Tec~onop/yrics, 98 ( 1983) I- 10 1

Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

PALEOMAGNETIC AND PLATE TECTONIC CONSTRAINTS ON THE MOVEMENT OF TIBET

MICHEL WESTPHAL and JEAN-PIERRE POZZI

Institut de Physique du Globe de Strasbourg, 5 rue R. Descartes, F- 67084 Strasbourg (France)

Institur de Physique du Globe de Paris, 4 place Jussieu, F-75230 Paris (France)

(Received March 28, 1983)

ABSTRACT

Westphal, M. and Pozzi, J.-P., 1983. Paleomagnetism and plate tectonic constraints on the movement of

Tibet. In: E. McClelland Brown and J. VandenBerg (Editors), Palaeomagnetism of Orogenic Belts.

Tectonophysics, 98: I- 10.

The paleomagnetic results from Tibet, north of the Yarlung-Zang bo suture zone, show that Tibet was

at about 15”-20”N in Middle Cretaceous time. It then moved south down to 7”-10”N in the Late

Cretaceous-Pa&gene. The oceanic crust of the Xigaze ophiolites was magnetized at 13ON but thereafter

migrated further south. This movement is compared with the relative movement of India and Asia as

deduced from magnetic anomalies and paleomagnetism. Experimental models on deformation help us to

explain how Tibet moved during the Late Cretaceous under the constraint of the Africa-Arabia indenter

and during the Upper Tertiary under the constraint of the Indian indenter.

INTRODUCTION

Tibet, Iran and Afghanistan are fragments of continents now situated north of the

most evident suture zones between Asia and Africa-India, i.e. the Zagros and Indus

Zang bo sutures. But in former times, in the Paleozoic and probably still in the

Triassic, they were parts of Gondwana (Soffel and Forster, 1980; Wensink, 1982).

The special problem with Tibet is that it has been strongly pushed about 1000-2000

km into Asia. Present paleomagnetic data and plate tectonic models give us more

information about these movements.

STRUCTURE OF TIBET

Tibet is a broad domain situated between the High Himalayan fold belt and the

great shear zones of Kunlun and Altyn Tagh (Sengb, 198 1; Tapponnier et al., 1981).

Tibet can be divided in two main parts. The first, larger one lies north of the

Yarlung Zang bo suture zone (Fig. 1). Its southwestern part is called the Lhasa

OO40-1951/83/$03.00 0 1983 Elsevier Science Publishers B.V.

4RABIA -* , I.

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. . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 1. A. General outline of South Asia with the main blocks, strike-slip faults and suture zones. The

large arrows indicate the relative movement of India, Arabia, Tibet and Indochina. Za = Zagros suture.

YZb = Yarlung Zangbo suture, MCT = Himalaya main central thrust.

B. Geological outline of Lhasa region. Black dots: main sampling zones. K = Takena and Linzizong

formation, G = Gangdeze granites and granodiorites, TJ = Triassic and Jurassic north of Yarlung Zang

bo, P = Paleozoic, Xf = Xigaze flysch, 0 = ophiolites, Hf = North Himalayan flyschs.

3

Block. Its Mesozoic history is mainly continental and it seems to have been attached

to Asia since the Jurassic. To the north, older sutures zones have been identified or

are suspected. The folding of this part, mainly Tertiary, is not very important. The

south of this domain is bordered by the Gangdeze and the Ladakh-Kohistan

batholith, interpreted as Andean margins.

South of the Yarlung Zang bo suture zone the structure is very different. The

sedimentation is mostly marine and the deformations, much more importantly,

began earlier than in the north. The age of the radiolarites just on the top of the

ophiolites is Aptian-Cenomanian (Nicolas et al., 1981). The structure is interpreted

as a slow accretion ridge with an opening direction estimated as N30”W in its

present orientation.

THE PALEOMAGNETIC RESULTS

Paleomagnetic results are available north of the Yarlung Zang bo zone for

Jurassic limestones, Middle Cretaceous redbeds and Late Cretaceous-Early Tertiary

volcanics and sediments in the vicinity of Lhasa. The Jurassic limestones have been

studied by Chinese paleomagneticians (Zhu Zhi Wen et al., 1981). The samples have

been demagnetized only by alternating fields up to 40 mT and only 10% of the initial

magnetization was removed. These rocks show a reversed direction with a nearly

equatorial position for the Jurassic limestones. As the samples have not been

completely demagnetized, a systematic error due to hard viscous components is

possible. The true paleolatitude may be higher. The Middle-Late Cretaceous red-

beds (Takena formations of Aptian age and younger) have been studied by several

authors and at several widely-spread locations (Zhu Xiang Yuan et al., 1977; Zhu

Zhi Wen et al., 1981; Achache et al., 1982; Pozzi et al., 1982; Westphal et al., 1983).

Chinese paleomagneticians used only weak alternating field demagnetization.

They could not unravel all the magnetic components. Our samples (Pozzi et al.,

1982; Westphal et al., 1983) have been thermally cleaned step-by-step. A stable,

characteristic magnetic direction is obtained often only above 590°C. The compo-

nent shows a significantly positive fold test. The corresponding paleolatitude is

about 20°N. It has been argued that some of the magnetic directions are more

Indian-like than Asian-like and that, therefore, Tibet was attached to India and that

these formations were remagnetized at low latitudes (from 20”s to 5ON). This has

already been discussed by Pozzi et al. (1982). It would mean that all the sites have

been remagnetized at very different periods even though some sites are very close.

We feel that this is very improbable and that the positive fold test is a strong

argument for primacy of the magnetization. This magnetization was acquired in

Middle Cretaceous or, at the latest, just at the beginning of the folding (Late

Cretaceous).

The field direction is D = 333”, I = 38”, ffgs = 8”. Achache et al. (1982) have

extended the sampling to the north and obtained similar results. These redbeds yield

4

a paleolatitude of 15” to 20’N. A first site of green and red sandstones just on top of

the Takena formation gives a shallower inclination of 24” with a declination of 3”. A

second site of volcanics, belonging to the Lingzizong formation and dated by K-Ar

as 48 Ma (in Westphal et al., 1983) gives an inclination of 11’ with a declination of

358’. Maluski et al. (1982) have dated another flow from the same formation by the

39Ar/40Ar method. This method gave a single plateau at 60 Ma.

In both sites, normal and reversed magnetizations are present (Westphal et al.,

1983). The presence of such a shallow inclination in Early Tertiary volcanics has

been confirmed by Achache et al. (1982). The paleolatitude was about 5”- 10”N.

On the western end of the Indus Yarlung Zang bo suture zone, but north of it,

Klootwijk et al. (1979) found a shallow paleolatitude for the Ladakh intrusion of

about 7”-10”N and for the Late Paleocene-Eocene material. Younger, secondary

components give higher paleolatitudes (Table I).

On the suture zone itself, we have sampled basalts and pillow lavas on top of the

ophiolites, radiolarites just above them, and several sites in the Xigaze flysch. The

paleomagnetic results are complicated, secondary magnetization being often pre-

dominant (Pozzi and Westphal, in prep.). Our most reliable results have been

obtained on a lava-radiolarites sequence near Xigaze (Table I, site 14R and 14B).

The radiolarites are dated as Albian-Cenomanian. They have thus most probably

been magnetized during the Cretaceous long normal polarity interval. They show a

paleolatitude of 13”N with a strong anticlockwise rotation of 90”-100”. This means

that the crust now abducted there did not become magnetized far from the Lhasa

TABLE I

Main paleomagnetic results from Tibet

Components D(O) 1(O) Paleo- References

latitude

North of the Indus - Yarlung Zang bo suture zone

Lhasa region

Upper Takena and Lingzizong 0 18 9”N Westphal et al. (1982)

Takena redbeds 333 38 20”N Westphal et al. (1982)

Jurassic limestones 175 2 1”S Zhu Zhi Wen et al. (1982)

Lndakh intrusions Klootwijk et al. (1979)

Component 6 3 15 7”N

Component 5 355 19 lOoN

Zang bo suture zone, Xigare region Pozzi et al. (in prep.)

Basalts, primary magnetization, site 14B 239 24 12”N

Radiolarites, primary magnetization, site 14R 278 28 14”N

Basalts, Primary magnetization?, site 21 309 - 10 50s

5

block. The accretion direction, corrected for the rotation, is about N60”-N70”E.

A probably younger magnetization (site 21) gives a shallower paleolatitude.

INTERPRETATION

The Late Tertiary movements of Tibet were certainly driven by the India-Asia

collision. India and Eurasia’s relative displacements can be checked by separate

methods.

(1) Paleomagnetic measurements on both Eurasia and India. We have recalcu-

lated the expected paleolatitudes for a point of coordinates 30°N and 90°E (near

Lhasa). For Eurasia we took first Irving’s polar wander curve (1977) and we checked

it against Harrisson and Lindh’s (1982) composite paleomagnetic curve for Europe

(Fig. 2A). In the Tertiary the expected paleolatitudes are always between 28” and

33”N. Irving’s curve showed a slight decrease in paleolatitudes in the Lower

Tertiary, which is not seen in Harrisson and Lindh’s curve.

For the Cretaceous the curves are different. These differences are mainly due to

the lack of good Middle Cretaceous European data and also to possible errors in the

transfer from the North and South American poles to Eurasia in the Cretaceous,

where rotation parameters are less precise. We think that these two curves should be

taken as extreme values.

For India we used Klootwijk’s (1979a, b) compilation of Indian data. During the

Mesozoic, India was far into the Southern Hemisphere. It started its northward drift

in the Late Cretaceous and crossed the equator about 45 Ma ago (Fig. 2A).

(2) Paleomagnetic measurements on D.S.D.P. cores from the Indian plate (Pierce,

1978). These measurements gave only inclination data that are difficult to transfer to

the 30”N, 90’E point. Klootwijk (1979a) estimated paleodeclinations and was then

able to compute virtual geomagnetic poles. These were used in Fig. 2B. Here India

crossed the equator about 50 Ma ago.

(3) The India-Asia movement can be deduced from marine magnetic anomalies

found in the West Indian Ocean (India-Africa movement) and Atlantic Ocean

(Africa-America-Europe movement) (Patriat et al., 1982).

Assuming Asia to be fixed, it is possible to compute the relative movement of

India at the same point (Fig. 2C). This gives the relative paleolatitudes of India.

Obviously they should be corrected for the absolute movement of Asia in order to

compare them to other curves. As seen in Fig. 2A, corrections are small for the

Tertiary but unfortunately dubious for the Cretaceous. In this last figure India

crossed the equator about 55 Ma ago.

(4) The movement of Tibet from paleomagnetic data. The Chinese Jurassic results

show a low latitude for the Lhasa block. The actual error margin is not known. In

the Middle Cretaceous the Lhasa block was about 15’-20”N. Younger formations

again gave lower paleolatitudes about 7”-1O”N (Fig. 2A). Before 50 Ma the Lhasa

block, and probably the whole northern part of Tibet, was far from India but close

Ma

India

/ - -v

3: ,30N

0

50s

30N

Ma 100

Fig. 2. Latitudinal positions of Asia, India, Lhasa block, Xigaze region and Ladakh with time and

transferred to the point 30”N, 90’E.

A. Paleomagnetic datas from Asia (black circles--Irving, 1977; white circles-Harrisson and Lindh, -+

to Asia. The Late Cretaceous-Early Tertiary southward drift of the Lhasa block is

stronger than the Asian movement expected from Irving’s curve and opposite to the

one expected from Harrisson and Lindh’s curve. This means that although Tibet was

close to Asia, in contact with it, there was still some uncoupling between them. It is

very difficult, on geological grounds, to imagine that a basin opened between Tibet

and Asia in the Late Cretaceous-Early Tertiary and closed afterwards. There are no

traces of it. If longitudinal movements occurred along an oblique margin, such

movements are easier to explain.

The final part of the movement of Tibet is explained by experiments performed

by Peltzer et al. (1982) and Tapponnier et al. (1982). They studied the deformation

of a plasticine block compressed by a rigid rectangular indenter coming from the

south. The plasticine block has its north and west side bounded, but the east side is

free. These experiments showed the formation of a triangle in front of the indenter,

only weakly deformed, pushed toward the northeast. Parts located to the east of the

indenter are pushed toward the southeast in a clockwise rotation. For our present

purpose the triangle represents Tibet and the side blocks Indochina and south China

(Fig. 1).

But the southward movement of Tibet in the Late Cretaceous and Early Tertiary

is not explained by the Peltzer model when applied to India. At that same time India

was still far into the Southern Hemisphere and probably separated from Asia by an

oceanic crust. The movements of Africa and India relative to Eurasia are shown in

Fig. 3. For India, this movement is shown for a point now at 30”N and 90’E (near

Lhasa) and for Africa-Arabia for a point at 30”N and 50”E (near Abadan in the

Persian Gulf) (Fig. 3A).

The movement of Africa-Arabia is slower than India’s, but much more continu-

ous, and compression between Arabia and Asia began as soon as the Early

Cretaceous (Fig. 2D). Arabia may thus have behaved as a rigid indenter, like India.

If we pull Tibet backwards to the southwest in the more shallower latitudes as

suggested by paleomagnetic observations, in a direction more or less parallel to the

Altyn Tagh and Tien Chan faults, according to the Peltzer et al. (1982) experiments

we find that in the Middle Tertiary Tibet was much closer to Arabia. It is then an

attractive hypothesis that Tibet, and probably Iran, have been pushed to the

southeast like Indochina now. This explains the southward movements of Tibet. We

should note that a southward movement of Central Iran of about 5”-10” can also be

1982), India (triangles-Klootwijk et al., 1979), Lhasa block and Xigaze region (five point stars-Zhu Zhi

Wen 1981 and Westphal et al., 1983), and Ladakh (small star-Klootwijk et al., 1979).

B. Data from D.S.D.P. cores in the Indian plate (Peirce, 1978 and Klootwijk, 1979a).

C and D. Relative movement of India (C) Africa-Arabia (D) and Asia. They are given for different

anomalies: 6 (20 Ma), 13 (35 Ma), 24 (55 Ma), 34 (80 Ma) and MO (110 Ma). The movement of India is

calculated for point 30”N, 90”E and the movement of Africa for point 30”N, 50°E. These relative

paleolatitudes should be corrected on the basis of the Eurasia curve (in A) to get true paleolatitudes. This

correction is small from 60 Ma until now.

Fig.

3.

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.

9

seen in Soffel and Forster’s (1980, table 4) results for the beginning of the Tertiary. The presence of young oceanic crust in Xigaze can be explained by distension not

only similar to the extension now observed in the Andaman Sea or in the South China Sea but also, and more probably, the northern tip of the rift that separated India from Africa in the Early-Middle Cretaceous.

CONCLUSION

The history of Tibet may be now summarized as follows: in the Trias, Tibet, like Iran, was probably near Gondwana lands in the Southern Hemisphere (Soffel and Forster, 1980; Wensink, 1982). It moved north during a yet unknown period but was near the southern margin of Asia, at 15”-20”N, in the Middle Cretaceous. When Africa began to move northward toward Asia in the Middle Cretaceous, Tibet was pushed to the southeast. It is possible that a first tectonic phase between the then rapidly rising India and the pelagic crust already took place. Continental collision between Tibet and India began only later on, probably in the Eocene (Fig. 3B). The movement of India was then the main driving force for Tibet; strike-slip motion along Altyn Tagh, Tien Chan and crustal thickening may account for the northward movement of Tibet (Fig. 3C).

ACKNOWLEDGEMENTS

The paleomagnetic work, sampling and measurements have been done under a contract between the French Centre National de la Recherche Scientifique and the People’s Republic of China Ministry of Geology and Academy of Sciences.

The authors gratefully thank M.M. Zhou Yao Xiu, Xing Li Sheng and Chen Xian Yao for their help both in the field and the laboratories. They are also grateful to C. Klootwijk for critical and helpful discussions and comments.

This is a contribution of CNRS Laboratoire Associt de Geophysique Inteme de Strasbourg (LA 323) and Equipe de Recherche Associe de Gtomagnttisme et Pal6omagnetisme (ERA 922).

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