JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104,NO. B5, PAGES 10,421-10,437,MAY 10, 1999
Quantifyingtectonicstrain and magmaticaccretionat a
slowspreadingridgesegment,Mid-AtlanticRidge,29øN
J.Escartin,•,2,
3P.A. Cowie,
2R.C.Searle,
• S.Allerton,
2'4
N. C.Mitchell,
•'5C.J.MacLeod,
6 andA. P.Slootweg
•
Abstract.High-resolution,
deep-towed
side-scan
sonardataareusedto characterize
faultingand
variations
in tectonicstrainalonga segment
of theslowspreading
Mid-AtlanticRidgenear29øN.
Sonardataallowusto identifyindividualfaultscarps,
to measure
faultwidthsandspacing,
andto
calculate
horizontal
faultdisplacements
(heave)andtectonicstrain.We findthatoverlongperiodsof time(> 1 Myr onaverage),
tectonicstrainis-10% onaverage
anddoesnotvarysignificantlyalongaxis. Thereis a markedasymmetry
in tectonic
strainthatappears
to belinkedto
asymmetric
accretion
alongthewholesegment,
indicated
by -50% lowertectonicstrainonthe
eastflank thanon the westflank. Thesevariationsin tectonicstraindo not correlatedirectlywith
changes
in faultspacingandheave.Faultspacingandheaveincrease
fromthecenterof thesegmenttowardthe end(insidecomer)on the westflank andfrom the outsideto the insidecomer
across
theaxis. Theseparameters
remainrelativelyconstant
alongthe segmenton theeastflank
andacrossthe axisat the segmentcenter. Tectonicstrainappearsto be decoupledfrom magmatic
accretionat timescales
> 1 Myr, asthe decreasein magmasupplyfromthe segmentcentertoward
the end (inferredfrom variationsin crustalthicknessalongthe axis) is not correlatedwith a complementaryincreasein tectonicstrain. Instead,tectonicstrainremainsrelativelyconstantalong
the axis at-7%
on the east flank and at- 15% on the west flank.
These results indicate that varia-
tionsin fault development
andgeometrymay reflectspatialdifferencesin the rheologyof the
lithosphereandnot changesin tectonicstrainor magmasupplyalongaxis.
1. Introduction
Frommarinegeophysical
andgeological
studiesand modeling
resultsit hasbeenproposed
thatmeltsupply[e.g.,KuoandForThe structureof the oceaniccrust and seafloormorphology
syth,1988;Lin et al., 1990;Detricket al., 1995],lithospheric
arethe resultof magmaticaccretion(i.e., dike injection,gabbroic
intrusions)
andextensional
processes
(i.e., brittlefaulting,ductile
deformation)taking place at mid-oceanridges (MORs). The
relative importanceof extensionalfaulting appearsto increase
with decreasing
spreading
rate,asindicatedby changes
in sizeof
fault-generated
abyssalhills [e.g.,Goffet al., 1997], or analyses
of fault populations
at fast and slowspreading
ridges[e.g.,Edwards et al., 1991; Carbotte and Macdonald, 1994]. It has also
beenproposedthat changesin lithosphericthickness[e.g., Forsyth,1992;Shaw, 1992], rheology[e.g.,Escartinet al., 1997b]
or magma supply [e.g., Laughtonet al., 1979; Searle and
Laughton,1981; Searle et al., 1988b] also controlthe style of
thickness
[e.g.,Parmentier
andForsyth,
• 1985;ShawandLin,
1996],andcrustalcomposition
[e.g.,Cannatet al., 1995]change
systematically
from segmentcenterstowardsthe ends. It has
beenhypothesized
thatvariationin meltsupplymaybe linkeddirectlyto tectonicstrain[e.g.,AlexanderandMacdonald,1996],
which would result in increasedtectonic strain from the segment
centertowardsthe ends, where melt supply is reduced[e.g.,
Jaroslow,1996; Allerton et al., 1996]. Either increasedlithos-
phericthickness
or reduced
faultstrength
dueto serpentinization
can resultin strainlocalizationfavoringthe formationof large
faultswidelyspaced
at theendof segments
[Francis,1981;Forsyth,
1992;
Shaw,
1992;
Shaw
and
Lin,
1996;
Escartinet al.,
faulting at MORs.
1997b],
independently
of
variations
in
tectonic
strain
alongthe
Detailedstudyof a slow spreadingridge may provideconridge
axis.
It
is
thus
necessary
to
determine
if
observed
variastraintson the effect of each of theseparameterson faulting.
tionsin faultpatterns
arelinkedto changes
in tectonic
strain.
In additionto along-axisvariationsin fault geometry(i.e.,
•Department
of Geological
Sciences,
University
of Durham,
Durham, fault spacing
andthrow)andmagmatic
supplyalonga ridge
England.
2Department
of GeologyandGeophysics,
University
of Edinburgh,
The GrantInstitute,Edinburgh,Scotland.
3Nowat Laboratoire
de P6trologie,
Universit6
Pierreet MarieCurie,
Paris, France
4Nowat RobertsonResearchInternational,Llandudno,Wales.
5Nowat Department
of EarthSciences,
Universityof Oxford,Oxford, England.
6Department
of EarthSciences,
University
of Wales,Cardiff,Wales.
Copyright1999by theAmericanGeophysical
Union.
Papernumber1998JB900097.
0148-0227/99/1998JB900097509.00
segment,
thereis alsoa marked
asymmetry
across
theridgeaxis
of slowspreading
ridges.Theinsidecorners
(ICs)of ridge-offset
discontinuities
arecharacterized
by elevatedterrainandthin crust
with respectto the outsidecorners(OCs) or segmentcenters
(SCs)[e.g.,Severinghaus
andMacdonald,
1988;Tucholke
and
Lin, 1994; Escartinand Lin, 1995] and frequentlycontainout-
cropsof serpentinized
peridotite
[Cannat,1993;Tucholke
and
Lin, 1994;Cannatet al., 1995]. At ICs, faultsare alsomoreir-
regular,larger,andmorewidelyspaced
thanat SCsor OCs[Tucholkeand Lin, 1994; Cannat et al., 1995; Escart•nand Lin,
1995;Jaroslow,1996]. Accordingto Jaroslow[1996], tectonic
strainestimated
from multibeambathymetrymay be -•50% larger
10,421
10,422
ESCARTIN
ET AL.: TECTONIC
STRAIN
at the IC than at the OC or SC. However, the differences in the
averagetectonicstrainbetweenthe segmentcenterandend(both
IC and OC) are small and vary amongthe three Mid-Atlantic
Ridge(MAR) segments
studied[Jaroslow,1996]. This variability amongsegmentssuggeststhat the effectsof magmasupply,
lithospheric
thickness,
or rheologyon faultingare poorlyunderstoodand requirefurther investigation. In particular,careful
characterization
of fault patternsand quantificationof tectonic
strain along a slow spreadingsegmentcan provide additional
constraints on these controls.
Tectonicstrainalonga ridge segmentis accommodated
near
the surfaceby the nucleation,linkageand growthof faultsin the
brittle domain[e.g., Alexanderand Macdonald, 1996]. At slow
spreading
ridges,faultsinitiatedwithin the axial valley floor are
shortand have small throws,and somedevelopinto large faults
along the rift-valley walls [McAllister and Cann, 1996]. Faults
may remainactiveat distancesof up to -•15-35 km from the ridge
axis, as indicated by the width of the seismically active zone
[e.g., Lin and Bergman,1990; Wolfeet al., 1995]. Studiesthat
quantify the proportionof the total plate separationaccommodatedby brittle deformationat MORs are scarceand are basedon
a wide variety of datasets.Estimatesof tectonicstrainat the slow
ON THE MID ATLANTIC
RIDGE
spreadingMAR vary between---18%(high-resolutionbathymetry, 37øN [Macdonald and Luyendyk,1977]), 10-20% (teleseismic earthquakes[Solomonet al., 1988]), ---25%(high-resolution
side-scansonar,24øN JAilertonet al, 1996]), and >30% (multibeam bathymetry and HMR1 side-scan sonar data, 27-28øN
[Jaroslow, 1996]). This disparity of estimatesmay arise from
differencesin the methodsusedto calculatefaulting strain,in addition to possibleregionalor local variationsin strain. The spatial resolutionof--100 m of sea surfacemultibeam bathymetry
data makesthem unsuitableto properlycharacterizesmall-scale
faulting (faults with lengths<2 km and displacements
<200 m),
and estimatesbasedon suchdata may be inaccurate[e.g., Cowie
et al., 1994].
The deep-towed side-scan sonar from the Towed-Ocean
Bottom Instrument(TOBI) provideshigh-resolutionsonar imagery(-•10 m spatialresolution),which allowsus to characterize
both large- and small-scalefaulting. The data presentedhere
were collected on board RRS Charles Darwin (see Searle et al.
[1998a] for cruisedetails). Sonar data coveredthe southerntwo
thirds of a slow spreadingridge segmentat the northernMAR
(29øN), extendingto---40 km off axis (---2.8Ma old crust),with a
track spacingof---2 km (Figure 1). This surveygeometrypro-
a)
25'N
29' 15'N
28' 45'N
b)
Figure 1. (a) Illuminatedsatellitegravity map [Sandwelland Smith, 1993] of the Mid-Atlantic Ridge betweenthe
AtlantisandKaneFractureZonesshowinglocationof the studyarea. Insetshowslocationof the mapin the northern Atlantic (tracesof transformand non-transformdiscontinuities
from Escart•n[1996]). (b) Multibeam bathymetricmap of the studyarea (100-m grid spacing,250-m contours). Continuouslines correspondto the TOBI
tracks,andthe dashedlinesto the boundariesbetweendifferenttectonicregions. IC, insidecorner;SC-W, segment
center,Westflank; SC-E, segmentcenter,Eastflank; OC, outsidecorner. The locationsof microseismicepicenters
from Wolfeet al. [1995] are alsoshown(circles).
ESCARTIN
ET AL'
TECTONIC
STRAIN
ON THE MID ATLANTIC
RIDGE
10,423
vides•-100% sonarcoverageof the areaand allowsus to better
sectionsacrossthe axis into IC andOC (segmentend) and SC-E
understand
geometryof seafloorstructures
withthetwo opposite andSC-W (segmentcenter,Figurelb).
lookingdirections
that are available(northand southlooking,
Figure 1). Previousstudiesusing TOBI or similar deep-towed 3. Data Description
instrumentshave coveredthe axial valley floor in detail [e.g.,
TOBI deep-towedside-scansonaroperatesat 30 kHz and has
Macdonaldand Luyendyk,1977; Kong et al., 1988; McAllister et
a scanwidth of•-6 km whenoperatedat 400-600 m from the seaal., 1995; Smith et al., 1995; McAllister and Cann, 1996; Allerfloor. The averagespatialresolutionat this towing altitude is
ton et al., 1996], but data over ridge flanks and away from the
---10 m (for more details on instrument specifications,see
ridge axis have been scant [e.g., Macdonald and Luyendyk, Flewellen et al. [1993] and le Bas et al. [1995]). The TOBI
1977]. Thissegmenthasalsobeenthe focusof severalgeologi- tracklineswere orientedE-W and spaced•-2 km apart(Figure lb;
cal and geophysicalstudiesin the past [e.g., Lin et al., 1990; seeSearleet al. [1998a] for cruisedetails). This surveygeomePurdy et al., 1990; Shaw, 1992; Rommeveauxet al., 1994; Semtry provided almost 100% overlap of backscatterdata between
pird et al., 1993, 1995; Mutton et al., 1995; Pariso et al., 1995;
adjacentlines in most of the survey area, which allowed us to
Smith et al., 1995], which provide information to constrain
obtainalmostcompletenorth- and south-lookingside-scansonar
variationsin crustalthickness,melt supply,thermalstructure,and mosaics(Plates 1 and 2). The use of mosaicswith uniform
crustalcompositionalongthe segment.
lookingdirectionfacilitatesthe identificationof tectonicfeatures
TOBI side-scansonardata allow us (1) to obtainstrainestiacrossTOBI tracks,andthe availabilityof two lookingdirections
matesbasedon high-resolution
fault interpretations,
and (2) to helpsto constrainthe geometryof thesefeatures.
determine
howtectonicstrainis coupledwith melt supplyalong
The backscatterdatawere processed
for noisereduction,slant
the axis. Tectonicstrain,fault spacing,and horizontalfault disrange,and gain correctionsusingthe methodsdescribedby le
placement(heave)estimates,as well as their spatialvariations, Bas et al. [1995]. The data were then locatedgeographically
are obtainedfrom the interpretedfault maps. Thesedata are usingestimatesof the TOBI positionand the attitudeof the vehicomparedwith existing models of crustal accretion at slow
cle (heading,pitch, and roll), projectedonto a horizontalplane
spreading
ridgesto constrain
the relationship
betweenmelt sup- below the vehicle. TOBI navigation was calculatedfrom the
ply andtectonicstrain.
ship'sposition,lengthof wire out, and depthof TOBI usingour
modification[CD99 ScientificParty, 1996] of Hussenoederet
al.'s [ 1995] implementation
of Trianta•llou and Hover's [ 1990]
2. Tectonic setting
cable-modeling
algorithm. The processedand remappeddata
wereseparated
intonorthandsouthlookingto createtwo digital
The studyfocuseson the secondMAR segmentsouthof the
mosaics
in
each
of the main directionsof insonification(north
Atlantis Fracture Zone, boundedby the 29ø23'N and 28ø51'N
andsouth,Plates1 and2, respectively).
nontransform
discontinuities[Sempdrdet al., 1993] (Figure l a).
The presentfull spreadingrateis •-26 mm/yr [e.g.,Semp•rdet al.,
1995], but plate separationis highly asymmetrical,with >60% of
the total plate separationoccurringon the east flank [Allerton,
1997;Searleet al., 1998a]. The morphologyof the rift valley is
typical of a slow spreadingridge segment,with a narrow and
shallowvalley at the centerthat widensand deepenstowardsthe
discontinuities
(Figure lb). This segmenthas been the site of
severalgeophysicalstudiesin the past that have describedits
morphology[Purdy et al., 1990; Sempdrdet al., 1993; 1995;
Shaw, 1992;Smithet al., 1995], gravityandcrustalstructure[Lin
et al., 1990; Rommeveauxet al., 1994], and tectonicevolution
[Searleet al., 1998a]. The crustis thickerat the SC and OC than
at theIC [Rommeveaux
et al., 1994],possiblyreflectingthe combinedeffect of focusedmagmaticaccretionon-axis[e.g., Kuo
and Forsyth,1988;Lin et al., 1990] andtectonicthinningat ICs
[Escartœn
and Lin, 1995]. The IC terrain is characterizedby
largerfaults(> 1 km in throw)thanat thethe OC or SC (<0.5 km)
[Shaw,1992; Shaw and Lin, 1993] and by diffusemicroseismic
activity[Wolfeet al., 1995] (Figure lb).
Tracesof the nontransform
discontinuities
boundingthe segmentto thenorthandsouth,asinferredfrom existingbathymetry
[Rommeveaux
et al., 1994] and satellitealtimetrydata [Escartœn,
1996],demonstrate
thatthe offsetshavemigratedalongthe axis
with time. To comparefaulting patternsand tectonicstrain in
differentpartsof the segment,we havedividedthe segmentinto
three zones (north, center, and south). The north-centerand
center-south
boundariescorrespond
to onethird andtwo thirdsof
the distancebetweenthe two boundingdiscontinuities
in a direction parallelto the ridgeaxis(•-010ø). Our datacovermainlythe
centerandsouthsectionsof the segment.To studyspatialvariability in faulting and tectonicstrain we have separatedthese
4. Fault Identification and Analysis
Quantification
of tectonicstrainaccommodated
by brittleprocessesrequiresspatialidentificationof faultsandtheir geometry,
and measurement
of both fault displacement
(verticalor horizontal) and spacing. Faults are identifiedin the side-scansonar
dataaszonesof relativelystrongbackscatter
intensity,with high
length to width ratios and characteristic
tectonic(i.e., linear
[McAllisterand Cann, 1996; Searle et al., 1998a]) ratherthan
magmatictexture(i.e., hummockyterrain,shingledareas[Smith
et al., 1995]). Backscatter
intensitydepends
on insonification
direction,fault geometry,and grazingangle, in additionto the
acousticreflectivityof the seafloor. Thus large variabilityin
backscatter
is expected
for differentfaultsacrossthe studyarea.
The useof imageswith two insonification
directionsprovidesin
most cases sufficient informationto determineif a highbackscatter
areadoescorrespond
to a steepscarp,and if so, to
constrainits geometryand orientation.Examplesof identified
faultsfor four small areasof the digitalmosaicsare shownin
Figure2, corresponding
to the IC, OC and SC (eastand west).
Only thosescarpspositivelyidentifiedas fault-relatedhavebeen
digitizedto createthefaultmapin Figure3, andusedin thefault
analysisdescribedbelow. Fault analysishas been performed
alongE-W transects
spacedevery0.5 km (Figure4), whichallowsus to calculatetectonicstrainwithoutrelyingon empirical
length-displacement
scalingrelationships
for faults[seeDawers
et al., 1993;Cowieet al., 1994].
Largerfaultsidentifiedin the backscatter
mosaicsare in good
agreementwith steep scarps as identified in the multibeam
bathymetric
maps(Figure5). The westwall of the rift valley in
10,424
ESCARTIN
ET AL ßTECTONIC
STRAIN
•.•
ß
ON THE MID ATLANTIC
'!•::•. .•*-,,..:**
...':•::.•......::..
ß.
........ ......:•:•::•::•:•:,•..,;
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RIDGE
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ESCARTIN ET AL.: TECTONIC STRAIN ON THE MID ATLANTIC
RIDGE
10,425
Figure
3. Fault
map
interpreted
from
thedigital
side-scan
sonar
mosaics.
Inward
facing
faults
aremarked
in
black,
and
outward
facing
faults
byoutlined
gray
areas
(see
also
Figure
4).The
ridge
axis
isindicated
bythedou-
bleline,
andthedashed
gray
lines
correspond
tothesouthern
nontransform
discontinuity
andtotheboundaries
between
thedefined
tectonic
regions
(see
Figure
1).Underlying
squares
mark
thelocation
ofthefourareas
shown
inFigure
2 (a;SC-W,
b;SC-E,
c;IC,d;OC)andthatofFigure
5.
the southcorresponds
to the largestidentifiedfault scarp(Plate1
and Figure 3) with >1000m of vertical relief (Figure 1). The
width of the fault as identified on the sonar data is in good
agreementwith that identified from the multibeambathymetry
(Figure5a). In otherareas(eastflank, e.g., Figure5, X=697 km),
model[DeMetset al., 1990]). Assumingthat fault displacement
is mostly downdip extensionalwith no substantialstrike-slip
component,
theerroroftheobserved
orapparent
(h*) versus
the
real heave(h) parallelto spreading(100ø) is <3% and therefore
negligible.
A potentiallymore importantsourceof error may arisefrom
bathymetricscarpscorrespondto severalfaults interpretedfrom
thebackscatter,
whichhavesmallheave(<500 m) andareclosely the interpretationof the bright backscatterunitsas representative
spaced(<1 km). These smallerfaults are not resolvedby the of the real heave. In somecasesthesebright areasmay include
highly reflectivetalusbuilt up by masswastingat the baseof the
multibeambathymetry.
To accuratelyquantifytectonicstrainandmagmaticaccretion, fault [Allerton et al., 1996], and the measuredheave will then
we havedeterminethe polarityof faults(inwardversusoutward correspondto an "apparentheave." The differencebetweenthe
facingfaults,Table 1), measuretheirspacingandheave(Tables2 apparentand the real heavewill dependon the dip of the fault,
talus slope,proportionof the fault height coveredby the talus,
and3), andcalculatetectonicstrain(Table4).
and erosionof the fault scarpby masswasting (scarpretreat),
amongother parametersand processes.Talus slopesmeasured
4.1. Fault Facing Direction
from near-bottombathymetryavailablein an adjacentareaof the
We determine for each fault whether the orientation of the
northernMAR [Tucholkeet al., 1997; Goff and Tucholke,1997]
fault scarpis towardthe ridge axis or away from it (inward and
yield valuesas high as 35ø. Assuminga talusangleof 30ø covoutwardfacing faults, respectively,Figure 3 and Table 1). As
ering --40% of an unerodedfault scarp,we can estimatethat h
mostof the faultsare subparallelto the ridgeaxis,with very few
canbe60%orequalto h* fordipsof faultequal
to 60øor30ø,
subperpendicular
to it, this distinctionis unambiguousin most
respectively(Figure6a). A dip of fault equalto --45ø, consistent
casesif boththe northand southlookingbackscatter
mosaicsare
with dipsestimatedfrom earthquakefocal mechanisms
[Thatcher
usedin the interpretation(Plates1 and2).
and Hill, 1995] and high-resolution
bathymetry[Macdonaldand
Luyendyk, 1977; Tucholke et al., 1997; Goff and Tucholke,
4.2. Fault heave h
1997],wouldgiveh=0.8h*. Anymass
wasting
resulting
infault
The width of a digitizedfault (Figure3) in the E-W direction scarp
retreat
mayalsomakeh* larger
thanh (Figure
6b). Other
is assumedto correspondto the horizontalcomponentof dis- processes
suchas partial burial of the fault scarpby lava flows
placementof the fault, or fault heaveh. The E-W transectsare [e.g.,Macdonald
etaL,1996]mayresult
in a reduction
of h* inonly 10ø off the spreadingdirection(---100
ø fromNUVEL-1 plate stead.
Figure 2. Comparisonof (top) illuminatedmultibeambathymetry,(middle) side-scansonardata, and (bottom)
detailedfault interpretation
for selectedareascorresponding
to the (a-c) SC-W, (d-f) SC-E, (g-i) IC, and (j-l) OC.
The outlineof digitizedfaultsis also shownon the shadedrelief mapsfor comparison
(100 m grid, Figure lb).
Backscatter
datafor the areasin the west flank (SC-W and IC, b and h) correspond
to the southlookingmosaic
(Plate 1), while datafor areason the eastflank (SC-E andOC, e andk) correspond
to southlookingmosaic(Plate
2). Locationsof theseareasare indicatedon the fault map in Figure3. We notethat the fault map (bottom)is interpretedfrom bothnorthwardand southwardlookingmosaics(seeFigure3), but for eachareathe backscatter
imageis shownfor only oneof the lookingdirections.
10,426
ESCARTIN ET AL.' TECTONIC
STRAIN ON THE MID ATLANTIC
RIDGE
29.228•
3230
ß,
'-l•;
f I
.• •l•; g•' •'•, ½."-", •'.•
E 3220
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!-- 3210
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=
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I
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710
]
7•0
'
'
730•
X-UTM (km)
Figure 4. Locationof faultmeasurements
alongE-W transects
usedin the faultandstrainanalysis.The location
of eachfaultmeasurement
is indicated
by solidor opensymbols
(inwardandoutwardfacingfaults,respectively).
Faultanalysis
hasonlybeendonefor thesouthern
insidecorner(IC, squares)
andoutsidecorner(OC, circles)and
thesegment
centerat theeastandwestflanks(SC-EandSC-W,triangles
andinvertedtriangles)
of the29øNsegment(biggersymbols);
faultscorresponding
to thesegment
immediately
to the southandto the segment
endtowardthenorth(smallersymbols)havenotbeenusedin theanalyses.
In this study we assumethat fault and talus geometry are out of the scopeof this paper. Lacking any additionalconthroughout
the studyareado not vary, andthereforewe useap- straints,we assumethat these processesare self-similarand do
parentheaveto providean upperboundof the tectonicstrainand
not vary spatially. Suchassumptionis implied in studiesthat use
to estimateits spatialvariability. Determination
of slopeof the backscatter
datato estimatestrain[e.g., Cowie et al., 1993]. In
talusramprequireshigh-resolution,
near-bottombathymetrythat Table2 we reportthe averageandcorresponding
standarddeviais not availablefor all our surveyarea. Processes
thatmay affect tion of measured(apparent)fault heavesfor all areas,calculated
theestimates
of faultheave,suchasscarpretreator burialof fault for all the faultsand for the inwardand outwardfacingpopulations(seeFigures7a and 7b).
scarpsby lava flows, are not well characterizedin this area and
''
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670
I ....
I ....
I ....
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' I ....
685
:1'' ' ' ' i '""'"'"'"'--"--"1
6•
695
7•
X-UTM (km)
Figure 5 (a) Comparison
of interpretedfault scarps(gray bars)•rom the backscaAcr
datawith multibeambathymctrydata. The bathymctric
profileis shownbothwith vc•ical (Sx) •d no vc•ical exaggeration
(thick•d thin
lines,respectively).(b) Locationof the bathymctricprofile in Figure5a on a relief map of the area(100 mcontours).(c) Faultsinterpreted
fromthe backscatter
data(scclocationin Figure3) for thesameareaasin Figure
5b.
ESCARTIN ET AL.' TECTONIC STRAIN ON THE MID ATLANTIC RIDGE
Table 1. NumberandPercentage
of Inward
andOurward FacingFaults
All
SC-W
SC-E
IC
OC
3119
520
1082
411
958
2934(94.1)
486 (93.5)
1004(92.8)
373 (90.8)
955 (99.7)
170(5.6)
28 (5.4)
71 (6.6)
36 (8.8)
3 (0.3)
Na:,total numberof faultsalongtransects
(Figure 4). Ni, numberof inward facing faults. No,
numberof outwardfacingfaults. Valuesin parenthesesare percentages.
10,427
4.4. Tectonic strain,
Tectonicstrainestimatesgiven in Table 4 are calculatedfrom
the measuredfault heaveand spacingusingthreedifferentmethods:(1) from the meanvaluesof h ands alongeachof the transects(Figures4 and 7), (2) from the slopesof the cumulative
fault heaveplots(m, Figure8), and (3) from the cumulativefault
heavealong eachtransectdivided by the lengthof the transect
(Y•h/L). Thesedifferentestimatesare givento determinethe variabilityin tectonicstraindependingon themethodused.
In the following we refer to "apparentheave"as "heave"and
to "apparenttectonicstrain" as "tectonicstrain," unlessotherwise
noted. Fault height cannotbe measureddirectly as extensive
high-resolution,deep-towedbathymetrydata are not available,
and smaller faults identifiable on the backscatter data cannot be
4.3 Fault spacing s
Fault spacings is measuredfrom the separationbetweencentersof adjacentfaults along E-W transects(Figures4, 7c and 7d)
or from the length (L) of the transectsextendingto the edge of
the bacscatterdata divided by the numberof faultsalong it (N/L,
Table 3). Estimatesof fault spacingbasedon numberof faults
per unit lengthare systematicallylower thanthe transectaverages
(Table 3), as the portionsat the end of each transectwith no
faults are includedin the calculation. In all casesthe average
fault spacingand corresponding
standarddeviationare given. As
discussedbelow, fault populations(heave and spacing)do not
follow Gaussiandistributions,so in both caseswe report the
standarddeviationinsteadof the standarderror, as this first pa-
recognized
on the shipboardmultibeamdata(seeFigure5). If
dip of fault, talusangle,and relativeheightof the talusare constantthroughout
the area,fault heavemay be usedas a proxyfor
fault height(seeFigure6).
Analysesof s, h and • have been carried out for each of the
tectonicprovinces(SC-W, SC-E, IC, andOC, Figure1), as well
as for the whole fault population,and for inward and outward
facingfaults(Figures3 and 4). The subscripts
a, i, and o are
usedto referto all, inwardandoutwardfacingfault populations
(seeTables2-4). Thosefaultswith unclearfacingdirection(single, sharpacousticreflectionin both lookingdirections)were
usedonly in the analyseswhich includedall the faults and were
disregardedin the analyseswhich treatedinward and outward
facingfaultsseparately.
rametermay be more indicativeof the variability in fault parameters than the second one.
5. Resultsof Fault and Strain Analyses
5.1. InwantJfersusOutwardFacingFaults
a)
b)
Almost all of the faults identified
Table
Figure 6. Illustrationof someof the possibleerrorsassociated
with estimates
of fault heavefrom backscatter
data. (a) Variationsin faultgeometry(dip of the fault) andtalus. The observed
fault heave(h*) may includetalus in additionto the actualfault
surface,andthereforemay overestimate
the real fault heave(h).
Errorwill dependon the relativeheightof the scarpcoveredby
thetalus,on the angleof the talus,andon the angleof the fault.
(b) Masswastingcanalsoresultin retreatof thefaultscarp,giving apparentheavesthatare largerthanthe realheave.
from side-scan sonar data
are inwardfacing(Figures2 and 5), with <6% of the total number of faultsbeing outwardfacing (Table 1). The largestconcentrationof outwardfacing faults is found at the west flank of
the IC (•9%), while they are almost non-existent at the OC
(<0.5%). Differencesacrossthe axis at the SC are not significant
(•6.6% at the SC-E, •5.5% at the SC-W). Multibeam bathymetry reveals numeroussteep slopesfacing away from the axis.
Theseareasshowbackscattertexturestypical of sedimentedvolcanic morphologies(i.e., hummocky terrain and seamounts
[Smithet al, 1995]), with no evidencefor bright and linear reflectorsexpectedfor outwardfacing faults,and are thereforeinterpretedasback-tiltedvolcanicterrain. Earlierstudiesalongthe
MAR indicatethat outwardfacing faults are not very common
[Macdonaldand Luyendyk,1977; Searle and Laughton, 1981;
All
SC-W
SC-E
IC
OC
2. Fault Heave Parameters
ha
hi
197 (293)
257 (278)
154 (146)
412 (618)
107 (103)
193 (296)
252 (272)
151 (147)
420 (641)
107 (103)
ho
270 (233)
384 (385)
204 (108)
354 (293)
259 (41)
In meters.ha, h•, and hocorrespondto the
average heave and the standarddeviation (in
parentheses)for all, inward, and outward facing faults,respectively.
10,428
ESCARTIN
ET AL.' TECTONIC
STRAIN ON THE MID ATLANTIC
West
RIDGE
East
lOO
lOO
10
•
ß
ß
ß
ß
ß
ß
2000
lOOO
0
3000
1000
2000
3000
Fault heave, h (m)
½)
1 ooo
1000
d)
lO0
100
10
lO
,t
•
ma•
mN
1
0
10
z•
ß
20
'
0
'
ß
•
'J' I '
10
ß
'
'
! I1= A
20
Fault spacing, s (km)
Figure 7. Log-linearplotsof the distributionsof (a-b) fault heaveh and (c-d) spacings for the differenttectonic
environments
(samesymbolconventionsas in Figure4). Only the populationsof inward(solidsymbols)andoutwardfacingfaults(opensymbols)are shown. The shadedlinescorrespond
to bestfit of an exponentialfunctionto
the data;the fit of the functionhasonly beendoneto the portionof the datathat showa lineardistributionin the
plot (seetext). Note the markedasymmetryin the slopesof the distributionsfrom westto east(left and right) and
the absenceof outward-facingfaultsat the outsidecorner. Meanfault heaveandspacingestimates
aregivenin Tables 2 and 3.
Kong et al., 1988], althoughdaroslow[1996] reportsthat up to
40% of the faultsare outwardfacing.
botteand Macdonald, 1994; Cowie et al., 1994]. The cumulative
distributions
of h ands at the IC, SC (eastandwest),andOC ter-
5.2. Fault Population Statistics
rain(Figure8) showthat at s < 9 km andh < 500 m, the populationscan be fitted with an exponentialdistribution(shadedlines
in Figure7) of the type
Cumulative size-frequencydistributionsof fault parameters
have often beenusedto characterizefault populations[e.g., Car-
N(h,s)=NT
e-3.h,s
Table 3. Fault SpacingParameters
s,,
All
SC-W
SC-E
IC
OC
1.77 (1.77)
1.53 (1.41)
1.40 (1.16)
2.59 (2.06)
1.55 (2.09)
si
1.70 (1.87)
1.54 (1.73)
1.50 (1.28)
2.69 (2.22)
1.55 (2.09)
So
4.64 (6.62)
1.20 (1.77)
7.20 (6.34)
0.96 (0.36)
-
L/Na
1.87 (1.28)
1.60 (0.64)
1.51 (0.64)
2.44 (1.35)
1.70(1.67)
L/Ni
2.10 (1.51)
1.77 (0.71)
1.74 (0.94)
2.62 (1.35)
1.71 (1.67)
L/No
0.48(1.51)
5.95 (3.74)
6.16(6.23)
5.78(2.76)
In kilometers.sa, si , andSo correspond
to the averagefault spacingandthe standard
deviation(in parentheses)
for eacharea.L/N corresponds
to the transectlength(L) divided
by the total numberof faults(N) alonga transect.Valuesare gi':en for all faults,inward,
andoutwardfacingfaults,respectively.
ESCART1N ET AL.' TECTONIC
STRAIN ON THE MID ATLANTIC
RIDGE
10,429
Table 4. TectonicStrain (%)
h /s
All
11.5(5.7)
16.6 (7.2)
8.0 (3.1)
14.8 (6.0)
6.8 (3.1)
SC-W
SC-E
IC
OC
10.7 (5.1)
15.1 (6.8)
7.2 (2.8)
14.0 (5.4)
6.8 (3.1)
m
0.8(1.4)
1.4 (2.9)
0.8 (0.8)
0.8 (2.2)
(0)
10.6(5.9)
17.0 (7.7)
8.9 (3.2)
10.2 (5.9)
7.3 (3.5)
Eh/L
9.9(5.0)
15.8 (7.4)
8.3 (3.0)
9.7 (4.8)
7.3 (3.5)
O.7(1.5)
1.2 (2.7)
0.6 (0.8)
0.5 (1.8)
(0)
10.53
16.06
10.19
16.88
6.29
9.19
14.23
8.67
16.00
6.26
2.58
6.45
1.26
6.12
(0)
Estimatesof tectonicstrain(c) from averageheaveand spacing,h / s , from slopesof cumulative
fault heavem (Figure8), and from the cumulativefault heaveand transectlengthEh/L. Numbersin parentheses
correspond
to the standarddeviations.In all cases,strainis calculatedfor all faults(Ca),inwardfacingfaults(c,),andoutwardfacingfaults(Co).
whereN(h,s) is the numberof faultswith heave(h) or spacing(s)
largerthana given value of h or s, andNT is the total numberof
faults.The reciprocalof •, is the characteristic
value of h or s,
which shouldcorrespondto the meanof h or s if the population
is properly characterizedby an exponentialdistribution. At
highervaluesof s and h (s > 9 km and h > 500 m) the cumulative
distributioncannot be describedsatisfactorilyby this simple
populationmodel. Consequently,fault spacingand heave have
been quantifiedusing the mean values of h and s insteadof the
characteristicvalues (1/•,) of the correspondingexponentialdistributions.
5.2.1.
in fault heave.
Fault heave estimates show
(Table2). FaultheaveattheIC islargest
(ha-MOO
m),whilethat
at theOC is smallest
(ha---100
m). The SC-WandSC-Efault
populationsshowintermediatefault heavescomparedto thoseof
SC-W
a)
Variations
significantvariationsamongthe differenttectonicenvironments
SC-E
-6
4
-4
2
0
0
6O
IC
OC
b) o
o
-30
- 15
0
15
30
45
60
Distance (km)
Figure8. Plotof cumulative
faultheaveZh0 versusdistance
fromthe axisfor all thetransects
(seeFigure4) for
thesegment
(a) centerand(b) end. BothinwardandoutwardfacingY•h
0 linesareshown(solidandopensymbols,
respectively).Shadedlinescorresponding
to 5% and 10% strainareplottedfor reference.Note the markedeastwestasymmetry
in theoverallslopeof the curves,evenat thesegmentcenter.The smallestamountof strainoccurs
at the OC, while the largestis foundat the IC. Samesymbolconventionhasbeenusedas in Figures4 and7. The
dashedshadedlineslabeledB/M mark the edgesof the centralmagneticanomaly(Brhunes-Matuyama
boundary).
Thereis a clearasymmetryaboutthe axis,with <40% total accretiontakingplaceon the westflank thanon the east
flank JAilerton,1997].
10,430
ESCARTIN
ET AL.: TECTONIC
STRAIN ON THE MID ATLANTIC
the IC andOC, but thereis a markedasymmetry;faultstendto be
RIDGE
usevaluesfor one of the estimates
of fault spacing,fault heave
lar_.y.
geron thewestflank(ha-250m) thanon theeastflank andtectonicstrain,but similarpatternswouldariseif otheresti(ha--•150
m), similarto theasymmetry
observed
at theendof the
segment(compareIC andOC, Table2).
5.2.2. Variations in fault spacing. Fault spacingis estimated from the averageof measuredspacingss, and from the
lengthof the transectdividedby the total numberof faultsalong
eachtransect(L/N, Table 3). Owing to the smallnumberof outwardfacingfaultsandto their clustering,particularlyon the west
flank (Figure 4), the averageof measuredspacingis lower than
that estimatedfrom the numberof faultsper unit length(e.g., at
the IC, So=0.96km, L/No-5.8 km). Discrepancies
are smallerin
the estimatesof spacingfor inward-facingand for all faults(Table 3), as they are more abundantand evenlydistributedacross
the area(Figure 4).
Faultsaremorewidelyspacedat theIC (T'a'a=2.59
km) thanin
the othertectonicprovinces(Table 3). There is someasymmetry
matesgiven in Tables2-4 were usedinstead.
6. Discussion
6.1. Fault Formation and Zone of Active Deformation
Variationsin fault spacingand heavewith distancefrom the
ridge axis allow us to constrainthe zone of active tectonic de-
formationwherefaultsform,develop,andremainactive. McAllisterandCann[1996],basedonTOBI datafromtheaxialvalley
floor of thissegment,suggested
thatfaultsareformedandlinked
within a "fault growthwindow." Accordingto this modelthis
"window" extendsalongthe wholelengthof the segmentand
hasa width of a few kilometersbeginningat -2 km from the
neovolcanic
zone. However,our analysisof the fault datadem-
that this model may be valid only in somesectionsof
at the segment
end,with the spacingat the OC (T'a'a=l.55
km) onstrates
the segmentstudied.Plates1 and2 andFigure3 showa marked
similarto thatof the SC-E (T'a'a=l.40
km) andSC-W (T•'a=l.53
kin). No significantasymmetryis observedat the SC (Table 3).
In all cases,averagefault spacingsare smallerthanthe 1-1.4 km
estimatedfrom similarfault studiesin the fast spreadingEastPacific Rise [Edwardset al., 1991].
near-axisasymmetryin fault spacing,heave, and overall fault
patternsthat suggests
significantvariabilityin the scaleand nature of strain localization.
Constraints
on the lateralextentof activefaultingmay be obtainedfrom microsesimicactivity andthe backscatter
characterof
the
sonar
data.
Microseismic
activity
over
a
period
of 41 days
5.3. Tectonic Strain Estimates
reportedby Wolfeet al. [1995] extendsto -10 km off axis at the
Tectonicstrainand its spatialvariationare estimatedfrom the
segmentend and is restrictedto the medianvalley at the segment
measured
fault parameters
h ands (Tables2 and3), andfromthe center(Figure lb). Thesedata are likely to give a minimum escumulativefault heave along transects(Figure 8). These esti- timateof the width of the activezonedueto the limitedrecording
mates of tectonic strain are apparentand not absolute(being time. Fault patternsand backscattercharactermay provide inbasedon apparentand not absoluteheave),but they providean stead constraintsfault formation and evolution at geological
timescales(-1 Myr). Examinationof the fault patternsin Plates
upperboundto actualtectonicstrain. In Table4 the estimates
of
the averagetectonicstrainfor each of the tectonicprovincesare
I and 2 and Figure3 reveals:(1) a lack of faultsnearthe ridge
given. Fault heave is measuredalong transectsspaced0.5 km
axis(within -2 km), (2) a well-developedzone-10 km wide of
apart(Figure4), thusadequatelycharacterizing
the displacement small-scalefaulting (fault lengths<5 km and s<0.5 km) on the
of faults <1.5 km in length. Using typical displacement-length eastflank alongthe whole segment,and (3) a lessdeveloped-5scalingrelationships[e.g., Cowie and Scholz, 1992; Dawers et
km-widezoneof small-scalefaultingat the segmentcenteron the
al., 1993], faultswith a length of-1.5 km would have displace- west flank that disappears
toward the segmentend (IC). The
ments of the order of--15 m, well below the resolutionof any
zoneof recent,unsedimented
volcanics(high reflectivityareain
shipboardmultibeamsystem. This suggests
that scalinglaws inferred from continentalareasneed to be calibratedbefore being
appliedto the oceanicenvironment.
Average tectonic strain for the study area is found to be
l•a-11%(Table4), depending
on the methodused,although
loII
cally it can be largerthan 20% (see slopesin Figure 8), qualitao
Averagestrain,i•= 11.5%
tively in agreement with results from earlier studies [e.g.,
O
Macdonald and Luyendyk, 1977; Solomonet al., 1988; ,4liefton
et al, 1996; daroslow, 1996]. Strain accommodated
by outward
facing faults is typically <3%, while that accommodatedby inward facingfaultsis >9% (Table 4). There is a markedasymmeI IIII II ii ii i illl ii i i ii i i iii
try east-westacrossthe axis both at the segmentcenterand end
L}
ff;=14.8% I]
•=6.8%
(Figure 8), indicatedby tectonicstrainson the west flank which
are a factor of 2 greater than on the east flank. While the OC
showsthe smallesttectonicstrain(-6-7%), the strainat the IC is
l!
Non-tranxf
orm
offset
similarto that at the SC-W (-11-17% and 16-17%, respectively,
Table 4). The averagetectonic strain decreasesslightly from Figure 9. Sketchshowingthe spatialvariationin fault heave,
Ill I I I I1I1I1i1i I i1i I•lli Iiiii1i1i1i1iii1i1i1
•
g=16.6%I!
• •=.26km,
g=l.5km
11•=.15km,
g=l.4km
7.2:'.
-12-13% at the segmentcenterto -9-12% at the segmentend,
dependingon the strain estimateused. We considerthat these
along-axisvariationsin tectonicstrainare not significanttectonically given that there is some variability dependingon the
methodusedin the estimation;in all caseswe reportthe maximumandminimumestimates
of thetectonicstrain(seeTable4).
The spatialvariationsof fault spacing,faultheaveandtectonic
strainat 29øN are summarizedin in Figure 9. In this sketchwe
.k?.
faultspacing,andtectonicstrainalongthe studiedsegment.The
nontransform
offsetsand the limit amongzones(dashedlines)
are shownhorizontalfor simplicity(see Figure 1). Valuesreportedcorrespond
to the averageheaveincludingall the faults
(Table2), averageof measured
spacing(Table 3), andthe strain
estimated
fromaveragespacingandheavealongtransects
(Table
4). The sameoverallspatialvariationsare observedif otherestimatesof faultspacing(Table3) or tectonicstrain(Table4) are
used instead.
ESCARTIN
ET AL.: TECTONIC
STRAIN
ON THE MID ATLANTIC
RIDGE
10,433
Plates1 and2) showsa markedasymmetry(-5 km and-3 km on 6.2. Asymmetric Tectonic Strain
the eastandwest flanks,respectively),consistentwith the asymThe differencesin fault patternsdescribedabove indicate a
metric spreadinginferredfrom the deep-towmagneticprofiles
marked E-W asymmetryboth in fault characteristics
(h and s),
(>60% and <40% of the total accretionon the east and west
and in tectonicstrain(g) that cannotbe explainedby variationsin
flanks,respectively[,411erton,
1997]).
melt supply along the ridge axis inferred from crustalthickness
On the east flank small-scale faults can be observed at disestimates[e.g., Lin et at., 1990; Wolfe et al., 1995]. The average
tancesof up to -15 km off-axis,andthe talusof largerfaultsat fault heave increasesfrom -100 m at the OC to -400 m at the IC,
distances
>20 km appearsto be sedimented.Theseobservations the spacingfrom-1.6 to -2.6 km (Tables2 and 3), andthe strain
indicate that faults are nucleated at-1-2
km from the axial vol-
canicridgeand continueto grow to 10-15 km off-axis. Strain
onlylocalizes
to formlargerfaults(h>200m) at > 10 km fromthe
axis. Thesefaultsmay remainactiveto distances
of <20 km offaxis, as indicatedby the lack of sedimentcoverof the talusand
scarps.Smallerfaultsmay becomeinactivebetween10 and 20
km off-axis and are not visible off-axis as they are coveredby
sediments.Faultingat distances<2 km from the axis may occur,
but volcanicactivity may partially or totally coverfaultsformed
in immediatevicinity of the axial volcanicridge [seeMacdonald
et al. , 1996].
On the west flank the processof fault formationand strainlocalization appearsto differ substantiallyfrom that on the east
flank. At the segmentend (IC), thereis no evidencefor a zone of
small-scalefaulting between the ridge axis and the bounding
fault (Figure 3), which has a vertical throw of-1.5 km (Figure
5). In this case strain localization must occur very close to the
ridge axis (<5 km). Most of the tectonicdeformationappearsto
be taken up by this fault, and may inhibiting the formationof
small-scalefaults. Seismogenicfaulting extendsto -10 km offaxis [WolJket al., 1995] on the IC, and the backscatterimages
show at distances<15 km unsedimentedfault scarps(i.e., highly
reflectivescarpswith no evidenceof a cover of poorly reflective
sediments).This observationsuggeststhat faults may be active
at <10 km from the ridgeaxisandbecomeinactiveat distances
of
10-15 km. Toward the segmentcenterthe fault patternis similar
to that of the east flank but with a narrower zone (<10 km) of
small-scalefaults (i.e., Y=3220-3230 km, X=675 kin, Figure 3).
from -7% to -15% (Table 4 and Figures9 and 10). A similar
patternbut with lessasymmetryis observedwhen the SC-W and
SC-E areasare compared(Tables 2-4 and Figure 8). The larger
asymmetryobservedat the segmentend is consistentwith that
reportedin bathymetryand gravity at the segmentendsof most
slow spreadingridge segments[e.g., Severinghausand Macdonald, 1988; Tucholkeand Lin, 1994; Escartœn
and Lin, 1995].
However,it is clearthat at this segmentthe asymmetryextendsto
the segmentcenter.
Part of the asymmetryin tectonicstrainmay be relatedto the
asymmetry in magmatic accretion inferred from deep-towed
magneticanomalies[,411erton.,1997] (see Figures8 and 10),
which is independentfrom variationsin melt supplyalong the
axis of the segmentas inferred from crustalthicknessestimates
[e.g., Lin et al., 1990; Wolfeet al., 1995]. Magmaticaccretion
over the last 1 Myr has been highly asymmetric,with 50-150%
more accretionon the eastthan on the west (see locationof the
centralanomalyboundaries
at eachsideof the axisin Figure8).
Cumulativetectonicstrain(g) is proportional
to thestrainrate(•)
andthe time lengthof activetectonicdeformation(t) as •= •/t.
In the caseof a mid-oceanridge,the time lengthof activedeformationof the crustwill dependon the width of the zone of deformation(w) and the spreadingrate (sr) as t=-sr/w. From our
observations we assume that the zone of active deformation is of
•tpproximately
the samewidthon bothsidesof the ridge(•-15-20
km). Lacking any additionalconstraints,we alsoassumethat the
rate of tectonic deformation is the same on both flanks.
In the
These results indicate that in some cases,such as the IC of this
segment,strainmay localizevery closeto the locusof magmatic
accretion,resultingin a large rift-boundingwall that accommodatesmost of the tectonicstrain early in the history of the oceanic crust. More generally (eastflank and segmentcenteron the
westflank), faulting is nucleatedover a relatively wide zone (1015 km), and strain localizesin one or more faults at distances>5
km from the axis. Faultsappearto becomeinactiveat 15-20 km
off-axis on both flanks, as indicatedby the backscatterimages,
fault distributions,and microseismicactivity. The with of this
zone is consistentwith the-20-km zone of teleseismicactivity
reportedby Lin and Bergman[1990]. Theseresultssuggestthat
the width of active zone is similar on both sides(i.e.,-15-20 km
from the ridge axis). At the fast spreadingEast Pacific Rise
(EPR), estimatesof the width of the zone of active deformation
vary from <10 km [e.g., Edwards et al., 1991] to -30 km about
the ridge axis [e.g., Alexander and Macdonald, 1996], with fault
spacingsof 1-1.4 km [Edwardset al., 1991], and scarpheightsof
<100 m [Alexanderand Macdonald, 1996]. The thicker brittle
layer at the ridge axis of slow spreadingridgesmay explain these
systematicdifferencesin fault spacingand height betweenthe
MAR andthe EPR. Although the width of the active zone of deformation is not well constrained at the EPR, at both fast and
slow spreadingridgesit may be stronglycontrolledby the thickening of the lithosphereoff-axis due to cooling [e.g., Alexander
and Russell,1991; Searleet at., 1998b].
Asymmetric
magmatic accretion
Srw
sr e
Zone of active
deformation
0
Distance from ridge axis
Srw = 0.5 x Sre
œw =2Xœe
Figure 10. Cartoonshowingasymmetrictectonicstraindue to
asymmetric
spreading.
The zoneof deformation,
wherefaultsnucleate and accumulatestrain, is relatively symmetricacrossthe
axis. We assumethat magmaticaccretionis asymmetric(for
simplicity,-50% on the westwith respectto the eastflank) and
that the rate of deformation
is constant.
As the crust on the west
flank remainson the zone of deformationduringa longerperiod
of time, the final cumulativetectonicstrainrecordedby the crust
will be largeron the westthanon the eastflank; srw and sre correspondto the relativespreadingratetowardsthe westand east,
respectively;
gw and gr correspond
to the cumulativetectonic
strainon the west and eastflank, respectively.
10,434
ESCARTIN
ET AL.: TECTONIC
STRAIN ON THE MID ATLANTIC
caseof the 29øN segment,if the accretionon the west is -50% of
that on the east, the observed tectonic strain on the west will
then be approximatelydouble of that on the east (Figure 10),
which is qualitativelyconsistentwith our observations.
Macdonaldand Luyendyk[ 1977] showedthat at 37øN thereis
an inverserelationshipbetweenthe senseof asymmetryand the
amountof tectonicextension. Spreadingrateto the west is -50%
of that to the east(7 versus13 km/Myr), but tectonicstrainon the
west is -60% smallerthan on the east(11% versus18%) [Mac-
donaldand Luyendyk,1977]. Thesediscrepancies
betweenthe
resultsfrom both studyareascould be due to variationsin some
of the unconstrained
parameters.In particular,at 37øN differences in the width of the active zone of deformation
rate of deformation
and/or the
between the east and the west flanks could
resultrelationships
betweenaccretionandtectonicstrainthat differ from thoseat 29øN. Additional constraintson theseparametersmay be obtainedwith studieson areasto determinethe local
variationsin tectonicstrainand fault patternsand their relationshipwith the asymmetryin plate accretion.
6.3. Partitioning Tectonic Strain and Magmatic Accretion
at the Segment Scale
RIDGE
segmentcenter(SC-W and SC-E, -12% versus-11%, respectively). Tectonicstrainobservedat the seafloorin the studyarea
is foundto be constantat -10%, abouttwice largerthan that re-
portedfor theEast-Pacific
Rise[e.g.,Cowieet al., 1993;Alexander and Macdonald,1996]. This impliesthat -90% of the total
plateseparation
mustbe accommodated
by magmaticprocesses.
Residualgravity anomaliesindicatethat the crustalthickness
alongthe ridgeaxis decreases
from-7.5 km at the segmentcenter(-30 mGalmantleBougueranomaly)to -4 km (--10 mGal)at
thesegmentend [Lin et al., 1990],suggesting
thatmagmaticaccretionis highly focused[Lin et al., 1990], and that melt supply
at the end of segmentis -50% of that at the segmentcenter.If
peridotitesare presentat the endsof the segment[Cannatet al.,
1995; Cannat, 1996], the geophysicallydefinedcrustalthickness
may overestimatethe actual magmaticcrustal thickness,and
variationsin melt supplyalong the segmentmay be even larger.
Small along-axisvariationsin magmaticaccretion(e.g., from
90% to 70% of the total plate separation)associatedwith variations in melt supply would result in a three-fold (10% to 30%)
increasein tectonicstrain (Figure 11). Such large variation in
tectonicstrain shouldbe recognizablebasedon the estimatesof
tectonicstraindeducedfrom the data analysispresentedhere, as
it would result in very large variationsin fault spacing,heave
Tectonic
strain
along
the29øNsegment
does
notvarysub-. and/or
geometry
thatarenotobserved
inthisarea.
stantially
inanaxis-parallel
direction.
Although
theIC shows These
results
suggest
that,in shallow
levels
oftheoceanic
thelargest
cumulative
tectonic
strain,
theaverage
strain
atthe lithosphere,
tectonic
strain
is decoupled
frommagmatic
procsegment
end(ICandOC)isonlymarginally
smaller
thanatthe esses.
Thisdecoupling
occurs
attime-scales
of -1 Myrover
Segment center
Segment end
Coupledmagmatism/ tectonism
===========
Uncoupled magmatfsm/ tectonism
Figure 11. Cartoonshowingthe differencebetweentectonicstraincoupledanduncoupledwith melt supplyto the
axis. •t corresponds
to tectonicstrain,andAm corresponds
to magmaticaccretion.Total plateseparationis •t+Am
(left), tectonicstrainis assumedto be takenup by brittle faulting(right;geometryof faultsshownis unconstrained
andfor illustrationpurposesonly). The thicknessof the crustat the ridgeaxisdecreases
from -7 km at the segment
centerto -4 km at the end [Lin et al., 1990]. Tectonicstrainaverages-10%, bothat the centerandend of segment
(top and bottom). If melt supply(i.e., crustalthickness)were directlylinked to tectonicprocesses
(center)we
wouldexpectthe tectonicstrain•t at the segmentendto be substantially
largerthanat the center. This is not supportedby the data. See text for discussion.
ESCARTIN ET AL.: TECTONIC STRAIN ON THE MID ATLANTIC RIDGE
which we have estimatedtectonicstrain,and over the lengthof
an individualridge segment.Thusvariationsin crustalthickness
alongthe axis of slow spreadingsegments[e.g., Lin et al., 1990;
Tolstoyet al., 1993; Detrick et al., 1995; Cannat, 1996] may reflect variationsin melt supplybut do not imply variationsin tectonic strain. As tectonicthinning of the crust can be important
off-axis [e.g., Escartin and Lin, 1995; 1998], crustalthickness
estimatesat these locationscannot be used to infer temporal
variationsin magmasupplyor tectonicstrain. In depththe accommodationof the plate separationamong tectonicextension
and magmaticaccretionmay vary substantially.Possibleprocessesoperatingat the baseof the lithospheremay includepassive
accretionof asthenospheric
material into the lithosphereas the
platesseparate[Cannat, 1996] and distributedbrittle processes,
in addition to localized
brittle
deformation.
Identification
and
10,435
from-10% at the ridge axis to ---40%at •-3 Ma old crustand remainsrelativelyconstantout to 20 Ma off-axis. This resultis not
consistent
with the fault patternsin our studyarea,aswe observe
no evidencefor an increaseof the numberof outwardfacing
faultstowardsthe eastandwest limits of the surveyarea(•-3 Ma,
Figures 1 and 3). If a similar processof formationof outwardfacingfaultsoperatesat the 29øN segment,it mustoccurat seafloor agesolder than 3 Ma, outsideof our surveyarea. Alternatively, someof the outwardfacingfaultsinterpretedby daroslow
[1996] may not be properlyconstraineddue to the coarserresolution of the multibeambathymetryand HMR1 backscatterdata.
They may correspondinsteadto volcanic terrain backtilted by
flexuralrotationcausedinwardfacingfaults,as seenin someareas at the 29øN segment. Analyses of coincidental highresolutionTOBI data, multibeambathymetry,and HMR1 backscatterdata would be required to unequivocallydeterminethe
quantificationof both passiveaccretionof asthenospheric
material and/or distributed deformation require further geological nature of these features and to establish if the formation and tecconstraints,and cannotbe donefrom the sea surfacebathymetric tonic evolutionof outwardfacing faultsare operatingfartheroffof backscatterdatapresentedhere.
axisat the 29øN segment.
Fault spacingand heavechangesubstantiallyalong axis even
thoughtectonicstrainis relativelyconstant. For example,fault 7. Conclusions
heaveat the west flank decreases
from -400 m (segmentend, IC)
We havepresentedthe resultsof a comprehensive
analysisof
to-250 m (segmentcenter,west, Table 2 and Figure 10), and
fault data interpretedfrom high-resolutionside-scansonar im-
fault spacingdecreases
from ---2.6to -1.5 km (alsofrom endto
segcenter,Table 3 and Figure 10). As thesechangesin fault char- agerycoveringtwo-thirdsof the lengthof a slowspreading
ment,
extending
out
to
-•3
Ma
old
crust.
These
results
reveal
that
acteristicsdo not correlatewith the estimatedlong-termtectonic
changes
in
fault
patterns
and
tectonic
strain
are
not
correlated.
strain,we infer that they are due insteadto changesin rheology
The zone of active deformation is estimated to extend to -10-20
inducedby the thickeningof the lithosphere
towardthe segment
km off-axis,and may be controlledby the thermalstructurebeends [Forsyth,1992; Shaw, 1992], by the weakeningof fault
planes[Escartinet al., 1997b] due to the presenceof serpen- low theridge. We inferthatvariationsin fault spacingandheave
primarilyby changesin the rheologyof faults,and
tinized materialsalong shearzones [e.g., Francis, 1981], or by arecontrolled
secondarilyby changesin lithosphericthickness. These results
the combinationof both effects. Lithosphericthickeningtoward
segmentendshas been invokedto explaintectonicpatternsobservedneardiscontinuities
[e.g.,Fox and Gallo, 1984], but thermal modelsof nontransformdiscontinuities[e.g., Parmentierand
Forsyth, 1985; Shaw and Lin, 1996] only predict small changes
in the depthsto isotherms.In addition,changesin fault spacing
and heavemay be more sensitiveto the rheologyof the oceanic
lithosphereand faults than to its thickness [Escartin et al.,
1997b]. Serpentinites
andgabbroshavebeendredgedat the base
of the largefault scarpboundingthe west flank of the rift valley
at the segmentend [Cann et al., 1997; CD99 ScientificParty,
1996] (Figure 3), suggestingthat the crustat the IC may be heterogeneous. Owing to the weak nature of serpentinites[e.g.,
Reinen et al., 1994; Escartin et al., 1997a] their presencemay
greatly enhancestrain localizationalong fault planes,resulting
both in larger displacements
and increasedfault spacings[Escartin et al., 1997b].
6.4. Formation of Outward Facing Faults
Outwardfacingfaults in our studyarearepresent< 10% of the
total numberof faults and accountfor a small proportionof the
total tectonicstrain. While at 23øN and 37øN this proportionis
estimatedto be -20% [Kong et al., 1988; Macdonald and Luyendyk, 1977], in an adjacentarea at 25.5ø-27.2øN,Jaroslow [1996]
reports that -40% of the total fault population correspondto
outward-facingfaults. This proportionof outward-facingfaults
reportedby Jaroslow [1996] is similar to that given for fast
spreadingridges[seeCarbotteand Macdonald, 1990, and referencestherein]. Jaroslow's[1996] study,basedon the interpretation of shipboardmultibeambathymetryand HMRI backscatter
data,showsthatthe proportionof outwardfacingfaultsincreases
indicatethat the amountof tectonicstrainis not the primary control on the developmentof faults. The major conclusionsof this
studyare as follows:
1. Approximately10% of the total strainobservedat the seafloor is accommodatedby brittle faulting. The rest of the plate
separation(90%) mustbe taken up by eithermagmaticaccretion
or diffuse tectonic deformation
within
the crust without
an ex-
pressionon the seafloorsurface. Estimatesof apparenttectonic
strainarean upperboundof the actualtectonicstrain.
2. Outward facing faults are scarceand accountfor <10% of
the total number of faults and tectonic strain.
3. Variations in tectonicstrain cannotbe resolvedalong the
axis and showno correlationwith changesin melt supplyas inferredfrom changesin crustalthicknessalongthe axis.
4. We observea markedasymmetryin tectonicstrainof up to
50% betweenthe eastand west flanks,both at the segmentcenter
and end. Asymmetric strain maybe partially explained by the
asymmetricmagmaticaccretionat the axis documentedby nearbottommagneticprofiles.
5. Differencesin fault geometry(heave and spacing)among
differentregionsof the segmentare not directly correlatedwith
total tectonicstrain but insteadmay reflect changesin lithospheric rheology (thickness,composition). Our study corroboratesthat faultsare largestand have the widestspacingat the IC,
while faults at the OC are smaller.
An increase in fault size and
spacingis observedfrom the segmentcentertoward the ends,
while tectonic strain remains constant.
These variations in fault
geometrymay be betterexplainedby rheologicalchangesalong
fault planesassociated
with the presenceof weak serpentinites,
as
changesin lithosphericthicknessalong a segmentpredictedby
thermalmodelsarenot very important.
10,436
ESCARTIN ET AL.: TECTONIC STRAIN ON THE MID ATLANTIC RIDGE
Acknowledgments. We thank the officers and crew of the RRS
Charles Darwin
cruise CD99
and the TOBI
team for their assistance.
We thankMathilde Cannat,Gary Jaroslow,JianLin, DebbieSmith and
Brian Tucholke
for discussions and comments that contributed to the
ideaspresentedin the paper. Thoroughreviewsby R. L. Carlson,D.
Fornariand K. C. Macdonaldhelpedclarify our argumentsand improve
our manuscript.The GMT software[Wesseland Smith, 1991] was extensivelyusedin this study. Fundingfor this work was providedby the
National ResearchCouncil (UK).
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29, 1998;
acceptedNovember23, 1998.)