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Transcript of Paleomagnetic Dating
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PaleomagneticDating
Kenneth L. Verosub
Departmentof Geology,Universityof California, Davis, California 95616
INTRODUCTION
The use of paleomagnetisms a datingmethod s based
on variationsof the Earth's magnetic ield. Under appropri-
ate circumstances, a record of the direction of the Earth's
magnetic ield is preservedby geologic materials, such as
rocks and sediments.Paleomagneticmethodscan be used o
recover his recordand to determine ts reliability. f the pat-
tern of variations in this record can be correlated with the
knownpattern or the general egionor for the approximate
time period, he recordcanbe used o determine he age of a
geologicunit. With regard o paleoseismic tudies, he most
likely uses of paleomagneticdating are correlation of
sequences f Plio-Pleistocenemarine and continentalsedi-
mentary ocks o the magneticpolarity ime scaleand corre-
lation of a sequenceof rapidly-depositedHolocene lacus-
trine sedimentso a regionalpatternof secularvariation. n
certaincircumstances,t is alsopossible o date a singlehori-
zon or an isolateddeposit.
THEORY OF DATING METHOD
Principles and Assumptions
The Earth's magnetic ield is a vector,and in a conven-
tional Cartesiancoordinatesystem ixed to a point on the
surfaceof the Earth, this vectorhas hree orthogonal ompo-
nents: up-down, north-southand east-west.The standard
convention s to take the down, north, and east components
as positive. n most circumstancest is more convenient o
considerhe vector n termsof a spherical oordinate ystem.
Again for a fixed point on the surfaceof the Earth, the three
components re the engthof the vector, he angleof the vec-
tor aboveor below the horizontalplane, and the deviationof
the horizontal componentof the vector from true north.
Thesecomponents re known, respectively, s the intensity
QuaternaryGeochronology: ethodsandApplications
AGU Reference Shelf 4
Copyright2000 by the AmericanGeophysicalUnion
(F), inclination (/), and declination (D). By conventio
downward inclination and an eastward declination are tak
as positive. The intensity, nclination, and declination
related o the north N), east E), andvertical V) compon
by the following equations:
F = (N*N+E*E+ V* V)ø.5
I = tan 1 V/(N*N+E*E) ø.5)
(
D = tan 1 (E/N) (
These elationships re also shown n Figure 1.
When sediment s deposited n a lake or ocean,whe
lava flow coolsat the surfaceof the earth,or when potter
fired in a kiln, the magnetic grains of the material beco
magnetizedparallel to the Earth's magnetic field. Un
favorable conditions, his magnetizationcan be prese
over geologically or archaeologically) ignificantperiod
time. The goal of mostpaleomagnetic tudies s the iden
cation and isolation of the primary or original directio
magnetizationof thesematehals (Collinson, 1983; Tarl
1983; Butler 1992). If the primary directionsare assoc
with fully-oriented samples, he directionscan be use
reconstruct history of variationsof the geomagnetic ie
The Earth's magnetic ield varies n different ways
on different ime scales.The largestscalevariationsare co
plete changesn the polarity of field, known as polarity tr
sitions or reversals. Mathematically one can describe
Earth's magnetic ield as the sum of a dipolar field an
non-dipolar ield. The dipolar ield, whichcorrespondso
field of a bar magnet,currentlyrepresents bout 80 perc
of the total field. The dipolar ield determineshe overallp
tern of the Earth's magnetic ield, and in its presentor n
mal polarity state, the dipolar field producesan ove
magnetic field that has downward inclinations in
Northern hemisphere and upward inclinations in
Southernhemisphere.n both hemispheres,he declinat
are generallynorthward. n the oppositeor reversed o
ity state, he inclinationsare upward n the Northern he
sphereand downward n the Southernhemisphere. n b
hemispheres,he declinations f a reversed ield are gene
ly southward.
339
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340 PALEOMAGNETIC DATING
Figure1. Relationshipetweenmagnetic omponentsn spherical
coordinatesD, I, F) and n Cartesian oordinatesN, E, V).
Over the past thirty years, the patternof normal and
reversed olarities asbeenextensively tudied, ndmostof
its featuresor thepast200 millionyears renowwell under-
stood CandeandKent, 1992).The pattern f polaritystates
is knownas he MagneticPolarityTime Scale MPTS) or the
Geomagneticeversal ime Scale GRTS).The timeduring
which he field remainsn a givenpolaritystatecan vary
from 50,000years o manymillionsof years.The time dur-
ing which the field is in the transitionalstatebetween he two
polaritystatess on the orderof severalhousandears. he
behavior f the ield during polarity ransitions not ully
understoodnd s the subject f intense tudy t thepresent
time (Laj and others, 1991, 1992; Valet and others, 1992;
McFadden and others, 1993).
The MPTS for the past 5.7 million years s shown n
Figure2. Based n recent ating sing dvancedotassium-
argon echniques,he dateof the ast ull-scale olarity ran-
sition s nowplacedat 780,000yearsago Baksiandothers,
1992). Previously, his boundarywas thought o be at
760,000 years Izett and others,1988), and evenbefore hat,
it wasplacedat 730,000years Mankinenand Dalrymple,
1979). The time period since his reversal s known as the
Brunhes ormalpolarityepochor Brunhes hron.The pre-
ceding eversed olarity nterval s called he Matuyama
reversed olarity poch r Matuyama hron. heMatuyama
chronbegan bout2.6 millionyearsagoandcontains ever-
al short ntervals f normalpolarity, ncludinghe Jaramillo
(0.99 -1.05 mya) andOlduvai 1.78-2.02mya)events r sub-
chrons Baksi, 1993).
If the Earth'smagneticield werepurelydipolar n
thisdipolewereoriented long he rotation xisof the Ea
then in the normal polarity state, he declinationswo
point precisely o the north and the inclinationswould c
form o a well-definedormulahat s a function nlyof
itude. Other orientations f the dipoleas well as the n
dipolar ortion f the ield ead o deviationsrom hisp
ly axialdipolar ieldconfiguration.ypicallyhese ariat
can be as large as _+40 in declinationand _+20 in incl
tion. Modem values of inclination and declination for No
Americaare shown n Figure3.
The non-dipolar ortionof the field is not static,an
a result, he patterns hownn Figure3 will varywith ti
At a fixedpointon the surface f the Earth, he chang
the non-dipolar ortion f the field produce hangesn d
lination and inclination that are known as secular variati
Curves f secular ariationn London ndParis or thep
400 yearsare shown n Figure4. Over longer nterva
time, thesecurveswould orm a series f loopsaround
TIME POLARITY POLARIT
(10 YEARS) EVENT EPOCH
1.0
2.0
3.0
4.0
5.0
2.60
3.55
Jaramillo
Cobb Mtn.
Olduvai
Reunion
Brunhes
Matuyam
Kaena Gauss
Mammoth
Cochiti
Nunivak
Sidufjall
Thvera
Gilbert
Figure 2. MagneticPolarityTime Scale (MPTS) for the last
million years.
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Figure3. Magneticdeclinationtop) and nclination bottom)over
North America in 1975. (Redrawn rom Defense Mapping Agency
Hydrographic enter,Charts 2 and30, 7th edition,June1975).
dipolar field direction.Each loop would have a diffe
shapealthough n many cases he loops show he gen
clockwisebehavior seen n Figure 4. The time required
the field to undergoa complete oop is on the order of 5
1,000 years.
Intermediate between secular variation and pola
transitionss a classof phenomena nown as geomagn
excursions.Geomagnetic xcursions an be characterize
short-term,high-amplitudedeviationsof the geomagn
field from the dipolardirection.No geomagnetic xcurs
have occurred in historic times, and the paleomagn
recordof them s difficult to decipher. he recordsof so
geomagneticxcursionseem o suggesthat heyare sim
large-scale ecularvariationwhile in other cases he e
dence suggestshat they represent bortedpolarity tra
tions (Hoffman, 1981). The problem s further complic
by the fact that geomagnetic xcursions o not consiste
appear n paleomagneticecordscovering he same ti
span Thouveny nd Creer, 1992). To someextent his
of consistencymay be due to inaccuraciesn dating an
hiatuses n the geologicrecord, and there is growing e
dence hat somegeomagnetic xcursions re at leastreg
al phenomenaHerrero-Berverand others,1994). Ther
also some evidence hat they may be related to period
low geomagnetic ield intensity (Valet and Meynad
1993).
Until recently, paleomagnetists ave not paid m
attention o the variations n the intensityof the geomag
60 ø
• 70ø
LONDON
ARIS
1900
- • 1600
I I I
340 ø 350 ø O* I00:330 ø
1900 -
1600 -
800 I I I
330 ø 340 ø 350 ø O*
I0'
DECLINATION
Figure . Secular ariation f thegeomagneticield n Paris ndLondonor thepast 00 years afterThellier, 981).
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342 PALEOMAGNETIC DATING
ic field even though he intensityhas changedby more than
5 percent n the last 150 years.Absolutedeterminations f
the paleointensity f the field have been made for many
yearsusing samples rom lava flows and from hearthsand
pottery. However, the methodology Thellier and Thellier,
1959) is very time-consuming, nd the percentage f sam-
ples hat give unsatisfactoryesults anbe quitehigh (Aitken
and others,1988). Primarily as a resultof the abor-intensive
nature of thesestudies, he databaseof absolutepaleointen-
sity determinationss relatively small.
For sediments,he intensityof magnetization f a sam-
ple is determinedboth by the intensityof the Earth's mag-
netic field at the time the sediment s deposited nd by the
concentration f magnetic arriers.The problemhasbeen o
find a satisfactoryway of separating hese wo effects.One
way to take account f the concentration f the magnetic ar-
hers is to produce a new magnetization n the laboratory
using a known magnetic ield (Opdyke and others, 1973;
Banerjee and Mellema, 1974; Levi and Banerjee, 1976;
Tucker, 1981). The ratio of the original ntensityof magneti-
zation o the intensityof a laboratory-induced agnetization
can be interpreted as a record of relative, rather than
absolute,paleointensity. or many years, there were ques-
tions about this approach Amerigian, 1977; Kent, 1982;
King and others, 1983), but recently, new techniques nd
new instrumentation ave addressed hese problems.More
importantly, here s growingevidence or the global coher-
enceof paleointensityeatures Tauxe,1993). These eatures
have time scaleson the order of a several thousandyears,
which is intermediatebetweensecularvariationand polarity
transitions.
Appropriate GeologicSettings
The useof paleomagnetism s a dating echniqueusual-
ly requiresa continuous equence f paleomagnetic irec-
tionsalthough n certaincircumstances atingcan be accom-
plishedusing he paleomagnetic irection rom a singlehori-
zon. The sequence f directions s then correlated o a pre-
existing, well-dated record of directions. n principle, the
undatedsequence f directions an be obtained rom either a
seriesof lava flows or from a sedimentary eposit. n prac-
tice, for paleoseismicstudies, he sequence f directions s
almost always obtained from sediments or sedimentary
rocks.
Time Range of Applicability
Becausepaleomagneticdating is a correlational ech-
nique, t can be used or any time range n which we have a
well-dated record of geomagnetic ield behavior, provided
the rate of sedimentation f the undatedsequence s high
enough to resolve the major features of the well-da
record.The two types of field behavior hat are most co
monly used or paleomagnetic atingare polarity ransit
(MPTS) and secularvariation. For studies hat involve co
lation to the MPTS, the undated sequence an be as old
200 million years.For studies hat involve the correlatio
features n secular ariation, he undatedsequences alw
less than 100,000 years old and is usually less than 10,
years old.
METHODOLOGY
Sample Collection
Material Type
For sedimentsand sedimentary ocks, the best pa
magnetic ecords ome rom relatively ine-grainedmate
deposited n quiet water. In general his means hat silts
siltstones nd mudsor mudstones re preferredalthough
isfactory resultscan often be obtained rom clays or cl
stones.Occasionallysandsor sandstones ill yield a sa
factorypaleomagnetic ecord,but this usuallyrequiresa
atively high fraction of finer-grainedmaterial. Limesto
tend to be weakly magnetized, ut when a magnetization
be measured, t is often very reliable. Within these c
straints t is not possible o determine n the field wheth
given sedimentaryunit will produce good paleomagn
results,and the best approachs to collect samples or p
studies rom as many units as possible.
Field Collection Methods
The specific techniqueused to collect paleomagn
samplesdependson the physicalstateof the material. T
most important consideration s that the sample which
returned o the laboratorybe fully orientedwith respect
geographiccoordinatesystem. n addition, if the sam
comes rom a tilted bed, the strike and dip of the bed sho
also be measured and recorded.
For well-consolidatedor lithified matehal, samples
be collected as cores drilled in the field using a porta
water-cooled,gasoline-powered iamond-coredrill. S
drills are availablecommercially, nd typically they prod
a core that is 2.5 cm in diameter and between 5 and 15
long. If the drill bit has been held straightand the mate
being sampled s not prone to fracturing, he drilling pro
dureremoves thin ring of matehaland eavesa solidcy
der attached t its base o the outcrop.A slottedbrassor a
minum tube s then slippedover the cylinder.The tube ha
platform at its top on which can be placed a comp
Determining he orientationnvolvesmeasuringhe angl
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the brass ube from the horizontalplane and the bearing of
the brass ube with respect o true North. Different laborato-
ries use different conventions for these measurements, and it
is important o find out in advancewhat these conventions
are. After the orientation has been measured and recorded, a
brasswire is placed n the slot on the tube. Movement of the
wire in the slot produces n index line on the rock cylinder.
The cylinder s freed from the outcropby tapping ightly on
a chiselplaced n the openspacebetween he sampleand the
outcrop. mmediately after the sample s removed rom the
outcrop, he index ine shouldbe scribedwith a diamondsty-
lus, and the direction nto or out of the outcropshouldbe
clearly marked.
In volcanic errain or in situationswhere power lines or
similar installations an affect the local magnetic ield, the
beatingasdeterminedwith a magnetic ompassmight not be
accurate. his can be checkedby taking readingson promi-
nent landmarksor by looking for changes n the compass
directionas one moves oward or away from an outcrop. f
there s a problemwith the directionsdeterminedby a mag-
netic compass,t may be necessaryo use a suncompass. s
its name mplies, a sun compass ses he positionof the sun
to determine true azimuth.The positionof the sundepends
on the longitudeand atitudeof the site, he time of day when
the reading s made, and the day of the year. Tableswhich
relate theseparameters o the positionof the sun are pub-
lishedannually.
An alternative o drilling samplesn the field is to collect
orientedhand samples. ypically suchsamples re fist-sized
or largerblocks.There are several chemesor obtainingori-
ented block samples.One of the simplest nvolvesbreaking
the sample rom the outcropand then putting t back in its
originalposition.A continuous orizontal ine is then marked
on two sidesof the block alongwith a north arrow.
For unconsolidated ediments, t is usually most conve-
nient to collect the samples n small plasticboxesabout 2.5
cm on a side. n the field, samples re usuallycollected rom
a fresh, clean vertical face. If the sediment can be carved
with a small knife, a pedestal of material, the size of the
insideof the box, is prepared, nd he box is slippedover he
pedestalwith relatively minor distortion of the sediment.
Simply pushing box into an outcropor hammering t in can
disturb he sediment nough o affect he magneticdirection
(Symonsand others, 1980).
The pedestaland ts box must be orientedwhile it is still
attached o the outcrop.One way to do this is to imagine an
arrow hrough he centerof the box into the outcrop.The ori-
entationdata consistof the bearing of the arrow, the small
deviation of the arrow above or below the horizontal, and the
small clockwiseor counterclockwiseotation of the top of
the box about the arrow.
After the orientation information has been recorded,
pedestaland its box are removed rom the outcrop.Exc
material s trimmedaway,and he box is capped or trans
to the aboratory. or sedimentwhich s only slightlycon
idated,particularlycoarsesilt and very fine sand, t may
desirableo use a resinor varnish o ensure hat the sam
doesnot disaggregate uring ransport.f a resin or varnis
used, t shouldbe checked o verify that it is non-magne
Preservation/Transportation
Some researchers elieve hat samplesshouldbe tra
ported n specialmagnetically-shieldedontainerso pro
them from strong magnetic fields. However, m
researchers elieve that sampleswhich become remag
tized through exposure o such fields probably would
have given reliable paleomagnetic esults anyway. He
they do not use thesecontainers.
For unconsolidated ediment, more importantcon
erationduring transport s that the sediments ot be allow
to dry out becausehe dryingprocess an ead to remagn
zation HenshawandMerrill, 1979). Keeping he sample
an air-tightplasticcontainer, erhapswith a piece of da
toweling, s usually sufficient or this purpose.
Laboratory Analysis
Preparation
No additionalpreparations needed or samples oll
ed in plasticboxes.For samples hat have been collecte
coresdrilled in the field, it is necessaryo cut the cores
2.5 cm long subsamples.Often these subsamples re ca
specimens.
Samplescollectedas orientedblocks are usually cas
plaster n a way that preserveshe original horizontalor
tation. Subsamples re obtainedby drilling vertically do
with a diamond-coredrill bit mounted in a drill pr
Various simple techniquesare used to transfer the no
arrow on the block to the subsample.Like drilling in
field, drilling in the lab requireswater coolingof the drill
Sometimes he water causesdisaggregation f the sam
When that happens,a rotary diamond saw, a band saw, o
wire saw can be used to cut the oriented block into recta
gular or cubic subsamples ithout the use of water.
In certain circumstances, cores from lakes or mar
environments ecomeavailable or paleomagnetic ampl
Again, small plastic boxesare normally used o collect
samples.n order o avoid problemsarising rom distor
associated ith the coringprocedure, amples re taken r
the interior of the core. As with unconsolidated mater
sampled n outcrop,samples re usuallycollectedby carv
pedestals f material and slippingplasticboxesover th
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344 PALEOMAGNETIC DATING
Unlessa corehasbeenazimuthallyoriented, he samples an
only be orientedwith respect o an arbitraryplane through
the long axis of the core. One technique or doing his is to
secure he core n a horizontalpositionso that t can not roll
and o stretcha string he lengthof the core.Again one mag-
ines an arrow thougheach box. The orientation nformation
consists f three small angles: the two that measure he devi-
ations of the arrow from vertical planes perpendicular nd
parallel to the axis of the core and the angle that measures
the rotation of the side of the box with respect o the long
axis of the core, as markedby the string.
Recently a new technique has been developed that
makes t possible o measurecontinuous amplesof sedi-
ment cores Nagy andValet, 1993, Weeks and others,1993).
The samples re collected n non-magnetic, lasticchannels
which are 2 cm high, 2 cm wide, andup to 1.5 m long. These
u-channelsamples re measuredwith a special ype of mag-
netometer,and very few of these nstrumentsare currently
available.However, he u-channel pproach as he potential
of revolutionizing he way paleomagnetic tudiesare done
over the next ten years.
Analysis
The mostbasic ype of paleomagnetic nalysis nvolves
determination f the directionof magnetization f a sample
with respect o a coordinatesystem ixed to the sample. n
the case of samples n plastic boxes, he axes would corre-
spond o the edgesof a box. For cylindricalsamples,he axes
of the coordinate ystemwould correspondo the index ine
on the side of the cylinder and two orthogonal ines on the
end of the cylinder. Using simple geometricrelationships
and the orientationangles, the measureddirection can be
transformed o give the direction elative o a geographic r
field coordinatesystem.A correctioncan also be made for
the tilt of the bedding.
The instrumentused to measure he magnetization s
calleda magnetometer.wo typesof magnetometersre cur-
rently used - the spinnerand the cryogenic.With a spinner
magnetometer, he sample is placed in a sample holder
mounted on a rotating shaft. In accordancewith Faraday's
Law, the two components f the magnetization erpendicu-
lar to the axis of rotationproduce voltage n a pick-upcoil.
The amplitudeof the voltage s proportional o the combined
intensityof the two components hile the phaseof the volt-
age is proportional o the ratio of the intensityof each com-
ponent.Different types of instruments se different detector
circuits o measure he amplitudeand he phaseand o deter-
mine the magnetization f eachcomponent.n order o mea-
sure the third componentof the magnetization, he sample
must be placed n the spinnermagnetometern a different
orientation. n principle, he two separatemeasurementsuf-
rice to measure he three componentsof magnetiza
However, many laboratories se a three-spinor a six-s
procedurewhich produceseither two or four indepen
measurements f each component. f there are signific
inhomogeneitiesn the sample, hey often showup as la
variationsn the measurement f the samecomponent.f
variations re small, he independentmeasurementsor e
component an be averaged ogether.
A cryogenicmagnetometer sessuperconductingo
incorporatednto detectors nown as superconductingu
tum interference devices or SQUIDs (Goree and Ful
1976). n order o operate s superconductors,he oopsm
be immersedn liquid helium.A sample s introducedn
room temperature pace hat is surrounded y and herm
insulated from the liquid helium. The current flow
througha loop is influencedby the component f magn
zation of the sampleperpendicularo the loop. Change
this current are detectedby the SQUID. Each SQUID
measureonly one componentof the magnetization.So
cryogenicmagnetometers ave one axial and two transv
SQUIDS so hat all threecomponents anbe measured t
same time. Other cryogenicmagnetometers ave only
transverseSQUID in addition o the axial one. These ns
mentsrequire a 90ø rotation of the sample o measur
threecomponents.s with the spinnermagnetometer,ed
dantmeasurementsre used o check or inhomogeneiti
the samples.
Because cryogenic magnetometers re about four
eight times more expensive han spinner magnetome
smaller aboratoriesend have spinnermagnetometers h
larger,better-establishedaboratories suallyhavecryog
magnetometers. owever, he precision f the measurem
from the two typesof instrumentss about he same,an
fact, the principal limitation on the measurement f
directionss the accuracyn determining he sampleorie
tion, which is typically on the orderof 1-2ø The real dif
encebetween he two typesof instrumentss the sensiti
that is, cryogenicmagnetometers an measuresamples
are one to two ordersof magnitudemore weakly magnet
than spinnermagnetometersan measure.Recently,a co
pany in the Czech Republichas startedmarketinga spin
magnetometerwith a sensitivityapproachinghat of a cr
genic magnetometer. everal companies re exploring
possibility f developing QUIDS thatusehigh-temper
superconductors, hich would make t possible o opera
cryogenicmagnetometer ith liquidnitrogen ather han
uid helium.
The initial magnetization f a samplebrought nto
laboratory s known as the naturalremanentmagnetiza
(NRM). It representshe superposition f the original or p
mary magnetizationwith all of the varioussecondarym
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netizations hat the samplemight have acquired. n order to
recover he primary magnetization,t is necessaryo remove
the secondarymagnetization. n paleomagnetic tudies, he
assumptions usually made that the most stable magnetic
carriersretain the primary magnetizationand that the sec-
ondary magnetization esideswith the less stablemagnetic
carriers. Removal of the secondarycomponents s called
demagnetization, nd n general, wo approaches re used.
The first of these s alternating ield demagnetizationn
which the sample s exposed o an alternatingmagnetic ield
which beginsat somepeak value and decreases niformly to
zero. In some instruments, he decreasing field can be
applied only along one axis of the sample at a time. To
achieve a complete demagnetizationat a given level, the
samplemustbe placed n the demagnetizerhree imes, each
time in a different orientation. With other instruments, a
complexsetof gears s used o continuallychange he orien-
tation of the sampleas it is exposed o the decreasing ield.
This instrument, known as tumbler, requires only one
demagnetization t each evel.
The ability of a magneticcarrier to respond o an exter-
nal magnetic ield is determinedby its coercive orce. The
basic principle involved n alternating ield demagnetization
is that all magnetic carrierswith a coercive orce less than
that of the peak field value will initially respond to the
applied field and will try to follow it. As the applied field
decreases, he magnetic carriers with the higher coercive
forces will no longer be able to follow the field, and their
magneticdirectionswill become mmobilized. At the end of
the demagnetization, he directionsof all of the magnetic
carriers hat responded nitially will be distributed n differ-
ent directions,and their net magnetizationwill be zero. In
this way, the alternating ield demagnetization erases he
contribution rom all of the magneticcarriers hat had coer-
cive forces ess han the peak applied ield value.
In practice,alternating ield demagnetizations a step-
wise process n which the sample s exposed o peak alter-
nating fields of increasingly higher value. In effect, the
demagnetization rogressively estroyshe magnetizationof
the sample,but the generalexpectations that the secondary
magnetization s removed irst, leaving behind the primary
magnetization.A typical sequencemight begin with a peak
field of 5 millitesla (mT) and ncrease y 5 or 10 mT steps o
a maximumof 60 or 80 mT. Betweeneachstep he direction
of the remaining magnetization s measuredwith a magne-
tometer.The resulting sequence f directionsshould eflect
the preferential emovalof the secondarymagnetization, ol-
lowed by removalof the primary magnetization.
The other commonmethod of demagnetizations ther-
mal demagnetization.Here the sample s first heated and
then cooled in a near-zero magnetic field. This method is
based on the fact that when a magnetic carrier is hea
above its Curie temperature, t loses ts ability to carr
magnetization. ust below the Curie temperature, he m
netic carder can still becomemagnetized, ut the magn
relaxation time is short and the magnetization quic
becomes andomized.At a lower temperature nown as
blocking emperature, he magnetic elaxation ime beco
sufficiently ong that the magneticcarrier can hold its m
netization or a geologicallysignificantperiod of time.
When a magneticcarder is cooled o room tempera
during hermal demagnetization,t acquiresa new magn
zation determinedby the near-zeroambientmagnetic ie
Thus, thermal demagnetization erases the contribut
from all magnetic carriers that have Curie temperat
lower than the maximum temperatureachieved n the h
ing. In this case, he assumptions made that the secon
magnetization esides n the carrierswith the lowest Cu
temperatures,rather than those with the lowest coerc
forces.
Like alternating ield demagnetization,hermal dem
netization s a step-wiseprocedure,beginning at 50øC
increasingn stepswhich often becomemore closelyspa
as the temperature ncreases. he procedureusually end
700øC which is above he Curie temperatureof all comm
magnetic minerals. The magnetizationof the sample
measuredafter each heating step, and the interpretatio
the changes n direction s similar to that usedwith alter
ing field demagnetization.
One problemwith thermal demagnetizations that h
ing may lead to chemical alterationof the sample and
changesn its magneticproperties.t is now considered
practice o check for these changesby measuring he m
netic susceptibilityof the samplesafter each heating s
Magnetic susceptibility is an induced magnetiza
acquired by samples n the presenceof a weak magn
field. Magnetic susceptibility epends n the magneticm
eralogy,and any significantchange n magneticsuscep
ity indicates hat there has been chemical alterationof th
minerals. Any data acquired after these changesbegin
occur shouldbe regardedas suspect. roblemswith ther
demagnetization re often encounteredn dealingwith s
mentsor poorly-lithified sedimentary ocks.
If a particular study results n the collection of a la
number of samples, t is not unusual o begin with a p
studyof a subsetof the samples.The goal of the pilot st
is to characterizehe generalbehaviorof the samples n
determinewhetheralternating ield or thermaldemagne
tion is the more appropriate echnique.Each sample n
pilot study is subjected o a complete alternating ield
thermaldemagnetization. ependingon the numberof s
pling horizonsand the number of samplesper horizon,
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346 PALEOMAGNETIC DATING
pilot studymight nvolveoneor two samplesrom eachhori-
zon or from every hird, fifth or tenthhorizon. t is oftenuse-
ful to compare he resultsof alternating ield and thermal
demagnetization n samples rom the samehorizon.
From the pilot study, t may be clear hat one methodof
demagnetizations moreeffective han he other n removing
the secondarymagnetization nd that all samples ehave n
about the same way during the demagnetization rocess. f
this is the case, it is acceptable o adopt an abbreviated
demagnetizationrocedureor the remainingsamples.f the
pilot studies howno consistent atternof behavior, t may
be necessaryo subjectall of the samples o a full demagne-
tization procedure.
Archival
The demagnetization rocesseads o the destruction f
the original magnetization f a sample.Therefore,archiving
of material is not a major issue n paleomagnetic tudies.
Nevertheless, t sometimesbecomesappropriate o conduct
additionalmineral magneticstudiesof samples, articularly
on material that has not been heated. For this reason, it is
considered oodpractice o keep paleomagnetic amples or
severalyearsafter a studyhasbeen completed.
Data Analysis
Data Reduction
As noted above, the orientation nformation gathered
when a sample s collected s used o convertdirections n
the laboratorycoordinatesystem o directions n the fi
coordinate ystem. his procedure ields directions f m
netization hat correspondo the actualgeographic nd g
logic setting.Although he resultscan be analyzed n te
of these directions, it is often more convenient to transfo
the data into virtual geomagnetic oles (VGPs). For a g
magnetic ield that is strictly dipolar, there is a one-to-
correspondence etween the inclination and declina
observed t a particularpoint on the surfaceof the Earth a
the longitudeand latitude of the axis of the dipole that p
duces he field. For example, he angulardistance p) of
pole from the point of observations givenby:
p = cot 1 (0.5 *tan I ) (
where is the inclination.The pole itself is located his d
tance along a great circle that passes hough the poin
observation in the direction of the declination.
Although he dea that the field is due o a dipolarsou
is clearly not consistentwith the existenceof secularva
tion, representation f data n terms of VGPs has prove
be a very convenientmathematicaldevice. In particula
providesa usefulway of comparingdirections rom site
different ocations. or example,Figure 5 gives heVGP r
resentation of the secular variation data from Paris
Londonshown n Figure 4. When the focus s on the pol
ty of the geomagnetic ield, VGPs are more effective h
directions n showing hat there s a bimodal distributio
the directions.
90øW
180 ø
/75øN
0 o
180 ø
1900
i 6oo /
,,soo 3, /
0 o
Figure5. Stereographicrojection f virtualgeomagneticoles VGPs)correspondingo secular ariationn Parisand
London or the past400 years after Thellier, 1981).
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VEROSUB
Assessment f Confidence
In paleomagneticstudies the quality of the data is
assessedy statistical ests,by field and aboratory ests,and
by mineralmagnetic ests.Paleomagnetic irections re sel-
dom analyzed as individual directions. nstead, they are
treatedas statistical ssemblages.n assemblagemight con-
sist of the primary directions rom all of the samples n a
given horizon. The mean direction or the horizon is com-
putedby giving each ndividualdirection he sameweight n
the averaging rocess.n effect,eachdirection s treatedas a
unit vector,and the meandirection s obtainedby summing
the individual vectors.
Although he mean direction s important, t is equally
important o know whether he directionsare tightly clus-
teredabout he meanor widely scattered. he quantities hat
are used o measure he scatterare the precisionparameter
(k) and he alpha-95 ot95). oth of thesequantities re based
on a statistical model known as a Fisherian distribution. This
distributions the analogon a sphereof a Gaussian istribu-
tion on a line.
The precisionparameter k) is a direct measureof the
scatter and can be estimated from the formula:
k = (N- 1)/(N-R) (5)
whereN is the numberof samples ndR is the lengthof the
resultantvector obtainedby adding the N unit vectors. f
there s considerable catter,R will be relatively small, .e.,
close o one or evenzero, so that k will be small.For tightly
clustereddirections,R will approachN and the value of k
will increasesignificantly.
The (z95 epresentswice the standard rror of the mean
and is expressedas a cone of confidence about the mean
direction.Specifically, here s a 95 percentchance hat the
truedirection or the assemblagealls within Z95 f themean
direction. As a rule of thumb, two mean directions are con-
sidereddistinct f their conesof confidence o not overlap
and are not considered istinct f their conesoverlapsignifi-
cantly. More precise ways of interpreting he amount of
overlap are also available (McFadden and Lowes, 1981;
Demarest, 1983).
In many situationst is more convenient o perform the
statistical nalysison the VGPs rather han on the directions.
The calculations re similaralthoughhe analogof the (Z95
for VGPs s designated s heA95.Groupsof meandirections
or of meanVGPs can alsobe analyzed n termsof precision
parameters nd conesof confidence. or example, t might
be of interest to know the mean direction or mean VGP of all
normal horizons n a particular nterval.
Statistical estsare used o assesshe quality of the data
that results rom the paleomagnetic nalysisof the samples.
The purposeof field and laboratory estsof stability s to
determinef samples cquired heir magnetization urin
shortlyafter they were deposited s sedimentor consol
ed as sedimentary ock (Verosub, 1977). The most comm
typeof field test s the fold testwhichcanbe performed
if samplescan be collected rom two limbs of a deforme
foldedbed. After the appropriate emagnetization,he
mary directionsof magnetization f the two limbs are co
pared before and after a correction s made for the effect
the folding. f the uncorrectedpost-folding) irection
more tightly clustered, t indicates hat the deformedmat
al acquiredts magnetization fter he folding. f the corr
ed (pre-folding)directions re more ightly clustered,t in
cates hat the magnetization redateshe folding.Two o
typesof field testsare the conglomerateest which invo
the directions f magnetization f clasts n a conglome
and the baked contact test which involves the directions
magnetizationof a lava flow and the baked and unbakeds
imentbelow t. Both of these estshavenot yet foundap
cation n paleoseismic tudies.
The mostcommon ype of laboratory est s the reve
test, which is only appropriate or studies hat involve co
lation to the MPTS. The presence of both normal
reverseddirections n a sedimentary equences usu
taken as strong evidence that the sequencehas not b
remagnetized.Furthermore, f the normal and reverseddir
tions are fully antipodal, the demagnetization roces
assumed o have been successfuln isolating he prim
directionof magnetization.
It is now considered ppropriateo includesomemin
al magnetic tudies spart of everypaleomagneticnves
tion. The purposeof these mineral magnetic studies,a
known asrock magneticstudies,s to determine he natur
the magneticgrains hat carry he paleomagneticignal.T
characterization f these grains involves specificatio
their mineralogy, articlesize and domainstate.Many d
ferent echniques nd nstruments anbe used n this ende
or, including severalnew ones that have been develo
quite recently (King and Channell, 1991; Verosub a
Roberts,1995). While a full discussion f the mineralm
netic parameterss beyond he scopeof this paper,a few
the most common ones are described below.
One importantmineralmagneticparameters the m
netic susceptibilitywhich, as noted above, is the indu
(temporary)magnetization cquired y a sample n the pr
ence of a weak magnetic ield. Magnetic susceptibili
usually measuredwith an inductance ridge that produ
weak alternating ields of high frequency.Magnetic sus
tibility is directly proportional o the quantityof magn
material n a sample.
Anhysteretic emanentmagnetization ARM) and
isothermal emanentmagnetization IRM) are two per
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348 PALEOMAGNETIC DATING
nentmagnetizationsroducedn the aboratory y exposing
a sampleo an externalmagneticield. n the caseof ARM,
the samples subjectedo a d.c.bias ield n thepresence f
a decreasinglternating agneticield.Usually hebias ield
is comparablen intensity o the Earth's magnetic ield.
ARM is particularly ensitiveo smallgrainswhereasmag-
netic susceptibility s more sensitive o larger grains.
Therefore, he ratio of magneticsusceptibilityo the ARM
susceptibilityanbe a usefulparameteror assessingaria-
tions n the amountof fine versuscoarsemagneticgrains n
geologicalmaterialsBanerjee nd others,1981;King and
others, 1982).
IRM is the magnetizationcquired y a sample hat s
exposedo a (strong) .c. magneticield.As the ntensity f
the field increases, he acquired magnetization ncreases
until the samplebecomes s magnetized s its mineralogy
and the laws of thermodynamics ermit. At this point, the
magnetizationf the sample s said o be saturated.f this
magnetizations measuredn theappliedield, t is called he
saturationmagnetization.f this magnetizations measured
after the applied ield is removed, t is called he saturation
remanence.The saturation emanence s always lower than
saturation agnetizationecause f thepartial ossof align-
ment of grains hat occurswhen he field is removed. he
saturation remanence is also called the saturation isothermal
remanentmagnetization SIRM). If the applied field is
cycledbetweenhigh valuesof both negativeand positive
polarity, he magnetization f the sample ollows what is
calleda hysteresisoop (Figure6). The point at which the
appliednegative ield drives he magnetizationrom satura-
tion back to zero is called the coercivity.The appliedback-
field that drives he remanence f the sample rom saturation
to zero is called the coercivityof remanence.
Some magnetic minerals, such as magnetite and
maghemite, aturaten applied ieldson theorderof 300 mT
while other magnetic minerals, such as hematite and
goethite, equire ields n excessof 2.5 T for saturationo
occur. In most laboratories the maximum field that can be
applied s on the order of 1-2 T. Thus, the presenceor
absence of saturation at these values can be used to differen-
tiate betweendifferent ypesof magneticcarriers.
Interpretation
Paleomagneticataareusually nterpreted t two evels.
The first level focuseson the behaviorof individual samples
during he demagnetizationrocess. he demagnetization
dataareusuallypresentedn termsof vectorcomponent ia-
grams,whichare alsoknownas Zijderveldplots,or simply
asZ-plots.The Z-plot is an attempt o providea two-dimen-
sionalrepresentation f the three-dimensionalehaviorof
the magnetization. his is doneby superimposingwo dif-
ferent graphs, sing wo differentsymbols Figure7). T
first graphalwaysportrays he evolutionof the north-s
component f the magnetization ersus he east-west o
ponentof the magnetization.n effect, his s a graphof
changesn declination uringdemagnetization.he sec
graphportraysheevolution f theverticalcomponent f
magnetization ersuseither the east-west omponent
north-south omponent, r the totalhorizontal ompone
the total horizontal omponents used, his s a graphof
changesn inclinationduringdemagnetization.f the e
westor north-south omponents used, his s a graphof p
jection of the inclinationon the appropriate erticalpla
The ordinateof the Z-plot represents oth the north-s
componentof the first graph)and the verticalcompo
(of the second raph).The abscissa f the Z-plot repres
the east-west omponentof the first graph)and either
north-south, ast-west, r total horizontalcomponent of
second raph). t should e noted hat somepaleomagn
restrict the use of the term Z-plot to graphs hat invo
orthogonal ectorcomponents. ecausehe total horizo
component oesnot satisfy his condition, ny graph
included his componentwould not be considered Z-p
by this definition.With practice, t becomes airy eas
visualize the three-dimensional ehavior of the magnet
tion by lookingat the Z-plot.
One of the main usesof Z-plots s to determine t w
level the secondary omponents f the magnetization
Saturation
Remanence
Appliedield
-0.1T -O. 5
½oercivity
Saturation
Magnetization
Figure 6. Typicalhysteresis urve showing elationship etw
saturationmagnetization, aturationemanence nd coercivity
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VEROSUB
Paleomagneticating
E,N, ; . I IE,
I I
N¾
...
- |E.H
Figure 7. Three typesof vectorcomponent iagrams or Z-plots). The closedsymbolsare plottedwith respect o the
north-south xis and he east-west xis; he open symbols re plottedwith respect o the up-downaxis and to the north-
south top left), east-west top right), or total horizontal bottom) axis. Some paleomagnetists ould classifyonly the
two upper graphsas Z-plots or Zijderveld diagrams.
been successfully emoved by the demagnetization roce-
dure. Often this can be doneby inspectionof the Z-plot. In
this case,at the initial demagnetizationevels, the direction
of magnetizationwill change s he secondarymagnetization
is preferentially emoved Figure 8). When only the primary
direction remains, the magnetizationwill show little or no
change n direction,and pointson both graphswill move in
straight ines oward he origin.Thesestraight ine segments
are often used o compute he primary directionof magneti-
zation. In other cases, t may be harder o separate he pri-
mary direction rom the secondary irection,and a sophisti-
catedcurve-fitting outine mustbe used Kirschvink, 1980).
Dependingon the natureof the study, he primary direc-
tions are treatedas ndividual data pointsor, if there are sev-
eral samples rom the same site or samplinghorizon, they
may be combined o determinea meandirectionand associ-
atedcz95.f the main nterest n the study s the patternof nor-
mal and reversedpolarities, he resultsare usually plotted as
a functionof stratigraphic osition Figure9). Although t is
possible o plot suchdata n terms of their inclinations, he
more commonparameter s the latitudeof the correspon
VGP as determined rom Equation4. From sucha plot, i
possible to determine the overall pattern of normal a
reversedpolarity intervals.
If the main interestof the study s the patternof sec
variation, he directions re often presented n an orthog
plot of declinationversus nclination,which is also calle
Bauer plot (Figure 4). Alternatively, he directionsor th
corresponding GPs are plotted on a stereographic ro
tion (Figure 5). The correlationbetween he secularvaria
features in the undated sequenceand those in the da
sequence s usually done visually althoughcomputer
tines which do this are now becomingavailable.
APPLICATIONS TO SEISMIC HAZARDS
Conventional
As noted above, he primary applications f paleom
netic dating o seismichazards nvolve correlationof the p
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350 PALEOMAGNETIC DATING
i i i
N,V N,V
E,H E,H
Figure 8. Vector component iagramsshowing emovalof secondary omponents uring demagnetization. arge sym-
bol is the initial direction.Primarycomponents f magnetization re normal left) andreversed right).
tern of paleomagnetic irectionsof an undatedsedimentary
sequence o either the Magnetic Polarity Time Scale or to a
known curveof secular ariation. n the caseof polarity tran-
sitions, he undatedpattern is called the magneticpolarity
zonation, and the correspondence etween t and the MPTS
is the magnetostratigraphicorrelation. f there is reason o
believe that the rate of sedimentation has been uniform, it is
possible o correlate he magnetic polarity zonation to the
MPTS primarily by matching the pattern of the polarity
intervals. However, a magnetic polarity zonation typically
containsbetween ive and ten polarity zones,and f there are
no other ime constraints n the undatedsequence,heremay
be several possible correlations o the MPTS. Thus, it is
important to have some prior estimate of the age of the
sequence.Moreover, the assumption hat the rate of sedi-
mentationhasbeenuniform may not be valid evenwhen the
sediments re quite homogeneous.When there is consider-
able lithologic variation, the assumption s almost certainly
inappropriate.For these easons,magnetostratigraphicorre-
lation usually requiresrelatively tight biostratigraphic on-
trol or at least one well-dated horizon, for example, an
interbedded ephra layer (Figure 10). The need for some
prior chronostratigraphic ontrol may make the paleomag-
netic datingappearunnecessary,owever, he magnetostrati-
graphiccorrelationprovidesan age for eachpolarity bound-
ary, and this usuallyresults n a much more refined chronol-
ogy and mportant nformationabout atesof sedimentation.
The entire procedure s simplifiedconsiderablyf there
is reason o believe hat the top of the sedimentary equence
representsmodem material. n that case, he uppermost or-
tion of the sequence houldbe of normal polarity, and that
polarity zone would correlatewith the Brunheschron. The
correlationof the remainderof the magneticpolarity zona-
• m Virtual Geomagnetic Pole
7-. .-J Latitude
ß
e•CO90ø Oø ß
2000- -- ß •
- %1
-
950- --
1900- •
1850- ß • •
- /
-
1800-•• • • J
=
1750- • , •• •
1700- -- • • •
Figure9. Determination f a polarityboundaryor a magnetic o
ity zonation (from Ensley and Verosub, 1982). Arrows indi
changesn latitudeof virtual geomagnetic olesduringdemag
zation. Solid circles indicate final latitudes for the two or three s
ples rom eachhorizon.Permissiono use his copyrightedmat
is granted y ElsevierScience-NL,Amsterdam, he Netherland
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VEROSUB
MPZ
MPTS MPTS
MPZ
2.60Ma •60Ma
Figure 10. Correlationof a magneticpolarityzonation MPZ) to the MagneticPolarityTime Scale MPTS) usinga dated
tephra ayer.
tion can be done by simply counting down through the
MPTS. This approachwas used n one of the few published
accounts f the use of paleomagnetismo date faulted mate-
rial (Davis and others, 1977). The study was done in con-
junction with a site survey for a proposednuclear power
plant nearBakersfield,California.The sitewasunderlainby
over 150 meters of sediment, he top 90 meters of which
were normallymagnetized.This normal zone was correlated
to the Brunheschron, and that proved that unfaulted strata
were at least 500,000 years old, the then-current riteria for
a capable ault.
The MPTS can evenbe used o obtainchronostratigraph-
ic information bouta singlehorizon. n particular,f the hori-
zon has a reversedpolarity, the horizon is almost certainly
more than780,000 yearsold basedon the age of the last tran-
sition rom reversed o normalpolarity.On the otherhand, f
the polarity s normal,one can not tell if the sampleacquired
its magnetization uring he presentBrunheschronor during
an earlier normal one. For datingbasedon secularvariation,
the well-established,well-dated sequenceof directions s
called a master curve of secular variation. Because secular
variationvarieson a regionalscale,differentmastercurvesare
needed or different egions.For the purposes f this discus-
sion,a region s an areaa few thousand ilometers cross. o
cover he continental nited Stateswould requireat leastsix
master curvesof secularvariation (northeast,southeast, orth
central,southcentral,northwest, nd southwest). t the pre-
sent time there are only two publishedmaster curves that
cover the entire Holocene in North America. One of these is a
composite ecord rom two lakes (Lake St. Croix and Kylen
Lake) in Minnesota Lund and Banerjee, 1985); the othe
from a single ake (FishLake) in Oregon Verosub ndoth
1986). Becauseof this paucityof data, hese ecords en
serve as the Holocene master curves for the central Unit
States ndwesternUnited States, espectively.
Both master curves are derived from what are cons
ered second-generationaleomagnetic tudiesof lake co
The hallmarks of these studies are the careful attention to
coringprocess, he collectionof replicatecores o asses
internalconsistency f the data, he detailed nvestigatio
the magnetic carriers and the magnetization process
availability of many high-quality radiocarbon dates,
independent alidationbasedon palynology, ephra stu
or historicaldata.For example, he Fish Lake study Vero
and others, 1986) was basedon a suite of eleven cores fr
five separateholes, distributedover an area of less than
m2 on the lake bottom.Six distinct ephra ayersand num
ous thin, distinctly colored bands were used to corre
between he holes.Age control was basedon 18 radiocar
dates from Fish Lake as well as 19 radiocarbon dates fr
two nearby akeswhich contained he samesix tephra ay
In addition,one of the tephra ayers was associatedwith
6,800 year old eruption of Mr. Mazama that led to the f
mation of Crater Lake, Oregon.Four hundredand fifty-f
paleomagnetic ampleswere collectedwith doubleor tr
overlap or all segments.Mineral magneticstudiesdem
strated hat the magneticcarrier was relatively fine-grai
magnetiteand that the magnetizationhad been acquireda
shortly after depositionof the sediment.The data show
very high degreeof serial correlationand excellent ag
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352 PALEOMAGNETIC DATING
mentbetweencorrespondingegments f overlapping ores.
Verification of the composite ecord was achievedby
comparingdirectionsat the Mazama tephrahorizon with
measurementst CraterLake andby comparing he direction
at a pollenchange ssociated ith European ettlement ith
historical measurements f the mag-netic ield in Oregon.
The final version of the Fish Lake master curves is shown in
Figure 11. Similar procedures ere used n developinghe
master curve for the central United States (Lund and
Banerjee,1985).
Dating of a sedimentary equence sing secularvaria-
tion involves correlation of secular variation features in the
undatedsequencewith thoseof a mastercurve.A primary
constraint n this approachs that the undated equencemust
represent nough ime and musthavea high enough ate of
sedimentation hat secularvariation featurescan actually be
resolved n the record. The lowest acceptable ate of sedi-
mentation s about0.1 mm/yr but a rate closer o 1.0 mm/yr
2000
4000
6000
8000
10,000
30o 45o 60 75
INCLINATION
i i I
3400 0ø 20
DECLINATION
Figure 11. Declination and inclination curve from Fish Lake,
Oregon from Verosuband others, 1986). Dashed ines represent
dated ephrahorizons.
would be much better.The time interval that shouldbe r
resented y the undated equence epends n the morph
gy of the secular ariationcurvebut, n general,several
dred o a thousand ears s probablynecessary. ecause
turesof different age on the mastercurve may have sim
morphologies, dditionalage constraints re alwaysus
and, n somecases,mandatory. f secularvariation eatu
of the undatedsequence anbe correlatedwith confidenc
the master curve, the age of the sequence an usually
determinedwith a resolutionof a few hundredyears w
respect o the chronologyof the mastercurve. Individ
horizons n the undatedsequence an oftenbe dated o a f
tens of years.However, n all cases, he accuracy s limi
by the accuracyof the datingof the mastercurve.
Dating of a singlehorizonusingsecularvariation s a
possibleunder favorablecircumstances. prerequisit
doing his s that the horizonhave a well-definedpaleom
netic direction. In addition, there must be sufficient n
paleomagnetic ge constraintso localize the paleomagn
direction o a single secularvariation oop. If the unda
paleomagnetic irection alls in a region of the loop wh
there are no ambiguities, a valid date can be obtai
However, f the paleomagnetic irection alls off the lo
overlapswith two portionsof the loop, or coincideswit
crossingpoint in the loop, the method can not provid
definitivedate (Figure 12).
90øW
180 ø
+ + 90O
o
Figure 12. Hypothetical esultof an attempt o date ndividualh
zons using secularvariation.The curve s the record rom F
Lake, Oregon, or the interval rom 8,000 to 6,000 yr B.P. rec
The triangles epresentwo possible utcomes, nly one of wh
yieldsan unambiguous ate.Shaded rea s the uncertainty ss
ated with each measurement.
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VEROSUB
A related applicationof secularvariation o paleoseis-
mic studiesnvolveshe question f whether wo deposition-
al units are contemporaneous.his approachassumes hat
the units were actually magnetized t the time they were
deposited.f that is the caseand f the two units have similar
paleomagnetic irections, here is a high probability hey
theywere ormedcontemporaneously.ecause f the repet-
itive natureof secular ariation urves, ontemporaneityan
neverbe provenusingpaleomagnetism. n the otherhand, f
the two units have distinctpaleomagnetic irections, t can
be taken as strongevidence hat they formed at different
times.
Experimental
Paleomagnetism as often been used to detect tectonic
rotations n a regional cale. n these tudies,he meanpale-
omagnetic eclinationor a geologicunit is comparedo the
expected eclinaton or that unit. Any significantdifference
is usuallyattributed o rotationabouta vertical axis (Horns
and Verosub, 1995). This approachcan also be used on a
local scale. For example, Salyards and others (1992)
attempted o assesshe importanceof non-brittledeforma-
tion at a site on the SanAndreas ault by looking or varia-
tions n declination longsedimentary orizons hat crossed
the fault at Pallett Creek. They provided evidence or as
much as 40 ø of rotation, which implied that non-brittle
deformation ad been ar more mportant han brittle defor-
mation. Nagy and Sieh (1993) recently showed hat there
might havebeenproblemswith samplingmethods sedby
Salyardsand others 1992). Using conventional ampling
techniques,in andothers 1991) showedhatpaleomagnet-
ic declinationsor a site on the Imperial Fault were consis-
tent with field observations of non-brittle deformation in the
1940 earthquake here. More work is needed o determine
the potential mportanceof this approach o paleoseismic
studies.
ADVANTAGES AND DISADVANTAGES
If a site hat s the subject f a paleoseismictudycon-
tains a continuously-depositedequence f relatively ine-
grainedsedimentarymaterial,paleomagnetic atingbased
on the MPTS can provide a rapid, inexpensivemeans of
establishinghe broadchronologicalrameworkof the site.
In addition, he procedureor collectinghe sampless rela-
tively simple and because he main goal is to determine
whether he samples re of normalor reversed olarity,high
precisions not requiredn orienting he samples. owever,
unless he youngestmaterial n the sequences known o be
modem, he sequencemust epresent sufficient ime inter-
val to encompasseveral olarity ntervals. n addition,some
otherchronostratigraphicnformations usually equire
order o make an unambiguousorrelation f the magn
polarityzonation o the MPTS. Anotherdisadvantages
the suitabilityof the material or paleomagnetictudy
only be determinedn the laboratory.
The situation ith respecto paleomagneticatingu
secular ariation s more problematical. espite he eff
described bove,questions ave be raisedaboutboth of
existingmaster urves or NorthAmerica.For example
master urve rom Lake St. Croix andKylen Lake should
respondclosely to a record of Holocene secular varia
from Elk Lake, also located in Minnesota (Sprowl
Banerjee, 1989). The chronologyof that lake is based
varve counting,and from all availableevidence, t too sho
have been an excellent recorder of the magnetic fi
Althoughhecorrespondenceetweenhe worecordss v
good or the ast5,000 years, he earlierpartsof the recor
not showgoodagreement nd are oftensignificantly ut
phase. hereare alsomajordiscrepanciesetween he ma
curve rom Lake St. Croix and Kylen Lake and a record
was previouslyproposedas a master curve for the cen
United States Creer and Tucholka,1982).
The Fish Lake study showsgeneralagreementwit
lower resolution record of secular variation obtained fr
Holocene lava flows in the western United Sta
(Champion, 980) andexcellent greement ith a high-
olution record of secular variation obtained from archa
logical features n the southwesternUnited States for
time intervalA.D. 750-1450 (Steinberg, 983;Verosub
Mehringer, 1984). Good correspondencean also be fou
between features in the Fish Lake record and those in
3,500-year-long record from Blue Lake in southwes
Idaho HannaandVerosub, 988;1989).However,he se
rate radiocarbon hronologiesrom the two lakesgive s
nificantlydifferent ges or the same eatures,ndicating
at leastone of the two chronologiess wrong.
Thesedisagreementsnddiscrepanciesemonstrate
therearesignificant roblemswith theradiocarbonatin
lacustrine ediments nd that there may also be probl
with the paleomagnetic ecordingprocessas well. Th
problems epresentnherentuncertaintiesn the method,a
they may explain why it is hard to find studies n which
dating was basedon correlation o mastercurvesof sec
variation obtained from lacustrine sediments. n fact,
archaeologicalstudies n the American Southwest,wh
secular variation dating has been successfully sed,
master urve s usuallya local one, derived rom nea
archaeologicalitesand spanning nly a few hundred e
(Eighmyand Steinberg,1990). Two otherpublished xa
ples of dating using secularvariation nvolve a ninetee
centurykiln (Dunlop and Zinn, 1980) and a seventeent
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8/18/2019 Paleomagnetic Dating
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354 PALEOMAGNETIC DATING
eighteenth-centuryava flow (Symons,1974). In both cases
the secularvariationcurve was extrapolated rom observato-
ry measurements nd the uncertainty n the age determina-
tion was about +_50 ears.
Thus, despite he fact that secularvariation dating is
often invoked n discussions f paleomagnetic pplications
(Verosub,1988), in practice, t is not commonlyused,and ts
ability to providehigh-resolution atesappears o be fairly
limited.
FUTURE DEVELOPMENTS
Recently there has been considerableprogress n the
developmentof a methodology or obtaining elative pale-
ointensityvalues rom sediments Tauxe, 1993). In addition,
there s growingevidence or globalcoherence f the relative
paleointensity ignal. For example,Tric and others 1992)
produced record rom the MediterraneanSea that extends
back to 80 kyr. This record is in agreementwith, and has
been calibratedagainst,paleointensitydata from lavas cov-
ering the period 0 - 40 kyr. The recordalso showssignificant
agreementwith earlier studies rom the westernequatorial
Pacific (Tauxe and Valet, 1989). Meynadier and others
(1992) extended he record back to 140 kyr in the Somali
Basin, and confirmatoryevidence elated to this time inter-
val has been providedby Schneider 1993) and Robertsand
others (1994).
The coherence f these ecordsmarks a significantstep
toward the establishment f a credible paleointensity efer-
ence curve for the last severalhundred housandyears and
raises he possibility hat the relativepaleointensity ould be
used for paleomagneticdating. The time scale or paleoin-
tensity variations falls in the range between 10,000 and
100,000 years which is intermediatebetween he resolution
provided by secular variation features and by magne-
tostratigraphy. he developmentof the u-channelsampling
techniqueand the availability of continuousmeasurement
magnetometers Nagy and Valet, 1993, Weeks and others,
1993) make it feasible o considerusingrelative paleointen-
sity measurements s a dating technique.
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