campo geomagnetco - johnson et al

8/11/2019 Campo Geomagnetco - Johnson Et Al.

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Recent investigations of the 05 Ma geomagnetic fieldrecorded by lava flows

C. L. JohnsonDepartment of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC,Canada V6T 1Z4 ([email protected])

C. G. Constable and L. Tauxe

Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, USA ([email protected];[email protected])

R. BarendregtDepartment of Geography, University of Lethbridge, 4401 University Drive, Lethbridge, Alberta, Canada TIK 3M4([email protected])

L. L. BrownDepartment of Geosciences, 611 North Pleasant Street, 233 Morrill Science, Center University of Massachusetts,Amherst, MA 01003-9297, USA ([email protected])

R. S. CoeEarth and Planetary Sciences Department, University of California, Santa Cruz, CA 95064, USA ([email protected])

P. LayerGeophysical Institute, 903 Koyukuk Drive, University of Alaska, Fairbanks, AK 99775-7320, USA([email protected])

V. MejiaUniversidad Nacional de Colombia, Manizales, Colombia ([email protected])

N. D. OpdykeDepartment of Geological Sciences, University of Florida, 241 Williamson Hall, P.O. Box 112120, Gainesville,FL 32611 USA ([email protected])

B. S. SingerDepartment of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street, Madison,WI 53706, USA ([email protected])

H. StaudigelScripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, USA ([email protected])

D. B. StoneGeophysical Institute, 903 Koyukuk Drive, University of Alaska, Fairbanks, AK 99775-7320, USA([email protected])

G3

G3Geochemistry

GeophysicsGeosystems

Published by AGU and the Geochemical Society

AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES

GeochemistryGeophysicsGeosystems

Article

Vo lu me 9, Number 4

2 3 A p ri l 2 0 08

Q0 4 0 3 2 , d o i:1 0 .10 2 9 /2 0 07 GC0 0 1 69 6

I SSN: 1 5 2 5 - 20 2 7

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[1] We p r es e nt a s y nt h es i s o f 0 5 M a p a le o ma g ne t ic d i re c ti o na l d a ta c o ll e ct e d f r om 1 7 d i ff e re n t l o ca t io n su n d er t h e c o l la b o r at i v e T i me Av e r a ge d g e o ma g n et i c F i e l d I n i t i a t i ve ( TA F I ) . W h e n c o m b i n e d w i t h r e g i on a lc o m pi l a t io n s f r o m t h e n o r t hw e s t U n i t e d S t a t es , t h e s o u t h w e s t U n i t e d S t a t e s , J a p a n, N e w Z e a la n d , H a w a i i ,M e x i c o , S o u t h P a c i f i c , a n d t h e I n d i a n O c e a n , a d a t a s e t o f o v e r 2 0 0 0 s i t e s w i t h h i g h q u a l i t y , s t a b l e p o l a r i t y ,

a n d d e cl i na t io n a n d i n cl i na t io n m e as u re m en t s i s o b ta i ne d . T h is i s a m o re t h an s e ve n fo l d i n cr e as e o v er s i mi l ar q u al i ty d a ta i n t h e e xi s ti n g P a le o se c ul a r Va r ia t io n o f R e ce n t L a va s ( P SV R L ) d a ta s e t, a nd h a sg r ea t ly i m pr o ve d s p at i al s a mp l in g . T h e n e w d at a s e t s p an s 7 8S t o 5 3N, and has sufficient temporal ands p at i al s a mp l in g t o a l lo w c h ar a ct e ri z at i on o f l a ti t ud i na l v ar i at i on s i n t h e t i me - av e ra g ed f i el d ( TA F ) a n dpaleosecular variation (PSV) for the Brunhes and Matuyama chrons, and for the 0 5 Ma intervalc om bi ne d. T he B ru nh es a nd M at uy am a c hr on s e xh ib it d if fe re nt TA F g eo me tr ie s, n ot ab ly s ma ll er d e pa r tu r es f r om a g e oc e nt r ic a x ia l d i po l e f i el d d u ri n g t h e B r un h es , c o ns i st e nt w i th h i gh e r d i po l e s t re n gt ho b se r ve d f r om p a le o in t en s it y d a ta . G e og r ap h ic a l v a ri a ti o ns i n P S V a r e a l so d i ff e re n t f o r t h e B r un h es a n dM a tu y am a . G i v e n t h e h i g h q u al i ty o f o u r d a t a s e t, p o la r it y a s ym m et r ie s i n P S V a nd t h e T AF c a nn o t b ea t tr i bu t ed t o v i sc o us o v er p ri n ts , b u t s u gg e st d i ff e re nt u n de r ly i ng f i el d b e ha v io r, p e rh a ps r e la t ed t o t h ei n f l ue n c e o f l o n g -l i v ed c o r e- m a n tl e b o u nd a r y c o n d i t i on s o n c o r e f l o w . P S V, a s m e a s ur e d b y d i s p er s i o n o f v i r t u a l g e o m a g n e t i c p o l e s , s h o w s l e s s l a t i t u d i n a l v a r i a t i o n t h a n p r e d i c t e d b y c u r r e n t s t a t i s t i c a l P S V m o d e l s ,o r b y p r ev i ou s d a ta s e ts . I n p a rt i cu l ar , t h e B r u n he s d at a r e po r te d h e re a r e c o m pa t ib l e w i t h a w i de r a ng e o f

m o d e l s , f r o m t h o s e t h a t p r e d i c t c o n s t a n t d i s p e r s i o n a s a f u n c t i o n o f l a t i t u d e t o t h o s e t h a t p r e d i c t a n i n c r e a s ei n d i sp er s i on w i th l a ti t ud e . D i sc r im i na t in g a m on g s u ch m o de l s c o ul d b e h e lp ed b y i n cr e as e d n u mb e rs o f l o w- l at i tu d e d a ta a n d n e w h i gh n o rt h er n l a ti t ud e s i te s . Te s ts w i th o t he r d a ta s e ts , a n d w i th s i mu l at i on s ,i n di c at e t h at s o me o f t h e l a ti t ud i na l s i gn a tu r e p r ev i ou s ly o b se r ve d i n V G P d i sp e rs i on c an b e a t tr i bu t ed t ot h e i n c l u s i on o f l o w - q u a l it y, i n s u f fi c i en t l y c l e a n ed d a t a w i t h t o o f e w s a m p l e s p e r s i t e . O u r M a t u ya m a d a t as h ow a s t r on g er d e pe n de n ce o f d i sp e rs i on o n l a ti t ud e t h an t h e B r un h es d a ta . T h e TA F i s e x am i ne d u s in gt h e v a ri a ti o n o f i n cl i na t io n a n om al y w i th l a ti t ud e . B e st f i t t w o- p ar a me t er m o de l s h a ve a x ia l q u ad r up o lec o n t r i b u t i o n s o f 2 4 % o f t h e a x i a l d i p o l e t e r m , a n d a x i a l o c t u p o l e c o n t r i b u t i o n s o f 1 5 % . A p p r o x i m a t e l y2 % o f t h e o c t u po l e s i gn a tu r e i s l i ke l y t h e r e su l t o f b i as i n cu r re d b y a v er a gi n g u n it v e ct o rs .

Components: 1 3, 9 38 w o r ds , 1 4 f i g ur e s, 1 3 t a b le s.

Keywords: p a l e om a g ne t i c ; t i m e -a v e r ag e d f i e l d; p a l e os e c ul a r v a r ia t i o n; l a v a s.

Index Terms: 1 5 2 2 G e om a gn e ti s m a n d P a le o ma g ne t is m : P a le o ma g ne t ic s e cu l ar v a ri a ti o n; 1 5 45 G e om a gn e ti s m a n dP a l e om a g ne t i s m: S p a t ia l v a r ia t i o ns : a l l h a r mo n i cs a n d a n o ma l i e s; 1 5 9 9 G e o m a g ne t i s m a n d P a l e om a g ne t i s m: G e n e ra l o r

miscellan eo u s.

Received 1 8 M a y 2 0 07 ;Revised 8 A u g us t 2 0 0 7; Accepted 2 2 O c t ob e r 2 0 07 ;Published 2 3 A p r i l 2 0 08 .

J o hn s on , C . L . , e t a l . ( 2 00 8) , R e ce n t i n ve s ti g at i on s o f t h e 0 5 M a g e om a gn e ti c f i el d r e co r de d b y l a va f l ow s ,Geochem.

Geophys. Geosyst., 9, Q0 4 0 3 2 , d o i:1 0 .1 0 29 /2 0 0 7 GC0 0 1 6 9 6 .

1. Introduction

[ 2] T he s tr uc tu re a nd e vo lu ti on o f t he m ag ne ti cf ie ld i s k ey t o u nd er s ta nd in g t he g eo dy na mo a ndE a rt h s d ee p i n te r io r, b u t m an y a s pe c ts o f i t r e ma i npoorly documented particularly on timescales of104 t o 1 06 y e a r s . W h i l e p a l e o m a g n e t i s t s h a v e m a d ee xt en si ve u se o f t he a pp ro xi ma ti on o f t he t im e-a v er a ge d g e om a gn e ti c f i el d b y a g e oc e nt r ic a x ia ld ip ol e ( GA D ), i t h as b ee n k no wn f or s om e t im e[Wilson, 1 9 7 0] , t h a t t h e r e a r e s y s t em a t i c d e p a r t ur e sf r om t h is s i mp l e m o de l . T h e n a tu r e o f d e pa r tu r esf ro m G AD ov er th e pa st fe w mi ll ion year s i sd eb at e d [Gubbins and Kelly, 1 993 ; Kelly andGubbins, 1 99 7; Johnson and Constable, 1 99 5,

1 9 9 7 ; Merrill et al., 1 99 6; McElhinny et al.,1996; Carlut and Courtillot, 1 99 8] ( se e r ev ie wg iv en b y Johnson and McFadden [ 1 99 7 ]) , a n d

c o nt r ov e rs y o v er f i el d s t r uc t ur e e x te n ds t o t i me -s ca le s o f 1 08 y ea rs a nd l on ge r [Piper and Grant,1989;Kent and Smethurst, 1 9 98 ;Van der Voo andTorsvik, 2 00 1; Torsvik and Van der Voo, 2 00 2;

McFadden, 2 00 4] , f or w hi ch d at a s et s a re e ve ns p a r s er i n t h e i r g e o g r a ph i c a n d t e m po r a l s a m pl i n g .E xp er im en ts w it h t he s iz e a nd n at ur e o f t he i nn er c or e, a nd w it h t he rm al b ou nd ar y c on di ti on s i nn um er ic a l d yn am o s i mu l at io ns [Hollerbach and

Jones, 1 99 3a , 1 99 3b ; Glatzmaier et al., 1 99 9;Kono and Roberts, 2 00 2; Christensen and Olson,

2 0 03 ] , s u gg e st t h at n o n- G AD f i el d s t ru c tu r e m a y

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persist over timescales governed by the longevityo f t he b ou nd ar y c on di ti on s a nd d em on st ra te t hen ee d t o q ua nt if y s uc h s tr uc tu re o n t he b as is o f m e a s ur e m e nt s o f E a r t h s m a g ne t i c f i e l d.

[ 3] M od el s o f t he g eo ma gn et ic f ie ld c an b e c on -s t r u ct e d u s i n g a s p h er i c a l h a r mo n i c r e p r es e n t at i o n .T he m ag ne ti c s ca la r p ot en ti al i n a s ou rc e- fr eer eg io n d ue t o a n i nt er na l f ie ld o be ys L ap la ce se q ua t io n , a nd c a n b e w r it t en a s

Y r; q;f; t aX1l1

Xlm0

a

r

l1gml t co s mf hml t sin mf

Pml co s q ; 1

where glm(t) and hl

m(t) a re t he S ch mi dt p ar ti al lyn o rm a li z ed G a us s c o ef f ic i en t s a t a t i me t, a i s t he

r a d i u s o f t h e E a r t h ,r,q andf a r e r a d i us , c o l a ti t u dea n d l o n gi t u d e r e s p ec t i v el y, a n dPlm a r e t h e p a r t ia l l y

n o r m al i z e d S c h mi d t f u n c ti o n s . T h e m a g ne t i c f i e l d,~B, i s t he g ra di en t o f t he p ot en ti al Y, a nd a f ie ld

m o de l i s s p ec i fi e d b y t h e G a us s c o ef f ic i en t sglm(t)

andhlm(t) . T h em = 0 t e rm s c o rr e sp o nd t o s p he r ic a l

h a rm o ni c f u nc t io n s w i th n o a z im u th a l s t ru c tu r e;t ha t i s, t he y a re a xi al ly s ym me tr ic o r z on al . H er e

g10, g2

0, a n d g30 a r e t h e G a u s s c o e f f i c ie n t s r e p r e s e n t-

i ng t he a xi al d ip ol e, a xi al q ua dr up ol e a nd a xi alo c tu p ol e t e rm s , r e sp e ct i ve l y. T im e -v a ry i ng f i el dm od el s c an b e c on st ru ct ed f ro m t im e s er ie s o f m a gn e ti c f i el d o b se r va t io n s u s in g a p a ra m et r iz a-

t i on i n t i me ( t yp i ca l ly c u bi cb splines). Regulariza-t io n o f m od el s i n b ot h t im e a nd s pa ce m in im iz ess tr uc tu re t ha t i s n ot r eq ui re d b y t he d at a [Parker,1 99 4] . O bs e rv at or y a nd s at el li t e m ea su re me nt sprovide (Bx,By,B z) , t h e n o rt h , e a st , a n d d o w n wa r dpointing, locally orthogonal magnetic field compo-n e nt s . P a le o ma g ne t ic o b se r va t io n s a r e t y pi c al l ym e as u re m en t s o f d i re c ti o n ( d ec l in a ti o n (D) andi n c l in a t i on (I) ) a n d /o r i n t e ns i t yjBj, w h er e

D tan1 ByBx

;

I tan1 BzB2xB2y 1

2

0B@ 1CA;

jBj B2xB2yB2z 1=2

: 2

[ 4] O ve r t he h is to ri ca l p er io d, 1 59 0 1 99 0 A D,s a t el l i te , o b s er v a to r y, a n d s u r ve y m e as u r em e nt so f t h e g e om a gn e ti c f i el d h a ve p e rm i tt e d t h e c o n-s t r u ct i o n o f a s p a t ia l l y d e t a i l e d, t e m po r a l ly v a r y in gm ag ne ti c f ie ld m od el , G UF M1 [Jackson et al.,2 0 0 0 ; s e e a l s o , e . g . ,Bloxham et al., 1 9 8 9 ;Bloxhamand Jackson, 1 9 92 ] . S t ar t in g i n t h e 1 9 70 s , s i mi l ar

t yp es o f m od el s h av e a ls o b ee n p ro du ce d o ve r m i ll e ni a l t i me s ca l es f r om c om p il a ti o ns o f a r ch e o-m a gn e ti c d i re c ti o na l a n d p a le o in t en s it y d a ta , a ndpaleomagnetic directional data from high accumu-

l a t i on - r a te s e d i me n t s [ e . g ., Hongre et al., 1 99 8]( fo r a r ev iew, s ee Constable [ 20 07 ]) ; t he m os t recent of these spans the past 7 ka [Korte andConstable, 2 00 5] . T im e- va ry in g m od el s, s uc h a sG U FM 1 a nd C A LS 7K .2 , c an b e u se d t o e xa mi net h e e v o l u t i on o f t h e g e o m a g ne t i c f i e l d . O u r i n t e r e s t h e r e i s t h e t i m e - a v e r a g e d s t r u c t u r e : a r e p r e s e n t a t i o no f t h i s i s o b t a i n e d b y a v e r a g i n g e a c h o f t h e m o d e l so ve r t he ir r es pe ct iv e t im e i nt er va ls . T he r ad ia lc o mp o ne n t o f t h e m a gn e ti c f i el d , Br, i s s ho wn i nF ig ur es 1 a a nd 1 b f or G UF M1 a nd C AL S7 K. 2,a f t e r d o w n w a r d c o n t i n ua t i on o f t h e s c a l a r p o t en t i a lt o t h e s u rf a ce o f E a rt h s c o re u n de r t h e a s su mp t io n

t ha t t he m an tl e i s a n i ns ul at or. G UF M1 s ho wss i gn i fi c an t n on -G A D s t ru ct ur e , w h ic h h as b ee ne x te n si v el y d i sc u ss e d e l se w he r e [Jackson et al.,2 0 00 ] , a n d i s g e ne r al l y c o ns i de r ed t o b e s t ro n gl yi n f l u e n c e d b y t h e p r e s e n c e o f t h e i n n e r c o r e a n d t h ea s so c ia t ed t a ng e nt c y li n de r. R e gi o ns o f i n cr e as e dr a di a l f l ux a t h i gh l a ti t ud es , c o mm o nl y r e fe r re d t oa s f lu x l ob es , h av e p er si st ed i n m uc h t he s am el oc at io ns f or 4 00 y ea rs . L ow r ad ia l f ie ld o ve r t hen o rt h p o le h a s b e en i n te r pr e te d a s a m a ni f es t at i ono f m ag ne ti c t he rm al w in ds a nd p ol ar v or ti ce sw it hi n t he t an ge nt c yl in de r [Hulot et al., 2 00 2;Olson and Aurnou, 1 99 9; Sreenivasan and Jones,2 0 05 , 2 0 06 ] . E q ua t or i al f l ux p a tc h es t h at a r e p r o-n ou nc ed i n t he t im e- va ry in g v er si on o f G UF M1a n d i n m o de r n s a te l li t e m o de l s [ e .g . , Hulot et al.,2 0 0 2] , a n d t h a t a p pe a r t o p r op a ga t e w e s t w ar d i n t h eA t l an t i c h e mi s ph e re , a r e a t t en u at e d i n t h e 4 0 0 y e ar t e m po r a l a v er a ge ( F i gu r e 1 a ). W h e n a v er a ge d o v er 0 7 k a , C A L S7 K . 2 ( F i g ur e 1 b ) s h o w s l o ng i t ud i na ls t ru ct ur e t ha t s ug ge st s t he p re se nc e o f f lu x l ob es s ee ni n t he h is to r ic al f ie ld . T h e r ad i al m ag ne ti c f i el d i sa t t en u at e d i n C A L S7 K . 2 c o mp a re d w i t h G U F M1 :t h e r es ol ut i on a nd a cc ur ac y i s c le ar ly i n fe ri or , b ut a v e ra g in g o f m i ll e nn i al - s ca l e s e c ul a r v a r ia t i on a l s o

plays a major role in subduing the structure.[ 5 ] T he t im e i nt er va l 0 5 M a i s o u r pr im a r yi n te r es t . T h re e q u it e d i ff e re n t, b u t r e pr e se nt a ti v e,f i e l d m o d e l s f o r t h i s i n t e r v a l a r e s e e n i n F i g u r e s 1 c ,1 d, a nd 1 e. T he s i mp le st o ne , F ig ur e 1 c, i s b as edo n t he p re mi se t ha t o nl y z on al s tr uc tu re c an b er e so l ve d , a nd s h ow s Br a t t h e c o re - ma n tl e b o un d -a r y d u e t o a n a x i a l d i p o l e , w i t h a z o n a l q u a d r u p o l econtribution, g2

0, of 0 .05g10. F ig ur es 1 d a nd 1 e a r e

c o ns t ru c te d u s in g t h e s a me t e ch n iq u es o f r e gu l ar -i ze d i nv er si on u se d f or G UF M1 a nd C AL S7 K. 2.T h es e p al e of i el d m o de l s a r e n ot c o nt i nu ou s ly


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v ar yi ng i n t im e, o wi ng t o t he i nt er mi tt en t r ec or dprovided by volcanic rocks and the poor spatiald a ta d i st r i bu t io n . I n st e ad , t i me - av e ra g ed f i el dd ir ec ti on s a re i nv er te d f or a t im e- av er ag ed f ie ldm od el ( se e r ev ie w b y Johnson and McFadden[ 19 97 ]) . I n F ig ur es 1 c 1 e , t he s tr uc tu re i n e ac hm od el d ep en ds o n t he d at a s et s u se d: t he g re at es t n u mb e r o f p a le o da t a c om e f r om s e di m en t p i st o n

c or es [Schneider and Kent, 1 99 0] w it h o nl y i nc li -n at i on d at a a nd l ar ge u nc er t ai n ti es . T he s e a rec om pa ti bl e w it h v er y s mo ot h m od el s l ik e t ha t s h ow n i n F i gu r e 1 c . W h en l a va f l ow o b se r va t io n sa r e c o mb i ne d w i th s e di m en t d a ta , t h ey s u gg e st t h em ut ed n on zo na l s tr uc tu re i n F ig ur e 1 d [Johnsonand Constable, 1 99 7] , a nd l av a f lo w d at a a lo nei n di ca t e m or e c om pl ex a ve r ag e f i el d s t ru ct ur e( F ig u re 1 e ) [Johnson and Constable, 1 9 95 ] .

[ 6] F ig ur e 2 s ho ws t he s ig na l e xp ec te d a t E ar th ss ur fa ce f or t he G UF M1 , C AL S7 K. 2 a nd L SN 1a ve ra ge f ie ld m od el s, i n t he f or m o f g eo gr ap hi c

v a r i at i o ns i n d e p ar t u r es o f i n c l in a t i on a n d d e c li n a -t i o n f r o m G A D p r e d i c t i o n s . T h e s e a r e g i v e n b y t h ei n c l in a t i on a n o ma l y (DI) a n d d e c li n a t io n a n o ma l y(DD) , w h er e

DI IIGAD DD D: 3

T h e s t ru c tu r e i n t h e a r ch e o a n d p al e of i el d a n om a -

l i es i s r a th e r s i mi l ar ( d es p it e t h e v e ry d i ff e re n t d a tad is tr ib ut io ns f ro m w hi ch t he y a re d er iv ed ) a ndc on tr as ts w it h t ha t s ee n i n G UF M1 . N ot e t ha t t hem ag ni tu de o f t he s ig na l d ec re as es o ve r l on ge r t i me s ca l es . F r om F i gu r e 2 f w e s e e t h at t h e a v e r ag ei nc li na ti on a no ma ly i n L SN 1 i s r at he r s ma ll , a ndw e c an e xp ec t t he l ar ge st s ig na l a t e qu at or ia ll a ti t ud e s. I f t h is v i ew o f t h e t i me - av e ra g ed f i el d i sa p p r ox i m a te l y c o r r e c t , t h e n a t m i d t o h i g h l a t i t u de si t w i ll b e d i ff i cu l t t o d e te c t d e p a rt u re s f r om G A Dw it ho ut l ar ge d at a s et s t ha t p ro vi de a cc ur at em e a s ur e s o f DI.

Figure 1. T i me - a ve r a ge d r a d ia l m a g n e t i c f i e l d , (Br) , a t t h e c o r e- m a nt l e b o u n d ar y ( C M B) , o n d i f f er e n t t i m e s c al e s .U n it s a r emT. ( a) H is t or i ca l f i el d : 1 5 90 1 9 90 , M o de l G UF M 1 [Jackson et al., 2 00 0] , ( b) A rc he o- Fi el d: 0 7 k a,M o d el C A L S7 K . 2 [Korte and Constable, 2 0 05 ] , ( c ) P a l e o- F ie l d: 0 5 M a , a x i a l d i p o le p l us a x ia l q u ad r up o le f i el d( s ee t e xt ) , ( d ) M o de l L S N1 [Johnson and Constable, 1 99 7] , a nd ( e) M od el L N1 [Johnson and Constable, 1 9 95 ] .


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[ 7] F ig ur es 1 and 2 p ro vi de t he m ot iva ti on f or g at he ri ng t he n ew l av a f lo w d at a p re se nt ed h er e,a n d f o r a s s e m b l i n g l a r g e r e g i o n a l d a t a s e t s f r o m t h epublished literature. Tantalizing similarities amongt h e m o de l s a v er a ge d o n q u it e d i ff e re n t t i me s ca l ess u gg e st p e rs i st e nt s t ru c tu r e i n t h e t i me - av e ra g edf ie ld i n t he f or m o f h ig h- la ti tu de f lu x l ob es , a nd

possible low radial field in polar regions. Suchs t r uc t ur e s a r e n o t i m pl a us i bl e o n s e ve r al g r ou n ds .T he i nn er c or e t an ge nt c yl in de r m ig ht i nf lu en cec or e f lo w i n s uc h a w ay a s t o p ro du ce p er s is t en tl yh i gh v a lu e s f o r Br i n t h e l a t i tu d e b a n d o c cu p ie d b yt h e c u r r e n t f l u x l o b e s , a n d l o w e r v a l u e s c l o s e t o t h epole. It is also possible that these lobes mighte xh ib it l on gi tu di na l b ia s i n t he ir l oc at io ns a s ar e su lt o f g eo gr ap hi ca ll y c om pl ex v ar i at io ns i nt h er m al c o nd i ti o ns a t t h e c o re - ma n tl e b o un d ar y.H o we v er , t h e m a jo r l i mi t at i on s i n u n de r st a nd i ngg e om ag n et i c f i el d b e ha vi o r o v er r e le va nt t i me -s c al e s ( 1 06 y ea rs a nd l on ge r) a re t he q ua li ty,

t e m p or a l d i s t r ib u t io n a n d s p a t ia l c o v er a g e o f e x i s t-i n g p a l eo m a gn e t ic d a t a.

[ 8] We e xa mi ne t he r ol e o f p al eo ma gn et ic d ir ec -t i on s f r om v o lc an i cs i n c h ar a ct e ri z in g t h e 0 5 M at i me - av e ra g ed f i el d ( TA F ) a n d i t s t e mp o ra l v a ri -a b il i ty o r p a le o se c ul a r v a ri a ti o n ( P SV ) . We f o cu so n t h es e d a ta f o r s e ve r al r e as o ns . F i rs t , v o lc a ni c s

provide geologically instantaneous recordings off i el d b e ha v io r, w i th o ut t h e t e mp o ra l a v er a gi n gi n h e re n t i n s e d i me n t ar y r e c o rd s . S e c on d , m e a s ur e -m e nt s o f a b so l ut e d e cl i na t io n p r ov i de i m po r ta n t c on st ra in ts o n l on gi tu di na l s t ru ct ur e i n t he f ie ld( s e e d i s c u s s i o n b yJohnson and Constable [1997]),a nd t he se a re o ft en u na va il ab le f ro m d ee p s eas ed im en t c or es . T hi rd , s uc h d at a a re w el l- s ui te dt o s t at i st i ca l i n ve s ti g at i on s o f t h e p a le o ma g ne t icf ie ld : t he y c an b e u se d t o t es t t he p re di ct io ns o f s ta ti st ic al m od el s s uc h a s t ho se p io ne er ed b yConstable and Parker [ 19 88 ], t he m os t r ec en t

Figure 2. ( a , c , e ) D ec l in a ti o n a n d ( b , d , f ) i n cl i na t io n a n om al i es i n d e gr e es ( d ev i at i on s f r om G AD d i re ct i on ) a t E a rt h s s u rf a ce p r ed i ct e d f r om m o de l s f o r t h e t h r e e t i me i n te r va l s i n F i gu r e 1 : F i gu r es 2 a a n d 2 b , M o de l G UF M 1;F i g u r e s 2 c a n d 2 d , C A L S 7 K . 2 ; a n d F i g u r e s 2 e a n d 2 f , M o d e l L S N 1 . T h e s c a l e b a r f o r t h e h i s t o r i c a l f i e l d i s t w i c e t h a t f o r t h e a r ch e o- f ie l d, a n d 4 t i me s t h at f o r p a le o fi e ld a n om al i es .


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r ef in em en t o f w hi ch i s t ha t o f Tauxe and Kent[ 2 0 0 4] ; t o c o m pa r e w i t h t h e s t a t i s t i ca l p r o p er t i e s o f d y na m o m o de l s [ e .g . , Bouligand et al., 2 00 5] ; o r u lt im at el y c an b e u se d t o i nv er t f or b ot h t he t im e-a v er a ge d f i el d a n d i t s t e mp o ra l v a ri a bi l it y ( s om eprogress in this area has been made byKhokhlov etal. [ 2 0 0 1, 2 0 0 6] ) . F i n a ll y, f r o m a p r a ct i c a l p e r s p e c -t i ve , d a ta c o ll e ct i on e ff o rt s b y s e ve r al g r ou p s o v er t he p as t d ec ad e h av e r es ul te d i n m an y n ew h ig h-

q ua li t y p al eo di r ec ti on s , t ha t c an c ol l ec ti ve l y b ec o mp a re d w i th t h os e u s ed t o g e ne r at e t h e m o de l ss ho wn i n F ig ur es 1 a nd 2 .

[ 9] I n P S V s tu di es o f l a v a f l ow s , a n e st im at e o f t he

paleofield at a given location and time is made bys am pl in g w ha t i s k no wn a s a si te . A t a g iv ens i te , m u lt i pl e s a mp l es a r e c o ll e ct e d t o r e du c e t h ei n f l ue n c e o f m e a su r e m en t e r r o r, a n d t h e s e s a m p l e sm u st b e d e ma g ne t iz e d i n t h e l a bo r at o ry t o r e mo v es e c o nd a r y r e m a n e n ce . S i t e m e a n v a l ue s o f D andIh a v e a n a s s o ci a t ed u n c er t a i nt y t h a t r e f l e c t s w i t h i n -s it e s ca tt er. T hi s i s u su al ly q uo te d a s t he 9 5%c on fi de nc e c on e a bo ut t he m ea n d ir ec ti on , a95,where a95 140/

ffiffiffiffiffiffiffikNs

p (for k g r ea t er t h an 2 5 )

and Ns, k ar e t he n um be r o f s am pl es a nd t hee s ti m at e o f t h e F i sh e ri a n p r ec i si o n p a ra m et e r, kr e s pe c t iv e l y. W it h i n- s i t e s c a tt e r i s s o m et i m es

d e s c ri b e d b y t h e w i t h in - s i t e d i s p e r s i on , sw, w he resw = 81/

ffiffiffik

p . M u l ti p le t e mp o r al l y- i n de p en d en t

s i te s a r e n e ed e d t o c h ar a ct e ri z e b o th t h e TA F a ndP S V a t a s i ng l e p l ac e. A s t at i st i c t r ad i ti o na l ly u s edt o d es cr ib e P SV i sSB, t he r oo t m ea n s qu ar e ( rm s)a n gu l ar d e vi a ti o n o f v i rt u al g e om a gn e ti c p o le s( V G Ps ) a b ou t t h e g e og r ap h ic a x is ,

SBffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1

N 1XNi1

D2i

S2wiNsi

vuut : 4

Table 1. D MA G 4 D at a o f McElhinny and McFadden[1997]a

Region N Superseded Reference

New Z ealan d 5 6 y es th is co mp ilatio n ( Tab le 3 )Japan 21 yes this compilation (Table 4)Reunion 27 yes Lawrence et al. [2006]Tahiti 135 yes Lawrence et al. [2006]SW USA 54 yes Tauxe et al. [2003]

NW USA 4 1 y es Tauxe et al. [2004b]Canaries 13 yes Tauxe et al. [2000]I celand 47 no . . .Total 394

aT h e se a r e p r e v i o us l y r e p o r t e d d a t a o f a d e qu a t e q u a l i t y ( D M A G 4 )

f o r TA F / PS V m o d el i n g [McElhinny and McFadden, 1 9 97 ] . N i s t o t a ln u mb e r o f D MA G 4 d a ta f o r r e g i on ( n o s e l e ct i on b a se d o nk ora95).S u p er s e d ed i n d i ca t e s w h e th e r d a t a f r o m McElhinny and McFadden[ 1 9 97 ] f o r t h a t r e g i on h a v e b e e n s u p er s e d ed b y n e w er c o m pi l a t io n sr e p o r t e d b y t h e c i t e d r e f e r e n c e . I n t h e c a s e o f t h e S W U S A , N W U S A ,

a n d t h e C a n ar i e s , t h e r e c en t p u b li s h ed d a t a c o m p i l at i o n s i n c l u de n e wd a t a c o l l e c te d a s p a r t o f t h e T A F I p r o j ec t .

Table 2. T AFI Studiesa

Region l, d e g f, deg Ntotal Ndated Ref er en ce

Aleu tians 54.4 190.1 89 31 Stone and Layer [2006]Nu n iv ak 5 3 .0 172.0 56 0 Coe et al. [ 2 0 00 ] , A p p en d i x A

Br itish Columbia 51.5 122.4 53 0 Mejia et al. [2002]S n a ke R i v er b 43.0 113.5 26 21 Tauxe et al. [2004b]San Fr an cisco Vo lcanicsb 35.3 111.9 37 0 Tauxe et al. [2003]Azo r esc 37.8 25.4 35 12 Johnson et al. [1998]L a P a l ma

b,c28.8 17.9 26 8 Tauxe et al. [2000]

Mexico 20.1 99.7 16 7 Mejia et al. [2005]Costa Rica 10.2 84.4 32 17 Appendix AEcuador 0 .4 78.3 63 13 Opdyke et al. [2006]Atacama 23.3 67.7 46 23 Appendix AE aster I slan d 27.1 109.2 62 0 Brown [2002]Ta t a r a S a n P e d ro , C h i le 36.0 70.9 193 22 Appendix AAu str alia 37.7 144.2 38 0 Opdyke and Musgrave [2004]Patag o n ia 47.0 71.1 36 34 Brown et al. [2004a]Patag o n ia 51.2 70.6 50 20 Mejia et al. [2004]McMu r d o 78.1 165.4 36 18 Tauxe et al. [2004a]17 studies total 894 226

aD e f i ni t i o ns a r e a s f o l lo w s : R e g i o n , g e o g r a ph i c al a r e a o f s t u dy ;l, m e a n s t u d y l a t i t u d e i n d e g re e s ( p o s i t i ve n o r t h) ;f, m e a n s t u d y l o n g it u d e i n

d e g r ee s ( p o si t i v e e a s t ) ; Ntotal, t o t a l n u m b er o f s i t e s w i t h d e c l i n at i o n a n d i n c l i n a t io n m e a s u r e me n t p a i r s ; Ndated, n u mb e r o f s i te s f o r w hi c h n e wradiometric (usually

40A r/

39A r ) d a te s w e re o bt a in e d; a n d R ef e re n ce , p u bl i ca t io n i n w hi c h t h e p a le o ma g ne t ic s t ud y i s r e po r te d . D a ta n o t y e t

published are reported in A ppendix A .b

D a t a f r o m t h e S a n F r a n c is c o v o l c a n ic s , S n a k e R i v e r p l a i n , a n d L a P a l m a w e r e i n c l ud e d i n t h e r e g i o n al c o m pi l a t io n s f o r t h e S W U S A , N WU S A , a n d C a n ar i e s , r e s p e c ti v e l y ( s e e T ab l e 1 ) .

cD a t a s e t s f r o m t h e A z o re s a n d L a P a l m a w e r e c o l le c t e d p r i o r t o t h e T AF I p r o j e c t b u t l a b p r o to c o ls f o l l ow e d t h o s e o f TA F I a n d s o t h e d a t a s e t s

a r e i n c l ud e d .


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Di r e pr e se nt s t h e a n gu l ar d e vi a ti o n o f t h e p o le f o r theit h s i t e f r o m t h e g e o g r a p h i c n o r t h p o l e , N i s t h en u m b e r o f s i t e s , a n dSB r e p r e s e nt s t h e g e o ma g n et i cs i g n al r e m a in i n g a f t e r c o r r ec t i n g f o r t h e w i t h i n- s i t edispersion Swi d e t e rm i n ed f r o mNsi s a m pl e s . ( N o t et h at t h is e q ua t io n c o rr e ct s a n e r ro r i n e q ua t io n ( 4 )of Lawrence et al. [ 2 00 6 ]) . T h e e s ti m at e k ford i re c ti o ns c an b e c o nv er t ed t o a n a p pr o xi m at i on t ot he p re ci si on p ar am et er f or V GP s u si ng t het r an s fo r ma t io n p r ov i de d b y Creer [ 1 96 2] . T he

s t at i st i ca l a pp r oa ch i n he r en t i n u s in g SB circum-v en ts th e la ck o f d et ai led a ge c on tr ol an d t hea bs en ce o f t im e s er ie s o f o bs er va ti on s t ha t a rei ne vi ta bl e i n w or ki ng w it h l av a f lo ws f ro mdisparate locations.

[ 10] W hi le t he re h av e b ee n p re vi ou s a tt em pt s t oc o mp i le g l ob a l d a ta s e ts s u it a bl e f o r TA F a n d P S Vm od el i ng [McElhinny and Merrill, 1 97 5; Lee,1 9 8 3 ; Quidelleur et al., 1 99 4; Johnson andConstable, 1 9 96 ] , m a ny i s su e s h a ve p l ag u ed i n te r -pretations, notably those of inadequate (or evenu n kn o wn ) t e mp o ra l s a mp l in g , p o or g e og r ap h ic a lc ov er ag e, a nd p oo r d at a q ua li ty r es ul ti ng f ro mi n fe r io r l a bo r at o ry m e th o ds . T h es e l i mi t at i on s a r et he c au se o f m uc h o f t he d is cu ss io n s ur ro un di ngt he l ev el o f c om pl ex it y i n t he 0 5 M a TA F. T hem o st c o ns e rv a ti v e v i ew i s a ff o rd e d b y McElhinnyand McFadden [ 1 99 7 ] ( h er e af t er M M 97 ) , w h o i na n a s se s sm e nt o f a g l ob a l p a le om a gn e ti c d a ta b as ec on cl ud ed t ha t o nl y 3 94 D, I p ai rs w ou ld m ee t m o de r n s t an d ar d s f o r p a le o ma g ne t ic r e se ar c h.T h es e d a ta ( d es i gn a te d b y a d e ma g ne t iz at i on c o de DM AG 4 i n w or k b y M M9 7) a re r es tr ic te d t o8 g l ob a ll y d i st r ib u te d l o ca t io n s, a n d h a ve p o or a g ec o nt r ol ( Ta b le 1 ) . T h is d o es n o t n e ce s sa r il y m e an

t h a t t h e o l d e r d a t a a r e u n u s a b l e , b u t h a s p r o v i d e d as t r o n g i n c e n t i v e t o u n d e r s t a n d t h e i n f l u e n c e o f d a t aq u al i ty i n t h ei r i n te r pr e ta t io n [ e .g . , Tauxe et al.,2003; Lawrence et al., 2 0 06 ] .

[ 11] I n t h i s p a pe r w e p ro vi de a s yn th es is o f o n g o-i n g w o rk t o wa r d n e w g l ob a l d a ta c o mp i la t io n s f o r TA F a nd P SV m od el in g. I n p ar ti cu la r, w e r ep or t t h e c o l l ec t i ve r e s u lt s o f a m a j or m u l ti - i n s ti t u t io n a le f f or t , t h e T i me - Av e r ag e d F i e ld I n ve s ti g at i on s( TA F I ) p r oj e ct , t o i m pr o v e t h e c h ar a ct e r iz a ti o n o f t h et i m e -a v e ra g e d g e o ma g n et i c f i e l d a n d p a l eo s e cu l a r v a ri a ti o n o v e r t h e p a s t 5 m i ll i on y ea r s. R e su l ts o f i nd iv id ua l s t ud ie s f ro m t hi s p ro je ct h av e b ee nr e po r te d i n s e ve r al p u bl i ca t io n s ( s ee Ta bl e 2 ) ; h e rew e i n ve s ti g at e n e w c o ns t ra i nt s t h es e d a ta p l ac e o ng l ob al p al eo f ie ld b eh av io r. We s u mm ar i ze t hen um be rs o f d at a c ol le ct ed , c om mo n f ie ld a ndl a bo r at o ry p r oc e du r es , a n d t h e r e su l ti n g t e mp o ra la nd s pa ti al d at a d is tr ib ut io n. We i nv es ti ga te t hei n fl u en c e o f d a ta q u al i ty a n d t r an s i ti o na l s i te s o ne s ti m at e s o f t h e T AF a nd P S V f r o m t h e T AF I d a ta .W e s u p p l e m e n t t h e T A F I d a t a w i t h r e c e n t l y p u b l i s h e d

r e gi o na l c o mp i la t io n s, a n d t w o n e w c o mp i la t io n sf or N ew Z ea la nd a nd J ap an t ha t w e r ep or t h er e.T h e r e su l ti n g d a ta s e t, w h il e n o t c o mp r eh e ns i ve ,provides a significant improvement over existingg l ob a l d a t a s e t s , a n d w e u s e i t t o i n ve s ti g at e z o na l( l a t it u d i na l ) s t r u ct u r e i n t h e T A F a n d P S V.

2. Time-Averaged Field Investigations(TAFI) Project

[ 12] P a le o ma g ne t ic d i re c ti o ns h a ve b e en o b ta i ne df r om 8 9 4 l a v a f l o w s a t 1 7 l o c a ti o ns ( F ig u re 3 ) ; w e

Figure 3. L o c at i o n s o f TA F I s i t e s ( r e d c i r cl e s , T ab l e 2 ) a n d r e g i o n al c o m pi l a t io n s f r o m l i t e ra t u re ( b l u e s t a r s, Tauxe

et al. [ 2 0 0 3 , 2 0 0 4 b ] ,Lawrence et al. [ 2 0 0 6 ] , a n d t h i s s t u d y , T a b l e 3 ) . O p e n c i r c l e d e n o t e s S p i t z b e r g e n s i t e , w h e r e d a t aa r e m o s tl y o l de r t h a n 5 M a a n d n o t r e p o rt e d h e r e.


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r ef er t o e ac h l oc at io n a s a s tu dy. T he TA FI s tu dyl o c a ti o n s w e r e c h o s e n t o i m p r ov e t h e g e o gr a p hi c a lc ov er ag e o f 0 5 M a p al eo ma gn et ic d ir ec ti on s a t

h i g h l a t i tu d e s ( S p i tz b e rg e n , A l e u ti a n s , A n t a rc t i c a) ,a nd i n t he S ou th er n H em is ph er e ( va ri ou s S ou thA m er i ca n l oc al e s, E a st e r I s la nd a n d A u st r al i a) .S am pl es i n f ou r s t ud ie s, A l eu ti an s [Stone and

Layer, 2 00 6; Coe et al., 2 0 00 ] , A n ta r ct i ca [Tauxeet al., 2 00 4a ], a nd E as te r I sl an d [Brown, 2 00 2] ,w e r e c o l l e c t e d i n t h e 1 9 6 0 s a n d 1 9 7 0 s , b u t o r i g i n a ll a bo r at o ry m e as u re m en t s i n vo l ve d m i ni m al d e -m a g ne t i z at i o n . F o r t h e s e s t u d i e s m o d er n d e m ag n e -t i za ti on t ec hn i qu es w er e a pp li ed t o e xi s ti ngpaleomagneti c cores. For the other 13 studies,n ew f ie ld w or k w as r eq ui re d. N ew r ad io me tr icd a te s , a l on g w i th 9 5 % u n ce r ta i nt i es w e re o b ta i ne df or 2 26 o f t he TA FI s it es ( Ta bl e 2 ). P re vi ou sl yo b ta in e d r a di o me t ri c d a te s a r e a va i la bl e f or 8 8a dd it io na l f lo ws . A ge s w er e a ss ig ne d t o t her e m a i n i n g s i t e s o n t h e b a s i s o f a b s o l u t e a n d r e l a t i v ea ge d at a p ro vi de d i n t he o ri gi na l TA FI p ub li ca -t io ns . I n s om e c as es o nl y t he p ol ar it y c hr on i sk n o w n, a n d a m e d i an c h r on a g e , w i t h u n c er t a i nt i e sc or re sp on di ng t o h al f t he c hr on l en gt h w er ea s si g ne d . T h us a s si g nm e nt s o f a g e a n d a g e u n ce r -t a in t y h a v e b e en m ad e f o r a l l T A FI s i te s . 8 8 3 s i te sh ave ag es l es s t han 5 Ma , t he ma jo ri ty a re o f B r u n h e s ( 6 7 % ) o r M a t u y a m a ( 2 6 % ) a g e ( F i g u r e 4 ) .

2.1. Paleomagnetic Field Procedures andLaboratory Methods

[ 13] S am pl in g w as r es tr ic te d t o l av a f lo ws o r t hi nd ik es : u ni ts t ha t c oo l q ui ck ly, a nd c an r ec or d t hei ns ta nt an eo us g eo ma gn et ic f ie ld a t t he t im e a ndl oc at i on o f e mp la ce me nt o f t he v ol ca ni c u ni t.S t a n da r d 2 . 5 - c m- d i a me t e r p a l e om a g ne t i c c o r e s ,a bo ut 1 0 c m l on g, w er e d ri ll ed i n t he f ie ld . E ac hs i t e w a s d e t e r m i n e d t o b e i n s i t u ; p o s t e m p l a c e m e n t t i l t i ng w a s a v o id e d . O u t c r o p s w e r e s u r v ey e d u s i n ga m ag ne ti c c om pa ss a nd /o r a B ar ti ng to n m ag ne -t o me t er t o m i ni m iz e s a mp l in g o f u n it s s u bj e ct e d t ol ig ht ni ng s tr ik es . F or e ac h s tu dy, a s m an y s it esw e re s a mp l ed a s p o ss i bl e d u ri n g t h e f i el d s e as o n,w it h a g oa l o f a t l ea st 1 0 s i te s f or a g iv en p ol ar it y.O n a v e r a g e , a b o u t 5 0 s i t e s w e r e o b t a i n e d p e r s t u d y

( Ta bl e 2 ). A m in im um o f 1 0 c or es w er e d ri ll edo ve r a s ev er al -m et er - ex te nt o f o ut cr o p t o a ll owa ss es sm en t o f o ri en ta ti on e rr or a t e ac h s it e. ( Th ed at a s et f ro m L a P a l ma [Tauxe et al., 2 00 0]preceded these standards and 5 12 samples pers i te w e re d r il l ed . ) E a ch s a mp l e w a s o r ie n te d u s in ga m ag ne ti c c om pa ss , a nd w he re p os si bl e a s unc o mp a ss a n d/ o r b a ck - s ig h ti n g ( s e e Tauxe et al.[ 2 0 0 3] f o r d e t ai l s ) .

[ 14] L a bo r at o ry m e th o ds v a ry s l ig h tl y a m on g s t ud -i e s, b u t a s e t o f m i ni m um c r it e ri a w e re a d op t ed : I na l l c as e s n at u ra l r e ma ne n t m ag ne t iz at i on s w e re

m ea su re d f or a ll s pe ci me ns , a nd o ne s pe ci me nper core for at least 5 cores (samples) per site,w as s ub je ct ed t o s te pw is e a lt er na ti ng f ie ld o r t h er m al d e ma g ne t iz a ti o n. A c co m pa n yi n g r o ck m a gn e t ic m e as u r em e nt s , s u ch a s s u s ce p t ib i l it ya nd C ur ie t em pe ra tu re w er e a ls o m ad e. N ew40Ar/39Ar r a di o me t ri c a g es w e re o b ta i ne d f o r l a vaf lo ws f ro m t we lv e s tu di es . D et ai ls o f t he p al eo -m a g ne t i c a n d a g e m e a s u r e me n t s , a n d r e s u l ti n g d a t af o r e a c h s t u d y a r e g i v e n i n t h e o r i g i n a l p u b l i c a t i o n so f p a le om a gn e ti c r e su l ts [Brown, 2 00 2; Brown etal., 2 0 0 4 a ;Johnson et al., 1 9 9 8 ;Mejia et al., 2 0 0 2 ,2 0 0 4, 2 0 0 5; Opdyke and Musgrave, 2 0 04 ;Opdyke

et al., 2 00 6; Stone and Layer, 2 0 06 ; Tauxe et al.,2 00 0, 2 00 3, 2 00 4a , 2 00 4b ]. I n t hi s p ap er w e u se s it em e a n d i r e c t i o n s a s f o l l o w s . O r i g i n a l d a t a , i n c l u d i n ga l l l ab or at or y a nd f i el d m ea su r em en ts , f r om t heA z or e s [Johnson et al., 1 9 9 8 ] , L a P a l m a [Tauxe etal., 2 0 0 0 ] , S o u t h e r n P a t a g o n i a [Mejia et al., 2 0 0 4 ] ,A n t a r c t i c a [Tauxe et al., 2 00 4a ], S na ke R iv er [Tauxeet al., 2 0 04 b ], S a n F r an ci s co Vo l ca n ic s [Tauxe et al.,2 00 3] , a nd C os ta R ic a ( C. G . C on st ab le e t a l. ,m a n us c r i pt i n p r e p ar a t i on , 2 0 0 8, a n d A p p e n d i x A )h a v e b e e n a r c h i v e d i n t h e M a g I C d a t a b a s e [Solheidet al., 2 0 0 2] ( h t t p: / / w w w. e a r t hr e f . o rg / M A G IC / ) .

Figure 4. A g e d i s t ri b u t i on f o r t h e 8 8 3 T AF I s i t e s w i t ha g e s l e s s t h a n 5 M a . A v e r a g e a g e a n d a n e s t i m a t e o f e r r o r i n t he a ge a re o bt ai ne d v ia o ne o f t he f ol lo wi ng :( 1 ) r a d i o m e t r i c d a t e s o b t a i n e d a s p a r t o f t h e T A F I p r o j e c t ( 2 2 6 f l o w s ) , ( 2 ) p r e v i o u s r a d i o m e t r i c a g e s m a d e d i r e c t l yo n T A F I s i t e s (9 0 f l o w s) , o r ( 3 ) i n f er r e d a g e : s t r at i g r ap h ya n d / or p o l a ri t y c h r o n ( r e m a in i n g f l o ws ) .


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S a mp l e- l ev e l d i r ec t io n s r e po r te d i n M a gI C u s eprincipal component analysis and require at least 4d e ma g ne t iz a ti o n s t ep s a n d a m a xi m um a n gu l ar d e vi a ti o n o f t h e d a ta a b ou t t h e s t ab l e c o mp o ne n t o f l es s t ha n 5. F i sh e r m ea n d i re c ti o ns w e re c o m-puted for all sites with at least 3 samples per site.O v er 8 0 % o f t h es e s i te s h a ve s i te m e an d i re c ti o nsd e r i v e d f r o m a t l e a s t 5 s a m p l e s . F o r a l l o t h e r s t u d i e s( A l e ut i a n s [Stone and Layer, 2 0 06 ] , N u n i va k [Coeet al., 2 00 0] ( Ap pe nd ix A ), B ri ti sh C ol um bi a [Mejia

et al., 2 0 0 2 ] , M e x i c o [Mejia et al., 2 0 0 5] , A t a c am aa n d T a ta r a S a n P e dr o , C h il e [Brown et al., 2 0 04 b ]( A pp e nd i x A ) , E a st e r I s la n d [Brown, 2 0 02 ] , P a ta -g o ni a [Brown et al., 2 00 4a ]) w e h av e u se d t he s it e

m e a n d i r e c t i o n s r e p o r t e d i n t h e o r i g i n a l p u b l i c a t i o no r i n A pp en di x A . I n t hi s p a p er w e u s e o nl yp al eo d ir ec ti on s , as ab s o lu t e pa l eo in te ns i t ym e su r em e nt s a r e s t il l u n de r wa y f o r s e ve r al o f t h eTA F I s t ud i es . F o r a l l TA F I s t ud i es , t h e s i te - le v elr e s u l t s u s e d i n t h i s p a p e r a r e a r c h i v e d i n t h e M a g I Cd a t a b a s e , i n c l u d i n g t h e d a t a r e p o r t e d i n A p p e n d i x A[Solheid et al., 2 0 0 2] ( h t tp : / /w w w. e a rt h r ef . o rg / MAGIC/).

2.2. Assessment of TAFI Data Quality

[ 15 ] A s h as b e en d i sc us s ed e xt e ns i ve ly [ e .g . ,

McElhinny and McFadden , 1 99 7; Johnson andConstable, 1 9 96 ;Merrill et al., 1 9 96 ] , a s se s sm e nt o f d a t a q u a l i t y i s c r i t i c a l t o a c c u r a t e i d e n t i f i c a t i o n o f pa leo se cul ar var ia ti on an d of non -G AD ti me -a v e ra g e d f i e ld s t r u ct u r e . Ty p i ca l l y s i t e m e a nd i r e ct i o ns w i t h a n a s s o ci a t e da95 g r ea t e r t h a n ( o r kl e ss t h an ) a c e rt a in v a lu e a r e e x cl u de d f r om a n al y se s .R e c e n t r e g i o n a l d a t a c o m p i l a t i o n s c o n s i s t o f s e v e r a lh u n d r e d s i t e m e a n d i r e c t i o n s , a n d e n a b l e a n a s s e s s -m e nt o f t h e b e ha v io r o f TA F a n d P S V e s ti m at e s a si n c r ea s i n gl y s t r i ng e n t d a t a q u a li t y r e q ui r e m en t s a r ei m p o s e d . W e u s e a s i m i l a r a p p r o a c h t o t h a t r e p o r t e dby Tauxe et al. [ 2 00 3] ( he re af t er T 0 3) a nd b y

Lawrence et al. [ 2 0 0 6 ] ( h e r e a f t e r L 0 6 ) .

[ 16] O ur m ea su re o f d at a q ua li ty f or i nd iv id ua ls i te s i s k. We u s e i n cl i na t io n a n om a ly, DI, ( e qu a -t i o n ( 3 ) ) t o c h a ra c t er i z e t h e TA F, a n d b e t w e e n -s i t eV G P d i s p er s i o n, SB, ( eq ua ti on ( 4) ), f or P SV. Wecalculate DI andSB u s i n g s i t e m e a n d i r e c t i o n s w i t hv a l u es o f k g r e at e r t h an a c u t- o ff v a lu e , d e no t ed b ykcut. I n cr e as i ng l y s t r in g en t d a ta q u al i ty c r it e ri ac o rr e sp o nd t o s u cc e ss i ve l y l a rg e r v a lu e s o f kcut.T h e a p p ro a ch i s d i sc u ss e d i n d e ta i l b y L 0 6 a n d w ed o n o t r e p e a t i t h e r e . T h e T A F I d a t a s e t d i f f e r s f r o mt ho se o f T 03 a nd L 06 , h av in g s it es f ro m a b ro ad

r an ge o f l at it ud es , a nd w e e xp ec t b ot h P SV a ndTA F e s ti m at e s t o i n cl u de a l a ti t ud i na l s i gn a tu r e.We i n ve s ti g at e t h e T A FI d a ta s e t b y b i nn i ng d a taf r om s i mi la r l at i tu de s, b ut r e ta in a d is t in ct io nbetween northern and southern latitudes. Sufficientd at a a re a va il ab le a t l at it ud es o f a pp ro xi ma te ly50N, 35N, 25S , 3 5S, an d 5 0S t o p er fo rm a ni n v e st i g a ti o n o f h o w DI andSB v a r y w i th kcut. Weu s e o n l y s i t e s f o r w h i c h t h e n u m b e r o f s a m p l e s p e r site, n, i s a t l ea st 5 b ec au se l ow er n l ea ds t o l es sr e l i ab l e e s t i ma t e s o f k ( s ee , e . g. , T 0 3) . T h e r e su l tsa re s ho wn i n F ig ur e 5 f or n or ma l p ol ar it y d at a

Figure 5. I n c l in a t i on a n o m a l y (DI) a nd V GP d is pe r-s i o n (S B) v e r s u s c u t - o f f v a l u e o f k (kcut) f o r f i v e l a ti t ud e

bins of the TAFI data set. Normal polarity data ares h ow n , a n d o n ly s i te s w i th n 5 ( a t l e a st f i ve s a mp l es

per site) are considered. Mean DI, S B ( b lu e c u r v es )a l on g w i th t h e b o ot s tr ap 9 5 % c o nf i de n ce i n te r va l s ( r edc ur ve s) a re s ho wn . G re en c ur ve s i nd ic at e n um be r o f

c o nt r ib u ti n g d a ta a t e a ch v a lu e o f kcut.


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o nl y, s in ce t he se d om in at e o ur d at a s et . N or ma lpolarity is defined here as site mean directions thath a ve a c o rr e sp o nd i ng p o si t iv e V G P l a ti t ud e ; t h usn o d i st i nc t io n b et w ee n s t ab l e p o la r it y a n d t r an s i-t i o n a l d a t a i s m a d e i n t h e e v a l u a t i o n o f d a t a q u a l i t y .

[ 17] Ta k en t o ge t he r, t h es e r e gi o na l a n al y se s i n di -c a te t h at i n cl i na t io n a n om a ly i s q u it e r o bu s t w i thr e sp e ct t o d a ta q u al i ty, e s pe c ia l ly i n r e gi o ns w i th

l a rg e d a ta s e ts . F o r e x am p le , t h e l a t i tu d e b a n d w i tht h e m o s t d a t a , c e n t e r e d o n 3 5S , s h ow s a n a v er a gei n cl i na t io n a n om a ly t h at i s s m al l (2) a nd p os i-t i ve , a n d d o es n o t c h an g e s i gn i fi c an t ly a s t h e v a lu eof kcut i s v ar ie d. F or o th er r eg io ns , t he re i s s om ec h an g e i n t h e e s t i ma t e o f DI a t h ig h v al ue s o f kcutbecause the smaller data sets yield less reliabler e su l ts . T h e b e ha vi o r o f SB with kcut v a r ie s f r omr eg io n t o r eg io n. Tw o l at it ud e b an ds ( 50N and

Figure 6. E q u a l a r e a p r o j ec t i on s o f s i t e d i r e ct i o n s f o r e a c h o f t h e 1 7 TA F I s t u d i es , f o r s i t e s w i t hn 5 . S o l i d ( o p e n )c i rc l es r e pr e se n t p r oj e ct i on s o n to t h e l o we r ( u pp e r) h e mi s ph e re . S i te s w it h e s ti ma t es o f t h e w i th i n- s it e F i sh e r

precision parameterk 5 0 a r e s h o w n i n g r e e n , a n d t h o s e w i t hk > 5 0 a r e s h o w n i n b l u e . N o V G P l a t i t u d e c u t - o f f i sa p pl i ed ; t h at i s , t r an s it i on a l d i re ct i on s , i f p r es e nt , a re i n cl u de d . T h e G AD f i el d d i re c ti o n a t t h e m ea n s i te l o ca t io n i sg iv en b y t he r ed t ri an gl e. N or th i s 0 declination.


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35S ) s ho w n o c ha ng e i n t he e st im at e o f SB a t l owk, t wo l at it ud e b an ds ( 25S and 50S) show ad e cr e as e i n SB of 2 6 a t v al ue s o f kcut around5 0, a nd o ne l at it ud e b an d ( 35N) shows a changei n b e ha v io r a t a r ou n dkcut = 1 0 0. T h e l o w- q ua l it yd a t a c a n r e s u l t i n o v e r e s t i m a t e s o f V G P d i s p e r s i o n .O ur c ur ren t da ta s et i s n ot abl e t o r es ol ve an y

l at it ud in al t re nd i n t hi s b ia s. F or l at it ud es 5 0

S,25S , 3 5N, values ofkcut o f 2 50 o r m o r e r e s ul t i nn o is y e s ti m at e s o f SB d ue t o t he s ma ll n um be r o f d at a r et ai ne d. A na ly si s o f r ev er se p ol ar it y TA FId a ta s h ow e d s i mi l ar b e ha v io r a s a f u nc t io n o f kcut,a l t h ou g h e s t i m a t es o f b o t hSB and DI d i s p l ay m o r es c at t er o w in g t o t h e m u c h s m al l er d a ta s e ts .

[ 18 ] O n t h e b as is o f t he s e r e s ul t s w e u se t h ef o l l o w i n g d a t a s e l e c t i o n c r i t e r i a f o r f u r t h e r a n a l y s e so f T A F I d a t a : w e r e q u i r e ( 1 ) n 5 , ( 2 )k > 5 0 . T h eo ne e xc ep ti on t o t hi s i s t he d at a s et f ro m E as te r I s la nd f or w hi ch o nl y 4 c or es w er e a va il ab le f or

s t ud y. I n t h is c a se w e r e qu i re n 4 , a nd k > 5 0 i no r de r t o r e ta i n g e og r ap h ic a l s a mp l in g . T h e TA F Id at a s et i s s um ma ri ze d i n F ig ur e 6 o n a s tu dy -b y-s tu dy b as is , s it es w it h k

5 0 a r e d i st i ng u is h ed

( g re e n) f r om t h os e w i th k > 5 0 ( bl ue ).

2.3. Effect of Transitional Data

[ 19] T ra d it i on a ll y, s i te s w i th l ow V G P l a ti t ud e sa r e r e m o ve d f r om s t ud i es o f P S V a n d t h e T AF. T h isa p pr o ac h h a s b e e n j u st i fi e d a s a m e an s o f c h ar a c-t e r i zi n g s t a b le p o l ar i t y a v e ra g e f i e l d g e o m e t r y a n di ts p al eo se cu la r v ar ia ti on , b ut i t i s i mp or ta nt t or e c o g n i z e t h a t i t m a y w e l l p r e j u d i c e o u r v i e w o f t h ephenomena we want to study. In addition, thec ho ic e o f w hi ch d at a t o e xc lu de i s a d h oc . We d on ot t ry t o r es ol ve t hi s i ss ue h er e, b ut s ho w h ow

d i ff e re n t c r it e ri a f o r i d en t if y in g ( a nd e x cl u di n g)t r a n si t i o na l d a t a a f f e c t e s t i m a t e s o f V G P d i s p er s i o nd et er mi ne d f ro m t he TA FI d at a s et . G iv en t het e mp o ra l d i st r ib u ti o n o f t h e TA F I d a ta ( F ig u re 4 ) ,w e a na l yz e B r un he s -a ge n or ma l p ol a ri t y d at a,M a tu y am a -a g e r e v e rs e p o la r it y d a ta , a n d 0 5 M ac om bi ne d n or ma l a nd r ev er se p ol ar it y d at a. F or e ac h TA FI s tu dy w e c om pu te SB u si ng t he d at aq ua li ty c ri te ri a a bo ve , f or 3 d if fe re nt c ho ic es o f V G P l at it ud e c ut -o ff ( Fi gu re 7 ). We c om pa re t heu se o f a c on st an t V GP l at it ud e c ut -o ff ( he re 4 5)w it h o n e t ha t i s p er mi tt ed t o v ar y w it h t h ee mp ir i ca ll y d er i ve d d is pe rs i on a s p r op os e d b y

Vandamme [ 1 99 4 ] . T h e Va n da m me c r i te r io nprescribes a VGP latitude cut-off as

lcut 9 0 1:8SB 5 : 5

SB i s c al cu la te d f ro m t he d at a s et , t he n s it es w it hV G P l at it ud es l es s t ha n lcut removed, SB recom-puted, and the procedure repeted; that is, all dataa r e i n c lu d e d.

[ 20] F i gu r e 7 a s h ow s t h at m o st B r un h es - ag e T A FId at a d o n ot i nc lu de l ow -l at it ud e V G Ps , a nd e st i-m at es o f SB a re i n a gr ee me nt f or a ny c ho ic e o f V G P c ut - of f . F i el d p r oc ed u re s w e re d e li b er a te l yf o cu s ed o n o b ta i ni n g s t ab l e p o la r it y d at a s o t h is i sn o t s u r p r i s i ng . F o r t h e t h r ee B r u n he s s t u d i e s w h e r es o m e l o w - V G P - l a t i t u d e d a t a a r e p r e s e n t , a s s e e n b yt h e h i gh e r e s ti m at e s o f SB f o r t h e g r e e n ( n o c u t- o ff a pp li ed ) v er su s b la ck o r b lu e ( cu t- of f a pp li ed )s ym bo ls , t he u se o f t he Va nd am me o r c on st an t V G P c u t -o f f c r i t e r i a d o e s n o t s i g n i f i ca n t ly a l t er t h ee s ti m at e s o f d i sp er s io n. F o r t h e M a tu ya m a- a ged a ta , s e ve r al s t ud i es s h ow h i gh SB w he n a ll V GPl at it ud e d at a a re i nc lu de d. ( No te t ha t SB f or t heC hi le an d at a a t 3 6S i s g re at er t ha n 3 5, and soplots off the scale of Figure 7, when either no VGP

Figure 7. E f fe ct o f V GP l a ti t ud e c u t- o ff o n e st i ma t eso f d i sp e rs i on f o r t h e TA F I s t ud i es f o r ( a ) B r un h es - ag en o r m a l p o l a r i t y d a t a , ( b ) M a t u y a m a - a g e r e v e r s e p o l a r i t y

d at a, a nd ( c) 0 5 M a c omb in ed n or ma l a nd r ev er sepolarity data. A constant 45 c ut - of f ( bl a ck ) , t h eVa n da mm e c r it e ri o n ( b lu e ), a n d n o c u t- o ff ( g re en ) a r eshown.


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c u t- o ff o r t h e Va n da mm e c r it e ri o n i s a p pl i ed . ) I na d di t io n , t w o o f t h es e s t ud i es , P a ta g on i a [Brown etal., 2 00 4a ] a nd C hi le ( Ap pe nd ix A ) [Brown et al.,2 0 04 b ], s h ow q u it e d i ff e re n t e s ti m at e s o f SB whent he Va nd am me v er su s t he c on st an t V G P c ut -o ff c r it e ri o n i s a p pl i ed . T h e u s e o f Va n da m me s c r it e -r i on a s su m es t h at l o w l a t i tu d e V G P s a r e o u t l ie r s i nt h e d i st r ib u ti o n; i f s e ve r al l o w- V G P- l at i tu d e s i te sa r e p r e s en t t h e Va n d am m e a l g or i t h m w i l l c o n ve r g ew it ho ut r em ov in g t he se s it es . I n t he c as e o f t heC h il e an d a ta s e t, m o st M a tu y am a- a ge s i te s s a mp l et h e M a tu y am a -B r u nh e s r e ve r sa l [Brown et al.,2 00 4b ], a nd i n t he c as e o f t he P at ag on ia d at a s et

[Brown et al., 20 04 a] 5 o f t he 14 s it es me et in go ur d at a q ua li ty s el ec ti on c ri te ri a h av e l ow V GPl at it ud es . A na l ys es o f t he 0 5 M a d at a s et s ho w s ev er al s tu di es f or w hi ch t he Va nd am mea n d c o n s t a n t c u t - o f f c r i t e r i a g i v e d i f f e r e n t e s t i m a t e sof SB. T hi s r es ul ts f r om c om bi ni ng n or ma l a ndr e ve r se p ol ar i ti es t ha t m ay i nd i vi du al l y h av ed i f f er e n t e s t i ma t e s o f SB a n d d i f f e r e n t V G P l a t i t u d epopulations.

[ 21] I n s um ma ry, F ig ur e 7 s ho ws t ha t m os t TA FIs t ud i es c o nt a in f e w o r n o l o w- V G P- l at i tu d e d a ta .H o we v er , i n t h e c a s e s w h e r e s u ch d a ta a r e p r e s en t

t h e Va n d am m e a n d c o n s ta n t V G P c u t - of f c r i t er i o nc an r es ul t i n d if fe re nt e st im at es o f SB, e s p e ci a l l yw he n n or ma l a nd r ev er s e p ol ar it y d at a a re c om -bined. Because we wish here to focus on stablepolarity field behavior, we choose to remove low-V GP -l at it ud e s it es . O n t he b as is o f t he a bo vea na ly se s, w e c ho os e t o u se t he c on st an t V G P c ut -o f f c r it e ri o n a n d e x cl u de s i te s w i th V G P l a ti t ud e sl e s s t h a n 4 5. I m po r ta nt l y, w e n o te t h at i n c om p ar -i n g t h e p r ed i ct i on s o f s t at i st i ca l m o de l s w i th d a tat he s am e c ri te ri on c an b e a pp li ed t o s i mu la ti on sf r om s ta ti st ic al m od el s a s t o t he d at a, a nd s o o ur

c h o i c e i s n o t r e s t r i c t i v e . A f t e r r e m o v a l o f l o w -k andl o w- V GP - la t it u de s i te s , o u r TA F I d a ta c o nt r ib u te6 6 1 s i te s .

3. Additional Data

[ 22 ] We s up pl em en t t he TA FI d at a w it h e ig ht r e gi o na l d a ta s e ts b as e d o n c o mp i la t io n s f r om t h el i te r at u re ; c o ns e qu e nt l y t h es e a r e m o re h e te r og e -n e o us i n a s s o ci a t e d a g e i n f o rm a t i on , s a m p li n g a n dl a bo r at o ry p r oc e du r es t h an o u r TA F I d a ta s e t. S i xo f t h e c o m p i l a t i o n s h a v e b e e n d e s c r i b e d e l s e w h e r e :paleomagnetic directions from the NW and SW

U S A (Tauxe et al. [ 2 0 0 4 b ] a n dTauxe et al. [2003],r e s p e c t i v e l y ) , M e x i c o [Mejia et al., 2 0 0 5 ;Lawrenceet al., 2 0 0 6 ] , H a w a i i , t h e S o u t h P a c i f i c a n d R e u n i o n[Lawrence et al., 2 00 6] . I n t hr ee c as es , t he S W U SA ,t h e N W U S A, a n d M e xi c o, t h e p u bl i sh e d r e gi o na lc o mp i la t io n s i n cl u de d d a ta c o ll e ct e d a s p a rt o f t h eT A F I p r o j e c t ( s e e T a b l e s 1 a n d 2 ) . I n t h i s p a p e r , w ea n a l y z e a l l TA F I d a t a t o g e t h e r b e c a u s e o f t he ir h om og en ei ty i n t er ms o f f ie ld w or k a nd l abprotocols. To avoid duplication, the TAFI data fromt h e S a n F r a n c i s c o v o l c a n i c s , S n a k e R i v e r p l a i n , a n dM ex ic o a re t hu s r em ov ed f ro m t he S W U SA , N WU S A a n d M e xi c an r e gi o na l c o mp i la t io n s r e sp e c-t i ve l y. We a d d n e w d at a s e ts f o r N e w Z e a l an d a n dJ a pa n, a nd s u mm ar i ze t he s e h er e. S am pl in g i nboth Japan and New Zealand has been concentratedo n B ru nh es o r M at uy am a a ge o ut cr op s, a nd s ow e i n ve s t i g at e t h es e p o la r i t y p e r i o ds o n ly.P a le o ma g ne t ic a n d a g e d a ta r e po r te d i n a l l s t ud i esc o n t ri b u t in g t o t h e r e g i on a l c o m pi l a t io n s a r e a v a il -a bl e f r om t he M ag IC d at ab as e a t E a rt hR e f. o rg(www.earthref.org/MAGIC).

[ 23] T h e v o lc a ni c c e n te r s o f t h e N o r t h I s l an d , N e wZ e a l an d h a v e b e e n e x t en s i v el y s a m p le d f o r s t u d ie so f p al e os e cu la r v ar i at i on , b eg i nn i ng w i th Cox

Table 3. N e w Z e a l a nd S t u d i e sa

Location l, d e g f, deg N Brunhes N Matuyama Ref er en ce

Au ck lan d v o lcan ic p r o v in ce 36.9 174.8 17 . . . Shibuya et al. [1992]S o u th A u c kl a n d v o l ca n i c p r ov i n ce 37.2 174.9 7 10 Briggs et al. [1994]T aup o ig n imb r itesb 38.5 176.5 15 . . . Shane et al. [1994]

No r th lan d 35.4 174.0 9 2 Shibuya et al. [1995]Cen tr al Tau p oc,d 38.5 176.0 39 14 Tanaka et al. [1996]T aup o , Ru ap eh u v o lcan od 39.3 175.6 27 . . . Tanaka et al. [1997]Total 114 26

aD e f i ni t i o ns a r e a s f o l l ow s :l, m e an s t ud y l a ti t ud e i n d e gr e es ;f, m e an s t ud y l o ng i tu de i n d e gr e es ; N Brunhes/N Matuyama, n u mb e r o f B r un h es /

M a t uy a m a s i t e s w i t h n 3 . T h er e a r e 9 2 B r un h es s i te s w i th k > 1 0 0 a n d n 5 , w i t h a m e a n d i r e c ti o n ,D = 8 . 2, I = 56.7 (IGAD = 5 7. 5) a n da95 = 2 . 1. T h e r e a r e 1 4 M a t y u a m a s i t e s w i t h k > 1 0 0 ,n 5 , w i t h a m e a n d i r e c ti o n ,D = 1 8 0 . 4,I = 6 2 . 1 (IGAD = 5 7 . 5) a n da95 = 6 . 2.

bM a m ak u i g n i mb r i t e i s e x c lu d e d s i n c e r e p or t e d d i r e c t i on s a r e a n o ma l o us a n d n o t w e l l u n d e r s t oo d .

cD i r e ct i o n s f r o m L a k e W h a k a m ur a a r e e x c l ud e d s i n ce p o s t e m p l a c e me n t t e c t on i c s r e p or t e d .

dD i r ec t i o ns s u p er c e d e a n d r e p la c e t h o s e o f Cox [1969, 1971].


3

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1 2 o f 3 1


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[ 1 96 9 , 1 9 71 ] . O u r c o mp i la t io n i n cl u de s p a le o di r -e ct io ns f ro m a ll s it es , e xc ep t t ho se s us pe ct ed o f t ec to ni c r ot at io n a nd t ho se w it h f ew er t ha n 3s am pl es . I t c om pr is es 1 40 s it es o f B ru nhe s or M a tu y am a a g e: f l ow s , d i ke s , w e ld e d t u ff s , w e ld e d

i g n i m b r i t e s o r s m a l l d o m e s ( T a b l e 3 ) . O f t h e s e 1 4 0s it es , 11 2 B ru nh es -a ge s it es a nd 2 2 M at uy am as it es h av e a t l ea st 5 s am pl es a nd a re u se d h er e( F ig u re s 8 a a n d 8 b ). I n m a ny s t ud i es , r a di o me t ri cd at es w er e o bt ai ne d f or s ev er al o f t he f lo ws s am -pled for paleomagnetic purposes. We use direc-t io na l a nd a ge d at a f ro m s it es f ro m t he A uc kl an dprovince [Shibuya et al., 1 9 92 ] , S o ut h A u ck l an dprovince [Briggs et al., 1 9 94 ] , N o rt h la n d [Shibuyaet al., 1 9 95 ] , a n d t h e Ta u po Vo l ca n ic Z o ne [Shaneet al., 1 99 4; Tanaka et al., 1 99 6, 1 99 7] . We t ak ec ar e t o a vo id s am pl es f ro m t he M am ak u i gn im -

brite, since the origin of anomalous directions fromt h is e x po s ur e i s d e ba t ed [Shane et al., 1 9 94 ] .

[ 24] T he d at a s et f or J ap an c om pr is es 1 76 s it esw it h a t l ea st 3 s am pl es p er s it e, o f w hi ch 1 72 a reB r u n h e s a g e ( T a b l e 4 ) . O f t h e s e , o n l y 1 2 6 B r u n h e s -a ge s it es h av e a t l ea st 5 s am pl es a nd a re u se d h er e( F i g u r e s 8 c a n d 8 d ) . A s w i t h t h e N e w Z e a l a n d d a t as e t, m an y o f t h e p a le o ma g ne t ic s t ud i es a l so r e po r t n ew r ad io me tr ic d at es . T he r eg io na l t ec to ni cs i sc o mp l ic a te d w i th b l oc k r o ta t io n s l i ke l y a f fe ct i ngs o me a r ea s i n cl u di n g t h e I z u P e ni n su l a [Kikawa etal., 1 98 9] , a nd w e t ak e c ar e n ot t o i nc lu de s uc hs i t e s . N o t a b l y , w e d o n o t i n c l u d e h e r e t h e s t u d i e s o f

Kono [ 19 68 , 1 97 1] t ha t w er e i nc lu de d i n o ur previous global PSV data compilation [Johnsonand Constable, 1 99 6] . I t i s p os si bl e t ha t s om e o f t h es e s i te s a r e t e ct o ni c al l y a f fe c te d. F u rt h er m or e ,

Figure 8. ( a , b ) N e w Z e a l an d a n d ( c , d ) J a p an e s e d a t a c o m p i la t i on s . A g e d i s t r i b u t io n s o f c o n t ri b u t in g s i t e s ( n 5,k > 1 0 0) a r e s h o wn i n N ew Z e al a nd ( F ig u re 8 a ), a n d J a p an ( F ig u re 8 c ). E q ua l a r ea p r oj e ct i on s ( f ig u re f o rm at a s i nF i gu r e 6 ) o f s i te d i re ct i on s a r e s h o w n i n N ew Z e al a nd ( F ig u re 8 b ) a n d J a p an ( F ig u re 8 d ).


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o t he r, m o re r e ce n t s t ud i es p r ov i de p a le om a gn e ti cd i re c ti o ns a t n e ar b y s i te s , a n d u s e m o de r n l a bo r a-t o r y m e t ho d s .

[ 25] We c on du ct ed a n a ss es sm en t o f d at a q ua li tyf o r t h es e n e w c o mp i la t io n s. E s ti m at e s o f DI weref ou nd t o b e r ob us t f or a ll v al ue s o f kcut. E s t i m at e so f P S V a r e r o b u s t f o r v a l u e s o f kcut u p t o a b o u t 2 5 0( b ey o nd w h ic h t h er e a r e i n su f fi c ie n t d a ta t o a s se s sP SV ), a nd g iv e SB 16. B ec au se t he d at a c ani n c l ud e s t u d ie s w i t h m i n i ma l d e m ag n e ti z a t io n , w ec o ns i de r s i te s w i th k > 1 00 i n o ur f ur th er i nv es -t i ga t io n s, f o r c o mp a ti b il i ty w i th t h e o t he r r e gi o na lc o m pi l a t io n s u s e d h e r e.

[ 26] P a le o di r ec t io n s f r om a l l c o nt r ib u ti n g s i te s a r es ho wn i n F ig ur es 8 b a nd 8 d. F or N ew Z ea la nd ,9 2 B r un h es - ag e s t ab l e p o la r it y s i te s h a ve k > 1 00and n 5 . T h e d i st r ib u ti o n o f t h es e d i re c ti o ns i sn o n - Fi s h e ri a n ( F i g ur e 8 b ) : T h e m e a n i n c l i n at i o n i sc om pa ti bl e w it h t ha t p re di ct ed b y G AD , b ut as i gn i fi c an t p o si t iv e d e cl i na t io n a n om a ly i s o b -s er ve d ( Ta bl e 3 ). O f t he 2 2 M at uy am a s it es w it hn 5 , 1 4 h a v e a v a l u e o f k > 1 0 0 , a n d t h e r e s u l t i n gm e an d i re c ti o n i s c o mp a ti b le w i th G A D ( Ta b le 3 ) .F o r J a p a n , 8 0 B r u n h e s - a g e s i t e s h a v en

5 a n dk >

1 0 0 . T h e d i s t r i b u t i o n o f d i r e c t i o n s i s n o n - F i s h e r i a n ,w i t h a m e a n d i r e c t io n i n d i st i n g ui s h a bl e f r o m G A D( Ta b l e 4 ) .

4. Toward a New Global DataCompilation: Latitudinal Variations inthe TAF and PSV

[ 27] We c om bi ne o ur TA FI d at a s et w it h t he n ewc om pi la ti on s f ro m J ap an a nd N ew Z ea la nd , a ndprevi ously publi shed compil ations for the SW

U SA [Tauxe et al., 2003], the NW USA [Tauxeet al., 2 00 4b ], a nd 2 0 l a ti t ud e [Lawrence et al.,2 00 6] . F or t he TA FI d at a w e r et ai n s it es w it h a t l ea st 5 s am pl es , v al ue s o f k g re at er t ha n 5 0 a ndV G P l a t i t u d e s h i g h e r t h a n 4 5. A s n o t e d p r e v i o u s l yt h e o n e e x ce p ti o n t o t h is i s E a st e r I s l a nd f o r w h i c hw e a ll ow 4 s am pl es p er s it e. F or t he r eg io na lc o m p i l a t i o n s w e r e t a i n s i t e s w i t h a t l e a s t 5 s a m p l e sa n d v a l u es o f k g r e a t e r t h a n 1 0 0 . T h e h i g h e r k c u t -o f f f o r t h e r e g i o n a l c o m p i l a t i o n s r e t a i n s c o n s i s t e n c yw it h t he o ri gi na l p ub li ca ti on s o f t he se d at a s et s,a nd i s m or e c on se rv at iv e b ec au se o f m in im al

d e m a g n e t i z a t i o n a t s o m e s i t e s i n o l d e r s t u d i e s . T h i sr e s u l t s i n a d a t a s e t o f 2 1 0 7 (D, I) p a ir s , c o mp a re dw it h 31 4 (D, I) p ai rs m ee ti ng b ot h t he se c ri te ri aa nd t he D MA G 4 c ri te ri a o f M M9 7. T he n ew d at as et s up er ce de s a nd r ep la ce s a ll o f t he M M9 7D MA G 4 s it es , e xc ep t t ho se f ro m I ce la nd , a nds p an s l a ti t ud e s f r om 7 8S t o 5 3N. In particular,t h e TA F I d a ta s i gn i fi c an t ly i m pr o ve c o ve r ag e i nt h e S ou t he r n H e mi s ph er e . T h is w a s p r ev i ou sl yr es tr ic te d t o 3 l oc at io ns ( Ta hi ti , R eu ni on a nd

New Zealand (Table 1), for which the numbers ofd at a h av e b ee n g re at ly i nc re as ed ( Ta bl e 5 )) a ndn o w i n c l u d e s T A F I d a t a f r o m 8 a d d i t i o n a l l o c a t i o n s

( T a b l e 2 ) . S i t e l e v e l a g e i n f o r m a t i o n i s a v a i l a b l e f o r a l l t h e r e gi o na l c om p il a ti o ns e x ce p t t h at f r om t h eSW U SA [Tauxe et al., 2 00 3] . I n t ha t s tu dy s it esw it h a ge s l es s t ha n 5 M a w er e r et ai ne d, b ut a gei n fo r ma t io n i s o n ly r e po r te d f o r t h os e s i te s d a te dr a di o me t ri c al l y; w e u s e o n ly t h es e s i te s ( Ta b le 5 ) .E xa mi na ti on o f t he t em po ra l d is tr ib ut io n o f o ur d a t a s e t ( F i g u r e 9 a n d T a b l e 5 ) s h o w s t h a t s a m p l i n gi s c o nc e nt r at e d i n t h e B r un h es , b u t s u gg e st s t h er ea r e a l s o s u f f ic i e nt d a t a t o e x a mi n e b e h a v i o r d u r i n gt h e M a t uy a m a c h r o n .

Table 4. J a p a n S tu d i e sa

L o catio n l, d e g f, deg N Brunhes N Matuyama Ref er en ce

Higshi-Izu volcano group 34.0 138.0 22 . . . Heki [1983]

Kagoshima Prefecture pyr oclastics 32.0 131.5 6 . . .

Heki [1983]Ashitaka dikes 35.2 138.8 35 . . . Tsunakawa and Hamano [1988]A s h it a k a v o l ca n o ; I z u -O s h im a I s lb 35.0 139.0 48 . . . Kikawa et al. [1989]Mount Fuji and Mount Oshima 35.4 138.8 6 . . . Tanaka [1990]Goto Islands, Tsushima Str ait 32.9 128.9 5 . . . Ishikawa and Tagami [1991]Z a o v o l ca n i c g r ou pc 38.1 140.5 11 4 Otake et al. [1993]Daisen volcano 35.4 133.6 2 . . . Tanaka et al. [1994]Shikotsu caldera 42.6 141.3 4 . . . Tanaka et al. [1994]Ontake Volcano 35.9 137.5 33 . . . Tanaka and Kobayashi [2003]Total 114 26

aTa b le f o rm a t i s a s i n Ta b le 3 .

bWe e x cl u de d s i te s f r om I z u p e n i ns u la o wi n g t o p o ss i bl e t e ct o ni c r o ta t io n. T he r e a r e 8 0 B r un he s s i te s w i th k > 1 0 0, w i th a m e an d i re c ti o n,

D = 3 5 8. 7, I = 5 4. 6 (IGAD = 5 4. 9) a nd a95 = 2 .6.c

We e x c l ud e d s i t e Z K 0 3, s i n c e o f J a r a mi l l o a g e . We a l s o e x c l u d ed t h e f o u r r e v e rs e p o l a r i t y s i t e s f r o m o u r c a l c u l a ti o n s.


3

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1 4 o f 3 1


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[ 28] T he n ew d at a s et a lt ho ug h s ti ll n ot g lo ba l i nd i s t r ib u t i on , o v e r wh e l ms p r e vi o u s c o m pi l a t io n s i nn u mb e r, q u al i ty a n d a g e c o ns t ra i nt s , a n d s u bs t an -t i a l ly i m p ro v e s g e o gr a p hi c a l c o v er a g e a n d t e m p o-r a l s a m p l i n g o f t h e B r u n h e s a n d M a t u y a m a c h r o n s .B ec au se t he re a re g ap s i n d at a c ov er ag e ( e. g. ,E u ro p e) w e r e st r ic t o u rs e lv e s t o i n ve s ti g at i on s o f l at it ud in al s tr uc tu re i n t he TA F a nd P SV. Wec o rr e ct f o r p l at e m ot i on s u s in g t h e m o de l N U V EL1 - A [DeMets et al., 1 9 9 4 ] a n d o u r e s t i m a t e s o f s i t eage.

4.1. PSV: Latitudinal Variations in VGPDispersion

[ 29] We f i r st e x am i ne l a ti t ud i na l v a ri a ti o ns i n d i s-persion recorded by the individual TAFI studiesa nd r eg io na l c om pi la ti on s ( Ta bl es 6 a nd 7 a ndF i gu r e 1 0 ). SB i s c al cu la te d u si ng e qu at io n ( 4) ,a n d 9 5 % c o nf i de n ce l i mi t s e s ti m at e d u s in g a b o ot -s t r ap r e sa m pl i ng t e ch n iq u e. F i gu r e 1 0 s h ow s l e ssl a ti t ud i na l v ar i at i on i n d i sp e rs i on d u ri n g t h eB r un h es t h an t h e M a tu y am a , a n d w i th t h e e x ce p -t i on o f t h e H a w a ii a n c o m p il a ti o n a t 2 0N, the datas e t f o r t h e B r un h es s u gg e st s a c o ns t an t d i sp e rs i onw i t h l a t i t u d e o f a r o u n d 1 6. T h e M a t u y a m a d a t a s e t s h ow s s e ve r al e s ti m at e s o f SB t h at a r e h i gh e r t h and u ri n g t h e B r un h es . T h is d i ff e re nc e i nSB betweenn or ma l a nd r ev er s e p ol ar it y c hr on s h as b ee n o b-s e rv e d m a n y t i me s p r ev i ou s ly a n d i t s o r ig i n i s s t il ln ot u nd er s to od [McElhinny et al., 1 99 6] . T heM a tu y am a d a ta d i sp l ay a s t ro n ge r d e pe n de n ce o nl a t i tu d e o f SB. I t i s u n c l e a r w h e t h e r SB i s s y m m e t r i ca b ou t t h e e q ua t or ; d i ff e re n t b e ha v io r i s s e en i n t h e

Northern and Southern Hemispheres, although thisc ou ld b e r el at ed t o d if fe re nc es i n s pa ti al a ndt e mp o ra l s a mp l in g . We n o te t h at t h e d a t a s e t f r om

E as te r I sl an d, f or w hi ch w e h av e o nl y 4 s am pl esper site , appear s to be compat ible with others t ud ie s . T h e s t ud y- b y- s tu dy c om p ar i so n o f SBi n di c at e s r e gi o na l v a ri a ti o ns i n SB, f or e xa mp le ,a t 5 3N [Coe et al., 2 0 0 0 ;Mejia et al., 2 0 0 2 ;Stoneand Layer, 2 0 0 6 ] a n d a t 5 0S [Brown et al., 2 0 0 4 a ;

Mejia et al., 2 0 0 4] . H o w e ve r , b e c a u s e t h e n u m be r so f d a t a f r o m i n d i v i d u a l s t u d i e s a r e r a t h e r l o w ( 7 o u t o f 1 3 M at uy am a TA FI s tu di es h av e l es s t ha n 1 0c o n t r i b u t i n g s i t e s ) d e t a i l e d c o m p a r i s o n s o f r e g i o n a lv a r i a t i o n s i nSB, a n d a s s e s s m e n t o f P S V m o d e l s a r e

n o t p o s s ib l e .

Table 5. N u mb e rs o f D a t a U s e d i n T h is S t ud ya

Compilation Nused N Bru N Mat NGau/Gil Ref er en ce

TAFIb,c 6 61 408 194 59 this paper

Japan 8 0 80 . . . . . .

t h i s p a p er New Z ealan d 1 0 6 9 2 1 4 . . . t h i s p a p er Hawaii 7 27 288 57 382 Lawrence et al. [2006]Mexico 1 37 44 44 49 Lawrence et al. [2006]South Pacific 2 09 1 163 45 Lawrence et al. [2006]Reunion 43 31 12 . . . Lawrence et al. [2006]

N. W. USA 8 9 6 2 1 1 1 6 Tauxe et al. [2004b]S. W. USA 55 31 12 12 Tauxe et al. [2003]Totals 2107 1037 507 563

aD e f i n i t i o n s a r e a s f o l l o w s : C o m p i l a t i o n , d a t a c o m p i l a t i o n r e p o r t e d b y c i t e d r e f e r e n c e ; Nused, a l l d a t a m e e t i n g o u r s e l e c t i o n c r i t e r i a ( s i t e s w i t h a t

l e a s t f i v e s a m p l e s p e r s i t e ,k > 1 0 0, a n d V G P l a ti t ud e g r ea t er t h an 4 5); N Bru, B r u nh e s ;N Mat, M a t uy a m a; NGau/Gil, G au s s a n d G i lb e rt a g e s i t e scombined.

b N ew data from M exico, the Snake R iver Plain, and the San Francisco volcanics are included in the TAFI summary numbers and are excluded

f r o m t h e s u m ma r y n u m b e r s f o r t h e M e x i c a n, N W U S A , a n d S W U S A c o m p i la t i o ns .c

TA F I d a t a s e t i n c l ud e s t h e E a s t er I s l a nd d a t a f o r w h i c h f o u r s a m pl e s p e r s i t e a r e a l l o we d ( s e e t e x t ).

Figure 9. A ge d i st r ib u ti o n f o r t h e c o m bi n ed d a ta s e t o f 2 1 07 p a ir s o f d e cl i na t io n a n d i n cl i n at i on m ea s ur e -m e n t s ( T A F I d a t a p l u s r e g i o n a l c o m p i l a t i o n s ) u s e d i n t h eremaining analyses.


3

G3


1 5 o f 3 1


16/31

[ 30] To i mp ro ve t em po ra l s am pl in g, w e b in o ur d a ta s e t i n to l a ti t ud e b a nd s g o ve r ne d b y t h e d i st r i-bution of sampling sites, and no more than 5 inspatial extent.SB i s r e p o r t e d f o r l a t i t u d e b a n d s w i t hm o re t h an 1 0 s i te s ; t h e e x c ep t io n i s t h e M a t u ya m ae s t im a te o f SB a t A nt ar ct ic a. T hi s h as o nl y 6c o nt r ib u ti n g s i te s , b u t t h e v a lu e o f SB i s s u p po r t e dby re ce nt me as ur em en ts on ad di ti on al da ta

( K . L a wr e nc e e t a l ., u n pu b li s he d d a ta , 2 0 08 ) , a n dt h is d a tu m p r ov i de s c r it i ca l l a ti t ud i na l c o ve r ag e.V G P d is p er s io n f o r t he l at i tu d in al l y b in n edB ru nh es , M at uy am a a nd 0 5 M a d at a s et s i ss ho wn i n F ig ur e 11 ( va lu es a re g iv en i n Ta bl e 8 ),a n d c o m pa r e d w i t h p r e d ic t i o ns f r o m t w o p a l eo s e c -u l ar v a ri a ti o n m o de l s: M o de l G [McFadden et al.,1 98 8] a nd T K0 3 [Tauxe and Kent, 2 00 4] . T he T K0 3predictions assume a TAF specified by GAD, and1 0, 0 00 s i mu la ti on s w er e m ad e a t l at it u de i nc r e-m e n t s o f 5. S i t e s w i t h V G P l a t i t u d e s l e s s t h a n 4 5

a r e e x cl u de d f r om t h e s i m u la t ed d a ta s e ts . I n a dd i -t i on , f o r e a c h t e m p or a l s u b s et o f t h e d a t a w e s h owt h e m e a n SB c a l cu l at e d f r om t h e d a ta . We n o te t h at t h e s t at i st i ca l m o de l o f Constable and Parker[1988]predicts almost constantSB w i t h l a t i tu d e .

[ 31] T h e B r u nh e s a g e d a t a d i s pl a y l i t tl e v a r i at i on i nSB v e r su s l a ti t ud e i n t h e N o rt h er n H e mi s ph e re ; t h eo n l y e x c e p t i o n i s l o w s e c u l a r v a r i a t i o n a t 2 0N that

r e s u l t s f r o m t h e H a w a i i a n d a t a s e t [Lawrence et al.,2 00 6] . O ve ra ll t he se d at a a re c om pa ti bl e w it h ar a ng e o f m o de l s, f r om t h os e t h at p r ed i ct a c o ns t an t SB w i t h l a t i t u d e t o t h o s e t h a t s h o w s o m e l a t i t u d i n a lv a ri a ti o n ( s uc h a s M o de l G a n d T K 0 3 ).

[ 32] T h e M a t uy a ma d a ta s h ow s o me d e pe n de n ce o f SB o n l a ti t ud e , a l th o ug h , g i ve n t h e r e st r ic t ed d a tas et , i t i s d if fi cu lt t o d is cr im in at e a mo ng P SVm o de l s. N o ne o f t h e e x is t in g m o de l s a p pe ar s a ti s -

Table 6. TA F I S t u d ie s : V G P D i sp e rs i o n V er s us L a t it u de f o r t h e 1 7 TA F I S t ud i esa

l, deg N Bru SB lo hi N Mat SB lo

hi Ntotal SB lo hi

78.0 27 20.616.124.7 6 26.618.833.2 34 21.117.124.6

51.5 9 16.210.023.7

. . . . . .

35 18.215.421.2

47.1 11 21.715.427.0 9 17.19.618.7 25 21.417.425.038.9 . . . . . . 21 12.38.812.6 35 13.810.215.636.0 102 16.814.818.6 6 19.715.524.3 111 16.714.718.327.0 40 12.88.416.8 4 0 1 2 .88.416.823.5 . . . . . . 12 17.413.321.2 27 19.615.422.90.5 13 14.411.917.8 26 14.711.319.3 46 14.011.617.010.0 18 15.210.6

20.2. . . . . . 25 17.412.5

22.5

20.2 . . . . . . 9 11.27.614.7 13 10.68.6

13.4

28.6 . . . . . . 11 16.012.819.7 17 15.612.5

18.8

35.4 12 17.613.419.7 11 11.37.6

14.9 24 14.411.216.2

37.8 19 16.612.319.8 9 17.213.8

20.4 30 16.513.618.8

43.1 10 19.214.822.9 6 18.512.2

23.2 20 17.114.320.4

51.5 50 17.014.219.6

. . . . . . 50 17.014.219.6

53.1 36 18.015.321.1 14 27.224.0

31.5 50 20.718.323.4

53.5 47 13.010.814.3 9 32.126.1

40.1 71 16.513.918.9

aD e f i ni t i o ns a r e a s f o l l ow s : N Bru, N Mat, and Ntotal, n u mb e r o f B r un h es - ag e n o rm a l p ol a ri t y, M a tu ya m a- a ge r e ve r se p o la r it y, o r 0 5 M a n o rm a l

a n d r e v e r s e p o l a r i t y d a t a c o m b i n e d ;l, m e a n l a t i t u d e i n d e g r e e s f o r t h e B r u n h e s , M a t u y a m a , a n d 0 5 M a c o m b i n e d d a t a s e t s ; SB lo hi, t h e b e t w e e n - s i t e

V G P d i s p e r s io n i n d e g re e s , a l o n g w i t h t h e 9 5 % c o n f i d e nc e l i m i ts .

Table 7. R e g io n al C o mp i la t io n s: V G P D i sp e rs i o n Ve r su s L a ti t ud e f o r t h e E i gh t R e gi o na l C o mp i la t io n s G i ve n i n

Ta b le 5a

l, deg N Bru SB lo hi N Mat SB lo

hi Ntotal SB lo hi

38.2 9 2 15.313.117.5 13 16.410.623.2 106 15.313.317.620.9 3 1 12.89.915.6 . . . . . . 43 14.512.216.917.9 . . . . . . 81 19.316.422.0 207 17.816.019.419.9 28 5 9.99.0

10.923 13.18.5

17.5681 14.313.5

15.1

23.6 44 17.313.621.0 21 14.610.8

18.3 123 15.914.017.7

35.2 80 15.113.017.0

. . . . . . 80 15.113.017.0

36.1 30 15.913.118.9 12 18.014.2

22.9 53 17.215.019.7

45.5 62 15.113.416.8

. . . . . . 78 14.613.316.2

aT A F I d a t a f r o m t h e S n a k e R i v e r P l a i n , S a n F r a n c i s c o v o l c a n i c s , a n d M e x i c o a r e e x c l u d e d f r o m t h e r e g i o n a l c o m p i l a t i o n s . T a b l e f o r m a t i s a s i n

T ab l e 5 .


3

G3


1 6 o f 3 1


17/31

f a c t o r y ; M o d e l G a n d T K 0 3 f i t s o m e d a t a , b u t d a t as et s a t a ro un d 2 0S and 53N show rather higherd is pe rs io n. T he d at a s et f ro m t he S . P ac if ic ( ap -proxima tely 17S ) c on ta in s a l ar ge n um be r o f t r an s it i on a l d a ta , w h os e e f fe c t r e ma i ns e v id e nt e ve n w he n t he l ow es t V GP l at it ud e d at a a ree x cl u de d ( o ur c u t- o ff o f 4 5) . S u c h o v e r s a m pl i n g

o f t r an s it i on a l d a ta s h ou l d n o t b e t h e c a s e h o we v er f or t he l at it ud e b in s a t 2 2S a n d 5 0N.

[ 33] F or t h e c o mb in ed 0 5 M a d at a s e t,SB appearst o s h ow l e ss l a ti t ud i na l v a ri a ti o n t h an p r ed i ct e d b ye i th er M o de l G o r T K 03 . R e as o na b le a g re e me n t o f t h e d a t a w i th M o de l G , a n d i n p a rt i cu l ar , T K 03 , i ss e e n a t m i d n o r t h e r n ( 2 0 t o 5 0) a n d m i d s o u t h e r n(20 t o 4 0) l a t i tu d e s, b u t SB e s t im a t es f o r d a t a s e t sf r om E cu ad or , C os ta R ic a, a nd t he S ou th P ac if ica r e h i gh e r t h an m o de l p r ed i ct i on s . E s ti m at e s o f SBa t C os ta R ic a a nd E cu ad or m ay b e i nf lu en ce d b y

t h e s m al l s a mp l e s i ze ; a dd i ti o na l d a ta a r e c l ea r lyr e qu i re d t o b e tt e r e s ti m at e P S V a t l o w l a ti t ud e s.

[ 34 ] O ve ra ll o ur d at a a pp ea r t o s ho w d if fe re nt l a t i tu d i n al s t r u ct u r e f o r t h e B r u n he s a n d M a t u y a m ad a t a , a n d a n a l y s e s o f c o m b i n e d n o r m a l a n d r e v e r s ed at a s et s a re t hu s d if fi cu lt t o

campo geomagnetco - johnson et al

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