Late Cenozoic landscape evolution along the Ailao Shan Shear Zone, SE Tibetan Plateau: Evidence from fluvial longitudinal profiles and cosmogenic erosion rates

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Earth and Planetary Science Letters www.elsevier.com/locate/epsl

Late Cenozoic landscape evolution along the Ailao Shan Shear Zone, SE Tibetan Plateau: Evidence from fluvial longitudinal profiles and cosmogenic erosion rates Yang Wang a,b,∗ , Lindsay M. Schoenbohm c , Bo Zhang a , Darryl E. Granger d , Renjie Zhou e , Jinjiang Zhang a , Jianjun Hou a a

The Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China Guangdong Provincial Key Laboratory of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-Sen University, Guangzhou 510275, China c Department Earth Sciences, University of Toronto, Toronto M5S 3B1, Canada d Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907-1396, USA e School of Earth and Environmental Sciences, The University of Queensland, St Lucia 4072 QLD, Australia b

a r t i c l e

i n f o

Article history: Received 19 October 2016 Received in revised form 25 January 2017 Accepted 19 May 2017 Available online xxxx Editor: A. Yin Keywords: Tibetan Plateau Ailao Shan Shear Zone fluvial longitudinal profile cosmogenic erosion rate landscape evolution

a b s t r a c t In tectonically active regions, geomorphic features, such as catchment slopes, terraces, and river profiles can be interpreted in the context of tectonic and climatic forcing; however, distinguishing tectonic impacts from other factors such as pre-existing geologic complexities and climate changes is challenging. We use fluvial longitudinal profiles, catchment slopes, and catchment mean erosion rates derived from insitu cosmogenic 10 Be and 26 Al to examine the late Cenozoic landscape evolution of the Ailao Shan Shear Zone (ASSZ) in the southeastern Tibetan Plateau. The trunk stream of the Red River, flowing along the eastern side of the shear zone, consists of three sections with distinct channel parameters, separated by knickzones (the Midu, Ejia, and Nansha sections from NW to SE). Tributaries to the Red River within the Ailao Shan Shear Zone in the Ejia and Nansha sections consistently display two channel segments (upper low-gradient and middle steep channel segments); a third set of lower, less steep channel segments are identified only along the tributaries in the Nansha section. Catchment mean erosion rates contrast sharply along strike: ca. 300 m/Myr in the Ejia section and ca. 100 m/Myr in the Nansha section. Collectively, our results provide strong evidence that: 1) two waves of incision induced by pulsed and declining regional uplift are propagating up the Red River shaping the background pattern of river incision; 2) vertical fault displacements, river reorganization and additional factors locally affect river profile morphology. Normalized steepness indices (k sn ), catchment slopes, and knickzone distribution vary systematically along the Ailao Shan Shear Zone, indicating long-wavelength regional surface uplift during plateau growth in the middle-late Miocene, which points to a tectonic model involving crustal thickening and diffuse or continuous deformation in the southeastern margin of the Tibetan Plateau. © 2017 Elsevier B.V. All rights reserved.

1. Introduction A principal goal of tectonic geomorphology is to decode information about the spatial and temporal patterns of active deformation in the landscape (e.g. Kirby and Whipple, 2001; Wobus et al., 2006a). Fluvial erosion along channel networks dictates the response of landscapes to local tectonic and climatic conditions (Whipple, 2004). A multitude of studies have demonstrated that fluvial longitudinal profiles are adjusted to balance erosion and

*

Corresponding author. E-mail address: [email protected] (Y. Wang).

http://dx.doi.org/10.1016/j.epsl.2017.05.030 0012-821X/© 2017 Elsevier B.V. All rights reserved.

uplift and have readily predictable morphology under different tectonic, climatic and lithologic conditions, making them particularly useful in diagnosing and quantifying patterns of surface uplift (e.g. Kirby and Whipple, 2001; Schoenbohm et al., 2004; Wobus et al., 2006a; Ouimet et al., 2009). However, deciphering the embedded tectonic information from other controlling factors is challenging. The question of how the southeastern margin of the Tibetan Plateau deforms in response to ongoing India–Eurasia continental convergence (Fig. 1A) – whether strain is localized along major fault systems bounding quasi-rigid blocks (e.g., Tapponnier et al., 2001), or operates diffusely within a continuously deforming medium (e.g., Houseman and England, 1993) – remains a vigorous

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Fig. 1. (A) Major structures in the Eurasia plate. The red rectangle indicates Fig. 1B. (B) Topography, major faults and rivers in the southeastern Tibetan Plateau (compiled from Tapponnier et al., 2001; Yin, 2010). Gray dots indicate catchment mean erosion rates (this study; Ouimet et al., 2009; Henck et al., 2011); black dashed lines indicate ranges for closely clustered samples. The light gray cross-hatched areas are relict landscape. The pink shaded area indicates the Red River watershed in Figs. 2A and 3A. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. (A) Digital elevation model (DEM) of the Red River watershed and river network derived from ASTER 30-m DEM. The Ailao Shan Shear Zone is indicated by pink shading. The Red River fault lies on the northeastern margin of the shear zone (compiled from Geological Map of Red River active fault zone, scale 1:50,000 (Guo et al., 2013) and field observations). The analyzed catchments are numbered from 0 to 24 from NW to SE. (B) The mean annual precipitation from TRMM 2B31 (1998–2009, processed by B. Bookhagen) in the study area. Precipitation increases slightly from NW to SE along the Red River. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

debate. Furthermore, no consensus exists concerning the relative roles of tectonics and climate in shaping orogenic topography in this area. Quantification of the spatial variations in geomorphic metrics and erosion rates provides a key to address these questions. The Red River drains the southeastern Tibetan Plateau (Fig. 1B). Most tributaries within the Ailao Shan Shear Zone, which is located on the southwest side of the Red River valley, are in disequilibrium (Schoenbohm et al., 2004). Their catchments are underlain by relatively uniform lithology and experience a similar climate and mean annual rainfall (Figs. 2A and 2B). Therefore, the Ailao Shan Shear Zone provides an ideal natural laboratory to use fluvial longitudinal profiles and catchment mean erosion rates to investigate its late Cenozoic tectonic and landscape evolution, to evaluate controlling factors shaping topography, and to differentiate deformation styles in the southeastern margin of the Tibetan Plateau. We present quantitative analyses of fluvial longitudinal profiles, catchment slopes, and catchment mean erosion rates determined from in-situ cosmogenic 10 Be and 26 Al. The results demonstrate that temporal change in regional uplift and associated river transient response have shaped the background pattern of river incision; vertical fault displacements, river capture, and additional factors affect profile morphology locally. A long-wavelength pattern of uplift is inferred to have occurred during plateau growth in the middle–late Miocene.

distributed generally between 2000 and 3000 m elevation (e.g., Schoenbohm et al., 2004). They are best preserved on the northern part of the Ailao Shan range and can be traced discontinuously toward the southeast along the crest. Tributaries draining the Ailao Shan range are deeply incised bedrock rivers that flow approximately perpendicular to the stem (Fig. 2A). They have their headwaters on the low-relief upland with knickpoints separating low-gradient channels above from steep bedrock channels below (Schoenbohm et al., 2004), suggesting ongoing headward river incision. A widely-cited interpretation is that such low-relief, upland topography above deeply incised valleys is remnant of a formerly continuous, low-elevation landscape undergoing increased regional uplift (e.g., Clark et al., 2005; Whipple et al., 2017). However, the timing of regional surface uplift in the southeastern Tibetan Plateau remains uncertain despite decades of study. Rapid cooling and exhumation inferred from thermochronology suggest that crustal thickening and regional uplift may have commenced ranging from the early Miocene (ca. 20–15 Ma) or earlier (e.g., Wang et al., 2012; Tian et al., 2014) to the late Miocene (<10 Ma) (e.g., Clark et al., 2005; Ouimet et al., 2010). Recent paleoaltimetry studies indicate present-day elevations may have been achieved in NW Yunnan by the middle Eocene (ca. 40 Ma) (Hoke et al., 2014), with a later onset of regional uplift (ca. 13 Ma) in SE Yunnan (Li et al., 2015).

2. Study area

2.2. Structural evolution

2.1. Topographic characteristics

The southeastern margin of the Tibetan Plateau has been extensively deformed as a result of India–Asia convergence (Fig. 1A; e.g., Tapponnier et al., 2001; Yin and Harrison, 2000). The Oligo– Miocene left-lateral Ailao Shan Shear Zone lies parallel to the trunk stream of the Red River. Together with the Xuelong Shan, Diancang Shan, and Day Nui Con Voi Shear Zones, it is interpreted as the northeastern boundary of the Indochina block, which accommodated the SE block extrusion in the early stages of collision (e.g. Peltzer and Tapponnier, 1988; Leloup et al., 1995, 2001;

A major topographic feature of the southeastern margin of the Tibetan Plateau is the presence of an elevated, low-relief landscape, which is continuous from the Tibetan Plateau and is deeply incised by the Salween, Mekong, Yangtze, Dadu, Red Rivers and their major tributaries (Fig. 1B; e.g., Clark et al., 2005; Liu-Zeng et al., 2008; Ouimet et al., 2010; Yang et al., 2016; Whipple et al., 2017). The low-relief surface patches in the Red River watershed are

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Yin and Harrison, 2000). Basement units in the Ailao Shan Shear Zone are typically amphibolite grade metamorphic rocks, which are spatially uniform along strike (e.g., Leloup et al., 1995, 2001). The Ailao Shan metamorphic massif may have experienced two phases of rapid exhumation in the Miocene. One cooling phase occurred during the left-lateral shearing from ca. 27 to 17 Ma based on 40 Ar/39 Ar dating (e.g., Harrison et al., 1996; Leloup et al., 2001). A second accelerated cooling episode commenced at ca. 14–13 Ma and lasted 2–3 Ma revealed by apatite fission-track and (U–Th)/He data (Bergman et al., 1997; Leloup et al., 2001; Wang et al., 2016). The Ailao Shan range may have risen to its modern elevation with high-relief topography developing due to river incision in this period. The Red River fault, which lies along the northeastern margin of the Ailao Shan Shear Zone today, shows right-lateral strike-slip movement (Fig. 2A). It has experienced 5.5 to 54 km of Plio–Quaternary right-lateral offset (e.g., Allen et al., 1984; Replumaz et al., 2001), with a normal component along strike and a thrust component in the major bend area (Fig. 2A; Wang et al., 2016). Regional surface uplift, river incision, exhumation of the shear zone, and displacement along the Red River fault have all interacted to shape the modern landscape of the Ailao Shan range. 3. Methods 3.1. Stream profile analysis Fluvial longitudinal profiles, along well-equilibrated bedrock channels with spatially uniform lithology, uplift rate, and climate, exhibit a power law relationship between slope (S) and drainage area ( A), given by:

S = k s A −θ

(1)

where k s is the channel steepness index and θ is the channel concavity index. Concavity indices (θ ) range between 0.4 and 0.6 when uplift and erosion are balanced (at steady state), and do not show a systematic variation with uplift rate (Whipple and Tucker, 1999; Wobus et al., 2006a; Kirby and Whipple, 2012). According to the stream power law, channel erosion rate (E) can be approximated as a function of contributing drainage area ( A) and local gradient (S):

E = K Am S n

(2)

where K is the erosional coefficient (set by climate and rock properties); m and n are positive exponents that describe the dependency of river incision rate on drainage area and channel slope (Whipple and Tucker, 1999). The elevation change (dz) of a point along a channel within time (dt) is accordingly determined by rock uplift (U ) and erosion (E):

dz dt

= U − E = U − K Am S n

(3)

At steady state, the elevation of a channel does not change over time, therefore solving for channel slope yields the following relationship:

 S=

U K

 n1 A

−m n

(4)

which has the same form as equation (1). Equation (4) reveals that steepness indices (k s ) should vary with uplift rate (U ) and erodibility (K ), allowing us to obtain the tectonic and climate signal. Although fluvial longitudinal profile analysis was developed under assumptions of steady state, it has been extended to transient systems as well (e.g., Schoenbohm et al., 2004; Harkins et al., 2007; Regalla et al., 2013; Whittaker and Walker, 2015). When alongchannel variations in lithology, climate, or uplift rate exist, anchored knickzones (usage after Regalla et al., 2013) in the profile

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would separate segments with different channel gradient, which exhibit sharp steps in log S-log A plots (Kirby and Whipple, 2012). Similarly, if a temporal change in regional uplift rate or climate occurs, a transient knickzone develops at the catchment outlet, and propagates upstream as an incisional wave, separating newly equilibrated lower reaches from upper reaches which have yet to respond and retain the characteristics of the preexisting state (e.g., Whipple and Tucker, 1999; Kirby and Whipple, 2012). We analyze the Red River stem and 25 main tributaries on the eastern flank of the Ailao Shan Shear Zone using a 30-m ASTER (Advanced Space borne Thermal Emission and Reflection Radiometer) DEM (Fig. 2A). Schoenbohm et al. (2004) previously analyzed tributaries throughout the Red River drainage using 90-m DEM data. Their results, however, were complicated by lithologic variability which plays an important role in determining k s and channel morphology. In this study, we therefore focus on a narrower selection of 25 evenly spaced, large tributaries, which lie almost entirely within the relatively uniform metamorphic rocks of the Ailao Shan Shear Zone, allowing us to eliminate the confounding factor of lithology. In addition, we identify the general form of the channel profiles by batch analyses of tributaries throughout the watershed. For the selected 25 tributaries, we utilize ArcGIS and Matlab scripts (www.geomorphtools.org) to derive longitudinal profiles, steepness and concavity indices. Steepness indices are determined using log S-log A plots following methods outlined by Wobus et al. (2006a), and normalized steepness indices (k sn ) are calculated using a fixed reference concavity of 0.45 in order to compare channels and channel segments of varying drainage area (Whipple and Tucker, 1999). We resample elevation data in 20 m contour intervals (30 m for the trunk stream) with a 250 m smoothing window to decrease data noise (Wobus et al., 2006a), and highlight the general form of the profiles. We further calculate χ -elevation profiles (plots of elevation vs. χ ) using the integrative method (Harkins et al., 2007; Perron and Royden, 2013) to facilitate our analyses of river profiles. Discrete river segments appear as piece-wise lines on χ -transformed profiles: the slope of χ plots equals channel steepness; abrupt transitions between quasilinear segments are knickzones (Perron and Royden, 2013). We use in-house scripts based on Topotoolbox (Schwanghart and Scherler, 2014) to derive χ -elevation profiles for tributaries throughout the Red River drainage in the Ejia and Nansha sections, respectively. We calculate average slope for the 25 tributary catchments along the shear zone using ArcGIS. We also calculate average slope for different reaches within each catchment, above and below the knickzones according to the following logic. If the knickzones propagate upstream according to equation (2), they would occupy the same elevation on all branches within a catchment (Wobus et al., 2006b). Therefore, a contour line passing through the analyzed tributary knickzone should roughly separate the relict landscape above from the active landscape below. Therefore, we divide the catchments by knickzone elevation, calculating the average slope for the landscape above and below this elevation, respectively. 3.2. Erosion rates determined from concentrations of 26 Al and 10 Be Cosmic rays interact with materials at the Earth’s surface, producing measurable quantities of rare radionuclides such as 26 Al and 10 Be in quartz. Cosmic rays attenuate rapidly in rocks, and thus the concentration of cosmogenic radionuclide in a sample is a measure of the time that sample has spent in the accumulation zone in the Earth’s near surface (Lal, 1991). Sediment collected from a stream contains grains that were originally sourced in an eroding hillside, accumulating cosmogenic nuclides as they passed upwards into the accumulation zone, approached the surface and eventually were eroded and transported to the sample site. If all parts of the catchment contribute sediment proportion-

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Fig. 3. (A) Shaded relief map of the Red River watershed with normalized steepness indices. (B) The longitudinal profile, the log S-log A, and χ -elevation plots for the Red River trunk stream, showing that the stem consists of the Midu, Ejia, and Nansha sections (from NW to SE), separated by knickzones with distinct channel parameters for each section. Smoothing window is 250 m and contour sampling interval is 30 m. Red squares are log-bin averages of the slope-area data. (C) Catchment mean erosion rates along the Red River, contrasting sharply along strike: ca. 300 m/Myr in the Ejia section and ca. 100 m/Myr in the Nansha section. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

ately to their erosion rates, and for the case that erosion rates are sufficiently fast such that radioactive decay can be ignored, the cosmogenic nuclide concentration in the sample can be used to calculate a spatially averaged erosion rate (Granger et al., 1996) according to the equation:

E =

 Li ∗  P i  Nsample

(5)

where  E  (cm/yr) is the spatially averaged catchment erosion rate, and L i is an effective cosmic ray attenuation length (cm). N sample (at/g) is the measured nuclide concentration in the multigrain sample and  P i  (at/g/yr) is the spatially averaged catchment production rate. The subscript i refers to production rates and penetration lengths for cosmogenic nuclide production by nucleon spallation, negative muon capture, and fast muon reactions. For simplicity, we parameterize production rates by muons following the single exponential approximation of Braucher et al. (2013) with an attenuation length of 5300 g/cm2 . A more complex model of muon production rates with depth would make little difference to the final result and is not justified here given other uncertainties. In our case, for rock of density of 2.65 g/cm3 , L 1 = 60 cm for neutron spallation and L 2 = 2000 cm as an approximation for muon production. Three samples for calculating catchment mean erosion rates were collected near the towns of Ejia, Yuanjiang and Nansha (Fig. 3A), taking care to avoid catchments with upstream sediment storage or recent mass wasting. Samples were analyzed in 2002–2003 at the Lawrence Livermore National Laboratory AMS facility (see supplementary material for sampling, preparation, and analysis). Production rates were estimated using ArcGIS and a Matlab script to calculate effective elevation, latitude and longitude

representative of each sampled catchment following the method of Portenga and Bierman (2011). Our study does not consider 10 Be and 26 Al production attenuation due to topographic shielding and surface cover since such effect would be negligible here (see the supplementary material for details). Calculations were made for both 26 Al and 10 Be in catchments 8 and 15, but unfortunately we were only able to report the 26 Al catchment mean erosion rate for catchment 2 because beryllium was lost during sample processing. 4. Results 4.1. River profiles Normalized steepness indices (k sn ) of streams in the Red River drainage range from <10 to >1000. The whole Red River drainage can be divided into three sections based on the trunk longitudinal profile, with conspicuous knickzones and distinct channel parameters for each section (Fig. 3B). For the trunk stream, the k sn of the upstream Midu section is 64.0 and the concavity is 0.46 ± 0.08; the central Ejia section has a higher k sn of 101.0 and a concavity of 0.84 ± 0.18; the downstream Nansha section exhibits a low k sn of 52.1, but a high concavity of 3.3 ± 1.7. The χ -elevation profiles for the tributaries throughout the Ejia and Nansha sections exhibit obvious differences (Figs. 4A and 4B). We highlight the general patterns on the χ -elevation profiles since we focus evolution of the whole watershed over a period of more than 10 Ma; stair-steps or spikes likely due to the quality of DEM data are ignored. Although there is some divergence on the plots, as expected, the χ -elevation profiles of tributaries in the Ejia section generally consist of two quasi-linear segments: a gentle gradient part above and a steep trend below (Fig. 4A). However, three segments are identified on

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Fig. 4. χ -elevation profiles for the tributaries throughout the Ejia section (A) and Nansha section (B). Tributaries in the Ejia section generally consist of two quasilinear segments: a gentle gradient part above and a steep trend below; three segments are identified on the χ -elevation profiles for the Nansha section: gentlegradient upstream and steep middle segments which are comparable to that in the Ejia section, as well as less steep downstream segments.

the χ -elevation profiles for the Nansha section: gentle-gradient upstream and steep middle segments which are comparable to that in the Ejia section, as well as less steep downstream segments (Fig. 4B).

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The 25 analyzed tributaries within the Ailao Shan Shear Zone in the Ejia and Nansha sections consist of two and three channel segments, respectively, consistent with the general trend revealed by the χ -elevation profiles. Upper segments are identified in all the analyzed tributaries (Figs. 5A–D, S1 and S2). These channel segments flow across the low-gradient relict landscape, and are characterized by low k sn , ranging from 26.8 to 128.0 (average = 65.2 in the Ejia section, average = 91.4 in the Nansha section; Table 1). The results are similar to previous study (Schoenbohm et al., 2004). Below the low-gradient segments are steep middle channel segments (Figs. 5A–D). They exhibit much higher k sn , ranging from 133.0 to 559.0 (average = 316.0 in the Ejia section, average = 195.0 in the Nansha section; Table 1). A third, lower channel segment is only identified along tributaries in the Nansha section, yielding lower k sn (average = 185.4; Table 1) compared to the steep middle segment; this contrasts with the findings of previous work (Schoenbohm et al., 2004). The k sn of upper segments in the 25 analyzed tributaries increases systematically from NW to SE (Fig. 6A). In contrast, the k sn of middle segments exhibit a decreasing trend toward the southeast (Fig. 6A). Lower segment k sn values are intermediate with no clear along-strike trend (Fig. 6A). Concavities for channel segments show normally observed values, with a few exceptions, without systematic variations along strike (Fig. 6B). Exceptionally high (e.g., Tribs. 0, 12, and 24) or negative (e.g., Tribs. 15 and 16) concavity indices may be a result of local fault displacement or river capture, as discussed below. Slope-break knickzones are identified along the Ailao Shan Shear Zone. The upper knickzones, which separate the lowgradient upper segments from the steep middle segment channels, are distributed across a wide range of elevations from 1279 to 2490 m along strike, and decrease systematically from NW to SE except the bend area (Fig. 6C, Table 1). Several knickzones that are clearly associated with fault displacement or river reorganization

Fig. 5. DEM and slope maps of the selected analyzed catchments (including three sampled catchments). We divide the catchments by knickzone elevations, indicated by the black dashed lines. The longitudinal profiles, the log S-log A and χ -elevation plots for the longest branch in each catchment are shown on the right. The knickzones are indicated by blue stars. Red arrow indicates the position of the fault cross the profile. Gray lines are smoothed profiles. The blue lines and orange lines are the profiles predicted by the regressed channel concavity (θ ) and the specified reference concavity (θ = 0.45), respectively. Smoothing window is 250 m and contour sampling interval is 20 m. Red squares are log-bin averages of the slope-area data. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Results of river longitudinal profile analyses.

Concavity

k sn

Average slope (◦ )

0 1 2 3 4 5 6 7 8 9 10 11 12 13 Average

0.22 ± 0.26 0.23 ± 0.15 0.29 ± 0.31 0.44 ± 0.18 0.54 ± 0.44 0.43 ± 0.10 0.49 ± 0.11 0.43 ± 0.34 0.30 ± 0.12 0.34 ± 0.08 0.47 ± 0.14 0.27 ± 0.08 0.94 ± 0.47 0.70 ± 0.18 0.44 ± 0.21

34.6 47.7 34.8 54.2 48.6 64.6 114.0 26.8 80.9 72.5 101.0 84.0 77.0 72.5 65.2

14 15 16 17 18 19 20 21 22 23 24 Average

0.10 ± 0.07 0.21 ± 0.11 0.42 ± 0.06 0.62 ± 0.15 0.29 ± 0.07 0.12 ± 0.97 0.11 ± 0.69 0.31 ± 0.13 0.30 ± 0.05 0.37 ± 0.05 0.34 ± 0.08 0.29 ± 0.22

70.9 74.1 110.0 33.3 122.0 63.6 47.7 118.0 116.0 128.0 122.0 91.4

a b c

Knickzone elevation (m)

Middle segment Concavity

k sn

Average slope (◦ )

12.3 15.5 13.9 15.5 18.5 19.2 21.3 14.6 18.0 18.6 19.4 16.9 17.3 20.3

2207 2209 2457 2456 2490 2213 2006 1976 1428 1279 1055c 1342c 1428c 1413

3.40 ± 2.80 1.10 ± 1.40 0.91 ± 0.98 0.45 ± 0.48 1.40 ± 1.20 0.49 ± 1.30 0.65 ± 0.19 0.55 ± 0.38 0.95 ± 0.72 0.15 ± 0.20 1.20 ± 1.60 1.70 ± 0.70 2.60 ± 0.63 1.00 ± 0.27 1.18 ± 0.92

384.0 266.0 559.0 325.0 342.0 279.0 230.0 280.0 266.0 244.0 327.0 246.0 377.0 299.0 316.0

24.1 27.1 24.8 22.3 26.3 23.9 23.7 22.7 21.8 24.2 22.6 19.8 18.4 21.1

25.8 18.4 12.9 18.4 18.4 19.2 20.2 21.9 21.4 22.9 26.1

2107 1717 1591 2582c 1836 1866 1706 1545 1418 1399 1490

0.36 ± 0.08 0.53 ± 0.32 0.59 ± 0.16 0.47 ± 0.09 0.85 ± 0.11 0.35 ± 0.05 0.40 ± 0.05 0.94 ± 0.35 0.44 ± 0.95 0.66 ± 0.48 0.74 ± 0.22 0.58 ± 0.26

191.0 191.0 133.0 252.0 251.0 129.0 123.0 235.0 290.0 174.0 177.0 195.0

20.7 17.3 18.8 23.7 20.7 19.5 20.6 22.4 19.0 19.7 20.2

Negative concavity is excluded in average calculation. k sn of channel segments with negative concavity is excluded in average calculation. Knickzones associated with vertical fault displacement or river capture.

Knickzone elevation (m)

952 977 794 894 920 862 813 806 761 836 765

Lower segment Concavitya

1.00 ± 0.84 −0.29 ± 1.60 −0.56 ± 3.20 0.74 ± 0.41 1.60 ± 1.40 −0.77 ± 0.50 −3.00 ± 4.10 1.50 ± 0.59 2.50 ± 2.40 0.90 ± 1.20 8.60 ± 5.70 2.40 ± 1.79

k sn b

167.0 248.0 233.0 189.0 231.0 220.0 226.0 196.0 196.0 154.0 165.0 185.4

Average slope (◦ )

17.58 21.64 24.41 23.33 23.92 19.77 20.09 15.56 15.94 20.12 19.39

Catchment average slope (◦ )

Drainage area (km2 )

14.6 17.6 16.6 18.0 22.2 20.7 22.2 16.7 19.1 19.5 19.8 17.7 17.6 20.5

74 73 30 26 44 52 49 201 145 457 210 119 158 159

20.6 18.4 16.4 23.1 20.6 19.5 20.4 20.4 19.4 21.2 21.2

61 81 195 130 97 103 231 188 164 361 304

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Trib. number

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No No No

(m/Myr)

290+110 /−60 280+39 /−30 115+12 /−10 Originally measured against standard KNSTD. Adjusted to revised value for 07KNSTD (Nishiizumi et al., 1997). Production rates for 10 Be from Balco et al. (2008). We assume that 26 Al production is 6.8 times higher than that of a

b

101.26 101.78 102.78 24.35 23.59 23.15 1270 693 262 101.3 101.9 102.84 24.38 23.65 23.21 – 47.7 ± 3.9 125.6 ± 7.7 2 8 15

458 ± 127 330 ± 40 683 ± 66

Catchment

Mean lat. (◦ ) Elev. (m) Long. (◦ )

[26 Al] (103 at/g) Al analyses.

[10 Be]a (103 at/g) Catchment

10

Catchment mean erosion rates in our study range from ca. 100 to 300 m/Myr, and contrast sharply along strike (Fig. 3C, Table 2). In the Ejia section, catchments 2 and 8 yield indistinguishable erosion rates from 280–290 m/Myr, with admittedly large uncer-

Table 2 Results of

4.2. Catchment mean erosion rates

Be and

26

are excluded in our analyses (Tribs. 10, 11, 12 and 17; Table 1). The lower knickzones, which are only identified in the Nansha section cluster at a relatively uniform range of elevation (between 761 and 977 m; Fig. 6C, Table 1). The catchment average slope of upper reaches where the upper channel segments flow ranges from ca. 12◦ to 26◦ (Figs. 4A–D, Table 1), and exhibits an increasing trend from NW to SE (Fig. 7A). In contrast, the catchment average slope of the middle reaches (corresponding to middle channel segments) decreases toward the southeast from ca. 27◦ to 20◦ (Figs. 4A–D, and 7A, Table 1). There is no obvious along-strike trend for the catchment average slope of lower reaches or the whole catchments (Fig. 7A and 7B).

Lat. (◦ )

Fig. 6. (A) Along-strike variations of normalized steepness indices (k sn ) for channel segments of tributaries within the Ailao Shan Shear Zone. The k sn of middle segments are significantly higher than those of upper segments, and decrease systematically from NW to SE, opposite to the trend for the upper segments. (B) Plot of concavity for each analyzed channel segment. Concavity indices for most channel segments exhibit normally observed values without systematic along-strike differences. Concavity and k sn of channel segments with negative concavity are excluded. (C) Vertical distribution of knickzones along strike. The upper knickzones decrease systematically in elevation from NW to SE except the bend area indicated in gray zone; the lower knickzones cluster at a relatively uniform elevation. Knickzones associated with vertical fault displacement or river capture are excluded.

Sample site

Mean long. (◦ )

10

2541 1786 1462

19.47 11.88 9.46

0.405 0.323 0.292

Be by both spallation and muon reactions.

– 285+25 /−22 2+6 /−5

Cover/ erosion correction

 E 26 Al

(m/Myr) Effective elev. (m)

P 10Be,spall. b (at/g/yr)

P 10Be,muon (at/g/yr)

 E 10 Be

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Fig. 7. Along-strike variations of catchment average slopes for separate reaches (A) and whole catchments (B). Catchment average slope of upper reaches increases from NW to SE, opposite to the trend for the middle reaches. There is no obvious alongstrike trend for average slope of lower reaches and whole catchments. The error bar represents the standard deviation for average slope.

tainties. The mean erosion rate for the three measurements can be determined from the error-weighted mean of the inverse erosion rate (the inverse erosion rate is used because the uncertainties are symmetric). The mean erosion rate for this section is 284+20 /−18 m/Myr. In contrast, in the Nansha section, the erosion rate for catchment 15 is ca. three times lower. The 26 Al and 10 Be data yield erosion rates of 115+12 /−10 and 92+6 /−5 m/Myr, respectively. The two estimates of erosion rate are different by 1.4 standard errors, which may be due to analytical uncertainty. There is no evidence from the field that the difference stems from prior burial, which would preferentially lower the 26 Al concentration. The tributary headwaters of these three catchments drain a high-altitude, deeply weathered, low-relief landscape (Figs. 5A–C). Erosion rates in steep channel segments should be much higher than that of the above slowly eroding landscape. However, it must be kept in mind that the erosion rates reported here are averaged over entire catchments. 5. Discussion 5.1. Controls on river morphology Longitudinal profiles of the Red River trunk stream and its tributaries within the Ailao Shan Shear Zone are segmented and exhibit different forms between the Ejia and Nansha sections; k sn , catchment slope, and knickzone elevation vary systematically along strike. We interpret these observations together with catchment mean erosion rates. Tectonic interpretations of river profile morphology require a priori knowledge of lithology, climate, drainage reorganization, and tectonic context. We exclude the former three as major causes for observed river profiles for the following reasons.

First, the basement unit exposed in the Ailao Shan Shear Zone is spatially uniform (e.g. Leloup et al., 1995, 2001), which indicates that different k sn between channels and channel segments cannot be attributed to lithologic variation. The resistant metamorphic rocks within the shear zone might lead to steep channel profiles (a decrease of K in equation (2), which can be achieved through stronger lithology); however, the tributaries developed on sedimentary rocks northeast of the shear zone also exhibit high k sn (Fig. 3A; Schoenbohm et al., 2004), excluding lithology as a controlling factor on channel morphology. A wide fault damage zone could increase bedrock erodibility, resulting lower k sn of channels near the fault (Roy et al., 2015). However, the damage zone of the Red River fault is asymmetric and mostly confined to the northeast side of the shear zone based on the following reasons. The Ailao Shan massif bounded by the Red River fault on its northeast side is generally in the footwall during normal vertical displacement, and the footwall tends to be the more intact side of the fault in normal fault settings (Roy et al., 2015). Further, tributaries draining the Ailao Shan Shear Zone are straight, closely spaced, and orthogonal to the fault zone, consistent with modeling results that suggest this drainage pattern is characteristic of the relatively intact side of a fault zone (Roy et al., 2015). Finally, our field observations indicate that the damage zone is limited to within tens or hundreds of meters of the range front, largely confined to the Red River valley. The lower segments along analyzed tributaries in the Nansha section are more than 5–10 km long, far exceeding the extent of the narrow damage zone within the Ailao Shan range, and therefore, are unrelated to the possible low-cohesion bedrock near the range front. Second, the late Miocene to present monsoon may have resulted in more precipitation in the southeastern Tibetan Plateau, leading to enhanced erosion at ca. 10–8 Ma (e.g., Allen and Armstrong, 2012). However, a climate-driven increase in erosion would cause a reduction in channel gradients (Bookhagen and Strecker, 2012), which is inconsistent with the high k sn of the middle tributary segments. We cannot fully exclude the possibility that a strengthened monsoon is responsible for the formation of the less-steep, downstream Nansha stem section and lower tributary segments. However, if this were the case a positive relationship between precipitation and erosion rate would be expected (higher precipitation would lead to faster catchment erosion), the opposite of what we observe, suggesting a minor role of climate. Third, stream capture as a result of strike-slip faulting may generate a transient knickzone by establishment of a new local base level after abrupt channel lengthening and shortening (Duvall and Tucker, 2015). Since we limit tributary analyses to the range front, and considering the low erodibility of Ailao Shan metamorphic rocks and ca. 5 mm/yr long-term average slip rate on the Red River fault (which would lower knickzone migration rate; Duvall and Tucker, 2015), downstream river reorganization likely only accounts for local deviations adjacent to the range front from overall patterns of tributary profile morphology. Upland low-relief surfaces could result from a rise in local base level following drainage area loss (Yang et al., 2015). The resulting low-relief patches are predicted to be distributed randomly in elevation, be surrounded by a rim of high-relief topography and vary significantly in relief (Whipple et al., 2017). However, the relict landscape along the Ailao Shan range crest generally decrease in elevation from NW to SE; these surface patches lack high-relief rims (only observed in catchment 17; Fig. S2); and average slope of the relict landscape shows an increasing trend toward the southeast. In addition, such processes are characterized by a concave curve below a gentle-slope linear part on the χ -elevation plots (Willett et al., 2014); we only identify a pattern matching this description in tributary 12 (Fig. S1). Therefore, the overall low-relief upland along the Ailao Shan range is a preexisting landscape instead of in situ formation of low-relief

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landscape patches by river reorganization, and can therefore be used to diagnose and quantify patterns of surface uplift. Tectonic forcing remains the most plausible controlling factor in the evolution of the Red River and its tributaries. When a temporal change in regional uplift rate occurs, a transient knickzone develops at the catchment outlet, propagating upstream as an incisional wave, working its way up the trunk and tributaries. Geological and thermochronological data provide insight into possible timing of changes in regional uplift rate. Apatite fission-track and (U–Th)/He data suggest an accelerated exhumation occurred at ca. 14–10 Ma along the Ailao Shan Shear Zone (Bergman et al., 1997; Leloup et al., 2001; Wang et al., 2016). Recent paleoaltimetric data also indicate that SE Yunnan achieved its modern elevation between 12.7 and 10 Ma (Li et al., 2015). This tectonic scenario explains the Red River stem and its tributary profiles as follows. Before the pulse of regional uplift, streams in the Red River watershed flowed across a continuous, if complex, low-relief landscape (Whipple et al., 2017). The Midu stem section and upper channel segments of the analyzed tributaries preserve the morphology of this stage. During the accelerated regional uplift (ca. 14–10 Ma), a transient knickzone propagated upstream from the outlet of the Red River watershed, migrating from the trunk into the tributaries. This incisional wave generated the central Ejia stem section and tributary middle segments, which are characterized by steep channels with high k sn . This process might have taken several millions years, but before the whole watershed reached equilibrium again, the pulse of uplift ceased. A new wave of incision as response to to decreasing regional uplift rate began to propagate up the river system, shaping the downstream Nansha section and forming the lower channel segments of the tributaries. This hypothesis is also supported by the cosmogenic erosion rates, which we discuss in detail in the next section. Although we are confident in our tectonic interpretations of river profile morphology, river profiles are confounded by additional complications. Vertical fault displacements are imposed on the regional uplift, which might have resulted in local and explainable deviations of river profiles and influenced knickzone distribution (Whittaker and Walker, 2015). For example, the vertical normal displacement near Ejia is about 750 m (Schoenbohm et al., 2004), resulting in locally steep gradients near the range front (e.g. Tribs. 0, 1 and 5). Near Yuanjiang, a fault within the Ailao Shan range cuts the middle segments of Tribs. 7–11 (Fig. 2A), causing a vertical step along the profiles (Figs. S1). The thrust component on the Red River fault in the bend area (Fig. 2A) results in high or negative concavity indices there (e.g. Tribs. 15 and 16; Figs. 5C and S2). The upper knickzones elevation within the bend area deviates from the general southeastward decreasing trend, probably associated with such kinematic differences as well. A decreasing gradient of regional uplift outward from the plateau interior might occur during plateau growth in the middle–late Miocene. Differences in fluvial erosion processes may arise from minor variance in precipitation, catchment topography, or vegetation coverage along strike. Moreover, river reorganization may occur randomly thorough the watershed. Lithology is more variable northeast of the Red River and totally different from the Ailao Shan Shear Zone. All these factors above would lead to variability in knickzones elevation and cause local steepened or flattened channels (revealed by some divergence of χ -elevation profiles) but would not have influenced the regional pattern of morphologic variation at the scale of the study area. 5.2. Cosmogenic erosion rates interpretation Our samples, which spread over 300 km along the length of the Red River stem, yield different erosion rates. The Ejia section exhibits the three times higher catchment mean erosion rates

9

than the Nansha section (Fig. 3C). To a first order this is consistent with our hypothesis of incisional waves resulting from a pulse of regional uplift; the Ejia section, associated with the increase in regional uplift rate, exhibits not only steeper channels, but also higher erosion rates. However, catchment mean erosion rate is also affected by additional factors, such as precipitation, lithology, catchment topography, vegetation coverage, and local vertical fault displacement (e.g., Portenga and Bierman, 2011). We exclude these as significant controlling factors for the following reasons. First, the major difference in erosion rates appears to be unrelated to the distribution of present-day precipitation since the minor variation in precipitation along strike cannot explain the large spatial variation in erosion rates (Fig. 2B). Furthermore, as we discuss above, slightly higher precipitation in the southeast should result in a higher erosion rate, which is inconsistent with our observations. Second, the uniform basement of the Ailao Shan Shear Zone excludes lithology as a controlling factor. Third, the sampled catchments are composite, tapping into both the relict landscape and the more active landscapes below (Figs. 5A–C). Long exposure in the presumably slowly eroding relict landscape, which would have experienced higher production rate because of its high elevation, will lead to higher concentrations of cosmogenic nuclide in sediment at the catchment outlet. Although the contribution of this sediment to the overall catchment sediment budget should be relatively low, it will nonetheless lower the basin-averaged calculated erosion rate (Granger et al., 1996). If this contribution were significant, we would expect catchment 15 yields higher erosion rates due to the smaller extent of the relict landscape compared with the other two sampled catchments (Figs. 5A–C), but instead the opposite is observed. Furthermore, the catchment average slopes are not significantly different across the sampled catchments (Table 2). Fourth, no obvious changes were found in the vegetation cover among sampled catchments based on examination of Google Earth images. Fifth, even though the normal fault component in the Ejia area as well as uplift gradient may account for the minor variation in erosion rates between catchments 2 and 8, they cannot explain the ca. three-fold decrease in erosion rates between catchments 8 and 15. We therefore argue that transient river response associated with temporal change in regional uplift is the major cause for the spatial variance of catchment mean erosion rates in the Red River watershed. Catchments 2 and 8, in the Ejia section, are equilibrated with the accelerated uplift conditions, although erosion rates may also be enhanced locally by normal fault displacement and uplift gradient along strike. Catchment 15, in the Nansha section, is adjusted to the post-rapid-uplift state. Previous studies also reveal northward increasing erosion rates in the Salween and Mekong Rivers drainages followed by a decrease in erosion rates upstream toward the unincised, low-relief parts of the Tibetan Plateau (Fig. 1B; Henck et al., 2011; Yang et al., 2016). These authors argued for a pulse of incision triggered by regional uplift in the wake of the northward advancing corner of the Indian indenter as the major cause (Yang et al., 2016). 5.3. Tectonic implications The incision and evolution of rivers traversing the southeastern plateau, including the Yangtze, the Mekong, the Salween, the Red Rivers and their tributaries (Fig. 1B), dictate the style and pace of landscape adjustment in response to the regional and local tectonic deformation. The southeastern margin of the Tibetan Plateau has been argued to experience long wave-length regional surface uplift, possibly as a result of the flow of weak middle to lower crustal material from beneath the Tibetan Plateau into its adjacent regions during the Miocene to Pliocene (e.g., Clark et al., 2005; Schoenbohm et al., 2006; Royden et al., 2008). This plateau ex-

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pansion scenario predicts outward migration of an erosional wave. However, accelerated incisional waves associated with pulsed regional uplift would propagate upstream toward the plateau interior during plateau growth. Therefore, the spatial variance of geomorphic metrics and erosion rates reflects the combined effects of transient response of river networks and broad patterns of regional uplift. Coupled river morphology, including steepness and concavity indices, catchment slopes, and catchment mean erosion rates along the Red River all point to a wave of accelerated incision induced by a pulse of regional uplift in the middle–late Miocene. Our data strengthen evidence for dominantly tectonic control on the regional pattern of river incision. Average slope of the upper reaches of each analyzed catchment within the shear zone increases systematically from NW to SE (Fig. 7A). The k sn of upper channel segments in analyzed tributaries exhibit a similar trend (Fig. 6A). Both types of data indicate that higher relief topography probably existed in the southeastern part of the Ailao Shan range prior to pulsed regional uplift since the relict part of each catchment characterizes topography at that stage. In contrast, catchment average slope of middle reaches, k sn of tributary middle segments, and upper knickzone elevations all decrease toward the southeast along the Ailao Shan range (Figs. 6A and 7A), suggesting a decreasing gradient of regional uplift outward from the plateau in the middle–late Miocene. Such a deformation pattern favors the plateau growth model which predict long wave-length, lowamplitude changes in surface uplift. 6. Conclusions The Red River watershed can be divided into the Midu (upstream), Ejia (central), and Nansha (downstream) sections. In the Ejia section, tributaries within the Ailao Shan Shear Zone consist of upper low-gradient and lower steep channels. Catchment mean erosion rate is 284+20 /−18 m/Myr in this section based on both 26 Al and 10 Be data from two catchments. In the Nansha section, the tributaries generally have three channel segments. The upper relict landscape channels and steep middle segments are comparable to the upper and lower tributary segments in the Ejia section; a third, lower-gradient segment is present only along tributaries in the Nansha section. Catchment mean erosion rate is three times lower in this section (115+12 /−10 and 92+6 /−5 m/Myr). In the context of climate and tectonic forcing, we find that middle–late Miocene regional uplift may have influenced the rate and style of landscape evolution along the Red River; two associated waves of incision are argued to be propagating upstream, driving relief evolving in the whole watershed. Fault displacement, river reorganization, and other factors may have shaped the river profiles and incision locally. Spatial variations in catchment slopes, k sn , and knickzone distribution favor the interpretation of long wave-length regional surface uplift in the southeastern margin of the Tibetan Plateau in the middle-late Miocene, such as predicted in plateau growth models with crustal thickening and diffuse deformation. Acknowledgements This work is supported by National Natural Science Foundation of China (No. 41272217; 41590861) and the Geological Survey of China (No. 12120115027101). We appreciate the Chinese Scholarship Council (CSC) for supporting joint PhD student Project. We thank Dr. Eric W. Portenga for providing Matlab scripts for effective elevation, latitude, and longitude calculation. We acknowledge discussions with Dr. Regalla and PhD candidate Erin Seagren. We sincerely thank two anonymous reviewers for their thorough, critical and constructive reviews. We also thank Editor An Yin for editorial handling and constructive comments.

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