, coarse grained, channeled, proximal turbidites (Figure 6) that occur in significant thicknesses and areal extent up to 50 km on either side of the triple junction (Figure 5).

The collisional marine basin facies has been

deposited in the 149øE Embayment, the Gira Waria Reentrant, and along the Markham submarine canyon system. We infer that these deposits will be coarser turbidites than those found in the trenches, based on their proximity to several major submarine canyons and the abundant channel structures cutting the basin strata (Figure 6). In addition, Krause et al. [1970] reported coarse conglomerate from the Solomon Sea west of the triple junction.

A Tectonostratigraphic Model of the Modern Environment

In the eastern and central Solomon Sea only pelagic sediments are subducted at the New Britain Trench [Davies et al., 1987a]. Turbidites occur in the western New Britain Trench and

along the Trobriand Trough. Collisional marine basin seismic facies (inferred by their thickness, intensive channeling, and lack of great continuity of individual reflectors) are present on either side of the triple junction. Fluvial sediments are deposited in the Markham valley (Figure 4). Following the models of Graham et al. [1975] and

Silver et al.: Collision Propagation in Papua New Guinea 867

0 ½q ½q • (D O0 0 ½q ½q • (D O0 0 ½q

I- zz

z

868

SS12 4

Silver et al.: Collision Propagation in Papua New Guinea

7

10

collisional basin facies Markham Canyon

Fig. 6. Detail of seismic line 12 showing character of collisional marine basin strata. Located on Figure 4. VE is vertical exaggeration.

SS2/3 5

S ' 1 kr• • pelagics

6 VE = 1.4 turbidites •--

9

lO

Fig. 7. Detail of seismic line 2/3 showing character of pelagics and turbidites within the Solomon Sea. VE is vertical exaggeration.

Crook [1989], these environments would be expected to prograde eastward with time, relative to both Australia and the Finisterre Terrane, as the site of initial collision migrates eastward. Assuming a simple model of continual frontal accretion, the order of accretion in the New Britain-Finisterre wedge will proceed with time from pelagic, through trench turbidite and marine collisional basin, to fluvial sediments.

Seismic profiles across the New Britain Trench support this order of accretion. Natsushima seismic lines across the central part of the New Britain Trench [Davies et al., 1987a] show an absence of sediment ponding in the trench east of 150øE. If any accretion occurs here, it involves only small thicknesses of pelagic sediments and oceanic basaltic rocks. To the west of 150øE both the UCSC and Natsushima profiles show westward thickening turbidires in the New Britain Trench, with accretion along the New Britain arc slope.

It is not possible to draw a sharp boundary between the trench fill turbidires and the collisional marine basin strata, as the former are

in part a distal facies of the latter. However, the area of collisional marine basin strata is characterized by thick (certainly rapid) deposition of strata associated with large submarine channels. The seismic stratigraphy within these deposits is often irregular as a result of migrating channel positions through the alepositional history of the strata.

OBSERVATIONS ONSHORE

The southern flank of the Finisterre mountains,

just north of the Markham valley (Figure 8), contains accreted material scraped off during subduction of the Solomon Sea plate and collision with the north margin of Australia. Assuming this collision has occurred in a steady state manner throughout its history, a sequence of accreted sediments similar to the tectonostratigraphic model described above should be found from north to south in this tectonic setting. Crook [1989] has organized the accreted material into four facies: (1) fluvial molasse, (2) marine molasse, (3)

Silver et al.: Collision Propagation in Papua New Guinea 869

Markham Valley Geological Provinces

--•_-• Sølømøn • Alluvium Sea

• • Finisterre Leron Volcanics and Formation Erap Complex

• ew Guinea Highlands

Fig. 8. Generalized geologic map of Markham valley area, showing locations of Erap canyon map (Figure 9) and Leron canyon cross section (Figure 10). L, Leron canyon; R, Rumu canyon; B, Bintia creek.

(conglomeratic) flysch, and (4) hemipelagics. For the purposes of this paper we will avoid use of the terms flysch and molasse in favor of the more descriptive terms we have used for the modern environments. These are: (1) fluvial, (2) collisional marine basin strata, (3) turbidites, (4) pelagics. Our fluvial sediments correspond to Crook's fluvial molasse, and our collisional marine basin and turbidite strata correspond to his conglomeratic flysch.

Tingey and Grainger [1976] described three formations parallel to the south flank of the Finisterre mountains. Farthest north, adjacent to the Finisterre volcanics, they mapped the Mebu beds, composed of volcanolithic argillites and graywackes. South of the Mebu beds are their Mena beds, distinguished by having micaceous sandstone and lesser volcanic components. The Leron Formation lies south of their Mena beds

and fronts onto the Markham valley. Our field work in the Leron canyon confirms the presence of the Mebu Formation, but its extent is much more restricted than indicated by Tingey and Grainger. While we also confirm the existence of the rocks with sedimentary characteristics of the Mena beds and Leron Formation, we have significant disagreements concerning their interpretation. We discuss these differences in the following two sections.

Erap Complex

The rocks lying north of the Leron Formation, in the area mapped by Tingey and Grainger as the Mena beds, are locally very disrupted, and we prefer to use the term Erap Complex, rather than Mena beds to describe them. The Erap Complex consists of two informal sandstone subunits, the Sukurum and Nariawang units, as well as basalt [Abbott and Silver, 1991]. The Sukurum unit is composed of micaceous turbidites with minor interbedded limestones. Foraminiferal ages of the Sukurum Unit are zones N19-N20, 2.8-4.8 Ma (Peter Thompson, written communication, 1990). Cobbles of quartz and schist are common and gamets are present in a few outcrops of the Sukurum, but these are not present in the other subunits of the Erap Complex.

The Nariawang Unit is composed of turbidites, lacks the abundant quartz and mica of the Sukurum, and was deposited in bathyal or greater depths (P. Thompson, written communication, 1990). Planktonic foraminifers (P. Thompson, written communication, 1990) and nannofossils (C. Muller, written communication, 1989) range in age from late Pliocene to Pleistocene (3.7-1.1 Ma). Almost all grains are volcanic, and cobbles are volcanic and limestone.

Erap Outcrop

Erap River Map

._•,•:• • Leron Alluvium Formation

Erap Complex

Fig. 9. Generalized geologic map of lower Erap fiver area. Location on Figure 8. Inset shows detail of melange outcrop near the Wongat Thrust. Contour interval 200 m.

870 Silver et al.: Collision Propagation in Papua New Guinea

The Nariawang Unit is in fault contact with the Leron Formation. The outcrop pattern of this fault shows it to be a thrust (Figures 9 and 10), which we have named the Wongat Thrust. Near the contact a melange block-in-matrix texture is present in several outcrops, wherein large (2-10 m) limestone tectonite blocks occur in a sheared argillite matrix (inset, Figure 9). Numerous outcrops of massive sheared argillite, with no obvious included blocks, are also visible in the upper Erap canyon along with blocks of basalt, sandstone, and conglomerate.

In Leron canyon (Figure 10) and Bintia creek (Figure 8) pervasively sheared sandstone occurs near the Wongat Thrust. We observed argillite matrix melange at two localities in the Leron canyon. One outcrop consisted of 10-30 cm long blocks of sandstone and basalt in a sheared

argillite matrix. The other outcrop contained rounded basalt blocks in a sheared argillite. Approximately 20 km up the Leron canyon we found one outcrop of pelagic limestone of early to middle Eocene age (J. Haggerty, oral communication, 1989).

Leron Formation

The Leron Formation lies south of the Erap Complex and north of the Markham valley. The type section, which is located in the Leron canyon (Figure 8), is composed of poorly indurated, interbedded lithic sandstone, pebble to cobble conglomerate, and mudstone layers [Tingey and Grainger, 1976]. The conglomerate clasts are well rounded and composed mostly of andesite, basalt, red chert and limestone. The sandstone and

conglomerate beds exhibit scoured bases and are between a few centimeters and 10 m thick.

Individual beds often pinch out within a few tens

to hundreds of meters along strike. The mudstone layers are generally thinner, particularly in Leron canyon, and many mudstone layers are only a few centimeters thick. Beds are poorly sorted and often lack internal grading, although both fining-up and coarsening-up beds occur. Small-scale trough cross bedding is the most common sedimentary structure in the sand units; hummocky cross stratification is present in a few sandstones exposed in Erap canyon. Most sedimentary sequences in Leron, Erap, and Rumu canyons (Figure 8) fine upwards. These sequences consist of conglomerate or sandstone with a scoured base and fine up to a thin mudstone unit. Crook [1989] presented detailed stratigraphic sections of several Leron Formation outcrops.

A 10 cm thick vitric volcanic ash is

interbedded with the Leron Formation in the

Rumu canyon. Abundant lignite occurs in the formation, and a few outcrops in Erap canyon are rich in organic debris and contain numerous leaf impressions. A coralline limestone interbed caps hills above Ohman creek (Figure 8).

The Leron Formation in the Leron, Erap, and Rumu canyons is covered unconformably by fluvial terrace deposits. These deposits are essentially horizontal and do not appear to be affected by the deformation of the underlying strata, except by vertical uplift. We observed one terrace tilted 18 ø north in Erap canyon, but this, too, was overlain by younger horizontal terraces. So far, it has not been possible to date the terrace deposits. The common occurrence of nearly horizontal terraces hundreds of meters high upstream in the Leron and Erap canyons, however, indicates that uplift is continuing them.

Deformation of the Leron Formation is

accomplished along north and northeast dipping thrust faults and associated folds which strike

2 / Ramu-Markham Wongat

Km 1 - 0

Erap Complex

i• Leron Formation I Nariawang Unit

-2

-1

-0

--1

---2

Km

i;• Finisterre •-----•---t Sukurum Unit Volcanics

Fig. 10. Interpreted structural cross section of lower Leron canyon area. From Abbott and Silver [1991]. Location on Figure 8.

Silver et al.: Collision Propagation in Papua New Guinea 87!

nearly parallel to both bedding and the range front. The Leron Formation strikes predominantly west to northwest and dips 400-60 ø north. Figure 10 is a structural cross section of the Leron

Formation and part of the Erap Complex in Leron canyon. In addition to the Wongat and Ramu- Markham thrusts, two other thrusts are present within the Leron Formation in the Leron canyon (Figure 10), both accompanied by fault propagation folds. The frontal thrust of the Leron Formation is not exposed, but we have assumed that its position is closely approximated by the mountain front. This interpretation implies that the stratigraphic thickness of the southern thrust sheet is approximately 1000 m. The middle and northern thrust sheets are about 800-900 rn thick.

The Leron Formation occupies two thrust sheets in Erap canyon, and both thrust sheets contain approximately 1100-1200 rn of stratigraphic section.

Tingey and Grainger [1976] considered the Leron Formation to be a Pliocene unit, but due to the southeastern migration of the collision we would expect lithologic units to be time transgressive and to exhibit a general younging to the southeast [Crook, 1989]. Dates from Leron Formation north of the Markham valley indicate that the formation there is Pleistocene.

Foraminifera from a limestone interbed in the Leron Formation at the west end of the Markham

valley in Ohman creek (Figure 8) are Quaternary in age, no older than 1.65 Ma (Zone N22-N23, P. Thompson, written communication, 1991). Farther to the east, in Leron canyon (Figures 8 and 10), the youngest nannofossils found are Pliocene (zones NN16-NN18, 1.65-3.5 Ma, C. Muller, written communication, 1990), but considerable reworking of nannofossils is apparent in all samples, with forms as early as Oligocene present (Hilyard et al., 1988; D. Bukry, written communication, 1988; C. Muller, written communication, 1989). Blocks in the Erap Structural Complex in the Leron canyon are as young as 1.15-1.32 Ma (see above) and are thrust over the Leron Formation. The Leron Formation

is poorly indurated and only moderately tectonized, in sharp contrast to the often highly sheared and generally well indurated Erap Complex. Its lesser disruption suggests that the Leron Formation is the younger of the two units. Sediments on the south flank of the Finisterre

mountains generally young from north to south toward the mountain front [Tingey and Grainger, 1976; Crook, 1989], a typical situation in accretionary wedges. This southward younging of

sediments also supports the inference that the Leron Formation is younger than the Erap Complex and places an upper age limit of 1.32 Ma on the Leron Formation in Leron canyon. By the same reasoning, the Leron Formation is also Quaternary in Erap canyon, where Crook [1989] has found Pleistocene (zone N22, 0.7-1.65 Ma) foraminifera in Erap Complex sediments (referred to as the Mena beds by Crook; see Abbott and Silver [1991] for a discussion of this terminology).

A sample of lignite from the Leron Formation in Erap canyon was dated using carbon 14 as greater than 40 Ka (Beta Analytic, Coral Gables, Florida, written communication, 1990). Crook [ 1989] reported a carbon 14 age of 21 Ka on an early postdepositional concretion from the Leron Formation at Lae, and Hilyard et al. [1988] found a carbon 14 age of 31 Ka on a charcoal log in the Leron Formation north of Lae. Thus the Leron

Formation is older at Erap canyon than it is near Lae, to the southeast. The preceding ages suggest that the Leron Formation is a Pleistocene unit in

all exposures north of the Markham valley and also support the inference of Crook [1989] that lithologic facies should young to the southeast.

Crook [1989] suggested that a suite of facies from fluvial to deep marine turbidite is present in the Leron Formation, with the deep marine turbidites being a nonfan variety and hence lacking classic Bouma sequences. Sedimentary structures and sequences present in the Leron Formation could be attributed to either fluvial or

channelized turbidite deposition. Several lines of evidence suggest, however, that the bulk of the Leron Formation in Leron, Erap and Rumu canyons is composed of fluvial sediments rather than turbidites.

Four varieties of terrestrial and freshwater snails

(B. Roth, written communication, 1990) are present in outcrops of Leron Formation from these three canyons. Roth believes that these snails inhabited forests along streams draining into the ocean or coastal lagoons. The relative abundance and diversity of the snails, some of which are nearly intact, argues for minimal transport of the fragile, thin-walled tests of these specimens (B. Roth, written communication, 1990). As noted above, marine nannofossils exhibit tremendous amounts of reworking. Marine macrofossils are rare in the Leron Formation, and all specimens we have collected are considerably more abraded than the land snails, indicating substantial transport and suggesting that they may have been reworked. One sample (PNG89A-96) contains fossils of three

872 Silver et al.: Collision Propagation in Papua New Guinea

marine organisms which would not be found together if deposited in sire (D. Lindberg, personal communication, 1990), strengthening the case for reworking of the marine fossils.

Crook [1989, location 3] noted the presence of thin carbonate tubules suggestive of caliche formation in a Leron Formation outcrop from Leron canyon. Carbonate concretions parallel to bedding may be incipient caliche horizons and are abundant throughout the unit. Palynology of samples from both Leron and Erap canyons (samples IOC 7 and IOC 113) suggests freshwater deposition near plant habitats [Hilyard et al., 1988]. Nannofossils are also present in the samples but display a great deal of reworking. Hilyard et al. [1988] suggested that this reworking may explain the presence of marine nannofossils amidst nonmarine palynomorphs. This explanation is consistent with our observation that even the

youngest nannofossils found in the Leron Formation are older than the likely depositional age of the unit and hence are reworked.

The Leron Formation section in Ohman creek

(Figure 8) contains outer neritic foraminifera (P. Thompson, written communication, 1990) and a coralline limestone interbed, showing that the Leron Formation contains shallow marine facies

here. A few outcrops with abundant organic debris and beds with hummocky cross stratification are present at the base of the Leron Formation section in one of the Erap canyon thrust sheets. These are likely lagoonal and shallow marine shelf sediments deposited during local sea level highstands in an environment similar to that inferred by Crook [1989] for the Leron Formation outcrops in and around the city of Lae. However, the bulk of the Leron Formation sediments exposed in the Leron, Erap, and Rumu canyons was deposited by a braided fluvial system and on associated humid alluvial fans, in a depositional environment very similar to the present-day Markham valley.

Estimate of Collision Propagation

For a steady state collision we can estimate the rate of collision propagation along the Markham valley from plate convergence vectors and assumed collision geometry (Figure 11). Taylor [1979] determined a convergence rate of 93 km/m. y. between the South Bismarck and Australian plates at the triple junction. Assuming the Australian margin was parallel to the present trend of the Markham valley but the island arc was parallel to eastern New Britain prior to collision, we find a minimum propagation rate of

Fig. 11. Vector plot of plate velocity and estimated velocity of collision propagation in eastern Papua New Guinea and western Solomon Sea. Au: Australia plate; Sb: South Bismarck plate. Line marking the trend of collision was drawn along the Ramu-Markham fault zone. Cross pattern indicates volcanic arc rocks; diagonal line pattern lumps all other rocks in PNG south of the Ramu-Markham fault zone. Geometries and

implications discussed in text.

120 km/m.y. If the arc maintained an E-W trend, similar to that of west New Britain, the propagation rate was about 180 km/m.y. (Figure 11).

We can constrain this geometrical interpretation using the dates of the youngest Nariawang Unit north of the Wongat Thrust. The Nariawang Unit above the thrust is collisional marine basin facies, which we interpret to have been deposited near the collision point. Beneath the thrust are fluvial sediments of the Leron Formation that must have

formed after the passage of the triple junction. The age of 1.32 Ma on bathyal sediments in the youngest part of the Erap Complex provides an approximate date on the passage of the triple junction. Leron canyon is 280 km NE of the present triple junction, implying a minimum rate of propagation of the collision point of 212 km/m.y.

If we base the rate of collision propagation solely on sedimentary facies, we infer an error of +50 km for the location of the triple junction using the present distribution of the collisional marine basin strata in the western Solomon Sea

(Figure 4). The fluvial strata occur 200 km west of the triple junction, producing a larger error in recognizing an ancient collision point. For a geometry similar to that of the present Solomon Sea collision, it is more accurate to seek the youngest occurrence of collisional marine basin strata than the oldest occurrence of fluvial strata.

Silver et al.' Collision Propagation in Papua New Guinea 873

DISCUSSION AND CONCLUSIONS

The sequence of accretion in the western Solomon Sea and that preserved onshore in the Erap and Leron canyons are similar, yet they differ quantitatively. The modem sedimentary sequence from east to west encompasses pelagic - > trench distal turbidite - > thick, channeled, collisional basin turbidites- > fluvial. Any given locati•on along the front of the Bismarck arc would be expected to accrete each of these packages of sediment in sequence. Of the marine sequences, the collisional marine basin strata are very prominent, with sediment thicknesses of > 5 km in the Gira Waria Reentrant and 3 km in the 149øE

Embayment. Onshore, we observe the Erap Complex, composed of a tectonized mixture of argillite, basalt, and sandstone, thin bedded sandstone and shale, and locally blocks of limestone and conglomerate. Structurally below the Erap Complex lies the fluvial Leron Formation.

Rocks corresponding to collisional marine basin origin lack prominence in the on-land sequence. Crook [1989] interpreted the Leron Formation to represent these rocks, but that interpretation does not withstand the findings of a dominantly fluvial origin for the Leron Formation. Sandstones and conglomerates in the Sukurum and Nariawang subunits of the Erap Complex may represent equivalents of the collisional marine basin setting. However, their outcrop width is small in Leron canyon (about 2 km). In contrast, the width of accreted thrust packages adjacent to the Gira Waria Reentrant is about 5 km (Figure 5, line 12).

Erosion along the Markham submarine canyon system acts to decrease the amount of sediment preserved on the front of the thrust belt in the collision zone. This process is clearly documented on line 10 (Figure 5). High rates of submarine erosion are likely in the Markham canyon, and thus significant thicknesses of material may be removed from the front of the wedge.

It is possible that the present thicknesses of sediment in the Gira Waria Reentrant and the

149øE Embayment do not represent the situation of 2 Ma. The Finisterre range is a young mountain range that is rapidly uplifting. Chappell [1974] determined uplift rates on terraces along the north coast of the Huon peninsula at 3 km/m.y. The high part of the range is composed of shallow water limestones coveting island arc volcanics,

documenting that the present elevation represents total uplift of the range. It is possible that during the time of deposition of the Erap Complex sediments the range was not sufficiently incised to produce depositional rates in the western Solomon Sea as high as they are today, resulting in lesser thicknesses of collisional marine basin strata 1-2 Ma.

The Leron Formation is widest in the region between the Leron and Erap canyons. The formation is narrower at most localities along the north side of the Markham valley. A possible explanation for this distribution of Leron Formation is the presence of the wide, fluvial embayment at the junction of the Owen Stanley range and the eastern Highlands (Figures 1 and 8), adjacent to the Leron and Erap canyons. South of the embayment, numerous faults trend normal to the mountain range. We suggest that this particular structural setting allowed the accumulation of a relatively thick and wide fluvial, alluvial, and possibly shallow marine basin during deposition of the Leron Formation. The last phase of convergence between the Finisterre Terrane and Australia resulted in the

accretion and uplift of the sediments within this embayment.

Acknowledgments. We thank Rick Rogerson, Greg Anderson, Geoff Francis, and others of the Geological Survey of PNG for their generous support and assistance. We are very grateful to Dave Bukry, Carla Muller, Peter Thompson, Janet Haggerty, Dave Lindberg and Barry Roth for their generosity in identifying fossils. We thank Greg Moore, Mary Mackay, and Paul Henkart for their support in processing the seismic data. Silver thanks Hugh Davies, Chris Pigram, Phil Symmonds, Jo Lock, and Dave Falvey of the Australian Bureau of Mineral Resources, who initially supported and stimulated his interest in PNG and the Solomon Sea. We are grateful to discussions with Casey Moore, Keyu Liu, Keith Crook, Phil Townsend, and the participants of the 1988 PNG Collision Zone conference. The paper was improved significantly as a result of the insightful review by Steve Lewis. The study was supported by NSF grants OCE-8614337 and OCE- 8722043 to Silver, a generous gift to UCSC by BP Australia, and a GSA graduate research grant to Abbott. Abbott was supported by an NSF Fellowship and an SEG scholarship.

874 Silver et al.: Collision Propagation in Papua New Guinea

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L. D. Abbott, B. Bernstein-Taylor, K. S. Kirchoff-Stein, D. L. Reed, and E. A. Silver, Earth Sciences Board and Institute of Marine

Sciences, University of California, Santa Cruz, CA 95064.

D. Hilyard, 82 Railway Ave., Stanmore, Australia 2046.

(Received July 23, 1990; revised March 4, 1991; accepted March 21, 1991.)