Origin of glacial–fluvial landforms in the Azas Plateau volcanic field, the Tuva Republic, Russia:...

2007) 352–366www.elsevier.com/locate/geomorph

Geomorphology 88 (

Origin of glacial–fluvial landforms in the Azas Plateau volcanicfield, the Tuva Republic, Russia: Role of ice–magma interaction

Goro Komatsu a,⁎, Sergei G. Arzhannikov b,Anastasia V. Arzhannikova b, Gian Gabriele Ori a

a International Research School of Planetary Sciences, Universita' d'Annunzio, Viale Pindaro 42, 65127 Pescara, Italyb The Institute of the Earth's Crust, Russian Academy of Sciences, Siberian Branch, Lermontova 128, 664033 Irkutsk, Russia

Received 7 August 2006; received in revised form 8 December 2006; accepted 11 December 2006Available online 25 January 2007

Abstract

The basaltic Azas Plateau volcanic field is located in the Tuva Republic of the Russian Federation. The area was glaciatedmultiple times, and the field is characterized by the formation of subglacial volcanoes called tuyas, but subaerial volcanoes and lavafields also exist. A combined study of remote sensing and field observations in the vicinity of the tuyas in the southeastern AzasPlateau volcanic field identified landforms that are best explained by the jökulhlaup hypothesis. The landforms include elongatedhills, paleochannels, and butte and basin topography. These landforms are hypothesized to have formed by both erosion anddeposition caused by high-energy water streams. The triggering for the hypothesized jökulhlaups was either melting of ice bysubglacial volcanism and/or destabilization of ice-dammed/subglacial reservoirs. The age estimation of the flood events is difficult,but they probably occurred during the ice ages of the Quaternary, as late as in the Middle-Late Pleistocene.© 2006 Elsevier B.V. All rights reserved.

Keywords: Siberia; Azas Plateau; Tuya; Jökulhlaup; Flood; Ice sheet

1. Introduction

The close relationships among glaciation, volcanism,and flooding have been important topics in Quaternarypaleohydrology. One place on Earth where these rela-tionships are currently observed is Iceland. There,modern occurrence of jökulhlaup and associated topo-graphic changes have been studied intensively (e.g.,Russell and Knudsen, 2002; Gudmundsson et al., 2004;Smith et al., 2006). Such floods must have occurred alsoin the past, and indeed recent studies are shedding lighton this aspect (e.g., Carrivick et al., 2004; Alho et al.,

⁎ Corresponding author. Tel.: +39 085 453 7507; fax: +39 085 453 7545.E-mail address: goro@irsps.unich.it (G. Komatsu).

0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.geomorph.2006.12.003

2005). However, there is limited understanding of paleo-jökulhlaup events in the rest of the world. The basalticAzas Plateau volcanic field is located in southern Siberia,and it contains a number of volcanic edifices includingtuyas, flat-topped mountains created by eruption of lavainto ice (Litasov et al., 2001; Komatsu et al., 2004a,2007-this volume). There are also subaerial volcanoesand lava fields. The Azas Plateau volcanic field has beenstudied in terms of ages of volcanics and petrologicalaspects (e.g., Yarmolyuk et al., 1999, 2001; Rasskazovet al., 2000; Litasov et al., 2001), but almost no attentionhas been paid to the flooding related to ice sheets andvolcanic eruptions.

Here in this paper, we present the first geomorpho-logical evidence for the past occurrence of catastrophic

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floods associated with meltwater of ice sheets thatcovered the Azas Plateau multiple times. Some floodswere likely induced by volcanic eruptions, but othersmay not be related to such events. The landformsdescribed in our study should serve as a basis for futurestudies. Our research was conducted using satelliteremote sensing data and by field observations. Theremote sensing data include a Canadian Radarsat image(SCAN SAR R120050130), and Landsat TM GeoCoverimage data (N47-50_2000), and airphotos. The digitalelevation models used in this study were derived fromthe SRTM (Shuttle Radar Topography Mission) data set.

2. The study area in the Azas Plateau volcanic field

The Tuva volcanic province is located in southernSiberia, and the largest lava field in the province is theAzas Plateau volcanic field. The Azas Plateau isadministratively situated in the eastern part of the TuvaRepublic within the Russian Federation. It is positionedalong the transition zone between the East Sayan

Fig. 1. Location of the Azas Plateau. Radarsat image (SCAN SAR R120050tuyas in the plateau (Shivit–Tagia, Derbi–Taiga, Ploskii, Kok–Hemskii,approximate position of the main figure (black rectangular area) in southern

mountains and the Todza Basin (Fig. 1). Russianscientists have conducted some important investigationsin the remote Azas Plateau volcanic field. This Cenozoicvolcanic field covers over 2000 km2, and the volume ofvolcanics is estimated to be as much as 600 km3

(Yarmolyuk et al., 1999). The compositions of the AzasPlateau volcanics include trachybasalt and basanite(Litasov et al., 2001). The chronology of eruptions inthe Tuva volcanic province has been studied using theK–Ar and Ar–Ar dating techniques (Yarmolyuk et al.,1999; Rasskazov et al., 2000; Yarmolyuk et al., 2001).The Azas Plateau volcanic field formed mainly duringthe Late Pliocene and the Quaternary. The subglacialvolcanic edifices in the Azas Plateau are concentrated inthe southeastern part of the volcanic field. Thesesubglacial volcanic edifices are Middle-Late Pleistocene(Neopleistocene in Russian time scale) in age, and theirformation coincided with extensive glaciation in theregion. Our study focused on an area where two largetuya edifices of Derbi–Taiga and Shivit–Taiga dominatethe landscape (Figs. 1, 2 and 3; Table 1). Shivit–Taiga,

130). The area of Fig. 2 is indicated by a white box. Positions of largeSorug–Chushku–Uzu, and Priozernyi) are shown. Inset shows theSiberia.

Fig. 2. The study area in the southeastern part of the Azas Plateau volcanic field. The area coverages of airphotos shown in this paper are indicated.Landsat TM GeoCover data (N47-50_2000).

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with maximum horizontal dimensions exceeding 10 km,is one of the largest subglacial volcanoes on Earth. Thearea is partially covered also by volcanic materialserupted subaerially (Fig. 2, Table 1).

3. Erosional and depositional landforms observednear Derbi-Taiga and Shivit-Taiga

A system of complex erosional and depositionallandforms occurs in the vicinity of subglacial volcanoesin the southeastern Azas Plateau volcanic field. Theproximity of such landforms and the volcanoes indicatesa close link between them.

3.1. Elongated hills, paleochannels, and butte andbasin topography near Derbi–Taiga

The surface of the volcanic plateau east of theDerbi–Taiga edifice (Fig. 3) is characterized by a seriesof NNE–SSW trending elongated hills and a paleo-

channel complex (Figs. 2 and 4). The elongated hillscovered by bush occur primarily on a hyaloclastic unit(Yarmolyuk et al., 2001; Litasov et al., 2001), and theyrange 100–600 m in length and 10–100 m in width(Fig. 5). The heights of these hills vary, but they aregenerally less than 5 m. They occur at various altitudi-nal levels on this plateau (Fig. 6A). Their long axes areoriented along the sides of the volcano, but in some casesthey point to the flanks of the volcano. Boulders up to afew meters in diameter are locally exposed at the flanksof these elongated hills (Fig. 6B), but it is not certain ifthe hills are made primarily of boulders or bedrocks. Atthe southeastern part of the volcanic plateau, a lava/lavaclastic unit is mapped by Yarmolyuk et al. (2001) and awell-defined paleochannel complex develops over thisunit. Sinuous channel reaches are visible between theupland hills that are similar to the elongated hills nearby.The channel pattern is braided or anastomosing.

The northern part of the volcanic plateau is mappedas either lava or hyaloclastite units (Yarmolyuk et al.,

Fig. 3. Topographic map of the study area. The coverage of this map is equal to that of Fig. 2. The unit is in meters. This map is based on the SRTMdata set.

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2001; Litasov et al., 2001). The surfaces of these unitsare characterized by the butte and basin topography(Figs. 7 and 8). The horizontal dimensions of the buttesand the basins are up to several hundreds meters long. Inthis topography, the basins are aligned in the NNW–SSE direction. However, locally this terrain has a dis-organized appearance without clear orientations of thedepressions.

3.2. Butte and basin topography near Shivit–Taiga

According to Yarmolyuk et al. (2001), an area (ap-proximately 1×4 km) southeast of Shivit–Taiga (Fig. 2)is lava/lava clastite of Kadyr–Sugskii volcano. Thesurface morphology of this unit is characterized by thebutte and basin topography and also some occurrence ofisolated semi-circular hills (Figs. 9 and 10A, B). Thehorizontal dimensions of the buttes and the basins aretypically less than 100 m long. Some of the buttes

exceed 10 m in height. Gravels are exposed on the sidesof the buttes in the area but no clear indication ofbedrocks is observed. In this butte and basin topography,there are hills with a shallow crescent-shaped depressionpositioned on the south side (Figs. 9 and 10A).

3.3. Elongated hills between Derbi–Taiga andShivit–Taiga

The most remarkable group of erosional/depositionallandforms in the study area occurs in the zone betweenDerbi–Taiga and Shivit–Taiga (Fig. 2). This zone isapproximately 15 km long and 6 km wide, and dippingto the northwest with a gradient range of about 0.02–0.03 (Fig. 3). Yarmolyuk et al. (2001) and Litasov et al.(2001) mapped it as two separate lava units, the olderone in the northwest and the younger one (Yurdawavolcano) in the southeast. Northwest-trending striationsare clearly visible in satellite images (e.g., Fig. 2). The

Table 1Types and ages of volcanic units in the study area

Name Main types of volcanics† Main mode of eruption† Age (ka) of volcanics† Age (ka) of volcanics§

Ulug–Arginskiia Scoria Subaerial• 48±20b N/AShivit–Taiga Hyaloclastite/Lava Subglacial 110±40, 130±40 370±40, 430±40 (upper lava)

1030±20, 1150±50 (base partc)Albine–Boldokd Hyaloclastite Subglacial 195±50e N/AYurdawa Lava Subaerial 290±40 N/AKadyr–Sugskii Lava/Lava clastite Subglacial 565±80 N/APlateau SE of Derbi–Taiga Lava/Lava clastite Subglacial 600±80 N/ADerbi–Taiga Hyaloclastite/Lava Subglacial 725±50, 760±50 1050±110 (upper lava)Lava plateau Lava Subaerial 2070±150, 2140±200 N/A

1210±80•

†Yarmolyuk et al. (2001).§Rasskazov et al. (2000).•Litasov et al. (2001).aThe listed data are for the Ulug–Arginskii volcanic cone described by Litasov et al. (2001) to be a scoria cone.bThis date was derived for the subaerial lava unit occurring to the south of the Ulug–Arginskii volcanic cone.cThese dates may represent ages of lava flows below the Shivit–Taiga edifice.dThe listed data are for the Albine–Boldok volcanic cone located in the eastern part of the Albine–Boldok volcanic complex.eThis date was derived from the low-lying hyaloclastite/lava edifice located in the western part of the Albine–Boldok volcanic complex.N/A Not Available.

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zone is roughly divided into two sections, one is thenorthwest section where NW-trending striations domi-nate the landscape (Fig. 11A). The southeastern sectionof the zone appears to have more chaotic appearancethan the northwestern part (Fig. 11B). This section

Fig. 4. An area southeast of Derbi–Taiga. Elongated hills and a system of palephoto in Fig. 5. Airphoto (location is shown in Fig. 2).

approximately coincides with the younger lava unit(Yurdawa volcano) in the zone.

Our inspection of airphotos revealed that the striationsvisible in the satellite images consist of NW-trendingelongated hills and channels running between them.

ochannels are developed. The black arrow indicates the direction of the

Fig. 5. Elongated hills east of Derbi–Taiga. These hills are about 100–600 m in length and 10–100 m in width. Paleochannels are visible beyond thesehills. Photographed by G. Komatsu (location is shown in Fig. 4).

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These elongated hills are normally 100–600 m long and10–100 m wide, but we currently lack data on theirheights. Their morphology varies locally. The southernhalf of the zone close to Derbi–Taiga is characterized bythe elongated hills that are light in color on the airphotoand the channels that are dark (Fig. 11A). The elongatedhills in the northern part of the zone close to Shivit–Taiga, in contrast, are covered by bushes, appearing darkin color (Fig. 12A). Some elongated hills near Shivit–Taiga have pronounced topography, and well-defineddrainage channels pass between them (Fig. 12B). Thelong axes of these hills point to Shivit–Taiga, originatingfrom the flanks of the volcano.

4. Glacial–fluvial origin of the observed erosionaland depositional landforms

The paleochannel system near Derbi–Taiga (Fig. 4)can be best interpreted as formed by fluvial processesjudging from its wide braided or anastomosing reach(∼1 km) that is smaller but similar in morphology to thetracts of the Channeled Scabland, the well-studiedcataclysmic flood-produced landscape in the northwest-ern USA (e.g., Bretz, 1928; Baker, 1973; Baker andNummedal, 1978). The channel pattern testifies for thehigh-energy nature of the floods that once created thesystem. The paleochannel system is oriented approxi-mately NNE–SSW, and the paleoflow direction is in-ferred to be from NNE to SSW based on the local slope(Fig. 3).

The butte and basin topography near Shivit–Taiga(Fig. 9) is also remarkably similar to scabland areascharacteristic of the Channeled Scabland (e.g., Bakerand Nummedal, 1978). This type of topography in theChanneled Scabland is characteristics of floodwaterpassing over jointed basaltic units. The plucking occursalong the joint lines and a complex series of buttes andbasins could form. The hills neighbored by a crescent-shaped depression on one side (Figs. 9 and 10A) can beproduced by floodwater passing an obstacle, forming adepression on the stoss side. It is interpreted to be a sortof giant obstacle mark, and the inferred orientation ofthe floodwater in the area is approximately towardsNNW, which is against the local slope (Fig. 3). Giantobstacle marks (due to ice-blocks) produced by a recentjökulhlaup are also observed in Iceland (Fay, 2002). Aflood origin is envisaged also for the butte and basintopography located in the northern part of the volcanicplateau near Derbi–Taiga (Fig. 7), and here the alignedbasins reflect a highly oriented current. Based on thetopographic gradient of the area (Fig. 3), the current isestimated to have passed over the plateau from NNW toSSE.

The origin of the elongated hills observed east ofDerbi–Taiga and in the area between Derbi–Taiga andShivit–Taiga (Figs. 11A, B and 12A, B) is moreproblematic. Large boulders are visible on somevegetation-free flanks of the hills, but we currentlylack data on the interiors of the hills and we do not knowif they are made mainly of bedrocks or sediments.

Fig. 6. Details of elongated hills east of Derbi–Taiga. A) Elongated hills (generally less than 5 m in height) observed at various altitudinal levels. Themountain massif in the background is Derbi–Taiga. Photographed by G. Komatsu. B) Boulder-dominated side view of an elongated hill. The bouldersare up to a few meters in diameter. Photographed by G. Komatsu.

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However, morphologically these hills resemble drum-lins that occur commonly at locations that were formallyunder ice sheets (e.g., Jansson et al., 2002). Drumlins aretypically interpreted as depositional and erosionallandforms of subglacial tills produced by the movementof ice sheets, but some researchers consider that theyformed by subglacial floods (e.g., Shaw and Sharpe,1987; Sawagaki and Hirakawa, 1997; Shaw, 2002). Acombination of these two processes may not beexcluded for the origin of drumlins.

For both the paleochannels and the butte and basintopography, there is no clear source of the flooding uphillof the features today (Figs. 2, 3 and 13). We propose thatthese features were formed by the floods originating in icesheets that covered the Azas Plateau and its surroundingsduring the Pleistocene (Fig. 14). The generation of theelongated hills, formed by ice sheet movement and/or bysubglacial floods, should also be attributed to the same icesheets. The extent of the ice sheet in Fig. 14, mapped

using positions of end moraines (Grosswald, 1965), wasthe maximum in the mapped area. The ice sheets couldhave been smaller when the floods occurred, but theyshould have been large enough to cover the areas ofsubglacial volcanoes.

Although our mapping of the erosional and deposi-tional landforms is still incomplete, their close spatialassociation with subglacial volcanoes in the AzasPlateau implies a genetic connection among them.Two mechanisms are possible for the generation of thefloods originating within ice sheets: volcanic and non-volcanic. These processes are discussed below.

5. Discussion

5.1. Subglacial volcanism-triggered flooding

Subglacial volcanism is a potential cause of theflooding in the study area. The proximity of various

Fig. 7. Butte and basin topography east of Derbi–Taiga. The buttes and basins are aligned. The black arrow indicates the direction of the photo inFig. 8. Airphoto (location is shown in Fig. 2).

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features discussed above to the subglacial volcanoes im-plies that eruptions into ice may have played a significantrole in the formation of these features. Such is the casewith the volcanic eruption-triggered jökulhlaup occur-rence in Iceland (Gudmundsson et al., 2004). Further-more, jökulhlaups could result in the genesis of landformsformed both under ice sheets and on nearby ice-freesurfaces.

Fig. 8. Butte and basin topography east of Derbi–Taiga. The lengths of the butby G. Komatsu (location is shown in Fig. 7).

The majority of the work in Iceland seems to havefocused on the later case. The depositional and erosionaljökulhlaup impacts within unconfined proglacial plains(Russell and Knudsen, 2002; for example, jökulhlaup-formed flood plains called sandar) and confined bedrockjökulhlaup routeways (Waitt, 2002; Carrivick et al.,2004) have been documented and examined by variousinvestigators. However, little is known about the

tes and the basins are up to several hundreds meters long. Photographed

Fig. 9. Butte and basin topography southeast of Shivit–Taiga. The small white arrow indicates an example of buttes with a crescent-shaped depressionon the south side (this butte is also indicated in Fig. 10A). The black arrow indicates the direction of the photo in Fig. 10A. Airphoto (location isshown in Fig. 2).

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jökulhlaup landforms formed under ice. This is probablybecause we cannot directly observe the flood processesunder the ice, and also the landforms formed under sucha circumstance may have been erased or reduced by theerosion of the ice itself. Russell et al. (2006) recognizeda lack of research on jökulhlaup impacts in sub- and en-glacial environments, and their study attributed someeskers and fracture-fill sediments to jökulhlaups passingunder the ice.

Braided channels in Icelandic sandar are thetestimony to outburst floods (Gomez et al., 2000), andthey form on ice-free surfaces. The paleochannels andthe butte and basin topography also commonly occur inthe Channeled Scabland, and they are also the exam-ples of landforms produced on ice-free surfaces. Like-wise, similar landforms in the Azas Plateau may haveformed on ice-free surfaces. Alternatively, these land-forms formed under ice as in the case of tunnel channelsand scablands that have been related to subglacial flood(Sjogren and Rains, 1995; Sjogren et al., 2002).

The long axes of the elongated hills close to Derbi–Taiga (Fig. 4) and Shivit–Taiga (Fig. 12B) are mostlyaligned along volcano flanks but some are towards thevolcano flanks. This close spatial association indicateshill formation in an environment in which meltwaterderived from the volcanic activity forms a lake that issurrounded by ice walls. Such a lake could drain rapidlythrough subglacial tunnels during jökulhlaup as in thecase of the 1996 Gjálp eruption within the Vatnajökull

ice cap (Gudmundsson et al., 2004). Rapid movement ofthe meltwater under the ice is expected in the vicinity ofthe volcanoes, possibly forming the elongated hills. Wepoint out that some elongated hills observed betweenDerbi–Taiga and Shivit–Taiga are not always traced tothe tuya edifices. Nevertheless, their proximity to thesevolcanoes may indicate their genesis linked to thesubglacial volcanism.

The distinction between thick ice and thin ice isrealized as an important factor controlling the meltwaterdrainage (Smellie, 2000). Thin glaciers (b150 m thick)are formed mainly of permeable snow and firn, allowingcontinuous escape of eruption-generated meltwater alongthe ice/bedrock surface. In the case of thick glaciers(N150 m thick) made of impermeable ice, the glacier maybecome buoyant when sufficient water is available at thebase of the glacier, allowing catastrophic releasing ofwater as jökulhlaup. The ice thicknesses estimated fromtuya edifices of the Azas Plateau are at least about a fewto several hundred meters thick (Yarmolyuk et al., 2001;Komatsu et al., 2007-this volume), making this jökulh-laup mechanism a possible explanation.

5.2. Outbursts of ice-dammed or subglacial reservoirs

Although the studied features are located in thevicinity of subglacial volcanoes, it is not absolutelynecessary that volcanic eruptions triggered the floods. Itis also hypothesized that jökulhlaups in Iceland could be

Fig. 10. Butte and basin topography southeast of Shivit–Taiga. A) The lengths of the buttes and the basins are typically less than 100 m long. Thesmall white arrow indicates an example of buttes with a crescent-shaped depression on the south side (this butte is also indicated in Fig. 9).Photographed by G. Komatsu (location is shown in Fig. 9). B) Some of the buttes exceed 10 m in height. Photographed by G. Komatsu.

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sudden outbursts of ice-dammed or subglacial lakes(Maizels, 1997), some of which form by high geo-thermal activity as in the case of Grímsvötn (Gud-mundsson et al., 2004), but not necessarily by volcaniceruption. As known from the existence of Lake Vostokin a non-volcanically active region under the AntarcticIce Sheet (Studinger et al., 2003), this type of lake doesnot require volcanic eruptions to maintain themselves.

Furthermore, the idea of subglacial lake outbursts hasbeen proposed for the Laurentide Ice Sheet (Rains et al.,1993; Shaw, 2002), for the Cordilleran Ice Sheet toexplain the Channeled Scabland in the northwestern USA(Shaw et al., 1999), and for the Antarctic Ice Sheet(Denton and Sugden, 2005). The subglacial flood pathsin the Laurentide Ice Sheet were inferred from giantflutings, drumlins, tunnel channels, and scoured bedrock

tracts. Although active discussions have emerged sincethis controversial proposal (e.g., Benn and Evans, 1998;Komatsu et al., 2000; Atwater et al., 2000), there is noclear consensus on the validity of the hypothesis. Thetriggering may be non-volcanic causes such as instabilityor surge, although the presence of subglacial volcanoes inthe area of the former Cordilleran Ice Sheet (Hickson,2000) keeps the possibility open for the involvement ofvolcanic eruption at least in the flood triggering.

5.3. Flood routes downstream

Based on topographic gradients of the region, themajority of floods should flow toward west through theTozda Basin along the Bii Khem and Azas River(Fig. 14). It is not certain about the impacts of such

Fig. 11. Morphology of elongated hills between Derbi–Taiga and Shivit–Taiga. A) Elongated hills distributed north of Derbi–Taiga. Airphoto(location is shown in Fig. 2). B) Elongated hills distributed northeast of Derbi–Taiga. Airphoto (location is shown in Fig. 2).

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floods further downstream but the floods could haveaffected a large part of the upper Yenisei drainage area.

To date, we have identified evidence for floods onlyin the study area. However, floods may have originatedat the sites of other Azas Plateau tuyas including Ploskii,Kok–Hemskii, Sorug–Chushku–Uzu, Priozernyi, and

Fig. 12. Morphology of elongated hills between Derbi–Taiga and Shivit–(location is shown in Fig. 2). B) Elongated hills distributed near the westernAirphoto (location is shown in Fig. 2).

smaller subglacial volcanic edifices (Fig. 1). Floodsfrom Ploskii would be particularly interesting since oneof the possible flood routes may have been southward tothe direction of the Belin River and the Kyzyl Khem(Fig. 14). The upper Yenisei River reaches along theKyzyl Khem, and along its downstream rivers called

Taiga. A) Elongated hills distributed west of Shivit–Taiga. Airphotoflank of Shivit–Taiga. Note that these hills orient towards the volcano.

Fig. 13. Geomorphological map and hypothesized flood directions (shown by the arrows) in the study area. The flood directions were inferred basedon airphoto and topographic analyses. The mapped coverage is equal to that of Fig. 2.

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Kaa Khem and Ulug Khem, record a number of floodlandforms (Rudoy, 1998; Grosswald, 1999). The floodsoriginated in the Azas Plateau may have affected theupper Yenisei River reaches, and contributed to theformation of at least some of these landforms. Furtherinvestigation is necessary to examine these possibilities.

5.4. Timing of the flooding

We lack direct age measurement of the flood-relatedlandforms in the study area, but we can use chronologyof the volcanics on which these landforms occur toconstrain the periods of their formation. The chronologyof the volcanics has been established primarily by twoRussian research groups (Yarmolyuk et al., 1999;Rasskazov et al., 2000; Yarmolyuk et al., 2001). Theages of the southeastern Azas Plateau volcanics mostlyfall in the Late Pliocene and Quaternary, but we cautionthat there are significant differences in these two age

estimates (Table 1). The ages described in this paper arederived from Yarmolyuk et al. (2001) because theyprovide a more complete age data set of volcanoes in thesoutheastern Azas Plateau volcanic field. The group ofthe elongated hills east of Derbi–Taiga appears to haveformed on a 725–760 ka hyalocalstic unit. Thepaleochannels near Derbi–Taiga formed on a lava/lavaclastic unit of 600 ka. The age of the lava unit with thebutte and basin topography near Derbi–Taiga isunknown, but this topography seems to occur also onthe hyaloclastite unit that is 725–760 ka. The age of thelava/lava clastic unit with the butte and basin topogra-phy near Shivit–Taiga is 565 ka (Kadyr–Sugskiivolcano). The elongated hills between Derbi–Taigaand Shivit–Taiga are positioned on two lava unitsapproximately ∼2.1 Ma (note that Litasov et al. (2001)mapped this unit as ∼1.2 Ma lava unit) and 290 ka(Yurdawa volcano) in age. The group of some elongatedhills that point towards Shivit–Taiga seems to reside on

Fig. 14. The reconstructed ice sheet over the drainage areas of the upper Yenisei River. Legend: (1) border of the Tuva Republic; (2) rivers; (3)volcanoes in the Azas Plateau; (4) the maximum extent of the ice sheet during OIS 2–5. Modified from Grosswald (1965). Kyzyl, the capital city ofthe Tuva republic and the area of Fig. 2 (black box) are indicated.

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a hyaloclastic and lava unit of the volcano, which is110–130 ka.

The ages of the landforms described in this paper arecertainly younger than the volcanic units on which theywere formed. This implies that the flood(s) most likelyoccurred during the Late Pliocene or Quaternary.Furthermore, the ages of the landform formation mighthave been quite recent considering their relatively pris-tine appearances. In the cases of the elongated hills thatpoint to Derbi–Taiga and Shivit–Taiga, their formationperiods are likely to coincide with the subglacialvolcanic processes.

The age information of the ice sheet coverage givesanother clue in understanding the timing of the flooding.The ice sheet mapped by Grosswald (1965) (Fig. 14)was at its maximum some time during Oxygen IsotopeStages 2–5 (10–120 ka). If the dating results ofYarmolyuk et al. (2001) are reliable, the ages of sub-glacial Shivit–Taiga (110±40, 130±40 ka) and anothersubglacial volcano Sorug–Chushku–Uzu (60±40 ka)east of Shivit–Taiga together with that of subaerial

Ulug–Arginskii (48±20 ka) narrow down the ending ofthe last extensive ice sheet coverage to sometime in theOIS 4–5. During the OIS 3 when Ulug–Arginskii wasprobably active, glaciations in the Azas Plateau werelikely confined in valleys and may not have covered alarge fraction of the plateau, or completely disappeared.We are unaware of the state of glaciation in the AzasPlateau during the OIS 2 due to a lack of extensivevolcanic eruptions. However, if the formation of thepurported flood landforms discussed in this paperrequires volcanic eruptions in addition to an ice-sheetcoverage larger than the areas of valley glaciers, theselandforms were most likely formed during the OIS 4–5.Older glaciations certainly existed prior to the OIS 5 asclearly indicated by the presence of subglacial volca-noes of earlier ages (e.g., Derbi–Taiga, Kadyr–Sugskii,and Albine–Boldok; see Table 1). Some flood land-forms may also be related to these earlier glaciations.However, it is not clear if such old landforms could havesurvived intense erosion by later glacier activities. Themore concrete age determination of the landform

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formation has to wait for further investigations includ-ing direct measurement of landform ages.

6. Conclusions

Our investigation in the Azas Plateau volcanic fieldyielded geomorphological evidence for the past occur-rence of jökulhlaups. The evidence includes erosionaland depositional landforms such as elongated hills,paleochannels, and butte and basin topography. Thesefloods were produced either in association withsubglacial volcanism and/or failures of ice-dammed/subglacial reservoirs without volcanic eruptions. Theexact timings of such flooding events are difficult to beconstrained since we have not conducted direct agemeasurement of the purported flood landforms. Theflooding probably occurred during the ice ages of theQuaternary, and some of the events may have happenedduring the Middle-Late Pleistocene.

Our study contributes to the understanding of howvolcanism and water interact in a large-scale tectono-magmatic framework of northern Eurasia (e.g., Komatsuet al., 2004b). The subglacial volcanism and thejökulhlaup occurrence in the Azas Plateau are alsopossible terrestrial analogues (Komatsu et al., 2006) forsimilar processes that might have operated on Mars.

Acknowledgements

Vic Baker and Takashi Oguchi provided commentsthat improved our manuscript. We thank Kirill Ershov,Dasha Zhigzhitov, Oshor Darzhinov, Alena Arzhanni-kova and our loyal and competent dog, Berta, for fieldassistance. We acknowledge financial support by theNATO Collaborative Linkage Grant, the RussianFoundation for Basic Research grant #04-05-64460,and the Italian Space Agency.

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