Mesozoic and Cenozoic tectonics of the northern edge of the

Tibetan plateau: fission-track constraints

M. Jolivet a,*, M. Brunel a, D. Seward b, Z. Xu c, J. Yang c, F. Roger d, P. Tapponnier d,J. Malavieille a, N. Arnaud e, C. Wu c

aLaboratoire GTS, CC 060, Universite Montpellier II, Place E. Bataillon, 34095 Montpellier cedex 5, FrancebGeology Institute, ETH Zentrum, Sonneggstrasse 5, CH-8092 Zurich, Switzerland

cInstitute of Geology, Chinese Academy of Geological Sciences, Beijing, ChinadLaboratoire de Geochronologie, Univ. Paris7-IPG Paris, CNRS-UMR 7578, 4 Place Jussieu, 75252 Paris cedex 05, France

eCNRS UMR6524, Magmas et Volcans, 63000 Clermont-Ferrand, France

Received 18 January 2001; accepted 3 September 2001

Abstract

Fission-track analysis on zircons and apatites yields new information about the timing of deformation of the northern Tibetan

plateau. Ages on zircons, ranging from 221 ± 22 to 96 ± 4 Ma are indicative of a general late Triassic–early Jurassic cooling

probably driven by the collision between the Qiantang and Kunlun blocks. Mid-Jurassic slow cooling is recorded also in the

apatites in regions not affected by later Cenozoic deformation. This Jurassic denudation was followed by a period of

sedimentation during the Cretaceous, except along the Altyn Tagh fault (ATF) zone, and in some restricted areas of the western

and eastern Qilian Shan. This long and relatively quiet period ended at about 40 ± 10 Ma along the major Altyn Tagh and Kunlun

strike-slip fault zones, which were activated by the India–Asia collision. This first movement along lithospheric faults resulted

in the eastward extrusion of the Tibet plateau, which was followed, in late Oligocene–Miocene times, by a major compression

event, initiating the formation of the high relief of north Tibet. A final compressional event took place at 9–5 Ma and is well

correlated with high sedimentation rates in the basins of this region. This compression induced continental subduction in the

Kunlun ranges, the Altun Shan belt, and possibly the Qilian Shan belt. D 2001 Elsevier Science B.V. All rights reserved.

Keywords: Fission-track analysis; Tibet; Exhumation rates; Mesozoic; Cenozoic

1. Introduction

Plate kinematics predict that India has moved north-

ward some 2500 km relative to Asia since they collided

about 50 Ma ago (Patriat and Achache, 1984; Besse et

al., 1984; Tapponnier et al., 1990b). This collision

appears to be diachronous along strike, starting in the

late Ypresian (� 52Ma) in the Zanskar–Hazara region

(western Himalaya) and probably after 45 Ma in the

eastern part of the Himalayan belt (Searle et al., 1997;

0040-1951/01/$ - see front matter D 2001 Elsevier Science B.V. All rights reserved.

PII: S0040-1951 (01 )00196 -2

* Corresponding author.

E-mail addresses: jolivet@dstu.univ-montp2.fr (M. Jolivet),

brunel@dstu.univ-montp2.fr (M. Brunel), diane@erdw.ethz.ch

(D. Seward), yangjsui@publicbtanetcn (J. Yang),

roger@ipgp.jussieu.fr (F. Roger), tappno@ipgp.jussieu.fr

(P. Tapponnier), arnaud@opgc.univ-bpclermont.fr (N. Arnaud).

www.elsevier.com/locate/tecto

Tectonophysics 343 (2001) 111–134

Rowley, 1998). The large amount of convergence

involves an important deformation of the Asian plate.

Despite differences in explaining the mechanism of its

formation, it is widely admitted that the main uplift of

the Tibet plateau results from the India–Asia continen-

tal collision (e.g. Dewey and Burke, 1973; Bird, 1978;

England and Houseman, 1986; Tapponnier et al., 1986;

Peltzer and Tapponnier, 1988; Molnar, 1988; Mattauer,

1990; Burg et al., 1995; Royden et al., 1997). The

northern edge of the Tibet plateau, between the Kunlun

and the Altyn Tagh lithospheric strike-slip faults (Tap-

ponnier and Molnar, 1977; Peltzer and Tapponnier,

1988; Wittlinger et al., 1998) is characterized by active

crustal shortening, leading to the formation of numer-

ous mountain ranges (Kunlun, Altun Shan, Qilian

Shan, etc. . .) separated by endoreic basins (Fig. 1).

These basins are filled with several kilometers of

Tertiary and Quaternary sediments (Metivier et al.,

1998; Vincent and Allen, 1999), implying very active

erosion over this time period. Meyer et al. (1998) and

Metivier et al. (1998) reported in detail on the tectonic

and sedimentation history of north Tibet. Meyer et al.

(1998) proposed a subduction of the Qaidam basin

underneath the Tibetan plateau along the Kunlun fault,

starting less than 15 Ma ago (see Fig. 10c in Meyer et

al., 1998). The Qilian Shan belt would thus have been

formed by thrusting in the Qaidam crust along a large

decollement level in the lower crust (Tapponnier et al.,

1990a), which implies a northeastward propagation of

the deformation in north Tibet, from the older Kunlun

and Qimantag ranges, to the young Qilian Shan (Meyer

et al., 1998; Metivier et al., 1998). In order to check this

hypothesis, we examined at the exhumation history of

the main mountain ranges in the area around the

Qaidam basin. The initial assumption was that if there

is a northward propagation of the deformation in north

Tibet, the exhumation of the southern ranges (the

Kunlun range, for example), should have occurred

before the exhumation of the northern ones (the Qilian

Shan belt). Further, in southern Asia, large movements

along lithospheric faults are generally associated with

high to medium temperature metamorphism in the rock

suites now exposed (e.g. Scharer et al., 1994; Leloup et

al., 1995; Roger et al., 2000). This does not seem to be

the case in north Tibet where no metamorphism due to

Cenozoic tectonic movements has been recognized

(Mock et al., 1999; Delville et al., in press; Sobel et

al., in press). Thus, to obtain information about exhu-

mation processes around the Qaidam basin, we used

fission track analysis on zircon and apatite. This low-

temperature method gives access to the later stages of

mountain building and apatite fission track analysis in

particular allows differentiation of vertical movements

in the upper 3 or 4 km of the upper crust. This paper

presents the results that yield new data on the Mesozoic

and Cenozoic tectonic history of north Tibet, between

the Altyn Tagh fault to the north, the Kunlun fault to the

south and west, and the Gobi desert to the east (Fig. 1).

2. Geological setting

North Tibet consists of several structural zones: (a)

the Tarim, (b) the Altyn Tagh fault system and

associated mountain system, including the Altun Shan

belt, (c) the Qaidam basin, (d) the Kunlun Fault

system with its associated mountain belt, and finally

(e) the Qilian Shan range (Fig. 1).

(a) The Tarim Basin: The following geological

description is summarized from Tian et al. (1989),

Lee (1985), Sobel (1995) and Hendrix et al. (1992).

The Tarim basement is composed of Archean and

Lower Proterozoic deeply metamorphosed gneiss.

Until Permian times, marine sediments were deposited

on this basement, but from the beginning of the

Mesozoic, sedimentation was dominantly continental.

Triassic sediments are composed of coarse clastic

piedmont deposits and shallow lake facies. The Juras-

sic sequence unconformably overlies these Triassic

sediments along the southern border of the basin.

They are coal-bearing continental formations (lower

to middle Jurassic), followed by coarse clastic depos-

its (upper Jurassic). Cretaceous and Paleogene series

are composed of red sandstones, gypsum, red clastic

sediments and argillaceous limestones from the east to

the west of the basin. During Oligo–Miocene times,

the Tarim basin subsided, and since Miocene times, a

huge thickness of conglomerates has been deposited.

Late Tertiary deposits, 6000–8000 m, are recorded in

the southwest depression, mostly composed of Plio-

cene conglomerates (Zhou et al., 1984; Sobel and

Dumitru, 1997).

(b) The Altyn Tagh fault (ATF) system and the

Altun Shan belt: The ATF is the longest (� 1800 km)

active strike-slip fault of Asia (Tapponnier and Mol-

nar, 1977; Peltzer and Tapponnier, 1988; Meyer et al.,

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134112

Fig. 1. Tectonic sketch of the north Tibet area. Topography is from DCW database, faults are drawn from field mapping, Landsat and Spot satellite photographs and published data (Gaudemer et al., 1995; Meyer et al., 1996, 1998; Lasserre et al., 1999). A.T.F—Altyn Tagh Fault; K.L.F.—Kunlun fault.

Displacement rates are from Avouac and Tapponnier (1992) and Van Der Woerd (1998). Moho depths are from McKenna and Walker (1990), Wu et al. (1996) and Meng and Cui (1997).

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134 pp. 113-114

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134 pp. 115-116

Fig. 2. Geological map of north Tibet simplified from Wang et al. (1993), with sample locations. Samples AT5 and AT32 had no apatites, only zircons. Samples AT68, AT140 and AT142 are from Jolivet et al. (1999).

1996; Sobel and Arnaud, 1999). It separates the Tarim

basin to the north from the Qaidam basin and Qian-

tang block to the south. This left-lateral strike-slip

fault is a major lithospheric boundary on the southern

edge of the Tarim, and its movement accommodates

the northeastward extrusion of the Tibetan plateau

(Tapponnier and Molnar, 1977; Peltzer and Tappon-

nier, 1988; Meyer et al., 1996). A 400 ± 60 km post-

Bajocian offset has been recognized along the western

segment of the ATF, using a piercing point consisting

of Aalenian–Bajocian deposits correlated on each

sides of the fault (Ritts and Biffi, 2000). Sobel et al.

(in press) calculate a 350 km post-Jurassic offset

along the same segment of the ATF, using cooling

ages obtained from 40Ar/39Ar diffusion modelling on

K-feldspars. The late Neogene rate of displacement

along the fault is still debated. On the basis of neo-

tectonic studies, Van Der Woerd (1998) estimated this

rate to be between 30 mm a� 1 west of the Qaidam

basin and 5 mm a� 1 near Dunhuang (Fig. 1). More

recently, Bendick et al. (2000), using GPS measure-

ments across the Altun Shan belt, calculated a strike-

slip rate of 9 ± 5 mm a� 1 along the ATF, a value two

to three times lower than the one reported by Van Der

Woerd.

Like most major strike-slip faults, the ATF is not a

discrete cut in the lithosphere but a very complex

structure. It is composed of several interconnected

segments with numerous associated thrust faults.

Several push-up structures have developed between

nearly parallel segments of the transcurrent fault.

From the western termination of the Kunlun fault to

the Gobi desert, on both sides of the fault corridor is

an associated mountain chain, the Altun Shan being

the best developed (about 100-km wide). Altitude

variation within the ranges is large, with a maximum

of nearly 4000 m on the northern side of the Altun

Shan. South of the fault corridor, these variations tend

to be lower (a maximum of 2000 m) because of the

already high mean level of the Qaidam basin (about

3000 m). Both mountain fronts, to the north and to the

south, are very active. Large rivers (especially in the

Altun Shan) cut through Quaternary sediments, indi-

cating nonmature topography. In this paper, the whole

structure around the ATF sensus stricto will be con-

sidered as the Altyn Tagh fault zone, except when

specified. A detailed geological description of the

ATF zone is given by the Chinese Bureau of Geology

(1981), Chen et al. (1985), Wang et al. (1993) and

more recently by Sobel and Arnaud (1999). Permian

and Triassic strata are absent. Coal-bearing nonmarine

lower to middle Jurassic sediments, unconformably

overlying Proterozoic and Palaeozoic igneous, meta-

morphic and sedimentary rocks are exposed in small

outcrops throughout the range. Cretaceous sediments

are not exposed. Neogene and Quaternary rocks are

continental conglomerates and sandstones, exposed in

small basins such as the Mangnai Zhen basin (Fig. 1).

Ophiolitic rocks are mapped along the fault, especially

near Hongliugou (Fig. 2), and are interpreted as early

Paleozoic oceanic material caught up in a post–early

Silurian to pre–middle Devonian suture (Sobel and

Arnaud, 1999).

(c) The Qaidam Basin: Because of its petroleum-

rich sedimentary series, the Qaidam basin has been

widely studied (e.g. Gu and Di, 1989; Wang and

Coward, 1990). The mean elevation of the basin is

between 2500 and 3000 m. The basement is com-

posed of a 1.5–2-Ga (Wang and Coward, 1990) suite

of quasi-flysch—quartzose sandstones—carbonate

formations, up to 18-km thick. During Paleozoic

times, and up to the end of the Triassic period,

sedimentation was alternately marine and continental.

From the end of the Triassic, it became exclusively

continental with lacustrine and fluviatile sediments.

Tertiary and Quaternary strata are thick (3200 m of

Quaternary sediments in some areas; Gu and Di,

1989). At the end of the Pliocene, the Qaidam basin

was uplifted by renewed tectonic movements (Metiv-

ier, 1996; Metivier et al., 1998).

The Cenozoic tectonic activity visible inside the

basin is weak, only marked by folding of the Meso-

zoic to Quaternary sedimentary cover in the northern

part. These folds trend NW–SE and are sometimes

developed into thrust faults (Fig. 1).

(d) The Kunlun range: The Kunlun fault system

marks the southern edge of the study area. This

lithospheric left-lateral strike-slip fault separates the

Qiantang block to the south from the Qaidam and

Tarim blocks to the north (Matte et al., 1996) (Fig. 1).

The slip rate near Xidatan (Fig. 1) is up to 12 mm a� 1

(Van Der Woerd et al., 1998). Meyer et al. (1998)

suggest that since about 15 Ma, the active southward

continental subduction of the Qaidam and Tarim

blocks under the Qiantang block induced deformation

on the southern edge of the Qaidam basin (Matte et

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134 117

al., 1996; Meyer et al., 1998), such as in the Qimantag

range (Fig. 1).

The geology of this largely inaccessible area is

mostly known along the Golmud to Lhasa road (Fig.

1) (Ministry of Geology and Natural Resources, 1980;

Chang et al., 1986; Kidd and Molnar, 1988). The

expedition to theUlughMuztagh area (Fig. 2) is provid-

ing some information on the western end of the Kunlun

system (Molnar et al., 1987; Burchfiel et al., 1989).

Basement rocks are middle Proterozoic gneiss and

granitoids (Harris et al., 1988; Mock et al., 1999).

Middle Paleozoic granites are exposed on the northern

side of the Kunlun fault along the whole southern

border of the Qaidam basin. Carboniferous sediments

and Permian limestones are covered by extensive

Triassic turbidites, deformed by E–W trending folds

(Burchfiel et al., 1989). Jurassic coal-bearing sedi-

ments are exposed on the western part of the Kunlun

fault around and west of the Ulugh Muztagh. Except at

a few localities, Cretaceous rocks are generally absent.

Outcrops of ophiolitic fragments are mapped along

the fault, SE of Golmud and around the Ulugh

Muztagh (Fig. 2). They are interpreted as remnants

of the late-Triassic Jinsha suture between the Kunlun

and Qiantang terranes (Harris et al., 1988; Pearce and

Mei, 1988). The extensive Triassic flysch now

thrusted on top of the Kunlun was deposited along

this subduction zone.

(e) The Qilian Shan belt: The Qilian Shan belt is a

500-km-wide fold and thrust belt, between the Qai-

dam basin and the Ala Shan massif (North China

Platform) (Fig. 1) (Tapponnier et al., 1990a). The

whole structure trends NW–SE and is limited to the

north by the ATF (Fig. 1). The Qilian Shan can be

divided into two blocks (Meng and Cui, 1997; Wu et

al., 1993, 1996):� The South Qilian block, limited to the south by

the Da Qaidam thrust zone and to the north by the Da

Xue Shan–Shule Nan Shan thrust zone (Fig. 1). It

consists of a Paleozoic metamorphic basement

intruded by large Ordovician and Silurian granites.

Small outcrops of Jurassic limestones appear along

the Da Qaidam thrust zone (Fig. 1), whereas Triassic

sediments are widely exposed north of the Qinghai

Lake (Fig. 1). Thick Cenozoic sediments accumulated

in small basins (about 5 km of Miocene and Pliocene

sediments in the Har Lake area) (Fig. 1) (Meng and

Cui, 1997; Xu et al., 1996). Along the Da Qaidam

thrust zone, outcrops of mafic to ultramafic rocks

indicate a possible Ordovician (Yang, personal com-

munication) suture zone between the Qaidam block

and the South Qilian block.� The North Qilian block, bounded to the north by

the Longshou Shan fault (Fig. 1). Its basement is

composed of a Precambrian metamorphic sequence

(Wu et al., 1993). Two Paleozoic blueschist facies

zones are exposed suggesting that an oceanic basin

existed in this area during the Ordovician (Wu et al.,

1993; Liu and Gao, 1998).

The 58–74-km-deep Moho under the inner and

western Qilian Shan (58–74 km) (Wu et al., 1996;

Meng and Cui, 1997) implies a large amount of

thickening of the crust, not taken into account in

many of the previous models. The seismic reflection

profile from Wu et al. (1996) crossing the northeastern

Qilian Shan piedmont thrust fault, south of Yumen

(Fig. 1), shows reflection planes dipping southward

between 15 and 30 km in the crust, and probably

down to the Moho at 45 km. The Moho itself may

have a discrete offset in this region, which could

represent the northern boundary of the Qinghai–Tibet

plateau lithosphere (Wu et al., 1996). The northeastern

Ala Shan block could be subducting under the Qilian

Shan belt along this major fault.

3. Summary of known Mesozoic and Cenozoic

large-scale tectonic events in NE Asia

Since Triassic times, several continental blocks

have been accreted to the southern margin of Eura-

sia. The thick Triassic flysch sequence south of the

Kunlun range is a remnant of the late Triassic

collision between the Kunlun and Qiangtang blocks

along the Jinsha suture zone (Fig. 1). This collision

is thought to have produced some tectonic move-

ments far to the north, along what is now the ATF

zone. A major discordance level occurs in the sedi-

ment deposits of the Tarim basin between the

Devonian and lower Jurassic, indicating a major

erosion/nondeposition period. The Jurassic sequence

evolved from lower Jurassic thin coal-bearing lake

deposits to middle Jurassic thick continental con-

glomerates, indicating the erosion of nearby relief

(Sobel, 1999; Sobel et al., in press). This Jurassic

sequence is also well documented in the Hexi

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134118

corridor, east of the Qilian Shan belt (Vincent and

Allen, 1999).

Evidence of N–S transtensive movements are

reported by Vincent and Allen (1999) in the Hexi

corridor between late Jurassic and early Cretaceous,

associated with coarse sedimentation. The late Juras-

sic–early Cretaceous period is marked by the collision

of the Lhasa block with Eurasia along the Bangong–

Nujiang suture zone (e.g. Allegre et al., 1984; Chang et

al., 1986; Matte et al., 1996). Nonetheless, in north

Tibet, the Cretaceous appears to be a tectonically quiet

period. Consistent with this, Coward et al. (1988)

reported that in the region of the Lhasa–Golmud road,

accretion of the Qiantang block was accompanied by

intense deformation, whereas later accretion of the

Lhasa block only weakly deformed the Qiantang block.

Cretaceous sedimentation was slow, with no more than

1000 m of deposits in the Hexi corridor. In late Creta-

ceous, the Tarim basin was near sea level, and a final

marine regression apparently occurred in the early

Oligocene (Sobel and Dumitru, 1997).

The India–Asia collision is dated between 52 and

40 Ma (e.g. Searle et al., 1997; Rowley, 1998; Zhang

and Scharer, 1999). However, the first movements are

recorded geochronologically in north Tibet around 30

Ma (40Ar/39Ar dating along the Kunlun fault near

Golmud) by Mock et al. (1999) and 25–20 Ma (apatite

fission track dating on sediments from the Tarim) along

the western Kunlun and in the Tian Shan by Sobel and

Dumitru (1997). The overall calculated sedimentation

rates (Metivier, 1996) increase very rapidly in both the

Qaidam basin and Hexi corridor in early Pliocene times

(from 0.07 ± 0.03 to 0.16 ± 0.09 mm a � 1 in Hexi

corridor and from 0.08 ± 0.03 to 0.74 ± 0.29 mm a� 1

in the Qaidam). Because there is no evidence of any

sediment exchange between the basins and the nearby

Tarim or Gobi basins, this increase in sedimentation

rates was due to renewed erosion indicating renewed

tectonics.

In summary, available data show tectonic activity

during late Triassic–early Jurassic, followed by wide-

spread active erosion during the Jurassic. The Creta-

ceous is described as a more tectonically quiet period

despite the collision of the Lhasa block with Eurasia.

The first evidence of the 60–40-Ma-old India–Asia

collision are recorded around 30–20 Ma along the

Kunlun and in the Tian Shan. Finally, an increase in

sedimentation rates in both Qaidam basin and Hexi

corridor indicates a late Miocene–early Pliocene

renewal of the tectonic activity. Since at least Oligo-

cene times (Ritts and Biffi, 2000), the Qaidam basin

has been a closed endoreic structure from which the

sediments cannot escape. This means that any varia-

tion in sedimentation rates inside the basin is induced

by change in erosion rates in the surrounding moun-

tain ranges and not by sediment exchange with other

basins, which would lead to the same results.

4. Sampling approach and methodology

4.1. Sampling approach

Sampling was carried out during the 1996 and 1997

Sino-French expeditions to north Tibet (INSU–Minis-

try of Chinese Geology collaboration project). Samples

were taken both along the large strike-slip AT and

Kunlun faults and within the Altun Shan, Qimantag

and Qilian Shan fold and thrust belts (Fig. 2).

Both major strike-slip faults and small associated

thrust faults were sampled. Such a sampling should

allow us not only to confirm the presence or absence of

vertical movement on the strike-slip faults themselves

but also to get an idea of the main activation periods of

the large faults by studying their effects on the smaller

related thrusts. We sampled the 1000-km length of the

AFT to the junction with the Kunlun with the hope of

describing the deformation at this point, to the north-

eastern termination of the ATF. The western Kunlun

fault was much more difficult to reach and we only

obtained samples south of the Aqqikkol basin (Fig. 2)

over a distance of about 100 km along strike.

Generally speaking, these samples were collected

along cross-sections perpendicular to the strike of the

main mountain belts, on the main thrust faults, but

also within the inter-mountain basins. We thus

expected to correlate the movements on the main

ranges with the movements in the basins.

Lithologically, the samples were from crystalline

rocks ranging in age from Precambrian (2 Ga) to Juras-

sic: granites, granodiorites, gneisses and syenites. The

altitude of most of the samples was measured using a

portable GPS coupled with an altimeter. When a GPS

measurement was not available, the altitude was calcu-

lated on the 1/500000 T.P.C. map provided by the

American Defense Mapping Agency or on the 1/

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134 119

200000 Chinese Topographic Map. In both cases, pre-

cision is of the order of ± 50 m.

4.2. Ages and genetic algorithm modelling of track

lengths

It is commonly accepted that fission tracks in

zircons record temperatures of approximately

250 ± 50 �C (Hurford, 1986). Recent studies show

that the zone of partial annealing may range from 370

to 190 �C (Yamada et al., 1995; Tagami et al., 1996;

Tagami and Shimada, 1996). In apatite, fission tracks

form with a constant length of about 16 mm (Gleadow

and Duddy, 1981; Gleadow et al., 1986a,b). For

temperatures higher than 110 ± 10 �C, these tracks

are immediately annealed, but for temperature below

110 ± 10 �C, the annealing rate decreases significantly

and tracks become more stable. Central fission-track

ages (calculated using the mean value of the log

distribution of ages, weighted by the error on each

age) have no simple geological meaning as in many

other geochronological systems. FT ages represent a

record of a complex history of past thermal events.

Access to such thermal events can be made by

measuring confined track lengths in the sample. The

statistics of the length distribution and the FT age can

be combined together using various modelling proce-

dures in order to investigate the possible thermal

histories that a rock has suffered. Because in apatite,

FT are immediately annealed at temperature higher

than 110 ± 10 �C and very slowly at temperatures

lower than 60 �C (‘‘immediately’’ and ‘‘slowly’’ refer

to geologically significant rates of the order of the

Ma), track length modelling can only describe the T– t

evolution of the sample in-between 110 and 60 �C.Using the Monte Trax program (Gallagher, 1995), we

performed this modelling for samples having sets of at

least 20 track-length measurements, but as often as

possible with sets of 100 measurements (Table 1). In

each model, only the 50 best T– t curves were taken

into account.

Because of some possible artifacts in the modelling

process, the last segment of the T– t curve can show a

dramatic increase in the cooling rate. To try to

discriminate between natural events and modelling

problems, these last segments will be taken into

account only if their starting point on the best T– t

curve is at a temperature higher then 40 �C. Exper-

imental data on fission track annealing and length

interpretation in zircon are not yet adequate to allow

any track-length modelling.

4.3. Laboratory processing

Fission track ages of apatites and zircons were

determined using the external detector method (Hur-

ford and Green, 1983). Apatite grains were mounted

on a glass slide, ground, polished and etched in 6.5%

HNO3 for 50 s at 20 �C, in order to reveal natural

fission tracks. Zircons were mounted in teflon,

ground and polished, and etched in the eutectic

mixture of NaOH and KOH (Gleadow et al., 1976)

at 210 �C for 12–15 h depending on their etching

rate (Yamada et al., 1995; Tagami et al, 1996; Tagami

and Shimada, 1996). The muscovite external detec-

tors were etched in 40% HF for 40 min at 20 �C to

reveal the induced fission tracks. Samples were

irradiated at the ANSTO facility, Lucas Heights,

Australia, with a nominal neutron flux of 1.0� 1016

n cm � 2 for apatites and 1.0� 1015 n cm � 2 for

zircons. The ages were calculated following the

method recommended by Hurford (1990), using the

zeta calibration method (Hurford and Green, 1983).

The CN1 zircon zeta is 93 ± 5 (M.J.) and CN5 apatite

zeta is 305 ± 25 (M.J.). Fission tracks were counted

on a Zeiss microscope, using a magnification of 1250

under dry objectives for apatites, and 1600 with oil

immersion for zircons. All ages are central ages and

are quoted at ± 2r.

5. Fission-track ages and their implications

5.1. Zircons ages

Twelve samples were analysed. The ages range

from 220.8 ± 21.8 to 96.4 ± 3.8 (Table 1 and Fig. 3).

Because the understanding of track-length distribu-

tions in zircons has not reached the level of under-

standing as apatites, we cannot«look into» the ages in

the same way. It has to be assumed here for simplicity

that the ages represent a cooling event that has not an

internal modification, e.g. a small heating and second

cooling phase. We conclude that all but one of the

zircon ages represent a Jurassic cooling event. There

is no relationship with the current altitude of the

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134120

Table 1

Fission track central ages

Number Altitude

[m]

Min. Number of

grains

Standard track

density� 104

cm�2 (counted)

qS� 104

cm�2

(counted)

qi� 104

cm�2

(counted)

U

concentration

[ppm]

P(v2)[%]

Var

[%]

Mean track

length [mm] ( ± 1r)(counted)

Standard

deviation

[mm]

Central age

( ± 2r)[Ma]

AT1 1600 Ap. 23 118.6 (6891) 191.0 (1925) 281.8 (2840) 29.7 25 7 12.58 ± 0.18 (100) 1.83 134.1 ± 4.8

AT6 2950 Ap. 20 109.1 (8991) 160.6 (1140) 262.1 (1861) 30.0 23 9 12.58 ± 0.14 (118) 1.55 101.2 ± 4.5

AT8 4000 Ap. 21 115.1 (6891) 19.3 (161) 229.9 (1921) 25.0 42 18 12.22 ± 0.46 (35) 2.73 16.9 ± 1.6

AT31 3600 Ap. 20 115.7 (8991) 158.8 (494) 187.1 (582) 20.2 31 12 12.43 ± 0.22 (100) 2.15 147.4 ± 10.2

AT49 3300 Ap. 20 125.5 (8991) 47.42 (414) 93.1 (813) 9.3 63 8 12.56 ± 0.18 (94) 1.73 97.3 ± 6.3

AT52 3100 Ap. 20 99.3 (8991) 36.6 (310) 71.1 (602) 9.0 80 0 12.41 ± 0.18 (99) 1.74 77.5 ± 5.5

AT53 3450 Ap. 20 129.5 (5188) 86.9 (563) 133.8 (867) 12.9 56 2 11.90 ± 0.19 (100) 1.92 127.0 ± 7.1

AT60 2800 Ap. 24 113.9 (6891) 36.8 (244) 41.6 (276) 4.6 60 1 11.73 ± 0.16 (100) 1.64 167.0 ± 14.8

AT68 * 2300 Ap. 21 111.6 (6891) 21.8 (108) 135.1 (669) 15.1 25 21 11.09 ± 0.31 (33) 1.77 30.3 ± 3.5

AT80 3950 Ap. 20 124.1 (5188) 236.8 (862) 425.0 (1547) 42.8 12 11 10.67 ± 0.23 (100) 2.29 105.0 ± 5.5

AT81 3950 Ap. 20 92.7 (8991) 50.8 (393) 53.0 (410) 7.2 88 0 12.17 ± 0.24 (100) 2.35 134.1 ± 9.6

AT101 4405 Ap. 23 145.5 (12670) 200.3 (2548) 259.4 (3300) 22.3 52 2 11.8 ± 0.17 (115) 1.807 166.9 ± 4.7

AT103 4377 Ap. 24 136.3 (8059) 16.7 (76) 68.5 (311) 6.3 45 18 11.97 ± 0.38 (38) 2.37 52.8 ± 7.2

AT113 5000 Ap. 20 107.9 (12670) 15.7 (124) 131.9 (1043) 15.3 90 0 13.56 ± 0.23 (80) 2.04 19.3 ± 1.8

AT118 5500 Ap. 23 109.3 (6891) 14.4 (100) 152.2 (1058) 17.4 46 6 12.45 ± 0.31 (35) 1.87 17.3 ± 1.8

AT120 4100 Ap. 25 144.0 (8059) 58.2 (251) 231.1 (996) 20.1 99 0 10.97 ± 0.32 (54) 2.32 55.1 ± 3.9

AT131 3400 Ap. 25 123.6 (12670) 13.5 (161) 142.6 (1701) 14.4 95 0 11.7 ± 0.39 (52) 2.82 17.6 ± 1.5

AT132 3500 Ap. 20 117.3 (12670) 120.4 (514) 152.2 (650) 16.2 100 0 11.8 ± 0.15 (87) 1.44 138.1 ± 8.2

AT140 * 2691 Ap. 20 132.1 (8991) 32.3 (169) 672.3 (3523) 63.6 14 17 13.49 ± 0.22 (61) 1.73 10.1 ± 0.9

AT142 * 1800 Ap. 19 128.5 (8059) 8.4 (62) 168.1 (1237) 16.3 69 0 9.8 ± 1.3

AT147 1980 Ap. 30 103.4 (6891) 102.4 (1217) 160.2 (1904) 19.4 16 11 11.29 ± 0.11 (120) 1.2 109.5 ± 4.9

AT148 1900 Ap. 20 139.2 (12670) 198.4 (962) 272.8 (1323) 24.5 36 5 13.3 ± 0.17 (100) 1.69 150.9 ± 6.8

BJ30 3780 Ap. 25 151.8 (8059) 23.8 (215) 113.3 (1024) 9.3 87 1 11.83 ± 0.20 (90) 1.85 48.4 ± 3.7

WQ122 4000 Ap. 24 106.9 (6891) 10.7 (95) 93.1 (824) 10.9 81 0 11.75 ± 0.41 (20) 1.85 20.7 ± 2.3

AT1 1600 Zr. 19 42.9 (2821) 2597 (2186) 242.3 (204) 226.2 100 0 205.0 ± 15.5

AT5 1600 Zr. 5 45.2 (2360) 3592 (241) 372.6 (25) 329.9 28 9 199.6 ± 42.9

AT8 3600 Zr. 20 48.5 (2821) 4368 (2250) 514.5 (265) 424.6 57 6 168.2 ± 11.8

AT32 3400 Zr. 19 421.5 (2360) 1981 (3493) 222.9 (393) 211.6 51 0 172.3 ± 9.8

AT52 3100 Zr. 8 38.1 (2360) 2673 (636) 302.6 (72) 317.6 99 0 155.1 ± 19.5

AT60 2800 Zr. 15 46.6 (2821) 1968 (1233) 185.1 (116) 158.9 99 0 220.8 ± 21.8

AT68 2300 Zr. 9 47.5 (2821) 4800 (738) 500.8 (77) 421.4 100 0 203.4 ± 24.7

AT80 3950 Zr. 12 37.1 (2360) 3557 (2864) 284.4 (229) 306.5 24 14 219.7 ± 19.8

AT113 5000 Zr. 20 33.88 (2507) 3038 (6486) 492.7 (1052) 581.7 31 3 96.4 ± 3.8

AT118 5500 Zr. 9 41.1 (2360) 3142 (1246) 370.7 (147) 360.5 61 0 160.6 ± 14.4

AT147 1980 Zr. 14 43.8 (2821) 2605 (1562) 2651 (159) 242.1 90 1 192.2 ± 16.4

WQ122 4000 Zr. 18 41.0 (2821) 2732 (1712) 3527 (221) 344.2 91 6 143.0 ± 10.9

Ap. = apatite; Zr. = zircon. qS and qi represent sample spontaneous and induced track densities; P(v2) is the probability of v2 for v degrees of freedom (where v= number of

crystals� 1). Zeta CN5 apatite = 305 ± 25 (M.J.) and zeta CN1 zircon 93 ± 5 (M.J.). Samples were irradiated at the ANSTO facility, Australia. Samples marked * are from Jolivet et al.

(1999).

M.Jolivet

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343(2001)111

–134

121

Fig. 3. Zircon fission track ages in Ma ( ± 2r error). Sample numbers are quoted in brackets. Sample AT68 is from Jolivet et al. (1999) (see Table 1 for detailed data).

M.Jolivet

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343(2001)111

–134

122

samples, implying tectonic disturbance since these

ages were set (Fig. 4). The onset of this cooling phase

cannot be established as there are no other thermo-

chronometric data available for these samples. None-

theless, Arnaud (1992) detected a major thermal event

in the Kunlun range between 200 and 150 Ma

(40Ar/39Ar dating on feldspars), which is consistent

with our zircon fission track data.

The ages fall into two groups: the first one, with

FT ages around 220–200 Ma, contains samples

located outside (AT1 and AT5) or near the border of

the areas intensely deformed during Cenozoic times

(AT60, AT68, AT80 and AT147). The second group

has ages in the region of 160 Ma (including error

margins); they are located on the hanging wall of

thrust faults connected to the main strike-slip faults

(AT8, AT32, AT52, AT118 and WQ122). Sample AT5

could also be included in the second group due to its

large error margins. Nonetheless, AT1 and AT5 were

collected from the same outcrop; therefore, we con-

sider that it is more likely that they fall together into

the first group.

5.2. Discussion

Zircon ages from the first group show that these

samples cooled through approximately 250 �C during

the Late Triassic to early Jurassic. The second group-

ing most likely represent the later exhumation during

this same phase, i.e. samples being exhumed from

deeper levels at a later time. These latter group of

samples are all from hanging wall structural positions

and are younger than the footwall sites, implying a

thrusting regime. It is not possible to derive from the

zircon data alone whether this thrusting happened

during the Jurassic or took place later during the

Cretaceous or the Cenozoic. Both sets of age groups

fit in well with the predicted higher temperature–time

patterns from the apatite modelling, as discussed

below. The Jurassic cooling/exhumation was a general

event affecting the whole north Tibet area.

5.3. Apatite ages

Twenty-four samples were analysed using the apa-

tite fission track method. Central ages range from

167.0 ± 14.8 (AT60) to 9.8 ± 1.3 Ma (AT142) (Table 1

and Fig. 5). Mean track lengths range from 10.67 ± 0.23

to 13.56 ± 0.23 mm. There is no obvious age/altitude

correlation between the samples, which indicates a

regionally complex cooling/tectonic history through-

out this large area.

These samples can also be divided into two broad

groups depending on their age and structural position.

Fig. 4. Age–altitude relationships. There is no correlation in the older samples, which indicates that they have a complex tectonic history

relative to one another. The correlation is only slightly better for the young samples.

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134 123

Fig. 5. Apatites fission track ages in Ma ( ± 2r error). Sample numbers are quoted in brackets. Samples AT68, AT140 and AT142 are from Jolivet et al. (1999) (see Table 1 for detailed

data).

M.Jolivet

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343(2001)111

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124

Group A will refer to samples with FT ages about

or older than 50 Ma, generally associated with areas

undeformed by the ATF or/and on the borders of the

Qilian Shan. Group B will refer to samples with FT

ages lower than 50 Ma, mostly structurally associated

with hanging wall situations (Fig. 5). Both group A

and group B show a strong correspondence with

structural position.

Group A can be divided in two subgroups.

(i) One group of older ages is situated along the

ATF zone, and samples are all from sites that are

undeformed by the ATF, that is basically undeformed

basement. Two of these sites, AT1 and AT60, have

corresponding zircon ages. The apatite modelling plus

the extension to higher temperatures of the zircon

(Fig. 6) show that these sites were exhuming since at

Fig. 6. Models of the oldest apatite fission track samples and their associated zircon fission track ages. These graphs show that the cooling rate

observed during the Jurassic period in the apatite models is consistent with the extension to ages obtained with zircons fission tracks.

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134 125

least 210 Ma. The timing of initiation of cooling

cannot be made from this data set as no higher

temperature geothermometer were used. They cooled

at about 3 �C/Ma from 210 to 120 Ma, and then

underwent a period of heating or stayed at constant

temperature until approximately 40 Ma. The timing of

this increase or stabilisation in temperature cannot be

clearly made from samples AT1 and AT60 (Fig. 6) as

it took place when the rocks were in the upper limit of

the P.A.Z., where the modelling procedure is not well

constrained. They were heated to temperatures of 60–

70 �C, and at about 40 Ma, they were finally exhumed

to the surface today.

All other samples with apatite ages older than 100

Ma, from the undeformed basement, show a cooling

period through the P.A.Z. in the Jurassic—sometimes

later, sometimes earlier, probably depending on the

position of sample relative to the surface at different

Fig. 7. Genetic algorithm modelling for samples with apatite central FT ages older than 50 Ma. The Jurassic exhumation period appears in every

sample, generally followed by either a slow reheating period or a slow exhumation during Cretaceous. Finally, a last exhumation started in some

samples around 40 ± 10 Ma.

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134126

times (Fig. 7). Even if isothermal residence in the

P.A.Z. cannot be fully excluded, many samples also

reveal a heating event after this phase of cooling,

always to temperatures of 60–70 �C. The exact timing

of this change is hard to assess, for the reasons

mentioned above, but sample AT53 suggests a time

of about 120 ± 20 Ma (Fig. 7). Cooling was reinitiated

at about 40 Ma. Some samples from this age group

have a simple noneventful cooling history, but with a

possible slight change in cooling rate also at this time

(e.g. samples AT101, AT31, Fig. 7), and some have a

possible mixture undefinable from the modelling

between the two, e.g. AT6, where the best fit is a

simple cooling history but the 50 best solutions

suggest also cooling, heating, cooling trend.

(ii) On both the western and eastern front of the

Qilian Shan belt, four other samples have apatite ages

older than 100 Ma. They constitute pairs of ages

representing present footwall and hanging wall struc-

tural positions (AT148, AT147 and AT81 and AT80,

respectively; Fig. 8). The hanging wall of both thrust

faults has a younger apparent age than the footwall. In

the footwall samples, only a single phase of exhuma-

tion is observed from at least 180 Ma to present. AT81

shows a hint of increase in cooling rate around Ma but

because it occurs at very low temperatures, it cannot

be fully interpreted as a true event. The hanging sites

show a general cooling period during the Jurassic as

previously recorded in samples from group A (Figs. 6

and 7). This cooling phase is followed by a reheating

event well documented in sample AT80 whereas

modelling of sample AT147 could also account for

an isothermal residence in the P.A.Z. This reheating or

isothermal phase ends at approximately 40 Ma on the

western front of the Qilian Shan and around 30 Ma on

the eastern front, suggesting that the actual thrusting

phase initiated during the Tertiary. Transtensive defor-

mation has been documented by Vincent and Allen

(1999) in the Hexi corridor during the Cretaceous,

leading to localized high sedimentation rates in small

pull-apart basins. Angular discordances between Cre-

taceous and Jurassic sediments in small basins inside

the Qilian Shan belt also indicate localized tectonic

activity in this area during the Cretaceous. The strong

Fig. 8. Hanging wall and footwall relationships in the Qilian Shan. Samples AT80 and AT81 are from western Qilian Shan; samples AT147 and

AT148 are from the eastern Qilian Shan.

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134 127

reheating event recorded by sample AT80 during the

same period might indicate that such transtensive

deformation also occurred further west along the

actual western front of the Qilian Shan belt. If con-

firmed, it could imply that the presently ongoing

deformation reactivates preexisting fault structures.

Group B contains samples with FT ages ranging

from 30 to 10 Ma, and situated dominantly in a

hanging wall structural position. Because the FT

history recorded by these samples is much shorter

compare to the one from samples in group A, the track

length modelling will give access to a much more

detailed T– t history. As illustrated in Fig. 9, there is

evidence in this group for cooling from sometimes

prior to 30 Ma. In some samples, there is evidence

that a reheating event at 22–18 Ma is present, with a

change to a final cooling phase between 9 and 5 Ma.

5.3.1. Discussion

The large spread in FT ages and mean track lengths

implies a complex temperature–time history of north

Tibet. Nonetheless, genetic algorithm modelling high-

lights some major events in the evolution of this area.

The widespread Jurassic cooling recorded on most of

the samples can be interpreted as a regional tectonic

event. Several authors have already reported evidence

for tectonic movements in north Tibet during Jurassic

times (Sobel, 1995; Sobel and Arnaud, 1996; Delville

et al., in press), and a discordance level is reported in

the sedimentary deposits, between late Triassic and

early Jurassic times in the Tarim basin (e.g. Sobel,

1999), indicating a period of erosion. Based on40Ar/39Ar and apatite fission track analysis, Sobel et

al. (in press) have documented a � 100–150 �Ccooling episode during Early to Middle Jurassic along

a transect from Mangnai Zhen to Ruoqiang (Fig. 1). A

similar cooling is recorded by a granite pluton, 350

km to the east and south of the ATF (Sobel et al., in

press). This cooling is interpreted as a major exhu-

mation event, the magnitude of which greatly

exceeded the Cenozoic cooling recorded by the same

samples. Early to mid-Jurassic sediments, both in the

Tarim basin and in the Hexi corridor (east Qilian

Shan) (Fig. 1), are conglomerates or sandstones,

indicative of active erosion processes (e.g. Vincent

and Allen, 1999). From early Jurassic to early Creta-

ceous times, several continental blocks were accreted

to the southern margin of the Asian lithosphere (e.g.

Matte et al., 1996). The late Triassic collision between

the Qiantang block and the Kunlun is followed,

further to the south, by docking between the Qiantang

and northern Lhasa blocks during middle to upper

Jurassic (e.g. Enkin et al., 1992).

Jurassic tectonic activity has been described along

the ATF and in the Tien Shan (e.g. Sobel, 1995; Sobel

and Arnaud, 1996; Delville et al., in press). A major

discordance is documented between the Devonian and

early Jurassic sediments in the southwest of the Tarim

basin, indicating a period of erosion probably asso-

ciated with relief building (Sobel, 1999). The zircon

FT data presented in this study indicate that the whole

north Tibet area was affected by exhumation pro-

cesses during the late Triassic–early Jurassic. None-

theless, the Jurassic cooling rates recorded by apatite

FT analysis are low (from 0.1 to 1.3 �C/Ma), implying

that erosion was also slow, which in turn is not

consistent with a very active orogen and high reliefs.

With the exception of the late Triassic belt along the

Jinsha suture zone that supplied a huge amount of

sediments to the Songpan Garze accretionary prism,

there is no indication of high erosion rates during

Triassic and Jurassic times. The Jurassic tectonic

activity, if documented by many data from the whole

north Tibet area, appears more like a long-term

regional deformation rather than a localized high relief

building process.

Although evidence only exists in small remanent

outcrops, the Cretaceous period was characterised by

sedimentation and little tectonic activity. This period

of sedimentation is possibly represented in our data by

slow reheating of 60 ± 10 �C (maybe 2 km of sedi-

ments) from 120 Ma in samples such as AT1 and

AT60. Other locations such as AT6, AT31, and

AT148, for example, have no reheating event in the

Cretaceous. It is improbable that over such a large

area, there were regions of differential subsidence as

well as others with continued slow exhumation, i.e.

there were small basins scattered over the region. This

can either indicate slow erosion of a previously

formed relief (during the Jurassic period) or continu-

ous weak tectonic activity localized along the south-

ern margin of the Tarim basin and into the Qilian Shan

belt. The first hypothesis is supported by the wide-

spread Jurassic tectonic movements and the fact that

the collision of the Lhasa block did not induce any

large deformation in the Qiantang block immediately

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134128

to its north (Coward et al., 1988). The second hypoth-

esis implies that the docking of the Lhasa block to the

south induced some deformation north of the Kunlun

ranges. This idea has already been proposed by

Vincent and Allen (1999) to explain the transtensive

movements observed in the Hexi corridor during early

Cretaceous. Cretaceous tectonic deformation has also

been reported in the Tien Shan (e.g. Hendrix et al.,

1992), or in southern Mongolia (e.g. Hendrix et al.,

1996; Lamb et al., 1999). We thus cannot exclude

tectonic deformation in north Tibet at that time.

Furthermore, Marshallsea et al. (2000), using apatite

Fig. 9. Genetic algorithm modelling for samples with apatite FT ages younger than 30 Ma. They show major, widespread tectonic activity since

mid-Oligocene times. There is no time– location pattern in the exhumation of these samples.

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134 129

fission track analysis, evidenced a cooling episode in

the northeastern Qilian Shan starting between 115 and

90 Ma.

As shown before, AT80 and AT81 (Fig. 8) also

seem to have recorded some movements during Mes-

ozoic times on the western front of the Qilian Shan. At

about 150 Ma, both samples were at a similar temper-

ature of 90 ± 10 �C, but AT80 cooled more rapidly

than AT81 after this time, possibly due to thrusting.

This period ended around 90 Ma when AT80 was

heated up again, possibly due to a combination of

sedimentation and burial by thrusting, whereas AT81

was still exhumed but at a lower rate. At 40 Ma,

sample AT80 started to cool again. An increase in

AT81 cooling rate is also observed from 40 Ma. These

three periods in the thermal history of sample AT80

might be interpreted in terms of fault activity.� From about 150 until 90 Ma, AT80 and AT81

were separated by a thrust fault, AT80 being exhumed

more rapidly than AT81. This movement may be the

same as that recorded by Vincent and Allen (1999),

during the Late Jurassic–Early Cretaceous period in

the Hexi corridor, induced by the docking of the

Lhasa block to the south.� From 90 to 40 Ma, the tectonic activity in the Da

Qaidam area changed from compression to extension,

allowing small pull-apart basins to form. Only small

outcrops of Cretaceous and Tertiary sediments seem to

remain west of the Qilian Shan but because the whole

series is represented by continental red sandstones, a

precise age cannot be assessed for each individual unit,

and the Cretaceous sediments might prove to be much

more extensive. Pull-apart basin formation was also

recorded during the Cretaceous in the Hexi corridor

(Vincent and Allen, 1999) along NW–SE trending

faults, parallel to the Da Qaidam thrust system.� Finally, around 40 Ma, the India–Asia collision

reactivated the fault again to a thrust system. The

Qilian Shan was affected by tectonic movements very

early in the India–Asia collision history.

The second exhumation event at about 40Ma is seen

in the «older» samples (Fig. 7). It is either represented

by a change from heating to cooling or by a small break

in slope of the continuous cooling curve, which would

normally be dismissed except for the fact that we see it

elsewhere. Using apatite fission track analysis, Sobel et

al. (in press) also record possible Paleocene (?)–

Eocene tectonism in the northern Qaidam basin. This

event represents intracontinental disruption following

the collision of India and Asia. The apatite fission track

modelling show a consistent increase of the cooling

rates along the large strike-slip faults (i.e. the Kunlun

and Altyn Tagh faults) around 40 Ma, indicating that

activation of these structures began in Eocene–early

Oligocene times, while nappe formation was very

active in the Himalayan belt (Treolar et al., 1989; Le

Fort, 1996). Strike-slip movements are documented

along the Red River Fault from 35 Ma (e.g. Harrison

et al., 1992; Scharer et al., 1994; Roger et al., 2000).

Using 40Ar/39Ar analysis on K feldspar, Mock et al.

(1999) demonstrate the occurrence of a significant

cooling event (9–15 �C/Ma) around 30 Ma in eastern

Kunlun. This cooling is localized along the actual

Kunlun fault and the Golmud batholith (Fig. 1) imme-

diately to the north is not affected. Finally, Delville et

al. (in press) show a faint metamorphic event on the

eastern end of the ATF around the same period. These

observations associated with our FT data could imply

that very early in the India–Asia collision, deformation

occurs far to the north along major mechanical boun-

daries in the Asian lithosphere.

The young apatite ages are situated within the

major mountain belts of north Tibet: on both sides

of the Qaidam basin, in the Kunlun ranges, in the

Qilian Shan and in the Altun Shan (Fig. 5). They tend

to be positioned on the hanging walls of major thrust

faults, which have deformed Neogene and Quaternary

sediments of the Tarim basin, the Qaidam basin and

the Hexi corridor. They all show cooling from prior to

30 Ma, which is consistent with the pattern in the

older ages. A second event is apparent in the young

ages with a change from cooling to heating around 20

Ma in some samples. Between 9 and 5 Ma, there is a

clear change to cooling. This may represent the last

rapid phase of exhumation which can be correlated to

the period of maximum sedimentation recorded in the

Qaidam basin during Cenozoic times (up to 0.7 km

Ma� 1; Metivier et al., 1998).

Finally, the Altun Shan belt on the northern edge of

the ATF zone has a slightly different evolution. Multi-

method geochronological analysis (Jolivet et al.,

1999) suggest that the Altun Shan has been a stable

block since at least 380 Ma with a mean cooling rate

of about 0.8 ± 0.3 �C/Ma. This very slow exhumation

ends around 10 Ma with a sharp increase in cooling

rates, up to 8 ± 2 �C/Ma. This 10-Ma-old range has

M. Jolivet et al. / Tectonophysics 343 (2001) 111–134130

been described by Wittlinger et al. (1998) and Jolivet

et al. (1999) as the consequence of the southward

subduction of the Tarim lithosphere underneath the

Altyn Tagh–north Tibet system. The ATF being the

southern boundary of this microbloc.

6. Conclusion

Fission track analysis has allowed identification of

the major tectonic events from the early Jurassic

onwards in north Tibet, between the Kunlun and the

Altyn Tagh faults.

(a) The first phase is represented by a period of

cooling interpreted as exhumation during the Jurassic.

The beginning of this event cannot be identified from

our data but must be situated sometimes during the

Triassic. This exhumation period may be the result of

the late Triassic collision between the Qiantang block

and the Kunlun block. The following period of heat-

ing by as much as 60 �C may represent a period of

localized sedimentation during the Cretaceous as

documented by Vincent and Allen (1999) in the Hexi

corridor.

(b) The first response to the India–Asia collision

appears in the Tertiary orogenic history, at about

40 ± 10 Ma. This cooling phase, localized along major

lithospheric block boundaries such as the Kunlun fault

or the ATF is consistent with several other arguments

leading to a possible early reactivation of mechanical

weakness in the Asian lithosphere, following the onset

of the India–Asia collision.

(c) Finally, track length modelling on samples from

group B records high cooling rates from late Oligo-

cene–Miocene times. This indicate large vertical

movements associated with the development of the

large-scale compressive structures throughout north

Tibet (there is no evidence of any major Tertiary

extensional faults north of the Kunlun fault). Because

apatite fission tracks do not ‘‘see’’ events occurring at

temperatures higher than 110 �C, we cannot assess theexact period of onset of this late compressive tectonic,

which is situated sometimes between Eocene and late

Oligocene.

Further investigations will be needed to fully

understand the kinematics of the deformation in north

Tibet. Fission track analysis inside the Qilian Shan

belt, at the eastern end of the ATF, might provide

some information about the relations between major

transcurrent movement and thrust fault activity.

Detailed cross-sections across the ATF and the Kun-

lun fault would allow a better understanding of the

kinematics along these major structures, and espe-

cially provide some information about the early stages

of the India–Asia collision-related deformation.

Acknowledgements

Field work was supported by INSU and the

Chinese Institute of Geology (Beijing). Analytical

work in University Montpellier has been supported by

ISTEEM. Constructive reviews by B.C. Burchfiel and

L. Ratschbacher helped to improve the manuscript.

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