Episodic crustal anatexis and the formation of Paiku composite leucogranitic pluton in the Malashan Gneiss Dome, Southern Tibet

The Paiku composite leucogranitic pluton in the Malashan gneiss dome within the Tethyan Himalaya consists of tourmaline leucogranite, two-mica granite and garnet-bearing leucogranite. Zircon U-Pb dating yields that (1) tourmaline leucogranite formed at 28.2±0.5 Ma and its source rock experienced simultaneous metamorphism and anatexis at 33.6±0.6 Ma; (2) two-mica granite formed at 19.8±0.5 Ma; (3) both types of leucogranite contain inherited zircon grains with an age peak at ～480 Ma. These leucogranites show distinct geochemistry in major and trace elements as well as in Sr-Nd-Hf isotope compositions. As compared to the two-mica granites, the tourmaline ones have higher initial Sr and zircon Hf isotope compositions, indicating that they were derived from different source rocks combined with different melting reactions. Combined with available literature data, it is suggested that anatexis at ～35 Ma along the Himalayan orogenic belt might have triggered the initial movement of the Southern Tibetan Detachment System (STDS), and led to the tectonic transition from compressive shortening to extension. Such a tectonic transition could be a dominant factor that initiates large scale decompressional melting of fertile high-grade metapelites along the Himalayan orogenic belt. Crustal anatexis at ～28 Ma and ～20 Ma represent large-scale melting reactions associated with the movement of the STDS.     

Early Oligocene anatexis in the Yardoi gneiss dome, southern Tibet and geological implications

The Yardoi gneiss dome is located to the easternmost of the North Himalayan Gneiss Dome (NHGD), southern Tibet. It consists of metapelite, garnet amphibolite, granite and leucogranite, and is a key subject to constrain the formation and tectonic evolution of NHGD. SHRIMP zircon U/Pb data on the leucogranite yield an age of 35.3±1.1 Ma, which is substantially older than that of the similar leucogranites to the west. Sr and Nd isotope systematics indicate that this leucogranite was derived from partial melting of the mixed garnet amphibolite and metapelite. Our data suggest that (1) during the early stage of Himalayan magmatism, amphibolite dehydration melting overwhelmed that of the metapelite; and (2) such a melting at middle-lower crust might be a major factor that initiated the movement along the Southern Tibetan Detachment System (STDS). Supported by National Natural Science Foundation of China (Grant No. 40673027), the Outlay Research Fund of Chinese Academy of Geological Sciences (Grant No. 20071120101125), and the Hundred Talent Program of Chinese Academy of Sciences     

Middle-Miocene transformation of tectonic regime in the Himalayan orogen

Understanding the multiple tectonic transformations during the Himalayan orogeny is significant in evaluating the evolution of Himalayan orogen.In the Gyirong area in south Tibet,deformed leucogranitic veins in the biotite-plagioclase gneisses of Greater Himalayan crystalline complex(GHC) constitute south-vergent asymmetric folds.The reconstruction of the veins shows that they experienced two generations of deformation under different tectonic regimes:an earlier top-to-north extension and a later top-to-south thrusting,implying a tectonic transformation from N-S extension to N-S shortening.Zircons LA-ICP-MS U-Pb dating of the leucogranite shows that it was emplaced during 21.03-18.7 Ma.The data suggest that the tectonic transformation occurred after 18.7 Ma.The chronological data of South Tibet detachment system(STDS) and North-South trending rift(NSTR) from Gyirong and other areas indicate that the Himalayan orogeny was in a period of tectonic transformation from N-S extension to N-S shortening during 19-13 Ma.The transformation of tectonic regime was probably controlled by the India-Asia convergence rate.An increase in the convergence rate resulted in N-S shortening of the orogen,thrusting and folding,with coeval formation of the NSTR in Tibet.A decrease in the convergence rate led to N-S extension and reactivation of the STDS.     

河南熊耳山北坡西段的变质核杂岩和拆离断层

河南熊耳山北坡西段的变质核杂岩核部,由太华群深变质岩系组成结晶基底,熊耳群未变质的火山岩系和第三系红层组成盖层。二者之间的滑脱拆离带下拆离盘由韧性变形的糜棱岩组成,具有典型的糜棱岩类岩石的基本特征。上拆离盘以脆性变形为主。盖层中发育了拆离断层特有的铲形断层,局部见同构造的花岗斑岩侵入其中。第三系红层向拆离断层反倾。滑脱拆离断层控制了本区热液矿床的分布,尤其是大型低品位金矿的分布。     

东营凹陷辛镇构造带构造变形的物理模拟研究

东营凹陷辛镇构造带底辟动力机制及"负花状"断裂形成的主控因素一直存有争议。通过地震资料精细构造解析,结合地层组合特征,设计含塑性层构造物理模拟实验,开展辛镇构造带构造沉积演化研究。根据构造带的地质特征及构造物理模拟结果,分析其深部底辟构造的动力机制,探讨伸展构造背景下塑性层的变形特征及其与上覆岩层的相互作用。取得了以下主要认识:(1)在盆地伸展构造背景下,坡坪式基底断层活动导致上盘岩层的地势起伏,后期同构造沉积产生的差异沉降诱发底辟发育;(2)辛镇构造带"负花状"断裂是底辟和区域伸展共同作用的结果,塑性层为断裂发育提供了良好的滑脱条件。     

Mesozoic thrusts and extensional structures in the Daqingshan orogen, Inner Mongolia, and their temporal and spatial relationship

Three large-scale thrusts developed in the Daqingshan orogen, Inner Mongolia, with a northeast-east trending from Louhuashan to Suletu are described in this paper. These northwest-directed thrusts, together with the four tectonic belts (or tectonic sheets) separated by these faults, formed a typical thrusting system. The originally defined Hohhot metamorphic core complex may be a tectonic sheet, making the uppermost tectonic unit of the thrusting system and truncated by the Daqingshan detachment fault on top. The existing tectono-chronological studies showed that the thrusting of this region possibly happened at the same time as the extension along the top detachment fault. This presents that the contraction and extension in Daqingshan orogen were probably simultaneous. The structural pattern of the Daqingshan orogen can be correlated to the Himalayan orogen, and models for simultaneous thrusting and detachment in Himalaya may give a hint for explaining the development of the Daqingshan orogen.     

Mesozoic thrusts and extensional structures in the Daqingshan orogen, Inner Mongolia, and their temporal and spatial relationship

Three large-scale thrusts developed in the Daqingshan orogen, Inner Mongolia, with a northeast-east trending from Louhuashan to Suletu are described in this paper. These northwest-directed thrusts, together with the four tectonic belts (or tectonic sheets) separated by these faults, formed a typical thrusting system. The originally defined Hohhot metamorphic core complex may be a tectonic sheet, making the uppermost tectonic unit of the thrusting system and truncated by the Daqingshan detachment fault on top. The existing tectono-chronological studies showed that the thrusting of this region possibly happened at the same time as the extension along the top detachment fault. This presents that the contraction and extension in Daqingshan orogen were probably simultaneous. The structural pattern of the Daqingshan orogen can be correlated to the Himalayan orogen, and models for simultaneous thrusting and detachment in Himalaya may give a hint for explaining the development of the Daqingshan orogen.     

U-Pb ages of Kude and Sajia leucogranites in Sajia dome from North Himalaya and their geological implications

~~U-Pb ages of Kude and Sajia leucogranites in Sajia dome from North Himalaya and their geological implications~~ FallMeetingSuppl.Abstr.,2001,0830h. 10.Lee,J.,Hacker,B.R.,Dinklage,W.S. et al.,Evolution of theKangmarDome, southernTibet:Structural, petrologic, and ther- mochronologic constraints,Tectonics,2000,19:872—895. 11.Parrish,R.R.,Roddick,J.C.,Loveridge,W.D. et al.,Ura- nium-lead analytical techniques at the geochronology laboratory,GeologicalSurvey ofCanada,Geol.Sur…     

Microstructural and deformational studies on mylonite in the detachment faults of Yalashangbo dome, North Himalayan domes zone

The Yalashangbo dome, located at the eastern end of North Himalayan domes zone, has a geometry and structure similar to those of a metamorphic core complex. Ductile shear zones formed the detachment system around the dome and these zones are composed of garnet-bearing phyllonite, mylonitic schist, mylonitic gneiss and mylonitic granite. Ductile shear fabrics developed well in mylonitic rocks, and penetrative lineation and foliation were formed by stretched quartz and feldspar and preferred orientation of mica. Polar Mohr diagram method is used to calculate the kinematic vorticity numbers of the shear zones in the detachment system. Results indicate that the shear zone is a thinned shear zone (thinning of 23%) in an extensional setting which underwent a general shear dominated by simple shear. Comparison of the vorticity numbers between the northern and southern flanks of the Yalashangbo dome shows that the dome is an asymmetric system formed by a north-northwest-directed detaching unanimously. Statistical fractal analysis shows that the shapes of dynamically recrystallized quartz grains in the mylonites have characteristics of self-similarity, with fractal numbers ranging from 1.05 to 1.18. From these fractal numbers, the strain rate of the rock was deduced from 10^-9.2 S^-1 to 10^-7.3 S^-1, the differential paleao-stress was 13.7-25.6 MPa during the deformation happened at a temperature over 500 ℃. The ductile shear zones in the detachment system around Yalashangbo dome were formed under a high green-schist grade condition or happened simultaneously with the intrusion of granite.     

Eocene high grade metamorphism and crustal anatexis in the North Himalaya Gneiss Domes,Southern Tibet

Determination of the timing and geochemical nature of early metamorphic and anatectic events in the Himalayan orogen may provide key insights into the physical and chemical behavior of lower crustal materials during the early stage of tectonic evolution in large-scale collisional belts.The Yardoi gneiss dome is the easternmost dome of the North Himalayan Gneiss Domes(NHGD),and contains three types of amphibolites with distinct mineral assemblage,elemental and radiogenic isotope geochemistry,as well as various types of gneisses.SHRIMP zircon U/Pb analyses on the garnet amphibolite and garnet-bearing biotite granitic gneiss yield ages of nearly peak metamorphism at 45.0±1.0 Ma and 47.6±1.8 Ma,respectively,which are 2 to 4 Ma older than the age for partial melting in migmatitic garnet amphibolite(43.5±1.3 Ma).Available data have demonstrated that ultra-high pressure metamorphism in the Tethyan Himalaya occurred at ～55 Ma,and high amphibolite facies to granulite facies metamorphism at 45 to 47 Ma.In addition,partial melting at thickened crustal conditions occurred at 43.5±1.3 Ma,which led to the formation of high Sr/Y ratios two-mica granites.The high-grade metamorphic rocks in the NHGD may represent the subducted front of the Indian continental lithosphere.In large collisional belts,fertile components in crustal materials could melt and form granitic melts with relatively high Na/K and Sr/Y ratios under thickened crustal conditions,significantly different from those formed by decompressional melting during rapid exhumation.     

黔东凯里—三都断裂结构及形成演化

通过宏观描述、地震解译、有限应变测量、显微构造分析及包裹体测温等方法对黔东凯里-三都断裂进行了研究。认为凯里-三都断裂是黔东南推覆体的前锋断裂,呈向NWW突出的弧形,总体表现为由数条断层组成的逆冲断裂带,主断裂倾向SEE,为黔南拗陷和雪峰古陆的分界线。断裂带以浅层脆性变形为主,断裂带上盘向NWW推覆的水平位移达12～15km,是多条断层多期挤压、拉张和平移剪切作用叠加的结果。断裂带经历了多期演化过程,燕山期以来的演化历史主要受太平洋构造域控制,早中燕山期为主挤压逆冲阶段、晚燕山期为拉张正滑阶段、喜马拉雅期为再次挤压-定型阶段。     

泌阳凹陷构造活动的沉积响应

总结了泌阳断陷湖盆的构造演化 ,以及盆地充填、沉积层序及沉积、沉降中心对构造活动的响应 ,并利用凹陷中沉积、沉降中心的迁移变化特征来确定边界断裂的活动时间 ,恢复湖盆的演化历史 .研究结果表明 :燕山运动的拉分作用、燕山运动之后的盆地边界断裂活动及喜山运动后的拗陷沉积作用形成了本区 3个盆地充填序列及构造层序 ;除了盆地形成、沉积充填序列受构造活动控制外 ,沉积层序的发育也受控于断层幕式活动引起的可容空间及沉积物供给变化的控制 .另外 ,对核二段沉积时期沉积、沉降中心研究可知 ,该时期凹陷内构造格局一直发生变化 ,至核二段一砂组沉积时期 ,两边界断层的活动都大大减弱或基本停止 ,凹陷内沉积格局稳定 ,沉降、沉积中心基本统一 ,断陷盆地真正进入萎缩阶段     

歧口凹陷隐性断裂带特征及其对油气聚集的控制作用

隐性断裂带(断裂趋势带)是指盆地沉积盖层内发育的弱变形(隐蔽性较强)构造带,属于断裂带形成演化早、中期阶段的产物,由于缺乏明显的断裂面(带)和显著的位移而难以识别。在对苏北盆地、渤海湾盆地隐性断裂带研究基础上,于歧口凹陷识别出基底断裂走滑型隐性断裂带,并分析其特征、形成机制、控藏作用等。该断裂趋势带其自南向北可以分3个区段:南段基底断裂与盖层断裂体系直接连接构成花状构造;中段古近系的花状构造终止于沙二段,新近系仅发生挠曲变形,成为东西侧不同构造单元、南北部不同构造特征的平衡点;北段新近系发育的花状构造深部收敛于古近系的重力滑脱断层,基底断裂通过影响滑脱断层控制盖层断裂体系。研究结果表明,古近系基底断裂扭动作用主要以直接传递、调节传递、间接传递等3种方式控制影响盖层中隐性断裂带的形成。深部隐伏断裂切入基底,有利于热作用传递而促进烃源岩成熟;盖层中的隐性断裂带控制了沉积体系和隐蔽性圈闭的形成和分布;隐性断裂带的观点丰富了构造地质学理论,为油气勘探提供了新认识。     

Genesis of granulite in Himalayan lower crust: Evidence from experimental study at high temperature and high pressure

Here we present an insight into the genesis of Himalayan granulitic lower crust based on the experimental studies on the dehydration melting of natural biotite-plagioclase gneiss performed at the temperatures of 770-980℃ and the pressures of 1.0-1.4 GPa. The experiments produce peraluminous granitic melt and residual phase assemblage (Pl+Qz+Gat+Bio+Opx±Cpx+Ilm/Rut±Kfs). The residual mineral assemblage is similar to those of granu-lites observed at the eastern and western Himalayan syntax-ises, and the chemical compositions of characteristic minerals-garnet and pyroxene in the residual phase and the granu-lite are identical. Additionally, the modeled wave velocities of the residual phase assemblage are comparable well with those of the top part of lower crust beneath Himalayas. Hence, we suggest that (1) the top part of lower crust beneath Himalayas is probably made up of garnet-bearing intermediate granulite; (2) the formations of granulite and leucogranites in Himalayas are interrelated as the results of crustal anatexis; and (3) dehydration melting of bio-tite-plagioclase gneiss is an important process to form granulitic lower crust, to reconstitute and adjust the crustal texture. Moreover, experimental results can provide constraints on determining the P-T conditions of Himalayan crustal anatexis.     

水城断陷的地质特征及构造演化

水城断陷属滇黔桂裂谷的次一级的构造单元，在大地构造演化，岩相古地理，对菱铁矿及铅锌矿等矿产的成矿控制方面均具十分重要的意义，水城断陷在伸展期的显著特点是不对乐变形，垭都－蟒洞断裂始于都匀运动，其伸展模式是以该断层为主拆离断层的单剪模式，水城断陷的演化可分为早期断裂，中期强烈沉降，后期基性岩喷溢，晚期封闭消亡和燕山期构造反转五个阶段。     

西藏札达盆地构造与沉积特征

札达盆地位于青藏高原南西部,是发育于喜马拉雅山内部的盆地。盆地北东侧以阿依拉日居断裂为边界,该断裂属逆冲性质。西北边界为曲松断裂,为一拆离断层。盆地西南主边界为藏南拆离系（STDS）,是盆地主控构造。盆地内部沉积了巨厚的第三纪地层,可以分为上下2个部分,即上部的湖相沉积和下部的河湖交互相沉积。湖相沉积是以泥岩为主的巨厚沉积;河湖交互相沉积以砾岩层的出现为标志,可分为3个沉积旋回。札达盆地的主控构造为伸展构造,但同时经受了垂直于造山带的挤压作用。     

伊春花岗质片麻岩锆石 U-Pb 年代学特征

通过1∶5万朱拉比拉河等图幅区域地质调查工作,在伊春地区发现了新元古代花岗质片麻岩。应用锆石 U-Pb 测龄方法对花岗质片麻岩进行了年代学研究,23个点的206 Pb／238 U表面谐和年龄均在850 Ma 左右,加权平均为850．2±2．1 Ma。该年龄代表花岗质片麻岩体的侵位时间为新元古代;而较新的499 Ma 年龄可能是后期构造事件改造的新生锆石,反映后期构造热事件的年龄;1518 Ma 的年龄应是本次岩浆作用过程中捕获的早期锆石的年龄,该锆石年龄可能代表本区花岗岩的继承性锆石年龄。伊春花岗质片麻岩所在的地区存在前寒武纪的古老微陆块。     

内蒙古中部乌拉山-大青山地区富铝片麻岩年代学特征

内蒙古中部乌拉山岩群是一套变质程度为高角闪岩相-麻粒岩相的早前寒武纪高级变质岩系,出露于内蒙古中部。为划分乌拉山-大青山地区富铝片麻岩的原岩形成时代,通过对侵入乌拉山岩群黑云石榴斜长片麻岩的石英闪长质片麻岩和紫苏花岗质片麻岩变质深成体进行年代学研究,本次研究涉及的实验样品共6件,分别于内蒙古中部乌拉山-大青山地区片麻岩中和变质深成岩中各取3件。分别为矽线石榴黑云斜长片麻岩(TWB19)、黑云斜长片麻岩(TWB20)、含橄榄石方解石大理岩(TWB18)和紫苏花岗质片麻岩(TW7)、石英闪长质片麻岩(TWB6)和花岗闪长质片麻岩(D1227)。结果表明：紫苏花岗质片麻岩(TW7)的加权平均年龄为2462±13Ma,花岗闪长质片麻岩(D1227)加权平均年龄为2512±24Ma,由于后期构造事件对岩石的改造,锆石的实际年龄应更大一些。可见,乌拉山富铝片麻岩原岩形成年代早于2.5Ga。石英闪长质片麻岩(TWB6)的变质年龄为2.2Ga和1.9Ga,指示乌拉山-大青山地区遭受古元古代早期和晚期构造热事件叠加改造。综上所述,乌拉山-大青山地区富铝片麻岩最晚形成时代限定在2.5Ga。     

Numerical simulation of GPS observed clockwise rotation around the eastern Himalayan syntax in the Tibetan Plateau

From Global Position System (GPS) measurements, there is a clockwise rotation around the eastern Himalayan syntax in the Tibetan Plateau. This phenomenon is difficult to be interpreted by simple two-dimensional modeling from a geodynamic point of view. Because of the extremely thick crust and the lower crust with relatively high temperature in the Tibetan Plateau, the lithospheric rheology in Tibet and surrounding areas present a complex structure. In general, the tectonic structure of the Tibetan Plateau consists of brittle upper crust, ductile lower crust, high viscosity lithospheric upper mantle, and low viscosity asthenosphere, the same as the case in many other continental regions. However, the lower crust in the Tibetan Plateau is much more ductile with a lower viscosity than those of its surroundings at the same depth, and the effective viscosity is low along the collision fault zone. In this study, we construct a three-dimensional Maxwell visco-elastic model in spherical coordinate system, and simulate the deformation process of the Tibetan Plateau driven by a continuous push from the Indian plate. The results show that the existence of the soft lower crust under the plateau makes the entire plateau uplift as a whole, and the Himalayas and the eastern Himalayan syntax uplift faster. Since the lower crust of surrounding blocks is harder except in the southeastern corner where the high-temperature material is much softer and forms an exit channel for material transfer, after the whole plateau reaches a certain height, the lower crustal and upper mantle material begins to move eastward or southeastward and drag the upper crust to behave same way. Thus, from the macroscopic point of view, a relative rigid motion of the plateau with a clockwise rotation around the eastern Himalayan syntax is developed. Supported by Knowledge Innovation Project of the Chinese Academy of Sciences (Grant No. KZCX2-YW-123) and National Natural Science Foundation of China (Grant Nos. 40774048 and 90814014)     

三塘湖盆地北西向构造变形带的正演

三塘湖盆地北西向构造变形带的形成与演化控制了盆地总的构造格局．平衡剖面的正演表明：控制盆地构造变形的滑脱面位于石炭系内部的泥岩层中,滑脱面的深度约４．４６ｋｍ;盆地北西向构造变形带是海西晚期和喜山期两期构造作用叠加的产物,变形带的扩展顺序呈前展式自南向北扩展;变形机制主要为受重力扩展作用控制的断弯褶皱作用;变形样式主要为断裂及相关褶皱．盆地总的缩短量最大约８．３ｋｍ,海西晚期最大缩短量约４．５ｋｍ,喜山期最大缩短量约５ｋｍ．