Abstract. A joint study of straln, anlsotropy of m-iiiiffi...

JOUBNÀI oF GLoPHYSICAL RESEARCH, yo t . g t , NO. 811, PAGES 1 t ,67 t -11 ,687, NoVEMBER 10, 1988

slRAIN, MACNETIC FABRIC, AND PALEOMÂCNETTSM OF THE DETORMED RED BEDS 0F ÎHÈPONT-REAN FORMÀTION, BRITTANY, FRANCE

Jcan-Paeca l Cogné

Lebore t .o l rê de Géophyc tque In !e rne , Un ivers l té de Rennes, France

Abstract . A jo int s tudy of s t ra ln, anlsotropyof m-ii i i ff i susceptibiltty (AMs), endpaleomagnet ism has been conducted in the deformad

rrd beds of the Ordovic lan Pont-Réan forme! ion,

B r l t t âny , F rance . t na l ys i s o f r esu l t s f r om 13

samp l l ng 3 l t es sho t r s t ha ! honogeneous sË ra in ,

es t lma red f r o rn che shapc o f e l l l p t i ca l r educ t i on

rpo t s t n each E l l e , con l r o l s t he devc lop rnen t o f a

n rgne t i c f ab r l c du r i ng t he Hc rcyn ian f o l d i ng o f

t h i c f o rma t i on . The good co r re l a t l on bê t t r een

s t râ l n and e l lS t enso r3 13 i n t e rP re l ed as ma in l y

â r t s i ng f r om a sc re i n - i nduced p rog ress l ve

ro taÈ ion o f p l ana r he tnâ t l t e t ha t t €nds co a l i gn

s l t h i n c l eavage p l ane . Th t s r o ta t i on o f r nagne t i c

ca r r l e rg i n t u rn i nduces â r o ta t l on o f ne t

neasu rab le p receccon j . c r emânenE dagne I i zâ t i on

toca rd c l eâvage . Because o f t he h i gh ang le

be t reen Ehè t n i t i a l l y subve r t i ca l o rdov i c l an

remenen! ûâgnet izat ion and che regional ly

subho r i zon ta l sho r t en ing d i r èc t l on , Eh i s

dêv1â t i on o f t he p re t cc l on i c megne I i ze t l on i s

ach ievcd r4 r i t hou t eny sca t l e r l ng buc w i t h e

p robab le c l us te r i ng e f f ec t uPon $ i r h i n - s l t e

l n i t l a l d i spe rs l ons . Fo r l he sâ rne reason l he

ove ra l l d i s c r i bu t i on o f mean -s l t e p re tec ton i c

negne t i zâ t i on vèc to r s i s ç l us te rêd ac t he i n s i t u

de fo rmed s ta te . t h l s r esu l t s i n an anoma lous

bchev io r o f t he c l ass l ca l f o l d t es t , t he t cou ld

e r roneous l y l ead t o ass i gnmenË o f a syn tec ton i c

t o pog t t ec ton i c âge t o t he ûagne t i zâ t l ons .

F tna l l y t hê sc re i n r enova l t echn ique .based upon

the pass i ve behav io r hypoches i s i s app l i ed t o t he

dev i . a ted vec lo r s . I t a l l o ss r ecove ry o f che

ln l t l a l d i r ec t i on o f t hese P re t , êc ton i c o rdov i c i . an

m â g n ê l l z a r l o n s ( D . 2 3 5 0 , I = 7 5 o , c 9 5 - 6 . 5 0 ) .

Introduct lon

A bâs l c l dea , co runon l y sdm i t t ed by a l l

r esea rche rs i n t he f i e l d o f pa leomagne t i sm , i s

t ha t t f a meSne t l zed s€dLmenÈary bed l s f o l ded ,

i t e nagne t i zâ t i on ' , t 111 be de f l ec ted away f r om l t s

o r l g l na l d i r ec t l on . Consequen t l y , l f one wan ts ! o

de te rm lne t he o r i g i na l d l r ec t i on o f nagnec i za t l on

w l t h i n a f o l ded a rea , he nus t f t r s t r ecove r t he

ro te t i on o r gequence o f r o ta t i ons l he t have

a f f ec ted each pa leomâgne t i c vec to r s t ud ied . The

more w ide l y used p rac t i ce t o Èh i s Pu rPose i s t o

consider that the g,eometry of the fo lded bedding

su r f ace , ass \ rmed t o be l n i t i a l l y p l ana r and

ho r l zon ta l , p rov l des good l nd i ca to r s o f t hese

ro te t ' 1on3 . F rom the desc r i p t l on o f t he poss ib l e

. r o ta t i on ( s ) unde rgone by t he bcdd ing su r f ace a t

each s l t e o f pa leomagne t l c sanp l i ng , an ove ra l l

n€ân d i r ec t i on o f P reÈec ton l c t i agne t i ze t i on can

Copyrlght 198E by the Amerlcan Geophysical Unlon

Paper nr:mber 88J4032400t48 -0227 | 88 / 88JB-3 240$0 5 . 00

then be recovered by apply lng the lnversero ta t l on ( s ) t ha t i hou ld un fo l d t he rockformat lon, Howcver, th ls t reatnent , knorn âs the' r ( b e d d l n g - ) t L l t c o r r e c r i o n t r I C r a h a m , 1 9 4 9 J ,bas l ca l l y r e l i es on t hê l np l l c i t assunp t i on cha tt he ro ta t i ons o f bedd ing p l ane ac tue l l y desc r i belhe ro l a t i ons o f pa leo rnagne t i c vecÈo rs . C lea r l y ,t . h i s t s t r ue on l y l f t hè megnec l c vecco r r ema lnsunchangcd w l t h i n t he bedd ing re fe rence f r ameth roughouc che f o l d l ng o f r ock un i t , l ha t i s ,vhen f o l d l ng i s ach ieved by r l g i d buck l l ng o f t hebeds , r r l t hou t any i n t e rna l de f , o rma t i on . I ne f f ecc , a rnong o the r g re l n - sca le de fo rna t i onnechan tsms , l t 19 now cs tab l l shed t ha t es t rE ln - l nduced ro ta t l on o f r aagnc t l c ca r r i e r s( " . g . , cee X ray d l f f r ac to rne t r y n€asu remen t , s byCogné and Gapa i s [ 1986 ] ) can g roduce s tgn l f l can tdev ia t l ons o f magne l l c vec to r s . t h t c has becnshown by ana log i ca l s imu la t i ons IOz i r na , 1980 ;Cogné , 1987b ; Anson end Koda rna , 1987 ] , n t rmc r i ca ln o d e l s l C o g n é e c a l . , 1 9 8 6 ; V a n d e r P l u l J r n ' 1 9 8 7 ;Kodama , 1988 ] and t h rough pâ leo rnegnê t l c ana l ys i s

o f d e f o r m e d r e d b e d s l f t r g f r e t a e t a l . , 1 9 E 1 ,

1 9 8 3 ; C o g n é a n d P e r r o u d , 1 9 8 5 ; C o g n é , 1 9 8 7 a J .

Th l s s t r a i n - i nduced dcv la t i on o f pa l ' eo rnagne t i c

vec to r s r nay be cons ide red as a second k l nd o f

r oÈâ t l on , w i t h r especC to Ehe i n t e rna l t e f e rence

f rame o f bedd ing p l ane . Consequen t l y , i n t he case

o f f o l d i ng ! t i ch l nÈe rna l de fo rmac ion ( f o r

examp le , when c l eavage deve lops ) t he desc r i p t i on

o f pa leomagnê t i c vec l o r r o !ac l ons 1n t e rms o f

bedd ing su r f ace geome t r y i s i ncomp leÈe , and t hus

conc lus l ons d rawn f r om the behEv lo r o fpa leomagneE ic vec to t popu lac i ons r t i t h r espec t t o

l he c l ass i ca l f o l d t es t mey be l nadequaÈe . Ï h i s

haa been c l ea r l y demons t raced on l he bas i s o f

some numer i ca l mode lg by Van de r P lu l j r n [ 1987 ]and Kodama [1988 ] , and by Cogné and Pe r roud

[1985 ] i n t he de fo r rned red beds o f t he Mar i t ime

. t l p s ( F r a n c e ) .

The p resen t , pape r add ressês t lÀ to nâ in Po in t s :( 1 ) co documen ! t he cha rac te r l za t i on o f s t r a i n

e f f ec t s uPon P re tec ton i c r e rnanen ! r negne t j . ze t i on

th rough Èhe s fudy o f s t r a i n , ân i soc roPy o f

nâgneÈ ic suscep r i b l l l t y (AMS) , and pa leomâ8ne t i 3m

n l t h l n de fo rmed red beds , and (2 ) t o de fend t he

ldee t hâ t a co r rec t l on f o r s t r a i n - i nduceddev la t i ons o f pa leo rnagne t i c vec to r s i s poss ib l e

ç i t h l n such f o rma t i ons ICogné and Pe r roud , 1987 ] '

t h i s wo rk i s t he t h i r d examp le i n a ee r i es o f

s lm i l a r i nves t i gâ t l ons we heve p rev i ous l ypub l i shed , f i r s t i n t he Pe r rn i an red bedE o f t he

t ' { a r i t i . ne a l ps [Cogné and Pe r roud , l 9E5 ] , t hen i nthe Perrnian red beds of the "Col du Somportr r int he Py renean Cha ln ICogné , 1987a1 .

Geo log l ca l Se t t i ng and Sa rnp l i ng S i t eg

the s tud led a rea i s s i t ua ted l n t he cen t ra lpa r t o f t he A rmor l can Mass t f , B r i t t eny , F rance(F tgu re 1 ) (mean pos i t l on : 4EoN, 2 . so t l ) and f o rms

11,671

15,674

pe r t o f mu l t l l a yc red sequence o f Pa leozo i cscd imen ts i n t he reg lon (F l gu re l a ) . The

Pon t -Réan red bed f o rn rac l on beg ins t he Pê leozoLcced imen ta t i on l n Cen t râ l B r i t t any , uncon fo r rnab l yove r l y l ng t he pe l l t i c B r l ove r i an basemen t , andover la in by the I 'Grès Armor icaln[ Forrnat ion. thePonc -Réan Fo rma t i on has no t ye t been da ted , buc apa leon to l og l ca l l y based A ren ig ( l owe r O rdov l c i an )

age has bcen p roposed by Cave t e t e l . [ 1gZg ] t o rthe lateral equivalent t tHoul in de Cha!eaupannsn

fo rma t l on , and a r ed iome t r i c age o f a65 + l - Ir ! .y . by U-Pb method has been obteined by Bonjoure t a l . [ 19SE ] on vo l can l c i nce rca la t l ons e j . t h i n

Cogné : De fo r rned Red Beds o f O rdov i c l an Pon t -Réan Fo rmê t ' 1on

F 1 g . 1 . ( a ) L o c a t i o n m : r p o f t h e s t u d i e d a r e a ( b o x e d ) ; d o c t e d a r e a , P a l e o z o i c c o v e r ;open ( so l i d ) s t a r : "Cap de l a Chèv re ' t ( t , ù l ou l i n de Cha teaupanne " ) Fo rma t i on . ( b )Loca t i on map o f t he samp l i ng s i t es ( l abe led do t s ) a round t he sync l i ne E t ruc tu re o f r hePon t -Réan Fo rma t l on (do t t ed a rea ) . Heavy l i , nes a re synsed imen ta r y no rmaL f au l t s . ( c )Schcma t i , c N -S c ross - sec t l ons 1 t o 3 es l ôca ted i n F i gu re l b ; t h ree f o rma l i ons a reshown : t he t iG rès A rmor i cê i n " Fo rne t i on ( sma l1 do t s ) , t he Pon t -Réan Fo rma t i on ( l a rgedo t s ) , end t hê undc r l y l ng f o tded B r i ove r i an basemen t . ( d ) Schemac i c c ross sec t i oni l l u s t r a t l ng t he s t r uc tu ra l con tex t du r i ng depos iL i on o f r he Pon t -Réan f o rma t i on I a f t e rB a l l a r d e t a l . . , l 9 E 6 ] ; 1 , B r i o v e r i a n b a s e r n e n t ; 2 , P o n r - R é a n F o r m a c i o n ; 3 , r ' G r è s

Armorlcalnr t Forrnat ion.

t he l a t e re l equ l va len t ' tCap de l a Chèv re t lFo rma t l on i n t he C rozon pen insu la (F l gu re 1a ) . t tseems l he re fo re r eaEonab le t o i n f e r a l owe rOrdov i c i an agc f o r t he Pon t -Réan Fo rma t l on . l hema jo r phase o f t he He rcyn ian o rogeny i n t heArmor i ca l n Mass l f i s r €spons ib l e f o r t he f o l d i ngo f t he P reca rnb r t an and Pa leozo i c f o rma t i ons o fC e n c r a l B r l t t ê n y ( F i g u r e s l b a n d l c ) [ L e C o r r e ,1 9 7 8 ; P e r c e v a u l t a n d C o b b o l d , 1 9 8 2 1 . F o r t h ePonc -Réan Fo rma t l on , t h l s r esu l t s i n an opensync l i ne s t r ucÈu re , r l t h subho r i zcn ta lapp rox lma te l y E -W t r end ing axes . Fo ld i ng 1saccompan ied by t he deve lopmen t . o f subve r t i ca l E -W

0 5 k m .l4J

r l end tng ex i e l p l âna r c l eevege . Sc ruccu ra lg tud les based upon s t r a l n r na rke r mêêsu remen ts I LeCor re and Le Théo f f ' 1976 ] , t exgu re d r ve loomen t[Gapa i s and Le Co r re , 19S ] . 1 , and me tamorph i s t r r[ H a n m c r e t a l . , 1 9 8 2 ] v e r e c o n d u c t e d i n ô e n r r a lB r i t t âny . They showcd a c l ea r s t r a i n g rad ien tf r om N t o S l n l he reg lon , l ead ing Gapa l s and LeCor re [ 1980 ] and Pe rcevau l t and Cobbo ld [ 1982 ] t oâ t t r l bu te t he He rcyn lan f o l d i ng o f t he pa leozo l c

cove r t o a c ruS !e l dEx t ra l shea r i ng p rocess .F ina l l y , r ecen t i nves r j . gec ions o f Ba l l a rd ec a l .[ 1986 ] and J . P . B run (pe rsona l co r rununLceÈ ton1987 ) based upon t he s tudy o f B r i ove r i an -Pa leoeo l c uncon fo rn l c l es , o f t h i c kness ve r i e t i onsin che Pont-Réan end Grès Armor icatn format lonse t c . , l ed Èhese au tho rs t o p ropose t ha tdcpos i t l on o f che Pon t -Réen Fo r rna t i on t ook p l ace

du r i ng en ex tens j , ona l even ! . t he sed imen ts we re

chus depos l t ed upon B r i ove r l an t l l t ed b l ocks i nsha l l bas ins o f ha l f g raben t ype (F i gu re 1d ) . One

can noce t ha t nÊa r co no rna l f au l t s , t he bedd ing

p lene rnay have had a s i gn i f t can t syn -depos i c i ond tp o f 10o t o 20o t owa rd t he cen te r o f t he bas in .We sha l l see l a t e r Èha r ch l s po ln t i s p robeb l y

! l gn l f l can t v r iÈh resp€cE t o t hè f i na ld i s t r t bu t l . ôn o f p r ima ry pa leomagne t i c d i r ec t i oûs .No te t ha t a s im l l a r syn -sed tmcnca ry d i pp ing was

suspec led by D io t t f 980 l i n t he Mou l i n deChateaupanne Format lon. th is was conf i r rned by

Pe r roud e t a l . [ 1986 ] , who sho red t hâ t t heOrdov l c l an pa leomagne t l c d i r ec t i on o f t h l sf o rmac lon i s mo re accu ra te l y de te r t n tned be fo reeny t i l t co r rec t . l on t han a f t e r .

the paleomagnet i .c study of three hand sarnplesf r om che Pon t -Réan Fo r rna t ' l on a l l owed Du f f [ 1979 ]t o i so l a re a s t eep l y d i pp ing nsgne t l za t i oncomponenC. t hese ve ry h i gh i nc l i na t i ons e recha rac te r i s t i c o f t he Ordov i c i ân magne t i c f i e l dl n t he A rno r i cân Mass i . f ( see comp l l a t i on byPc r roud [ 1985 ] ) . We cou ld t hus suspec t t hecx l s t ence o f a p recec ton i c r emanen t mâgne t i za t i onrr t . th in thc Pont-Réân red bed format ion, which wasve r l f i . ed by ou r p re l im i . na ry r esu lEs obca ined l nt h l s f o n a â t i o n [ C o g n é e t a l . , ' 1 9 8 6 ] .

A t oca l . o f 13 s i t es , i nc l ud ing chose o f t hep re l lm ina ry s t udy , have been samp led a1 l a roundthe sync l i ne s t r uc tu re (F i gu re I b ) . as noced l np rev i ous s im i l a r s t ud ies ICogné and Pe r roud ,1 9 8 5 ; C o g n é , 1 9 8 7 a 1 , ! n o r d e r t o a s s l g n e a c hpa leomagne t i c dL rec t l on a g i ven s t r a l n s t a te , er r samp l i ng s l t e r r has t he s t r i ng ,enÈ de f i n i t i on o fthe nâxlmurn rock voLume in which strâ in can beassr: rned to be macroscopical ly concinuous andhomogeneous . F i e l d c r l t e r l a f o r s t r a i nhomogene i t y a re ( 1 ) i n i t t â1 l y p l ana r su r f acesre rna ln p l ana r a f t e r dc fo r f l r âÈ ion , ( 2 ) i n i t t a l l ypa ra l l e l p l anes rema in pa ra l l e l . The re we re 177

cores (25 nrm in d lameter, 50 to 100 nua long)d r l l l ed end o r t enced i n s i t u us i ng rnagne l l c andeu r r conpasses . A long w iCh t h i s paLeomagne t i c

samp l l ng , sc ruc tu re l measu remen ts ( o r i en ta t i on

and d l p o f bedd lng and c l eavage p l anes ,

s t r e t ch i ng l i nea t i on , e t c . ) e re re r nade a t eachs iÈe . A9 i n o the r r ed f o r rna t i oas , some sma l lg reen e l l l p t i ca l zones , known as reduc t i on spo t s ,p r o v l d e d s t r a i n m a r k e r s [ " . g . , W o o d e t a l . , 1 9 7 6 ;G r a h a m , 1 9 7 8 ; K l i g f i e l d e t a 1 . , 1 9 8 1 J . A t l e e s c

one o r i en ted b l ock con ta i n i ng such reduc t l onspo t s ( gene ra l l y o f m i l l l r ne t r l c d i r nens lons ) was

samp led i n each s i t e ( excep t SH) t n o rde r t o

es t i neÈe t he mean s t r a i n va l ues o f t he s i t e .

1 Z A 1 q

S t r a i n a n d A n i s o t r o p y o f M a g n e t i c S u s c e p t i b i l i c y

lechniques and measurernents

Because o f che s rna l l d imens ion o f r educ t i ons p o t s , t h e i r a x i a l r e t l o a n d o r i e n t a È i . o n w e r emeasu red on pho tog raph i c en le rg€men ts o f po l l shed

sec t l ons o f b l ocks Ea red f o l l ow ing che t h reep r l nc i pa l s t r â i n p l anes . t hese neasu remen ts hâve

shovn cha t n i t h i n t he 1 lm iÈs o f expe r imen ta l

e r r o r , t h e p r t n c l p a l s t r a i n d i r e c t i o n s ( t h a t i s ,

t he ave rage d l r ec t l on o f e l l l p t i ca l ma rke rs axes )

a t each s l l e a r€ coax ia l . r l t h c l eavage and

l l nea t i on d l r ec t i ons . Th i . s obse rva t {on , wh i ch 1s

cons l sÈen t w i ch t hose o f Le Co r re t 1978 ] and Ler h é o f f I f 9 7 7 ] [ s e e P e r c e v a u l . t . a n d C o b b o l d , 1 9 8 2 ] ,neans chac t he po le t o c l eavage i s . t he sho r t en ing(o r r a i n tmum e longa t i on ) d i . r ec t l on , l he c l eavagei s i he l l ) , 2 p r i nc tpa l p l ane , and t he s t r e r ch ingl i neac lon i s t he nax imum e longê t i on d i r ec l i on .l bese a re t hus t he e i genvec to r s , o r so - ca l l edp r i n c i p e l d i r e c t i o n s , o f t h e m e e n s t r ê i n t è n s o ra ! each s i ce i n t he geog raph i ca l r e fe rence f r ame .

The two -d i r nene iona l s t r a i n w i t h i n eachp r i n c l p a l p l a n e i s c l a s s l c a l l y d e t e r m i n e d b yu s i n g c h e R f / 0 t , e c h n l q u e o f D u n n ê t 1 1 9 6 9 1 [ " . g . ,Dunne t and S lddans , 19711 Grahan , 1978 ; Ramsaya n d H u b e r , 1 9 E 3 ; K l l g f t e l d e t . a 1 . , 1 9 8 1 , 1 9 8 3 ;C l e n d e n c n e c a l . , 1 9 8 8 ] . S u c h a t e c h n i , q u e w e sused he re f o r t he f ou r s l t ee o f t he p ré l i r n i na ry

s t u d y ( P M , M B , L C , C B l s e e C o g n é e r a I . , 1 9 8 6 J ) .Howeve r , assum lng t ha t i n an i so t r op i c r ned j . un t hereducÈ ion spo t s shôu ld have a sphe r i ca l shape , i tcan be cons lde red t ha t t he mean âspec ! r a t i o o fe l l i p t l c a l s p o È s p r o v i d e s a n e s t i m e t i o n o f s l r a i nr â t i o l r i t h i . n e a c h p r l n c i p a l p l a n e I e . g . , R a m s a ya n d H u b e r , 1 9 8 3 , p . 7 3 1 . T h i s a s s u m p t i o n i spa r t l y ve r i f l ed by t he f ac t t ha t Èhe o r i enca t i ono f l ong axes o f e l l i p ses i s r ough l y cons tan !l t i t h i n each s tud ied su r f ace . Consequen t l y , as i r ap le a r l t hne t i c mean o f t he neasu re r ren t s o fa x l a L r a t i o s w l t h i n e a c h s e c t i o n ( t y p i c a l l y 2 0 t o30 neasu re rnen t s pe r sec t i on ) was used i n t her e m € i n i n g s i t e s t o e g t i m a t e t h e a x i a l r a t i o o ftwo -d imens tona l s t . r a i n . S tanda rd dev ia t i ons o ft f te mean ( o) are used as error barE on thesees t i nê tes . I n Èhe s l t es whe re s t r a i , n r e t i , os l r e re

de te rm ined f r om R f /O de [e , e r ro r ba rs have beenrough l y es t ima ted f r o rn t he range o f R f / 0 cu r vestha ! , can be supe r imposed on da ta .

P r i n c i p a l s t r a i n s À 1 , À 2 , a n d 1 3 h a v e b e e nca l cu la ted f r om che two ra t i os À I /À2 end ) tZ l l t , 3 ,assunlng there have been no voluma changes dur ingd e f o r m a t i o n , i . e . , À 1 ^ 2 À 3 = 1 . F r o n r h e d i a g o n a lm a c r i x S r l i o f p r l n c i p a l s t r a i n s , a n d t h et rans fo rmaÉ ion na t r i x a1 ; o f p r i nc i pa ld i r e c t i o n s [ . . g . , B o u r n e â n d K e n d a l l , 1 9 7 7 ] , t h esy f i ne t r i ce l s t r a i n t enso r o f each s i t e cân t henbe cornputed fo l lor , r lng the t rensformat ion ru le forsecond o rde r t enso rs :

s g 1 = a i l . a 3 r n . S t k r n ( i , J , k , m ' 1 , 2 , 3 )

An i . so t r opy o f r nagne t l c auscep t i b i l i t y wasmeasu red on sÈanda rd pa leomagneÈ lc spec lmens(cy l i nde rs 21 mm long and 25 rnm i n d i ame te r )us i ng che D ig i co en i so t ropy de l i nea to r I seeCo l l i n son , 1983 ] i r np roved by ehe add i t i on o f af l l t e r / a r n p l i f L e r o n t h e p i c k - u p c o l l s s i g n a lc l r cu i t , and ca l i b ra ted f o l l ow ing t he co r rec t t onfo r i ns t r umen ta l e r ro r as p roposed by H rouda e t

Cogné : De fo r rned Red Beds o f o rdov l c i an 'Pon t -Réan Fo rma t i on

r7 A '7É, Cogné: Deformed Red Beds o f Ordov lc ian Ponc-Réan Formac ion

TABLE 1 . Mean -s l t e S t ruc tu ra l De ta i n t he Pon t -Réen Fo tmae ion

S i t e sgr I r 2 À 3

S D ) t , u e t

sÀS BS Dq Ë

) I J )

269 3E280 19299 30146 138 7 2 090 26

112 990 3766 2856 33

I35 ?2280 56

262 EI277 80

) t é z252 88

77 9087 90

270 7I261 81278 7988 8691 E0b t ô è

101 86

79 1827E 7

) t L 5

7L 23257 L28"7 09 0 0

262 4' t t )89 L295 22

247 20101 0

l. .400 1 .0001 . 4 E 6 1 . 1 8 91 . 5 2 7 0 . 8 4 8L . 4 7 2 1 . 0 9 01 . 1 9 1 0 . 9 1 6L . 2 7 8 1 . 0 6 5

1 . 3 3 9 0 . 8 6 41 . 6 5 2 1 . 1 E 0I . 4 4 9 1 . 1 1 51 . 2 0 6 1 . 0 9 6I . 3 9 2 0 . 9 2 8r . b 1 6 L . 4 t L

1 , 0 0 1 . 8 0Q . 2 3 2 . 3 56 . 0 0 1 . 9 00 . 4 6 2 . 1 0

- I . J U

0 . 4 5 I . 6 5

- L . ) J

0 . 3 1 7 . 7 00 . 3 7 2 . 1 0v . L / . l - . ) )

2 . 5 0 1 . 7 00 . 0 4 3 . 6 0

u . ) 0 b0 . 7 7 I0 . 6 2 30 . 9 1 60 . 730

0 . 8 6 40 . 5 1 30 . 6 1 90 . 7 5 60 . 7 7 4

B e d d i n g ( S O ) e n d c l e e v a g e ( S 1 ) p l a n e s e r e g i v e n a s 5 , s c r i k e ; D , d i p(downrard in che d l rec t lon S+90o) ; l inea l ion ( f ) ig g iven as De,d e c l l n a t i o n l I , d o v n w a r d i n c l l n a t i o n ; t r 1 , À 2 , a n d l 3 a r e p r i n c l p a ls ! râ insres t imated f ro rn ax ia l rac io rneasurenencs o f reduc t ion spots ; K ,

- s h a p e p a r a r n e t ê r , K - ( À 1 / f Z - l ) 1 0 , , 2 1 X 3 - 1 ) ; r : i n t e n s i l y p a r a m e È l r , r) . 1 / À 2 + r 2 l r 3 - 1 .

a 1 . [ 1 9 8 3 ] a n d v e i t c h e È a l . [ 1 9 8 3 ] . T h e k r "c o m p o n e n t o f t h e s u s c e p t i b i l t t y t e n s o r ( i . e . , t h es u s c e p t l b l l i t y a l o n g t h e a x l s o f l h e c y l l n d r i c â lspec lmen ) l r as r neasu red w i t h t he D ig l co bu l ks u s c e p t l b l l i t y n e t e r . F r o m t h e N m e a s u r e dspec lmens ac each s i t e , a mean -no rna l i zedsuscep r l b i l i t y t , enso r has been conpuced by us l ngl h e t e n s o r s t e c l s l t c s o f J e l l n e k [ 1 9 7 8 ] d e r i . v e df t o m H e x t t 1 9 6 3 1 . F o l l o w t n g t h i s a n a l y s i s , e a c htenso r de te rm ined by a neasu remenc i s exp ressedln ê coûmon re fe rence f r a rne (e .g . , geog raph i c )a n d l s n o r m a l i z e d È o i t s p r i n c i p a l i n v a r i a n c ( k 1 1

+ k g 2 + k 3 3 ) / 3 , i n o r d e r t o e l i m i . n a t e t h e e f f e c [ so f t h e f l u c t u a c i n g s p e c l m e n b u l k s u s c e p t i b i l i t yupon t he es t lmec ion o f t he s iÈe -mean l enso r . Theneân t enso r and i t s con f l dence l im i t s â re t h€nes t ! . ne têd by su r rn i ng t he N no r rna l i zed t enso rs o fa s l ce and compu t i ng t he re l evan t cova r i ancema t r i x , f r om wh tch a re de r i ved l he va r i anc . ( o2 )

o f t he no rma l l zed p r i nc l . pa l suscep t i b i l l t i e s andt h e s e m i - e n g l e s ( a , b ) o f t h e e l l i p s e o fcon f i dence a t t he 95% p robab i l i t y l eve l a roundthe p r i nc i pa l d i r ec t , i ong . l o co rnp le te t hedecc r i p t i on o f AMS, t he nean -s i t e bu l ks u s c e p t l b i l i t y l s c a l c u l a c e d a s k = ( k 1 + k 2 +k 3 ) 1 3 , w h e r e k 1 , k 2 , k 3 e r e l h e e l g e n v a l u e s o ft hc unno rnâ l , i zed mean t enso r ( compu ted as above ,bu t l r i t hou t no rme l i z i ng each t enso r by i t sp r l n c i p a l i n v e r i â n t ( k 1 1 + k 2 2 + k 3 3 ) / 3 )

Resu l t s

Meen E t ruc tu ra l da ta ob ta i ned i n t he Pon t -Réanred beds a re g l ven i n Tab le I : bedd ing (56 ) andc leavage (S1 ) a re cha rac te r i zed by t he i r s t r i ke(S , dec l t na t l on o f t he ho r i zon ta l l l ne o f t hep1âne ) and d i p (D , max lmum < iocnward i nc l i naÈ lono f t he p l ane t n t he d i r ec t i on S+90o ) . t hes t r e t c h l n g l l n e a t i o n L ( D e , d e c l i n a t l o n ; I ,

i n c l i n a t l o n ) , a s o b s e r v e d i n t h e f l e l d a n dve r t f i ed f r o rn e l l i p t i ca l ma rke r measu lemencs ono r i e n c e d b L o c k s , i s s y s t e m a t i c a l l y a t c h eb e d d i n g / c l e â v a g e i n t e r s e c t i o n . S t r a i n e s L i r n e ! e sa r e g l v e n i n T a b l e 1 a s | 1 , À 2 , À 3 , p r l n c i p â ls t r e i n s l K , r : s h a p e I F l i n n , 1 9 6 2 J a n d i n t e n s l t y[ w a t t e r s o n , 1 9 6 8 ] p e r a m e Ê e r s . M e a n A M S d a r a a r eg i v e n i n T a b l e 2 w h e r e e a c h n o r m a l i z e d p r i n c i p a ls u s c e p t l b i l l c y k 1 , k 2 , k 3 , i s c h a r a c t e r i z e d b yi t . s i n t e n s i c y a n d s c a n d a r d d e v i a t i o n , i t sd e c l i n a t i o n a n d i . n c l i r i a r i o n ( D / I ) , a n d r h e 1 / 2a p e r t u r e a n g l e s ( a / b ) o f t h e 9 5 % c o n f i d e n c ee l l i p s e s a r o u n d È h e m e a n d i r e c i i o n s . l h e b u l ks u s c e p t i b i l i t y i s g i v e n i n d i m e n s i o n l e s s S Iun i r s , 7 .An i s ( k1 / k3 -1 ) x100% [ van De r voo andK loo t l r i j k , 19721 .

t he p r l nc i pa l d i r ec t i ons o f s c ra i n and AMS a reshown i n F i gu re 2 . t he homogene i r y o f AMS w i rh i ne a c h s i t e 1 s u n d e r l i n e d b y t h e g e n e r a l l y s r n a l ld i m e n s i o n o f t h e 9 5 7 . e l l i p s e s o f c o n f i d e n c ea r o u n d p r l n c i p a l s u s c e p t i b i l i t y d i r e c t i o n s . U p o nexani .n ing lhe data, ! {e see that the rnaximums u s c e p t i b i l l t y k 1 i s g e n e r a l l y s u b c o a x i a l r ' l t ht h e b e d d i n g / c l e a v a g e l n È e r s e c È i o n , t h a t i s , l l i t ht he max imurn e l onga t i on ax i s À1 . The m in imums u s c e p t i b i l l t y k 3 i s , o n t h e a v e r a g e , p a r a l l e l . t ot he po le t o c l eavage r13 . The re a re , hoveve ! , somes i g n i f i c â ô t d l f f e r e n c e s i n s i t e s S G a n d S I , w h e r ek2 and k3 show i n t , e rmed ia te d i t ecc i ons be tweenpo le t o c l eavage and po le t o bedd ing . Th i s t endsto i nd i ca te t ha t magne t i c f ab r i c s ave rage t hee f f e c t s o f t h e s u p e r i m p o s i t i o n o f t e c t o n i c s t r a i nupon an l n i t i a l compac t i on - i nduced f ab r i c[G raham, 1978 ; Ramsay and Hube r , 19E3 ] . Such aninÈe rp re te t i on i s co r robo raÈed by t heexcep t i onna l da ta o f s i t e SF , whe re a l t houghc l e a v a g e i s a l r e a d y o b e e r v a b l e , k 3 l s n o È y e treo r i en !ed t owa rd c l eavage po le bu t r ema insp a r a l l e l t o b e d d l n g p o l e . I t c a n f u r t h e r b e n o t e d

ô a < N r i n N r l f t O n ô \ O C OF t Ê { n N N N ô a d N d d

< É ô Ê : t r ( 9 : É H ' ' r É E ( ' ) I OVt <h q)V2 Q @.A s7 (â E Ê{ J (J

l t , 6 7 7

t h â t i n t h l s s i t e , e s w e l l a s i n s i t e L C , w h e r e

e l l i p ses e round k2 and k3 show tha t t hêsed t r c c t i o n s e r e i l l - d e f i n e d i n t h e k 2 k 3 p l a n e , t h c

s t r uc tu re l s o f penc l l t ype [G raham, 1978 ] , t hac

i s , t h e f a b r l c i s m a c r o s c o p i c a l l y c o n s t r i c t l v e .The e l l i p so ld shape and i n t ens i t y o f mean

s t re l n and A l lS a re l l l u s t r a ted by t he F l i nn

[1965 ] d l ag rams o f F i gu re 3 . Po ln t . s a re

d i sc r l bu ted l n t he cons t r i c t i on end f l e t t en lng

a reas f o r s t r a l n es we l l as f o r AMS. ' I nÈens i t i es

â re qu l t e l ow w i t h r <2 .5 and 7 .An <6% l n nos t

9 i t es . G rea t€ r i n t ens i t . i e s e re r eached f , o r po in t s

s i t u a t e d i n t h e f l a t t e n i n g f i e l d ( s i c e s S B , C B

f o r c x a n p l e ) . I n F i g u r e 3 , a c l e a r d i f f e r e n c eeppeâ rs beÈwecn t he shape o f sc ra i n e l l l p so ldsand t he shape o f AHS e l l i p so ids . As a ma t t e r o f

f ac t , A l ' l S e l l i p so ids e re , on t he ave rage , r no re

consÈr i c t l ve t han s t r â i n onês (Eee f o r exa rnp le ,s l t es PM, SF , SG) . Tbo na in r easonE can be evoked

t o e x p l a i n t h l s d i f f e r e n c e . T h e f i r s t o n e l s t h a tnagneÈ ic f sb r i c r eco rded t he compec t i on o f r edbeds s t r onge r t han reduc t i on spocs d i d . I t cou ldbe i n f e r red t hac reduc t i on spo t s deve lop du r i nglace s l ages o f compac t i on . A second andequ i va lenc r cason cou ld be t hÂ t A l lS evo lu t i on i s

s l owe r t han t o tâ1 s t i a l n evo lu r i on as es t l rT :e tedf r o m r e d u c t i o n s p o t s . I n b o È h c a s e s , s t r a i ne l l t pso tds shou ld be more f l ac tened t han AMSo n e s , d u r i n g p r o g r e s s i v e d e f o r m a l i o n , f o l l o w t n gÈhe pâ th desc r i bed by G raha rn [ 1978 ] o r Ramsay andH u b e r [ 1 9 E 3 ] . f r t s , h o w e v e r , d i f f i c u l . t È o s t r e s st h e a c t u a l p h y s i c a l r e a s o n f o r t h i s d i f f e r e n c e ,

and even ! . o assess i t s s l gn i f i cance . As a ma t t e r

o f f ac t , i f r . r e t ake i nco accoun ! t he l ow

ln tens l t i e s o f s t r a i . n and AMS, s rna l l va r i e t i ons

o f i n re rmed ia te -ax l s va l ues ( t 2 o r no r rne l i zed k2 )

a round un i ! y can l nduce s l gn l f i can t va r i a ! 1ons o f

r e l a t i v e v a l u e s b e t w € e n l 1 / À 2 a n d À 2 / À 3 , a n dbeÈween k1 / k2 and k2 / k3 . T t r e re fo re 1 f we a t t emp t

! o d raw a co r re l a t l on be tween s t r e i n and AMS

e l l i p s o i d s s h a p e s a n d l n t e n s i t i e s , i t a p P e e r sbe [ t e r ! o l ook f o r a co r re l e t . l on be tweenp r i n c i p a l v a l u e s , r a È h e r t h a n b e t ï e e n p r i n c i p a l

r â t i o s .To t h l s pu rpose , and f o l l o { i ng l he

r e l a t i o n s h i p p r o p o s e d b y K l i g , f i e l d e t a l . [ 1 9 8 1 ,1 9 8 3 ] , w e h a v e c o m p u t e d t h e l i n e a r t e g r e s s l o nb e t w e e n t h e p r i n c i p a l s u s c e p c i b i l i - t y - d t f f e r e n c è s ,M i = ( k i - k o ) / k o w i r h k o = ( k r . k 2 . k 3 ) r / J , a n dn a È u r a l s t r a l n s r € i = l n ( l i ) . t h r s i s s h o w n i nF igu re 4 . t he co r re l aÈ ion eppee rs qu i t e good( c o r r e l ê t l o n c o e f f i c i e n t r = 0 . 9 4 3 ) , a n d c h e b e s tf l c l l n e e i = 1 3 . 6 M 1 - 0 . 0 4 8 , o e s t h r o u g h t h e o r l g i n(F lgu re 4 ) . Th i s means t haÈ va r l a t i ons o f AMSa r e , o n t h e a v e r a g e , c o n t r o l l e d b y s t r a l nvâ r i eÈ lon3 . The t emarks made above , ebou ! Ehed i s c r i b u t i o n o f d a t a i n F l i n n d i a g r a m s ( F i g u r e

3 ) , a p p e a r j u s t l f i e d , â n d a r e v e r i f i e d b y t h ed l s t r i b u t l o n o f i n t e r m e d i a t . e v a l u e s ( 1 2 a n d M 2 :t r l ang les ) a round ze ro i n t he d l ag ram o f F i gu re

F ina l l y , a l l t he above obse rve t i ons can bec lass i ca l l y i n t e rp re l ed i n t e rms o f p rog ress i vede fo rma t i on , as beau t l f u l l y desc r i bed by G raham[ f978 ] i n t he Mar i t ime A lps r ed beds . I n t hein i t i a l sed lmen ta r y s t e ta , compecC ion i nduces ab e d d l n g f a b r i c s h o w i n g o b l a c e e l l i p s o l d s w i t h À 3coax la l w l t h t he po le t o bedd ing . Th i s s t e te i sno t ac tua l l y obse rved i n t he Pon t -Réan Fo rmaÈ ionbu t can be i n f e r red f r om the o r t hogone l i t y o f k3and bedd lng i n s l t e SF . Du r i ng ducc i l e t ec ton l c

Cogné: Deformed Red Beds of Ordov lc lan Ponr -Réan Format ion

o\ $ -t F È \o .f .'\ CO .t q\ \ô o

o \O 6 tt1 N N (n n rr..' \t \O d

1 1 N ( v ! N O N - ? c 1 O 6 t t l È \ O\ \ \ \ \ \ \ \ \ \ - \ \O \ t \ O \ O - ? ô O c ) r ^ r N h ( 1" 4 d N d n d

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o o o n f \ c ) 6 o o o o o o

€ r o . Y ! " i | o € d o F d 6 Nd F { d r . $ \ t d d\ \ \ \ \ \ \ \ \ \ \ \ \

6 O ' / r 6 O O O 6 6 û \ ' ô C ) 6

o o \ N i \ o f r N d . f i @ . t o c o6 @ 6 È . 1 6 @ o \ æ . i l t ^6 F a Ê t F t O d F i d ( 1 ( r l

fl € @ (1 .ir c.t ('r N fr n aO O\ c)o o o o o o o o ô o o o N

- t O À C O f l € \ 1 È ô \ O c O \ O \ O6 \ O t s \ O c O @ c O @ È r r È no\ g\ o1 êÀ o\ o\ 6\ o\ o\ o\ 6|r o\ o\

o. ' l Fl ôt N.S \ t .1 . t N S \o È d\ \ \ \ \ \ \ \ \ \ \ \ \C) n \O d rtl O c) o rtt \o N 6 c'\â d à . - l d N d n r i

6 c ) c ) 6 d C T O O O O O O O

ô l N . S \ g c O O ( v , | < ô . t , r - f i dÈ r r È . 1 È , 1 È € P C O \ O È\ \ \ \ \ \ \ \ \ \ \ \ \o | l ' 1 6 c ) ' ^ r f | o c ) ' / r o o c ) t t l

r r 1 . t N 6 \ o â d o d r o \ t o \ 6( i . ) ô r @ ô \ O ô r @ v ' l { O rN N r " - ( Ê l é m N d ô l

1 1 . t ô r n | r . l d \ t . n n F ! \ O r G l

o o ( ) o o o o o o o o o o

ô l È N C O N F È ( n r . t \ O \ t @o \ d o o o o \ o \ o \ a À o o \ @ F aO \ O O \ O ô o | \ O \ o À C F O o \ O r O

O Ê i O d â O O O ( ) d O O Ê

\ C | 6 t s 6 - N 6 É 1 n 6 6 d \ Oo o o o o o o o o o o i -

.'t N o ô ôr .$ co o o\ o\ \o o \oN N O N d d i N N d N \ t - t

i r { d d à d / à d À À à À

' / \ 6 O O O f i 6 O O t ô 6 O O

6 \ t ô l 6 ( 1 6 f i d N N \ O d 6 \À d À d N d\ \ \ \ \ \ \ \ \ \ \ \ \r ô O o O 6 O O O O O 6 O < f

N O ô d c O O \ . J ' d ô c O j ' / r Oc O O \ O c O r l € @ È ( ) O r r n O \

d N N N d N

N ('t N \ô cr] N cl d'l d .t .t \o at\ \ \ \ \ \ \ \ \ \ - \ \6 |tt.t Ê{ 6 \O 6 rJ' {' È r N CÀ

\o o\ o 6 rr r^ o\ fi \o.3' \o c) .J'N $ \ t N â r { O Ê t È O . l Ê ( 1- i d É t 4 , 1 à r - . t O i H Ê F {

| t , 678

de fo rma t i on , Èhe reg lona l l y subho r i zon !a1sho r t en ing resu l t s i n a cons t i l c t l ve shape o f t het o t a l s l r a l n e l l i . p s o l d s n i t h J o i n t d e v e l o p m e n c o fs t r e t c h l n g a n d m a x i m u m s u s c e p t t b i l ! t y â t t h eb e d d i n g / c l e â v a g e i n t e r s e c t i o n . T h e n , w i t hl n c r e a s l . n g d e f o r m a t i o n , e l l i p s o i d s r e t u r n i n t h ef l ac ten lng f t e l d w i ch 13 as t he no rma l r oc l eavage . t h l s sequence l s accompan ied by ap rog ress i ve reseÈ t t ng o f t he m in imumsuscep t i b i l i t y k3 f r o rn t he no rma l t o bedd ing( s t t e S F ) t o t h e n o r m a l t o c l e a v a g e ( s i t e s S e ,S B , S D , S E , S H , S J , P M , H B , C B ) , r h r o u g hl n t e r m e d l a t e r n e g n e t i c f a b r l c s ( s i t e s L C , S G , S t ) .

One can f u r t he r noÈe l ha t t h i s sequencercugh l y desc r l bes a s t r a i n g rad ien t f r om no r t h o ft he s tud led e rea t owa rd t he sou th . Th i s r esu l t sl n l owe r s t r a i , n i nÈenE i t i es i n t he no the rn pa r t( e e e l a b 1 e 1 ; s i t e s S F , S G , S I ) t h a n i n c h eE o u t h e r n p a r t ( s l t e s C B , S B , S J , M B ) . T h i so b s e r v a t i o n 1 s c o n s i s t e n ! w i t h t h e d e s c r l p t i o n o fGapa l s and Le Co r re [ 19E0 ] and Pe rcevau l t andC o b b o l d [ 1 9 8 2 ] , a n d w i t h È h e i r i n t e r p r e t â t i o nt h e t m o s t o f H e r c y n i a n f o l d l n g o f P a l e o z o i cf o r rna t l ons i n Cen t re l B r l t t any i s con t ro l l ed bydcx t ra l t r ânscu r ren t movemen t on c rus ta l f au l t s .

Pa leomagne È l c Ana l ys 1s

Naturs l Remanent H6 n e È i z a t l o n

Cogné : De fo rmed Red Beds o f O rdov i c i an Pon ! -Réan Fo rma t i on

F i g . 2 . S l e r e o g r a p h i c p r o j e c c i o n s i n t h e l o ç e r h e m i s p h e r e o f m e a n - s j . t e s L r u c t u r a l â n dA l ' t S d l r e c t i o n s . S 6 , b e d d i n g ; S 1 , c l e a v a g e ; s È r e t c h i n g l i n e a t i o n i s a t r h eb e d d i n g / c l e a v a g e t n t e r s e c i i o n . P r i n c i p a l s u s c e p t i b i l i t i e s w i r h t h e i r 9 5 7 . c o n f i d e n c er e g t o n s â r e s q u â r e s , k 1 ( m a x ) ; t r i a n g l e s , k 2 ( i n t e r m e d i e È e ) ; a n d c i r c l e s , k 3 ( m i n ) .

Natura ll n È e n s i t l € s

remânent nag,nc t lze t lon (Nru l )(F igure 5) range beLTeen 0 .001 and

0 . 0 1 e / m , ç h l c h a r e t y p i c a l v a l u e s f o r r e d b e d s .A l t h o u g h w e a k , t h e s e i n t e n s i t i e s a r e s u f f i c i e n tf o r l h e r e m e n e n t m a g n e t i z e È l o n c o b e a n a l y z e dth rough demag ,ne t i za t , t on p rocedu res , and measu redu s i n g t h e S c h o n s t e d t ^ D S M - I s p i . n n e r m e g n e È o m e t e r( s e n s i È i v i t y 1 0 - 9 t u n Z ) . N R H d i r e c r i o n s , s h o w n a sl n s i t u l n t h e d e n s i È y p l o t o f F i g u r e 5 , d i s p l a yan e l onga ted pa t . h be t l r een SSW s l i . gh t l y i nc l i nedd l r e c t i o n s t o w a r d W S W h i g h l y i n c l i n e d o n e s . W i t hre fe rence t o pa leomagne t i c dâ ta f r om theA r m o r i c e n M a s s i f I P e r r o u d , 1 9 8 5 ] , i È c a n b es u r m l s e d t h a t t h l s d i s t r i b u t i o n a r i s e s f r o m ap a r t i a l r e n e g n e ! 1 z e È i o n o f p r i m a r y h i g h l yi nc l i ned Ordov l c i an d i r ec t i ons , i n che S1 . ls l i g h e l y i n c l l n e d C a r b o n i f e r o u s m â g n e C i c f i e 1 d ,du r i ng rhe He rcyn lan o rogenes i s .

DemaSne È i zâ ! i . on

In o rde r t o cesc f o r t he o r l g i n and na tu re o fNRM, vec to r ana l ys i s o f r nagne t i zâ t l . on t h roughs tepw ise t he rma l demagne t l za t i on was conduc !ed on1 0 0 s p e c i m e n s . f h e b e h a v i o r o f m a g n e t i z a r i o nd u r l n g d e m a g n e ! i z a t i o n w a s g e n e r a l l y s i m p l e . T l ot ypes o f demagneE iza t i on cu rves ( i n o r t hogona lp r o j e c t i o n s I Z t j d e r v e l d , 1 9 6 7 ] ) w e r e o b r e i n e d(F igu re 6 ) . ( 1 ) On t he one hand , when NRMi n l t i a l l y e x h i b i t s h i g h i n c l i n a r i o n s , r h em e g n e È l z a È i o n l n t e n s i t y p r o g r e s s l v e I y d e c r e a s e sn l t h i nc !eâs in8 t empe raÈu re , w i t hou t any changei n d l r e c E l o n ( F i g u r e 6 a ) . T h i s i s c h a r a c t e r i s t i co f un i vec to r i a l nagne t i ze t ' 1on . One can no te t he t

FLATlENINC

Cogné: Deformed Red Beds of Ordovic ian Pont-Réan FormaEion r a Â ? O

M io o s = { k t _ k o L k e

l n t he5 , a n d

1 ro5 't.l

F l g . 3 . F 1 . i n n [ 1 9 6 5 ) d l a g r a r n s f o r r n e a n s t r a i n

and AHS da ta . L i nes o f s t r e i n l n t ens l t i e s 7=2 , 3 ,

and 4, and of 7.en = 5, 10, and 15% are drawn.

E r ro r ba t s oo 3 t r â i n r s t l os a re t he scanda rd

dev la t l ons on a r i t hn€ t l c nean ; e r ro r ba rs on

suscep t ! b111 t y r e t i os a re de r i ved f r on s tanda rd

d e v l â t i o n s o n m e a n s u s c e p t i b l l i t l ê s I a f t e rJ e l i n e k , 1 9 7 8 1 .

unb lock i ng t empe râ tu res e re sepa re ted i n t l t o

g roups : a Pa r t o f t he rnagne t l za t l on i sp rog ress i ve l y unb tocked be t 'ween 20o and 600oC ,

then iÈ scab i l i zes be t reen 6000 and 650oC , and i s

f i na l l y unb locked be tween 6500 and 680oC , t he

C u r i e p o i n t o f h e m a È t t e . ( 2 ) T h e s e c o n d k i n d o f

behav io r was obse rved ma in l y on spec imens whe re

NRM ças l n l t i a l l . y e i che r s t i gh t l y o r

i n t e rmed ieÈe l y i nc l l ned SW. I c i s che rac te r i zed

by È r ro magne t i ze t i on componen ts (F i gu re 6b ) : a

t ow-Èempera tu re ( LT ) co rnPonenc nh i ch i s unb locked

up to ebout 600oC and shows a shal low

sou thwesce r l y d i r ec t l on . The second magne t i za t i on

componen t i s mo re s teep l y i nc l i ned l ' ' t o s l , l , and

appee rs s l r n i l a r t o t he 'megne t i ze t i on comPonen t

dâsé r l bed i n t he case o f un i vec to r i a l behav io r .

Th i s h i gh - t empe recu re (HT ) componen t i s unb locked

zUt l ooU- 3

F i g . 4 . P r l n c t p a l s u s c e p t i b l l i t y d i f f e r e n c e s( M 1 ) v e r s u s n a t u r a l s t r a i n s ( e i ) . T h e b e s t f l t

1 l ne l s sho rm and reg ress lon equac ion ove r Po in t si s g i ven . E r ro r g r i d l s de r l ved f r om s tanda rd

d e v i e t i o n s o n m e a n s u s è e p t i b i l i t i e s a n d s t r a i n

ax la l r ac l os . Same symbo l s as i n F l . gu re 2 . '

i n t he 650 -6800 l e rnpe racu le r ânge . No te , howeve r ,

t ha ! e good sepa ra t t on o f t hese componen ts i s no t

a lways ensu red , as sho . , r n i n F l gu re 6b , spec imen

5404 . A sepâ ra t i on i n t ' o such LT and HT comPonen ts

w a s o b t a i n e d i n f i v e s i t e s ( S A , S D , S H , S I , L C ) ;

i n o l h e r s i t e s , a u n i q u e s t e e p l y i n c l i n e d

mag ,ne t i za t i on , cove r i ng t he who le t emPera tu re

spec t run , i s dom inan t .The b imoda l d i s t r i bu t i on o f unb lock i ng

tempc ra tu res po in t s Èo t he P resence o f t ço na in

mâgne t i c ca r r l e r s . Howeve r , some i so l he rma l

renanenÈ megne l t zâ t l oo ( I r u l ) expe r imen ts a l l owed

i d e n t . l f i c a c i o n o f o n l y o n e k i n d o f m a g n e t i c

r n i n e r a l , w l t h h l g h c o e r c i . v i c i e s , t y p i c a l o f

hema t l ! e . The range o f unb lock i ng ÈemPerâcu res o f

LT co rnpone i rÈs i s cons i s t en t n iÈh u l t r a f i ne i ed

heme t l t . e p i gnen t , wh i l e h i gh t e rnpe ra tu res cou ld

b e r e l a t e d t o l a r g e r h e m a t l c e c r y s t a l s , p o s s l b l y

s p e c u l a r i t e . F o l l o w i n g E h i s i n È e r p r e t a t i o n '

sha l l ow LT componen ts cou ld r esu l ! f r om chem ica l

r e m o b i l i z â t i o n o f p l S m e n t , m o r e g e n s i t i v e C o

f l u l d c l r c u l a c i o n d u r i n g t h e H e r c y n i a n o r o g e n y '

and cou ld t hus ac tua l l y be seconda ry .

0.901 o.01 o.'l A I mI N T E N S I T Y

F lg . 5 . ( Le f t ) F requency p l o t o f NRM in tens i t y '

l owe r hem lsphe re o f NRH d tSec t l ona l n geog raph i c

107 . o f r he àa ta i n ' 17 . o f ' t hà hem isphe re su r f ace '

1 8 0( n t g l t ) E q u a l - a r e a d e n s i t y p l o t

c o o r d i n a t e s ; c o n t o u r s : L , 2 . 5 ,

1 1 ,580 Cogné : De fo r rned Red Beds o f O rdov i c i an Ponc -Réan Fo rmaÈ ion

SJ 106

590 -665

f , f ,u

S Down

F i g . 6 . t h e r m a l d e m a g n e t i z a t i o n o f r e p r e s e n È â c i v e s p e c i . m e n s i n Z i j d e r v e l d [ 1 9 6 7 ]o r t h o g o n a l p r o j e c t i o n s . C l o s e d ( o p e n ) s y m b o l s a r e p r o j e c t i o n o n t o t h e h o r i z o n t a l( v e r t i c a l ) p l a n e . F o r e x p l a n a t i o n o f F i g u r e s 4 a a n d 4 b , s e e t e x L .

Àoa l ys t s o f Remanen t Magne t l za t i on D i recÈ ions

Low- tempe ra tu re componen ts . Mean -s iÈed i r ec t i ons o f LT componen ts a l e l i s t . ed as i n s i t u( IS ) and t l l t co r rec ted (Tc ) i n Tab le 3 and d rawnl n F i g u r e 7 . I n t h e i n s i t u c o o r d i n a t e s , t h e s ed i r e c t l o n s s h o w a q u i t e c o n s t a n t d e c l i n a t i o n o fa b o u t 2 0 0 0 , b u È a g r e e t s c a t t e r i n i n c l i n â t i o n .T h l s d i s p e r s i o n i s n o t r e d u c e d b y t h e c l a s s i c a lt i l t c o r r e c t i o n I G r a h a m , 1 9 4 9 ] . T h l s s i t , u a t i o ncan pa r t l y a r l se f r om a syn fo l d i ng age o fmagne t l za t i on acqu l s t t i on . Howeve r , i n t he i ns i t u coo rd i na tes , po ln t s show a g rea t - c i r c l e

d l s t r l bu t l on be tween t he s teep l y l nc l i nedformat ion mean of h igh-temperaiure componenls(Tab le 4 ) and sha l l os Ca rbon i f e rons and Loçe r

P e r m i a n d i p o l e f i e l d d i r e c t l o n s ( F i g u r e 7 ) . L Tmegne t i zaÈ ion componenÈs â re t hus p robab l ys y n t . e c r o n i c t o p o s È t e c c o n i c p a r t i a lr emagne t . i zâ t i ons , bu t poo r l y sepa ra ted by t he rma ld e m a g n e t i z a t i o n s , d u e È o t h e o v e r l a p p i n g o fb l o c k i n g t e m p e r a t u r e s p e c t r a . T h i s b a d s e p a r a t i o ndoes no t a l l ow us t o compu te an accu ra te meând t r e c t i o n o f m a g n e t i z a t i o n f o r È h i s s e c o n d a r ycomponenc .

H igh t . emqe râ tu re componen ts . Meân -s l t ed i r ec t i on o f magne t i zaÈ ion HT componen ts a requo ted i n l ab1e 4 and d rawn i n F i gu re 8 as i ns i t u ( I S ) a n d c L a s s i c a l l y t i l È c o r r e c r e d ( T C ) . e sn o È e d e b o v e , È h e s e d a t a d i s p l a y W t o 5 H

d e c l i n a t l o n s a n d s t e e p d o w n w a r d i n c l i n a t i o n s .S u c h d i r e c t i o n s a r e q u i t e s i m i l a r t o t h e

Cogné : De fo rmed Red Beds o f

ÎABLE 3 . Mean -s i t e Mag re t i ze t i " on Low- l empe ra tu reComponen ts I so l a ted by The rna l Demagne t l zâ t l on

S l È e N I S T Ck o 9 5

Ordov l c i an Pon t -Réan Fo rma t i on 1 1 A . e 1

S È r ê i n - i n d u c e d d e v l a t i o n o f R M . H e r e , a s i n

o re i f 6 l î cases , Èhese anoma l i es can be suspecced

i o a r l s e f r o m a d e v i a t i n g e f f e c t o f s t r a i n u p o n

the h i gh unb lock i . ng t e tnpe ra tu re componenÈ o f

remanen t l nâgne t i zâ t i on (R t ' t ) . The p roposed

in te rp re te t l on i s shovn schemac l ce l l y i n l i gu re

9 . f he sed imen ta r y beds a re supposed t o be

t n i t l a l l y s u b h o r i z o n È a 1 a n d h a v e a s u b v e r t i c a l

RM. Sc ra i n du r i ng f o l d i ng o f t he f o ima t i on

induces t he deve lopmen t o f a subve rc i ca l ax i a l

p l ana r c l eavage . Tak ing i n t o accoun t . t he h tgh

ang le be ta reen t he subho r i zon ta l sho r t en ingd i r e c ! i o n a n d t h e s u b v e r È i c a l i n i t i a lnegneÈ iza t l on , t he dev ia t i on o f r emânen t vec to r s

t . owa rd t he f l a t t en ing p l ane ( t he cLeavage )r e s u l t s i n b o t h e f f e c È s d e s c r i b e d a b o v e .

1 . A t t h e s c a l e o f e a c h s i t . e w h e r e s t r a i n i shomogeneous (F i gu re 9b ) t he dev ia t i on o fr naSneÈ izaÈ ion i s ach ieved w i t houc any l oss i nw i t h i n - s i t e g r o u p i n g o f m a g n e t i . z a t i o n , e s w a s È h ec a s e i n t h e M a r l t l m A l p s r e d b e d s [ K t i g , f i e l d e ta t . , 1 9 8 1 , 1 9 8 3 ; C o g n é a n d P e r r o u d , 1 9 8 5 1 . I ncon t râs t , de fo rna t l on p robab l y t ends t o c l us te rt he da te , âs r r as obse rved i n expe r lmen ta ld e f o r m e t i o n o f s y n t h e t i c s a m p l e s I C o g n é , 1 9 8 7 b ] ,when such e case o f a h i gh ang le be tweens h o r t e n i n g d i r e c t i o n a n d i n i È ! a l d t r e c t i o n o fmâBne t i za t i on L ras s imu la ted . The anoma lous l y h i ghc l u s t e r i n g o f w i c h i n - s i È e d i s È r i b u t i o n s d e s c r i b e dabove can t hus be exp la i ned as Èhe resu l t o fs t r a l n ac t i ng on p reCecÈon i c r emenen tm a g n e t l z e t i o n .

2 . S ince t he remânen t mâgne t i za t i on i si n i t i a l l y c l o s e c o c h e f l a t t e n i n g p l a n e , t h edev ia t i on t ova rds c l eavage i nduces an ove ra l lc l u s t e r o f m a g n e t l z a t i o n v e c È o r s a t t h e s c a l e o ft he f o l ded a rea ( f i gu re 9b ) . Th i s exp la i ns Èhegood g roup ing o f da ta i n t he i n s i t u coo rd i ne tes ,and t he i nev i cab le d l spe rs ton nhen app l y i ng chec l a s s l c a l t i l t c o r r e c t i o n ( F l g u r e 9 c ) .

Obv ious l y , t h i s h tgh l y schema t . i . c desc r i pc i oni s conven len t on l y t o exp la i n na in f ea tu res o fÈ h e r e s u l t s t h a t a r e ( 1 ) t h e h i g h w i t h i n - s i . t ec l us te r o f pa leomagnec i c da ta and (2 ) t he goodg r o u p i n g o f b e t w e e n - s i t e d i s t r i b u t i o n i n t h e i ns i t u c o o r d i n a t e s . H o w e v e r , t h e s i m p l i f y i n ga s s u m p È i o n s u s e d i n t h e d e s c r i p t i o n o f F i g u r e 9

F i g . 7 . M e a n - s i t e d j . r e c È i o n o f l o w - t e m p e r a È u r ecomponen ts w iÈh t he i r c i r c l es o f . 95 "L con f i dence .I S , i n s i t u ; T C , t i l t c o i r e c t e d . I n t h e I Sp r o j e c t l o n , a s t . e r i s k a n d c L o s e d a n d o p e n s t a r sa re t he f o rma t i on mean o f HT componen ts ( guo ted

1n l ab le 4 ) and t he Ca rbon l f e rous and t he Loçe rP e i m l a n f i e l d d i r e c t i o n s , r e s p e c t i v e l y ; t h e g r e a tc i r c l e l s t h e b e s t f l t p l a n e o v e r t h e s e t h r e ed i r e c È i o n s . C l o s e d ( o p e n ) s y m b o l s a r e p r o j e c t i o ni n t he l owe r ( uppe r ) hem lsphe re . S te reog raph i cp r o j e c t i o n s .

SAS DSH5 L

4349< q

l 2l-u

201 3200 6202 7.3163 54I U O ' L

204 -14201 25200 -?Lg6 t+5208 10

I S r l n s i t u ; T C , t i l t c o r r e c t e d ; D e , d e c l i n a -t i o n ; I , i n c l i n a t l o n ; k a n d d 9 5 , F l s h e r t s [ 1 9 5 3 ]s te t i s t i c s pa ra$e te r s ; and N , number o f en t r i est n c h e s c a t i s t i c a .

cha rac te r i sÈ i c r emanen t t nagne t i za t l on d i r ec ! , i on(R l { ) ob ta i ned by Pe r roud e t a l . [ 19E6 ] i n t heOrdov i c i an t tMou l l n de Cha teaupanne r i r ed f o rma l i on( D * 2 2 8 0 , I = 8 I o ) , a n d b y P e r r o u d a n d V a n d e rvoo [ 1985 ] i n che t he La te o rdov i c i an i n t r us i veThoua rs l . { ass i f ( r eve rsed d i r ec t i on D * 1600 , I =

E 3 o ) . I n t h e l S p r o j e c t l o n o f F i g u r e 8 , o u rd l r e c c i o n s a r e a l s o c l e a r l y d i f f e r e n t f r o r n t h ep r e s e n ! - d a y d i p o l e f i e l d a x i s . S i n c e w e d o n o Èk n o r . r a n y s t e e p l y i n c l i n e d f i e l d d i r e c t i o n i n È h eA r m o r i c a n M a s s { f s i n c e 0 r d o v l c i a n t i m e s I P e r r o u d ,1 9 8 5 ] , i t i s a s s u m e d t h a r t h e s e h i g h u n b l o c k i n gtempe re tu re componen ts , i so l a ted by t he rna ld e n a g n e L i z a t i o n , e r e a c t u a l l y t h e p r l m a r yp re tec l on i c mâgne t i ze t i on componen t o f t hePon t -Réan Fo r rna t i on .

Upon exam in ing t hc da la o f Tab le 4 and F igu reE , two remarks can be made .

1 . l t t h e s c a l e o f e a c h s i t e , m e g n e t i z e È l o nv e c t o r s a r e e x c r e m e l y w e l l c l u s t e r e d , w i t h t h ee x c e P t i o n o f s i t e s C B a n d S A , v h l c h v i l l b e

d i s c u s s e d l a t e r . T h e k D a r a m e t e r o f t h e F i s h e r r s

t 1 9 5 3 ] s t a t i s t i c s o f c h e w i t h i n - s i c ed i s t r i b u t i o n s i s o f t e n h i g h e r t h a n 1 0 0 ( s o m e t i m e s

2 0 0 ) , a n d i n t h e c a s e o f s i È e S D , i t i s h i g h e rt han 1000 . Such va lues , wh i ch a re qu i t e coûmonw iÈh in vo l can i c r ocks r r he re t he rmoremanen tr l l âSne t i zâ ! i on i s an i ns !an taneous reco rd i ng o fm e g n e t l c f i e l d d l r e c t i o n , c â n b e r e g a r d e d a sanoma lous l y h i gh f o r â sed imen tâ r y f o rma t i onç h e r e w i t h l n - s l t e k v a l u e s c l a s s i c a l l y r a n g e f r o m5 0 t o 1 0 0 .

2 , The second remark dea l s l t i t h t hec o n p a r i s o n o f i n s l t u a n d È i 1 È - c o r r e c È e db e t l . t e e n - s i t e d i s t r i b u t i o n s ( F i g u r e 8 ) . I t i sobv ious t ha t t he s imp le È i l t co r rec t i on i nduces al e r S e s c a t È e r i n g o f t h e q u i t e v e l l - c l u s t e r e d i ns i t u d â t â , l n r e l a È i o n w i È h È h e s t r u c t u r â lpos i t i on o f s i t es 1n t he sou the rn o r no r t he rnl l m b o f t h e s y n c l i n e . F o l l o w i n g t h e c l a s s i c a li n c e r p r e l a t i o n o f t h e f o l d t e s t , t h i s s h o u l d l e a du6 t o i n t e rp re t t hese pa leomagne t i c d i r ecÈ ions aspo6 t f o l d i ng . i . l e have t hus an anoma lous behev io ro f magne t . l za t l on du r i ng , un fo l d i ng , and t he samepa radox i ca l s i t ua t i on as i n t he Mar i t lme A lps :t i l E c o r r e c t i o n È e n d s c o d i s p e r s e d â t â w h i c h e r e

n o t l n t e r p r e t a b l e a s s y n f o l d l n g o r p o s t f o l d i n gr e m a g n e t l z e t i o n s .

180180

11,682 Cogné : De fo r rned Red Beds o f O rdov l c i an Pon t -Réan Fo rmâ t i on

ÎABLE 4 . Mean -S i t e H lgh -Tempera tu re Componen ts o f MegneE izâc ion I so l a tedby The rma l Demagne r i za t l on i n Geog raph i c ( I n S i t u ) and Bedd ing (Un fo l ded )

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n /N , r a t i o o f nu rnbe r o fa n a l y z e d s p e c i m e n s ; D e , I ,

*Da ta r e j ec ted f r om che

( i n i t i a l t y h o r i z o n t a l b e d d l n g p l a n e a n d v e r t i c a lm e g n e È 1 z a l i o n , h o r i i o n t a l s h o r l e n i n g ) a r e f a rf r o rn bc i ng exâc t l y obeyed i n na tu re . Oned l sc repancy be tween t he p red i c t ed and rea l f i na ld i s t r i buÈ ions mus t be pa r t i cu l a r l y no ted . F romt h e s c h e m a t l c d e s c r l p t i o n o f F i g u r e 9 , w e s h o u l do b c a i n r h e f o l l o w i n g f e a È u r e s ( F i g u r e 1 0 a ) : i nt hè sou th d i pp ing beds o f s i t es f r om che no rÈhe rn1 l m b , p a l e o m a g n e t i c v e c t o r s s h o u l d b e l o c a t e d o nthe no r t he rn s i de o f t he c l eavage (o r magne t i cf o l i a t j . on ) p l ane , i ^ ' h i l e t hey shou ld be i n chesou the rn s i de o f t h i s p l ane i n t he s iÈes f r om thesôu the rn l imb . A l t hough t h i s s i t uaÈ ion i sg e n e r a l l y m € t l n t h e l a t t e r c a s e ( s i t e s S E , S D ,C B , S B ) , l h i s i s n o t n o t t h e c a s e f o r s o m e s l t e s

e n t . r i e s i n t h e s t a t i s L l c a È o n u m b e r o fk , a n d o 9 5 , " . r . a s i n T a b l e 3 .f o r m a t i o n m e e n e s t i m â t i o n s ( r n e a n ) .

F t g . 8 . M e a n - s i t e d i r e c t i o n sc o m p o n e n t s w i t h t h e i r c i r c l e sSqua res shoç d l r ec t i ons f r oms i t e s . S a m e c o n v e n È i o n s a s i n

f r o m t h e n o r t h e r n l i m b ( F i g u r e 1 0 b ) . T h i s i sp a r l i c u l a r l y o b v i o u s i n s i t e s L C , M B , S H , a n d S J .ï he re fo re t hese f ou r s i t es do no t obey t he g l oba lbehav io r desc r i bed above . Howeve r , t h i s anoma lyd i s a p p e a r s i f w e c o n s i d e r t h e t m a g n e È i z a t i o n w â sn o t a c q u i . r e d b y h o r i z o n r a l b e d s b u t b y a l r e a d ys o u t h e r l y d i p p i n g b e d s . T h i s h y p o r h e s i s i s

bmff iw/,<l-\ li\F+-n0{__#+___}

rc\-_l_/ \ï_/F i g . 9 . S c h e m a t i c i n t e r p r e t a t i o n o f s t r a l ne f f ecÈs upon p r ima ry magne t i za t i on w l t h i n r edbeds o f t he Pon t -Réan Fo rma t i on . Fo r exo lana t i ono f F l g u r e s 9 a , 9 b , a n d 9 c , s e e t e x t .

I e ô

o f h l gh - t enpe raÈu reo f , 95L con f i dence .

the southern l i rnbF l g u r e 7 .

Cogné : De fo rmed Red Beds

/<-\ ,/î-\n , < - - \ / )(-ffiJ R=-:-\ / $'----7

o ( o / o

F l g . I 0 . ( a ) l t r e o r e t i c a l a n g u l a r r e l a t i o n s h l p so f m e a n - s i t e p a l e o m a g n e t i c v e c t o r ( s t a r ) , b e d d i n g( S g ) , a n d c l e a v a g e ( s 1 ) f o r s i r e s f r o r n È h eno rÈhe rn (N ) and sou the rn (S ) l tmbs o f t hes y n c l i n e . ( b ) O b s e r v e d r e l a t i o n s h i p s i n l w o s i t e s(PH and SH) f rorn the northern l lmb. AMSd t rec t i ons w l t h t he i r e l l l p ses a f , 957 . con f i dencea re d raçn \ r l t h sane conven l l ons as i n F i gu re 2 .

c o n s i s t e n t ç i t h e h e d e s c r i p t i o n b y B a l l a r d e È a l . .

[ 1 9 8 6 ] ( s e e F i g u r e l d ) o f a s y n s e d i m e n t a r y d l p o fbedd ing p l anes t owe rd che cen te r o f bas in , andw i l l b e f u r t h e r v e r i f i e d b y t h e r e s u l È s o f s t r a i nl emova l app l i ed t o t hese pa leomagne t l c vecco rs ,a s d i s c u s s e d a t c h e e n d o f t h i s p a p e r .

W i t h r e s p e c È È o t h e g l o b a l b e h a v i o r d e s c r i b e da b o v e , l h e w e a k w i t h i n - s l t e g r o u p l n g o f s i c e s S Aand SB can be cons ide red âs ânoma lous . Houeve r ,H T m a g n e t i z a t i o n c o m p o n e n t s o f b o È h s i t e s ( F i g u r e

11 ) shos a non -F l she r i an , e l onga ted pa th f r o rnh t g h l n c l i n a ! 1 o n s t o ç a r d S W l o w e r i n c l i n a t ! o n s .I t l s t hus p robabLe t ha t p r ima ry O rdov i c i an andseconda ry Ca rbon i f e rous megne t i zeÈ ion componen tshave been poo r l y sepa ra ted by t he rna ldenagne t i za t i on w l t h i n t hese spec imens , as show ii n F i g u r e 6 b . S i n c e t h e y a r e p r o b a b t y b a d l ys e p a r a t e d d i r e c L i o n s , t h e p o o r q u a l i t y o f t h e i rv l t h l n - s i t e c l u s t e t d o e s n o È i n t e r f e r e l r i t h t h eabove ana l ys l . s , and t he da ta f r om s i t es SA and CBw e r e r e j e c t e d f r o m t h e f o l l o w i n g .

Mechan l sm o f RM dev iaÈ ions . The i n s l ! ud l s t r i bu t i on o f pa leomâcne t i c vec to r s i s he re i nsha rp conc ras t t à wha t i as obse rved i n t heM e r i t i m e A l p s r e d b e d s , w h e r e s t r a i n h a s ascaÈ te r l ng e f f ec t upon popu la t i ons o f r emenen tmagneC ize t i on vec to r s . l . l e mus t , ho reve r , po in tou t t ha t i t i s t he same fundamen ta l mechan i smt h é t c a u s ê s s u c h d i f f e r e n t f i n a l s t a t e s : t h edev ia t l on o f spec i r nen rnagne t i zec i on Èoward t hes t r a i n f l a c t e n i n g p l a n e . A s d e s c r l b e d e a r l l e rI C o g n é a n d P e r r o u d , 1 9 8 7 ] , l h e d e v l a t l o n o fç i t h i n - s l t e d i r e c t i o n E a n d t h e i r c h a n g e i ng roup ing t hen depend upon t he ang le be tweens t ra i n sho r t en ing ax i s and i n i t i ê1 magne t i zâ t i ond t recc i on aÈ t he sca le o f each s i t e o f homogenousd e f o r m a t i o n , w h i l e t h e o v e r a l l d i s t r i b u t i o n o fmean -s i t e d i r ecC ions depends upon s t r a i ndLs t r i bu t i on ac ross t he f o l ded a rea . Theseobse rvae lons po in ! t o Èhe need o f cons ide r i ngf l r s t t he e f f ec t s o f de fo rma t i on e t a sca le whe res t ra l n i s homogeneous , be fo re e t t emp t l ng t odesc r l be t hem a ! l he sca le o f a who le f o l dedf o r m a t l o n , w h e r e s t r a i n i s È y p l c a l l yheCerogeneous . Ï he on l y common po in t a t t he

o f O rdov i c l an Pon t -Réan Fo rma t i on 1J ,631

l a t c e r s c a l e i s t h e f a i l u r e o f t h e c l a s s i c a l f o l dtes t when us l ng pa leomeSne t i c da ta f r om s l r e i nedf o l d e d b e d s .

t he roÈac ion o f ne t measu rab le r emenen tm a g n e t i z a c i o n p r o b a b l y r e s u l t s f r o m t h e r o c e È i o no f e l emen te r y mâgne t l c m i c romomenÈs and Èhus f r omthe roÈa t i on o f hema t iÈe pâ r t i c l es t hâ t ca r r yt hem. t h i s assumed mechan i sm i s l n good ag reemen tr { l . t h Èhe ana l ys l s o f A l , lS da ta made above . As ame l ce r o f f ac t , i f , we cons ide r t ha t A l tS l s ma in l ydom ina ted by hema t i t e an i so t ropy , t he goodco r re l a t i on es tab l l shed be iween AMS and s t r â i nshows t ha t s t r e i n conÈro l s che deve lopmen t o fhema t i t e basa l p l ane p re fe r red o r i enÈa t . i on . T t r i gp re fe r red o r i êû tâ t l on deve lop rnen t r nay be ach ievedi n p a r t t h r o u g h r e c r y s ! a l l i z e t i o n o f o r l e n t e dhema !1 te , as unde r l l ned by che occu r rence i n somes l t es o f a seconda ry r nagne t i c ove rp r i n t i ngre l âced co t he He rcyn ian o rogeny . 0n t he o ! , he rh a n d , ! h e r e i g n o c l e a r d i f f e r e n c e i n s t r e i n / A M Sr e l a c i o n s h l p s t n s i t e s s h o r l n g o n l y t h e p r i m a r y

P re tec ton i c r emânenc megne t l zâ t i on . I t see rnsthe re fo re r easonab le Eo assu rne e r o ta t i onmechan i sm fo r t he reo r i enÈaÈ ion o f p r ima ryhema t i t . e pâ r t i c l es wh i ch ca r r y t he p re tec ton l cremanen t megneÈ iza t l on . No te , howeve r , t ha t . w l t ht h e e v l d e n c e a t h a n d , i t ' i s n o t p o s s i b l e È o s È a È ecacego r l ca l l y t h i s r o te t i on mechan l sm fo r p r ima ryh e r n a t i t e . N e v e r t h e l e s s , t h e c o n s i s È e n c y o f t h ed e s c r i b e d p a l e o m a g n e t l c v e c t o r b e h a v l o r w i t hp r e v i o u s r e s u l t s I C o g n é a n d P e r r o u d , 1 9 8 5 ; C o g n é ,1987a ] vhe re such a mechan i sm has been shown[Cogné and Gapa i s , 1986 ] o r mode led lCogné ,1 9 8 7 b 1 m a k e s 1 t h i g h L y p r o b a b l e .

Co r rec t i ons f o r S t ra i n - I nduced Dev ia t i ons o fP r e È e c e o n l c R e m a n e n t M a g n e t i z a t i o o .

Techn lque o f S t ra l n Re rnova l

t he ana l ys i s o f p re tec ton l c magne t i za t l ond l r e c t i o n a l d i s t r i b u È i o n h a s s h o r r n t h a t i c a r i s e sf rom one e l emen ta r y mechan i sm: t he dev ia t i on o fsPec imen remanenÈ megne t i ze t i on vec to r f r om i t . sl n i È l a l d i r e c t t o n t o w a r d t h e f l a t t e n i n g p l a n e o fhomogeneous i nce rna l de fo rma t i on . Th l s behav io ri s r ough l y cons l s t en t ç1 th t he behav io r o fp a s s i v e l i n e , f o l . l o w i n g t h e m o d e l o f M a r c ht1932 ] . Th i s mode l i s t hug used as a wo rkLnghypo thes i s i n o rde r t o es t imE te t he p r ima ryd i r ecc i on o f magne t i zâ t i on â t t he unde fo r rnedE t .â !e . Th l s t echn ique has been p roposed by Cognée t a l . I I 9 E 2 ] a n d d e s c r i b e d b y C o g n é a n d P e r r o u d[ 1 9 8 5 ] . I t c a n b e s r a u n a t i z e d a s f o l l o w s . I n t h et h e o r y o f M a r c h [ 1 9 3 2 ] , t h e f i n a l o r l e n È a c i o n o fa p a s s i v e l i n e i s c o n n e c t e d t o i t s i n i t i a l

180F tg . 11 . Spec imen I IT componenÈs o f magne t l zâ t i ona n d t h e i r m e e n s ( a s t e r i s k s ) i n s l t e s S A a n d C B .SÈereog raph l c p roJec t l ons i n geog raph i cc o o r d i n a t e s .

.^+sA ràr .'.) cB

1 t , 6 u

o r l en te t i on t h rough t he t o tâ1 de fo rma t i on t enso r

1 . I h i s t enso r i . s decomposed i n t o two pa r t s I see

R a m s a y , 1 9 6 7 ] : a n i n t e r n a l d e f o r m a t l o n , o r

s t r a i n , l e n s o r S , a n d a r i g l d - b o d y r o t a t i o n

tênso r R . I n . an i denÈ i ca l way , we cân decompose

the t o te l dev la t i on o f pa leomagne t i c vec to r s i n t o

t e o p â ! c s : ( 1 ) d e v i a t i o n d u e t o s c r a i n a n d ( 2 )

r l g i d -body roÈa t i on . Fo l l o * i ng rhe i dea chaÈ

p a l e o m a g n e t i c v e c c o r s c l o s e l y f o l l o w t h e m o d e l o f

M â r c h [ 1 9 3 2 J , r e a s s u m e t h â t â n e s t i n a t e o f t h e

l e n s o r o f s t r a l n - i n d u c e d d e v i a t i o n o fpa leo rnagne t i c vec to r s i s g i ven by t he s t r â i n

t e n s o r S i c s e l f . A n e s t i m e t i c n o f t h e r i g i d - b o d y

r o t a t i o n i s p r o v i d e d , a s l n a l l o t h e r

p a l e o m a g n e l i c w o r k s , b y t h e s t ; i k e a n d d i p o f

b e d d t n g s u r f a c e a t e a c h s i t e - H o w e v e r , i n

de fo rmed beds t he bedd ing p l ane has a l so su f f e red

f t o m d l s t o r t i o n a l s È r a i n . I t l s t h u s n e c e s s e r y t o

c o r r e c t b e d d i n g t i l t e n d d i r e c t i o n f o r c h e s e

e f f e c l s l 0 t r e n s , 1 9 7 3 ; C o g n é a n d P e r r o u d , 1 9 8 5 ] .

Then , l he r i . g t d r o tâ t l on i s e3 l lmâced f r om

u n s t r a i n e d b e d d i n g p l a n e s . N o t e ! h a È p e r f o r m i n g

the t i l t co r recc i on f r om s t r i ke and d i p o f t he

u n s t r a l n e d b e d d i n g p l â n e o n l y c o r ! e c t s f o r t h e

pa r t o f r i g i d -body ro ta t i on abou t t he ho r i zon ta l

a x l s a È e a c h s i t e . T h i s m e t h o d d o e s n o c a 1 l o w t o

e s t i m a t e f o r r o t a t i o n s a r o u n d v e r I i c a l a x e s -

N e v e r t h e l e s s i n t h e P r e s e n t c a s e ' m a g n e È i z e t i o n

v e c t o r s a r e s t e e p l y i n c l l n e d , a n d t h e r e f o r e È h l s

l l r n i t a t i o n h a s l i t t l e e f f e c t .

1o summar l ze , t he t o l a l i n ve rse de fo rma t i on o f

each , magne t i za t i on d i r ec t i on l s pe r f o rmed i n two

s t e p s : ( 1 ) u n s t r a i n i n g p a l e o m a g n e l i c v e c t o r u s i n g

t h e r e l e v a n È l n v e r s e s t r a l n t e n s o r ; ( 2 ) i n v e r s e

r i , g i d -body ro ta t i on by t i l t co r rec t i on o f t he

u n s t r a i n e d v e c t o r , u s i n g s t r i k e a n d d i p o f t h e

u n s t r a i n e d b e d d i n g p l a n e o f t h e s i t e -

E s t i m a t e o f M e a n S t r a l n a t E a c h S i t e

Measu remen ts o f AMS ne re used t o imp rove t he

è s t l m a t e s o f m e a n s t r a i n w i t h l . n e a c h s i t e b y

c o n s l d e r i n g t h a t m e a n s u s c e p t i b l l i t y a x e s a r e t h e

p r i n c i p a l s t r a i n d l r e c t i o n s , a n d b y r e s e È t i n g

p r l n c i p a l s t r a i n v a l u e s f r o m t h e c o r r e l a t i o n

e { = 1 3 . 6 M t - 0 . 0 4 , e s t a b l i s h e d a b o v e . Ï T t e r e a s o n

f o r t h i s r e e 3 t i m a t i o n o f s t r a i n i s t w o f o l d .

1 . We can cons ide r t ha t r on t he eve rage t

p r L n c l p a l s u s c e p t i b i l l t i e s a n d s t r a i n s a r e

c o a x i a l o v e r a l l t h e s t u d i e d a r e a ( F i g u r e 2 ) .

Cogné : De fo rmed Red Beds o f 0 rdov i c i an Pon t -Réan Fo rma t i on

1 8 0 1 8 0

F ig .12 . l , t ean -s i t e HT componenÈs o f magneÈ lze t i on and t he re l evan t f o r r na t i on means

( a s t e r i s k s ) . I S , i n s i t , u ; î C , t i l t c o r r e c t e d ; S R T C , a f t e r s t r a i n r e m o v a l a n d C i l t

co r rec t l on f r o rn t he uns l r a i ned bedd lng p l ane . S le reog raph i c p ro j ec t i ons .

A n g u l a r d e p a r c u r e s f r o m c h i s c o a x i e l i È y w i t h i n

e a c h s i c e a r e a s s u m e d t o â r i s e r n a i n l y f r o m e r r o r s

t n f i e l d m e a s u r e n e n t . s o f c l e a v a g e , l i . n e a c i o n ,

e t c . I n c o n t r a s t , t h e p r e c i s i o n o f c o r e

o r l e n È a t i o n u s i n g p a l e o m a g n e t i c s a m p l l n g

t e c h n i q u e s , a n d t h e s È â t i s t i c e l p r o c e s s i n g o f A X S

d a t a , s u g g e s E E h a t p r i . n c l p a l s u s c e p t l b i l i l y

d l r e c ! i o n s a r e d e f i n e d w i c h b ê t t e r e c c u r â c y .

T h e r e f o r e È h e e i g e n v e c t o r s o f t h e s È r a i n t e n s o r

a t e a c h s i È e h a v e b e e n s e t p a r a l l e l l o t h e

e i g e n v e c t o r s o f t h e A M S t e n s o r .

2 . P r i n c i p a l s È r a i n v a l u e s h a v e b e e n

r e e s Ë i m a ! e d f o l l o w i n g s i r n i l a r a r g u m e n c s - 0 n t h e

o n e h a n d , p r i n c i p a l s t r e i n s h a v e b e e n e s È i m e t e d

f r o m t h e m e a n a x i a l r â È i o o f e l l i p t i c e l r e d u c l i o n

s p o t s m e a s u r e d . r i t h i n o r i e n È e d h a n d s a m p L e s - T h e

e x a c ! s t a t i s t i c a l m e a n i n g o f s u c h m e a s u r e m e n t s

w i t h r e s p e c ! t o t h e m e a n s È r â i n o f a s i t e i s

o u i t e d l f f i c u l È t o a s s e s s . 0 n t h e o t h e r h a n d '

i a l e o m a g n e t i c s a n r p l i n g i s m o r e w i d e l y d i s t r i b u t e d

w i t h l n â s i È e , a n d t h u s i t i s c o n s i d e r e d t o b e

b e t t e r r e p r e s e n t a t i v e o f m e a n A M S e È e a c h s i t e .

H e r e t o o , È h e r e i s a g o o d c o r r e l a t i o n b e t l t e e n

p r i n c i p a l s t r a i n a n d s u s c e p t i b l l i t y v a l u e s a t t h e

s c a l e o f c h e w h o l e a r e a ( F i g u r e 4 ) , a n d l o c a l

depa rcu res a re assumed t o a r i se f r om Lhe l owe r

â c c u r â c y o f s t r a i n d e t e r r n i n a t i o n . P r i n c i p a l

s L r e i n s w e r e c h u s r e c a l c u l a t e d u s i n g t h e e q u a t i o n

o f r e g r e s s i o n l i n e , a n d m e a n A M S d a t a o f e a c h

s t t e ( T a b l e 2 ) .

R e s u l t s o f S t r a l n R e r n o v a l

M e a n - s i t e d i r e c t i o n s o f p r e È e c È o n i c r e m a n e n f

m a g n e t i z a t i o n o b t a i n e d b y s c r a i n r e m o v a l a n d t i l t

co r rec t i on f r om the uns t ra i ned bedd ing p l ane a re

g i v € n i n T a b l e 4 a n d d r a w n i n F i g u r e 1 2 . l n t h i s

f i gu re , t hey e re comPâred w i t h t he i n s i t u and

c l a s s i c a l l y t i l t - c o r r e c t e d d â È a . O n e c a n s e e t h â t

t h e p o p u l a t i o n o f c o r r e c t e d d i r e c t i o n s i s

r e a s o n n a b l y w e l l c l u s t e r e d a t t h e u n d e f o r m e d

s t â t e . T h e p a r a d o x i c a l b e h a v i o r o f t h i s

p o p u l a t i o n u p o n s i m p l e u n f o l d i n g t h u s v a n i s h e s ,

a n d t h e h y p o t h e s i s o f a p r e t e c t o n i c m a g n e t i z a t i o n

i s v a l i d a t . e d . T h e c o n s i s t e n c y o f o v e r a l l m e a n

d i recÈ lon o f nagne ! i zâ t i on â t t he unde fo rmed

s t e t e w i t h o r d o v i c i a n p a l e o m a g n e t i c d i r e c t i o n

known i n t he A r rno r i can Mass i f lPe r roud , 1985 ]

a l l o w s u s t o d e f e n d t h e u s e f u l n e s s o f t h e s t r a i n

remova l t echn ique app l l ed t o t . he dev ia ted

F lna l Es t ima t l - ons o f Fo rmac ion MeanM a g n e t i z a t l o n

1 t , 6 8 5

been re j ec l cd f r o rn t he f i na l es t i r î a t i ons o f

f o rma t i on mean rnagne t l zec i on t he t a re quo ted i n

l ab le 5 and d rawn i n F i gu re 13 .

I t l s t he unde fo r rned d i r ec t l on o f Tab le 5 t ha t

I c o n s l d e r t o b e t h e c h a r a c t e r i s È i c p r e t e c c o n i cpa leomagne t i c d i r ecc i on o f t he Ordov i c i an redb e d s o f t h e P o n t - R é a n F o r m a L i o n . S e a t i s t i c a lt es t s o f McELh i . nny [ 1964 ] show the t t he kpe ra r î e te r o f t he unde fo rmed mean i s s i gn i f i cane l yh ighe r t han k o f bo th i n s i t u and t i l t - co r rec tedmeans . l t r e unde fo rned d i r ec t i on i s i den t i ca l t ot h e O r d o v i c i a n m a g n e t i c d i p o l e f i e l d d i r e c t i o n( s ta r i n F i gu re 13 ) compu ted f o r t he l oca t i on48oN , 2 .5q ,1 f r om rhe Ordov i c i an vGP o f t heA r n o r i c a n M e s s i f a f È e r P e r r o u d [ 1 9 8 5 J .

Conc lus ions

T t re na in r esu l t s ob ta i ned i n t h i s scudy can be

summar i zed as f o l l ows .I F ^ 1 r f - ^ ^ r . L e P o n ! - R é a n F o r m a È i O n l s^ v r s r . , é v r s r ,

accompan led by t he deve lopTnen t o f an ax i a l p l ana r

c l eavage . T t r iE i nd i ca tes t he occu r rence o f s t r a i np t o c e s s e s d u r i n g f o l d i n g . S t r a i n i n d u c e s t h edeve lopmen t o f an an i so t ropy o f t he megne t i c

s u s c e p f l b l l i t y . S t r a i n a n d A M S t e n s o r s s h o w agood co r re l a t i . on be te . r een t he i r p r i nc i pa ld i r e c t i o n s a s w e l l a s p r i n c i p a l v a l u e s . W e c a n

thus cons ide r t ha t s t r a i n conÈro l s t . he magne t i c

f ab r l c . Tak ing i n t o accoun ! t he P resence o f ap r e t e c t o n l c r e m a n e n t m a g n e t i z ê t i o n , es t r a i n - l n d u c e d r o t a t i o n m e c h a n i s m i s i n f e r r e d f o rp r i m a r y h e m a t i t e t . h a t c â r r i e s t h i s m a g n e E i z a t i o n .t h i s r nechan i s rn i s t hough t Èo be qu iÈe s im i l a r co

the one desc r i bed i n t he Mer i t . ime A lps r ed beds

on t he bas i s o f X ray d i f f r ac tome t r y measu re rnen l s

l C o g n é a n d G a p a i s , 1 9 8 6 ] .? . The p rog ress i ve a l i gnnen t o f hemaÈ ice

w i t h i n t he c l eavage p l ane i nduces a r o t . a t i on o fn e t m e a s u r a b l e s p e c i m e n r e m â n e n È m a g n e t i z a t i o ntowa rd t h i s p1ane . A t . t he sca le o f eachh o m o g e n e o u s l y d e f o r r n e d s i È e , t h i s r e s u l t s i n adev la t i on o f t he mean magneÈ iza t i on t ôwa rdc leavage . Due t o Che h i gh ang le be tweens h o r t e n i n g a n d i n i t i a l m a g n e t i z e È i o n d i r e c t l o n s ,t h e w h o l e d e v i a t i o n i s a c h i e v e d w i t h e c l u s t e r i n ge f f e c t u p o n t h e i n i t i a l d i s p e r s i o n . T h i s i s i nc l . ose ag reemenÈ \ r i t h r esu l t s o f numer i ca l ande x p e r i m e n t a l s i r n u l a t i o n s I C o g n é e t a 1 . , 1 9 8 6 ;C o g n é , 1 9 8 7 b 1 . A t t h e s c a L e o f t h e f o l d e d a r e a ,

F i g . 1 3 . F i n a l f o r m a t i o n m e a n d i r e c t i o n s f o r t h e

P o n t - R é a n F o r m a t l o n . I S , T C , S R T C ( d o t t e d c i r c l e )a s l n F i g u r e 1 1 . A s t e r i s k i s t h e O r d o v i c i a nd i p o l e f i e l d d i r e c t i o n i n t h e A r m o r i c a n M a s s i f

I a f t e r P e r r o u d 1 9 8 5 ] ; k i s d i s p e r s l o n p â r a m e È e r

o f F i s h e r ' s [ 1 9 5 3 ] s t a t i s t i c s .

Cogné : De fo rned Red Beds o f O rdov i c l ân Pon t -Réan Fo rna t i on

TABLE 5

Imum 9s

I n 3 1 t uT i l t c o r r e c t e dUndeformed

r emanen t . megne t l za t l on d t r ec t i ons o f t he de fo rmed

red beds o f t he Ponè -Réan Fo rma t i on . Us ing t he

s tâc l sÈ i cs o f l ' l cE lh i nny [ 1964 ] , t he k Pa re rne te ro f t he unde fo r rned meân ePPea rs g ree te r a t t he 95%

probab i l i t y l eve l Èhan l n t he un fo l ded mean , bu t

l s no t d l f f e ren t f r o rn t he i n s i t u mean . The

iden t l ca l g roup ing o f l n s i t u and unde fo rnedm e a n s i s n o l s u r p r l s i n g , s l n c e t h e e f f e c t o f

s t r a i n , a s d e s c r i b e d a b o v e a n d i n F l g u r e 9 , l s È o

c lus te r mag f i e t i ze t ' i on vecLo rs . I t i s Èhusp robab le t ha t ' pâ l eomâSneL i c vec lo r s have s im i . l a r

o r be t ce r g roup ing a f t e r de fo rme t i on t han i n t he

i n i t i â 1 , p r e d e f o r m e d , s t a t e .Ho reove r , a mo re t ho rough exam ina t i on o f t he

resu l ! s shows cha t uns t re i ned pâ leomagne t l c

d i r e c t i o n s ( F i g u r e 1 2 , s R T c ) c l u s t e r l n t o t w o

g r o u p s . T h e f i r s t g r o u p o f s e v e n s i t e s ( P M , S B ,

S D , S E , S F , S G , S I ) h a s i n c l i n a t i o n s h i g h e r t h a n

7 0 o . w h i l e i n t h e e e c o n d ( L c , M B , s H , s J ) ,

i n c l i n a c l o n s a r e l o w e r c h a n 6 0 0 . W i t h r e f e r e n c e

Èo known Ordov i c i an dÂ tâ , Èhe l a t t e r eppea r t oo

l o w , a n d È h i s g i v e s a s l i g h c l y e l o n g a t e d

d i s c r i b u t i o n o f p o i n e s i n t h e N - S d i r e c t i o n . l h e

ano rna l y o f t hese f ou r s i t es i s r e l a ted t o l he

a n o m a l y o f t h e i r l n s i t u d a t a , b u ! l s e a s i e r t o

i n t e r p r e È a t t h e u o d e f o r m e d s t a t e , a s r e s u l t i n g

f r om a bad es t ima l i on o f t he r i g i d -body ro ta t i on

dece rm ined f r om the d i p o f uns t ra i ned bedd ingp l e n e . I n e f f e c t , a s n o t e d i n t h e i n t r o d u c t i o n ,

B a l l a r d e È a l . [ 1 9 E 6 ] h a v e s h o w n È h a t t h e

P o n t - R é a n f o r m a c i o n w a s d e p o s i t . e d d u r i n g t i l t i n g

o f B r i o v e r i a n b l o c k s ( s e e F i g u r e l d ) . N e a r t h e

no rma l f au l t s , che sed imen te t i . on p l ane may have

h a d a s i g n i f i c a n t i n i t i a l d i p o f 1 O o t o 2 0 o

t o w a r d t h e c e n t e r o f t h e b a s i n s . S i t e s L C ' M B ,

S H , a n d S J a r e a c t u a l l y l o c a t e d n e a r s u c h

s y n s e d i m e n t a r y f a u l t s ( s e e F l g u r e 1 b ) , t h e i r

sh i f t f r o rn t he oche r g roup eve râges abou t 15o '

and t he d i r ec t i on o f t h i s sh l f t t oge rd t he sou th

i s c o n s i s t e n t w i È h t h e d i r e c t l o n o f s u s p e c t e d

s y n s e d i m e n È a r y d i p p i n g . I h e d i s c r e p a n c y o f t h e s efou r s i t es cou ld t hus a r i se f r om an i nco r rec t

t i l È c o r r e c t i o n f o r t h e s e i n i t i a l l y d i p p i n gs i t e s . N o t e t h a c t h i s s i t u a t i o n i s n o t d i r e c t l y

connec ted t o ou r sÈ ra in r emova l t echn ique bu t t o

t h e c o m m o n l y a s s u m e d , a l b e i t d i f f i c u l t t o v e r i f y ,

i d e a t h a t s e d i m e n c a r y b e d s a r e d e p o s i È e dh o r i z o n t a l l y . A s i m i l a r p r o b l e m w a s e n c o u n t e r e d

in t he l a t e ra l equ i va len t t 'Mou l i n de

Cha teaupanne r t r ed bed f o rma t . i on , whe re Pe r roud eÈ

a1 . [ 1986 ] showed t ha t t he Ordov i c i an remanen tmagneÈ iza t i on 1 tâs acqu i r ed by beds a l r eady

t i l t e d , p r o b a b l y d u r i n g s e d i m e n t a È i o n .F ina l l y , a l t hough re have some a rgumen ts Èo

e x p l a i n t h e s l i g h r l y d e v i a t e d p a l e o m a g n e c i c

d l r e c t i o n s o f s i t e s L C , M B , S H , a n d S J , w e h a v e

no p rec i se e l emen ts t o co r rec t f o r t hese

a n o m a l i e s . C o n s e q u e n t l y , t h e s e f o u r s i t e s h a v e

?54238235

o /t ut )

13,686 Cogné : De fo rmed Red Beds o f

3 t r e i n i n d u c e s a c l u s t e r i n g o f b e t w e e n - s i t ed i s t r i bu t i on o f mean pa leomagne t ! c vec to r s . I hec l a s s i c a l a p p l i c a t i o n o f r h e f o l d r e s t s h o u l dhave t hus l ead t o en e r roneous ass tgnmen t o f as y n c e c È o n i c t o P o s t t e c t o n i c e g e t o c h e s ep re tec ton i c pa leomagne t i c vec to r s . I n Èhe t e rmsused i n t he l n t r oduc t i on o f Èh i s pepe r , t h i srneans t haC the desc r i p t i on o f pa leomagne t i cv e c t o r s r r o t e ! i o n f r o m o n l y b e d d i n g p l a n eg e o m e t r y i s , l n e f f e c ! , i n a p p r o p r i a t e w i t h i ns t r a i ned beds , and conc lus i ons d rawn f r om th i sdesc r i p t i on a re i nadequa te . t h i s con f i rms t hes im i l a r ana l ys l s we nade t n t he Mar i t ime A lps r edbeds lCogné and Pe r roud , 1985 ] , and t heconc lus i ons o f Van de r P lu i Jm [1987 ] and Koda rna

[1988 ] d rawn f r om nune r l ca l - s imu la t i ons . Th i spo in t i s impo r tan t because we can suspec t châ t ess o o n e s c l e a v a g e d e v e l o p s , a n a l y s i s o fpa leo rnagne t i c vec to r popu la t i ons ' , r i t h t he he lp o ft h e c l a s s i c a l f o l d t e s t I G r e h â m , 1 9 a 9 ] c a n b eb i a s e d .

3 . l t r e p r o b a b l e r i g i d r o t a t i o n o f h e m a t i t e

P a r t l c l e s t h a È c a r r y t h e p r e t e c t o f l i cmagne t l za t i on i nduces dev ia t i ons o f RM. Thesedev ia t i ons appea r r ough l y cons i s l enC a r iÈh t hed e v i a t i o n s o f p a s s i v e l i n e s f o l l o w i n g t h e m o d e lo f M a r c h [ 1 9 3 2 ] . T h e p a s s i v e l i n e n r o d e l h a s t h u sbeen used as a wo rk i ng hypo thes i s i n an a t t emp tt o c o r r e c t f o r s t r a i n - i n d u c e d d e v i a t i o n s o fp a l e o m a g n e t i c v e c t o r s . A p p l i c a t i o n o f c h e s t r a i nr e m o v a l t e c h n i q u e l C o g n é e t a l . , 1 9 8 2 ] t o c h eremanen t megneÈ iza t i on vecÈo rs o f t he Pon t -RéanFo rma t i on has been shown t o be success fu l . Nos i g n i f i c a n t o v e r c o r r e c t i o n c o n n e c t e d w i t h t h eb e h a v i o r m o d e l o f M a r c h [ 1 9 3 2 ] h a s b e e n o b s e r v e d .A f : e r s i m i l a r r e s u l t ' s o b t a i n e d i n t h e M a r i t i r n e

Â lps red beds lCogné and Pe r roud , 1985 ] and t heP y r e n e a n r e d b e d s I C o g n é , ] . 9 8 7 a 1 , t h i s t h i r dexamp le con f i rms che use fu l ness o f t he s t r a i nr e n o v a l t e c h n i q u e i n p a l e o m a g n e t i c s E u d i e s o fm o d e r a t e l y d e f o r m e d r e d b e d s .

F i n a l l y , È h e d e t â i l e d s t u d y o f s t r a i n / m e g n e t i -z a t i o n r e l a t i . o n s h i p s a l l o w e d t h e d e È e r m i n a t i o n o fa p r e t e c C o n i c d i r e c t i o n o f m a g n e C i z a t i o n f r o r o ade fo rmed f o rmac ion cha t shou ld have been re i ec tedb y c l a s s l c a l p a l e o m a g n e t i c a n a l y s i s .

Acknov ledgmen ts . P re l im ina ry samp l i ng andmeasu remen ts by M . P . l ex i e r encou raged me t oc o m p l e È e t h i s r e s e a r c h . I a l s o a p p r e c i a t e ds t i m u l â t i n g s u g g e s È i o n s â n d d i s c u s s i o n s w i t h C .L e C o r r e , J . P . B r u n , P . C o b b o l d . , a n d H .Pe r roud . R . K l i g f i e l d and K . P . Koda rna p rov i dedc o n s t r u c t i v e c r i ! i . c i s m s o n a f l e a r l y d r a f t . l h i sl s a c o n È r i b u t l o n o f C A E S S ( U n i t é P r o p r e d eReche rche du CNRS no 214 ) .

Re f e r enc e s

A n s o n , G . L . , a n d K . P . K o d a r n a , C o m p a c t i o n -i n d u c e d i n c l i n a t i o n s h a l l o w i n g o f t h ep o s t - d e p o s i t i o n a l r e m a n ê n È m a g n e t i z a t i o n i n as y n t h e t i c A e d i m e n t , G e o p h y s . J . R . A s È r o n .S o c . , 8 8 , 6 7 3 - 6 9 2 , 1 f f i

B a l l a r d , J . F . , J . P . B r u n , a n d J . D u r a n d , L ad i s c o r d a n c e B r i o v é r i e n - P a 1 é o z o i q u e i n f e r i e u re n B r e t a g n e c e n t r a l e : s i g n a t u r e d r u n é p i s o d ed e d l s t e o s i o n o r d o v i c i e n n e , C . R . A c a d . S c i .P a r i s , 3 0 3 , 1 3 2 7 - 1 3 3 2 , 1 9 8 6 . -

E o n j o u r , J . L . , J . J . P e u c a t , J . J . C h a u v e l , F .P a r i s , a n d J . C o r n i c h e t , U - P b z l r c o n d a t i n g o f

O rdov t c i an Pon t -Réan Fo rma t i on

t h e E a r l y P a l e o z o i c ( A r e n i g i a n ) È r a n s g r e s s i o ni n l . l e s t e r n B r i t t a n y ( F r a n c e ) : a n e n c o n s c r a i n tf o r t h e L o r r e r - P a l c o z o i c t i m e - s c a l e , C h e m .ceo l . I so topes Geosc i . , 72 , 329 -336 , -1968 ' .

B o u r n e , D . E . , a n d P . C . K e n d a l l , V e c t o r a n a l y s i sâ rd ca rces ian Èenso rs , Znd ed . , - 25 t rFp - ; * -

f f i ob i , 1977 .C a v e t , P . , J . J . C h a u v e l , H " L a r d e u x , a n d J .

B I a i s e , P a l é o z o i q u e d u d o m a i n e l i g é r i e n e n t r eA n c e n i s e ! C h a l o n n e s , B u l l . S o c , G e o l . M i n e r .

! :9 !3939, 11 ,61-65, 1 -C l e n d e n e n , I ^ 1 . S . , R . K l i g f i e l d , A . M . H i r t , a n d

W . L o ç r i e , S t r a i n s t u d l . e s o f c l e a v a g edeve lopmen t i n t he Che lms fo rd Fo rmac ion ,S u d b u r y B a s i n , O n c e r i o , T e c È o n o p h y s i c s , 1 4 5 ,191-211 , 19E8

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-- - f : :F:-Togné, Centre Arrnor icain d 'Etude

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( R e c e i v e d J a n u a r y 2 8 , 1 9 8 8 ;r e v l s e d J u n e 3 0 , 1 9 8 8 ;

a c c e p t e d - I u l y 1 2 , f 9 8 8 . )

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