The geological structure background and the crustal structure in the northeastern margin of the Qinghai-Tibetan plateau

The geological structure background, the crustal structure and the shape of Moho in the northeastern margin of the Qinghai-Tibetan plateau are studied. Based on artificial seismic sounding profile as well as geological data. The main results are summarized as follows: (1) The geotectonic subdivisions and the characteristics of main deep and large faults in the northeastern margin of the Qinghai-Tibetan plateau are presented; (2) The general features of the Moho are obtained mainly based on artificial seismic sounding data; (3) There exists well corresponding relation between surface faults and some features of the Moho, which suggests that such complex crustal structure might be the preparation environment of strong earthquakes.

Formation and Evolution of the Late Mesozoic-Cenozoic Ruo'ergai Basin,Northeastern Margin of the Qinghai-Tibet Plateau

The Middle Triassic Ladinian-Upper Triassic Norian series in the Mesozoic-Cenozoic Ruo’ergai basin of Songpan area is characterized of large thick shallow marine-deep marine fine grained clastic.The strata are regionally unconformable between each adjacent two of the Middle-Late Triassic fine grained clastic,the Jurassic coal-containing clastic,the Cretaceous-Paleogene variegated coarse clastic。

Analysis of Doppler Radar Data about a Super Monomer Hailstorms in the Northeastern Qinghai-Tibet Plateau

[Objective] The Doppler radar data about a super monomer hailstorms in the northeastern Qinghai-Tibet Plateau in the Zhongchuan Airport in the Lanzhou City on September 6,2010 was studied.[Method] By dint of routine data and radar data,the low vortex shear line type and the super monomer hailstorm around the Zhongchuan Airport in the Lanzhou City on September 6,2010 were expounded.Basic product and secondary product of Doppler radar were used in this process to reflect the characteristics of strong convection weather.Some characteristics of this process shall be explored.[Result] A small gush of cold air from the cold vortex of 500 hPa in the middle and high layer provided impacts.The warm shear line provided water vapor and energy in the 700 hPa.There was strong convective weather in the upper air.Such 10 minutes of hailstorm was rarely seen in the drought land in the northwest.The characteristics of the strong convection were distinct and typical.The front showed no echo form.However,it can not be reflected in 'strong wedge' in another form.In this process,characteristics of BWER and middle scale cyclone were distinct.And this was a typical hailstorm process caused by super monomer.[Conclusion] The study provided some helpful references for the forecast of strong convection weather in the Zhongchuan Airport in Lanzhou City.

Present-day tectonic movement in the northeastern margin of the Qinghai-Xizang (Tibetan) plateau as revealed by earthquake activity

Characteristics of present-day tectonic movement in the northeastern margin of Qinghai-Xizang plateau (Tibetan) are studied based on earthquake data. Evidence of earthquake activity shows that junctures between blocks in this area consist of complicated deformation zones. Between the Gansu-Qinghai block and Alxa block there is a broad compressive deformation zone, which turns essentially to be a network-like deformation region to the southeast. The Liupanshan region, where the Gansu-Qinghai block contacts the Ordos block, is suffering from NE-SW compressive deformation. Junction zone between the Ordos and Alxa block is a shear zone with sections of variable trend. The northwestern and southeastern marginal region of the Ordos is under NNW-SSE extension. The above characteristics of present-day tectonic deformation of the northeastern Qinghai-Xizang plateau may be attributed to the northeastward squeezing of the plateau and the resistance of the Ordos block, as well as the southeastward extrusion of the plateau materials.

Lateral variation of Pn velocity beneath northeastern marginal region of Qinghai- Xizang plateau

Pn arrival time data are collected from the bulletins of both national and regional seismological network in China. These data are tomographically inverted to map the lateral variation and anisotropy of Pn velocity in the northeast-ern marginal region of Qinghai-Xizang plateau. The average Pn velocity in this region is 8.09 km/s, being a little higher than the average for whole China. Higher velocity is found in tectonically stable Qaidam basin, while lower velocity is seen in and around tectonically active Shanxi graben. The region where the 1920 Haiyuan great earth-quake occurred shows a slightly low Pn velocity. A noticeable result is that, differing from the tectonically com-pressive Tianshan region, where Pn velocity is low, the Qilianshan region, where the Neotectonic deformation is also primarily compressive, shows high Pn velocity. In the uppermost mantle beneath the Ordos plateau Pn veloc-ity is inhomogeneous, varying from higher velocity in southwestern part to lower one in northeastern part. This may be attributed to possible movement of the Ordos block, as there are strong earthquakes all around the block.

青藏高原东北缘清水河流域地貌特征及其构造意义

新生代以来,受印度板块与欧亚板块剧烈碰撞汇聚的影响,青藏高原快速隆起,其构造变形不断向北东扩展、挤出,造就了清水河流域及其周缘独特的构造地貌格局。通过对清水河流域构造地貌的研究,可以揭示新生代晚期以来青藏高原东北缘变形机制和地貌演化过程。本文基于30 m分辨率DEM数据,利用ArcGIS 10.2软件对清水河流域地貌参数进行提取,精确计算了22条子流域的河长坡降指标(SL)及Hack剖面、面积-高程积分(HI)、流域不对称度(AF)以及盆地延伸率(Re)这4种地貌形态指数,定量获取了清水河流域的地貌形态特征,再结合4个参数求得各子流域的相对活动构造指数(Iat),对研究区的相对构造活动性程度进行分析。得到以下结论:(1)研究区SL值分布在70.1~353.9之间,表明整个流域内构造活动性存在区域差异且整体活动变形程度较高;(2)受海原断裂带和香山-天景山断裂带的影响,研究区西岸流域HI和AF指数的平均值大于东岸,Re指数的平均值小于东岸,表明西岸构造活动性大于东岸;(3)研究区相对活动构造指数(Iat)处于第一、二级的面积达到80%,主要分布在西岸,表明整个流域的构造活动性强烈,且展现由东向西活动性逐渐增强的趋势。同时结合相关地质资料综合评估流域构造活动状态,并据此对河流发育阶段以及构造运动进行分析。

Geomorphologic evidence of phased uplift of the northeastern Qinghai-Tibet Plateau since 14 million years ago

A typical sequence of fluvial terraces and aeolian deposits overlying these ter- races were multidisciplinary investigated. New evidences for uplift process of the northeastern Qinghai-Tibetan Plateau in the past 14 million years were obtained. At least 11 river terraces along Huangshui, the first-class tributary of Yellow River, at the Xining-Huzhu region are identi- fied. While the first one (T1) is classified as an accumulation terrace, the others are all basement river terraces, which consist of the Tertiary sandstone and siltstone bedrock, fluvial gravel and pebbles and the overlying aeolian loess-Red Clay deposit. Samples from the aeolian deposits were examined for paleomagnetic stratigraphic reconstruction (1030 samples), luminescence dating (16 samples), magnetic susceptibility and grain-size distribution (more than 4000 sam- ples). The luminescence dating and stratigraphic correlation suggest that terraces of Tll, T10, T8, T7, T3, T2, T1 were formed at 14, 11.3, 1.55, 1.2, 0.15, 0.07 and 0.01 million years ago, re- spectively. Sedimentological analysis and geomorphological observation indicate that formation and evolution of these terraces were mainly driven by tectonic uplift. Therefore, the terrace se- quence provides an ideal geological record, of the uplift process of the northeastern during the past 14 million years, and the timings of the terraces formation are regarded as the timings of tectonic uplift. The significant uplifting events took place at 14, 11.3, 1.2 and 0.15 mil- lion years ago, respectively. The fluvial incision at the Xining-Huzhu region is less than 100 m during a period of ~12 million years in the Miocene era (between the Tll and T9), while the Huangshui River had incised 432 m during the past 1.2 million years (from T7 to the present floodplain). The river incision process clearly demonstrates that accelerated rising of the north- eastern Qinghai-Tibet Plateau during the late Cenozoic, and provides new evidence of previous thoughts. There was a significant readjustment of the fluvial catchment during 1.55--1.2 million years ago: before this time, the paleoriver flowed to southwest. After this time the Huangshui River flows to southeast. A tectonic movement dominates reorganization of this fluvial system.

青藏高原东北边缘云杉-巴山冷杉林火烧迹地植物群落演替的数量研究

[目的]分析青藏高原东北边缘云杉-巴山冷杉林火烧迹地不同演替阶段植物群落物种组成和多样性变化规律,为火烧迹地植物群落的科学管理提供依据。[方法]采用空间代替时间方法,在迭山北坡,选取4个不同演替阶段的植物群落,即火后演替5年(5 a)、火后演替15年(15 a)、火后演替23年(23 a)和森林顶级群落(Climax)作为一个演替序列,采用样方法调查植物群落的物种组成,计算物种重要值、多样性指数和群落相似系数等指标,以揭示火烧迹地植物群落演替的数量特征。[结果]1)随着火烧迹地植物群落演替时间的推移,群落物种丰富度下降,个体数量逐渐减少,其中5 a群落物种丰富度最大,个体数最多(4008±298株·(400 m^(2))^(-1)),Climax的物种丰富度最小,个体数量也最少(1759±90株·(400 m^(2))^(-1));草本层5a群落、灌木层和乔木层Climax群落的Margalef指数和Shannon-Wiener指数最大。2)依据重要值,5 a、15 a、23 a和Climax 4个不同演替阶段的群落可命名为扁刺蔷薇+箭竹+密生薹草、唐古特忍冬+黑水柳+密生薹草、青海云杉-红桦+扁刺蔷薇+密生薹草群落和云杉-巴山冷杉+唐古特忍冬+密生薹草群落。3)不同演替阶段草本植物群落异质性指数5 a(0.07)<15 a(0.12)<23 a(0.19)<Climax(0.22)。4)5 a、15 a和23 a群落的多度为几何级数分布型,表明火烧迹地环境条件有利于草本植物生长,生态位确定性物种在群落构建过程中占据主要地位;而Climax群落的多度呈“S”型分布,“中间类型”物种占据了群落的绝大部分,属于演替晚期群落。5)根据β多样性的差异,可将4个不同演替阶段群落的灌木层划分为初建灌木层(5 a)和稳定灌木层(15 a,23 a,Climax),草本层划分为剧烈变化草本层(5 a和15 a)和稳定草本层(23 a和Climax)。[结论]随着植物群落演替时间越长,青藏高原东北边缘云杉-巴山冷杉林火烧迹地物种丰富度呈递减趋势,群落间的相似性越加接近,群落为正向演替,演替过程为灌木林--落叶阔叶林--针叶林--云杉-巴山冷杉林。

青海化隆县独特的峡谷丹山地貌景观及其科学价值

“峡谷丹山”是内外地质营力共同作用而形成的多成因地貌组合,不仅具有非常高的景观价值,而且具有重要的科学价值。青海省化隆县拉木峡为黄河一级支流,发育典型的角度不整合和丹霞地貌,因而成为青藏高原隆升和黄河形成演化研究的良好区域。以该地貌组合类型的形态特征、成因机制及形成年代研究为目标,采用野外地质调查等方法,对组成峡谷的岩石及地层、构造背景开展研究,系统总结与分析拉木峡地貌景观的形成动力和演化过程,进而探讨其对青藏高原隆升及黄河演化的指示意义。拉木峡由约900 Ma前的新元古代化隆岩群片麻岩及角度不整合于其上的约54 Ma前的新生代古近纪西宁群红色砂砾岩层组成。约8 Ma前青藏高原快速隆升,丹霞地貌雏形开始形成。自1.1 Ma黄河贯通化隆盆地后,受拉木峡流水强烈侵蚀下切和重力崩塌的持续作用,发育“峡谷丹山”地貌组合景观。峡谷险峻幽深、丹霞栩栩如生,成为极具典型性、独特性和稀有性的国家级地质遗迹,对青藏高原隆升、黄河形成演化和新生代气候环境的研究具有重要价值。

采用新方法计算b值评估中长期地震危险性——以青藏高原东北缘为例

1研究背景。古登堡-里克特(G-R)关系描述了地震活动震级-频度关系(Gutenberg&Richter,1944;Utsu,1965)。随后,通过一系列岩石力学实验和地震学研究表明(Scholz,1968;Geobel et al,2013;Amitrano,2003;Scholz,2015;Wu,2018),G-R关系中b值与地壳应力存在反比关系,即较低的b值代表区域应力处于一个较高的水平,b值的降低代表区域应力的升高。因此,b值作为应力水平的示踪剂对于评估该地区地震危险性具有指示意义。随后,b值被广泛的应用到天然地震和诱发地震的研究中(Wyss et al,2000;Schorlemmer etal,2005;Nandanetal,2017;Nanjoetal,2019;Petruccellietal,2019;Finzietal,2020),相关研究表明,大震通常发生在低b值区域(Kulhanek et al,2005;EI-Isa et al,2014;Shcherbakov et al,2019),并且大震前在震源区范围内b值会表现出随时间的降低,反映了震前应力水平的积累。

青藏高原东北部黄土区灌木柠条锦鸡儿单根抗拉力学特性研究

[目的]柠条锦鸡儿(Caragana korshinskii Kom.)是青藏高原东北部和黄土高原地区最为广泛的护坡和水土保持灌木之一,开展该种植物单根抗拉力学特性试验研究,为定量评价该种植物固土护坡效果提供理论依据。[方法]以根径为2.33~9.16 mm的中龄期柠条锦鸡儿根系为研究对象,通过单根拉伸试验对比分析了不同断裂模式和不同根径级别条件下,柠条锦鸡儿单根抗拉力学特性的差异性及其与根径之间的关系,并进一步探讨了单根断裂破坏模式及其应力—应变特征。[结果]柠条锦鸡儿单根存在周皮断裂和整体断裂2种断裂模式,2种断裂模式单根抗拉力和单根抗拉强度分别在(2,4],(4,6]和(6,8]mm以及(6,8]mm根径级别条件下存在显著性差异;相同断裂模式不同根径级别之间,4个单根抗拉力学特性指标差异程度各不相同。周皮断裂和整体断裂模式条件下,单根抗拉力随着根径的增大分别呈指数函数和幂函数增长趋势;单根抗拉强度、弹性模量和极限延伸率随着根径增大而降低。2种断裂模式的产生取决于单根受拉伸作用时,周皮、中柱鞘薄壁组织和次生维管组织的应力状态;对应的单根应力—应变曲线表现为弹性变形、弹塑性变形、应变硬化及周皮(断裂)破坏4个阶段。[结论]在采用单根拉伸试验来获取木本植物根系单根抗拉力学特性指标时,有必要考虑根径和断裂模式对单根抗拉力学特性的影响。

青藏高原东北缘龙首山晚新生代多阶段构造隆升的盆地记录

龙首山地区位于青藏高原与阿拉善地块的结合部位,其盆山演化过程和断裂带扩展模式的恢复对揭示青藏高原新生代北东向生长具有重要意义。本文基于龙首山南北两麓3个新近系沉积剖面的构造特征和5个砂岩样品的碎屑锆石U-Pb同位素LA-ICP-MS年龄结果,重建了青藏高原东北缘龙首山地区晚新生代的盆山演化过程。研究结果显示,龙首山地区晚新生代沉积-构造演化经历了3个主要阶段:(1)~14Ma,龙首山南缘断裂带活化,阿拉善地块南缘沿此断裂逆冲于张掖盆地之上,张掖盆地开始接收阿拉善地块的物源;(2)~5Ma,龙首山南缘断裂带的次级断裂开始发育,使龙首山南侧山麓白垩系开始剥露并为张掖盆地提供物源,同时,龙首山北缘断裂带活化并导致潮水盆地发生挠曲沉降,接收龙首山的物源;(3)5~2.5Ma,龙首山北缘断裂带的次级断裂开始发育,导致龙首山北麓白垩系开始剥露并为潮水盆地提供物源。通过对龙首山周缘盆地沉积相和物源开展分析,本研究反演了龙首山地区中新世以来的断裂带活动和山体隆升过程,表明在14~2.5Ma,随着龙首山南缘断裂、北缘断裂的活化和次级断裂的发育,龙首山经历了3次强烈的隆升,这对揭示青藏高原北东向的扩展过程具有重要的指示意义。

青藏高原东北边缘云杉-巴山冷杉林火烧迹地草本植物群落构建机理

群落构建机制是生态学研究的核心论题。生态位理论和中性理论是阐明群落构建的两种主要理论,但这两种理论分别强调的环境过滤和扩散限制对群落构建的影响尚存争议。该研究以青藏高原东北边缘云杉-巴山冷杉林火烧迹地不同演替阶段草本群落为研究对象,应用扩散-生态位连续体指数(DNCI)和校正随机率(MST),分析不同演替阶段草本植物群落的主导构建机制。研究结果显示:1)5、15和23 a群落与Climax群落之间的DNCI分别为-22.6169、-21.7507和-8.2594,都远小于0,表明不同演替阶段草本植物群落构建的主要驱动作用是扩散过程;2)5 a-Climax组和23 a-Climax组的MST值均高于50%阈值线,这表明随机性过程在两组群落构建中均占支配地位;对于15 a-Climax组,15 a的MST值为0.6664,Climax的MST值等于0.4927,说明15 a植物群落构建以随机性为主,而Climax群落构建以确定性为主;由于15 a的MST值远高于50%,而Climax的MST值较接近于50%的阈值,也可说明15 a-Climax组群落构建仍以随机性为主。在青藏高原东北边缘云杉-巴山冷杉林火烧迹地,随机扩散主导了草本植物群落的构建过程。

宽角反射/折射剖面揭示的祁连造山带莫霍面深度

祁连造山带位于青藏高原东北缘,距南侧的喜马拉雅碰撞带前缘1500 km,以一个宽阔的(东西长约1000 km,南北宽200~400 km)、NW走向的造山带的形式被夹持于北侧的河西走廊盆地与南侧的柴达木盆地之间,西侧被NEE走向的阿尔金左行走滑断裂带所截切,北缘以青藏高原北缘断裂带,祁连山北缘断裂带和祁连山东缘断裂带与河西走廊盆地相邻,南东方向与西秦岭造山带相接,东缘与鄂尔多斯地块相邻.记录了新生代以来印度板块和亚洲大陆板块碰撞和青藏高原边缘造山和地壳变形的重要过程.对其地壳深部结构的探测是研究青藏高原隆升和向北扩展,理解印度与欧亚大陆碰撞的大陆内部构造作用的关键手段.自1980年代以来,前人在研究区实施了多条宽角反射/折射剖面,以揭示祁连造山带及周缘的地壳深部结构.本文通过对这些宽角反射/折射剖面的收集汇总和梳理分析,以探讨祁连造山带不同区段下方莫霍面起伏及深度差异,研究结果显示:祁连造山带莫霍面埋深整体自西向东变浅,最深的莫霍面位于北祁连造山带内的哈拉湖附近;结合其他地质与地球物理资料,本文推测莫霍面深度的起伏及变化状态揭示了祁连造山带由西向东不同的地壳缩短方式,其中西段最深的莫霍面可能由大陆俯冲的“底垫作用”所引起;中段的壳内低速体和低阻体反映了该区上下地壳解耦变形,地壳的持续缩短主要靠变形解耦面以上发育的大型逆冲断裂带调节;而莫霍面深度最浅的东段累积应力则主要靠左行走滑的海原断裂和壳内发育的逆冲断裂调节.

青藏高原东北缘地震动衰减模型及其在地震影响场评估中的应用

青藏高原东北缘活动断裂分布密集、构造变形活动强烈、地震频发.建立更符合该地区地震动特征的地震动衰减模型,能够在本地区震害防治和震后应急救援等方面发挥重要作用.首先,基于青藏高原东北缘地震构造特征,有针对性地选择强震数据库,构建考虑断层类型、近断层效应及场地条件等因素的区域性地震动衰减模型,运用随机效应回归法计算出模型系数;结合国内强震动记录通过残差分析对模型预测结果进行检验,并比较与NGA模型的差异;最后,运用震动图快速生成系统产出2022年门源Mw6.6地震理论烈度图并与现场调查烈度进行对比.结果表明:本文模型与NGA模型的预测偏差与场地条件和距离关系不大,而主要与震级相关,预测偏差随震级增大而明显增大;残差分析显示本文模型与青藏高原东北缘地区地震记录更接近;基于震动图快速生成系统产出的理论烈度图与现场调查结果比较接近,运用地震动衰减模型可有效提高台网稀疏地区强震Shakemap和地震影响场的预测精度和效率.

青藏高原东北缘ALOS AW3D30高程模型和EIGEN-6C4自由空气重力异常模型精度分析

利用EGM96全球重力场模型对ALOS AW3D30全球数字高程模型的高程系统进行转换,结合实测GNSS数据,分析ALOS AW3D30模型在青藏高原东北缘地区的精度.利用实测重力数据计算青藏高原东北缘地区的自由空气异常,并与EGIEN-6C4自由空气异常模型进行对比,分析其精度.结果表明:青藏高原东北缘地区ALOS AW3D30模型与264个GNSS测点的差异标准差为3.4 m,该地区ALOS AW3D30精度优于4 m;青藏高原东北缘地区自由空气重力异常整体呈负值,局部为正值,变化范围为-177×10^(-5)~166×10^(-5)m·s^(-2),实测重力异常与模型结果空间分布基本一致,存在局部差异;EIGEN-6C4自由空气重力异常变化范围为-163×10^(-5)~142×10^(-5)m·s^(-2),实测结果与模型结果差异为-119×10^(5)~63×10^(-5)m·s^(-2),平均值为-20×10^(-5)m·s^(-2),差异标准差为29×10^(-5)m·s^(-2),实测结果与模型结果差异较大,青藏高原东北缘地区真实地球重力场应该通过实测获得.最后,本文基于青藏高原东北缘地区的实测重力异常与EGIEN-6C4重力异常模型,利用消去-恢复法,将实测重力异常与模型结果进行融合,提高了青藏高原东北缘地区,尤其是祁连山东缘、海原断裂及周边地区的重力异常精度,可以为区域重力场研究提供参考.

High-Pressure Eclogite-Blueschist Metamorphic Belt and Closure of Paleo-Tethys Ocean in Central Qiangtang, Qinghai-Tibet Plateau

The high-pressure metamorphic belt （HPMB） of eclogite-blueschist in Central Qiangtang （羌塘） lies in the Longmu Co （龙木错）-Shuanghu （双湖） suture zone. To the west, the HPMB extends 500 km from Hongjishan （红脊山） to Caiduochaka （才多茶卡）, east of Shuanghu; to the east it extends to Baqing （巴青） and Jitang （吉塘） in Qamdo （昌都）, and then bends southward to Yunnan （云南） Province. Including the Lancangjiang （澜沧江） blueschist belt, the entire HPMB is about 2 000 km long. In Central Qiangtang, the belt is mainly composed of blueschist and eclogite, whereas in West Yunnan it contains only blueschist. The Baqing-Jitang segment is dominated by garnet phengite schist. 40Ar-39Ar dating of glaucophane and phengite from the blueschists yielded plateau ages ranging from 223 to 215 Ma, whereas SHRIMP U-Pb dating of zircon from the eclogites gives metamorphic ages of 243-217 Ma. The calculated metamorphic conditions for the blueschists are 410-460 ℃ and 0.67-0.75 GPa, and for the eclogites, 〈500℃ and 1.56-2.35 GPa. The metamorphic ages suggest that the Longmu Co-Shuanghu suture closed in the Late Triassic. The region south of the Longmu Co-Shuanghu- Lancang suture consists of the pan-African basement overlain by Condwana sedimentary and metasedimentary rocks, whereas the region north of the suture is dominated by the Jinning （晋宁） basement and Yangtze sedimentary and metasedimentary rocks. The Qiangtang HPMB marks the closure of the paleo-Tethys Ocean.

近代青藏高原东北部商业发展与农业种植结构变迁研究

青藏高原东北部是各民族交往交流交融发展的重要地区,近代商业的活跃,引起该地区农业种植结构发生了变迁。文章从粮食商品化程度加深与种植面积扩大、经济作物种植比例增长与种植结构多样化、家庭园艺养殖业的市场化与多元结构等方面分析了农作物种植多样化且稳增,农户家庭农业经营领域结构实现了多元化,具有了商品生产的性质,自然经济开始解体。种植结构的变迁使人口得到增长,为农业提供更多劳动力,城市商业更加繁荣,为农产品销售提供了广阔的消费市场,引起区域社会经济整体发展。

Geomorphometric Features of the Alluvial Fans around the Chaka-Qinghai Lake in the Northeastern Tibetan Plateau

This article aims to study the geomorpometric features of alluvial fans since they act as a small-scale geomorphic unit response to tectonics and climate changes around the Chaka-Qinghai Lake area in the northeastern Tibetan Plateau. We quantitatively extracted geomorphic parameters, such as the surface area and slope of alluvial fans adjacent to the Qinghai Nan Shan and Ela Shan. Alluvial fans in the Chaka Lake partition area, south of the Qinghai Nan Shan, are featured by a small area and short length, but the largest slope. Geomorphic parameters of the alluvial fans in Ela Shan area are in- termediate in size, and the alluvial fans in the Qinghai Lake partition area north of Qinghai Nan Shan have the gentlest slope. Together with the regional faulting activity analysis, we suggest that the alluvial fans with the high slopes in the south of Qinghai Nan Shan are mainly controlled by the reverse faulting along the Qinghai Nan Shan faults, and the strike-slip movement of the Eia Shan fault zone plays a weak role. In contrast, due to the lack of active faults, the alluvial fans near the Qinghai Lake area north of the Qinghai Nan Shan only respond to regional erosion, transportation, and deposition proc- esses, thereby forming relatively gentle geomorphic units.

青藏高原东北缘—鄂尔多斯地壳上地幔地震层析成像研究

根据横跨青藏高原东北缘—鄂尔多斯地区的流动宽频带地震台阵和区域地震台网的走时数据 ,利用地震层析成像方法研究了该地区 4 0 0km深度范围内地壳上地幔的P波速度结构 .结果表明 :青藏高原东北缘—鄂尔多斯地区地壳上地幔结构呈明显的横向不均匀性 .这种不均匀性体现在不同块体之间 ,同时也存在于块体内部 ;青藏高原东北缘上地幔P波平均速度低 ,而鄂尔多斯地块的P波平均速度高 ,这与两个地块的地质构造活动特征相吻合 .在青藏高原东北缘与鄂尔多斯地块之间的上地幔存在宽约 2 0 0km的过渡带 ,该过渡带在地表的界限在兰州—海原之间 ,在上地幔表现为向东 4 5°左右的倾斜条带 ,基本结构特征表现为高速与低速物质的混杂 ;青藏高原内部的柴达木地块平均速度偏低 ,而祁连地块的上地幔的平均速度偏高 ,两者相差约 8% ;在泽库、兰州和海原地区的上地幔顶部有明显的低速体侵入特征 .