Vaikrita Thrust Research Articles

Defining the Himalayan Main Central Thrust in Nepal

An inverted metamorphic field gradient associated with a crustal-scale south-vergent thrust fault, the Main Central Thrust, has been recognized along the Himalaya for over 100 years. A major problem in Himalayan structural geology is that recent workers have mapped the Main Central Thrust within the Greater Himalayan Sequence high-grade metamorphic sequence along several different structural levels. Some workers map the Main Central Thrust as coinciding with a lithological contact, others as coincident with the kyanite isograd, up to 1–3 km structurally up-section into the Tertiary metamorphic sequence, without supporting structural data. Some workers recognize a Main Central Thrust zone of high ductile strain up to 2–3 km thick, bounded by an upper thrust, MCT-2 (= Vaikrita thrust), and a lower thrust, MCT-1 (= Munsiari thrust). Some workers define an ‘upper Lesser Himalaya’ thrust sheet that shows similar P – T conditions to the Greater Himalayan Sequence. Others define the Main Central Thrust either on isotopic (Nd, Sr) differences, differences in detrital zircon ages, or as being coincident with a zone of young (<5 Ma) Th–Pb monazite ages. Very few papers incorporate any structural data in justifying the position of the Main Central Thrust. These studies, combined with recent quantitative strain analyses from the Everest and Annapurna Greater Himalayan Sequence, show that a wide region of high strain characterizes most of the Greater Himalayan Sequence with a concentration along the bounding margins of the South Tibetan Detachment along the top, and the Main Central Thrust along the base. We suggest that the Main Central Thrust has to be defined and mapped on strain criteria, not on stratigraphic, lithological, isotopic or geochronological criteria. The most logical place to map the Main Central Thrust is along the high-strain zone that commonly occurs along the base of the ductile shear zone and inverted metamorphic sequence. Above that horizon, all rocks show some degree of Tertiary Himalayan metamorphism, and most of the Greater Himalayan Sequence metamorphic or migmatitic rocks show some degree of pure shear and simple shear ductile strain that occurs throughout the mid-crustal Greater Himalayan Sequence channel. The Main Central Thrust evolved both in time (early–middle Miocene) and space from a deep-level ductile shear zone to a shallow brittle thrust fault.

Younger hanging wall rocks along the Vaikrita Thrust of the High Himalaya: A model based on inversion tectonics

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 129

Out-of-sequence thrusting in Himalaya: Modification of wedge extrusion and channel flow models

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 130

Younger hanging wall rocks along the Vaikrita thrust of the high Himalaya: A model based on inversion tectonics

In sharp contrast to the common observed characteristic of areas of thrust tectonics, where older rocks are thrust over younger, along the Vaikrita Thrust in the High Himalaya younger hanging wall rocks (i.e. Vaikrita Group—Late Mesoproterozoic to Early Neoproterozoic) lie above the older footwall rocks (i.e. Munsiari Formation—Paleoproterozoic). The phenomenon is explained by an inversion tectonics-based model where normal faulting and metamorphism were followed by thrusting, in which the thrust displacement was less than the displacement during the earlier normal faulting. The present day hanging wall tilt towards north may have been caused by a later thrust, initiated as a piggy back sequence, accompanied by folding and Himalayan metamorphism.

Three terrane-defining thrusts of the Kumaun Himalaya: controversies on position and nomenclature

Three of the five thrust fault systems defining the boundaries of four lithotectonic terranes of the Himalaya and involved in controversies related to their positions and nomenclature, are the objects of discussion in this paper. Youngest of the five terrane-defining faults, the Himalayan Frontal Fault (HFF) is a series of reverse faults that demarcates the boundary of the Siwalik front of the Himalayan province with the alluvial expanse of the Indo-Gangetic Plains. Over large tracts, it is either concealed under younger sediments or has as yet not reached the ground surface and is therefore a blind fault. The nature of this frontal fault varies along its length. Where the hidden ridges of the Indo-Gangetic basement impinge the Himalaya, the mountain front is ruptured and the HFF is repeatedly reactivated. In the sectors intervening these ridges, it is not expressed on the surface, but the ground of the adjoining Indo-Gangetic Plain is sinking, the rivers are shifting their courses and large tracts of land are waterlogged and characterized by The Vaikrita Thrust is the plane that marks pronounced metamorphic break and abrupt change in style and orientation of structures within the succession of crystalline rocks that build the bulk of the snowy ranges in the Kumaun Himalaya. Not only is there a jump of pressure of the order of 4 kb and a temperature rise of >200 oC, but also is there a conspicuous change of neodymium isotope value across the tectonic plane that separates the low-grade metarnorphics in the lower part from the high-grade metamorphic rocks of the upper part of the Great Himalayan succession. The Vaikrita Thrust is therefore recognised as the Main Central Thrust (MCT). While the basal low-grade metamorphic assemblage comprises 1900±100 Ma old highly tectonised porphyritic granite characterised by low initial strontium isotope ratio, the upper high -grade metamorphic group is intruded by 20±1 Ma old anatectic granites characterised by garnet, kyanite, sillimanite and cordierite, and a high but variable value of strontium isotope ratio. Moreover, the anatectic Lower Miocene granites are singularly absent in the succession of the Lesser Himalayan nappes. Probably it is this thrust that has flexed downwards the plane of decoupling and displacement between the under thrusting Indian plate and the overlying Himalayan mass.
 The terrane-defining fault between the high-grade metamorphics with granitic rocks of the Himadri (i.e., Great Himalaya) and the Tethyan sedimentary pile was recognised as the Malari Thrust Fault in the northern Kumaun Himalaya in the early seventies, as the South Tibetan Detachment System in southern Tibet adjoining north-eastern Nepal in the early eighties and as the Zanskar Shear Zone in north-western Himachal Pradesh in the late eighties. Not only is there an abrupt change in the metamorphic grade across the tectonic plane, but also an attenuation and wholesale elimination of some lithostratigraphic formations of the hanging wall besides the difference in the style of deformation. Exhibiting predominant dip-slip movement in the central sector of the Himalayan arc, the Trans-Himadri Fault (T-HF) was formed as a consequence of the Tethyan sedimentary cover detaching from its rigid foundation of the basement complex that was squeezed up following blocking or slowing down of tectonic movements related to India-Asia convergence. The sedimentary cover lagging behind the thrust­ up basement complex slid down and toppled over northward and gave rise to back-folds and back-thrusts. In Kashmir, western Himachal Pradesh, central Nepal and north-western Bhutan, in sharp contrast, there was very strong compression, so that the Tethyan sedimentary rocks-along with a slice of the low-grade metamorphic basement in the hanging wall advanced southwards across the Himadri and were emplaced as nappes and klippen south of the Main Central Thrust. The T-HF movement occurred approximately around 20.9 Ma, although the movement had started quite earlier in some places. Quaternary reactivation resulted in river ponding and development of huge lakes such as the Garbyang palaeolake in the Kali valley and the ~ 40,000 year-old Goting palaeolake in the Western Dhauli Valley.

Timing, quantification and tectonic modelling of Pliocene–Quaternary movements in the NW Himalaya: evidence from fission track dating

Variable exhumation rates, deduced from the Pliocene–Quaternary FT zircon–apatite ages from the Himalayan Metamorphic Belt (HMB) of the NW Himalaya along the Sutlej Valley in Himachal Pradesh, have been modelled in the tectonic framework of fast exhumed Lesser Himalayan windows, which caused lateral extensional sliding of the metamorphic nappe cover along the well-known Main Central Thrust (MCT) and differential movements along thrust zones as well. In the northern belt of the Higher Himalayan Crystallines (HHC), two distinct clusters of the FT apatite ages have been deciphered: apatite ages having a weighted mean of 4.9±0.2 Ma (1 σ) in basal parts on the hanging wall of the MCT, and 1.49±0.07 Ma (1 σ) in the hanging wall of a newly, recognized NE, dipping Chaura thrust further north. Fast exhumation of the Chaura thrust hanging wall has been inferred at a rate of 4.82±0.55 mm/yr from the zircon–apatite cogenetic pairs during 1.54 Ma and 0.97 Ma, and 2.01±0.35 mm/yr since 1.49 Ma. In comparison, its foot wall has been exhumed at a much slower rate of 0.61±0.10 mm/yr since 4.9 Ma. The overlying Vaikrita Thrust zone rocks reveal an exhumation rate of 1.98±0.34 mm/yr from 2.70±0.40 Ma to 1.31±0.22 Ma and 2.29±0.66 mm/yr since 1.31±0.22 Ma. Using these data, a vertical displacement of ca. 2.08±0.68 km has been calculated along the Chaura thrust between 4.9 and 1.50 Ma on an average rate of 0.6 mm/yr. It is of the order of 1.18 km from 2.70 Ma to 1.54 Ma along the Vaikrita Thrust, and 0.78 mm/yr from 1.31 Ma to 0.97 Ma, and has behaved as an extensional normal fault during these periods. Tectonic modelling of the exhumation rates in the NW Himalaya reveals fastest uplifting Himalayan domes and windows like the Nanga Parbat in Pakistan, Suru and Chisoti domes in Zanskar and Kishwar–Kulu–Rampur Window axis in SE Kashmir and Himachal Pradesh during Pliocene–Quaternary. These windows appear to have caused lateral extensional sliding of the Himalayan metamorphic nappes in the lower parts. The middle parts of the HHC belt have witnessed both overthrusting and extensional faulting due to complex and variable exhumation patterns within the hanging and foot walls of the MCT and Vaikrita Thrust along the Sutlej Valley, thus causing movement of upthrust crustal wedge between the extensional ones. Thus, FT zircon–apatite ages provide evidence for the presence of a number of crustal wedges having distinct tectonothermal history within the HHC.

Isotopic constraints on the structural relationships between the Lesser Himalayan Series and the High Himalayan Crystalline Series, Garhwal Himalaya

Nd and Sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences. The potential of the technique is illustrated by this isotopic study of the Main Central thrust system of the Himalaya. Nd isotope data from the Garhwal Himalaya indicate that metasedimentary rocks from the Vaikrita Group (Nd = –14 to –19) correlate closely with those from the High Himalayan Crystalline Series, which constitutes the hanging-wall lithologies of the Main Central thrust. In contrast, metasedimentary rocks from the Munsiari Group (Nd = –23 to –28) show marked similarities to the Lesser Himalayan Series in the footwall of the Main Central thrust. Sr isotopes support the correlations in that the Vaikrita Group shows partial reequilibration at 500 Ma, whereas the Munsiari Group has not undergone Sr isotope homogenization since 1800 Ma. Thus, the Vaikrita thrust that juxtaposes these two formations is recognized as the Main Central thrust in Garhwal Himalaya. The thrust coincides, approximately, with the location of the kyanite isograd, confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the Main Central thrust. Along the Tons thrust (known locally as the Srinagar thrust) 50 km south of the Main Central thrust, low-grade quartzarenites with Nd-Sr isotope and trace element characteristics typical of Lesser Himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the High Himalayan Crystalline Series. The field relationships most probably result from out-of-sequence thrusting in which Lesser Himalayan Series rocks to the north were emplaced over low-grade equivalents of the High Himalayan Crystalline Series preserved in the external part of the orogen. This study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts.

Geochemistry of Precambrian mafic magmatic rocks of the Western Himalaya, India: petrogenetic and tectonic implications

Precambrian sequences of the Higher Himalayan Crystallines (Vaikrita Group) and the Lesser Himalaya (Chail, Jutogh and Jaunsar Groups), in the Garhwal and Himachal regions of the Western Himalaya, include abundant metamorphosed mafic lavas and dykes (amphibolites). A gabbroic body within the Chail Group has been dated at 1907±91 Ma (initial 87 Sr/ 86Sr ratio of 0.7022±0.0008) by the whole rock Rb–Sr method. This age is consistent with several age data (1800–2000 Ma: whole rock Rb–Sr method), assigned to the granitoids and gneisses of this region. These amphibolites and gabbros exhibit low-Ti tholeiite characteristics. The Lesser Himalayan samples are enriched in light rare earth elements (LREE) and large ion lithophile element (LILE), with distinct negative Nb, Sr, P and Ti anomalies. Conversely, the Vaikrita Group samples are characterized by less enriched LREE–LILE and absence of the above anomalies but have distinct positive Sr anomalies. The chemical characteristics of the Lesser Himalayan samples are remarkably similar to the basal Aravalli volcanics of the NW Indian shield, inferred to reflect significant components from enriched mantle sources and crustal contamination. Samples of the Vaikrita Group appear to have been influenced by an asthenospheric mantle and are less contaminated as indicated by the absence of negative Nb, P and Ti anomalies. These distinct geochemical characteristics are used to demarcate the Main Central Thrust along the Vaikrita Thrust. The Chail and Jutogh Groups are affiliated with the Lesser Himalaya and the Chail Group gabbro body may represent portion of one among the numerous magma chambers that fed a large Precambrian magmatic province in the south of the Main Central Thrust. The similar emplacement ages (1800–2000 Ma) for the mafic and felsic magmatic rocks, suggest that this region experienced a major episode of crustal generation and evolution in a rift environment.

Geology and tectonics of the Central Crystallines of northeastern Kumaun Himalaya, India

Four major geo-tectonic domains are encountered in the northeastern Kurnaun Himalaya between the Ramganga and Kali rivers. These tectonic domains are: (1) Sedimentary Terrain of the south, (2) Lower Central Crystallines, (3) Upper Central Crystallines, and (4) Tethyan Supergroup in the north. The Sedimentary Terrain comprises of the Berinag, Damtha and Tejam groups. The Lower Central Crystallinel are divided into Rungling and Munsiari formations, separated by a thin lensoidal wedge of sedimentary sequence of the Sirdang Sedimentary Zone, which is the continuation of the Nawakot Complex in western Nepal. The rocks of the Nawakot Complex wrap around the Chhiplakot Crystallines of the Munsiari Formation in Nepal and join with the Berinag Formation in northeastern Kumaun region. The Munsiari Formation has been divided into Phalyati, Baram, Chhiplakot and Khet members.The Upper Central Crystallines are represented by the rocks of the Vaikrita Group, and form the basement for the Tethyan sedimentary sequence. The Vaikrita Group is divided into four formations namely Joshimath, Pandukeshwar, Pindari and Martoli. The Joshimath Formation is further subdivided into Dar and Sela members whereas the Martoli Formation is subdivided into Budhi Schist and Dugtu members.
 The southern contact of the Munsiari Formation with the Lesser Himalayan sedimentary sequence forms a tectonic boundary designated as the Munsiari Thrust or MCT I. The southern contact of the Rungling Formation with the rocks of the Sirdang Sedimentary Zone forms another tectonic boundary designated as the Rungling Thrust or MCT II. The Munsiari Thrust (MCT I) has been interpreted to terminate near Chainpur in western Nepal at its branch point. The contact between the Upper Central Crystallines and the Lower Central Crystallines is designated as the Vaikrita Thrust or Main Central Thrust (MCT). The quartzite of the Berinag Formation exposed in the Sirdang Sedimentary Zone thrust over the Munsiari Formation along the Pangla Thrust. The northern part of the Martoli Formation of the Vaikrita Group is characterised by concordant relationship with the Ralam Conglomerate and also the Garbayang Formation of the Tethyan Super Group. Either side of this contact zone is characterised by a series of minor southward thrusts and associated shear planes, which are subsequently cut by a series of normal faults and related shear planes exhibiting down to the NE oblique slip displacement. The shear zone containing contractional and superposed extensional structures at the northern portion of the Vaikrita Group and at the basal part of the Tethyan Supergroup is designated as Tethyan Shear Zone.

Structure of the Chamba nappe and position of the Main Central Thrust in Kashmir Himalaya

The Chamba nappe, composed of an approximately 8 km thick sequence of Late Precambrian to Jurassic age rocks is located between the Higher Himalaya Crystallines (HHC) and the Lesser Himalayan (LH) formations of Panjal Imbricate Zone (PIZ) in the Kashmir Himalaya. To the south, the Panjal Thrust, demarcating the base, brings the Chamba nappe rocks over the Panjal Imbricate Zone. To the north, the Chamba nappe rocks lie over the metamorphic HHC along the south dipping Chenab Normal Fault (CNB). A pervasive stretching lineation defined by a mineral lineation, stretched pebbles and felspar phenocrysts plunges NE–NNE and occurs on the foliation/cleavage surface. This lineation is related to southward displacement of the Chamba nappe. The Chamba nappe is folded by regional scale fold, viz. the Chamba, Tandi and Bharmor synclines and the Tisa anticline. These NW–SE trending folds structures were developed synchronously with southward thrusting of the Chamba nappe. The Chamba nappe results from southwestward sliding of cover rocks from their basement (HHC) due to topographic uplift. The Main Central Thrust (MCT) in Kashmir Himalaya is different from that of the Kumaun and Nepal Himalaya. The MCT (Vaikrita Thrust) does not extend west of the Beas river, but it is exposed in the Rampur Window and the Kishtwar Window separating the HHC from the underlying LH rocks. Southward propagation of the MCT from the window zone, up-cutting the overlying HHC, is transferred to the Panjal Thrust which transports the Chamba nappe to the south over the Lesser Himalayan formations.

Rupture history and seismotectonics of the 1991 Uttarkashi, Himalaya earthquake

The 19 October 1991 Uttarkashi, India earthquake occurred in the main thrust zone of the Himalaya. With a moment magnitude of 6.8, this event is characteristic of the present-day motion on the thrust fault system. We examine this earthquake using different sets of data in order to understand better the faulting process of a major earthquake in the Himalayan region. Firstly, the modeling of the teleseismic records indicates that the mechanism is similar to the published CMT and indicates a shallow (between 10 and 15 km depth) low-angle thrust event. In the vicinity of the source, the earthquake was recorded by a network of accelerometers run by the University of Roorkee. Six three-component accelerometers were triggered within a radius of 60 km. Two of them were very close to the surface projection of the fault. Forward modeling of those records shows that the rupture propagated toward the west. This forward modeling gives us the possibility to confirm the epicenter location and to evaluate the timing of the accelerograms. The accelerogram records are inverted to obtain the distribution of slip on the fault plane. The results show a complex rupture process. The slip maxima (1.5 m) occurred 10 km west and 15 km southwest of the hypocenter. The slip source function obtained with near-field data is similar to the function obtained from teleseismic records and shows a low moment release at the beginning of the rupture and a maximum rate of moment release 4 seconds after. The relation between the slip distribution obtained by inversion, isoseimals, mapped faults and the aftershocks location is then discussed and we finally propose a seismotectonic interpretation of this earthquake. The Uttarkashi earthquake probably occurred along the detachment surface which coincides with the upper surface of the subducting Indian lithosphere. This detachment surface is gently dipping under the Lesser Himalaya and south of the Vaikrita thrust. The Vaikrita thrust marks the line separating the very shallow-dipping detachment (along which big earthquakes like the Uttarkashi earthquake could occur) from the steeper-dipping, aseismic basement thrust. This observation is important for correctly estimating the seismic hazard in the Uttarkashi region.

Pressure, temperature and time constraints on metamorphism across the Main Central Thrust zone and High Himalayan Slab in the Garhwal Himalaya

Abstract Following the early Eocene collision of India and Asia, continental subduction occurred on the northward-dipping Main Central Thrust (MCT). In western Garhwal, N. India, upper amphibolite-facies gneisses on the High Himalayan Slab are thrust southwards over unmetamorphosed to greenschist facies quartzites, carbonates and metabasics of the Lesser Himalaya. In the Bhagirathi valley, the MCT forms a c. 10 km thick shear zone composed of mylonitic augen gneiss, amphibolite and metasediments. Metamorphic grade increases both northwards and with structural height. The MCT zone is bounded to the north by the Vaikrita (roof) Thrust and to the south by the Munsiari (floor) Thrust. The Vaikrita Thrust is a diffuse high-temperature shear zone, whereas the Munsiari Thrust is a relatively discrete fault formed under brittle-ductile conditions. North of the MCT zone, at the top of the High Himalayan Slab a northward-dipping extensional shear zone, the Jhala normal fault, was responsible for the downthrow of the Tethyan sediments to the north with respect to the uplifting High Himalayan Slab gneisses to the south. Thermobarometic transects reveal a sudden increase in both pressure and temperature across the Vaikrita Thrust from south to north but with subsequent decreases accompanying structural height in the High Himalayan Slab. Temperatures increase going up-structural section from about 500° C to 770° C across the MCT zone, but then decrease again to the north varying between about 550 and 640° C. Similarly, pressures increase sharply up-structural section across the MCT zone from 6 to 12 kbar, then decrease towards the top of the slab to between 7 and 8.9 kbar. The inverted P-T gradient across the MCT zone changes to approximately isothermal and isobaric conditions in the top 9 km (horizontal distance) of the High Himalayan slab. Cooling rates for the upper MCT zone determined from 40 Ar/ 39 Ar (hornblende) and K-Ar (muscovite and biotite) cooling ages suggest a return to erosion-controlled denudation following extension at the top of the High Himalayan Slab. Additional K-Ar (muscovite) cooling ages from a transect through the MCT zone and High Himalayan Slab are progressively younger towards the south, reflecting the southward propagation of the deformation sequence with time. Hornblende 40 Ar/ 39 Ar cooling ages from the MCT zone suggest that structurally lower rocks have not been heated above c . 500° C since the Precambrian, whilst a 19.8 ± 2.6 Ma hornblende age from the MCT zone dates the latest high-temperature shearing at higher structural levels in the MCT zone and places a minimum age constraint on Himalayan metamorphism in the Garhwal sector of the Himalaya.