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A new vision of the intracontinental evolution of the eastern Kunlun Mountains, Northern Qinghai-Tibet plateau, China
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Radiation Measurements 36 (2003) 357 – 362 www.elsevier.com/locate/radmeas
A new vision of the intracontinental evolution of the eastern Kunlun Mountains, Northern Qinghai-Tibet plateau, China W.-M. Yuana;∗ , X.-T. Zhangb , J.-Q. Donga , Y.-H. Tanga , F.-S. Yua , S.-C. Wanga a Institute
of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China b China 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 Kunlun mountains, Qinghai-Tibet Plateau, China, this paper shows that (1) the FTA in di=erent blocks increases with the distance from the South-Kunlun fault and Mid-Kunlun faults, respectively, indicating the control of the main faults on the tectonic evolution of this region; and (2) the thermal histories are characterized by slow cooling from ∼160◦ C to ∼80◦ C at ∼240 to ∼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
2. Geological setting
The Eastern Kunlun Mountains (EKM) is located in the northern Qinghai-Tibet Plateau and consists of a Paleozoic– Triassic collision belt, rejuvenated during the Cenozoic India-Asia collision (Matte et al., 1996). They represent the southern margin of the Qaidam basin (Fig. 1). To the south of the Middle-Kunlun fault zone (MKFZ), the middle– upper Proterozoic Wanbaogou Group in Middle-Kunlun Block (MKB) yielded a muscovite 40 Ar= 39 Ar age of ca. 160 Ma and the lower Silurian Nachitai Group in the MKB revealed a biotite 40 Ar= 39 Ar age of ca. 110 Ma that was overprinted by a very-low-grade event at 60 –40 Ma (Liu et al., 2000). Mock et al. (1999) concluded that the Mesozoic plutons had undergone an important cooling period around 140 –120 Ma, coeval with ductile deformation along the Xidatan fault. According to the Xidatan fault zone, four major stages of tectonic movements have been distinguished, which are 240 –200, 150 –140, 120-110, and 20 Ma (Li et al., 1996). Strike slipping was accompanied with the uplifting of the crust, and, as a result, this caused the formation of the speci9c structural pattern of the EKM.
The EKM are an epicontinental active belt in geological history. The MKFZ and the South-Kunlun fault zone (SKFZ) are paleosuture zones, dipping to the north and stretching more than 1000 km in the EWW extension. The EKM can be subdivided into three blocks; designated as the North-Kunlun Block (NKB), Middle-Kunlun Block (MKB) and South-Kunlun Block (SKB), respectively, having the MKFZ and the SKFZ as their limits (Fig. 1). The EKM were subjected to multiepisode tectonic activities. During the Caledonian epoch, crustal stretching was dominant, and a primary small ocean basin (i.e. archipelago) was formed. Subsequent to this, the crust was compressed in the majority of this area, resulting in uplifting, and the primary ocean basin was closed during the Hercynian epoch. The EKM were in a strong orogenic period during the Indo-Sinian epoch. The intense compression resulted in both the SKB and the MKB being subducted to the north continuously with strike-slip, and the south relict Tethyan Ocean was entirely closed. The uplifting caused by continent–continent collision took place during the Jurassic and Cretaceous, periods meanwhile, there were strong sinistral strike slips along both the MKFZ and the SKFZ. Since the Cenozoic period, inhomogeneous imbalanced elevation and subsidence resulted in basin-and-range
∗ Corresponding author. Tel.: +86-10-88233189; fax: +8610-882386. E-mail address: [email protected] (W.-M. Yuan).
c 2003 Published by Elsevier Ltd. 1350-4487/03/$ - see front matter 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-Chahanwusu fault, F8: Chachaxuema-Lakagongma fault, F9: Wahongshan-Wenquan fault.
terrain formation. Igneous rocks developed, controlled by the subduction of both the MKB and the SKB to north. The occurrence of igneous rocks (especially granitoid) accounts for about 90% of the area in the NKB. The secondary faults with a southwest dip formed after the continent–continent collision.
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 for 9ssion track analysis from a transect in the eastern part of the EKM, from Dulan to Buqingshan mountains (Fig. 1). These mainly consist of di=erent kinds of granitoid and sandstones. Apatite separates from a total of 41 samples were obtained by using standard heavy liquid and magnetic separation techniques. The individual apatite grains were mounted in epoxy and polished to expose internal grain surfaces. Spontaneous tracks were revealed by etching in 7% HNO3 for 30 sec at 25◦ C. Low-uranium muscovite in close contact with these grains served as an external detector during irradiation. After irradiation in the 492 Light-Water Reactor of Beijing, the muscovite external detectors were detached
and etched in 40% HF for 20 min at 25◦ C Track densities for both natural and induced 9ssion track populations were measured with a dry objective at 100 × 15 magni9cation. Neutron Luency was determined by using the dosimeter glass CN5. Fission track ages (FTAs) were measured using the IUGS-recommended Zeta calibration approach. The Zeta values used in this study have been determined from repeated measurements of standard apatites (Hurford and Green, 1983; Hurford, 1990). The weighted mean Zeta value obtained in this paper is 322:1 ± 3:6(1). The length of horizontal con9ned 9ssion tracks were measured exclusively in prismatic apatite crystals because of the anisotropy of 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. The 41 apatite FTAs of these samples from a south–north cross-section of the EKM lie between 25 and 130 Ma and the mean track lengths for these samples range from 9.4 to 12:1 m, with respective standard deviations of the track length distributions of 2.9 –1:9 m. The samples with older apparent apatite ages were analyzed from the northern NKB
W.-M. Yuan et al. / Radiation Measurements 36 (2003) 357 – 362
359
Table 1 Apatite 9ssion track analytical results of eastern Kunlun mountains Sample no. Elevation Distance in SN Rock type Era (no. of grains) (m) (km)
s ( 105 cm−1 ) i ( 105 =cm) P(x2 ) FT age ±1 Length (m) ± SD (Ns ) (Ni ) (%) (Ma) (n)
XT73 (2) XT72 (7) XT70 (11) XT69 (27) XT68 (14) XT66 (15) XT63 (14) XT64 (13) XT4 (4) XT4-1 (12) XT5 (10) XT7 (7) XT60 (6) XT59 (21) XT10-2 (17) XT11 (24) XT58-2 (17) XT12 (13) XT13 (19) XT14 (16) XT15 (11) XT20 (34) XT16 (8) XT19 (15) XT17-1 (4) XT24-2 (14) XT26 (18) XT51 (16)
8.57 (48) 7.99 (123) 3.43 (93) 9.29 (514) 6.37 (290) 11.61 (324) 9.01 (318) 8.88 (356) 5.54 (62) 4.43 (182) 4.43 (215) 5.16 (110) 1.36 (41) 7.78 (507) 11.01 (337) 3.92 (340) 10.92 (416) 4.12 (205) 3.67 (133) 4.89 (201) 3.26 (116) 2.36 (400) 2.61 (30) 4.11 (167) 2.14 (27) 11.09 (386) 20.70 (766) 4.38 (160)
3266 3244 3369 3473 3314 3175 3267 3140 3224 3224 3180 3236 3330 3335 3346 3369 3391 3385 3429 3448 3467 3450 3488 3490 3513 3581 3593 3614
141.4 118.6 110.0 103.1 96.0 92.8 87.3 87.1 83.3 83.3 81.6 78.6 76.8 74.2 72.1 69.5 68.8 66.7 62.1 60.9 59.8 58.9 57.8 56.8 56.0 52.1 46 44.2
Granite
3
Granite
5 − 1
Granite
5 − 1
Granite
5 − 1
Granite
5 − 1
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Diorite
4 − 3
Diorite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite Tectonic breccia Granitic vein
4 − 3 O-S
Granite
4 − 3
4 − 3
Sandstone T1 Sandstone T1 Sandstone T1
18.57 (104) 11.69 (180) 9.70 (263) 16.76 (927) 10.33 (470) 14.80 (413) 14.08 (497) 19.00 (762) 21.43 (240) 13.53 (556) 11.18 (542) 18.59 (396) 5.3 (160) 24.60 (1604) 35.01 (1102) 11.80 (1023) 27.51 (1048) 12.33 (614) 19.92 (721) 24.18 (994) 19.78 (704) 13.14 (2224) 11.22 (129) 26.97 (1095) 13.17 (166) 26.41 (919) 65.95 (2440) 28.88 (1054)
94 96 99 31 51 80 9 10 38 96 37 16 68 34 96 83 29 11 86 40 55 15 99 15 49 23 0 18
77 ± 14
10:8 ± 2:0 (104)
114 ± 14 57 ± 3 93 ± 6 103 ± 6
10:7 ± 2:3 (102)
130 ± 10 107 ± 8 78 ± 5 43 ± 6 57 ± 5 66 ± 6 47 ± 5
11:2 ± 2:8 (100) 11:1 ± 2:7 (105) 9:8 ± 2:6 (99)
45 ± 8 53 ± 3 51 ± 4 56 ± 4 66 ± 4 56 ± 5 31 ± 3 34 ± 3 28 ± 3 30 ± 2 43 ± 6
10:7 ± 2:5 (92) 10:3 ± 2:4 (67) 11:9 ± 2:5 (100) 11:5 ± 2:3 (100) 11:8 ± 2:4 (86) 11:9 ± 2:4 (104) 10:3 ± 1:9 (34)
27 ± 2 27 ± 6 70 ± 5 55 ± 6 25 ± 2
10:1 ± 2:9 (55) 9:4 ± 2:9 (72)
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W.-M. Yuan et al. / Radiation Measurements 36 (2003) 357 – 362
Table 1 (continued) Sample no. Elevation Distance in SN Rock type Era s ( 105 cm−1 ) i ( 105 =cm) P(x2 ) FT age ±1 Length (m) ± SD (Ni ) (%) (Ma) (n) (no. of grains) (m) (km) (Ns ) XT27 (20) XT28 (15) XT30 (15) XT47 (17) XT31 (18) XT46 (18) XT33 (12) XT34 (17) XT35 (23) XT41 (20) XT40 (19) XT38 (14) XT36 (14)
3702 3707 3722 3757 3768 3815 3806 3804 3806 4292 4352 4426 4630
41.2 40.5 38.7 37.4 36.6 35.7 34.7 34.0 33.5 15.8 12.9 10.7 10.0
Sandstone
P2
Sandstone
P2
Sandstone
P2
Sandstone
P2
Sandstone
T1
Sandstone
T1
Sandstone
T1
Sandstone
T1
Sandstone
T1
Sandstone
P2
Sandstone
P2
Sandstone
P2
Sandstone
P2
18.99 (1075) 6.49 (480) 19.28 (671) 12.27 (476) 26.43 (769) 19.90 (770) 12.04 (507) 6.96 (553) 9.52 (729) 9.81 (456) 7.93 (558) 6.47 (628) 5.20 (307)
51.64 (2923) 28.61 (2117) 50.57 (1760) 44.10 (1711) 99.59 (2898) 64.16 (2483) 45.13 (1900) 30.01 (2386) 35.74 (2738) 34.30 (1595) 23.25 (1637) 11.97 (1162) 12.20 (709)
1 27 16 38 8 4 8 26 32 24 13 0 54
67 ± 4 38 ± 2 64 ± 3 47 ± 3 44 ± 2 50 ± 3 45 ± 2 39 ± 2 45 ± 2 48 ± 3 57 ± 3 89 ± 8 72 ± 5
10:6 ± 2:2 (101) 9:9 ± 2:9 (113) 11:6 ± 2:1 (107) 11:1 ± 2:0 (103) 11:8 ± 2:1 (107) 12:1 ± 1:9 (101) 10:7 ± 2:3 (130) 10:8 ± 2:2 (135)
s and i are fossil track density and induced track density, respectively. Standard track density and the track number for standard track are 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 near the MKFZ. Both the longest and shortest mean track lengths originated in the same 9rst-grade blocks (e.g. the NKB and MKB). The mean lengths and the length deviations decrease and increase towards the north in the MKB fault and the SKB, respectively. Almost all of the apatite FTAs is signi9cantly younger than their host rocks. This con9rms that the age grains analyzed belong to a single population of ages, and hence the samples experienced a thermal event which resulted in complete annealing of the original tracks subsequent to the formation of their host rocks. It is demonstrated that three positive correlation lines can be qualitatively traced out according to the distribution trend of the samples in a diagram of the 9ssion track age vs. the relative distance perpendicular to both the MKFZ and the SKFZ (Fig. 2). The dotted line (a) indicates that the 9ssion track age expresses approximately 1 Ma for each 0:51 km distance from the fault zone to the north. This result indicates that the samples from the NKB detect the thermal e=ects of the activity of the MKFZ. In other words, the thermal evolution of the NKB is mainly controlled by intracontinental subduction 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 into the present surface, due to a higher uplifting rate near the fault zone than at points distant from it. On the other hand, since there is a higher geothermal gradient near the fault zone, the samples far from the fault zone display higher AFT ages than those near the fault zone, although these samples were collected at similar elevations. The regional tectonization was mainly related to the continent–continent collision between India and Asia, which orogenesis within the continent plays a key role for the uplifting of the Qinghai-Tibet plateau (Copeland et al., 1995; Harrison et al., 1995). Similarly, another two positive correlations (evolution trends) that parallel each other between the FTA and the distance can be seen between the SKFZ and the MKFZ (Fig. 2, dot lines (b) and (c)). Their FTAs increase by 1 Ma per approx. 0:22 km (a trend which is 100% higher than the trend (a)). According to the distribution relationship between samples and faults, one can deduce that, the trends (b) and (c) are controlled by the Chakarita–Chaigenagena fault (F3) and the SKFZ (F2), respectively. The Chakarita– Chaigenagena fault (F3) played an important role in intracontinental evolution, in concert with the MKFZ and the
W.-M. Yuan et al. / Radiation Measurements 36 (2003) 357 – 362
361
140 (a)
F4
F2
120 Fission track age (Ma)
F3 100 (c)
80 60
(b)
40 1
20 0 0
20 40 60 80 100 120 140 Distance perpendicular to both Mid-Kunlun and South-Kunlun faults (km)
160
Fig. 2. Relationship between the 9ssion track age and the distance perpendicular to 9rst-grade fault zones. F2, F3 and F4 are the locations of 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. 240
200
160
120
80
40
240
0
(XT4-1)
40
40
80
80
120
120
160
160
200
200
240
200
160
120
80
40
0 40
200
160
120
80
40
0
0
0
0
( XT13)
240
200
160
120
80
40
0
0
(XT34) 40
80
80
120
120
160
160
200
200
(XT36)
Fig. 3. Fields of possible thermal histories for the samples from the eastern Kunlun mountains calculated by inverse modeling of the observed apatite 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 the solid 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 southwestern side of the F1, and the trend mainly controlled by the F1 that dips southwest. Based on the annealing model of Ketcham et al. (1999), thermal histories of samples from di=erent blocks in the EKM are modeled using the Monte Carlo method selected by Ketcham et al’s AFTSolve Program. Modeling conditions are determined according to the observed apatite 9ssion track parameters. The modeling results, which are quite a good-9t, are shown in Fig. 3 in the whole. There are some
similar characteristics in the thermal histories of the samples, i.e., the model thermal histories are characterized by slow cooling from ∼160◦ C to ∼80◦ C at ∼240 to ∼20 Ma, followed by rather rapid cooling to near-surface temperatures (Fig. 3). The modeling data indicate that there were at least two episodes of regional tectonic events in the EKM, i.e. 9rst, intracontinental subduction of the MKB along the MKFZ, and subduction of the SKB along the SKFZ in the late Triassic (Yuan et al., 2000); and second, rapid cooling and denudation of basement rocks since around 20 Ma (Harrison et al., 1992; Mock et al., 1999).
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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 collected from the NKB 9rst rapidly cooled from ∼150◦ C to ∼95◦ C at ∼240 to 120 Ma, and then residence at temperature of ∼90◦ C until 20 Ma, followed by a rapid cooling after ∼20 Ma mentioned above; (2) sample XT13 near the margin of the NKB experienced a slow cooling from ∼160◦ C to 110◦ C to 80◦ C between ∼240 and ∼110 to 12 Ma, followed by a rapid cooling after 12 Ma; (3) sample XT34 located at the MKB had a thermal 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 experienced 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, especially in the NKB, reLects the long -duration of the intracontinental orogenesis. The earlier the rapid cooling in the NKB started after ∼20 Ma, the further the distance from the MKFZ. The MKFZ (F4) and the SKFZ (F2), the 9rst-grade faults with NWW strike and SSW dips, are a suite of active faults during continent–continent collision. It is the two-paleosuture zones that controlled the intracontinental evolution in the eastern Kunlun area, resulting in the positive FTA correlation with distance from the faults (Fig. 2).
Acknowledgements This work was supported by Nature Science Foundation of China (No. 40072068 and 10175076). The author wishes to thank Prof. A.G.W. Gleadow, Dr. B.P. Kohn and Dr. A. Raza of the School of Earth Sciences, the University of Melbourne for their help in the 9ssion track measurement.
References Copeland, P., Harrison, T.M., Yun, P., 1995. Thermal evolution of the Gangdese batholith, southern Tibet: a history of episodic unroo9ng. 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, A qualitative 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 of the Nyainqentanghla shear zone: implications for uplift of the southern Tibet Plateau. Tectonics 14, 658–676. Hurford, A.J., 1990. Standardization of 9ssion track dating calibration: recommendation by the Fission Track Working Group 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 9ssion track dating. Isot. Geosci. 1, 285–317. Ketcham, R.A., Donelick, R.A., Carlson, W.D., 1999. Variability of apatite 9ssion-track annealing kinetics III: extrapolation to geological time scales. Am. Mineral. 84, 1235–1255. Li, H., Xu, Z., Chen, W., 1996. Southern margin strike-slip fault zone of the east Kunlun mountains: an important consequence of intracontinental deformation. Cont. Dyn. 1, 146–155. Liu, Y., Genser, J., Neubauer, F., 2000. Geochronology of 40Ar/39Ar dating in the basement rocks in the eastern Kunlun mountains 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 of western Tibet, between the Tarim and the Indus. Earth Planet. Sci. Lett. 142, 311–330. Mock, C., Arnaud, N.O., Cantagrel, J.-M., 1999. An early unroo9ng in northeastern Tibet? Constraints from 40 Ar/39 Ar thermochronology 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 Indosinian tectonic setting from the granitoid of eastern Kunlun mountains. Geol. Rev. 46, 203–211 (in Chinese with English abstract).
Radiation Measurements 36 (2003) 357 – 362 www.elsevier.com/locate/radmeas
A new vision of the intracontinental evolution of the eastern Kunlun Mountains, Northern Qinghai-Tibet plateau, China W.-M. Yuana;∗ , X.-T. Zhangb , J.-Q. Donga , Y.-H. Tanga , F.-S. Yua , S.-C. Wanga a Institute
of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China b China 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 Kunlun mountains, Qinghai-Tibet Plateau, China, this paper shows that (1) the FTA in di=erent blocks increases with the distance from the South-Kunlun fault and Mid-Kunlun faults, respectively, indicating the control of the main faults on the tectonic evolution of this region; and (2) the thermal histories are characterized by slow cooling from ∼160◦ C to ∼80◦ C at ∼240 to ∼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
2. Geological setting
The Eastern Kunlun Mountains (EKM) is located in the northern Qinghai-Tibet Plateau and consists of a Paleozoic– Triassic collision belt, rejuvenated during the Cenozoic India-Asia collision (Matte et al., 1996). They represent the southern margin of the Qaidam basin (Fig. 1). To the south of the Middle-Kunlun fault zone (MKFZ), the middle– upper Proterozoic Wanbaogou Group in Middle-Kunlun Block (MKB) yielded a muscovite 40 Ar= 39 Ar age of ca. 160 Ma and the lower Silurian Nachitai Group in the MKB revealed a biotite 40 Ar= 39 Ar age of ca. 110 Ma that was overprinted by a very-low-grade event at 60 –40 Ma (Liu et al., 2000). Mock et al. (1999) concluded that the Mesozoic plutons had undergone an important cooling period around 140 –120 Ma, coeval with ductile deformation along the Xidatan fault. According to the Xidatan fault zone, four major stages of tectonic movements have been distinguished, which are 240 –200, 150 –140, 120-110, and 20 Ma (Li et al., 1996). Strike slipping was accompanied with the uplifting of the crust, and, as a result, this caused the formation of the speci9c structural pattern of the EKM.
The EKM are an epicontinental active belt in geological history. The MKFZ and the South-Kunlun fault zone (SKFZ) are paleosuture zones, dipping to the north and stretching more than 1000 km in the EWW extension. The EKM can be subdivided into three blocks; designated as the North-Kunlun Block (NKB), Middle-Kunlun Block (MKB) and South-Kunlun Block (SKB), respectively, having the MKFZ and the SKFZ as their limits (Fig. 1). The EKM were subjected to multiepisode tectonic activities. During the Caledonian epoch, crustal stretching was dominant, and a primary small ocean basin (i.e. archipelago) was formed. Subsequent to this, the crust was compressed in the majority of this area, resulting in uplifting, and the primary ocean basin was closed during the Hercynian epoch. The EKM were in a strong orogenic period during the Indo-Sinian epoch. The intense compression resulted in both the SKB and the MKB being subducted to the north continuously with strike-slip, and the south relict Tethyan Ocean was entirely closed. The uplifting caused by continent–continent collision took place during the Jurassic and Cretaceous, periods meanwhile, there were strong sinistral strike slips along both the MKFZ and the SKFZ. Since the Cenozoic period, inhomogeneous imbalanced elevation and subsidence resulted in basin-and-range
∗ Corresponding author. Tel.: +86-10-88233189; fax: +8610-882386. E-mail address: [email protected] (W.-M. Yuan).
c 2003 Published by Elsevier Ltd. 1350-4487/03/$ - see front matter 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-Chahanwusu fault, F8: Chachaxuema-Lakagongma fault, F9: Wahongshan-Wenquan fault.
terrain formation. Igneous rocks developed, controlled by the subduction of both the MKB and the SKB to north. The occurrence of igneous rocks (especially granitoid) accounts for about 90% of the area in the NKB. The secondary faults with a southwest dip formed after the continent–continent collision.
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 for 9ssion track analysis from a transect in the eastern part of the EKM, from Dulan to Buqingshan mountains (Fig. 1). These mainly consist of di=erent kinds of granitoid and sandstones. Apatite separates from a total of 41 samples were obtained by using standard heavy liquid and magnetic separation techniques. The individual apatite grains were mounted in epoxy and polished to expose internal grain surfaces. Spontaneous tracks were revealed by etching in 7% HNO3 for 30 sec at 25◦ C. Low-uranium muscovite in close contact with these grains served as an external detector during irradiation. After irradiation in the 492 Light-Water Reactor of Beijing, the muscovite external detectors were detached
and etched in 40% HF for 20 min at 25◦ C Track densities for both natural and induced 9ssion track populations were measured with a dry objective at 100 × 15 magni9cation. Neutron Luency was determined by using the dosimeter glass CN5. Fission track ages (FTAs) were measured using the IUGS-recommended Zeta calibration approach. The Zeta values used in this study have been determined from repeated measurements of standard apatites (Hurford and Green, 1983; Hurford, 1990). The weighted mean Zeta value obtained in this paper is 322:1 ± 3:6(1). The length of horizontal con9ned 9ssion tracks were measured exclusively in prismatic apatite crystals because of the anisotropy of 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. The 41 apatite FTAs of these samples from a south–north cross-section of the EKM lie between 25 and 130 Ma and the mean track lengths for these samples range from 9.4 to 12:1 m, with respective standard deviations of the track length distributions of 2.9 –1:9 m. The samples with older apparent apatite ages were analyzed from the northern NKB
W.-M. Yuan et al. / Radiation Measurements 36 (2003) 357 – 362
359
Table 1 Apatite 9ssion track analytical results of eastern Kunlun mountains Sample no. Elevation Distance in SN Rock type Era (no. of grains) (m) (km)
s ( 105 cm−1 ) i ( 105 =cm) P(x2 ) FT age ±1 Length (m) ± SD (Ns ) (Ni ) (%) (Ma) (n)
XT73 (2) XT72 (7) XT70 (11) XT69 (27) XT68 (14) XT66 (15) XT63 (14) XT64 (13) XT4 (4) XT4-1 (12) XT5 (10) XT7 (7) XT60 (6) XT59 (21) XT10-2 (17) XT11 (24) XT58-2 (17) XT12 (13) XT13 (19) XT14 (16) XT15 (11) XT20 (34) XT16 (8) XT19 (15) XT17-1 (4) XT24-2 (14) XT26 (18) XT51 (16)
8.57 (48) 7.99 (123) 3.43 (93) 9.29 (514) 6.37 (290) 11.61 (324) 9.01 (318) 8.88 (356) 5.54 (62) 4.43 (182) 4.43 (215) 5.16 (110) 1.36 (41) 7.78 (507) 11.01 (337) 3.92 (340) 10.92 (416) 4.12 (205) 3.67 (133) 4.89 (201) 3.26 (116) 2.36 (400) 2.61 (30) 4.11 (167) 2.14 (27) 11.09 (386) 20.70 (766) 4.38 (160)
3266 3244 3369 3473 3314 3175 3267 3140 3224 3224 3180 3236 3330 3335 3346 3369 3391 3385 3429 3448 3467 3450 3488 3490 3513 3581 3593 3614
141.4 118.6 110.0 103.1 96.0 92.8 87.3 87.1 83.3 83.3 81.6 78.6 76.8 74.2 72.1 69.5 68.8 66.7 62.1 60.9 59.8 58.9 57.8 56.8 56.0 52.1 46 44.2
Granite
3
Granite
5 − 1
Granite
5 − 1
Granite
5 − 1
Granite
5 − 1
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Diorite
4 − 3
Diorite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite
4 − 3
Granite Tectonic breccia Granitic vein
4 − 3 O-S
Granite
4 − 3
4 − 3
Sandstone T1 Sandstone T1 Sandstone T1
18.57 (104) 11.69 (180) 9.70 (263) 16.76 (927) 10.33 (470) 14.80 (413) 14.08 (497) 19.00 (762) 21.43 (240) 13.53 (556) 11.18 (542) 18.59 (396) 5.3 (160) 24.60 (1604) 35.01 (1102) 11.80 (1023) 27.51 (1048) 12.33 (614) 19.92 (721) 24.18 (994) 19.78 (704) 13.14 (2224) 11.22 (129) 26.97 (1095) 13.17 (166) 26.41 (919) 65.95 (2440) 28.88 (1054)
94 96 99 31 51 80 9 10 38 96 37 16 68 34 96 83 29 11 86 40 55 15 99 15 49 23 0 18
77 ± 14
10:8 ± 2:0 (104)
114 ± 14 57 ± 3 93 ± 6 103 ± 6
10:7 ± 2:3 (102)
130 ± 10 107 ± 8 78 ± 5 43 ± 6 57 ± 5 66 ± 6 47 ± 5
11:2 ± 2:8 (100) 11:1 ± 2:7 (105) 9:8 ± 2:6 (99)
45 ± 8 53 ± 3 51 ± 4 56 ± 4 66 ± 4 56 ± 5 31 ± 3 34 ± 3 28 ± 3 30 ± 2 43 ± 6
10:7 ± 2:5 (92) 10:3 ± 2:4 (67) 11:9 ± 2:5 (100) 11:5 ± 2:3 (100) 11:8 ± 2:4 (86) 11:9 ± 2:4 (104) 10:3 ± 1:9 (34)
27 ± 2 27 ± 6 70 ± 5 55 ± 6 25 ± 2
10:1 ± 2:9 (55) 9:4 ± 2:9 (72)
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 ( 105 cm−1 ) i ( 105 =cm) P(x2 ) FT age ±1 Length (m) ± SD (Ni ) (%) (Ma) (n) (no. of grains) (m) (km) (Ns ) XT27 (20) XT28 (15) XT30 (15) XT47 (17) XT31 (18) XT46 (18) XT33 (12) XT34 (17) XT35 (23) XT41 (20) XT40 (19) XT38 (14) XT36 (14)
3702 3707 3722 3757 3768 3815 3806 3804 3806 4292 4352 4426 4630
41.2 40.5 38.7 37.4 36.6 35.7 34.7 34.0 33.5 15.8 12.9 10.7 10.0
Sandstone
P2
Sandstone
P2
Sandstone
P2
Sandstone
P2
Sandstone
T1
Sandstone
T1
Sandstone
T1
Sandstone
T1
Sandstone
T1
Sandstone
P2
Sandstone
P2
Sandstone
P2
Sandstone
P2
18.99 (1075) 6.49 (480) 19.28 (671) 12.27 (476) 26.43 (769) 19.90 (770) 12.04 (507) 6.96 (553) 9.52 (729) 9.81 (456) 7.93 (558) 6.47 (628) 5.20 (307)
51.64 (2923) 28.61 (2117) 50.57 (1760) 44.10 (1711) 99.59 (2898) 64.16 (2483) 45.13 (1900) 30.01 (2386) 35.74 (2738) 34.30 (1595) 23.25 (1637) 11.97 (1162) 12.20 (709)
1 27 16 38 8 4 8 26 32 24 13 0 54
67 ± 4 38 ± 2 64 ± 3 47 ± 3 44 ± 2 50 ± 3 45 ± 2 39 ± 2 45 ± 2 48 ± 3 57 ± 3 89 ± 8 72 ± 5
10:6 ± 2:2 (101) 9:9 ± 2:9 (113) 11:6 ± 2:1 (107) 11:1 ± 2:0 (103) 11:8 ± 2:1 (107) 12:1 ± 1:9 (101) 10:7 ± 2:3 (130) 10:8 ± 2:2 (135)
s and i are fossil track density and induced track density, respectively. Standard track density and the track number for standard track are 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 near the MKFZ. Both the longest and shortest mean track lengths originated in the same 9rst-grade blocks (e.g. the NKB and MKB). The mean lengths and the length deviations decrease and increase towards the north in the MKB fault and the SKB, respectively. Almost all of the apatite FTAs is signi9cantly younger than their host rocks. This con9rms that the age grains analyzed belong to a single population of ages, and hence the samples experienced a thermal event which resulted in complete annealing of the original tracks subsequent to the formation of their host rocks. It is demonstrated that three positive correlation lines can be qualitatively traced out according to the distribution trend of the samples in a diagram of the 9ssion track age vs. the relative distance perpendicular to both the MKFZ and the SKFZ (Fig. 2). The dotted line (a) indicates that the 9ssion track age expresses approximately 1 Ma for each 0:51 km distance from the fault zone to the north. This result indicates that the samples from the NKB detect the thermal e=ects of the activity of the MKFZ. In other words, the thermal evolution of the NKB is mainly controlled by intracontinental subduction 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 into the present surface, due to a higher uplifting rate near the fault zone than at points distant from it. On the other hand, since there is a higher geothermal gradient near the fault zone, the samples far from the fault zone display higher AFT ages than those near the fault zone, although these samples were collected at similar elevations. The regional tectonization was mainly related to the continent–continent collision between India and Asia, which orogenesis within the continent plays a key role for the uplifting of the Qinghai-Tibet plateau (Copeland et al., 1995; Harrison et al., 1995). Similarly, another two positive correlations (evolution trends) that parallel each other between the FTA and the distance can be seen between the SKFZ and the MKFZ (Fig. 2, dot lines (b) and (c)). Their FTAs increase by 1 Ma per approx. 0:22 km (a trend which is 100% higher than the trend (a)). According to the distribution relationship between samples and faults, one can deduce that, the trends (b) and (c) are controlled by the Chakarita–Chaigenagena fault (F3) and the SKFZ (F2), respectively. The Chakarita– Chaigenagena fault (F3) played an important role in intracontinental evolution, in concert with the MKFZ and the
W.-M. Yuan et al. / Radiation Measurements 36 (2003) 357 – 362
361
140 (a)
F4
F2
120 Fission track age (Ma)
F3 100 (c)
80 60
(b)
40 1
20 0 0
20 40 60 80 100 120 140 Distance perpendicular to both Mid-Kunlun and South-Kunlun faults (km)
160
Fig. 2. Relationship between the 9ssion track age and the distance perpendicular to 9rst-grade fault zones. F2, F3 and F4 are the locations of 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. 240
200
160
120
80
40
240
0
(XT4-1)
40
40
80
80
120
120
160
160
200
200
240
200
160
120
80
40
0 40
200
160
120
80
40
0
0
0
0
( XT13)
240
200
160
120
80
40
0
0
(XT34) 40
80
80
120
120
160
160
200
200
(XT36)
Fig. 3. Fields of possible thermal histories for the samples from the eastern Kunlun mountains calculated by inverse modeling of the observed apatite 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 the solid 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 southwestern side of the F1, and the trend mainly controlled by the F1 that dips southwest. Based on the annealing model of Ketcham et al. (1999), thermal histories of samples from di=erent blocks in the EKM are modeled using the Monte Carlo method selected by Ketcham et al’s AFTSolve Program. Modeling conditions are determined according to the observed apatite 9ssion track parameters. The modeling results, which are quite a good-9t, are shown in Fig. 3 in the whole. There are some
similar characteristics in the thermal histories of the samples, i.e., the model thermal histories are characterized by slow cooling from ∼160◦ C to ∼80◦ C at ∼240 to ∼20 Ma, followed by rather rapid cooling to near-surface temperatures (Fig. 3). The modeling data indicate that there were at least two episodes of regional tectonic events in the EKM, i.e. 9rst, intracontinental subduction of the MKB along the MKFZ, and subduction of the SKB along the SKFZ in the late Triassic (Yuan et al., 2000); and second, rapid cooling 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 collected from the NKB 9rst rapidly cooled from ∼150◦ C to ∼95◦ C at ∼240 to 120 Ma, and then residence at temperature of ∼90◦ C until 20 Ma, followed by a rapid cooling after ∼20 Ma mentioned above; (2) sample XT13 near the margin of the NKB experienced a slow cooling from ∼160◦ C to 110◦ C to 80◦ C between ∼240 and ∼110 to 12 Ma, followed by a rapid cooling after 12 Ma; (3) sample XT34 located at the MKB had a thermal 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 experienced 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, especially in the NKB, reLects the long -duration of the intracontinental orogenesis. The earlier the rapid cooling in the NKB started after ∼20 Ma, the further the distance from the MKFZ. The MKFZ (F4) and the SKFZ (F2), the 9rst-grade faults with NWW strike and SSW dips, are a suite of active faults during continent–continent collision. It is the two-paleosuture zones that controlled the intracontinental evolution in the eastern Kunlun area, resulting in the positive FTA correlation with distance from the faults (Fig. 2).
Acknowledgements This work was supported by Nature Science Foundation of China (No. 40072068 and 10175076). The author wishes to thank Prof. A.G.W. Gleadow, Dr. B.P. Kohn and Dr. A. Raza of the School of Earth Sciences, the University of Melbourne for their help in the 9ssion track measurement.
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