SLJPERFICI&L STRUCTURES AND STRESS REGIMES OF THE DOWNBO~M3 PLATE

ASSOCICITED WITH SUBDUCTION-COLLISION IN THE CENTRAL NEW HEBRIDES i 1 ARC (VANtJC\TU).

GPO Bcsx 376? &u-~t;et-ra ACT 2&01 I Acrstral i. a

it* Office de 1 a Recherczhe Scienti.f i que et T’ochni que OLrt.re-Mer,

AESTRACT

]: 1-1 .k, E! 1” p r- e t; ;I.;. j, c; r-j c:ns . sel 5lTil. c: t-e -+ 1. (A? (1:. .t j, (3 i-, prcsf i les co1 lected across t.he

canvergent plate buw-~dary i ri the central. New Hebrides Arc display

structures that can account + c;j t-. the observed absence of a deep

geotectoni c: trench, these incl ~tcle: (1) the pt-otrasi on of the

Santa-Ma1 akirl a b 1 oc 1:: at the leading edge of the overriding plate;

(2) col. 1 i si on of ridges nn t.he downgoing plate with this block;

(3) 51 Ump i ITg C3f sed i m6x-i t 5 from the insular wall of the trench,

ai-td / CJY- the tectuni c dli spl. acement C) ,J: material outward to

compfksate .f or ad-j acent indentation by co]. I iding features; (4)

compressi ve updciTT;i ng of t.he downgoing pl.ate adjacent to the zone

tnf plate C~~i~itiZCt 5 (5) and sed i ment fill.ing of incipient

trenkhez. The super-f i c i al. structures indic:ate that there are three

major z uT’\g>-s Clf CICiTli nal-lt Stl”ETSS af f ectiog thz top of the dowgoing

plate in this region. Nort.h of the North D’Entrecasteaux Ridge and

scic.itt-i 1 a I:. i t CI d e I 7 degrees LXiUt I7 !, tensi onal str-rss regimes are

r-e]. ated t. u t h e passive bending of the plate under the weight of

the downgoi ng slab. There i r_ weak coupling between the plates and

subduct i on is mainly control led by creep ., between t. h e North

D’ Entrecasteaux Ri dge and t..at.i tude 17 degrees south a

compressi unal. stress regime indicates relatively strong coupling

between t h e pl. atrs. The hi. yhest compre5si onal stresses occur at

the Nor- th D’ Entrecasteaux Fii dge and at the Bougai nvi 11 e Spur,

co1 1 i 5i on points that c a 1-i be related to 1 ocal i sed shall ow

asperities i i-i the Beq j, 0-f .i: 2: c.7 r-l e . The transitions between these

regidns of constt-at i ng stI’eE>&. regime that occur at. 1.7 degrees

south and al. cing the northern f 1 ar,iI.:: C:I+ the D’ Entrecasteaux Zone are

probably areas of cross--hat-.iz(~~;t,aI shearing.

I f

* sU~!a.cI.jyc2~7.84

INTRODUCTION

I- h e New Hebrides I s.1. and Alr C mar 1::s t h k? ~.!ndert.171~I.i~;lt.j. ng Of t hi5

’ north--eastern margin 0-f: 1: h e Indo-Austral. i an j:s 1 at e b en e a t h t. h e

young F acti vel y-spreadi rig North Fiji Basi n . Subdue t i. on is

proceeding at 10 cm/i/r along a slip vector o+ N 75 degrees (Dcnbois

and ot.her-6, 1.9’77) * A conventional assoc i at. i on OS: ocean basin,

geotectonic trench Y arc-trench -gaP, arc S and back-arc basin has

developed north of 1 at i tude 13 degrees south and south of latitude 0

17 degrees south. Between these latitudes the structure of the arc

ir, more complex (Fig. 3 ) n T’he underthrusting plate carries with it

the West Torres Massif F the West Santo basin Y andthe

D’ EIntrecasteaux Zone? rel. i ct geol ogi cal features of poorly

understood origin (Landmess;er and others, 1975)) as well as the

ocean i c North Loya.1. t.y Baki n. The is1 ands of Espiri tu Santo and

Ma1 akul a, which are composed of 01 i gocene-Mi occne volcanic

basement r 0 c 1:: 5 (Mi tchel 1 and Warden, 1971; Carney and Macf arlane,

19821 ,) +orm an ~Lrf)prgp-).t t;1. ock protruding (.i) S”, 6;: 1(” the expected

location of the trench and the arc-trench gap (Mammer-ickx and

others, 1971; Daniel arid Katz 1981; Collot and others? in press).

The act.i ve arc 1 including the islands of Sarit.a Maria (Qua) s Aoba,

. and Fimbrym, runs down the center of the deep sedimentary North and

Socrth koba basins. The eastern boundary of this basin is flanked

by volcanic i sl ands of Mi ocene-Pl i ocene age, Maewo and F’entecost

Islands (Mitchell and Warden, 1971; Carney and Macf arlane, 1982) .

Collot and Daniel (1983) and Collot and others (in press) found

t.hat the front. of. the over-r-i di ng plate, which forms the inner

(east 1 wall of t h E; trench to the north and south, is displaced

westwaipd in this c@ntraI. part of the arc &~d general I y f c51. I. ows the

i risul ar slope o-f the Sarlto-,.lYtal.ai::LIla block:. They suggest that I. oc:al

- -.7 -_ 4

sopacl. jyc2.i.G

featLi,res borne on the downgoing pl. ate in this region.

Mareover r they descr-i be large seal e compressive deformation

affecting the central New Hebrides Arc and propose a simple mode?

of the ma i n tect.onic features and stress regimes of the New

Hebrides Arc. However I the superf i c i al deformation of the

downgoing plate has not been well documented.

The absence of a deep geotectonic trench is noted on bathymetric

charts (Fig. 2, Mammerickx and others? 1.971; Collot and others, in

press) and has beer1 discussed by Daniel and Katz (1,981) and Collot

and others (i. n press) . However) Greene and others (19831 suggested

that the New Hebrides Trench in fact ox tends aCT’cJsS much Of this

.central. TO.TlE?, a.nd is on1 y absent between 15 degrees and 15 degrees

Tel esiei ami c data confirm that east facing subduction is taking

place in t h 6.2 c:e;ltral New Hebri de!5 Arc (Paxal and others, 1778;

Louat and o t h e r s Y 1982). However) this information relates to the

large seal e behaviour of the Indo-fiustral. ian plate. The tectonic

processes affecting the upper part Of the plate may not be

representative 0.f: the plate behaviour overal. I 3 and may give rise .

to very localired deformation. The purpose of this paper is to

study al 1 available seismic reflection data in an at.tempt to shed

light on the tectonic regime control 1 ing the superficial

deformation of the plates and to explain the general abscence of a

deep geatectcrni. c trench along the central New Hebr-ides Arc.

.._. 4 -

DATA

Figure 3 show’s the location of cruise t.racks along which seismic

ref.1 ect.iol-7 prufiles wet-f obtained and are used in t.hi s report.

Pub1 i shed profiles i ncl ude those obtained by Woods Hole

Oceanographic Inst i tut.i on (LuyendyC:: and others, 1974) ; the

Austradec cruise programme (Ravknne and others, 1977) ; the

Georstom cruise .programme (Daniel and others, 1977) g Gulf and

Mnbil (Daniel and Katz, 1981).

Most of: the new data pub1 i shed here come from a j 0 i n t.

ORSTOM--CCUF’/ SOF’AC cruise (GEOVAN I 1: ) uridertal::en on the PI. 0.

CORIOL.IS between 8-22 October 1982. Navigation was by fully

automat i c 5,atellite sy5t em :, and the seismic source was a water

cannon firing every 15 seconds Ibut-ne and Tiffin, 1982). These are

a~.igmfzn t ccl by several profiles made across the plate contact zone

by R.V. LEE (Greene and others, 19831 and one profile across t.he

D’ Entrecasteaux Zone made by the t? m V e KANA KE:Ot< I (Tay 1. c)r I i n

press) as part of the 1982 Tr-iparti te cruise programme (Exori,

1982).

Daniel and Kat: z (1981) concluded that the protrusion of the rigid

Santo+al a&u1 a bl ocC:: at the leading edge of the overriding plate

has prevented the development of a deep geotectonic trench in the

cent.ral New--Hebrides. However Collot and ot.hers (in press) showed

the importance Of t hE var i ous topographical features of the

sC;bduct i ng plate i n determining the nature of deformation at the

plate contact zone. We will therefore describe the superficial

struct.ures o+ these features of the downgoi ng plate (Fig. 1)

before’ analysing their deformation at the plate boundary.

i s intermediate between continental. and oceani c crust. Mini mum

water depths of 1100 m are reported by LJc;uanr~i~ (1975) and Collot

and others (in press) . Profile GEOV XOOti (Fig 4. and fSd) across the

massi f shows a sm211 F perched sedimentary basin containing about

0.4 set TWT of f au1 ted and def urmed sediments abut.tj.ng a non

1 ayered structural high to the south I) The de+ ormat i on affects both

sediments and basement 7 especial 1 y on the northern and southern

f I. an E:: s of t h e b as i 1-i . North of the basin, appruximatcl y 10 km from

the niJr.~,~;zr-n end c;.f tJy<< py-csf j, 1 e, the de.f:ormed basemfrk is capped

-by rel at i vel y undi sturbed s&i i;ierits; . The 1 uiiier- p a r t. 5 of t. he

soutl3erri f 1 an i:: at-e over1 ai I‘I by slope deposi ts wi th poor-? y defined

1 ayer j. rig . This unit c: a ri bi? traced beneath the sediment!s of the

West. Sant~.o Basin to a depth of 8.8 set TWT, i.e. at least 6.6 km.

The oc ear7 floor west. OS t h e west Tort- es Massi f (The North

D’Entrecas.teaux Basin of Maillet and others (1983)) has a thin 0.2

c;pc: - . TWl sedimentary cover that. is in faii1.t contact with the West .

Sar1t.o Easi n (Fig 3).

2. Eastern_ ~‘Q-i$,r-~c,as;te~u~ Z]one.

The originally vague1 y defined “D’Ent.recasteaux Zone“ now refers

t: 0 the arcuate camp 1. ex of ridges a r-i d sedimentary basins that

ES X t f3 I7 d 5 f P’ om the north of New Caledonia to abut the New Hebrides

&..- c: west uf Espi r i tu SarIto and Malakc~l. a !Dani.~‘?. and others, 1977) .

i. C~I”~E between the western Mai 1 I et. and others ( 1.983) noted di sti ncrt

-. 6 -

a r, d e .-. <:’ , .?..I t-. r, C? ._: c.7 sectors (.. 4: t ti c. zone. 11-r the west a sedimentary basin

wi tt-l 1..1 iI3 t. 0 2.3 SfZC TWT elf sedi mentar-y +%I1 is bisected by a

C E? f”l t. 1.” ii I. I-lorst . The northerr; half of the bazi n is deformed into, a

FliOilCDC: 1. i T’iC aI Secondary bursts cxc:c:ur along the boundary between this

and t. h e North Loyal. ty Basin. Collot and others (in presss) have

r6xugri.i sed the following features running frum north to south

acr-055 the eastern sector of the Zone (Fig 1):

-Th_e West Sant.o Basin

-The North D’E:r;trecasteaux Fi’idge

-The Central D’ Entrecasteacrx Basin

-The South D’ Entrecasteaux Chain

a- The West Santa Hasi n .

This basin lies i n normal oceani f water depths of 5CKK) m, bLit

ront.ai ris I.1 p t 0 ! u 7 set TWT of sediments. It has been interpreted

a5 an 01 d t r- en c h -. I. j. k: e feature by Lapocri 1l.e and Dugas (15’75) and

Coll.ot. and others (in press).

In t..hE western part of this eastern sector of the D’Entrecasteaux

zone t l-l e We5.t: c&(nto Basin con5iists of a r-elatively thick

sedimentary 5eCjLi~~RiZk? deporsi ted i n a graben bounded by conjugate

f au1 t5 with a deformed sediment f ill s which attains a thick::ness of

1 I 5 5 Ei c: TWl (Fj.g !%I). To the east the overall deformation in the

basin decreases and the faulting of the southekn margin of the

basi 17 i.s replaced by monoclinal. folding (Fig 5bI. The sediments of

this b a 5 :i. r-i show clear unccmformity with the southern flank: of the

West Torres P’lassif (Fig 4 and 5d) l

Ba5;ement structures consistent. with t. he underthrusti ng of the

f 1 our 0-f t.tje We5t Santa Basin beneath the North D’Entrecasteaux

F? i. d (2 e a 1” e vi 5 5. b I. e on prof:ile GEOV 1Wb (Fig.4 and Sd), GEOV 1010

(Fig.Sb) and GE0 104 (Daniel. and others9 1977, their figure 5) .

-. 7 -

c;oqac I I j ~~2.7.84 --

b- The North I:’ Ent.recasteaux Ridge.

The morph01 ogy and el rvati on of this double hot-St structure,

which is bounded b y a major scarp on the north side, varies

cansi det-abl y from we 5 t.. t 0 east. A small basin lying between the

horsts contains a slightly deformed sediment fill reaching up to

0. 8 SEC TWT in the east.ern region (Fig.4 and 5d) Y GE0 104. in

Dan i e 1. and others (1977) I whereas westward it is free of sediments

(Fig.!!%>. Maillet and other5 (1983) have dexri bed 37 my. Mid

Oceanic Ridge type ba saltes dredged from these hor-stz;.

C - The Central D’ Entrqcasteaux Basin.

.This ifi; a shall ow (wat et-. depth of around 3--.4(j(j(j m ) yt-ahen

C0Rta.i ni l:g 0. C t. 0 Il.2 set: TWT of def armed and f aul. ted sediment..

F a u 1. t j. n g e:.: tends down t. 0 the basement and the thickness of the

s;ed i mm t f i 1. 1 i net-eases east.ward (Fig 5a, c and d,Fig 11 and Fig

i2d).

d - South D’Entrei-rasteaux Chain,

l-hi 5 5eamoun t chain I we1 1. devel opped eastwards, displays the

Sabine Em I:: (Fig.11 which rises to only 7 m below sea level and a .

ma1 1 23cHj m deep seamount west of the Sabine Bank (Fig 2). These

structural highs are general.1.y non;layered (Fig 4 and Sa,c,d,e).

However t. he western flank of the Sab i. ne Ban I:: (GEOV 1017

profil.e, (Fig 13 , and 148) shows slope deposits with poorly defined

1 ayeri rig, contrasting w i t h the i rregcil at-. eastern slope. The

seaimouri t 5 ire i ntet-pt-eted a6 volcanic edifices on the basis of

thei r morphology (COI 1 cit. and others, in pre55) w To the east., the

lbugai nvi 1 1 e SpLtt” (F-j.g 1) * morphologically connected to the Sarito

-.. 8 -., .

.

bas@,;)er)t that local.ly cr.ops out and is overlain by sediments that

m&i ;y’ attain more than 1 se5 TbJT in thickness (Fig Sa, d and a). The

depth of the basin decreases eartwards, and t.his corresponds with

an increase in deformation, particularly faulting.

DSDP site 286 (Fig 3) sampled the sediments of the North Loyalty

Eas j. II (FSndrEws and ottier5!, 1975). The cored sedimentary sectian

/ c:ciri6j. sted of 650 m of F’l i ocene-+‘I. ei stocene ash I , Cl1 i gac:ene and

upper Eocsne silts and oozes, underlain by Pliddle Encene vitric

sandstones9 si. I. tstones and vol. cani c agglomerates that directl. y

civet- 1 i e a VC! car-ii c basement. e This location was crossed by the R. V.

Lapou i :i. I. e (19821 has suggested nn the basis of paleomagneti c

d a t a t.hat the North Loyalty Basin and the North D’Entrecasteaux

Basi rl were ariginal ly a con t i guouEj basin and originated at a

spread i ng cent.er to the SOl.it h . The basin was later broken by

t e i:: t f) n i c mizvemen t s along t.he D’ Entrfcasteaux Zone, thought by

D ici ri i e I. and others 11’777) to be a t.rans.f:orm fracture zone. Maillet

a il d uthers (1983) suggest the alternative possibility that the ,

D’ Ektrecasteacrx Zone marks an EIocene zone of plate convergence,

wh j. ch was modif ied by a middle Miocene tensional phase that gave

rise to tlie morphal ogy o+ the D’Entrecast.eat\x ZOllE?. Recent.1 y

C:k:i:l. 11, ot: and others ( i n press) hive conc:l uded that the eastern

D’Entrec:asteaux Zone may De interpreted, as previ uctsl y suggested by

smooth ba5ement 3 thy’ thin and slightly deformed sedimkntary cover

of the North D' En:rr-ecasteaL~>: Easi n. This WCJLI 1 d either argue

agai n5t the 5Ll&jCj~~t i on Of Lapouille (19821 that they formerly

ccxi5t.i tuted a single basin, or sugger;t that, aster their

formation, they have undergone different stress regimes. The Wkst

San tc:, Basi n, interpreted as a former trench by Co1 lot and others

( i l-l press) has f eatut-es consistent with a trench formed at the

‘Z. i t. e of a .f or mer sout.herly directed subduction zone. We suggest

that the West Santo E;asi.n is the eastward extension of the trench

thatI i 5 defurmed a 17 d bisected by the main hGrst in the Western

D’E:Titlt-eca6tBaL.~:.: Zorse. aw- data at-e cc3nsi. stent wi. th t h e

i. riterpretat i on of Ma i. 1. :I. e t. and others (19831, which i ES that the

hlC3rth D’ En trecasteaux <i dge

mlddlri’

is a 37 ma. old ocean flonr probably

up 1. i. f ted cl L.I t-’ i n g ’ \ _ Miocene. The Sc?ut.h D’ Entrerasteaux Chain

lTli3jf represent the remnants of an Eocene proto-island arc formed

contemporaeously w i t l-i subdue t i on al any t.he northern I. ine of the

13” Entrecasteaux Zone, .

variat1csn along the zone is: described bEI.lDw::

1 = - - -. - -! --. -. -.. Plate Cortact East of: the Wezt-.T’gryez Ciass&,f -. - ..- - I -.

co1 1 ot and others ( i n press) have shcwn that the topographical

t r en c I-I between t pi e west. To;-yes _ : Mac,c;. i .f and Espiri tu Santa

represents aF; area of n 0 r m a I ocean floor depth isolat.ed between

t.wCJ tcq3ngr aphi c&.l t-1 i g l-l 5 ,, ‘l”piq,J fol 1GW Daniel and Katz (1981 1 in

sugqes;t’i. rig that the sk.[r$ai:~ expres;F,iGn o$ the subducti. on zone is;

ma s I:: E? d b ‘r’ the protr ccsi c);‘i C! ,f Sar-!tcJ Ma1aC::~tl.a El.oc:C::, and that, in

consequenC:e!, r’lcl C,JE!Ut.ECt.C5;'ii C:. t; j-- E rl c: PI j. 5 cj )J 5 e f .' \i e cl i n this area.

co1 1 ot a n d other-, ii.n pres.::..) .r t.het-efore, place the j i..lricti. on zone

at the c:ontact between the E.m(IGth abyssa? plain and the disturbed

t 0 e Of t. h (2 wes.t err1 Wi3.1. I. C2+ t ITiS? Santa Ma]. a!~:c;l. K& bl ocb:. Gr-eene and

other--c, ( 1985) have coni:: 1. UdE~cl that f Gn evi dence G+ the LEE prof i les

3 8 and 40 (Fig 6) the nort.hern New Hebrides geotectoni t trench

extends a 5 far as 15 degrees south. However I these profiles both

show ti h i3 t t. h e t.rench i n question is a ~jteep r;i decl decl. i vi ty

between the Massif s of Espiritu Santu and Wc?st Torrer, and 1 ies

i3bGV@ t: h f? mean abyssal cjcean floor depth in this region: maximum

depth sha 1. I. ctws frGm 5550 m (LEE 40) to 4125 m ILEE 38). There is

nL7 e-v,j, der,c:e of either de+Grmation of the West. Tarres Massif or of

t, I- e I”i I-: 1-1 .f’ i 1. 1 c;eq;lence. -j-!-IQ inner wall c3.f the trenchI1 i ke feature

,",I A.." clgtg &or~t:gc:L 6Jdjagent-tp %hE b&zt-San&a-E$si-n,

co:1 l 0 t and others (in press) found that the trench topographically

di sappeared &c-J j ac.er1.t to the West Safito Etasin. Profile GEXYJ lOW7

(Fig. 7 and 8) f wh.i ch cro%.s.je:; the pl. ate boundary in thi E, region

(Fig 2 and 3 ) 3 shclws a sl ight 6UCl m depressi.arl of the sea floor

adj acent t.0 Espiri tu Santa. l”her~~ is deformation of both sediments

and bas~~m6?:-l t indicating .f 1. e j.: ,Ji’” c< and downwarping. The def armed

s&2<“j j, trj(Sri t !‘- 2 a I” e c,.%/et-. 1 24 i r) h v ” I u .5 secf: TWT o.f su!:,-,horizorital I

undeformed “t.reriL:t; f i 1. 1. ” sczcj j, ;fieyts-3. The total depth of the

“tt-~~~r;c-h” (tc~pographic:: deprec-sion plcrs “trench $il 1”) is thus about

,j, ;:;; (1.) (,j in I The 1. c!pg+r s l 0 p e 0 f the west wal. 3. oi: Espiri ti.i Santa i 5

i ).-,?.(qul e,y” ~ and 64.1 t: h o u g h n o c 1. +..a)+ ref lectorr; are SEerI 9 i t i 5

possible that 5 1 ij ;:; e deposi t.s !rave slumped down over sediments of

t. he “trench fill”” Alternatively the ‘outward deformation of the

5 I. 13 p e ad j acent. t C) t I? 42 b a.5 i ri E! Y could be t h fz? resul. t of the

horizontal. impact 0.f a ridge and consequent 1 ateral. westward

bul gi ng of the roc.:C::s on either side a+ the ridge as suggested by

Co1 1 C7t. and other-s iin press) ; these bulging rocks may mask the

deepest part of t t-i E geotectonic trench. Despite the lack of a

pronoiinced topograp;hical. trench F the super-f i ci al s;tructurec; shown

i. n prof i 1 k3 1 (1) !:I 4 (Fig 7 and 8) are consistent with f lexut-e

a,szoci ated wi t.h c:r~:~tal. dawnwarp at. a subduction zurte;

e.xpresai cln of t, t-i in p :I. at e c.cx-lta.c.t z ori e f 01 1. owz. the tect.or!:i. cal. 1. y

cli st.ur-becJ saddl Q t 17 a t separates t.he North D’ Entrecastfaux Ridge

,f i“ 0 fCi the Wc)usi Ef a rj I..: (Fig. 1 al-id 2) T a submarine promontory of

Espiri tu Santa.

CORIOLIS profile 1011 (Fig.9 and 10) crossed this boundary,and

show5 about 1.5 set TWT of deformed sediment in a small faulted

basin within the North D’EntrecactF?aux Ri dye. The ridge is an

extensive strtrctr..ire w i t h i rreyul ar topography, and teas either a

Cal. 1. at. arid others ( j, n press) 5 u g g @ s I: that li !::e the West Sant.o

Basin, the Central. D” E:.‘ntrecast.eaux Easi n is undergoing normal

subdue t i on beneath Espi r-i tu Santo. Greene and others 11%X5) have

suggested t.hat the snuthern New Hebrides trenc:h is continuclus this

far north.

CcJRIOLIS lines 1015 (Fig. 11 and 12d) 7 i612,1013, and LEE line 12

(Fig. 12 a-c ) which c:ros~ this boundary show deformed and f au1 ted

basement of the Central. D’ Entrecasteaux Basin downwarping to the

east m This i 5 over 1 ai n b y Lip to 1.5 set TWT of deformed and

east et- 1 y downwarped sediment, A thin .3 t a .5 set TWT “trench

fill.” sediment: sequence over]. ies these downwarped sedimersts at the

base gf .t, i-j @J 5 t &J e p f 1 c’;r~ I:: of the New Hebv-i derl arc. These “trench

fi 11 I’ sedi ment.5 are folded sync1 inal ly on LEE: 1 :i.ne 12. The Island

_- 1 3 -.

.’ .

C’ Entrecasteaux ]E! a 5 j. I-I f-gpi’leatp, the New Hebrides Arc are not

accompanied by 61 pt~~Ol3OUl3CC2d tOjIlOCjr-aphi C al. trench, but a minor 450m

depressi on of t h e Oceari floor. The total rel i ef Of the

feature, including the thickness af the “t.rench- f il. 1. ” is less than

1 (:)(:)t&j ”

_.

5. Ihe_ El $tE ~O~g-l~~a~y~~~E~is~ ~f:_the_spu~h_DLEgtl~e~a~t~a~~~

C:~~,ajr~._,a~.d_th_e_Boiiqa~l-i~i &I. E Ejp~!t-.,

Daniel and t:zatr f1981) e x a m i n e d GULF line NHlOD (Fig. 14b) and

proposed that the surf:ace e:.: pressl on of the plate baundary 1 ay

east of the Eougainville Spur, Collot and others (in press) cancur

w i t. h this definition and draw a transitional boundary marking the

di:fference between a topographic: hi gh P capped by secl i men t r; of

s i mi I. ar characters tu f”p,(-JsQ ot: the &Jjac:ent. ocean f lsor,and t.he

rugged acoustic basement Cl4 the south ex tensi on 0.f: Espiri tu

Sar;tn, whi ch i s co’v’er ed by a t.hin series of disturbed sediments.

Th i. s junction WGiS crossed by CORICL:LS line I.017 <Fig. 13 and 14a) .

in a west-east direction,north o+ the Bougainville Spur .Qn this

pt-,of i 1 e upwarping o+ sediments west. of the Sabine bank is similar

t c.f that seen west of the Eaugainvi I. le Spur on Gulf 1 ine NHlOD

(Fig. 14b). A wide and &$0(&n deep depression between the Sabine

B a n I:: and New Hebri.des kt-c s?ope is underl.ain by a thin sequence of

deformed ’ sedimentrr . . This depression,which is abruptly terminated l

tu \ the Eiouth by the fIanC:: c!f t.he E~x..(gainvi lie Spur, is simi I.ar to

t t1 e deprcssi c:ln i dent; j, .f i & as the plate boundary east of the North

- 1. 4 -.-

B cj L.1 g ;i j. ri tq j 11. 1 p Spur and onto- the rugged toe 0-F the west slope of the

NE?W t-1 e t:::t r i d e 5 k-c. The morphological contrast;. between the Spur and

t t1e At-. c 51. ope i s we1 1 marked; moreover the northern and southern

f 1 aI7 1:: 52 Clf the Spur are bounded by very steep faults. The northern

3 t3Ul ted boundary appears t. 0 separate two a r e a 5 of differing

sed i mient at, i on si mi 1 ar to t. t105;e described al. ong the prof i 1. NHl!If)

( F i g I 1. rib f II Thus:, the s1.w+ ace expressi or: of the p 1 & t;, (<f

boundary, I. ayi ng east of the Euugai nvi 1. 1 e Spur and recugni c,ed north

IIsl and Gt.. (.I $51 ape* I.-!c.;wever- P thi 5 area remai nc; tectoni cal ly compl ei;

and e v i d 6 n c: e 5 f 0 r t h e p r ec i. se location of this plate contact zone

are p CL3 c.i r I. $; (5 i n d i c: iii .k, e cj i n F i g t-k y- e 2 by both dcktE?d 1 ines $01 lowing

ihe su<Gi+:sted p I. 2, t e I. i mi t b at 1 east: b two e:.:tre~~e pos;sibi 1 i ties,

e :.: i 5 t f:rom a cant i nuous arcuat.e line to a jump of that. line

generated by a stri ke sl i p f au1 t running appros: i mate1 y along the

Nrsrthern E.rZiSr-p of the B<x.igai nvi 11 e Spur.

N#i- t h c> .f:

this the i"j 1 $ 1.. ii,, boundary is mi;t-l.::ed either- by a shal low

deFirE?SESi. OF1 tit- by an abrc.cpt transition between the abyssal plain

and the disturbed toe of the insular slope. Despite the contention

C?f Gt-E?f2flE~ and others ( 1.5T?3) that the trench ezt.ends across this

area. n rio tqiographi ca3. t r e:i E:: I-I i s visible on LEE line 10 (Fig.151

other t. h an a na.rrc!w 3i](lni deep depr-essi un V whi ch occurs at the

J 1.i ,-I CI: t i L’j !‘-I bet:wf.zen the aby.;,sa?. plain and insular sl ape. The abyssa!

13 1 it i n j. 5 3nc:lerI. ai n by Lip to 1 5 (2 c ‘TWT of f au1 ted, deformed

sed i men t s i’ ,t.. h i c I:: en f. n g e a 5 .\;, 6 a ;- d and over~l yi ng f au1 ted and def armed

basement V Dar) i e 1 and 1.::: <. t 2: (1981.1 have descri bed Gu.1 + f i ne NH1 1

(Fig. 15) m Again TiG C? eat- morpholag:i. CI+S:~ trench is vi sib1 e. On both

section5 the st.eep west; em slope o.f the i s;l and arc ex i bi ts

i r r e g u 1. 3 i- t cipography; t l-i i E i 5 es.pecn: :i al 1 y apparent at the toe bf

t.h i s r, 1 Ci(JE (Gul s NH1 I j and is consi.s.tent with an accumulation of

sl ape debris over the cie(.j j, me r, t.. s of the dowgoing plate.

DISCUSSION

-A The atjsencE: -.-“z .______-I.

n.i’ -.-.___: --........ k3 dce~_geotectonic trench, A”...” -..-._.- ---------we-

Desp i te t. he conti ricii ty ~4 east facing subduction alcmg the plate

b~w-iclar): in the c: e (“i t y- (3 1 New Hebrides Arc, the superficial

5 t r u c:: t 1-t r e S 0 f the 61. ate roritact zone between 17 degrees sauth and

14 degrees ~c:utt~ differ Ki a r I:: e d 1. y fram t.he cnnventional. deep’ trench

syst EiNi encountered s cl l.l ,t. h cl .f 17 degrees SGiJth IFi g. lbh) . However

,

by the arc trench i~ap i 5 a maj or factor in the suppression of

trench development t h t- 0 IA g h i.1 I..; .I~ the region. The western portion of

the block actual 1 y lies over the expected position of the deep

geotectoni c trench a5 extrapolated f rum the axis of the adjacent

trenches CFig.1). ?-he ~01. I i si an of topographic features on the

,downgoi ng plate with the Santo Ma1 akul a b 1 GC I:: has completely

suppr zs5ed trench development in the region of plate contact

ad j acent t 0 the North E” Er;treca5teau~~* ,. Ridge and the south

D’Entrec_asteaux Chair? iFi tZ] m 1tjC and f) , Thi 5 collisic3n is only

s;iperf i c::i al 8) al"lil doers i?iZt inhibit over-al 1 m..:bdt.tczt i or; processe?3

! co1 I ot. ~i~ij ot.hers, in 13 I?’ (S 5 5 ) w The west Torres Massi+ (Fig. iba)

,?J .f the Will 1.S i3.f these ~ieati.ir-eS i 5 tensi onal and di splays normal

faulting. -I- 1.3 e i nterveni i?ij decl. i vi t.y, c71 though topographi call y a

t r en c: h !, :i. -, not. a getntectonic trench, bcit rather a;-~ area of normal

0 C e 63 n .i i: Cl E p .t 1'1 i sol ated by the approach of .the West Tcs-I~‘@~s Massif

<co1 1 ot and others, in press). Fig.16~ shows a later stage in the

process where t l-7 e North D’ Entrecasteaux Ridge has

collided w i t, h the Sant.o i~la1aC::~~l.a block, and is separated from it

by a &ha1 1 uw biASi fl contai rii ng deformed sedimenkz. Co1 lot and

other-s I i n pr-e5!5 1 considered that the LII tima-te stage 0.f this

* represent a Piece uf t t; e Nczr I-. h 13’ Entrecasteaux Ridge t.hai: has

become incc:)rporated with ,the Santo Malakul a bloc!:: by this proceis.

The j unct i on between %he North Loyalty Easin and the BougBinville

Spur shows st.rongl y deformed and uplifted basin 5ediments at the

margin of the Spur, and neither evidence of geotectonic trench nor

trench f i-1 1 sedi merits. Twu po55ible explanation5 .f or these

structures are given her-e (Fig. Ibe and .f). -1) either the sediment

cover of the North L..oyal ty Easin has become separated frnm the .

basement and has averthrust a protruding wedge Of the sarlto

Mdl ak:Lil a block (f-ig. l&s!) 5 or- .i. 1 .- ‘7 alternatively, intense

comprf55i onal doming uf fha Nc;rth Loyalty Basin as it approaches

i-kc u 17 d e r t h r- u 55 ,t~. i n g z OR(2 0 f t I-1 e Santa) i’lal akul a block has rersul ted

i rt dr-acturing of the 3truCture, and up1 if t of a “keystone” shaped

block of the downgoi ng plate, which then collide5 with 9 and

a,~i;er”~.~ %o the fror;t of the overriding plate (Fig. 16f 1. An earlier

+ . :y .-. 5 c z! ‘,, i;’ of the latter p r C) iz a 5 5 is obixrved west of North Malakula

IFig. l6g and Fig.171 where there i5 marked compressional updoming

of the North Loyalty Basin sediments adjacent to the plate contact

rme. Here the pi-jss;ible 1. ocat. i c;n c2.f the trench is obscured by

disorgani5ed sediments slumped from the steep insular slope.

The Central D’ Entrecasteaux Masi n develop5 an incipient

geo{cect(3r,i c trench at i t:z contact with the Santa Malakula blucl:

<Fig. l&d) camp 1 ete with a sl.ightly deformed “trench fill”. Tklere

is no evidence for compression o.f the overriding plate,’ rather the

insular slope c3f the E5;c:,iri tu Santa i5 affected by normal

f .4ctl t i r-iq . However- . . r. .“,~_ ,I : riompi kai:p.t on 0.f ~sedi met-its cm the downgui r,g pl. ate

;-sflect the influence of .the II’ Er:t,rec:astea;..i:,: Zi3ne.

. . \ i-z Si il?i 1 i3r i n c :i. p i en t trei-ic:h hais (“J $2 .,,: Q 1 ::; ‘- “3 I kJr:.(.j at the plate b:)cILindary

de,f or-mat i n1-l !., - f t h e diljwngoi r;g plate is by tensi unal normal.

f a Li 1 t i i-1 4 , I...! @ i’ cq a”. the Iax- t h 75 degrees dip voctnr oI the dc;wngoir7g

plate means that the absence o,# a deep trench cannot be attributed

to the i nd 1 uence of the D’ Entrecasteaux Zone, which 1 ies to the

south I :I 1-l this case the absence of a trench must be explained by

the protrusion af the santo Ma1 akul a block over the hinge of

f lexure 0.f the downgoing plate. This effect m<-ist be emphasised by

di sorgani s-4 sediments sl. umi;ed .f rom the steep insular slope and by

the hurirontal displacement outwards 0.f the slope of ,the Santo

Malakula bi ock to compensate .for its adjacent indentation by the

colliding North D’Entreeastea~tx Ridge.

Thus five 5pQi: i f i C j3 ir 0 (1 2 fli 5 62 S can b e clef i ned to show why a deep

geotectonic trefich did KitJt develop along the Central New Hebrides

Arc; Those W-ES :

“?- L. Cal 1 i 5i on of eupographical feature5 carried on the

downgoi ~~~~ p 11. <i t @ with t h iG? l cad i ng edge o.f the overriding plate

(Fig. 18~~ and f) .

-7 .J .- /Abundant 51 Lii’Gpi ITCJ ,from the insular slope, and / or the

tectonic displacement c?f materi al outward to compensate for

adjacent indentation by colliding features IFig. lSb,d,g).

4- Llpdomi ng of the downgoing plate adjacent to the zone of

plate contact (Fig.16.f and g).

s.... F i 1 1 i n g o-f any incipient trench with r;ediments (Fig. i&b

ar;d d).

The super-f i ci al structures described abiJve can be used -td provide

an insight into the stress reg i me at the top of the downgoing

plate. This may be radically di f .f erent from the regime in the

downgoi ng plate as a who1 e. F-’ i ii ,-I ,‘. e 1’7 shows the variation in

stress regime along the Central New Hebrides plate contact zone as

inferred from the sei smi c pr(3f i ‘I. es. South o.f lati tude 17 degrees

sclut h there is a tez-,i onal t-;‘eg i me which coi nci des with the

presence of a deep geOteC.t.i::mic ‘trench and a atypical arc trench

gap= There is a I. d i: a 1 a r e a =-f prrJbab1 y , cc;moressi ve stress

a-,s?oci ated wi %h the i mpac t of ,:I -;;;nal ] . ~~F;l~ilNllL~iTt r4.l th the overriding

plate at latitude 17 degrees 25’ south (COI. Ict ar;d zthers, in

press).

North of 1 ati tude 17 degrees scx..ith there is a ccjtni:,r”es~:~i vcs stress

regime, which gets msJr e intense northwards l,f’i i-;. 16 and F-i g I 17) m

The compresr,i ve strezs field decreases i 11 the Central

D’ Entrecasteaux Easi l-1 ) but still exist, a5 is indicated IJY

deformation in the sediments of the basin <Fig. I.bd) q This region

of superficial compr essi ve tectoni c:s c(3:i. nci des wi th the absence of

a deep trench, and with the presence of the Sal;to Malakula block

at the leading edge of the cj v (2 r r i d :i. 1-1 g p :1 at:. e . The cumpressi ve

stress bet omes most i ntensl y c)e.+ze:i c;pecj where the North

overriding plate.

area does no% ;:I: ~3 j. ~1 c j, (:j <+? 4i th the presence C)f either a deep

geotect.onic trench zr with a normal arc trench gap.

‘l-h Ll5 the dominant stress patterns affecting the upper parts of the

downgni ng plate ad..i acent to the plate boundary may be grouped into

three major ar pLqs (Fig. 171 m North o+ ,the North D’Entrecasteaux

Ridge and south uf 1 at i tude 17 south, the dominant tensional

regime is probably related to the passive bending of the downgoing

plate under t h e weight of the sl at3 I According to Isacks and .

others (1%31) t h e ccxtpl ing between the plates in r,uch an area

shoc.11 d be weak: a;-id sctbduction should be cor;trolEd mainli/ by creep.

Between the Nor-t h D’Entrecasteaux Ridge &nd 1 ati tude 17 degrees

<south evidence af a de,f ormed weak sed i men t at-y 1 ayer over 1 yi ng a

more competant el ast i c plate suggest that the origin uf the

stresses is shall ow and cnmpressi c&al.

.Thi 5 may ref l.ect a relatively strong coupling of the plate beneat.h

D’ Entrecasteau%.a ,I Zone. Al though i t is di+fic~tlt to qualitatively

a5sess variatians in the intensity of stresses, we suggest that

relative i/ar- i a.t i on in horizontal cumprei~sive stress does occur at

tfle top the downgoing plate in front of the central New Hebrides

Wc: The greatec.t apparent stress c.. c c 1.1 r across the North

D’ Entrecasteaux Ridge and adjacent to the Sougai nvi II e Spur since

deformation is more intense. ‘These have been i nkerpreted by Co1 lut

and others ( i n prC%Gf a 5 area of high horizontal 5tre5s

concentrated at impact points, which may be considered as areas o,f

high coup1 i ng tl 2 t w e e II .t h e pl at.es ,where ,I: /y 25 f- e is evidence u f:

?. cxa!. i sed _ a and 3h a I. 1 C)W raughnesE; cx- i r- r e g (..I I a r surface of t t-i e

-,hould be pl dCES where ,the duwriguing plate is affected by cross

horizontal shearing (Fig. 1.7). Some evidence for this exists at

both locations. At latitude 17 degrees south, where deformation of

the plate boundary and disappearence of the trench are observed,

the downgoing plate displays an E-W topographic .feature delineated

by the !XKKUn isabath (Callot and others3 in press, their, Fig.2);.

Along the northern limit of the D"Entrecasteaux Zone there is even .

inor e striking evidence fur cross horizontal shearing (Fig.%,b and

d) with relatively strong deformation at the transition between

the West Santa Eiasin and the North D”Entrecasteaux Ridge. Thus

this boundary which has been interpreted as the trace of a

pre-Miocene subdue t i on zone (Callot and others, in press) may be \

reactivated at the present time as a strike slip fault.

.

. REFERENCES

ANDREWS y J = E , r PACKHAM T 13 y H. and at hers !, ‘1975 -" i:iit,e 28by in " Ini.tial report.5

of the deep sea drilling project”,vol.30,Washington ‘(US Goverm

nt Printing Office),pp.69,131.

BURNE,R.V. ,and TIFFIN,D.L. y 1982 -- The EVA 10 !GEOVAN II) Geophysical Sur

vey in Vanuatu Waters. Cruise Report N”. 79, CCOP SOPAC, Suva 20

ages.

CARNEY 9 J. and MACFARLANE, A., 1982 -- Geological evidence bearing on the

miocene to recent structural evolution of the New Hebrides arc.

Tectonuphysics, 87, pp. 147-1’75.

COLLOT, J.Y. and DANlEl,, J., 1983 -- Stress and ,strain in Central New Hebr

des : e-f f ($-z-at 11,s 1J.f the subduc’t. i cjn co1 1 i sion process of the D’ Ent

I~eE:d!Lti2iS.UX Zi3ne -. E. 0. $5. -Transact i ai?s American Geophysical Union

COLLOT, J. Y. , DAN:I:fX.!, J. y and BURNE, I?. V. 3 in prerzs ‘-* Recent tectoni c5

associated eri -th the subduct.ion co1 1 i si ~117 of the D’ Entrerasteaux

Zone in the Central New Hebrides. Tectonuphysics.

DANIEL, J., JUUANNXC, C., LARUE, B.,and RECY, J., 1977 - Interpretatiun

o.f D’ Entreca5teaux Zone (Nor-th o.f New Caledunia). International

Symposi l.lin of Geodynamics in South-F’acif ic,Noumea 1976,ed. Technip

, Faris, pp” 117-124.

DANIEL, J. and KATZ, t-l.R.,l98j. - D’Entrecast.eaux Zone, trench and western

chain of t.he Central New Hebrides island arc : their significance

and tectonic relationship - Sea-Marine Letters, 1, pp. 213-219.

DUBOI~~, J., LAUNAY, J., RECY, J., and MARSHALL, J., 1977 - New Hebrides

tt-er;ch: Subduction rate from associated lithospheric bulge. Can.

J” Earth, SC i I :, 1.4. 3 pp l 2X>--,255.

DUF’ONT 3 J ,I .9 and RECY 3 J I ? :t 98 I - I.-e E;c,~ij.,...C)i!es,t f”al=i f i ;d;le: d,--,yir?&E+5 cjtrLlCtL\r*<A

,.,.. ‘-8 -7 L. . .._I

Geophysi cs, Canberra. pp. X5-36.

GREENE, H.G., WONG, F.L.. 9 and athers, 15’83 I- Hydrocarbon ResiIc\rce Studi es

in the Southwest Pac i f i c Y 1782. !Jni t.ed States Geulagi cal Survei/

Open-Fi le Repnr k ~33-293, 24 pages.

ISACKS, B.L. f CARDWELL, R 5::: 0 n 7 CHATELAIN, :J. L. r BGRAZANGI g M. Y MARTHELOT,

J.l’j., CHINN, I). Y ai-;d i::tY.JDER-r 3 E. 3 1. 78:l - Sei smi ci ‘ty and tecttD,ni cs

of %he cxntral IQeg E&r-ides island +J~c, in Earthquake Predictiun- I I

An international review. Maurice Ewing Series 4, AG1.J.

JOUANNIC, J. 3 1?75 - Carte bathym&trique de la region Rennell- D’Entrecast

@dUK ORSTUM No~.tmki~, t.tr;pLibl i shed.

LANDMESSER 9 C. W. I ANDREWS, 3. Em 7 and PACKHAM Y G. Y l’i’75 - Aspect5 uf the Geolo

yy of the eastern Coral Sea and the western New Hebrides Basin.

*In Andrews, J. E. Y PACKHAM, G. V and uthers. Init.ial Report.5 of the

Deer-, Sea Dr i 1 1 i 114 Pra j ec t 7 ‘401 ume 3C!g Wash i ng ton (US Government

Printing Off ice) pp. 647-662.

Lapouille, A., 1382 - Etude de5 bassins marginaux ,fnssiles du Sud-Ouest

Pacif i Cp.iS: bassi n Nr:x-d 11’ Lirl.t.rec:asteau:.: 1 bassi n Nord LoyautB, bar3

TOM de Nourn&. Ccxltr i bciti an A 1 ’ &tude gkadynami que du Sud-Ouest

Pacifique. Trav. et, Due:. QRSTOM N” 147, pp.409-438.

LAPOUILLE, A. jand DUGAS,F. 7 %5”75 .- Geolagical evolution of the New Hebrides

and Loyalty Airsas. B~1.l~ A;;st, Sot. Explor. Geophys. v.h,N”Z 3,~.

MAILLET, P. y 1’10NZ1ER,M. 3 SELCJ, M. ,and STURZEH, D. y 1983 - The D’Entrecas

teacri: ZcIne (South-West Paci+ic) - a petrolagical and geachrunolo

gical reappraisal. Marine Geolagy, 53. pp. 175-198.

MAMMERICKX, J. T CHASE, T,E.,SMITH,S,M. r and TAYLOR, I.L. Y 1971 - (Revised

1974) Bathymetry of the South F’acific. Chart N”lZ- Scripps Insti .

tution of OFeanagraphy.

MITCHELL, A.H.G. r and WARDEN$A. J. r 1971 -- Geological evolution of the New

Hebrides island arc. J. Geol. ‘Sot. London, 127, pp. 501-529.

PASCAL, G. p iSACKS, B.L. 5 .EARAZANGI,M. r and DUBOXS, 3. F i978 - Precise

rel acat i On5 of earthquakes and seismckectcx-tics cif the New Hebri I

des island arc. J. Geophys. Res. ?R3, pp.4957-4975.

RAVENNE, C. , PASCAL, G. 3 DUE{0 IS, J . z DtJGAS, F. r and MONTADEHT, I-. I J 1977 *-

Pli3d21 af a young i ntra-oi.eai?i c arc :: The New Hebrides island arc.

Internati anal Sympnsi urn 04 Gei;dyr;ami cs in South-West Pacific,

Noum&a 1975, Ed. Technip, Paris, pp. 53-78.

TAYLOR, B. 3 in press ‘.- Geophysical survey of the Waodlark - Solomans

area. Circum Pacific Earth Science Series,AAPG.

.,,

FIGURE CWTIONS

Figure 1: Diagrammatic locality map of the central. New Hebrides‘ showing

: regi canal setti ng (inset) ; structural units; and si mpl if: i ed geol

..__ ‘711’ ,.- _(._ %..I

,

k

Fi gut-e 2: Simplified bathymetric chart after Jouannic (i979) ; Ravenne

and others ! 1977) j and Colltst and others (in press). Isabath in

km. Heavy or dotted line is the surface expression of the plate

boundary.

Figure 3: Location of seismic reflection profilincj tracks. CH: R.V.

CHAIN cruise (1971); NH: GULF cruise (1973); GEO: GEORSTQM I

and II cruises i1973,1974); R.V LEE, R.V.KANA KEOKI: Tripartite

J r, cr-uise programme ~19G2) ; GV: GEClVr?M I I crui se ( 1982‘

Figure 4: Ciriginal seismic prafil GEU’V’ :LOO5 across the West

(left) and the D’E1.7trec~~tt6a(..(:.: Zane (right).

Tar-res Massif

Figure 5: Traced sei tsmic prof i 1 es acrasss the D’ Entrecasteaux Zone. Lcxa

ticms uf the profiles are shcrwn in Figure 3. Vertical scale is

i n set TWT I Abbrevi at i on5 as in +igure 1.

F i g 1-t r e 5: Traced seismic sect.ions acrc2s.s the West Torrer, Mas;si,f--Espiritu

Santa plate boundary.

Fi gure 7: Original seismic profil GECIV l(X)9 acrossi the junction between

the west Santcl Basin and Espiritu Santts.

Figure 8: Tracer:J sei smi c section acrG--,s the plate buundary between the .‘

i west Sa;;f-o Basin and Espi ri tu Santa.

Figure 9: Original seismic profile GEOV 1011 across the junction between

the Nclrth II’ Entr-ecasteaux Ki dye and Espi ri tu Santa.

F i g Ill t- e 3. (::‘I : Traced seismic: section acrass ,the jur;cticsn between ‘the Pkxth

au:.: Chain and the West f lar;!:: of Espirit.u Santa.

Figure 14: Traced seismic sections ~rciss the South D”Entrecasteaux Chiain.

(a) and the Bougainville Spur !b,c).

Figure 1.5: Traced seismic section acr(3ss the junction between the North

Loyalty Basin and the Santo Pialakula black.

Fi gut-e 16: Diagrammatic representaticx? of viAriou5 subduction !XEX"driGS .

observed along the Central New t-lebrides. For location of exemples

see Figure 17. Abbrevi at i c;ns as. i I-I Fi gure; 1. _.

Figure 17: Diagrammatic representation of stress regimes at the top of

the downgoing plate showing compressive regime (swl id arrows;

left in.set) in central region, flushed by tensicmr71 regimes to

the narth and south lopen arrows; right inset) I S-tippled areas

are tretiches and sedimentary basins, hachured areas massifs and

ridges.

Lower inset - Di agrammati c representat i nn c3f xari at i on of

her i z mtal compressi ve cstreS!s at the tnp af the dr;wngoing plate

along the Central New Hebt-ides plate contact z-one. t/al UGFS are

strictly qua1 i tati ve and show relative intensities only. Maxi

mum compressive stresses are related to the co1 l.iding +eatures

of the North D’Entrecasteaux Ridge and the bouqainville Spur.

._- 9 -7 - .c. /

h .I

-L - 15.

>

- 16.

NFE

m Western Belt

m Central Chain

tiEI Eastern Bett

m Arc

m Mossifs _ Ridges

m Basins

m Trench I

\--,

I 6

DSDP 2c86 o

1660 yn 167e -

V.E.xl9

0 a

0 N

OS 06 07 06 05 04 03 02 01 00 23 22 21 20 19 18 17 16 15 14 13 12 11 10 21, I 1 I 1 I 81 I.1 I I

V.E. x 8.8

0 b

V.E.x 6.6 0 S

05 06 07 06 OS IO 11 12 13 1.S 15

9

t NNE

ssw

‘O @ V.E.x 8.6 17 16 15 14 13 12 11 10 09 08 07 1

2 3

V.E.x 13.3 4 5

6

0 e 7

1

2

6

5

6

12 13 14 15 16 17 18 19

. LEE 40

.

20 Km

E

7654321

LEE

(_ 20Kn

E

r

ua 0 0

. --

> 0 w

c3

1 0 23 22 21 20 19 18 17 16 15

I I I I I I I I I I I I

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0 20Km wsw . ENE i 1

L . . I

-

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-

- \ ‘\

’

O* 08 09 10 11 12 13 14 15 I I I I I I I I

l- GEOV tOI1 V.E.x 10

2-

--..

W

(c W

---

+-.-

’ “! . .

L i 1

18 17 16 -1

GEOV 1012 -

/ e -a

-- 0 10 Km

/

SW NE

‘@ V.E.x 9.8

~~ 1 I I

GEOV 1013 L I I 0 10 Km

/

ENE -

0 b V.E.x 9.8

18 19 20 21 22 23 00 01

LEE 12

Km

ESE

0 C V.E.x 16.6 08 07 06 05 04 03 02 01 00

2 [' I I I I I I I I

3 - GEOV 1015 V.E.x 9.3

4-

5- T-c,,r4C . -- B-M-, I

I 6- I 1 0 d 7- wsw 0 20 Km ENE

.

a. c Ln C

.

1 13 1C 15 16 17 18 19 20 , , I I I 1 1 1

7 GEOV 1017 V.E. x 9.5

6-

0 0 20 Km

a w E

15 11 0

13 12 11 10 09 08 07 I I

06 1 I I I I I I I

1 l- NH10 V.E.x 6.3 2-

3-

L-

5-

L I ’ 20Km

O- b 1 wsw ENE

0 18 19 20 21 22 23 00 01 ,

03 ,

04 05 I I

06 I I I I I 1 I I

l-

2-

3-

c-

5-

LEE 10 V.E. x 15.7

19 20 21 22 23 00 01 02 03 04 0~ I I I I I I 1 I !

'- NH 11 V.E. x 6.7

2-

3-

4-

5-

I

20 Km

COB

NLB

Ql No trench

a Yo trench OfI

Q Normel foufting at mergtna of WTM and Sent0

@ No evidence for comprerefon

0 Smell trench

a No deformation of trench ffff l edlment@

0 Norm81 faufting on Sento’mergfe

@ Slight normal teufting of borh l dbmenta end baaemont

8 No evidence for comproaeioa of W8B

@ No trench

0 Deformed sediment. betww blocko

0 Deformetion of sedimrnto l @ mergie of HDn

@ Some evidence for compros*ion

0 Slight trench

a Slightly deformed trench flW

‘0 Deformed eedimonto es4 beewent l f CD0

@ Normal faulting on margin et Santa

‘8 No evidence for conProra4ee of l rorridfrg pleto.ovidenCO

for slight comprosrfor of 8odimoato on downgoing pIat0

No trench

Separation of sediment cover tram beeeeknt et lunctfon wfth

Evidence for compreeoion Santo’block

No trench

Strongly deformed and uplltted sedtmonta of 8. spur

Evidence for compression

@ No trench

a Doming of NLB sediment8

0 Deformation of toe of !nnerwall

t---@ Evidence for compression

@ Possible accretionary prism

@ No evidence for comprenaion

3mpressive Stress b+

0 N

SANTO MALEKULA

/ Y 1

I