Apatite fission track evidence for Miocene denudation history in the Gangdese conglomerate belt and Yarlung Tsangpo River: Implications for the evolution of Southern Tibet

Journal of Asian Earth Sciences 160 (2018) 159–167

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Apatite fission track evidence for Miocene denudation history in the Gangdese conglomerate belt and Yarlung Tsangpo River: Implications for the evolution of Southern Tibet

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Shiyu Song, Daiyong Cao , QingChao Zhang, Anming Wang, Yangwen Peng College of Geoscience & Surveying Engineering, China University of Mining & Technology, Beijing 10083, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Gangdese conglomerate belt Yarlung Zangbo River Apatite fission track Evolution history

Low-temperature thermochronology is used widely in the Tibet plateau uplift. Some researches, however, have defined the time of rapid denudation as simply rock uplift and have neglected the fact that the rock denudation recorded by fission track (FT) data was controlled by both surface incision and rock uplift. The incision of the Yarlung Zangbo River had a significant influence on uplift history inversion in Southern Tibet. This paper simulated the bedrock denudation and river incision histories using apatite fission track (AFT) data sampled from the Gangdese conglomerate belt, which is located in the middle of Southern Tibet, and analyzed the geological meaning of the AFT age of each sample. The results showed the following: (1) In the early Miocene (22–16 Ma), both the value of the denudation rate and the incision rate were high (0.56 mm/yr and 0.24 mm/yr). (2) In the middle-late Miocene, the incision rate (0.12 mm/yr) was similar to the denudation rate (0.09–0.11 mm/yr). (3) The historical model between river incision and bedrock denudation revealed a significant difference in the denudation rate during the period ca. 8–6 Ma. Combining these data with previously published thermochronological ages and synthesizing these ages with regional geological, we arrived at the following conclusions: (1) In the early Miocene, the denudation event probably was caused by a combined result of Indian plate rollback and the incision of the Yarlung Zangbo River. (2) In the middle-late Miocene, the denudation rate was consistent with the incision rate, which suggested that the denudation episode was caused by climate change associated with Asian monsoon intensification. (3) After 8 Ma, the stable and slow incision rate indicated that regional drastic uplift had ceased. The paleo-elevation of the research area had approached, and even exceeded, the present-day elevation in the late Miocene.

1. Introduction Because of the collision of the India-Asia plate, the Gangdese conglomerate belt formed a significant tectonic unit in Southern Tibet. The regional topography is characterized today by river valleys and upland incision surfaces that are occupied by the eastward-flowing Yarlung Zangbo River, which flows through the Gangdese arc and the Xigaze basin. The Gangdese conglomerate belt demonstrates the history of this uplift and the denudation of the Gangdese arc and Tethyan Himalayas. The process by which the Gangdese conglomerate belt was formed, filled, buried, and eventually uplifted to modern elevations and exhumed, however, are not well understood and remain controversial (Wang et al., 2000; Aitchison et al., 2002; DeCelles et al., 2011). Low-temperature thermochronology is used widely in the Tibetan plateau uplift. The fission track (FT) age can record the timing at which the rock volume passed through the isothermal closure surface. The

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Corresponding author. E-mail address: [email protected] (D. Cao).

https://doi.org/10.1016/j.jseaes.2018.04.021 Received 18 January 2018; Received in revised form 4 April 2018; Accepted 19 April 2018 Available online 21 April 2018 1367-9120/ © 2018 Elsevier Ltd. All rights reserved.

rock volume passing through the isothermal closure surface in a rapid period is obviously larger than that in slow denudation period, which could lead to a peak being formed by the AFT ages in the accumulation area. Thus, AFT age could be a proxy for rock denudation. In addition to the rock uplift, rock denudation also is caused by surface erosion, such as rivers, glaciers, wind, and structural activity. Additionally, the AFT age reflects the rock denudation, which contains information about rock uplift and surface erosion. The research area is located in the Gangdese conglomerate belt, which is in the middle of Southern Tibet. Remnant surfaces of the research area are rugged and discontinuous, because as remnant surfaces have not yet adjusted to modern uplift and Yarlung Zangbo River incision. These surfaces act as passive markers of the vertical motion of the Earth’s surface. The timing of accelerated river incision rate can be used as a proxy of the timing of plateau uplift (Clark et al., 2005). As a mixture, modern detritus, contains geological information of different

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it was deposited during the late Oligocene to early Miocene (ca. 26–23 Ma) (DeCelles et al., 2011; Wang et al., 2013; Carrapa et al., 2014; Li et al., 2017a,b). In the Xigaze area, the Gangdese conglomerate has three distinct members. The lowermost unit is characterized by alluvial fan deposits that contain material sourced from the immediately underlying basement (Aitchison et al., 2011). The middle and topmost units consist of mingled gravel, which was deposited in braided-river environment and alluvial fan environment, respectively (Li et al., 2017a,b). Deposition of the Gangdese conglomerate has been attributed to an extension associated with Indian slab shearing (DeCelles et al., 2011; Carrapa et al., 2014) and the initiation of the Paleo-Yarlung River (Wang et al., 2013, Wang et al, 2015, Li et al., 2017a,b). The Xigaze forearc basin was formed by the continuous northward subduction, along the Gandese arc. The existing residual basin mainly crops out from Xigaze to Saga (Fig. 2). The basin is dominated by Cretaceous flysch sediments known as Angren formation, which recorded early exhumation of the Gangdese arc (Wu et al., 2010). All samples collected from the Southern Gangdese conglomerate belt were spread along the Yarlung Zangbo River and distributed in the range of E87° 26′–88° 32′; N29° 20′–29° 24′. In the Xigaze area, the bedrock samples Q003 and Q027, were sampled from the Qiuwu formation. In the Qiuwu area, bedrock samples are Q010 and Q009, both of which were sampled from the Qiuwu formation in the Gangdese conglomerate belt and the Angren formation in the Xigaze basin, respectively. A detrital sample, Q005, was derived from the Yarlung Zangbo catchment area of ∼315 km2 (Fig. 3).

locations form the source region (Reiners et al., 2006) and provides an opportunity to study the timing of regional denudation and inferred incision related to processes following the India-Asia collision. This detritus can be denudated from the bedrock in the catchment area (Saylor et al., 2013; Zhang et al., 2013). Therefore, research on the uplift process of Southern Tibet using low-temperature thermochronology data must consider the effect of the incision of the Yarlung Zangbo River on acquired age data. Only by recognizing the real geological meaning of this age data, can we understand the uplift process and mechanism of the Tibetan plateau. On the basis of the detritus apatite fission track (AFT) data of the Yarlung Zangbo River and the topography elevation data acquired by a catchment area’s digital elevation mode (DEM), we established the ageelevation relationship to reflect the incision history of the Yarlung Zangbo River. Then by comparing the incision history of the Yarlung Zangbo River and the denudation history of bedrock AFT data and an analysis of published low-temperature data of the Gangdese conglomerate belt, this paper studied the uplift and denudation process of the entire Gangdese conglomerate belt.

2. Geological setting and sample sites The research area is situated at the middle of Southern Tibet, Xigaze to Qiuwu, which features the juxtaposition of three tectonic units, from north to south: the Gangdese arc, the Gangdese conglomerate belt, and the Xigaze forearc basin (Fig. 1). The Gangdese arc is located in the Southern Lhasa terrane and was formed during the Mesozoic to Cenozoic as a result of the continuous northward subduction of the Neo-Tethyan Ocean and the early stage of the India-Asia collision (Mo et al., 2007; Chen et al., 2015; Wang et al., 2015). The Gangdese Arc consists primarily of late Triassic-Miocene (ca. 205–10 Ma) intrusions (Ji et al., 2009; Chen et al., 2015), early to middle Jurassic (190–174 Ma) volcanics of the Yeba formation (Ding et al., 2014), Cretaceous (136.5–95.4 Ma) volcanics of the Sangri Group (Zhu et al., 2009), and late Cretaceous (68–43 Ma) Linzizong volcanic successions (He et al., 2007). The Gangdese conglomerate belt, which is exposed along the southern margin of the Gangdese arc, extends from the Kailas area in the west to Nang County in the east. Various formation names have been assigned to these rocks, including the Kailas formation (Aitchison et al., 2002) and the Qiuwu formation (Liu et al., 1988). Published Zicon U-Pb dating results from the Gangdese conglomerate indicate that

3. AFT methods and results We used the confirmed apatite gains for FT data by the external detectors method and the Zeta (ξ) calibration method (Hurford and Green, 1982). We separated the apatites by standard magnetic and heavy liquid techniques. The etched condition of the apatites was 7% nitric acid (HNO3) for 30 s at 25 °C. The external detectors were low-u mica, and the etched condition was 40% HF for 40 min at 25 °C. FT densities were manually measured at 1000× magnification under the AUTOSCAN system at the Institute of High Energy Physics of the Chinese Academy. We measured the horizontal confined FT lengths (Gleadow et al., 1986) in each sample. In bedrock samples, we used the χ2-test to evaluate the congruence degree between single apatite age and all apatite ages in a sample

Fig. 1. General geologic map of Southern Tibet. Modified from Yin (2006) and Lee et al. (2011). 160

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Fig. 2. Geologic map of the Xigaze area, showing locations of the sampling site.

4. Incision historical model of the Yarlung Zangbo River

(Galbraith and Laslett, 1993; Sobel et al., 2005). In the results of bedrock samples, we divided samples into two groups. The first group included Q009, Q010, and Q027 and passed the χ2-test (P(χ2) > 5%); thus, we used the pooled ages—that is, 12.3 ± 1.4 Ma (Q009), 16.5 ± 2.3 Ma (Q010), and 10.9 ± 2 Ma (Q027). The second group included the sample Q003 and failed the χ2-test (P(χ2) < 5%); thus, we used the central age—that is, 12 ± 1 Ma. All samples’ track lengths were less than the initial track length (16.3 μm). This result indicated that all samples were part annealing and that the AFT ages were less than the strata age. The experimental results are shown in Table 1 and Fig. 7. Unlike bedrock samples, a detrital sample is a mixture that contains some information about the bedrock denudation in the Yarlung Zangbo River catchment area; thus, we cannot calculate the single-grain obtained from the experiment for the χ2-test. In a detrital sample, the central age and pooled age used in the bedrock sample usually do not have any geological significance. The intuitionistic result is an age probability curve that consists of single-grain ages. The peak of the curve represented the rapid cooling period of the geological body (Enkelmann et al., 2011), and the range of peak age usually corresponded to the denudation period that the bedrocks experienced as a result of river incision (Brewer et al., 2003). The character of the catchment area topography is the result of river incision, which means that the feature of the age probability curve generally is confined to the catchment area topography. Thus, the distribution of the age probability curve could reflect the incision history of the catchment area. The observed results are shown in Table 2 and Fig. 3.

4.1. Method of incision history simulation As mentioned, the detrital age probability curve is controlled by the area topography, which could be reflected by the river detritus contribution that eroded from bedrocks in the catchment area. These characteristics reflect the incision historical of a catchment area. On the basis of the consistency of lithology, two factors influence the river detritus contribution in the catchment area. The first factor is the size of the exposed surface area—that is, the larger the exposed surface area of a certain elevation, the larger the corresponding detritus contribution (Brewer et al., 2003). The second factor is the slope angle. The yield of sediments in a basin depends mainly on the incision rate of slope surface, and also the average incision rate of the slope surface is greater than that of the river valley (Bennett et al., 2013). Therefore, in the drainage system, apatites of river sand are more inclined to come from the slope area. That is, the steeper the slope angle, the larger the corresponding detritus contribution (Palumbo et al., 2011). We can calculate the river detritus contribution according to the formula proposed by Sun et al. (2015). We then can build the probability density curve of the detritus contribution, as follows: n

Q h = kt ∑

i=1

⎡ sinai ⎢Ai ⎢ 1− tanai Sc ⎣

( )

2

⎤ ⎥, ⎥ ⎦

(1)

where Qh (km3) is the river detritus contribution; k is a constant; t is time when a single pixel has been denuded to the present; n is the number of the pixel in a certain elevation range; Ai is the catchment area represented by a single pixel (km2); Ai = xy / cosai ; ai is a slope

Fig. 3. Observed detrital sample age probability density plot (black-dotted line). 161

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Table 1 Observed apatite fission track results of bedrock samples. Sample Elevation (m) Grains (n) ρs (105/cm2) (Ns)

ρi (105/cm2) (Ni)

ρd (105/cm2) (N)

P(χ2) (%)

Central age (Ma ± 1σ)

Pooled Age (Ma ± 1σ)

L (μm) (N)

Mean Dpar (Range) (μm)

Q009 Q010 Q003 Q027

12.031 (1839) 8.103 (1225) 33.19 (5132) 35.656 (4178)

11.52 (6587) 12.796 (6587) 11.946 (6587) 12.371 (6587)

52.5 52.4 2.5 36.9

12 ± 1 16.5 ± 2.3 12.2 ± 0.8 11.6 ± 1.1

12.3 16.5 10.8 10.9

13.1 12.8 13.3 14.2

2.70 2.37 2.70 2.53

4156 4418 4183 4013

39 42 35 35

0.628 0.509 1.468 1.536

(96) (77) (227) (180)

± ± ± ±

1.4 2.3 0.6 2.0

± ± ± ±

2.1 (15) 2.3 (25) 2.3 (87) 2.1(102)

(1.98–3.96) (1.49–3.47) (1.49–3.49) (1.48–3.71)

0.12 mm/yr and then the tectonic environment remained stable to 17 Ma. At 17–14 Ma, the incision rate sharply increased at 0.24 mm/yr. The incision rate was maintained at about 0.06 mm/yr in the range of 14–12 Ma and 8–2 Ma, but it increased to 0.12 mm/yr in the range of 12–8 Ma.

angle of the pixel (°); and Sc is a critical value determined by the friction coefficient. In a catchment area, if the river incision rate is consistent, a linear relationship will exist between the detrital age and elevation, and the slope is the river incision rate (Braun et al., 2006). Then, by using the linear relationship, we could randomly set a variety of possible ageelevation curves. The probability density curve of the detritus contribution can be transferred to the probability density curve of detrital age. Last, using the χ2-test of the observed and calculated detrital age probability density curve, we selected the most probable age-elevation relationship, which represents the actual incision history in the catchment area.

5. Denudation historical model of bedrock AFT data 5.1. Method of denudation history simulation AFT is a continuous process, and each track in minerals records information about a specific stage of the entire thermal history of the sample below the closure temperature. To obtain further information about the denudation history of our bedrock samples in the Gangdese conglomerate belt, we completed a denudation historical model using HeFTy software following the annealing model of Ketcham et al., (2007). We used Monte Carlo simulation in our modeling operation and took the Dpar values into consideration. We excluded the sample Q003 in our denudation modeling because it failed the χ2-test (P(χ2) < 5%) and might give an ambiguous result. On the basis of our AFT analysis and relevant studies, we defined four groups of time-temperature constraints. (1) Both Q010 and Q027 were collected from the Qiuwu formation, which had a depositional age of 26–23 Ma (DeCelles et al., 2011; Wang et al., 2013; Carrapa et al., 2014; Li et al., 2017a,b). Accordingly, the initial constraints should have a temperature range of 0–20 °C when deposited at 26–23 Ma. In addition, Q009 was collected from the Angren formation, which was deposited in the late Cretaceous (Wu et al. 2010; An et al., 2014). We set a constraint as a range of ages higher than AFT ages and set corresponding temperature higher than 120 °C. (2) All sample’ AFT ages were younger than their depositional ages, suggesting that they completed annealing after deposition. Accordingly, we set a second constraint as a range of ages older than AFT ages and set the corresponding temperature higher than 120 °C. (3) The third constraint is set by the AFT age of each bedrock sample and the corresponding partial annealing zone (60–120 °C). (4) The final constraint is based on the present-day mean surface temperature at the elevation at which we collected the samples (typically 0–20 °C). Each bedrock sample acquired the best fit, and the modeling graph is shown in Fig. 7.

4.2. Results of incision history simulation In this paper, we used the DEM data (STRM 30 m) of the Qiuwu section in the Yarlung Zangbo River catchment area (Fig. 4). In the formula, Ai sinai can be converted to xytanai and the value of xy representing a pixel area is fixed. Therefore, we calculated the elevation of each pixel and the slope angle tangent of the pixel position in the catchment area using the ArcGIS software (Fig. 4). We calculated the vertical projection area of the catchment area in different elevations and the entire catchment area using MatLab software with the formula proposed by Sun et al. (2015). We divided the value of the former by the value of the latter to arrive at the detritus contribution ratio of this elevation. We also eliminated the constant k and t. The result is shown in Fig. 5. The elevation range of the Qiuwu section in the Yarlung Zangbo River drainage is 3998–5904 m, and the maximum range of the detritus contribution is 4430–4710 m. We established the age-elevation relationship and found that denudation age generally increases with the elevation. To ensure the consistency of the age range between the theoretical and the observed, according to the range of observed detrital age and the range of elevation with the drainage, we set the maximum elevation in the elevation-age relationship map to be 5904 m, which corresponds to the maximum detrital single-grain age of 22.4 Ma and the minimum elevation of 3998 m, which corresponds to the minimum detrital singlegain age of 2.96 Ma as the starting point and end point, respectively. After determining these two points, we produced 96 × 98 control points in elevation-age relationship by using 20 m and 0.2 Ma as the interval of elevation and time, respectively. Then we can get the 96 × 98 curves on the age-elevation relationship. We conducted the χ2test on each cure. The best-fit age-elevation curve is shown in Fig. 6. The incision history of the Qiuwu section in the Yarlung Zangbo River has periodic characteristics. At 22–20 Ma, the incision rate was

5.2. Results of denudation history simulation The denudation model’s history revealed a cooling record from ∼120 °C to 60 °C. The modeling graphs are shown in Fig. 7. The Q027 was collected from the Qiuwu formation in the north of Xigaze. The

Table 2 Observed apatite fission track result of the detrital sample. Sample

Elevation (m)

Grains (n)

Age range/Ma

ρs (105/cm2) (Ns)

Ρi (105/cm2) (Ni)

ρd (105/cm2) (N)

P(χ2) (%)

L (μm) (N)

Q005

4047

43

2.9–22.4

0.624 (213)

17.524 (5986)

14.498 (6587)

46.2

13.3 ± 1.9 (100)

AFT Age Data: Grains – number of AFT analyzed grains; ρs – spontaneous fission track density; Ns – total number of fission tracks counted in ρs; ρi–induced fission track density; Ni – total number of fission tracks counted in ρi; L – mean track length. Dpar –arithmetic mean diameter of fission-track etch figures parallel to the crystallographi c-axis. 162

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Fig. 4. Digital topographic maps of the Yarlung Zangbo River in Qiuwu section.

that the cooling rates had periodic changes. The first rapid denudation period occurred during the time from ca. 17 Ma to 15 Ma, with a cooling rate of 17 °C/ Ma. After this rapid denudation period, a slow denudation period lasted until ca. 8 Ma, with a cooling rate of 2.4–3.5 °C/Ma. The second rapid denudation stage occurred in the period of 8–6 Ma, which was recorded in samples taken from the Qiuwu formation. Assuming the paleo-geothermal gradient is 30 °C/km (Clark et al., 2003; Li et al., 2015) in the research area, we transferred the cooling rate to the denudation rate and the results showed the following: (1) from 17 Ma to 15 Ma, the denudation rate was 0.56 mm/yr; (2) at the period of 12–8 Ma, the denudation rate was 0.09–0.11 mm/ yr; (3) the denudation rate was 0.43–0.67 mm/yr, which is recorded in the sample collected from the Qiuwu formation from 8 Ma to 6 Ma; and (4) the bedrock denudation rate has maintained a low level since 6 Ma. Fig. 5. Relationship between elevation and river detritus relative probability.

6. The Multi-Age data in the Gangdese conglomerate belt and the tectonic significance

AFT age was 10.9 ± 1.0 Ma. The modeling graphs show that the cooling rate was slow (ca. 3 °C/Ma) during the period in the middle-late Miocene (ca. 12–8 Ma). Then the cooling rate increased sharply, up to 20 °C/Ma. Two other samples were collected from the Qiuwu area, which was located in the west of Xigaze. The Q010 sampled in the Qiuwu formation yielded the oldest AFT age (16.5 ± 2.3 Ma). The modeling graph shows a rapid denudation stage during the early Miocene (ca. 21–16 Ma), and the cooling rate was ca. 17 °C/Ma. Then the cooling history was similar to the cooling history of Q027, for which a stable cooling rate lasted until the late Miocene (ca. 8 Ma) and was followed by a rapid denudation stage (ca. 13 °C/Ma) in the late Miocene. The Q009 sampled in the Angren formation was 12.3 ± 1.4 Ma. The simulated denudation history is slightly different than the others. The cooling rate was low (ca. 2.4 °C/Ma) in the late Miocene. Then a relatively rapid cooling rate (10 °C/Ma) continued to the present day. The results of the denudation historical model generally showed

By comparing historical models of bedrock denudation and detrital incision and combining the information with recently published lowtemperature data (Copeland et al., 1995; Carrapa et al., 2014; Carrapa et al., 2017; Dai et al., 2013; Li et al., 2016; Li et al., 2017a,b), magmatic-tectonic activity, and regional geological events (Harrison et al., 2000; Clift et al., 2008; Murphy and Harrison, 1999; DeCelles et al., 2011), we were able to analyze the intensity of each geological event, understand the denudation history of the Gangdese conglomerate belt, and examine the uplift process of Southern Tibet. As shown in Fig. 8, the low-thermochronology data in the margin and inside the Gangdese conglomerate belt showed that signals of FT age are gathered in two periods: the early Miocene (22–16 Ma) and the middle-late Miocene (11–9 Ma).

Fig. 6. (A) The best-fit curve on age-elevation relationship; (B) Comparison of simulated and observed detrital ages. The purple line is best-fit simulated data and the dotted line is observed data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 163

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Fig. 7. (A) Single grain apatite fission track age distribution of the sample presented in radial plots. (B) Results in the temperature–time diagram are indicated by two colors indicating matching between data and model: purple envelopes indicate a good match (fit N 0.5 estimated using Kolmogorov-Smirnov test and Kuiper's Statistic) and green envelopes indicate acceptable fit (between 0.05 and 0.5). (C) The histograms of the apatite fission track length distributions.

sharp uplift. If this viewpoint is exactly correct, the northern Gangdese arc should record the same early Miocene signature. Overall, however, low-temperature thermochronology ages from the northern Gangdese arc are much older than ages observed in this study (Dai et al., 2013; Li et al., 2015). Li et al. (2015) thought that the Gangdese conglomerate was a relatively “soft” unit and that it was more sensitive to the effect of an India-Asia convergence (e.g., thrusting or folding) compared with the more distal and relative “rigid” Gangdese arc farther north. These findings indicated that a stronger denudation and a higher magnitude of incision existed in the research area compared with the northern Gangdese arc, which might account for the incision in the Yarlung Zangbo River (0.24 mm/yr) in the early Miocene. Thus, the denudation in Southern Tibet and along the Gangdese conglomerate belt likely were the combined result of Indian plate rollback and incision along the Yarlung Zangbo River in the early Miocene.

6.1. Early Miocene In the early Miocene (22–16 Ma), the earliest age is 16.5 ± 2.3 Ma, which was recorded from the Q009 sample, and this age signal in the early Miocene can be found in the entire Gangdese conglomerate belt from east to west (Copeland et al., 1995; Dai et al., 2013; Li et al., 2016). The bedrock thermochronology historical model shows that the denudation rate was 0.56 mm/yr in the early Miocene and the incision historical model shows that the incision rate was 0.24 mm/yr at the same time. As noted earlier, the timing of the accelerated incision rate can be used as a proxy for the timing of the plateau uplift (Clark et al., 2005). The drastic uplift event can be recorded by the incision history of the Yarlung Zangbo River in Southern Tibet. Thus, by combining the widely distributed samples and the history modeling results of bedrock denudation and incision, these results indicate that the early Miocene denudation is regional and that it affected an area ∼1500 km along the strike length of the Gangdese conglomerate belt and the neighboring regions. The early Miocene denudation event usually has been explained by the viewpoint (DeCelles et al., 2011) that northward underthrusting of the Indian plate was followed by a rollback and breaking off of a portion of the Indian slab, which caused the regional extension and led to

6.2. Middle-late Miocene In the middle-late Miocene (12–8 Ma), the younger ages of bedrock samples were developed in the eastern Gangdese conglomerate belt based on the Qiuwu to the Qushui formation. In addition, Dai et al. (2013) and Li et al. (2016) found that samples with younger ages were 164

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Fig. 8. (A) The AFT data of the Gangdese Conglomerate belt. (B) Major geological events (1,6-after Harrison et al., 2000 2-after Murphy and Harrison, 1999; 3-after Clift et al., 2008; 4,5-after DeCelles et al., 2011;). (C) A comparison of the historical model of bedrock denudation and detrital incision.

6 Ma. The incision rate remained stable between ca. 8 Ma and 6 Ma; however, the denudation rate increased sharply at the same time, which is recorded in the bedrock sampled only from the Qiuwu formation. The stable and slow incision rate represented the establishment of a regional relatively low-relief landscape at high elevation (Rohrmann et al. 2012; Li et al., 2015). The paleo-elevation research conducted in basins located in Southern Tibet also showed that the paleo-elevation of these basins in the late Miocene approached, and even exceeded, the present-day elevation (Table 3). Thus, the drastic uplift had stopped in Southern Tibet, has been proven by the stable incision rate of the Yarlung Zangbo River since the late Miocene. Thus, the drastic denudation reflected by bedrock samples may be induced by the local tectonic activity. The time of this denudation event is consistent with the regional east-west extension, which occurred in Southern Tibet in the late Miocene from 8 Ma to 6 Ma. For example, Ya Dong–Yangbajing rift activity mainly increased ca. 8 Ma (Edwards and Harrison, 1997; Carrapa et al., 2017).

developed in the Gangdese arc and Liuqu conglomerate located in the north and south of the Gangdese conglomerate belt, respectively. These sample distributions have remarkable characteristics, which mainly are distributed in the river valley. The incision historical model of the Yarlung Zangbo River shows that the incision rate increased to 0.12 mm/yr at 12–8 Ma. Additionally, the denudation historical model of the bedrock samples showed that the uplift rate was 0.09–0.11 mm/ yr at 11 Ma, which was roughly consistent with the incision rate during the same period. DeCelles et al. (2011) thought the climate of Southern Tibet had been influenced by the Asian monsoon from research about oxygen isotopes in carbonates since the late Oligocene. Stable isotopes studied from the sediment in the Siwalik basin indicated that a dramatic environmental shift from C3 to C4 plants began as early as 11–7 Ma, which was interpreted to represent the intensification of the Asian monsoon at that time (Quade et al., 1989; Sanyal et al., 2010). These studies suggested that the climate underwent significant changes associated with the Asian monsoon in the middle-late Miocene. Such drastic intensification of the Asian monsoon would enhance the efficiency of fluvial incision and would induce more precipitation onto Southern Tibet, which influenced a higher discharge and stronger incision, as the dominate driver of the middle-late Miocene denudation.

7. Conclusions From historical models between river incision and bedrock denudation, we arrived at the following conclusions: (1) The first rapid denudation stage occurred in the early Miocene (22–16 Ma). This event was probably the combined result of the Indian plate rollback and the Yarlung Zangbo River incision along the Gangdese conglomerate belt. As a result, the AFT ages of the

6.3. Late Miocene The historical model between river incision and bedrock denudation show a significant difference in the denudation rate from ca. 8 Ma to Table 3 Paleo-elevation data of Southern Tibet. Basins

Tectonic location

Age

Research method

Sample

Result

Reference

Linzhou Basin Zhada Basin

On the north of Gangdese Arc In the north of India Block

Nanmulin Basin

On the north of Gangdese Arc

60–48 Ma 9.2 Ma 9–4 Ma 15 Ma 15 Ma

Thermodynamic model Thermodynamic model Parameter △47 Thermodynamic model Foliar physiognomy

Paleosol, lacustrine carbonates, marl Gastropod shells Lacustrine carbonates Soil carbonate Leaf fossil

4500 ± 400 ＞6000 m (5400 ± 500) m (5200 ± 1370) m (4689 ± 895) m

Ding et al., 2014 Saylor et al., 2009 Huntington et al., 2014 Currie et al., 2005 Spicer et al., 2003

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early Miocene were recorded in only the Gangdese conglomerate belt and neighboring regions. (2) In the middle-late Miocene, samples that recorded the signature of this period were found only in the river valley. The consistency between the denudation rate and incision rate reflected the fact that the bedrock denudation was caused predominantly by river incision, which was a response to the intensification of the Asian monsoon. This finding indicated that there was no drastic uplift in the research area in the middle-late Miocene. (3) After 8 Ma, the stable and slow incision rates indicated that regional drastic uplift already had ceased. The paleo-elevation of the research area in the late Miocene approached, and even exceeded, the present-day elevation in the late Miocene. A rapid cooling rate reflected by the denudation model indicated a response to the tectonic denudation associated with regional east-west extension.

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