Magnetization of the La Palma Seamount Series: Implications for Seamount Paleopoles

26 Pages·2017·3.15 MB·English

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. B7, PAGES 11,743-11,767, JULY 10, 1993
Magnetizationo f the La Palma SeamountS eries:
Implicationsf or SeamountP aleopoles
JEFFG F_1H1,t1m ERTS TAUDIG2I(cid:127)L L,I SAT AUX3(cid:127), ANDT HOMAPSI CK
ScrippsIm titutlono f OceanographLya, Jolla, California
YVES GALLET
Institutd e Physiqued u G?obed e Paris
Paleopolesd eterminedf rom seamounmt agnetica nomaliesc onstitutet he major data sourcef or the
Pacific apparentp olar wander path, but relatively little is known about the processeso f remanence
acquisitioni n seamounts.S ince magnetica nomaliesr eflect both natural remanence( NRM) and the
inducedf ield, it is importantf irst to assessw hethert he NRM is likely to representa n original field
directiona nd secondt o constraint he magnitudeo f the inducedc omponent.T o this end, we present
paleomagneticd ata from an uplifted, subaeriallye xposeds ectiont hrougha seamounto n La Palma,
Canary Islands. The PlioceneS eamountS erieso f La Palma comprisesa >6 km sequenceo f alkalic
extrusivesa ndi ntrusivesw hichi ncludesa ll lithologiesli kely to be volumetricallyim portanitn seamounts.
The structuratli lt of the SeamounSt eriesa llowss eparationo f early thermalo r chemicalr emanencefr om
magnetizationc omponentasc quireda fter tilting (e.g., viscousr emanence).T he NRM providesa poor
indicationo f the originalm agnetizationd irection,a lthought he characteristimc agnetizationo f manyL a
Palma samplesis compatiblew ith the originalp retilt direction.H ydrothermaal lterationh as resultedin
the productiono f Ti-poorm agnetitea nd an increasingc ontributiono f hematitew ith increasingd egreeo f
alteration.M ore importantlyw, ell-definedm agnetizatiodni rectionsw hich deviatef rom any reasonable
geomagnetidci rectiona t La Palmac anb e attributedto hydrothermaall terationin a differentp olarityt han
prevalentd uringt he originalm agnetizationB. asedo n a comparisono f the magnitudeo f low-stability
componentosf magnetizatioann dl aboratorya cquisitioonf viscousre manencaen d previouse stimates
of the inducedm agnetizationw, e concludeth at viscousa nd inducedm agnetizationp robablya ccount
for 15-25% of the total magnetizationo f seamountsT. he resultingp aleopoleb ias is a functiono f the
polaritya nd paleolatitudeo f the seamounat nd rangesf rom 4ø to 16ø for Cretaceouss eamountisn the
Pacific.
INTRODUCTION plex anomaly patterns may be accommodatedb y arbitrarily'
subdividingt he seamounti nto regionso f uniform magnetiza-
Magnetic anomaliesa ssociatedw ith seamountsh ave long
tion [e.g., McNutt, 1986], the spatialc oherenceo f residuals
been recognized as a source of tectonic information, and
suggestst he standardl east squaresi nversioni s based on an
seamountp aleopolesh ave been particularlyu seful in r<on-
improper statisticalm odel [Parker, 1988]. The seminorm
structingt he history of the Pacific plate [e.g., Francheteau
minimizationt echniqueo f Parker et al. [1987] incorporates
et al., 1970] where traditional, land-basedp aleomagnetic
both uniforma nd nonuniformc omponentsp,r ovidingt he most
sampling is rarely possible. Although paleomagneticd ata
uniform model compatible with the observed anomaly and
from drillcores, sedimentd istributionp atterns,a nd skewness
thereby estimatingt he minimum degreeo f nonuniformityi n
of lineatedc rustala nomaliesp rovidec omplementaryte ctonic
the magnetizationo f the seamoundt emandedb y the data. The
information[ Gordona nd Cox, 1980], seamounpt olesr emain
lognormal distributiono f magnetizationi ntensitieso bserved
the principals ourcef or the Pacifica pparenpt olar wanderp ath
in seamountr ocks [e.g., Kono, 1980; Gee et al., 1989]
and, indeed, are sometimesh eld as the standardb y which
suggestst hat nonuniformityi n intensity is probably the rule
thesea dditionald ata arej udged [e.g., Gordon,1 990].
rathert hant he exception.I n recognitiono f this largei ntensity
The nonuniquenessi nherent in the determination of
variation,P arker [1991] developeda n ideal body modeling
seamount paleopoles requires that some assumptionsb e
t(cid:127)hnique for seamountm agnetizationw hich constrainst he
madec oncerningth e magnetizations ources(s eeP arker et al.
solutiont o have uniform directionw hile allowing intensityt o
[ 1987] for a review). Early modelsu tilized the assumptiono f
vary. Directional informationc an still be recoveredd espite
uniform magnetizationt o determinet he best fit magnetization
relaxation of the intensity constraints,b ut the location of
in a least squaress ense [Vaquier, 1962]. Although com-
seamountp aleopolesi s much more poorly constrainedt han
commonly assumed.
Although these recent models have made significant
(cid:127)Now atL amont-DoheErtayr thO bservatoPrya,l isadeNse, wY ork.
progressi n addressingt he nonuniformityo f seamountm ag-
2Nowa t Vrije UniversiteAitm sterdamFa, culteidt erA ardweten-
schappen. netic sources,b ias introducedb y viscousa nd inducedc ontri-
3Alsoa t FortH oofddijPk aleomagneLtiacb ,U trecht. butionsi s difficult to incorporatein the models.C omparison
of modeling results from normal and reversely magnetized
seamounts[H ildebranda nd Staudigel,1 986; Bryan and Gor-
Copyright1 993 by the AmericanG eophysicaUl nion.
don, 1991] suggestsa combinedc ontributiono f up to 25%
Papern umber9 3JB00932. for induced and viscous components. A similar bias from
0148-0227/93/93 JB-00932505.00. induced magnetization( ~15% of total magnetization)h as
11,743
11,744 GEEE TA L.:M AGNErlZATIOONF L A PALMAS EAMOUNSTE RIES
beens uggestefdo r large seamountws ith a significanpt ortion layereds equenceo f entirely submarinee xtrusivesa nd volcani-
of intrusive material based on results from the La Palma clastics,t hree generationso f alkalic dikes/sills,a nd a basal
SeamountS eries [Gee et al., 1989]. In addition,t he presence plutonisce quenc(Fei gurelb ). Thes ubmarineex trusivsee ries
of multiplep olaritiesis often difficultt o assessfr om models. is consistentlbye ddedw ith a dip of 48o alonga n azimuth
For example,t he ideal body solutionf or JasperS eamount of 230o [Staudigel,1 981], which representsa tilt roughly
doesn ot requirem ixed polarity [Parker, 1991] despitet he radial with respectt o the presentc enter of intrusivea ctivity
demonstrablylo ng duration( >7 nay.) of volcanismw hich in the southeasternan d centralp ortionso f the caldera. The
stronglys upportsth e presenceo f both normala nd reversed extrusives eriesc onsistsp redominantloyf pillow lavas( 50%),
polarityr ocks [Pringlee t al., 1991]. Finally, the effects pillow fragmentb reccias,s ubmarines coriaceousb recciasa nd
of' progressivhe ydrothermaal lteration( perhapsin different hyaloclastite(ste rminologyo f Fishera nd Schminck[e1 984])
polarity intervals)o n the original magnetizationa re poorly and ranges in compositionf rom alkalic basalt to mugearite
understood. and rare trachyte. Three generationso f dikes and sills are
In this paper we discusst he origin of the magnetization observedin the basemenct omplex[ Staudigeal nd Schmincke,
in an uplifted, tilted sectiont hrough a seamounte xposed 1984; Staudigele t al., 1986]. The earliest generationo f
on La Palma, Canary Islands. The La Palma Seamount dikes (Group I) is orienteda pproximatelyo rthogonalt o the
Seriesp rovidesa uniqueo pportunityt o studyt he magnetic bedding of the layered extrusive series. On the basis of
propertieasn d origino f the magnetizatioinn seamountsIn. their orientation and the similarity in both compositiona nd
contrastt o dredginga nd drilling, which sample only the alteration history to the extrusive series, these dikes are
outermostp art of the volcanic edifice, the exposureso f interpreteda s feedersf or the extrusives eries.S ills (Group1 1)
the SeamountS eries provide about 4 km of stratigraphic occur within the layered extrusives eries,p arallel to bedding,
relief and include a wide range of extrusivesa nd intrusives and also form a 2 km sheeted sill swarm which underlies the
which encompassa ll lithologiesl ikely to be importanti n extrusives equence. Crosscuttingr elationshipss uggestt hat
seamounts. Age constraintsfo r the SeamountS eries and mosto f the sillsa rey oungerth ant he GroupI dikes[ Staudigel
the overlyings ubaeriafl lows allow generalc orrelationto the et al., 1986]. GroupI II dikes generallyh ave steepd ips, may
geomagneticre versalt ime scale and facilitatee valuationo f crosscutt he overlying Cobefta lavas and are typically less
the role of multiple polarities in generatingi nhomogeneous alteredt han the countryr ock. These dikes are interpreteda s
magnetizationT.h e tiltingo f the seamounste quenc(e~ 50ø ) memberso f a radial dike swarm feeding the Cobeftal avas.
allows separationo f pretilt thermalo r chemicalc omponents Isotopica ged atings uggestasn ager angeo f 0.51 to 0.80 Ma
from later viscous, chemical and thermal magnetizations for the radial dikesi n the Barrancod e las Angustias[S taudi-
acquireda fter tilting. While the magnitudeo f tilting in La gel et al., 1986]. The basalp lutonicc omplexi s exposedin
Palma is probably atypical, the effect is analogoust o the the Calderad e Taburientep rimarily as numerouss, mall (10s
latitudinal translation common in Pacific seamounts. Results to 100s of m) gabbro,l eucogabbroa nd ultramafici ntrusions,
from La Palma indicate that most assumptionsm ade in some of which are probably associatedw ith the subaerial
modelings eamounta nomaliesm ay be questionableb, ut our Cobeftal avas. Small leucocraticg abbroi ntrusionsa lsoo ccur
data alsop rovidea n estimateo f the bias inherenti n seamount within the extrusivep ortiono f the sequenceS. taudigeal nd
paleopolesw hichm ay ultimatelyc ontributeto a morea ccurate Schmincke [1984] refer to the extrusives, feeder dikes and
Pacific apparentp olar wanderp ath. sills as the SeamountS eries. For simplicity, and because
such rocks are expectedt o occur in seamountsa s well, we
GEOLOGY OF THE LA PnLMA SEAMOUNT SE(cid:127)ms
will also use the term to include the alkalic gabbroso f the
La Palma is the northwesternmosits land of the Canary basalp lutonicc omplex.
island chain and is locatedo n Jurassico ceanicc rust [Hayes The SeamountS eries has been subjectedt o hydrothermal
and Rabinowitz, 1975]. Schmincke [1976] has summarized alteration[ Hernandez-Pachechoa nd Fernandez-Santin,1 975;
the geology of the Canary Islands and a recent review Staudigel, 1981]. The upper part of the sequencei s
of the geologyo f La Palma is given by Staudigeal nd characterizedb y low-temperaturea ssemblageso f smectite,
Schmincke[ 1984]. The oldest rocks exposedo n La Palma analcime,c arbonatea nd low-temperaturez eolites. Alteration
are biostratigraphicaldlya ted at 3-4 Ma [Staudigel,1 981] increasesb, oth in pervasivenesasn d extent, with stratigraphic
and volcanisme xtendst o historict imes [e.g., Middlemost, depth. Interlayeredc hlorite/smectitegsi ve way to epidote
1972]. The island consistso f two major structural units. and (locally) actinolite( Figure 1). The upper stabilityl imit
The Pleistocene subaerial lavas of the Coberta Series cover of analcime correspondst o a temperatureo f approximately
more than 90% of the island and unconformablyo verlie the 200ø C [Liou, 1971]. The first appearanceo f epidote
Pliocenes ubmarinea nd plutonicb asemenct omplex[ Afonso, in hydrothermal systems occurs at temperatures greater
1974]. The oldest lavas of the Coberta Series are dated than 200o to 250øC and actinolite denotes temperatures
at 1.61 Ma [Abdel-Moneme t al. 1972] (recalculatedu sing in excess of ~300øC [Bird et al., 1984]. The silicate
the decay constantso f Steigera nd J(cid:127)iger [1977]). The assemblagesth erefore suggesta fossil geothermalg radient
basementc omplexi s a >6 km sectiono f alkalic extrusives of approximatel2y0 0øC/km. The alterationa ssemblages
and intrusivese xposedw ithin the Calderad e Taburientea nd are similar to those found in upwelling regions of present
the steep-walledc anyon( Barrancod e las Angustias)w hich day hydrothermals ystemsi n basaltic lavas (e.g., Iceland
drains the caldera (Figure la inset). Erosion rates in the [Palmassone t al., 1979] and Reunion Island [Demangee t
canyon are high (1-2 mm/yr as determinedb y erosiono f al., 1989]). The absenceo f similar alteration in the Cobefta
1973 drillholes), providing extremely fresh exposuresa long lavas and in recent pyroclasticd epositso n the flanks of the
the canyon bottom. canyon indicatest hat the metamorphismo f the extrusives,
The basementc omplex comprisest hree units: an approx- Group I dikes, and the sill complex was associatedw ith
imately 2500 m (~1800 m prior to inflationb y intrusives) seamountc onstructionL. ater localizedh ydrothermaal ctivity,
GEE El' AL: MAGNErlZATIONO F LA PALMA SEAMOUNTS ERIES 11,745
17.91øW 17.90 ø 17.89 ø 17.88 ø 17.87 ø
A
28.70 ø
N
835-844
0
846
847
0 0
28.69 ø 0
O0
0
865
792 pro-out
28.68 ø
'/
750
calderraix n
road
28.67 ø stream bed
subaerialla vas
0 1 km Fuencaliente(cid:127)/
met. isograd
B
SW NE
(cid:127) plutOffXcs
- 600 m
(cid:127) ...?. :' t%1%. ...
-400
'X/(cid:127)S :(cid:127)://':: % % ..:.:.: ::::::::::::::::::::::
..(cid:127) .. .... ::. .: % % % . (cid:127)s(cid:127);-
..(cid:127):.(cid:127):. :.:' ..;::(cid:127),::. (cid:127) % ..(cid:127):.:. [(cid:127):(cid:127):-- %
- 200
I i I i
0 2 4 6 km
Fig. 1. Paleomagnetsica mplelo cationsa nds chematiccr osss ectiono f La PalmaS eamounSt eries.( a) Samplelo cations
ands implifiedg eologys hownf or westernmoCsta lderad e Taburientaen dB arrancod e las Angustia(ss eei nset).S quares
indicated etaileds ill samplings itesa ndo nly sitesd iscusseidn the text are labeled.A n additiona1l 8 sitesa re widely
dispersewd ithint he calderato the northeasotf them appedre gion.P lutonicss, ill swarma nde xtrasiveasr ei ndicatedb y
hatchedp atterng, reya ndw hite,r espectivelyN.o tet hats outheastemrna rgino f canyonh asn otb eenm appedM. etamorphic
isogradms odifiedfr omS taudige[l1 981]b asedo n presenst amplec ollectionL. abels:a n, analcimep; ro,p umpellyitea;b ,
albite;e p, epidotea; ct, actinolite(. b) Simplifiedc rosss ectiona pproximateplya ralletl o the canyonb ottom.E xtrasives
are consistentlbye ddeda ndd ip ~50 ø to SW. Crosscuttinrge lationshiposf GroupI feederd ikes( solid),G roupI I sills
(grey), and GroupI II verticald ikes( white) and plutonicss howns chematically.
as evidencedb y sulfur-richh ydrothermalw aters,i s currently be locally quite steep. For example,s teep-sided(5 0-600)
presenti n the southeasterna nd centralp ortionso f the caldera. pillow walls (10-20 m high) have been describedf rom the
The tilt of the extrusive series plays a central role submergedp ortion of Kilauea's east rift zone [Fornari et
in determining the origin of the magnetization in the al., 1978] althought he slopeso f seamountasr e typically
Seamount Series. Although bedding is less obvious in <20o [e.g., Lonsdalea nd Batiza, 1980; Baronea ndR yan,
volcanic sequencest han in sediments, layering in fine- 1990]. The orientationo f geopetasl tructure(se .g., the single
grained hyaloclasfites,f lattened pillow bottoms overlying drainedp illow surfacei n Table 1) and the consistenbt edding
bedded hyaloclastites,a nd the upper surfaceso f partially- of fine-grained (sand-sized)h yaloclastitesin the Seamount
drainedp illowsy ield a consistenatt titudeo f 48o dip toward Series suggeststh at the presentb eddinga pproximatesth e
230o (Table 1). Constructionasll opeso f seamountms ay paleohorizontal.
11,746 GEE ET AL.: MAGNETIZATIOON F LA PALMA SEXMOO(cid:127) SERIES
TABLE 1. BeddingM easurementfso r the SeamounSt eries therefore yield four expected magnetizationd irections (in
presentd ay coordinates)f or comparisonw ith the observed
paleomagneticd ata. All magnetic data in this paper are in
Attitude
presentd ay coordinatesu nlesso therwisen oted. We refer
Dip/Dip Estimated Measurement Measurement
to the normal( 0ø , 48ø ) and reversedp olarity( 180ø , -48ø )
Direction Error & Type Location#
GAD directions as the expectedp osttilt directionsa nd the
44/228 + layeredh yaloclastite unit 2 correspondingG AD directionsc orrectedf or the tilt of the
55/235 +++ pillow bottom baseo f unit 5 SeamounSt eriesa st hee xpectedp retiltn ormal( 19ø , 10ø ) and
55/232 +++ pillow bottom baseo f unit 5 reversed(1 99ø , -10ø ) directionsW. e estimateth e uncertainty
58/245 ++ pillow bottom baseo f unit 5 of the pretilt directionsto be --,10ø owing to the possible
54/235 ++ layeredh yaloclastite top of unit 10 errors in the structural correction. This error is similar to
40/240 ++ hyaloclastitein unit 10 the dispersionin paleomagnetidci rections(- -,12ø ) expected
pillow interstices at La Palma based on the latitudinal dependenceo f virtual
41/232 +++ pillow bottom baseo f unit 11 geomagnetipco le dispersionfr om paleoseculavra riation[ Cox,
45/228 +++ pillow bottom baseo f unit 11 1970; McFaddena nd McElhinny, 1984].
45/225 +++ pillow bottom baseo f unit 11
45/238 +++ pillow bottom baseo f unit 11 SAMPLE COLLECTION
25/225' + layeringi n pillow unit 12
Over 1200 oriented cores (-,,200 paleomagnetics ites)
stringerb reccia
were collectedf rom the La Palma SeamountS eriesd uring
35/190' + layeringi n pillow unit 12
two samplingt rips in 1985 and 1988 (Table 2). Standard
stringerb reccia
paleomagneticc oresw ere drilled and orientedw ith a magnetic
50/220 +++ pillow bottom baseo f unit 12
compassS. omec oresw ere alsoo rientedw ith a sunc ompass.
50/240 +++ pillow bottom baseo f unit 12
Comparisono f the two methods indicated no bias in the
58/251' ++ pillow bottom baseo f unit 12
magneticc ompasso rientations,a nd the sun compassr eadings
53/259* ++ pillow bottom baseo f unit 12
indicatea magneticd eclinationn early identicalt o the IGRF
53/195' ++ hyaloclastite top of unit 13
declinationa t the site (-10ø). More than9 0% of the sitesw ere
50/225 ++ layeringi n pillow baseo f unit 13
fragmentb reccia located in the layered extrusive series and sill/dike swarm
43/206* ++ pillow bottom baseo f unit 14 (Figure 1a ). Approximately9 0 sitesw ere drilled in dikes/sills,
58/219' ++ pillow bottom baseo f unit 14 includingt hree localitiesw ith 10-15 dikes/sillse ach. Half of
52/224 ++ layeredh yaloclastite top of unit 15 thesew ere identifieda s Group II sills, primarilyo n the basis
48/220 +++ pillow bottom baseo f unit 15 of their attitude( meand ip/directiono f dip 54ø/223ø),w ith
48/222 +++ pillowb ottom baseo f unit1 5 equaln umberso f samplesf rom GroupI and GroupI II dikes.
40/225 +++ partially drained unit 17 More than 60 pillow sites span the range of compositions
pillow and alteration temperaturesf ound in the SeamountS eries.
48/246 ++ pillow bottom baseo r'unit 19 The volcaniclasticu nits sampled (25 sites) were primarily
48/245 +++ layeredh yaloclastite unit 20 pillow fragmentb reccias,l apillistonesa nd scoriaceoupsi llow
50/244 +++ layeredh yaloclastite unit 20 breccias. Gabbrosw ithin the extrusives eries and gabbro
45/215 + - water channel screensw ithin the sill complexw ere sampledw herep ossible.
48/220 + - water channel The remaining1 0% of the sitess ampledg abbrosa nd dikes
45/222 + hyaloclastitein - at 18 widely spaceds ites within the caldera (not shown in
ankaramiticp illows Figure la).
48/230 weightedm ean
USE OF THE SUM OF VECrOR
* Measurementsf rom this study, remaining data from DIFFERENCE MAGNITUDES
Staudigel[ 1981] .
The SeamountS eries samplese xhibit both normal and
# With the exceptiono f the last threed ata,a ll measurements
reversedp olarity characteristicm agnetizationsa nd most sam-
are from the base of the Barrancod e las Angustias. Unit
ples have a low-stabilityc omponenot f normalp olarity,w hich
numbersfr om Staudigeal nd Schminck[e1 984].
often significantlyr educes the initial remanent intensity of
& Qualitativee rrore stimate+: ++(5ø),+ +(5-10ø), +(> 10ø).
reversedp olarity samples( Figure 2a). As a result, param-
eters such as blocking temperaturea nd median destructive
We have usedt he observedl ayeringo f the extrusives eries field, which are basedo n the naturalr einanentm agnetization
as the basisf or our structuralc orrection.P aleomagneticd ata (NRM), are biassedb y polarity. To amelioratet his problem
from the least alteredp illow lavas in the SeamountS eries we have used the vector difference sum (VDS) as a basis for
provide independents upport for this structural correction. calculatingm agneticp arameterss ucha sb lockingt emperature.
The mean characteristicm agnetization direction obtained The sum of the vectord ifferencem agnitudesb etweens ucces-
from theset en sites( 1.60, 50.3ø;t ilt correctedi)s statistically sive demagnetizations tepsg ives an estimateo f the sample's
indistinguishablefr om the expectedg eocentrica xial dipole original NRM, assumingt hat the secondaryc omponenits
(GAD) directiona t La Palma (0ø , 48ø). While original acquiredb y reorientationo f momentsi n existingg rainsr ather
paleoslopesc annot be excluded, reasonables lopes (up to than net addition of magneticm aterial. This is in effect the
10ø ) would have only a minor effect on our structural sum of each componenta lignedt o a commond irection.T he
correction,a ltering the azimuth of the rotational axis by a VDS is, of course, also artificially increasedb y any noise
maximumo f 150 with little changei n the dip. The GAD in the directional measurementa nd artificially reducedi f an
direction at La Palma and the tilt of the Seamount Series insufficient number of demagnetizations teps were made to
GEE ET AL: MAGNETIZATIOON F LA PALMA SEAMOUNTS ERIES 11,747
TABLE 2. Summaryo f MagneticP ropertieso f La PalmaL ithologies
Number Number Low-stability Characteristic
of of NRM, Koenigsberger Component, MDF, Tb, Remanence
Lithology Specimens Sites A/m Ratio, Q & % VDS mT øC Directions
Gabbro s 229 16 7.28 3.05 14.9 18.1 605
In extrusives/sills 65 5 8.55 2.66 22.4 21.6 610 pretilt
In caldera 164 11 6.78 3.21 12.1 17.2 605 posttilt
Dikes/Sills 799 88 2.46 1.88 35.5 20.6 595
GroupI (feeder) 176 22 2.14 1.79 40.3 14.1 585 pretilt
GroupI I (sills) 372 44 2.47 1.72 29.6 26.0 605 pretiltt o posttilt
GroupI ll (vertical) 251 22 2.67 2.19 44.6 16.4 575 posttilt
Pillow lavas # 731 66 3.87 5.29 21.7 34.0 615
No epidote 130 13 8.43 11.58 25.4 19.1 595 pretilt
Ep-pm 227 18 4.87 5.62 16.9 39.6 625 pretilt
Pm-act 330 29 1.89 3.00 19.7 34.0 620 pretilt
>Actinolite 44 6 0.22 2.22 40.3 54.9 635 pretilt
Volcaniclastics 376 25 3.11 5.45 15.0 26.1 605
Pillow fragments 240 25 4.38 6.19 13.5 23.5 605 random
Hyaloclastit@e 136 36 0.86 4.15 17.1 30.5 600 pretilt
Subaerial lavas 86 12.12 18.69 - - -
Collectiono f fully orienteds amplesfo rms subseto f data reportedb y Gee eta/. [1989]. Arithmeticm eansa re used
throughout.N RM is naturalr emanentm agnetization.L ow-stabilityc omponentm, ediand estructivefi eld (MDF), and
maximumb locking temperature( Tb) all basedo n vectord ifferences um (VDS) as discussedin text.
& Q is the ratio of the remanentto inducedm agnetization.
# Subdivisionbsa sedo n metamorphiics ograds(e p, epidotei n; pm, pumpellyiteo ut; act, actinolitein ) in Figure 1.
Actinoliteg roupi ncludesr esultsf rom keratophyres.
@ Includesin terstitiahl yaloclastitme ateriailn pillowu nitsa ndp illow fragmenbt recciasa s well asb eddedla pillistones.
A w, Up
400 B
2.5
500øC
2.0
1.5
300
620
S / N 0.5
200
2(cid:127) 4(cid:127) 6(cid:127)
c
E,D own I 2(cid:127) ' 460 ' 660
Temperature(ø C)
Fig. 2. Illustratioonf theu seo f thev ectodr ifferencseu m( VDS).( a) Vectoer ndpoindti agramo f thermadl emagnetization
of a dike samplefr om La Palma. The dike hasa reversedp olarityc haracteristmica gnetizatio(n5 00ø-620øC)a nd a
significannto rmalp olarityo verprin(t1 00ø-300øC)T. he low-temperatucreo mponenist estimatedas 65% of the VDS
usingt he techniqude escribeidn the text. Solid( open)c irclesa re projectionosf the vectoro ntot he horizonta(lv ertical)
plane.( b) Thermadl ecayo f the NRM (solidc irclesa) ndV DS (openc ircles).N otet he significandti fferencbe etween
the originala ndr econstructeredm anenint tensities(.c ) Histogramo f blockingte mperaturedse rivedf romt he VDS. The
componenret mainingab ove6 00øC( solid)i s useda s an estimatoe f the importancoef hematitew hent hisc omponent
exceeds 5% of the VDS.
11,748 GEE ET AL: MAGNETIZATIONO F LA PALMA SEAMOUNTS ERIES
clearly delineatet he full complexity of the remanencev ectors. have been previously reported [Gee et al., 1989]. Two
The effect of noise is small in most casesa nd is partially polaritiesa re evidenti n the NRM data and thesed atac an be
offsetb y the incompletes eparationo f the two componentss o arbitrarilys eparateda long the E-W vertical plane and their
that the VDS probablyp rovidesa reasonablee stimateo f the distributionsc ontouredo n an equal area projection( Figure
originalN RM. 3). The northernh emisphere(n ormalp olarity)d irectionsa re
The vector differencem ethod also allows a quantitative skewed toward the expectedp retilt normal direction,b ut the
estimateo f the magnitudeo f the variousc omponentsI.n the densitym aximum( 11.2%) is locatedn ear the posttiltn ormal
examplei n Figure2 , the low-stabilityc omponen(t3 59.4ø , GAD direction( Figure 3a). The reversedd irections( Figure
44.6ø ) is well definedb etween1 00ø and 300øC and the char- 3b) have a maximum which lies between the pretilt and
acteristicm agnetizatio(n1 70.0o , -30.5o ) is isolatedb etween posttilt reversedd irections. A secondarym aximum occurs
500o and 620øC. Using the principalc omponendt irections slightly to the east of the reversedG AD directiona nd can
for these well-defined segments,t he vector differencesi n the largelyb e attributedto samplesfr om gabbroicin trusionsT. he
intervalf rom 300o to 500øC can be expresseads a linear expectedp retilt directionsa re based on the bedding attitude
combinationo f the two known directionsi n a least squares observeidn the layeredp ortiono f the SeamounSte riesa nd
sense[ Presse t al., 1986, p. 59]. A similar procedurefo r thuss trictlya pplyo nly to the layerede xtrusives eries,G roup
estimatingt he magnitudeo f componentsf rom demagnetiza- I dikes and the sills. Nonetheless, deviations from these
tion data was suggestedb y Zijderveld[ 1967]. We will show expectedd irectionsa re a generali ndicationo f the severity
later that the low-stability componentp ostdatesti lting of the of secondarym agnetizationin the sequence. Clearly, the
SeamounSt eriesa ndc anr easonablyb e attributedto a viscous original pretilt directionsf or the SeamountS eriesa re not well
remanence(V RM) acquireda t low to moderatet emperature. representebdy the NRM directionsa s assumedin modeling
Sincet emperatureis muchm ore efficientt hana lternatingfi eld of seamount anomalies.
treatmentf or removingV RM [Dunlop, 1973], we have used The effect of hydrothermala lterationo n the NRM in-
thermald emagnetizatiocnu rvese xclusivelyfo r calculatingth e tensity is most clearly demonstratedin pillow lavas, where
percentageo f secondarym agnetization.F or this studye ach complexitiesd ue to differencesi n cooling rate and original
thermallyd emagnetizeds pecimenw as typically subjectedto magneticm ineralogys houldb e minimal (Figure 4). The
15-20 steps, sufficientf or the purposeo f calculatingt he fossil hydrothermals ystem exposedo n La Palma (Figure
VDS. 1) is similar to presentlya ctive basalt-hostedh ydrothermal
systemsin Iceland [Palmasson,1 979] and on ReunionI sland
RESULTS
[Dernangee t al., 1989] and recordsa geothermagl radient
Natural RemanentM agnetization
of ,-,200øC/km.S amplesfr om the least-alterepdo rtiono f
The magnetic anomaly of a seamountm easureda t the the sectionh aveN RM intensitiesc omparableto dredgeda nd
sea surface arises from the combined induced and remanent drilled extrusivesf rom seamounts[ e.g., Kono, 1980; Gee
magnetizationsw, ithout benefit of magneticc leaning. Thus, et al., 1988]. Progressiveh ydrothermaal lterationr esultsi n
the NRM intensitya nd directiona re the most pertinentd ata approximatelytw o orderso f magnituder eductioni n the NRM
for comparisonw ith anomalym odels. The NRM intensities intensityo ver the temperaturera nge (<2000 to >300øC)
and inducedm agnetizationo f the La PalmaS eamounSt eries indicatedb y the silicate assemblages.
A N B N
+
normal polarity mode
Fig. 3. Equal-areac ontourp lot of NRM directions(i n geographicco ordinates()a. ) Lower hemispherper ojectiono f the
northernh emispher(en ormal)d irectionsT. he maximumd ensity(1 1.2% ) is locatedn eart hep osttiltG AD (squarea) ndt he
presentf ield (triangle)d i-rectionsfo r La Palmar athert hann eart he expectedp retiltd irection( circle). (b) Upper hemisphere
projectiono f (cid:127)outhern hemisphere(r everaed)d irectionsT. he maximumd ensity( 7.2%) li(cid:127) betweent he GAD reversed
direction( square)a nd the expectedp, retilt reveraedd irection( circle). One percenti sodenistyc ontoursa re shown.T he
antipodeis plottedf or upperh emispherdei rectionsin Figure3 a and lower hemispherdei rectionsin Figure3 b.
GEE ET AL: MAONE'rrzATIONO F LA PALMA SEAMOUNT SERIES 11,749
10- a circular standard deviation of more than 20 ø have been
a excluded from consideration.
actinolite
The range of demagnetizationb ehavior in samplesf rom
n=44
La Palma is illustratedi n Figure 5. Approximately2 0% of
the samplesh ave univectorialb ehavior, as definedb y a low-
stabilityc omponen<t 10% of the VDS. Univectoriald irections
compatible with a posttilt magnetizationa re most common
in gabbroic intrusions( Figure 5a) and vertical Group lIl
I i I I I dikes (Figure 5b) althoughG roup II sills and a few extmsive
-2 -I 0 1 2
sites also have single component posttilt magnetizations.
b Directions compatiblew ith a pretilt magnetizationa re more
50- pmou -a t cint ';':r¾.-'--'(cid:127).-.(cid:127)(cid:127) typical for the extrusivep ortion of the SeamountS eries as
40- n= 331 r:-(cid:127)[l(cid:127)(cid:127)'(cid:127)'(cid:127)(cid:127)(cid:127)(cid:127):(cid:127) (cid:127).(cid:127)....-(cid:127) well as many of the Group I and II feederd ikes and sills.
Nearly univectoriapl retilt directionsa re particularlyc ommon
2t0-
in the pillow lavaso f the upperp ortiono f the SeamounSt eries
2t3- (Figure 5c). Low-stabilityc omponentso f variablem agnitude
10- characterizem any sitesa lthought he shallowp retilt inclination
can be isolated( Figures5 d and 5e). A leucocraticg abbro
I i I I
from within the extrusives eries( Figure 50 illustratesa more
-2 -1 0 1
complex demagnetizationb ehaviorb ut a pretilt magnetization
' 40- is also indicatedb y the final shallow inclination.
A significant number of samples from all lithologies
ep in- pm out
-½z 30 - n = 227 display characteristicm agnetizationd irectionsw hich deviate
(cid:127) _ from either the expectedp retilt or posttilt directions( Figure
o 20- 6). In many suchs amplesa, discrepancyis observedb etween
the results of AF and thermal demagnetization(c ompare
E 10
Figures6 a and 6b). Note that the north and up component
= -
in the thermallyd emagnetizesda mple(5 20ø -540ø C ) appears
I !
-2 -1 0 1 2 as the highestc oercivityc omponentin the AF sample. We
30- suggestt hat this inconsistencyr esults from the superposition
d
noep idote :(cid:127) of reverseda nd normal magnetizationsw hich are inadequately
separatedb y either technique.T he demagnetizatiodni agrams
20- n= 2 27 (cid:127)-'-.-J
of the pillow lavas in Figures 6c and 6d illustratet hat these
- anomalousd irections( NW, up and W, up) are well-defined.
The inflection in the inclination curve of LP865b2 at 200øC is
also typical of many samplesf rom the SeamountS eries.T he
_ , i.'. .......[.2....(cid:127):...:.. ......c.o.n.si.st.e.not rientationo f the low-stabilityc omponentp arallel
to the presentf ield directiona t La Palma (e.g., Figure 6d)
-2 -1 0 1 2 rules out sample misorientationa s an explanation of these
log NRM (A/in) anomalous directions.
Results from a volcaniclasticu nit (Figures 6e and 6f)
Fig. 4. NRM intensitiesfo r pillow lavasa s a functiono f alteration.
suggestt hat these aberrant directions may be the result of
Note the reductioni n intensityb y nearly two orderso f magnitude
alteration.T he samplei n Figure 6e is an amoeboidapl illow
overt he temperaturrea nge( <200 ø- >300øC) indicatedb y the silicate
assemblagesG. roupingsa re basedo n the isogradss howni n Figure which has a pretilt normal magnetizationc ompatiblew ith
1. Actinolites amplesin cludet wo keratophyres creens. the in situ high temperaturee mplacemenot f the amoeboidal
pillows in this unit [Staudigela nd Schmincke,1 984]. The
water-saturatedm atrix probably never reached temperatures
DemagnetizatioRn esults
> 100øCa nd may haveb eenm echanicalldyi sturbedd uring
Approximately1 000 specimensw ere subjectedto stepwise pillow emplacement.B oth AF and thermald emagnetization
thermal or alternatingf ield (AF) demagnetizationw, ith a of the interstitial hyaloclastitem aterial (Figure 60 yield a
minimum of three samplesf rom each site. The majority of northward and up components imilar to that noted in other
samplesw ere thermally demagnetized,a s this method often lithologies.T heser esultss uggestth att he highg lassc ontento f
providesm orer eliabler esultsi,n 50øC stepsf rom 100ø to the hyaloclastiteh asr esultedi n significanta lterationw hile the
500øC followedb y 20øC stepsu ntil the directionsb ecame amoeboidalf ragment maintainedi ts original magnetization,
randomo r the samplei ntensityf ell below 1% of the NRM albeit with a significantv iscouso verprint.T he origin of these
value. The uniformity of temperatures tepsw as maintained anomalousd irectionsw ill be discussedm ore fully below.
to facilitate comparison of blocking temperature spectra. Blockingt emperaturdea taf rom the demagnetizatiosntu dies
and maximum unblocking temperatures. Componentso f also yield a first order characterizationo f the magnetic
magnetizationw ere definedb y principalc omponenta nalysis mineralogy.T hermal demagnetizatioonf isothermarl einanent
[Kirschvink,1 980] wheneverp ossible. Specimensfo r which magnetization,p etrographico bservations,m icroprobed ata
no stablec haracteristicm agnetizationc ould be isolatedw ere and Curie temperatured eterminationsp rovide additional
analyzedu singt heg reatc irclem ethodd escribebdy McFadden constraintosn the magneticm ineralogyh; owevert, heb locking
and McElhinny[ 1988]. A smalln umbero f sites( < 10%) with temperatured ata are particularly useful since this data is
11,750 GEEE l'A L.:M AGNETIZATIOOFNL A PALMAS EAMOUNSTE RIES
B
A W Up
300 S (cid:127) ß- I
5.0 A/m
NRM
560øC
LP704g2
gabbro
580
/
N LP8061c
dike _x (cid:127)
NRM '
E, Down
E, Down
D W, Up
c
LP743al
WU, p pillow $(cid:127) I N
8.0 A/m
S
LP740b2 '(cid:127)x
NRM
-0 A.4/ mdi ke (cid:127) NRM
E, Down
E, Down
W, Up
E
NRM F LP855dl
gabbro W, Up
680
LP805fl -(cid:127)$
pillow
450 - 400oc
(cid:127) 580
6OO
s N
250
- 1.0 Aim
0.1 A/m
E, Down
E, Down
Fig. 5. Representatviveec toer ndpoindti agram(sg eographcioco rdinatefrso)m L a PalmaN. earlyu nivectoridael mag-
netizatioinn (a) reversepdo larityg abbroan d( b) normapl olarityd ike.B othd irectionasr ep osttilt(.c ) Normapl olarity
directionw hichp redatetsil ting. (d) Shallown, ormapl olarityp retiltc omponewnti th moderatleo w-stabilitcyo mpo-
nent.( e) Reversepdr etiltd irectiown iths ubstantniaol rmapl olarityo verprin(tf.) Compledxe magnetizatbioenh avioinr
leucogabbwroit hinth ee xtrusivsee riesS. olid( openc) irclesa rep rojectioonfs t hev ectoor ntot heh orizont(avl ertical)
plane.
universallya vailable. We define the maximumb locking Intrusivefsro mL a Palmaa red ominatebdy Tb near6 00øC
temperatur(eT b) as the temperaturset ep at which the (Table 2). Gabbrosh ave Tb rangingf rom 580ø-680øC,
remanenitn tensityd ropst o below2 % of the VDS intensity althought he majorityo f samplesh ave peak unblocking
or the directionb ecomesr andom. Peak unblockingm ay, and overa veryd iscretien terva(l5 40ø-600øCc)o mpatibwlei th
oftend oeso, ccura t a temperatu<reT b. A Tb of <600øC the presenceo f magnetite. Severalg abbrosh ave a small
is thereforec ompatiblwe ith the presencoef magnetitaes the percentagoef remanencreem aininagt 600øC( up to 18%),
remanenccea rryingp hase,w hereash igherv aluesa pparently but in only one case (Figure 50 is the high temperature
requireth ep resencoef a highetre mperatuprhea se(h ematite). directiond istincftr omt hed irectiona t temperatur<es6 00o C .
The justificationfo r, and limitationso f, our mineralogical Them ajority(7 4%)o f dikes/silhlsa veT b betwee5n0 0øand
interpretatioonf theT b dataa red ealtw ith in a laters ection. 6000C , buth igheTr b (upt o 640øC)o ccura nda rep articularly
GEE ET AL: MAGNETIZATIONO F LA PALMA SEAMOUNT $ERIE(cid:127) 11,751
A B
LP604al LP604a2
W, Up W, Up
gabbro gabbro
Iloo3 5 (cid:127) ..(cid:127)
s t (cid:127)5, (cid:127)-1(cid:127)
N s5 80/ _ N
540øC
-2 A /m 20(cid:127) 0' NRM
E, Down NRM
E, Down
W,U p LP847c2
pillow
C
D
350
W,U p LP865b2 350
pillow '
_ (cid:127)(cid:127)(cid:127)(cid:127)(cid:127)200øC
1.0A /m NRM 500øC
E, Down NRNI
660
S I N
0.3 A/m
E
W, Up
F LP792c 1
S
hyaloclastite
620 W, Up
40 2m05 T
200øC
NRM
100
0.3 A/m LP792kl (cid:127) (cid:127)l- - N
amoeboidal 0.1A/m
NRM
E, Down pillow
E, Down
Fig. 6. Vectore ndpoindt iagramisll ustratinagn omalourse manencdei rection(sg eographcico ordinatesN).o rthwarda nd
shallowup wardfi nald irectioinn (a)A F-demagnetizseadm plceo rrespontdos a ni ntermedia(t5e2 0ø-540øCc)o mponeinnt
thec orrespondinCgo )t hemally demagnetizesda mple.( c) Well-defineda nomalourse manencdei rectionw ith characteristic
inflectionin inclinatiopnr ojectioant ,,,200øC(. d) An extremee xampleo f an anomaloudsi rection(W , Up) with a low-
stabilityo verprinst imilart o the presendt ay fielda t La Palma. (e) Amoeboidapli llowf ragmenmt aintainas shallow,
pretiltd irectionw, herea(sf) thes urroundinhgya loclasfisthe owasn omalounso rtha ndu p directionsS.o lid( openc) ircles
arep rojectionosf the vectoro ntot he horizonta(lv ertical)p lane.
commoni n sills. In contrastt o gabbros amplesd, ikes/sills in accoraw ith a moreT i-richt itanomagnetiatelt hougha high
often have a distributedb lockingt emperatures pectrumw hich blocking temperaturep hase is always present. Both Tb
includess ignificantu nblockinga t temperatureos f 250ø - and the mediand estructivefi eld increasew ith the degreeo f
400øC. alteration( Table 2). Pillow samplesf rom the more altered
Perhapst he most unexpectedr esult from the study of portiono f the SeamounSt eriesm ay havea significanatm ount
the SeamountS eries is the uniformly high Tb (500o - of remanencree maininga t 600øC( as high as 32%) andT b
680øC)i n pillow lavas,i n view of the Ti-richc omposition which extendt o near the N6el temperaturoef hematite( e.g.,
of primary titanomagnetitien rapidly-cooledb, asic igneous Figure 6d).
rocks (Fe3-zTizO4, with 0.50 < z < 0.85 [Carmichael,
Low-StabilityC omponenatn d lhscousR emanence
1967; Carmichaela nd Nicholls, 1967; Haggerty, 1976]).
Although only one third of the samplesi s completely The majority of samplesf rom La Palma have a low-
demagnetizebdy 600øC, the majorityo f samplese xhibit stabilitym agnetizationc omponenot f normal polarity, the
peaku nblockingb etween5 00o and6 00øC,c onsistenwt ith a magnitudeo f which (as a percentageo f the VDS) varies
remanenmt agnetizatiocna rriedp rimarilyb y low-Ti magnetite. considerably(F igure 7). Becauset he magnitudeo f the
Somes ampleasl soh aves ignificanutn blockinagt temperatures low-stabilityc omponenits inverselyc orrelatedw ith the VDS,
11,752 GEE ET AL: lVIAGNE(cid:127)IZATIONO F LA PALMA SEAMOUNTS ERIES
A B
ß .
./.. ....G.....a....b.....b. ros
I I I I I I I I I I I I
0 20 40 60 80 0 20 40 60 80 100
C D 25-
(cid:127) 20-
ß ' ' 20 Volcaniclastics
(cid:127) (cid:127) 15 -
15-
c(cid:127) 10
10-
5-
0 20 40 60 $0 100 0 20 40 60 80
Low-StabilityC omponen(t% ) Low-StabilityC omponen(t% )
Fig. 7. Histogramosf the percentagoef the low-stabilitcyo mponenotf magnetizatiofonr differenlti thologiesT. he
magnitudoef the low-stabilityc omponenist inverselyre laledt o the vectord ifferences um( VDS). The magnitudoef
the low-stabilityc omponenwt eightedb y the VDS providesth e beste stimateo f the low-stabilityc ontributiofno r each
lithology (Table 2).
a weightedm ean was calculatedt o providet he best estimate at 70 mT immediatelyp rior to the starto f the experiment
of the low-stabilityc ontributionf or each lithology (Table to minimize thesee ffects. The remainings amplesw ere in:
2). Moderate low-stabilityc omponentsa re commoni n an undemagnetizedst ate and provide a comparisonb etween
severall eucogabbrosa,l thought he majorityo f gabbrosh ave NRM and AF-demagnetizesdp ecimenfsr om the samec ore.
relatively minor secondaryo verprints. The most substantial Althoughs omes amplese xhibitn earlyl inearV RM acquisi-
secondarym agnetizations( 36%) are in dikes/sills from La tion with respectt o log (time), the majoritys howa n increase
Palma. Pillow lavas typicallyh ave lower percentageosf after approximately1 00 hours (Figure 9a). Such nonlinear
secondarym agnetizationt han do the dikes and sills; however, behaviorh as been attributedt o inhomogeneitiyn the grain
larger low-stabilityc omponentasr e presenti n somep illow
lavas,p articularlyth osef rom the moreh ighlya lteredp ortion N
of thes equenc(eT able2 ). Pillowf ragmentasn dh yaloclastites
alsoh ave small secondarym agnetizations.
Principacl omponendt irections(m aximuma ngulard evia-
tion <20ø ) for thel ow-stabilitcyo mponenotf magnetization
indicateth att hisc omponenwt asa cquireda ftert ilting( Figure
8). Apart from a smalln umbero f reversedd irections(~ 20),
which are not shown in the lower hemispherep rojection
in Figure 8, the distributioni s compatiblew ith a recent
magnetization. The more than 600 directionsi n the normal
modey ield a Fisherianm eano f 352.3ø , 46.8ø (c(cid:127)95 = 1.8ø )
which lies betweent he posttiltn ormalG AD directiona nd the
presenfti eld directiona t La Palma( 351ø,40ø).T his direction
is compatiblew ith a VRM of relativelyr ecento rigin, an
interpretatiogne nerallys upportebdy resultsfr om a laboratory
viscousa cquisitione xperiment.
The potentialc ontributiono f viscousr emanencein the La
Palma samplesw as evaluatedt hrougha VRM acquisition
experimenitn which 80 samplesw ere placedi n a vertical0 .1
mT (4- 3%) field for 5600 hours( ~8 months).T he samples
werep eriodicallyre movedfr om thef ield, measured(t ypically
less than one minute), and returned to the field. Low-
field storagea nd prior demagnetizationa re known to affect
Fig. 8. Lowerh emispheerqe uaal reap rojectioonf thel ow-stability
VRM acquisitionin multidomains amples[ e.g., 7iveya nd
componenfrto mL a Palmas amplesT. wo percenits odensitcyo ntours
Johnson1, 984]. Thereforet,h e previouslyA F-demagnetized
are shown. The GAD and presentf ield directionsa t La Palma are
samples( 61) were again subjectedto AF demagnetization indicatedb y the squarea ndt riangler,e spectively.