Geometry and kinematics of recent deformation in the Mondy–Tunka area (south-westernmost Baikal rift zone, Mongolia–Siberia)

A. Arjannikova,1 C. Larroque,2 J. -F. Ritz,3 J. Deverchere,4 J. F. Stephan,2 S. Arjannikov1 and V. San’kov11Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, Lermontova street 128, 66 Irkutsk, Russia; 2Laboratoire

Geosciences Azur, UMR 6526 – Universite de Nice Sophia Antipolis, CNRS-bat. 4, 250 Av. A. Einstein, 06560 Valbonne, France; 3Laboratoire

Dynamique de la Lithosphere, UMR 5573 – Universite de Montpellier-2, 34095 Montpellier cedex 5, France; 4Institut Universitaire Europeen

de la Mer, UMR 6538 – Universite de Bretagne Occidentale, Place N. Copernic, 29280 Plouzane, France

Introduction

A major concern regarding continen-tal collision zones is to understandwhat is happening within the trans-ition zone, between transtension andtranspression areas, and how thistransition evolves with time. Interestin the Mondy–Tunka area comes fromits situation at the transition betweenthe Baikal transtension and the Mon-golian transpression (Fig. 1).Since 50 Ma, a large part of the

Asian plate has been affected bycompressive stresses that propagatenorthward from the India–Eurasiacollision zone (Tapponnier and Mol-nar, 1979) while extension started inthe Baikal zone at around 35 Ma(Logatchev, 1993). The onset of com-pressional deformation in Mongolia isnot well determined but is consideredto have started during the Neogene(Baljinnyam et al., 1993; Cunninghamet al., 1996; Bayasgalan, 1999; Ritzet al., 2003) while the extension star-ted in the Baikal zone at around35 Ma (Logatchev, 1993). One major

aim is to determine whether the tworegimes (compression and extension)have always coexisted or if there is achronology between them.A preliminary field analysis sugges-

ted that the normal faults of theTunka area are now undergoing com-pression (Ritz et al., 2000; Larroqueet al., 2001). However, the geometry,distribution in space, kinematics and amore precise timing of these recenttranspressional features were not wellestablished. Here we present new datafrom satellite images, airborne photo-graphs and field investigations withinthe Mondy–Ikhe Ukhgun area, a faultsystem connecting the Tunka basin tothe Khubsugul basin at the south-westernmost Baikal rift zone.

Seismotectonic setting

The Tunka–Mondy area is boundedto the north by the Sayan Mountainsand to the south by the Khamar–Daban Mountains (Fig. 1). Westof Lake Baikal, the E–W-trendingTunka basin is about 200 km longand 30 km wide. To the north, thebasin is bounded by the 200-km-longnorth Tunka normal and left-lateralfault (NTF, Riazanov, 1978; McCal-pin and Khromovskikh, 1995). Del-vaux et al. (1997) suggested that thesouthern Tunka basin underwent

strike-slip faulting during the Pliocene(STF, Fig. 1). The development of theTunka basin is contemporaneous withthe opening of the south Baikal riftsince the Oligocene, and its formationoccurred under a sinistral strike-slipstress regime with r3 trending NW–SE (San’kov et al., 1997). To the west,the basin connects to the Khubsugulgraben, corresponding to N–S-trend-ing normal faults. Extension began inthe Late Miocene in the Khubsugulrift and in the Middle Pliocene in theDarkhat and Busingol rifts (Logat-chev, 1993, 2001). Southwards, thearea is bounded by the Bolnai andTsetserleg left-lateral strike-slip faults.The Baikal continental rift zone is

highly seismogenic; 13 earthquakeswith magnitude over than 6.5 haveoccurred here in the last 300 years(Solonenko, 1981; Deverchere et al.,1993; Solonenko et al., 1997). Theseismicity pattern of the westernBaikal rift zone is heterogeneous(Deverchere et al., 2001). Trends ofepicentres appear along active faults,and earthquakes cluster in the SouthBaikal, in the Busingol basin and westof the Tunka basin. In the last cen-tury, the region was struck by twomajor earthquakes: the 1950 (Mw ¼6.9) Mondy earthquake and the 1991(Mw ¼ 6.3) Busingol earthquake(Fig. 1). The strain regime is complex,

ABSTRACT

We used satellite imagery and field data to investigate thesouth-westernmost Baikal rift zone. We focus our study in theMondy and Ikhe Ukhgun valleys, site of an Mw ¼ 6.9 seismicevent in 1950. Surface deformations are observed along theE–W-trending Mondy strike-slip fault and along the IkheUkhgun thrust. The Mondy fault system is 80 km long and iscomposed of four segments 10–15 km long. These segmentsare characterized by subvertical planes with left-lateral move-ments. The Ikhe Ukhgun thrust is 20 km long, dips 40� to thesouth and shows reverse movement with a left-lateral compo-

nent. These observations are consistent with the present-dayregional NNE–SSW compression and with the focal mechanismof the 1950 Mondy earthquake that was recently re-evaluated.These features, like those observed in the Tunka basin,demonstrate a recent change of regional strain regime fromtranstension to transpression that we place before the LatePleistocene.

Terra Nova, 16, 265–272, 2004

Correspondence: Christophe Larroque,

Laboratoire Geosciences Azur, UMR

6526 – Universite de Nice Sophia Antipo-

lis, CNRS-bat. 4, 250 Av. A. Einstein,

06560Valbonne,France.E-mail: larroque@

geoazur.unice.fr

� 2004 Blackwell Publishing Ltd 265

doi: 10.1111/j.1365-3121.2004.00565.x

with various focal mechanisms, cor-responding to reverse, normal andstrike-slip faulting (Misharina et al.,1983; Doser, 1991). The regional stressanalysis shows a clear overall evolu-tion from dominant NE–SW compres-sion in Mongolia to dominant NW–SE extension in the Baikal area. Thecomplex transition zone is character-ized by N30�E P-axes of earthquakesleading to a strike-slip regime locatedbetween the Tunka and Khubsugulbasins (Petit et al., 1996; Delouiset al., 2002). To the west and to

the south, from the Darkhat andBusingol basins up to the Bolnai fault,the area shows a transpressive stressregime.From Landsat MSS images, Tap-

ponnier and Molnar (1979) mappedout normal and left-lateral strike slipfaulting along the NTF and normalfaulting associated with the Khubsu-gul, Darkhat and Busingol grabens.Sherman and Ruzhich (1973) reportat least 11 km of Neogene–Quater-nary left-lateral movement along theSayan Fault and several hundred

metres of left-lateral movement alongthe E–W-trending NTF. Along theNTF, McCalpin and Khromovskikh(1995) determined a long-term(500 kyr) vertical slip rate of 0.08–0.16 mm yr)1 from terrace age esti-mates and fault scarp heights, andthey determined a rate of0.5 mm yr)1 from the morphologyof the Tunka range front. Recently,trenching studies near Arshan(Fig. 1) pointed out compressionalfaulting of Holocene age at the footof the Tunka ranges (Chipizubov

Fig. 1 Topography of the western Baikal rift from Gtopo30 data. Elevations are from �500 m (green) to �3000 m (grey). Blacklines are main rivers and lake contours, bold lines are main faults. Focal mechanisms of earthquakes (M > 4.0) since 1950 arefrom Delouis et al. (2002) and the BEMSE catalogue. STM, Sayan–Tunka Mountains; KDR, Khamar–Daban Mountains; SF,Sayan fault; NTF, north Tunka fault; STF, south Tunka fault; BOF, Bolnai fault; TSF, Tsetserleg fault. The star in centre of theTunka basin indicates the location of GPS site BADA. Relative location of Fig. 2 is given by the rectangle. Inset: the relativelocation of Fig. 1, where major tectonic features are from Tapponnier and Molnar (1979).

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et al., 2003). However, the onset ofthis compressional deformation isdifficult to establish because theMondy–Tunka region was stronglyaffected by glacial and periglacialprocesses during the Zirian(100–50 ka) and probably Sartan(25–10 ka) periods (Olunin, 1965;Ufimtsev et al., 2003).

Imagery and field analysis of theMondy–Ikhe Ukhgun fault sytem

Figure 2 shows the studied area be-tween the Tunka basin to the east andthe Khubsugul basin to the west. It ischaracterized by steep mountains tothe north (Eastern Sayan), whereasthe topography is more gentle to thesouth (Khamar Daban and Khubsu-gul). The morphology between thetwo topographic domains is markedby several fault scarps belonging tothe Mondy fault system, which aredescribed below.Along its eastern part

(101�15¢E)101�30¢E, Fig. 2), theMondy fault system trends N100�Eand is characterized by a southernuplifted compartment. As suggestedby Loukina (1989), this segment prob-ably extends eastwards and couldmeet the N100�E south Tunka Fault(Larroque et al., 2001). Between101�10¢ and 101�15¢, north of theIrkut river, hills aligned E–W between100 and 250 m high form a counter-slope scarp (Fig. 3). North of these

hills, the fault plane dips 75� south-wards.Further to the west, the Mondy

valley is a large alluvial and asymmet-rical valley of glacial origin incisingthe morphology and cutting throughthe Mondy fault system. Between101�00¢E and 101�10¢E, along thenorthern slope of the valley, elongatedridges trending N95�E and left-lateraloffsets of rivers with displacements of160–600 m are observed (Fig. 3). Thelinear trend of the scarps in map viewis indicative of the near-vertical dipangle of this segment (Figs 2 and 3).From 101�00¢E up to 100�50¢E, thefault scarp disappeared, either coveredor eroded by fluvio-glacial deposits(Kame terraces, moraines). West ofMondy, near 100�58¢, on the southernside of the fault, we observed strathterraces perched 13 m above the pre-sent Irkut river bed, suggesting arecent uplift consistent with a smallreverse component along the Mondyfault. A preliminary radiocarbon dateof the palaeosoil at the top of theterrace gives an age of 8105–8026 calyears BP (J. Michelot, personal com-munication). This yields a preliminaryuplift rate of 1.5 mm yr)1 for thehangingwall.After the 4 April 1950 event (Mw

6.9), Treskov and Florensov (1952)have reported only local ground rup-tures of a few hundred metres to thenorth of Mondy and about 1 km tothe south of the main fault scarp

(Fig. 3). Open fractures up to 1 mwide striking N110–140�E in fluvio-glacial terraces with a maximum ver-tical displacement of 0.8 m and ahorizontal left-lateral strike-slip com-ponent of 15 cm were described.These fractures are on the top of aterrace and parallel to its topographicboundary.From 100�45¢E to 100�30¢E, in the

westwards continuity of the Mondyfault (Fig. 2), several parallel trendsare observed having a mean strike ofN90–100�E (Fig. 4A,B). Most of thestreams descending from Monku Sar-dyk ridge (3491 m a.s.l.) attest to theleft-lateral kinematics along this seg-ment, and offsets range from 300 to1000 m (Fig. 4A).Westwards, a clear fault scarp is

observed between 100�30¢E and100�15¢E (Fig. 2). This 15-km-longscarp steps 1 km to the north withrespect to the previous fault scarp. Ittrends N90�E and is found at the baseof the range. Between 100�25¢ and100�30¢, we measured the fault dip at70� northward and streams show left-lateral offsets of up to 70 m (Fig. 4C).North-west of Lake Khubsugul, a

NE–SW scarp underlines the normalfault that bounds the half-graben.This scarp stops a few kilometressouth of the Mondy fault, suggestingthat these two faults are not connectedat the surface (Fig. 2).The morphology of the Ikhe Ukh-

gun valley is mostly of glacial origin.

Fig. 2 Landsat-TM mosaic image of the Khubsugul–Tunka area (lt513502 and lt413602; 21 August 1990 and 18 September 1989).White triangles underline major faults. NTF, north Tunka fault. The dotted circle indicates the epicentral area of the Mw 6.9,4 April 1950 Mondy earthquake. Focal mechanism is from Delouis et al. (2002). Rectangles show the relative location ofFigs 3(A), 4(A) and 5(A).

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The valley is 500 m wide and strikesE–W to the west and N120�E to theeast (Fig. 2). Along the western part

and on the southern slope, three en-echelon cliffs trending N95�E form a3.5-km-long scarp (Fig. 5). These

cliffs have heights increasing fromeast to west, from several metres toseveral tens of metres (Arjannikova

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and Arjannikov, 1999) and offsetstreams and slopes. At the base ofthe cliffs, a sheared zone, dipping40�S and showing reverse–left-lateralslickensides with 45–60�W pitches,were observed.From 101�10¢E to the east, the

valley changes its direction toN120�E (Fig. 2); discrete scarps canbe followed for 10 km along thesouthern slope as far as the easterntermination of the Ikhe Ukhgun val-ley. Beyond 101�15¢E, no morphotec-tonic features are visible, as the zone iscovered by unconsolidated lacustrine

sediments of Holocene age and LatePleistocene fluvio-glacial deposits.

Interpretation and discussion

TheMondy fault is at least 80 km longand appears to be segmented into fourparts, the longest part reaching 30 km(Fig. 2 and no. 2 on Fig. 6). The dip ofthe fault is almost vertical, but withvariations along strike, typical ofstrike-slip faults: it dips southwardalong segments 1 and 2 and northwardwithin segments 3 and 4. West of100�15¢E, no traces can be seen in the

morphology, either on images or in thefield (Fig. 2). Eastwards, the extensionof the Mondy fault south of the Tunkabasin remains unclear. East of101�30¢E, there is no morphotectonicfeature attesting to recent deforma-tion. Therefore, we suggest that thefault either dies out within the southTunka basin or is still active but notvisible at the surface, hidden beneathrecent sedimentary deposits.The morphotectonic features ob-

served along the Mondy fault (streamoffsets, scarps, uplifted ridge and ter-races) allow us to conclude that since

Fig. 4 (A) Landsat-TM image (lt413602; 18 September 1989): detail of Munku–Sardyk ridge. (B) Topography and morphologicalinterpretation showing parallel trends of fault traces; black arrows numbered 1, 2 and 3 correspond to left-lateral offsets ofwatercourses, which range from 700, 350 and 1000 m, respectively (legend as in Fig. 3). (C) View to the NW (see location onFig. 4B): E–W-trending fault trace is underlined by white triangles. Stream (white arrows) is offset 70 m left-laterally. Dip of thefault (70� northward) has been measured in the present-day watercourse (black arrow).

Fig. 3 (A) Panchromatic SPOT image (K241-J245; 28 August 1994): detail of Mondy area. The star indicates surface breaksobserved in 1950 following the Mondy earthquake. (B) Topography and morphological interpretation (elevations from 1 : 100 000Russian topographic map, with elevation contours every 200 m): (1) boundary of Cenozoic and Quaternary deposits, (2) alluvialfans, (3) conglomerates, (4) kame terraces, (5) Irkut alluvial terraces, (6) moraines, (7) lakes, (8) surficial traces of the Mondy fault,(9) crest lines. (C) View to the east (see location on Fig. 3B) showing the trace of the Mondy fault near 101�05¢E (black triangles)and the counterscarp developed by uplift of the hangingwall. Black arrows numbered 1, 2 and 3 indicate left-lateral stream offsetsof 160, 200 and 600 m, respectively.

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the Pleistocene the kinematics of theMondy fault is mainly left-lateral witha small reverse component at somelocations.The Ikhe Ukhgun fault is 20 km

long and is segmented into two parts(Fig. 2 and A,B on Fig. 6). Analysisof the morphology along the E–Wsegment attests to reverse and left-lateral movement along a southward-dipping fault and suggest that thefault was active during the Holocene.We therefore propose that the Mon-dy and Ikhe Ukhgun faults define acoupled fault system accommodatingoblique-slip deformation (Fig. 6).Their geometry suggests a faultjunction at a depth of 8–12 km.The left-lateral and reverse kinemat-ics proposed for the Mondy–IkheUkhgun fault system is consistentwith the seismological analysis madeby Delouis et al. (2002). With aregional compressional stress regimetrending N30�E, the local deforma-tion appears partitioned between theMondy left-lateral fault and the IkheUkhgun fault, mainly reverse.The four segments of the Mondy

fault reach a total length of 80 km

(Figs 2 and 6). As demonstrated, forinstance, for the 1992 Landers earth-quake (Bouchon et al., 1998), all thesesegments could have been activatedtogether during one strong event andtherefore be consistent with an Mw ¼6.9 earthquake. From scaling laws, astrike-slip earthquake with Mw ¼ 6.9should develop a surface rupture lengthof about 35 km and a downdip rupturewidth of about 12 km with an averagedisplacement of 0.8 m (Wells and Cop-persmith, 1994). Nevertheless (1) thesurface fractures reported after theearthquake (Treskov and Florensov,1952) reach only a few hundred metresand (2) they are located on the top of aterrace and parallel to its topographicboundary; this location therefore al-lows us to propose that these surfacefractures are secondary cracks. Unfor-tunately, this zone is covered by denseforest, and the analysis of aerial pho-tographs, taken 10 years after theearthquake, does not reveal any surfacebreaks. Therefore, a question remains:did the 1950 Mondy event break thesurface? Taking into account the relat-ively thick seismogenic layer found inthe crust of the Baikal rift zone [i.e. a

seismogenic thickness of about 35 kmand a transition from brittle to ductiledeformation at about 20–25 km depth,Deverchere et al. (2001)], and consid-ering the focal depth of around 14 km(Delouis et al., 2002) and the downdiprupture width of 12 km, it is possiblethat the rupture did not reach thesurface.Within the Tunka basin, the pre-

sent-day compressional style of defor-mation has been present since at leastthe Holocene. Indeed, Chipizubovet al. (2003) found Holocene ages forpost-glacial transpressional features inthe eastern part of the basin. Thetranspressional structures observed inthe Mondy–Ikhe Ukhgun area areconsistent with those observed inTunka. There, the amount of cumu-lative deformation suggests that thisnew regime started earlier in the Pleis-tocene. The rate of deformation in theMondy area is not easily quantifiablebut it is certainly low, as there is nomajor post-glacial deformation. If weconsider that the uplift rate estimatedfor the hangingwall, south of Mondy,corresponds to the vertical slip ratealong the Mondy fault, the minimum

Fig. 5 (A) Panchromatic SPOT image of the western Ikhe Ukhgun valley (K241-J245; 28 August 1994). (B) Topography andmorphological interpretation (legend as in Fig. 3). (C) View to the SE showing the major Ikhe Ukhgun scarp near 101�07¢E. (D)Field detail of the Ikhe Ukhgun scarp showing fault trace (white line) and left-lateral tectonic offset of watercourse (whitetriangles).

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250 m of cumulative vertical offsetdefined by the large-scale counterslopescarp along the fault represents atleast a 150-ka-old relief. Eastwards,the vertical slip rate of about1 mm yr)1 determined for the last500 kyr by McCalpin and Khromovs-kikh (1995) along an ENE–WSWsegment of the north Tunka fault isnot directly transferable to the Mondyfault, but it attests to a low deforma-tion rate as well. With regard tohorizontal slip, the GPS measure-ments (Calais et al., 2002) indicate arate of less than 1 mm yr)1 eastwardsfor the site BADA (Fig. 1) relative toEurasia that must be distributed, atleast, along the Sayan and Mondy–Tunka faults.

Conclusion

Within the transition zone betweentranspression in Mongolia and trans-tension in Baikal, the Mondy–IkheUkhgun fault system has been affected

by transpressional deformations sinceat least the Late Pleistocene. Thesetranspressional deformations post-date the transtensional regime thatwas active since the Oligocene. Thisrecent change in strain regime appearsas a regional phenomenon as it is alsoobserved within the eastern part of theTunka basin. At present, the bound-ary between transtension and trans-pression is located at the eastern partof the Tunka basin, at the southern tipof the Siberian platform.

Acknowledgements

We thank Valentina Melnikova for discus-sions about the different focal solutionsproposed for the Mondy earthquake. Care-ful reviews by F. Audemard and especiallyby J. Van der Woerd greatly improved anearlier version of the manuscript. The workof A.A. in France has been sponsored bythe French Ministry of Foreign Affairs(EGIDE grant 278849G) and INTAS grantYSF 2002-338. Fieldwork was funded byInterieur de la Terre INSU-CNRS project.

This is the UMR Geosciences Azur publi-cation no. 653 and the IUEM (InstitutUniversitaire Europeen de la Mer) contri-bution no. 916.

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Received 4 February 2004; revised versionaccepted 25 May 2004

Recent deformation in the Mondy–Tunka area • A. Arjannikova et al. Terra Nova, Vol 16, No. 5, 265–272

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272 � 2004 Blackwell Publishing Ltd