I Tectonophysics, 212 (1992) 213-241 213 Elsevier Science Publishers B.V., Amsterdam wI *c

'1 I Geology of the d'Entrecasteaux-New Hebrides Arc collision rh zone: results from a deep submersible survey

J.-Y. Collot a, S. Lallemand b, B. Pelletier ', J.-P. Eissen ') G. Glaçon d, M.A. Fisher e, H.G. Greene e, J. Boulin f, J. Daniel and M. Monzier I' ORSTOM -BP 48, 06230 fillefranche /mer, France Université P. et M. Curie, Laboratoire de Géologie Structurale, 4 Place Jussieu, 75252 Paris, France QRSTQM-BP A5, Nouniia, New Caledonia c Université de Provence, Laboratoire de Stratigraphie et Paléoécologie, Place Victor Hugo, 13000 Marseille, France US.Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA -1 Uniuersité de Marseille III Saint Jérôme, Laboratoire de Géologie Structurale, 13397 Marseille, France (Received August 16,1991; revised version accepted April 27,1992)

ABSTRACT

Collot, J.-Y., Lallemand, S., Pelletier, B., Eissen, J.-P., Glaçon, G., Fisher, M.A., Greene, H.G., Boulin, J., Daniel, J. and Monzier, M., 1992. Geology of the d'Entrecasteaux-New Hebrides Arc collision zone: results from a deep submersible survey. Tectonophysics, 212 213-241.

During the SuBPSOl cruise, seven submersible dives were conducted between water depths of 5350 and 900 m over the collision zone between the New Hebrides island arc and the d'Entrecasteaux Zone (DEZ). The DEZ, a topographic high on the Australian plate, encompasses the North d'Entrecasteaux Ridge (NDR) and the Bougainville guyot, both of which collide with the island-arc slope. In this report we use diving observations and samples, as well as dredging results, to analyse 4"' the geology of the Bougainville guyot and the outer arc slope in the DEZ-arc collision zone, and to decipher the P mechanisms of . These data indicate that the Bougainville guyot is a middle Eocene island arc volcano capped with limestones that appear to have been deposited during the Late Oligocene to Early Miocene and in Miocene-Pliocene times. This guyot possibly emerged during the Middle and Late Miocene, and started to sink in the New Hebrides trench after the Pliocene. The rocks of the New Hebrides arc slope, in the collision zone, consist primarily of Pliocene-Recent volcaniclastic rocks derived from the arc, and underlying fractured island-arc volcanic basement, possibly of Late Miocene age. However, highly sheared, Upper Oligocene to Lower Miocene nannofossil ooze and chalk are exposed at the toe of the arc slope against the northern flank of the NDR. Based on a comparison with cores collected at DSDP Site 286, the ooze and chalk can be interpreted as sediments accreted from the downgoing plate. East of the Bougainville guyot an antifonn that developed in the arc slope as a consequence of the collision reveals a 500-m-thick wedge of strongly tectonized rocks, possibly accreted from the guyot or an already subducted seamount. The wedge that is overlain by less deformed volcaniclastic island-arc rocks and sediments includes imbricated layers of Late Oligocene to Early Miocene reef and micritic limestones. This wedge, which develops against the leading flank of the guyot, tends to smooth its high-drag shape. A comparison between the 500-m-thick wedge of limestones that outcrops southeast of the guyot and the absence of such a wedge over the top of the guyot, although the top is overthrust by island-arc rocks and sediments, can be interpreted to suggest that the wedge moves in the subduction zone with the guyot and facilitates its subduction by streamlining.

Introduction underthrusts an island arc provide important keys to understanding both the tectonic processes that The lithology and structure of collision zones shape island arcs and the mechanical properties k.F at plate boundaries where a ridge or a seamount of the island arc crust. One such collision zone occurs in the SW Pacific between the New Hebrides island arc and the d'Entrecasteaux n Correspondence to: J.-Y. Collot, ORSTOM, BP 48, 06230, Zone, a major submarine chain carried by the Villefrauche/mer, France. Australian plate. The New Hebrides island arc

.w ' 0040-1951/92/$05.00 Q 1992 - Elsevier Science Publishers B.V. All rights reserved ;s

P 114 J.-Y.COLLOT ET AL marks the boundary along which the Australian ( 19821, this subduction initiated in the early Late plate to the west underthrusts the Pacific plate Miocene. The d'Entrecasteaux Zone (DEZ) ex- and the North Fiji basin to the cast (Fig. 1). The tends eastward from the northern New Caledonia convergence rate between the Australian and Pa- ridge to where the DEZ collides with the New cific plates is about 10 cm/yr (Minster and Jor- Hebrides island arc (Pascal et al., 1978; Daniel dan, 1978; Louat and Pelletier, 1989) in an east- and Katz, 1981; Collot et al.. 1985). Near the ward direction (N7h"E & 11") and the Wadati- trench the DEZ trends slightly oblique (14") to Benioff zone dips uniformly about 70" east be- the plate-convergence direction, so that DEZ neath the New Hebrides island arc (Isacks et al.. creeps northward along the trench at 7.5 cm/yr 1981). According to Carney and Macfarlane (Fig. 1). The complex 3-D structure of the arc

166"E 167" 1

14's

15-

16"

17'

Fig. 1. Location of the study :ireil within the central New Hebrides islrind arc. NDR = North d'Entrecasteaux Ridgc; .SDC = South d'Entrec:ìsteaux Chain. Large arrow's show the relative plate motion hehveen the Australisn md the North Fiji Risin plates. The + bathymetric contour interval is 1 hn (after Krornke et al.. 1W31. GEOLOGY OF THE d'EN7'RECASTEAL'\-NEW lrEL3RIDS ARC COLLISION ZONE 215

slope that results from this collision has been The DEZ separates the West Santo basin to described from Seaheam bathymetric data (Daniel the north from the North Loyalty hasin to the et al.. 1986; Collot and Fisher, 1989, 1991) and south (Fig. I). The West Santo hasin underlies multichannel seismic reflection data (Fisher, 1986; water as deep as 5000 m and contains about 1.5 Fisher et al., 19S6, 1991a,b). However. little was km of sediments that overlie of known about the lithology, age and rocks de- unknown age (Pontoise and Tiffin. 1986; Collot formed within this collision zone. and Fisher, 1991). In March 1989, during the SUBPSOI cruise, The North Loyalty basin. under 3500 m of the DEZ-New Hebrides island arc collision zone water, formed as a marginal hasin during the was surveyed using the deep- submersible Late Paleocene to Late Eocene (Andrews et al., Nautik (Collot et al., 1989). This cruise was jointly 1975; Weissel et al., 1982). Drilling conducted aboard the R.K Níidir by the Institut Project (DSDP) Site 286 (Andrews et al., 1975). Français de Recherche Scientifique pour le located 50 km south of the DEZ (Fig. 1). revealed Développement en Coopération (ORSTOM) and 650 m of sediments overlying basaltic pillow lavas the Institut Français de Recherche pour I'ExpIoi- and gabbro. Drilling results show that, during the tation de la Mer (IFREMER). The main objec- Middle and Late Eocene. alternating siltstone tives of the dives were: ( 1) to collect rock samples and sandstone containing pumice and volcanic ;Icross the arc slope to resolve its lithology and glass ot andesitic affinity was rapidly deposited origin: (2)to study the plate contact zone: and (3) on the oceanic basement. This volcanic series to characterize the tectonic style of both the arc includes ;i coarse andesite conglomerate. suggest- slope and the DEZ in the collision zone. ing that. during the Middle Eocene. the location of Site 286 was proximal to an island arc (Maillet et al.. 1983). During the latest Eocene sedimenta- Geologic setting of the DEZ and New Hebrides tion changed sharply from the volcanic series to island arc overlying nannofossil ooze and chalk, with minor amounts of ash. The calcareous sedimentation West of the New Hebrides island arc. the DEZ continued during Oligocene times. suggesting a strikes east-west and bifurcates into the North deepening of the sea floor. This deepening may d'Entrecasteaw Ridge (NDRI and the South have intensified during the latest Oligocene or d'Entrecasteaux Chain (SDC) (Fig. 1). The NDR Early Miocene, as indicated by the deposition of rises 3000 m locally above the and abyssal red clays. The absence of Miocene sedi- consists of horsts of latest Paleocene to Early ments points to either non-deposition or erosion Oligocene mid-ocean ridge basalt (MORB) (Mail- hefore deposition of Pliocene and Pleistocene let et al.. 1983). which is overlain by stratified deep-water sediments. volcaniclastic sediments (Fisher ct al., 1991a). The The central New Hehrides island arc includes SDC includes two flat-topped , the three north-trending belts of islands: the western Bougainville guyot and the Sabine bank. which helt exposed along the Espiritu Santo and are aligned with two smaller conical seamounts. Malakula Islands, the central belt represented by The Sahine hank rises nearly to , whereas Aoba and Ambrym islands, and the eastern belt the Bougainville guyot lies at a water depth of evident on Maewo rind Pentecost islands (Fig. 1). 1000-1500 m and clogs the trench. This guyot is All three helts have foundations composed of L 301313 m high and is topped by a reefal platform island-arc volcanic rocks. The western belt is

that is tilted 5O ;ircward (Daniel et al., 19%). thought to have originated during the Late Multichannel seismic retlection data have rc- Oligocene (?)-Early Miocene along a subduction vealed that this platform is approximately 700 m zone that faced east (Mitchell and Worden, 1971; thick and that a 2-km-thich. well stratified debris Carney and Macfarlane. 1982; Greene et al.. apron underlies the guyot' s flanks (Fisher et al., 1088). During the Early to lower Middle Miocene 19%. 19cllh). (Fig. 2). this subduction gave rise to volcanos that .- 216 J.-Y. COLLOT ET AL

were flanked by fringing reefs and surrounded by c N12O0-trendingstrike-slip lineaments north of the thick volcaniclastic deposits (Mallick and Green- NDR; (2) the highly uplifted Wousi bank east of baum, 1977). Volcanic intrusions dated from the the NDR and (3) E-W-trending normal faults latest Early to the early Middle Miocene were south of the ridge. South of the NDR, the ongo- associated with this phase of volcanism (Carney ing collision with the Bougainville guyot has pro- , et al., 1985). During the Middle Miocene, vol- duced a 10-km indentation in the arc slope caniclastic sediment that was derived from up- (Daniel et al., 1986). Uplift and thrust faulting lifted volcanic basement, as well as reefal lime- concomitant with this indentation have produced -! stone, was deposited in subsiding basins along the an antiform in the arc slope that towers 800 m western belt. From the end of the Middle Miocene over the guyot (Fisher et al., 1986, 1991b). to the latest Miocene the western belt was up- lifted and eroded. Detrital material derived dur- Lithology, age and structure of rocks in the colli- c ing this erosion was probably deposited along sion zone what is now the trench slope that lies west of Espiritu Santo island. When the North Fiji basin Geologic data collected during the dives across began to form during the Late Miocene (Mala- major seafloor scarps of the DEZ-New Hebrides hoff et al., 1982; Auzende et al., 19881, the vol- island arc collision zone were augmented by sam- canic axis shifted from the western to the eastern ples that were dredged from along the NDR belt; this shift may have been caused by a flip of during the GEORSTOM III cruise (1975) and subduction polarity, from east-facing to west-fac- along the Bougainville guyot during the SEAPSO ing (Chase, 1971; Falvey, 1975). The present ac- 1 cruise (1985). In the next four sections, the cretionary complex could have begun to form NDR and Bougainville guyot collision zones are following this polarity flip. By the end of the described. For each collision zone, we first pre- Miocene, a marine transgression had deposited sent and interpret the geologic data from the hemipelagic and pelagic calcareous sediments un- downgoing plate, and then from the adjacent arc conformably on top of the older rocks of the slope. In this report we use the sediment and western belt (Macfarlane et al., 1988). During the sedimentary rock classification employed by the Pliocene, when the volcanism shifted westward Ocean Drilling Program (Mazzullo et al., 1987). from the eastern belt to the central belt, the Lithologic, age and geochemistry data are pre- western belt again started to rise and became sented in Tables 1, 2 and 3. covered by successive reef terraces (Mallick and The North d’Entrecasteaux Ridge (NDR) Greenbaum, 1977; Carney and Macfarlane, 1980). This uplift, which may have resulted from colli- The summit of the North d’Entrecasteaux sion with the DEZ, reached a maximum rate of Ridge was explored during dive 2 (Fig. 2). The 5-6 mm/yr during the Holocene (Taylor et al., ridge has a smooth morphology and is blanketed 1980, 1985, 1987; Jouannic et al., 1980) and with a silty clay of Late Pleistocene to Recent age formed high (1800 m) on Espiritu ( 1); however, Oligocene and Miocene Santo island. Consequently, sediment eroded pelagic microfossils are common in this clay. Sev- from Espiritu Santo island since the Pliocene eral rock samples dredged from the flanks of the could have contributed significantly to deposits of NDR about 120 km west of the collision zone the eastern trench slope. (Maillet et al., 1983) revealed Late Pliocene and 4 . West of Espiritu Santo island, the trench slope Late Pleistocene clay (Go314-315 in Table 2). has been severely deformed by the collisions of <’ . the NDR and the Bougainville guyot (Fig. 2). The The arc slope in front of the North d’Entrecasteaux oblique collision of the NDR against the arc has Ridge produced an asymmetric tectonic pattern in arc slope rocks (Collot and Fisher, 1991). This pat- During dive 1 of the Nautile, the upper arc tern is characterized by three features: (1) slope adjacent to the NDR was surveyed along 15 S

15"3C

Pleistocene-Recent uplifted limcstonc

Pliocene and Pleistocene MALA KULA I. terrigcnous scdimcnt

hliddle Mioccnc ___ graywacke and lulitc

Lower Mioccnc to

volcaniclasf ics

Uppermost Lower to

Oligocene rcd nmudstone

Fig. 7. Simplified geologic map of Espiritu Santo and Malakula islands (after Carney and Macfarlane, 1982) and Seabeam bathymetric map of the collision zone benveen the d'Entrecasteaux Zone and the New Hebrides island arc (after Daniel et al., 1956). The location of this area is shown in Fig. I. Dive 1 indicates the location of a dive conducted during the SUBPSOl cruise: D1 indicates a dredging site from the SEAPSOl cruise. MO GEOLOGY OF THE d'ENTRECASTEAUX-NEW HEBRIDES ARC COLLISION ZONE 221

TABLE 1 Description and biostratigraphy of samples collected in the d'Entrecasteaux Zone-New Hebrides island arc collision zone during the SUBPSOl cruise (1989). Pliocene (Miocene) = Pliocene sediment including reworked Miocene; biostratigraphy is based on B'erggren et al. (1985); lithology is based on Mazznllo et al., (1987); carbonate facies are from L. Montaggioni; scleractinian determination from G. Faure; MSR = mixed sedimentary rock

Sample Depth Description Age a no. (m) North d'Entrecasteaux Ridge Zone Dive I 101 2084 Gray polygenetic volcanic breccia ? 102 2086 Reefal limestone ? 103 2087 Weathered reddish andesite ? 104 2090 ? 105 2074 Weathered reddish andesite ? 106 2042 Gray coarse polygenetic volcanic breccia ? 107 1956 Gray polygenetic volcanic breccia ? 108 1954 Reddish gray andesitic basalt ? 109 1903 Gray volcanic sandstone ? 110 1880 Greenish gray volcanic sandstone ? 111 1835 Gray highly phyric andesitic basalt 7.3 f 0.36 Ma (K/Ar) 112 1742 Greenish gray volcanic sandstone ? Dive 2 201 3060 Greenish brown calcareous silty clay Late Pleistocene-Holocene NN2l(early Middle Miocene "4-"5) (N, P) 202 2870 Greenish brown calcareous silty clay Late Pleistocene-Holocene NN 20-21(early Middle Miocene NN4-NN5, Pliocene) (N, P) 203 2870 Greenish brown calcareous silty clay Pleistocene "20-21 (Latest Miocene "11, early Middle Miocene "5) (N, P) 204 ? Greenish brown calcareous silty clay Late Pleistocene "20-21 (Oligocene, early Middle Miocene NN4-NN5, Pliocene) (N, P) Dive 3 301 3426 Dark gray volcanic sandstone ? 302 3372 Pale brownish silty clay foram MSR Late Pliocene "18 (N, P) with nannofossils 303 3364 Dark grayish brown poorly consolidated ? volcanic breccia 304A 3365 Light brown calcareous volcanic clayey Latest Miocene NNllb (early Middle Miocene NN4- siltstone "5) (N),Early Pliocene (P) 304B 3365 Light brown calcareous volcanic clayey Late Miocene NNlla (N, P) siltstone 305 3262 Dark gray unconsolidated volcanic breccia ? 306 3267 Brown calcareous volcanic silty clay with Late Pliocene-Quaternary (Pl, Latest Miocene NNllb volcanic and chalk clasts (NI 307 3260 Light gray silty clay nannofossil foram MSR Early Pleistocene "19 (Pliocene, Middle Miocene) (N, P) 308 3211 Light brown silty clay nannofossil foram Late Pliocene "16 (N), Late Quaternary (P) MSR 309 3191 Grayish calcareous volcanic clayey siltstone Late Pleistocene "20 (Pliocene) (N, P) 310 3194 Light gray sandy biograinstone Pliocene-Quaternary (P) 311 3134 Light brown calcareous volcanic clayey Latest Miocene "11 (early Middle Miocene NN4- siltstone "5) (NI, Late Quaternary (P) 312 3022 Brownish silty clay foram MSR with Early Pleistocene "19 (N, P) nannofossils 313A 3023 Light grayish brown foram clayey silt MSR Early Pleistocene "19 (N, P) ---717 .I.-Y. COLLOT ET AL.

TABLE 1 (continued)

Sample Depth Description Age no. (m) 313B 3023 Light hrown gray foram clayey silt MSR Late Pleistocene "21 (Late Miocene. Pliocene) (N, P) 314 2826 Brownish gray foram silty clay MSR Late Pleistocene NNZ0-21 (Pliocene) (N) 318 2836 Brownish volcanic hreccia with basaltic clast: 16.04 0.8 Ma (Ki/&) clasts

Dir.r 4 40 1 5349 Light gray silty clay foram MSR Early Pleistocene "19 (Pliocene) (N, P) 402 5159 Very pale hrown clayey nannofossil ooze Middle Oligocene NP 23 (Eocene) (NI Late Pliocene-Quaternary (P) 403 5108 Very pale brown clayey nannofossil ooze Early Miocene "2 (Oligocene) (NI, Late Pliocene-Quaternary (P) 401 5071 Pale brown clayey nannochalk with silt spicule Early Miocene "1-2 (N). Late Miocene-Recent (PI 405 5068 Light olive gray silty clay foram MSR Late Pliocene "18 (NI, Early Pliocene (P) 406 5065 Very pale brown clayey nannofossil chalk with Uppermost Oligocene, Early Miocene NN2 (N), spicule Late Pliocene-Quaternary (P) 407 5003 Pale hrown silty clavcy nannofossil ooze Middle Oligocene NPZ (N), Pliocene (P) 408A 4882 Pale hrown nannofossil foram clayey silt MSR Pleistocene (P), Oligocene, Early Miocene (N) 408B 4882 Light gray nannofossil foram clayey silt MSR Late Pliocene "16 (N, P) 4n9 48 1o Grey calcsreous silty clay with clasts of Late Pliocene NNlh (Oligocene-Miocene) (N. P) very pale brown chalk 4lOA 471 15 Light gray clayey silt nannofossil foram MSR Late Pliocene "17 (N, P) 4lOB 4705 Light gray silty clay nannofossil foram MSR Early Pliocene "15 (N), Late Pliocene-qua tern^ì^ iP) 4l0C 4705 Light olive gray calcareous volcanic sandy Late Pleistocene "20-21 (Middle-Late Miocene, clayey siltstone Pliocene) (N, P) 41 1 4523 Pale hrown clayey nannochalk with silt spicule Early Miocene NN1 (N),Late Miocene-Recent (PI

Bougainville Zone Dire 5 501 Encrusted reef grainstone to wackstone Middle Eocene to Present (B) 502 Coral colony fragment Q 503 Coral colony fragment 504 White fine limestone with form 505 Reddish yellow foram volcanic coarse Late Eocene to Early Oligocene (Ca) semi-consolidated sandstone 506 White finc limestone with foram 1 507 Brownish white foram packstone with Early Oligocene NP 21 (N). late Oligocene-early plagioclases Miocene (Late Eocene) (BI 50s Grayish white hiopachtone with large foram ?, filling: Late Pliocene-Quaternary (P) and ooze filling 509 Olive gray calcareous volcanic sandstone with Early Pliocene "15 (N, P) foram 510 Olive gray calcareous volcanic clayey siltstone Late Pleistocene "21- Holocene (N, P) with foram and spicule

Dirt- O bit 1 White reef wackstone with ooze filling Miocene-Pliocene (B), filling: Pliocene (PI 602A Pale hrown clayey limestone with foram Miocene-Pliocene (N. P) and spicule hO2R Light olive gray calcareous volcanic clayey Latest Miocene "11 (early Middle Miocene "5) (N), siltstone Pliocene-Pleistocene (PI hn3A microlitic hasalt 9.42 2 11.47 Ma (K/Ar) h03B Pale hroun clayey nannofosbil limestone Nith Late Oligocene NP25 (N. P) foram and spicule 603C White foram packstone Late Oligocene NP25 (N.P) (hliddle Eocene) (P) 5: GEOLOGY OF THE d'ENTRECASTEAUX-NEW HEBRIDES ARC COLLISION ZONE 223

J TABLE 1 (continued)

Sample Depth Description Age no. (m)

~ 604A 1602? Olive gray calcareous volcanic sandy siltstone Late Pliocene "16 (N, Pl (Miocene-Pliocene) (P) 604B 1602? Very pale brown clayey nannofossil chalk with Early Miocene "3 (NI, Late Miocene-Recent (Middle foram and spicule Oligocene) (P), Late Miocene-Pliocene (O) 604C 1602? Pumice boulder ? 605 1590? Olive gray calcareous volcanic sandy siltstone Quaternary (Latest Miocene "1 1) (NI, i. Late Pliocene to Recent (Late Miocene) (PI 606 1590? Serpentinized lherzolite ? 607 1240? Gray sandy biograinstone (rhodolithe) Early Miocene (B)

Dioe 7 70 1 2258 Dark olive gray calcareous silty clay Late Pleistocene "21 (Mio-Pliocene) (N, P) 702 2213 Dark olive gray volcanic sandstone ? 703 2000 Gray volcanic sandstone ? z 704 1974 Grayish brown calcareous volcanic siltstone Late Pleistocene-Holocene "20-21 (Mio-Pliocene) (N) 705 1974 Phyric andesite from a conglomerate 16.6-17 0.8 Ma (K/Ar) 706 2000 Olive gray calcareous silty clay with chalk Late Pleistocene-Holocene "21 (N, P) (Oligocene, and volcanic clasts Pliocene) (N) 707 2000 Dark gray volcanic sandstone ?

a K/Ar ages (fide H. Bellon); N planktonic nannofossils (fide C. Muller); P planktonic foramminifers (fide G. Glaçon); B large benthic foraminifers (fide J. Butterlin); Ca: Camerinidae (fide A. Blondeau), O: Ostracods (fide J. F. Babinot).

166"31E 166"32E DIVE 1 112 -1700111 V.E. :i.O N=+ 1' 111 - 1800 1Y265

106 - 1900 .

O 0.5 1 km

Volcanic sandstone Lavas Volcanic breccia ,...... Sandy dayey sediment Fig. 3. Interpreted geologic cross-section of a scarp of the New Hebrides arc slope along the south flank of the Wousi bank (dive 1); location in Fig. 2; inset is Nautile traverse; arrows with numbers indicate location of samples described in Table 1. r 214 J.-Y.COLLOT ET AL the south flank of the Wousi hank. During dives Tlie uppcr arc slope erist of the NDR 7. 3 and 4 the toe of the arc slope was explored Dive 1 began on a subhorizontal platform and east and north of the NDR (Fig. 21. rose along a steep (30"). south-facing slope (Fig.

TABLE 1 Description and biostrstigraphy of samples dredged along the d'Entrecasteatn Zone. Go 314-315 are from the GEORSTOM 111 Nord (1975) cruiw: D1. DZ. D3 are from the SEAPSOl (1W) cru¡%: for legend see Table 1 ..

Dredge Sample Depth Description Age ' 110. no. (m) North d'Entrecasteaux Ridge. Go314 DIO Light hrown calcareous clay Late Pleistocene NN?I (N. P) Go315 D15 Light hrown foraminiferal clay Late Pliocene NNlh (N, P)

Bougainville guyot DI I\ Volcanic breccia B Olivine porphyritic hasalt I c Olivinc pnrphyritic lmalt D Gray volcanic microcnnglornerate E Porphyritic hasaltic andesite , F Olivine porphyritic habalt G Microcryst;illine basalt H Porphyritic tiasaltic niidesite I5.(K1 f (1.8 (K,;,-\r) (altered sample) I \'olennic breccia. chalhy matrix Uppermost hliddle EOCCII~NP17 (NI J Volcanic. c:trhonste cong1nmer;ite Middle Oligocene (N),Late (3ligocene- karly hliocenr (H) K Volcanic hrcccia. carbnn,ìte matrix , L Carbonate hreccia Uppermost hliocene-Early Pliocene (ß). Late hficidle Oligcicene (NI. Late C)ligocene-E:trly Pliocene (PI hi Scleractinian (Leptastrea J Oligocene to Recent

D' A Gras white nannufmil foram I :ìle Pleizlocenc NN2.11 (N.PI chalk and clay B1 Gras white iiaiinotcwil fnram Lntr Fleistocene NNIO (N. PI. (Lrite Eacene- chalk and clay Early Miocencl CP) I32 Gray white nannofossil fiiram Late Pleibtocene "11) (N. Pl chalk and clay c Gray beddcd chert , D Grrenish lirown silt! cliiy L..ite Pleistocene "31 (N. P) E Cord fragment F Coral fragment (!'I;ìt>gyra) Eocene to Recent ci Recf packstone Lile )Iigoccnc-E.trl> hlioceno (ßl I1 Coral fragment , 1 Reef packstone Late Fticcne to Recent (BI .I Frcsli corrtl tragments Eiicene tc~Recent

D.? :\ tircian silty clay and nercric foram R Reef prrcbtone with h;tlimctl;t

c Reef grainstone i p.ickstcine D Reef p:icLstulle E Reef grsinstonc pirt-hhtcinc

E; Ar ages (fide H. ßc1lc.m): N: plmktonic naniintii~ds(fide C. tiliillcrl: 1': phnktonic tor:imminifer~(t'ide i. Glaçiinl: R: I:rrge herithic foruninifcrb (fide J. Hutterlinl

- GEOLOGY OF THE d'ENTRECASTEAUX-NEW HEBRIDES ARC COLLISION ZONE 225

3). The platform is covered with mud, sand and The contact zone and the toe of the arc slope transported boulders of andesite, coral, reefal aboue the NDR limestone and volcanic breccia, together with The contact zone between the arc slope and wood and land-born vegetation debris. These ob- the summit of the NDR is marked by a west-fac- servations suggest debris flow deposition. The ing, l-2-m-high scarp that trends N155" and ex- south-facing slope shows a series of flats inter- poses slightly indurated clay (dive 2, Fig. 2). At spersed with southward-dipping sedimented this location, the sea floor over the toe of the arc slopes, and 40-70" steep scarps that trend N20- slope dips 25-45" W and is blanketed by a frac- 90". Rock outcrops along the scarps reveal frac- tured, silty clay of Pleistocene to Holocene age tured volcanic breccia, volcanic sandstone and (Table 1). lava. Several outcrops show large blocks (1-2 m) About 4 km east of the contact zone, a major of lavas in a chaotic volcaniclastic mass that sug- 250-m high, north-facing scarp reveals the inter- gests olistostrome deposits; however, some mas- nal structures of the arc slope above the sub- sive outcrops of lava (Plate 1-11 could represent ducted part of the NDR (Dive 3, Fig. 4). Obser- coherent lava flows that are interbedded with vations made during dive 3 indicate that the scarp volcaniclastic layers. Most these lavas are domi- towers above a gently north-dipping seafloor that nantly plagioclase-rich, one or two pyroxene, is incised by N20-4W-trending channels. This sea phyric andesites; island-arc basalt tholeiite and floor consists of stratified clayey siltstone that is dacite fragments were also recovered from the overlain by a debris cone. The major scarp slopes breccias (Table 3). 0ne.andesite (111, Table 1) 25" north and has walls as steep as 70-80". was radiometrically dated and yielded a Late Exposed at the scarp are Late Pliocene to Miocene age. Massive lava and volcaniclastic beds Quaternary, calcareous, deep-water rocks that locally dip 30-40"NE and strike N120", and are have a volcaniclastic component; these rocks are affected by N30-50"-trending fractures that dip interbedded with volcanic breccias (dive 3, Table 20"N to subvertical. 1). A clast of island-arc tholeiite (sample 318 in

Ififi"l9F 166'20

4.

Volcanic silty clay to Silty clay nanno foram mixed clayey siltstone El sedimentary rock Volcanic breccia ,...... Sandy clayey sediment

Fig. 4. Interpreted geologic cross-section of a scarp of the New Hebrides arc slope above the North d'Entrecasteaux Ridge (dive 3); location in Fig. 2; inset is Nautile traverse; arrows with numbers indicate location of samples described in Table 1.

GEOLOGY OF THE d'ENTRECASTEAUX-NEW HEBRIDES ARC COLLISION ZONE 221

Table 3) obtained from a volcanic breccia, yielded older than Early Miocene. Because of the consid- an early Middle Miocene K/Ar age (Table 1) erable amount of nannofossils in >thesamples, we that is coeval with the volcanism of the western infer that Oligocene-Miocene clayey nannofossil belt of the New Hebrides island arc (Fig. 2). ooze and chalk outcrop at the sea floor (Fig. 5). Structural data show that, along the scarp, Nevertheless, a Late Pliocene sedimentary rocks are fractured and locally folded on a small melange including clasts of Oligocene-Miocene scale; these rocks strike N120" and dip 20-40"N. chalk is evident near the depth of 4800 m (409, This northward dip could have been caused as Table 1). rocks that dipped trenchward were progressively Where the lower northern flank of the NDR uplifted as the NDR crept north during the colli- enters the subduction zone, arc slope rocks dip sion. Subvertical fractures trending N120-140" arcward 45-70" and are strongly tectonized. were observed in rock outcrops and appear to Rocks are sheared and fractured (Plate 1-2h shear parallel the N120" trending strike-slip faults that zones that dip east (Fig. 5) often parallel the have been interpreted from Seabeam bathymetric bedding plane and are interpreted as thrusts. data (Collot and Fisher, 1991). Near 5050 m, a subvertical fault trending N110" is subparallel to both the N120-140" directions of Tlie toe of the arc slope north of the NDR subvertical fractures measured upslope (dive 3) The arc slope adjacent to the lower northern and the strike-slip faults interpreted from flank of the NDR has a lobate morphology (Fig. Seabeam morphologic data. Structural observa- 2) and appears to be offset left-laterally along a tions such as fresh fractures and slump scars are N120" trending strikeTslipfault (Collot and Fisher, indicative of present-day tectonic activity (Greene 1991). The lower northern flank of the NDR rises et al., 1992). Age and lithologic data, together gently (8") southward, is overlain by sand, and with structural observations, suggest that a se- scattered boulders and gravels (dive 4, Fig. 5). quence of both Late Pleistocene siltstone and The toe of the arc slope rises 650 m southeast- Early Pliocene to Quaternary mixed sedimentary ward at an angle of 18" and shows an uneven rocks is sandwiched between the Oligocene- morphology with steep (50-70") scarps. The mor- Miocene nannofossil ooze and chalk (Fig. 5). phology suggests that outcrops were sculpted by Rock outcrops frequently exhibit white miner- down slope transport of sediment (Greene et al., alized patches and veins. These mineralizations 1992). may relate to dewatering process of the accre- Rock outcrops at the toe of the arc slope tionary complex (Plate 1-3). consist of deformed, layered Pliocene to Quater- nary calcareous volcanic siltstone and mixed sedi- The Bougaiiwille guyot mentary rocks, and Oligocene-Miocene clayey nannofossil ooze and chalk (dive 4, Table 1). This The platform and the shallow part of the SE last age is questionable because -planktonic flank of the Bougainville guyot were explored foraminifers recovered from the ooze and chalk during dives 6 and 5; furthermore, three dredging provide Pliocene-Quaternary ages, whereas nan- hauls were obtained along the south flank of the nofossils from the same samples indicate ages guyot (Fig. 2).

Plate I Fractured, massive andesite exposed along the south flank of the Wousi bank (dive 1, 1954 m). Fractured and sheared calcareous volcanic siltstone at the lower arc slope against the northern flank of the North d'En- trecasteaux Ridge (dive 4, 4650 m). Sheared and veined brown ooze and chalk at the toe of the arc slope against the northern flank of the North d'Entrecasteaux Ridge (dive 4, 5103 m). The white patch may indicate mineral deposits that result from compressional dewatering. Miocene-Pliocene reef limestone on the summit platform of the Bougainville guyot; note the dissolution features (dive 6, 1480 m). AlIll 55.711 4X.SII 34,711 5tl.h.5 5 1 .ho .5.U If I 54.211 5-1.411 48.t15 42.05 46.80 11.75 0.sh I.hll (1.111 0.45 11.'45 O.% ILS7 (1.41 1.15 0.frX l1.3h IO.nl1 17.21 14.7'4 1.1 I 17.41 I 17.75 1'4.42 1h.S' 17.35 I tl.55 15.78 11.47 s Ïx 7.77 lII.tlX 3.24 5.14 X.XII 7.77 7.47 1.58 q.1q X.2'4 7.77 0.1.5 11.1' 11.15 l1.(14 (1.1 I') 11.13 11.12 11.1 1 (1.117 11.1 1 0.14 11.1 I 4.Xh ?.hl tl.75 35.10 2.117 3h4 3.54 3.77 3.115 2.78 3.4h 1 -1.irii S.5.i 7.47 0.117 11.57 5.w 7.40 9.12 kXï 7.20 4.146 1 h.lN s.7ii 3.(Jtj .i.71 2 81 11.117 4.11'4 3.711 3.78 3.fll 4.68 4.3 2 70 1.71 11.3h I 27 (I 71 11.114 1.3s O.hl 11.34 0.h2 11.45 0.511 (1.63 i1.2h 11.114 11.15 ll.Il1 11.1 Il I 11.115 O.Ill 11.15 11.15 11.15 1.70 11.4Il o. 1 Il 2.711 1.311 -.-7 -l7 lLfl5 4.27 3.87 1.0s 3.411 1.114 1 .cl7 7.55 3.58 11-72 I ).'ln 1.25 l.lQ 1.7h 1.17 I.(lÏ l.hl l..il 2.10 1 .?X 3.47 __ - - - - iï4m m W.58 m m q7jr-77 I oms m 'lY.1tY '49.25 44.14 u').33

1.411 2.3 I 1.23 1l.lh I .íl2 1 .h7 1.711 I .53 1.49 2.h4 I .74 I .Sh 11.43 52.V 411.5 5fI. 1 S'4.tl 41.lJ 45.h 4SSl 511.0 51.4 37.2 -17.4 453 78.5

LI 11 7 IO 12 41 IO I' 37 1h 14 13 Rh h x 7 23 I' 1 11 7 4 h x SI 171 5.30 4hh Ahï 144 554 43 4 41 Il I 448 3ltl

H 'I 711 I 3s 35 "1 I .Cl IO Ill5 1 lh 75 Xi SI1 7s- 115 \ -, I '37 21 17 lYll 274 21 '3 I4 210 21 IS &_ 212 c I 45 2( I I5 -7 14s 2'51 I y Ill 21 Ill 272 c Il 30 21 27 24 514 X e1 14 25 41 311 1X NI 31 21 2s 23 '44 '11511 23 21 'Il 21J 1 fl2 c LI 71 13s tl'l 5.1 4' III 41 53 tll Al4 S4 Zn 72 XS 57 x2 h3 35 td 85 tlh 1117 XI \e' 3'4.4 3x.5 '2.8 175 3fl.(l 11.1 24,s 25.b 21.V 2'4.4 28.3 ? l(J.0 2'4.4 1 f1.N 3.3 34.4 11.1 18.3 24.'4 '11.11-- 27.3 17.1 Zr 41 1114 41 (J(j S.¡ 4 44 71 13 1115 511 Nh 1.2 2.1 1 .Il 3.5 1.1 1.11 1 ..I 31 4.' I (1 1.11

1 I \ - I.- I .I '? GEOLOGY OF THE d'ENTRECASTEAUX-NEW HEBRIDES ARC COLLISION ZONE 229

166'15E 166O16E

Clayey nannofossil ooze and chalk Q/ calcareous volcanic siltstone

r Melange and shear zones Silty clay nannofossil, foram m.mixed sedimentary rock ...... Sandy clayey sediment - Concentration of heavy minerals Fig. 5. Interpreted geologic cross-section of the toe of the New Hebrides arc slope north of the North d'Entrecasteaux Ridge (dive 4); location in Fig. 2; inset is Nautile traverse; arrows with numbers indicate location of samples described in Table 1.

Close to the contact zone between the arc and Fig. 6). In contrast, the upper SE flank of the the guyot, the platform of the guyot gently dips 8" guyot (dive 5; Fig. 7) dips uniformly 27" eastward. arcward and shows a 150-m-high bump (dive 6; Limestone outcrops discontinuously between .7 166Y3E 166"44 15'58 SmQ

DIVE 6 V.E. :1.0 Fgoom1000 m

Slum€

Guyot's platform I I I 1 1700 \ 170n

L Volcanic clayey Siltstone to sandstone Whitish VolCaniclastic &bris flow Synsedimentaty folds 14;1 Calcareous volcanic sinstone Foraminiferal packstone Cross stratification -. Coral Limestone - Sandy clayey sediment Clayey nanno chalk Fig. 6. Interpreted geologic cross-section of the toe of the New Hebrides arc slope that towers above the platform of the Bougainville guyot (dive 6); location in Fig. 2; inset is Nautile traverse; arrows with numbers indicate location of samples described in Table 1.

J

GEOLOGY OF THE d'ENTRECASTEAUX-NEW HEBRIDES ARC COLLISION ZONE 231

1450 m on the platform of the guyot (dive 6) and ville guyot is an island-arc volcano (Fig. 8). Only 2350 m along its SE flank (dive 5). Massive sub- one sample (DlH, Table 2) was tentatively dated, horizontal limestone beds are exposed to a height and yielded a 15 Ma K/Ar minimum age. This over of 100 m along the eastern slope of the age appears very young when compared to the bump (Fig. 6). At the upper SE flank of the guyot Middle Eocene age of the volcanic breccia recov- (Fig. 7), rare limestone beds, which are 10 cm to 1 ered from the guyot, although the 15 Ma age m thick, dip consistently 20-30" parallel to the might indicate late guyot magmatism. slope. Reef limestones (Plate 1-41 and fragments of The arc slope in front of the Bougainville guyot coral colony, studied by Montaggioni et al. (1991), attest to a previous reef environment on the We first present the geology of the arc slope guyot during the Late Oligocene to Early Miocene 15 km north of the indentation (dive 7). Then we and Latest Miocene to Early Pliocene (Tables 1 summarize the geologic features of the contact and 2). Moreover, strong evidence for diagenetic zone and focus on two closely spaced cross-sec- alteration of and reef limestones indicates tions (dives 5 and 6) that illustrate the lateral that the guyot was emergent in the past (Montag- variation in the geology and tectonics of the gioni et al., 1991). Bougainville guyot-New Hebrides island arc col- The oldest rock recovered from the Bougain- lision zone. ville guyot was dredged along the guyot's south- ern flank (DlI, Table 2). This rock, dated from The arc slope north of the guyot's indentation the uppermost Middle Eocene (40-42 Ma), is a At the site of dive 7 (Fig. 21, the arc slope is coarse volcanic breccia with basaltic clasts and a smooth and dips 25"W. This slope is cut by shal- white chalky matrix. The basaltic clasts appear to low (10 cm-3 m) gullies that trend N50-60" and be mineralogically identical to the samples of by 10-m-high scarps that trend N140". The flanks porphyritic island-arc tholeiite D1B and D1C of the gullies reveal well stratified, 1-20-cm-thick (Tables 2 and 3). Other clasts found in the con- layers of volcaniclastic rocks that dip 20-30" glomerate D1J and the breccia D1L (Table 2) trenchward (Plate 11-5, Fig. 9). These rocks, which indicate deposition of nannofossil ooze on the were deposited during Late Pleistocene and guyot's flank during the Middle Oligocene. Holocene times, show evidences of clasts of lavas All igneous samples dredged at site D1 (Fig. 2, and nannofossil chalk, and reworked Oligocene Table 2) can be grouped into moderately por- and Miocene-Pliocene foraminifers. An andesite phyritic olivine basalt, microcrystalline basalt and clast recovered from a volcanic conglomerate (705, highly porphyritic basic andesite. Geochemical Table 3) yielded an Early-Middle Miocene radio- analysis suggest that these igneous samples have metric age (Table 11, the same age as volcanic an island-arc origin and include three main com- rocks on Espiritu Santo and Malakula Islands ponents: Island-Arc Tholeiite (LAT), basic an- (Mallick and Greenbaum, 1977). Nevertheless, a desite (BA), and andesite (A) (Table 3). Hence, clast of Oligocene nannofossil chalk (706, Table petrologic data clearly indicate that the Bougain- 1) recovered from a calcareous silty clay is un-

Plate II 5 Late Pleistocene volcanic siltstone and sandstone beds dipping 20-30" trenchward along the New Hebrides Island Arc slope (dive 7, 2018 m). 6 A synsedimentary fold at the toe of the arc slope east of the Bougainville guyot; sediments are stratified calcareous volcanic siltstone and sandstone (dive 6, 1611 m). 7 Fractured fine limestones in a wedge of accreted rocks near the toe of the arc slope in the collision zone between the Bougainville guyot and the New Hebrides island arc (dive 5, 2175 m). 8 Steeply dipping layers of white limestone interc'aiated with brown sheared sediments that may indicate a zone of tectonic contact in the wedge of accreted rocks east of the Bougainville guyot (dive 5, 2076 m). 232 J.-Y. COLLOT ET AL. known on the nearby islands and might have an locations (dives 2,s and 6) along the contact zone exotic origin. probably indicate recent tectonic activity.

The contact zone hetween arc and guyot The toe of the arc slop east of the Bougainuille The contact zone between the guyot's platform SUYOt and the arc slope (dive 6, Fig. 6) lies along a The western flank of the arc slope antiform 300-m-wide that is draped with that towers above the guyot's platform dips 35" ., sandy-clayey sediment; no scree was ohserved in trenchward and has a rough morphology (dive 6, this depression. The seaward flank of this depres- Fig. 6). The sea floor shows 5-50-m-high scarps sion exhibits NE-facing, vertical scarplets that are dipping 40-80" westward, fresh slump scars and 20-40 cm in height. Along the SE flank of the NE-trending deep canyons. guyot (dive 5, Fig. 7), the contact zone forms a The western flank of the antiform exhihits 10-m-wide, flat-bottomed canyon floored by fine- spectacular outcrops that consist of Late Pliocene grained sediment. A N-S-trending, 20-cm-high, to Quaternary interbedded volcanic siltstone scarplet cuts across Recent sediment at the deep- (Plate 11-6) and sandstone (dive 6, Table 1) with est point of the canyon. Scree partially covers whitish volcaniclastic debris flows (Fig. 6). Vol- both flanks of the canyon (dive 5). Limestone canic siltstones sampled along this part of the sea blocks on the western flank of the canyon are floor contain reworked early Middle Miocene and topped with dark gray clay, whereas boulders uppermost Miocene nannofossils. similar to those deposited along the eastern flank of the canyon recovered along the arc slope near the NDR. are clay-free. This difference in clay cover sug- A few samples of sedimentary rock collected at gests active tectonics along the arc side of the the base of the flank differ in lithology from the canyon. The scarplets that were observed at three rest of the flank. Among these samples are a

166'466 166 47

2100 -

2200 -

2300 -

1 km 2400 -

Micrlic limestone Volcante clayey slitstone to sandstone B foramlniferai padtstone Coral limestone Calcareous volcanic siltstone Sandy clayey sediment Fig. 7. Interpreted geologic cross-section of the toe of the New Hehrides arc slope southeastward of the Bnugainville guyot' s flank (dive 5): location in Fig. 2; inset is Ntzrri'ilt. traverse: arrows with numbers indicate location of samples descrihed in Table I. GEOLOGY OF THE. d'ENTRECASTEAUX-NEW HEBRIDES ARC COLLISION ZONE 233

Ti/lBB Miocene-Pliocene pelagic limestone, with sponge spicules (602A, Table 1) that suggests a deep- water, fore-reef environment, and Late Oligocene to Early Miocene nannofossil chalk and lime- stone (603B and 604B, Table 1). These rocks are very hard, and show shear distortion and an in- cipient foliation that indicate tectonic deforma- tion. A pebble of basalt (603A, Table 1) was sam- pled from the whitish debris flow at the toe of the antiform; this basalt has a MORB affinity (Table 3) that contrasts with island-arc lavas sampled near this area. The source origin of both this

vvvvvv"vv sample and the deformed calcareous samples will Zr Sr/2 be discussed further below. Fig. 8. Compositions of volcanic rocks from the d' Entre- The oldest age determined from rocks any- casteaux Zone region reported in a geotectonic triangular plot where along the arc slope within the study area is (Pearce and Cam, 1973). Delineated fields are: A = island-arc late Middle Eocene (46-41 Ma). This age is based tholeiite; B = calc-alkali basalt; C = ocean-floor basalt (MORB). Symbols are: closed triangle = samples from the on Globigerinatheka subconglobata euganea, which New Hebrides arc slope (SUBPSO1); the closed triangle in is reworked in a Late Oligocene reef packstone field C is sample 603A (Table 3); open triangle = samples (603C, Table 1). from the Bougainville guyot (SEAF'SOl); open circle = Arc slope rocks that form the western flank of samples from the NDR east of 164"E (GEORSTOM 3; Mail- the antiform east of the guyot's platform are let et al., 1983); cross = samples from the NDR west of 1WE (GEORSTOM 3); asterisk = DSDP 286 basement (analyses tilted wholly arcward and show numerous struc- from Stoeser, 1975). tures such as folds, slumps and cross-stratifica-

166"41E 166'42E

Volcanic sandstone Calcareous volcanic silty clay to clayey siltstone Volcanic breccia and conglomerate __..,... Sandy clayey sediment

Fig. 9. Interpreted geologic cross-section of a possible slump scar at the New Hebrides arc slope, north of the Bougainville guyot (dive 7); location in Fig. 2; inset is Nautile traverse; arrows with numbers indicate location of samples described in Table 1. 234 J.->. COLI CrT ET AL. tions. Within the lower half of the section (Fig. Rocks of the lower two-thirds of the arc slope 6). the rocks dip monoclinally 30-60" arcward, explored during dive 5 itre highly tectonized. The whereas rocks forming the upper half dip alter- bedding strikes primarily NW-SE and dips vari- - nately arcward and seaward. Either tectonic com- ably between 50 and S0"NE. These rocks are pression or slumping of rocks could account for fractured and show discrete zones of intense the apparently folded layers. Although no clear shearing that are interpreted as reverse and thrust evidence for shearing was observed within the faults (Plate 11-8. Fig. 7). Samples of micritic deformed sedimentary mass, faults between lay- limestone and reef packstone hear the mark of - ers of opposite dip cannot he excluded. However. folding and stretching. Two groups of subvertical numerous folds ranging in size from 10 cm to faults and fractures that trend respectively NN" several meters affect the arcward dipping layers and E-W suggest conjugate faulting. The trends of the lower half of the section (Plate 11-6). The of faulting are consistent with a maximum hori- attitude of these folds suggests that they are zontal compression oriented NM"E k lo", synsedimentary structures that formed prior to whereas the plate convergence direction is N76' . the eastward tilting of the sedimentary mass. We 4 11" (Isacks et al.. 1983). Horizontal striations suggest that these folds developed íis sediment on a vertical fault plane (slickensides?) suhstanti- deposited and slumped on a paleosurface that ate strike-slip motion along the fault. The tec- dipped southwestward. tonic deformation strongly decreases above 1850 m in the upper third of thc section. Thc m-í-slops SE of the Borrgoinrdlt* giiyot Southeast of the guyot, where the antiform narrows and deepens (dive 5. Fig. 7), the toe of Discussion tho arc slope is severely eroded and exhibits the deep structures of the antiform. At this location, the sea floor dips as steeply as 43'' west. flit. Boiiguinrdle guyot: ci Midrllt. Eocene isliinti- The upper third of the cross-section consists of arc i deuno . well-stratified, volcanic siltstone that was de- The dredged volcanic breccia DI1 (Table 2) posited during Late Pleistocene and Holocene shows that the Bougainville guyot formed as an times. Most of the layers dip WE. but near the island-arc vo1c;ino prior or during the latest Mid- summit of the rintiform they dip 30"s. dle Eocene. Corch collected at DSDP site 286, 50 C;iIcareous rocks (Plate 11-71 outcrop in the km SW of the guyot, revealed an uppermost lower two-thirds of the section. Locally. these Middle Eocene andesitic cunglomerate that over- rocks arc in tectonic contact with reddish-yellow lay$ Middle Eocene vitric andesitic siltstone and rocks of uncertain origin. Nevertheless, a sample sandstone that was deposited on the basaltic of volcanic and foraminiforal sandstone (505, oceanic crust (Andrews et d., 1975: Stoeser. Table 1) collected from iì scree slope (Fig. 7)may 1075). Multichannel seismic reflection data that represent these reddish-yellow rocks. This sand- tie the area of Site 280 to the South d'En- stone formed in a reef environment during the trecasteaux Chain (SDC) show that stratas. in- Late Eocene to Early Oligocene (Montaggioni et cluding the andesitic conglomerate. thicken from al.. 19911. The calcarecxis rocks include. very hard. this site toward the SDC (Fisher. 1986: pp. ' micritic limestone (504 and 506. Tahle 1) and reef 10.379-10.380) suggesting that these rocks were packstone (507 rind 508. Table 1). Similar to the derived from the SDC. Hence. the breccia reef packstone collected on the guyot. sample 50s dredged :ìt the Bougainville guyot. together with

shows evidences for meteoric diagenesis (Mon- the scqurncr drilled at DSDP Site 286, suggest 3- taggioni et al.. 1WI). Reef prickstone 507. which that the guyot formed during the Middle Eocene. includes reworked Late Eocene large benthic The petrology and Middle Eocene age of the foraminifers. is of Late Oligocene to Early Bougainville guyot imply that when the North Miocene age (Montaggioni et al.. 19'11 1. Loyalty basin was an active marginal basin (Weis- - GEOLOGY OF THE d'ENTRECASTEAUX-NEW HEBRIDES ARC COLLISION ZONE 235 sel et al., 1982; Andrews et al., 1975), island-arc determined from volcanic rocks in SE Malakula volcanism was active along its northern edge. island (Macfarlane et al., 1988). Andesite and This volcanism occurred in conjunction with a island-arc tholeiite clasts sampled from conglom- possible south-dipping subduction zone that ex- erates and breccias along the arc slope (samples tended along the north side of the NDR (Burne 318 and 705, Table 3) could also have been de- et al., 1988). The SDC grew on the DEZ'S crust, rived from the New Hebrides island arc. These which is scarcely thicker (12-14 km, Collot and samples yielded an Early-Middle Miocene radio- Fisher, 1988) than that of the North Loyalty basin metric age (Table 21, which is consistent with the (10-12 km, Pontoise et al., 1982) and is not Early and lower Middle Miocene island-arc vol- typical of a well developed island-arc crust (24-25 canism which is known to have occurred on Es- km, Ibrahim et al., 1980). Therefore, the SDC can piritu Santo and Malakula islands (Mallick and be regarded either as an island arc that aborted Greenbaum, 1977). early or as the poorly developed western end of a A microlitic basalt with MORB affinity col- mature island arc that is now subducted beneath lected at the toe of the arc slope near the plat- the New Hebrides arc. form of the guyot (603A, Table 3, Fig. 8) may Montaggioni et al. (1991) used the facies and substantiate the hypothesis of accretion from the biostratigraphic analysis of the carbonate samples downgoing plate. Indeed no a such type of basalt to suggest that during the Middle and Late is known on the New Hebrides Island Arc, but Miocene the deposition of reef limestones was the basement of the DEZ is MORB (Fig. 8). interrupted by a phase of low sea level or cooling. Although it is likely that this basalt was accreted The deposition of reef limestones on the guyot from the oceanic crust of the DEZ, the 9.4 Ma appears to have ceased some time after the Early K/Ar age of the sample, which is a minimum Pliocene. This cessation is in agreement with the age, does not appear realistic with respect to the recent drowning of the guyot, which was trans- 36-56 Ma age of the DEZ basement (Maillet et ported from the sea level down to the trench as al. 1983). suggested by the models of Dubois et al. (1988). A lherzolite fragment (606, Table 3) recovered at the base of the arc slope east of the Bougainville guyot (dive 6) is of uncertain geo- Geology of the forearc slope offshore Espiritu Santo graphic origin. The only peridotites known on the island New Hebrides islands are exposed along the east- ern belt, on Pentecost island (Mallick and Neef, Four major groups of lithologies were identi- 1974). This fragment could have been transported fied along the forearc slope west of Espiritu Santo from the eastern belt area along a sediment path- island, and consist of (1) lavas and peridotite, (2) way that was later closed off by the uplift of the volcaniclastic rocks and sediments, (3) nannofos- western belt. Alternatively, the location of the sil ooze and chalk, and (4) lagoon and reef lime- lherzolite sample on the forearc slope could indi- stones. The first two groups include lithologies cate that the two converging have that mainly originated from the New Hebrides been deeply affected by the tectonic effects of the island arc, whereas the other two may have been collision; in this respect, the sample could have accreted from the downgoing plate, as discussed been evenly thrust up from the deep crust of the further below. downgoing plate or from that of the western belt. Massive andesites and andesitic basalts en- Pliocene-Quaternary volcaniclastic rocks and countered in the upper part of the arc slope (dive sediments appear to be locally important in volu- 1) probably represent the volcanic New Hebrides metric terms, because they were observed over arc basement. However, the unique 7.3 Ma K/Ar hundreds of meters of thickness at all of the dive date obtained from an andesitic basalt has no sites. The composition of these rocks and sedi- equivalent on Espiritu Santo island, but this date ments suggests that they derive from the nearby is similar to the 10.7-7.5 Ma ages that were island arc. For example, volcanic sandstone en- ??b cases island-arc volcanic clasts and results locally ooze and uppermost Eocene chalk cored íit DSDP from an almost pure accumulation of island-arc Site 286 (Andrews et al., 1975). The deep-water volcanic debris (301 and 703, Tables 1 and 3). rocks and sediments collected at the arc slope The Pliocene-Quaternary volcaniclastic rocks and at Site 286 both contain sponge spicules and and sediments include early Middle Miocene a small amount of volcanic silt. Although the nannofossils (samples 304A, 31 1 and 602B, Table Early Miocene(?) abyssal red clays cored at Site 1 1) and latest Miocene clasts of chalk (sample 306) 286 were not recovered at the toe of the arc that could either be reworked from the island arc slope, we suggest that the Middle Oligocene to .i or from sediments that were accreted from the Early Miocene deep-water sediments were ac- downgoing plate. However, these Miocene nan- creted from the Australian plate to the arc slope. nofossils and clasts of chalk are difficult to relate Reef and micritic limestones are the major to sediments from the downgoing plate, because components of a 500-m-thick wedge accreted at DSDP site 286 most of the Miocene is absent within the arc slope antiform east of the and only questionable Early Miocene abyssal red Bougainville guyot (dive 5). This wedge also in- clay is present (Andrews et al.. 1975). In contrast, cludes minor volcanic siltstone and sandstone that the reworked early Middle Miocene nannofossils was tectonically emplaced hetween thrust sheets are coeval with a thick greywacke formation that of reef and micritic limestones (Fig. 7). A Late outcrops on Espiritu Santo island (Macfarlane et Oligocene to Early Miocene reef limestone (507) al.. 1988) and the latest Miocene clasts of chalk collected within this wedge is equivalent in age resemble the Late Miocene to earliest Pliocene and litholobr to those dredged from the guyot, calcarenite exposed on this same island (Fig. 2; but the limestone from the wedge could also be Mallick and Greenbaum, 1977; Carney et al., correlated with Late Oligocene to Early Miocene 1985). Therefore the miocene microfossils and lenses of reef limestones exposed on the western clasts reworked in Pliocene-Quaternary volcani- belt of islands (Mallick and Greenbaum, 1977; clastic rocks and sediments could be derived from Mitchell, 1966, 1971). However, the reef lime- the island arc. stone recovered from the wedge includes re- .. The total absence of reworked late Middle to worked Late Eocene large benthic foraminifers, early Late Miocene nannofossils in rock samples indicative of reef material similar to that discov- that contain early Middle ("4-5) and latest ered about 200 km further east, on Maewo island Miocene (NN 1 1 nannofossils (Table 1) is consid- within an Early Miocene conglomerate (Coleman, ered significant of a hiatus that extended between 1969). This reef material may have been trans- 14 and 8 Míì. In this hypothesis the hiatus roughly ported from Maewo island to the forearc prior to correlates in time to the 11-8 Ma uplift and the uplift of the western belt. This hypothesis is erosion phase that affccted the western belt (Car- unlikely. since these reefal debris are not present ney et al., 1985. Miicfarlane et al., 1988) and to in the western belt. Such Late Eocene the Middle-Late Miocene hiatus recorded at foraminifers were not recovered from the DSDP Site 286 (Andrews et al.. 1975). Bougainville guyot either. However, a Middle Clayey nannofossil ooze. chalk and limestone Eocene age was determined from the guyot (DlI, of Middle Oligocene to Early Miocene were en- Table 2) and the toe of the arc slope (603C. Table countered rit the toe of the arc slope, north of the l), but is unknown on the New Hebrides island NDR (dive 4) and to a lesser extent east of the arc. Hence, because of equivalent Late Oligocene Bougainville guyot (dive 6). These sediments lo- to Early Miocene reef limestone and similar Mid- cally contain Eocene nannofossils and differ from dle Eocene age found on both the guyot and the the uppermost Oligocene to Lower Miocene, toe of the arc slope, and because the wedge of coarse volcaniclastic deposits exposed on Espiritu imbricated reef limestone was only encountered Santo island (Mrtllick and Greenbaum. 1977). In in front of the guyot, we suggest that this wedge contrast. these deep-water sediments are similar accreted from the guyot or ;I previously sub- in age and facies to the Oligocene nannofossil ducted one. GEOLOGY OF THE d’ENTRECASTEAUX-NEW HEBRIDES ARC COLLISION ZONE

Guyot Trench Island arc slope

. a .

4 -- W Reef limestone . Volcanic core of the guyot Sedimentary apron Arc slope deposits

Fig. 10. Schematic diagram showing the early stages of subduction of a guyot. The heavy line is the inter-plate décollement. (a) The arc slope shortens by internal thrusting. (b,c) Limestones of the guyot’s cap and sediments of the debris apron that flanks the guyot are thrust up and form an imbricated wedge beneath the arc slope; this wedge tends to smooth the slope breaks of the leading flank of the guyot. (d) Insert showing the imbricated wedge accreted to the arc and overthrusting the guyot’s cap; this situation was not observed during the dives. (e) An alternate situation interpreted from dives 5 and 6 the décollement shifted upward from the guyot’s flank to the top of the wedge, so that the imbricated wedge does not accrete to the arc slope; this process would deform the guyot into an elongated feature and facilitate its subduction.

1 (1 í%e early stage of collision between a seamount or commodates the ,earlx stages of seamount or ridge a ridge and arc subduction (Fig, .lo). For example, where the The models of arc-seamount collision pro- lower northern. flank of the NDR enters the a posed by Lallemand and Le Pichou (1987), Ya- subduction (Fig. 51, highly sheared and upturned r mazaki and Okamura (1989) and Van Huene and rocks and sediments with fresh fractures suggest Lallemand (1990) predict that, in the early stages active shortening of the arc slope by collision.

h of the collision, compressive thickening affects Fresh slump scars and steep slopes incised by . the toe of the arc slope. Geologic data collected deep canyons ,support active erosion along the in the DEZ-arc collision zone support this model deformation front. and emphasize mechanisms such as shortening A mechanism that facilitates seamount sub- h and tectonic erosion, by which the arc slope ac- duction is offsetting of the seamount by normal 238 .I -Y COLLOT ET AL . faults induced by flexure of the oceanic plate flection data collected across the arc slope, east hefore subduction (Fryer and Smoot. 1985, Bal- of the guyot, image thrust faults that cut through lance et al.. 1989: Pelletier and Dupont, 1990). highly reflective rocks with parallel bedding, sug- * For example. the Daiichi-Kashima seamount that gesting that sedimentary layers may have been is subducting in the Japan trench is split into two stripped from the debris apron of the guyot and halves hy a 1600-m-high normal fault (Kobayashi incorporated into the imbricating layers (Fisher . et al.. 1987: Cadet et al.. 1987). Although the et al.. 1991h). Sandstone pinched behveen the . Bougainville guyot is analogous to the Daiichi- accreted limestones (Fig. 7) may be evidence for Kashima seamount in volume and position with sediments stripped off the guyot apron. although respect to the trench, the guyot is not dissected the sandstone may be derived from the arc. De- by such a large normal fault. Fisher et al. (1991hL spite this uncertainty, material accreted east of 8 drawing an analogy between features of glacial the guyot forms imbricating layers that tend to origin and the subduction of the Bougainville smooth out slope breaks in the leading flank of guyot, suggest that the guyot's subduction may he the guyot (Fig. 10c). facilitated by an eventual evolution of the high- Whether the imbricating layers joined rocks of drag shape of the guyot into a more streamlined the island arc or became plastered against the shape during the collision process. One feature of leading flank of the guyot and now move with it the high-drag shape of the guyot that is eventu- remains a critical problem (Fisher et al., 1991b). ally smoothed during the collision is the steep Conceptually, if the 500-m wedge of imbricating (40-45"). 700-m-high scarp that hounds the layers that outcrops southeast of the guyot was guyot's carbonate cap and towers over the gently accreted to the arc slope, the wedge should have sloping (5-10") lower flank of the guyot. The heen thrust up along the décollement until the nature and extent of smoothing and deformation wedge overthrust the carbonate cap of the guyot caused by the collision depends on the contrast in (Fig. Iod). This cap extends for about I km east- compressibility between guyot and arc rocks, as ward beneath the antiform, ;ìS indicated by multi- discussed by Fisher et al. (1991b). Geologic data channel seismic data (Fisher et al., 199lh); never- .. presented in this report support the shape evolu- theless. observations made during dive h (Fig. n) tion of the guyot during the collision. as discussed do not show evidence for imbricating layers ex- hereafter. posed at the western flank of the antiform but, Geologic samples and observations collected rather. show steep. several hundred meters thick during dive 5 (Fig. 7)indicate a 500-m-thick wedge outcrops of layered volcanic sediments derived of accreted reef limestones that imbricated within from the arc. However. the few sheared calcare- the arc slope antiform, east of the guyot. Exami- ous rocks that were sampled (602A. h03B and nation of rock deformation at the location of dive h04B, Table 1) near the contact zone between the 5 shows that the intense tectonic deformation arc and the platform of the guyot suggest that that affects the imhricating layers strongly de- minor (10 cm-1 m(?) thick). imbricating layers creases near 1850 m water depth. in the well- could he buried beneath sandy-clayey deposits layered, calcareous, volcanic siltstone and sand- and pinched at the very toe of the arc slope. stone of the upper third of the observed section. Therefore, the absence of a thick wedge of imhri- This upward decrease of the intensity of defor- criting layers overthrusting the guyot'$ cap can be mation suggests that the major thrust fault. i.e. interpreted to suggest that the imbricating layers . the inter-plate d&ollement, is located near the plastered against the lcading flank of the guyot base of the imbricating layers. Hence. the reef and. conversely. that the dCcollenient migrated limestones that probably derived from the guyot's upward from the guyot's flank to the top of the rh cap were probably scraped off und thrust up the wedge (Fig. lue). In this interpretation. the plas- guyot's leading flank, dong the inter-plate décol- tered wedge of imhricating layers deforms the lement, as the guyot enters the subduction zone guyot into an elongated feature and hence should * (Figs. 103-cl. Moreover, multichannel seismic re- fxi1it:ite its subduction. Sandbox experimental GEOLOGY OF THE d'ENTRECASTEAUX-NEW HEBRIDES ARC COLLISION ZONE 239 modeling (Malavieille and Calassou, 1992) sup- limestones probably accreted from the guyot or a ports the idea of a wedge plastered between arc previously subducted one. This wedge that accu- and seamount that tends to smooth the décolle- mulates ahead of the guyot beneath the lower arc ment. slope rocks can be interpreted as moving down- ward with the guyot, and thus facilitating its sub- Conclusions duction by streamlining.

Observations and geologic samples collected in Acknowledgements the collision zone between the New Hebrides island arc and the DEZ provide constraints about the age and lithology of the Bougainville guyot We gratefully acknowledge the captain and and forearc slope rocks, as well as support for the crew of the R.V. Nadir and the pilots of the submersible Nautile for their invaluable help'dur- mechanisms that control the subduction of a ridge ing the cruise. We also thank J.M. Auzende, C. or a seamount. The Bougainville guyot consists of Blot, Uyeda and Ogawa for their scientific an island-arc tholeiite and andesite core formed S. Y. during the Middle Eocene, which was later comments and help in improving the manuscript. capped by reef limestones. The guyot has had a complex and still partially known story, References the major elements of which deduced from dredg- ing are as follows: two periods of reef limestone Andrews, J.E. et al., 1975. Site 286. Init. Rep. DSDP, 30 deposition (Late Oligocene to Early Miocene, 69- 131. and latest Miocene to Early Pliocene), which Auzende, J.-M., Lafoy, Y. and Marsset, B., 1988. Recent appear to be separated by a Middle-Late geodynamic evolution of the North Fiji Basin (southwest Pacific). Geology, 16 925-929. Miocene period of emergence. The forearc slope Ballance, P.F., Scholl, D.W., Vallier, T.L., Stevenson, A.J., in the DEZ-New Hebrides island arc collision Ryan, H., Quiterno, P., Blome, C., Barron, J.A., Bukry, zone is characterized by various lithologies and D., Cawood, P.A., Chaproniere, G.C.H., Herzer, R.H. and structures, and bears the marks of the collision. Tappin, D.R., 1989. Subduction of a late Cretaceous The upper arc slope consists of fractured Miocene seamount of the Louisville ridge at the - accretion, arc fragmentation, and accelerated subduction basement and Pliocene-Pleistocene erosion. Tectonics, 8: 953-962. trenchward-dipping, volcaniclastic deposits that Berggren, W.A., Kent, D.V., Flynn, J.J. and Van Couvering, appear to derive from onshore. The time of the J.A., 1985. Cenozoic geochronology. Geol. Soc. Am. Bull., volcaniclastic deposition coincides with the 96 1407-1418. Pliocene to Recent uplift phase of Espiritu Santo Burne, R., Collot, J.-Y. and Daniel, J., 1988, Superficial island. In the lower part of the arc slope, arc-de- structures and stress regimes of the downgoing plate asso- ciated with subduction-collision in the central New He- rived volcaniclastic sediments adjacent to the brides arc (Vanuatu). In: H.G. Greene and F.L. Wong NDR were fractured, tilted northward and eroded (Editors), Geology and Offshore Resources of Pacific Is- as a result of the collision. Above the Bougainville land &=-Vanuatu Region. Circum-Pacific Council for guyot's platform, volcaniclastic sediments were Energy and Minerals Resources, Houston, Texas, pp. 357- strongly tilted arcward; those sediments that were 376. Cadet, J. -P., Kobayashi, K., Aubouin, J., Boulegue, J., De- fractured may have been removed, by erosion, plus, c., Dubois, J., von Huene, R., Jolivet, L., Kanazawa, producing a 10-km indentation in the arc slope. T., Kasahara, J., Koizumi, K., Lallemand, S., Nakamura, At the toe of the arc slope, against the northern Y., Pautot, G., Suyehiro, K., Tani, S., Tokuyama, H. and flank of the NDR, highly sheared, Middle Yamazaki, T., 1987. The Japan trench and its juncture Oligocene to Early Miocene nannofossil ooze and with the Kurile trench: Cruise results of the Kaiko project, chalk probably accreted from the downgoing Leg 3. Earth Planet. Sci. Lett., 83: 267-284. Carney, J.N. and Macfarlane, A., 1978. Lower to middle plate. Similarly, in front of the Bougainville guyot, Miocene sediments on Maewo, New Hebrides, and their a 500-m-thick wedge of imbricating layers of Late relevance to the development of the outer Melanesian arc Oligocene to Early Miocene reef and micritic system. Bull. Austral. Soc. Explor. Geophys., 9 123-130. 240 J.-Y. COLLOT ET AL,

Carney. J.N. and Macfarlane, A., 1980. A sedimentary basin (Vanuatu): mechanical and geodynamic implications. in the central New Hebrides arc. UN ESCA€', CCOP/ Tectonophysics, 14s): 11 1-1 10. SOPAC, Tech. Bull., 3: 109-120. Falvey. D.A.. 1975. Arc reversals, and a tectonic model for the Carney, J.N. and Macfarlane, A.. 1982. Geological evidence North Fiji Basin. Bull. Austral. Soc. Explor. Geophys., 6: bearing on the Miocene to Recent structural evolution of 47-49. the New Hehrides arc.Tectonophysics. 87: 147-175. Fisher, M.A., 1986. Tectonic processes at the collision of the Carney, J.N., Macfarlane, A. and Mallick. D.I.J., 1985. The d'Entrecasteaux zone and the New Hebrides island arc. J. Vanuatu Island arc: an outline of the stratigraphy, struc- Geophys. Res., 91: 10. 470-10. 486. ture, and petrology. In: A.E.M. Nairn, Frances G. Stehli Fisher, M.A., Collot, J.-Y. and Geist, EL., 1991a. The colli- and S. Uyeda (Editors). The Ocean Basins and Margins, sion zone hetween the north d'Entrecasteaux ridge and 7A. Plenum Press, New York, N.Y., pp. 683-718. the New Hebrides island arc: part 2, structure from multi- Chase. C.G., 1971. Tectonic history of the Fiji . Geol. channel seismic data. J. Geophys. Res., 9h: 479-4495. Soc. Am. Bull., 87: 3087-3109. Fisher, M.A.. Collot, J.-Y. and Geist, E.. 199lh. Structure of Coleman. P.J., 1969. Derived Eocene larger foraminifera on the collision zone between the Bougainville guyot and the Maewo. eastern New Hebrides, and their southwest Pa- accretionary wedge of the New Hebrides island arc. South- cific implications. In: New Hebrides Anglo-French Con- west Pacific. Tectonics, 10: 887-903. dominuim. Annual Report of the Geological Survey for Fisher, M.A., Collot, J.-Y. and Smith, G.L., 1986. Possible the year 1067, pp. 36-37. causes for structural variation where the New Hebrides Collnt. J.-Y. and Fisher, M.A., 1988. Crustal structure, from island arc and the d'Entrecasteaux zone collide. Geology. gravity data. of a collision zone in the central New He- 14: 951-054. brides island arc. In: H.G. Greene and F.L. Wong (Edi- Fryer, P. and Smoot, N.C., 1985. Processes of seamount tors), Geology and Offshore Resources of Pacific Island subduction in the Miriana and Izu-Ronin trenches. Mar. Arcs-Vanuatu Region. Circum-Pacific Council for En- Gcol., 64 77-90. erby and Minerals Resources, Houston. Texas, pp. 125- Greene. H.G., Macfarlane, A. and Wong. F.L., 1988. Geology 140. and offshore resources of VHnuatu-introduction and Collot. J.-Y. and Fisher, M.A., 1989. Formation of forearc summary. In: H.G. Greene and F.L. Wong (Editors), Ge- basins hy collision between seamounts and accretionary ology and Offshore Resources of Pacific Island Arcs- wedges: an example from the New Hebrides subduction Vanuatu Region. Circum-Pacific Council for Energy and zone. Geology, 17 930-933. Minerals Resources. Houston. Texas, pp. 1-25. Collot, J.-Y. and Fisher. M.A., 1991. The collision zone he- Greene, H.G.. Collot J. -Y., Pelletier B. and Lallemand S.. tween the north d'Entrecasteaux ridge and the New He- 1997. Ohservations of forearc seafloor deformation along brides island arc: part l: Seaheam morphology and shallow the North dEntrcciisteaux Ridge-New Hehrides Island structure. J. Geophys. Res.. 06: 4.57-4478. arc Collision Zone from Nautile submersible. Init. Rep. Collot, J.Y.. Daniel, J. and Burne. R.V., 1085. Recent tecton- Ocean Drilling Program, Leg 134. College Station, Texas, ics associated with the subduction/collision of the d'En- pp. 25'3-546. trecasteawv zone in the central New Hebrides. Tectono- Ibrahim, A.K., Pontoise. B.. Latham, G., Lirue, M., Chen, T.. physics, 117: 37.5-356. Isacks. B.. RCcy, J. and Louat. R.. 1980. Structure of the Collot, J.-Y.. Pelletier, B., Boulin, J., Daniel, J., Eissen, J.-P., New Hebrides arc-trench system. J. Geophys. Res., 85: Fisher, MA.. Greene, H.G., Lallemand. S. and Monzier, 253-266. M.. 1989. Premiers rhltats des plongees de la campagne Isacks. R.L., Cardwell, R.K., Chatelain, J.-L. ßarazangi, M.. SLIBPSOl dans la zone de collision des rides d'En- Marthelot, J.-M., Chinn. D. and Louat. R.. 1981. Seismic- trecasteaux et de l'arc des Nouvelles-Htbrides. C. R. Acad. ity and tectonics of the central New Hehrides island arc. Sci., Paris. 309: 1937-1954. In: D.W. Simpson and P.G. Richards (Editors), Earth- Daniel, J. and Katz. H.R., 1981. d'Entrecasteaux zone. trench quake Prediction and International review. American and western chain of the central New Hebrides island arc: Geophysical Union, Washington, D.C., pp. 93-1 16. their significance kind tectonic relationship. Ceo. Mir. Jouannic. C.. Taylor, F.W., Bloom, A.L. and Bemat, M., 1Y80. Letts., I: 113-219. Lite Quaternmy uplift history from emerged reef terraces Daniel, J.. Collot. J.-Y., Monzier, M., Pelletier, ß.. Butscher, on Santo and Malekula Islands. LIN ESCAP. CCOP/ J., Deplus, C.. Dubois, J.. Girard, M.. Maillet, P., Mon- SOPAC Tech. Bull., 3: 91-108. jaret, M.-C., Ricy J.. Renard, V., Rigolot P. and Temakon. Kchayashi, K.. Cadet. J.-P., Auhouin, J., RouEgue. J., Dubois. S.J., 1986. Subduction et collisions le long de l'arc des J.. von Huene, R., Jolivet. L., Kanazawa. T., Kasahara. J.. Nouvelles-Hhides (Vanuatu): risultats préliminaires de Koisumi, K.,Lallemand. S., Nakamura, Y.P.. Suyehiro. K.. la campagne SEAPSO (Leg I). C. R. Arad. Sci.. Paris, Tani. S.. Tokuyama. H. and Yamazaki, T., 1987. Normal 303: 81)5-81(1. faulting of the Daiichi-Kashima seamount in the Japan Dubois, J.. Deplus, C.. Diament, M.. Danicl, J. and Collot, trench revealed hy the Kaiko I cruise, Leg 3. Earth Planet. J.-Y.. 1 988. Suhduction of the Bougainville seamount Sci. Lett.. 83: 37-766. GEOLOGY OF THE d'ENTRECASTEAUX-NEW HEBRIDES ARC COLLlSION ZONE 241

Kroenke, LW., Jouannic, C., and Woodward, P., 1983. G., Lauriat-Rage A., Vénec-Peyré M.T., Blondeau A., of the Southwest Pacific. Geophysical Atlas of Lozouet P.. Vacelet J. and Babinot J.-F., 1991. Résultats the Southwest Pacific, 1, pp. 2 sheets. des plongées SUBPSOl du Nautile: Signification Lallemand, S. and Le Pichon, X., 1987. Coulomb wedge géodynamique des calcaires de plate-forme le long de la model applied to the subduction of seamounts in the zone de subduction des Nouvelles-Hébrides (Sud-Ouest Japan trench. Geology, 15: 1065-1069. Pacifique). C. R. Acad. Sci., Paris, 313: 661-668. Louat, R. and Pelletier, B., 1989. Seismotectonics and pre- Pascal, G., Isacks; B.L., Barazangi, M. and Dubois, J., 1978. sent-day relative plate motions in the New Hebrides- Precise relocations of earthquakes and seismotectonics of North Fiji basin region. Tectonophysics, 167: 41-55. the New Hebrides island arc. J. Geophys. Res., 83: 4957- Macfarlane, A., Carney, J.N., Crawford, A.J. and Greene, 4973. H.G., 1988. Vanuatu-a review of the onshore geology. Pearce, J.A. and Cam, J.R., 1973. Tectonic setting of basic In: H.G. Greene and F.L. Wong (Editors), Geology and volcanic rocks determined using trace element analysis. Offshore Resources of Pacific Island Arcs-Vanuatu Re- Earth Planet. Sci. Lett., 19: 290-300. gion. Circum-Pacific Council for Energy and Minerals Pelletier, B. and Dupont J., 1990. Effets de la subduction de Resources, Houston, Texas, pp. 45-91. Ia ride de Louisville sur l'arc des Tonga-Kermadec. Maillet, P., Monzier, M., Selo, M. and Storzer, D., 1983. The Oceanol. Acta, 10 57-76. d'Entrecasteaux zone (Southwest Pacific): a petrological Pontoise, B., Latham, G.I. and Ibrahim, A.K., 1982. Sismique and geochronological reappraisal. Mar. Geol., 53: 179-197. réfraction: structure de la croûte aux Nouvelles-HEbrides. Malahoff, A., Reden, R.H. and Fleming, H.S., 1982. Magnetic Trav. Doc. I'ORSTOM, France, 147: 79-90. anomaly and tectonic fabric of marginal basins north of Pontoise, B. and Tiffin, D., 1986. Seismic refraction results New Zealand. J. Geophys. Res., 87: 4109- 4125. over the d'Entrecasteaux zone west of the New Hebrides Malavieille, J., Calassou, S., Larroque, C., Lallemand, S. and arc. Géodynamique, 2 109-120. Stéphan, J.-F., 1991. Experimental modelling of accre- Stoeser, D.B., 1975. Igneous rocks from leg 30 of the deep sea tionary wedges. EUG VI, Strasbourg, March 24-28. Terra drilling project. Init. Rep. DSDP, 30, College station, Abstracts, 3: p. 367. Texas, pp. 401-414. Mallick, D.I.J. and Greenbaum, D., 1977. Geology of south- Taylor, F.W., Jouannic, C. and Bloom, A.L., 1985. Quaternary ern Santo. New Hebrides Geological Survey Report, New uplift of the Torres Islands, northern New Hebrides frontal Hebrides, pp. 1-84. arc: Comparison with Santo and Malekula Islands, central Mallick, D.I.J. and Neef, G., 1974. Geology of Pentecost. New New Hebrides frontal arc. J. Geol., 93: 419-438. Hebrides Geological Survey Report, New Hebrides, pp. Taylor, F.W., Frohlich, C., Lecolle, J. and Strecker, M., 1987. 1-103. Analysis of partially emerged corals and reef terraces in Mazzullo, J.M., Meyer A. and Kidd R., 1987. New sediment the central Vanuatu arc: comparison of contemporary classification scheme for the Ocean Drilling Program: coseismic and nonseismic with Quaternary vertical move- handbook for shipboard sedimentologist. In: J.M. Maz- ments. J. Geophys. Res., 92 4905-4933. zullo and A.G. Graham, ODP Tech Note, College Station, Taylor, F.W., Isacks, B.L, Jouannic, C., Bloom, A.L. and Texas, pp. 45-67. Dubois, J., 1980. &seismic and Quaternary vertical tec- Minster, J.B. and Jordan, T.H., 1978. Present-day plate mo- tonic movements, Santo and Malekula Islands, New He- tions: J. Geophys. Res., 83: 5331-5354. brides island arc. J. Geophys. Res., 85: 5367-5381. Mitchell, A.H.G., 1966. Geology of South Malekula. New von Huene, R. and Lallemand, S., 1990. Tectonic erosion Hebrides Geological Survey, New Hebrides, pp. 1-42. along the Japan and Peru convergent margins. Geol. Soc. Mitchell, A.H.G., 1971. Geology of Northern Malekula. New Am. Bull., 102 704-720. Hebrides Geological Survey, New Hebrides, pp. 1-56. Weissel, J.K., Watts, A.B. and Lapouille, A., 1982. Evidence Mitchell, A.H.G. and Warden, A.J., 1971. Geologic evolution for Late Paleocene to Late Eocene seafloor in the south- of the New Hebrides island arc. J. Geol. Soc. Lond., 127: ern New Hebrides basin. Tectonophysics, 87: 243-251. 501-529. Yamazaki, T. and Okamura, Y., 1989. Subducting seamounts Montaggioni, L,Butterlin J., Glagon G., Collot J.-Y., Monzier and deformation of overriding forearc wedges around M., Pelletier E., Boulin J., Lallemand S., Daniel J., Faure Japan. Tectonophysics, 160: 207-229.