Table of Content

29 June 2011, Volume 33 Issue 2

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Brief Report

LATE CENOZOIC AND PRESENT TECTONIC DEFORMATION IN THE PAMIR SALIENT,NORTHWESTERN CHINA

CHEN Jie, LI Tao, LI Wen-qiao, YUAN Zhao-de

2011, 33(2):  241-259.  DOI: 10.3969/j.issn.0253-4967.2011.02.001

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The northern margin of the Pamir salient indented northward by ～300km during the late Cenozoic,however,the spatiotemporal evolution of this process is still poorly constrained.Regional deformation within the Pamir salient is asymmetric.Previous work has shown that deformation along the western flank of the Pamir was accommodated by northwest-directed radial thrusting and associated anticlockwise vertical axis rotation of the Pamir over the eastern margin of the Tajik Basin,along with a component of left-slip faulting along the Darvaz Fault.In contrast,subduction of the Tajik-Tarim Basin beneath the Pamir along the MPT was absorbed along the eastern margin of the salient by dextral-slip along the Kashgar-Yecheng transfer system,accompanied with Oligocene-Miocene northward underthrusting, thickening and widespread melting of the middle and lower crust beneath the Pamir,eventually led to east-west extension along the Kongur Shan extensional system at ～7～8Ma.The slip rate of the KYTS decreased substantially from 11～15mm/a to 1.7～5.3mm/a since at least 3～5Ma,termination of slip along the northern segment of the Karakorum Fault occurred almost at the same time.Late Quaternary and present active deformation in the Pamir is dominated by east-west extension along the Kongur Shan extensional system and north-south contraction along the PFT and the Atux-Kashi fold belts in the southern margin of Tianshan.

COSEISMIC SURFACE RUPTURES OF MULTI SEGMENTS AND SEISMOGENIC FAULT OF THE TASHKORGAN EARTHQUAKE IN PAMIR,1895

LI Wen-qiao, CHEN Jie, YUAN Zhao-de, HUANG Ming-da, LI-Tao, YU Song, YANG Xiao-dong

2011, 33(2):  260-276.  DOI: 10.3969/j.issn.0253-4967.2011.02.002

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Based on the interpretation of satellite images,combined with field geomorphic and tectonic investigations and surveys,we get the parameters of surface rupture zones of the 1895 Tashkorgan earthquake,such as the geometry,the types of rupture,the displacements and their distribution and so on.And on these grounds,we estimate the possible magnitude,the epicenter and seismogenic fault of this earthquake.The south segment of Muztag Fault and the whole Taheman Fault were ruptured by the Tashkorgan earthquake.The length of the surface rupture zone is 27km.The rupture zone strikes NNE,and it changes from N25°W in the north to N25°E in the south segment.The surface rupture zone is composed of consequent or obsequent normal fault scarps,represented by horst,graben,and step-like structure on the profile,and distributed in patterns as en echelon,parallel,convergent and parallel cross shaped and so on in the plane.The surface ruptures are dominated by pure dip-slip,with little lateral displacement.The general width of these overlapping surficial fault rupture strands is ca.30～60m, and the largest may come to 825m.The largest co-seismic displacement of a single scarp is 4.2±0.2m. The surface ruptures are composed of three independent secondary segments.The seismogenic fault of this earthquake is Taheman Fault.The south segment of Muztag Fault was also ruptured.Moreover,we find a younger fault scarp which may be induced by the 1895 earthquake in the small basin between the two above-mentioned faults.

THE DISCORVERY AND GEOLOGIC SIGNIFICANCE OF THE THRUST AT SOUTH LIMB OF THE WULAGEN ANTICLINE, SOUTH MARGIN OF THE SOUTHWESTERN TIANSHAN

LI Tao, CHEN Jie, XIAO Wei-peng, Burbank D W, Thompson J, YANG Xiao-dong

2011, 33(2):  277-288.  DOI: 10.3969/j.issn.0253-4967.2011.02.003

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The western Tarim Basin is a convergent zone of the Southwestern Tianshan and the Pamir,and there have been big debates about its exact boundary.However,in the Mayikake Basin,the boundaries of two tectonic systems are very clear: the north-vergent Pamir Front Thrust is the leading edge of the Pamir,and south-vergent thrust at south limb of the Wulagen anticline,which was discovered in recent field study,is the south margin of the Southwestern Tianshan.The thrust created 7.5～17.6m high scarps on the Tk3(the high terrace of the Kezilesu river)and Tb3(the high terrace of the Bieertuokuoyi river),with an occurrence of 6°∠15°.To the west,the thrust cuts all terraces of the Bieertuokuoyi river and the underlying youngest alluvial fans ultimately.The total length of thrust trace is about 12km.As activity of the thrust,lots of subparallel flexural-slip scarps are formed on terrace surfaces,which make terrace surfaces obviously differential back-tilted(tilted to south),and the locations of tilted degrees changed are corresponding to locations of flexural-slip faults.Shortening at south limb of the Wulagen anticline is absorbed by the thrust and flexural-slip faults,which is about ～71.4m since abandonment of the high terrace.Regional correlation indicates the high terrace is the same surface as the T2 located at north limb of the Mushi anticline with the age of～16ka,which indicates the average shortening rate of south limb of the anticline in late Quaternary is～4.5mm/a.

QUATERNARY DETACHMENT FOLDING AND PROPAGATION OF NORTH LIMB FAULT OF MUSHI ANTICLINE,NORTHERN MARGIN OF THE PAMIR

XIAO Wei-peng, CHEN Jie, LI Tao, LI Wen-qiao, Thompson J

2011, 33(2):  289-307.  DOI: 10.3969/j.issn.0253-4967.2011.02.004

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The Mushi anticline locates at the frontier Pamir arcuate nappe tectonics belt(PFT),which is a detachment fold with a gentle south limb and steep north limb,and its earth crust minimum shortening is ～0.7km with uplift up to 1.5km.The north limb fault of Mushi anticline is composed of a series of obsequent slope fault scarps,and the distribution of vertical displacements among different fault scarps presents a pattern of one increasing and the other decreasing.No matter of the entire western segment of the northern limb faults or a single fault,the displacement distribution is asymmetric,that is,high in the east and low in the west,and the same to displacement gradient.This may reflect the late Quaternary folding of Mushi anticline as being intensive in the east and feeble in the west.The fault may be a shallow,rootless secondary fault formed during the growth process of the anticline in order to accommodate the constantly decreased space of anticline nucleus as the fold tightened gradually.The late Quaternary shortening rate of the fault is 0.8mm/a,absorbing only one fifth of the nowadays crustal shortening rate of the region.The growth of Mushi anticline and the north limb fault of Mushi anticline both are in accordance with global fault dataset scaling relationship,that is,fault length is over 100m.The power-law regression scaling exponent of west segment of the northern limb fault of Mushi anticline is n=1.37(R²=0.88),and its specific value(k)of maximum fault displacement and fault length is far less than that of the Mushi anticline,which is ～4.3％,but 1～2 orders of magnitude larger than that of global fault dataset(10^-4～10^-5). This may show that the northern limb fault of Mushi anticline is the offshoot of several moderate strong earthquakes,and it is still in initial stages.

USING DEFORMATION TERRACES TO CONFINE THE SHOR-TENING,UPLIFT AND LATERAL PROPAGATION OF THE MUSHI ANTICLINE,NORTHERN MARGIN OF THE PAMIR

LI Tao, CHEN Jie, XIAO Wei-peng, Burbank D W, HUANG Ming-da, Thompson J

2011, 33(2):  308-322.  DOI: 10.3969/j.issn.0253-4967.2011.02.005

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Locating at eastern end of the Pamir Front Thrust(PFT),the Mushi anticline grows initiating from early-Pleistocene till now.The anticline,with a gentle south limb and steep north limb,outcrops Pliocene Atushi formation and lower-Pleistocene Xiyu formation.Topographic profiles and drainage pattern indicate the lateral growth of the anticline from west to east.Combining mapping data and seismic profiles from the neighboring area,we find the Mushi anticline is a detachment fold,with a total shortening of ～0.7km and a total uplift up to～1.5km.Northern part of the anticline is dominated by a series of wide,flat terraces.According to OSL samples,the age of the terrace T2a,T3and T4 is 15.8±2.4ka,55.1±10.3ka and 131.4±23.9ka respectively.Correlating with Marine Isotopic stages(MIS),the formation of terraces has some relationship with global climate change.As growing of the anticline,terrace surfaces deformed obviously,which is characterized by fault scarps,surface tilting or back-tilting,folding scarps and lateral tilting.Deforming patterns of the terrace surfaces indicate the Mushi anticline grows by limb rotation in late-Pleistocene.Using calculating models,we can confine the minimum shortening rate is 1.6±0.3mm/a and the minimum uplift rate is 1.9±0.3mm/a. Longitudinal profiles of terraces indicate the Mushi anticline grows laterally through limb rotation.According to relationship between uplift and lateral propagation,we can acquire a faster eastward lateral propagation rate of the anticline during the period of 131～16ka,with a rate about 14.6±3.6mm/a; however,since 16ka,the rate reduced to 1.7±0.3mm/a,implying the anticline tip stopped propagating to the east,and growing of the anticline was mainly dominated by lateral limb rotation in late Quaternary.

TECTONIC AND CLIMATIC SIGNIFICATION OF TERRACES AT THE OUTLET OF MOLEQIEHE RIVER, THE WESTERN SEGMENT OF ALTYN TAGH FAULT ZONE

ZHENG Rong-zhang, XU Xi-wei, MA Wen-tao, LI Jian-ping

2011, 33(2):  323-334.  DOI: 10.3969/j.issn.0253-4967.2011.02.006

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Based on the interpretation of satellite image,field investigation and geomorphic survey and sample dating of surface,the strath terrace and fill terrace at the outlet of Moleqiehe River on the western segment of Altyn Tagh Fault zone(ATF)are used to study the tectonic uplift rate,uplift model and aggradation rate,and cooperated with data of the regional climate,the response to climate evolution of development of terrace are discussed.The previous studies of the terraces related with the Altyn Tagh Fault zone are mainly focused on the horizontal offset in order to obtain sinistral-slip rate,but few studies involve the uplift using terraces.As a structural zone with strike-thrust characteristic,the ATF is a boundary structure of the northern fringe of Qinghai-Tibet Plateau,and its thrusting and uplifting movement is of significance for controlling the uplift of the northern fringe of the Plateau.Therefore, the study of uplift of the ATF will be helpful for understanding the uplift model and mechanism and promoting the kinematic study of the northern fringe of the Plateau.The formation of strath terrace is closely related with tectogenetic movement,the landform age of the terrace represents the starting time of uplift.Based on the height and strath and landform age,the uplift rate can be calculated.The fill terrace is formed by climate forcing,the surface age represents the end time of one aggradation event.If the starting time of aggradation is obtained,the aggration rate can be calculated.There are four stream terraces at the outlet of Moleqiehe River(T₄,T₃,T₂,and T₁). T₄ and T₃ are strath terraces,T₂ is fill terrace,and T₁ are fill-cut terraces.The landform ages of T₃,T₂,and T₁ are 18.98±1.42ka BP,13.08±1.01ka BP,and 5.72±0.43ka BP,respectively.The existence of T₃ reveals the uplift rate of 6.66±0.50mm/a since 18.98±1.42ka BP.The existence of T₃ and T₂ reveals the time of fast uplift movement and aggradation events between 18.98±1.42ka BP to 13.08±1.01ka BP,the uplift rate is bigger than 20mm/a and the aggradation rate bigger than 10mm/a.The model of tectonic uplift shows tilted uplift from south toward north across the ATF,and this model is one of the types of the Qinghai-Tibet Plateau extending toward north.The aggradations,that constructed the T₂,are the result of the coactions of fast uplift and deglaciation climate between 15ka BP to 12ka BP.

LATE QUATERNARY FAULTED LANDFORMS AND DETERMINATION OF SLIP RATES OF THE MIDDLE PART OF KOUQUAN FAULT

XU Wei, LIU Xu-dong, ZHANG Shi-min

2011, 33(2):  335-346.  DOI: 10.3969/j.issn.0253-4967.2011.02.007

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Kouquan Fault is located in the north part of Shanxi graben which controls the west edge of Datong Basin.Two M6 1/2 earthquakes happened in the west side of basin in historical time,and there has been a concern about the future hazard of the fault.However,previous researches on Kouquan Fault were limited only in several points,especially,there was lack of measurements and dating data.Based on the 1 :50000 geological mapping of Kouquan Fault,the paper investigates the late Quaternary faulted landforms of its middle part(the part between Shangshenquan village and Yangjiayao village),combining with remote sensing interpretation of Spot image and field validating of the study area,and finally obtains the late Quaternary dip-slip rates of this fault.Five stratiform landforms can be found from piedmont to riverbed.The topmost part(the fifth geomorphic surface)is piedmont erosion surface which might be the planation surface of Tangxian period; the fourth geomorphic surface,which formed in the end of the middle Pleistocene to the early stage of late Pleistocene,consists of T₃ terrace of big rivers and diluvium mesas developed on piedmont; the third and the second geomorphic surface can be found in valleys and are represented as T₂ and T₁ terrace,respectively.Diluvium mesas of the same period formed in the end of the late Pleistocene and the middle stage of Holocene are distributed in different parts in front of mountains.The first geomorphic surface is flood plain and modern alluvial fans at mountain front.According to OSL dating and radiocarbon dating of different terraces,we obtained the ages below: T₃,no less than 70ka; T₂,about 33ka; T₁,4～8ka.The characteristic of faulted landform of the research area is different due to the fault activity of different parts.In the segment with intense faulting,the fault trace is obvious,and we could see fault scarps and triangular facets in the field,low river terraces such as T₁ and T₂ had been faulted; In the segments with less activity,the fault trace is unclear,the older fault scarps have gentle slope due to river erosion and reverse slope,and there is no evidence of faulted low terraces.Based on faulted landforms of the different terraces,we divide the middle part of Kouquan Fault into three sections by Baipo village and Chanfang village.At south of Baipo village,the diluvium mesas corresponding to the period of T₃ were faulted,but there is no evidence found dislocating the younger geomorphic surface.This indicates that this part has not been active since Holocene; T₂ and the older terraces were faulted between Baipo village and Chanfang village; the evidence of offset of T₁ terrace could be seen at the north of Chanfang village,especially in the part between Xiaoyukou village and Louzikou village.In the section north of Chanfang village,the fault throw of T₁ terrace is 50cm in Dayukou village,over 3m in the part between Xiaoyukou village and Louzikou village,and 25～30cm in the part between Shijing village and Ermaokou village; the fault throw of T₂ terrace is 5.7m in the part between Chanfang village and Dayukou village,over 17.5m in the part between Xiaoyukou village and Louzikou village,and 13m in the part between Shijing village and Ermaokou village. We calculated the slip rates combining with the fault throw of T₂ terrace at different sites,and the results are as follows: >0.53mm/a between Xiaoyukou village and Louzikou village,0.4mm/a between Shijing village and Emaokou village and 0.17mm/a between Chanfang village and Dayukou village.These maybe indicate that the late Quaternary activity of the fault was centered on the part between Xiaoyukou village and Louzikou village,and became weaker towards both sides.

PALAEO-EARTHQUAKE STUDY ON THE NORTHERN YUMUSHAN ACTIVE FAULT

JIN Qing, HE Wen-gui, SHI Zhi-gang, YUAN Dao-yang

2011, 33(2):  347-355.  DOI: 10.3969/j.issn.0253-4967.2011.02.008

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The northern Yumushan Fault located on the northern flank of Qilian fold system is an active fault in Holocene.The fault is about 60km long,trending NWW.It is a trust fault with left-lateral strike-slip component.The activity of the fault produced a series of scarps along the mountain front.The fault zone is divided into three segments,and the middle part is the most active.In this paper,palaeo-earthquake events on the fault are studied.With the study of trench profiles,two palaeo-earthquake events are determined.Event I occurred at 4.066±0.086ka BP,and event II is between 6.852±0.102ka BP and 6.107±0.082ka BP.The last palaeo-earthquake event on this fault occurred in 4.066±0.086ka BP. So,the northern Yumushan Fault is not the seismogenic fault of the M 7 1/2 Biaoshi earthquake of 180 AD.The elapse time from the latest event has been 4000yr,so the possibility of generating destructive earthquake in future should be recognized sufficiently.

AN EXPERIMENTAL STUDY OF EVOLUTION OF PHYSICAL FIELD AND THE ALTERNATIVE ACTIVITIES DURING STICK-SLIP OF 5°|BEND FAULT

YUN Long, GUO Yan-shuang, MA Jin

2011, 33(2):  356-368.  DOI: 10.3969/j.issn.0253-4967.2011.02.009

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The spatial-temporal evolution process of strain field and acoustic emission(AE)events was investigated during the deformation of 5° bend faults,with 96-channels strain acquisition system and 16-channels distributed AE acquisition system in the laboratory.The loading was applied by controlling the Y-displacement and holding the X-load in a biaxial servo-control loading system, and the Y-loading rate was altered by 0.5μm/s,1μm/s,0.5μm/s and 0.1μm/s in sequence.The observation results show that: (1)quasi-periodic stick-slip always occurred under different loading rates, and the smaller the loading rate,the greater the period and stress drop; (2)low energy AE events increased before faults slid,but high energy AE events appeared as faults slid.AE events distributed near the bends and the upper and lower fault segments which were located by arrival time of AE wave.From the AE location results,AE sources mostly scattered in bend zones,and upper and lower fault segments,and the fault instability appeared first near bend point,then the alternative activities happened between upper and lower fault segments.Large instability took place in the lower fault segment,finally; (3)High strain concentration zone located near bend point and fault segment.And it is significantly different that mean strain and maximum shear strain increment changed alternately at the inside and outside of bend during strain accumulation and release stage; (4)Strain observation results illustrate that mean strain release first occurred near the bend,then released in the whole fault.It would be a critical instability condition for a bend fault.The observation to bend faults is important and helpful to investigate fault activity state.

MODELING THERMAL HISTORY DURING LOW TEMPERATURE BY K-FELDSPAR MDD AND FISSION TRACK:EXAMPLE FROM MESO-CENOZOIC TECTONIC EVOLUTION IN SAISHITENGSHAN IN THE NORTHERN MARGIN OF QAIDAM BASIN

WAN Jing-lin, ZHENG De-wen, ZHENG Wen-jun, WANG Wei-tao

2011, 33(2):  369-382.  DOI: 10.3969/j.issn.0253-4967.2011.02.010

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K-Feldspar MDD(Multiple Diffusion Domain)and fission track are two commonly-used methods in low closure temperature thermal chronometry.By modeling both the feldspar ³⁹ Ar/⁴⁰ Ar data and the fission track age and track-length data,the thermal history that sample underwent can be revealed and the effective temperature range of both feldspar ³⁹ Ar/⁴⁰ Ar method and fission track method is extended.Because of the multiple resolution of modeling,it is important to restrict the modeling process to gain a reasonable result,though it seems difficult.The possible problem in modeling thermal history is presented in this paper,and the helpful method that can be used to improve the result is illustrated by the sample collected along Saishitengshan in the northern margin of Qaidam Basin.Three rapid cooling events,occurring at 130～150Ma,30～40Ma and 5～10Ma respectively,in northern margin of Qaidam Basin are revealed by feldspar MDD method and fission track method.

THE CALENDAR DATE OF PALEO-EARTHQUAKE EVENT ESTIMATED BY DEPTH-AGE WIGGLE MATCH

YIN Jin-hui, CHEN Jie, LU Yan-chou, ZHENG Yong-gang

2011, 33(2):  383-390.  DOI: 10.3969/j.issn.0253-4967.2011.02.011

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This paper presents a comparison of the calendar range of individual paleoearthquake calculated by conventional calibration,Bayesian analysis of series of peat samples as well as depth-age wiggle matching.The results suggest that: (1)The Bayesian analysis provides an excellent means to enhance chronological resolution when applied to a series of radiocarbon dates from sections with clear stratigraphic relationships.Such application can assess systematic errors when combined with independent chronological information,and determine the optimum chronological information for specific events and contexts.(2)The calendar ranges of series of samples collected from the same peat but different sampling thickness were compared with that of depth-age wiggle matching.The result shows that the chronological resolution by depth-age wiggle matching for the peat sample with thickness less than 2cm could achieve less than 100a.Therefore,the condition to obtain a high resolution calendar age is to have 5～6 samples with 0.5～2cm sampling thickness each and the interval of samples is 0.5～1cm.(3)The calendar range of paleoearthquakes constrained by the Markov chain Monte Carlo(MCMC)Bayesian analysis model,which constructs the order of samples according to their position in the strata with series of radiocarbon dates,was compared with calendar range by the conventional method.The results show that Bayesian analysis could obtain high-precision calendar dates for closely spaced samples.(4)Therefore,the calendar interval of the paleoearthquake event on the middle segment of Huaiyuan Fault was inferred as 1340～1114cal a BP(2σ)by the above systematical comparsion.

THE AGES AND IMPLICATION OF YANGTZE RIVER HOLOCENE TERRACE

LEI Sheng-xue, CHEN Jie, LIU Jin-feng, YIN Jin-hui, HOU Kang-ming, WANG Chang-sheng

2011, 33(2):  391-401.  DOI: 10.3969/j.issn.0253-4967.2011.02.012

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The fluvial terrace has plenty of paleoclimate and paleoenviromental information which play an important role in paleoclimate and paleoenviromental researches.In this paper,we drilled a 42.85m-long core(N06S2)in 2007,which was located in the south bank of Yangtze River at Shifuqiao in Qixia district of Nanjing City.Firstly,fine quartz grains(4～11μm)were extracted from bulk samples in dark room,and the quartz purity tests were conducted.The results show that purity can satisfy the experiment.All measurements were performed on an automated Daybreak 2200 TL/OSL system with blue(470±5nm)light stimulation and U-340 luminescence detection filters.Thick source alpha counting(TSAC)was used to measure the uranium and thorium concentrations.The potassium content was determined using flame spectrophotometer.Secondly,the preheat plateau test and dose recovery test were performed on one sample using the SAR protocol.The results indicate that the fast component dominates the OSL signals.In preheat plateau test,identical D_e in the thermal treat from 160～260℃ was observed,thus we use the preheat temperature of 220℃ for 10 seconds.The recuperation ratios of zero point are below 2％and the recycling ratios lie between 0.9 and 1.1.In dose recovery test,OSL signal sensitivity changes are well corrected.Tests of luminescence characteristics confirm the suitability of the material for OSL dating.At last,samples from this drilling core were systematically dated by optically stimulated luminescence(OSL)dating method,and samples which contain organic matters were dated by AMS¹⁴ C.The results show that the D_e values from the two methods accord with each other very well.However,OSL dating results show that there is a hiatus in this core,and the hiatus,which ranges from 1.9ka to 7.9ka,may be the result of change of Yangtze River's channel or the erosion of the river.The ages of the fluvial deposition mainly range from 0.26ka to 1.9ka and 7.9ka to 9.1ka,it could be attributed to the results of paleoclimate and sea levels change.Meanwhile,study results on pollen and spores show that the paleoclimate has changed from warm wet to temperate wet during the stage of hiatus.

OPTICAL DATING OF PALEOEARTHQUAKE SIMILAR TO THE 12 MAY WENCHUAN EARTHQUAKE, AT XIAOYUDONG SURFACE RUPTURES ZONE

YANG Hui-li, CHEN Jie, RAN Yong-kang, LIU Jin-feng, WANG Chang-sheng, LI Feng

2011, 33(2):  402-412.  DOI: 10.3969/j.issn.0253-4967.2011.02.013

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In historical records,no earthquake of magnitude comparable with that of the Wenchuan M_S 8.0 earthquake has ever been reported in Chengdu and Longmenshan regions.The penultimate event similar to the 12 May 2008 M_S 8.0 was revealed by the surface ruptures in the vicinity of Xiaoyudong.It is important,therefore,to date accurately when the prior large earthquake happened.This paper presents 7 optically stimulated luminescence(OSL)ages from Xiaoyudong trench.We investigate the use of simplified multiple aliquot regenerative-dose(SMAR)protocol and single aliquot regenerative-dose(SAR)protocol from fine-grain quartz to date deposits associated with earthquake.The results of SMAR and SAR protocols are consistent.The behavior of quartz to different internal consistency checks of SAR protocol(preheat plateau,thermal transfer,recycling ratio,recuperation and growth curves)and of SMAR protocol(dose recovery)used for the dose estimation method was satisfactory.Various internal consistency tests of the measurement protocols indicate that the dose estimates from the fine grain quartz are accurate and the optical ages are reliable.The preliminary OSL ages indicate that the last large earthquake happened between 1.7～2.2ka and the slip rate of Xiaoyudong Fault was 1.0±0.08mm/a at least in Holocene.

OPTICAL DATING OF FINE QUARTZ FROM THE 12 MAY 2008, M_S 8.0 WENCHUAN EARTHQUAKE RELATED SEDIMENTS

YANG Hui-li, CHEN Jie, LIU Jin-feng, YU Song

2011, 33(2):  413-420.  DOI: 10.3969/j.issn.0253-4967.2011.02.014

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The research of optical dating about modern strong earthquake sediments will be useful for understanding geological significance of paleoearthquake related sediments,improving dating precision and accuracy,and better understanding the law of earthquake occurrence.This article chooses some typical sediments related with 2008 Wenchuan earthquake, such as dammed lake deposits,floodwater deposits and ejected sands, to do optical dating of fine-grained quartz.Preliminary results from optical dating of fine-grained quartz extracted from 11 samples suggest that fine-grained quartz is sensitive and D_e values obtained from the three sections indicate residual D_e up to 10Gy. Caution should be taken for optical dating of sediments related to paleoearthquake with a recurrence interval of hundreds or thousands of years,but the effect can be neglected for the paleoearthquake with a recurrence interval of ten thousands or hundred thousands of years.Residual D_e values less than 0.2Gy are observed in two samples collected from pre-earthquake surface.Therefore,we should collect samples on pre-earthquake surface to limit the event time.

OPTICAL DATING OF THE HOLOCENE TERRACE SEDIMENTS AND THE INCISION RATE IN THE UPPER REACH OF THE YARKANT RIVER IN XINJIANG

LIU Jin-feng, CHEN Jie, WANG Chang-sheng

2011, 33(2):  421-429.  DOI: 10.3969/j.issn.0253-4967.2011.02.015

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The Tashkurgan-Yarkant River,which is the upper reach of the Tarim River,originates from the Karakoram Mountains in the west syntaxis of the Tibetan Plateau.Thick terrace sediments are widespread along the Tashkurgan-Yarkant River.These deposits contain geologic fingerprints that allow identification of the environmental changes and geologic hazards.However,few geochronological data was available on these sedimentations to allow us to build an irreproachable age model.4 samples of fine grains from one terrace profile were dated by optically stimulated luminescence(OSL)dating method.In darkroom,fractions of fine grains(4～11μm)were extracted from the bulk samples.OSL signal measurements were performed on an automated Daybreak system.Identical D_e plateau in the thermal treat was observed in preheat plateau test.Tests of luminescence characteristics confirm the suitability of the material for OSL dating.Our results indicate the deposition age for these sediments is between 4.3～7.3ka.After the river terrace deposited,the river has deeply incised for 27±5m,with the incision rate 6.3±1.2mm/a.The thermochronologic data show that the magnitude of exhumation decreases from upstream(west)to downstream(east)along the Tashkurgan-Yarkand River.These data may reflect the the active uplift process of Mustagh Ata antiform.

GEOLOGICAL CHARACTERISTICS AND GENESIS OF BASALT PLATFORM IN BEILIKE,INNER MONGOLIA

CHEN Sheng-sheng, FAN Qi-cheng, ZHAO Yong-wei, SUI Jian-li

2011, 33(2):  430-439.  DOI: 10.3969/j.issn.0253-4967.2011.02.016

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Cenozoic basalts distribute along NW-SE direction in Xilinguole,Inner Mongolia.According to volcanic geology,they are divided into three volcanic fields,namely,Abaga basalt,Beilike basalt and Dalinuoer basalt.The Beilike basalt is distinctive to the other two basaltic areas,for its characteristics of simple petrology of tholeiite,no volcanic cones or vents surviving from aging,and mantle derived xenoliths free.Geographically,Beilike basalt forms four steps of lava platforms at different heights: 1034～1057m,1070～1120m,1150～1200m and 1280m,respectively.Isotopic dating reveals the ages of the Beilike basalt varying from middle Pliocene to middle Pleistocene,and the upper lavas are older than the lower ones.According to this fact,we can tell the scenario of the Beilike basalt as that the Cenozoic volcanic eruptions form the lava shield,then it is elevated by episodic uplift of the Neotectonic movement.

MORPHOLOGICAL CHARACTERIZATION AND TERMINAL VELOCITY OF PUMICE PARTICLES ERUPTED DURING THE MILLENNIUM ERUPTION OF CHANGBAISHAN TIANCHI VOLCANO,CHINA

YU Hong-mei, XU Jian-dong, LIN Chuan-yong

2011, 33(2):  440-451.  DOI: 10.3969/j.issn.0253-4967.2011.02.017

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The shape parameter of particles is an important parameter in the formula of the terminal settling velocity,but it was usually assumed to be spherical or ellipsoidal without detailed study.According to the morphological characterization of the tephra particles erupted during the Millennium eruption of Changbaishan Tianchi volcano,we got some new recognition: The tephra particles consist mainly of pumice,which have irregular shapes,varying from very equant to elongate and from subrounded to angular.With decreasing of the particle size,the elongation of particles becomes significant,and the proportion of elongated particles increases from 15.02％to 47.5％.Moreover,the smaller the size of particles is,the more angular the shape of particles.An average shape parameter F=0.72 has been obtained through the morphological characterization of the pumice particles.The terminal velocity of the pumice particles,which were assumed to be spherical(F=1),erupted during the Millennium eruption of the Changbaishan Tianchi volcano is about 1.52 times the terminal velocity of the pumice particles of F=0.72,and the terminal velocity of the pumice particles,which were assumed to be ellipsoidal(F=0.5), is about 0.89 times the terminal velocity of the pumice particles of F=0.72.When the wind speed is taken to be 10m/s,the isopach map of tephra particles with different shape parameters shows that the dispersal range expands with the decrease of the shape parameter F.The aforementioned results show clearly that the morphology of particles influences strongly both the terminal settling velocity and dispersal range,and hence should be fully considered when simulating tephra dispersion and forecasting disasters in the future.

MUD VOLCANO DISASTER AND GENETIC MECHANISM IN RAMREE ISLAND

WAN Yuan, XU Jian-dong, YU Hong-mei, ZHAO Bo, PAN Bo

2011, 33(2):  452-461.  DOI: 10.3969/j.issn.0253-4967.2011.02.018

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This paper mainly discusses the mud volcano disaster and its genetic mechanism in Ramree Island.Combined with the geological background of Ramree Island and the present situation of the mud volcano,grading analysis and micro-morphology analysis have been made by taking samples from three sampling sites of the northeast,the middle and the western part of Ramree Island.The finding is that the particle sizes of samples between 1～100μm show a unimodel normal distribution,with similar grain size mid-value,concentrated size distribution,poor sorting,and quite positive skewness in grading symmetry; The samples' matrix are mainly argillaceous,containing a lot of argillaceous clasts,tiny crystal clasts and a few pores,and having a little strongly altered plagioclase phenocryst and pyroxene phenocryst,from which,we can judge that the sampling sites have the same provenance.Thus,it can be deduced that there is a uniform argillaceous layer as the unified provenance of the Ramree Island mud volcano.

SURFACE RUPTURE FEATURES OF THE 2008 YUTIAN M_S 7.3 EARTHQUAKE AND ITS TECTONIC NATURE

XU Xi-wei, TAN Xi-bin, WU Guo-dong, CHEN Jian-bo, SHEN Jun, FANG Wei, SONG He-ping

2011, 33(2):  462-471.  DOI: 10.3969/j.issn.0253-4967.2011.02.019

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High-resolution satellite image interpretation and field investigation indicate that the surface rupture zone produced by the Yutian M_S7.3 earthquake is～25km long along a NS-trending fault at the western piedmont of a snow-covered range at the upper reach of the Yurungongkash River,about 20km south of the Ashikule Volcanoes.The surface rupture zone consists of different striking ruptures with both normal and left-lateral faulting components.The maximum left-lateral and vertical co-seismic slips measured in the field are～1.8m and～2.0m,respectively.Its seismogenic NS-trending fault belongs to the secondary structure at the NE-trending tensile area of the southwestern end of the Altyn Tagh Fault,which conforms to the eastward escape of the Kunlun-Qaidamu-Qilian block,relative to the Western Kunlun block.

INVESTIGATION OF TSUNAMI DEPOSITS IN ISHINOMAKI PLAIN,MIYAKI COUNTY,JAPAN

HE Hong-lin, RAN Yong-kang, SONG Fang-min, CHEN Li-chun, CHEN Jie

2011, 33(2):  472-479.  DOI: 10.3969/j.issn.0253-4967.2011.02.020

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To carry out the project "Study on paleo-tsunami in east and southeast seashore area of China" supported by China Ministry of Science and Technology,we made a study tour to Japan in April,2007.In this visit,we investigated roughly the tsunami deposits in Ishinomaki Plain,Miyaki County,Japan,where a huge earthquake of M_W 9.0 occurred at March 11,2011.This earthquake caused a great tsunami along the northeast coast of Honsyu Island,Japan,bringing lots of death and huge economic loss.To understand the tsunami history in this area and the methods of investigating tsunami deposits,it is necessary to introduce briefly our investigation in Ishinomaki Plain,Miyaki County,Japan.Our investigation results demonstrated three tsunami events occurred in this area. The latest one occurred before 915 AD,when the Towada volcano erupted and the tephra from this eruption covered almost all of the Northeast Japan,corresponding to the 869 AD Jogan earthquake tsunami.

TERRESTRIAL IN SITU COSMOGENIC NUCLIDES BURIAL DATING AND ITS APPLICATION IN GEOSCIENCES

YUAN Zhao-de, CHEN Jie, ZHANG Hui-ping

2011, 33(2):  480-489.  DOI: 10.3969/j.issn.0253-4967.2011.02.021

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Terrestrial in situ cosmogenic nuclides burial dating has a promising application in dating of late Cenozoic detrital sediments,for example,cave sediments,fluvial sediments and moraine.This method relies on a pair of cosmic-ray-produced nuclides that are produced in the same rock or mineral target at a fixed ratio,but have different half-lives.For example, ²⁶ Al and ¹⁰ Be are produced in quartz at ²⁶ Al :¹⁰ Be=6.75 :1.The ratio is not affected by latitude and altitude.After sediments are buried,the ratio would become less as time goes.Therefore, ²⁶ Al/¹⁰ Be ratio can be used as a geological clock.The dating range can be from several hundreds of thousand years to five million years.In this article,we introduce four methods and their applications: exposure-burial diagram method,depth profile method,isochron method, ²⁶ Al-²¹ Ne and ¹⁰ Be-²¹ Ne method.Exposure-burial diagram method is often applied to cave sediments dating, for exposure-burial history of cave deposits is easy.Depth profile method is applied to fluvial sediments dating.There is a good application for isochron approach in till-paleosol sequences in North America. ²⁶ Al-²¹ Ne and ¹⁰ Be-²¹ Ne method has a great potential applicaton in future for its larger dating time and less uncertainty than other methods.The dating method still has many problems.Firstly,there are no exact half-lives.For example,there is still controversy for ¹⁰ Be half-life.Its estimate is 1.51±0.06Ma or 1.36±0.07Ma.Secondly,it is also a debate how to determine nuclides' production rates.In addition,post-burial or preburial erosion rate,inheritance nuclides concentration,post-burial nuclide production,effect of post-burial or preburial muonic production,sediment rework,complicated exposure-burial history of sediments all bring great challenges to cosmogenic nuclides dating.

RESEARCH ADVANCES IN ESR GEOCHRONOLOGY OF QUATERNARY DEPOSITS

LIU Chun-ru, YIN Gong-ming, GAO Lu, HAN Fei, ZHANG Hui-ping

2011, 33(2):  490-498.  DOI: 10.3969/j.issn.0253-4967.2011.02.022

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Because of lack of Quaternary volcano activity in China,Quaternary sediments become the main dating material in the study of geological structure, topographic feature and environment evolution,etc.ESR is a potential dating method for the sediments older than 200ka.After sunlight bleaching or heating,the quartz ESR signals,including E'-,Ge-,Al-,Ti-center,can attenuate or be reset.The sediments deposited during Quaternary period only have the effect of sunlight bleaching before the last burial time.Therefore,the sunlight bleaching characteristics of ESR signal centers is one of the most important factors in ESR dating.In this study,the paper firstly makes a simple introduction on the ESR theoretical basis and the measuring process of dose rate(D) and equivalent dose(ED),and then,reviews the sunlight bleaching characteristics and the applications in Quaternary geochronology of different ESR signal centers.The E'-center ESR signal increases with the sunlight bleaching during first 72 hours,it is not suitable for the sediment dating.Ge-center ESR signal is bleachable and can be reset after several hours sunlight bleaching,so,it is the most light sensitive signal center.However,it is very difficult to measure the Ge-center ESR signal in laboratory because it is very weak.Al-center can attenuate 20 percent after 2 hours sunlight bleaching and after tens to hundreds of hours bleaching it still maintains a stable residual signal,50-80 percent.The remnant signals are not equal under different sediment environment.We usually gain a bigger age using Al-center ESR signal for the uncertain remnant.Ti-center ESR signals can be totally bleached after tens to hundreds of hours sunlight bleaching,and this ESR signal also has enough intensity for measurements.According to the review of all the ESR signal centers' sunlight bleaching characteristics and several successful application examples,we suggest that Ti-center ESR signal is more suitable than others for the ESR dating of Quaternary sediment.