Available online at www.sciencedirect.com

Radiation Measurements 36 (2003) 357–362www.elsevier.com/locate/radmeas

A new vision of the intracontinental evolution of the easternKunlun Mountains, Northern Qinghai-Tibet plateau, China

W.-M. Yuana ;∗, X.-T. Zhangb, J.-Q. Donga, Y.-H. Tanga, F.-S. Yua, S.-C. WangaaInstitute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China

bChina University of Geosciences, Beijing 100083, China

Received 21 October 2002; received in revised form 7 February 2003; accepted 29 April 2003

Abstract

Based on apatite 9ssion track ages (FTA) of 41 samples collected from a south–north transect of the eastern Kunlunmountains, Qinghai-Tibet Plateau, China, this paper shows that (1) the FTA in di=erent blocks increases with the distancefrom the South-Kunlun fault and Mid-Kunlun faults, respectively, indicating the control of the main faults on the tectonicevolution of this region; and (2) the thermal histories are characterized by slow cooling from ∼160◦C to ∼80◦C at ∼240to ∼20 Ma, followed by rather rapid cooling to surface temperatures.c© 2003 Published by Elsevier Ltd.

Keywords: Intracontinental evolution; Fission track; Apatite; Tectonics; Eastern Kunlun; Qinghai-Tibet plateau

1. Introduction

The Eastern Kunlun Mountains (EKM) is located in thenorthern Qinghai-Tibet Plateau and consists of a Paleozoic–Triassic collision belt, rejuvenated during the CenozoicIndia-Asia collision (Matte et al., 1996). They represent thesouthern margin of the Qaidam basin (Fig. 1). To the southof the Middle-Kunlun fault zone (MKFZ), the middle–upper Proterozoic Wanbaogou Group in Middle-KunlunBlock (MKB) yielded a muscovite 40Ar=39Ar age of ca.160 Ma and the lower Silurian Nachitai Group in the MKBrevealed a biotite 40Ar=39Ar age of ca. 110 Ma that wasoverprinted by a very-low-grade event at 60–40 Ma (Liu etal., 2000). Mock et al. (1999) concluded that the Mesozoicplutons had undergone an important cooling period around140–120 Ma, coeval with ductile deformation along theXidatan fault. According to the Xidatan fault zone, four ma-jor stages of tectonic movements have been distinguished,which are 240–200, 150–140, 120-110, and 20 Ma (Li etal., 1996). Strike slipping was accompanied with the uplift-ing of the crust, and, as a result, this caused the formationof the speci9c structural pattern of the EKM.

∗ Corresponding author. Tel.: +86-10-88233189; fax: +86-10-882386.

E-mail address: yuanwm@mail.ihep.ac.cn (W.-M. Yuan).

2. Geological setting

The EKM are an epicontinental active belt in geologi-cal history. The MKFZ and the South-Kunlun fault zone(SKFZ) are paleosuture zones, dipping to the north andstretching more than 1000 km in the EWW extension. TheEKM can be subdivided into three blocks; designated as theNorth-Kunlun Block (NKB), Middle-Kunlun Block (MKB)and South-Kunlun Block (SKB), respectively, having theMKFZ and the SKFZ as their limits (Fig. 1).The EKM were subjected to multiepisode tectonic ac-

tivities. During the Caledonian epoch, crustal stretchingwas dominant, and a primary small ocean basin (i.e.archipelago) was formed. Subsequent to this, the crust wascompressed in the majority of this area, resulting in up-lifting, and the primary ocean basin was closed during theHercynian epoch. The EKM were in a strong orogenic pe-riod during the Indo-Sinian epoch. The intense compressionresulted in both the SKB and the MKB being subductedto the north continuously with strike-slip, and the southrelict Tethyan Ocean was entirely closed. The upliftingcaused by continent–continent collision took place duringthe Jurassic and Cretaceous, periods meanwhile, there werestrong sinistral strike slips along both the MKFZ and theSKFZ. Since the Cenozoic period, inhomogeneous imbal-anced elevation and subsidence resulted in basin-and-range

1350-4487/03/$ - see front matter c© 2003 Published by Elsevier Ltd.doi:10.1016/S1350-4487(03)00151-3

358 W.-M. Yuan et al. / Radiation Measurements 36 (2003) 357–362

Fig. 1. Regional geological sketch map and sample locations. Q: Quaternary, N: Tertiary, T: Triassic, P2: Late Permian, C: Carboniferous, Pt :Proterozoic, Y5: Granitoid in Yanshanian epoch, Y4: Granitoid in Indo-Sinian epoch, F1: Manite-Haerguole fault, F2: South-Kunlun fault, F3:Chakarita–Chaigenagena fault, F4: Middle-Kunlun fault, F5: Awate-Nagedang fault, F6: Chaidamuhe-Xiangride fault, F7: Dulan-Chahanwusufault, F8: Chachaxuema-Lakagongma fault, F9: Wahongshan-Wenquan fault.

terrain formation. Igneous rocks developed, controlled bythe subduction of both the MKB and the SKB to north. Theoccurrence of igneous rocks (especially granitoid) accountsfor about 90% of the area in the NKB. The secondary faultswith a southwest dip formed after the continent–continentcollision.

3. Samples and methodology

In order to constrain the tectonic evolution in the EKM,a series of samples of about 2 kg each were collected for9ssion track analysis from a transect in the eastern part of theEKM, from Dulan to Buqingshan mountains (Fig. 1). Thesemainly consist of di=erent kinds of granitoid and sandstones.Apatite separates from a total of 41 samples were ob-

tained by using standard heavy liquid and magnetic separa-tion techniques. The individual apatite grains were mountedin epoxy and polished to expose internal grain surfaces.Spontaneous tracks were revealed by etching in 7% HNO3for 30 sec at 25◦C. Low-uranium muscovite in close con-tact with these grains served as an external detector duringirradiation. After irradiation in the 492 Light-Water Reactorof Beijing, the muscovite external detectors were detached

and etched in 40% HF for 20 min at 25◦C Track densi-ties for both natural and induced 9ssion track populationswere measured with a dry objective at 100× 15 magni9ca-tion. Neutron Luency was determined by using the dosime-ter glass CN5. Fission track ages (FTAs) were measuredusing the IUGS-recommended Zeta calibration approach.The Zeta values used in this study have been determinedfrom repeated measurements of standard apatites (Hurfordand Green, 1983; Hurford, 1990). The weighted mean Zetavalue obtained in this paper is 322:1± 3:6(1�). The lengthof horizontal con9ned 9ssion tracks were measured exclu-sively in prismatic apatite crystals because of the anisotropyof annealing of 9ssion tracks in apatite (Green et al., 1986).

4. Results and geological signi�cance

Fission track analysis results are listed in Table 1. The41 apatite FTAs of these samples from a south–northcross-section of the EKM lie between 25 and 130 Ma andthe mean track lengths for these samples range from 9.4 to12:1 �m, with respective standard deviations of the tracklength distributions of 2.9–1:9 �m. The samples with olderapparent apatite ages were analyzed from the northern NKB

W.-M. Yuan et al. / Radiation Measurements 36 (2003) 357–362 359

Table 1Apatite 9ssion track analytical results of eastern Kunlun mountains

Sample no. Elevation Distance in SN Rock type Era �s ( 105cm−1) �i ( 105=cm) P(x2) FT age ±1� Length (�m)± SD(no. of grains) (m) (km) (Ns) (Ni) (%) (Ma) (n)

XT73 141.4 8.57 18.57 94 10:8± 2:0(2) 3266 Granite �3 (48) (104) 77± 14 (104)XT72 118.6 7.99 11.69 96(7) 3244 Granite �5− 1 (123) (180) 114± 14XT70 110.0 3.43 9.70 99(11) 3369 Granite �5− 1 (93) (263) 57± 3XT69 103.1 9.29 16.76 31(27) 3473 Granite �5− 1 (514) (927) 93± 6XT68 96.0 6.37 10.33 51 10:7± 2:3(14) 3314 Granite �5− 1 (290) (470) 103± 6 (102)XT66 92.8 11.61 14.80 80(15) 3175 Granite �4− 3 (324) (413) 130± 10XT63 87.3 9.01 14.08 9(14) 3267 Granite �4− 3 (318) (497) 107± 8XT64 87.1 8.88 19.00 10(13) 3140 Granite �4− 3 (356) (762) 78± 5XT4 83.3 5.54 21.43 38(4) 3224 Granite �4− 3 (62) (240) 43± 6XT4-1 83.3 4.43 13.53 96 11:2± 2:8(12) 3224 Granite �4− 3 (182) (556) 57± 5 (100)XT5 81.6 4.43 11.18 37 11:1± 2:7(10) 3180 Granite �4− 3 (215) (542) 66± 6 (105)XT7 78.6 5.16 18.59 16 9:8± 2:6(7) 3236 Granite �4− 3 (110) (396) 47± 5 (99)XT60 76.8 1.36 5.3 68(6) 3330 Granite �4− 3 (41) (160) 45± 8XT59 74.2 7.78 24.60 34(21) 3335 Granite �4− 3 (507) (1604) 53± 3XT10-2 72.1 11.01 35.01 96 10:7± 2:5(17) 3346 Granite �4− 3 (337) (1102) 51± 4 (92)XT11 69.5 3.92 11.80 83 10:3± 2:4(24) 3369 Diorite �4− 3 (340) (1023) 56± 4 (67)XT58-2 68.8 10.92 27.51 29(17) 3391 Diorite �4− 3 (416) (1048) 66± 4XT12 66.7 4.12 12.33 11 11:9± 2:5(13) 3385 Granite �4− 3 (205) (614) 56± 5 (100)XT13 62.1 3.67 19.92 86 11:5± 2:3(19) 3429 Granite �4− 3 (133) (721) 31± 3 (100)XT14 60.9 4.89 24.18 40 11:8± 2:4(16) 3448 Granite �4− 3 (201) (994) 34± 3 (86)XT15 59.8 3.26 19.78 55 11:9± 2:4(11) 3467 Granite �4− 3 (116) (704) 28± 3 (104)XT20 58.9 2.36 13.14 15(34) 3450 Granite �4− 3 (400) (2224) 30± 2XT16 57.8 Tectonic 2.61 11.22 99 10:3± 1:9(8) 3488 breccia O-S (30) (129) 43± 6 (34)XT19 56.8 Granitic 4.11 26.97 15(15) 3490 vein �4− 3 (167) (1095) 27± 2XT17-1 56.0 2.14 13.17(4) 3513 Granite �4− 3 (27) (166) 49 27± 6XT24-2 52.1 11.09 26.41 23 10:1± 2:9(14) 3581 Sandstone T1 (386) (919) 70± 5 (55)XT26 46 20.70 65.95 0 9:4± 2:9(18) 3593 Sandstone T1 (766) (2440) 55± 6 (72)XT51 44.2 4.38 28.88 18(16) 3614 Sandstone T1 (160) (1054) 25± 2

360 W.-M. Yuan et al. / Radiation Measurements 36 (2003) 357–362

Table 1 (continued)

Sample no. Elevation Distance in SN Rock type Era �s ( 105cm−1) �i ( 105=cm) P(x2) FT age ±1� Length (�m)± SD(no. of grains) (m) (km) (Ns) (Ni) (%) (Ma) (n)

XT27 41.2 18.99 51.64 1 10:6± 2:2(20) 3702 Sandstone P2 (1075) (2923) 67± 4 (101)XT28 40.5 6.49 28.61 27(15) 3707 Sandstone P2 (480) (2117) 38± 2XT30 38.7 19.28 50.57 16 9:9± 2:9(15) 3722 Sandstone P2 (671) (1760) 64± 3 (113)XT47 37.4 12.27 44.10 38(17) 3757 Sandstone P2 (476) (1711) 47± 3XT31 36.6 26.43 99.59 8 11:6± 2:1(18) 3768 Sandstone T1 (769) (2898) 44± 2 (107)XT46 35.7 19.90 64.16 4(18) 3815 Sandstone T1 (770) (2483) 50± 3XT33 34.7 12.04 45.13 8 11:1± 2:0(12) 3806 Sandstone T1 (507) (1900) 45± 2 (103)XT34 34.0 6.96 30.01 26 11:8± 2:1(17) 3804 Sandstone T1 (553) (2386) 39± 2 (107)XT35 33.5 9.52 35.74 32 12:1± 1:9(23) 3806 Sandstone T1 (729) (2738) 45± 2 (101)XT41 15.8 9.81 34.30 24(20) 4292 Sandstone P2 (456) (1595) 48± 3XT40 12.9 7.93 23.25 13 10:7± 2:3(19) 4352 Sandstone P2 (558) (1637) 57± 3 (130)XT38 10.7 6.47 11.97 0(14) 4426 Sandstone P2 (628) (1162) 89± 8XT36 10.0 5.20 12.20 54 10:8± 2:2(14) 4630 Sandstone P2 (307) (709) 72± 5 (135)

�s and �i are fossil track density and induced track density, respectively. Standard track density and the track number for standard trackare 1:04× 106 cm−1 and 2607. Ns and Ni are fossil track and induced track. P(x2) is x2 probability (Galbraith, 1981).

with lower elevations and, the younger samples from nearthe MKFZ. Both the longest and shortest mean track lengthsoriginated in the same 9rst-grade blocks (e.g. the NKBand MKB). The mean lengths and the length deviationsdecrease and increase towards the north in the MKB faultand the SKB, respectively. Almost all of the apatite FTAsis signi9cantly younger than their host rocks. This con9rmsthat the age grains analyzed belong to a single populationof ages, and hence the samples experienced a thermal eventwhich resulted in complete annealing of the original trackssubsequent to the formation of their host rocks.It is demonstrated that three positive correlation lines can

be qualitatively traced out according to the distribution trendof the samples in a diagram of the 9ssion track age vs. therelative distance perpendicular to both the MKFZ and theSKFZ (Fig. 2). The dotted line (a) indicates that the 9ssiontrack age expresses approximately 1 Ma for each 0:51 kmdistance from the fault zone to the north. This result indicatesthat the samples from the NKB detect the thermal e=ects ofthe activity of the MKFZ. In other words, the thermal evo-lution of the NKB is mainly controlled by intracontinentalsubduction of the MKB along the Middle-Kunlun fault zone.The reason why the AFT ages are directly correlated to the

distances from the fault zone is as follows: on the one hand,the deep part with lower age near the fault zone uplifted intothe present surface, due to a higher uplifting rate near thefault zone than at points distant from it. On the other hand,since there is a higher geothermal gradient near the faultzone, the samples far from the fault zone display higher AFTages than those near the fault zone, although these sampleswere collected at similar elevations. The regional tectoniza-tion was mainly related to the continent–continent collisionbetween India and Asia, which orogenesis within the conti-nent plays a key role for the uplifting of the Qinghai-Tibetplateau (Copeland et al., 1995; Harrison et al., 1995).Similarly, another two positive correlations (evolution

trends) that parallel each other between the FTA and thedistance can be seen between the SKFZ and the MKFZ(Fig. 2, dot lines (b) and (c)). Their FTAs increase by 1 Maper approx. 0:22 km (a trend which is 100% higher thanthe trend (a)). According to the distribution relationship be-tween samples and faults, one can deduce that, the trends(b) and (c) are controlled by the Chakarita–Chaigenagenafault (F3) and the SKFZ (F2), respectively. The Chakarita–Chaigenagena fault (F3) played an important role in intra-continental evolution, in concert with the MKFZ and the

W.-M. Yuan et al. / Radiation Measurements 36 (2003) 357–362 361

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140 160

Distance perpendicular to both Mid-Kunlun and South-Kunlun faults (km)

Fiss

ion

trac

k ag

e (M

a)

F2F4

F3

(a)

(b)

(c)

1

Fig. 2. Relationship between the 9ssion track age and the distance perpendicular to 9rst-grade fault zones. F2, F3 and F4 are the locationsof the South-Kunlun fault zone, Chakarita–Chaigenagena fault zone and Middle-Kunlun fault zone. The evolution lines (a), (b) and (c)were, respectively, controlled by the faults F4, F3 and F2. The trends ◦1 was related to the activities of the F1.

0

40

80

120

160

200

04080120160200240

(XT4-1)

0

40

80

120

160

200

04080120160200240

( XT13)

0

40

80

120

160

200

04080120160200240

(XT34)

0

40

80

120

160

200

04080120160200240

(XT36)

Fig. 3. Fields of possible thermal histories for the samples from the eastern Kunlun mountains calculated by inverse modeling of the observedapatite 9ssion track parameters, based on the annealing model (Ketcham et al., 1999). The X - and Y -coordinate reLect the 9ssion track age(Ma) and the temperature (◦C), respectively. The 9eld between dashed lines is the goodness-of-9t result predicted by the model, and thesolid line is the best-9t result.

SKFZ. Meanwhile, there is an inverse correlation trend line◦1 in Fig. 2, of which the samples are located at the south-western side of the F1, and the trend mainly controlled bythe F1 that dips southwest.Based on the annealing model of Ketcham et al. (1999),

thermal histories of samples from di=erent blocks in theEKM are modeled using the Monte Carlo method selectedby Ketcham et al’s AFTSolve Program. Modeling condi-tions are determined according to the observed apatite 9s-sion track parameters. The modeling results, which are quitea good-9t, are shown in Fig. 3 in the whole. There are some

similar characteristics in the thermal histories of the sam-ples, i.e., the model thermal histories are characterized byslow cooling from ∼160◦C to ∼80◦C at ∼240 to ∼20 Ma,followed by rather rapid cooling to near-surface tempera-tures (Fig. 3). The modeling data indicate that there were atleast two episodes of regional tectonic events in the EKM,i.e. 9rst, intracontinental subduction of the MKB along theMKFZ, and subduction of the SKB along the SKFZ in thelate Triassic (Yuan et al., 2000); and second, rapid cool-ing and denudation of basement rocks since around 20 Ma(Harrison et al., 1992; Mock et al., 1999).

362 W.-M. Yuan et al. / Radiation Measurements 36 (2003) 357–362

Some details in di=erent tectonic sub-units (Fig. 3)are separately discussed as follows: (1) sample 4-1 col-lected from the NKB 9rst rapidly cooled from ∼150◦Cto ∼95◦C at ∼240 to 120 Ma, and then residence attemperature of ∼90◦C until 20 Ma, followed by a rapidcooling after ∼20 Ma mentioned above; (2) sample XT13near the margin of the NKB experienced a slow cool-ing from ∼160◦C to 110◦ C to 80◦C between ∼240and ∼110 to 12 Ma, followed by a rapid cooling af-ter 12 Ma; (3) sample XT34 located at the MKB had athermal history with accelerating cooling rates, 9rst from∼155◦C to ∼114◦C between ∼255 and 120–60 Ma,then to 80◦C until ∼19 Ma; and (4) sample XT36 ex-perienced simple cooling to ∼75◦C from 260 to ∼8 Ma(Fig. 3).The long span from ∼120 to ∼20–12 Ma for the

residence of temperature ∼110◦C to 90◦C, especiallyin the NKB, reLects the long -duration of the in-tracontinental orogenesis. The earlier the rapid cool-ing in the NKB started after ∼20 Ma, the further thedistance from the MKFZ. The MKFZ (F4) and theSKFZ (F2), the 9rst-grade faults with NWW strikeand SSW dips, are a suite of active faults duringcontinent–continent collision. It is the two-paleosuturezones that controlled the intracontinental evolutionin the eastern Kunlun area, resulting in the posi-tive FTA correlation with distance from the faults(Fig. 2).

Acknowledgements

This work was supported by Nature Science Founda-tion of China (No. 40072068 and 10175076). The authorwishes to thank Prof. A.G.W. Gleadow, Dr. B.P. Kohnand Dr. A. Raza of the School of Earth Sciences, theUniversity of Melbourne for their help in the 9ssion trackmeasurement.

References

Copeland, P., Harrison, T.M., Yun, P., 1995. Thermal evolutionof the Gangdese batholith, southern Tibet: a history of episodicunroo9ng. Tectonics 14, 223–236.

Galbraith, R.F., 1981. On statistical models for 9ssion track counts.Math. Geol. 13, 971–988.

Green, P.F., Duddy, I.R., Gleadow, A.J.W., Tingate, P.T., Laslett,G.M., 1986. Thermal annealing of 9ssion tracks in apatite, 1, Aqualitative description. Chem. Geol. 59, 237–253.

Harrison, T.M., Copeland, P., Kidd, W.S.F., 1992. Rising Tibet.Sci. 255, 1663–1670.

Harrison, T.M., Copeland, P., Kidd, W.S.F., 1995. Activation ofthe Nyainqentanghla shear zone: implications for uplift of thesouthern Tibet Plateau. Tectonics 14, 658–676.

Hurford, A.J., 1990. Standardization of 9ssion track datingcalibration: recommendation by the Fission Track WorkingGroup of the I.U.G.S. Subcommission on Geochronology. Chem.Geol. (Isotope Geoscience Section) 80, 171–178.

Hurford, A.J., Green, P.F., 1983. The Zeta age calibration of 9ssiontrack dating. Isot. Geosci. 1, 285–317.

Ketcham, R.A., Donelick, R.A., Carlson, W.D., 1999. Variabilityof apatite 9ssion-track annealing kinetics III: extrapolation togeological time scales. Am. Mineral. 84, 1235–1255.

Li, H., Xu, Z., Chen, W., 1996. Southern margin strike-slip faultzone of the east Kunlun mountains: an important consequenceof intracontinental deformation. Cont. Dyn. 1, 146–155.

Liu, Y., Genser, J., Neubauer, F., 2000. Geochronology of40Ar/39Ar dating in the basement rocks in the eastern Kunlunmountains and its tectonic implications. Earth Sci. Front. 7(Suppl.), 227.

Matte, P., Tapponnier, P., Arnaud, N., Bourjot, L., Avouac, J.P.,Vidal, P., Liu Qing, Pan Yusheng, Wang Yi, 1996. Tectonics ofwestern Tibet, between the Tarim and the Indus. Earth Planet.Sci. Lett. 142, 311–330.

Mock, C., Arnaud, N.O., Cantagrel, J.-M., 1999. An earlyunroo9ng in northeastern Tibet? Constraints from 40Ar/39Arthermochronology on granitoids from the eastern Kunlun range(Qinghai, NW China). Earth Planet. Lett. 171, 107–122.

Yuan, W., Mo, X., Yu, X., Luo, Z., 2000. The record of Indosiniantectonic setting from the granitoid of eastern Kunlun mountains.Geol. Rev. 46, 203–211 (in Chinese with English abstract).