Shortening and Erosion Across the Indus Valley, NW Himalaya

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Squeezing river catchments Squeezing river catchments through tectonics: Shortening and erosion across the Indus Valley, NW Himalaya

H.D. Sinclair1,†, S.M. Mudd1, E. Dingle1, D.E.J. Hobley2, R. Robinson3, and R. Walcott1,4 1School of GeoSciences, The University of Edinburgh, Drummond Street, Edinburgh, EH8 9XP, UK 2Department of Geological Sciences/Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, UCB 399, 2200 Colorado Avenue, Boulder, Colorado 80309-0399, USA 3Department of Earth Sciences & Environmental Sciences, University of St. Andrews, Irvine Building, St. Andrews, KY16 9AL, UK 4Department of Natural Sciences, National Museums Scotland, Chambers Street, Edinburgh, EH1 1JF, UK

ABSTRACT lake development. Conglomerates beneath and Molnar, 2001). Similarly, the river catch- some of the modern alluvial fans indicate ments of the Southern Alps of New Zealand are Tectonic displacement of drainage divides a northeastward shift of the Indus River understood to have been deformed to their pres- and the consequent deformation of river channel since ca. 45 ka to its present course ent shape during oblique convergence (Koons, networks during crustal shortening have against the opposite side of the valley from 1995; Castelltort et al., 2012). Tectonically been proposed for a number of mountain the Stok thrust. The integration of structural, induced changes in catchment shape may be fur- ranges, but never tested. In order to pre- topographic, erosional, and sedimentological ther modified by river capture and progressive serve crustal strain in surface topography, data provides the first demonstration of the migration of drainage divides in response to fac- surface displacements across thrust faults tectonic convergence of drainage divides in tors such as variability in rock strength (Bishop, must be retained without being recovered by a mountain range and yields a model of the 1995), changing river base levels (Mudd and consequent erosion. Quantification of these surface processes involved. Furbish, 2005), and ridge-top glaciation (Dortch competing processes and the implications for et al., 2011a). The competition between tec- catchment topography have not previously INTRODUCTION tonic deformation of river catchments and the been demonstrated. Here, we use structural response of the rivers is highlighted across the mapping combined with dating of terrace The topography of active mountain ranges Himalaya, where all of the big rivers are charac- sediments to measure Quaternary shortening records surface uplift in response to crustal terized by steepened reaches and more localized across the Indus River valley in Ladakh, NW thickening countered by erosion (e.g., Dahlen, knick zones as they respond to variable rock Himalaya. We demonstrate ~0.21 m k.y.–1 of 1990). The horizontal velocities that drive uplift fields (Seeber and Gornitz, 1983; Wobus horizontal displacement since ca. 45 ka on crustal thickening are commonly an order of et al., 2006a). The smaller river catchments the Stok thrust in Ladakh, which defines magnitude higher than the vertical velocities, near the foothills of the Himalaya exhibit vari- the southwestern margin of the Indus Val- and so it is expected that this should be recorded able catchment geometries in response to lateral ley catchment and is the major back thrust by the topography (Pazzaglia and Brandon, advection over thrust ramps (Champel et al., to the Tethyan Himalaya in this region. We 2001; Willett et al., 2001; Miller and Slinger- 2002; Miller et al., 2007). Large-scale catch- use normalized river channel gradients of the land, 2006). Model experiments have indicated ment deformation has broad implications for the tributaries that drain into the Indus River to that the broad asymmetry of many small moun- topographic form of active mountain ranges and show that the lateral continuation of the Stok tain ranges, such as the Southern Alps of New the distribution of erosion and transported sedi- thrust was active for at least 70 km along Zealand, the Pyrenees, and ranges in Taiwan, ment to surrounding sedimentary basins. Any strike. Shortening rates combined with fault may be explained by the horizontal translation modification of catchment shape also has impli- geometries yield vertical displacement rates of deforming rock from the side of the range cations for the scaling of upstream catchment that are compared to time-equivalent ero- dominated by accretion toward the opposing area with channel length and hence the long pro- sion rates in the hanging wall derived from side (Willett et al., 2001; Sinclair et al., 2005; file of rivers (Whipple and Tucker, 1999; Willett published detrital 10Be analyses. The results Herman and Braun, 2006). et al., 2014). demonstrate that vertical displacement rates It is reasonable to suggest that such large- Fluvial erosion into bedrock can be approxi- across the Stok thrust were approximately scale forcing of topography must also play a mated by a power-law relationship between twice that of the time-equivalent erosion role in determining the geometry of river catch- channel slope and river discharge (Howard rates, implying a net horizontal displacements and their channel courses. At the largest et al., 1994; Whipple and Tucker, 1999). In this ment of the surface topography, and hence scale, it is proposed that the extraordinarily stream power model, the fault offset generates narrowing of the Indus Valley at ~0.1 m elongate forms of the rivers draining eastern an oversteepened channel reach (knickpoint or k.y.–1. A fill terrace records debris-flow em- Tibet (Salween, Mekong, and Yangtze) repre- knick zone) that migrates upstream as a kine- placement linked to thrust activity, result- sent highly strained forms of previously more matic wave. Additionally, the model predicts ing in damming of the valley and extensive regularly shaped catchments in response to dis- that sustained differential rock uplift across a tributed crustal shortening and rotation around fault will generate increased channel steepness †hugh.sinclair@ed.ac.uk the eastern corner of the Indian indentor (Hallet (for a given upstream area) on the upthrown

GSA Bulletin; Month/Month 2016; v. 128; no. X/X; p. 1–15; doi: 10.1130/B31435.1; 11 figures; Data Repository item 2016164.; published online XX Month 2016.

GeologicalFor Society permission of to America copy, contact Bulletin [email protected], v. 1XX, no. XX/XX 1 © 2016 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Sinclair et al. block. Analysis of channel steepness has been sedimentological data provides the first quanti- The Indus molasse records sedimentation in a used to assess fault activity in mountain ranges fication of the tectonic convergence of drainage forearc basin that evolved into an intramontane (e.g., Hodges et al., 2004; Kirby and Whipple, divides in a mountain range and yields a model basin following continental collision (Garzanti 2012), with relative rock uplift in the hanging of the surface processes involved. and Van Haver, 1988; Searle et al., 1990; Sin- wall of a thrust fault leading to increased stream clair and Jaffey, 2001). The Ladakh batholith power generated by channel steepening. REGIONAL BACKGROUND forms part of the Gangdese batholith complex Little is known of the interaction between at the boundary between the northern mountains thrust shortening and the consequent deforma- The Indus River of Ladakh flows northwest- of the Himalaya and the Tibetan Plateau. It rep- tion of catchment shape, as opposed to the off- ward (Fig. 2) between the highly deformed resents the magmatic arc prior to continental set of individual channels by faults. As yet, there Cretaceous to Miocene sediments of the Indus collision and consists of a succession of grano- has been no demonstration of the horizontal molasse, which are thrust northeastward against dioritic rocks overlain by a volcanic succession convergence of drainage divides in response to the relatively undeformed Cretaceous and Paleo- that forms the southern wall of the Shyok Valley shortening on a thrust fault that bisects a catch- gene Ladakh batholith complex (Figs. 2 and 3). to the north (Weinberg and Dunlap, 2000). ment. The challenge that this sets is the require- ment to quantify both the shortening and the time-equivalent erosional response. A The objective of this study is to test whether SW NE rates of horizontal displacement across a thrust drainage drainage fault are capable of driving the horizontal con- divide vergence of opposing drainage divides when divide moderated by the erosional response to fault Mean topographic displacement. We examine the Indus River val- slope (α) ley in Ladakh, NW India, which is one of the x x x largest longitudinal river catchments of the x x x Himalaya, with an average width of around x Indus x x x 35 km and a length of ~200 km parallel to the x x x River x x x mountain range in this region. The aim is first to River long x x x x x x x test for the presence of active shortening across x x x proﬁle x the Indus Valley, which has never been demon- x x x x x strated. This is regionally significant because the valley follows the line of the main back thrust in the region, carrying Tethyan Himalaya units northeastward toward the Gangdese batholith (van Haver, 1984; Searle et al., 1990). Large B portions of the Indus and Tsangpo Rivers further east in the Himalaya also follow this struc- Displacement of thrusted tural feature. Thrust displacement rates were α hanging-wall measured using mapped and dated alluvial and lacustrine terraces, and by documenting dis- V v placement of these terraces across faults. Sec- ond, having presented evidence for Quaternary deformation, we compare the vertical compo- V nent of rock displacement in the hanging wall V of the main back thrust relative to the magnitude β h of erosion at similar time scales (Fig. 1); this is this ratio that determines the signal of topographic change across the valley. Erosion rates For measured horizontal displacement rate (Vh), vercal are presented using published low-temperature displacement rate (Vv) = Vh(tanα + tanβ) thermochronology (Kirstein et al., 2006, 2009) and detrital cosmogenic nuclides (Dortch et al., Figure 1. Cartoons illustrating the mechanics of rock displacement by a thrust fault bound- 2011a; Munack et al., 2014; Dietsch et al., ing a longitudinal valley and the erosional response required to sustain a steady-state topog 2014). In addition, the distribution of changing raphy. (A) Schematic cross section across the Indus Valley. (B) A geometric representation erosion rates in response to thrust displacement of the Indus Valley enabling the application of trigonometric relationships between main is inferred regionally through an analysis of parameters. For a horizontal displacement rate across the fault (Vh), the vertical displace- river channel steepness of catchments that drain ment rate (Vv) at any point in the hanging wall is a function of the slope of the thrust plane into the Indus Valley. Third, sedimentological (β) and the mean topographic slope (α). In order to retain a steady-state topography fol- evidence for valley damming in response to lowing shortening, the vertical rock displacement must be countered by an equal amount of fault movement, and for the migration of the erosion (gray shaded area). For a topographic narrowing of the valley to occur, the vertical main Indus channel, is presented. The integra- displacement rate must be greater than the mean erosion rate on similar time scales in order tion of structural, topographic, erosional, and to sustain a component of the horizontal displacement and translation of the drainage divide.

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Figure 2. (A) Regional setting of study, showing the cross Aʹ Fig. 3 Tibetan section A-A′ in part B and the region in Figure 3. (B) Re- Plateau A 30°N A gional cross section A-A′ through the northwestern Hima laya showing the geological setting of the upper Indus River valley. The Stok thrust (Fig. 3) represents the major northeastward-vergent back thrust immediately southwest of the Ladakh batholith; this thrust is comparable in structural context to the Great Counter thrust recorded further 0 500 km east (e.g., Murphy and Yin, 2003). MBT—Main Boundary 75°E 90°E thrust; MCT—Main Central thrust.

B A Aʹ Stok Thrust

75°E km

The Indus molasse of the Stok Range is that carries steeply tilted Miocene molasse suc- the youngest sediments around 20 Ma (Sinclair intensely deformed, with fold-and-thrust struc- cessions in its hanging wall over Quaternary and Jaffey, 2001). The extent to which deforma- tures verging to the northeast and southwest. alluvial-fan deposits; we term this the Stok tion has continued since this time has not been At the boundary with the Ladakh batholith, the thrust (Fig. 3), which laterally correlates to documented. Cretaceous succession locally onlaps the margin the Great Counter thrust further east (Murphy The Ladakh batholith contains crystallization of the batholith (Van Haver, 1984), but the main and Yin, 2003). The bulk of deformation of the ages ranging from ca. 103 to 47 Ma (Honegger topographic boundary is defined by a thrust fault Indus molasse has occurred since deposition of et al., 1982; Weinberg and Dunlap, 2000), and it is overlain by a volcanic succession along 77.2°E 77.4°E 77.6°E 77.8°E its northern margin that is tilted steeply northeastward (Weinberg et al., 2000). This rotation is thought to have occurred in the hanging wall Karakorum Fault of a thrust fault that dips northeastward under the batholith, and which was active during early Miocene times (Kirstein et al., 2006); this struc- 34.2°N ture is comparable to the Gangdese thrust near Phey Lhasa (Yin et al., 1994). Thermochronologi- 34.2° Leh cal analyses using apatite and zircon U-Th/He dating and apatite fission-track dating indicate 40 Fig. 4A rapid cooling of ~25 °C/m.y. around 22 Ma, followed by a deceleration to rates <3.5 °C/m.y. 80 since then (Kirstein et al., 2006). Detrital cosmogenic 10Be analysis across 34.0°N 35 85 the Ladakh batholith indicates erosion rates of 34.0°N ~0.04–0.09 m k.y.–1 for the main tributaries on the northeastern side and 0.02–0.05 m k.y.–1 on the southwestern side (Dortch et al., 2011a; Munack et al., 2014). Smaller, side tributaries 42 on the southwestern side of the batholith record Hemis –1 33.8°N rates as low as 0.008 m k.y. (Dietsch et al., 60 2014); these represent the slowest rates recorded 20 km from the Himalaya. These measurements aver- 77.2°E 77.4°E 77.6°E 77.8°E age over tens of thousands of years and record an asymmetry in erosion rates associated with Figure 3. Hillshade image of the Indus Valley in the Ladakh region with principal geological greater degrees of glaciation on the northern features shown. The Ladakh batholith is highlighted by a lighter transparency. The drain- side of the Ladakh batholith driving glacial age divides that define the margins of the Indus Valley are shown with thick dashed lines. headwall erosion and migration of the drainage White stars are the location for published apatite fission-track samples (Kirstein et al., 2006, divide toward the southeast over this time period 2009; Clift et al., 2002). The locations of Figures 4A and 8 are shown by dotted and dashed (Jamieson et al., 2004; Dortch et al., 2011a). lines, respectively. Small (~1-km-long) glaciers are still present at

Geological Society of America Bulletin, v. 1XX, no. XX/XX 3 Sinclair et al. the drainage divide around 5500 m elevation, 77.5° E with significant glacial erosion having occurred A down to ~4700 m on the southwestern side of the batholith (Hobley et al., 2010). Dating of 31.0±0.7 33.4±1.2 boulders on moraines in the Ladakh region has 40.3±1.3 demonstrated multiple glaciations in this region 50.8±5.4 (Owen et al., 2006; Owen and Dortch, 2014), with the oldest significant glaciation being ca. 35.6±2.7 300 ka (Dortch et al., 2013). On the southwest- 40.0±5.2 ern margin of the Indus Valley, erosion rates from the Indus molasse successions of the Stok

Range are faster than on the batholith, with 10Be Spituk concentrations implying millennial erosion rates Fig. 5B –1 Fig. 5C of 0.07–0.09 m k.y. (Munack et al., 2014). Fig. 5A In the Leh region of the valley, the northwest- y 11.7±0.7 Markha erly flowing Indus River is bounded by large Valle 24.7±1.7 19.1±0.7 alluvial fans draining the Indus molasse from the 22.0±1.3 southwest. These fans appear to force the pres- 34.1° N ent river channel to bank up against the interfluve ridges of the batholith to the northeast (Fig. 3). A terrace containing evidence of lake sedimentation forms the distal margin of these alluvial fans (Fig. 4), and other terraces in the valley testify to B a history of damming of the Indus River (Bür- gisser et al., 1982; Fort, 1983; Phartiyal et al., T1 terrace ﬁll 2005; Blöthe et al., 2014). The presence of broad regions of alluvium in the lower reaches of the T2 terrace ﬁll tributaries draining the batholith (geomorphic domain 3 of Hobley et al., 2010, 2011) encour- aged Jamieson et al. (2004) to suggest that an asymmetry in deformation and erosion across the Indus Valley has resulted in northeastward trans- Figure 4. (A) Detailed hillshade image of the lower Leh Valley using 30 m, 1 arc second lation of the valley over the batholith. However, Shuttle Radar Topography Mission (SRTM) data. White areas record exposures of the upper evidence for ongoing structural deformation and T1 terrace fill, and dark areas record exposures of the lower T2 terrace fill. The reconstructed relative displacement of the Indus molasse has lake level at the time of the end T1 terrace fill is shown as a dotted line. Dated ages (ka) used not been recorded (Dortch et al., 2011a) and is in this analysis are shown in light boxes: normal text from the eastern exposures near Spituk therefore a key focus of this study. As the valley shows radiocarbon ages from Phartiyal et al. (2005); italicized numbers show ages generated is traced northwestward from the village of Phey, from optically stimulated luminescence (OSL) analysis in this study; underlined age in the 10 so the river’s course cuts a large gorge into the west represents a Be exposure age from Dortch et al. (2011b). (B) Lateral tracing of the T1 deformed molasse, and the long profile exhibits (unfilled circles) and T2 (gray triangles) terrace fills from Spituk in the east to the Markha a broad steepening downstream of the alluviated Valley junction in the west (Fig. 4A). These data were generated using a laser range finder reach in the Leh Valley (Jamieson et al., 2004). plotted relative to the height of the modern river (filled squares).

EVIDENCE FOR QUATERNARY the edge of ancient lakes; this sedimentologi- of the valley (Fig. 4). Field mapping of terrace SHORTENING cal transition is associated with a geomorphic successions using a laser range finder was sup- Fan Terrace Data break recording the approximate coastline of ported by Google Earth satellite imagery and the paleolake. In order to distinguish these fea- the 1 arc second Shuttle Radar Topography- Geomorphic fill terraces usually record tures from classic fill terraces (e.g., Wegmann Mission digital elevation model (DEM); 1 arc abandoned floodplain surfaces that parallel and Pazzaglia, 2009), we refer to these as “fan second equates to approximately 30 m horizon- the modern river channel, and they can usu- terraces.” One of the best documented sections tal resolution. The top surface of the higher fan ally be correlated across the landscape, and through a fan terrace succession is in the Spituk terrace (T1) is at an average elevation of around so can be used to assess evidence of ongoing region near Leh, where radiocarbon dates yield 3250 m and represents the dissected remnant deformation since formation (e.g., Lavé and ages from ca. 51 to 31 ka (Phartiyal et al., of an alluvial fan, with lacustrine sediments at Avouac, 2001; Pazzaglia and Brandon, 2001; 2005). Several terrace successions also con- a downslope break in topography (Figs. 4 and Wegmann and Pazzaglia, 2009). The terraces tain extensive soft sediment deformation that 5). A lower fan terrace succession is capped in the Leh region of the Indus Valley represent has been interpreted as a record of seismicity by a surface (T2) at around 3200 m elevation the abrupt downslope termination of alluvial- throughout the region (Phartiyal and Sharma, and is evident throughout the region. This level fan surfaces into a 20–80 m succession of bed- 2009). We mapped two terrace fill successions forms the break of slope between alluvial fans ded sandstones and laminated siltstones that around the northwestern part of the Leh Valley that drain the Stok Range and the modern Indus record floodplain and shoreline settings around that could be correlated across the two sides River floodplain in the Leh Valley (Fig. 4).

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The sedimentology of the T1 infill is best A exposed around Spituk (Fig. 4), where at least Lake sediments within 50 m of silts, sands, and gravels are present (Fig. Ladakh batholith T1 terrace ﬁll DR11), recording marginal lake sedimentation (Bürgisser et al., 1982; Phartiyal et al., 2005). The lower portions of the section are dominated T1 by coarse-grained, fining-upward event beds Indus molasse delivered from marginal deltaic feeder systems. This thick succession underlying the T1 surface can be traced at the same elevation downstream for at least 10 km (Fig. 4B). The lower T2 infill T2 is exposed in the cliffs on the southwest side of the valley opposite Spituk. This succession is ~20 m thick and dominated by poorly bedded T2 Indus River coarse gravels and breccias typical of alluvial- fan sedimentation. Approximately 2–4 m below the fan surface, there is a succession of well- bedded, fine to medium sands with some planar lamination, and some evidence of rootlets, grass B blades, shells, and other organic material. There is also a 40 cm unit of finely laminated siltstones, similar to the lacustrine deposits of the T1 fill (Fig. DR2 [see footnote 1]). This interval is interpreted as an episode of lacustrine and Indus molasse marginal floodplain sedimentation that defined the base level for the alluvial fans that drain the Stok Range (Fig. 4A). In contrast to the T1 fill a succession, downstream tracing of the T2 ter- Stok thrust race fill demonstrates a reduction in elevation Dissected T1 surface that is parallel, but ~25 m above the modern Indus River. As the Indus River continues downstream to the northwest, so it changes course from flow- Indus River ing at the boundary between the Indus molasse Lacustrine sediments conglomerates and the Ladakh batholith to flowing within, and within alluvial fan b along the strike of the Indus molasses, where it forms a steep gorge (Figs. 4 and 5A). On C a b either side of this gorge, the two terraces fills 500 m are clearly visible, with the T1 fill characterized 3500 Indus River conglomerates underlying alluvial fan by light, cream-colored lake sediments, and T2 3400 3300 Dissected Modern Indus having a more pink tone where the sediment T2 surface forms a bench in the gorge. Near to the turn- 3200 River valley ing for the Markha Valley, the T1 fan terrace is Lacustrine silts deformed by thrusting, folding, and extensive Figure 5. (A) View up the Indus River valley from the junction with the Markha Valley. Two irregular soft sediment deformation (Fig. 6). At terraces are evident at this location—a lower bench representing the younger T2 terrace the southwestern extent of the terrace, it is over- marked by dots and characterized by a pinky cream siltstone, and the upper T1 terrace, thrust by the Indus molasse on a fault dipping which contains a lacustrine deposit (labeled) and forms the dipping fan surface in the middle 37° to the southwest. Thinly bedded alluvium ground above this deposit. The far mountains are part of the Ladakh batholith. (B) View is folded into a broad syncline in the footwall over the dissected T1 terrace surface immediately east of the Markha Valley junction. Sec- of the fault with a wavelength of ~200 m (unit tion a-b shown in C is located. (C) Topographic cross section a-b across Stok thrust showing 1; Figs. 6 and 7). Within this lower succession, exposures of conglomerates deposited by an older Indus River channel draped by modern there are meter-scale thrust faults and folds that alluvial-fan sediments sourced from the Indus molasse. are draped by overlying beds and hence are syndepositional. An unconformity divides this folded succession into two, recording a phase the final phase of folding. These folded allu- by finely laminated pale siltstones that are inter- of erosion and renewed sedimentation prior to vial sediments are truncated by a structureless preted as lake deposits (unit 3; Figs. 6 and 7). breccia with meter-scale blocks of the Indus These siltstones are capped by gravels of the 1GSA Data Repository item 2016164, OSL methodology and data, is available at http://www molasse that is interpreted as a surficial debris- abandoned T1 alluvial-fan surface (unit 4; Figs. .geosociety.org /pubs /ft2016 .htm or by request to flow deposit that ranges from 2 to 5 m thick 6 and 7); this surface has since been dissected editing@geosociety.org. (unit 2; Figs. 6 and 7). This debris flow is draped by a dense network of modern river channels

Geological Society of America Bulletin, v. 1XX, no. XX/XX 5 Sinclair et al.

~50 m

B unit 4 unit 3 unit 3 Indus molasse unit 2

C unit 4 unit 3 Indus molasse unit 2

unit 1

Figure 6. Deformed Quaternary terrace sediments near Markha Valley junction (Fig. 4). (A) Photographic montage of T1 terrace fill exposed along the road track (note circled small car for scale). The four stratigraphic units that make up the terrace are described in the text. (B) Drawing of photograph in A showing location of optically stimulated luminescence (OSL) samples (dots) and ages. Circled numbers refer to stratigraphic units labeled in A. (C) Projected section through the terrace fill enabling total shortening to be calculated, where each component of faulting and folding is accounted for with a length in meters. The lower unit 1, composed of bedded alluvial gravels, contains an unconformity recording the progressive motion on the thrust during this interval. The last stage of deformation is truncated by the debris flow (unit 2), which is then draped by lacustrine sediments (unit 3). Figure 7 is a measured sedimentary section through this succession.

(Fig. 5B). In comparison, the lower T2 fill is date the deformed T1 terrace sediments near the surements, and the remaining material was used undeformed. Markha Valley using optically stimulated lumi- for dating. Analysis of luminescence behavior, These exposures are interpreted as a record nescence (OSL) on quartz, and to compare this dose rate estimation, and age calculations were of syndepositional thrust faulting that caused against the range of radiocarbon ages at Spituk, conducted at University of St. Andrews using progressive deformation of Indus Valley allu- and against new ages for the other terraces. the protocol outlined in King et al. (2013). vium. Coseismic slope instability resulted in The analytical details and results (with tables extensive landslide material, which was sub- OSL Methodology and figures) are presented in the supplemen- sequently reworked during a storm to deposit We collected 20 samples of medium- and tary material (see footnote 1). Only 17 of the a debris flow that dammed the valley, leading fine-grained sand and silt layers for OSL dating 20 samples were dated, and two ages are based to lake formation. Folding and intraformational of quartz grains, and most samples were derived on a low number of aliquots (Zansk2011-1 and unconformities in the footwall of the thrust indi- from units that were interbedded with coarse- Nimmu2011-1). cate that this records fault propagation folding grained or conglomeratic deposits of fluvial and with associated growth strata (e.g., Suppe et al., alluvial-fan origin (see supplementary mate- OSL Results 1992). Soft sediment deformation has been rec- rial for full description [see footnote 1]). Other T1 terrace fill. The four samples from the ognized elsewhere in this T1 terrace fill, as well deposits that were sampled record lacustrine deformed T1 terrace succession near the Markha as a fault offset between the batholith granites environments, and reworked horizons overly- junction generated ages, in ascending strati- and lake sediments near Spituk (Phartiyal and ing mass-flow deposits. Samples were collected graphic order, of 35.6 ± 2.7 ka, 73.0 ± 0.7 ka, Sharma, 2009). in copper tubes (2.5 cm diameter, 12 cm long) 40.0 ± 5.2 ka, and 77.2 ± 11.7 ka (Figs. 6 and 7); The exposures in the region of Spituk, near that were tapped into the target deposits parallel given the observed stratal sequence, these cannot Leh (Fig. 4), were dated using four radiocar- to the stratigraphic orientation. The tubes were all represent true depositional ages. Having con- bon ages that range from ca. 50.8 ± 5 ka at the sealed with black tape to avoid light penetra- fidently correlated the T1 succession from Spituk base to ca. 31.0 ± 0.7 ka near the top (Fig. 3; tion and to minimize any moisture loss within to the Markha junction (Fig. 4B), we would Phartiyal et al., 2005). Given the significance of the tubes. At least 2 cm of sediment from both expect the ages to fall within the time interval thrust shortening of the T1 terrace, we chose to ends of each tube were used for dosimetry mea- of 50.8 ± 5.4–31.0 ± 0.7 ka based on the radio-

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A B 60m Angular gravels of modern Modern dissected alluvial fan alluvium unit 4

Finely laminated pink/ 50m cream siltstones with Lake interbedded micaceous sedimentaon ﬁne sandstone. unit 3 40m 40m 30.980±0.69 ka

Thick unit with erosive base, Debris flow 33.440± Finely laminated pink/ Suspension fall-out angular blocks of Indus 30m truncang 30m 1.16 ka cream siltstone, into lake with molasse up to 2 m. Muddy unit 2 lted gravels interbedded fine intermient delivery gravel matrix. sandstone with cross from rivers. Planar bedded gravels laminated tops and vc sandstone. 20m Alluvial gravels 20m 40.33±1.33 ka Homogeneous pink to sourced from 40.0±5.2 ka cream siltstone, locally Indus molasse 0.5–1m thick graded sst. laminated with thin gravelly with intermient beds Extensive so 10m unit 1 interbeds 10m River-fed hyperpycnal . ponding of water 50.79± sediment deformaon Parallel strafied pebbly flows carrying sands gravels and find sst. 5.37 ka 35.6±2.7 ka Interbedded medium into lake. Interbeds. Clasts rounded sandstone. Beds with 0m and angular up to 30cm. 0m laminated pink and si ms gr Very poorly sorted. si ms gr cream mudstone/ Imbricated clasts indicate siltstone. eastwardly flow.

Figure 7. Sedimentary sections through the T1 terrace at the Markha junction and Spituk. (A) Sedimentary section through the T1 fill exposures near the Markha Valley junction illustrated in Figure 6. The succession records the impact of thrust activity on the Stok thrust, which caused progressive deformation of unit 1 and the ultimate emplacement of a mass-flow unit of Figure 2 that resulted in damming of the valley and lake formation (unit 3). The two starred ages are the optically stimulated luminescence (OSL) ages that were complementary to the radiocarbon ages from Spituk (Phartiyal et al., 2005); sst—sandstone. (B) Approximately time-equivalent sedimentation at the Spituk site recording subaqueous deposition dominated by laminated lake sediments punctuated by event beds that record hyperpycnal discharge from the mountain rivers. The black stars show sites of radiocarbon ages (Phartiyal et al., 2005). carbon ages at Spituk (Phartiyal et al., 2005). In terrace levels downstream near Nimu and Basgo shortening of at least 9.5 m. Based on the total order to be confident of the OSL correlation to suggest that this was a period of widespread horizontal displacement (folding and faulting) the radiocarbon ages, we also ran a sample from sediment aggradation throughout this part of the since deposition of unit 1 of 9.5 m, and the old- the top of the Spituk T1 succession and obtained Indus Valley. est age for the deformed alluvium of 45.2 ka, we an age of 27.5 ± 3.0 ka, which is within error of estimate a mean shortening rate from that time the radiocarbon age. Consequently, our interpre- Horizontal Displacement Rates to the present of at least 0.21 m k.y.–1. tation of the ages at the Markha junction locality Based on the stratigraphic location of the T1 is that the two ages that are significantly older Markha junction samples (Fig. 6B), deforma- Topographic Expression of Shortening than the radiocarbon age bracket record age tion of this succession must have started during overestimation. Inheriting older ages is common accumulation of the alluvial deposits of unit 1 Whether the activity on the Stok thrust was in fluvial systems where sediment grains were with ages of 35.6 ± 2.7 ka and 40.0 ± 5.2 ka; in localized or regional is significant in the context not fully exposed during transport and deposi- order to convey conservative estimates of short- of its impact on orogenic topography. Therefore, tion, meaning that their luminescence “clocks” ening rates, we use the oldest possible age for we used fluvial topography to test the lateral had not been reset (incomplete bleaching; Wall- unit 1 of 45.2 ka. The horizontal shortening at extent of thrust activity in the Indus molasse inga, 2002). This is particularly common where the Markha junction locality was measured from (e.g., Kirby and Whipple, 2012). coarse sands were deposited by short-lived, tur- three components of the deformation (Fig. 6C): It has been recognized for over a century bid flows and mass flows, which are typical in (1) the minimum value of 2.3 m of horizontal that erosion rates in bedrock channels should alluvial-fan settings. displacement of the Indus molasse at the thrust increase with increasing channel gradient and T2 terrace fill. The T2 terrace fill was sampled (the molasse is not observed in the footwall of water discharge (e.g., Gilbert, 1877). If other on the opposite side of the valley from Spituk this thrust, so the value could be much greater); factors are equal, for example, rock hardness at the margins of the large alluvial fans that dip (2) a line-length restoration of the folding in the or local uplift rates, channel gradients should gently northward into the Indus Valley (Fig. 4). lowermost unit 1, beneath the unconformity, decrease as discharge (or its proxy, drainage The samples were taken ~20 m above the mod- amounting to 4.5–5.5 m of shortening; and area) increases, and so any topographic analysis ern floodplain and consisted of sands and grav- (3) a line-length restoration of the folding of the that uses channel gradients as a proxy for ero- els with finer-grained intervals (Fig. DR2 [see upper part of unit 1, totaling 2.0–2.5 m, which sion rates must take into account drainage area. footnote 1]). The three samples (Dung2011-01, was then truncated by the debris flow (Fig. 6C). Several authors have used a scaling relationship Dung2011-02, and Dung2011‑03) yielded ages The errors in these line-length restorations are to explore changes in erosion rates along bed- –q of 22.0 ± 1.3 ka, 19.1 ± 0.7 ka, and 11.7 ± 0.7 ka based on uncertainties in projected fold limbs. rock channels: S = ksA , where S is the topo-

(Table DR3 [see footnote 1]). Similar ages rang- By combining the midpoint values of these three graphic slope, ks is a steepness index regressed ing between 22.0 ± 1.3 ka and 8.8 ± 0.8 ka from shortening measurements, we obtain a total from slope and area data, A is the drainage area,

Geological Society of America Bulletin, v. 1XX, no. XX/XX 7 Sinclair et al. and q describes the rate of change of slope or [dimensionless] that Mudd et al. (2014) called analytical work of Royden and Perron (2013), concavity of the long river profile, (Wobus et al., Mc, or the gradient in c-elevation space: demonstrating that the chi method could dis-

2006b). If q is set to a fixed value, the steepness 1/ n tinguish varying erosion rates in transient land- index k becomes a normalized steepness index,  E  scapes. Changes in M may be due to factors s Mχ =  m . (4) c K(A0)  ksn, and this index has been applied to a num- other than changing erosion rates; for example, ber of regions of active tectonics; importantly, changes in channel erodibility could force In channels with fluvial incision that can be it can identify differential rock uplift fields that changes in M . The Mudd et al. (2014) method described with the stream power law (Eq. 2), the c are bordered by faults that have not been histori- is agnostic with regards to the cause of chang- gradient in c-elevation plots is closely related to cally active, and so aid seismic hazard aware- ing M values, it simply finds segments with the normalized channel steepness (k ), which c ness (Kirby and Whipple, 2012). However, the sn different M values that may be differentiated is simply the steepness index (k ) that has been c selection of q and identification of reaches with s statistically. calculated using a reference concavity, q (e.g., statistically different values of k can be diffi- Because channel steepness using M or k is sn Hodges et al., 2004; Kirby and Whipple, 2012): c sn cult with noisy slope and area data. viewed through the lens of a detachment-limited 1/n Our topographic analysis of river long pro- U m/n incision model (i.e., Eq. 2), channels compared M   k A− . (5) files normalizes for drainage area by integrating χ =  m = sn 0 with these methods should be detachment and K(A0)  drainage area over flow distance. This method, not transport limited. Channels to the south of first suggested by Royden et al. (2000), pro- Other models have been proposed for chan- the Indus, upstream of the fans, are bedrock, duces a transformed coordinate, c (chi), which nel incision, including those that incorporate but channels to the north of the Indus contain has dimensions of length (Perron and Royden, the role of sediment supply (Sklar and Dietrich, coarse sediment (Hobley et al., 2010). However, 2013). The elevation of the channel can then be 1998) and erosion thresholds (e.g., Snyder et al., Hobley et al. (2011) demonstrated through both plotted against the c coordinate, and the gra- 2003). However, even if the stream power inci- field measurements and numerical modeling dient of the transformed profile inc -elevation sion model is an imperfect description of channel that the evolution of these channels could be space provides a steepness indicator that can be incision (cf., Lague, 2014), Gasparini and Bran- best described with a detachment-limited model used to compare channel segments with differ- don (2011) demonstrated that Equation 2 works of channel incision, and should therefore be ent drainage areas. as an approximation of the proposed incision amenable to Mc or ksn analysis.

The transformed coordinate is calculated with model. At a minimum, both Mc and ksn can still To calculate both segments and Mc values, the be calculated and allow a qualitative compari- transformation of Equation 1 requires values for x m/n  A0  χ= dx, (1) son of the steepness of channel segments rela both A0 and m/n to be selected. The reference ∫xb A(x)   tive to their upstream area from different parts drainage area simply scales c, so it changes the where x [length] is the flow distance from the of the channel network. Both chi-analysis and absolute of Mc but not relative values. The m/n outlet, with dimensions henceforth denoted as the normalized steepness index (ksn) have been ratio is, on the other hand, determined statisti- found to correlate well with erosion rates in the cally, following the method of Devrani et al. length [L] and time [T] in square brackets, xb [L] is the flow distance at the outlet, A [L2] is Yamuna River, which is a basin to the south of (2015), in which target basins are selected (in 2 Ladakh (Scherler et al., 2014), and many other our case, 14 basins), and in each basin, 250 sen- the drainage area, A0 [L ] is a reference drainage area introduced to ensure the integrand is studies have shown a correlation between steep- sitivity analyses are run in each of the 14 basins dimensionless, and m and n are empirical con- ness index and erosion rate (Ouimet et al., 2009; to determine the range of m/n ratios in each basin stants, and where –m/n = q. DiBiase et al., 2010; Miller et al., 2013; Kirby and to determine a regional m/n value to be used The choice of the integrand in Equation 1 is and Whipple, 2012; Harel et al., 2016). in calculating Mc values. Of the 14 basins, seven informed by a simple model of channel incision We use a method developed by Mudd et al. were located in the Ladakh batholith, and seven called the stream power model (e.g., Howard (2014) to determine the most likely locations in the molasse, and in each, we ran the collinear- and Kerby, 1983; Whipple and Tucker, 1999): of channel segments, defined as sections of the ity algorithm of Mudd et al. (2014). This algo- channel that have distinct slopes in chi-elevation rithm both selects the most likely segments in E = KAmSn, (2) space. Our method is aimed at using channel the channel network and also seeks to maximize profiles to infer differences in erosion rates and the collinearity of the main stem and tributaries, where E [L T–1] is the erosion rate, S [dimen- thus has the same objectives as previous studies which is expected if the most plausible m/n is sionless] is the slope, and K is an erodibility that used (k ). The chi method differs from pre- selected (Perron and Royden, 2013). The Mudd coefficient with dimensions that depend on the sn vious studies based on slope-area information in et al. (2014) algorithm takes several parameters: exponent m. Royden and Perron (2013) demon- that it uses elevation rather than slope, which is the minimum length of a segment, the maxi- strated that in landscapes where channel inci- less noisy (e.g., Perron and Royden, 2013). mum number of nodes in a subsection of profile sion could be described by Equation 2, changes The Mudd et al. (2014) method tests all pos- to be analyzed, the uncertainty of the elevation, in erosion rates at the base of channels would sible contiguous segments in a channel network and a parameter that determines how frequently result in upstream-migrating “patches” or seg- and selects the most likely segment transitions sections are sampled by a Monte Carlo sam- ments of constant slope in -elevation space, c using the Akaike information criterion (AIC; pling routine (each m/n analysis is the result of given constant bedrock erodibility and local Akaike, 1981), which is a statistical technique several hundred resampling runs of the channel uplift rates. These segments can be described by: that rewards goodness-of-fit while at the same network). The m/n ratio can change depending E 1/ n time penalizing overfitting. Mudd et al. (2014) on these parameters, so we ran 250 iterations z x B   , (3) ( ) = χ +  m χ used both field examples and numerical models across parameter values and selected the most K(A0)  to show the method could distinguish channel likely m/n ratio. Thus, the m/n ratio was selected where z(x) [L] is elevation. Equation 3 is a linear segments of varying erosion rates via detection based on 250 × 250 × 16 implementations of the equation with an intercept of B [L] and a slope of varying Mc values; their results followed the Mudd et al. (2014) algorithm.

8 Geological Society of America Bulletin, v. 1XX, no. XX/XX Squeezing river catchments

In the Ladakh region, it has been previously ated catchments (Fig. 8B), we derived a range tributaries that drain from the glaciated drainage noted that river concavity (i.e., m/n) varies of concavity values with a mean of 0.4 for both divide of the Ladakh and Stok Ranges. These in relation to the degree of upstream glacia- the Indus molasse and the Ladakh batholith; variable Mc values link directly to the three geo- tion (Hobley et al., 2010), which suggests that i.e., there was no significant difference between morphic domains associated with glacial ero- local channel slopes are not a simple function them. Once we determined the regional m/n sion, incision into glacial moraines, and alluvial- of rock uplift or lithology. In order to avoid the ratio, we then applied this to all the river net- fan growth identified by Hobley et al. (2010). influence of glaciation, we selected those catch- works in the region to map Mc values of chan- Therefore, these larger catchments were not ments where moraines, valley widening, and nels draining into the Indus from both the north used for the evaluation of variable erosion rates channel slope reduction due to glacial erosion and south. across the region; we speculate that the variation were absent—these being the characteristics The channel steepness for all rivers across may be linked to the sediment flux–dependent of the upper glaciated domain of Hobley et al. the region demonstrated a high degree of vari- channel incision processes documented in a (2010). Based on 14 of these smaller, nonglaci- ability (Fig. 8A), particularly within the larger number of these valleys by Hobley et al. (2011).

34°10′0″N 78°0′0″E

Figure 8. Analysis of river steepness using the chi parameter for catchments draining both sides of the Indus Valley.

34°10′0″N (A) Mχ values for all catchments showing highest values in glaciated upper reaches and lowest in alluvial stretches near valley floor, calculated using A θ = 0.4. (B) Catchments selected where there is no impact 34°10′0″N 78°0′0″E of glaciers, and where channel gradient is solely a function of

fluvial processes. Mχ values for these are plotted in Figure 9C, where the data from each num- bered catchment are identified. Black line indicates Stok thrust overthrusting to northwest. Dashed lines represent drainage divides. 34°10′0″N

Geological Society of America Bulletin, v. 1XX, no. XX/XX 9 Sinclair et al.

However, the smaller unglaciated catchments, Indus, because if this were the case, the vari- be driving the difference in channel steepness, 10 which range from 4 to 18 km in length, provide ability in Mc would be mirrored in the Ladakh but Be erosion rates are significantly higher in

Mc values that can be compared throughout the batholith. Variability is more likely caused by the molasse, suggesting that differential erosion region (Fig. 8B). changes in channel erodibility or changes in plays a role in setting the differences in channel

We compared Mc values for opposing catch- local erosion rates. Changing drainage areas due steepness across the two geologic units. ments on either side of the Indus River valley to divide migration can also alter Mc values, but We then turn our attention to possible struc- (Fig. 9C), which, due to the proximity of their changing drainage area is unlikely to cause dis- tures (i.e., the Stock thrust) within the molasse outlets, have the same local base level (i.e., the continuities in middle reaches of channels such that might lead to either increased local rela-

Indus River). Mc values are consistently higher, as those seen in Figure 9D. Erodibility could tive uplift (i.e., increased uplift relative to the and more variable, on the southwestern margin of the valley on the Indus molasse compared to SW NE the opposing tributaries that drain the batholith. Distance across valley (m) A 0 5000 10000 15000 20000 25000 30000 35000 Within the batholith, the relatively constant Mc 6500 6000 values suggest that there is little spatial varia- Indus tion in local uplift rates, channel erodibility, or 5500 Valley 5000 erosion driven by base-level changes; it is also ridgline noticeable that there is no change in the chan- 4500 e nels as they pass across the transition from Elevaon (m) 4000 incised canyons (which Hobley et al. [2011] 3500 valley proﬁl showed were best described with a detachment- 3000 Molasse Batholith ) limited erosion model) to alluvial-fan sedimen- 50 0.10 –1 tation. In contrast to the batholith catchments, B 10Be erosion rate range (n=5) 0.08 there is a high degree of variability in the 40 Be (m k. y. molasse catchments, which also have higher 30 0.06 10

M values for opposing catchments. There are age (Ma) c 20 0.04 also changes associated with mapped structures 10 AF T Be erosion rate range (n=10) within the Indus molasse such as the Choksti 10 0.02 thrust (Sinclair and Jaffey, 2001). It is unlikely 0 0.00 that the variation in Mc values in the molasse has Erosion rate from been caused by variations in base level along the

C Figure 9. Analysis of the asymmetry of erosion and topography plotted as a transect across the Indus Valley. (A) Maximum, minimum, and median lines of elevation across the Indus Valley with location of main thrust faults. Values are mean values from a 10-km-wide swath (for location of transect, see Fig. DR3 [see text footnote 1]). (B) Apatite fission-track ages (AFT; black circles) projected onto line of swath transect in A. Locations of samples are shown in Figure 2 (ages from Kirstein et al., 2006, 2009; Clift et al., 2002). While apatite and zircon U-Th/He ages are recorded for the batholith, only fission-track data extend D across the valley. Gray boxes show the range of values of erosion rates calculated from the detrital 10Be cosmogenic nuclide analysis from Dortch et al. (2011a) and Mu- nack et al. (2014). Numbers of catchments measured are given in each of the boxes.

(C) Mχ values plotted for each of the catchments in B against their distance from the Indus Valley floor. Overall, figure demonstrates faster erosion rates, younger fission- track ages, and steeper and more irregular river channels over the Indus molasse of the Stok Range.

10 Geological Society of America Bulletin, v. 1XX, no. XX/XX Squeezing river catchments

Ladakh batholith) or changes in drainage area. are seen in isolated locations higher up the fan, around 20 Ma (Kirstein et al., 2006), possibly If the molasse is being thrust toward the north- ~1.2 km from the modern Indus River channel linked to southward-vergent thrusting of the east, leading to motion of the drainage divide and 120 m higher (Fig. 5C). batholith. However, the lower-elevation inter- relative to the Indus, it would truncate the drain- Overlying the boulder conglomerate, there is fluve promontories on the southwestern margin age area at the base of the catchment at the point a poorly structured gravel that includes angular of the batholith nearest to the modern Indus of the thrust fault but would not affect the drain- clasts of the Indus molasse. These gravels are River have older ages (Fig. 9). For example, the age area upstream. This is because the entire very poorly sorted, with some clasts greater apatite fission-track ages range between ca. 35 catchment would be advected to the north. On than 1 m. The vague bedding dips gently down and 30 Ma (Kirstein et al., 2009); this increase the other hand, if there were internal deforma- the direction of the dissected fan surface. In the in age at lower elevations on the southern mar- tion within the molasse, in which drainage middle of these gravels, there is a light cream- gin of the batholith remains to be explained. areas were systematically declining within the colored bedded and laminated siltstone, with Published apatite fission-track ages from the molasse, then according to Equation 1, c would interbeds of the gravels. Indus molasse in the Zanskar Gorge record cen- increase while elevation remained relatively This upper succession represents deposits of tral ages of 13.7 ± 3.2 Ma and 13.8 ± 1.9 Ma constant, leading to a decrease in Mc, which the ancient alluvial fan interbedded with lake (Clift et al., 2002). An additional age from fur- is the opposite of what we observe. A vertical sediments that are traceable into the deformed ther east was reported as being between 7 and component of thrusting would lead to increases T1 lake sediments described previously 9 Ma (Sharma and Choubey, 1983). Assuming in channel gradients and erosion rates across ~3.7 km west-northwest from this location as similar geothermal gradients across the Indus any faults, which is consistent with our observa- unit 4 (Figs. 6 and 7). Underlying the alluvial Valley, the contrast in ages (Fig. 9B) from the

tion of greater Mc values in the molasse. This is gravel, the boulder conglomerates must repre- Indus molasse (ca. 14 Ma) to the southwestern corroborated by data from cosmogenic 10Be (see sent the course of the Indus paleochannel prior margin of the Ladakh batholith (ca. 30–35 Ma) later section on cosmogenic nuclides). to ca. 51 ka (oldest age of the T1 terrace from and into the core of the batholith (ca. 20 Ma) We therefore find the most likely interpreta- Phartiyal et al., 2005). The implication is that implies that the long-term erosion rates in the tion of the contrasting Mc values between the the modern Indus River channel has migrated Indus molasse have been at least twice as fast molasse and the Ladakh batholith is the pres- northeastward since ca. 51 ka. relative to those in the batholith since at least ca. ence of at least one active thrust fault within the 14 Ma. The absolute erosion rates are difficult molasse. We cannot rule out the possibility that EROSION RATES ACROSS to assess due to lack of multiple thermochro- erodibility differences have caused the observed INDUS VALLEY nometers and vertical profiles, but assuming a patterns, but given that erosion rates and fission- geothermal gradient of ~30°/km, and closure track data appear to correlate with the patterns Having calculated rates of structural dis- temperature of 110 °C (e.g., Reiners and Bran- in channel steepness (Fig. 9), we believe thrust- placement across the Stok thrust, we synthe- don, 2006), then the likely erosion rates have ing is the most likely explanation. We propose sized published erosion rates from the upthrown been of the order of 0.1–0.3 m/k.y. We interpret that the northeastward-vergent Stok thrust, as side of the fault in order to evaluate the balance the higher longer-term erosion rates in the Indus identified in the deformed terraces (Fig. 6), can between vertical displacement rates and erosion molasse to have resulted from Miocene to recent be traced as an active structure along the range rates. In addition, we compared these rates to deformation, and the development of the Stok front at the head of the large alluvial fans that the time-equivalent erosion on the opposite side Range (Sinclair and Jaffey, 2001). feed the Indus Valley (Fig. 8B), and that there of the Indus Valley from the Ladakh batholith, is likely to be additional active displacement since the river morphologies suggest lower ero- Cosmogenic Nuclides across other structures within the Indus molasse sion rates. Published data on bedrock thermo- such as the Choksti thrust (van-Haver, 1984; chronology and detrital cosmogenic 10Be are Concentrations of cosmogenically induced Sinclair and Jaffey, 2001). presented as a record of long-term (>106 yr) and radionuclides such as 10Be and 26Al in quartz are short-term (<105 yr) data. routinely used for dating the period of exposure Sedimentary Evidence of Northward of a rock at the surface (Lal, 1991). Applications Migration of the Indus River Channel Thermochronology include dating boulders on glacial moraines (e.g., Brown et al., 1991) and fluvial bedrock In addition to the deformed terraces and Thermochronology data on the cooling histo- strath terraces (e.g., Burbank et al., 1996b). steepened river profiles, there is sedimentary ries of rock samples within the top few kilome- Additionally, cosmogenic radionuclides mea- evidence to indicate that the course of the main ters of Earth’s surface in most mountain ranges sured from quartz sand in river catchments can Indus River channel has migrated northeastward can be used as an approximation of erosion be used to estimate averaged catchment-wide through time. rates (Reiners and Brandon, 2006). Apatite fis- erosion rates (Lal and Arnold, 1985; Brown The dissection of the T1 fan surface, described sion-track and apatite and zircon U-Th/He data et al., 1995). This method fills the gap between previously (Fig. 5B), exposes the internal stra- have been extensively published from across traditional erosion estimates determined from tigraphy of the fan, which reveals a unit com- the Ladakh region, with the majority of the measured river sediment loads (Schaller et al., posed of coarse boulder conglomerates with analyses on the Ladakh batholith (e.g., Kirstein 2001) and long time scales approximated using very well-rounded clasts up to 1.5 m in diameter, et al., 2006, 2009). We integrated these data thermochronology. including multiple lithologies but with grano with published values from the Indus molasse in Analysis of cosmogenic 10Be from sediment diorite from the Ladakh Range being dominant. the Stok Range (Clift et al., 2002; Sharma and across the Ladakh batholith has demonstrated Boulders and pebbles show strong imbrication Choubey, 1983). catchment-averaged erosion rates ranging from indicating flow toward the northwest (i.e., paral- The age-elevation data from the center of ~0.02 to 0.08 m k.y.–1, with ~10% error (Dortch lel with and downstream from the modern Indus the Ladakh batholith as recorded by all three et al., 2011b). These rates are slow compared to River). Exposures of this boulder conglomerate thermochronometers indicate rapid cooling at the mean for the Himalaya mountain range as

Geological Society of America Bulletin, v. 1XX, no. XX/XX 11 Sinclair et al. a whole, which is ~1.0 m k.y.–1 (Lupker et al., into the Indus Valley relative to the opposing ~8° (Fig. 9A). Therefore, a horizontal displace- 2012). The catchments along the southern side tributaries that drain the Ladakh batholith. The ment rate of 0.21 m k.y.–1 equates to a vertical of the Ladakh batholith have calculated mean steepened river channels enable the erosional displacement rate of ~0.19 m k.y.–1. averaged erosion rates of between ~0.02 m response to variable rock uplift relative to the The catchments that drain the hanging wall k.y.–1 and 0.04 m k.y.–1 (Fig. 10; Dortch et al., Indus River valley to be mapped to the east of of the Stok thrust are sourced from the Indus 2011b; Munack et al., 2014). Further detrital the observed thrust, indicating that the Stok molasse, where the erosion rates measured from 10Be data from the Stok Range record rates of thrust has been active along the northeastern 10Be concentrations range from ~0.07 to 0.09 m 0.07–0.09 m k.y.–1 (Munack et al., 2014); this margin of the mountain front. k.y.–1. This implies that more than half of the supports the fission-track thermochronology by Whether the horizontal displacement rate vertical component of displacement, and pro- indicating erosion rates of the Indus molasse across the Indus Valley is sufficient to perma- portionately half of the horizontal component that are approximately twice as fast as those nently offset the drainage divides depends on of displacement, on the fault is converted into over the batholith on the opposing side of the the ability of the erosive processes to counter the topographic displacement at the surface, with Indus Valley (Fig. 9B). topographic displacement induced by the defor- the rest being eroded. The implication is that mation. In bedrock channel networks, erosion is the drainage divide that forms the spine of the INTERPRETATION OF RESULTS enhanced by the propagation of knick zones (or Stok Range must be migrating toward the Indus (FIG. 11) knickpoints) up to the head of the catchments River at approximately half the rate of horizon- These results confirm that structural short- and the consequent response of hillslopes. In tal displacement on the Stok thrust, equating to ening is taking place across the present Indus geometric terms, a river catchment’s ability to ~0.1 m k.y.–1. However, given the presence of Valley, with horizontal displacement rates of at fully recover its form during shortening across a knick zones up the Stok Range catchments (Fig. least 0.21 m k.y.–1, which represent just a small thrust fault can be simplified to a ratio of the ver- 9C), the 10Be concentrations are likely record- 10 fraction (<2%) of the total shortening across tical rock displacement rate (Vv; this being dis- ing a mixture of higher erosion rates (lower Be the Himalaya in this region. An assumption in placement relative to the footwall block) versus concentrations) below the knick zones and lower this calculation is that the recurrence interval the erosion rate in the hanging wall of the thrust rates (higher 10Be concentrations) above the between episodes of fault slip on the Stok thrust (Fig. 1). The vertical rock displacement rate is a knick zones, where the kinematic wave of accel- are approximately equal to the period since the function of the horizontal displacement rate (Vh) erated incision has not yet reached. For this addi- last displacement. If the recurrence intervals are multiplied by the combined tangents of the dip tional reason, it is possible that the calculation longer, then the time-averaged slip rate on the of the thrust (b) and the mean topographic slope for divide migration is an underestimate, and that thrust would be slower. (a) from ridgeline to tributary outlet (Fig. 1). the true value is likely to lie somewhere between The deformation of the Stok Range has The Stok thrust has a measured dip at the sur- 0.1 m k.y.–1 and the horizontal fault displacement resulted in a steepening of river channels with face of 37°, and the mean surface slope of the rate of 0.21 m k.y.–1. If the knickpoints were higher erosion rates and a higher sediment flux Stok Range from ridge crest to valley floor is able to propagate to the drainage divide prior to subsequent shortening on the fault, then the averaged erosion rates for the catchments would 77.2° 77.4° 77.6° 77.8° be considerably higher, and they could fully Dortsch et al., 2011 reequilibrate the channel profile and so counter Munack et al., 2014 the fault displacement rates. However, the presence of multiple channel segments of variable steepness values (Fig. 9D) bounded locally by 34.2° knickpoints (Fig. 9C) suggests reequilibration of 0.04 the channels is not achieved between episodes 0.03 0.02 34.2° 0.02 0.03 Leh of shortening across the fault. Greater fault displacement rates compared 0.07 to erosion rates in the hanging wall of the Stok 0.02 0.02 thrust demonstrate that this topographic form is 0.07 evolving, and that the elevation contrast from 0.08 34.0° 0.03 outlet to drainage divide across the Stok Range

34.0° (catchment relief) is likely to be increasing over long time scales. If we assume the elevation of 0.07 0.02 the Indus Valley is constant, then it would sug- 0.09 gest that catchment relief is growing at a simi- N lar rate to the divide migration rate, i.e., ~0.1 m 0.02 –1 33.8° k.y. . However, the elevation of the Indus Valley N relative to the surrounding tributaries has fluc- 33.8° 20 km tuated, as recorded in the documented alluvial terraces, but the present elevation of the Indus 77.2° E 77.4° 77.6° 77.8° 78.0° River is up to 100 m lower than it was ca. 50 ka Figure 10. Detrital 10Be-derived erosion rates for tributary catchments draining into the (Fig. 4B). Based on this evidence, it is hard to Indus River from Dortch et al. (2011a) and Munack et al. (2014). Data demonstrate clear conclude whether the long-term elevation of the asymmetry of erosion rates, with higher values from catchments draining the Stok Range Indus River channel is rising or falling relative to (values in ovals) versus those draining the Ladakh batholith (values in rectangles). the deforming Indus molasse of the Stok Range.

12 Geological Society of America Bulletin, v. 1XX, no. XX/XX Squeezing river catchments

As sediment flux increases with relief and Deforming Indus River Ladakh batholith channel steepening, so must have the alluvial Indus molasse channel fans that drain into the Indus Valley off the Stok A Range expanded. The expansion of alluvial fans from this side of the valley has forced the present-day Indus channel to migrate laterally toward the opposing valley margin against the rock promontories of the batholith (Fig. 11). This interpretation is supported by the presence of Indus River boulder conglomerates exposed beneath the present fans that drain the Indus molasse (Figs. 5B and 5C). Another consequence of the asymmetry in erosion and sedi- B ment flux across the Indus Valley is the aggradation of alluvial fans within the valleys of the Ladakh batholith. This aggradation has resulted in some isolated hills or inselbergs of granodio- rite that once formed parts of interfluve ridges, but are now buried in alluvium and topographi- debris ﬂows resulng in valley cally detached from the range. damming and lake development While this study has focused on the tectonic driver for topographic narrowing of the Indus C Valley, it is clear that asymmetry of erosion rates ca. 35 ka driven by lithology and climate will also influence divide migration. In the case of the Indus Valley, the divide that runs along the Ladakh batholith has also experienced a strong asym- Increased relief, channel Lateral shi of metry in glacial erosion, with headwall retreat steepness and sediment Sediment –1 Indus channel rates of 0.18–0.6 m k.y. in the northeastward- yield aggradaon facing glaciated catchments (Dortch et al., D Fill terraces recording over batholith 2011a). A similar asymmetry exists across present valley damming the drainage divide of the Stok Range, with glaciers only present on the northeast-facing catchments; however, the rates of headwall retreat have not been measured in these catchments. A working assumption is that the rates of headwall retreat across these two drainage Figure 11. Interpreted evolution of the Indus Valley near Leh since Miocene times. (A) divides is comparable. Dating the onset of Broad valley and the early formation of the Stok Range and a stable Ladakh batholith with glaciations in these settings has not been pos- slow erosion rates as derived from the thermochronology (Kirstein et al., 2006). Dotted relief sible, but 10Be exposure ages extend back to ca. shows position of mountain range in B. (B) Relative uplift of the Stok Range due to shorten- 300 ka in the region (Dortch et al., 2013), and ing generates erosion and sediment flux that outpace the flux from the batholith, leading to it is probable that glaciers have existed in this the migration of the Indus channel toward the northwest. (C) Around 35 ka, motion on the setting for a great deal longer. Therefore, the Stok thrust generates mass flows off the Stok Range that cause damming of the Indus Valley, tectonic convergence of drainage divides must leading to formation of a large lake with deltas feeding in from the margins. (D) Present con- have been modified by the glacial signal. In the figuration with high-relief and high-gradient catchments over the Indus molasse and high case of the Indus Valley, if the two divides on sediment flux forcing the Indus River channel against the batholith. Sediment aggradation the batholith and Stok Range are equally influ- outpaces river incision in the lower reaches of the batholith, leading to sediment accumula- enced with the same sense of motion, then the tion of the lower interfluve ridges and local isolation to form inselbergs. Terrace remnants impact on the relative convergence between the record episodic damming of valley due to thrust activity. drainage divides is zero. However, it is recognized that this remains a component of signifi- leading to debris flows and consequent dam- structurally defined, strike-parallel (longitu- cant uncertainty. ming of the valley. The younger T2 terrace can dinal) valleys in any large mountain range. In An additional consequence of the thrust also be correlated downstream to similar age the Himalaya, rivers such as the Tsangpo and deformation driving topographic shortening deposits that incorporate numerous mass-flow Shyok, and the upper reaches of the Kosi, Sutlej, across the valley is the increased susceptibility deposits (e.g., Blöthe et al., 2014). and Karnali all run parallel to structures that to damming of the valley (e.g., Bürgisser et al., Here, we have documented the structural, may be actively modifying catchment form in 1982; Blöthe et al., 2014). We have been able topographic, and surface process response to a similar way to the Indus case presented here. to demonstrate that the thickest lacustrine ter- slow horizontal displacements across a single This is particularly relevant where active short- race in the Leh Valley was most likely caused valley. We would expect similar processes to ening occurs in regions of relatively low erosion by thrust motion at ca. 38 ka on the Stok thrust, take place simultaneously across numerous rates, such as in the lee of the Himalaya.

Geological Society of America Bulletin, v. 1XX, no. XX/XX 13 Sinclair et al.

CONCLUSIONS Trust for the Universities of Scotland. The authors are of the Ladakh Range, India: Earth Surface Processes thankful to Fida Mitoo Hussein for field logistics. Fida and Landforms, v. 40, no. 3, p. 389–402, doi:10 .1002 has sadly since died, and we would like to acknowl- /esp.3640. (1) Through OSL dating and analysis of Qua- edge his help for much of the work in this region. We Dortch, J.M., Owen, L.A., Schoenbohm, L.M., and Caffee, ternary terraces in the Indus Valley, Ladakh, we M., 2011a, Asymmetrical erosion and morphological thank Mark Brandon, Frank Pazzaglia and an anony- development of the central Ladakh Range, northern demonstrated that the southwestwardly dipping mous reviewer for valuable comments on the initial India: Geomorphology, v. 135, p. 167–180, doi:10.1016 Stok thrust, which represents a lateral continu- submission. /j.geomorph.2011.08.014. ation of the Great Counter thrust in the Hima- Dortch, J.M., Dietsch, C., Owen, L.A., Caffee, M., and REFERENCES CITED Ruppert, K., 2011b, Episodic fluvial incision of rivers laya, has been active from ca. 42 ka, resulting in and rock uplift in the Himalaya and Transhimalaya: ~10 m of shortening. 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