In this article, we will discuss Physiographic Divisions of India (The Himalayan Region). So, let’s get started.
Physiographic Divisions of India
Physiography deals with the study of the surface features and landforms of the Earth. On the basis of tectonic history, stratigraphy, and physiography, India may be divided into the following four physiographic divisions:
1. The elevated Peninsular region;
2. The mighty Himalaya and their associated young folded mountains,
3. The Indo-Gangetic-Brahmaputra Plains;
4. The Deserts.
5. The Coastal Plains and Islands
The Himalayan Region
The Himalaya consist of four lithotectonic mountain ranges, namely (i) the Trans-Himalaya or the Tethys Himalaya, (ii) the Greater Himalaya, (iii) the Lesser Himalaya, and (iv) the Shiwalik or the Outer Himalaya. The Indian Himalaya extend from the eastern boundary of Pakistan to the border of Myanmar for about 2500 km with a varying width of about 500 km in the west and about 320 km in the east. They lies to the north of the Ganga-Brahmaputra Plains and are separated from the plains by the Himalayan Front Fault (HFF). They include parts of Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Nepal, Sikkim, Bhutan and Arunachal Pradesh. Their offshoots run in a north-south direction along the India-Myanmar boundary through Nagaland. Manipur, and Mizoram.
Origin of the Himalaya
The origin of the Himalaya has been a point of contention among the geologists and geomorphologists. It is a complex mountain system having rocks from the Precambrian and Eocene periods. Mostly forned of sedimentary and metamorphic rocks, it has been subjected to intense folding and faulting, The main theories about the origin of the Himalays are as under.
(i) The Geosynclinal Origin
The main supporters of the geosynclinal origin of the Himalaya are Argand, Kober, and Suess. According to these geologists, the disintegration of Pangaea, about 200 million years back, led to the formation of a long Tethys Sea between the Lauratian Shield (Angaraland) of the north and the Gondwanaland of the south. This sea was occupying the region of Himalaya during the Mesozoic Era (180 million years ago). At the end of the Palaeozoic and beginning of the Mesozoic Eras, the Tethys almost girdled the whole Earth running from Europe in the west to China In the east. Eroded material from the two land masses (Eurasian Shield-Angaraland and Gondwanaland) was deposited in the Tethys Sea and assumed considerable thickness due to the sinking nature of the seabed. During the Cretaceous Period, the bed of the sea started rising which led to the folding of three successive ranges of the Himalaya. The first upheavalled to the formation of the Greater Himalaya during the Eocene Period (about 65 million years back). Similarly, the second upheaval took place during the Miocene Period (about 45 million years back) resulting in the formation of the Lesser Himalaya, and the third upheaval started in the Pliocene period (about 1.4 million years back) resulting in the formation of the Shiwaliks or the Outer Himalaya.
(ii) The Plate Tectonic Origin of the Himalaya
The theory of Plate Tectonics was put forward by WJ. Morgan of Princeton University in 1967. This theory is based on the concept of “Sea-Floor Spreading” advocated by H.H Hess. According to this theory, about 70 or 65 million years ago there was an extensive geosyncline, called the Tethys, in place of the Himalaya. About 65–30 million years ago the Indian plate came very close to the Asian plate and started subducting under the Asian plate. This caused lateral
compression due to which the sediments of the Tethys were squeezed and folded into three parallel ranges of the Himalaya. It has been estimated that this convergence has caused a crustal shortening of about 500km in the Himalayan region and is compensated by sea floor spreading along the oceanic ridge in the Indian ocean region. Since the northward movement of the Indian plate is still continuing, the height of the Himalayan peaks is increasing. The Indian Plate is moving northward and the center of rotation is constantly changing. The northward drift of the Indian Plate and the subcontinent of India have been shown in.
The continent-to-continent collision between the Indian and the Asiatic plates started around. 65 million years ago and caused the Himalaya to rise from the Tethys geosyncline. Thus, the first major phase of uplift in the Himalaya occurred around 65 million years ago. This orogenic movement elevated the central axis of ancient crystalline and meta-sedimentary rocks which have been intruded by large masses of granite. It is believed that the first major phase of uplift initially produced the Ladakh and Zaskar ranges of the Trans-Himalaya before the formation of the Great Himalaya. Hence, it is to be realized that except the Kashmir part of the Himalaya, the Himalayan ranges have not developed from a geosyncline and are made up of elements formerly connected to the marginal parts of the Indian shield. During the main Himalayan orogeny, this continuous geosynclinal sedimentation led to the underthrusting of the Indian shield against the Tibetan Massif which buckled down the geosynclinal deposits, resulting in the outflow of a large amount of ultrabasic rocks known as ophiolites. These ophiolites are seen as exotic blocks on the Ladakh and Zaskar Ranges of the Trans-Himalaya. The end effect of the buckling of the geosyncline was not only the crustaline thrust effect on Ladakh and Zaskar leading to their rise as ranges, but also the creation of the sharp tectonic line of the Indus suture along which large geosynclinal areas disappeared. The intermontane basins in the Indus suture zone of Ladakh continued to receive molasses sedimentation in this period. The second major uplift which took place around 45 million years ago caused the rapid uplift of the southern mountain front of the Lesser Himalaya, giving rise to the extremely rugged and youthful Pir-Panjal, Dhauladhar, Karol, and Mahabharat Ranges abruptly and steeply. The Greater Himalaya and the Lesser Himalaya are separated by the Main Central Thrust (MCT). These spurs of the Lesser Himalaya again formed in their turn, the intermontane basins of Kashmir, the Karol-basin, Dun Valley (Uttarakhand), and the Kathmandu Valley of Nepal. The foredeep which was formed further away received the thick sequence of terrestrial sediments called Shiwaliks from the middle Miocene to the middle-Pleistocene periods, covering a span of about 14 million years. The Lesser Himalaya and the Shiwaliks are separated from each other by the Main Boundary Thrust (MBT). The 5000 m thick Shiwaliks dominated by boulder and conglomerate, reflect the progressive uplift of the Himalaya from which they have been derived as a result of the third major phase of uplift. The Shiwaliks are separated from the Northern Plains of India by the Himalayan Front Fault or HFF.
The Shiwaliks form the normal jura type of strutures with wider basin-like synclines alternating with steep.,often faulted, asymmetric anticlines . At present, the Himalayan Front Fault (HFF) is quite active recording frequent tremors and earthquakes.
Physiographic Division of the Himalaya
For a systematic study of the physiography and relief, the Himalaya may be divided into the following four divisions from north to south:
The Trans-Himalaya
The Greater Himalaya
The Lesser Himalaya
The Shiwaliks or the Outer Himalaya.
The Trans-Himalaya
The Trans-Himalaya are about 40 km wide. They contain the Tethys sediments. The rocks of this region contain fossils bearing marine sediments which are underline by “Tertiary granite”. It has partly metamorphosed sediments and constitutes the core of the Himalayan axis. It has a great accumulation of debris in the valleys of defeated streams which could not maintain their southerly course across the rising barrier of the Himalaya.
The Greater Himalaya
The Greater Himalaya rise abruptly like a wall north of the Lesser Himalaya. The MCT separates
the Greater Himalaya trom the Lesser iimalaya. The Greater Himalaya are about 25 km wide
with an average height above 6l00 m (Wadia, D.N.J. Almost all the lofty peaks of the Himalaya,
Mt. Everest, Kanchanjunga, Nanga-Parbat, Gasherbrum, Manasu, Dhaulagiri, Annapurma,
Gosainthan, Cho-Cyu, Nanda-Devi, Kamet, Badrinath, Nanda Devi, etc, lie in this zone. The
Greater Himalaya are composed of crystalline, igneous or metamorphic rocks (granite, schists,
and geneiss). The basal complex of the Himalaya is Archaean. At places, due to heavy thrust, older rocks are found overlying the newer rocks. The Greater Himalaya are almost a contiguous range. The range has very tew gaps mainly provided by the antecedent riverS. The Greater Himalaya receive less rainfall as compared to the Lesser Himalaya and the Shiwaliks. Physical weathering is pronounced. Erosion is, however, less effective over the Greater Himalaya as compared to the Lesser Himalaya. Being lotty, they have very little forest area.
The Lesser Himalaya
The width of the Lesser Himalaya is about 80 km with an average height of 1300-4600m.
It consists, generally, of unfossiliferous sediments or metamorphosed crystalline. The main
rocks are slate, limestone and quartzites. Along the southern margin ot the Lesser Himalaya
lies the autochthonous belt of highly compressed Upper Palaeozoic to Eocene rocks, often
containing volcanic material. Examples of autochthonous belts are found between Murree
and Panjal thrust in Kashmir, Giri thrusts in the Shimla region and Krol and MBT in Garhwal
region. This region is subjected to extensive erosion due to heavy rainfall, deforestation, and
urbanisation.
The Shiwaliks or Outer Himalaya/Sub-Himalaya
The Shiwaliks extend from Jammu Division of Jammu and Kashmir territory to Assam. In width,
ShiwalikS vary from 8 km in the east to 45 km in the west with an average elevation of about
900-1500 m above sea level., It is not a continuous range. It is broader in the west and narrows
down in the east. Between the Shiwaliks and the Lesser Himalaya are longitudinal valleys called
Doons/Duns. Some of the important Duns are Dehra Dun, Poti, Kothri, Kathmandu, Chumbi, and Kyarda. The Shiwaliks are mainly composed of sandstones, sand-rocks, clay, conglomerates and
limestones, mostly belonging to the Upper Tertiary Period.
