SEAPSO cruise (16 October 1985-29 January 1986)

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S E A P S O C R U I S E (ON BOARD THE R,V, JEAN CHARCOT) MAIN INVESTIGATORS I FREMER O RSTOPI INSTITUT FRANCAIS DE RECHERCHE 66, AV, D'IÉNA - 75116 PARIS, -p+sk€ae kq., 213, RUE LA FAYETTE INSTI~UT FRANÇAIS DE RECHERCHE POUR L'EXPLOITATION DE LA MER SCIENTIFIQUE POUR LE DÉVELOPPEMENT EN COOPÉRATION 75480 PARIS CÉDEX 10, fooCHFR %-?. - j , , , , . DANlCL 3-m*-m\Qp, &ssm Tats- L€ty ' @yMAAA¿ bt4TO'hLE GIS "OCÉANOLOGIE E T GÉODYNAMIQUE" UNIVERSITÉ DE BRETAGNE OCCIDENTALE . 6, AV, LE GORGEU - 29287 BREST EÉPEX, UNDER PATRONAGE OF CCOP/SOPAC COMMITTEE FOR CO-ORDINATION OFJOI:.IT PROSPECTING FOR MLNERAL RESOURCES I N SOUTH PACIFIC OFFSHORE AREAS,

Transcript of SEAPSO cruise (16 October 1985-29 January 1986)

S E A P S O C R U I S E

(ON BOARD THE R , V , JEAN CHARCOT)

M A I N I N V E S T I G A T O R S

I FREMER O RSTOPI

I N S T I T U T FRANCAIS DE RECHERCHE

66, A V , D'IÉNA - 75116 PARIS, - p + s k € a e kq., 213, RUE LA FAYETTE

I N S T I ~ U T FRANÇAIS DE RECHERCHE POUR L'EXPLOITATION DE LA MER S C I E N T I F I Q U E POUR L E

DÉVELOPPEMENT EN COOPÉRATION

75480 PARIS CÉDEX 10, fooCHFR %-?. - j,,,, . D A N l C L

3-m*-m\Qp, &ssm Tats- L€ty

' @ y M A A A ¿ b t4TO'hLE G I S "OCÉANOLOGIE E T GÉODYNAMIQUE"

UNIVERSITÉ D E BRETAGNE OCCIDENTALE . 6, A V , LE GORGEU - 29287 BREST EÉPEX,

UNDER PATRONAGE OF CCOP/SOPAC

COMMITTEE FOR CO-ORDINATION OFJOI:.IT PROSPECTING

FOR MLNERAL RESOURCES I N SOUTH P A C I F I C OFFSHORE AREAS,

1.1. "7

INTRODUCTION : The SEAPSO project.

Origin and evolution of the SEAPSO project.

preliminary proposals for work i n the Southwest Pacific within the framework of the JEAN CHARCOT world cruise programme were p u t forward independently by ORSTOM and IFREMER. The objectives, notably i n the n o r t h Fi j i basin, were suff ic ient ly similar, b o t h from a geographic and a s c i en t i f i c point of view, t h a t the sc i en t i f i c commission for evaluation of cruise proposals recomnended t h a t the two organisations work together. The projects proposed by ORSTOM and IFREMER had, moreover, complimen- tary log is t ic requirements which made the organisation of a common project imminen- t l y suitable.

This project was christened SEAPSO (SEABEAM Pacifique Sud-Ouest). I t follows the programmes en t i t l ed Hydrothermal ism and EVA respectively mounted by IFREMER and ORSTOM. The Hydrothermal ism programme has the object of describing and quant i fy ing the changes in matter and energy linked t o the circulation of seawater in the oceanic crust , particularly in the neighbourhood of oceanic spreading centres and more generally around submarine volcanoes. Up unti l now the programme has largely been concerned w i t h the study o f mechanis'ms of ocean crust formation and the hydrothermal phenomena t h a t accompany i t i n the axial portions of several mid ocean ridges in the large ocean basins ( the Atlantic, the Pacific and the Indian Oceans) w i t h rates of expansion t h a t d i f f e r on a , large scale (2.5cm/year t o 17cm/year). W i t h the SEAPSO project i t was proposed t o explore hydrothermal pheno- mena i n the different tectonic and magnetic context of basins.formed behind island arcs and subduction zones.

The EVA programe, i t s fu l l t i t l e i s "Evolution of island Arcs in time and space", i s concerned w i t h the phenomena linked t o subduction. Several stages of evolution are represented i n the active and fossi l island arc systems of the southwest Pacific. After studying normal subduction routines, i t was decided t o study subduc- tion zones affected by the arr ival o f ridges or seamounts which modify the s t r e s s regime controlling such phenomena as up l i f t of the forearc, formation of in t ra -arc basins by flexure and subsidence, formation of trenche;, and of back-arc basins by extensi o n .

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Because of the exceptional variety of subduction zones and marginal basins in different stages of evolution, the southwest Pacific constitutes a natural geogra- phic focus for the two programmes originally separately conceived and planned.

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The SEAPSO project i s organised around three principal s c i en t i f i c themes ,:

- Tectoniics a t the junction of aseismic ridges with subduction zones - Tectonics and magmatism of basins situated behind island arcs a t d i f fe ren t

- Ocean accretion and hydrothermalism of back-arc basins.

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stages o f forma ti on

The three themes come under the umbrella theme of the evolution of convergent plate margins and associated geological phenomena which constitutes one o f the overall ideas behind the world tour of the N / O JEAN CHARCOT.

OBJECTIVES.

The cruise programme i s composed of 5 legs the principal objectives are : ,

Leg 1 - From Noumea t o Port Vila. Chief Scient is t : J . Daniel, ORSTOM. ( i ) Tectonic study of the interaction between t he Loyalty Islands Ridge and the New Hebrides subduc t ion zone ; ( i i ) Tectonic study o f the interaction between the d'Entrecasteaux Ridge and the New Hebrides subduction zone. ( L N H , D N H ) .

Leg 2 - From Port Vila t o P o r t Vila. Chief Scient is t : J . Recy, ORSTOM. Nature and o r i g i n of the New Hebrides back arc basins (Coriolis T r o u g h ) . ( A A N H ) .

Leg 3 - From Port Vila t o S u v a . Co-chief Scient is ts ; J.M. Auzende, IFREMER ; J.P. Eissen, ORSTOM. J

Processes of oceanic accretion and regional investigation o f hydrothermal ac t iv i ty in the North F i j i basin. (BNF).

Leg 4 - From Suva t o Nukualofa. Chief Scient is t : J.P. Foucher, IFREMER. Processes of oceanic accretion and regional investigation of hydro- thermal ac t iv i ty in the Lau basin. ( B L ) .

Leg 5 - From Nukualofa t o Papeete. Chief Scient is t : B . Pontoise, ORSTOM - Tectonic study o f the interaction between the Louisville Ridge and

- Testing of a unique method of detecting locating and evaluating the Tonga island arc ( L V T K ) .

submarine mountains by analysis of SEASAT Sa te l l i t e al t imetric d a t a .

Major themes.

1.3.

Tectonics of the interaction between aseismic ridges and subduction zones.

Tectonics and magmatism of back a rc basins i n early stages of development.

Magmatism, oceanic accretion and hydrothermalism i n back arc basins.

Techniques.

M u l t i narrow-beam bathymetry (Seabeam), gravity, magnetics , 3.5 kHz echosounding, single channel seismic ref lect ion, sediment coring, rock dredging , hydrocasts, sea-floor photographic observations.

Cruise schedule

Leg. 1 - 16.10.1985 - 07.11.1985 NOUMEA - PORT VILA Leg. 2 - 10.11.1985 - 30.11.1985 PORT VILA - PORT VILA Leg. 3 - 02.12.1985 - 23.12.1985 PORT VILA - SUVA Leg. 4 - 29.12.1985 - 10.01.1986 SUVA - N U K U ' A L O F A Leg. 5 - 13.01.1986 - 28.01.1986 N U K U ' A L O F A - PAPEETE.

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J . RECY ORSTOM.

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SCIENTIFIC REPORT

SEAPSO C R U ISE

LEG I

J . D A N I E L Chief S c i e n t i s t

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SEAPSO LEG I : SCIENTIFIC STAFF

Jacques DAN I EL

John BUTSCHER

Jean-Yves COLLOT

Christine DEPLUS

Jacques DUBOIS

Martine GERARD

Patrick FAILLET

Marie-Claire MONJARET

Michel MONZI ER

Bernard PELLETIER

Jacques RECY

Vincent RENARD

Patrick RIGOLOT

S . Jules TEMAKON

ORSTOM Nouméa y chief sc ien t i s t

ORSTOM Nouméa y draughtsman

ORSTOM Nouméa , geophysicist

Un i vers i t é d ' Orsay , geophys ic i s t

Un i vers i t é d Orsay geophys ic i s t

Universi t é d'Orsay, geologist

ORSTOM Nouméa y geol og i s t

U B O / O R S T O M , geologist

ORSTOM Nouméa, geol ogi s t

ORSTOM Nouméa, geologist

ORSTOM Nouméa y geol ogi s t

IFREMER Brest, geophysicist

U B O / ORSTOM y geol ogi s t

Dept. of Geology, Mines and Rural Water Supply of Vanuatu, geologist

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L I - 2.

I - Objectives of the SEAPSO Leg I - (fig. 1 )

The first leg of the J . Charcot SEAPSO cruise was devoted to the study o f the contact of ridges and other irregularities of the dipping plate with the New Hebrides island arc.

Two main targets were identified :

( 1 ) LNH: Loyalty ridge/NH island arc interaction :

the objective was to recognize and map the deformations caused along the inner and outer .trench slopes by the approach of the Loyalty ridge, and to deduce the vatiations in nature and orientation of stress regimes affecting this area.

(2) DNH: d’Entrecasteaux zone / NH island arc interaction :

the objective was to study in detail 3 areas that appear to have been influenced by the subduction of the d’Entrecasteaux zone.

a) interaction between the North d’Entrecasteaux ridge (NDR) and the Santo block : collision, indentation and possible accretion of a crustal block of the ridge (the Wousi bank) to the fore-arc.

b) interaction between the South d’Entrecasteaux chain with the southern submarine prolongation of Santo : structure and deformation of the ”Bougainville Spur” and nature of the transition zone between subduction and collision northwards and southwards of the spuy.

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c) study of the transition between the tensional and compressive regimes characterizing the dipping plate, southwards of Malekula (17 ”s ) .

E I I - LEG I chronology ( f i g . 2 ) 3 I 1

- Oct. 16 Depart NOUMEA New Caledonia I

- Oct. 17 t o Oct. 23 LNH box : bathymetry (sea-beam and 3.5 Khz) , magnetics , gravity and single channel seismic reflection - 34 profiles recorded.

1 - Oct. 23 t o Oct. 24 t r a n s i t

- Oct. 24 t o Oct. 28 BOUGAINVILLE box : bathymetry (sea-beam and 3.5 K h z ) , magnetics, gravity and single channel seismic reflection - 42 prof i 1 es recorded.

- Oct. 28 t o Oct. 31 WOUSI box : bathymetry (sea-beam a n d 3.5 Khz) , magnetic, gravity and single channel seismic reflection - 53 profiles recorded.

- Nov. 1 dredging : 3 successfu l l / 5 attemps.

- Nov. 2 t o Nov. 3 t r a n s i t

- Nov. 3 t o Nov. 7 EMAE box : bathymetry (sea-beam and 3.5 Khz) , magnetics , gravity and single channel seismic reflection - 30 prof i 7 es recorded.

- Nov. 7 Arrive Port Vila ( V a n u a t u ) .

I I I - Prel iminary resu l t s

1 - The Loyalty ridge /New Hebrides a rc col l is ion

From the precise bathymetry (sea-beam) and seismic reflection da ta , i t i s possible t o identify some features :

- on the outer wall of the trench, presence of folding, s t r ike s l i p fau l t s and normal faul t ing

- on the inner wall , normal faul t ing whereas evidence of folding i s absent

- shallowing and eastwards shif t ing of the trench a t the contact zone.

A four stages geodynamic history i s proposed ( f i g . 3) :

1st stage : normal subduction o f the North Loyalty basin : normal f a u l t i n g on the outer wall and accretion o n the inner wall.

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2d stage : the toe of the ridge arr ives near the trench : increasing normal fault ing of the ridge.

3d stage : the ridge s t a r t s to subduct : u p 1 i f t ing of the inner wall , compression and folding; shallowing of the trench.'

4 th stage : down faul t ing of the ridge flank and collapse of the lower p a r t of the inner and outer walls, the trench i s shifted eastwards.

2- The South d ' Entrecasteaux chain / New Hebrides arc col 1 i sion ( f i g . 4 )

a ) nature of the "Bougainville feature".

Because of i t s posit ion, apparently a part of the S a n t o block, th i s shallow was named "Bougainville S p u r " . From the morphology : a flat-topped seamount , the gravity and the nature of the sampled rocks (o1 i vine basal t s , pyroclastic breccia, coral 1 imestone) , i t must be actually named Bougainville

b ) flexure of the lithosphere.

The depth of t h 2 roof of the Bougainville guyot ( a b o u t 1000 m) as a function of i t s distance t o the trench i s consistent w i t h the deflexion curve of the lithosphere deduced from the uplifted a to l l s of the Loyalty islands. the seamount due t o the co l l i s ion . In t h a t scenario, the age of the Bougainville guyot oldest coral reefs could be a b o u t 2 Ma .

I t shows tha t there i s no evidence for any change i n the course of

c ) collision and subduction.

The guyot, whose summit platform i s dipp'ing 4" t o the eas t , i s involved in the subduction. There i s no evidence df >normal faul t ing comparable with those observed on the Kashima and Erimo seamounts subducting in the Japan trench. O n the other hand , seismic study shows folded deformations on the inner wall. This situation could be due t o the existence of a proeminen't frontal arc which is collided by the g u y o t . Moreover, as the dominent s t ress affecting the downgoing plate i s compressional , normal fault ing does n o t appear on the outer

3- The North d'Entrecasteaux ridge / New Hebrides a r c col1 is ion.

a ) nature of the North d'Entrecasteaux ridge

The ridge i s 40 km wide. Magnetic and gravimetric data , suggest that i t s basement can be basa l t ic . As we k n o w , by dredging, the existence

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of MORB type basal ts westwards on the ridge, we conclude tha t th i s . r idge i s a piece of uplifted oceanic c rus t overlapped by volcani-clastic sedi- ments . ..

b ) subduction of the ridge.

Unlike the s i tuat ion of the Bougainville guyot/New Hebrides contact, the deformation f ront i s n o t c lear ly shifted t o the east. That means tha t the subduction of the ridge i s quite easy. This i s confirmed by the SOPAC I I cruise seismic reflection data showing the ridge underthrus- ting the inner wall a t l e a s t 25 km eastward of the deformation front .

c ) nature of the Wousi bank

The Wousi bank, i s a shallow being now a p a r t of the Santo block. B u t i t could be a p a r t of the ridge tha t was unable t o subduct and which has been f ina l ly accreted t o the Santo block. T h a t means t h a t the main deformation front m i g h t have been previously east t o the Wousi bank. This hypothesis i s supported by the magnetic da ta , which show tha t the Wousi b a n k can be of the same nature than the North d’Entrecasteaux ridge.

4- The t rans i t ion from compressive s t r e s s regime t o a tensional s t ress regime between Malekula and Efate islands.

a ) the subducting plate

The North Loyalty basin l i e s on the leading edge of the subducting plate. The oceanic crust i s 1 2 km thick. The pre,sence of the anomaly 23, previously ident i f ied , has been confirmed b u t the outline i s perturbed by the interference of the ORSTOM seamount lying on a fracture zone perpendi- cular t o the magnetic anomalies. The seamount has been down faulted by re- activated f a u l t s and the main f a u l t const i tutes the northern l i m i t of the trench.

North of anomaly 23 the downgoing plate i s deformed by compressional forces t h a t may be induced by the presence of the Malekula block.

b ) the plate boundary

The south New Hebrides trench forms the plate boundary. The trench reaches depth exceeding 6000 m i n the south-central parts b u t shallows t o

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4800 m i n the north. The trench i s abruptly interrupted a t 17 '05s . 'North of th i s l ine i t becomes less d i s t inc t as i t i s replaced by a ser ie of thrust f au l t s t h a t diverge t o the west. The superficial expression of the plate boundary i s located a t the base of the inner slope of the Malekula block.

c ) the fore-arc

The structure of the fore-arc region, inferred from bathymetry and seismic d a t a , i s largely controlled by the original configuration ( a t the end of the Miocene) of €he proto-volcanic a rc . A second factor which modifies t h e fore-arc i s the sediment supply from the arc . I t appears as t h o u g h the supply from Efate would have a more s ignif icant consequence t o sediment f i l l i n g t h a t the smaller island of Epi, which i s , in addition, located well t o the eas t .

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SCIENTIFIC REPORT

SEAPSO C R U I S E

L E G I I

J . R E C Y

Chief Scient is t .

? L 11-1.

SEAPSO LEG I I : SCIENTIFIC TEAM.

Jacques RECY G i I les AUCLAIR : IFREMER Brest, geologist

Catherine BALDASSARI : ORSTOM Nouméa , techni cian Jean Michel B O I R A T : BRGM Brest, geologist Glenn R . BROWN John BUTSCHER : ORSTOM Nouméa, drawer Philippe CHARVIS Jean Yves COLLOT Jacques DANIEL Marti ne GERARD Rémy LOUAT Marie-Claire MONJARET : UBO/ORSTOM, geologist Michel MONZIER Bernard PONTOISE : ORSTOM Paris , geophysi ci s t Etienne RUELLAN : UBO/ORSTOM, geologist

: ORSTOM Nouméa, geologist, Chief Scient is t

: HIG (Hawaii), geologist

: ORSTOM Nouméa , geophys i ci s t : ORSTOM Nouméa, geophysicist : ORSTOM Nouméa, geologist : Université d’Orsay/ORSTOM, geologist : ORSTOM Nouméa, geophysicist

: ORSTOM Nouméa, geologist

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x BRGM : Bureau de Recherches GéolGgfques e t Minières. x U B O : Université de Bretagne Occidentale x HIG : Hawaii Ins t i tu te o f Geophysics.

+ L 11-2.

SEAPSO Cruise

L E G 2

Noumea-Port Vi l a (10-30 november 1985).

I - INTRODUCTION A N D OBJECTIVES OF L E G 2 .

Leg 2 of the Seapso cruise began on 10 november 1985 a t Port Vila (Vanuatu) and finished 30 november also a t P o r t Vila. This Leg was for the most part devoted t o the s tudy of the back arc area of the Vanuatu arc (Figure 1) .

The troughs a t the rear of the New Hebrides island arc were discovered i n 1966 during the crqise of the ORSTOM RJV Coriolis (de Chalvron and others , 1966 ; Puech and Reichenfeld, 1968, 1969) . These troughs are shown on the bathymetric maps of Scripps Inst i tu te of Oceanography (Mammeri ckx and others, 1971) , on the ORSTOM map (Monzier and others, 1984), and on the USGS maps constructed fo r the ANZUS Tri parti te-CCOP/SOPAC investigations (Chase a n d others, i n press) . In the northern and southern parts of the arc , these t roughs separate the island arc and the North F i j i Basin ; they are absent i n the central par t of the a rc , east of the islands of Maewo and Pentecost i n front o f the coll ision of the D'EntrecasteauxRidge w i t h the arc (Figure 2 ) .

Karig and Mammerickx (1971) interpreted the back-arc structures 'as en-eche- lon inter-arc basins and Luyendyk and others (1974) believed them t o be extensional features, al t h o u g h they could locate' no new oceanic crust . Dubois and others (1975, 1978) proposed t h a t €lese troughs formed by deep-seated magmatic a n d tectonic processes t h a t involved considerable faul t ing. The most recent work i n the area suggests t h a t these troughs were caused by a change in the island a r c ' s s t r e s s f i e l d , possibly'due t o the i n i t i a l coll ision of the DEZ (Collot and others, 1985 ; Burne and others , i n press) .

The back-arc troughs south of the coll ision of the DEZ a n d arc include two grabens w i t h a separating horst , or a simple graben.

The back-arc troughs nor th of the col l is ion zone of the DEZ and arc are n o t as well known as the southern ones. The northern troughs appear complex in t h a t there may be formed by sub-parallel troughs.

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Seismic refraction d a t a from the southern troughs (Pontoise e t a l , 1982) indicate t h a t 2 t o 3 km of low-velocity ( 2 . 0 t o 3 .5 km/s) rocks overlie a high-veloci ty (6 .0 km/s) basement. In these troughs, single-channel seismic l ines show weak disorganized reflections which appear t o resu l t from the heterogeneity and complexity of the tectdnized s t ructures . Thus , l o w velocities indicated by refraction d a t a could represent the average velocities of rocks of varied origin and nature. Gravity modeling, (Collot and Malahoff, 1982) do n o t indicate a significant difference between the crust of the t roughs and the crust of the North Fi j i Basin, b o t h of which are different t h a n the c rus t of the New Hebrides island arc.

Shallow earthquakes occur along these troughs (Coudert and others, 1981 ; Chatelain, pers. com.). Well-defined hypocenters l i e between 10 and 20km depth, s l igh t ly deeper t h a n mid-ocean ridge earthquakes. Locating events recorded by the ORSTOM-Corne11 network i s s t i l l in process (Chatelain, pers. comm.), b u t Coudert and others (1981) report t h a t focal mechanisms show normal fault ing on the southern flank of the t r o u g h .

A recent petrological study of dredged samples collected along the back- arc troughs shows t h a t most of the samples are related t o island a r c volcanism (Val l o t , 1984). However, some of the basal t s dredged a t 18”s la t i tude along the eastern scarps of the southern trough migh t be marginal- basin tho le i t i c basalts. To the south of Fu tuna Island and on the eastern scarp of the t r o u g h , samples dredged a t 20”s lat i tude range from ankara- mites a n d olivine basalts t o plagioclase-rich basalts. These samples are geochemically similar t o those derived from the volcano on Tanna Island and may resu l t from an independent volcanic complgx l ike t h a t which crops o u t on F u t u n a Island.

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The back-arc troughs of the New Hebrides a r c might represent examples of a incipient crustal extension between an active island a r c and the active, marginal North F i j i Basin. These troughs may have developed by the onset of a double-axis spreading system in the marginal basin or from a jump of the axis of expansion.

The objectives of the Seapsocruise was t o know i f these troughs represent the i n i t i a l extension of thecrus t in a purely oceanic environment.

For t e s t ing th i s hypothesis we needed t o know

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1. The shallow structure of the t r o u g h , 2 . The deep ( c rus t a l ) s t ructure of the t r o u g h , 3 . The location and love1 of seismic ac t iv i ty , 4 . I f there i s a specif ic magmatism related t o these troughs, 5 . If there i s hydrothermalism related t o th i s volcanism.

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The Seapso cruise would allow us t o answer the questions Questions 2 and 5 will need additional document which will be collected during 1 a t e r crui ses .

3 and 4.

I I - PRELIMINATY RESULTS.

1) EMAE Box

The d a t a collected from th is portion o f Leg 2 permits us t o complement the survey done on the frontal arc during the leg one and t o characterise the subducting and MALLICOLO.

p la te , the plate boundary, and the fore arc zone between EFATE

a . The subducting plate

The North Loyalty Basin l i e s on the leading edge of the subducting plate. We have confirmed the presence of anomaly-23 i n the northwest of the box. A seamount l i e s on a N25E trending fracture zone which i s perpendicular t o the magnetic anomalies. The seamount has been downfaul ted by re-activated normal fault ing as i t approached the subduction zooe. The principal f au l t in the fracture zone consti tutes the northern l imit of the trench and can be extrapolated under the inner wall of the fore-arc.

North of anomaly-23 the downgoing plate i s deformed by the compressional forces induced by the presence of the "Mallicolo Block". The convergence motion i n the direction o f N70E i s translated by a system of thrust and s+nij- t r a l s t ike-s l ip fau l t s which bifurcates from the northern end of the trench.

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b . The plate boundary

The South New Hebrides Trench forms the plate boundary. The trench rea- ches depths exceeding 6000 meters i n the south-central parts b u t shallows t o 4800 meters i n the North. The trench i s abruptly interrupted a t 17'05' S lati tude. North o t th is l ine i t becomes less d i s t inc t as the deformation f r o n t diverges t o the West ( f i g 2 ) . We consider t ha t the sur f ic ia l expression of the plate boundary i s located a t the base o f the inner slope o f the Mallicolo Block.

c . The fore-arc.

Three morpho-tectonic zones have been identified on the fore-arc : the accretionary prism, the arc slope, and the fore-arc (accretionary) basins. Seismic reflection data does n o t show d i s t inc t boundaries between these areas.

The accretionary prism is pronounced i n the central part of the arc complex. The prism i s bounded on the North and Sou th by apparent NE-SW

trending s t r ike-s l ip t o n e s ( s in i s t e r in the North, dextral in the S o u t h ) . Between the two zones the prism doubles i n w i d t h .

The seismic data suggests t h a t the s t ructure o f the fore-arc i s largely controlled by the original configuration ( a t the end of the Miocene) of the proto-volcanic a rc . The mode of subduction however, constant along the course o f the a rc , i s unable t o explain the i r regulat ies i n the morphology of the proto-arc. A second factor which modifies .th? fore-arc i s the sedi- ment supply from the a rc . I t appears as t h o u g h the supply from the relatively large island of Efate would have a more s ignif icant consequence t o sediment loading i n the central fore-arc t h a n the smaller island o f Epi, located well t o the East of the fore-arc area.

2 ) COMPRESSIONAL BACK ARC A R E A .

The data collected d u r i n g this part of the survey back t o the islands of Maewo and Pentecost allows US t o make the following observations on the s ta te o f deformations caused by the Arc-Ridge col l is ion in the central NeN Hebrides.

I

1 1 i l

i i

!

I

l

,/ I

L JI-6.

*

a . The seismic profile a long the slope of the arc east of Epi and Ambrym, does not show any tectonic perturbations which could be related t o the main transverse fracture zone. However a strong positive magnetic anomaly ( t1260nT) w i t h a half wavelengthof 25 km and associated with 2 minima (-600 and -450 n T ) l i e s i n the direction of elongation of Ambrym Island ( N 105"E). I t could be due t o intrusions along the same fracture zone which runs through Ambrym Island.

b. The profiles in the East of Pentecote and Maewo indicate from West t o East :

. an a b r u p t high-angle escarpment probably caused by normal faul t ing,

. a weak compressive deformation be l t a t the base of the slope, characte- rized by westward thrusting and folding t h a t suggests an incipient tectonic accretion,

. the abyssal plain of the North F i j i Basin t o be 3300 meters in depth.

The western margin o f the bas in has an i r regular and fractured basement covered by layered sediments of varying thickness. Close t o the deformation zone the sediment basin thickens. The observation i s in good accordance with a weak negative free-air anomaly ( r e l a t ive amplitude of -40 mgals) corresponding t o the base of the slope.

Seismic refraction profiles from the North Fi j i Basin east of Maewo indicates a deepen proposed by COLLOT 25 km deep.

ng of the Moho towards the West. The grav and FISHER ( i n prep.) confirm a 9" d i p of

'.

Y

seismicity i n the area, focal mechanisms , quaternary up l i f t of the nearby islands i t appears as t h o u g h

ty model the Moho u p t o

Considering the and plio- the eastern

edge of the central New Hebrides is undergoing compression. The west dipping deformation front l i e s between the North F i j i Basin and the nascent accretio- nary prism a t the toe of the is lands. This model i s consistent with the hypothesis of a deflection i n the lithosphere prior t o the inception of a 200 km long subduction zone eas t of the Maewo-Epi l ine .

L 11-7.

3 . North VOT TANDE Box.

Two main volcanoes were found i n the "boite Nord". They developed i n the New Hebrides back-arc extension zone and appear t o be of different ages. The area i s characteri s t i zed by :

- an absence of a well defined trough s t ructure , contrary t o what i s observed towards the South i n the Coriolis Trough ;

- lavas dredged from the central volcano appear t o be younger t h a n those sampled from the off-center volcano ;

- a free-air gravity anomaly amplitude suggesting a r i g i d behavior of the 1 i thosphere under the vol canoes ;

- l o w temperature hydrothermalism.

The volcanoes could be explained by :

- back-arc bas in volcanism occuring i n the zone of extension. However there is currently a lack of support t o confirm the existence of oceanic accretion. The results of petrographic and geochemical studies will help t o shed l i gh t on this question ;

- displacement of island-arc volcanism to the East of the vo as there i s a 4" gap i n arc volcanism between the Banks Islands and Tinakula i n the North ;

I .

Y

canic l i ne i n the South

- a transverse volcanic alisrmentin the direction of N 70" E from the islands of Mitre and Tikopia i n the East t o the "boite Nord" i n the West suggesting a migration of volcanism w i t h time (i.e. h o t spot-like 'volcanism).

- intermediate vol canism between the island-arc and back-arc regimes similar t o for other back-arc basins ( T A Y L O R and KARNER, 1983).

the Okinawa Trough (MAURY e t a l . , pers. C o m m . j , and proposed

A study of structures and volcanic ac t iv i ty i n the back-arc of the New Hebrides necessi ta tes additional complementary data such as seismic refract ion, heat f l o w , and submersible d i v i n g .

L 11-8. ?

ri 4. VOT TANDE Box.

A r i f t graben s t ructure i s present i n the study area. Collapse and extension has been su f f i c i en t t o allow the emplacement of a discontinuous volcanic ridge in the axis and a volcanic cone in the Southeast. Although the rock samples dredged from the volcanic axis of the graben and i t s flanks show no signs of active volcanism (due t o the presence of ferromanganese c rus t s ) , the lack of a l te ra t ion and the t h i n crusts suggests recent volcanism.

The relatively t h i n and regular sediment layer found i n the graben indicates recently active extension tectonism. Intense perturbations of the bot tom by faul t ing, intrusions, and extrusions have upset the original sedi ment 1 ayers.

The observed s t ructures are clearly products of recent extension and re 1 a ted vol cani sm .

5. HAZEL HOLME Box.

The Hazel-Holme fracture zone i s ident i f ied i n the West more by i t s gravi ty and magnetic signature than by ilts morpho1 ogical expression. The original morpho-structure of the zone could have been modified i n the West by the formation o f the island arc.

B u t i s the zone act ive today ? Shallow seismicity seems t o indicate t h a t the zone i s active (HAMBURGER a n d ISACKS, i n press). However i t i s d i f f i c u l t t o affirm i f there has been 15 t o 20 km of s in i s t r a J s t r ike-s l ip motion between the South and North troughs because NE-SW \rending folds suggests a dextral movement.

I t i s possible t h a t the extension associated with the opening, o f the troughs has reactivated the ancient fracture zone w i t h an apparent dextral movement. This would mean the location of the Sou th Trough t o the East in relation t o the North Trough i s primary and n o t the resul t o f s t r ike-s l ip mo- vemen t .

L 11-9.

6 . FUTUNA Box.

The F u t u n a t r o u g h which range from 19'20 t o 20" i s the larger and the deeper one. I C

In the Northern p a r t o f the box the cruise has discovered two recent volcanoes which closes the t r o u g h toward Southward

North. the main t r o u g h ends by a succession of narrow en-echelon troughs.

In the central part .the Seabeam discovered many small edifices no more t h a n 100 t o 200m high which might be interpreted as small volcanoes. Dred- g i n g on one o f th i s ed i f ice collected ankaramitic rocks which are also known on the island around. So we d o n ' t know i f the small edifices i n the troughs are the t o p subsided faulted arc blocks or volcanoes resulting from a specific magmatic phasis intruding the bot tom o f the troughs in so many places t h a t i t can be considered as recent stage of crust fabr ic .

Chemical analysis o f the samplings collected will a l l o w t o solve th i s problem.

7. ERROMANGO Box.

This box was so well surveyed as the F u t u n a box. Nevertheless i t appears clearly t h a t the bot tom of the t r o u g h i s made of several subsided faulted and t i t t e d ea r l i e r stage of evolution than the F u t u n a t r o u g h . I t i s a graben where volcanic act ivi ty is n o t so strong did n o t begun.

a rc blocks. So we guess t h a t th is t rough is a t a

1.

9

8. VATE Box.

Few profiles have been done in th i s box where the troughs appear as two bens separated by a central horst .

The quarter o f the time cruise of th i s leg was devoted t o dredging. 31 dredginphave collected 4 tons of rock sampling in the back arc on the flanks of these troughs. The on board observations d i d n o t give the answer t o the questions concerning the nature and origin of the magmatism which are essential for the understanding the origin and possible evolution o f the troughs.

troughs o r

12"s-

NORTH

BOX VOT TANDE

VOT TANDE BOX

14'

16"

18"

20"- 2O"lO -

o / t

SEAPSO Leg 2

HOLME !%S

BO x

CK MAEWO PENTECOST BOX

ER n

167"lOE lao l iF

L. I I . 10. Y

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4A SOX

- f ig*l - "30

Sketch map of the s t ress pattern alon? the a rc inferred from the model developed by C O L L O T e t a l . ( i n press).

Dotted areas are D'Entrecasteaux Fracture Zone and \!est Torres Massif. Ruled areas are back-arc troughs. Solid l ines show western a n d eastern boundaries of the arc. Long dashed l ines indicate Str ike-s l ip fau l t s from focal mechanism solutions are represented with

the radiating s t r e s s pattern.

sense of shear. Filled arrows show the zone of compressive s t r e s s and open arrows the tensional zones. The 2-km isobath emphasizes the protruded shape of the central block. f i a .2

SCIENTIFIC R E P O R T

SEAPSO CRU ISE

L E G I I I

J.M. AUZENDE Co. Chief s c i e n t i s t J .P. EISSEN Co. Chief Sc ien t i s t

L.111-1.

S E A P S O L E G I I I .

SC I ENTIF IC STAFF.

Jean-Marie A U Z E N D E : IFREMER, Brest, Co-chief s c i en t i s t Jean-Philippe EISSEN : ORSTOM, Nouméa, Co-chief s c i en t i s t Marie-Paule CAPRAIS Jean-Pierre C H O P I N Pascal GENTE Stephanie GUENELEY François HARMEGNIES Yves L A G A B R I E L L E André L A P O U I L L E Christian LEFEVRE Patrick MAILLET Jean-Pierre MAZE Hélène ONDREAS André SCHAFF Rajendra SINGH

: IFREMER, Brest, chemist : G E N A V I R , Toulon, technician : U B O , géologist : U B O , chemist : IFREMER, Brest, ingeneer : U B O , geologist : ORSTOM , Nouméa , geophys i cis t : Univ. de Li l le , geologist : ORSTOM , Nouméa , geol ogi s t : IFREMER , Brest , techni ci an : IFREMER, Brest, technician : U B O , geologist : Mineral Resources Department, F i j i , geophysicist.

L.111-2. c

% SEAPSO Project

I

L E G I I I

NORTH FIJI BASIN.

The North F i j i Basin (NFB) i s a re la t ively old and complex marginal basin. I t s formation and spreading has been induce by the s t r icking of the Salomon-Vitiaz subduction and i t s inversion with the creation of the Vanuatu subduction some 10 My ago (Falvey 1978 ; Colley e t al 1984) . The HIG and the German worked recently i n the North and the North-East of the NFB and the ORSTOM of Noumea collected in 1983 some data south of 20"s. B u t , very few detailed oceanological data has been collected in the central p a r t of the NFB.

1 - Objectives :

The objectives of the leg I I I of the SEAPSO cruise was :

- t o study the geology of the active spreading ridge of the central pa r t of the NFB ;

- t o evaluate the hydrothermal potentiel of t h i s spreasing ridge ; - t o survey a seismically active zone of the eastern par t of the NFB, bo rd ing the Viti Levu island ( F i j i ) .

Thus a seabeam, s ingle chane1 sismic ref lect ion, magnetic, gravimetric and sampling survey has been conducted between la t i tude 17"30 S t o 21" S , and longitude 173" E t o 177" E .

I

I

2 - The active spreading ridge of the central NFB :

A wide survey of the spreading axis was done along 12 geophysical and seabeam profiles of abou t 100 nautical miles long, centered on the assumed axis location. A t the extremity of 4 o f these prof i les , one Kuhlenberg core was done t o sample the sediment. Then so r t e r profiles were realized, which allow us t o precise the location of the a x i s . These prof i les was interrupted twice for the realisation of 2 "boxes" (10 by 30 nautical miles, Fig. 1 a n d 2 ) with a ful l seabeam coverage. These surveys was then completed by seabottom water sampling ( w i t h Mn and s i l i c a on board analyses), hard rock dredging, and near bot tom photographies using the deep towed package RAIE of IFREMER (see the Table).

L. I 11-3.

I

The d a t a collected allow us t o descibe some along s t r i k e morphologico-struc- tural variations. A preliminary interpretation of the s t ructure of the axïal area (Auzende e t a l , 1986a, Fig. 3B) show t h a t the axis i s located on a simple, N-S trending spreading ridge between 21's and 17'40 S . The axial ridge presents there an axial dome a few kilometers wide and 200 t o 300m high (Fig. 3A). An axial graben, u p t o 500m wide and u p t o 40m deep, i s sometime observed. Between 17's

and 17'40S, the axial area i s marked by a more complex pattern o f oblique structures (N45 dominan t ) as observed on the bathymetric map of Chase e t al (1982) . In the northernmost area surveyed between 16's and 17'S, we observe a complex alternance o f ridges and lows. There i s no c lear spreading axis b u t two main ridges (one o f them i s shallower than 2500m. depth) separeted by a deep graben ( u p t o 4000m. depth). Our f i r s t interpretation of a double spreading ridge i s however very speculative and will probably be modified when complementary d a t a will be collected i n th i s area.

Table : l i s t of the sampling s ta t ions .

Hard rocks :

Nber Lati tude

R A I E 19'54.8

D R 1 19'57.5 DR 2 18'12.2

DR 3 18'13.4

DR 4 18'33.0 DR 5 16'18.2

DR 6 16'58.1 DR 7 17'28.6

Sediments : KG 1 17'07.3 KG 2 18'23.4 KG 3 20'03.4

KG 4 21'24.6 KG 5 17'39.9

DR 5 16'18.2 DR 6 16'58.1 DR 7 17'28.6

Longitude

173'23. O

173'22.1

173'38.3

173'25. O

173'31.2 173'31.2

175'39.1 175'58.1

174'17.3

172O45.5 174O21.4 174'31.2 175'57.4

173O31.2 175O39.1 175'58.1

Depth

2700- 2800

2700- 2800

2550- 2800

2770- 2940 2800-2870 3920-3530

2525-2200

41 1 O- 3600

2990

3071 3333

3 2 5 1 3728

3920-3 530 2525-2200 4 1 10- 3600

Location

axial graben axial graben N45 accident seamount axial graben central graben N60 ,ac&dent wes te+n graben

deep graben N60 accident western graben

Nature

60 kg basalts 50 kg basalts 25 kg basalts 80 k g basalts 50 kg basalts 60 kg basalts 2 kg pumices 80 kg basalts

5.52 meters 6.47 meters 5 .25 meters 4.47 meters 6.37 meters 3 fragments traces traces

Bottom water sampling : H Y 1 19'58.7 173'22.4 2750 HY 2 19'51.2.- 173'24.3 2900 H Y 3 18'34.2 173'30.4 2800 H Y 4 18'09.3 173'29.0 2800 H Y 5 16'18.3 173'32.9 3800 HY 6 16'19.3 173'33 . O 2400 H Y 7 17'18.0 173'33 . O 3700

Bottom photographies :

RAIE 19'54.8 173'23. O 2700- 2800

L. I I 1-'4. //

4

axial graben axial graben axial graben axial graben central graben eastern ridge western graben ,

western side -F axial graben

DR = dredge ; KG = Kuhlenberg core sampler ; H Y = Rosette (water sampler) ; RAIE = camera r u n .

A f i r s t interpretat ion of the magnetic data give a half spreading rate of 37mm/y calculated on the Jaramillo anomaly, and 4lmm/y a t 18's or 3lmmly a t 20's calculated on the anomaly 2 (-1.7 My).

Aside from the NS s t ructures direct ly re la ted t o the recent behavior o f the spreading axis , we observe on the seabeam profi l es several transversal directions of N45 and N25 orientations ( F i g . 3B). The f i r s t ones could be related t o an early phase of opening of the NFB between 10 t o 3 My. The second ones m i g h t correspond t o the recent reorganization of the NFB to the general strengh f i e l d of this area of the Pacific (Maillet e t a l . 1986).

The preliminary resu l t s of the hydrothermal acti-vi'ty of the spreading ridge of the NFB from the Mn and s i l i c a onboard analyses gave negative results on 4 s i t e s and very s l igh t ly positive resul ts on 3 s i t e s near 19"50S and 16'20s. Those results have been subsequently confirmed by ashore C H 4 complementary' anaJyses.

3 - The deformation area of the eastern NFB :

The area mapped d u r i n g the l a s t phase of this leg consists of a square of about l O O k m of s ide between la t i tude 17'10s t o 18'S, a n d between longitude 175'40 t o 176'40E ( F i g . 4A). The pourcentage of seabeam coverage was superior t o 70% and 20 single chane1 sismic reflection prof i les completed the data on t h i s area. Some sediment, hard rock and bottom water sampling have also been performed.

L.111-5.

The major structures shown by the bathymetyric map (Fig. 4A) are two arcuated large grabens separated by a central plateau limited t o i t s northern end by an oblique accident of N60 orientation. The western graben, limited by scars u p t o 1000m. h i g h , i s about 15km wide, reaches 4400m. of maximum depth and i s double a t i t s southern end. The eastern graben, s l i gh t ly less well defined t h a t the western one, b u t nevertheless reaches 375Om. of maximum depth, and i s 10 t o 20km wide.

The sismic profiles enhanced the complicated carac te r i s t i s of the thin sedimentary cover of t h i s area (Fig. 4B). The most recent sediments are perturbated by the tectonic ac t iv i ty , the ent i re area being cut by numerous faul ts . Some oblique reflectors on the central plateau are interpreted as over thrusting. Discontinous re f lec tors , deformated and cut by fau l t s are observed i n this plateau down t o 800m. depth below the seafloor (Fig. 4B). The basement could represent oceanic crust of the early stage of formation of the NFB covered by deformated vol cano-sedimen tary terrains .

The figure 5A shows the main topographic directions of the south-eastern p a r t of the NFB completed by some direction from focal mecanism of superficial seisms observed west o f Viti Levu (Hamburger and Isacks, 1.985).

The formation of these grabens i s probably associated t o the ro ta t ion of the Fi j i islands (James and Falvey, 1979 ; Malahoff, 1982). We proposed t h a t the N-S grabens have f i r s t opened i n a system of N160 dextrial s t r ike s l ipe faults followed by the arcuated deformation of these grabens i n resul t of their subsequent rotation i n th i s system (Fig. 5B). B u t th is N160 s t r ike s l ipe system i s i t s e l f controled by the general N45 dextral s t r i k e s l i pe f a u l t s as the North Fij i and Hunter Fracture Zones.

4 - Conclusion.

The leg I I I of the SEAPSO project allow us :

1. t o survey the central NFB spreading center between 16"s and 21"S, and thus give us a s t ructural frame of a l l th i s area ;

2 . t o map and sample i n detail 2 areas of t h i s spreading centers near 18"s and 20°S, t o caracteri ze th i s spreadi ng center ;

3. t o detect s l i gh t signs of hydrothermal ac t iv i ty near 16'20s and 19'5s ;

L . I II-'6. 4

4. to map and study the deformation area, sismically act ive, of the eastern

5. t o recognize t h a t these s t ructures and deformations are associated t o

6. t o understand much be t te r the present ac t iv i ty and the history of formation

7 . t o prepare subsequent oceanographic cruises for even more detailed studies

NFB ;

active s t r ike s l ipe motion ;

of the NFB ;

of the NFB.

Publications on the preliminary resu l t s of the leg I I I of the SEAPSO project :

J.M. Auzende, J.P. Eissen, M.P. Caprais, P . Gente, S. Gueneley, F. Harmegnies, Y. Lagabrielle, A. Lapouille, C . Lefevre, P . Maillet, J.P. Maze, H . Ondreas, A . Schaff e t R . S i n g h - 1986 - Accretion oceanique e t déformation dans la par t ie

meridionale du Bassin Nord-Fidjien : Résultats préliminaires de l a campagne SEAPSO I I I du N/O Jean Charcot (décembre 1985). Comptes Rendus Acad. Sc. Paris 303, 93-98.

J . M . Auzende, Y . Lagabrielle, A . Schaaf, P . Gente e t J .P. Eissen - 1986 - Tectonique intraocéanique décrochante à 1 'ouest des Iles F i d j i (Bassin Nord- Fidj ien) . Campagne SEAPSO I I I du N/O Jean Charcot. Comptes Rendus Acad. Sc. Pari s 303 , 241-246.

Fi gure captions .

Figure 1 - Seabeam Bathymetric map (100% coverage) of ,the spreading ridge of the central NFB between 19'50s and 20'20S. .. Isobaths interval i s 20 meters.

Figure 2 - Seabeam bathymetric map (100% coverage) of the spreading, ridge of the central NFB between 18'06s and 18'39s. Isobaths interval i s 20 meters.

Figure 3 - ( a f t e r Auzende e t al . 1986a) A : West-East section of the spreading centre near 20'10S.

L . 111-7.

Vertical exageration = 5

B : Preliminary s t ructural diagram of the central p a r t o f the NFB ; 1. Spreading axis ; 2. transverse ridge ; 3. transverse graben.

Figure 4 - (a f te r Auzende e t a l . 1986b

A : Seabeam bathymetric map (70% coverage) o f the eastern NFB (west of Viti Levu) between 17'14s and 18's. Isobaths interval i s 50 meters.

B : Interpreted sismic prof i le . Soft sediment are shaded.

Figure 5 - ( a f t e r Auzende e t a l . 1986b)

A : Structural sketch o f the S o u t h Eastern p a r t of the NFB. The double l ine shows the spreading centre a f t e r th i s study ; the single 1 i nes represent the sei smi c and topographi c 1 i neati ons a f t e r Chase e t a l . (1982) study ; BNF = North F i j i Basin ; ZNF = North F i j i Fracture Zone ;

FNH = Vanuatu (ex New Hebrides) Trench ; ZFH = Hunter Fracture Zone.

Hamburger and Isacks (1986), and t h i s

B : model of formation of the grabens a - early stage of creation of the grabens. b - stage of deformation o f the grabens, more recently.

I

C : Simplified s t ructural sketch of the suryeyed area (see F i g . 4 A ) . Black arrows indicate distension. White arrows show s t r ike s l i p mot ion .

L .'I 11-8. b

L . I I I - ~ . jl Il!

f i g . 3 A - - * I

E W Graben

2600

2800

3000

Eraqiration verticale : 5

: 2800

I

3000

I I l I I t 1 I l I l I l

4 O 2 6 8 I 1 t I

10

Fig, I 3B t

12 14 I6 km

=.

fig93

I

Y L.111- 11.

I

Fig. 48

I E

mi 5 00 400 3 CO 7 O0 W I

f iu.4

L. 4 11-12.

Fig

. ii

5 A

B N -

5c I 1

1 7 O 2 o'

17" 40' '

/

18 O

00'-

176'00' 176O20' I l

f ig*5

SCIENTIFIC REPORT

SEAPSO CRUISE

LEG IV

I .

I

\ I

I

J.P. FOUCHER C h i e f Scient is t .

i

!

L.IV-1

SUMMARY REPORT OF THE SEAPSO L E G IV CRUISE,

SUVA - NUKUALOFA,

30 december 1985 - 11 january 1986.

Prepared by the Shipboard Sc ien t i f ic party.

Jean Paul FOUCHER, P h i 1 i ppe BOUYSSE , Jean Pierre CHOPIN, Marti ne DAVAGNIER, Jacques DUPONT , André GOURMELON, Jean P h i 1 i ppe EISSEN , Y ves FOUQUET , Bernard GUEGUEN , Stéphanie G U E N E L E Y , François HARMEGNIES, Jacques H E R V E O U , André L A P O U I L L E , Janet MORTON , Pascal O G E , Hé1 ène ONDREAS , Yvon PENEAUD, Jean Claude SIBUET,

IFREMER BREST, Chief Scient is t BRGM Or1 éans , geol ogi s t GENAVIR Toulon, technician IFREMER Brest , U B O , geophysicist ORSTOM Pari s , geol ogi s t GENAVIR Brest , Seabeam techni ci an ORSTOM Noumea, petrologist IFREMER Brest , geologist GENAVIR Brest , Seabeam techni ci an U B O , chemist IFREMER Brest , hydrocas t engineer G E N A V I R Brest,seismics engineer ORSTOPI Noumea, geophysi ci s t USGS Menlo Park, geophysicist GENAVIR Brest, Seabeam technician IFREMER Brest , ‘cechni ci an GENAVIR Brest, seismics technician IFREMER Brest, geophysicist.

0

L.IV-2.'" ,

SUMMARY REPORT OF THE SEAPSO L E G IV CRUISE.

The Lau Basin i s an active back arc basin between the Lau ridge (remnant a rc , active i n middle and upper miocene) and the Tonga ridge (volcanic a r c , active from eocene t o present). Identification of the l inear magnetic anomalies in the central p a r t of the basin, between 17 and 20" S , suggests t h a t the Lau Basin i s less t h a n 6 MY old. Yet, the geometry and time history o f i t s opening remain poorly understood. The nature o f such major structures as the Peggy Ridge i n the northern p a r t o f the Lau Basin and the Valu Fa ridge i n i t s southern p a r t have n o t been t o now sa t i s fac tor i ly explained. A t a more detailed scale , oceanic accretion processes, including hydrothermal systems and m i neral i sation occurences associated with the expansion of the Lau Basin are s t i l l a t an i n i t i a l stage of exploration.

.OBJECTIVES OF THE SEAPSO L E G IV CRUISE.

The main objectives of the SEAPSO Leg IV cruise were :

1) t o carry o u t regional bathymetric and geophysical profi 1 es in order t o investigate the geological s t ructural and history of formation o f the Lau Basin ;

2) t o map act i ve vol cani c systems , including oceani c spreading centers i n the southern Lau Basin ;

3) t o search fo r hydrotherma ac t iv i ty .

Because o f the shortness of the cruise - 11 days from p o r t t o p o r t - the three objectives could n o t be considered with the same high pr ior i ty .

The second objective was given a higher pr ior i ty because the cruise was preliminary intended t o bring the basic bathymetric and geological information needed t o p l a n a future submersible diving program focusing on in s i t u detai l e d observation of tectonic, voq canic and hydrothermal processes active i n the Lau Basin.

0 L. IV-3. c

TECHNIQUES.

The investigation techniques used during the SEAPSO Leg IV cruise i ncl uded mu1 t i narrow beam (Seabeam) and 3., 5 KHz echosoundi n g , gravi ty , magnetics, single channel seismic ref lect ion, rock dredging, hydrocasts and seafloor photographic observations.

C R U I S E D A T A ,

GEOPHYSICAL PROFILES.

53 geophysical prof les (figure 1) were run with simultaneous ty , magnetics and recording of multibeam and 3 , 5 KHz echosounding, g r a v

single channel seismic ref lect ion.

The structura

format i on spreading allow one

primary purpose of the geophysical profi 1 es was t o obtain morpho--

and geophysical data t o further constrain the history and mode of of the Lau Basin, and more specif ical ly , t o locate active oceanic boundaries. Of par t icular i n t e re s t , the Seabeam bathymetric d a t a t o determine along the sh ip ' s tracks the sea bot tom structural

directions, the network of which i s useful t o delimit sea floor domains t h a t were formed during one spreading episode. Using these structural di recti ons also helps in correlating magnetic anomalies from profi le t o prof i le .

1

>

Profiles 12 t o 53 were intended t o define the northern extension, i f any, o f the Valu Fa ridge in the southern Lau basin, I and i t s structure.

Profiles 1 t o 1 2 , on the way from Suva t o the southern Lau basin, were intended t o bring fur ther s t ructural information on the Peggy Ridge and within the region around 17'30 S o f a postulated " t r i p l e point", in the Central Lau basin.

I' ? - .

BATHYMETRIC SURVEYS.

Full Sea beam bathymetric coverage was obtained of the inferred northern extension of the Valu Fa Ridge, between 21'15 S and 21'40 S. The mapped area i s about 25 miles long and 5 miles wide.

r

L !I U,-4.

A smaller survey located 3 miles west o f the f i r s t survey discovered a "secondary" volcanic ridge between Valu Fa Ridge and Tofua volcanic arc between 21'20 S and 21'30 S. The second mapped area i s a b o u t 10 miles long and 3 miles wide.

ROCK A N D NATER SAMPLING STATIONS.

Table 1 summarizes the s ta t ions occupied during the SEAPSO Leg 4 cruise.

Tab. 1 .Lis t o f s ta t ions .

station Nr

H Y O1

DR O1

HY O2

DR O2

D R 03

HY 03

DR 04

R 86-01

Da t e

31.12.85

04.01.86

06.01 e 86

06.01.86

07.01.86

07 .O1 e 86

07.01.86

08.01.86

Latitude S

16'32' ,92

19'18' ,18

21'26' $60

21'26' ,O6

21'23' ,37

21'18' ,20

21'18' ,50

D 21'23',30

F 21'26' ,95

D R : Dredge

H Y : Hydrocast

R : Sea Bottom Photography

Longitude W

177'14' ,67

175'26' ,46

176'23' ,70

176'23 ' , 54

176'16' ,30

176 '21 ' ,26

176'20' ,18

176'22' ,GO

176'21 ' ,65

I

Depth (m)

751

2100-1100

2105

2190-2030

247 5- 1700

2085

207 5

2100

L. I V - 5 .

Geological sampling had a low prior i ty compared w i t h bathymetric and geophysical profiling because a primary goal of the cruise was t o provide the needed back ground bathymetric and geophysical information prior t o proposed submersible studies i n the Lau Basin.

A summary description of the dredged rock samples follows.

DRI - located on t o p of a volcanic ridge a t 19'17 S - Recovered rocks : 40 kg of andesi t e s , pumi ce, hemipelagi c sediment, iron oxides and hydroxi des.

D R 2 - located on t o p of the Valu Fa ridge a t 21'26 S - Recovered rocks : fragments of andesites and pumice.

DR3 - located on the western flank of a seamount between the Valu Fa ridge and the Tofua ridge. Recovered rocks : 120 kg of vesicular andesi t e s , pumi ce and pelagic sediments.

DR4 - located i n the inner graben o f the Valu Fa ridge a t 21'19 S - Recovered rocks : 50 kg of vesicular andesites, pumice and sediments.

In a d d i t i o n , rock samples including two 25 kg and 15 kg pieces of andesite and a doler i te (? ) fragment, were accidently dredged by the towed camera system a t the camera s t a t i o n R 1 i n the inner graben o f Valu Fa ridge, between 21'23' S and 21'27's. I

1

The water column was sampled a t s ta t ion HY1, on t o p of the Peggy ridge, and a t stations H Y Z and HY3, in the axial region of the Valu Fa ridge, for ashore laboratory chemical analysis.

M A I N R E S U L T S ,

STRUCTURAL ZONATION.

Four dominant structural directions in the Lau Basin are identified from the Seabeam bathymetric data :

I - The area n o r t h o f 17'30 S .

The PU50 direction i s dominant across the Peggy ridge and i n basins flanking i t , as well as across the Donna ridge, along profiles 1 t o 4.

- The central Lau basin, south of 17'30 S .

Dominant N-S s t ructural directions are observed along profi les 10, 12 and 1 7 i n the central Lau basin, south o f 17'30 S , w i t h i n a domain about l O O k m wide. The extension of this domain, s o u t h o f 2OoS, is uncertain.

- The S o u t h eastern basin (including Valu Fa ridge)

N 20 directions are observed i n the south eastern Lau basin, s o u t h of 17'30 S , w i t h i n a domain, about l O O k m wide, adjacent t o the Tonga arc.

- A domain w i t h transverse directions around 19's.

A narrow s t r i p e of sea f loor w i t h N45 directions cuts the central domain around 19's.

The above reported zonation may re la te to different episodes of formation o f the Lau basin. I f s o , Valu Fa ridge i n the southern basin and the spreading center i n the central basin do n o t belong t o the same oceanic accretion system i n h e Lau basin.

MAGNETIC ANOMALY PATTERNS. 7

Magnetic anomalies along prof i le 10 were intrepreted using a Vine and Matthews type synthetic prof i le , assuming a 3.7 cm/yr half spreading rate . The inferred location o f the spreading center i s close t o tha t 7roposed by Weissel for the study area from previous magnetic data. The Jamarillo event (. 97my) and anomalies 2 (1.87my) and Z'(2.8my) are clearly identified west of the spreading centre. The identification of the symetrical anomalies, east of the spreading centre i s hypothetical.

s c'

L . IV-7.

THE V A L U FA VOLCANIC SYSTEM.

Discovered d u r i n g the Tr ipar t i te program from R/V Lee in 1982, then mapped in considerable detai l between 21'51 S and 22'42 S from F/S Sonne i n 1984, Valu Fa ridge i s an act ive volcanic system of the Southern Lau basin, tha t remains poorly unders t o o d .

Bathymetric and geophysical data gathered d u r i n g the cruise indicate tha t Valu Fa ridge extends north wards a t l eas t to 21'25 S.

We carried o u t a f u l l bathymetric Seabeam coverage of a segment of Valu Fa ridge, between 21'15 S and 21'40 S (figure 2 ) .

W i t h i n the mapped area, the ridge t h a t trends N20, i s morphologically characterized by an uplifted area about 7-10 r e l i e f . A t 21°26N, a small (E I k m ) l e f t la teral o f fse t separates two

wide, w i t h a 200-500 m

segments of the ridge w i t h d i f fe ren t axial morphologies. From a single ridge system w i t h a marked r e l i e f south of the of fse t area, the c res t evolves w i t h a double ridge system, w i t h a subdued r e l i e f , n o r t h of this area.

Fresh l o o k i n g glassy andesites were dredged along the Valu Fa :

ridge c res t .

C O N C L U S I O N S a

1. indicates a multistage formation.

The network of s t ructural directions identified i n the Lau Basin f

! 2 . southern Lau b a s i n , trending N15-N20. I t extends from 22'45 S tr, n o r t h of 21'15 S . Detailed bathymetric mapping of the ridge was carried o u t from 21'15 S t o 21'40 S.

The Valu Fa ridge i s a major act ive l inear volcanic structure of the

3. Valu Fa ridge : a newly developed spreading center or segment o f the Tonga arc , w i t h a high potential fo r hydrothermal act ivi ty and mineralisation ?

!5s

IS'

I I'

IS'

19'

20'

2 I'

2 I'

! 4'

,

I'

L. IV-9. i '

I

i

I I

1

7

{ 1800:T

1800-2000

B 2000-2100,

2100-2300

a 2300-2500 1 1

Simolifled bathymetric map of the Valu Fa _ _

Ridge between 21'15 S and 21"40 S .

f ia.2

PROFIL 18 - 0401.86

VALU FA I

PROFIL 25 - 06.Ol.86 I

W ! n- llhm ! lohaa

I PROFIL 2s - os.m.86 I

I I ESE

I

W I

I

Rx)FIL 19 - 05 O1 86 !

f ia.3 Seismic s e c t i o n s acron t h e Valu Fa Ridge

between 21"15 S and 21 '40 S . I

SCIENTIFIC REPORT

SEAPSO CRU ISE

LEG V .

&- B. PONTOISE

Chie f Scient is t

h

SEAPSO L E G V : SCIENTIFIC TEAM.

Bernard PONTOISE

Jean A U B O U I N

Ni colas BAUDRY

René BLANCHET

John BUTSCHER

Pierre CHOTIN

Michel DIAMENT

Jacques DUPONT

Jean-Phi 1 i ppe EISSEN

Jacky FERRIERE

André GOURMELON

Bernard GUEGUEN

Jacques HERVEOU

Rick HERZER

André LAPOUILLE

Rémy LOUAT

Pascal OGE

Laurent D ' OZOUV ILLE

Bernard PELLETIER

Y.von PENEAUD

Sione SOAKAI ,

Andy STEVENSON

: ORSTOM Paris , geophysicist, Chief s c i en t i s t .

: U n i v . P . & M . Curie-Paris, geologist

: ORSTOM Nouméa , geophysicist

: U n i v . Bret. Occid. Brest, geologist

: ORSTOM Nouméa, drawer

: U n i v . P . & M. Curie-Paris, geologist

: Univ. Paris Sud-Orsay, geophysicist

: ORSTOM Paris, geol og i s t

: ORSTOM Nouméa,geologist

: U n i v . de Li l le I , geologist

: GENAVIR Brest , SEABEAM techni cian

: GENAV IR Brest , SEABEAM techni ci an

: G E N A V I R Brest, ingeneer

: N . Z . Geol. Survey, Lower H u t t , geologist

: ORSTOM Nouméa, geophysicist

: ORSTOM Nouméa , ggophysi c i s t

: G E N A V I R Brest, SEABEAM technician

: CCOP/SOPAC-Fi j i , geologist

: ORSTOM Nouméa, geologist

: GENAVIR Brest , techntci an

: Ministry o f Lands, Survey and Natural

Resources, Tonga , geologist

: U.S. Geol. Survey, Menlo Pa rk , geophysicist

V L v-2 . > Y

*

SEAPSO Cruise

L E G V

N u k u ' Alofa-Papeete (13-28 january 1986).

(from translation of Leg V report by Dr R. Herzer).

1. INTRODUCTION A N D OBJECTIVES OF L E G V.

Leg V of the SEAPSO campaign began on 13 january 1986 a t Nuku'Alofa (Kingdom of Tonga) and finished on 28 january 1986 a t Papeete (French Polynesia).

This Leg was for the most pa r t devoted t o the study of the interaction between the Louisville aseismic ridge and the Tonga Arc (Figure 1 ) .

Carried by the Pacific plate which i s subducted beneath the Australo- Indian Plate along the length of the Tonga Island Arc, the Louisville Ridge

' oriented a t 160" obliquely intersects the zone of plate convergence which i s oriented a t 020". Thus i t "sweeps" the a rc from n o r t h t o south. Suspected of significantly modifying the morphology and tectonic regime of the a rc , t h i s ridge whose nature and origin are perhaps s t i l l controversial, interacts w i t h the a r c around la t i tude 26"s. The strategy of studying th i s type of phenomenon i s therefore c lear . One should look a t i t before i t has occured south of 26"S, while i t i s occurring 26"S, and a f t e r i t has occurred, n o r t h of 26"s.

t

Bearing in mind the relat ive brevity of t h i s Leg,'which lasted only 15 days, of which 9 were spent on the convergence zone, t h e r e s t being taken u p by t r ans i t between Tonga and Tahiti, only two grid SEABEAM surveys were done (Figure 1) . The f i r s t n o r t h of 26"s in the post-interaction zone, the second centred on the contact between the end o f the Louisville Ridge and the arc .

Tele 'a, a Tongan word meaning trench, was chosen as a name for the northern g r i d survey. The objective of this survey was t o study the post interaction phase in a n area where existing bathymetric charts showed a flexure in the direction of the trench. This dislocation could be interpreted as the p a t h o f the return t o equilibrium a f t e r passage of the perturbation caused by the interaction o f the ridge and the arc.

L v73. I

Fetaulaki , again Tonga, s ignifyies meeting. The Fetaulaki grid survey was, as i t s name suggests, designed t o study the zone o f contact between the ridge and the arc. Because of shortage o f time we were unable t o carry o u t a reference survey of the i n i t i a l or pre-interaction s t a t e south of the junction. We had t o be content w i t h the southern for evidence o f th i s .

part of the Fetaulaki g r i d survey

Osbourn Seamount, (f igure 1 ) the l a s t important seamount on the Pacific Plate a t the northern end o f the ridge, was precisely surveyed by'SEABEAM. Two dredge s t a t i o n s were made on i t s western flank, one without success, the other recovering only a few samples.

Dur ing the t r a n s i t t o Tahiti several reconnaissance surveys were made on seamounts in the AUSTRAL chain. The seamounts, uncharted on existing maps, were detected by the SEASAT a l t imet r ic geoide. This unique method of detecting, locating and estimating the volume o f seamounts promises t o be very e f f i c i en t .

Leg V of the SEAPSO mission ended i n T a h i t i on the 28 o f january 1986, a f t e r a mass of data were obtained which have already been i n i t i a l l y interpreted and which will provide a great deal of very good sc i en t i f i c resu l t s . We hope t h a t the t a s t e o f ' ) too l i t t l e l ' o r more exactly of ''too br ief" t h a t Leg V leaves in the mouth of a l l the participants, will be transformed t o " l e t s come back" because already there are ca l l s for projects of deep submersi.blesdives and deep se dri 11 i n g .

I I . CRUISE MAIN RESULTS. *.

1. TRANSIT TONGATAPU TO T E L E ' A G R I D SURVEY. 1

The t r a n s i t from Tongatapu t o the s i t e o f the f i r s t SEABEAM grid survey involved a r a p i d reconnaissance of the morphology of the Tonga trench without losing t o o much time. To th i s end s i x prof i les were carried o u t , four of which cut across the trench. As well a s SEABEAM,single channel seismic reflec- t ion, magnetics a n d gravity were collected.

Looking only a t the SEABEAM d a t a we can get a glimpse of the morphology of the inner and outer trench walls.

k

L v-4. i

A quick scan of the prof i les reveals the following p o i n t s :

a. Profiles 5003-5006 cut the trench a t different depths b u t show progressive deepening towards the s o u t h (9800 m , 9400 m , 9800 m , 10300 m ) w i t h the exception o f the zone traversed by profile 5004 which forms a s i l l . The direction of the axis of the trench between 22's and 24's i s 028'.

b . Although our prof i les only climb a short way u p the outer trench wall (downgoing Pacific P1 a t e ) , one n o t i ces that the d i rect i on o f structures sub-paral le1 t o the trench (about 028') changes an approximate north-south direction (007"). The feature w i t h the 007' orientation corresponds t o a more- or-less horizontal area (par t o f a s t a i r t read) . I t does n o t influence the direction of the ax is o f the central part o f the trench. On the inner trench wall there i s above a l l a s t ructural grain parallel. to the trench, offset here and there in an east-west direction .

I n conclusion, there are two structural trends on the down-going plate :

a primary trend parallel t o the trench (028") imparted perhaps by the subduction process ; a secondary trend approximately north-south (007") which could be the direction o f the original structural grain of the Pacific plate . On the inner trench wall the principal structural grain is parallel t o the trench w i t h east-west o f f se t s .

2 . T E L E ' A S U R V E Y .

The Tele'a g r i d survey, contained w i t h i n 23'45's and 24'30's la t i tude and 174'45'W and 175'45'W 'longitude covers the central : p a r t of the Tonga trench.

The Louisville Ridge will have been subducted obliquely (figure 1 ) into the Tonga trench w i t h a north-south sweeping m o t i o n , as the convergence vector of 300" for the two plates suggests. One would expect, therefore, tha t the morphology of the arc w i 1 1 show some characterist ic evidence of t h i s . The questions t h a t come t o mind are :

- What does the morphology become , - Did the subduction o f the ridge leave deformation trends linked t o i t s

trend (160") ?

Does the ridge-arc interact ion cause irreversible or reversible changes i n the morphology, and i n th is case i n what time frame ?

-

We will attempt t o resolve a number of questions by SEABEAM surveys as well as by the classical methods of seismic ref lect ion, gravity and magnetics.

A t t h i s stage i t i s possible t o make tentative conclusions from the Tele'a grid survey (however these have had to be made in the l ight of the conclusions drawn from the Fetaulaki survey).

Following i s a summary o f our interpretat ion of the structural elements revealed by the g r i d survey (figure 2 ) .

a ) The oceanic plate has a north-south s t ructural grain of large horsts and grabens induced by flexure of the lithosphere as i t enters the subduction zone. There i s an east-west magnetic g ra in underlying the forearc and trench which strongly suggests deri v a t i o n from the ori gi na1 , spreading geometry. This would suggest in t u r n t h a t the Pacific plate in this area i s n o t 80 t o 120 Ma old. This hypothesis needs t o be verified by a wider magnetic survey.

I

b ) Contrary t o what has been indicated by previous, bathymetric mapping, the trench i s l inear and trends uniformly 020'in the survey area (which includes the region of the previously mapped o f f s e t ) . This poses the problem of accommodating the trend of the trench mapped in the Fetaulaki survey for i t requires a bend i n the trench axis somewhere beween the surveys.

C ) The inner trench wall appears uplifted i n the south, transit ional in the centre. a n d downdropped in the n o r t h , strongly suggesting t h a t i t represents a return t o equilibrium of the subduction system a f t e r being affected by the interaction of the ridge and the a rc . The relat ively high

southern region of the inner wall i s exactly in l ine w Louisville Ridge trend (340") . I t i s surprising t o see straightforward subduction regime so spectacularly man

L V-6. I

t h the projected the return t o a fes t e d .

3 - FETAULÁKI G R I D S U R V E Y .

The junction zone between the Louisville Ridge and the Tonga a rc were surveyed w i t h seabeam by the vessel Jean Charcot between 26"25'-26"15'S and 175"50'-174"50'W. The junction has a twofold morphology, t h a t of the inner trench slope where the structure paralells the general 020" allignment of the trench and t h a t of the outer trench slope where the structure shows a great deal o f north-south lineation i n the form of well developed depressions.

A - The oceanic plate which forms the often steep outer slope of the trench has two categories o f re l ie f :

- Seamounts, represented here by Osbourn Seamount, which peaks a t a dept of 1920m, and a smaller seamount, already i n the trench, with a summit a t a depth o f 5120m (figure b o t h are distinguished by strong magnetic anomalies.

3 ) . The l a t t e r we have named Mo'unga seamount ;

Osbourn Seamount i s surmounted by a platform t h a t seismic profiles show t o be a sedimentary layer o f 0.5 sec. average thickness. The detailed morphology resembles kars t with many closed depressions. The section agrees well with the dredging resul ts of G N I B I D E N K O e t a l . (1985) who reported carbonate sands and basal ts . The thickness o f the sediment cover could mean khat this p a r t of the LouisvilleRidge was i n the "West Pacific hiatus" zone.

- North-south f a u l t scarps t h a t describe a..horst and graben system by which the Louisville Ridge decends , sometimes abruptly, into the trench. This s e t of fau l t s s t r ikes into Osbourn Seamount, a guyot whose width i s far greater than the spacing of the fau l t s . By contrast Mo'unga Seamount, w,hich has been carried into the trench on a horst block, i s small enough t o f i t within this fau l t spacing.

This f a u l t system, which extends more than 30 miles o u t from the trench, ref lects the flexure of the oceanic plate as i t begins t o descend into the subduction zone and i s n o t a reactivation of the s t ructural grain of the oceanic plate . On the eastward t r ans i t l ine towards Tahiti these effects were seen t o f i na l ly die o u t almost 60 miles eas t of the trench, af ter which new linements appeared with azimuths of 80", 110" and 140" which were encountered fa i r ly constantly from then o n .

L v-7. v,

The f au l t offsets die o u t and disappear t h r o u g h Ozbourn Seamount ; no d o u b t they will become accentuated as the seamount approaches the subduction zone as i n the case of Erimo and Kashima Seamounts which are decending into the Japan Trench.

B - The inner trench slope i s composed of successive N-S t o 020' trending slope breaks, some of which contain small perched closed basins. The morphology and seismic profiles suggest downdropped and rotated f au l t blocks. There i s nothing t o suggest the presence of compressional features such as faul ts and thrusts .

There does n o t appear t o be any morphology indicative of active accretion i n the Tonga Trench, even a t the junction with the Louisville Ridge. The Tonga arc i s therefore of the extensional convergent margin type.

The northern third of the inner trench slope i n the Fetaulaki study area, where the coll ision w i t h Mo'unga Seamount i s taking place, i s on average 2000 m higher than the area t o the south, and shows evidence of back-ti l t ing of the lower slope. This may be the e f fec t of up l i f t due t o coll ision w i t h other seamounts and t he i r eventual passage beneath the margin. The southern third of the inner trench slope i n the survey area i s characterized by a major depression of the lower trench slope, which seismic profiles suggest i s due t o the foundering o f the slope into the trench. We canno t exclude the possi bi 1 i ty , however, t h a t t h i s depressed area belongs t o the trench i t s e l f (see bel ow) .

I

C - The trench comprises the fol lowing secti-ons I , :

- a northern section which widens progressively t h r o u g h fault ing on the oceanic s ide.

- a central section which contains Mo'unga Seamount, perched on a horst block of oceanic crust . The actual subduction contact i s west of the seamount on a l i e s j u s t east o f the seamount on the oceanic side (6000 m). The subduction contact in th i s case i s thus n o t a t the deepest p o i n t in the bathymetric

a t a depth of 5600 m. This i s higher than the graben t h a t

trench.

Mo'unga Seamount, and the horst which supports i t , i s an obstacle t o subduction which can evidently be easi ly accomodated w i t h minor up l i f t o r

! I L V-8.

foundering of the inner m a r g i n , f o r n o t h i n g i n the profiles suggests any change i n the tensional nature of the faul t ing t h a t surrounds the seamount. S imi la r observations have been made where Erimo and Kashima Seamounts are being subducted i n the Japan Trench.

- a southern section where the main trench is very s t ra ight and corres- ponds exactly w i t h a graben on the decending plate. A parallel and shallower graben on the oceanic s ide is destined t o become the future trench when the graben forming the present trench axis i s subducted.

However, one cannot exclude the possi b i 1 i ty t h a t the apparently subsi des region forming the base of the inner slope (see above) may, i n f ac t , belong t o the oceanic plate w i t h a small graben and a horst forming the basal par t of the inner trench wall. The s i tua t ion could be analogous t o t h a t of Molunga Seamount where the subduction contact i s higher than,and west of , the deepest p a r t of the oceanic p la te .

The al t i tude of the subduction contact may thus vary s ignif icant ly along the length of the trench, dropping t o the level of the graben floors and rising t o the level of the horsts on the oceanic plate.

D - The collection of data presented here i s compatible w i t h a zone of convergence accompanied by a destral sweep of the foot of the inner trench wall by structure on the down-going plate .

Only the short section of the northern part pf the study area, t ha t is i n the region of Mo'unga Seamount, i s there an active junction w i t h a volcanic edif ice; further nor th ithere i s no longer such a junction, while fur ther sou th such a junction has ye t t o form. Osbourn Seamount, which i s not yet i n coll ision will be so t o the southwest a f t e r the different grabens of the oceanic plate have successively occupied the trench axis.

The general direction of the trench i s defined by the shape of the margin of the Tonga arc and n o t by the subduction. This observation, which has a l s o been noted a t the junction of the Japan and Kurile Trenches, leads us to suspect t h a t a r i g i d basement of non-oceanic origin underlies the Tonga arc.

c L Y-9. *J

In th i s area, which i s one of col l is ion on a grand scale w i t h an

oceanic ridge, no accretion appears t o be taking place. Instead, the 'trench slope i s broken by extensional faul t ing as the oceanic plate i s subducted. The Tonga arc would thus belong t o the category of an extensional convergent margin, which in t h i s case i s unique for the dextral (southward) sweep of i t s point of coll ision w i t h a ridge on the down-goint plate , and which i s particu- la r ly well displayed thanks t o the thinness of the sedimentary cover.

As w i t h the Marianas and Central America Trenches - where the model of extensional convergence was developed - and the Japan Trench, the Tonga Trench i s also a good example of an accretion-free margin. I t contrasts with a compressive convergent margin where active accretion i s apparently due t o a great thickness of sedimentary section.

GEN ERAL CONCLUSIONS.

Despite i t s brevity, Leg Y of the SEAPSO campaign has made i t s possible t o gather new data o f great i n t e re s t on two well defined problems :

i

- the interaction of an ase island arc (The Tonga - Kermadec

smic ridge (the Louisville Ridge) w i t h an a r c ) .

- the t e s t of a unique method of detecting, locating and evaluating submarine mountains by analysis of SEASAT s a t e l l i t e al t imetric d a t a . The predicted location of the seamounts emphasized by th i s method was confirmed by Seabeam survey. 2

W i t h regard t o the f i r s t problem, there was a need for a quan tum leap i n the scale of observation. Previous data could n o t futher resolve 'the problem a n d Seabeam held the only promise of advance. This proved t o be the case and even t h o u g h , the d a t a acquired in the two survey areas i s only par t ia l ly analysed, structural and tectonic interpretations are already well advanced. The orientation of the trench i s defined by the form of the edge of the arc and n o t by the subduction geometry.

The Tonga-Kermadec arc belongs t o the category of accretion-free extensional convergent margins l ike the Mariannas, Central America a n d Japan Tranches. This par t icu l ia r arc i s unique in t h a t an aseismic ridge on the

9

,\ L v-10. *

down going plate , oriented a t an oblique angle to the a rc , i s sweeping down the arc as the plate i s subducted. A model for the interaction of the ridge w i t h the arc must involve continual tectonic erosion of the lower trench wall. With the development of such a tectonic model, new objectives come t o mind which deep submersible dives should help t o real ize .

With regard t o detection of seamounts, the l a s t p a r t of Leg V has shown t h a t although some refinements are s t i l l necessary in the e l ac t i c thickness model used for detecting submarine features , the locations of features predicted by SEASAT data are confirmed. One the most satisfying sc i en t i f i c studies i s coming t o a conclusion as , a f t e r the early analytical phase, the detection o f submarine features by sate11 i t e al timetry will become a routine method.

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TRANSIT TO TELE'A I Box

Relative positions of the Louisville lidge, the Tonga Ridge and Tongatapu from tbc present to four million y e a n ago with the Australian plate fixed. syeading In the Lau Basin Is taken as 4 m.y. and the Autral ia and Pacific plate motiom w from Stock and Moolnar (1982).

The duration of I

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SEAPSOI~ I3 -281 1186

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S c h e m a s t r u c t u r a l de la boite TELEA' (Provisoire)

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IFREMER ORSTOM

SEAPSO CRUISE 16 October 1985 - 29 January 1986

GIS Oceanologie e t Geodynamique

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under patronage of CCOP/SOPAC