Glacial advances constrained by 10Be exposure dating of bedrock landslides, Kyrgyz Tien Shan

Quaternary Research 76 (2011) 295–304

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Quaternary Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / y q r e s

Glacial advances constrained by Tien Shan

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Be exposure dating of bedrock landslides, Kyrgyz

Katia Sanhueza-Pino a, Oliver Korup b,⁎, Ralf Hetzel a, Henry Munack b, Johannes T. Weidinger c, Stuart Dunning d, Cholponbek Ormukov e, Peter W. Kubik f a

Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany Institut für Erd-und Umweltwissenschaften, Universität Potsdam, 14476 Potsdam, Germany Erkudok Institute, K-Hof Museums, 4810 Gmunden, Austria d Division of Geography, Northumbria University, Newcastle Upon Tyne NE1 8ST, UK e Kyrgyz Institute of Seismology, 720060 Bishkek, Kyrgyzstan f Laboratory of Ion Beam Physics, ETH Zurich, CH-8093 Zurich, Switzerland b c

a r t i c l e

i n f o

Article history: Received 11 November 2010 Available online 6 August 2011 Keywords: Landslide Rock avalanche 10 Be exposure dating Quaternary glaciations Tien Shan

a b s t r a c t Numerous large landslide deposits occur in the Tien Shan, a tectonically active intraplate orogen in Central Asia. Yet their significance in Quaternary landscape evolution and natural hazard assessment remains unresolved due to the lack of "absolute" age constraints. Here we present the first 10Be exposure ages for three prominent (N10 7 m3) bedrock landslides that blocked major rivers and formed lakes, two of which subsequently breached, in the northern Kyrgyz Tien Shan. Three 10Be ages reveal that one landslide in the Alamyedin River occurred at 11–15 ka, which is consistent with two 14C ages of gastropod shells from reworked loess capping the landslide. One large landslide in Aksu River is among the oldest documented in semi-arid continental interiors, with a 10Be age of 63–67 ka. The Ukok River landslide deposit(s) yielded variable 10Be ages, which may result from multiple landslides, and inheritance of 10Be. Two 10Be ages of 8.2 and 5.9 ka suggest that one major landslide occurred in the early to mid-Holocene, followed by at least one other event between 1.5 and 0.4 ka. Judging from the regional glacial chronology, all three landslides have occurred between major regional glacial advances. Whereas Alamyedin and Ukok can be considered as postglacial in this context, Aksu is of interglacial age. None of the landslide deposits show traces of glacial erosion, hence their locations and 10Be ages mark maximum extents and minimum ages of glacial advances, respectively. Using toe-to-headwall altitude ratios of 0.4–0.5, we reconstruct minimum equilibrium-line altitudes that exceed previous estimates by as much as 400 m along the moister northern fringe of the Tien Shan. Our data show that deposits from large landslides can provide valuable spatio-temporal constraints for glacial advances in landscapes where moraines and glacial deposits have low preservation potential. © 2011 University of Washington. Published by Elsevier Inc. All rights reserved.

Introduction Several recent studies have helped constrain the regional Quaternary glacial chronology in the Tien Shan and neighboring mountain ranges of Central Asia (Heuberger and Sgibnev, 1998; Solomina et al., 2004; Zech et al., 2005; Abramowski et al., 2006; Narama et al., 2007; Koppes et al., 2008; Kong et al., 2009; Narama et al., 2009). In the Kyrgyz Tien Shan, Koppes et al. (2008) used 10Be exposure dating to regionally specify the timing and extent of glacier advances during the local Last Glacial Maximum (LGML), which may have predated that in North America or Europe by at least 20 ka. Nevertheless, a compilation of proposed glacial stages highlights significant spatio-temporal variability (Xu et al., 2010). This is partly

⁎ Corresponding author. Fax: +49 331 977 5700. E-mail address: [email protected] (O. Korup).

due to the limited number of samples per moraine, as well as the low preservation potential of glacial deposits especially in the downstream parts of steep, dissected river valleys incising in response to active tectonic uplift (Thompson et al., 2002; Koppes et al., 2008). As a result, the regional chronology awaits further refinement. Here we explore the potential of exposure dating of prominent landslide deposits as an independent and hitherto underexplored proxy for glacial advances in the Tien Shan and elsewhere. Inferring the timing of large catastrophic rock-slope failures from concentrations of in-situproduced cosmogenic 10Be and 36Cl in deposit boulders and detachment scars has produced promising results (Mitchell et al., 2007; Hormes et al., 2008; Dortch et al., 2009; Ivy-Ochs et al., 2009). Most of these studies compared inferred failure ages to those of moraines in order to explain rock-slope response to deglacial debuttressing. However, none has explored whether glacially undisturbed landslide deposits may serve as geomorphic markers for constraining former glacial extents. Hewitt (2009) used geomorphic evidence to suggest that scars of large bedrock

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landslides in the Karakoram occur at, or below Last Glacial Maximum (LGM) ice limits, thus indicating ice-free conditions during failure, but these scars and deposits have not been dated to compare to existing glacial chronologies. Our objective here is to use 10Be exposure dating of boulders from deposits of three large (N10 7 m 3) bedrock landslide deposits to further constrain the timing and extent of late Quaternary valley glaciers in the northern Tien Shan. These deposits (i) are preserved to varying degree but are clearly distinguishable from glacial deposits according to the criteria of Hewitt (1999); (ii) had blocked major rivers in steep mountain valleys that were glaciated repeatedly during the Quaternary (Koppes et al., 2008; Narama et al., 2009); but (iii) show no evidence of subsequent glacial modification or reworking. We surmise that the exposure ages of these landslide deposits yield minimum ages of maximum ice-margin positions following landslide emplacement. Regional setting The Tien Shan is a tectonically active intraplate mountain belt that is ~1500 km long and up to 500 km wide. It formed due to convergence between the Tarim Basin and the Kazakh Shield as a result of the IndiaAsian collision (Sadybakasov, 1972; Molnar and Tapponier, 1975; Chedia, 1986; Bullen et al., 2003). Reverse faulting has formed roughly E–W trending mountain ranges, i.e. basement-cored anticlines up to 100 km long, 10–40 km wide, up to 7 km high, and dissected by 1–2 km deep gorges (Burbank et al., 1999; Bazhenov and Mikolaichuk, 2004; Fig. 1). GPS data indicate an orogen-normal shortening rate of ~ 20 mm yr − 1 across the Tien Shan (Abdrakhmatov et al., 1996; Reigber et al., 2001), half of the convergence rate between India and Eurasia. Several historic earthquakes with magnitudes M N 7 have occurred along the fringes of the Tien Shan (Korjenkov et al., 2002; Abdrakhmatov et al., 2003; Arrowsmith et al., 2005). Late Quaternary slip rates on thrust faults such as the Issyk-Ata fault at the northern front of the Kyrgyz Range, and the Akchop Hills and South Kochkor faults along the northern margin of the Karakaty and Terskey Ranges are of the order of 1–2 mm yr − 1 (Thompson et al., 2002; Fig. 1). Basement rocks consist mainly of metamorphic rocks and granitoid intrusions that formed during Paleozoic orogenies. During the

Mesozoic these older mountain belts were largely eroded. A preOligocene erosion surface provides a distinct marker for neotectonic deformation (Makarov, 1977; Burbank et al., 1999), constraining relationships between rates of rock uplift, fluvial incision, and glacial cirque retreat (Oskin and Burbank, 2005; 2007). Many of the headwaters host cirque glaciers, while lower valleys are filled with glacigenic debris. Abundant periglacial landforms, particularly rock glaciers and earth lobes, indicate sporadic alpine permafrost between ~2700 and ~4000 m above sea level (asl; Gorbunov and Titkov, 1992; Gorbunov and Seversky, 1999; Marchenko et al., 2007). Hillslopes are steep, though frequently mantled by thick (N1 m) blankets of loess, reworked loess colluvium or fan deposits and screes up to ~2500 m asl. Despite its continental setting, the alpine topography of the northern Tien Shan intercepts considerable amounts of atmospheric moisture transported by Atlantic cyclones along the westerly drift. While north-facing slopes of the Kyrgyz Front Range record up to 1500 mm of annual orographic precipitation, south-facing slopes receive only 400–500 mm (Aizen et al., 1996). Precipitation decreases with increasing continentality, creating distinct latitudinal and altitudinal gradients and glacier equilibrium line altitudes (ELAs; Aizen et al., 1996, 1997; Böhner, 2006). Present ELAs rise with increasing continentality given that they are a function of moisture availability in the region (Kotlyakov et al., 1991; Koppes et al., 2008). Characteristics of the landslide deposits Alamyedin rockslide The Alamyedin landslide deposit (42°37′ N, 74°40′ E) is situated between 1730 and 1850 m asl, and extends for ~700 m along the Alamyedin River valley, one of the major transverse rivers dissecting the northern flanks of the Kyrgyz Range. The deposit is a compact but unlithified debris pile with an estimated volume of 10–15 × 10 6 m 3. Its eroded flank is perched above the shoulder of a ~ 30-m deep bedrock gorge at 1730 m asl. Ridgelines attaining 2600–2800 m asl could have allowed a vertical drop of ~1000 m for the landslide from the eastern valley flank. The deposit is characterized by granitic massive angular rock debris with pervasive fragmentation down to silt size. The tightly interlocked jigsaw texture of undissagregated clasts with radial

Figure 1. Shaded relief of the northern Tien Shan, Kyrgyzstan, with main active thrust faults. Red stars are locations of Aksu and Alamyedin landslides, Kyrgyz Range; and Ukok landslide, Terskey Range.

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fractures and no visible pore spaces are diagnostic features of dynamic fragmentation during motion consistent with emplacement by a large (N10 6 m 3) catastrophic rockslide or rock avalanche (Davies and McSaveney, 2009). Most of the deposit surface is littered by angular boulder nests coated by desert varnish (Figs. 2a, b). Fragmented angular granite cobbles floating in a ~ 3-m thick loess-rich matrix that caps the downstream flank of the landslide deposit indicate local reworking by loess flows. The deposit surface shows no signs of glacial overprint such as erosion marks, polishing, clast striations, rounding, or distinctive downstream asymmetry. Projecting the surface of the deposit in the runout direction indicates that the river had been blocked by the landslide. Upstream of this former dam, the Alamyedin River occupies a steep narrow valley, and is constricted by large toe-trimmed tributary fans impinging on the valley floor in a zipper-like manner, which would have buried any former backwater sediments, and certainly glacigenic sediments. Hence we infer that there has been no glacial advance beyond this site after the landslide had been emplaced. Aksu rockslide With a volume of ~1.5 × 109 m 3 the Aksu rockslide deposit (42°32′ N, 74° E; ~1560–2130 m asl) is among the largest reported for the Tien Shan (Strom and Korup, 2006). It is a compact debris mound of Paleozoic granite and metasediments, located in the deeply incised

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Aksu River that drains the Kyrgyz Range through a steep-sided valley (Fig. 1). The landslide descended from the eastern valley flank and filled the trunk river and several minor tributaries with pervasively fragmented debris over a length of N2.5 km. The crest of this formerly river-blocking deposit lies ~400 m above the river channel (Figs. 3a, b). Large transverse ridges on the deposit surface and a steep undisturbed deposit front indicate rapid deceleration during emplacement (Dufresne and Davies, 2009). Exposures of the deposit interior show comparable evidence of dynamic fragmentation typical of large catastrophic rock avalanches as found at Alamyedin. The boulderlittered deposit surface is locally smoothed by fines. Slide scars buttressed by debris cones indicate episodic instability of the undercut deposit margin (ls, Fig. 3b), possibly during triggering events such as the M ~7 Belovodsk earthquake in 1885 after which the Aksu River was blocked for a short period (Strom and Korup, 2006). Ukok landslide The Ukok landslide deposit (42°6′ N, 75°54′ E; ~3000–3100 m asl) sits at a right-angle bend in the Ukok valley, and impounds a 2.8-km long and ~ 20-m deep lake (Fig. 4a). The Ukok River drains the Terskey Range south of the Kochkor Basin, crossing the active Akchop Hills and South Kochkor thrust faults. Four distinct and partly nested detachment scars ~ 0.02–0.04 km 2 in size attest to repeated rockslope failure at this location (I–IV, Fig. 4b). The largest scar is ~500 m

Figure 2. (a, b) Google Earth images of Alamyedin landslide deposit (white dashed line) and locations of boulders sampled for 10Be exposure dating (samples 08K11 to 08K14). White arrows mark detachment area of the landslide deposit; blue arrows indicate N flow direction of Alamyedin river. (c) View of the boulder from which sample 08K12 from was taken. The 2-m-high boulder is 3 m long and the sampled surface has a dip angle of 25°. The boulder shows perpendicular joints typical for jigsaw structures indicating that the boulder was deposited and has remained in a stable position. (d) Photograph of reworked loess at the northern margin of the landslide. Note the presence of numerous granite clasts in the reworked loess. We took two gastropod shells from ~1.5 m below the ground surface for 14C dating.

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Figure 3. (a, b) Google Earth images of Aksu landslide deposit (white dashed line) with locations of boulders sampled (08K7 to 08K10); white arrow shows landslide runout direction. A smaller landslide (ls) adjacent to the Aksu river was triggered by the Ms N 7 Belovodsk Earthquake in 1885. Blue arrows show flow direction of Aksu river. (c, d) Granitic boulders from which samples 08K7 and 08K8 were taken. The boulders are 4 m and 4.5 m long, respectively, protruding ~ 1.3 m and ~ 2 m above ground. The sampled surfaces dip at angles of 10° and 5°, respectively.

high and 30°–35° steep, exposing cross-cutting sets of discontinuities prone to wedge failures. Abundant clasts with epidote-coated slickenslides indicate that failure took place in a tectonic fault zone. The hummocky deposit mainly contains granite, though some discrete debris bands and lobes feature metasediments, correlating with major lithological boundaries exposed in the source areas. Fractured but undissagregated and wedged clasts attest to fragmentation during rock avalanching and abrupt termination of motion. Frost weathering has exploited fragmentation cracks, created patches of granitic grus, and caused in situ collapse of some boulders. The debris spreads down-valley for N2 km but individual lobes show characteristics of several superimposed catastrophic rockavalanche deposits; one distinct lobe is partly submerged. One rock avalanche ran up N20 m on the opposite valley flank before continuing down-valley. The deposit forms a N60-m high dam that largely sustains a characteristic carapace of angular boulder openwork (Mitchell et al., 2007) ruling out any glacial modification of the deposit. The downstream dam face grades into a 3-km long tongueshaped valley-floor deposit interspersed by patchy fines and soil cover, perched ponds and anabranching creeks (Fig. 4a), resembling a deadice landscape in which lithological mixing and clast roundness increase downstream. Scour pits beneath several boulder nests indicate fluvial reworking, implying that one long-runout rock avalanche could have either entrained material from the valley floor or rafted sediments from the source hillslope. Overall, the well-defined sharp-edged detachment areas, the surface texture of the deposit(s),

and the surface geomorphology suggest that the Ukok landslide postdates those at Alamyedin and Aksu. Sampling strategy, analytical procedures, 14 C dating

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Be exposure and

To constrain the failure age of the three investigated bedrock landslides we sampled the upper 2–4 cm of granitic boulder surfaces located on the highest parts of the landslide deposits. We sampled four boulders at both Alamyedin and Aksu landslides, and six boulders at the Ukok landslide. The boulders were locked in at the base into jigsaw texture, i.e. they were wedged with other large fragmented clasts, precluding any toppling following primary emplacement. We excluded boulders on scree mantling the detachment areas and proximal fringes of the rock-avalanche deposit(s) at Ukok in order to avoid underestimated ages. One of the boulders at Ukok (08K5) is somewhat unique as it has conspicuously rounded edges (Fig. 4d), which we interpret to have resulted from glacial and/or fluvial processes before the landslide occurred. Without any further traces of erosional reworking of the deposit, we infer that this boulder would have been part of the original land surface, thus predating failure. All sampled boulders are 3–10 m long and only partly covered by lichen and weathering rinds 0.2–1 mm thick. All samples were crushed, washed and sieved, and the 125–500 μm size fraction was split into a magnetic and a non-magnetic fraction. The latter was purified according to procedures introduced by Kohl and

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Figure 4. (a) Google Earth image of Ukok landslide deposits (white dashed line) impounding a lake; I–IV are detachment scars of bedrock landslides; blue arrow indicates flow direction of Ukok river. (b) Close-up view of the deposit and locations of samples 08K1 to 08K5 (sample 08K6 was taken from a boulder ~ 1.5 km further W). The upper limit of the scar surface consists of several segments marked by white arrows. Note grayish color in NW part of the scar surface, which is underlain by granite; darker color further to the SW is exposed metasedimentary rocks. (c, d) Angular granitic boulders; the length of boulder sampled (08K3) is 6 m, its surface dip angle is 25°. The boulder stands ~ 2.5 m above the ground surface. View of the boulder where sample 08K5 was taken. The boulders length is 7.5 m and it protrudes up to ~ 3 m above ground. The surface dip angle is 40°. The granitic scar surface produced by the landslide can be recognized in the background.

Nishiizumi (1992), with 2–4 additional etching steps in aqua regia and HF as described by Goethals et al. (2009). After addition of ~ 0.3 mg of Be carrier solution, the quartz samples (9 to 36 g each) were dissolved and Be was separated with successive anion and cation exchange columns. Finally, Be was precipitated as Be(OH)2, transformed to BeO and pressed into targets, which were analyzed by accelerator mass spectrometry at the ETH Zurich (Kubik and Christl, 2010). The measurements were normalized to the standards S555 and S2007 with nominal 10Be/ 9Be ratios of 95.5 × 10 − 12 and 30.8 × 10 − 12, respectively (Kubik and Christl, 2010). These secondary standards were calibrated to the ETH standard material BEST433. All 10Be exposure ages were calculated with the CRONUS-Earth 10 Be– 26Al web calculator, version 2.2.1 (Balco et al., 2008; http://hess. ess.washington.edu). The calculator uses a 10Be half-life of 1.387 Ma (Chmeleff et al., 2010; Korschinek et al., 2010) and corrects for the different half-life and standard material used at ETH Zurich. We used the time-dependent scaling scheme of Lal (1991) and Stone (2000), accounting for temporal variations in the magnetic field intensity (Dunai, 2001). Using other available scaling schemes (Dunai, 2001; Desilets and Zreda, 2003; Lifton et al., 2005; Desilets et al., 2006) results in exposure ages that differ by b10% from those in Table 1, with the exception of sample 08K11 from the Alamyedin landslide (Table S1). Notably, the differences in age for the samples from the oldest of the three landslides (Aksu landslide) are b3% (Table S1). All 10Be exposure ages were calculated assuming no erosion and no shielding by snow or ice. Hence the ages are minimum ages, and presented with internal and external errors (Table 1). Internal errors allow comparing

different exposure ages within a given region, whereas external errors include the uncertainty of the local 10Be production rate relevant for comparing exposure ages obtained by different methods (Balco et al., 2008). In addition to the samples for 10Be dating, we took two samples for 14C dating from the reworked loess deposits that are spatially associated with the Alamyedin landslide (Figs. 2a, b). The loess contains angular granite clasts up to 20 cm in size and was presumably deposited during or shortly after the landslide event. The two samples consist of cm-sized gastropod shells that were taken ~ 1.5 m below the ground surface within a horizontal distance of 1–2 m (Fig. 2d). The samples were analyzed at the Leibniz Labor für Altersbestimmung und Isotopenforschung, University of Kiel, Germany, and yielded uncalibrated 14C ages of 11,055 ± 60 and 12,690 ± 90 14C yr BP (Table 2). Land snails incorporate carbon that is depleted in 14C (Goodfriend and Stipp, 1983). Hence, most modern snails yield radiocarbon ages, expressed as conventional 14C yr BP, that range from a few hundred to about two thousand years (Goodfriend, 1987; Quarta et al., 2007; Romaniello et al., 2008), although for small gastropods this age anomaly can be smaller (Pigati et al., 2010). In order to correct radiocarbon ages obtained on fossil snails for this effect, the age anomaly must be subtracted from the uncalibrated 14C ages (Goodfriend and Ellis, 2000; Quarta et al., 2007). The age anomaly of the two gastropod samples from the reworked loess at the Alamyedin site is unknown. Hence we use a conservative estimate of 1000 ± 500 yr, which we subtracted from the uncalibrated 14C ages. The estimated

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Table 1 Sample locations, Sample ID

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Be concentrations, and

Latitude (N)

Alamyedin landslide 08K11 42°36.535′ 08K12 42°36.632′ 08K13 42°36.663′ 08K14 42°32.540′ Aksu landslide 08K7 42°32.487′ 08K8 42°32.492′ 08K9 42°32.398′ 08K10 42°32.540′ Ukok landslide 08K1 42°6.127′ 08K2 42°6.143′ 08K3 42°6.177′ 08K4 42°6.107′ 08K5 42°6.110′ 08K6 42°6.300′

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Be exposure ages of granitic boulders from three landslides in the Tien Shan. Dip anglea (°)

Shielding factorb

Sample thicknessc (cm)

5.5 3 3 3.5

25 25 30 15

0.945 0.939 0.933 0.946

4 3 3 3

2132 2159 2113 2104

4 4.5 6.5 4

10 5 45 40

0.993 0.993 0.914 0.973

3103 3088 3094 3091 3101 2894

10 12 6 6 7.5 5.5

45 20 25 10 40 30

0.916 0.984 0.975 0.985 0.939 0.936

Longitude (E)

Elevation (m)

74°39.960′ 74°39.958′ 74°39.988′ 74°39.945′

1869 1851 1852 1854

73°59.707′ 73°59.542′ 73°59.565′ 73°59.808′ 75°54.325′ 75°54.443′ 75°54.392′ 75°54.228′ 75°54.122′ 75°52.928′

Length of sampled boulder (m)

10 Be concentrationd (104 at g− 1)

10 Be agee (ka)

Internal 1σ error (ka)

External 1σ error (ka)

13.9 ± 0.7 21.3 ± 1.0 24.0 ± 1.5 28.0 ± 1.2

7.30 11.3 12.8 14.7

0.39 0.5 0.8 0.6

0.73 1.1 1.4 1.4

4 3 3 3

168.4 ± 8.9 171.5 ± 5.2 145.0 ± 4.5 82.7 ± 3.6

67.1 66.5 63.0 34.1

3.8 2.1 2.1 1.6

6.8 6.1 5.8 3.3

3 3 3 3 2 3

5.9 ± 1.2 1.61 ± 0.69 160.8 ± 5.0 25.7 ± 1.4 34.9 ± 1.9 1.45 ± 0.53

1.47 0.41 35.3 5.89 8.21 0.44

0.28 0.16 1.2 0.31 0.46 0.14

0.32 0.18 3.2 0.59 0.84 0.16

a

Dip angle of sampled boulder surface. The shielding factor includes the correction for skyline shielding and the shielding caused by the dip of the sampled boulder surface. Shielding was quantified using the formulas given in Niedermann (2002). c We assumed a rock density of 2.7 g cm− 3, which was inserted into the CRONUS-Earth 10Be–26Al calculator, version 2.2.1 (Balco et al., 2008; http://hess.ess.washington.edu). d Blank-corrected 10Be concentrations. Propagated errors (1σ) include the analytical error based on counting statistics and the error of the blank correction. e Exposure ages were calculated with the CRONUS-Earth 10Be–26Al calculator, version 2.2.1 (Balco et al., 2008; http://hess.ess.washington.edu) assuming no erosion and using the time-dependent scaling scheme of Lal (1991), Stone (2000). Internal uncertainties include errors from the counting statistics and the blank correction, whereas external uncertainties also include the error of the production rate introduced by the scaling model. b

error of ± 500 yr was taken as 1σ error and combined with the analytical 1Σ error, σ1 (in years), to give the total error σt (Goodfriend and Ellis, 2000): σt =

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi σ12 + 5002 :

ð1Þ

The resulting 14C ages of 10,055 ± 504 14C yr BP and 11,690 ± 508 14C yr BP were calibrated with CALIB 5.01 (Stuiver et al., 2009) and the IntCal04 calibration curve (Reimer et al., 2004), yielding ages of 11.7 ± 0.9 and 13.6 ± 0.6 cal ka BP (Table 2). Discussion Interpretation of

10

Be concentrations

The 10Be concentrations in the four samples from the Alamyedin landslide boulders correspond to ages of 14.7–7.3 ka (Table 1). With the exception of the youngest sample (08K11), these ages are consistent with the 14C ages of 11.7 ± 0.9 ka and 13.6 ± 0.6 ka cal BP obtained from gastropod shells in the loess-flow deposits overlying the landslide debris, indicating a landslide occurrence between 15 and 11 ka. The younger 10Be age of sample 08K11 may either indicate that

the boulder has been tilted after its emplacement or that it was part of a subsequent smaller rockfall onto the deposit. The 10Be exposure ages of three boulders of the Aksu landslide range between 67.1 and 63.0 ka and agree within internal error bounds (Table 1). The fourth boulder (08K10) yielded a much younger age of 34.1 ka. This boulder sits on a ridge below the crest of the deposit and is the nearest to its undercut face, though we did not observe any evidence of rotational slumping or fluvial scour marks from dam breach that could have caused local instability (Figs. 3a, b). Nevertheless boulder tilting by ~ 90° would explain its low 10Be concentration as in vertical surfaces the production rate of cosmogenic nuclides is halved (e.g. Niedermann, 2002). Discarding this youngest age, and using the three boulders that appeared unaffected by reworking we obtain a mean age of ~66 ka for the Aksu landslide. By worldwide comparison this age is one of the oldest reported for terrestrial, non-volcanic landslides with a volume N10 9 m 3 (Figs. 5; Korup and Clague, 2009). The 10Be ages of the boulders at Ukok landslide are by far the youngest we obtained and confirm the relatively fresh appearance of the deposit(s). However, these ages also have the highest scatter, ranging from ~0.4 to ~35.3 ka (Table 1). Boulders 08K1, 08K2, and 08K6 had fresh rock surfaces with abundant slickenslides and little desert varnish and were expected to yield minimum ages of slope failure. While 08K1

Table 2 Radiocarbon ages of gastropod shells from reworked loess-flow deposits capping the downstream flank of the Alamyedin landslide. Sample ID

Amount of carbon analyzed (mg)

Percent of modern carbon (corrected)a

Conventional (14C yr BP)

KIA38778 KIA38779

1.3 1.6

25.26 ± 0.18 20.60 ± 0.23

11055 ± 60 12690 ± 90

a

14

C ageb

δ13Cc (‰)

Calibrated 14C aged (cal ka BP)

−6.68 ± 0.18 −7.38 ± 0.21

11.7 ± 0.9 13.6 ± 0.6

Percent of modern carbon (1950) corrected for fractionation using the 13C measurement. Conventional 14C ages were calculated according to Stuiver and Polach (1977) with a δ13C correction for isotopic fractionation based on 13C/12C ratio measured by the AMSsystem simultaneously with the 14C/12C ratio. Reported error limits are 1σ. c Includes the fractionation that occurs during sample preparation as well as that associated with the AMS measurement. The value can therefore not be compared to a massspectrometer measurement. d The calibrated ages were calculated with the CALIB 5.01 software (Stuiver et al., 2009) and the IntCal04 calibration curve (Reimer et al., 2004) assuming an age anomaly of 1000 ± 500 yr, which was subtracted from the conventional age before calibration. b

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Figure 5. Box-and-whisker of age–size distribution of n = 393 bedrock landslides that mobilized N 106 m3 compiled from published data. Gray crosses are raw data, and black ellipses are landslides dated in this study. Box height encompasses 25th and 75th percentiles; whiskers are 18th and 83rd percentiles; open circles are outliers. Box width is proportional to the square root of sample number log-binned by age interval. The deposits of the Aksu, Alamyedin, and Ukok landslides have escaped significant erosion for longer periods than one would expect from the overall scaling of deposit volume with age, attesting to a high preservation potential compared to more humid mountain belts.

and 08K6 yielded consistent ages of ~0.4 ka at a distance of ~2 km apart, boulder 08K2 may have been emplaced earlier (~1.5 ka). We can exclude rockfall onto the deposit surface from the adjacent valley walls for these boulders, given the wedged-in position of the boulders at their base, and the long runout needed. Samples 08K4 and 08K5 yield mid-

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Holocene ages of ~5.9 ka and ~8.2 ka, respectively. Given its rounded appearance which stands out amidst similar-sized angular debris of comparable lithology, we interpret that 08K5 has a fluvial and/or glacial overprint. With the surrounding debris being a decisively angular boulder carapace any fluvial reworking during lake overtopping or glacial advance onto the landslide deposit can be excluded. We thus infer an inherited 10Be component in the boulder from which sample 08K5 was taken, which therefore yields a maximum age of slope failure (Fig. 4d). This interpretation requires that the boulder was pre-exposed to abrasion and rounding on the landslide detachment slope, and rafted on top of the moving rock avalanche during runout. Comparable occurrences of such “inherited boulder rafts” have been reported from the early Holocene Flims rockslide in the Alps (Ivy-Ochs et al., 2009). Another characteristic of rock-avalanche dynamics is that large blocks detaching from the source ridgelines are rafted to proximal parts of the deposit where source-zone stratigraphic relationships are maintained (Hewitt, 1999). Therefore, the highest apparent 10Be age of 35.3 ka (08K3) obtained for a prominent boulder in the central and highest part of the deposit(s) may also indicate an inherited 10Be component, providing a maximum age of catastrophic rock-slope failure affecting this location (Fig. 4c). We infer that one rock avalanche has occurred between 1.5 and 0.4 ka, i.e. in the Late Holocene. The rock avalanche could have overridden at least one earlier mid-Holocene deposit, spreading a thin veneer of debris for some 3 km downstream and/or entraining glacifluvial sediments, which are now being reworked by water courses shifting laterally along the longitudinal ridges and boulder trains, issuing from springs inside the dam. While we cannot rule out on the basis of sedimentological evidence an emplacement onto a glacier snout, the estimated deposit thickness of N20 m renders an entirely supraglacial deposition unlikely, given that such deposits

Figure 6. Compilation of radiometric ages of glacial landforms, i.e. mainly moraines, and inferred glacial advances (boxes) in the Tien Shan and adjacent northern Pamir. Note that the inferred detachment ages of Aksu and Ukok landslides dated in this study appear to broadly lie within previously proposed glacial advances.

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rarely exceed 5 m in thickness (e.g. Jibson et al., 2006). In either case, the scatter of our 10Be ages can be reconciled with the runout dynamics of rock avalanches and the possibility of multiple failure events for which the nested source scars offer unambiguous support.

Implications for glacial advances in the northern Tien Shan Our results show that 10Be exposure dating of large bedrocklandslide deposits has significant potential to augment existing records of Quaternary landscape evolution in the Tien Shan. There are complications due to the possibility of superimposed deposits, inheritance, and/or subsequent reactivation; deposits at higher elevations may further be subject to periglacial reworking. However, at sites of multiple superimposed landslide deposits, substantial scatter of ages may be explained by combined geomorphic, sedimentological, and weathering characteristics, which are indispensable for identifying primary emplacement features of large catastrophic bedrock landslides. Ages of older low-lying deposits deliver consistent ages, and may be substantiated by 14C dating of gastropod shells from stratigraphically related loess-flow deposits that are ubiquitous in the subalpine parts of the Tien Shan. The 10Be ages help constrain the timing and extent of late Quaternary glacier advances in the northern Kyrgyz Tien Shan. The inferred timings of large catastrophic rock-slope failure at Aksu and Ukok overlap, within external error bars, with the proposed ‘Aksai’ and ‘Ala Bash’ glacial advances of Koppes et al. (2008), respectively. This overlap may simply reflect the broad error margins and uncertainties in the regional glacial chronology, as large bedrock instabilities are typically, though not exclusively, associated with interglacial or at least ice-free conditions (e.g. Hewitt, 2009). Comparing our data with those of Koppes et al. (2008) supports this notion, as the inferred failure ages occupy local minima in the probability density of ages associated with glacial advances. In this context Alamyedin and Ukok may be considered as postglacial, and

Aksu as an interglacial landslide despite dating to Marine Oxygen Isotope Stage 4 (MIS 4). The reconstruction of former ELAs remains only crudely resolved based on regional interpolation from a small number of 10Be and OSL ages of moraines per drainage basin, and faces the problem of distinguishing potential outliers, different assumptions on erosion rates, and use of varying calculation schemes (Koppes et al., 2008; Narama et al., 2007, 2009; Xu et al., 2010; Fig. 6). We suspect that the uncertainty of this interpolation is highest along the northern margin of the Tien Shan, where datable glacial deposits are scarce, and climatic variability due to incoming moist westerly airflow is highest. There, in the Ala Archa valley Koppes et al. (2008) noted a number of remnant benches that were interpreted as till or outwash, suggesting that Pleistocene glaciers could have advanced to ~ 1400 m asl. In the adjacent Alamyedin valley, which shares all the major topographic characteristics, the maximum altitude to which valley glaciers could have advanced without significantly reshaping the Alamyedin landslide deposit and its preserved base after 15–11 ka is 1730 m asl. Hence, this landslide is clearly postglacial. Using the drainage basin's maximum headwall elevation of 4850 m asl, which we derived from SRTM90 digital elevation data, and assuming a toeto-headwall altitude ratio (THAR) of 0.4 and 0.5, we infer a minimum ELA of 2980 m and 3290 m, respectively (Fig. 7). Similarly, ice could not have extended below ~ 1800 m asl after ~63 ka in the Aksu valley based on the external errors of the three consistent 10Be ages (Table 1). In fact, the Aksu landslide deposit is located near the proposed ELA of the LGML advance in the northern Kyrgyz Range during Marine Oxygen Isotope Stage 4 (Fig. 6; Koppes et al., 2008). Yet the landslide predates the glacial advance. Depending on whether we assume a THAR of 0.4 or 0.5, the ELA required to advance glaciers to the upstream face of the dissected rockslide dam is 2820 and 3080 m, respectively (Fig. 7). Given the consistency of our 10 Be ages at Aksu we infer that any LGML glacier advances in this valley would have to either predate ~ 63 ka or remained much less extensive with an ELA as much as 400 m higher than earlier estimates

Figure 7. Topographic swath profiles across the Kyrgyz and far western Terskey Ranges, featuring locations of Alamyedin and Aksu, as well as Ukok landslide deposits (black polygons labeled LS), respectively. Horizontal lines are recent equilibrium line altitudes (ELAs) with gray shading and whiskers indicating estimated errors. Dark gray areas labeled LGML are ELAs during “Local” Glacial Maximum reconstructed by Koppes et al. (2008); whisker denotes estimated error range. Black boxes encompass range of ELAs reconstructed using assumed toe-to-headwall altitude ratios (THAR) of 0.4 and 0.5 from the upstream base of landslide deposits (white labels are inferred 10Be ages).

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(based on a THAR of 0.4; Koppes et al., 2008). Moreover, it seems that thus inferred post- and interglacial positions (based on Alamyedin and Aksu, respectively) of the ELA in the northern Kyrgyz Range are similar. Finally, Lake Ukok at 3050 m asl roughly marks the maximum position that glaciers could have reached in the mid Holocene. The necessary ELA to promote this is 3570 m (3700 m) for THAR = 0.4 (THAR = 0.5). Within error bars, this coincides with the Holocene ELA proposed by Koppes et al. (2008) for the Terskey Range (Fig. 7). Conclusions Our 10Be exposure ages attest to a high residence time of large (N10 7 m 3) deposits of formerly river-blocking landslide debris despite breaching and pronounced fluvial bedrock incision in the Tien Shan (Fig. 5). This persistence makes these post- and interglacial deposits promising and independent geomorphic markers for refining the spatio-temporal extent of Quaternary glacial advances in confined mountain valleys where river incision has censored the glacial sedimentary record. Our results show that glaciers did not extend anywhere near the northern front of the Kyrgyz Range after ~63 ka. This is inconsistent with the proposed timing of the LGML at ~ 49 ka in the northern Tien Shan (Koppes et al., 2008), unless the last maximum glacier extent along the northern fringe of the mountain belt was indeed much earlier. Moreover, previously estimated ELAs for this northern fringe of the Tien Shan may need to be corrected upwards by as much as 400 m. The MIS 4 age of Aksu landslide coincides with a regional interglacial, and supports the notion that glaciations in the Tien Shan are distinctly asynchronous with regard to glaciations in Europe and North America. Supplementary data to this article can be found online at doi:10. 1016/j.yqres.2011.06.013. Acknowledgments We acknowledge funding by the International Working Group on Natural Hazards in the Tien Shan (“NATASHA”/EU INCO-SSA contract no. 026363), CCES at ETH Zurich (Project “COGEAR”), and the German Research Foundation (DFG Heisenberg Program). References Abdrakhmatov, K.Y., et al., 1996. Relatively recent construction of the Tien Shan inferred from GPS measurements of present-day crustal deformation rates. Nature 384, 450–453. Abdrakhmatov, K., Havenith, H.-B., Delvaux, D., Jongmas, D., Trefois, P., 2003. Probabilistic PGA and Arias Intensity maps of Kyrgyzstan (Central Asia). Journal of Seismology 7, 203–220. Abramowski, U., et al., 2006. Pleistocene glaciations of Central Asia: results from 10Be surface exposure ages of erratic boulders from the Pamir (Tajikistan), and the AlayTurkestan range (Kyrgyzstan). Quaternary Science Reviews 25, 1080–1096. Aizen, V.B., Aizen, E.M., Melack, J., 1996. Precipitation, melt and runoff in the northern Tien Shan. J. of Hydrol. 186, 229–251. Aizen, V.B., Aizen, E.M., Melack, J.M., Dozier, J., 1997. Climatic and hydrologic changes in the Tien Shan, Central Asia. American Meteorological Society 10, 1393–1404. Arrowsmith, J.R., Crosby, C.J., Korjenkov, A.M., Mamyrov, E., Povolotskaya, I.E., 2005. Surface rupture of the 1911 Kebin (Chon-Kemin) earthquake, Northern Tien Shan, Kyrgyzstan. Eos Trans. AGU 86/52 Fall Meet. Suppl., Abstract T51F-05. Balco, G., Stone, J.O., Lifton, N.A., Dunai, T.J., 2008. A complete and easily accessible means of calculating surface exposure ages or erosion rates from 10Be and 26Al measurements. Quaternary Geochronology 3, 174–195. Bazhenov, M.L., Mikolaichuk, A.V., 2004. Structural evolution of Central Asia to the North of Tibet: a synthesis of paleomagnetic and geological data (in Russian). Geotectonics 5, 68–84. Böhner, J., 2006. General climatic controls and topoclimatic variations in Central and High Asia. Boreas 35, 279–295. Bullen, M.E., Burbank, D.W., Garver, J.I., 2003. Building the Northern Tien Shan: integrated thermal, structural and topographic constraints. Journal of Geology 111, 149–165. Burbank, D.W., McLean, J.K., Bullen, M.E., Abdrakhmatov, K.Y., Miller, M.M., 1999. Partitioning of intramontane basins by thrust-related folding, Tien Shan, Kyrgyzstan. Basin Research 11, 75–92. Chedia, O.K., 1986. Morphostructures and Neotectonics of the Tien Shan. Ilim Publishers, Frunze. 315 pp.

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