Thermotectonic history of the Chiplakot Crystalline Belt in the Lesser Himalaya,Kumaon, India: Constraints from apatite fission-track thermochronology

Fission-track ages and confined track length distribution of apatite samples separated from the Chiplakot Crystalline Belt (CCB) of the Lesser Himalayan Crystalline (LHC) zone, located to the south of the Main Central Thrust (MCT)/Munsiari Thrust (MT) in Kumaon, India, have been determined. Ages from the CCB along the Kali and Darma valleys fall in two distinct groups. In the northern part of the CCB, the ages range from 9.8 ± 0.6 to 7.6 ± 0.6 Ma with a weighted mean of 9.6 ± 0.1 Ma, while in the southern part the ages vary from 17.9 ± 0.9 to 12.9 ± 1.1 Ma with a weighted mean of 14.1 ± 0.1 Ma. The bimodal distribution of track lengths indicates that the ages are mixed ages, rather than simple cooling ages. The apatite fission-track (AFT) ages and already published structural data of the CCB suggest a complex erosional, denudation history within the upper 3–4 km of the crust of the CCB. The ages further indicate that the CCB was thrust into place earlier than the Middle Miocene i.e. at the time of development of the MCT. Since, then these rocks have remained within the upper 3 km of crust and were affected by only moderate to slow erosion and exhumation. These results have important implications for the tectonic evolution of the LHC zone to the south of the MCT/MT. The exhumation of the LHC zone in different parts of the Himalaya was not uniform. In the Kumaon Himalaya, it was not controlled, as in the Himachal Himalaya, by any major tectonic event, since it was thrust over the Lesser Himalayan Meta-sedimentary (LHMS) zone, and underwent moderate to slow erosion and exhumation.     

Exhumation history of the NW Indian Himalaya revealed by fission track and 40Ar/39Ar ages

New fission track and Ar/Ar geochronological data provide time constraints on the exhumation history of the Himalayan nappes in the Mandi (Beas valley) – Tso Morari transect of the NW Indian Himalaya. Results from this and previous studies suggest that the SW-directed North Himalayan nappes were emplaced by detachment from the underthrusted upper Indian crust by 55 Ma and metamorphosed by ca. 48–40 Ma. The nappe stack was subsequently exhumed to shallow upper crustal depths (<10 km) by 40–30 Ma in the Tso Morari dome (northern section of the transect) and by 30–20 Ma close to frontal thrusts in the Baralacha La region. From the Oligocene to the present, exhumation continued slowly.Metamorphism started in the High Himalayan nappe prior to the Late Oligocene.High temperatures and anatexis of the subducting upper Indian crust engendered the buoyancy-driven ductile detachment and extrusion of the High Himalayan nappe in the zone of continental collision. Late extrusion of the High Himalayan nappe started about 26 Ma ago, accompanied by ductile extensional shearing in the Zanskar shear zone in its roof between 22 and 19 Ma concomitant with thrusting along the basal Main Central Thrust to the south. The northern part of the nappe was then rapidly exhumed to shallow depth (<10 km) between 20 and 6 Ma, while its southern front reached this depth at 10–5 Ma.     

Structural events and metamorphic consequences in Almora Nappe,during Himalayan collision tectonics

Almora Nappe in Uttarakhand, India, is a Lesser Himalayan representative of the Himalayan Metamorphic Belt that was tectonically transported over the Main Central Thrust (MCT) from Higher Himalaya. The Basal Shear zone of Almora Nappe shows complicated structural pattern of polyphase deformation and metamorphism. The rocks exposed along the northern and southern margins of this nappe are highly mylonitized while the degree of mylonitization decreases towards the central part where the rocks eventually grade into unmylonitized metamorphics.Mylonitized rocks near the roof of the Basal Shear zone show dynamic metamorphism (M₂) reaching upto greenschist facies (～450 °C/4 kbar). In the central part of nappe the unmylonitized schists and gneisses are affected by regional metamorphism (M₁) reaching upper amphibolite facies (～4.0–7.9 kbar and ～500–709 °C). Four zones of regional metamorphism progressing from chlorite–biotite to sillimanite–K-feldspar zone demarcated by specific reaction isograds have been identified. These metamorphic zones show a repetition suggesting that the zones are involved in tight F₂ – folding which has affected the metamorphics. South of the Almora town, the regionally metamorphosed rocks have been intruded by Almora Granite (560 ± 20 Ma) resulting in contact metamorphism. The contact metamorphic signatures overprint the regional S₂ foliation. It is inferred that the dominant regional metamorphism in Almora Nappe is highly likely to be of pre-Himalayan (Precambrian!) age.     

Late Quaternary–Holocene evolution of Dun structure and the Himalayan Frontal Fault zone of the Garhwal Sub-Himalaya,NW India

Dun structures are common in the Sub-Himalayan zone of the Himalaya bounded by the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT). They are broad synclinal longitudinal valleys formed as a consequence of the exhumation of the range front of the Himalaya. In the Garhwal Sub-Himalaya, these structures have grown since 0.5 Ma, with the peak activity postdating ∼100 ka. A series of out-of-sequence deformation structures have been identified within the MBT-HFT-bounded Dun structures. They are identified on the basis of geomorphic, post-100 ka stratigraphic, and structural expressions, with activity as young as the early Holocene. To the south of the range front of the Himalaya, uplift has been observed in the Piedmont Zone, with peculiar active tectonic geomorphic expressions. Piedmont sediments of 15–5 ka, determined by Optically Simulated Luminescence (OSL), have been affected by the above uplift. The complete tectonic scenario has been analyzed and an attempt has been made to delineate the sequential evolution of these structures during the post-100 ka period (Late Quaternary–Holocene) in the Himalayan range front.     

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.     

The longest voyage: Tectonic,magmatic, and paleoclimatic evolution of the Indian plate during its northward flight from Gondwana to Asia

The tectonic evolution of the Indian plate, which started in Late Jurassic about 167 million years ago (~ 167 Ma) with the breakup of Gondwana, presents an exceptional and intricate case history against which a variety of plate tectonic events such as: continental breakup, sea-floor spreading, birth of new oceans, flood basalt volcanism, hotspot tracks, transform faults, subduction, obduction, continental collision, accretion, and mountain building can be investigated. Plate tectonic maps are presented here illustrating the repeated rifting of the Indian plate from surrounding Gondwana continents, its northward migration, and its collision first with the Kohistan–Ladakh Arc at the Indus Suture Zone, and then with Tibet at the Shyok–Tsangpo Suture. The associations between flood basalts and the recurrent separation of the Indian plate from Gondwana are assessed. The breakup of India from Gondwana and the opening of the Indian Ocean is thought to have been caused by plate tectonic forces (i.e., slab pull emanating from the subduction of the Tethyan ocean floor beneath Eurasia) which were localized along zones of weakness caused by mantle plumes (Bouvet, Marion, Kerguelen, and Reunion plumes). The sequential spreading of the Southwest Indian Ridge/Davie Ridge, Southeast Indian Ridge, Central Indian Ridge, Palitana Ridge, and Carlsberg Ridge in the Indian Ocean were responsible for the fragmentation of the Indian plate during the Late Jurassic and Cretaceous times. The Réunion and the Kerguelen plumes left two spectacular hotspot tracks on either side of the Indian plate. With the breakup of Gondwana, India remained isolated as an island continent, but reestablished its biotic links with Africa during the Late Cretaceous during its collision with the Kohistan–Ladakh Arc (~ 85 Ma) along the Indus Suture. Soon after the Deccan eruption, India drifted northward as an island continent by rapid motion carrying Gondwana biota, about 20 cm/year, between 67 Ma to 50 Ma; it slowed down dramatically to 5 cm/year during its collision with Asia in Early Eocene (~ 50 Ma). A northern corridor was established between India and Asia soon after the collision allowing faunal interchange. This is reflected by mixed Gondwana and Eurasian elements in the fossil record preserved in several continental Eocene formations of India. A revised India–Asia collision model suggests that the Indus Suture represents the obduction zone between India and the Kohistan–Ladakh Arc, whereas the Shyok-Suture represents the collision between the Kohistan–Ladakh arc and Tibet. Eventually, the Indus–Tsangpo Zone became the locus of the final India–Asia collision, which probably began in Early Eocene (~ 50 Ma) with the closure of Neotethys Ocean. The post-collisional tectonics for the last 50 million years is best expressed in the evolution of the Himalaya–Tibetan orogen. The great thickness of crust beneath Tibet and Himalaya and a series of north vergent thrust zones in the Himalaya and the south-vergent subduction zones in Tibetan Plateau suggest the progressive convergence between India and Asia of about 2500 km since the time of collision. In the early Eohimalayan phase (~ 50 to 25 Ma) of Himalayan orogeny (Middle Eocene–Late Oligocene), thick sediments on the leading edge of the Indian plate were squeezed, folded, and faulted to form the Tethyan Himalaya. With continuing convergence of India, the architecture of the Himalayan–Tibetan orogen is dominated by deformational structures developed in the Neogene Period during the Neohimalayan phase (~ 21 Ma to present), creating a series of north-vergent thrust belt systems such as the Main Central Thrust, the Main Boundary Thrust, and the Main Frontal Thrust to accommodate crustal shortening. Neogene molassic sediment shed from the rise of the Himalaya was deposited in a nearly continuous foreland trough in the Siwalik Group containing rich vertebrate assemblages. Tomographic imaging of the India–Asia orogen reveals that Indian lithospheric slab has been subducted subhorizontally beneath the entire Tibetan Plateau that has played a key role in the uplift of the Tibetan Plateau. The low-viscosity channel flow in response to topographic loading of Tibet provides a mechanism to explain the Himalayan–Tibetan orogen. From the start of its voyage in Southern Hemisphere, to its final impact with the Asia, the Indian plate has experienced changes in climatic conditions both short-term and long-term. We present a series of paleoclimatic maps illustrating the temperature and precipitation conditions based on estimates of Fast Ocean Atmospheric Model (FOAM), a coupled global climate model. The uplift of the Himalaya–Tibetan Plateau above the snow line created two most important global climate phenomena—the birth of the Asian monsoon and the onset of Pleistocene glaciation. As the mountains rose, and the monsoon rains intensified, increasing erosional sediments from the Himalaya were carried down by the Ganga River in the east and the Indus River in the west, and were deposited in two great deep-sea fans, the Bengal and the Indus. Vertebrate fossils provide additional resolution for the timing of three crucial tectonic events: India–KL Arc collision during the Late Cretaceous, India–Asia collision during the Early Eocene, and the rise of the Himalaya during the Early Miocene.     

Isotopic and structural constraints on the late Miocene to Pliocene evolution of the Namche Barwa area,eastern Himalayan syntaxis,SE Tibet

Within the Namche Barwa area, SE Tibet, the Indus–Yarlung suture zone separates the Lhasa terrain in the north from the Himalayan unit including the Tethyan (sedimentary and volcanic rocks), Dongjiu (greenschist to lower amphibolite facies), Namche Barwa (granulite facies), Pei (amphibolite facies) and Laiguo (greenschist facies) sequences in the south. Two fault systems were distinguished in the Namche Barwa area. The former includes a top-down-to-the-north normal fault in the north and two top-to-the-south thrust zones in the south named as Upper and Lower Thrusts, respectively. The Namche Barwa and Pei sequences were exhumed southwards from beneath the Dongjiu sequence by these faults. Thus, the fault system is regarded as a southward extrusion structure. Subsequently, the exposed Dongjiu, Namche Barwa, Pei and Laiguo sequences were displaced northwards onto the Lhasa terrain by the top-to-the-north fault system, thus, marking it as northward indentation structure. Monazite TIMS U–Pb dating demonstrates that the normal fault and the Lower Thrust from the southward extrusion system were probably active at ~ 6 Ma and ~ 10 Ma, respectively. Zircon U–Pb SHRIMP and phlogopite K–Ar ages further suggest that the Upper Thrust was active between 6.2 ± 0.2 Ma and 5.5 ± 0.2 Ma. The northward indentation structures within the core portion of the eastern Himalayan syntaxis were perhaps active between 3.0 Ma and 1.5 Ma, as inferred by published zircon U–Pb SHRIMP and hornblende Ar–Ar ages. The monazite from upper portions of the Pei sequence dated by U–Pb TIMS indicates that the precursor sediments of this sequence were derived from Proterozoic source regions. Nd isotopic data further suggest that all the metamorphic rocks within eastern Himalaya (εNd = ? 13 to ? 19) correlate closely with those from the Greater Himalayan Sequences, whereas the western Himalayan syntaxis is mainly comprised of Lesser Himalayan Sequences. The two indented corners of the Himalaya are, thus, different.     

Isotopic ages for alkaline igneous rocks,including a 26 Ma ignimbrite,from the Peshawar plain of northern Pakistan and their tectonic implications

New isotopic ages on zircons from rocks of the Peshawar Plain Alkaline Igneous Province (PPAIP) reveal for the first time the occurrence of ignimbritic Cenozoic (Oligocene) volcanism in the Himalaya at 26.7 ± 0.8 Ma. Other new ages confirm that PPAIP rift-related igneous activity was Permian and lasted from ∼290 Ma to ∼250 Ma. Although PPAIP rocks are petrologically and geochemically typical of rifts and have been suggested to be linked to rifting on the Pangea continental margin at the initiation of the Neotethys Ocean, there are no documented rift-related structures mapped in Permian rocks of the Peshawar Plain. We suggest that Permian rift-related structures have been dismembered and/or reactivated during shortening associated with India–Asia collision. Shortening in the area between the Main Mantle Thrust (MMT) and the Main Boundary Thrust (MBT) may be indicative of the subsurface northern extension of the Salt Range evaporites. Late Cenozoic sedimentary rocks of the Peshawar Plain deposited during and after Himalayan thrusting occupy a piggy-back basin on top of the thrust belt. Those sedimentary rocks have buried surviving evidence of Permian rift-related structures. Igneous rocks of the PPAIP have been both metamorphosed and deformed during the Himalayan collision and Cenozoic igneous activity, apart from the newly recognized Gohati volcanism, has involved only the intrusion of small cross-cutting granitic bodies concentrated in areas such as Malakand that are close to the MMT. Measurements on Chingalai Gneiss zircons have confirmed the occurrence of 816 ± 70 Ma aged rocks in the Precambrian basement of the Peshawar Plain that are comparable in age to rocks in the Malani igneous province of the Rajasthan platform ∼1000 km to the south.     

Neotectonic fault analysis by 2D finite element modeling for studying the Himalayan fold-and-thrust belt in Nepal

This paper examines the neotectonic stress field and faulting in the fold-and-thrust belt of the Nepal Himalaya using the 2D finite element technique, incorporating elastic material behavior under plane strain conditions. Three structural cross-sections (eastern, central and western Nepal), where the Main Himalayan Thrust (MHT) has different geometries, are used for the simulation, because each profile is characterized by different seismicity and neotectonic deformation. A series of numerical models are presented in order to understand the influence of a mid-crustal ramp on the stress field and on neotectonic faulting. Results show that compressive and tensional stress fields are induced to the north and south of the mid-crustal ramp, and consequently normal faults are developed in the thrust sheets moving on the mid-crustal ramp. Since the shear stress accumulation along the northern flat of the MHT is entirely caused by the mid-crustal ramp, this suggests that, as in the past, the MHT will be reactivated in a future large (M_w > 8) earthquake. The simulated fault pattern explains the occurrence of several active faults in the Nepal Himalaya. In all models, the distribution of the horizontal σ₁ (maximum principal stress) is consistent with the sequence of thrusting observed in the fold-and-thrust belt of the Himalaya. Failure elements around the flat–ramp–flat coincide with the microseismic events in the area, which are believed to release elastic stress partly during interseismic periods.     

Mapping crustal structure beneath southern Tibet: Seismic evidence for continental crustal underthrusting

Receiver function imaging along a temporary seismic array (ANTILOPE-2) reveals detailed information of the underthrusting of the Indian crust in southern Tibet. The Moho dips northward from ~ 50 km to 80 km beneath the Himalaya terrane, and locally reaches ~ 85 km beneath the Indus–Yalung suture. It remains at ~ 80 km depth across the Lhasa terrane, and shallows to ~ 70 km depth under the Qiangtang terrane. An intra-crustal interface at ~ 60 km beneath the Lhasa terrane can be clearly followed southward through the Main Himalaya Thrust and connects the Main Boundary Thrust at the surface, which represents the border of the Indian crust that is underthrusting until south of the Bangong–Nujiang Suture. A mid-crustal low velocity zone is observed at depths of 14–30 km beneath the Lhasa and Himalaya terranes probably formed by partial melt and/or aqueous fluids.     

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.     

Detrital zircon U–Pb geochronology,trace-element and Hf isotope geochemistry of the metasedimentary rocks in the Eastern Himalayan syntaxis: Tectonic and paleogeographic implications

The origin of the Greater Himalayan Sequence in the Himalaya and the paleogeographic position of the Lhasa terrane within Gondwanaland remain controversial. In the Eastern Himalayan syntaxis, the basement complexes of the northeastern Indian plate (Namche Barwa Complex) and the South Lhasa terrane (Nyingchi Complex) can be studied to explore these issues. Detrital zircons from the metasedimentary rocks in the Namche Barwa Complex and Nyingchi Complex yield similar U–Pb age spectra, with major age populations of 1.00–1.20 Ga, 1.30–1.45 Ga, 1.50–1.65 Ga and 1.70–1.80 Ga. The maximum depositional ages for their sedimentary protoliths are ~ 1.0 Ga based on the mean ages of the youngest three detrital zircons. Their minimum depositional ages are ~ 477 Ma for the Namche Barwa Complex and ~ 499 Ma for the Nyingchi Complex. Detrital zircons from the Namche Barwa Complex and Nyingchi Complex also display similar trace-element signatures and Hf isotopic composition, indicating that they were derived from common provenance. The trace-element signatures of 1.30–1.45 Ga detrital zircons indicate that the 1.3–1.5 Ga alkalic and mafic rocks belt in the southeastern India is a potential provenance. Most 1.50–1.65 Ga zircons have positive ε_Hf(t) values (+ 1.2 to + 9.0), and most 1.70–1.80 Ga zircons have negative ε_Hf(t) values (− 7.1 to − 1.9), which are compatible with those of the Paleo- to Mesoproterozoic orthogneisses in the Namche Barwa Complex. Provenance analysis indicates that the southern Indian Shield, South Lhasa terrane and probably Eastern Antarctica were the potential detrital sources. Combined with previous studies, our results suggest that: (1) the Namche Barwa Complex is the northeastern extension of the Greater Himalaya Sequence; (2) the metasedimentary rocks in the Namche Barwa Complex represent distal deposits of the northern Indian margin relative to the Lesser Himalaya; (3) the South Lhasa terrane was tectonically linked to northern India before the Cambrian.     

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.     

The youngest eclogite in central Himalaya: P–T path,U–Pb zircon age and its tectonic implication

Relict omphacite inclusions have been discovered in mafic granulite at Dinggye of China, confirming the existence of eclogite in central Himalayan orogenic belt. Detailed petrological studies show that relict omphacite occur as inclusions in both garnets and zircons, and the peak mineral assemblage of eclogite-facies should be garnet, omphacite, rutile, muscovite and quartz which was strongly overprinted by granulite-facies minerals during the exhumation. Phase equilibria modeling and associated geothermometer predict that the minimum P–T conditions for peak eclogite-facies stage are 720–760 °C and 20–21 kbar, and those of overprinted granulite-facies are 750 °C and 7–9 kbar in water-undersaturated condition. Thus, a near isothermal decompression P–T path for central Himalayan eclogite has been obtained. Zircon SHRIMP U–Pb dating of two studied eclogite samples at Dinggye yields the peak metamorphic ages of 13.9 ± 1.2 Ma and 14.9 ± 0.7 Ma, respectively, which indicates that the Dinggye eclogite should be the youngest eclogite in Himalayan orogenic belt. Geochemical characteristics and zircon analyses show that the protoliths of eclogite in Dinggye are predicted to be continental rift-related basaltic rocks. The eclogite at Dinggye in central Himalaya should be formed by the crustal thickening during the long-lasting continental overthrusting by Indian plate beneath Euro-Asian continent, and its exhumation process may be related with channel flow and orogen-parallel extension. In the middle Miocene (~ 14 Ma), Indian continental crust had reached at least ~ 65 km depth in southern Tibet.     

A new kinematic evolutionary model for the growth of a duplex — an example from the Rangit duplex,Sikkim Himalaya,India

The Lesser Himalayan duplex (LHD) is a prominent structure through much of the Lesser Himalayan fold–thrust belt. In the Darjeeling - Sikkim Himalaya a component of the LHD is exposed in the Rangit window as the Rangit duplex (RD). The RD consists of ten horses of the upper Lesser Himalayan Sequence (Gondwana, Buxa, Upper Daling). The duplex varies from hinterland-dipping in the north, through an antiformal stack in the middle to foreland-dipping in the south. The Ramgarh thrust (RT) is the roof thrust and, based on a balanced cross-section, the Main Himalayan Sole thrust is the floor thrust at a depth of ~ 10 km and with a dip of ~ 3.5° N.Retrodeformation suggests that the RD initiated as a foreland-dipping duplex with the Early Ramgarh thrust as the roof thrust and the RT as the floor thrust. The RT became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent RT reactivation. This kinematic history best explains the large translation of the overlying MCT sheets. The restoration suggests that RD shortening is ~ 125 km, and the original Gondwana basin extended ~ 142 km northward of its present northernmost exposures within the window.     

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.     

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 .…     

Crustal shear-wave velocity structure beneath northeast India from teleseismic receiver function analysis

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong–Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show ∼34–38 km thick crust beneath the Shillong Plateau which increases to ∼37–38 km beneath the Brahmaputra valley and ∼46–48 km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2–8 km along with a low velocity zone within ∼8–13 km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range ∼10–18 km and low velocity zone within ∼18–36 km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 30⁰ toward northwest.     

Early Tertiary deformation of the Zhongba–Gyangze Thrust in central southern Tibet

As the boundary thrust between India and Asia in southern Tibet, the Zhongba–Gyangze Thrust (ZGT) emplaced the Yarlung Zangbo Suture Zone (YZSZ) units in the hanging wall southward onto Tethyan Himalaya sequences (THS) of the northern Indian continental margin in the footwall. Detailed field investigation, electron backscatter diffraction (EBSD) analysis, detrital zircon U–Pb geochronology and ⁴⁰Ar–³⁹Ar thermochronology were conducted to understand the evolution of the ZGT in Sangsang area, central southern Tibet. The shear zone of ZGT is located within the sedimentary-matrix mélange of YZSZ that is mainly composed of matrix of blueschist with meta pelagic–hemipelagic siliceous and siliciclastic rocks and blocks of basalt, limestone and sandstone. Penetrative F1 foliation and kink band structure were recorded within the matrix both on outcrop and under microscope. Strong lattice preferred orientation (LPO) fabric initiated by the low-temperature (350−450 °C) (010)[001] slip system was detected by EBSD in the sodic amphiboles of the blueschist. The ⁴⁰Ar–³⁹Ar ages of the phengites from blueschist and sericites from the phyllite in the shear zone indicate that the activity of ZGT occurred between 71 and 60 Ma. In the THS, a newly documented younger unit preserving detrital zircons from the southern Asian margin lies above the Triassic–Cretaceous sequences that carries only detrital zircons from the Indian continent. This unit is dated to be ~ 61 Ma by the detrital zircon ages, similar to the Sangdanlin Formation and representing the Yarlung Zangbo Foreland Basin (YZFB) system. The ZGT had probably been active due to the initial India–Asia collision and acted as the frontal thrust controlling the development of YZFB.     

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.