Paleomagnetism of Borneo

Mike Fuller a, *, Jason R. Ali b, Steve J. Moss c, Gina Marie Frost a, Bryan Richter d,Achmad Mah®e

aHawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii, 2525 Correa Rd,

Honolulu, HI 96822 U.S.A.bDepartment of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong, China

cRobertson Research, 69 Outram Street, West Perth, Western Australia 6005, AustraliadExxon Research Laboratory, Houston, Texas, U.S.A.

eSub-Department of Geophysics, Department Physics, Gajah Madah University, Jogyakarta, Indonesia

Received 4 August 1997; accepted 20 October 1998

Abstract

The paleomagnetism of Borneo remains controversial, although the preponderance of results, both from the island itself and

from the surrounding regions, suggest that counterclockwise (CCW) rotation has taken place. CCW rotations are seen in minorintrusions in Sarawak, Sabah and Kalimantan, which increase systematically with the age of the intrusion to a maximum valueof 51.8823.78. The rotation can be no older than 25 Ma, which is the age of the intrusion showing the maximum rotation. Therotation appears to have neared completion by 10 Ma. Similar CCW rotations are seen in sites from Peninsular Malaysia

through Borneo to Sulawesi, the Celebes Sea and Palawan in the Philippines, but the ages of these rotations are, for the mostpart, unknown. In Mesozoic rocks in Kalimantan and Sarawak, a stronger declination rotation of nearly 908 CCW is recordedat seven sites, including sites which pass fold and reversal tests. This strong rotation is no older than youngest Cretaceous, and

although seen over a wide region in Borneo, it is not seen in Peninsular Malaysia, nor in the Celebes Sea or Palawan, whereonly the weaker CCW rotation is seen. The widespread occurrence of this strong rotation in Western Borneo suggests that it isessentially a rigid plate, or microplate rotation, and not a series of local rotations caused by distributed shear in limited

deformation zones. The rotation of Borneo appears to be a consequence of convergence between the Australian and Eurasianplates, which is accommodated by subduction along the northwest margin of Borneo. # 1999 Elsevier Science Ltd. All rightsreserved.

1. Introduction

Borneo lies in a central position in SE Asia andhence is a key element of tectonic models and paleo-geographic reconstructions of the region. The ro-tational history of Borneo is controversial. Variouspossible rotations with respect to stable Eurasia havebeen suggested: No rotation (Lee and Lawver, 1993,1995); CW rotation (Rangin et al., 1990); CCW ro-tation (Haile et al., 1977; Hall, 1996; Hamilton, 1979;Schmidtke et al., 1990); Mixed rotations (Briais et al.,1993).

To the north of Borneo lies the South China Seamarginal oceanic basin, to the east the Sulu andCelebes Seas marginal oceanic basins and the

Makassar Straits and Sulawesi (Fig. 1). To the south

and southwest are the islands of Java and Sumatra. In

the west lie the Sunda Shelf and ultimately the

Palaeozoic and Mesozoic continental crust of

Peninsular Malaysia. The tectonostratigraphic frame-

work of Borneo bears little resemblance to that of

Peninsular Malaysia and previous workers have postu-

lated a Mesozoic terrane boundary within the Sunda

Shelf (Metcalfe, 1991). Thus, Borneo is surrounded to

the north, east, and south by plate boundaries, mar-

ginal ocean basins and arc systems, which are presently

active, or which have been active during the Tertiary,

and to the west by an under-explored shelf region. It is

therefore only by establishing the paleomagnetic his-

tory of Borneo that we can hope to determine its ro-

tation history.

Journal of Asian Earth Sciences 17 (1999) 3±24

1367-9120/99 $ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0743-9547(98 )00057 -9

PERGAMON

* Corresponding author.

Fig.1.A.Geologic

mapofSEAsia.B.Geologic

mapofBorneo.

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±244

2. Geological background for paleomagnetic studies

An overview of the geology of Borneo is given inFig. 1B. Much of the southern half of the island con-sists of a Sundaland cratonic core of Triassic andJurassic metamorphics and volcanics. To the north ofthis lie the arcuate tectonostratigraphic belts of theCretaceous to Paleogene Rajang±Embaluh Groupsand beyond it the turbidite sequences of the Oligoceneto Miocene Crocker-Temburong Formations, usuallyconsidered to have an accretionary complex origin,although some parts of the Embaluh Group haverecently been re-interpreted (Moss, 1998). For themost part, these units are not suitable for paleomag-netic work, but there are substantial sections in theCrocker Formation that are not structurally disturbedon a ®ne scale and have yielded useful paleomagneticdata. In South Kalimantan, the Meratus and Bobarisophiolites and volcanic island arc rocks are exposed inthe Meratus mountains (Fig. 1B), having beenemplaced by the late Cretaceous (Sikumbang, 1986,1990). Several phases of NNE±SSW oriented strike-slip deformation have a�ected these rocks during boththe Cretaceous and Tertiary (Kusuma and Darin,1989; Sikumbang, 1986, 1990). Following ophioliteemplacement, arc volcanism in SE Kalimantan thenjumped outboard to the Sulawesi arc system. Theregion is almost untouched paleomagnetically, but ispotentially important in de®ning the southern bound-ary of any rotated regions within Borneo.

The ®rst paleomagnetic studies in Borneo were inthe Schwaner Mountains in Western Kalimantan(Haile et al., 1977). They consist of Lower to UpperCretaceous granites and tonalites intruded intoPalaeozoic metamorphic rocks of the Sundaland core.The Schwaner mountains granites have been radio-metrically dated mainly by K±Ar techniques at 130±80Ma (Amirruddin, 1989; Amirruddin and Trail, 1993;Keyser and Rustandi, 1993; Pieters and Sanyoto, 1993;Williams et al., 1988). They have been recognized geo-chemically as volcanic arc granites (Amirruddin, 1989).Other smaller granitic plutons of Cretaceous age arelocated north of the Schwaner Mountains inKalimantan (Fig. 1B).

During the Middle Eocene the Melawi±Ketungaubasins, the Upper Kutai Basin, and the Kutai Basin(Fig. 1B) formed along the southern margin of theRajang±Embaluh belt. Paleomagnetic studies havebeen reported from these basins (Lumadyo et al.,1993) and others are underway. The Barito, Pasir,Asem-Asem, Muara and Tarakan sedimentary basinsalso were formed at this stage. This period has forsome time been recognised as a time when basin open-ing and extension a�ected much of the region (Daly etal., 1990, 1991; Hall, 1996; Van der Weerd and Armin,1992). Sedimentation continued throughout most of

the Neogene in these basins, although sedimentation in

the Melawi±Ketungau basins had ceased by the end of

the Oligocene. The Melawi and Kutai basins have

yielded paleomagnetic data and further work is under-

way.

At least three phases of igneous activity a�ected

Borneo during the Tertiary. Three suites of volcanic

and intrusive rocks are recognised within the Tertiary

of Kalimantan: the Nyaan Volcanics, the Sintang

Intrusive Suite and the Metulang Volcanics (or Plateau

Basalts). The felsic Nyaan Volcanics in the Upper

Kutai Basin (Fig. 1B) have K±Ar ages of 48±5021

Ma (Pieters et al., 1987, 1993). The basaltic to andesi-

tic Muller Volcanics, north of the eastern extremity of

the Schwaner Mountains (Fig. 1B), have a K/Ar radio-

metric age of 4120.4 Ma (Pieters et al., 1993). These

represent an important paleomagnetic target for absol-

utely dated paleomagnetic directions from the Eocene.

The Sintang Intrusive Suite of Kalimantan and its

equivalent in Sarawak have been sampled paleomagne-

tically (Haile, 1979; Moss et al., 1997; Schmidtke et

al., 1990) and more work is underway. They are

de®ned as Late Oligocene to Middle Miocene,

although a wider range of K/Ar radiometric ages (41

to 8 Ma) has been obtained from rocks assigned to

this suite (Heryanto et al., 1993; Pieters et al., 1993).

The Sintang intrusives have a ma®c to felsic compo-

sition and consist of diorites, microdiorites, dacites,

microgranites and andesites. Intrusions of the Sintang

Intrusive Suite are widely distributed as plugs and

other hypabyssal intrusions throughout parts of west,

central and east Kalimantan, often forming conical

hills. The ®nely crystalline nature and limited contact

aureoles of these intrusions imply a high level of

emplacement. Andesitic breccias and agglomerates are

typically associated with the plugs. Pieters et al. (1993)

correlate the Sintang intrusives with the composition-

ally similar coeval Oligo-Miocene intrusives of Kirk

(1968) in Sarawak. In Sabah in northern Borneo, the

intrusive which forms Mount Kinabalu, the highest

mountain in the region, is of a similar age to the

younger Sintang intrusives. A satellite stock of the

Kinabalu intrusive with a K/Ar age of 13.325.3 Ma

has been sampled paleomagnetically (Table 5).

Pliocene±Pleistocene volcanics of the Metulang Suite

are common throughout the centre of Kalimantan and

parts of Sarawak and have been termed the Metulang

or Plateau Basalts. They have been studied paleomag-

netically by Lumadyo et al. (1993). The Metulang

Volcanics have K/Ar radiometric ages of between 2.4

and 1.7 Ma (Pieters et al., 1993). The basalts of the

Nuit Volcanics in West Kalimantan, with a K/Ar

radiometric age of 4.920.1 Ma (Supriatna et al.,

1993), belong to this suite.

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 5

3. Present tectonic models for Borneo

For much of the last decade the extrusion tectonicmodel (Peltzer and Tapponnier, 1988; Tapponnier etal., 1982, 1986) has dominated tectonic and paleogeo-graphic discussions of East and Southeast Asia. Thismodel predicts small CW rotations and southeastwardextrusion of Southeast Asia as a consequence of thecollision of India with Eurasia during the Eocene.Current paleogeographic and plate tectonic reconstruc-tions for the region, therefore, show Borneo rotatingclockwise as a portion of an extruding Sundalandblock (Briais et al., 1993; Daly et al., 1990, 1991; Leeand Lawver, 1993, 1995; Rangin et al., 1990;Tapponnier et al., 1982, 1986). The extrusion hypoth-esis has been widely accepted, but the relationship ofBorneo to the extruding Indochina block has rarelybeen questioned.

Prior to the extrusion tectonic hypothesis, Haile etal. (1977), Haile (1979) and Hamilton (1979) proposedplate wide CCW rotations in Borneo. Several sub-sequent models followed Hamilton and Haile in treat-ing Borneo and eastern Peninsular Malaysia as aseparate microplate dominated by CCW rotations(Haile et al., 1983; Holcombe, 1977; Richter, 1996;Schmidtke et al., 1990). Interestingly, small (10±208)CCW rotations of Borneo have also been shown inreconstructions based dominantly upon the extrusionmodel (Briais et al., 1993). The di�culty, however, ofunderstanding the substantial (45±908) CCW rotationsindicated by the paleomagnetic data have hamperedattempts to merge the two contrasting hypotheses.Current tectonic reconstructions (Hall, 1996; Richter,1996) incorporate a rigid body CCW rotation of all ofBorneo, but recognize that this approach leads to di�-culties in accommodating the rotation of PeninsularMalaysia and Borneo: the Thai and Malay basinsappear to preclude rotations as late as 30 Ma, whilethe absence of stuctural evidence for a break betweenPeninsular Malaysia and Borneo make later rotationof Borneo alone di�cult.

4. Paleomagnetic data from Borneo

We now review the currently available paleomag-netic data from Borneo and surrounding areas. Thequoted results are, for the most part, already pub-lished. While many of the studies were made beforethe advantages of modern paleomagnetic techniques ofmeasurement and analysis were available, several ofthe earlier studies were in lithologies which proved tobe excellent paleomagnetic recorders and the results oflater studies have been remarkably consistent with thedata in the pioneering work of Haile (1977),McElhinny et al. (1974) and others.

As these results are discussed, it is worth remember-ing that (1) high sensitivity paleomagnetic measure-ments are by now relatively routine, so that very fewrock types are too weakly magnetized to be measured,(2) paleomagnetism is not good at detecting smallchanges in latitude, i.e., 1000 km displacement requiresvery accurate studies, (3) paleomagnetism is good atdetecting tectonic rotations, but it is often not clearwhether the rotations took place in local shear zonesor are plate wide, and (4) to interpret a paleomagneti-cally determined rotation, the age of magnetization,which is not necessarily simply the age of the rock, iscritical. If the age is known, then the rotation musthave taken place after that time.

In the tables and ®gures, the results are presented inuncorrected in situ coordinates if there is no tectoniccorrection, as is the case with most intrusives, or if theresults fail a fold test. They are presented in tectoni-cally corrected coordinates if the results pass a foldtest or regional attitude test. A true fold test is distin-guished from a regional attitude test because only inthe former are the sites known to come from a singlefold, whereas in the latter the timing of the acquisitionof the observed structural attitude is not necessarilysynchronous.

The paleomagnetic measurements were made in avariety of laboratories over a period of more than twodecades so that they are by no means a data set of uni-form quality. However, in all cases some indication ofthe response to AF and thermal demagnetization isgiven and reported here.

The statistics of the paleomagnetic data aredescribed by the con®dence interval a95 and k, the esti-mate of Fisher's precision parameter k. The former isthe cone about the mean direction de®ning the areawithin which there is a 95% probability of ®nding thetrue mean of the population sampled. The latter is ameasure of the variance in direction obtained from theformula (N-1)/(N-R), where N is the number ofsamples or sites and R is the resultant vector obtainedby adding all of the unit vectors. Clearly, if the N unitvectors are perfectly aligned, k is in®nite; if they arerandom, it is zero. Values of k less than 10 re¯ectlarge scatter and are generally regarded as too badlyscattered to be useful. Values of k in excess of 100 forsamples at a site, or of sites in a collection, representvery tightly grouped distributions. Values much largerthan this in sedimentary rocks suggest remagnetizationand, in igneous rocks, indicate that dispersion due tosecular variation is not being recorded.

In more recent studies, demagnetization results havebeen analysed using the principal component analysistechnique introduced by Kirschvink (1980). With thistechnique the directions of lines of best ®t along thedemagnetization paths are estimated and assigned ameasure of collinearityÐthe Maximum Angular

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±246

Deviation or MAD angle. Values of MAD angles ofless than 58 are well de®ned lines, but with increases tolarger than 108, the line is poorly de®ned and largerthan 208 the results are not useful.

In describing the paleomagnetic data from Borneo,we present results in terms of following regions: (1)Northwest Borneo, (2) West KalimantanÐSchwanerMountains, (3) Central and East KalimantanÐBaritoand Kutai Basins (4) South KalimantanÐMeratusMountains, and (5) Sabah (Fig. 2). For each region,we describe the results beginning with the youngestand working back in time. The results are summarizedin Fig. 3, 4, 5, 6, 7, 8 and 9 and in Table 1, 2, 3, 4 and5; the reader should refer to the original publicationsfor more details.

5. Northwest Borneo

This area is predominantly in Sarawak, but includessome of West Kalimantan. It has been studied paleo-magnetically in some detail (Schmidtke et al., 1990)and a�ords a coherent record which provides a frame-work against which records from elsewhere in the

island can be compared. The area consists of the meta-morphic Sundaland Craton upon which Mesozoic clas-tic and limestone sediments lie (Fig. 1). In theCenozoic, the Melawi and Ketungau basins developedand a suite of shallow Oligocene-Miocene stocks wereintruded; these are known as the Sintang intrusives inKalimantan. Results are discussed in groups organizedas follows: Oligocene-Miocene shallow intrusives,Silantek Formation, Cretaceous intrusives, Jura-Cretaceous: Bau Limestone, Pedawan and KedadomFormations, and Triassic volcanics; the data are listedin Table 1 and shown in Fig. 3.

5.1. Oligocene±Miocene shallow intrusions

These intrusives are small enough and were intrudedat shallow enough depths to have cooled relativelyrapidly and are excellent paleomagnetic recorders giv-ing in most cases a single stable direction of magneti-zation by either AF or thermal demagnetization(Schmidtke et al., 1990). The results, shown in Fig. 3aand Table 1, are either unrotated and oriented parallelto the present ®eld or to the reversed ®eld direction, orare rotated counterclockwise. As discussed in a later

Fig. 2. Location of sites in Borneo used in this study, shown as ®lled triangles, data for sites in South Sulawesi are taken from Haile (1978),

Mubroto (1988) and unpublished data from Ali. Refer to Tables 1±5 for details.

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 7

section, there is a progression of the maximum ro-tation found in rocks of a particular age; however,there are also older intrusives which are not rotated.

5.2. Silantek Formation

The youngest sediments, apart from the recent allu-vium, are the Plateau Sandstones in the Tertiarybasins. Below this are the Late Cretaceous and EoceneKayan Sandstone and its equivalent the SilantekFormation, a sequence of clastics including redbeds.Results from the Silantek Formation are shown inFig. 3b and Table 1; all are from red mudstones withthe exception of one site, which was a grey ¯uvialmudstone. The most e�ective demagnetization methodwas thermal, which either isolated a single directioncarried by both magnetite and hematite, or yieldedtwo directions. In samples for which two directionswere isolated, the direction blocked between 4508 and6308C was similar to that isolated as a single directionin other samples, whereas the direction blockedbetween 1508 and 4508C was the less strongly rotateddirection. The more strongly rotated direction isshown in Fig. 3b, as is one site at which directions par-allel to the reversed present ®eld direction were found.The bedding dips in the Silantek are weak, so that noconvincing fold or regional attitude test is possible.

5.3. Cretaceous intrusives

Throughout Northwest Borneo there are scatteredCretaceous intrusives. In Sarawak, samples from theseCretaceous intrusions carried a weak and scatteredmagnetization and no useful paleomagnetic record wasobtained from them. However, in Kalimantan about20 km northwest of Sanggau, an intrusion and a dykecutting the intrusion gave results with MAD anglesbetween 28 and 158 after thermal demagnetization(Fuller, unpublished data, 1998); a K/Ar date of93.223.5 Ma was obtained for the dyke. The mean of6 samples from the dyke was Dec. 280.88, Inc., 7.28,a95 10.98, k 38.7 (Table 1, Fig. 3c).

5.4. Jura-Cretaceous: Bau Limestone, Pedawan andKedadom formations

These formations overlap in age and grade laterallyinto each other, respresenting a period of basin devel-opment. The Kedadom is the nearshore equivalent ofthe Bau Limestone; both grade upward into the domi-nant basin-®lling clastic Pedawan Formation. Thebasin was inverted and the Kayan Sandstone of LateCretaceous±Early Eocene age was deposited uncon-formably upon it. The most extensively studied for-mation is the Bau Limestone from which 11 of 21 sitesyielded interpretable paleomagnetic data. Thermaldemagnetization was again the most e�ective treat-ment. Sometimes, a low blocked phase was demagne-tized between 508 and 1008C and then the vectorstabilized between 2508 and 4508C, while in othersamples there was no low blocking fraction. Theresulting data fell into three groups, which were inti-

Fig. 3. Results from Northwest Borneo: (a) Oligocene-Miocene shal-

low intrusions show mixed results; (b) Silantek Formation mudstones

suggest a weak CCW rotation of about 408 since the Eocene; (c)

Mesozoic results suggest a strong CCW rotation of about 908 since

80 Ma. Refer to Table 1 for data.

Fig. 4. Results from West Kalimantan±Schwaner Mountains. Data

has been divided into three groups, showing unrotated, weakly

rotated and strongly rotated mean directions. Refer to Table 2 for

data.

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±248

mately mixed spatially. Group 1 exhibited present ®eldor reversed ®eld directions. Group 2 consists of ®vesites in the Bau Limestone and two in the Kedadomformation, which show an average de¯ection of about308 CCW; these results failed a fold test and are there-fore secondary and post Late Cretaceous. Group 3 isrepresented by two sites on a syncline near Bunuk andpassed a fold test; two sites in the Pedawan also passeda fold test. A third site yielded results that, after tec-tonic correction, were antipodal to the direction fromthe fold test sites. These ®ve sites de®ne a CCW ro-tation of some 908 and are illustrated in Fig. 3c andlisted in Table 1.

From Kalimantan, a declination of 267.18 and incli-nation of 2.68 with a95 13.48, k 21 was reported fromJurassic sediments (Sunata and Wahyono, 1991),which were analysed both with AF and thermaldemagnetization.

5.5. Triassic volcanics

In Sarawak, the Triassic Serian Volcanics are alwaysfound to be metamorphosed, or the ®eld relations aresu�ciently unclear that they are not useful for paleo-magnetism. In contrast, in Kalimantan their equiva-lents are less deformed and have yielded paleomagneticdata (Sunata and Wahyono, 1991). Two results arefrom Suti Semarang and Gunung Bawan, whichyielded almost 908 of CCW rotation after AF andthermal demagnetization (Fig. 3c, Table 1).

5.6. Summary of results from Northwest Borneo

The paleomagnetic record from the Northwest

Borneo domain is relatively straightforward. Intrusions

are increasingly CCW rotated with age, reaching a

maximum of 518 in an intrusion dated by K/Ar to be

25.821.9 Ma. The Late Cretaceous±Eocene Silantek

Formation gives 418 of CCW rotation. The eight

Mesozoic results which come from the Bau Limestone,

the Kedadom and Pedawan Formations in Sarawak

and from the Jura-Cretaceous and Triassic sites in

Kalimantan, all indicate a strong rotation of about 908

Fig. 5. Results from Central and Eastern Kalimantan: (a) Plateau

Basalts of Pliocene±Pleistocene age show no rotation; (b) Oligocene±

Miocene igneous rocks show mixed results; (c) Eocene sediments

suggest a weak rotation; (d) Jurassic sediments show a strong ro-

tation. Refer to Table 3 for data.

Fig. 6. Results from South Kalimantan±Meratus Mountains: (a)

Miocene microdiorite suggests weak rotation; (b) Tanjung Fm. sand-

stones suggest weak rotation; Kuaro Formation basalts passed a fold

test; (c) Cretaceous ultrabasic rocks suggest weak rotation. Refer to

Table 4 for data.

Fig. 7. Results from Sabah. Refer to Table 5 for data. Eo-

Mio=Eocene-Miocene.

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 9

which took place sometime after 93 Ma, the age of theCretaceous dyke. Among these results there are unro-tated directions in the sediments and the intrusives. Atleast in the Bau Limestone and the SilantekFormation, it is very likely that they are secondarymagnetizations and do not constrain the rotations atthe time of formation of the rocks. In the intrusives, itis less clear that they are necessarily secondary direc-tions and further work is being undertaken to test thissuggestion.

6. West Kalimantan: Schwaner Mountains

The Schwaner Mountains were studied by Haile etal. (1977) in their pioneering paleomagnetic and geo-chronological study. They obtained sixteen K/Ar ages,which ranged from 75 to 112 Ma, mainly from thegranitic plutonic rocks, and were able to get satisfac-tory paleomagnetic results from 39 of 48 hand samplescollected from dykes, tu�s, lavas and intrusives. Theoverall mean of the paleomagnetic results they

Fig. 8. Summary of rotations for Borneo; declinations shown as pie slices projected on a compass face, the angle of which represents the associ-

ated a95. (a) Tertiary results <10 Ma; Central and East Kalimantan: Plateau basalts (refer to Table 3 for data); (b) Tertiary results >10 Ma;

NW Borneo: 1=shallow intrusions, no rotation, 2=shallow intrusions, weak rotation, 3=Silantek Formation, no rotation, 4=Silantek

Formation, moderate rotation (refer to Table 1 for data); Central and East Kalimantan: 1= igneous rocks, 2=clastic sediments, Eocene (refer

to Table 3 for data); South Kalimantan: 1=diorite, 2=Tanjung and Kuaro Formations (refer to Table 4 for data); Sabah: 1=Kapa Quarry

intrusion, 2=Crocker Formation (refer to Table 5 for data); (c) Mesozoic results; NW Borneo: 1=Cretaceous intrusives, 2=Bau limestone,

Pedawan and Kedadom formations, Group 1, 3=Bau Limestone, Pedawan and Kedadom formations, Group 2, 4=Bau Limestone, Pedawan

and Kedadom formations, Group 3 (refer to Table 1 for data); West Kalimantan; 1=Schwaner Mountains, no rotation, 2=Schwaner

Mountains, weak rotation, 3=Schwaner Mountains, strong rotation (refer to Table 2 for data); Central and East Kalimantan: Clastic sediments,

Jurassic (refer to Table 3 for data); South Kalimantan: Ultrabasic ophiolite (refer to Table 4 for data); Sabah: Telupid Ophiolite (refer to Table 5

for data).

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±2410

obtained was similar to those obtained earlier fromPeninsular Malaysia and they concluded that the tworegions have behaved as a unit since the main intrusiveevent at about 80±85 Ma.

We present the results in a slightly di�erent manner.We divide them into three groups: unrotated, weaklyrotated and strongly rotated. The strongly rotatedgroup is de®ned as having rotations greater or equalto the smallest rotation seen in the strongly CCWrotated group from the Northwest Borneo domain.The latter two groups are shown in Fig. 4 and arelisted in Table 2.

The di�erence in interpretation of these resultsbetween the original authors and ourselves turns onthe question whether the two datasets are sampling thesame population or two separate populations. Giventhe present data set, it is probably not wise to pressthe case too strongly. Rather the suggestion should betested with a new collection using modern techniques.Nevertheless, there are directions comparable with thestrongly and weakly rotated directions seen in theNorthwest Borneo domain.

6.1. Summary of results from West Kalimantan

In the Schwaner Mountains results, we see a similarpattern of directions to that seen in the NorthwestBorneo domain with unrotated, weakly and stronglyrotated directions all represented. However, the resultsare based on a collection of only 48 hand samples, sothat additional studies are needed to substantiate theseresults.

7. Central and Eastern Kalimantan

This region includes the Barito and Kutai basins,which were formed during the Middle Eocene in a

period of widespread extension and basin formation.Paleomagnetic results have been reported by Lumadyoet al. (1993), Moss et al. (1997) and Sunata andWahyono (1991).

7.1. Plio-Pleistocene

The study by Lumadyo et al. (1993) of Plio-Pleistocene sites in the Plateau ¯ows from the Kelian,Magerang and Bigung localities in East Kalimantanreported that all sites carried a reverse polarity magne-tisation indistinguishable from the reversed present®eld. The magnetic recorders are shown to be ®nemagnetite. No tectonic correction is applied (Fig. 5a,Table 3).

7.2. Oligo-Miocene

Lumadyo et al. (1993) reported unrotated declina-tions in seven Late Oligocene±Early Miocene igneousrock sites (Dec. 178.88, Inc. ÿ4.78, a95 13.48, k 21).The behaviour of the magnetic recorders was good;magnetisation was interpreted as primary because of apositive fold test. The age of the folding is not known,but probably was Early to Middle Miocene and coinci-dent with a series of structural inversions in the KutaiBasin where the study sites were located (Chambersand Daley, 1995; Cloke et al., 1999).

An unrotated result (Dec. 183.88, Inc 4.68, a95 5.28)was also reported by Sunata and Wahyono (1991)from 22 samples taken from two Oligo±Miocene basal-tic sills which intrude the Late Eocene Mandai volca-nics near Nanga Raun on the Mandai river. Thereported paleomagnetic information on the magnetiza-tions suggests that the remanence is simple and yieldsstatistically consistent data. No fold test was reported.

A reversed weakly de¯ected result (Ali, unpublisheddata) comes from a diorite intrusion in the Long

Fig. 9. Plot of rotation versus age for sites with absolute age determinations (see Tables 1±5).

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 11

Table

1

Paleomagnetic

data

from

NorthwestBorneo.References:1=

Schmidtkeet

al.(1990);2=

MikeFuller

(unpublished

data);3=

Sunata

andWahyono(1991).See

Fig.3forplots

ofdata

Shallowintrusions,Oligocene±Miocene

Area

Locality

Latitude

Longitude

Rock

type

Age

N(sites)

n(samples

Dec

(8)

Inc(8)

a 95(8)

kRef.

Norotation

Sarawak

JKR

Quarry-A

irport

1.428N

110.338E

Microtonalite

Oligo-M

io1

62.7

2.8

5.8

176

1

Sarawak

KuchingQuarry

1.388N

110.308E

Tonalite

Oligo-M

io1

8358.9

ÿ2.1

7.2

114

1

Sarawak

Bau-LunduRd.

1.428N

110.158E

Diorite

Porphyry

Oligo-M

io1

6358.1

23.5

2.4

787

1

Sarawak

JalanRock

1.508N

110.328E

Diorite

Porphyry

Oligo-M

io1

6185.4

15.8

14

24

1

Weakrotation

Kalimantan

BukitGem

bah

0.108N

111.808E

Dacite

16.4

20.6

m.y.

110

333.9

ÿ16.1

7.8

39

2

Sarawak

Siburan

1.368N

110.388E

Microtonalite

17.2

21.9

m.y.

18

338.1

15.3

7.8

97

1

Kalimantan

B.BungaSinik

0.308N

111.78E

Dacite

18.0

20.7

m.y.

17

164.6

ÿ22.1

4224

2

Sarawak

GunungSerapi

1.558N

110.258E

Granodiorite

25.8

21.9

m.y.

17

128.2

ÿ0.2

3.7

269

1

Sarawak

BukitStabar

1.428N

110.338E

Microtonalite

Oligo-M

io1

6352.0

44.4

136

1

Sarawak

Mr.Choo'sGarden,Kuching

1.398N

110.318E

?Oligo-M

io1

5154

19

13.7

32

1

Sarawak

HuaSunQuarry

1.368N

110.388E

Microtonalite

Oligo-M

io1

10

168.5

ÿ21.8

7.4

43

1

Sarawak

Sem

engoQuarry

1.398N

110.318E

Microtonalite

Oligo-M

io1

8170.7

ÿ11.8

3.6

446

1

SilantekForm

ation

Norotation

Sarawak

1km

no.Sim

unjam

Jcn.

1.108N

110.858E

Red

mdst.

Eocene

14

193.3

ÿ30.6

6.1

229

1

Moderate

rotation

Sarawak

2km

no.Sim

unjam

Jcn.

1.108N

110.858E

Red

mdst.

Eocene

16

134.9

ÿ27.0

11.8

33.4

1

Sarawak

2km

e.Sim

unjam

Jcn.

1.088N

110.908E

Red

mdst.

Eocene

17

321.3

15.1

776.2

1

Sarawak

BatangUndup

1.108N

111.508E

Fluvialmdst.

Eocene

19

319.6

30.8

9.6

29.9

1

Mesozoic

results

Cretaceousintrusives

Kalimantan

DusunBunutQuarry

0.308N

110.38E

Dyke

93.2

23.5

m.y.

16

280.8

7.2

10.9

38.7

2

BauLim

estone,

PedawanandKedadom

Fms:

Group1Ð

norotation

Sarawak

Baudistrict

1.48N

110.28E

BauLim

estone

J±K

431

358.6

ÿ12.7

14.6

3.9

1

BauLim

estone,

PedawanandKedadom

Fms:

Group2Ð

weakrotation

Sarawak

Baudistrict

1.48N

110.28N

BauLim

estone

J±K

536

301.6

11.5

14.1

30.4

1

Sarawak

Serian-TebeduRd.(K

ed.)

1.138N

110.448E

MarlyLim

estone

J±K

321

311.1

ÿ2.1

22.7

2.9

1

BauLim

estone,

PedawanandKedadom

Fms:

Group3Ð

strongrotation

Sarawak

Bunukfold,Penrissen

area

1.48N

110.28E

BauLim

estone

J±K

217

272.0

ÿ8.6

5.3

45.7

1

Sarawak

Bau-LunduRd.(Ped.fold)

1.58N

109.98E

Mudstone(Ped.)

J±K

215

87.9

16.0

7.1

14.5

1

Sarawak

Pedawan

1.188N

110.288E

Lim

estonelens

J±K

18

271.9

ÿ15.9

13.3

18.4

1

Kalimantan

Tenguwe

0.708N

110.18E

Black

mudstone

Jurassic

11

40

267.1

2.6

6.9

Ð1,3

Kalimantan

SutiSem

arang

0.98N

109.88E

Blackshale

Triassic

121

278.5

ÿ20.8

11.8

Ð3

Kalimantan

GunungBawan

1.28N

109.68E

Basalt&

shale

Triassic

130

286.7

31.7

11.5

Ð3

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±2412

Bagum region north of the Kiham Haloq gorge on theupper Mahakam River in central Kalimantan (Dec.1648, Inc ÿ38, a95 8.68, k 61). Similar high-level intru-sions in the vicinity have been dated as 18 Ma oldusing K±Ar techniques (Pieters et al., 1993). Reversedresults are also recorded from lava ¯ows along theMalnyu river (northern edge of the Kutai basin; seeMoss et al., 1998) in which three sites gave a Dec.156.98, Inc. 228, a95 13.18 and k 89) after tectonic cor-rection (Ali, unpublished data). These lavas have K±Ar ages of 23.6±21.3 Ma (Moss et al., 1998).

Along the Telen River on the northeastern edge ofthe Kutai Basin, two sites in minor intrusives yieldedreliable data. These samples were collected by theLondon University SE Asia Research group andmeasured in the UCSB laboratory by C. Anderson. In

a hornblende andesite, a soft overprint was completlyremoved by 1508C. The single direction blocked abovethis temperature gave a site mean in situ direction ofDec. 143.68, Inc., 45.48 with an a95 of 11.68 and k 34.A biotite microgranite about 30 km upstream was alsowell behaved with a soft overprint removed by 15 mT,leaving a single component. With one outlier dis-carded, the in situ mean was Dec. 342.68 and Inc.42.48 with a95 9.08 and k 73 (Fig. 5b, Table 3).

7.3. Eocene

The University of London SE Asia Research Groupmembers collected dark grey ®ne sandstones ofEocene±Oligocene age which were analysed at UCSBby C. Anderson. They showed multicomponent beha-

Table 2

Paleomagnetic data from West Kalimantan. Reference: 1=Haile et al. (1977). See Fig.4 for plot of data

Locality Latitude Longitude Rock type Age N (sites) n (samples Dec (8) Inc (8) a95 (8) k Ref.

No rotation

Shwaner Mtns. 1.58S 1108E Tu�s, Porphyry Cretaceous 4 4 358.5 16.4 11.7 63 1

Weak rotation

Schwaner Mtns. 1.58S 1108E Tu�s, Dykes, Volcanics, Granites Cretaceous 37 37 311.7 0.3 7.8 10 1

Strong rotation

Schwaner Mtns. 1.48S 110.08S Metatu� Cretaceous Ð 1 98.0 ÿ18.0 Ð Ð 1

Schwaner Mtns. 1.48S 110.08E Hornfels Cretaceous Ð 1 93.0 17.0 Ð Ð 1

Schwaner Mtns. 1.38S 110.08E Dyke Cretaceous Ð 1 282.0 29.0 Ð Ð 1

Schwaner Mtns. 1.38S 110.08E Dyke Cretaceous Ð 1 284.0 22.0 Ð Ð 1

Schwaner Mtns. 1.28S 110.78E Dyke Cretaceous Ð 1 284.0 ÿ25.0 Ð Ð 1

Mean direction for W. Kalimantan±Schwaner Mtns. Cretaceous 1 5 280.1 ÿ5.6 24.5 10.7

Table 3

Paleomagnetic data from Central and Eastern Kalimantan. References: 1=Lumadyo et al. (1993); 2=Sunata and Wahyono (1991); 3=Jason

Ali (unpublished data); 4=Moss et al. (1997); 5=Fuller et al. (1991). See Fig. 5 for plots of data

Locality Latitude Longitude Rock type Age N (sites) n (samples Dec (8) Inc (8) a95 (8) k Ref.

Plateau basalts, Plio±Pleistocene

No rotation

Kelian 0.208N 115.348E Basalt Plio±Pleist 5 34 174.0 ÿ8.8 14.2 30 1

Magerang 0.308N 115.408E Basalt Plio±Pleist 5 33 186.0 ÿ31.3 5.3 213 1

Bigung 0.208N 115.178E Basalt Plio±Pleist 6 46 182.0 ÿ3.1 9 56 1

Igneous rocks, Oligo±Miocene

Mixed results

Nakan 0.258N 115.258E Basalt, Andesite Oligo-Mio 7 38 178.8 ÿ4.7 13.4 21.2 1

Nanga Raun 0.68N 113.28E Basaltic sills Oligo-Mio 2 22 183.8 4.6 5.3 Ð 2

Long Bagun 0.698N 115.48E Diorite intrusion 18 Ma 1 6 164.0 ÿ3.0 8.6 61 3

Mainyu 1.38N 116.78E Basalt, Andesite 23.6 Ma 3 15 157.0 22.0 13.1 89 3

Telen River 1.28N 116.78E Andesite intrusion Oligo-Mio 1 4 144.0 ÿ35 11.6 34 4

Telen River 1.38N 116.68E Biotite microgranite Oligo-Mio 1 6 343 42 9 73 4

Telen River 1.198N 116.78E Hbl-Andesite intrusion Oligo-Mio 1 5 120.0 ÿ33.1 7.3 86 3

Sediments, Eocene

Weak rotation

Kalasin 0.18N 114.28E Ss., Sltst., minor Ls. Eocene 1 27 322.8 0.6 13.9 Ð 2

Telen River 1.68N 116.08E Turbidites Eocene 1 4 345.0 ÿ18.6 14.7 40 3

Sediments, Jurassic

Strong rotation

Tiong Cihan (Keramu Fm.) 0.88N 114.38E Ss., Sltst., minor Ls. Jurassic 5 39 284.5 2.4 10.7 Ð 2,5

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 13

viour, but a single direction was isolated withKirschvink's method (Kirschvink, 1980) giving a meanin situ Dec. 333.88 and Inc. ÿ18.68 with a95 15.58 andk 36. When corrected for tilt, the direction becomesDec. 345.28, Inc. ÿ18.68, a95: 14.7, k: 40.2.

Sunata and Wahyono (1991) reported results fromEocene sandstones and siltstones of the Kalasin regionfrom the tributaries of the Murung River. Thirty-eightsamples were collected from which, after AF demagne-tization, 27 gave a mean Dec. 322.88 and Inc. 0.68,with a95 13.98.

7.4. Summary of results from Central and EasternKalimantan

The results from this area are the most puzzlingfrom Borneo. As in other regions, we see CCW rotatedresults of some 308 but there are also unrotated direc-tions in rocks of the same age. Among these are direc-tione which pass a fold test reported by Lumadyo etal. (1993).

8. South Kalimantan: Meratus Mountains

This important region is undersampled paleomagne-tically. Results from a granodiorite have been reportedby Sunata and Permanadewi (1998) and results from

the Eocene Tanjung Formation and from the ophioliteare given by Sunata and Wahyono (1998). Lumadyo etal. (1993) reported results from two sites in basaltic¯ows.

8.1. Miocene

A microdiorite located north of Gunung Kukusanin the Batulicin area with a K/Ar age of 19.620.76Ma was sampled for paleomagnetism by Sunata andPermanadewi (1998). AF demagnetization revealedessentially single component magnetization with ®vesamples yielding Dec. 3238 and Inc. 18, with a95 7.78and k 65.9 (Fig. 6a, Table 4).

8.2. Eocene

Sunata and Wahyono (1998) also reported resultsfrom the Late Eocene Tanjung Formation. The for-mation is widely distributed and passes up from abasal conglomerate into a sandstone. Seven of 14 sitesstudied yielded reliable data after AF demagnetizationwith a mean Dec. 3448 and Inc. ÿ68, a95 7.48 and k50.8.

Results from two sites from the Meratus region inSouth Kalimantan were reported by Lumadyo et al.(1993). They were in basaltic ¯ows of the KuaroFormation in the northern part of the Pasir Basin; the

Table 4

Paleomagnetic data from South Kalimantan: Meratus Mountains. References: 1=Sunata and Permanadewi (1998); 2=Sunata and Wahyono

(1998); 3=Lumadyo et al. (1993). See Fig. 6 for plots of data

Locality Latitude Longitude Rock type Age N (sites) n (samples Dec (8) Inc (8) a95 (8) k Ref.

Diorite, Miocene

Weak rotation

Gunung Kukusan 3.28S 116.08E Microdiorite 19.620.76 Ma 1 5 323.0 1.0 7.7 65.9 1

Tanjung and Kuaro Fms, Eocene

Mixed results

Tanjung 3.38S 116.08E Sandstone Eocene 7 Ð 344 ÿ6 7.4 50.8 2

Muru (Kuaro Fm.) 1.88S 116.08E Basalt Eocene 1 5 188.0 ÿ3.8 11.8 43.0 3

Tebruk (Kuaro Fm.) 1.88S 116.08E Basalt Eocene 1 5 1.0 ÿ16.8 12.3 39.4 3

Ultrabasic ophiolite, Cretaceous

Weak rotation

Batulicin 3.48S 115.98E Ultrabasic Cretaceous 1 5 321.0 ÿ13.0 14.4 18.9 2

Table 5

Paleomagnetic data from Sabah. References: 1=Fuller et al. (1991); 2=Mike Fuller (unpublished data). See Fig. 7 for plot of data

Locality Rock type Age N (sites) n (samples Dec (8) Inc (8) a95 (8) k Ref.

Intrusion near Mt. Kinabalu, Miocene

Kapa Quarry Adamellite 13.325.3 Ma 1 9 348.9 1.0 2.4 422 1

Crocker Fm., Eo-Miocene

NW coast Red mdst., tilt corr. Eocene±Miocene 1 7 277.5 2.8 3.7 227 1

Telupid ophiolite, Cretaceous

Telupid Chert Cretaceous 1 9 281.0 0.0 6.0 66 1

Telupid Spilite Cretaceous 6 48 313.3 ÿ9.1 27.4 7 2

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±2414

dating of the ¯ows is based on biostratigraphy. Onlytwo sites are reported and, although a fold test waspositive, the result had poor statistics (Dec. 184.68, Inc6.58, a95 49.28). These results are presented after tec-tonic correction (Fig. 6b, Table 4).

8.3. Mesozoic

Sunata and Wahyono (1998) reported one site fromthe ultrabasic of the ophiolite. Five samples gave a sitemean Dec. 3218 and Inc. ÿ138, with a95 14.48, and k

18.9 (Fig. 6c, Table 4). Further work is underway withadditional sites in the ophiolite.

8.4. Summary of results from South Kalimantan

This area is clearly undersampled, but the limitedresults seem to show a similar pattern to Central andEastern Kalimantan with rotated and unrotated direc-tions mixed. The weak rotation is seen in all threeunits, which Sunata and Wahyono (1991) report, i.e.,the Miocene microdiorite, the Eocene Tanjung

Fig. 10. Summary of rotations for Palawan. For data, see Almasco (1996) and Almasco et al. (1999).

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 15

Formation and the Cretaceous ultrabasic rocks; thereis no report of the strong rotation from this region.Since there is a range in age from Mesozoic toMiocene in these units, there has either been remagne-tization of the older units in the direction of the intru-sion, or there was no rotation here between theMesozoic and the Miocene. The two sites reported byLumadyo et al. (1993) from Eocene Kuaro Fm. basaltsare unrotated, so that they are inconsistent with theresults of Sunata and Wahyono (1998).

9. Sabah

In Northern Sabah, the NE/SW strike of theCrocker Belt, which is consistent for several hundredkm into Sarawak, turns almost E/W (Tongkul, 1990).This presents the opportunity to test paleomagneticallywhether the con®guration is brought about by defor-mation of an earlier continuously NE/SW striking fea-ture. To the east of the Kinabalu intrusion, there areoutcrops of the Cretaceous chert-spilite unit of theTelupid ophiolite, but it is badly dismembered so thatstructural continuity is hard to establish. In the norththe chert-spilite unit is again seen. Although a numberof paleomagnetic results are available for the region,their interpretation is still far from clear.

9.1. Eo-Miocene

The youngest site sampled is at the Kapa Quarry ina small satellite stock of the Mount Kinabalu intru-sion; the K/Ar age is 15.025.3 Ma. The rock is anexcellent paleomagnetic recorder and gives Dec. 348.98and Inc. 18 with a95 2.48 and k 422 (Fig. 7, Table 5).

Much of the Eocene±Miocene Crocker Formationconsists of turbidites, including relatively coarse sand-stones which give a present ®eld directions with someminor CW rotation (Fuller, unpublished data). Amongthe Crocker sandstones are red mudstones which areexcellent recorders; these give the strongly rotatedCCW de¯ected Dec. 277.58 and Inc. 2.88, with a95 3.78and k 227, after tectonic correction (Fig. 7, Table 5).Before tectonic correction the result is Dec. 309.38 andInc 49.78, with a95 3.48 and k 311. Work is underwaywith further collections from the mudstones.

9.2. Cretaceous chert-spilite unit

Collections were made from the chert-spilite unit inthe vicinity of Telupid in central North Sabah. Thechert gave a strongly rotated direction after tectoniccorrection of Dec. 2818, Inc 08 with a95: 68, and k: 66.Tectonic correction has little e�ect on this direction. In

contrast, the spilites fail a regional attitude test givingbadly scattered results after correction. The before-cor-rection value, after elimination of 2 outliers is Dec.313.38 and Inc. ÿ9.18, with a95 27.48 and k 7 (Fig. 7,Table 5).

9.3. Summary of results from Sabah

Sabah is a potentially important area for the tec-tonics of Borneo but requires much more paleomag-netic work. The few results which are available suggestthat the magnetization carried by the intrusion at theKapa Quarry is younger than most of the rotation ofthe region. The Crocker results are too few and tooscattered to provide useful data yet, but additionalresults from the red mudstone facies could prove im-portant, if they are the youngest units showing thestrongly rotated direction. The spread in directions ofthe spilite magnetizations, from the strongly rotateddirection towards the present ®eld, suggests remagneti-zation. There are insu�cient results from north of theregion, in which the structure swings from NE/SW toE/W, for interpretation.

10. Discussion of paleomagnetic results from Borneo

In Fig. 8, the Borneo results are presented in theform of maps showing the rotations for the varioussites. The ®gure is in three parts: Fig. 8a shows theresults from the youngest sites with ages less than 10Ma, represented by the Plio±Pleistocene Plateaubasalts. Fig. 8b presents all Tertiary sites older than 10Ma. Finally, in Fig. 8c the results from Mesozoic sitesare shown. From this ®gure it is clear that the rotatedand unrotated directions are intimately mixedspatially.

In Fig. 9, the results from igneous rocks are shownin the form of a plot of rotation versus age for sites atwhich we have absolute age determinations and thedata are not interpreted as remagnetizations. TheCenozoic sites which show directions not seen inyounger rocks exhibit a trend which gives a rate of ro-tation of approximately 28 per million years. However,as we have noted elsewhere, there are other well-datedCenozoic sites of this age which show no rotation.Among the older dated material which is primarilyCretaceous, as well as the strongly rotated directionsshown here, there are unrotated and weakly rotateddirections. The strong rotation is carried by an intru-sive and a cross-cutting dyke of Cretaceous age fromNorthwest Kalimantan; the dyke has been dated by K/Ar as 93.223.5 Ma (Table 1). The second Mesozoicresult comes from metatu�s (Table 2) in the contactzone of adamellite dated at 86.222.0 Ma (Haile et al.,

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±2416

1977). The weak rotation is carried by the youngestgranites and associated dykes (see Tables 1±5). Theoccurrence of rotated and unrotated directions is,therefore, intimately mixed in age as well as in locality.Nevertheless, there is a clear progression of the maxi-mum rotations shown by rocks of each particular age.

Although not shown in Fig. 9, there are also resultsfrom sedimentary sites which show a similar pattern ofunrotated, weakly and strongly rotated. Using Fig. 9as a key to direction as a function of age we can, inprinciple, interpret possible ages of magnetization forsediments. At present this is only useful for age rangeswithin which we have good control, i.e., from about10 to 30 Ma. For example, the unrotated sites in theSilantek Formation, Bau Limestone, and Pedawan andKedadom formations must have been remagnetizedafter about 10 Ma. Similarly, the Silantek Formationrotated direction and the weakly rotated direction inthe Bau Limestone and Kedadom and Pedawan for-mations must have been remagnetized prior to 30 Mabut after about 80 Ma.

11. Summary of discussion of paleomagnetic resultsfrom Borneo

The Sintang results and other data for sites youngerthan 30 Ma de®ne a CCW rotation trend with a rateof approximately 28 per million years. The Mesozoicstrongly-rotated results are also shown, and while theyare insu�cient to de®ne a trend, it is clear that the ro-tation rate of 28/Ma was not maintained throughoutthe Cenozoic. In the face of this evidence of systematictrends in the observed rotations, we provisionally in-terpret the data to indicate a rotation of theSundaland core of Borneo, which can be divided intoa strong rotation shown by rocks older than 80 Ma,and a weaker rotation seen in rocks between 30 andabout 10 Ma. As a working hypothesis, we proposethat all directions in rocks that are also seen inyounger rocks are secondary magnetizations and donot carry information about the rotation of Borneo atthe time of formation of the rock. They may, however,record particular important periods of remagnetizationand reveal the rotation history for that time. Forexample, it appears that there may have been an im-portant period of remagnetization during a reversedchron in the Plio±Pleistocene, coincident with theextrusion of the Plateau Basalts (Table 3), whichwould account for the common reversed present ®elddirection seen in many sites of diverse age. Anotherperiod of remagnetization appears to have taken placeafter about 80 and before 30 Ma because the weak ro-tation is seen in many Mesozoic rocks.

12. Paleomagnetic data from regions surroundingBorneo

If the paleomagnetic results from Borneo are to beinterpreted in terms of CCW rotation, then it is im-portant to see how this behaviour ®ts into local paleo-geography and in particular how consistent it is withthe paleomagnetic history of nearby regions.Unfortunately, the paleomagnetism of nearby regionsis incomplete, but important relevant results are nowdiscussed. The most immediately relevant region is thewestern part of the south arm of Sulawesi, which isseparated from Borneo only by the Makassar Straits(Fig. 1). From this region a number of studies havebeen reported and are discussed below. To the north-east lies Palawan and again there are paleomagneticresults from that island. While work is underway inJava and Sumatra, the results are presently not su�-ciently advanced to warrant inclusion. FromPeninsular Malaysia there is a considerable paleomag-netic database, although the absence of a Tertiary sec-tion makes interpretation di�cult. We now discuss theresults from Sulawesi, Palawan, and PeninsularMalaysia and give a brief overview of work underwayin Sumatra and Java.

12.1. Sulawesi

Western South Sulawesi and Borneo have been e�ec-tively attached throughout the Cenozoic (Fig. 1B).Although extension between the islands took placeduring the middle Cenozoic (Situmorang, 1982), exten-sive oceanic lithosphere was not generated along theMakassar Straits and constraining the relative motionbetween these two crustal fragments is relativelystraightforward. The opening of the Makassar Straitsreduces CCW rotations in Sulawesi compared withtheir equivalents in Borneo for data older than theopening event. However, the amount is within theerrors of presently available results for sites antedatingthe opening of the Makassar Straits , i.e., Mesozoic orearly Cenozoic data.

There have been several paleomagnetic studies car-ried out on SW Sulawesi (Ali, unpublished data; Haile,1978; Mubroto, 1988; Mubroto et al., 1994, andSasajima et al., 1980). In this review, we restrict ouranalysis to those sites from the western side of theSouth Arm (Fig. 1B and 2); this avoids the problem oflocal tectonic complexities associated with the collisionof the allochthonous terranes to the east. Ali (unpub-lished study) has identi®ed nine paleomagnetic sitesfrom the late Miocene Camba Formation of SouthSulawesi which have yielded meaningful directionaldata. The sites are from mainly igneous rocks (thindykes, lava ¯ows and diorite bodies) but also include

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 17

siltstones and claystones (van Leeuwen, 1981).Demagnetisation experiments suggest that their magne-tisation is primary. Combined with data from eightpreviously reported sites from Haile (1978) andMubroto (1988) the new in situ formation mean direc-tion is D 171.68 and I 12.28 with a95 5.88 and k 38.3;correction for tilt results in a mean direction of D171.38 and I 10.28 with a95 5.38 and k 46.8. The tiltcorrected data indicate a small (098) CCW rotationand negligible translation of the region since 010 Ma.These results are consistent with those from Borneo.

Preliminary unpublished work by Ali on the UpperCretaceous Balang Baru Formation from three sec-tions in the Doidoi region of SW Sulawesi gave Dec.341.78, Inc. ÿ21.58, a95 6.58, and k 73, with marginallybetter in situ statistics. This direction is similar to thedirection recorded in the younger rocks on Sulawesi.Similar results were obtained by Haile (1978), whoobtained Dec. 3258 and Inc. ÿ58, with a95 7.28 and k60, from brown radiolarian cherts from the ParingRiver in the Dera Valley. In this same paper, twostrongly rotated results are reported, which are similarto those seen in Kalimantan and Sarawak. One camefrom a dyke which gave Dec. 1058 and Inc. 118, witha95 4.98 and k 624; the other was from grey calcareoussiltstone from the same valley with NRM directions ofDec. 1218 and Inc. 328, with a95 14.98 and k 38.6.While these last two results suggest a strongly rotatedMesozoic direction similar to those seen in Borneo, itis clear that more data are badly needed.

12.2. Celebes and Sulu Seas

Shibuya et al. (1991) have reported a period ofCCW rotation in late Eocene and early Oligocene timefor the Celebes Sea. This result was obtained withorientation of core using the soft magnetization, whichwas interpreted to be in the present ®eld direction. Themethod (Fuller, 1969) is not always reliable, but hasbeen checked elsewhere against the formational micro-scanner and was found to give similar results(Cisowski et al., 1990). In contrast, samples from thetop 220 m at Site 769 on the southern ¯ank of theCagayan Ridge in the Sulu Sea showed no evidence ofCCW rotation from about 8.9 Ma (Hsu et al., 1991).The latter result is consistent with results from Borneo,which suggest that the weak rotation was almost com-pleted by this time. The result from the Celebes Sea isinconsistent with the Borneo results, in that the timingis much earlier than the 30 to 10 Ma seen in Borneo.Thus, the CCW rotated regions in Borneo have adi�erent rotation history from the Celebes Sea duringthis time.

12.3. Palawan

The elongate island of Palawan lies to the north east

of Borneo between the South China Sea and Sulu

Seas. It is divided into the Northern and Southern

Palawan blocks. The former is dominated by a

Mesozoic section, whereas the latter is dominated by

ophiolites. Paleomagnetic studies are reported by

Almasco (1996) and Almasco et al. (1999). Fig. 10

shows that the directions in the North Palawan Block

are predominantly CW rotated, whereas those from

the South are predominantly CCW rotated. There is a

region at the northern margin of the ophiolites where

there is large between-site scatter, probably re¯ecting

local deformation.

The results from the Jurassic Busuanga cherts and

the Cretaceous Guinlo Formation on the island of

Busuanga in the North Palawan Block and from the

Guinlo Formation on the main island of Palawan

failed a regional attitude test and gave indistinguish-

able directions. When combined, the 11 sites yield a

declination of 50.98 and inclination of 45.38, with a k

of 26 and an a95 of 9.18, giving a VGP of 183.48E and

39.38N and a paleolatitude of 26.8824.68. This VGP

is similar to those found in regions of pervasive

Cretaceous remagnetization in South China.

Results from the Cretaceous Espina Basalts of the

Calatuigas Ophiolite in the Southern Palawan Block

pass a fold test, carry an NRM with AF demagnetiza-

tion characteristics consistent with a primary TRM,

include Normal and Reversed directions, and yield a

mean normal direction of 283.98 and an inclination of

5.88, with a k of 37.7 and an a95 of 12.68, which gives

a paleolatitude of 38N268. The declination is similar

to that reported from the Celebes Sea by Shibuya et

al. (1991).

Results from the North Palawan Block are consist-

ent with its proposed South China origin, from whence

it has moved southward and rotated clockwise. The

Espina Basalts of the South Palawan Block have

moved northward and experienced counterclockwise

rotation. Their origin is less clear, but suggest a

Cretaceous spreading center to the south, possibly re-

lated to that at which the Sabah Ophiolite was formed.

While these paleomagnetic results require di�erent tec-

tonic histories for those parts of the North and South

Palawan blocks studied, both the North and South

Palawan block basements probably have a common

origin, having been moved southward by South China

Sea spreading. Subsequently ophiolites were thrust

over the Southern Block. The ophiolites in Palawan

have an age similar to those of Sabah to the south in

northernmost Borneo.

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±2418

12.4. Peninsular Malaysia

Peninsular Malaysia is dominated by the MainRange granites of predominantly Late Triassic age(Hutchison, 1989). They are deep seated in the eastand become more epizonal to the west. In contrast, theEastern Belt is an epizonal calc-alkaline series of LatePermian to Late Triassic age. Between the two rangeslies the Central Basin separated from the Main Rangein the west by the Raub±Bentong suture (Fig. 11).Spectacular uplift associated with the IndosinianOrogeny produced extensive intermontane basins ®lledwith the Jurassic±Cretaceous redbed sequences, whichhave yielded excellent paleomagnetic results. The otherprincipal sources of late Mesozoic paleomagnetic datahave been the Kuantan dykes and the Segamat basalts.The former were intruded into the Permo-Triassicintrusives of the Eastern Belt centered aroundKuantan. Towards the southern end of the CentralBasin lie the Segamat basalts of probable LateCretaceous age. The Cenozoic section in the peninsula

is con®ned to a single coal basin at Batu Arang nearKuala Lumpur (Fig. 11) and scattered minor outcropsof coarse sediments.

The published paleomagnetic database forPeninsular Malaysia was reviewed by Haile and Briden(1982). It is primarily the pioneering work of NevilleHaile and others (Haile, 1981; Haile et al., 1983; Haileand Khoo, 1980; McElhinny et al., 1974). CCW decli-nations were found with moderate inclinations (3308,408) in the Cretaceous Kuantan dykes and Segamatbasalts (Haile et al., 1983; McElhinny et al., 1974).CCW rotated declinations with moderate inclinationswere also found in Jurassic±Cretaceous redbeds nearMaran and Kluang (Haile and Khoo, 1980). For com-parison, the present latitude spread of PeninsularMalaysia gives shallow inclinations from 28 to 128.Additional results have been described by Richter andFuller (1996) and in general there is excellent agree-ment between the directions listed above and the newdata. Fig. 11 gives the results from Jurassic±Cretaceous redbeds, from the Cretaceous Kuantan

Fig. 11. Summary of rotations for Peninsular Malaysia. For data on the Kuantan dykes and Segamat basalts, see Haile et al. (1983) and

McElhinny et al. (1974); for the Jura±Cretaceous redbeds, see Haile and Khoo (1980) and Richter and Fuller (1996).

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 19

dykes and the Segamat basalts. The youngest rocksthat have been studied give CCW rotations.Unfortunately the absence of a signi®cant Tertiary sec-tion means that the age of these rotations cannot beconstrained and we have no information aboutTertiary rotation history. There are, however, coalbasins such as Batu Arang near Kuala Lumpur(Fig. 11), which give the promise of some Tertiaryresults. These rocks are very weakly magnetized whichhampered earlier studies, but they are being studied atpresent.

12.5. Java and Sumatra

Despite a number of paleomagnetic studies in Javaand Sumatra, it is still too early to use these results tohelp interpret the results from Borneo. In both areaswork is underway and older studies are available. InJava, paleomagnetic results have been reported byHirooka et al. (1980) and Mah® (1984). Hirooka et al.(1980) reported results from Bayat which were all ofnormal polarity and close to the present ®eld, butthese were NRM directions before demagnetization.From West Java, they reported results after AFdemagnetization, which were widely dispersed. Mah®(1984) reports results from Bayat, Kalissongo andKarang Sambung, where windows through the blan-keting ¯ows which cover the island, permit access toolder sections. There is again a mixture of rotated andunrotated sites but no sytematic pattern of rotationwith age has emerged comparable with that seen inBorneo. The dominant results from Sumatra give CWrotations (Haile and Briden, 1982) but the situationremains unclear. The results from Sumatra are poten-tially particularly important because there are plentifulTertiary intrusives which will aid in the interpretationof the results from Peninsular Malaysia.

12.6. Summary of paleomagnetic data from regionssurrounding Borneo

Results from the Late Miocene of Western Sulawesiare su�ciently well determined and geologically con-strained to provide a useful comparison with theresults from Borneo itself and the results are consistentin the two regions, showing the weak rotation. Thissame direction is seen in some of the Mesozoic rocks,whereas the more strongly rotated direction is seen ina dyke and in Jurassic±Cretaceous siltstones.

The results from North Palawan clearly represent avery di�erent geological history from Borneo. TheNorth Palawan block has a paleomagnetic history con-sistent with an origin associated with South China andcarries the typical CW rotations of that region. Asnoted above the South Palawan basement probablyhas a similar history, but the ophiolites which were

thrust over that basement have the CCW clockwise ro-tations similar to the weak rotation of Borneo and theCCW rotation reported in the Celebes Sea (Shibuya etal., 1991).

The comparison between Borneo and PeninsularMalaysia is hampered by the absence of a signi®cantTertiary section in the peninsula. However, theJurassic±Cretaceous redbeds, the Kuantan dykes andthe Segamat basalts all show CCW rotations similar tothat seen in Borneo in the Sintang intrusives. The pro-blem in interpreting these results is that we have no in-dication of the age of the rotation. It could have takenplace at the same time as the weak rotation in Borneo,or it could have been much older. Clearly PeninsularMalaysia cannot have rotated coherently with Borneothroughout the Cenozoic because it shows only theweaker rotation seen in the 30 Ma aged units inBorneo.

13. Discussion

Despite obvious shortcomings, there is now a sub-stantial paleomagnetic data base from Borneo and thesurrounding regions. The chief di�culty in interpretingthese paleomagnetic data and particularly those fromBorneo, is that rotated and unrotated sites are inti-mately mixed in age and in location. The fundamentaldi�erence in resulting tectonic models and paleogeo-graphic reconstructions lies in the interpretation of theunrotated sites. One view assumes that these are faith-ful records of the ®eld at the time of origin of the rockand that, because there are unrotated directions aswell as CCW rotated directions, the island of Borneocannot have rotated as a single block. Rather theisland must have been predominantly ®xed with therotated sites recording local rotations in shear zones,or are anomalous or spurious directions. A second in-terpretation assumes that the unrotated directions aresecondary magnetizations and therefore do not con-strain rotations since the time of formation of therock. In this view, the CCW rotational history of theisland is recorded by only those samples and siteswhich are not remagnetized, i.e. those that show themaximum rotation for each particular age.

The chief di�culty faced by the suggestion thatBorneo has remained ®xed with respect to stableEurasia and indeed with respect to the GeomagneticAxial Dipole from the Mesozoic (e.g., Lumadyo et al.,1993; Lee and Lawver, 1993, 1995) is the internal con-sistency of the paleomagnetic results. If the island hasnot rotated, why are similar rotations seen over such awide region? For example, the strongly rotatedMesozoic results are seen in the Jurassic of Sarawak,the Cretaceous±Jurassic intrusions of the SchwanerMountains, sediments in Central Kalimantan, and in

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±2420

preliminary results from the chert-spilite unit in north-ern Sabah. It seems very unlikely that similar rotationswould be seen if the inferred rotations come fromsimply anomalous or spurious paleomagnetic results. Itseems equally unlikely that the observed rotation onlytook place in localized shear zones because, given theintimate mixing of the locations of rotated and unro-tated sites, it is hard to see how zones of distributedshear could thread their way through the island a�ect-ing rotated sites and avoiding the unrotated.

The intimate mixing throughout Borneo of rotatedand unrotated results is easier to explain in terms ofremagnetization than by a special distribution of localshears. This suggests that there was pervasive remagne-tization in Borneo through which a paleomagneticrecord has partially survived. We take as a workinghypothesis, to be tested by further work, that theCenozoic rocks and Mesozoic rocks which are unro-tated, or show the weak 30 Ma rotation, are remagne-tized. The identi®cation and detailed understanding ofremagnetization remains a major problem in paleo-magnetism despite many studies (e.g., Elmore et al.,1987; Fuller et al., 1988; Hart and Fuller, 1988;McCabe and Elmore, 1989; McClelland-Brown, 1982;Reynolds et al., 1990). That sedimentary rocks are fre-quently remagnetized is well known, so that there is nodi�culty in accepting a major role of remagnetizationin the sedimentary rocks studied even though criteriawhich might have been used to establish remagnetiza-tions were not, for the most, part used. The interpret-ation of remagnetization in small shallow stocks, suchas the Singtang intrusives poses more di�cult pro-blems. Preliminary investigations of the magnetizationof the minor intrusions in Kalimantan revealed somedistinctions between the blocking temperature distri-butions of the magnetizations which were rotated, andthose that were unrotated. Further work is planned toinvestigate the cause of these blocking spectra, but thepresence of such distinctions may be the key to dis-tinguishing primary and secondary magnetizations inthis suite of rocks.

If we accept that the paleomagnetic results require acoherent rotation of Borneo, there still remain twopossible modes of rotationÐa rigid plate rotation, orrotation brought about by distributed deformation inone or more shear zones. One possibility raised byHolcombe (1977) in a discussion of the rotation ofPeninsular Malaysia is that a series of concentric sli-vers rotate. In Peninsular Malaysia the rotation isCCW about a pole to the northeast. A somewhat simi-lar kinematic scheme was advocated by Rangin et al.(1990) in describing CW rotation in SE Asia in re-sponse to the India±Eurasia collision. In that examplethe CW rotation was about a pole at the eastern syn-taxis in Assam. It is possible that a similar kinematicscheme might explain the paleomagnetic observations

in Borneo. The structure of the island does de®ne aseries of roughly arcuate zones, which might be inter-preted as rotating slivers. While it is not clear how onecould refute such an explanation with paleomagneticdata alone, from a broader geological viewpoint themodel is open to criticism. First, there is no evidenceof the necessary faults to accommodate such move-ment, and second there are features which appear tocrosscut the arcuate boundaries, thereby precludingthe proposed movement. Still another model whichcould be used to interpret CCW rotations acrossBorneo is that of Wood (1985) who considered thatthe dominant tectonic features were a series of NW/SEshears which cut across Borneo. These could thenform a region of left lateral shears which would gener-ate CCW rotations across Borneo. However, the evi-dence for these shears with their associated distributeddeformation is not very convincing. We therefore fallback to an essentially rigid plate model with much ofKalimantan, Sarawak and southern Sabah participat-ing in a rotation of about 508 CCW between 30 and10 Ma, and in an earlier rotation of about 408 some-time between 80 and 30 Ma. The underlying cause ofthese rotations is convergence between the AustralianPlate and Eurasia, which is accommodated in thesoutherly subduction which gave rise to the accretion-ary wedge in Sarawak and volcanism in Kalimantan.

14. Conclusion

The paleomagnetic results from Borneo give con-siderable support to the suggestion of CCW rotation,which is consistent with earlier discussions (Haile,1978; Hamilton, 1979) and with aspects of more recenttectonic models and paleogeographic reconstructionsHall (1996) and Richter (1996). There are also resultswhich are inconsistent with this idea, but the simplestinterpretation of the paleomagnetic data is that thereis a CCW rotation of some 508 since about 30 Ma.Directions corresponding to this rotation are also seenin Mesozoic rocks and are interpreted as a secondarymagnetization. Other Mesozoic rocks show a strongerrotation of an additional 408 giving a total of almost908. The timing of this earlier rotation is not clear, butit cannot be older than 80 Ma, the age of the youngerCretaceous intrusions in the Schwaner Mountainswhich show the rotation. In comparing the results fomBorneo with those from the surrounding regions, we®nd CCW rotations from Western Sulawesi, theCelebes Sea, Palawan and Peninsular Malaysia.However, these rotations are not as large as the ro-tation in Borneo, so that the region has not rotatedcoherently as Borneo appears to have done. The ro-tation of Borneo is a consequence of convergencebetween the Australian and Eurasian plates which is

M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 21

accommodated by subduction along the northwestmargin of Borneo.

Acknowledgements

MF wishes to express thanks to the many peoplewho have taken part in the work reported here andthose who made it possible. In particular, we acknowl-edge this manuscript is prepared with aid from ACS.Earlier work was supported by the NSF Tectonics pro-gram and a consortium which, at various times, com-prised Conoco, Texaco, Marathon, Chevron, Unocal,Amoco, and Exxon, of which Chevron OverseasPetroleum continues to fund our work. JRA and SJMfunding came from the University of London SE Asiaconsortium (Arco, Canadian Petroleum, Exxon,LASMO, Mobil and Union Texas). Dr Irwan Bahar,the Director of the Geological Research andDevelopment Center, Bandung, Indonesia, is gratefullyacknowledged for his assistance in organizing ®eld ex-peditions. Dharma Satria Nas and H. Wahyono arethanked for their help and friendship in the ®eld. Thisis HIGP publication number 1027.

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