geomorphological evidence of active faulting in low …...regional tectonic geomorphology studies...

Conclusions

• KinematicsoftheValleyofLakesfaultsfitwithintheregionaltec-tonicleft-lateralstrike-slipregime.

• AllfaultsintheValleyofLakescanaccommodateM~7earthqua-kes,andhavebeenactivesincePleistocenetimes.

• Verticalslipratesaccummulateto0.31±0.12mm/yr,whichisofthesameorderasupliftalongtheBogdfault;however, reccur-renceintervalsareslowincomparisontothe3-5kyrestimatedintheGobiAltai.

• ItremainsunclearhowthestrikeslipsystemsoftheGobiAltiai,ValleyofLakesandHangayconnectatdepth.Likely,thesurfaceexpressionofactivefaultingisrelatedtotheoverprintingofpre-existingcrustalstructures.

• MiddlePleistocenetomoderndrainagenetworkevolutionoftheTuynGolValleyreflectstectonicactivityaswellaslakelevelvari-ationsintheOrogNuurBasin.

• ThestrongtectonicimprintontheTuynGoldepositsanddrainagereorganisationshighlights the importanceof regional, long-termstudiestoassesstheseismichazardinareasoflowinstrumentalseismicity.

Geomorphologicalevidenceofactivefaultinginlowseismicityregions-examplesfromtheValleyofLakes,southernMongoliaJorienL.N.vanderWal1,V.Nottebaum2,G.Stauch2,F.Lehmkuhl2,K.Reicherter1

1NeotectonicsandNaturalHazards2PhysicalGeographyandGeoecologyRWTHAachenUniversity,Germany

ReferencesHeidbach,O.,Rajabi,M.,Reiter,K.,Ziegler,M.,WSMTeam(2016).WorldStressMapDatabaseRelease2016,V.1.1.GFZDataServicesNottebaum,V.,Stauch,G.,vanderWal,J.L.N.,Zander,A.,Reicherter,K.,Batkhishig,O.,Lehmkuhl,F.(inreview).SedimentarysysteminteractioninendorheicbasinsduringthelateQua-ternary:theOrogNuurBasininsouthernMongolia,submittedtoEarthSurfaceProcessesandLandforms,2020Rizza,M.,Ritz,J.F.,Braucher,R.,Vassallo,R.,Prentice,C.,etal.(2011).SliprateandslipmagnitudesofpastearthquakesalongtheBogdleft-lateralstrike-slipfault(Mongolia).Geophy-sicalJournalInternational,186(3),897–927Vassallo, J.F.Ritz,R.Braucher,M.Jolivet,S.Carretier,etal.,2007;Trans-pressionaltectonicsandstreamterracesoftheGobi-Altay,Mongolia.Tectonics,AmericanGeophysicalUnion(AGU),2007,26(5),pp.TC5013Wells,D.L.,&Coppersmith,K.J.(1994).NewEmpiricalRelationshipsamongMagnitude,RuptureLength,RuptureWidth,RuptureArea,andSurfaceDisplacement.BulletinoftheSeis-mologicalSocietyofAmerica,84(4),974–1002

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Fig. 1 A)TectonicsettingofMongoliainrelationtostructureswithintheAsiancontinentandtheIndia-Eurasiacollision;cut-outofregionalhistoricandinstrumentalseismicityofMongolia(1900-2000;adaptedfromDugarmaaandSchlupp,2003);B)MorphotectonicmapoftheTuynGolValleyhighlightingmultiplepreviouslyunmappedfaults(kinematicsinblack:trianglesindicatethrusting,arrowsindicatestrike-slipmovement,linesindicatenormalfaulting),andalluvialplainsassociatedwiththeTuynGol(TG),includingapaleo-channel(PC).GreyoutlinesindicateOrogNuur(ON)lakelevelhighstandsdatedbyNottebaumetal(inreview).BlackN-Slineindicatesthelocationofthecrosssectiondiscussedin‚Structuralsetting‘;SHmaxisinferredfromtheWorldStressMap(Heidbachetal.,2016);C)Timelineoflandscapeevolutionsuggestingthetimingofterraceandfaultactivity,andincludinglakelevelhighstands;see‚Landscapeevolution‘forfurtherexplanationofourinterpretations.

Characterisationoffaultactivity

TheValleyofLakesfaults(Fig.1B)caneachaccommodateM~7earthquakeswithupto~1.7moffsetperearthquake(Wells&Coppersmith,1994).Verticaloffsetsmea-suredacrosstheT3surfaceeastoftheTuynGolriver(seetable)reflectcumulativedeformationalongthefaultscarps.Wedatedthissurfacetobe~400ka(blackcirclesinFig.1B),anddeterminedassociatedslipratesandrecurrenceintervals.

Background&Methods

Asaresultoffar-fieldstressesrelatedtotheIndia-Eurasiacollision,activefaultinginsouthernMongolialocalizesonthemarginsoftherigidHangaydome,inthetranspressionalAltaiandGobiAltairanges.TheAltaisandtheHangayaccom-modatedfourM~8earthquakesinthelastcenturyhowevertheyareseparatedbytheseismicallyquiescentValleyofLakes.QuaternarydepositsoftheTuynGolriver,thatflowsintotheendorheicOrogNuurintheValleyofLakes,arecrosscutbyE-WstrikingfaultscarpsthatcouldnothaveformedonlybytheminorM~4earthquakesrecordedbyinstrumentalseismicity(Fig.1A).ToassessandquantifythetectonicactivityalongtheseValley of Lakes faults, weevaluatedtheregionaltectonicgeomorphology.Weanalysedfaultkinematics,quantifiedverticaloffsetsandestimatedslipratesfromagesofassociatedfanlevelsdeterminedbyBe10cosmogenicnuclidedating.Additionally,wecombinedtectonicandgeomorpho-logicalmappingtoreconstructlandscapechangesoftheTuynGolValleysinceMiddlePleistocenetimes.

Structuralsetting

Thekinematicsof theValleyofLakesfaults(Fig.1B)fitwellwiththeNW-SEdirectionofmaximumprincipalstress.Althoughlateraloffsetscannotbequantifiedfrommorphotectonicobservations,weinterpretthefaultstoreflectastrike-slipregime(seefigurebelow)similar to,howevermorewidespread, than in theGobiAltai.Theregionalspreadofdeformation is likely related todifferentialsub-surfacestructuresthatareoverprintedbytheCenozoictomoderntranspressionalregimeinsouthernMongolia.

Landscapeevolution

Assuming that alluvial fan abandonment and incision occurs at glacial-interglacialtransitionsthatoccurevery~100kyrclimatecycle(suggestedfortheGobiAltaibyVassalloetal.,2007andRizzaetal.,2011),wecancombineourgeomorphologicalmapping,tectonicinterpretationsandcosmogenicnuclidedating(Fig.1B)todetermi-neatimelineforlandscapeevolutionintheValleyofLakes(seeFig.1C).• NoremnantsofT3arefoundsouthofF1,soF1musthavebeenactivepriororcoevaltoT3deposition.

• T2depositionisconfinedbyF2,implyingpriororcoevalfaultactivity,relativetode-position.

• Paleochannelactivitylikelystartedduringthefollowingglacial-interglacialtransiti-on.ThepreservationofT2terraceremnantswithinT1impliesthatF3wasactiveduringthistime.ActivityofthechannellasteduntilaftertheMIS5(130-80ka)lakelevelhighstandofOrogNuur(ON;Nottebaumetal.,inreview):fluvialsedimentsattheoutletofthepaleochannelinterferewithbeachridgesassociatedwiththelakelevelhighstand.

• Deflectionfromthepaleochannel(PC)totheactiveTuynGolchannel(TG),withsubsequentformationofHolboliinNuurbyactivityalongF2followed.

• DepositionofT0musthaveoccurredpriortothemid-Holocenelakelevelhighstand(~7ka;Nottebaumetal.,inreview).

Results

Mainconclusions

FaultsintheValleyofLakesaretectonicallyactiveatratessimilartosurroundingsystemsintheGobiAltaiandHangay,andtheyhaveplayedamajorroleindrainagenetworkevolutionsinceMiddlePleisto-cenetimes.

Regionaltectonicgeomorphologystudiesthatspantimescales longer than thepaleoseismological re-cordareessentialforseismichazardassessmentinslow-slipregionswithlittleinstrumentalseismicity.

A B

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JorienL.N.vanderWal,MScTel.:+49(0)2418098906E-Mail:j.vanderwal@nug.rwth-aachen.de