Deep structure and lithospheric shear faults in the East Kunlun-Qiangtang region,northern Tibetan Plateau

The Cenozoic magmatic rocks of shoshonitic series in the eastern Qinghai-Tibet Plateau include potassic alkaline plutonic rocks, volcanic rocks, lamprophyres and acidic porphyries. Analytical results show that these different lithological rocks are extremely similar in Sr, Nd and Pb isotopic compositions with the range of 0.705 187– 0.707 254 for⁸⁷Sr/⁸⁶Sr, 0.512 305–0.512 630 for¹⁴³Nd/¹⁴⁴Nd, 18.53–18.97 for²⁰⁶Pb/²⁰⁴Pb, 15.51–15.72 for²⁰⁷Pb/²⁰⁴Pb and 38.38–39.24 for²⁰⁸Pb/²⁰⁴Pb. They are isotopically similar to the EMII end-member. This indicates that mantle metasomatism must have taken place in their source region. The formation of these particular rocks is related to crustal thinning and mantle upwelling in a large-scale strike-slip and pull-apart fault zone at about 40 Ma in northern and eastern Qinghai-Tibet Plateau     

Sounding of electrical structure of the crust and upper mantle along the eastern border of Qinghai-Tibet Plateau and its tectonic significance

Sounding and study on electrical structure of the crust and upper mantle within the eastern border region of Qinghai-Tibet Plateau by using the magnetotelluric sounding (simply MT) method permitted us to understand the characteristics of specific electrical structure in the region. The sounding result clearly revealed that: (1) The Xianshuihe fault zone represents a large-scale lithospheric fault and is an important boundary fault of the rhombic Sichuan-Yunnan block. (2) The sounded region is a strong earthquake-prone zone. The different crustal media of blocks on both sides of the fault became an important deep background for the strong seismo-active zone. (3) A large-scale low-resistivity layer is found to exist at a depth more than ten kilometers beneath the northern part of the rhombic Sichuan-Yunnan block. Its electrical resistivity is only several to tens Ω?m. The layer northeastward extends down at an angle of 45°. It is related to an obstacle to the lateral squeeze of Qinghai-Tibet Plateau and eastward flow of mass by the rigid block. It is inferred from the characteristics of electrical property of deep media that the northern part of the recent rhombic Sichuan-Yunnan block is in a thermal state and is one of the recently fairly active blocks. (4) The lithosphere in the sounded region is gradually thickened from the western segment (northern Sichuan-Yunnan block) to east (Yangtze block).     

青藏高原羌塘东部治多县左支——失多莫卜辉长岩带地球化学特征及构造意义

青藏高原羌塘东部治多县左支——失多莫卜辉长岩带形成于晚二叠世,由单一辉长岩组成。岩石富碱,Na2O〉K2O为钙碱性系列。微量元素特征表现为大离子亲石元素（LILE）富集,高场强元素分异,显示板内玄武岩特征。轻稀土元素高度富集,δEu不显亏损,为弱负异常到正异常。（87Sr／85Sr）i较低,变化于0．70419～0．70471之间,εNd（t）值较高,变化于4．3～4．9之间,显示了略亏损的地幔源区特征。该辉长岩带应形成于板内伸展扩张构造环境。     

九寨沟地震发震断层属性及青藏高原东南缘现今应变状态讨论

九寨沟地震（M_S7.0或M_W6.5）震中位于青藏高原巴颜喀拉块体东缘东昆仑断裂带东端塔藏断裂、岷江断裂和虎牙断裂交汇部位,中国地震局相关科研机构的研究人员曾将该震中区判定为玛沁-玛曲高震级地震危险区.地震应急科学考察期间没有发现地震地表破裂带,但地震烈度等震线长轴方位、极震区基岩崩塌和滑坡集中带、重新定位余震空间展布和震源机制解等显示出发震断层为NNW向虎牙断裂北段,左旋走滑性质,属东昆仑断裂带东端分支断层之一.此外,汶川地震后,在青藏高原东缘和东南缘次级活动断层上发生了包括2017年九寨沟地震（M_W6.5）、2014年鲁甸（M_W6.2）、景谷（M_W6.2）、康定（M_W6.0）等多次中强地震,显示出青藏高原东缘至东南缘各块体主干边界活动断层现今处于中等偏高的应变积累状态,即在巴颜喀拉、川滇等块体主干边界活动断层上具备了发生高震级（M_W ≥ 7.0）地震的构造应力-应变条件,未来发生高震级地震的危险性不容忽视.

     

Discovery of the Longriba fault zone in eastern Bayan Har block,China and its tectonic implication

Re-measured GPS data have recently revealed that a broad NE trending dextral shear zone exists in the eastern Bayan Har block about 200 km northwest of the Longmenshan thrust on the eastern margin of the Qinghai-Tibet Plateau. The strain rate along this shear zone may reach up to 4-6 mm/a. Our interpretation of satellite images and field observations indicate that this dextral shear zone corresponds to a newly generated NE trending Longriba fault zone that has been ignored before. The northeast segment of the Longriba fault zone consists of two subparallel N54°±5°E trending branch faults about 30 km apart, and late Quaternary offset landforms are well developed along the strands of these two branch faults. The northern branch fault, the Longriqu fault, has relatively large reverse component, while the southern branch fault, the Maoergai fault, is a pure right-lateral strike slip fault. According to vector synthesizing principle, the average right-lateral strike slip rate along the Longriba fault zone in the late Quaternary is calculated to be 5.4±2.0 mm/a, the vertical slip rate to be 0.7 mm/a, and the rate of crustal shortening to be 0.55 mm/a. The discovery of the Longriba fault zone may provide a new insight into the tectonics and dynamics of the eastern margin of the Qinghai-Tibet Plateau. Taken the Longriba fault zone as a boundary, the Bayan Har block is divided into two sub-blocks: the Ahba sub-block in the west and the Longmenshan sub-block in the east. The shortening and uplifting of the Longmenshan sub-block as a whole reflects that both the Longmenshan thrust and Longriba fault zone are subordinated to a back propagated nappe tectonic system that was formed during the southeastward motion of the Bayan Har block owing to intense resistance of the South China block. This nappe tectonic system has become a boundary tectonic type of an active block supporting crustal deformation along the eastern margin of the Qinghai-Tibet Plateau from late Cenozoic till now. The Longriba fault zone is just an active fault zone newly-generated in late Quaternary along this tectonic system.     

Petrogenesis and tectonic setting of the Queershan composite granitic pluton,eastern Tibetan Plateau: Constraints from geochronology,geochemistry and Hf isotope data

The Queershan composite granitic pluton is located in the north of the late Paleozoic Yidun arc collision-orogenic belt, eastern Tibetan Plateau. The main rock types are coarse-grained porphyritic alkalic-monzonite granite with minor fine-grained porphyritic monzogranite and granodiorite distributed in the eastern and southwestern regions. Here we report their zircon U-Pb ages and geo- chemical data. The intrusive contact relations indicate that granodiorite was formed earlier than the alkalic-monzonite granite(105.9±1.3 Ma) and monzogranite(102.6±1.1 Ma). These suggest that the Queershan composite granitic pluton was formed through three-stage magmatic events. The alkalic-monzonite granite(105.9±1.3 Ma) and monzogranite(102.6±1.1 Ma) are characterized by high SiO2(73.5%–77.7%), K2O+Na2O(6.9%–8.5%), Ga/Al ratios(2.6–3.4) and low Al2O3(11.8%–14.5%), CaO(0.25%–1.5%), MgO(0.18%–0.69%), negative Ba, Sr and Eu anomalies, showing A-type granite affinities. The granodiorite exhibits lower SiO2, P2O5 and K2O+Na2O contents, but higher Al2O3, CaO and MgO contents than alkalic-monzonite granite and monzogranite, showing I-type granite affinity. 176Hf/177 Hf ratios of the alkalic-monzonite granite and the monzogranite are 0.282692–0.282749 and 0.282685–0.282765, respectively, and with similar ?Hf(t) values(?0.56 to 1.43 and ?0.87 to 1.90 respectively). They also present similar TDM2 model ages(1.04–1.22 and 1.07–1.2 Ga respectively), indicating they may be sourced from a similar rock source, mostly like Kangding Complex. The homogeneity of the Hf isotopic compositions and the absence of the MMEs demonstrate that little depleted mantle materials have contributed to the source. We propose that the Mesoproterozoic crust materials of the Yangtze Craton exist beneath the Yidun arc terrane and support it was a dismembered part of the Yangtze Craton. The A-type granites of Queershan composite granitic pluton are most probably related to the closure of the Bangong-Nujiang Tethys ocean.     

Study of charge structure and radiation characteristic of intracloud discharge in thunderstorms of Qinghai-Tibet Plateau

The comprehensive observations on lightning discharges were conducted in Naqu area of Qinghai-Tibet Plateau in summer of 2002. The electric structures of thunderstorms and the characteristics of lightning discharges at initial stage were analyzed by using the observation data. The results show that most of intracloud (IC) lightning flashes were polarities inverted in thunderstorms with tripole electric charge structure and occurred between negative charge region located in the middle of the thunderstorm and positive charge region located at the bottom of the thunderstorm. The radiation characteristics of discharge processes in cloud with longer lasting time involved in Cloud-to-Ground (CG) lightning flashes were similar to that of IC discharges.A lot of radiation pulses were produced in these discharge processes. Because the IC discharges took place at the bottom of thundercloud and were near the ground, they may produce more serious damage to equipment on the ground therefore should not be neglected in lightning protection.     

应力释放模型在青藏高原地区的应用

介绍了应力释放模型、耦合应力释放模型及其在时、空两个方面经过改进的模型。利用改进后的模型研究了青藏高原地区的地震活动,结果显示,在青藏高原地区改进后的应力释放模型仍然适用,并且改进后的耦合应力释放模型要优于简单的应力释放模型,其预测地震的效能和精度都要优于泊松模型。     

Geochemistry and Sr-Nd-Pb isotopic characteristics of the Mugouriwang Cenozoic volcanic rocks from Tibetan Plateau:Constraints on mantle source of the underplated basic magma

The Mugouriwang Cenozoic volcanic rocks exposed in the north Qiangtang Block of Tibetan Plateau are mainly composed of basalt and andesitic-basalt,both characterized by the lower SiO2 (51%―54%),high refractory elements (i.e. Mg,Cr,Ni) as well as the moderate enrichment in light rare earth elements (LREE) relative to a slight depleted in Eu and high strength field elements (HFSE,i.e. Nb,Ta,Ti). Be-sides,the fairly low Sm/Yb value (3.07―4.35) could signify that the rocks should be derived directly from partial melting of the spinel lherzolite at the upper part of the asthenosphere. These rocks have radiogenic Sr and Pb (87Sr/86Sr = 0.705339 to 0.705667; 208Pb/204Pb = 38.8192 to 38.8937; 207Pb/204Pb = 15.6093 to 15.6245; 206Pb/204Pb = 18.6246 to 18.6383),and non-radiogenic Nd (143Nd/144Nd = 0.512604 to 0.512639; εNd = 0.02 to -0.66) in agreement with those values of the BSE mantle reservoir. The DUPAL anomaly of the rocks can be evidently attested by the △8/4Pb = 66.82 to 74.53 ,△7/4Pb = 9.88 to 11.42,△Sr>50,implying that the Mugouriwang volcanic rock is likely to be generated by partial melting of a Gondwana-bearing asthenospheric mantle ever matasomatised by the fluid from subduction zone. Depending on the previous study on the high-K calc-alkaline intermediate-felsic volcanics in the study area,this paper proposed that the fluids derived from the subducted Lhasa Block metasomatised the asthenosphere beneath the Qiangtang Block,and induced its partial melting,and then the melt under-plated the thickened Qiangtang lithosphere and caused the generation of the Cenozoic adakite-like felsic magmas in the Qiangtang region.     

Alkali-rich intrusive rocks in the Qinghai-Xizang Plateau and its vicinities as exemplified by Kuganzi and Taihe plutons

The alkali-rich intrusive rocks in the Qinghai-Xizang Plateau can be divided into two series in terms of dark-colored rock-forming minerals, petrochemical composition and trace elements: ∣) sodic alkaline series rocks consisting of alkaline rocks and alkali granite which contain dark-colored minerals, such as aegirine augite, aegirine and riebeckite, and being petrochemically enriched in alkali and sodium but low in Ca, Sr and Ba; ║) potassic alkaline series rocks consisting of alkaline rocks and alkali granite which contain non-alkaline dark-colored minerals (diopside, edenite), and being petrochemically enriched in alkali, K and Ca with high abundances of Sr and Ba.     

Thermal regime transition in eastern North China and its tectonic implication

Many evidences published in recent years reveal that the thickness,chemical composition and thermal state of lithosphere in eastern North China have ex-perienced dramatic transition during the Phanero-zoic.Comparative study[1—8]of early Paleozoic dia-mond-and xenoliths-bearing kimberlites to Cenozoic mantle peridotite xenoliths-bearing alkali basalts indi-cates that the early Phanerozoic lithosphere is thick and stable to depths within the diamond stability field(180–200km)with depleted ma…     

青藏高原东缘岩石圈物性结构特征及深部构造涵义

青藏高原东缘是研究青藏高原地壳物质向东侧向挤出的焦点地区.为探索青藏高原东向挤出其东部壳幔结构响应及深部地质构造依据,本文利用卫星测高重力数据、数字地震台网（"喜马拉雅"项目一期）634个台站的观测数据、以及跨越龙门山构造带、攀西构造带的两条长周期大地电磁测深资料,获得了青藏高原东缘视密度物性结构、P波速度异常结构、以及电性成像结构.物性成像结果表明：（1）松潘地块、川滇地块中-下地壳、上地幔具有低密度、低速、高导的韧性物性结构,部分地区这种韧性物性结构甚至可到达150 km处;（2）四川盆地下方扬子克拉通岩石圈具有稳定的高密度、高速、高阻的刚硬物性结构,其结构向下可延伸至150 km深处;（3）青藏高原东缘横向和垂向的物性结构差异,为揭示龙门山构造带、川滇地块隆升机制提供了物质基础和动力学依据;（4）岩石圈物性结构中,沿岷山—龙门山—锦屏山—玉龙雪山构造带一线存在明显的密度、速度梯级带,其东西两侧呈明显物性二元结构,该物性梯级带可能反映了中上扬子地块西边界位置.     

Alkali-rich igneous rocks and related Au and Cu large and superlarge deposits in China

The alkali-rich igneous rocks in China occur as fifteen linear distributed belts of each extending several hundreds to several thousands kilometers in length. These include most types of alkali-rich igneous rock categories discovered worldwide. The related Au, Cu large and superlarge deposits or metallogenic focus-areas include Dongping, Guilaizhuang, Yulong, etc. Direct and indirect genetic links have been found between alkali-rich igneous rocks and Au, Cu mineralization. The petrogenesis and metallogenesis of the alkali-rich igneous rocks are mainly controlled by (1) mantle enrichment, (2) strong interaction between mantle and crust, (3) lower contens of sulfur, high and high contents of volatiles, (4) the significant turn of regional tectonic framework from compress to extension and (5) the strong change of regional lithosphere structure.     

青藏高原东缘下地壳流动的地震学证据

在2000年完成的穿过川西高原和四川盆地的深地震测深剖面揭示了川西高原的地壳结构具有地壳增厚(主要是下地壳增厚)、地壳平均速度低等特点,显示地壳的缩短与增厚的碰撞变形特征。根据川西高原上设置各爆炸点的记录截面图共同呈现PmP(莫霍界面反射波)弱能量的特点,推断在川西高原的下地壳介质具有强衰减(Qp=100~300)的性质。利用我国西部地区的宽频带地震台站的面波资料反演青藏高原及其邻区的地壳上地幔S波三维速度结构,在周期T=29.2s和T=42.9s的Rayleigh波群速度分布图上,显示了青藏高原东部(包括川西高原)呈现大范围的低速异常。多方面的结果表明,地震学方法为当前流行的下地壳流动模型提供了深部证据。     

青藏高原北部地区b值短临异常特征

在青藏高原北部地区活动地块划分的基础上,以地块为研究单元,对1970年以来的b值进行全时空扫描,以研究其短临时间特征,结果表明,虽然受地域和地质构造的影响,各地块b值的短临异常与5级地震的对应率不同,但绝大多数地块的6值正、负异常起始时间、最大(最小)值时间和结束时间通常出现在震前1-6个月内。对其结果统计检验,本地块R≥R97.%的合格率为93％,内邻地块合计R≥R97.5%的合格率为100％,可信度检验率较高。     

青藏高原及周缘地壳浅层构造应力场量值特征分析

以"中国大陆地壳应力环境基础数据库"中的实测地应力数据为基础,合理筛选出地理空间范围为21°N—40°N,73°N —110°N的近2000条数据,深度范围0~2 km.通过将研究区内实测地应力扣除重力影响,并考虑数据样本数量沿深度分布不均匀的问题,分析构造应力场的作用.重力影响的扣除采用海姆假说与金尼克假说两种模式估算其下限与上限,给出了青藏高原及周缘及青藏地块、南北地震带北、中、南段上构造应力和构造差应力随深度分布的特征和量值范围.结果显示：（1）青藏高原及周缘地区最大水平应力σH、最小水平应力σh随深度D呈线性增加：σH=22.115D+5.761、σh= 14.893D+3.269;最大水平构造应力σT、最小水平构造应力σt的量值估算范围分别为4.609 < σT < 15.522D+4.609、3.121 < σt < 6.366D+3.121（D >0）;构造差应力σT-σt=7.222D+2.492,地表值（D=0 km）为2.5 MPa左右,随深度增加以7.2 MPa·km-1的梯度增大;（2）在测量深度范围内,青藏地块、南北地震带北、中、南段研究区σT、σt、σT-σt随埋深均呈线性增大;D=1 km时,各地块σT的统计回归值中最大为30.1 MPa,最小为17.6 MPa,量值由大到小排序依次为：青藏地块、南北带北段、南北带中段、南北带南段;D=1 km时,各地块σT-σt的统计回归值中最大为15.8 MPa,最小为8.9 MPa,量值由大到小排序依次为：青藏地块、南北带北段、南北带中段和南北带南段.总体表现为青藏地块强、南北带较弱的基本特征.（3）与南北带相比较,青藏地块地壳在从南向北的挤压作用下呈现出明显的"浅弱深强"特点.     

青藏高原冻土带天然气水合物的形成条件与分布预测

冻土带是天然气水合物发育的两个重要地质环境之一.青藏高原平均海拔在4000m以上,多年冻土面积约1.4×106km2.本文根据青藏高原冻土层厚度和地温梯度特征,运用天然气水合物的热力学稳定域预测方法,确定中低纬度高海拔区冻土带天然气水合物的产出特征.青藏高原多年冻土带热成因天然气水合物形成的热力学相平衡反映,水合物顶界埋深约27~560m,底界埋深约77~2070m.初步计算表明,青藏高原冻土带水合物天然气资源约1.2×1011~2.4×1014m3.在冻土层越厚、冻土层及冻土层之下沉积层的地温梯度越小的地区,最有利于天然气水合物的发育.气温的季节性变化对天然气水合物影响不大.在全球气温快速上升的背景下,青藏高原天然气水合物将处于失稳状态,天然气水合物顶界下降、底界上升,与冻土带的退化相似,分布区逐渐缩小,最终将完全消失.     

Cenozoic volcanism and lithospheric tectonic evolution in Qiangtang area, northern Qinghai-Tibet Plateau

Accompanying with the shortening,thickening and uplifting of the lithosphere,a series of Cenozoic potassic volcanic rock zones are developed in the northern Qinghai-Tibet Plateau.From south to north,the volcanic rocks can be divided into three volcanicrock belts:Qiangtang-Nangqian volcanic belt,Middle Kunlun-Hoh Xil volcanic belt and Western Kunlun-Eastern Kunlun volcanic belt[1].Spatiotemporal evolu-tion of the volcanism and the origins of magmas con-strains on the pulsing uplifting and …     

Isotopic feature and uranium dating of the volcanic rocks in the Okinawa Trough

1 Introduction The Okinawa Trough(OT) extends for ~1200 km between Taiwan and Kyushu, and is an incipient back-arc basin bounded by the Ryukyu Arc to the south and east, and by Diaoyudao Uplift Belt[1,2] (Tai- wan-Sinzi Belt[3]) to the north and west. Drastic faults and volcanic activities are the typical features in the OT. Generally, the OT is divided into three segments (north, middle and south) by the Tokara Strait and Kerama Gap[4]. The geological features, such as tec- tonics, m…