V. C. Thakur

Uplift and convergence along the Himalayan Frontal Thrust of India

Along the Himalayan thrust front in northwestern India, terrace deposits exposed 20 to 30 m above modern stream level are interpreted to have been uplifted by displacement on the underlying Himalayan Frontal Thrust. A radiocarbon age limits the age of the terrace to ≤1665±215 calendar BC (≤3663±215 radiocarbon years before present), yielding a vertical uplift rate of ≥6.9±1.8 mm/yr. In combination with published studies constraining the dip of the Himalayan Frontal Thrust fault to about 30° in the study area, the observed uplift rate equates to horizontal shortening across the Himalayan Frontal Thrust of ≥11.9±3.1 mm/yr and the slip rate of the Himalayan Frontal Thrust of ≥13.8±3.6 mm/yr. This is similar to previously reported rate estimates along the Himalayan arc based on displacement of older Plio-Miocene age rocks, or the much shorter records of geodesy and historical seismicity. The similarity is consistent with the idea that convergence across the Himalayan front has occurred at a relatively steady rate through time. The seismic expression of this deformation includes several great (M∼8) historical earthquakes which, due to lack of surface rupture during those events, have been attributed to their occurrence on blind thrusts. Yet, the occurrence of a possible fault scarp in the field area indicates that past earthquakes have been sufficiently large to rupture to the surface and produce coseismic scarps. These observations suggest a potential for earthquakes along the Himalayan Frontal Thrust larger than those observed historically.

Tectonic framework of the Indus and Shyok suture zones in Eastern Ladakh, Northwest Himalaya

Abstract Three different phases of plutonic-volcanic activity, viz. 1. (1) Cretaceous volcanics of the Dras and Luzarmu formations. 2. (2) Early Tertiary granitoids of the Ladakh plutonic complex. 3. (3) Oligocene acid volcanics and dykes of the Khardung Formation have been recognised in the Ladakh magmatic arc. The Dras Formation of the Indus suture zone and the Luzarmu Formation of the Shyok suture zone were one stratigraphic unit into which granitoids of the Ladakh plutonic complex intruded. The rocks of the Shyok suture zone are interpreted to represent the tectonised remainder of a marginal (back-arc) basin. The closure of the marginal basin occurred at the Eocene-Oligocene time boundary, and crustal shortening was responsible for generation of Neogene 2-mica granites of the Karakoram plutonic complex and regional metamorphism of the Pangong Tso group.

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

Abstract 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.

Tectonics of the Central crystallines of Western Himalaya

The crystalline sheet of the Higher Himalaya, referred to as the Central Crystallines, is a continuous lithotectonic unit which can be traced from the River Kali of eastern Kumaun in the east to Sankoo in the Suru River valley of Kashmir in the west. The principal lithostratigraphic units of this zone are pelites, psammites, gneisses, amphibolites, migmatites and leucocratic granites. The rocks of this zone show progressive regional metamorphism of normal as well as reverse types, the metamorphic grade ranging from chlorite to sillimanite zone. The Main Central Thrust, which demarcates the southern boundary of the Central Crystallines, has brought the crystalline rocks to rest over the sediments of Deoban Group in Kumaun and Garhwal and over the Outer Crystallines (=Chail-Jutogh Nappe) in Himachal Pradesh. The evidence obtained from metamorphism, deformation and radiometric dating indicate that the Central Crystallines is an old Precambrian basement which has been reactivated during Caledonian and Alpine orogenic movements.

Plate tectonic interpretation of the Western Himalaya

Abstract The Western Himalaya, which includes the regions of Kumaun, Garhwal, Himachal, Jammu and Kashmir in India displays a complete cross-section of the Himalayan orogenic belt encompassing the principal tectonic zones of Outer Himalaya, Lesser Himalaya, Higher Himalaya, Tethys Himalaya, and Trans Himalaya. The rock formations of the Lesser Himalaya, which are correlatable to those of the northern part of the Indian Peninsular shield, represent deposits on the northern edge of the Indian craton. The Phanerozoic sequence of the Tethys Himalaya zone is interpreted as representing sedimentary deposits of the continental margin. The Central Crystallines of the Higher Himalaya zone, underlying the Phanerozoic sequence of the Tethys Himalaya zone, have been exposed in their present position as a result of uplift and southward movement along the Main Central Thrust. The Trans-Himalaya zone consists of the Indus Suture, Ladakh magmatic arc, Shyok Suture and Karakoram subzones. The Nidar ophiolite and the Shergol and Zildat ophiolitic melanges of the Indus Suture are the tectonised remnants of the Tethys ocean floor. The Ladakh plutonic complex, the Dras volcanics and other related Cretaceous volcanics belong to the plutonic-volcanic series of the Ladakh magmatic arc. The Shyok Suture is interpreted as a back-arc basin, and the Palaeozoic-Mesozoic sequence of the Karakoram subzone represents continental margin deposits of the Karakoram block. The molasse sedimentary deposits of the Outer Himalaya zone are interpreted as representing post-collision sedimentation in a trough that developed in front of the rising Himalaya. The tectonic evolution of Northwestern Himalaya has been involved in the following sequence of events. 1. (a) Subduction of the Indian oceanic plate under the Karakoram (Tibet) block in the Cretaceous, giving rise to the Ladakh magmatic arc and arc-trench gap sedimentation. 2. (b) Collision of the Indian plate with the Ladakh magmatic arc in the middle Eocene forming the Indus palaeosubduction zone. 3. (c) Closure of the Shyok back-arc basin and collision of the Ladakh magmatic arc with the Karakoram block. 4. (d) Suturing of the lithospheric blocks as a result of collision. The penetrative and non-penetrative deformation, regional metamorphism, generation of S-type granites in the Higher Himalaya zone and the formation of nappes and thrusts owe their origin to crustal shortening in the Oligocene-Miocene.

Microstructures and strain variation across the footwall of the Main Central Thrust Zone, Garhwal Himalaya, India

Abstract The microstructural variation with a progressive change in the strain pattern are described in the rocks occurring across the footwall of the Main Central Thrust (MCT) in an area of the Garhwal Himalaya. In the western Garhwal Himalaya, the MCT has brought upper amphibolite facies metamorphic rocks southward over the greenschist facies rocks of the Lesser Himalaya. The progressively increasing flattening strain towards the MCT changes either to plane strain or in some cases to constrictional strain. This change in strain is well recorded in the microstructures. The zone dominated by flattening strain is expressed as bedding parallel mylonites. The grain reduction in this zone has occurred by dynamic recrystallization and quartz porphyroclasts were flattened parallel to the mylonite zone. The maximum finite strain ratio observed in this zone is 2.2:1.8:1. The zone, where the flattening strain changes either to plane strain or constrictional strain, record an increase in finite strain ratio up to 3.8:1.9:1. This zone represents deformation fabrics like S–C microstructures simultaneously developed during mylonitization in an intense ductile shear zone. The above zone is either near the MCT or adjacent to crystalline klippen occupying the core of the synforms in the footwall of the MCT. The microstructural evolution and the finite strain suggest that the MCT has evolved as the result of superposition of southward directed simple shear over the flattening strain. The simple shear has played an active role in the rapid translation which followed the mylonitization at deeper levels.

Development of major structures across the northwestern Himalaya, India

The Himalaya originated as a result of subduction of the Tethyan oceanic crust that lay between India and Tibet followed by continent-continent collision. A petrotectonic assemblage of the subduction complex consisting of the Shergol ophiolitic melange, the Nidar ophiolite, the Nindam flysch Formation and blue-schist rocks were obducted over the Indian margin during initial collision of the Indian continent with Tibet in the middle Eocene. The collision was followed by continued convergence of India against Tibet resulting in crustal shortening both in the Indian and the Tibetan margins. This crustal shortening produced regional metamorphism and polyphase deformation of the Higher Himalaya zone, the Tso Morari Crystallines and the Pangong Tso group. Continued convergence produced major folds, nappes and thrusts of the Higher, Tethys and Lesser Himalayas on the Indian plate and the Indus, Shyok and Karakoram zones of the Trans-Himalaya. The major structures include the Main Karakoram Thrust, Zanskar Thrust, Tso Morari dome, Zanskar synform, Sum dome, Kashmir-Chamba nappe, Panjal Thrust and MBT. The compressional phase was followed by an uplift phase in the early Miocene which formed a foredeep to the south in front of the uplifted cordillera. The Siwalik group sediments were deposited in this foredeep and were later folded and faulted. The folds and thrusts in the Outer Himalaya belt are younger than those of other tectonic zones of the Himalaya.

Pan-African magmatism, and sedimentation in the NW Himalaya

Correlation of early Palaeozoic, Pan-African (500 ± 50 Ma) granites that intruded the Chail, Salkhala, Haimanta Formations in the Lesser Himalaya, Zanskar crystallines, and Lower Taglang La of Tso-Morari crystallines in the northwestern Himalaya, is based on the field relationship, tectonic setting, mineralogical, and geochemical characteristics, and isotope dating of the granites. These granite plutons exhibit identical petrographical, and geochemical character. The mineralogical composition of the granites is quite similar, consisting of quartz, K-feldspar, plagioclase feldspar, biotite, muscovite, garnet, tourmaline, ± cordierite, andalusite, and sillimanite fibrolite. The granite which are massive, and inequigranular in the core of the plutons, show strongly foliated character indicating development of ductile shear zone at the margins. These are peraluminous S-type granites having high A/CNK value (> 1). Presence of normative corundum, rounded shape of zircon, and high initial Sr ratio suggest crustal source of the granites. Mantle normalized spider-diagram exhibits similar characters for all these granitoids. The intrusion of the Pan-African granites mark an abrupt end of the sedimentation that continued virtually uninterrupted from Palaeoproterozoic. The sudden break in sedimentation towards the terminal phases of the Lower Cambrian has been observed in almost all parts in Lesser as well as the Tethys Himalaya. Occurrences of large number of plutons along different tectonic belts of northwestern Himalaya are indicative of widespread tectono-thermal event during early Palaeozoic (500 ± 50 Ma). The bracketing of the two features like, the break in sedimentation during post-Late Cambrian, and the intrusion of granites around 500 ± 50 Ma, is considered to be the result of a strong diastrophic orogenic event correlatable to the late phases of the Pan-African Orogeny in Africa.

Regional framework and tectonics of the Yarlung Zangbo suture in southern Tibet

Abstract A synthesis of regional framework and tectonic characteristics of the Yarlung-Zangbo Suture in Southern Tibet has been attempted. The Yarlung-Zangbo Suture lies between the Tethys Himalayan zone to the south and the Lhasa Terrane to the north. In the northern part of the Tethys Himalaya, a belt of Neogene granites and older (Cambrian and Pre-Cambrian) orthogneiss is located. The Xigaze ophiolite, the ophiolite tectonic breccia and the Xigaze Group are the principal tectono-stratigraphic units of the Yarlung-Zangbo Suture. The Xigaze Group represents the remanant of the Neotethyan oceanic crust of Jurassic-Cretaceous age, deposited in a fore-arc basin. Magmatism of the Gangdise plutonic-volcanic arc began around 90 Ma and paroxysmal activity took place between 50–60 Ma. The magmatic arc owes its origin to northward Tethyan subduction beneath the Lhasa block. In Lhasa Terrane, the Precambrian Nyainqentanglha group crystallines are succeeded by Carboniferous to Cretaceous sequence of sedimentaries and volcanics. The Yarlung-Zangbo Suture contains crustal fragments of the Neotethys ocean that originated through rifting in Triassic and opening of the ocean basin in the Jurassic-Cretaceous times. The suture was closed as a result of the collision between the Indian continent and the Lhasa block at 45 Ma.

Geology of the area around Dharmsala, Kangra District, H. P., India

ZusammenfassungIn der vorliegenden Arbeit wird der Versuch unternommen, die Geologie des Gebietes von Dharmsala im Kangra-Distrikt (Provinz Himachal Pradesh) zu beschreiben; außerdem wird die stratigraphische Stellung des Dharmkot Limestone und der Dharmsala Traps diskutiert.RésuméLe mémoire présenté comporte la description de la géologie dans la région de Dharmsala, district de Kangra, Himachal Pradesh. Une attention particulière est accordée à la position stratigraphique de certains horizons problématiques, le Dharmkot Limestone et le Dharmsala Trap.AbstractIn the present paper an attempt has been made to describe the geology of the area around Dharmsala, Kangra District, Himachal Pradesh. The stratigraphic position of the problematic horizons, i. e., Dharmkot Limestone and Dharmsala Traps is discussed in detail.Краткое содержаниеАвтор делает попытку описать геологию Дар мсалы в дистрикте Кангра (про винция Гимахал-Прадеж). Обсуж дается стратиграфич еское положение известняк а Дармкот и траппов Дар мсала.