MAIN CENTRAL THRUST ZONE IN THE KATHMANDU AREA, CENTRAL NEPAL, AND ITS TECTONIC SIGNIFICANCE

MAIN CENTRAL THRUST ZONE IN THE KATHMANDU AREA, CENTRAL NEPAL, AND ITS TECTONIC SIGNIFICANCE1　AritaK ,LallmeyerRD ,TakasuA .TectonothermalevolutionoftheLesserHimalaya ,Nepal:constraintsfrom 4 0 Ar/3 9AragesfromtheKathmandunappe[J].TheIslandArc ,1997,6 :372～ 384. 2　RaiSM ,GuillotS ,LeFortP ,etal.Pressure temperatureevolutionintheKathmanduandGosainkundregions ,CentralNepal[J].JourAsianEarthSci ,1998,16 :2 83～ 2 98. 3　SchellingD ,KArita .…     

Himalayan main central thrust and its implications for Himalayan inverted metamorphism

Within the scheme of south-directed orogenic polarity in the Himalayan region, the Main Central Thrust (MCT) developed at an intermediate stage between the collisional tectonics at the Indus-Tsangpo suture in the north and the latest underthrusting at the Main Boundary Thrust (MBT) in the south. The proposition that the MCT marks the base of the High Himalayan Central Crystalline zone against the Inner sedimentary belt of the Lower Himalayas creates confusion in its definition, and its location in Kashmir and especially in the Eastern Himalayas. The problem can be resolved by defining the MCT as the basal thrust of the crystalline nappe sequences either rooted at or detached as klippe from the Central Crystalline zone.Characteristically, the nappe sequences show inverted metamorphism which generally starts at the basal thrust, the redefined MCT, from the chlorite-biotite grades and reaches the kyanite-sillimanite grades with migmatites and anatectic granites in the highest tectonic levels. The metamorphic rank of both the substrate and the overthrust rock units in the vicinity of the MCT is generally low. The inverted metamorphism may be explained by intracontinental underthrusting and resultant downbowing of the isotherms. The redefined MCT does not appear to represent the thrust zone along which the continental underthrusting was initiated. It developed to the south of this zone as a possible sole thrust of the nappe stack of the crystalline rocks that overrode the parautochthonous sedimentary sequences of the Lower Himalayas.The sequence of events associated with the MCT would probably be repeated in the MBT.     

Structural set up in northern part of Kumaun Himalaya

The Main Central Thrust (MCT) and the Main Boundary Thrust (MBT) are the two major thrusts in Kumaun, the MCT forming the boundary between highly sheared, deformed and mylonitized rocks of the Great Himalayan Central Crystallines and the Lesser Himalayan metasedimentaries. While in the Central Crystallines four-folding episodes are observed of which two are of the Precambrian age, the Lesser Himalayan rocks show only two phases of folding. MCT has its own distinctive structural history and the crystalline mass comprises an integral part of peninsular India.     

The two intracrustal boundary thrusts of the Himalaya

The series of four different, steeply inclined thrusts which sharply sever the youthful autochthonous Cenozoic sedimentary zone, including the Siwalik, from the mature old Lesser Himalayan subprovince is collectively known as the Main Boundary Thrust (MBT). In the proximity of this trust in northwestern and eastern sectors, the parautochtonous Lesser Himalayan sedimentary formations are pushed up and their narrow frontal parts split into imbricate sheets with attendant repetition and inversion of lithostratigraphic units. The superficially steeper thrust plane seems to flatten out at depth. The MBT is tectonically and seismically very active at the present time.The Main Central Thrust (MCT), inclined 30° to 45° northwards, constitutes the real boundary between the Lesser and Great Himalaya. Marking an abrubt change in the style and orientation of structures and in the grade of metamorphism from lower amphibolitefacies of the Lesser Himalayan to higher metamorphic facies of the Great Himalayan, the redefined Main Central Thrust lies at a higher level as that originally recognized by A. Heim and A. Gansser. They had recognized this thrust as the contact of the mesozonal metamorphics against the underlying sedimentaries or epimetamorphics. It has now been redesignated as the Munsiari Thrust in Kumaun. It extends northwest in Himachal as the Jutogh Thrust and farther in Kashmir as the Panjal Thrust. In the eastern Himalaya the equivalents of the Munsiari Thrust are known as the Paro Thrust and the Bomdila Thrust. The upper thrust surface in Nepal is recognized as the Main Central Thrust by French and Japanese workers. The easterly extension of the MCT is known as the Khumbu Thrust in eastern Nepal, the Darjeeling Thrust in the Darjeeling-Sikkim region, the Thimpu Thrust in Bhutan and the Sela Thrust in western Arunachal. Significantly, hot springs occur in close proximity to this thrust in Kumaun, Nepal and Bhutan. There are reasons to believe that movement is taking place along the MCT, although seismically it is less active than the MBT.     

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.     

Spectral analysis and Euler deconvolution technique of gravity data to decipher the basement depth in the Dehradun-Badrinath area

The stratigraphic and tectonic setting in the northwest part of Himalayan belt is complex and thrusted due to the collision of Indian plate and the Eurasian plate. During the past, the Himalaya is divided into four parts; these are Outer Himalaya, Lesser Himalaya, Greater or Higher Himalaya and Tethys Himalaya. The appearance of basement rocks played a significant role in the Himalayan periphery for stratigraphic, structural and tectonic movement. The deformation pattern of the crustal rocks causing the rise of basement rocks which constitutes an integral part of crustal configuration during the evolutionary stages of the Himalaya. In this study, an attempt has been made to estimate the basement depth configuration using spectral analysis and Euler deconvolution technique of gravity data in the northwest Himalaya region. The elevation increases continuously from 500 m to 5100 m in SW to NE direction, however, Bouguer gravity anomaly decreases continuously from ?130 mGal to ?390 mGal in SW to NE direction due to the isostaic adjustment. Gravity anomaly is very low near Harsil, Badrinath and Joshimath area and observed higher elevation due to the deep rooted basement. However, there are extrusion of crystalline basement in and around the Badrinath area due to the resettlement of geologic process which are overlaid to the top surface of the sedimentary layers. Euler deconvolution technique has been applied to detect the direct basement depth and results show a good correlation with the average depth of the spectral analysis and other works carried by different authors. Three gravity profiles are selected in appropriate places orienting SW-NE direction with a profile length of 160 km, 150 km and 140 km respectively in the study area for calculating the average depth of the basement rock. The average basement depth calculated is around 11.27 km using the spectral analysis technique and results are well correlated with the results of various workers. Euler deconvolution studies along the three selected profiles have been interpreted. It has been observed that there are more number of cluster points falling between depth ranges of 10 to 15 km, dipping in SW to NE direction. Euler’s study shows deep rooted connection near Main Frontal Thrust (MFT), Main Boundary Thrust (MBT), Main Central Thrust (MCT), Bearing Thrust (BT) and Vaikrita Thrust (VT) locations as per profile study. Based on these studies three geological models have been prepared along the profiles showing different tectonic resettlement and depth of crystalline basement. Crystalline rocks exposed at the surface may be due to uplift along the shear in the MCT zone by kinetic flow basically, Munsiayari Thrust (MT) and VT in the of NW-Himalaya region.     

METAMORPHISM IN THE LESSER HIMALAYAN CRYSTALLINES AND MAIN CENTRAL THRUST ZONE IN THE ARUN VALLEY AND AMA DRIME RANGE (EASTERN HIMALAYA)

METAMORPHISM IN THE LESSER HIMALAYAN CRYSTALLINES AND MAIN CENTRAL THRUST ZONE IN THE ARUN VALLEY AND AMA DRIME RANGE (EASTERN HIMALAYA)1　BrunelM ,KienastJR . tudep啨ｔｒｏ structuraledeschevauchementsductileshimalayenssurlatrans versaledel’Everest Makalu (N啨ｐａｌｏｒｉｅｎｔａｌ) [J].CanadianJ .EarthSciences,1986 ,2 3:1117～ 1137. 2　LombardoB ,RolfoF .TwocontrastingeclogitetypesintheHimalayas :implicationsfortheHimalayanorogeny…     

THRUST PACKAGES OF 1.68 Ga INDIAN SUPRA-CRUSTAL ROCKS IN THE MIOCENE SIWALIK BELT,CENTRAL NEPAL HIMALAYAS

THRUST PACKAGES OF 1.68 Ga INDIAN SUPRA-CRUSTAL ROCKS IN THE MIOCENE SIWALIK BELT,CENTRAL NEPAL HIMALAYAS     

GEOLOGY OF THE TAPLEJUNG WINDOW AND FRONTAL BELT, FAR EASTERN NEPAL HIMALAYA

GEOLOGY OF THE TAPLEJUNG WINDOW AND FRONTAL BELT, FAR EASTERN NEPAL HIMALAYA1　PecherA .Deformationandmetamorphismeassociesaunezonedecisaillement :Exampledugrandchevauchementcen tralHimalayan (MCT) [M ].Thesed’Etat,UnivGrenoble ,France ,1978.35 4. 2　RaiSM .LesnappesdeKathmandudtduGosainkund ,HimalayaduNepalcentral[M ].Thesededoctoral,Univ .Grenoble ,France ,1998.2 44 . 3　SchellingD ,AritaK .Thrusttectonics ,crustalshortening ,andthestructureofthe…     

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…     

Post-emplacement kinematics and exhumation history of the Almora klippe of the Kumaun–Garhwal Himalaya,NW India: revealed by fission track thermochronology

International Journal of Earth Sciences - Tectonically transported crystalline thrust sheet over the Lesser Himalayan meta-sedimentary zone along the Main Central Thrust (MCT) is represented by...     

Crustal architecture of the Himalayan metamorphic front in eastern Nepal

The Himalayan Metamorphic Front consists of two basinal sequences deposited on the Indian passive margin, the Mesoproterozoic Lesser Himalayan Sequence and the Neoproterozoic–Cambrian Greater Himalayan Sequence. The current paradigm is that the unconformity between these two basinal sequences coincides with a crustal-scale thrust that has been called the Main Central Thrust, and that this acted as the fundamental structure that controlled the architecture of the Himalayan Metamorphic Front. Geological mapping of eastern Nepal and eight detailed stratigraphic, kinematic, strain and metamorphic profiles through the Himalayan Metamorphic Front define the crustal architecture. In eastern Nepal the unconformity does not coincide with a discrete structural or metamorphic discontinuity and is not a discrete high strain zone. In recognition of this, we introduce the term Himalayan Unconformity to distinguish it from high strain zones in the Himalayan Metamorphic Front. The fundamental structure that controls orogen architecture in eastern Nepal occurs at higher structural levels within the Greater Himalayan Sequence and we suggest the name; High Himal Thrust. This 100–400 m thick mylonite zone marks a sharp deformation discontinuity associated with a steep metamorphic transition, and separates the Upper-Plate from the Lower-Plate in the Himalayan Metamorphic Front. The high-T/moderate-P metamorphism at  20–24 Ma in the Upper-Plate reflects extrusion of material between the High Himal Thrust and the South Tibet Detachment System at the top of the section. The Lower-Plate is a broad schistose zone of inverted, diachronous moderate-T/high-P metamorphic rocks formed between  18 and 6 Ma. The High Himal Thrust is laterally continuous into Sikkim and Bhutan where it also occurs at higher structural levels than the Himalayan Unconformity and Main Central Thrust (as originally defined). To the west in central Nepal, the Upper-Plate/Lower-Plate boundary has been placed at lower structural levels, coinciding with the Himalayan Unconformity and has been named the Main Central Thrust, above the originally defined Main Central Thrust (or Ramgarh Thrust).     

Late Miocene movement within the Himalayan Main Central Thrust shear zone, Sikkim, north-east India

In the Sikkim region of north‐east India, the Main Central Thrust (MCT) juxtaposes high‐grade gneisses of the Greater Himalayan Crystallines over lower‐grade slates, phyllites and schists of the Lesser Himalaya Formation. Inverted metamorphism characterizes rocks that immediately underlie the thrust, and the large‐scale South Tibet Detachment System (STDS) bounds the northern side of the Greater Himalayan Crystallines. In situ Th–Pb monazite ages indicate that the MCT shear zone in the Sikkim region was active at c. 22, 14–15 and 12–10 Ma, whereas zircon and monazite ages from a slightly deformed horizon of a High Himalayan leucogranite within the STDS suggest normal slip activity at c. 17 and 14–15 Ma. Although average monazite ages decrease towards structurally lower levels of the MCT shear zone, individual results do not follow a progressive younging pattern. Lesser Himalaya sample KBP1062A records monazite crystallization from 11.5 ± 0.2 to 12.2 ± 0.1 Ma and peak conditions of 610 ± 25 °C and 7.5 ± 0.5 kbar, whereas, in the MCT shear zone rock CHG14103, monazite crystallized from 13.8 ± 0.5 to 11.9 ± 0.3 Ma at lower grade conditions of 525 ± 25 °C and 6 ± 1 kbar. The P–T–t results indicate that the shear zone experienced a complicated slip history, and have implications for the understanding of mid‐crustal extrusion and the role of out‐of‐sequence thrusts in convergent plate tectonic settings.     

Evidence for structural discordance in the inverted metamorphic sequence of Sikkim Himalaya: Towards resolving the Main Central Thrust controversy

Inverted metamorphism in the Himalayas is closely associated with the Main Central Thrust (MCT). In the western Himalayas, the Main Central Thrust conventionally separates high grade metamorphic rocks of the Higher Himalayan Crystalline Sequence (HHCS) from unmetamorphosed rocks of the Inner sedimentary Belt. In the eastern Himalayas, the Inner sedimentary Belt is absent, and the HHCS and meta-sedimentary Lesser Himalayan Sequence (LHS) apparently form a continuous Barrovian metamorphic sequence, leading to confusion about the precise location of the MCT. In this study, it is demonstrated that migmatitic gneisses of the sillimanite zone in the higher structural levels of the HHCS are multiply deformed, with two phases of penetrative fabric formation (S_1HHCS and S_2HHCS) followed by third folding event associated with a spaced, NW-SE trending, north-east dipping foliation (S_3HHCS). The underlying LHS schists (kyanite zone and lower) are also multiply deformed, with the bedding S₀ being isoclinally folded (F_1LHS), and subsequently refolded (F_2LHS and F_3LHS). The contact zone between the HHCS and LHS is characterized by ductile, top-to-the southwest shearing and stabilization of a pervasive foliation that is consistently oriented NW-SE and dips northeast. This foliation is parallel to the S_3HHCS foliation in the HHCS, and the S_2LHS in the LHS. Early lineations in the HHCS and LHS also show different dispersions across the contact shear zone, implying that pre-thrusting orientations of the two units were distinct. The contact shear zone is therefore interpreted to be a plane of structural discordance, shows a shear sense consistent with thrust movement and is associated with mineral growth during Barrovian metamorphism. It may well be considered to represent the MCT in this region.     

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.     

U-Pb zircon ages and Sm-Nd isotopic characteristics of the Lesser and Great Himalayan sequences,Uttarakhand Himalaya,and their regional tectonic implications

Detrital zircons (DZ) and Nd isotopic characteristics constraint maximum depositional ages of two distinct Paleoproterozoic and Neoproterozoic terranes across the Main Central Thrust zone (Munsiari Group) in the Himalaya. New DZ ages and Nd isotopic characters are reported from the Inner Lesser Himalaya (iLH) sedimentary belt (Berinag Group quartzite) and the Munsiari Group through the Great Himalayan Sequence (GHS–Vaikrita Group) across the MCT to the lower parts of the Tethyan Himalayan Sequence (THS) along the Alaknanda–Dhauli Ganga valleys, Uttarakhand Himalaya. The iLH Berinag Group quartzite yielded nearly unimodal DZ U-Pb ages between 2.05 and 1.80 Ga with ε_Nd(0) values of −17 and −23, while the overthrust Munsiari Group, bounded by the Munsiari Thrust at the base and the Vaikrita Thrust (MCT) at the top, represents the Proterozoic magmatic arc with ∼1.95 and 1.89 Ga U-Pb zircon age population with an average of −25 ε_Nd(0) value; the arc developed during the Columbia Supercontinent assembly. In contrast, overthrust Great Himalayan Sequence (GHS–Vaikrita Group) above the MCT is characterized by entirely new Neoproterozoic 1.05–0.85 Ga zircon population, which appears for the first time in this sequence, and has higher ε_Nd(0) values (average −16). Tectonically overlying the GHS, the Tethyan Himalayan Sequence (THS) has first appearance of the Early Paleozoic detrital zircons, with its ε_Nd(0) values like the GHS. Broadly, these characters persist throughout the Himalayan belt from Himachal to NE Himalaya. The iLH sediments were possibly derived from northernly ∼1.9 Ga magmatic arc, and southern the Archean–Proterozoic Aravalli–Bundelkhand nuclei of the Indian craton. Potential sources for the GHS sediments may be a northerly ‘destroyed’ Neoproterozoic magmatic arc whose remnants exists within the Himalaya as the Neoproterozoic granitoids, and possibly be the iLH sedimentary belt, an ‘In-board’ Aravalli–Delhi Fold Belt (ADFB)–Central Indian Tectonic Zone (CITZ) in the south.     

Seismic Activity in the Himalayan Orogenic Belt and Its Related Geohazards

Himalayan orogenic belt is the highest and largest continental collision and subduction zone on the Earth. The Himalayan orogenic belt has produced frequent large earthquakes and caused several geohazards due to landslides and housing collapse, having an impact on the safety of life and property along a length of over 2500 km. Here we took three earthquake clusters as examples, which occurred at Nepal Himalaya, eastern Himalayan syntaxis and western Himalayan syntaxis, respectively. Here we calculated the earthquake locations and fault plane solutions based on the waveform data recorded by seismic stations deployed in source areas by the Institute of Tibetan Plateau Research, Chinese Academy of Sciences. We found that at the Nepal Himalayan, the Main Himalayan Thrust is the major tectonic structure for large earthquakes to occur. At the eastern Himalayan syntaxis, most earthquakes are of the reverse or strike-slip faulting. The major tectonic feature is the combination of the NE-dipping thrust with the southeastern escape of the Tibetan plateau. At the western Himalayan syntaxis, intermediate-depth earthquakes are active. These observations reveal the geometry of the deep subduction of the continental plate with steep dipping angle.     

喜马拉雅造山带地震活动及其相关地质灾害

喜马拉雅造山带是地球上海拔最高、规模最大的陆陆板块俯冲碰撞带在这条长达2 500 km的板块边界上,近年来多次发生破坏性地震,造成大规模的滑坡、房屋倒塌等次生灾害,给人民生命和财产安全造成严重的威胁。分别选取尼泊尔喜马拉雅、喜马拉雅东构造结和喜马拉雅西构造结地区近期发生的3个地震震群作为研究实例,基于中国科学院青藏高原研究所在研究区架设的区域流动地震台站记录的波形资料,对地震的震源位置和震源机制解进行计算。结果表明,在尼泊尔喜马拉雅地区,主喜马拉雅逆冲断裂是大地震的主要发震构造;东构造结地区的地震以逆冲和走滑型为主,表明印度板块向北东方向的逆冲推覆和青藏高原向东南逃逸的侧向挤出是该地区的主要构造背景;西构造结地区中深源地震多发,揭示了高角度大陆深俯冲的几何形态。     

STRUCTURAL EVOLUTION OF THE KULU-RAMPUR AND LARJI WINDOW ZONES, WESTERN HIMALAYA, INDIA

STRUCTURAL EVOLUTION OF THE KULU-RAMPUR AND LARJI WINDOW ZONES, WESTERN HIMALAYA, INDIA     

Sedimentologic attributes of the Proterozoic siliciclastic packages of the Garhwal–Kumaun Lesser Himalaya,India: Implication for their relationship and palaeobasinal conditions

The present work addresses the long-standing issues on the characterization aspect of the Proterozoic siliciclastic successions exposed in the central part of the Lesser Himalaya, restricted between the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT). Geologic, sedimentologic, and petrographic study divides the Lesser Himalaya in two zones- northern Palaeo- Mesoproterozoic Inner Lesser Himalayan (ILH) and southern Neoproterozoic Outer Lesser Himalayan (OLH) zones. The major lithofacies recognized from the zones are - (i) coarse grained siliciclastic (CGS), (ii) interbedded medium and fine-grained siliciclastic (IMFS), (iii) argillite (ARG), and (iv) siliciclastic–argillite rhythmites (SAR). Amongst all these facies, the nearshore IMFS facies shows consistent presence in both OLH and ILH zones. From the facies distribution pattern, a northwest–southeasterly trending palaeo- shoreline has been envisaged. The CGS facies in the ILH hints towards an alluvial fan setting during 1.8 Ga rifting phase associated with penecontemporaneous basic magmatism. Compositionally, the siliciclastics of both the zones (ILH and OLH) are arenite and wacke types with minimal variation in their detrital proportions, derived from the early Proterozoic (between 2.4-1.6Ga) Aravalli-Delhi Supergroup provenance. Nearly matching types and content of detrital modes and the lithofacies pattern of the ILH and OLH siliciclastics probably conclude the derivation from the rising (nearby) Aravalli-Delhi orogen and deposition in a foreland like situation.