Quaternary Channel-Focused Rapid Incision in the Phung

Journal of Asian Earth Sciences 129 (2016) 243–253

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Journal of Asian Earth Sciences

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Full length Article Quaternary channel-focused rapid incision in the Phung Chu-Arun River in Central Himalaya: Implications for a Quaternary capture event ⇑ An Wang a,b,c, , Kyoungwon Min c, Guocan Wang a,b a School of Earth Sciences, Center for Global Tectonics, China University of Geosciences, Wuhan, China b State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China c Department of Geological Sciences, University of Florida, Gainesville, FL, USA article info abstract

Article history: The Phung Chu-Arun River, one of the few major trans-Himalaya rivers, consists of a large drainage area Received 24 May 2016 captured in the northern slope of the Himalaya, and provides a key to understand tectonic, surface pro- Received in revised form 18 August 2016 cesses and their interplay in topographic development of the Himalaya. In this study, we carried out ther- Accepted 22 August 2016 mochronologic and topographic analysis along the Phung Chu-Arun River in Central Himalaya to reveal Available online 24 August 2016 its incision development. Apatites from a downstream section along its major tributary yield very young ﬁssion track and (U-Th)/He ages of 0.8–1.5 Ma. These cooling ages, combined with previously reported Keywords: data for higher elevations, reveal an abrupt change of the slope in the age-elevation plot suggesting an Phung Chu-Arun River accelerated channel incision initiated at between 1.5 and 2.5 Ma. The age-elevation relationship yields Central Himalaya Channel topography a high apparent exhumation rate (AER) of 2.6–3.8 mm/a during Quaternary, which is spatially focused Apatite ﬁssion track dating along the mainstream of the Kharta-Chentang reaches and the associated nearby tributaries. Topographic Apatite (U-Th)/He dating analysis suggests that this channel-focused incision acceleration was probably facilitated by a Quaternary capture event in upper drainage of the palaeo Arun River. This capture event greatly enhanced the effective discharge draining southward, causing an accelerated incision wave along the mainstream and tributaries of the palaeo Arun River. Furthermore, thrusting at physiographic transition zone is also required to sustain high channel gradient and the long-term rapid incision. Channel morphology presenting common knick points in major tributaries suggests that the drainage basin in Ama Drime Range area likely remain in pre-steady state adjustment of this capture event. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction et al., 2002; Zeitler et al., 2001). In this model, the rock uplift might be dynamically sustained though a positive feedback between It remains a mystery that how Himalaya sustains its high topo- surface process and thermally-weakened deep crustal rocks graphic front while remain under a long-term, rapid surface (Beaumont et al., 2001; Willett, 1999). For example, the Pliocene- denudation. A range of chronologic data (Herman et al., 2010; Quaternary exhumation of the Ama Drime Range was suggested Huntington et al., 2006; Streule et al., 2012; Thiede et al., 2004; to be enhanced by denudation of the Arun River gorge (Jessup Whipp et al., 2007) and erosion data (Burbank et al., 2012; Galy et al., 2008). An alternative model suggests that the high topogra- and France-Lanord, 2001; Garzanti et al., 2007) indicate a rapid phy has been maintained by a protective effect of surface regional denudation at 1–2 mm/a in the Himalaya (Thiede processes, such as glacial damming developed at high altitudes, et al., 2005; Thiede and Ehlers, 2013; Wobus et al., 2003). Indeed, which prohibits rivers from headward incision (Korup and the Himalaya topographic front would have been completely Montgomery, 2008; Owen, 2008). In this case, a northward eroded within 3–5 Myr if there was no tectonic force. decrease in bedrock denudation is required to maintain the high One possible explanation for the high topography is that the relief across the Himalayan topographic front. focused surface denudation has been tectonically compensated Across the Himalaya, only a limited number of rivers have suc- by corresponding rapid rock uplift (Beaumont et al., 2001; Koons cessfully dissected through the orogenic divide, capturing significant drainage areas of the northern slope. These trans-Himalaya rivers occur as a key to understanding the nature and development ⇑ Corresponding author at: School of Earth Sciences, Center for Global Tectonics, China University of Geosciences, Wuhan, China. of geomorphic system of the Himalaya, where its erosional topog- E-mail address: [email protected] (A. Wang). raphy results from an interplay between tectonic input and http://dx.doi.org/10.1016/j.jseaes.2016.08.017 1367-9120/Ó 2016 Elsevier Ltd. All rights reserved. 244 A. Wang et al. / Journal of Asian Earth Sciences 129 (2016) 243–253 erosional output (Beaumont et al., 2001; Hodges et al., 2004; et al., 2010; Murphy and Harrison, 1999). Interestingly, the STDS Zeitler et al., 2001). However, the erosional developments of these yielded clustered cooling ages of 16–12 Ma from AFT (Apatite Fis- trans-Himalaya rivers remain poorly understood. In this study, we sion Track; Searle et al., 1997), ZFT (Zircon Fission Track; Streule performed (U-Th)/He and fission track dating as well as topo- et al., 2012;A.Wang et al., 2010) and 40Ar/39Ar systems (Leloup graphic analysis for the Phung Chu-Arun River to investigate its et al., 2010; Macfarlane, 1993; Wang et al., 2006; Zhang and Guo, incision development and its topographic implications for the 2007). These thermochronologic data highly suggest fast cooling Himalaya. of the footwall of the STDS by tectonic denudation. The STDS activity was likely to have ceased in Middle-Late Mio- 2. Geology and topography of the Phung Chu area cene after its transition from ductile to brittle deformation. This is supported by a number of undeformed leucogranite veins with zir- The Phung Chu-Arun drainage is located in the Central Hima- con U/Pb ages of 17–15 Ma intruding the shear zone (Hodges laya, which transects upstream across the Himalaya into Tibet. et al., 1998; Leloup et al., 2010). In the Phung Chu area, the STDS Its upper drainage (to the north of the main Himalayan divide), has been clearly deformed and offset by the N-S fault system which is locally referred to as Phung Chu River, covers a large (Fig. 1c), indicating an orogenic structural transition from the NS region in Tibet, and is drained northward by several highest peaks to the EW extension (Kali et al., 2010; Leloup et al., 2010). P-T-D- in the Himalayas including the Everest (Fig. 1a and b). Along the t (Pressure-Temperature-Deformation-time) data in the STDS foot- western flank of the Ama Drime Range, the Phung Chu River runs wall suggest that the system became inactive since 13.5–12 Ma, southward reaching Nepal, where it is known as Arun River. and this timing is considered as the upper limit for the initiation of the EW extension (Kali et al., 2010; Leloup et al., 2010). 2.1. Orogenic parallel structures The Main Central Thrust (MCT), defining the bottom of GHS in regional, occurs as a key boundary fault (mainly ductile) The Phung Chu drainage covers two major tectono-stratigraphic accommodating hundreds of kilometers shortening between the units of the Himalaya: the Tethyan Himalaya Sequence (THS) in the Indian Plate and Tibetan Plateau during Neogene (Robinson et al., north and the Greater Himalayan Sequence (GHS) in the south. 2003; Schelling, 1992). The specific location of the MCT across These two tectono-stratigraphic units are divided by a north dip- the Arun River remains unresolved although a good number of ping extensional fault, the South Tibetan Detachment System studies have been carried out in this area (Searle et al., 2008). (STDS), which was activated in Middle Miocene (Burchfiel et al., The MCT was initially identified by a kyanite isograd in the upper 1992; Leloup et al., 2010; Wang et al., 2006). reaches of the Arun River, located 5–10 km to the south of Chen- The THS is composed of Phanerozoic unmetamorphosed sedi- tang (Bordet, 1961). Similar location was accepted in subsequent mentary sequences developed in passive continental margin of studies (Goscombe et al., 2006; Schelling, 1992; Upreti, 1999). the Indian Plate. In contrast, the GHS to its south, is composed of Goscombe et al. (2006) later termed it as ‘High Himal Thrust’ high-grade metamorphic rocks evolved from Precambrian Indian (HHT), which divided the GHS into the upper and lower plates. basement (Pan et al., 2012; Yin, 2006). In its upper part, the GHS However, based on strain data, Searle et al. (2008) suggested a was commonly intruded by a number of Miocene leucogranites much lower structural level for the MCT, which is located at Tum- (Harris, 2007; Searle and Godin, 2003; Searle et al., 1997), which lingtar village, 50 km to the south of the HHT. In this study, we have been interpreted as intra-crustal melting due to either crust preserve the location of HHT as MCT. It is interesting that in many thickening or decompression by extensional movement of the locations investigated, the MCT (root zone) is located at the foothill STDS (Searle et al., 1997). of Higher Himalaya, coinciding with a physiographic transition The STDS, placing the unmetamorphosed THS over GHS of high- zone across the Himalayan topographic front, which may bear an grade Precambrian gneissic rocks, can be well traced over the implication of out-of-sequence tectonics (Hodges et al., 2004; entire orogenic belt. In the Phung Chu and Everest regions, Wobus et al., 2005, 2003; Yin, 2006). the STDS is characterized by a ductile shear zone over several The timing of ductile deformation of MCT, from a regional kilometers in thickness (Carosi et al., 1998). Syntectonic leucogran- perspective, occurred from the Early to Middle Miocene (Arita ite veins are widely deformed indicating normal faulting (Leloup et al., 1997; Hodges, 2000; Johnson et al., 2001; Yin, 2006). In some

Tarim Basin 40 c Phung Chu River

Everest Tibetan Plateau 28 Katmandu

30 v i

India n

u b r 27 A a b 70 80 90 100 110 86 87 88 89

Fig. 1. Simpliﬁed geological and topographic map of the Phung Chu and Ama Drime Range area. Upper insets show the (a) general location and (b) Phung Chu-Arun drainage in Central Himalaya. In (c) the geologic map letters indicate chronostratigraphy. Q is Quaternary; N is Neogene; K is Cretaceous; J is Jurassic; T is Triassic; P is Permian; C is Carboniferous; D is Devonian; O is Ordovician; e is Cambrian; Pt is Proterozoic; gc is granite. Thick lines indicate channel gradient anomalies along the trunk and major tributaries (also see Figs. 5 and 6). Blue circles indicate data previously published, while black indicate data obtained in this study. Blue and red numbers next to samples indicate AFT and AHe ages in Ma respectively. See context for other abbreviations. A. Wang et al. / Journal of Asian Earth Sciences 129 (2016) 243–253 245

Fig. 1 (continued)

localities, a Late Miocene-Pliocene activation was proposed based fault, the Kharta Fault (KF), developed a fault valley extending on chronologic data (Catlos et al., 2004; Harrison et al., 1997). In 60 km from Moguo at the northern tip of the Ama Drime Range addition, brittle deformation has also been commonly identiﬁed to its southern at Kharta (Fig. 1c). Surface ruptures, which extend within MCT zone (Searle et al., 2008; Searle and Godin, 2003). along the foot of many giant triangular facets with heights of 1.6–2 km (Fig. 2c and d), widely offset Quaternary moraines 2.2. Ama Drime Horst and alluvial fans developed at foothill. In southern segment of the Kharta valley, the KF ruptured a second strand, showing an In structure, the Ama Drime Range corresponds to an active NS en echelon pattern of dextral step (Fig. 1c). This western strand horst (Figs. 1c and 2) accommodating the EW extension in Hima- extends SW into the Higher Himalaya and phases out at high alti- laya (Jessup et al., 2008; Kali et al., 2010). Its western bounding tudes, showing a southward attenuation in fault throw. 246 A. Wang et al. / Journal of Asian Earth Sciences 129 (2016) 243–253

Kharta Fault Dinggye Fault

a b

Kharta Valley Yo Ri Gorge

Kharta fault Moguo Valley

c d

Fig. 2. Surface rupture, fault scarp of (a) Kharta fault and (b) Dinggye fault, which offset foothill Quaternary alluvial and glacial deposits. (c) A glacial valley in the Kharta valley, indicated by dashed curve, dissects in the triangular facet and hangs high above the current valley. Heights of these giant triangular facets are estimated to be 1.6– 2 km. Note that topographic exposure of these giant triangular facets is not solely due to normal faulting, but requires rapid incision at the fault valley. (d) The Yo Ri gorge, an anomaly course, has been locally trapped in the uplifting footwall of Ama Drime Range, while both its upstream and downstream are in ﬂat fault valley. Note that the topographic dam between the upstream Moguo valley and the downstream Kharta valley is only 400 m.

The eastern range-flanking active fault, Dinggye Fault (DF), longer and wider, and are almost evenly spaced. In contrast, most extends straight in SSW and ruptures foothill Quaternary glacial valleys in the eastern flank are short and narrow displaying an deposits (Figs. 1c and 2b). The surface rupture of DF dies out south- apparent weaker surface erosion (Fig. 1c). As a result, the western ward in Higher Himalaya at similar latitude as the KF. flank of the Ama Drime Range is much wider (22 km) than the Along both flanks of the Ama Drime Range, ductile shear zones eastern (<5 km). were developed, in which S-C fabrics, grain tails and lineations are The differential erosion is also manifested in the swath topo- common, and suggest consistent ductile deformation of dip-slip graphic profile across the Ama Drime Range, which show apparent normal sense without apparent oblique slip components (Jessup contrasts in relief and minimum topography between the range et al., 2008; Kali et al., 2010; Langille et al., 2010). These ductile flanks (Fig. 3). It is interesting that the maximum topographic line shear zones are crosscut by high-angle brittle deformation associ- (Fig. 3), represented by EW ridges, presents an overall flat trend ated with the KF and DF. The ductile and brittle deformations are across the majority of the range, implying an EW symmetrical considered as a coherent extensional structure developing the topographic pattern. This topographic flat trend is consistent with horst structure at different crustal level (Jessup et al., 2008; Kali the main divide of the Ama Drime Range striking NS. These topo- et al., 2010; Kapp et al., 2008). graphic highs share similar elevations of 6 ± 0.5 km. The overall The NS normal faulting in the Ama Drime area was initiated consistent elevations of these topographic highs and the symmet- prior to 11 Ma, following the ductile deformation and rapid cool- rical pattern are compatible with a low-relief topography of the ing between 17 and 12 Ma in the STDS (Leloup et al., 2010, 2009; palaeo Ama Drime Range (possibly analogous to the topography Murphy and Harrison, 1999; Wang et al., 2010, 2006). There are lit- in hinterland of Tibetan Plateau), which was dissected and destruc- tle direct constraints on the magnitude of displacement of these ted by a young channel incision event, and transformed into the normal faults. Assuming the STDS as a pre-deformational marker, current high relief topography. horizontal offsets by the KF and the DF are 15 km and 35 km, respectively (Kali et al., 2010; Leloup et al., 2010), which indicates 3. Methods an apparent less partition in extension along the KF. 3.1. Apatite Fission Track (AFT) 2.3. Ama Drime Range topography Apatite fission track dating was performed at the State Key Lab- The horst structure of the NS-trending Ama Drime Range is oratory of Geological Processes and Mineral Resources in China characterized by a couple of normal faults, which appear conju- University of Geosciences. Samples were prepared following the gate. The topography, in contrast, presents an asymmetrical pat- standard external detector method (Hurford and Green, 1983). tern (Figs. 1c and 3), which implies an apparent contrast in Apatite grains were mounted in epoxy and polished to make surface erosion between the eastern flank and the western flank. scratch-free surfaces, and the mount was etched in 5 N HNO3 at In its western flank, EW glacial and fluvial valleys are relatively room temperature for 21 s. Thermal neutron irradiation was car- A. Wang et al. / Journal of Asian Earth Sciences 129 (2016) 243–253 247

7 Divide of Ama Drime Range Dinggye fault (Northern tip) Kharta fault 6

5 Maximum topography 4 Mean topography Main divide 3 Minimum topography Phung Chu River

Altitude (km) Altitude Topographic relief 2

1 West East 0 0 20406080 Horizontal distance (km)

Fig. 3. EW swath topographic profile across the Ama Drime Range. See Fig. 1 for location. Note that the maximum topographic line shows an approximate symmetrical pattern within the Ama Drime Range, where its central segment indicated by dark area fluctuates at around 6000 m. This is contrasted to the minimum topographic line showing asymmetrical pattern, which indicate that the western flank of the Ama Drime Range is favored by erosion. The relief between the maximum and the minimum topography, an approximate proxy for local erosion magnitude, strengthens downstream and peaks at near the Kharta fault over 2 km. The NS-striking main divide of the Ama Drime Range indicated by dashed line is plotted for comparison. ried out at the China Institute of Atomic Energy with a nominal extracted gas was purified and spiked with 3He, followed by iso- neutron fluence of 8 Â 1015 cmÀ2, which was monitored by CN5 topic measurements using a quadrupole mass spectrometer. All standard glasses mounted at ends of each sample column. A zeta the samples were re-extracted at least once to ensure complete value of 331.11 ± 11.52 was obtained using the Durango apatite degassing. The degassed sample packets were spiked and dissolved standard from multi-irradiated sample columns. Fission tracks in 5% nitric acid at 120 °C overnight. The sample solutions’ U-Th- were counted under a Zeiss Axioplan 2 microscope at a magnifica- Sm isotopic compositions were measured using an Element2 ICP- tion of 1000. MS.

3.2. Apatite (U-Th)/He (AHe) 3.3. Channel morphology processing

Apatite (U-Th)/He dating was performed at University of Flor- For topographic analysis, we used the data from ASTER Global ida. All apatite grains were examined under a binocular stereomi- Digital Elevation Model (2nd version released in 2011). These data croscope at a magnification of 160 to avoid samples with have an average precision of 2.4 arc-sec in horizontal and 15 m in significant inclusions. Alpha ejection (FT) factor for each grain vertical, with substantial quality improvements compared to the was calculated based on measured linear dimensions (Farley previous version (Tachikawa et al., 2011). Processing of channel et al., 1996). Single or multiple apatite grains were wrapped in Pt longitudinal profiles was carried out using the software package tubes, and heated using a diode laser under high vacuum. The of Arc Hydro on the platform of ArcGIS.

6.0 MJAD13 0.63 ± 0.09 km/Ma 5.5 MJAD28 3536-1 2580-1 5.0 MJAD29 MJAD30 MJAD14 P44

) MJAD36 4.5 0.78 ± 0.08 km/Ma m MJAD10

k P42

(

e P40

u 4.0

t ~0.45 km/Ma AFT(this study)

t Underneath STDS

l P37 AHe (this study)

A 3.5 0.29 ± 0.06 km/Ma MJAD1 Fault Dinggye AFT (Liu et al., 2005) R = 0.98 AHe (Jessup et al., 2008) 3.0 2.5 ± 1.6 km/Ma AHe (Kali et al., 2010) P35 R = 0.85 AFTAER (this study) 2.5 3.8 ± 1.6 km/Ma P34 R = 0.92 AHe AER (this study) 2.0 02468101214 Age (Ma)

Fig. 4. Age-elevation plot of AFT and AHe data in the Phung Chu and Ama Drime Range area. AERs indicated by slope coefﬁcients are interpreted based on local transects to avoid possible misleading in AER analysis over large horizontal space. Solid and short dashed lines are reinterpreted AERs for AHe data of Jessup et al. (2008) and Kali et al. (2010). Note that exhumation of these sampled rocks is a combined result of two exhumational scenarios. See context for details. 248 A. Wang et al. / Journal of Asian Earth Sciences 129 (2016) 243–253

4. Results information of Ama Drime Range rather than NS faulting of the KF and the DF. 4.1. Thermochronology 4.1.2. Apatite (U-Th)/He (AHe) 4.1.1. Apatite Fission Track (AFT) The AHe ages (Table 2) range from 0.8 to 13.2 Ma, and show a Six samples collected on valley bottom along the Chentang similar positive correlation with elevation (Fig. 4). Samples P40 tributary yielded AFT ages between 1.1 and 13.6 Ma and P42 yielded dispersed ages ranging from 2.8 to 26.0 Ma and (Figs. 1c and 4; Table 1). These ages generally increase 4.9 to 11.2 Ma, respectively. This may be due to unidentiﬁed upstream/northward, which represent a positive correlation high-U-Th inclusions, U-Th zonation, or He implantation from out- with elevation. The sample P44, which is located in the hang- side of the grains. Considering that these ages are older than their ing wall of DF, yielded an exceptionally older age than others. AFT ages, and are apparently in odd with existing AHe ages at This may be due to a lower erosion rate in hanging wall than higher elevations in the Ama Drime range (Jessup et al., 2008; in footwall of normal fault. Therefore, the sample P44 is Kali et al., 2010), we precluded these ages from apparent exhuma- excluded from the exhumation-rate calculation for the Ama tion rate (AER) calculation. AHe ages of P34, P35 and P37 are inter- Drime Range. It is worthy to note that all samples to its nally consistent within their uncertainties, and cope well with south are essentially unaffected by the normal faulting of their AFT ages, suggesting that they can provide more reliable con- DF. (1) Surface rupture along the DF terminates to the north straints for documenting exhumational processes. of the sample P42, and faulting-associated triangular facets die Sample P44 in the hanging wall of DF yields an average age of out in concert. This indicates an overall southward attenuation 13.2 ± 0.7 Ma, which is slightly younger than its AFT age of in fault throw to the north of sample P42. (2) Field investiga- 13.6 ± 0.9 Ma. These ages are consistent with a number of Middle tion along the upper reaches of the Chentang tributary iden- Miocene cooling ages obtained at equivalent structural locations tiﬁed no evidence of active faulting. (3) These young cooling at the top of GHS and below STDS in Central Himalaya ages are much younger than the timing of KF and DF which (Macfarlane, 1993; Searle et al., 1997; Wang et al., 2010, 2006), initiated in the middle Miocene. Thus samples to the south of which indicate syntectonic denudation by the STDS (Wang et al., P44 most likely preserve a complete and original exhumation 2010, 2006).

Table 1 Samples information and AFT data resultsa.

Sample Latitude (°) Longitude (°) Ele (m) N qS/Ns qi/Ni qd/Nd U (ppm) P(v2) (%) Age Ma ± 1r P44 28.0553 87.6919 4654 14 3.43E5/397 4.77E6/5518 1.139E6/3776 50.9 ± 1.9 2.9 13.6 ± 0.9 P42 27.9399 87.6408 4354 15 8.89E4/113 4.68E6/5952 1.123E6/3751 50.7 ± 1.8 70.6 3.5 ± 0.4 P40 27.9032 87.5816 4165 15 2.18E4/24 1.59E6/1750 1.115E6/3738 17.3 ± 0.9 65.5 2.5 ± 0.6 P37 27.9120 87.5275 3771 17 4.76E4/89 5.82E6/10,872 1.099E6/3713 64.5 ± 2.0 73.1 1.5 ± 0.2 P35 27.8757 87.4582 2933 21 6.96E4/102 1.16E7/16,975 1.091E6/3701 129.2 ± 3.7 75.0 1.1 ± 0.1 P34 27.8522 87.4473 2553 21 2.60E4/71 4.04E6/11,048 1.083E6/3688 45.4 ± 1.4 98.4 1.2 ± 0.2

a À2 À2 N is number of grains analyzed. qs is spontaneous track density (cm ); Ns is number of spontaneous tracks; qi is induced track density (cm ); Ni is number of induced À2 tracks; qd is track density on ﬂuence monitor (cm ); Nd is tracks counted on ﬂuence monitor. U ± 2r is the average uranium concentration (ppm). P(v2) is Chi-squared probability. Ages (Ma) are determined using the Zeta method and calculated using the computer program and equations by Brandon (1992). All listed ages are pooled ages with 1r error. See context for other lab parameters and processes.

Table 2 Apatite (U-Th-Sm)/He data results.

a b 4 c d Sample N FT He (fmol) U (ppm) Th (ppm) Sm (ppm) Age (Ma) 1r (Ma) Mean SD P44 1 0.84 61.48 46.0 1.6 207.2 12.4 0.3 13.2 0.7 1 0.87 121.77 57.6 1.8 265.7 13.8 0.3 1 0.87 187.70 77.0 4.3 297.2 13.2 0.3 P42 1 0.90 165.87 46.0 3.5 237.4 9.7 0.3 4.9e 0.3 1 0.92 590.81 73.2 9.9 258.3 11.2 0.3 1 0.86 17.20 41.6 2.5 243.5 4.9 0.1 P40 1 0.87 11.41 9.8 4.6 251.6 5.8 0.2 2.8e 0.2 1 0.84 3.22 12.4 3.0 248.1 2.8 0.1 1 0.82 59.06 28.1 26.8 136.1 26.0 0.7 P37 1 0.85 7.39 72.3 4.5 300.2 1.2 0.03 1.1 0.1 1 0.84 2.10 30.0 0.5 242.7 1.2 0.03 3 0.78 7.21 99.0 4.7 442.9 1.0 0.03 P35 1 0.82 3.99 86.4 1.5 214.4 0.8 0.02 1.0 0.2 2 0.78 5.38 103.1 11.0 282.3 0.8 0.02 3 0.76 7.54 98.4 3.8 263.9 1.2 0.03 P34 1 0.85 5.55 46.4 2.4 224.7 0.8 0.02 0.8 0.1 1 0.85 1.90 27.5 2.0 191.0 0.6 0.02 1 0.85 4.87 48.2 2.5 240.7 0.8 0.02

a Number of apatite grains analyzed in a single packet. b Alpha recoil correction factor calculated from linear dimensions of each grain. c Analytical errors added 4% relative error by alpha correction factor. d Standard deviation. e For samples P40 and P42, which are poor in age replication, minimum age packet is given as reference. See context for details. A. Wang et al. / Journal of Asian Earth Sciences 129 (2016) 243–253 249

4.2. Channel morphology downstream reaches are characterized by lower channel gradient. It is interesting that the lower break in channel gradient, marking a 4.2.1. Mainstream major physiographic transition in the Arun River, coincides with The longitudinal profile of the Phung Chu-Arun River main- the location of the MCT (Figs. 1c and 5). This coincidence simply stream shows multiple channel segments with contrasting channel implies an active tectonic significance if it is no accident. gradient anomalies (Fig. 5). The upstream-most high-gradient anomaly is located at the Yo Ri gorge, which connects upstream 4.2.2. Tributaries the Moguo valley and downstream the Kharta valley. Both the As is consistent with previous observations (Robl et al., 2008), Moguo and the Kharta segments are characterized by flat braided tributaries developed along the Phung Chu River show an interest- channels with lower channel gradient. An unusual aspect of the ing style in channel longitudinal profile. Most of these tributaries Yo Ri gorge, as has long been noticed, is its planar course, which show a convex pattern, which means that channel gradients is locally trapped in erosional-resistant footwall of the KF forming increase downstream overall (Fig. 6). This is contrasted with a sharp bedrock relief over 1.5 km (Armijo et al., 1986; Kali et al., graded channel longitudinal profiles, which in order to maintain 2010; Wager, 1937). Note that the relief of the topographic barrier a constant erosion rate along longitudinal profile, channel gradi- between the Moguo and the Kharta valley is only <400 m (Fig. 2d), ents attenuate downstream due to the effect of downstream accu- which is much less than that in the Yo Ri gorge. Apparently, this mulation in discharge (Brookfield, 1998; Burbank and Anderson, peculiar gorge course is inconsistent with a formation of upstream 2011). Note that in first order, most of these tributaries consist of progression by headward incision, but may imply a capture event linear segmented reaches. These reaches have almost constant (Armijo et al., 1986). channel gradients, instead of a gradual change in gradient, which The second high-gradient anomaly coincides with the Kharta- indicate apparent existence of knick points. Segments downstream Chentang gorge, which covers a longitudinal length of 50 km knick points show high average gradients of 6.5–8.1%, which are (Figs. 1c and 5). Both its upstream (the Kharta fault valley) and significantly higher than those upstream of 1.4–5.0%.

5 6 Yo Ri gorge Kharta-Chentang gorge Original channel Moguo valley Kharta valley 5 4 1 Fault

MCT h

Adjustment to faulting 4 n

Gadong e 3 Kharta l

DEM data deficiency a 1 2 3 d

2 t

( Altitude (km) Altitude 2 %

) Chentang 1 1 3.5% 1.8% 0 0 0 50 100 150 200 250 300 Longitudinal Distance (km)

Fig. 5. Longitudinal profile of the Phung Chu-Arun River. Histogram indicates local channel gradient, of which high-anomaly zones are indicated by shaded rectangle. Percentage numbers indicate average channel gradients of specified zones. Note that the Kharta-Chentang anomaly zone is characterized by an approximate constant channel gradient, indicated by short dashed line. Dashed circle marks an apparent deficiency in DEM data. Top-right inset illustrates the response of channel longitudinal profile to channel uplift, in which gradient anomaly forms in uplifting wall (modified from Brookfield, 1998). Note that the downstream boundary of the channel gradient anomaly zone predicts the location of fault.

5.5 T8 9.6% 5.0% 5.0% T9 Phung Chu River T6 5.0 T5: 9.8% 2.2% 5.1% T10 T4:6.5% 2.6% 4.5 T3:6.7% 2.0% 3.6% T2 1.4% 4.0 T7 5.0% T1 3.5 7.9% Altitude (km) Altitude 7.0% 3.0 Ama Drime Range 2.5 8.1% West East 2.0 50 40 30 20 10 0 102030405060 Longitudinal Distance (km)

Fig. 6. Longitudinal proﬁles of major tributaries of the Phung Chu River. Circles mark interpreted knick points. Percentage numbers indicate average channel gradients for speciﬁed segments of tributary, which are indicated by dashed lines. See Fig. 1c for locations of labelled tributaries. 250 A. Wang et al. / Journal of Asian Earth Sciences 129 (2016) 243–253

5. Discussion power law (Burbank and Anderson, 2011; Howard et al., 1994; Whipple and Tucker, 1999), bedrock channel incision rate is a 5.1. Early-Quaternary strengthened exhumation power law function of channel gradient, which means that the channel gradient anomaly corresponds to high channel incision. In age-elevation diagram our newly obtained and the previ- The spatial coincidence between the young cooling ages and the ously reported AHe data show two linear segments, which presents high channel incision rate, as predicted by the stream power law an abrupt slope change at between 2.5 and 1.5 Ma (Fig. 4). The (Burbank and Anderson, 2011; Howard et al., 1994; Whipple and strengthened AER since 1.5 Ma also occurs in the AFT data sets, Tucker, 1999), allows us to hold that it is the focused channel inci- which shows an identical trend with a contemporaneous slope sion dominated the cooling of channel bedrocks and formation of change. Three low elevation/southernmost samples from the Chen- gradient anomaly. tang tributary yield AERs (Apparent Exhumation Rates) of 3.8 mm/ Robl et al. (2008) proposed that the downstream increase in a and 2.5 mm/a for AHe and AFT, respectively. In contrast, the AFT channel gradient of tributaries is driven by the erosional-driven and the AHe data from higher elevation samples (>3.7 km) define rock uplift, which strengthens downstream toward the main- AERs of 0.29 mm/a and 0.63 mm/a, respectively, which are signifi- stream, i.e., the ‘‘river anticline effect” (Montgomery and Stolar, cantly lower. 2006; Robl et al., 2008). We notice that most tributary longitudinal The strengthened exhumation in Quaternary is apparent, if we profiles in the study area contain knick points, and both its employ the recently implemented analytical methods (Willett upstream and downstream are characterized by linear segments and Brandon, 2013) to convert cooling ages to time-averaged ero- with almost constant gradients (Fig. 6). These features are incon- sion rates. Samples P34, P35, P37 at downstream of the Chentang sistent with a continuous downstream increase in channel gradient tributary yield high erosion rates of 4.0–2.3 mm/a and 4.2– predicted by the ‘‘river anticline effect” (Montgomery and Stolar, 1.8 mm/a for AFT and AHe, respectively; while upstream samples 2006; Robl et al., 2008). Therefore, we interpret the convex longi- P40 and P42 yield much lower average erosion rates of 1.2– tudinal profile as a result of strengthened channel incision at sur- 0.75 mm/a, assuming a geothermal gradient of 35 °C/km and an face rather than the ‘‘river anticline effect” a tectonic feedback to average local elevation of 3500 m. fluvial erosion. Jessup et al. (2008) reported an overall AER of 1 mm/a for the The Kharta channel segment is bounded by the Yo Ri and period between 1.4 and 3.0 Ma based on datasets with AHe ages of Kharta-Chentang gorges (Figs. 1c and 5), both of which are of rapid 2.3–4.2 Ma (elevation = 4418–5580 m) from EW valleys in the incision due to high channel gradient. Therefore, we suggest that Ama Drime, and one young AHe age of 1.44 Ma (eleva- the fluvial erosion in the Kharta valley is passive, and may have tion = 3435 m) from the entrance to the Kharta-Chentang gorge been dominated by its local base level lowering at the Kharta- (Figs. 1c and 4). Worthy to note that the authors realized that these Chentang gorge. This implies that the Kharta valley probably data might reflect a possible slower exhumation prior to 2–3 Ma shared a similar incision rate to those at the bounding gorges. acknowledging that data was limited. By synthesizing subsequent Rapid erosion in the Kharta valley is consistent with the young AHe ages (3.3–3.8 Ma) obtained from the eastern limb of Ama AHe cooling age of 1.44 Ma (Jessup et al., 2008) obtained from Drime Range at similar elevations (5217–5419 m; Kali et al., the southern tip of the Kharta valley (the entrance to the Kharta- 2010), our reinterpretation of these data as a whole yields a lower Chentang gorge; Fig. 1c). It is worthy to note that the topographic average AER of 0.63 mm/a between 2.3 and 4.2 Ma (0.78 mm/a outcropping of many giant triangular facets with height of 1.6– between 1.5 and 4.2 Ma when taking into account the sole lowest/- 2 km, is unlikely a sole result of tectonics by normal faulting, but southernmost young AHe data). might be greatly facilitated by rapid fluvial erosion in the Kharta The pre-Quaternary lower exhumation rate is also supported by valley. This is because the time-averaged slip rate across the KF an AFT age of 8.2 Ma (elevation = 5040 m; Liu et al., 2005) and an is only 0.12–0.36 mm/a, assuming that the vertical offset of STDS AHe age of 3.3 Ma (3959 m; Jessup et al., 2008) obtained from a is 1.3–4 km since 11 Ma when the NS horst initiated (Kali et al., leucogranite outcropped at the northern tip of the Ama Drime 2010; Leloup et al., 2010, 2009). This tectonic rate is much lower Range. These data yield an overall AER of 0.45 mm/a between than the exhumation rates suggested by AFT and AHe cooling ages 8.2 and 3.3 Ma, assuming a temperature gap of 40 °C between at the Ama Drime Range, which requires extra surface erosional effective closure temperatures of the AFT and AHe, and a geother- contribution to the exhumation of footwall. The suggested erosion mal gradient of 35 °C/km. episode in the Kharta valley further likely promoted the develop- Acknowledging that AERs over non-vertical transects may pos- ment of many EW sub-valleys sculpting the western flank of the sibly deviate from true exhumation rates (Braun et al., 2012; Valla Ama Drime Range, which progressively migrated the Ama Drime et al., 2010), we suggest that the Ama Drime Range area experi- Rang divide eastward forming asymmetric range flanks. enced a slower exhumation (AER < 0.8 mm/a) prior to 2.5– 1.5 Ma, followed by an abrupt strengthened exhumation 5.3. Driving force of Quaternary incision (AER = 2.6–3.8 mm/a) at between 2.5 and 1.5 Ma. Factors that could enhance channel incision rate include climate (e.g., heavy precipitation, glaciation: Burbank et al., 2003; 5.2. Associated erosional topography Huntington et al., 2006), channel uplift or base-level lowering by tectonics (Brookfield, 1998; Lang and Huntington, 2014), and drai- In this section, we argue that the Quaternary strengthened nage reorganization by capturing (Clark, 2004; Snyder et al., 2003). exhumation has been driven by fluvial process at surface, of which Climate factors such as heavy precipitation and glaciation are rapid channel incision dominated the development of gorge land- often of large spatial scale in regional, of which associated incision forms along reaches from Chentang, Kharta to Yo Ri gorge. This strengthening is likely to occur over the entire drainage. However, channel-incision associated erosional landforms also involve many the rapid incision observed occurs exclusively along the main- EW valleys dissecting the western flank of the Ama Drime range stream, and appears to be radiating peripherally upstream along due to base level lowering at the mainstream. tributaries (Fig. 1c). This is inconsistent with the climate change As noted, the very young cooling ages along the Chentang tribu- of a drainage-scaled precipitation enhancement, which appears tary coincide in space with the high channel gradient zone down- to steepen channel gradients at the whole drainage scale rather stream the knick point (Figs. 1c and 6). According to the stream than locally at certain reaches. Furthermore, glaciation appears to A. Wang et al. / Journal of Asian Earth Sciences 129 (2016) 243–253 251 drive rapid erosion at high altitudes where glacial and periglacial 1996). Coincidentally, both the Zhada (Saylor et al., 2010, 2009) landforms prevail. The spatial coupling between the young cooling and the Thakkhola-Mustang basins (Adhikari and Wagreich, ages and the high channel gradient suggests a fluvial dynamic for 2011; Colchen, 1999) remained closed until Quaternary, when they channel incision instead of glaciation related erosion in high alti- were finally incorporated into the Ganges drainage. These young tudes. Therefore, we conclude that climate change is not the dom- capture events favor that the Himalaya topographic front repre- inating force for the observed incision rate change, although it is a sents a dynamic result between active tectonics and surface most common factor enhancing surface erosion in Quaternary. denudation, and that the majority drainages of trans-Himalaya riv- We propose that the Quaternary rapid incision was most likely ers remain in topographic pre-steady state adjustment. driven in potential by tectonic movements, and was facilitated by a Quaternary capture event. According to the channel gradient response to faulting (Fig. 5 inset), the high channel gradient zone 6. Conclusions of the Kharta-Chentang gorge (Figs. 1c and 5) can be simply explained by an active thrust fault lying around the MCT, where The Phung Chu-Arun River, one of the major trans-Himalaya channel gradient leaps greatly upstream. By southward thrusting rivers, consists of a large drainage captured from the northern along this out-of-sequence fault, the rapid incision and high chan- slope of the Himalaya. Apatite fission track and (U-Th)/He data nel gradient can be well tectonically maintained over long term. provide cooling and exhumation histories during Pliocene- This suggestion is supported by a growing number of ther- Quaternary, and suggest an abrupt increase of fluvial incision rate mochronologic and topographic data, as well as field evidence, occurred at 1.5–2.5 Ma with a four-fold increase in apparent indicating active thrusting in the Central Himalaya, at the foothill exhumation rate. of Higher Himalaya along strike coinciding the physiographic tran- Topographic analysis suggests that the Quaternary rapid inci- sition zone (Harrison et al., 1997; Hodges, 2000; Hodges et al., sion is spatially focused along channels in reaches downstream 2001; Huntington et al., 2006; Wobus et al., 2005, 2003). Indeed, the Yo Ri gorge. Cooling ages, channel morphology and associated without such a tectonic compensation, the Himalaya topographic erosional landforms favor that this Quaternary rapid incision most front could not maintain its high relief and would have disap- likely reflects a Quaternary capture event at the Yo Ri gorge. Sus- peared within 2 Myr at a given incision rate of 2 mm/a. tained out-of-sequence thrusting at the physiographic transition We notice that the abrupt incision rate hike (between 1.5 and zone (near the MCT) is required to serve as a long-term potential 2.5 Ma) appears occurred long after the onset of the out-of- force maintaining high channel gradient and rapid incision across sequence thrusting in regional, which is dated as late Miocene- the Higher Himalaya. Pliocene inferred from thermochronologic data and modeling results (Catlos et al., 2004; Harrison et al., 1997). If the timing of Acknowledgements out-of-sequence tectonics is correct, then an addition triggering force is required for the incision rate hike. Recognizing that the This work was supported by the China Geological Survey Insti- peculiar course of the Yo Ri gorge is unlikely formed by a normal tute [grant numbers 1212011121261, 1212010610103]; and the headward incision progression, but might reflect a capture event Natural Science Foundation of China [grant numbers 40902060, (Armijo et al., 1986), we suggest that the Quaternary incision 41202144]. We express special appreciation to Yanyuan Liu, Jiang- was greatly facilitated by a capture event, rather than solely driven wei Wei, Pengfei Jiang and Wenyuan Zhang who provided assis- by tectonics of thrusting at the physiographic transition zone. In tance in field investigation and in apatite (U-Th)/He analysis. We this capture event, large drainages previously draining the Tibet are also grateful to two anonymous reviewers whose suggestions was redirected to the Arun River, reaches at and below the Yo Ri well improved this paper. gorge subjected strengthened incision due to a significant increase in effective discharge and sediment load. This can be envisaged from the fact that the current drainage area upstream the Moguo References (EW stream dominated) is 2.2 Â 104 km2, which is 1.9 times of its downstream (NS stream dominated). This suggestion is sup- Adhikari, B.R., Wagreich, M., 2011. Provenance evolution of collapse graben fill in ported by the 1.44-Ma young AHe age obtained at the southern- the Himalaya—the Miocene to Quaternary Thakkhola-Mustang Graben (Nepal). Sed. Geol. 233, 1–14. http://dx.doi.org/10.1016/j.sedgeo.2010.09.021. most/lowest of the Kharta valley, and is consistent with the Arita, K., Dallmeyer, R.D., Takasu, A., 1997. 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