Himalayas (3)
- 1. Amit K. Mishra Assistant Professor Department of Geology Banasthali University Himalayas An introduction
- 2. • Digital Elevation Model showing the Himalaya, Karakoram, and Tibetan Plateau with locations of the major peaks.
- 3. • The Himalaya, located on the southern fringe of the Tibetan Plateau, form a mountain arc (convex toward the south) about 2400 km long and 250-300 km wide; they are bounded by two structural bends on the northwest (Nanga Parbat, the Indus gorge) and northeast (Namcha Barwa, the Tsangpo gorge). The Himalayas are the loftiest and still rising mountains in the world. • Indeed, the Tibetan Plateau and its surrounding mountains form the largest topographic high on earth. All of the mountain summits taller than 7000 m are located in this part of our planet, and several large rivers, which rise in these highlands, provide fresh water for nearly three billion people in Asia (almost half of world population). • Sediments shed from the Himalaya have also formed extensive agricultural plains in northern India, Pakistan, Nepal, and Bangladesh which have supported civilizations for millennia.
- 4. • The Himalaya act as a topographic barrier to the summer-time warm monsoon winds coming from the Indian Ocean and the winter-time cold winds coming from Siberia, and have thus caused cold dry climate in Tibet but torrential rainfall on the valleys and plains to the south. • Rich diversity of flora and faunas extending from the almost sea-level plains upward the Higher Himalaya is also significant for ecology. The Himalaya offer splendid peaks and valleys for mountaineers, tourists and artists. Major earthquakes that occasionally visit the Himalayan region can indeed be devastating for people. • Last but not least, these mountains are a natural laboratory for geologists to study how mountains of similar type on Earth form and influence the environment. For all these reasons, scientific studies of the Himalaya are significant and necessary.
- 5. The Anatomy of the Himalaya • It is interesting that geologists sometimes employ biological terms to describe mountains; for example, ‘anatomy’ for tectonic framework and structure of mountains, or ‘evolution’ for the geologic history and formation of mountains. To understand how the Himalaya formed we need to consider its structure the types and ages of the rocks and the nature of faults that have fashioned these mountains.
- 6. • Geological mapping in various parts of the Himalaya over the past century has led the geologists to divide these mountains into six major longitudinal zones or northwest- northeast trending belts. Although local differences exist to some degree, the overall consistency of these zones along the entire length of the Himalaya is significant, and points to a single tectonic explanation for the mountain formation. The six zones of the Himalayans are described below from north to south.
- 8. • The term ‘Trans-Himalaya’ for a mountain range to the north of the Indus and Yarlung-Tsangpo rivers was first used by Alexander Cunningham in his book Ladak (1854), but it was Sven Hedin who popularized the name in his book Trans- Himalaya (1909-1912) which documents Hedin’s exploration of this region. • Geologically speaking, the Trans-Himalaya is made up of granitic and volcanic rocks formed from 110-40 million year ago (Ma). These igneous rocks intruded the metamorphic and sedimentary rocks of the southern Tibetan block. • The Trans-Himalaya is geographically divided into several areas: Kohistan (to the west of Nanga Parbat), Ladakh (between Nanga Parbat and the Karakoram strike-slip fault), Kailash, Gangdese, Lhasa (all in southern Tibet), and Mishimi (to the east of Namche Barwa).
- 10. • A ‘suture zone’ is a tectonic line along which two continental plates collide and join each other. • Prior to the collision, an ocean separates the two continents but it gradually shrinks as the ocean floor subducts beneath one of the continents. The collision thus occurs after the intervening sea completely subducts and disappears, and the moving continental plates crash into each other. • The Indus-Tsangpo Suture Zone marks the boundary between the Indian and Asian plates. It was so named by the Swiss geologist Augusto Gansser in his Geology of the Himalayas(1964) because the upper river valleys of the Indus and Tsangpo lie along this suture zone. Here, we find ocean- floor volcanic rocks, subduction-related high-pressure metamorphic rocks (greenschist rocks), and deep-sea sediments of the Tethys Ocean.
- 11. • This zone consists of Cambrian through Eocene sediments (sandstone, shale and limestone) which were deposited on the continental shelf of the Tethys Ocean. These sedimentary rocks contain many kinds of fossils including the Cretaceous-age ammonite fossils which are called ‘shaligram shila’ in Nepal and are used as amulets (dark stones devoted to Lord Shiva) by the Himalayan Hindus. • Since most of this zone lies within southern Tibet, it is also sometimes called the Tibetan Himalaya. • In northwest India, the Tethyan sediments are exposed in Kashmir, Zanskar, Chamba, and Spiti. With a width of about 100 km, the Tethys Himalaya is the widest zone of the Himalaya. • The stratigraphic thickness of sediments in this zone reaches up to 12 km. Within the Tethyan Himalaya, there is also a series of dome-shaped structures made up of Cambrian- age (about 500-450 Ma) granite gneiss or Miocene-age (18-9 Ma) white granites (leucogranites), collectively referred to as the North Himalayan Granite Gneiss Domes. These rocks and structural domes have been uplifted by a thrust fault called the North Himalayan Thrust, situated at their base. • Further south, the Tethys Himalaya is separated from the Higher Himalaya by a major normal fault called the South Tibetan Detachment, first mapped in the 1980s in southern Tibet and Zanskar.
- 12. • The Higher Himalaya, the backbone of the Himalayan mountains, is made up of 10-20 km-thick metamorphic rocks (schist and gneiss) and granites situated at altitudes of 3000 m to over 8000 m. • These rocks are Proterozoic-Cambrian age (2,000-500 Ma) and belong to the continental crust of the Indian plate. In addition we also find white granites (leucogranites) of Miocene-age (24-17 Ma) in the upper parts of the Higher Himalaya. • The South Tibetan Detachment constitutes the northern boundary of the Higher Himalaya where the highest summits of the Himalaya (over 7000 m) lie. • Along its southern boundary, the Higher Himalaya is thrust over the Lesser Himalaya along the Main Central Thrust, so named by Augusto Gansser who first mapped this structure in Kumaon (reported in Central Himalaya: Geological Observations of the Swiss Expedition, 1939).
- 13. • French geologists working in Nepal have also called this zone the Tibetan Slab because it forms the basement for the thick pile of the Tethyan (Tibetan) sedimentary rocks. Still other geologists also called it the Central Crystalline zone. ‘Crystalline’ refers to an igneous or metamorphic rock, in which minerals were crystallized in the rock itself; unlike a sedimentary rock which contains minerals derived from other rocks.
- 14. • To the south of the Higher Himalaya (the Main Central Thrust) lies the Lesser Himalaya which consists of metamorphosed sedimentary rocks (quartzite, marble, slate, phyllite, schist and gneiss) and minor volcanic and granitic rocks of Proterozoic-Cambrian age (2000-500 Ma). • The stratigraphic thickness of the rock sequence in this zone is 10- 20 km. In a plate-tectonic setting, the Lesser Himalaya, like the Tethyan Himalaya and the Higher Himalaya, belongs to the northern margin of the Indian plate. • Unlike the rich fossiliferous sediments of the Tethyan Himalaya, the sedimentary rocks in the Lesser Himalaya barely yield fossils partly because they are much older (dating back to a period when life forms were not abundant or diverse) and partly because they have been metamorphosed. Nevertheless, metamorphism in the Lesser Himalaya was less severe (under lower temperatures) compared to the Higher Himalayan metamorphic rocks.
- 15. • Within the Lesser Himalaya, geologists have also mapped transported, isolated parts of the Higher Himalayan (Central) crystalline rocks; these are called the ‘outer crystalline zone’ or the Higher Himalayan ‘klippe’. ( ‘Klippe’ is a German word which literally means ’cliff’). These rocks were transported on the back of the Main Central Thrust onto the Lesser Himalaya, but were later isolated from their root zone in the Higher Himalaya. The Almora klippe in Kumaun is a classic example.
- 16. • With elevations of 2000-3000 m, the Lesser Himalaya is home to many hilly resorts and forests. In Nepal, the Lesser Himalayan zone geographically corresponds to the Mahabharat Range. • The southern boundary of the Lesser Himalaya is the Main Boundary Thrust which has uplifted this zone atop the Sub- Himalayan (Siwalik) rocks. The Main Boundary Thrust was first mapped (and named) in 1864 by Henry B. Medlicott, a British geologist working for the Geological Survey of India.
- 18. • The Sub-Himalaya (or the Outer Himalaya) geographically corresponds to the Siwalik range (or the Churia range in Nepal) � foothills ranging in elevation from 250-800 m. • This zone is made up of 10-km thick succession of sandstone and mudstone shed from the Himalayan mountains, and carried and deposited by rivers, especially since the Miocene (over the past 24 million years). • These sediments have yielded abundant fossils of mammals (elephants, horses, etc.), who then lived in the Himalayan forests. (The Shivalik Fossil Park in Saketi, Himachal Pradesh houses examples of these fossils and recreates the ancient environment in which the animals lived.
- 19. • The Siwalik foothills were surveyed by Captain Proby Cautley of the East India Company in the 1840s-1850s, when he was constructing the Ganga Canal in the Doab region of north India. Cautley’s geologist colleague Hugh Falconer examined the rocks and fossils excavated from the hills, which he called ‘Siwalik’, after the pilgrimage site at Haridwar, dedicated to Lord Shiva.
- 20. • The Siwalik foothills were uplifted approximately one million years ago along high-angle reverse faults (still active), called the Himalayan Frontal Fault (or Thrust) and first mapped in India and Nepal by the Japanese geologist T. Nakata in 1972. The Himalayan Frontal Fault marks the boundary between the Siwalik Range and the north Indian plains. Similar faults have also been mapped in Pakistan by Robert Yeats in the 1980s. The Himalayan Frontal Fault is a series of ‘blind’ faults (the fault plane is not visible at the surface), and can be mapped from the topographic features they have created. These faults are often concealed under an apron of debris, called Bhabhar, washed down from the hillsides.
- 21. • As the Himalayan Frontal Fault began uplifting the Siwalik range, a series of river valley were formed within the Sub- Himalayan zone; these tectonic basins are called dun (doon), and some well-known examples include Dehra Dun in India and Chitwan in Nepal.
- 22. India, Gondwana and Tethys • About 500 million years ago, a supercontinent comprised by Australia, Antarctica, India, Africa, and South Africa, was assembled in the southern hemisphere. Geologists have called it Gondwana, after the Gond tribes in central India, where the sediments of this supercontinent were first studied in the late nineteenth century. • Within Gondwana, India was adjacent to Madagascar-Africa on the west and to Antarctica on the east. • The Paleo-Tethys Ocean bordered the northern margin of Gondwana. • During the Permian (275-260 Ma) a series of continental fragments consisting of Turkey, Iran and Tibet drifted away from Gondwana, thus opening the Neo-Tethys Ocean on their trails. • When a continent splits apart, a large volcanic eruption affects some part of it. The Permian-age Panjal Traps (basalts) exposed in the Pir Panjal Range of Kashmir are believed to be the volcanic outpouring of continental rifting between India and the Tibetan block.
- 23. • Gondwana largely remained intact until the late Jurassic when this supercontinent started to fragment, part of a long global process which eventually created the present-day continents and oceans. • India together with Madagascar was drifted away from Africa in the late Jurassic (about 165 Ma); afterwards, India was separated from East Antarctica at about 135 Ma and began its northward journey across the Neo-Tethys Ocean, at the same time as the Indian Ocean was opening behind India. • Around 85 Ma, a further split occurred between India and Madagascar. • The Cretaceous-Cenozoic boundary at 65 Ma marks an important event in Earth’s history. The dinosaurs, ammonites and many other species were wiped out probably because a huge meteorite struck our planet and played havoc with the environment. • At this time, India was passing over the Reunion hotspot which caused a large volume of volcanic rocks � the Deccan traps � to flood the central western India; this also caused the Seychelles- Mauritius fragment to separate from India.
- 25. • As India drifted northward, the Neo-Tethys ocean floor began to subduct beneath the southern margin of Asia (along the Karakoram and Tibet). This usually happens because the oceanic crust (mainly basalt) is heavier than the continental crust (mainly granite), and if pushed the oceanic crust subducts into an oceanic trench. The subducting slab then partially melts (dehydrates), and produces a large volume of granitic and volcanic rocks in the form of an island arc (Aleutian type) or a continental margin igneous belt (Andes type).
- 26. • In the northwestern part of the Himalaya, we have evidence of Cretaceous-age Kohistan-Ladakh island arc made up of volcanic rocks similar to those found today on the Aleutian Islands in the northern Pacific Ocean. • At about 85 Ma or so, the Kohistan-Ladakh island collided with the Karakoram margin as the oceanic floor between the two subducted and closed. Therefore, in the northwestern Himalaya there is a Northern Suture Zone between the Karakoram and the Kohistan-Ladakh blocks; this suture zone has also been called the Chalt Suture in Kohistan and the Shyok Suture in Ladakh, after the two valleys where the Chalt and Shyok volcanic rocks were mapped. • After the Northern Suture formed, the subduction-related igneous activity continued along an Andes-type continental margin from Kohistan and Ladakh all along the southern Tibetan block. The age of granites found in this Trans- Himalayan belt have been determined by the radiometric methods to range from 110 Ma through 40 Ma.
- 27. The India-Asia Collision • When did India collide with Asia? Several types of data appear to point to the early-middle Eocene period. • First, reconstruction of the speed of the Indian plate in the geologic past indicates that the plate motion drastically decreased in the Eocene. During Cretaceous and Paleocene times (85-55 Ma), India moved northward at rates of 15-20 cm per year but slowed down to only about 5 cm per year by 45 Ma. (These data come from examination of magnetic anomalies of rocks on the Indian Ocean floor and from paleomagnetic analyses of rock samples from the Trans-Himalaya by various scientists.) • Second, the youngest granites in the Trans-Himalaya, produced by the subduction of the Neo-Tethys Ocean floor beneath Asia, have been dated by radiometric methods to be about 40 Ma. This indicates the ending of subduction of Neo-Tethys sometime before 40 Ma. • Third, the sedimentary record in the Himalaya show that after the lower- middle Eocene (55- 45 Ma) the depositional environment changed from purely marine to continental.
- 28. • In short, data of different kinds have led many geologists to believe that at first the northwest corner of the Indian plate collided with Asia, probably at 55 Ma or even 60-65 Ma (as some studies suggest); then the Neo-Tethys ocean floor closed in an oblique manner and India moved in counterclockwise rotation until its northeastern corner docked with Asia at about 45 Ma. The Indus-Tsangpo Suture Zone marks the line of this plate collision.
- 29. • Block diagram illustrating the structure of the Himalaya. STD is the South Tibetan Detachment—the low angle normal fault that bounds the northern margin of the Greater Himalayan metamorphic rocks and granites (magenta color); MCT is the Main Central thrust that places the metamorphic rocks south over un-metamorphosed rocks of the Lesser Himalaya. MBT is the Main Boundary thrust, the active southern margin of the Himalaya and the fault zone along which earthquakes are triggered.
- 30. The Evolution of the Himalaya • The Himalaya are products of 55 million years of tectonic compression and structural deformation. • Examination of geochronologic, structural, petrologic and sedimentary data from various parts of the Himalaya indicates that at least five major episodes or phases were milestones in the geologic history of these mountains. • Here a question may arise: While the motion of the Indian plate has been steady and constant (about 5 cm per year) over the past 45 million years, how is it possible that the uplift of the Himalaya has not been at a uniform rate but in distinct episodes? To answer this, consider an earthquake prone area undergoing tectonic stress; the stress builds up gradually over time, and then the area ruptures along a fault which releases the stress as it surpasses the strength of the rock. On a larger scale too, it takes time for major tectonic events to occur.
- 34. • Even before the India-Asia continental collision, tectonic deformation was taking place in the leading margin of the Indian plate, near the Neo-Tethys subduction zone and the continental margin of southern Tibet. However, the Himalayan drama began after the collision.
- 35. • It is conceivable that the first mountain range to emerge was the Trans-Himalayan igneous arc, and that the Indus and Tsangpo were the first rivers to arise and transport sediments in the Eocene period. Geochronologic analyses of granites from the Trans-Himalaya indicate that these rocks rapidly cooled and eroded at about 40 Ma. And, there are basins in the southern front of the Trans-Himalaya filled with continental sediments (for example, in Kargil, Kailash and Lhasa). • Mapping in southern Tibet by An Yin reveals the existence of a south-directed thrust fault which he has called the Gangdese Thrust, probably part of a deep-seated structure responsible for the uplift of the Trans-Himalaya. (Along most of the Indus-Tsangpo Suture, geologists, as early as Ausgsot Gansser in 1930s, have mapped northdirected backthrusts, which are probably later and shallower structures compared to the Gangdese Thrust.)
- 36. • Partly concurrent with and partly after the uplift of the Trans- Himalayan range, tectonic deformation also affected the Tethyan Himalaya. Interestingly, the Tethyan sediments in most of the Himalaya have escaped metamorphism (they are still sedimentary rocks) but they are highly folded and faulted. The emergence from the sea and folding of these sediments probably occurred shortly after the India- Asia collision and continued for millions of years. The North Himalayan Thrust and the uplift of the granite gneiss domes within the Tethyan Himalaya began during this phase (40-35 Ma). The Eocene-age tectonic deformation in the Himalayan region may be called the ‘Eo- (early) Himalayan’ episode.
- 37. • Since the India-Asia collision occurred in a diachronous (variable time), oblique manner, both the Trans-Himalayan and Eo-Himalayan events first began in the northwestern Himalaya (in Pakistan and Ladakh) around 55-45 Ma, and then affected the eastern parts of the Himalaya at 35 Ma. • The fact that the Trans-Himalaya and the Tethyan Himalaya were uplifted prior to the Higher Himalaya is evident from the ‘antecedent’ (flowing before) pattern of many of the Himalaya river (the Indus, Satluj, Ganga, Kali Gandaki, Arun and Brahmaputra); these river courses rise behind the Higher Himalaya (predating its uplift) and cut deep gorges through it to flow southward.
- 38. • There is ample evidence that during the early Miocene (24-17 Ma), the Higher Himalayan rocks, which were then buried 20-25 km deep and metamorphosed and partially melted under temperatures of 600-800 degrees Celsius and pressures of 6-10 kilobars, began a rapid uplift along the Main Central Thrust. • Concurrent with the activity of this fault, the Tethyan Himalayan zone was detached from the Higher Himalaya along a normal fault, named the South Tibetan Detachment. This fault, which runs parallel to the Main Central Thrust and forms the northern boundary of the Higher Himalaya, caused tectonic extension (spreading), gravitational gliding and back- folding of sediments in the Tethyan Himalaya (the down-thrown block), while uplifting and exhuming the Higher Himalayan rocks (the upthrown block). • Moreover, a series of white granites (leucogranites) formed during 24-17 Ma were emplaced and rapidly uplifted along the South Tibetan Detachment. These granites constitute the core or the summit of the highest peaks in the Himalaya including Manaslu and Annapurna in Nepal and Badrinath and Shivling in Kumaun. It is interesting that these granites are located along the upthrown edge of the South Tibetan Detachment, indicating that their formation (possibly by decompressional melting of deeply buried rocks) and exhumation to the surface were related to the activity of this fault.
- 39. • Along the Main Central Thrust we do not find any Miocene-age granites. Nevertheless, the Main Central Thrust emerged as a major structure across which the topography of the Himalaya markedly increases from south to north. The slip on the Main Central Thrust is more than 150 km as this thrust has transported the Higher Himalayan metamorphic rocks onto the Lesser Himalaya (where we find them as klippe outliers), some 150 km distance their root zone. • With the uplift of the Higher Himalaya beginning in the early Miocene, increasing amounts of sands were deposited not only in the Siwalik and Indus-Ganga basins but were also transported by rivers to the Arabian Sea (forming the submarine Indus Fan) and the Bay of Bengal (the Bengal Fan).
- 40. • During the late Miocene (11-7 Ma), sedimentation rates in the Siwalik basin drastically increased, and the mineral kyanite which, in the Himalayan region is found only in the Higher Himalayan metamorphic rocks (to the north of the Main Central Thrust), first appeared in the Siwalik sediments. These data indicate rapid erosion of the Himalaya at that time. There is also limited geochronologic evidence from Pakistan that the Main Boundary Thrust began its activity in the late Miocene. • Geochemical analyses of samples from the Indian Ocean drilled cores and from the Siwalik ancient-soils indicate that the Monsoon seasonality related to the Himalayan topographic barrier also began in the late Miocene, indicating that by 7 Ma, the Himalaya had attained sufficient heights.
- 41. • Some geologists including Augusto Gansser and K. S. Valdiya have highlighted the significance of the Himalayan uplift over the past 2.5 million years or so, while certain geologists who confine their research to the Miocene geochronology of the Himalaya have ignored or downplayed the Quaternary events as if the maximum topography of the Himalaya was reached in the Miocene and since then only erosion has shaped these mountains. • Considering various lines of evidence I consider the Quaternary period as an important renewed phase of tectonic uplift in these mountains. The uplift of the Siwalik range along the Himalayan Frontal Fault over the past one million years, geochronologic evidence (fission-track ages of 1-3 Ma) of rapid erosion from various parts of the Higher Himalaya (Nanga Parbat, Zanskar, Gangotri, Khumbu Himal, Namche Barwa, etc.), presence of active faults and large earthquakes, deep and narrow gorges in the Higher Himalaya, uplifted terraces of young sediments, and enormous amounts of coarse-grained fluvial sediments in the Quaternary record of the Himalayan foreland basins all testify to the intense neotectonics in this region.
- 42. The Himalaya’s Adventure Continues • Geophysical surveys by the seismic (sound wave) reflection and gravity methods indicate that the crustal thickness beneath the Himalaya is about 70 km, which is twice that of a normal continental crust (for example, the Indian Peninsula). In other words, the Himalaya are higher because these mountains are sitting on an over-thickened continental crust, produced by compression, thrusting, and folding. • Geologists believe that all major faults in the Himalaya, including the Normal Himalayan Thrust, the South Tibetan Detachment, the Main Central Thrust, the Main Boundary Thrust, and the Himalayan Frontal Fault, are all joined at depth to a major shallow-dipping structure, called the Main Himalayan Thrust or Decollement (a French word meaning Detachment). • This structural model together with the available geologic data discussed in this article implies that as the Indian plate has pushed northward, thrust faults have developed sequentially from north to south and that as a new thrust fault has ruptured, the tectonic push has added some elevation to the entire Himalaya. In other words, the present elevation of the Himalaya is a cumulative effect of 55 million years of continental collision, a magnificent drama of episodes which geologists have painstakingly pieced together and are beginning to make sense of it. • A period of 55 million years or so since the India-Asia collision may seem a long time from human perspective, but by Earth’s standards, the Himalaya are still young, rising mountains.