buton of sundaland

7/28/2019 Buton of Sundaland

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S E S u n d a l a n d a c c r e t io n : p a l a e o m a g n e t i c e v i d e n c e o f l a r g eP i i o - P l e i s t o c e n e t h i n - s k i n r o t a t i o n s i n B u t o n

J A S O N R . A L I 1, J O H N M I L S O M 2, E D W A R D M . F I N C H 2 & B U N D A N M U B R O T O 31 D e p a r t m e n t o f O c e a n o g r a p h y , T h e U n i ve r si ty , S o u t h a m p t o n , U K .2 D e p a r t m e n t o f G e o l o g i c a l S c i e n c e s , U n i v e r s it y C o ll e g e , L o n d o n , U K .3 G e o l o g i c a l R e s e a r c h a n d D e v e l o p m e n t C e n t r e, B a n d u n g , I n d o n e s ia .

Abstract: The Tukang B es i P l a t form, an Aust ra l ian m ic rocontinen ta l f ragment , began do ckingwith Sundaland in the Pl iocene, impact ing on east Buton (SE Sulawesi) . Fortuin et al. (1989;Journal of SE A sian Earth Sciences,4, 107-124) postula ted that south Bnton had rota ted c lock-wise throug h about 60 re la t ive to centra l /north Buto n in response to the Tuk ang Bes i col l i sion.A palaeomagnet ic invest igat ion was carr ied out to test this model . Some 41 (of 72) palaeo-magne t i c s it es f rom the u pper Neogen e Tondo and Sampolakosa Format ions on Buton y i e ldedinterpretable data . S i tes from south Buton record local ly consistent di rect ions, but a t sampl inglocal i t ies < 25 km apart deflect ions are between 0 and 35 c lockwise . The re is no t rend in thesedata wi th respect to the broad geotectonic se t t ing. In centra l Buton decl inat ion offsets arenegl igible . In north Bu ton local ized (ki lom etre scale) large (30-60 ) c lockw ise and coun ter-c lockwise decl inat ion offsets are observed. Effect ively the Buton data provide spot markers oncover sequences tha t have been loca l ly de formed as ' t h in- sk in ' shee ts . T he unde r ly ing basemen tmay have exper i enced the l a rge re l a tive mot ions proposed by For tu in et al. (1989) but the upperCenozoic cove r has no t been d e formed in such a s imple way . Thi s s tudy demonst ra t e s tha t t h in-skin sheets associa ted wi th cont inental col l ision may un dergo 3 0-60 rota t ions wi thin very shortintervals (< 2-3 Ma).

T h e I n d o n e s i a n a r c h i p e l a g o h a s b e e n a z o n e o fa c t i v e c o n v e r g e n c e t h r o u g h o u t th e T e r ti a ry . S i n c ea t l e a s t t h e e a r l y M i o c e n e , t h e t e c t o n i c d e v e l o p -m e n t o f th e a r e a h a s b e e n d o m i n a t e d b y t h e i n t er -a c t io n s b e t w e e n t h e S u n d a l a n d m a r g i n o f E u r a s i aa n d t h e I n d o - A u s t r a l i a n a n d P h i l i p p i n e S e a p l a t e s .C o n v e r g e n c e o f t h e s e p l a t es o n t h e i r tr i pl e j u n c t i o ni n e a s t e rn I n d o n e s i a h a s l e d to o b d u c t i o n , s u b -d u c t i o n , fr a g m e n t a t i o n a n d s h u f f li n g o f n u m e r o u sm i c r o p l a t e s , t y p i c a l l y c o v e r i n g 1 04 t o 1 05 k m 2 ,w i t h i n a n e v e r - d e c r e a s i n g a r e a . T h e b o u n d a r i e sb e t w e e n th e s e f r a g m e n t s h a v e p r o b a b l y b e e ng e n e r a l l y s h o r t - l iv e d ( < 5 M a ) a n d o v e r t h e n e x t1 0 - 2 0 M a m a n y w i l l b e i n c o r p o r a t e d i n t o e i th e rS u n d a l a n d o r A u s t r a li a . T h i s p r o c e s s i s a l r e a d yw e l l a d v a n c e d i n t h e w e s t e r n B a n d a S e a w h e r eA u s t r a l i a n m a r g i n f r a g m e n t s s u c h a s t h e B a n g g a ia n d S u l a Is l a n d s , B u t o n a n d t h e T u k a n g B e s iP l a t f o r m h a v e c o l l i d e d w i t h t h e S u n d a l a n d m a r g i nr e p r e s e n t e d b y t h e l a r g e i s l a n d o f S u l a w e s i . Aq u a n t it a ti v e u n d e r s t a n d i n g o f t h e d e v e l o p m e n to f th i s m a r g i n w i l l p r o v i d e v a l u a b l e i n s i g h t sc o n c e m i n g t h e p r o c e s s e s ( a n d r at e s) i n v o l v e d i nt e r r a n e a c c r e t i o n a t c o n t i n e n t a l m a r g i n s . I n t h i sp a p e r n e w p a l a e o m a g n e t i c d a t a ar e p r e s e n t e d f r o mu p p e r N e o g e n e r o c k s o n B u t o n a n d t h e r e s u lt sr e l a t e d to t h e P l i o - P l e i s t o c e n e c o l l i s i o n p r o c e s s e s i nt h e r e g i o n .

B u t o n

B u t o n m e a s u r e s 1 5 0 k m f r o m n o r t h t o s o u t h a n da p p r o x i m a t e l y 6 0 k m a c r o s s a t i ts w i d e s t p o i n t( F i g . 1 ) . T o p o g r a p h i c a l l y , i t d i f f e r s m a r k e d l y f r o mM u n a I s l a n d to t h e w e s t a n d t h e i s l an d s o f th eT u k a n g B e s i p l a t f o r m t o t h e s o u t h e a s t , b e i n gg e n e r a l l y m o u n t a i n o u s , w i t h p e a k s i n e x c e s s o f1 0 0 0 m . G e o l o g i c a l l y , t h e is l a n d a t t r a c te d e a r l yi n t e r e s t b e c a u s e o f t h e o c c u r r e n c e o f d e p o s i t s o fa s p h a l t w h i c h a r e w o r k e d c o m m e r c i a l l y a t an u m b e r o f l o c a l i t i e s . B e c a u s e o f t h i s , t h e g e o l o g yo f t h e is l a n d i s w e l l k n o w n , a t le a s t b y c o m p a r i s o nw i t h m o s t o f t h e o t h e r i s l a n d s o f e a s t e r n I n d o n e s i a .P r e s e n t u n d e r s t a n d i n g i s b a s e d o n t h e p u b l i s h e dw o r k o f W i r y o s u j o n o & H a i n i m ( 1 9 7 8 ), S m i t h( 1 9 8 3 ) , F o r t u i n et al. ( 1 9 8 9 ) a n d S m i t h & S i l v e r( 1 9 9 1 ), a s w e l l a s u n p u b l i s h e d w o r k b y M . E . M .d e S m e t ( p e rs . c o m m . 1 9 9 2 ) . I n a d d i t i o n , a c o n -s i d e ra b l e a m o u n t o f la n d a n d m a r i n e s e i s m i c w o r kh a s b e e n c a r d e d o u t b y o i l c o m p a n i e s e x p l o r i n g f o rh y d r o c a r b o n s i n th e r e g i o n (D a v i d s o n 1 9 9 1 ) . O t h e rm a r i n e s e i s m i c , g r a v it y a n d m a g n e t i c s u r v e y s h a v ef o r m e d p a r ts o f b r o a d e r s c a le i n v e s t ig a t i o n s o f t h eB a n d a S e a b y U S o c e a n o g r a p h i c i n st it u te s . T h e r eh a v e b e e n o n s h o r e g r a v i ty s u r v e y s o n B u t o n , M u n aa n d o n e o f th e T u k a n g B e s i i s l an d s a n d i n th ee x t r e m e s o u t h e a s t o f S u l a w e s i b y a j o i n t U n i v e r s i t y

From Hall , R. & Blun del l , D. (eds) , 1996, Tectonic Evolution o f So utheast Asia,Geological Society Special Publ icat ion No. 106, pp. 431-4 43. 4 3 1


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4 3 2 J . R . A LI ET AL.

S U L AWE S Ir - - -41,"NI ,

4o30

TanjungBatu

R

BU TON

5o30

Traverse

O

I

o ~ [ 1 ~ ] A l lu v iu mQuaternarypost- . , . , .: WapolakaFormation (reef limestone)collision [ U Plioceneunits pper Miocene~ SampolakosaFormation (chalk and marl)[Middle o Upper~ TondoForm ation (fine - coarse clastics)W Miocene

~ Pre-Neogene ormations

5 O k m

/ (, ~ 250km

Fig. 1. Geological map of Buton, SE Sulaw esi (based on Sm ith & Silver 1 991) with palaeomagnetic sample si tes:1, Bu ngi River; 2, Sampolakosa R iver (no reliable palaeo ma gnetic data obtained, ND); 3, K em beru River;4, Sampolakosa Bridge, (ND ); 5, Warum bia River, Lapodi; 6, W abiau River; 7, Wakalumbe River, (ND); 8, WaulalaRiver; 9, Kawau R iver; 10, Lon gito River; 11, Loa ngk um be River; 12, Siloi River; 13, Walue River.


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SE SUNDALAND ACCRETION 4 3 3o f L o n d o n / G e o l o g i c a l R e s e a r c h a n d D e v e l o p m e n tC e n t re t e a m (Mi l som 1992) a nd t he se l i nk t oe a r li e r w o r k b y m e m b e r s o f U C S a n ta C r u z i n S ES u l a we s i (S i l ve r e t a l . 1983).

The s t y l e a nd s e t t i ng o f s e d i me n t a t i on on B u t onha s be e n de sc r i b e d wi t h i n a f r a me wor k o f r if t -d ri f tf o l l o w e d b y c o l l i s i o n a n d a m a l g a m a t i o n ( e . g .F o r t u i n e t a l . 1989) . The p re -Ne oge ne s e c t i onr e c o r d s s e d i m e n t a t i o n o n a m i c r o - c o n t i n e n t a lf r a g m e n t w h i c h w a s r i f t e d a w a y f r o m n o r t h e r nAus t ra l i a i n t he Me soz o i c . The b l oc k d r i f t e d a he a dof t he Aus t ra l ia n c on t i ne n t a nd e nc oun t e r e d t he S ES u l a we s i subduc t i on z one i n t he e a r l y Mi oc e ne ,unde rgo i ng de fo rma t i on a s t he z one ' c hoke d ' a ndsubduc t i on c e a se d (F i g . 2) . C onve rge nc e c on t i nue dbe t we e n S unda l a nd a nd o t he r Aus t ra l i a n f r a gme n t si n t he B a nda S e a , a c c ommoda t e d by subduc t i one a s t o f B u t on un t i l t he mi d -P l i oc e ne , whe n t heT u k a n g B e s i p l a t f o r m e n t e r e d t h e t r e n c h o f fsou t he rn B u t on , g i v i ng r i s e t o a s e c ond de fo rm-a t i ona l e ve n t (F i g . 2 ) . F o r t u i n e t a l . ( 1 9 8 9 )sugge s t e d t ha t c on t i nue d c onve rge nc e fo l l owi ngt h i s s e c o n d e v e n t c o u l d e x p l a i n t h e e l e v a t e dQua t e rna ry r e e f s i n sou t he rn B u t on , t he pa t t e rn o f

major faul t s , fo lds and thrus t s ac ross the i s land, andt he 50 -60 c ha nge i n r e g i ona l s t r i ke be t we e n i t sno r t he rn a nd sou t he rn ha l ve s . C l oc kw i se ro t a ti on o ft he sou t h w i t h r e spe c t to t he no r t h wo u l d i ne v i t a b l yl e a d t o e x t e ns i on s e a wa rd o f c e n t ra l B u t on , a nd a ne x t e ns i ona l ba s i n doe s e x i s t i n a pp rox i ma t e l y t hee xpe c t e d pos i t i on . The pa t t e rn o f B ougue r g ra v i t yc on t ou rs , wh i c h ' wra p a round ' sou t he rn B u t on a ndMuna , i s a l so pe r sua s i ve e v i de nc e fo r d i f f e re n t i a lro ta t ion .

In a dd i t i on t o the s e d i me n t a ry roc ks , ou t c rops o fa d i sme mbe re d oph i o l i t e a re s c a t t e re d t h roughw e s t e r n B u t o n , t h e i r p r e s e n c e b e i n g g e n e r a l l ya t t r ibuted to the f i rs t of the two col l i s ions (e .g .F o r t u i n e t a l . 1989) . Gravi ty surveys have fa i led ,howe ve r , t o l oc a t e a ny ma j o r a noma l i e s a s soc i a t e dwi t h t he se roc ks , o r a ny s i gn o f a h i gh de n s i t y roo tz o n e ( M i l s o m 1 9 9 2) . A g r a v i t y m a p o f th eB u t on -Tuka ng B e s i r e g i on i s shown i n F i g . 3 .The se obse rva t i ons a re e v i de nc e i n f a vour o f t hei n t e rp re t a t i on o f t he s t ruc t u re a nd de fo rma t i on o fB u t on i n t e rms o f a t h i n - sk i nne d t h rus t mode l ;suc h a mode l ha s a l so be e n p ropose d on pu re l yge o l og i c a l g rounds (H . Ma nur , pe r s. c omm . 1993) .

IwNwtSea Level EOC ENE - OLIGOC E NE

~ ~ .......... : .....................+ + + + + + ~ l ~ t ~ t ~ ~ ~ . - - ~ , . . ~ - - - - , x . ~ . . ~ . . . ' . ~ E ~ ~+ + + + + + ~~1~ ~ -~ ~ '~ ~+ + + + + ~ ~ ' ~ / - - - - ~+ ~ / BUTON MICRO - CONTINENTMUNA SE .f f' ~' //SULAWESI - /CONTINENTALCRUST

BUTON KAPANTOREHSTRAITS OPHIOLITEBASIN .. ~ BULU BASIN MI OC EN E

~ _ . . ~ _ _ _ _ / ~ ~ - " TUKANG BESI MICRO - CONTINENT

UPLIFTED ~ ~ FOLD & THR UST BELTMUNA/S E SULA WESICONTINENTAL CRUST

P LIOC ENE / P LEIS TOC ENE-- ,,

ACCRETED B UTON ACTIVE FOLD & THRUST BELT/ / MICRO - CONTIN ENTI /Fig. 2. Cartoons to illustrate the mid to late Cenozoic developm ent of the SE Sulawesi region, based on Fortuin et al .(1989) and Davidson (1991).


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434 J .R. ALI ET AL.

5" S

123E 124EFig. 3. Bouguer anomalies, Buton, Muna and the TukangBesi Platform. Reduction density = 2.67 Mg m 3.

Mu na and the T ukang Besi platformMuna, which l ies to the west of Buton, f rom whichit is separated by a narrow seaway, is an uplif tedcarbonate plat form of general ly low rel ief . Anarrow limestone ridge which runs close to theeastern coastl ine is interpreted as a former barrierreef. Uplif t in the extreme south has been strongerthan in the remainder of the island, although lessstrong than in the adjacent parts of Buton, whereflights of coral terraces are impressive features ofthe landscape. Basem ent rocks are exposed only ina t iny area of schist outcrop at Tanjung B atu, on thewest coast.The Tukang Besi platform, east of southernButon, is largely submerged but some moderatelylarge islands r ise a few metres above sea-level.These seem to be ent i rely bui l t up of upperNeogene or Quaternary l imestone. Gravi ty datafrom Wangi Wangi, the largest of the islands, andfrom marine surveys (Fig. 3) , suggest that thecrustal thickness in the area approaches that of anormal continent, supporting the view expressed byHamil ton (1979) that the plateau is anotherfragment of Australian-related continental crust .Seismic reflection data obtained using relativelylow power sources on oceanographic cruises haveimaged only very thin sedimentary sect ions and i tseems un l ike ly tha t t he re a r e th i ck Neogenesediments analogous to those exposed on Butonanywhere on the plateau. The same seismic datasuggest a clear structural break betw een the plateauand Buton i t sel f , the in tervening seaway beingconsiderably deeper than the strait between Butonand Muna.

Formations sampled for palaeomagnetismThe aim of the project was to use palaeom agnet ismto quant i fy the proposed di f ferent ia l rotat ionbetween north and south Buton (Fortuin et al.1989). As the collision of Tukang Besi Platformwith Buton is believed to have started in themid-Pl iocene, sam pl ing was conf ined to two upperNeogene format ions exposed across much o f Buton(the stratigraphy of Buton is summa rized in Fig. 1) .Older format ions were not sampled as they wereconsidered to have complex pre-Miocene mot ionhistories: isolating their record of Plio-Pleistocenedeformation would be diff icult , if not impossible.The upper Miocene Tondo Format ion comprisesterrigenous clastic rocks ranging from breccias tomudstones. The sediments were eroded followingthe upl i f t associated wi th the Buton-Sundalandcol l i s ion. By Mio-Pl iocene t imes the region hadstabilized, and pelagic l imestones and marls of theSampolakosa Format ion were being deposi ted.The Tondo and Sampolakosa Format ions a r eunconformably over l a in by Qua te rnary r ee fdeposi ts of the Wapulaka Format ion (Sikumb ang &Sanyoto 1981). Reefal l imestones are usually veryweak ly magne t i zed , and because o f th i s theWapulaka Format ion was not sampled for th ispalaeomagnetic study.

In addi t ion to the palaeomagnet ic s tudies,nannofossi ls were examined f rom 32 of the palaeo-magnetic si tes, thus providing information on themaximum age of the magnet ic remanence. Thenannofossi ls have been assigned to the Mar t ini(1971) zones which in turn have been linked to thegeomagnet ic t imescale of Cande & Kent (1992)using the magnetochron-biozone correlations ofBerggren et al. (1985). Unfortunately some of theassemblages from this study lack the nominatetaxon for the Martini scheme, however the totalrecorded as semb lages are suf f ic ient to enableassignment to a particular zone(s) .

PalaeomagnetismM e t h o d sThe major i ty of specimens were obtained using agasoline-powered rock-dril l which was used to cut2 .5 cm diam eter mini-cores. The cores wereorienta ted to _+ 2 usin g a m agn etic com pass-inclinometer. A palaeomagnetic si te comprises 6-9or i en ta t ed min i - cores spann ing 10-30cm ofsection within a bed. A number of orientatedblocks (c. 30 0 cm 3) were also retr ieved. W herev erpossible, si tes were collected with the aim ofutil izing the fold and reversal tests to constrain theage of magnet izat ion.The stabi l ity of magnet izat ion of each specimenwas assessed after using either stepwise alternating


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SE SUNDALAND ACCRETION 43 5f i e ld ( A F ) d e m a g n e t i z a t i o n o r c o n t i n u o u s t h e r m a ld e m a g n e t i z a t i o n t o i s o l a te t h e v a r io u s c o m p o n e n t so f m a g n e t i z a t i o n h e l d w i t h i n t h e r o c k s . T h eA F - p r o c e s s e d s p e c i m e n s w e r e a n a l y s e d a tS o u t h a m p t o n , u s i n g e i t h e r a ' M o l s p i n ' s p i n n e rm a g n e t o m e t e r i n ta n d e m w i t h a ' M o l s p i n ' d e -m a g n e t i z e r , o r a ' 2 G E n t e r p r i s e s ' c r y o g e n i cm a g n e t o m e t e r w h i c h h a s a n i n - l i n e 3 -a x isd e m a g n e t i z i n g u n it . C o n t i n u o u s t h e r m a l d e -m a g n e t i z a t i o n s w e r e p e r f o r m e d o n a n u m b e r o fs p e c i m e n s i n t h e l a b o r a t o r y o f M . F u l l e r a t S a n t aB a r b a ra , u s i n g a ' 2 G E n t e r p r i s e s ' c r y o g e n i c m a g -n e t o m e t e r w i t h a n i n - li n e t h e rm a l d e m a g n e t i z i n gu n i t ( D u n n & F u l l e r 1 9 8 4 ) . I n p r a c t i c a l l y a l l c a s e st h e r e m a n e n c e o f t h e T o n d o a n d S a m p o l a k o s aF o r m a t i o n s w a s f o u n d t o b e d o m i n a t e d b y a s i n g lec o m p o n e n t s u g g e s t i n g a s i m p l e m a g n e t i z a t i o nh i s t o r y . C h a r a c t e r i s t i c c o m p o n e n t s o f m a g n e t i s a -t i o n w e r e i d e n t i f i e d f r o m v e c t o r e n d - p o i n t p l o ts( Z i j d e r v e l d 1 9 6 7 ) , a n d c a l c u l a t e d u s i n g aK i r s c h v i n k ( 1 9 8 0 ) b a s e d s o f t w a r e p a c k a g e . S i t em e a n d i r e c t i o n s ( T a b l e 1 ) h a v e b e e n c a l c u l a t e du s in g t h e s t a t i s t i c s o f F i s h e r ( 1 9 5 3 ) . T h e s t a t i s t i c so f M c F a d d e n & R e i d ( 1 9 8 2 ) h a v e a l so b e e n u s e d t oc a l c u l a t e m e a n i n c l i n a t i o n s ; i n a n u m b e r o f c a s e st h e s i t e s a r e f r o m i s o l a t e d o u t c r o p s a n d a v e r a g i n gs i t e d i r e c t i o n s u s in g F i s h e r ( 1 9 5 3 ) s t a t i s t i c s ma yh a v e b e e n i n c o r r e c t . I t i s w o r t h n o t i n g , h o w e v e r ,t h a t i n a l l c a s e s t h e d i f f e r e n c e b e t w e e n t h e F i s h e r( 1 9 5 3 ) a n d M c F a d d e n & R e i d ( 1 9 8 2 ) v a l u e s i s le s sth a n 0 . 5 ( s e e T a b l e 1 ).Isothermal remanent magnetizationI s o t h e rm a l r e m a n e n t m a g n e t i z a t io n ( I R M ) a n a ly s isw a s c a r r i e d o u t o n r e p r e s e n t a t i v e s p e c i m e n s f r o ms i t e s y i e l d i n g r e l i a b l e d a t a t o p r o v i d e i n f o r m a t i o no n t h e i r p r i n c i p a l r e m a n e n c e c ar r ie r s . T h e I R M w a sg e n e r a t e d u s i n g a 'M o l s p i n ' p u l s e m a g n e t i s e r ( u pt o 0 .8 6 T ) a n d w a s m e a s u r e d b e t w e e n s t e p s u s i n ga ' M o l s p i n ' m a g n e t o m e t e r . T h e s h a p e o f t h e I R Mc u r v e ( a s w e l l as t h e p e a k I R M v a l u e ) w a s u s e d t oe v a lu a t e t h e c h a r a c t e r i s t i c r e ma n e n c e c a r r i e r ( s ) .T h e I R M r a ti o , d e f i n e d b y A l i ( 1 9 8 9 ) , is t h e r a ti oo f t h e IR M a t 0 .3 T t o t h e I R M a t 0 . 8 6 T . A sa g e n e r a l g u i d e , v a l u e s a b o v e 0 . 9 i n d i c a t e t h a tm a g n e t i t e i s t h e d o m i n a n t r e m a n e n c e c a r r i e r ,w h e r e a s v a l u e s b e l o w 0 . 9 i n d i c a t e t h a t t h er e m a n e n c e m a y b e d u e t o o t h e r m i n e r a l s . A l l o ft h e s p e c i m e n s h a v e I R M r a t i o s g r e a t e r t h a n 0 . 9 5( T a b l e 1 ) s u g g e s t i n g t h a t t h e r e m a n e n c e o f t h eT o n d o F o r m a t i o n a n d t h e b as a l S a m p o l o k o s aF o r m a t i o n i s ca r r i e d b y m a g n e t i t e .

Palaeom agnet ic results: south ButonBungi River, BungiF o u r d r i l l si te s a n d o n e h a n d s p e c i m e n w e r es a m p l e d f r o m t h e l o w e r p a r t o f t h e S a m p o l a k o s a

F o r m a t i o n a t f i v e o u tc r o p s e x p o s e d a l o n g t h e B u n g iR iv e r , a b o u t 5 k m E S E o f B u n g i ( F ig . 1 ; 1 ). T h es i t e s w e r e f r o m b e d s w h i c h d i p t o t h e n o r t h a n dw e s t a t 1 0 - 1 5 . T h e f o r m a t i o n i s d o m i n a t e d b yc o m p a c t e d m a r l s i n b e d s t y p i c al l y 3 0 - 8 0 c m t h ic k .S a m p l e s f r o m t h r e e s i t e s w e r e a n a l y s e d f o r n a n n o -f o s s il s , b u t t h e f au n a s w e r e t o o p o o r l y p r e s e r v e d t oy i e l d a r e li a b l e a g e ; a M i o - P l i o c e n e a g e is a s s u m e d .

S p e c i m e n s f r o m S i t e s B U 1 , 3 , 4 a n d 5 h a v eN R M i n t e n s i ti e s b e t w e e n 0 . 0 7 a n d 1 .4 m A m -1( T a b le 1 ). A F d e m a g n e t i z a t i o n o f t h e s e s i te si n d i c a t e s t h a t t h e r e m a n e n c e i s c a r r i e d b y a s i n g l eh i g h - c o e rc i v i ty c o m p o n e n t . S i te B U 2 . H , f r o m t h eT o n d o - S a m p o l a k o s a t r a ns i ti o n , r e v e a l e d q u i t ed i f fe r e n t m a g n e t i c b e h a v i o u r ; t h e r e m a n e n c e b e i n gc a r r i e d b y a r e l a t i v e l y s o f t s i n g l e c o m p o n e n t o fm a g n e t i z a t i o n . S p e c i m e n s f r o m S i t e s B U 1 , 2 a n d 5w e r e t h e s u b j e c t o f I R M s t u d i e s ( T a b le 1 ). A l l t h r e es p e c i m e n s y i e l d e d I R M r a ti o s > 0 . 9 5 s u g g e s t i n gt h a t t h e r e m a n e n c e i s p r o b a b l y c a r r i e d b y m a g -n e t i t e . T h e f i v e s i t e s h a d a me a n in situ d i r e c t i o n o fD = 3 57 .9 , I = -2 3 .5 , a95 = 7 .7 , K = 98 .4 (Tab le1 , F i g . 4 a ) , a n d a t i l t c o r r e c t e d d i r e c t i o n o fD = 2 .7 , I = -31 .3 a95 = 9 .6 , K = 63 .8 (F ig . 4b) .A n o r m a l p o l a ri t y m a g n e t i z a t i o n i s a s s u m e d .

T h e r e m a n e n c e d a t a c a n b e i n t e r p r e t e d i n t w ow a y s . A s s u m i n g t h e i n d i v i d u a l s i t e c o r r e c t i o n sa c c o u n t f o r a l l o f t h e d e f o r m a t i o n e x p e r i e n c e d b ye a c h s i te , t h e n t h e i n s i t u a n d t i l t c o r r e c t e d a 95 a n dK v a l u e s s u g g e s t t h a t t h e r e m a n e n c e p o s t - d a t e st i l t i n g . H o w e v e r , t h e p r e s e n t - d a y g e o c e n t r i c a x i a ld i p o l e i n c l i n a t i o n fo r t h e B u n g i R i v e r a r e a is- 1 0 . 7 , w h i c h s u g g e s t s t h a t t h e r e m a n e n c e p r e -d a t es d e f o r m a t io n . T h e a g e o f m a g n e t i z a t i o n o f t h eS a m p o l a k o s a F o r m a t i o n i n t h e B u n g i R i v e r is c o n -s i d e r e d a m b i g u o u s . T h e d e c l i n a t i o n d a t a , w h e t h e ri n t e r p r e t e d a s e i t h e r p r e - o r p o s t - f o l d i n g , i n d i c a t en e g l i g i b l e r o t at i o n .

Kemberu River, MasiriN u m e r o u s o u t c r o p s o f t h e T o n d o F o r m a t i o n w e r ee x p o s e d i n t h e d r y b e d o f t h e K e m b e r u R i v er , n e a rM a s i r i ( F ig . 1 ; 3 ) . T h e f o r m a t i o n i s d o m i n a t e d b ym a s s i v e l y b e d d e d c o n g l o m e r a t e s a n d i m m a t u r es a n d s t o n e s ; l i t h o l o g i e s n o t p a r t i c u l a r l y s u i t e d f o rp a l a e o m a g n e t i c i n v e s t i g a t io n s . T h e b e d s a r e f o l d e da b o u t a x es a l i g n e d N N E - S S W . D i p s a r e t y p i c a l l y2 0 - 4 0 a l t h o u g h v a l u e s o f 6 0 - 7 0 w e r e l o c a ll yr e c o r d e d . E i g h t d r i l l a n d t w o h a n d s i t e s w e r es a m p l e d f r o m 1 0 - 2 0 c m t h i c k m u d s t o n e s a n ds i lt s to n e s . T h r e e s i te s y i e l d e d d a t a w i t h a m e a n insitu d i r e c t i o n o f D = 3 5 5 . 6 , I = - 1 4 . 2 , a 95 = 1 8 . 4 ,K = 4 5 . 8 , a n d a t il t c o r r e ct e d d i re c t i o n o fD = 3 5 9 . 6 , I = - 1 4 . 8 a 9 5 = 1 5 . 3 , K = 6 5 . 4( T a b l e 1 , F ig . 4 c /d ) . T h i s d i r e c t i o n i s i n t e r p r e t e da s a p r i m a r y n o r m a l p o l a r i t y m a g n e t i z a t i o n w i t hn e g l i g i b l e n e t r o t a ti o n .


6/13

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8/13


9/13

SE SUNDALAND ACCRETION 439

is D = 3 1 . 0 , I = - 2 9 . 8 a 9 5 = 2 1 .0 , K = 1 9 . 9( T a b l e 1 , F ig . 4 f ). A l th o u g h a 9 5 a n d K im p r o v e w i thth e c o r r e c t i o n f o r d ip , t h e s c a t t e r o f t h e v e c to r ss u g g e s t s t h a t t h e l o c a l t e c t o n i c s a r e m o r e c o m p l e xt h a n s i m p l e S - S W t i lt i n g.

Wabiau River, WalumpoE a s t B u t o n i s c l o s e t o t h e s u p p o s e d T u k a n g B e s iP l a t f o r m - B u t o n B l o c k s u t u r e . T h e W a b i a u R i v e r( F ig . 1 ; 6 ) i s a d j a c e n t t o t h e T o n d o F o r m a t i o n t y p ea r e a a n d c o n t a i n s s p l e n d i d e x p o s u r e s o f b o t h t h eT o n d o F o r m a t i o n a n d t h e b a sa l p a r t o f t h eS a m p o l a k o s a F o r m a t i o n . T h e r o c k s a r e m i l d l yd e f o r m e d o n l y w i t h d i p s g e n e r a l l y < 1 5 . N i n eT o n d o F o r m a t i o n a n d t w o S a m p o l a k o s a F o r m a t i o nd r i ll s i te s w e r e s a m p l e d a t s i x o u tc r o p s . T h e e l e v e ns i te s y i e l d e d d a t a w i t h a m e a n in situ d i r e c t i o n o fD = 356 .8 , I = -1 1 .9 , a95 = 5 .4 , K = 72 .9 , and at il t c o r r e c t e d d i r e c t i o n o f D = 3 5 7 . 0 , I = - 6 . 4 ,a 95 = 8 .1 , K = 3 2 . 8 ( T a b l e 1 , F ig . 4 g /h ) . A l th o u g hth e in situ d i r e c t i o n s a r e s l i g h t l y m o r e c l u s t e r e dt h a n t h e t i l t c o r r e c t e d v e c t o r s , s u g g e s t i n g t h a tt h e r e m a n e n c e p o s t -d a t e s d e f o r m a t i o n , t h e m a g n e t -i z a t i o n o f t h e W a b i a u R i v e r T o n d o - S a m p o l a k o s aF o r m a t i o n s e q u e n c e i s c o n s i d e r e d t o b e p ri m a r y .D e m a g n e t i z a t i o n i s o l a t e d e s s e n t i a ll y s i n g lec o m p o n e n t l o w - c o e r c i v i t y r e m a n e n c e s ( a ls o s e eT a b l e 1 I R M d a t a) . F i e l d o b s e r v a t i o n s s u g g e s t th a tt h e r o c k s h a v e u n d e r g o n e n o t h i n g m o r e d r a m a t i cth a n b u r i a l a n d s l i g h t s t r u c tu r a l u p h e a v a l . I t i sp o s s i b l e t h a t t h e a s s u m p t i o n o f a t i l t c o r r e c t e d m e a nd i r e c t i o n b a s e d o n s i m p l y r e s t o r i n g t h e b e d s t oh o r i z o n t a l i s in c o r r e c t , a n d t h a t f a u l ts b e t w e e n e a c ho f t h e o u t c r o p s m a y h a v e a c c o m m o d a t e d l o c a ls m a l l - s c a l e d e f o r m a t i o n . T h e t i l t c o r r e c t e d m e a nd i r e c t i o n s u g g e s t s n e g l i g i b l e ro t a t i o n o f t h e s i tes i n c e t h e s t a rt o f t h e P l i o c e n e .

Waulala River, LaweleS u p e rb e x p o s u r e s o f th e T o n d o a n d S a m p o l a k o s aF o r m a t i o n s a r e p r e s e n t i n t h e W a u l a l a a n d L o k oR iv e r s , a b o u t 5 k m s o u th o f L a w e le ( F ig . 1 ; 8 ) . T h eg e o l o g y is d o m i n a t e d b y a n u m b e r o f m a j o rN E - S W t r e n d i n g fa u lt s l i m i t in g t h e ar e a to w h i c hp a l a e o m a g n e t i c d e c l i n a t io n d a t a o b t a i n e d m i g h t b ea p p l i e d . F o u r d r i l l s i t e s a n d o n e h a n d s p e c i m e nw e r e s a m p l e d f r o m t h e T o n d o F o r m a t i o n , t o g e t h e rw i t h o n e d r i l l s i t e f r o m t h e S a m p o l a k o s aF o r m a t i o n . O n l y s i t e B U 4 6 . H , t h e h a n d s p e c i m e nf r o m t h e T o n d o F o r m a t i o n , y i e l d e d r e l i ab l e d a t aw i t h a r e v e r s e p o l a r i t y t i l t c o r r e c t e d d i r e c t i o n o fD = 2 1 5 . 1 , I = 4 . 7 ( T a b l e 1 ), s u g g e s t i n g c lo c k -w i s e r o t a t i o n i n e x c e s s o f 3 0 H o w e v e r , a s t h e s i tei s a d j a c e n t to a m a j o r f a u l t s y s t e m , i t is s u g g e s t e dt h a t t h i s d e f l e c t i o n r e f l e c ts l o c a l d e f o r m a t i o n .

Palaeom agnet ic results: centra l Bu tonKawau River, BubuT h e T o n d o F o r m a t i o n i s e x p o s e d o v e r a k i l o m e t r eo f c o n t i n u o u s o u t c r o p i n t h e K a w a u R i v e r , 5 k min l a n d o f B a u b a u ( F ig . 1 ; 9 ) . S e v e n d r i l l s i t e s w e r es a m p l e d f r o m b e d s t h a t d i p t o w a r d s t h e e a s t a n ds o u th a t b e tw e e n 1 0 a n d 3 0 . S i t e s B U 4 9 , 5 0 , 5 2 ,5 3 a n d 5 5 r e c o r d n o r m a l p o l a r i t y m a g n e t i z a t i o n s ;B U 5 1 a n d 5 4 r e c o r d r e v e r s e p o l a r i t y m a g n e t i z -a t ions (Table 1) . The in situ m e a n d i r e c t i o n i sD = 355 .0 , I = -1 7 .3 , a95 = 14 .6 , K = 17 .8 (Table1 , F ig . 4 i ) . A p p l i c a t i o n o f t h e t i l t c o r r e c t i o n sp r o d u c e s a d i r e c t i o n o f D = 3 5 5 . 1 , I = - 7 . 4 ,a9 5 = 1 0 . 0 , K = 3 7 . 2 ( F ig . 4 j ) , w h ic h s u g g e s t st h a t th e r e m a n e n c e p r e -d a t e s f o l d i n g . T h e p r e s e n c eo f b o t h n o r m a l a n d r e v e r s e p o l a r i t y s i t es i n as i n g l e o u t c r o p i s h i g h l y s u g g e s t i v e o f a p r i m a r ym a g n e t i z a t i o n . T h e m e a n d i r e c t i o n i n d i c a t e sn e g l i g i b l e r o t a t io n o f t h e s i te s i n c e f o r m a t i o n .

Longito River, BubuT h e K a m u n t e R i v e r , a t r i b u t a r y o f t h e L o n g i t oR i v e r , w a s a c c e s s e d f r o m t h e r o a d l i n k i n g B u b u t oP u r e ( F ig . 1 ; 1 0 ) . F iv e s i t e s w e r e s a mp le d f r o m a6 0 0 m l o n g e x p o s u r e o f t h e T o n d o F o r m a t i o n i n t h ed r y r i v e r b e d . T h e r o c k s , m a i n l y m u d s t o n e s a n ds i l ts t o n e s in b e d s 1 0 - 3 0 c m t h i c k , ar e d e f o r m e di n t o a n o p e n s y n f o r m . T h e T o n d o F o r m a t i o ns a m p l e s a r e w e a k l y m a g n e t i z e d w i t h i n i ti a l N R Mi n t e n s i t ie s b e t w e e n 0 . 0 6 a n d 0 . 2 0 m A m -1 . R e l i a b l ed a t a w e r e o b t a in e d f o r t h r e e s i t e s . S c a t t e r f o rs p e c i m e n s f r o m t h e s e s i t e i s l a r g e w i t h v a l u e s f o ra 95 a n d K ty p i c a l l y 2 5 a n d 2 0 r e s p e c t i v e ly .H o w e v e r , t h e o u t c r o p c o n t a i n s b o t h n o r m a l ( B U 5 8a n d B U 5 9 ) a n d r e v e r s e ( B U 5 6 ) p o l a r i t y s i t e s ,w h i c h s u g g e s t s t h a t t h e r e m a n e n c e i s p r i m a r y .T h e in situ m e a n d i re c ti o n i s D = 3 5 9 . 8 ,I = -15 .7 , a95 = 20 .4 , K = 37 .3 (Table 1 , F ig . 4 k) .A p p l i c a t i o n o f t i l t c o r r e c t i o n s p r o d u c e s a m e a nd i r e c t i o n o f D = 1. 7 , I = - 9 . 8 , a 95 = 1 2 . 7 ,K = 9 5 . 2 ( F i g . 4 1), w h i c h s u g g e s t s th a t t h em a g n e t i z a t i o n p r e - d a t e s f o l d i n g , w i t h n o r o t a t io n .

Palaeom agnet ic results: north Bu tonLoangkumbe River, LoangkumbeT h e T o n d o a n d S a m p o l a k o s a F o r m a t i o n s a r e w e l le x p o s e d a l o n g t h e L o a n g k u m b e R i v e r ( Fi g . 1 ; 11 ).T h i s i s o n e o f t h e m o s t i m p o r t a n t a r e a s o f B u t o nf o r t h e c o l l e c t i o n o f p a l a e o m a g n e t i c s a m p l e s ,p o t e n t i a l l y p r o v i d i n g t h e M i o - P l i o c e n e r e f e r e n c ed i r e c t i o n f o r t h e n o r t h e r n p a r t o f t h e i s l a n d .U n f o r t u n a t e l y t h e s t r at i g ra p h i c i n t e r v a l f r o m w h i c hp a l a e o m a g n e t i c s i t e s c a n b e s a m p l e d i s l i m i t e db e c a u s e t h e L o a n g k u m b e R i v e r f l o w s p a r a l l e l t o


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44 0 J.R. ALI ET AL.

t h e g e n e r a l s t r i k e o f r o c k s i n t h e a r e a . T h e l o w e rr e a c h e s o f t h e fi v e r e x p o s e m a r l s f r o m t h e l o w e rp a r t o f t h e S a m p o l a k o s a F o r m a t i o n b u t o n l y as i n g l e s i t e w a s s a m p l e d ; d e t e r m i n i n g a c c u r a t e d i p sa n d s t r i k e s i n t h e f o r m a t i o n i s d i f f i c u l t b e c a u s eo b v i o u s s i gn s o f b e d d i n g h a v e b e e n o b l i te r a t e d b ye r o s i o n . E i g h t d r i l l s i t e s a n d o n e h a n d s p e c i m e nw e r e s a m p l e d f r o m t h e T o n d o F o r m a t i o n .

S e v e n T o n d o F o r m a t i o n s i t e s , f r o m t h r e eo u t c r o p s , h a v e y i e l d e d d a t a ( T a b l e 1 , F i g . 4 r n /n ) .S i te s B U 6 2 a n d B U 6 3 f r o m a n o u t c r o p c . 2 0 0 mu p s tr e am o f t he L o a n g k u m b e - M o u s o C a b a n gy i e l d e d t il t c o r r e c t e d d i r e c t i o n s o f D = 1 5 3 . 9 ,I = 8 .9 , a95 = 8 .3 , K = 66 .5 , N = 6 an d D = 3 37 .5 I = 3 . 1 a 95 = 1 7 . 5 , K = 1 7 . 5 , N = 4 . T h e p r e se n c eo f r e v e r s e a n d n o r m a l p o l a r i ty s i t es i n o n e o u t c r o ps u g g e s t s t h a t th e r e m a n e n c e i s p r i m a r y . T h ed e c l i n a t i o n s i n d i c a t e a b o u t 2 5 c o u n t e r - c l o c k w i s er o t a t i o n .

F i v e s i te s , b e t w e e n 3 a n d 5 k m u p s t r e a m o f S i te sB U 6 2 a n d 6 3 , y i e l d e d d ir e c ti o n a l da ta . B U 6 6 h a s at il t c o r r e c t e d m e a n d i r e c t i o n o f D = 2 2 6 . 7 ,I = -17 .8 , a95 = 8 .4 , K = 65 .3 , N = 6 , i nd ica t inga b o u t 4 5 o f c l o c k w i s e r o t a ti o n . S i t e B U 7 0 , a b o u t6 0 0 m u p s t r e a m , a l s o c a r r i e s r e v e r s e p o l a r i t ym a g n e t i z a t i o n , w i t h D = 1 8 8 . 0 , I = - 2 7 . 8 ,a 95 = 1 0 . 3 , K = 5 6 . 0 , N = 5 , i n d i c a t i n g n e g l i g i b l er o t at io n . S i t e B U 6 8 is n o r m a l l y m a g n e t i z e d w i t h n od e c l i n a t i o n o f f se t . F r o m t h e s a m e o u t c r o p , h a n ds a m p l e s B U 6 9 H . a a n d B U 6 9 H . c r e c o r d c . 3 5 o fc l o c k w i s e r o t a t i o n .

D e c l i n a t i o n d a t a f r o m t h e L o a n g k u m b e R i v e rs u g g e s t t h a t ro t a t i o n s f o r t h i s p a r t o f n o r t h B u t o na r e s i g n i f i c a n t , b u t a p p e a r t o b e l o c a l ( s u b -k i l o m e t r e s c a l e ) . U s i n g t h e s t a t i s t i c s o f M c F a d d e n& R e i d ( 1 9 8 2 ) t h e m e a n t i lt c o r r e c t e d i n c l i n a t i o n is+ 9 . 0 , a s s u m i n g a n o r m a l p o l a ri ty , s u g g e s t i n gd e p o s i t i o n a t a l a t i t u d e c . 4 . 5 N . H o w e v e r , b e c a u s eo f t h e l a r g e a 95 c o n f i d e n c e c i r c l e ( 1 2 . 9 ) a s s o c i a t e dw i t h t h i s i n c l i n a t i o n , i t c a n o n l y b e s t a t e d w i t hc o n f i d e n c e t h a t t h is p a r t o f B u t o n w a s c l o s e t o t h ee q u a t o r d u r i n g M i o - P l i o c e n e t i m e s.

Siloi River, RontaT h e S i lo i R i v e r , a c c e s s e d f r o m t h e r o a d w h i c h l in k sR o n t a t o M a l i g a n o , e x p o s e s t h e T o n d o F o r m a t i o n( F i g . 1 ; 12 ). T h e g e o l o g y o f t h e a r e a i s c o m p l i c a t e db y a s e r i e s o f m a j o r f a u l ts j u x t a p o s i n g t h e s e u p p e rM i o c e n e r o c k s w i th a n u m b e r o f p r e - N e o g e n ef o r m a t i o n s . D u r i n g f i e l d w o r k , e x p o s u r e s o f th eT o n d o F o r m a t i o n w e r e s e e n o n l y i n t h e r i v e r b e d ,s e r i o u s l y h a m p e r i n g t h e p a l a e o m a g n e t i c s a m p l i n g .H o w e v e r , a s i t e i n t h e T o n d o F o r m a t i o n w a ss a m p l e d 4 k m d o w n s t r e a m f r o m t h e r o a d b r i d g eo v e r t h e S i l o i R i v e r . T h e s i t e ( B U 7 1 ) i s i n am e d i u m - g r a i n e d s a n d st o n e a d j ac e n t t o a N E - S Ws t r i k i n g f a u l t t h a t j u x t a p o s e s t h e f o r m a t i o n w i t h t h e

O g e n a F o r m a t i o n ( l o w e r J u ra s s ic ; D a v i d s o n 1 9 9 1) .T h e in situ s it e m e a n d i r e c t io n i s D = 3 5 5 . 1 ,1 = 0 . 9 , a 95 = 1 8 .1 , K = 2 6 . 6 , N = 4 . ( T a b l e 1 ).A p p l i c a t i o n o f t h e ti lt c o r r e c t i o n p r o d u c e s ad i r e c t i o n o f D = 3 5 4 . 5 , I = - 1 6 . 5 . T h e r e m a n e n c ei s c a r r i e d a s a n o r m a l p o l a r i t y , b u t t h e a g e o f t h er e m a n e n c e ( d e p o s i t i o n a l / p o s t d e f o r m a t i o n ) i s n o tk n o w n .

Walue River, KobakobaE x p o s u r e s o f t h e T o n d o F o r m a t i o n i n t h e W a l u eR i v e r a r e s c a r c e a n d j u s t o n e d r i ll s it e , B U 7 2 , w a ss a m p l e d ( F i g . 1 ; 1 3 ). T h e m i n i c o r e s w e r e o b t a i n e df r o m a d j a c e n t s a n d s t o n e a n d s i l t s t o n e b e d s w h i c hd i p 8 0 t o t h e e as t . T h e s p e c i m e n s f r o m b o t h t h es a n d s t o n e a n d s i l t st o n e h a d s i m i l a r in i t ia l N R Mi n t e n s i ti e s o f a b o u t 1 0 m A m - 1. T h e s it e h a s a nin situ m e a n d i r e c t io n o f D = 7 . 0 , I = - 3 3 . 7 ,a95 = 12 .7 , K = 28 .6 , N = 6 (Ta ble 1) and a t i l tc o r r e c t e d d i r e c t i o n o f D = 3 1 7 . 6 , I = - 1 9 . 6 . I t i sa s s u m e d t h at t h e r e m a n e n c e i s p r im a r y b e c a u s e t h ein situ d i r e c t i o n is m u c h s t e e p e r th a n t h e p r e d i c t e dg e o c e n t r i c a x i a l d i p o l e i n c l i n a t i o n f o r t h e W a l u eR i v e r a r e a ( -8 . 9 ) . T h e s t e e p d i p o f t h e s e ro c k s , a n dh e n c e t h e a m o u n t o f d e f o r m a t i o n t h a t si te B U 7 2h a s u n d e r g o n e , m u s t l i m i t t h e a re a l e x t e n t f o r w h i c ht h e d e c l i n a t i o n o f f s e t c a n c o n f i d e n t l y b e u s e d t ou n r a v e l r o t a t i o n s r e l a t i v e t o n o r t h . H o w e v e r , t h et i l t c o r r e c t e d i n c l i n a t i o n a n g l e c a n b e u s e d t oh e l p d e f i n e t h e l a t i t u d e o f f o r m a t i o n ( 1 0 _ 7 . 5 S ,a s s u m i n g t h a t t h e r e m a n e n c e w a s a c q u i r e d d u r i n g an o r m a l p o l a r i ty g e o m a g n e t i c f ie l d) .

Summary of the palaeomagnetic dataL a t i t u d i n a l i n f o r m a t i o n f r o m a l l o f t h e p a l a e o -m a g n e t i c s i t e s y i e l d i n g i n t e r p r e t a b l e d a t a i s s u m -m a r i z e d i n F i g . 5 . T h e y s u g g e s t n e g l i g i b l e (c . 1 o N )n o r t h w a r d m o t i o n o f B u t o n a n d , b y i m p l ic a t io n , o fS E S u l a w e s i s i n c e t h e l a t e M i o c e n e .

T h e p a l a e o m a g n e t i c d e c l i n a t i o n d a t a f r o m b o t ht h e u p p e r M i o c e n e T o n d o F o r m a t i o n a n d t h eM i o - P l i o c e n e S a m p o l o k o s a F o r m a t i o n o n B u t o na r e s u m m a r i s e d i n F i g . 6 . S i t e s f r o m s o u t h B u t o nr e c o r d l o c a l l y c o n s i s t e n t d e c l i n a t i o n o f f s e t s , b u t a ts a m p l i n g l o c a l it i e s l e s s t h a n 2 5 k m a p a r t t h ed e f l e c t i o n s v a r y b e t w e e n 0 a n d 3 5 c l o c k w i s e .T h e r e a p p e a r s t o b e n o s i m p l e p a t t e r n t o t h e s e d a t aw i t h r e s p e c t t o t h e r e g i o n a l t e c t o n i c s e t t i n g . T h ea g e o f m a g n e t i z a t i o n o f th e S a m p o l a k o s aF o r m a t i o n s i t e s f r o m t h e B u n g i R i v e r i s u n c e r t a i n .T h e f i v e s i t es s h o w m a r g i n a l l y ti g h t e r in situ ( r a t h e rt h a n t i lt c o r r e c t e d ) c lu s t e r i n g . H o w e v e r t h e in situi n c l i n a t i o n o f - 2 3 . 8 i s to o s t e e p t o b e t h e r e s u l t o fa r e c e n t w e a t h e r i n g i n d u c e d c h e m i c a l r e m a n e n c e ,w h e r e a n i n c l in a t i o n o f a r o u n d - 1 0 . 7 w o u l d b ep r e d i c te d . R e g a r d l e s s o f t h e a g e o f t h e r e m a n e n c e ,


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SE SUNDALAND ACCRETION 44 110 - M E A N N O R T H W A R D

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0 6 12 18Apparen t Nor thward Dr i f t (c)Fig. 5. Ap pare nt formation latitude shift for the Tondoand S ampolakosa Formation sites, Buton.

I ' I ' I

-18 -12 -6

the Bungi River area has exper ienced negl igiblerotation since the magnetization was acquired. Inthe Kem beru River, 25 km to the south of Bung i,the Tondo Formation sites also record negligiblerotation. In the Warumbia River, 25 km to the eastof Bungi, four upper Tondo Formation sites recordclockw ise rotations of c. 30 . East Bu ton was theone part of the is land where large rotations mighthave been predicted because i t is closest to theTukang Bes i P la t fo rm-Buton impac t po in t .However , the large number of s i tes f rom theWabiau River r evea led neg l ig ib le dec l ina t ionoffsets . The sing le reliable Tondo Formatio n site inthe Waulala River has a large declination offset(35 clockwise). Large rotations migh t be expec tedhere as this s i te is from a block located in themiddle of several NE -S W s t r iking faul ts .

In central Buton, declinations for the TondoFormat ion s i tes in both the Kawau and Longi toRiver s ind ica te neg l ig ib le ro ta t ion . The twosections include both normal and reverse polaritysites suggesting that the reman ence is primary. Alsoin both sections there is a clear clustering of the sitevectors following application of the t i l t correctionsto the in situ data.In nor th Buton , dec l ina t ion da ta f rom theLoangkum be River sugges t local (< km) large-scale, clockwise and counter-clockwise rotations. Asingle site from the Tondo Formation in the SiloiRive r records ne gligible rotation, but the age o f themagnet izat ion is uncer tain . One Tondo Format ionsite from the Walue River, records a counter-clockw ise rotation of o ver 40 . However, the sitewas sampled f rom s teeply dipping beds , and thedeclination deflection probably reflects localizeddeformation.

Fig. 6. Sum mary of the palaeomagnetic declination datafrom the Tondo and Sampolakosa Formations, Buton.The o rientations of the arrows illustrate the declinationoffsets. Note each arr ow has an error typically 10-1 3(see Table 1).

DiscussionThe Buton r eg ion con ta ins a r ecord o f therelat ively recent success ive accret ion of two micro-cont inents to the edge of a cont inent , and aquan t i t a t ive under s t and ing o f the p roces sesinvolved provides a valuable analogue for oldersystems where the rock record is less complete.Pr ior to the present s tudy no palaeomagnet icinformation was available for the area and theinit ial aim of the research was to test the model ofFortuin et aL (1989) which proposed large-scale(c. 60 ) Pliocene clo ckw ise rotation of south Butonrelative to central and north B uton as a result of theTukang Bes i P la t fo rm co l l i s ion . Al though i tremains poss ible that the under lying basementexperienced these large relative motions, the studyhas shown that the upper Cenozoic cover has notbeen deformed in such a simple way (Fig. 6) .In recent publications (e.g. Davidson 1991) thegeology of Buton has been interpreted in terms of


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4 4 2 J . R . A LI E T A L .t h i n - s k i n n e d d e f o r m a t i o n p r o c e s s e s b u t n o i n s ig h t sh a v e b e e n p r o v i d e d i n t o t h e r o ta t io n s w h i c h t h e t h i ns h e e t o v e r t h r u s t s m i g h t h a v e e x p e r i e n c e d . S t r i c t l ys p e a k i n g , t h e p a l a e o m a g n e t i c s t u d y h a s n o b e a r i n go n t h e q u e s t i o n o f t h in - s k i n v e r s u s t h i c k - s k i nt e c t o n i c s b u t i t i s p e r h a p s e a s i e r t o e n v i s a g e l a r g er e l a t i v e r o t a t i o n s o f s m a l l b l o c k s t a k i n g p l a c e i n at h i n - s k i n r a t h e r t h a n a t h i c k - s k i n c o n t e x t .R e s o l v i n g t h e f i n e s c a l e d e t a i l o f t h e t e c t o n i c s o fB u t o n i s b e y o n d t h e s c o p e o f t h is o r i n d e e d a n yp r o j e c t , b e c a u s e o f t h e l a c k o f e x p o s u r e , b u t i t h a sb e e n d e m o n s t r a t e d b y t h e p a l a e o m a g n e t i c r e s u l t st h a t i n a c o l l i s i o n s e t t in g t h i n o v e r t h r u s t s m a y w e l lb e r o t a t e d b y 3 0 - 6 0 w i t h i n 2 - 3 M a .

T h e p a l a e o m a g n e t i c s t u d y h a s a l s o p r o v i d e ds u p p o r t f o r t h e t w o - s t a g e m o d e l f o r c o l l i s i o n ,w h i c h i s s t i l l c o n t r o v e r s i a l ; i f t h e T u k a n g B e s iP l a t f o r m w e r e p a r t o f t h e B u t o n b l o c k p r i o r t o i tsc o l l i s i o n w i t h S E S u l a w e s i i n t h e M i o c e n e , t h e r ew o u l d b e l i tt le r e a s o n f o r t h e l a r g e P l i o c e n e t oR e c e n t r o t a t i o n s w h i c h a r e a c t u a l l y o b s e r v e d .H o w e v e r , t h e a m o u n t o f t h i n -s k i n n e d r o t a ti o nd o e s n o t s e e m t o h a v e b e e n a s i m p l e f u n c t i o n o fp r o x i m i t y t o t h e m i c r o c o n t i n e n t a l s u t u r e . R o t a t i o n si n n o r th B u t o n , w h i c h i s f u r th e r f r o m T u k a n g B e s i ,a r e g r e a t e r t h a n r o t a t i o n s a t s i t e s , su c h a s t h eW a b i a u R i v e r , w h i c h a r e c l o s e r to it . T h e d i f f e r e n c ei n d e f o r m a t i o n s t y l e b e t w e e n n o r t h a n d s o u t h i sa l s o t h e r e v e r s e o f e x p e c t a t i o n . I n t h e s o u t h ,e a c h i n d i v i d u a l t r a v e r s e s e c t i o n , u p t o 1 5 k m i nl e n g t h , s h o w s a c o n s i s t e n t d e c l i n a t i o n d e f l e c t i o n ,s u g g e s t i n g t h e p r e s e n c e o f c o h e r e n t s h e e t s a t l e a s t1 5 k m a c r o s s , w h e r e a s i n t h e n o r t h r o t a t io n sa p p e a r t o h a v e o c c u r r e d o n a m u c h m o r e l o c al( c . 1 k m ) s c a l e .

F i n a l l y , t h e r e s u l t s p r e s e n t e d h e r e e m p h a s i z e t h ei m p o r t a n c e o f c o m p r e h e n s i v e s a m p l i n g i n a n y

p a l a e o m a g n e t i c p r o g r a m m e i n S E A s i a . I f j u s t th eW a u l a l a a n d W a r u m b i a s i te s h a d b e e n s a m p l e d i ns o u t h e r n B u t o n , t h e F o r t u i n e t a l . ( 1 9 8 9 ) r o t a t i o nh y p o t h e s i s w o u l d p r o b a b l y h a v e b e e n r e g a r d e d a sc o n f i r m e d , a l t h o u g h w i th r o ta t i o n s s o m e w h a ts m a l l e r t h a n t h o s e o r i g i n a l l y s u g g e s t e d . I f , o n t h eo t h e r h a n d , t h e s e s i t e s h a d n o t b e e n v i s i t e d a n dw o r k h a d b e e n c o n f i n e d t o th e B u n g i , K e m b e r u a n dW a b i a u R i v e r s , it w o u l d h a v e b e e n c o n c l u d e d t h a tr o t a t i o n s w e r e a b s e n t i n s o u t h e r n B u t o n . N e i t h e rc o n c l u s i o n i s s u p p o r t e d b y t h e f u l l d a t a s e t a c t u a l l yo b t a i n e d .

Co n c l u s i o n sT h i s p a l a e o m a g n e t i c s t u d y o f th e S E S u l a w e s i /B u t o n / T u k a n g B e s i a c c r e t io n c o m p l e x h a sp r o v i d e d a q u a n t i t a t i v e i n s i g h t i n t o t h e p r o c e s s o fc r u s t a l d e f o r m a t i o n a t a c o n t i n e n t - m i c r o c o n t i n e n ta c c r e t i o n s i t e . I n t h e B u t o n r e g i o n , a s i g n i f i c a n tc o m p o n e n t o f th e d e f o r m a t i o n h a s t a k e n p l a c e ast h i n - s k i n o v e r t h r u s t s . T h e p a l a e o m a g n e t i c d a t a s e ts u g g e s t s t h a t t h e r o t a ti o n o f t h e t h i n - s k i n s h e e t s isc o m p l e x ; t h e r e i s n o o b v i o u s r e l a t i o n s h i p b e t w e e nt h e r o t a t i o n o f a s h e e t a n d i t s p r o x i m i t y t o t h em i c r o c o n t i n e n t i m p a c t p o in t . R o t a t i o n s o f u p to3 0 - 6 0 m a y b e g e n e r a t e d . R o t a t i o n r a te s a r ec o n s i d e r a b l e ; t h e T u k a n g B e s i P l a t f o r m - B u t o nc o l l i s i o n b e g a n i n t h e P l i o c e n e , 2 - 3 M a .Thi s s tudy was funded by the Unive rs i ty o f LondonGeological Research in SE Asia Consort ium. Specialthanks are given to Tony Barber , John Davidson, KateDavis, Michael de Smet , Mike Ful ler , Ernie Hai lwood,Hendry Manur and Graham Rose for t he i r i nput t o th i sproject . The const ruct ive comm ents of H. Wensink, A. R.Fortuin and an anonymous referee great ly improved thismanuscript.

R e f e r e n c e sALI,J. R. 198 9. Mag netostratigraphy o f early Palaeogene

sediments from NW Europe. PhD thesis, Universi tyof Southampton.BERGGREN, W . A ., KE NT , D. V., FLYNN, J. J. & VANCOUVERING, J. A . 19 85 . Ce noz oic geoc hron olog y.Bulletin of the Geological S ociety o f America, 96,1407-1418.CANDE,S. KENT,D. V . 1992 . A new geom agne t i c t imescale for the Late Cretaceous a nd T ertiary. Journalof Geophysical Research, 97, 13917-13951.DAVIDSON,J. W. 19 91. The g eology and prosp ect ivi ty ofButon I s l and , SE Sulawesi, Indones i a . Proceedingso f the Indones ian Pe t ro leum Assoc ia t ion 20 thAnnual Convent ion, 209-231 .DUNN, J. R . & FULLER, M. 1984 . Therm al demag net iz-a t ion wi th measurements a t high temperature usinga SQUID magne tomete r . LOS Transact ions AGU,65, 863.FISHER,R. A. 1953 . Dispersion on a sphere . Proceedingsof the Royal Society of London, Ser ies A, 217,295-305 .

FORTUIN, A. R., DE SMET, M . E. M ., HADIWASASTRA,S.,VAN MA RLE, L. J., TROELSTRA, S. R . &TJOKROSAPOETRO, S. 1 98 9. Late Ceno zoic sedi-menta ry and t ec ton ic h i s to ry of sou th Buton .Journal o f SE Asian Earth Sciences , 4, 107-124.HAMILTON, W. 19 79 . Tectonics of the Indonesian region.US Geol . S urvey Professional Paper , 1078.KIRSCHVlNK,J. L. 198 0. Th e least squares l ine and planea n a l y s i s o f p a l e o m a g n e t i c d a t a . Geophys ica lJournal of the Roya l Astronomical Society, 62,699-718 .MARTINI, E. 197 1. Stand ard T ertiary and Quaternary cal-careous nannoplankton zonat ion. In : FARINACCI,A.(ed.) Proceedings of the H Planktonic Conference,Roma 1970 . Ediz ion i Tecnosc ienza , Rome , 739-85MCFADDEN, P. L. & REID, A. B. 1982. A naly sis o f palaeo-magnet ic incl inat ion data . Geophysical Journal ofthe Ro yal Astronomical Society, 69, 307-319.MtLSOM, J. 199 2. St ructure and col l i sion history o f theButon cont inen ta l f ragment , eas t e rn Indones i a(abstract). AA PG Bull et in , 76, 1117.


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S E S U N D A L A N D A C C R E T IO N 443SIKUMBANG,N . & SANYOTO,P. 1981. Geolog i c map o f theButon and Muna Quadrangle, southeast Sulawesi ,Sca l e 1 :250 ,000 . G e o l o g i c a l R e s e a r c h a n dD e v e l o p m e n t C e n tr e , B a n d u n g .SILVER, E. A., MCC AFFRE, R. & SMITH, R. B. 1983.

Col l i s ion , r o t a t i on an d t he i n i t i a ti on o f subduc t i on i nt he evo l u t i on o f S u l awes i , I ndones i a . Journa l o fGeophysical Research, 8 8 , 9 4 0 7 - 9 4 1 8 .

SMITH, R. B . 1983 . Sed imen to logy and t ec ton i cs o f aMiocene co l l i s i on complex , and over l y ing l a t eorogen i c c la s t i c s t ra ta , Bu ton I s l and eas t ernIndones ia . P h D t h e s is U n i v e r s i t y o f C a l i f o r n iaS a n ta C r u z , U S A .

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c o l l i s i o n c o m p l e x , B u t o n , e a s t e r n I n d o n e s i a .Bul l e ti n o f t he Geo log i ca l Soc i e t y o f Amer i ca , 103 ,6 6 0 - 6 7 8 .WIRYOSUJONO, W. & HAINIM, J. A. 1978 . C ain oz oicsed i ment a t i on i n But on I s l and . In : WIRYOSUJONO,W. & SUDRAJAT, A. (eds ) Proceed ings Reg iona lCon ference on t he Geo logy and M inera l Resourcesof Southeast Asia , Jakarta , Au gust 1975. I n d o n e s i a nA s s o c i a t i o n o f G e o l o g i s t s , J a k a r ta , I n d o n e s i a ,1 0 9 - 1 1 9 .ZUDERVELD, . D . A . 1967 . AC dem agne t i za t i on o f r ocks :ana l ys i s o f r e su lt s . In : COLLINSON,D. W . , CREER,K .M . & RUNCORN, S . K . ( eds ) . M e t h o d s i nPa laeomagnet i sm. E l s ev i e r, N e w Y o r k, 2 5 4 - 2 8 6 .

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