Crozet Archipelago, south

18 Pages·2017·0.39 MB·English

Geomagnetic paleosecular variation recorded in
Plio-Pleistocene volcanic rocks from Possession Island
(Crozet Archipelago, southern Indian Ocean)
Pierre Camps, Bernard Henry, Michel Prevot, Liliane Faynot
To cite this version:
Pierre Camps, Bernard Henry, Michel Prevot, Liliane Faynot. Geomagnetic paleosecular varia-
tion recorded in Plio-Pleistocene volcanic rocks from Possession Island (Crozet Archipelago, south-
ern Indian Ocean). Journal of Geophysical Research: Solid Earth, 2001, 106 (B2), pp.1961-1971.
￿10.1029/2000JB900370￿. ￿hal-00003976v2￿
HAL Id: hal-00003976
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JOURNAOL FG EOPHYSICARLE SEARCVHO, L.1 06N, O.B 2,P AGES19 61-197F1E,B RUAR1Y0 ,2 001
Geomagnetic paleosecular variation recorded in
Plio-Pleistocene volcanic rocks from Possession Island
(CrozetA rchipelago,s outhernI ndian Ocean)
Pierre Camps
LaboratoireG (cid:127)ophysique, Tectoniquee t S(cid:127)dimentologie, CNI(cid:127)S and ISTEEM, Universit((cid:127)
Montpellie2r, MontpellierF, rance
Bernard Henry
LaboratoireG domagndtismee t Pal(cid:127)omagn(cid:127)tisme, CNRS and IPGP, Paris, France
Michel Pr(cid:127)vot and Liliane Faynot
LaboratoireG (cid:127)ophysique, Tectoniquee t S(cid:127)dimentologie,C NRS and ISTEEM, Universit(cid:127)
Montpellie2r , Montpellier,F rance
Abstract. PossessionIs land, in the Crozet Archipelago,c onsistso f volcanicu nits
eruptedm ainlyb etween- .(cid:127) 5 and 0.5 Ma. A paleomagnetsica mplingw asc arried
outa longs everasl ectionsd istributedn eart he northern,e asterna, nd southeastern
coastsA. total of 45 independenftl owsw eres ampled( 320 samples).F or eachf low
a preciseldye finedc haracteristirce manencdei rectionw asu suallyis olateda fter a
carefupl rogressivcel eaningin alternatingf ields. Howeverp, articularlyc omplex
remanenceb ehavior is often observed.T he magnetostratigraphyo f the lava pile is
quites implew, ith reversedro cksin the lowerp art and normaul nitsi n the upper
parto f two sectionsA. third sectionis of normalp olarityt hroughouitt s whole
thicknesisn, cludingth reee xcursionadli rectionsW. e did not find any intermediate
directionbse tweent he normal and reversem agnetozonesT.h us we haven o evidence
fort he recordingo f the Matuyama-Brunhetrsa nsitione xpectedfr oma previous
study[ Watkinse t at., 1972].T he amplitudoef paleosecuvlaarr iatione,s timated
fromb etween-flodwis persiofnro mt he fieldo f an axiald ipolei,s 11.8øwith95 %
confidenceli mits between 9.3øand 14.0ø . This value is consistentw ith the general
anisotropsicta tisticaml odelf or paleoseculvaar riationo f Constablaen dJ ohnson
1. Introduction tion between the Pacific and Atlantic hemispheres,a nd
moreover,i f there are differencesin paleoseculavra ria-
The existence of broad fluctuations in direction and
tion between the Northern and Southern Hemispheres
magnitudes,p anninga variety of spatiala nd tempo- as it is the casei n the moderng eomagnetifcie ld (for
rals calesis, a fundamentacl haracteristiocf the Earth's a review,s ee Merrill ei al. [1996]). Theseq uestions
magnetifci eld. Theseo scillationsp rovidee ssentiailn - will undoubtedlyr emain unanswereda s long as paleo-
putsf or a better understandingo f the field-generating magnetica nalysedso n ot allowt he developmeonft r eli-
procesosc curringin the Earth's outer corea nd they able observationsb,o th in terms of directiona nd inten-
maya lsop rovidein formationo np ossibllea teralc hanges
sity,f roms itesg eographicaldlyis persedIt. is important
at the core-mantleb oundary.Y et there are still many to assesas goodg eographicaclo veragep,r eferablyp ro-
uncertaintiiens t hem orphologiccahl aracteristiocfst hese
vided by volcanicr ocksw hich are, in general,a more
fluctuationsa,s illustratedb y many controversiadle -
reliable recordert han sedimentaryr ocks.
bates. For example,t he questiono f whethero r not In examinindgi rectionadla tabaseosf v olcanirce cords,
thet ransitionavli rtualg eomagnetpico les( VGP) are
McElhinnya nd McFadden[1 997]n otea predominance
preferentialclyo nfinedto two sectorso f opposeldo ngi- of dataa cquireddu ringa n erab eforeth em odernm eth-
tudeo, rt heq uestiono f differenceins paleoseculvaar ria- odso f principacl omponenatn alysiws erei ntroduced
intot hep rocedurfeosr d emagnetizatioofnr o cksT. here-
Copyri2g0h0t1 b yt heA mericGane ophysUicnaiol n. fore it is feared that some of these data are sullied
with errorsd ue to the presenceo f secondaryc ompo-
Papenru mbe2r0 00JB900370.
0148-0227/01/2000JB900370509.00 nentso fm agnetizatiownh ichh aven otb eenc ompletely
1961
1962 CAMPS ET AL.: GEOMAGNETIC PALEOSECULAR VARIATION IN PLIO-PLEISTOCENE
eliminated. For this reason, McElhinny and McFad- LowerB runhes)T. he thickt op plateauf lowsa, ssigned
den [1997]s uggestetdh at someo f theses tudiesw ould to phasIeV , weree ruptedd uringv olcaniacc tivitya long
be worth repeating using modern experimental tech- a wide rift oriented along a NW-SE azimuth of 135ø .
niques. At first glance,t his remark is particularly rel- They yield radiometrica gesf rom 0.70 4- 0.15 to 0.53
evant to the volcanic data coming from the southern 4- 0.09 Ma (Brunhes).F ollowinga glaciale pisoded ur-
Indian Ocean, since they were obtained mainly at the ing whichU -shapedv alleysw eref ormed,t he third cycle
beginningo f the 1970s [Watkinse t al., 1972; Watkins (phaseV ) producedre centS trombolianc onesa nds ome
and Nougier, 1973; Watkins et al., 1974; Amerigian et sparsela vaf lows( differentiateds uitef rom felsicb asalts
al., 1974;W atkinse ta!., 1975].T hesed ata nowa ppear to phonolites)T. his phases eemsre latedt o an impor-
crucialb ecauset hey comef rom geographicallyis olated tant tectonic event marked by the formation of horsts
islandsa nd thus representr are field observationsp ossi- and rifts and the collapseo f a large westernp art of the
ble in a wide area devoid of data. stratovolcano.
Spurred on by these considerations,w e carried out
a new paleomagnetics tudy of the volcanic sequences 3. Paleomagnetic Sampling
from PossessioIns land (Crozet Archipelago,s outhern
Indian Ocean). This work was made possibleb y the For paleomagnetics ampling, we looked for strati-
presenceo f the French scientificb ase Alfred Faure on graphic sectionsn ot affectedb y tectonic eventsa nd
PossessionIs land, which enabled us to have essential presumedt o encompassa large time interval. One of
logisticals upport for the paleomagnetics ampling. We Watkinse t at.' s [1972]p aleomagnetisce ctionsn eart he
alsoh oped to be able to sample the full spectrumo f the southeastercno asta t Criqued e Noel (CN), and a new
field fluctuations, since a reversed to normal polarity one alongt he northern coasti n the Petit Caporalv al-
boundaryt hought to correspondto Matuyama-Brunhes ley (PC) (Figure1 ) were chosenb ecauseth ey include
was detected and because some intermediate directions volcanic sequenceso f flows from phasesI I and III and
haveb eenp reviouslyre portedo n this island [Watkins becauseo ne K-Ar age is available from each of them.
et al., 1972]. Unfortunately,o wingt o very difficult field workingc on-
ditions,w e had to stop samplingb eforer eachingt he top
of the PC section,l eavingt he upperp art of the phaseII I
2. Geological Setting
sequenceu nsampled.A third section,w ith goodq uality
The Crozet Archipelagoi s composedo f five main outcrops,i s located near the eastern coast on Alouette
islandsi ncludingP ossessionIs land. This archipelago Mountain (AL). This volcanics equenceis exclusively
lies east of the Southwest Indian Ridge on an oceanic composedo f phase II lava flows. Becauset heset hree
plateauw hichw as createda t 54 Ma duringa n episode sectionsa re several kilometers apart, a stratigraphic
of anomalousv olcanismo n the flank of the ridge, prob- correlationf rom one sectiont o the other was impossible
ably linked to a slows preadingp hase [Gos!ine t al., independenot f that providedb y the magneticd ata. It is
1981].I slandv olcanice dificesa re beingf ormeda st his noteworthyt hat we could not resamplee ither Watkins
oceanicp lateau passedo ver the Crozet hotspot. et al.' s[ 1972s] ectionn earP ort Alfred,w hichi sn owin a
tailed geologicawl orks [Chevallier1, 981; Chevalliere t protecteda rea for seab irds, or his Morne Rouges ection
al., 1983] indicatet hat PossessioIns land corresponds wheren o goodo utcropsw eref ound. Instead,w et riedt o
to a stratovolcanob uilt through three main cycles,i n- sampleo ne sectiond own from the Alfred Faure basea t
cludingf ive volcanicp hases( Figure 1). The first cycle Bollard( BO) but couldo nly collectc oresf rom a single
of activity, which likely lasted more than 7 Myr, con- flowb elongingto phaseII . Alonge achs ectionw, ec ol-
stitutes the basemento f a large stratovolcanoo f -(cid:127) 40 lecteda n averageo f sevenc oresf rom each consecutive
km diameter. Basaltic flows were first eraplacedu nder- volcanicu nit usinga gasoline-powerpedo rtabled rill.
water (phaseI) and later as subaeriaol r intrusiveu nits Samplesw ere oriented using a magnetic compassc or-
(phaseI I). No age couldb e obtainedf rom the very al- rectedf or locala nomalyb y sightingth e Suna ndk nown
landmarks.I n all, we collected3 20 orientedc oresfr om
tered basalts from phase I. For the subaerial phase II
45 units.F lowsa reu suallyth in ands eldome xcee1d0
flows,p otassium-argo(nK -Ar) ageso f 8.1+ 0.6 Ma and
2.7 + 0.8 Ma were obtained [Chevalliere t al., 1983] m in thickness. Becauset hey are horizontal or near
near the middle and the top of the sequence,r espec- horizontal, no tilt correctionh as been made.
tively, whereasa n age of 1.3 + 0.4 Ma was measured
on a dyke. These ages correspondt o severalc hrons
4. Experimental Procedure
precedingth e Matuyama chron.
After a period of erosiont he secondc yclel astedf or Determination of the direction of characteristic reina-
0.5 Myr. Conglomeratea nd differentiatedfe lsicb asalts nent magnetization( ChRM) in the laboratorye ncoun-
of phaseI II lie, often unconformablyo, ver the phase tereds everadl ifficultiesF.i rst,t he 15-daym agnetivci s-
cositiyn dex[T helliearn dT hellie1r9, 44P; rivot1,9 81]
II formations. Phase III includesl ava with K-At ages
between1 .03 4. 0.4 and 0.72 4. 0.11 Ma (Matuyama- was estimated on two thirds of the collection by mea-
CAMPS ET AI ' GEOMAGNETIC PALEOSI'_'CUi.ARV ARIATION IN PLIO-PLEISTOCENE 1963
(cid:127) I.L I
N(cid:127)
1964 CAMt)SE T AL.: GEOMAGNETICP ALEOSECULARV ARIATIONI N PLIO-PLEISTOCENE
a b
W
w
60m To,.o .4m0 T
(cid:127) (cid:127)..(cid:127).2r0n T
(cid:127) 'o... 10m T
x,(cid:127) 0'...5 rnT 5 ,,(cid:127)
10 mT (cid:127)', -(cid:127)
S. .... ! 90mT N
,
.Down
d Downo'
W
10mT
32O50(cid:127)(cid:127)(cid:127) ©
2O
(cid:127)'0 480 400 / kT(cid:127),a/'
S i
580 (cid:127)m -o--o -- o--- -o- 'i..-1- IO
Down o'
E Down o
f
w 9, ! !
10 mT !
,(cid:127), 6mT ! !
!5 m (cid:127)-o. 4 mT !
!
(cid:127) NRM !
1:t25m T
(cid:127)45 mT
(cid:127) ti50m T
(cid:127)o,(cid:127)90 m']
LT
s N N
Down
Down
Figure 2. Orthogonavl ectorp lotso f stepwispe aleomagnetcicle aningo f representativsea mples
(corec oordinates)E. xampleo f samplesw ith no significanotv erprin(ta ) flowA Ll4, specimen
193C,a nds trongv iscouso verprint( b) flowP C2, specime1n4 C.D emagnetizatiodni agramso f two
specimenfsro mt he samec ore( flowC N15)i llustrateth e poorq ualityo f thermalt reatmenot f (c)
specimen3 10B comparedto AF cleaningo f (d) specimen3 10A and the acquisitiono f spurious
GRM componenotf (e) flowP C1, specime0n0 5A whichi s successfulrleym oved(f ) specimen
005C followinga specialp rocedure(s eet ext for explanation).
We recommend that our results do not complete but ter criterion. This method,n o longeru sedi n current
supersedteh e onesd escribedb y Watkinse t al. [1972] researchc, onsistedo f AF-demagnetizinsgp ecimenast
becauset he directions do not corroborate each other, at variouss teps( maximum3 0 mT in this case)a ndt hen
least for the CN section which was resampled exactly combiningt he core directionso btained at any demagne-
at the same place, and because the former directions tizations tepst o calculateth e averaged irectiony ielding
were calculated using the method of the minimum scat- the minimum within-flow scatter. First, it is obvious
CAMPSE TA L.: GEOMAGNETPICA I(cid:127)EOSECULAVARR IATIOINN PLIO-PLEISTOCENE 1965
suringt he remanentm agnetizationf irst after 2 weeks each demagnetizations tep for two reversedp ositions
of storagew ith the ambientf ield parallelt o the posi- of the sampleth en averagedth e two measuremenftosr
tivec ylindricaal xiso f eachs pecimen(M 1), andt hen eachd uplicateds tepi n order to reducet he ARM effect
aftera nothetrw o-weeks toragein zerof ield( M2). The on the calculatedd irectiono f the remainingN RM.
viscositiyn dex( F) is expresseidn percenbt y Finally, in a few casesf or which the demagnetiza-
tion curved oesn ot decayt o the origin( Figure2 e), we
- M2I
suspectth at an artificial gyroremanenmt agnetization
IM21 (GRM) wasi nducedd uringA F cleaning.I n princi-
ple, GRM is acquiredb y particularlya nisotropisci ngle
with
domaing rains [Edwards1, 982; Ropercha nd Taylor,
M1 = NRM+VRMzab 1986],i n a directionp erpendiculator botht he applied
AF axisa ndt he easya xiso f magnetization[S tephen-
M2 = NRM
son, 1980]. The GRM magnitudei s maximumw hen
whereN RM is the stable natural remanentm agneti- the angleb etweenA F and anisotropya xis is 45øand
zation,w hich of coursei ncludess omev iscousr einanent zero when these two axes are parallel or perpendicu-
magnetizatio(nV RM) fractiona cquiredp riort o thev is- lar to each other. ThereforeG RM is suppressebdy
cosityt est, and VRMLab the VRM acquiredd uring demagnetizinga nd measurings uccessiveleya chc ompo-
the first 2-week storagei n the ambiant field, and nor- nent at eachs tepo f the AF processing[ Dankersa nd
mallyd estroyedd uringt he 2-weeks toragein zerof ield. Zijderveld1, 981].T his measuremenptr ocedureg reatly
Viscosityin dex measuredf rom the Possessiobna salts increasedt he quality of our demagnetizationd iagrams
rangefsr om <1% to >100%. It showsa lognormadl is- (Figure2 f). The overallc omplexr emanenceb ehavior
tribution characterizedb y a median value of 5% and prevented us from using the remanence intensities as
a geometrica verageo f 15 4- 3% (95% intervalf or the proxy for paleointensitya s was donew ith Icelandica nd
mean)w hichi s rather high comparedto the meanv alue Hawaiianv olcanisce quence[sC ampsa ndP r(cid:127)vot, 1996].
of 6.1 4- 0.7% foundf or someu pper Tertiary and pre- Nor could we determine the absolute intensity of the
Bruhnesq uaternarys ubaerialv olcanicr ocks [Prgvot, paleofield,s incev ery few sampless atisfiedt he usuals e-
1981].T he highv iscousin dexc ouldb e duet o the pres- lection criteria.
encei n Possessiobna saltso f nearly superparamagnetic
single-domaing rains as it has been shownf or subaerial
5. Results
lava [Pr(cid:127)vot, 1981]o r for syntheticm agnetite [Dun-
lop,1 983]. The large numbero f highlyv iscouss am- Followingc arefullyc hosen( to avoids puriousr ema-
plesw arnsu st hat significanVt RM overprintsa re likely, nence)a nd very detailedA F demagnetizationp roce-
sinceth e ratio of VRM acquiredi n situ sincet he begin- duresi nvolvingu p to 18 cleanings teps,t he ChRM was
ning of the Bruhnes polarity epoch, to the ChRM is successfulilsyo latedf rom significants econdaryc ompo(cid:127)
estimatedf or subaerial lava to be -(cid:127) 3-4 times as large nents( Figure4 ) for almost9 0% of samplesW. e think
ast he viscosityin dex [Prdvot,1 981]. Becausew e be- that the natural magnetico verprinti s likely of viscous
lieve,a s has beens hownb y one of us [Prdvot,1 981], origin becauset he componentsw ith low unblocking
that heating in a zero field is the most efficientc lean- fieldsa re generallyd irecteda longt he present-dayfi eld
ing method for VRM, we treated one pilot samplef rom direction. Thus we observet hat the geometric average
eachr ocku nit by stepwiseth ermal demagnetizatiouns - intensityo f NRM is larger for the normal (6.2 4. 0.4
ing a noninductive PYROX furnace with a residual field A/m, 95%i ntervacl onfidencfoer the averaget)h anf or
<20 nT. Surprisinglyt,h is treatment providedd emag- the reversely(2 .2 4. 0.3 A/m) magnetizedro cks.T his
netizationd iagramso f poor quality whichw ered ifficult interpretationis further validatedb y the rather large
to interpret( Figure2 c). This behaviors, omewhautn - viscosityin dicesr eporteda bove. We determinedt he
usualf or young basalts, could be explainedb y large ChR.Mb ym eanso fa least-squaremse thod[ Kirschvink,
instabilityo f samplesu ponh eatingw hichi s alwayso b- 1980].N earlya ll directioncso mputedp rovidea maxi-
servedfo r the few low-fields usceptibilitym easurements muma nguladr eviationo f < 2ø . Amongt he 10%o f re-
performeudn derv acuum( < 10- 2 mbar) (Figure3 ). jecteds ampleasr ea lsot hosew hichg avea significantly
Consequentwlye, decidedto usea lternatingfi elds( AF) differentd irectionf rom that of the other samplesf rom
cleaningu singa laboratoryb uilt AF demagnetizeinr the samef low if field notes indicated possiblem isorien-
whicht he samplei s stationarya nd subjectedto fields tation. The flow averaged irectionsa re listed together
up to 140 roT. in Table 1 with the parametersf rom the Fisher'ss tatis-
Secondd, uringA F processingm, any samplesre - tics. Onlyf or flowP C7, no means tabled irectionc ould
vealeda n atypical easinestso acquirea spuriousa n- be successfudlleyt erminedW. ith the exceptioonf flow
hysteritirce manenmt agnetizatio(nA RM) duet o the PC1, the ChRM directionasr e well clustereidn each
presenceo f a small direct field in our device not pre- flow with rather small valueso f the 95% confidencec one
viouslyd etected.T hus we repeated,f or theses amples, aboutt he meand irection( a95), all (!0 ø.
!966 CAMPS ET AL.' GEOMAG_(cid:127)TIC PALEOSECULAR VARIATION IN PLIO-PLEISTOCENE
300 1.00 , I , , I , , , I ,, , I ,
(cid:127) FlowC N14
__.__l
7C301
250
0.75
200
(cid:127)50 ßr - 0.50 , ,
(cid:127)00
0.25 ............
50-
, ' ' ' ' I .... ' ' ' ' I ..... ' ' ' '(cid:127) '1'' (cid:127)- 0.00 0 1i0 20 , 30i ß 40 , 50i 60 70 80
200 400 600
Ordered min AF treatment (mT)
Temperature (øC)
Figure 5. Cumulatived istributionfu nctiono f thef irst
Figure 3. Examt)leo f thermal dependencoef weak AF value used in the least squaresa nalysist o compute
fiel(! magnetics usceptibilityu nder vainmin. Heating the direction of ChRM.
m(cid:127)(t cooling( n(cid:127)rvesa re indicatedb y arrows.F or the few
san(cid:127)t'fiesm easuredt,h e curvesa re alwaysi rreversible.
rectionsin itially describedb y Watkinse t at. [1972c] or-
respondt o reversedd irectionsw hich would have been
that s(cid:127)ch a znett(cid:127)od is not suitable for rockso verprinted
incompletelyc leanedo f their present-dayf ield viscous
})yl arges econdacryo mp(mentass,f orm anyo ft heP os-
overprint. The principal component analysisu sedi n
sessiolns lm.t has. alts;a processinogf 30 mT mayn ot be
the presents tudy providesm ore reliable estimateso f
s(cid:127)ffii(:ientto retrieveth e prinmryc omponen(Ft igure5 ).
the paleofieldd irections.
Sec(.M,t hism ethodis not efficienitn detectingth e ac-
(l(cid:127)fisiito, no fs puriotAasR M andG RMc omponenfrtosm
A F t,r'eatmmda,s low as 10 roT, as occursi n the present 6. Discussion
st,(cid:127).ty.F ore xamplew, eb elieveth att hei ntermediadtei-
It seemsv ery speculativet o correlate one sectionto
the others from the paleomagneticr esults. The main
reasonis that the magnetics equenceosb taineda rev ery
1.00 ,
simple,a nd thust hey are not constraininign termo f
magnetostratigraphFyo.r s ectionPsC andC N,r eversed
polarityu nitsa re foundi n the lowerp art andn ormal
unitsa re foundi n the upperp art. SectionA L is ofn or-
0.75
malp olarityt hroughouitts wholet hicknesse,x cepfto r
three consecutivfelo wsy ieldinge xcursionadl irections.
A notable result is the absence of intermediate direc-
tions between the normal and reverse magnetozones.
(cid:127) 0.50 We have,h owevesr,o mea rgumenttso suggestht att he
sampledse ctiondso n ot duplicatee acho thera ndt hat
altogethethr eyc overa timei ntervallo nge nougfho r
providinag reliablere cordo f the historyo f fieldf luc-
;(cid:127) 0.25 '
tuationsa t PossessioIsnla nd( Figure6 ). The phase
III units from the CN and PC sectionsa re of norma]
polaritya ndt hereforseh ouldh avee ruptedd urintgh e
Brunheesp ochc, onsistewnti th the K-Ar ageo f 0.72
0.00 Mao btaineind t hiss equenc[Ceh evallieetra l.,1 983].
0 20 40 60 80 100 120 140 160
Fort hep hasIeI basalttsh er eversepdo laritoy btained
OrderedA ngle (NRM-GhRM)
in theP C sectioinn thet opf lowsw ithK -Ara geo f2 .7
:k0 .8M a [Chevallieetv a l.,1 983c] ouldc orrespoton d
Figure4 . Cumulativdei stributiofnu nctionfo rt hea n-
thee ndo ft heG ilberet pocho r to theb eginninogft he
glesb etweenth e ChRMa ndN RM directions.
CAMPSE T AL.' GEOMAGNETIPCA LEOSECULAVRA RIATIONIN PLIO-PLEISTOCENE 1967
Matuyamoan e. Thent he bottomp arto f PC section,f romB Os ectioann dt hel owerp arto f CN sectionB. e-
beinga lsoo nly of reversedp olarity,s houldh aveb een causet hey are, like the phaseI I PC sectiono, f reversed
eruptedd uringt he samee poch.W e haven o cluesf or polarity, we assumedte ntatively that they alsob elong
assigninag precisem agneticc hront o the normalp o- to the beginningo f Matuyamao r to the end of Gilbert
larityz onec orrespondintog the wholes ectionA L. We chron. Notwithstandinga ge uncertaintiesi,t is likely
arec ertain,h owevert,h at flowsb elongingto the same that the three sectionsc omplemente ach other with no
chronh aven ot beens amplede lsewhersei ncep haseII (or small)o verlappinign time (Figure6 ). Nevertheless,
unitsf rom the other sectionsa re of reversep olarity. givent he episodicn ature of volcanismt,h e questionr e-
The only doubt we have concernst he phaseI I units mainsw hethert he time elapsedb etweens uccessivlea va
Table 1. Directional Results
Flow Phase n/N f D c(cid:127)95 Longitude Latitude Jo J(cid:127)o
Petit Caporasl ection(L atitude:-J6.$ø7; L ongitude:51.ø7)6
pc13 III 7/7 -55.1 349.3 6.4 90 11.2 76.6 5.53 2.57
pc12 III 7/7 -76.1 357.6 3.7 269 235.3 72.7 5.86 2.91
pcll II 7/7 50.0 186.5 3.3 330 251.9 -73.6 2.35 1.25
pclO II 6/7 66.4 189.0 5.7 139 347.3 -83.4 2.63 0.87
pc9 II 8/8 68.8 180.3 2.4 528 49.9 -84.2 2.58 1.76
pc8 II 9/9 65.8 174.1 4.4 !38 116.9 -85.7 2.75 1.00
pc7b II 0/7 ............ ...... 3.88 1.21
pc6 II 7/7 67.4 180.1 7.6 54 68.7 -82.8 1.25 0.80
pc5 II 7/7 65.2 171.6 4.3 198 129.9 -84.2 1.14 1.00
pc4 II 5/7 63.1 186.8 9.7 63 303.7 -84.9 1.37 0.86
pc3 II 5/5 65.9 197.1 8.0 92 336.9 -78.3 1.64 1.09
pc2 II 7/7 68.2 188.4 7.6 64 6.8 -82.6 1.85 0.60
pcl II 5/7 55.5 196.1 16.0 24 286.9 -74.1 2.15 0.86
Alouettem ountains ection( Latitude:-J6.38;ø L ongitude:517.8 ø )
8116 II 8/8 -61.0 350.5 2.2 619 350.8 81.9 6.68 3.74
8115 !I 7/7 -53.1 8.8 2.6 444 82.9 75.6 4.64 3.04
8114 II 7/7 -58.6 11.1 4.5 183 104.7 79.2 4.95 2.12
8113 II 8/8 -70.0 5.3 4.4 158 209.6 81.7 8.51 3.48
8112 II 7/7 -75.8 244.5 2.6 525 260.1 30.8 5.47 2.80
alll II 8/8 -74.6 252.7 8.6 43 264.8 32.4 7.79 2.39
all0 II 4/4 -62.5 278.3 4.3 458 292.4 35.0 5.16 2.17
819 II 7/7 -66.0 347.7 2.4 617 304.2 81.5 6.79 4.60
818 II 10/10 -63.3 352.4 2.6 340 335.3 84.5 4.67 2.14
817 II 6/7 -71.1 1.2 4.1 272 237.8 73.6 11.55 3.44
816 II 7/7 -68.6 357.8 4.0 232 245.5 84.3 7.05 4.56
815 II 12/12 -70.1 8.5 3.2 186 199.8 80.6 5.84 2.32
814 II 2/3 -69.8 7.8 ...... 200.1 81.2 9.08 6.59
813 II 7/7 -69.0 356.5 4.0 228 250.9 83.5 6.48 3.76
812 II 7/7 -68.5 8.8 2.8 455 188.1 82.1 7.62 3.16
all II 7/7 -50.7 332.2 3.8 255 346.9 63.9 3.52 1.14
Bollards ection(L atitude:-J6ø.J; 3L ongitude:51.87ø)
bol II 7/8 63.4 189.0 4.6 130 311.9 -83.5 7.27 1.78
Criqude eN oks'Ie ctio(nL atitude:-J6.L,(cid:127)6oøn;g itude:51.83ø)
cn15 III 5/7 -74.0 329.4 7.1 79 273.2 67.5 3.10 2.56
cn14 III 6/7 -66.2 7.7 4.6 153 166.9 84.4 5.30 . . .
cn13 III 3/4 -66.0 22.0 4.5 327 157.0 75.0 5.25 0.95
cn12 III 3/5 -75.4 47.8 5.1 254 190.3 58.9 9.06 3.27
cn11 III 6/7 -61.7 341.0 6.1 87 329.8 76.1 11.99 5.20
cn10 III 8/9 -63.2 !3.5 9.1 29 136.2 80.4 7.05 1.72
cn9 II 7/8 74.7 234.8 6.5 66 7.7 -55.7 0.51 0.40
cn8 II 3/7 46.3 228.9 8.9 83 313.1 -47.5 2.68 2.90
cn7 II 5/7 45.5 210.1 6.5 91 292.9 -59.3 5.53 2.57
cn6 II 7/7 56.7 215.7 4.4 144 315.5 -62.2 5.14 2.26
cn5 II 7/7 50.1 231.0 4.3 150 318.6 -48.1 1.96 1.08
1968 CAMPS ET AL.' GEOMAGNETIC PALEOSECULAR VARIATION IN PLIO-PLEISTOCENE
Table 1. (continued)
Flow Phase n/N
I D c(cid:127)9(cid:127) n Longitude Latitude J0 J(cid:127).o
cn4 ii 5/7 62.2 205.0 5.2 !47 321.3 -72.1 2.73 1.71
(:n3 II 7/8 50.5 147.6 2.7 379 161.8 -60.8 3.14 0.94
on2 II 7/7 46.8 148.9 5.9 70 168.0 -59.5 2.00 0.96
cnl Ii 7/7 52.7 159.6 3.7 287 174.9 -69.6 2.84 1.67
(cid:127) Plowsa re listedi n stratigraphico rderw ith the youngesot n top, oldesto n the bottom.T he volcanic
phaseasr ei nferretd¾ omth eg eologicmala p( Figure1 ) [Chevallie1r9, 81a] ndf ieldo bservationn/sN; is
the numbero f samplesu sedi n the analysis/totaln umbero f samplesc ollectedI; and D are the meani n-
elinationp ositived ownwarda ndd eclinatione asto f north,r espectivelya;9 (cid:127) is 95%c onfidencceo nea bout
averaged.i rectionL; ongitudea nd Latitudec orrespontdo VGP positionJ; o and J2oa re the geometric
(cid:127)nean remanencien tensitiesin A/m of NI'iM and after 20 mT alternatingfi eldt reatmentr, espectively.
(cid:127)' Very scattereedl irections.
fi((cid:127)ws is long enough c(cid:127);mpared to the rate of secular variation was estimated by the total angular standard
variatim(cid:127) of the geomagneticfi eld. !f not, oversampling deviation( asd) expresseidn degreesa s (cid:127)qT in
of the same field directions fi'om lava flows clustered
i,empr(cid:127)rally will skew the estimate of the paleosecular
variation. Unfortunately, without very accuratea ged e-
terminations for each flow, we have no objective means
of aw(cid:127)raging or discardings imilar successivdei rections,
whereN is the numbero f data and 5i is the angular
since the rate of secular variation is itself variable, and
distance between the ith field direction or VGP posi-
thus similar paleo(cid:127)nagneti,c (liteorionsm ay be due to
tion and the direction or VGP position about whicht he
rea,1s tasiso f sem(cid:127)lar variation [Love,2 000]. Thus we
dispersioni s computed, correctedf or the experimental
havet o as, s(cid:127)mm, t,e ntatiwdy,t hat t,h e paleodirectionos b-
errors by calculatingt he between-flowa sd expresseidn
tained correspondto a reasonablera ndo(cid:127)n samplingo f
degreesa s $B in
l,h{(cid:127) field fiuctuat,i m(cid:127)s over the Plio-Pleistocene period.
The characterizati{mo f the paleosecularv ariation was
(h(cid:127)ne by meanso f sta, tistical tech(cid:127)fiquesc ommoni n pa-
lcomagnetismW. e processetdh e data by reversingth e where fi is the mean number of sample cores per lava,
directirmso f reversedp olarity and removingt hoseh aw $w (cid:127) 81ø/v/(cid:127), and (cid:127) the meanp recisiopna rameter.
ing a VGP latitudel esst han an arbitrarilyc hosenc ut,- The dispersions tatisticso f localf ield directionsa ndc or-
off t,o avoid inclusion of transitional data. The secular respondingV GPs are given in Tables 2 and 3, respec-
I !(cid:127) 0.7+2/0- .11
1.03 +/- 0.4
PC 12-13
CN 10-15 Erosion
-2
Phase II
BO 1
-3
-4
PC 1-11
AL 1-16
-5
_6
,,. -7
Phase I
Figure 6. Relationshibpe tweenth e sampledse ctionsth, eg eomagnetpico larityt imescalfeo r the
last7 Myr [Candea ndK ent, 1995],a ndt he chronostratigrapehviocl utioonf PossessiIosnla nd
[Chevallieert al., 1983].( Seete xtf ore xplanation.)
CAMPSE T AL.: GEOMAGNETIPCA LEOSECULAVRA RIATIONIN PLIO-PLEISTOCENE 1969
Table 2. DispersioSnt atisticosf Flow-AveragDeir ections
VGP > 45 ø VGP > 55 ø
N 41 39
Mean inclination _64.4ø _64.7ø
Mean declination 5.2ø 1.9ø
Precision parameter kappa 37.2 46.5
a95 3.7 ø 3.4 ø
Mean of the within-flow asd Sw 5.6ø 5.5ø
Dispersiofnr om the DipoleF ieldD irection( In = -64.6 D,(cid:127) = 0.0)
Total asd $:, 13.6ø(1 3.0) 12.0ø(11.6)
Between-flow asd $s 13.4ø(12.9) 11.8ø(11.5)
95% confidence limits on $s 10.4-16.2 ø 9.3-14.0 ø
Dispersionf rom the Mean Direction
Total asd$ (cid:127), 13.4ø ( 12.9) 12.0ø ( 11.5)
Betweenfl owa sd$ (cid:127) 13.2ø(12.7) 11.7ø(11.3)
95% confidenceli mits on $(cid:127) 10.4-15.ø9 9.2-14.1ø
a N, number of lava flow includedi n the analysis;a sd, angulars tandardd eviation; see text for expla-
nation of subscriptsT , W, and B. The 95% confidenceli mits are computedu singt he jackknifem ethod
[Efrona nd Tibshirani1, 986].N umbersin parenthesecso rrespontod asdc alculatedfr om Watl(cid:127)inse t aI.' s
[1972]d ata set.
Table 3. Dispersions tatisticso f Virtual GeomagnetiPco les
VGP > 45 ø VGP > 55 ø
Possession Island
N 41 39
AveragVeG Pl atitude 86-4ø 88'0ø
AveragVeG Pl ongitude 157.3ø 179-2ø
Precisiopna rametKear ppa 19.6 24.1
a95 5.2ø 4.8ø
Totaal sdS T 18'9ø(17'2) 16'8ø(15'5)
Meano f thew ithin-floaws d$ w forV GP 7-8ø 7-6ø
Between-flaoswd$ B 18'6ø 16'5ø
95%c onfidenlicmei tso n$ B 14.6-22.4ø 13'2-19'6ø
MarionI sland[ Amerigiaent aI., I97J]
N 21 20
Totaal sdS T 20'3ø 18'7ø
Meano ft hew ithin-floawsd S wf orV GP 10.9ø 11'0ø
Between-faloswdS B 19'9ø 18'3ø
95%c onfidelnimceit osn S B 14'8-24'2ø 13'9-21'9ø
Possessiona nd Marion Combined
N 62 59
Totaal sdS T 19'2ø 17'3ø
Meano ft hew ithin-flaoswdS wf orV GP 8-5ø 8'4ø
Between-aflsod$w B 18'9ø 17'0ø
95%c onfidelnimceit osn $ B 15'9-21'7ø 14'4-19'3ø
(cid:127)' N,n umbeorfl avafl owin cludiendt hea nalysaiss;d a, ngulsatra ndadrde viatiosne;et e xtf ore xpla-
natioonf s ubscriTp,t Ws , andB .T he9 5%c onfidelnimceit sa rec omputuesdi ntgh ej ackknimfee thod
[E/toann dT ibshira1n9i8, 6N].u mbeinrps a renthecsoerrse sptoona sdd c alculaftreodm W atkientsa l.'s
[1972]d ata set.