Active tectonics of the Himalaya

The Himalayan mountains are a product of the collision between India and Eurasia which began in the Eocene. In the early stage of continental collision the development of a suture zone between two colliding plates took place. The continued convergence is accommodated along the suture zone and in the back-arc region. Further convergence results in intracrustal megathrust within the leading edge of the advancing Indian plate. In the Himalaya this stage is characterized by the intense uplift of the High Himalaya, the development of the Tibetan Plateau and the breaking-up of the central and eastern Asian continent. Although numerous models for the evolution of the Himalaya have been proposed, the available geological and geophysical data are consistent with an underthrusting model in which the Indian continental lithosphere underthrusts beneath the Himalaya and southern Tibet. Reflection profiles across the entire Himalaya and Tibet are needed to prove the existence of such underthrusting. Geodetic surveys across the High Himalaya are needed to determine the present state of the MCT as well as the rate of uplift and shortening within the Himalaya. Paleoseismicity studies are necessary to resolve the temporal and spatial patterns of major earthquake faulting along the segmented Himalayan mountains.     

Geologic and tectonic evolution of the Himalaya before and after the India-Asia collision

The geology and tectonics of the Himalaya has been reviewed in the light of new data and recent studies by the author. The data suggest that the Lesser Himalayan Gneissic Basement (LHGB) represents the northern extension of the Bundelkhand craton, Northern Indian shield and the large scale granite magmatism in the LHGB towards the end of the Palæoproterozoic Wangtu Orogeny, stabilized the early crust in this region between 2-1.9 Ga. The region witnessed rapid uplift and development of the Lesser Himalayan rift basin, wherein the cyclic sedimentation continued during the Palæoproterozoic and Mesoproterozoic. The Tethys basin with the Vaikrita rocks at its base is suggested to have developed as a younger rift basin (～ 900 Ma ago) to the north of the Lesser Himalayan basin, floored by the LHGB. The southward shifting of the Lesser Himalayan basin marked by the deposition of Jaunsar-Simla and Blaini-Krol-Tal cycles in a confined basin, the changes in the sedimentation pattern in the Tethys basin during late Precambrian-Cambrian, deformation and the large scale granite activity (～ 500 ± 50 Ma), suggests a strong possibility of late Precambrian-Cambrian Kinnar Kailas Orogeny in the Himalaya. From the records of the oceanic crust of the Neo-Tethys basin, subduction, arc growth and collision, well documented from the Indus-Tsangpo suture zone north of the Tethys basin, it is evident that the Himalayan region has been growing gradually since Proterozoic, with a northward shift of the depocentre induced by N-S directed alternating compression and extension. During the Himalayan collision scenario, the 10–12km thick unconsolidated sedimentary pile of the Tethys basin (TSS), trapped between the subducting continental crust of the Indian plate and the southward thrusting of the oceanic crust of the Neo-Tethys and the arc components of the Indus-Tangpo collision zone, got considerably thickened through large scale folding and intra-formational thrusting, and moved southward as the Kashmir Thrust Sheet along the Panjal Thrust. This brought about early phase (M1) Barrovian type metamorphism of underlying Vaikrita rocks. With the continued northward push of the Indian Plate, the Vaikrita rocks suffered maximum compression, deformation and remobilization, and exhumed rapidly as the Higher Himalayan Crystallines (HHC) during Oligo-Miocene, inducing gravity gliding of its Tethyan sedimentary cover. Further, it is the continental crust of the LHGB that is suggested to have underthrust the Himalaya and southern Tibet, its cover rocks stacked as thrust slices formed the Himalayan mountain and its decollement surface reflected as the Main Himalayan Thrust (MHT), in the INDEPTH profile.     

Early Cretaceous Tectonics and Evolution of the Tibetan Plateau

Selected geological data on Early Cretaceous strata, structures, magmatic plutons and volcanic rocks from the Kunlun to Himalaya Mountains reveal a new view of the Early Cretaceous paleo-tectonics and the related geodynamic movement of the Tibetan Plateau. Two major paleo-oceans, the Mid-Tethys Ocean between the Qiangtang and Lhasa blocks, and the Neo-Tethys Ocean between the Lhasa and Himalayan blocks, existed in the Tibetan region in the Early Cretaceous. The Himalayan Marginal and South Lhasa Seas formed in the southern and northern margins of the Neo-Tethys Ocean, the Central Tibet Sea and the Qiangtang Marginal Sea formed in the southern and northern margins of the Mid-Tethys Ocean, respectively. An arm of the sea extended into the southwestern Tarim basin in the Early Cretaceous. Early Cretaceous intensive thrusting, magmatic emplacement and volcanic eruptions occurred in the central and northern Lhasa Block, while strike-slip formed along the Hoh-Xil and South Kunlun Faults in the northern Tibetan region. Early Cretaceous tectonics together with magmatic K2O geochemistry indicate an Early Cretaceous southward subduction of the Mid-Tethys Oceanic Plate along the Bangoin-Nujiang Suture which was thrust ~87 km southward during the Late Cretaceous-Early Cenozoic. No intensive thrust and magmatic emplacement occurred in the Early Cretaceous in the Himalayan and southern Lhasa Blocks, indicating that the spreading Neo-Tethys Oceanic Plate had not been subducted in the Early Cretaceous. To the north, terrestrial basins of red-beds formed in the Hoh-Xil, Kunlun, Qilian and the northeastern Tarim blocks in Early Cretaceous, and the Qiangtang Marginal Sea disappeared after the Qiangtang Block uplifted in the late Early Cretaceous.     

First-order kinematics of wedge-scale active Himalayan deformation: Insights from Darjiling–Sikkim–Tibet (DaSiT) wedge

The Indian Plate has collided with the Eurasian Plate along an arcuate boundary over the last 55–60 million years defining the Himalayan Mountain belt. The geometry of the collision boundary is wedge-shaped; the base of this wedge is defined by a decollement named the Main Himalayan Thrust (MHT). In the Darjiling–Sikkim–Tibet (DaSiT) Himalayan wedge, a crustal-scale fault-bend fold (Kangmar Anticline) and the Lesser Himalayan Duplex (LHD) are dominant structures that have built taper and controlled the foreland-ward propagation of the thrust sheets. A frontal physiographic half-window has eroded through the Main Central Thrust (MCT) sheet to expose the LHD in the DaSiT wedge. Preliminary data suggest that active tectonics and seismicity in the DaSiT wedge may be concentrated in the half-window; this suggests that LHD may be an active structure. High-precision Global Positioning System measurements in the DaSiT wedge suggest that a minimum of 12 mm/yr convergence is being accommodated in the Darjiling–Sikkim Himalaya out of which ∼4 mm/yr convergence is being taken up in the LHD. Given that decollement earthquakes with minimum internal deformation in a deforming wedge occur when it attains critical taper, continued deformation within the DaSiT wedge and the lack of great decollement earthquakes indicate that the DaSiT Himalayan wedge is presently sub-critical and in the process of building taper. The sub-critical nature of the DaSiT wedge is probably the result of low topographic and decollement slopes, weaker rocks and pronounced erosion in the frontal part of the wedge.     

喜马拉雅琼嘉岗超大型伟晶岩型锂矿的发现及意义

近年来,喜马拉雅新生代淡色花岗岩的"高度分离结晶、异地深成侵入"成因,及其具有良好的稀有金属成矿潜力而倍受关注。已有野外调查和资源勘查工作表明该花岗岩带可能成为我国稀有金属重要的战略储备基地。目前带内金属组合以铍-铌-钽(锡-钨)组合为主(如错那洞大型锡-钨-铍矿床),但尚未发现工业锂矿体的产出。本次工作在高喜马拉雅琼嘉岗地区发现了超大型伟晶岩型锂矿,并初步揭示该伟晶岩型锂矿的基本地质特征。琼嘉岗伟晶岩属于过铝质LCT型伟晶岩,稀有金属(REL)类REL-Li亚类钠长石-锂辉石型。含矿伟晶岩呈串珠状、囊状体产出在前寒武系肉切村群大理岩中,伟晶岩具有一定分带,目前主要包括细粒钠长石带、文象结构带、分层细晶岩带和块体微斜长石+锂辉石带,赋矿主体结构带为后两者。矿石矿物主要为锂辉石、铌铁矿-铌锰矿,以及少量锡石和绿柱石。59件样品中44件Li₂O含量在工业品位(0.80%)之上,平均1.30%。4条伟晶岩脉群资源量估算表明琼嘉岗锂资源可达超大型规模,琼嘉岗是喜马拉雅首例具有工业价值的伟晶岩型锂矿,其发现证实我国高喜马拉雅地区具有找寻大型-超大型花岗伟晶岩型锂(铍)矿的潜...     

Surface displacement estimation along Himalayan frontal fault using differential SAR interferometry

The Himalayan region has been studied extensively during the past few decades in terms of present ongoing deformations. Various models have been proposed for the evolution of the Himalaya to explain the cause of earthquake occurrences and to understand the seismotectonics of the Himalayan collision zone. However, the information on displacements from field geodetic surveys is still too scarce in time and spatial domains so as to provide convincing evidences. Moreover, classical Probabilistic Seismic Hazard Approaches also fail due to paucity of data in higher magnitude range, thus emphasizing the need of spatial level displacement measurements. It is in this context that the present study has been carried out to estimate the surface displacement in a seismically active region of the Himalaya between Ganga and Yamuna Tear using Differential SAR interferometry. Three single-look complex images, obtained from ASAR sensor onboard ENVISAT satellite, have been used. A displacement rate of 8?C10?mm per year in N15°E direction of Indian plate has been obtained in this three-pass SAR interferometry study. It has been noted that the estimated convergence rate using Differential SAR interferometry technique is relatively low in comparison with those obtained from previous classical studies. The reported low convergence rate may be due to occurrence of silent/quite earthquakes, aseismic slip, differential movement of Delhi Hardwar ridge, etc. Therefore, in view of the contemporary seismicity and conspicuous displacements, a study of long-term observations of this surface movement has been recommended in future through a time-series SAR interferometry analysis.     

GEOLOGY OF THE NORTHERN ARUN TECTONIC WINDOW

GEOLOGY OF THE NORTHERN ARUN TECTONIC WINDOW1　BordetP .Recherchesg啨ｏｌｏｇｉｑｕｅｓｄａｎｓｌ’ＨｉｍａｌａｙａｄｕＮ啨ｐａｌ,r啨ｇｉｏｎｄｕＭａｋａｌｕ[R].EditionsduCNRS ,Paris ,196 12 75 . 2　BordetP .G啨ｏｌｏｇｉｅｄｅｌａｄａｌｌｅｄｕＴｉｂｅｔ (Himalayacentral) [J].M啨ｍｏｉｒｅｓｈｏｒｓｓ啨ｒｉｅｄｅｌａＳｏｃｉｅｔ啨ｇ啨ｏｌｏｇｉｑｕｅｄｅＦｒａｎｃｅ,1977,8:2 35～ 2 5 0 . 3　BurcfielBC ,ChenZ ,HodgesKV ,etal.TheSouthTibetanDetachmentSystem ,Hima…     

The devastating Muzaffarabad earthquake of 8 October 2005: New insights into Himalayan seismicity and tectonics

The recent earthquake of 8 October 2005 in the Muzaffarabad region in western Himalaya destroyed several parts of Pakistan and the north Indian state of Jammu and Kashmir. The earthquake of magnitude 7.6 claimed more than 80,000 lives, clearly exposing the poor standards of building construction — a major challenge facing the highly populated, earthquake prone, third world nations today. In this paper, we examine variations in the stress field, seismicity patterns, seismic source character, tectonic setting, plate motion velocities, GPS results, and the geodynamic factors relating to the geometry of the underlying subsurface structure and its role in generation of very large earthquakes. Focal mechanism solutions of the Muzaffarabad earthquake and its aftershocks are found to have steep dip angles comparable to the Indian intra-plate shield earthquakes rather than the typical Himalayan earthquakes that are characterized by shallow angle northward dips. A low p-value of 0.9 is obtained for this earthquake from the decay pattern of 110 aftershocks, which is comparable to that of the 1993 Latur earthquake in the Indian shield — the deadliest Stable Continental Region (SCR) earthquake till date. Inversion of focal mechanisms of the Harvard CMT catalogue indicates distinct stress patterns in the Muzaffarabad region, seemingly governed by an overturned Himalayan thrust belt configuration that envelops this region, adjoined by the Pamir and Hindukush regions. Recent developments in application of seismological tools like the receiver function technique have enabled accurate mapping of the dipping trends of the Moho and Lithosphere–Asthenosphere Boundary (LAB) of Indian lithosphere beneath southern Tibet. These have significantly improved our understanding of the collision process, the mechanism of Himalayan orogeny and uplift of the Tibetan plateau, besides providing vital constraints on the seismic hazard threat posed by the Himalaya. New ideas have also emerged through GPS, macroseismic investigations, paleoseismology and numerical modeling approaches. While many researchers suggest that the Himalayan front is already overdue for several 8.0 magnitude earthquakes, some opine that most of the front may not really be capable of sustaining the stress accumulation required for generation of great earthquakes. We propose that the occurrence of great earthquakes like those of 1897 in Shillong and 1950 in Assam have a strong correlation with their proximity to multiple plate junctions conducive for enormous stress build up, like the eastern Himalayan syntaxis comprising the junction of the India, Eurasia plates, and the Burma, Sunda micro-plates.     

Drainage morphometry of Himalayan Glacio-fluvial basin,India: hydrologic and neotectonic implications

Morphometric analysis, being widely used to assess the drainage characteristics of the river basins, has been found to be a useful tool to delineate the glacial till covered overburden material as well as to identify areas prone to flash floods in present studies. A number of parameters including the stream frequency, drainage density and drainage texture suggest that the unconsolidated, unstratified and highly permeable glacially deposited overburden till material facilitates the infiltration of snowmelt and rainwater in the Pindari glacio-fluvial basin, Eastern Kumaun Himalaya, India. Likewise, other till overburden covered glacial and proglacial areas of Higher Himalayan regions have been contributing to the groundwater budget. The shape parameters further suggest that the sub-basins with higher form factor are more prone to flash floods. Besides this, the anomalies in the morphometric parameters have been found to be a useful tool to delineate zones of active tectonics in such areas.     

Some elements of continental subduction along the Himalayan front

Detachment tectonics, in which the subducting basement is not internally affected by thrusting; and a steady state system, in which the pattern of deformation and topography are invariant, uplift is balanced by erosion and the material can experience subsequent “phases” of deformation by migrating through the system, are proposed to be the fundamental mechanisms of convergence at the Himalayan arc. Both surface and subsurface data are more consistent with these concepts than with more familiar concepts widely used in current models: intracrustal thrusting and evolutionary tectonics, where superimposed phases of deformation are interpreted as distinct phases. The belt of intermediate-magnitude thrust-earthquakes, the topographic front at the High Himalaya and the Main Central Thrust (MCT) are associated with the same fundamental element, the basement thrust front (BTF), which traces a small circle in the central portion of the Himalayan arc. Fault plane solutions indicate thrusting in the radial direction of this arc. This radial convergence at the BTF implies that Tibet is extending laterally at a rate similar to the rate of convergence across the BTF. This extension cannot be unidirectional and must be parallel to the BTF, if the circular shape of the BTF is invariant.     

青藏高原构造动力及变形机制的弹性有限元分析

利用现今青藏高原地质和地球物理研究成果,本文建立了垂直高原总体构造走向的南北向直立剖面的有限元模型,其根据实际资料,划分成分层和有限单元。在此模型基础上进行弹性材料的计算模拟和分析。 印度板块向北运动挤压、高原北部岩石圈阻碍及软流圈拖曳是青藏高原北移变形、隆升和地壳增厚的动力机制;重力及其均衡调整作用是地体间相对运动和地体内差异运动的主要动力,另外青藏高原还受地壳和上地幔结构构造的影响。计算模拟还得到了一些有实际意义的结果,如活动的地质构造和地球物理现象的分带集中、主边界和雅鲁藏布江等地体边界断裂的逆冲性质、各地体南部地表的南倾正断层及喜马拉雅山南坡向南的重力推覆等。     

藏南定结淡色花岗岩--基底隆升降压熔融成因的地质证据

西藏南部定结地区高喜马拉雅结晶基底中淡色花岗岩体紧靠藏南拆离断层内部产出．野外地质和岩相学特征显示其为S型、分两期侵入的淡色花岗岩体——早期的黑云母淡色花岗岩和晚期的白云母淡色花岗岩。基底副变质岩中广泛分布淡色花岗岩脉体．在基底副变质岩中的淡色花岗岩脉体中发现紫苏辉石暗色麻粒岩残留体，这表明本区高喜马拉雅淡色花岗岩源岩为基底副变质岩，且基底副变质岩是在基底快速隆升降压的条件下发生缺水熔融生成的淡色花岗岩岩浆。     

喜马拉雅地体的泛非-早古生代造山事件年龄记录

喜马拉雅地体是55±10Ma以来印度陆块与欧亚大陆碰撞而形成的增生地体,位于其中的高喜马拉雅与特提斯-喜马拉雅构造单元的变质基底主要由角闪岩相的富铝变质沉积岩和花岗质片麻岩组成。对两类岩石中锆石的SHRIMPU-Pb测年结果表明,除了记录了20Ma以来的构造事件年龄外,主要保存了529-457Ma的变形和变质事件记录,另外还保存了更早期(>835Ma)的年龄信息。根据20Ma以来崛起的喜马拉雅挤出岩片中包含早期强烈褶皱和向南的斜向逆冲构造以及伴随的角闪岩相变质作用记录,结合岩石测年所获得的大量泛非-早古生代年龄和奥陶纪底砾岩的发现,说明曾位于南半球印度陆块北部的变质基底岩石经历过泛非-早古生代造山事件,同位素年代学数据表明:(1)原始喜马拉雅山是泛非-早古生代造山事件的产物;(2)印度陆块早-中元古代变质基底的再活化在原始喜马拉雅山形成中起重要的作用;(3)现在的喜马拉雅山是在泛非-早古生代造山事件基础上再造山的结果。     

Environmental geoinformaicts of fast growing Himalaya’s foothill towns and surrounding: concept and implementation

Himalaya foothill zone have fragile geo-environment due to active tectonics and dynamic hydrological process and its associated reshaped geomorphology. Rapid urbanization and development of new colonies leads to high rate of land use change and natural resource degradation since last three decades which has been accumulating the fragility of the area. Consequently entire Himalaya foothill zone requires a comprehensive local level geo-environmental appraisal for effective sustainable development planning of the region. Keep in view this; the Ramnagar Himalayan Foothill area (RHFA) in district Nainital, Uttarakhand (India) has been selected for the case illustration. The main objective of the study was to develop a GIS database on Ramnagar environmental geo-informatics (REGI) to facilitate all the concern line departments to implement their socioeconomic developmental activities in the most suitable and safe places throughout the study area. REGI suggests, although landscape of the study area have few favorable conditions for the inhabitant (i.e. maximum proportion (92 %) of the study area enjoys sub-tropical to sub-temperate climatic conditions, easy approachable, thick vegetation cover, rich water resources) but the active tectonics and dynamic hydrological process and its associated reshaped geomorphology poses a stressed geo-environment which need to be consider to formulate a decision support system (DSS) for sustainable development planning.     

2015尼泊尔大地震及喜马拉雅造山带未来地震趋势

2015年4月25日尼泊尔M_s 8.1级大地震是发生在喜马拉雅造山带中段的低角度逆冲断层运动, 特点是震源很浅, 震中烈度达Ⅺ度, 震害严重。破裂带走向北西西—南东东, 穿越尼泊尔首都加德满都, 使首都建筑遭受严重破坏。该震是1934年以来尼泊尔最大地震, 标志着喜马拉雅带自1950年以来半个世纪的平静期已经结束。自2005年进入新活动期, 至2015年尼泊尔大地震发生已达到活动高潮。预计将持续十到几十年。根据历史地震资料分析, 今后可能沿喜马拉雅带走向发生纵向迁移, 将在喜马拉雅带东段发生更大的地震, 从而使地震高潮达到顶峰而结束, 可能对我国西藏东南、不丹和印度边界产生破坏。另外还可能沿着与喜马拉雅带走向垂直方向向北迁移(即横向迁移), 在几年之内即可在西藏、青海引起破坏性地震, 需要相关省市做好监测预报和防灾工作。      

36<Superscript>th</Superscript> International Geological Congress - 2020 India: A broad perspective

Lichenometry is an extremely useful technique in dating moraine ridge and recent glacier retreat in polar and alpine regions. The study relates the size of the lichen thallus to the minimum age of the exposure of the surface on which it grows which consequently helps in assessment of the age of the boulders. The Lichenometric studies are carried out in Kupup and Thangu area of eastern Himalaya in Sikkim and Thajiwas glacier in Ganderbal district of north western Himalaya of Jammu and Kashmir with the help of diameters of a common crustose lichen Rhizocarpon geographicum growing luxuriantly on exposed boulder. The Thajiwas glacier showed retreat of 200 m in 279 years while in Thangu and Kupup area of eastern Himalaya the retreat was estimated as 200 m in 100 and 91 years respectively. The rate of retreat was slightly more faster in eastern Himalayan (20 mm/century) than the north-western Himalaya region (18.5mm/century).     

藏南吉隆地区早古生代大喜马拉雅片麻岩锆石SHRIMP U-Pb年龄、Hf同位素特征及其地质意义

藏南吉隆地区眼球状片麻岩是大喜马拉雅结晶岩系的一部分,其矿物组成为石英、斜长石、钾长石、黑云母和少量的白云母。片麻岩中的锆石具有核边结构,由继承碎屑锆石核和具有同心环带结构的岩浆锆石边组成,SHRIMP U Pb测年显示,边部岩浆锆石加权平均年龄为（4989±44） Ma,表明片麻岩的原岩为早古生代的花岗岩,εHf加权平均值为-83±095,暗示片麻岩原岩为壳源,可能是印度大陆北部地壳部分熔融的产物。文中和现有的地质年代学数据表明,喜马拉雅造山带是一个复合造山带,经历了早古生代的造山作用,早古生代的喜马拉雅造山带是原特提斯洋向冈瓦纳大陆北缘俯冲的结果,是冈瓦纳大陆拼合之后在其边缘形成的安第斯型造山带,而不属于冈瓦纳超大陆聚合过程中陆陆碰撞形成的泛非造山带的一部分。     

Tectonic stress pattern in the Chinese Mainland from the inversion of focal mechanism data

The tectonic stress pattern in the Chinese Mainland and kinematic models have been subjected to much debate. In the past several decades, several tectonic stress maps have been figured out; however, they generally suffer a poor time control. In the present study, 421 focal mechanism data up to January 2010 were compiled from the Global/Harvard CMT catalogue, and 396 of them were grouped into 23 distinct regions in function of geographic proximity. Reduced stress tensors were obtained from formal stress inversion for each region. The results indicated that, in the Chinese Mainland, the directions of maximum principal stress were ～NE–SW-trending in the northeastern region, ～NEE–SWW-trending in the North China region, ～N–S-trending in western Xinjiang, southern Tibet and the southern Yunnan region, ～NNE–SSW-trending in the northern Tibet and Qinghai region, ～NW–SE-trending in Gansu region, and ～E–W-trending in the western Sichuan region. The average tectonic stress regime was strike-slip faulting (SS) in the eastern Chinese Mainland and northern Tibet region, normal faulting (NF) in the southern Tibet, western Xinjiang and Yunnan region, and thrust faulting (TF) in most regions of Xinjiang, Qinghai and Gansu. The results of the present study combined with GPS velocities in the Chinese Mainland supported and could provide new insights into previous tectonic models (e.g., the extrusion model). From the perspective of tectonics, the mutual actions among the Eurasian plate, Pacific plate and Indian plate caused the present-day tectonic stress field in the Chinese Mainland.     

Tectonochemistry and p-t Conditions of Ramgarh and Almora Gneisses from Askot Klippe,Kumaun Lesser Himalaya

The chemical and petrological correlation of metamorphic nappes and klippes overlying the Proterozoic sedimentary units in the Kumaun Himalaya is still debated. The Ramgarh and Almora gneisses, not previously distinguished in the Askot Klippe, show distinct field, petrological and chemical signatures markedly similar to the tectonostratigraphic disposition of the Almora Nappe. A negative Eu anomaly in the Ramgarh granitic gneisses indicates lesser plagioclase fractionation while the Eu anomaly in the Almora pelitic gneisses is likely to have been controlled by feldspar crystallization in restites. During the anatexis at 776°C temperature and 6.6 kbar pressure, the melt moved slightly away to its crystallization sites. The Rb/Sr ratio ?0.54 and Nb ?10 ppm is consistent with the granodioritic composition. The negative Sr anomaly in the underlying Ramgarh granitic gneisses indicates a distinct mantle derived source/plagioclase fractionation with a notable correspondence to other late orogenic granites, particularly the basement Ulleri gneisses from the Nepal Himalaya. Ramgarh gneisses plot in the late-and post-COLG field. The Askot ensemble is likely to be the tectonometamorphically reworked basement, viz. the Ramgarh Group along with its metapelitic cover o f the Almora Group, together comprising southward thrust remnants of the leading edge of the Indian Plate that collided with Tibet during the Tertiary Himalayan orogeny.     

An overview of the stratigraphy and tectonics of the Nepal Himalaya

Nepal can be divided into the following five east–west trending major tectonic zones. (i) The Terai Tectonic Zone which consists of over one km of Recent alluvium concealing the Churia Group (Siwalik equivalents) and underlying rocks of northern Peninsular India. Recently active southward-propagating thrusts and folds beneath the Terai have affected both the underlying Churia and the younger sediments. (ii) The Churia Zone, which consists of Neogene to Quaternary foreland basin deposits and forms the Himalayan mountain front. The Churia Zone represents the most tectonically active part of the Himalaya. Recent sedimentologic, geochronologic and paleomagnetic studies have yielded a much better understanding of the provenance, paleoenvironment of deposition and the ages of these sediments. The Churia Group was deposited between ∼14 Ma and ∼1 Ma. Sedimentary rocks of the Churia Group form an archive of the final drama of Himalayan uplift. Involvement of the underlying northern Peninsular Indian rocks in the active tectonics of the Churia Zone has also been recognised. Unmetamorphosed Phanerozoic rocks of Peninsular India underlying the Churia Zone that are involved in the Himalayan orogeny may represent a transitional environment between the Peninsula and the Tethyan margin of the continent. (iii) The Lesser Himalayan Zone, in which mainly Precambrian rocks are involved, consists of sedimentary rocks that were deposited on the Indian continental margin and represent the southernmost facies of the Tethyan sea. Panafrican diastrophism interrupted the sedimentation in the Lesser Himalayan Zone during terminal Precambrian time causing a widespread unconformity. That unconformity separates over 12 km of unfossiliferous sedimentary rocks in the Lesser Himalaya from overlying fossiliferous rocks which are >3 km thick and range in age from Permo-Carboniferous to Lower to Middle Eocene. The deposition of the Upper Oligocene–Lower Miocene fluvial Dumri Formation records the emergence of the Himalayan mountains from under the sea. The Dumri represents the earliest foreland basin deposit of the Himalayan orogen in Nepal. Lesser Himalayan rocks are less metamorphosed than the rocks of the overlying Bhimphedis nappes and the crystalline rocks of the Higher Himalayan Zone. A broad anticline in the north and a corresponding syncline in the south along the Mahabharat range, as well as a number of thrusts and faults are the major structures of the Lesser Himalayan Zone which is thrust over the Churia Group along the Main Boundary Thrust (MBT). (iv) The crystalline high-grade metamorphic rocks of the Higher Himalayan Zone form the backbone of the Himalaya and give rise to its formidable high ranges. The Main Central Thrust (MCT) marks the base of this zone. Understanding the origin, timing of movement and associated metamorphism along the MCT holds the key to many questions about the evolution of the Himalaya. For example: the question of whether there is only one or whether there are two MCTs has been a subject of prolonged discussion without any conclusion having been reached. The well-known inverted metamorphism of the Himalaya and the late orogenic magmatism are generally attributed to movement along the MCT that brought a hot slab of High Himalayan Zone rocks over the cold Lesser Himalayan sequence. Harrison and his co-workers, as described in a paper in this volume, have lately proposed a detailed model of how this process operated. The rocks of the Higher Himalayan Zone are generally considered to be Middle Cambrian to Late Proterozoic in age. (v) The Tibetan Tethys Zone is represented by Cambrian to Cretaceous-Eocene fossiliferous sedimentary rocks overlying the crystalline rocks of the Higher Himalaya along the Southern Tibetan Detachment Fault System (STDFS) which is a north dipping normal fault system. The fault has dragged down to the north a huge pile of the Tethyan sedimentary rocks forming some of the largest folds on the Earth. Those sediments are generally considered to have been deposited in a more distal part of the Tethys than were the Lesser Himalayan sediments.The present tectonic architecture of the Himalaya is dominated by three master thrusts: the Main Central Thrust (MCT), the Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT). The age of initiation of these thrusts becomes younger from north to south, with the MCT as the oldest and the MFT as the youngest. All these thrusts are considered to come together at depth in a flat-lying decollement called the Main Himalayan Thrust (MHT). The Mahabharat Thrust (MT), an intermediate thrust between the MCT and the MBT is interpreted as having brought the Bhimphedi Group out over the Lesser Himalayan rocks giving rise to Lesser Himalayan nappes containing crystalline rocks. The position of roots of these nappes is still debated. The Southern Tibetan Detachment Fault System (STDFS) has played an important role in unroofing the higher Himalayan crystalline rocks.