High pressure garnet amphibolites in ophiolitic mélange from the Changning-Menglian suture zone, southeast Tibetan Plateau: P-T-t path and tectonic implication

The garnet amphibolites from the newly identified Wanhe ophiolitic mélange in the Changning-Menglian suture zone(CMSZ)provide a probe to elucidate the evolution of the Triassic Palaeo-Tethys.An integrated petrologic,phase equilibria modeling and geochronological study of the garnet amphibolites,southeast Tibetan Plateau,shows that the garnet amphibolites have a peak mineral assemblage of garnet,glaucophane,lawsonite,chlorite,rutile,phengite and quartz,and a clockwise P-T path with a prograde segment from blueschist-facies to eclogite-facies with a peak-metamorphic P-T conditions of 2000–2100 MPa and 495–515℃,indicating a cold geothermal gradient of about 240–260℃/GPa.Theretrograde metamorphic P-T path is characterized by nearly isothermal decompression to lower amphibolite-facies and subsequent cooling to greenschist-facies.The metamorphic zircons have fractionated HREE patterns and significant negative Eu anomalies,and therefore the obtained zircon U-Pb age of 231±1.5 Ma is interpreted to be the timing of the amphibolite facies metamorphism occurrence.The present study probably indicates that the garnet amphibolites in the Wanhe ophiolitic mélange was the retrograded highpressure eclogite-facies blueschist,instead of the previously proposed eclogites,and the garnet amphibolites recorded the subduction and exhumation process of the Palaeo-Tethys Oceanic crust in the Triassic.

Petrology and metamorphism of glaucophane eclogites in Changning-Menglian suture zone, Bangbing area, southeast Tibetan Plateau: An evidence for Paleo-Tethyan subduction

High/ultrahigh-pressure(HP/UHP)metamorphic complexes,such as eclogite and blueschist,are generally regarded as significant signature of paleo-subduction zones and paleo-suture zones.Glaucophane eclogites have been recently identified within the Lancang Group characterized by accretionary mélange in the Changning-Menglian suture zone,at Bangbing in the Shuangjiang area of southeastern Tibetan Plateau.The authors report the result of petrological,mineralogical and metamorphism investigations of these rocks,and discuss their tectonic implications.The eclogites are located within the Suyi blueschist belt and occur as tectonic lenses in coarse-grained garnet muscovite schists.The major mineral assemblage of the eclogites includes garnet,omphacite,glaucophane,phengite,clinozoisite and rutile.Eclogitic garnet contains numerous inclusions,such as omphacite,glaucophane,rutile,and quartz with radial cracks around.Glaucophane and clinozoisite in the matrix have apparent optical and compositional zonation.Four stages of metamorphic evolution can be determined:The prograde blueschist facies(M_(1)),the peak eclogite facies(M_(2)),the decompression blueschist facies(M_(3))and retrograde greenschist facies(M_(4)).Using the Grt-Omp-Phn geothermobarometer,a peak eclogite facies metamorphic P-T condition of 3000–3270 MPa and 617–658℃ was determined,which is typical of low-temperature ultrahigh-pressure metamorphism.The comparison of the geological characteristics of the Bangbing glaucophane eclogites and the Mengku lawsonite-bearing retrograde eclogites indicates that two suites of eclogites may have formed from significantly different depths or localities to create the tectonic mélange in a subduction channel during subduction of the Triassic Changning-Menglian Ocean.The discovery of the Bangbing glaucophane eclogites may represent a new oceanic HP/UHP metamorphic belt in the Changning-Menglian suture zone.

Characteristics of recent temperate glacier fluctuations in the Parlung Zangbo River basin,southeast Tibetan Plateau

Little is known about recent variation of temperate glaciers on the Tibetan Plateau,although they are of particular theoretical and practical interests in terms of climate change and water supply.The study of glacier mass balance and terminus variation for six temperate glaciers in the Parlung Zangbo River basin,southeast Tibetan Plateau,shows the facts of the overall negative mass balance and the continued terminus retreat from 2005/2006 to 2007/2008.The mass balances of smaller glaciers were more negative compared to larger glaciers.Referring the trend of glacier mass balance in the Hengduan Mountains,the Himalayas and glacier shrinkage in southeast Tibetan Plateau,the mass loss and recession of temperate glacier in this region are expected if the current climate condition continues.

Study of altitudinal lapse rates of δ^(18)O in precipitation/river water with seasons on the southeast Tibetan Plateau

Seasonal δ 18O variation in water on the southeast Tibetan Plateau has been studied, showing the consistent variation pattern of δ 18O with altitude indicative of relevant atmospheric circulation processes. Study shows a similar variation pattern of fixed-site river water δ 18O with that of the precipitation δ 18O in southeast Tibet. δ 18O in regional rivers in southeast Tibet demonstrates a gradual depletion with increasing altitude, though the rates vary seasonally. The most depleted river 18O occurs during the monsoon period, with the lowest δ 18O/altitude lapse rate. The river 18O during the westerly period is also depleted, together with low δ 18O/altitude lapse rate. The pre-monsoon rivers witness the most enriched 18O with least significant correlation coefficient with the linear regression, whilst the postmonsoon rivers witness the largest δ 18O/altitude lapse rate. Different coherence of seasonal δ 18O variation with the altitude effect is attributed to different moisture supplies. Though sampling numbers vary with seasons, the δ 18O-H linear correlation coefficients all reach the 0.05 confidence level, thus witnessing the variation features of δ 18O in seasonal river water due to the influence of atmospheric general circulation and land surface processes revealed from the altitudinal lapse rates.

The Indian monsoonal influence on altitude effect of δ^(18)O in surface water on southeast Tibetan Plateau

The altitude effect of δ18O is essential for the study of the paleo-elevation reconstruction and possible to be solved through modern process studies. This study presents new δ18O reults from southeast Tibetan Plateau along two transects, the Zayu transect and the Lhasa-Nyang transect, with δ18O data from June to September representative of monsoon period and δ18O data during the rest of the year of non-monsoon period. Altitude effect outweighs the longitude and latitude effects in determining regional δ18O variation spatially. Relevant δ18O data from previous studies in the nearby region have also been combined to comprehensively understand the influence of different moisture sources on δ18O from local scale to regional scale. The δ18O in surface water in the southeast Tibetan Plateau and its nearby regions influenced by the Indian summer monsoon shows that single dominant moisture source or simple moisture sources lead to smaller altitudinal lapse rate, whilst growing contributions from local convection to precipitation enlarge δ18O-altitude rate. It thereupon reveals the significance of the Indian summer monsoon to the altitude effect of δ18O in surface water, and the complicated effect of local convection or westerlies evolution to the variation of altitudinal lapse rate. Paleo-monsoon evolution therefore should be considered when altitude effect is applied to paleo-elevation reconstruction for the Tibetan Plateau.

Gravity pattern in southeast margin of Tibetan Plateau and its implications to tectonics and large earthquakes

There are many active faults in the southeast margin of Tibetan Plateau,where three large active faults zones,the Longmenshan,Xianshuihe and Anninghe,merge to form a"Y"shape.Strong crustal deformation and a complicated fault distribution accompany strong earthquake activity in this zone.In this paper,we investigate a multi-scale gravity anomaly in the southeastern margin of the Tibetan Plateau using the wavelet transform;we find that the pattern of the gravity field is closely related to the fault system in the study area.Analyzing the characteristics of this Bouguer gravity anomaly at different orders indicates that the eastern Himalayan syntaxis has produced a strong eastward push during its northward movement,resulting in a shortening of the crust from west to east and a rapid uplift of the Tibetan Plateau.The Songpan–Garzêand Sichuan–Yunnan blocks have been forced to slip and extrude southward and eastward laterally.The distributions of seven large earthquakes from 1970 to 2018 reflects the relationship between large earthquakes and characteristics of the gravity anomaly.Comparing the tectonic backgrounds of several earthquakes reveals that the large earthquakes occur usually in the high gravity anomaly gradient zone,which corresponds in general to the boundary zones of the blocks.We infer that large earthquakes occur primarily in high Bouguer gravity anomaly zones in the upper crust,while low Bouguer gravity anomalies encompass the lower crust and the uppermost mantle.

Crustal thicknesses and Poisson's ratios beneath the Chuxiong-Simao Basin in the Southeast Margin of the Tibetan Plateau

In the Southeast Margin of the Tibetan Plateau, low-velocity sedimentary layers that would significantly affect the accuracy of the H-κ stacking of receiver functions are widely distributed.In this study, we use teleseismic waveform data of 475 events from 97 temporary broadband seismometers deployed by ChinArray Phase I to obtain crustal thicknesses and Poisson's ratios within the Chuxiong-Simao Basin and adjacent area, employing an improved method in which the receiver functions are processed through a resonance-removal filter, and the H-κ stacking is time-corrected.Results show that the crustal thickness ranges from 30 to 55 km in the study area, reaching its thickest value in the northwest and thinning toward southwest, southeast and northeast.The apparent variation of crustal thickness around the Red River Fault supports the view of southeastern escape of the Tibetan Plateau.Relatively thin crustal thickness in the zone between Chuxiong City and the Red River Fault indicates possible uplift of mantle in this area.The positive correlation between crustal thickness and Poisson's ratio is likely to be related to lower crust thickening.Comparison of results obtained from different methods shows that the improved method used in our study can effectively remove the reverberation effect of sedimentary layers.

滇藏铁路沿线重要活动断裂带晚第四纪活动性初步研究

滇藏铁路位于青藏高原东南缘板块碰撞和构造活跃的地形急变带,沿线活动断裂发育,地震频发,地震烈度强,且地质灾害频繁,其规划施工建设和长期运营面临的地质安全风险问题严峻。滇藏铁路香格里拉—波密段沿线发育了哈巴—玉龙雪山东麓断裂带、中甸—龙蟠断裂带、德钦—中甸—大具断裂带、维西—乔后断裂带、金沙江断裂带、巴塘断裂带、澜沧江断裂带、怒江断裂带、边坝—洛隆断裂带、嘉黎—察隅断裂带等10余条活动断裂带。受断裂黏滑位错、蠕滑变形和引发强震风险的突出影响,迫切需要厘定区域活动断裂带的几何展布和活动性。基于前人资料、遥感解译和野外调查,本文总结分析了沿线10余条活动断裂带的几何展布、运动性质、滑动速率和古地震历史等,以期为滇藏铁路等国家重大工程规划建设与安全运营提供基础资料。

青藏高原东南部及东坡雨滴谱特征的研究

为研究青藏高原东南部地区及东坡的云微物理特征,利用2021年青藏高原东坡峨眉山站点以及青藏高原东南部林芝站点雨滴谱仪的观测资料,分析了青藏高原东南部及其东坡不同季节的3类降水云的雨滴谱特征及典型个例降水与西南涡之间的关系。结果表明:峨眉山站点和林芝站点的主要降水类型都为层状云降水,年累计降水量较大,降水持续时间短,降水频次多;峨眉山站点4个季节对应粒子数浓度峰值的粒径为1.5 mm左右,而林芝站点4个季节对应粒子数浓度峰值的粒径为1 mm左右,其中层状云雨滴谱浓度最低,主要贡献在小粒径上。峨眉山地区的3类云降水在Gamma拟合中,与实测相比模拟较好的粒径区间在1.5~4.0 mm;而林芝站点的拟合曲线与实测曲线的趋势较为相似,模拟较好的粒径区间在1.3~4.5 mm;因此,峨眉山站点和林芝站点的雨滴谱在适当范围内是能够运用Gamma分布模拟的。伴随较深厚的气旋性涡旋出现的降水强度较大,48 h累积雨量较大,雨滴谱分布的谱宽较宽,粒子数浓度在小粒径时更高,西南涡的发生发展造成的降水对本次研究区域的雨滴谱分布有很大的影响。

20世纪初以来青藏高原东南部岗日嘎布山的冰川变化

采用地形图、航空摄影相片、中巴资源卫星和Landsat TM数字影像, 对青藏高原东南部岗日嘎布山区20世纪初以来的冰川变化进行了研究, 分析了该地区冰川对 20 世纪后期全球变暖的响应.结果表明: 20世纪初期至1980年, 研究区的冰川基本处于退缩状态, 期间冰川面积减少了13 8%, 储量减少了9 8%, 储量减少量相当于249 2×108 m3水当量, 因冰川萎缩导致其对河川径流的调节作用减弱了一半左右. 1980年以来, 本区气候表现出升温和降水增加, 在此气候变化背景下, 冰川总体呈面积减小的退缩状态, 但有一定数量的冰川处于前进之中, 这可能与不同规模冰川对气候变化的响应特点和响应时间有关.

1980-2005年藏东南然乌湖流域冰川湖泊变化研究

基于1980年地形图和1988年、2001年Landsat数据以及2005年中巴资源卫星数据,对藏东南然乌湖流域1980—2005年25 a来冰川和湖泊的面积变化进行了研究.结果表明:1980—2005年间,冰川面积从496.64 km2减少到466.94 km2,冰川萎缩了29.7 km2,萎缩速率为1.19 km2.a-1,萎缩量占冰川总面积的5.98%,冰川面积占流域总面积的比例从22.42%减小到21.08%.区域冰碛湖泊面积则从1980年29.79 km2增大到2005年33.27 km2,湖泊面积扩大了3.48 km2,增加的速率为0.14km2.a-1,扩大面积占湖泊总面积的11.68%,湖泊面积占流域总面积的比例从1.34%增加到1.5%.其中,冰川面积在1980—1988年萎缩速率为1.73 km2.a-1,1988—2001年为0.82 km2.a-1和2001—2005年为1.3 km2.a-1.而湖泊面积在1980—1988年扩涨速率为0.11 km2.a-1,1988—2001年为0.12 km2.a-1,2001—2005年为0.27 km2.a-1,湖泊逐年加速扩涨.从流域内的气象数据来看,温度升高,是该区域冰川萎缩的根本原因,湖泊加速扩涨主要受到冰川萎缩,冰川融水量加大的影响.

青藏高原东南部地区瑞雷波相速度层析成像

本研究收集了＂中国地震科学探测台阵-南北地震带南段＂项目325个流动宽频带台站于2011年8月至2012年9月记录的远震垂直向资料,利用双台法测得了3594条独立路径上的瑞雷波相速度频散曲线,反演得到了青藏高原东南部地区周期10~60s瑞雷波的相速度分布图像.空间分辨尺度图表明,在台站覆盖范围内的绝大部分地区横向分辨率达到50km.2D相速度分布图显示,青藏高原东南部地区地壳上地幔S波速度结构存在较明显的横向非均匀性.短周期（如10s）的相速度分布主要受地表沉积层厚度的影响.绝大多数地震发生在周期15s相速度图上的低速区或高低速的陡变梯度带附近,充分说明该区的强震活动与中上地壳速度结构的变化有直接关系.中等周期（如20~30s）的相速度分布主要与中下地壳速度结构、地壳厚度密切相关,小江断裂、松潘—甘孜块体呈现最显著的低速,可能暗示这两处的中、下地壳存在低速层.较长周期（如40~60s）的相速度分布与上地幔顶部热状态和构造活动（如岩浆作用）有关.滇西南地区表现为大范围的显著低速,可能暗示滇西南地区上地幔顶部物质存在部分熔融.不同构造块体下方的频散曲线,具有不同的相速度特征.腾冲火山下方的频散曲线在10~60s一直为较低的速度,尤其是到40s以后,相速度随周期的变大增速明显放缓,至60s比其他任何块体速度都低,暗示腾冲火山区下方的低速至少来自上地幔顶部（约100km）.

稻城冰帽区更新世冰川测年研究

通过ESR和14C测年对典型冰碛物进行直接和间接定年,并结合冰碛地貌形态和位置,确定稻城冰帽区自中更新世以来经历了6次较大规模的冰川前进,依次与MIS 16、MIS 6、MIS 3中期、MIS 2早期、全球意义上的末次盛冰期(MIS 2)和冰后期对应,冰川作用规模基本上逐渐减小.最早、规模最大的冰川作用发生在570kaBP前后,意味着本段高原面在570kaBP之前已进入了冰冻圈,且自那时起,该区经历了较大规模的抬升作用.全新世到来之前,稻城冰帽全部消失了,它是一个随着气候冷暖波动逐渐后退的过程,不是以死冰的方式突然消亡.值得注意的是,末次冰期大间冰阶中期(MIS 3b)的冰川前进规模超过了末次盛冰期(MIS 2),表明末次冰期最盛期在全球范围内的不同步性.其不同步的原因可能是:MIS 3中期,北半球夏季太阳辐射相对较低,但其海陆分布状况能够诱发较强的南亚夏季风,它给以季风降水为主要补给的海洋性冰川区带来较多的降水,结合该时段较低的温度,有利于冰川较大规模前进;末次盛冰期时,气候严寒,但夏季风微弱,降水稀少,冰川平衡线下降程度反而不及MIS 3b.

利用面波频散与接收函数联合反演青藏高原东南缘地壳上地幔速度结构

青藏高原东南缘对于青藏高原的隆升、增厚和物质逃逸等问题有着重要的研究价值.本文对研究区内布设的大型流动地震台阵的观测记录进行处理,联合反演面波频散与接收函数数据,获得了地壳厚度、沉积层厚度分布情况以及地壳上地幔高精度S波速度结构.联合反演的结果表明:(1)研究区域内地壳厚度变化很大,从西北往东南方向地壳厚度逐渐变薄;(2)沉积层厚度与研究区内沉积盆地的分布情况较为一致;(3)在研究区中下地壳内由北向南呈条带状分布有两条主要的壳内低速体,其中一条从川西北次级块体向南延伸,穿过丽江断裂到达滇中次级块体下方,另一条低速体沿小江断裂分布,向南延伸到24°N左右,两条低速体在中地壳范围被四川盆地及峨眉山大火成岩省内带下方的高速异常所隔开.

近30余年来盛夏东亚东南季风和西南季风频率的年代际变化及其与青藏高原积雪的关系

利用地面观测资料和NCEP/NCAR再分析资料集,使用相关分析、合成分析等方法,在将地面风分为东南季风和西南季风的基础上,分析了近30余年来盛夏东亚季风频率的年代际变化特征。结果表明:盛夏东南季风、西南季风频率和前期春季青藏高原积雪均在21世纪初期发生了显著的年代际变化;东南季风、西南季风频率由较少改变为较多,春季青藏高原积雪则由深变浅。由于青藏高原积雪厚度发生了年代际变浅,说明青藏高原发生了年代际变暖和南亚高压变强,南亚高压的年代际变强,使得其下游对流层低层(18°~28°N,108°~118°E)的反气旋性环流异常增强,有利于东亚西南季风频率的增加;同时,由于高原发生湿反馈作用,使得淮河地区降水发生年代际变多,由Sverdrup涡度平衡关系,降水的异常增多通过潜热释放,使得东亚副热带高压异常加强,而副热带高压异常变强则有利于盛夏东亚东南季风频率发生年代际增加。

夏季青藏高原东南部水汽收支气候特征及其影响

采用1961—2005年NCEP/NCAR再分析资料,研究了夏季青藏高原东南部水汽收支的气候特征及其影响效应。结果表明:夏季青藏高原东南部总体上是一个水汽汇区,平均总收入为39.9×106kg/s。东亚夏季风的建立、推进对青藏高原东南部的水汽输入有重要影响,而青藏高原东南部的水汽输出则与夏季我国东部雨带的推进过程密切相关。该区对周边地区的水汽收支有重要影响,是向我国西北地区东部、长江中下游地区输送水汽的重要通道,青藏高原东南部的水汽"转运站"效应是长江中下游流域洪涝和北方夏季干旱异常的关键因子之一。青藏高原东南部东、北边界夏季水汽收支均具有准两年周期振荡特征,并分别与长江中下游、西北地区东部夏季降水的准两年振荡特征具有一定的联系。

1979年5月东南亚夏季风的建立和青藏高原的作用

东南亚夏季风开始于５月，它是大气环流向夏季环流过渡的一个重要阶段。本文用ＦＧＧＥ－ＩＩｂ全球网格点资料，分析１９７９年４月２６日到５月２５日，大范围（４０°Ｓ～５０°Ｎ，３０～１６０°Ｅ）温度、湿度和风场变化特征，计算了垂直速度、辐散风场、热源和水汽汇收支，研究了东南亚夏季风的性质和来源，其中着重分析了青藏高原的热力和动力作用与东南亚夏季风建立的关系。东南亚夏季风建立的主要因素是中纬度的环流形势，来自热带海洋的西南气流和青藏高原的作用。东南亚夏季风开始时，高空大气环流发生调整，青藏高原上空为波脊，两侧为波槽。低空东南亚夏季风区的辐合气流有四个来源：１）同经度范围的偏南辐散风，２）高原南侧的偏西辐散风，３）高原东侧的偏北辐散风，４）西太平洋上的偏东辐散风。分析表明，东南亚夏季风降水受高原西南侧的中纬度西风带波动影响。低空西南气流来源于８０～１２０°Ｅ的南半球热带地区，西南气流输送水汽并且加强低空辐合。主要的热源位于东南亚，主要是潜热释放形成的，它是驱动东南亚夏季风环流的主要机制。整个分析期，青藏高原是一热源，抬升的感热加热和动力作用形成和维持了高原上空的高压脊和两侧的波槽，从而有助于东南亚夏季风的建立。

藏东南尼洋河流域降水径流水量平衡问题

为了深入分析藏东南尼洋河流域水文特征及产汇流机理,对该流域降水径流水量平衡问题进行研究。基于尼洋河流域工布江达、巴河桥、更张等水文站日降水、流量等水文气象观测资料,对采用传统泰森多边形法计算的尼洋河流域面平均雨量小于径流量这一现象进行分析,提出降雨径流水量平衡问题;针对尼洋河流域地形地貌特征、降水空间分布规律和测站分布的实际情况,提出尼洋河流域面平均雨量分布式计算方法,并进行了实例计算。计算结果表明:目前对尼洋河流域降水空间分布特征缺乏正确认识;采用传统的泰森多边形法计算面平均雨量是导致出现水量不平衡问题的主要原因。指出尼洋河流域面平均雨量计算应考虑降水的垂直分布差异。

大理地区所代表的青藏高原东南缘降水日变化类型的分析

用谐波分析法对大理地区各气象站2005—2014年的小时降水量做日变化分析,结果表明,各站点降水量日变化可用前3个谐波做拟合,方差贡献率≥30%的谐波是显著谐波,降水量日变化呈现4种形态:洱海盆地内呈夜间单峰型,后半夜降水高峰由持续≥3 h的降水事件引起;哀牢山起始西侧和点苍山西侧峡谷低地呈夜间峰值是午后峰值1.5倍的双峰型,后半夜降水高峰多由持续≥6 h的长时降水事件引起,傍晚降水次高峰一般由持续≤5 h的短时降水事件造成;区域北部高山峡谷区属午后峰值是夜间峰值1.5倍的双峰型,午后至傍晚的降水峰值由持续≤10 h的降水事件引起,夜间降水次峰值由持续4~10 h的降水事件造成;南部和西部区域呈午后峰值与夜间峰值持平的双峰型,持续≤4 h的短历时降水事件易引发午后峰值,而持续≥5 h的降水事件则造成夜间降水高峰。这些降水特性的区域差异主要是由于太阳辐射日变化和复杂地形因素引起的湖陆风和山谷风效应造成。

南海夏季风建立的数值模拟试验

利用中国科学院大气物理研究所全球谱模式(T42L9)对南海夏季风建立的过程进行了数值模拟试验.试验结果表明:通过青藏高原的动力作用和东南亚地区强大的潜热加热产生印度洋上跨赤道的偏南辐散风、高原南侧的偏西辐散风、高原东侧的偏北辐散风和西太平洋上的偏东辐散风,这4支散度流加强南海海域大尺度低空辐合,南海的雨季和西南季风爆发.