~
OCEANOLOGICA ACTA, 1990, VOL. SPÉCIAL 10
----~
Impact of oceanic asperities
on the tectogenesis
of modem convergent margins
Subduction
Oceanic asperities
Tectogenesis
Tectonic crosion
Underplating
Subduction
Aspérités océaniques
Tectogenèse
Érosion tectonique
Accrétion subcrustale
Serge LALLEMAND a, Jean-Yves COLLOT b, Bernard PELLETIER c,
Claude RANGIN a Jean-Paul CADET a
a Laboratoire de Géologie Structurale, URA CNRS 1315, Université Pierre et Marie Curie,
4 place Jussieu, 75252 Paris Cedex 05, France.
b Station de Géodynamique sous-marine, Antenne ORSTOM, BP 48, 06230 Villefranchesur-Mer, France.
c ORSTOM, Centre de Nouméa, BP AS, Nouméa Cedex, Nouvelle-Calédonie.
Reçu le 15/09/89, révisé le 29/06/90, accepté le 03/07/90
ABSTRACT
This paper presents sorne of the implications of oceanic asperities in subduction zones,
based on examples gathered during the world circumnavigation of RN Jean Charcot. The
study of these examples provides a new overview of active margin modelling in response
to asperity ubduction. It is conc1uded that both erosion and accretion rnay be govemed by
the oceanic plate even though frontal tectoruc erosion is better docurnented because il is
accessible to conventional surveys. Most of the time, erosion is due to the relaxation of the
arc slope in the wake of asperity subduction.
Oceanologica Acta, 1990, volume spécial 10, Actes du colloque Tour du Monde Jean
Charcot, 2-3 mars 1989, Paris, 17-30.
RÉSUMÉ
Rô1e des aspérités d 1a croûte océanique dan la tectogenèse des marges
convergente
A partir des données géophysiques récoltées lors du Tour du monde du N/O Jean Charcot,
nous présentons une première synthèse des effets produits par la subduction d'aspérités sur
la structuration des marges convergentes. Les exemples de zones de subduction étudiées
sur le pounour de l'Océan Pacifique (Japon, Philippines, Nouvelles-Hébrides, TongaKermadec et Amérique Centrale) ont permis de reconnaître quatre types d'effets: impact
morphologique, éro ion tectonique, déformation interne de la marge et accrétion
subcrustale. Des liens élroits existent entre la morphologie de la plaque en s ubduction et la
structure du rebord de la plaque chevauchante. L'érosion tectonique des marges actives est
de mieux en mieux documentée el emble e produire principalement dans le sillage des
aspérités en subd uction. En revanche, l'accrétion subcrustale de lambeaux sédimentaires ou
océaniques est oupçonnée dans plusieurs ca mais reste encore difficile à prouver.
Oceanologica Acta, 1990, volume spécial 10, Actes du colloque Tour du Monde Jean
Charcot, 2-3 mars 1989, Paris 17-30.
17
S. LALLEMAND t t al.
ll\'TRODUCTION
aeeretio/Jary prism in the text bceause, in our opinion.
Ihis laSI tenn is nOI appropriale 10 the consuming margins
Ihal we descrihe. Among the discussed areas. it can only
he uscd for Ihe Nankai Trough.
Fro m 1984 10 1987. se ven RN Jeall Charcot cruises
werc dcvOlcd to Ihe study of the subduction zones around
the Paci fic rim where numero us aspcrit ies of the oceanic
plaIe arc subductcd. Chronologically. these cruises were
(Fig. 1): Kaiko 1 off Japan (from JUlie 10 August 1984);
Pop Il off Mindoro (No vembcr 1984); Nanhai off Luzon
( March 1985); Seapso 1 off New Hebrides (Oc tobcr
1985); Seapso V off Tonga (January 1986); Seaperc off
Peru (July 1986) and Seamal off Middle America (from
June 10 July 1987). Sorne of Ihem. !ike Kaiko 1or Scapso 1.
werc followcd by in situ o bservations and mo nitoring
from Ihe Nautile submersible: the Kaiko Il (from June to
August 1985), Subpso J (March [989) and Kaiko-Nankui
(Aug usi-Se ptembcr 1989) Nadir cruises. The occanic
asperitics ean he fault scarps. seamounts, hot-spot chains.
spreading ridges. ascismic ridges. cominenlal p1alforms.
oceanie platcaus o r fraelUre zones.
MORPHOLOGICAL IMPRlNT
When an oeeanic asperity subducts, the margin adapls
itself o n the undcrthrusting lopography even whc n the
aspe rities are small fauil scarps. Scabeam and singlechannel seismic data rccovered from RN Jeall Charcot
surveys provide an excellent support for diseussing this
topie.
Ckcanic faulls
During Kaiko 1 erui se in the no rthe rn Japan Trench
(Cadet et al., 1987). we have shown that IWO sets of
oeeani e plaI e fault s arc re ae t ivated bene ath th e
continental margin, and that grnvÎty sliding consequently
dcveloped in th e upper pla tc alon g Ihese oeeanl e
directions. They a re N-S trending horst and graben s.
related to the bending of the oceanic plate when enlering
the subduc li o n zo ne, on the one hand , and anc ie nl
normal fault s sel N 65 0 inhc ritcd from the original
struelure of the occanic erust, on the other hand (Fig. 2).
The N 65 0 fault set para!lel 10 the oceanic mag netic
lineati ons seems 10 be reae ti va ted into vert ical (or
We reeognizc four effeels of subdueting asperities on the
tcetogencsis of the murgins:
• Morphologiea! imprint:
• Tcctonie erosion;
• lnlemal defonnation of the margin:
• Underplating o r aeerelion.
Wc shall use te nns like margin eomplex. comillel/lal or
island arc II/argin, upper plate o r wedge instead of
"'" z?
?
~:1
'.
, ~
i
1"
'\,\ "-J
&l
'~
,
" ' .0 1
1
10 ::_ _ ~
1
r,~
...... '
l
PA CIFie
.•
OC E AN
•
,
.',
Figure 1
Location of th~ PuC/fic Qrras sun''}'~d dUFl/lg Ih~ ClrcUmntll'/KQlion 01 Hi\' Jtan Clwrclll and dlSClISsed i/l lM lUt.
LocalISation de) l.OIlCS reconnues lors du Tour du Monde du NIO Jean Charcot et discutt:es dans te IUle.
18
'f
ACfIVE MARGIN TECfOGENESIS 1OCEANIC ASPERmES
reverse) faults wilh a major righl-Iateral compone nt .
Apparent vertic al o ffset cao reach 50 to 200 m and
appare nt laierai offse l of a fe w kilom etres can be
c hecked a long the tre nc h axi s for ex ample ( Fi g. 2,
Lallemand et af., 1986). The N 65° Ijneaments can he
traced 25 km landward of the trench axis al a point where
the cn/stal thickness of the margio reaches 8 km. Thus.
we can, al least partly, relate the N-S cliffs of the
landward slope to the passage of the N-S oceanic nonnal
fau ll scarps. Thi s relat ion impli es thal the oceanic
grabens are not enürely fill ed by slumps or IUrbidilcs.
AJlematively, cliffs of the landward slopc may be caused
by oceanic faults that are reactivated below the margin.
as sho wn by sorne focal mechanisms of carthquakcs.
The horst and graben morphology dcvelopcd aeross the
plungi ng Pac ific plate on the Tonga and Ke rmadec
tre nc hes seaward s lopes (Fig. 1 and 3) pro ba bly
COostÎtutes the best example of ex tens io nal faulting
induced by be nd ing of Ihe downgo ing Iithos phe re .
Multibeam mappings at different places in the Tonga
Irc nc h a rea (Seapso V: Jean C ha rcot c ruise and T.
Washington transit) indicate that the bending-induced
scarps are frequent ly up 10 more th an 1 000 m and
somelÎmcs reach up 10 1 500 In in vertical displacemenl
(Po ntoise et al .• 1986; Lo nsdalc. 1986; Pelletier and
Dupont, Ihis volume). Near the deepest part of the Tonga
Trench (mo re than 9 500 m decp), thcse spcetaeular
ho rs ts and g rabens trend N 15° to N-S. They a re
di scontinuous along s trike and Întc rrupted by small
transverse scarps Ihat trend N 600-70<'. These transverse
features probably correspond to the grain of the occanic
cruSI, because Ihey pamllel the magnctie lioeations. The
Ireneh axis shows "en-éche lon" basins almost parallel 10
the major scarps of the seaward slope and surrounded by
stcep slopes; the landward side bcing a straighl scarp of
A
Figu~
2
A- Siruelu,al map . 1 _ Slrue"'ral aligflmCI1lS or "/inCUflI/tI1lS·
subIJU,.all~/lo
occanic magnctie /incalions. 2 - Vcrtical fau llS. J - Normalfaults.
4 - Apparent rn·ersc f uults 011 !il"ismie projilt"s. 5 _ Upfa" lled bloc/:. (5 • 1J0wnfaultcd hlack.. 7 . Inlersec/ion PQint btt ...·UII N (55~ IilleUM/';I1IS and an
é· 1V sd !imie projilc. 8· Fuull wilh ullellSl OIlC important r'ghf·lalcral c(>/llf'Oncm. 9 - Pacifie plaie motion rdali.·c 10 Iht! Japantst plaM (N 55 'W).
10· Escarp_fIlsfaciflg Iht! Irr flch lUis (aftu Lallcmand el al.. 1986).
8- 7-OOm Oll l~ Stabeam hoJhytIYlry o[ /~Japan 1....nch showing a seclor ,.·huc JOffllI N 65°lùleamf'I1IS an aligllcd across Ihe ITCllch ax,s (in dolled lilles).
Jwbtuh.1 caeh 20 m.
A- Catte ~truelUrale. 1- Alignc rlle UlS Stl\lC1Urau~ ou "linéaments" subparnll~ les nUll linéations magnétiques. 2- Failles venicales.)- Failles normales.
4- Failles d'aspect in ver$C s ur les prolils s ismiques. S- Bl oc soulevé. 6- Bloc ab;ai ~sé. 7- Imerscction entre les li néaments N 65° el 1C!l profil s
sismi~ues E-W. 8- Faille ayant au moi ns un rejet dextre important. 9. Mouvement re lat if de la platj Uf! Paci fique par rapport à la plaque Ja ponaise
(N S5 W). t ().. Escatpements rcgaro.:u1l1a fosse d'après Lallemand /';1 al.• 1986. B-Gros plllfl de la balhylTll!trie Seabcam de la fO$se du Japon montranl
un SCCICur o ù des linéaments N 65° traversent l'axe de la fosse (en pointi llés). Intervalle e ntre les isobathes : 20 m
s. LALLE:"IAND II al.
700· 900 m height. Si Ils (or bascmcnt highs) between
bas ins arc observed where slightly oblique horsts and
grabens or transverse struclUrcS enter into the subduction
ronl!:. The landward slope exhibits a stepped morphology
suggesting a back-tilted fault -block ICClOnics. Scarps
rcilch 200 to 800 m in heighl ilnd strike N-S 10 N 15°,
parallel to Ihe Hench and SCilward slopc major scarps.
These data clcarly suggest that the morphology of the
lower landward s iope is largely governed by Ih e
Slruc turc s of the s ubdu cting plate (Lonsdale. 1986:
Pelletier and Dupont. this volume), Possibly. as in the
Japan Trencll , structure. collapse and erosion of the
Tonga landwa rd s lope is contro lled by the passage
undemeath of thcse spcetacu lilr bcnding-induccd scarps,
panly subductcd in the southem Jupan Trench. rcsulting
in a 7 km reentrant and several hundrcds of metres of
uplift of the continental margin. This produces a lrough
paralle l 10 the Irench and 20 km landward of il. The
mo rphological impri nl of [he s ubducted pan of the
seamount on the margin is accommodated in thr.ce ways:
the oceanic plate is depressed by more than 100 m due tO
the load of the scamount and the landward pan of Ihe
seamounl is down -fault ed up to 1, 7 km; the lower
land ward slope is uplifted in rcsponse 10 the local
thickening of the IOC of the prism: and the oversteepened
lowcr slope collapses and the debris from mass-wasling
subducts as suggested by the Jack of filling in the Irench
(Lallemand el al., 1989 ),
ln the Manila Trcnch (Nanhai eruise), Pautot and Rangin
(1989) noticed thm the inherited fabric of Ihe subducled
occanic c rust had no major structural imprint on the
land ward si ope fabri c except the trans form fault s
morpho logy whi e h g uide s s lightly the trend of the
dcfonnation front at the base of the landward slope.
At the junclion bclwcen the Japan and Kuril trenchcs, it
has been alsa demollstrated by Lallemand and ChamOIRooke ( 1986), mainly on the bas is of ilS magnetic
signature, that a subdueting volcano is responsible for Ihe
20 km reentrant of the land ward slope. lnasmueh as Ihe
scamoum is entircly buried bclow the lower landward
slope. ilS size has been deduced from a 3-D magnetic
modcl and corresponds 10 30 km in diametcr and 1.8 km
in height. that is 10 say about 2500 km). The margin,
forme rly upliftcd by 1,6 km above Ihe top of the
seamoun!. (corresponding 10 about 2000 km 3 of positive
topographie anomaly), eollapsed behi nd Ihe Irailillg flank
of the seamoullt producing an indentation (corresponding
tO about 500 km 3 of negative topographie anomaJy) of
Ihe lower arc slope (sec cross-section H-G on Fig. 4 and
Fig. 5).
SeamQunl"
The western Pile ifi c plaie supports many seamounts,
some of Ihem just beginning 10 subduel al lrellches.
The Kaiko programme has studied several seamounls
that arc now at various stages of subduclion in the Japan
Trenetl ( Fi g, 4). Th e Daii c hi Ka shima seamOullt,
approximately 60 km in diameter and 3.5 km in heighl, is
Flgu re 3
Stru('(ural sAc/(h of Ihe SLV Pacifie f('gion
al. /9871.
sho"'I11,~
th"
~tudlfd
urcu. Isoballis each km (modifitd /rOIll Collai el
Schéma Struclul'lll du Su d·Ouest PacifIque momrant la lOllC d'étude, Les isob.1thes sonl comourte$ 10US [e5 km
(modIfié d':lprês COIIOII.'I al .. 1987).
20
ACfIVE MARG IN TECI"OGENES IS 1OCEANIC AS PERITlES
, • • 1Ii< ,lolO
,lot.,
MOI..., •• " " .1
•••• 1tK
~
"'"""
Oep l ll
SeC/iD'"
'0 km
II.E.- ' .5
A
B
~
O R / 1'·4
D
t'!::
E
F
~
l(6,i~"
,.,
Figure 4
l..ocDÛOllS of Stanwullls Ilt/U lM }DfXJn Trtnch uxis Dnd SÎmpfijiN dtplh sutiollS alollg SQn~
stismir /intS al differtn/ s/Dgn 0/ StanwulII subduction. Shudtd bund rtprntnlS arta 0/
StatrlOallf dmi" subduction. Circ/td Ilumbers rt/er 10 di//tr(llt sta/nounlS plOfltd a/ollg Iht
rrtnch (a//er IAllem.:lM DM U Picho". 19lJ7).
Loçalisation des monts sous·marins le long de la rosse du Japon CI sections pro rondeurs
simplifiées le long dt: qudques lignes l>ismiques 11; différen ts SUIdes de subduction de mon ts
$Ous·marins. La bande en grisé n:pn!:s.ente la projection liOUS le prisme de la chaine volcanique
$Ous·marine. Les nuffil!ros cerclés renvoient aux monlS sous·marins reportés s ur la carle (d·a~
Lallemand Ct Le Pichon, 1987).
21
S. LALLEMAt\'D tt al.
N4120
•
• •
•
•
o
•
5
10l<m
--'------"
'--,
Figure 5
Structural nw(' "fl" t'unClian bm"un Ih~ Ja(UJn alld Kurillundru. DaJh~d line 01 ba$~ oflundwl,rd Ilo~: det~JI
fXCurfenc~ af'~ lop
acauslic hascmcn/lmder lurhUli/es. Sufid llafl: Ilu: /Op of ,'olcano. Open SlarJ : base oflile seamounJ
(at m'uage deplh of 7S0 m. dklmelf!f J2 km). BOIh conlOurs au en/lui)" tkduau from mllg/Ullc motklling a/ur lJJ/{emand
und Chnmol·Rooke. 1986. A and B are lhe locatloM of NAUfILE dil"u. Gr~ pal/un : «curunee of lUnch fill. CU""es ...,llh
long barbs : c/ijJ.I. S/rai8ht IilU!S ...ilh small barbs: QCtunit.: IWrmal jaullS. PI 10 P9 : locations of single·challJll!/uismic 1i1U!S.
huel: J . Ireneh " Jupon Tff!neh. K. 1. " Kuri/Tuneh.
Cane slIUClurale de La joonioo des (06)eS Japon t\ Kouriles. PointillEs l ia base du mur interne: profondeur maJlimale de la
croûte. ÉIoilei pleines ; toit du voICôIIl. Etoiles 4!vidfc:s : base du vo\e;an subduit (profondeur mO)'Cflne: TISO m. diamèue: 32 km).
Ces oomours SOIIt emièremenl d&luilS du modèle: rtI3~UC d'aprh Lallemand et OJamot·Roon. 1986. A el B localisent les
plongées du Nautile:. Zone grisée : sédirnel1ts de rempli~ de la fosse. Courbes avcc de longues barbelun:s ; c:scarpcments.
Vrones !l' 'CC de peilleS twbelu~ ; fail les ll()I"Tl"I.1Ic5 oœ.2niqucs.. 1'1 11'9 ; locaIlsw.lon des prufll~ tk: ,ilol llÎljUl: "".u...u""".
Encan: J. trcnc;h ... Fmsc du Japon. K. l " f-osse des KouriIe$..
Figure 6
Central Ne ...'· // ebrides is/and "TC. AfP: Aus/ra/ia· /ndia Plaie. Ilarlleu line shows Ih~ !il!apoor trur;~ of tllf in/erp/ott
dccol/('melll , barbs jhow d/J ....ndip direction . HaIlJ)"m~,,!c comour inur\"a/ is 1 km . W.H . • LVous! Bank. Locu/iun of fig. 7
(al'u Cal/ot and f"is~r. 1989).
Arc insulaire des NuuveJles· lIébrük s centrales. AIr>: Plaque ludo·australiennc. La li gne a,·cc les uianglc$ sou ligne la tlTlCC
sur le) fond , muin, du d~coll~mem im~rplaquc. Ics mangles montrant la direclion d'c nfonce menl. Les contours
balhyrœtnques som reponés tous les ~m. W.B." Banc de \Vousi. Localisation de la I-ï g. 7 (d'après CoLlot CL Fisher. 1989).
22
ACTI VE MARGlN TECTOGENESIS / OCEANIC ASPERITIES
Lallemand and Le Picho n (1987) conside red that the
Japan Trench margin cvolved as a homogeneous wedge
of defonnable, noncohesive Coulomb material during
seamount subduction, and show that the application of
th e Davis. Suppe and Dahl en modcl (1983) could
acco unt fo r the observed deformat io n. Thc ir mode l
pred icted th al the fir st con sequence of seamount
subduction is a compressive thickening of the toc of the
wedge above the advancing flank of the seamount
because of the oversteepening of the rigid base of the
wedge; this agrees with our observations in the Daiichi
Kashima area. This thickening shifts landward across the
wedge with the advancing flank of the seamount. Erosion
dom inales the wedge above the trailing flank of the
scamount because of the corresponding understeepening
of the rigid base of the wedge. A reentrant is thus created
in the toc of the wedge; il is largest when the oceanic
f1ank of the seamount coincides with the base of the
landward slope. At this time, the trench will probably
have an abnormally large amount of sediments, as
illustrated at the junction between the Japan and Kuril
trenches (Fig. 5). After the subduction of the seamount.
the edge of the wedge should progressively move baek to
its initial location, because no significant reentrants along
the Japan Trench margin have becn obscrved, despite the
probable past subduction of seamounts.
stage of subduction as the Daiiehi Kashima seamount in
the Japan Trench (Fig. 6). Funhennore, its size is similar
10 that of Kashima. The area was surveyed by RN Jean
Charcot during the Seapso 1 c ruise and the follo wing
observations were made: no clear normal faulting affects
the seamount. Ihe margin is uplifted abave the advancing
flank of the seamount and the margin presents a 10 km
reentrant (Daniel et al., 1986). Immediately south of this
seamoun! lies the deep subcircular Malekula rcentrant in
the lower arc slope (Fig.7). Us proximity as well as the
simîlarily in morphology between Ihis rccntranl and that
al Ihe junclion between Japan and Kuril trenches suggest
thal il îs also fonned by the collision of a scamount with
the arc (Collot and Fisher, 1989). No clear magnetÎc
signature proves Ihat a seamount i5 subducling, but a
strong and nearly circular uplift of the nonhem e nd of
Malekula island (30 to 40 km land ward of the rcentmnt
along the direction of plate convergence) has been
recorded. Consequently, il is probable Ihat a subducted
seamount forms an asperity and caused both the recent
uplift and the recenl reentrant. Using Seabeam data,
Collot and Fisher ( 1989) propose that after fonnation of
the rcentrant. IwO styles of defonnation cooperale to
rcstore the arc slope: one involves eollapse of the slope
rocks surrounding the reentrant; the other includes the
fonnation of a new aecrelÎonary wedgc aeross the mouth
of Ihe reentrant (Fig. 7). These authors suggeSI that such
a restorat ion process co uld lead to formation of a
s ubc irc ular for ea rc ba s in; s uc h a bas in may be
exemplified by the basin ensconced in the Efate reentrant
(Fig. 6 ).
ln the New - He brid es Trenc h off Es peritu Santo
(Vanuatu), the Bougainville seamount is no w at the same
During the SEAMAT e ruise (Fig. 1), which occu rred
along the Middle America Trench, a topographic high
Part of the missing slope material is sedimented in the
Ireneh, as suggested by the unusual trench fill thickness
reaching locally 700 m (Lallemand, 1987) and the rcst
might he subducted.
167"30
Î
;~
.....
~
....
,-,-
-
Slumps
"'~.
In\erp/ale oecollemOnl
FoIej'ancHtlrust b&lt
Uplilt cont ou~ (m)
Figure 7
Structural ifllerpret(J.tio~ of the morp~logy of tM Malekula ru lllmlII. which is outlined by tM ho.% i/l figure 6.
COIIIOurS on Ma/ekula ISlaM are derlYed from coralterraces and show the magnitude of up/ift during tM /965
eurthquuke. The '!laldula ~Utllralll. and the area of maximwn uplifl are a/igned along th e plaie convergence
~ecior. Bathymetrlc COlllOur If/urvalls 500 m (ufter Col/ol and Fisher. /989).
Interprétation structurale de ta morphologie du rentran t de Maletula. défini par une boÎ\e dans la Figure 6. lA!s
oon to~~ sur l'île de Malekula sont déduilS des terrasses corallie nnes et montrent l'amplitude du sou lèvement lors
du sé Isme de 1965. Le rentrant de Malckul a et la 7.orIC de SOtl lè~emenlllla.l.imaJ sont al ignés parallèlement au
~ecteur coo~ergeoce. L'intervalle entre les contours bathymétriques est de 500 m (d'après Collot cl Fisher, 1989).
23
S. LALLEM AND ri of.
obscrved at the base of the landward slope juSt nonh of
the junction of the trench with the Tehuantcpec ridge was
intcrpreted as an uplift related to the passage of a volcano
in the s ubdu ctio n zone. As a matter of fact, thi s
prom inent feature is IOC3ted in Ihe continuation of a
yolcani<: <:hain Iying on the vecani<: <:rust (Seamat cmise
r<:pon, unpubli shcd).
margin gcne rating a 30 km reen trant. Ihe tip of the
indenter being at present si tu a ted under the upper
acc refio nary pri sm. The general balhymetri c map
confinns that subduction of the ridge docs not produce
colli sion effccts sî mil ar 10 those induced by the IzuBonin lidge. The main reasons are probabl y that this
ridge is much narrower, deeper and colder than the IzuBonin ridge. However. Ihe quitc pcculiar configuration of
Ihe margin ( Fig . 8) shows an asymm et ric indenting
effect. The asymmetry is obviously related to Ihe present
obliquity of the morphologiea! axis wilh respect to the
N 3 \00 subduction \'cctor.
Ridgcs
Ge ne rally, s ublinear ridges cree p laterally along
subduction zones because Illey trend obliquely to the
plate convergence veetor and the trench ax is.
The Nanhai cnlise took place al the junction between the
extinet South China Sea spreadillg ridge and the Manila
Trench off Luzon (Pautot and Rangin, 1989). Seamounts
(e.g. Scarborough seamounl chain) are present westward
ail along the spreading ax is. They arc 30-40 km in
diamcter and 1.5 10 3 km in heighl, and they might have
been injected in the rift struc ture after cessation of
spreading (Taylor and Hayes, 1980) during the Late
Miocene. The N 3100 Irending convergence between the
South China Sea oceanic plate and the Luzon Arc along
the N-S Manila Trench suggests tha! the ridge creeps
slowly southward along thc subduction zone (Fig. 9).
The double curvature of the Ireneh is interpreted as a
During Kaiko 1 (li rst phase. 1984). the junction belween
the Nankai Trough and the Ryukyu Trendl was surveyed
( Le Pichon et al., 1987; F ig . 8). ft is mark cd by
subduction of Ihe NNW-S SE Kyushu-Palau ridge which
is a remnant volcank island arc, inactive sinee 28 My
(Karig, 197 1). The nonhemmoSI extremity of the ridge is
topogrJ.phically less pronounccd than funher south; it is
approximatcly 20 km wide, and 500 m high. The trend of
th e Nanka i Trough in thl s area is NE-SW and the
subduction vector is oriented N 3100 50 Iha! the ridge is
crecping slowly soulhwestwa rd along the subduction
zone. The ridge appears 10 il CI as an indenler on the
.... 0
. ~.
LIa 1
,
_.
o.
0
, 0
2 0
3
-4
.l....l...1. 5
Figure 9
/lIIt rprtII11n"t
*"11 map 0{ lM ManilD
cr-. and lM
T,, ~ ~
__~ ""'" ,. lM ~ bt,..__ lM
Imri. MlJiltf~ _ pIauJ COtnbinuIg ~ and
SIll1:/HIwwttI :stIS1tUC dao /: stll'lOl/llU: 1_ /rtItt:1I Jill: J" fQlran: basUI: 4= lIOf1rr'JI
fa../t rr/Dud ICI llit JPrtlJdutg hiJlory If rlle S<lwlr CiWwI St..: 5 .. - - - ' fa t.J/ MIJI /ilt
Imllh and ,,"",d IIJ fIuJ.n C/w..-u p/tIte.lsobiMlut«1t 500 '" rPa410land R""IU.. /9891ScItrN UlIetfdUlÎf de ta lD/1e n:conooe de la f_ de ~lanilk lia jJrcion ~ ta ch.ûroI:
Sarto:wugh et ta f~ Les lnIItS ~ iCfII rqoté5 et dMlil$ ~ SeaI:am et de la
!;ismiquc: 1I"(lI"(lD"aœ. l "1T1ID.i ~ : 2 • .i6di_ de rtmpIis§agt de la foo.e : 3 •
8awn a\o1nl~ ; J ,. F:oillc IIOl....aIe b6e l t"e. p;wIKIII de la Met de O\ine du Sud : S •
P.alIo lI!mlaIe ~ de ta rom el b6e lia fbl,R de ta pb:jw:: en S1llW.1ion. Les ~
~l(U;;b!JOOIQ\P3IlIaelltalp. t98'.l).
~Ir
Figure 8
S/rwc/wrul skelch mo.p of Ihe sur....eyed orto al the june/ion ber..-u n Iht
Kywshw·Palau ,idgt and Ille Nankal Tl"Ow,h . R.T. '" R)"ukyu Trench; N."!: '"
N/lnJ;a. Trw.J(1r (ufler Le Pichon li al . /987/.
SclK!ma structural de la wne leconnue ~ ta jo/K"UOIl tlllre la ride PalauKyusl"lu CI la rOS!iC de N:uwlJ. R.T•• Fosse: de R)'uk)u; N.T. '" Fo.1oI: de
Nank.aJ (d"aptèsl.c Pichon et al.. 1987).
24
ACTIVE MARGIN TECTOGENESIS J OCEANIC ASPERITIES
PaJau-Kyushu ridge. no indentation is recognized in the
New Hebrides margi n in front of the North
d' Entrecasteaux ridge (Co llot and Fi sher, in press),
However, the major fealure of slope morphology in the
central pan of the "collision" zone is the broad protrusion
Ihal cutS Iransversely aeross the slope and culminates in
the Wousi Bank, which almOSI reaches sea level. This
strongly uplifted area (1500-2500 m) is c1early due la the
passage of the crest of the North d'Entrecasteaux ridge.
rcs ult of the indentation swceping of the ridge, The
subducted axial seamount chain characterized by a strong
relief has left a clear impact in the fore-arc area. The
most evident of these structures is the Stewart Bank,
which can be traced 50-60 km eastward on the landward
s lope of th e tren c h, The fore-a rc bas in is clearly
interrupled in front of the ridge (Fig. 9) and two distinct
s ubbas in s are known: the Wes t and North Luzon
Troughs, The first of these is not yet affected by the ridge
subduction, while the second (exlCnding north of 18°N,
out of the Fi g. 9) is c ha racterized by a n irregular
topography. Between th ese two basin s lies a broad
structurd.1 high corresponding la the Stewart Bank.
The N 155 0 Lou isv ille Rjdge. a Late Cretaceous and
Cenozoîc hot -spot c hain of guyot s and seamounts,
obliquely enters the N 20° Tonga-Kennadec subduction
zone at 26 °S, eausi ng a major s ill whi c h mark s
geographically thc trans ition betwee n the Tonga and
Kcnnadec Trcnches (Fig, 10). Extinct volcanoes of the
Louisville ridge are distributed aJong a 75 km-wide band
(Lonsdale, 1988). The nonhemmosl occurrence of the
ridge before subduclion is the Osboum guyot which is 30
km in width al its base and 3 km in heighl, culminaling at
1900 m wat er de pth , Taking inl o account th e
convergence motion bctween the Paci fi e-plaIe and the
Tonga-Kennadec arc-platelet, the ridge swept the Tonga
Trench (Dupont , 1979) and c reeps rapidly southward
along the Kermadec Trench at a minimum raIe of 10
cm/yr (Pelletier and Dupont, this volume). Severallargescale dra!;tic changes occ ur west of the ridge-trench
junclion (Fig. 10):
The curvil inear d'En trecasteaux Zone is at prese nt
subducting below the New Hcbrides Island arc. Il is a 80
km wide submarine complex of ridges and seamounts
Ihat extends from the non hem New Caledonia ridge to
Ihe Central New Hebrides (Fig. 6), and comprises IwO
eas l-I re nding ridge s 2 ta 4 km hi gh : Ihe North
d'Entrecasteaux ridgc and the South d'Entrecasteaux
chain (Collot et al., 1985), The convergence bclwccn Ihe
Australia-India plate and the Pacifie plate supponing Ihe
island arc occurs approximately along a NSO° direction.
The E-W d'Entrecasteaux Zone bcing slightly oblique ta
the plate convergence, it crceps slowly northward paraUc1
to the Ircnch al an average rate of about 2.5 em/yr. Unlike
the indentati on created in the Nankai margin by the
•
.'~
.. .
.... .
C
• ' :.
.
,
'0 •
Cl i..;
.•.~"
'B
;J
!J. •
,
.'li .•
~
..
:!
'! O:
onou AN
~ij:'
\..
•
~
.
'
•
"S' .,
Figure
...
",
tO
Simplified /xlth)'melric and structural maps of Ihe Tonga·Kermadec island ale sySltm, il/us/raling Iht! t!fft CI$ of Ihe Loui$l'i/Ie ridge $ubduclion. Dtp//u
aft in km. Full cirdes aft emuged and jubmerged vo/cunocs idenlified from balhymelry or jingle dUJ1lnel ~iJmic data. Projection of the prcviously
suhducud porrion of lM ridge assuming Il N 15j Dtrend is .IMwn. Can vffge nCt! motion (l/ II~ Loui$l'i/le ridge ·/ft/ICh intersection is fro m Pel/etier and
Loua/(/989), (oJoptrd from Pelletier and /Juponl, this volume).
Can es balhym:!lriquc CI stl1lclllralc simplifiœs du syslème d'arc ins ulaire Tooga-Kermadcc ill usl rant les effets de la su bduclioo de la ride de Louisville.
Les pro~OIl~urs sonl en km. Les œr<:lcs. pleins CQfTCSpor~1II au~ moOlIIS SOU~-mari IlS émergés el immergés identifiés à ~ir de la OOlhy nJétrie el des
profils sIsmIques HlOIlotraces. La proJecuoo de la panle déjà SlIbduite de la mie N t55 Dest reportée. Le 1l1OllVCmcnl re 'auf des plaques au niveau de
l'intersect ion ride -fosse est lin! de l'cllelier el Lou ai ( t989), (aduplé de PcJlelierel Dupont, ~ volume).
25
S. LALLEMAND 1'/ al.
• The trench presents a segment Ihal slrikes N-S and
links the N 20o-parallel Tonga and Kennadec Trenches.
rcsulling in a misalignement and a 50 km offsel belween
the trenches;
• The Tonga Platfonn located between the active arc and
the trench disappears southward and is replaced by the
Kermadec ridge including active volcanoes and a
seaward sloping fore-arc basin:
• The Tonga active volcanic arc is also misaligned and
offset land ward with respect to the Kennadec volcanic
island chain;
• The baek-arc domain is shon and presents a major sin
be t ween the Lau Basi n and th e Hav re Trough s.
Moreover, both rates of convergence at the trench and
back-arc opening increase drastically nonh of the ridgcIrench junclion (Pelletier and Louat, 1989).
Von Huene and Lallemand ( 1990) have estimated the
magnitude of frontal erosion along the northem l apan
and Peru margins supcrior to 50 and 27 km respectively
during the lasl 20 Ma. Ind uding subcrustaJ erosion, this
provides minimal estimated rates of erosion through a
cross-section of the l apan and Peru trenches of 50 and 27
kml/ Ma respective ly. The eros ion rate reached 44
km 2/Ma during the last 8 Ma. Von Huene and Lallemand
noticed that one agent of such erosion is the subduction
of asperities on the subducting lower plate. Rather than
abrading, the positive topographic fearures break up the
base of the slopc by e1cvating and ovcrsteepening it to a
condition of general gravit y fai lure. The disaggregated
slump debris are transponed down the subduction zone
on the descending plate, as the absence of a significant
trench fil! suggests.
The Louisville ridge-trench intersection was surveyed
during the Seapso V cruise (Ponto ise et al. , 1986;
Pelletier and Dupon t. thi s vo lume). Sout h of the
interseclion. where the flank of the ridge arrives at the
subduction zone. the Kermadec Trench shallows and
migrates westward due 10 a change in trend from N 20"
to N-S. Thi s land ward shift observed in the restricted
Seabeam survey is 17 km, but reaches 50 km from the
end of the N 20 Kennadcc Trench to the junction point.
Simultaneously, the landward slopc of the trcnch axis
steepens, short ens and uplifts (1 500 m) due 10 the
landward dipping thrust faults erealed in response to the
advancing ridge. As memioned above at the intersection.
the treneh is marked by a major sill reaching 5 500 m in
depth. 3 500 m above the mean depth of the Tonga and
Kennadec Trenches. This sill occurs just west of a small
satellite scamount of the Osboum guyot , that panly clogs
the trench. In front and in the extension of the ridge,
although a small hill eould be an offscraped volcano al
the base of the landward slope, major clongated hills
(reach ing 2 600 m water depth) and depressions are
likely pans of the land ward slopc locally uplifted and
collapsed ovcr the relief of the subducted ridge. North of
the junction, immediately after the eastcm flank of the
rid ge cnte red th e s ubduction zone. the tren e h is
abnormal ly deepened by more than 10 000 m and the
Irench axis shows an arcward vi rgation. The landward
s lope deepe ns rapidly northward, has a s tep ped
morpho log y and is affec ted by collapse and masswasting along twO nomlal-faull sets, one parallel to the
treneh and the seaward slopc bcndîng-induccd scarps and
the othcr tmnsvcrsc to thc trench.
Tectonic erosion related to ridge subduction has been
proposed along the Tonga-Kennadec Trench (Pelletier,
1989; Pelletier and Dupont, this volume). As discusscd
above. the Tonga volcanic island chain and treneh are
shifted 50 km to the west with respect to the Kennadcc
system (Fig. 10). Consequently, the distance between the
trench and the summit of the arc (now inactive) shortens
and the land ward trench slope steepens from Kennadec
to Tonga. Because these changes suggest a disappcaranee
of a lowcr part of the landward slope and occur at the
latitude of the Louisville ridge-french intersection, it
cou Id be coneluded that the ridge erodes during its
oblique passage a pan of the front of the upper plate and
is easily subducted - exeepted some portions locally and
temporaril y accreted - under the Tonga-Kermadec
margins (Pelletier, 1989: Pelletier and Dupont. thi s
volume). Erosion occu rs before a nd after the ridge
passage following diffe rent processes. Soulh of the
ju nc tion. the base of the arc margin is shortened and
crodcd by underthrusting; indeed, the Tonga platfonn and
the shifts of the trench and the volcanic island chain
aJready exist immediately IlOnh of a line parallel to the
convergence motion and c rossing the ridge-trench
intersection (Fig. 10). The land ward flank of the ridge
seems 10 abrade the cumbcrsomc part of the landward
slope before undenhrusting the matgin. Nonh of the
junetion. the land ward slopc collapses and debris from
mass- wasting subduc ts (Pelletier and Dupont, Ihi s
volume): indeed, the deepest pari and the larges t
reentrant of the Tonga Trench occ ur just north of the
intersection between the lrench and the eastem flank of
Ihe ridge. and probably resull from an aceelerated erosion
duc to the relaxation of the slopc in the wake of the ridge
subduc tion. The s ize of Ihe eroded part between Ihe
Kennadee and Tonga margins and consequently the 50
km westward retreat of lhe treneh and volcanic arc are a
function of the dip of the lower trench slopc and of the
height of the facing ridge.
0
TECTONIC EROSIO
Most of the margins registering aspcrities subduction arc
characterized by tectonic erosion. Active lectonic erosion
mcans a loss by faultin g, slumping and draggi ng of
continental or arc material al the front of the margins. We
will only empha size the role of th e s ubducti on of
asperities on tectonic erosion.
South of the North d'Entrecasteaux ridge, Collot and
Fisher ( 1989b) and Fisher el al. (in press), describe a
vigorous erosion of the arc slope along large normal
fault s accompanied by numcrous slump masses due to
26
ACIlVE MARGIN TECTOGENESIS 1OCEAN IC ASPERITIES
the rclaxalion of the arc slope in the wake of the ridge.
The area of the "coll ision" zone that is affeeted by
lectonic erosion and mass-wa.<;ting is about twice as large
as the width of the ridge and the authors estimate Ihe
lime required 10 heal the morpho log ic disturbances
caused by the ridge subduction in Ihe order of 0.8 Ma.
slope mi ght weLi correspond to thrusting; s mall-seale
fo lding of Pleistocene layers, with an axis paralle! to the
trench, was observed along the cross-section. These
layers also exhibit a pseudocleavage (c1ose1y spaced
joints) along mainly transverse directions, paralle! to the
local vector of plate convergence.
Two dives went to the rim of lhe reentram, located in the
northernmost Japan Trcnch (Fig. 5), due to the recem
collision with a seamount, re vealed a fraclured and
deformed volcano-detritÎc sequence affccted by normal
faults parallel to Ihe trcnch (gravity sliding abave the
traili ng flank of the s ubdu c ted seamou nt ) and a
pseudocleavage parallel to the subduction vector like Ihat
prev iously memioned.
lNTERNAL DEFORMATION OF THE MARGlN
We can now examine Ihe Icctogenesis of Ihe margin in
terms of internaI deformalion. Seismic reflection data
inte rpretations and jn situ observations by submersible
give information for Ihis.
Shortening and thickening of the New Hebrides margin
complex were also observed in fronl of the Bougainville
seamount, where muhichanncl seismic data (Fisher et al .•
1986; Collot and Fisher. 1988) indicate that west-verging
Ihrust faults and folds defonn the aecrctionary complex.
Structur.d directions transverse 10 the arc slope were aJso
reported in the same area (Daniel et al., 1986). During
the Subpso 1 cruise, Nautile dives were condueted in this
collision zone. and will provide sorne lIew data.
Beca use in the Cou lomb wedge model . the crit ical
topogmphic slope changes much slower than the dip of
Ihe r igid base. the drama tic cha nges in dip of the
subducling plaie thal accompany the subduct ion of a
seamounl induce a characleri Slie sequence of lectonic
evenls. which Lallemand and Le Pichon (1987) have
ide ntified in the Japan Treneh . As Ihe s ubduc ted
sea mounl progresses be low the wedge, a zone of
shorlen ing and thickening that overlies the landward
fl ank of the seamount progresses at the same veioc ilY;
Ihis zone is followed by a zone of erosion and slumpi ng
that overlies the trailing flank of the seamoun!.
Ridge subduction can cause deformation even across the
entire width of the island arc. This is exemplified with
the d'Entrecasteaux Zone which seems responsible for
the compressive deformation recorded across the Central
New-Hebrides Arc just in front of the subducted part of
the ridge (Collot et al .• 1985; Daniel et al., 1989).
The shorten ing of lower Irench slopes in "colli sion"
zones can reach several lens of ki lomelres mainly as a
function of the size of the subducling body. Concerning
seamounts, we observe a 7 to JO km reentrant in front of
Oaiichi Kashima and Bougainville seamounts, which are
partly engaged in the s ubduction zone. ReenlTan ts
obscrved in the Immediate wake of subducted seamounts,
al Ihe junction between Japan and Kuri l trenches and
south of Bougainville. have typically a diameter of 20
km. The indentations relalcd 10 Ihe impingement of large
scale ridges are of the order of scver.ù tens of kilometrcs
(50 km in front oflhe Louisville Ridge for example).
A particularly interesting case of convergence between a
continental platform (seaward slope) and a margin was
studied during Pop n cruise off Mindoro (Philippines)
(Rangin et al .• 1988). Both South China Sea basin and
North Palawan continental block subduct at the southem
Manila Trench (Fig. Il).
When th e ocean ic c ru s t is s ubduc ted . a s im ple
aecretionary prism-fore-arc basin pattern is deve1oped.
Conversely, where the continental margin of this basin is
subducted. internai defonnation is randomly distributcd
through the major part of Ihe fore -arc area which is
fragme nted into various crustal microblocks. In the
"collision" zone, the upper part of the landward slope is
eharac terized by structures which are oblique 10 the
general trend of the deformation front , most of the fo lds
trcnding N-S. Imponant thrusi fault and strike-slip fault
zones delineate distinct crustal mieroblocks present in
the fore-arc area. These microblocks are also affectcd by
internai deformation.
Such a shonening is aecommodatcd along Ihrusl faults
within Ihe lower lrench slope and c reales a consequent
thickening of the prism during the subduction of the
posi ti ve feature (see the amplitud e of uplift in the
previous section. from a few hundreds of metres 10 a few
kilometres) and then a relaxation by mass-wasting.
As a malter of fact, IWO promincnt thrusts are recognized
along Ihe P-844 Shell multichannel scÎsmic Hne in fron l
of the Daiic hi Kashima seamount (Lallemand et al .•
1989). They c rop out 8 to 9 km landward of the trench
axis and separale an area of eXlensionaJ deformation on
the s lope from a frontal wcdge under compressive
deformation (see cross-section J-J on Fig. 4). The frontal
wedge material is deformed by imbricatcd thrusting and
fo lding. This se ismic Interpreta ti on was confirmed
during Nau tile dives (Kaiko Il c ru ise; Pautot et al.,
1987). The slcppcd morphology in the lower pan of Ihe
UNDERPLATlNG OR ACCRETION
ln the following discussion, underplating does not refeT
any magmatism, but ralher to subcrustal ofTscrapping
proccss.
10
27
S. LAUEMAND nal.
Many arguments, invo lving gravit)' anomalies.
morphology or scismic inlcrprelations, were used by
Paurot and Rangin (1989) for proposing (hat Stewarl
Bank (Fig. 9) is a piece of the subd ucted Scarborough
Seamount Chain which has protrudcd through the forearc of Manila Trendl in very recent lime. This can be
interpre ted as the res ult of underplating of rid ge
fragment s which upliftcd Ihe forc-arc dunng slowing or
recem cessai ion of s ubdu c tion . Both uplifting and
sli venng of the oceanic crust fringing the Stewart Bank
indicate Ihat il is also thrust faulted at ils base and in the
process of being incorporated by underplating to Ihe
land ward slope of the Ircnch. Traces of more advanccd
accreted stages could be round northward in the Vigan
High area (Iocated juSt out of Ihe upper righl limit of Fig. 9
in the extension of the sulxJucting ridge), whcre the ridge
has already swepl the land ward slope of the trench.
volcano from the ridge accreted al the foot of Ihe
landward slope (Pellel ier and Dupont, Ihis volume).
Immedialely north of Ih e j un ctio n poinl, the
1Il0rphological charncleristics of the trench and land ward
slopc s uggesl a development of a small accrelionary
pris m where the Loui sv ille turbidite apron is bcing
subduct ed ( Lonsdale, 1986). ln Ihi s area. lat es t
crClaCCOUS pelagic sediment rccovered by a dredge haul
ind icate Ihal some of Ihe LouÎ svi lle c ha in has been
accreted to Ihe lower Irench slope (Scholl et al .. 1987).
I-I oweve r. accre tion appears limited in size and
momentaneous (1 10 2 Ma) sÎnce collapse and masswastÎng oceur subsequently in Ihe wnke of the ridge
sulxluction.
Undc rplal ing is rcq ui rcd to ex p ia in Ihe profo und
morphologica l c han ges from Kermadec 10 Ton ga,
especiall)' the presence of the Tonga Plmform in a high
position. Underplating of Loui sville Ridge fragments ,
though likcly, has not yel becn definitively proved. In
contrast, underplatÎng of arc-slope rocks crOOcd in front
of the underthru Sled rid ge seems al le ast partl y
responsiblc for the presence of the high-standing Tonga
platfonn Ihat is developed north of Ihe lalitude of the
present-da)' ridge- trench junclion, and already ex ists
south of the supposed position of Ihe subducted ridge
be nea lh the Tonga marg in ( Fi g. 10) (Pelletier and
Dupont, Ihis volume).
The Wousi Bank prouusion observed a !iule southward
o f Ihe exten sion of the Nort h d'Entrecas teaux ridge
be low the New I-Ie brides margi n (Fig. 6) suggesls a
similar intcrpretation inasmuc h as magnClic anomalies
wh ich ou llin e Ihe s ubduC ICd ridge are offsel ri gh tlaterally (Collot and Fisher, in press).
Accretion of pans of the Louisville guyot-chain and its
sUITounding apron has becn documcntcd along the Tonga
land ward trench slope ncar the present-day ridge-lrench
junc l ion. Mo rpho logy. se i smic interpretat io n and
magnel ic anomalies suggest that a small dome-shaped
hill. located just west of the sill of the trench in from of
Ihe satellite seamoullI of the Osboum guyOt, is possibly :l
----i
i<
i
.. ~......
.....~ ",
~
,
0,
•
.,
0
Thcre are also sorne arguments in the soulhem Japan
Trench for underplating (Lallemand et al., 1989). The
Oniic h; Ka s hima Se:lmount helongs 10 an cxlÎn Ct
.
- - .......""
\
\
\
"
A
1
c<
•0
?
",
0
1
2
3
4
•
•
/ •• .7
~
•
6
•
"
Figure 11
tmapf?/{Jtion of the sowherlllip oflhe Mallila trtnch. IYMB = Wesl Milllloro Hlocfl. ; t::M8 _ Easl MiNÛ>ro Block. 1 _ SIf?J$ IrIljtCIOry ; l '" h)"potheljcol
ntl.JtiQII of the EMD alld the lV...m: J :: seme of motioll of J/riu·s/ip fouit: -1 :: fl.tU/uioll diœctioll ill Luballg /losin : 5 _ 111fIiS/ foul/; 6 '" deforma/ion
front of Ihe Malli/a Trellch; 7· axis oftht: Lu:on I"OlcIJnic IJrc; 8 '" OOUlldary of major h/oc/(s (IYM/l and EMB) aftu Hangill CI al.. 1988.
lmerpr~ull ioo de la terminaison mérid ionale de la fosse de Manille. WMB c Bloc Ouesl Mindoro: EMS :: Bloc Est Mindoro: t = traje<:lDires des
COIltflintes : 2 _ MouvcmenlS hypodltliquc s de EMB el WMS : 3 " sens \lu mou"elMnl le long des détrochernents : 4 " direction d'c~tensîon dans le
bassin de Lubang ; 5 '" chevauchement : 6 • Fronl de défoml.ll Îoo de la fosie de Manille : 7 "axe de I"an- "okaniIlUC de Lu"lOll : 8 '" frontière cn~ les
principaux blocs (W MB ct EMB) d'après RlUlgin tl a/ .. 1988.
28
ACTIVE MARG IN TECTOGENES IS I OCEAN IC AS PERITIES
volcanic chain oriented ENE-WSW and other seamounts
may have preceded tne Oaiichi Kashima. A knoll locatcd
landward of it (clearly evidenced on the Seaheam map at
the western end of cross-section I-J on Fig. 4) is bounded
by two pronounced hcadlcss canyons perpend icu lar to
the trench. One small canyon cuts across the fronl of the
knolJ and appears to predate the devclopment of the knoll
itself, thus indicating a recem uplift. A swann of small
ea rt hq ua kes below Ine k no ll was re lat ed to the
subduction of a seamount (Kikuchi and Sudo, 1985). The
P 844 multichannel seismic line crosses th is area and
snows a zone of poor refl eclions bclow wh ich we can
follow rather easily to the decollement (Fig. 4 , crosssection 1-1). Allthe above fealures might be explained by
underpl atin g of a part of a seamounl. Subc rusta l
accretion could have produced the uplift of the knoll, the
oversteepening of the slope, and the res ulting listric
fau lts, in the wake of the accreted seamount in front o f
Ihe Kashima, compcnsatîng the mass cxcess.
ophiolilic body. At present. this Îs the mere speculation
bccause it is not already engaged in the subduction zone.
CONCL USION
The review of the di fferent structural contcx ts wnere
oceanic asperities subduct in trenches allows us 10 draw
sorne general conclusions.
• During subd uc tion, al least the lower slopc of the
conti nental or island-arc margin, models itself o n the
oceanic topography even if the irregulari ties are smallamplitude faul t scarps.
• A compressive thickeni ng o f the IOC of the margin is
observcd above the advancing flank of a subducting
seamount and collapse occurs above the trailinj: flank.
• Loss of continental or island-arc male rial Îs at leasl
partly due [0 Ihe relaxalion of the arc slope in the wake
of the asperily follow ed by tne debris subduction. Sorne
observations show that 10ss of upper plate material seems
also due to the abrading (or downward forc ing) of the
deep margÎn complex in front of the asperity subd uction .
T hus, Ihe subduction of positive features would he one of
the major components o f frontal teclonic erosion.
• Transverse fa ultin g or joi nti ng of the lowc r slope
material in collision zones scems common among to the
different situations.
• Underpl ati ng o r s ubcrustal accret ion of ridges or
seamoullls to the uppe r plate is proposed for mos t
regions studied, but very difficult to prove.
ln Mindoro Is la nd, the Midd le Miocene bo undary
between the Nonh Palawan block (Eurasian plate). and
the Mindo ro block to the cast, Îs OUllined by obducted
Middle Ol igocene ophiolites (Rangi n el al. , 1985),
interpreted as a jammed fragme nt o f Ihe soulh China Sea
oceanic crust between the Nort h Pa lawan and Luzon
co n ve rgent rerra ne s (Fig. II). T hi s s uture zone is
prcsently inactive auesting ta a rapîd rcarrangcment of
the suture zones within the collision area (Rangin et al.,
1988).
The Zenisll ridge off the nOfthern Nankai Trough may
correspond to an incipient accretion o f a ponion of the
oceanic crust 10 Ihe margin (Lallemant et al., 1989). T here
exists geological evidence for a repetitive southward j ump
of the plate boundary d ur ing the Neogene, leadi ng 10
successive block accretions and a future j ump of the plaIe
boundary to the so ut h cou ld be respo nsib le for tne
accretion of the Zenisu ridge 10 the Japancse margin as an
Acknowledgcmcnts
The authors are indebted to H. Beicrsdorf and G. Boillot fOf
constructive suggestions through their reviews of this parer.
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30