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 .…     

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.     

Inverted metamorphism in the Pre-Siwalik foreland basin sediments beneath the crystalline nappe, western Nepal Himalaya

One of the Pre-Siwalik foreland basin sedimentary units, the Dumri Formation, is tectonically covered by the Lesser Himalayan Crystalline nappe and the Kuncha-Naudanda thrust sheet. It is narrowly distributed in the eastern margin of the Karnali klippe along the NNE–SSW trending Chakure Fault. The whole sequence of the fluvial Dumri Formation attaining 1500 m in thickness is weakly metamorphosed to muscovite phyllite and foliated phyllitic sandstone. The metamorphic grade decreases stratigraphically downward and underlying Nummulitic limestone of the middle Eocene Bhainskati Formation is converted into a slaty limestone. No metamorphic mica is detected from the late Cretaceous to Paleocene Amile Formation below the Bhainskati Formation. These facts indicate that the Tansen Group has undergone inverted metamorphism.A ⁴⁰Ar/³⁹Ar plateau age of 25.69±0.13 Ma was obtained from garnetiferous biotite gneiss in the lower part of the crystalline nappe. Another ⁴⁰Ar/³⁹Ar age spectrum from muscovite phyllite of the Dumri Formation suggests that metamorphism occurred at 16–17 Ma. The origin of the inverted metamorphism limited to the uppermost part of the Lesser Himalayan autochthon can be attributed to heat from the hot crystalline nappe and shearing along the sole thrust of the Kuncha-Naudanda thrust sheet. The depositional age of the Dumri Formation is estimated to be 26–17 Ma.Provenance of the Dumri Formation is considered to be from the Naudanda Quartzite, the Kuncha Formation and the Tibetan Tethys sediments, because the sandstone contains orthoquartzite pebbles, phyllitic lithic fragments and a sparry calcite cement. The sedimentary facies indicates deposition by meandering rivers on flood-plains in the distal part of the foreland basin. No proximal facies, such as alluvial fan and pebbly braided river deposits, could be detected from the formation, though it is near the Main Central Thrust (MCT). The northern continuation of the foreland basin sediments must be concealed beneath the Higher Himalayan Crystalline. Judging from the present distribution of the Dumri Formation from the south of the Main Boundary Thrust (MBT) to near the MCT and from the shortening of the Lesser Himalayan sediments by thrusts and folds, the width of the foreland basin where the Dumri Formation was deposited is estimated to have been more than 300 km.     

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…     

River profiles along the Himalayan arc as indicators of active tectonics

Longitudinal profiles along sixteen major transverse Himalayan rivers add important constraints to models of active continental subduction and its evolution. These profiles are characterized by a zone of relatively high gradient that cannot be associated with differential resistence to erosion in all cases. The base of the zone of increased gradients correlates with (1) the topographic front between the Lesser and High Himalayas, (2) the narrow belt of intermediate-magnitude thrust earthquakes, (3) the Main Central Thrust zone (MCT). These features define a small circle in the central portion of the Himalayan arc. These correlations suggest that the discontinuity in the river profiles and the other features are controlled by a major tectonic boundary between the rising High Himalayas and the Lesser Himalayas. No sharp increases in gradient are observed near the Main Boundary Thrust (MBT), except on a few rivers, such as the Jhelum or Kundar, where the MBT lies close to both the MCT and the seismic belt. Thus, it is unlikely that the MBT is a major tectonic boundary. The diversion of river courses along the MBT and around anticlines in the Sub Himalayas has probably been caused by aggradation near the rosion-deposition boundary, upstream of uplifts in the Mahabharat range and Sub Himalayas.A parallel is drawn between the Himalayas and New Guinea based on the hypothesis that continent-arc collision, of the type occurring in northern Australia, preceded continent-continent collision in the Himalayas. The present sedimentary/tectonic phase in New Guinea resembles the Subathu (Paleocene-Eocene) phase in the Himalayas. Incipient counterparts of the major Himalayan structures, including the MCT and the MBT, are recognized in New Guinea. The drainage patterns in the Himalayas and in New Guinea bear a similar relation to major structures. This suggests that (1) the tectonic evolution of the Himalayas has been rather uniform since early stages of collision, and (2) the Himalayan drainage was also formed at these early stages and is therefore antecedent to the rise of the High Himalayas.     

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.     

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).     

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…     

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.     

An overview of the metamorphic evolution in Central Nepal

This paper summarizes the studies of the metamorphic evolution of Central Nepal carried out by Nepali and international teams in the last 25 years. In Central Nepal, three metamorphic units are recognized. (1) The southernmost zone is the Lesser Himalaya, which is characterised by an inverted mineral zoning towards the Main Central Thrust (MCT) zone; (2) the Kathmandu nappe corresponds to an early (<22 Ma) out-of-sequence thrusting zone over the Lesser Himalaya along the Mahabharat thrust (MT) and is characterised by a Barrovian metamorphic evolution; (3) the Higher Himalayan Crystalline unit (HHC) is bounded at its base by the MCT and at its top by the South Tibetan Detachment system (STDS). It is characterised by successive tectonometamorphic episodes during the period spanning from 35–36 Ma to 2–3 Ma. Recent investigations suggest that the apparent metamorphic inversion througout the MCT zone does not reflect geothermal inversion. Instead, these investigations suggest successive cooling of the HHC along the MCT and the local preservation, above the MCT, of high-grade metamorphic rocks. The overall metamorphic history in Central Nepal from Oligocene to Pliocene, reflects the thermal reequilibration of rocks after thickening by conductive and advective heating and partial melting of the middle crust.     

DIFFERENT VARIETIES OF MIOCENE LEUCOGRANITE IN THE ARUN VALLEY—EVEREST—MAKALU AREA:FIELD RELATIONS, PETROLOGY AND ISOTOPE GEOCHEMISTRY

DIFFERENT VARIETIES OF MIOCENE LEUCOGRANITE IN THE ARUN VALLEY—EVEREST—MAKALU AREA:FIELD RELATIONS, PETROLOGY AND ISOTOPE GEOCHEMISTRY1　AritaK .OriginoftheinvertedmetamorphismoftheLowerHimalayas,CentralNepal[J] .Tectonophysics,1983,93:4 3～6 0 . 　BarbarinB .Areviewoftherelationshipsbetweengranitoidtypes,theiroriginsandtheirgeodynamicenvironments[J] .Lithos,1999,4 6 :6 0 5～ 6 2 6 . 3　BurchfielBC ,ChenZ,HodgesKV ,etal.TheSou…     

Tectonic and polymetamorphic history of the Lesser Himalaya in central Nepal

The Lesser Himalaya in central Nepal consists of Precambrian to early Paleozoic, low- to medium-grade metamorphic rocks of the Nawakot Complex, unconformably overlain by the Upper Carboniferous to Lower Miocene Tansen Group. It is divided tectonically into a Parautochthon, two thrust sheets (Thrust sheets I and II), and a wide shear zone (Main Central Thrust zone) from south to north by the Bari Gad–Kali Gandaki Fault, the Phalebas Thrust and the Lower Main Central Thrust, respectively. The Lesser Himalaya is overthrust by the Higher Himalaya along the Upper Main Central Thrust (UMCT). The Lesser Himalaya forms a foreland-propagating duplex structure, each tectonic unit being a horse bounded by imbricate faults. The UMCT and the Main Boundary Thrust are the roof and floor thrusts, respectively. The duplex is cut-off by an out-of-sequence fault. At least five phases of deformation (D₁–D₅) are recognized in the Lesser Himalaya, two of which (D₁ and D₂) belong to the pre-Himalayan (pre-Tertiary) orogeny. Petrographic, microprobe and illite crystallinity data show polymetamorphic evolution of the Lesser and Higher Himalayas in central Nepal. The Lesser Himalaya suffered a pre-Himalayan (probably early Paleozoic) anchizonal prograde metamorphism (M₀) and a Neohimalayan (syn- to post-UMCT) diagenetic to garnet grade prograde inverted metamorphism (M₂). The Higher Himalaya suffered an Eohimalayan (pre or early-UMCT) kyanite-grade prograde metamorphism (M₁) which was, in turn, overprinted by Neohimalayan (syn-UMCT) retrograde metamorphism (M₂). The isograd inversion from garnet zone in the Lesser Himalaya to kyanite zone in the Higher Himalaya is only apparent due to post-metamorphic thrusting along the UMCT. Both the Lesser and Higher Himalayas have undergone late-stage retrogression (M₃) during exhumation.     

Tectonothermal evolution of the High Himalayan Crystalline Sequence, Langtang Valley, northern Nepal

The High Himalayan Crystalline Sequence in north-central Nepal is a 15-km-thick pile of metasediments that is bound by the Main Central Thrust to the south and a normal fault to the north. The Langtang section through the metasediments shows an apparent inversion of metamorphic isograds with high-P, kyanite-grade rocks exposed beneath low-P, sillimanite-grade rocks. Textural evidence confirms that the observed inversion is a result of a polyphase metamorphic history and phase equilibria studies indicate that thermal decoupling has occurred within a mechanically coherent section of crust. Rocks now exposed at the base of the High Himalayan thrust sheet underwent Barrovian regional metamorphism (M1) prior to 34 Ma in the early stages of the Himalayan orogeny, recording metamorphic conditions of T= 710 ± 30° C, P= 9 ± 1 kbar. After the activation of the Main Central Thrust, which emplaced these metapelites southwards onto the lower grade Lesser Himalayan formations, the upper part of the thrust sheet was overprinted by a second heating event (M2), resulting in sillimanite-grade metamorphism and anatexis of metapelites at T= 760 ± 30° C, P= 5.8 ± 0.4 kbar between 17 and 20 Ma. Crustally derived, leucogranite magmas have been emplaced into low-grade Tethyan sediments on the hangingwall of the normal fault that bounds the northern limit of the metapelitic sequence. The cause of the selective heating of the upper section of the metasediments during M2 cannot be reconciled with either post-thrusting thermal relaxation or advection models. The cause of M2 remains problematical but it is suggested that heat focusing has occurred at the top of the High Himalayan Crystalline Sequence as a result of movement on the normal fault blanketing metapelites of high heat productivity with low-grade sediments of low thermal conductivity. This model implies that the normal fault was active before M2, consistent with decompression textures that formed during, or shortly after, sillimanite-grade metamorphism.     

Tectonometamorphic evolution of the Himalayan metamorphic core between the Annapurna and Dhaulagiri, central Nepal

The metamorphic core of the Himalaya in the Kali Gandaki valley of central Nepal corresponds to a 5-km-thick sequence of upper amphibolite facies metasedimentary rocks. This Greater Himalayan Sequence (GHS) thrusts over the greenschist to lower amphibolite facies Lesser Himalayan Sequence (LHS) along the Lower Miocene Main Central Thrust (MCT), and it is separated from the overlying low-grade Tethyan Zone (TZ) by the Annapurna Detachment. Structural, petrographic, geothermobarometric and thermochronological data demonstrate that two major tectonometamorphic events characterize the evolution of the GHS. The first (Eohimalayan) episode included prograde, kyanite-grade metamorphism, during which the GHS was buried at depths greater than c. 35 km. A nappe structure in the lowermost TZ suggests that the Eohimalayan phase was associated with underthrusting of the GHS below the TZ. A c. 37 Ma ⁴⁰Ar/³⁹Ar hornblende date indicates a Late Eocene age for this phase. The second (Neohimalayan) event corresponded to a retrograde phase of kyanite-grade recrystallization, related to thrust emplacement of the GHS on the LHS. Prograde mineral assemblages in the MCT zone equilibrated at average T =880 K (610 °C) and P =940 MPa (=35 km), probably close to peak of metamorphic conditions. Slightly higher in the GHS, final equilibration of retrograde assemblages occurred at average T =810 K (540 °C) and P=650 MPa (=24 km), indicating re-equilibration during exhumation controlled by thrusting along the MCT and extension along the Annapurna Detachment. These results suggest an earlier equilibration in the MCT zone compared with higher levels, as a consequence of a higher cooling rate in the basal part of the GHS during its thrusting on the colder LHS. The Annapurna Detachment is considered to be a Neohimalayan, synmetamorphic structure, representing extensional reactivation of the Eohimalayan thrust along which the GHS initially underthrust the TZ. Within the upper GHS, a metamorphic discontinuity across a mylonitic shear zone testifies to significant, late- to post-metamorphic, out-of-sequence thrusting. The entire GHS cooled homogeneously below 600–700 K (330–430 °C) between 15 and 13 Ma (Middle Miocene), suggesting a rapid tectonic exhumation by movement on late extensional structures at higher structural levels.     

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.     

Thermal model for the Zanskar Himalaya

ABSTRACT Crustal thickening along the northern margin of the Indian plate, following the 50 Ma collision along the Indus Suture Zone in Ladakh, caused widespread high-temperature, medium-pressure Barrovian facies series metamorphism and anatexis. In the Zanskar Himalaya metamorphic isograds are inverted and structurally telescoped along the Main Central Thrust (MCT) Zone at the base of the High Himalayan slab. Along the Zanskar valley at the top of the slab, isograds are the right way-up and are also telescoped along northeast-dipping normal faults of the Zanskar Shear Zone (ZSZ), which are related to culmination collapse behind the Miocene Himalayan thrust front. Between the MCT and the ZSZ a metamorphic-anatectic core within sillimanite grade rocks contains abundant leucogranite-granite crustal melts of probable Himalayan age. A thermal model based on a crustal-scale cross-section across the Zanskar Himalaya suggests that M₁ isograds, developed during early Himalayan Barrovian metamorphism, were overprinted during high-grade MCT-related anatexis and folded around a large-scale recumbent fold developed in the hanging wall of the MCT.     

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...     

青藏高原典型地区的地貌量化分析——兼对高原“夷平面”的讨论

尽管过去150年以来,人们对于喜马拉雅造山带有很长的一段研究历史,但是对其几何特征、运动方式、动力学演化仍然理解不深。这种情况的出现,主要是因为人们持续关注的是喜马拉雅造山带的二维构造空间特性,并将某些研究程度较高地区的地质关系向外推广到造山带其他地区。就地理、地层及构造划分而言,概念的混淆和误解在有关喜马拉雅的文章中也大量存在。为了阐明这些问题,并为那些有兴趣探究喜马拉雅造山带地质演化过程的人们提供一个新的平台,文中系统地综述了以前的基本观察。我的综述主要是强调沿走向变化的喜马拉雅地质格架在喜马拉雅剥露、变质和前陆沉积方面所起的作用。文章的主要目的是阐明占据造山带核部的大喜马拉雅结晶岩带(GHC)的侵位历史。因为喜马拉雅大部分地区是由主中央冲断层(MCT)和藏南拆离系(STD)之间的GHC所组成,所以在地图和剖面观察上确定这些一级喜马拉雅构造之间的关系是非常关键的。中喜马拉雅出露的平面模式表明,MCT具有断坪-断坡的逆断层的几何特征。南部的逆冲断坪携带了一个GHC的板片(Slab)叠置在小喜马拉雅层序之上(LHS),并形成了一个在MCT逆冲断层带之南延续100km的巨大上盘断弯褶皱。在西喜马拉雅造山带地区,东经约77°处,MCT呈现为横向逆冲断坡(Mandi倾向逆冲断坡)。在其西边,MCT将低级变质的特提斯喜马拉雅层序(THS)叠置到低级变质的小喜马拉雅之上;而在其东边,MCT将高级GHC叠置到低级LHS之上。这种沿走向变化的地层叠置和横穿MCT的变质等级表明,逆冲断层的断距向西减小,可能是由于地壳短缩总量沿着喜马拉雅造山带向西减小所致。在所有出露的地方,STD大致都沿着THS底部的同一地层面,呈现出一个长度>100km的上盘断坪。这种关系说明:STD可能沿着一个先期存在的岩石接触面,或者沿中部地壳近水平的脆性—韧性转换带而发生。虽然喜马拉雅造山带藏南拆离系的上盘都有THS发育,但是至今没有找到THS切断STD下盘的证据。这样使得估算STD的滑动距离非常困难。STD最南端地层或与MCT(即,Zanskar)相交,或者位于MCT前端1～2km的范围内(不丹),这两种可能都暗示MCT与STD在它们向南的上倾(up-dip)方向有可能结合。虽然这种几何特征在现有的模型中几乎被忽略,但对于整个喜马拉雅造山带的变形和剥露历史具有重要的指示作用。     

Geothermobarometry and development of inverted metamorphism in the Darjeeling-Sikkim region of the eastern Himalayan

ABSTRACT The Darjeeling-Sikkim region provides a classic example of inverted Himalayan metamorphism. The different parageneses of pelitic rocks containing chlorite, biotite, garnet, staurolite, kyanite, sillimanite, plagioclase and K-feldspar are documented by a variety of textures resulting from continuous and discontinuous reactions in the different zones. Microprobe data of coexisting minerals show that X_Mg varies in the order: garnet < staurolite < biotite < chlorite. White mica is a solid solution between muscovite and phengite. Garnet is mostly almandine-rich and shows normal growth zoning in the lower part of the Main Central Thrust (MCT) zone, and reverse zoning in the upper part of the zone. Chemographical relations and inferred reactions for different zones are portrayed in AFM space. In the low-grade zones oriented chlorites and micas and rolled garnets grew syntectonically, and were succeeded by cross-cutting chlorites and micas and garnet rims. In the upper zones sillimanite, kyanite and staurolite crystallized during a static inter-kinematic phase. P-T contitions of metamorphism, estimated through different models of geothermobarometry, are estimated to have been 580°c for the garnet zone to a maximum of 770°c for the sillimanite zone. The preferred values of pressure range from 5.0 kbar to 7.7 kbar. Models to explain the inverted metamorphism include overthrusting of a hot high Himalayan slab along a c. 5 km wide ductile MCT zone and the syn- or post-metamorphic folding of isograds.     

Magnetotelluric investigations for imaging electrical structure of Garhwal Himalayan corridor,Uttarakhand, India

Magnetotelluric investigations have been carried out in the Garhwal Himalayan corridor to delineate the electrical structure of the crust along a profile extending from Indo-Gangetic Plain to Higher Himalayan region in Uttarakhand, India. The profile passing through major Himalayan thrusts: Himalayan Frontal Thrust (HFF), Main Boundary Thrust (MBT) and Main Central Thrust (MCT), is nearly perpendicular to the regional geological strike. Data processing and impedance analysis indicate that out of 44 stations MT data recorded, only 27 stations data show in general, the validity of 2D assumption. The average geoelectric strike, N70°W, was estimated for the profile using tensor decomposition. 2D smooth geoelectrical model has been presented, which provides the electrical image of the shallow and deeper crustal structure. The major features of the model are (i) a low resistivity (<50Ωm), shallow feature interpreted as sediments of Siwalik and Indo-Gangetic Plain, (ii) highly resistive (> 1000Ωm) zone below the sediments at a depth of 6 km, interpreted as the top surface of the Indian plate, (iii) a low resistivity (< 10Ωm) below the depth of 6 km near MCT zone coincides with the intense micro-seismic activity in the region. The zone is interpreted as the partial melting or fluid phase at mid crustal depth. Sensitivity test indicates that the major features of the geoelectrical model are relevant and desired by the MT data.