U-Th)/He Dating

Journal of Earth Science, Vol. 31, No. 4, p. 735–742, August 2020 ISSN 1674-487X Printed in China https://doi.org/10.1007/s12583-020-1328-4

Cenozoic Uplift of the Central Yunnan Fragment, Southwestern China, Revealed by Apatite (U-Th)/He Dating

Ke Wu , Youpu Dong *, Jiaxin Duan, Xin Ru, Dongyue Zhang, Dan Wang Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming 650093, China Ke Wu: https://orcid.org/0000-0003-4915-242X; Youpu Dong: https://orcid.org/0000-0002-2829-7585

ABSTRACT: The age of central Yunnan fragment uplift has long been debated, with estimates ranging from the Late Eocene to about 1 Ma. To determine the central Yunnan fragment uplift time in the Ce- nozoic, apatite (U-Th)/He (AHe) was used to analyze the low-temperature thermochronology of samples from the Jiaozi Mountain area of the eastern central Yunnan fragment. The sampling area is located in the Dongchuan District of Kunming, Yunnan Province, near the Xiaojiang fault zone. The results show that AHe ages from the eastern part of central Yunnan fragment were mainly concentrated around 25.7–37.9 Ma, and intensive uplift had happened before 36.5 Ma. Together with previous low- temperature thermochronology research on the western and eastern central Yunnan fragment, we concluded that the Yunnan Plateau uplifted prior to 36.5 Ma, in a west to east sequence. The uplift caused a change in paleo-geographical terrain, which may have altered the ancient river systems of the southeast Tibetan Plateau. KEY WORDS: central Yunnan fragment, (U-Th)/He dating, uplift, Cenozoic.

0 INTRODUCTION Tapponnier, 1988; Tapponnier et al., 1986, 1982; Tapponnier Since the Early Cenozoic, the collision of Indo-Asian con- and Molnar, 1976). And the second is that the eastern Tibetan tinents has led to the shortening of the Tibetan Plateau crust. Plateau can be explained by the eastward flow of the hot and The Qiangtang terrane in the middle of the Tibetan Plateau had weak lower crust material (Copley, 2008; Royden et al., 1997, risen to its current height before 40 Ma (Wang C S et al., 2008), Royden, 1996; Bird, 1991; Englannd and Houseman, 1986). and as the Indian-Asian continental collision continued, the Yunnan Plateau, named for its average elevation above plateau gradually expanded towards the north and the south, 2 000 m, is along the southeast margin of Tibetan Plateau and respectively. The southern edge of the Qinghai-Tibet Plateau is high in the northwest and low in the southeast. The Yalong had risen to its present heights in the Mid-Miocene (Zhang et River thrust fault represents the Tibetan Plateau boundary and al., 2018; Rowley et al., 2001), while the Hoh Xil Basin divides the Yunnan Plateau into two parts (Liu et al., 2009). reached its present height in the Late Pleistocene (DeCelles et The southeast side of the Yalong River fault is the central al., 2007). The northeastern part of the Tibetan Plateau gradu- Yunnan fragment, and the west side of the Yalong River fault ally uplifted in the Pliocene and Quaternary, forming the belongs to the Tibetan Plateau. The uplifting age of the central present Tibetan Plateau with Qilian Mountains uplift in the Yunnan fragment uplift has been debated, with estimates rang- Late Cenozoic (Tapponnier et al., 2001). ing from the Late Eocene to about 1 Ma, and Yang et al. (2010) The southeastern margin of the Tibetan Plateau has dis- argued that the central Yunnan fragment was formed a million tinct geomorphological features with a gradual and slow loss of years ago. Wu et al. (2018) considered that Northwest Yunnan altitude from northwest to southeast, bearing no significant began to rise gradually in the Late Eocene by reconstructing the geomorphological differences (Fig. 1). Two main deformation ancient elevation and environment. Hoke et al. (2014) con- modes have been proposed to explain the formation of the cluded that the central Yunnan fragment reached its current southeast margin of the Tibetan Plateau: the first is that the altitude as early as the Late Eocene by calculating the ancient IndoChina terrane and the Sichuan-Yunnan fragment extruded elevation and river profile of the Red River Basin. to the southeast along a large strike-slip fault with large-scale Previous assessments of central Yunnan fragment formation clockwise rotation (Huangfu et al., 2016; Clift et al., 2006; were not very consistent, however, so it is still controversial and Leloup et al., 2001; Avouac and Tapponnieer, 1993; Peltzer and other means are needed to obtain comprehensive estimates. Low-temperature thermochronology can record when a section *Corresponding author: [email protected] of crust cooled through an orrotemperature as it was exhumed © China University of Geosciences (Wuhan) and Springer-Verlag towards the surface. Because it has a relatively low closure GmbH Germany, Part of Springer Nature 2020 temperature, apatite (U-Th)/He dating is a useful way to analyze and document the latest stages of topographic evolution Manuscript received November 20, 2019. and cooling within the uppermost kilometers of the crust (Wu et Manuscript accepted April 06, 2020. al., 2017). Previous studies have been conducted in the western

Wu, K., Dong, Y. P., Duan, J. X., et al., 2020. Cenozoic Uplift of the Central Yunnan Fragment, Southwestern China, Revealed by Apatite (U-Th)/He Dating. Journal of Earth Science, 31(4): 735–742. https://doi.orgg/10.1007/s12583-020-1328-4. http://en.earth-science.net 736 Ke Wu, Youpu Dong, Jiaxin Duan, Xin Ru, Dongyue Zhang and Dan Wang

Figure 1. Geological and tectonic settings of the Tibetan Plateau and Yunnan Plateau region. Topography, active faults, and major suture zones in the Yunnan Plateau. SYF. Sichuan-Yunnan fragment; WSF. western Sichuan fragment; LXF. Lijiang-Xiaojinhe fault; YTB. Yalong thrust belt. and central parts of the central Yunnan fragment (Jiang et al., sandstone samples collected along vertical profiles (Fig. 3). 2018; Liu et al., 2018; Chen et al., 2016; Li, 2012; Wang G et We used apatite (U-Th)/He to explore the cooling history al., 2008; Fu, 2005). However, there are not good estimates for of these samples and its relationship to topographic evolution. the eastern part. Moreover, the eastern part is far from the Ti- The low closure temperatures of apatite (U-Th)/He (AHe, betan Plateau, which can better reflect and constraint the uplift 60±20 ºC; Farley, 2002) make it possible to measure cooling of time. Our sampling locations were in the Jiaozi Mountain, lo- the shallow crust that is too small in magnitude to be recorded cated in the eastern part of the central Yunnan fragment and by higher temperature systems in regions undergoing slow along the western edge of the Yangzte Craton (Pan, 2009). exhumation, and to offer more detailed information for very There have been no prior reports on low-temperature thermal young cooling events or relief change. chronological research in this area. Samples collected by the institute were 400–500 m in height, a vertical sampling interval of 80–100 m, and a weight 1 GEOLOGICAL SETTING AND METHODS of about 3–4 kg. The position of each sampling point was rec- The basement of the central Yunnan fragment is Protero- orded with a portable GPS (Fig. 4). zoic metamorphic rock, which is overlaid by Late Palaeozoic All experiments were conducted at the 40Ar/39Ar and and Mesozoic continental deposits (Wang and Wang, 2005). (U-Th)/He Laboratory of the Institute of Geology and Geo- The fragment is bounded by the IndoChina Terrane, South physics of the Chinese Academy of Sciences (IGGCAS). The China Terrane, the Sichuan-Qinghai fragment, and the Tibetan sandstone samples were crushed and sieved to obtain fragments Plateau to the south, the east, the north, and the west, respec- between 280 and 450 μm in diameter. The apatite crystals were tively. The sampling area is Jiaozi Mountain area located in the manually selected directly from the fragments and carefully eastern part of Central Yunnan fragment. The Cenozoic, Me- inspected under a high-powered microscope to meet (U-Th)/He sozoic, Palaeozoic, and Proterozoic strata are well exposed. dating criteria (Reiners and Farley, 2001). Most apatite grains Samples were collected from the Lower Proterozoic metamor- were characterized by different degrees of roundness, which phic sandstone (Yunnan Provincial Bureau of Geology and indicates that they experienced erosion and transport for vari- Mineral Resources, 1990). The Jinsha River runs through the ous distances before deposition. To meet the (U-Th)/He dating whole region from west to east. The eastern side is the Xian- criteria, less-rounded grains were selected. shui River-Xiaojiang fault zone (Fig. 2). It is an ideal place to Based on the clarity and morphology, hand-picked grains limit uplift on the eastern central Yunnan fragment. To better were wrapped in platinum capsules. Length and width mea- understand the history of denudation in the region, we used surements for α ejection correction (Farley, 2002) were taken low-temperature thermochronology to study four metamorphic for each grain. The analytical protocol adopted in this study

Cenozoic Uplift of the Central Yunnan Fragment, Southwestern China, Revealed by Apatite (U-Th)/He Dating 737

Figure 2. Geological map of the sampling area. E. Palaeogene; Q. Quaternary; T. Triassic; K. Cretaceous; Є. Cambrian; P. Permian; Pt. Proterozoic; Cg. con- glomerate; Ms. mudstone; St. siltstone; Ls. limestone; Mss. metamorphic sandstone; Dol. dolomite; Sl. slate; Qz. quartzite; Ph. phyllite.

Figure 3. (a) Geomorphology of the Jiaozi Mountain region; (b) swath profiles at the latitudes of Jiaozi Mountain sampling sites, showing the max, min and average topography in the direction of N10°E and roughly perpendicular to the Yangtze River, showing incision depths of major rivers, and perched low relief surfaces. It also shows sampling locations projected on profiles.

followed those described by Foeken et al. (2006). (U-Th)/He dates were calculated using standard procedures developed by Meesters and Dunai (2002). Total analytical uncertainty was calculated as the square root of squares of weighted uncertain- ties of U, Th, and He measurements, and included the estimated additional variations of ±7% determined after repeating analyses of Durango apatite.

2 RESULTS Four metamorphosed sandstone samples were selected at different elevations for apatite (U-Th)/He dating (Table 1). Single-grain apatite He ages show no clear relationships with the contents of effective uranium ([eU]=[U]+0.235×[Th]), nor a large difference in the retentivity of grains with different amounts of radiation damage (Fig. 5a). No relationship between the length and age of the apatite grains was observed, i.e., there was no correlation between grain size and age, implying

Figure 4. Sampling locations near Jiaozi Mountain superimposed on the that grain size did not affect (U-Th)/He age (Fig. 5b). geological map and shaded relief, respectively. Circles with red filling The apatite He ages of the samples range from 50.1 to denote samples. 25.7 Ma and are much younger than the depositional ages.

738 Ke Wu, Youpu Dong, Jiaxin Duan, Xin Ru, Dongyue Zhang and Dan Wang

Table 1 Test results for apatite (U-Th) /He age in the Jiaozi Mountain region

Sample ID Latitude (N) Longitude (E) Elevation (m) Lithology Mean AHe (Ma) PDS03 26°13′16.87″ 102°57′10.48″ 3 606 Metamorphic sandstone 37.9 JDS04 26°14′07.05″ 102°57′17.73″ 3 332 Metamorphic sandstone 50.1 PDS05 26°13′50.16″ 102°57′12.23″ 3 290 Metamorphic sandstone 36.5 PDS06 26°14′09.76″ 102°57′04.64″ 3 220 Metamorphic sandstone 25.7

Figure 5. (a) (U-Th)/He ages versus effective uranium content eU; (b) (U-Th)/He ages versus length.

This indicates that the apatites were exposed to higher temper- which may have affected tectonic activity in the region. Xiao- atures than the apatite He closure temperature after deposition, jiang fault is the southern part of the Xianshui River-Xiaojiang at which point all samples had undergone complete or partial fault belt that is made up of several faults: Ganzi-Yushu and annealing. He ages are cooling ages that are usually related to a Xianshui River faults in the northwest, Anning River fault in tectonic uplifting event (Ehlers and Farley, 2003; Dodson, the north-south direction after extending to Shimian County, 1973). There was an obvious turning point in the altitude-He and Zemu River fault in the northwest direction (Zhang, 2008). age plot, and the cooling rate from 37.9 to 36.5 Ma was much The Xianshui River-Xiaojiang fault zone sequence is north to higher than 36.5–25.7 Ma (Fig. 6 and Table 1), implying that south (Schoenbohm et al., 2006). Ganzi-Yushu and Xianshui intensive uplift occurred prior to 36.5 Ma. River faults were initiated at 12.6 and 9 Ma, respectively One much older age (50.1 Ma from sample S04) was dif- (Zhang et al., 2017; Wang et al., 2009), which implied that ficult to explain. It may be because, in some cases, the Xiaojiang fault activity should have occurred after 9 Ma. The unaccounted-for effects of specific types of U and Th zonation AHe ages from our study area were older than 23 Ma, on the within apatite can lead to older ages when the Ft correction is other hand. Thus, in the eastern part of the central Yunnan applied under a homogeneous distribution assumption, result- fragment, tectonic activity did not appear to account for the ing in a >30% difference in age (Farley et al.,1996). uplift prior to the Neogene.

3 DISCUSSION 3.2 Central Yunnan Fragment Uplift 3.1 Xiaojiang Fault Zone and Central Yunnan Fragment Sample data from the Jiaozi Mountain area in the eastern Uplift part of the central Yunnan fragment allowed us to plot an The sampling locations were very close to Xiaojiang fault, elevation-age map. As age increased, elevation also increased in a positive linear relationship. The cooling rate from 37.9 to 36.5 Ma was much higher than 36.5–25.7 Ma, implying that intensive uplift occurred prior to 36.5 Ma. Other research conducted in the central part of the central Yunnan fragment (Xiangyun, Sazhi, Xinping, and Dali) observed that a rapid uplift occurred during 20–43 Ma (Jiang et al., 2018; Wu et al., 2018; Hoke et al., 2014; Fu, 2005). The western part of the central Yunnan fragment, west of the Ya- long River, uplifted prior to 40 Ma according to studies of stratigraphy and sedimentation, and low temperature and thermal chronological data were concentrated within 33–56 Ma (Liu et al., 2018; Li, 2012; Wang C S et al., 2008) (Fig. 7). In conclusion, it can be inferred that central Yunnan fragment uplift occurred from west to east, and that eastern uplift Figure 6. Terranes of apatite (U-Th)/He (AHe) ages against elevation for may have occurred prior to 36.5 Ma. It can be explained by the samples. India-IndoChina oblique convergence in the Cenozoic. The

Cenozoic Uplift of the Central Yunnan Fragment, Southwestern China, Revealed by Apatite (U-Th)/He Dating 739

Figure 7. Sampling locations and schematic geologic map of the Yunnan Plateau.

Indian Block indented against the Asian Plate during its nor- geographical framework, impacting the southeast Tibetan Pla- theastward drifting (Molnar and Stock, 2009; Bertrand and teau river systems. Prior to 35 Ma, there was no connection Rangin, 2003), and the eastern Himalaya syntaxis has wedged between the ancient Jinsha and Yangtze rivers, but the ancient into Eurasia Block before 60 Ma (Zhong and Ding, 1996), and Chuan River might meet with Jinsha River in the Shigu area, the India Block showed a counterclockwise rotation compared and flowed into the ancient the Red River (Zheng, 2016). The with the Eurasian Block before 37 Ma (Lee and Lawver, 1995). ancient Sichuan River changed flow direction from westward So the west part of the central Yunnan fragment would be af- to eastward around 36–23 Ma (Fig. 8) (Zheng et al., 2013). The fected earlier than the east part. The extrusion model inferred ancient Red River, which connected the Tibetan Plateau with that the extrusion of IndoChina Block initiated at about 35 Ma the South China Sea prior to 35 Ma, was cut off and lost its (Socquet and Pubellier, 2005), and its main extrusion period northern source after 35 Ma (Chen et al., 2017; Clark et al., was from 27 to 17 Ma (Gilley et al., 2003; Leloup et al., 1995). 2006, 2004; Clift et al., 2006; Brookfield, 1998). Although The channel flow model concluded that the southeast of Tibe- many works had been done (Wu et al., 2018; Gourbet et al., tan Plateau deformed before 13 Ma (Clark et al., 2005). There- 2017; Xu et al., 2016), there is no convinced evidence to de- fore, together with all the deformation ages described above, it scribe the paleoaltitude of the southeast of Tibetan Plateau due could be inferred that there had been an obvious uplift in the to the enormous error of modern research methods. Study re- southeast of the Tibetan Plateau before the extrusion or the sults that ancient rivers system changed their incipient course material flow happened. were consistent with our research. It is likely that the central Multiphase uplifts are considered as an important form in Yunnan fragment uplift altered the ancient river systems of the Tibetan Plateau (Zhong and Ding, 1996), and in this paper, the southeastern Tibetan Plateau. uplift before 36.5 Ma could represent the initiative uplift in the southeastern margin of the Qinghai-Tibet Plateau in Cenozoic. 4 CONCLUSIONS There are two strong cooling events around 13–10 and 5 Ma in In this study, we used low-temperature thermochronology to this area (Jing et al., 2018; Wang and Lu, 2014; Wang et al., determine the timing of the eastern central Yunnan fragment 2013; Wang and Wang, 2005). uplift. AHe ages were mainly concentrated from 25.7–37.9 Ma Central Yunnan fragment uplift may have also altered the and intensive uplift occurred prior to 36.5 Ma, implying that the

740 Ke Wu, Youpu Dong, Jiaxin Duan, Xin Ru, Dongyue Zhang and Dan Wang

Figure 8. Simplified tectonic map of paleo-Red River and paleo-Chuanjiang River during the Early Cenozoic. Tectonic reconstructions and the positions of the terranes are based on Zheng et al. (2013) and Chen et al. (2017). sampling area was less likely to be affected by Xiaojiang fault Geochronological and Sedimentological Study of the Simao Basin, movement. Together with previous studies conducted in the cen- Yunnan: Implications for the Early Cenozoic Evolution of the Red tral and western parts of the central Yunnan fragment, we con- River. Earth and Planetary Science Letters, 476: 22–33. cluded that the uplift sequence occurred from west to east. The https://doi.org/10.1016/j.epsl.2017.07.025 central Yunnan fragment uplift altered the geographical frame- Clark, M. K., House, M. A., Royden, L. H., et al., 2005. Late Cenozoic work, which may have changed river system courses. Other re- Uplift of Southeastern Tibet. Geology, 33(6): 525–528. searches on the ancient Sichuan and Red rivers were consistent https://doi.org/10.1130/g21265.1 with our study. Clark, M. K., Royden, L. H., Whipple, K. X., et al., 2006. Use of a Regional, Relict Landscape to Measure Vertical Deformation of the Eastern Ti- ACKNOWLEDGMENTS betan Plateau. Journal of Geophysical Research: Earth Surface, This study was supported by the National Natural Science 111(F3): F3002. https://doi.org/10.1029/2005jf000294 Foundation of China (Nos. 41802215, 41762017). We are also Clark, M. K., Schoenbohm, L. M., Royden, L. H., et al., 2004. Surface Uplift, grateful for the helpful comments from the anonymous review- Tectonics, and Erosion of Eastern Tibet from Large-Scale Drainage Pat- ers. The final publication is available at Springer via terns. Tectonics, 23(1): 1–20. https://doi.org/10.1029/2002tc001402 https://doi.org/10.1007/s12583-020-1328-4. Clift, P. D., Blusztajn, J., Nguyen, A. D., 2006. Large-Scale Drainage Cap- ture and Surface Uplift in Eastern Tibet-SW China before 24 Ma In- REFERENCES CITED ferred from Sediments of the Hanoi Basin, Vietnam. Geophysical Re- Avouac, J. P., Tapponnier, P., 1993. Kinematic Model of Active Deforma- search Letters, 33(19): 027772. https://doi.org/10.1029/2006gl027772 tion in Central Asia. Geophysical Research Letters, 20(10): 895–898. Copley, A., 2008. Kinematics and Dynamics of the Southeastern Margin of https://doi.org/10.1029/93gl00128 the Tibetan Plateau. Geophysical Journal International, 174(3): Bertrand, G., Rangin, C., 2003. Tectonics of the Western Margin of the Shan 1081–1100. https://doi.org/10.1111/j.1365-246x.2008.03853.x Plateau (central Myanmar): Implication for the India-Indochina Obli- DeCelles, P. G., Quade, J., Kapp, P., et al., 2007. High and Dry in Central que Convergence since the Oligocene. Journal of Asian Earth Sciences, Tibet during the Late Oligocene. Earth and Planetary Science Letters, 21(10): 1139–1157. https://doi.org/10.1016/s1367-9120(02)00183-9 253(3/4): 389–401. https://doi.org/10.1016/j.epsl.2006.11.001 Bird, P., 1991. Lateral Extrusion of Lower Crust from under High Topo- Dodson, M. H., 1973. Closure Temperature in Cooling Geochronological graphy in the Isostatic Limit. Journal of Geophysical Research, 96(B6): and Petrological Systems. Contributions to Mineralogy and Petrology, 10275. https://doi.org/10.1029/91jb00370 40: 259–274. https://doi.org/10.1007/BF00373790 Brookfield, M. E., 1998. The Evolution of the Great River Systems of Ehlers, T. A., Farley, K. A., 2003. Apatite (U-Th)/He Thermochronometry: Southern Asia during the Cenozoic India-Asia Collision: Rivers Methods and Applications to Problems in Tectonic and Surface Draining Southwards. Geomorphology, 22(3/4): 285–312. Processes. Earth and Planetary Science Letters, 206: 1 – 14. https://doi.org/10.1016/s0169-555x(97)00082-2 https://doi.org/10.1016/s0012-821x(02)01069-5 Chen, X. Y., Liu, J. L., Wu, W. B., 2016. The Exhumation and Uplift of the England, P., Houseman, G., 1986. Finite Strain Calculations of Continental Southern Shigu Complex since Early Cretaceous Evidenced by Zircon Deformation: 2. Comparison with the India-Asia Collision Zone. and Apatite Fission Track. Geological Bulletin of China, 35(5): Journal of Geophysical Research: Solid Earth, 91(B3): 3664–3676. 727–737 (in Chinese with English Abstract) https://doi.org/10.1029/jb091ib03p03664 Chen, Y., Yan, M. D., Fang, X. M., et al., 2017. Detrital Zircon U-Pb Farley, K. A., 2002. (U-Th)/He Dating: Techniques, Calibrations, and Ap-

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