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

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, Australia dExxon 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 minor intrusions in Sarawak, Sabah and Kalimantan, which increase systematically with the age of the intrusion to a maximum value of 51.8823.78. The rotation can be no older than 25 Ma, which is the age of the intrusion showing the maximum rotation. The rotation 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 most part, unknown. In Mesozoic rocks in Kalimantan and Sarawak, a stronger declination rotation of nearly 908 CCW is recorded at 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, where only the weaker CCW rotation is seen. The widespread occurrence of this strong rotation in Western Borneo suggests that it is essentially 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 Eurasian plates, which is accommodated by subduction along the northwest margin of Borneo. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Makassar Straits and Sulawesi (Fig. 1). To the south and southwest are the islands of Java and Sumatra. In Borneo lies in a central position in SE Asia and the west lie the Sunda Shelf and ultimately the hence is a key element of tectonic models and paleo- Palaeozoic and Mesozoic continental crust of geographic reconstructions of the region. The ro- Peninsular Malaysia. The tectonostratigraphic frame- tational history of Borneo is controversial. Various possible rotations with respect to stable Eurasia have work of Borneo bears little resemblance to that of been suggested: No rotation (Lee and Lawver, 1993, Peninsular Malaysia and previous workers have postu- 1995); CW rotation (Rangin et al., 1990); CCW ro- lated a Mesozoic terrane boundary within the Sunda tation (Haile et al., 1977; Hall, 1996; Hamilton, 1979; Shelf (Metcalfe, 1991). Thus, Borneo is surrounded to Schmidtke et al., 1990); Mixed rotations (Briais et al., the north, east, and south by plate boundaries, mar- 1993). ginal ocean basins and arc systems, which are presently To the north of Borneo lies the South China Sea active, or which have been active during the Tertiary, marginal oceanic basin, to the east the Sulu and and to the west by an under-explored shelf region. It is Celebes Seas marginal oceanic basins and the therefore only by establishing the paleomagnetic his- tory of Borneo that we can hope to determine its ro- * Corresponding author. tation history.

1367-9120/99 $ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0743-9547(98)00057-9 4 .Fle ta./Junlo sa at cecs1 19)3±24 (1999) 17 Sciences Earth Asian of Journal / al. et Fuller M.

Fig. 1. A. Geologic map of SE Asia. B. Geologic map of Borneo. M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 5

2. Geological background for paleomagnetic studies the Neogene in these basins, although sedimentation in the Melawi±Ketungau basins had ceased by the end of An overview of the geology of Borneo is given in the Oligocene. The Melawi and Kutai basins have Fig. 1B. Much of the southern half of the island con- yielded paleomagnetic data and further work is under- sists of a Sundaland cratonic core of Triassic and way. Jurassic metamorphics and volcanics. To the north of At least three phases of igneous activity a€ected this lie the arcuate tectonostratigraphic belts of the Borneo during the Tertiary. Three suites of volcanic Cretaceous to Paleogene Rajang±Embaluh Groups and intrusive rocks are recognised within the Tertiary and beyond it the turbidite sequences of the Oligocene of Kalimantan: the Nyaan Volcanics, the Sintang to Miocene Crocker-Temburong Formations, usually Intrusive Suite and the Metulang Volcanics (or Plateau considered to have an accretionary complex origin, Basalts). The felsic Nyaan Volcanics in the Upper although some parts of the Embaluh Group have Kutai Basin (Fig. 1B) have K±Ar ages of 48±5021 recently been re-interpreted (Moss, 1998). For the Ma (Pieters et al., 1987, 1993). The basaltic to andesi- most part, these units are not suitable for paleomag- tic Muller Volcanics, north of the eastern extremity of netic work, but there are substantial sections in the the Schwaner Mountains (Fig. 1B), have a K/Ar radio- Crocker Formation that are not structurally disturbed metric age of 4120.4 Ma (Pieters et al., 1993). These on a ®ne scale and have yielded useful paleomagnetic data. In South Kalimantan, the Meratus and Bobaris represent an important paleomagnetic target for absol- ophiolites and volcanic island arc rocks are exposed in utely dated paleomagnetic directions from the Eocene. the Meratus mountains (Fig. 1B), having been The Sintang Intrusive Suite of Kalimantan and its emplaced by the late Cretaceous (Sikumbang, 1986, equivalent in Sarawak have been sampled paleomagne- 1990). Several phases of NNE±SSW oriented strike- tically (Haile, 1979; Moss et al., 1997; Schmidtke et slip deformation have a€ected these rocks during both al., 1990) and more work is underway. They are the Cretaceous and Tertiary (Kusuma and Darin, de®ned as Late Oligocene to Middle Miocene, 1989; Sikumbang, 1986, 1990). Following ophiolite although a wider range of K/Ar radiometric ages (41 emplacement, arc volcanism in SE Kalimantan then to 8 Ma) has been obtained from rocks assigned to jumped outboard to the Sulawesi arc system. The this suite (Heryanto et al., 1993; Pieters et al., 1993). region is almost untouched paleomagnetically, but is The Sintang intrusives have a ma®c to felsic compo- potentially important in de®ning the southern bound- sition and consist of diorites, microdiorites, dacites, ary of any rotated regions within Borneo. microgranites and andesites. Intrusions of the Sintang The ®rst paleomagnetic studies in Borneo were in Intrusive Suite are widely distributed as plugs and the Schwaner Mountains in Western Kalimantan other hypabyssal intrusions throughout parts of west, (Haile et al., 1977). They consist of Lower to Upper central and east Kalimantan, often forming conical Cretaceous granites and tonalites intruded into hills. The ®nely crystalline nature and limited contact Palaeozoic metamorphic rocks of the Sundaland core. aureoles of these intrusions imply a high level of The Schwaner mountains granites have been radio- emplacement. Andesitic breccias and agglomerates are metrically dated mainly by K±Ar techniques at 130±80 typically associated with the plugs. Pieters et al. (1993) Ma (Amirruddin, 1989; Amirruddin and Trail, 1993; correlate the Sintang intrusives with the composition- Keyser and Rustandi, 1993; Pieters and Sanyoto, 1993; ally similar coeval Oligo-Miocene intrusives of Kirk Williams et al., 1988). They have been recognized geo- (1968) in Sarawak. In Sabah in northern Borneo, the chemically as volcanic arc granites (Amirruddin, 1989). intrusive which forms Mount Kinabalu, the highest Other smaller granitic plutons of Cretaceous age are mountain in the region, is of a similar age to the located north of the Schwaner Mountains in younger Sintang intrusives. A satellite stock of the Kalimantan (Fig. 1B). Kinabalu intrusive with a K/Ar age of 13.325.3 Ma During the Middle Eocene the Melawi±Ketungau basins, the Upper Kutai Basin, and the Kutai Basin has been sampled paleomagnetically (Table 5). (Fig. 1B) formed along the southern margin of the Pliocene±Pleistocene volcanics of the Metulang Suite Rajang±Embaluh belt. Paleomagnetic studies have are common throughout the centre of Kalimantan and been reported from these basins (Lumadyo et al., parts of Sarawak and have been termed the Metulang 1993) and others are underway. The Barito, Pasir, or Plateau Basalts. They have been studied paleomag- Asem-Asem, Muara and Tarakan sedimentary basins netically by Lumadyo et al. (1993). The Metulang also were formed at this stage. This period has for Volcanics have K/Ar radiometric ages of between 2.4 some time been recognised as a time when basin open- and 1.7 Ma (Pieters et al., 1993). The basalts of the ing and extension a€ected much of the region (Daly et Nuit Volcanics in West Kalimantan, with a K/Ar al., 1990, 1991; Hall, 1996; Van der Weerd and Armin, radiometric age of 4.920.1 Ma (Supriatna et al., 1992). Sedimentation continued throughout most of 1993), belong to this suite. 6 M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24

3. Present tectonic models for Borneo As these results are discussed, it is worth remember- ing that (1) high sensitivity paleomagnetic measure- For much of the last decade the extrusion tectonic ments are by now relatively routine, so that very few model (Peltzer and Tapponnier, 1988; Tapponnier et rock types are too weakly magnetized to be measured, al., 1982, 1986) has dominated tectonic and paleogeo- (2) paleomagnetism is not good at detecting small graphic discussions of East and Southeast Asia. This changes in latitude, i.e., 1000 km displacement requires model predicts small CW rotations and southeastward very accurate studies, (3) paleomagnetism is good at extrusion of Southeast Asia as a consequence of the detecting tectonic rotations, but it is often not clear collision of India with Eurasia during the Eocene. whether the rotations took place in local shear zones Current paleogeographic and plate tectonic reconstruc- or are plate wide, and (4) to interpret a paleomagneti- tions for the region, therefore, show Borneo rotating cally determined rotation, the age of magnetization, clockwise as a portion of an extruding Sundaland which is not necessarily simply the age of the rock, is block (Briais et al., 1993; Daly et al., 1990, 1991; Lee critical. If the age is known, then the rotation must and Lawver, 1993, 1995; Rangin et al., 1990; have taken place after that time. Tapponnier et al., 1982, 1986). The extrusion hypoth- In the tables and ®gures, the results are presented in esis has been widely accepted, but the relationship of uncorrected in situ coordinates if there is no tectonic Borneo to the extruding Indochina block has rarely correction, as is the case with most intrusives, or if the been questioned. results fail a fold test. They are presented in tectoni- Prior to the extrusion tectonic hypothesis, Haile et cally corrected coordinates if the results pass a fold al. (1977), Haile (1979) and Hamilton (1979) proposed test or regional attitude test. A true fold test is distin- plate wide CCW rotations in Borneo. Several sub- guished from a regional attitude test because only in sequent models followed Hamilton and Haile in treat- the former are the sites known to come from a single ing Borneo and eastern Peninsular Malaysia as a fold, whereas in the latter the timing of the acquisition separate microplate dominated by CCW rotations of the observed structural attitude is not necessarily (Haile et al., 1983; Holcombe, 1977; Richter, 1996; synchronous. Schmidtke et al., 1990). Interestingly, small (10±208) The paleomagnetic measurements were made in a CCW rotations of Borneo have also been shown in variety of laboratories over a period of more than two reconstructions based dominantly upon the extrusion decades so that they are by no means a data set of uni- model (Briais et al., 1993). The diculty, however, of form quality. However, in all cases some indication of understanding the substantial (45±908) CCW rotations the response to AF and thermal demagnetization is indicated by the paleomagnetic data have hampered given and reported here. attempts to merge the two contrasting hypotheses. The statistics of the paleomagnetic data are

Current tectonic reconstructions (Hall, 1996; Richter, described by the con®dence interval a95 and k, the esti- 1996) incorporate a rigid body CCW rotation of all of mate of Fisher's precision parameter k. The former is Borneo, but recognize that this approach leads to di- the cone about the mean direction de®ning the area culties in accommodating the rotation of Peninsular within which there is a 95% probability of ®nding the Malaysia and Borneo: the Thai and Malay basins true mean of the population sampled. The latter is a appear to preclude rotations as late as 30 Ma, while measure of the variance in direction obtained from the the absence of stuctural evidence for a break between formula (N-1)/(N-R), where N is the number of Peninsular Malaysia and Borneo make later rotation samples or sites and R is the resultant vector obtained of Borneo alone dicult. by adding all of the unit vectors. Clearly, if the N unit vectors are perfectly aligned, k is in®nite; if they are random, it is zero. Values of k less than 10 re¯ect 4. Paleomagnetic data from Borneo large scatter and are generally regarded as too badly scattered to be useful. Values of k in excess of 100 for We now review the currently available paleomag- samples at a site, or of sites in a collection, represent netic data from Borneo and surrounding areas. The very tightly grouped distributions. Values much larger quoted results are, for the most part, already pub- than this in sedimentary rocks suggest remagnetization lished. While many of the studies were made before and, in igneous rocks, indicate that dispersion due to the advantages of modern paleomagnetic techniques of secular variation is not being recorded. measurement and analysis were available, several of In more recent studies, demagnetization results have the earlier studies were in lithologies which proved to been analysed using the principal component analysis be excellent paleomagnetic recorders and the results of technique introduced by Kirschvink (1980). With this later studies have been remarkably consistent with the technique the directions of lines of best ®t along the data in the pioneering work of Haile (1977), demagnetization paths are estimated and assigned a McElhinny et al. (1974) and others. measure of collinearityÐthe Maximum Angular M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 7

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.

Deviation or MAD angle. Values of MAD angles of island can be compared. The area consists of the meta- less than 58 are well de®ned lines, but with increases to morphic Sundaland Craton upon which Mesozoic clas- larger than 108, the line is poorly de®ned and larger tic and limestone sediments lie (Fig. 1). In the than 208 the results are not useful. Cenozoic, the Melawi and Ketungau basins developed In describing the paleomagnetic data from Borneo, and a suite of shallow Oligocene-Miocene stocks were we present results in terms of following regions: (1) intruded; these are known as the Sintang intrusives in Northwest Borneo, (2) West KalimantanÐSchwaner Kalimantan. Results are discussed in groups organized Mountains, (3) Central and East KalimantanÐBarito as follows: Oligocene-Miocene shallow intrusives, and Kutai Basins (4) South KalimantanÐMeratus Silantek Formation, Cretaceous intrusives, Jura- Mountains, and (5) Sabah (Fig. 2). For each region, Cretaceous: Bau Limestone, Pedawan and Kedadom we describe the results beginning with the youngest Formations, and Triassic volcanics; the data are listed and working back in time. The results are summarized in Table 1 and shown in Fig. 3. in Fig. 3, 4, 5, 6, 7, 8 and 9 and in Table 1, 2, 3, 4 and 5; the reader should refer to the original publications 5.1. Oligocene±Miocene shallow intrusions for more details. These intrusives are small enough and were intruded at shallow enough depths to have cooled relatively 5. Northwest Borneo rapidly and are excellent paleomagnetic recorders giv- ing in most cases a single stable direction of magneti- This area is predominantly in Sarawak, but includes zation by either AF or thermal demagnetization some of West Kalimantan. It has been studied paleo- (Schmidtke et al., 1990). The results, shown in Fig. 3a magnetically in some detail (Schmidtke et al., 1990) and Table 1, are either unrotated and oriented parallel and a€ords a coherent record which provides a frame- to the present ®eld or to the reversed ®eld direction, or work against which records from elsewhere in the are rotated counterclockwise. As discussed in a later 8 M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24

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.

Fig. 3. Results from Northwest Borneo: (a) Oligocene-Miocene shal- low intrusions show mixed results; (b) Silantek Formation mudstones 5.3. Cretaceous intrusives suggest a weak CCW rotation of about 408 since the Eocene; (c) Mesozoic results suggest a strong CCW rotation of about 908 since Throughout Northwest Borneo there are scattered 80 Ma. Refer to Table 1 for data. Cretaceous intrusives. In Sarawak, samples from these Cretaceous intrusions carried a weak and scattered magnetization and no useful paleomagnetic record was obtained from them. However, in Kalimantan about section, there is a progression of the maximum ro- 20 km northwest of Sanggau, an intrusion and a dyke tation found in rocks of a particular age; however, cutting the intrusion gave results with MAD angles there are also older intrusives which are not rotated. between 28 and 158 after thermal demagnetization (Fuller, unpublished data, 1998); a K/Ar date of 5.2. Silantek Formation 93.223.5 Ma was obtained for the dyke. The mean of 6 samples from the dyke was Dec. 280.88, Inc., 7.28,

The youngest sediments, apart from the recent allu- a95 10.98, k 38.7 (Table 1, Fig. 3c). vium, are the Plateau Sandstones in the Tertiary basins. Below this are the Late Cretaceous and Eocene 5.4. Jura-Cretaceous: Bau Limestone, Pedawan and Kayan Sandstone and its equivalent the Silantek Kedadom formations Formation, a sequence of clastics including redbeds. Results from the Silantek Formation are shown in These formations overlap in age and grade laterally Fig. 3b and Table 1; all are from red mudstones with into each other, respresenting a period of basin devel- the exception of one site, which was a grey ¯uvial opment. The Kedadom is the nearshore equivalent of mudstone. The most e€ective demagnetization method the Bau Limestone; both grade upward into the domi- was thermal, which either isolated a single direction nant basin-®lling clastic Pedawan Formation. The carried by both magnetite and hematite, or yielded basin was inverted and the Kayan Sandstone of Late two directions. In samples for which two directions Cretaceous±Early Eocene age was deposited uncon- were isolated, the direction blocked between 4508 and formably upon it. The most extensively studied for- 6308C was similar to that isolated as a single direction mation is the Bau Limestone from which 11 of 21 sites in other samples, whereas the direction blocked yielded interpretable paleomagnetic data. Thermal between 1508 and 4508C was the less strongly rotated demagnetization was again the most e€ective treat- direction. The more strongly rotated direction is ment. Sometimes, a low blocked phase was demagne- shown in Fig. 3b, as is one site at which directions par- tized between 508 and 1008C and then the vector allel to the reversed present ®eld direction were found. stabilized between 2508 and 4508C, while in other The bedding dips in the Silantek are weak, so that no samples there was no low blocking fraction. The convincing fold or regional attitude test is possible. resulting data fell into three groups, which were inti- M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 9

Fig. 5. Results from Central and Eastern Kalimantan: (a) Plateau Basalts of Pliocene±Pleistocene age show no rotation; (b) Oligocene± Fig. 6. Results from South Kalimantan±Meratus Mountains: (a) Miocene igneous rocks show mixed results; (c) Eocene sediments Miocene microdiorite suggests weak rotation; (b) Tanjung Fm. sand- suggest a weak rotation; (d) Jurassic sediments show a strong ro- stones suggest weak rotation; Kuaro Formation basalts passed a fold tation. Refer to Table 3 for data. test; (c) Cretaceous ultrabasic rocks suggest weak rotation. Refer to Table 4 for data. mately mixed spatially. Group 1 exhibited present ®eld or reversed ®eld directions. Group 2 consists of ®ve 5.6. Summary of results from Northwest Borneo sites in the Bau Limestone and two in the Kedadom formation, which show an average de¯ection of about The paleomagnetic record from the Northwest 308 CCW; these results failed a fold test and are there- Borneo domain is relatively straightforward. Intrusions fore secondary and post Late Cretaceous. Group 3 is are increasingly CCW rotated with age, reaching a represented by two sites on a syncline near Bunuk and maximum of 518 in an intrusion dated by K/Ar to be passed a fold test; two sites in the Pedawan also passed 25.821.9 Ma. The Late Cretaceous±Eocene Silantek a fold test. A third site yielded results that, after tec- Formation gives 418 of CCW rotation. The eight tonic correction, were antipodal to the direction from Mesozoic results which come from the Bau Limestone, the fold test sites. These ®ve sites de®ne a CCW ro- the Kedadom and Pedawan Formations in Sarawak tation of some 908 and are illustrated in Fig. 3c and and from the Jura-Cretaceous and Triassic sites in listed in Table 1. Kalimantan, all indicate a strong rotation of about 908 From Kalimantan, a declination of 267.18 and incli- nation of 2.68 with a95 13.48, k 21 was reported from Jurassic sediments (Sunata and Wahyono, 1991), which were analysed both with AF and thermal demagnetization.

5.5. Triassic volcanics

In Sarawak, the Triassic Serian Volcanics are always found to be metamorphosed, or the ®eld relations are suciently unclear that they are not useful for paleo- magnetism. In contrast, in Kalimantan their equiva- lents are less deformed and have yielded paleomagnetic data (Sunata and Wahyono, 1991). Two results are from Suti Semarang and Gunung Bawan, which yielded almost 908 of CCW rotation after AF and Fig. 7. Results from Sabah. Refer to Table 5 for data. Eo- thermal demagnetization (Fig. 3c, Table 1). Mio = Eocene-Miocene. 10 M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24

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).

which took place sometime after 93 Ma, the age of the 6. West Kalimantan: Schwaner Mountains Cretaceous dyke. Among these results there are unro- tated directions in the sediments and the intrusives. At The Schwaner Mountains were studied by Haile et least in the Bau Limestone and the Silantek al. (1977) in their pioneering paleomagnetic and geo- Formation, it is very likely that they are secondary chronological study. They obtained sixteen K/Ar ages, magnetizations and do not constrain the rotations at which ranged from 75 to 112 Ma, mainly from the the time of formation of the rocks. In the intrusives, it granitic plutonic rocks, and were able to get satisfac- is less clear that they are necessarily secondary direc- tory paleomagnetic results from 39 of 48 hand samples tions and further work is being undertaken to test this collected from dykes, tu€s, lavas and intrusives. The suggestion. overall mean of the paleomagnetic results they M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 11

Fig. 9. Plot of rotation versus age for sites with absolute age determinations (see Tables 1±5). obtained was similar to those obtained earlier from period of widespread extension and basin formation. Peninsular Malaysia and they concluded that the two Paleomagnetic results have been reported by Lumadyo regions have behaved as a unit since the main intrusive et al. (1993), Moss et al. (1997) and Sunata and event at about 80±85 Ma. Wahyono (1991). We present the results in a slightly di€erent manner. We divide them into three groups: unrotated, weakly 7.1. Plio-Pleistocene rotated and strongly rotated. The strongly rotated group is de®ned as having rotations greater or equal The study by Lumadyo et al. (1993) of Plio- to the smallest rotation seen in the strongly CCW Pleistocene sites in the Plateau ¯ows from the Kelian, rotated group from the Northwest Borneo domain. Magerang and Bigung localities in East Kalimantan The latter two groups are shown in Fig. 4 and are reported that all sites carried a reverse polarity magne- listed in Table 2. tisation indistinguishable from the reversed present The di€erence in interpretation of these results ®eld. The magnetic recorders are shown to be ®ne between the original authors and ourselves turns on magnetite. No tectonic correction is applied (Fig. 5a, the question whether the two datasets are sampling the Table 3). same population or two separate populations. Given the present data set, it is probably not wise to press the case too strongly. Rather the suggestion should be 7.2. Oligo-Miocene tested with a new collection using modern techniques. Nevertheless, there are directions comparable with the Lumadyo et al. (1993) reported unrotated declina- strongly and weakly rotated directions seen in the tions in seven Late Oligocene±Early Miocene igneous Northwest Borneo domain. rock sites (Dec. 178.88, Inc. 4.78, a95 13.48, k 21). The behaviour of the magnetic recorders was good; 6.1. Summary of results from West Kalimantan magnetisation was interpreted as primary because of a positive fold test. The age of the folding is not known, In the Schwaner Mountains results, we see a similar but probably was Early to Middle Miocene and coinci- pattern of directions to that seen in the Northwest dent with a series of structural inversions in the Kutai Borneo domain with unrotated, weakly and strongly Basin where the study sites were located (Chambers rotated directions all represented. However, the results and Daley, 1995; Cloke et al., 1999). are based on a collection of only 48 hand samples, so An unrotated result (Dec. 183.88, Inc 4.68, a95 5.28) that additional studies are needed to substantiate these was also reported by Sunata and Wahyono (1991) results. 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. The reported paleomagnetic information on the magnetiza- 7. Central and Eastern Kalimantan tions suggests that the remanence is simple and yields statistically consistent data. No fold test was reported. This region includes the Barito and Kutai basins, A reversed weakly de¯ected result (Ali, unpublished which were formed during the Middle Eocene in a data) comes from a diorite intrusion in the Long 12

Table 1 Paleomagnetic data from Northwest Borneo. References: 1=Schmidtke et al. (1990); 2 = Mike Fuller (unpublished data); 3=Sunata and Wahyono (1991). See Fig. 3 for plots of data

Shallow intrusions, Oligocene±Miocene Area Locality Latitude Longitude Rock type Age N (sites) n (samples Dec (8) Inc (8) a95 (8) k Ref.

No rotation Sarawak JKR Quarry-Airport 1.428N 110.338E Microtonalite Oligo-Mio 1 6 2.7 2.8 5.8 176 1 Sarawak Kuching Quarry 1.388N 110.308E Tonalite Oligo-Mio 1 8 358.9 2.1 7.2 114 1 Sarawak Bau-Lundu Rd. 1.428N 110.158E Diorite Porphyry Oligo-Mio 1 6 358.1 23.5 2.4 787 1 Sarawak Jalan Rock 1.508N 110.328E Diorite Porphyry Oligo-Mio 1 6 185.4 15.8 14 24 1

Weak rotation 3±24 (1999) 17 Sciences Earth Asian of Journal / al. et Fuller M. Kalimantan Bukit Gembah 0.108N 111.808E Dacite 16.420.6 m.y. 1 10 333.9 16.1 7.8 39 2 Sarawak Siburan 1.368N 110.388E Microtonalite 17.221.9 m.y. 1 8 338.1 15.3 7.8 97 1 Kalimantan B. Bunga Sinik 0.308N 111.78E Dacite 18.020.7 m.y. 1 7 164.6 22.1 4 224 2 Sarawak Gunung Serapi 1.558N 110.258E Granodiorite 25.821.9 m.y. 1 7 128.2 0.2 3.7 269 1 Sarawak Bukit Stabar 1.428N 110.338E Microtonalite Oligo-Mio 1 6 352.0 4 4.4 136 1 Sarawak Mr. Choo's Garden, Kuching 1.398N 110.318E ? Oligo-Mio 1 5 154 19 13.7 32 1 Sarawak Hua Sun Quarry 1.368N 110.388E Microtonalite Oligo-Mio 1 10 168.5 21.8 7.4 43 1 Sarawak Semengo Quarry 1.398N 110.318E Microtonalite Oligo-Mio 1 8 170.7 11.8 3.6 446 1 Silantek Formation No rotation Sarawak 1 km no. Simunjam Jcn. 1.108N 110.858E Red mdst. Eocene 1 4 193.3 30.6 6.1 229 1 Moderate rotation Sarawak 2 km no. Simunjam Jcn. 1.108N 110.858E Red mdst. Eocene 1 6 134.9 27.0 11.8 33.4 1 Sarawak 2 km e. Simunjam Jcn. 1.088N 110.908E Red mdst. Eocene 1 7 321.3 15.1 7 76.2 1 Sarawak Batang Undup 1.108N 111.508E Fluvial mdst. Eocene 1 9 319.6 30.8 9.6 29.9 1 Mesozoic results Cretaceous intrusives Kalimantan Dusun Bunut Quarry 0.308N 110.38E Dyke 93.223.5 m.y. 1 6 280.8 7.2 10.9 38.7 2 Bau Limestone, Pedawan and Kedadom Fms: Group 1Ðno rotation Sarawak Bau district 1.48N 110.28E Bau Limestone J±K 4 31 358.6 12.7 14.6 3.9 1 Bau Limestone, Pedawan and Kedadom Fms: Group 2Ðweak rotation Sarawak Bau district 1.48N 110.28N Bau Limestone J±K 5 36 301.6 11.5 14.1 30.4 1 Sarawak Serian-Tebedu Rd. (Ked.) 1.138N 110.448E Marly Limestone J±K 3 21 311.1 2.1 22.7 2.9 1 Bau Limestone, Pedawan and Kedadom Fms: Group 3Ðstrong rotation Sarawak Bunuk fold, Penrissen area 1.48N 110.28E Bau Limestone J±K 2 17 272.0 8.6 5.3 45.7 1 Sarawak Bau-Lundu Rd. (Ped. fold) 1.58N 109.98E Mudstone (Ped.) J±K 2 15 87.9 16.0 7.1 14.5 1 Sarawak Pedawan 1.188N 110.288E Limestone lens J±K 1 8 271.9 15.9 13.3 18.4 1 Kalimantan Tenguwe 0.708N 110.18E Black mudstone Jurassic 11 40 267.1 2.6 6.9 Ð 1,3 Kalimantan Suti Semarang 0.98N 109.88E Blackshale Triassic 1 21 278.5 20.8 11.8 Ð 3 Kalimantan Gunung Bawan 1.28N 109.68E Basalt & shale Triassic 1 30 286.7 31.7 11.5 Ð 3 M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 13

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 18.0 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 25.0 Ð Ð 1 Mean direction for W. Kalimantan±Schwaner Mtns. Cretaceous 1 5 280.1 5.6 24.5 10.7

Bagum region north of the Kiham Haloq gorge on the a hornblende andesite, a soft overprint was completly upper Mahakam River in central Kalimantan (Dec. removed by 1508C. The single direction blocked above

1648, Inc 38, a95 8.68, k 61). Similar high-level intru- this temperature gave a site mean in situ direction of sions in the vicinity have been dated as 18 Ma old Dec. 143.68, Inc., 45.48 with an a95 of 11.68 and k 34. using K±Ar techniques (Pieters et al., 1993). Reversed A biotite microgranite about 30 km upstream was also results are also recorded from lava ¯ows along the well behaved with a soft overprint removed by 15 mT, Malnyu river (northern edge of the Kutai basin; see leaving a single component. With one outlier dis- Moss et al., 1998) in which three sites gave a Dec. carded, the in situ mean was Dec. 342.68 and Inc.

156.98, Inc. 228, a95 13.18 and k 89) after tectonic cor- 42.48 with a95 9.08 and k 73 (Fig. 5b, Table 3). rection (Ali, unpublished data). These lavas have K± Ar ages of 23.6±21.3 Ma (Moss et al., 1998). 7.3. Eocene Along the Telen River on the northeastern edge of the Kutai Basin, two sites in minor intrusives yielded The University of London SE Asia Research Group reliable data. These samples were collected by the members collected dark grey ®ne sandstones of London University SE Asia Research group and Eocene±Oligocene age which were analysed at UCSB measured in the UCSB laboratory by C. Anderson. In by C. Anderson. They showed multicomponent beha-

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 8.8 14.2 30 1 Magerang 0.308N 115.408E Basalt Plio±Pleist 5 33 186.0 31.3 5.3 213 1 Bigung 0.208N 115.178E Basalt Plio±Pleist 6 46 182.0 3.1 9 56 1 Igneous rocks, Oligo±Miocene Mixed results Nakan 0.258N 115.258E Basalt, Andesite Oligo-Mio 7 38 178.8 4.7 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 3.0 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 35 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 33.1 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 18.6 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 14 M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24

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 6 7.4 50.8 2 Muru (Kuaro Fm.) 1.88S 116.08E Basalt Eocene 1 5 188.0 3.8 11.8 43.0 3 Tebruk (Kuaro Fm.) 1.88S 116.08E Basalt Eocene 1 5 1.0 16.8 12.3 39.4 3 Ultrabasic ophiolite, Cretaceous Weak rotation Batulicin 3.48S 115.98E Ultrabasic Cretaceous 1 5 321.0 13.0 14.4 18.9 2 viour, but a single direction was isolated with the Eocene Tanjung Formation and from the ophiolite Kirschvink's method (Kirschvink, 1980) giving a mean are given by Sunata and Wahyono (1998). Lumadyo et in situ Dec. 333.88 and Inc. 18.68 with a95 15.58 and al. (1993) reported results from two sites in basaltic k 36. When corrected for tilt, the direction becomes ¯ows.

Dec. 345.28, Inc. 18.68, a95: 14.7, k: 40.2. Sunata and Wahyono (1991) reported results from 8.1. Miocene Eocene sandstones and siltstones of the Kalasin region from the tributaries of the Murung River. Thirty-eight A microdiorite located north of Gunung Kukusan samples were collected from which, after AF demagne- in the Batulicin area with a K/Ar age of 19.620.76 tization, 27 gave a mean Dec. 322.88 and Inc. 0.68, Ma was sampled for paleomagnetism by Sunata and with a95 13.98. Permanadewi (1998). AF demagnetization revealed essentially single component magnetization with ®ve

7.4. Summary of results from Central and Eastern samples yielding Dec. 3238 and Inc. 18,witha95 7.78 Kalimantan and k 65.9 (Fig. 6a, Table 4).

The results from this area are the most puzzling 8.2. Eocene from Borneo. As in other regions, we see CCW rotated results of some 308 but there are also unrotated direc- Sunata and Wahyono (1998) also reported results tions in rocks of the same age. Among these are direc- from the Late Eocene Tanjung Formation. The for- tione which pass a fold test reported by Lumadyo et mation is widely distributed and passes up from a al. (1993). basal conglomerate into a sandstone. Seven of 14 sites studied yielded reliable data after AF demagnetization

with a mean Dec. 3448 and Inc. 68, a95 7.48 and k 8. South Kalimantan: Meratus Mountains 50.8. Results from two sites from the Meratus region in This important region is undersampled paleomagne- South Kalimantan were reported by Lumadyo et al. tically. Results from a granodiorite have been reported (1993). They were in basaltic ¯ows of the Kuaro by Sunata and Permanadewi (1998) and results from Formation in the northern part of the Pasir Basin; the

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 9.1 27.4 7 2 M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 15

Fig. 10. Summary of rotations for Palawan. For data, see Almasco (1996) and Almasco et al. (1999). dating of the ¯ows is based on biostratigraphy. Only 18.9 (Fig. 6c, Table 4). Further work is underway with two sites are reported and, although a fold test was additional sites in the ophiolite. positive, the result had poor statistics (Dec. 184.68, Inc

6.58, a95 49.28). These results are presented after tec- 8.4. Summary of results from South Kalimantan tonic correction (Fig. 6b, Table 4). This area is clearly undersampled, but the limited 8.3. Mesozoic results seem to show a similar pattern to Central and Eastern Kalimantan with rotated and unrotated direc- Sunata and Wahyono (1998) reported one site from tions mixed. The weak rotation is seen in all three the ultrabasic of the ophiolite. Five samples gave a site units, which Sunata and Wahyono (1991) report, i.e., mean Dec. 3218 and Inc. 138, with a95 14.48, and k the Miocene microdiorite, the Eocene Tanjung 16 M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24

Formation and the Cretaceous ultrabasic rocks; there contrast, the spilites fail a regional attitude test giving is no report of the strong rotation from this region. badly scattered results after correction. The before-cor- Since there is a range in age from Mesozoic to rection value, after elimination of 2 outliers is Dec.

Miocene in these units, there has either been remagne- 313.38 and Inc. 9.18, with a95 27.48 and k 7 (Fig. 7, tization of the older units in the direction of the intru- Table 5). sion, or there was no rotation here between the Mesozoic and the Miocene. The two sites reported by Lumadyo et al. (1993) from Eocene Kuaro Fm. basalts 9.3. Summary of results from Sabah are unrotated, so that they are inconsistent with the results of Sunata and Wahyono (1998). 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 suggest that the magnetization carried by the intrusion at the 9. Sabah Kapa Quarry is younger than most of the rotation of the region. The Crocker results are too few and too In Northern Sabah, the NE/SW strike of the scattered to provide useful data yet, but additional Crocker Belt, which is consistent for several hundred results from the red mudstone facies could prove im- km into Sarawak, turns almost E/W (Tongkul, 1990). portant, if they are the youngest units showing the This presents the opportunity to test paleomagnetically strongly rotated direction. The spread in directions of whether the con®guration is brought about by defor- the spilite magnetizations, from the strongly rotated mation of an earlier continuously NE/SW striking fea- direction towards the present ®eld, suggests remagneti- ture. To the east of the Kinabalu intrusion, there are zation. There are insucient results from north of the outcrops of the Cretaceous chert-spilite unit of the region, in which the structure swings from NE/SW to Telupid ophiolite, but it is badly dismembered so that E/W, for interpretation. structural continuity is hard to establish. In the north the chert-spilite unit is again seen. Although a number of paleomagnetic results are available for the region, 10. Discussion of paleomagnetic results from Borneo their interpretation is still far from clear. In Fig. 8, the Borneo results are presented in the form of maps showing the rotations for the various 9.1. Eo-Miocene sites. The ®gure is in three parts: Fig. 8a shows the results from the youngest sites with ages less than 10 The youngest site sampled is at the Kapa Quarry in Ma, represented by the Plio±Pleistocene Plateau a small satellite stock of the Mount Kinabalu intru- basalts. Fig. 8b presents all Tertiary sites older than 10 sion; the K/Ar age is 15.025.3 Ma. The rock is an Ma. Finally, in Fig. 8c the results from Mesozoic sites excellent paleomagnetic recorder and gives Dec. 348.98 are shown. From this ®gure it is clear that the rotated and Inc. 18 with a 2.48 and k 422 (Fig. 7, Table 5). 95 and unrotated directions are intimately mixed Much of the Eocene±Miocene Crocker Formation spatially. consists of turbidites, including relatively coarse sand- In Fig. 9, the results from igneous rocks are shown stones which give a present ®eld directions with some in the form of a plot of rotation versus age for sites at minor CW rotation (Fuller, unpublished data). Among which we have absolute age determinations and the the Crocker sandstones are red mudstones which are data are not interpreted as remagnetizations. The excellent recorders; these give the strongly rotated Cenozoic sites which show directions not seen in CCW de¯ected Dec. 277.58 and Inc. 2.88,witha 3.78 95 younger rocks exhibit a trend which gives a rate of ro- and k 227, after tectonic correction (Fig. 7, Table 5). tation of approximately 28 per million years. However, Before tectonic correction the result is Dec. 309.38 and as we have noted elsewhere, there are other well-dated Inc 49.78, with a 3.48 and k 311. Work is underway 95 Cenozoic sites of this age which show no rotation. with further collections from the mudstones. Among the older dated material which is primarily Cretaceous, as well as the strongly rotated directions 9.2. Cretaceous chert-spilite unit shown here, there are unrotated and weakly rotated directions. The strong rotation is carried by an intru- Collections were made from the chert-spilite unit in sive and a cross-cutting dyke of Cretaceous age from the vicinity of Telupid in central North Sabah. The Northwest Kalimantan; the dyke has been dated by K/ chert gave a strongly rotated direction after tectonic Ar as 93.223.5 Ma (Table 1). The second Mesozoic correction of Dec. 2818, Inc 08 with a95:68, and k: 66. result comes from metatu€s (Table 2) in the contact Tectonic correction has little e€ect on this direction. In zone of adamellite dated at 86.222.0 Ma (Haile et al., M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 17

1977). The weak rotation is carried by the youngest 12. Paleomagnetic data from regions surrounding granites and associated dykes (see Tables 1±5). The Borneo occurrence of rotated and unrotated directions is, therefore, intimately mixed in age as well as in locality. If the paleomagnetic results from Borneo are to be Nevertheless, there is a clear progression of the maxi- interpreted in terms of CCW rotation, then it is im- mum rotations shown by rocks of each particular age. portant to see how this behaviour ®ts into local paleo- Although not shown in Fig. 9, there are also results geography and in particular how consistent it is with from sedimentary sites which show a similar pattern of the paleomagnetic history of nearby regions. unrotated, weakly and strongly rotated. Using Fig. 9 Unfortunately, the paleomagnetism of nearby regions as a key to direction as a function of age we can, in is incomplete, but important relevant results are now principle, interpret possible ages of magnetization for discussed. The most immediately relevant region is the sediments. At present this is only useful for age ranges western part of the south arm of Sulawesi, which is within which we have good control, i.e., from about separated from Borneo only by the Makassar Straits 10 to 30 Ma. For example, the unrotated sites in the (Fig. 1). From this region a number of studies have Silantek Formation, Bau Limestone, and Pedawan and been reported and are discussed below. To the north- Kedadom formations must have been remagnetized east lies Palawan and again there are paleomagnetic after about 10 Ma. Similarly, the Silantek Formation results from that island. While work is underway in rotated direction and the weakly rotated direction in Java and Sumatra, the results are presently not su- the Bau Limestone and Kedadom and Pedawan for- ciently advanced to warrant inclusion. From mations must have been remagnetized prior to 30 Ma Peninsular Malaysia there is a considerable paleomag- but after about 80 Ma. netic database, although the absence of a Tertiary sec- tion makes interpretation dicult. We now discuss the results from Sulawesi, Palawan, and Peninsular Malaysia and give a brief overview of work underway in Sumatra and Java. 11. Summary of discussion of paleomagnetic results from Borneo 12.1. Sulawesi The Sintang results and other data for sites younger than 30 Ma de®ne a CCW rotation trend with a rate Western South Sulawesi and Borneo have been e€ec- of approximately 28 per million years. The Mesozoic tively attached throughout the Cenozoic (Fig. 1B). strongly-rotated results are also shown, and while they Although extension between the islands took place are insucient to de®ne a trend, it is clear that the ro- during the middle Cenozoic (Situmorang, 1982), exten- tation rate of 28/Ma was not maintained throughout sive oceanic lithosphere was not generated along the the Cenozoic. In the face of this evidence of systematic Makassar Straits and constraining the relative motion trends in the observed rotations, we provisionally in- between these two crustal fragments is relatively terpret the data to indicate a rotation of the straightforward. The opening of the Makassar Straits Sundaland core of Borneo, which can be divided into reduces CCW rotations in Sulawesi compared with a strong rotation shown by rocks older than 80 Ma, their equivalents in Borneo for data older than the and a weaker rotation seen in rocks between 30 and opening event. However, the amount is within the about 10 Ma. As a working hypothesis, we propose errors of presently available results for sites antedating that all directions in rocks that are also seen in the opening of the Makassar Straits , i.e., Mesozoic or younger rocks are secondary magnetizations and do early Cenozoic data. not carry information about the rotation of Borneo at There have been several paleomagnetic studies car- the time of formation of the rock. They may, however, ried out on SW Sulawesi (Ali, unpublished data; Haile, record particular important periods of remagnetization 1978; Mubroto, 1988; Mubroto et al., 1994, and and reveal the rotation history for that time. For Sasajima et al., 1980). In this review, we restrict our example, it appears that there may have been an im- analysis to those sites from the western side of the portant period of remagnetization during a reversed South Arm (Fig. 1B and 2); this avoids the problem of chron in the Plio±Pleistocene, coincident with the local tectonic complexities associated with the collision extrusion of the Plateau Basalts (Table 3), which of the allochthonous terranes to the east. Ali (unpub- would account for the common reversed present ®eld lished study) has identi®ed nine paleomagnetic sites direction seen in many sites of diverse age. Another from the late Miocene Camba Formation of South period of remagnetization appears to have taken place Sulawesi which have yielded meaningful directional after about 80 and before 30 Ma because the weak ro- data. The sites are from mainly igneous rocks (thin tation is seen in many Mesozoic rocks. dykes, lava ¯ows and diorite bodies) but also include 18 M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 siltstones and claystones (van Leeuwen, 1981). 12.3. Palawan Demagnetisation experiments suggest that their magne- tisation is primary. Combined with data from eight The elongate island of Palawan lies to the north east previously reported sites from Haile (1978) and of Borneo between the South China Sea and Sulu Mubroto (1988) the new in situ formation mean direc- Seas. It is divided into the Northern and Southern tion is D 171.68 and I 12.28 with a95 5.88 and k 38.3; Palawan blocks. The former is dominated by a correction for tilt results in a mean direction of D Mesozoic section, whereas the latter is dominated by 171.38 and I 10.28 with a95 5.38 and k 46.8. The tilt ophiolites. Paleomagnetic studies are reported by corrected data indicate a small (098) CCW rotation Almasco (1996) and Almasco et al. (1999). Fig. 10 and negligible translation of the region since 010 Ma. shows that the directions in the North Palawan Block These results are consistent with those from Borneo. are predominantly CW rotated, whereas those from Preliminary unpublished work by Ali on the Upper the South are predominantly CCW rotated. There is a Cretaceous Balang Baru Formation from three sec- tions in the Doidoi region of SW Sulawesi gave Dec. region at the northern margin of the ophiolites where there is large between-site scatter, probably re¯ecting 341.78, Inc. 21.58, a95 6.58, and k 73, with marginally better in situ statistics. This direction is similar to the local deformation. direction recorded in the younger rocks on Sulawesi. The results from the Jurassic Busuanga cherts and Similar results were obtained by Haile (1978), who the Cretaceous Guinlo Formation on the island of obtained Dec. 3258 and Inc. 58, with a95 7.28 and k Busuanga in the North Palawan Block and from the 60, from brown radiolarian cherts from the Paring Guinlo Formation on the main island of Palawan River in the Dera Valley. In this same paper, two failed a regional attitude test and gave indistinguish- strongly rotated results are reported, which are similar able directions. When combined, the 11 sites yield a to those seen in Kalimantan and Sarawak. One came declination of 50.98 and inclination of 45.38,withak from a dyke which gave Dec. 1058 and Inc. 118,with of 26 and an a95 of 9.18, giving a VGP of 183.48Eand a95 4.98 and k 624; the other was from grey calcareous 39.38N and a paleolatitude of 26.8824.68. This VGP siltstone from the same valley with NRM directions of is similar to those found in regions of pervasive Dec. 1218 and Inc. 328,witha95 14.98 and k 38.6. Cretaceous remagnetization in South China. While these last two results suggest a strongly rotated Results from the Cretaceous Espina Basalts of the Mesozoic direction similar to those seen in Borneo, it Calatuigas Ophiolite in the Southern Palawan Block is clear that more data are badly needed. 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

12.2. Celebes and Sulu Seas 5.88,withak of 37.7 and an a95 of 12.68, which gives a paleolatitude of 38N268. The declination is similar Shibuya et al. (1991) have reported a period of to that reported from the Celebes Sea by Shibuya et CCW rotation in late Eocene and early Oligocene time al. (1991). for the Celebes Sea. This result was obtained with Results from the North Palawan Block are consist- orientation of core using the soft magnetization, which ent with its proposed South China origin, from whence was interpreted to be in the present ®eld direction. The it has moved southward and rotated clockwise. The method (Fuller, 1969) is not always reliable, but has Espina Basalts of the South Palawan Block have been checked elsewhere against the formational micro- moved northward and experienced counterclockwise scanner and was found to give similar results rotation. Their origin is less clear, but suggest a (Cisowski et al., 1990). In contrast, samples from the Cretaceous spreading center to the south, possibly re- top 220 m at Site 769 on the southern ¯ank of the lated to that at which the Sabah Ophiolite was formed. Cagayan Ridge in the Sulu Sea showed no evidence of While these paleomagnetic results require di€erent tec- CCW rotation from about 8.9 Ma (Hsu et al., 1991). The latter result is consistent with results from Borneo, tonic histories for those parts of the North and South which suggest that the weak rotation was almost com- Palawan blocks studied, both the North and South pleted by this time. The result from the Celebes Sea is Palawan block basements probably have a common inconsistent with the Borneo results, in that the timing origin, having been moved southward by South China is much earlier than the 30 to 10 Ma seen in Borneo. Sea spreading. Subsequently ophiolites were thrust Thus, the CCW rotated regions in Borneo have a over the Southern Block. The ophiolites in Palawan di€erent rotation history from the Celebes Sea during have an age similar to those of Sabah to the south in this time. northernmost Borneo. M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 19

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).

12.4. Peninsular Malaysia is con®ned to a single coal basin at Batu Arang near Kuala Lumpur (Fig. 11) and scattered minor outcrops Peninsular Malaysia is dominated by the Main of coarse sediments. Range granites of predominantly Late Triassic age The published paleomagnetic database for (Hutchison, 1989). They are deep seated in the east Peninsular Malaysia was reviewed by Haile and Briden and become more epizonal to the west. In contrast, the (1982). It is primarily the pioneering work of Neville Eastern Belt is an epizonal calc-alkaline series of Late Haile and others (Haile, 1981; Haile et al., 1983; Haile Permian to Late Triassic age. Between the two ranges and Khoo, 1980; McElhinny et al., 1974). CCW decli- lies the Central Basin separated from the Main Range nations were found with moderate inclinations (3308, in the west by the Raub±Bentong suture (Fig. 11). 408) in the Cretaceous Kuantan dykes and Segamat Spectacular uplift associated with the Indosinian basalts (Haile et al., 1983; McElhinny et al., 1974). Orogeny produced extensive intermontane basins ®lled CCW rotated declinations with moderate inclinations with the Jurassic±Cretaceous redbed sequences, which were also found in Jurassic±Cretaceous redbeds near have yielded excellent paleomagnetic results. The other Maran and Kluang (Haile and Khoo, 1980). For com- principal sources of late Mesozoic paleomagnetic data parison, the present latitude spread of Peninsular have been the Kuantan dykes and the Segamat basalts. Malaysia gives shallow inclinations from 28 to 128. The former were intruded into the Permo-Triassic Additional results have been described by Richter and intrusives of the Eastern Belt centered around Fuller (1996) and in general there is excellent agree- Kuantan. Towards the southern end of the Central ment between the directions listed above and the new Basin lie the Segamat basalts of probable Late data. Fig. 11 gives the results from Jurassic± Cretaceous age. The Cenozoic section in the peninsula Cretaceous redbeds, from the Cretaceous Kuantan 20 M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 dykes and the Segamat basalts. The youngest rocks thrust over that basement have the CCW clockwise ro- that have been studied give CCW rotations. tations similar to the weak rotation of Borneo and the Unfortunately the absence of a signi®cant Tertiary sec- CCW rotation reported in the Celebes Sea (Shibuya et tion means that the age of these rotations cannot be al., 1991). constrained and we have no information about The comparison between Borneo and Peninsular Tertiary rotation history. There are, however, coal Malaysia is hampered by the absence of a signi®cant basins such as Batu Arang near Kuala Lumpur Tertiary section in the peninsula. However, the (Fig. 11), which give the promise of some Tertiary Jurassic±Cretaceous redbeds, the Kuantan dykes and results. These rocks are very weakly magnetized which the Segamat basalts all show CCW rotations similar to hampered earlier studies, but they are being studied at that seen in Borneo in the Sintang intrusives. The pro- present. blem in interpreting these results is that we have no in- dication of the age of the rotation. It could have taken 12.5. Java and Sumatra place at the same time as the weak rotation in Borneo, or it could have been much older. Clearly Peninsular Despite a number of paleomagnetic studies in Java Malaysia cannot have rotated coherently with Borneo and Sumatra, it is still too early to use these results to throughout the Cenozoic because it shows only the help interpret the results from Borneo. In both areas weaker rotation seen in the 30 Ma aged units in work is underway and older studies are available. In Borneo. Java, paleomagnetic results have been reported by Hirooka et al. (1980) and Mah® (1984). Hirooka et al. (1980) reported results from Bayat which were all of 13. Discussion normal polarity and close to the present ®eld, but these were NRM directions before demagnetization. Despite obvious shortcomings, there is now a sub- From West Java, they reported results after AF stantial paleomagnetic data base from Borneo and the demagnetization, which were widely dispersed. Mah® surrounding regions. The chief diculty in interpreting (1984) reports results from Bayat, Kalissongo and these paleomagnetic data and particularly those from Karang Sambung, where windows through the blan- Borneo, is that rotated and unrotated sites are inti- keting ¯ows which cover the island, permit access to mately mixed in age and in location. The fundamental older sections. There is again a mixture of rotated and di€erence in resulting tectonic models and paleogeo- unrotated sites but no sytematic pattern of rotation graphic reconstructions lies in the interpretation of the with age has emerged comparable with that seen in unrotated sites. One view assumes that these are faith- Borneo. The dominant results from Sumatra give CW ful records of the ®eld at the time of origin of the rock rotations (Haile and Briden, 1982) but the situation and that, because there are unrotated directions as remains unclear. The results from Sumatra are poten- well as CCW rotated directions, the island of Borneo tially particularly important because there are plentiful cannot have rotated as a single block. Rather the Tertiary intrusives which will aid in the interpretation island must have been predominantly ®xed with the of the results from Peninsular Malaysia. rotated sites recording local rotations in shear zones, or are anomalous or spurious directions. A second in- 12.6. Summary of paleomagnetic data from regions terpretation assumes that the unrotated directions are surrounding Borneo secondary magnetizations and therefore do not con- strain rotations since the time of formation of the Results from the Late Miocene of Western Sulawesi rock. In this view, the CCW rotational history of the are suciently well determined and geologically con- island is recorded by only those samples and sites strained to provide a useful comparison with the which are not remagnetized, i.e. those that show the results from Borneo itself and the results are consistent maximum rotation for each particular age. in the two regions, showing the weak rotation. This The chief diculty faced by the suggestion that same direction is seen in some of the Mesozoic rocks, Borneo has remained ®xed with respect to stable whereas the more strongly rotated direction is seen in Eurasia and indeed with respect to the Geomagnetic a dyke and in Jurassic±Cretaceous siltstones. Axial Dipole from the Mesozoic (e.g., Lumadyo et al., The results from North Palawan clearly represent a 1993; Lee and Lawver, 1993, 1995) is the internal con- very di€erent geological history from Borneo. The sistency of the paleomagnetic results. If the island has North Palawan block has a paleomagnetic history con- not rotated, why are similar rotations seen over such a sistent with an origin associated with South China and wide region? For example, the strongly rotated carries the typical CW rotations of that region. As Mesozoic results are seen in the Jurassic of Sarawak, noted above the South Palawan basement probably the Cretaceous±Jurassic intrusions of the Schwaner has a similar history, but the ophiolites which were Mountains, sediments in Central Kalimantan, and in M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 21 preliminary results from the chert-spilite unit in north- in Borneo. The structure of the island does de®ne a ern Sabah. It seems very unlikely that similar rotations series of roughly arcuate zones, which might be inter- would be seen if the inferred rotations come from preted as rotating slivers. While it is not clear how one simply anomalous or spurious paleomagnetic results. It could refute such an explanation with paleomagnetic seems equally unlikely that the observed rotation only data alone, from a broader geological viewpoint the took place in localized shear zones because, given the model is open to criticism. First, there is no evidence intimate mixing of the locations of rotated and unro- of the necessary faults to accommodate such move- tated sites, it is hard to see how zones of distributed ment, and second there are features which appear to shear could thread their way through the island a€ect- crosscut the arcuate boundaries, thereby precluding ing rotated sites and avoiding the unrotated. the proposed movement. Still another model which The intimate mixing throughout Borneo of rotated could be used to interpret CCW rotations across and unrotated results is easier to explain in terms of Borneo is that of Wood (1985) who considered that remagnetization than by a special distribution of local the dominant tectonic features were a series of NW/SE shears. This suggests that there was pervasive remagne- shears which cut across Borneo. These could then tization in Borneo through which a paleomagnetic form a region of left lateral shears which would gener- record has partially survived. We take as a working ate CCW rotations across Borneo. However, the evi- hypothesis, to be tested by further work, that the dence for these shears with their associated distributed Cenozoic rocks and Mesozoic rocks which are unro- deformation is not very convincing. We therefore fall tated, or show the weak 30 Ma rotation, are remagne- back to an essentially rigid plate model with much of tized. The identi®cation and detailed understanding of Kalimantan, Sarawak and southern Sabah participat- remagnetization remains a major problem in paleo- ing in a rotation of about 508 CCW between 30 and magnetism despite many studies (e.g., Elmore et al., 10 Ma, and in an earlier rotation of about 408 some- 1987; Fuller et al., 1988; Hart and Fuller, 1988; time between 80 and 30 Ma. The underlying cause of McCabe and Elmore, 1989; McClelland-Brown, 1982; these rotations is convergence between the Australian Reynolds et al., 1990). That sedimentary rocks are fre- Plate and Eurasia, which is accommodated in the quently remagnetized is well known, so that there is no southerly subduction which gave rise to the accretion- diculty in accepting a major role of remagnetization ary wedge in Sarawak and volcanism in Kalimantan. in the sedimentary rocks studied even though criteria which might have been used to establish remagnetiza- tions were not, for the most, part used. The interpret- 14. Conclusion ation of remagnetization in small shallow stocks, such as the Singtang intrusives poses more dicult pro- The paleomagnetic results from Borneo give con- blems. Preliminary investigations of the magnetization siderable support to the suggestion of CCW rotation, of the minor intrusions in Kalimantan revealed some which is consistent with earlier discussions (Haile, distinctions between the blocking temperature distri- 1978; Hamilton, 1979) and with aspects of more recent butions of the magnetizations which were rotated, and tectonic models and paleogeographic reconstructions those that were unrotated. Further work is planned to Hall (1996) and Richter (1996). There are also results investigate the cause of these blocking spectra, but the which are inconsistent with this idea, but the simplest presence of such distinctions may be the key to dis- interpretation of the paleomagnetic data is that there tinguishing primary and secondary magnetizations in is a CCW rotation of some 508 since about 30 Ma. this suite of rocks. Directions corresponding to this rotation are also seen If we accept that the paleomagnetic results require a in Mesozoic rocks and are interpreted as a secondary coherent rotation of Borneo, there still remain two magnetization. Other Mesozoic rocks show a stronger possible modes of rotationÐa rigid plate rotation, or rotation of an additional 408 giving a total of almost rotation brought about by distributed deformation in 908. The timing of this earlier rotation is not clear, but one or more shear zones. One possibility raised by it cannot be older than 80 Ma, the age of the younger Holcombe (1977) in a discussion of the rotation of Cretaceous intrusions in the Schwaner Mountains Peninsular Malaysia is that a series of concentric sli- which show the rotation. In comparing the results fom vers rotate. In Peninsular Malaysia the rotation is Borneo with those from the surrounding regions, we CCW about a pole to the northeast. A somewhat simi- ®nd CCW rotations from Western Sulawesi, the lar kinematic scheme was advocated by Rangin et al. Celebes Sea, Palawan and Peninsular Malaysia. (1990) in describing CW rotation in SE Asia in re- However, these rotations are not as large as the ro- sponse to the India±Eurasia collision. In that example tation in Borneo, so that the region has not rotated the CW rotation was about a pole at the eastern syn- coherently as Borneo appears to have done. The ro- taxis in Assam. It is possible that a similar kinematic tation of Borneo is a consequence of convergence scheme might explain the paleomagnetic observations between the Australian and Eurasian plates which is 22 M. Fuller et al. / Journal of Asian Earth Sciences 17 (1999) 3±24 accommodated by subduction along the northwest Elmore, R.D., Engel, M.H., Crawford, L., Nick, K., Imbus, S., Sofer, margin of Borneo. Z., 1987. Evidence for a relationship between hydrocarbons and authigenic magnetite. Nature, London 325 (6103), 428±430. Fuller, M., 1969. Magnetic orientation of borehole cores. Geophysics 34 (5), 772±774. Acknowledgements Fuller, M., Cisowski, S.M., Hart, M., Haston, R., Schmidtke, E., Jarrard, R., 1988. NRM:IRM(S) demagnetisation plots; an aid to the interpretation of natural remanent magnetisation. Geophysical MF wishes to express thanks to the many people Research Letters 15 (5), 518±521. who have taken part in the work reported here and Fuller, M., Haston, R., Lin, J.L., Richter, B., Schmidtke, E., Almasco, those who made it possible. In particular, we acknowl- J., 1991. Tertiary paleomagnetism of regions around the South edge this manuscript is prepared with aid from ACS. China Sea. Journal of Southeast Asian Earth Sciences 6, 161±184. Earlier work was supported by the NSF Tectonics pro- Haile, N.S., 1978. Reconnaissance palaeomagnetic results from Sulawesi, Indonesia, and their bearing on palaeogeographic recon- gram and a consortium which, at various times, com- structions. Tectonophysics 46 (12), 77±85. prised Conoco, Texaco, Marathon, Chevron, Unocal, Haile, N.S., 1979. Rotation of Borneo microplate completed by Amoco, and Exxon, of which Chevron Overseas MioceneÐpalaeomagnetic evidence. Warta Geologi (Newsletter of Petroleum continues to fund our work. JRA and SJM the Geological Society of Malaysia) 5 (2), 19±22. funding came from the University of London SE Asia Haile, N.S., 1981. Paleomagnetism of Southeast and East Asia. In: McElhinny, M.W., Valencio, D.A. (Eds.). Paleoreconstruction of consortium (Arco, Canadian Petroleum, Exxon, the Continents. Geodynamics Series 2. American Geophysical LASMO, Mobil and Union Texas). Dr Irwan Bahar, UnionÐGeological Society of America, pp. 129±135. the Director of the Geological Research and Haile, N.S., Beckinsale, R.D., Chakraborty, K.R., Abdul, H.H., Development Center, Bandung, Indonesia, is gratefully Tjahjo, H., 1983. Paleomagnetism, geochronology and petrology acknowledged for his assistance in organizing ®eld ex- of the dolerite dykes and basaltic ¯ows from Kuantan, West Malaysia. Bulletin of the Geological Society of Malaysia 16, 71±85. peditions. Dharma Satria Nas and H. Wahyono are Haile, N.S., Briden, J.C., 1982. 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