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Geodynamic history of Uttarakhanda Himalaya
For citation: Valdiya K.S., 2011, Geodynamic history of the Uttarakhand Himalaya, available in www.ngs.org.np (website of Nepal Geological Society).
Geodynamic history of the Uttarakhand Himalaya
K S Valdiya
Jawaharlal Nehru Centre for Advanced Scientific Research Bangalore-560 064, India
The state of Uttarakhand, embodying the divisions of Kumaun and Garhwal, encompasses the central sector of the Himalayan arc. This article comprises relevant excerpts culled from the recent work of the author "The Making of India: Geodynamic Evolution", Macmillan, Naida- 201 301, 2010, 816 p.
1. EARLY PROTEROZOIC SEDIMENTATION
In the Early Proterozoic a large part of the northern continental margin of the Indian Shield subsided to give rise to more than 2400-km long basin of sedimentation, now represented by the. Himalaya province. The Lesser Himalaya terrane of Uttarakhand comprises autochthonous sedimentary succession thrust over by sheets or nappes of metamorphic rocks associated with granites occurring as prominent components all through the length and breadth of the thrust sheets (Figures 1 and 2). There are three packages or units in the Lesser Himalaya:
Figure 1. Inset shows the diagrammatic structural map of the central sector of the Himalaya, encompassing Uttarakhand. HFF- Himalayan Frontal Fault; MBT- Main Boundary Thrust; MCT: Main Central Thrust Diagramatic cross section shows the main thrust planes that define the boundaries of the terranes- the Siwalik, the Lesser Himalaya, the Great Himalaya and the Tethys Himalaya. The Indus-Tsangpo Suture demarcates the northern boundary of the Indian plate against Asia.
Figure 2. Sketch map of Uttarakhand Lesser Himalaya showing various lithotectonic units. Inset depicts the profile of the structure of the Lesser Himalaya and Himadri in Kumaun
(1) Flysch and flyschoid assemblages of sedimentary rocks including turbidites making the lower part of the autochthonous succession comprising the Damtha and Tejam Groups. The basement of the succession is nowhere seen (Figure 2 and 3 Lower). (2) Low-grade (epimetamorphic) rocks comprising metaflysch, almost invariably associated with 1900±100 m.y. old porphyritic granites and quartz porphyries which have overwhelming presence through the length and breadth of the tectonic sheet known as the Chaii-Ramgarh. This tectonic unit is characterized by pronounced facies variation -from predominant sublitharenite to subgreywacke and shale to quartzarenite interbedded with 1487±45 Ma-old basic volcanics (Figure 2). (3) Medium-grade or mesometamorphic rocks intruded by 500±25 Ma old granodiorite-granite bodies, such as the 500±25 Ma Champawat Granodorite and A/mora Granite, and also associated at the base impersistently by 1900±100 Ma old augen gneisses forming the Munsiari-Aimora-Jutogh nappe (Figure 2). The 6000- to 10,000-metre thick lithotectonic slab of the Great Himalaya (Himadri) is made up of high-grade metamorphic rocks known as the Vaikrita Group (Figures 1, 2 and 4 Lower). The thick homoclinal terrane is split up by a number of intraformational thrust planes. The metamorphism took place under amphibolite to locally lower granulite facies conditions. The Vaikrita succession is intruded by Early Miocene granites of anatectic origin, such as the Badarinath Granite. They have thrown out a network of dykes and veins of adamellite, aplite and pegmatite. Migmatites occur on an extensive scale around the granitic bodies.
Figure 3. Lower: Generalized lithological column shows the Proterozoic sedimentary formations in Uttarakhand Lesser Himalaya. Middle: Lithological column of the Paleogene Foreland Basin, in the western sector. Upper: Lithological succession of the Neogene-Quaternary Siwalik Basin in the western sector.
2. NEOPROTEROZOIC DEVELOPMENTS
2.1 Flyschoid Sediments and Volcanics
In the Lesser Himalaya the tectonic windows and half windows carved out by r rivers in the thrust sheets of epimetamorphic and mesometamorphic rocks expose Proterozoic sedimentary succession (Damtha-Tejam Groups), the larger part of which belongs to the upper Mesoproterozoic-Neoproterozoic, and extending upto Lower Cambrian (Figures 2 and 3 Lower). The Proterozoic rocks of the Himalaya represent a thick and extensive deposit on the distal part of the passive continental margin of the Indian plate.
The quartzites of the epimetamorphic assemblage of the Lesser Himalaya are intimately associated with basalts and tuffs, representing volcanism penecontemporaneous with sedimentation in shallow-shelf platform. The lava eruption is related to rifting of the sialic crust.
It may be mentioned that in the central and western sectors, the outer Lesser Himalayan part was uprooted and thrust 4 to 23 km southward during Tertiary revolution, forming parautochthonous Krol Belt which is made up of preponderant Neoproterozoic to Early Cambrian rocks.
2.2 Carbonate Sedimentation
The Palaeoproterozoic to Early Mesoproterozoic argilloarenaceous sediments pass upwards to preponderant calcareous and argillocalcareous sediments laid down in shallow-shelf platforms and lagoons under tectonically tranquil conditions. The carbonates form an extensive succession of cherty, siliceous dolomites intercalated with bands and beds of blue limestone and grey slates. The carbonate horizon is associated with the Middle Riphean and Upper Riphean (around 1100-900 Ma) stromatolitic bioherms and large and small lentiform deposits of crystalline magnesite of synsedimentary origin between the Kali and Mandakini rivers.
The dominant carbonate sediments give way to a very thick argillo-calcareous succession, locally dominated by black carbonaceous shales and in the Krol Belt to phosphatic shales and lense-shaped units of diamictites occurring at several levels. The diamictites represent debris flows, presumably triggered by tectonic disturbances. These tectonic movements were precursors of the impending tectonic upheaval that terminated protracted Proterozoic sedimentation.
2.3 Appearance of Shelly Fauna
In the Krol Belt the uppermost horizon of the argillocalcareous succession is characterized by the occurrence of small shelly fossils heralding the coming of a variety of organism such as conodonts, trilobites, brachiopods hexactinellid and siliceous sponge speculates of Lower Cambrian epoch.
After life had diversified in various forms and proliferated in the Himalayan province, and sedimentation proceeded in environments varying from open shelf to anoxic lagoons, there was a tectonic upheaval. The tectonic movements resulted in wholesale cessation of sedimentation in the entire Lesser Himalayan subprovince and caused pronounced interruption in basin-filling in the Tethys domain (Figures 3 Lower and 4 Middle). The tectonism manifested itself not only in the deformation of rocks but also widespread granitic activity in the period 500±25 million years as represented by the Champawat Granodiorite and A/mora Granite, occurring extensively, as already stated, in the upper part of the low- and medium-grade metamorphic nappes of the Lesser Himalaya and in the lower part of the high-grade metamorphic rocks of the Great Himalayan succession.
Among the many crucial developments in the Tethys domain (Figures 1 and 4 Middle), one phenomenon is related to the emplacement of the Muth Formation made up of strikingly snow-white Muth Quartzite overlying the Cambrian succession with intercalations of brown-weathering dolomite. The Muth sediments representr elongate offshore bars and shoal complex developed during the Silurian time, and mark an epoch of tectonic stability in the period. The Carboniferous time witnessed restriction of water circulation in the sea causing development of anoxic conditions as reflected in the accumulation of thick black carbonaceous shales and black limestone, all through the extent of the Tethys terrane (Figure 4 Middle).
Figure 4. Lower: Lithological succession of the metamorphic and associated granitic rocks that make the Great Himalaya (Himadri) in Uttarakhand. In Figure
(1) is kyanite-sillimanite-garnet psammitic gneiss and subordinate schist,
(2) is kyanite-garnet schist interbedded with gneissic biotite-quartzite,
(3) is marble and calc-silicate fels, rich in pyroxenes and amphiboles,
(4) is migmatite and related gneisses,
(5) is augen gneisses,
(6) is biotite-porphyroblastic calc-schist.
Middle: Generalized lithostratigraphic column showing the Palaozoic succession with intervening hiatures in the Tethys Basin.
Upper: Generalized lithostratigraphic column of the Mesozoic period in the Tethyan domain of Uttarakhand.
3. HERCYNIAN REVOLUTION
Towards the end of the Palaeozoic era, the southern ·part peripheral to the Lesser Himalaya-Siwalik boundary came under the sway of seawaters -for the first time after the Middle Cambrian retreat. The sedimentary assemblages with Gondwanic elements including Lower Permian including fauna in the Lansdowne region in southern Garhwal represent that transgression of the Gondwanic sea. Significantly, this Lesser Himalayan Permian unit is associated intimately with the pyroclastics.
4. SUBSIDENCE OF NORTHERN CONTINENTAL MARGIN
As a large part of the continental margin of the Peninsular India came under the sway of seawater, the leading edge of the northward moving India started subsiding. The sea continued to deepen progressively along the northern periphery. The deep see is represented by chert betds, limestones and shales with phosphatic nodules and glauconite occurring locally (Figure 4 Upper). A unique, most conspicuous and persistent horizon of the Jurassic time consists of strikingly golden reddish ferruginous oolite within the Spiti Shale and contemporary formations (Figures 4 Upper and 5).
By Early Cretaceous, the sea floor had sunk considerably to form a deep depression in which thick deposits of flysch accumulated all along the periphery of the passive continental margin of India. Steepening of the shelf slope occasionally triggered submarine slides and generated debris flows and attendant turbidity currents forming wild flysch characterized by olistostrome of chaotic texture and structure (Figures 4 Upper and 5).
Figure 5. Sketch map of the Tethys domain in Uttarakhand showing full succession of sedimentary formations from Late Proterozoic to Upper Cretaceous. The section shows the succession and structural features along the Kali River in its upper reaches.
5. DOCKING OF INDIA WITH ASIA
The convergence of continents brought about docking of continents and culminated in the welding of India with Asia. It is inferred that the complete closure of the zone of docking took place in the interval 55-50 Ma. The northwestern edge of Asia first docked along with the Kohistan complex, at about 65 Ma, and in the east India touched the Cuofiang area in southern Tibet at about 68 Ma. The resistance the fast moving Indian plate encountered due to its docking with Asia gave rise to the Gurla Mandhata Dome in the Kailas-Mansarovar region NE of Kumaun.
6. DEFORMATION, METAMORPHISM AND MAGMATISM
The collision of continents was so strong that the entire assemblage of volcanic islands, sediments of the arc-basins and deep sea were chaotically folded and split into multiple stacks of mutually overlapping slices. These were then squeezed up and thrown out - obducted - onto the continental margin. It is in the Malia Johar area where the eloquent signs of the obduction is seen. The zone of collision, made up of tectonized melange-bearing zone, is known as the Indus Tsangpo Suture (ITS).
As the leading edge of the Indian plate subducted beneath the Asian plate, there was large-scale differential melting of the crustal rocks, giving rise to an anatectic g·ranitic magma along the southern margin of Tibet as discordant bodies such as batholiths and stocks of the granite-granodiorite-tonalite composition forming the Andean-type magmatic arc. The arc is represented by the Kailas Range. The granitic activity occurred in the period 110 Ma to nearly 42 Ma, the peak being in the 60-45 Ma interval.
Even as the northern edge of the Indian plate bulged up, the zone of welding of India and Asia sagged down in the Late Eocene time, 34-30 Ma ago. A wide depression was formed along vyhat is today occupied by the valleys of the Sindhu and Tsangpo rivers in which was deposited enormous volumes of coarse detritus derived from brisk erosion of the emerging Himalaya. The oceanic flysch thus passes upwards into a very thick succession of fluvial deposits -the Kailas Molasse.
7. BREAKING OF HIMALAYAN CRUST
The revival of tectonic movements in the Early Miocene time accentuated the deformation of synclinoria! Tethys Himalayan terrane and its basement, the Himadri complex. Many of the tightened folds were split by faults along their axial planes and subsequently displaced or thrust southwards as much as 30 to 80 km (Figures 1 and 2). The buckled up Himalayan crust broke up around 21±2 Ma along what was to become the Main Central Thrust, culminating in the emergence of the Great Himalaya or Himadri terrane delimited at the base by the Main Central Thrust. Nearly at the same time when the Himalayan crust broke along the Main Central Thrust, the thick Tethyan sedimentary cover was detached from the hard unyielding crystalline basement. The plane of detachment is described as Trans-Himadri Fault. In Nepal it is known as the South Tibetan Detachment System (Figures 1 and 58). The fault zone is characterized by northeast-directed backfolds and back thrusts produced probably during gravitational collapse of the thrust stack. The breaking up of the crust was accompanied by stong deformation and high pressure-high temperature metamorphism, the temperature being higher than 600°C and the pressure more than 7-8 kbar. The pressure and temperature had risen locally so high in the early Miocene time that some parts of the metamorphic rocks melted differentially or partially, giving rise to molten material of granitic composition. The anatectic magmas were emplaced in the period 24-22 Ma in the form of batholiths, stocks, laccoliths, sills, dykes and veins intimately associated with migmatites throughout the length of the Great Himalaya terrane.
8. DEVELOPMENT OF LESSER HIMALAYA TERRANE
Repeated translation or thrusting of the Himadri rock pile onto the Lesser Himalayan terrane threw the more than 7000-m thick Lesser Himalayan sedimentary and metasedimentary successions into a series of folds (Figures 1 and 2). Close to the MCT, the squeezing and tightening of folds resulted in southward overturning and toppling over of folds, accompanied by their splitting by a multiplicity of faults along axial planes, giving rise to a duplex or schuppen zone of imbricating lithotectonic stacks. Increasing compression resulted in the uprooting of the entire folded pile under the MCT, and their 80-100-km displacement southwards in the form of thrust sheets or nappes (Figures 1 and 2). As already stated elsewhere, there are two principal lithologically and structurally distinctive nappes -the lower epimetamorphic Ramgarh-Chail Nappe, made up of low-grade metamorphics integrally associated with 1900±100 Ma mylonitized porphyritic granite and porphyry and the upper Almora-Munsiari-Jutogh Nappe comprising medium-grade metamorphics intruded by 500±25 Ma granite-granodiorite plutons.
9. HIMALAYAN FORELAND BASIN
Following crustal thickening in the Lesser Himalayan along its southern margin, a flexural depression developed immediately south of the emerging young Himalaya giving rise in the Early Palaeocene to a foreland basin-Sirmur Basin.
One of the very momentous developments recorded in the foreland-basin succession is the drastic reversal of drainage in the Palaeocene time from N and NW to S and SE. The rivers and stream started flowing southwards. The second significant event was the beginning of the fluvial sedimentation for the first time which is dated at about 31 Ma. Evidently the whole of the Himalayan province had risen up as a highland and subjected to denudation, resulting in the generation of enormous volumes of detritus, which the rivers and streams carried to the foreland basin.
10. FORMATION OF MAIN BOUNDARY FAULT
As the Lesser Himalayan thrust sheets advanced southwards, they encountered resistance. Consequently, the Himalayan crust broke at about 11 Ma along the 2400-km length of the Himalayan province in a series of faults collectively known as the Main Boundary Thrust (Figures 1 and 2 Inset). As the crust broke along the Main Boundary Thrust, it sagged immediately south of the line of breaking. An elongate depression - the Siwalik Basin - overlapping onto the Indian shield came into existence.
The Siwalik Basin, a great repository (- 7000 m) of detritus brought down from the fast-rising youthful Himalaya mountain. The Himalayan rivers eventually converted the Siwalik Basin into a vast floodplain. Tawards the close of the Pliocene and the beginning of the Pleistocene about 1.5 to 1.7 Ma ago, the entire Siwalik subprovince, was overwhelmed by sudden excessive influx of gravelly detritus and avalanches of muddy debris. The resultant deposits are represented by the 2800- to 1800-m thick Boulder Conglomerate.
Much later towards the end of the Pleistocene at about 1.6 Ma, revival of tectonic movement on the Main Boundary Thrust and associated faults brought the Lesser Himalayan rocks riding over and trampling upon the Siwalik. The intensity of deformation was strongest close to the Main Boundary Thrust zone where the folds were tightened and split along axial planes and squeezed out as imbricated stacks. The southern boundary of the Siwalik is demarcated by a series of discontinuous reverse faults and imbrication structures in a 0.5 to· 1 km wide zone of brittle deformation of the Himalayan Frontal Thrust, the tectonic boundary of the Siwalik against the Indo-Gangetic Plains (Figure 1).
11. TECTONIC RESURGENCE IN THE QUATERNARY PERIOD
The Quaternary period represents morphogenic phase of the evolution of the Himalaya. It was during this period that the Himalaya rose up to attain its present spectacular height and form. There is an overwhelming evidence of acceleration of erosion in the temporal interval 0.8 to 0.9 Ma in Tibet, in the Himalaya and in the Indo-Gangetic Plains. The rapid rise has resulted in the development of a very lofty mountain barrier characterized by formidable scarps, soaring peaks and pronounced incisions in river valleys.
In many sectors the middle belt of the Siwalik has a remarkably gentle topography characterized by aggradational flat expanses called. duns. The duns represent wide synclinal valleys filled up thickly by gravel deposits during the Later
Pleistocene to Early Holocene time. Movements on active faults entailing uplift or strike-slip displacement of the footwall caused blocking of river and stream flows, resulting in the formation of lakes upstream of the fault crossings in Siwalik the Lesser Himalaya and Tethys domain. Where the rivers and streams carried larger quantities of sediment load and the pending lasted longer temporal period, the lakes were filled up and converted into flat stretches of sediments. These represent. the palaeolakes, comprising predominant clays and muds with lenses of gravels of fluvial and/or colluvial origin.
12. STRAIN BUILD-UP AND HOLOCENE MOVEMENTS
As the Indian plate is moving towards mainland Asia a larger part of the convergence is stored as elastic strain in the structural frameworks of the Himalaya, particularly where the leading edge of the Indian plate such as represented by the Aravali Ridge and subsurface Bundelkhand Ridge impinges against the Himalaya. Periodic acceleration of continental convergence results in reactivation of long, deep ancient faults, particularly those that define the terrane boundaries and tear faults. This manifested itself in the occurrence of earthquakes, with attendant geomorphic changes and drainage aberration including change of courses and pending of rivers.
It is noticeable that there is high concentration of epicentres of earthquakes of 5 in a 50 km wide zone lying south of but close to the Main Central Thrust in the zone of imbricate pack of thrust sheets and.slabs of the schuppen zone related to the MCT. Focal mechanism solutions indicate that most of the events were related to shallow (10-20 km) thrusting.
The seismic slip rate of the underthrusting of the Peninsular shield under the Uttarakhand Himalayan pile of the order of 20±3 mm/yr. This is confirmed by GPS geodesy, indicating that the Indian shield is moving towards Himalaya at the rate of 0-18 mm/yr. A large number of palaeoseismic events just south of the Himalayan Frontal Fault of mega magnitude occurred in BC 400 AD, 260, 800, 1294, 1423, 1500 and 1700 AD.
This article is based on works carried out by a large number of earthscientists, mainly in the last sixty years. The references can be seen in the author's recent book "The Making of India: Geodyanmic Evolution", Macmillan, Naida - 201 301, 2010, 816 p
15 Bhadra 2074 (Thursday)