| The ongoing collision between India and Eurasia has caused widespread Cenozoicdeformation resulting in uplift of the Himalayas, the Tibetan Plateau and other mountain ranges incentral Asia and abrupt environmental change in Asia. The research on uplift of the TibetanPlateau and its dynamic mechanism have been the focus in the past many years. Recent studies onthe Damxung-Yangbajian graben have very important role to resolve the extensional tectonic, theextensional mechanism and the uplift of the Tibetan Plateau. But the ongoing debate about thecauses and magnitude of late Cenozoic east-west extension in Tibet reflect a lack of informationon the timing of initiation of east-west extension and the distribution and age of sediments ingrabens. Because of the lack on the distribution and age of sediments in grabens, the slip rate offaults are often inferred based on offset geomorphic features which were assumed to have formedafter the Last Glacial Maximum. In this paper, based on the detailed study the distribution and ageof the deposits in grabens and field mapping of active faults in Damxung-Yangbajain, we furtherexplore the distribution and age of the deposits in grabens, the normal faulting and left-lateralstrike slip along the Nyainqentanglha south-eastern piedmont fault zone (NSPF).1. Geologic setting and site descriptionThe studied area is located in the middle of Lhase terrane. The stratigraphic system f theterrane includes Proterozoic-Paleozoic matemorphic rock, Paleozoic mate-sedimentary rock,Mesozoic marine sedimentary, volcanic and igneous rock, Tertiary volcanic and igneous rock andQuaternary deposits. Pre-Neogene structures are mainly NWW and EW-trending thrust-fold zonein the region. Quaternary structures cut all preexisting structures, and are mainly NNE -trendingand south-north-trending normal faulting, NW-trending right-lateral strike-slip fault andNE-trending left-lateral strike-slip fault. The geophysical evidence including seismic reflectionand refraction, gravity and Magnetotelluric data suggest the Moho is 70～80km depth over most ofTibet, there is a partially molten in middle Tibetan crust, the lower crust is low viscosity, hot andweak, the India lithospheric mantle is underthrusting beneath the southern Tibet, the Asialithospheric mantle is underthrusting beneath the northern Qinghai-Tibetan plateau and the shearwave anisotropy polarization is NEE in central Tibet.2. Distribution and ages of basin-fill stratigraphyThe oldest observed deposits interpreted as graben fill are undated fluvial coarseconglomerates and lacustrine mudstone and sandstone, and mainly is exposed locally in easternJidaguo valley, south-western Yangbajain valley and the south-west and north-east of Nyinzongvalley. Based on morphostratigraphy, relative weathering and few sporopollen data, we suggestthese deposits were formed during Pliocene-early Pleistocene.Based on morphostratigraphy and relative weathering, we were able to divide the glacialsediments into four sets of Pleistocene glacial deposits Along both the south and north feet and inthe valleys of West Nyaiqentanglha Mountains. The first set of moraines is locally distributed atQianbuquan located in the north-east of Nyinzong valley, constituting the high piedmont moraineplatforms of 100-150m above the valley bottoms. The ESR age results of the moraines suggest theglacial stage occurred during 900～800ka B.P.. The second set of moraines is distributed at the feetof Nyaiqentanglha, constituting the high piedmont moraine platforms of 160-250m above the valley bottoms. On their front margins, there are always simultaneous high fluvioglacial platforms.Along the east and west sides of the outlets of Nia valley north of Damxung County, thefluvioglacial platform was displaced by the NE direction piedmont normal fault. On the slope ofthe fault and the top of fluvioglacial platform, two dating samples were collected. Their ESR datesare 593±260kaBP and 678±307kaBP, respectively. The third set of moraines is mainlydistributed in the glacial troughs cutting the mountains and their outlets, constructing the highlateral and terminal moraines of 40-80m higher than the valley bottom. The ESR and U-seriesdating of moraine indicates the moraines were formed during 300～112ka B.P.. At the northeast ofLargen valley, the simultaneous fluvioglacial platform cut the first set of moraine, formingplatform of 40-60m higher than the valley bottom. The ESR date of calcareous cement collectedfrom the central part of the platform is 205±54kaBP. The U-series date of calcareous cementcollected from the high lateral moraine of Laqu River of Ningzhong basin is 143.4±16.3 kaB.P.The fouth set of moraines is also distributed in the glacial troughs, constituting the low lateral andterminal moraines of 15-35m higher than the valley bottom. The U-series and OSL dating ofcalcareous cement and sand on the top of fluvio-glacial or proluvial platform in Yangbajain andNingzhong is 72.1±6.1kaB.P. and 25.4±8.7kaBP, respectively. These results of dating show thatthe ages of the three sets of glacial deposits along the southeastern foot of Nyaiqentanglha inDamxung County should be correspond to the Nieniexiongla Glaciation, Guxiang Glaciation andBaiyu Glaciation in the south Tibetan Plateau. The latter two glacial periods maybe able to dividedinto two-three stages.Nam Co is the biggest and highest great lake (4718m a.s.l.) in central Tibet plateau. Levellingof 19 profiles along the coasts of Nam Co shows that there are 6 lake terraces of 1.5-8.3m,8.3-15.6m, 14.0-19.9m, 18.7-25.8m, 26.0-37.3m and 38.3-47.6m and high lacustrine sedimentsfrom 48m to 139.2m above the lake level. Below 27m a.l.l.,8 to 30 coastal levees and an erosionaltrough at the height of 17.5-19.8m which is as high as the waterdivide between the Nam Co basinand the Ren Co-Jiuru Co basin are discovered. 12 carbonate-bearing sediments samples oflacustrine and beach facies from 7 profiles along the coasts of Nam Co and 3 samples from theadjacent lakes have been dated by an isochron technique of total-sample-dissolution. Result ofU-series dating indicates that the highstand lacustrine sediments, lake terraces of Nos.6, 5, 4, 3, 2and 1 were formed in ca. 115.9-71.8 ka B.P., 53.7 ka B.P., 41.2-39.5 ka B.P., 36.1-35.2 ka B.P.,32.3 ka B.P., 28.2-11.2 ka B.P. and 11.2-4.2 ka B.P., respectively, and the beach conglomerate ashigh as the lake eroded trough, a little older than 29.3±2.7kaBP.There are a great many alluvial and proluvial sediments formed during late Pleistocene inDamxung-Yangbajain basin and valleys located in Pangduo moutains. In theDamxung-Yangbajain basin, the late Pleistocene sediments constitute alluvial and proluvial fansor platforms 10～25m higher than valley floor and are formed about 25kaB.P. simultaneous withthe latter deposits of the third set of moraines and glaciofluvial deposits. The Holocene depositscontitute fluvial flat, river terrace and proluvial fans distributed in valley cutting Nyaiqentanglhamountains and the center of basin. At eastern Wumatang, the date of silt collected from top ofriver terrace,～6m above valley floor, is 3.9±0.3kaB.P.. At north-west of Yangbajain, the dates ofpeat and sand collected from upper of river terrace,～4m above valley floor, are 4.26±0.17kaB.P.and 3.7±0.7kaB.P. respectively. The dates of terraces, 4～6m higher than valley floor, indicate thatthe incision of the terraces is correspond to Neoglaciation. In the Pangduo mountains, there are 6river terraces of 3-5m, 15-25m, 30-45m, 55-70m, 80-90m and 100-110m above the valley bottem formed since middle Pleistocene. there are some travertine platforms located in the Gulu,Damxung, Nyinzong, Laduogan, Yangbajain ang Jidaguo valley. ESR and U-series datingindicates travertine were formed since 500kaB.P..3. Paleo-climate and Paleo-environment during QuaternaryGlacial successions and dating of moraines in the Nyainqentanglha mountains indicated thatthere were multi-stage glaciations including Qianbu Nyainzong glacial period, Paran glacialperiod and Laqu glacial period occurred during approximately 900～800ka B.P., 700～600ka B.P.,300～120ka B.P., 75～15ka B.P. in the Pleistocene respectively, broadly associated with marineisotope stage (MIS)-26～22, MIS-18～16, MIS-8～6 and MIS-4～2, and Neoglaciation and Little IceAge occurred during Holocene. In south-eastern piedmont of Nyainqentanglha, There were 2～3glacial advances occurred during MIS8～6 and MIS 4～2.Based on the mentioned-above data, this paper divides the lake development of Nam Co intothe three stages: (1) Qiangtang palaeo great lake of the early-middle Late Pleistocene before40kaBP, (2)outflow lake between 40 and 30kaBP and (3) Nam Co since 30kaBP. In the stage ofpalaeo great lake, a series of lakes, such as Nam Co, Siling Co and other big and moderate lakes inthe southeast Qiangtang Plateau were joined together into a great lake whose range might overstepthe water divide between the inland lakes and the Nujiang River system, but not a number oflakes joined each other by rivers. We name the palaeo great lake the "East Qiangtang Lake" whichmight be joined together with the other palaeo great lakes in the central-southern and thesouthwestern Qiangtang Plateau, formed a united palaeo great "Qiangtang Lake".Based on the data of age results, terraces, lacustrine sediments and sporopollen analysis, thepalaeovegetation, palaeoclimate and lake-level fluctuation of Nam Co since approximate 120 kaB.P. are discussed in this paper. The change of palaeovegetation, palaeoclimate and lake-levelfluctuation of Nam Co since Last Interglacial Stage can be divided into the five stages: (1) In116-78ka B.P., lake-level fluctuated between 140m and 88m above lake level. The climate waswarm and slightly moisture. The vegetation was forest-steppe and steppe with sparse-forest. (2) In78-53ka B.P., lake-level strikingly dropped and fluctuated between 48m and 36m above lake level.The climate was colder and drier than that at previous. The vegetation was steppe with a few trees.(3) In 53-32ka B.P., lake-level fluctuated between 15m and 28m above lake level. The climate waswarmer and wetter than that of present and there were 3 stages of warm corresponded to thelake-terraces of Nos. 5,4,and 3. The vegetation was forest and forest-steppe. Particularly in periodof 36-35ka B.P., the coniferous and deciduous broad-leaf mixed forest occurred. It shows, in theperiod, the climate is the warmest and wettest during the Last Glacial Stage and also is more warmand wet than that of the present. (4) In 32-12 ka B.P., lake-level strikingly dropped and fluctuatedbetween 17m and 12m above lake level. The climate was cold and dry. The vegetation was steppeor steppe with sparse trees. The lowerest lake-level is 8m above lake level in the period. (5) In11.8-4.2ka B.P., lake-level fluctuated between 2m and 9m above lake level. The climate was warmand wet The vegetation was forest and steppe with a few trees. Especially, the coniferous anddeciduous broad-leaf mixed forest occurred during 8.4-4.2ka B.P. which is the warmest andwettest during Holocene. The most lower lake-level is under present lake level in the period.Acomparison of the climate change and lake-level fluctuation of Nam Co with the data of oxygenisotopic curve from the Guliya ice core in West Kunlun Mountains and the ice core of GRIP2shows the cooling and warming change of Climate in Nam Co area is corresponded to the climatechange of Tibet plateau and northern globe. Especially, the striking lake-level dropping revealed by lake terraces formed is broadly associated with Henrich events.Based on the data of 14C, TL, OSL and U-series age results, the geological evidences fromlacustrine sediments and sporopollen analysis of lake terraces T1, glacier fluctuation,paleo-cryoturbation, palaeosol and palaeo-carbonaceous sediments indicate that the chang of thepalaeovegetation, palaeoclimate and palaeoenvironment during Holocene in Nam Co and itsadjacent area can be divided into the three stages:(1) In 11.8～8.4ka B.P.. The climate was relative warm with a few wet to the previous period.The lower part of Nam Co lake terrace T1 was formed. The vegetation was Artemisia steppe with afew Pinus and Betula trees around the Nam Co, and alpine Cypressaceae meadow in themountains of Damxung's adjacent area. The lake level fluctuation and carbonaceous sedimentswere not prominent in the period.(2) In 8.4～4.2 ka B.P.. The Holocene Megaathermal occurred in the period. The climate ofthe period was the warmest and wettest in Holocene. The middle-upper part of Nam Co laketerrace T1 was formed. The vegetation was coniferous and deciduous broad-leaf mixed forest orforest-steppe with Pinus, Betula and Artemisia around the Nam Co, and alpine Cypressaceaemeadow with a few subalpine shrubbery in the mountains of Damxung's adjacent area. thetemperature was～5℃higher than that of the present and the precipitation was～100～200mmhigher than that of the present. The lake level of Nam Co was rise and probably higher 10.2m thanthe present lake level. The extent and deposited rate of carbonaceous sediments increased in theNam Co and its adjacent area. In addition, the black gray carbonaceous fossil soil, the pluvial fanin east-southern Damxung and the river terrace T1 of～4m above river level in theDamxung-Yangbajain basin were formed in the period.(3) Since 4.2 ka B.P. The climate was colder and drier than that during 8.4～4.2 ka B.P.. Thevegetation was alpine Cypressaceae meadow in the mountains of Damxung's adjacent area.Lake-level of Nam Co dropped and terraces T1 was formed. The Lake-level experienced 12～17fluctuation between 2m and 9m above lake level. At the same time, the glaciers of Xibu haveexperienced 12 fluctuation in Nyainqentanglha. There are 5 fluctuation during Neoglaciation, 6fluctuation during Little ice age and 1 fluctuation since 1970A.D. The cryoturbation oflate-Quaternary sediments the grey-yellow silty loam occurred in the period. Though the climatewas cold in the period, it obvious have been warmer since 1970A.D. Because the glaciers of Xibuhave retreated approximately 120～200m and the Lake-level of Nam Co rise about 2m since1970A.D.4. Geometric pattern, characteristics of deformation and seismieity along NSPFThe～240km long and south-east facing Nyainqentanglha south-east piedmont fault zone iscomprised of the～50km long west boundary fault of Gulu graben striking N10～15°E and dippingeast to the north, the～130km long north boundary fault of Damxung-Yangbajain graben strikingN30～50°E and dipping south-east to the center and the～60km long graben-bounding fault strikingsouth-north to the south. The NEE and south-north trending northern and southern fault segmentsare dominantly normal-slip, whereas the NE-trending central fault segment is comprised of someleft-lateral enéchelon array normal faults striking NNE-NE and three NEE-trending left-lateralstrike-slip faults. The strike-slip faults which connect overstepping segments of parallel or enéchelon normal faults and "transfer" the displacements across the enéchelon array normal faultszone are transfer fault.The NSPF offsets river terraces, glacial lateral-moraine, and alluvial fan at the piedmont. The displacements of Quaternary geological units and geomorphic surface with different age isobvious different. Based on 214 data of the offset terraces, glacial lateral-moraines, alluvial fans,gullies and beheaded channels, the displacements range from～500-0.7m. The variations indisplacements presumably reflects the relative ages of the offset geomorphic features with theleast incised and smoothest (i.e., Youngest) and relatively lower alluvial or glacial surfacesdisplaying the smallest offsets. Based on the dates of the deposits determined by ESR, U-seriesand OSL (Table 1) and the relationship between the glacial or glaciofluvial deposits and glacialcycle in Tibet plateau, The age range of the offset Q22-1gl-fgl, Q22-2gl-fgl, Q23-1gl-fgl, Q23-2gl-fg, Q32gl-fgl,Q33fgl-pal, Q41pal and Q42pal is between 400～340kaB.P., 300～240kaB.P., 200～170kaB.P.,160～125kaB.P., 75～58kaB.P., 32～15kaB.P., 12～SkaB.P. and 8～4kaB.P. respectively. Sodisplacements of offset Q21gl-fgl, Q23gl-fgl, Q32gl-fgl, Q33fgl-pal and Q41pal represents the cumulativethrows since 400～340kaB.P., 300～240kaB.P., 200～170kaB.P., 160～125kaB.P., 75～58kaB.P.,32～15kaB.P. and～12kaB.P.. The scarp height of different ages indicates the fault verticalslip-rates are approximately 0.8～1.3mm/a, 0.7～1.3mm/a, 0.4～1.3mm/a, 0.2～1.8mm/a,0.4～2.8mm/a and 0.6～5.3mm/a since 400～200kaB.P., 170～160kaB.P., 125～75kaB.P., 58～32kaB.P.,15～12kaB.P. and 8～4ka B.P. respectively. The fault vertical slip-rates are approximately 0.86mm/a,1.14mm/a, 1.33～2.44mm/a, 0.34～0.72mm/a, 0.11～1.04mm/a, 0.21～2.53mm/a and 0.6～5.3mm/aduring 340～240 ka B.P., 240～170 ka B.P., 170～125 ka B.P., 125～58 ka B.P., 58～15 ka B.P., 15～4ka B.P. and 4～0 ka B.P. respectively. The left-lateral strike-slip rate is 1～4mm/a along strike-slipfault in northern Damxung-Yangbajain graben. This corresponds to a extensional rate 2.0±1.2mm/a across Gulu-Yangbajain-Jidaguo rift.The Damxung-Yangbajain and adjacent area is a region of strong seismicity. The twodocummented great earthquakes of magnitude 8 and 7.5 occurred at northern Yangbajain andwestern Gulu in 1411 A.D. and 1952 A.D. respectively. The surface ruptures associated with 1411A.D. Ms.8 of Yangbajain earthquake is distributed along central and southern segments of NSPF.The maximum vertical offsets are 8～10m at northern Yangbajain valley. The surface rupturesassociated with 1952 A.D. Ms.7.5 of Gulu earthquake is distributed along northern segment ofNSPF. The maximum vertical offsets are 4～5m at south-western Gulu valley. The magnitude 4～8earthquakes occurred along NSPF during 1900～2000 A.D. indicate the recurrence interval ofMs.5.7～6.9 earthquakes is 15±5a. But most earthquakes occurred along the southern andnorthern segments of fault zone. The central segment of fault has not produced an≥Ms.6earthquke during 80 years of historical record. The colluvial wedge and the convex break of faultscarp and the multi-terraces close to fault on footwall indicate that there are at least 6 to 7paleo-earthquakes (Ms.7 to 8) since approximately 18～15ka B.P. and 2～3 paleo-earthquakes (Ms.7to 8) since approximately 4～5ka B.P.. Based on dating of buried soil in colluvial wedge and offsetterraces, we suggest that there were 3 paleo-earthquakes (Ms.7 to 8) occurred in approximately 7.4±0.7 ka B.P., 4.5±0.3 ka B.P. and 2.3±0.2 ka B.P., and the recurrence interval of Ms.7 to 8earthquake is 2300±700a and 1700±200a along northern and central segments of NSPF since8ka B.P. and 5ka B.P. respectively.5. Characteristics and mechanism of east-west extensional deformation during late CenozoicThe basin-range tectonic geomorphic system and the geometric pattern and sense of motiondifferent trending active faults correspond with east-west extensional deformation atDamxung-Yangbajain basin and adjacent area. Analysis of topography acrossDamxung-Yangbajain graben shows that high topography ocurrs along graben margins, the uplift rate of Nyainqentanglha mountains is fastest, the grabens are only 5～15 wide, and the wavelengthof the flank uplift is narrow (20～40km). The features correspond with flexural rift flank uplifts inelastic upper crust under vertical loading, resulting from regional isostatic compensation of lowdensity graben. The existence of a hot and low-viscosity lower crust suggest isostaticcompensation is taking place within this ductile lower crust. The geometric pattern and sense ofmotion of NSPF and extensional ductile shear zone of Nyainqentanglha south-east piedmontshows that the high-angle and parallel or enéchelon army normal faults distributed in brittleshallow crust result from the development of subhorizontal detachment faults in the ductile middlecrust. Thermochronology data on Yangbajain granite, Nyinzong granite, Nyainqentanglha graniteand extensional ductile shear zone of Nyainqentanglha south-east piedmont shows that the age ofonset of east-west extension is constrained to be during 18.3～11Ma, the rapid denudation ofmountains around Damxung-Yangbajain basin since～10Ma results from rift extension. Thecharacteristics of late Cenozoic extension deformation in central Tibet indicate that the distributedand brittle east-west extension deformation of upper crust is driven by rheological and plasticlower crust eastward extruding or flowing without significant increase in height because a hot,low-viscosity and weak lower crust and buoyancy force associated with the exaggerated thicknessof Tibet crust inhibits further crust thickening and WNW-ENE extension stress became theminimum principle stress when the thickness of crust is two times of normal crust in process ofthe convergence between India and Eurasia.
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