Paleomagnetic Dating

8/18/2019 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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VEROSUB

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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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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VEROSUB

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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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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VEROSUB

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,


8/18


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).


9/18

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


10/18


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


11/18

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


12/18


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


13/18

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


14/18


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.


15/18

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


16/18


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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Paleomagnetic Dating