RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES5002, doi:10.2205/2008ES000303, 2008

2. Geotectonic Model of North India Region

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Figure 1
[6]  Himalaya is the result of convergent movement of Indian plate moving to the northward and under thrusting to the south moving Eurasian plate (Figure 1). The result of this convergence has formed the arcuate mountain belt, stretching from Nanga Parbat in the west to the Namche Barwa in the east. The collision of these two plates has formed the widespread Tertiary transgressions and development of major thrusts and nappes, directed towards the converging plate (Indian subcontinent). It is the earth's largest and youngest mountain range comprising the rock types from the Palaeoproterozoic to the Quaternary age. Its northern margin is demarcated by the Indus-Tsangpo Suture zone (ITSZ) and the southern margin by the Indo-Gangetic plains. In north-south direction, the Himalaya can be divided geotectonically into four parallel zones by the four principal intracrustal thrust. The Himalayan Frontal thrust (HFT), the Main Boundary Thrust (MBT), the Main Central Thrust (MCT) and the ITSZ are the major tectonic discontinuities, which separate these zones from each other.

[7]  The HFT or MFT (Main Frontal Thrust) is the southern most, younger and neotectonically active thrust of the Himalayan region giving a topographical break of the Himalaya (the Outer Himalaya), against the alluvial Indo-Gangetic Plains. The Outer Himalaya comprises the low altitude Siwalik Hills with flat-floored structural valleys consisting of about 9500 m thick Cenozoic sedimentary pile characterized by folds and faults [Thakur, 1992]. The Indo-Gangetic plains of thick alluvial lie to the south of this discontinuity that is formed due to thick deposited sediments by the Indus, the Ganga and other Rivers. The Delhi-Hardwar ridge is the important structural feature in the Ganga basin which is tectonically and seismically active as compared to the rest of the basin.

[8]  The evolutionary model of Himalaya [Le Fort, 1975] states that MBT is the younger tectonic discontinuity as compare to MCT, which is more active currently. However, both the MCT and MBT has been treated as the contemporaneous features in the steady-state model of Seeber and Armbruster [1981] and these merge with each other at depth with a common detachment surface. The MBT is the tectonic boundary between the Lesser Himalaya nappes lying to its north and the Tertiaries of the Outer Himalaya foreland basin to the south. The majority of the earthquakes in the NW Himalaya are concentrated in the zone between MBT and MCT of shallow focal depth and great Himalayan earthquakes are originated at the surface of the detachment, which represents the upper surface of the underthrusting Indian plate with apparent northward dip of about 15o [Ni and Barazangi, 1984].

[9]  The MCT is the major tectonic discontinuity, which divide the two contrasting structures as Lesser and Higher Himalaya having contrasting stratigraphic and tectonic features. The relative movement of the blocks across this tectonic discontinuity has caused the development of crustal buckles in which the Palaeogene and Neogene sediments have deposited. At depth the MCT represents a ductile shear zone and tectonically it is a duplex shear zone having three distinct thrust planes as MCT I, MCT II and MCT III. According to the degree of metamorphism, lithostratigraphy and tectonic setting, Valdiya (1980) has named these thrusts as Chail (MCT I, lower thrust), Jutogh (MCT II, middle thrust) and Vaikrita (MCT III, upper thrust).

[10]  The ITSZ is the discontinuity along which the initial contact between the two plates occurred in the Early Eocene (60-70 my ago). The Tethys Sea, which was existed between two continents, vanished to the end of Eocene and after that the ITSZ converted to active plate boundary. The region between ITSZ and the MCT is composed of the Tethyan sedimentary zone in the north and the central crystalline zone in the south. The suture is made up of imbricated mélanges of flysch sediments, pillow lavas, volcanics and the basic and ultrabasic rocks, which are cut by steep faults [Allegre et al., 1984]. According to Burg et al., [1984], the Tethyan slab consists of several thrust sheets with distinct ages and deformations. Mainly normal faulting is existed in the high Himalaya and the normal fault zone separates the highly metamorphosed crystalline rock and a transitional zone from the overlying sedimentary sequence.


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Citation: Gitis, V., E. Yurkov, B. Arora, S. Chabak, N. Kumar, and P. Baidya (2008), Analysis of seismicity in North India, Russ. J. Earth Sci., 10, ES5002, doi:10.2205/2008ES000303.

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