Structurally the Himalayas may be reg, as intermediate between the Alpine and Jura typeoj fold mountain structure. In the western Himi with wider and lower range, as in the Spiti region, 1 structure is simple Jura type (A. Gansser, 1964,1 253). But the Simla, Garhwal and Kumaun has have more complex and multiple nappe stru characterised by several thrusts or klippen and wit dowse highly constricted, metamorphosed root z injected with igneous masses and rock slices of nappes.
Another structural feature of the Himi distinguishing it from the other mountains of the world is a number of syntaxis or ‘elbow’ or ‘knee’ bends where the Indus, Jhelum and the Brahmaputra cross the mountain wall. In all these situations there is a succession of Pleistocene, Lesser Pliocene, Oligocene and Eocene beds extending mountain- ward from the plain-ward margin as parallel bends on either side of the syntaxine.
It is the western and central Himalayas which have been subjected to relatively detailed studies, i.e. Kashmir region by D.N. Wadia, Simla region by W.D. West and G.E. Pilgrim, Garhwal area by J.B. Auden, and Kumaun Himalayas by A. Heim and A. Gansser.
The Eastern Himalayas particularly eastof Bhutan are a sort of terra incognita. Besides the lack of enthusiasm by scholars the impassable difficult terrain and intensity of folding activity leading to recumbent folds and nappes formation make the job more difficult.
For the sake of convenience of visualising the structure of the Himalayas these may be conveniently divided into following four parallel zones.
(a) The Tibetan Zone-this is about 40 km wide and consists of fossil bearing marine sediments which are underlain 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 Himalayas.
(b) The Greater Himalayan Zone-this zone rises abruptly like a wall north of the Lesser Himalayas. It is about 25 km wide with average height above 5000 m.
It houses almost all the lofty peaks of the Himalayas along with extensive snowfields and glaciers. The zone is mainly composed of crystalline igneous or metamorphic rocks (granites, schist’s and gneisses) and is regarded as the root zone of the nappes.
At places due to heavy thrust older rocks are found overlying the newer rocks. It is more or less a continuous range characterised with sharp ridges and broad spurs. The range has very few gaps mainly provided by antecedent rivers. Erosion is also less effective over this range, mostly confined to river valleys and basins.
(c) The Lesser Himalayan Zone-it is about 80 km wide with average height between 1300 to 5000 m. It generally consists of unfossiliferous sediments or metamorphosed crystallines, constituting the main nappe zone in the Kashimr, Himachal and Garhwal sections.
The sequence of rocks is the oldest ancient pre-Cambrian or Cambrian crystallines lying at the top, occasionally as klippen, succeeded by Palaeozoic sediments exposed through structural windows. The main rocks are slate, limestones and quartzites.
Along the southern margin of this zone lies the autochthonous belt of highly compressed Upper Palaeozoic to Eocene rocks, often containing volcanics, which in contrast to the nappe zone have not undergone horizontal movement. Examples of autochthonous belts are found between Murree and Panjal thrusts in Kashmir, Giri and Krol thrusts in the Shimla region and Karol thrust and MBT in Garhwal region. This zone is subjected to excessive erosion due to heavy rainfall.
(d) The Sub-Himalayan Zone-this is an 8 to 45 km wide zone with average height below 1300 m. This is not a continuous range which is broader in the west but narrows down towards the east. The Siwalik Range forms the southern part of the zone and the intervening area is generally occupied by the tectonic longitudinal valleys called the Duns, viz., Dehra, Kotah, Patli, Kothri, Chumbi and Kyarda.
The zone largely consists of fossiliferous sedimentary rocks like sandstones, sand rocks, clay, conglomerate and limestones mostly belonging to the Upper Tertiary period.