# Rapid incision of the Mekong River in the middle Miocene linked to monsoonal precipitation

Junsheng Nie, Gregory Ruetenik, Kerry Gallagher, Gregory Hoke, Carmala Garzione, Weitao Wang, Daniel Stockli, Xiaofei Hu, Zhao Wang, Ying Wang, et al.

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Junsheng Nie, Gregory Ruetenik, Kerry Gallagher, Gregory Hoke, Carmala Garzione, et al. Rapid incision of the Mekong River in the middle Miocene linked to monsoonal precipitation. Nature Geo-science, 2018, 11 (12), pp.944-948. 10.1038/s41561-018-0244-z. insu-01896683

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# Rapid incision of the Mekong River in the middle Miocene linked to monsoonal precipitation

Junsheng Nie<fnref n="1" />*, Gregory Ruetenik², Kerry Gallagher³, Gregory Hoke D², Carmala N. Garzione⁴, Weitao Wang D⁵, Daniel Stockli⁶, Xiaofei Hu<fnref n="1" />, Zhao Wang<fnref n="1" />, Ying Wang⁵, Thomas Stevens D⁷, Martin Danišík⁸ and Shanpin Liu<fnref n="1" />

The uplift of orogenic plateaus has been assumed to be coincident with the fluvial incision of the gorges that commonly cut plateau margins. The Mekong River, which drains the eastern Qiangtang Terrane and southeastern Tibetan Plateau, is one of the ten largest rivers in the world by water and sediment discharge. When the Mekong River was established remains highly debated—with estimates that range from more than 55 to less than 5 million years ago—despite being a key constraint on the elevation history of the Tibetan Plateau. Here we report low-temperature thermochronology data from river bedrock samples that reveal a phase of rapid downward incision (>700 m) of the Mekong River during the middle Miocene about 17 million years ago, long after the uplift of the central and southeastern Tibetan Plateau. However, this coincides with a period of enhanced East Asian summer monsoon precipitation over the region compared with the early Miocene. Using stream profile modelling, we demonstrate that such an increase in precipitation could have produced the observed incision in the Mekong River. In the absence of an obvious tectonic contribution, we suggest that the rapid incision of the Tibetan Plateau and the establishment of the Mekong River in the middle Miocene may be attributed to increased erosion during a period of high monsoon precipitation.

Steep gorges incised by rivers and related fluvial terraces are common geomorphological features on the margins of orogenic plateaus (Fig. 1). Such features are often used to infer the timing and amplitude of surface uplift<fnref n="1" />,². However, climatic changes, such as an intensified monsoon precipitation and increased amplitudes of glacial-interglacial climatic fluctuations, could also potentially enhance fluvial incision³⁻⁶. The potential for feedbacks⁷ means that to deconvolve the relative roles of tectonics, climate or autogenic processes in fluvial incision is both a fundamental question and a huge challenge⁸⁻¹¹ for the earth sciences. The ability to discriminate between these roles has major ramifications for understanding large-scale landscape evolution, the timing and mechanisms of plateau formation and the collateral effects on monsoon initiation and, more generally, global climate change³,¹¹⁻¹³.

The middle Miocene climate optimum (MMCO, ~17–14 million years ago (Ma)) is characterized by the warmest climate and highest atmospheric CO₂ concentrations¹⁴,¹⁵ over the past 20 Myr (Fig. 2a,b). The MMCO corresponds to a phase of enhanced East Asian summer monsoon precipitation with large amplitude variability, as recorded in the magnetic susceptibility of loess deposits¹⁶,¹⁷ and potentially in offshore Southeastern Asian sediment accumulation rates<fnref n="1" /> (Fig. 2c,f). However, geological evidence suggests that Qiangtang–southeastern Tibet experienced protracted deformation and surface uplift, including crustal thickening during the Cretaceous–Eocene¹⁸⁻²⁴, southeastward extrusion during the Oligocene–Miocene²⁵⁻²⁷ and potentially lower crustal flow since the late Miocene². Currently, no tectonic model suggests a kilometre-scale surface uplift for Qiangtang–southeastern Tibet

during the MMCO¹⁸,¹⁹,²⁶. Thus, the Qiangtang Terrane and southeastern Tibetan Plateau provide an ideal opportunity to test whether changes in the precipitation amount during the MMCO could generate kilometre-scale fluvial incision.

The Mekong River drains the eastern Qiangtang and southeastern Tibetan Plateau (Fig. 1) and is characterized by a kilometre-scale fluvially incised gorge. With estimates that range from the pre-Eocene to the Pleistocene (see below), there is no agreement on when the deeply incised Mekong River valley formed.

Significant sediment accumulation in offshore basins adjacent to the modern Mekong outlet since at least the late Eocene²⁸ has led some to propose that the Mekong River formed before the India–Asia collision (before 55 Ma)²⁹. However, the dominantly lacustrine facies in the Palaeogene basins of east–central Tibet²¹ indicates that the upper reaches of the Mekong River were not integrated until the Neogene, after the cessation of lacustrine deposition. Others argue that the Mekong River formed during the early Miocene because the river valleys appear to be affected by shear zones active at this time²⁵. Yet another hypothesis suggests that the non-dendritic southeastern Asian rivers were tributaries of a palaeo-Red River that was progressively dismembered during the late Miocene to Pliocene through river capture³⁰.

We report 20 apatite (U-Th)/He (AHe) age data from six granite samples along an altitude transect that spans 1,270 m of relief in the Triassic–Early Jurassic Linchang granite belt bounded and cut by the lower Mekong River³¹ (Figs. 1 and 3, Supplementary Figs. 1–3 and Supplementary Tables 1 and 2). We also compiled the available AHe age data from the upper and middle Mekong River (Fig. 3 and

<footnote n="1">¹Key Laboratory of Western China's Environment System (Ministry of Education), College of Earth and Environmental Sciences, Lanzhou University, Lanzhou, Gansu, China. ²Department of Earth Sciences, Syracuse University, Syracuse, NY, USA. ³Géosciences, Université de Rennes 1, Rennes, France. ⁴Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY, USA. ⁵State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, China. ⁶Department of Geological sciences, University of Texas, Austin, TA, USA. ⁷Department of Earth Sciences, Uppsala University, Uppsala, Sweden. ⁸John de Laeter Centre, The Institute for Geoscience Research, School of Earth and Planetary Sciences, Curtin University, Perth, Western Australia, Australia. *e-mail: jnie@lzu.edu.cn</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1** | Topographic map of the study area and major active faults and rivers. The main map corresponds to the black rectangle in the index map. Sample locations mentioned in the text are marked in the plot. The faults are synthesized from the literature<sup>27,40</sup>, EHS, East Himalayan syntaxis. White dashed lines are terrain boundaries.</figcaption></figure>

Supplementary Figs. 4–7). The results, coupled with stream profile modelling (Fig. 4 and Supplementary Figs. 8 and 9), indicate that synchronous river incision over a large portion of the modern Mekong River valley was coincident with an intensified East Asian summer monsoon in the middle Miocene<sup>4,16</sup>.

## Middle Miocene incision of the upper and lower Mekong

The two highest Linchang granite samples yield AHe ages of 27.4 and 38.5 Ma, whereas the lower four samples have younger AHe ages that range from 19.6 to 15.5 Ma with an average of ~18 Ma. This pattern of dispersed ages at a higher elevation and a tighter age clustering at a low elevation typically indicates the onset of increased rock cooling rates that correspond to the age of the lower samples<sup>32</sup>. Given that the samples within the middle Miocene age range span ~700 m in elevation, at least that amount of incision occurred rapidly, consistent with the river profile modelling (Fig. 4) and AHe data thermal history modelling (Fig. 3). Our inverse thermal history modelling (Fig. 3 insets and Supplementary Figs. 1–3) reveals a ~15°C Myr<sup>-1</sup> middle Miocene cooling for the altitude transect samples.

Two published AHe ages (15 and 16.3 Ma at 3,888 and 3,673 m elevation, respectively) of igneous rocks from the upper reaches of the Mekong River on the central Tibetan Plateau<sup>33</sup> (Fig. 1) reflect a similar timing of the faster cooling to that in the lower reaches reported here. A headwater sample of the Salween River<sup>33</sup> of similar altitude also yields an AHe age of ~15 Ma (Supplementary Fig. 4). By contrast, other AHe ages on the central Tibetan Plateau, further away from the main river<sup>20,33</sup>, range from 75 to 36 Ma (Fig. 3). We also analysed ten apatite grains from a bedrock sample of the upper Mekong River near the river bed in the area where the 15 Ma and the 16.3 Ma AHe ages were reported (Fig. 3 (green stars) and Supplementary Table 3). With one exception (31.7 Ma), the AHe

<figure><img src="image_4.png" /><figcaption>**Fig. 2** | A comparison of Asian summer monsoon, atmospheric CO₂, Mekong River AHe age distribution and benthic oxygen isotope records between 20 and 5 Ma. **a**, Benthic oxygen isotope record<sup>14</sup>. **b**, Atmospheric CO₂ record<sup>15</sup>. **c**, Magnetic-parameter-based weathering record from the Qaidam Basin<sup>39</sup>. **d**, Normalized probability distribution of AHe ages from the Mekong River drainage area<sup>33–35</sup>, this study. **e**, Loess magnetic susceptibility (MS) record from the Chinese Loess Plateau (CLP)<sup>16</sup>. **f**, Southeastern Asian offshore sedimentary accumulation rate (SR) record<sup>4</sup>. χ²/d/HIRM, frequency-dependent magnetic susceptibility/hard isothermal remanent magnetization. Purple curves represent East Asian summer monsoon proxy records.</figcaption></figure>

ages have a range of 14.7–20.9 Ma with an average of 17.1 Ma (Supplementary Table 3). Combined, the data from the lower and upper Mekong, and the inverse thermal history modelling, indicate a synchronous incision during the middle Miocene (Fig. 3 insets and Supplementary Figs. 2, 3, 6 and 7).

## Middle Miocene incision of the middle Mekong

An AHe age elevation profile for the middle reach of the Mekong River is also available<sup>34</sup>. In contrast to the upper and lower reaches of the Mekong River, AHe ages for samples in the lower half of the profile<sup>34</sup> are significantly younger (Fig. 3), and range from ~6 to 4 Ma, similar to the AHe ages from another study in the region<sup>35</sup>. The middle and upper profile samples of the middle reaches<sup>34</sup> yield AHe ages of ~10-9 and ~19–16 Ma, respectively. We disregard one old outlier AHe age (81.8 Ma) from this region<sup>35</sup> (Fig. 3).

The young ages at the lower and middle profile in the middle reach of the Mekong River reflect locally higher rates of exhumation that are associated with a large knickzone, as well as steep hill and channel slopes within the middle reaches (Fig. 3). The channel slope and river discharge are the primary controls on the total stream power and a strong correlation exists between the mean catchment hill-slope angle and the erosion rate until a critical threshold value is achieved<sup>36,37</sup>. Thus, the timing of the initial incision recorded at or near the river level in the thermochronometric data from the upper and lower reaches is not recorded over about a 500 km segment of the middle reach where latest Miocene-to-Quaternary rock uplift has occurred (Fig. 3).

However, the uppermost three samples in the middle Mekong age profile, which span from 4,529 to 4,974 m, do show middle Miocene ages (Fig. 3) and indicate that the middle reach Mekong River also experienced a phase of increased incision rate, prior to the renewed downcutting since 10 Ma. These middle Miocene age samples span 445 m and provide a minimum constraint on the total amount of faster middle Miocene incision of the middle reach Mekong River as some portion of the rocks with middle Miocene ages might have been eroded away. This >445 m relief between the middle Miocene
<figure><img src="image_5.png" /></figure>

**Fig. 3** | Vertical profiles of Mekong River AHe ages and thermal history modelling reveal increased incision rates during the middle Miocene. Mekong River profile, thick black line; maximum topography, thin grey dashed line; local hill-slope, crosses; AHe ages: squares, Liu-Zeng et al.<sup>34</sup>; triangles, this study; hexagons, Yang et al.<sup>35</sup>; stars, Dai et al.<sup>33</sup>. AHe ages during, older than and younger than the middle Miocene are in green, dark blue and light blue, respectively. Vertical dashed lines divide the Mekong River into the upper, middle and lower reaches. Insets: expected thermal histories of the lower (bottom right) and upper (top left) Mekong reaches inferred from the elevation transect data using a radiation damage model (Methods and Supplementary Information).

<figure><img src="image_6.png" /></figure>

**Fig. 4** | Modelled stream profile and predicted river profile variations based on an upwind finite-difference model. During the modelled MMCO, kilometre-scale erosion occurred over a period of 3 Myr, and a slow erosion occurred in the remaining 14 Myr. Supplementary Table 4 gives the parameter values.

age profile samples in the middle Mekong is similar to the amplitude of our stream profile modelling and that preserved for the upper and the lower Mekong reaches (Fig. 3). Together, these data provide a record of enhanced (~15°C Myr⁻¹) exhumation driven by middle Miocene incision along the entire Mekong.

A mechanism is required, however, to balance the faster and more recent erosion and maintain the high elevation observed in the middle reaches³⁸. We suggest that lower crustal flow or isostatic

adjustment associated with high erosion rates might result in a focused rock uplift³⁰ and that the active rock uplift since 10 Ma was probably driven by another phase of monsoon precipitation intensification, as is recorded in the monsoon proxy records¹⁶,³⁹ (Fig. 2c,e,f and Supplementary Information). Two active reverse faults⁴⁰ (the Chenghai and Jinshajiang faults) are present near the elevation profile in the middle reach (Fig. 1) and we propose that these faults facilitated rock uplift post 10 Ma.

## Forcing for the middle Miocene incision of the Mekong River

Tectonic uplift, monsoon intensification or a combination of both could potentially have caused a faster incision in the Mekong River in middle Miocene time. However, a review of the tectonic and surface uplift history of the study region suggests that this is a less likely cause for this faster phase of fluvial incision. The headwaters of the Mekong River are near the Nangqian-Yushu region in the eastern Qiangtang Terrane of central Tibet, for which detailed sedimentological, stratigraphical and structural research indicate that the basins in this region are contemporaneous with the India-Asia collision and sedimentation ceased by the Late Eocene²¹. Fine-grained lacustrine facies dominate and provenance data indicate that this part of the Tibetan Plateau had developed a low relief similar to present-day conditions during the Eocene, which makes the existence of a large river, such as the Mekong, less likely²¹. Available low-temperature thermochronology ages²⁰,³³ in this region (Supplementary Fig. 4) are Palaeogene or older with the exception of those near the Mekong and Salween river valleys. Further west of this region, in central Tibet, palaeoelevation estimates⁴¹⁻⁴⁵ reveal that near-modern elevations were established by 40–26 Ma. This inference is consistent with the occurrence of significant crustal shortening related
to thrust faulting between the Cretaceous and Early Tertiary in the south part of the Qiangtang Terrane²², and intense Eocene-to-Oligocene deformation in the Hoh Xil basin along the northern part of the Qiangtang Terrane⁴⁶. Taken together, these observations show that the surface uplift of the Qiangtang Terrane occurred during the Eocene-Oligocene, much earlier than the observed faster incision of the Mekong River in the middle Miocene.

The middle and lower reaches of the Mekong River drain the part of the southeastern Tibetan Plateau and Southeast Asia bounded by the Ailao Shan-Red River fault to the east and the Sagaing Fault to the west²⁵,⁴⁷ (Fig. 1). Sandwiched between India and South China, continental deformation in this part of the Tibetan Plateau and the Southeast Asian continent was controlled by a northward movement of the East Himalayan syntaxaxis, which generates crustal shortening, strike-slip faulting and clockwise rotation²⁵,⁴⁷. It is clear from previous research that the Ailao Shan-Red River fault was left-lateral during the Oligocene-early Miocene and the current right-lateral feature did not form until the early Pliocene²⁶,²⁷. There is no evidence of an intensified tectonic activity during the middle Miocene. The right-lateral Sagaing fault and the Gaoligong fault are the conjugate shear systems to the Ailao Shan-Red River shear zone in some models¹⁷, ⁴⁰Ar/³⁹Ar and K/Ar dates of biotites and muscovite from the mylonitic gneisses within the Gaoligong shear zone²⁶,⁴⁷ reveal active faulting during 24-11 Ma. Palaeoelevation studies¹⁸,¹⁹ reveal near modern elevations since the Eocene-Oligocene in the middle reaches of the Mekong River region based on revised geochronology³⁸ (Fig. 1).

It is clear from the above review that there was no regional surface uplift nor tectonic deformation in the drainage area of the Mekong River during the MMCO. Without a tectonic cause for the observed increase in the middle Miocene incision rates and the formation of the Mekong River, and probably the Salween River, could strong monsoon precipitation during the middle Miocene be responsible for the enhanced river downcutting (Fig. 2)?

Several independent lines of evidence support a climatic forcing for the middle Miocene Mekong River incision. First, stream profile modelling of the river incision in response to an intensified MMCO monsoon precipitation under a constant rock uplift provides enough power to cause the observed simultaneous incision across the entire river profile (Fig. 4 and Supplementary Figs. 8 and 9b), consistent with our AHE data. In contrast to the observed simultaneous profile-wide cooling during the MMCO, modelled profiles with only an increased tectonic uplift exhibit limited incision during the MMCO and a diachronous pattern of AHE ages that decrease upstream following the migrating wave of erosion (Supplementary Fig. 9a). We note that the widespread river incision is in response to surface tilting, but not necessarily with knickpoint retreat, as has been predicted in a passive margin setting⁴⁸,⁴⁹. However, on the western flank of the Andes, where a gradient in surface uplift exists, all the rivers contain knickpoints⁵⁰. Therefore, whether this inference from a passive margin setting is directly applicable to a compressional setting, such as in our study area, is an open question. Second, a comparison of AHE age distributions in the Mekong River with East Asian monsoon records reveals age modes that align well with periods of strong monsoon intensity during the MMCO over the length of the river and then post 10 Ma within the middle reach (Fig. 2). Third, no major thrust fault is observed in the upper and lower reaches of the Mekong River study sites (Fig. 1). Although we cannot conclusively rule out all alternative explanations, our results are most congruent with an enhanced monsoonal precipitation during the MMCO as the most likely cause of the uniform middle Miocene river incision of the Mekong, as well as of other rivers that record a similar signal.

In summary, the sedimentological, thermochronological and tectonic evidence implies that the Mekong River in its modern extent probably did not exist prior to 17 Ma, despite a high elevation on the central and southeastern Tibetan Plateau (Supplementary Fig. 10a). Drainage integration combined with a synchronous incision of the

Mekong River across its present course most probably occurred during the period 17–14 Ma in response to intensified monsoon precipitation (Supplementary Fig. 10b). With the exception of the middle reach of the Mekong River, where faults locally control rock uplift (Supplementary Fig. 10c), the river profile did not change significantly despite further monsoon intensification post 10 Ma.

## Online content

Any methods, additional references, Nature Research reporting summaries, source data, statements of data availability and associated accession codes are available at https://doi.org/10.1038/s41561-018-0244-z.

Received: 22 November 2017; Accepted: 11 September 2018; Published online: 15 October 2018

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## Acknowledgements

We thank J. Dai for clarification of the sample elevations of the upper Mekong in his work, S. Ji for assisting with sampling, Z. Zhang for analytical help, and H. Geng for discussion. This work was financially supported by the National Key Research and Development Program of China (2016YFE0109500), the (973) National Basic Research Program of China (grant no. 2013CB956400), the National Natural Science Foundation of China (grant nos 41422204 and 41672157) and the US National Science Foundation (grant nos 1348005 and 1545859). M.D. was supported by Australian Research Council Discovery funding scheme (DP160102427) and Curtin Research Fellowship.

## Author contributions

J.N., G.H., K.G. and C.N.G. designed the experiments. J.N., G.R., D.S., K.G., M.D., Y.W., Z.W. and S.L. performed the experiments. All authors analysed the data. J.N., G.R., K.G., G.H. and W.W. wrote the manuscript with help from the other authors.

## Competing interests

The authors declare no competing interests.

## Additional information

Supplementary information is available for this paper at https://doi.org/10.1038/s41561-018-0244-z.

Reprints and permissions information is available at www.nature.com/reprints.

Correspondence and requests for materials should be addressed to J.N.

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© The Author(s), under exclusive licence to Springer Nature Limited 2018
## Methods

**AHe dating.** The experimental procedure follows standard procedures<sup>51-53</sup>. After crushing, heavy liquid and magnetic separation, inclusion-free, uhedral, non-fractured apatite grains were hand-picked using a high-power stereo-zoom microscope under reflected and transmitted light. Single apatite crystals were then put into 1 mm Pt tubes and He gas was extracted at ~1,050 °C for 5 min and analysed for ⁴He, followed by a second extraction to ensure complete degassing and to monitor the He release from more retentive U- and Th-bearing inclusions in the analysed apatite. After this step, the samples were dissolved in HNO₃, and the U and Th contents were detected by inductively coupled plasma mass spectrometry. Most of the samples were measured in the thermochronology lab at the University of Texas at Austin. We also double checked the result using one sample (YN4197) in the thermochronology lab at the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration. The results for this sample are comparable between the two laboratories.

**One-dimensional stream profile modelling.** For stream profile modelling we used the linear stream power formulation with a critical threshold<sup>54,55</sup>:

$$ \frac{dz}{dt} = U + k \times A(x)^m \times \left(\frac{dz}{dx}\right)^n - T_c \quad \text{when} \\ k \times A(x)^m \times \left(\frac{dz}{dx}\right)^n > T_c \quad (1) $$

$$ \frac{dz}{dt} = U \quad \text{when} \quad k \times A(x)^m \times \left(\frac{dz}{dx}\right)^n \le T_c \quad (2) $$

where z is elevation, t is time, k is erodibility, U is the uniform uplift rate, A is the drainage area, m is an exponent that depends on both the drainage basin dynamics and the erosion dynamics, n determines the relative importance of the slope and T_c is the critical stream power threshold for erosion. Previous studies<sup>58</sup> showed that for the linear-form stream power formulation above, when n=1 the value of m is typically around 0.5. In other settings, the critical threshold was shown from experiments to vary by several orders of magnitude<sup>57</sup>, ranging from ~0.001 to ~6 m yr<sup>-1</sup>. We adopted the relatively lower threshold of 0.001 m yr<sup>-1</sup>.

We extracted the cumulative drainage area A(x) for the Mekong and uplands using the SRTM3 PLUS digital elevation model<sup>58</sup> and TopoToolbox 2 (ref. <sup>59</sup>). We then smoothed A(x) using a 30 km averaging kernel to improve the model stability.

To simulate the stochastic precipitation variability, we used an exponential function that describes the measured precipitation and hialtal interval distributions in other areas<sup>60</sup>. For simplicity, precipitation varies only through time but is uniform in space, and is coupled with erodibility by the relation $k_i = (P^n/P^*)^m \times k_i^*$, where $k_i^*$ is the erodibility when the perturbed precipitation rate P^* is equal to the background precipitation rate P^*. Any change in erodibility, therefore, can reflect a change in precipitation $P^*/P^* = (k_i/k_i^*)^{1/m}$. The absolute value of erodibility and erodibility threshold are poorly constrained<sup>61,62</sup>, but we tuned the erodibility and erosion threshold to produce both a similar degree of relief in the equilibrium profile, with similar observed mean erosion rates to the Mekong of 100 m yr<sup>-1</sup> during non-monsoonal periods. Using a random real number r, which ranges from zero to one inclusive, our stochastic precipitation function was $p = -6 \times \ln(r)$ during the interval 17–14 Ma, which corresponds to the duration of the MMCO, and $p = -3 \times \ln(r)$ during all other times. This means that the precipitation mean is doubled for our value of m=0.5. Note that because p is ultimately coupled with $k_i$, the absolute values of p are effectively arbitrary.

Starting with an initial profile as a straight line with a uniform slope of ~0.001, we then integrated the stream power equation through time using an upwind finite-differencing scheme. The model takes 20 Myr of the 40 Myr run to reach a pseudo-steady state. From 23–26 Myr (17–14 Ma), we doubled the mean precipitation, as mentioned above, which resulted in a kilometre-scale surface lowering across much of the profile. Afterwards, the mean erosion rates returned to the prior values, except at the downstream end, where a knickpoint formed and rapid erosion occurred.

For comparison, we also include the (non-threshold) stream-power-based model (for example, $T_c=0$ in equation (1)) which uses both n=2 and n=4 (equation (1)) to represent different degrees of non-linearity. Although n=2 is thought to be most representative of many streams when this no-threshold/non-stochastic approach is used, n=4 is also feasible<sup>63</sup>. These models do not require stochastic rainfall and therefore do not require assumptions over any sort of rainfall and/or flood-frequency distribution. Here precipitation and erodibility are kept constant through time with $k_i$ tuned so that the steady-state stream profile matches the approximate present-day profile, except for the period from 17 to 14 Ma, at which point precipitation is doubled. We show that this induces a similar magnitude of maximum incision of 600 m to 1.5 km for the stream profile from 17 to 14 Ma among the two models—albeit the spatial patterns of the 'background' incision from 14 Ma to the present are slightly different, with less background incision in the uplands.

**Inverse thermal history modelling.** We modelled the thermal histories for the suite of AHe data jointly for the Linchang granite transect and the upper reach Mekong transect. We used the software QTQt, which allowed us to treat the samples as a vertical profile with the lowest elevation sample always being hotter than the highest elevation sample, and all the intermediate samples with temperature histories between these two limiting cases<sup>63</sup>. The modelling approach

was based on Bayesian transdimensional Markov chain Monte Carlo (MCMC) and allows us to sample a wide range of potential thermal history models<sup>63</sup>. The aim of the transdimensional sampling is to try to balance model complexity with fitting the data, that is we want to avoid over-fitting the data with overly complex models. The Bayesian approach deals with this implicitly.

We considered the effect of the range of the prior distribution on time from about twice the oldest age (26 ± 26 Ma) to the range of 100 ± 100 Ma broadly implied by various higher-temperature thermochronometric systems (for example, zircon U/Pb, 250–200 Ma; biotite Rb/Sr, 100–200 Ma; biotite Ar/Ar, 150–100 Ma; K-feldspar <sup>40</sup>Ar/<sup>39</sup>Ar, 120 Ma)<sup>61</sup>. The initial results implied that the data do not require a protracted thermal history implied by the higher-temperature systems, consistent with the river profile modelling that suggests a limited incision at the first ~20 million years. However, we did add an explicit constraint to force the thermal histories to start at 160 ± 30 °C, that is, above the range of temperature sensitivity for the data, at the time 60 ± 30 Ma.

We ran simulations without radiation damage effects and with two radiation damage models<sup>65,66</sup>, both based on modifying the diffusivity of He in apatite as a function of the effective uranium for each single-grain analysis. We do not show all the results of this modelling exercise here, but select the maximum likelihood and expected models as an appropriate summary. Note that all the modelling is based on AHe ages not corrected for α ejection. The maximum likelihood model is, by definition, the sampled model that fits the data the best, but it tends to be more complex than most of the other models as we may be partly fitting noise in the data by adding complexity in the thermal history to improve the data fit. The expected model is often used to represent the range of models sampled by the MCMC sampler. It is the weighted average of all the models accepted during the MCMC sampling (models from between up to 200,000 post-burn-in iterations were used to calculate this average model). The weighting was provided by the posterior probability distribution inferred as part of the inverse modelling process.

## Data availability

The authors declare that all data supporting the findings of this study are available within the article and its Supplementary Information.

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