GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO. 19, PAGES 3631-3634, OCTOBER 1, 1998
Direct measurement of magnetic reversal polarity boundaries
in a cross-section
of oceanic crust
Maurice
A. Tivey,
• H. PaulJohnson,
2Corinne
Fleutelot,
3Stefan
Hussenoeder,
•
Roisin
Lawrence,
4CherylWaters,
• andBeecher
Wooding
•
Abstract. Magnetic field measurements
made by submersible
define the cross-sectional
geometryof a magneticpolarityreversal boundaryand the verticalvariationof crustalmagnetizationin
upperoceaniccrust. Measuredpolarityboundariesshowa systematicpatternof shallowdip towardsthe spreadingaxis within
the upper extrusivelavas,and steeperdip in the lower extrusive
lavas. This geometryis a consequence
of the emplacementof
extrusivelava at a midoceanridge. Reversalboundarygeometry
and magnetizationestimatesare usedto calculatethe magnetic
contributionof the extrusivelava sequenceto the overlyingmarine magneticanomalysignal. From the forwardmodeling,the
highly magnetizedextrusivelavas contributethe majority (5075%) of the observedsea surfacemagneticanomaly,consistent
with the extrusive crust forming the primary sourcelayer for
youngmarinemagneticanomalies.
anomaliesprovide informationon the lateral extent of magnetization and have been used to hypothesize dipping polarity
boundarieswithin the crust [Atwaterand Mudie, 1973; Blakely
and Lynn, 1977; Cande and Kent, 1976; Arkani-Hamed, 1988;
Wilsonand Hey, 1981], thesemeasurements
are nonuniqueand
provide no constraintson the vertical polarity structureof the
crust. By combining magnetic field measurementsat several
levels, including outcrop polarity measurements,a strong case
was madefor the dip of polarity boundariestowardsthe spreading axiswithin the extrusives[Macdonaldet al., 1983]. Conceptual modelsof the geometryof a polarity boundaryprovide an
interpretational
framework[Kidd, 1977; Schoutenand Denham,
1979], but prior to this study,no direct observationshave been
made of the three-dimensionalcross-sectionalgeometry of a
magneticpolarityreversalboundaryin oceancrust.
Data Collection and Analysis
Introduction
We used the submersibleALVIN to collect near-bottommag-
A cornerstone
of plate tectonictheoryis the Vine and Mat-
netic profiles up a steepscarpface that exposesupper oceanic
crust of Jaramillochron age (0.99-1.07 Ma [Cande and Kent,
thews[ 1963] hypothesis,which statesthat coherentmarinemagnetic anomaliesare a recordof the recurringpolarity reversalsof 1995]).Thesurvey
focused
ontheBlar•co
Scarp
atthewestern
Earth's magnetic field preservedin oceanic crust by seafloor end of the Blanco fracture zone (BFZ) in the northeast Pacific
spreadingat midoceanridges. While the spatialvariationin (Figure 1), which offsetsJuande FucaRidgefrom GordaRidge.
magneticanomalieshasbeenusedwith greatsuccess,
thevertical Between0.2 and 0.4 Ma, a northwardjump of 20 to 40 km ocdistributionof magnetizationwithin the crust and the relative curred at the western end of the BFZ that resulted in the formacontributionof each stratigraphiclayer to the anomalysignalhas tion of WestBlancodeepandexposureof relativelynormal,non-
remaineda topicof considerable
discussion
[e.g., Talwaniet al.,
1979; Harrison, 1987].
Oceanic crustal drilling directly samplesthe vertical dimension, but hasbeen limited by a lack of significantbasementpenetration, poor core recovery,and ambiguousdirectionalinformation [Talwani et al., 1979]. Early drilling resultsfrom the Atlantic found multiple magneticreversalswithin the extrusivelavas
44' 30'N
130' 15'W
130' 10'W
I
I
130'
5'W
!
suggestingsignificantoverlap of magneticreversalboundaries
that reducethe effective contributionof the extrusivelayer to the
overlyinganomalies[Johnsonand Merrill, 1978; Talwaniet al.,
1979]. Overlappingpolarityboundaries
highlightthe importance
of spatialvariationsin crustalmagnetization,a circumstance
for
which singlehole drilling is poorly suited. While the shapeand
44' 25'N
transition widths of sea surface and near-bottom magnetic
•Woods
HoleOceanographic
Institution,
Woods
Hole,Massachusetts.
2Sch.
ofOceanography,
University
ofWashington,
Seattle,
Washington.
3Dept.
GEOTER,
Universita
diUdine,
Udine,
Italy.
4Dept.
ofGeology,
DukeUniversity,
Durham,
NorthCarolina.
5Dept.
ofGeology,
University
ofNorthCarolina,
Chapel
Hill,North
Carolina.
Copyright1998 by the AmericanGeophysicalUnion
Papernumber98GL02752.
0094-8534/98/98GL-02752505.00
-32-28-24-20-16-12
-8
-4
0
4
8
12
16
20
24
48
Figure 1. Bathymetry map of the BLANCOVIN survey area
(contourinterval 250 m) with color-shadedmagnetization. Submersibledive tracksare shownby bold black lines annotatedwith
dive numbers. A deep-towedmagnetic survey line (DTM) is
shown by bold line along the top of the scarp. The trace of the
Blanco transformfault is shownby the dashedline.
3631
3632
TIVEY ET AL.' MEASUREMENT
OF MAGNETIC
BLANCO SCARP - Phase shiftedTotal Magnetic Field Profiles
-1.5 o• cb m o• (o >. m o
i
,•
•
(o cu
i , , , , , , , , I
-3.5
-4
Figure 2. Observedtotal field magneticanomalyprofilesplotted
versusdepthand phase-shifted
(70ø) to removethe skewhess
due
to the slopingscarpgeometryandmagneticinclination.
REVERSAL POLARITY BOUNDARIES
can be correctedto an accuracyof-200 nT, trivial relative to
-10,000 nT anomaliesthat mark the polarity boundaries. Total
field was then calculatedfrom the componentdata and corrected
for the 1995 InternationalGeophysicalReferenceField [IAGA,
1996] for the area. Subsequentanalysisof the ALVIN magnetic
profilestreatedthe horizontallayeringof oceaniccrustasdipping
40ø relative to the observationplane and subparallelscarpface
[Tivey, 1996]. This angleof the sourcebodies,treatedas a semiinfinite slabs,generatesa phaseshiftexactlyasif the observation
planehadbeenhorizontalandsourceslabshadbeendipping. Inversionfor magnetizationuseda modifiedFouriertransformapproachfor the analysisof dippingtabularbodies,assumingthe
sourceextendsto infinite depthwith an averagedistancefrom the
sensorto the scarpface of 5 meters(Figure3) [Tivey, 1996]. A
short-wavelength
filter cutoffof 100 m wasusedto filter out high
frequencyanomaliescontaminatedby bathymetrysources. No
corrections were made for talus, which is assumed to be non-
fracturezone crustby a steep(-40ø), south-facingscarp[Embley magneticdue to the randomorientationof blocks,however,talus
and Wilson,1992]. The 2.3 km high BlancoScarpcleanlytrun- doesattenuatethe magneticsignalby placingthe sensorfurther
cateswell-defined, highly-lineatedBrunhesand Jaramillomag- from the source.Figure3 showsthe upperlimit of the main talus
netic anomaliesthat formed at the Juande Fuca spreadingcenter fan, whichis thoughtto progressively
maskmoreof the extrusive
(Figure 1). Submersibleobservations
of the scarpfacerevealthat layer towardsthe westernedgeof the survey. The zero-levelof
the exposedextrusivecrustcan be dividedinto a relativelyintact, the profilesaredefinedby adjustingtheDC-levelof the magnetiundeformedsectionof pillow lavas that overlies more massive zation contrastat the top of the scarpto obtainzero magnetizaand stronglyjointed basaltlava with cross-cuttingfeederdikes. tion for seawater. The zero-crossingsdefine the boundariesof
An intrusivediabasesequenceunderliesthe extrusiverocksbut is
the main positiveandreversepolaritychronsin the magnetization
obscuredby talus, excepttowardsthe easternend of the study profiles (Figure 3). We have interpretedonly the main reversal
area [Juteau et al., 1995].
patternsthat correlate laterally between profiles, becausesmall
Magneticfield datawere obtainedusinga 3-axis fluxgatemag- local anomalies are either associated with dikes, which can have
netometerattachedto ALVIN (Figure 2). A deep-towedmagne- oppositepolarity to the host extrusives,or are due to tectonic
tometerprofile was also obtainedacrossthe top of the scarp,to
complexitysuchas faulting. We testedfor two-dimensionalityof
magneticfield
tie the ALVIN profiles to the overlying sea surfaceanomalies the main anomaliesutilizing the three-component
(Figure 1). The observedthree-component
magneticfield data data corrected for submersible orientation [Rutten, 1975]. A
were first correctedfor the magnetismof ALVIN usinga Nelder- minimum amplitudein the componentsparallel to scarpface is
Meade minimization approach[Tivey, 1996]. Calibration by obtainedparallel to the strike of the assumedsourcebodiesand
spinningALVIN in the water coluntoindicatesits 2000 nT field
scarp azimuth (110ø). Also, a good match between the scarp-
BLANCOSCARP- Magnetization Profiles
-1.5
-2
E -2.5
•
-3
-4
20
22
24
26
28
30
32
Distance from axis [km)
Figure 3. Magnetizationprofilesversusdepthplottedat approximate
lateraldistancefrom the spreadingaxis. Dive
numbersrefer to ALVIN or NAUTILE (BN prefix) dives. Positivelymagnetizedcrustis shadeddark gray with
profiles filled in black. Reverselymagnetizedcrustis shownin light gray and profiles are filled with white (J:
Jaramilloand B: Brunheschron). Bold dashedline indicatesthe approximateboundarybetweenupperpillow lavas
(UPL) and lower lava (LL) sequence.Bold solid line indicatesupper limit of talus below which the polarity units
and extrusivecrust are unshadedand the boundariesand base of extrusivesdashedto indicate greaterlevel of
uncertainty.
34
,.
TIVEY
ET AL.: MEASUREMENT
OF MAGNETIC
REVERSAL
POLARITY
BOUNDARIES
3633
perpendicularcomponentand the transformedtrack-parallel [Johnsonand Salem, 1994]. Also, while lateral variability in
componentconfirmsthat the major anomaliesare generallytwo- polarity structureis correlatableat -0.5-1 km profile spacing,
dimensional
perpendicular
to the profiles(i.e. alongthe scarp magnetizationintensityappearsto vary on a smallerlength-scale
face).
Results
The magneticresultsshowa remarkablycoherentpictureof the
horizontaland verticalmagneticstructureof upperoceaniccrust.
Magneticpolarityboundaries
of the normalJaramillochronsystematicallydip towardsthe spreading
axiswith a shallowdip (<
5ø) in theupper150 m of extrusivecrustandsteeperdip (30ø to
45ø) in thelowerextrusives
(Figure3). The shallow-dipping
polarity boundariesoccurwithin the undeformedand recognizable
pillow lava section,while the steeply-dipping
boundariesoccur
within the more massive and tectonized lower extrusives.
This
suggestsa geneticlink betweenlava emplacementand polarity
boundarygeometryas suggestedby others [e.g. Atwater and
Mudie, 1973; Kidd, 1977; Schoutenand Denham, 1979; Mac-
donaldet al., 1983]. The totalhorizontalextentof the dipping
polarity boundariesfor both the young and old edge of the
Jaramillochron is -3-4 km, with the largestproportion(3 km)
occurringwithin the shallowupperlava sequence.
Magnetizationis highly variable within the extrusive lavas,
varyingfrom zero to -20 A/m, but averages-12 A/m over the
entire extrusivelava thickness,consistentwith paleomagnetic
rock measurements
[Tivey, 1996]. Our magnetization
estimates
of the underlyingintrusivedikesare not well-constrained
because
of talus cover, but appear to be an order of magnitudeless
1000
........
a) •'
Seasurface
level
-
•
-1000
b)
•
andis likely to be ultimatelyof a fractalnature[Tivey, 1996].
The mean magnetizationand mapped polarity boundarygeometry(Figure 3) can be usedto model the magneticfield anomaly and estimate the contribution of the extrusive lavas to the
overlying sea surfacemagneticanomalysignal. We used a twodimensional(2D) polygonroutine[Won and Bevis,1987] to calculatethe magneticanomaliesat both deeptow(-2.1 km) and sea
surfacelevelsusingthe mappedgeometryand estimatedmagnetization intensity. The forward model assumessourcebodiesthat
extendto infinity perpendicularto the profile. The boundariesof
the Jaramillo polarity are well-definedby our submersibleprofiles, but the adjacentBrunhes anomaly boundaryis not wellknown. We used two previous profiles (BN18, BN19) from
Tivey [ 1996] and assumedthe Brunhesboundaryhad a geometry
similar to the Jaramillofor the forward modeling(Figure 3). We
comparedthe calculatedanomalieswith the observeddeeptow
anomalyand a representativesea surfacemagneticanomalyprofile 5 km northof the scarp,well awayfrom anytopographic
edge
effectsof the scarp(Figure4). Initial calculations,usingonly the
extrusivecrustalsectionexposedabove the talus fan (Figure 4),
producedanomaliesonly 50% of the observedseasurfaceamplitude. To correctfor this discrepancy,we assumedthat talusconcealsthe extrusivebaseat the westernend of the surveyand extrapolatedthe reversalboundariesto a sourcedepth of 3400 m
depth, producingapprox. 75% of the amplitudeof the Jaramillo
anomalyobservedat both the deeptowand sea surfacelevel and
-...........
,......
,
,
,
,
I
, [I ----'
Observed
-Model full 2A thickness
2000Near-boffomlevel-2.1 km
0
-1.5
C)
........
Mognetizotion
Geometry
-2
'
-3.5............
-4
]6
I
I
I
I
I
I
I
I
18
20
22
24
26
28
30
32
34
Distance from spreading •xis [km)
Figure 4. (a) Magnetizationpolaritygeometry(Fibre 3) usedin forw•d modelof magneticfield. Magnetization
is 12 Nm, J is J•a•11o andB is Bmnheschron. Computedmagneticfield for exposedlayer 2A thickness(dotted)
andfull layer2A thickness(solid)comp•ed to (b) the observeddeeptowmagneticprofile at -2.1 km depth(dashed)
and(c) observedseasurfacemagnetic•omaly (dashed).
3634
TIVEY ET AL.:MEASUREMENT
OFMAGNETICREVERSAL
POLARITYBOUNDARIES
closely
approximating
theposition
andshape
of theobservedReferences
anomalies
(Figure
4). Thisresult
indicates
thattheextrusive
la- Arkani-Hamed,
J.,Remanent
magnetization
of theoceanic
uppermantle,
Geophys.
Res.Lett.,15,48-51,1988.
vas,although
highlyvariable
in magnetization
intensity,
neverT., andJ.D.Mudie,Detailed
near-bottom
geophysical
studyof
theless
constitute
theprimary
source
of theseasurface
marine Atwater,
GordaRise,J. Geophys.
Res.,78, 8665-8686,1973.
magnetic
anomaly
signal.
Theoldedge
oftheJaramillo
anomalyBlakely,R.J.,andW.S.Lynn,Reversal
transition
widthsandfastshows
a slightdiscrepancy
in shape
compared
to theobserved spreading
centers,
EarthandPlanet.
Sci.Lett.,33,321-330,
1977.
imposed
bytheshape
ofmarine
anomaly
(Figure
4). Thismayberelated
tocomplexities
in the Cande,S.C.,andD.V. Kent,Constraints
magnetic
anomalies
onthemagnetic
source,
J. Geophys.
Res.,81,
vertical
magnetization
pattern
andfaulting
atthislocation.
The
4157-4162, 1976.
dipofthepolarity
boundaries
suggests
thatthelower
extrusiveCande,S.C.,andD.V. Kent,Revised
calibration
of thegeomagnetic
posection
contributes
thegreater
proportion
of themagnetic
anom-
laritytimescale
forthelateCretaceous
andCenozoic,
J. Geophys.
alysource
because
thepolarity
boundaries
have
steeper
dipand Res.,100, 6093-6095, 1995.
G.L., G.M. Purdy,andG.J.Fryer,Seismicconstraints
on
greater
effective
magnetization
contrast
compared
totheshallowChristeson,
shallow
crustal
emplacement
processes
at thefastspreading
EastPadipping
polarity
boundary
intheuppermost
extrusives.
cificRise,J. Geophys.
Res.,99, 17,957-17,973,
1994.
Thechange
in slope
of thepolarity
boundary
fromshallow
to Embley,
R.W.,andD. Wilson,
Morphology
of theBlanco
Transform
steeply
dipping
andthecorresponding
lithologic
break
between faultzone-NE Pacific:Implications
for its tectonic
evolution,
Mar.
Geophys.
Res.,14, 25-45,1992.
theupper
andlowerlavascanbeexplained
bya bimodal
lava
emplacement
process
[e.g.,Schouten
andDenham,
1995;
Hooft
Harrison,
C.G.A.,Marinemagnetic
anomalies
- Theoriginof thestripes,
Ann. Rev. Earth Planet. Sci., 15, 505, 1987.
et al., 1996]. Lavaserupted
anddeposited
directly
on-axis
are Hooft,E.E.E.,H. Schouten,
andR.S.Detrick,Constraining
crustal
emrapidly
buried
andintruded
bydikefeeding
subsequent
eruptions placement
processes
fromthevariation
inseismic
layer2Athickness
andformthetectonized
lowerlavasequence.
Lavadeposited
1 to
at the EastPacificRise,Earth and Planet.Sci.Lett., 142, 289-309,
1996.
2 kmoff-axis,
either
byeruption
off-axis
orbylateral
transport
of
Association
of Geomagnetism
andAeronomy
(IAGA),Dilavasfromtheaxisarerelatively
undeformed
andcomprise
the International
vision
V, Working
Group
8,International
geomagnetic
reference
field,
upper
lavasequence.
Thedeposition
ofoff-axis
lavas
hasbeen 1995revision,
Geophys.
J. Int,125,318-321,1996.
suggested
asa mechanism
forthenear-axis
increase
inthickness
Johnson,
H.P.,andR.T. Merrill,A directtestof theVine-Matthews
hypothesis,
EarthandPlanet.
Sci.Lett.,40,263-269,
1978.
oflayer2A[Christeson
etal.,1994].If thismodel
iscorrect,
the
H.P.,andB.L. Salem,Magnetic
properties
of dikesfromthe
transition
widthof theshallowdippingpolarityzone(3 km) can Johnson,
oceanic
upper
crustal
section,
J. Geophys.
Res.,99,21,733-21,740,
beinterpreted
astheapproximate
maximum
distance
thatlavas 1994.
canflowfromthespreading
axisorcanbeerupted
off-axis.
The Juteau,
T., D. Bideau,
O. Dauteuil,
G. Manac'h,
D.D.Naidoo,
P. Nehlig,
lowerlavashavea narrower
transition
width(1 to 2 km) which
reflectsthehalf-widthof the axialrift valley,consistent
withthe
H. Ondreas,
M.A. Tivey,K.X. Whipple,
andJ.R.Delaney,A submersible
studyin thewestern
BlancoFracture
Zone,N.E. Pacific:
Lithostratigraphy,
magnetic
structure,
andmagmatic
andtectonic
evolution
duringthelast1.6Ma, Mar. Geophys.
Res.,17, 399-430,
present-day
JuandeFucaspreading
axis[Tivey,
1994].The
transition
widthof thelowerlavasis alsocomparable
to previous
1995.
studies
at similarspreading
ratemidocean
ridges[e.g.Atwater Kidd,R.G.W.,A modelfortheprocess
of formation
of theupperoceanic
crust,Geophys.
J. R.Astr.Soc.,50, 149-183,1977.
and Mudie,1973;Macdonald
et al., 1983],whilethe wider
K.C., S.P.Miller, B.P. Luyendyk,
T.M. Atwater,andL.
transition
in theupper
lavassuggests
a greater
variability
in the Macdonald,
emplacement
ofoff-axis
flows
possibly
duetolocal
topography.
Conclusions
Shure,Investigation
of a VineMatthews
magnetic
lineation
froma
submersible:
The sourceandcharacter
of marinemagneticanomalies,
J. Geophys.
Res.,88, 3403-3418,1983.
Rutten,K., Two-dimensionality
of magnetic
anomalies
overIceland
and
Reykjanes
ridge,
Mar.Geophys.
Res.,2, 243-263,
1975.
H., andC.R.Denham,
Modeling
theoceanic
magnetic
source
Submersible
magnetic
profiles
of an exposed
crustal
section Schouten,
showthat the horizontalvariationin crustalmagnetization
as a
function
ofdepth
canbemapped
anddirectly
related
totheover-
lyinglineated
anomaly
signal
measured
attheseasurface.
Dippingpolarity
boundaries
withintheextrusive
sequence
arecom-
layer,in DSDPResults
in theAtlantic
Ocean:
Ocean
Crust,
Maurice
EwingSer.,edited
byM. Talwani,
C.G.A.Hamson,
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2,
pp.151-159,
AGU,Washington,
DC, 1979.
Schouten,
H., andC. Denham,Virtualoceancrust,EOSTrans.AGU,
76(17), S48, 1995.
Talwani,
M., C.G.A.Harrison,
andD.E.Hayes,DSDPResults
in theAt-
patible
withthelocation
ofJaramillo
normal
polarity
chron.
Po- lanticOcean:OceanCrust,MauriceEwingSer.,2, AGU, Washinglarityboundaries
haveashallow
dipwithin
theupper
undeformed ton, DC, 1979.
pillowed
lavas
anda steeper
dipintheunderlying
massive
and Tivey,M.A.,Thevertical
magnetic
structure
ofocean
crust
determined
tectonized
extrusive
lavasequence.
Fromforward
models,
using
fromnear-bottom
magnetic
fieldmeasurements,
J. Geophys.
Res.,
themapped
geometry
andmeasured
magnetization
intensity,
we
101, 20,275-20,296,1996.
neticanomaly
amplitude
to account
forthemajority
(50-75%)
of
crustal
structure,
J. Geophys.
Res.,99,4833-4855,1994.
M.A.,Thefine-scale
magnetic
anomaly
fieldovertheSouthern
find thatthe extrusive
lava sequence
generates
sufficient
mag- Tivey,
JuandeFucaRidge:Theaxialmagnetic
lowandimplications
for
theobserved
seasurface
magnetic
anomaly
signal.Intriguingly, Vine,F.J.,andD.H.Matthews,
Magnetic
anomalies
overoceanic
ridges,
theremaining
source
(25%)mustbe accounted
for by deeper
Nature,199, 947-949, 1963.
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axialmagnetic
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crustalunits,eitherwithinthedikeor gabbrosections.We con- Wilson,
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fortheEmperor
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andfora two-
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primary
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andFortran
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Acknowledgments.
We thankthecrewof Atlantis
II andWHOIDSOGfor theireffortsin makingtheAlvin magneticsurveys
a success.
We thankD. Van PattenandD. Naidoofor theirhelpon thecruise.We
Geophysics,
52, 232-238,1987.
M.A.Tivey,
Woods
HoleOceanographic
Institution,
Woods
Hole,MA
alsothank3 anonymous
reviewers
fortheircomments
whichhelped
im- 02543-1542(email:[email protected])
prove
themanuscript.
M.A.Tiveywassupported
byNSFgrant
OCE-
9400623;H.P. Johnson
wassupported
by NSFgrantOCE-9405078.(ReceivedFebruary20, 1998;revisedJuly13, 1998;
WHOI Contribution #9778.
accepted
August14, 1998.)