JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. tOO, NO. Btt, PAGES 22,509-22,522, NOVEMBER
t0, 1995
Quantitative analysisof abyssalhills in the Atlantic Ocean:
A correlation
between inferred
crustal thickness
and extensionalfaulting
John A. Goff
Universityof TexasInstitutefor Geophysics,Austin
Brian E. Tucholke,JianLin, andGary E. Jaroslow
WoodsHole Oceanographic
Institution,WoodsHole, Massachusetts
Martin C. Kleinrock
VanderbiltUniversity,Nashville,Tennessee
Abstract. A recentcruiseto the Office of Naval ResearchAtlanticNaturalLaboratoryobtained
-100% Hydrosweepbathymetriccoverage,>200% Hawaii MR1 (HMR1) sidescancoverage,
gravityandmagneticsover an areaspanningthreeridge segmentsalongaxis (--25ø25'Nto
-27 ø10'N), andcrustalagesfrom0 to 26-30 Ma (-400 km) on thewestflank of theMid-Atlantic
Ridge. This datasetrepresents
a first opportunityfor an extensiveregionalanalysisof abyssalhill
morphology
createdat a slowspreading
ridge. The primarypurposeof thiswork is to investigate
the relationshipbetweenabyssalhill morphologyandthepropertiesof the ridgecrestat whichthey
were formed. We applythe methodof Goffand Jordan[1988] for the estimationof twodimensionalstatisticalpropertiesof abyssalhill morphologyfrom the griddedHydrosweep
bathymetry.Importantabyssalhill parameters
derivedfrom thisanalysisincluderoot-meansquare(rms) height,characteristic
width, andplan view aspectratio. The analysisis partitioned
into two substudies:
(1) analysisof near-axis(< 7 Ma) abyssalhills for eachof the threesegments
and(2) analysisof temporalvariations(-2-29 Ma) in abyssalhill morphologyalongthe run of the
southsegment.The resultsof thisanalysisare comparedandcorrelatedwith analysisof the
gravitydataandpreliminarydetermination
of faultingcharacteristics
basedon HMR1 sidescan
data. Principalresultsof thisstudyare:(1) Abyssalhill morphologywithinthe studyregionis
stronglyinfluencedby theinside-outside
cornergeometryof the mid-oceanridge segments;
abyssalhills originatingat insidecomershavelargerrmsheightandcharacteristic
width and
smallerplan view aspectratiothanthoseoriginatingat outsidecorners.(2) The residualmantle
Bouguergravityanomalyis positivelycorrelatedwith intersegment
andalong-flow-linevariations
in rmsheightandcharacteristic
width,andit is negativelycorrelatedwith planview aspectratio.
From thisresult,we infer thatlower-relief,narrower,andmoreelongatedabyssalhills are
producedwhenthe crustbeinggenerated
is thicker. (3) Intersegment
variationsin near-axisrms
heightnegativelycorrelatewith averagefault densityasdeterminedfrom analysisof HMR1 side
scanimagery.
1. Introduction
Neumannand Forsyth, 1995]. Many studieshave investigated
The processof abyssalhill formationat mid-oceanridgecrests the geologic structureassociatedwith axial faulting and volcanof abyssalhill formation
is a poorlyunderstood
componentof the ridgesystem.Abyssal ism, which are the primarymechanisms
(seeShawand Lin [1993] andTucholkeand Lin [1994] for up-tohills are complexand chaoticin natureand as suchpresenta
uniquesetof challengesfor geologicand quantitativecharacteri- dateandthoroughreferencelistsfor Mid-AtlanticRidge(MAR)
zation and for physicalmodelingof the processesthat create studies). Few studies,however,haveyet attemptedto formulate
them. Quantitative
characterization
of abyssalhillshaslongbeen physicalmodelsof abyssalhill formationwhich satisfyquantitastudied,but it hasreceivedmuchmoreattentionin recentyears tive and/or geologicalobservationalconstraints[Schoutenand
[e.g., Krause and Menard, 1965; Menard and Mammerickx, Denham, 1983; Malinverno and Gilbert 1989; Malinverno and
Cowie, 1993; Shaw and Lin, 1993]. This is due in part to the
1967; McDonald and Katz, 1969; Bell, 1975, 1978; Fox and
Hayes, 1985; Gilbert and Malinverna,1988;Gaff and Jordan, complicatednatureof abyssalhill faulting and volcanismand in
1988; Malinverno and Packalny, 1990; Gaff, 1991, 1992; part to the dearth, until recently, of high-resolutionoff-axis
Malinverna,1991;Hayes and Kane, 1991; Gaff et al., 1993; bathymetryand geophysicaldata which help constrainthe relationshipbetweenabyssalhills and axial conditionsat the time of
accretion. This paper continuesthe work of quantitativecharacCopyright1995by theAmericanGeophysical
Union.
terizationof abyssalhill morphologyandprovidesconstraints
for
physicalmodelsof abyssalhill formation.Previouscontributions
Papernumber95JB02510.
0148-0227/95/95 JB-02510505.00
have relatedabyssalhill morphologyto spreadingrate variations
22,509
22,510
GOFFET AL.: ABYSSALHILLS IN THE ATLANTICOCEAN
49øW
48øW
47øW
46øW
45øW
44øW
49øW
48øW
47øW
46øW
45øW
44øW
Figure 1. Simplifiedbathymetry(0.8-km contourinterval)of the surveyarea,with seafloorisochronsshownat 5m.y. intervals. Isochronsare brokenat segmentboundaries,whichare shownin Figure 2.
[Menard, 1967; Malinverno, 1991; Hayes and Kane, 1991;Goff,
1991, 1992] andrelatedabyssalhill morphologyto ridgesegmentation at fast spreadingrates [Goff, 1991; Goffet al. 1993]. Our
studyexpandson this work by relatingabyssalhill morphologyto
ridge segmentationand temporal variability at slow spreading
rates.
Our principal data set consists of Hydrosweep swath
bathymetry[Chayeset al., 1991] collectedon a recentgeophysical cruise to the Office of Naval Research (ONR) Atlantic
Natural Laboratory [Tucholke et al., 1992] aboardR/V Maurice
Ewing. This surveyobtainednearly 100% Hydrosweepbathymetric coverageas well as gravity, magnetic,and>200% Hawaii
MR1 (HMR1) side scansonarcoveragein a region extending
from nearthe Mid-Atlantic Ridge (MAR) axiswestward-400 km
Near-Axis
49 øW
48 øW
47 øW
to 26-30 Ma crust. The ridge-flankcrustin thisregionconsistsof
threemain segments(south,center,and north segments;Figures
1 and 2) which have developedbetweenlong-lived, small-offset
(_<30 km) ridge axis discontinuities. The southsegmentis the
longestlived (> 29 m.y.) and exhibitsa continuousrecordof relatively stableevolution. The centerand north segmentshave had
shorterlives (_<20 m.y.), their boundingdiscontinuitieshave
small (to zero) offsets(Figure 1), and they are tectonicallymore
complexthan the southsegment.
We usethe methodologyof Goffand Jordan[1988, 1989] and
Goff [ 1990] to estimatestatisticalparameters
of abyssalhill morphology in the survey area through formalized inversion of
bathymetric data. Principal estimatedparametersinclude the
second-order statistical parameters root-mean-square (rms)
height, characteristicwidth, and plan view aspectratio. The
Data
46 øW
SouthSegmentData
45 øW
49øW
48øW
47øW
46øW
45øW
27øN
27 øN
26øN
••t Segment
• ,•: /
26øN
25 øN
Figure 2a. Map of simplified segmentboundariessurveyedduring the R/V Ewing cruise EW9208. Locationsof the near-axis
swathsusedfor parameterestimationare indicatedby line segmentsterminatedby dots. The swathdata setsare sampledfrom
gridded bathymetricdata and are 7.5 km in width. The dashed
line in the westernpart of the southernsegmentlocatesa smalloffset,secondarydiscontinuity.The long-shortdashedlinesindicatelikely pseudofaults
createdby propagatingridges.
25øN
Figure 2b. Locations of off-axis swathsused within the south
segment(see Figure 2a for details). With minor, short-termexceptions, all offsets at the discontinuitiesboundingthe south
segmentare right lateral (as is the offsetat the secondarydiscontinuity at 29-21 Ma); thus inside corner crust is in the southern
part of the segment.
GOFF ET AL.' ABYSSAL HILLS IN THE ATLANTIC OCEAN
betweenthe two mid-oceanridgesgo well beyondsimplescaling
(althoughit remainsto be determinedpreciselywhat thoseaddi-
analysisis partitionedintotwo substudies
(Figure2): (1) analysis
of near-axis(< 7 Ma) bathymetricdatafor eachof the three segments,enablinga comparison
betweenthe present-dayridgeaxis
and the most recently createdabyssalhills and (2) analysisof
bathymetricdata within the long-livedsouthsegment,enabling
an analysisof temporalvariability(from ~2-29 Ma) in abyssal
tional differences are).
A significantamountof variation in abyssalhill morphology
not attributableto spreadingrate variationalso occurson a regional scale (i.e., tens to hundredsof kilometers),. Studiesby
Goff [1991] and Goff et al. [1993] on multibeamdata from the
hill morphology.
Resultsof the abyssalhill parameterestimationare compared
with other geophysicaldata from the survey area, including
residualmantle Bouguergravity anomaly (RMBA), and fault
identificationsfrom side scansonardata. The RMBA provides
an estimateof crustalthickness[e.g.,Kuo and Forsyth,1988;Lin
et al., 1990; Lin and PhippsMorgan, 1992; Tucholkeand Lin,
1994], andby inferencethe temperature
structureof the spreading
axis at the time of crustalaccretion[e.g., $uet al., 1994]. This
informationhelps to identify changesin the axial environment
that mighthavecausedchangesin abyssalhill morphology.The
fault identificationsfrom side scandata provide an independent
measureof faulting,oneof the two principalabyssalhill forming
flanks of the EPR show that much of this variation
is controlled
by segmentationof the mid-ocean ridge crest. In particular,
abyssalhill rms heighttendsto be largesttowardthe endsof a
segment(boundedby first- or second-order
discontinuities)and
smallesttoward the middle. This patternis well correlatedwith
what is interpretedfrom morphologyandgravityconstraintsto be
focusedmantleupwellingbeneaththe ridge axis [e.g., Cochranet
al., 1993; Scheirer and Macdonald, 1993; Wang and Cochran,
1993], implying an associationof larger-throwfaults with colder
axial mantletemperaturesnear segmentends[Goff, 1991; Goff et
al., 1993].
The present study demonstratesthat MAR intrasegment
abyssalhill variationis primarilycorrelatedto inside-outside
cor-
processes.
ner location [Tucholke et al., 1992; Lin et al., 1992; Tucholkeand
Quantitativeanalysisof abyssalhill morphologyhasbegunto
place observationalconstraintson the processof generationof
seafloorand crustat mid-oceanridge systems.Severalprevious
studies[Menard, 1967; Malinverno, 1991; Hayes and Kane;
1991; Goff, 1991, 1992] showedthat abyssalhill rms heightsare
generallylargerat slowerspreading
rates.Thiscorrelationcanbe
attributed,at least in part, to the spreadingrate dependenceof
axial thermalstructure[PhippsMorgan et al., 1987; Shawand
Lin, 1994] andthe consequent
effecton lithosphericstrengthand
faulting(seediscussion
by Goff [ 1991]). The dependence
of rms
heighton spreadingrate is, however,only one small part of the
relationshipbetweenabyssalhills andthe ridge systemat which
they were formed. For example,Goff [ 1991] found that abyssal
hills formedat the fast spreadingEastPacificRise (EPR) possess
shapeandtexturecharacteristics
that are fundamentallydifferent
from thoseproducedat the slowspreadingMAR. This observation suggeststhat differencesin abyssalhill forming processes
Lin, 1994] ratherthan to focusedmantleupwellinginferredfrom
along-axis variations in crustal thickness[e.g., Detrick et al.,
1995] or dynamicmodeling[e.g.,Sparkset al., 1993]. Variations
in MAR abyssal hill morphology along the run of segments
(Figure3) andbetweensegments
arefoundto be large,implying
strongtemporaland segment-to-segment
variations,respectively,
in axial processes.These variationsin abyssalhill properties
(most notably rms height) are correlatedto variations in the
RMBA (consistentwith earlierfindingsby Shawand Lin [ 1993]
andNeumannand Forsyth[ 1995]) and,wherenotundulymasked
by sedimentcover, to fault densityas interpretedfrom HMR1
side scansonardata. We infer from this a correlationof abyssal
hill faulting with the temporal and spatialvariationsin crustal
thicknessand/or mantle temperaturestructureat the ridge axis.
Both this interpretationandearlieronesfrom EPR studiessuggest
that the strengthandthicknessof the lithosphereare the dominant
physicalfactorswhich controlthe formationof abyssalhills.
•
ß,. /
22,511
/
I
i
/
/
,%
%,
/
/
\
Second-order
Discontinuity
I \
I
.. '• \
X
// I / X l \
oc
Second-order
\
/---I
/
•1
\
\
/
-%
---I
/
%, /
\
Transform
Discontinuity
\
Figure 3. Schematicsketchdefininggeometryof insidecomercrust(IC, stippledregion), outsidecomer crust
(OC), segmentlength and run for ridge segmentsboundedby transformand nontransformdiscontinuitiesin
slowspreading
crust[from Tucholkeand Lin, 1994].
22,512
2. Stochastic
GOFFETAL.' ABYSSALHILLSIN THEATLANTICOCEAN
Characterization
theseparameters
cannotdo at presentis tell us, for example,that
a certainregion is dominatedeither by extensionaltectonismor
volcanism. Theseprocesses
are dissimilar,andit
tics of abyssalhill morphologyand the procedurefor inverting constructional
to expectthat the morphologies
they produce
modelparametersfrom multibeambathymetricdataare detailed is not unreasonable
will have characteristic differences which can be detected with
by Goffand Jordan [1988, 1989] and Goff[1990]. The method
takesas input a swathof multibeamdata,or the griddeddataset, the estimatedstochasticmodel parameters.However,to make
and producesas outputestimationsof relevantphysicalparame- this distinctionwe will needto "ground-truth"the statistics,i.e.,
ters with estimateduncertainties.To obtainoptimaldataorienta- match statistical characteristicswith geologic characteristics.
in part with the HMR1 sidescansonar
tion relative to segmentboundaries,only gridded Hydrosweep This canbe accomplished
data (griddedat 200-m spacing)were used for this analysis. data, from which can be identified steep,unsedimentedscarps
Sectionsof the gridded data chosenfor analysisconsistedof interpretedas normal faults (G. Jaroslowet al., manuscriptin
elongaterectangularareas7.5 km wide and 50-100 km long, ori- preparation,1995).
In addition to the second-orderstatisticalparameterslisted
entedsubparallelto plate flow lines (seeFigure2).
above,
estimatescanbe madeof higher-order(i.e., non-Gaussian)
For simplicity,we refer to each elongated-boxsampleof the
gridded data set as a "swath"of data (this term also emphasizes statistical parameters such as skewness, tilt, and kurtosis
similarityto multibeamsampling). Two constraints
are placedon (peakiness). In practice,however,for a regionalstudysuchas
parametersrmsheight,linswathschosenfor input:(1) They are straightandlongenoughto presentedhere, only the second-order
producewell-resolved estimationsof the statisticalparameters eamentazimuth, characteristicwidth and plan view aspectratio
[Goff and Jordan, 1989], and (2) the morphologysampleddoes are well enoughresolvedthat their variationdueto geologicpronot includemajor seamountsor major segmentboundariesas de- cesseswithin the study area exceeds their resolution errors.
terminedfrom bathymetry,gravity, and magneticdata. Because Hence this studyfocuseson only theseparameters(with the exceptionof lineamentazimuth,which is the subjectof a separate
of differences in decorrelation distances (the distance between
two points required for each to be consideredan independent study).
The stochastic model used to characterize second-order statis-
sample),longerdata recordsare requiredto make reasonableparameterestimationsin the Atlantic comparedto the Pacific. For
example, whereasa 20- to 25-km-long swath of data might be
sufficient for flanks of the EPR, a 60- to 75-km-long swath is
necessaryfor the flanksof the MAR.
Sampleswathsare orientedgenerallyalongflow line pathsfor
two reasons:(1) This orientationmaximizesresolutionof alongisochronvariability associatedwith ridge segmentation,and (2)
the along-flow-linedirectionrepresents
the directionof shortest
decorrelationdistanceand thereforemaximizesparameterresolution versussamplesize. Whereverheterogeneity
existson scales
shorterthanthe lengthof the swath,the resultsrepresentan average of the propertiessampled. For example,propagatorpseudofaults are contained within some of the swaths used for this anal-
ysis (Figure 2) (B. Tucholke et al., manuscriptin preparation,
1995). These swathslikely containa mixtureof seafloortypes,
includingpseudofaultmorphology,abyssalhills adjacentto the
pseudofaultwhich are influencedor modifiedby the pseudofault,
andabyssalhills unaffectedby the pseudofault.The spatialresolution of the statistical estimation is insufficient
3. Near-Axis Analysis
Figure 2a showsthe pathsof individual swathsusedfor parameter estimation
in the near-axis
data set. These data extend
from -1-2 Ma to -6-7 Ma crustal ages. We assumethat the
abyssalhills sampledby theseswathswere formedby processes
similarto thosethat are now active at zero age. Hencethesedata
allow us to correlateabyssalhill characteristics
with knownridge
axis processes[Tucholke and Lin, 1994; B. Tucholke et al.,
manuscript
in preparation,1995].
RMS Heights
RMS heights estimatedfrom near-axis data (Figure 2a) are
plottedwith 1-orerrorbarsin Figure4. There are two principal
observations.First, variation in rms heightswithin each ridge
segmentis asymmetric,with greaterrms heightsskewedtoward
the southernparts of the segment. Each of thesesegmentsis
to make these
distinctions,so care must be taken in interpretationof results.
The outputphysicalparameters
includethe following:
1. The rmsheightis the averagevariationof bathymetryfrom
the meandepth.
2. The lineamentazimuthis trendof abyssalhill lineaments.
3. The characteristic
scaleis the dominantlengthof features
alonga profile. It is definedby the width of the covariancefunction along the profile direction [Goff and Jordan, 1988].
Perpendicularto the grain, it is calledthe characteristic
width of
abyssalhills, andparallelto the grain,it is calledthe characteris-
Near-Axis
•
Data
NorthSegment
ß CenterSegment
0 SouthSegment
I
tic length.
4. The aspectratio is the characteristic
planview shapeof the
abyssalhills, defined by the ratio betweencharacteristiclength
and width.
5. The Hausdorff (fractal) dimensionis a measureof roughness. It is definedby the asymptoticpropertiesof the covariance
functionat smalllag. An increasein fractaldimensionrepresents
an increasein roughness,
with the limiting casesof 2 and3 correspondingto a randomfield with continuousderivative(Euclidean
surface)and one which is spacefilling (Peanosurface),respectively.
Theseparametersand their uncertaintiesform an objectivebasis to distinguishdifferencesfrom one regionto another. What
IC
25000 '
25o20'
OC
25o40'
IC
26o00 '
OC
26o20 '
IC
OC
26o40 '
27000 '
Latitude,Deg N
Figure 4. RMS heightof abyssalhills estimatedfrom the nearaxis swaths(Figure 2a), plottedas functionsof averagelatitude
overthe lengthof the swath. Vertical barsrepresent1-orerrorestimates.
"IC" and "OC" indicate locations of inner comers and
outercomers,respectively.
GOFF ET AL.: ABYSSALHILLS IN THE ATLANTIC OCEAN
boundedby right-lateraldiscontinuities
(Figure 1), so the southern portionsare insidecornercrest,and the northernportionsare
outsidecomercrest(Figure3). This observationdiffersfrom that
of Shaw [1992], who statedthat largerfaults (and consequently
largerabyssalhills) tendto occurtowardbothendsof MAR segments. However, the Shaw [1992] study,which did not consider
inside-outsidecornerasymmetry,demonstrated
this observation
by comparinga singleprofile throughthe centerof a segment
with a singleprofile over an insidecomer,andthe latterwasused
to demonstratethe behaviorof segmentendsin general. RMS
heightsdo not increasemonotonically
goingfrom outsideto insidecomers. Within the southsegment,rmsheighttendsto reach
a minimumtowardthe middle of the segment,similarto observed
variationsalongthe EPR flank [Goff, 1991; Goffet al., 1993]. In
contrast,within the centersegment,maximum valuesare in the
middle of the segment,and the sparselysamplednorth segment
showsno clearpattern.
The secondprincipalobservationis the large differencein rms
heightsbetweenthe southsegmentandthe northandcentersegments. Neglectinginsideandoutsidecomerposition,rmsheights
within the southsegmentare approximatelyhalf the rms heights
within the center and north segments. This differenceis very
striking;southsegmentrms heightsare amongthe lowestvalues
seenin near-axis Atlantic abyssalhill morphology,while rms
heightswithin the adjacentcentersegmentare amongthe largest
[seeGoff, 1991].
Characteristic
22,513
Near-Axis
•
Data
NorthSegment
ß CenterSegment
0 SouthSegment
IC
25000 '
25020 '
25040 '
26o00 '
26o20 '
26o40 '
2 7o00'
Latitude,Deg N
Figure 6. Plan view aspectratiosof abyssalhills estimatedfrom
the near-axisswaths(Figure2a), plottedas a functionof average
latitudeover the lengthof the swath. Vertical barsrepresent
error estimates.
"IC" and "OC" indicate locations of inner cor-
nersandoutercomers,respectively.
southsegment,which occursin the middleof the segment,is distinctlylessthanthatof the centerandnorthsegments.
Width
Characteristic
widths estimated
from the near-axis
data are
shownwith 1-(• error barsin Figure 5. This parameteris not as
well resolvedas rmsheight,sointerpretation
of variationare less
clear. Nevertheless,
we observethataswith rmsheights,characteristic widths display an inside-outsidecomer asymmetry,and
the largest characteristicwidths tend to occur at inside comers
(southernpart of segments). The center segmenthas a nearly
monotonic increase in characteristic width from outside to inside
Plan View Aspect Ratio
Plan view aspectratios estimatedfrom the near-axisdata are
shownwith 1-tyerror barsin Figure 6. Again, the variationin
this parameteris dominatedby inside-outside
cornerposition,
with the lowestaspectratiostendingto be at insidecorners.For
both the southand centersegments,the aspectratio nearthe inside comers approachesunity; i.e., there is no discerniblelineationto the fabric. Neglectingthe samplesnear the insidecorner,however,we find thataspectratiosattaintheirhighestvalues
at the centerof the outsidecomerpartof the southsegment.
corner. In contrast,the southsegmenthas a well-resolvedminimum in the middle of the segment.
Unlike rms heights,thereis little contrastin averagecharacteristic width betweenthe southsegmentand the centerand north
ComparisonWith Other GeophysicalConstraints
segments. However, the minimum characteristicwidth for the
To examinethe causesof variationsin abyssalhill parameters
notedabove,here we considercorrelationswith other geophysical and geologicaldata from the samearea and with resultsand
Near-Axis Data
interpretationsfrom other studies. The mostconsistentvariation
in all of our parametersis the inside-outsidecomer asymmetry
- • NorthSegment
(Figures 4-6), which is well recognizedfrom other studiesof
ß CenterSegment
0 SouthSegment
slow spreading ridges [e.g., Karson and Dick, 1983;
Severinghausand Macdonald, 1988; Blackmanand Forsyth,
1991; Tucholke et al., 1992; Tucholke and Lin, 1994; Escartin
and Lin, 1995]. Theseauthorsnote the following asymmetries:
inside comer crest is more elevated than outside corner crest;
IC
25000 '
OC
IC
OC
I
I
I
I
25o20 '
25o40 '
26000 '
26020 '
IC
OC
26040 '
27o00 '
Latitude,Deg N
Figure 5. Characteristicwidthsof abyssalhills estimatedfrom
thenear-axisswaths(Figure2a), plottedasa functionof average
latitudeoverthe lengthof the swath. Verticalbarsrepresent
error estimates.
"IC" and "OC" indicate locations of inner cor-
nersand outercomers,respectively.
faultingat insidecomersis of higheramplitudeandmoreirregular in patternthanat outsidecomers,wherefaultsgenerallyparallel the ridgeaxis;RMBAs are largeroverinsidecomercrestthan
over outside corner crest, implying that inside corner crest is
thinner; and basementsamplesof inside cornerscontainperidotitesand gabbrosmorefrequentlythanoutsidecomers. These
observationssupporta model in which rifting toward segment
endsis accommodated
in a low-angledetachmentfault, with the
insidecomer constitutingthe footwall and the outsidecomer the
hangingwall [Dick et al., 1981; Karsonand Dick, 1983; Karson,
1990;Tucholkeand Lin, 1994]. In thisscenario,extrasiveserupting at the ridgeaxis are emplacedprincipallyon the outsidecorner hangingwall, whereasthe insidecomerfootwallis primarily
deep crustal or mantle rocks [Karson, 1990; Tucholkeand Lin,
22,514
GOFF ET AL.: ABYSSAL HILLS IN THE ATLANTIC OCEAN
1994]. Both hangingwall and footwall are subjectto secondary,
high-anglenormalfaulting.
30
A second
primary
observation
is thelargecontrast
in rms
25
heightsbetweenthe southsegmentandthe centerandnorthsegments(Figure4). Characterof the segments
at the spreading
axis
shows
nostrong
correlation
withthisobservation.
Eachsegment • 20
isapproximately
thesame
length
and
has
thesame
sense
ofoffset
(Figures
1and
2).Offset
length
atboth
ends
ofthesouth
and <m
15
center
segments
is~10-15
km,while
offset
length
atthenorthern
endof thenorthsegment
is ~5 km. Depthprofilesof theseseg-
ments
along
theMARaxisareshown
inFigure
7. Thesouth
and
center segmentshave similar, asymmetricprofiles. The north
segment
profilereaches
shallower
depths
andismoresymmetric.
None of thesemorphologicvariationsconvincinglydistinguish
the southsegmentfrom the centerandnorthsegments.
The corresponding
along-axisRMBA field for thesethree
segments
(J. Lin et al., manuscriptin preparation,1995)is shown
in Figure 8. In theseprofiles,we do seemarkeddifferencesbetween the south segmentand the center and north segments.
Over the southsegment,thereis a deeplow in the RMBA which
is roughlycircularin two-dimensionalplanform(a "bull'seye"
gravitylow [Kuo and Forsyth, 1988;Lin et al., 1990]). RMBA
signalssuchas theseare interpretedto indicateeither focused
mantle upwelling and/or thickenedcrustbeneaththe segment
center [Kuoand Forsyth,1988;Lin et al., 1990;Lin andPhipps
Morgan, 1992;Sparkset al., 1993]. Both are likely truebecause
thickercrustin the middle of the segmentimpliesgreatermelt
productionandthuselevatedmantletemperatures
[e.g.,Sparkset
al., 1993;Suet al., 1994]. In contrast
to the southsegment,
the
5
South
0
25ø30'N
Center
Noah
Segment
Segment
Segment
25ø•5'N
26ø60'N 26'i5'N
26'•0'N
26'45'N
Latitude
Figure 8. ResidualmantleBougueranomaly(RMBA) (J. Lin et
al., manuscriptin preparation,1995) sampledalongthe axis of
theneovolcanic
zoneof theMid-AtlanticRidgeat the easternend
of the surveyarea. RMBA is derivedby subtractingfrom the
free-airanomalythefollowingpredictivecomponents:
(1) theeffects of water-crustand crust-mantleinterfaces,assuminga
model crust of 6 km thicknessand water, crust, and mantle densi-
tiesof 1.03,2.70,and3.30Mgtm3, respectively,
and(2) theeffectsof mantledensitychanges
dueto lithospheric
cooling[Kuo
and Forsyth,1988;Lin et al., 1990] basedon a three-dimensional
modelof passivemantleupwelling[PhippsMorganandForsyth,
1988].
center segmentexhibits a much weaker RMBA low at its center
(Figure8). The RMBA profileacross
thenorthsegment
is highly
uniqueamongMAR segmentsin that it hasa deeplow (lower
eventhanthe southernsegmentanomaly),but the low is centered
at the end of the segmentrather than the middle. The causeof
this asymmetryis uncertain.
The mostconvincing
evidencethatthe southsegment
is or was
in therecentpastdifferentthanbothcenterandnorthsegments
is
provided by off-axis RMBA [Lin et al., 1992; J. Lin et al.,
manuscriptin preparation,1995] (Figure9). Over the spansampledby thenear-axisswaths,theRMBA for thesouthsegment
is
on average~10-14mGal lowerthanfor thecenterandnorthsegments,implyingthat the southernsegmenthasaccretedcrustaveraging~1 km thickerfor the past~7 to 1 Ma. Again,thisimpliesthatthemantletemperature
duringaccretion
wasgreaterfor
the intersegmentcomparisona negative correlationbetween
mantle temperature/crustalthicknessand the rms height of
abyssalhills, anda positivecorrelationbetweenmantletemperature/crustalthicknessandplan view aspectratio.
Fault densityinterpretedfrom the HMR1 sidescansonardata
is also markedlydifferentbetweenthe southsegmentand the
center and north segments(G. Jar.slow et al., manuscriptin
preparation,1995). Through the middle of each segmentand
spanningcrustalages~ 1-7 Ma, the southsegmentexhibitsa fault
density of 1.18 + 0.07 faults/km, whereasthe center and north
Neat-Axis Gravity
thesouthsegment
thanfor thecenterandnorthsegments
during
•
this spanof time [e.g., Suet al., 1994]. We thereforeinfer from
-
NorthSegment
l,
ß CenterSegment
0 SouthSegment
il/'
-3000
-3500
-4000
IC
•
4500
25000 '
South
Segment
Center
Segment
North
Segment
I
I
I
25020 '
25040 '
26000 '
OC
I
26020 '
IC
oc
I
26040 '
27000 '
Latitude,Deg N
Figure 9. RMBA values associatedwith the near-axisswaths
(Figure
2a); each plotted value representsthe averageof the
25ø30'N 25',i5'N 26'60'N 26'iS'N
26'•0'N
26ø45'N
RMBA
field
(seeFigure8 for details)calculatedoverthe length
Latitude
of eachnear-axisswath. Data from J. Lin et al. (manuscript
in
Figure 7. Bathymetryalong the axis of the neovolcaniczone of
preparation, 1995). "IC" and "OC" indicate locationsof inner
the Mid-AtlanticRidgeat the easternendof the surveyarea.
comersandoutercomers,respectively.
-5000
GOFFETAL.: ABYSSALHILLSIN THEATLANTICOCEAN
segments have significantly smaller values of 0.76 + 0.03
faults/km and 0.70 + 0.02 faults/km,respectively. Comparing
these values with center-of-segment parameter estimates in
Figures4-6 implies a negativecorrelationbetweenfault density
and both rms heightsand characteristic
width, as well as a positive correlationbetweenfault densityandplan-viewaspectratio.
SouthSegment4-5 m.y. Averages
4. Analysis of South Segment
Figure 2b showspositionsof data swathsusedto analyzethe
full run of the southsegment. Thesedata representa -29-m.y.
record of a continuouslydevelopedspreadingsegment. There
are, however, severalsecond-ordercomplexitiesin the evolution
of the segment. At -22 Ma, the plate spreadingdirectionrotated
counter-clockwise
by -9 ø [Tucholkeet al., 1992; B. Tucholkeet
al., manuscriptin preparation,1995). Prior to thistime, a distinct
but small-offsetridge axis discontinuitydivided the southsegment in two (dashedlines,Figure 2). This boundaryhasa much
weaker bathymetricexpressionthan the primary, larger-offset
discontinuities
boundingthe southsegment(solidlines,Figure2).
As we will show later, this boundaryhad someeffect on abyssal
hill morphology,but this effect was subsidiaryto the grossinside-outsidecornerasymmetryof the full southsegment. In the
following text, we apply the term "secondarydiscontinuity"to
identifythis structure.
At youngercrustalagesof ~ 16 Ma and -11 Ma, structurallineamentscrosscutthe abyssalhill fabric and offset the magnetic
anomalies(Figures 1 and 2). Thesestructuresappearto be the
resultof rift-propagationevents(B. Tucholkeand M. Kleinrock,
manuscriptin preparation,1995), andthey mostlikely have some
effect on our estimatesof abyssalhill statisticalpropertiesas discussed below.
In the following analysis,we averageabyssalhill parameters
estimated from all swaths combined within each of a series of 4-
to 5-m.y. crustalage ranges(age bins). This averagesout variability associated
with inside-outside
comerasymmetry,allowing
us to investigatetemporalvariationsin abyssalhill morphology.
Also, the two southernmost(inside corner) sampleswithin each
age bin are excludedfrom the averaging. This maintainsour focus on "normal" abyssalhills, i.e., thosecreated on what is assumedto be full-thicknesscrustwith well-developed,ridge-parallel faultingIn addition, we examine rms height, swath by
swath,within eachage bin, to determinealong-isochronvariation
of abyssalhill characterwithin the segment.
22,515
!
30 Ma
-49O00'
25 Ma
-48030 '
I
30 Ma
-49o00 '
-48000 '
20 Ma
-47030 '
15 Ma
-47000'
10 Ma
-46030 '
5 Ma
-46000 '
0 Ma
-45ø30 '
-45000 '
I
25 Ma
-48030 '
-48000 '
20 Ma
-47030'
15 Ma
-47000'
10 Ma
-46030 '
5 Ma
-46ø00 '
-45030 '
0 Ma
-45000 '
Longitude,Deg West
Figure 10. (a) RMS heightsaveragedwithin4- to 5-m.y. crustal
age bins over the southsegmentswaths(Figure 2b) (excluding
the southernmost two swaths, i.e., those associated with inside
RMS Height
corners) and plotted as a function of average longitude. (b)
RMBA alongthe run of the southsegment,averaged,aswith rms
heights above, to emphasizetemporal variations' each plotted
value representsan averageof the RMBA field (seeFigure 8 for
details)calculatedboth over the lengthof eachswathand over all
swaths(exceptingthe southernmosttwo) in each age bin. Data
are from J. Lin et al. (manuscriptin preparation,1995). Vertical
bars represent1-rr error estimateson the averages. Horizontal
bars are approximatelongitudinalspanof the samples. Crustal
agesare indicated.
RMS heightsestimatedalongthe run of the southsegment,averagedin the mannerdescribedabove,are plottedin Figure 10a.
There is significant variation in observedrms height on time
scalesrangingfrom 5 to -15 m.y. The largerms heightvaluesin
the 12-7 Ma bin appearto be influencedby the propagatorpseudofault running through this region (Figure 2b). However, the
pseudofaultin the 17-12 Ma bin hasno comparableeffect; rms
heightshereare amongthe lowestvaluesobserved.
RMS heightsfrom individualswathswithin the southsegment
are plottedas a functionof swathlatitudein Figure 11 for the set
of six agebins. The samplesin the 29-25 and 25-21 Ma agebins
crossthe secondarydiscontinuitywithin the westernpart of the
segment(Figures2b, 11a, and 1lb). Along-isochronvariationsin
rms heightdisplaysimilar characterwithin eachof thesesix age
bins. As with the near-axis data (Figure 4), there is a clear
asymmetryassociatedwith the inside-outsidecomer positionin
the segmentas a whole, with larger rms heightsin the southern,
insidecomerpart;thisis particularlynotablein the 29-25 Ma, 2521 Ma, and 21-17 Ma age bins. (The large error bars on these
plots are somewhatmisleading,becausewe are making observations regardingthe populationof estimationsratherthan a single
estimation.For example,a weightedlinear inversionof the rms
heightestimationsdisplayedin Figure 1la, 1lb, or 1lc resolvesa
negativeslope(left to right) at -2rr or better.) In the 17-12 Ma
agebin the trendis still present(mostlydue to the singlelow rms
estimateof the northernmostsample),but high rms heightsalso
appearnearthe outsidecomer,possiblybecauseof the propagator
pseudofault.Similar propagatoreffectsmay accountfor the lack
of any trend in the 12-7 Ma age bin.
Thoughthe grossinside-outsidecomerpatternis the same,the
southsegmentrms heightsplottedfor agebins 29-25 Ma, 25-21
Ma, 21-17 Ma (Figures 11a-11c) displaya structuredifferentin
detail from that of the near-axis rms heights (Figure 4).
Proceedingfrom outsideto insidecomer (northto south),the rms
heightsincreaseand then decreasemarginallytoward the middle
before increasingagain toward the inside corner. Though the
error bars are generally large relative to this small "step"in rms
22,516
GOFFET AL.: ABYSSALHILLS IN THEATLANTICOCEAN
a
29-25 Ma
Secondary
Segment
Discontinuity
IC
25o00 '
25o20 '
25o40 '
26o00 '
26o20 '
OC
26o40 '
27o00 '
b
oc
25-21 Ma
!C
25o00 ,
25o20 ,
25o40 ,
Secondary
Segment
Discontinuity
I
i
26o00 '
26o20 '
OC
I
26o40 '
27o00 '
21-17 Ma
IC
•
25000 '
OC
I
I
I
I
I
25020 '
25040 '
26000 '
26020 '
26040 '
27o00 '
Latitude,Deg N
Figure 11. Swath-by-swathestimatesof rms heightsfor six successive
4- to 5-m.y. crustalagebinsin the south
segment,plottedas a functionof swathlatitude. Vertical barsrepresentestimated1-• errors. The spanof swaths
whichsamplethe small-offsetsecondary
discontinuity
with thesouthsegment(seetext)aremarkedby the shaded
region. "IC" and "OC" indicatelocationsof innercomersandoutercomers,respectively.
GOFF ET AL.: ABYSSALHILLS IN THE ATLANTIC OCEAN
22,517
d
17-12 Ma
oc
IC
25000 '
I
I
I
I
I
25020 '
25040 '
26000 '
26020 '
26040 '
i
I
i
i
25040 '
26000 '
26020 '
26040 '
26020 '
26040 '
27000 '
e
12-7Ma
oc
IC
I
25000 '
25020 '
27000'
f
7-2 Ma
IC
OC
I
25000 '
25020 '
25040 '
26000 '
27000'
Latitude,Deg N
Figure 11. (continued)
height, the consistencyof this feature at all three age bins suggeststhat it is real. In the 29-25 and 25-21 Ma agebins,the step
is spatiallycoincidentwith the secondarydiscontinuityidentified
by B. Tucholkeet al. (manuscriptin preparation,1995) (Figures
1la and 1lb). The continuation
of thisstepstructureinto the 21-
17 Ma agebin (Figure 1l c) suggests
that the secondarydiscominuity influencedtopographyin this youngercrest. However,
thereis no gravity,magnetic,or bathymetricevidencefor continuation of the boundary(J. Lin et al., manuscriptit preparation,
1995;B. Tucholkeet al., manuscript
it preparation,1995). RMS
22,518
GOFFETAL.:ABYSSAL
HILLSIN THEATLANTICOCEAN
heightswithin the 17-12 Ma age bin (Figure 1l d) displaya behavior similarto that of older agebins,i.e., a southwardincrease
in rmsheight,followedby a decrease,andthenan increaseagain
toward the inside corner. However, the initial increaseis large
and abrupt,andit is followedby a lengthydecreasetowardthe
insidecomer;thusit doesnot give the appearance
of a "step"in
an overall outside-to-insidecomer increasein rms height. This
characteris differentfrom thatat the secondary
discontinuity,
and
we infer thatit is relatedto the presenceof thepropagator
pseudofault.
Characteristic Width and Plan-View Aspect Ratio
Characteristicwidths estimatedfrom the southsegmentdata
set and averagedfor each 4- to 5-m.y. age bin are plottedin
Figure 12a. Though somedetailsdiffer, the grossvariationin
thisparameteris similarto thatof rmsheights(Figure10a):Both
parameters
startoff largein the 29-25 Ma agebin, generallydecreaseto the 25-21 Ma, 21-17 Ma, and 17-12 Ma age bins, increaseto maximum value in the 12-7 Ma agebin, anddecreaseto
minimum value in the 7-2 Ma agebin.
Individual
estimations of characteristic width between 29 Ma
and 12 Ma are, like rms heights,generallylargerat insidecomers
thanat outsidecomers. However,thereis no obvioussteptoward
the middle of the segmentas there appearsto be in the rms
heights. The degreeof positivecorrelationbetweenrms height
and characteristicwidth is mostobjectivelyquantifiedby the corSouthSegment,4-5 m.y.Averages
i
I,
30 Ma
30 Ma
-49o00 '
, 25Ma
I 20Mr
151Ma
110Ma
I 5MaI,
.48o30 '
-48o00 '
25 Ma
-48o30 '
-48o00 '
-47o30 '
20 Ma
-47030 '
-47o00 '
-46o30 '
15 Ma
-47000 '
-46o00 '
-45o30 '
5 Ma
-46030 '
between these two parametersis p = +0.48 with confidence
>99%. Although this coefficient is not very high, it is well resolved,andit representsa minimumvaluebecausethe actualcorrelationis degradedby randomerrorsin bothparameters.Hence
it is likely that characteristicwidth and rms heightare responding
to similar physicalmechanismswhich control the formationof
abyssalhills.
Figure 12b plots plan view aspectratios estimatedfrom the
southsegmentdata and averagedin the mannerdescribedearlier.
The patternexpressedin this plot is similar and oppositeto the
plots of averagedrms height and characteristicwidth in Figures
10a and 12a. Over both the near-axis and south segmentdata
sets,rms heightand plan view aspectratio have a correlationcoefficientof p = -0.32 with confidence>99%. Overjust the south
segmentdata set, the coefficientis p =-0.28 with confidence=
97%.
As with the near-axissamples(Figure 6), individual estimationsof plan view aspectratiosover the entire southsegmentare
almost uniformly smallestnear the inside corner. To quantify
this observation,we calculated the average aspectratio for the
southerntwo swaths in each segmentand over every age bin
shown on Figure 2, as well as the average over all remaining
swaths. The near-inside-cornerswathshave an average aspect
ratio of 2.04 + 0.18, and the remaining sampleshave an average
aspectratio of 3.70 + 0..21,almosta factor of 2 difference.
ComparisonWith Other GeophysicalConstraints
,1,
-49o00 '
relation coefficient [see Press et al., 1989]. For either the entire
data setor just the southsegmentdata, the correlationcoefficient
-46000 '
-45030 '
-45o00'
0 Ma
-45000 '
Longitude,Deg West
Figure 12. (a) Characteristicwidthsand (b) planview aspectratios averagedwithin each4-5 m.y. crustalagebin overthe south
segmentswathsamples(Figure 2b), plottedas a functionof average longitude. Vertical barsrepresent1-rr error estimateson the
averages.Horizontalbarsrepresentthe approximatelongitudinal
spanof the samples.Crustalagesare alsoindicated.
As in our analysisof near-axisabyssalhill morphology,we
find a strong(thoughnot perfect) positive correspondence
betweenalong-flow-linevariationsin RMBA andrmsheightsover
the run of the southsegment(Figures10a and 10b). Figure 10b
showsaveragedRMBA over the run of the southsegment(data
from Linet al., [1992] and J. Lin et al. (manuscriptin preparation, 1995)). The averagingscheme,which is essentiallyidentical to that usedfor RMS heightsin Figure 10a, subduesRMBA
variationsassociatedwith segmentation(i.e., focusedupwelling,
inside/outsidecorner asymmetry)while emphasizingvariations
associatedwith temporal variability in the accretionaryprocess.
The principal difference betweenFigures 10a and 10b is that
residualgravity in the 17-12 Ma age bin is large relativeto the
correspondingaveragerms height. This may indicate that the
propagatorpseudofaultrunningthroughthisregion(Figure2) has
moreexpressionin gravity signaturethanit doesin bathymetry.
Along-flow-line variations in averaged characteristic width
(Figure 12a) and plan view aspectratio (Figure 12b) (with a
negativecorrespondence)
also displaysimilaritiesto the RMBA
plottedin Figure 10b.
The RMBA is interpretedprimarily to reflect the thicknessof
the crust[e.g., Kuo and Forsyth,1988; Lin et al., 1990; Lin and
Phipps Morgan, 1992; Tucholkeand Lin, 1994]: RMBA lows
suggestthicker crust, and RMBA highs imply thinner crust.
Variations in thicknessof the crust,to a first order, may in turn
reflect variations in melt production and hence in mantle
temperatureat the axis at the time of emplacement[Lin et al.,
1990; Suet al., 1994]. Thus we infer that the quantitative
characteristicsof abyssal hills are correlated with temporal
variationsin both crustalthicknessand mantle temperatureat the
ridge axis;wheremantletemperatureis relativelyhot andthicker
crustis produced,abyssalhills generallyhave reducedheightand
are narrower
and more lineated,
and where the mantle
temperatureis cooler and thinnercrustproduced,abyssalhills are
generallyhigher,wider, and lesslineated.
The correlationbetweenRMBA and abyssalhill characteristics
can alsobe demonstratedon unaveragedestimates.Over the en-
GOFF ET AL.: ABYSSAL HILLS IN THE ATLANTIC OCEAN
22,519
mated sedimentthicknessover the southsegmentas derived by
Webb and Jordan [1993]. Webb and Jordan [1993] created a
highly innovative approachto modeling sedimentaccumulation
correlationis also consistentwith inside-outsidecomer asymme- based on numerical sedimentationalgorithms. Comparing the
try (largerrms heightsandthinnerci'ustat insidecorners). modelwith observedbathymetry,they foundthat sedimentloads
Though less robust, well-resolvedcorrelationscan also be do not increasemonotonicallywith age within the surveyarea,
demonstratedbetweenRMBA and both characteristicwidth (p =
but ratherpeak at an estimated50 m in the range20-15 Ma before decreasingto -25 rn at -25 Ma. This observationis consis+0.31, confidence= 99%) andplan-viewaspectratio (p = -0.33,
tent with actual accumulations observed in seismic reflection data
confidence= 99%).
The negativecorrespondence
notedearlierbetweenabyssalhill collectedin the sameregionand may be explainedby a hypothestatisticalparametersand fault density(as interpretedfrom the sized deepeningof the calcite compensationdepth in the late
HMR1 side scan data) cannot be demonstrated for the entire
Miocene [Tucholkeand Vogt, 1979; Webb and Jordan, 1993].
southsegment.Figure 13a plotsaveragefault densityfor each The variationin sedimentload plottedin Figure 13b matches,in
agebin calculatedalonga flow line swathwithin the centerof the reverse,the fault densitiesinterpretedfrom sidescansonardataat
southsegment.The largeincreasein fault densityfrom the 12-7 crustalages>-7 Ma (Figure 13a).
We note that the ~25-m increase in sediment load from ~25
Ma to the7-2 Ma agebinsis consistent
with thelargeincreases
in
rmsheightandcharacteristic
widthandthedecrease
in apectratio Ma to ~17 Ma (Figure 13b) may partially explain the variationin
over the sameage bins (Figure 10a), but theserelationshipsdo rms heightover corresponding
age bins (Figure 10a). However,
not continuefor older crustalages.
H. Webb (personalcommunication,
1993)estimatesthat,evenfor
The discrepancyat olderagesmay be relatedto the accumula- the moststronglypondedsedimentswithin the studyarea,an avtion of sediment cover, which can mask smaller faults and thus
eragesedimentload of L will causea decreasein topographicrms
decreaseobservedfault density. Figure 13b plots averageesti- height at mostby L and more likely by ~L/2. Thereforesedimentationis not likely a significantfactorin the observed~100-m decreasein averagerms heightfrom-29 Ma to ~17 Ma.
tire data set, the correlation coefficient between RMBA and rms
heightis p = +0.57 with confidence
>99%. In additionto reflecting along-flow-linecorrelationsin gravity and rms height,this
SouthSegment,
4-5 m.y.Averages
5. Results
30 Ma
25 Ma
,
20 Ma
I
,
15 Ma
I
10 Ma
•
5 Ma
0 Ma
I
,
?49o00
' -48o30
' .48o00
' .47o30
' .47o00
' -46030
' -46000
' -45030
' -45000
'
and Discussion
Principal results of statistical analysis of the Hydrosweep
bathymetrywithinthe ONR AtlanticNaturalLab areasfollows:
1. Near-axisintrasegment
variationin abyssalhill morphology
is dominatedby asymmetrybetweeninside and outsidecorners;
abyssalhills producedon inside corner crust have larger rms
heightand characteristicwidth and smallerplan view aspectratiosthanabyssalhills producedat outsidecomers.
2. Insideto outsidecomerasymmetryin abyssalhill morphology generallypersistsin crustof the southsegmentover the full
rangeof crustalagessampledin the survey. However,within the
agebins 29-25 Ma and 25-21 Ma, the outsideto insidecomerincreasein rms heightsis slightlyoffsetby a negativestepnearthe
segmentcenter,and this stepappearsto be associated
with a secondary, small-offsetdiscontinuity. The stepin rms height persistsinto the 21-17 Ma age bin, althoughthe secondarydiscontinuity has no signaturein gravity, magnetics,or crustal geomorphology.
3. Along-flow-lineandintersegmentvariationsin abyssalhill
morphologycorrelatewith variationsin RMBA, consistentwith
earlier results from Shaw and Lin [1993] and Neumann and
Forsyth [1995]. These variationsappearto correlateto variations in crustalthicknessand thusby inferenceto melt generation
and temperatureof the mantle at the time of crustal accretion;
lower-relief, narrower, and more lineated abyssalhills are produced when the crust being generatedis thicker (i.e., when the
mantle beneaththe axis is hotter).
30 Ma
.49o00 '
25 Ma
-48o30 '
.48o00 '
20 Ma
.47o30 '
15 Ma
.47o00 '
10 Ma
.46o30 '
5 Ma
-46o00 '
-45o30 '
0 M•
.45000 '
Longitude,Deg West
Figure 13. (a) Fault densitiesaveragedwithin each4- to 5-m.y.
crustalage bin in the southsegment(Figure 2b), plotted as a
functionof averagelongitude. Fault densitieswere calculated
from HMR1 sidescansonarinterpretations
alonga flow line corridor approximatelyat the centerof the southsegment. Vertical
bars representl-ix error estimateson the averages. Horizontal
barsrepresentthe approximatelongitudinalspanof the samples.
Crustalagesare alsoindicated. (b) Estimatedaveragesediment
thicknessesover the south segment[from Webb and Jordan,
1993].
4. Near-axisestimatesof averagefault density(as determined
from analysisof HMR1 sidescanimagery)computedthroughthe
centerof eachsegmentcorrelatewith colocatedestimatesof statistical parameters: negatively with both rms height and
characteristic
width andpositivelywith planview aspectratio.
We discuss these results below.
AbyssalHill Segmentation
Goff [1991] documentednumeroustextural differencesbetweenMAR- and EPR-producedabyssalhills. To thesedifferenceswe add another:the along-isochronasymmetryof abyssal
hill characterin responseto ridge segmentation.Along the EPR,
abyssalhills appearto changein responseto the upwellingstruc-
22,520
GOFF ET AL.: ABYSSALHILLS IN THE ATLANTIC OCEAN
ture and consequentvariationin mantletemperaturebeneatha
ridge segment,with smallerrms height towardthe middle of a
segmentandlargerrmsheighttowardthe ends[Goff, 1991;Goff
et al., 1993]. AlongnorthernEPR segments,
characteristic
width
and aspectratio tend to find their maximum and minimum, respectively,towardthe middleof the segment[Goff, 1991]. Along
southernEPR segments,little or no along-isochronvariationin
eithercharacteristic
widthor aspectratiois observed[Goffet al.,
1993]. In contrastto theseEPR observations,inside-outsidecor-
rms heightvariationsseenin the 17-12 Ma and 12-7 Ma agebins
(Figures 1ld and 1l e) are relatedto the propagatorpseudofaults
traversingthoseregions(Figure 2b).
AbyssalHills, Crustal Thickness,and Axial Mantle
Temperature
As with intrasegment
variationsin abyssalhill morphology,
intersegmentand along-flow-linevariationscan be correlatedto
variations
in relative
crustal thickness inferred
from RMBA
ner asymmetriesappearto dominateabyssalhill development
within segmentsof the slow spreadingMAR [Tucholkeet al.,
1992; Tucholke and Lin, 1994]. This is clearly documentedin
along-axis variation in abyssalhill rms height, characteristic
width, and aspectratio (Figures4-6).
The differentcharacterof MAR andEPR abyssalhills in relation to ridge segmentationis very similar to differencesin other
geologicand geophysicalparameters.Only MAR segmentsdisplay significant inside-outsidecorner asymmetryin elevation,
gravity signature,apparentcrustalthickness,and bulk composition [Tucholkeand Lin, 1994]. This asymmetryis thoughtto be
causedby low-angle, detachmentfaulting in the slow spreading
(Figures10 and 12). Both correlationsindicatethatlarger,wider
abyssalhills are generatedin thinnercrust,suggesting
that lithosphericstrength,as modulatedby crustalthickness,playsa primary role in controllingextensionalfaulting and the resulting
abyssalhill size and shape. However, while it is hypothesized
that inside-outside
comerasymmetryin crustalthicknessis a resultof tectonicthinningby low-angledetachment
faulting[Dick
et al., 1981; Brown and Karson, 1988; Karson, 1990; Tucholke
and Lin, 1994], intersegmentand flow line variationsin crustal
thicknesslikely resultfrom spatialvariations[Lin et al., 1990]
and temporalvariations[Tucholkeand Lin, 1994], respectively,
in axial mantletemperatureandmagmatism.Thusaxial thermal
MAR crust [Dick et al., 1981; Karson and Dick, 1983, 1984;
structure,either directly or indirectly,may alsocontrolthe charKarson, 1990; Tucholkeand Lin, 1994]. Two implicationsof the acterof abyssalhills. The effectsof crustalthicknessand axial
detachment model which will affect inside-outside comer surface
mantletemperatureare expectedto be similar:Eitherlower axial
morphologycanbe consideredin the contextof our observations. mantle temperature[e.g., McNutt, 1984] or a thinnercrustassoFirst, crustal thickness will have an effect on the mechanical
ciatedwith reducedmelt generationin the presenceof lowertemstrengthof the lithosphere,andin particular,thickercrustmay reperatureswould imply greaterlithosphericstrength. It is theresult in weaker lithosphere[e.g., Shaw and Lin, 1994]. From fore difficult to separatethe two effects.
simplified theoreticalwork relating faulting and morphologyto
An apparentcorrelationof abyssalhill structurewith axial
lithosphericstrength[e.g., Malinverno and Cowie, 1993; Shaw thermal structureis also observedwithin segmentsat the EPR,
and Lin, 1993], we would predict faults with larger throw and but there is little variationbetweensegments.RMS heightson
wider spacing(and consequentlylarger and wider abyssalhills)
EPR flanks tend to be smallesttoward the middle of segments
on thinned, inside comer crust, which is consistent with our oband increasetoward the ends;this is a patternwhich correlates
servations. Second,volcanicedificesare likely to be more comwith the inferred focusingof mantleupwelling beneaththe ridge
mon on outsidecorner crust (hangingwall of detachmentfault)
axis [Goff, 1991; Cochranet al., 1993; Goff et al., 1993]. MAR
than on inside corner crust [Tucholke and Lin, 1994]. Because abyssalhills alsomay be influencedby focusedmantleupwelling
rms heightsincreasefrom outsidecomers(more volcanic)to inbeneathridge segments,but if so, this signalis mostlyobscured
side comers(less volcanic), it appearsthat roughnesscreatedby
by the inside-outsidecornerasymmetry. The small minimum in
faulting mustdominateover roughnesscreatedby volcanoes.
rmsheightandmoresignificantminimumin characteristic
width
in the near-axis swathsof the south segment(Figures4 and 5)
Division of the South Segment
may be one example of abyssalhill formationrespondingto inB. Tucholke et al. (manuscript in preparation, 1995) have
side-outsidecorner asymmetrysuperimposedon focusedmantle
identifieda small-offset,secondarydiscontinuityspanningcrustal upwelling. We anticipatethat as the averagetemperatureof the
ages29-21 Ma within the southsegment. This raisesa question mantle at the axis increases,abyssalhill formationwill respond
aboutthe suitabilityof treatingdatafrom bothnorthandsouthof
more to mantle upwelling and less to the factorscontrollinginside-outsidecomer asymmetry.
this boundaryas observationsfrom a singlesegment. It is interesting,however,that over the full run of the southsegment,there
is a positivetrend in rms height going from north to south(from Abyssal Hills and Faulting
outside to inside corner; Figures 1 l a-1 ld, 1l e). Thus the south
segment, as defined by the larger-offset discontinuitiesat its
northernand southernmargins,hasbehavedoverall as a complete
inside-outside corner system throughout its history.
Superpimosed
on the positivetrendof rmsheightin the 29-25 Ma
and 25-21 Ma age bins we do seea smallbut consistentnegative
stepwhich seemsto be associatedwith the crossingfrom inside
to outsidecomer acrossthe secondarydiscontinuity(Figures1l a
and 1lb). Thus it appearsthat even suchminor offsetsmay influence lithosphericpropertiesthat control faulting and abyssal
hill formation,althoughthe influenceis reducedas offsetlength
is reduced.
The smallnegativestepin rmsheightwithin the interiorof the
southsegmentpersiststhroughcrustalages21-17 Ma, despitethe
lack of any discordancein crustal geomorphology,gravity, or
magnetics. Whether this persistenceis somehowrelatedto the
prior secondarydiscontinuityor to someotherunrelatedfactoris
presentlyunknown. We believethat the complexstructureof the
Two kinds of circumstantialevidence have been presentedin
the resultsand above discussionfavoring the assertionthat normal faulting, as opposedto volcanic edifice formation,is the
principalabyssalhill formingprocessat slowspreadingrates:(1)
Assumingthe validityof the detachment
model(in particular,that
volcanic edifices are more common on outside comer crust), in-
side-outsidecomerasymmetryin abyssalhill characteristics
(rms
heightsare larger on inside corner crust) can only be accounted
for if faulted morphologyis much larger than volcanicedifice
morphology;and (2) fault density(as interpretedfrom sidescan
sonardata)computedwithin the centerof eachsegmentcorrelates
with colocatedestimatesof statisticalparameters(negativelywith
both rms height and characteristicwidth and positively with
aspectratio) in the near-axis (low sediment)regions. We note
that the latter evidencecould have other explanations. For example,in the eventthat volcanismwere dominant,faultingcould
be controlledby the locationsof volcanicedifices,andthe corre-
GOFF ET AL.: ABYSSAL HILLS IN THE ATLANTIC OCEAN
lationbetweenstatisticalparametersandfault densitycouldbe an
artifice of the separatecorrelationof each with volcanicedifice
formation. However, we believe the first evidenceis a strong
constraintand sufficientfor our interpretation;the secondevidencedoesnot contradictthisinterpretation.
Whether MAR abyssalhills are generatedpredominantlyby
faultingor volcanicprocesses
hasbeendebatedvigorouslyin the
literature(seealsoGoff[1991] andShawand Lin [1993] for more
extensive discussionand references). Rona et al. [1974] concludedfrom a studyof abyssalhills eastof the MAR near 23øN
that both volcanicand tectonicprocesses
play an importantrole
in their formation. Kong et al. [1988] and Pockalnyet al. [1988]
observedthat a large volcanicedifice which formed in the rift
valley near 23øN was similar in size and shapeto abyssalhill
structuresoff axis and thusproposedthat abyssalhills in this region are extinct volcanicedificeswhich have been rafted away.
However, beforea volcanicedifice canbe transferredto the ridge
flank, it mustpassoverthe rift mountains,andin the process,it is
subjectedto extensive normal faulting with individual fault
throws of the order of hundreds of meters [Macdonald and
Luyendyk,1977]. Smith and Cann [1990] suggestthat such
faulting so drasticallyalterssurfacemorphologythat volcanic
edificesconstructedwithin the medianvalley can no longerbe
identified as distinct featuresfollowing the faulting process.
While many volcanicfeaturesmay exit the rift valley relatively
intact [Jaroslow et al., 1994], theseearlier studies,like our data,
are consistent
with the assertionthatnormalfaultingis the dominantabyssalhill formingmechanism.
22,521
sideandoutsidecorners,abyssalhill morphologyalsoresponds
to temporal and intersegment variations in crustal thickness
and/or thermal structure at the axis.
4. Assumingthe validity of the detachmentmodel, insideoutsidecomerasymmetryin abyssalhill characteristics
canonly
be accounted
for if faultedmorphologyis muchlargerthanvolcanicedifice morphology.This interpretationis consistentwith
theobservation
thatin near-axisdata,faultdensity(asdetermined
from analysisof HMR1 sidescanimagery)correlates
negatively
with variationsin both rms height and characteristicwidth and
positivelywith variationsin aspectratio. However,thesecorrelationsare not generallyobservedoff axis alongthe mn of the
southsegment.This maybe attributableto sedimentcover.
Acknowledgments. This researchwas supportedby the Office of
Naval Researchunder grants N00014-92-J-1214, N00014-94-I-0197,
N0014-90-J-1621, andN0014-94-1-0466. G.E.J.was supported
by ONR
AASERT grant N00014-93-I-1153, and additional supportto J.L. was
providedby NSF grant OCE93-00708. We are indebtedto M. Orr, J.
Kravitz, and M. Badiey for their supportin developmentof the research
plan for the ONR Atlantic Natural Laboratory. We also thank T. Reed
and P. Lemmond for their efforts in processingand display of the
Hydrosweepdata on which this study was basedand to the crew and
scientific party on Ewing Cruise 9208 for their assistancein data
acquisitionand processing. Thoughtful reviews from M. Cannat, J.
Karson,and an anonymousassociateeditor on an earlier draft helped
improvethismanuscriptconsiderably.UTIG contribution1154. WHOI
contribution 9004.
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(ReceivedOctober3, 1994;revisedAugust3, 1995;
accepted
August10,1995)