Tectonic evolution of structures in Southern Sindh Monocline,Indus Basin,Pakistan formed in multi-extensional tectonic episodes of Indian Plate

There are a number of structures and structural styles found in extensional tectonic settings of the world,and it is a big challenge to study the evolution of these structures. Evolution of structures formed in extensional tectonic settings have been studied by researchers on different extensional basins of the world. Southern Sindh Monocline lies on the western corner of Indian Plate and the tectonic history of Indian plate has also experienced different extensional episodes, and its journey rifted from Gondwanaland to its final welding to Asia. The aim of this study is to figure out the evolution of structures in the subsurface of Southern Sindh Monocline, Pakistan using the seismic data interpretation and flattening of horizons approach. Structures within the subsurface of Southern Sindh Monocline have been characterized by different tectonic episodes of Indian plate while rifting from Gondwanaland, rifting from other plates at different geological times and to its collision with the Asia. Basic structures within study area are classified into nine types while the structural styles have been classified into six types as horst and grabens, dominos, crotch, synthetic and antithetic, negative and flashlight structural style. The structures within the study area revealed evidence for three major structural episodes which can be characterized as Episode 1: Structures associated with rifting of Indian plate from Gondwanaland during Late Jurassic to Early Cretaceous, Episode 2: Modification and reactivation of previous structures while Madagascar rifted from Indian Plate during the Middle Cretaceous and during Episode 3: Inversion and reactivation of structures occurred when Indian Plate collided with Asia during Early Eocene.

Magnetotelluric Constraints on the Occurrence of Lower Crustal Earthquakes in the Intra-plate Setting of Central Indian Tectonic Zone

Lower crustal earthquake occurrence in the Central Indian Tectonic Zone （CITZ） of the Indian sub-continent was investigated using magnetotelluric （MT） data. MT models across the CITZ, including the new resistivity model across the 1938 Satpura lower crustal earthquake epicenter, show low resistive （〈80 Ωm） mid-lower crust and infer small volume （〈1 vol%） of aqueous fluids existing in most part of lower crust. This in conjunction with xenoliths and other geophysical data supports a predominant brittle/semi-brittle lower crustal theology. However, the local deep crustal zones with higher fluid content of 2.2%-6.5% which have been mapped imply high pore pressure conditions. The observation above and the significant strain rate in the region provide favorable conditions （strong/ moderate rock strength, moderate temperature, high pore pressure and high strain rate） for brittle failure in the lower crust. It can be inferred that the fluid-rich pockets in the mid-lower crust might have catalyzed earthquake generation by acting as the source of local stress （fluid pressure）, which together with the regional stress produced critical seismogenic stress conditions. Alternatively, fluids reduce the shear strength of the rocks to favor tectonic stress concentration that can be transferred to seismogenic faults to trigger earthquakes.

Plate convergence in the Indo-Pacific region

The Indo-Pacific convergence region is the best target to solve the teo remaining challenges of the plate tectonics theory,i.e.,subduction initiation and the driving force of plate tectonics.Recent studies proposed that the Izu-Bonin subduction initiation belongs to spontaneous initiation,which implies that it started from extension,followed by low angle subduction.Numerical geodynamic modeling suggests that the initiation of plate subduction likely occurred along a transform fault,which put the young spreading ridge in direct contact with old oceanic crust.This,however,does not explain the simultaneous subduction initiation in the west Pacific region in the Cenozoic.Namely,the subduction initiations in the Izu-BoninMariana,the Aleutian,and the Tonga-Kermadec trenches are associated with oceanic crusts of different ages,yet they occurred at roughly the same time,suggesting that they were all triggered by a maj or change in the Pacific plate.Moreover,low angle subduction induces compression rather than extension,which requires external compression forces.Given that the famous Hawaiian-Emperor bending occurred roughly at the same time with the onset of westward subductions in the west Pacific,we propose that these Cenozoic subductions were initiated by the steering of the Pacific plate,which are classified as induced initiation.Induced subduction initiation usually occurs in young ocean basins,forming single-track subduction.The closure s of Neo-Tethys Oceans were likely triggered by plume s in the south,forming northward subductions.Interestingly,the Indian plate kept on moving northward more than 50 Ma after the collision between the Indian and Eurasian continents and the break-off of the subducted oceanic slab attached to it.This strongly suggests that slab pull is not the main driving force of plate tectonics,whereas slab sliding is.

Voyage of the Indian subcontinent since Pangea breakup and driving force of supercontinent cycles: Insights on dynamics from numerical modeling

Recent advances in three-dimensional numerical simulations of mantle convection have aided in approximately reproducing continental movement since the Pangea breakup at 200 Ma. These have also led to a better understanding of the thermal and mechanical coupling between mantle convection and surface plate motion and predictions of the configuration of the next supercontinent. The simulations of mantle convection from 200 Ma to the present reveals that the development of large-scale cold mantle downwellings in the North Tethys Ocean at the earlier stage of the Pangea breakup triggered the northward movement of the Indian subcontinent. The model of high temperature anomaly region beneath Pangea resulting from the thermal insulation effect support the breakup of Pangea in the real Earth time scale, as also suggested in previous geological and geodynamic models. However, considering the low radioactive heat generation rate of the depleted upper mantle, the high temperature anomaly region might have been generated by upwelling plumes with contribution of deep subducted TTG(tonalite-trondhjemite-granite) materials enriched in radiogenic elements. Integrating the numerical results of mantle convection from 200 Ma to the present, and from the present to the future, it is considered that the mantle drag force acting on the base of continents may be comparable to the slab pull force, which implies that convection in the shallower part of the mantle is strongly coupled with surface plate motion.

Upper mantle anisotropy inferred from shear wave splitting beneath the Eastern Indian Shield region

We estimate the shear wave splitting parameters vis-a-vis the thicknesses of the continental lithosphere beneath the two permanent seismic broadband stations located at Dhanbad(DHN) and Bokaro(BOKR) in the Eastern Indian Shield region. Broadband seismic data of 146 and 131 teleseismic earthquake events recorded at DHN and BOKR stations during 2007-2014 were analyzed for the present measurements.The study is carried out using rotation-correlation and transverse component minimization methods. We retain our "Good", "Fair" and "Null" measurements, and estimate the splitting parameters using 13 "Good" results for DHN and 10 "Good" results for BOKR stations. The average splitting parameters(φ,δt)for DHN and BOKR stations are found to be 50.76°±5.46° and 0.82±0.2 s and 56.30°±5.07° and0.95 ± 0.17 s, and the estimated average thicknesses of the anisotropic layers beneath these two stations are ～94 and ～109 km,respectively. The measured deviation of azimuth of the fast axis direction(0)from the absolute motion of the Indian plate ranges from ～8° to 14°. The measured deviation of azimuth of the fast axis direction(0) from the absolute motion of the Indian plate ranges from ～8° to 14°. The eastward deviation of the fast axis azimuths from absolute plate motion direction is interpreted to be caused by induced outflow from the asthenosphere. Further, the delay time found in the present analysis is close to the global average for continental shield areas, and also coherent with other studies for Indian shield regions. The five "Null" results and the lower delay time of ～0.5-0.6 s might be indicating multilayer anisotropy existing in the mantle lithosphere beneath the study area.

Tectonics of the Indo-Australian Plate Near the Ninetyeast Ridge Constrained from Marine Gravity and Magnetic Data

Although the Indo-Australian plate near the Ninetyeast Ridge is important for understanding the formation of new plate boundaries, its tectonic problems are complex and most of them are poorly known. This paper made a detailed tectonic analysis based on the data of bathymetry, gravity and magnetics. Bathymetry and gravity maps show morphological features of many folds, which are related to the intraplate deformation of the Indo-Australian plate due to the collision between the Indian and Asian plates. Gravity anomalies show the structure of fracture zones, which are caused by the seafloor spreading and transform faulting. The characteristics of the folds and fracture zones are consistent with the hypothesis that diffuse plate boundaries and redefined plate components would occur within the Indo-Australian plate. In addition, compiled magnetic data demonstrate magnetic lineations, abandoned spreading centers, southward ridge jumps and plate motions. These features provide useful information for rebuilding the tectonic evolution history of the study area. Magnetic anomalies suggest that an additional plate boundary of transform fault type is developing.

印度-欧亚板块主碰撞带全地壳尺度相互作用关系研究

印度板块自新生代早期开始沿雅鲁藏布江缝合带向北与欧亚板块碰撞以来,印度板块的深俯冲过程一直是大家关注的焦点,而垂向上主碰撞带全地壳尺度由深及浅的相互作用关系研究程度相对较弱,主要归咎于之前缺少全地壳尺度高分辨率数据资料,从而也阻碍了对主碰撞带巨厚地壳成因机制及深部地球动力学过程的了解。在本次研究中,分别基于横穿主碰撞带中部和东部的180 km及100 km长的深反射地震剖面进行精细构造地球物理学分析,揭示了主碰撞带全地壳尺度由深及浅的相互作用关系:(1)横向上,印度板块下地壳存在北向俯冲,且俯冲前缘有限的存在于南拉萨地体南缘。上覆的南拉萨地体则出现透明反射结构和中拉萨地体统一北倾的反射结构。(2)垂向上,印度地壳主要表现下地壳俯冲、中上地壳双冲构造回返结构。南拉萨地体四分之三的地壳表现为透明反射。占据另外四分之一的上地壳顶部表现为统一的南倾结构形态;中拉萨地体则以下地壳北倾、上地壳上拱反射结构为主。三者皆在垂向上出现差异性变形。(3)主碰撞带上地壳顶部表现为统一的后展式顶板逆冲推覆构造,该逆冲推覆系统可一直从南拉萨地体北边界的洛巴堆—米拉山断裂向南越过南倾的雅江缝合带延伸至北喜马拉雅穹窿背斜北翼。结合大地电磁数据所揭示的南拉萨地体高熔体百分比区域沿俯冲印度下地壳顶边界发生的南移现象,研究结果揭示南拉萨地体巨厚地壳主要由新特提斯构造域幕式岩浆作用所形成的新生地壳物质易挤压变形引起。同时,南拉萨地体幕式岩浆作用在印度与欧亚板块的相互碰撞作用过程中发生了热量的向南运移。该过程引发北喜马拉雅构造带深熔作用的同时减弱了北喜马拉雅构造带地壳机械强度,从而使中上地壳物质的双冲构造回返主要表现为短波长背型堆垛结构,并垂向增厚了俯冲印度地壳厚度。同时,背型堆垛构造形变过程所导致的北喜马拉雅穹窿带的加速出露给主碰撞带区域上地壳顶部带来北向的构造挤压推覆作用,最终使主碰撞带区域在北喜马拉雅穹窿区域以北展现为统一的后展式顶板逆冲推覆结构构造。印度-欧亚板块主碰撞带的圈层相互关系是造成该区域巨厚地壳的关键,其上地壳顶板逆冲推覆过程亦降低了主碰撞区域的地形起伏。

印度尼西亚北苏拉威西岛弧岩浆作用及其对板块俯冲起始的制约

板块俯冲起始是板块构造理论的核心内容,但却研究最为薄弱。印度尼西亚北苏拉威西岛弧是始新世期间开始发育的一个印度洋洋内弧,其记录了多期次(包括印度洋、马鲁古海和西里伯斯海)的且处于不同演化阶段的俯冲作用及相应的俯冲起始过程,因此该岛弧是研究板块俯冲起始的天然实验室。本文通过回顾板块俯冲起始研究进展,结合北苏拉威西岛弧内已有的野外调查、锆石U-Pb定年、岩石地球化学特征等,以厘清区域内岛弧岩浆的岩石类型、形成时代、空间分布和岩浆活动节律,制约岛弧岩浆的构造背景和岩浆源区性质等,并着重关注该岛弧内可能与板块俯冲起始相关的地质和岩石记录,如弧前玄武岩、玻安岩、俯冲带(SSZ)型蛇绿岩、变质底板等。在此基础上,综合限定北苏拉威西岛弧记录的印度洋洋壳俯冲历史,探讨其俯冲起始的时间与地球动力学机制。本文仅抛砖引玉,期待更多学者参与到北苏拉威西岛弧板块俯冲起始的研究中来。

青藏高原东北部天然地震探测与岩石圈深部特征

为了研究青藏高原东北部块体构造变形的深部驱动机制 ,笔者对青藏高原东北部的天然地震观测数据进行地震层析反演 ,并结合同一剖面的接收函数及各向异性结果进行讨论。介绍了ACH方法的基本原理 ,论述了由于印度板块向北俯冲的强大的持续作用力 ,造就了NE向俯冲到柴达木盆地之下的昆仑造山带 ,并发现在巴颜喀拉地体下方壳幔内的仰冲活动。壳幔内的低速体十分显著地出现在阿尼玛卿缝合线以北 ,深度可达 30 0km。推断该低速体可能与昆仑断裂在深层的剪切作用有关。深部资料显示该区莫霍界面由北向南逐渐加深 ,这与青藏高原东北部的岩石圈减薄现象一致 ,而且与印度板块向北运动的远程效应有关。另外 ,地震层析结果及各向异性分析也支持青藏高原东北部主应力方向转为NE向的观点。

印度板块-古亚洲板块碰撞及亚洲恐龙的出现

印度板块在5500万年前的古近纪撞击了亚洲板块,造就了东西走向的雅鲁藏布江水系和喜马拉雅山系。根据近年来的研究,笔者又提出一个新观点:印度板块在约2亿年前与亚洲板块发生碰撞时,将一个发源于非洲、南美洲的“冈瓦纳恐龙动物群”送上亚洲大陆,在中国西南地区形成以禄丰龙、蜀龙、峨眉龙、马门溪龙为特征的恐龙类动物群。这一事件不仅使亚洲首次出现了恐龙动物,而且板块相撞还造就了南亚地区南北走向的横断山脉和伊洛瓦底江、怒江、澜沧江、金沙江等纵向水系,以及四川盆地的形成。

青藏高原的金属矿产

青藏高原地处印度板块与亚欧板块的衔接部位,长期处于岗瓦纳大陆边缘,大约在65 Ma时印度板块与亚欧板块发生碰撞,对过去构造格局和金属矿产等改造,同时产生新的构造格局和金属矿产,因此青藏高原的金属矿产别具一格。本文讨论青藏高原的铬、铁、铜钼、铅锌、锡、金、锑、铯等优势矿产。

青藏高原岩石圈三维结构及高原隆升的液压机模型

青藏地区可以昆仑断裂和雅鲁藏布缝合线为界分为3个岩石圈地球物理特征各不相同的区域:青海高原、藏北高原和藏南高原。青海高原位于昆仑山脉以北,是重力高和重力低毗连出现的盆山结构。藏南高原位于雅鲁藏布江以南,是印度板块分布的地区,其上是印度板块的陆缘沉积。它的地壳结构是一个向南运动的逆冲推覆系统。INDEPTH反射剖面在藏南发现的主喜马拉雅逆冲断层(MHT)与宽角反射地震扇形剖面得到的T4震相反射面完全吻合。两种地震测深方法得到的结果之间不存在矛盾。T4震相在高喜马拉雅地区没有显示,MHT向南延伸到高喜马拉雅只是一个推论,因而MHT是否为印度板块的俯冲带仍有待于获取新的证据。在昆仑山脉以南到雅鲁藏布缝合带为藏北高原,是广泛发生局部熔融的强流变岩石圈。局部熔融地区呈漏斗状。在藏北广泛存在的深度为15～20km的上部地壳内的低速层是一个最富于流变性能的局部熔融层,它的埋藏深度平坦稳定,可能含大量水质流体。紧挨着上述上部壳内局部熔融层,在藏北岩石圈大范围出现分布不均匀的网状局部熔融。局部熔融体的底部从雅鲁藏布江地区的80km向北逐步加深到200km。漏斗的漏管处位于羌塘—可可西里。藏北局部熔融体的形成是由于印度板块向北运移,受到亚洲板块的阻挡,沿雅鲁藏布缝合带向青藏高原高角度俯冲,在弧后羌塘—可可西里地区产生高热流上升地幔所致。根据卫星重力异常、航空磁测、地震接收函数研究、地球化学资料以及地表地质均揭示,印度板块沿雅鲁藏布缝合带的俯冲仅发生在亚东—唐古拉一线以西的西藏西部。在亚东—唐古拉一线以东,印度板块与西藏块体间仅仅发生碰撞,但没有发生俯冲。高原的整体隆升是由液压效应所造成。青藏高原的隆升像一台液压机。印度板块对青藏俯冲过程中产生的各种应力,通过局部熔融体,传递到地壳深15～20km处的熔融层,在其下形成一个等压面。在这个等压面的驱使下,在低速层以上未被局部熔融的地壳的底部均匀受力,将它们同步向上抬升。高原隆升期后的跨塌,使上部地壳向四周流动。在青海高原,造成毗连阿尔金断裂的一系列由西南向北东方向推动的叠瓦构造。在雅鲁藏布江以南地区,形成一系列向南凸出的弧形逆冲断层。在昆仑山脉与雅鲁藏布缝合带之间,向东的流动便形成上部地壳的滑脱构造。虽然青藏高原的形成是由于印度板块的俯冲,但它的隆升机制不单纯是一个刚体力学问题,更重要的要考虑到流体的作用,简单的用以刚体假设为前提的板块学说去解释高原的隆升机制是青藏高原研究中的误区。西藏高原的深部是一个大热库,西藏热储的开发利用是一个重大的研究课题。

中国周边板块的相互作用及其对中国应力场的影响——(Ⅱ)印度板块的影响

本文利用地震资料并结合地质资料,讨论了印度板块与欧亚板块在中国周边的相互作用及其对中国应力场的影响,指出两板块在喜马拉雅山前断裂地区碰撞,碰撞边界向西延续到35°N,74°E附近,其主要挤压方向为NNE,并形成SE方向的物质流动.帕米尔地区有强烈的构造运动,并存在俯冲带形态的构造.在26.5°N,97°E附近,板块边界的走向发生突变,并形成东倾的缅甸山弧俯冲带,但印度板块挤压造成的主压应力方向为NNE向.在安达曼—尼科巴—苏门答腊—爪哇岛弧,印度板块俯冲于欧亚板块之下,在中国南海一带形成NNW向或近Ns向的主压应力.

利用接收函数反演青藏高原西部地壳S波速度结构

相对于宽阔的腹地,青藏高原西部南北向宽度仅约600km,却记录了印度和欧亚板块汇聚的深部过程及其响应.本文用22台宽频带流动地震台站在西缘构建了一条南北向探测剖面（～80°E,TW-80试验）.利用接收函数反演剖面下方S波速度结构,综合西部已有的宽频带探测结果,分析认为：印度板块向北俯冲可能已到达班公湖—怒江缝合带附近,俯冲过程中下地壳发生榴辉岩化;喀拉昆仑断裂带、班公湖—怒江缝合带、阿尔金断裂带均为切穿地壳的深断裂,莫霍面发生错断;喀拉昆仑断裂带和龙木错断裂带之间的中上地壳没有发现连续的S波低速体,说明可能缺乏解耦层,支持青藏高原西部地壳为整体缩短增厚模式.

滇西哀牢山构造带:结构与演化

哀牢山构造带是藏东(东南亚)地区的一条重要线性构造,它分隔了扬子—华南地块与印支地块,并保存了多阶段复杂大地构造演化的记录。哀牢山构造带内由东向西依次发育了晚太古代—新元古代深变质岩系、新生代构造-岩浆活动带(剪切带)、金平—沱江晚二叠—早三叠世裂谷带残余和哀牢山早石炭世—早三叠世混杂岩带。具有不同特点的地质单元间被以新生代为主发育的断裂构造所间隔;而不同时期异地就位或混合岩化成因的花岗质岩石在构造带中普遍存在。哀牢山构造带在不同地质历史阶段具有多重大地构造属性,总体上经历了3个重要大地构造演化阶段:前特提斯演化、特提斯演化和新生代陆内演化阶段。前特提斯演化时期,主体部分(尤其是其东部带)具有亲扬子地块的属性,保留了自晚太古代到新元古代地壳演化的记录。一直到早古生代时期,哀牢山构造带的大地构造属性与扬子—华南地区依然具有密切的亲缘关系。自晚古生代—早中生代时期古特提斯洋打开之后,该带与华南-扬子板块之间分化成2个属性不同的构造域,始于早石炭世打开的哀牢山洋与始于早二叠世打开的金平—沱江洋依次消亡。特提斯洋的闭合,一方面形成了古哀牢山造山带,同时使得扬子—华南地块与印支地块回复到一个统一的陆内环境中;印度—欧亚板块之间的交互作用,对这一地区有着深刻的影响,相继形成了早新生代哀牢山造山带、晚渐新世—早中新世造山后区域性伸展与高钾碱性岩浆活动性和晚渐新世—早中新世印支地块的大规模南东向逃逸、哀牢山大型左行走滑剪切作用及伴生的钙碱性岩浆活动性。

西藏谢通门斯弄多地区新生代逆冲推覆构造的基本特征

谢通门县斯弄多地区发育大型新生代逆冲推覆构造,逆冲断层面倾向200°左右,断层倾角15～25°。逆冲断层上盘主要由上石炭统昂杰组灰岩、白云质灰岩构成,形成陡峭高耸的推覆前缘断崖;下盘以始新统年波组火山岩为主。断层全长约60km,断层带宽2～3km。逆冲推覆上盘推覆体向北北东方向滑移,根据飞来峰的相对位置估算,推覆体推覆距离约15km。逆冲推覆构造的形成可能与印度板块向北俯冲导致冈底斯地区强烈的陆内挤压作用有关。

印度板块北边界地震活动和中国大陆地震

本文研究了印度板块北边界地震活动与我国大陆地震活动的关系,给出了两者在时间起伏上的一致性,以及印度板块北边界兴都库什和缅甸两端点附近中深地震对中国大陆强震活动所具有的前兆性和相关性。这些特点,对于中国大陆强震活动的预测、尤其是地震大形势预测是十分有意义的。文中还应用了中国大陆强震孕育发生的计算机模型的研究结果,讨论了强震活动的时空分布特征,对于理解中国大陆作为一个孕震系统在周围地质构造块体动力作用下所出现的强震轮回活动及其主体活动地区转移等特点有一定的启迪作用。

印度板块的北缘在哪里?

通过对INDEPTHII在雅鲁藏布江南的2条南北向深地震反射剖面资料的进一步处理,观察到主喜马拉雅逆冲断裂带(MHT)形成的反射向北逐渐倾没于藏南地壳之下。这一反射一直可延伸至康马穹隆北、浪卡子南,在向北延伸的过程中,断裂带向北倾角逐渐加大,可以看到MHT反射最北端的反射同相轴向北倾斜的角度到达27°30'～29°,最深处的双程走时达到22.5s左右。根据深地震反射资料并结合大地电磁(MT)资料,提出印度板块在雅鲁藏布江南30～40km(大约28°50'N)处沿MHT俯冲到了藏南的地壳之下,即在地壳范围内印度板块的最北部边缘位于雅鲁藏布江南30～40km处。

陆-陆碰撞对南亚地区油气的控制作用

南亚地区主体为印度板块,裂谷期(4 570～166 Ma)时为东冈瓦纳大陆的一部分,自晚侏罗世(166 Ma)经过从南向北的长距离漂移,从始新世(49 Ma)开始,该板块向欧亚板块俯冲,早-中中新世(16 Ma)至今喜马拉雅山脉和印度-缅甸山脉快速隆升,形成了5个构造单元。前晚白垩世板块北部被动边缘型沉积被破坏,而东、西两侧和南缘沉积遭受抬升剥蚀程度较小,油气保存条件较好,板块内部在裂谷期仅发生微弱的沉降,晚碰撞期烃源岩未成熟。油气主要富集在沿着印度板块边缘分布的被动大陆边缘、前陆盆地、弧前-弧后盆地和夭折裂谷系中,以中、新生代沉积为主,典型的油气运聚模式依次为复合砂体运聚模式、构造褶皱-冲断运聚模式、断-坳垂向运聚模式和断裂-岩性运聚模式。

印度板块向欧亚板块俯冲碰撞的新模式及其对青藏高原构造演化的影响

青藏高原南部与北部的断裂带走滑方向不同,断裂带在高原东、西两侧的展布规律和构造运动方式也存在极大的差异。通过高原南缘横向地球物理特征的对比,发现该地区东、西两侧深部岩石圈结构存在明显不同,表明印度次大陆向欧亚大陆俯冲碰撞的方式和强度在横向上差异明显。在充分利用高原地质研究最新成果的基础上,提出印度次大陆向欧亚大陆碰撞前地壳增厚及斜向碰撞是造成上述地质现象的根本原因,并由此对青藏高原构造演化的一些特点作了新的解释和探讨。