Main Central Thrust

The Main Central Thrust is a major geological fault where the Indian Plate has pushed under the Eurasian Plate along the Himalaya. The fault slopes down to the north and is exposed on the surface in a NW-SE direction (strike). It is a thrust fault that continues along 2200 km of the Himalaya mountain belt.

The generally accepted definition of the Main Central Thrust is that it is a ductile shear zone along which the High-grade Great Himalayan Crystalline complex was placed above the low-grade to unmetamorphosed Lesser Himalayan Sequence. However, this definition is not perfect because of many difficulties and complications defining the Main Central Thrust.

Many geologists have researched the Main Central Thrust using various different criteria such as lithology, metamorphic isograd, geochronology, geochemistry, and strain magnitude. None of these are reliable if used independently. Furthermore, there is uncertainty because of the differences along-strike in the active ages of the Main Central Thrust. It was not all formed at the same time.

The Himalayan mountain belt was produced by the collision of the Indian Plate and the Eurasian Plate. It is structurally dominated by three north-dipping, fault-bound geological units stacked on each other. The major faults are South Tibetan Detachment, the Main Central Thrust, the Main Boundary Thrust and the Main Frontal Thrust. These units (figure 1), from south to north, are:

Knowledge of the kinematics of the Himalayan fault system is not as ideal as it has long been debated. To help understand the structural position the Main Central Thrust and role it played in the tectonic evolution of Himalaya, there are three general kinematic models: extrusion model, channel flow model, tectonic wedging model. for the fault system of Himalaya shown in shown in figure 2.

Although the general definition of the Main Central Thrust has been given, it is not enough due to the complication and difficulties in defining the Main Central Thrust.

For long, many researchers have defined the Main Central Thrust by different criteria, including by lithology that differs between the hanging wall and the footwall, by metamorphic grade changes from the hanging wall to the footwall, by the different Uranium-Lead (U-Pb) detrital zircon ages, by the different Neodymium isotope compositions, by different strain, etc. Some of these criteria have also been combined. However, none of these criteria are reliable if they are used by themselves. Meanwhile, these criteria are not all be satisfied together. The dominant problems are:

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