TECTONICS, VOL. 5, NO. 7, PAGES 1145-1160,
DECEMBER1986
TECTONICALLY
CONTROLLED
ORIGIN
OF THREE UNUSUAL
ROCK SUITES
IN THE WOODLARK
BASIN
Dallas Abbott and Martin Fisk
Collegeof Oceanography,
OregonStateUniversity,
Corvallis,Oregon
Abstract. We proposealternativemechanisms
for theoriginof threeunusualrocksuites,high-Mg
andesites,NaTi basalts,and arclike rocks,that have
beendredgedfrom the Woodlark basin,southwest
PacificOcean. We showthatthehigh-Mgandesites
andNaTi basaltsareassociated
with anunusually
coolridgeenvironment.The coolingis dueto
increased
hydrothermal
circulation,
stimulated
by an
unusuallyhighcrustalpermeability.The high
permeabilityis mainlydueto crackingof the
Woodlarkbasinlithosphere
asit passes
overthe
flexuralbulgein front of the subductionzone,
althoughlocalprocesses,
suchasfaultingin fracture
zones,may also make somecontribution.This
increased
convectivecoolingaffectsmagma
dynamicsandchemistryat theridgecrest.The
high-Mgandesites
occurwherethehydrothermal
circulation
lowersthetemperatures
in theupper
oceaniccrust,causingthemagmachamberto sitin
themantleratherthanin thecrustandpromoting
the
interaction
of basaltmagmawith harzburgite.
The
NaTi basalts are also the indirect result of increased
crustalcooling,whichcausesanomalously
low
degrees
of partialmeltingof theirdepletedmantle
source.The arclikerocksarecausedby interaction
withvolatilesfromlithosphere
thatwasemplaced
beneath
theedgeof thebasinlessthan6 m.y. agoby
a now inactive subduction zone to the north of the
currentlyactiveone.Simplethermalmodelsindicate
Copyright1986
by the AmericanGeophysicalUnion.
Papernumber6T0474.
0278-7407/86/006T-0474510.00
thatthisformerlysubducting
plateis still cold
enoughto retainvolatilesandto remainseismically
active. As the old platelosesvolatiles,theyriseinto
the mantle convection cell which feeds the Woodlark
basinridge crest. Becauseridge subduction
increases
the likelihoodof ophioliteobduction,these
observations
andexplanations
of Woodlarkbasin
tectonicsarepotentiallyimportantfor ophiolites.
INTRODUCTION
The Woodlark Basin, locatedin the southwest
PacificOceaneastof PapuaNew Guinea,contains
an activelyspreadingridge,thatis beingsubducted
beneaththe New Georgiaislandarc to the northeast
(Figures1 and 2). A secondlithosphericslab,a
relic from a previousperiodof southward
subduction
of thePacificplate,is emplacedbeneath
the island arc from the north.
Althoughoceaniccrustfurtherthan 150km from
the subductionzoneis normalmid-oceanridge basalt
(MORB), the other rocksfrom the Woodlark basin
spreading
centerbelongto threedifferentsuites:(1)
arclikerocks,basaltsthroughdaciteswith a high
contentof arclikecomponent;(2) high-Mg andesites,
andesites
with a highmagnesiumcontent,and(3)
NaTi basalts,basaltswith a high sodiumand
titaniumcontent[Perfitet al., 1986]. The high-Mg
andesites and NaTi basalts formed at transform
zones within 50 km of the trench. The arclike rocks
were generatedat ridgecrestswithin 150 km of the
trench[Taylor and Exon, 1986].
The threespatiallyrestrictedsuitesareall unusual
in theirtectoniclocation.High-Mg andesites
have
beenpreviouslyfoundin islandarc andforearc
regions[Johnsonet al., 1983; ReaganandMeijer,
AbbottandFisk:TectonicallyControlledRockSuites
•55
156
157
I
KEY
&
_
DREDGE
•TRENCH
•
LOCATION
AXIS
RIDGE
CREST
ol•':•'J
_< 2. M.Y.OLD
[7"/'/"/•>
2 M.Y. OLD
I0
155
156
157
158 ø
Fig. 1. The Woodlarkbasin:an areawherelithospheric
flexuremaybe affectinghydrothermal
and
volcanicactivity(mapafterTaylorandExon [ 1986]). The spreading
centermarkedby double
dashedlines(Ghizoridge)is eruptingislandarc-typetholerites,dacites,andandesites
andis within
the flexuralbulge. Trianglesaredredgehaullocations.Dredges31 and32 areon the Simbofracture
zoneandtransformfault.Dredge31 recoveredhigh-Mgandesites
andarclikerocks. Dredge32
recoveredNaTi basaltsandarclikerocks. Dredge26 recoverednormalMORBs [afterTaylor,
1986].
1984], but have never before been found on a
transform fault.
NaTi basalts have been found at
five other sites,four of which are far from
subducti6nzones[CAYTROUGH, 1979;Schilling
The factorswhichdistinguish
theWoodlarkbasin
from othermarginalbasinsarethe subduction
of
very youngoceaniccrest(0-5 m.y.), a rapid
convergence
rate(> 10 cm/yr), anda polarityreversal
et al., 1983; Cousenset al., 1984; W.G.Melson and
of subduction(late Miocene) [Colemanand
T. O'Heam, unpublishedcompilationof glass
analysesof MORB, 1985]. All four of thesesites
are (or were)ridgesegments
bracketedat bothends
by relativelyold (10-30 m.y.)oceaniclithosphere.
TheNaTi basaltsfromtheWoodlarkbasinerupted
alonga transformfault with an agecontrastof only
2.6 m.y., muchyoungerthanin the otherlocations.
Finally,the arclikerocksarecommonlyrestrictedto
Kroenke, 1981;Weisselet al., 1982;Taylor, 1986;
Figure2]. The presenceof ridgesubduction,
the
rapidconvergence
rate,andthespatialrestriction
of
thedifferentrocktypesimpliesthattheunusual
volcaniccharacterof theWoodlarkbasinmaybe
causedby tectonicprocesses.Therefore,we
examinedtwo processes
whichcouldrestrict
unusualrocktypesto thevicinityof thetrench:(1)
unusuallystrongcoolingof the lithosphereby
hydrothermalcirculation,whichcouldbe
responsible
for the high-Mg andesites
andNaTi
basalts,and (2) volatile lossfrom the relict
subducting
plate,whichcouldbe responsible
for the
back-arc basinsand island arcs, and the Woodlark
basin is neither.
The origin of theserocksis relevantnot only in
thecontextof Woordlarkbasinpetrology,but alsoto
the muchmoregeneralconcernof whetherterrestrial
ophiolitesreallyrepresent
typicaloceaniccrustal
rocks[Miyashiro,1973;MooresandJackson,1974;
Mooreseta!., 1984]. Ridge subduction,suchas is
occurringin theWoodlarkbasin,may increasethe
chancesof ophioliteemplacement
[Mooreset al.,
1984]. Consequently,
theremaybe petrologicand
chemicalsimilaritiesbetweenophiolitesandthe
unusual Woodlark basin rocks.
arclike rocks.
LITHOSPHERIC
COOLING
PLATE FLEXURE
AND
The oceaniccrestcreatedat mid-oceanridgesis
cooledby a combinationof conductionand
AbbottandFisk: TectonicallyControlledRock Suites
1147
valuesat depthat a rate of 0.222 kbar/km. We
assume this model in the discussion to follow.
Lithosphericflexureis onesourceof tensile
stress.Lithosphericplatesbendastheyenter
subductionzones. This flexureproduceslarge
tensionalstresses
in the upperhalf of the subducting
oceanicplate (Figures3 and4). Thesehigh tensile
stresses can cause new cracks to form on the
ß.-.: /..o ß ,.
surfaceof the flexuralbulge. Thesecrackswill
propagatedownwarduntil the breakingstrengthof
the rock exceedsthe tensilestresses
inducedby
Young Plate
o
o-
flexure (Table 1). These new cracksadd to cracks
causedby thermoelasticstresses.
Old Plate
o
,60
Woodlark
Plate
;$o
Pacific
460
Plate
Fig. 2. SeismicitybeneaththeNew Georgiaisland
arc [afterCooperandTaylor, 1985]. Both the old,
previouslysubductingPacificplate(fight) andthe
presentlysubducting
Woodlarkbasinplate(left) are
clearlyvisible. The seismicdataimply thattheolder
subducting
plate is intactandhasnot brokenapart.
convection. The relativeimportanceof conduction
andconvectiondependsprimarilyon thebulk
permeabilityof the crust-sediment
system.
Becausemarinesedimentsare usuallyseveralorders
of magnitudelesspermeablethanbasalt[Abbottet
al., 1981; Andersonet al., 1985], the depositionof
200-300 metersof sedimenton top of thebasalt
basementnormally causescrustalcoolingto be
dominatedby conduction
ratherthanconvection
[Andersonet al., 1979]. However, the intensityof
Increasedhydrothermalcirculationproduceslarge
chemicalandthermalchangeswithinthe oceanic
crust. If flexurally inducedcrackingandfaulting
increases
thepermeabilityor thetopographic
roughness
of an unsealedplate,hydrothermal
circulationwill intensify. Althoughoceaniccrestas
old as 80 m.y. is sometimeshydrothermallyactive
[Embleyet al., 1983], theseolder,hydrothermally
active areas are located in oceanic areas with
unusuallylow sedimentation
rates. Because
sedimentation
ratesneartrenchesareusuallyquite
high,the sedimentcoveron mostsubducting
oceanic
crust reaches a thickness of 200-300
meters when
the subductingplateis only 0-20 m.y. old. The
subducting
platein the Woodlarkbasinis 0-5 m.y.
old and is coveredby lessthan200 metersof
sediment;thushydrothermalcirculationand
consequent
lithosphericcoolingshouldbe intensified
by flexurallyinducedcracking.
EVIDENCE FOR INC•ASED
CRACKING
DUE TO LITHOSPHERIC
FLEXURE
Three observations indicate that new cracks form
hydrothermal
convection
increases
with increasing
totalavailableheat,topographic
roughness,
and
systempermeability[Stefanson,1983;Fehnet al.,
1983]. Thus, changingany one of thesevariables
canchangethe amountof convectivehydrothermal
coolingat mid-oceanridges.
Fracturingincreases
thenumberof interconnected
at flexuralbulges: normalfault earthquakes,
new
fault blocks,andporewaterheliumanomalies.
Many trencheshavereactivatedfault blockson the
flexurallyformedseawardtrenchslope[Schweller
voids within oceanic crust and thus increases the
alignedparallelto the trendof thetrench,ratherthan
perpendicular
to theoriginalspreading
direction
permeabilityof the oceaniccrust. The increased
permeabilitystimulates
hydrothermal
circulation,
especiallyin thelowerlayersof theoceaniccrust.
The primarycauseof thesecracksis thoughtto be
tensilethermoelastic
stresses
inducedby the cooling
andcontraction
of agingoceaniccrust[Lister,1974;
Bratt et al., 1985]. Laboratoryexperimentson small
mineralsamplesindicatethatcrackscanpropagate
at
cleviatoficstresses
of 500 barsor greater[Martin,
1972]. However, the strengthof the oceaniccrustis
probablylessthan500 bars,sinceanysufficiently
largepieceof materialwill containmanypreexisting
and Kulm, 1978; Watts et al., 1980]. In the
Peru-Chile trench at 220-27 ø S and in the Guatemala
trenchat 12ø-14ø N, someof thesefault blocks are
[Von Huene and Aubouin, 1982; Schwelleret al.,
1981,Figure6]. Theseobservations
imply that
lithosphere
flexurecancausetheformationof new
faultsandthatthedeviatoricstresses
in theupper
partof theflexuralbulgeexceedthetensilestrength
of therock. Becausecrackspropagateat roughly
half the stressthatis requiredto breaktherock
[Verhoogenet al., 1970], new crackswill form at
greaterc•epth•s
thannewfaults.
The OHe/'*Heratio of gascontainedin mid-ocean
ridgerocksderivedfrom the mantlesourceof
flaws and zones of weakness. Bodine et al. [1981]
normal
MORBis~1.15x 10-5 [Lupton
andCraig,
modeledthe strengthof the crustundertensionas
increasingfrom nearzeroat the surfaceto higher
1975;R. Barnes,unpublished
manuscript,1986].
When the rocksarecracked,juvenileheliumis
1148
AbbottandFisk: TectonicallyControlledRock Suites
go has height A
trench
I
• bulge
atkx=3w/4
-,.,'"P•.'."•
"':•i!i!i!i:i:i:i:i:!:i:i:i:i:'
......
"' '
I" ::i:i:!:i:i•'',it
Fig. 3. Diagramof lithospheric
flexureat a trench(notto scale):h, elasticthickness;
x, horizontal
distancefrom the load;z, distanceaboveor belowtheneutralsurface;y(x), flexureof theplate;A,
maximumheightof thebulgeabovethe surfaceof zerodeflection(measured
from theseawardside
of theplate). The bulgehasits maximumheightat kx = 3•r/4. Themaximumstressis at kx = •r/4.
The upperhalf of theplateis in tension;thelowerhalf of theplateis in compression.
concentration,
of H•erelativeto therockandhasthe
depthcouldcomefrom newly crackedrock [R.
Barnes,unpublishedmanuscript,1986].
al., 19801,the releaseof heliumby the crackingof
•ewly,emplacedrock causesanomaliesin the
TENSILE STRESS CAUSED
LITHOSPHERIC
FLEXURE
ventsat mid-oceanridges. Fracturingof older,
previouslyuncrackedoceaniclithospherewill also
releasejuvenileheliumgasinto the surrounding
water. The onlyDeep SeaDrilling Project (DSDP)
sitedrilled on top of a flexuralbulge,site436,
locatedon 110-m.y._-old
lithosphereenteringthe
Thickerplatesaremoredifficultto bend,and
consequently,
stresses
andcrackinginducedby
flexuralbendingof theplateareproportional
to the
thicknessof theplate. Becauseplatesincreasein
thicknessasthey age,maximumflexuraltensile
stresses
increasewith increasingageof theplate.
the porewater [R. Barnes,unpublishedmanuscript,
1986]. To producethisratio, 10% of theporewater
heliummustcomefrom newly crackedrocks. As
partof theplatewhichbehaveselastically.The
mechanicalthickness
of theplateis thethickness
of
thelayerthatbreaksratherthanflowsunderthe
influenceof largeflexural bendingstresses,
andis
approximately
givenby thedepthto the600 øC
released.
Because most seawater has a low
atmosphericOHePHe
ratioof 1.4x 10-6[Lupton
et
BY
He/•4He
ratioofpore
fluids
emitted
athydrothermal
Japan
trench,
hasa 3He/4He
ratioof4.40x 10-6in
thel•eliu•ndiffusesslowlythrough
thesediments,
the •He/'*He ratio of the pore water decreases.
Consequently,100% of theporewaterheliumat
These flexural tensile stresses are concentrated in the
isotherm [McNutt, 1984]. Mechanical thickness
shouldnot be confusedwith the plate'sthermal
thickness,whichis roughlyequalto thedepthto the
1o
a•
1300 øC isotherm [Parsonsand Sclater, 1977]. If
o
5
i
o
1oo
DISTANCE,
2oo
km
Fig. 4. The tensilestressat thetopof theplate(z
= 1/2 h) versusdistancefrom thebeginningof the
trenchwithinlithospheric
platesof differentages(in
millionsof years). Note thatthemaximumtensile
stressexceedsthe breakingstrengthof the rock (1
kbar)andthecrackpropagation
stressevenin the
youngestsubducting
plates.Seeappendixfor
derivationof the figure.
tensionalcracksanddeepwatercirculationare
present,the convectivetransferof heatwill increase
thedepthto the 600 øCisothermandwill
consequently
increasethemechanicalthickness.
The portionof theplatethathasincreased
surface
tensilestresses
inducedby flexureis calledthe
flexuralbulge. Our calculations
(seeappendixfor
details)of the tensilestressat thetopof theplate
within the flexuralbulgeindicatethatstresses
will
locallyexceedthestrengthof therock (Figure4).
The depthof crackingvarieswith plateage. For a
5-m.y.-oldplate,cracksmay extendto 3.7 km depth
nearthe flexuralbulge (Table 1). Figure5 shows
thedepthof crackingandconsequent
hydrothermal
alterationwhichcanbe producedby lithospheric
flexure. On platesolderthan20 m.y., thisdepthis
greaterthanthe 4- to 8-km depthof hydrothermal
AbbottandFisk:TectonicallyControlledRock Suites
1149
TABLE 1. Elastic Half-Thickness,Maximum TensionalStressat the Top of the Plate near the
FlexuralBulge,andMaximum Depthof Flexure-Induced
Cracking,of the OceanicLithosphereasa
Functionof PlateAge*
PlateAge, m.y.
ElasticHalf-Thickness,
km
1
5
10
20
40
80
140
2.1
4.6
6.6
9.3
13.1
18.6
24.6
MaximumStress,kbar
3.1
4.2
5.2
6.2
7.2
8.6
10.5
Depth,km
1.8
3.7
5.1
6.9
9.2
12.3
15.6
*The plateis assumed
to be conductively
cooled.
alteration inferred from marine seismic studies
[Lewis, 1978; Abbott andLyle, 1984] and from
ophiolites[GregoryandTaylor, 1981; Cockeret al.,
1982]. This impliesthatthe crackingof the oceanic
crustin the vicinity of flexuralbulgeswill very
quicklyexceedthenormaldepthof the hydrothermal
cracking and will resultin thereleaseof juvenile
3He. Ourcalculations
aretherefore
consistent
with
the observationof porewaterheliumanomaliesin
the sedimentoverlyingthe flexuralbulgenext to the
Japantrench.
CALCULATION
HYDROTHERMAL
OF TIlE DEPTH
CIRCULATION
OF
Hydrothermalconvectionlowersthe temperatures
in the upperpart of the plate (Figure6). Becausethe
mechanicalthicknessof theplateis determinedby
the depthto the 600øC isotherm,theselower
temperatures
will alsoincreasethethickness
of the
HALF-ELASTIC
•
•o
o
I
I
I
I
I
0
30
60
90
120
I
PLATE AGE, m.y.
Fig. 5. Elastichalf-thickness
of a conductively
cooledplateandcrackedthickness
of the sameplate
at the apexof theflexuralbulgeversusageof the
platein millionsof years. The crackedlayerhas
flexurallygeneratedtensilestresses
whichexceedthe
breakingstrengthof therock. The dashedline
indicatesthe approximateincreasein elastic
half-thickness
of theplatecausedby heatremoval
duringhydrothermal
circulation. Seeappendixfor
derivationof the figure.
layer whichhastensilestresses
(Figure5). Tensile
stresses
will occurin the upperhalf of thislayer,
which is boundedroughlyby the 300ø C isotherm.
Althoughwe haveno dataon the temperatures
within ridge crestcrustaffectedby flexure,Fehnet
al. [ 1983] modeledthe intensityof hydrothermal
circulationat the CostaRica rift by usingcrustal
modelswith differentratesof exponentialdecrease
of permeabilitywith increasingdepth. Subsequent
in situ measurements
in the same area found that the
permeabilityof the oceaniccrustdoesdecrease
exponentiallywith increasingdepth[Andersonet al.,
1985]. Fehn et al. [1983] foundthat a changein
thicknessfrom ~2.5 km to ~4.2 km of the layer with
a highenoughpermeabilityto supportactive
hydrothermalcirculationchangedtheratio between
minimum
and maximum
values of heat flow in one
off-ridgeconvectioncell from about1:2 to about1:8
(Figure7). The modelwith a maximumdepthof
circulationof ~4.2 km producedthebestfit to
surfacemeasurements
of heatflow. Subsequent,
independentobservationsof the heatflow in this
areaindicatea maximumdepthof hydrothermal
coolingof 4-6 km [Hobart et al., 1985]. If we use
the isothermsin Fehn'smodels,the depthto the
300ø C isothermon crustlessthan 1 m.y. old with
activehydrothermal
circulationis at least3 km,
equivalentto the thicknessof the layerwith tensile
flexuralstresses
within a conductivelycooled
5-m.y.-old plate (Figure 5). The maximumdepthof
significanthydrothermalcirculationis shownby the
streamlinesto be 4.2 km. Becausehydrothermal
crackingis causedby the temperaturecontrast
betweenthe water andthe surrounding
rock [Lister,
1983], thisgreaterdepthis actuallya betterestimate
of the maximumdepthof cracking.
We can estimate the effect of flexure on the
hydrothermalcirculationby notingthatlithospheric
flexureroughlydoublesthenumberof largefaults
on the trenchwall. Otherfactorsbeingequal,a
doublingof surfacefaultswouldquadruplethe
averagesurfacepermeabilityof theoceaniccrustand
115o
AbbottandFisk: TectonicallyControlledRock Suites
Temperature
600
1200
i
Oceanic Crust
With
maximum deviatoric stresses near transform faults
couldbe ashighas200-400bars:a significant
fractionof thetotalstress
requiredto propagate
cracks.
Because
of thesmallsizeof theplate,theregional
stress field in the Woodlark basin has not been
Hydrothermal
Circulation
T
Temperature
600
1200
Oceanic Crust
With No
Circulation
T
Fig. 6. Schematiccartoonshowingthechangein
thethermalprofile(fight) andconsequent
increase
in the elasticthickness,
E, of younglithosphere
(left)
asa resultof hydrothermal
circulation.Circulating
fluidsareestimated
to havea maximumtemperature
includedin globalsolutions
of intraplatestress
[Richardson
et al., 1979]. Theseglobalsolutions
indicatethatslabpull andridgepushcontribute
significantly
to intraplatestresses
andthatslabpullis
about2-3 timesstrongerthanridgepush.
Consequently,
theregionalaxisof tensionalstress
withintherapidlyconverging
Woodlarkbasinplate
is mostlikely within 30ø of the trendof the Simbo
transform
fault. Thisorientation
of theregional
stress field near the transform trace would cause
localtensilestresses
thatexceedtheregional
tensionalstresses
by asmuchas50-100% [Fujita
andSleep, 1978]. Therefore,tensionalstresses
alongthe Simbotransformfault will havethree
additivecomponents:
thefirstinducedby
therm•lasticstresses,
thesecond
by lithospheric
flexure,andthe thirdby thetransformorientation.
A ridgecrestor transformlocatedon a flexural
bulgeshouldhaveanincreased
depthof
hydrothermal
circulation
andincreased
coolingof the
crust. The cool crust would result in short-lived
crustalmagmachambers,
andconsequently,
any
steadystatemagmachamberswouldlie withinthe
mantle.Deepmagmachambers
mayoccuralong
of 400ø-500øC. In this cartoon,the total thermal
thickness,
T, doesnot change.If flexurallyinduced
coolingis profound,the thermalthicknesswill also
•o
ß
C
increase.
woulddoublethe maximumdepthof crack
penetration[Lister, 1983]. This, in turn, would
roughlydoublethe maximumdepthof hydrothermal
circulationandalterationto 8.4 km (Figure5).
bJ
T O k(z)=
5 x10-15exp(-z/434)m
2
'
TRANSFORM
FAULTS AS A SECONDARY
CAUSE OF HIGH PERMEABILITY
In additionto flexurallyinducedtensilestresses,
transform faults can cause local increases in tensile
stresswhichdependuponthe orientationof the
transformwithin theregionalstressfield. If the
regionaltensilestressaxisis parallelto the
orientationof the transform,the maximum tensile
stressnearthe transformwill be 50% greaterthan
the regionalstress[Fujita andSleep,1978].
However, if the transform fault is oriented at an
angleof 30ø to theregionaltensilestress,the
maximum
tensile stress near the transform will be
twice the regionalstress[Fujita andSleep,1978].
The regionaldeviatoricstressfieldswithin large
plateshavea maximummagnitudeof 100-200bars
[Richardsonet al., 1979]. This impliesthat
Fig. 7a. Top: Calculatedheatflow versusdistance
from theridgecreston a ridgewith active
hydrothermalconvection.Solidlines:totalheat
flow.
Dashed lines: conductive heat flow.
Bottom:
Streamlinesof water flow (dashedlines) and
isotherms(solidlines)versusdistancefrom theridge
crest. Streamlinesare unevenlyspacedto better
delineate
areasof weakcirculation.Thefunction
in.,
themiddleof thefigureis thepermeability,k, in m•,
versusdepth,z, in the oceaniccrust(in meters)
[after Fehn et al., 1983].
AbbottandFisk:TectonicallyControlledRock Suites
increased
hydrothermal
coolingalongthe Simbo
transform
E
•
fault.
The otherknownoccurrences
of high-Mg
andesites
in arcvolcanossupporttheideathata
magmachamberin contactwith harzburgite
can
producehigh-Mg andesites.Many islandarcsand
most continental arcs have crustal thicknesses in
moJk(z)=5
T
x10-15exp(-z/.724)m
2
excessof 30 km [Gill, 1981]. In thosearcs,magma
chambers
at lessthan30 km wouldnot bringbasaltic
magmaintocontactwith harzburgite,
andhigh-Mg
andesites
couldnot form by themechanism
proposedby Fisk [1986]. The two arcswith
documented
occurrences
of high-Mgandesites
(boninites),theIzu-Boninarc [HickeyandFrey,
1982] and the New Britain arc [Johnsonet al.,
1983], have crustal thicknessesof 12-15 km and
0
10
DISTANCE,
20
Km
Fig. 7b. SameasFigure 7a, exceptfor different
permeabilityversusdepthfunction. This model
providesthe bestfit to heatflow measurements
from
the CostaRica rift, an area of normal,unflexed
oceanic crust [Fehn et al., 1983].
20-30 km, respectively[Gill, 1981].
Consequently,bothof thesearcshavelocations
wherebasalticmagmacouldequilibratewith
harzburgiteat pressures
of lessthan 10 kbar.
ORIGIN
OF NaTi BASALTS
Dredgehaul32 (Figure1) sampledvolcanic
rocks formed at the intersection of the Simbo
the Simbo transform where it crosses the flexural
bulge(station31, Figure 1). Recentvolcanismon
the Simbo transform occurs at two locations: near
transform
with theWoodlarkbasinspreading
center,
about33 km fromthetrench.Theflexuralbulge
produced
by a 1- to 2-m.y.oldsubducting
plateis
the ridge-transformintersectionandat the Simbo
volcano.This "leak•" behaviorimpliesthatthe
50-60 km wide (Figure3); thusthe Simbo
ridge-transform
intersectionis within the flexural
bulge. This dredgerecoveredrockswith an unusual
Simbotransformis not likely to haveunusuallythin
crust,whichcouldalsoproducea subcrustal
magma
lessthan0.3%K20. Perfitetal. [1986]hav•
chamber. Simbo volcano has been characterized as a
forearcvolcano,butthelocationsof deep
earthquakes
indicatethatit is seawardof the trench
[CooperandTaylor, 1985]. As discussed
earlier,
the Simbotransformis in theproperorientationto
magnifythe regionaltensilestresses
inducedby
subduction.
A DEEPER
MAGMA
CHAMBER
AND
THE
FORMATION OF HIGH-Mg ANDESITES
Fisk [1986] foundthattheinteractionof n-type
MORB andharzburgiteat pressures
of lessthan 10
kbar canproducehigh-Mg andesites[Johnsonet al.,
1986]. He infersthat a subcrustal
magmachamber
canresultin thegenerationof high-Mgandesites
with boniniticaffinities. High-Mg andesites
with
alkalienrichments
similarto theexperimentally
producedandesiteswere foundin a dredgehaul
northwest of the Simbo volcano in the Woodlark
basin [Johnsonet al., 1986]. In addition,
comparison
of experimentallyproducedandesites
[Fisk, 1986] with naturallyoccurringoceanic
andesites
(Figure8) showsthattherearemajor
elementchemicalsimilarities. Consequently,
the
Woodlarkbasinhigh-Mg andesitescouldhave
formedin a subcrustal
magmachamberproducedby
composition:
over4% Na?.O,over1.5%TiOg,and
termed these rocl•s NaTi basalts. We use a less
restrictive definition for NaTi basalt of over 3.4%
Na2 O,over1ß5%TiO2 , andlessthan0.5%K20.
TheNaTi basaltmustalsohavea Mg number
indicatingthatit is notextensively
fractionated
anda
low (lessthan 1%) water content. Basedon this
definition,all of thebasaltglasses
dredgedfromthe
Caymanspreading
centerin theCaribbeanareNaTi
basalts[CAYTROUGH, 1979]. The Cayman
spreading
centeris 110km longandis bounded
by
two transformsover 1000km long. At the
ridge-transformintersections,
eachof these
transforms
juxtaposesnew oceaniclithosphere
against34-m.y.-oldlithosphere,producingage
offsetsof 34 m.y. [Fox and Gallo, 1984]. Because
the Caymanridgeis relativelyshort,muchor all of
theridgecrestwill "see"the thermaleffectsof the
two boundingtransformfaults. The old oceanic
lithosphereon the othersideof the transformfaults
couldindirectlyproducerapidcoolingof any
shallowmagmachambersaswell ascoolingof the
local asthenosphere.
Enhancedcrustalcoolingis alsoassociated
with
the unusualbasaltglassesfrom the 40-km-longridge
segmentin theCharlieGibbsfracturezone(Figure
9). The ageoffsetsof the two adjoiningtransform
segmentsare 16 m.y. 'and10 m.y. [Searle, 1981].
1152
AbbottandFisk: TectonicallyControlledRock Suites
6
5
-
•xe\.•Q_
• • Fieldforlower ,•'
%t',•/ •,%,.oXx/)
//•: alkali
series
of.,'
•o•
•t•
• / Woodlark
Basin
0
_
4
,,-'
+
2
-I
40
I
45
I
50
....
5•5
I
60
I
65
7•0
SiO2 wt %
Fig. 8. Alkali-silicadiagramfor oceanicandesites
andexperimentally
synthesized
high-magnesium
andesites.Triangles,Hart [ 1971];squares,Byerly et al. [1976];diamonds,experimentally
producedliquidsat 1250ø C; starredhexagon(MORB), whichwasthe startingbasaltcomposition
in experiments,Fisk [1986]. Field for low silica andesitesof Johnsonet al. [1986] is shown.
Fractionalcrystallization
of olivinealonefromtheexperimental
andesite
produces
thesilicaand
alkali enrichmentshownby the line of crosses.
The threeglasssamplesfrom the CharlieGibbs
ridgesegment
have3.4-3.7%Na20, 1.6-2.0%
TiO , and0.3-0.5%K20 [Schillinget al., 1983].
The2water
contents,
which
range
from
0.1to0.6%,
aretoolow for thesamplesto havebeenextensively
altered. The samplesalsohaveMg numbersfrom
57 to 63, andthuscouldnot havebeenderivedby
fractionation•Consequently,
two of the documented
occurrences
of NaTi basaltsarefrom smallridge
segmentssandwichedbetweentwo longtransform
faults. In additionto the coolingeffectscausedby
thelong ageoffsets,the hightopographic
relief of
long ageoffsettransforms[Fox andGallo, 1984]
will alsoincreasetheintensityof hydrothermal
circulation [Stefanson,1983] and hencethe
efficiencyof coolingof theoceaniclithosphere.
Another nonsubduction zone area where NaTi
basaltshavebeenfoundis at two topographic
steps
on theSouthwest
Indianridgewherethetriple
junctionhasbeenmigratingeastwardin successive
ridgejumps. The largetopographic
steps[Anderson
et al., 1977] andanomalously
gmatdepths[Sclater
et al., 1981]observedat theboundarywith the
Antarcticplateon thispartof theridgecrestare
evidencethatsuchridgejumpshaveoccurred.
NaTi basaltglass(8.5% FeO, 7.6% MgO, 4.2%
Na:•O,0.2% K2 O, 1ß7% TiO2 ) [WßGß Melsonand
T. O' Heam, unpublished
compilationof glass
analysesof MORB, 1985] occursat 28.85ø S,
61.92øE, in an areawheretriplejunctionmigration
anda ridgejump havejuxtaposed
crustof about
anomaly19 andanomaly8 ages[Sclateret al.,
1981]. The jumping SW Indianridgesegmentwas
thusthermallyequivalentto a shortridgesegment
sandwichedbetweentwo piecesof 19-m.y.-old
oceaniclithosphere.NaTi basaltglass(8.5%FeO,
7.6%MgO,3.7%Na20,0.2%K20,and1.5%
TiO2.) [W ' G ß Melson andT ' O' I-Fearn,unpublished
compilation
of glassanalyses
of MORB, 1985] also
occursat 26.62ø S, 67.53øE, wherethetriple
junctionmigrationboundaryliesbetweencrustof
anomaly7 and 5 age [Sclateret al., 1981]. This
locationwasthermallyequivalent
to a smallridge
segmentbetweentwo transforms
with ageoffsetsof
18 m.y. Thus, the NaTi basaltson the Southwest
Indianridgewerealsogenerated
at a ridgesegment
betweentwo piecesof unusuallyold crust.
Reducedpartialmeltingof a depletedmantle
sourcehasbeenproposedas a causeof theNaTi
basaltsin the Woodlark basin [Perfit et al., 1986].
A mid-ocean
ridgetype,depleted
[%urce
forthese
rocksis supportedby measuredœ1•uvaluesof +9.2
to + 10.0 [Staudigelet al., 1986]. The reduced
partialmeltingof themantlecouldbeindirectly
causedby theincreased
crustalcoolingwhichis
inferred for all three of the tectonic locations of NaTi
basaltsmentionedpreviously.However,if one
acceptsall of theCaymantroughglassesasNaTi
basalts,
thereducedpartialmeltingmustoccurin the
mantleupwellingcell beneaththeridgecrest.
AbbottandFisk:TectonicallyControlledRock Suites
1153
theflexuralbulgeof the subducting
Explorerplate
[Riddihoughet al., 1980; Cousenset al., 1984],is
divertedby the Paul Reveretransformridge (age
offset, 4.5 m.y.), which parallelsthe trench. Thus,
the hydrothermalsystemin the Explorerrift should
not be sealed,andflexurallyinducedcrackingand
increasedcoolingcannotbe ruledout. One sample
from the Explorerrift hasa low watercontent
(0.11%), an unfractionated
Mg number(61), anda
CAYMAN
TROUGH
N-MORB
Nap_O,%
Fig.9. Ti02 versus
Na20content
of NaTi basalt
glassesfrom differentareascomparedto normal
MORB with the sameMg number. Dottedline:
Field of analysesfrom the Caymantrough.
Triangles:Woodlarkbasin. Crosses:CharlieGibbs
ridge. Squares:SouthwestIndianridge. Open
circle: Explorerridge.
The ridge segmentin the Caymantroughis 110
km long,but conductivecoolingof theoceaniccrust
near transform faults is calculated to extend less than
15 km from ridge-transformintersections
[Forsythe
andWilson, 1984]. However,convectivecooling
of theupwellingcell in the asthenosphere
near
transformfaultscanextendto distancesequalto the
thicknessof the transformlithosphere[Forsytheand
Wilson, 1984]. On a ridgesegmentsandwiched
betweentwo transformswith ageoffsetsof 34 m.y.,
theconvectioncell beneathanentire110-km-long
ridgesegmentcouldbe affected. Consequently,
the
upwellingcell or cellsfeedingtheridgecrestin the
Caymantroughupwellmoreslowlythanthose
beneathmore typicalridge segments,
thereby
producingdecreased
decompressive
meltingof their
MORB source(Figure 10). However, the age offset
on the Simbotransformis only 2.6 m.y., muchless
thanat the Caymantroughtransforms,
the Charlie
Gibbs transforms, and the old SouthwestIndian
ridgetriplejunction. If increased
coolingcauses
reducedpartialmeltingof the asthenosphere,
some
other tectonic factor must act to cause anomalous
crustalcoolingon the Simbotransform.
Two features of the Woodlark
basin could cause
anomalous
crustalcoolingalongtheSimbo
transform:the longdurationof subduction
in this
areacouldcoolthe underlyingasthenosphere
[Perfit
et al., 1986] or the additionalhydrothermalcracking
andcoolinginducedby lithosphericflexureand
transform fault orientation could increase the
thickness
of thelithosphere
at theridge-transform
intersection.The relativeimportanceof thesetwo
mechanisms
couldbe testedby drillingfor Na-Ti
basaltsat flexuralbulgeswhicharecoveredby over
300 m of sedimentandareunequivocally
hydrothermallysealed.
Much of the Explorerplate,locatedoff the coast
of southwestern
Canada,is coveredby over300 m
of sediment. However, the continentalsediment
supplyto the Explorerrift, a ridgesegmentwithin
NaTi basaltcomposition
of 0.28%K20, 3.52%
Na20, and1.82% TiO2 [Cousens
et al., 1984].
-The secondhypothesisfor the originof NaTi
basalts,increasedasthenospheric
coolinginducedby
long term subduction,couldalsoproduceNaTi
basaltson the Explorerridge. However, the cooling
inducedby longterm subduction
is not requiredto
remainconfinedwithin theflexuralbulgeandshould
not be affectedby thick sedimentcover. Despite
extensiverecentstudyof the Gorda-Juande
Fuca-Explorerplatesystem[Clagueet al., 1984;
International
N.E. Pacific Activities Consortium
(INPAC), 1985], we know of no NaTi basalts
dredgedat locationsoutsidetheflexuralbulgewith
thin ridge crestsedimentcoveror at locationswithin
the flexuralbulgewith thickridgecrestsediment
cover. Therefore,additionalasthenospheric
cooling
causedby lithosphericflexureand/ortransformfault
orientationis the bestexplanationfor theWoodlark
basinandthe ExplorerridgeNaTi basalts.This
increasedcoolingwouldcausethe SimboandPaul
Reveretransformsto behavethermallylike much
longertransformsandwouldproducetheNaTi
basalts.
VOLATILES
THE RELICT
AND SEISMICITY
WITHIN
SUBDUCTED
PLATE
Perfit et al. [ 1986] suggested
thatthe arclike
rocksin theWoodlarkbasinarecausedby volatile
additionto theWoodlarkbasinasthenosphere
during
thepreviousphaseof subduction
to thenorth
(Figure2). To testthisidea,we calculatedthe
lengthof time thatvolatilescouldbe heldin stable
mineralphasesin a descending
slab. The mantle
retention times of volatiles contained within a
subducting
plateare a functionof the ageof theplate
at the initiation of subduction,the thicknessof the
volatile-richlayer, andthe breakdowntemperatures
andpressures
of volatile-richminerals. The
maximumthicknessof the volatile-richlayer of
normal, unflexed oceanic crustis ~8 km [Abbott and
Lyle, 1984]. Oncethisvolatile-richlayerreachesa
temperatureand/orpressuresufficientto causeeither
partialmeltingor completebreakdownof
volatile-richphases,the subducting
platecanno
longercontributevolatilesto theoverlyingmantle.
The mantle retention time is defined here as the total
lengthof theportionof the subducting
slabwhich
retainsvolatilesdividedby the convergence
rate.
We have calculated mininum mantle retention
AbbottandFisk:TectonicallyControlledRockSuites
PRODUCES
NA-TI
RIFT
VALLEY
I
I^
A ?
A
BASALT
have shorter mantle residence times than in other
AXIS
platesof the samethermalage. However,these
largehorizontaltemperature
gradientswill not be
presentin the deeperportionsof a rapidly
subducting
platewhichsuddenlyceased
convergence.Consequently,we canusea thermal
modeldevelopedfor fasterconvergingplatesto
predictthe thermalbehaviorof a platewhich
suddenlyceasedconvergence.
r'- 4x -n • A I
+
'
•
^AI
TF
I
I
T.,.F,
';::.':'
;:':-f;:':'f-::
':'.')]:'
:':
•:'
•:5':':
.::-':':
)2;':
.:.'.'..':
.:L-':.".:'
;:."4':
!.:i'
;:-:5:'
:.:
.f:'5:'.::.i.
:':':!
.:".
':!
.5'
:.i.5
':•
.:'.:
:•.
!:':.:
.:(!.:.:_(:.!.:)::.:((:!((:i':?](:'
PRODUCES
N-MORB
RIFT
VALLEY
AXIS
T.F-t t t t t ttT. F.
ttttttt
ttttttt
In order to facilitate observational tests of our
model, we have also calculatedminimum mantle
"retentiontimes"for seismicitywithin a subducting
plate. Seismicity"retentiontimes"dependon the
timerequiredto heatthecenterof theplateto a
temperature
toohighfor brittlefailureto occur.
Althoughthebrittle-to-ductile
transitionis clearly
dependenton strainrates,the transitionfrom seismic
to aseismicbehaviorcanbe described
reasonably
accurately
by usinga cutoffpotentialtemperature
of
750ø C at depthsof 0-125 km [Molnar et al., 1979;
Jarrard, 1986; Chen and Molnar, 1983; Weins and
Stein,1983]. Becausethecenterof a subducting
plateis usuallyabout100-125km beneaththe
volcanicline of an island arc [Gill, 1981; Jarrard,
1986],seismicactivitywill not be presentbeneath
the arcif the entireplateis heatedabove750øC.
Volatileretentiontimesin theplatedependon the
pressureand temperatureof breakdownof
volatile-rich
mineralsand/orinitiationof partial
melting. The top 8 km of a subducting
plateis
usuallyabout 100 km beneaththe arc. At 30 kbar
(roughly100 km), partialmeltingandthe
..T:.•:::..::::.:.:.;`..•.::::z....•.:.:.:(.:.•.::?:.:....:.;.•:.:........
;..
...:.5.•...:....:.t:....•:.:....:.?..::•.:.:?:&.!•.i?.h?.:•.th.i&?.•:.?.i?.!?.i?.i?.!?.!
Fig. 10. Schematiccartoonsof slowed
asthenospheric
upwellingbeneaththe axialvalleyof
a mid-oceanridgeproducingNaTi basalts(top)
comparedto fasterasthenospheric
upwellingbeneath
a normalridgecrestproducingMORBs (bottom).
Boundariesare transformfaults (TF). Area inside
blockslabeledTF represents
theadditional
lithosphericthicknessin the transformfault zones.
Dottedareais the zoneof partialmeltin the
asthenosphere.
Lengthof arrowsis proportionalto
thespeedof asthenospheric
upwelling.
timesfor volatilesin a subducting
plateusingthe
thermal model of McKenzie [1969] and an
8-km-thickvolatile-richlayer. This thermalmodel
doesnot includethe effectsof the thermalgradients
in the overlyinglithosphericplate,whichwould
increasetheretentiontime. Althoughtheactual
retentiontimesare somewhatlongerthanour
calculatedvalues,relative retentiontimes are
portrayedquiteaccurately.Becausehorizontalheat
conductionalongthe axisof the slabmakesan
insignificantcontributionto reheatingof rapidly
subducting
plates,minimumretentiontimes
calculatedfrom thismodelusingconvergence
rates
from 2 to 12 cm/yr areconvergence-rate-independent
to within 1% for 10- to 100-m.y.-oldplatesand to
within 10% for 1-to 10-m.y.-old plates. The
reheatingof slabssubducting
at lessthan2 cm/yris
compressed
into a relativelysmallregionby the slow
convergence,
producinglargehorizontaltemperature
gradientsalongthe axisof the slab. Becauseof
theselargehorizontaltemperature
gradients,
horizontal and vertical heat conduction are both
important,and volafilesin slowlysubductingslabs
breakdown of richterite-tremolite
commence at
~1000øC [Green,1973;Hariya andTerada,1973].
Thus,within a subducting
platewhichsuddenly
ceasedconvergence,
the lastvolatilesin theportion
of theplatebeneaththe volcanicline wouldbe lostat
temperatures
of ~ 1000øC, andthelastseismicity
at
temperatures
of ~750ø C.
ORIGIN
OF THE ARCLIKE
ROCKS
Thevolatileandincompatible
element
enrichments in the arclike lavas from the Woodlark
basinhavebeenproposedto be theresultof a
polarityreversalof thetrench-arcsystem[Perfitet
al., 1986] about 8- to 10-m.y.ago [Colemanand
Kroenke, 1981; Cooperand Taylor, 1985].
However,the relictslabcontinuedto convergeand
to producethrustingon the Solomonislandarcuntil
at least6 m.y. ago [ColemanandKroenke, 1981].
Thus,thepolarityreversalof the arcandsubduction
of therelict slabendedlessthan6 m.y. ago.
The model of Perfit et al. [ 1986] doesnotrely
uponfluids comingdirectlyfrom the older,
previouslysubductingslabandenteringtheisland
arc magmas,but insteadreliesuponsubduction
and
associated
mantlecontamination
priorto thepolarity
AbbottandFisk: TectonicallyControlledRock Suites
o
750oc
x•
•)o_
'•
• O0
(center)
/.-"melt,•ng
Aor
amph!b.o.
le.
out
• uo-
Subducting Plate Age, m.y.
Fig. 11. Minimum mantleretentiontimesof
volatilesand seismicitywithinplateswhichhad
differentagesat the onsetof subduction.Dotted
curve:Time neededto warm centerof a subducting
plateto 750ø C, as a functionof subducted
plateage.
Seismicactivityis thoughtto ceaseabovethis
temperature.Hachuredcurve:Time neededto warm
the upper8 km of theplate above1000ø C. Most
hydrothermallyemplacedvolatileswill havebeen
lostwhentheplatereachesthistemperature.
Minimum mantleretentiontimesarecalculatedusing
theformulain McKenzie [ 1969]. The trianglewith
errorbarsshowsthe estimatedage andtime since
polarityreversalof thepreviouslysubducting
Pacific
platewhichlies beneaththeNew Georgiaarc.
reversaland consequentcessationof subduction.
Our simplethermalmodels(below)indicatethatan
old, previouslysubducting
platecanremaincold
enoughto contributevolatilesto the surrounding
mantlefor millionsof yearsaftersubduction
has
ceased.Consequently,the slabwhich stopped
subducting
lessthan6 m.y. agocouldbe presently
contributing
volatilesto themantleconvection
cell
thatfeedstheWoodlarkbasinridgesegmentclosest
to the trench.
The minimum
the seismicactivitynearthevolcanoesis shallowand
sparseandcanbe attributedto surfacedeformation
ratherthanto a subductingplateat depth
[Drummondet al., 1981; Kellogg et al., 1985]. The
subducting
platesin thesethreeareasare all
Pleistoceneto Miocenein age [HeezenandFornari,
1976; Sugi and Uyeda, 1984], consistentwith our
modelpredictions.
Thesemodelsalsoimply thatvolatilescould
continueto be derivedfrom the old, nonsubducting
slabin zonesof polarityreversalof the arc. The
previouslysubducting
Pacificplatewhichunderlies
theNew Georgia/Solomon
islandarcswasof Lower
Cretaceousage (100-136 m.y.) at the initiationof
(8km)
,,--,
•:x
•
[
•
1000øC
80 120 160a&O
• -m ,
40
1155
mantle retention times for volatiles
andseismicityat depthsof 100-125km show
severalinterestingtrendswhichprovide
observational
testsof our results(Figure 11). Older
subducting
plateswill remainseismicallyactivefor a
longertime thantheyretainvolatiles. Conversely,
youngersubductingplates(lessthan30 m.y. old
uponenteringthe trench)will retainvolatilesfor a
longertime thantheyremainseismicallyactive.
Volatilesderivedfrom the subducting
plateare
widelybelievedto be an importantcontributorto
islandarc volcanism[McBimey, 1969;Boettcher,
1973;Nichols andRingwood, 1973; Kushiro,
1973]. Consequently,theseresultsimply that
young,activelysubducting
plateswill sometimes
produceactiveislandarcswithoutthepresenceof
subcrustal
seismicitybeneaththevolcanicline. The
Cascadesarc and the Andes in southernChile,
whichhaveno seismicactivitybeneaththe volcanic
line [BarazangiandDorman, 1969], areexamplesof
this. In southernColumbia and northernEcuador,
subduction[Heezen et al., 1976]. The minimum
volatileretentiontime in a plateof thisageis 6 m.y.
(Figure 8). The relict slabceasedsubductingless
than6 m.y. ago. Consequently,
the incompatible
element enrichments in the rocks of the Woodlark
basinspreadingcentermay be theresultof active
volatileandincompatibleelementlossby an
immobileslabof Pacificplate(Figure2) andarenot
requiredto be causedby a previousphaseof mantle
enrichment.
CONCLUSIONS
Lithosphericflexureat trenchescausestensionin
theupperpart of the subducting
plate,whichcracks
new rock and forms new fault blocks. Further
idenceforflexurallyinduced
cracking
is thehigh
e contentof porefluidsfrom DSDP site436,
locatedon the crestof the flexuralbulgeof
110-m.y.-oldoceaniccrustenteringthe Japan
trench.If the flexedplateis youngandhasthin
sediment cover, the new cracks and increased
permeabilityshouldalsoincreasetheintensityof
hydrothermalactivity. This increasein hydrothermal
activitywouldbe mostpronounced
on a ridgecrest
or transformfault sittingon a flexuralbulge.
Increasedridgecresthydrothermal
circulationwould
lower near-surface isotherms and could cause a
deepermagmachamber.A magmachamberlocated
in the uppermostmantle,wherebasaltmagmacould
reactwith harzburgite,explainsthe anomalous
high-Mg andesiteson Simbotransformfault, which
lieswithin the flexuralbulgeof theWoodlarktrench.
The NaTi basalts,also extrudedon the Simbo
transform,maybe theresultof eruptionthrough
anomalously
thick,youngoceaniclithosphere.The
coldneighboring
platescausemantleupwellingto
slow,thusproducingdecreased
decompressive
partialmeltingof therisingasthenosphere.
NaTi
basaltsalsooccurat shortridgesegments
sandwichedbetweenlong ageoffsettransformsand
at largeageoffsetridgejumps,bothof whichare
logicallocationsfor slowedmantleupwellingdueto
anomalouslycoldneighboringlithosphere.
Mathematical
models of minimum mantle
1156
AbbottandFisk:TectonicallyControlledRock Suites
retentiontimesof seismicityandvolatilesare
consistentwith the followingobservations:
(1) active
Holocenevolcanismandno deep,subarcseismicity
APPENDIX:
CALCULATION
OF STRESS
AND THE WDTH
OF FLEXURAL
BULGE
in the Cascades,northernEcuador, southern
Colombia, and the southemAndes; and (2)
At smalldeflections
of theplate,theelastic
thickness
is approximately
equalto themechanical
thickness.Thisassumption
is particularlygoodfor
incompatibleelementenrichmentin therocksof the
Woodlarkbasinspreadingcenterlessthan6 m.y.
aftertherelictPacificplatestoppedsubducting.
Consequently,
theincompatibleelementenrichments
in the rocksof the Woodlarkbasinspreadingcenter
arenot requiredto be causedby a previousphaseof
mantle enrichment but could continue to be derived
from the old, previouslysubducting
plate.
The unusualrock typesfoundin theWoodlark
basinmay alsobe foundin ophiolitesemplacedasa
resultof ridge subduction
and/ortrenchreversal.
The Troodosophiolite,whichis postulatedto have
formedas a resultof a combinationof ridge
subduction
andseafloorspreadingbehinda trench
[Moores et al., 1984], has somearclike rocks
trenches[McNutt, 1984]. Therefore, all of the
followingcalculations
aremadeassuming
thatthe
elasticandmechanicalthickness
areroughlyequal.
The thickness,
h, whichis equivalentto thedepthto
the600ø C isotherm[McNutt, 1984],is proportional
to the squareroot of the plate age:
h(t)= 7.4x 10-2t1/2cm
(1)
wheret is theageof the oceanicplatein seconds.
The loadat thetrenchcausestheplateto deformwith
thefollowingflexure,y(x):
[Miyashiro,1973;Robinsonet al., 1983]. The
NaTi basalts found in the Woodlark basin would be
easyto dismissasspilitesand/oralteredbasaltif
theywerefoundin anophioliteandarenotlikelyto
havebeenreported.Boninitesoccurin thepillow
lavasof the Troodosophiolite[Cameronet al.,
1979;Robinsonet al., 1983]. Consequently,
two of
the unusualrock typesreportedfrom theWoodlark
basin,thehigh-Mg andesites
(boninites)andthe
arclikerocks,arefoundin an ophioliteemplacedasa
resultof ridge subductionand trenchreversal. The
researchon theWoodlarkbasinshouldbe applicable
to otherophiolitesemplacedduringridgesubduction
A(O
y(x) =
cos[k(t) x] exp[ -k(t) x]
(2)
0.067
where k(t) is the flexural wavenumber,x is distance
from the trench,andA(t) is the maximumflexure
inducedby the load (which occursat kx=3x/4)
(Figure3) [Caldwell and Turcotte,1979]. We
estimateA(t) from McNutt's [1984] data:
and/or trench reversal.
A(t)= 5.2x 10-4 t1/2cm
Thesemodelsof the effectsof lithospheric
flexurecanbe testedby dredgingandby heatflow
surveysin anotherareaof activeridge subduction.
The Woodlarkbasinis not the optimumlocationfor
relatedultimatelyto theplatethickness
throughthe
such a test because of the small amounts of sediment
relations:
covetingthe flexuralbulgeandbecauseof the
complicatingeffectsof thepolarityreversalof the
arc. The requirementsfor sucha testareaare
k(t)= [Apg/4D]
1/4
50-200 m of sediment cover in the area of the
(3)
where t is in seconds. The flexural wavenumber is
(4)
flexuralbulgeanda ridgeorientationnearlyparallel
to the trench. The Peru-Chileridgeis mostlike the
whereAp is the differencein densitybetween _
Woodlark basin in its orientationto the trench, and it
alsohas sufficientsedimentcover (50-100 m) for a
heat flow survey. Furthermore,due to somewhat
is theacceleration
dueto gravity,andD is theplate's
flexuralrigidity,givenby:
unusualcircumstances,
areaswith roughlyequal
sedimentcoveron the sameageof oceaniccrustcan
be foundbothwithin andoutsidethe flexuralbulge
seawater
andmantle
(assumed
tobe2.3gm/cm3),
g
D = Y h3(t)/ 12(1-v2)
(5)
[Cande and Leslie, 1986]. Other areasof active
ridge subductionhavethe wrongsediment
distributionandridgeorientation(theExplorerridge
[INPAC, 1985]), are undergoinga ridgejump (the
eastPacific rise at 18ø N [Luhr et al., 1985]), or are
perpendicular
to thetrenchaxis(Ayu trough
[Weissel and Anderson, 1978]).
where Y is Young'smodulus(assumedto be 1.8 x
12
2,
,
10 dynes/cm ) and v is Poissons ratio (assumed
to be 0.25). The flexural wavenumberis therefore:
k(t)= 5.4x 10-2t-3/8cm-1
AbbottandFisk: TectonicallyControlledRock Suites
The flexure inducesstress,(5,in the lithosphere:
(6)
(5= (•+2g)zO2y/ Ox
2
1157
Anderson, R. N., M.D.
Zoback, S. H. Hickman,
andR. L. Newmark, Permeabilityversusdepth
in the upperoceaniccrust:In situmeasurements
in DSDP hole 504B, easternequatorialPacific,
J. Geor)hvs.Res., 90, 3659-3669, 1985.
Barazangi;M.,
andJ.Dorman,
Worldseismicity
where• andg are.tJ•eLam• colqstants
(assumed
•+2g = 2.17x 10•z dynes/cm
z) andz is distance
abovethe plate'sneutralsurface(thatis, middle).
The maximum stress,which occursat z=h/2 and
kx=•/4
is then:
mapscompiledfrom ESSA, CoastandGeodetic
Survey,epicenterdata, 1961-1967, Bull.
S½ismol.Soc. Am,, 59, 369-380, 1969.
Bodine, J. H., M. S. Steckler, and A. B. Watts,
Observationsof flexureandtherheologyof the
oceaniclithosphere,J. Geophys.Res., 86,
3695-3707, 1981.
Oma
x= 1.16tl/4bars
(7)
Boettcher,A. L., Volcanismandorogenicbelts--The
origin of andesites,Tectonophysics,
17,
223-240, 1973.
The stressdecreases
linearly with depthin the
lithosphere,while the strengthof therock increases
linearlywith depth. Thusthe toppartof the
1/thosphere
is crackeddown to somedepthwherethe
stressand strengthare equal. Using the Bodine et
al. [1981] estimateof 0.222 bars/mfor the strength,
the depthof crackingis:
Bratt, S. R., E. A. Bergman,and S.C. Solomon,
Thermoelasticstress:How importantas a cause
of earthquakes
in youngoceaniclithosphere?,
J.
Geophys.Res., 90, 10,249-10,260, 1985.
Byerly, G. R., W. G. Melson, andP. R. Vogt,
Rhyodacites,andesites,ferrobasalts,and
oceanictholeiitesfromtheGalapagos
spreading
center,Earth Planet. Sci. Lett., 30. 215-221,
d= 3.7x 10-2t1/2{1-[14100t-1/4+1]
-1} cm (8)
Caldwell, J. G., andD. L. Turcotte,Dependenceof
thethicknessof theelasticoceaniclithosphere
on age, J. Geophys.Res,, 83, 7572-7579,
1976.
Equations7 and 8 areplottedin figures4 and 5.
1979.
Cameron, W. G., E.G. Nisbet, and V. J. Dietrich,
Acknowledgments.We thankM. Petit, R.
Barnes,B. Taylor, andH. Staudigelfor providing
uswith preprintsof their unpublishedmanuscripts.
We thank W. White, W. Menke, S. Hoffman, P.
Hoffman, B.Taylor andN.Sleep for their critical
comments.Acknowledgmentis madeto theDonors
of the PetroleumResearchFund, administeredby
theAmericanChemicalSocietyfor thesupportof
Abbottduringthisresearch.Fisk was supportedby
OCE84-12395.
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(ReceivedJune23, 1986;
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