Tertiary paleomagnetism of regions around the South China Sea

Journal of Southeast Asian Earth Sciences, Vol. 6, No. 3/4, pp. 161-184, 1991 Printed in Great Britain

0743-9547/91 $3.00 + 0.00 Pergamon Press Ltd

Tertiary paleomagnetism of regions around the South China Sea M. FULLER,* R. HASTON,* JIN-LU LIN,* B. RICHTER,* E. SCHMIDTKE* a n d J. ALMASCO~" *Department of Geological Sciences, University of California, Santa Barbara, California; and tDepartment of Geological Sciences, University of Illinois, Chicago, Illinois, U.S.A. (Received 4 September 1990; accepted for publication 5 May 1991) AMtraet--Paleomagnetic data from the Philippine Sea Plate (PSP) reveal a history of plate-wide clockwise (CW) rotation and northerly translation since the late Eocene about a nearby pole to the east. The motion has generated left lateral oblique convergence between the Philippine Sea Plate and S.E. Asia. Paleomagnetic data from Luzon in the northern Philippines show early Miocene CCW rotation followed by late Miocene CW rotation. In contrast, the Southern and Central Philippines display early Miocene CW rotation and unrotated late Miocene directions. These results define two different paleomagnetic domains with distinct post early Miocene histories. Pre-Miocene CCW rotation is suggested by data from Zambales, the Visayans and the Celebes Sea. In Borneo, a history of Tertiary CCW rotation has been found in Sarawak, and Sabah. Conflicting results have been reported from Kalimantan, some show no rotation with respect to Eurasia, while others give CCW rotations. In the Malaysian peninsula, the Segamat basalts and Kuantan dykes, of probable late Cretaceous early Tertiary age show CCW rotations similar to those seen in Sarawak. To the north in peninsular Thailand, CW rotations have been found in two Miocene non-marine basins. Late Tertiary basalts from Northern and Central Thailand yield similar CW rotations while coeval flows on the Khorat plateau in Eastern Thailand are unrotated. The tectonic implications of the results remain problematical. In particular, the relative importance of true plate rotations and localized rotation of upper crustal blocks in distributed shear zones is unclear. The substantial region of CCW rotation in Borneo, the Celebes Sea and the Philippines is consistent with the broad features of the Holloway model, although the timing of the rotations precludes simple coherent rotation. The model must also be modified to include the effect of the left lateral oblique convergence between the PSP and Eurasia. The CW rotations seen in peninsular Thailand and Malaysia are consistent with the propagating extrusion tectonic model, but we cannot yet preclude a local transtensional origin for these rotations. The lack of Tertiary rocks in the peninsula has hampered attempts to relate these late Tertiary CW rotations to the late Mesozoic and Tertiary CCW rotations seen in Malaysia and Borneo.

INTRODUCTION THE TECTONICS of S.E. Asia has fascinated successive generations of geologists, who have used the region's modern tectonics as an analog for processes interpreted in the geological record. In the past decade, the model of extrusion tectonics (e.g. Molnar and Tapponnier 1975, Tapponnier et al. 1982) has emerged as the predominant model for the tectonics of S.E. Asia. Two invaluable references for the geological background of the regions discussed are Hamilton (1979), in which the geology was placed for the first time in a modern plate tectonic setting, and Hutchison (1989), which provides a modern compilation of the regional geology. Recently, comprehensive paleogeographic reconstructions have been described (Jolivet et al. 1989, Rangin et al. 1990). In the long run, paleomagnetic data from the region around the South China Sea are likely to make contributions to three aspects of the tectonics of S.E. Asia. The first concerns the points of origin of the various blocks, or microcontinents. Did they originate as a part of Gondwanaland and if so, where? The second concerns their pre-collision translation and accretion to Eurasia. Can we establish from the paleomagnetic record when and where these events took place? Did these blocks cross Tethys? The final aspect is the history of the blocks after they were accreted to Eurasia. What is their subsequent history of intracontinental deformation? Is there a paleomagnetic record of the effects of the

collision of India with Eurasia? In this paper, we focus upon the last of these three topics. Paleomagnetic data may eventually provide critical tests of the extent of the role of indentor, or extrusion tectonics (e.g. Molnar and Tapponnier 1975, Tapponnier et al. 1982) in the tectonics of S.E. Asia. However, the interpretation of paleomagnetism in such active regions is difficult. The primary paleomagnetic vector may be modified by subsequent tectonic effects, such as stress and temperature changes, or fluid migration. PaleomagneticaUy detected rotations may sometimes reflect local rotations related to shear zones (e.g. Freund 1974, Ron et al. 1984, Luyendyk et al. 1985, Wells and Coe 1985, Kissel and Laj 1988, Jackson and Molnar 1990), they can also be caused by local deformation in thrust sheets, or in arc related deformation. Coherent rotations of plates, or microplates, cannot be assumed. An important aspect of the interpretation of the paleomagnetic data of S.E. Asia is therefore to understand the origin of the paleomagnetically observed rotations. What is the extent in time and space of particular rotations? Are there criteria we can establish to distinguish plate rotations from upper crustal block rotations? There have been two substantial reviews of paleomagnetic data from S.E. Asia (Jarrard and Sasajima 1980, Haile and Briden 1984). The latter gives a data set of paleomagnetic directions for S.E. Asia. Our studies have for the most part been consistent with earlier work where they overlap. Indeed, the achieve-

161

162

M. FULLER et al.

D

,'7 Philippine Sea Plate Saipan Guam

China Sea qm

Palau Peninsular Malaysia b

/~ I-Ialmahera 0*

Java

I 100°

I 110 °

-0 °

1,0 o

o

I o 140

Fig. 1. Map of S.E. Asia showing locations of study areas.

ment of these earlier workers in this difficult area was remarkable, considering the difficulties in processing large numbers of specimens before the modern era of paleomagnetism. In this paper we discuss our results from the regions bordering the South China Sea (Fig. 1) and compare them with other relevant data. We begin in the north with S. China and then move clockwise around the South China Sea to the Philippine Sea Plate, the Philippines, Borneo, Malaysia and Indochina. We present directional means for time intervals if they are available, if not we use formation means. For the most part, the results discussed are already published and for more detailed analyses of individual sites, the original papers should be consulted. Some new data are presented, but these are a minor part of the paper. An Appendix provides preliminary descriptions of the new sites. The principal aim of the paper is to discuss the available data, so that those interested in the tectonics of S.E. Asia can gain an impression of the likely reliability of the various data sets. Controversy arises primarily over the interpretation of particular magnetization directions, rather than whether the magnetization was correctly determined. Typical problems are to establish whether the magnetization was acquired when the rock was formed, or whether it was a remagnetization, what the age of the rock and its magnetization are, whether the site suffered local rotation. Fold tests, structural criteria and characteristics of the magnetization are potentially keys to such determinations. An attempt is made to point out where these have been used. Where there are disagreements, the nature of the dis-

agreement is discussed, e.g. is it in interpretation, or in raw data?

RELATION OF S.E. ASIA TO STABLE EURASIA

A prerequisite for a plate tectonic reconstruction of S.E. Asia is an Apparent Polar Wander (APW) path for Eurasia. In this way, the possible locations of the various mobile units can be fixed relative to stable Eurasia. Unfortunately and somewhat surprisingly, the APW path for stable Eurasia is not very well determined. However, recently Besse and Courtillot (in press) have presented a new path taking advantage of paleomagnetic poles from other continents, which can be rotated into the Eurasian reference frame using the sea floor anomaly record of relative motion. Figure 2 shows their preferred Eurasian path.

PALEOMAGNETISM OF SOUTH CHINA The South China Sea is presently part of the Eurasian plate. Numerous studies (e.g. Lin et al. 1985, Zhao and Coe 1987) have established that the North and South China Blocks (NCB and SCB) completed their collision during the late Triassic and early Jurassic and that the Chinese Mongolian assemblage, which includes North and South China, Indochina and Mongolia was finally accreted to the Siberian Platform in late Jurassic to early Cretaceous time (Lin 1988, Kuzmin et al. 1989). Thus,

163

Tertiary paleomagnetism of regions around the South China Sea Greenwich

Greenwich

90°E

90* E

SIT[

N UK 11"1I..~ttlNA

180 °

(x~)

BLOCK Fig. 2. APW path for Eurasia (Besse and Courtillot in press). The 95% confidence intervals are not shown, but are mostly between 5 and 10.

during its formation, the South China Sea north of the spreading was a part of Eurasia, but may have been subjected to intracontinental deformation. To investigate the configuration of the South China Sea during its opening between 30 and 17 Ma, we therefore need Tertiary data from South China. Late Tertiary paleomagnetic data from several localities in North China and Inner Mongolia are internally consistent (Lin et al. 1985, Zhao et al. in press). These poles are close to the present pole and poles from Northern Eurasia. Paleomagnetic data for the Tertiary of South China are few and not internally consistent (Table 1, Fig. 3). There is a study of Eocene redbeds from Central South China (Li and Ye 1965), which gives a pole suggesting substantial CCW rotation. This result is consistent with that of Lin et al. (1985). In contrast to these results, a recent study by Zhuang et al. (1988) gives a pole intermediate between the present pole position and the Cretaceous pole. The poles reported by Li and Ye (1965) and Linet al. (1985) may well be anomalous. They differ from other poles from the SCB and from NCB (Fig. 3) implying improbable rotations for a lithospheric plate. Nevertheless, while the former result comes from Hubei and Hunnan, the latter comes from nearly 1000 km east in

~iUU | It I~I'IINA

BLOCK

Fig. 3. Tertiary APW path for North and South China. The anomalous poles from South China are to the west of the APW paths for North and South China implying CCW rotation.

Zhejiang. The anomalous results could be ascribed to local rotations in an extensive left lateral shear. However, this interpretation is not very satisfactory until the limits of the region showing the rapid rotation are established and the necessary structural information are available. Clearly more Tertiary data are required from China, if its paleomagnetism is to contribute to our understanding of the South China Sea.

PHILIPPINE SEA PLATE The Philippine Sea Plate has been the site of a substantial paleomagnetic effort (Kobayashi et al. 1971, Larsen et al. 1975, Louden 1977, Aoki 1972, Fuller et al. 1980, Kodama 1981, Keating and Helsley 1985, Haston et al. 1988, Adachi et al. 1987, Haston and Fuller 1991). Part of this work has been directed at the interpretation of the present form of the Marianas A r c ~ i d it originate as we see it today, or was it deformed from a simpler configuration (Vogt 1973)? There is also a strong motivation to establish the paleomagnetic history of the plate because standard methods of establishing present plate motion do not work well, as the plate is surrounded by subduction zones. The paleo-

Table 1. Paleomagnetic results from China. Kis the Fisher precision parameter. A95 is the radius of the 95% error circle about the mean pole, or the semi-angle of the 95% error about the mean direction when in parentheses. N gives the number of sites, R signifies a reversal test and F a fold test. References (1) Lin et aL 1985, (2) Li and Ye 1965, (3) Zhuang et al. 1988, (4) Kent et al. 1986, (5) Zhao and Coe 1987 VGP Location South China Block

North China Block

Age Neogene Eocene-Miocene Middle and late Eocene Early and middle Cret. Neogene Middle Cret.

N 8 5 6 l0

°N. Lat. 74.2 71.0 85.2 76.3

°E. Long. 36.5 49.3 174.6 172.6

6 8

80.6 69.0

183.1 182.0

K 148 97 46 23 91 193

A95 7.6 7.8 (11.9) 10.3 7.1 4.0

Test R R R F R R

Ref. (1) (2) (3) (4) (1, 5) (1)

164

M. FULLERet al.

magnetic record can then aid understanding of the nature of the eastern boundary of S.E. Asia and may distinguish between the various models of the origin of the West Philippine Sea Basin. In particular, does it have a back arc origin as suggested by Lewis et al. (1982), Seno and Maruyama (1984), or is it trapped oceanic plate, e.g. Uyeda and Ben-Avraham (1972). Figure 4 gives locations of the islands situated on the Philippine Sea Plate and the observed declination anomalies. We now review these results. P a l e o m a g n e t i c data

Palau, in the West Carolines, lies at the southern extremity of the Palau-Kyushu ridge (Fig. 4). It affords a unique paleomagnetic opportunity to determine the motion of the West Philippine Sea Basin (WPSB) because it has been a part of the WPSB since the Eocene. It has been isolated from the active arc which gave rise to the Parece Vela Basin and the Marianas and Bonins. Hence, it alone can provide a direct paleomagnetic record of West Philippine Sea Plate motion. Palau, like the other islands, of the Philippine Sea Plate consists of a volcanic core with a younger cover of limestones. The first paleomagnetic results from Palau were reported by Aoki (1972) and suggested CW rotations.

A preliminary account of a more recent study, which also reveals CW rotations has been given by Adachi et al. (1987). Haston et al. (1988) presented results giving rotations of approximately 55 ° CW for the early Miocene Arakabesan unit and 70 ° for the mid-Oligocene Medorm unit. CW rotations of 100 ° have been determined from a smaller collection from the early Oligocene Aimelik formation (Table 2, Fig. 5a). The magnetization directions from the late Eocene Babeldaup formation are probably remagnetized--they are similar to those observed in the mid-Oligocene Medorm unit and moreover the magnetization of one of the sites has characteristics typical of secondary magnetizations. The results from Palau also indicate northward motion with time. Paleomagnetic data from Guam (Kobayashi et al. 1971, Larsen et al. 1975, Keating and Helsley 1985, Haston and Fuller 1991) provide a reliable result from a single formation--the Oligocene Alutom Formation (Table 2, Fig. 5b). Both fold and reversal tests are satisfied and a CW rotation of more than 60 ° is evident. The Eocene Facpi Formation gives scattered results some of which indicate strong CW rotations. The paleomagnetic interpretation of the results from this formation is not straightforward. The scatter is far too large to be caused simply by secular variation. Yet many of the individual site statistics are very good. We have

0

1000 km

I

I

40°N

I

Bonin Is.

30° N

NeckerRidge

2930

O. West Philippine Basin "" ~ ~

29o0

J,~¢~

11

\

20° N

0 '~ II 4e~o45

,,o,,,

°Fx. b ..i"J Guam

10°N

Caroline Ridge

Pal

120°E 130°E 140°E 150°E Fig. 4. PhilippineSea Plate. The pie slicesare 95% confidenceintervalsabout the directionsof magnetization.The numbers refer to DSDP and ODP drill sites.

165

Tertiary paleomagnetism of regions around the South China Sea

Table 2. Paleomagnetic results from the Philippine Sea Plate. References (1) Haston and Fuller (1991), (2) Kodama (1981), (3) Kodama et al. (1983), (4) Keating et al. (1983). All directions are corrected for structural tilt except post-folding remagnetized directions which are indicated by * Location Palau Gaum Saipan

Bonins Chichijima Anijima and Nishijima

Rock type Basaltic, dacite flows, and sill Tuff Limestone Basalt flows and mudstone Limestones Basaltic flows and limestone Dacite flows and ash

Age E. Miocene Oligocene Miocene* Oligocene Plio-Pleistocene Miocene Eocene

N 5 3 3 9 4 6 10

Inc. 3.5 - 11.8 - 23.1 15.1 8.6 -30.7 -12.0

Dec. 54.5 95.2 175.7 66.1 350.1 208.1 43.0

ct95 17.4 10.3 4.2 I1.1 22.5 8.4 12.5

k 20.3 20.4 877.5 20.1 17.5 64.6 16.0

Basalt flows Basalt flows

Late Eocene Late Eocene

27 22

9.9 -0.8

105.6 91.7

11.6 16.9

7.0 4.9

excluded the result from the tectonic analysis on the grounds of excessive between-site-scatter; the estimate of the precision parameter k is only 4.5. Despite studies by ourselves and others of the younger limestones, no reliable result has emerged. Hence, we are left with the Oligocene Alutom result.

Ref. (1) (1) (1) (1) (1) (1) (1) (2, 3, 4) (4)

Results from Saipan (Fuller et al. 1980, Keating and Helsley 1985, Haston and Fuller 1991) provide a well determined CW rotation of approximately 30 ° for the Miocene Fina Sisu Formation flows. This is indistinguishable from the Miocene Tagpochau Limestone Formation result. The early Oligocene Hagman For-

N

N

90 o

(b) Middle Oligocene N

-I-

._r_.....,,,.,,,,~

900

90 °

N

900

Fig. 5. Paleomagnetic results from islands of the Philippine Sea Plate. (a) Palau; (b) Guam; (c) Saipan. Crosses indicate positive, or downward, inclination and open symbols upward, or negative, inclination. The shaded areas give the 95% confidence intervals.

166

M. FULLER et al.

(b)

Minimu~ ¢. . . . M0f

(a)

,°

Present ,,,",,

_ 30°N

,Bomn b

Bonin Is ,,-",

I k,

.-- Saipan Guam _ 10°N

~

_5°N _ 0

I

i1310°E

Fig. 6. Philippine Sea Plate reconstruction. (a) Present configuration; (b) Restoration of P a l a u - K y u s h u ridge according to m i n i m u m space criteria.

mation volcanics give scattered results which we are unable to interpret, but the late Eocene Sankakuyama Formation gives a shallow negative inclination and CW rotation of approximately 40 ° (Table 2, Fig. 5c). The results from a large number of sites in the islands of the Philippine Sea Plate have yielded a very low percentage of usable paleomagnetic data. Nevertheless, there is a consistent history of CW rotations observed from all of the islands (Table 2, Fig. 4), including the Bonins which were studied by Kodama (1981), Keating et al. (1983), and Keating and Helsley (1985). The key question is whether the paleomagnetic results are a record of local rotations associated with deformation at the margin of the arc, or whether they are plate-wide. The following test was developed to distinguish between the two possibilities (Haston and Fuller 1991). The extent to which the observed rotations are the result of plate-wide rotation should be systematically revealed by correcting the results from older rocks from Guam, Saipan and the Bonins for subsequent arc splitting and migration which has carried them away from the ancestral Palau-Kyushu ridge. Conversely, if the

(a)

120

results are due to deformation of the arc and do not reflect plate motion, there should be no systematic rotation seen from all of the islands. In Fig. 6a, the Philippine Sea Plate is drawn in its present configuration. In Fig. 6b, the various arcs are returned to their initial positions on the proto-PalauKyushu Ridge, according to a visual fit with the minimum space between the various topographic arc features. Comparing Figs 7a, b reveals that a systematic rotation is indeed revealed in the reconstructed configuration. The rotations therefore appear to be plate wide and there is no need to invoke local deformation of the arc to explain the observed CW rotation. The paleomagnetic data from the islands around the Philippine Sea Plate indicate CW rotation and northward motion. The northward motion (Fig. 8) comes from results reported from DSDP and ODP studies (e.g. Louden 1977), as well as from the islands. A particularly important result, which has emerged from Leg 126 paleomagnetic studies is that the Shikoku basin has rotated CW by approximately 90 ° since the early Oligocene (Cisowski et al. 1990). The rotation reported 120

f

lOO

•

(b)

In Situ

i

i

,

i

,

i

•

Minimum Space Model 100

80

80

-+=%

C 60

C 60 T

40

"-- 40

0 g : -20

0

0I+

n Palau • Gaum • Saipan Bonin

20

10

20

30 Age (Ma)

u • a A

2O

40

50

0

10

20

30

Palau Saipan Bonin Guam

40

Age (Ma)

Fig. 7. Paleomagnetic declination results from Philippine Sea Plate. (a) In present site locations; (b) with relocation of sites to P a l a u - K y u s h u ridge. Errors in declination are 95% confidence intervals. Errors in age are estimated.

50

167

Tertiary paleomagnetism of regions around the South China Sea

I 20 [ 10, "1"

i ]

120°E

--20°N

o DSDP • On-land

e o

I 1250E

1 Zamb

- - 15°N

,a -10

Bata~

Min(

i -20 -30 -40 0

10

20

30

40

50

60

Age (Ma)

Fig. 8. Inclination data from the Philippine Sea Plate. Errors in inclination are 95% confidence intervals. Errors in age are estimated.

for the Shikoku Basin is therefore consistent with those for the various islands. Moreover, as the result comes from the interior of the oceanic plate, it is not susceptible to local deformations at the margin. It therefore appears that the paleomagnetism of the Philippine Sea Plate records plate wide rotations. An APW path for the Philippine Sea Plate is given in Fig. 9.

PALEOMAGNETISM

OF T H E P H I L I P P I N E S

Paleomagnetic studies in the Philippines have proved difficult and have given rise to different interpretations by the various groups which have worked there (e.g. Hsu 1972, De Boer et al. 1980, McCabe et al.'1982, Fuller et al. 1983, Fuller 1985, McCabe et al. 1987, Fuller et al. 1989). The geology of the Philippines records the complicated history of a mobile belt between two major plates (Fig. 10). Papers on Philippine geology appeared at the end of the last century (Becker 1901). Since that time, regional descriptions have been attempted (Corby et al. Greenwich

90°E

180 0 Fig. 9. APW path for Philippine Sea Plate. Light dotted shading-Palau. Dark dotted shading--Saipan. Cross hatched s h a d i n g ~ u a m . Vertical bar shading--Bonins.

Fig. 10. Philippines: sample locations.

1951). During the 1960s, a comprehensive geologic map at a scale of 1:1,000,000 was produced by the Philippine Bureau of Mines and a tectonic synthesis appeared (Gervasio 1973). Subsequently, in 1982 the Bureau of Mines and Geo-sciences published a review of Philippine Geology entitled "Geology and Mineral Resources of the Philippines". In the past decade, the tectonic history of the Philippines has been reinterpreted by a variety of workers including Holloway (1981), Karig (1983), Hawkins and Evans (1983), Rangin et al. (1985), Mitchell et al. (1986), Sarewitz and Karig (1986), Jolivet et al. (1989), Rangin et al. (1989), Bischke et al. (1990), Defant et al. (1990), Forster et al. (1990) and Marchadier and Rangin (1990). Paleomagnetism can, in principle, provide critical tests of the various models for the origin of the Philippines. For example, if the Holloway (1981) model is correct, the paleomagnetism of Philippines should reveal a long history of CCW rotation and northward translation. In contrast, models, such as that of McCabe and Cole (1986), which place the Philippines on the leading edge of the Philippine Sea Plate, predict consistent CW rotation and northward translation. Models which involve accretion from the Philippine Sea Plate and subsequent northward translation along left lateral shears (e.g. Karig 1983, Sarewitz and Karig 1986) predict an early paleomagnetic history of CW rotation and northward rotation followed by either CCW rotation and northward motion, or simply northward motion. The paleomagnetic results can provide timing for observed rotations, whether they are local, or on a large scale. Finally, if paleomagnetic histories are established for the various islands, they will reveal to what extent the islands have a common history. Luzon

The pioneering paleomagnetic studies of Hsu (1972) were primarily on the island of Luzon, the northernmost

168

M. FULLER et al.

and most accessible of the islands of the archipelago (Fig. 10). De Boer et al. (1980) describe results from Bataan west of Manila Bay in Luzon. Most of our work has also been in Luzon. The island is the product of multiple arcs and presently lies between the active Manila and Luzon subduction systems. Ophiolites form the oldest basement. There are two prominent sedimentary basins--the Central Basin and the Cagayan basin to the north east. The Cordillera Central forms a north-south uplifted range west of the Cagayan Valley and the Sierra Madre Mountains lie to the east. The Philippine fault cuts across the island with a NW/SE strike. The Zambales ophiolite and the overlying sediments afford the possibility of a paleomagnetic record over a region of approximately 100 by 20 km in Western Luzon ranging in age from late Eocene to late Miocene. The youngest material studied was a small intrusion which had an easterly declination anomaly (Table 3, Fig. 1la). In contrast, Miocene sediments show westerly declination (Fig. 1lb), while Oligocene sediments show stronger westerly anomalies (Fig. l lc). The results from the ophiolite itself are inconsistent, spreading over a declination range of almost 90 ° (Fig. lid). Assigning the polarity from the sediments, the directions appear to be reversed and extend from the reversed Miocene to reversed Oligocene declinations. Dykes at Coto mine, dated at 46.6 + 5.1 Ma by K/Ar whole rock determination, suggest still stronger westerly declinations of as much as 120° (Fig. lle). A similar result comes from a sill at Sual whose age is 44.3 ___3.5 Ma, although a single sill does not of course average secular variation. The easterly declination of the intrusion is also seen as a secondary direction in the sediments, which may be a reflection of the effect of the hydrothermal system around the intrusions. The results from the Miocene and Oligocene sediments reveal CCW rotation increasing progressively with age. We interpret the magnetization of the dykes to be the oldest recorded by the ophiolite. Although the late Eocene age from a whole rock K/Ar may not be reliable, the agreement between three ages from different parts of the ophiolite and the fact that the overlying sediments are late Eocene to Oligocene encourage us to think that the ophiolite is indeed Eocene and that the direction seen in the dykes is Eocene. Other results from the ophiolite yield declinations ranging from

(a)

Late Miocene/Pliocene +

(c)

(e) Eocene +

Fig. 11. Paleomagnetic results from Western Luzon-Zambales: (a) Plio-late Miocene; (b) early Miocene; (c) Oligocene; (d) late Eocene ophiolite; (e) late Eocene Coto dykes. Symbols as in Fig. 5.

these to the Miocene directions, suggesting that magnetization was"acquired later, possibly during uplift. East of the Zambales region across the Central Valley of Luzon in the Central Cordillera, a similar paleomagnetic history of early Miocene CCW rotations followed by late Miocene CW rotation is interpreted (Table 3, Fig. 12, Appendix). The Miocene Agno intrusive and mid and early Miocene dykes in the Bued River show CCW rotations. However, no reliable data have been obtained from older units, such as the Eocene Pugu volcanics. To the northeast of Manila in the Southern Sierra Madre westerly declinations are again found (Table 3, Fig. 13, Appendix) in dykes previously interpreted to be of Miocene age. A recent K/Ar age of 26.3 + 2.4 Ma

Table 3. Paleomagnetic results from the Philippines. References (1) Fuller et al. (1983), (2) McCabe et al. (1987), (3) Fuller et al. (1989). All directions are corrected for structural tilt except post-folding remagnetized directions which are indicated by * Location Rock type Age N Inc. Dec. ct95 k Ref. Luzon Zambales Basaltic flows Plio-Pleistocene 7 15.0 355.3 7.9 60 (1, 2) Dacite intrusion Late Miocene-Pliocene 6 26.6 16.0 9.5 51 (1, 2) Sediments Middle Miocene 8 11.4 312.9 14.2 16 (3) Sediments Early Oligocene 4 15.0 259.4 15.4 371 (3) Gabbros/Anorthosites Late Eocene 12 9.2 234.0 9.4 23 (3) Dykes Late Eocene 5 4.7 50.6 8.0 71 (3) 6 -21.5 57.2 9.6 64 (3) Central Cordillera Plio-Pleistocene Flows and siltstone 3 16.5 356.8 9.5 171 (1, 2) Late Miocene-Pliocene Ignimbrite and flows 8 24.3 014.8 9.9 33 (1, 2) Early and mid-Miocene Dykes and intrusions 6 13.5 318.8 16.4 18 (1) S. Sierra Madre Flows and tufts Plio-Pleistocene 2 21.2 357.6 22.2 128 (2)

Tertiary paleomagnetism of regions around the South China Sea

169 N

N

90o

90

N

Fig. 14. Paleomagneticresults from Talahib flows, Batangas. Symbols as in Fig. 5.

confirms this Miocene age, so that similar CCW rotations in early middle Miocene rocks are now demonstrated in Baguio and in the Southern Sierra Madre. In Batangas, the Talahib flows show westerly declinations and from the positive inclination direction, it is clear that this implies CCW rotation (Appendix, Fig. 14). Stratigraphic evidence suggests that it is late Miocene to Pliocene. A recent K/Ar determination gives a late Miocene age of 7.0 ___0.8 Ma. All of these data are consistent with a prolonged CCW rotation, followed by a CW rotation from late Miocene time. As is evident from Figs 11, 12, 13 and 14, there is no substantial change in inclination with time.

is continental and thought to have been rifted from South China during South China Sea spreading. The southwesterly half is predominantly ophiolitic. A whole rock K/Ar age of 43.8 + 2 . 2 M a was obtained for basaltic flows from the ophiolite, so that the age of these flows is indistinguishable from that of the Zambales ophiolite. However, stratigraphic evidence suggests that they may be Cretaceous. Work in progress in the ophiolites of Southern Palawan has established a region of consistent westerly declination anomalies in the far south of the island (Appendix, Fig. 15a). These results marginally pass a regional attitude test, that is their scatter is reduced by structural correction, although no formal fold test was possible. To the north, towards the border with the continental half of the island the directions are inconsistent (Appendix, Fig. 15b). We provisionally interpret the direction from the southernmost part of the island as indicative of a CCW rotation because of the positive inclination. A similar direction is seen in the north and suggests a coherent rotation of much of the ophiolite. However, the northern results are complicated in a way not yet understood, but possibly related to the right lateral shear on the boundary.

Palawan

Summary of results from the Philippines

The elongate island of Palawan is divided by its geology into two distinct halves. The northeasterly half

These paleomagnetic data from the Philippines include substantial declination anomalies implying CW and CCW rotations. The first step is to see whether there are any patterns of the declination anomalies in time or space. The results from Luzon do not divide into regions of CW and CCW rotations. Not only do nearby sites of different age show the two senses of rotation, but so do individual samples. Hence in Luzon, the sense of rotation changes with time. However, there are systematic differences in rotation history between Luzon and the regions to the south, defining two paleomagnetic domains. The vast majority of Plio-Pleistocene rocks studied in Luzon do not show systematic declination anomalies, being magnetized either parallel to the present field, or to its reversed equivalent (Fuller et al. 1983, McCabe et al. 1987). Westerly declination anomalies suggesting

900

Fig. 12. Paleomagneticresults from Luzon Central Cordillera: (a) late Miocene; (b) early Miocene. Symbols as in Fig. 5.

N ,I

0 90o

Fig. 13. Early Miocene Paleomagnetic results from Southern Sierra Madre. Symbolsas in Fig. 5.

170

M. FULLERet

-.1-

al.

.-]--90'

Calamiancontinentalfragment I

270"

90'

1$0 °

Fig. 15. Paleomagnetic results from Palawan: Eocene Cretaceous ophiolite basalts: (a) Southern block; (b) Northern block. Symbols as in Fig. 5.

CCW rotations in rocks of as little as 1-4 Ma have been described by De Boer et al. (1980). However, they have not been confirmed by other workers (McCabe et al. 1984) and may be related to recent local deformation behind the Manila arc. In Luzon, the youngest pattern of general tectonic significance appears to be the easterly declination anomalies seen as primary magnetizations in the rocks of late Miocene, or early Pliocene age (Fig. 16a). This direction is also seen as a secondary direction in sites in Zambales and Baguio. With one exception the CCW, or westerly directions are always seen in the older rocks of mid Miocene, or older age (Fig. 16a and b). The one exception is the Talahib flow of Batangas. We therefore consider that in

Luzon mid Miocene or earlier CCW rotations are followed by CW rotations. It remains to be seen whether the CCW rotation in Batangas is later than elsewhere, or whether the date is unreliable. McCabe, in a series of papers of which he was senior author, has argued against the interpretation of any CCW rotation in Luzon, e.g. McCabe et al. (1985, 1986, 1987). Because the sense of rotation is so important and because this is the principal source of controversy in Philippine paleomagnetism, the problem is reviewed in some detail. The controversy here is not principally about the raw data, it is the interpretation of the sense of rotation which is contentious. In particular, it concerns the sense of the rotation of the results from the Coto dykes (b)

Fig. 16. Paleomagnetic domains in the Philippines: (a) late Miocene; (b) early Miocene. The pie slices represent 95% confidence intervals on the directions.

Tertiary paleomagnetism of regions around the South China Sea (Fig. lie). It is our position that the progressively increasing CCW rotation seen in the early Miocene and Oligocene sites continues to give the maximum CCW rotation seen in the dykes. In this way, we use the standard technique of building up the sequence of paleomagnetic results from youngest to oldest sites. Additional results obtained since the publication of McCabe et al. (1987), in Zambales (Fuller et al. 1989), Baguio (this paper) and the Southern Sierra Madre (this paper) lead us to reassert the original suggestion of the CCW rotation of Luzon throughout much of the Tertiary (Fuller et al. 1983). The timing of the rotation is sufficiently imprecise that we cannot preclude an important role for local rotations at various times, nor indeed for some earlier CW rotation, but it appears that the dominant sense of rotation in Luzon, prior to the late Miocene was CCW. The CCW rotations appear in the oldest rocks we have studied which are Eocene. In these rocks, we estimate CCW rotation of approximately 120° in Luzon. In the early Oligocene formations in the Zambales region about 90 ° is seen and finally in a variety of Miocene sites about 45 ° is evident. Results reported by McCabe et al. (1985) and Cole et al. (1989) from the Visayans and other southern and eastern parts of the Philippine archipelago are very different from those seen in Luzon. They observed little, or no rotation, in rocks back to late Miocene age and prior to that minor CW rotation is seen in older Cenozoic rocks (Fig. 16). The distinctive patterns of results (Fig. 16) permits definition of paleomagnetic domains in the archipelago based upon differing paleomagnetic records. (1) The Northern Philippine paleomagnetic domain (NPPD) consists of those parts of Luzon studied (with the exception of southernmost Bicol). This domain has a history of late Miocene CW rotation of approximately 20 ° since 10 or 15 Ma. Prior to this it experienced CCW rotation. (2) The Southern and Central Philippine paleomagnetic domain (SCPPD) includes the remainder of the Philippine archipelago. It gives no indication of rotation since late Miocene (e.g. Cole et al. 1989). Prior to this, the domain was making a smaller CW rotation of approximately 25 °. Negros and Cebu, in the western part of the domain appear to have experienced substantial CCW rotation in the Paleogene. The boundary between the two paleomagnetic domains appears to cut across southernmost Luzon. It includes Batangas, which may account for the CCW rotations of late Miocene age there. It is not yet clear whether there are geological features which coincide with this paleomagnetic boundary. Possible candidates are the newly recognized strand of the Philippine fault (Bischke et al. 1990), the Macolod rift (Forster et al. 1990) and the Verde Passage (Karig 1983, Marchadier and Rangin 1990). It has commonly been stated that the inclination data from the Philippines indicate northward motion of the archipelago throughout the Tertiary (Hsu 1972, Fuller et al. 1983, Fuller 1985, McCabe et al. 1987). However, $EAES 6J3-4~B

171

with the acquisition of new data, the evidence for northward motion has weakened. While the means of the Eocene results from Zamabales and Palawan and the early Miocene results from all sites are lower than the present dipole field inclination there is statistical overlap. There may be slight northward motion, but at present it is within the errors of our results. We can be confident that the magnetizations we have studied were not acquired substantially south of the magnetic equator.

PALEOMAGNETISM OF THE CELEBES SEA Shibuya et al. (1989) have reported a period of CCW rotation in late Eocene and early Oligocene time. This result is potentially very important because it establishes a history of rotation of a plate to the west of the present Philippines. Moreover, because it comes from the interior of an oceanic plate it is unlikely to have suffered the local rotations so frequently suspected on land. Inclination data suggest little change in paleolatitude. Unfortunately this result was not obtained with independently oriented core, but by using soft magnetization interpreted to give the present field direction. This method (Fuller 1969) is not entirely reliable because soft magnetizations in directions other than that of the present Earth's field may be present. Nevertheless, the method has been tested against the formational microscanner in the Shikoku Basin (Cisowski et al. 1990) and gave similar results, so some confidence can be placed in its effectiveness in soft sediment cores. The result is similar to those from Palawan and from Negros and Cebu in the Central and Southern Philippines (McCabe et al. 1987). All are consistent with CCW rotation in the Paleogene. Possibly, even more importantly the result demonstrates that the use of oriented ocean sediments is a powerful technique for studying the tectonic history of S.E. Asia.

PALEOMAGNETISM OF BORNEO

Borneo lies in a central position so that to establish its paleomagentic history is an important aspect of tectonic models of S.E. Asia. Paleomagnetic studies have been carried out in East Malaysia, in both Sabah and Sarawak, and in Kalimantan, Indonesian Borneo. Prior to 1941, the Dutch mapped extensively in Kalimantan (e.g. Van Bemmelen 1949). Sarawak and Sabah were mapped by British geologists and by the Malaysian Geological Survey. Hamilton (1979) was the first to interpret the geology of Borneo in a plate tectonic framework. Recently important mapping and gravity data have been reported from West Kalimantan (Williams et al. 1988). The geology of Western Sabah and Northern Sarawak is dominated by the Crocker accretionary complex, a series of deep water turbidite sediments. The complex is

172

M. FULLER et al.

separated from the Tertiary basins and older mrlanges of Southern Sarawak by the Lupar Line. The complex is imbricated with large thrust wedges dipping to the southeast and younging to the northwest. There is considerable lateral continuity, so that stratigraphy can be traced for kilometers (e.g. Hamilton 1979). The Mt Kinabalu adamellite and associated smaller bodies intruded the complex about 10 Ma. To the southeast, lie older units of the accretionary complex such as the Rangau Formation, which can also be traced southwestward into Sarawak to the Lupar Line. The oldest unit in Sabah is the Chert-Spilite Formation (Fitch 1956) which outcrops in Central Sabah in the form of isolated ophiolitic fragments with associated overlying cherts. Numerous small late Oligocene to Miocene intrusions occur throughout Sarawak and in the neighboring part of western Kalimantan. They have I-type granitoid geochemical characteristics. They were thought to be post tectonic by Kirk (1968), to be a last phase of activity associated with subduction from the north by Hamilton (1979), while Williams and Harahap (1987) argue for deep crustal remelting in a passive, post subduction environment. The magmatic activity began in the south and moved northward. The bulk of our samples come from the Northwest Kalimantan Block (Williams et al. 1986), which may be an allochthonous terrane accreted in the Cretaceous. The Upper Eocene to Miocene Silantek Formation is part of the Ketungau forearc basin to the east of the Northwest Kalimantan Block, and unconformably overlies mrlange and accretionary deposits. No mrlange or deep water sediments are found younger than Eocene. Thus the key Tertiary results we have obtained come from igneous rocks intruded into a passive post-subduction environment and sediments from basins unconformably overlying earlier mrlange and accretionary prism.

Sabah

The results from Sabah are only preliminary (Schmidtke et al. 1985). A small intrusion close to the main Mt Kinabalu body revealed a slight CCW deflection of declination (Table 4). A whole rock K/Ar age for this Kapa intrusion gave 13.3 Ma + 5.3. Results from the Crocker Formation (Table 4) have been disappointing. The sandstone members are almost uniformly reset in the present field direction, leaving the red shales as the most promising paleomagnetic material. In the shales, westerly declination anomalies of almost 90 ° were found with inclinations close to zero. The Chert-Spilite Formation has yielded several individually reliable paleomagnetic directions both in the chert and the basalts. Before tectonic correction, a particularly good result from the cherts is consistent with the Crocker Formation redbed result. The spilites are for the most part poorly controlled structurally and give westerly declination anomalies in situ. They appear to fail a fold test, so that the magnetization of the Chert-Spilite Formation may have been largely reset, possibly some time in the early Tertiary. The chert spilite also outcrops in the north, in the Kudat Peninsula. Here it has an easterly declination. We present these results as raw data of possible interest to others, but do not interpret them in detail. Any interpretation of the data must take into account the structural complexity of the region which includes the orocline noted by Hamilton (1979) and the major NW/SE striking left lateral shear zone which passes through northern Sabah. Sarawak

In Sarawak, reliable results have been obtained from the Tertiary intrusions and continental sediments dis-

Table 4. Paleomagnetic results from Borneo. References (1) Schmidtke et al. (1985), (2) Schmidtke et al. (1990), (3) Lumadyo et al. (1990), (4) Sunata et al. (1987), (5) Wahyono and Sunata (1987), (6) Haile et aL (1977), (7) Sunata and Wahyono (1987). All directions are corrected for structural tilt except post-folding remagnetized directions which are indicated by * Location

Rock type

Age

N

Inc.

Dec.

~%

k

Sabah

Kapa Adamellite Crocker Fm.

Late Miocene Oligocene/Miocene

Chert/Sp. Hyp. Int. (a) Unrotated (b) Int. Rot. (c) Max. Rot. Silantek Fro. Bau Lst. Bau Lst. Pedawan Fro. Kedadom Fm. Basalts Basalts, Shale, Andesites Nanga Raun. Basalt sills. Kalasin Fro. Schwaner Mtns. Plutonic/Volc. Tiong Cihan sediment

Cretaceous Mio

9 7 6 5 9

1.0 2.8 -5.0 2.5 0.0

348.9 277.5 289.9 282.0 281.0

2.4 3.7 10.6 10.4 6.0

422 227 40 79 66

(l) (1) (l) (1) (1)

4 4 1 3 2 5 3 3 16 9

2.1 7.0 0.2 24.3 -8.6 - 11.5 16.0 -2.1 - 13.8 -2.2

1.3 344.8 308.2 318.7 272.0 121.6 87.9 311.1 180.4 173.5

19.2 17.0 3.7 15.7 5.3 14.1 7.1 22.7 7.8 11.0

24 211 269 62 45 30 14 3 24 23

(2) (2) (2) (2) (2) (2) (2) (3) (3)

2

4.6

183.8

5.2

--

(4)

Eocene Cret.

27 39

0.6 0.0

322.8 311.0

13.9 8.3

-8

(5) (6)

Jur.

39

2.4

284.5

10.7

Sarawak

Kalimantan

Eocene/Miocene Jur/Cretaceous Jur/Cretaceous Jur/Cretaceous Jur/Cretaceous Plio/Pleistocene Eocene/Miocene Oligocene/Miocene

Ref.

(5)

Tertiary paleomagnetism of regions around the South China Sea

173

PALEOMAGNETIC SAMPLING LOCATIONS - SARAWAK

Fig. 17. Site location map for Sarawak. cussed above and also from Mesozoic shelf limestones (Schmidtke et al. 1990). Figure 17 gives site localities. The results reveal a pattern of CCW rotation which according to the intrusive record took place after 26 Ma and was underway at 17 Ma (Table 4, Fig. 18). While the results from the intrusives give an indication of the precise time of rotation, they do not provide an indication of paleohorizontal and so results from intrusions alone are unreliable. However, in this case, results from the Silantek formation indicate the same CCW rotation. Although the other results from Sarawak are from Mesozoic formations, they are relevant to the present discussion in that they exhibit secondary magnetizations similar to the Tertiary directions seen in the younger rocks. They also provide further evidence of CCW rotation in the area. The Jurassic Bau limestone yields a variety of directions including remagnetization in recent fields and the Silantek direction. The Kedadom formation is magnetized in the Silantek direction. A fold test indicates that the Bau direction giving about a 90 ° declination anomaly is at least pre-late Cretaceous, and may be a primary Jurassic magnetization (Fig. 19a). A similar direction with a fold test is found in the Pedawan formation (Fig. 19b). These results suggest that the CCW rotation seen in the Tertiary rocks was preceded by earlier CCW rotation in the late Mesozoic.

timing of the rotation. Our results from the Silantek suggests that it is rotated. We found sites with the unrotated reversed direction Haile found, but we think

270" ~

9

0

~

"

R85-7

0 °

Summary of paleomagnetism of Borneo In Fig. 20, the results from Borneo are presented on a much simplified map of the island after Hamilton (1979). The results which we have obtained from Sarawak in Tertiary and Jurassic formations are for the most part consistent with those reported by Haile et al. (1977) from Sarawak and Kalimantan. However, we differ from Haile (1979) in our interpretation of the

~

180°

Fig. 18. Tertiary Paleomagnetic results from Sarawak, East Malaysia: (a) Neogene intrusion; (b) Silantek formation. Symbols as in Fig. 5.

174

M. FULLER et al.

that it is a secondary magnetization because it is blocked at low temperature in samples which carry the rotated direction blocked at higher temperatures. Moreover intrusions as young as 26 and 17 Ma show rotated directions. The rotation does not therefore appear to have been completed by early Miocene, but rather was underway during the Miocene. Our results from Sarawak are also consistent with those from the Eocene Kalasin Formation (Wahyono and Sunata 1987) and the late Cretaceous Schwaner Mountain intrusions (Haile et al. 1977) in Kalimantan. Moreover, the Bau/Pedawan direction agrees well with results from the Jurassic in Kalimantan (e.g. Wahyono and Sunata 1987). In contrast, the Nanga Raun direction (Sunata et al. 1987) shows no significant rotation. Lumadyo et al. (1990) have also reported unrotated directions from southeastern Borneo, for 16 PlioPleistocene sites and nine Eocene to early Miocene sites. The results from the older units are very puzzling because, like those from the Nanga Raun, they conflict with results coming from a very similar tectonic and sedimentary environment. It is also somewhat remarkable to find a result indistinguishable from the present dipole axis in an area of such complexity. For the present the two different sets of results remain a puzzle. The results from Sarawak and Kalimantan are used to construct an APW path for this part of Borneo (Fig. 21). There are two branches. First, there is a progression of poles revealing a consistent path moving over Eastern Europe to Africa. The Jurassic poles cannot be explained by rotation in place because there is a significant paleolatitude change. Hence at least N

N

(a)

90o

N

/

/

~ ~

SR88-23 (S. Fold Limb)

2 0

180v

N

/ SR88-23 / ~ (S. FoldLimb) FOLD

1800

Fig. 19. Jurassic results from Sarawak: (a) Pedawan formation fold test; (b) Bau Lst. formation fold test. Symbols as in Fig. 5.

Fig. 20. Paleomagneticresults from Borneo: (a) Tertiary; (b) PreTertiary. Arrows indicate mean directions reported.

the Northwest Kalimantan Block was south of the equator and strongly CCW rotated by late Cretaceous time. The Cretaceous-Tertiary poles can however be explained by rotation in place, whether this is local, or a record of a major block rotation remains to be seen. The agreement between the various Jurassic results suggests that at least all of these sites must have experienced the same rotation in the Tertiary. Second, there are the Nanga Raun results from Sunata et al. (1987) and those from Lumadyo et al. (1990). The latter are not distinguishable from the Eurasian path, nor the present pole.

PALEOMAGNETISM OF PENINSULAR MALAYSIA AND PENINSULAR THAILAND The principal contributions to the paleomagnetism of Peninsular Malaysia have come from the work of Haile and McElhinny. Geological reports for the region go back to the middle of the last century (Logan 1848). However, it was Scrivenor who laid the foundations of Malaysian geology with his description of the ore deposites (Scrivenor 1928) and his Geology of Malaya (Scrivenor 1931). The most recent summary is the text Geology of the Malay Peninsula: West Malaysia and Singapore (Gobbett and Hutchison 1973).

Tertiary paleomagnetismof regions around the South China Sea

175

Greenwich

180 0 Fig. 21. APW path for Sarawak and Kalimantan.The open symbolsindicate older results. The solid symbolsrepresent the recent results from Lumadyoet al. (1990).

The principal paleomagnetic results from Peninsular Malaysia and Thailand. We have sampled the Krabi Thailand come from Bunopas and his colleagues from Basin of Southern Thailand and a small unnamed basin the Department of Mineral Resources, Bangkok. near Petchaburi (Fig. 22). The sedimentary rocks are Reports on Thai geology appeared during the last century, but it was the "Geological reconnaissance of mineral deposits of Thailand" by Brown (1951) which provided the first comprehensive summary. Bunopas (1981) gives an important synthesis recognizing the role of plate tectonics. BURM~ " ~ii!ii!ii?: The geology of Peninsular Malaysia and Thailand ~i~iii~i is dominantly continental. Metcalfe (1990) places the western province of Malaysia and the Thai Peninsula in the Sibumasu, or Shan Thai block. The eastern part HE,~ of Thailand, including the Khorat plateau is in the I TertiaryBasiv Indochina block. The eastern part of the Malaysian -12 0 Peninsula is in a second block called East Malaya which Andaman lies immediately south of Indochina (Fig. 22). The two '~" are joined along the Raub-Bentong line, an interpreted Triassic suture along which there may also have been translational motion. To the east of the Raub-Bentong -- 8° PhslU~kd- ' ~ ~ IBentongRaub line lies the Central Valley, in which there are well [ Tenia~Basin-A ~ ~_~Line Langkawi ~ developed sections of Mesozoic redbeds. The Thai peninsula is dominated by the Permian Ratburi Limestone which is correlative with the Chuping _4 ° Limestone of Malaysia. There are also prominent Triassic and Cretaceous intrusions. The peninsula is cut by a number of NE/SW faults such as the Ranong faults in the south and further north by NW/SE striking Three Pagodas fault. The sense and timing of displacement on these faults is poorly known. Fig. 22. Samplingsites in Peninsular Malaysiaand Thailand. Tertiary sections are very limited in Peninsular

Sea ~/A~

176

M. FULLERet North (0°)

al.

North (0°)

90°

o

180

180°

North (0°)

(b)

s •....

sins-26^ In S i t u

North (0~

.,27 M88-26B

+

zorn4-3 ~

} 9d3

90o

j

KOD84-

SM88-

KOD84-7/

1800

I

1800

Fig. 23. Paleomagnetic results from Peninsular Malaya and Thailand: (a) Tertiary basins; (b) Cretaceous Tertiary dykes and flows. Symbols as in Fig. 5.

non-marine sandstones and claystone of marine and transitional continental environments (Hutchison 1989). There are plentiful Mesozoic sedimentary sections and intrusions, which have provided the bulk of our data. There are also Paleozoic sections in northernmost Malaya, which are particularly well developed in Langkawi island. These Mesozoic and Paleozoic sections are of relevance to the present discussion because secondary magnetizations in the older formations are potential sources of Tertiary paleomagnetic data. The youngest material studied comes from the two Tertiary basins. Work is not completed in either basin. However, the preliminary results are included (Appendix, Fig. 23a). They are the first examples we have found in Malaysia, or Thailand, of possible primary directions of magnetization showing CW

rotations. All sites show evidence of CW rotations, but the scatter is substantial. We will return to these sites and other Tertiary basins to make further collections in the near future. A similar direction is seen as a secondary component of magnetization in the Setul Limestone of Langkawi island (Appendix, Fig. 23a). The next youngest material studied was the Segamat Basalt Formation, whose age is 65 Ma. A southeasterly upward inclined direction was found (Fig. 23b). An equivalent direction of opposite polarity was found in the Kuantan dykes, although this direction has somewhat steeper inclination (Fig. 23b). To the west of the Raub-Bentong line we have no reliable magnetizations from Tertiary material. However, the Kodiang limestone carries a secondary magnetization which is similar to that of the Segamat and Kuantan (Table 5, Fig. 23b).

Table 5. Paleomagnetic results from Peninsular Malaysia. References (1) McElhinny (1974), (2) Schmidtke et al. (1990), (3) Haile et al. (1983). All directions are corrected for structural tilt except post-folding remagnetized directions which are indicated by * Rock type Age N Inc. Dec. ~5 k Ref. Segamat basalts Paleogene 16 -31.0 136.0 9.4 16.3 (1, 2) Kuantan Dykes Cretaceous 22 40.0 333.0 5.6 28.0 (2, 3) -Kodiang Lst.* Permian 6 -30.1 122.3 14.1 42.3 (2) et al.

Tertiary paleomagnetismof regions around the South China Sea

177

Greenwich

90°E Thailand Malaysia

180 o

Fig. 24. APW path for Sarawak and Peninsular Malayaand Thailand.

Summary Thailand

o f results from

Peninsular Malaysia

and

The few Tertiary results are the first reported from the peninsula and are consistently rotated CW. A CCW direction similar to that seen in Tertiary and late Cretaceous units from Borneo was found in Peninsular Malaysia and Thailand. Similar results were obtained earlier by McElhinny et al. (1974). At present, work is still in progress with the aim of finding out the extent and timing of the CW rotation. While similar CCW rotations have been found through the whole region, the CW rotation has only been found as far south as Langkawi in Northern Malaysia. The results from Peninsular Malaysia and Thailand are presented in the form of an APW path (Fig. 24). There are two branches again, as in the plot for Borneo. In this instance, for at least the northern part of the area, the path tracks from the poles in the Bering Straits to those in Europe. The youngest section of the path shows some convergence between Eurasia and northern Malaysia and Thailand, while the older shows major CCW rotation with respect to Eurasia.

the recognition of substantial CW rotation, but the timing of the rotation is controversial. Initially the rotation was thought to be Triassic (e.g. Bunopas 1981, Achache et al. 1983). A recent study by McCabe et al. (1988) on late Neogene flows from the Korat Plateau and from the region to the west indicated CW rotation of the western region of 13.5+5.8. While some individual sites among the eastern group exhibited similar rotations the mean declination was indistinguishable from the geocentric dipole and hence showed no significant rotation. The results from Indochina have recently been reinterpreted by Yan and Courtillot (1989) in terms of widespread Tertiary remagnetization, which according to them took place prior to the rotations due to the IndiaEurasia collision and hence preserve a record of them. The young basalts studied by McCabe et al. (1988) are assigned an older age by Yan and Courtillot (1989), consistent with the timing of the rotation predicted by indenter tectonics. The key to resolving the Indochina data may lie in the inclination change between Tertiary and Mesozoic results showing clockwise rotations. However, at present the resolution is inadequate.

PALEOMAGNETISM OF INDOCHINA Numerous studies of the paleomagnetism of Indochina have appeared with the main emphasis on the Khorat Plateau and lesser efforts in Northern Thailand and Vietnam (Bunopas 1981, Barr et al. 1978, Maranate 1982, Achache et al. 1983, Giang 1984, Achache and Courtillot 1985, Maranate and Vella 1986, McCabe et al. 1988, Yan and Courtillot 1989). The principal result is

THE TECTONIC IMPLICATIONS OF THE PALEOMAGNETIC DATA We now review the results from the various regions and assess their tectonic significance. Modern paleomagnetic techniques ensure that the directions of magnetization measured in rock units are not likely to differ very much from laboratory to laboratory. Different

178

M. FULLER et al.

a

10-15 m a ~

b

20-30 ma ~

C 40-60 ma

Fig. 25. Summary of results. (a) late Miocene. (b) early Miocene. (c) Eocene Oligocene. The arrows represent reported mean directions. There is considerable variability in the data represented by each arrow. In some instances it is a single site, as in the Celebes Sea. In others such as Peninsular Malaysia tens of sites are represented.

laboratories may place emphasis upon different techniques of analysis, but where the principal difficulties arise is no longer in the measurement. The paleomagnetism of S.E. Asia does, however, face two profound difficulties. These are: (l) determination of the ages of the magnetizations observed; and (2) determination of the role of local rotations in zones of distributed shear. These two difficulties are general problems faced by paleomagnetists, but are particularly severe in S.E. Asia due to its tectonic complexity. It must be admitted that the problems are far from solved and that S.E. Asia is a severe challenge to paleomagnetists, so that it is not wise to be too dogmatic about interpretations. Nevertheless, there are patterns in the data which warrant interpretation. The results are presented on a base map of S.E. Asia (Wood 1985). Although no two geologists are likely to agree on the details of the faults shown, the map serves as a reminder of the tectonic complexity of the region (Fig. 25). The tectonic implications of the results from the Philippine Sea Plate are relatively straightforward. The strong clockwise rotation of the Philippine Sea Plate invalidates the basic premise of the Uyeda and Ben-Avraham (1972) model accounting for the initiation of subduction along a former N/S transform to generate the Palau-Kyushu ridge: the ridge would not have originated N/S and therefore would not have been parallel to the Emperor Seamount chain at the time of initiation of subduction. However, the idea of initiation of subduction along an earlier transform is still attractive. The transform would have been oriented approximately WNW/ESE and be between the relatively young

West Philippine Sea basin and an older "Pacific" plate to the northeast. Convergence between the Pacific plate and the CW rotating Philippine Sea plate would require the initiation of subduction from the time of change of motion of the Pacific Plate. The paleomagnetic data do not require deformation of the arc to explain them, so that they do not require models such as those proposed by Vogt (1973), or by McCabe and Uyeda (1983). However, some such deformation may have taken place without inducing paleomagnetically detectable rotation. Unfortunately, the data do not distinguish clearly between the back arc, or trapped ocean model of origin for the West Philippine Sea basin, although the initial spreading was roughly parallel to the S.E. Asian margin, which might be regarded as slightly favoring the back arc origin of Lewis et al. (1982). The paleomagnetic data from the Philippine Sea Plate are broadly consistent with the paleogeographic reconstructions of Seno and Maruyama (1984), Jolivet et aL (1989), Teng (1990) and Rangin et al. (1990), although according to our results the plate would be more strongly rotated than is generally shown and hence implying a pole of rotation nearer to the Philippine Sea Plate. Figures 26 and 27 give 10 and 30Ma reconstruction cartoons, respectively. The results from the Philippine Sea Plate imply oblique convergence with sinistral shear as the eastern boundary condition for S.E. Asia throughout much of the Tertiary, a feature common to most reconstructions. It is in the Philippines that the difficulties noted at the beginning of this section prove most severe. Before the paleomagnetic results from the Philippines can be used

179

Tertiary paleomagnetism of regions around the South China Sea

10 Ma Reconstruction D = 30°CW Latitude = 5 °N

300 Bonin Is.

d %

Marcus Necker Ridge

200

Saipan

t Philippine Basin Central Basin

10

Spreading Center

0

Caroline Ridge

Palau Is. 0

100okm

I

130 o

I

140 o

0

I

0

150 0

Fig. 26. Reconstruction of the Philippine Sea Plate at I0 Ma.

Fault (Cole et al. 1989). The situation may be analogous to behavior along appropriately oriented straight segments of major strike slip faults, where relative motion across the fault does not involve extensive local rotations about a vertical axis. Hence declination anomalies from such regions should not be assumed to be caused by local rotations, records of plate motion may also be preserved. The paleomagnetic results suggest that the Philippine archipelago consists of at least two paleomagnetic domains with different records of rotation. During the

for tectonic interpretation, we must know whether they are due to local rotations of crustal blocks in the sinistral shear, generated by the oblique convergence between S.E. Asia and the Philippine Sea Plate, or whether they reflect coherent rotations of lithospheric plates and fragments of plates. The predominantly CCW sense of the paleomagnetically determined rotations is indeed consistent with block rotation in the sinistral shear zone. However, Cenozoic results from the southern Philippines carry a record of CW rotation and give no evidence of CCW rotations despite straddling the Philippine I

I

I

I

30 Ma Reconstruction D=70°CW oN Latitude =

0

I

I

10ookm

I

I

20

I

d % Marcus Necker Ridge

Bonin Is.

Pacific Plate

10'

~ e s p t C d ~ ' < e 2 x -.~l ~ -;, /

Guam

0°

Caroline Ridge

r ~'~'2/ /¢.,,¢$ I

130 0

I

I

140 0

I

I

150 0

I

I

160 0

Fig. 27. Reconstruction of the Philippine Sea Plate at 30 Ma.

I

I

170 o

180

M. FULLERet al.

late Miocene there was some form of plate boundary between the NPPD and the SCPPD, which was locked between subduction zones to the west and east. The boundary between the two domains may have been a region of diffuse crustal deformation. Despite recognizing the strength of arguments that Luzon did not rotate coherently (Karig 1983, Sarewitz and Karig 1986, Jolivet et al. 1989), we still think that the simplest interpretation of the paleomagnetic data is that the NPPD, which is much of Luzon, rotated coherently CW from about 10 Ma and that before that it rotated CCW. The principal reason is that the paleomagnetic history of Zambales in the west of the Central Basin and of the Central Cordillera and the Southern Sierra Madre to the east are all similar. It seems unlikely that this would be the case, if the rotations were in small blocks within multiple shear zones. Nevertheless, it must be admitted that our knowledge of the age of the various magnetizations is not good enough to preclude local rotations at different times. The young CW rotation of the NPPD is most easily explained by recognizing that it may have been part of the Philippine Sea Plate at this time. The boundary between the NPPD and the SCPPD would then have been the transform along which motion between the Philippine Sea Plate and the southern Philippines was taken up. There are two candidates to explain the rapid Miocene CCW rotation of the NPPD. One is the impingement of the Palawan microcontinent, as suggested by Holloway (1981) and initially used to explain the CCW motion of the Luzon data (McCabe et al. 1982, Fuller et al. 1983). A second possibility arises from the proposed E/W configuration of the archipelago (Holloway 1981). Interaction with the Philippine Sea Plate would be likely to deform the eastern extremity, possibly rotating it CCW and carrying it north, in a manner analogous to that proposed for disruption of the leading edge of the Australian plate along the Sorong Fault (e.g. Hamilton 1979, Hall and Nichols 1990). If this analogy with the Sorong Fault is appropriate then it suggests that lithospheric plate fragments are involved. The SCPPD did not take part in the Miocene CCW rotation in the north, but made a smaller CW rotation during this time. The rotation may be related to the impact of the Palawan microcontinent (McCabe et al. 1982). Prior to early Miocene time, the various parts of the Philippine archipelago studied, and the Celebes Sea, appear to have experienced CCW rotation. This is superficially consistent with the Holloway (1981) model. However, other parts of the Philippines, such as Eastern Mindanao, which remain to be studied paleomagnetically, may well have originated on the Philippine Sea Plate and been accreted, as suggested by Hall and Nichols (1990). In comparing the paleomagnetic data with the various paleogeographic reconstructions, the central problem is to establish how much of the archipelago has Eurasian

affinity and how much has come from the Philippine Sea Plate. It is our interpretation that both the minimal change in paleolatitude of the archipelago and its predominantly CCW rotation suggests that it has principally Eurasian affinity. This is again broadly compatible with the Jolivet et al. (1989) and Rangin et al. (1990) reconstructions, although we would argue that more of Luzon is related to Eurasia than they suggest. Conversely, in our interpretation, less of the archipelago is derived from the Philippine Sea Plate and we see less evidence for a major mobile belt with a long history of moving northwards. Like the paleomagnetism of the Philippines, the paleomagnetism of Borneo raises difficult interpretational problems. It should also be remembered that in a region the size of France, we have roughly six Tertiary and four Mesozoic independent results. Moreover, the bulk of these results come from a single geological setting--the Kuching Zone of Haile (1974). The simplest and most consistent pattern is one of CCW rotation. Yet, as was discussed in detail above, there are exceptions in Kalimantan. A possible escape from the dilemma is to acknowledge that the CCW rotation may not have taken place coherently. However, the CCW rotations should not be ignored because some unrotated results have appeared. Both results need to be re-examined. We may also finally find a tectonic model, which accommodates both sets of results. Provisionally we retain our interpretation of substantial CCW rotation on the grounds that our Tertiary results are paleomagnetically reliable and come from a region, which was tectonically stable at the relevant time. Although our interpretation of CCW rotation and the stasis approach of Lumadyo et aL (1990) for Borneo are poles apart, they have in common incompatibility with that part of the kinematics of Rangin et al. (1990), which derives from Le Pichon's suggestion of a CW rotation about a pole at 13°N, 95°E in response to the Indian collision (Le Pichon 1988). In this instance the paleomagnetic results are in conflict with the recent reconstruction models. Lumadyo's stasis approach is not directly in conflict with the extrusion model of Molnar and Tapponnier (1975). Indeed, even the CCW rotations need not necesarily be in conflict with the model-Borneo may simply be too far south to exhibit the same behavior as regions nearer to the collision. Nevertheless the CCW rotations and northward motion are more consistent with the reconstructions following the approach of Taylor and Hayes (1980), Ru and Pigott (1986) and Williams et al. (1988). The paleomagnetism of Peninsular Malaysia and Thailand yields CW rotations in the Tertiary basins studied and CCW rotations in the Cretaceous Tertiary Segamat basalts and Kuantan dykes. The CW rotations have been found at the north end of the Peninsula and at Krabi south of the Ranong fault zone, and as secondary magnetizations as far south as Langkawi. It remains to be seen whether these occur throughout the peninsula and so define a substantial region of similar CW rotation. This result is consistent with the

Tertiary paleomagnetism of regions around the South China Sea

predictions of indenter tectonics, although the precise timing of the rotation is not established. The earlier CCW rotation has been found as a primary direction only in the southern part of the Malay Peninsula, but similar directions can be traced up to the north of the Thai Peninsula as secondary magnetizations, in Triassic and Jurassic rocks, although their age is poorly constrained. As noted in the discussion of the results from Borneo, the tectonic interpretation of this direction is problematical. One must recognize the possibility of block rotation on some scale. At present there are key faults which could play the role of boundaries between regions with different paleomagnetic histories, but we do not have sufficient coverage to test the suggestion. The interpretation of the paleomagnetic data from Indochina is so problematical that one cannot test paleogeographic reconstructions very meaningfully.

181

Plate and Eurasia. To the west of the Philippines, the pervasive CCW rotations in Celebes Sea, Southern Palawan, Borneo and Sundaland seem to require lithospheric plate rotation. The younger CW rotations in Peninsular Malaysia and Thailand may be an expression of extrusion tectonics, but we cannot yet be sure how important is the role of crustal block rotations in the dextral shear between the Indian-Australian and Eurasian plates. The paleomagnetic results are consistent with many aspects of recent paleogeographic reconstructions, although there is severe conflict over the rotational history of Borneo. Admittedly interpretation of the paleomagnetic record remains problematical, but it is probably not wise to develop tectonic models and paleogeographic reconstructions which totally ignore this record. Acknowledgements--It is a great pleasure to acknowledge the help we

CONCLUSION We conclude that while small block rotations may obscure the interpretation of the paleomagnetic data from S.E. Asia, the effect does not make the data uninterpretable. Similarly, although the profusion of secondary magnetizations makes it difficult to assign ages to observed magnetizations, and indeed sometimes to isolate individual magnetizations, there is a useful paleomagnetic record. Our mode of operation, in attempting to document this record, has been to make widespread collections to establish patterns of magnetization directions and to place limits on the ages of these magnetizations by finding the youngest units which exhibit particular directions. A most remarkable feature of the record is that throughout much of the region there is a history of CW rotation following CCW rotation. In Luzon the CW rotation is from about 10 Ma. In the southern Philippines the CW rotation is earlier, but still apparently preceded by CCW rotation. In Sarawak, we have not seen the CW rotation, but neither have we sampled units younger than 10 Ma. Both in Luzon and Sarawak the CCW rotation is between about 30 and 10Ma. In Peninsular Malaysia and Thailand the timing of the rotations is poorly known, but again the CW is preceded by CCW. The easterly declination anomalies implying CW rotation are predominantly weaker (20-30°), while the westerly declinations are stronger (30 and 50°). A relatively small number of intermediate directions are found, suggesting that the rotations were not continuous, but stop and go. The key question remains the separation of true lithospheric plate rotations, from rotations of superficial crustal blocks, in response either to continuous deformation below, or due to interaction with neighboring blocks. In the case of the Philippine Sea Plate, we can be confident that the lithospheric plate is rotating. In the Philippines, there may be both lithospheric plate rotation and there may also be crustal block rotation in distributed sinistral shear between the Philippine Sea

have had from our many friends in S.E. Asia and the Western Pacific. Wherever we have worked, we have received support in the field. In the Marianas and Western Carolines, members of the faculty of The University of the Marianas at Guam and Gait Siegrist from the University of Maryland assisted us. In the Philippines, John Wolfe has helped us from our earliest trips, as have many members of the Philippine Bureau of Mines and Geosciences and in particular Guilmo Balce. Jim Hawkins, Cindy Evans, Dan Karig, Dan Sarewitz, Steve Lewis and Ed Geary have all helped in planning fieldwork and discussed the interpretation of results. In Malaysia, we have had generous help from Achmed Samsudin of the Universiti Kebangsaan Malaysia, and members of the University College, London Group led by Mike Audley-Charles and Robert Hall, Charles Hutchison at University of Malaya, members of the Geological Survey of Malaysia and the Surveys in Sarawak and Sabah. Throughout our efforts in Borneo and Malaysia Neville Haile has been a major source of information. In Thailand, help from Sangad Bunopas and his group from the Department of Mineral Resources has been invaluable. We thank Patricia Fuller for help in the field in the Marianas, Malaysia and Singapore. We are grateful to Stan Cisowski for a collection from Peninsular Thailand made in conjunction with colleageus from the Department of Mineral Resources. Finally, we thank Ellie Dzuro for preparation of the tables. The project has been supported by the NSF and we are very grateful for the funding in these hard times. In addition, we have received support from a consortium of oil companies among whom Marathon, Conoco, Unocal, Chevron, Exxon, Mobil, Sohio and Amoco have at various times been members. This has permitted a far more extensive effort than would otherwise have been possible. We are also very grateful for this support.

REFERENCES Achache, J. and Courtillot, V. 1985. A preliminary Upper Triassic pole for the Khorat plateau (Thailand): consequences for the accretion of Indochina against Eurasia. Earth planet. Sci. Lett. 73, 147-157. Achache, J., Courtillot, V. and Besse, J. 1983. Paleomagnetic constraints on the late Cretaceous and Cenozoic tectonics of southeastern Asia. Earth planet. Sci. Lett. 63, 123-136. Adachi, Y., Inokuchi, H., Otofuji, Y., Isezaki, N. and Yaskawa, K. 1987. Rotation of the Philippine Sea Plate inferred from the paleomagnetism of Palau and Yap Islands. Rock Magnetism and Paleogeophysics 14, 72-75. Aoki, Y. 1972. A post depositional magnetization mechanism of sediments and its application to geophysical problems-Paleosecular variation in Japan. Unpublished Ph.D. thesis, University of Tokyo, 101 pp. Barr, S. M., McDonald, A. S. and Haile, N. 1978. Reconnaissance paleomagnetic measurements of Triassic and Jurassic sedimentary rocks from Thailand. Geol. Soc. Malaysia Bull. 10, 53-62. Becker, G. F. 1901. Report on the geology of the Philippine Islands. U.S. Geol. Survey 21st Annual Report 3, 493-644.

182

M. FULLERet al.

Besse, J. and Courtillot, V. 1988. Paleogeographic maps of the Indian Ocean bordering continents since the Upper Jurassic. J. geophys. Res. 93, 11791-11808. Besse, J. and Courtillot, V. in press. Revised and synthetic polar wander paths of the African, Eurasian North American and Indian plates, and true polar wander since 200 Ma. J. Geophys. Res. Bischke, R. E., Suppe, J. and Pilar, R. 1990. A new branch of the Philippine fault system as observed from aeromagnetic and seismic data. Tectonophysics 183, 243-264. Brown, G. F. 1951. Geologic reconnaissance of the mineral deposits of Thailand. U.S. Geol. Survey Bull. 984. Bunopas, S. 1981. Paleogeographic history of western Thailand and adjacent parts of Southeast Asia: a plate tectonics interpretation. Geological Survey Paper Special Issue 5, Department of Mineral Resources of Thailand, Bangkok. Cisowski, S. M., Fuller, M., Haston, R. B., Koyama, M. and the Shipboard Scientific parties of ODP legs 125 and 126, 1990. Paleomagnetic evidence from land-based and ODP cores for clockwise rotation and northward translation of the Philippine Sea Plate (Abstract). Fifth Circum-Pacific Energy and Mineral Resources Conference 33. Cole, J., McCabe, R., Moriarty, T., Malicse, J., Delfin, F. G., Tebar, H. and Ferrer, H. P. 1989. A preliminary Neogene data set from Leyte and its relation to motion on the Philippine Fault. Tectonophysics 168, 205-220. Corby, G. W., Merchant, F. E. and Taylor, E. F. 1951. Geology and oil possibilities of the Philippines. Department of Agriculture and Natural Resources, Technical Bulletin, Bureau of Printing, Manila. DeBoer, J., Odom, L. A., Ragland, P. C., Snider, F. G. and Tilford, N. A. 1980. The Bataan orogen: eastward subduction, tectonic rotations, and volcanism in the Western Pacific (Philippines). Tectonophysics 67, 251-282. Defant, M. J., Maury, R. C., Joron, J.-L., Feigenson, M. D., Leterrier, J., Bellon, H., Jacques, D. and Richard, M. 1990. The geochemistry and tectonic setting of the northern section of the Luzon arc (the Philippines and Taiwan). Tectonophysics 183, 187-205. Fitch, F. H. 1956. Problems of stratigraphy and geotectonics in North Borneo. Proc. 8th Pacific Science Con[] 11, 537 553. Forster, H., Oles, D., Knittel, U., Defant, M. J. and Torres, R. C. 1990. The Macolod Corridor: a rift crossing the Philippine island arc. Tectonophysics 183, 265-271. Freund, R. 1974. Kinematics of transform and transcurrent faults. Tectonophysics 21, 93-134. Fuller, M. 1969. Magnetic orientation of borehole cores. Geophysics 34 (5), 772-774. Fuller, M. 1985. Paleomagnetism in an accretionary margin, Luzon, Northern Philippines. J. Geodynamics 2, 141-158. Fuller, M., Dunn, J. R., Green, G., Lin, Jin-lu, McCabe, R. J., Toney, K. and Williams, I. 1980. Paleomagnetism of Truk Islands, eastern Carolines and of Saipan, Marianas. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 1 (Edited by Hayes, D. E.). Am. Geophys. Union Mon. 27, 235 246. Fuller, M., Haston, R. and Almasco, J. 1989. Paleomagnetism of the Zambales ophiolite, Luzon, northern Philippines. Tectonophysics 168, 171 203. Fuller. M., McCabe, R., Williams, I. S., Almasco, J., Encina, R., Zanoria, A. S. and Wolfe, J. A. 1983. Paleomagnetism of Luzon. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 2 (Edited by Hayes, D. E.). Am. Geophys. Union Mon. 27, 79-94. Gervasio, F. 1973. Geotectonic development of the Philippines. In: The Western Pacific, Nedlands, Western Australia (Edited by Coleman, P. J.). University of Western Australia Press, Nedlands, pp. 307 324. Giang, N. T. 1984. Paleomagnetic studies of Cenozoic basalts in Vietnam. In: Paleomagnetic Research in Southeast and East Asia. CCOP Technical Publication 13, 25-46, United Nations Building Bangkok. Gobbett, D. J. and Hutchison, C. S. 1973. Geology' of the Malay Peninsula: West Malaysia and Singapore. John Wiley Interscience, New York. Haile, N. S. 1974. Borneo. In: Mesozoic-Cenozoic Orogenic Belts: Data for Orogenic Studies (Edited by Spencer, A. M.), Spec. Pubis. geol. Soc. Lond. 4, 337-347. Haile, N. S. 1979. Rotation of Borneo microplate complete by Miocene: paleomagnetic evidence. Warta Geol. 5, 19-22. Haile, N. S., Beckinsale, R. D., Chakraborty, K. R., Hussein, A. H. and Hardjono, T. 1983. Paleomagnetism, geochronology and petrology of the dolerite dykes and basaltic flows from Kuantan, West Malaysia. Bull. Geol. Soc. Malaysia 16, 71-85.

Haile, N. S. and Briden, J. C. 1984. Past and future paleomagnetic research and tectonic history of East and Southeast Asia. In: Paleomagnetic Research in Southeast and East Asia. CCOP Technical Publication 13, 25-46, United Nations Building, Bangkok. Haile, N. S., McElhinny, M. W. and McDougall, I. 1977. Paleomagnetic data and radiometric ages from the Cretaceous of West Kalimantan (Borneo) and their significance in interpreting regional structure. J. geoL Soc. Lond. 133, 133-144. Hall, R. and Nichols, G. 1990. Terrane amalgamation in the Philippine Margin. Tectonophysics 181, 207-222. Hamilton, W. 1979. Tectonics of the Indonesian Region. U.S. Geol. Surv. Prof. Pap. 1078, 345pp. Haston, R. and Fuller, M. 1991. Paleomagnetic data from the Philippine Sea Plate and their tectonic significance. J. geophys. Res. 96, 134, 6073~078. Haston, R., Fuller, M. and Schmidtke, E. 1988. Paleomagnetic results from Palau, West Caroline Islands: A constraint on Philippine Sea plate motion. Geology 16, 654-657. Hawkins, J. W. and Evans, C. A. 1983. Geology of the Zambales Range, Luzon, Philippine Islands: ophiolite derived from an island arc--backarc basin pair. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 2 (Edited by Hayes, D. E.). Am. Geophys. Union Mon. 27, 95-123. Holloway, N. H. 1981. The North Palawan block Philippines: its relation to the Asian mainland and its role in the evolution of the South China Sea. Geol. Soc. Malaysia Bull. 14, 19-58. Hsu, I. 1972. Magnetic properties of igneous rocks in the northern Philippines. Unpublished Ph.D. dissertation, Washington U., St Louis, p. 165. Hutchison, C. S. 1989. Geological Evolution of Southeast Asia, p. 368. Clarendon Press, Oxford. Jackson, J. and Molnar, P. 1990. Active faulting and block rotation in the Western Transverse Ranges, California. J. geophys. Res. 95 (B 13), 22,073-22,078. Jarrard, R. D. and Sasajima, S. 1980. Paleomagnetic synthesis for Southeast Asia: constraints on plate motions. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 1 (Edited by Hayes, D. E.). Am. Geophys. Union Mon. 27, 293-316. Jolivet, L., Huchon, P. and Rangin, C. 1989. Tectonic setting of Western Pacific marginal basins. Tectonophysics 160, 23-47. Karig, D. E. 1983. Accreted terranes in the northern part of the Philippine archipelago. Tectonics 2, 211-236. Keating, B. I-[. and Helsley, C. E. 1985. Implications of island arc rotations to the studies of marginal terranes. J. Geodynamics 2, 159-181. Keating, B. H., Kodama, K. and Helsley, C. E. 1983. Paleomagnetic studies of the Bonin and Mariana Island Arcs. In: Arc Volcanism; Physics and Tectonics (Edited by Shimozoru, D. and Yokoyama, I.), pp. 243 259. Terra Scientific Publishing, Tokyo. Kent, D. V., Xu, G., Huang, K., Zhang, W. Y. and Opdyke, N. D. 1986. Paleomagnetism of Upper Cretaceous rocks from S. China. Earth planet. Sci. Lett. 79, 1179 1184. Kirk, H. J. C. 1968. The igneous rocks of Sarawak and Sabah. Borneo Region Malaysia Geol. Surv. Bull. 5. Kissel, C. and Laj, C. 1988. Paleomagnetic rotation and continental deformation. NATO, ASI series, Maths. Phys. 254, 516pp. Kobayashi, K., Hoshino, M. and Aoki, H. 1971. Reconnaissance paleomagnetic and rock magnetic study of igneous rocks of Guam, Mariana and related sites. In: Izu Peninsula, pp. 385-390. Tokai Univ. Press, Japan. Kodama, K. 1981. A paleomagnetic reconnaissance of the Bonin Islands. Bull. Earthquake Res. Inst., Tokyo Univ. 56, 347-365. Kodama, K., Keating, B. H. and Helsley, C. E. 1983. Paleomagnetism of the Bonin Islands and its tectonic significance. Tectonophysics 95, 25-42. Kuzmin, M. I., Filipova, I. and Zonenshain, V. 1989. Evolution of Mongol-Okhotsk. In: 28th Int. Geol. Cong. Abstracts 2, 244 245. Larsen, E. E., Reynolds, R. L., Ozima, M., Aoki, Y., Kinoshita, H., Zasshu, S., Kawai, N., Nakejima, T., Hirooka, K., Merrill, R. and Levi, S. 1975. Paleomagnetism of Miocene volcanic rocks from Guam and the curvature of the southern Mariana Island arc. Geol. Soc. Am. Bull. 86, 346-350. LePichon, X. 1988. Cours au College de France: Geodynamique-Resume des cours et travaux, pp. 135-144. Lewis, S. D., Mzorowski, C. L. and Hayes, D. E. 1982. The origin of the West Philippine Basin by inter-arc spreading. In: Geology and Tectonics of the Luzon-Mariana Region (Edited by Balce, G. R. and Zanoria, A. S.). Proc. CCOP-SEATAR Workshop, pp. 31-51.

Tertiary paleomagnetism of regions around the South China Sea Li, Huamei and Ye, Su-jan. 1965. Paleomagnetism of some Redbeds in Yichang of Hubei and in Hengyang of Human. Geotectonics Science Press, Beijing. Lin, Jin-lu. 1988. Paleomagnetic constraints on accretion and continental growth of eastern Asia. In: Tectonics of Eastern Asia and Western Pacific Continental Margin (Edited by Kono, M, et al.), pp. 61-62. Tokyo. Lin, Jin-lu, Fuller, M. and Zhang, Wen-you. 1985. Preliminary Phanerozoic polar wander paths for the north and south China blocks. Nature 311, 444-449. Logan, J. R. 1848. Sketch of the physical geography and geology of the Malay Peninsula. J. Indian Archipelago 2, 83-138. Louden, K. E. 1977. Paleomagnetism of DSDP sediments, phase shifting of magnetic anomalies and rotations of the West Philippine Basin. J. geophys. Res. 82, 2989-3002. Lumadyo, E., McCabe, R., Harder, S., Merrill, D. and Lee, T. 1990. Borneo: A stable protion of the Eurasian Margin since the Eocene (Abstract). EOS 71 (43), 1642, Luyendyk, B. P., Kamerling, M. J., Terres, R. R. and Hornafius, J. S. 1985. Simple shear of southern California during Neogene time suggested by paleomagnetic declinations. J. geophys. Res. 90 (12), 454--466. Maranate, S. 1982. Paleomagnetism of the Khorat Group Northeast Thailand. Unpublished M.Sc. thesis, Victoria University of Wellington. Maranate, S. and Vella, P. 1986. Paleomagnetism of the Khorat group, Mesozoic, Northeast Thailand. J. SE Asian Earth Sci. 1, 23-3 I. Marchadier, Y. and Rangin, C. 1990. Polyphase tectonics at the southern tip of the Manila Trench Mindoro-Tablas Islands, Philippines. Tectonophysics 183, 273-287. McCabe, R., Almasco, J. and Diegor, W. 1982. Geologic and paleomagnetic evidence for a possible Miocene collision in western Panay, central Philippines. Geology 10, 325-329. McCabe, R., Almasco, J., Shibuya, H., Zanoria, W., Pagnitan, H. and Torii, M. 1984. Evidence against Late Neogene rotations of the island of Luzon. In: Paleomagnetic Research in Southeast and East Asia (Edited by Briden, J.). CCOP Bangkok, pp. 117-129. McCabe, R., Almasco, J. and Yumul, G. 1985. Terranes of the Central Philippines. In: Tectonostratigraphic Terranes of the Circurn-Pacific Region (Edited by Howell, D. H.). Circum-Pacific Energy Miner, Resources Earth Sci. ser. 1,421-436. McCabe, R., Celaya, M., Cole, J., Han, H.-C., Ohnstadt, T., Paijitprapapon, T. and Thitipawarn, V. 1988. Extension tectonics: The Neogene opening of the north-south trending basins of Central Thailand. J. geophys. Res. 93 (B10), 11,899-11,910. McCabe, R. and Cole, J. 1986. Speculations on the Late Mesozoic and Cenozoic evolution of the Southeast Asian margin. Transactions of 4th Circum-Pacific Energy and Mineral Resources Conf., pp. 375-394. McCabe, R., Kikawa, E., Cole, J. T., Malicse, A. J., Baldauf, P. E., Yumul, J. and Almasco, J. 1987. Paleomagnetic results from Luzon and the central Philippines. J. geophys. Res. 92, 555-580. McCabe, R. and Uyeda, S. 1983. Hypothetical model for the bending of the Mariana arc. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and lslands, Part 2 (Edited by Hayes, D. E.). Am. Geophys. Union Mon, 27, 281-293. McElhinny, M. W., Haile, N. S. and Crawford, A. R. 1974. Paleomagnetic evidence shows Malay Peninsula was not part of Gondwanaland. Nature 252, 641~i45. Metcalfe, I. 1990. Allochthonous terrane processes in Southeast Asia. Phil. Trans. R. Soc. Lond. A 33, 625-640. Mitchell, A. G., Hernandez, F. and Dela Cruz, A. P. 1986. Cenozoic evolution of the Philippine Archipelago. J. SE Asian Earth Sci. 1, 3-22. Molnar, P. and Tapponnier, P. 1975. Cenozoic tectonics of Asia, effects of continental collision. Science 189, 419-426. Rangin, C., Jolivet, L., Pubellier, M. and the Tethys Working Group, 1990. A simple model for the tectonic evolution of southeast Asia and Indonesia region for the past 43 m.y. Bull. Soc. geol. France (8), t VI, No. 6, 889-905. Rangin, C., Muller, C. and Porth, H. 1989. Neogene geodynamic evolution of the Visayan region. Geol. ,lb. bT0, 7-27. Rangin, C., Stephan, J. F. and Muller, C. 1985. Middle Oligocene

183

oceanic crust of South China Sea jammed into Mindoro collision zone (Philippines). Geology 13, 425-428. Ron, H., Freund, R., Garfunkel, Z. and Nur, A. 1984. Block rotation by strike-slip faulting: Structural and paleomagnetic evidence. J. geophys. Res. 89, 6256-6270. Ru, K. and Pigott, J. D. 1986. Episodic tiffing and subsidence in the South China Sea. Am. Assoc. Petrol. Geol. Bull. 70, 1136-1155. Sarewitz, D. R. and Karig, D. E. 1986. Processes of allochthonous terranes, Mindoro Island, Philippines. Tectonics 5, 526-552. Schmidtke, E., Dunn, J. R., Fuller, M. and Haston, R. 1985. Preliminary paleomagnetic results from Sabah, East Malaysia (Abstract). EOS 66, 46, 864. Schmidtke, E., Fuller, M. and Haston, R. 1990. Paleomagnetic data from Sarawak, Malaysian Borneo, and the Late Mesozoic and Cenozoic tectonics of Sundaland. Tectonics 9, 1, 123-140. Scrivenor, J. B. 1928. The Geology of the Malayan Ore Deposits. Macmillan Press, London. Scrivenor, J. B. 1931. The Geology of Malaya. Macmillan Press, London. Seno, T. and Maruyama, S. 1984. Paleogeographic reconstruction and origin of the Philippine Sea. Tectonophysics 102, 53-84. Shibuya, H., Hsu, V., Merrill, D. and ODP Leg 124 Scientists, 1989. Paleomagnetic results of ODP Leg 124, Celebes and Sulu Seas (Abstract). EOS 70, 43, 1365. Sunata, W. and Wahyono, H. 1987. APWP untuk Kalimantan Barat (Abstract). GRDC Colloquium, Bandung. Sunata, W., Wahyono, H. and Sanyoto, P. 1987. Magnet Purba Dari Beperapa Batuan Beku di Daerah Putussibau, Kalimantan Barat (Abstract). GRDC Colloquium, Bandung. Tapponier, P., Peltzer, G., Le Dain, A. Y., Armijo, R. and Cobbold, P. 1982. Propagating extrusion tectonics in Asia, new insights from simple experiments with plasticene. Geology I0, 611~516. Taylor, B. and Hayes, D. E. 1980. The tectonic evolution of the South China Sea Basin. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 1 (Edited by Hayes, D. E.). Am. Geophys. Union Mort. 22, 89-104. Teng, L. S. 1990. Geotectonic evolution of late Cenozoic arc-continent collision in Taiwan. Tectonophysics 183, 57-76. Uyeda, S. and Ben-Avraham, Z. 1972. Origin and development of the Philippine Sea. Nature, Lond. 240, 176-178. Van Bemmelen, R. W. 1949. The Geology of Indonesia, Vol. IA, 732pp. Government Printing Office, The Hague. Vogt, P. R. 1973. Subduction and aseismic ridges. Nature 241, 189. Wahyono, H. and Sunata, W. 1987. Posisi lintang purba Kalimantan sejak Mesozoikum awal berdasarken hasil penelitian magnet purba. Assoc. lndon. Geophysics 12 Ann Meeting. Wells, R. and Coe, R. S. 1985. Paleomagnetism and geology of Eocene volcanic rocks of southwest Washington, implications for mechanisms of tectonic rotation. J. geophys. Res. 90, B20, 1925-1947. Williams, P. R., Supriatna, S. and Harahap, B. 1986. Cretaceous Melange in West Kalimantan and its implications. Bull. Geol. Soc. Malaysia 119, 69 78. Williams, P. R. and Harahap, B. H. 1987. Preliminary geochemical and age data from postsubduction intrusive rocks, northwest Borneo. Aust. J. Earth. Sci. 34, 405-415. Williams, P. R., Johnston, R. A. and Simamora, W. H. 1988. Late Cretaceous to Early Tertiary structural elements of West Kalimantan. Tectonophysics 148, 279-297. Wood. B. G. M. 1985. The mechanics of progressive deformation in crustal plates--a working model for S.E. Asia. Geol. Soc. Malaysia Bull. 18, 55 99. Yan, C. and Courtillot, V. 1989. Widespread Cenozoic (?) remagnetization in Thailand and its implications for the India-Asia collision. Earth planet. Sci. Lett. 93, 113-122. Zhao, Xi-xi and Coe, R. S. 1987. Paleomagnetic constraints on collision and rotation of North and South China. Nature 327, 141-144. Zhao, Xi-xi, Coe, R. S., Zhou, Yao-xiu, Wu, Harouo and Wang Yie. in press. New paleomagnetic results from China: collision and suturing with Soviet plates. Tectonophysics. Zhuang Zhonghai, Tian Duanxiao, Ma Xinghua, Ren Xifei, Jiang Xingzhi and Xu, Shujin. 1988. Cretaceous-lower Tertiary paleomagnetism of the Ya'an-Tianquan section in the Sechuan basin. Geophys. Geochem. Expl. 12, 224-227.

Petchaburi Basin

Thailand Krabi Basin

Malaysia Langkawi Island

Central Palawan

T90.34 T90.35 T90.36

T90.16

$86.62 $86.63

P86.2 P86.3 P86.5 P86.11 P86.21 P86.22

P88.1 P88.2 P88.7 P88. I0 P88.12

P83.5 P84.7 P84.5 P84.7 P84.8 P84.9

Batangas

Palawan S. Ophiolite

P89.30 P89.31

P88.1 P88.2 P88.9 P88.12

Site

Southern Sierra Madre

Phillippines Luzon Central Cordillera

Location

13.1 13.1 13.1

8.1

6.2 6.2

9.9 9.9 9.9 9.9 9.8 9.8

9.3 9.3 8.8 8.8 8.9

13.7 13.7 13.7 13.7 13.7 13.7

14.9 14.9

16.4 16.4 16.5 16.5

Lat. (°N)

99.8 99.8 99.8

98.9

99.8 99.8

118.7 118.7 118.7 118.7 118.7 118.7

118.2 118.2 117.7 117.7 117.8

121.2 121.2 121.2 121.1 121.1 121.1

121.2 121.2

120.9 120.9 120.7 120.7

Long. (°E)

Mio(?) Mio(?) Mio(?)

Mio(?)

Silurian Silurian

Eoc/K Eoc/K Eoc/K Eoc/K Eoc/K Eoc/K

Eoc/K Eoc/K Eoc/K Eoc/K Eoc/K

Mio/Pli Mio/Pli Mio/Pli Mio/Pli Mio/Pli Mio/Pli

Miocene Miocene

Miocene Miocene Miocene Miocene

Age

Dyke Dyke Dyke Dyke

Seds Seds Seds

Seds

Lst. Lst.

Basalt Basalt Basalt Basalt Basalt Basalt

Basalt Basalt Basalt Basalt Basalt

Andesite Andesite Andesite Andesite Andesite Andesite

Flows Flows Flows Flows Flows Flows

Flows Flows Flows Flows Flows

Talahib Talahib Talahib Talahib Talahib Talahib

Basalt Dyke Basalt Dyke

Basalt Basalt Basalt Basalt

Rock type

5 5 6

8

7 6

7 7 6 6 7 7

8 11 6 12 7

5 9 11 9 9 13

7 7

5 5 6 5

N

APPENDIX

3.8 -25.4 - 14.3

15.9

17.0 13.9

39.8 -53.2 - 55.9 - 50.3 15.4 28.6

-34.3 - 10.1 -6.3 11.7 9.9

-4.8 5.3 19.3 -23.4 -28.3 -29.5

12.0 14.6

35.7 32.8 - 1.3 5.2

I

7.6 212.7 195.4

213.2

36.5 39.1

311.6 207.9 255.4 86.3 56.6 34.4

119.0 117.5 295.0 298.1 298.4

129.8 140.5 319.8 144.9 156.5 160.2

295.1 350.9

300.8 326.7 313.8 326.4

D

7.4 26.0 16.0

12.8

9.9 11.7

6.8 10.9 I 1.5 6.3 10.4 14.9

7.1 5.1 13.7 4.4 11.1

6.8 4.1 1.8 8.2 2.3 2.6

3.8 3.0

6.8 8.9 7.7 8.0

~95

157 10 19

20

60 34

79 31 35 I 13 35 17

62 81 25 100 31

127 162 647 40 516 248

248 414

284 76 76 93

k

342/33N 352/10N 020/14E

330/28N

347/40E 170/29W

290/66N 290/66N 290/66N 290/66N ---

215/30W 185/12W 170/20W -100/25S

075/35S 075/35S 075/35S 119/14S 119/14S 119/14S

---

014/24W 014/24W ---

Struct. Corr.

-1.8 -18.7 - 15.0

40.1

- 13.5 34.4

1.0 12.6 -- 2.6 - 33.9 ---

--4.5 -0.5 -22.6 11.7 16.3

-32.0 -26.3 49.2 -28.6 -35.3 -37.7

12.0 14.6

12.8 13.8 ---

I'

13.1 215.7 199.1

205.3

36.1 29.0

336.3 205.1 227.5 156.7 ---

120.2 117.6 298.2 298.1 292.7

122.0 137.6 306.4 138.7 150.4 153.3

295.1 350.9

301.0 320.4 ---

D'

2

t-

,-13

~r

OO