North Himalayan Research Articles

The Eastern prolongation of the Zanskar Shear Zone (Western Himalaya)

This paper aims to describe the possible SE extension of the Zanskar Shear Zone and its relation with other extensional and compressional structures. New structural and metamorphic data were collected in the Baralacha La, Yunam, Lingti region and a new geologic map of the studied area is proposed. The new data reveal that an E-verging syncline (the Kenlung Serai fold), formerly interpreted as a late backfold is, in fact, related to an early northward underthrusting of India below Asia before the main NE movement related to nappes emplacement. This succession of movements is coherent with the displacement of India postulated by several authors from paleomagnetic data. The well defined Zanskar Shear Zone can be followed towards the SE in a zone of low angle normal faults (the Tapachan normal fault zone) located at the front of the North Himalayan nappes. The change in style from a ductile shear zone to a set of low angle normal faults is related to a change of tectonic level and metamorphic grade. The location of these extensional structures at the front of the North Himalayan nappes as well as their tectonometamorphic history is totally similar to what is known farther NW for the Zanskar Shear Zone. High angle normal faults such as the Sarchu faults are late structures related to vertical extrusion accommodated by doming. Late NE-verging backfolds develop mainly in the hanging wall of the high angle normal faults and are therefore associated with the doming event. The new data are incorporated into a kinematic model for the “Crystalline nappe”.

Crustal channel flows: 1. Numerical models with applications to the tectonics of the Himalayan‐Tibetan orogen

Plane strain, thermal‐mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens. In the models, radioactive self‐heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large‐scale horizontal flow. Channels can be exhumed and exposed by denudation focused on the high‐relief transition between plateau and foreland. We interpret the Himalaya to have evolved in this manner. Channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws, and well developed if there is an additional reduction in viscosity to 1019 Pa s. This reduction occurs from 700°C to 750°C in the models and is attributed to a small percentage of in situ partial melt (“melt weakening”). Model HT1 provides an internally consistent explanation for the tectonic evolution of many features of the Himalayan‐Tibetan orogenic system. Erosional exhumation exposes the migmatitic channel, equivalent to the Greater Himalayan Sequence (GHS), between coeval normal and thrust sense ductile shear zones, corresponding to the South Tibetan Detachment and the Main Central Thrust systems. Outward flow of unstable upper crust rotates these shears to low dip angles. In the model both the GHS and the Lesser Himalayan Sequence are derived from Indian crust, with the latter from much farther south. Similar models exhibit a range of tectonic styles, including the formation of domes resembling north Himalayan gneiss domes. Model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau.

U-Pb ages of Kude and Sajia leucogranites in Sajia dome from North Himalaya and their geological implications

U-Pb ages of zircon, monazite and xenotime from the Kude and the Sajia leucogranites in the Sajia dome of the North Himalayan tectonic unit are measured using TIMS method. The results show that the magma emplacement ages for the Kude and the Sajia leucogranite are 27.5 f 0.5 and 14.4±0.2 Ma, respectively. Combined with published U-Pb data from the North Himalayan leucogranites, the time-span for the formation of the North Himalayan leucogranites is updated from 27.5 to 10 Ma, instead of previous 15 to 10 Ma. According to the U-Pb ages, the petrogenesis for the North Himalayan leucogranites can be constrained. It is suggested that the North Himalayan leucogranites have variety in their petrogenesis.

Exhumation history of eastern Ladakh revealed by 40 Ar/ 39 Ar and fission-track ages: the Indus River–Tso Morari transect, NW Himalaya

Fission-track and 40 Ar/ 39 Ar ages place time constraints on the exhumation of the North Himalayan nappe stack, the Indus Suture Zone and Molasse, and the Transhimalayan Batholith in eastern Ladakh (NW India). Results from this and previous studies on a north–south transect passing near Tso Morari Lake suggest that the SW-directed North Himalayan nappe stack (comprising the Mata, Tetraogal and Tso Morari nappes) was emplaced and metamorphosed by c . 50–45 Ma, and exhumed to moderately shallow depths ( c . 10 km) by c . 45–40 Ma. From the mid-Eocene to the present, exhumation continued at a steady and slow rate except for the root zone of the Tso Morari nappe, which cooled faster than the rest of the nappe stack. Rapid cooling occurred at c . 20 Ma and is linked to brittle deformation along the normal Ribil–Zildat Fault concomitant with extrusion of the Crystalline nappe in the south. Data from the Indus Molasse suggest that sediments were still being deposited during the Miocene.

Glaucophane and barroisite eclogites from the Upper Kaghan nappe: implications for the metamorphic history of the NW Himalaya

Abstract This paper presents the results of a petrographical and mineral chemical study of glaucophane- and barroisite-bearing eclogites from the Upper Kaghan nappe in the Higher Himalayan Crystallines of the Pakistan Himalaya, and discusses the implications of the P-T path recorded in such rocks for the tectonic and metamorphic history of the NW Himalaya. The eclogites described here come from a previously undescribed outcrop at Gittidas, but belong to the same suite as the garnet-omphacite-phengite-quartz-rutile eclogites previously described elsewhere in the Upper Kaghan nappe. The metamorphic peak assemblage is garnet-omphacite-rutile-quartz in glaucophane eclogite and garnet-omphacite-zoisite-rutile±kyanite±phengite±ankerite in barroisite eclogite. Most samples contain a significant amount of amphibole, white mica and quartz. White mica may be present either as part of the peak assemblage (phengite) or as a retrogressive phase after kyanite (paragonite). Amphibole is later than the metamorphic peak assemblage and is barroisite in most samples, but in relatively Fe-rich eclogite it is glaucophane with significant Na in the A site, Ca in the M4 site and tetrahedral A1. Garnet displays strong prograde zoning in the barroisite eclogites, with Mg increasing and Fe decreasing from core to rim. The iron-rich core is crowded with mineral inclusions of the peak assemblage, but inclusions of earlier paragonite, green and blue-green amphibole were also found. Peak metamorphic conditions in the barroisite eclogites have been estimated at T = 610 ± 30 °C and P = 24 ± 2 kbar from Fe/Mg partition in garnet-omphacite pairs, and from the garnet-omphacite-phengite barometer. These values are close to the equilibration conditions estimated for the eclogites of the North Himalayan Tso-Morari Dome. Strong similarities between metamorphic evolution of the Upper Kaghan nappe and metamorphic evolution of the eclogite-bearing units of the Neelum valley just to the east of the Kaghan valley indicate that the eclogite occurrences of the Kaghan-Neelum area define an eclogite-bearing terrane of regional extent, which was subjected to high pressure metamorphism in middle Eocene times. A comparison of the metamorphic evolution recorded in the eclogites of the NW Himalaya with that of the granulitized eclogites recently discovered in the E Himalaya suggests the possibility of a Himalaya-wide eclogitic metamorphism of pre-Miocene age. Therefore, the main difference between the Higher Himalayan Crystalline nappes of the NW Himalaya and those of the E Himalaya appears to lie less in the early part of their metamorphic evolution than in the different P-T paths they followed during exhumation.

Weathering processes in the Ganges–Brahmaputra basin and the riverine alkalinity budget

We present river chemistry data for a network of rivers draining the western and central Nepal Himalaya. Our sampling locations cover the system from the sources of rivers in Tibet to the Gangetic plain. Water samples were collected throughout the year, including the monsoon season, for rivers in Nepal and for the Ganges and Brahmaputra in Bangladesh. Rivers draining the North Himalaya are characterized by low discharge under a cold and arid climate. Main stream waters have δ13CDIC near 0‰ and high [SO42−]/([SO42−]+[HCO3−]) ratios (XSO4) with values around 40 Eq% and high [Cl−]. Ca is the dominant cation (Ca2+/∑cations=55 to 75 Eq%, after correction of sodium by chloride). Dissolved sulfate is produced in waters from the Tethian Sedimentary Series whereas chlorine is related to thermal waters. Dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution. δ13CDIC values of up to 3.9‰ show that metamorphic CO2 is an important weathering agent. Rivers of the North Himalayan basins have about 50% of their alkalinity derived from carbonate dissolution, 20% from biogenic activity and 30% from metamorphic CO2. On the south flank of the Himalaya, rivers are more depleted in 13C and have, on average, lower XSO4. Most rivers have δ13CDIC and XSO4 values compatible with a simple mixing between soil CO2 and sulfate derived from sulfide oxidation. During the monsoon, discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced. XSO4 and δ13C decrease while Ca/∑cations increases, implying enhanced dissolution of pedogenic calcite. As a whole, the G–B riverine flux of alkalinity derived from silicate weathering is around 2.7×1011 mol/year, modest at the global scale. Sulphuric acid controls 20–30% of the weathering reaction in the Brahmaputra and 6 to 9% in the Ganges. Na+ and K+ balance 60 to 65% of the silicate-derived alkalinity flux, and the long term CO2 consumption by Ganges and Brahmaputra is near 6.4×1010 mol/year. The flux of metamorphic CO2 converted to alkalinity via weathering reactions is ca. 1×1010 mol/year.

Tertiary deformation history of southeastern and southwestern Tibet during the Indo-Asian collision

Geologic mapping and geochronological analysis in southwest (Kailas area) and southeast (Zedong area) Tibet reveal two major episodes of Tertiary crustal shortening along the classic Indus-Tsangpo suture in the Yalu River valley. The older event occurred between ca. 30 and 24 Ma during movement along the north-dipping Gangdese thrust. The development of this thrust caused extensive denudation of the Gangdese batholith in its hanging wall and underthrusting of the Xigaze forearc strata in its footwall. Examination of timing of major tectonic events in central Asia suggests that the initiation of the Gangdese thrust was approximately coeval with the late Oligocene initiation and development of north-south shortening in the eastern Kunlun Shan of northern Tibet, the Nan Shan at the northeastern end of the Altyn Tagh fault, the western Kunlun Shan at the southwestern end of the Altyn Tagh fault, and finally the Tian Shan (north of the Tarim basin). Such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the Eurasian plate and the Indian and Pacific plates. The younger thrusting event along the Yalu River valley occurred between 19 and 10 Ma along the south-dipping Great Counter thrust system, equivalent to the locally named Renbu-Zedong thrust in southeastern Tibet, the Backthrust system in south- central Tibet, and the South Kailas thrust in southwest Tibet. The coeval development of the Great Counter thrust and the North Himalayan granite-gneiss dome belt is consistent with their development being related to thermal weakening of the north Himalayan and south Tibetan crust, due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between India and Asia or frictional heating along major thrusts, such as the Main Central thrust, beneath the Himalaya.

A model for the origin of Himalayan anatexis and inverted metamorphism

The origin of the paired granite belts and inverted metamorphic sequences of the Himalaya has generally been ascribed to development of the Main Central Thrust (MCT). Although a variety of models have been proposed that link early Miocene anatexis with inverted metamorphism, recent dating studies indicate that recrystallization of elements of the MCT footwall occurred in the central Himalaya as recently as ∼6 Ma. The recognition that hanging wall magmatism and footwall metamorphism are not spatially and temporally related renders unnecessary the need for exceptional physical conditions to explain generation of the High Himalayan leucogranites and North Himalayan granites, which differ in age, petrogenesis, and emplacement style. We suggest that their origin is linked to shear heating on a continuously active thrust that cuts through Indian supracrustal rocks that had previously experienced low degrees of partial melting. Numerical simulations assuming a shear stress of 30 MPa indicate that continuous slip on the Himalayan decollement beginning at 25 Ma could trigger partial melting reactions leading to formation of the High Himalayan granite chain between 25 and 20 Ma and the North Himalayan belt between 17 and 8 Ma. The ramp‐flat geometry we apply to model the Himalayan thrust system requires that the presently exposed rocks of the hanging wall resided at middle crustal levels above the decollement throughout the early and middle Miocene. Late Miocene, out‐of‐sequence thrusting within the broad shear zone beneath the MCT provides a mechanism to bring these rocks to the surface in their present location (i.e., well to the north of the present tectonic front) and has the additional benefit of explaining how the inverted metamorphic sequences formed beneath the MCT. We envision that formation of the MCT Zone involved successive accretion of tectonic slivers of the Lesser Himalayan Formations to the hanging wall and incorporate these effects into the model. The model predicts continued anatexis up to 400 km north of the Himalayan range, consistent with the timing and geochemistry of leucogranites exhumed on the flank of a south Tibetan rift.

Crustal structure of the Himalayan orogen at ∼90° east longitude from Project INDEPTH deep reflection profiles

International Deep Profiling of Tibet and the Himalaya (INDEPTH) deep seismic reflection profiles show that the Indian lithosphere is underthrusting the central Himalaya along a gently north dipping decollement that is traceable northward beneath the Tethyan belt to ∼28.6°N and to a depth of about 45 km. The decollement carries in its hanging wall a near‐crustal‐thickness slice of internally deformed Indian continental basement and cover represented in outcrop by the Greater Himalayan belt and structurally overlying Tethyan belt. Geometric relationships suggest that the decollement probably ramps downward beneath the northern Tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from Indian mantle lithosphere that presently underlies southern Tibet to the north. The North Himalayan anticlinorium (Kangmar dome) appears to be a large duplex ramp anticline in the hanging wall of the decollement. The South Tibetan Detachment appears to have been imaged in two areas. In the Wagye La area it appears to follow the Tethyan belt/Greater Himalayan belt contact northward in the subsurface and to have been folded over the Kangmar dome. In the Zherger La area it appears to have been active relatively recently as a ∼30° north dipping normal fault that cuts deeply into the Greater Himalayan belt allochthon. Palinspastic reconstruction indicates a minimum of about 326 km of shortening of the Indian basement across the Himalaya since the initiation of the Main Central Thrust. Hence Indian continental lithosphere, largely stripped of its overlying crust, can extend northward beneath the Tibetan plateau to at least 32°N, where earthquake seismological observations show that the properties of the upper mantle change markedly [e.g., McNamara et al., 1995]. Taken together, the seismic reflection and earthquake seismological observations lend support for the view that the Indian mantle lid mechanically underplates roughly the southern half of the Tibetan plateau. The INDEPTH reflection data do not show that Indian crust attached to this lithosphere extends north of southernmost Tibet (Kangmar dome), nor do they negate this possibility. Similarly, the reflection data do not yield direct evidence for or against lower‐crustal subduction beneath the Himalaya.

Distribution of Physical Erosion in Himalaya from River Particle Geochemistry

The Himalayan range is exposed to a large variety of erosion processes such as glaciation, landsliding and soil development under varying climatic regimes. While it is clear from river profile geometry that intense physical erosion is concentrated on the south flank of the high range (Seeber and Goemitz, 1983) the proportion of erosion between the different zones (North Himalaya, High Himalaya, Lesser Himalaya) remains largely unknown, as do the importance of the different erosion processses. The study of eroded material in river loads or sedimentary basins is one way to adress these problems. The overall composition of the different Himalayan formations are similar, primarily variably metamorphosed mixed clastic and carbonate strata. Lithology does not provide diagnostic test for the origin of river particles. In contrast several geochemical tracers have distinct values in different formations which acan be used to decipher the origin of detrital particles. We present a geochemical study of river bedload and suspended load in rivers from the Narayani watershed in Central Nepal Himalaya and Ganges and Brahmaputra in Bangladesh. The geochemical characteristics of the main geological formations are mostly known from previous geochemical studies. Small watersheds draining single geological formations have been sampled to improve our understanding of the end members. Main rivers in Nepal have been sampled from their source in Tibet down to the floodplain. For bedload samples, analyses have been caried out on whole decarbonated bedloads, leachable fractions, and <2~tm clay fractions. Analyses includes major and trace elements, Sr, Nd and O isotopic compositions.

Le bassin Stephanien de St-Etienne ne résulte pas d'une extension tardi-hercynienne généralisée: c'est un bassin pull-apart en relation avec un décrochement dextre

The St-Etienne area (Stephanian coal-basin and Mt Pilat metamorphic massif) has been taken as one of the examples of late orogenic extension in the Variscan belt. The key structure was the Mt Pilat ductile normal fault which was considered as the southern boundary of the St-Etienne coal basin. We show with precise field observations and taking into account the old subsurface data obtained from coal exploitation that this structural interpretation must be modified. The Stephanian coal-basin is a pull apart basin on a NE-SW strike-slip fault, which is distinct from the Mt Pilat detachment. We compare the latter to the North Himalayan normal fault and we discuss the evolution of this part of the Hercynian belt from the Monts du Lyonnais to the Cévennes al lithospheric scale.

Glaucophane-bearing eclogites in the Tso Morari dome (eastern Ladakh, NW Himalaya)

Lenses of glaucophane-bearing eclogites have been discovered within the Cambro-Ordovician metagranitic basement and in the Permian metasedimentary cover of the Tso Morari dome (eastern Ladakh, India). Petrological and thermobarometrical evidence shows that these rocks underwent a high-pressure event (P> 16 kbar, probably near 20±3 kbar) associated with relatively low-temperature conditions (T = 580±60 °C). The beginning of decompression was isothermal down to l 1 ± 2 kbar. Subsequently, temperature increased to 610±70°C under amphibolite-facies conditions. This record of high-pressure and relatively low-temperature metamorphism in the Himalayan belt implies that an early subduction of the whole NW part of the Indian continental margin to a minimum depth of 70+10 km occurred during the India-Asia convergence. Two different units can be distinguished in the Higher Himalaya of Ladakh-Zanskar, recording two successive and distinct orogenic events: (i) the North Himalayan Massif related to the continental subduction and (ii) the High Himalayan Crystalline slab related to the intracontinental subduction.

New insights into the origin of two contrasting Himalayan granite belts

Research Article| October 01, 1997 New insights into the origin of two contrasting Himalayan granite belts T. Mark Harrison; T. Mark Harrison 1Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095 Search for other works by this author on: GSW Google Scholar Oscar M. Lovera; Oscar M. Lovera 1Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095 Search for other works by this author on: GSW Google Scholar Marty Grove Marty Grove 1Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095 Search for other works by this author on: GSW Google Scholar Geology (1997) 25 (10): 899–902. https://doi.org/10.1130/0091-7613(1997)025<0899:NIITOO>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation T. Mark Harrison, Oscar M. Lovera, Marty Grove; New insights into the origin of two contrasting Himalayan granite belts. Geology 1997;; 25 (10): 899–902. doi: https://doi.org/10.1130/0091-7613(1997)025<0899:NIITOO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The two parallel belts of Miocene granite that extend along much of the Himalaya differ in age, petrogenesis, and emplacement style. We suggest that their origin is linked to shear heating on a continuously active decollement that cuts through previously metamorphosed Indian supracrustal rocks that were transformed into basement during the initial stages of the Indian-Asian collision. Numerical simulations assuming a shear stress of 30 MPa indicate that initiation of slip on the Himalayan thrust at 24 Ma could trigger discontinuous melting reactions leading to formation of the High Himalayan granite chain from 24 to 20 Ma and the North Himalayan belt from 18 to 12 Ma. This model result agrees well with observation as do model predictions regarding emplacement style and location. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Travel time tomography inversion and the deep structure of the southern Tibet

2312 P and some S arrival times were picked from 80 earthquakes recorded by 51 seismographs deployed on a profile that followed the southern Tibetan highway during 1992 Sino-French seismic experiment. We analysed the azimuthal and distance variations of P, PKP residuals and S-P travel time residuals along the profile and their tectonic implication, and finally, we applied the "ACH" inversion scheme to calculate the velocity perturbation in the crust and upper mantle. The initial results from our study are: (1) the upper mantle velocity beneath Tibetan Plateau is lower than that of normal continents, the lowest mantle velocity is found beneath QiangTang block. The early PKP residuals in the north (north to Sangxiong) of the profile indicates a shallower Moho depth. (2) The abrupt increase of S-P travel time residuals northward from Indus-Tsangpo Suture (ITS) and the low velocity crust feature in this area from tomography inversion evidenced the existence of partial melting within the crust. (3) Karakorum Jiali Fault Zones (KJFZ) may be the southern branch of Banggong-Nujiang Suture (BNS), the southern boundary of Qiangtang block. (4) The high velocity anomaly beneath north Himalaya revealed by tomography inversion may represents the the front of the underthrusting Indian lithosphere. The lower velocity anomaly beneath the center of the profile may reflect the physical state change related to high temperature.

Granites from Mustang and Surrounding Regions (Central Nepal)

Two Himalayan leucogranite plutons of northern central Nepal have been recognized, mapped with the help of satellite imagery, sampled and analyzed. The 1600 km Dolpo-Mugu may be the largest High Himalaya leucogranite, and the 325 km2 Mustang leucogranite is a North Himalaya circular pluton cut in half by the Thakkhola graben. Both are very similar to the Manaslu granite. They have provided much of the filling material for the graben after a high rate of denudation.

Detection of inherited monazite in the Manaslu leucogranite by [formula omitted] ion microprobe dating: Crystallization age and tectonic implications

Although leucogranites are among the least petrologically variable of all igneous rocks, ironically they are among the most difficult to reliably date. The High Himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt. For a variety of reasons linked to the minimum melt composition of these leucogranites, these dating studies have not been entirely successful. We report results of a new ion microprobe dating method based on the decay of 232Th to 208Pb in monazite that has directly revealed the presence of inherited Pb in monazite from the Manalsu granite, casting doubt on its previously accepted age. Monazite ages obtained from this leucogranite yield two distinct populations, a large number of ages with a normal distribution and mean age of 22.4 ± 0.5 Ma (±2 S.E.) that we interpret to be the crystallization age, and a smaller inherited fraction with an age of ca. 600 Ma. Because formation of the granite is thought to be related to slip on the Main Central Thrust, both this date and a second less precise result from a structurally similar pluton near Mt. Everest indicate that the Main Central Thrust was active in the interval 24-22 Ma, but do not constrain its initiation. Together with crosscutting relationships, these data require that movement on the North Himalayan Fault occurred prior to 22 Ma at both locations.

Contact metamorphism and depth of emplacement of the Manaslu granite (central Nepal). Implications for Himalayan orogenesis

The Manaslu massif (central Nepal) provides a well-exposed example of a deeply eroded pluton: its contact aureole can be followed from the base to the top along medium- to low-grade (mesozonal to anchizonal) Tethyan metasediments. Contact metamorphic mineral assemblages and thermobarometric estimations suggest that the granite was emplaced at 18–21 km for the base and 9–13 km for the roof. Calculated temperatures within the aureole, at 550 ± 40°C, are compatible with the intrusion temperature of a leucogranitic magma. Microstructural evidence shows that the temperature remained high (> 500°C) at the base of the massif during and after granite emplacement, whereas towards the top of the granite deformation proceeded rapidly at lower temperature. Heating of the abundant calcareous rocks in the contact aureole released a local CO 2-rich fluid, whereas a H 2O- and boron-rich fluid seems to have pervaded the whole zone; this fluid is probably exsolved from the granitic melt during its crystallization. The depth of emplacement of the massif has an important implication for the reconstruction of the Himalayan geodynamic evolution, implying a burial of the Tethyan metasediments by a major refolding of the sedimentary cover or, more probably, by extensive development of the North Himalayan nappes towards the south for more than 70 km, before granite emplacement, i.e., before the Miocene. The young age of the granite and its depth of emplacement suggest a rapid tectonic denudation, in the order of 1 mm a −1, probably by normal faulting north of the massif during its cooling.

VB-3: A PROMISING BLACK GRAM VARIETY WITH WIDE ADAPTABILITY

VB-3 black gram, is a spontaneous mutant selection from the variety Type-9. It matures in 65 days. It is an erect dwarf with an average height of 25 to 30 cm. It is resistant to yellow mosaic virus disease, suitable for kharif season. The average grain yield is about 1000kg per ha. It has wider adaptability to varied climatic zones such as South, Central and North Himalayan zones of India. Based on the yield superiority over the check varieties, VB-3 has been identified by the Central varietal identification Committee for release in 1995.

The Himalayan passive margin from Precambrian to Cretaceous times

The Himalaya passive margin should be separated into the Indian Himalaya and the Pakistan Himalaya, divided by the Nanga Parbat-Haramosh basement uplift in the northwestern syntaxes. The Indian passive margin shows a complex history involving splitting of microcontinents off the northern Gondwana margin from early Paleozoic times until the Jurassic. In contrast, the Pakistan Himalaya formed part of the stable Indian shelf until separation from Africa started in the Jurassic. Each margin has a distinct stratigraphic history until the mid-Mesozoic after which they show similar histories until the Tertiary.The Himalaya mountains in India consist of three distinct tectonic units juxtaposed during the Neogene by southward thrusting of the northern Indian Precambrian to Mesozoic passive margin. The Lesser Himalaya on the south forms a late Precambrian passive margin usually directly overlain by late Cretaceous to Tertiary clastics: its stratigraphy is closest to that of the Indian Shield. In contrast, the High Himalaya has a thin Ordovician to Carboniferous shelf sequence deposited after early Ordovician deformation and granite intrusion. Above this, thick Permian and Mesozoic shelf sequences mark the separation of continental blocks off the northern Indian margin and the opening of the Neotethys ocean. The North Himalaya formed the slope and basin of this ocean and consists of reactivated Paleozoic gneiss domes overlain by thick Mesozoic sediments which pass, in the northeastern Himalaya, into an enormously thick sedimentary sequence resting on oceanic crust.Paleomagnetic and structural evidence indicates at least 500 km of southward thrusting along the Main Central Thrust between the Lesser and High Himalaya. This thrusting, together with a further 250 km of erosion at the front of the nappes, has removed the entire inner shelf of the Himalaya passive margin and can explain the startling contrast between the High and Lesser Himalaya stratigraphic sequences. But there are still discrepancies, particularly in the eastern Himalaya, where Precambrian basement is juxtaposed with the North Himalaya Mesozoic slope. These discrepancies can be partly resolved by strike-slip movements roughly parallel to the Himalaya which have removed parts of the northern Indian passive margin. Such faults of the requisite orientation, displacement and age occur in southeast Asia where their cumulative displacements can add up to several thousand kilometres. Similar faults, of undoubted pre-Tertiary age, occur in the Pakistan Himalaya. Here, the passive margin shows a much simpler history of Jurassic to Miocene subsidence.

Deformation and motion along the southern margin of the Lhasa block (Tibet) prior to and during the India-Asia collision

Field studies in the Lhasa block (central and eastern S-Tibet, long. 88° to 95°E and lat. 29.3° to 30°N) demonstrate N-S shortening with coeval and consecutive E-W extension and wrenching. Cretaceous, pre-collisional deformation was both coaxial and non-coaxial with top-to-S displacement in central S-Tibet and top-to-SE displacement in eastern S-Tibet. Post-Eocene, probably Miocene shortening as calculated from restored sections across the Qiuwu molasse and the Xigaze forearc basins caused ⩾31% N-S contraction by folding and ⩾65% by folding, faulting and ductile deformation, with significant coeval E-W extension. Reduced stress tensors calculated from fault-striae data yield σ 1 trending subhorizontally 010°(±15°) and σ 2 trending subhorizontally 101° (±17°, azimuths of 22 sites in central S-Tibet). σ 2 is tensional. Post-Eocene structures related to N-S shortening are overprinted by structures indicating E-W compression in eastern S-Tibet ( σ 1: 068°±11°, σ 3: 155°|=-16°, 6 sites) and by distributed deformation involving rifting and predominantly dextral wrenching ( σ 3: 40°±17°, 9 sites in eastern S-Tibet; 075°±11°, 6 sites in eastern S-Tibet). The eastern syntaxis forms an anticline (axis 011°/61°), shows dextral-oblique normal faulting between Indian-plate and Lhasa-block rocks, and is cut in the NE by the active Karakoram-Jiali-Parlung fault zone. We discuss the geodynamics of the southern Lhasa block using all the data abailable: Pre-collisional deformation resulted from shortening during northward subduction and magmatic activity in an Andean-type continental margin. A major phase of exhumation in the early Miocene is correlated with major Tertiary folding and faulting. Associated strain accumulated pre-dominantly coaxilly, involved (major) subvertical and (minor) subhorizontal stretching, and reflects both thickening of Tibetan crust and bulk orogen-parallel (E-W) stretching. E-W extension is concentrated along distinct zones which coincide with margins of tectonostratigraphic units. The coaxially deformed part of S-Tibet is probably bound to the S by the Miocene, dextral-oblique North Himalayan (normal) Fault Zone. This zone connects the western and eastern syntaxes and allowed the Miocene deformation of the Himalaya-Tibet system to be partitioned into a Himalayan zone thickened by simple-shear stacking and a Tibetan zone thickened by predominantly coaxial flattening-type shortening. At present strong eastward flow of the area S of the Karakoram-Jiali-Parlung fault zone is inhibited by pinning at the eastern syntaxis. The E-W compression in eastern S-Tibet results from the “dead corner” W of the syntaxis. Recent strike-slip faulting truncates the syntaxis and will probably relieve the locking of S-Tibet.